2012 Volume 91 Issue 10 Pages 976-984
In this study, thermal conduction and gas generation during pyrolysis of biomass in a packed bed were investigated experimentally and numerically. The setting temperature of the furnace, TS, was varied between 673 K and 1073 K, while the diameter of the biomass particles, DP, was varied between 0.34 mm and 1.13 mm. The heating rate of the furnace was 400 K/h. Experimental results for thermal conduction showed that the average gas temperature, TG, in the packed bed increased steeply from 500 K to 800 K with time. This sudden increase in gas temperature was steeper when the particle diameter was increased. The time course of the average gas temperature could be partially reproduced by numerical simulation which takes into account the change in porosity during pyrolysis. However, the sudden increase in gas temperature observed experimentally did not agree with that obtained by the simulation. The sudden increase in gas temperature was considered to be due to larger heat transfer of the radiation at the surface of the packed bed than that of the thermal conduction from the heated wall caused by volume reduction during pyrolysis. Experimental results for gas generation indicated that the mass flow rate of the generated gas had a maximum at a certain time, tmax, for TS ⟩ 773 K due to secondary decomposition of tar. tmax became longer as the diameter of the biomass decreased. The reasons are as follows. In the packed bed, not only heat transfer due to volume reduction, but also an endothermic reaction would simultaneously occur. The sudden increase in gas temperature for smaller particles could be less steep than that for larger particles due to this endothermic reaction. Therefore, the time required for decomposition of tar is longer for smaller particles. It was found that the calculated gas flow rate, taking into account the effect of the temperature distribution in the packed bed, agreed with the experimental results except for the gas flow rate due to tar decomposition.
