Control of Reactivity of Formed Coke from Torrefied Biomass by Its Washing with Torrefaction- derived Acidic Water

Torrefaction, pulverization, hot briquetting, and carbonization in sequence successfully produce highstrength coke from woody biomass. This method was further improved by introducing washing of torrefied biomass with acidic water from torrefaction before briquetting. The primary purpose of the washing was to remove alkali, and alkaline-earth metallic species of which catalyses were responsible for high reactivity of the coke. The acidic water (AW) from 275 degrees C torrefaction of Japanese cedar contained 12, 0.9, and 39.4 mass% of acetic and formic acids, and the other organic compounds, respectively. A simulated AW (SAW) was prepared with the same composition as that of AW. SAW with pH of 1.95 removed 96-97% of K, Mg, and Ca and 48% of Na from the torrefied cedar. These removal rates were higher than those by washing with an aqueous solution of acetic acid, hydrogen chloride, or oxalic acid with pH of 2.35, 1.05, or 0.77, respectively. Organic compounds dissolved in SAW helped water and acids penetrate the matrix of the cedar. The washing with SAW increased the tensile strength of coke from 16 to 21 MPa by promoting volumetric shrinkage of the briquette during the carbonization and then particle bonding and coalescence. More importantly, the washing greatly reduced coke reactivity. The times required for gasifying 50% and 99% of coke with 50 kPa CO2 at 900 degrees C, t(0.50) and t(0.99), respectively, were extended by factors of 24 and 46, respectively. It was thus demonstrated that the coke reactivity was controllable over such a wide range.