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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Trends in biohydrogen production: major challenges and state-of-the-art developments a
a
a
Sanjay Kumar Gupta , Sheena Kumari , Karen Reddy & Faizal Bux
a
a
Institute for Water and Wastewater Technology, Durban University of Technology , PO Box 1334, Durban , 4000 , South Africa Published online: 08 Oct 2013.
To cite this article: Sanjay Kumar Gupta , Sheena Kumari , Karen Reddy & Faizal Bux (2013) Trends in biohydrogen production: major challenges and state-of-the-art developments, Environmental Technology, 34:13-14, 1653-1670, DOI: 10.1080/09593330.2013.822022 To link to this article: http://dx.doi.org/10.1080/09593330.2013.822022
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, Nos. 13–14, 1653–1670, http://dx.doi.org/10.1080/09593330.2013.822022
Trends in biohydrogen production: major challenges and state-of-the-art developments Sanjay Kumar Gupta, Sheena Kumari, Karen Reddy and Faizal Bux∗ Institute for Water and Wastewater Technology, Durban University of Technology, PO Box 1334, Durban 4000, South Africa
Downloaded by [Nipissing University] at 06:09 08 October 2014
(Received 9 April 2013; final version received 28 June 2013 ) Hydrogen has shown enormous potential to be an alternative fuel of the future. Hydrogen production technology has gained much attention in the last few decades due to advantages such as its high conversion efficiency, recyclability and nonpolluting nature. Over the last few decades, biological hydrogen production has shown great promise for generating large scale sustainable energy to meet ever increasing global energy demands. Various microorganisms, namely bacteria, cyanobacteria, and algae which are capable of producing hydrogen from water, solar energy, and a variety of organic substrates, are explored and studied in detail. Current biohydrogen production technologies, however, face two major challenges such as low-yield and high production cost. Advances have been made in recent years in biohydrogen research to improve the hydrogen yield through process modifications, physiological manipulations, through metabolic and genetic engineering. Recently, cell immobilization such as microbes trapping with nanoparticles within the bioreactor has shown an increase in hydrogen production. This review critically evaluated various biological hydrogen production technologies, key challenges, and recent advancements in biohydrogen research and development. Keywords: algae; cyanobacteria; biohydrogen; biophotolysis; photo-fermentation; dark-fermentation
1. Introduction Energy is one of the basic requirements needed to sustain life on earth and 80% of current global energy demand is met from fossil fuels and the remaining 20% from solar, wind, hydro energy, nuclear power, and biomass processing.[1] It is postulated that the ever increasing demand in global energy will eventually lead to foreseeable depletion of limited fossil energy resources in the next few decades. Moreover, greenhouse gases (GHGs) such as COx , Sox , and NOx emitted during the combustion of fossil fuel are recognized as one of the primary contributors to the increased global warming, which is a major challenge of this decade. Thus, many researchers are currently focusing on exploring sustainable and cost effective alternative renewable energy substitutes such as biofuels to meet future global energy demands.[2–6] These biofuels mainly include biodiesel, bioethanol, bio-methane, and biohydrogen produced from various renewable organic biomasses. Among these, first-generation biofuels, such as biodiesel and bioethanol produced from vegetable oils and sugar cane, impose an imbalance between economicsand agricultural-based foods.[7,8] However, technological innovations lead to the production of second-generation biofuels from lignocellulosic waste materials such as agricultural wastes and from bioprocessing organic wastes.[9] The pilot scale production of second-generation biofuels by the use of low-cost lignocellulosic materials and bio-waste ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
aims to establish energy security as well as environmental and economic sustainability. However, low energy gain, high production costs, reduction in CO2 emission, etc. are still some of the barriers which need vigorous technological inputs. Improving the conversion rate and yield also require process integrations and optimization of thermochemical conversion routes.[10] However, there are still some techno-economic challenges in commercial scale production and application of biofuels at the compatible cost of conventional fossil fuels, the major issues such as low energy density, high GHGs emissions and more importantly high production cost has been whispered to be solved. Presently, algal biofuel has emerged as a next generation biofuel and has drawn considerable attention as its cultivation does not require agricultural land or the use of portable water.[11] However, lower biomass yield (1–7 g/L/D) even in superlative conditions [12] and the requirement of mammoth water volume is the major technical barrier in the pilot scale algal biofuel production. Apart from this, the climatic conditions such as freezing temperature over a long period in a year is also one of a major geographical challenge.[13] The details of algal and other biofuels are not in the scope of this review. The state-of-the-art developments in biohydrogen production and associated challenges are the main focus of this review. Hydrogen, one of the potential energy carriers of the future, has advantages over classical hydrocarbon fuels
Downloaded by [Nipissing University] at 06:09 08 October 2014
