Microbial Conversion of Bimass For Hydrogen EconomyThis is a featured page



Introduction

The phenomenon of biological hydrogen production was observed one century ago. When the oil crisis broke out in 1970s, the technology started receiving attention, especially in hydrogen production by photosynthetic process. However these works are in laboratory scale and the practical applications still need to be demonstrated. Biological hydrogen production can be classified into five different groups (Levin et al., 2004):
  • Direct biophotolysis,
  • Indirect biophotolysis,
  • Biological water–gas shift reaction,
  • Photo-fermentation and
  • Dark fermentation

Table: General reaction implicated in the microbial conversion of Biomass
Process General reaction Microorganism
1 - Direct biophotolysis 2 H2O + light → 2 H2 + O2 Microalgae
2 - Photo-fermentations CH3COOH + 2 H2O + light → 4 H2 + 2 CO2 Purple bacteria,
Micro-algae
3 - Indirect biophotolysis (a) 6 H2O + 6 CO2 + light → C6H12O6 + 6 O2
(b) C6H12O6 + 2 H2O → 4 H2 + 2 CH3COOH + 2 CO2
(c) 2 CH3COOH + 4 H2O + light → 8 H2 + 4 CO2
Microalgae, Cyanobacteria
Overall reaction: 12 H2O + light → 12 H2 + 6 O2
4 - Water Gas Shift Reaction CO + H2O → CO2 + H2 Fermentative bacteria, Photosynthetic bacteria
5 - Two-Phase H2 + CH4
Fermentations
(a) C6H12O6 + 2 H2O → 4 H2 + 2 CH3COOH + 2 CO2
(b) 2 CH3COOH = 2 CH4 + 2 CO2
Fermentative bacteria + Methanogenic bacteria
6 - High-yield Dark
Fermentations
C6H12O6 + 6 H2O → 12 H2 + 6 CO2 Fermentative bacteria


All processes are controlled by the hydrogen-producing enzymes, such as hydrogenase and nitrogenase. The major components of nitrogenase are MoFe protein and Fe protein. Nitrogenase has the ability to use magnesium adenosine triphosphate (MgATP) and electrons to reduce a variety of substrates (including protons). This chemical reaction yields hydrogen production by a nitrogenase-based system where ADP and Pi refer to adenosine diphosphate and inorganic phosphate, respectively: 2e- + 2H+ + 4ATP → H2 + 4ADP + 4Pi Hydrogenases exist in most of the photosynthetic microorganisms and they can be classified into two categories: (i) uptake hydrogenases and (ii) reversible hydrogenases. Uptake hydrogenases, such as NiFe hydrogenases and NiFeSe hydro genases, act as important catalysts for hydrogen consumption. Reversible hydrogenases, as indicated by its name, have the ability to produce HYDROGEN as well as consume hydrogen depending on the reaction condition. The biological hydrogen production process, which has favorable operational costs, can be classified as photo- and dark-fermentation processes. To produce the hydrogen from a photo-fermentation process, hydrogenase enzyme must be synthesized and activated under dark anaerobic condition. The activated enzyme catalyzes the production of hydrogen from water, or organics, under light anaerobic condition. However, oxygen is inevitably evolved from this process due to photosynthesis, so the hydrogen evolution efficiency is markedly reduced by the oxygen. Maness and Weaver (Maness et al., 1999) reported that 1% of the oxygen content in the headspace of a reactor reduced the efficiency by 50%.

