Energy & Fuels 2005, 19, 87-93
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Numerical Modeling of Tar Species/VOC Dissociation for Clean and Intelligent Energy Production Georgios Taralas*,† and Michael G. Kontominas Laboratory of Food Chemistry and Technology, Department of Chemistry, University of Ioannina, GR-45 110, Ioannina, Greece Received May 19, 2004
Secondary pyrolysis of vaporized unsaturated hydrocarbons in the presence of water vapor and oxygen gas was studied, using toluene and benzene as tar-derived species and/or volatile organic compounds (VOCs). Toluene and benzene have been chosen as model formulas for reactive one-ring species determined from tar constituents present in the gas derived from gasification and pyrolysis technologies. The experiments were performed in a plug-flow-reactor at atmospheric pressure via the introduction of dynamic steady-state assumptions, a temperature of 1098 K, and residence times of τ e 5 s. The gas-phase molar ratios were [H2O + O2]air/[C7H8] ) 3.52 and [H2O + O2]air/[C6H6] ) 3.53 in toluene and benzene, respectively. The experimental observations have been evaluated in terms of a chemical kinetics model. The proposed model interpreted the experimental trends and reproduced the experimental data. Benzyl radical species (C7H7) can be considered as a key component of the thermal dissociation of toluene. Comparison of the results from computer simulations shows that hydrogen influences the destruction of toluene. In the presence of water vapor and hydrogen, the intermediate compound C7H7 undergoes decomposition. During VOC dissociation, the oxidized environment enhances the formation of H2, CO, and CO2.
Introduction Thermal gasification technologies that use fossilsand, particularly, nonfossilssolid fuels as renewable energy sources for heat and electric power generation are currently being introduced in energy markets to establish their general economic and technical viability.1,2 The permanent gas that is produced (H2, CO, CH4, steam (H2O), CO2, etc.) contains tar that can plug the downstream process equipment, such as filters, internal combustion (IC) engine suction channels, gas turbine blades, flue gas compressors, and other operating devices (i.e., fuel cell and intelligent hybrid technologies).3-5 Tar abatement is generally the key problem.6,7 Compared with solid fossil fuels, nonfossil fuels (such as woody biomass, refuse-derived fuel (RDF), municipal solid waste (MSW), agricultural residues, and lignocel* Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Also associated with the ESCOR Company. (1) Maniatis, K.; Millich, E. Energy from Biomass and Waste: The Contribution of Utility Scale Biomass Gasification Plants. Biomass Bioenergy 1998, 15 (5), 195-200. (2) Bridgwater, A. V. The Technical and Economic Feasibility of Biomass Gasification for Power Generation. Fuel 1994, 74 (5), 631653. (3) Corella, J.; Narva´ez, I.; Orı´o, A. Criteria for Selection of Dolomites and Catalysts for Tar Elimination from Gasification Gas: Kinetic Constants. In New Catalysts for Clean Environment; Maijanen, A., Hase, A., Eds.; VTT Symposium 163; Julkaisija and Utgivare Publishers: Espoo, Finland, 1996; pp 177-184. (4) Taralas, G. Cyclohexane-Steam Cracking Catalysed by Calcined Dolomite. In Developments in Thermochemical Biomass Conversion, International Energy Agency-IEA Bioenergy; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, U.K., 1997; pp 1086-1100. (5) Taralas, G. Decomposition of CH3OH to H2 over Copper-Containing Catalysts in a Temperature Gradient-Free Reactor for Hybrid Automobile Applications. Master’s Science Thesis, Royal Institute of Technology (KTH), Department of Chemical Engineering and Technology, Stockholm, Sweden, 1986 (Thesis No. TRITA-KT-1986:01).
