Hydrothermal processes

Hydrothermal processes (HTP) enable diverse and attractive applications. HTP efficiently convert biomass - and in particular water-containing, biogenic residual and waste materials - under increased pressure and temperature into high-quality refined solid, liquid or gaseous carbon carriers. A large potential of starting materials can be used to make a wide range of products available. They thus make a sustainable contribution to resource protection and are an essential component of the bioeconomy.

 


Hydrothermal processes can be divided into hydrothermal carbonisation (HTC), hydrothermal liquefaction (HTL) and hydrothermal gasification (HTG), since different products are created with increasing temperatures and pressures.

The work at the DBFZ mainly concentrates on carbonisation and liquefaction. Due to the use of numerous test facilities, a large number of research projects have already been successfully carried out. The expertise acquired at the DBFZ is based primarily on the use of biogenic residues.

Hydrothermal carbonisation (HTC)

Hydrothermal carbonisation (HTC) converts wet biogenic residues into so-called HTC char. The process uses the changed properties of liquid water at high temperatures (180-280 °C) and high pressure (10-65 bar). In addition to its properties as a solvent, water also functions as a reaction partner and heat carrier.

Biomass is rich in chemically bound oxygen. During HTC, so-called dehydration reactions take place, in which the chemically bound oxygen is split off in the form of water. What remains is the carbon contained in the biomass, now in significantly increased concentration, which is why we can speak of coal.

The resulting HTC char can be used for various applications. In addition to energetic use (combustion or gasification), it can also be used for material purposes, e.g. as activated carbon or soil additive. In addition, the HTC process enables the recovery of valuable nutrients such as nitrogen and phosphorus when using nutrient-rich, biogenic residues such as sewage sludge or liquid manure. This residues are the largest domestic sources of phosphorus for agriculture, but their application on agricultural land causes high nitrogen and phosphorus emissions to water and air. Nutrient recovery would help to sustainably close material cycles and protect the environment.

During HTC, biobased basic chemicals are also produced in the process water. Through controlled process conditions such interesting compounds as 5-hydroxymethylfurfural (HMF), furfural and levulinic acid can be obtained. These are suitable, for example, for the production of plastics or serve as fuel precursors or additives and can replace fossil products. The HTC process water can also be used for biogas production.

Hydrothermal liquefaction (HTL)

Compared to HTC, hydrothermal liquefaction takes place at higher temperatures (250-400 °C) and pressures (30-250 bar) as well as shorter reaction times (5-60 min). Due to the much more drastic reaction conditions, the biomass is almost completely liquefied. This liquid consists of two phases: an aqueous and an oily phase, also called bio-crude. While the aqueous phase contains mainly organic acids, the oily phase contains non-polar and aromatic hydrocarbons. The bio-crude oil can thus reach a calorific value of between 30 and 36 MJ kg-1 depending on the starting material used. However, the oil still contains an oxygen content between 10 and 20 %. By removing the oxygen with hydrogen (hydrogenation) and subsequent fractionation (rectification), the bio-oil can be processed into various fuels (kerosene, petrol, diesel etc.).

 

Copyright: Biowaste: Schumacher, DBFZ / Cow: Großmann, pixelio / Water treatment plant: nonameman, fotolia.com / Biogas plant: wulff, pixelio.de / Coal: Römerscheid, DBFZ / Nutrient recycling: berwis, pixelio.de / Gasoline pump: Lione, pixelio.de