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Effect of Catalysts in the Quality of Syngas and By-Products Obtained by Co-Gasification of Coal and Wastes. 2: Heavy Metals, Sulphur and Halogen Compounds Abatement
Abstract
This paper analyses the formation of sulphur and halogens compounds during co-gasification of low grade coals with different types of wastes that include: pine, petcoke and polyethylene (PE) with the aim of taking profit of waste energy value with the smallest possible impact on the environment. The influence of different types of catalysts or sorbents was studied: calcined dolomite, dolomite enriched with nickel, olivine, nickel and magnesium oxides, zinc oxide and cobalt and molybdenum oxides. The presence of dolomite led to the lowest HCl, HF and H2S concentrations in the syngas produced. ZnO also gave rise to significant decrease in H2S concentration, though higher concentrations were obtained than those with dolomite. It was found that when catalysts or sorbents were used, a significant fraction of sulphur, halogens and heavy metals were trapped in the solid residue left behind in the bed and cyclone. Higher temperatures increased the volatility of some metals and sulphur. Leachability assays of solids showed that small quantities of and Cl− could be released and most metals were not leachable, although the use of dolomite slightly increased metals leachability, because of the higher alkaline nature of residual solids produced.
... Generally, there are organic sulphur and inorganic sulphur compounds in coal, and the amount of sulphur does not depend on coal rank but the origin of coal. During gasification process, some sulphur present in coal might pass to the gas phase, and the environmental implications should be carefully evaluated because many countries impose emission limits to minimize the impact on the environment [7] . Moreover, sulphur in coal may cause the formation of compounds in fuel gas that could give rise to corrosion and to deposition, hence increasing maintenance costs [7,8]. ...
... During gasification process, some sulphur present in coal might pass to the gas phase, and the environmental implications should be carefully evaluated because many countries impose emission limits to minimize the impact on the environment [7] . Moreover, sulphur in coal may cause the formation of compounds in fuel gas that could give rise to corrosion and to deposition, hence increasing maintenance costs [7,8]. Therefore, the process and environmental control mechanisms of coal gasification are getting increasingly important for the provision of cheap, clean and reliable energy. ...
... Sulphur transformation can be influenced by several parameters such as fuel property, temperature, pressure, heating rate and so on [5,[7][8][9][10][11] . Till now, some researchers have studied the effects of these parameters on sulphur transformation through experimental and theoretical methods with different feedstocks. ...
To promote the utilization efficiency of coal resources, and to assist with the control of sulphur during gasification and/or downstream processes, it is essential to gain basic knowledge of sulphur transformation associated with gasification performance. In this research we investigated the influence of O2/C molar ratio both on gasification performance and sulphur transformation of a low rank coal, and the sulphur transformation mechanism was also discussed. Experiments were performed in a circulating fluidized bed gasifier with O2/C molar ratio ranging from 0.39 to 0.78 mol/mol. The results showed that increasing the O2/C molar ratio from 0.39 to 0.78 mol/mol can increase carbon conversion from 57.65% to 91.92%, and increase sulphur release ratio from 29.66% to 63.11%. The increase of O2/C molar ratio favors the formation of H2S, and also favors the retained sulphur transforming to more stable forms. Due to the reducing conditions of coal gasification, H2S is the main form of the released sulphur, which could be formed by decomposition of pyrite and by secondary reactions. Bottom char shows lower sulphur content than fly ash, and mainly exist as sulphates. X-ray photoelectron spectroscopy (XPS) measurements also show that the intensity of pyrite declines and the intensity of sulphates increases for fly ash and bottom char, and the change is more obvious for bottom char. During CFB gasification process, bigger char particles circulate in the system and have longer residence time for further reaction, which favors the release of sulphur species and can enhance the retained sulphur transforming to more stable forms.
... Hence, the characteristics and composition of these byproducts are strongly affected by the properties of the feed material and the operating conditions of the specific gasification technology adopted, such as feedstock dimensions and the operating temperature and pressure of the gasification chamber [5,6]. Gasification bed ash is generally characterized by a high content of glassy compounds (SiO 2 and Al 2 O 3 ) related to the mineral constituents of the coal and, from an environmental point of view, by an overall limited release of potentially toxic metals compared to other types of thermal treatment residues, due to its vitreous character [7,8]. However, depending on the coal conversion grade of the gasification technology employed, the slag sample may present various percentages of a carbon-rich fraction, called char. ...
... However, both residues presented a much lower hydrogen and nitrogen content than that detected for the feed coal, due to a high degree of particle carbonization, especially for the bed ash, at the operating temperatures of the gasification process, in agreement to what reported in [13]. Moreover, both types of ash, the BA in particular, exhibited a low sulfur content, indicating a low retention of this element in the solid phase which, as suggested by [8], may be ascribed to the low iron content of olivine employed to make up the bed of the gasifier. ...
... The Si concentration showed to be similar, for both residues around 8 mg/l, while Mg and Na release resulted almost three times higher in the eluate of the mixed ash sample, for which concentrations of 11 and 4 mg/l were retrieved, respectively. Apart from Cr release, that in the mixed ash sample showed to be slightly higher than the requirement for reuse (0.05 mg/l), the overall leachability of the regulated elements resulted low for both residues, in agreement with the findings of other studies [8,30]. ...
... Therefore, catalysts should be used to enhance the kinetics. ZnO, Al 2 O 3 CuO, Fe 2 O 3 and so on are wildly used as the catalysts in the pyrolysis of municipal solid waste [8][9][10][11]. ZnO plays an active role in reducing the H 2 S emission and enhancing the pyrolysis kinetics [8,10], and Fe 2 O 3 is preferred to increase in yields of liquid product. Great progresses on coal char, biomass char gasification with CO 2 using catalyst study have been made in recent years [12][13][14][15][16][17][18]. ...
... ZnO, Al 2 O 3 CuO, Fe 2 O 3 and so on are wildly used as the catalysts in the pyrolysis of municipal solid waste [8][9][10][11]. ZnO plays an active role in reducing the H 2 S emission and enhancing the pyrolysis kinetics [8,10], and Fe 2 O 3 is preferred to increase in yields of liquid product. Great progresses on coal char, biomass char gasification with CO 2 using catalyst study have been made in recent years [12][13][14][15][16][17][18]. ...
With thermogravimetric apparatus (TGA), X-ray diffraction (XRD) and Scanning electron microscopy (SEM), the catalytic pyrolysis and gasification characteristics of waste textiles (Polyester Fiber (PF), Cotton and Woolen) using several common metallic oxide as catalysts were studied at atmospheric pressure under CO2 atmosphere, several parameters including catalysts' types, loadings and reaction temperatures were also conducted. Results showed that the pyrolysis reaction of the three textiles was best enhanced by 5 wt% ZnO while the gasification reaction was best enhanced by 5 wt% Fe2O3.·The pyrolysis and gasification processes were also quantitatively evaluated by using shrinking core model. The apparent pyrolysis activation energies for textile with 5 wt% ZnO loading were 65.1 kJ/mol, while the apparent gasification activation energies for textile char with 5 wt% Fe2O3 loading were 89.0 kJ/mol, which is significantly lower than un-catalytic textiles in our previous study. Besides, a Zn-Fe composite catalyst was prepared by sol gel method, and the catalytic effect of the composite catalyst was better than that of the single catalyst. The findings reported in the manuscript are beneficial to the development of energy utilization of waste textile under carbon dioxide atmosphere.
... This increase in syngas composition might be due to the fact that dolomite has more capability to reform tar into the gaseous part as compared to the limestone. The presented results are consistent with the study of Pinto et al. [24] conducted on co-gasification of biomass in a fluidized bed using several natural catalysts. It is also reported that dolomite is more efficient in terms of syngas concentration. ...
... It is also reported that dolomite is more efficient in terms of syngas concentration. Additionally, it minimizes the production of HCl, HF, and H 2 S, which normally harm the environment and machinery [13,23,24]. ...
Catalytic co-gasification is an important tar reforming technique, which may appreciably improve the quality of syngas through tar reforming reaction. In this study, wood chips (WC) were co-gasified with two coconut wastes, namely coconut shells (CS) and coconut fronds (CF), in a downdraft gasifier. The dolomite and limestone were used as tar reforming mediums. The effect of the blending ratio, catalyst type, biomass type and catalyst to biomass loading on gas composition and heating value of the syngas was investigated for different WC/CS and WC/CF blends. The results revealed that the WC/CS blending ratio of 70:30 produces the highest H2 amount (11.70 vol.%), which was 31% higher than the H2 amount of the other blends. The HHVsyngas of 70:30 blend was measured about 4.96 MJ/Nm³, which was also higher among all the tested blends. The co-gasification of 70:30 blend of WC/CS, when compared with same blending ratio WC/CF, produced two times higher CO, 60% higher H2 and 75% higher HHVsyngas. During catalytic co-gasification of WC/CS blends with dolomite and limestone, the dolomite yielded 24%, 13.8% and 25.6% increment in CO, H2, and CH4, respectively. It is concluded that the coconut wastes can be substituted or co-gasified with wood after carrying out some major changes in a gasifier geometry.
... The presence of ion-exchangeable alkali and alkaline earth metal species in coal, however, appeared to not only change the aromatic/aliphatic composition of the produced tars, but also promote the decomposition of the tar and heavy hydrocarbons formed, whether by incomplete gasification or pyrolysis . Also, the use of inorganic additives, containing Ca, Fe, Mg, and Si compounds could diminish the release of sulfur compounds in both the tar and the gas-phase (Pinto et al., 2008). ...
... Kosminski et al. (2006a) indicted that sulfur concentrations have no influence on alkali vapor formation under gasification conditions. The presence of alkali metals in the char after pyrolysis, however, has a positive catalytic effect on its subsequent gasification or combustion (Pinto et al., 2008). ...
