Mechanism of Tar Generation during Fluidized Bed Gasification and

Article pubs.acs.org/IECR

Mechanism of Tar Generation during Fluidized Bed Gasification and Low Temperature Pyrolysis Ute Wolfesberger-Schwabl,*,†,§ Isabella Aigner,‡ and Hermann Hofbauer‡ †

Test Laboratory for Combustion Systems and ‡Institute of Chemical Engineering, Vienna University of Technology (VUT), Getreidemarkt 9/166, A-1060 Vienna, Austria § Bioenergy 2020+ GmbH, Wienerstraße 49, 7540 Güssing, Austria ABSTRACT: This work focuses on the comparison of two different thermal conversion processes with respect to different tar amounts and tar compositions at different temperatures and process parameters. Steam gasification experiments were carried out in a 100 kW dual fluidized bed steam gasifier at a temperature range of 700 to 870 °C with straw and wood. Pyrolysis experiments were conducted in a rotary kiln reactor at temperatures between 600 and 630 °C and straw as fuel. For better understanding of tar formation during thermo-chemical conversion of biomass, the tar content and composition in the product gas were analyzed with a gas chromatograph coupled with a mass spectrometer. The main observation was that at higher temperatures the tar composition is shifted to higher molecular tars such as polyaromatic hydrocarbons (PAH). Key tar components at lower temperatures (pyrolysis) are phenols. The tar content and composition in the product gas of steam gasification of straw and wood as well as the tar yields and composition of low temperature pyrolysis of straw are presented in the following work.

1. INTRODUCTION 1.1. Background of the Work. The world’s energy demand as well as its CO2 emission is increasing rapidly. For the future a reduction of greenhouse gas emissions is a major goal of many countries worldwide. Biomass gasification and biomass pyrolysis provide an opportunity to use a renewable energy carrier. At present, renewable energy has a share in the world’s total primary consumption of 11%. It is expected that the renewable energy share will increase to 13% by 2015.1 To optimize the biomass gasification and biomass pyrolysis processes, research has to be carried out in many fields. This work should be a contribution to clarify the difficult subject of tar formation during these processes. The knowledge of tar formation is essential for process stability, as tar condenses and polymerizes in pipes and filters and therefore leads to blockages. In gasification tar reduction is the major goal, whereas in pyrolysis a goal to maximize the pyrolysis oil yields is desired. Therefore, the tar formation, tar content, and tar composition at different process conditions are important features for these processes. Tar formation is based on a series of complex thermochemical reactions, like depolymerization, homolysis, oxidation, polymerization, and cycloaddtion.2 The reaction conditions have a strong influence on tar formation. Tar can consist of over 100 different substances. This work shows the influence of different temperatures on tar component concentrations. Furthermore the different gasification and pyrolysis processes are evaluated due to their tar formation mechanisms. The comparison of the separate processes was chosen because in the gasification processes the pyrolysis step also occurs. To provide an insight into the different processes, the same measurement technique and data evaluation were used for this comparison. Hence, according to the analysis, the results of straw/wood gasification and straw pyrolysis are well comparable. This approach could © 2012 American Chemical Society

lead to a better insight of tar formation and its mechanism. This work is intended to summarize and analyze the various data collected at the VUT. Fuels and bed materials that were used during conversion of biomass are as follows. For gasification wood pellets, straw pellets, pellets with wood and straw blends were used and the bed material was Olivine (which is known for it is catalytic activity3). For pyrolysis, indoor stored wheat straw was used. 1.2. Tar Definitions and Nature. Generally, tars are hydrocarbon containing mixtures, which can form liquid or highly viscous to solid deposits by cooling of the gaseous phase down to ambient temperature.4 Besides the main stoichiometric elements carbon (C) and hydrogen (H), other elements like oxygen (O), nitrogen (N), or sulfur (S) are found in tar. It has to be mentioned that owing to their different tar nature, the oxygen content is significantly higher in pyrolysis tar than in gasification tar. Tars are generally assumed to be largely aromatic. Various ways have been proposed in literature for tar classification,5−7 for instance the division in primary, secondary, and tertiary tar shown in the work of various researchers. According to Milne, et al.,5 so-called primary tar emerges from the pyrolysis process. The three main components of wood and straw cellulose, hemicellulose, and lignin can be identified as a source for the primary tar. Cellulose and hemicellulose, which contain a lot of oxygen, form mainly oxygen-rich primary tar products like alcohols, ketones, aldehydes, or carbon acids. On the contrary bi- and trifunctional monoaromatics, mostly substituted phenols, occur from lignin. Verifiable substances Received: Revised: Accepted: Published: 13001

