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Tar reduction through partial combustion of fuel gas
Abstract
A partial combustion burner is introduced as a cleaning system for the tar content of gaseous (bio) fuel. The results of experiments, using a synthetic low calorific gas mixture, demonstrate the effectivity of the proposed process. In these experiments naphthalene is added as a model tar component. The effect of partial combustion of the fuel gasmixture on the naphthalene is examined for different air/fuel ratios (λ) and varying hydrogen-methane fuel concentrations. For a fuel gasmixture with high methane concentrations or for higher λ-values the total tarcontent slightly decreases. In this case the naphthalene polymerises, i.e. forms higher ring components and sometimes even turn into soot. At lower λ's and higher hydrogen concentrations the tarcontent strongly decreases. Moreover, the naphthalene is now cracked, i.e. converted into lighter tars and permanent gases. It is found that, for fuel gases representative for biogasification products and at a λ of 0.2, the presented burner reduces the tar content of the gas with over 90% by cracking. The paper ends with a short discussion on the conditions that may determine the cracking/polymerisation mechanism.
... For the effect of other major gas species in syngas on soot formation, H 2 can reduce the reactivity of vinyl and acryl groups by reacting with OH radicals and aliphatic hydrocarbons, and inhibit the dehydrogenation and condensation [52]. Thus, H 2 chemically inhibits soot formation at all stage, and the rate of nucleation and PAH growth slow down [53,54]. CO seems to have complex effect on soot formation. ...
... For CH 4 , there is some divergence in its role in soot generation among literature. Some studies argue that the methyl radicals produced by methane interacted with other fuel components, producing more PAHs and soot than expected [53]. Meanwhile, the results of Baroncelli et al. showed that the aromatics reduced drastically when methane was added [29]. ...
... Moreover, H 2 strongly inhibited the formation of soot, while the influence of steam was limited. Houben et al. studied the effect of partial oxidation of naphthalene under different H 2 concentrations [53]. The naphthalene ring structure was decomposed by H 2 addition, even broken into permanent gases. ...
Biomass gasification is a promising technology in current and future low carbon energy systems. Soot formation is a great technical challenge for the industrialization of biomass gasification that is inevitable at high temperature and fuel rich conditions. In this review, a comprehensive summary of soot formation in biomass gasification is provided with special focus on entrained flow technologies. The topics covered the state of the art knowledge of soot formation in different gasifiers, the fundamental knowledge, experimental methods and recent control strategies. Soot generation and oxidation mechanism are discussed while the relationship between soot, tar and char in biomass gasification are analyzed in detail. Reaction models for soot formation coupled to the gasification process are introduced, including (semi-)empirical and detailed models. Effect of biomass components and ash forming elements on soot formation are highlighted. This is followed by a detailed description of in-situ and ex-situ experimental measurements, such as the optical diagnostics, aerosol particle mass analyzer and mass spectrometer. Soot formation characteristics and properties in different types of gasifiers are then addressed in detail with an emphasis of entrained flow gasifiers. Finally, the soot control strategies in biomass gasification are reviewed and evaluated. This review concludes by summarizing the available knowledge and challenges in soot formation during biomass gasification.
... European commission (DG XVII) and US DoE have defined tar as hydrocarbons with molecular weight higher than benzene [58]. Tars may be classified into four classes, (a) primary products, derived from cellulose, hemicelluloses and lignin, (b) secondary products such as phenolics and olefins, (c) alkyl tertiary products which are basically methyl derivative of aromatic series, and (d) condensed tertiary poly-aromatic hydrocarbons (PAH) without substituent [59,60]. Tar concentration in fuel gas basically depends on the type of gasifier in use. ...
... Thermal cracking uses high temperature (1100-1300 C) to decompose large organic compounds into small non-condensable gases [65]. For example, naphthalene may be reduced by more than 80% in about 1 s at 1150 C, but it takes 5 s at 1075 C [66,60]. In another experiment it was shown that only 0.5 s is required for tar to be reduced effectively at 1250 C [67]. ...
... In another experiment it was shown that only 0.5 s is required for tar to be reduced effectively at 1250 C [67]. The disadvantage of thermal cracking is that as it approaches to downstream of gasifier, it may increase the soot production, which may also increase the particulate load on the processing equipment [60,68]. ...
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 study elucidated that postgasifier addition of thermal cracking is more economical than the use of sophisticated gasifiers for high-temperature processes. However, postgasifier thermal cracking usually increased soot production resulting in an increased concentration of particulate matter (Chen et al., 2009;Houben et al., 2005). Extensive production of soot and PAHs has been observed in a fluidized bed gasifier equipped with postgasifier indirect heating (Chen et al., 2009). ...
... Extensive production of soot and PAHs has been observed in a fluidized bed gasifier equipped with postgasifier indirect heating (Chen et al., 2009). Similar results were also observed during the partial oxidation of naphthalene using high air proportion (Houben et al., 2005). Efficient control of H 2 :CH 4 ratio is favorable for thermal cracking by eliminating soot formation; however, this process required low CH 4 and high H 2 contents. ...
Considering global climate changes induced by anthropogenic activities, there is an impetus to expand away from fossil fuels as part of attempts to diminish greenhouse gas emissions. At present, the share of renewable biofuel production in the overall fuel demand is insufficient to substitute fossil fuels. Microalgal biomass has shown great potential to produce a sustainable and complementary biofuel podium with various considerable benefits. This work presents a review of the aspects of the conversion of microalgae into syngas via gasification. Syngas is main product of gasification and is a combination of H2 and CO. Syngas can be further treated by means of the Fischer-Tropsch method into methanol, dimethyl ether, and other chemical feedstocks. Moreover, the authors relatively discuss the crucial parameters, including the temperature, addition of catalyst, particle size, and type of gasification agent affecting quantity and quality of syngas production. Tar formation, catalyst deactivation, and syngas cleaning to attain the purity are also discussed comprehensibly as a means to stimulate this technology. This chapter also debates syngas industrial processing prerequisites.
... A partial combustion burner was introduced as a cleaning system for the gaseous fuel [22] . In these experiments, partial combustion was stopped by isolating the combustion gas before the flashover of the fire. ...
... Different pre-exponential factors and reaction mechanisms are apparent for different temperature ranges. The mechanisms correspond to the thermal decomposition of the soluble extract, hemicellulose, cellulose, and lignin in the sawdust and pine shells [21,22] . Therefore, the corresponding pre-exponential factor accurately characterizes the combustion characteristics of each component and the combustion process of sawdust with increasing temperature. ...
Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to show that isolated sawdust particles, ∼1.5 μm in size, are enriched in sulfur (up to 6.8 wt%). Differential scanning calorimetry data of sawdust and pine shells were used to analyze the distribution and variation of the pre-exponential factor, and the kinetic parameters of the combustion process in air were studied using the iso-conversional Friedman method. It is known that while the combustion process of biomass fuel can be divided into two main stages (smoking and flame burning), the burning of elemental sulfur occurs mainly during the smoking stage. The smoking and flame burning stages can be accurately distinguished by analyzing the variation and the first-derivative curve of the natural logarithm of the pre-exponential factor. By terminating the combustion at the transition stage between smoking and flame burning, the desulfurization of biomass can be completed, and the best total desulfurization efficiency could reach approximately 80%.
... Coal powder obtained because the coal feed for fixed bed gasification reactor must have a size above 20 mm. Meanwhile, tar is a high molecular weight hydrocarbon compounds [2], [3], [4]. Tar will condense at low temperatures, which cause clogging and blocking of pipes, valves, filters, engine and fuel cell stack. ...
... Increased temperature gasification is a promising approach to reduce the formation of tar [2]. Houben et al. [3] carried out tar reduction through partial combustion burner of a combustible gas experiment. In his study, naphthalene is added as a model tar component. ...
Fixed bed gasification of coal generate some by-products, such as tar and coal powder. Coal powder obtained because the coal feed for fixed bed gasification reactor must have a size above 20 mm. Meanwhile, tar is a high molecular weight hydrocarbon compounds that will condense at low temperatures, which cause clogging and blocking of pipes. In this study, experiment on combustible gas generation from co-gasification of tar and coal powder was conducted in an auto-thermal reactor to determine the temperature of the process, combustible gas composition, and efficiency of the process. From calculation and experiments about oxidizing reactor operation, 20 kg/hr of tar was more promising to operate and can reach the optimal temperature which was 1900°C. The energy from oxidizing reactor used for the reduction reaction of tar and pyrolysis of coal powder and produce combustible yield gas. The coal powder that can be conversion was about 14.4 kg/hr and produce approximately 84.52 kg/hr combustible gas. The calorific value of combustible yield gas amounted to 783.62 Cal/g. Combustible yield gas has advantages levels of hydrogen gas (H2) as high as 19.2%, which is already exceeding the levels of hydrogen gas from coal gasification is only < 10%. Cold gas efficiency (CGE) has a value which was still low at 26.31%. It is caused by two factors, namely the calorific value fuel gas produced was still low and the remainder of the conversion of coal powder were still mostly in the form of charcoal.
... Conventional hot gas cleaning technologies for removing tar from producer gas include thermal cracking, catalytic cracking, plasma application, and physical separation. Thermal cracking is a simple way to reduce tar; however, high temperatures (1100e1300 C) [12] and production of soot or particulate matter [13] are potential disadvantages. Catalytic cracking involves reactions at lower temperatures than thermal cracking; mineral catalysts (dolomite, olivine), transition-metal-based catalysts (Ni, Fe), and zeolite-based catalysts have been the most effective over 500e900 C [4,14]. ...
... Dry gas yield, CCE, Tar amount, and LHV changes before and after the gas cleaning with molten tin. ½CH 4 % þ CO% þ CO 2 % þ nðC n H m %Þ Â 12=22:4Mass flow rate of fuelðkg=hÞ Â C% Â 100%(13) ...