[edit] Direct Photo-biological hydrogen production

The photo-biological production of hydrogen is an area of long-term research and is covered in a IEA Task (IEA Hydrogen Agreement Task 15, Photobiological Production of Hydrogen). Direct biophotolysis of hydrogen production is a biological process using microalgae photosynthetic systems to convert solar energy into chemical energy in the form of hydrogen: 2H2O + Solar energy → 2H2 + O2 Two photosynthetic systems are responsible for photosynthesis process: (i) photosystem I (PSI) producing reductant for CO2 reduction and (ii) photosystem II (PSII) splitting water and evolving oxygen. In the biophotolysis process, two photons from water can yield either CO2 reduction by PSI or hydrogen formation with the presence of hydrogenase. In green plants, due to the lack of hydrogenase, only CO2 reduction takes place. On the contrary, microalgaes, such as green algae and Cyanobacteria (blue-green algae), contain hydrogenase and, thus, have the ability to produce hydrogen. In this process, electrons are generated when PSII absorbs light energy. The electrons are then transferred to the ferredoxin (Fd) using the solar energy absorbed by PSI. The hydrogenase accepts the electrons from Fd to produce hydrogen as shown in figure 10. Since hydrogenase is sensitive to oxygen, it is necessary to maintain the oxygen content at a low level under 0.1% so that hydrogen production can be sustained (Hallenbeck et al., 2002).
Schematics of molecular direct photolysis

Schematics of molecular direct photolysis

This condition can be obtained by the use of green algae Chlamydomonas reinhardtii that can deplete oxygen during oxidative respiration (Melis et al., 2000). However, due to the significant amount of substrate being respired and consumed during this process, the efficiency is low. Benemann (Benemann et al., 1998) estimated the cost of direct biophotolysis for hydrogen production to be 16 €/GJ assuming that the capital cost is about 50.5 €/m2 with an overall solar conversion efficiency of 10%. Hallenbeck and Benemann (Hallenbeck et al., 2002) performed similar cost estimation and reported that the capital cost of 83 €/m2. In their estimation, some practical factors were neglected, such as gas separation and handling. A variety of microalgae and cyanobacteria are able to split water into hydrogen and oxygen with the aid of absorbed light energy. A bottleneck in this "direct biophotolysis" process is that the enzyme responsible for hydrogen production (hydrogenase) is inhibited by the oxygen produced. As a consequence, the photochemical efficiency of this process is thus far much lower (<1% on solar light) than theoretically possible (10%). Several process variants are under development in which the inhibition of hydrogen production is prevented by separating the hydrogen from the oxygen production phase in space or time ("indirect biophotolysis"). The photo-biological processes are at a very early stage of development with so far low conversion efficiencies obtained. There are various biological processes by which hydrogen is released or appears as an intermediate product. One can basically separate these into two process types: Photosynthesis, for which light is required and fermentation, which occurs in darkness. These processes are actually at the point of technical system development, although many biochemical fundamental questions also remain unresolved. An Algae-bacteria-system seems to be the best candidate for the first technical application. The photosynthetic production of hydrogen employs micro-organisms such as cyanobacteria, which have been genetically modified to produce pure hydrogen rather than the metabolically relevant substances (notably NADPH2) The conversion efficiency from sunlight to hydrogen is very small, usually under 0.1%, indicating the need for very large collection areas. The current thinking favours ocean locations of the bio-reactors. They have to float on the surface (due to rapidly decreasing solar radiation as function of depth), and they have to be closed entities with a transparent surface (ex: glass), in order than the hydrogen produced is retained and in order for sunlight to reach the bacteria. Because hydrogen build-up hinders further production, there further has to be a continuous removal of the hydrogen produced, by pipelines to a shore location, where gas treatment and purification can take place. These requirements make it little likely that equipment cost can be kept so low that the very low efficiency can be tolerated. A further problem is that if the bacteria are modified to produce maximum hydrogen, their own growth and reproduction is quenched. There presumable has to be made a compromise between the requirements of the organism and the amount of hydrogen produced for export, so that replacement of organisms (produced at some central bio-factory) does not have to be made at frequent intervals. The implication of this is probably an overall efficiency lower than 0.05%. In a life-cycle assessment of bio-hydrogen produced by photosynthesis, the impacts from equipment manufacture are likely substantial. To this one should add the risks involved in production of large amounts of genetically modified organisms. In conventional agriculture, it is claimed that such negative impacts can be limited, because of slow spreading of genetically modified organisms to new locations (by wind or by vectors such as insects, birds or other animals). In the case of ocean biohydrogen farming, the unavoidable breaking of some of the glass- or transparent plastic-covered panels will allow the genetically modified organisms to spread over the ocean involved and ultimately the entire biosphere. A quantitative discussion of such risks is difficult, but the negative cost prospects of the bio-hydrogen scheme probably rule out any practical use anyway.

