Numerical study of Heat transfer and chemical reaction in the

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					Numerical study of Heat transfer and chemical reaction in the Packed Bed of Woody Biomass
Particles during Pyrolysis


                       Ken-ichiro Tanoue, Takahiro Suetomi, Tatsuo Nishimura
                                    Yamaguchi University, Ube, Japan


                                  Miki Taniguchi and Ken-ichi Sasauchi,
                                     Chugai Ro Co.ltd, Sakai, Japan


ABSTRACT
Heat transfer and chemical reaction undergoing the pyrolysis in the packed bed of biomass has been
investigated experimentally on accounting for the particle size of the biomass. For Dp = 0.38 mm, the
gas flow rate increase with time until 90min, and then the flow rate decrease with time. As the average
temperature at 90min shows about 380 ºC for Dp = 0.38mm, the decomposition of lignin suggest to
occur mainly. On the other hand, for Dp = 0.78 mm and 1.13mm, the maximum value of the gas flow
rate appeared at 70 min when the average temperature was about 400 ºC. It suggests that the chemical
reaction undergoing pyrolysis in the packed bed doesn’t depend on the particle size. The volume
fraction for Dp = 0.38mm was about 0.84 while the fraction for Dp = 1.13mm was about 0.932. The
thermal diffusivity of gas is about 20.8 mm2/s at 20 ºC while the diffusivity of biomass is about 0.06
mm2/s. Therefore, the smaller particle size got the higher average temperature in the packed bed and
the higher gas generation rate.


                                         1. INTRODUCTION


  As Japan has many mountains with steep slopes, it is difficult to transport the felled trees. So that,
the development of a compact gasifier of high quality, which can cope with variations both in the
amount of biomass collected and in energy demand, is necessary. Furthermore, in order to ensure that
the system is used in the most efficient manner, it is necessary to increase our understanding of the
reaction mechanisms involved in both decomposition of biomass and heat transfer in the packed bed
of biomass.
  It has been well known that the woody biomass decomposed to char, gas with having the middle
heating value and tar undergoing the pyrolysis. There are many chemical reaction models of
pyrolysis1). The heat transfer with including the chemical reactions undergoing the pyrolysis has been
also investigated by Koufopanos et al 2), Won et al    3)
                                                            and etc. The roughly mechanism of the heat
transfer could be predicted by the numerical simulation 4). However, most of reports assumed that
the volume didn’t change undergoing the pyrolysis and there is no report how the thermal conduction
in the bed changes with the effect of the volume reduction.
  In our previous reports5,7,8), the numerical simulation of the unsteady thermal conduction without
the chemical reaction has been conducted and compared with the experimental one. The yield of solid
component could be reproduced by the analysis using the model by Miller et.al9) even if the heating
rate and the lignin content changed5). Furthermore, the volume in the packed bed of the biomass
undergoing the pyrolysis can be estimated by using the model by Miller et.al9) and the dependence of
gas volume in the bed on the temperature5).
  In this report, heat transfer and chemical reaction undergoing the pyrolysis in the packed bed of
biomass has been investigated experimentally on accounting for the particle size of the biomass.


