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Journal of Scientific & Industrial ResearchVol. 63, September 2004, pp. 729-738

Biohydrogen production as a potential energy resource – Present state-of-art

Kaushik Nath and Debabrata Das*

Department of Biotechnology, Indian Institute of Technology, Kharagpur 721 302

Biological hydrogen production is one of the most challenging areas of technology development for sustainablegaseous energy generation. The future of biological hydrogen production depends not only on research advances, i.e.improvement in efficiency through genetically engineered microorganisms and/or the development of bioreactors, but alsoon economic considerations, as compared to fossil fuels, social acceptance, and the development of hydrogen energysystems. The present study critically updates various biohydrogenation processes with special references to their merits anddemerits. Different approaches towards improvement of the bioprocesses are also outlined.

Keywords: Hydrogen, Biophotolysis, Photofermentation, Light energy, Dark fermentation, Energy conversion efficiency IPC Code: Int. Cl.7: C 12 P 1/02

1 IntroductionHydrogen holds much promise as a dream fuel of the future against the projection of the global energy crisis.

In the past few years the concept of the hydrogen economy has been put forth as a clean and efficientreplacement for the petroleum based economy we now live under. It is a versatile energy carrier with thepotential for extensive use in power generation and in many other applications. About half of all hydrogenproduced is used in the manufacture of ammonia, which is itself mostly used in making fertilizers; a further 37per cent is used in oil refineries for removal of impurities or for upgrading heavier oil fractions into lighter andmore valuable products; 8 per cent in methanol production and4 per cent in a wide variety of chemical, metallurgical and other uses. 1 per cent of hydrogen production is usedin the world’s space programmes1 (Fig. 1). The future widespread use of hydrogen is likely to be in thetransportation sector where it will help reduce pollution. Vehicles can be powered with hydrogen fuel cells,which are three-times more efficient than a gasoline powered engine. As on today, in all these areas hydrogenutilization is equivalent to 3 per cent of the energy consumption, but it is expected to grow significantly infuture2.

More than 500 b cu m of hydrogen are produced each year, for use in a wide variety of processes. Thisamount of hydrogen could produce 6.5 EJ of energy, equivalent to about 1.5 per cent of world energyconsumption3. 99 per cent of this hydrogen is produced from fossil fuels, primarily natural gas, with chemicalproduction and renewable energy sources accounting for the rest. At present, all of the CO2 generated is releasedto atmosphere. Catalytic steam reforming of naphtha or natural gas, gasification of coal, and electrolysis of waterare some of the classical methods of hydrogenmanufacturing currently in vogue. But all these methodsare highly energy intensive, thereby incurring highercost and not always environmentally benign. On thecontrary, biological processes are particularly useful forthis application because they are catalyzed bymicroorganisms in an aqueous environment at ambienttemperature and pressure. Furthermore, these techniquesare well suited for decentralized energy production insmall-scale installations in locations where biomass orwastes are available, thus avoiding energy expenditureand costs for transport1. From thermodynamicperspective, as the organic substrates dissolved and

Fig.1—Different uses of hydrogen



J SCI IND RES VOL 63 SEPTEMBER 2004730

diluted in wastewater are in a high entropy state, it is somewhat difficult to obtain their combustion enthalpy bymechanical means4. Here comes the importance of biological hydrogen production process wheremicroorganisms can recover and concentrate the energy from high water content organic resources such as,industrial wastewater and sludges in a usable form. Thus, biohydrogenation, in a sense, an entropy reducing

process, which could not be realized by mechanical or chemical systems4

.Interest in biohydrogen started getting prominence in early 90s, when it became apparent that atmosphericpollution by fossil fuels is not only unhealthy locally, but might also cause significant climate changes globally.As a result, biological hydrogen production became a focus of Governmental support, particularly in Germany,the US and Japan, with least efforts in the other countries. The present paper briefly describes state-of-the-art of various biological hydrogen production processes. Attempts have also been made to highlight both theadvantages and bottlenecks of each process towards improvement of production and process efficiency.