lulosic materials of processing wastes from the food production industry) are characterized by a higher reactivity of char and a higher content of tar than fossil fuels.8-10 Care must also be taken to achieve complete destruction of tar and/or volatile organic compounds (VOCs) by ensuring higher process efficiency, which is traced to increasingly strict legal requirements for low environmental emissions.2,11 The generic word tar, which refers to material collected at ambient temperature, is defined as a continuous mixture of condensable aromatic and polycyclic aromatic hydrocarbons (AH and PAH, respectively), along with their substituted derivates.12 Benzene, tolu(6) Taralas, G.; Kakatsios, X. Gasification of Biomass for Energy Production Proposes: Development of Catalytic Gasification Technology (in Greek). In Implementing Renewable Energy SourcessNational Priorities and European Strategies. National Technical University of Athens, Unit of Renewable Energy Sources, November 30-December 2, 1998, Athens, Greece; pp 476-482. (7) Taralas, G.; Kontominas, M. G. Combined Heat and Electric Power (CHP) Generation from Non-Fossil Fuel-Biomass in Greece. Future Prospects and Improvements. In Management of Solid Waste for Sustainable Development in Front of 21st Century, February 28March 1, 2002, Athens, Greece. (8) Taralas, G.; Kakatsios, X. Industrial Solid Waste Management. European Commission DG XXIIsEducation, Training and Youth, LEONARDO DA VINCI Programme, AIOLOS Project, Contract No. EL/98/1/68126/PI/I.1.1.B/FPC, June 2001. (9) Taralas, G. Co-Generated Thermal Power Systems Utilizing Biomass and Natural Gas in Greece. Presented at The 12th European Conference and Technology Exhibition on Biomass for Energy, Industry, and Climate Protection, Amsterdam, The Netherlands, June 1721, 2002. (10) Guanxing, C.; Sjo¨stro¨m, K.; Bjo¨rnbom, E. Pyrolysis/Gasification of Wood in a Pressurized Fluidized Bed Reactor. Ind. Eng. Chem. Res. 1992, 31, 2764-2768. (11) Taralas, G.; Koukios, E. G.; Aznar, M. P.; Orio, A.; Corella, J. Progress Toward Processing Fuel Gas Cleanup Systems. Continuing Developments from Biomass Gasification for Combined Heat and Power (CHP) Production and other Implementations. In Proceedings of the European Congress on Renewable Energy Implementation, CRES: Athens, Greece, 1997; pp 816-825.
10.1021/ef040048o CCC: $30.25 © 2005 American Chemical Society Published on Web 11/13/2004
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ene, and xylenes (BTX) are major VOCs of commensurate quantity, representing >60% of the total tar from fluidized-bed gasification.13 A complete mechanistic model of the tar minimization would require a detailed knowledge of the composition of the tar. This would be impossible because, generally, the real detailed tar composition is not experimentally available. In thermochemical gasification systems, tar at the exit is subjected to homogeneous gas/gas reactions and heterogeneous solid/gas reaction paths during pyrolysis and partial oxidation with the gasifying agent (usually air, oxygen, steam, or mixtures of them). Sampling (off-line and online), as well as preparation and analysis of the samples (i.e., extraction, enrichment, preliminary separation, etc.), must be standardized.14-16 Nevertheless, much research has been concentrated on thermal and catalytic tar elimination studies of tar-derived model substances.17-23 This study is focused on the thermal dissociation of vaporized toluene (C7H8), which is used as a tar model compound, relevant to hot tarry fuel gas at temperatures upward of 1098 K. The thermal dissociation reactions have been investigated in the presence of water vapor and oxygen gas ([H2O + O2]air/[C7H8] ) 3.52 mol/mol), i.e., in an oxidizing environment. Because toluene is not only the primary tar compoundsbenzene (C6H6) and other products of the reactions also are producedsparallel thermal dissociation of vaporized C6H6 as another surrogate tar-related one-ring compound has also been performed ([H2O + O2]air/[C6H6] ) 3.53 mol/mol). The proposed chemical reaction mechanism during thermal dissociation of toluene is compared (12) Vassilatos, V.; Taralas, G.; Brage, C.; Sjo¨stro¨m, K. Analys och Kracking av Biomassa-tja¨ra. In Proceedings Fo¨ rbra¨ nningsTeknisk Projekt-Expose Nr 3, Energiforskningsna¨ mnden/UTR 1990: 5, Lund, Sweden, 1990; Vol. 2, 142-151. (13) Taralas, G.; Kontominas, M. G.; Kakatsios, X. Modelling the Thermal Destruction of Toluene (C7H8) as Tar-Related Species for Fuel Gas Cleanup. Energy Fuels 2003, 17, 329-337. (14) Moersch, O.; Spliethoff, H.; Hein, K. R. G. Tar Quantification with a New Online Analysing Method. Biomass Bioenergy 2000, 18, 79-86. (15) UT-BTG. An overview of these standards can be found on the Internet at http://btg.ct.utwente.nl/Projects/55, 2001. (16) Taralas, G. Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases. Mid-Term Report Prepared for the European Commission (DGXII), Netherlands Agency for Energy and the Environment (NOVEM), Swiss Federal Office of Education and Science, U.S. Department of Energy (DoE) and National Resources Canada, May 2001. (Research conducted in the framework of the Energy Project EEN5-1999-00507.) (17) Taralas, G. Modelling the Influence of Mineral Rocks, Active in Different Hot Gas Conditioning Systems and Technologies, on the Production of Light a-olefins. Can. J. Chem. Eng. 1999, 77, 12051214. (18) Simell, P. A.; Leppa¨lahti, J. K.; Kurkela, E. A. Tar-Decomposing Activity of Carbonate Rocks under High CO2 Partial Pressure. Fuel 1995, 74 (6), 983-945. (19) Bangala, D. N.; Abatzoglou, N.; Martin, J.-P.; Chornet, E. Catalytic Gas Conditioning: Application to Biomass and Waste Gasification. Ind. Eng. Chem. Res. 1997, 36, 4184-4192. (20) Caballero, M. A.; Aznar, M. P.; Gil, J.; Martin, J. A.; Frances, E.; Corella, J. Commercial Steam Reforming Catalysts to Improve Biomass Gasification with Steam-Oxygen Mixtures. 1. Hot Gas Upgrading by the Catalytic Reactor. Ind. Eng. Chem. Res. 1997, 36, 5227-5239. (21) Corella, J.; Aznar, M. P.; Gil, J.; Caballero, M. A. Biomass Gasification in Fluidised Bed. Where to Locate the Dolomite to Improve Gasification? Energy Fuels 1999, 13, 1122-1127. (22) Corella, J.; Orio, A.; Toledo, J. M. Biomass Gasification with Air in a Fluidized Bed: Exhaustive Tar Elimination with Commercial Steam Reforming Catalyst. Energy Fuels 1999, 13, 702-709. (23) Corella, J.; Orio, A.; Aznar, P. Biomass Gasification with Air in Fluidized Bed: Reforming of the Gas Composition with Commercial Steam Reforming Catalysts. Ind. Eng. Chem. Res. 1998, 37, 46174624.
Taralas and Kontominas
to the overall mechanism, which has been recently studied experimentally and numerically with sufficient reliability in the presence of hydrogen ([H2O + H2]/ [C7H8] ) 3.52 mol/mol), i.e., in a reducing environment.13 Experimental Section Testing Rig and Procedure. The plug-flow-reactor (PFR) and the investigational procedure applied under dynamic steady-state (SS) approximation conditions are described elsewhere.13 Perturbation experiments on toluene dissociation supplemented with experiments on dissociation of benzene were performed with gas-phase molar ratios of [H2O + O2]air/ [C7H8] ) 3.52 and [H2O + O2]air/[C6H6] ) 3.53 in toluene and benzene, respectively, at a total pressure of 101.3 kPa and residence times of τ e 5 s. Experiments with toluene covered the following range of variables: temperature, 1023 K < T < 1123 K; toluene partial pressure, P[C7H8] ) 0.93 kPa; and [H2O + O2]air partial pressure, P[H2O+O2]air ) 22.9 kPa (where [H2O/ O2]air ) 3.0 mol/mol). In the runs with benzene, the experiments covered the following range of variables: temperature, 1123 K < T < 1223 K; partial pressure of benzene, P[C6H6] ) 0.93 kPa; and [H2O + O2]air partial pressure, P[H2O+O2]air ) 22.9 kPa (where [H2O/O2]air ) 3.0 mol/mol). In the experiments, diluted gas mixtures were used, to avoid interference of the hydrocarbon disappearance with subsequent bimolecular reactions17 and maintain some isothermal conditions along the reaction zone.13 Moreover, the experimental conditions were also formulated to minimize sooting effects, whereas the impact of the oxidizing agents on the conversion of the toluene and benzene was ignored.13
Results and Discussion Experimental data under dynamic SS approximation conditions13,24 are shown in Table 1. The following methods were used to evaluate the degree of conversion of the aromatic hydrocarbon species k and residence time τ:
τ (s) )
( )
∫0V
〈(Tref/TRZ) exp{-(Ej/R)[(1/TRZ) - (1/Tref)]}〉 dVR
Xk (mol %) ) R
∑i
Cgas,outlet x
CxHy,inlet
× 100
(1)
∑r Vr(Tref,pR) (2) Nonisothermal data were converted to pseudo-isothermal conditions, using the equivalent reactor concept.25 There are several possible reactions that can occur at high temperatures. The most important types of these reactions are presented in Table 2. Reactions R1-R9 in the table can be considered irreversible. The amounts of the formed and/or reacted products may match the (24) Taralas, G. Effects of MgO, CaO and Calcined Dolomites on Model Substance Cracking and Conversion of Tar from Biomass Gasification /Pyrolysis Gas, Lic. of Engineering Dissertation Thesis, Royal Institute of Technology (KTH), Department of Chemical Engineering and Technology, Stockholm, Sweden, 1990. (ISBN 91-7170043-9.) (25) Taralas, G.; Vassilatos, V.; Delgado, J.; Sjo¨stro¨m, K. Thermal and Catalytic Cracking of n-Heptane in the Presence of CaO, MgO and Calcined Dolomites. Can. J. Chem. Eng. 1991, 69, 1413-1419.