This chapter presents a comprehensive review of the effects of the physical and chemical properties of coal on the gasification process. These include coal reactivity (rank, macerals, porosity, surface area, mineral matter contents, etc.); ash and slag properties (mineral matter transformation, ash fusibility, slag viscosity, ash agglomeration, fouling, clinkering, etc.); coal particle size and fragmentation; and caking and swelling. Different considerations, such as gasifier types, syngas compositions and ash-related problems during gasification are also discussed. Moreover, methods for preventing ash-related problems during gasification are summarized.
... Regarding the ash content, a low percentage of it will minimize the production of fly and the bottom ash. In general, these solids contain significant amounts of unreacted carbon and sulfur [22]. ...
... d Pb concentrations in the fly ash comparing with bottom ash, during wood gasification in fixed bed; however, Cr remained in the bottom ash. Pinto et al. [22] detected higher Pb concentrations in the ash captured by cyclones comparing with bottom ash, confirming the above mentioned studies. ...
The characterization analysis of three agro-industrial wastes was performed in order to study its thermal gasification. Some analyses such as determination of Ca, K and Mg concentration and determination of three representative toxic metals concentration Cd, Cr and Pb in all its oxidation states and the fundamental state, were carried out. The heavy metals concentration was also determined in the ashes obtained during the gasification process. The mobility of these elements was studied through three leaching tests. The behavior of heavy metals, sulfur and chlorine compounds, was predicted considering the presence of water vapor, syngas, Ca, Mg, K, Si, Al and other ash components. The heavy metals are not more concentrated in the gasification ash; these pollutants are released during this process. Ca, Mg and K presence in these residues would promote the pollutants retention. The ash of the studied waste can be disposed in controlled landfills or used in road construction, according to the obtained results during the leaching test DIN-DEV S4. The obtained results in the leaching test EPA 1311 TLCP classify these gasification ashes as no toxic waste.
... Regarding the ash content, a low percentage of it will minimize the production of fly and the bottom ash. In general, these solids contain significant amounts of unreacted carbon and sulfur [22]. ...
... d Pb concentrations in the fly ash comparing with bottom ash, during wood gasification in fixed bed; however, Cr remained in the bottom ash. Pinto et al. [22] detected higher Pb concentrations in the ash captured by cyclones comparing with bottom ash, confirming the above mentioned studies. ...
The characterization analysis of three agro-industrial wastes was performed in order to study its thermal gasification. Some analyses such as determination of Ca, K and Mg concentration and determination of three representative toxic metals concentration Cd, Cr and Pb in all its oxidation states and the fundamental state, were carried out. The heavy metals concentration was also determined in the ashes obtained during the gasification process. The mobility of these elements was studied through three leaching tests. The behavior of heavy metals, sulfur and chlorine compounds, was predicted considering the presence of water vapor, syngas, Ca, Mg, K, Si, Al and other ash components. The heavy metals are not more concentrated in the gasification ash; these pollutants are released during this process. Ca, Mg and K presence in these residues would promote the pollutants retention. The ash of the studied waste can be disposed in controlled landfills or used in road construction, according to the obtained results during the leaching test DIN-DEV S4. The obtained results in the leaching test EPA 1311 TLCP classify these gasification ashes as no toxic waste.
... A few papers specifically report on co-gasification in a fluidized bed containing nickel for enhancing the gas yield and tar abatement [9,14,15]. Pinto et al. [14,15] studied the use of dolomite and Nidolomite beds during the co-gasification of coal and olive oil industry waste in steam and air atmospheres. The authors report that the most effective catalysts in tars abatement are nickel based, especially a synthetic catalyst mainly composed of nickel and magnesium oxides. ...
... A few papers specifically report on co-gasification in a fluidized bed containing nickel for enhancing the gas yield and tar abatement [9,14,15]. Pinto et al. [14,15] studied the use of dolomite and Nidolomite beds during the co-gasification of coal and olive oil industry waste in steam and air atmospheres. The authors report that the most effective catalysts in tars abatement are nickel based, especially a synthetic catalyst mainly composed of nickel and magnesium oxides. ...
The paper addresses the combined gasification of biomass and brown coal in an internal circulating fluidized bed (ICFB) for generating a valuable producer gas.A primary method for tar abatement and gas improving was adopted, consisting in a Ni-based catalyst in the bed together with sand. The hydrogen concentration in the producer gas was high (up to 35% in the best case, > 20% typically) demonstrating the validity of the ICFB process for co-gasification of coal with biomass. Higher H2 and CO contents and lower tar yields were obtained with greater coal loadings. The tar was reduced by up to three times when changing from inert to a partly catalytic bed, under steam gasification conditions. However, this good figure is contrasted by a rather fast decay in catalytic activity, and a rather low carbon conversion even in presence of the catalyst.A mathematical model based on thermodynamic computations predicts the trend and values of the measured gas concentrations at different steam/fuel ratios, providing useful indications for its optimal choice. The findings of the research reinforce the idea that dual bed gasification is suitable even when a relatively low reactivity fuel is used, such as the brown coal in this study.
... The direct addition of this oxide or calcite or dolomite in the gasification bed during the gasification of coal or other alternative fuels has also been investigated [11] not only to obtain a clean gas, but also to produce hydrogen [12]. The absorption of H 2 S contained in gasification gases to remove it from this gas by means of these oxides has also been studied [13][14][15][16][17][18][19][20]. ...
... In the study described herein, the product chosen for H 2 S absorption was dolomite as it works well under calcining (1173 K) and non-calcining(1123 K) conditions according to previous studies by Adanez [18,19], Pinto et al., [20] and Álvarez-Rodríguez and Clemente-Jul [28]. Furthermore, all the aforementioned studies on CaS oxidation have been carried out with purchased or prepared CaS. ...
Dolomite reacts with H2S to produce calcium sulphide and has been broadly investigated as a desulphurisation agent due to its low-cost and favourable properties.Because CaS reacts with water or water vapour in the environment to regenerate hydrogen sulphide and, therefore, disposal is problematic and the chemical cannot be uses as a landfill material. One of the methods used to make this material inert is oxidation to convert calcium sulphide into calcium sulphate or calcium oxide.In our study, tests were carried out using dolomite from Granada, Spain, that was previously calcined and sulphurised at high temperature with a gas similar to that produced in gasification facilities. To approximate real-scale results, a relatively large amount of substance was used for each sample (100–150g) and the samples were used in a fixed-bed position.The influence of different conditions, such as grain size, composition of the oxidation gas, gas velocity, bed length and temperature, was them investigated. The final solid products were characterised by X-ray diffraction and chemical analysis and the CO2, SO2, H2S and COS concentrations in the gases produced during oxidation were analysed by gas chromatography.The results showed that the most influential factor was grain size and that the best oxidant was O2 mixed with nitrogen.The presence of water vapour increases the residual concentration of CaS in the end product, but increased the CaO contentThe higher the oxygen concentration and the higher the gas velocity, the lower the residual content of CaS. CO2 used alone oxidises CaS to produce SO2 and COS, but at very low rates. It also produces some CS2. Water vapour used alone can also oxidise the CaS to produce H2S and SO2 but also at very low velocity.At higher oxidation temperature, between 700°C and 850°C, lesser residual CaS is obtained in the oxidised product.
... Historically, there have been a few studies that have focused on the performance of MSW and coal co-gasification. Pinto et al. [8,9] performed a co-gasification experiment on coal blending with wastes and researched the effects of catalysts on the tars, heavy metals, and sulfur produced from co-gasification; the gasifying agent was a mixture of steam and oxygen. It was found that the presence of catalysts facilitates the reduction of hydrocarbons and tars, heavy metals, and H2S. ...
An integrated CO2/O2 co-gasification system of municipal solid waste (MSW) and bituminous coal (BC) with CO2 capture was developed and simulated by the Aspen plus, which mainly consisted of three processes: air separation unit, co-gasification system, and CO2 absorption unit. In addition, raw syngas composition, cold gas efficiency (CGE), and overall energy efficiency (OEE) of the entail system were evaluated in detail with respect to the main operating parameters (gasification temperature, T; oxygen equivalence ratio, Ro; mole of CO2 to carbon ratio, Rc; and the MSW blending ratio, RM). The results indicated that the addition of BC improved the gasification of MSW. Higher gasification temperature increased CGE and OEE. Increasing the Rc ratio led to the decrease of H2 mole fraction due to the enhanced reverse water-gas shift reaction. In addition, the CGE and OEE of the system decreased with increasing RM. From the analyses of the parameters, the most optimal operating conditions were set as T = 900 °C, Ro = 0.2, Rc = 0.5, and RM = 0.6, and the corresponding OEE of the system reached 0.57. The system can achieve a large processing capacity of MSW at the cost of the efficiency loss of this condition.
... Liu et al. (2017) reported that A. donax used for phytoremediation purposes could be successfully used to produce biochar with stabilised heavy metals through pyrolysis as a method for contaminated biomass disposal. On the other hand, Werle et al. (2017) showed a potential use of M. × giganteus, S. hermaphrodita, P. virgatum and S. pectinata-contaminated biomass in energy generation via gasification, which is suggested in the literature as a safe method for HMC biomass conversion due to the capacity to control the fate of the heavy metals during the process (Pinto et al. 2008;Nzihou and Stanmore 2013). In addition, Pogrzeba et al. (2018a) showed a potential use of ashes after S. hermaphrodita gasification process as a soil amendment, where the permissible level of HM is not exceeded. ...
