- Email: [email protected]
Detailed kinetics in the mathematical model of fixed bed gasifiers
20th European Symposium on Computer Aided Process Engineering – ESCAPE20 S. Pierucci and G. Buzzi Ferraris (Editors) © 2010 Elsevier B.V. All rights reserved.
Detailed kinetics in the mathematical model of fixed bed gasifiers R. Grana, S. Sommariva, T. Maffei, A. Cuoci, T. Faravelli, A. Frassoldati, S. Pierucci, E. Ranzi* Politecnico di Milano, CMIC Department Piazza Leonardo da Vinci, 32, 20131 Milano, Italy * Corresponding author: [email protected]
Abstract This work presents a general mathematical model of a fixed bed gasifier, where both the transport resistances and the chemical kinetics are important at the reactor and the particle scale. Pyrolysis and gasification of the solid fuel (coal, plastics, biomasses or wastes) is only the first step of the whole process. Successive gas phase reactions of the released species are described with a detailed kinetic scheme. Furthermore, an accurate description of heat and mass transport between gas and solid phases allows a proper characterization of combustion and gasification of the solid fuel. This work summarizes several facets of this problem with examples and validations. Examples relating to coal and biomass gasification allow evaluating the feasibility and limitations of the proposed approach. Keywords: Gasifier, detailed kinetics, coals, biomasses, wastes
1. Introduction The environmental concerns towards the combustion of fossil fuels drive the interest in gasification processes of biomasses, coals, plastics, and refuse derived fuels (RDF). Gasification of solid fuels is nowadays a promising alternative to direct combustion, both electric and thermal energy are viable products, together with chemicals. The mathematical description of such processes is rather difficult due to the complex phenomena involved, such as modelling solid devolatilization, gas phase reactions and gas-solid interactions. However, it is worth to emphasize the importance of these models, which could ease the scale-up and the optimization of the gasifier and could improve the understanding of the whole process. In this work, we proposed the methodology for solving such problems, showing the approach for the main facets involved in solid fuels gasification. Finally, an application example of a fixed bed gasifier model is provided, emphasizing the thermal features of the reactor as well as the role of feedstock characterization.
2. Devolatilization of Coals, Plastics, Biomasses and Refused Derived Fuels (RDF) 2.1. Solid fuel characterization The different solid fuels are described with a limited number of reference compounds and for each of them a multistep kinetic scheme was developed. While plastics, such as poly-ethylene (PE), poly-propylene (PP) and poly-styrene (PS) have a very well-defined structure and composition, the available information about coals and biomasses is usually limited to the elemental composition in terms of C/H/O. Degradation of plastics
R. Grana et al.
was already discussed by [Marongiu et al., 2007]. Biomass composition , if biochemical analysis is available, is simply defined in terms of humidity, ash, cellulose, hemicelluloses and lignin. If only the elemental analysis is available, then a suitable combination in terms of reference species is derived by atomic balance. [Ranzi et al., 2008]. Similarly, three reference species (COAL1, COAL2 and COAL3) are used to describe the composition and reactivity of the different coals. COAL1 (C12H11) , together with pure carbon (CHARC), is useful to describe anthracitic coals with different degree of aromaticity. COAL2 (C14H10O) lies in the middle of bituminous coals, while COAL3 is highly oxygenated (C12H12O5) and is representative of lignitic coals [Sommariva et al., 2010]. In a very similar way, Refused Derived Fuels (RDF), typically with heating values of 4500-5000 kcal/kg, are described in terms of a proper combination of plastic wastes (15-30%), lignocellulosic material (30-50%), ash, and humidity. Fig. 1 shows typical compositions of different RDF in terms of C and H (wt% ash free) 16
RDFelementalcomposition Phyllisdatabase:www.ecn.nl/phyllis
14
PE
% H [w/w]
12 H2O
10
8
6 Cellulose Lignin
4
2 40
50
60
70
80
90
% C [w/w]
Figure 1 Typical compositions of different RDF in terms of C and H (wt% ash free)
2.2. Solid fuel devolatilization The devolatilization of solid particles (mixture of reference compounds) is always considered as a linear combination of the pure component devolatilization. The overall kinetic scheme of RDF devolatilization is simply the combination of the multistep devolatilization models of Biomass [Ranzi et al., 2008], plastic [Marongiu et al., 2007] and coal [Sommariva et al., 2010]. The peculiarity of this approach is that all these schemes consist of a limited number of devolatilization reactions, which are able to describe not only the solid residue, but also the detailed composition of released gas and tar species. Thus, the novelty of this kinetic model, when compared with the majority of the available ones in the literature, is the effort and the challenge to define lumped stoichiometries of the devolatilization reactions with a detailed characterization of the gas and the tar fractions. Thus, Fig. 2 shows the lumped reference components of tar products released by the reference coals. Operating conditions affect the devolatilization selectivity and yields; in particular this multistep approach allows also to describe the chemical and morphological evolution of the solid phase in terms of composition and reactivity.
