Preparation of Coke from Hydrothermally Treated Biomass in Sequence of Hot Briquetting and Carbonization

Abstract

A sequence of briquetting of biomass solids (bamboo, larch and mallee) at temperature and mechanical pressure of 130–200°C and 114 MPa, respectively, and carbonization at 900°C produces coke with tensile strength (TS) of 5–19 MPa. Introduction of heat treatment in hot-compressed water (i.e., hydrothermal treatment; HT) of the biomass prior to the briquetting increases TS up to 44, 57 and 42 MPa for the bamboo, larch and mallee, respectively. TS of coke is correlated well and positively with the coke/briquette bulk density ratio, and HT increases the ratio if operated under appropriate conditions. The efficacy of HT is attributed primarily to increase in the coke yield on a basis of the briquette mass. HT hydrolytically removes highly volatile cellulosic material (i.e., cellulose and hemicellulose), transforms it into solid that contributes to coke as effectively as lignin, and thereby increases the mass yield of coke by a factor of 1.4 to 2.1. HT also enhances the plasticizability of the biomass during the briquetting by degradation of the lignin to reasonable extent, and then promotes particles’ coalescence/fusion and densification of the briquettes. Applying mechanical pressure over a range of 12–114 MPa to the briquetting of a solid from HT of the bamboo at 240°C successfully results in production of coke with TS of 41–44 MPa.