Hot gas cleaning of producer gas generated from a gasification process has many advantages in terms of thermal efficiency, gas-quality improvement, compact gas-cleaning devices, and economic feasibility. In this study, the characteristics of molten tin as a working fluid for hot gas cleaning were examined. To evaluate the hot gas cleaning performance of molten tin, the producer gas generated from the gasification of empty fruit bunch pellets was tested with a molten-tin-based gas cleaning system connected to the downstream of the gasifier. Gas chromatographic analysis of the producer gas shows that the removal efficiencies of hydrogen sulfide and non-condensable tar were about 97% and 80%, respectively, in a molten tin bed maintained at 400 °C. The results suggest that molten tin could be used as a multifunctional gas-cleaning medium for the simultaneous removal of tar and hydrogen sulfide from the producer gas.
... Tar treatment is mainly carried out to decompose heavy aromatic tar species to lighter and less problematic non-condensable gases by means of catalytic cracking or thermal cracking. Thermal cracking is decomposition of large organic molecules to lighter gases by heating for a certain residence time at temperature range from 1000 to 1300°C [128], with higher temperatures needing shorter residence time [129]. High temperatures can be generated (i) using high temperature gasifiers [98,130] (ii) by heating the gas stream via heat exchangers (iii) by introducing air or O 2 downstream of gasifier, and (iv) using energy efficient radio frequency [131,132]. ...
... High temperatures can be generated (i) using high temperature gasifiers [98,130] (ii) by heating the gas stream via heat exchangers (iii) by introducing air or O 2 downstream of gasifier, and (iv) using energy efficient radio frequency [131,132]. Thermal cracking increases soot production when applied downstream of the gasifier [129] which hampers the SOFC performance. Detailed discussion on thermal tar cracking is available elsewhere in literature [128,133]. ...
The combination of biomass gasification with fuel cells, especially high temperature Solid Oxide Fuel Cells (SOFCs) promises sustainable and highly efficient (decentralized and modular) energy conversion systems. This review encompasses the components of biomass integrated gasification-SOFC technology including biomass characteristics, the thermochemical conversion in gasifiers and the factors affecting the gasification process, the cleaning technologies for raw producer gas and its conditioning and finally the integration of gasifier with SOFCs. The influence of impurities present in biomass producer gas such as particulates, tar, H2S, HCl and alkali compounds based on recent experimental studies and their tolerance limits towards SOFCs are presented. Even though analysis based on the probable tolerance limits of impurities towards SOFCs and a comprehensive overview of the cleaning technologies for producer gas impurities indicate that producer gas cleaning at various temperatures using current technologies to meet SOFC requirements is possible, more experimental studies are still needed to acquire the detailed information on the tolerance limits of impurities for SOFCs. The recent theoretical modeling and experimental studies of biomass integrated gasification-SOFC systems are also presented.
... Naphthalene is a typical PAH component. 41,46 It can be seen that in the partial oxidation process of anisole, the generation of naphthalene happens mainly through 2C 5 H 5 = C 10 H 8 + 2H, that is, the RSR mechanism of cyclopentadienyl. The cracking reaction of naphthalene occurs mainly through the reaction with H and OH free radicals, and OH is the main reaction. ...
Methods of partial oxidation for biomass tar conversion were studied based on their detailed reaction mechanism. The good accuracy of the modeling results compared with the experimental data indicate that the model was reasonable. Anisole was chosen as the tar model component for partial combustion with equivalence ratios (ER) from 0 to 0.8. The results show that oxygen promotes the pyrolysis of anisole and thereby the tar conversion rate. An appropriate amount of oxygen could crack tar into flammable small-molecule gases (H2, CO) and inhibit the generation of polycyclic aromatic hydrocarbon (PAH) compounds. In addition to the introduction of active free radicals, partial oxidation could also improve tar cracking by exothermic oxidation to produce amounts of heat. Typical PAH production was studied based on the rate of product formation (ROP). The results show that active radicals, such as H and OH, promote tar cracking. A detailed reaction pathway for tar conversion was built. Staged oxygen supply benefited the cracking of tar into small-molecule gases and inhibited the formation of PAHs.
... 4 The heavier fraction of the formed bio oils (that is, tar) accounts for up to 70% of the total yield. 5 The amount of tar produced can be controlled through optimization of reactor design and reactor operating conditions, such as peak temperature, pressure, heating rate and residence time. 6 The major components of tar are polycyclic aromatic hydrocarbons (PAHs) and their formation involves bond breaking and cross-linking reactions. ...
Pyrolysis provides a route for the conversion of lignocellulosic biomass into solid, liquid and gaseous energy vectors or platform chemicals. Polycyclic aromatic hydrocarbons (PAHs) generated in the vapour phase of the biomass pyrolytic reaction may condense to form tars, which are difficult to further upgrade and cause process inefficiency. Control of tar production requires optimization of reactor design and careful control of reactor operating conditions. In this study, a vertical resistively-heated fixed-bed reactor is used to study the effect of pyrolysis peak temperatures and holding period (at peak temperature) on the formation of PAHs during pyrolysis of two lignocellulosic biomass samples, walnut shells (WS) and almond shells (AS). ‘In situ’ planar laser induced fluorescence (PLIF) is used to optically detect 3-to-5 ring PAHs in the vapour phase immediately above the reactor bed. Results show that the PAH PLIF signal appeared at ∼275 °C for the biomass samples and peaked at ∼400 °C for WS and ∼375 °C for AS, which is in agreement with previous ‘off situ’ analysis conducted by the authors. Beyond 400 °C, the PLIF signal was observed to reduce significantly and almost disappear at 550 °C. Initial PAH formation was attributed to condensation reactions occurring due to the drop in temperature along the sample bed. The detection of the PAH PLIF signal itself and its changing intensities, close to the bed, signified the rapid changes the products released from biomass undergo and emphasised the importance of using online techniques for pyrolysis studies. The detailed understanding of the temperature dependent characteristics of PAH formation from this study could help improve reactor design.
... To have an idea of the cooling effect of the oxidant, we performed a preliminary test in which we set the temperature of the electric heating system to 845 • C (controlling TH2, see Figure 1) and injected air at 450 • C. In this case, the temperature measured by TH1, which was just 13 cm below TH2, was about 580 • C. Figure 3 reports the average temperature distributions within the reactor in the three tests performed. Most of the works in the literature [34,35,37,38,42] show a close link between the amount of oxidant injected within the reactor and the temperature reached inside it. The temperature depends on the energy developed by the combustion reactions, which in turn depends on the amount of oxidant available for the reactions. ...
This work aims to study the influence of an oxidant agent on syngas quality. A series of tests using air and steam as oxidant agents have been performed and the results compared with those of a pyrolysis test used as a reference. Tests were carried out at Sapienza University of Rome, using an updraft reactor. The reactor was fed with hazelnut shells, waste biomass commonly available in some parts of Italy. Temperature distribution, syngas composition and heating value, and producible energy were measured. Air and steam gasification tests produced about the same amount of syngas flow, but with a different quality. The energy flow in air gasification had the smallest measurement during the experiments. On the contrary, steam gasification produced a syngas flow with higher quality (13.1 MJ/Nm3), leading to the best values of energy flow (about 5.4 MJ/s vs. 3.3 MJ/s in the case of air gasification). From the cold gas efficiency point of view, steam gasification is still the best solution, even considering the effect of the enthalpy associated with the steam injected within the gasification reactor.
... High temperature can be obtained by using high-temperature gasifiers [54,78], heating the gas stream through the heat exchanger, introducing oxygen or air in the downstream gasifier and employing energy efficient radio frequency [79,80]. However, thermal cracking process increases the soot production when downstream gasifier is used which is the obstacle to SOFC operation and performance [81]. Some catalysts can reform the tar into gas through a reaction of a certain temperature. ...
Solid oxide fuel cells (SOFC) is in focus to integrate with biomass gasification technologies to have a single and highly efficient system; combining the benefits of renewable energy sources and hydrogen energy systems. Combined heat and power (CHP) for houses is highly efficient (>90%). Micro CHP for space heating in Europe and Japan is already been popular using natural gas as fuel. Biomass derived syngas μ-CHP is more interesting. A brief overview of the systems, the present status and the future prospect of the technology has been discussed in this review to offer the guideline of its way for implementation. Since SOFC exposes to syngas derived from biomass gasification, SOFC material needs to be developed, particularly anode part, for the better sulfur tolerance and inhibit carbon deposition. The different anode materials have been discussed to have a guideline for future development. By the development of SOFC material, the entire energy integration system seems to offer greater energy efficiency for sustainable and renewable energy route.
... During the operation of the abovementioned industries, high-temperature dusty gas such as furnace gas, reactor burning and high-temperature flue gas will be generated by coal combustion, etc, usually releasing temperatures above 220°C [7]. At present, mechanical filtration [8,9], electrostatic filtration [10] and barrier filtration are the main methods of high-temperature industrial dust removal [11,12]. The bag dust removal method is defined as a form of barrier filtration; it utilizes nonwoven fabrics to capture solid particles from dusty gas to form a filter dust cake, further to filtering fine dust particles through the filter dust cake, for the purpose of efficient dust removal [13,14]. ...
Filtering of industrial PM2.5 is a major challenge for global environmental and animal protection. Filtering of materials with excellent thermal stability and other comprehensive performances is required for the removal of fine particles in high-temperature operating industries such as steel, cement, metallurgy, incineration, etc. In this study, a meta-aramid/polysulfone-amide (PMIA/PSA) composite nanofibrous filtration membrane is prepared via solution electrospinning for the development of high-temperature-resistant filtering products. To maximize the merits of each component, PMIA/PSA composite nanofibrous membranes with different mass blending ratios are prepared to determine the optimal balance. It is found that the PMIA/PSA composite nanofibrous membranes show excellent thermal stability and thermal shrinkage performance. They also maintain superb mechanical retention ratios after 200 h treatment at 200 °C. In addition, they exhibit excellent removal efficiency of polystyrene aerosol (PSL) particles of various sizes. It is found that the removal efficiency of PMIA/PSA (3/7) is 96.7% for 0.1 μm, 98.3% for 0.2 μm and 99.6% for 0.3 μm particles and it possesses optimal filtration resistance (79 Pa), while other composite membranes can reach a removal efficiency of over 99.7%. Our experimental results illustrate that the filtration efficiency for PM2.5 of PMIA/PSA (7/3), (5/5) composite nanofibrous membranes is still kept as high as 99.9% even after being treated at 200 °C for 120 h. It indicates that the prepared composite nanofibrous membranes have potential for applications where high-efficiency filtration is desired, such as bag dust filters for use under high temperatures.
