Coal creation mechanism uncovered

Max Lloyd, Penn State's assistant research professor of geosciences, tested samples from all over the globe. The left-hand side shows wood, lignite and subbituminous, as well as bituminous, coal. Credit: Patrick Mansell at Penn State
A team of researchers discovered that one of the initial stages of coal production was not what we thought. This is due to the fact that methane fuel recovery may be affected by microbes.

Researchers looked at coal samples from all over the globe for methoxyl groups and used stable isotopes in order to prove that organic material can eventually become coal by microbial action.

A methoxyl group is composed of a carbon atom and three hydrogen atoms that are attached to an oxygen. An oxygen atom can attach at any place in a larger molecule. It attaches to a carbon-atom in one of the coal's rings.

Max K. Lloyd assistant research professor of geosciences at Penn State said, "If you polled Geochemists they would all say that coal was made by temperature, acids, or catalysts." Our results do not support these mechanisms. These results show that microbes directly consume coal methoxyl group, transform coal, and make methane.

When plant matter from wetland forests is dissolved in the water, coal forms and is quickly buried. Peat is the first form of organic material. It then becomes lignite, subbituminous bituminous, and finally anthracite. As it gets buried deeper, it becomes more carbon-rich. Anthracite coal is mainly carbon while lignite remains very vegetal.

Lloyd says that most of the coal currently used in countries like India and China is lignite or subbituminous. These coals are the most likely to produce large amounts of greenhouse gases. The researchers today in Science note that methane wells within these coal beds, called coal bed methane (CBM), are attractive options for a way to bridge the gap from fossil fuels. However, "CBM production wells often have short life spans", they say.

Lloyd stated that the challenge in coal bed methane production lies in the fact that wells are very costly and may not be able to produce enough gas for a month. We don't know the reason. Producers can add more microbes and more nutrition (for microbes), but this only works if these are the limiting factors, and not if coal is the limiting factor.

This is the process microbes use in order to make a methane precursor from coal. The pore spaces between the coal and anaerobic microbes are where they live. They secrete enzymes into the pore space to attach the coal structure. This enzyme releases a methoxyl group, forming a methane precursor molecular. Credit: Max Lloyd, Penn State

Lloyd had originally been looking at the abundance of methoxyl group in dead or living trees when he met Elizabeth Trembath, a Caltech graduate student who was studying microbes that eat methyl groups in coal. Lloyd began looking for the same thing in all kinds of coal around the globe after they had confirmed their observations using two different methods.

Researchers discovered that methane is formed from methoxyl compounds in coal. However, it is not well understood how methane is made from coal. The researchers studied stable carbon isotopes in the methoxyl group left behind to better understand the process.

Stable isotopes, which are non-radioactive elements that have varying numbers of neutrons within their nucleus, are the nonradioactive forms. Although the isotopes carbon 13 and 12 contain similar amounts of neutrons, carbon 13 is somewhat heavier. The preference of biological organisms for one isotope will be the same. This is because what is left over in the original source may differ from the normal percentages.

Lloyd and his colleagues examined methoxyl groups from wood to bituminous coal, and found that their profiles did not match those expected if methane was created by heat, acidity, or catalytic reactions. However, they were consistent with the patterns expected to be generated from microbial activity.

Lloyd stated that although aerobic microbes can degrade the coal rings, anaerobic microbes cannot. "So, the only thing left for anaerobic microbes to do is remove the methoxyl portions."

These methoxyl groups that have been liberated are then converted into methane. Once all of the methoxyl radicals have been removed from the rings, microbes can no longer access them and the reaction ceases.

Lloyd said that the most interesting part is that microbes release enzymes to remove methoxyl. "They are destroying the structure extracellularly. This is very limited because coal is not an option and the microbe cannot easily reach all parts of the coal structure.

Researchers believe that methane production is limited by the depletion in methoxyl groups within coal over time. Adding more nutrients or microbes will not increase methane production. A different approach is needed.

Learn more Measuring methane in Pennsylvania from coal and natural gas

Further information: Max Lloyd. Methoxyl stable Isotopic Constraints on the Origins and Limits of Coal-bed Methane. Science (2021). Journal information: Science Max Lloyd, Methoxyl stable isotopic constraints on the origins and limits of coal-bed methane,(2021). DOI: 10.1126/science.abg0241