- Email: [email protected]
CO Ratio under Different Operating Conditions
Jiří Jaromír Klemeš, Petar Sabev Varbanov and Peng Yen Liew (Editors) Proceedings of the 24th European Symposium on Computer Aided Process Engineering – ESCAPE 24 June 15-18, 2014, Budapest, Hungary. Copyright © 2014 Elsevier B.V. All rights reserved.
Mathematical Modelling of Coal and Biomass Gasification: Comparison on the Syngas H2/CO Ratio under Different Operating Conditions Michele Corbetta,a Flavio Manenti,a,* Fiona Soares,b Zohreh Ravaghi-Ardebili,a Eliseo Ranzi,a Carlo Pirola,c Guido Buzzi-Ferraris,a Sauro Pieruccia a
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Piazza Leonardo da Vinci 32, 20133 Milano, Italy b INPT-ENSIACET, Génie des procédés, 4 allée Emile Monso 31000 Toulouse, France c Università degli Studi di Milano, Dipartimento di Chimica, Via Golgi 13, 20133 Milano, Italy [email protected]
Abstract Gasification is a thermo-chemical process aiming at the production of high heating value syngas, starting from a solid fuel such as biomass or coal. From a chemical point of view the process results in a partial oxidation by means of sub-stoichiometric air or oxygen and/or steam. According to the solid fuel used as a feedstock and to the operating parameters of the process, the quality and the chemical composition of the produced syngas is differently affected. This gas is mainly composed by carbon monoxide and hydrogen, while carbon dioxide, water, methane and small hydrocarbons are minor components. The final applications of syngas include the power generation and the production of chemicals, with special reference to methanol synthesis and Gasto-Liquid technologies. For these last catalytic processes it is necessary to provide syngas with a specific H2/CO ratio, and for this reason in the present work we apply our comprehensive mechanistic approach to the description of the gasification process, highlighting the sensitivity of the key operating parameters on syngas quality. Moreover a comparison between coal and biomass gasification is proposed, as well as the validation of the kinetic model with some experimental data. Keywords: coal gasification, biomass gasification, detailed kinetics, syngas.
1. Introduction The gasification of cheap solid fuels is gaining more attention, due to the possibility to efficiently exploit the calorific value of solid fuels in a greener fashion. Beyond the applications in the power generation field, the syngas produced from this kind of processes could provide an interesting platform for the production of fuels and chemicals with a lower environmental footprint. Actually, several Gas-to-Liquids technologies are available for the production of hydrocarbons (e.g. Fischer-Tropsch synthesis (Pirola et al., 2009)) and oxygenated chemicals (e.g. MeOH/DME synthesis, (Manenti et al., 2013)), and for these applications it is crucial to focus the attention on the quality of the syngas produced, mostly in terms of H2/CO ratio. In fact the downstream catalytic processes need to be fed with a syngas with a proper composition, usually in the range of H2/CO § 1-2. For this reason it is of utmost importance to be able to predict the performance of a gasifier, not only in terms of the overall efficiency but also in relation to the chemical characterization of the syngas produced. In this work we
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aim to describe a mathematical modeling approach, which is based on the solution of a multi-scale problem spanning from the particle scale up to the full reactor scale. The novelty of the proposed approach, on the contrary of a conventional thermodynamic and equilibrium approach (Emun et al., 2010), consists in a greater chemical detail of the pyrolysis and devolatilization process of coal and biomass, considering a large number of lumped tar and gas species. In fact, both for coal and biomass feedstocks a multi-step kinetic model for the pyrolysis of solid fuels was developed and embedded within the particle model, along with gas-solid reactions of char gasification and secondary gasphase reactions of the released volatiles (Ranzi et al., 2013). The chemical evolution of the system is predicted with a mechanistic approach, and it is thus possible to infer the behavior of gasification units also in case of scarce experimental data availability. On the other hand, the mathematical model is applied to the case of coal and biomass gasification in order to find out the sensitivity of the system to the key operating parameters (e.g. oxygen/fuel and steam/fuel ratios, temperature, composition, etc.). An additional comparison is made between the responses of the two different kind of solid fuels keeping constant the geometry of the gasifier and the energy flux associated with the solid feed stream. With respect to coal gasification, biomass gasification occurs at lower temperatures, due to the higher reactivity of the feedstock. On the other hand biomass is more heterogeneous and difficult to characterize, leading to a demanding flexibility of reactors and processes. We also provide some comparisons of the model predictions with the experimental data on the available literature in order to validate the consistency of our approach for both biomass (Di Blasi et al., 1999) and coal (Grieco and Baldi, 2011) gasification.
