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Release and Transformation of Inorganic Elements in Combustion of a High-Phosphorus Fuel
Abstract
The release and transformation of inorganic elements during grate-firing of bran was studied via experiments in a laboratory-scale reactor, analysis of fly ash from a grate-fired plant, and equilibrium modeling. It was found that K, P, S, and to a lesser extent Cl and Na were released to the gas phase during bran combustion. Laboratory-scale experiments showed that S was almost fully vaporized during pyrolysis below 700 °C. Sixty to seventy percent of the K and P in bran was released during combustion, in the temperature range 900–1100 °C. The release of K and P was presumably attributed to the vaporization of KPO3 generated from thermal decomposition of inositol phosphates, which were considered to be a major source of P and K in bran. The influence of additives such as CaCO3, Ca(OH)2, and kaolinite on the release was also investigated. Ca-based additives generally increased the molar ratio of the released K/P, whereas kaolinite showed an opposite effect. Thermodynamic modeling indicated that the fly ash chemistry was sensitive to the molar ratio of the released K/P. When the molar ratio of the released K/P was below 1, KPO3 and P4O10(g) were the main stable K and P species at temperatures higher than 500 °C. Below 500 °C, the KPO3 and P4O10 (g) may be converted to H3PO4(l), which may cause severe deposit build-up in the economizers of a grate-fired boiler. By increasing the molar ratio of the released K/P to above 2, the equilibrium distribution of the K and P species was significantly changed and the formation of H3PO4(l) was not predicted by thermodynamic modeling.
... Detailed studies on the fate of inorganic constituents during biomass pyrolysis and combustion can be found in the literature [48][49][50][51][52][53]. It is well understood that inorganics can substantially influence combustion reactions [23], lead to undesired emissions and cause corrosion of downstream equipment [22]. ...
... For P, the lab-scale experiments showed minimal release, which is similar to the results found for the thermal decomposition of bran under combustion conditions up to 900 • C [52]. In contrast, in two of the farm-scale processes, FA-BS and FI-CP, only 42.7-71.0% of P could be accounted for in the biochar. ...
... For a large fraction of the inorganic constituents, there is no clear trend visible, when comparing the present results with previous investigations. Although the release of K and Mg has been reported at well below <10% for bran and wheat straw under grate-fired combustion conditions up to 700 • C [52,54], for grass the release is generally >10% for both compounds at temperatures >450 • C. When looking at the autothermal farm-scale experiment (FA-BS), both retentions of K and Mg of 32% and 39%, respectively, appear comparable to grate combustion at temperatures ≥1100 • C [52,54]. Such high temperatures were not reached in FA-BS, suggesting that higher retention of K and Mg may be possible with optimized process design. ...
Grass and other herbaceous biomass are abundant, but often under-or not utilized as a renewable resource. Here, the production of biochar from extensive late-harvest grass via multiple thermochemical conversion technologies was investigated at lab and farm scale for use in soil applications. While biochar is a product with highly diverse potential applications, it has a multitude of benefits for agricultural usage as a soil amendment, if the quality adheres to certain limit values of potentially toxic constituents. The results show that the biochar can adhere to all limit values of the European Biochar Certificate (EBC) for utilization in agriculture. Generally, the contents of heavy metals were well below the proposed EBC limits and very low PAH concentrations in the biochar were achieved. The high ash content in the grass of 7.71 wt% db resulted in high nutrient concentrations in the biochar, of benefit in soil applications, but the ash also contains chlorine, nitrogen and sulphur, which presents a challenge for the operation of the thermochemical processes themselves due to corrosion and emission limits. In the farm-scale processes, ash retention ranged from 53.7 wt% db for an autothermal batch process, reaching up to 93.7 wt% db for a batch allothermal process. The release of Cl, N and S was found to differ substantially between processes. Retention ranged from 41.7%, 22.9% and 27.6%, respectively, in a continuous allothermal farm-scale pyrolysis process, to 71.7%, 49.7% and 73.9%, with controlled lab-scale pyrolysis at 450 • C, demonstrating that process optimization may be possible.
... [15][16][17] As a distinctive feature, agricultural biomass typically has higher contents of P, K, and Si than traditional commonly used woody biomasses, which could have undesirable influences on ash transformation pathways. 14, [18][19][20] In several studies, it has been shown that using P-rich agricultural fuels in thermal conversion units can cause severe ash-related problems such as deposits and slag formation 10, 21 , corrosion 22 , and particulate matter emissions 23 . However, the ash transformation reactions occurring in thermal conversion of P-and K-rich agricultural biomass assortments specifically, are not fully understood 14 and rarely described in the literature. ...
... The release of P during the thermal conversion of biomass is strongly dependent on the fuel composition and temperature. 18,34,40,41 The previous studies 34, 40,41 have suggested that the most important elements that influence P release and retention are: (1) K, that can produce volatile and melted K-phosphates, (2) Ca, that can form nonvolatile phosphate phases, and (3) Si, that can, together with reducing gas or carbon, decrease the required temperature and conditions for producing volatile P. ...
... According to the previous studies, the P found in biomass and sludge needs relatively high temperature to be released to the gas phase. As an example, previous results have shown that the P release occurs at temperature above 900 °C for bran 18 and algae 34 or even at higher temperatures (> 1100 °C) for sludge 41 . ...
The utilization of different biomass feedstocks in thermal conversion
systems can contribute towards mitigation of global warming. However,
the formation of different ash fractions (i.e., bottom ash, and fly ash)
during thermal conversion of biomass can cause several ash-related
problems such as deposit formation, slagging, and particle emissions, all of
which may limit its usage as an energy source. It has been found that
phosphorus (P), even in relatively low concentrations, can play a vital role
in the abovementioned ash-related problems. However, the ash
transformation reactions occurring in the thermal conversion of P-bearing
biomass assortments are not fully understood and rarely described in the
literature. Therefore, an understanding of the phenomena associated with
ash transformations with a special focus on P is crucial.
The overall objective was to determine the ash transformation and release
of P during single-pellet thermochemical conversion of different types of
agricultural and forest fuels in the low to medium temperature range (600-
950 °C). Different agricultural biomasses (poplar, wheat straw, grass, and
wheat grain residues), as well as forest residues (bark, twigs, and a mixture
of bark and twigs) were used. The bark and poplar fuels represent a fuel
rich in K and Ca with minor P contents. The wheat straw, grass, and
twigs represent a typical Si- and K-rich fuel with minor and moderate P
contents. The wheat grain residues represent a typical K- and P-rich fuel
with a considerable amount of Mg. The produced residual materials, i.e.
chars and ashes, were characterized by SEM-EDS, XRD, and ICP-OES.
The experimental results were interpreted with support from
thermodynamic equilibrium calculations (TECs).
The overall findings are that the majority of P (>80%) in all the studied
fuels remained in the final condensed residues, and that the main fraction
of P release occurred during the devolatilization stage. The chemical form
of P in the residues is strongly dependent on the relative concentrations
of other major ash-forming elements such as K, Ca, and Si, as well as the
type of association of P in the pure fuel. For woody-based fuels rich in
ii
Ca and K (poplar, bark, and twigs in this study), P in the ash is generally
found in the form of crystalline hydroxyapatite. For herbaceous fuels rich
in Si and K (wheat straw and grass), P in the ash is generally found in
Ca5(PO4)3OH, Ca15(PO4)2(SiO4), KCaPO4, and K-Ca/Mg
phosphosilicate melts. For wheat grain residues rich in P, K, and Mg, P
in the ash is found in crystalline forms K4Mg4(P2O7)3, K2MgP2O7,
K2CaP2O7, and KMgPO4, as well as amorphous K-Mg/Ca phosphates.
The obtained new knowledge can be used to find practical measures to
mitigate ash-related problems during thermochemical conversion of Pbearing
biomass fuels. It can also be used to find optimal pyrolysis process
conditions to obtain biochar suitable as alternative fuels and reducing
agents in the metallurgical industry.
... of P in fine ash particles can be observed in previous works [31,32]. During the combustion of bran, Wu et al. [31] found that 60-70% of K and P were released to gas phase and subsequently transformed into fine ash particles. ...
... of P in fine ash particles can be observed in previous works [31,32]. During the combustion of bran, Wu et al. [31] found that 60-70% of K and P were released to gas phase and subsequently transformed into fine ash particles. Liaw et al. [32] proposed that phosphorus was presented in PM 1 in the form of either NaPO 3 /KPO 3 or P 4 O 10 during the combustion of a P-rich fuel. ...
... (7). Ref. [31] also illustrate that powder KCl and Ca 3 (PO 4 ) 2 reacting to form Ca 10 K(PO 4 ) 7 , as listed in Eq. (6), which is consistent with the XRD results. ...
The emission of fine particulate matters with an aerodynamic diameter of less than 1 µm (PM1) is usually high from straw biomass combustion, resulting in great danger to atmospheric environment and public health. In this work, the effect of three calcium phosphate additives on PM1 emission from cornstalk combustion was investigated using a lab-scale reactor. The addition of Ca(H2PO4)2, CaHPO4 and Ca3(PO4)2 reduced PM1 emission by 1.5–50.6%, 22–55.6% and 23–53.7%, respectively. For Ca(H2PO4)2, PM1 reduction rate reached its maximum values of 50.6% at P/K molar ratio equal to 1 and then decreased significantly with further increasing of P/K molar ratio. For both CaHPO4 and Ca3(PO4)2, PM1 reduction rate increased approximately linearly with increasing the amount of additives under the current operating conditions. Analyses of the collected particulate matters and residual ashes indicated that phosphorus was mainly transformed into PM1-10 and residual ash in the form of K-Ca/Mg phosphates and Ca/Mg phosphates, respectively. The PM1 reduction mechanism was proposed based on the characterization results. Finally, economic analysis showed that the addition of Ca3(PO4)2 is a potentially promising method to reduce PM1 emissions during straw biomass combustion.
... Wu et al. [41] suggested that during bran combustion K, Mg and P were presumed to be present as phytic acid or as other inositol phosphates, and when heated, KPO 3 was likely formed by the thermal degradation process and K, as well as P, evaporated presumably as KPO 3 when the temperature was between 900 and 1000°C. In the same work, simultaneous thermal analysis on KH 2 PO 4 showed a mass loss corresponding to the formation of KPO 3 at temperatures between 200 and 400°C. ...
... • KH 2 PO 4 melts, decomposes with H 2 O leaving, and forms KPO 3 at temperatures below 400°C [23,41,48,49]. The melting point of KPO 3 is 800-810°C [23,41,48,49] and it has been suggested that KPO 3 is probably present in a polymer form with varying chain lengths that can influence the melting temperature [48]. ...
... • KH 2 PO 4 melts, decomposes with H 2 O leaving, and forms KPO 3 at temperatures below 400°C [23,41,48,49]. The melting point of KPO 3 is 800-810°C [23,41,48,49] and it has been suggested that KPO 3 is probably present in a polymer form with varying chain lengths that can influence the melting temperature [48]. Evaporation starts at around 1000°C [41]. ...
Phosphorus has been observed to cause agglomeration during fluidized bed combustion of phosphorus-rich biomass. Phosphorus can react with alkali during combustion, forming low melting phosphates that glue the bed particles together, or form phosphates that react with the bed material. In this work, seven different phosphate compounds (KH2PO4, NaH2PO4, CaHPO4, NH4H2PO4, K3PO4, Na3PO4, Ca3(PO4)2) were tested. Experiments were performed in an electrically heated laboratory-scale fluidized bed reactor at 750, 800, 850 and 900 °C and the feeding of the phosphates continued until defluidization occurred or until 10 g of a phosphate-compound had been added. Defluidization was detected by a sudden decrease in the pressure drop over the bed and a simultaneous increase in the temperature difference between the upper and lower bed. Samples of the bed material were taken after each test and cast in epoxy, ground and polished to get a cross-section and then analyzed with SEM/EDX. KH2PO4, NaH2PO4, and NH4H2PO4 were found to cause defluidization by melt formation whereas K3PO4 and Na3PO4 caused defluidization via reaction with the quartz bed material forming alkali silicate.
... The release behavior of P during the thermal process is strongly dependent on the fuel composition and temperature [96,[105][106][107]. It seems that the most effective elements on P release and retention are K to produce volatile melted K-phosphate, Ca to form nonvolatile phosphoric mineral, and Si in the form of SiO 2 together with reducing gas or carbon for decreasing the required temperature for the reaction that produces volatile P [96,106,107]. ...
... The release behavior of P during the thermal process is strongly dependent on the fuel composition and temperature [96,[105][106][107]. It seems that the most effective elements on P release and retention are K to produce volatile melted K-phosphate, Ca to form nonvolatile phosphoric mineral, and Si in the form of SiO 2 together with reducing gas or carbon for decreasing the required temperature for the reaction that produces volatile P [96,106,107]. The P release takes place at high temperature (> 900°C) for bran and algae or even at higher temperatures (> 1100°C) for sludge. ...
... The K release during pyrolysis of biomass is strongly dependent on the chemical composition of biomass, especially presence and abundance of Cl, Si, and S in the biomass. According to the literature, it is expected that more than 80 wt% of K in the original biomass will remain in the slow pyrolyzed charcoal [96,100,[107][108][109]. ...
This paper provides a fundamental and critical review of biomass application as a reducing agent and fuel in integrated steelmaking. The basis for the review is derived from the current process and product quality requirements that also biomass-derived fuels should fulfill. The availability and characteristics of different sources of biomass are discussed and suitable pretreatment technologies for their upgrading are evaluated. The existing literature concerning biomass application in bio-coke making, blast furnace injection, iron ore sintering and production of carbon composite agglomerates is reviewed and research gaps filled by providing insights and recommendations to the unresolved challenges. Several possibilities to integrate the production of biomass-based reducing agents with existing industrial infrastructures to lower the cost and increase the total efficiency are given. A comparison of technical challenges and CO2 emission reduction potential between biomass-based steelmaking and other emerging technologies to produce low-CO2 steel is made.
... Phosphorous can react with the alkali metals to form molten alkali phosphates (e.g. KPO 3 ) [124] which volatilise at low temperatures [125] and contribute to fouling and ash deposition [126]. However, P can also react with the alkali metals to form stable, high-temperature melting, alkaline earth metal-rich, alkali phosphates (e.g. ...
... Studies which address the release of P are limited, since the concentration of P in coal and most commonly used terrestrial biomass fuels is generally low. Wu et al. [125] measured the release of P and K from bran during pyrolysis and combustion in a laboratory-scale, fixed-bed reactor. The majority (60 -70%) of the total P was released by 1100 °C during combustion, whereas less than 10% of the total P was released during pyrolysis at the same temperature. ...
... The release of K was proportional to that of P under both pyrolysis and combustion conditions, within the temperature interval 700 -1100 °C. Based on this result, Wu et al. [125] inferred that P is released by vaporisation of molten KPO 3 . Bourgel et al. [140] modelled the release of P during the gasification of sewage sludge by means of thermodynamic equilibrium calculations. ...
Algal biomass has gained recent interest as an energy source due to diminishing reserves of fossil fuels and growing pressure to reduce emissions of greenhouse gases. The most developed technologies for converting biomass to energy and fuels are based on thermochemical processes, particularly pyrolysis, combustion, and gasification. However, little is known about the behaviour of micro- and macroalgae in these processes. The aim of this thesis was to characterise the thermochemical fuel behaviour of micro- and macroalgal biomass, with emphasis on the following areas of fuel particle conversion: char reactivity; oxidation of carbon; conversion of fuel-N; occurrence of the inorganic elements; bed agglomeration; release of volatile inorganic elements; and mobilisation of trace elements. A range of laboratory-scale experiments was carried out in order to address each of these areas.
Char reactivity was characterised by gasifying four samples of algae in a thermobalance in pure CO2 at 850 °C, following in situ drying and devolatilisation of the algal samples. The reactivity of the chars varied for different species of algae and for different cultivation regimes. The oxidation of carbon and conversion of fuel-N to NO were studied by monitoring the concentrations of gas phase species released during fixed-bed combustion. Conversions of C to CO and CO2 exceeded 84% for all of the tested algae. In most cases, a greater proportion of the total C was released during devolatilisation rather than during char oxidation, which is consistent with the high volatile matter contents reported in the literature for algal biomass fuels. The total conversion of fuel-N to NO ranged between 6 – 21g of N / 100g of fuel-N and was found to diminish with increasing fuel-N content. In most cases, emissions of NO were predominately released during devolatilisation. These results provide a basis for the development of control measures needed to minimise emissions of NO in combustion processes. The char reactivity measurements and partitioning of released C between volatiles and char, collectively enable improved predictions of the extent of carbon burnout in industrial-scale thermochemical processes.
