Impact of hydrothermal carbonization on combustion properties of residual biomass

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.