Detailed kinetics in the mathematical model of fixed bed gasifiers
OH
HO OH O
OCH 3 OH
OH OCH 3
TAR1
TAR2
C12H11
C14H10O
TAR3 C11H10O2
Figure 2 Lumped reference components of tar products from reference coals
3. Combustion and gasification reactions of residual char The gasification and combustion of char, i.e. the set of heterogeneous reactions of oxygen and steam with the solid residue coming either from coal, biomass or plastics, are responsible for the autothermic behaviour of the whole gasification process. These reactions and related kinetic parameters are summarized in Table 2 [Groeneveld and van Swaaij, 1980, Kashiwagi and Nambu, 1992]. Table 1: Char gasification and combustion reactions. Units are: kmol, m3, K, kcal, s. k CHAR + O2
ĺ CO2
CHAR + 0.5 O2 ĺ CO CHAR + H2O
ĺ CO + H2
5.7u109 exp (-38200/ RT) [O2] 0.78 5.7u1011 exp (-55000/ RT) [O2] 0.78 7.9u109 exp (-52000/ RT) [H2O] 0.7
4. Detailed kinetics of secondary gas phase reactions The volatile components released during the pyrolysis undergo successive decomposition or combustion reactions in the surrounding gas phase. As clearly stated by Li et al 2004, a pure thermodynamic or equilibrium approach is not able to properly predict the composition of the gas mixture in a gasifier. The equilibrium approach largely over predicts the heating value of flue gases; in particular, higher amount of hydrogen is predicted, while only trace amounts of methane and hydrocarbons are obtained. On this basis, it is clear the utility to develop a detailed or semi-detailed kinetic scheme for the gas phase. An existing kinetic scheme for pyrolysis and oxidation of hydrocarbon species, already validated in a wide range of operating conditions [Ranzi et al., 2001], has been extended to properly describe successive gas phase reactions of released species. Due to the modularity of the detailed kinetic scheme, it is only necessary to describe the primary initiation, decomposition and H abstraction reactions of the new species. The overall gas-phase kinetic model, together with thermodynamic properties, is available at the website http://www.chem.polimi.it/CRECKModeling/. The number of new species included in the gas phase is a compromise between accuracy and computational efforts, maintaining the ability of the model to describe the
R. Grana et al.
gas composition and reactivity in a wide range of operating condition: in particular tar species are grouped into pseudo-components representative of a set of similar species with similar reactivity. As already discussed in a previous paper [Sommariva et al., 2010], the secondary gas phase reactions can also explain relevant temperature and pressure effects during coal devolatilization, not only related to the solid phase reactions.
