Experimental and Numerical Investigation of Biomass Pyrolysis Process Focusing on Intra-Particle Heat Transfer

Abstract

The purpose of this research was to investigate biomass pyrolysis process focusing on intra-particle heat transfer. Thermal decomposition characteristics of wood cylinder with a diameter of 8mm were studied experimentally and numerically. In an experiment, a thermobalance reactor was used to investigate weight loss of wood cylinder during the pyrolysis. Three K-type thermocouples with a diameter of 0.5 mm were placed in the sample to measure the intra-particle temperature. Wood cylinders were heated by infrared furnace under inert gas at 1 Ks−1 and 30 Ks−1. In a calculation, unsteady two-dimensional heat and mass transfer equations were discretized by using Finite Volume Method with first order implicit scheme. The reaction kinetics of biomass pyrolysis were modeled by using a multi-step kinetic scheme. To investigate the effect of intra-particle heat transfer, calculations with considering temperature gradient and with uniform temperature were carried out. As a result, the calculation results with considering intra-particle heat transfer were in good agreement with experimental ones, while calculation results without considering temperature gradient were quite different from experimental ones at high heating rate. Intra-particle heat transfer mechanism at low heating rate was quite different from that at high heating rate. Both numerical and experimental results showed that there was a distinct peak of intra-particle temperature due to strong exothermic reaction at low heating rate. Meanwhile, endothermic reactions were dominant at high heating rate, and there was no temperature peak. Moreover, an increase in the slope of temperature history observed at high heating rate. It was difficult to explain the slope increase by only weak exothermic reactions. This was because that heat capacity was decreased significantly during pyrolysis. When the heating rate was high, the yield of volatile matter whose heat capacity was quite less than that of char or wood was increased. It was shown that volatile and char formation characteristics were strongly related with intra-particle heat transfer characteristics.