The Hydrothermal Carbonization Process for Waste Valorisation: a Study on the Effect of Process Conditions on the Yield and Properties of Hydrochars from Municipal Solid Waste

Hydrothermal carbonization (HTC) has received considerable attention in recent years for its potential to process heterogeneous organic wastes with high moisture contents. Unlike other thermochemical processes, HTC does not require drying of the feedstock as a pre-treatment. Performed in aqueous conditions at moderate temperatures under autogenous pressure, HTC produces a carbon rich solid phase, referred to as hydrochar, which can be directly used as a solid fuel, or it can be better exploited for new applications. In the present work the potential valorization by HTC of a waste stream from mechanical biological treatment of mixed municipal solid waste, namely the under-sieve fraction from the mechanical treatment stage, was explored. This stream does not have any potential for material recovery, and it is usually landfilled after aerobic biostabilization. HTC tests were carried out in a laboratory-scale reactor. The joint effect of different process parameters on the yield and physicochemical properties of the hydrochar produced was investigated. The design of experiments (DoE) / response surface methodology (RSM) approach was used for the analysis of the influence of temperature, time and solid load on the mass yield and properties of hydrochar. Quantitative relationships between responses and process parameters were determined. The results obtained demonstrated the feasibility of HTC for mixed municipal solid waste, suggesting HTC as a promising alternative treatment path to current waste management.