BtL fuel

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BtL fuels are synthetic fuels ( XtL fuels ) that are produced from biomass by means of thermo-chemical conversion . The abbreviation BtL stands for English biomass to liquid , German biomass liquefaction . The processes for BTL production are under development and are not competitive.

Straw compressed into round bales, which can be used as a raw material for BtL fuel production
Short rotation culture from hybrid poplars , which can provide raw materials for BtL fuel production

Principle and application

(see article XtL fuel and Fischer-Tropsch synthesis )

The most important steps of the most common manufacturing process are the gasification of the biomass, in which the so-called synthesis gas is generated, and the subsequent synthesis using the Fischer-Tropsch process or the methanol-to-gasoline process (MtG). The end product can be fuels that are chemically different from conventional fuels such as gasoline or diesel , but can also be used in gasoline or diesel engines . BtL fuels are second generation biofuels . This means that they have a wider range of raw materials than biodiesel or bioethanol . B. cellulose-rich biomass such as straw and wood can also be used. The fuel yield per hectare of cultivated area could thus be increased.

Some other fuels, also produced by biomass liquefaction, are generally not classified as BtL fuels. These are z. B. the fuels produced by bioconversion from sugar , starch or cellulose bioethanol and cellulosic ethanol and furanics . In the case of pyrolytic direct liquefaction of biomass to pyrolysis oil , this can be converted into fuel after processing.

Several process steps are necessary in the production of BtL fuels:

The provision of the raw materials (step 1) differs from the other XtL fuels, which are made from gas or coal. The gasification and gas cleaning steps also differ significantly from the gas to liquid production process, but are similar to the CtL (Coal to Liquid) production process. The synthesis, on the other hand, can proceed in the same way for all XtL preparations, e.g. B. by using the Fischer-Tropsch synthesis .

While the processes for the production of CtL and GtL fuels are established on an industrial scale, processes for the production of BtL fuel are still in development or in an early phase of practical testing. Research is currently being carried out mainly on the production of BtL diesel fuels.

Historical background

(see article Fischer-Tropsch synthesis )

CtL fuels were produced on an industrial scale in the German Reich as early as the 1940s and after the Second World War until today in South Africa. The production of GTL fuels has also been established since the 1990s. In the course of the energy transition, renewable energies and thus also biofuels such as biodiesel , bioethanol and BtL came into focus. Driven by rising prices for fossil fuels at the turn of the millennium, concern about economic dependency on politically unstable producing countries and accompanied by a discussion about a CO 2 -free energy industry, capacities for first-generation biofuels (e.g. biodiesel, bioethanol) were increased in many industrialized countries . built up. As second-generation biofuels, BtL fuels are politically strongly promoted, especially in Europe, but have not yet achieved an economic breakthrough.

Comparison of biofuels in Germany
Biofuel Yield / ha Fuel equivalence
[l]		 Fuel equivalent
per area [l / ha]		 Mileage
[km / ha]
Vegetable oil (rapeseed oil) 1590 l 0.96 1526 23300 + 17600
Biodiesel (rapeseed methyl ester) 1550 l 0.91 1411 23300 + 17600
Bioethanol (wheat) 2760 l 0.65 1794 22400 + 14400
Biomethane (with corn) 3540 kg 1.4 4956 67600
BtL (from energy crops) 4030 l 0.97 3909 64000
BtL (made of straw) 1361 l 0.97 1320 21000
  1. ↑ 1 l of biofuel or 1 kg of biomethane corresponds to this amount of conventional fuel
  2. ↑ without by-products
  3. ↑ separate calculation, not based on the other data
  4. ↑ a b c with biomethane from by-products rapeseed cake / stillage / straw
  5. ↑ a b based on FT fuels


