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June 2011, Volume 2, No.3International Journal of Chemical and Environmental EngineeringDevelopments in Hydrogen Productio...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective     Table 1. Comparison of energy ...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective The merits and demerits of the bio...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective reduced ferredoxin to generate H2 ...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective  serves as the electron acceptor w...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective  input, convert dissolved organic ...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective  (stage 1) are temporally separate...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective                                   ...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective                                   ...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective                                   ...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective 8. Optical properties of light abs...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective  not yet matured and the infrastru...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective  of economic security to develop a...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective  that as the technologies develop ...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective [15].    Kumar, N., & Das, D. Enha...
Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective [48].    Mahyudin, A. R., Furutani...
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  1. 1. June 2011, Volume 2, No.3International Journal of Chemical and Environmental EngineeringDevelopments in Hydrogen Production throughMicrobial Processes; Pakistan’s ProspectiveAbdul Waheed Bhutto1, †, *, Aqeel Ahmed Bazmi2,3, Muhammad Nadeem Kardar2 and Muhammad Yaseen2,Gholamreza Zahedi3 and Sadia Karim1Department of Chemical Engineering, Dawood College of Engineeringand Technology, M.A.Jinnah Road, Karachi-Pakistan2 Biomass Conversion Research Centre (BCRC), Department of Chemical Engineering,COMSATS Institute ofInformation Technology, Defence Road, Off Raiwind Road, Lahore-Pakistan.3 Process Systems Engineering Centre (PROSPECT), Chemical Engineering Department, Faculty of ChemicalEngineering, Universiti Teknologi Malaysia, Skudai 81310, Johor Bahru (JB), Malaysia.† Affiliated member BCRC* Corresponding Author Email: abdulwaheed27@hotmail.comAbstractCurrently, hydrogen (H2) is primarily used in the chemical industry as a reactant, but it is being proposed as future fuel. H2 has greatpotential as an environmentally clean energy fuel and as a way to reduce reliance on imported energy sources. A combination of theneed to cut carbon dioxide emissions, the prospect of increasingly expensive oil and the estimated growth in the worlds vehicle fleetindicates that only H2 can plug the gap. There are many processes for H2 production. The key issue to make H2 an attractivealternative fuel is to optimize its production from renewable raw materials instead of the more common energy intensive processessuch as natural gas reforming or electrolysis of water. With such vision, this paper reviews developments in microbial processes for H2production followed by a road map to H2 economy in Pakistan. The H2 economy potentially offers the possibility to deliver a range ofbenefits for the country; however, significant challenges exist and these are unlikely to be overcome without serious efforts.Keywords: At least five1. Introduction  At the start of the 21st century, we face significant being used worldwide. Electricity is a convenient form ofenergy challenges. The concept of sustainable energy, which can be produced from various sources anddevelopment is evolved for a livable future where human transported over large distances. Hydrogen is anotherneeds are met while keeping the balance with nature. clean energy source as well as energy carrier. H2Driving the global energy system into a sustainable path economy has often been proposed by researchers asis progressively becoming a major concern and policy another clean, efficient and versatile renewable energyobjective. sources as well as energy carrier [1-3], but the At the present, world’s energy requirement is by large transformation from the present fossil fuel economy to abeing fulfilled by fossil fuels which serve as a primary H2 economy will need the solution of numerous complexenergy source. Fossil fuel has delivered energy and scientific and technological issues. The provision of costconvenience, in our homes, for transport and industry. competitive hydrogen in sufficient quantity and quality isHowever, the overwhelming scientific evidence is that the the groundwork of a hydrogen energy economy. Presentlyunfettered use of fossil fuels is causing the world’s H2 is not an alternative fuel but only an energy carrierclimate to change, with potential disastrous effect on our produced from H2-rich compounds. H2 holds the promiseplanet. The dramatic increase in the price of petroleum as a dream fuel of the future with many social, economicare also forcing for the search for new energy sources and and environmental benefits to its credit. It has the long-alternative ways. World is in search of convenient, clean, term potential to reduce the dependence on foreign oil andsafe, efficient and versatile energy source as well as lower the carbon and criteria emissions from theenergy carrier that can be delivered to the end user. transportation sector as depicted in Table 1.Electricity is one of the energy carriers which is already
  2. 2. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective Table 1. Comparison of energy and emissions of combustible common energy intensive processes Water splitting by fuels [4] artificial photosynthesis, photobiological methods based Fuel type Energy Energy Kg of per unit per carbon on algae, and high temperatures obtained by nuclear or mass volume release per concentrated solar power plants are promising approaches (MJ/kg) (MJ/l) kg of fuel (approx.) used [5]. The H2 economy is an inevitable energy system of the H2 gas 120 2 0 future where the renewable sources will be used to H2 liquid 120 8.5 0 generate H2 and electricity as energy carriers, which are Coal 15–19 — 0.5 capable of satisfying all the energy needs of human (anthracite) Coal (sub- 27–30 0.7 civilization. However nearly all H2 produced today for the — bituminous) industrial sector, is largely by thermal processes with Natural gas 33–50 9 0.46 natural gas as the H2 feedstock. Thus the development of Petrol 40–43 31.5 0.86 Oil 42–45 38 0.84 alternative and renewable pathways for producing H2 Diesel 42.8 35 0.9 fuels is of utmost importance. The purpose of this paper is Bio-diesel 37 33 0.5 to provide a brief summary of significant current and Ethanol 21 23 0.5 developing biological H2 production technologies. A Charcoal 30 — 0.5 Agricultural vision for H2 economy in Pakistan is also discussed. 