Endothermic and Exothermic Reactions During Pyrolysis of a Woody Biomass Particle Ken-ichiro Tanoue, Shoma Murata, Toshihide Irii, Tatsuo Nishimura, Yamaguchi University, Ube, Japan Yoshimitsu Uemura Universiti Teknologi PETRONAS, Tronoh, Perak, Malaysia Miki Taniguchi and Ken-ichi Sasauchi, Chugai Ro Co.ltd, Sakai, Japan Introduction Compared to other countries, Japan is particularly rich in forests; the percentage of Japan’s landmass that is forested is 67%, which is the third highest in the world. Only 20% of all wood is effectively utilized as timber; the rest, including waste wood (4 million tons/year) and natural forests (10 million tons/year), is unused. In recent years, there has been increasing demand for quickly produced, environment-friendly energy resources; in particular, there is wide interest in the production of energy from woody biomass, using wood resources which are currently underutilized. The use of biomass to provide energy has many merits: it is renewable, abundant, easy to store, and carbon-neutral. 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 biomass layer. 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 pyrolysis5). The heat transfer with including the chemical reactions undergoing the pyrolysis has been also investigated by Koufopanos et al 3), Won et al10) and etc. The roughly mechanism of the heat transfer could be predicted by the numerical simulation 10). Wood biomass consists of Hemicellulose, Cellulose, Lignin and ash. However, there is no report to take the effect of the above components on the pyrolysis into consideration. In this study, fast pyrolysis of wood isolated spheres have been conducted experimentally and numerically to investigate the influence lignin content on heat transfer and chemical reaction during pyrolysis of wood biomass. Experimental set up and procedure A kind of wood and diameter was cornel and 19.8 mm, respectively. The cornel particle was connected with a thermocouple to measure the temperature at the center of it. The diameter of the thermocouple was 1mm. 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. A diameter and a height of the tubular reactor were 106 mm and 230 mm, respectively. The furnace had a maximum output of 1.5 kW, and the wall temperature of the reactor could be set to temperatures up to setting temperature, TS. The TS was varied from 673 K to 973 K. 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 1 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. Fast pyrolysis was started by entering the particle with having the thermocouple into the tubular reactor. Generated gas flow rate and the temperature at the center of the particle were measured simultaneously during pyrolysis up to 10min. Furthermore, in order to investigate the gas generation by tar decomposition, a nitrogen career-gas was adopted during pyrolysis. The flow rate of the career gas, QC, was 0 L/min or 4 L/min. Numerical Simulation Chemical reaction model during pyrolysis with tar decomposition When a biomass particle was pyrolyzed around the pyrolysis temperature, TPy, it could be partly converted to an activated biomass, char, tar and gas. Figure 1 shows the proposed chemical reaction model during pyrolysis by arranging Miller’s one 4). The model could reproduce the pyrolysis of the main components (hemicellulose, cellulose and lignin) of biomass 6, 7). Firstly, every main component decomposes to the activated one. Then the tar, char and gas are formed by the successive reactions. Furthermore, the char and gas increased with the secondary decomposition of tar 8, 9). Material balance for the component i is given by: GasH2 TarH1 Hemicellulose Activated Hemicellulose βH CharH + (1-βH) GasH CharH2 GasC2 TarC1 Wood Cellulose Activated Cellulose CharC2 βH CharH + (1-βH) GasH GasL2 TarL1 Lignin Activated Lignin CharL2 βL CharL + (1-βL) GasL Fig.1 Proposed chemical reactions during pyrolysis with the effect of tar decomposition. βH = 0.6, βC = 0.35, βL = 0.75. 