Hydrothermal liquefaction of sewage sludge under isothermal and fast conditions

- First application of fast HTL and different organic solvents for sewage sludge.
- Showed that water loading in reactor can influence product yields.
- Highest biocrude yield and energy recovery achieved with dichloromethane.
- Fast HTL produces biocrude with higher H/C and O/C ratios and lower N/C ratio.
- Additives (K2CO3, Na2CO3, MoO3-CoO/γ-Al2O3 and Ru/C) decrease the biocrude yield.

We investigated the effects of water loading, sludge moisture content, recovery solvent, and additives on the product yields and compositions from isothermal (673 K, 60 min) and fast (773 K, 1 min) hydrothermal liquefaction (HTL) of sewage sludge. The water loading (which affects pressure within the reactor) plays a small role in product yields. The sludge moisture content had a larger effect with the highest biocrude yields (26.8 wt% from isothermal HTL and 27.5 wt% from fast HTL) being produced from sludge that was 85 wt% moisture. The HTL biocrude from sludge mainly contains long-chain aliphatic hydrocarbon moieties and aliphatic acids. Dichloromethane recovered more biocrude and energy content (∼50%) than did the other solvents tested. Added K2CO3, Na2CO3, HCOOH, CH3COOH, MoO3-CoO/γ-Al2O3 and Ru/C significantly decrease the biocrude yield when their loadings are 50 wt% of the dried sludge. The additives, excepting carbonates, enhance gasification of sludge.