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due to its high conversion efficiency, recyclability, and non-polluting nature.[14] It has a high energy content (122 kJ/g); yields 2.7 times higher than other hydrocarbons fuels and has potential to be used in fuel cells for the generation of electricity.[3,15] Hydrogen is, therefore, regarded as one of the best high energy, non-polluting clean fuel with a wide range of applications such as use as transport fuel and for electricity generation.[16,17] These superior qualities of hydrogen over fossil fuels, surpasses its limitations in production technologies.[18–20] Hydrogen gas production technologies have gained special attention during the last 50 years due to the constantly increasing energy demand. The consumption and need for hydrogen as an alternative energy source is continuously increasing and expected to contribute 8–10% in the energy market by year 2025.[21] As per the BP statistical review of world energy June 2012, a 5.1% growth in global energy consumption was estimated in 2010 and 2.5% in 2011. A decline of 0.8% in overall energy consumption was noticed in Organisation for Economic Co-operation and Development (OECD) countries whereas a 5.3% increase was noticed in non-OECD countries consumption in 2012.[22] Various methods are used for hydrogen production such as solar gasification, thermo-chemical gasification, and super critical conversions. Steam reforming of natural gases accounts for nearly 50% of global hydrogen demand, whereas industrial oil and naphtha reforming yields about 30%. Aside from this hydrogen is also produced through coal gasification (18%), electrolysis of water (3.9%), and other sources (0.1%).[23] However, most of these conventional methods are highly energy-intensive processes and are of major economic and environmental concern when adopted for large-scale hydrogen production. For example, hydrogen produced by the electrolysis of water is entirely electricity dependent, and almost 80% of the production cost is spent towards its operation.[24] Similarly, hydrogen production by pyrolysis generates lots of carbonaceous by-products, waste oil, and minerals, making the waste removal process expensive.[24] Another highly expensive technology is hydrogen fuel cells that are used to convert chemical energy to electricity. The above challenges have compelled researchers to explore other avenues such as hydrogen production from renewable sources. Hydrogen production through photosynthetic and fermentative processes by green algae, cyanobacteria, and anaerobic bacteria is popularly known as biohydrogen and has gained momentum during the last few decades.[1,25,26] Biohydrogen production processes are less energy intensive and can work at ambient temperature and pressure which makes it viable option for large-scale production.[27] Compared with the first-generation biofuels such as biodiesel produced from vegetable oils and bioethanol from sugar cane, second-generation biofuels including biohydrogen are quite economic as they are mainly produced from lignocellulosic biomass and organic wastes.[7,8] Nevertheless, the
global research for biohydrogen is still in its infancy, and only laboratory scale experiments have been reported till date. Optimization strategies are in progress to overcome the challenges and to obtain the desired biohydrogen levels, find new and alternate resources, design hydrogen storage vessels and finally to improve its contribution to the present energy demand.[2,28] In this review, efforts have been made to critically evaluate the latest trends in biohydrogen production technologies such as direct and indirect biophotolysis as well as darkfermentation process. Merits and demerits of different types of reactors, and their hydrogen production efficiencies, have also been briefly covered. The types of microbes and reactors that are being currently used and future prospects are also discussed in brief. The major challenges and key issues of these procedures have been discussed in light of the state-of-the-art developments. 2. Biohydrogen production technologies Biological hydrogen production processes can be broadly grouped into two major categories, light dependent also known as photo-biological processes and light-independent processes or a combination of both.[1,15,29,30] Photobiological processes include direct and indirect biophotolysis and photo-fermentation depending on the nature of reactions and microbes involved. Light-independent process involves dark-fermentation by anaerobic microorganisms. 2.1. Photo-biological hydrogen production 2.1.1. Direct and indirect biophotolysis Photo-biological hydrogen production by microalgae is in operation since the early 1970s [31] and has received much scientific interest in the last few decades. The fundamental concept of the photolysis process is to use microorganisms as a catalyst to convert solar energy and water into hydrogen with oxygen as a by-product. The procedure depicts the photosynthetic course within the plant and algal cells, however, differs due to the end result leading to the creation of hydrogen instead of biomass accumulation [1,32] (Figure 1). Direct photolysis by green algae and cyanobacteria has advantages over other bioprocesses, as it can produce biohydrogen through dissociation of water by the use of solar light.[33] The photosystems (PS-I and PS-II) present in the chloroplast of green algal cells absorb solar energy and convert it into redox energy by adenosine diphosphate (ADP) phosphorylation and nicotinamide adenine dinucleotide phosphate (NADP) reduction resulting in carbohydrate synthesis. The hydrogenase enzymes present in the chloroplast catalyze proton reduction by ferredoxin and thereafter generate molecular hydrogen.[34,35] Although the process is attractive and theoretically efficient, as hydrogen is produced from water using solar energy, studies have revealed that oxygen generated during the photosynthetic
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Sun Light
Photons
NAD(P)H
Photosystem II
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H20
Photosystem I
e-
FERREDOXIN
NAD(P)
O2 ADP
H+
ATP
Hydrogenase
2H+
H+
Figure 1.