5. Mathematical model of fixed bed gasifier The mathematical model of the fixed bed gasifier consists of two models. The first one at the particle scale and the latter at the reactor scale. This approach is discussed in details elsewhere [Pierucci and Ranzi, 2008; Dupont et al., 2009]. The particle model provides an internal description in order to account for intraparticle heat and mass resistances. The accuracy of this description depends on the number of discretization sectors (N). This feature becomes fundamental for the analysis of the gasification of thermally thick solid fuel particles. The equation 1 summarizes the main terms of material and energy balances for the particle: dm j ,i J j 1,i J j ,i R j ,i ° dt °° NCP ® NCP NCP ° d ¦ m j ,i Cp j ,iT j ° i1 JC j 1 ¦ J j 1,i h j 1,i JC j ¦ J j ,i h j ,i HR j dt i 1 i 1 ¯°
(1)
where m is the mass and j refers to the jth particle sector and i to the ith component. t is the time variable and J the mass flux (Fick and Darcy’s laws contributions) and R the kinetic term. Analogously, the energy balance settle the particle temperature T and accounts for conduction contribution (JC) as well as the convective contribution (Jh). HR is the heat of reactions involved in the solid phase. This particle model together with the kinetic models previously described is embedded in the fixed bed reactor model (equation 2). The reactor is considered as a series of elemental units (Fig. 3) which exchange mass and heat to each others. The single unit accounts for gas-solid interactions with particular attention to the inter-phase resistances. The balance equations (2) refer to a perfectly stirred reactor in dynamic conditions, where Rg,i are the gas phase reactions and G0,i and Gi are the inlet and outlet gas feeds [Pierucci and Ranzi, 2008; Dupont et al., 2009]. dg i ° dt G0 ,i J NR ,iK Rg ,i Gi ° NCP ® g i CpiTg NCP NCP NCP °d ¦ G0 ,i hg 0,i ¦ J NR ,i hNR ,iK JC NRK HR g ¦ Gi hg i ° i1 ¦ dt i 1 i 1 i 1 ¯
(2)
The term JNR,iK refers to the gas-solid mass exchange (JNR,i) multiplied by the number K of particles in the bed. Four terms contribute to gas phase temperature (Tg) evolution: the enthalpy convective terms (Ghg) between adjacent reactor layers, the gas-solid heat exchange (JNR,iK), the enthalpy diffusion term (JNR,ihNR,iK), and finally the contribution due to gas phase reactions (HRg).
Detailed kinetics in the mathematical model of fixed bed gasifiers
More than one thousand balance equations are obtained, when considering ~10-15 solid species, 100 gas-phase components, 10 reactor layers and 5 discretizations of the solid particle. The numerical problem is structured in tridiagonal blocks. The resulting ODE system is solved by using BzzMath Library (www.chem.polimi.it/homes/gbuzzi/).
6. Application examples Panel (a) Coal gasification 10 1400
8th layer
9 1300
Temperature (K)
Reactor layer
8 7 6
Coal
5 4
Gas phase
3
Solid surface
2
Solid core
9th layer
1200 1100
10th layer
1000 900 800 0
0.2
0.4
0.6
0.8
1
Normalized radius
1 0
400
800
1200
1600
Temperature (K)
Internal temperature profiles in the solid particles of the three top layers. Char combustion explains the internal maximum temperaturein the 8th layer.
Panel (b) Biomass gasification 10 1400
8th layer
9 1300
Temperature (K)
Reactor layer
8 7 6
Biomass
5 4
Gas phase
3
Solid surface
1100
10th layer 1000 900 800 0
Solid core
2
9th layer 1200
0.2
0.4
0.6
0.8
1
Normalized radius
1 0
400
800
1200
Temperature (K)
1600
The gradient in the 8th layer is more pronounced in biomass case than in coal case due to the minor presence of char on surface.
Figure 3: Predicted gas and solid temperature profiles of a counter-current fixed bed gasifier.
Fig. 3 shows the predicted gas and solid temperature profiles of a counter-current fixed bed gasifier fed with biomass (panel a) and with coal (panel b). Solid particles of 3 cm are treated with air at equivalence ratio 0.25-0.4 and with steam/fuel ratio of about 0.30 wt/wt. Typical contact times of gases are of a few seconds while several hours are necessary to treat solid particles. The model characterizes the solid and gaseous streams, giving a proper emphasis to secondary gas phase reactions of tar and gas components. Typical molar composition of flue gases is about 11-15% of H2, 13-17% of CO, 10-12% of CO2, 15-20 % of H2O and ~1-3% of CH4 and hydrocarbons.
R. Grana et al.
Maximum gas phase temperature is higher when treating biomasses, due to the largest extent of volatilization and the highest oxygen availability. Large temperature gradients are predicted in the first top layers of the bed. Char combustion in the third layer from the top of the gasifier justifies the maximum internal temperature of coal particles (~1100 °C), while this effect is less evident when biomass is treated in the gasifier, due to the lower presence of residual char. Depending on the ash content in the solid fuel, the shrinking of the bed is usually rather larger than 50%. These simulations require several hours of CPU time, due both to the stiff nature of the gas phase kinetics and to the dynamic approach to the steady solution. For these reasons a further model validation strongly demands for further simplifications in the solid and gas phase description.
7. Acknowledgements Authors gratefully acknowledge the financial support of EUROPOWER as well as of CNR-DET in the behalf of the Clean Coal Project.
