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Sequestration: Part 5 - In Coal

I get to do some of this in my research with colleagues here at Penn State and elsewhere. Coal is a wonderful material! While it looks solid, if you select the appropriate rank of coal the structure is full of very tiny holes, so small they are too small to be holes so we call them pores. The very small pores are known as micropores. There are larger pores, and very large pores or cracks in the coal which is very useful for increasing the permeability (speed of access of the gases into the coal).

Source:JPM Coal can have a complex interconnected cleat system (cracks) and lots of gas storage potential indicated by the green spheres within a molecular model of coal (from my Doctoral thesis!)—I knew it would have a use!

In the gas phase (there are three phases: solid, liquid and gas for most things) the molecules or CO₂ are bouncing around. We can increase the pressure (more gas molecules in the same volume as before) but that takes work. The molecules like their space and anyway we are decreasing their entropy. But if we put a molecule of CO₂ into a coal micropore there is an attraction between the matrix and the CO₂ molecule. Now the CO₂ does not need as much space, it wants to associate with the coal and thus we can put in more CO₂ into coal than we could into the same volume of empty space (bloody marvelous!)

There are a couple of other advantages too. Remember that methane is often in these pores too (if we are at the bituminous rank range) well CO₂ acts like an invading army and kicks out the methane (it competitively replaces the methane molecules). Thus, you could sequester CO₂ into a deep coal seam, collect the methane that is released from the coal (coalbed methane), combust that and put the CO₂ formed back into the coal seam. Thus forming a closed loop of emission free (CO₂ anyway) electricity for as long as the methane lasts. As we have lots of coal, much of it is close to large point sources of CO₂ (such as a power plant) then this all makes sense.

Source:JPM The three gases shown Nitrogen (blue), methane (gray and green) and carbon dioxide are similar in size.

There are only a couple of minor details, the main one being cost. If the pumping and separation of the CO₂ costs $ then the price of the electricity generated will increase. We are not yet in a competitive region for any of the sequestration technologies (maybe enhanced oil recovery). Selling methane certainly helps to lower the cost of sequestering CO₂ into coal but the cost is still too high. There are a couple of locations where we are currently pumping CO₂ into coal.

One advantage of doing this in coal fields that have coalbed methane is you have enhanced coalbed methane recovery that can help off-set the cost of sequeastration.