Biohydrogen
As bio-hydrogen (H2 ) is called, which is produced from biomass or means of living biomass. Produced by Kvaerner process, hydrogen from natural gas, the usual form of the industrial production of hydrogen will not be referred to as bio-hydrogen, even though the energy required originates from biomass.
Hydrogen is an energy- rich gas which can be used eg in fuel cells for power generation, as a fuel in internal combustion engines or in the chemical industry. Currently, the energy use plays no economically relevant role. As part of the energy revolution hydrogen is discussed as a storage and transport form of energy in a so-called hydrogen economy.
Production
The production of hydrogen requires energy derived in biohydrogen from either the raw material used as biomass or solar energy that is absorbed by living biomass during photosynthesis. Secondly, the element hydrogen is required. This comes from the biomass used as a raw material or is supplied as a part of the water production, or production process.
Production from biomass
The production of hydrogen from biomass can be accomplished by biological and chemical processes:
Fermentation
In the laboratory - scale can be formed by fermenting bacteria in addition to CO2 and oxidized organic compounds and H2 from energy-rich organic compounds in the biomass (eg carbohydrates, fats, proteins). In this anaerobic process may be developed by the bacteria only a portion of the energy contained in the biomass, since oxygen is not available as an oxidizing agent. The bio-hydrogen formed may thus contain a large proportion of the remaining energy.
Thermochemical processing
On an industrial scale can biohydrogen from biomass ( wood, straw, grass clippings, etc.) but also from other sources of bioenergy (biogas, bioethanol, etc. ) by thermochemical processing ( gasification or pyrolysis ) and subsequent or direct steam reforming ( "steam reforming " ) made be. The synthesis gas formed during gasification is, according to the used raw material, from different proportions of carbon dioxide (CO2 ), carbon monoxide (CO), methane (CH4 ), hydrogen and other components. In the steam reforming reaction between the water vapor and synthesis gas components are held, whereby the hydrogen yield can be increased.
This method of hydrogen production is a large scale in the production of hydrogen from natural gas for use, for example for the production of ammonia for nitrogen fertilizer (Haber -Bosch process ).
As final products as essentially hydrogen ( with a conversion efficiency of about 78%), carbon dioxide and mineral ash. To start up the system the energy-rich synthesis gas can be used. Subsequently, the wear process is by exothermic reactions energetically self. The hydrogen production on the basis of thermochemical biomass gasification is in the experimental stage.
Pros and Cons of biohydrogen from biomass
The use of bio-hydrogen offers different advantages and disadvantages. The evaluation depends in detail on the raw materials used, the manufacturing process and the type of use. This is particularly the assessment by the lack of practical experience and so far lack of relevance of Biowasserstofferzeugung.
Benefits
In the production of hydrogen by thermochemical biomass processing ( " gasification ") of the process can be controlled so that charcoal -like biochar pellets obtained, which is introduced along with the accrued mineral ash in biomass - farmland, soil fertility and water holding particular assets improved from sandy soils.
Simultaneously, the carbon dioxide content of the atmosphere is reduced by this procedure. Without the biochar soil incorporation only as much CO2 would be released, as has been previously recorded in the formation of biomass. The carbon cycle would be closed and therefore classify this type of energy to be almost carbon neutral. However, all previous chains and energetic costs and emissions of the overall process ( plant cultivation, fertilizing, processing, transportation, etc. ) are taken into account for the creation of a proper air balance.
The dependence on energy imports is reduced when biomass and bio- hydrogen are produced regionally.
It is a controversial issue, which modification effort is needed to toughen up the gas supply system for the transport of hydrogen to the end-use locations. It should be noted that the town gas generated earlier by coking plants by coal gasification has existed for around 60 % of hydrogen.
Disadvantages
The treatment of biomass, in the production of intermediate products and the final product is expensive. When recovery and recycling of nutrients from processed biomass in the form of mineral ash on the acreage can be lost certain elements such as nitrogen and sulfur. These must be replaced with appropriate fertilizer additions. Most processes for bio-hydrogen production have so far been successfully tested only in pilot plants. The foundation stone for a larger demonstration plant Blue Tower Herten was laid in 2009. The proposed plant should produce 150 cubic meters of hydrogen per hour. However, the main investor Solar Millennium went into bankruptcy in late 2011, the project was abandoned.
Production by biomass
For bio-hydrogen production and living biomass (eg cyanobacteria, algae ) can be used. In some metabolic processes (eg, photosynthesis, nitrogen fixation) by certain enzymes (eg nitrogenases, hydrogenases ) can arise hydrogen. Distinction can be drawn between oxygener and anoxygener photosynthesis.
Oxygenic photosynthesis
Typical photosynthesis, such as land plants and algae, as is oxygen (oxygen forming, see oxygenic photosynthesis ), since the product of the water-splitting oxygen is released:
Purpose of photosynthesis is the provision of energy. However, the release of high-energy biohydrogen means a loss of energy. Therefore, these processes occur only under certain circumstances:
- Cyanobacteria are capable of nitrogenase by the essential nutrient nitrogen from the poorly accessible form N2 ( eg dissolved air in the present or in water ) into biologically available compounds. Base, this reaction is nitrogen fixation:
- Green algae also operate oxygenic photosynthesis. In certain circumstances provided for the photosynthetic water splitting energy electrons will not be used for the reduction of carbon dioxide, but in some kind of idle react with protons ( from the surrounding aqueous phase ) to give the hydrogen molecules. This catalyzed by hydrogenase reaction is induced, for example, in the absence of oxygen.
The absorbed solar energy is thus not initially stored in biomass, but can be directly converted into hydrogen. An attempt is made to harness this process in hydrogen bioreactors.
Anoxygenic photosynthesis
In anoxygenic photosynthesis can be formed from organic substrates or reduced sulfur compounds by phototrophic bacteria using solar energy H2 and CO2 or oxidized sulfur compounds.
Advantages and disadvantages of bio-hydrogen from solar energy
The production of bio-hydrogen from solar energy through metabolic processes differ significantly or completely from the production of biomass. Thus, other pros and cons are given.
In algae reactors or photobioreactors cultured photosynthetic algae a significantly higher energy productivity per area can have as a plant. In the photosynthetic production of hydrogen, solar energy is converted directly into a final energy. Conversion losses against the production and use of carbon-based biomass (wood, bioethanol, biodiesel, biogas, etc.) could theoretically be reduced.
The cultivation of algae and bacteria is associated with high investment and operating costs. A commercial production of hydrogen via biomass is currently lacking. The metabolic processes in which hydrogen is produced, occur only to a small extent or under special conditions ( stress ) in nature. A transfer from laboratory to production conditions is not yet foreseeable.
Perspective
The method for producing biohydrogen are still in development or in prototype use. Practical experience in large-scale applications are still missing. Therefore, a final assessment is not possible at present.