The occurrence of the main inorganic elements in algal biomass was studied by means of chemical fractionation. Scanning electron microscopy and X-ray diffraction analyses were used to aid interpretation of the results. The relative proportions and speciation of the main inorganic elements were largely dependent on the cultivation regime. A high level of inorganic, water-soluble, alkali salts was found in all of the tested algae. These salts are likely to cause operational problems in industrial reactors such as fouling, ash deposition, corrosion, and in the case of fluidised-bed technologies, bed agglomeration. In order to gain insights into bed agglomeration, interactions between algal ashes and quartz bed material were simulated by heating pellets consisting of algal ashes mixed with quartz particles in a muffle furnace at 850 °C in air. Analysis of cross-sections of the resultant pellets indicated that bed agglomeration follows a non-reactive mechanism, involving the binding of bed particles with an ash-derived melt. Based on this outcome, it is expected that bed agglomeration will be largely controlled by the formation of molten ash on inert bed particles during combustion, rather than the ash reacting with the bed particles.
The release of Cl, S, P, K, and Na was characterised by preparing char and ash samples in a fixed-bed reactor, at a range of temperatures (500 – 1100 °C) and under different gas atmospheres (N2, 2% O2, and CO2) relevant to pyrolysis, combustion and gasification processes. The extent to which these elements volatilise was determined for different species of algae by means of mass balances based on elemental analyses of the char and ash residues. Results for the different species of algae were compared and explained in terms of mechanisms existing for coal and terrestrial biomass fuels. Differences in the volatile behaviour of Cl, S, K, and Na were significant between marine and freshwater species but were only minor between micro- and macroalgal species. The volatile behaviour of P was similar for all of the tested algae. If volatilised, the studied inorganic elements may cause fouling, ash deposition, and corrosion. The results from this study therefore provide essential knowledge for the prediction and mitigation of these problems.
The potential for mobilisation of 11 environmentally important trace elements (As, Be, Co, Cu, Mn, Ni, Pb, Sb, Se, V, Zn) was assessed during the thermal conversion of two samples of algae which had been cultivated in ash-dam water at a coal-fired power station. The volatility of the trace elements was studied in the same experimental setup as that used to study the volatility of Cl, S, P, K, and Na. Se and As were substantially volatilised at low temperatures (<500 °C), under pyrolysis, combustion, and gasification gas atmospheres. Zn, Pb, and Sb were also substantially volatilised, but at higher temperatures (700 – 1000 °C). Batch leaching tests were carried out in order to assess the stability of the trace elements in the char and ash residues. The trace elements were generally more stable following thermal conversion with the exception of V, which was up to 4 – 5 times more leachable in the combustion ashes than in the corresponding algal feedstock. The trace elements were generally more stable in residues prepared under pyrolysis and gasification conditions than in residues prepared under combustion conditions. The results from this study show that several trace elements have potential to be released into the environment in significant quantities when ash-dam cultivated algae are thermally processed. Appropriate control measures would need to be implemented to minimise the release of these elements in industrial-scale thermochemical processes.
The outcomes of this thesis collectively provide an improved understanding of the potential for operational and environmental problems associated with the thermochemical conversion of micro- and macroalgal biomass. This will help in the development of commercial processes for the utilisation of these resources.
... The precise mechanisms that govern the release of phosphorus (P) during algae pyrolysis remain an area of very rudimentary level of understanding. However, the research indicates that P is liberated during pyrolysis through the creation of molten-phase phosphate compounds, which subsequently undergo vaporization [135]. The melting properties of these compounds are primarily influenced by the quantity of alkaline earth metals, particularly calcium and magnesium, that are present in the ashes [135]. ...
... However, the research indicates that P is liberated during pyrolysis through the creation of molten-phase phosphate compounds, which subsequently undergo vaporization [135]. The melting properties of these compounds are primarily influenced by the quantity of alkaline earth metals, particularly calcium and magnesium, that are present in the ashes [135]. Algae generally contain significant quantities of calcium and magnesium, which help to retain some of the P within the char and ash residues, especially at temperatures below 1100 • C [126]. ...
... These problems are often associated with potassium and its reactions with other ash forming elements at elevated temperatures. Potassium salts, i.e., KCl, K 2 SO 4 and K 2 CO 3 are often formed during combustion of agricultural residues with high concentrations of K, Cl and S. Potassium phosphates are often observed in the ash residues produced during combustion of agricultural residues rich in phosphorus [9]. In addition, a large portion of potassium reacts with Si in the fuel, leading to formation of different potassium silicates. ...
... Depending on major compositions of studied additives, they can be grouped into: (1) Al-silicates based additives, (2) sulfur based additives, (3) calcium based additives, and (4) phosphorus based additives [8,10,[13][14][15][16][17][18][19]. Using additives can: (1) increase ash melting points by altering ash chemical compositions and transformation chemistry, (2) chemically bind potassium containing compounds and convert them into inert compounds, and (3) capture problematic ash species (i.e., KCl) by means of physical adsorption [9,10,15,20]. However, it is often an arduous task to find an additive that is cheap, has high anti-sintering ability and gives minor impacts on ash handling and disposal as well. ...
The understanding of ash sintering during combustion of agricultural residues is far from complete, because of the high heterogeneity of the content and composition of ash forming matters and the complex transformation of them. In order to make agricultural residues competitive fuels on the energy market, further research efforts are needed to investigate agricultural residues' ash sintering behavior and propose relevant anti-sintering measures. The aim of this work was to investigate the ash characteristics of rye straw and effects of additives. Three additives were studied regarding their abilities to prevent and abate rye straw ash sintering. Standard ash fusion characterization and laboratory-scale sintering tests were performed on ashes from mixtures of rye straw and additives produced at 550 °C. Ash residues from sintering tests at higher temperatures were analyzed using a combination of X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX). High sintering and melting tendency of the rye straw ash at elevated temperatures was observed. Severe sintering of the rye straw ash was attributed to the formation and fusion of low temperature K-silicates and K-phosphates with high K/Ca ratios. Among the three additives, calcite served the best one to mitigate sintering of the rye straw ash. Ca from the calcite promoted formation of high temperature silicates and calcium rich K-phosphates. In addition, calcite may hinder aggregating of ash melts and further formation of large ash slag. Therefore, the chemical reactions and physical restraining effects arose by calcite addition contributed to reduction of ash melts and sintering degree. Upon addition of kaolin, compositions of rye straw ash shifted from low temperature melting K-silicates to high temperature melting K-Al-silicates. The changes of ash chemistry were favorable for reducing sintering of the rye straw ash. As the Ca-sludge was added, reduction of sintering of the rye straw ash was less pronounced. Only K4CaSi3O9 and a small amount of KCaPO4 were identified in the rye straw ash as Ca-sludge was added.
... Bottom ash from combustion of biomass with high growth rates (e.g., agricultural crops) is often rich in P. It has been suggested that P availability decreases with increasing combustion temperature. However, at high temperatures P compounds may vaporise in a simple form, e.g., as KPO 3 (Wu et al., 2011), and precipitate again when the gas temperature decreases. The final composition is nevertheless dependent on the concentrations of ash-forming elements and can be difficult to predict (Boström et al., 2012). ...
... The ash transformation reactions that determine the speciation of phosphates is largely decided by the relative concentrations of ash-forming elements in the feedstock and kinetics, and to some extent by the combustion process used (Boström et al., 2012). The apparent, high P availability in the ashes (compared to TSP) could imply that P compounds were converted to more available forms during combustion, to for example simple gaseous compounds like KPO 3 before precipitation in the ash (Wu et al., 2011). However, only a very small proportion of P is watersoluble in these ashes (Table 1). ...
(Free access to this article until June 6, 2016 via this link: http://authors.elsevier.com/a/1SuM3B8ccYuno) For biomass combustion to become a sustainable energy production system, it is crucial to minimise landfill of biomass ashes, to recycle the nutrients and to minimise the undesirable impact of hazardous substances in the ash. In order to test the plant availability of phosphorus (P) and cadmium (Cd) in four biomass ashes, we conducted two pot experiments on a P-depleted soil and one mini-plot field experiment on a soil with adequate P status. Test plants were spring barley and Italian ryegrass. Ash applications were compared to triple superphosphate (TSP) and a control without P application. Both TSP and ash significantly increased crop yields and P uptake on the P-depleted soil. In contrast, on the adequate-P soil, the barley yield showed little response to soil amendment, even at 300-500kgPha(-1) application, although the barley took up more P at higher applications. The apparent P use efficiency of the additive was 20% in ryegrass - much higher than that of barley for which P use efficiencies varied on the two soils. Generally, crop Cd concentrations were little affected by the increasing and high applications of ash, except for relatively high Cd concentrations in barley after applying 25Mgha(-1) straw ash. Contrarily, even modest increases in the TSP application markedly increased Cd uptake in plants. This might be explained by the low Cd solubility in the ash or by the reduced Cd availability due to the liming effect of ash. High concentrations of resin-extractable P (available P) in the ash-amended soil after harvest indicate that the ash may also contribute to P availability for the following crops. In conclusion, the biomass ashes in this study had P availability similar to the TSP fertiliser and did not contaminate the crop with Cd during the first year.
... The interaction of P with bottom ash slag formation is comparatively less covered in literature [215,[224][225][226][227][228]. Consequently, P is usually not considered in the ash transformation reactions. ...
In the present dissertation, ash-related aspects during the combustion of rice husk (RH) and rice straw (RS), were investigated to produce biogenic silica. In this respect, firstly, an extensive literature review was conducted in the field of high-quality biogenic silica obtained from the combustion of RH and RS including chemical fuel pre treatment. The review showed that the main properties of the silica-rich ashes such as chemical composition, silica purity, whiteness, specific surface area, pore volume, carbon content, and morphology are mainly influenced by the combustion temperature and chemical fuel pre-treatment. Furthermore, the review revealed that slag formation is a serious problem, especially for the combustion of RS, and fuel pre treatment is a powerful strategy to mitigate the issue. Then, in the present research project, ash transformation process including evolution of the chemical composition, biogenic silica quality and slag formation behavior of untreated, blended, water washed and citric acid leached rice husk and rice straw from Italy were investigated theoretically and experimentally. In the experiments, newly developed laboratory methods as well as the spectroscopy and diffractometry methods were employed to characterize the ashes produced at different temperatures in terms of the ash quality and slag formation. Furthermore, thermodynamic equilibrium calculations as well as fuel indexes and viscosity calculations were employed to predict the silica quality and slag formation tendency of the studied materials. With the help of experimental and theoretical investigations, the ash transformation mechanism of the silica-rich biomass assortments was revealed, and some novel findings were highlighted. For the first time, it was shown that the chemical composition in the bulk and on the surface of the ashes is not identical, and the deposition of metallic impurities on the surface initiates the ash related issues on the surface causing agglomeration and reduction of the ash porosity. However, acid leaching followed by combustion at a suitable temperature generated an amorphous ash with almost 100 weight percentage silica purity on the ash surfaces. Quantitative phase analysis by Rietveld refinement of X-ray diffraction (XRD) data and porosity analysis via nitrogen sorption method, for the first time, revealed a distinct threshold in the crystallinity fraction (i.e. crystalline fraction ~ 10 wt.%), and combustion and pre-treatment factors have only impact below this crystallinity threshold. The proposed ash atomic structural model as well as the viscosity calculation clarified the role of alkali and alkaline earth metals on the slag formation. The results showed that the impact of alkali metals (i.e. Na, K, etc.) on the slag formation is significant as compared to the alkaline earth metals. Further investigation was performed by developing a novel python computer code to define new fuel indexes, as practical simple ash quality indicators, to predict the slag formation in silica-rich biomass fuels. The selected new fuel index, (K + Na + Mg) / P [mol/mol], showed high potential for slag formation and ash quality prediction, and it was able to classify the ashes based on their chemical backgrounds. Furthermore, the newly defined fuel index had higher coefficient of determination as compared to the already defined fuel indexes in literature. The results of present Ph.D. thesis provide a detailed insight on the quality and slag formation tendency of Si-rich bottom ashes. This information is highly required to produce biogenic silica with high quality (i.e. silica with higher purist and porosity, and lower crystallinity fraction) without slag formation issue from RH and RS. Furthermore, findings of the present thesis can be employed to efficiently design an optimized biomass boiler.
... It is known that different thermal conversion procedures may impact CEC, as well as elemental availability in the produced biochar due to the complexity of reactions occurring in different temperature ranges (Li and Jiang, 2017). Wu et al. (2011) reported that during combustion of bran at 900-1100°C, 60-70 % of K and P was released in gaseous form and speculated that this could be due to vaporization of KPO 3 which has a melting point of around 800°C. Furthermore, Karim et al. (2017) reported that utilization of banana peduncle waste biomass through thermal plasma processing, significantly modified the characteristics of biochar and K enrichment status. ...
The established practices of intensive agriculture, combined with inadequate soil Κ replenishment by conventional inorganic fertilization, results in a negative environmental impact through the gradual exhaustion of different forms of K reserves in soils. Although biochar application as soil amendment has been established as an approach of integrated nutrient management, few works have focused on the impact of biochar application to soil K availability and crop uptake. This review provides an up-to-date analysis of the published literature, focusing on the impact of biochar in the availability of potassium in soil and crop growth. First, the effect of biomass type and pyrolysis temperature on potassium content of biochar was assessed. Second, the influence of biochar addition to the availability of potassium in soil and on potassium soil dynamics was examined. Finally, alternative methods for estimating available K in soils were proposed. The most promising biomasses in terms of potassium content were grape pomace, coffee husk and hazelnut husk however, these have not been widely utilized for biochar production. Higher pyrolysis temperatures (>500 °C) increase the total potassium content whereas lower temperatures increase the water-soluble and exchangeable potassium fractions. It was also determined that biochar has considerable potential for enhancing K availability through several distinct mechanisms which eventually lead directly or indirectly to increased K uptake by plants. Indirect mechanisms mainly include increased K retention capacity based on biochar properties such as high cation exchange capacity, porosity, and specific surface area, while the direct supply of K can be provided by K-rich biochar sources through purpose-made biochar production techniques. Research based on biochar applications for soil K fertility purposes is still at an early stage, therefore future work should focus on elucidating the mechanisms that define K retention and release processes through the complicated soil-biochar-plant system.
... Although a majority of the available studies on ash transformation of agricultural biomass limited their focus on the fate of Si, K, Ca, S, and Cl under combustion conditions, 2,17−24 there are some valuable studies 3,22,24,25 on ash transformation of specific types of agricultural biomass with special focus on P, which can have a significant influence on the overall ash transformation. However, the ash transformation reactions occurring in fixed-bed combustion of Pand K-rich agricultural biomass assortments are still not fully understood and rarely described in the literature. 2 Therefore, there is a need for further studies to fully understand the ash transformation reactions occurring during the combustion of different P-and K-bearing agricultural biomass assortments to overcome the above-mentioned barriers. ...
In this study, ash transformation during fixed-bed combustion of different agricultural opportunity fuels was investigated with a special focus on potassium (K) and phosphorus (P). The fuel pellets were combusted in an underfed fixed-bed pellet burner. Residual ashes (bottom ash and slag) and particulate matter were collected and characterized by scanning electron microscopy–energy-dispersive X-ray spectroscopy, X-ray diffraction, inductively coupled plasma, and ion chromatography. The interpretation of the results was supported by thermodynamic equilibrium calculations. For all fuels, almost all P (>97%) was found in residual-/coarse ash fractions, while K showed different degrees of volatilization, depending on fuel composition. During combustion of poplar, which represents Ca–K-rich fuels, a carbonate melt rich in K and Ca decomposed into CaO, CO2, and gaseous K species at sufficiently high temperatures. Ca5(PO4)3OH was the main P-containing crystalline phase in the bottom ash. For wheat straw and grass, representing Si–K-rich fuels, a lower degree of K volatilization was observed than for poplar. P was found here in amorphous phosphosilicates and CaKPO4. For wheat grain residues, representing P–K-rich fuels, a high degree of both K and P retention was observed due to the interaction of K and P with the fuel-bed constituents, i.e., char, ash, and slag. The residual ash was almost completely melted and rich in P, K, and Mg. P was found in amorphous phosphates and different crystalline phases such as KMgPO4, K2CaP2O7, K2MgP2O7, and K4Mg4(P2O7)3. In general, the results therefore imply that an interaction between ash-forming elements in a single burning fuel particle and the surrounding bed ash or slag is important for the overall retention of P and K during fuel conversion in fixed-bed combustion of agricultural biomass fuels.