References A. Cuoci, T. Faravelli, A. Frassoldati, S. Granata, G. Migliavacca, E. Ranzi, S. Sommariva, 2007, A General Mathematical Model of Biomass Devolatilization, 30th Meeting of the Italian Section of the Combustion Institute. C. Dupont, L. Chen, J. Cances, J.-M. Commandre, A. Cuoci, S. Pierucci, E. Ranzi, 2009, Biomass Pyrolysis: Kinetic Modelling and Experimental Validation Under High Temperature and Flash Heating Rate Conditions, Journal of Analytical and Applied Pyrolysis, 85, pp. 260-267. M.J. Groeneveld and W.P.M. van Swaaij, 1980, Gasification of Char Particles with CO2 and H2O, Chemical Engineering Science, 35, pp 307-313. T. Kashiwagi and H. Nambu, 1992, Global kinetic Constants for Thermal Oxidative Degradation of a Cellulosic Paper, Combustion and Flame, 88, pp. 345-368. X.T Li, J.R. Grace, C.J. Lim, A.P. Watkinson, H.P. Chen, J.R. Kim, 2004, Biomass Gasification in a Circulating Fluidized Bed, Biomass and Bioenergy, 26, pp. 171-193. A. Marongiu, T. Faravelli, E. Ranzi, 2007, Detailed Kinetic Model of the Thermal Degradation of Vynil Polymers, Journal of Analytical and Applied Pyrolysis, 78, pp. 343-362. S. Pierucci, E. Ranzi, 2008, A General Mathematical Model for a Moving Bed Gasifier,Computer Aided Chemical Engineering, 25, pp. 901-906. E. Ranzi, A. Cuoci, T. Faravelli, A. Frassoldati, G. Migliavacca, S. Pierucci, S. Sommariva, 2008, Chemical Kinetics of Biomass Pyrolysis, Energy&Fuels 2008, 22, 4292–4300. E. Ranzi, M. Dente, A. Goldaniga, G. Bozzano, T. Faravelli, 2001, Lumping procedures in detailed kinetic modeling of gasification, pyrolysis, partial oxidation and combustion of hydrocarbon mixtures, Progress in Energy and Combustion Science, 27, pp. 99-139. S. Sommariva, T. Maffei, G. Migliavacca, T. Faravelli, E. Ranzi, 2010, A Predictive Multi-Step Kinetic Model of Coal Devolatilization, Fuel, 89, pp. 318-328.
Detailed kinetics in the mathematical model of fixed bed gasifiers R. Grana, S. Sommariva, T. Maffei, A. Cuoci, T. Faravelli, A. Frassoldati, S. Pierucci, E. Ranzi* Politecnico di Milano, CMIC Department Piazza Leonardo da Vinci, 32, 20131 Milano, Italy * Corresponding author: [email protected]
Abstract This work presents a general mathematical model of a fixed bed gasifier, where both the transport resistances and the chemical kinetics are important at the reactor and the particle scale. Pyrolysis and gasification of the solid fuel (coal, plastics, biomasses or wastes) is only the first step of the whole process. Successive gas phase reactions of the released species are described with a detailed kinetic scheme. Furthermore, an accurate description of heat and mass transport between gas and solid phases allows a proper characterization of combustion and gasification of the solid fuel. This work summarizes several facets of this problem with examples and validations. Examples relating to coal and biomass gasification allow evaluating the feasibility and limitations of the proposed approach. Keywords: Gasifier, detailed kinetics, coals, biomasses, wastes
1. Introduction The environmental concerns towards the combustion of fossil fuels drive the interest in gasification processes of biomasses, coals, plastics, and refuse derived fuels (RDF). Gasification of solid fuels is nowadays a promising alternative to direct combustion, both electric and thermal energy are viable products, together with chemicals. The mathematical description of such processes is rather difficult due to the complex phenomena involved, such as modelling solid devolatilization, gas phase reactions and gas-solid interactions. However, it is worth to emphasize the importance of these models, which could ease the scale-up and the optimization of the gasifier and could improve the understanding of the whole process. In this work, we proposed the methodology for solving such problems, showing the approach for the main facets involved in solid fuels gasification. Finally, an application example of a fixed bed gasifier model is provided, emphasizing the thermal features of the reactor as well as the role of feedstock characterization.
