Abstract
Hydrothermal carbonization (HTC) is a promising approach for the fuel upgrading of moist biomass wastes. However, its effects on high-ash municipal sewage sludge (HA-MSS), which is produced largely in China, are unclear. In this study, HA-MSS was treated at various temperatures from 170 °C to 350 °C. The fuel characteristics of hydrochar products, namely, chemical compositions; heat value; water-holding capacity; mineral components; and chemical bonding forms of carbon, nitrogen, and sulfur, were comprehensively discussed. Low-temperature HTC below 260 °C decreased the amounts but only slightly changed the species of mineral components in HA-MSS. However, temperatures exceeding 290 °C removed some alkali and alkaline earth minerals completely and formed a new species, namely, anorthite (CaAl2Si2O8). In terms of fuel characteristics, 230 °C was determined to be the optimal temperature for the HTC of HA-MSS. At this temperature, the HA-MSS was upgraded to a hydrochar with a large higher heating value and good dewaterability. When the hydrochars were combusted, SO2 and NOx were mainly released during the devolatilization stage of fragile volatiles. With the increase in HTC temperature, the releasing peak of SO2 decreased. However, for hydrochars obtained at HTC temperatures lower than 260 °C, the releasing peaks of NOx showed only a small difference.
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