2 Biohydrogenation – State-of-the-ArtHydrogen metabolism is primarily the domain of bacteria and micro-algae. Within these groups, it involves

many microbial species, including significantly different taxonomic and physiological types, various enzymesand metabolic pathways. Table 1 summarizes various biological hydrogen production processes with general

overall reactions involved therein, broad classification of microorganisms used and their relative advantages.

Table 1 — Different biological hydrogen production processes with their advantages

Process General reactions and broad classification of microorganisms used

Advantages

1 Direct biophotolysis 2 H2O + light = 2 H2 + O2

Micro-algae

(i) Can produce H2 directly from water andsunlight(ii) Solar conversion energy increased by 10- foldsas compared to trees, crops

2 Indirect biophotolysis 6 H2O + 6 CO2 + light = C6H12O6 + 6 O2

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

2 CH3COOH + 4 H2O + light = 8 H2 + 4 CO2

Overall reaction:12 H2O + light = 12 H2 + 6 O2

Microalgae, cyanobacteria

(i) Can produce H2 from water

(ii) Has the ability to fix N2 from atmosphere

3 Dark fermentation

C6H12O6 + 6 H2O = 12 H2 + 6 CO2

Fermentative bacteria

(i) It can produce H2 all day long without light(ii) A variety of carbon sources can be used assubstrates(iii) It produces valuable metabolites such asbutyric, lactic, and acetic acids as by products(iv) It is anaerobic process, so there is no O2 limitation problem

4 Photo -fermentationCH3COOH + 2 H2O + light = 4 H2 + 2 CO2

Purple bacteria,Microalgae

(i) A wide spectral light energy can be used bythese bacteria(ii) Can use different waste materials like distilleryeffluents, waste, etc.

5 Hybrid reactor system(combined dark andphoto- fermentation)

Stage IC6H12O6 + 2 H2O = 4 H2 + 2 CH3COOH + 2 CO2

Stage IICH3COOH + 2 H2O + light = 4 H2 + 2 CO2

Fermentative bacteria followed by anoxygenicphototrophic bacteria (PNS)

Two stage fermentation can improve the overallyield of hydrogen



NATH & DAS: BIOHYDROGEN PRODUCTION AS A POTENTIAL ENERGY RESOURCE 731

2.1 Direct Biophotolysis

Direct biophotolysis employs the dissociation of water molecules under sunlight in the presence of

microalgae. It uses the same process found in plantsand algal photosynthesis, but adapts them for thegeneration of hydrogen gas, instead of carboncontaining biomass5 (Fig. 2). Green microalgaepossess the genetic, enzymatic, metabolic, andelectron transport machinery to photoproducehydrogen gas. Several researchers have reported algal photolysis using various species like Scenedesmus

obliquus, Chlamydomonas reinhardii, C. moewusii (ref.5). Biophotolysis is an inherently attractive process sincesolar energy is used to convert a readily available substrate, water to oxygen, and hydrogen. This approach, if successful, would allow virtually unlimited production of hydrogen from the earth’s most plentiful availableresources −water and light6.

Hydrogen production by green algae requires several minutes to a few hours of anaerobic incubation in the

dark to induce the synthesis and/or activation of enzymes involved in hydrogen metabolism, including areversible hydrogenase enzyme. Since hydrogenase activity is extremely oxygen sensitive the concurrentproduction of O2 poses a serious limitation. This is the major bottleneck of the process. Sweeping out theoxygen, as it is produced, could mitigate this inhibition to a great extent7. Although this would not be practicalfor large-scale operation. Some other attempts to overcome this problem include the use of O2 absorbers, bothreversible and irreversible, and the use of O2 tolerant hydrogenase enzymes. But these attempts have met withlimited success8. One promising solution is to develop microalgae with an O 2 insensitive hydrogenase reaction.Apart from these there are tremendous biological and engineering challenges to be overcome in realizing thisgoal. Direct biophotolysis requires entire production area to be enclosed in photobioreactor, which is able to bothproduce and capture H2 and O2. But this approach seems to be economically not viable as handling of H 2 /O2 mixtures in large volumes and over large areas would likely be impractical6. The reducing power generated byphotosynthesis must be produced as close as possible to the maximal possible solar conversion efficiency of

about 10 per cent and then efficiently transferred to hydrogenase. Currently, photosynthetic organisms likehigher plants capture only 3-4 per cent of sunlight’s available energy7.

