Liquefaction of sewage sludge in ethanol-water mixed solvents for bio-oil and biochar products

Abstract

Liquefaction of sewage sludge (SS) is a promising technology for the preparation of bio-oil (target product) and biochar (by-product). The liquefaction of SS in ethanol-water cosolvent were preliminarily investigated in this study. There existed synergetic effects between ethanol and water on the liquefaction of SS. Higher bio-oil yield (33.5–36.1 wt%) were obtained in ethanol-water cosolvent, compared to those in pure water (32.2 wt%) and pure ethanol (26.8 wt%). Furthermore, the bio-oil obtained in ethanol-water cosolvent contained higher content of ester components, suggesting higher bio-oil quality. The addition of Na2CO3 further increased the yield of bio-oil (37.5 wt% versus 34.9 wt%), but lowered the quality of bio-oil. The main two types of coke oils in bio-oils were diesel oils and lubricating oils. Elemental, proximate and micromorphology analyses of biochars further indicated that ethanol-water cosolvent had better degradation/transformation effects on SS. The biochars obtained from the liquefaction of SS in ethanol-water cosolvent possessed similar specific surface area (23.0–26.0 m2/g) and pore volume (0.118–0.141 cm3/g) to SS pyrolysis biochar. In view of the yield and quality of liquefaction products, ethanol-water cosolvent may be more proper for the liquefaction of SS, compared to pure water/ethanol.