Biomass gasification

The articles biomass gasification and wood gas overlap thematically. Help me to better differentiate or merge the articles (→ instructions ) . To do this, take part in the relevant redundancy discussion . Please remove this module only after the redundancy has been completely processed and do not forget to include the relevant entry on the redundancy discussion page{{ Done | 1 = ~~~~}}to mark. DF5GO • ☎ • 02:26, 15 Sep. 2013 (CEST)

Wood gasifier Güssing (2006)

Wood gasification plant Villach, 15.5 MW BWL, (2010)

Biomass gasification refers to a thermo-chemical conversion of biomass into a combustible product gas ( fuel gas ) with the help of a gasification or oxidizing agent (mostly air , oxygen , carbon dioxide or water vapor ).

Since wood is primarily used as biomass both historically and currently, it is usually referred to as wood gasification.

Via gasification , the biomass present as solid fuel can be converted into a gaseous secondary fuel , which can be used in various options such as power generation or as a power and fuel (fuel gas), or for use as synthesis gas for chemical synthesis can be used more efficiently. Similar processes also exist for other solid fuels, especially for the gasification of coal ( coal gasification ).

Ultimately, every combustion process of biomass is preceded by a gasification process, since it is not the biomass itself that is combustible, but basically only the gases emerging from the biomass.

history

Wood gasifier on an Opel P4 (1940)

Wood gasifier locomotive in the Bavarian Railway Museum in Nördlingen. The vehicle was built by Deutz in 1916 for the benzene operation and converted by Imbert in Cologne in 1935

Wood gasification was developed by Philippe Lebon at the end of the 18th century .

In the 19th century many important raw materials for the chemical industry such as methanol (wood spirit), acetone and acetic acid were produced through charring . The main product of charring, charcoal , still finds important areas of application as activated charcoal .

Especially in times of war and crisis when there is a lack of fuel, vehicles are usually equipped with an improvised wood gasifier on their own initiative. Even the Deutsche Reichsbahn tested the use of charcoal gasifiers on shunting locomotives of the Köf II series in the 1930s and 1940s (see also: gas engine ).

Adler Diplomat 3 GS with wood gas generator (1941)
Wood gas drive in Berlin (1946)
Front left view

Wood gas was used, among other things, internal combustion engines of motor vehicles to drive . The generators were built on the outside of the body or carried as trailers. The technical system for this, the wood gasifier, was filled with firewood and functioned as a fixed bed gasifier. When the wood was heated, the combustible gas mixture ( wood gas ) was released, the components of which consisted mainly of the non-combustible nitrogen in the air, carbon dioxide , combustible carbon monoxide (together approx. 85%) and methane as well as smaller proportions of ethylene and hydrogen . Until the early 1950s, a number of small trucks with a special driver's license were in use in Germany for which only tested and approved beech logs could be used. About one liter of gasoline could be replaced by the amount of gas obtained from 3 kg of wood.

Opel Kadett City with wood gas drive
Ford Model A 1928 with wood gas drive
Wood gas powered truck
Chevrolet HS 157D truck built in Sweden in 1937
Modern car with wood gasifier

In the context of the discussion about the increasing use of renewable raw materials at the end of the 20th and the beginning of the 21st century, wood gasification and the gasification of other organic substances, especially organic residues, for the production of gaseous fuels for heat and power generation were revived discussed and implemented in individual demonstration systems. Based on this purely energetic use, the use of the product gas as a raw material for the chemical synthesis of biofuels and products of the chemical industry was targeted and is to be implemented in the near future, especially for BtL fuels , dimethyl ether and methanol. Subsequent methanation and processing means that it can also be fed into the natural gas network as Substitute Natural Gas (SNG). High-quality product gases that contain more than 50% hydrogen are also referred to as biohydrogen .

