Fate of biomass inorganic elements during hydrothermal carbonisation: an experimental study on several agrofood waste

The distribution of inorganic elements between solid and liquid phases during biomass hydrothermal carbonization (HTC) is a topic still poorly investigated despite its importance for process optimization. To fill in this gap, in this study, the distribution of inorganic elements and their form were determined using ICP-OES, colorimetry methods and XRD for three agrofood waste feedstocks converted at HTC temperatures of 180, 220 and 260 °C during 12 hours. At 180 °C, over 90% of P, Mg, Ca, K and Na were removed from hydrochars whatever feedstock. At higher temperatures, these elements were partly reincorporated into hydrochars, possibly due to the formation of insoluble precipitates. Some minor elements, i.e. Cu, Zn and Al, mostly remained in the hydrochars, whatever temperature. Si showed different removal behaviors according to feedstock and temperature. These results show the possibility to optimize the removal of inorganic elements from hydrochars using different temperatures.