March 28, 2012 July 31, 2012 September 9, 2012 September 10, 2012 dx.doi.org/10.1021/ie300827d | Ind. Eng. Chem. Res. 2012, 51, 13001−13007

Industrial & Engineering Chemistry Research

Article

Figure 1. Transition of tars dependent on temperature5

Table 1. Classification of Tars Based on Molecular Weight9 tar class

class name

1 2

GC-undetectable heterocyclic aromatics

3

light aromatic (1 ring)

4

light PAH compounds (2−3 rings) heavy PAH compounds (4−7 rings)

5

property

representative compounds

very heavy tars, cannot be detected by GC tars containing hetero atoms, highly water-soluble compounds usually light hydrocarbons with single ring; do not pose a problem regarding condensability and solubility 2 and 3 rings compounds; condense at low temperature even at very low concentration larger then 3 rings; these components condense at hightemperature at low concentration

2. EXPERIMENTAL SECTION 2.1. Dual Fluidized Bed Steam Gasification. The 100 kW dual fluidized bed gasifier (DFB) at VUT belongs to the group of circulating fluidized bed reactors. The basic idea of this reactor type is the separation of a steam fluidized gasification zone and an air fluidized combustion zone to generate a hydrogen-rich and nitrogen-free product gas. Bed material is continuously circulating between these two zones to provide the heat for gasification. Biomass is fed directly into the gasification zone (bubbling fluidized bed). To supply enough heat in the test facility the char in the bed material is burned together with additional oil in the combustion zone (fast fluidized bed). Gas flows into the gasification zone (steam) and into the combustion zone (air) are measured separately. The main gas components beside tar are measured with a Rosemount NGA 2000. In Table 4 selected characteristic parameters of the DFB reactor and of the used fuels are shown. In Figure 2 the main principle of the DFB gasifier is shown; more technical details are found in refs 13−15. A demonstration plant (8 MWth) based on the same technology also exists in Güssing, Austria.16 2.2. Low Temperature Pyrolysis Biomass Pilot Plant. The pyrolysis tests were conducted in an indirect heated rotary kiln reactor at the low temperature biomass pyrolysis pilot plant Dürnrohr (Energie Versorgung NiederösterreichEVN) with a capacity of about 3 MW. The heat needed in the rotary kiln reactor is supplied indirectly via the reactor wall. The pyrolysis gas temperature at the outlet of the rotary kiln is between 450 and 650 °C. The reactor is a cylindrical vessel, inclined slightly

Table 2. Classification of Tars Based on Molecular Weight10 class name

definition

naphthalene

single ring aromatics naphthalene aromatics with higher molecular weight as naphthalene

indene; naphthalene; methylnaphthalene; biphenyl; acenaphthalene; fluorene; phenanthrene; anthracene fluoranthene; pyrene; chrysene; perylene; coronene

Each tar component has a functional group, which is responsible for its characteristic chemical behavior. Each sample analyzed gives a value for gravimetric tar and GC− MS tar. The GC−MS tar can be discussed as a sum GC−MS tar, and as well the single GC−MS components can be considered. For a better and easier interpretation of the results the measured GC−MS components were combined to substance groups12 shown in Table 3. The substance groups were chosen due to the related chemical structure and properties and molecular weight as well as functional groups. Also displayed in Table 3 are the 45 calibrated components. For the sum GC−MS tar the measured single components are summed up. The relative content of each substances group is based on the sum GC−MS tar in percentage.

