Page 207 - Advances in bioenergy (2016)

Different biological methods toward hydrogen production processes are (1) direct
biophotolysis, (2) indirect biophotolysis, (3) photo-fermentation, (4) dark fermentation, and
(5) hybrid reactor system (combined dark and photo-fermentation). Comparing with thermal,
thermochemical, and electrolytic processes, the photolytic process or biological hydrogen
production (BHP) processes are found to be more eco-friendly and less energy intensive. The
term ‘Biohydrogen’ has much significance and it can be produced from water, renewable
organic wastes or biomass, either biologically (biophotolysis and fermentation) or
photobiologically (photodecomposition) and photochemically. BHP processes are much
dependent on the presence of a hydrogen-producing enzyme. Hydrogenases (two subcategories,

hydrogenases and reversible hydrogenases) and nitrogenases are two known enzymes that
catalyze biological hydrogen production. 20-22,33,69,73,81-85

Biological process and photocatalytic hydrogen production from water and biomass
derivatives such as different saccharides are under extended research as it may be through
homogeneous or heterogeneous catalytic routes. Methane production from an anaerobic
condition (fermentation) broadly proceeds through three steps. In first stage, all large
molecules of organic compounds such as carbohydrates, proteins, and fatty oils are
decomposed to smaller molecules such as monosaccharides, amino acids, and higher fatty
acids by acid-producing bacteria and those are further decomposed to lower fatty acids (e.g.,
propionic acid and butyric acid), lactic acid and ethanol. Further, all these <C to CH 4
6
molecules are converted into hydrogen, while acetic acid is converted to H by the hydrogen-
2
producing bacteria. Finally, they are transformed into methane and carbon dioxide by the
methanogenic bacteria. The obtained methane can be further converted into hydrogen by either
thermal decomposition or cracking into pure hydrogen and carbon nano-tubes as a by-product.
Alternately, conventional steam reforming and the water gas shift reaction are other options
with dry reforming or bi-reforming which utilize CO to produce syngas in bulk. As this
2
fermentation proceeds at ambient temperature, the reaction rate is low for H production, but in
2
a decentralized manner in a remote location it can be achieved to a larger extent for self-

sufficiency in domestic use. 73,83-94

CONCLUSION

Hydrogen production from biomass is a promising method. Different reaction schemes
involved in cracking, reforming, pyrolysis, and co-pyrolysis processes can be
thermodynamically controlled after understanding the enthalpy associated with them.
Physicochemical properties of steam, supercritical water, different solvents associated with
reforming, and co-gasification are largely different from those of gas and liquid at normal
condition, which not only make these solvents to associate in the reaction as a reactant but also

affect the reaction pathway as a catalyst. Design of suitable reactor systems such as
microchannel reactors or monolith reactors might be useful for reforming reactions. High
effectiveness factor can be obtained by using the thin layer of the catalyst on these reactors.
This overall review depicts about the thermochemical processes for hydrogen production as