2. Devolatilization of Coals, Plastics, Biomasses and Refused Derived Fuels (RDF) 2.1. Solid fuel characterization The different solid fuels are described with a limited number of reference compounds and for each of them a multistep kinetic scheme was developed. While plastics, such as poly-ethylene (PE), poly-propylene (PP) and poly-styrene (PS) have a very well-defined structure and composition, the available information about coals and biomasses is usually limited to the elemental composition in terms of C/H/O. Degradation of plastics
R. Grana et al.
was already discussed by [Marongiu et al., 2007]. Biomass composition , if biochemical analysis is available, is simply defined in terms of humidity, ash, cellulose, hemicelluloses and lignin. If only the elemental analysis is available, then a suitable combination in terms of reference species is derived by atomic balance. [Ranzi et al., 2008]. Similarly, three reference species (COAL1, COAL2 and COAL3) are used to describe the composition and reactivity of the different coals. COAL1 (C12H11) , together with pure carbon (CHARC), is useful to describe anthracitic coals with different degree of aromaticity. COAL2 (C14H10O) lies in the middle of bituminous coals, while COAL3 is highly oxygenated (C12H12O5) and is representative of lignitic coals [Sommariva et al., 2010]. In a very similar way, Refused Derived Fuels (RDF), typically with heating values of 4500-5000 kcal/kg, are described in terms of a proper combination of plastic wastes (15-30%), lignocellulosic material (30-50%), ash, and humidity. Fig. 1 shows typical compositions of different RDF in terms of C and H (wt% ash free) 16
RDFelementalcomposition Phyllisdatabase:www.ecn.nl/phyllis
14
PE
% H [w/w]
12 H2O
10
8
6 Cellulose Lignin
4
2 40
50
60
70
80
90
% C [w/w]
Figure 1 Typical compositions of different RDF in terms of C and H (wt% ash free)
2.2. Solid fuel devolatilization The devolatilization of solid particles (mixture of reference compounds) is always considered as a linear combination of the pure component devolatilization. The overall kinetic scheme of RDF devolatilization is simply the combination of the multistep devolatilization models of Biomass [Ranzi et al., 2008], plastic [Marongiu et al., 2007] and coal [Sommariva et al., 2010]. The peculiarity of this approach is that all these schemes consist of a limited number of devolatilization reactions, which are able to describe not only the solid residue, but also the detailed composition of released gas and tar species. Thus, the novelty of this kinetic model, when compared with the majority of the available ones in the literature, is the effort and the challenge to define lumped stoichiometries of the devolatilization reactions with a detailed characterization of the gas and the tar fractions. Thus, Fig. 2 shows the lumped reference components of tar products released by the reference coals. Operating conditions affect the devolatilization selectivity and yields; in particular this multistep approach allows also to describe the chemical and morphological evolution of the solid phase in terms of composition and reactivity.
Detailed kinetics in the mathematical model of fixed bed gasifiers
OH
HO OH O
OCH 3 OH
OH OCH 3
TAR1
TAR2
C12H11
C14H10O
TAR3 C11H10O2
Figure 2 Lumped reference components of tar products from reference coals
3. Combustion and gasification reactions of residual char The gasification and combustion of char, i.e. the set of heterogeneous reactions of oxygen and steam with the solid residue coming either from coal, biomass or plastics, are responsible for the autothermic behaviour of the whole gasification process. These reactions and related kinetic parameters are summarized in Table 2 [Groeneveld and van Swaaij, 1980, Kashiwagi and Nambu, 1992]. Table 1: Char gasification and combustion reactions. Units are: kmol, m3, K, kcal, s. k CHAR + O2
ĺ CO2
CHAR + 0.5 O2 ĺ CO CHAR + H2O
ĺ CO + H2
5.7u109 exp (-38200/ RT) [O2] 0.78 5.7u1011 exp (-55000/ RT) [O2] 0.78 7.9u109 exp (-52000/ RT) [H2O] 0.7
4. Detailed kinetics of secondary gas phase reactions The volatile components released during the pyrolysis undergo successive decomposition or combustion reactions in the surrounding gas phase. As clearly stated by Li et al 2004, a pure thermodynamic or equilibrium approach is not able to properly predict the composition of the gas mixture in a gasifier. The equilibrium approach largely over predicts the heating value of flue gases; in particular, higher amount of hydrogen is predicted, while only trace amounts of methane and hydrocarbons are obtained. On this basis, it is clear the utility to develop a detailed or semi-detailed kinetic scheme for the gas phase. An existing kinetic scheme for pyrolysis and oxidation of hydrocarbon species, already validated in a wide range of operating conditions [Ranzi et al., 2001], has been extended to properly describe successive gas phase reactions of released species. Due to the modularity of the detailed kinetic scheme, it is only necessary to describe the primary initiation, decomposition and H abstraction reactions of the new species. The overall gas-phase kinetic model, together with thermodynamic properties, is available at the website http://www.chem.polimi.it/CRECKModeling/. The number of new species included in the gas phase is a compromise between accuracy and computational efforts, maintaining the ability of the model to describe the