Direct photolysis process (modified from Hallenbeck and Benemann).[26]
process inhibits hydrogenase activity resulting in reduced hydrogen production.[15] Various problems of direct biophotolysis such as inhibition of hydrogenase activity by oxygen and generation of highly explosive H2 –O2 mixtures, led to the proposal of indirect, two-stage light-driven processes (indirect biophotolysis). This process includes two phases: (i) growing green algae and cyanobacteria in carbohydrate rich biomass by photosynthetic assimilation of CO2 into starch (in green algae) and glycogen (in cyanobacteria) and (ii) conversion of stored carbohydrates to hydrogen by the action of reversible hydrogenases, in dark and possibly light-driven anaerobic fermentation processes.[15,36,37] By separating these two phases from each other, indirect biophotolysis could overcome the inhibition of the hydrogenase activity by oxygen and also limit the production of potentially highly explosive hydrogen and oxygen mixtures during the reaction.[31] The hydrogen production during the two-stage indirect biophotolysis process can be explained by the following reactions: Light Energy
6H2 O + 6CO2 −−−−−−→ C6 H12 O6 + 6O2 , [38] Light Energy
C6 H2 O6 + 6H2 O −−−−−−→ 6CO2 + 12H2 . [38]
Cyanobacteria possess key enzymes (nitrogenase and hydrogenase) that carry out metabolic functions in order to achieve hydrogen generation.[39] Some of these enzymes produce hydrogen, as one of its by-products during nitrogen fixation. They also have the ability to consume hydrogen, this however depends on the type of microbial strain used [40–42] (Figure 2). 2.1.2. Photo-fermentation The photo-fermentation process includes conversion of organic substrate (acids) to biohydrogen by a diverse group of anaerobic photosynthetic bacteria in the presence of light. The advantages of this process include (i) high substrate conversion rate, (ii) no oxygen inhibition, (iii) use of wider wavelength of light, and (iv) ability to use waste organic substrates for biohydrogen production. Hydrogen gas production capabilities of some purple non-sulphur (PNS) photosynthetic bacteria such as Rhodobacter, Rhodopseudomonas, and Rhodospirillum have been investigated to some extent.[43,44] The PNS photosynthetic bacteria can be photoheterotrophs, photoautotrophs, or chemoheterotrophs subject to the available cultural conditions.[44,45] Numerous simple organic compounds serve as suitable substrates for PNS bacteria,
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Sunlight
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Photons
Figure 2.
Indirect biophotolysis process (modified from Hallenbeck and Benemann).[26]
including lactate, acetate, butyrate, propionate, and succinate. Alcohols, ethanol and propanol, and other substrates such as aromatic acids are also preferred.[45,46] However, the substrate requirement for biohydrogen production is purely strain-specific and the metabolic pathways involved are uncertain for many of these substrates.[46] During the photo-fermentation process, hydrogen production takes place under anaerobic conditions with light illumination and is catalyzed by the key enzyme nitrogenase. Nevertheless, this enzyme is inactive in the presence of oxygen and also at high ammonia concentrations.[44] Another major factor affecting the photo-fermentation process is light intensity. Although an increase in light intensity has shown some stimulatory effect on the overall hydrogen production rate of photosynthetic micro-organisms, an adverse effect was also reported on their light conversion efficiency at high light intensities.[34,35] However, the light conversion efficiency can be improved by genetic manipulation of the light-harvesting antennae, thereby reducing the saturation effect of light.[47,48] 2.2.