Thus, direct biophotolysis, although theoretically attractive as a hydrogen production process, suffers from themajor limitations of oxygen sensitivity, low light conversion efficiency and gas separation (Table 2). Futurestudies will pursue the possibility of "sweeping" oxygen from the system or development of a hydrogenaseengineered to be insensitive to oxygen inactivation. In addition, this method requires enclosure of the solarcapture area within a photo-bioreactor, which creates great engineering challenges.

2.2 Indirect Biophotolysis

Cyanobacteria (also known as blue-green algae, cyanophyceae, or cyanophytes) are a large and diverse groupof photoautotrophic microorganisms, which can evolve hydrogen by indirect biophotolysis of water (Fig. 3).Photosystem II utilizes the energy of sunlight in photosynthesis to extract electrons from water molecules.

Electrons released upon the oxidation of water are transported to the Fe-S protein ferredoxin on the reducingside of photosystem I. The hydrogenase in the stroma of the algal chloroplast accepts electrons from reducedferredoxin and donates them to two protons to generate one H2 molecule5. For photobiological hydrogenproduction, cyanobacteria have been adjudged as one of the ideal candidates since they have simple nutritionalrequirements as they can grow in air (N2 and CO2), water and mineral salts, with light as the only energysources9. Hydrogen production by cyanobacteria has been studied for over three decades and has revealed thatefficient photoconversion of H2O to H2 is influenced by many factors. Hydrogen production has been assessed invarious species and strains, within at least 14 genera, under a vast range of culture conditions10. The need of lightfor hydrogen evolution, nitrogenase, oxygen sensitivity, and lower hydrogen evolution vs acetylene reduction are some

Fig. 2—Direct biophotolysis




Fig. 3—Indirect biophotolysis

of the important factors of biophotolysis10. The cultivation of cyanobacteria in nitrate-free media under air andCO2, followed by incubation in light under argon and CO2 atmosphere, has rapidly become standard, since itresults in immediate hydrogen production. Localization of nitrogenase in heterocysts provides an oxygen-freeenvironment and the ability of heterocystous cyanobacteria to fix nitrogen in air11. Hydrogenase and nitrogenaseinhibitors are used in an attempt to screen for aerobic hydrogen evolution potential. It has been observed thatthese inhibitors allow for hydrogen to be released from aerobic cultures in amounts similar to those in argon11.

Photobiological technology holds great promise but because oxygen is produced along with the hydrogen thetechnology must overcome the limitation of oxygen sensitivity of the hydrogen-evolving enzyme systems.

Table 2—Major bottlenecks of the processes and various approaches to overcome

Processes Major limitations Approaches to overcome References

1 Direct (a) O2 sensitivity of hydrogenase (a)

biophotolysis enzyme (i) Use of O2 absorbers, both irreversible (glucose-oxidase, dithionite) and reversible (hemoglobin) 6

(ii) Use of O2 tolerant uptake hydrogenase 13

(b)Low light conversion efficiency (b)

(i) Genetic manipulation of light gathering antenna 14

(ii) Optimization of light input into photobioreactor 15

2 Indirectbiophotolysis

(a) Enzyme inhibition by O2 (a) To achieve O2 tolerant hydrogenase activity byclassical mutagenesis

13

(b) H2 consumption by an uptakehydrogenase

(b) Genetic modification of strains to eliminate uptakehydrogenase

16

(c) Overall low production rate (c) Genetic modification to increase levels of bidirectional hydrogenase activity

17

3 Dark fermentation (a) Relatively lower achievableyields of H2

(a) Metabolic shift of biochemical pathways to arrest theformation of alcohol and acids

