Hydrothermal carbonation

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The hydrothermal carbonization (HTC process, for example: "aqueous carbonization at elevated temperature"), the product also referred to as hydrocarbon , is a chemical process for the simple production of brown coal substitute, synthesis gas , liquid petroleum precursors and humus from biomass with release of energy and water. The process that technically imitates the lignite formation (" coalification ") that takes place in nature in 50,000 to 50 million years within a few hours was researched by Friedrich Bergius and first described in 1913.

motivation

In the processes used up to now for converting biomass into fuels, the carbon efficiency , i. H. the proportion of the carbon initially contained in the biomass, which is later contained in the usable end product, is relatively low: in alcoholic fermentation the carbon efficiency is 67%, in anaerobic conversion to biogas 50% and in charcoal production by charring approx. 30% ; in the production of humus through composting , the carbon efficiency is only 5 to 10%. The unused portion escapes into the atmosphere as carbon dioxide or, during fermentation, as methane ; both gases are considered harmful to the climate . In addition, these processes release heat that has not previously been used.

The problem with producing biodiesel from oil plants is the fact that only the energy contained in the fruit can be used; If, on the other hand, the whole plant is used for fuel production , then when growing fast-growing plants such as willow wood , poplar , Chinese reed , hemp , reed or forest wood , the energy yield can be increased by a factor of three to five with the same cultivation area while reducing energy and fertilizer and herbicide use and the possibility of using - for previous energy crop cultivation - too poor soils. The 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 production.

procedure

In a pressure vessel , biomass , in particular plant material, (described in the following reaction equation as sugar with the formula C 6 H 12 O 6 ) is heated together with water to 180 ° C. The pressure rises to around 1 MPa (10 bar). During the reaction, oxonium ions are also formed, which lower the pH value to pH 5 and below. This step can be accelerated by adding a small amount of citric acid . It should be noted that at low pH values, more carbon is transferred into the aqueous phase. The reaction taking place is exothermic ; H. energy is released. After 12 hours, the carbon in the starting materials has completely reacted, 90 to 99% of the carbon is in the form of an aqueous sludge of porous lignite spheres (C 6 H 2 O) with pore sizes between 8 and 20  nm as a solid phase, the remaining 1 to 10 % Carbon is either dissolved in the aqueous phase or has been converted to carbon dioxide . The reaction equation for the formation of lignite is:

The reaction can be terminated in several stages if the elimination of water is incomplete, with different intermediate products being obtained. If the process is terminated after a few minutes, liquid intermediates, lipophilic substances, are formed, which are very difficult to handle because of their high reactivity. Subsequently, polymerize these materials and are formed torfähnliche structures present as intermediates for about 8 hours.

Theoretically, the reaction could be catalyzed with certain metal particles , but these would be added very quickly with the products and lose their function.

Efficiency

The exothermic reaction of the hydrothermal carbonization releases about 3/8 of the calorific value of the biomass in relation to the dry matter (with a high lignin, resin and / or oil content still at least 1/4). With skilful process management, it could be possible to use this waste heat to produce dry biochar from wet biomass and to use part of the converted energy to generate energy.

In the large-scale implementation of the hydrothermal carbonization of sewage sludge , it was demonstrated that around 20% of the fuel energy content contained in the fuel energy content is required for the process to supply heat to produce 90% fully dried HTC coal. Furthermore, around 5% of the generated energy content is necessary for the electrical operation of the system. The HTC process has proven to be particularly advantageous that mechanical dewatering can achieve a dry matter content of more than 60% in the raw coal and thus the energy and equipment expenditure for the final drying of the coal is low compared to conventional drying processes for this sludge.

The energy requirement of the HTC compared to a sewage sludge digestion with subsequent drying is around 20% of the electrical energy and around 70% of the thermal energy lower. The amount of energy produced, which is available at HTC as storable coal, is also 10% higher. Compared to the conventional thermal drying of sewage sludge, the HTC saves 62% in electricity and 69% in thermal energy due to the significantly easier dewatering.

Use

An exothermic process design would be advantageous in which the carbon content remains biologically, chemically or thermally convertible without further oxidation of the biomass . This could be for specific reduction of CO 2 - release lead.

According to Markus Antonietti , the most important point is "... that you have a simple method in hand to convert atmospheric CO 2 into a stable and harmless storage form, a carbon sink, via the detour of biomass." With the process of hydrothermal carbonization As with other methods of coking biomass, a large amount of carbon could be stored decentrally all over the world. With sufficient chemical stability of the coal, it could also be used very well to improve soils (see also Terra preta ).

The artificially produced humus could be used to re- green eroded areas. The increased plant growth in this way could bind additional carbon dioxide from the atmosphere, so that in the end a carbon efficiency greater than 1 or a negative CO 2 balance could be achieved. The resulting coal sludge could be used for combustion or for operating new types of fuel cells with an efficiency of 60%, as is currently being researched at Harvard University . To generate conventional fuels, the carbon-water mixture would first have to be heated more intensely, so that so-called synthesis gas , a gas mixture of carbon monoxide and hydrogen , is created:

This synthesis gas could be used to produce gasoline using the Fischer-Tropsch process . Alternatively, the liquid intermediates that result from the incomplete conversion of the biomass could be used for fuel and plastics production.

In addition, the resulting coal sludge can be briquetted and marketed as environmentally friendly - because it is carbon dioxide-neutral - "natural coal", which, compared with the starting biomass, should be able to be dried by means of separation or filtering or pressing with lower energy input and due to its higher energy content per volume or mass cause less transport costs and require smaller storage space.

