Researchers at the US Department of Energy's National Energy Technology Laboratory (NETL) have carried out initial investigations into the potential environmental impacts of carbon dioxide sequestration in unmineable coal seams, using carbon dioxide to displace methane from the seam – sequestering the carbon dioxide, and at the same time freeing up natural gas, which could then be used as a power source.

The research team collected coal samples from 250 coal beds across 17 US states, to test the availability of methane (natural gas) in the coal. They found that the amount of methane contained in the coal varied from seam to seam. Some sources of coal harbour vast quantities of natural gas – and low-volatile rank coals generally average the highest methane content, at 13 cubic metres per tonne of coal.

The key question is whether methane can be tapped from the unmineable coal seams and replaced permanently with huge quantities of carbon dioxide. If so, deep coal seams could represent a vast sink for carbon dioxide produced by industry. The study points out that worldwide, there are almost 3 trillion tonnes of storage capacity for carbon dioxide in such coal seams.

But the study also says that, while the idea is a good one in principle, a number of problems will need to be solved before the technique could find a real world application.

The researchers found that the depth from which a coal sample is taken reflects the average methane content. Deeper seams contain less methane, making them less useful for injection sequestration – although the researchers stress that the research is only at an early stage, and relies on data collected only in the US.

The second problem comes with the injection process itself.

To replicate actual geological conditions, the researchers built a Geological Sequestration Core Flow Laboratory (GSCFL) to conduct a wide variety of carbon dioxide injection experiments in coal (and other rock cores, such as sandstone) under in situ conditions of triaxial stress, pore pressure and temperature.

Preliminary results obtained from one source of the coal, Pittsburgh No. 8 coal, indicate that the permeability of the seam decreases (from micro-darcies to nano-darcies – extremely low flow properties) with increasing carbon dioxide pressure, with an increase in strain associated with the triaxial confining pressures restricting the ability of the coal to swell. The already existing low pore volume of the coal is decreased, reducing the flow of carbon dioxide, measured as permeability.

The upshot of this is that the more carbon dioxide that is pumped in, the less permeable the coal becomes, making it more difficult to displace the methane and therefore sequester more carbon dioxide.

The researchers say that this is the most significant problem that will have to be overcome if coal seam sequestration is to be widely used.

In the second study, the research team also investigated some of the possible side effects of sequestering carbon dioxide in coal mines. They tested a high volatility bituminous coal with produced water and gaseous carbon dioxide at 40C and 50 times atmospheric pressure, using microscopes and X-ray diffraction to analyse the coal after the reaction was complete.

The study found that some toxic metals originally trapped in the coal were released by the process, contaminating the water used in the reaction.

“Changes in water chemistry and the potential for mobilising toxic trace elements from coal beds are potentially important factors to be considered when evaluating deep, unmineable coal seams for carbon dioxide sequestration, though it is also possible that, considering the depth of the injection, that such effects might be harmless," the researchers said.

“The concentrations of beryllium, cadmium, mercury and zinc increased significantly, though both beryllium and mercury remained below drinking water standards."

However, toxic arsenic, molybdenum, lead, antimony, selenium, titanium, thallium, vanadium and iodine were not detected in the water, although they were present in the original coal samples.