Anthropogenic impacts, for example residues from mining, industrial processes such as smelting or overuse of pesticides and fertilisation, are causing degradation and elevated heavy metal concentrations in farmland soils. Food or feed crops grown on this land can become contaminated with heavy metals with their consumption potentially leading to the accumulation of contaminants in human or animal populations, causing both chronic and acute health problems. Arable soils contaminated with heavy metals have a negative influence on regional economies by restricting sustainable agricultural development and the trade of goods. Second-generation bioenergy crops, based on perennial lignocellulosic crop species, are considered to be the future of the bioenergy industry and are the focus of intense research. Perennial energy crops have a low demand for nutrient inputs and higher lignin and cellulose contents than the biomass of annual crops. Moreover, they appear to be a viable economic alternative to food or feed production at heavy metal-contaminated arable lands. Besides offering an immediate cash crop for polluted soils, their deployment may eventually lead to the future recovery of those areas for arable crops thanks to their capacity for phytoremediation.
... [114]. The effect of different type of catalysts such as: dolomite, Ni-dolomite, G 72-D, C49TRX, olivine, and Ni-Mg was reported on the concentrations of HCl, HF, and H 2 S in the syngas during the co-gasification of petcoke, pinedust, and polyethlylene with coal [115]. The highest reduction in H 2 S content around 90% was achieved when dolomite was used as catalyst/sorbent followed by Ni-dolomite, G 72-D, C49TRX, olivine, and Ni-Mg. ...
... Petroleum coke is the most abundant byproduct of oil refining with over 100 million tons global production reported in the year 2010. Efforts have been directed towards petroleum coke utilization in the fields of combustion and gasification to generate electric power or produce syngas [5]. The applications for petroleum coke, however, have been limited mainly due to high Sulphur content (̴ 6 wt%) and low surface area (<5 m 2 /g). ...
... The presence of bed material reduces the temperature of gasification and prevents solid agglomeration tendency of the bed. Pinto et al. [54] found that use of dolomite catalyst helped to reduce H 2 S and to retain more than 90% sulfur in solid phase during co-gasification of Puertollano coal mixed with pine, petcoke, and polyethylene. The dolomite catalyst also reduced the formation of gaseous chlorine. ...
Electricity generation through coal–thermal route is one of the highest contributors to environment pollution through greenhouse gas emission, which has given rise to issue of climate change risk. Among different alternatives of renewable energy, an important source is biomass-based energy. Utilization of biomass for energy production in coal-fired power plants is essentially in terms of partial substitution of coal feed with biomass. Major challenge in this route is fluctuating supply and varying compositions of biomass. It can be overcome by adopting co-gasification technology (using mixed feed of biomass and low-grade coal) for power generation. In this chapter, we have presented a critical review and analysis of the literature in the area of co-gasification of biomass and coal. Analysis in this paper touches upon several facets of co-gasification process such as effect of biomass/coal ratio, the composition (proximate/ultimate analyses of biomass/coal), gasification media, temperature and heating rates on the gasification kinetics, producer gas composition, and yield. The synergistic effects between gasification of coal and biomass have been reviewed. The alkali/alkaline earth metal content in the ash of biomass catalyzes the kinetics of the gasification of coal char. However, if coal has high silica content, adverse reaction between silica and potassium oxides can deactivate the catalytic effect. Actual chemical mechanisms related to this synergy have also been described and discussed. Finally, a brief review of the literature on gasification of coal/biomass blends in bubbling/circulating fluidized bed gasifiers has also been presented.
... A low percentage of it ash content will minimize the production of fly and the bottom ash and affect positively the high heating value (HHV) [33]. Dermibas and Arin [38] In general, these solids contain significant amounts of unreacted carbon and sulfur [43], but the studied agro-industrial wastes contain very small amounts of this last element in their compositions. The high content of organic matter makes these wastes very suitable for thermal treatment [44]. ...
... Effect of catalyst or sorbent type on Cl À formation during gasification of coal at 850 C[125]. ...
The product gas produced from gasification of solid fuel contains various impurities such as particulates, toxin gases, tar, vapours of heavy metals, etc. Presence of tar is a major issue which requires to be addressed before the use of gas product in the downstream process. Tar causes problems in the process equipment like flow channels, power generating units, etc. Generally gasification technology is adapted for the utilization of low grade coal, municipal solid waste, agro-waste, bio-waste, etc., which generates toxic and emits various hazardous compounds of chlorine, sulphur, nitrogen and heavy metals like Mn, Cd and Hg. Various alkali metals like Na, K, etc., generated through the gasification of wastes also create problem in the downstream processes when condensed at low temperature. The key challenge to commercializing gasification technology is to generate a clean fuel gas which meets the global emission standards. This paper provides a comprehensive overview of the fuel gas cleaning methods those are used to remove the contaminants and gas impurities generated from various types of reactor for gasification of coal or biomass.
... This might be explained by the fact that accompanied by the rising temperature due to the higher oxygen content, some inorganic sulfides like FeS, PbS and ZnS are more released or decomposed as gaseous sulfides, leading to a decrease of sulfur concentration in the bottom ash, which is consistent with the increase of H 2 S discussed above. Pinto et al. [30] investigated the co-gasification of coal and different kinds of biomass wastes, and it was observed that the rise in temperature also allowed a decrease in sulfur retention in the condensed phase. Thus, the results obtained in the present study show similar trends with other conclusions. ...
This study concerns the distribution of sulfur species during the gasification of non-woody biomass. Experiments were carried out in a downdraft fixed bed gasifier using air and steam as oxidizing agents. Based on RSM (response surface methodology), the study investigated three operating parameters, including the ER (equivalence ratio), steam to biomass mass ratio (S:B) and biomass particle size. The independent factors were tested in the range of 0.2–0.4, 0.8–1.2 and 2.5–8.5 mm for the equivalence ratio, steam to biomass mass ratio and biomass particle size. The response variables investigated were gaseous sulfides, including H2S (hydrogen sulfide), COS (carbonyl sulfide), CH3SH (methyl mercaptan) and SO2 (sulfur dioxide), as well as the condensed phase of sulfur that retained in the fly ash and the bottom ash. The results indicate that the three operating conditions significantly affect the distribution of sulfur compounds, and the equivalence ratio is found to be the most important factor.
... However, petcoke also contains significantly more sulphur. Pinto et al. [21] have examined the influence of different catalytic materials on removal of sulphur and halogens compounds during co-gasification of coals with different types of wastes, such as pine, petcoke and polyethylene. They reported that the presence of dolomite in the bubbling fluidised bed led to the highest sulphur reduction. ...
... In a study of the cogasification of Puertollano coal mixed with pine, petcoke, and polyethylene (PE), Pinto et al. [42] found that the use of dolomite catalyst helped to reduce H 2 S and to retain sulfur in solid phase by more than 90%. It also reduced the formation of gaseous chlorine. ...
Recently, there has been significant research interest in cogasification of coal and various types of biomass blends to improve biomass gasification by reducing the tar content in the product gas. In addition, ash present in biomass catalyzes the gasification of coal. However, due to the fibrous nature of biomass and the large difference in gasification temperature of coal and biomass, cogasification in existing systems presents technical challenges. This paper documents research studies conducted on the cogasification of various types of coal and biomass using different types of gasifiers under various sets of operating conditions. In addition, the influence of cogasification on upstream and downstream processing is presented.
... Velez et al. [37] claimed that because of high differences between density of biomass and coal, the appropriate percentages of biomass were 6% and 15% for rice husk, sawdust and coffee husk. Pinto et al. [38] determined that the biomass composition should not exceed 10% as a result of high surface tension of edible oil waste in cogasification with coal. According to Nasrin et al. [39] the blending ratio depended on the specifications of fuel and the calorific value and they suggested that the maximum biomass contents should be around 5e10 %. ...
Kinetics of bituminous coal (BC) and palm shells (PSs) were evaluated using thermo-gravimetric analysis under different environments (N2/CO2/O2). The observed percent mass loss of biomass was higher than mass loss percentage of coal because biomass had higher reactivity and volatile matter content. Ignition temperatures of pure coal, biomass and their blends were also investigated and it was observed that biomass blends had improved ignition properties in both air and oxy-fuel environments. However, the combustion mechanism wasn’t affected. Different mixtures of CO2/O2 were also used on 10% PS–90% coal samples and compared with air as the reference. At the same composition of oxygen in oxy-fuel as that of simulated air, ignition temperatures were slightly higher and mass loss percentages were marginally lower. However, this difference due to heat capacities of N2 and CO2 was meager and was considered negligible. Upon increasing O2 content, lower ignition temperatures were observed. Kinetics of coal, PS and their blends were determined at different gas mixture compositions using Doyle’s and Coats-Redfern’s models. For both models, E was found to decrease with increasing PS composition in coal as well as increasing O2 concentration in oxy-fuel. However, a reverse trend was observed for the pre-exponentional factor (A).
... This suggests to investigate the possibility to feed in the gasifier a combination of different fuels since the possible synergy between their products and intermediates could lead to maximize the process performance, to reduce the carbon losses (in both particulate and tar fractions) and to increase the energy content of syngas. The scientific literature about co-gasification of plastics with wood and coal in fluidized bed reactors is limited and the behaviour of different mixtures of fuels and their possible synergic effects are far from being sufficiently understood even though very good experimental studies are available [6][7][8][9][10][11]. In particular, the effect of the fuel type, i.e., of its chemical structure and composition, on the gasification process performance and on the production of tar and particulate should be deeper investigated. ...
Seven mixtures of coals, plastics and wood have been pelletized and fed into a pre-pilot scale fluidized bed gasifier in order to investigate the main aspects of the co-gasification of these materials. The main components of the obtained syngas (CO, H2, CO2, N2, CH4, CnHm) were measured by means of on-line analyzers and a gas cromatograph. The performance of the gasifier was evaluated on the basis of syngas composition, carbon conversion efficiency, energy content of syngas, cold gas efficiency and yield of undesired by-products (tar and soot-like particulate). The results of a first series of experimental tests showed the effect of gas fluidizing velocity and that of equivalence ratio on the main performance parameters for a specific coal–plastics mixture. A second series of tests has been carried out by changing the mixture composition keeping fixed the gas velocity and equivalence ratio. The presence of wood and coal in the mixture with plastics contributed to reduce the tar production even though it is accompanied by a lower syngas specific energy.
