Biohydrogen Production through Fermentative Technology
The human race since its inception has been faced with the dilemma of fulfilling its energy requirements. It has led to economic competition and in extreme case scenarios, even wars. Hydrogen gas has always remained a promising option for fulfilling the energy requirements. However, currently hydrogen production suffers from many hurdles such as high economic costs, technical limitations and difficulty in storage. Hydrogen is present abundantly in nature but not in its natural form and is found as compounds such as water, methane and biomolecules.
The use of hydrogen as a future energy fuel holds great promise because it is a clean fuel (generates only water vapour and energy). Another advantage is that it has the highest energy content per unit weight- 122 kJ/g and is environmentally safe(Das & Veziroglu, 2008).
Photosynthesis and fermentation are the chief biological methods of hydrogen production with fermentation process clearly holding an advantage over the latter in having better rates of hydrogen production and the option of utilising different wastes.
Mark D. Redwood et al. 2009 have reviewed and suggested using a combination of micro-organisms for maximising H2 production(Redwood, Paterson-Beedle, & Macaskie, 2008).
However, present processes are neither economically viable nor the yield is sufficient for large scale economic applications. Another potential disadvantage pointed out sometimes is that hydrogen is a dangerous fuel but so is gasoline if not handled properly and hydrogen can serve as a safer fuel if treated with care (Pudukudy, Yaakob, Mohammad, Narayanan, & Sopian, 2014).
Hydrogen can be produced by the following processes-
Non-renewable (fossil fuel) technologies include SMR (Steam Methane Reforming). SMR is currently the commercially viable method suitable for hydrogen production(Lam & Lee, 2011).
The SMR reaction can be depicted by the following equations(Lam & Lee, 2011)-
From renewable sources, hydrogen can be produced via two major pathways- light dependent and light independent. Light dependent pathways include direct and indirect photolysis and photo-fermentation while indirect pathway includes dark fermentation. These processes are briefly presented in Table 1
|Type of pathway||Rate of H2 production (mol H2/l/h)||Organisms||By products||Advantages/Disadvantages|
|Direct biophotolysis||2.5- 12||Algae||O2||No extra energy required, sunlight provides energy.|
|Indirect biophotolysis||Algae||O2||Can fix N2 too.|
|Photo-fermentation||Algae and purple bacteria||CO2||Wide spectrum of light is utilised|
|Dark fermentation||12- 83||Fermentative bacteria||CO2, CH4, H2S, CO, Acetate etc.||No light input needed. Oxygen limitation is not an issue here.VFA accumulates in the effluent.|
|Hybrid Systems (Both dark and photo fermentation)||10- 15 X 103||Fermentative bacteria and phototrophs||CO2||Can contribute towards maximising overall yield and take advantage of both processes.|
Table ref- Hallenbeck, Abo-Hashesh, & Ghosh, 2012,Kothari, Singh, Tyagi, & Tyagi, 2012, Argun & Kargi, 2011, Dasgupta et al., 2010 and S. Srikanth, S.V. Mohan, M.P. Devi, D. Peri, P.N. Sarma, 2009.
Sunlight is converted into chemical energy here and green algae have this ability. Direct biophotolysis can be represented by the following equation-
2H2O → 2H2 + O2 (Energy Source- sunlight) +1498 kJ
The reaction is rather slow because of the large amount of energy that needs to be overcome first (Perera, Ketheesan, Gadhamshetty, & Nirmalakhandan, 2010). Other disadvantages are the high bioreactor cost and costs involved in the separation of hydrogen and oxygen. The process can become feasible in future if photon conversion can significantly improved upon (Show, Lee, & Chang, 2011). In theory, two moles of H2 should be produced from two moles of water but in actual the yield is much lower partly because of enzyme inhibition by oxygen (Dasgupta et al., 2010).
O2 produced acts as an inhibitory agent of all hydrogen productivity and affects gene expression, leads to unstable mRNA and also affects catalytic enzyme activity (Eroglu & Melis, 2011).
Biophotolysis process has not received much attention largely due to the fact that uniform distribution of light and effective penetration is not easy to maintain, effective light collection would require a very large surface area(Show et al., 2011) and unless direct sunlight is utilised, the process will not be economically feasible(Kothari et al., 2012).
Indirect biophotolysis can also result in hydrogen production from water. The reaction can be represented by the following reactions depending on the substrate being employed-
12H2O → 12H2 + 6O2 (Energy Source- sunlight)
Dark fermentation employs anaerobic bacteria without using sunlight. Dark fermentation products are typically hydrogen (H2), carbon dioxide (CO2), methane (CH4), carbon monoxide (CO) and dihydrogen sulfide (H2S) (Muhamad, Johan, Isa, & Kutty, 2011). The maximum yield from dark fermentation equals 4 moles of hydrogen whenever acetate is produced but in practice butyrate and propionate are also formed that brings down the yield. Advantages of dark fermentation include no sunlight requirement, multi-varied substrate utilisation and simple reactor design but overcoming low H2 yield remains an issue. Reactions can be represented as follows-
Acetate C6H12O6 + 2H2O → 2CH3COOH + 4H2 + 2CO2
Butyrate C6H12O6 + 2H2O → 2C3H7COOH + 2H2 + 2CO2
Propionate C6H12O6 + 2H2 → 2C2H5COOH + 2H2O
Photo fermentation is the process wherein hydrogen and carbon dioxide production occurs with energy being provided by sunlight to breakdown organic compounds (Argun & Kargi, 2011). It may be represented by the following equation-
2CH3COOH + 4H2O → (Energy source- Sunlight) 8H2 + 4CO2
Photo fermentation has the advantages of high hydrogen yield (theoretically a maximum of 12 moles), no oxygen production, and the ability to employ varied spectra of light.
Shortcomings include limited efficiency in the conversion of light, nitrogenise is energy intensive enzyme and the higher costs of bioreactors (Muhamad et al., 2011). Future strategies aim towards combining both fermentation technologies in order to maximise hydrogen production.
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