Effect of transition metals and homogeneous hydrogen producers in the hydrothermal liquefaction of sewage sludge

Hydrothermal liquefaction (HTL) of sewage sludge (SS) was performed in the presence of metallic Zn, Fe and Ni to investigate their effect on the performances of the process in terms of product yields and quality. Experiments were performed in subcritical water at 350 ◦C for 10 min using each metal individually and in the presence of homogenous hydrogen producers like formic acid (FA) and KOH. Interesting results were obtained with Zn that when used alone or in the presence of KOH, increased the cumulative biocrude (BC) yields and resulted in energy recoveries (ER) higher than 100% with respect to initial energy content of the biomass, thus indicating that HTL of SS is globally endothermic. Moreover, when Zn was used with FA it strongly enhanced gas yield leading to a significant hydrogen production that was determined to be generated mainly from SS or water. Ni chips were used in two consecutive HTL experiments with no decrease in BC yields and ER. Collected results suggest that metallic powders can be interesting catalyst to improve yield and quality of BC in particular when FA acid was used as hydrogen vector.