Two "biochars" from different production processes are used in the TOPKO project: hydrochar and biochar.

Hydrochar is produced using the hydrothermal carbonisation process, or HTC for short. Here, the (moist) biomass is mixed with water and heated to around 180 to 240°C in a pressure-tight vessel and "cooked" for several hours. The liquid and solid phases can be separated by simple filtration. The peat to coal-like product has similar properties to peat: it has a low pH value (around pH 5), low nutrient and salt content depending on the biomass used and has many chemical compounds on the surface so that nutrients and water are available to plants. Hydrochar is usually mineralised after a few years (decomposition by bacteria and fungi to CO₂ and nutrients) and is therefore only used for short-term carbon storage. The main reactants for the hydrochars at the end of the project will be residual materials. These include, for example, growths from paludiculture (reeds, cattails, sedges) or grass-like growths from landscape conservation areas. However, plant (residues) such as Miscathus or shives from fibre nettles also offer great potential here.

Biochar is usually produced using the pyrolysis process. Biomass (mainly wood or wood-like) with a low water content is "partially burnt" in a pyrolysis furnace at temperatures between 500 and 900 °C, i.e. part of the biomass carbon is burnt and provides the high process temperatures. The other part of the biomass carbon is turned into coal at high temperatures and in the absence of oxygen. Pyrolysis charcoal differs greatly from hydrochar: the pH value is well above pH 7, the nutrient and salt content is high, biochar is very stable against degradation by bacteria and fungi and can therefore contribute to long-term (centuries) carbon storage. With biochar, too, attention is paid to the origin of the source materials. The focus here is on wood cuttings from landscape conservation or regionally occurring fermentation residues from biogas plants, which are produced in large quantities but have so far had few lucrative uses.

Images: Grass-like landscaping material (left) is converted into hydrochar (centre). The hydrochar compost substrate in a planting trial with marigolds (right). M. Röhrdanz