Improvement of Charcoal Yield and Quality by Two-Step Pyrolysis on Rice Husks

The utilization of waste rice husks for biochar production has received much attention due to the energy concerns and environmental issues. But how to increase the charcoal yield and quality remains a big challenge. In this work, a two-step pyrolysis of rice husks was carried out at atmospheric pressure, consisting of a first low heating rate step followed by a high heating rate step. The effects of operation conditions at each step, including temperature, heating rate and residence time, on changes in charcoal yield and quality based on fixed-carbon yield were investigated. At the first step, switch temperature and heating rate were the predominant factors that influenced charcoal and fixed-carbon yields. The increase of charcoal and fixed-carbon yields at the first step was attributed to the rate-determined decomposition of hemicellulose. Therefore, the switch temperature should be set at those lower than the decomposition temperature of celluloses. However, at the second step, decreasing final pyrolysis temperature and shortening residence time benefited the increase of charcoal yields. Whereas, a maximum value of fixed-carbon yield was observed at the final pyrolysis temperature of 600 °C, which could result from the aromatics formed from cellulose-derived volatiles. Therefore, the switch temperature should be lower than cellulose decomposition temperature in order to reduce the cycle time for biochar production. Moreover, the charcoal and fixed-carbon yields obtained by the optimized two-step pyrolysis were higher than those by one-step pyrolysis.