Synergistic effect of hydrothermal co-carbonization of sewage sludge with fruit and agricultural wastes on hydrochar fuel quality and combustion behavior.

In order to improve fuel quality of sewage sludge, fruit and agricultural wastes have been selected for hydrothermal co-carbonization. After hydrothermal co-carbonization, organics retention was facilitated, while O/C and H/C atomic ratios of hydrochars were substantially upgraded. Particularly, hydrochar from hydrothermal co-carbonization of sewage sludge with peanut shells at mass ratio of 1:3 (denoted as "SS:PS = 1:3") showed the highest fuel ratio of 0.79 and its carbon content was increased to 50.0% with significantly decreased O/C and H/C atomic ratios. Furthermore, higher heating value of hydrochars from hydrothermal co-carbonization was increased by nearly 2.65-fold and reached 21.72 MJ/kg. Moreover, the most favorable aromatization occurred when sewage sludge and peanut shells blending ratio was 3:1 or 1:1, whereas hydrothermal co-carbonization induced more CO and OH than COOH in hydrochars due to synergistic decarboxylation. A relatively higher value of point of zero charge for hydrochars from hydrothermal co-carbonization implied improved hydrophobicity. Combustion kinetics results indicated that hydrothermal co-carbonization balanced activation energies of hydrochars in devolatilization/combustion stage and char combustion process, rendering a more stable and lasting combustion profile. Hydrochars "SS:PS = 1:3" demonstrated desirable combustion performance. Therefore, hydrothermal co-carbonization can realize sustainable utilization of organic solid wastes towards superior hydrochar solid biofuels.