Hydrothermal co-carbonization of sewage sludge and fuel additives: Combustion performance of hydrochar

Hydrothermal treatment improves dewaterability of sewage sludge, but its solid product (hydrochar) requires enhancement for energy production. Hydrothermal co-carbonization (co-HTC) of sewage sludge and fuel additives could be a successful solution, and in addition boost dewaterability. Thus, sewage sludge with charcoal (10% db), oak sawdust (10% db) and fir sawdust (10% and 20% db) was hydrothermally carbonized. Prior to and after the process, the physical and chemical properties of samples were analyzed and compared. Capillary suction time and filtration tests were conducted in terms of dewaterability. The fuel properties of hydrochars, were determined, namely ultimate and proximate analyses, higher heating values and thermal analysis. Based on the ash composition the operating risk indexes were found. Additionally, the combustion kinetic and comprehensive combustibility indexes were calculated. Concluding, the addition of biomass to the co-HTC process halved the time required for the filtration process and improved dewaterability to 41% moisture content. The higher heating value of hydrochar derived from sewage sludge and 20% fir addition, increased by approximately 6%. Moreover, all additives are believed to provide a more stable combustion process demonstrated by higher values of carbon content (from 34.9% to 37.9%) and lower values of volatile matter (from 56.4% to 40.7%).