... High temperature processes have been recently reviewed by Meng et al. [3]. Although several metallic compounds have been tested (Zn, Mn, Cu and Fe), calcium compounds are still the most widely used sorbents for hot gas desulphurisation in gasification and co-gasification processes [7][8][9]. Among calcium containing compounds, the most effective seems to be dolomite, which can reach desulphurisation efficiencies of up to 90% [2,10]. ...
H2S is an important constraint for the final use of gas from coal or biomass gasification. Its content varies depending on the sulphur present in the feedstock raw material. In parallel, char and ash from biomass and coal gasification or combustion usually have significant amounts of metals, some of which have shown activity towards H2S abatement. Thus, these materials could be a feasible and cheap alternative for H2S removal, as they are generated inside the gasification process. This work evaluates the feasibility of using ash and char from several materials (lignite, bituminous coal and sewage sludge) for H2S removal. Experiments were carried out in a fixed bed reactor at 700–900 °C, using a synthetic gas with 0.5 vol.% of H2S (similar to that obtained by air gasification of sewage sludge).
... In particular, gasification of some fuels produces a large amount of particulate while that of others fuels generates a large amount of tars (Arena et al., 2009;Mastellone et al., 2010a). This suggested to investigate the possibility to utilize a co-gasification process (Mastellone et al., 2010a;Pohorely et al., 2006;Pinto et al., 2007Pinto et al., , 2008, i.e. to feed in the gasifier a combination of different fuels since the possible synergy between their products and intermediates could lead to maximize the process performance, to reduce the carbon losses (in both particulate and tar fractions) and to increase the energy content of syngas. Few studies are available in the literature about the effect of different gasification agents on the gasification process and only a small part of these investigations are carried out with fluidized bed reactors. ...
The effect of oxygen-enriched air during fluidized bed co-gasification of a mixture of coal, plastics and wood has been investigated. The main components of the obtained syngas were measured by means of on-line analyzers and a gas chromatograph while those of the condensate phase were off-line analysed by means of a gas chromatography-mass spectrometer (GC-MS). The characterization of condensate phase as well as that of the water used as scrubbing medium completed the performed diagnostics. The experimental results were further elaborated in order to provide material and substances flow analyses inside the plant boundaries. These analyses allowed to obtain the main substance distribution between solid, gaseous and condensate phases and to estimate the conversion efficiency of carbon and hydrogen but also to easily visualise the waste streams produced by the process. The process performance was then evaluated on the basis of parameters related to the conversion efficiency of fuels into valuable products (i.e. by considering tar and particulate as process losses) as well as those related to the energy recovery.
รายงานฉบับสมบูรณ์ โครงการการประเมินสิ่งแวดล้อมระดับยุทธศาสตร์ สำหรับพื้นที่ก่อสร้างโรงไฟฟ้าถ่านหินภาคใต้ The completed report of the Strategic Environmental Assessment Project (SEA) for the construction area of the coal-fired power plant in the south of Thailand.
The search for alternatives to fossil energy traditional sources led to the development of a set of energy conversion processes, which include biomass thermochemical conversion technologies, such as torrefaction, pyrolysis, hydrothermal liquefaction, or gasification. These conversion technologies have shown significant evolutions, and there are already several examples available of application on an industrial scale. Biomass gasification processes have also presented significant developments, mainly when associated with the production of syngas with potential for energy recovery or to produce synthetic fuels, but mainly due to its potential to be used as a sustainable hydrogen production technology. In the present work, a bibliographic review of the current state-of-the-art of the biomass gasification is carried out, focusing in the gasification technologies, syngas cleaning processes, simulation methodologies on process parameters. Finally, future developments and possibilities are also analyzed and discussed, with the introduction of a new approach to hydrogen production based on the use of an adapted combustion process with air deficit.
A decoupled dual loop gasifier (DDLG) has been developed in which biomass gasification and hot gas upgrading are separated into two parallel loops and so that could be optimized individually. In the gasification loop, the gasifier is so designed that the contact between volatiles and char is restrained and therefore the steam gasification of char is enhanced. In the upgrading loop, both desulfurizer and tar reforming catalyst are used for desulfurization and tar destruction, respectively. As in-bed desulfurizer, an iron-bearing olivine supported ZnO (Zn/olivine) was prepared and tested in a fixed bed reactor. H2S sorption over ZnO, adversely affected by H2O, was accompanied by evident reduction of ZnO and vaporization of Zn at 550 oC. By contrast, no obvious ZnO reduction was observed at the same condition over Zn/olivine. The reduction-resistance of Zn/olivine was illustrated by temperature programmed reduction and powder X-ray diffraction. In DDLG test with pine sawdust as feedstock and Zn/olivine+Ni/olivine as upgrading bed materials, a synergy was found between desulfurization and tar destruction. The H2O-involved reactions such as steam gasification of char and steam reforming of tar/hydrocarbons were intensified at elivated gasification temperature and in presence of Ni/olivine. As a result, the decrease of H2O favored H2S sorption by Zn/olivine, which in turn alleviated sulfur-poisoning of Ni/olivine. Under the gasifier temperature of 850 oC, steam to biomass mass ratio (S/B) of 0.3 and upgrading reactor temperature of 600 oC, H2O and tar contents were effectively decreased to 8.8% and 1.5 g/Nm3, respectively. In the 2 h test, during which Zn/olivine experienced about 4 cycles of sulfidation/regeneration, H2S in product gas was lowered to 1.7 ppmv.
For the production of bio-SNG (substitute natural gas) from syngas of biomass steam gasification, trace amounts of sulfur and tar compounds in raw syngas must be removed. In present work, biomass gasification and in-bed raw gas upgrading have been performed in a decoupled dual loop gasifier (DDLG), with aggregation-resistant nickel supported on calcined olivine (Ni/olivine) as the upgrading catalyst for simultaneous desulfurization and tar elimination of biogenous syngas. The effects of catalyst preparation, upgrading temperature and steam content of raw syngas on sulfur removal were investigated and the catalytic tar reforming at different temperatures was evaluated as well. It was found that 850 °C calcined Ni/olivine was efficient for both inorganic-sulfur (H2S) and organic-sulfur (thiophene) removal at 600–680 °C and the excellent desulfurization performance was maintained with wide range H2O content (27.0–40.7%). Meanwhile, tar was mostly eliminated and H2 content increased much in the same temperature range. The favorable results indicate that biomass gasification in DDLG with Ni/olivine as the upgrading bed material could be a promising approach to produce qualified biogenous syngas for bio-SNG production and other syngas-derived applications in electric power, heat or fuels.
The presence of HCl, HF, tar, and sulfur compounds in fuel gas from the fluidized bed (FB) gasification of coals and wood was studied. The combined effects of coal char with special ash composition (higher Ca and Fe content) had beneficial effect on HCl, HF, and on S-compounds emissions (particularly thiophene). The less reactive bituminous coals (South African (SA)-coal) with high content of SiO3 and Al2O3 in ash could exert elevation of HCl emission during time of gasification with accumulation of coal char in bed. However, the S-compounds concentrations in producer gas always decreased during time of FB gasification. The presence of calcined dolomite together with sand in FB gasification had prevailing positive effect on emission reduction of HCl, HF, sulfur compounds, and tar for reactive coals (German coal). Even emissions of organic sulfur compounds (thiophene and benzothiophene) were reduced. Heavier tar compounds concentrations in fuel gas from FB gasification of reactive subbituminous coal (German coal) were lower than corresponding tar compounds concentrations in FB gasification of wood or less reactive bituminous (SA) coal. Accumulation of coal-char in FB significantly reduced tar compounds concentration in producer gas from reactive coal gasification. Emissions in FB gasification of wood on sand particles could be reduced by the presence of calcined dolomite particles (dust). This is an abstract of a paper presented at the CHISA 2012 - 20th International Congress of Chemical and Process Engineering and PRES 2012 - 15th Conference on Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction (Prague, Czech Republic 8/25-29/2012).
The chapter describes the state-of-the-art of fluidized bed gasification of solid fuels, starting from the key role played by hydrodynamics, and its strong correlation with physical and chemical phenomena of the process and operating performance parameters of the reactor. The possible configurations of fluidized bed gasification plants are also assessed, and an analysis of the main methods for syngas cleaning is reported. Finally, the chapter describes some of the most interesting commercial experiences. The analysis indicates that the gasification of biomass and also of municipal and industrial solid wastes appear to be the most interesting sectors for the industrial development and utilization of fluidized bed gasifiers.
In the present study, in situ removal of ammonia (NH3) and hydrogen sulphide (H2S) in the producer gas has been investigated for steam gasification of woody biomass in a 100 kW dual fluidised bed (DFB) gasifier. The removal measures consist of optimisation of operation conditions and application of catalytic bed materials. The operation conditions investigated were gasification temperature in a range of 750 to 850 °C, steam to fuel ratio from 0.6 to 1.4 kg/kgdry, and mean gas residence time from 0.19 to 0.25 s. The catalytic bed materials tested included silica, ilmenite and calcined olivine sands which were applied in the DFB steam gasifier. The concentrations of NH3 and H2S in the producer gas were measured and the conversions of fuel nitrogen (N) and sulphur (S) to form NH3 and H2S were calculated based on the experimental results. The results showed that NH3 and H2S concentrations and conversions increased with the gasification temperature and the steam to fuel ratio. However, with increase in the mean gas residence time, the NH3 concentration and conversion increased whereas the H2S concentration and conversion slightly decreased. In addition, ilmenite sand and calcined olivine sand as the bed materials showed stronger influence on reduction of NH3 and H2S concentrations as well as fuel-N and fuel-S conversions in comparison with silica sand.