... As a result, ash melting behavior of a phosphorus-poor biomass ash can be commonly evaluated with the three remaining compounds in the K 2 O(+Na 2 O) -CaO(+MgO) -SiO 2 ternary system [34][35][36]. It is believed that the role of P, especially its interaction with biomass ash melting, is less covered in literature [32,[37][38][39][40][41]. In general, P is disregarded from the proposed biomass ash chemistry, particularly for biomass fuels with minor P content in their fuel ash. ...
Biomass is an alternative energy resource to fossil fuels because of its potential to reduce greenhouse gas emissions. However, ash-related problems are serious obstacles for this development, especially for the use in combustion plants. Thus, design and operation of biomass boilers require detailed understanding of ash transformation reactions during thermochemical conversion. To evaluate ash transformation in silica-rich biomass fuels, rice husk and rice straw were selected because of their abundance, limited utilization conflicts with the food sector, as well as their potential in both energy and material applications. This paper reveals ash transformation mechanisms relevant for the ash melting behaviour of silica-rich biomass fuels considering chemical and phase composition of the ashes. In this regard, several advanced spectroscopic methods and diffractometry were employed to characterize the materials. The ash transformation reactions and the viscosity were simulated using thermodynamic equilibrium calculations and a slag viscosity modeling toolbox. The results illustrate the impact of impurities on the atomic structure of the silica resulting in an altered ash melting behaviour and viscosity of the silica-rich ashes. Chemical water washing, acid leaching, and blending of rice straw with rice husk strongly influenced the chemical composition of the ashes and improved ash melting behaviour. The analysis also revealed the correlation between the crystalline fraction and the porosity in silica-rich biomass ashes, as well as a crystallinity threshold. These findings are highly relevant for future investigations in boiler designs and production of biogenic silica for material applications.
... Previous work has suggested, though, that the majority of P is expected to form hydroxyapatite (Ca 5 (PO 4 ) 3 OH) or Ca-whitlockite (β-Ca 3 (PO 4 ) 2 ) during thermal conversion of forest-based biomass [21]. Most studies on the release and ash transformation of critical elements in biomass during thermochemical conversion limited their focus to the fate of Si, K, Ca, S, and Cl under combustion conditions [22][23][24][25][26]. Considerably fewer studies [27][28][29] were conducted under pyrolysis and gasification conditions, and these studies are mainly limited to a specific type of agricultural biomass such as straw from wheat, rice, and corn. ...
The phosphorus and potassium contents of the char obtained from thermal conversion of forest residues can limit its utilization as an alternative fuel and reducing agent to substitute coal/coke in the steelmaking industry. In this study, ash transformation and release of K and P during single-pellet thermal conversion of different types of forest residues (i.e., bark, twigs, and bark+twigs) were investigated with the aid of a vertical tube furnace (Macro-TGA) at different temperatures (i.e., 600, 800, and 950 • C) and within and after different fuel conversion stages, i.e., devolatilization and char gasification. The residual char before and after full devolatilization, and ash after char gasification were characterized by SEM-EDS, XRD, and ICP-OES with the support of thermochemical equilibrium calculations. The concentrations of K (7970-19500 mg/kg) and P (1440-4925 mg/kg) in the char produced after devolatilization were more than four times higher than in coke and pulverized coal frequently used in metallurgical processes. A low amount of P and K (≤15%) were released from all fuels. K and P were evenly distributed within the char residues, and no crystalline compounds containing K and P were found. In ash residues of bark, K was found in K 2 Ca 2 (CO 3) 3 , and K 2 Ca(CO 3) 2. K in ash residues from twigs and bark+twigs was mainly found in the amorphous part of ash, most likely in the form of K-Ca rich silicates. Apatite was found as the main P crystalline compound in all ashes at all temperatures. Estimations show that a release of more than 80% is needed for the studied forest residual assortments to reach K and P concentrations typical of blast furnace coals and cokes.
... Most of the available studies on the ash transformations of agricultural biomass limit their focuses onto the fate of Si, K, Ca, S, and Cl under combustion conditions [1,[13][14][15][16][17][18][19][20]. Fewer studies [21][22][23] can be found performed under gasification conditions, and the few available are limited to a specific type of agricultural biomass such as straw from wheat, rice, and corn. ...
Agricultural biomasses and residues can play an important role in the global bioenergy system but their potential is limited by the risk of several ash-related problems such as deposit formation, slagging, and particle emissions during their thermal conversion. Therefore, a thorough understanding of the ash transformation reactions is required for this type of fuels. The present work investigates ash transformation reactions and the release of critical ash-forming elements with a special focus on K and P during the single-pellet gasification of different types of agricultural biomass fuels, namely, poplar, grass, and wheat grain residues. Each fuel was gasified as a single pellet at three different temperatures (600, 800, and 950 °C) in a Macro-TGA reactor. The residues from different stages of fuel conversion were collected to study the gradual ash transformation. Characterization of the residual char and ash was performed employing SEM-EDS, XRD, and ICP with the support of thermodynamic equilibrium calculations (TECs). The results showed that the K and P present in the fuels were primarily found in the residual char and ash in all cases for all studied fuels. While the main part of the K release occurred during the char conversion stage, the main part of the P release occurred during the devolatilization stage. The highest releases – less than 18% of P and 35% of K – were observed at the highest studied temperature for all fuels. These elements were present in the residual ashes as K2Ca(CO3)2 and Ca5(PO4)3OH for poplar; K-Ca-rich silicates and phosphosilicates in mainly amorphous ash for grass; and an amorphous phase rich in K-Mg-phosphates for wheat grain residues.
... As the blend ratio increases, more flake minerals appear, obscuring the crystalline structures as shown in Fig. 2 (blends). Ash samples are also exposed to a spot electron beam in the EDS (Wu et al., 2011). The EDS elemental characteristics of the X-ray energy spectrum were analyzed and expressed as an average percentage (in weight) of specific elements for 10 well-spaced spots on a particular ash sample, as shown in Fig. 3. ...
Combustion of biomass or coal is known to yield aerosols and condensed alkali minerals that affect boiler heat transfer performance. In this work, alkali behavior in the pressurized oxyfuel co-combustion of coal and biomass is predicted by ther- modynamic and chemical kinetic calculations. Existence of solid minerals is evaluated by X-ray diffraction (XRD) analysis of ashes from pressure thermogravimetric combustion. Results indicate that a rise in pressure affects solid alkali minerals negligibly, but increases their contents in the liquid phase and decreases them in the gas phase, especially below 900 °C. Thus, less KCl will condense on the boiler heat transfer surfaces leading to reduced corrosion. Increasing the blend ratio of biomass to coal will raise the content of potassium-based minerals but reduce the sodium-based ones. The alkali-associated slagging in the boiler can be minimized by the synergistic effect of co-combustion of sulphur-rich coal and potassium-rich biomass, forming stable solid K2SO4 at typical fluidized bed combustion temperatures. Kinetics modelling based on reaction mechanisms shows that oxidation of SO2 to SO3 plays a major role in K2SO4 formation but that the contribution of this oxidation decreases with increase in pressure.
... With an excess of Ca in the overall fuel ash composition, Ca phosphates such as whitlockite and hydroxyapatite are more stable than the K−Ca phosphates and will preferably be the final product. Furthermore, Wu et al. 5 found that the release of P and K was negligible in the temperature range of 300−700°C, while 60−70% of P and K was released at 1100°C during the combustion of residual bran from ethanol production. They suggested that the vaporization of KPO 3 generated from the thermal decomposition of organically bound phosphate contributed to the release of K and P. ...
In this study, ash transformation and release of critical ash-forming elements during single-pellet combustion of different types of agricultural opportunity fuels were investigated. The work focused on potassium (K) and phosphorus (P). Single pellets of poplar, wheat straw, grass, and wheat grain residues were combusted in a macro-thermogravimetric analysis reactor at three different furnace temperatures (600, 800, and 950°C). In order to study the transformation of inorganic matters at different stages of the thermal conversion process, the residues were collected before and after full devolatilization, as well as after complete char conversion. The residual char/ash was characterized by scanning electron microscopy−energy-dispersive X-ray spectroscopy, X-ray diffraction, inductively coupled plasma, and ion chromatography, and the interpretation of results was supported by thermodynamic equilibrium calculations. During combustion of poplar, representing a Ca−K-rich woody energy crop, the main fraction of K remained in the residual ash primarily in the form of K 2 Ca(CO 3) 2 at lower temperatures and in a K−Ca-rich carbonate melt at higher temperatures. Almost all P retained in the ash and was mainly present in the form of hydroxyapatite. For the Si−K-rich agricultural biomass fuels with a minor (wheat straw) or moderate (grass) P content, the main fraction of K remained in the residual ash mostly in K−Ca-rich silicates. In general, almost all P was retained in the residual ash both in K−Ca−P−Si-rich amorphous structures, possibly in phosphosilicate-rich melts, and in crystalline forms as hydroxyapatite, CaKPO 4 , and calcium phosphate silicate. For the wheat grain, representing a K−P-rich fuel, the main fraction of K and P remained in the residual ash in the form of K−Mg-rich phosphates. The results showed that in general for all studied fuels, the main release of P occurred during the devolatilization stage, while the main release of K occurred during char combustion. Furthermore, less than 20% of P and 35% of K was released at the highest furnace temperature for all fuels.
... The viscosity of biomass slags is strongly dependent on the incorporation of alkaline metals (Na, K) [24]. Because alkaline metals are volatile at high temperatures, alkali fractionation is an influencing process to the slag viscosity [25][26][27][28][29][30]. In straw biomass, K is the main alkali metal in the organic matrix and in inorganic salts such as KCl or K 2 CO 3 [16,[31][32][33][34]. ...
Pressurized entrained-flow gasification (PEFG) of straw biomass is currently being studied as a potentially sustainable and economically viable process to produce fuels and other vital chemicals. In the process chain the gasification is integrated and straw is converted via pyrolysis into a bioslurry consisting of a liquid, tar-rich phase and char. Afterwards, the slurry is gasified into a tar-free, low-methane syngas which is a basic reactant for the synthesis of biofuels. At the high temperatures over 1200 • C the ash constituents of the char in the bioslurry melt and flow down the inner wall as slag. The slag viscosity has to be in a certain range to form a protective layer at the reactor wall and to guarantee a continuous removing. For this reason, the composition of the molten ash at the reactor wall has to be well known. Due to several fractionation processes in the gasifier the composition of the slag at the reactor wall does not correspond directly with the slurry ash. Therefore, experiments were conducted to identify depletion and enrichment processes in the gasifier. Finally, the composition of the slag at the reactor wall is obtained and can be used for the adjustment of the viscosity.
... These groups were also confirmed by FTIR-ATR (Fig. 1). According to Wu et al. [48], during calcination of K and P-rich biomass KPO 3 formation may occur at low temperatures. When calcium compounds are present, KPO 3 reacts with them to form calcium phosphate (KCaPO 4 ), as observed by XRD. ...
In this work, waste cupuaçu seeds were calcined for 4 h at 800 °C and evaluated as a heterogeneous catalyst for the biodiesel synthesis. The catalyst (CCS) was characterized by X-ray powder diffraction (XRD), wavelength dispersive x-ray fluorescence (WDXRF), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric and differential thermal analysis (TG-DTA) and soluble alkalinity. The catalytic activity was evaluated by CCS-catalyzed ethanolysis of soybean oil and the process was optimized using response surface methodology (RSM) and analysis of variance (ANOVA). The significance of the different process parameters and their combined effects were established through a central composite design (CCD) and the optimum process (catalyst loading of 10% (w/w) relative to oil mass, reaction time 8 h, ethanol:oil molar ratio 10:1 and temperature 80 °C) resulted in a conversion of 98.36% with good agreement with predicted conversion, 97%. The catalyst was recycled, maintaining its great catalytic activity and resulting in conversions close to 98% in the first two cycles. The high potential of CCS as a catalyst for biodiesel production was demonstrated.
... The impact of phosphorous can be significant when burning materials such as agricultural/animal/domestic residues. Preliminary investigations in this field were carried out by Wu et al. [159]. ...
This dissertation deals with the development of a co-firing advisory tool capable of predicting the effects of biomass co-firing with coal on the ash deposition and thermal performance of pulverised fired (pf) boilers. The developed predictive methodology integrates
a one-dimensional zone model of a pf boiler to determine the heat transfer conditions and midsection temperature profile throughout the boiler, with the phase equilibrium–based ash deposition mechanistic model that utilises FactSageTM thermo-chemical data. The designed
model enables advanced thermal analysis of a boiler for investigating the impact of fuel switching on boiler performance including the ash deposition effects. With respect to the ash deposition predictive model, the improved phase equilibrium approach, adjusted to the pf boiler conditions was proposed that allows the assessment of the
slagging and high temperature fouling severity caused by the deposition of the sticky ash as well as low-temperature fouling due to salts condensation. An additional ash interaction phase equilibrium module was designed in order to estimate the interactions occurring in the furnace between alumino-silicate fly ash and alkali metals originating from biomass. Based on the developed model, the new slagging/fouling indices were defined which take into account the ash burden, slag ratio in the fly ash approaching the tube banks as well as the slag viscosity corresponded to the conditions within the pf boiler.
The developed model was validated against field observations data derived from semiindustrial pf coal-fired furnace as well as a large scale 518 MWe pf boiler fired with a blend of imported bituminous coals and biomass mix composed of the various quality biomass/residues, such as meat and bone meal, wood pellets and biomass mix pellets produced on-site: the power plant typically fired up to 20wt% coal substitution. Good agreement has been found for the comparison between predictions and slagging/fouling
observations. Based on the validated model the fuel blend optimisation was performed up to 30wt% co-firing shares revealing highly non-additive ash behaviour of the investigated fuel blends.
... Vanadium, a hard, ductile, silvery grey malleable transition metal can exist in a number of different oxidation states (2)(3)(4)(5) which is quarried in South Africa, Russia and China [12]. The com- mon viable form is vanadium pentoxide (V 2 O 5 ; CAS No. 1314-62- 1), -a pentavalent state of vanadium -brown/yellow crystalline powder, can be synthesized by different techniques [13]. ...
... Nutrient availability in soil amended with ash or biochar is related not only to the feedstock and the temperature during thermal processing (Trippe et al. 2015), but also depends on soil properties (Crane-Droesch et al. 2013). Wu et al. (2011) reported that 60-70% K and P was released in gaseous form during combustion of bran at 900-1100°C, probably from vaporisation of KPO 3 which has a melting point of around 800°C. The complex reactions occurring at different temperature ranges during different thermal conversion procedures may impact elemental availability in the produced ash/char. ...
The expansion of the bioenergy sector and adoption of novel thermal conversion technologies produce increasingly large amounts of biomass ashes and biochars. Before returning such products to agricultural soil, the plant availability of nutrients when mixing with soil should be assessed. The objective of this study was to evaluate the potassium (K) availability in various types of biomass ashes and gasification biochars (GBs) derived from straw, wood, sewage sludge and poultry manure when mixed with soil. A 16-week incubation study was conducted with three contrasting soils of variable pH (5.8–7.8) and clay contents (3–17%). Exchangeable K in the product-soil mixture was determined, and the K recovery rate from the applied products varied from 31 to 86%. The relative recovery compared to applied KCl was used to indicate K availability and was 50–86% across all soils, but lower for two sewage sludge-based GBs in the alkaline soil. Most of the K release occurred within the first week, with insignificant release thereafter. Wet storage of GB did not affect K availability. We conclude that the tested biochars and ashes can be used as K fertilisers with slightly lower short-term availability than KCl.
The inorganic fractions of biochar are influenced by feedstock and pyrolysis conditions. They act as plant nutrients but can also be harmful to the environment. Pyrolysis alters the content and availability of nutrients, including nitrogen, phosphorus, and potassium. Biochars contain trace elements, such as calcium, magnesium, sulfur, and silicon as well as heavy metals. We consider the sources of heavy metals in biochar and their bioavailability, focusing on ensuring agricultural application safety. Biochar-derived nanoparticles can transport inorganic compounds, posing environmental threats. Balancing positive contributions while minimizing the negative impacts of inorganic fractions is essential for optimizing agronomic benefits with biochar.