are, for example, phenol, dimethylphenol, and cresol. These primary tars are formed at temperatures starting from 200° to 500 °C. Due to increasing temperature and the presence of an oxidant (oxygen, air, or steam) a part of the cellulosecontributed primary tars reacts to small gaseous molecules. The residual primary tars form secondary tars, which are composed of alkylated mono- and diaromatics including heteroaromatics like pyridine, furan, dioxin, and thiophene. Tertiary tars can be found over 800 °C. Tertiary tars are also called recombination or high temperature tars. Typical tertiary tars are naphthalene, phenanthrene, pyrene, and benzopyrene (polynuclear aromatic hydrocarbons PAH). Tertiary tar structures cannot be found in natural biomass, which was used in the experiments. They can emerge from small molecule fragments as allyl-, aryl-, and alkyl radicals, which result from homolytic cleavage of the secondary tar. Tertiary tar can be also formed at lower temperatures, and a possible mechanism is the 2 + 4 cycloaddition according to the Diels−Alder reaction,8 which forms additional cyclohexene rings followed by an aromatization due to dehydrogenation and respectively dehydration. Figure 1 shows the transition of tars as a function of process temperature as discussed in Milne et al.5 Another approach for classification of tars by Li et al.9 is based on molecular weight of the compounds summarized in Table 1. A different classification also based on molecular weight described by Hüttler et al.10 is displayed in Table 2.

1 2 3

pyridine; phenol; cresols; quinoline; isoquinoline; dibenzophenol toluene; ethylbenzene; xylenes; styrene

As mentioned in the “Tar Guidline”11 tar can be categorized into gravimetric tar and GC−MS tar. Since all classifications interleave at the boundaries there is not one true definition but rather all classifications are an attempt for better understanding of the tar nature. The approach for this work is to classify by analysis method, according to the molecular weight and the chemical properties. 13002

dx.doi.org/10.1021/ie300827d | Ind. Eng. Chem. Res. 2012, 51, 13001−13007


Article

Table 3. Measured GC−MS Components Combined in Substance Groups12 substance groups

component

phenols furans aromatic compounds aromatic nitrogen compounds naphthalenes polyaromatic hydrocarbons (PAH) (without naphthalenes) guaiacols

phenol; 2-methylphenol; 4-methylphenol; 2,6-dimethylphenol; 2,5-dimethylphenol; 2,4-dimethylphenol; 3,5-dimethylphenol; 2,3dimethylphenol; 3,4-dimethylphenol; 2-methoxy-4-methylphenol; catechol benzofuran; 2-methylbenzofuran; dibenzofuran phenylacetylene; styrene; mesitylene; indene isoquinoline; indole; carbazole; quinoline naphthalene; 2-methylnaphtalene; 1-methylnaphtalene biphenyl; acenaphtylene; acenaphtene; fluorene; anthracene; phenanthrene; 4.5-methylphenanthrene; 9-methylanthracene; fluoranthene; pyrene; benzo[a]anthracene; chrysene; benzo[b]flouranthene; benzo[k]flouranthene; benzo[a]pyrene; benzo[g,h,i] perylene; indeno[1,2,3-cd]pyrene guaiacol; eugenol; isoeugenol

pretreatment step for cocombustion of straw in a coal power station. Therefore, maximizing of oil yield is not the aim for this application. The applied fuel was indoor stored wheat straw. Further technical information and detailed results can be found in Wolfesberger.15 2.3. Measuring Techniques. The analysis, calculation, and sampling for GC−MS tar as well as gravimetric tar are based on the method as described in CEN/TS 15439 Biomass GasificationTar and Particles in Product GasesSampling and Analysis,11 which is used as guideline at the VUT. An isocinetical taken product gas slipstream is run through a particle separating system, which consists of a cyclone and a glass wool filled filter cartridge. To reduce condensation, the probe and all parts of the particle separating system are traceheated (≥300 °C). The particle-free gas is led through six impinger bottels, five filled with a defined volume of solvent and one impinger bottle without content for safety reasons. All impinger bottles are situated in a cooling bath at −8 °C for better absorption and condensation of the tars in the solvent. After a defined sampling time, the temperature and the gas flow rate is recorded. The solvent phase is unified, the water phase is separated, and a GC−MS sample is taken and analyzed. After evaporation of the solvent from the remaining sample the gravimetric tar can be determined. Condensation on the particle fraction can occur, therefore the tar is extracted from the particles with Soxhlet extraction. This sample undergoes the same procedure as the unified solvent phase. However, the method has been modified since toluene is used as solvent instead of 2-propanol in order to account for the high moisture content in the product gas from the steam gasification. Owing to the immiscibility of toluene and water, the water content of the product gas can be determined. The schematic diagram of the tar sampling system is shown in Figure 3. 2.3.1. GC−MS Equipment. The GC−MS tar analysis system consists of an Autosystem XL GC (manufacturer: PerkinElmer) and a TurboMass MS (manufacturer: Perkin-Elmer) with positive electron impact ionization. The oven temperature program starts at an initial temperature of 60 °C and ends at 290 °C with various temperature ramps in between. The GC column is a DB-17MS MS-capable capillary column (manufacturer: Agilent). The capillary column has a length of 30 m with an inner diameter of 250 μm and a film thickness of 5 μm. To quantify the different substances, selected ion recording (SIR) is used as the scanning mode.