10–17 — 0.5 residue Wood 15 — 0.5 2. Industrial Applications of Hydrogen Approximately 49% of hydrogen produced is used for H2 has some unique characteristics which make it the manufacture of ammonia, 37% for petroleum refining,suitable for H2 economy, namely: H2 is one of the most 8% for methanol production and about 6% forplentiful elements on Earth and in the Cosmos miscellaneous smaller-volume uses [6]. It is also used inCombustion of molecular H2 with oxygen produces heat. the petrochemical manufacturing, glass purification,H2 has the highest energy content per unit weight of any semiconductor industry and for the hydrogenation ofknown fuel (142 KJ /g or 61,000 Btu/lb) H2 can be unsaturated fats in vegetable oil [7]. In metallurgicalproduced from and converted into electricity at a processes, hydrogen mixed with N2, is used for heatrelatively high efficiency. The only byproduct is water, treating applications to remove O2 as O2 scavenger. Thewhile burning of fossil fuels generates CO2 and a variety future widespread use of hydrogen is likely to be in theof pollutants. H2 may be completely renewable fuel It can transportation sector, where it will help reduce stored as liquid, gas It can be transported over large Vehicles can be powered with hydrogen fuel cells, whichdistances using pipelines, tankers, or rail trucks. It can be are three times more efficient than a gasoline-poweredconverted into other forms of energy in more ways and engine [8, 9].more efficiently than any other fuel, i.e., in addition to 3. Current Hydrogen Productionflame combustion (like any other fuel) H2 may beconverted through catalytic combustion, electro-chemical Worldwide, H2 is being considered as a fuel for theconversion, and hydriding. future. It is an environmentally benign replacement for Some vehicle manufacturers have already gasoline, diesel, heating oil, natural gas, and other fuels indemonstrated that H2 can be used directly in an internal both the transportation and non-transportation sectors.combustion engine, and fuel cell-powered prototype cars Although abundant on earth as an element, H2 combineshave also been constructed. H2 can be transported for readily with other elements and is almost always found asdomestic/industrial consumption through conventional part of some other substances, such as water, biomass andmeans. hydrocarbons like petroleum and natural Gas. Currently Production of H2 from petroleum product or natural 500 billion cubic meters H2 are produced annuallygas does not offer any advantage over the direct use of worldwide. Presently, 40 % H2 is produced from naturalsuch fuels while Production from coal by gasification gas, 30 % from heavy oils and naphtha, 18 % from coal,techniques with capture and sequestration of CO2 could and 4 % from electrolysis and about 1 % is produced frombe an interim solution [5]. The key issue to make H2 an biomass [8, 10] Currently, the most developed and mostattractive alternative fuel particularly for the used technology is the reforming of natural gas/transportation sector is to optimize the production process hydrocarbon fuels [11]. Each method of H2 productionfrom renewable raw materials instead of the more requires a source of energy, i.e., thermal or electrolytic. 190
  3. 3. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective The merits and demerits of the biomass processes are All microbial conversions can be carried out at ambientdiscussed in Table 2. conditions, however lower rate of H2 production and low yield are chief drawbacks. All processes are controlled by Table 2. Advantages and disadvantages of different H2 the hydrogen-producing enzymes, such as hydrogenase production processes from biomass [7, 12] and nitrogenase. Hydrogenases exist in most of the Process Advantages Disadvantages photosynthetic microorganisms and they can be classified Thermochemical (i)Maximum (i)Significant gas gasification conversion can be conditioning is into two categories: (i) uptake hydrogenases and (ii) achieved required reversible hydrogenases. Uptake hydrogenases, such as (ii)Removal of tar NiFe hydrogenases and NiFeSe hydro genases, act as Pyrolysis (i)Produces (i)Chances of important catalysts for hydrogen consumption. Reversible carbonaceous catalyst deactivation material along hydrogenases, as indicated by its name, have the ability to with bio-oil, produce H2 as well as consume hydrogen depending on (ii)chemicals and the reaction condition. The major components of minerals nitrogenase are MoFe protein and Fe protein. Nitrogenase Solar gasification (i)Good H2 yield (i)Required effective collector has the ability to use magnesium adenosine triphosphate plates (MgATP) and electrons to reduce a variety of substrates Supercritical (i)Can process (i)Selection of (including protons). This chemical reaction yields conversion sewage sludge, supercritical hydrogen production by a nitrogenase-based system which is difficult medium to gasify where ADP and Pi refer to adenosine diphosphate and inorganic phosphate, respectively 2e- + 2H+ + 4ATP ---->H2 + 4ADP + 4Pi4. Biological H2 production processes The processes of biological H2 production can be Producing H2 using conventional methods defeats the broadly classified into following distinct approaches forpurpose of using H2 as a clean alternative fuel. The include: 1) Direct biophotolysis 2) Indirect biophotolysisproduction of H2 from non-fossil fuel sources has 3) Photofermentation 4) Dark fermentation 5) Microbialbecomes central for better transition to H2 economy. fuel cell (MFC) (bioelectrohydrogenesis )Certain microorganisms can produce enzymes that can 4.1. Direct Biophotolysisproduce H2 provides an attractive option to producehydrogen through microbial process. A large number of The process of photosynthetic H2-production withmicrobial species, including significantly different electrons derived from H2O [18, 22] entails H2O-taxonomic and physiological types, can produce H2. oxidation and a light-dependent transfer of electrons toDiversity in microbial physiology and metabolism means the [Fe]-hydrogenase, leading to the synthesis ofthat there are a variety of different ways in which molecular H2. The concerted action of the twomicroorganisms can produce H2, each one with seeming photosystems of plant-type photosynthesis to split wateradvantages, as well as problematic issues [13]. From an with absorbed photons and generate reduced ferredoxin toengineering perspective, they all potentially offer the drive the reduction of protons to hydrogen, is carried outadvantages of lower cost catalysts (microbial cells) and by some green algae and some cyanobacteria as shown inless energy intensive reactor operation (mesophilic) than (Fig. 1). The two photosynthetic systems responsible forthe present industrial process for making hydrogen (steam photosynthesis process are: (i) photo system I (PSI) whichreformation of methane) [14]. produces reductant for CO2 and (ii) photo system II (PSII) The H2 metabolism of green algae was first discovered which splits water to evolve O2. The two photonsin the early 1940s by Hans Gaffron. He observed that obtained from the splitting of water can either reduce CO2green algae (under anaerobic conditions) can either use by PSI or form H2 in the presence of hydrogenase. InH2 as an electron donor in the CO2-fixation process or plants, due to the lack of hydrogenase, only CO2evolve H2 in both dark and the light [15-17]. Although the reduction takes place. On the contrary, green algae andphysiological significance of H2 metabolism in algae is cyanobacteria (blue-green algae) contain hydrogenase andstill a matter of basic research, the process of thus have the ability to produce H2 [23]. In thesephotohydrogen production by green algae is of interest organisms, electrons are generated when PSII absorbsbecause it generates H2 gas from the most plentiful light energy, which is then transferred to ferredoxin. Aresources, light and water [18-21]. reversible hydrogenase accepts electrons directly from the 191