Combustion is an effective disposal method for sewage sludge (SS), but the corresponding condensable particulate matter (CPM) emission poses a big risk to the environment. To investigate the CPM characteristics from pelleted SS combustion, tests were conducted in a tube furnace experimental system, and the effects of combustion temperatures on CPM generation and the impacts of sampling temperatures on CPM migration were studied. Results showed that the inorganic components took up the majority (60.75–86.24%) of the total CPM. Significant PO4³⁻ was first detected in CPM mainly due to the remarkable phosphorus content in SS. The CPM exhibited distinct acidity, and SO4²⁻ was the primary water-soluble ion in CPM, followed by PO4³⁻ and Cl⁻. Ca, Na, and K accounted for the vast majority of metal elements in CPM. Alkanes and esters were the predominant organic species of CPM. With the combustion temperature increasing, the inorganic CPM increased by the enhanced volatilization of relevant elements, and the organic CPM decreased owing to the improved combustion conditions. As the sampling temperature decreased, more CPM precursors converted to FPM, resulting in a decrease in both inorganic and organic components of CPM. Particularly, a drastic cooling process contributed to the enlargement of CPM by strengthening the heterogeneous condensation and agglomeration. CPM from SS incineration could do great harm to the environment and human health for its fine particle size and complex compositions, and combustion control and proper flue gas cooling devices would be conducive to CPM control.
Oxygen carriers, in the form of metal oxide particles, are bed materials that transport oxygen in solid form in fluidized bed applications, such as in chemical-looping applications. In biofuel applications, it is well known that alkali from the fuel ash can react with the bed material and cause, among other operation issues, agglomeration. When using oxygen carriers in a fluidized bed, it is likely that the bed material is either a mixture of different metal oxide materials or partly diluted with sand. This is to improve or combine the chemical properties of the materials used or simply for economic reasons.
This work investigates how three potassium salts K2CO3, K2SO4 and KH2PO4 interact with the oxygen carriers: Steel converter slag (LD slag), ilmenite, mixtures of the two and each carrier diluted with silica sand. The salts were used as model compounds that can occur in biofuel ash. The set-up used was a fixed bed where a small sample of bed material is mixed with a potassium salt equivalent to 4 wt-% of potassium. The mixture was then exposed to reducing (H2 in steam) conditions at 900 °C during several hours in a tubular furnace. This provides a worst-case scenario for solid–solid interaction in a fluidized bed. If a solid–solid reaction does not take place in this setup, it will most likely never occur in a fluidized bed.
When LD slag and ilmenite were combined, the potassium from the salts would prefer to accumulate in the ilmenite rather than the LD slag. Ca from LD slag interacted with KH2PO4 resulting in a less severe agglomeration than when ilmenite was used separately with the same salt. When the oxygen carriers were diluted with silica sand, potassium salt interaction resulted in agglomeration for both the oxygen carriers with all potassium salts. K2CO3 and K2SO4 formed potassium silicates, while KH2PO4 formed a phosphorus-containing melt. When LD slag was present, phosphorus was located in a K-Ca-P phase that was not present if ilmenite was present.
Phosphorus (P) from biomass can cause operational problems in thermal conversion processes. In order to explore the release mechanism of P to the gas phase, carbothermic reduction of meta-, pyro-, and orthophosphates of ash elements commonly found in biomass; sodium, potassium, magnesium, and calcium was investigated. Mixtures of each phosphate and activated carbon were heated to 1135 °C in a laboratory-scale reactor, with the CO and CO2 evolving from the sample monitored, and the chemical composition of selected residues analyzed to quantify the release of P. Thermal gravimetric analysis was also performed on selected samples. The alkaline earth phosphates were reduced in steps, following the sequence meta → pyro → ortho → alkaline earth oxide. However, the alkali metaphosphates appear to be reduced in one step, in which both alkali and P are released. Alkali pyro- and orthophosphate appear to undergo a two-step process. In the first step, mainly alkali is released and in the second step both alkali and P. An intermediate is produced in the first step, which has a K:P:O atomic ratio of about 2:1:2.7, indicating it might be a phosphite with the overall stoichiometry; K4P2O5. The reduction of alkaline earth phosphates could be interpreted using available thermodynamic data, whereas thermodynamic equilibrium calculations for the alkali phosphates did not correspond well to the experimental observations. Kinetics were derived for the different reduction reactions, and can be used to compare the reactivity of the phosphates. The work suggests that carbothermic reduction reactions are important for the release of P in the temperature range 850–1135 °C and relevant for biomass combustion, pyrolysis and gasification.
To study the transformation and release of fuel K, the combustion of corn stalk and wheat straw were performed in a reactor with a fixed bed between 400 and 1000°C. Measurements of weight and elemental composition, chemical fractionation analysis, and low-temperature ashing coupled with X-ray diffraction (XRD) analysis were conducted on biomass and residual samples. The effects of biomass species, temperature and duration time of combustion, oxygen concentration, and pretreatment of water washing were evaluated. The results show that compared with wheat straw, corn stalk has a lower K release, which is much more sensitive to the combustion temperature than to the oxygen concentration. The inorganic potassium occurs as KClO3 and KClO4 in wheat straw and KCl and KClO4 in corn stalk. K2SO4 appears through sulphation of KCl as combustion happens at 800°C or above. When the combustion temperature reached 900 C for corn stalk and 1000 C for wheat straw, some KCl changed into K2Si2O5, which contains insoluble K. K2Ca(SO4)2 appears as corn stalk and wheat straw are burnt at 1000 and 900°C, respectively. Along with the proceeding of wheat straw combustion, more ion-exchangeable and water-soluble K transform to the insoluble K with the increasing temperature. The pretreatment of water washing removes nearly all the water-soluble K from the corn stalk and significantly decreases the K release from 3.26–0.27 mg g⁻¹ in quantity and from 26.74–14.84% in ratio at 1000°C, respectively.
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Ash melting behaviour is an important issue regarding operation of boilers, for instance, to avoid the agglomeration of the fluidized bed. Previous studies have shown that ash melting behaviour of the blends could differ from that of single parent fuels. This study focuses on the melting behaviour of fuel ash blends of reed, pine wood pellets and Douglas fir wood chips. For that purpose, simultaneous thermal analysis, heating microscope and scanning electron microscopy with energy dispersive X-ray analysis, X-ray diffraction, and FactSage modelling were used. The ash sample was heated up to the final temperatures stepwise, and the morphology and composition of the ash samples were examined at different temperatures. The results revealed that the melting onset for reed and woody fuels blend ashes was related to KCl-K2SO4 eutectic. The melting started at 700 °C and chlorine was released. The decomposition of CaCO3 was observed for the ashes containing wood pellets. In the case of reed and wood pellet ash, both processes performed simultaneously. The second mass loss and sulphur release were observed at 1100 °C, which is related to decomposition of K2SO4. In reed and woody fuels blend ashes, the gas release was accompanied with the melting of other substances (e.g. Si-Ca or Si-Ca-K system) and the gases were trapped into melt. The feldspars abundant in Douglas fir wood chips, could cause the sluggish melting behaviour of ash material. The FactSage modelling indicated the melting onset at lower temperatures compared to the results obtained by the other methods applied.
The effects of alkali and alkaline earth metals (AAEM) on the component and distribution of products, gasification reactivity, catalytic mechanism and char-ash/slag transition during biomass gasification were reviewed. Biomass is mainly composed of cellulose, hemicellulose and lignin, and has a much higher AAEM species content than coal. The influence of inherent AAEM on biomass gasification is closely related to the gasification agent and operating conditions. To remove tar and reduce soot, interest has grown on the subject of AAEM catalysis for biomass gasification. Therefore, component and distribution of products and gasification reactivity in inherent and loaded AAEM catalytic biomass gasification were reviewed. Furthermore, the effects of AAEM on biomass conversion can be divided into homogeneous and heterogeneous catalytic effects based on the phase states involved in chemical reactions. Three main mechanisms including oxygen transfer mechanism, active site mechanism and catalytic desorption mechanism are summarized to understand the heterogeneous catalytic process. The attribution to the promotion of the homogeneous reaction is concluded as the volatile gaseous AAEM catalyzed the water-gas shift reaction and the hydrocarbon reforming reaction. In addition, the effects of AAEM content on char-ash/slag transition in biomass gasification were also reviewed.
Oxygen carriers (OCs) with perovskite structure are attracting increasing interests due to their redox tunability by introducing various dopants in the structure. In this study, LaNixFe1-xO3 (x=0, 0.1, 0.3, 0.5, 0.7, 1.0) perovskite OCs have been prepared by a citric acid-nitrate sol-gel method, characterized by means of X-ray diffraction (XRD) analysis and tested for algae chemical looping gasification in a fixed bed reactor. The effects of perovskite types, OC/biomass mass ratio (O/B), gasification temperature and water injection rate on the gasification performance were investigated. Lower Ni-doped (0≤x≤0.5) perovskites crystalized in the rhombohedra system which was isostructural with LaNiO3, while those with composition 0.5≤x≤1 crystalized in the orthorhombic system. Despite the high reactivity for LaNiO3, LaNi0.5Fe0.5O3 (LN5F5) was found to be more stable at a high temperature and give almost as good results as LaNiO3 in the formation of syngas. The relatively higher syngas yield of 0.833 m³‧kg⁻¹‧biomass was obtained under the O/B of 0.4, water injection rate of 0.3 ml‧min⁻¹ and gasification temperature at 850 oC. Continuous high yield of syngas was achieved during the first 5 redox cycles, while a slight decrease in the reactivity for LN5F5 after 5 cycles was observed due to the adhesion of small grains occurring on the surface of OCs. However, an obvious improvement in the gasification performance was attained for LN5F5 compared to raw biomass direct gasification, indicating that LN5F5 is a promising functional OC for chemical looping catalytic gasification of biomass.
This paper investigates the effect of phosphorus (P) on char structure and reactivity of char prepared from the fast pyrolysis of purposely-prepared P-loaded biomass samples at 1000 °C in absence of other inorganic species. Biomass was first acid-washed then loaded with P of three different occurrence forms (one organic P i.e. phytic acid, and two inorganic P i.e. orthophosphoric acid and polyphosphoric acid) at the same P content of 0.8 wt%. Experimental results show that both organic and inorganic P substantially increase char yields during pyrolysis from 6.2% for the biomass sample without P to 23.0–26.0% for P-loaded samples due to the enhanced crosslinking by P-containing structures in char, leading to increases in the char C and H contents and decrease in O content. The presence of P in biochars from fast pyrolysis of various P-loaded biomass samples plays important role in the evolution of char structure and intrinsic reactivity measured during low-temperature oxidation at 500 °C in air under chemical-reaction-controlled regime. After pyrolysis and subsequent char oxidation, all P in biomass either as organic or inorganic P are found to be present in forms of acid-insoluble organic structures. For char prepared from acid-washed wood, char reactivity increases with char conversion due to the increasing pore surface area at higher conversion. Comparatively, for char prepared from acid-washed wood loaded with various P at char conversion below 60%, the presence of P increases char intrinsic reactivity due to the enhanced crosslinking of reactive carbon structures and reduced condensation of char structures. However, at conversions above 60%, P-containing species in char lead to a significant decrease in char reactivity, due to the formation of abundant CO-P bonds, that is highly resistant to the oxidation in air, in the reacting chars.
Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases.
To understand the influence of P-containing compounds on particulate matter (PM) emissions from the combustion of agricultural residues, the combustion of cornstalk was performed with the addition of a phosphorus-based additive, namely ammonium dihydrogen phosphate (NH4H2PO4), in a fixed bed combustion system. Simultaneously the ash samples, including PM collected by DLPI and residual ash, were analysed with variant analytical techniques. It was found that NH4H2PO4 addition could reduce PM0.1 largely by 18.03-48.5%, but against an increase in PM1-10 yield of 22.42-46.97%. The influence of NH4H2PO4 addition on PM0.1-1 had a more complex effect. PM0.1-1 first decreased largely (>40%) when the P/K molar ratio was lower than 1 but increased slightly by 7.81% with P/K molar ratio over than 1. The addition of NH4H2PO4 clearly increased the P content in both PM1 and PM1-10 with more K phosphates (presumably KPO3 with relatively lower volatility) and K-Ca/Mg phosphates formed. This reduced the evaporation and subsequent condensation of volatile K-containing species, which contributes dominantly to the formation of PM1. In addition, the residual ash after combustion was rich in K- and P-containing species, indicating a potential utilization as fertilizer. It showed that the addition of NH4H2PO4 is a promising approach to reduce PM1 emissions during the combustion of agricultural biomass.
This study reports the release and transformation of phosphorus (P) in products of pyrolysis of a P-rich biomass (rice bran) at 400–900 °C. The focus is the effect of reactor configurations, including slow heating pyrolysis in a fixed-bed reactor, and fast heating pyrolysis in drop-tube/fixed bed reactors with continuous and pulse feeding, respectively. For slow heating pyrolysis in the fixed-bed reactor, little P was released in the studied temperature range. For fast heating pyrolysis in the drop-tube/fixed-bed reactors, the release of P is small but appreciable at 400–700 °C. However, as temperature increases further to 900 °C, the release of P increases significantly to 8.1% and 22.4% under continuous feeding and pulse feeding configurations, respectively. The results clearly show that the interactions between volatiles and char enhance the release of P. Further investigation shows that P in rice bran is dominantly (∼96%) organic potassium phytate. The results also show that such organic phosphorus in biomass is thermally unstable and transformed into acid-soluble inorganic P even at temperatures as low as 400 °C. At a temperature above 800 °C, acid-soluble inorganic P can react with other inorganic species in char and is transformed into acid-insoluble inorganic P species during rice bran pyrolysis. In the volatiles generated in situ during rice bran pyrolysis, more than 95% of P are presented in tar as organically-bound P that can be completely converted into water-soluble P or phosphorus oxides during in situ combustion at 950 °C.
The formation of nitrogen oxides (NO and N2O) during raw and demineralized biomass char combustion and the reduction of NO over biomass char were investigated. The biomass fuels were pine wood, straw, waste wood, bran, dried distillers grains with solubles (DDGS), and sunflower seed. Fixed bed combustion experiments were performed at 800°C in 10vol% O2, while NO reduction experiments were conducted at temperatures from 800 to 900°C and NO inlet concentrations from 400 to 1500 ppmv. The chars were characterized by means of ICP-OES, BET, SEM-EDX, and XPS. The conversion of char-N to NO decreased with an increase in the initial char-N content, partly explained by the increased inherent conversion of char-N to N2O. The reduction of NO over char exhibited no correlation to the surface functionalities and content of nitrogen and oxygen at the investigated conditions. The NO reduction reactivity was strongly dominated by the content and association of ash forming elements in the chars. The NO reduction reactivity of pine wood, waste wood, and straw chars correlated reasonably well with the (K+Ca)/C molar ratio, while the chars with a high phosphorous content, i.e., bran, DDGS, and sunflower seed chars, differed by showing a significantly lower reactivity. The inhibition effect of phosphorous on NO reduction reactivity was likely caused by the formation of less catalytically active potassium species (such as KPO3) in biomass char.
Quantification of phosphorus (P) and determination of the occurrence forms of P in solid fuels are important to thermochemical processing of P-containing solid fuels. This study has developed a new three-step method for separating and quantifying the total P in a solid fuel into five major P-containing species. These include three organic P-containing species (i.e. acid-soluble organic P, P in lipid and P in nucleic acids) and two inorganic P-containing species (i.e. acid-soluble inorganic P and acid-insoluble inorganic P). The first step of this new method uses cold 0.6M HClO4 to extract the acid-soluble P species. The extracted solution is then neutralised, followed by selectively converting the acid-soluble organic P in the solution into orthophosphate (PO43-), pyrophosphate (P2O74−) and tripolyphosphate (P3O105-) by UV irradiation in the presence of H2O2. The second step of this new method uses an ethanol/chloroform mixture to extract the residue from the first step to yield a solution for quantifying the contents of P in acid-insoluble phospholipid by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The third step of this new method uses buffered NaCl solution to extract P in nucleic acids from the residue of the second step into a solution for subsequent quantification using ICP-OES. The acid-insoluble inorganic P in the residue is then quantified using ICP-OES after HF/HNO3/H2O2 acid digestion. The new method is validated with a comprehensive set of standard samples loaded with known amounts of single or mixed P-containing species with known occurrence forms. Compared to the conventional the Standards, Measurements and Testing method, complete P recovery is achieved with minimal errors. Further applications of the new method to seven different solid fuels also achieve close to 100% mass balance of P. The results demonstrate that the new method is suitable for quantifying various forms of P in solid fuels.