Table 4. DFB Reactor Parameters height [m] diameter [m] fuel feed air flow steam flow temperature [°C] bed material pressure nominal power [kW] ash content [wt %db]

gasification zone

combustion zone

2 0.31 biomass = 20 kg/h dry fuel

5 0.15 light fuel oil (auxiliary fuel) = 2 kg/h 55 N m3

21 N m3 700−870

800−950 olivine atmospheric 100

wood = 0.3

straw/wood = 1.6

Figure 2. The schematic diagram of the DFB gasifier at VUT.14

to the horizontal, which rotates slowly around its axis. As the reactor rotates, material gradually moves down toward the lower end, and may undergo a certain amount of stirring and mixing. This reactor is used for low temperature pyrolysis because of the longer residence times for maximizing the solid yield and an unproblematic continuous feeding and discharge of the solid components. Pyrolysis is carried out as a 13003



Article

Figure 3. The schematic diagram of the tar sampling system.

Table 5. Experimental Parameters for Figures 4−6 (wt %) expt no. 1 fuel gasification temperature steam to fuel ratio [kg/kg]

expt no. 2

expt no. 3

87% wood pellets 13% straw pellets 776 1.2

expt no. 4

expt no. 5

expt no. 6

mix pellets 80% wood 20% straw 802 1.5

3. RESULTS AND DISCUSSION 3.1. Tar Content and Composition of Product Gas of DFB Steam Gasification. The steam gasification experiments were carried out with different fuels. Because of the low ash melting point of straw, only straw/wood blends could be gasified at temperatures above 700 °C. Two different kinds of blends were used. On the one hand wood pellets and straw pellets were fed separately into the gasifier (mixture ratio 13% straw/87% wood, experiment numbers 1−3). On the other hand pellets containing straw and wood were used (mixture ratio 20% straw/80% wood and 40% straw/60% wood, experiment numbers 4−11). Table 5 displays the main experimental parameters, as the type of fuel and temperature for the experiments. Figure 4 shows an average of the sum GC−MS tar content in steam gasification product gas and the corresponding gas-

expt no. 7

expt no. 8


expt no. 9

expt no. 10

expt no. 11


The lower sum GC−MS tar content can be associated with the lower steam to fuel ratio (0.8) in wood gasification. This effect is underlined by the lower CO2 content in the product gas that is shown in Table 6. For the gravimetric tar such effects Table 6. Average Gas Composition for 40%/60% Straw/ Wood and Wood H2 CO CO2 CH4

40%/60% straw/wood [vol%]

wood17 [vol%]

38 20 23 10

36 28 18 10

cannot be seen which would indicate that molecules of higher molecular structure as are found in the gravimetric tar are not as sensible to a changing steam to fuel ratio as the GC−MS tars. Another suggestion for the lower sum GC−MS tar content could be the increasing amount of inorganic particles (6.5 g/ Nm3, wood; 14.0 g/Nm3, 40%/60% straw/wood; Nm3 = cubic meter at 273.15 K and 101 325 Pa) in the product gas, derived from the higher ash content of the fuel (Table 4). The effect could be explained with catalytical reactions by contact of the fine ash particles with the formed tars in the bed and the freeboard of the gasifier. However, the main influencing parameter is the temperature as can be seen in the 13%/87% straw/wood experiments. Figure 5 shows the different mixtures of straw and wood according to their relative content of GC−MS substance groups. There is no significant change in the relative GC−MS tar content. The main influence is the temperature, which is shown in experiments 1−3 (Figure 5). In Figure 6 the gravimetric tar content for straw/wood blends is displayed and different straw/wood blends mean different gravimetric tar content. The experiments 6−11 show a similar result according to the gravimetric tar content. On the contrary the experiments 4 and 5 have a significantly lower gravimetric tar content that can be explained by the higher circulation rate of the bed material during these experiments.