Indirect Biophotolysis or Photofermentation

According to Gaudernack (Gaudernack et al., 1998), the concept of indirect biophotolysis involves the following four steps as illustrated in figure 12: (i) biomass production by photosynthesis, (ii) biomass concentration, (iii) aerobic dark fermentation yielding 4 mol hydrogen/mol glucose in the algae cell, along with 2 mol of acetates, and (iv) conversion of 2 mol of acetates into hydrogen. In a typical indirect biophotolysis, Cyanobacteria are used to produce hydrogen. Photofermentations are processes in which organic compounds, like acetic acid, are converted into hydrogen and CO2 with sunlight by bacteria. This process takes place under anaerobic conditions and can be combined with the dark hydrogen fermentation described in chapter 3.2.5. In the dark hydrogen fermentation acetic acid is one of the end products. A photofermentation can be employed as the second stage in a two-stage biohydrogen production process (refer figure below), where the organic substrate is completely converted into hydrogen and CO2.
Process design for biological hydrogen production in two steps (dark and light steps)



Process design for biological hydrogen production in two steps (dark and light steps)
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 utilisation of solar collectors. Again, a drawback of these collector systems are the high production costs with the currently available technology. Although many types of photobioreactors have been designed, there is currently only one type of photobioreactor realised in practice that could be used for biological hydrogen production. The IGV (Institut für Getreideverarbeitung GmbH, Germany) constructed a tubular reactor with a ground surface area of 1.2 hectare at a total investment of 8 million € or 660 €/m2. This system can be compared to a 3 hectare roof structure equipped with photovoltaic cells with investment costs of 580 €/m2. For the bio-hydrogen production system conversion of hydrogen into electricity at 50% efficiency in a fuel cell was assumed. The comparison shows that both the investment costs per m2 and the overall efficiency (sunlight to electricity) of the photovoltaic and the photobiological system are comparable. Photoproduction of hydrogen at a rate of about 12.5 ml H2/gcdw h (cdw: cell dry weight) was found. The hydrogen production rate through indirect biophotolysis is comparable to hydrogenase-based hydrogen production by green algae. The estimated overall cost is 8 €/GJ of hydrogen (Hallenbeck et al., 2002). However, it should be pointed out that indirect biophotolysis technology is still under active research and development. The estimated cost is subject to a significant change depending on the technological advancement. The energy potential of photobiological hydrogen production is assessed on the basis of the amount of sunlight received. At maximum light conversion efficiency (10%) a 1,000 hectare photobioreactor system in The Netherlands could produce 21,300 tons of hydrogen per year, equivalent to 3 PJ. A system of the same size in the south of Spain or in the desert of Australia could produce 4.6 PJ and 5.3 PJ of hydrogen energy per year respectively due to the higher solar irradiance in these areas. The estimate shows that the potential energy production of a photobiological system per hectare is 10-fold higher than for energy crops, such as Miscanthus which would yield approximately 0.3 PJ of energy on a 1,000 hectare surface. An additional advantage of the photobiological system is that it produces clean hydrogen (with 10-20% CO2) that can be transported easily and used directly in fuel cells. Preliminary cost estimates in the literature indicate that photobiological hydrogen could be produced in large-scale systems (100 ha and more) at a cost of 10-15€/GJ. These estimates are based on preliminary data and favourable assumptions, and indicate the major directions for further development. The development of low-cost photobioreactors and optimisation of photosynthetic light conversion efficiency are the major R&D issues in this field.

