Biohydrogen production

Hydrogen gas is a high energy ( 122 KJ/g) clean fuel which can be used for many different purposes. Hydrogen can be produced by steam reforming of hydrocarbons and coal gasification. Hydrogen production from renewable resources by fermentation is a more promising method among the other alternatives. In accordance with sustainable development and waste minimization issues, biological hydrogen production, known as “green technology”, has received considerable attention in recent years. Biohydrogen production can be realized by dark and light anaerobic fermentations. Hydrogen is produced as a by product in anaerobic digestion of organic wastes.In order to increase the amount of hydrogen production under anaerobic conditions, the activity of methanogenic bacteria should be reduced/inhibited, or the system should be operated under acidogenic conditions. Photosynthetic hydrogen production basically takes place by two pathways. 1. Algae breaks down water to H ₂ and O ₂ in the presence of light energy. 2. Photo-heterotrophic bacteria utilize organic acids such as acetic, lactic and butyric acid to produce H ₂. The major advantages of the later method is more favorable process economy, reduced operating problems and higher rate of H ₂ gas production. Organic acids produced in the acidogenic phase of anaerobic digestion of organic materials may be used as the substrate for production of hydrogen by the photo-heterotrophic organisms. Therefore, the hydrogen yield may be improved by using a two-stage process such as dark and photo-fermentations or by their combinations.

Sustainable production of hydrogen gas requires availability of sustainable raw materials such as renewable energy sources or biomass.Cellulose or starch containing biomass can be used as a reliable and renewable raw material for hydrogen gas production. A three step process scheme consisting of pre-treatment-hydrolysis, dark fermentation and foto-fermentation can be used for this purpose. The first step of pre-treatment includes grinding, acid hydrolysis, neutralization and nutrient balancing to produce carbohydrate solution from the biomass. Fermentable sugars are converted to organic acids, CO ₂ and hydrogen in the dark fermentation phase. Light-fermentation is used for production of hydrogen from organic acids under anaerobic conditions in the presence of light.

The major challenge in biohydrogen production by dark and light fermentation is to improve the rate and the yield of hydrogen production for an economic process. Biological and engineering studies must be concentrated on these issues. Raw material cost is another concern in biohydrogen fermentations. Therefore, waste materials and renewable resources such as biomass should be used for this purpose. Key words: Hydrogen gas, renewable resources, biomass, dark and photo-fermentations