  4. 4. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective reduced ferredoxin to generate H2 in the presence of Even though photosynthetic hydrogen production is ahydrogenase. theoretically perfect process with transforming solar energy into hydrogen by photosynthetic bacteria, applying it to practice is difficult due to the low utilization efficiency of light and difficulties in designing the reactors for hydrogen production [26-28]. 4.2. Photofermentation Photofermentation also requires input of light energy for hydrogen production from various substrates, in particular organic acids, by photosynthetic bacteria (Fig .2). Photosynthetic bacteria have long been studied for their capacity to produce hydrogen through the action of their nitrogenase system. Fermentative hydrogen production has the advantages of rapid hydrogen production rate and simple operation. Photosynthetic bacteria have long been studied for their capacity to produce significant amounts of hydrogen due to their high substrate conversion efficiencies and ability to degrade a wide range of substrates.   The photosynthetic bacteria have been shown to produce hydrogen from various organic acids and food processing and agricultural wastes [13]. Although pure substrates have usually been used in model studies, some success in using industrial wastewater as substrate has been shown [29, 30]. In general, rates of hydrogen production by photoheterotrophic bacteria are higher when the cells are immobilized in or on a solid matrix, than when the cell is free-living. However, pre-treatment may be needed prior to photosynthetic biohydrogen gas production due to either Figure-1. Direct Biophotolysis (green algae – cyanobacteria) [14] the toxic nature of the effluent, or its color/ opaqueness. Since hydrogenase is sensitive to oxygen, it isnecessary to maintain the oxygen content at a low level(under 0.1 %) so that the hydrogen production can besustained [13]. This process results in the simultaneousproduction of O2 and H2 with a H2: O2 = 2:1 ratio [24].This mechanism holds the promise of generatinghydrogen continuously and efficiently through the solarconversion ability of the photosynthetic apparatus. In theabsence of provision for the active removal of oxygen,this mechanism can operate only transiently, as molecularoxygen is a powerful inhibitor of the enzymatic reactionand a positive suppressor of [Fe]-hydrogenase gene Figure-2. Photofermentation (Photosynthetic bacteria) [14]expression. At present, this direct mechanism has 4.3. Dark fermentationlimitations as a tool of further research and for practicalapplication, mainly due to the great sensitivity of the [Fe]- In dark fermentation, H2 production is inherently morehydrogenase to O2, which is evolved upon illumination by stable since it takes place in the absence of oxygen. Thethe water-oxidizing reactions of PSII [25]. Nevertheless, oxidation of the substrate by bacteria generates electronssuch H2 co-production can be prolonged under conditions which need to be disposed off in order to maintain thedesigned to actively remove O2 from the reaction mixture. electrical neutrality. Under the aerobic conditions O2 192
  5. 5. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective serves as the electron acceptor while under the anaerobicor anoxic conditions other compounds, such as protons,act as the electron acceptor and are reduced to molecularH2. Hydrogen can be produced by anaerobic bacteria,grown in the dark on carbohydrate-rich substrates. While direct and indirect photolysis systems producepure H2, dark-fermentation processes produce a mixedbiogas containing primarily H2 and carbon dioxide (CO2),but which may also contain lesser amounts of methane(CH4), CO, and/or hydrogen sulfide (H2S). The gascomposition presents technical challenges with respect tousing the biogas in fuel cells. In order for hydrogenproduction by dark fermentation to be economicallyfeasible and sustainable, a two-step/hybrid biologicalhydrogen production process would be necessary. Higher overall substrate conversion efficiency ispossible by combining the anaerobic and photosyntheticsteps, as shown in Fig. 3. The photosynthetic microbes Figure 3. Dark fermentation (Clostridia, Enterobacteracae) [14]can degrade the soluble metabolites from the fermentativestep using sunlight to overcome the energy barrier. Dark fermentation reactions can be operated at 4.4. Microbial fuel cell (MFC)mesophilic (25 –40°C), thermophilic (40–65°C), extremethermophilic (65–80°C), or hyperthermophilic (80°C) It is based on the concept and practice of a microbialtemperatures. fuel cell (MFC). Fact the idea is to add a little electrical Biohydrogen production by dark fermentation is potential to that generated by a microbial fuel cell, thushighly dependent on the process conditions such as reaching a sufficient force to reduce protons to hydrogen,temperature, pH, mineral medium formulation, type of in a process that can be called bioelectrohydrogenesis. Aorganic acids produced, hydraulic residence time (HRT), MEC consists of four parts: first, the anodic chamber withtype of substrate and concentration, hydrogen partial the anode; second, the cathodic chamber with cathode;pressure, and reactor configuration [31]. third, an external electrical power source; and fourth, an electronic separator [32, 33] as shown in Fig. 4.Thus the Since organic substrates are the ultimate source of cell could be called a microbial electrohydrogenesis cellhydrogen in photofermentations or indirect biophotolysis (MEC). Acetate is typically used as the electron donorprocesses, it can be argued that it should be simpler and and it is oxidized according to the following reaction [34]:more efficient to extract the hydrogen from such Acetate - + 4H2O  2HCO3- + 9H+ +9e-substrates using a dark fermentation process [13]. The pH at the anode surface has a strong tendency to decrease, as one proton is produced per electro transferred [35, 36]. At the cathode the hydrogen evolution reaction takes place, in which protons and electrons are combined to form hydrogen: 2H2 + 2e  H2 The reaction can be catalyzed by microorganisms or by a chemical catalyst like platinum or nickel. When microorganisms are used as catalyst these reactions are essentially anaerobic respirations where the external electron acceptor is an electrode instead of the more usual oxidized compound (nitrate, TMAO, fumurate, etc.). Thus bioelectrohydrogenesis utilizes electrochemically active micro-organisms which, with a small to moderate voltage 193