Biological water-gas shift reaction

The use of microorganisms to perform the shift reaction is of great relevance to hydrogen production because of the potential to reduce carbon monoxide levels in the product gas far below the level attained using water gas shift catalysts and, hence, eliminate final CO scrubbing for fuel cell applications. Some photoheterotrophic bacteria, such as Rhodospirillum rubrum can survive in the dark by using CO as the sole carbon source to generate ATP by coupling the oxidation of CO to the reduction of H+ to H2 (Kerby et al., 1995): CO + H2O ↔ CO2 + H2 ∆G0 = -20,1kJ/mol In equilibrium, the dominating products are CO2 and HYDROGEN. Therefore, this process is favorable for hydrogen production. Organisms growing at the expense of this process are the gram-negative bacteria, such as R. rubrum and Rubrivax gelatinosus, and the gram-positive bacteria, such as Carboxydothermus hydrogenoformans. Under anaerobic conditions, CO induces the synthesis of several proteins, including CO dehydrogenase, Fe–S protein and CO-tolerant hydrogenase Biological water–gas shift reaction for hydrogen production is still under laboratory scale and only few works have been reported. The common objectives of these works were to identify suitable microorganisms that had high CO uptake and to estimate the hydrogen production rate. The maximum hydrogen production activity was found to be 27 mmol/g cell/h, which is about three times higher than R. rubrum. Recently, Wolfrum et al. (Wolfrum et al., 2003) have conducted a detailed study to compare the biological water–gas shift reaction with conventional water–gas shift processes. Their analysis showed that biological water–gas shift process was economically competitive when the methane concentration was under 3%. The hydrogen production cost from biological water–gas shift reaction ranged from 1,45€/kg (12.1€/GJ) to around 1,87 €/kg (15,7€/GJ) for a methane concentration between 1% and 10%. Compared with thermochemical water–gas shift processes, the cost of biological water–gas shift processes are lower due to the elimination of reformer and associated equipment.

