Hydrothermal carbonisation of mixed agri-food waste: process optimisation and mechanistic evaluation of hydrochar inorganic chemistry

In this study, low-quality mixed agri-food waste (MAFW) was upgraded into homogenous high-quality solid fuel (i.e., hydrochar) via hydrothermal carbonisation (HTC). Response surface methodology (RSM) and principal component analysis (PCA) were used to study the effect of temperature (190–230 °C), residence time (1–5 h) and solid loading (5–20 %) on hydrochar fuel characteristics and provide mechanistic insights into hydrochar inorganic chemistry. In addition, HTC operating conditions were optimised to maximise hydrochar yield and calorific value and minimise ash content. Results from RSM revealed that reaction temperature and solid loading had most influence on the studied responses. The process optimisation of the HTC of MAFW resulted in hydrochar yield, calorific value and ash content of 52.25 %, 24.56 MJ/kg and 6.20 % (further validated within 3 % error), respectively at optimum operating condition of ⁓212 °C, 5 h and ⁓7.8 % solid loading. Furthermore, analysis by PCA revealed solid loading had a more significant impact on the fate of inorganic elements compared to reaction temperature and residence time. The results also suggests that inorganics are less concentrated in hydrochar at low solid loading and medium reaction severity. Compared to the raw-MAFW, the likelihood of fouling and slagging during combustion was reduced in the hydrochar.