EGEE 101
Energy and the Environment

Coalbed Methane

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As coal ages, it undergoes chemical and physical structural changes. You will see below some of the proposed structural changes that occur. I love this stuff! This is very much why I am Dr. Mathews: 6 years of study and the generation of a few structures like these. The structures here are simplistic representations of some of the structural features found in coals of those ranks. We still do not know the structure of coal, because it is highly variable and complex. As the structure changes one of the gases it produces is Coalbed methane, which supplies on the order of 8% of the domestic natural gas production and so is an important unconventional resource (shale gas is also an unconventional resource and a much larger contribution to the domestic natural gas supply).

Look at each of these three models. They are very different in appearance. Look at the hydrogen and oxygen atoms -there are fewer as the coalification proceeds. The anthracite coal is the most carbon-rich. These changes influence calorific value.

The make-up of coal
Diagram of atoms for low-rank coal, bituminous Vitrinite, and Anthracite coal
Credit: (in order): Leigh Clemo, Australia's CRC for Clean Power from Lignite; JPM, PSU Energy Institute; Peter Pappano, PSU Energy Institute

Coalification Changes

A coal lamp
The Police sang about
a "Canary in a Coal
Mine", but if you don't
have one handy the
coal miners' lamp is
a pretty useful tool
for detecting methane.
Any guesses as to
why it does not
cause an explosion?
Credit: JPM

Okay, a few structural changes are evident as coalification proceeds from low-rank coal to anthracite:

  1. Lots of moisture is associated with low-rank coals (water is actually part of the structure, up to 60% of the mass of some coals).
  2. Less oxygen (which is why the calorific value increases over much of the rank range).
  3. Less hydrogen, particularly in the anthracite structure (this is why on reaching anthracite rank the calorific value decreases slightly).
  4. Larger aromatic structures; when transitioning between bituminous coal and anthracite, the number of aromatic rings in a layer (or the size of the aromatic portion of the molecule) increases from a few rings (around 3) to perhaps 100's. These chicken-wire like aromatic structures are becoming more like graphite and is less reactive during combustion.
  5. The structure is becoming more carbon-rich.

The only way for the structure to become more carbon-rich is to either 1) have carbon added (not likely 100's of feet down), or 2) lose other material. The latter is the case here. As coal matures, the oxygen is lost probably as water and hydrogen as methane (as most of the oxygen formed water there is not enough oxygen left and so the hydrogen generated from the coalification process is lost as methane). The methane will either migrate to the surface or into other structural traps, with a considerable quantity being retained in the coal.

For over a century, this coalbed methane has been problematic. In the right mixture with air, it is explosive. Coal mine methane explosions are among the worst mining accidents. Methane in the mine causes ventilation problems trying to prevent the methane content from building up. If the levels are too high in modern mines (in the US) work has to shut down for hours before the miners can return, causing huge losses in productivity.

In the (4:13) video below, Dr. Mathews describes the perils of being a miner and the cues miners used to avoid serious danger (and explains why the lamp doesn't explode!)

Perils of Being a Miner
Click Here for Transcript of Perils of Being a Miner Video

[Video opens with Dr. Mathews standing next to a pillar of coal.] Dr. Mathews: One important thing about mine safety is the gases. Now, there are several things that can go wrong in the mine. The roof can fall in on you, you could have an explosion, or you could get trapped or crushed by any of the machinery or the explosive devices you are using. One of the key things, however, is the gases. And this is a very important miner's safety piece of equipment. [Dr. Mathews hold up a lamp-like object that is in his hands.] Dr. Mathews: This is a Davey's Miner's Lamp. Probably about eighty years old by the looks of it. What I would have is some fuel source, probably kerosene back at about 1920. Prior to that, we may have been using Sperm Whale oil. And there is a naked flame. One thing you don't want to have in a coal mine is a naked flame. And so it is protected by this wire mesh. When I was in the mine, if I was to lift it up, and I would see a bluish tinge, and if the light would get brittle, I would know that there was methane in the coal mine. Now methane now-a-days is a good thing. It is a very valuable resource. But if you are in a coal mine, one spark, which is very easily done, is going to start an explosion, if it is in the right mixture. And that could kill everybody. So certainly there have been coal mine explosions where more than 100 people have died. It is one of the main reasons coal miners today will still wear a very thick leather belt with their name stamped on it so you can find their remains. But of course, there are other gasses in the mines as well. If you lowered your miner’s lamp and the light was to go dim, when you put it toward the bottom of the mine, it means there is probably not a lot of oxygen there. There is probably either carbon monoxide or carbon dioxide. Carbon monoxide will kill you. Carbon dioxide means there is a lot less oxygen in the air and you might want to leave. It is one of the good ways of staying alive. The other methods of using are of course high tech equipment which we have now, but in the good old days, you would use a canary. A canary has very small lungs and breathes very fast. [A picture of a miner holding a canary in a small cage.] Dr. Mathews: If your canary stops singing, and is lying in the bottom of his cage, it is probably a good time to leave. If you are in a drift mine, that is a mine that goes into the slope of a mountain, then there is probably going to be rats there as well. If they start leaving it means either the roof is going to come down, or it is the presence of dangerous gasses. Thank goodness for rats and canaries. [Video ends]

Credit: JPM

Methane is also a greenhouse gas (more on that in unit 3). So now that natural gas has value, so does Coal Bed Methane. Coal Bed Methane is a danger to the miner, as methane is an explosive gas (when it is present with air at the correct concentration levels). It is there because of the maturation of the coal and the changes in the molecular construction of the coal. With age, the coal becomes more carbon-rich by losing oxygen and hydrogen in the form of methane. Now methane has more value, the methane in coal has value and is now being extracted. This can extend the lifetime of our natural gas supply. Locations with bituminous coal are extracting the methane from abandoned coal mines and from deep unminable coal seams. They also look to extract the methane from coal seams that haven't been mined yet, which have the added benefits of creating mining operations later that are safer and less expensive, because the air flow into the mine can now be minimized, saving time, money, and overall productivity. We will see in unit 3 that we might be able to store CO2 (sequestration) within deep coal seams as a climate change mitigation option.