... There exist some systems, such as the so-called the gas reformers, for the disposal of tar by water washing, thermal decomposition by a catalyst, and partial combustion. In the partial combustion-type gas reformer, an oxidizer gas is flushed into the producer gas 8) . An inverse diffusion flame, which has fuel and oxidizer positions opposite to those in the ordinary diffusion flame, is formed. ...
An inverse diffusion flame is formed during the partial combustion of the reformed gas for tar reduction in the producer gas generated by the gasification of woody biomass. The polymerization and decomposition of tar occur simultaneously in the vicinity of this inverse diffusion flame. The combustion reaction of producer gas proceeds in the diluted phase. In order to decompose tar without it polymerizing into soot, it is necessary to understand the flame structure. Therefore, this study is aimed at understanding the flame structure of an inverse diffusion flame. In particular, in order to analyze the influence of the diluent, the effect of the concentration of carbon dioxide as an oxidizer on the flame structure and tar decomposition was investigated by observing the CH* chemiluminescence, the planar laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAHs), and the laser-induced incandescence (LII) of soot. The results showed that the peak intensities of CH* chemiluminescence, LIF signals from PAHs, and LII signals from soot are distributed in the stated order in a radial direction from the central axis. While PAHs are formed in the upstream of the flame and decrease gradually along the mainstream direction, the relative volume fraction of primary soot particles continued to increase along the mainstream direction. Further, a high carbon dioxide concentration resulted in a longer flame. At the same time, it led to a large volume fraction of soot downstream of the flame. As the concentration of carbon dioxide in the oxidizer increased, carbon yield decreased, suggesting an increase in soot formation.
... catalytic) treatments can reform tar at very low temperature, but shortcomings such as catalyst deactivation due to deposited carbon and H 2 S still exists. Thermal tar cracking seems a viable approach for larger scale pyrolysis units, which may not however reduce tar yields sufficiently or even generate undesirable soot [33]. Phuphuakrat et al. [34] investigated pyrolysis tar removal, by the two-step function of thermal decomposition and physical adsorption. ...
A pilot scale investigation of co-pyrolysis of biological dairy sludge and spruce wood chips and pyrolysis of spruce wood chips solely was carried out. Pyrolysis was tested as a waste treatment method aiming to reduce the volume of dairy sludge while producing a pyrolysis gas suitable for an internal combustion engine. Pyrolysis tests were carried out in a continuously fed, pilot scale rotating retort type of facility in the temperature range between 700 and 770 °C. Feedstock feeding rates were between 40.9 – 68.6 kgd.a.f. h⁻¹. Tar yields and composition was measured by means of the solid phase adsorption method in order to assess gas quality with regard to the specified tar limits given for downstream applications. The yields of total gas chromatography detectable tar produced from the dairy sludge and spruce wood chips blend was in the range between 7.25 - 10.98 gtotal tar Nm⁻³dry raw gas, while spruce wood chips solely produced yields between 11.18 - 13.31 gtotal tar Nm⁻³dry raw gas. Composition wise, the main difference was a number of nitrogen-containing tar compounds reflecting the high nitrogen content in dairy sludge feedstock with 2-butenenitrile, pyridine and 1H-pyrrole being the most abundant nitrogen-containing tar compounds. Raw pyrolysis gas from the two feedstocks tested did not meet the requirements regarding tar limits given in the manufacturer’s specification for their internal combustion engine. The raw pyrolysis gas contained excessive amounts of 3 and 4+ aromatic ring tars. Therefore tar removal is required prior to combustion in the engine. The proposed tar removal strategy includes a thermal tar reformer using air as a reforming agent followed by adsorption using wood chips, or in-process generated bio-char, or torrefied biomass as a viable adsorbent.
... Such processes as a multi-stage gasification combining pyrolysis and gasification [13], combination of pyrolysis and gasification at different locations [14] or combination of gasification with a partial oxidation stage [15] promise high process efficiency, sufficiently pure syngas with low tar concentration or even tar reduction by partial oxidation. ...
Recycling of organic waste is an increasingly hot topic in recent years. This issue becomes even more interesting if such processing leads to a source of hydrogen or syngas for fuel production. A process of high-temperature decomposition of lignite was studied on the plasma gasification reactor PLASGAS, where water-stabilized plasma torch was used as a source of high-enthalpy plasma. The plasma torch power was 120kW and allowed heating of the reactor to more than 1000°C. The material feeding rate in the gasification reactor was 30 or 60kg per hour that is comparable to a small industrial production process. The efficiency evaluation of the thermal decomposition process was performed. Energy balance of the process was carried out as well as an influence of the lignite particle size and the addition of methane (CH4) on the synthesis gas composition. The ratio H2/CO was in the range of 1.5–2.5 depending on the experimental conditions.
... Therefore, tar removal from syngas is necessary. Several tar elimination methods have been developed, such as physical treatment (electrostatic precipitation, inertial separation) [12], wet or dry scrubbing [13,14], plasma cracking [15,16], thermal cracking [17][18][19] and catalytic cracking [20][21][22][23]. Cleaning processes can be classified in two types: primary methods aim at limiting the tar formation during the gasification reactions, and secondary methods consist in the collect or decomposition of tar already formed [24]. ...
The catalytic activity of pyrolysis chars from bio-waste was investigated for the cracking of model tar compounds (ethylbenzene and benzene). Two pyrolysis chars were produced at 700 °C from (1) used wood pallets (UWP), and (2) a 50/50 dry% mixture of food waste (FW) and coagulation-flocculation sludge (CFS). Steam activation at 850 °C was used to study the influence of the porous structure. While coke deposition is known to be responsible for the deactivation of carbonaceous chars and metal catalysts during tar cracking reactions, the deactivation of complex materials such as bio-waste chars has scarcely been studied. For this reason, special attention was paid on the relationships between the physicochemical properties of the chars, the operating conditions, and the deactivation mechanisms. To this aim, the cracking tests were performed over a wide temperature range: 400–650 °C for the ethylbenzene cracking, and 850–950 °C for benzene cracking. After the ethylbenzene cracking tests at 650 °C, the characterisations performed with SEM, BET, FTIR and Raman revealed that coke deposition was responsible for the char’s deactivation. The high specific surface area of activated chars explained their higher catalytic activity, and mesoporous catalysts were proved to be more resistant to coke deactivation than microporous catalysts. For these reasons, the higher ethylbenzene conversion (85.8%) was reached with the activated char from food waste and sludge (ac.FW/CFS). For benzene cracking at higher temperature (850 and 950 °C), the chars from food waste and sludge (FW/CFS) were the most active catalysts, despite their deactivation by the melting, diffusion and sintering of the inorganic species. This original deactivation mechanism, reported for the first time, led to the formation of an inorganic layer composed of P and Ca species at the char surface, with some areas rich in KCl and NaCl. Non-activated char from food waste and sludge (c.FW/CFS) was surprisingly proved to be more resistant to deactivation by inorganic species than the activated char (ac.FW/CFS) during the benzene cracking tests at 950 °C. This extended catalytic activity was explained by the activation of the non-activated char (c.FW/CFS) with the CO2 contained in the syngas which simultaneously developed the porosity and created new available active sites. This study marks a step forward in the understanding of the relationships between the deactivation mechanisms, the physicochemical properties of the chars, and the cracking temperature. Finally, a proposal for process integration is presented to consider the possibility to valorise the chars as catalysts to decompose the tar generated in the same pyro-gasification process.
... Apart from DFBG, partial combustion has also attracted an increasing deal of interest in recent years as a method to achieve thermal tar conversion. Air/fuel ratio, hydrogen concentration, methane concentration, temperature, and free radicals produced during the combustion, influence the cracking or polymerization reactions of the tar components (Houben et al., 2005; Anis and Zainal, 2011; Gómez-Barea, et al., 2013b). ...
Gasification is an efficient process to obtain valuable products from biomass with several potential applications, which has received increasing attention over the last decades. Further development of gasification technology requires innovative and economical gasification methods with high efficiencies. Various conventional mechanisms of biomass gasification as well as new technologies are discussed in this paper. Furthermore, co-gasification of biomass and coal as an efficient method to protect the environment by reduction of GHG emissions has been comparatively discussed. The increasing attention to renewable resources is driven by the climate change due to GHG emissions caused by conventional fossil fuels, while biomass gasification is considered as a potentially sustainable and environmentally friendly technology. Social and environmental aspects should also be taken into account in the design, to guarantee the sustainable use of biomass. This paper also reviews life cycle assessment studies on the biomass gasification, considering different technologies and various feedstocks.
... C 6 H 5 Cl and C 6 H 5 CH 2 Cl were the two main chlorinated hydrocarbons from PVC pyrolysis. Other traditional kinds of tar [27][28][29] had also been found, such as toluene, o-xylene, p-xylene, indane, indene, naphthalene and so on. ...
Dioxins emission is a serious problem during thermal utilization of municipal solid waste (MSW). The key structure in dioxins is CCl bond, so controlling the formation of chlorinated hydrocarbons, which contain CCl bond, is very important. PVC, a main source of chlorine in MSW, was used as fuel in this work. Chlorinated hydrocarbons were released from PVC pyrolysis process. A hot-rod reactor combined with a homogeneous conversion stage was established in an attempt to realize the concept of utilizing the hydrogen, which was also released from PVC pyrolysis process, to control the formation of chlorinated hydrocarbons. In the experiments, the influences of temperature, concentration of hydrogen and residence time in the homogeneous conversion stage on the formation of chlorinated hydrocarbons were investigated. Some results were concluded as follows, first, during PVC pyrolysis progress, H2, Cl2, HCl, hydrocarbons and chlorinated hydrocarbons would be released so that the dechlorination reaction and the chlorination reaction occurred in the homogeneous conversion stage. Second, the concentration of hydrogen released during PVC pyrolysis process was enough as opposed to the release of chlorine. Third, elevating the temperature of homogeneous conversion could promote the dechlorination reaction. Increasing the concentration of hydrogen had the same effect. Fourth, enough residence time in the homogeneous conversion stage could ensure the dechlorination reaction completely to occur. In a word, the results suggested that utilizing hydrogen released from PVC pyrolysis could control the formation of chlorinated hydrocarbons effectively when the temperature of the homogeneous conversion stage was over 700°C with residence time more than 1.5 s. © 2016 American Institute of Chemical Engineers Environ Prog, 2016
... However, tar emission still remained an issue for attainment of low-temperature gasification. Many researchers have reported on methods to reduce tar, such as partial oxidation [4], [5], catalytic decomposition [6]- [10], and char utilization [11]- [22]. Char utilization has advantages such as the avoidance of catalyst deactivation by impurities in the coal, such as sulfur and chlorine, and low cost. ...