Light-independent procedures or dark-fermentation Dark-fermentation is a complex process of converting organic biomass to biohydrogen through a series of biochemical reactions catalyzed by a diverse group of fermentative micro-organisms.[29] Dark-fermentation proves to be superior over photo-fermentation as this requires no light and the energy produced is relatively higher, due to the fermentation of sugar and carbohydrates. The process is initiated by the hydrolysis of organic polymers to
monomers, thereafter acetogenic conversion of monomers to organic acids, alcohols, and release of hydrogen. Although biohydrogen production by dark-fermentation is promising and advantageous over photo-fermentation [49] however, the requirement of organic biomass as a feedstock makes this process quite expensive.[50] Furthermore, the quality and quantity of hydrogen produced during darkfermentation also dependent on various factors such as the type of feedstock, inoculums, and reactors. Sugars and carbohydrate rich biomass are reported to be the most suitable feedstock for the formation of biohydrogen from darkfermentation.[51] It is reported that glucose, isomers of hexoses, or polymers in the form of starch or cellulose could release different amounts of hydrogen depending on the chosen fermentation pathway and the end-products.[15] Mesophilic and thermophilic dark-fermentative processes have been validated to have superior benefits during biohydrogen production. Although the mesophilic process has shown great potential over thermophilic process in terms of energy requirement and production rate, the requirement of higher quantity of feedstock make mesophilic process quite expensive. On the other hand, thermophilic is an economically viable option with reference to feasibility, feedstock requirement, and pre-treatment.[52] The downfalls of the thermophillic process include expenses involved in the assets needed for operation as well as low productivity. However in some instances, thermophilic fermentation has shown to have better performance than mesophilic fermentation,[53] and can be further improved by modifying the pre-treatment methods. For instance, Cakir et al. [54] concluded that acid-hydrolyzed wheat starch generated the highest amount
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Photofermentation
Biophotolysis
High cost
High cost Low H2 productivity Expensive photobioreactors
Expensive bioreactors
Major challenges of biological hydrogen production
Needs high intensity light Low solar energy utilization
Large surface area requirement
Formation of explosive gas mixer
Complex photobioreactor design
Less than 10% solar energy utilization
Low productivity of nitrogenases
Oxygen intolerance H2 producing enzymes
Problematic practical applications
Low photosynthetic conversion efficiencies
Low photosynthetic conversion efficiency
Problems in hydrogen recovery from reactor
Oxygen intolerant photo-biological enzymes
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Dark fermentation
Figure 3.
Low chemical oxygen demand (COD) removal
Low H2 production
Incomplete substrate conversion
High amount by products
Difficult fermentive substrate utilization
Production of organic acids/ alcohols
Inefficient and costly pretreatment methods
Thermodynamically unstable at higher H2 yield
Major challenges in current biological hydrogen production technologies.
of hydrogen when compared with the quantity produced during mesophilic fermentation. The dark-fermentation process has a dual advantage of being functional for water and wastewater treatment as well as energy production from waste. Due to the ease of process, application of dark-fermentation for biohydrogen production on pilot scale will be relatively easy in comparison with photo-fermentation. Though biohydrogen production through dark-fermentation technologies is not yet established at pilot scale, it has however shown tremendous potential at laboratory scale. It can be considered as a futuristic technology due to its low energy demand, easy operation, and maintenance. These remarkable benefits can lead to a promising future of hydrogen energy.
3.
Major challenges of biohydrogen production and the way forward Maximizing hydrogen production efficiencies is a major challenge affecting the biohydrogen industry (Figure 3). Currently, the hydrogen economy is fading due to its complications in use, storage, distribution, transport, and also its high production costs. Financial experts claim that the production costs can be put down on mass production through appropriate technologies.[55] However, major technological innovations and improvement is crucial for increasing the hydrogen yield through both photo- and dark-fermentation processes. The inhibitory effect of oxygen on the key enzymes such as [FeFe]-hydrogenases and [NiFe]-hydrogenases is a major concern during photo-biological reactions.[15] Oxygen stable hydrogenase production is probably not possible on a thermodynamic basis; hence, efforts are being made to overcome the sensitivity of these enzymes to oxygen.
Oxygen absorbers like glucose-oxidase have also been used to overcome the problems of hydrogenase inactivation by simultaneous oxygen generation during the photolysis in green algae.[24] Apart from this, low hydrogen yield (
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Trends in biohydrogen production: major challenges and state-of-the-art developments a
a
a
Sanjay Kumar Gupta , Sheena Kumari , Karen Reddy & Faizal Bux
a
a
Institute for Water and Wastewater Technology, Durban University of Technology , PO Box 1334, Durban , 4000 , South Africa Published online: 08 Oct 2013.
