ABSTRACT
Energy has played a very important role in our life since the beginning of human history. Mankind’s fuels have continually evolved as better, more ef‚cient, safer, and cleaner fuels. From wood, to animal fat, to coal, to petroleum, to natural gas, to hydrogen, a clear trend to lighter and cleaner fuels is apparent. Fossil fuels have unanimously ruled the world for more than one century. The indiscriminate use of fossil fuels has gone to such an extent that it has not only polluted the environment but also exhausted the limited fuel reserves, necessitating the quest for an alternative energy. Hydrogen (H2) is most promising in the succession of fuel evolution, with several technical, socioeconomic, and environmental bene‚ts to its credit. It has the highest energy content per unit weight of any known fuel (142 kJ g−1) and can be transported for domestic/industrial consumption through conventional means. H2 gas is safer to handle than domestic natural gas. H2 is now universally accepted as an environmentally safe, renewable energy resource and an ideal alternative to fossil fuels that does not contribute to the greenhouse effect. Presently, 40% H2 is produced from natural gas, 30% from heavy oils and naphtha, 18% from coal, 4% from electrolysis, and about 1% from biomass. However, today, biological H2 production processes are becoming important mainly due to two reasons: utilization of renewable energy resources and usually operation at ambient temperature and atmospheric pressure. The microbial production of hydrogen by fermentation can be broadly classi‚ed into two main categories-one is light independent and the other is light dependent. The light-independent fermentation processes, commonly known as dark fermentation, employ both obligate and facultative anaerobic bacteria for the production of H2 from a variety of potentially utilizable substrates, including refuse and waste products. It generally gives a high rate of H2 evolution and does not rely on the availability of light sources. In contrast, in photofermentation, small-chain organic acids are used by photosynthetic bacteria as electron donors for the production of H2 at the expense of light energy. Both green algae and blue-green algae (cyanobacteria) can convert water to hydrogen. This chapter deals with the fundamental of these biohydrogen production processes.