R. Grana et al.
gas composition and reactivity in a wide range of operating condition: in particular tar species are grouped into pseudo-components representative of a set of similar species with similar reactivity. As already discussed in a previous paper [Sommariva et al., 2010], the secondary gas phase reactions can also explain relevant temperature and pressure effects during coal devolatilization, not only related to the solid phase reactions.
5. Mathematical model of fixed bed gasifier The mathematical model of the fixed bed gasifier consists of two models. The first one at the particle scale and the latter at the reactor scale. This approach is discussed in details elsewhere [Pierucci and Ranzi, 2008; Dupont et al., 2009]. The particle model provides an internal description in order to account for intraparticle heat and mass resistances. The accuracy of this description depends on the number of discretization sectors (N). This feature becomes fundamental for the analysis of the gasification of thermally thick solid fuel particles. The equation 1 summarizes the main terms of material and energy balances for the particle: dm j ,i J j 1,i J j ,i R j ,i ° dt °° NCP ® NCP NCP ° d ¦ m j ,i Cp j ,iT j ° i1 JC j 1 ¦ J j 1,i h j 1,i JC j ¦ J j ,i h j ,i HR j dt i 1 i 1 ¯°
(1)
where m is the mass and j refers to the jth particle sector and i to the ith component. t is the time variable and J the mass flux (Fick and Darcy’s laws contributions) and R the kinetic term. Analogously, the energy balance settle the particle temperature T and accounts for conduction contribution (JC) as well as the convective contribution (Jh). HR is the heat of reactions involved in the solid phase. This particle model together with the kinetic models previously described is embedded in the fixed bed reactor model (equation 2). The reactor is considered as a series of elemental units (Fig. 3) which exchange mass and heat to each others. The single unit accounts for gas-solid interactions with particular attention to the inter-phase resistances. The balance equations (2) refer to a perfectly stirred reactor in dynamic conditions, where Rg,i are the gas phase reactions and G0,i and Gi are the inlet and outlet gas feeds [Pierucci and Ranzi, 2008; Dupont et al., 2009]. dg i ° dt G0 ,i J NR ,iK Rg ,i Gi ° NCP ® g i CpiTg NCP NCP NCP °d ¦ G0 ,i hg 0,i ¦ J NR ,i hNR ,iK JC NRK HR g ¦ Gi hg i ° i1 ¦ dt i 1 i 1 i 1 ¯
(2)
The term JNR,iK refers to the gas-solid mass exchange (JNR,i) multiplied by the number K of particles in the bed. Four terms contribute to gas phase temperature (Tg) evolution: the enthalpy convective terms (Ghg) between adjacent reactor layers, the gas-solid heat exchange (JNR,iK), the enthalpy diffusion term (JNR,ihNR,iK), and finally the contribution due to gas phase reactions (HRg).
Detailed kinetics in the mathematical model of fixed bed gasifiers
More than one thousand balance equations are obtained, when considering ~10-15 solid species, 100 gas-phase components, 10 reactor layers and 5 discretizations of the solid particle. The numerical problem is structured in tridiagonal blocks. The resulting ODE system is solved by using BzzMath Library (www.chem.polimi.it/homes/gbuzzi/).
6. Application examples Panel (a) Coal gasification 10 1400
8th layer
9 1300
Temperature (K)
Reactor layer
8 7 6
Coal
5 4
Gas phase
3
Solid surface
2
Solid core
9th layer
1200 1100
10th layer
1000 900 800 0
0.2
0.4
0.6
0.8
1
Normalized radius
1 0
400
800
1200
1600
Temperature (K)
Internal temperature profiles in the solid particles of the three top layers. Char combustion explains the internal maximum temperaturein the 8th layer.