  6. 6. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective input, convert dissolved organic matter into hydrogeninside an electrochemical cell/microbial fuel cell viacoupled anode-cathode reactions. Expressed per amountof organic matter, the MEC can achieve much higherhydrogen yields (80–100%) [37] compared tofermentative hydrogen production (<33%). This isbecause the MEC uses electricity to overcome theenergetic barrier for acetate oxidation. Figure 5. Indirect biophotolysis [13] One elaboration of this concept [37] involved four distinct steps: 1. Production in open ponds at 10% solar efficiency of a biomass high in storage carbohydrates. 2. Concentration of the biomass from the ponds in a settling pond. 3. Anaerobic dark fermentation to yield 4H2 /glucose stored in the algal cells, plus 2 acetates. 4. A photobioreactor in which the algal cells would Figure-4. Layout of the microbial electrolysis cell. convert the two acetates to 8 mol of H2.Microorganisms present on the anode catalyze the oxidation ofsubstrate to bicarbonate, protons and electrons. The production of After this last step the algal biomass would be returned tohydrogen on the cathode may be catalyzed by a chemical catalyst or the ponds, to repeat the cycle. Support systemsby microorganisms (biocathode) [32] included the anaerobic digestion (methane fermentation) of any wasted biomass (assumed at 10% The performance of a MEC is determined on the one for each cycle), an inoculum production system tohand by the physiology of the microorganisms, and on the provide make-up biomass and a gas handling andother hand by the physical chemical transport processesinvolved. There remains a great challenge to reduce the separation system (to recycle the CO2 from the H2overpotential at both the bioanode and biocathode [32]. A back to the ponds) [13].typical application of a MEC would be wastewater Genetic modifcation of strains to eliminate uptaketreatment, in which the organic compounds in the hydrogenases and increase levels of bidirectionalwastewater serve as electron donors for the bioanode [38, hydrogenase activity may yield signi6cant increases in39]. MEC could also produce hydrogen from H2 production.agroindustrial residues containing biopolymers likecellulose and starch. 4.6. Two-stage System4.5. Indirect Biophotolysis Photosynthetic O2 formation and H2 evolution occur Indirect biophotolysis processes involve separation of simultaneously in green algae as electrons and protonsthe H2 and O2 evolution reactions into separate stages released from photosynthetic H2O oxidation are used in(Fig. 5), coupled through CO2 fixation/evolution. Indirect the hydrogenase catalysed H2 evolution [30, 40]. In thisbiophotolysis, consists of two stages in series: one-stage process, H2 evolution is transient and cannot bephotosynthesis for carbohydrate accumulation, and dark sustained due to strong deactivation of hydrogenasefermentation of the carbon reserve for hydrogen activity by O2 (at as low as 2% partial pressure) evolvedproduction. In this way, the oxygen and hydrogen from photosynthesis [41]. This mutually exclusive natureevolutions are temporally and/or spatially separated. This of the O2 and H2 photoproduction reactions has halted theseparation not only avoids the incompatibility of oxygen development of H2 production process by green algaeand hydrogen evolution (e.g., enzyme deactivation and under ambient conditions [41].the explosive property of the gas mixture), but also makes To overcome this problem, a two-stage protocol hashydrogen purification relatively easy because CO2 can be been developed to evolve H2 from green algae, in whichconveniently removed from the H2/CO2 mixture. photosynthetic O2 evolution and carbon accumulation 194
  7. 7. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective (stage 1) are temporally separated from the consumption Cells from Reactor 1 are transferred to Reactor 2,of cellular metabolites and concomitant H2 production which is maintained under anaerobic conditions. Cells(stage 2) [18, 42]. H2 evolution strongly depended upon entering Reactor 2 already have suppressed PS-IIthe duration of anaerobic incubation, deprivation of systems, so they will not cause Reactor 2 to go aerobic.sulphur (S) from the medium and the medium pH [43]. Any residual oxygen is quickly consumed by the algae in It has been reported that inhibition of the hydrogenase Reactor 2. Finding themselves under anaerobicby oxygen can be partially overcome by cultivation of conditions, the cells will start producing hydrogenase andalgae under sulfur deprivation for 2–3 days to provide subsequently, H2. The transition step that consumes theanaerobic conditions under the light [26, 44]. Melis et al. oxygen in solution in the batch system is avoided by[42] and Ghirardi et al. [25] devised a mechanism to having Reactor 2 already anaerobic. At the same time,partially inactivate PSII activity to a point where all the some cells are continuously removed from Reactor 2. TheO2 evolved by photosynthesis is immediately taken up by effect is that the cells are removed from Reactor 2 beforethe respiratory activity of the culture. This mechanism is they completely stop producing H2. Successful operationbased on a two-step process. The steps, growth mode and has been shown with a dilution rate of 0.5/day, which isH2 production mode, are initiated by cycling between equivalent to an average residence time of 2 days for thesulfur-containing and sulfur-free culture medium. This cells. Because Reactor 2 is a continuously-stirred reactorresults in a temporal separation of net O2- and H2- (like Reactor 1), the average residence time is 2 days, butevolution activities in the green alga Chlamydomonas some individual cells removed from the reactor may havereinhardtii. This discovery eliminates the need for a purge been there longer or shorter times. With an averagegas, but introduces the need for careful sulfate controls in residence time of 2 days, one would expect a H2the aqueous medium. production rate lower than the initial production rate of The absence of sulfur nutrients from the growth the batch system, but higher than the production rate atmedium of algae acts as a metabolic switch, one that the end of a batch production cycle.selectively and reversibly inhibits photosynthetic O2production. Thus, in the presence of S, green algae donormal photosynthesis (H2O-oxidation, O2-evolution andbiomass accumulation) [45]. In 2002, NREL researchers developed a system usingtwo continuous-flow reactors for producing H2continuously for periods of up to several weeks [46]. Thecontinuous H2 production process involves using twocontinuously-stirred tanks. Fig.6 shows the tankconfiguration. In Reactor 1, cells are cultured in mediacontaining minimal levels of sulfur. PS-II is slowed andoxygen production remains lower than oxygenconsumption for cellular respiration, but by bubbling the Figure 6. Continuous H2 Productionsolutions with carbon dioxide and a small amount ofoxygen, the cells are able to remain in Reactor 1 The merits and demerits of each biological process areindefinitely, obtaining some energy from photosynthesis discussed in Table 3.and some energy through respiration of acetate insolution. 195
