Catalytic hydrothermal liquefaction of municipal sludge assisted by formic acid for the production of next-generation fuels

Hydrothermal liquefaction (HTL) of municipal sludge (MS) was studied at 325 C and 30 min at 10 and 30% w/w concentration of MS using formic acid (FA) as a green liquid hydrogen donor both in the absence and in the presence of heterogeneous catalysts. Pellets of commercial NiMo/Al2O3, CoMo/Al2O3 and felt of activated carbon ACF 1600 were used as catalysts. The addition of FA at high MS loading allowed to increase the yields in BC from 41 to 62% and its H/C ratio from 1.80 to 2.01 leading to energy recovery (ER) higher than 100%.When heterogeneous catalysts were used together with FA, a marked improvement of BC yields and quality was obtained at 10% MS loading. This behavior was not observed at 30% w/w concentration probably for the fouling of the catalyst surface. Experimental results obtained in this study indicates that FA can be an effective additive to improve BC yields and quality in the HTL of MS at high loading, that is a very promising result in the perspective of industrial utilization of the process.