Coal is a witches’ brew of chemicals including hydrocarbons, sulphur, and other elements and molecules. Burning it is a dirty business, producing toxic and carcinogenic emissions including arsenic, selenium, cyanide, nitrous oxides, particulate matter, and volatile organic compounds. Coal plants also produce large amounts of carbon dioxide, thus contributing to climate change. That said, some coal plant designs can reduce both toxic and climatically relevant emissions to a considerable extent. Given concerns about energy security – coupled with the vast coal reserves in the United States, United Kingdom, China, and elsewhere – giving some serious thought to cleaner coal technology is sensible.
Integrated Gasification Combined Cycle (IGCC) plants are the best existing option for a number of reasons. Rather than burning coal directly, they use heat to convert it into syngas, which is then burned. Such plants can also produce syngas from heavy petroleum residues (think of the oil sands) or biomass. One advantage of this approach is that it simplifies the use of carbon capture and storage (CCS) technologies, which seek to bury carbon emissions in stable geological formations. This is because the carbon can be removed from the syngas prior to combustion, rather than having to be separated from hot flue gases before they go out the smokestack.
The problems with IGCC include a higher cost (perhaps $3,593 per kilowatt, compared with less than $1,290 for conventional coal) and lower reliability than simpler designs (this diagram reveals the complexity of IGCC systems). In the absence of effective carbon sequestration, such plants will also continue to emit very high levels of greenhouse gasses. If carbon pricing policies emerge in states that make extensive use of coal for energy, both of these problems may be reduced to some extent. In the first place, having to pay for carbon emissions would reduce the relative cost of lower-emissions technologies. In the second place, such pricing would induce the development and deployment of CCS.
One way or another, it will eventually be necessary to leave virtually all of the carbon that is currently trapped in coal in the ground, rather than letting it accumulate in the atmosphere. Whether that is done by leaving the coal itself underground or simply returning the carbon once the energy has been extracted is not necessarily a matter of huge environmental importance (though coal mining is a hazardous business that produces lots of contamination). That said, CCS remains a somewhat speculative and unproven technology. ‘Clean coal’ advocates will be on much stronger ground if a single electricity generating, economically viable, carbon sequestering power plant can be constructed.