Hydrothermal carbonization characteristics of sewage sludge and lignocellulosic biomass. A comparative study

Abstract This study evaluates and compares the responses of two kinds of lignocellulosic biomass (pine and acacia) and sewage sludge to their hydrothermal carbonization treatment. All materials were subjected to HTC processes in an aqueous environment at 200 °C for 4 h with a 1:8 biomass to water ratio, using a Zipperclave Stirred Reactor. The physical, chemical analyses and, additionally, fibre, mercury porosimetry analyses and optical microscope images applied to both raw materials and to hydrochars, proved that HTC enhances the energy related properties of lignocellulosic biomass. Whereas, for sewage sludge, the HTC effect is negligible, because most of the carbon is transferred to the HTC liquid phase, confirmed by high COD and high ash content in the sewage sludge hydrochar. Subsequently, the thermo-chemical conversion kinetics of the raw and HTC materials were examined in the TGA analyser through nonisothermal runs. Finally, kinetics analyses were performed by two methods based on the TGA results. The estimated apparent activation energy values obtained by the Coats & Redfern method were higher than those given by the Arrhenius method, which also gave almost similar values for all samples, shedding doubt on the accuracy of this method for the current experiments. The Coats & Redfern analysis gave activation energy values which were lower by one half for sewage sludge hydrochars compared to lignocellulosic hydrochars. This indicates the potential of the HTC process for generating sewage sludge hydrochars suitable for co-combustion or co-gasification with other biomass solid fuels.