An experimental analysis on property and structure variations of agricultural wastes undergoing torrefaction

Three agricultural wastes, consisting of coffee residue, sawdust and rice husk, undergoing torrefaction are investigated to evaluate the potential of biomass as solid fuel. Two different torrefaction temperatures (240 and 270 °C) and durations (0.5 and 1 h) are considered in the study, and the properties and structures of the raw and torrefied wastes are extensively investigated by means of proximate, elemental, fiber, calorific, thermogravimetric, SEM and FTIR analyses. A high-volatile bituminous coal and a low-volatile one are also regarded for comparison. By virtue of more hemicellulose contained in the coffee residue, it is the most active biomass to torrefaction and its higher heating value (HHV) is improved up to 38%. The empirical atomic formula of the raw wastes is expressed by CH1.54–1.76O0.65–0.89 and it changes to CH1.02–1.57O0.26–0.64 after undergoing torrefaction. The torrefied biomasses approach high-volatile coal when the torrefaction temperature and duration increase. From fuel point of view, the improved properties and changed molecular structure are conducive to the applications of biomass in industrial furnaces such as boilers and blast furnaces.

► Coffee residue, sawdust and rice husk undergoing torrefaction are investigated.
► A high-volatile bituminous coal and a low-volatile one are also regarded for comparison.
► Coffee residue is the most active biomass to torrefaction.
► Torrefied wastes approach the high-volatile coal as increasing temperature and duration.
► Torrefaction is conducive to the applications of biomass in industrial furnaces.