18

(b) As yields increase, H2 fermentation becomesthermodynamically unfavorable

(b) Maintaining low partial pressure of H2 19

(c) Product gas mixture containsCO2 which has to be separated

(c) Efficient removal of gases 20

4 Photofermentation (a) Light conversion efficiency is (a)

very low, only 1-5 per cent (i) Elimination of competing microorganisms (e.g.microalgae) using light filters

21

(b) Inhomogeneity of lightdistribution

(ii) Co-cultures of photo heterotrophic bacteria withdifferent light utilization characteristics

22

(c) O2 is a strong inhibitor of

hydrogenase

(iii) Control of photosynthetic protein expression to

allow efficient absorption of light energy

6

(b) Improvement in photoreactor design with lightdiffuser

6

5 Hybrid reactorsystem

Relatively newer approach,techno-economic feasibility is yetto be studied

− 23




Researchers are addressing this issue by screening fornaturally occurring organisms that are more tolerant of oxygen, and by creating new genetic forms of theorganisms that can sustain hydrogen production in the

presence of oxygen

12

. A new system is also beingdeveloped that uses a metabolic switch (sulphurdeprivation) to cycle algal cells between aphotosynthetic growth phase and a hydrogen productionphase12.

2.3 Photofermentation

The efficiency of light energy used for the productionof hydrogen by photosynthetic bacteria is theoreticallymuch higher compared with cyanobacteria. Theadvantages of phototrophic bacteria can be attributed to the following facts:

(i) High theoretical conversion yields,(ii) Lack of O2 evolving activity, which otherwise causes O2 inactivation problems in different biological

systems,(iii) Ability to use wide spectral light energy, and(iv) Ability to consume organic substrates derived from wastes in association with wastewater treatment.

These organisms have an important role in the anaerobic cycling of organic matter – as producers(photoautrophs, utilizing CO2 as their carbon source and H2 as their electron donor), and as consumers(photoheterotrophs, using organic molecules as their carbon source and electron donor)24. Production of hydrogen by photosynthetic bacteria takes place under illumination and in the presence of an inert anaerobicatmosphere (such as argon or helium), from the break down of organic subtracts like, malate and lactate (Fig. 4).These anions of organic acids are preferred substrates. Apart from different organic acids and carbohydrates,several wastewaters have also been attempted to explore their suitability to be used as substrates for PNS

bacteria to produce hydrogen.The efficiency of phototrophic bacteria for biological production of hydrogen is closely associated with

several important variables. Photochemical efficiency is one of those. A generalized expression forphotochemical efficiency has been put forward by Akkerman et al.12

Efficiency (per cent) =

H2 production rate ×Hydrogen energy content

Absorbed light energy

For the photoautotrophic hydrogen production, photochemical efficiencies are only 3-10 per cent when theoxygen is totally and immediately removed. Such low efficiencies of photoautotrophic process pose a major

limitation towards its commercial acceptance. On the other hand, photochemical efficiency of photoheterotrophic bacteria is comparatively higher than that of photoautotrophs. However, this is essentiallybased on artificial light, the efficiency of which can reach 10 per cent or even more by only at low lightintensities with low hydrogen production rates12. The energy conversion efficiency of light energy into hydrogenin the presence of phototosynthetic bacteria varies under different light sources. It is believed that the hydrogenproduction by photosynthetic bacteria may depend on the spectral distribution, since the bacteria utilize thespecific light wavelengths for photosynthesis. An approach for the improvement of hydrogen production byphotosynthetic bacteria is the control of photosynthetic protein expression to allow efficient absorption of lightenergy. A method for the enhancement of the bacterial light-dependent hydrogen production is proposed byMiyake et al.4 by rearrangement of light harvesting systems. Genetic manipulation of photosynthetic pigmentcontent of bacteria by ‘promoter competition method’ can be controlled for making the light penetration easy4.

