Co-hydrothermal carbonization of digested sewage sludge and cow dung biogas residue: Investigation of the reaction characteristics

Highlights: • Digested sewage sludge (DSS) and cow dung biogas residue (CDBR) were recovered energy using co-HTC method. • The reducing O/C and H/C atomic ratios revealed that the dehydration was main reaction during the co-HTC. • The interaction of DSS and CDBR can be confirmed for TOC value of mixture was far exceeded that of the raw materials. • The combustion properties of hydrochar were improved. -- Abstract: In order to recovery energy from digested sewage sludge (DSS) and cow dung biogas residue (CDBR), co-hydrothermal carbonization (co-HTC) was employed to obtain the hydrochar for the use as solid fuel. Carbon content and higher heating value of hydrochars were increased, while the O/C and H/C atomic ratios reduced, revealing the dominance of the dehydration reaction during the co-HTC of DSS and CDBR. The interaction between DSS and CDBR or their derivate intermediates took place during the co-HTC for the TOC value exceeded far more than that in the raw materials and the intensity of mixture was lower than pure material. The CDBR can notably promote hydrochar formation during the co-HTC with DSS, resulting in a high fraction of nitrogen content in hydrochar of DSS. The combustion properties of hydrochar were improved notably when CDBR was added, evidencing by the increasing ignition temperature and the comprehensive devolatilization index. The activation energy of the hydrochar from co-HTC increased notably especially the hydrochar with 75% CDBR. This study suggested that co-HTC of the mixed feedstocks was a promising strategy to prepare a high-quality hydrochar for recovery energy.