2. Kinetic models During the gasification process, the chemical evolution of the system has been described by means of detailed kinetic schemes for solid fuel devolatilization and pyrolysis, residual solid (char) gasification with steam, CO2 and oxygen, and finally, secondary gas phase reactions, as outlined in Figure 1. At first, the solid fuel, either biomass or coal, is heated up until devolatilization and pyrolysis occur. TAR HEAT FUEL
CHAR
GAS O2/H2O
PYROLYSIS
CO H2
GAS-SOLID REACTIONS
CO H2 CO2 H2O CH4
SYNGAS
SECONDARY GAS-PHASE REACTIONS
Figure 1. Solid fuel devolatilization and gasification. Pyrolysis: dotted box and arrows; Gas-solid reactions: solid box and arrows; secondary gas-phase reactions: dashed box and arrow.
Mathematical Modelling of Coal and Biomass Gasification: Comparison on the Syngas H2/CO Ratio under Different Operating Conditions
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During this first stage the solid fuel is progressively converted to three main product groups: light gases, tar species and a residual solid, mainly composed by ashes and char, with a residual content of volatiles trapped within the porous matrix (i.e. metaplastic phase). In the model this stage is described using the multi-step kinetics reported in (Ranzi et al., 2013) for biomass pyrolysis and in (Sommariva et al., 2011) for coal pyrolysis. For both kinetic schemes, the solid feedstock is characterized as a mixture of reference lumped species, for which first-order kinetics are provided. After this first step, the carbonaceous residual solid is partially subject to gas-solid gasification reactions with steam and oxygen, with the preferential production of carbon monoxide and hydrogen. Moreover, volatiles, especially heavy tar species, as soon as are released in the gas phase, are cracked to lighter gaseous species through gas-phase secondary reactions (Ranzi et al., 2012). A detailed kinetic scheme has been adopted, the POLIMI_1310, available on the creckmodeling.chem.polimi.it website. This kinetic scheme, including the pyrolysis and combustion of tar and oxygenated species and successive reactions of aromatic and polycyclic-aromatic species, involves more than 450 species and about 15,000 reactions.
3. Particle and reactor model In order to model and simulate the gasification process, a suitable particle and reactor model is mandatory for the description of both kinetic and transport phenomena aspects. This lead to the solution of a multi-scale dynamic system, spanning from the description of kinetic and transport aspects at the particle scale, up to the description of mass and energy transfer as well as secondary reactions at the reactor scale (Ranzi et al., 2013). To further increase the complexity, the system is intrinsically multi-phase, due to the combined presence of a gas, liquid and solid phase, which can exchange mass and energy among themselves (Mettler et al., 2012). The structure of the system is outlined in Figure 2, where it is possible to highlight the presence of three main scales. At the particle scale, the system evolves along the radial coordinate, as well as through time.
Figure 2. Multi-scale gasifier
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The successive scale (elementary reactor layer) accounts for the coupling between isotropic solid particles with an external gas phase, considered homogeneous and perfectly mixed. Finally, at the reactor scale, several elementary reactor layers are adopted and interconnected to reproduce different reactor configurations. For instance the counter-current fixed bed gasifier (i.e. updraft) is reproduced through a cascade of elemental reactor layers. In this configuration, which has been selected for the successive studies below in the paper, the solid fuel is fed from the top of the reactor where it encounters a rising gas stream, fed from the bottom of the tower. During the residence within the gasifier, particles are progressively dried, pyrolyzed and gasified, leading to a residual solid stream withdrawn from the bottom and to a gas stream rich in hydrogen and carbon dioxide exiting from the top. The complete set of model equations is provided elsewhere (Ranzi et al., 2013).
