Waste Reduction and Bioenergy Generation from Secondary Sludge Using Hydrothermal Liquefaction

Given its global accessibility and high organic content, biogenic waste such as sewage sludge currently represents a valuable renewable resource for energy production. Hydrothermal liquefaction (HTL) stands out as one of the most suitable technologies to convert these feedstocks into biocrude, a valuable biofuel precursor. This process operates at moderate temperatures and high pressure in the presence of water, eliminating the need for energy-intensive preliminary dewatering steps when treating high-moisture feedstocks. This study focuses on investigating the effects of various reaction temperatures and holding times on biocrude yield and quality obtained from batch HTL conversion of digested secondary sludge derived from paper mill facilities. Experiments were conducted at temperatures of 280, 300 and 330 °C, with holding times ranging from 0 to 35 minutes. Optimal conditions were identified at 300 °C and holding times between 10 and 35 minutes, resulting in biocrude yields of 20-21%, higher heating values (HHV) of 35 MJ kg-1 and energy recovery of 54-59%. The findings underscore the potential application of HTL in waste biomass disposal cycles, contributing to waste minimisation and enhancing the bioenergy recovery.