Hydrogen production by biological processes: a survey of literature
Introduction
Today global energy requirements are mostly dependent on fossil fuels (about 80% of the present world energy demand). This will eventually lead to the foreseeable depletion of limited fossil energy resources. Presently, the utilization of fossil fuels are causing global climate change mainly due to the emission of pollutants like , soot, ash, droplets of tars and other organic compounds, which are released into the atmosphere as a result of their combustion. In order to remedy the depletion of fossil fuels and their environmental misdeeds hydrogen has been suggested as the energy carrier of the future. It is not a primary energy source, but rather serves as a medium through which primary energy sources (such as nuclear and/or solar energy) can be stored, transmitted and utilized to fulfil our energy needs.
Hydrogen is the most plentiful element in the universe, making up about three-quarters of all the matter. The atmosphere contains about 0.07% hydrogen, while the earth's surface contains about 0.14% hydrogen. Hydrogen is the lightest element. The mass of 1 l of hydrogen is , while the mass of 1 l of air is about 1.2 g. The higher heating value of hydrogen is 3042 cals/m3 (considering water as a product). In combustion, water is the main product, thus, hydrogen is regarded as a clean non-polluting fuel. As compared to other gaseous fuels like water gas, hydrogen is harmless to humans and the environment [1], [2].
Today environmental pollution is a great concern to the world, mainly due to rapid industrialization and urbanization. So, increasing focus is being placed on clean energy alternatives for satisfying growing energy demand. Hydrogen has various other uses [3], [4], [5], which can be broadly divided into the following categories:The above stated areas of hydrogen utilization is equivalent to 3% of the energy consumption today, and is expected to grow significantly in the years to come.
At present hydrogen is produced mainly from fossil fuels, biomass and water. The methods of hydrogen production from fossil fuels areMethods of hydrogen production from biomass areMethods of hydrogen production from water areOut of the above listed processes, nearly 90% of hydrogen is produced by the reactions of natural gas or light oil fractions with steam at high temperatures (steam reforming). Coal gassification and electrolysis of water are other industrial methods for hydrogen production. These industrial methods mainly consume fossil fuel as energy source, and sometimes hydroelectricity [6], [7], [8], [9], [10]. However, both thermochemical and electrochemical hydrogen generation processes are energy intensive and not always environment friendly. On the other hand, biological hydrogen production processes are mostly operated at ambient temperatures and pressures, thus less energy intensive. These processes are not only environment friendly, but also they lead to open a new avenue for the utilization of renewable energy resources which are inexhaustible [11], [12], [13], [14], [15]. In addition, they can also use various waste materials, which facilitates waste recycling. The objective of this paper is to review literature on different biological hydrogenation processes and make a comparative analysis.
Section snippets
Biological hydrogen production processes
Biological hydrogen production processes can be classified as follows:2.1 Biophotolysis of water using algae and cyanobacteria. 2.2 Photodecomposition of organic compounds by photosynthetic bacteria. 2.3 Fermentative hydrogen production from organic compounds, and 2.4 Hybrid systems using photosynthetic and fermentative bacteria.
Microbiology
Different microorganisms participate in the biological hydrogen generation system such as green algae, cyanobacteria (or blue–green algae), photosynthetic bacteria and fermentative bacteria, which are tabulated in Table 1. About 50 years ago Gaffron et al. discovered that the eucaryotic unicellular green algae, Scenedesmus obliquus, is able to evolve molecular hydrogen by means of a hydrogenase in the light under anaerobic conditions [16]. This is called direct biophotolysis. It is possible to
Major enzymes
There are three fundamentally different hydrogen producing and metabolizing enzymes found in algae and cyanobacteria:(1) the reversible or classical hydrogenases, (2) the membrane-bound uptake hydrogenases, and (3) the nitrogenase enzymes.
Genetic manipulation of microorganisms
Cyanobacteria in general, possess three hydrogen metabolizing enzymes: nitrogenase, membrane-bound hydrogenase, and soluble hydrogenase. Hydrogenases are mostly involved to utilize hydrogen. So, attempts have been made to maximize the amount of hydrogen production by manipulation of metabolic scheme, namely by maximizing the hydrogen-producing nitrogenase and minimizing that consumed by the so called hydrogenase. So, hydrogenase negative gene has been found to be useful for the hydrogen
Theoretical considerations
Little information is available on the kinetics of biological hydrogen production processes. However, Kumar et al. studied the cell growth and substrate degradation kinetics of Enterobacter cloacae IIT-BT 08 with the help of Monod model [76]. Substrate and biomass concentration profiles of the experimental and simulated data are significantly different from each other. This might be due to substrate and/or product inhibition. Since in the hydrogen generating system the product is gaseous
Typical results obtained from the biological hydrogenation processes
The purpose of biological hydrogen studies is to develop commercially practical hydrogen production processes by exploiting hydrogen producing ability of microorganisms through modern biotechnology. Attempts have already been made by several researchers to find out the suitability of different biological processes. Some important research works are discussed herebelow in order to understand present-state-of-art.
Energy analysis
The yield of hydrogen from sucrose is sucrose. Assuming overall fuel cell efficiency as 80% [85], Gibb's free energy of hydrogen as , the lower heating values of hydrogen and sucrose as 58.3 and , respectively, the following energy analysis was done:
Comparative studies
So far, as microbial hydrogen production is concerned, Table 3 has been prepared to give the comparative studies on the different microbial hydrogen-producing systems. It is clear that the rate of fermentative hydrogen production is always faster than that of the photosynthetic hydrogen production. The merits and demerits of the different biological processes are presented in Table 4. It has been found that most of the biological processes are operated at an ambient temperature (30–40°C) and
Purification of hydrogen
The gases produced by biological processes mostly contain hydrogen (60–90% v/v). However, different impurities like CO2 and O2 are present in the gas mixtures. CO2 acts as fire extinguisher. This is sparingly soluble in water. Scrubbers can be used to separate CO2. Fifty percent w/v KOH solution is a good CO2 absorbent. So, it can be used for CO2 removal. The presence of O2 in the gas may cause a fire hazard. Water solubility of O2 is less as compared to that of CO2. Alkaline pyrogallol
Acknowledgements
Financial assistant received from the Department of Biotechnology, Government of India as a Biotechnology Overseas Associateship to Dr. D. Das is thankfully acknowledged.
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