Efficient boilers with wood gasifiers are now also being offered for private households.

process description

The gasification of biomass begins after drying at temperatures of 150 ° C, with water vapor and oxygen first escaping. At higher temperatures, the solid components of the biomass, especially the lignin and cellulose , are gasified. This gas ignites as soon as secondary air is supplied, the ignition temperature is 230 ° C to 280 ° C.

Technical biomass gasification involves partial combustion with the aid of a gasification agent or oxidizing agent (usually air , oxygen , carbon dioxide or water vapor ) without ignition at temperatures of 700 ° C to 900 ° C, which is not the case with combustion to carbon dioxide (CO ₂ ), but is essentially oxidized to carbon monoxide (CO) . Further components of the resulting gas are hydrogen (H ₂ ), carbon dioxide (CO ₂ ), methane (CH ₄ ), water vapor (H ₂ O) and, depending on the biomass used and the gasification process, a number of organic substances in different concentrations. As a solid residue remains ashes and residues of biochar . When the temperature of the process gas drops, the water vapor, mixed with organic components, condenses to a tar or to an organically contaminated wood gas condensate .

The combustible product gas can be further oxidized in a subsequent process by combustion (fuel gas) or chemical synthesis ( synthesis gas ) with the release of energy ( exothermic process ). If the gasification with air, which thereby is with nitrogen diluted product gas often called lean gas designated (LCV, low calorific value gas).

Hydrothermal gasification

Hydrothermal gasification is a special case of biomass gasification, in which wet biomass is to be converted into hydrogen and methane. The bioconversion takes place at temperatures from 400 ° C to 700 ° C and pressures from 200 bar to 300 bar through the reaction with supercritical water , whereby an almost complete conversion of the organic components of the biomass is achieved.

Excess air ratio

The excess air ratio of the gasification process and hydrothermal gasification is less than one and greater than zero, while it is greater than or equal to one for complete combustion and zero for pyrolysis . ${\ displaystyle \ lambda}$

raw materials

The raw materials used in biomass gasification are mainly lignocellulose-rich agricultural raw materials , as well as residual forest wood , residual wood , but also z. B. sewage sludge , ditten and horse droppings into consideration. The first is primarily stalk-like biomass such as cereal and corn stalks and other residual products from agricultural cereal cultivation. In addition, there are energy crops such as giant Chinese reed ( Miscanthus × giganteus ) as well as poplar and willow wood from short rotation plantations . The use of organic waste streams from industry and household z. B. waste wood and green waste (kitchen waste, garden waste, lawn cuttings, leaves, shrub and tree cuttings) for biomass gasification is under discussion and even waste (plastics) materials can be recycled.

Previous biomass gasification plants are designed for the gasification of wood in the form of forest wood and residual wood, which are supplied as wood chips . This form of biomass gasification is called wood gasification.

The biomass must be pretreated for gasification. This happens after the provision by drying and crushing the biomass into particles, which should have the largest possible surface and a small volume in the gasifier. In particular for use in the entrained-flow reactor, the particles need to dust and finely ground in a slurry are transferred.

technology

Most wood gasification systems generate the energy required for gasification by partially burning the wood with a lack of air. Depending on the gasification agent used, different product gases with correspondingly different quality are created, which are important for the subsequent use. When using air (21% oxygen, 79% nitrogen), the product gas contains a very high nitrogen content, which does not contribute to the calorific value of the gas and reduces the hydrogen yield. In contrast, when using oxygen and water vapor, the product gases do not contain nitrogen and accordingly have a higher calorific value and a high hydrogen yield.

The generated gas is cooled, whereby water vapor and hydrocarbons condense, and filtered, after which it is sent to its use. The processing and cleaning of this condensate is an essential part of the system technology in large plants, as it contains organic pollutants ( phenols or ammonium compounds ) that have to be disposed of in a targeted manner, for example in a sewage treatment plant or thermal post-combustion .

Allothermal and autothermal gasification

On the basis of the type of heat provision for the gasification process, gasifier types can be divided into allothermal and autothermal gasifiers. With the allothermal heat supply, the process heat is supplied from the outside, while with the autothermal variant it is generated through partial combustion of the feedstock.