Fig. 4—Schematic of hydrogen production by photosyntheticbacteria




Inhomogeneity of the light distribution in the reactor also contributes to lower the overall light conversionefficiency. To enhance the total efficiency of light to hydrogen conversion in a photobioreactor, light energyshould be equally distributed throughout the reactor. The spectrum of light in the reactor also affects hydrogenevolution. An important point of the application of solar energy is the efficiency of the conversion. It is said that

the plant photosynthesis is done with energy conversion efficiency as low as 1 per cent

4

.Another limitation of photofermentation is lower achievable yield of hydrogen. Koku et al.25 have proposedseveral conditions for maximum hydrogen production. Among these maintaining a maximal activity of nitrogenase and minimal activity of hydrogenase, a favorable molar ratio of carbon source to nitrogen source andavailability of uniform distribution of light through the culture are important. Efforts to improve hydrogenproduction by photosynthetic bacteria also include elimination of competing microorganisms, such as microalgae, using light filters, as proposed by Ko and Noike 21. The use of co-cultures of photoheterotrophic bacteriawith different light utilization characteristics22, novel photobiorector designs26 and use of specific waste streamsas substrates for photo-fermentation27 have also been explored as alternative options for improving the efficiencyof photosynthetic processes.

The main bottleneck for practical application of photobiological hydrogen production is the required scaling-up of the system. A large surface area is needed to collect light. Construction of a photobioreactor with a large

surface/volume ratio for direct absorption of sunlight is expensive. A possible alternative is the utilization of solar collectors. Again, a drawback of these collector systems is the high production cost with the currentlyavailable technology.

2.4 Dark Fermentation

Carbohydrates, mainly glucose, are the preferred carbon sources for fermentation process, whichpredominantly give rise to acetic and butyric acids together with hydrogen. Here, pyruvate the product of glucose catabolism is oxidized to acetyl-CoA, which can be converted to acetyl phosphate and results ingeneration of ATP and excretion of acetate. Pyruvate oxidation to acetyl-CoA requires ferredoxin (Fd)reduction. Reduced Fd is oxidized by hydrogenase, which generates Fd and releases electrons as molecularhydrogen.

Pyruvate + CoA + 2 Fd(ox) → Acetyl-CoA + 2Fd(red) + CO2 2 Fd(red) → 2 Fd(ox) + H2

Despite having higher evolution rate of hydrogen the yield of hydrogen from fermentation process is lowerthan that of other chemical/electrochemical processes, and thus the process is not economically viable in itspresent form. The pathways and experimental evidences cited in literature reveal that a maximum of four moleof hydrogen could be obtained from one mol of glucose . The relatively low yield of hydrogen duringfermentation is a natural consequence of the fact that fermentations have been optimized by evolution to producecell biomass and not hydrogen. Thus, a portion of the substrate (pyruvate) is used to produce ATP giving aproduct (acetate), which is excreted. Moreover, in many organisms the actual yields of hydrogen are reduced byhydrogen recycling owing to the presence of one or more uptake hydrogenase, which consume a part of hydrogen produced6.

The generation of hydrogen by fermentative bacteria also accompanies the formation of organic acids asmetabolic products, but these anaerobes are incapable of further breaking down the acids. Accumulation of theseacids causes a sharp drop of culture pH and subsequent inhibition of bacterial hydrogen production28,29. Bacteriacannot sustain at pH smaller than 5.0 and this necessitates to evolve a way to reduce acid production or to carryout certain biochemical reactions which reduces the proton concentration on the outside of the cell in proportionto the culture pH (ref. 30). The use of an aciduric facultative anaerobe of which the lower limit of pH for H2 production is as low as possible to reduce alkali consumption might be an option. Another approach to improvethe hydrogen yield is to block the formation of these acids through redirection of metabolic pathways 18,31. Thehydrogen yield is reportedly increased to 3.8 mol/(mol glucose) by blocking the pathways of organic acidformation by proton-suicide technique18 using NaBr and NaBrO3.




Gas sparging has been found to be a useful technique to reduce hydrogen partial pressure in the liquid phasefor enhancement of its yield. Mizunoet al.19 have observed that specific hydrogen production rate has increased from 1.446 mL hydrogen/min/gbiomass to 3.131 mL hydrogen/min/g biomass under nitrogen sparging conditions. With N2 sparging at a flow

rate approx 15-times the hydrogen production rate the hydrogen yield was 1.43 mol H 2 /(mol glucose). Thisshows about 50 per cent increase in hydrogen yield with nitrogen sparging.