... This is reflected by an increasing gap between toluene conversion T η and syngas production ox sg η efficiencies. This could be partially explained by the fact that the rate of hydrogen oxidation is an order of magnitude greater than that of CO and various organics [33,34].Therefore, with rising oxygen content hydrogen consumption increases rapidly through combustion and its participation in saturation of double bonds to alleviate steam reforming and prevent polymerization reactions [35] becomes very limited. ...
A complete decomposition of all organic molecules generated as a result of biomass gasification into simple inorganic species like CO and H 2 (syngas) means both an improved energy efficiency and technological robustness of the process of renewable synthesis gas (syngas) production. The syngas could be then converted into synthetic fuels, chemicals or electricity. A contamination of the produced syngas with polyaromatic hydrocarbons (PAH) promotes an origination of tar which, when the temperature of the gas becomes lower, deposits on the inner surfaces of the downstream equipment inducing its fouling and possibly its failure. A non-catalytic method of tar decomposition includes mixing of gasification products with plasma gas (usually air) generated by its heating to extremely high temperatures (5000 0 C and higher) in a high-voltage electrical field. Combustion products at temperatures of the oxy-fuel flames could be also considered as plasma due to a presence of ionized particles, radicals and free electrons. The effect of continuous injection of the hydrogen/oxygen high-temperature flame into the blend of gases containing toluene in order to decompose the latter has been studied in the present work. Toluene in the mixture with nitrogen and steam (reaction mixture) was selected as a tar model compound because it has been regarded by many researchers as a precursor of heavier polyaromatic compounds in high temperature gasification processes. The experimental results indicate that treatment of the reaction mixture with the stoichiometric hydrogen/oxygen flame leads to the reforming of toluene into H 2 and CO. A certain excess of oxygen in the flame promotes almost complete toluene decomposition into syngas in the regime of its partial oxidation. In spite of some reduction in the amount of syngas produced, that excess oxygen in the hydrogen/oxygen flame could be efficiently applied to the syngas contaminated with tars; this allows the original syngas heating value to remain unchanged. A mechanism of hydrogen/oxygen flame influence on tar destruction/decomposition pathways is also discussed. Hydrogen and oxygen could be technically produced by electrolysis of water. The efficiency of electricity utilization to make the hydrogen/oxygen flame in order to decompose volatile organic compounds and tars will be a subject for our following studies.
... The syngas partial combustion also allows for the cracking of the tar into lighter HCs, but a relatively high amount of useful compounds can be found in the flue gas. 18 In this context, tar destruction through the complete combustion of the syngas might be an interesting alternative. A suitable technology to achieve this may be the combustion of the syngas in porous media. ...
The cleaning of syngas is one of the most important challenges in the development of technologies based on gasification of biomass. Tar is an undesired byproduct because, once condensed, it can cause fouling and plugging and damage the downstream equipment. Thermochemical methods for tar destruction, which include catalytic cracking and thermal cracking, are intrinsically attractive because they are energetically efficient and no movable parts are required nor byproducts are produced. The main difficulty with these methods is the tendency for tar to polymerize at high temperatures. An alternative to tar removal is the complete combustion of the syngas in a porous burner directly as it leaves the particle capture system. In this context, the main aim of this study is to evaluate the destruction of the tar present in the syngas from biomass gasification by combustion in porous media. A gas mixture was used to emulate the syngas, which included toluene as a tar surrogate. Initially, CHEMKIN was used to assess the potential of the proposed solution. The calculations revealed the complete destruction of the tar surrogate for a wide range of operating conditions and indicated that the most important reactions in the toluene conversion are C6H5CH3 + OH ↔ C6H5CH2 + H2O, C6H5CH3 + OH ↔ C6H4CH3 + H2O, and C6H5CH3 + O ↔ OC6H4CH3 + H and that the formation of toluene can occur through C6H5CH2 + H ↔ C6H5CH3. Subsequently, experimental tests were performed in a porous burner fired with pure methane and syngas for two equivalence ratios and three flow velocities. In these tests, the toluene concentration in the syngas varied from 50 to 200 g/Nm3. In line with the CHEMKIN calculations, the results revealed that toluene was almost completely destroyed for all tested conditions and that the process did not affect the performance of the porous burner regarding the emissions of CO, hydrocarbons, and NOx.
To address the impacts of climate change, it is imperative to significantly decrease anthropogenic greenhouse gas emissions. Biomass-based chemicals and fuels will play a crucial role in substituting fossil-based feedstock...
Among the methods to reduce tar emission, the partial oxidation (POX) of biomass gasification tars has been studied both experimentally at a pilot-scale and numerically. The gasification producer gas was obtained at a temperature of 800 °C in an air-blown fluidized bed with an equivalent ratio (ER) of 0.25. For the POX unit, two secondary ER values were selected: 0.05 and 0.10, with the option of pre-heating air or not. Multiple advanced analytical methods were employed to provide a detailed composition of the producer gas, tars and acid gases. The POX unit demonstrated the ability to reduce tar levels by 60 to 90% depending on the secondary ER (from 6.5 to 2.4 and 0.72 gtars/Nm3, excluding benzene). The lighter tars were almost completely eliminated. The permanent gases were barely modified while the light hydrocarbons (except C2H2) and benzene were significantly reduced. Consequently, there was a slight decrease in the lower heating value. These results were compared to an isothermal plug flow reactor model, which utilized a detailed radical kinetic scheme constructed from various sources to account for all the species measured during the experiments as well as soot mass yield. The model provided relatively accurate predictions of the hydrocarbon species variations, even though it did not consider the mixing between air and syngas at the inlet of the POX unit.
The horizontal fixed bed pyrolysis method was used in this study to examine the reaction parameters of K in-situ catalytic pyrolysis of the cotton stalk at 600 °C. The pyrolysis conversion mechanism of cotton stalk under the influence of K was investigated in conjunction with gas chromatography analysis, FT-IR analysis, and GC-MS analysis. According to the findings, the gas production of a mixture of 1 g cotton stalks grew from 215 mL (0.0 %- K2CO3) to 275 mL (7.5% -K2CO3), but it was inhibited to 263 mL when K2CO3 addition was at 10.0%. According to the results of the characterization, K2CO3 might accelerate the breakdown of oxygen-containing rings in cellulose and hemicellulose, encourage the conversion of furan structure into ketones, and prevent the transformation of furan into long-chain alkanes. The addition of K2CO3 introduces more K into the cotton stalk. Under the influence of K, long-chain alkanes, phenols, and esters will be further cracked and polymerized to create more stable aromatic hydrocarbons. According to quantum chemical calculations, xylose's oxygen-containing ring opened first without the presence of K, then H transfer, dehydrogenation, dehydration, and cyclization to generate the cyclopentanone structure. The oxygen-containing groups in the xylose side chain preferentially bind to K in the presence of K, and the bond length between the O and C rings of the side chain is lengthened, while without K, the C–O bond length of the preferred ring opening is shortened.
La minimisation des impacts causés par le changement climatique impose de substituer des énergies fossiles par des énergies faiblement émettrices de CO₂. L’hydrogène est vu comme un vecteur énergétique permettant de décarboner une partie de l’industrie et des usages de transport et de mobilité. Pourtant, l’hydrogène est produit aujourd’hui quasi-exclusivement à partir d’énergies fossiles pour des usages industriels.Ces travaux s’intéressent à la production d’hydrogène à partir d’une ressource renouvelable, les plaquettes de bois produits secondaires de l’industrie forestière. Compte tenu de la nature du combustible utilisé, des petites unités de valorisation sont envisagées (zone d’approvisionnement limitée, transport de la ressource à courte distance). Les procédés de pyrogazéification permettent la transformation de cette ressource en un gaz de synthèse (CO, H₂, CH₄, CO₂) sous l’effet d’un apport de chaleur (pyrolyse) ou d’un agent oxydant (gazéification) constitué d’oxygène et de vapeur d’eau.Pour juger de la pertinence de ces procédés de pyrogazéification, ils sont étudiés et modélisés avec Aspen Plus. Une attention particulière est apportée à la chaîne de traitement du gaz de synthèse produit. Ce syngaz contient des goudrons qu’il convient de réduire pour l’utilisation ultime du gaz. Dans ce but, une unité d’oxydation partielle est envisagée et modélisée à partir de mécanismes de cinétique radicalaire. Le gaz épuré peut alors être enrichi en H₂ avec des réacteurs de Reformage Catalytique et de Water Gas Shift. La séparation de l’hydrogène produit est une autre étape cruciale et les technologies classiques ne sont pas toujours adaptées au gaz produit. Quand une seule technologie n’est pas à même de réaliser la séparation, un procédé hybride combinant des technologies membranaire et d’adsorption est adopté. La chaleur produite par le procédé est valorisée dans un réseau de chaleur. Afin de juger de la pertinence de ces options, tant d’un point de vue financier que du développement durable, une analyse technico-économique est réalisée ainsi qu’une analyse de cycle de vie. Ces procédés offrent clairement une alternative vertueuse pour la production de différents vecteurs : hydrogène, chaleur, voire biochar. Mais dans les conditions actuelles de marché, ces filières ne sont pas en mesure d’atteindre l’équilibre financier sans un soutien public.
