Biohydrogen production

Biohydrogen may produced by steam reforming of methane (biogas) produced by anaerobic digestion of organic waste. In the latter process, natural gas and steam react to produce hydrogen and carbon dioxide.

1. BIOHYDROGEN PRODUCTION

2. Introduction Bio-hydrogen Bio-hydrogen is hydrogen that is produced biologically. Clean fuel and rapidly produced from biomass. Molecular H2 has the highest energy content per unit weight among the known gaseous fuels (143GJton−1). The only carbon-free fuel which ultimately oxidizes to water as a combustion product. Does not contribute to greenhouse emission, acid rain or ozone depletion. Use of fossil fuel is increased (transportation , power & heat generation) Increased CO₂ level

3. Why Hydrogen? It can be produced from renewable feed stocks using non-fossil energy sources. Water is the only byproduct and does not produce any green house gas. Electricity can be produced directly via fuel cells Good automotive fuel. Since fossil fuels contribute massive carbon emission so we need to explore sustainable energy sources like hydrogen and methane

4. Bio hydrogen production methods :

5. PHOTOFERMENTATION Photo fermentative hydrogen production is a microbial process in which electrons and protons generated through oxidation of organic compounds are used to produce molecular hydrogen under anaerobic, nitrogen-limited conditions, utilizing light as energy source  Photo fermentation differs from  dark fermentation  because it only proceeds in the presence of  light .

6. Phototropic bacteria Phototropic bacteria  produce hydrogen gas via photofermentation , where the hydrogen is sourced from organic compounds. Photolytic producers Photolytic  producers are similar to phototrophs, but source hydrogen from water molecules that are broken down as the organism interacts with light. Photolytic producers consist of algae and certain photosynthetic bacteria.       (algae)     (photolytic bacteria) LIGHT-DEPENDENT PATHWAYS

7. Advantage Disadvantage Possible Solution Uses light energy to produce hydrogen from otherwise unusable substrates •Poor light-conversion efficiency •Create antenna mutants that harvest light more effectively •Requires hydrogen impermeable transparent photobioreactors •Develop high-tech, low-cost materials Hydrogen production is catalyzed by nitrogenase and hence irreversible •Nitrogenase requires excess energy (ATP) and thus reduces efficiency •Replace nitrogenase with FeFe hydrogenase Can be used as the second stage to derive more hydrogen from dark fermentation effluents •Extensive treatment of effluent to remove inhibition necessary •Direct photofermentation of sugars

8. Product yield may be higher. The growth rate may be higher. Mixed cultures are able to bring about multistep transformations that would be impossible for a single microorganism. Compounds made by a mixture of microorganisms often complement each other and work to the exclusion of unwanted microorganisms. Mixed cultures permit better utilization of the substrate.. Mixed cultures offer more protection against contamination. In mixed-culture fermentations phage infections are reduced. Mixed-culture fermentations enable the utilization of cheap and impure substrates Mixed cultures can provide necessary nutrients for optimal performance. APPLICATIONS

9. DARK FERMENTATION

10. Dark Fermentation – another way to get hydrogen Dark Fermentation is the fermentative conversion of organic substrate to hydrogen It is a complex process manifested by diverse group of bacteria involving series of biochemical reactions, similar to anaerobic conversion In this, fermentative hydrolytic microorganisms hydrolyze complex organic polymers to monomers, which are further converted to a mixture of lower molecular weight organic acids and alcohol by acidogenic bacteria

11. Microbes involved in Dark Fermentation Hydrogen producing Bacteria – These includes class: Clostridia - e.g. C. thermocellum, C. acetobutylicum Bacilli- e.g. B. thuringiensis, Enterobacter faecium Bacteriode- e.g. Bacteriodes capillosus Mollicutes- e.g. Acholeplasma laidlawi Gammaproteobacteria - e.g. Escherichia coli Actinobacteria - e.g. Slackla heliotrinireducens

12. Substrates used in Dark fermentation Industrial Waste - Dairy industry, Distillery effluent, Food processing waste water , Food processing waste water Agricultural waste- Corn straw, Wheat straw, Rice bran, Grass silage Others- Synthetic waste water, Sewage waste water and sludge , Food Waste, Kitchen waste

13. Significance of Dark fermentation Reduction of waste is enhanced along with the production of hydrogen and methane Inherently more stable Ecofriendly, Low-tech, Low capital cost and does not require inputs of more energy Produces valuable metabolites like acetic, butyric and lactic acid High rate of H2 evolution

14. COMBINED FERMENTATION The combination of dark and photo fermentation provides an integrating system for maximization of an hydrogen yield. The idea of combined fermentation takes into an consideration the very fact of relatively lower achievable yield of H2 in dark fermentation. The non utilization fermentation. of acid produced in dark fermentation. Mechanism Stage 1 Dark fermentation : Anaerobic fermentation of carbohydrate produces intermediates such as low molecular weight organic acids and Co2 along with hydrogen. Stage 2 Light fermentation : The low mol wt . organic acid in stage 1 are converted to hydrogen by photosynthetic bacteria. 2CH3COOH + 4H2O  CH3COOH + 2Co2 + 4H2

15. Advantages Two stage fermentation can improve the overall yield of hydrogen and overcomes the major limitation of dark fermentation. Drawbacks Relatively new ap proach techno economic feasibility is yet to studied.