The KOH-capture reaction by coal fly ash at suspension-fired conditions was studied through entrained flow reactor (EFR) experiments and chemical equilibrium calculations. The influence of KOH-concentration (50–1000 ppmv), reaction temperature (800–1450 °C), and coal fly ash particle size (D50 = 6.03–33.70 μm) on the reaction was investigated. The results revealed that, at 50 ppmv KOH (molar ratio of K/(Al + Si) = 0.048 of feed), the measured K-capture level (CK) of coal fly ash was comparable to the equilibrium prediction, while at 250 ppmv KOH and above, the measured data were lower than chemical equilibrium. Similar to the KOH-kaolin reaction reported in our previous study, leucite (KAlSi2O6) and kaliophilite (KAlSiO4) were formed from the KOH-coal fly ash reaction. However, coal fly ash captured KOH less effectively compared to kaolin at 250 ppmv KOH and above. Studies at different temperatures showed that, at 800 °C, the KOH-coal fly ash reaction was probably kinetically controlled. At 900–1300 °C it was diffusion limited, while at 1450 °C, it was equilibrium limited to some extent. At 500 ppmv KOH (molar ratio of K/(Al + Si) = 0.481), and a gas residence time of 1.2 s, 0.063 g K/(g additive) and 0.087 g K/(g additive) was captured by coal fly ash (D50 = 10.20 μm) at 900 and 1450 °C, respectively. Experiments with coal fly ash of different particle sizes showed that a higher K-capture level were obtained using finer particle sizes, indicating some internal diffusion control of the process.
The effect of atmosphere and chlorine on potassium release under air and oxy-fuel combustion conditions of biomass in a fluidized bed was investigated using a temporal measurement system. The results showed that the release proportion of potassium in air atmosphere was bigger than oxy-fuel environment at the temperatures below 900 °C. The overall activation energy for potassium release under air atmosphere is less than that under oxy-fuel atmosphere. The influence of chlorine on potassium release behavior also revealed an obvious difference in the two kinds of combustion environments. In general, the enhancement degree of chlorine to potassium release under air atmosphere was higher than the case under oxy-fuel atmosphere, probably caused by variations produced in chlorine species by massive CO2 under oxy-fuel atmosphere. Thermodynamic analysis was performed to determine final potassium species in air and oxy-fuel environment. The impact of mass CO2 under oxy-fuel condition on release mechanism of potassium was presented in the form of a route map graph.
A novel two-stage alumina reactor system is developed for studying particulate matter (PM) emission from in situ volatiles combustion. It enables the generation of in situ volatiles at different pyrolysis temperatures (up to 1300 °C) and the subsequent combustion of in situ volatiles in air and oxyfuel at 1300 °C. It is found that the PM emitted from volatiles combustion contains only PM with aerodynamic diameter <1 μm (PM1) and has a unimodal distribution. An increase in pyrolysis temperature from 1100 to 1300 °C results in a substantial increase in PM1 yield and a shift of fine mode diameter from 0.043 to 0.108 μm. The PM1 emitted from the volatiles generated at 1100 °C mainly consists of Na, K, S, and P. For PM1 emitted from the volatiles generated at 1300 °C, there are substantial increases in the yield of Na, K, and P; in addition, Mg and Si are present in PM1 because of the release of these inorganic species from biosolid into the volatiles. For trace elements, increasing pyrolysis temperature from 1100 to 1300 °C not only increases the As and Cd yields in PM1 but also results in the presence of Cr, Cu, and Mn in PM1 because of increasing As and Cd volatility and the release of Cr, Cu, and Mn from biosolid into the volatiles. The yields of V and Pb remained unchanged, and there is no Ti, Ni, and Co present in the PM1. Changing the combustion atmosphere from air to oxyfuel causes a slight increase in PM1 yield due to increased formation of alkali sulfates and enhanced formation of P4O10 but results in no changes in the yields and particle size distributions of trace elements. Further analysis indicates the Na, K, S, and Cl are present in the PM1 emitted from the combustion of volatiles produced at 1100 °C in the form of Na and K sulfates and Cl while P is present in the form of P4O10. The P in PM1 is present in the forms of Na, K, and Mg metaphosphates and P4O10 where higher proportion of P4O10 is formed in PM0.1–1 when pyrolysis temperature increases to 1300 °C. It is also evident that (Na, K)PO3 and P4O10 vapors can react with the alumina reactor tube to form alkali aluminophosphate glass which is then retained in the furnace, leading to only a fraction of Na and K in the volatiles being collected as PM1 after combustion.
Pulverized rice husks are co-fired with natural gas in a 100 kW (rated) down-fired oxy-fuel combustor (OFC) under two conditions: 1) air combustion (denoted as Air); 2) oxy-combustion with 70% O2 and 30% CO2 in the inlet oxidant gas (denoted as OXY70). Studied in this paper are: 1) mechanisms governing the partitioning of inorganic matter within the fly ash aerosol, and 2) how these affect mechanisms of deposition on heat transfer surfaces. In each case, the ash aerosol particle size distributions (PSDs) were determined using electric mobility/light scattering instruments (SMPS/APS) and a Berner Low Pressure Impactors (BLPI), where the latter collected size segregated aerosol for subsequent analysis. The ash deposition rate was measured experimentally using a specially designed probe, and its relationship with aerosols was explored. The properties of the deposits were also investigated using a laser diffraction particle size analyzer for PSDs, and SEM/EDS and XRD for compositions. The data show that, compared to Air, OXY70 produces greater amounts of submicron aerosols, due to increased mineral vaporization at the higher flame temperature. The Cl and P are combined with K to form KCl and KPO3 in the submicron, but not in the super-micron, aerosols. For both conditions, the particle sizes within the more loosely bound outside deposits are much larger than those within the tightly bound inside deposits. Except for S and Cl, the deposit compositions do not differ much between Air and OXY70. Inside deposition rates show a positive correlation with concentration of submicron particles, which is consistent with previous findings on coal combustion.
This study reports a systematic investigation into ash slagging behaviour during combustion of barley straw and barley husk pellets with or without additives in a residential pellet burner. The slagging tendencies of the pellets were evaluated based on the amount, chemistry, mineralogy and morphology of inlet ash formed as slag and sintering degrees of residual ash. The barley straw and husk pellets showed high slagging tendencies with 39 wt % and 54 wt % ingoing ash formed as slag. Analyses using X-ray fluorescence (XRF) and scanning electron microcopy combined with Energy-dispersive X-ray spectroscopy (SEM/EDX) revealed high concentrations of K, Si, Ca but minor amount of P in barley straw slag. The slag mainly contained melted potassium silicates, directly observed by X-ray diffraction (XRD). For the barley husk, high ash slagging tendency was observed, mainly attributed to the formation and melting of potassium phosphates, potassium silicates and complex mixtures of the two mineral phases. Addition of marble sludge completely eliminated ash slagging during combustion of barley straw and husk pellets because it led to the formation of high temperature melting calcium potassium phosphates, calcium rich potassium silicates and oxides. Addition of calcium lignosulfonate showed a less pronounced ability to mitigate ash slagging issues during pellets combustion, although it promoted the formation of calcium rich silicates and phosphates (both with high-melting points) in barley straw and husk ash, respectively. This process was accompanied by considerable reduction in the amount and sintering degree of the formed barley straw and husk slag.
A three-stage pyrolysis/combustion reactor was used to demonstrate the importance of volatile–char interactions in inorganic particulate matter (PM) emission from the combustion of biosolid volatiles. It consists of a two-stage quartz reactor (including an inner drop-tube/fixed-bed pyrolyser as Stage I and an outer fixed-bed as Stage II) cascaded into a large drop-tube furnace (DTF, Stage III). The unique reactor design enables the volatiles that are produced in situ from the fast pyrolysis of cellulose, polyethylene or acid-washed biosolid in Stage I to pass through a preloaded bed of slow-pyrolysis biosolid char in Stage II then be immediately combusted (achieving complete combustion) in the DTF as Stage III at 1300 °C. Limited by quartz maximum working temperature (in Stages I and II), two temperatures (800 or 1000 °C) were considered for preparing the bed of char and generating the in situ volatiles. The results clearly show that volatile–char interactions lead to significant changes in the particle size distributions (PSDs) of PM emitted from the combustion of volatiles produced in situ from cellulose, polyethylene or acid-washed biosolid pyrolysis. The volatile–char interactions increase the yield of PM1 (i.e. PM with aerodynamic diameter <1 µm), dominantly PM0.1 (i.e. PM with aerodynamic diameter <0.1 µm). The results show that small non-oxygenated reactive species (especially H free radicals) in the fresh volatiles can react with the chars to enhance the release of alkalis (Na and K) as well as P and S in the chars. The released Na, K, P and S can react to form alkali metaphosphate and sulphate which subsequently form PM1 during volatiles combustion. It is also evident that volatile–char interactions convert some of Pb and Cr in the biosolid chars into volatile forms which are released and then contribute to PM1 emission.
Volatiles and char were prepared from the pyrolysis of a biosolid (with a phosphorus content of ~2.3 wt%) at 1000 ○C and then combusted separately in air and oxyfuel (30% O2 in CO2) in a drop-tube furnace at 1300 ○C. The aim is to understand the effect of oxyfuel conditions on the emission of particulate matter with aerodynamic diameters ≤10 µm (PM10) from separated combustion in homogeneous and heterogeneous phases, respectively. For volatiles combustion in the homogenous phase that leads to only PM1 (dominantly PM0.1) emission, a change from air to oxyfuel results in an increase in PM1 emission due to higher yield of Na, K, S and P, likely resulted from enhanced sulfation of alkali species under oxyfuel conditions (with a higher O2 content), but leads to negligible effect on the release of trace elements (As, Cd, Pb, V, Zn). On the contrary, for char combustion in the heterogeneous phase that contributes to both PM1 and PM1-10 emission, a change from air to oxyfuel conditions leads to a reduction in PM1 emission but little change in PM1-10 yield. Such a reduction in PM1 is contributed by reductions in the yields of Na, K and P, most likely due to part of the P volatilized to react with CaO to form non-volatile Ca3PO4. For trace elements during char combustion in heterogeneous phase, oxyfuel conditions lead to reductions in As and Cr released as PM1 (most likely due to the enhanced formation of Al/Fe/Ca arsenate and iron chromate) but has little effect on the release of Co, Cu, Mn, Ti and V. The results show that P plays an important role in PM10 emission. For volatiles combustion in homogeneous phases, P is present in PM0.1 (contributes to most of PM1) in the forms of both (Na, K)PO3 and P4O10 that is slightly favored under oxyfuel condition due to higher O2 content. However, for char combustion in heterogeneous phase, P are present in PM0.1 dominantly as (Na, K)PO3 with little P4O10 under both air and oxyfuel conditions. The phosphorus in PM1-10, which is only produced during char combustion, are in the forms of Mg3(PO4)2 and Ca3(PO4)2 under both air and oxyfuel conditions.
Energy crops share many characteristics with other solid fuels, such as coal and wood. Nevertheless, they are clearly differentiated with regard to the fuel chemical properties and, particularly, the ones linked to the inorganic matter.
Properties of biomass relevant to combustion are briefly reviewed. The compositions of biomass among fuel types are variable, especially with respect to inorganic constituents important to the critical problems of fouling and slagging. Alkali and alkaline earth metals, in combination with other fuel elements such as silica and sulfur, and facilitated by the presence of chlorine, are responsible for many undesirable reactions in combustion furnaces and power boilers. Reductions in the concentrations of alkali metals and chlorine, created by leaching the elements from the fuel with water, yield remarkable improvements in ash fusion temperatures and confirm much of what is suggested regarding the nature of fouling by biomass fuels. Other influences of biomass composition are observed for the rates of combustion and pollutant emissions. Standardized engineering practices setting out protocols of analysis and interpretation may prove useful in reducing unfavorable impacts and industry costs, and further development is encouraged.
Combustion aerosols were measured in a 22 MW (thermal energy) municipal waste incinerator. Different types of waste fractions were added to a base-load waste and the effect on aerosol formation was measured. The waste fractions applied were: PVC plastic, pressure-impregnated wood, shoes, salt (NaCl), batteries, and automotive shredder waste. Also, runs with different changes in the operational conditions of the incinerator were made. Mass-based particle size distributions were measured using a cascade impactor and the number-based size distributions were measured using a Scanning Mobility Particle Sizer. The plant is equipped with flue gas cleaning and the penetration through this was determined. The particle morphology was investigated by Transmission Electron Microscopy (TEM) and chemical analysis of the aerosol particles was made by Energy Dispersive X-ray Spectroscopy (EDS). The mass-based particle size distribution was bimodal with a fine mode peak around 0.4 µm and a coarse mode peak around 100 µm. The addition of NaCl, shredder waste, and impregnated wood increased the mass concentration of fine particles (aerodynamic diameter below 2.5 µm). In general the mass concentration was stable and close to the reference PM2.5-value of 252 ± 21 mg/m (std.T,P). The total number concentration deviated during runs and between runs spanning from 43 · 10 to 87 · 10 #/cm(std.T,P). The aerosols formed were mixtures of dense and aggregated particles in all tests. The fine particles are mainly composed by alkali salts, zinc, and lead. The heavy metals Cu, Cd, Hg, and Pb are significantly enriched in the fine particles.
A growing interest has been observed for the use of cereal grains in small- and medium-scale heating. Previous studies have been performed to determine the fuel quality of various cereal grains for combustion purposes. The present investigation was undertaken in order to elucidate the potential abatement of low-temperature corrosion and deposits formation by using fuel additives (calcite and kaolin) during combustion of oat. Special emphasis was put on understanding the role of slag and bottom ash composition on the volatilization of species responsible for fouling and emission of fine particles and acid gases. The ash fractions were analyzed with scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), for elemental composition, and with X-ray diffraction (XRD) for identification of crystalline phases. The previously reported K and Si capturing effects of kaolin additive were observed also in the present study using P-rich biomass fuels. That is, the prerequisites for the formation of low melting K-rich silicates were reduced. The result of using kaolin additive on the bottom ash was that no slag was formed. The effect of the kaolin additive on the formation of submicrometer flue gas particles was an increased share of condensed K-phosphates at the expense of K-sulfate and KCl. The latter phase was almost completely absent in the particulate matter. Consequently, the levels of HCl and SO2 in the flue gases increased somewhat. The addition of both calcite assortments increased the amount of formed slag, although to a considerably higher extent for the precipitated calcite. P was captured to a higher degree in the bottom ash, compared to the combustion of pure oat. The effect of the calcite additives on the fine particle emissions in the flue gases was that the share of K-phosphate decreased considerably, while the content of K-sulfate and KCl increased. Consequently, also the flue-gas levels of acidic HCl and SO2 decreased. This implies that the low-temperature corrosion observed in small-scale combustion of oat possibly can be abated by employing calcite additives. Alternatively, if problems with slagging and deposition of corrosive matter at heat convection surfaces are to be avoided, kaolin additive can be utilized, on the condition that the higher concentrations of acidic gases can be tolerated.
Samples of 29 common foods and 10 feedstuffs of tropical origin were analysed for their total and phytate phosphorus (P) contents. In cereal grains, oilseeds and grain legumes, high levels of phytic acid were obtained, and phytate P constituted the major portion (60–82%) of total P. The various roots and tubers contained moderate amounts of phytic acid and phytate P accounted for 21–25% of the total P in this food group. Leafy greens contained negligible amounts of phytate P. In rice bran and the various oilseed meals, phytate P constituted 56–77% of the total P. Phytic acid contents were highest for gingelly (3·87%), gingelly meal (3·76%) and rice bran (3·65%).
