Effect of Pre-Stage Thermal Hydrolysis on the Yield, Chemical Composition, and Thermal Decomposition of Sewage Sludge Hydrochar

Abstract

Hydrothermal carbonization has attracted considerable interest in sewage sludge treatment. However, it suffers from a low degree of carbonization due to limited hydrolysis of carbohydrates and proteins. In this study, pre-stage thermal hydrolysis was adopted to increase the degree of carbonization and obtain high quality hydrochar with low nitrogen and sulfur content. The nitrogen and sulfur content in hydrochars with pre-stage thermal hydrolysis decreased to 1.46-1.63 from 1.71, and to 0.38-0.41 from 0.53, respectively. Thermal decomposition stability of hydrochars were improved with thermal hydrolysis as well. The two major reactions occurred during thermal hydrolysis at 90 oC and 125 oC were decomposition of particulate matter and further oxidation by free radicals, which enhanced organic matter solubilization, reduced the polymerization degree of sewage sludge, and promoted hydrolysis, dehydration, and the Diels-Alder reaction during hydrothermal carbonization, resulting in lower hydrochar yield and higher H/C and O/C atomic ratio. The Maillard reaction occurred during pre-stage thermal hydrolysis at 155 oC led to the formation of large molecular refractory compounds, which retained in the hydrochar and increased the hydrochar yield. Compared to hydrochars with pre-stage thermal hydrolysis at 90 oC and 125 oC, the energy retention of hydrochar with pre-stage thermal hydrolysis at 155 oC was higher, because of higher C, H, N, and O contents and hydrochar yield. Furthermore, pre-stage thermal hydrolysis can accelerate pyrolysis reaction of hydrochars, thus reducing energy consumption. The results of this study could provide valuable, quantitative information for a deeper understanding of hydrothermal carbonization process for sludge treatment.

Keywords: sewage sludge, thermal hydrolysis, Hydrothermal carbonization, hydrochar yield, chemical composition, thermal stability