With allothermal gasification, the heat required for the gasification process is introduced via a heat exchanger. More recent developments work with heat pipes , which have a high heat flow density. The main advantage of this process is the generation of process gas with a high calorific value (mainly hydrogen and carbon monoxide). Due to the allothermal heat input, the process gas is not exposed to additional flue gas from the combustion.

A similar process is the introduction of heat through water vapor or through the introduction of particles. In most cases, the gasification residues ( coke ) are fed to the steam generator as an energy medium. This increases the cold gas efficiency considerably.

Carburetor types

Different technical gasifiers can be used for biomass gasification , which differ mainly in the type of contact between biomass and gasification agent (air, oxygen or water vapor). As a rule, three basic reactor types are used:

Fixed bed gasifier

Principle of the fixed bed gasifier boiler

In the fixed bed gasifier, the fuels lie on a grate like in a normal furnace .

Wood gas production by means of a countercurrent process

Wood gas production using the co-current process

In a countercurrent process , the air is sucked through the grating and the burning wood. The layers of wood above burn only partially and smolder into product gas, which is sucked off at the top of the furnace. Air and product gas move in the opposite direction (in countercurrent) to the slowly sinking wood. The resulting gas has a relatively low temperature of usually around 100 ° C and, due to the drying and charring of the wood, contains a correspondingly large amount of water vapor and organic components, which condense to wood gas condensate when further cooling . The condensate is usually quite acidic with a pH value of around 3, which is mainly caused by components of formic and acetic acid .

In the cocurrent process , the air is fed directly above the grate into the hot gasification zone of the furnace and extracted under the grate. Product gas and air move in the same direction in the area of the grating (in direct current ). The temperature of the product gas is much higher here (several hundred ° C) and because the gas has a very high temperature before it leaves the furnace , it contains significantly fewer organic components in the condensate. The condensate here has slightly basic pH values, which can be traced back to ammonium compounds that arise in the reducing atmosphere of the hot zone (due to the lack of oxygen) .

Fluidized bed gasifier

The fluidized bed gasifier is in principle a fluidized bed furnace that is operated with a lack of air and thus delivers the desired product gas due to the incomplete combustion of the wood as waste gas. The fuels are introduced into the combustion chamber with a particle size of less than 40 millimeters, i.e. in the form of wood chips or sawdust , and a water content of at least 25%, and mixed with hot sand. The product gas is produced at a temperature of around 900 ° C.

This technology is mainly used in energy systems in the power range from 1.5 to 3 MW, the electrical efficiency is around 30% and thus significantly higher than with conventional, biomass-fired steam power plants.

Entrained flow carburetor

In the entrained flow gasifier, the fuel is introduced into the gasification chamber as dust, slurry or paste via a burner, whereby the gasification processes take place in a so-called dust cloud. This form of supply requires appropriate pre-treatment of the biomass in order to be introduced into the gasifier via a pneumatic system and to be gasified there in a very short time.

Gas use

The gas produced in the biomass gasification can be used both energetically and materially.

Energetic use through combustion

The currently common use for the gas mixture from biomass gasification is combustion in appropriate incineration plants to generate heat (steam) and electricity , with a very high degree of energy conversion efficiency being achieved through a combined heat and power system.

Alternatively, the gas mixture from biomass gasification can be converted directly into electricity in solid oxide fuel cells . The design of the gas cleaning depends on the gasification process used and the type of solid oxide fuel cell used. The operating principle of this technology was already proven in 2004 in tests with countercurrent gasifiers and planar solid oxide fuel cells.

Except for the direct combustion of the still hot gas (implemented in modern wood-burning systems in two-stage combustion , in which the wood is gasified in the first stage with a lack of oxygen, in the second stage with an excess of oxygen for complete and low-pollutant combustion) there is no gas cooling all other usage processes in not inconsiderable amounts (approx. 0.5 liters per kg of wood used) wood gas condensate , which must be disposed of in an environmentally friendly manner in our own facilities, e.g. B. in sewage treatment plants.