Figure 4. Average sum of GC−MS tar content for wood pellets and different straw/wood blends during gasification.

ification temperature. It can be seen that the type (wood/straw pellets or straw pellets and wood pellets) of blend is an insignificant influence compared to the temperature. The average sum GC−MS tar content for wood pellets is displayed for comparison (Figure 4). Wood pellets have a higher sum GC−MS tar content at higher temperature as the blends 20%/ 80% and as the 40%/60% straw/wood blend. 13004



Article

Figure 8. Gravimetric tar content for wood pellets.17

Figure 5. Relative content of GC−MS substance groups for mixtures of straw and wood in gasification.

wood gasification and wood gasification is given. Because of the lower steam to fuel (S/F) ratio (0.8 wood compared to 1.7 straw/wood) in the wood gasification experiments, a decrease of the CO2 and an increase of the CO in comparison to the 40%/60% straw/wood gasification experiments can be noted. More information about the product gas composition for smaller molecules like CO, CO2, H2, CH4, and higher gaseous hydrocarbons can be found in refs 18−20. A detailed discussion of the product gas composition is not part of this publication. 3.2. Tar Content and Composition of Low Temperature Pyrolysis. The temperature dependency of the sum of GC−MS tars is shown in Figure 9, and that of the gravimetric Figure 6. Gravimetric tar content for mixtures of straw and wood in gasification.

However, the influence of the circulation rate on the relative content of GC−MS substance groups is insignificant. Standard wood pellets were gasified at temperatures between 800 and 850 °C.17 The change of the relative tar content of the substance groups is seen in Figure 7. The relative contents of naphthalenes and PAHs are significantly increasing and the relative content of aromatic compounds and phenols are decreasing. Figure 9. Temperature dependency of sum GC−MS tars for straw during pyrolysis.

tar is shown in Figure 10 displayed. During pyrolysis operation the sum of the GC−MS tars ranges between 27 and 42 g/Nm3 and the gravimetric tar between 38 and 72 g/Nm3. The

Figure 7. Relative content of naphthalenes, aromatic compounds, PAHs, furans, and phenols for wood pellets.17

Further the temperature dependency of the gravimetric tar was investigated as seen in Figure 8. A decrease of the gravimetric tar content due to the higher temperature can be noted. As an exsample for the gas concentration, an average gas composition of the product gas (Table 6) for 40%/60% straw/

Figure 10. Temperature dependency of gravimetric tar for straw during pyrolysis. 13005



Article

pyrolysis has been optimized for cofiring of the volatiles but not in direction of maximizing the oil yield. Due to the lower temperature and the lack of a gasification agent the tar contents obtained in pyrolysis (Figures 9 and 10) are much higher than in the gasification product gas. As observed for gasification also a decrease in the tar content with increasing temperatures is noted. Figure 11 shows the relative tar composition of pyrolysis product gas at a temperature range from 578 to 632 °C.

Figure 12. Comparison of relative tar composition of pyrolysis and steam gasification at different temperatures.

ification the group of the aromatic nitrogen compounds can be found at a very low level (0.2% relative content). Aromatic nitrogen compounds decompose at higher temperatures.

