Valorization of Food Waste via Torrefaction: Effect of Food Waste Type on the Characteristics of Torrefaction Products

Torrefaction is considered to be an important pretreatment method to enhance the fuel quality for efficient valorization of food waste. In this study, four typical food wastes (leftover rice, leftover pork, leftover cabbage, and watermelon peel) were torrefied in a fixed bed furnace at 200-300 degrees C to compare their torrefaction performances by characterizing their torrefaction products (i.e., solid, liquid, and gas). Torrefaction is found to greatly decrease the O/C and H/C molar ratios and increase the higher heating value (HHV) of torrefied food waste, especially for leftover rice. Due to different components in the four food wastes, their yields of solid products are different, following a trend of leftover pork > leftover cabbage > watermelon peel > leftover rice. Their tar products also consist of different compounds as major components, i.e., anhydrosugars for leftover rice, aldehydes/ketones for left pork, and acids/aldehydes/ketones for leftover cabbage and watermelon peel. Our results also show that significant oxygen removal takes place during torrefaction, mainly via dehydration and decarboxylation to form water and CO2, respectively. Depending on the torrefaction temperature, up to 77.6, 56.9, 74.3 and 82.4% of oxygen can be removed from leftover rice, leftover pork, leftover cabbage, and watermelon peel, respectively. Unlike oxygen, the majority of carbon (71.8-82.3%) is retained in the solid product, resulting in a high HHV of torrefied food waste. This study also provides useful strategies for efficient valorization of food waste via torrefaction.