To cite this article: Sanjay Kumar Gupta , Sheena Kumari , Karen Reddy & Faizal Bux (2013) Trends in biohydrogen production: major challenges and state-of-the-art developments, Environmental Technology, 34:13-14, 1653-1670, DOI: 10.1080/09593330.2013.822022 To link to this article: http://dx.doi.org/10.1080/09593330.2013.822022
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, Nos. 13–14, 1653–1670, http://dx.doi.org/10.1080/09593330.2013.822022
Trends in biohydrogen production: major challenges and state-of-the-art developments Sanjay Kumar Gupta, Sheena Kumari, Karen Reddy and Faizal Bux∗ Institute for Water and Wastewater Technology, Durban University of Technology, PO Box 1334, Durban 4000, South Africa
Downloaded by [Nipissing University] at 06:09 08 October 2014
(Received 9 April 2013; final version received 28 June 2013 ) Hydrogen has shown enormous potential to be an alternative fuel of the future. Hydrogen production technology has gained much attention in the last few decades due to advantages such as its high conversion efficiency, recyclability and nonpolluting nature. Over the last few decades, biological hydrogen production has shown great promise for generating large scale sustainable energy to meet ever increasing global energy demands. Various microorganisms, namely bacteria, cyanobacteria, and algae which are capable of producing hydrogen from water, solar energy, and a variety of organic substrates, are explored and studied in detail. Current biohydrogen production technologies, however, face two major challenges such as low-yield and high production cost. Advances have been made in recent years in biohydrogen research to improve the hydrogen yield through process modifications, physiological manipulations, through metabolic and genetic engineering. Recently, cell immobilization such as microbes trapping with nanoparticles within the bioreactor has shown an increase in hydrogen production. This review critically evaluated various biological hydrogen production technologies, key challenges, and recent advancements in biohydrogen research and development. Keywords: algae; cyanobacteria; biohydrogen; biophotolysis; photo-fermentation; dark-fermentation
1. Introduction Energy is one of the basic requirements needed to sustain life on earth and 80% of current global energy demand is met from fossil fuels and the remaining 20% from solar, wind, hydro energy, nuclear power, and biomass processing.[1] It is postulated that the ever increasing demand in global energy will eventually lead to foreseeable depletion of limited fossil energy resources in the next few decades. Moreover, greenhouse gases (GHGs) such as COx , Sox , and NOx emitted during the combustion of fossil fuel are recognized as one of the primary contributors to the increased global warming, which is a major challenge of this decade. Thus, many researchers are currently focusing on exploring sustainable and cost effective alternative renewable energy substitutes such as biofuels to meet future global energy demands.[2–6] These biofuels mainly include biodiesel, bioethanol, bio-methane, and biohydrogen produced from various renewable organic biomasses. Among these, first-generation biofuels, such as biodiesel and bioethanol produced from vegetable oils and sugar cane, impose an imbalance between economicsand agricultural-based foods.[7,8] However, technological innovations lead to the production of second-generation biofuels from lignocellulosic waste materials such as agricultural wastes and from bioprocessing organic wastes.[9] The pilot scale production of second-generation biofuels by the use of low-cost lignocellulosic materials and bio-waste ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
aims to establish energy security as well as environmental and economic sustainability. However, low energy gain, high production costs, reduction in CO2 emission, etc. are still some of the barriers which need vigorous technological inputs. Improving the conversion rate and yield also require process integrations and optimization of thermochemical conversion routes.[10] However, there are still some techno-economic challenges in commercial scale production and application of biofuels at the compatible cost of conventional fossil fuels, the major issues such as low energy density, high GHGs emissions and more importantly high production cost has been whispered to be solved. Presently, algal biofuel has emerged as a next generation biofuel and has drawn considerable attention as its cultivation does not require agricultural land or the use of portable water.[11] However, lower biomass yield (1–7 g/L/D) even in superlative conditions [12] and the requirement of mammoth water volume is the major technical barrier in the pilot scale algal biofuel production. Apart from this, the climatic conditions such as freezing temperature over a long period in a year is also one of a major geographical challenge.[13] The details of algal and other biofuels are not in the scope of this review. The state-of-the-art developments in biohydrogen production and associated challenges are the main focus of this review. Hydrogen, one of the potential energy carriers of the future, has advantages over classical hydrocarbon fuels
Downloaded by [Nipissing University] at 06:09 08 October 2014
