In this study, a series of seven residual biomass feedstock was treated by hydrothermal carbonization (HTC) at temperatures of 180 °C, 210 °C, 240 °C, and 270 °C and residence times of 0.5 h, 2 h, and 4 h. The processed samples were analyzed with focus on properties that are relevant for the combustion of a fuel. Temperature was found to have the highest impact on fuel properties. HTC has a positive effect on the energy density of the material, increasing lower heating values typically by 10–15% at 180 °C and 47–59% at 270 °C. At the same time, mass yield was decreasing for increasing treatment temperature. The hydrothermal treatment was found to have a profound impact on the inorganic composition of the fuels, lowering significantly the alkali metal and chlorine content while increasing silicon and phosphorous concentrations in the ash. These transformations lead to improvements in ash melting temperatures and in molar S/Cl ratio, an indicator commonly used to assess the risk of high-temperature corrosion in biomass combustion. HTC is also expected to have a positive impact on fine particle emissions upon combustion due to lowered concentrations of elements responsible for aerosol formation after HTC treatment. On the other hand, HTC leads to higher nitrogen contents in the fuel, thereby potentially increasing the risk for higher NOx emissions upon combustion of HTC-treated fuels. Overall, HTC clearly shows a positive effect on combustion properties, but the effects are fuel specific and especially interesting for biogenic waste that originates from lignocellulosic material. Applying the criteria of this study, the fuel properties of sewage sludge could not significantly be improved. For feedstock like this, the advantage of utilizing HTC as treatment lies in improved dewatering, storage, and feedstock logistics.

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