Considering global climate changes induced by anthropogenic activities, there is an impetus to expand away from fossil fuels as part of attempts to diminish greenhouse gas emissions. At present, the share of renewable biofuel production in the overall fuel demand is insufficient to substitute fossil fuels. Microalgal biomass has shown great potential to produce a sustainable and complementary biofuel podium with various considerable benefits. This work presents a review of the aspects of the conversion of microalgae into syngas via gasification. Syngas is main product of gasification and is a combination of H2 and CO. Syngas can be further treated by means of the Fischer-Tropsch method into methanol, dimethyl ether, and other chemical feedstocks. Moreover, the authors relatively discuss the crucial parameters, including the temperature, addition of catalyst, particle size, and type of gasification agent affecting quantity and quality of syngas production. Tar formation, catalyst deactivation, and syngas cleaning to attain the purity are also discussed comprehensibly as a means to stimulate this technology. This chapter also debates syngas industrial processing prerequisites.
Les procédés de gazéification permettent la valorisation énergétique de biomasses solides par leur transformation en un gaz de synthèse riche en H2 et CO, valorisable pour la production d’énergie. Le gaz de synthèse contient également de nombreux polluants et son épuration reste un des freins majeurs au développement industriel de cette technologie. Parmi ces polluants, les goudrons sont des composés organiques qui condensent à partir de 350°C et encrassent les équipements en aval du gazéifieur. Leur condensation impacte la fiabilité de ces procédés du fait d’une maintenance récurrente et de la réduction de la durée de vie de certains équipements. L’utilisation du charbon pour l’épuration du gaz de synthèse a beaucoup été étudiée sur des molécules modèles à l’échelle laboratoire, mais beaucoup moins sur des goudrons réels provenant directement d’un gazéifieur. Cette thèse vise l’étude de la conversion des goudrons et du gaz de synthèse à travers un lit de charbon.D’un point de vue méthodologique, une étude expérimentale a été menée en s’appuyant sur un réacteur original de conversion des goudrons développé et mis au point dans le cadre de cette thèse. Ce réacteur catalytique a été couplé à un réacteur commercial de gazéification de technologie à lit fixe co-courant.Dans les conditions opératoires de référence pour la conversion des goudrons (800°C ; 2s), et pour une teneur en goudrons de l’ordre de 9 g/Nm3, le taux de conversion est de 51%. La part du craquage thermique sur la conversion des goudrons est dans ces conditions de 11%.Des essais ont également été menés pour étudier l’influence de la teneur en air et en eau, du temps de séjour, et de la température sur la conversion des goudrons et du gaz de synthèse. Une augmentation de ces paramètres favorise la conversion des goudrons à l’exception de la teneur en eau. Une teneur en vapeur d’eau de 19% diminue le taux de conversion des goudrons à 40%, contre 50% pour une teneur en eau de 12%. L’ajout de 11,2% d’air dans le gaz de synthèse permet d’augmenter le taux de conversion des goudrons à 70% contre 40% sans air à 800°C et 2s. Dans ce cas, le PCI du gaz de synthèse diminue de 12,6% malgré un taux de production de H2 de 31,9% à travers le lit de charbon. Pour une faible teneur en goudrons de 5,7 g/Nm3 et un temps de séjour de 1,82s à 800°C, un gaz de synthèse composé majoritairement de benzène, toluène, éthylbenzène, xylènes (BTEX) est obtenu. Dans ces conditions, la teneur en goudrons (hors BTEX) de 151 mg/Nm3 et le point de condensation des goudrons de 40°C après craquage permettent d’envisager un couplage direct avec un moteur. Pour des temps de séjour inférieurs à 0,6s à 800°C, des composés HAP sont formés malgré un taux de conversion des goudrons de 40,5%, se traduisant par une augmentation du point de condensation des goudrons à travers le lit de charbon.Et enfin, un jeu de paramètres opératoires permettant l’élimination de l’ensemble des goudrons problématiques a été proposé et testé en vue d’un passage à l’échelle industrielle.
This study investigates the partial combustion technique for tar conversion using a modified experimental set up comprising a packed bed reactor with bed-inside probe for air supply. Simulated producer gas (SPG) and benzene were selected as a real producer gas alternative and model tar component respectively. The benzene conversion was investigated under different experimental conditions such as reactor temperature (650 – 900 °C), packed bed height (0 – 12 cm), residence time (1.2 – 1.9 s), air fuel ratio (0.2 and 0.3) and SPG composition. The results showed insignificant effect of temperature over benzene conversion while air fuel ratio of 0.3 caused high benzene conversion than at 0.2. Absence of packed bed lead high benzene conversion of 90% to polyaromatic hydrocarbons (PAHs) compared to similar low PAHs free benzene conversion of 32 % achieved at both packed heights. In SPG composition effect, H2 and CH4 had a substantial inverse effect on benzene conversion. An increase in H2 concentration from 12 to 24 Vol% increased the benzene conversion from 26 to 45% while an increase in CH4 concentration from 7 to 14 Vol% reduced the benzene conversion from 28 to 4%. However, other SPG components had insignificant impacts on benzene conversion.
Although rapid pyrolysis affords higher volatile yield than slow pyrolysis, the change in the yield of components at different heating rates have not been reported in detail. Moreover, few studies have assessed the changes in the tar component yield with respect to biomass type. Therefore, from a practical point of view, this study quantitatively determined the effects of the heating rate and biomass type on the tar component yield through gas chromatography-mass spectrometry. In addition, the mechanism of tar formation was investigated. The main results of the study are as follows: (1) Changes in the yield and component of biomass tar due to increase in the heating rate by a factor of 100 were quantitatively determined. An increased heating rate resulted in a higher yield of aromatic compounds and induced the formation of benzene, toluene, and other compounds. At slow heating rates, the yield of odorous components such as vanillin, furfural, acids, and aldehydes increased. At the middle heating rate (1.0 K/s), a significant increase in the amount of phenols containing OH and O groups was observed. (2) For woody biomass, acetic acid, cellulose-derived glucose, catechol, phenols, and furfural were identified as the major tar components. (3) The tar components volatilized from the wood trunk, bark, and grass are affected by the primary content of the biomass constituents.
Drop tube furnace is a high-efficiency pyrolysis reactor, which is helpful for fundamental research of coal base poly-generation technology. In this paper, the characteristics of coal pyrolysis product at various temperature using drop tube furnace were investigated. A typical Chinese lignite Pingzhuang coal was used in current research, pyrolysis temperature ranged from 600 to 1000 °C with a step of 100 °C. The compositions of pyrolysis gas, coal tar and char were characterized and discussed in detail. Results showed that volatiles and moisture content decreased after pyrolysis, hydrophilic functional groups like carboxyl and hydroxyl also decreased, pyrolysis was a dehydration upgrading process for lignite. Specific surface area and total pore volume increased from 1.65 m²/g and 11.57 mm³/g in raw coal to 199.10 m²/g and 191.95 mm³/g in 1000 °C char, respectively. As for pyrolysis gas, when temperature increased, H2 and CO increased obviously, other compositions like CO2, CH4 and C2∼C3 hydrocarbons decreased, consequently, the gross heating value of pyrolysis gas decreased from 22.5 MJ/m³ at 600 °C to 13.1 MJ/m³ at 1000 °C. High temperature pyrolysis had a high content of syngas (H2+CO), which can be utilized as raw material in synthetic chemical industry. There were high content of toluene in low temperature coal tar. There were more polycyclic aromatic hydrocarbons and less aliphatic chain hydrocarbons in high temperature coal tar.
The detailed reaction mechanism of OH radical destroying toluene, phenol and naphthalene was studied through quantum chemical calculations in the research. Theoretical results indicate that for phenol and toluene, OH radical preferentially attack the ortho C atom due to the functional group on the benzene ring. But for naphthalene, OH radical preferentially attack the para-position C atoms because of its inherent benzo structure. To further study of the kinetics, the rate constant was calculated by the transition state theory. The comparison shows that the theoretical reaction rate constants for the degradation of tar by the OH radical were consistent with those obtained from literature experiments. And the rate constant of destructing naphthalene by OH radical was larger than that of destructing toluene, but lower than that of destructing phenol. The degradation sequence of OH radical to tar is: phenol>naphthalene>toluene. Because of the activation of hydroxyl group in benzene ring, phenol is the most easily attacked and destroyed by OH radical. The theoretical results can provide theoretical basis and data reference for further research on the removal of biomass tar and aromatics by OH radical.
This paper aims to investigate the hydropyrolysis behavior of pulverized bituminous coal in a drop-tube furnace (DTF) with a temperature of 800–1000 °C. The effects of temperature and hydrogen on the compositions (gases and tar) and the structure of products (char) were investigated. Hydropyrolysis between 800 and 1000 °C resulted in a high yield of methane, and tar production decreased with increasing temperature. Besides, the light components of tar, especially the naphthalene, were significantly increased with hydrogen. The methane yield firstly increased to a peak value of 72.83 ml/g(air-dried coal) at 900 °C and then decreased with increasing temperature during hydropyrolysis. Hydrogen significant improved the yield of gaseous hydrocarbon (C2H2, C2H4, and C2H6) when the temperature below 900 °C, while rarely change that at a higher temperature. At the temperature above 900 °C, the content of graphite in the hydropyrolysis char was lower than that of pyrolysis char in nitrogen. The graphite contents in the char decreased to a minimum at 950 °C and then gradually increased with increasing temperature. The char from hydropyrolysis has a higher condensation degree structure and higher saturation. The small aromatic molecules transformed into large aromatic molecules with increasing temperature under both pyrolysis and hydropyrolysis.
Results are presented of experimental studies of the conversion of products formed in gasification of wood, peat, coal-wastes, and shale rock via the partial oxidation by air in a flow-through filtration converter filled with various kinds of porous packing. Charcoal ZPS zeolite and Raschig porcelain rings served as a packing. It was possible to experimentally select for all kinds of packing the modes in which the combustion heat of the conversion products is not lower than that of the starting gaseous products. The conversion of pyrolysis tars was 60–85% and depended insignificantly of the type of a packing. The experimental results were compared for products formed in gasification of wood and coal waste with the data calculated in terms of the thermodynamic conversion model. The satisfactory correspondence between the results of a thermodynamic calculation and experimental data was obtained for products formed in gasification of wood, in which the content of pyrolysis tars is rather high.