This review deals mainly with co-gasification studies of different kinds of coal and biomass and the effect of biomass and coal contents in the fuel blends. The percentage of biomass or coal in co-gasification is one of the most important parameters that affect the gasification process. In this paper, the effect of gasifying various kinds of biomass, coal and their blends on the production of H2, CO2, CO, CH4 and other hydrocarbons is reviewed. In addition, other characteristics of the co-gasification process such as carbon conversion, gas yield, calorific value, cold gas efficiency, tar, H2S and NH3 contents, which are influenced by changing the kinds and percentages of fuel in the mixture of coal and biomass is elaborated upon.
Co-gasification of coal and biomass has recently attracted great attention because it has potential to offer
combined benefits of coal for high energy density and biomass for clean and renewable resources. However, the presence of ammonia (NH3) and hydrogen sulfide (H2S) in the producer gas is one of the main
technical challenges because they poison the catalysts used in the downstream applications. This
research investigated the influence of lignite to fuel (L/F) mass ratio on the NH3 and H2S concentrations
in the producer gas as well as the conversions of fuel-nitrogen (N) and fuel-sulfur (S) to form NH3 and
H2S. Experiments were performed in a 100 kW dual fluidized bed (DFB) steam gasifier at 800 �C with
blended lignite and radiata pine wood with the L/F ratio ranging from 0% to 100%. The blends were in
pellet form while the pure lignite was tested in particles. The results showed that the NH3 and H2S concentrations increased exponentially with the L/F ratio. This influence was attributed to higher contents of
N and S in lignite compared with those in wood. Moreover, non-linear relationships between the conversions of fuel-N or fuel-S and the L/F ratio were observed. The non-linear relationships of the NH3 and H2S
concentrations, and the fuel-N and fuel-S conversions as a function of the L/F ratio showed a synergetic
effect of the blended lignite and wood pellets.
Gasification has been considered as a potential thermo-chemical method of conversion to effectively utilize the carbon (organic content) present in the feedstock. Preparation and handling methods of some of the potential feedstocks (i.e., coal, petroleum residue, biomass, and municipal solid waste (MSW) to produce the synthetic gas) for various applications are discussed. The pre-treatment process for feedstock constitutes the steps that must be imposed on the raw material in preparation for use in a gasification reactor. The properties of fuel that influence the gasification are energy content, moisture content, particle size and distribution, form of the fuel, bulk density of the fuel, volatile matter content, ash content, and composition and reactivity of the fuel.
Petroleum coke (petcoke) is an abundant resource that can potentially be converted to catalyst support materials through activation to increase the surface area and reduce the sulphur content. In this work, potassium hydroxide (KOH) catalysed activation was employed with petcoke to produce activated carbons, which were characterised with nitrogen physisorption, X-ray diffraction, scanning electron microscopy and temperature-programmed reduction. With activation temperatures between 500 and 800°C, the surface area increased from 4 m2/g to between 200 and 2400 m2/g while the sulphur content was reduced from 6.6 wt% to between 1 and 0.2 wt%. Nickel catalysts (nominally 5 wt%) were prepared on the activated carbon supports using wet impregnation. The activities of these catalysts were measured for toluene hydrogenation in a plug-flow reactor with a toluene liquid hourly space velocity of 2.4/h, a pressure of 1.38 MPa, and a H2/toluene mole ratio of 90. The catalytic activity varied between zero for nickel supported on petcoke to 98% conversion, with essentially 100% to methylcyclohexane for nickel supported on carbon activated at 750°C. Thus, activated carbon from petcoke was a suitable support for Ni-based catalysts when used for toluene hydrogenation as a model reaction. © 2011 Canadian Society for Chemical Engineering
Presently, sewage sludge from wastewater treatment systems has become a critical problem in many regions of the world because it can inflict harm on human beings and the environment. Gasification technology is widely held to be a suitable and convenient approach to convert waste materials to energy with minimal greenhouse gas emissions. In a pilot-scale experiment on sewage sludge gasification ultimately aimed at generating electricity, the reactor temperature profile, syngas characteristics, and performance of the syngas in the production of electricity were tied to the equivalence ratio (ER) and syngas flow rate (i.e., 100, 150, or 180 N m3 h–1). An increase of the ER resulted in an increase of the temperature inside the gasifier, causing a variation in syngas characteristics and the level of tar and dust contamination. Calorific values at different syngas flow rates were found to vary from 4.20 to 4.87 MJ N–1 m–3. The syngas obtained at flow rates of 150 and 180 N m3 h–1 could be used in an engine–generator set to generate 21 or 47 kW of electrical power, respectively, whereas at the flow rate of 100 N m3 h–1, the syngas could only run the engine without electrical load. The specific sewage sludge consumption, which is the amount of feedstock required to generate electricity, decreased with an increase in the syngas flow rate. The performance evaluation of the sewage sludge gasification system, i.e., gasification efficiency, engine–generator set efficiency, and electrical efficiency, showed that these were in the range of those obtained from biomass, such as agricultural residues. Overall, sewage sludge can serve as a feedstock for electricity generation using a pilot-scale downdraft gasification system.
Nowadays there is a great interest in producing energy through co-gasification of low grade coals and waste blends to increase the use of alternative feedstocks with low prices. The experimental results showed that the viability of co-gasification to process such blends and that by the right manipulation of coal and biomass or waste blends, syngas treatment and upgrading may be simplified and the cost of the overall process may be reduced. Blends of three different coal grades (sub-bituminous coal from Puertollano mines, South African bituminous coal and German brown coal) with two different types of biomass (pine and olive oil bagasse) or polyethylene (PE) were co-gasified. Blend co-gasification showed to be beneficial to reduce the negative characteristics of some coals, such as the high ash and sulphur contents, especially of Puertollano coal. Syngas obtained by these blends was hot cleaned and undesirable syngas components (tar, NH3 and H2S) were measured along the hot treatment tested, which proved to be suitable to treat syngas produced by a wide range of feedstocks. Different routes for syngas cleaning were analysed to reduce unsuitable components to values required by most common end-uses. The results obtained showed that the type of feedstock to be gasified is a key outcome on initial syngas composition, affecting greatly syngas cleaning needs, its application and the economic viability of the overall process.
Halide species are one of the most abundant and damaging impurities in syngas derived from coal. Aside from environmental concerns, halides can cause irreversible damage to downstream processes such as particulate filters, catalysts, gas separation membranes, fuel cells and turbines. The current trend towards high-temperature, dry gas cleaning is making solid sorbents increasingly favourable, and sorbent development must take into consideration the chemical, physical, efficiency and economic aspects of sorbent performance. This article reviews the origins of halides in coals, cleaning requirements and efforts to develop solid sorbents for halide removal for gasification processes. Most research effort to date has concerned (1) the performance and optimization of basic sorbents such as calcium and sodium carbonate and (2) determining the tolerances of catalysts, filters, fuel cells and other plant components to halide impurities. In addition, thermodynamic calculations have been used to compare the performance of a large number of potential sorbents examined using criteria such as conversion, speciation, volatility and regenerability. This exercise confirms alkali and alkaline earth carbonates exhibit the greatest halide conversion at the temperatures demanded by various downstream processes, but these species are not regenerable at high temperatures in the same manner as desulfurization sorbents. Although not previously investigated, scope exists for the investigation of barium carbonate for halide sorption. Halide removal is best performed preceding sulfur removal due to the irreversible inhibition of regenerable desulfurization sorbents by halide species. Copyright © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
The behavior of Cynara cardunculus L. was studied during fluidized-bed (FB) combustion and gasification. The Cynara had a low moisture content and considerable lower heating value (LHV). Cynara presented significant quantities of S, Cl, and ash, which contained high levels of Na, K, P, Ca, and Si. The fuel N conversion to NO x was high because of the large release of NH 3 and HCN during pyrolysis. The conversion of the fuel S to SO 2 was low because of S retention mainly as alkali sulfates. HCl emissions were higher than the usual legal limits imposed in European Union (EU) countries, although retentions of 40− 55% fuel Cl could be estimated. The co-combustion of Cynara with eucalyptus was tested with benefits regarding process conditions, pollutant emissions, and ash behavior, but still, the HCl concentration surpassed the legal limit. The tendency for bed agglomeration was also observed during the gasification of cardoon. Two strategies were carried out to minimize this adverse effect: (1) co-gasification of cardoon with eucalyptus and (2) addition of natural minerals to the gasification bed. The results of the first strategy caused a decrease in H 2 levels, while tar, hydrocarbon, and CO amounts were found to increase. On the other hand, the addition of natural minerals did not lead to any significant change in the major gas components, although some tar and hydrocarbon abatements were observed, with olivine being the most effective. Dolomite and ZnO gave rise to a greater reduction in HCl and sulfur compounds in the gas phase, respectively.
Numerous reviews have been found related to individual studies on coal gasification (CG) and biomass gasification (BG). However, this review deals mainly with the CO-gasification (COG) of numerous types of coal and biomass and then compares their results with those obtained using coal and biomass gasification in detail. There are several process parameters which have a direct effect on the gasification process and among them temperature is the most significant one. In this paper, the production of H2, CO2, CO, CH4, and other hydrocarbons in CG, BG and COG with the variation of temperature is reviewed in detail. As it mainly influences the gaseous products and their characteristic behaviour, this review takes into account the effect of temperature on various other parameters such as carbon conversion, gas yield, calorific value, cold gas efficiency as well as tar and char contents in various kinds of coals, biomasses and their mixtures under catalytic or non-catalytic conditions.
