The fossil fuels where formed over millions of years from plant biomass. Rapid use of the fossil fuels is releasing carbon from these fuels in the form of carbon dioxide. Hence, there is a increase in the carbon dioxide concentration in the earths atmosphere. With the growing energy demands and the depleting fossil fuel reserves, we are in need of alternative renewable sources of energy.
A large-scale transition to renewable energy is not possible in the short term due to the current technology for harnessing the alternative sources not being cost effective. Use of biomass for meeting the needs of the energy has been explored with significant success. Gaseous forms of fuel products from biomass such as biohydrogen and biomethane are considered as good sources due to their portability and efficiency.
Hydrogen can be produced by the electrolysis or from fossil fuels in either a small or a large scale.Large scale production from fossil fuels has the advantage of being able to capture the carbon dioxide to be utilized for stimulating plant growth or for storage in chemical form such as carbonates or in underground reservoirs.
Methane production through anaerobic process of digestion of wastewater and residues involves hydrogen as an intermediate product which is rapidly taken up and converted to methane by methane producing micro-organisms. The degradation of organic matter to methane and carbon dioxide in the absence of oxygen by microorganisms is called as Anaerobic microbial digestion. This digestion occurs in several phases involving many microbes. The complex organic compounds are first degraded to simple molecules. In the second phase the molecules are degraded into organic acids and hydrogen. The last step involves organic acids and hydrogen being converted into methane.
Biophotolysis involves many microalgae and cyanobacteria which are able to split water into hydrogen and oxygen with the aid of absorbed light energy. However, this process is limited by the efficiency of the enzyme involved in the conversion process. The enzyme is inhibited by the oxygen produced in the process of splitting water. Several variants of this process are being developed to separate the hydrogen and oxygen production steps.
Organic compounds like acetic acid are converted into hydrogen and carbon dioxide with sunlight by bacteria in what is known as photofermentations. However, this process is difficult to scale up as it requires a large surface area to capture the light needed for the driving the process.