G. ANTONOPOULOU, I. NTAIKOU, K. STAMATELATOU and G. LYBERATOS, University of Patras, Greece

Abstract: This chapter discusses all the biological hydrogen production processes such as indirect and direct water biophotolysis, biological water gas shift, photo and dark fermentation and hydrogen production through microbial electrolysis cells. Dark fermentation or fermentative hydrogen production is focused on this chapter, since it is considered as the most promising compared to all biological hydrogen production methods. However, there are significant remaining barriers to practical application. The chapter includes the limitations of each process and suggests several methods that are aimed at overcoming these barriers.

Key words: biohydrogen, biological hydrogen production processes, fermentative hydrogen production, advantages and limitations.

13.1 Hydrogen

Hydrogen is a colorless, odorless gas that accounts for 75% of the universe mass. It is also the simplest element in the periodic table, since its atom consists of only one proton and one electron. Despite its simplicity and abundance, hydrogen does not exist naturally as a gas, but is found in water, biomass and fossil fuels (gasoline and natural gas), where it is always combined with chemical bonds with other elements such as oxygen, carbon and nitrogen. In order to get hydrogen into a useful form, it must be extracted and separated from these substances. These ‘extraction’ processes are often quite energy intensive. For this reason, many efforts have been invested on the exploration and development of cost-effective and efficient methods of hydrogen production.

Apart from being a very useful reagent for the production of many chemicals, hydrogen is also the most clean and environmentally friendly fuel, which produces water instead of greenhouse gases when burned and possesses a high energy yield of 122 kJ/g, which is 2.75 times greater than that of hydrocarbon fuels. Hydrogen is indeed considered a viable alternative fuel and the ‘energy carrier’ of the future.

Today, hydrogen finds a wide range of industrial applications being a widely used feedstock for the production of chemicals, hydrogenation of fats and oils in food industry, production of electronic devices, processing steel and also for desulfurization and re-formulation of gasoline in refineries. Furthermore it is used in NASA’s space programme as fuel for the space shuttles and in fuel cells for heat and electricity generation. Proton exchange membrane fuel cells (PEMFC) fed with hydrogen are

believed to be the best type of fuel cell that could be used as power sources in vehicles and have the potential to replace the gasoline and diesel in internal combustion engines (http://www. fctec. com). Beyond its use in fuel cells, hydrogen could be directly burned in a fossil internal combustion engine (very similar to petrol or gas-fired engines) to produce mechanical energy without producing CO2 at the point of use. According to the National Hydrogen Program of the United States, the contribution of hydrogen to the total energy market is projected to be 8-10% by 2025 (Armor, 1999). In Fig. 13.1, a hydrogen station in Tsurumi of Japan is depicted.