Manufacturing

Process scheme for the production of BtL fuels

Provision and preparation of raw materials

The production of BtL usually begins with drying the highly water-containing biomass. Biomass waste such as straw or residual wood as well as useful plants specially grown for fuel production ( energy crops , e.g. in short rotation plantations ) can be used as the starting material . After the plant parts have been shredded and cleaned, depending on the process and system technology, gasification takes place. It should be noted that often only the calorific values ​​of the substances used are considered. However, since these are determined on a mass-related basis, the different densities of the substances, which, for example in the case of straw, lead to significantly larger volumes of substance to be transported and processed, are usually not taken into account when considering. For example, beech and spruce have almost the same calorific value of around 15 MJ / kg, but the density (volume) differs significantly: 0.77 and 0.44 kg / dm 3, respectively . Taking into account the large volumes that must necessarily be transported and processed, the processing of residues, fast-growing biomass or straw must also be viewed critically.

gasification

In the synthesis processes discussed here, the first step is a thermal cleavage of varying degrees, the pyrolysis . At temperatures of approx. 200 ° C to over 1000 ° C, the physical and chemical structure of the biomass is transformed. Long molecular chains are split by the influence of heat. Numerous different liquid and gaseous hydrocarbons with shorter chain lengths and, as they progress, more and more carbon monoxide , carbon dioxide , carbon and water are formed. While a lack of (air) oxygen prevents complete oxidation to carbon dioxide and water, the other properties of the pyrolysis products can be influenced not only by the primary process conditions of temperature, pressure and residence time in the reactor but also by the chemical reactants and catalysts added . Further variants of the gasification are possible. If the reaction is carried out in a liquid solution that is also a reactant, it is also referred to as solvolysis , whereas in a hydrogen atmosphere it is referred to as hydrogenolysis .

Carbo-V process

This special process is based on a two-stage process, whereby the lumpy biomass is first broken down into coke ( biocoke ) and tar-containing carbonisation gas at 400–500 ° C. While the biochar is being discharged, entrained flow gasification of the carbonization gas takes place at approx. 1500 ° C, so that the longer-chain hydrocarbons can be broken down into simple molecules and thus into a tar-free synthesis gas . The high temperature of this gas is then used to the removed and crushed biochar, now at 900 ° C to also gasify . This means that the raw material can be used more extensively than with other processes. The resulting raw gas is tar-free and, after dedusting and washing, is of a quality similar to that of synthesis gas produced from natural gas.

liquefaction

If the pyrolysis is less complete, a liquid product is created instead of a gas, which is also known as pyrolysis oil . This method could e.g. B. can be used for raw materials with low density, such. B. straw to increase the transportability . Gasification can then take place at the BTL production plant.

synthesis

The next step is the synthesis step, in which the fission products in the synthesis gas are processed into BTL fuel through a chemical reaction. Usually a synthesis based on the Fischer-Tropsch process takes place to produce the BtL fuels.

This procedure was used in the pilot plant of Choren Industries GmbH . The Carbo-V process for the production of biogas was combined with Shell Middle Distillate Synthesis , a further developed Fischer-Tropsch process . Shell is already producing GtL fuel from natural gas on a large industrial scale in Bintulu, Malaysia, and adding it to its “V-Power” fuel.

Another plant on a smaller scale is the plant in Güssing (Austria). Synthesis gas is produced here with a wood fluidized bed gasification system , which is currently still burned in an engine. Work is in progress on the installation of a Fischer-Tropsch system. From spring 2007 there should be gaseous fuel at a gas station. Liquid fuels are to be offered from around autumn 2007.

Product preparation

The product of the synthesis is a mixture of various hydrocarbons. To enable use as a fuel, a treatment is necessary, the z. B. uses processes from petroleum refining (e.g. distillation , rectification ). So z. B. the extraction of BtL gasoline and BtL diesel from the synthesis product. The synthesis can be controlled within a limited framework so that e.g. B. a preferred production of BtL diesel is possible.

Other manufacturing processes

BtL fuel can also be produced using other processes, but these usually have their own name to distinguish them. Sometimes there is no synthesis gas as an intermediate product, but a liquid (pyrolysis oil):

With these processes, a product liquid ( called bio crude oil or biocrude oil ) is created which can mainly contain lipophilic (water-insoluble) substances. In a further step, processing into biofuel takes place with the help of common petrochemical processes .

