Hydrothermal processing of fermentation residues in a continuous multistage rig – Operational challenges for liquefaction,salt separation,and catalytic gasification

Fermentation residues are a waste stream of biomethane production containing substantial amounts of organic matter, and thus representing a primary energy source which is mostly unused. For the first time this feedstock was tested for catalytic gasification in supercritical water (T ≥ 374 °C, p ≥ 22 MPa) for methane production. The processing steps include hydrothermal liquefaction, salt separation, as well as catalytic gasification over a ruthenium catalyst in supercritical water.In continuous experiments at a feed rate of 1 kg h⁻¹ a partial liquefaction and carbonization of some of the solids was observed. Significant amounts of heavy tars were formed. Around 50% of the feed carbon remained in the rig. Furthermore, a homogeneous coke was formed, presumably originating from condensed tars. The mineralization of sulfur and its separation in the salt separator was insufficient, because most of the sulfur was still organically bound after liquefaction.Desalination was observed at a salt separator set point temperature of 450 °C and 28 MPa; however, some of the salts could not be withdrawn as a concentrated brine. At 430 °C no salt separation took place. Higher temperatures in the salt separator were found to promote tar and coke formation, resulting in conflicting process requirements for efficient biomass liquefaction and desalination. In the salt separator effluent, solid crystals identified as struvite (magnesium ammonium phosphate) were found. This is the first report of struvite formation from a supercritical water biomass conversion process and represents an important finding for producing a fertilizer from the separated salt brine.