16. BIO-PHOTOLYSIS Bio-photolysis  is the process by which water dissociates into molecular hydrogen and oxygen in biological systems in the presence of light. Bio-photolysis is the production of hydrogen from water by sunlight energy using biological systems. Photoautotrophic organisms such as microalgae and cyanobacteria are capable of oxygenic photosynthesis. Photosynthetic bacteria could be used for hydrogen production from wastes. Bio-photolysis provides a sustainable and environmentally friendly way to produce clean energy from renewable resources. 4H2O + light energy 2O2 + 4H2

17. TYPES OF BIOPHOTOLYSIS DIRECT BIOPHOTOLYSIS INDIRECT BIOPHOTOLYSIS

18. Direct Bio-photolysis Direct bio-photolysis is a process of simple water splitting producing biohydrogen by either green algae or cyanobacteria. Very high energy input from solar radiation is demanded in water splitting reaction. Direct bio-photolysis is the same process that can be found in algal photosynthesis and plants but instead of adapting biomass containing carbon the process is manipulated for biohydrogen generation. In direct bio-photolysis, the electrons derived from the light energy-mediated water splitting are transferred through photosystem II (PS II) and photosystem I (PS I) to ferredoxin (Fd) as an electron carrier. Subsequently, the reduced Fd reduces a hydrogenase enzyme that is responsible for H 2  production 2H +  + 2Fd(re) ↔ H 2  + 2Fd(ox) Major limitation for this process is simultaneous production of oxygen .

19. Indirect Bio-photolysis In indirect bio-photolysis, photosynthesis converts light energy to chemical energy in the form of a carbohydrate, which is reused to produce H 2 . Indirect biophotolysis was design to address the oxygen inhibition of biohydrogen production problem in direct biophotolysis. This process involves two stages: biohydrogen production and oxygen separation in space or time. Oxygen separation in space method consist of two phases , where in first phase the photosynthesis into carbohydrates and oxygen from atmospheric CO2 is taking place in an open pond . In the second phase anaerobic and dark conditions are applied in closed bioreactor where carbohydrates are degraded to acetic acid and biohydrogen.

20. Photobiological hydrogen production by Microalgae ( Hydr ogenase-dependent hydrogen production ) The oxygenic photosynthetic microorganisms such as green microalgae ( Scenedesmus obliquus ,  Chlamydomonas reinhardtii ,  Chlorella , and  Scenedesmus etc ) use this process that requires only water and sunlight. Reversible hydrogenase, the key enzyme that catalyzes the biohydrogen production in algae is very sensitive to presence of oxygen(below 0.1% ). (FeFe) hydrogenase in green algae drives the evolution of H 2 . I n direct bio-photolysis, the electrons derived from the light energy-mediated water splitting are transferred through photosystem II (PS II) and photosystem I (PS I) to ferredoxin (Fd) as an electron carrier . S ubsequently, the reduced Fd reduces a hydrogenase enzyme that is responsible for H 2  production . 2H +  + 2Fd(re) ↔ H 2  + 2Fd(ox)

21. The other hydrogen producing enzymes in cyanobacteria are  Hydrogenases ; they occur as two distinct types in different cyanobacterial species. One type of them, uptake hydrogenase (encoded by  hupSL ) has the ability to oxidize hydrogen and the other type of hydrogenase is reversible or bidirectional hydrogenase (encoded by  hoxFUYH ) and it can either take up or produce hydrogen.  Uptake hydrogenase   enzymes are found in the thylakoid membrane of heterocyst from filamentous cyanobacteria, where it transfers the electrons from hydrogen for the reduction of oxygen via the respiratory chain in a reaction known as oxyhydrogenation or Knallgas reaction . The hydrogen formed is usually re oxidized by an uptake hydrogenase via a Knallgas reaction and hence there is no net H 2  production in strains with uptake hydrogenases under ambient conditions. The biological role of  bidirectional or reversible hydrogenase , is thought to control ion levels in the organism. Reversible hydrogenase is associated with the cytoplasmic membrane and likely functions as an electron acceptor from both NADH and H 2. Unlike uptake hydrogenase, reversible hydrogenases are helpful in hydrogen production.

22. Because the production of H2 by cyanobacteria occurs in the heterocyst, and the oxygenic photosynthesis is microscopic indirect bio-photolysis, which is concomitant with CO2 fixation in the vegetative cell, the highly O2-sensitive nitrogenase is protected, resulting in the production of H2. N2 + 8e− + 8H+ + 16ATP → 2NH3 + H2 + 16ADP + 16Pi . However, H2 production by (FeFe)-hydrogenase and oxygenic photosynthesis cannot occur simultaneously in green algae. Thus, to obtain sustainable H2 production, elemental sulfur (S°) deficiency, which causes a severe (≈90%) reduction in photosynthesis, occurred with cells grown on acetate, resulting in a drastic decrease in the oxygen production rate coupled with the improved respiration caused by the existence of residual acetate. In this condition, the cells grow in anaerobic conditions to produce H2 by using some of the electrons from the residual water-splitting mechanism (direct biophotolysis ) and the reserved carbon (indirect biophotolysis ).

23. Microbial electrolysis cell ( MEC) Bio-electrochemically Assisted  Microbial  Reactor, is an ecologically clean, renewable and innovative technology for  hydrogen production.  Microbial electrolysis cells produce hydrogen  mainly from waste biomass assisted by various bacterial strains . Electrogenic microorganisms  consuming an energy source ( acetic acid ) release electrons and protons, creating an  electrical potential  of up to 0.3 volts. In a conventional MFC, this voltage is used to generate electrical power. In a MEC, an additional voltage is supplied to the cell from an outside source. The combined voltage is sufficient to  reduce  protons, producing hydrogen gas.

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