Hydrothermal carbonization of oily sludge for solid fuel recovery – investigation of chemical characteristics and combustion behaviour

Abstract

The application of hydrothermal carbonization (HTC) as a feasible technology to remediate hazardous oily sludge and generate a renewable solid fuel resource was evaluated in this study. The effect of process variables such as temperature, residence time and solid load on carbonization were investigated. 75.7–78.9 % of energy and 74.81–78.62 % of carbon were retained in hydrochar. The decrease of O/C and H/C ratios to 0.21 and 0.691 respectively was due to the dehydration and decarboxylation reactions. Hydrochar exhibited fuel properties close to lignite at temperatures of 250 °C. FTIR analysis showed that aliphatic species decomposed via hydrolysis, dehydration and decarboxylation reactions while aromatic groups increased due to condensation and polymerization reactions. HTC resulted in higher ignition and burnout temperatures. The hydrochars activation energy in stage 2 and stage 3 were 19.03–25.58 kJ/mol and 43.027–59.31 kJ/mol, respectively. HTC efficiently modified the fuel properties and ignition behavior of oily sludge ensuring sustainable utilization.