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due to its high conversion efficiency, recyclability, and non-polluting nature.[14] It has a high energy content (122 kJ/g); yields 2.7 times higher than other hydrocarbons fuels and has potential to be used in fuel cells for the generation of electricity.[3,15] Hydrogen is, therefore, regarded as one of the best high energy, non-polluting clean fuel with a wide range of applications such as use as transport fuel and for electricity generation.[16,17] These superior qualities of hydrogen over fossil fuels, surpasses its limitations in production technologies.[18–20] Hydrogen gas production technologies have gained special attention during the last 50 years due to the constantly increasing energy demand. The consumption and need for hydrogen as an alternative energy source is continuously increasing and expected to contribute 8–10% in the energy market by year 2025.[21] As per the BP statistical review of world energy June 2012, a 5.1% growth in global energy consumption was estimated in 2010 and 2.5% in 2011. A decline of 0.8% in overall energy consumption was noticed in Organisation for Economic Co-operation and Development (OECD) countries whereas a 5.3% increase was noticed in non-OECD countries consumption in 2012.[22] Various methods are used for hydrogen production such as solar gasification, thermo-chemical gasification, and super critical conversions. Steam reforming of natural gases accounts for nearly 50% of global hydrogen demand, whereas industrial oil and naphtha reforming yields about 30%. Aside from this hydrogen is also produced through coal gasification (18%), electrolysis of water (3.9%), and other sources (0.1%).[23] However, most of these conventional methods are highly energy-intensive processes and are of major economic and environmental concern when adopted for large-scale hydrogen production. For example, hydrogen produced by the electrolysis of water is entirely electricity dependent, and almost 80% of the production cost is spent towards its operation.[24] Similarly, hydrogen production by pyrolysis generates lots of carbonaceous by-products, waste oil, and minerals, making the waste removal process expensive.[24] Another highly expensive technology is hydrogen fuel cells that are used to convert chemical energy to electricity. The above challenges have compelled researchers to explore other avenues such as hydrogen production from renewable sources. Hydrogen production through photosynthetic and fermentative processes by green algae, cyanobacteria, and anaerobic bacteria is popularly known as biohydrogen and has gained momentum during the last few decades.[1,25,26] Biohydrogen production processes are less energy intensive and can work at ambient temperature and pressure which makes it viable option for large-scale production.[27] Compared with the first-generation biofuels such as biodiesel produced from vegetable oils and bioethanol from sugar cane, second-generation biofuels including biohydrogen are quite economic as they are mainly produced from lignocellulosic biomass and organic wastes.[7,8] Nevertheless, the
global research for biohydrogen is still in its infancy, and only laboratory scale experiments have been reported till date. Optimization strategies are in progress to overcome the challenges and to obtain the desired biohydrogen levels, find new and alternate resources, design hydrogen storage vessels and finally to improve its contribution to the present energy demand.[2,28] In this review, efforts have been made to critically evaluate the latest trends in biohydrogen production technologies such as direct and indirect biophotolysis as well as darkfermentation process. Merits and demerits of different types of reactors, and their hydrogen production efficiencies, have also been briefly covered. The types of microbes and reactors that are being currently used and future prospects are also discussed in brief. The major challenges and key issues of these procedures have been discussed in light of the state-of-the-art developments. 2. Biohydrogen production technologies Biological hydrogen production processes can be broadly grouped into two major categories, light dependent also known as photo-biological processes and light-independent processes or a combination of both.[1,15,29,30] Photobiological processes include direct and indirect biophotolysis and photo-fermentation depending on the nature of reactions and microbes involved. Light-independent process involves dark-fermentation by anaerobic microorganisms. 2.1. Photo-biological hydrogen production 2.1.1. Direct and indirect biophotolysis Photo-biological hydrogen production by microalgae is in operation since the early 1970s [31] and has received much scientific interest in the last few decades. The fundamental concept of the photolysis process is to use microorganisms as a catalyst to convert solar energy and water into hydrogen with oxygen as a by-product. The procedure depicts the photosynthetic course within the plant and algal cells, however, differs due to the end result leading to the creation of hydrogen instead of biomass accumulation [1,32] (Figure 1). Direct photolysis by green algae and cyanobacteria has advantages over other bioprocesses, as it can produce biohydrogen through dissociation of water by the use of solar light.[33] The photosystems (PS-I and PS-II) present in the chloroplast of green algal cells absorb solar energy and convert it into redox energy by adenosine diphosphate (ADP) phosphorylation and nicotinamide adenine dinucleotide phosphate (NADP) reduction resulting in carbohydrate synthesis. The hydrogenase enzymes present in the chloroplast catalyze proton reduction by ferredoxin and thereafter generate molecular hydrogen.[34,35] Although the process is attractive and theoretically efficient, as hydrogen is produced from water using solar energy, studies have revealed that oxygen generated during the photosynthetic
Environmental Technology
1655
Sun Light
Photons
NAD(P)H
Photosystem II
Downloaded by [Nipissing University] at 06:09 08 October 2014
H20
Photosystem I
e-
FERREDOXIN
NAD(P)
O2 ADP
H+
ATP
Hydrogenase
2H+
H+
Figure 1.
