Hydrothermal carbonization

The hydrothermal carbonization ( approximately: " aqueous charring at elevated temperature") is a chemical process for the simple production of brown coal, the synthesis gas, liquid petroleum precursors and humus from biomass under release of energy. The process, which technically mimics the running in nature into 50,000 million to 50 million years lignite formation ( " coalification " ) within a few hours, was explored by Friedrich Bergius and first described in 1913.

Motivation

The problem in the manufacture of biodiesel from vegetable oil is the fact that only the energy contained in the fruit can be used; could, however, use the whole plant for fuel production, as could be the cultivation of fast-growing plants such as willow, poplar, miscanthus, hemp, reeds or forest wood energy yield for the same acreage by a factor of three to five increase with simultaneous reduction of energy, fertilizer - and the use of herbicides and the ability to use - for existing energy crops - to poor soils. The hydrothermal carbonization allows it - similar to the biomass-to- liquid process - almost to use all of the carbon contained in the biomass for fuel production.

Expiration

In a pressure vessel is biomass, especially plant material ( described as sugar with the formula C6 H12 O6 of simplicity in the following reaction equation ) together with water at 180 ° C heated. The pressure rises to about 1 MPa ( 10 bar). During the reaction also oxonium ions are formed, which lower the pH to pH 5 and below. This step can be accelerated by adding a small amount of citric acid. It must be noted that, at low pH values ​​more carbon goes into the aqueous phase. The ongoing reaction is exothermic, i.e. energy is released. After 12 hours, the carbon of the starting materials is fully implemented, 90 to 99 % of the carbon is present as aqueous slurry of porous lignite beads ( C6H2O ) with pore sizes of 8-20 nm as the solid phase before the remaining 1-10 % of carbon are either dissolved in the aqueous phase, or have been converted to carbon dioxide. The reaction equation for the formation of brown coal is:

The reaction can be terminated in several stages of incomplete elimination of water to yield different intermediates. If you cancel after a few minutes arise liquid intermediates, lipophilic substances whose handling due to their high reactivity, however, is very difficult. Following that polymerize these materials and form peat -like structures are present after 8 hours as intermediates.

Theoretically, the reaction could be catalyzed by certain metal particles, this would be rather quickly added with the products and lose their function.

Efficiency

Due to the exothermic reaction of the hydrothermal carbonization about 3/ 8 of the calculated on the dry mass calorific value of the biomass are released (with high lignin, resin and / or oil content is still at least 1 /4). With skillful litigation could succeed, by means of this waste heat from wet biomass to produce biochar dry and use a portion of the converted energy for power generation.

In large-scale implementation of the hydrothermal carbonization of sludge has been demonstrated that for the production of a finally dried to 90 % carbon about 20 % of the HTC contains fuel energy content required to heat the process. Further about 5% of the generated energy content for the electrical operation of the system are necessary. Particularly advantageously, has proven the HTC method that with a mechanical dewatering already more than 60 % dry matter content in the raw coal can be achieved, and thus the energy and equipment costs for the final drying of the coal compared to traditional drying process of this sludge is low.

The energy demand of HTC is lower compared to a sludge digestion followed by drying at approximately 20 % of the electrical energy and approximately 70 % of the thermal energy. The produced amount of energy that is present as a storable coal in HTC, is also higher by 10 %.

Benefit

Would be advantageous to design an exothermic process, in which the carbon content is maintained biologically, chemically or thermally convertible without further oxidation of the biomass. This could lead to the specific reduction of CO2 release.

According to Markus Antonietti is the most important point, " ... that one has in hand a simple method to transform atmospheric CO2 via the detour of biomass in a stable and safe form of storage, a carbon sink. " With the process of hydrothermal carbonization as well as with other methods for carbonization of biomass, would be almost anywhere in the world decentralized a large amount of carbon store them permanently. Much safer than the currently discussed liquid or gaseous sequestration of carbon dioxide. With sufficient chemical stability of the coal it could also very well be used to improve soils ( see also Terra preta ).

The humus artificially produced could be used for revegetation of eroded areas. Due to the increased plant growth in this way additional carbon dioxide could be bound from the atmosphere, so that, in effect, a carbon efficiency would be greater than 1 or a negative CO2 balance reached. The resulting coal slurry could be used with an efficiency of 60 %, as they are currently being researched at Harvard University for combustion or for the operation of new types of fuel cells. For the production of conventional fuel -water mixture, the carbon may have to be initially heated more, so that so-called synthesis gas, a gas mixture of carbon monoxide and hydrogen is generated, :

For this synthesis gas could be produced gasoline via the Fischer- Tropsch process. Alternatively, the liquid intermediates that arise from the incomplete conversion of the biomass, for fuel and be used for plastics production.

In addition, the resulting coal slurry can briquetted and as environmentally friendly - because carbon neutral - " natural charcoal " are marketed, which should be dry with lower energy consumption in comparison with the initial biomass by means of deposition or filters or pressing and by their higher energy content per volume or mass would cause less transport costs and require smaller storage areas.

One advantage of the hydrothermal carbonization is that the availability of plant biomass is not limited to plants with low moisture contents and the recoverable without carbon dioxide emissions energy is not reduced by measures necessary drying and is available directly to the drying of the final products as required. Thus, even so far hardly usable plant material as waste from gardens and serve urban green areas for energy production, while at the same carbon dioxide is saved, which otherwise - would be incurred in the implementation of the bacterial biomass - together with the still klimaschädlicheren methane.

Problems

The big problem in the production of synthesis gas from biomass tar formation, which could be at hydrothermal process control in fact avoided. However, it is then no reason why this detour is to be gone through biochar. A biomass slurry should be under supercritical conditions at 400 ° C and a pressure of at least 221.2 bar ( critical temperature of water is 374 ° C) can be decomposed into CO2 and H2, which, however, a large amount of energy due.

Unclear at this issue are a suitable process control as well as problems in the collection, transport and storage incurred biomass. These processes also require power, this should be less than that released by the hydrothermal carbonization.

An advantage over dry thermal process of refining of biofuels with a low moisture content is not as easy recognizable. At the end of the 19th century was a weak pyrolysed charcoal, which still contains at least 4/5 of the calorific value of the wood, propagated for thermal processes.