To well understand the effect of char property on tar catalytic reforming and support the development of two-stage gasification technology for the clean fuel gas production, the roles of pore structure and metal oxide in tar removal behavior were examined and compared on a fluidized bed two-stage reactor. The results show that although both of micro pore and meso pore had a remarkable influence on tar removal efficiency, fuel gas component and distilling fractions in tar, perhaps micro pore played a much more important role. Moreover, the tar removal efficiency did not have a good proportional relationship with the specific surface area of char. For the spent char, the activation treatment in steam was very beneficial to renew the catalytic reforming on tar by increasing the surface area of micro pore and maintain its high gasification activity. Compared to the demineralized char, the char samples that loaded metal oxide by impregnation approach largely promoted the tar removal efficiency, following the rank of Na 2 O > Fe 2 O 3 > CaO > MgO. For Na 2 O, it not only had the best catalytic activity on tar, but also greatly promoted the generations of light tar and effective fuel gas, such as H 2 and CO. Finally, considering the different effect of pore structure and metal oxide on tar catalytic reforming, a brief mechanism of tar catalytic reforming by char was proposed.
Partial combustion is applied for tar reduction in the gasified-gas of woody biomass. An inverse diffusion flame was formed in partial-combustion-type gas reformer. Pyrolysis and polymerization of tar occur simultaneously at the vicinity of the combustion reaction zone where exothermic reaction is active. Since soot formation could be a significant problem in the partial-combustion-type gas reformer, it is desirable to suppress soot formation. Temperature is one of the most important properties in the process of reforming tar, and it governs the decomposition of tar and the formation process of soot. In this study, we tried temperature measurement for inverse diffusion flame formed in partial-combustion-type gas reformer by two color pyrometry. A calibration method of two color method using radiation from thermocouple are proposed. Furthermore, the influence of the concentration of carbon dioxide and steam in the oxidizer agent on the temperature field of the inverse diffusion flame are investigated by using the partial combustion gas reformer. The flame structure taking into consideration the flame temperature of the inverse diffusion flame was clarified. Results show that the presence of carbon dioxide and steam in the oxidizer affects the temperature field of inverse diffusion flame. With the concentration of carbon dioxide and steam in the oxidizer increases, the flame temperature decreases. We also find that adding carbon dioxide or steam into the oxidizer affects the accuracy of the temperature result which used two color pyrometry.
Varying lignin and cellulose contents in agro-waste cause feed-stock to respond differently during their thermochemical conversion. The effect of pyrolysis temperature (400, 500, 600oC) and feedstock composition on product yields and gas composition of Olive-Kernel (OK) and Corn-Cobs (CC) was investigated in a lab-scale, fix bed reactor under a 20mL/min of nitrogen flow at atmospheric pressure. Results were compared to those obtained in the same pyrolysis set up from model synthetic mixtures of cellulose and lignin, simulating the composition of real feedstocks. Experimental results showed how lignin influences the thermochemical process and how non-negligible synergistic effects among lignin and cellulose are affecting the process outcomes. Lignin affects the increase in char yields from synthetic mixtures more than it does in real feedstock. Similarly higher yield of CO2 in produced gas is reported from pyrolysis of synthetic mixtures compared to that obtained from real feedstock containing the same amount of lignin. Thus the pyrolysis behaviour of raw feedstock cannot be satisfactorily predicted by the behaviour of their main components in an ‘additive’ rule.
This work investigates the fate of polycyclic aromatic hydrocarbons (PAH) in relation to the process severity in the steam- and H2-containing reaction environment of steam gasification of biomass. The focus is on the regimen of tertiary tar formation during the gasification in a fluidized bed gasifier; the tertiary tar is tar that is predominantly aromatic compounds. The process severity reflects the following operating conditions: temperature; gas residence time in the reactor; and contact time between the gas phase and catalytic bed material. The conducted experiments employed a raw gas upgrading process downstream of the gasifier. A mature tar-containing raw gas produced in the Chalmers 2−4-MWth dual fluidized bed biomass gasifier was upgraded in a bench-scale, bubbling fluidized bed reactor, in which inert silica sand and a naturally occurring ilmenite catalyst were used as the bed materials. The obtained results show that following the increased process severity, the growth of PAH can either enhance or suppress. For the growth of PAH being suppressed, it is required that the process severity is sufficient to convert steam and H2 into the reactive hydrogen intermediates that prevent the combination of the carbon-containing species. To ensure this, the application of silica sand as bed material requires an operating temperature of 850°C and a gas residence time of >11.5 s, while the use of ilmenite requires an operating temperature of 800°C and a gas residence time of >3.4 s, together with a gas-solid contact time of about 0.7 s. In particular, the results obtained for ilmenite encourage the use of naturally occurring catalysts in fluidized bed gasifiers, despite that their catalytic activities are lower than that of synthetic catalysts.
Producer gas, derived from biomass gasification, contains a wide variety of compounds organic as well as inorganic, apart from the gas species and particulate matters. The hydrocarbon compounds present in the raw gas, which have comparatively lower molecular weights, act as fuel in gas turbine or gas engines. Hydrocarbons with higher molecular masses are collectively called tars. Relatively simpler tars often polymerize into more complex structures. These heavier species quickly condense, some even solidify, and choke the particulate filters and other restrictions and valves in the gas paths, causing serious obstruction to continuous operation of the application components. Some other impurities, like sulphides and halides, too cause damages to the materials of downstream equipment. It is, therefore, essential to remove the tars and impurities in the product gas to the extent possible. Tars also pollute the environment if discharged untreated. If, however, tars could be cracked and converted to permanent gas species, the producer gas calorific value could be improved substantially. Tars can be eliminated or effectively converted or their production rates can be reduced by certain measures. They include installing separation devices, modifying the conditions and parameters of gasification, modifying the gasifier design, using additives and catalysts. This chapter discusses these measures or processes that are aimed at tackling the tars.
Biofuels from biomass gasification are reviewed here, and demonstrated to be an attractive option. Recent progress in gasification techniques and key generation pathways for biofuels production, process design and integration and socio-environmental impacts of biofuel generation are discussed, with the goal of investigating gasification-to-biofuels’ credentials as a sustainable and eco-friendly technology. The synthesis of important biofuels such as bio-methanol, bio-ethanol and higher alcohols, bio-dimethyl ether, Fischer Tropsch fuels, bio-methane, bio-hydrogen and algae-based fuels is reviewed, together with recent technologies, catalysts and reactors. Significant thermodynamic studies for each biofuel are also examined. Syngas cleaning is demonstrated to be a critical issue for biofuel production, and innovative pathways such as those employed by Choren Industrietechnik, Germany, and BioMCN, the Netherlands, are shown to allow efficient methanol generation. The conversion of syngas to FT transportation fuels such as gasoline and diesel over Co or Fe catalysts is reviewed and demonstrated to be a promising option for the future of biofuels. Bio-methane has emerged as a lucrative alternative for conventional transportation fuel with all the advantages of natural gas including a dense distribution, trade and supply network. Routes to produce H2 are discussed, though critical issues such as storage, expensive production routes with low efficiencies remain. Algae-based fuels are in the research and development stage, but are shown to have immense potential to become commercially important because of their capability to fix large amounts of CO2, to rapidly grow in many environments and versatile end uses. However, suitable process configurations resulting in optimal plant designs are crucial, so detailed process integration is a powerful tool to optimize current and develop new processes. LCA and ethical issues are also discussed in brief. It is clear that the use of food crops, as opposed to food wastes represents an area fraught with challenges, which must be resolved on a case by case basis.
Thermochemical conversion of biomass or coal by gasification produces mainly gases such as H2, H2O, CO, CO2, and CxHy but also carbon rich particulate matter. Impurities such as tars, sulfur compounds, alkali, halide, nitrogenous compounds, and trace elements are also present in biomass-derived producer gases. Efficient and effective gas cleaning is needed to reduce impurities to an acceptable level given by specific downstream applications. The electrochemical conversion of producer gas to electricity by a fuel cell (e.g. solid oxide fuel cells (SOFC)) is shown as an example but the gas cleaning process is also valid for other conversion processes involving catalysts, such as methanation or liquid fuel synthesis. Particle removal from the product gas is needed to protect downstream process units from fouling. There are two examples in Europe where catalytic tar reforming is applied at industrial scale combined heat and power (CHP) plants: Kokemäki in Finland and Skive in Denmark.
Fixed bed gasification is a simple and suitable technology for small scale power generation. One of the disadvantages of this technology is producing tar. So far, tar is not utilized yet and being waste that should be treated into a more useful product. This paper presents a thermodynamic
analysis of tar conversion into gas producer through non-catalytic auto-thermal reforming technology. Tar was converted into components, C, H, O, N and S, and then reacted with oxidant such as mixture of air or pure oxygen. Thus, this reaction occurred auto-thermally and reached chemical equilibrium. The sensitivity analysis resulted that the most promising process performance occurred at flow rate of air was reached 43% of stoichiometry while temperature of process is 1100°C, the addition of pure oxygen is 40% and preheating of oxidant flow is 250°C. The yield of the most promising process performance between 11.15-11.17 kmol/h and cold gas efficiency was between 73.8-73.9%.The results of this study indicated that thermodynamically the conversion of tar into producer gas through non-catalytic auto-thermal reformingis more promising.
Energy from biomass based gasifier-engine integrated systems are becoming more popular for power generation applications in rural and urban driven societies. The quality of producer gas from the down draft gasifiers plays a significant role in power generation aspects. During gasification, tar is produced and its magnitude depends on the type of gasification process and biomass feedstock used which can vary from biomass to municipal solid waste (MSW) available. The pollutants generated from gasification include particulate matter, tars, and char and acid gases. A major challenge for commercializing the gasification process is to reduce tar. In order to address these tar related problems a cleaning and cooling system has been developed in house that facilitates tar removal to acceptable levels tolerated by the internal combustion (IC) engine and meets emission standards as well. The main objective of the present work is to reduce tar level and develop control strategies for improving the performance and emission of diesel engines. Results showed that the brake thermal efficiency of the dual fuel engine used was increased by 2-4% and tar was reduced from325 mg/Nm3 in the gasifier to 60-50 mg/Nm3entering the engine. In addition, the emission levels such as hydrocarbon, carbon monoxide were reduced comparatively with developed cooling-cleaning system provided in the conventional gasifier-engine system. The biomass consumption rate was40kg/h. Air and gas flow rates were measured to be 18.8 m3/h and 20.12 kg/h respectively. The temperature of the gas after developed cooling and cleaning system was found to be 34 °C.