4. Results 4.1. Validation of the model The mathematical model has been validated with some available experimental data from the scientific literature, mainly in terms of gas composition (H2/CO ratio). 4.2. Comparisons between coal and biomass gasification model predictions Following the partial validation of the model, a comparison has been performed between coal and biomass gasification, keeping constant in the two case studies both the gasifier geometry and the inlet solid energy flux (in terms of heating value multiplied by the mass flowrate). For sake of conciseness, we report below (Figure 3) just the complete summary of the simulation of coal gasification, considering a reactor 4 m high with a section of 1 m2, and a coal flowrate of 10,000 kg/h. In the following Table 2 we summarize the main results and comparisons between the two different simulations for coal and biomass gasification. In Table 2, cold gas efficiency (CGE) is considered as the heating value of the produced syngas at room temperature multiplied by its flowrate all divided by the heating value of the solid fuel multiplied by its flowrate.
CGE
m SYNGAS HHVSYNGAS m FUEL HHVFUEL
(1)
Table 1. Comparison between coal (Grieco and Baldi, 2011) and biomass (Di Blasi et al., 1999) gasification experimental data; and model predictions
H2/CO CO2 CH4
EXP 0.58 4.1 % dry 1.4 % dry
Coal MODEL 0.70 4.60 0.00
Deviation -0.12 -0.54 1.30
EXP 0.24 7.0 % mol 1.8 % mol
Biomass MODEL 0.36 8.2 % mol 2.0 % mol
Deviation -0.12 -1.20 -0.20
Table 2: Comparison between coal and biomass gasification
Fuel mass flowrate H2/CO CGE
Coal 10,000 kg/s 0.820 0.70
Biomass 18,000 kg/s 0.906 0.59
Mathematical Modelling of Coal and Biomass Gasification: Comparison on the Syngas H2/CO Ratio under Different Operating Conditions wt % COAL1 COAL2
31.35 46.87
COAL3
13.9
ASH
5.9
Moisture 2 F = 10000 kg/h
COAL d = 5 mm wt fract. Porosity = 20 % C 0.849 T = 300 K H 0.058 O 0.093 sub-bituminous ʌ = 1350 kg/m3 updraft gasifier HHV coal 35.51 MJ/kg POWER 99.4 MW
H2 CO CO2 5 T5=1630K
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SYNGAS mol fract. wt fract. flow (kg/s)dry mol % 0.28 0.03 0.21 37.8% 0.34 0.48 3.22 45.9% 0.09 0.21 1.38 12.2%
H2O
0.24
0.22
1.49
-
CH4 H2/CO
0.03 0.824
0.02 POWER
0.14 70.09
4.1% MW
4 T4=2098K 3 T3=1232K 2 T2=1032K
H=4m A = 1 m2 V = 4 m3 ɸ = 50%
1 T1=1003K
SOLID RESIDUE 8 % of coal -> 800kg/h ASH content = 5.9 % wt.
INLET GAS oxydizer mol fract. flow (kg/s)oxy. /coal 0.32 1.83 0.65 O2 H2O 0.68 2.18 ʄ 0.25 ʔ Start-up T = 1300 K Steady state T = 1000 K
0.78 4.0
Figure 3. Summary of the coal simulation
4.3. Sensitivity analysis In order to exploit the possibilities of the proposed methodology, we performed some sensitivity analysis on the key operating parameters, such as inlet gas temperature. Here we propose just a few results referring to the case study of the coal gasification reported above. After reaching a steady-state hot solution of the gasifier, summarized in Figure 3, the simulation was restarted with stepwise input changes on some operating conditions. For instance, in Figure 4 it is possible to see the effect of the inlet oxidizing gas temperature on temperature profiles, mass flowrates of the produced gas components and residual solid. From Figure 4 it is possible to point out that increasing the temperature of the oxidizing gas, the amount of produced gas increases, leading to a lesser solid residue. However the quality of the gas decreases because CO increases more than hydrogen. Moreover axial thermal profiles show a decrease in the temperature gradient increasing the inlet gas temperature.