  8. 8. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective Table 3. Comparison of important biological H2 production processes [12, 47]Process General reaction Advantages Disadvantages Maximum Reference reported rate (mmol H2 /L h)Direct 2 H2O + light → 2 H2 + O2 -Can produce H2 directly -Requires high intensity of light 0.07 [48, 49]biophotolysis from water and sunlight -Simultaneous production of O2 -Solar conversion energy and H2. O2 can be dangerous for increased by ten folds as the system compared to trees, crops -Hydrogenase (green algae) is highly sensitive to even moderately low concentrations of O2 -Lower photochemical efficiencyIndirect (a) 6H2O + 6CO2 + light → -Cyanobacteria can -Uptake hydrogenase enzymes are 0.36 [50, 51]biophotolysis C6H12O6 + 6O2 produce H2 from water to be removed to stop degradation (b) C6H12O6 + 2H2O→ 4H2 + -Has the ability to fix N2 of H2 2CH3COOH + 2CO2 from atmosphere -About 30% O2 present in gas (c) 2CH3COOH + 4H2O + mixture light → 8 H2 + 4CO2 Overall reaction 12H2O + light → 12 H2 + 6O2Photo- CH3COOH + 2H2O + light → -A wide spectral light -Production rate of H2 0.16 [52]fermentation 4H2 + 2CO2 energy can be used by is slow these bacteria -O2 has an inhibitory effect on -Can use different organic nitrogenase wastes -Light conversion efficiency is -High substrate very low, only 1–5% conversion efficiencies -Pre-treatment may be needed due -Degrade a wide range of to either the toxic nature of the substrates. substrate (effluent), or its color/opaqueness. -Large reactor surface areas requirement -Expensive equipmentDark C6H12O6 + 2H2O → -Simpler, less expensive, -O2 is a strong inhibitor of 75.60 [53, 54]Fermentation 2CH3COOH + 4H2 + 2CO2 and produce hydrogen at hydrogenase much higher rate -Relatively lower achievable yields 64.50 -It can produce H2 all day of H2 long without light -As yields increase H2 -A variety of carbon fermentation becomes sources can be used as thermodynamically unfavorable substrates -Product gas mixture contains CO2 -It produces valuable which has to be separated metabolites such as butyric, lactic and acetic acids as by products -It is anaerobic process, so there is no O2 limitation problemMicrobial C6H12O6 + 2H2O → 4H2 + -Energy available in -Metabolic pathways involved arefuel cell 2CO2 + 2CH3COOH waste streams can be not clear (MFC) directly recovered as -MEC studies have been carried Anode: CH3COOH + 2H2O → electricity (MFC) or out only with mixed cultures, often 2CO2 + 8e- + 8H+ (15) hydrogen (MEC). using those already enriched and Cathode: 8H+ + 8e- → 4H2 promising future active in microbial fuel cells approach to hydrogen (MFC). generation from -Power densities at the electrode wastewater, especially for surface are low, which translates effluents with low organic into low volumetric hydrogen content. production. -Higher yields require increased voltage, adversely affecting energy efficiency.Two-stage 51.20 [15, 55]fermentation(dark + 47.92photo) 197
  9. 9. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective Other barriers to microbial based, large-scale5. Barriers for biohydrogen production production of H2 include [57] [Maness el al 2009] (a) The diffuse nature of solar energy and the consequent inherent properties of the microbes that precludelow energy density places severe economic restrictions on continuity and efficiency of H2 production; (b) underlyingpotential light-driven processes for biological conversion limitations of photosynthetic efficiency; and (c)of solar energy to hydrogen [13]. limitations of the hydrogenase catalytic function. Major challenges need to be overcome for the smooth Scientific and technical barriers for biohydrogentransition from the fossil fuel based economy to the H2 production have been summarized in Table based economy and may be outlined as follows[56]: 6. Immobilization The yield of H2 from any of the processes defined One of the largest challenges of optimizing molecular above is low for commercial application. H2 production by Chlamydomonas reinhardtii cells is the The pathways of H2 production have not been transfer of the cells from sulfur deficient conditions to identified and the reaction remains energetically sulfur rich conditions (for regenerative purposes) and then unfavorable. back to sulfur deficient conditions (for further H2 There is no clear contender for a robust, industrially production). Recent research in immobilization has capable microorganism that can be metabolically provided a new technique to eliminate this challenge. engineered to produce more than 4 mol H2/mol of Prior to the development of immobilizations, cells were glucose. suspended in aqueous media with either sulfur rich or Several engineering issues need to be addressed which deficient conditions present. This posed a problem forinclude the appropriate bioreactor design for H2 scientists because the cells had to be filtered out of theproduction, difficult to sustain steady continuous H2 media to be transferred to the next media in the cycle ofproduction rate in the long term, scale-up, preventing molecular H2 production. The filtration process was veryinterspecies H2 transfer in non sterile conditions and time consuming and so was not feasible on an industrialseparation/purification of H2. scale. Another dilemma that plagued the free suspension Sensitivity of hydrogenase to O2 and H2 partial pressure in liquid media technique was the inability to make theseverely disrupts the efficiency of the processes and adds media with cells very concentrated. This restricted theto the problems of lower yields. Insufficient knowledge amount of light that could interact with the cellson the metabolism of H2 producing bacteria and the levels decreasing the overall yield of molecular H2. To avoidof H2 concentration tolerance of these bacteria. difficulties with media transition or cellular concentration immobilization techniques were developed [58]. Table 4. Scientific and technical barriers for biohydrogen production [7] Type of barrier Barrier Putative Solution Bacteria do not produce more than 4 Isolate more novel microbes and combinational mol H2/mol glucose naturally screen for H2 production rates yields, and Organism durability. Genetic manipulation of established Basic science bacteria. Hydrogenase over expression not Greater understanding of the enzyme regulation Enzyme stable and expression. (hydrogenase) O2 sensitivity Mutagenic studies. H2 feed back inhibition Low H2 partial pressure fermentation. High cost of suitable feedstock Renewable biomass as feedstock. Feedstock (glucose) Co-digestion/use of microbial consortia which can Fermentative Low yield using renewable biomass increase the yield Lack of industrial-suitable strain Development of industrially viable Strain strain(s)/consortia 198