Numerical Modeling of Tar Species/VOC Dissociation
Energy & Fuels, Vol. 19, No. 1, 2005 89
Table 1. Experiments for Formed Products in Toluene and Benzenea parameter
value Toluene 1023 15.0
temperature, T (K) time of run, t (h) residual conversion, Rek (mol %)b average CO (mol/mol of toluene reacted) H2 (mol/mol of toluene reacted) CO2 (mol/mol of toluene reacted) C6H6 (mol/mol of toluene reacted)
53.9
0.143 × 0.163 × 102 0.522 × 101 1.046 × 10-1
0.369 × 0.144 × 102 0.329 × 101 0.860 × 10-2
0.363 × 101 0.139 × 102 0.325 × 101 0.843 × 10-2
1173 14.9
1223 15.5
60.8
41.3
0.197 × 102 0.365 × 102 0.198 × 101
0.248 × 102 0.373 × 102 0.208 × 101
Benzene 1123 15.7 [H2O + O2]/[C6H6] ) 3.53 mol/mol 83.8
residual conversion, Rek (mol %)b average CO (mol/mol of benzene) H2 (mol/mol of benzene) CO2 (mol/mol of benzene)
1123 14.8
[H2O + O2]/[C7H8] ) 3.52 mol/mol 89.6 101
temperature, T (K) time of run, t (h)
1073 14.6
0.148 × 102 0.304 × 102 0.303 × 101
31.4 101
a Residence time was τ ) 0.40-0.90 s. Because the experiments are conducted with a constant volume rate of the reactants at lower and higher dissociation temperatures, the residence time τ is longer and shorter, respectively. b The residual conversion is given by: Rek ) 100 - Xk.
Table 2. Possible Reactions of Toluene and Benzene -∆H°298K (kJ/mol)
reaction number
-869.1 -581.2 -458.1 -704.8
R1 R2 R3 R4
Steam Dealkylation C7H8 + H2O f C6H6 + 2H2 + CO -164.2 -123.1 C7H8 + 2H2O f C6H6 + 3H2 + CO2
R5 R6
reaction Steam Reforming C7H8 + 7H2O f 7CO + 11H2 C7H8 + 14H2O f 7CO2 + 18H2 C6H6 + 12H2O f 6CO2 + 15H2 C6H6 + 6H2O f 6CO + 9H2
C7H8 + 10H2 f 7CH4 C6H6 + 9H2 f 6CH4
Hydrocracking
Hydrodealkylation C7H8 + H2 f C6H6 + CH4
573.9 532.0
R7 R8
41.9
R9
Dry Reforming C7H8 + 7CO2 f 14CO + 4H2 -1090 C7H8 + 11CO2 f 18CO + 4H2O -1200 -950 C6H6 + 6CO2 f 12CO + 3H2 C6H6 + 9CO2 f 15CO + 3H2O -1070 C7H8 f 7C + 4H2 C6H6 f 6C + 3H2 CO f 1/2C + 1/2CO2
R10 R11 R12 R13
Carbon Formation 50.0 82.9 86.2
R14 R15 R16
Disproportionation C7H8 f 1/2C6H6 + 1/2m-C8H10 C7H8 f 1/2C6H6 + 1/2o-C8H10 C7H8 f 1/2C6H6 + 1/2p-C8H10
-0.08 -0.96 -0.44
R17 R18 R19
Water-Gas Shift CO + H2O T CO2 + H2
41.1
R20
206.1
R21
-131.3 -90.0
R22 R23
-171.4
R24
123.6 165.0
R25 R26
Methanation CO + 3H2 T CH4 + H2O H2O + C f H2 + CO 2H2O + C f CO2 + 2H2 CO2 + C f 2CO
reactions R21-R26 are assumed to occur in the presence of a catalyst.26-28 The residence time τ in eq 2 for the benzene thermal reactions was obtained by an iterative technique25 used to evaluate the activation energy, Ej, from the rate constants at different temperatures. Some other values for Ej and mixed reactor data from literature are shown in Table 3. Simulated equilibrium conversions of VOC and/or tarderived species, relative to varied temperature for reactions R1-R9, are shown in Figures 1 and 2. According to these figures, the equilibrium conversion of the dealkylation of toluene and benzene (reactions R7R9) in the presence of hydrogen and water vapor increases as the temperature decreases. As an example, the equilibrium conversion for reaction R9 becomes almost 98% at 673 K (for the stoichiometric molar ratio of H2/C7H8 ) 1). Moreover, the calculated equilibrium conversion of toluene in reactions R5 and R6, at 800 K, is 86%, 90%, and 99.8% for [H2O]/[C7H8] molar ratios of 1, 2, and 5, respectively.