Direct photolysis process (modified from Hallenbeck and Benemann).[26]
process inhibits hydrogenase activity resulting in reduced hydrogen production.[15] Various problems of direct biophotolysis such as inhibition of hydrogenase activity by oxygen and generation of highly explosive H2 –O2 mixtures, led to the proposal of indirect, two-stage light-driven processes (indirect biophotolysis). This process includes two phases: (i) growing green algae and cyanobacteria in carbohydrate rich biomass by photosynthetic assimilation of CO2 into starch (in green algae) and glycogen (in cyanobacteria) and (ii) conversion of stored carbohydrates to hydrogen by the action of reversible hydrogenases, in dark and possibly light-driven anaerobic fermentation processes.[15,36,37] By separating these two phases from each other, indirect biophotolysis could overcome the inhibition of the hydrogenase activity by oxygen and also limit the production of potentially highly explosive hydrogen and oxygen mixtures during the reaction.[31] The hydrogen production during the two-stage indirect biophotolysis process can be explained by the following reactions: Light Energy
6H2 O + 6CO2 −−−−−−→ C6 H12 O6 + 6O2 , [38] Light Energy
C6 H2 O6 + 6H2 O −−−−−−→ 6CO2 + 12H2 . [38]
Cyanobacteria possess key enzymes (nitrogenase and hydrogenase) that carry out metabolic functions in order to achieve hydrogen generation.[39] Some of these enzymes produce hydrogen, as one of its by-products during nitrogen fixation. They also have the ability to consume hydrogen, this however depends on the type of microbial strain used [40–42] (Figure 2). 2.1.2. Photo-fermentation The photo-fermentation process includes conversion of organic substrate (acids) to biohydrogen by a diverse group of anaerobic photosynthetic bacteria in the presence of light. The advantages of this process include (i) high substrate conversion rate, (ii) no oxygen inhibition, (iii) use of wider wavelength of light, and (iv) ability to use waste organic substrates for biohydrogen production. Hydrogen gas production capabilities of some purple non-sulphur (PNS) photosynthetic bacteria such as Rhodobacter, Rhodopseudomonas, and Rhodospirillum have been investigated to some extent.[43,44] The PNS photosynthetic bacteria can be photoheterotrophs, photoautotrophs, or chemoheterotrophs subject to the available cultural conditions.[44,45] Numerous simple organic compounds serve as suitable substrates for PNS bacteria,
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S.K. Gupta et al.
Sunlight
Downloaded by [Nipissing University] at 06:09 08 October 2014
Photons
Figure 2.
Indirect biophotolysis process (modified from Hallenbeck and Benemann).[26]
including lactate, acetate, butyrate, propionate, and succinate. Alcohols, ethanol and propanol, and other substrates such as aromatic acids are also preferred.[45,46] However, the substrate requirement for biohydrogen production is purely strain-specific and the metabolic pathways involved are uncertain for many of these substrates.[46] During the photo-fermentation process, hydrogen production takes place under anaerobic conditions with light illumination and is catalyzed by the key enzyme nitrogenase. Nevertheless, this enzyme is inactive in the presence of oxygen and also at high ammonia concentrations.[44] Another major factor affecting the photo-fermentation process is light intensity. Although an increase in light intensity has shown some stimulatory effect on the overall hydrogen production rate of photosynthetic micro-organisms, an adverse effect was also reported on their light conversion efficiency at high light intensities.[34,35] However, the light conversion efficiency can be improved by genetic manipulation of the light-harvesting antennae, thereby reducing the saturation effect of light.[47,48] 2.2.
