Hydrogen is the first element in the periodic table, a colourless, odourless gas which is the most plentiful element in the universe. Hydrogen has been used extensively in the chemical industry in the manufacture of ammonia, methanol, petrol, heating oil, fertilizers, vitamins, cosmetics, lubricants, cleaners, margarine and as a rocket fuel. Hydrogen has been put forward as a new energy carrier in a system known as the ‘hydrogen economy’, which was first mentioned in 1972. In the hydrogen economy, hydrogen would be used as a fuel and to transport and store energy in the way that electricity is used (Winter, 2005; Clark and Rifkin, 2006). Hydrogen as an energy source has many advantages as it is non-toxic, high in energy, on combus­tion yields only water and it can be used both in fuel cells and internal combustion engines. Hydrogen has three times the energy content of petrol and methane at

141.9 MJ/kg but because of its low density it has very low energy content per unit volume (Table 5.4). Table 5.4 compares the energy per unit mass and unit volume for hydrogen, petrol and other gaseous fuels. It is clear that both methane and hydrogen in the gaseous state have low energy per unit volume but in hydrogen in liquid form the energy per unit volume was still low. However, there are disadvan­tages in the production, storage and flammability of hydrogen which have been used to question the adoption of hydrogen as an energy carrier (Hammerschlag and Mazza, 2005).

There are a number of chemical and biological routes for the production of hydrogen, but only some processes are renewable and sustainable. These methods are listed below where only the first three methods are operated at an industrial scale.

Non-renewable:

• Steam reformation of methane (natural gas).

• Coal gasification.

• Partial oxidation of heavy oil.

• Thermocatalytic treatment of water.

Renewable:

• Electrolysis of water using electricity, only renewable if sustainable electricity sup­ply used such as wind or solar power.

• Photocatalytic splitting of water using TiO2.

• Gasification of food waste, sewage sludge, biomass.

• Pyrolysis of biomass.

• Biological processes:

1. anaerobic metabolism (dark fermentation);

2. photosynthetic hydrogen production (direct biophotolysis);

3. indirect hydrogen production;

4. photo-fermentation;

5. carbon monoxide metabolism, water-gas-shift reaction.

The most widely used process for producing hydrogen is steam reforming of natural gas which is the least expensive process and produces 97% of the hydrogen made. The other two commercial processes are the gasification of coal (Chapter 4) and the partial oxidation of heavy oils. Clearly none of these processes are renewable and sustainable.

There are a number of renewable methods of producing hydrogen but to date these are all at the experimental stage (Kapdan and Kargi, 2006). The theme of the book is biofuels and the production of energy from biological materials and there­fore this section will concentrate on the biological production of hydrogen. As can be seen from the list there are a number of biological processes which result in hydrogen.