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| Methanization of Fossil Fuel: A Possible Sustainable Future Energy Source for Mankind? |
| Phil M. Oger* |
| Laboratory of Geology of Lyon, CNRS UMR 5276, University of Lyon, ENS Lyon, 46 Allée d’Italie, Lyon, France |
| *Corresponding author: |
Phil M. Oger Ph.D
Laboratory of Geology of Lyon,
CNRS UMR 5276
University of Lyon, ENS Lyon
46 Allée d’Italie, Lyon, France
E-mail: philippe.oger@ens-lyon.fr |
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| Received April 19, 2012; Accepted April 20, 2012; Published April 23, 2012 |
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| Citation: Oger PM (2012) Methanization of Fossil Fuel: A Possible Sustainable
Future Energy Source for Mankind? Ferment Technol 1:e110. doi:10.4172/
fmt.1000e110 |
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| Copyright: © 2012 Oger PM. This is an open-access article distributed under the
terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
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| One of the main human challenges over the ages has been to find
and exploit the best and most efficient source of energy. Mastering
of wood, coal, petroleum and nuclear powers have marked out the
path to the progress. Today, petroleum and coal represent the most
extensively used energy source worldwide and the resources tend
to be depleted. The large scale burning of fossil fuels also comes at a
high environmental cost and we are still trying to imagine the next
generation energy source. It is not expected that renewable energies
based on the harvest of solar or wind energy will supplant fossil fuels
in the near future. Thus, we are stranded with fossil fuels for at least
the next few decades and need to find ways to make our use of fossil
fuels cause the least environmental impact. One approach to reducing
the environmental impact of fossil fuel usage is to lower CO2 emission
per Kj of energy, a claimed advantage of biogenic methane. Thus, in
time of declining discovery of conventional fossil fuel reserves and the
ongoing issues relating to security of energy supply and global warming
methane may represent the future for fossil fuels. |
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| Fossil fuels, e.g. oil and coal, are mainly composed of hydrocarbons,
whether linear, cyclic or chained, that originates from the geochemical
and microbiological transformation of buried macromolecular organic
matter. Hydrocarbons are non polar and have low aqueous solubilities.
Accordingly, they are some of the most reduced and chemically inert
organic compounds in natural ecosystems. Saturated hydrocarbons,
n-alkanes and isoprenoids, are the most abundant compound in
petroleum, which also contains a substantial fraction of low molecular
weight aromatic compounds such as benzene, toluene, ethylbenzene
or xylene and low molecular weight polyaromatic compounds (PAHs)
such as napthalenes, phenanthrenes, or biphenyls. Comparatively,
coal contains a heavier proportion of insoluble polycyclic compounds.
Hydrocarbons are distributed ubiquitously from surface environments
through the shallow subsurface to the deep subsurface. Methane
is often present in these carbon rich environments. It has long been
considered a by-product of the evolution of coal and oils under the
high temperature and pressure needed for their maturation. In recent
years, it has become obvious from carbon isotopic data that in many
settings biogenic methane represented from a significant fraction to
the majority of the gas present in the reservoirs [1]. With evidence
for microbial oil- or coal-derived methane generation accumulating,
one can ascertain today that any coal bed or petroleum reservoir at
temperatures less than 80°C contains microbial communities capable
of generating biogenic methane, a process called methanogenesis.
Questions about whether methanogenesis is active and/or significant
in situ today is still under question. |
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| Methanogenesis is the formation of methane by microbes known
as methanogens. All methanogens known to date belong to the domain
Archaea. Methanogenes use carbon as a terminal electron acceptor.
Carbon can occur only in a very small number of organic compounds,
all with low molecular weights, with 1 or two carbons at best. The two
best described pathways involve the use of carbon dioxide and acetic
acid. The production of methane is an important and widespread form
of microbial metabolism. In most environments, it is the final step in
the decomposition of biomass. In absence of electron acceptors other than carbon, low molecular weight carbons suitable for methanogen
respiration originate from the fermentation of the biomass by consortia
of different bacterial species, which rely on methanogens for the removal
of fermentation by products such as hydrogen and CO2. Anaerobic
fermentation/methanogenesis is the archetypic example of a syntrophic
relationship between different groups of microorganism. Although the
fermenters are not strictly dependent on the syntrophic relationship,
they gain profit from the scavenging of hydrogen by methanogens,
since they gain maximum energy yield when protons are used as
electron acceptor with concurrent H2 production. Methanogenesis is
beneficially exploited to treat organic waste in biogas fermentors and
the methane produced is collected and used as biogas. |
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| Although evidence for aerobic hydrocarbon-degradation have been
known for nearly a century, and the enzymes and mechanisms they
employ are well understood, it is only very recently that evidence for
strictly anaerobic degradation of hydrocarbons were obtained and the
first consortia of syntrophic bacteria and methanogenic archaea could
be described [2]. Besides the presence of methane, which origin could
be debated, direct evidence for anaerobic hydrocarbon degradation
comes from the detection of intermediate metabolites that are present
in reservoir-degraded oils from around the world, but not present in
‘‘pristine’’ or aerobically degraded oils [3,4]. However, we still know
little of the mechanisms employed by these anaerobes to overcome
the chemically inert nature of hydrocarbons. Ecological studies
have identified the systematic presence of Clostridia, Thermotoga,
Bacteroidetes and other obligate fermenters suggestive of a fermentative
capacity within the microbial community. In the absence of exogenous
electron acceptors, methanogenesis is thought to play an important
part in the degradation of hydrocarbon. Methanogenic incubations
with coal or oil as a sole carbon source have been performed [5,6].
