Comparison of Fuels and Effluents Originating from Washing and Hydrothermal Carbonisation of Residual Biomass

A series of four different biomass feedstock was washed and hydrothermally carbonized at temperatures of 50 °C and 150–270 °C for four hours, respectively. For the first time both the resulting solid and liquid products were characterised and evaluated in a comprehensive study. Concerning fuel properties, HTC had a higher impact on the fuel quality than washing. HTC yielded hydrochar with higher carbon content than the starting material leading to a significant increase in heating value, while washing only had a minor effect on elemental composition and heating value. Treatment temperature was found to have the highest impact on LHV and elemental composition. Both washing and HTC proved effective in reducing potassium and chlorine content, while earth alkaline, phosphorous and silicon removal was limited. Process water characterisation revealed that filtrates from washing and HTC are acidic, with acidity being increased by HTC. Electrical conductivity of the effluent was found to correlate with the amount of electrolytes Na, K, Mg and Ca in the feedstock, thereby being feedstock dependent. COD, BOD₅ and TOC values determined revealed that effluent from both washing and HTC is strongly contaminated by organic matter. The organic load was significantly higher in HTC effluents. Feedstock type was found to be the main influencing factor on effluent characteristics rather than HTC temperature. Nutrients were found in low concentrations. Graphic Abstract: [Figure not available: see fulltext.]