(17b) The Suppression of Tar Evolution during Biomass Pyrolysis by Using Simple Air Treatment Method

Treatment methods of biomass were investigated in order to suppress tar formation during pyrolysis by changing the precursor structure at low temperature region. Treatments in inert gas and air by slowly heating biomass samples to the temperatures between 240 and 340°C were conducted. Then pretreated samples were pyrolyzed in a flash mode at high temperature to analyze the products. From the result of thermogravimetric analysis, it was found that biomass treated with air started to decompose at lower temperature than samples treated with inert gas. Due to the cross-linking reaction, more char yields were gained by combining air treatment at low temperature and subsequent pyrolysis. Moreover, we successfully reduced the tar yield from about 50 wt% to less than 20 wt% by oxidizing the sample at lower heating rate (0.2 ? 1 K•min^-1) up to between 260 and 300°C by air. At the same treatment temperature, the lower heating rate caused a decrease in tar yield, but an increase in CO and CO₂ gas yields. This indicates that the structure of solid precursors was changed little by little before reaching the temperature to release tar since the cross-linking reaction gradually proceeded during slow air treatment. These pretreated solid precursors also resulted in almost no tar released during subsequent flash pyrolysis at high temperature. With increasing holding time of the treatment at the fixed temperature, the yield of tar released during air treatment showed little difference, but solid precursors treated with longer holding time hardly released tar during subsequent flash pyrolysis due to its cross-link structure. For tar analysis by GPC method, tar obtained during this air treatment at low temperature seems to be easily decomposed by steam reforming because of its lower molecular weight. This above result suggests that the method can be useful as a simple method to suppress tar evolution in an on-site gasification/gas engine system for biomass utilization.