Reducing agent from biomass sludges using hydrothermal carbonization and upgrading

Hydrothermal carbonization (HTC) is a thermochemical process which utilizes liquid phase to convert biomass into char like material. This material is also known as hydrochar or HTC-char and resembles lignite coal. Biosludges are biomass originating from plants and/or animals such as digestate and industrial side streams. Biosludge itself is a challenging raw material to utilize as it has a high moisture content and poor properties which limit the applications. Using moderate temperatures (180–280 °C) and in high autogenous pressure HTC can produce solid biocoal and compound rich process water which can further be used in different applications. Biocoals are under active research, and they have the potential, for example, to replace fossil fuels such as coal, coke, and oil in steelmaking industries. Ferrochrome is the main ingredient of producing stainless steel. Growing demand of steel together with the carbon neutrality targets drives the industry to develop more environmentally friendlier ways to produce steel. Using HTC-char as a reducing agent and a fuel would give environmental benefits for steel making process as well as give a way to utilize biosludges giving them more value. As HTC process depends on the feed material, it is hard to predict what kind of hydrochars can be obtained. Pyrolysis could be one option to upgrade the hydrochar with poorer properties to reach the standard for steel industry. Direct pyrolysis of wet biosuldges is not considered feasible as the energy is mostly used for the drying of biosludge before pyrolysis can occur. Thus HTC offers a possibility to utilize hardly used biosludges and produces highly valuable biocoal that has many possible applications.