[edit] Dark fermentation

The production of hydrogen from a dark fermentation process is not only stable, but also fast, compared to a photo-fermentation process. To produce hydrogen from a dark fermentation metabolism, the blocking of the methanogenesis in the anaerobic pathway is one of the key considerations, due to the conversion of hydrogen to methane in this step. The inhibition of the methanogenic activity can be achieved by controlling various parameters, such as the pH, the loading rate and the solids retention time (SRT), for acidogenesis (Fang and Liu, 2001; Noike and Mizuno, 2000 and Tanisho et al., 1998). Of the various parameters, the pH is considered the most useful. In general, the inhibition of methanogens has been reported under weak acidic pH conditions. Another important consideration for the production of hydrogen, from anaerobic fermentation, is the type of fermentation. It is known that hydrogen is not produced in propionate fermentation pathway. Conversely, butyrate and ethanol fermentations have the potential for the production of hydrogen (Moat, 1979 and Thauer et al., 1977). McCarty and Mosey (McCarty et al., 1999) suggested a new model for anaerobic acidogenesis, which was connected with the competition between propionic and butyric acid-producing bacteria. The production of hydrogen from the butyrate–acetate fermentation contains this risk, which may be pH related. Conversely, ethanol–acetate fermentation appears to be more stable than that of butyrate with respect to pH control, as only acetic acid is produced as an acid in this pathway. In addition, the theoretical yield of hydrogen is 2 mol hydrogen/mol glucose, which is the same as that of the butyrate–acetate fermentation (Moat, 1979 and Thauer et al., 1977). The 4 mol hydrogen production/mol glucose cannot be achieved because the end products normally contain both acetate and butyrate. Despite its potential possibility, information on this fermentation for the production of hydrogen is very deficient, as the research on this fermentation have focused on the production of ethanol as a fuel ( Yokoi et al., 2001 and Zaldivar et al., 2001). The amount of hydrogen production by dark fermentation highly depends on the pH value, hydraulic retention time (HRT) and gas partial pressure. For the optimal hydrogen production, pH should be maintained between 5 and 6. Partial pressure of hydrogen is yet another important parameter affecting the hydrogen production. When hydrogen concentration increases, the metabolic pathways shift to produce more reduced substrates, such as lactate, ethanol, acetone, butanol or alanine, which in turn decrease the hydrogen production. Besides the pH value and partial pressure, HRT also plays an important role in hydrogen production. Ueno et al. (Ueno et al., 1994) have reported that an optimal HRT of 0.5 day could effect maximum hydrogen production (14 mmol/g carbohydrate) from wastewater by anaerobic microflora in the presence of chemostat culture. When HRT was increased from 0.5 day to 3 days, hydrogen production rate was reduced from 198 to 34 mol l-1 day-1, while the carbohydrates in the wastewater were decomposed at an increasing efficiency from 70% to 97%. Due to the fact that solar radiation is not a requirement, hydrogen production by dark fermentation does not demand much land and is not affected by the weather condition. Hence, the feasibility of the technology yields a growing commercial value. Hydrogen production by thermophilic bacteria occurs freely on glucose, xylose, oligosaccharides and starch. Another essential difference is that complex organic compounds in the feedstock are converted to simple molecules not during the digestion process, but rather in a separate process preceding the fermentation. This pre-treatment and hydrolysis process is performed by means of physical/chemical methods (e.g. extrusion) and/or treatment with (industrial) enzymes. Cellulose can be fermented to hydrogen with low conversion rates. For employing lignocellulosic biomass, research has also addressed pretreatment and hydrolysis. Extrusion and enzymatic hydrolysis have been employed to provide fermentable feedstock for the first fermentation. A conceptual design has been made for a hydrogen production plant where potato steam peels at 800 kg/h are fermented to 57 kg H'2/h. The required volume for a thermoreactor is 450 m3 'and 12 ha for a tubular photoreactor. The hydrogen production cost has been estimated at 3.10 €/kg H2 or 22 €/GJ H2.
The anaerobic process has been developed for the efficient treatment of waste and high organic wastewater. The one of advantages of the anaerobic process is the recovery of the useful matters such as solvents, VFA and methane. In general, the recovery of these useful matters in the anaerobic process has been focused on methane only, which is the final product in the anaerobic process. However, the production and utilization of methane has the several problems such as the green house effect and the storage of methane. Conversely, hydrogen, whose recovery from anaerobic fermentation was suggested by several researchers, has been considered as ideal fuel, because residues are not produced in its utilization, except water (Fang and Liu, 2001 and Zoetemeyer et al., 1982). In addition, the development of two phase anaerobic process (TPAP), which is operated separating acidogenesis (fermentation) and methanogenesis, has suggested the potential to produce hydrogen and methane simultaneously. However, research on the anaerobic bio-hydrogen production has been relatively deficient, as most of studies on TPAP have focused on the role of hydrogen to the thermodynamic conversion of volatile fatty acids (VFAs) for the production of methane only. Therefore, in this study, only the production of hydrogen from an anaerobic fermentation was studied. The two-stage bioprocess, comprising a dark fermentation followed by a photofermentation, is a focal point in the current bio-hydrogen research in The Netherlands. A preliminary design and cost estimate for a smallscale, two-stage bio-hydrogen production process with a hydrogen production capacity of 425 Nm3/hour (from 1 metric tonne of biomass/hour) is presented (BioHydrogen and Biomethane, 2003). The economic evaluation indicates that bio-hydrogen could be produced at a cost of 19 €/GJ. Based on the potentially available feedstock in The Netherlands (mainly residues) it is estimated that sufficient bio-hydrogen could be produced to provide 9% of the Dutch households with electricity. The two-stage bioprocess with hydrogen as the sole final product shows potential for sustainable hydrogen production. Further development is however required, particularly for the photofermentation stage. As discussed before, the development of a bioprocess for the production of hydrogen through dark fermentation followed by a second stage for the conversion of acetic acid into methane, is also possible. Such a bioprocess for combined production of bio-hydrogen and bio-methane could well fit into a transition application. The combination of dark hydrogen fermentation and methane production seems technically feasible in the near term, whereas a longer development trajectory is anticipated for the twostage "hydrogen-only" process. Several issues for further development have been identified. A major challenge is to optimise feedstock pretreatment, especially the mobilisation of fermentable substrates from lignocellulosic biomass. The available physical/chemical and enzymatic pre-treatment methods need to be optimised with respect to efficiency, cost and energy consumption. Another major R&D issue is to enhance hydrogen production rates by increasing the concentration of active biomass and improving the efficiency of hydrogen separation. The latter is required because the rate of fermentative hydrogen production is inhibited by the hydrogen produced.

References

  • EurObserv‘Er, W. E. B.
    Systemes solaires n o 169
    October 2005
  • Levin, D. B., Pitt, L. & Love, M.
    Biohydrogen production: prospects and limitations to practical application
    International Journal of Hydrogen Energy 29, 173-185, 2004


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