Hydrothermal liquefaction of sewage sludge: towards the effects of transition metals in the presence of homogeneous hydrogen producers

Waste carbonaceous matrices, such as sewage sludge (SS) seem to have a great potential, because they are namely costless matrices compared to the microalgae and hydrothermal liquefaction (HTL) can be considered an interesting route to dispose and valorize them [1-5]. However, SS are characterized by high amount of non-biogenic fractions, such as plastics, salts and metals. In particular SS contains a high amount of inorganics [5], such as Al, Ba, Ca, Cr, Cu, Fe, Ni, K, Mg, Mn, Na, P, S, Ti, Zn, Si which are distributed in the products at the end of the reaction. The presence of heavy metals represent a big challenge for the implementation of HTL on the industrial scale as they can affect product quality and/or the performances of the process. One of the hurdles of the transfer of the HTL from the lab to an industrial plant fed by SS is represented by the fact that the metals in the feedstock can be transferred to the biocrude (BC) affecting its placing on the market [6]. Some studies analyzed the distribution of metals in the phases produced by HTL of real SS and the main evidence was that the higher the reaction temperature, the higher is the fraction of metals entrapped in the produced BC [5, 7]. However, metals can positively affect the process behavior acting as catalysts [5]. Among the metals contained in residual biomass [6-8], Ni, Zn and Fe were selected as model compounds in this work and a systematic investigation was conducted to study their effect, individually and combined with homogenous hydrogen producer as formic acid (FA) and KOH, on the biocrude yield and quality.