  10. 10. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective Commercially feasible product yield Hybrid system (photo + dark fermentation) Incomplete substrate utilization Link fermentation to a second process that makes Sustainable process both economically possible Process Sterilization Application and utilization of fermentation tools such as continuous culture Development of low-cost stream sterilization technology/process that can bypass sterilization Engineering Lack of kinetics/appropriate reactor Incorporation of process engineering concepts to design for H2 production Light develop a suitable reactor for the defined Reactor intensity in case of photo-bioreactor strain/process, flat panel or hollow tube reactor can be employed Thermodynamic barrier Reverse electron transport to drive H2 production Thermodynamic NAD(P)H → H2 (+4.62 kJ/mol) past barrier Selection absorption of CO2 /H2S H2 H2 purification/separation Storage Basic studies on H2 storage7. Maximum possible yield of H2 by green algae Application of the two-stage photosynthesis and H2 production protocol to a green alga mass culture could Even though the catalytic activity of the various provide a commercially viable method of renewable H2enzymes differs enormously, there is no evidence for the generation.quantity of hydrogen-producing enzyme being the Table 5 provides preliminary estimates of maximumlimiting factor. Indeed, in many microbial systems, possible yield of H2 by green algae, based on thepotential catalytic activity far surpasses the amount of luminosity of the sun and the green algal photosynthesishydrogen produced, suggesting that other metabolic characteristics. Calculations were based on the integratedfactors are limiting [13]. luminosity of the sun during a cloudless spring day. In The use of light attenuation devices that transfer mid-latitudes at springtime, this would entail delivery ofsunlight into the depths of a dense algal culture is an approximately 50 mol photons m 2 d 1 (Table 5). It isapproach to overcoming the light saturation effect in light generally accepted that electron transport by the twodriven processes. The simplest approach is to arrange photosystems and via the hydrogenase pathway for thephotobioreactors in vertical arrays to reduce direct production of 1 mol H2 requires the absorption andsunlight. Of course, this arrangement also proportionally utilization of a minimum of 5 mol photons in theincreases the area of required photobioreactors, which is photosynthetic apparatus (Table 5). On the basis of thesethe limiting economic factor in any photobiological fuel- “optimal” assumptions, it can be calculated that greenproduction process. algae could produce a maximum 10 mol (20 g) H2 per m2 Another alternative is the use of optical fiber culture area per day. If yields of such magnitude could bephotobioreactors, in which light energy is collected by approached in mass culture, this would constitute a viablelarge concentrating mirrors and piped into small and profitable method of renewable H2 production.photobioreactors with optical 1bers [13]. Table 5. Yield of H2 photoproduction by green algae (Estimates are based on maximum possible daily integrated irradiance and algal photosynthesis characteristics.) [20, 59-61] Photoproduction Characteristics Comments on Assumptions Made Maximum photosynthetically active radiation, 50 mol photons m 2 Daily irradiance can vary significantly depending on season and cloud d 1 (based on a Gaussian solar intensity profile in which the peak cover. It can be greater than 50 mol photons m 2 d 1 in the summer and solar irradiance reaches 2,200 µmol photons m 2 s 1) much less than that on cloudy days and in the winter. [29]. Theoretical minimum photon requirement for H2 production in green Based on the requirement of 10 photons for the oxidation of two water algae: 5 mol photons/mol H2 molecules and the release of four electrons and four protons in photosynthesis [30, 31] Theoretical maximum yield of H2 production by green algae: 10 mol Assuming that all incoming photosynthetically active radiation will be H2 m 2 d 1 (20 g H2 m 2 d 1; ~80 kg H2 acre 1 d 1) absorbed by the green algae in the culture and that it will be converted into stable charge separation. 199
  11. 11. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective 8. Optical properties of light absorption by Photosynthetic H2 production by green algae involves green algae water splitting to produce H2 and oxygen. Unfortunately, Light absorption by the photosynthetic apparatus is H2 production by this process is quite ineffective since itessential for the generation of H2 gas. However the simultaneously produces oxygen, which inhibits theoptical properties of light absorption by green algae hydrogenase enzyme. Thus, during light reaction, H2impose a limitation in terms of solar conversion evolution ceases due to an accumulation of oxygen.efficiency in the algae chloroplast. This is because wild- Therefore the prerequisite for photohydrogen productiontype green algae are equipped with a large size light- by green algae is that they have to adapt to an anaerobicharvesting chlorophyll antenna to absorb as much sunlight they can. Under direct and bright sunlight, they could By exposing the cells to specific conditions scientistswaste up to 60% of the absorbed irradiance [47, 62]. This are able to modify photosynthesis so that oxygen will notevolutionary trait may be good for survival of the act as the final electron carrier of the electron transportorganism in the wild, where light is often limiting, but it chain; rather H2 will allow the cells to release molecularis not good for the photosynthetic productivity of a green H2 as opposed to molecular oxygen.algal mass culture. This optical property of the cells could Melis [45] estimates that, if the entire capacity of thefurther lower the productivity of a commercial H2 photosynthesis of the algae could be directed toward H2production farm. production, 80 kilograms of H2 could be produced The analysis up to this point has shown that H2 commercially per acre per day. The yield of H2production can be limited by the photons available or the production currently achieved in the laboratorycapacity of algae to process the photons into H2. Another corresponds to only 15 to 20% of the measured capacityobservation is that the number of photons absorbed is of the photosynthetic apparatus for electron transportmuch higher than the algae’s ability to process the [63].photons. By reducing the number of excess photons In a laboratory, Melis [45] worked with low-densityabsorbed and let them reach deeper into the liquid, it may cultures and have thin bottles so that light penetrates frombe possible to produce more H2. By reducing the size of all sides. Because of this, the cells use all the light fallingthe algae’s light collecting antennae, but not