HYDROGEN PRODUCTIONDr. Ruchika Yadu

IntroductionHydrogen is a valuable gas as a clean energy source and as feed stock for some industries.

It is a non-pollutant gas in environment

Demand on hydrogen production has increased considerably in recent years.

Hydrogen gas is a high energy ( 122 KJ/g) clean fuel which can be used for many different purposes.

Low conversion efficiencies of biological systems can be compensated for, by low energy requirements and reduced initial investment costs.

Need of hydrogen gasThere is a concern that the emission of gases will decrease the ozone layer and increase temperatures, which lead to the global warming.

Hydrogen becomes a promising alternative energy carrier to fossil fuels since it is clean, renewable, contains high energy content and does not contribute to green house effect.

Demand on hydrogen is not limited to utilization as a source of energy.

Safer to handle

Hydrogen has the highest energy content per volume of any known fuel and can be transported for domestic/industrial consumption through conventional means

Need of hydrogen gas (Continued)Hydrogen gas is a widely used feedstock for the production of chemicals,

hydrogenation of fats and oils in food industry, production of electronic devices,processing steel and also for desulfurization and re-formulation of gasoline inrefineries.

Biomass and water can be used as renewable resources for hydrogen gasproduction.

Due to increasing need for hydrogen energy, development of cost-effective andefficient hydrogen production technologies has gained significant attention inrecent years.

The only carbon free fuel H2 upon oxidation produces water alone

At present hydrogen is produced mainly from fossil fuels, biomass andwater.The methods of hydrogen production from fossil fuels are

(a) Steam reforming of natural gas.

(b) Thermal cracking of natural gas.

(c) Partial oxidation of heavier than naphtha hydrocarbons.

(d) Coal gassification.

Methods of hydrogen production from biomass are

(e) Pyrolysis or gassification (which produces a mixture of gases, i.e., H2; CH4; CO2; CO; N2).

Methods of hydrogen production from water are

(f) Electrolysis.

(g) Photolysis.

(h) Thermochemical process.

(i) Direct thermal decomposition or thermolysis.

Biological production.

Biological Methods in hydrogen gas production

Biological hydrogen production methods can be classificated as below:

Direct biophotolysis

Indirect biophotolysis

Photo fermentation

Dark fermentation

Two stage process (integration of dark and photo fermentation)

Biocatalyzed electrolysis

Biological hydrogen productionProduction of clean energy source and utilization of waste materialsmake biological hydrogen production a novel and promisingapproach to meet the increasing energy needs as a substitute forfossil fuels

Biological hydrogen production stands out as an environmentallyharmless process carried out under mild operating conditions, usingrenewable resources. Several types of microorganisms such as thephotosynthetic bacteria, cyanobacteria, algae or fermentativebacteria are commonly utilized for biological hydrogen production

Hydrogen gas production from water by algae: Direct biophotolysis

Algae split water molecules to hydrogen ion and oxygen via photosynthesis. In this process solar energy is directly converted to hydrogen via photosynthetic reactions

2H2O + ‘light energy’→ 2H2 + O2.

The generated hydrogen ions are converted into hydrogen gas by hydrogenaseenzyme.

Chlamydomonas reinhardtii is one of the well-known hydrogen producing algae

Hydrogenase activity has been detected in green algae, Scenedesmusobliquus,in marine green algae Chlorococcum littorale.

Hydrogen gas production from water by algae: Direct biophotolysisThe advantage of this method is that the primary feed is water, which is inexpensive andavailable almost everywhere

Hydrogen production by direct photolysis using green algae is currently limited by followingparameters:

solar conversion effciency of the photosynthetic apparatus;

strong inhibition effect of generated oxygen on hydrogenase enzyme is the major limitation forthe process.

Low hydrogen production potential and

no waste utilization

bioreactor design and cost.

S.No. Algal species containing hydrogenase Species

1. Blue-Green Algae Anabena azollae, A. cylinderica, Anacystis elongata, Nostoc muscorum, Spirulina platensis, Synechoccuselongatus

2. Green algae Chlamydomonas moewusii, Chlorella fusca, C. homosphaera, C. kessleri, C. sorokiniana, Ulva lactuca

3. Brown algae Ascophyllum nodosum

4. Red algae Ceramium rubrum, Chondrus crispus, Corallina officinalis, Porphyra sp. Porphyridium cruentum

Purple Bacteria: A potential source of hydrogen

Halobacterium: H. halobium, H. curtirubrum

Rod shaped, halophilic, Require high concentration of salt (3-4M NACl) andlow amount of oxygen

Bacteriorhodopsin(bR)-Purple pigment in cell membrane. Acts as a lightdriven proton pump.