4. CONCLUSION At Vienna University of Technology (VUT) the tar compositions and concentrations have been measured over a long period of time and for different processes. For this work results from a 100 kW dual fluidized bed (DFB) gasifier and a low temperature 3 MW pyrolysis biomass pilot plant were chosen. For such experiments the exact same parameters are not doable because of various reasons. One main reason is the huge difference in the processes itself, but there are also other reasons which one might not think about at first glance. First the fuel biomass is not a perfect homogeneous fuel and second each fuel also leads to certain process limitations. For straw gasification for instance, the low ash melting temperature of straw limits the experiments in a DFB gasifier used in this work. In this process the ash not only has to withstand the temperatures in the gasification zone but also those in the combustion zone which can reach up to 900 °C. Nevertheless the importance of the comparison is the uniformity of measurement technique as well as evaluation, which normally is not given if one compares results from two different processes. Hence it is impressive that despite all this major differences some main trends can be seen in both processes. They are summarized as follows: • The main influence parameters are the temperature and the availability of a oxidation reactant. • The GC−MS tar substance groups are shifted from the phenols (pyrolysis) to the naphthalenes and PAHs (gasification) with increasing temperature. • Main tar components for straw/wood cogasification are naphthalenes. Their relative content is rising with increasing temperatures. • Phenols are the main components (60%−75%) in pyrolysis. With increasing temperatures the phenols are decreasing, and with a reaction partner (as in steam gasification) a reduction to less than 5% relative content in the gasification product gas is notable. • Aromatic compounds are increasing from 10% in pyrolysis up to 28% in straw/wood steam gasification. • The gravimetric tar content shows a clear tendency to lower content at higher temperatures (pyrolysis at 600 °C, 72 g/Nm3; straw/wood steam gasification at 800 °C, up to 9 g/Nm3).

Figure 11. Temperature dependency of relative tar content of phenols, naphthalenes, and aromatic compounds, aromatic nitrogen compounds, PAH, and furans for straw pyrolysis.

The decrease of the phenols (Figure 11) with increasing temperature can be explained by the primary tar transition to secondary tar, thus leading to a reduction of the phenols. In Figure 11 no significant change of the furans and aromatic nitrogen compounds can be observed. On the contrary, an increase in the amount of aromatic compounds, naphthalenes, and PAHs can be observed. This increase occurs at higher temperatures because secondary tars (aromatic compounds) as well as tertiary tars (naphthalenes and PAHs) are formed at these temperatures. 3.3. Comparison of Gasification Tar with Pyrolysis Tar. The tar composition in pyrolysis product gas is different from the tar composition in steam gasification. The main pyrolysis tar component, phenol, is only a byproduct in gasification. The reason is that phenols are a primary tar component, which is reformed to smaller (gaseous) or larger components at higher temperatures. At pyrolysis temperatures from 600 to 630 °C a slight increase of aromatic compounds is noted. Aromatic compounds are mainly secondary tars, which begin to form during pyrolysis. If a reactant is available and the temperature is increasing to higher temperatures (800−850 °C), mainly tertiary tars, as naphthalenes and PAH, are formed. Therefore, the higher concentration of naphthalenes in gasification product gas, as well as their increase with rising gasification temperatures, is a logical consequence. Figure 12 shows the average tar composition for straw pyrolysis, wood/straw, and wood pellets steam gasification. For each process two typical temperatures were chosen. On average the relative concentration of furans remains the same for gasification and pyrolysis as shown in Figure 12. The concentration of aromatic nitrogen compounds is relatively low with only one percentage in pyrolysis product gas, and no aromatic nitrogen compounds occurred during the wood gasification experiments. In comparison in straw/wood gas13006



Article

(12) Wolfesberger, U. First test runs and tar analyses of a low temperature pyrolysis biomass pilot plant. Master thesis, Vienna University of Technology, 2009. (13) Aigner, I. Einfluss des Brennstoffwassergehalts auf den Betrieb eines Zweibettwirbelschichtvergasers. Master thesis, Vienna University of Technology2008. (14) Pfeifer, C.; Koppatz, S.; Hofbauer, H. Steam gasification of various feedstocks at a dual fluidised bed gasifier: Impacts of operation conditions and bed materials. Biomass Convers. Biorefin. 2011, 1, 39− 53. (15) Wolfesberger, U.; Koppatz, S.; Pfeifer, C.; Hofbauer, H. Effect of iron supported olivine on the distribution of tar compounds derived by steam gasification of biomass. ICPS11: International Conference on Polygeneration Strategies; Vienna, 2011; pp 69−76. (16) Pfeifer, C.; Hofbauer, H. Development of catalytic tar decomposition downstream from a dual fluidized bed biomass steam gasifier. Powder Technol. 2008, 180, 9−16. (17) Koppatz, S.; Pfeifer, C.; Hermann, H. Primary tar reduction in a dual fluidised bed gasification system by means of Fe-supported olivine. 17th European Biomass Conference and Exhibition; Hamburg, 2009; pp 932−935. (18) Wolfesberger, U.; Aigner, I.; Hofbauer, H. Tar content and composition in producer gas of fluidized bed gasification of woodInfluence of temperature and pressure. Environ. Prog. Sustainable Energy 2009, 28, 372−379. (19) Kreuzeder, A.; Pfeifer, C.; Soukoup, G.; Cuadrat, A.; Hermann, H. Increased Fuel Flexibility of a Dual Fluidized-Bed Gasification Process. In Proccedings 15th European Biomass Conference and Exhibition; Berlin, 2007; pp 913−920. (20) Aigner, I.; Pfeifer, C.; Hofbauer, H. Co-gasification of coal and wood in a dual fluidized bed gasifier. Fuel 2011, 90, 2404−2412. (21) Pröll, T.; Hofbauer, H. H2 rich syngas by selective CO2 removal from biomass gasification in a dual fluidized bed systemProcess modelling approach. Fuel Process. Technol. 2008, 89, 1207−1217.