                               2. EXPERIMENTAL PROCEDURE7,8)


  The rig consisted of a nitrogen gas supply, a tubular reactor, cold traps for tar and water, a
gas-sampling bag, and a gas flow meter. The furnace had a maximum output of 1.5 kW, and the wall
temperature of the reactor could be set to temperatures up to about 800ºC. The cold traps, which were
situated in an ice bath, were two 500-mL Erlenmeyer flasks filled with glass wool and solid CaCl2.
During the pyrolysis, tar and water were adsorbed by the traps, and the gas generated in the reaction
flowed through the traps and filled the sampling bag (GL Science Co., Ltd.) for measurement of the
gas component. Gas flow rate was measured using a wet-type gas meter (Shinagawa Co., Ltd.,
W-NK-2B). An O2 monitor was placed at the exit of the reactor. Before the experiment, air in the
reactor was replaced by nitrogen gas, which was allowed to flow into the reactor until the
concentration of O2 at the exit was less than 1%, after which time the nitrogen gas supply was stopped.
Woody biomass particles (sample weight W0 = 100 g, Dp,50 = 1.1 or 11 mm) were introduced into the
tube reactor, and the wall of the reactor was heated to 800ºC by the electric furnace at heating rate of
400 ºC h-1. The temperature of the reactor wall was then maintained at 800ºC for about 60 min. Three
samples with different particle sizes were used, and for each sample, two pyrolysis experiments were
conducted: in the first, the temperature profile of the packed bed of biomass, the flow of gas generated,
and the material balance were measured; and in the second, the components of the generated gas were
analyzed. The carbide material remaining in the reactor is defined as char. Samples of the gas
generated in the reaction were collected in the sampling bag every 20 min and analyzed quantitatively
by gas chromatography (GL Science Co., Ltd., GC323) to determine the concentrations of H2, CH4,
CO and CO2.


                                  3. NUMERICAL SIMULATION


3.1 Thermal conduction without the heat of the chemical reaction
The tubular reactor can be divided into two layers; the upper layer is filled with nitrogen gas, while
the lower layer is filled with the packed bed of biomass. If we neglect the process of convective heat
transfer and the heat source of the chemical reaction, the energy equations for gas phase and solid
phase are as follows.
 (For gas phase)



    g CP,g Tg       1         T       T 
                            g r g    g g   hSaS TS  Tg                         (1)
         t            r r 
                                  r   z 
                                              z 
                                                   


 (For solid phase)


  1    S C P,S TS  1                 T              T 
                                 1   S r S    1   S S   hS aS TS  Tg     (2)
           t               r r              r   z           z 


The initial and boundary conditions of the reactor are given by the following equations.


  t  0 : Tg,0  TS,0  T0                                                                   (3)



t  0:
(For gas phase)
      Tg
            0 at r  0
       r
      Tg  TW ( z ) at r  R                                                                 (4)
      Tg
             0 at z  0
       z
      Tg
             0 at z  z exit
       z


  (For Solid phase)
       TS
             0 at r  0
        r
       TS  TW ( z ) at r  R                                                                 (5)
        TS
               0 at z  0
         z

                    z
                                            
        1   S TS  h p Tg - TS   e TW 4  TS 4      at z  z top surface



  The governing equations and the boundary conditions were descretized over a control volume using
the finite difference method. The calculation program was made originally by using fortran language.
The calculation has been conducted by using not only the dependence of the physical properties on
the temperature bat also the change of the position of the top in the packed bed. The temperature
profile in the packed bed and the temperature at the top of the packed bed are solved by the SOR
method and Newton-Raphson method, respectively. The r-component and z-component grid sizes
were r = z = 1 mm. The total grid numbers were 53  230 = 12190. In this calculation, the void
fraction in the packed bed was assumed to be constant:  = 0.92, which was obtained based on the
initial conditions of the packed bed of biomass. The dependence of the temperature for all physical
properties was written in our previous report8).


3.2 Chemical reaction undergoing pyrolysis of biomass
  There are many models for pyrolysis of biomass. In this study, three models were chosen in these
models. In this study, the chemical reaction model, which was reported by Miller et. al 6), has been
used. The model 6) shows that Hemicellulose, Cellulose and Lignin decompose parallel to gas, tar and
char. The effect of the char formation due to the secondary decomposition by tar was ignored. The
yield of solid component, tar component, gas component and the generation rate of the gas every
control volume were calculated by the Runge Kutta method. Initial mass fraction of Hemicellulose,
                                                        9)
Cellulose and Lignin in the Pseudotsuga menziesii            was assumed to be 0.29, 0.47 and 0.29,
respectively.