Coal mixed with different types of wastes was co-gasified in a pilot-scale installation. The syngas produced was hot treated in two catalytic fixed-bed reactors. In the first one, dolomite was used and in the second reactor, a nickel-based catalyst was employed. Two different grade coals were tested, Puertollano and Colombian. Puertollano coal had high ash and sulphur contents, 42.5% and 2.4%, respectively, while ash and sulphur contents of Colombian coal were, respectively, 12.7% and 0.9%. Pine, bagasse, RDF and PE were the wastes mixed with both coals. After dolomite fixed-bed reactor, H2S and NH3 contents in syngas were much lower than those of the gas leaving the gasifier. For most coal and waste blends, NH3 reductions changed between 30% and 50% depending on feedstock nitrogen content, while H2S reductions achieved values from 68% to 74%, also depending on H2S concentration in syngas. After syngas had gone through the nickel-based catalyst, it presented H2S and NH3 contents that allowed its use in boilers and gas engines for most coal and waste blends. The overall syngas treatment led to H2S and NH3 reductions higher than 97%. For most experiments, final H2S and NH3 concentration in syngas were below 20ppmv and 30ppmv, respectively.
Co-gasification of poor quality coals mixed with wastes has the advantage of diversifying energy resources and of decreasing the dependency on imported fossil fuels. However, the use of wastes like plastics increased the production of tar and gaseous hydrocarbons. Although, the correct adjustment of gasification experimental conditions, like temperature and air flow, may lead to some reduction of undesirable compounds, this procedure is not usually enough to accomplish effective reductions of tar and hydrocarbons, thus obliging to the use of further gas treatment. Co-gasification studies were undertaken in a pilot-scale installation. The syngas produced, after going through a cyclone to decrease particulates content, was further treated in two catalytic fixed bed reactors. In the first fixed bed reactor was used a low cost catalyst, like dolomite, to reduce H2S content in the gas and also to promote some tar destruction. In the second fixed bed reactor, Ni based catalysts were employed to achieve effective reduction of tar and other undesirable compounds. After the second fixed bed reactor, H2 content was much higher than that of the gas leaving the gasifier, values higher than 50% were obtained, while gaseous hydrocarbons contents were much lower, particularly CnHm contents were quite low, usually below the detection limit of the method used. The presence of tar was never detected after the second fixed bed reactor.
Steam gasification of plastic materials was studied in a dual fluidized bed gasification pilot plant (DFB). Several types of plastics, which are available in large amounts in waste streams, were investigated: PE, PP, and mixtures of PE + PS, PE + PET and PE + PP. It was found that the product gas from PE was rich in CH4 and C2H4 and had a LCV of 25 MJ/N m3. About 22% of PE was converted to the monomer C2H4. Different mixtures of PE with other polymers showed, that the concentrations of CH4 and C2H4 increased with an increasing proportion of PE and that they were the main decomposition products of PE. The product gas from pure PP contained more CH4 and less C2H4 compared to the product gas from PE. The polymer mixtures behaved differently from the pure substances. Significantly more H2 and CO were generated from PE + PP and PE + PS. It can be assumed that the decomposition products of the two polymers in the mixture interacted strongly and alternately influenced the gasification process. More water was converted, so the gas production increased. The reforming reactions were enhanced and yielded H2 and CO at the expense of CH4 and C2H4. The mixture of PE + PET differed from the other polymers because of the high oxygen content of PET. Thus, 28% of CO2 were measured in the product gas. By contrast, CO2 was in the range of 8%, when oxygen-free polymers were gasified and CO2 was only produced from reactions with steam. Gasification of polymers resulted in significantly high tar loads in the product gas in the range of 100 g/N m3. The GCMS analysis of tars showed that tars from polymers mainly consisted of PAH and aromatics. Naphthalene was the most important tar compound.
It is predictable that energy demand will greatly increase in years to come, due to the continuous growth of world population,
together with the quest to improve living standards. CO2 emissions are hence expected to increase significantly. Gasification is a mature technology for energy production that permits
an easier separation of CO2 for its storage. As modern societies are producing everincreasing amounts of wastes with negative impact on the environment,
new technologies have been developed to co-gasify these wastes either with coal or alone, thus resolving a serious problem
of waste disposal. Wastes gasification reduces the dependence on fossil fuels and co-gasification with coal could provide
the benefit of security in fuel supply, as the availability of wastes and biomass fuels could vary from region to region and
show seasonal changes. Gasification experimental conditions and technologies and syngas cleaning methods are key issues for
the production of a clean gas that could find a wide range of applications. This chapter will concentrate on syngas end-uses,
focusing on new ones, like gas turbines or engines in IGCC, synthesis of methanol, ethanol and dimethyl ether, Fischer–Tropsch
synthesis, and hydrogen production. The role of gasification in CO2 sequestration will also be discussed.
Syngas normally contains a series of contaminating gases, depending on the raw materials used, the most abundant one usually
being H2S, accompanied by COS and, also, HCl, HF, etc. Normally, purification should be performed before its combustion in the gas
turbine (in the case of combined cycle plants) and the classic procedure, as performed at present in some installations, uses
the wet process, which demands a reduction in the temperature of the gas to be purified and, therefore, gives rise to a series
of thermodynamic losses. The trend is to research high-temperature purification processes that avoid or reduce this loss in
performance. In particular, there are two research lines for sulphur compounds: (i) The use of low-value metal adsorbents
that may be discarded once they have been stabilised and without contaminating properties, such as calcium compounds that
may produce CaO that captures the hydrogen sulphide and (ii) the ‘important value’ adsorbents that therefore require the ability
to be regenerated and reused, and whose base are metals with high affinity with sulphur, such as Zn, Fe and Cu but whose cost
is much higher than the previously mentioned calcium compounds.
In this work, the thermolysis of composite binary mixtures of refinery or coal-processing waste with waste biomass and D-grade
(long-flame) coal was analyzed in order to increase the efficiency of the cothermolysis of chemically different organic wastes
mainly because of the synergism of the thermolysis of mixture components and, correspondingly, the selectivity of formation
of high-quality by-products (solid, gaseous, or liquid). A new approach to the analysis of thermogravimetric data was proposed
and developed as applied to complex binary mixtures of carbon-containing materials. This approach was based on (1) the preliminary
separation of the thermal degradation of individual carbon-containing mixture components into individual structural constituents
and (2) the monitoring of the conversion of each particular structure fragment as a constituent of the mixtures in the course
of the cothermolysis of the mixtures of starting components. Based on the approach developed, data on the main synergism effects
in the course of cothermolysis in the binary test systems were obtained: the temperature regions of the appearance of these
effects were distinguished, the main conclusions were made with respect to particular structure fragments in complex organic
wastes responsible for the interaction of components in composite systems, and the directions (positive or negative) of changes
in the yields of solid by-products and the degrees of effects (difference between the yields of cothermolysis by-products
in each particular region of the appearance of synergistic effects in the systems) were determined. Additionally, the influence
of alkali metal carbonate additives on synergistic effects in the interaction between binary system components under the process
conditions of cothermolysis was analyzed.
The paper proposes a critical assessment of municipal solid waste gasification today, starting from basic aspects of the process (process types and steps, operating and performance parameters) and arriving to a comparative analysis of the reactors (fixed bed, fluidized bed, entrained bed, vertical shaft, moving grate furnace, rotary kiln, plasma reactor) as well as of the possible plant configurations (heat gasifier and power gasifier) and the environmental performances of the main commercially available gasifiers for municipal solid wastes. The analysis indicates that gasification is a technically viable option for the solid waste conversion, including residual waste from separate collection of municipal solid waste. It is able to meet existing emission limits and can have a remarkable effect on reduction of landfill disposal option.
New coal gasification processes that can generate electricity with high thermal efficiency either in a combined gas-turbine, a steam-turbine cycle (IGCC), or in a fuel cell (MCFC) are being developed. Both of these new coal-to-electricity pathways require that the coal-derived fuel gas be at a high temperature and free of potential pollutants, such as sulfur compounds. Unfortunately, some high-sulfur Illinois coals also contain a significant amount of chlorine, which converts into hydrogen chloride (HCl) in the coal gas. In this study, two simulated gasifier-product streams were contacted with the zinc titanate desulfurization sorbent in a bench-scale atmospheric fluidized-bed reactor at temperatures ranging from 538 to 750 C (1,000--1,382 F). The first set of experiments involved treating a medium-Btu fuel gas (simulating that of a Texaco oxygen-blown, entrained-bed gasifier) containing 1.4% H{sub 2}S and HCl concentrations of 0, 200, and 1,500 ppmv. The second set of experiments evaluated the hot-gas desulfurization of a low-Btu fuel gas (simulating the product of the U-Gas air-blown gasifier), which had a 0.5% H{sub 2}S content and with HCl concentrations of 0, 200, and 800 ppmv. The results of the experiments at 538 and 650 C at all the HCl concentrations revealed no deleterious effects on the capability of the sorbent to remove H{sub 2}S from the fuel-gas mixtures. In most cases, the presence of the HCl significantly enhanced the desulfurization reaction rate. Some zinc loss, however, was encountered in certain situations at 750 C when low-steam operating conditions were present. Also of interest, a portion of the incoming HCl was removed from the gas stream and was retained permanently by the sorbent.
Pyrolysis of the bituminous and brown coals was studied in the circulating fluidised bed-reactor. Influence of air/coal ratio on the product yields and composition was investigated. The pyrolysis was performed at temperature of 800–900 °C, which results from partial gasification of circulated coal char. The process was investigated in a scale of 300 kg/h of coal throughput. Particularly, the effect of dolomite addition on polyaromatic organic compounds, sulphur and chlorine distribution was investigated and discussed.