5. Conclusions We outlined a multi-scale mathematical model for the simulation of solid fuels thermochemical conversion processes. The novelty of this approach relies on a kinetic modelling approach, which can characterize, with a reasonable detail, also the devolatilization and pyrolysis steps, as well as the secondary gas phase reactions. The best characteristic of the present model relies on an intrinsic flexibility to handle different feedstock and to account for new available experimental data. This model has been here applied to the gasification process of biomass and coal, considering the counter current fixed-bed reactor. Comparisons with experimental data show the viability of the approach although some further comparisons should be done in order to improve the reliability of the model. Finally, the sensitivity analysis shows the strong effect of operating parameters on the quality of the produced syngas, and could provide a tool to optimize them in order to reach the desired target.
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0.865
3.5
0.860 0.855
2.5
CO
CO2
0.850
2
H2
CH4
0.845
1.5
H2/CO
Mass flowrate [kg/s]
3
0.840 0.835
Particle diameter 5 mm Humidity 7%
1
0.830
0.5
0.825
0
0.820 300
500
700
900
300
Inlet gas temperature [K]
500
700
900
Inlet gas temperature [K]
5
Inlet gas temperature
4
25 Solid residue [%]
Number of reactor layer (axial coordinate)
30
3
20 15 10
Particle diameter 5 mm Humidity 7%
5
2
0 300 1
500
700
900
Inlet gas temperature [K] 0
1000
2000
Bulk temperature [K]
Figure 4. Sensitivity analysis. Effect of the inlet gas temperature on the gasifier bulk temperature, composition, and residual solid percentage.
References C. Di Blasi, G. Signorelli, G. Portorico, 1999,Countercurrent fixed-bed gasification of biomass at laboratory scale, Industrial Engeneering Chemical Research, 38, 2571-2581. F. Emun, M. Gadalla, T. Majozi, D. Boer, 2010, Integrated gasification combined cycle (IGCC) process simulation and optimization, Computers & Chemical Engineering, 34, 3, 331-338. E. M. Grieco, G. Baldi, 2011, Predictive model for countercurrent coal gasifiers, Chemical Engineering Science, 66, 5749-5761. F. Manenti, A. R. Leon-Garzon, G. Bozzano, 2013, Energy-Process Integration of the GasCooled/Water-Cooled Fixed -Bed Reactor Network for Methanol Synthesis, Chemical Engineering Transactions, 35, 1243-1248. M. S. Mettler, D. G. Vlachos, P. J. Dauenhauer, 2012, Top ten fundamental challenges of biomass pyrolysis for biofuels, Energy Environmental Science, 5, 7797-7809. C. Pirola, C.L. Bianchi, A. Di Michele, P. Diodati, S. Vitali, V. Ragaini, 2009, High Loading Fesupported Fischer–Tropsch Catalysts: Optimization of the Catalyst Performance, Catalysis Letters, 131, 294-304. E. Ranzi, M. Corbetta, F. Manenti, S. Pierucci, 2013, Kinetic modeling of the thermal degradation and combustion of biomass, Chemical Engineering Science, DOI: 10.1016/j.ces.2013.08.014. E. Ranzi, A. Frassoldati, R. Grana, A. Cuoci, T. Faravelli, A.P. Kelley, C.K. Law, 2012, Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels, Progress in Energy and Combustion Science, 38, 4, 468-501. S. Sommariva, R. Grana, T. Maffei, S. Pierucci, E. Ranzi, 2011, A kinetic approach to the mathematical model of fixed bed gasifiers, Computers and Chemical Engineering, 35, 928-35.
Mathematical Modelling of Coal and Biomass Gasification: Comparison on the Syngas H2/CO Ratio under Different Operating Conditions Michele Corbetta,a Flavio Manenti,a,* Fiona Soares,b Zohreh Ravaghi-Ardebili,a Eliseo Ranzi,a Carlo Pirola,c Guido Buzzi-Ferraris,a Sauro Pieruccia a
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Piazza Leonardo da Vinci 32, 20133 Milano, Italy b INPT-ENSIACET, Génie des procédés, 4 allée Emile Monso 31000 Toulouse, France c Università degli Studi di Milano, Dipartimento di Chimica, Via Golgi 13, 20133 Milano, Italy [email protected]
Abstract Gasification is a thermo-chemical process aiming at the production of high heating value syngas, starting from a solid fuel such as biomass or coal. From a chemical point of view the process results in a partial oxidation by means of sub-stoichiometric air or oxygen and/or steam. According to the solid fuel used as a feedstock and to the operating parameters of the process, the quality and the chemical composition of the produced syngas is differently affected. This gas is mainly composed by carbon monoxide and hydrogen, while carbon dioxide, water, methane and small hydrocarbons are minor components. The final applications of syngas include the power generation and the production of chemicals, with special reference to methanol synthesis and Gasto-Liquid technologies. For these last catalytic processes it is necessary to provide syngas with a specific H2/CO ratio, and for this reason in the present work we apply our comprehensive mechanistic approach to the description of the gasification process, highlighting the sensitivity of the key operating parameters on syngas quality. Moreover a comparison between coal and biomass gasification is proposed, as well as the validation of the kinetic model with some experimental data. Keywords: coal gasification, biomass gasification, detailed kinetics, syngas.