An investigation on the complex formation equilibria between divalent metal ions Me (with Me=Mn, Co, Ni, Cu, Cd, and Pb) and phytic acid (H12L) is presented. Experiments were performed through a potentiometric methodology by measuring, at 25 °C, the proton and, in some cases (Cu2+, Cd2+, and Pb2+), also the metal ion activity at equilibrium in solutions containing, besides the metal and the ligand, 3 M NaClO4 as the ionic medium. Unhydrolyzed solutions of the metal ion at millimolar concentration levels were titrated with solutions of about 10 mM sodium phytate, until the formation of a solid phase took place (always at pH ≈2.5, except in the case of Cu2+, which formed soluble complexes up to pH ≈3.3). Coulometry was employed to produce very dilute solutions of either Cu2+, Cd2+, or Pb2+ of accurately known composition. The emf data were explained by assuming, in the acidity interval explored, the formation of the complexes of general stoichiometry MeH5L5– and Me2H3L5–. Coordination compounds in the solid state were also synthesized and characterized by elemental analysis, thermal analysis, and ICP spectroscopy. The solids had a general stoichiometry Me6Ht
LClt
·x H2O, with the following t and x values for each metal investigated: Me (t; x) = Mn (4; 2); Co (4; 2); Ni (4; 2); Cu (2; 2.5); Zn (2; 1); Sn (6; 6).
A commercial V2O5–WO3–TiO2 corrugated-type SCR monolith has been exposed for 1000 h in a pilot-scale setup to a flue gas doped with KCl, Ca(OH)2, H3PO4 and H2SO4 by spraying a water solution of the components into the hot flue gas. The mixture composition has been adjusted in order to have P/K and P/Ca ratios equal to 2 and 0.8, respectively. At these conditions, it is suggested that all the K released during biomass combustion gets captured in P–K–Ca particles and the Cl is released in the gas phase as HCl, thus limiting deposition and corrosion problems at the superheater exchangers during biomass combustion. Aerosol measurements carried out by using a SMPS and a low pressure cascade impactor have shown two distinct particle populations with volume-based mean diameters equal to 12 and 300 nm, respectively. The small particles have been associated to polyphosphoric acids formed by condensation of H3PO4, whereas the larger particles are due to P–K–Ca salts formed during evaporation of the water solution. No Cl has been found in the collected particles. During the initial 240 h of exposure, the catalyst element lost about 20% of its original activity. The deactivation then proceeded at slower rates, and after 1000 h the relative activity loss had increased to 25%. Different samples of the spent catalyst have been characterized after 453 h and at the end of the experiment by bulk chemical analysis, Hg-porosimetry and SEM-EDX. NH3-chemisorption tests on the spent elements and activity tests on catalyst powders obtained by crushing the monolith have also been carried out. From the characterization, it was found that neither K nor Ca were able to penetrate the catalyst walls, but only accumulated on the outer surface. Poisoning by K has then been limited to the most outer catalyst surface and did not proceed at the fast rates known for KCl. This fact indicates that binding K in P–K–Ca compounds is an effective way to reduce the negative influence of alkali metals on the lifetime of the vanadia-based SCR catalysts. On the other hand, P-deposition was favoured by the formation of the polyphosphoric acids, and up to 1.8 wt% P was accumulated in the catalyst walls. Deactivation by polyphosphoric acids proceeded at about 0.2% day−1. About 6–7% of the initial activity was lost due to the accumulation of these species. However, the measured relative activity reached a steady-state level during the last 240 h of exposure indicating that the P-concentration in the bulk reached a steady-state level due to the simultaneous hydrolysis of the polyphosphoric acids.
The content and composition of inositol phosphate phosphorus (InsP-P) in maize, wheat, barley and
heat treated soybean meal, rapeseed meal and sunflower meal was determined by high-performance
ion chromatography (HPIC). Approximately 0.88–0.96 of the InsP-P in the feedstuffs was present in
the inositol hexaphosphate (InsP6) form, whereas the rest was in the inositol pentaphosphate (InsP5)
form and for oilseeds a very small amount was present as inositol tetraphosphate (InsP4). Rapeseed
differed from this pattern by having as much as 300 and 60 g InsP4-P/kg of the total InsP-P pool.
The effect of pelleting (90 ◦C) and extrusion cooking (130–140 ◦C, 6.5MPa) on the composition of
InsP-P was investigated. Neither treatment had any major effect on the total content of InsP-P in
the feedstuffs. However, as indicated by the statistically significant effects on the proportion of the
inositol phosphates, extrusion cooking shifted the inositol phosphates from InsP6-P towards InsP5-
P both in cereals (P=0.002) and in oilseeds (P<0.001), which show a slight degradation of phytate
during this treatment. The degradation of InsP6 to InsP5 appeared to be unspecific with regard to
isomers in all feedstuffs, indicating that the degradation was non-enzymatic, i.e. a result of the high
temperature and pressure during the extrusion cooking. The degradation of InsP6 in the feedstuffs during extrusion is too limited to have any nutritional effect on the availability of phosphorous and
minerals.
Straw of various types of rape, wheat and barley have been studied with respect to the formation of crystalline compounds and high-temperature reactions in ash, as well as sintering and melting behaviour. During the low-temperature ashing process simple, crystalline compounds such as carbonates, sulphates and chlorides were formed. A significant part of the ash from wheat and barley straw was amorphous whereas rape ash was found to be mainly crystalline. The large content of potassium compounds present in wheat and barley straw ash contributes to their low melting points. The ash components primarily formed are reactive. Solid state reactions at temperatures above 800°C lead to the formation of secondary products such as oxides and silicates. Minerals such as kaolin and dolomite have been suggested as fuel additives to give the ash a higher melting point. High-temperature reactions between straw ash and kaolin, Al2Si2O5(OH)4, or dolomite, CaMg(CO3)2, respectively, were therefore investigated. Kaolin was found to be the more effective additive. The reaction between kaolin and potassium salts in straw ash gave KAlSiO4 and KAlSi2O6. A laboratory study of reactions involving K2SO4 or KCl and kaolin showed that several products are possible, one of which is KAlSiO4. The potassium capture by kaolin partly explains the higher melting point of the ash-additive mixture. Dolomite added to wheat and barley ash reacted with silica to form silicates. No reaction between dolomite and potassium compounds could be detected. The observed enhancement of the melting point caused by dolomite is probably an effect of dilution or adsorption.
A total of 285 samples representing 51 feedstuffs used in Belgian feed-mills were quantitatively analysed for phytase activity, phytate-P and total P. It was concluded that the number of feedstuffs showing significant phytase activity (more than 100 units kg−1) is rather limited (n = 13). Of the cereals analysed, only rye (5130 units kg−1), triticale (1688 units kg−1), wheat (1193 units kg−1) and barley (582 units kg−1) were rich in phytase. Wheat by-products, such as fine bran meal (4601 units kg−1) or pellets (2573 units kg−1)], middlings (4381 units kg−1), feed flour (3350 units kg−1) and bran (2957 units kg−1) were very phytase rich. Pelleted wheat fine bran samples were, on average, only 56% as active as non-pelleted wheat fine bran. Malt sprouts pellets (877 units kg−1) as well as corn distillers (385 units kg−1) also showed a moderate phytase activity. All other feedstuffs analysed showed zero or extremely low phytase activity. Phytase activity was not related to total P content or phytate-P content.A significant linear relationship was found between phytate-P and total P, for two feedstuff classes—wheat plus wheat by-products (R2 = 0.953; RSD = 0.060) and maize plus maize by-products (R2 = 0.928; RSD = 0.042)—but not for cereals or oil meals. Phytate-P content, as a percentage of total P content, was higher in cereals and in wheat by-products than in oil-seed meals and legume seeds. In malt sprouts, corn distillers and moist ensiled maize, phytate-P seems to be totally or partly hydrolysed as a result of processing or ensiling. In feedstuffs derived from roots and tubers, as well as in citrus pulp, cocoa shells, soyabean hulls, flax chaff, maize cob meal, dehydrated alfalfa and a dried mycelium sample, phytate-P was not detectable.
Sodium transformation mechanisms during pulverized coal combustion were investigated as functions of combustion temperature and of the modes in which sodium occurred in the parent coal. The combustion experiments took place in a 17-kW laboratory downflow combustor which was designed to form a link between bench-scale reactor studies and commercial-scale combustors. Three different coals were burned under excess air conditions, and size-segregated fly-ash samples were extracted far from the combustion zone. Atomic absorption on each ash sample yielded bulk composition data. Computer-controlled scanning electron spectroscopy (CCSEM) yielded composition data of individual ash particles and allowed inference of mechanisms governing the formation of these individual particles. It was demonstrated that partitioning of sodium in the vapor, and the subsequent fraction of sodium in the submicron fume, is greatly reduced by the presence of silicates and high temperatures. This was attributed to reaction, of sodium in the vapor with solid silicates. The CCSEM analysed showed that reaction to form sodium aluminum silicates was preferred over reactions forming sodium silicates, alone, although both are formed. Some physical condensation also occurs. Both included and excluded alumino-silicates are effective in reactively scavenging sodium. The maximum amount of sodium scavenged in any one particle was consistent with formation of Na2O·Al2O3·2SiO2. It was demonstrated that sodium from coal reacted with kaolinite additives mixed with the coal, suggesting that this is a strategy to reactively scavenge vaporized sodium. Additional bench-scale experiments and equilibrium calculations suggest that the mechanism of sodium reaction with alumino-silicates involves sodium as sodium hydroxide.
Co-firing straw with coal in pulverized fuel boilers can cause problems related to fly ash utilization, deposit formation, corrosion and SCR catalyst deactivation due to the high contents of Cl and K in the ash. To investigate the interaction between coal and straw ash and the effect of coal quality on fly ash and deposit properties, straw was co-fired with three kinds of coal in an entrained flow reactor. The compositions of the produced ashes were compared to the available literature data to find suitable scaling parameters that can be used to predict the composition of ash from straw and coal co-firing. Reasonable agreement in fly ash compositions regarding total K and fraction of water soluble K was obtained between co-firing in an entrained flow reactor and full-scale plants. Capture of potassium and subsequent release of HCl can be achieved by sulphation with SO2 and more importantly, by reaction with Al and Si in the fly ash. About 70–80% K in the fly ash appears as alumina silicates while the remainder K is mainly present as sulphate. Lignite/straw co-firing produces fly ash with relatively high Cl content. This is probably because of the high content of calcium and magnesium in lignite reacts with silica so it is not available for reaction with potassium chloride. Reduction of Cl and increase of S in the deposits compared to the fly ashes could be attributed to sulphation of the deposits.
Inorganic components in low-rank coals can be incorporated as discrete mineral particles, ion-exchangeable cations or coordination complexes. The mode of incorporation may influence the way in which the metals behave in utilisation processes. This paper describes the mode in which Al, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Si, Sr and Ti were found in a North Dakota lignite, a Texas lignite and a Montana sub-bituminous coal. The three coals showed significant differences not only in amounts of the elements present but also in how they occur in the coal.
myo-Inositol-1,2,3,4,5,6-hexakisphosphate (Ins P(6)) was first described as an abundant form of phosphorus in plant seeds and other plant tissues and dubbed "phytic acid". Subsequently it was found to be a common constituent in eukaryotic cells, its metabolism a basic component of cellular housekeeping. In addition to phosphate, myo-inositol (Ins) and mineral storage and retrieval in plant organs and tissues, other roles for Ins P(6) include service as a major metabolic pool in Ins phosphate and pyrophosphate pathways involved in signaling and regulation; possibly as an effector or ligand in these processes; as a form of energy currency and in ATP regeneration; in RNA export and DNA repair; and as an anti-oxidant. The relatively recent demonstration that pyrophosphate-containing derivatives of Ins P(6) can function as phosphate donors in the regeneration of ATP is reminiscent of the proposal, made four decades ago in studies of seed development, that Ins P(6) itself may serve in this function. Studies of Ins P(6) in non-plant systems rarely include the consideration that this compound might represent a significant fraction of cellular P; cellular phosphate nutrition has been viewed as either not interesting or of little importance. However, there may be few fundamental differences among diverse eukaryotes in both the metabolic pathways involving Ins P(6) and the spectrum of possible roles for it and its metabolites.
A brief description of the problems involved with the combustion of low-rank coals was presented. Two samples of lignite, from the Beulah mine, North Dakota, and the Bryan mine, Texas, and one sample of subbituminous coal from the Rosebud mine, Montana, were collected and analyzed. The samples were extracted using ammonium acetate and hydrochloric acid, and the extracted portion analyzed. The data of the chemical analysis were presented in tabular form. Several conclusions were derived from the data: (1) 48% of the total inorganics of the Beulah are removed with ammonium acetate, by far the highest of the 3 coals; (2) Bryan, Texas, lignite consists of mostly extraneous mineral matter including clays and quartz minerals; (3) the Rosebud subbituminous has higher percentages of Mg, Ca, and Sr associated as carbonates, revealed in the HCl extractions.
This paper is part I in a series of two describing the fate of alkali metals and phosphorus during cocombustion of rapeseed cake pellets in a 12 MW thermal CFB boiler. In paper I the results of using the mixture of wood chips and wood pellets as a base fuel are described. Up to 45% on energy basis of rapeseed cake was cocombusted during a 4 h test. Two approximately 12 h tests with energy fractions of rapeseed cake of 12 and 18% were performed with limestone as a varying parameter. Fuels were characterized by means of chemical fractionation and standard methods. Elemental mass balances were calculated for ingoing and outgoing streams of the boiler. In addition SEM/EDX analyses of ashes were performed. Gaseous (KCl + NaCl) as well as HCl and SO2 were measured upstream of the convection pass, where deposit samples were also collected with a deposit probe. The deposit samples were analyzed semiquantitatively by means of SEM/EDX. The elemental mass balances show accumulation of alkali metals and phosphorus in the boiler. Analyses of bed material particle cross sections show the presence of phosphorus compounds within a K-silicates matrix between the agglomerated sand particles, indicating a direct attack of gaseous potassium compounds on the bed surface followed by adhesion of ash particles rich in phosphorus. Build-up of deposit during the cocombustion tests mainly took place on the windward side of the probe; where an increase of K, Na, and P has been observed. Addition of limestone prevented formation of K-silicates and increased retention of phosphorus in the bed, most probably due to formation of high-melting calcium phosphates. During the tests with limestone, an increase of potassium chloride upstream of the convection pass and a decrease of phosphorus in the fly ash fraction could be noticed. Agglomeration and slagging/fouling when cofiring wood with rapeseed cake may be linked to its high content of organically bonded phosphorus—phytic acid salts—together with high contents of water-soluble alkali metals chlorides and sulfates in the fuel mixture.
A future shortage of biomass fuel can be foreseen. The production of rapeseed oil for a number of purposes is increasing, among others, for biodiesel production. A byproduct from the oil extraction process is rapeseed meal (RM), presently used as animal feed. Further increases in supply will make fuel use an option. Several energy companies have shown interest but have been cautious because of the scarcity of data on fuel properties, which led to the present study. Combustion-relevant properties of RM from several producers have been determined. The volatile fraction (74 ± 0.06%wt ds) is comparable to wood; the moisture content (6.2−11.8%wt) is low; and the ash content (7.41 ± 0.286%wt ds) is high compared to most other biomass fuels. The lower heating value is 18.2 ± 0.3 MJ/kg (dry basis). In comparison to other biomass fuels, the chlorine content is low (0.02−0.05%wt ds) and the sulfur content is high (0.67−0.74%wt ds). RM has high contents of nitrogen (5.0−6.4%wt ds), phosphorus (1.12−1.23%wt ds), and potassium (1.2−1.4%wt ds). Fuel-specific combustion properties of typical RM were determined through combustion tests, with an emphasis on gas emissions, ash formation, and potential ash-related operational problems. Softwood bark was chosen as a suitable and representative co-combustion (woody) fuel. RM was added to the bark at two levels: 10 and 30%wt ds. These mixtures were pelletized, and so was RM without bark (for durability mixed with cutter shavings, contributing 1%wt of the ash). Each of these fuels was combusted in a 5 kW fluidized bed and an underfed pellet burner (to simulate grate combustion). Pure RM was combusted in a powder burner. Emissions of NO and SO2 were high for all combustion tests, requiring applications with flue gas cleaning, economically viable only at large scale. Emissions of HCl were relatively low. Temperatures for initial bed agglomeration in the fluidized-bed tests were high for RM compared to many other agricultural fuels, thereby indicating that RM could be an attractive fuel from a bed agglomeration point of view. The results of grate combustion suggest that slagging is not likely to be severe for RM, pure or mixed with other fuels. Fine-mode particles from fluidized-bed combustion and grate combustion mainly contained sulfates of potassium, suggesting that the risk of problems caused by deposit formation should be moderate. The chlorine concentration of the particles was reduced when RM was added to bark, potentially lowering the risk of high-temperature corrosion. Particle emissions from powder combustion of RM were 17 times higher than for wood powder, and the fine-mode fraction contained mainly K-phosphates known to cause deposits, suggesting that powder combustion of RM should be used with caution. A possible use of RM is as a sulfur-containing additive to biomass fuels rich in Cl and K for avoiding ash-related operational problems in fluidized beds and grate combustors originated from high KCl concentrations in the flue gases.