In order to remove hydrogen sulfide from a synthetic gas produced in a gasifier, a two-stage process for the ultra hot gas cleaning was set up in this study. In the first stage the sulfur content of the gasified fuel gas was reduced from 10,000ppmv to several ppmv in a fluidized-bed reactor. A fixed-bed reactor was used in the second stage for the removal of the residual hydrogen sulfide that remains less than 5ppmv levels in the fuel gas. The suitable sorbents with high sulfur-removing capacity and long-term durability for each stage were also developed and their reactivity was investigated in the ultra hot gas desulfurization process incorporated with the two-stage process. Near-zero emission was achieved as a result of experiment in this study.
The feasibility of adding iron compounds with the coal feed was examined as a means to capture sulfur in situ during the partial oxidation of coal in a Texaco gasifier operating in the slagging mode. Overall, our studies indicate that iron additives have good potential as sorbents for in situ desulfurization with oxygen gasification. The resultant slag is stable and has a low viscosity with good sulfur capture capability. Our data indicate that the required iron dosage decreases as the operating temperature decreases due to an increasing sulfur capture efficiency.
Mechanisms governing the fate of trace metals during coal combustion are reviewed, and new theoretical results interpreting existing data are presented. Emphasis is on predicting the size-segregated speciation of trace metals in pulverized coal-fired power plant effluents. This facet, which determines how trace metals originally in coal impact the environment, is controlled by fuel composition and combustion conditions. Multicomponent equilibrium calculations are used to predict vaporization/condensation temperatures for antimony, arsenic, beryllium, cadmium, chromium, lead, mercury, nickel, and selenium compounds in coal combustion flue gases, for a representative Illinois No. 6 coal. Experimental data show that equilibrium provides a good guide on the effect of chlorine on the partitioning of pure nickel, cadmium, and lead salts, introduced separately into a gaseous turbulent diffusion flame within an 82 kW combustor. Metal nuclei coagulation mechanisms are examined using existing computer codes, and these predict that coagulation does not allow condensed metal nuclei to be scavenged by existing coal ash particles. Rather, literature data on trace metal enrichment on small particles are consistent with processes of reactive scavenging of metals by larger particles, and it is suggested that these processes might be exploited further to convert these metals into environmentally benign forms.
The mechanisms which control the emission of trace metals from waste combustion systems were examined. Important phenomena include particle entrainment, chemical interactions, vaporization, condensation, particle coagulation and particle collection by flue gas cleaning equipment. A model based on these phenomena was developed to estimate their relative importance and to assess the potential impact of waste combustor operating parameters on metals emissions. The results of this assessment were used to develop a more accurate method of assessing the ability of waste combustion devices to control the emission of toxic metals than is currently proposed by the Environmental Protection Agency. The method is based on use of metals spiking and was tested in a pilot scale rotary kiln incinerator. The tests indicated that the spiking method could effectively be used to estimate metals emissions.
The capacity of solid slaked lime and limestone for binding HCl from a gas phase has been investigated in the temperature range 60-1000-degrees-C. The binding capacity is largest in the range 500-600-degrees-C. However, for slaked lime in the presence of water, a large binding capacity is observed also below 150-degrees-C. This is ascribed to the formation of a partially liquid product phase. At temperatures exceeding 500-degrees-C the binding capacity is limited by chemical equilibrium between gas and solid. For the range of properties studied the binding capacity is independent of particle size and only depends slightly on specific surface area. The kinetics of the binding reaction is governed by diffusion in the solid phase which is proved to follow an unreacted grain-core model. Diffusion coefficient for mass transport in the grain is reported for the temperature range 80-250-degrees-C.
Metal oxide sorbents with high surface area for ultra hot gas cleanup were prepared by a matrix-assisted method. A granular type of activated carbon was used as a matrix in order to increase the surface area of the metal oxide. Zinc was loaded on the surface of activated carbon by an impregnation method. Metal oxide maintained in the form of a matrix structure was observed by scanning electron microscope (SEM). A Wurtzite structure of ZnO was also confirmed by XRD measurement of the prepared granular particles. Particle sizes of metal oxide were distributed in the range of 5–50 nm in transmission electron microscope (TEM) images. The surface area of the zinc oxide calcined at 500 °C was around 56 m2/g. The values of the particle size and the surface area were directly related to the temperatures of calcination. The curves of sulphidation rates for the zinc oxides with high surface areas were measured by a Cahn balance at several different calcination temperatures. It was found that a catalyst with a large surface area showed a high activity in the desulphurization. Not only sulphur content in a simulated coal gas was completely removed by the ZnO with high surface area but also COS formed by a secondary reaction was not detected in the sulphidation tests performed in a fixed-bed reactor system. It was believed that the high surface area ZnO prepared in this study is a suitable sorbent for the ultra hot gas cleaning from the experimental results.
The catalytic effects of Na, Ca and Fe on the formation of HCN and NH3 during pyrolysis of nitrogen-containing model chars and on the emission of NOx during the char combustion have been investigated in a fixed bed reactor. The results show that fuel-type nitrogen is mainly retained in char under the pyrolysis conditions (∼900 °C). The presence of Na favors the transformation of char-nitrogen to volatile-nitrogen at high temperature, but the influence of Ca and Fe is negligible. The NH3:HCN ratio under catalytic pyrolysis conditions is higher than that under non-catalytic pyrolysis conditions. It was also found that the emission of NOx was restrained under catalytic conditions at high combustion temperature, but was favored at low temperature. The conversion of char-N to NOx depended on a number of factors including the properties of char, the types of catalysts and the combustion conditions. The influence of catalysts on the emission levels of NOx reflects the relative importance of the catalytic effect on char-N oxidation and on NO-char reaction during the combustion of nitrogen-containing char.
During devolatilization of coal, the sulphur present is distributed into solid, liquid, and gaseous products depending on the type and quantity of the coal sulphur and the processing conditions (e.g., temperature, pressure, and heating rate) used. In this study, a series of coals (mainly high volatile bituminous) was devolatilized at a relatively low temperature (500 °C) in a fixed-bed reactor in an inert atmosphere. Primarily bituminous coals were used in this study as these are shown to yield the most liquid product during pyrolysis. The distribution of sulphur in solid, liquid, and gaseous products was monitored. Influence of peak devolatilization temperature on sulphur distribution in products was determined for a high volatile bituminous coal (Pittsburgh No. 8). The sulphur content of the pyrolysis liquids generated at 500 °C correlates well with the total coal sulphur. The total sulphur of the char can be correlated with the pyritic sulphur content of the coal. Total gaseous sulphur content (sum of H2S and COS) or sulphur content of tar increases with increase in organic sulphur of coal, and relatively good correlations were obtained. An increase in pyrolysis temperature increases the total gaseous sulphur yield at the expense of char sulphur. Based on sulphur distribution data for 32 bituminous coals, equations have been developed to correlate the sulphur yield in products with the total sulphur of the feed coal. The correlations developed in this study are expected to be applicable for predicting the sulphur distribution in products during conversion of bituminous coals using low temperature pyrolysis technology, and to estimate sulphur products in the exit gases and tars when coal is converted in a fixed-bed gasifier in which initial devolatilization occurs at a relatively low temperature.
Zinc oxide based sorbents doped with CuO or TiO2 have been studied in 5-cycles tests in a fixed-bed reactor, as regenerable sorbents for desulfurization of coal gas at high temperature (600°C). TiO2 increases the stability of ZnO and zinc ferrite, under the reducing power of coal gas, and the sorbent porosity. However, it also increases the H2S concentration in the outlet gas before breakthrough, and the sorbent reactivity is not substantially modified. In addition, the presence of TiO2 makes more difficult the formation of mixed oxides and the behaviour of the fresh sorbents is usually different than that of the first regenerated samples. The presence of CuO increases sorbent reactivity and the efficiency for the first 1–3 cycles is excellent. Unfortunately, neither CuO nor TiO2 can prevent the excessive decay in performance of the studied sorbents as the number of cycles increases. This feature appears correlated, not with structural changes as shown by XRD, but with a decrease of the sorbent porosity due to progressive thermal sintering. The presence of iron oxides in the sorbent composition causes different behaviour with the appearance, after breakthrough, of COS in sulfidation and H2S and elemental sulphur in regeneration.
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.
Combustion tests have been carried out in a fluidised bed boiler to investigate the fate of trace elements during co-combustion of wood and municipal sewage sludge. The approach was to collect fuel and ash samples and to perform thermodynamic equilibrium calculations for gasification (reducing) and combustion (oxidising) conditions. Trace elements are found in the ash. Even most of the highly volatile Hg is captured in the bag filter ash. The bag filter ash offers higher surface area than the secondary cyclone ash and enhances the capture of Hg. There is no obvious correlation between capture and parameters investigated (sludge precipitation agent and lime addition). As, Cd, Hg, Pb, Se, Sb and Tl are predicted by equilibrium calculations to be volatile in the combustion chamber under oxidising conditions and Hg even at the filter temperature (150 °C). Reducing conditions promote, in some case more than others, the volatility of As, Cd, Pb, Sb, Se, Tl and Zn. The opposite effect was observed for Cu and Ni. Data points to the necessity of including bag filter in the gas cleaning system in order to achieve good removal of toxic trace elements.
The fate of trace element pollutants in integrated gasification combined-cycle (IGCC) systems is closely tied to their volatilization in the gasifier and subsequent condensation or reaction with fine fly ash particles. The results of an experimental bench-scale programme to measure the partitioning of selected trace elements during atmospheric pressure entrained flow gasification of Illinois No. 6 bituminous coal are presented. The experimental data are used in conjunction with thermodynamic equilibrium calculations to investigate the downstream behaviour of the volatile elements. It is shown that a significant mass fraction of each of the elements As, Se, Sb, Pb and Hg vaporizes during gasification of this coal, whereas Cd, Cr, Co, Mn, Ni, U and Th are relatively non-volatile. Under the reducing conditions associated with the gasifier product gas, As, Sb, Pb, Hg, and Se remain in the vapour phase at temperatures of 773 to 873 K, the operating range of most hot gas removal systems.
