Hydrogen By Microorganisms
BioCycle January 2004, Vol. 45, No. 1, p. 13
Q: How do microorganisms produce hydrogen from organic residuals?
A: Many people consider hydrogen gas (H2) as the energy source of the future because it can be used in fuel cells to efficiently generate electricity without producing greenhouse gas emissions. Furthermore, it can be derived from renewable energy sources like organic residuals. Currently, most hydrogen is produced by “steam reforming” of methane, which predominantly relies on natural gas as a source of methane and energy. In this process, natural gas plus a chemical catalyst is mixed with steam at high temperatures and pressure to form hydrogen and carbon dioxide (CO2). The chemical formula is:
CH4 + 2H2O ‘ 4H2 + CO2
Ultimately, the goal is to replace the fossil fuels used in hydrogen production with renewable energy sources. Thus, a great deal of research and development activity is occurring on processes that produce hydrogen directly from energy crops, woody biomass, agricultural residues, food residuals and municipal solid waste.
As Kenneth Brown’s article on page 54 describes, generating hydrogen gas from biological materials does not necessarily require a biological process. For example, pyrolysis and gasification employ heat, pressure and/or chemical reactions to “liberate” the hydrogen from biomass (i.e. organic compounds). These physical/thermal/chemical technologies currently offer nearer term commercial potential than biologically produced hydrogen. However, as Brown outlines, and research by the U.S. Department of Energy’s National Renewable Energy Laboratory indicates, the prospects for biologically-produced hydrogen also are promising.
Hydrogen is a natural, though transient, by-product of several microbially driven biochemical reactions, including anaerobic digestion and fermentation. In addition, certain microorganisms can produce enzymes that can derive hydrogen from water given an outside energy source, like sunlight. Specific ways in which microorganisms produce hydrogen are described in the following paragraphs. The first three processes rely on organic feedstocks in the hydrogen-generation process. In the latter two processes, organic feedstocks are not theoretically necessary to produce hydrogen. However, organic compounds can supply nutrients for growing the desired cultures and/or contribute to other stages in a total hydrogen production system (e.g. cell biomass production).
“Dark” fermentation: During fermentation of carbohydrates, in the absence of sunlight, anaerobic bacteria produce hydrogen along with carbon dioxide and an organic by-product like acetic acid (CH3COOH). The chemical formula is:
C6H12O6 (glucose) + 2 H2O ‘ 2CH3COOH + 4H2 + 2 CO2
In a “natural” anaerobic system, other microorganisms quickly consume the hydrogen produced. Therefore, the composition of microflora in the reactor has to be limited to the hydrogen-producing varieties. One idea being investigated is using hyperthermophilic conditions (>70°C;160°F) to select for a particular species of hydrogen-yielding bacteria. For the same reason, the early decomposition steps (e.g. hydrolysis) require a separate pretreatment stage or reactor. Another challenge is that the accumulated hydrogen gas tends to inhibit the generation of more hydrogen. Hence, to keep the process going, it has to be continually separated and removed from the substrate.
Photofermentation: With light, and under anaerobic conditions, reduced organic compounds, like acetic acid, can be converted to hydrogen and carbon dioxide (CO2) by certain microorganism, such as purple bacteria. The chemical formula is: CH3COOH + 2 H2O + light ‘ 4H2 + 2CO2
Given the simple organic compounds involved in this reaction, organic wastes might be used as an economical feedstock. Photo-fermentation can also be used as subsequent stage to dark fermentation, to generate more hydrogen from the organic by-products of that process.
Indirect biophotolysis: Blue green-algae (cyanobacteria) can produce hydrogen in a two step process that starts with photosynthesis and the formation of sugar, followed by a second light-induced process in which the sugar and water yield H2 and CO2. The overall chemical formula is:
6H2O + 6CO2 + light ‘ C6H12O6 + 6O2
C6H12O6 + 12 H2O + light ‘ 12H2 + 6CO2
While many types of green algae can perform photosynthesis, species of cyanobacter also have the ability to fix nitrogen from the atmosphere and produce enzymes that can catalyze the second, hydrogen-generating step.
Direct biophotolysis: Green algae can use energy obtained from sunlight to produce hydrogen from water: H2O ‘ H2 + O2
These algae can synthesize the enzyme hydrogenase, which is the key in a series of metabolic and electron transport reactions that lead to the formation of hydrogen gas. One problem is that hydrogenase is inhibited by the presence of oxygen, a particular problem when oxygen is a product of both photosynthesis and the hydrogen-generation reactions. Therefore, some means of separating the oxygen from the hydrogen-generating reactions needs to be developed. With a particular species of green algae, Chlamydomonas reinhardtii, oxygen production is suppressed when sulfur is unavailable. When light is provided in the absence of oxygen these microalgae begin to produce hydrogen gas.
Water-gas shift reaction: Again in the absence of light, certain bacteria in the family Rhodospiririllaceae can use carbon monoxide (CO) as a carbon source and in doing so generate hydrogen and carbon dioxide, sacrificing some energy in the process: CO + H2O ‘ CO2 + H2
These organisms are otherwise photoheterotrophic (obtain energy from light, carbon from their surroundings). As in dark fermentation, the desired microorganisms need to be maintained, and the undesirable organisms excluded, from the reactor.
Commercial production of hydrogen via microbiological processes currently is far from being economically viable. Although these processes are sound in theory, and there has been success in the laboratory, large hurdles lie ahead. These hurdles relate to scale-up, gas separation, microbial cultural requirements, inhibitory compounds and collection of sufficient solar energy. However, researchers are hoping that gains in knowledge, technology, experience and biotechnology will make biologically produced hydrogen practical in the near future. The combination of using biological and biomass-based hydrogen production technologies holds particular importance for the agricultural, forest products, food processing, solid waste management and wastewater treatment sectors of our economy.
For more information on renewable biobased hydrogen, consult the DOE’s Energy Efficiency and Renewable Energy web site of the National Renewable Energy Laboratory, http://www.nrel.gov/. Another source of information is the Minnesota Renewable Hydrogen Initiative located in the Energy Office of the Minnesota Department of Commerce. Contact Linda Limback at 651-296-1883 or linda.limback@state.mn.us.