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.

Biochar derived from pyrolysis has received much attention recently as a soil additive to sequester carbon and increase soil fertility. Hydrochar, a brown, coal-like substance produced via hydrothermal carbonization, has also been suggested as a beneficial soil additive. However, before soil application, both types of char need to be tested for potential toxic effects. The aim of this study was to develop simple, inexpensive, and easy-to-apply test procedures to identify negative effects of chars but not to provide false-negative results. The following tests, based partly on ISO norm biotoxicity test procedures, were chosen: (i) cress germination test for gaseous phytotoxic emissions; (ii) barley germination and growth test; (iii) salad germination test; and (iv) earthworm avoidance test for toxic substances. Test reproducibility was ensured by carrying out each test procedure three times with the same biochar. Several modifications were necessary to adapt the tests for biochars/hydrochars. The tested biochar did not induce negative effects in any of the tests. In contrast, the beet-root chip hydrochar showed negative effects in all tests. In an extension to the regular procedure, a regrowth of the harvested barley shoots without further nutrient additions yielded positive results for the hydrochar, which initially had negative effects. This implies that the harmful substance(s) must have been degraded or they were water soluble and leached. Tests with a biochar and hydrochar showed that the proposed modified quick-check test procedures provide a fast assessment of risks and effects of char application to soils within a short period of time (<2 wk).

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Hydrothermally converted biomass (hydrochar) is evaluated as a carbon-rich soil amendment in addition to pyrogenic biochar. After assessing the suitability of hydrochar for use in agriculture, its environmental safety and comparing its chemistry with that of biochar, we describe a field trial established at Halle (Germany) under natural conditions for a temperate climate and without further external management practices. The main objective of our study was to analyse the stability and hence the C sequestration potential of composted chars over a period of 2 years. Four treatments (no amendment control, compost, co-composted hydrochar and co-composted biochar) in fourfold field replication were chosen to make a direct comparison of biochar and hydrochar under field conditions. The total organic carbon and total N increased in all treatments in comparison with the control but only in biochar-amended treatments were N concentrations more stable. Composted biochar showed significantly more black carbon content in topsoil, sampled some months after application, compared with all other treatments. We show that hydrochar is less suitable for long-term C sequestration in comparison with biochar but has potential for soil amelioration because it delivers essential nutrients. On the other hand, biochar is richer in polyaromatic C than hydrochar and therefore is more stable in the long term. We assessed biochar stability using the black carbon analysis of the different soil samples.