H2S and HCl released during biomass gasification can decrease the performance of high-temperature fuel cells in an Integrated Biomass Gasification Fuel Cell power-generating system. In this study, the behaviors of such poisonous gases during the gasification of different biomass fuels at various temperatures ranging from 673 to 1473 K were predicted using an equilibrium calculation approach. The predictions showed not only a difference in emission behaviors of HCl and H2S among the biomass fuels, but also a low HCl emission (below 10 ppmv) for a few of the fuels at any temperature. In addition, the influence of biomass metal composition and gasification temperature on emission behavior was investigated by analyzing the distribution of chlorine and sulfur compounds and the phase diagram of selected elements such as silicon and aluminum. Finally, we suggest that the addition of a potassium-rich biomass to a potassium-poor biomass has the potential to reduce the HCl emission during gasification and then to maintain the HCl concentration in gas phase below the tolerance concentration of the fuel cells.
Biomass gasification is a possible alternative to the direct use of fossil fuel energy. Biomass, a CO2 neutral source of renewable fuel, can contribute to the demand for heat, electricity and synthesis gas. However, there are inefficiencies in the technology, which at present render biomass gasification economically unviable. The presence of condensable organic compounds and methane in the product gas renders the gas unsuitable for specific applications. Elimination of the condensable organic compounds and methane by a suitably cheap technology will enhance the economic viability of biomass gasification. This paper contains an extensive literature review of the three main groups of catalysts, which have been evaluated for the elimination of these hydrocarbons. These three groups of catalysts are dolomite, alkali metals and nickel.
Edible oil wastes (EOW) are generally used in the production of soap and/or disposed in waste water treatment stations. In Europe, their use for energy is almost exclusively for the production of biodiesel. However, the nature and quality of EOW may turn their use for biodiesel not always suitable. Therefore, for environmental reasons, it is important to investigate other options like co-gasification with coal. Gasification is generally considered more environmentally friendly and its impact is considerably less polluting than other thermochemical processes. Co-gasification requires that synergy exists between coal and EOW to take profit of their complementary advantages. Co-gasification runs were undertaken on a bench-scale atmospheric fluidised bed gasifier, using both steam and air-steam mixtures as gasification medium. Operating conditions like gasification temperature, steam/air ratio and oil content in feedstock blends were varied to check their effect with the aim of optimising the gasification process. Some difficulties in feeding the blend of coal with EOW were observed when the oil content increased, which also raised hydrocarbons content in the gas produced. Both the rise of temperature and of air flow rate allowed lower tars and gaseous hydrocarbons concentrations. Higher gas yields were obtained at higher gasification temperatures, the gas being richer in hydrogen content at the expense of hydrocarbons. Solid residues (ashes and char) produced by gasification of blends of coal and EOW were also analysed to understand their nature and to evaluate their impact on the environment.
Toxic metals appear in the effluents of many combustion processes, and their release into the environment has come under regulatory scrutiny. This paper reviews the nature of the problems associated with toxic metals in combustion processes, and describes where these problems occur and how they are addressed through current and proposed regulations. Although emphasis in this paper is on problems associated with metals from incineration processes, conventional fossil fuel combustion is also considered, insofar as it pertains to mechanisms governing the fate of metals during combustion in general. This paper examines the release of metals into the vapor phase, with the particle dynamics of a nucleating, condensing, and coagulating aerosol that may be subsequently formed, and with the reactive scavenging of metals by sorbents.
A novel hot gas cleanup method to decompose a low concentration of NH3 to N2 with metal cations present inherently in low rank coals has been studied with a quartz reactor under the conditions of 750–850 °C, 0.1 MPa and high space velocity of 45,000 h−1. Each coal is pyrolyzed at 900 °C to prepare the char, which is subjected to the decomposition of 2000 ppm NH3 after pretreatment with H2. All of five chars examined promote NH3 decomposition in inert gas, but the promotion effect depends strongly on the kind of char and can correlate more closely with the Fe content than with the Ca content. This result may indicate that the Fe plays a crucial role in the reaction. A commercial activated carbon with a very low Fe content of <0.05 wt% exhibits lower conversion of NH3 to N2 than five chars. The TEM pictures reveal the formation of nanoscale particles of Fe and Ca components on a brown coal char that provides the largest catalytic performance. The char maintains the high conversion level of 80% during 25 h reaction at 750 °C and achieves the complete decomposition of NH3 at 850 °C. The co-feeding of a mixture of H2, CO, and CO2 does not change significantly the catalytic activity of the char at a steady state, whereas the coexistence of 2000 ppm H2S lowers it in the whole range of time on stream. It is proposed by combining the XRD and TPD observations with our previous results that the catalytic decomposition of NH3 in inert gas with the chars derived from low rank coals proceeds through two cycle mechanisms involving iron metal, iron nitrides, CaO and CaCN2.
In co-operation with a Dutch company (NV Afvalzorg) and the Dutch agency for energy and environment (Novem), ECN has successfully tested 10 different biomass residues in its circulating fluidized-bed gasification facility. Among the fuels used are demolition wood (both pure and mixed with sewage sludge and paper sludge), verge grass, railroad ties, cacao shells and different woody fuels. Railroad ties turn out to contain very little (heavy) metals. Initially, fuel feeding problems often impeded smooth operation. Contrary to feeding systems, the circulating fluidized-bed gasification process itself seems very flexible concerning the conversion of different kinds of biomass fuels. The fuel moisture content is one of the most important fuel characteristics. More moisture means that more air is needed to maintain the process temperature resulting in better carbon conversion and lower tar emission but also lower product gas heating value and lower cold gas efficiency. So, for a good comparison of the gasification behaviour of different fuels, the moisture content should be similar. However, the moisture content should be defined on an ash-free basis rather than on total mass (the usual way). Some of the ashes produced and retained in the second cyclone were analysed both for elemental composition and leaching behaviour. It turned out that the leaching rate of Mo and Br, elements only present in small concentrations, are preventing the ash to be considered as inert material according to the Dutch legislation for dumping on landfill sites.
The behaviour of gaseous chlorine and alkali metals of three sorts of biomass (Danish straw, Swedish wood, and sewage sludge) in combustion or gasification is investigated by the chemical equilibrium calculating tool. The ranges of temperature, air-to-fuel ratio, and pressure are varied widely in the calculations (T=400–1800 K, λ=0–1.8, and P=0.1–2.0 MPa). Results show that the air excess coefficient only has less significant influence on the release of gaseous chlorine and potassium or sodium during combustion. However, in biomass gasification, the influence of the air excess coefficient is very significant. Increasing air excess coefficient enhances the release of HCl(g), KOH(g), or NaOH(g) as well as it reduces the formation of KCl(g), NaCl(g), K(g), or Na(g). In biomass combustion or straw and sludge gasification, increasing pressure enhances the release of HCl(g) and reduces the amount of KCl(g), NaCl(g), KCl(g), or NaOH(g) at high temperatures. However, during wood gasification, the pressure enhances the formation of KOH(g) and KCl(g) and reduces the release of K(g) and HCl(g) at high temperatures. During wood and sewage sludge pyrolysis, nitrogen addition enhances the formation of KCN(g) and NaCN(g) and reduces the release of K(g) and Na(g). Kaolin addition in straw combustion may enhance the formation of potassium aluminosilicate in ash and significantly reduces the release of KCl(g) and KOH(g) and increases the formation of HCl(g).
Ni-Al coprecipitated catalysts promoted with magnesium have been prepared using the rising and the constant pH techniques, two precipitant agents [(1) KOH and K(2)CO(3), and (2) NH(4)OH)] and different metal contents. Catalyst characterization by temperature-programmed reduction and CO(2) reforming of methane as a test reaction served to select the appropriate catalysts for use in the steam gasification of biomass. The catalysts selected were NiMgAl(2)O(5), prepared at constant pH and precipitated with KOH and K(2)CO(3); NiMgAl(4)O(8) and NiMgAl(1.24)O(3.86), both prepared at increasing pH with NH(4)OH. Biomass steam gasification experiments were carried out at 700 degreesC and at atmospheric pressure using different steam/biomass (S/B) and catalyst weight/biomass flow rate (W/B) ratios. From an analysis of the results obtained, the initial activity and stability of the catalysts have been studied. The NiMgAl(2)O(5) catalyst presents the best performance showing the highest initial activity and stability. This work evidences an improvement of the NiMgAl(2)O(5) catalyst with respect to the previously studied NiAl(2)O(4) catalyst.
Catalytic activities and deactivation characteristics of oxides-supported nickel catalysts for the reaction of methane reforming with carbon dioxide were investigated. The dynamic carbon deposition on various nickel catalysts was also studied by a thermogravimetric method. Among the catalysts prepared, Ni/La2O3, Ni/alpha-Al2O3, Ni/SiO2, and Ni/CeO2 showed very high CH4 and CO2 conversions and moderate deactivation whereas Ni/MgO and Ni/TiO2 had lower conversions when the Ni reduction was conducted at 500 degrees C. When Ni/MgO catalyst was reduced at 800 degrees C, it exhibited not only comparable conversions of CH4 and CO2 with other active catalysts but also much longer period of stability without deactivation. The amount of carbon deposited in Ni-based catalysts varied depending on the nature of support and followed the order of Ni/La2O3 > Ni/alpha-Al2O3 > Ni/SiO2 > Ni/MgO > Ni/CeO2 at 700 degrees C. The carbons formed on the catalyst surface showed different structural and chemical properties, and these in turn affected the catalytic activity of the catalysts.