Ultimate and Proximate Correlations for Estimating the Higher Heating Value of Hydrothermal Solids

Hydrothermal conversion processes aiming at the generation of renewable fuels have attracted much attention in recent years, because they require no predrying and limited pretreatment of the biomass. At relatively low temperatures, hydrothermal carbonization (HTC) produces a coal-like solid fuel (HTC-char), that reaches higher heating values (HHV) which are comparable to the HHV of hard-coal. Nevertheless, until now it has not been examined if the HHV of these HTC-chars could be estimated adequately using available (semi)empirical correlations, that had been established on the basis of other fuels. In this work both ultimate and proximate correlations for the estimation of HHV were studied and applied to chars generated in HTC-experiments. HTC runs were performed for poplar, straw, and grass at final temperatures between 180 and 240 C and residence times between 2 and 6 h. The proximate correlations considered in this investigation lead to average bias errors of 17.0% and 12.8%. Ultimate correlations lead to significantly lower average bias errors of 0.4% and 4.0%. A new proximate correlation was suggested for HTC-chars with an average bias error of -1.1%: HHV = 0.4108.FC + 0.1934.VM 0.0211.ash.