Water Gas
Boudouard
Side Reactions CO + H2 f 1/2CH4 + 1/2CO2 CO2 + 4H2 f CH4 + 2H2O
stoichiometry of the individual reactions R5 and R6, in the case of toluene, and R3 and R4 for benzene. Conversely, reactions R10-R19 might not occur, whereas
(26) Taralas, G. Catalytic Steam Pyrolysis of a Selected Saturated Hydrocarbon on Calcined Mineral Particles. Can. J. Chem. Eng. 1998, 76, 1093-1101. (27) Taralas, G. Modelling the Characteristics of the Endothermic Reaction Potential of Tar for Flue Gas Cleanup in Advanced Thermochemical Conversion Processes. In Progress in Thermochemical Biomass Conversion, International Energy Agency-IEA Bioenergy; Bridgwater, A. V., Ed.; Blackwell Science: Oxford, U.K., 2001; pp 176-187. (28) Taralas, G.; Kontominas, M. G. Kinetic Modelling of VOC Catalytic Steam Pyrolysis for Tar Abatement Phenomena in Gasification/Pyrolysis Technologies. Fuel 2004, 83, 1235-1245. (29) Szwarc, M. The C-H Bond Energy in Toluene and Xylenes. J. Chem. Phys. 1948, 16 (2), 128-136. (30) Pamidimukkala, K. M.; Kern, R. D.; Patel, M. R.; Wei, H. C.; Kiefer. J. H. High-Temperature Pyrolysis of Toluene. J. Phys. Chem. 1987, 91, 2148-2154. (31) Brouwer, L. D.; Mu¨ller-Markgraf, W.; Troe, J. Thermal Decomposition of Toluene: A Comparison of Thermal and Laser-Photochemical Activation Experiments. J. Phys. Chem. 1988, 92, 4905-4914. (32) Smith, R. D. A Direct Mass Spectrometric Study of the Mechanism of Toluene Pyrolysis at High Temperatures. J. Phys. Chem. 1979, 83, 1553-1563. (33) Blades, H.; Blades, A. T.; Steacie, E. W. R. The Kinetics of the Pyrolysis of Toluene. Can. J. Chem. 1954, 32, 298-311. (34) Price, S. J. The Pyrolysis of Toluene. Can. J. Chem. 1962, 40, 1310-1317. (35) Bruinsma, O. S. L.; Geertsma, R. S.; Bank, P.; Moulijn, J. A. Gas-Phase Pyrolysis of Coal-Related Aromatic Compounds in a Coiled Tube Flow Reactor. Fuel 1988, 67, 327-333.
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Table 3. Activation Energies and Frequency Factors for Thermal Cracking of Tar-Derived Species [H2O + O2]a
[H2O + H2]b
activation energy (kJ/mol)
reference
pre-exponential factor (s-1)
356.0 324.3 376.6 340.0 315.0
13 29 33 34 35
2.30 × 1015 N/Gc N/Gc N/Gc 1.23 × 1013
468.0
35
5.48 × 1019
activation energy (kJ/mol)
pre-exponential factor (m3 mol-1 s-1)
reference
Toluene 250.0 N/Ad N/Ad N/Ad N/Ad
3.34 × 1010 N/Ad N/Ad N/Ad N/Ad
13
Benzene N/Ad
N/Ad
a Partial pressure of [H O + O ] is
