Efforts to reduce methane emissions from coal mining have focused on the recovery of coal mine methane (CMM). Coal seams, however, can also be utilized to reduce atmospheric concentrations of greenhouse gases – by sequestering carbon dioxide (CO2).
About a third of all CO2 emissions is generated by the burning of fossil fuels to generate electricity. Increasing the efficiency of these facilities can be achieved for example, by switching from coal, to natural gas. But CO2 emissions could also be reduced substantially by capturing and permanently sequestering the CO2 in secure locations from which it can no longer be emitted to the atmosphere.
Geologic formations, such as oil and gas reservoirs, unmineable coal seams, and deep saline reservoirs, are structures that have stored crude oil, natural gas, brine and CO2 over millions of years.
These geologic formations are attractive candidates for the permanent sequestration of CO2. Moreover, many large industrial emitters of CO2 are located near geologic formations that are amenable to CO2 sequestration. In many cases, injection of CO2 into a geologic formation can enhance the recovery of hydrocarbons, providing value-added byproducts that can offset the cost associated with CO2 capture and sequestration.
Coal seams typically contain large amounts of methane-rich gas that is adsorbed onto the surface of the coal. The current practice for recovering coal bed methane (CBM) is to depressurize the coal seam, usually by pumping water out of the reservoir.
An alternative or complimentary approach for recovering CBM is to inject CO2 into the coal seam.
Recent laboratory and field studies (http://www.coal-seq.com) have demonstrated that coal adsorbs roughly twice as much CO2 as methane. Although much more work is necessary to understand and optimize this process, it offers the potential for CO2 to displace the adsorbed methane and remain sequestered in the coal bed. The methane, which can be recovered and utilized, would provide a value added revenue stream to the sequestration process.
The cost of geological storage in coal seams is site-specific, depending on the characteristics of the storage formation, especially location and depth. Costs are uncertain, as CO2 injection in coal seams for enhanced coal bed methane production is not yet commercial. Well costs are expected to be the major factor driving costs.
The US has considerable potential for CO2 sequestration in unmineable coal seams. US coal resources are estimated at 6 trillion tons but 90% of this coal is considered unmineable due to unfavorable geological characteristics including seam thickness, depth, and structural integrity.
Many large unmineable coal seams are located near electricity generating facilities that are significant point sources of CO2. Integration of coal bed methane with a coal-fired electricity generating system can provide an option for additional power generation with low emissions.
More research and development on geologic sequestration is required to establish its credibility and acceptability as a safe, reliable, long-term option for reducing greenhouse gases in the global atmosphere. A key priority for development of CO2 capture and sequestration technology is cost reduction, as projections based on current technologies indicate that sequestration would be very expensive.
* Based on article that appeared in the June 2004 edition of Coalbed Methane Extra.