Hydrothermal Carbonization: A Pilot-Scale Reactor Design for Bio-waste and Sludge Pre-treatment

The purpose of the paper is to illustrate the basis of the design of a pilot-scale reactor built to convert putrescent and high-water content biowaste into a stabilized product by using the Hydrothermal Carbonization process (HTC). The hydrothermal carbonization of selected biowaste has been previously studied in a bench-scale reactor to optimize the process parameters such as the temperature, reaction time, water-to-dry matter ratio and then scaled up at a scale 30 times larger. The new pilot-scale reactor has a volume of 0.1 m3 and has been designed and certified to be operated at 300 °C and 86 bar, allowing a wide range of operating conditions. The design has been structured in two steps: process design (a) and mechanical design (b). The main results of the process design step have been: the installed heat power, the method to provide and control the heating, the minimum reaction time necessary to reach a given yield. The mechanical design focused on the scalability of the reactor, the extraction of reaction products from the reactor at the end of process and increasing of reliability and safety. The designed reactor has been then built, commissioned, and operated in such a way to validate the design criteria and hypotheses. The comparison between the experimental results and the design input dataset confirmed the correctness of the design data input but highlighted that the thermal efficiency of the pilot scale plant was low so indicating the need to enhance it for the demonstrative plant. Graphical Abstract: [Figure not available: see fulltext.]