Methods of thermal cracking, partial oxidation and char bed conversion on tar destruction has been investigated by a two stage fixed-bed reactor, effects of fuel type, temperature, residence time, char particle size and char type on tar destruction are considered. The result indicates that tar conversion efficiency increase with the second stage reactor temperature increasing in all three kinds of conversion methods. Partial oxidation and char bed conversion is more effective in tar destruction compared to thermal cracking. Associated with partial oxidation and thermal cracking, char bed can get least tar yield. Three kinds of biomass tar yield in the experimental condition of 1000°C is: rice straw 0.43%, corn straw 0.61% and fir sawdust 1.15%, and the corresponding tar conversion efficiency is 98.28%, 97.23% and 96.29% respectively. Tar yield content of each conversion methods are decreasing with reactor temperature increase. It is really difficult to removal all tar completely in these experiments due to complex tar composition and experimental conditions. The results also show that biomass tar destruction feasibility are: rice straw > corn straw > fir sawdust, and on obvious diversity is obtained between different char species on tar destruction.
Technological parameters such as composition of reactant gas, size distribution of raw material and temperature of the upper fluidized bed were investigated in order to conclude their effects on tar content and tar formation mechanism. Results from experiments indicate tar content decrease as the ratio of steam and oxygen and biomass particle size decrease while temperature of upper fluidized bed increase. Highest hydrogen yield of 66.42g/kg biomass, highest total composition of H 2 and CO of 54.82%, and tar content less than 2g/Nm 3 dry gas are obtained under the best experimental condition, and the lower tar content is good for raw gas catalytic steam reforming. The relationship between tar formation and SOR is very close. At high SOR, tar comes from biomass pyrolysis, and its component is very complex, main containing Oxygen-contaning compounds; At relative lower SOR, Oxygen-contaning compounds disappeared, and the structure of benzene with substituent groups is reduced, and the structure of benzene with less chain is increased; while SOR decrease to an extent, poly-aromatic compounds obviously increased.
Experimental study for clarifying the effect of combustion mechanism on reforming of producer gas was carried out. The inverse diffusion flame was formed in the actual gas reformer by partial combustion of producer gas. Direct observation and laser diagnostics were applied to the inverse diffusion flame formed in the modeled gas reformer. Experimental parameters are steam concentration in supplied model producer gas (Xsteam) corresponding to moisture content of wood, and the flame configuration (flame at the nozzle and lifted flame). The main results are as follows. In the condition of 20% concentration of oxygen, flames at the nozzle and lifted flames were formed in the same supply condition. One might identify a type of hysteresis in flame formation that determines its configuration. When the flames at the nozzle were formed, the exergy yield was increased with the increase of Xsteam because soot formation was suppressed and steam reforming of model tar can be promoted. In contrast, when the lifted flames were formed, the exergy yield was decreased with the increase of Xsteam because of formation of the particulate matter such as the soluble organic fraction and the young soot.
During the process of biomass gasification tars are formed which exit the gasifier in vapor phase. Tar condensation creates problems like fouling and plugging of after-treatment, conversion and end-use equipment. Gasification tars consist mainly of Polycyclic Aromatic Hydrocarbons (PAHs). Former research has shown the possibilities and difficulties of tar conversion by partial combustion. Basic studies to investigate the oxidation of tars in non-premixed combustion processes are expected to give more insight in this problem. In this paper the ability of the flamelet-generated manifold (FGM) approach to numerically model multi-dimensional, laminar, non-premixed flames with the inclusion of PAH chemistry is investigated. Modeling detailed PAH chemistry requires the employment of large reaction mechanisms which lead to expensive numerical calculations. The application of a reduction technique like FGM leads to a considerable decrease (up to two orders) in the required computation time. A 1D numerical validation shows that the improvements achieved by implementing a varying Lewis number for the progress variable Y are significant for PAH species with a large Lewis number, such as C10H8. Considerable improvements are found near the flame front and on the fuel side of the flame. A comparison has been made of FGM results with qualitative Planar Laser Induced Fluorescence (PLIF) measurements. A laminar CH4/N-2-air co-flow flame has been doped with two dopants, benzene and toluene, at three different concentrations. A set of filters was used in order to qualitatively distinguish the small (1-2 rings) and large (3 or more rings) aromatic species. The results show that the model is able to capture the major flame characteristics typical for PAH formation in multi-dimensional laminar non-premixed flames.
One way to reduce tar is by oxidative and thermal cracking by means of partial combustion of the producer gas in the gas reformer, an apparatus stage subsequent to the woody biomass gasifier. During the partial combustion process of the producer gas, inverse diffusion flame is formed as oxidizer is supplied to the hot producer gas. Cracking and polymerization of tar occur simultaneously at the proximity of the inverse diffusion flame. Polymerized tar grows into soot passing through polycyclic aromatic hydrocarbons (PAH). Several growth mechanisms of PAH have been proposed, which are common to the primary PAH growth followed by abstraction of a hydrogen-atom from the reacting hydrocarbon by a hydrogen radical. We can, therefore, point to the possibility of suppression of the soot formation by controlling the hydrogen concentration at the proximity of the inverse diffusion flame. In the present study, hydrogen concentration at the proximity of the inverse diffusion flame has been controlled by the small amount of hydrogen addition to the oxidizer. Soot formation is suppressed by the chemical effect of a small amount of additional hydrogen (approximately 0.5 % in the total enthalpy of the model producer gas), rather than by a slight change in oxygen concentration and the flow velocity of the oxidizer.
This paper deals with the modelling and the commissioning of a high temperature reactor (Turboplasma®) devoted to the thermal cracking of tars held in the product gas leaving a RDF/biomass gasification unit. High temperatures are reached in the reactor thanks to a plasma torch that can reach up to 5,000 K. In a first section, information on the model that was derived to design the reactor is given. This model relies on the CFD Software Ansys Fluent. A specific reaction pathway and the associated kinetics were set up in the software to characterize the degradation of model tars (naphthalene, toluene and benzene). Specific physical properties were also used to characterize the properties of the plume of plasma downstream the torch. The overall model was used to check the efficiency of several reactor configurations and then to build the experimental reactor. This reactor was tested downstream a fluidized bed gasifier and information on the operation of the overall process is provided. In a second section, the results of the CFD computation as well as the results of the experimental campaign are provided. These last ones show that the efficiency of the real reactor is better than the computed ones. Comparison of both results reveals discrepancies, which led us to modify the reaction pathway model in order to take into account the gasification of char produced in the fluidized bed reactor and entering the Turboplasma®.
A co-current moving bed gasifier with internal recycle and separate combustion of pyrolysis gas has been developed with the aim of producing a design suitable for scaling-up downdraft gasifiers while maintaining a low tar content in the producer gas. Using wood chips with a moisture content of 7–9 wt% (db) as a fuel at a rate of 20 kg h−1, this system produced a gas with a heating value of 4500 kJ ms−3 and a very low tar content of < 0.1 gms−3.
In the gasification of biomasses for production of methanol it is desirable to reduce the contents of methane and other higher hydrocarbons in the gas.
The kinetics of the thermal conversion of aromatic hydrocarbons in the presence of hydrogen and steam were studied, using anphthalene, toluene and benzene as model compounds. The experiments were performed in a tubular flow reactor at a total pressure of 160 KPa, temperatures of 700–1400°C, residence times of 0.3–2 s and different gas-phase concentrations of hydrogen, steam and the aromatics. The mechanisms of primary and consecutive reactions are presented as reaction schemes that are supported by kinetic calculations. The following order of reactivity is obtained: toluene ⪢ naphthalene > benzene. Besides gaseous organic cracking products such as methane and ethene, condensed products and a carbonaceous residue (soot) is formed, principally from naphthalene. Soot formation is strongly inhibited by hydrogen. Steam has only a little influence on the conversion of the aromatics. Under the given reaction conditions, neither the soot primarily formed nor the organic cracking products such as methane are completely converted by steam to carbon monoxide and hydrogen, even at the highest temperature investigated (1400°C).
The effects of various gaseous additives on soot formation in strained diffusion flames are reported. The additives N2, Ar, He, H2, and CO were introduced with fuels C2H4, C3H8, and C4H10, and were selected to isolate the effects of dilution, temperature, preferential diffusion, and active chemical participation resulting from the additive. Special emphasis was placed on understanding the mechanisms by which CO and H2 addition influence soot inception. Measurements were made of the limiting strain rate for complete suppression of soot, i.e., the soot-particle inception limit, Kp, in the counterflow diffusion flame. Some laser-extinction measurements of soot volume fraction were also made in the coflow flame to determine the applicability of the results to this geometry. The addition of inerts to the fuel decreases the sooting limit due to the reduction in fuel concentration and temperature. Concentration modification due to preferential diffusion enhances the suppressive effect of He, causing it to be the most effective additive considered. The behavior of the reactive additives is more complex. The addition of H2 increases flame temperature but decreases Kp for the fuels considered. Preferential diffusion is partially responsible for this behavior, however direct chemical suppression may also play a role in the strongly suppressive effects of this additive. The chemical role of H2 is discussed in the context of Frenklach's H abstraction/C2H2 addition model for PAH formation. Carbon monoxide addition to C2H4 results in a monotonic decrease in Kp that is primarily a consequence of dilution. For CO addition to the alkanes there is initially an increase in Kp followed by a decrease for XCO > 0.5, suggesting a small chemical enhancement. Coflow results tend to support these findings: For C2H4 the results are consistent with dilution while for C3H8 a small chemical enhancement combined with suppression due to dilution nets a weak suppression of soot formation. This finding, that CO can enhance inception chemistry in alkanes, requires further study.
This handbook has been prepared by the Solar Energy Research Institute under the US Department of Energy /bold Solar Technical Information Program/. It is intended as a guide to the design, testing, operation, and manufacture of small-scale (less than 200 kW (270 hp)) gasifiers. A great deal of the information will be useful for all levels of biomass gasification. The handbook is meant to be a practical guide to gasifier systems, and a minimum amount of space is devoted to questions of more theoretical interest.
