Economic sustainability of hydrothermal liquefaction of sewage sludge: From a conceptual analysis to a practical verification

Introduction: Hydrothermal liquefaction (HTL) could be a promising technology to produce biocrude from wet biomass. A conceptual analysis on the HTL of microalgae assisted by the use of solar heat demonstrated that even if microalgae are one of the most productive and investigated feedstock, their high cost hinders the economic sustainability of the process1. To overcome this issue a zero-cost wet waste biomass like sewage sludge (SS) could be adopted with comparable efficiency. Furthermore, the use of a proper catalyst, promoting the in-situ upgrading of the biocrude, still represents a big challenging opportunity for industrial HTL development. Aim: The outcomes of the conceptual analysis and the need to find out the economic sustainability of HTL prompted us to study of HTL of SS both in batch reactors and in a continuously operated plant reproducing operative conditions more similar to an industrial plant also to investigate the benefits of solar heating assisted HTL of SS. Methods: SS provided by the wastewater treatment plant of Karlsruhe, Germany were used in HTL experiments. HCOOH and KOH were selected as catalysts. A slurry at 10%w/w of dry SS was used as feedstock and the catalyst was added at 10%w/w (based on dry SS). Batch runs were performed at 350°C for 10 min in an AISI 316Ti high-pressure reactor with an internal volume of 25mL, processing 10 g of slurry in each experiment. A sand bath was used to heat the reactor until 350°C. A continuous plug-flow reactor (350 mL of volume) was implemented to build a HTL plant able to work at 20 MPa and 350°C with a flow rate of slurry of 2 kg/h. The procedures adopted to separate the products downstream of HTL experiments were an optimization of those used in a previous work. Using them a hydrocarbon fraction can be separated from the biocrude and quantified.2 Results: The highest yield of biocrude (27% w/w dry ash-free) and yield of gaseous phase (20% w/w) was obtained in batch experiments using KOH as catalyst. In all catalytic experiments an increase of CH4 mol% in the produced gas phase was detected. Preliminary results with the continuous plant showed that, when KOH was added to the slurry, a stable and easily pumping suspension was obtained. Moreover, using reaction conditions similar to those considered in the conceptual analysis1 we found that it is possible to reduce the minimum fuel selling price of the biocrude of about 38%. Conclusion: The results collected from this practical verification are in agreement with those obtained by the already published conceptual analysis on HTL of microalgae assisted by solar heat pushing forward the practical development of HTL of SS on the industrial scale.