affecting the on them. But in a commercial bioreactor, where denseorganism’s ability to process the photons to produce H2, algae cultures would be spread out in open ponds underone gets deeper light penetration for the same cell the sun, the top layers of algae absorb all the sunlight butconcentration, which means more photons are available at can only use a fraction of it [63].the lower depths for H2 production. Further research and development aimed at increasing While regular green algae absorb most of the light rates of synthesis and final l yields of H2 are essential.falling on them, algae engineered to have less chlorophyll Optimization of bioreactor designs, rapid removal andlet some light left through. In University of California, puri6cation of gases, and genetic modifcation of enzymeBerkeley, Melis and his colleagues are designing algae pathways that compete with hydrogen producing enzymethat have less chlorophyll so that they absorb less sunlight systems offer exciting prospects for biohydrogen systems[63]. When grown in large, open bioreactors in dense [48]. Increase in the rate of H2 would reduce bioreactorcultures, the chlorophyll-deficient algae will let sunlight size dramatically to overcome the engineering challengespenetrate to the deeper algae layers and thereby utilize of scale up, and create new opportunities for practicalsunlight more efficiently [64]. applications. The critical enzymatic component of this 9. H2 Economyphotosynthetic reaction is the reversible hydrogenase A typical energy chain for sustainable H2 comprisesenzyme, which reduces protons with high potential the harvesting of sunlight into H2 as energy carrier, theenergy electrons to form H2. During normal storage and distribution of this energy carrier to the end-photosynthesis, algae focus on using the sun’s energy to device where it is converted to power. The key market forconvert carbon dioxide and water into glucose, releasing fuel cells has always assumed to be the automotiveoxygen in the process. Only about 3 to 5 percent of industry. The great expectation that hydrogen fuel-cellphotosynthesis leads to H2. Because hydrogenase is powered vehicles will displace gasoline and dieselsensitive to oxygen, this H2 production must be carried powered vehicles has not materialized for a variety ofout in an anaerobic environment reasons, but primarily because fuel cell technology has 200
  12. 12. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective not yet matured and the infrastructure required for cells connected directly to wind turbines are a convenienthydrogen storage, transportation, and refueling has been way to balance out local fluctuations in the availability ofslow to develop. Consumer energy applications will wind power. The development of fuel cells and a H2require delivery systems that can supply H2 as readily as economy will provide new market opportunities and newgasoline and natural gas are supplied today. Higher- jobs. Present knowledge indicates that H2 as an energypressure gaseous storage and non-conventional storage carrier will involve little environmental risk. Alltechnologies will be used to meet the requirements of renewable hydrogen production technologies face thetransportation applications (storage at 350–700 bar common challenge of integration with hydrogencompared to the 200 bar storage pressure commonly used purification and storage [65].in normal merchant gas systems) [65]. 10. Present energy scenario of Pakistan Gas purity requirements are important for the H2energy market. They very much depend on the energy Pakistan is basically an energy deficient country.conversion device used, as well as on the storage Pakistan’s per capita energy consumption, 3894kWh astechnology. Combustion systems are much less sensitive against the world average of 17620kWh, gives it ato impurity levels, however, fuel cells are very sensitive ranking of 100 amongst the nations of the world [70]. Theto CO and sulfur poisoning. demand for primary energy in Pakistan has increased The U.S. Department of Energy has developed a considerably over the last few decades and the country ismultiyear plan with aggressive milestones and targets for facing serious energy shortage problems. The energythe development of H2 infrastructure, fuel cells, and supply is not increasing by any means to cope with thestorage technologies. The targeted H2 cost is $2–4 kg-1 rising energy demands. As a result the gap between the(energy equivalent of 1 gallon of gasoline) delivered [66, energy demand and supply is growing every year. The67] country is meeting about 86% of oil demand from imports A rollout of such a sustainable H2 chain in developed by spending around US$6.65 billion per annum [71].countries could go either gradually via a H2 economy Pakistan’s future energy system looks ratherbased on fossil fuels or discontinuously in the case of uncertain. In recent years, the combination of rising oilinventions of disruptive technologies. For developing consumption and flat oil production in Pakistan has led tocountries the situation may be different. Introduction of rising oil imports from Middle East exporters. Thesuch H2 chains for their fast-growing primary energy balance recoverable reserves of crude oil in the country asdemands might enable them to skip the stage of on January 1st 2010 have been estimated at 303.63conventional, fossil fuel-based technologies and markets million barrels [72].and leapfrog directly to a sustainable H2 economy [68]. Natural gas accounts for the largest share ofThe salient features of a H2 economy will be as follows Pakistan’s energy use, amounting to nearly 43.7 percent[69]: of total energy consumption. As on January 1, 2010, the A H2-based energy system will increase the balance recoverable natural gas reserves have beenopportunity to use renewable energy in the transport estimated at 28.33 trillion cubic feet. The averagesector. This will increase the diversity of energy sources production of natural gas during July- March 2009-10and reduce overall greenhouse gas emissions. H2 in the was 4,048.76 million cubic feet per day (mmcfd) [72]. Astransport sector can reduce local pollution, which is a the demand of natural gas exceeds the supply, country ishigh priority in many large cities. already facing shortage of natural gas and during the peak The robustness and flexibility of the energy system demand most of the gas fired generating units arewill be increased by the introduction of H2 as a strong shutdown while duel fuel units are fired by oil. Pakistan isnew energy carrier that can interconnect different parts of presently facing shortage of around 300-350 MMCDF ofthe energy system. The targets for reducing vehicle noise natural gas which is likely to go up because of risingmay be met by replacing conventional engines with H2- needs and slowing down of supplies at home [73].powered fuel cells. Fuel cells for battery replacement and According to The Energy Security Action Plan of thebackup power systems are niche markets in which price Planning Commission, Pakistan will be facing a shortfalland efficiency are relatively unimportant. Sales in this in gas supplies rising from 1.4 Billion Cubic Feet (BCF)market will drive the technology forward towards the per day in 2012 to 2.7 BCF in 2015 and escalating to 10.3point at which fuel cells will become economic for the BCF per day by the year 2025 [74]. It is therefore a matterintroduction into the energy sector. H2 electrolysers/fuel 201