Numerical flow modeling has become an increasingly important design and analysis tool for improving the air distribution to power plant burners. Uniform air distribu- tion allows the burners to perform as designed to achieve the lowest possible emissions and best fuel burn-out. Modifications can be made internal to the existing windbox to improve the burner-to-burner and burner pe- ripheral air distributions. These modifications can include turning vanes, flow splitters, perforated plate, and burner shrouding. Numerical modeling allows the analysis of design trade-offs between adding flow resistance, fan power, and windbox modification construction cost. Nu- merical modeling has advantages over physical modeling in that actual geometric scales and air temperatures are used. Advantages over a field data based study include the ability to quickly and cheaply analyze a variety of design options without actually modifying the windbox, and the availability of significantly more data with which to interpret the results. Costs to perform a numerical study are generally one-half to one-third of the cost to perform a physical flow model and can be one-fourth of the cost to perform a field study. The continued development of affordable, high speed, large memory workstations and reliable, commercially available computational fluid dy- namics (CFD) software allows practical analyses of power plant windboxes. This paper discusses (1) the impact of air distribution on burner performance, (2) the methodol- ogy used to perform numerical flow modeling of power plant windboxes, and (3) the results from several windbox analyses including available post-modification observa- tions. Introduction This paper discusses experience learned while perform- ing numerical flow analyses of power plant windboxes. A windbox is a system of ducts and plenums which create the conduit to transport air to the burners. Equal distribu- tion of air to the burners is required for optimum perfor- mance. Simple modifications can be made to redistribute the air and correct existing air unbalances. These modifi- cations require knowledge of the existing flow patterns and the ability to predict the post-retrofit performance. Numerical modeling of power plant windboxes has been shown to be a viable flow prediction tool at a fraction of the cost of either physical flow modeling or a field data based study.
A kinetic mechanism, previously developed and successfully applied to the prediction of the formation of benzene and aromatics in different flame conditions, was applied to assess the importance of the various benzene and aromatic formation pathways in non-premixed flames. Four sets of data were tested: the methane flame and the same flame doped with toluene, ethylbenzene, and tert-butylbenzene, as studied by Anderson and co-workers. The model predicts, with good accuracy, the growth of hydrocarbons and the formation of benzene and aromatic species. The modeling shows that in the undoped methane flame, benzene formation is controlled by propargyl radical combination. Acetylene addition to C4 radicals contributes a moderate amount, whereas toluene decomposition is insignificant. The predictions are almost unaffected by the fulvene pathway. Benzene is strongly perturbed by dopant addition to methane. Predictions agree quite well with benzene concentrations in the undoped flame and agree with the increase in benzene concentration when alkylbenzenes are added. Key reactions leading to the formation of naphthalene are the propargyl addition to benzyl radicals, and, to a lesser extent, the hydrogen-abstraction acetylene-addition mechanism. Cyclopentadienyl radical combination, which is the dominant route in premixed and partially premixed flames, is insignificant in these flame conditions.
Although there have been many studies of soot formation, the processes involved are still not fully understood. Recently, particular attention has been directed at the rate of growth of soot particles when rich premixed hydrocarbon-oxygen mixtures are burned in a flat flame, but an alternative approach is to compare the reported growth rates for soot particles in flames with those predicted from the rate constants for pyrocarbon growth in the absence of soot nucleation. In this paper it is shown that in the absence of soot formation there is no difference in the growth processes of soot particles and pyrocarbon. The observed discrepancy in the growth rate constants for a soot surface and a quartz filaments (4.5 times for methane and 2-3 times for other hydrocarbons) results from the appearance of a rough surface on the quartz. It is known that when soot is formed in the course of methane pyrolysis, the growth rate of particles is two order higher than the pyrocarbon growth rate in the absence of soot formation, and the same effect has been observed with other hydrocarbons. This acceleration in the soot particle growth at low hydrocarbon concentration is caused by destruction of hydrocarbon radicals on the growing surface. Acceleration of pyrocarbon growth in the thermal decomposition of methane, acetylene, and benzene is also found at temperatures up to 1400{degrees} C in pipes of 1 mm diameter, and soot formation is prevented with the rate of gas flows of 100 ms{sup {minus}1}. The present calculations were made for typical mixtures compositions in premixed sooting flames for which detailed data on the growth of soot particles has been presented.
The chemistry of the formation of methane from the hydrogasification of naphthalene, substituted naphthalenes and toluene has been investigated using a flow tube. Temperatures were varied between 650–1050 °C (depending on the aromatic) and pressures ranged over 0.5–2 MPa. The results show that methane yields increase with increasing hydrogen pressure. For naphthalene the methane yield increases linearly with temperature for a given pressure. Methyl substituents are lost from aromatic rings, in a reaction which is insensitive to hydrogen pressure, to form 1 mole of methane and the parent aromatic. At these hydrogen pressures the phenolic group in 1-naphthol is removed predominately as H2O to form naphthalene and the methane yields from this species parallel those from naphthalene. Analyses of the condensed products demonstrate that increased hydrogen pressure results in a reduction in the amounts of high molecular weight condensation products resulting in increased yields of methane.
Parametric tests on tar formation, varying temperature, equivalence ratio, and residence time, were performed on a bench-scale, indirectly-heated fluidized bed gasifier. Prepared tar samples were analyzed in a gas chromatograph (GC) with a flame ionization detector, using a capillary column. Standard test mixtures containing the dominant tar species were prepared for GC calibration. The identified peaks included single-ring hydrocarbons, such as benzene, to five-ring hydrocarbons, such as perylene; these compounds represent about 70–90% (mass basis) of the tar constituents. The influences of the above-mentioned gasification parameters on tar formation were analyzed.
Parametric tests are performed on an indirectly heated, fluidized bed biomass gasifier. The test system allows feedstock, oxygen, nitrogen, and steam flow rates, and temperature to be controlled independently. Gas residence time, temperature, equivalence ratio, and steam:biomass ratio are varied, and product gas composition and select gasification parameters are evaluated and compared with theoretical predictions.
Experimental results are presented on the effect of methane content in a non-aromatic fuel mixture on the formation of aromatic hydrocarbons and soot in various fundamental combustion configurations. The systems considered consist of a laminar flow reactor, a laminar co-flow diffusion flame burner, and a laminar, premixed flame burner, all of which operate at atmospheric pressure. In the flow reactor, the experiments are performed at 1430 K, constant C-atom flow rates, 98% nitrogen dilution, C/O ratio = 2, and with fuel mixtures consisting of ethylene and methane. The diffusion flames are performed with fuel mixtures of methane and ethylene diluted in nitrogen to maintain a constant adiabatic flame temperature. The premixed flame experiments are performed with n-heptane and methane mixtures at a C/O ratio = 0.67 with nitrogen-impoverished air. The results show the existence of synergistic chemical effects between methane and other alkanes in the production of aromatics, despite reduced acetylene concentrations. This effect is attributable to the ability of methane to enhance the production of methyl radicals that will then promote production channels of aromatics that rely on odd-carbon-numbered species. Benzene, naphthalene, and pyrene show the strongest sensitivity to the presence of added methane. This synergy on aromatics trickles down to soot via enhanced inception and surface growth rates by polycyclic aromatic hydrocarbon condensation, but the overall effects on soot volume-fraction are smaller due to a compensating reduction in surface growth from acetylene. These results are observed under the very fuel-rich environments existing in the flow reactor and diffusion flames. In the premixed flames, however, instabilities did not permit investigation of conditions with sufficiently high equivalence ratios to perturb the aromatic and soot-growth regions.
Tar formation is one of the major problems to deal with during biomass gasification. Tar condenses at reduced temperature, thus blocking and fouling process equipments such as engines and turbines. Considerable efforts have been directed on tar removal from fuel gas. Tar removal technologies can broadly be divided into two approaches; hot gas cleaning after the gasifier (secondary methods), and treatments inside the gasifier (primary methods). Although secondary methods are proven to be effective, treatments inside the gasifier are gaining much attention as these may eliminate the need for downstream cleanup. In primary treatment, the gasifier is optimized to produce a fuel gas with minimum tar concentration. The different approaches of primary treatment are (a) proper selection of operating parameters, (b) use of bed additive/catalyst, and (c) gasifier modifications. The operating parameters such as temperature, gasifying agent, equivalence ratio, residence time, etc. play an important role in formation and decomposition of tar. There is a potential of using some active bed additives such as dolomite, olivine, char, etc. inside the gasifier. Ni-based catalyst are reported to be very effective not only for tar reduction, but also for decreasing the amount of nitrogenous compounds such as ammonia. Also, reactor modification can improve the quality of the product gas. The concepts of two-stage gasification and secondary air injection in the gasifier are of prime importance. Some aspects of primary methods and the research and development in this area are reviewed and cited in the present paper.
This study has shown that injecting secondary air to a fluidised-bed at temperatures over 830–850°C decreases the tar contents of the gasifier offgas. Tests carried out at a laboratory scale show that the formation of tars strongly depends on the raw material type and the operating conditions, especially the gasification temperature. It is also shown that an optimum ratio of secondary to primary air is about 20% for forest residue and about 10% for lignite, obtaining a tar removal efficiency of about 90 and 80%, respectively. The offgas composition and its low heating value were also analysed with and without secondary air injection for forest residue gasification. CO, H2, gaseous hydrocarbon contents and the low heating value of the gas decreased and N2 increased with secondary air. However, the CO2 contents kept at a close level, 16.5–18.5%, when increasing the secondary air ratio from 0 to 50 vol.%. At the optimum ratio for the secondary to primary air, a stoichiometric secondary air gasification is proposed to estimate the mixed gas composition. The expression developed in this study is independent of the intermediate stages of the process and it is applicable to forest residue gasification.
Straw gasification in a two-stage gasifier
- Jd Bentzen
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Thermal conversion of biomass into secondary products the case of gasification and pyrolysis
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Tar reduction by partial oxidation
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Effects of temperature and gas composition on model tar compounds decomposition kinetics
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