Release of potassium (K) during biomass combustion, may cause significant operating problems in terms of ash deposition and high-temperature corrosion of superheater tubes. Other ash-forming elements, such as calcium (Ca), silicon (Si), and phosphorus (P), may to a certain degree control the K release. The aim of this work was to study the release of K from simple systems, to obtain information on the retaining effects of the elements Ca, Si, and P. Further objectives were to investigate the effects of temperature, the presence of water vapor, the speciation of K and Ca, and the sample size on the release rate of K, from the simple ternary systems K−Ca−Si and K−Ca−P. Well-defined mixtures of K, Ca, and Si (or P) species were heat-treated in a reactor, at constant temperature (900 or 1000 °C), in a gas flow of 4 nL/min N2 containing 2% (v/v) H2O. Average release rates were calculated from weight measurements of the samples after every 15 min of the heat treatment (and subsequent cooling to room temperature). The presence of water in the gas flow was found to significantly enhance the K-release rate, from both the K−Ca−Si system and the K−Ca−P system. For the K−Ca−Si system, a significantly higher release rate was observed at 1000 °C compared to 900 °C. Furthermore, doubling the Ca/Si molar ratio K2CO3−CaO−SiO2 mixture strongly enhanced the K-release rate (by about 2 times) at 1000 °C. This suggests that SiO2 preferentially reacts with CaO, so that more K is being released to the gas phase instead of being incorporated into the silicate structure. For the K−Ca−P system, with K2CO3 as the K source, the Ca/P molar ratio had a strong effect on the K-release rate: a decrease in the Ca/P molar ratio (or increase in the P content) significantly decreased the K-release rate from the K−Ca−P mixtures. As opposed to the K−Ca−Si system, it thus seems that K is preferentially incorporated in (nonvolatile) (K2O)k·(CaO)l·(P2O5)m structures. The effects of temperature and Ca source on the K-release rate from this system were limited but most pronounced for the mixtures with the highest Ca/P ratio (lowest P content). Furthermore, the sample size had a strong influence on the K-release rate. In the case of K−Ca−P mixtures containing KCl as the K source, the K-release rate was significantly higher at 1000 °C compared to 900 °C in the first 15 min of the heat treatment, whereas the Ca/P ratio had no effect on the K-release rate. Selected samples of the K−Ca−Si and K−Ca−P mixtures, before and after the heat treatment, were studied by scanning electron microscopy (SEM) in combination with energy-dispersive X-ray (EDX), to investigate the morphological and compositional changes. Moreover, selected samples of the K−Ca−P mixtures were heated from room temperature to 1400 °C, in a simultaneous thermal analyzer (STA), to investigate the melting and gas-phase release behavior. Both methods confirmed the effects of the Ca/Si and Ca/P ratios and the speciation of K and Ca on the release behavior observed in the heating experiments.
The global production of rapeoil is increasing. A byproduct is rapeseed meal that is a result of the oil extraction process. Presently the rapeseed meal mainly is utilized as animal feed. An interesting alternative use is, however, energy conversion by combustion. This study was undertaken to determine the combustion properties of rapeseed meal and bark mixtures in a bubbling fluidized bed, with emphasis on gas emissions, ash formation, -fractionation and -interaction with the bed material. Due to the high content of phosphorus in rapeseed meal the fuel ash is dominated by phosphates, in contrast to most woody biomass where the ash is dominated by silicates. From a fluidized bed combustion (FBC) point of view, rapeseed meal could be a suitable fuel. Considering FBC agglomeration effects, pure rapeseed meal is in level with the most suitable fuels, as earlier tested by the methods utilized in the present investigation. The SO2 emission, however, is higher than most woody biomass fuels as a direct consequence of the high levels of sulfur in the fuel. Also the particulate matter emission, both submicron and coarser particles, is higher. Again this can be attributed the high ash content of rapeseed meal. The high abundance of SO2 is apparently effective for sulfatization of KCl in the flue gas. Practically no KCl was observed in the particulate matter of the flue gas. A striking difference in the mechanisms of bed agglomeration for rapeseed meal compared to woody biomass fuels was also observed. The ubiquitous continuous layers on the bed grains found in FBC combustion of woody biomass fuels was not observed in the present investigation. Instead very thin and discontinuous layers were observed together with isolated partly melted bed ash particles. The latter could occasionally be seen as adhered to the quartz bed grains. Apparently the bed agglomeration mechanism, that obviously demanded rather high temperatures, involved more of adhesion by partly melted ash derived potassium−calcium phosphate bed ash particles/droplets than direct attack of gaseous alkali on the quartz bed grains forming potassium−calcium silicate rich bed grain layers. An explanation could be found in the considerable higher affinity for base cations of phosphorus than silicon. This will to a great extent withdraw the present basic oxides from attacking the quartz bed grains with agglomeration at low temperatures as a result.
The ash behavior during suspension firing of 12 alternative solid biofuels, such as pectin waste, mash from a beer brewery, or waste from cigarette production have been studied and compared to wood and straw ash behavior. Laboratory suspension firing tests were performed on an entrained flow reactor and a swirl burner test rig, with special emphasis on the formation of fly ash and ash deposit. Thermodynamic equilibrium calculations were performed to support the interpretation of the experiments. To generalize the results of the combustion tests, the fuels are classified according to fuel ash analysis into three main groups depending upon their ash content of silica, alkali metal, and calcium and magnesium. To further detail the biomass classification, the relative molar ratio of Cl, S, and P to alkali were included. The study has led to knowledge on biomass fuel ash composition influence on ash transformation, ash deposit flux, and deposit chlorine content when biomass fuels are applied for suspension combustion.
Emissions from the combustion of agricultural biomasses have not been studied extensively. In this study, the effects of different biomasses and mixed fuels on fine particle (PM1) and gas emissions from a residential cereal burner were investigated. The cereal seeds of oat and rape, rape bark pellets, and wood pellets were the main fuels. In addition, oat was mixed with peat and kaolin and wood with kaolin. The gas emissions of NOx, SO2, and HCl were clearly higher from cereal or mixed-cereal fuels than from pure wood fuel. The emissions of carbon monoxide (CO), organic gaseous carbon (OGC), PM1 and the particle numbers from the cereal fuels did not differ significantly from the emissions of wood fuels, although the fuel ash contents were substantially higher. The release of alkali metals varied substantially between different fuels, probably due to large differences in ash chemical compositions. In contrast to wood fuels, phosphate contributed significantly to the formation of fine particles in the cereal fuels. In rape fuels, probably due to high S/Cl and S/K ratios, all of the fuel chlorine was released in the gas phase and was not enriched in the fine particles. At least partly due to this, the PM1 and alkali metal emissions from the combustion of rape seeds were low, considering the high ash content (4.4%) and high alkali metal contents in the fuel. The addition of 5 wt % of kaolin to oat grains seemed to decrease the alkali metal emissions but slightly increased the emissions of incomplete combustion. It seems that the formation of chlorides (e.g., KCl) affects significantly the emission of fine ash particles. Moreover, sulfation of alkali metals seems to decrease the emission of fine alkali metal particles.
This paper is part 2 in a series of two papers describing the fate of alkali metals and phosphorus during cocombustion of rapeseed cake pellets with different fuels in a 12 MWth CFB boiler. In the first part (Piotrowska, P.; Zevenhoven, M.; Davidsson, K.; Hupa, M.; Åmand, L.-E.; Barišić, V.; Coda Zabetta, E. Energy Fuels 2010, 24, 333−345), wood was applied as a base fuel for the cocombustion tests. In this second paper, coal was used. Cocombustion with coal has been proven to be a strategy to improve the combustion of rapeseed cake. This paper presents the fate of alkali metals and phosphorus during successful cocombustion of up to 25% of rapeseed cake pellets on an energy basis with coal. Tests with and without addition of limestone were performed. The fuels were analyzed according to standard fuel analyses and chemical fractionation. Elemental analyses of outgoing streams were performed by means of wet chemical analysis. In addition, SEM/EDX analyses of outgoing ashes and deposit samples collected with a deposit probe were performed. The SO2 and HCl emissions were analyzed. Mass balances were calculated for all cocombustion tests. Gaseous alkali chlorides were measured before the convective pass at a flue gas temperature of 800 °C using an in situ alkali chloride monitor (IACM). At the same place HCl and SO2 were measured, and deposit samples were collected with a deposit probe. Rapeseed cake cocombustion caused an increase in alkali metals and phosphorus. However, no heavy bed agglomeration or deposits were observed. This is due to interactions between alkali metals and aluminum silicates from coal. No formation of gaseous alkali metal chlorides was detected in the beginning of the convection pass by means of IACM. Phosphorus was present in the deposit samples up to about 9 wt %P2O5 in the leeward side of the deposit probe when no lime was supplied to the combustion chamber. Addition of limestone resulted in a higher deposition rate and lowered emissions of HCl and SO2.
A residential cereal burner (20 kW) was used to study the slagging characteristics of cereal grains with and without lime addition. The deposited bottom ash and slag were analyzed using X-ray diffraction (XRD), to identify the crystalline phases, and environmental scanning electron microscopy, coupled with energy-dispersive X-ray spectroscopy (ESEM/EDS), to study the morphology and elemental composition. Phase-diagram information was utilized to extract qualitative information about the behavior of cereal grain ashes under combustion conditions. Chemical equilibrium model calculations were used to interpret the experimental results. In addition, investigations of the melting behavior of the produced slags were conducted. The results showed significant differences in slagging characteristics between the fuels that were used. The slags consisted of high-temperature melting crystalline phases (calcium/magnesium potassium phosphates) and a potassium-rich phosphate melt for all cereal grains. For oat and barley, cristobalite was also identified in the slag. Furthermore, in these cases, the slags most probably contained a potassium-rich silica melt. The differences in the melting behaviors of the slags had a considerable effect on the performance of the burner. The addition of lime reduced the formation of slag for barley and totally eliminated it for rye and wheat. This occurs because lime contributes to the formation of high-temperature melting calcium potassium phosphates.
The influence of six sorbents on aerosol formation during the combustion of straw in a 100 MW boiler on a Danish power plant has been studied in full-scale. The following sorbents were studied: ammonium sulfate, monocalcium phosphate, Bentonite, ICA5000, clay, and chalk. Bentonite and ICA5000 are mixtures of clay minerals and consist mainly of the oxides from Fe, Al, and Si. The straw used was Danish wheat and seed grass. Measurements were also made with increased flow of primary air. The experiments showed between 46% and 70% reduction in particle mass concentrations when adding ammonium sulfate, calcium phosphate, Bentonite, ICA5000, and clay. The addition of chalk increased the aerosol mass concentration by 24%. Experiments in a laminar flow aerosol condenser with the six sorbents were carried out in the laboratory using a synthetic flue gas to avoid fluctuations in the alkali feeding. These experiments showed similar reductions to the ones found in full-scale. When feeding ammonium sulfate, the aerosol mass concentration increased as a result of the feeding method. The chemical compositions of the fine particles suggest that there is chemical equilibrium in the gas for the sulfation reaction at temperatures above 812 °C.
The transformation of inorganic constituents in annual biomass was experimentally investigated at grate-combustion conditions. A laboratory fixed-bed reactor was applied to obtain quantitative information of the release of Cl, K, and S to the gas phase from six distinctively different annual biomass fuels. Samples of 4.0 g of biomass were combusted at well-controlled conditions at temperatures from 500 to 1150 °C. The elemental release was quantified by analysis of the residual ash and a mass balance on the system. The experimental results revealed that potassium was released to the gas phase in significant amounts at combustion above 700 °C. The potassium release increased with the applied combustion temperature for all biomass fuels; however, the quantity released was largely determined by the ash composition. At 1150 °C, between 50 and 90% of the total potassium was released to the gas phase. The biomass fuels with an appreciable content of silicate showed the lower release of potassium. Between 25 and 70% of the fuel chlorine was released below 500 °C; the residual chlorine was released by evaporation of KCl, mainly between 700 and 800 °C. Above 800 °C, the fuel chlorine was completely released to the gas phase for all of the samples. Between 30 and 55% of the fuel sulfur was released at 500 °C. The samples rich in K and Ca, but low in Si, displayed only a minor increase in the sulfur release as the combustion temperature was further increased. On the contrary, the sulfur release increased abruptly above 700−800 °C for the Si-rich samples. On the basis of the release quantification, the overall transformations of the ash-forming elements are discussed at grate-combustion conditions.
The high-temperature phenomena exhibited by KH2PO4 and RbH2PO4 have been investigated by differential thermal analysis, thermogravimetric methods, and thermo-polarizing microscopy. The thermal transformations which appear at Tp = 196 °C in KH2PO4 and Tp = 96 °C in RbH2PO4 are endothermic in addition to showing weight loss. On heating further to beyond Tp, the thermal transformation shows several endothermic peaks and there is weight loss in KH2PO4 and RbH2PO4. It has been observed by thermo-polarizing microscopy that until Tp is exceeded, uniaxial interference figures are exhibited by crystals of KH2PO4 and RbH2PO4, with cracks and chemical change at the surface of KH2PO4 near Tp~192 °C and RbH2PO4 near Tp~92 °C. The high-temperature phenomena exhibited by KH2PO4 and RbH2PO4 near Tp could indicate not changes from tetragonal to monoclinic structure but chemical decomposition at the surface of the crystals such as that described by nMH2PO4→MnH2PnO3n + 1 + (n-1)H2O (M = K, Rb)
Commercial vanadia-based SCR monoliths have been exposed to flue gases in a pilot-scale Setup into which phosphoric acid has been added and the deactivation has been followed during the exposure time. Separate measurements by SMPS showed that the phosphoric acid formed polyphosphoric acid aerosols, which were characterized by particle number concentrations in the order of 1 x 10(14) #m(3) at 350 degrees C and diameters <0.1 mu m. Three full-length monoliths have been exposed to flue gases doped with 10, 100 and 1000 ppmv H3PO4 for 819, 38 and 24 h. respectively. At the end of the exposure the relative activities were equal to 65, 42 and 0%, respectively. After exposure, samples of the spent monoliths have been characterized by ICP-OES. Hg-porosimetry. SEM-EDX and in situ EPR. The results showed that the polyphosphoric acids chemically deactivate the vanadia-based catalysts by decreasing the redox properties of the catalyst surface and by titrating the number of V(V) active species. When plate-shaped commercial catalysts have been wet impregnated with different aqueous solutions of H3PO4 obtaining P/V ratios in the range 1.5-5, the relative activity for the doped catalysts in the whole P/V range was 0.85-0.90 at 350 degrees C. These results show that the presence of phosphor compounds in the flue gas may be much more harmful than indicated by simple wet chemical impregnation by phosphoric acid. The reason has been found in the nature of the polyphosphoric acid aerosol formed in the combustion process, which cannot be reproduced by the wet impregnation process.
Commercial vanadia-based full-length monoliths have been exposed to aerosols formed by injection of K3PO4 (dissolved in water) in a hot flue gas (T > 850 °C) from a natural gas burner. Such aerosols may form when burning fuels with high K- and P-content, or when P-compounds are mixed with biomass as a K-getter additive. The formed aerosols have been characterized by using both a SMPS system and a low pressure cascade impactor, showing a dual-mode volume-based size distribution with a first peak at around 30 nm and a second one at diameters >1 μm. The different peaks have been associated with different species. In particular, the particles related to the 30 nm peak are associated to condensed phosphates, whereas the larger particles are associated to potassium phosphates. Two monoliths have been exposed during addition of 100 and 200 mg/Nm3 K3PO4 for 720 and 189 h, respectively. Overall, deactivation rates up to 3%/day have been measured. The spent catalysts have been characterized by bulk chemical analysis, Hg-porosimetry and SEM-EDX. NH3-chemisorption tests on the spent elements and activity tests on catalyst powders obtained by crushing the monoliths have also been carried out. The catalyst characterization has shown that poisoning by K is the main deactivation mechanism. The results show that binding K in K–P salts will not reduce the rate of catalyst deactivation.