Biocells: Sandwich purple membrane and lipid between 2 platinumelectrode. Photovoltages are generated as a result of photochemicalconversion of purple pigment after illumination. Therefore due to increasingenergy demand utilization of solar energy by biocells offer a great potential

Indirect Biophotolysis

Involve separation of the H2 and O2 evolution reactions into separate stages, coupledthrough CO2 fixation/evolution

Cyanobacteria have the unique characteristics of using CO2 in the air as a carbon source andsolar energy as an energy source

12H2O + 6CO2 + ‘light energy’→ C6H12O6 + 6O2

The cells take up CO2 first to produce cellular substances, which are subsequently used forhydrogen production

C6H12O6 + 12H2O + ‘light energy’→ 12H2 + 6CO2

Cyanobacteria possess key enzymes (nitrogenase and hydrogenase) that carry outmetabolic functions in order to achieve hydrogen generation

Photo fermentation Photosynthetic bacteria have long been studied for their capacity to produce significant amounts of hydrogen.

H2 production by purple non-sulfur bacteria is mainly due to the presence of nitrogenase under nitrogen-deficient conditions using light energy and reduced compounds (organic acids).

CH3COOH + 2H2O + ‘light energy’→ 4H2 + 2CO2

Advantages and disadvantages of Photo fermentationAdvantages

The advantage of this method are that oxygen does not inhibit the process

These photoheterotrophic bacteria have been found suitable to convert lightenergy into H2 using organic wastes as substrate

Disadvantages

The disadvantages are the limited availability of organic acids, the nitrogenaseenzyme is slow, the process requires a relatively high amount of energy, andhydrogen re-oxidation

Another major factor affecting the photo-fermentation process is light intensity

Dark fermentationHydrogen can be produced by anaerobic bacteria, grown in the dark oncarbohydrate-rich substrates.

Bacteria known to produce hydrogen include species of Enterobacter, Bacillus, andClostridium (C. buytricum, C. thermolacticum, C. pasteurianum, C. paraputrificum M-21 and C. bifermentants)

Carbohydrates, mainly glucose, are the preferred carbon sources for fermentationprocesses, which predominantly give rise to acetic and butyric acids together withhydrogen gas

During dark fermentation, sugars are converted to H2, CO2 and short-chain organicacids with a theoretical maximum hydrogen yield of 4 mol of H2/mole of hexosesugar, when all sugars are fermented to acetate, CO2 and H2.

While direct and indirect photolysis systems produce pure H2, dark fermentationprocesses produce a mixed biogas containing primarily H2 and carbon dioxide(CO2), but which may also contain lesser amounts of methane (CH4), CO, and/orhydrogen sulfide (H2S).

Dark-fermentation proves to be superior over photo-fermentation as thisrequires no light and the energy produced is relatively higher, due to thefermentation of sugar and carbohydrates.

The process is initiated by the hydrolysis of organic polymers to monomers,thereafter acetogenic conversion of monomers to organic acids, alcohols, andrelease of hydrogen

According to reaction stoichiometry, bioconversion of 1 mol of glucose intoacetate yields 4 mol H2/mol glucose but only 2 molH2/mol glucose is formedwhen butyrate is the end product

C6H12O6 + 2H2O →2CH3COOH + 2CO2 +4H2

C6H12O6 + 2H2O→2CH3CH2COOH + 2CO2 +2H2

Although biohydrogen production by dark-fermentation is promising andadvantageous over photo-fermentation. However, the requirement of organicbiomass as a feedstock makes this process quite expensive

Hydrogen synthesis pathways are sensitive to H2 concentrations and are subject to end-product inhibition. As H2 concentrations increase, H2 synthesis decreases

Sugars and carbohydrate rich biomass are reported to be the most suitable feedstock for the formation of biohydrogen from dark fermentation

The type of substrates used in hydrogen gas production by dark fermentation are

i) use of simple sugars

(ii) use of starch containing wastes

(iii) use of cellulose containing wastes

(iv) use of food industry wastes and waste water

(v) use of waste sludge

Two stage process with integration of dark and photo fermentation

With external energy supply (photon-energy in photofermentation) theoretically 12 moles ofhydrogen per mole of glucose can be produced. However this process cannot be operated inthe absence of light.