  13. 13. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective of economic security to develop alternative H2 resources economy, and will have a positive impact on theto avoid mid century energy crises in the country. environment in which atmospheric pollution is all but Natural gas is used in general industry to prepare alleviated and the so-called greenhouse effect isconsumer items, produce cement, fertilizer and generate mitigated.electricity. At present, the power sector is the largest user To ensure a sustainable energy future for Pakistan, itof gas accounting for 33.5 percent share followed by the is necessary that the energy sector be accorded a highindustrial sector (23.8 percent), household (18.1 percent), priority. In Pakistan efforts to reduce reliance on fossilfertilizer (15.6 percent), transport (5.4 percent) and fuels through increasing the share of renewable energy incement (0.9 percent) [75]. Natural gas is used in the the energy supply systems have met with little success sotransport sector in the form of CNG. There are about far. Mirza et al. [77] and Sahir and Qureshi [78] have3,116 established CNG stations in the country and discussed the barriers to development of renewableapproximately 2 million vehicles are using CNG. Pakistan energy. Mirza, et al. [77] has broadly classified thesehas become the largest CNG consuming country among barriers as policy and regulatory barriers, institutionalNatural Gas Vehicle (NGV) countries. According to barriers, fiscal and financial barriers, market-relatedPetroleum Policy 1997; the use of CNG in vehicles was barriers, technological barriers and information and socialencouraged by Government to reduce pressure on barriers. They have also suggested better coordinationpetroleum imports, to curb pollution and to improve the among various stakeholders and indigenization ofenvironment [75]. renewable energy technologies to overcome these Transport sector is one of the major consumers of barriers.commercial energy in Pakistan. It accounted for about Sahir, and Qureshi [78] has suggested an integrated28% of the total final commercial energy consumed energy planning approach, consistency in government(33.95 MTOE) and 55.8% of the total petroleum products policies and rational policy instruments to deal with theconsumed (15 MTOE) in the country. techno-economic and socio-political barriers are the pre- requisites for long-term sustainable development of the11. H2 Production in Pakistan renewable energy technologies. In Pakistan H2 is largely produced in the fertilizer There is little doubt that power production byindustry from natural gas, which is used for the renewable energies, energy storage by H2, and electricproduction of anhydrous ammonia. All urea plants in the power transportation and distribution by smart electriccountry are based on natural gas as feedstock. On an grids will play an essential role in phasing out fossil fuels.average, the fertilizer sector consumes 15.6 per cent ofnatural gas produced in country. The government 12. Conclusionsprovides an indirect subsidy to fertilizer manufacturers by Concerns about global warming and environmentalselling feedstock gas at rates ranging up to $1.0 against pollution due to the use of fossil fuels, combined withcommercial rates of $4.0 per MMBTU. The return on projections of potential fossil fuel shortfall toward thepaid-up capital in the fertilizer industry is about 80-100 middle of the 21st century, make it imperative to developper cent per annum [73]. The current energy scenario in alternative energy sources that would clean, renewable,the country, already discussed above , identifies the and environmentally friendly.transport sector and fertilizer sector as key sectors where It is important to note that hydrogen can be producedthe H2 gas can be immediately employed as substitute to from a wide variety of feed stocks available almostfossil fuel. anywhere. There are many processes under development Mirza et al. [76] has presented complete road map to which will have a minimal environmental impact.H2 economy in Pakistan. They have concluded that the H2 Development of these technologies may decrease theeconomy potentially offers the possibility to deliver a world’s dependence on fuels that come primarily fromrange of benefits for the country including reducing unstable regions. The ‘‘in house’’ hydrogen productiondependence on oil imports, environmental sustainability may increase both national energy and economic security.and economic competitiveness. In medium term advent of The ability of hydrogen to be produced from a wideH2 will bring about technological developments in many variety of feedstocks and using a wide variety offields, including power generation, agriculture, the processes makes it so that every region of the world mayautomotive industry, and other as yet unforeseen be able to produce much of their own energy. It is clearapplications. It will increase employment, stimulate the 202
  14. 14. Developments in Hydrogen Production through Microbial Processes; Pakistan’s Prospective that as the technologies develop and mature, hydrogen Non-incorporation of renewable energy issues in themay prove to be the most ubiquitous fuel available. regulatory policy and lack of awareness among regulators The vision for a H2 future is one based on clean restrict technology penetration. There is a lack ofsustainable renewable energy supply of global financial resources and proper lending facilities,proportions that plays a key role in all sectors of the particularly for small-scale projects in country. Ineconomy. addition, the absence of a central body for overall Microbial Processes provides an attractive option to coordination of energy sector activities results inproduce H2 at ambient conditions. A large number of duplication of R&D activities. Unfortunately privatemicrobial species, including significantly different sector especially transports and fertilizer sector has madetaxonomic and physiological types, can produce H2, no contributions to promote research activities to produceDiversity in microbial physiology and metabolism means H2 from renewable resources.that there are a variety of different ways in whichmicroorganisms can produce H2, each one with seeming REFERENCESadvantages, as well as problematic issues. Lower rate ofH2 production and low yield are chief drawbacks. From [1]. Kırtay, E. Recent advances in production of hydrogen froman engineering perspective, they all potentially offer the biomass. Energy Convers. Manage., (2011). 52: 1778-89.advantages of lower cost catalysts (microbial cells) and [2]. Momirlan, M., & Veziroglu, T. 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