Effects of solvent participation and controlled product separation on biomass liquefaction: A case study of sewage sludge

In this study, the effects of product separation on the distribution of liquid products and the energy efficiency of sewage sludge liquefaction in supercritical alcohol and supercritical alcohol–water mixtures were investigated. While considering alcohol participation in the liquefaction reaction (6–47 wt%), the effects of process parameters such as temperature (300–400 °C), residence time (10–120 min), concentration (9.1–25.0 wt%), and type of supercritical fluid (water, methanol, ethanol, water–alcohol mixture) on the yield and properties of bio-oils were examined. Accounting for alcohol participation and product separation allowed the bio-oil yield, energy recovery, and energy efficiency to be newly defined. Application of the new separation protocol developed in this study realized a 10–25 wt% increase in bio-oil yield because light fractions were efficiently captured. When supercritical methanol was used, the light fractions consisted primarily of methylated short-chain esters, whereas ketones and alcohols were the major species when supercritical ethanol was used. Liquefaction at 400 °C and 20 wt% sewage sludge in a mixture of water–methanol (80:20, v/v) resulted in a bio-oil with a high calorific value (35.8 MJ kg−1), achieving 155% energy recovery and 106% energy efficiency. Computational fluid dynamics (CFD) analysis of bio-oil combustion conducted in a commercial boiler demonstrated that cofiring with a mixture of petroleum heavy oil and bio-oil resulted in a high firing temperature of 1570 °C and a heat transfer rate, which were comparable to that obtained from conventional heavy oil firing.