2.5 Hybrid Reactor System

The idea of combined dark and photofermentation system takes into consideration the very fact of relativelylower achievable yield of hydrogen in dark fermentation and also the non-utilization of the acids producedtherein. The combination of photosynthetic bacteria with that of anaerobic could provide an integrated systemfor maximization of hydrogen yield32. In such a system, anaerobic fermentation of carbohydrate (or organicwastes) produces intermediates such as, low molecular weight organic acids, which are then converted tohydrogen by photosynthetic bacteria in the second step in a photobioreactor. Lee et al.23 have studied thecombination of purple nonsulphur photosynthetic bacteria and anaerobic bacteria for efficient conversion of wastewater into hydrogen. In this study, effluents from three carbohydrate-fed reactors (CSTR, ASBR, andUASB) have been used for hydrogen production. In another study, Kim et al.

33 have combined dark fermentation

with photofermentation to improve hydrogen productivity from food processing wastewater and sewage sludge.The conversion efficiency of light energy to hydrogen, with the supply of an appropriate carbon source, is one

of the key factors for hydrogen production by biological systems. Anaerobic bacteria decompose carbohydratesto obtain both energy and electron. Because reaction with only negative free energy could be possible, organicacids formed by the anaerobic digestion could not be decomposed to hydrogen any more. Complete degradationof glucose to hydrogen and carbon dioxide is virtually impossible by anaerobic digestion. But photosyntheticbacteria could use light energy to overcome the positive free energy of reaction (bacteria can utilize organicacids for hydrogen production)5. The conversion of malate and lactate to hydrogen by photosynthetic bacteria(mainly purple non-sulphur) is well documented22, 25. The main products of anaerobic fermentation are acetic andbutyric acids. Thus, further conversion of these acids into hydrogen by photofermentation underlines the synergyof the integrated process. This combination of both kinds of bacteria not only reduces the light energy demand of the photosynthetic bacteria but also enhances the hydrogen yield as well5,34.

Dark hydrogen fermentation is an incomplete oxidation, yielding not only hydrogen and CO2, but also organicacids like, acetic acid. Therefore, for an economically sound process the remaining carbonaceous compounds areto be converted, either in a photo-bioreactor to H2 and CO2 or in a methane reactor to CH4 and CO2. If the dark hydrogen fermentation is not followed by further conversion the H2 yield will not warrant economic feasibility.

3 Energy Potentials of Biohydrogen and other Bioenergy ResourcesBiohydrogen, bioethanol, and biomethane have many comparable points of resemblance, both in terms of

renewablity of their raw materials for production and their end-uses as fuels. However, bioethanol is a liquidfuel, which unlike any gaseous fuel requires the process of distillation and dehydration during production.Considering yields of ethanol from glucose as 1.6 mol/mol glucose, lower heating value of ethanol and glucoseas 296 and712 kcal/mol, the distillation energy of ethanol as50 per cent of the energy of the produced ethanol produced35, therefore,

Energy recovered as ethanol from glucose

= ( )0.50

296 1.6 100 per cent712

× × ×

= 33.25 per cent

A simplified stoichiometric equation for producing hydrogen by utilization of glucose as substrate can bewritten as:

C6H12O6 + 6H2O = 12H2 + 6CO2 ... (1)




The stoichiometric coefficients of the two gasses on the right hand side indicate that the volumetric ratio of

hydrogen to carbon dioxide should be 2 to 1. It is interesting to note that half the produced hydrogen comes fromglucose and half from water, which is split in this reaction.

The gross heating value of hydrogen is 68.6 kcal/mol, so that the energy yield per mol of glucose reacted canbe calculated as follows:

Hydrogen energy yield = 68.6 kcal/mol ×12 mol= 823.2 kcal

This can be compared to the energy yield of methane from anaerobic digestion, with reaction:

C6H12O6 = 3CH4 + 3CO2 ... (2)

In Eq. (2), 180 g of glucose produces 48 g of methane and 132 g of CO2. The gross heating value of methaneis 212.27 kcal/mol.

