Comparison on solid biofuel production from wet and dry carbonization processes of food wastes

In this research, three categories of food waste (dried cabbage, chicken, and rice) were separately subjected to hydrothermal carbonization (HTC) under 180 °C, 200 °C, and 220 °C and pyrolytic carbonization (PC) under 200 °C, 250 °C, 300 °C, 350 °C, and 400 °C for solid biofuel production, respectively. Carbonization pathways, solid yield, high heating value (HHV), the transformation of nitrogen and ash, and char reactivity were detailly compared. After both pre-treatments, the ratio of fixed carbon (FC) to volatile matter (VM) of solid increased with the increase of the temperature, except for HTC of chicken. HTC of chicken led to more VM in solid product with very low solid yield and significantly lower N content. HTC-chars showed higher VM even at the same temperature as PC despite more severe carbonization (higher C content). Even though the N content can be reduced by both HTC and PC, HTC showed superiority than PC in regards to reducing the N content, indicating the potential to mitigate the N-related emission. Even though the HHV of HTC char was higher than that of PC char at the same pretreatment temperature, the product yield of the former one was much lower, resulting in lower energy yield in HTC char. The alkaline index (AI) can be used as the main factor for comparisons of gasification reactivity between (1) HTC and PC-chars and (2) Among HTC-chars from different temperatures. PC-chars presented higher reactivity than HTC-chars for all types of feedstock when considering at the same pre-treatment temperature at 200 °C. In conclusion, the key parameters including VM, the solid yield, HHV, the N content reduction, and char reactivity can be adopted to well evaluate the solid biofuel qualities production from different processes of HTC and PC, respectively.