On the other hand, in the absence of external energy (in the case of dark-fermentation),oxidation of glucose by fermentative bacteria results in other by-products also and maximum4 moles of hydrogen are produced per mole of glucose consumption with acetate as the soleby-product.

C6H12O6 + 2H2O → 4H2 + 2CO2 + 2CH3COOH

Acetate produced in the dark-fermentation stage can be oxidized by photosynthetic bacteriato produce hydrogen

CH3COOH + 2H2O + ‘light energy’→ 4H2 + 2CO2

Hence continuous production of hydrogen at maximum yield can be achieved by integratingdark- and photo-fermentation methods.

Biocatalyzed electrolysis Another way of oxidizing the acetate (or the effluent of dark fermentationprocess) to produce hydrogen is to provide external energy in the form ofelectrical energy instead of solar energy.

In this approach, the bioreactor containing acetate forms the anodiccompartment of an electrolyzer cell and protons and electrons produced bybacteria are collected at cathode (a platinum electrode catalyzing hydrogenevolution reaction). Anodic and cathodic reactions are as follows:

Anode: 2CH3COOH+2H2O→2CO2 +8H+ +8e_

Cathode: 8H+ + 8e_ → 4H2

Major routes and sites of production of hydrogen

BY THE ACTION OF HYDROGENASE ENZYME BY THE ACTION OF NITROGENASE ENZYME

Hydrogenase, the enzyme responsible for this hydrogen production, catalysesthe following reaction:

(2H+ + 2Xreduced 6 H2 + 2Xoxidized)

The electron carrier, X, is thought to be ferredoxin. Since ferredoxin is reducedwith water as an electron donor by the photochemical reaction, green alga aretheoretically water-splitting microorganisms.

Electron donors (organic compounds) transfer electrons to PSI of chloroplastfrom where electrons are received by electron carriers (eg. Ferredoxin).Hydrogenase accepted electrons from the electron carrier. In the visible lighthydrogenase separates high energy electrons from ferredoxin and facilitatestheir transfer to H+ ultimately H2 is evolved.

Hydrogen production catalyzed by nitrogenase occurs as a side reaction at a rate of one-third to one-fourth that of nitrogen-fixation

Molecular nitrogen is reduced to ammonium with consumption of reducing power (e' mediated by ferredoxin) and ATP. The reaction is substantially irreversible and produces ammonia:

N2 + 6H1+ + 6e- 2HN312ATP 12(ADP+Pi)However, nitrogenase catalyzes proton reduction in the absence of nitrogen gas (i.e. in an argon atmosphere).2H+ + 2e- H24ATP 4(ADP+Pi)

Nitrogenase itself is extremely oxygen-labile.

Unlike in the case of hydrogenase, however, Cyanobacteria have developedmechanisms for protecting nitrogenase from oxygen gas and supplying it withenergy (ATP) and reducing power.

The most successful mechanism is the localization of nitrogenase in the heterocystsof filamentous cyanobacteria (Fig. 5-3).

Vegetative cells (ordinary cells) in filamentous cyanobacteria carry out oxygenicphotosynthesis. Organic compounds produced by CO; reduction are transferred intoheterocysts and are decomposed to provide nitrogenase with reducing power.

ATP can be provided by PSI-dependent and anoxygenic photosynthesis withinheterocysts.

Advantages and disadvantages of different hydrogen production processes

Process Advantages Disadvantages

Direct biophotolysis Can produce H2 directly from water and

sunlight

Solar conversion energy increased by ten

folds as compared to trees, crops

Requires high intensity of light

O2 can be dangerous for the system

Lower photochemical efficiency

Indirect biophotolysis Cyanobacteria can produce H2 from water

Has the ability to fix N2 from atmosphere

Uptake hydrogenase enzymes are to be

removed to stop degradation of H2

About 30% O2 present in gas mixture

Photo-fermentation A wide spectral light energy can be used by

these bacteria

Can use different organic wastes

O2 has an inhibitory effect on nitrogenase

Light conversion efficiency is very low, only

1–5%

Dark fermentation It can produce H2 all day long without light

A variety of carbon sources can be used as

substrates

It produces valuable metabolites such as

butyric, lactic and acetic acids as by

products

It is anaerobic process, so there is no O2

limitation problem

O2 is a strong inhibitor of hydrogenase

Relatively lower achievable yields of H2

As yields increase H2 fermentation becomes

thermodynamically unfavorable

Product gas mixture contains CO2 which has

to be separated