Utilizing Hydrothermal Carbonization for Sustainable Processing and Reuse of Oily Sludge

Abstract

Hydrothermal carbonization (HTC) is emerging as a promising technology for converting waste biomass into a dense coal-like product known as hydrochar. Oily sludge (OS), known for its toxic, carcinogenic, and mutagenic effects, is classified as hazardous waste, posing negative implications for both humans and the ecosystem if improperly disposed. Consequently, the conversion of hazardous oily sludge into safe and renewable products offers significant benefits. To investigate the HTC process further, a design matrix consisting of 20 experimental runs was utilized, integrating results obtained from HTC experiments as input for response surface methodology (RSM). The variables were restricted to a temperature range of 175°C–250°C, a time span of 0.5–2 h, and a solid load range of 3%–8%. Analyzing the data, it was observed that raw sludge initially exhibited a hydrogen-to-carbon (H/C) ratio of 1.33 and an oxygen-to-carbon (O/C) ratio of 0.59. However, following HTC processing, these ratios decreased significantly to 0.18 (at 250°C, 2 h, 3% solid load) and 0.691 (at 250°C, 1 h, 5.5% solid load), respectively. These decreases can be attributed to dehydration and decarboxylation reactions that occurred during the HTC process. The model equation developed using RSM was: carbon content = 66.29 + 6.98A + 0.3515B–2.60C – 0.0412AB + 0.1730AC – 0.0037BC – 1.83A2 + 0.8005B2 – 0.945C2. Carbon densification increased from 1.09 to 1.45 with hydrochar retaining 66.5%–77.65% of the carbon. HTC holds promise as a sustainable technology for carbon and energy recovery.