Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 188

Biohydrogen is hydrogen produced biologically (by biological process), most commonly by bacteria from waste organic materials.

Hydrogen (H₂) is not available independently in the atmosphere, but is always combined with other elements. Hence, before being used, it has to be extracted from various compounds, e.g., via the syngas route. Currently, hydrogen is produced mainly from natural gas and to a lesser extent from oil derivatives (totally 77% of all hydrogen production), while its production from biomass is new – due partly to the more sophisticated manufacturing of syngas from biomass compared to natural gas, but also to the much lower relative hydrogen content of biomass vice-versa natural gas.

Hydrogen can be produced from biomass by pyrolysis, gasification, steam gasification, steam-reforming of bio-oils, and enzymatic decomposition of sugars. The yield of hydrogen that can be produced from biomass is relatively low, 16 to 18% based on dry biomass weight. In the pyrolysis and gasification processes, water-gas shift is used to convert the reformed gas into hydrogen, and pressure swing adsorption is used to purify the product. Gasification coupled with water gas shift is the most widely practiced process route for biomass to hydrogen. In general, the gasification temperature is higher than that of pyrolysis and the yield of hydrogen from the gasification is higher than that of the pyrolysis. Modeling of biomass steam gasification to synthesis gas is a challenge because of the variability (composition, structure, reactivity, physical properties, etc.) of the raw material and because of the severe conditions (temperature, residence time, heating rate, etc.) required. Hydrogen can be produced via steam gasification of biomass materials. The yield of H₂ from steam gasification increases with increasing water-to-sample (W/S) ratio. The yields of hydrogen from steam gasification increase with increasing temperature.

Several concepts have promise for near-term to long-term process development for biohydrogen production. For example, the microbial shift reaction (analogous to one step of the steam methane reaction in coal gasification), operates at ambient temperatures in a single-stage process, compared to the two-stage, high-temperature, chemical catalyst processes currently used. Process development is just beginning, but this concept appears promising for near- to mid-term practical applications.

Hydrogen yields from the fermentation of organic wastes are typically less than 20% (on a heating value basis) compared to methane production. Higher yields might be possible at elevated temperatures, with nutrient limitations, and through metabolic engineering of the bacteria.

Photofermentation, the conversion of organic substrates to hydrogen by nitrogen-fixing photosynthetic bacteria, achieve high yields of hydrogen, but low solar conversion efficiencies. The inefficiency of the nitrogenase enzyme suggests that biohydrogen processes must be based on reversible hydrogenases.

Larger-scale biohydrogen production requires biophotolysis processes, such as hydrogen production from water and sunlight. Photobioreactor costs and solar conversion efficiencies are main challenges in the development of practical processes. Direct biophotolysis couples the reductant produced by photosynthesis directly to hydrogenase, producing oxygen and hydrogen simultaneously, while indirect processes separate these basically incompatible reactions through intermediate carbon dioxide fixation. Direct, but not indirect, biophotolysis processes would require hydrogenase activity in the presence of high oxygen levels.

The photosynthetic production of hydrogen employs microorganisms such as cyanobacteria, which have been genetically modified to produce pure hydrogen rather than the metabolically relevant substances. The conversion efficiency from sunlight to hydrogen is small, usually under 0.1%, indicating the need for large collection areas. However, hydrogen build-up hinders further production and there has to be a continuous removal of the hydrogen produced, by pipelines to e.g., a shore location, where gas treatment and purification can take place.

Furthermore, if the bacteria are modified to produce maximum hydrogen, their own growth and reproduction are quenched. There presumably has to be a compromise made between the requirements of the organism and the amount of hydrogen produced for export, so that replacement of organisms (produced at some central biofactory) does not have to be made at frequent intervals.

Hybrid indirect processes using both algae and photosynthetic bacteria have been proposed and tested. The simplest indirect process would use the same algal cells for carbon dioxide fixation, oxygen evolution, and hydrogen production, at separate times or even in different reactors. The hydrogen reactions would take place both in the dark and light. Light-driven hydrogen evolution requires suppression of the oxygen evolution process. The biophotolysis process must achieve the highest possible solar conversion efficiencies, which will require development of algal strains with reduced light-harvesting pigment content.

See also: Biological Hydrogen Production, Hydrogen Production.