Studies show that growth rates of methanogenic hydrocarbondegrading
communities in the laboratory are slow. However, a large
range of hydrocarbons can be fully degraded to CH4 when inoculated
with production waters from oil well or formation waters from coal
seams. Methanogenesis rates in the laboratory can be increased by
substrate amendment [1,7-9]. There is today ample evidence that
methanogenesis also occurs in situ in deep and shallow petroleum
reservoirs [8], as well as coal beds [10]. Conversion rates to methane
in situ are extremely slow [11], rendering the exploitation of biogenic
gas possible only because of the geologic time-long accumulation of gas in the reservoirs. Less mature coal, lighter oil, and immature kerogen
are the preferred substrate for the syntrophic-methanogenic consortia. |
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| The wide distribution of methanogens in both shallow and deepsubsurface
environments offers the potential to harness methanogenesis
to transform unexploited or unexploitable petroleum reservoirs or coal
beds into methane at an economic scale. In situ microbial conversion
of residual oil, which is typically more than 50% of an oil field potential,
to methane would allow the recovery of large amounts of energy [12].
It can also help transform inaccessible coal beds and immature source
rocks to exploitable methane resources [13]. However, technological
challenges must be overcome before biogenic methane from source
rocks can make a serious contribution to global energy supply. One
challenge is the engineering of the subsurface environments to
effectively stimulate methanogenic consortia and recover the gas
produced. The engineering challenges include the logistics of moving
fluids, gases, and biostimulants through heterogeneous reservoir
systems and the development of tools to monitor the process in situ.
The key microbiological challenge is the stimulation of the indigenous
or the establishment of an exogenous hydrocarbon-degrading
microbial consortium within the target rocks. Energy recovery from
proven biologically active target rocks would best rely on accelerating
the natural methanogenic biodegradation rates. Several laboratories
have shown that methanogenesis could be increased substantially
in the laboratory by the addition of nutrients, demonstrating that
bioaugmentation could be an important strategy for implementing
enhanced in situ methanogenesis in source rocks. Tested nutrient
additions included hydrogen, CO2, ammonia, phosphate, yeast extract,
tryptone, milk, agar, trace metals, and vitamins, or inorganic nutrients.
In addition to chemical stimulation of microbial conversion of coal
to methane, some authors have suggested and experimented with the
addition of selected microbial consortia. Examples of consortia used
for methanogenic inoculation with coal in laboratory settings include
a cultivated consortium indigenous to studied coal, a consortium
obtained from termite guts, and a consortium obtained from an
abandoned coal mine used as sewage disposal. However, this approach
has some clear technical challenges. The first is linked to the logistics
of producing an inoculum in sufficient amounts for the target system.
The second is linked to the survival of the syntrophic methanogenic
consortium during injection. It is a well known fact that the syntrophic
partnerships between fermenters and methanogens is a physical
association that small perturbation may disrupt [14]. The last challenge
relies with the in situ survival of the methanogenic consortium in the
target system, its ability to displace the indigenous flora and establish
itself in situ. Very few such microbial introductions have been
successful thus far, regardless of their target. Thus this may be the main
challenge facing this approach. |
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| The path to in situ methane transformation of coal or petroleum
is still long. Several challenges remain before this approach can
be economically sound. First it is essential that we become able to
determine what fraction of the resource can be ultimately converted
to methane, and therefore determine the influence of the geologic
context, exploitation history and the microbiology in situ on the rate
of methanogenesis that can be expected. Second, in order to improve
and control its rates, we need to understand the initial steps of the fermentation process that generate the substrates for the methanogens.
Coupled approaches of analytical chemistry and omics will help shed
light on these rate-limiting initial steps. Third, as in many industrial
processes, we need to understand how to best scale our cubic centimeter
microcosms observations up to the scale of the cubic kilometer scale of
the in situ reactor, that the petroleum reservoir or coal beds definitely
are, before being able to aid field-scale microbial methane stimulation.
Last, we need to evaluate whether the stimulated subsurface methane
conversion rates using nutrients and specialized microbes will be
enough to convert vast global fossil fuel reserves into economically
fit gas resources. Despite the complexity and challenge, there is little
doubt about the benefits of enhancing methanogenic hydrocarbon
degradation in situ. Extrapolation of laboratory methane production
data to a realistic field-scale application estimated a 3–13 billion cubic
feet of methane per day for the sole known US oil reservoirs [7], e.g. ca
17% of current natural gas consumption in the United States. |
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| References |
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