Hydrothermal carbonization of Opuntia ficus-indica cladodes: Role of process parameters on hydrochar properties

•Opuntia ficus-indica cladodes possible hydrothermal carbonization biomass feedstock.•Multivariate linear regression demonstrates impact of process variables on yields.•Solid yield and secondary char depend on temperature, residence time, biomass:water.•Hydrochar HHV maximal and volatile matter minimal at intermediate biomass:water.•Calcium oxalate in biomass remains in char, gasifies it at high temperature.

Opuntia ficus-indica cladodes are a potential source of solid biofuel from marginal, dry land. Experiments assessed the effects of temperature (180-250 °C), reaction time (0.5-3 h) and biomass to water ratio (B/W; 0.07-0.30) on chars produced via hydrothermal carbonization. Multivariate linear regression demonstrated that the three process parameters are critically important to hydrochar solid yield, while B/W drives energy yield. Heating value increased together with temperature and reaction time and was maximized at intermediate B/W (0.14-0.20). Microscopy shows evidence of secondary char formed at higher temperatures and B/W ratios. X-ray diffraction, thermogravimetric data, microscopy and inductively coupled plasma mass spectrometry suggest that calcium oxalate in the raw biomass remains in the hydrochar; at higher temperatures, the mineral decomposes into CO2 and may catalyze char/tar decomposition.