1. Introduction The gasification of cheap solid fuels is gaining more attention, due to the possibility to efficiently exploit the calorific value of solid fuels in a greener fashion. Beyond the applications in the power generation field, the syngas produced from this kind of processes could provide an interesting platform for the production of fuels and chemicals with a lower environmental footprint. Actually, several Gas-to-Liquids technologies are available for the production of hydrocarbons (e.g. Fischer-Tropsch synthesis (Pirola et al., 2009)) and oxygenated chemicals (e.g. MeOH/DME synthesis, (Manenti et al., 2013)), and for these applications it is crucial to focus the attention on the quality of the syngas produced, mostly in terms of H2/CO ratio. In fact the downstream catalytic processes need to be fed with a syngas with a proper composition, usually in the range of H2/CO § 1-2. For this reason it is of utmost importance to be able to predict the performance of a gasifier, not only in terms of the overall efficiency but also in relation to the chemical characterization of the syngas produced. In this work we
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M. Corbetta et al.
aim to describe a mathematical modeling approach, which is based on the solution of a multi-scale problem spanning from the particle scale up to the full reactor scale. The novelty of the proposed approach, on the contrary of a conventional thermodynamic and equilibrium approach (Emun et al., 2010), consists in a greater chemical detail of the pyrolysis and devolatilization process of coal and biomass, considering a large number of lumped tar and gas species. In fact, both for coal and biomass feedstocks a multi-step kinetic model for the pyrolysis of solid fuels was developed and embedded within the particle model, along with gas-solid reactions of char gasification and secondary gasphase reactions of the released volatiles (Ranzi et al., 2013). The chemical evolution of the system is predicted with a mechanistic approach, and it is thus possible to infer the behavior of gasification units also in case of scarce experimental data availability. On the other hand, the mathematical model is applied to the case of coal and biomass gasification in order to find out the sensitivity of the system to the key operating parameters (e.g. oxygen/fuel and steam/fuel ratios, temperature, composition, etc.). An additional comparison is made between the responses of the two different kind of solid fuels keeping constant the geometry of the gasifier and the energy flux associated with the solid feed stream. With respect to coal gasification, biomass gasification occurs at lower temperatures, due to the higher reactivity of the feedstock. On the other hand biomass is more heterogeneous and difficult to characterize, leading to a demanding flexibility of reactors and processes. We also provide some comparisons of the model predictions with the experimental data on the available literature in order to validate the consistency of our approach for both biomass (Di Blasi et al., 1999) and coal (Grieco and Baldi, 2011) gasification.
2. Kinetic models During the gasification process, the chemical evolution of the system has been described by means of detailed kinetic schemes for solid fuel devolatilization and pyrolysis, residual solid (char) gasification with steam, CO2 and oxygen, and finally, secondary gas phase reactions, as outlined in Figure 1. At first, the solid fuel, either biomass or coal, is heated up until devolatilization and pyrolysis occur. TAR HEAT FUEL
CHAR
GAS O2/H2O
PYROLYSIS
CO H2
GAS-SOLID REACTIONS
CO H2 CO2 H2O CH4
SYNGAS
SECONDARY GAS-PHASE REACTIONS
Figure 1. Solid fuel devolatilization and gasification. Pyrolysis: dotted box and arrows; Gas-solid reactions: solid box and arrows; secondary gas-phase reactions: dashed box and arrow.