Light-independent procedures or dark-fermentation Dark-fermentation is a complex process of converting organic biomass to biohydrogen through a series of biochemical reactions catalyzed by a diverse group of fermentative micro-organisms.[29] Dark-fermentation proves to be superior over photo-fermentation as this requires no light and the energy produced is relatively higher, due to the fermentation of sugar and carbohydrates. The process is initiated by the hydrolysis of organic polymers to
monomers, thereafter acetogenic conversion of monomers to organic acids, alcohols, and release of hydrogen. Although biohydrogen production by dark-fermentation is promising and advantageous over photo-fermentation [49] however, the requirement of organic biomass as a feedstock makes this process quite expensive.[50] Furthermore, the quality and quantity of hydrogen produced during darkfermentation also dependent on various factors such as the type of feedstock, inoculums, and reactors. Sugars and carbohydrate rich biomass are reported to be the most suitable feedstock for the formation of biohydrogen from darkfermentation.[51] It is reported that glucose, isomers of hexoses, or polymers in the form of starch or cellulose could release different amounts of hydrogen depending on the chosen fermentation pathway and the end-products.[15] Mesophilic and thermophilic dark-fermentative processes have been validated to have superior benefits during biohydrogen production. Although the mesophilic process has shown great potential over thermophilic process in terms of energy requirement and production rate, the requirement of higher quantity of feedstock make mesophilic process quite expensive. On the other hand, thermophilic is an economically viable option with reference to feasibility, feedstock requirement, and pre-treatment.[52] The downfalls of the thermophillic process include expenses involved in the assets needed for operation as well as low productivity. However in some instances, thermophilic fermentation has shown to have better performance than mesophilic fermentation,[53] and can be further improved by modifying the pre-treatment methods. For instance, Cakir et al. [54] concluded that acid-hydrolyzed wheat starch generated the highest amount
Environmental Technology
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Photofermentation
Biophotolysis
High cost
High cost Low H2 productivity Expensive photobioreactors
Expensive bioreactors
Major challenges of biological hydrogen production
Needs high intensity light Low solar energy utilization
Large surface area requirement
Formation of explosive gas mixer
Complex photobioreactor design
Less than 10% solar energy utilization
Low productivity of nitrogenases
Oxygen intolerance H2 producing enzymes
Problematic practical applications
Low photosynthetic conversion efficiencies
Low photosynthetic conversion efficiency
Problems in hydrogen recovery from reactor
Oxygen intolerant photo-biological enzymes
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Dark fermentation
Figure 3.
Low chemical oxygen demand (COD) removal
Low H2 production
Incomplete substrate conversion
High amount by products
Difficult fermentive substrate utilization
Production of organic acids/ alcohols
Inefficient and costly pretreatment methods
Thermodynamically unstable at higher H2 yield
Major challenges in current biological hydrogen production technologies.
of hydrogen when compared with the quantity produced during mesophilic fermentation. The dark-fermentation process has a dual advantage of being functional for water and wastewater treatment as well as energy production from waste. Due to the ease of process, application of dark-fermentation for biohydrogen production on pilot scale will be relatively easy in comparison with photo-fermentation. Though biohydrogen production through dark-fermentation technologies is not yet established at pilot scale, it has however shown tremendous potential at laboratory scale. It can be considered as a futuristic technology due to its low energy demand, easy operation, and maintenance. These remarkable benefits can lead to a promising future of hydrogen energy.
3.
Major challenges of biohydrogen production and the way forward Maximizing hydrogen production efficiencies is a major challenge affecting the biohydrogen industry (Figure 3). Currently, the hydrogen economy is fading due to its complications in use, storage, distribution, transport, and also its high production costs. Financial experts claim that the production costs can be put down on mass production through appropriate technologies.[55] However, major technological innovations and improvement is crucial for increasing the hydrogen yield through both photo- and dark-fermentation processes. The inhibitory effect of oxygen on the key enzymes such as [FeFe]-hydrogenases and [NiFe]-hydrogenases is a major concern during photo-biological reactions.[15] Oxygen stable hydrogenase production is probably not possible on a thermodynamic basis; hence, efforts are being made to overcome the sensitivity of these enzymes to oxygen.
Oxygen absorbers like glucose-oxidase have also been used to overcome the problems of hydrogenase inactivation by simultaneous oxygen generation during the photolysis in green algae.[24] Apart from this, low hydrogen yield (
