The effects of conventional and microwave torrefaction on waste distiller's grains and its steam gasification characteristics

High moisture content and low energy density of distillers' grains (DG), along with the corrosion and clogging issues caused by high tar yield during gasification, are the main barriers to effective energy conversion and clean utilization of waste DG. Microwave torrefaction is a novel, low-cost, and efficient pretreatment method to enhance biomass quality and reduce tar yield during gasification. In this study, the effects of microwave and conventional torrefaction methods on the product yield (mass yield-Y-mass and energy yield-Y-energy) and fuel property (higher heating value-HHV) of DG were systematically evaluated under various conditions of temperature (180-270 degrees C), time (5-19 min), and microwave power (200-1000 W). Moreover, steam gasification experiments were performed on DG treated by both microwave and conventional torrefaction, specifically examining how these methods affect gasification characteristics, product yields, and particularly tar components. Microwave torrefaction yielded torrefied samples with a higher HHV and lower H/C and O/C ratios compared to the conventional method, albeit with about 30 % lower Y-mass and 20 % lower Y-energy. Response Surface Methodology identified optimal microwave torrefaction conditions of DG sample at 200 degrees C with 448 W for 8 min. Under these conditions, a 72.55 % Y-mass and an 81.66 % Y-energy were achieved, with an energy consumption of only 36.40 kJ/g. The gasification efficiency of microwave-torrefied DG (MTDG) was significantly higher than that of both raw DG and conventionally torrefied DG (CTDG). Steam gasification of MTDG also generated more hydrogen-rich syngas and reduced tar yield. Additionally, the energy utilization efficiency of the microwave torrefaction combined with biochar steam gasification (MT-BCSG, 82.1 %) process was 4.9 % higher than that of the conventional torrefaction combined with biochar steam gasification (CT-BCSG, 77.2 %) process. In summary, microwave torrefaction provides a superior and more cost-effective method than conventional torrefaction for pretreating and valorizing high-moisture waste biomass, showing great potential for industrial applications.