Mathematical Modelling of Coal and Biomass Gasification: Comparison on the Syngas H2/CO Ratio under Different Operating Conditions
1671
During this first stage the solid fuel is progressively converted to three main product groups: light gases, tar species and a residual solid, mainly composed by ashes and char, with a residual content of volatiles trapped within the porous matrix (i.e. metaplastic phase). In the model this stage is described using the multi-step kinetics reported in (Ranzi et al., 2013) for biomass pyrolysis and in (Sommariva et al., 2011) for coal pyrolysis. For both kinetic schemes, the solid feedstock is characterized as a mixture of reference lumped species, for which first-order kinetics are provided. After this first step, the carbonaceous residual solid is partially subject to gas-solid gasification reactions with steam and oxygen, with the preferential production of carbon monoxide and hydrogen. Moreover, volatiles, especially heavy tar species, as soon as are released in the gas phase, are cracked to lighter gaseous species through gas-phase secondary reactions (Ranzi et al., 2012). A detailed kinetic scheme has been adopted, the POLIMI_1310, available on the creckmodeling.chem.polimi.it website. This kinetic scheme, including the pyrolysis and combustion of tar and oxygenated species and successive reactions of aromatic and polycyclic-aromatic species, involves more than 450 species and about 15,000 reactions.
3. Particle and reactor model In order to model and simulate the gasification process, a suitable particle and reactor model is mandatory for the description of both kinetic and transport phenomena aspects. This lead to the solution of a multi-scale dynamic system, spanning from the description of kinetic and transport aspects at the particle scale, up to the description of mass and energy transfer as well as secondary reactions at the reactor scale (Ranzi et al., 2013). To further increase the complexity, the system is intrinsically multi-phase, due to the combined presence of a gas, liquid and solid phase, which can exchange mass and energy among themselves (Mettler et al., 2012). The structure of the system is outlined in Figure 2, where it is possible to highlight the presence of three main scales. At the particle scale, the system evolves along the radial coordinate, as well as through time.
Figure 2. Multi-scale gasifier
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The successive scale (elementary reactor layer) accounts for the coupling between isotropic solid particles with an external gas phase, considered homogeneous and perfectly mixed. Finally, at the reactor scale, several elementary reactor layers are adopted and interconnected to reproduce different reactor configurations. For instance the counter-current fixed bed gasifier (i.e. updraft) is reproduced through a cascade of elemental reactor layers. In this configuration, which has been selected for the successive studies below in the paper, the solid fuel is fed from the top of the reactor where it encounters a rising gas stream, fed from the bottom of the tower. During the residence within the gasifier, particles are progressively dried, pyrolyzed and gasified, leading to a residual solid stream withdrawn from the bottom and to a gas stream rich in hydrogen and carbon dioxide exiting from the top. The complete set of model equations is provided elsewhere (Ranzi et al., 2013).
4. Results 4.1. Validation of the model The mathematical model has been validated with some available experimental data from the scientific literature, mainly in terms of gas composition (H2/CO ratio). 4.2. Comparisons between coal and biomass gasification model predictions Following the partial validation of the model, a comparison has been performed between coal and biomass gasification, keeping constant in the two case studies both the gasifier geometry and the inlet solid energy flux (in terms of heating value multiplied by the mass flowrate). For sake of conciseness, we report below (Figure 3) just the complete summary of the simulation of coal gasification, considering a reactor 4 m high with a section of 1 m2, and a coal flowrate of 10,000 kg/h. In the following Table 2 we summarize the main results and comparisons between the two different simulations for coal and biomass gasification. In Table 2, cold gas efficiency (CGE) is considered as the heating value of the produced syngas at room temperature multiplied by its flowrate all divided by the heating value of the solid fuel multiplied by its flowrate.
