Methane {CH4} is produced through a variety of sources. Human activity has been known to produce copious amounts of methane through rice farming, livestock farming, landfills, biomass burning/digesting, coal mining and drilling for oil/gas.
Natural sources include cellulose-disgesting bacteria, or methanogens, in the gut of termites or in the wetlands.
Methane is produced through two major pathways under anaerobic conditions, where
- carbon dioxide is reduced with hydrogen, fatty acids, or alcohols as hydrogen donors.
- transmethylation of acetic acid (CH3COOH) or methyl alcohol (CH3OH).
Abiotic Generation of Methane;
[Link to Abstract] Over the last 30 years, geochemical research has demonstrated that abiotic methane (CH4), formed by chemical reactions which do not directly involve organic matter, occurs on Earth in several specific geologic environments. It can be produced by either high-temperature magmatic processes in volcanic and geothermal areas, or via low-temperature (<100°C) gas-water-rock reactions in continental settings, even at shallow depths.
The isotopic composition of C and H is a first step in distinguishing abiotic from biotic (including either microbial or thermogenic) CH4.
Herein we demonstrate that integrated geochemical diagnostic techniques, based on molecular composition of associated gases, noble gas isotopes, mixing models, and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic CH4 in natural gases, which are frequently mixtures of multiple sources.
Although it has been traditionally assumed that abiotic CH4 is mainly related to mantle-derived or magmatic processes, a new generation of data is showing that low-temperature synthesis related to gas-water-rock reactions is more common than previously thought.
This paper reviews the major sources of abiotic CH4 and the primary approaches for differentiating abiotic from biotic CH4, including novel potential tools such as clumped isotope geochemistry. A diagnostic approach for differentiation is proposed.
Key Points
- Abiotic CH4 occurs in specific geologic areas under a wide range of temperature
- Updated global CH4 isotopic diagram is provided
- Integration of geochemical-geological data necessary to determine abiotic origin
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/rog.20011
Methane is Abundent
Although this phenomenon is well understood, calculating the actual net amount is a bit complicated since methane can be consumed in overlying aerobic layers. Nevertheless, current estimates put methane production on the order of millions of tonnes. [4]
One source in particular has gained much attention over the past few years. Methane clathrate, or more colloquially known as "fire ice", is found as an ice crystal with natural methane gas locked inside, formed through the combination of low temperatures and high pressure. One cubic meter of the compound releases about 160 cubic meters of gas, making it a highly energy-intensive fuel. [5] A large portion of methane can be found distributed primarily within permafrost regions and in continental slode sediments. [6] Evidence shows that these deposits are enormous, with more energy stored in methane hydrates compared to the world's oil, coal and gas combined. Methane produces energy in the form of heat when ignited through oxidative pyrolysis. The following reaction equations describe this process: [4]
CH4 + O2 → CO + 2 H2O (oxidative pyrolysis)
2 H2 + O2 → 2 H2O + ENERGY (heat)
2 CO + O2 → 2 CO2 + ENERGY (heat)
CH4 + 2 O2 → CO2 + 2 H2O + 891kJ/mol (STP)
Recent efforts especially by Japan have proven to be fruitful, where there is an estimated 6 trillion cubic meters of methane hydrate in sedimentary basins nearby. [7]
References
[1] "BP Statistical Review of World Energy 2016," British Petroleum, Jun 2016
[2] B. Bhandari et.al., "Mathematical Modeling of Hybrid Renewable Energy System: A Review on Small Hydro-Solar-Wind Power Generation," Int. J. Precis. Eng. Man. Tech. 1, 157 (2014).
[3] K. L., Cashdollar et.al., "Flammability of Methane, Propane, and Hydrogen gas," J. Loss Prev. Process Ind. 13, 327 (2000)
[4] L., Milich, "The Role of Methane in Global Warming: Where Might Mitigation Strategies Be Focused?," Global Environ. Change 9, 179 (1999)
[5] S-Y., Lee, G.D., Holder, "Methane Hydrates Potential as a Future Energy Source," Fuel Process. Technol. 71, 181 (2001)
[6] L.D. D. Harvey and Z. Huang, "Evaluation of the Potential Impact of Methane Clathrate Destabilization on Future Global Warming," J. Geophys. Res. 100, 2905 (1995)
[7] H. Koide and K., Yamazaki, "Subsurface CO2 Disposal With Enhanced Gas Recovery and Biogeochemical Carbon Recycling," Environ. Geosci. 8, 218 (2001)
[8] A. R., Moss, J-P. Jouany, and J. Newbold, "Methane Production by Ruminants: Its Contribution to Global Warming," Ann. Zootech. 49, 231 (2000)