Corresponding systems currently exist at various locations in Germany. a. at the "Department of Process Engineering" of the HAW Hamburg (pilot plant scale), as well as at some commercial operators ("HP-DoS" process, pre-production).

Other institutes are also working on the development of manufacturing processes, such as B. at the research center Karlsruhe with the bioliq process .

Microbiological production is also conceivable. For example, researchers at the University of Exeter have genetically engineered the Escherichia coli bacterium so that it can produce alkanes and alkenes of various lengths by adding fatty acids, depending on the genetic makeup. Depending on the composition, fuel replicas can then be generated.

Fuel properties

There are currently only measured values ​​from pilot plants. Fischer-Tropsch fuels will have a 7% lower volumetric energy content compared to diesel, a lower viscosity and a significantly higher cetane number .

The emissions from BtL fuel are lower than from fossil fuels (see paragraph environmental impact). BtL diesel or gasoline can be used in common diesel or gasoline engines without extensive conversion.

Environmental impact

Mechanized harvest of a short rotation plantation with a converted maize chopper

In terms of environmental impacts, a distinction can be made between the consequences of biomass cultivation and the emissions resulting from the use of the BtL by burning. For an overall balance, however, the complete process including the complex production must be considered.

Cultivation

The environmental impact of the production of BTL fuel primarily depends on the type of biomass used. As with other biofuels, the use of waste or forest wood tends to reduce the environmental impact. If energy crops are used, however, the environmental impact is higher and the greenhouse gas savings are lower. Another important criterion is the degree of conversion, which also depends on whether the process is still producing electricity and heat. So there is a trade-off between high fuel yields per area on the one hand and the lowest possible environmental impact on the other.

Around 5 to 10 kilograms of wood are necessary to produce 1 kg of BtL. According to initial optimistic estimates, enough biomass can be grown on one hectare of arable land that around 4000 liters of BTL fuel can be produced from it annually. More recent calculations in a European research project resulted in a maximum of 2300 kg BtL fuel per hectare when using short rotation wood . The degree of conversion and area yields vary depending on the process and type of biomass.

use

Theoretically, every diesel engine can also be operated with BtL fuel. The first practical measurements showed a reduced emission of (unburned) hydrocarbons (by 40%) as well as carbon monoxide and soot particles during combustion. The reason is, in contrast to fossil fuels, the lack of aromatic compounds. Since it does not contain any sulfur compounds, emissions are also reduced here. However, a slight loss of performance also had to be accepted. For a complete assessment, however, the additional emissions of these pollutants during fuel production must also be taken into account, which in part offset these lower emissions during combustion.

Advantages and disadvantages

(for general advantages and disadvantages of bioenergy see also article bioenergy )

advantages

  • Improvement of the foreign trade balance in many countries dependent on oil imports, by reducing oil imports.
  • BtL fuels have advantages similar to other renewable energies, such as
    • Reduction of fossil CO 2 emissions
    • Conservation of fossil resources
    • greater independence from energy imports
    • Strengthening the regional economy
  • In theory, any available plant biomass can be used for BtL production, such as plant waste, leaves, residual wood and other previously unused biomass. This also avoids competition for use (e.g. for the use of raw materials for food production) and land competition . However, this also limits the potential of BtL fuels.
  • The annual wood growth in Germany is around 65 million m 3 , which corresponds to a 4% increase in mass. This could theoretically cover around a quarter of the annual diesel demand. However, there is competition for use z. B. for material use ( timber ) and for the production of wood pellets or cellulosic ethanol.
  • The common diesel or gasoline engines can use the corresponding BtL fuels without retrofitting, while other biofuels (ethanol, vegetable oil) may require an adaptation. The existing infrastructure (petrol stations) can still be used.

