doi:10.1016/j.enconman.2006.08.018
Copyright © 2006 Elsevier Ltd All rights reserved.
Most sensitive parameters in pyrolysis of shrinking biomass particle
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A.S. Chaurasiaa, 
, 
, and B.D. Kulkarnib
aChemical Engineering Department, MAEER’s, MIT’s, Maharashtra Academy of Engineering (MAE), Pune 412 105, Maharashtra, India
bNational Chemical Laboratory (NCL), Dr. Homi Bhabha Road, Pune 411 008, Maharashtra, India
Received 9 February 2006;
accepted 27 August 2006.
Available online 10 October 2006.
Abstract
In the present study, the impact of shrinking and non-shrinking biomass particles on pyrolysis is studied employing a kinetic model coupled with a heat transfer model using a practically significant kinetic scheme consisting of physically measurable parameters. The numerical model is used to predict the effects of the most important physical and thermal properties (thermal conductivity, heat transfer coefficient, emissivity, reactor temperature and heat of reaction number) considering cylindrical geometry. A finite difference pure implicit scheme utilizing the tri-diagonal matrix algorithm (TDMA) is employed for solving the heat transfer model equation. The Runge–Kutta 4th order method is used for the chemical kinetics model equations. The computer simulations are performed for wide ranges of particle size and temperatures. The results obtained are in excellent agreement with many experimental studies, much better than the agreement with earlier models reported in the literature. The most dominant design variable is reactor temperature and exothermic reaction. The applications of these findings are important and useful for optimum design of biomass gasifiers and pyrolysis reactors.
Keywords: Modelling; Pyrolysis; Shrinkage; Dominant; Biomass
Nomenclature
- A1, A2, A3
- frequency factor, s−1
- b
- geometry factor (slab = 1, cylinder = 2, sphere = 3)
- B
- virgin biomass
- G1
- (gases and volatiles)1
- C1
- (char)1
- G2
- (gases and volatiles)2
- C2
- (char)2
- CB
- concentration of B, kg m−3
- CG1
- concentration of G1, kg m−3
- CC1
- concentration of C1, kg m−3
- CG2
- concentration of G2, kg m−3
- CC2
- concentration of C2, kg −3
- Cp
- specific heat capacity, J kg−1 K−1
- d
- pore diameter, m
- D1, D2
- constants defined by expressions of k1 and k2, respectively, K
- E3
- activation energy defined by expression of k3, J mol−1
- h
- convective heat transfer coefficient, W m−2 K−1
- H
- modified Biot number
- k
- thermal conductivity, W m−1 K−1
- k1, k2, k3
- rate constants, s−1
- l
- axial length of cylinder, m
- L1, L2
- constants defined by expressions of k1 and k2, respectively, K2
- M
- mass, kg
- n1, n2, n3
- orders of reactions
- N
- total number of equations used in simulation of model
- Q
- defined by Eq. (35), m3 kg−1
- r
- radial distance, m
- R
- radius for cylinder and sphere; half thickness for slab, m
- Rc
- universal gas constant, J mol−1
- t
- time, s
- T
- temperature, K
- V
- total particle volume, sum of volume occupied by pores and by solid phase, m3
- Vg
- volume occupied by pores, i.e. by gases and volatiles, m3
- VS
- solid phase (wood and char) volume, m3
- VS0
- initial effective solid volume, m3
- x
- dimensionless radial distance
- X
- conversion of biomass
Greek letters
- ΔH
- heat of reaction, J kg−1
- Δτ
- axial grid length
- Δx
- radial grid distance
- ρ
- density, ρ0 at initial condition, kg m−3
- α
- thermal diffusivity, m2 s−1
- τ
- dimensionless time
- θ
- normalized temperature
- ε
- emissivity coefficient
- ε″
- void fraction of particle as defined by Eq. (26), ε0″ at initial condition
- σ
- Stefan Boltzmann constant, W m−2 K−4
- η
- reaction progress variable
- α′
- shrinkage factor (defined by Eq. (19))
- β′
- shrinkage factor (defined by Eq. (20))
- γ′
- shrinkage factor (defined by Eq. (22))
Dimensionless numbers
- Bi
- Biot number
- Pr
- Prandtl number
- Q″
- Heat of reaction number
- Re
- Reynolds number
Subscripts
- g
- gas
- 0
- initial
- f
- final
- B
- wood
- C
- char
- eff
- effective
- m
- mean
Fig. 1. Temperature profile as a function of time at the centre of the cylindrical pellet (R = 0.003 m, T0 = 303 K, Tf = 780 K, x = 0).