One advantage of hydrothermal carbonization is that the usability of plant biomass is not limited to plants with low moisture contents and the energy that can be obtained without carbon dioxide emissions is not reduced by the necessary drying measures, but can be used directly to dry the end products if required. So even previously hardly usable plant material such as clippings from gardens and urban green spaces can be used to generate energy, while at the same time saving carbon dioxide, which would otherwise - together with the even more climate-damaging methane - be produced during the bacterial conversion of the biomass.

In recent years, hydrothermal carbonization has also been used as a digestion process for recovering phosphorus from sewage sludge, which is intended to increase the recovery rate significantly.

Problems

The major problem in the production of synthesis gas from biomass is the formation of tar , which could indeed be avoided with hydrothermal process management. However, it cannot then be seen why the detour via biochar should be taken. A biomass slurry should break down into CO 2 and H 2 under supercritical conditions at 400 ° C and a pressure of at least 221.2 bar ( critical temperature of water is 374 ° C) , which, however, requires a high amount of energy.

A suitable process management as well as problems with the collection, transport and storage of biomass are not clarified. These processes also require energy; this should be less than is released by the hydrothermal carbonization.

An advantage over dry thermal processes for refining biofuels with a low moisture content is not so easily recognizable. Already at the end of the 19th century, only weakly pyrolyzed charcoal, which still contains at least 4/5 of the calorific value of wood, was propagated for thermal processes.

Current application advances

In Relzow near Anklam ( Mecklenburg-Western Pomerania ), a hydrothermal carbonation plant was officially put into operation in the local "Innovationspark Western Pomerania" in mid-November 2017. According to the company involved, the HTC plant in Relzow represents a “new stage in the field of hydrothermal carbonization” and is currently “the world's first industrial plant that produces so-called biochar”. In the summer of 2016, an HTC plant for processing sewage sludge was put into operation in Jining (China), which, according to the manufacturer, has been processing 14,000 tons annually into regenerative coal for the local power plant.

Thomas Maschmeyer , Professor of Chemistry at the University of Sydney , is working on a catalytic hydrothermal reactor that converts plastic into new raw materials within 20 minutes with little energy. A first industrial plant is to be built in East Timor .

See also

Web links

literature

  • Tobias Helmut Freitag: Hydrothermal carbonization. Student work, Grin, 2011, ISBN 978-3-656-07822-7 .
  • XJ Cui, M. Antonietti, SH Yu: Structural Effects of Iron Oxide Nanoparticles and Iron Ions on the Hydrothermal Carbonization of Starch and Rice Carbohydrates . In: Small . tape 2 , no. 6 , 2006, p. 756-759 , doi : 10.1002 / smll.200600047 .
  • SH Yu, XJ Cui, LL Li, K. Li, B. Yu, M. Antonietti, H. Colfen: From Starch to Metal / Carbon Hybrid Nanostructures: Hydrothermal Metal-Catalyzed Carbonization . In: Advanced Materials . tape 16 , no. 18 , 2004, p. 1636-1640 , doi : 10.1002 / adma.200400522 .

Individual evidence

  1. ↑ Friedrich Carl Rudolf Bergius: Use of high pressure in chemical processes and the reproduction of the process of formation of hard coal. W. Knapp, Halle aS 1913, OCLC 250146190 .
  2. ↑ Peter Brandt: The "hydrothermal carbonization": a remarkable possibility to minimize or even avoid the formation of CO 2 ? In: J. Verbr. Lebensm. 4 (2009): pp. 151-154, doi: 10.1007 / s00003-009-0472-7 .
  3. ↑ Marc Buttmann: Climate-friendly coal through hydrothermal carbonization of biomass . In: Chemical Engineer Technology . tape 83 , no. 11 , 2011, p. 1890-1896 , doi : 10.1002 / cite.201100126 .
  4. ↑ P. Jeitz, O. Deiss: New ways in sewage sludge treatment. In: Aqua & Gas. No. 4, 2012, pp. 42-45.
  5. ↑ T. Kläusli: Study confirms the advantages of hydrothermal carbonization of sewage sludge. In: Garbage and Garbage. March 2014.
  6. ↑ Tobias Wittmann: Refining biomass into fuel. ( Memento from September 11, 2012 in the web archive archive.today ), In: Energy 2.0. Edition 01/2011.
  7. ↑ German Phosphor Platform eV: TerraNova® Ultra process. In: www.deutsche-phosphor-plattform.de. Deutsche Phosphor Plattform eV, May 1, 2018, accessed on March 26, 2019 .
  8. ↑ Official launch of the AVA HTC plans in Relzow , ava-htc.com, Nov. 20, 2017
  9. ↑ The energy revolution starts in Western Pomerania , Nordkurier, Nov. 16, 2017
  10. ↑ Dorothee dos Santos: Positive interim results for TerraNova Ultra process for sewage sludge treatment. In: EUWID New Energy News. November 17, 2017. Retrieved March 26, 2019 .
  11. ↑ ABC: Chemical recycling plant to open in Timor-Leste , May 17, 2019 , accessed on May 17, 2019.
  12. ↑ Brisbane Times: East Timor at the forefront of fixing the global recycling crisis , May 17, 2019 , accessed May 17, 2019.
  13. ↑ University of Sydney: A new plastic recycling technology converts a liability into an asset , accessed May 17, 2019.