Use as synthesis gas

In addition, a product gas composed of carbon monoxide and hydrogen can be used as a synthesis gas for the chemical synthesis of various products . The material use of synthesis gas from biomass gasification is currently still in development; such systems are currently only found on a laboratory and demonstration scale. The large-scale production and use of CO / H ₂ synthesis gas currently takes place exclusively on the basis of natural gas and other fossil fuels such as coal and naphtha .

The chemical-technical options for use are primarily the production of hydrogen and the subsequent production of ammonia using the Haber-Bosch process , methanol synthesis , various oxo syntheses and the production of biofuels ( BtL fuels ) and other products via fishermen -Tropsch synthesis :

Process scheme for the production of BtL fuels

in ammonia synthesis using the Haber-Bosch process
- ${\ displaystyle \ mathrm {N_ {2} +3 \ H_ {2} \ longrightarrow 2 \ NH_ {3}}}$
in methanol synthesis
- ${\ displaystyle \ mathrm {CO + 2 \ H_ {2} \ longrightarrow CH_ {3} OH}}$
in oxo synthesis
- ${\ displaystyle \ mathrm {R {-} CH {=} CH_ {2} + CO + H_ {2} \ longrightarrow R {-} CH_ {2} CH_ {2} CH {=} O}}$
in the Fischer-Tropsch synthesis
- ${\ displaystyle \ mathrm {n \ CO + (2n + 1) \ H_ {2} \ longrightarrow C_ {n} H_ {2n + 2} + n \ H_ {2} O}}$

In addition to these chemical-technical areas of application, synthesis gas can also be used biotechnologically via synthesis gas fermentation . Products of this option can e.g. Alcohols like ethanol , butanol , acetone , organic acids and biopolymers . This use is currently still in the development stage and is accordingly not yet used on a large scale.

Biofuels

In the production of biofuels, too, the product gas produced in the gasification process is used as synthesis gas in the synthesis processes already described. The focus is on gaseous fuels such as bio-hydrogen , substitute natural gas (methane, SNG) and dimethyl ether as well as liquid fuels such as methanol and BTL fuels.

Biohydrogen is obtained from the synthesis gas by means of steam reforming , methane can be produced by methanation of the gas. For the production of methanol and dimethyl ether is methanol synthesis employed. BTL fuels are produced using the Fischer-Tropsch synthesis , whereby both gasoline and diesel fractions can be produced due to the process parameters .

literature

Hermann Hofbauer, Alexander Vogel, Martin Kaltschmitt : Gasification. In: Martin Kaltschmitt, Hans Hartmann, Hermann Hofbauer (Hrsg.): Energy from biomass. Basics, techniques and procedures. Springer Verlag, Berlin and Heidelberg 2009, pp. 599-669, ISBN 978-3-540-85094-6 .
Heinz Hiller among others: Gas Production. In: Ullmann's Encyclopedia of Industrial Chemistry . Wiley-VCH, Weinheim 2005, doi : 10.1002 / 14356007.a02_143.pub2 .
Christopher Higman, Maarten van der Burgt: Gasification. 2nd edition, Gulf Professional Publishing 2008. ISBN 978-0-7506-8528-3 .
T. Metz: Allothermal gasification of biomass in indirectly heated fluidized beds. Technical University of Munich, 2006, online (PDF; 2.76 MB), at energetische-biomassenutzen.de, accessed on January 15, 2017.
Ralf Röger : Environmental issues in the construction of wood gasification plants. In: Reinhard Hendler, Peter Marburger, Michael Reinhardt, Meinhard Schröder (ed.): Yearbook of environmental and technology law. 2005, pp. 361-381, ISBN 978-3-503-08753-2 .

