Effect of Torrefaction on Physicochemical Properties and Steam Gasification Reactivity of Chars Produced from the Pyrolysis of Typical Food Wastes

This study compares the physicochemical properties and steam gasification reactivity of the chars produced from the pyrolysis of the raw and torrefied food wastes at 800 degrees C, including leftover rice (LR), leftover pork (LP), leftover cabbage (LC), and watermelon peel (WP). Particularly, the release and transformation of various forms of alkali and alkaline earth metallic (AAEM) species during the pyrolysis of the raw and torrefied food wastes are studied in details. Steam gasification results of the chars from the raw food wastes indicate that the LC and WP chars have much higher gasification reactivity than the LR and LP chars, mainly as a result of the higher char surface areas, higher contents of AAEM species, and less condensed carbon structures for the LC and WP chars. However, the gasification reactivity of the torrefied LR and LP chars is reduced, mainly as a result of the decreased surface areas and increased large aromatic ring (>= 6 rings) structures as well as the decreased active K content for the torrefied LP char. In contrast, the gasification reactivity of the torrefied LC and WP chars is slightly increased, most likely as a result of the less condensed carbon structures and increased contents of active AAEM species for those chars. The results demonstrate that the effect of torrefaction on char gasification strongly depends upon the type and properties of food waste, especially the contents of active AAEM species (i.e., the water- and CH3COONH4-soluble AAEM species) in food waste. For the food wastes of low contents of active AAEM species, such as LR and LP, co-torrefaction with other food wastes of high contents of active AAEM species is an important strategy to enhance the overall gasification efficiency of all food wastes.