Panel (b) Biomass gasification 10 1400
8th layer
9 1300
Temperature (K)
Reactor layer
8 7 6
Biomass
5 4
Gas phase
3
Solid surface
1100
10th layer 1000 900 800 0
Solid core
2
9th layer 1200
0.2
0.4
0.6
0.8
1
Normalized radius
1 0
400
800
1200
Temperature (K)
1600
The gradient in the 8th layer is more pronounced in biomass case than in coal case due to the minor presence of char on surface.
Figure 3: Predicted gas and solid temperature profiles of a counter-current fixed bed gasifier.
Fig. 3 shows the predicted gas and solid temperature profiles of a counter-current fixed bed gasifier fed with biomass (panel a) and with coal (panel b). Solid particles of 3 cm are treated with air at equivalence ratio 0.25-0.4 and with steam/fuel ratio of about 0.30 wt/wt. Typical contact times of gases are of a few seconds while several hours are necessary to treat solid particles. The model characterizes the solid and gaseous streams, giving a proper emphasis to secondary gas phase reactions of tar and gas components. Typical molar composition of flue gases is about 11-15% of H2, 13-17% of CO, 10-12% of CO2, 15-20 % of H2O and ~1-3% of CH4 and hydrocarbons.
R. Grana et al.
Maximum gas phase temperature is higher when treating biomasses, due to the largest extent of volatilization and the highest oxygen availability. Large temperature gradients are predicted in the first top layers of the bed. Char combustion in the third layer from the top of the gasifier justifies the maximum internal temperature of coal particles (~1100 °C), while this effect is less evident when biomass is treated in the gasifier, due to the lower presence of residual char. Depending on the ash content in the solid fuel, the shrinking of the bed is usually rather larger than 50%. These simulations require several hours of CPU time, due both to the stiff nature of the gas phase kinetics and to the dynamic approach to the steady solution. For these reasons a further model validation strongly demands for further simplifications in the solid and gas phase description.
7. Acknowledgements Authors gratefully acknowledge the financial support of EUROPOWER as well as of CNR-DET in the behalf of the Clean Coal Project.
References A. Cuoci, T. Faravelli, A. Frassoldati, S. Granata, G. Migliavacca, E. Ranzi, S. Sommariva, 2007, A General Mathematical Model of Biomass Devolatilization, 30th Meeting of the Italian Section of the Combustion Institute. C. Dupont, L. Chen, J. Cances, J.-M. Commandre, A. Cuoci, S. Pierucci, E. Ranzi, 2009, Biomass Pyrolysis: Kinetic Modelling and Experimental Validation Under High Temperature and Flash Heating Rate Conditions, Journal of Analytical and Applied Pyrolysis, 85, pp. 260-267. M.J. Groeneveld and W.P.M. van Swaaij, 1980, Gasification of Char Particles with CO2 and H2O, Chemical Engineering Science, 35, pp 307-313. T. Kashiwagi and H. Nambu, 1992, Global kinetic Constants for Thermal Oxidative Degradation of a Cellulosic Paper, Combustion and Flame, 88, pp. 345-368. X.T Li, J.R. Grace, C.J. Lim, A.P. Watkinson, H.P. Chen, J.R. Kim, 2004, Biomass Gasification in a Circulating Fluidized Bed, Biomass and Bioenergy, 26, pp. 171-193. A. Marongiu, T. Faravelli, E. Ranzi, 2007, Detailed Kinetic Model of the Thermal Degradation of Vynil Polymers, Journal of Analytical and Applied Pyrolysis, 78, pp. 343-362. S. Pierucci, E. Ranzi, 2008, A General Mathematical Model for a Moving Bed Gasifier,Computer Aided Chemical Engineering, 25, pp. 901-906. E. Ranzi, A. Cuoci, T. Faravelli, A. Frassoldati, G. Migliavacca, S. Pierucci, S. Sommariva, 2008, Chemical Kinetics of Biomass Pyrolysis, Energy&Fuels 2008, 22, 4292–4300. E. Ranzi, M. Dente, A. Goldaniga, G. Bozzano, T. Faravelli, 2001, Lumping procedures in detailed kinetic modeling of gasification, pyrolysis, partial oxidation and combustion of hydrocarbon mixtures, Progress in Energy and Combustion Science, 27, pp. 99-139. S. Sommariva, T. Maffei, G. Migliavacca, T. Faravelli, E. Ranzi, 2010, A Predictive Multi-Step Kinetic Model of Coal Devolatilization, Fuel, 89, pp. 318-328.