Web links

Commons : wood gasification - collection of pictures, videos and audio files

Biomass gasification on energie-lexikon.info, accessed on January 15, 2017.
Market overview of log gasifiers 2012 (PDF file; 3.05 MB) Agency for Renewable Raw Materials FNR
Wood gasifiers for combined heat and power plants - Article by Fraunhofer-Gesellschaft contains FhG contact address
Gasification of biomass Working group of the support company for renewable energies
Opel Kadett and Ford Model A, powered by wood gas, exemplify the effort
Report Joos-Vergaser (scientific endurance test by engineering office)

Individual evidence

↑ ^a ^b ^c ^d ^e ^f Hermann Hofbauer, Martin Kaltschmitt, Thomas Nussbauer: Thermo-chemical conversion processes. In: Martin Kaltschmitt, Hans Hartmann, Hermann Hofbauer (Hrsg.): Energy from biomass. Basics, techniques and procedures. Springer Verlag, Berlin and Heidelberg 2009; Pp. 377-378. ISBN 978-3-540-85094-6 .

↑ ^a ^b ^c ^d Sven Geitmann: Renewable energies. Hydrogeit-Verlag, Oberkrämer 2010; Pp. 149-150. ISBN 978-3937863146

↑ FTP: Energy of the Future: Electricity from Fir and Spruce ( Memento from September 15, 2013 in the web archive archive.today ), April 28, 2013, accessed on September 15, 2013.

↑ The first gas heating on wasistwas.de, accessed on September 15, 2013.

↑ The construction of a wood gasification boiler . on holzvergaser.org. Archived from the original on November 27, 2015. Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved May 16, 2015. @1@ 2

↑ Annex on sewage sludge gasification (PDF; 486 kB), on aoew.de, accessed on January 18, 2017.

↑ Biogas from horse droppings on energiezukunft.eu, accessed on January 18, 2017.

↑ Generating synthesis gas from waste plastics (PDF; 992 kB), on bine.info, accessed on January 18, 2017.

^ Hermann Hofbauer, Alexander Vogel, Martin Kaltschmitt: Vergasung. Gasification technology. In: Martin Kaltschmitt, Hans Hartmann, Hermann Hofbauer (Hrsg.): Energy from biomass. Basics, techniques and procedures. Springer Verlag, Berlin and Heidelberg 2009; Pp. 600-601. ISBN 978-3-540-85094-6 .

↑ Florian Nagel: Electricity from wood through the combination of gasification and solid oxide fuel cells. Dissertation, Eidgenössische Technische Hochschule ETH Zurich, No. 17856, 2008, doi : 10.3929 / ethz-a-005773119 .

^ Hermann Hofbauer, Alexander Vogel, Martin Kaltschmitt: Vergasung. In: Martin Kaltschmitt, Hans Hartmann, Hermann Hofbauer (Hrsg.): Energy from biomass. Basics, techniques and procedures. Springer Verlag, Berlin and Heidelberg 2009; Pp. 599-600. ISBN 978-3-540-85094-6 .

[Kaltschmitt_378-1] ↑ ^a ^b ^c ^d ^e ^f Hermann Hofbauer, Martin Kaltschmitt, Thomas Nussbauer: Thermo-chemical conversion processes. In: Martin Kaltschmitt, Hans Hartmann, Hermann Hofbauer (Hrsg.): Energy from biomass. Basics, techniques and procedures. Springer Verlag, Berlin and Heidelberg 2009; Pp. 377-378. ISBN 978-3-540-85094-6 .

[Geitmann-2] Sven Geitmann: Renewable energies. Hydrogeit-Verlag, Oberkrämer 2010; Pp. 149-150. ISBN 978-3937863146

[3] FTP: Energy of the Future: Electricity from Fir and Spruce ( Memento from September 15, 2013 in the web archive archive.today ), April 28, 2013, accessed on September 15, 2013.

[4] The first gas heating on wasistwas.de, accessed on September 15, 2013.