Microwave-assisted torrefaction and pyrolysis of rice straw pellets for bioenergy

Abstract

The highlights of this study involve the use of microwave heating for torrefaction and pyrolysis of pelletized rice straw for the generation of high-quality bio-char pellets, and thorough energy assessment of the process. The effects of microwave power and final temperature on the product yields and their quality were thoroughly assessed. At 600 W power, the bio-char yield followed the trend: 33.7 wt.% (800°C) < 35.5 wt.% (600°C) < 55.6 wt.% (300°C). Bio-char yield was unaffected by microwave power at the final temperature of 300°C. High yield of moisture (23.5 wt.%) with a minimal amount of bio-oil (10.5 wt.%) and gases (10.7 wt.%) were obtained at 600 W and 300°C. The major functional groups in the bio-oil were phenols (∼40%), cyclic oxygenates (∼16%), furan derivatives (∼20%) and aliphatic oxygenates (16%). Bio-char obtained from torrefaction at 600 W and 300°C possessed a calorific value of 17.2 MJ/kg with limiting oxygen index of 36.7%, and power generation potential of 4.8 kWh/kg. Overall, microwave-assisted torrefaction is shown to be a promising process intensification strategy to derive high-quality bio-char in a short time of 5–10 min with high process efficiency (51–56%) and comparable biomass conversion efficiency (∼83%) with respect to conventional pyrolysis.