Fig. 2. Temperature profile as a function of radial distance with cylindrical pellet (R = 0.011 m, T0 = 303 K, Tf = 643 K, t = 11 min).
Fig. 3. Conversion profile as a function of time with cylindrical pellet (R = 0.011 m, T0 = 303 K, Tf = 753 K).
Fig. 4. Average char yield as a function of temperature for particle half thickness of 0.0000125 m.
Fig. 5. Average product concentrations and pyrolysis time as functions of heat of reaction number (Q″) for exothermic reaction with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 6. Average product concentrations and pyrolysis time as functions of heat of reaction number (Q″) for exothermic reaction with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 7. Temperature profile as a function of radial distance for different values of heat of reaction number (Q″) for exothermic reaction with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 8. Temperature profile as a function of radial distance for different values of heat of reaction number (Q″) for exothermic reaction with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 9. Average product concentrations and pyrolysis time as functions of heat of reaction number (Q″) for endothermic reaction with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 10. Average product concentrations and pyrolysis time as functions of heat of reaction number (Q″) for endothermic reaction with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 11. Temperature profile as a function of radial distance for different values of heat of reaction number (Q″) for endothermic reaction with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 12. Temperature profile as a function of radial distance for different values of heat of reaction number (Q″) for endothermic reaction with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 13. Average product concentrations and pyrolysis time as functions of thermal conductivity with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 14. Average product concentrations and pyrolysis time as functions of thermal conductivity with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 15. Temperature profile as a function of radial distance for different values of thermal conductivity with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 16. Temperature profile as a function of radial distance for different values of thermal conductivity with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 17. Average product concentrations and pyrolysis time as functions of convective heat transfer coefficient with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 18. Temperature profile as a function of radial distance for different values of convective heat transfer coefficient with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 19. Average product concentrations and pyrolysis time as functions of convective heat transfer coefficient with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 20. Temperature profile as a function of radial distance for different values of convective heat transfer coefficient with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 21. Average product concentrations and pyrolysis time as functions of emissivity with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 22. Average product concentrations and pyrolysis time as functions of emissivity with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 23. Temperature profile as a function of radial distance for different values of emissivity with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 24. Temperature profile as a function of radial distance for different values of emissivity with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K, Tf = 900 K).
Fig. 25. Average product concentrations and pyrolysis time as functions of final temperature with cylindrical pellet without shrinkage (R = 0.011 m, T0 = 303 K).
Fig. 26. Average product concentrations and pyrolysis time as functions of final temperature with cylindrical pellet with shrinkage (R = 0.011 m, T0 = 303 K).
Table 1.
Mathematical model
Table 2.
Dimensionless groups
Table 3.
Values and correlations used in the numerical solution of the model
Table 4.
Comparison of present model results with shrinkage with those of earlier models for various stages of pyrolysis (time) at the centre of cylindrical pellet
(R = 0.003 m, T0 = 303 K, Tf = 780 K).
PZ, Pyle and Zaror [41] (Experimental); PS, Present model with shrinkage [7]; BC, Babu and Chaurasia’s Model [6]; JS, Jalan and Srivastava’s Model [43].
Table 5.
Comparison of present model results with shrinkage with those of earlier models for pyrolysis time = 11 min
(R = 0.011 m, T0 = 303 K, Tf = 643 K).
PZ, Pyle and Zaror [41] (Experimental); PS, Present model with shrinkage [7]; BC, Babu and Chaurasia’s Model [6]; JS, Jalan and Srivastava’s Model [43]; BCM, Bamford, Crank and Malan’s Model [18].
Table 6.
Sensitivity analysis for pyrolysis with ±50% variation to reference values of properties with cylindrical geometry with shrinkage
Corresponding author. Present address: Imperial College London CPSE, Department of Chemical Engineering, South Kensington Campus, London SW7 2AZ, UK. Tel.: +44 20 7594 6624; fax: +91 20 27185759.
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