Methane energy yield = 212.27 kcal/mol × 3 mol= 636.81 kcal

Therefore, theoretically it is evident that the energetic of hydrogen production compares favourably withmethane production.

Anaerobic conversion of carbohydrates into methane gas is well known. In the biomethanation process theappropriate residues produce volatile acids in such a manner that it allows methanogens to build up, resulting inthe formation of CH4 and CO2. Several studies with energy crops producing methane are found in literature withalgae, kelp and water hyacinth 36, 37. It has been observed that conversion efficiencies of various crops grown formethane production are generally under 50 per cent, depending on degree of mixing and the content of celluloses/hemicelluloses. Energy recovery in biomethanation of distillery waste, both with and without

recycling of spent slurry varies within 56-65 per cent

38

. In another study, using live stock waste 32 GJ of methane was obtained from an overall energy input of 82.5 GJ39. This shows that overall energy conversion of this process of biomethanation is only 38.78 per cent. A comparison of production costs, conversion efficiency,and net CO2 emissions for different hydrogen production processes are given in Table 3. In the biologicalhydrogen production from biomass, CO2 is one of the products. Besides CO2 and H2, no other gaseous productsare expected from the dark fermentation.




Hydrogen is currently more expensive than other fuel options, so it is likely to play a major role in theeconomy in the long run, if technology improvements succeed in bringing down costs. Biological hydrogenproduction, employing renewable biomass may be a potential answer to overcome some of the economicconstraints to fulfill many of our energy needs. There is scope to use sugarcane juice; molasses or distilleryeffluent as substrates, because they contain sugar in significant quantities. Therefore, production as well as unitenergy cost of biohydrogen would be reduced drastically. However, a rigorous techno-economic analysis isnecessary to draw a cost-effective comparison between biologically produced hydrogen and the various otherconventional fossil fuels. But economic survey, based on fuel cost estimation, turns out to somewhatcomplicated when applied in practical terms. This is because of the intervening several other techno-economicparameters. The socially relevant cost of bringing any fuel to market must also include such factors as pollutionand other short and long-term environmental cost, as well as direct and indirect health cost. When these factors

are taken into consideration, together with its initial cost competitiveness, hydrogen is surely the most logicalchoice for a worldwide energy medium.

4 Concluding Remarks

It is widely acknowledged that hydrogen can offer tremendous potentials as a clean and renewable energy

currency. As a consequence, biological hydrogen production has been the subject of basic and applied research

for several decades. Many research works are available on the biochemistry, enzymology, and process

technology of biohydrogen production processes. Each process has its pros and cons in terms of technology and

productivity. Most of these works focus on the enhancement of hydrogen yield and also energy efficiencies of

the respective processes. But unfortunately, all these processes have yet to be evaluated rigorously in terms of

the cost for commercialization. Therefore, it is particularly imperative to address several techno-economic

challenges for cost-effective production as well as commercial application of biohydrogen.

AcknowledgementFinancial assistance obtained from the Department of Biotechnology, Government of India, is acknowledged.

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Table 3 — Comparison of hydrogen production costs, conversion efficiency and net CO2 emissions

Production processes Conversion efficiency Production costs CO2 emissions References(per cent) (US$/Nm3 H2) (kg/Nm3 H2)

Natural gas reforming 75 0.45 0.8 40Electrolysis of water with conventional electricity N A 0.32 1.8 40

Electrolysis with electricity from wind turbines ~75 0.35 0 40

Steam-reforming of bio-methane ~20 0.45 0 40

Photobiological hydrogen ~ 10 ~10** 0 10

Electrolysis with electricity from photovoltaic cells N A 4.13 0 40

2-stage bioprocess for hydrogen from biomass 15 0.35 0 40

Indirect micro-algal biophotolysis ~7 10* N A 41,42

Cyanobacterial biophotolysis N A 15* N A 43

Fermentative hydrogen ~22 ~ 40** N A 5

Hydrogen from coal/biomass N A 4** N A 44

* US$/GJ

** US$/MBTU

N A − Not available




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