disadvantage

  • The potential of previously unused biomass is limited. A comprehensive expansion of BtL production would therefore also lead to increased competition for land and use, since agricultural and forestry land would have to be used more for this purpose.
  • The production costs for BtL are estimated to be high, so that with the current production processes, competitiveness with conventional fuels only appears possible with financial support.
  • Another biofuel that is under development is cellulosic ethanol . For its production, raw materials similar to those for BtL would be required. It is unclear which procedure is more suitable.
  • During thermal conversion, depending on the process and by-products (electricity, heat, naphtha ), 30 to 60% of the energy stored in the biomass is lost. The fuel yield per hectare is therefore not necessarily higher than with other biofuels and can vary greatly depending on the starting material and process. In addition, the effort for harvesting, transport, shredding and other things must be taken into account.
  • Since the energy density of the planned raw materials (straw, reeds, bamboo, fast-growing woods, ...) is low, the volumes to be processed are significantly larger compared to fossil fuels, but also to slow-growing hardwoods.
  • The limited catchment areas for the large-volume raw materials (transport costs) require smaller producer units with lower efficiency.

Production and launch

In 2005 Choren Industries agreed with the mineral oil company Shell to build the world's first large-scale production plant for 18 million liters of BTL fuel per year. On July 6, 2011, preliminary insolvency administration was ordered over the assets of Choren Industries. The Carbo V biomass gasification process developed by Choren Industries was acquired from the insolvency estate of Linde AG and further developed.

The industrial policy framework is generally considered to be important, such as E.g. the tax concessions for particularly eligible biofuels such as BTL, cellulosic ethanol and biomethane , which have only been promised until 2015 under the Energy Tax Act .

See also

literature

  • Martin Kaltschmitt, Hans Hartmann and Hermann Hofbauer (eds.), Energy from biomass. Basics, techniques and procedures. Springer Verlag (2009), 2nd edition, pp. 685-690, ISBN 978-3-540-85094-6
  • Norbert Schmitz, Jan Henke, Gernot Klepper: Biofuels: A Comparative Analysis . Ed .: Agency for Renewable Raw Materials . 2nd Edition. Gülzow 2009 ( fnr-server.de [PDF; 2.0 MB ; accessed on January 13, 2017]).

Web links

Individual evidence

  1. ↑ a b c d ESU services: Life cycle assessment of the use of synthetic biofuels ( Memento of the original from March 6, 2009 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / www.esu-services.ch
  2. ↑ a b c d Biofuels Basic Data Germany, as of October 2009 Fachagentur Nachwachsende Rohstoffe e. V. (FNR), Gülzow, 2009, 14-page brochure, available as pdf
  3. ↑ a b c d Biofuels basic data Germany, as of January 2008 Fachagentur Nachwachsende Rohstoffe e. V. (FNR), Gülzow, 2008, brochure, no longer available as a pdf due to the updated version
  4. ↑ ( Page no longer available , search in web archives: European Center for Renewable Energy Güssing ).@1@ 2Template: Dead Link / www.eee-info.net
  5. ↑ Research report 2004/2005. (PDF; 2.0 MB) HAW-Hamburg , p. 33; 38 ff. , Accessed on January 13, 2017 .
  6. ↑ Willner, Th .: Direct liquefaction of biomass using the example of developments at the Hamburg University of Applied Sciences. Gülzower Expert Discussions, Volume 28, Ed. Fachagentur Nachwachsende Rohstoffe, Gülzow 2008, pp. 54–86
  7. ^ Research Center Karlsruhe - Renewable Energies Program
  8. ^ Howard, TP. et al. (2013): Synthesis of customized petroleum-replica fuel molecules by targeted modification of free fatty acid pools in Escherichia coli . In: PNAS 110 (19); 7636-7641; PMID 23610415 ; doi: 10.1073 / pnas.1215966110 ; PDF (free full text access)
  9. ↑ RENEW homepage .
  10. ↑ FNR: Biofuels Basic Data Germany. October 2008 (PDF; 526 kB).
  11. ↑ a b Life Cycle Assessment of BtL-fuel production: Inventory Analysis (PDF; 3.3 MB).
  12. ↑ BtL information platform .