CGE
m SYNGAS HHVSYNGAS m FUEL HHVFUEL
(1)
Table 1. Comparison between coal (Grieco and Baldi, 2011) and biomass (Di Blasi et al., 1999) gasification experimental data; and model predictions
H2/CO CO2 CH4
EXP 0.58 4.1 % dry 1.4 % dry
Coal MODEL 0.70 4.60 0.00
Deviation -0.12 -0.54 1.30
EXP 0.24 7.0 % mol 1.8 % mol
Biomass MODEL 0.36 8.2 % mol 2.0 % mol
Deviation -0.12 -1.20 -0.20
Table 2: Comparison between coal and biomass gasification
Fuel mass flowrate H2/CO CGE
Coal 10,000 kg/s 0.820 0.70
Biomass 18,000 kg/s 0.906 0.59
Mathematical Modelling of Coal and Biomass Gasification: Comparison on the Syngas H2/CO Ratio under Different Operating Conditions wt % COAL1 COAL2
31.35 46.87
COAL3
13.9
ASH
5.9
Moisture 2 F = 10000 kg/h
COAL d = 5 mm wt fract. Porosity = 20 % C 0.849 T = 300 K H 0.058 O 0.093 sub-bituminous ʌ = 1350 kg/m3 updraft gasifier HHV coal 35.51 MJ/kg POWER 99.4 MW
H2 CO CO2 5 T5=1630K
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SYNGAS mol fract. wt fract. flow (kg/s)dry mol % 0.28 0.03 0.21 37.8% 0.34 0.48 3.22 45.9% 0.09 0.21 1.38 12.2%
H2O
0.24
0.22
1.49
-
CH4 H2/CO
0.03 0.824
0.02 POWER
0.14 70.09
4.1% MW
4 T4=2098K 3 T3=1232K 2 T2=1032K
H=4m A = 1 m2 V = 4 m3 ɸ = 50%
1 T1=1003K
SOLID RESIDUE 8 % of coal -> 800kg/h ASH content = 5.9 % wt.
INLET GAS oxydizer mol fract. flow (kg/s)oxy. /coal 0.32 1.83 0.65 O2 H2O 0.68 2.18 ʄ 0.25 ʔ Start-up T = 1300 K Steady state T = 1000 K
0.78 4.0
Figure 3. Summary of the coal simulation
4.3. Sensitivity analysis In order to exploit the possibilities of the proposed methodology, we performed some sensitivity analysis on the key operating parameters, such as inlet gas temperature. Here we propose just a few results referring to the case study of the coal gasification reported above. After reaching a steady-state hot solution of the gasifier, summarized in Figure 3, the simulation was restarted with stepwise input changes on some operating conditions. For instance, in Figure 4 it is possible to see the effect of the inlet oxidizing gas temperature on temperature profiles, mass flowrates of the produced gas components and residual solid. From Figure 4 it is possible to point out that increasing the temperature of the oxidizing gas, the amount of produced gas increases, leading to a lesser solid residue. However the quality of the gas decreases because CO increases more than hydrogen. Moreover axial thermal profiles show a decrease in the temperature gradient increasing the inlet gas temperature.
5. Conclusions We outlined a multi-scale mathematical model for the simulation of solid fuels thermochemical conversion processes. The novelty of this approach relies on a kinetic modelling approach, which can characterize, with a reasonable detail, also the devolatilization and pyrolysis steps, as well as the secondary gas phase reactions. The best characteristic of the present model relies on an intrinsic flexibility to handle different feedstock and to account for new available experimental data. This model has been here applied to the gasification process of biomass and coal, considering the counter current fixed-bed reactor. Comparisons with experimental data show the viability of the approach although some further comparisons should be done in order to improve the reliability of the model. Finally, the sensitivity analysis shows the strong effect of operating parameters on the quality of the produced syngas, and could provide a tool to optimize them in order to reach the desired target.
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0.865
3.5
0.860 0.855
2.5
CO
CO2
0.850
2
H2
CH4
0.845
1.5
H2/CO
Mass flowrate [kg/s]
3
0.840 0.835
Particle diameter 5 mm Humidity 7%
1
0.830
0.5
0.825
0
0.820 300
500
700
900
300
Inlet gas temperature [K]
500
700
900
Inlet gas temperature [K]
5
Inlet gas temperature
4
25 Solid residue [%]
Number of reactor layer (axial coordinate)
30
3
20 15 10
Particle diameter 5 mm Humidity 7%
5
2
0 300 1
500
700
900
Inlet gas temperature [K] 0
1000
2000
Bulk temperature [K]
Figure 4. Sensitivity analysis. Effect of the inlet gas temperature on the gasifier bulk temperature, composition, and residual solid percentage.
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