                                   4. RESULTS AND DISCUSSION


  Fig.1 shows (a) time course of the generated gas flow rate and (b) time course of the average
temperature in the packed bed. For Dp = 0.38 mm, the gas flow rate increase with time until 90min,
and then the flow rate decrease with time. As the average temperature at 90min shows about 380 ºC
for Dp = 0.38mm, the decomposition of lignin suggest to occur mainly. On the other hand, for Dp =
0.78 mm and 1.13mm, the maximum value of the gas flow rate appeared at 70 min when the average
temperature was about 400 ºC. It suggests that the chemical reaction undergoing pyrolysis in the
packed bed doesn’t depend on the particle size. The volume fraction for Dp = 0.38mm was about 0.84
while the fraction for Dp = 1.13mm was about 0.932. The thermal diffusivity of gas is about 20.8
mm2/s at 20 ºC while the diffusivity of biomass is about 0.06 mm2/s. Therefore, the smaller particle
size got the higher average temperature in the packed bed and the higher gas generation rate.
  The results for the numerical simulation will be reported in the presentation.


                                           5. CONCLUSIONS


Heat transfer and chemical reaction undergoing the pyrolysis in the packed bed of biomass has been
investigated experimentally. The larger particle size of the packed bed got the higher gas flow rate by
by the decomposition of biomass because of the apparent thermal diffusivity.
 a) Generated gas flow rate




 b) Average temperature in the packed bed




 Fig.1 Experimental results for the time course of heat transfer and chemical reactions undergoing
 the pyrolysis in the packed bed.


Acknowledgements
This work was supported in part by a Grant-in Aid for Scientific Research C (No. 19560174) from the
Japan Society for the Promotion of Science. Our special thanks are extended to Dr. Morihisa Yokota
of Ube Industries, Ltd., for helpful discussions on the work.


References
1.   Okumura, Y., (2011),” Modeling of Pyrolysis and Gasification Reactions of Many Types of Biomass” , J. Jpn.
     Inst. Energy, 90 (2), 122-131 (in Japanese).
2.   Koufopanos, C. Papayannakos, A., N., Maschio, G. and Lucchesi, A.(1991), “Modelling of the
     Pyrolysis of Biomass Particles. Studies on Kinetics, Thermal and Heat Transfer Effects,” Can. J.
     Chem. Eng., 69, pp.907-915.
3.   Won, C. P, Arvind, A. and Howard, R. B. (2010)”Experimental and theoretical investigation of
     heat and mass transfer processes during wood pyrolysis” , Combust. Flame, 157, 481-494.
4.   Di Blasi, C. (2008), “Modeling chemical and physical processes of wood and biomass
     pyrolysis,” Prog. Energy Comb. Sci., 34, 47-90.
5.   Tanoue, K., Yamasaki, K., Nishimura, T., Taniguchi, M., Sasauchi, K., “A Relationship between the
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     during Pyrolysis”, J. Jpn. Inst. Energy, in press (in Japanese).
6.   Miller, R. S., Bellan, J. (1997)” A Generalized Biomass Pyrolysis Model Based on
     Superimposed Cellulose, Hemicelluloseand Liqnin Kinetics” , Combust. Sci. Technol., 126,
     97-137.
7.   Tanoue, K., Hinauchi, T., THAUNG, O., Nishimura, T., Taniguchi, M. and Sasauchi, K.(2007),
     “Modeling of heterogeneous chemical reactions caused in pyrolysis of biomass particles,” Adv.
     Powder Technol., 18-6, pp.825-840.
8.   Tanoue, K., Widya, W., Yamasaki, K., Kawanaka, T., Yoshida, A., Nishimura, T., Taniguchi, M., Sasauchi, K.,
     (2010) “Numerical Simulation of the thermal conduction of packed bed of woody biomass particles
     accompanying volume reduction induced by pyrolysis”, J. Jpn. Inst. Energy, 89 (10), 948 (in Japanese).
9.   Ayhan D. (1997),” Calculation of higher heating values of biomass fuels,” Fuel, 76-5, 431-434.

				
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