2 d Wi = Ri dt (1) The kinetic parameters for tar decomposition proposed by C. Di Blasi 1) were used. Thermal conduction during pyrolysis If the volume change of the particle and the convective heat transfer by the formation of tar and gas can be neglected, the energy equation of a biomass particle is given as follows: [ 1 ∂ (WB,tot C B + W AB,tot C AB + WC, tot C C + WT, tot C T + WG, tot C G ) T V0 ∂t 1 ∂  1 2 ∂T  Qreac. = 2  λeff r + ∂r  V0 r ∂r  λeff = WB,tot + W0 WT, tot W0 λB + WAB,tot λT + WG, tot W0 W0 λB + WC, tot W0 ] (2) λC (3) 13.5σT 3 d λG + e The last term on the right side of Eq. 3 shows the effect of radiation heat transfer through the pores . The governing equation and the boundary conditions were discretized over a control volume using the finite difference method. The calculation program was made originally by using FORTRAN language. The calculation has been conducted in accounting the dependence of the physical properties on the temperature. The total grid numbers along radius direction were 600. 10) Algorithm for solving heat and mass transfer during pyrolysis For the mass transfer, the yields of solid component, tar component, gas component and the generation rate of the gas in every control volume were calculated by solving the material balance using the 4th order Runge Kutta method. On the other hand, for the heat transfer, the temperature profile in the particle and the temperature at the surface of the particle were solved by the THOMAS algorithm and the Newton-Raphson method, respectively. Fig. 2 shows the algorithm for solving heat and mass transfer during pyrolysis. 3 Fig. 2 Flow chart of numerical simulation for the thermal conduction during pyrolysis of a biomass particle. Results and discussion Fig. 3 shows the dependence of heat transfer and chemical reaction on the setting temperature, TS. Plots and lines show the temperature at the center of the wood, TC, and the temperature at the ambient around it, TA, respectively. For TS = 673 K and t < 4 min, the TC increased monotonously with time. As time elapsed, for t > 4min, the TC was higher than the TA due to the exothermic reaction. For TS = 773 K and t < 2.5min, the TC increased also monotonously with time. There was an inflection point at T = 650K due to endothermic reaction. The TC increased dramatically at t > 3min due to the exothermic chemical reaction. After that, the TC was higher than that the TA and then approached a constant temperature. On the other hand, for TS = 873 K and t < 2 min, the TC increased monotonously with time and increased suddenly at about 700 K. Then it approached a constant temperature. The tendency of the time course of TC for TS = 973 K was almost as same as that for TS = 873 K. For 873 K < TS < 973K, there was no inflection point which was reported by Won et al. The reason is that heat transfer can be controlled by thermal conduction rather than heat of reaction for 873 K < TS . It was reported that decomposition of cellulose, hemicellulose and lignin occurred at 513 K < T < 623 K, at 513 K < T < 623 K and at 503 K < T < 773 K, respectively 5). Pyrolysis of cellulose and hemicellulose follows the similar circumstances that are listed collectively as the holo-cellulose. Therefore, it was found that the inflection point of temperature could be occurred by pyrolysis of holo-cellulose. By the way, it was reported that the secondary decomposition of tar occurred for T > 773K 2). As the exothermic behavior was observed also around T = 773K, the decomposition of lignin or secondary tar decomposition could cause the 4 TS = 973 [K] TS = 873 [K] TS = 773 [K] TS = 673 [K] Fig. 3 Time course of temperature at the center of the biomass particle during fast pyrolysis exothermic reaction. In order to clear the effect of secondary tar decomposition, measurements of temperature and gas flow rate were conducted with supplying career gas. The gas flow rate was 4 L/min. Fig. 4 shows time course of the experiment results of heat and mass transfer during pyrolysis. The setting temperature was 773 K. Fig. 4 (a) shows time course of the TC and the temperature at the TA during pyrolysis. For QC = 4 L/min and t < 2.5 min, the TC increased monotonously with time. For 2.5 min < t < 3.5 min, the infection point of the temperature was observed. As time elapsed, for t > 4min, the TC was higher than the TA due to the exothermic reaction and then approached a constant