Hydrothermal carbonization (HTC) is a thermochemical pretreatment process where biomass is treated under hot compressed water to produce hydrochar. Hydrochar is a stable, hydrophobic, friable solid product, which has a fuel value similar to that of lignite coal. Among its other advantages, its capability to handle wet feed makes the HTC process most attractive. The complex reaction chemistry of HTC offers a huge potential for producing a variety of products, from fuel to supercapacitors, from carbon nanospheres to low cost adsorbents, from fertilizers to soil amenders. Hydrochar opens possibilities for replacing coal in existing coal-power plants. Its high surface area and adsorption characteristics make it compatible for use in supercapacitors. Hydrochar also contains high amounts of stable carbon and other nutrients, which are essential for soil amendment. Moreover, the HTC process liquid, especially if a short retention time is used, contains potentially toxic substances like phenols, furfurals, and their derivatives, which open opportunities for anaerobic digestion to produce biogas. This review paper gives an overview of the HTC process parameters, reactions, and the use of hydrochar for energy and crop production.

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This experimental work investigates anaerobic digestion of waste water from hydrothermal carbonization of maize silage comparing a continuously stirred-tank reactor (CSTR) and an anaerobic filter (AF). Both reactors were operated for 91 days at a constant organic loading rate of 1 g COD L -1 d . Consecutively lower degradation rates were assumed to be caused by a significant lack of sulfur and phosphorus due to a precipitation by ferrous iron. Over the whole time the AF proved to be more stable. Very small concentrations of phenol compounds contained in the waste water were nevertheless degraded by up to 80 %.

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Process waters obtained from hydrothermal carbonization (HTC) of wheat straw, a biogas digestate derived thereof, and four woody biomass feedstocks were quantified regarding the total organic carbon (TOC) and selected organic compounds. HTC runs revealed that TOC loads were largely unaffected by process severity or type of feedstock whereas the C2–C6 fatty acids, determined by GC, displayed clear effects of temperature and feedstock. HPLC demonstrated simultaneously the initial increase and subsequent consumption of cellulose‐derived furfural and 5‐hydroxymethylfurfural as well as the increase of the lignin‐derived 2‐methoxyphenol. 2‐Methylbenzofuran, an example for a substance potentially harmful to aquatic biota, was observed in high concentration in the HTC liquor from wheat straw‐based feedstocks.

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