...or at least is a deceptive term, according to the UK Advertising Standards Authority:
The ASA understood that the phrase "clean coal" was the term given to a branch of research and innovation aimed at reducing the environmental impact of using coal, such as filtering out particulates and preventing or neutralising the emission of waste gases. However, we also understood that this technology was not able to prevent CO2 from being emitted during the use of coal, relying instead on carbon capture and storage, and that although emissions such as sulphur dioxide were reduced, they were still produced. We also noted that the line immediately following this claim stated "We call it Advanced Energy for Life. Because clean, modern energy is the solution for better, longer and healthier lives" and considered that consumers were likely to assume that this referred to Peabody Energy's 'solution' of "clean coal". Although we noted that the ad stated "clean coal" technologies would "improve emissions", we considered that this was not sufficient to make clear the nature of this technology, particularly in the context of the word ”clean”. Notwithstanding the fact that "clean coal" had a meaning within the energy sector, we considered that without further information, and particularly when followed by another reference to "clean, modern energy", consumers were likely to interpret the word ”clean” as an absolute claim meaning that "clean coal" processes did not produce CO2 or other emissions. We therefore concluded that the ad was misleading.
The ad breached CAP Code (Edition 12) rules 3.1 and 3.3 (Misleading Advertising), and 11.1, 11.2 and 11.3 (Environmental Claims).
...which is interesting in light of the fact that "clean coal" has not been ruled out as a possible source of green energy here in Ontario. Although you might argue that while the province might hypothetically approve some form of coal-with-carbon-capture facility, it isn't terribly likely.