Phase transitions and the thermal decomposition of KH2PO4 have been examined from room temperature to above 300°C by means of hot-stage microscopy, isothermal gravimetry and differential scanning calorimetry. Phase transitions at 198 and 242°C are confirmed, with corresponding enthalpy changes 4.2 and 2.3 kJ mole−1, but no evidence has been found of a transition reported near 110°C. The thermodynamic and other evidence suggest a structural change at 198°C while the change at 242°C is less profound, perhaps involving only changes in the form of the hydrogen bonding. Thermal decomposition occurs in four stages, under conditions of free vapour escape, with the loss of one-quarter of a mole of water per formula unit of KH2PO4 in each stage. The products of each stage of decomposition are tentatively identified.
The composition of common buckwheat ( Fagopyrum esculentum Moench) seed milling fractions depends upon the relative abundance of various seed tissues in each. Fancy (light) flour contains mainly central endosperm, while the bran milling fraction has seed coat and some embryo tissues. Phytate, found in protein bodies of embryo and aleurone cells, is the major storage form of phosphorus, potassium, magnesium and some microelements in seeds. Phytic acid (35–38 g kg ⁻¹ ) and minerals are concentrated in bran, a milling fraction with high concentrations of phytate‐rich tissues. Polyphenolics, including condensed tannins (proanthocyanidins), are also concentrated in bran (11–15 g kg ⁻¹ ). Rutin is concentrated in the hull of common buckwheat (0.8–4.4 g kg ⁻¹ ). Rutin concentration is low (0.2–0.3 g kg ⁻¹ ) in groats of common buckwheat but higher (0.7–0.8 g kg ⁻¹ ) in bran containing hull fragments. Rutin is 300‐fold more concentrated (81 g kg ⁻¹ ) in groats of tartary buckwheat ( Fagopyrum tataricum (L) Gaertn) than in groats of common buckwheat. Only small amounts of quercetin were detected. Bran is a concentrated source of phytic acid and tannins, a consideration in consumption of large amounts of buckwheat bran for nutritional or medicinal purposes.
© 2001 Society of Chemical Industry
The reactions between gaseous potassium chloride and coal minerals were investigated in a lab-scale high temperature fixed-bed reactor using single sorbent pellets. The applied coal minerals included kaolin, mullite, silica, alumina, bituminous coal ash, and lignite coal ash that were formed into long cylindrical pellets. Kaolin and bituminous coal ash that both have significant amounts of Si and Al show superior potassium capture characteristics. Experimental results show that capture of potassium by kaolin is independent of the gas oxygen content. Kaolin releases water and forms metakaolin when heated at temperatures above 450 °C. The amounts of potassium captured by metakaolin pellet decreases with increasing reaction temperature in the range of 900–1300 °C and increases again with further increasing the temperature up to 1500 °C. There is no reaction of pre-made mullite with KCl at temperatures below 1300 °C. However, the weight gain by mullite is only slightly smaller than that by kaolin in the temperature range of 1300–1500 °C. A simple model was developed for the gas–solid reaction between potassium vapor and metakaolin pellet at 900 °C.
Phosphorus compounds have been reported to be a cause of increased deactivation rates of selective catalytic reduction (SCR) DeNOx catalysts in coal fired power plants. The deactivating behaviour of phosphorus was previously verified during lab and bench scale investigations. The results have been in good compliance with other research activities on similar topics.In order to compare the results of lab and bench scale tests with the actual operating conditions, measurements at two full scale power plants were carried out. The plants co-fired either meat and bone meal or sewage sludge as secondary fuel, while both represent high phosphorus fuels with a considerable amount of volatile fractions.The results of fuel and fly ash analysis showed an increased content of phosphorus during the co-combustion conditions. The analysis of the fine particulate matter showed a high phosphorus concentration on particles smaller than 0.9 μm. A considerable phosphorus concentration on particle fractions smaller than 0.05 μm is a clear indication of the presence of gaseous phosphorus compounds. These are expected to have a high deactivation potential on the SCR catalysts.
The fate of several trace elements in the thermal conversion of coal has been investigated, assuming global equilibrium and using an in-house database and a Fortran-77 computer code for the calculations. The format and content of the database DGFDBASE, containing reduced data on ΔG°fi(T) for approximately 800 chemical species of the elements Al, As, B, Be, Br, C, Ca, Cd, Cl, Co, Cr, F, Fe, Ga, Ge, H, Hg, K, Mg, N, Na, Ni, O, P, Pb, S, Sb, Se, Si, Sn, Ti, V and Zn are described. Results of thermodynamic equilibrium calculations performed using ‘the total Gibbs free energy minimization’ program MINGTSYS on simple systems containing one of the trace elements As, B, Be, Cd, Co, Cr, Ga, Ge, Hg, Ni, P, Pb, Sb, Se, Sn, Ti, V and Zn are presented and compared with results from the literature. Combustion as well as gasification conditions have been considered.At oxidizing conditions all the trace elements considered form at least one stable condensed phase in the temperature range from 300–2000 K. Regarding the condensed phase being stable at the lowest temperatures, the trace elements can be divided into two groups, the first of sulfate forming elements (this group includes the elements Be, Cd, Co, Cr, Hg, Ni, Pb, Sb, Sn, V, and Zn) and the latter of oxide-hydroxide forming elements (this group includes the elements: As, B, Ga, Ge, P, Se and Ti).At reducing conditions, the behavior of the trace elements considered is complex, and no simple classification of the elements is possible.
The effect of phytic acid on the solubility of mineral elements in oat bran was studied by digesting phytic acid with phytase enzyme. The combined effect of phytase treatment and the addition of three chelating agents common in food were also tested. Starch and proteins were digested enzymatically. The sample was dialysed, using an equilibrium dialysis method, and the soluble mineral elements were analysed from the dialyzate. The minerals studied were calcium, magnesium, iron, manganese, zinc, potassium and phosphorus. The chelating agents used were citric and malic acids and glucose. The phytase treatment increased the solubility of minerals less than expected. Citric acid was the most efficient chelating agent. The effect of malic acid was small. The results confirmed that the minerals were tightly bound to the dietary fibre of oat bran and were only partially released when the influence of phytic acid was reduced by degradation.
A twofold study using thermodynamic equilibrium calculations was carried out to study corrosion in MSW incinerators. Corrosion was associated with the amount of alkalis and trace metals gaseous chlorides. Firstly, a two-level factorial experimental design combined with a data analysis were used to determine the main and interaction effects for various alkalis and trace metals gaseous chlorides responses. The factors studied were Na, K, S and Cl concentrations. The results provided a picture of the controlling parameters and insight about the processes taking place. Secondly, the efficiency of two corrosion-fighting additives (ammonium sulphate and silica) was investigated. Calculations confirmed experimental results and brought further insight on differentiated results for Na, K, Pb and Zn but also on capture mechanisms.
This paper is concerned with the sequestration of mercury, cadmium, lead, sodium and potentially other volatile and semi-volatile metals by high-temperature mineral, non-carbon based dispersed sorbents. The focus here is on kaolinite and lime powders (for Pb, Cd, and Na), and on intimate kaolinite/calcite/lime mixtures (for Hg), both of which undergo morphological and chemical changes when exposed to high temperatures. These changes play critical roles in the metal capture mechanisms, initially enhancing metal capture, either through a eutectic melt on the surface, or through some other transformation, but ultimately causing sorbent de-activation through a catastrophic melt that causes pore closure.
DeNOx catalysts for the selective catalytic reaction (SCR) in coal fired power plants are deactivated by catalyst poisons such as As, Na, K, P, etc. The deactivation rate depends on the fuel quality and the concentration of catalyst poisons in the fuel. However, operational parameters such as O2 content, residence time, combustion and flue gas temperature also have an influence on decreasing catalyst activity.Equilibrium calculations were carried out to identify possible catalyst deactivation reactions. The calculations concentrated on the elements phosphorus and sodium as primary catalyst poisons in animal residues or sewage sludge. A set of deactivated catalyst samples from different power plants were analysed with respect to surface area, chemical composition and permeability. From the results obtained, deactivation mechanisms were derived which seem to cause increased catalyst deactivation due to P-rich secondary fuels. The samples showed a high P and alkali concentration on the surface. The surface area and the pore volume decreased. Compared to pure coal combustion this decrease was significant for meat and bone meal co-combustion and was also noticed for samples exposed to sewage sludge co-combustion. A correlation between relative activity and Na and P concentration is established. The results of the surface analyses indicate pore condensation as a major deactivation mechanism.
The behaviour of different ashes was predicted by the combination of extended fuel analysis with advanced global thermodynamic equilibrium calculations. The extended fuel analysis is a fractionation method that consists of sequential leaching of a solid fuel with water, ammonium acetate and hydrochloric acid. In order to cover a broad spectrum of fuels a coal, a peat, a forest residue and Salix (i.e. willow) were studied. The last was taken with and without soil contamination, i.e. with a high and low content of silica, respectively.Results from the fractionation showed clear differences in mineral distribution in the fuels. More ash-forming elements were present as included minerals in the older fuels. In relatively young fuels, almost half of the inorganic material was found in the soluble fractions after leaching with water and ammonium acetate. Fouling and slagging predictions based on the combined use of the extended fuel analysis and the advanced global equilibrium analysis indicated that no ash-related problems should be expected in FBC boilers firing the studied coal. The peat that was studied could cause minor ash depositions in the flue gas channel at temperatures above 700°C. The studied forest residue could form fly ash deposits in the flue gas channel at temperatures between 600 and 860°C. The Salix could cause fly ash depositions at temperatures between 840 and approximately 1000°C. If soil contamination was present as well, Salix could cause bed sintering at temperatures above 1030°C.
The release of inorganic elements, mainly K, Na, Zn, Pb, S, and Cl, from a number of well-characterized biofuels (wood chips, bark, waste wood, and straw) was quantified as a function of temperature in a lab-scale fixed-bed reactor, and as a function of residence time in a lab-scale entrained flow reactor. The fuels were characterized by use of wet chemical analysis, and also by advanced techniques: chemical fractionation analysis (of the raw fuels) and simultaneous thermal analysis (of ash samples derived from the fuels at 550 °C). In parallel to the experimental release investigation and fuel characterization, global equilibrium analysis, simulating the experimental combustion conditions, was performed.
Kaolinite, bauxite and emathlite have been found suitable for alkali removal from hot flue gases in coal conversion systems. The effect of temperature on the kinetics and mechanism of alkali adsorption/reaction on these sorbents was studied under a simulated flue gas atmosphere. Kaolinite and emathlite reacted irreversibly with the alkali; however for bauxite, 10% of the total weight gained was due to physisorption. Kaolinite was found to have the highest capacity and the largest activation energy for alkali removal. The overall sorption process is not just physical and non-selective, but rather a combination of physical and chemical processes, which are dependent on the temperature and sorbent chemistry. The reaction product of alkali with emathlite has a melting point of approximately 1270 K, while kaolinite and bauxite form compounds with a melting point of about 1870 K. Consequently, kaolinite and bauxite are more suitable for in situ removal of alkali, while all three can be used for downstream alkali removal.
In the important efforts to decrease the net CO2 emissions to the atmosphere, new, alternative fuels are being included in the fuel mixes used in utility boilers. However, these fuels have ash properties that are different from those of the traditionally used fuels and in some cases technical problems, such as ash fouling and corrosion occur due to this. Therefore, diagnostic and predictive methods are developed and used to avoid such problems. Determination of the chemical association forms of important elements, such as potassium and sodium, in the fuel by chemical fractionation is a method well defined for coal and biofuels, such as wood pellets, bark and forest residues. Chemical fractionation is a step by step leaching method extracting water soluble salts in the first step, ion exchangeable elements, such as organically associated sodium, calcium and magnesium in the second step and acid soluble compounds such as carbonates and sulfates in the third step. The solid residue fraction consists of silicates, oxides, sulfides and other minerals. The compound extracted in the two first steps is considered reactive in the combustion with a few exceptions. In this work, it has been applied to some waste fuels, i.e. sewage sludge, straw and refuse derived fuel (RDF), as well as to coal and wood. The present work also includes results from combustion tests in a fluidised bed boiler where three blends of the investigated fuels were used. The fractionation results for the fuel blends are weighted results of the fractionations of the pure fuels discussed above which are compared with fractionations of their corresponding fly ashes. The co-combustion strategy gave very good results in reducing ash problems. Possible chemical mechanisms involved are discussed in the article.
Properties of biomass relevant to combustion are briefly reviewed. The compositions of biomass among fuel types are variable, especially with respect to inorganic constituents important to the critical problems of fouling and slagging. Alkali and alkaline earth metals, in combination with other fuel elements such as silica and sulfur, and facilitated by the presence of chlorine, are responsible for many undesirable reactions in combustion furnaces and power boilers. Reductions in the concentrations of alkali metals and chlorine, created by leaching the elements from the fuel with water, yield remarkable improvements in ash fusion temperatures and confirm much of what is suggested regarding the nature of fouling by biomass fuels. Other influences of biomass composition are observed for the rates of combustion and pollutant emissions. Standardized engineering practices setting out protocols of analysis and interpretation may prove useful in reducing unfavorable impacts and industry costs, and further development is encouraged.
Phosphorus (P), phytic acid (myo-inositol hexakisphosphate or Ins P6) and mineral storage were studied in grains homozygous for barley (Hordeum vulgare L.) low phytic acid (lpa) mutations named lpa1-1, M 635, M 955, and lpa2-1. In wild-type (WT) grain both the embryo and rest-of-grain (aleurone layer) contained about 10 and 90% of whole-grain total P and Ins P. Most of the inositol phosphate (Ins P) was phytic acid. Proportional reductions in both embryo and aleurone layer Ins P contributed to whole-grain Ins P6 reductions in M 635 and M 955, with little effect on embryo or aleurone layer total P. In terms of total P these mutations show no grain-tissue-specificity. In contrast, whole-grain Ins P6 reduction in lpa1-1 and lpa2-1 is solely or largely aleurone layer specific. In lpa1-1 the distribution of total P shifted in part, from rest-of-grain to embryo, with a net reduction in total P. Electron microscopy showed a general reduction, as compared with WT, in electron-dense globoids in both aleurone and scutellum cells in all mutants except lpa2-1, whose globoid morphology appeared indistinguishable from WT. Energy-dispersive X-ray analyses of P, K, Mg, Ca, Fe, and Zn indicated that, generally, relative levels were similar in mutant and WT aleurone layer and scutellum tissues. Likely inorganic P found in lpa grains is stored as salts of K and Mg, as is Ins P6 in WT grain.
Second-rate cereals, unsuitable for food, can be used as fuel for small-scale production of heat and hot water. However, there are more problems related to cereals than to woody fuels. This work aims at characterising the particle emission from residential combustion of oat grain and its potential reduction by addition of limestone or kaolin with the fuel. Then, to a large extent, the potassium supplied by the fuel is expected to be found in coarse particles, leaving the boiler as bottom ash, instead of being emitted to the air in the form of submicron particles. Combustion experiments were performed on a residential boiler, using filter sampling and low-pressure impactors to measure the mass and number concentrations and size distributions of the emitted particles. The particles and the bottom ash were subsequently analysed for inorganic material. To check the combustion conditions and basic emissions from combustion of cereals, the flue gas was analysed with respect to gaseous O2, CO2, CO, NOx, TOC (total organic carbon), HCl and SO2. Furthermore, thermodynamic equilibrium analysis was used to support the experimental data. Finally, it is concluded that the particle emission can be lowered by supplying kaolin, while there was no effect of limestone.
Wheat phytase was purified to investigate the action of the enzyme toward its pure substrate (phytic acid - myo-inositol hexakisphosphate) and its naturally occurring substrate (phytate globoids). Phytate globoids were purified to homogeneity from wheat bran, and their nutritionally relevant parameters were quantified by ICP-MS. The main components of the globoids were phytic acid (40% w/w), protein (46% w/w), and several minerals, in particular, K > Mg > Ca > Fe (in concentration order). Investigation of enzyme kinetics revealed that K(m) and V(max) decreased by 29 and 37%, respectively, when pure phytic acid was replaced with phytate globoids as substrate. Time course degradation of phytic acid or phytate globoids using purified wheat phytase was followed by HPIC identification of inositol phosphates appearing and disappearing as products. In both cases, enzymatic degradation initiated at both the 3- and 6-positions of phytic acid and end products were inositol and phosphate.
Anonymous 20 by 2020 Europe's climate change opportunity; Commission of the European Communities Effects of sewage sludge and meat and bone meal co-combustion on SCR catalysts
- K R G Hein
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Fate of Alkali Metals and Phosphorus of Rapeseed Cake in Circulating Fluidized Bed Boiler Part 1: Cocombustion with Wood Combustion properties of biomass
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