temperature. As the time course of the TC for QC = 0 L/min was almost as same as that for QC = 4L/min, heat transfer could not be affected by the tar decomposition. Fig. 4 (b) shows time course of the generated gas flow rate during pyrolysis of the woody biomass. For QC = 0 L/min and t < 1.5 min, the generated gas flow rate increased monotonously with time. The generated gas flow rate had a maximum value at t = 2 min and then decreased monotonously with time for t < 4 min. On the other hand, for QC = 4 L/min, the maximum value of the generated gas flow rate was lower than that for QC = 0 L/min. Although secondary decomposition of tar could not influence on heat transfer during pyrolysis, it has a significant effect for gas generation. Therefore, it was suggested that the exothermic behavior at 650K < TC < 800K controlled by the pyrolysis of lignin content. Conclusions Heat transfer and chemical reactions during fast pyrolysis of wood biomass has been studied experimentally. The setting temperature of the furnace, TS, was changed from 673 K to 973 K. The diameter of biomass particle was about 19.8 mm. Some conclusions were obtained as follows. 1) From time course of the temperature at the center, TC, at 673 K < TS < 773 K, there were four regions. Firstly, heat transfer was controlled by thermal conduction of wood. Secondary, there was an inflection point at 650K due to endothermic reaction. Next, the TC was higher than that the temperature at the ambient around wood due to exothermic reaction. Finally the TC approached a constant temperature. On the other hand, for 873 K < TS < 973K, there was no inflection point as mentioned above. The reason is that heat transfer can be controlled by thermal conduction rather than heat of reaction for 873 K < TS. 2) In order to the effect of tar decomposition on heat and mass transfer, the TC during fast pyrolysis with adopting career gas was measured. Although secondary decomposition of tar could not influence on heat transfer during pyrolysis, it has a significant effect for gas generation. 5 a) Temperature b) Generated gas flow rate Fig. 4 Time course of the experiment results of heat and mass transfer during fast pyrolysis. References 1. Di Blasi, C. and Branka, C.,” Kinetics of Primary Product Formation from Wood Pyrolysis,” Ind. Eng. Chem. Res., 40-23, 5547-5556 , 2001. 2. Fagbemi, L., Khezami, L., Capart, R., “Pyrolysis products from different biomass : application to the thermal cracking of tar”, Applied Energy, 69, pp. 293-306, 2001. 3. Koufopanos, C. Papayannakos, A., N., Maschio, G. and Lucchesi, A., “Modelling of the Pyrolysis of Biomass Particles. Studies on Kinetics, Thermal and Heat Transfer Effects,” Can. J. Chem. Eng., 69, pp.907-915, 1991. 4. Miller, R. S., Bellan, J.” A Generalized Biomass Pyrolysis Model Based on Superimposed Cellulose, Hemicelluloseand Liqnin Kinetics” , Combust. Sci. Technol., 126, 97-137, 1997. 5. Okano, K. and Sofue, N., Mokuzai kagaku handbook, Asakura shoten, 117, 2006. 6. Tanoue, K. , Yamasaki, K, Nishimura, T. , Taniguchi, M. and Sasauchi, K. , “A Relationship between the Formation 7. Rate of Solid Component and Volume Reduction of Packed Bed of Woody Biomass Particles during Pyrolysis,” J. Jpn. Inst. Energy, 90-11, pp. 1031-1037, 2011. Tanoue, K. , Suetomi, T. , Nishimura, T., Taniguchi, M. and Sasauchi, K. , “Thermal conduction and gas 6 8. 9. 10. generation undergoing pyrolysis in the packed bed of woody biomass,” J. Jpn. Inst. Energy, 91-10, pp. 976-984, 2012. Antal, M. J. , “Effects of reactor severity on the gas-phase pyrolysis of cellulose- and kraft lignin-derived volatile matter, ” Ind. Eng. Prod. Res. Dev., 22, 989. 366-375, 1983 Antal, M. J. , “A review of the vapor phase pyrolysis of biomass derived volatile matter,” In: Overend RP, Milne TA, Mudge LK, editors. “Fundamentals of biomass thermochemical conversion”. Elsevier, pp. 511-537, 1985. Won, C. P, Arvind, A. and Howard, R. B.”Experimental and theoretical investigation of heat and mass transfer processes during wood pyrolysis” , Combust. Flame, 157, pp. 481-494, 2010. Acknowledgements This work was partly supported by Electric Technology Research Foundation of Chugoku and Tokuyama Co, Ltd. 7