Microwave-induced carbonization of rapeseed shell for bio-oil and bio-char: Multi-variable optimization and microwave absorber effect

Abstract

Abstract Microwave carbonization induced by microwave absorber was investigated to convert rapeseed shell to bio-oils and bio-chars. The effects of microwave power, vacuum pressure and absorber (SiC) to biomass mass ratio on heating profile and bio-oil production were investigated. Response surface methodology based on BOX-BEHNKEN design was used to study the significance of process parameters on product yields. The conditions (600.00 W, 19.21 kPa and 0.51) for obtaining high-yield liquid products (42.74%) were more stringent; namely, lower process parameters (492.54 W, 16.44 kPa and 0.49) could acquire more organics (23.61%), in which microwave absorber played the most significant role. Furthermore, composite absorbers with different TiC to SiC mass ratios were introduced under the maximum organics production condition. With the increase of TiC to SiC ratio from 5% to 40%, the organic yield first increased and then decreased and the char production was just the opposite. The GC–MS and 1H/13C NMR analysis showed that the high-yield bio-oil retained more aliphatics from cellulose and hemicellulose and lowered the relative content of aromatics. The SEM and N2 adsorption-desorption characterizations indicated that the char obtained using 20TS had an obvious porousness carbon-skeleton structure and the highest surface properties. Appropriate amount of TiC effectively regulated the temperature rise, which made rapeseed shell be preheated and carbonized modestly. Therefore, microwave-carbonization of rapeseed shell under vacuum environment by using 20TS as absorber is a promising and competitive approach for production of bio-oil with high yield, and by-product of bio-char also had higher availability value.