POTENTIAL OF THE HYDROTHERMAL LIQUEFACTION PROCESS IN THE ENERGY VALORISATION OF TANNERY SLUDGE

Around 600 million m3 of wastewater and 6 million tonnes of leather solid wastes are generated annually worldwide, with a chromium content of 1 to 4%. In this context, the thermochemical valorisation of tannery sludge (TS) by hydrothermal liquefaction (HTL) process represents a promising route both for the reduction of the material to dispose in landfill and for the production of an energy carrier. In fact, HTL process produces bio-crude from wet biomasses in a hot pressurised water environment, thus avoiding the energy-intensive drying step commonly associated to other thermochemical processes. Moreover, HTL, not aiming at the complete oxidation of the organic component, potentially avoids the oxidation of Cr in its harmful hexavalent form. In this study, first, a TS was characterised by a combination of techniques (proximate, ultimate, higher heating value, inorganic elements analyses) and, then, it was originally investigated as substrate for HTL carried out in a 500 mL batch reactor to obtain a bio-crude for energy purposes and reduce the material destined to disposal. More in particular, HTL has been carried out for temperatures in the range 250–350°C and for times up to 1 h. By means of a purposely developed separation protocol, it was possible to determine, as a function of the HTL operating conditions, values of yields for the main products. Calorimetric analyses, finally, allowed to evaluate the energy recovery of the process.