Hydrothermal carbonization (HTC) (also referred to as "aqueous carbonization at elevated temperature and pressure") is a chemical process for the conversion of organic compounds to structured carbons. It can be used to make a wide variety of nanostructured carbons, simple production of brown coal substitute, synthesis gas, liquid petroleum precursors, and humus from biomass with the release of energy. Technically, the process imitates, within a few hours, the brown coal formation process (German "Inkohlung", literally "coalification"), which takes place in nature over enormously longer geological periods of 50,000 to 50 million years. It was investigated by Friedrich Bergius and first described in 1913.
Motivation The carbon efficiency of most processes for converting organic matter into fuel is relatively low, i.e., the proportion of carbon contained in the biomass, which is later contained in the usable end product, is relatively low:
In poorly designed systems, the unused carbon escapes into the atmosphere as carbon dioxide, or, when fermented, as methane. Both gases are greenhouse gases with methane even more climate-active on a per molecule basis than CO2. In addition, the heat that is released in these processes is not generally used. Advanced modern systems capture nearly all the gases and use the heat as part of the process or for district heating. The problem with the production of biodiesel from oil plants is that only the energy contained in the fruit can be used. If the entire plant could be used for fuel production, the energy yield could be increased by a factor of three to five with the same cultivation area when growing fast-growing plants such as willow, poplar, miscanthus, hemp, reeds or forestry, while simultaneously reducing energy, fertilizer and herbicide use, with the possibility of using – for current energy plant cultivation – poor soil. Hydrothermal carbonization makes it possible - similar to the biomass-to-liquid process – to use almost all of the carbon contained in the biomass for fuel generation. It is a new variation of an old field (biomass conversion to biofuel) that has recently been further developed in Germany. It involves moderate temperatures and pressures over an aqueous solution of biomass in a dilute acid for several hours. The resulting matter reportedly captures 100% of the carbon in a "charcoal" powder that could provide a feed source for soil amendment (similar to biochar) and further studies in economic nanomaterial production.
Process Biomass is heated together with water to 180 °C (356 °F) in a pressure vessel, in particular vegetable material (in the following reaction equation, simplified as sugar with the formula C6H12O6). The pressure rises to about 1 megapascal (150 psi). During the reaction, oxonium ions are also formed, which reduce the pH to pH 5 and lower. This step can be accelerated by adding a small amount of citric acid. In this case, at low pH values, more carbon passes into the aqueous phase. The effluent reaction is exothermic, that is, energy is released. After 12 hours, the carbon of the reactants is completely reacted, 90 to 99% of the carbon is present as an aqueous sludge of porous brown coal spheres (C6H2O) with pore sizes between 8 and 20 nm as a solid phase, the remaining 1 to 10% of carbon is either dissolved in the aqueous phase or converted to carbon dioxide. The reaction equation for the formation of brown coal is:
C 6 H 12 O 6 → C 6 H 2 O + 5 H 2 O Δ H = − 1.105 k J / m o l {\displaystyle \mathrm {C_{6}H_{12}O_{6}} \quad \rightarrow \quad \mathrm {C_{6}H_{2}O} +\mathrm {5\ H_{2}O\qquad \Delta H=-1.105\ \mathrm {kJ/mol} } }
The reaction can be stopped at several stages, leading to incomplete elimination of water and yielding different intermediate products. After a few minutes, liquid intermediate lipophilic substances form, but they are very difficult to handle due to their high reactivity. Subsequently, these substances polymerize and form peat-like structures, which are present as intermediates after about 8 hours.
Efficiency As a result of the exothermic reaction of hydrothermal carbonization, about 3/8 of the calorific value of the biomass based on the dry mass is released (with a high lignin, resin and/or oil content of at least 1/4). If the process is managed properly, it is possible to use this waste heat from wet biomass to produce dry biocoal and to use some of the converted energy for energy generation. In a large-scale technical implementation of hydrothermal carbonization of sewage sludge, it has been shown that about 20% of the fuel energy content contained in 90% end-dried HTC coal is required to heat the process. Furthermore, approximately 5% of the generated energy is required for the plant's electrical operation. It has proved particularly beneficial in the case of the HTC process that, with mechanical dehydration, more than 60% of the dry substance content can be achieved in the raw carbon, and thus the energy and equipment expenditure for the final drying of the coal is low compared to conventional drying methods of these slurries. Compared to sludge digestion with subsequent drying, the HTC requires approximately 20% less electrical energy and approximately 70% less thermal energy. The amount of energy produced by the HTC as a storable coal is simultaneously 10% higher. Compared to conventional thermal drying of sewage sludge, the HTC saves 62% of electricity and 69% of thermal energy due to its significantly simpler drainage.
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