Effect of Carbonaceous Deposit on Bed Particles During Rapid Pyrolysis on Gasification Rate of Biomass Char in Fluidized Bed

Gasification characteristics were evaluated for various biomass samples such as softwoods and hardwoods. They were pyrolyzed at 1000 °C in a small scale fluidized bed under rapid heating condition with N2 flow. After keeping 10 min at the temperature, the produced char was gasified with 25% CO2 at 1000 °C. Char gasification characteristic was investigated by monitoring CO producing rate and the effects of woody biomass species on gasification rate and its time variation were examined. The conversion rate, dX/dt was plotted against the value of conversion. As a result, the following characteristic curves were observed mainly for softwoods; at nearly conversion X = 0.05, high reactivity peak was found, CO production was decreased first rapidly and then slowly until X = 0.5, and then almost followed volume reaction behavior. In order to elucidate the cause of this phenomenon, the high temperature char with bed material of alumina in the bed was cooled down under the N2 flow to the room temperature and the char and alumina particles were separated by sieving. The alumina particles became black which suggests existence of some carbonaceous materials. Then each sample was introduced into the fluidized bed and separately gasified in the same bed. The gasified gas analyses showed that gasification rapidly proceeded for alumina particles only at the first stage of its gasification which explained the peak of gas evolution at low conversion of the continuous in situ pyrolysis/gasification experiments. In case of hardwoods, the amount of carbonaceous materials remaining in bed materials was quite small and it does not affect the results of the in situ pyrolysis/gasification experiments. Gasification difference of the char separately collected depended on their own properties rather than the common properties in their group.