It was also shown that the process itself, reaction temperature, and concentration of reaction agent have a more significant influence on the tar concentration and tar composition than the used fuel. As discussed, the analyses and measurment method used in the experimental test runs was very important and allowed, if used in a correct way, processes even as different as pyrolysis and gasification to link, whereby impulses for further research were given to both research fields. Further experiences at VUT show that results for plants of this small size are representative also for larger gasification plants of the same type and, therefore, extremely valuable for the up scaling of gasification processes mentioned in Pröll et al.21

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the team of the Test Laboratory for Combustion Systems especially the division Analytics, Stefan Koppatz at the Institute of Chemical Engineering at VUT, and the Energie Versorgung Niederösterreich (EVN) at Dürnrohr.

■

REFERENCES

(1) International Energy Agency. World Energy Outlook; OECD/IEA: Paris, 2007. (2) Egsgaard, H.; Larsen, E. Thermal Transformation of Light TarSpecific Routes to Aromatic Aldehydes and PAH. In Proceedings of the 1st World Conference and Exhibition on Biomass for Energy and Industry, June 5−9, 2000; pp 1468−1471. (3) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Pretreated olivine as tar removal catalyst for biomass gasifiers: investigation using naphthalene as model biomass tar. Fuel Process. Technol. 2005, 86, 707−730. (4) Devi, L.; Ptasinskia, K. J.; Janssena, F. J. J. G.; van Paasenb, S. V. B.; Bergmanb, P. C. A.; Kielb, J. H. A. Catalytic decomposition of biomass tars: Use of dolomite and untreated olivine. Renewable Energy 2005, 30, 565−587. (5) Milne, T. A.; Abatzoglou, N.; Evans, R. J. Biomass Gasifier “Tars”: Their Nature, Formation and Conversion; National Renewable Energy Laboratory (NREL): Golden, CO, 1998; Report No. NREL/TP-570− 25357. (6) Kübel, M. Teerbildung und Teerkonversion bei der Biomassevergasung. Ph.D. Treatise, University Stuttgart, Germany, 2007, pp 1− 165. (7) Rabou, L. P. L. M.; Zwart, R. W. R.; Vreugdenhil, B. J.; Bos, L. Tar in biomass producer gas; the Energy Research Centre of the Netherlands (ECN) experience: An enduring challenge. Energy Fuels 2009, 23, 6189−6198. (8) Cuncliffe, A. M.; Williams, P. T. Composition of oils derived from the batch pyrolysis of tyres. J. Anal. Appl. Pyrol. 1998, 44, 131− 152. (9) Li, C.; Suzuki, K. Tar property, analysis, reforming mechanism and model for biomass gasificationAn overview. Renew. Sustain. Energy Rev. 2009, 13, 594−604. (10) Hüttler, H.; Rauch, R.; Hofbauer, H. Tar formation in a dual fluidised bed biomass steam gasification demonstration plant: Quantification and influencing parameters. BioEuro Success and Visions for Bioenergy, Salzburg, Austria, March 2007. (11) CEN/TS 15439 Biomass Gasification−Tar and Particles in Product Gases−Sampling and Analysis; European Committee for Standardization: 2005. 13007


Mechanism of Tar Generation during Fluidized Bed Gasification and

Recommend Documents