Clean coal – we hear this term a lot. Unlike many other energy/environment terms, however, this one’s confusing because of the range of circles in which it is used. The fossil fuel industry is a big fan of it, as are some environmental organizations and politicians on both sides of the aisle. We’ve heard it called a savior and an oxymoron, but those discussions almost never tell us what it is and how it works.
The term clean coal actually refers to a range of technologies that can reduce the environmental impact of coal combustion. Overwhelmingly, the technology that people refer to when talking about clean coal is Carbon Capture and Sequestration (CCS). While CCS is just one of the relevant technologies, it’s the one that gets (and arguably deserves) the most attention. CCS is the process of capturing carbon dioxide (CO2) as it’s emitted from a fossil fuel-burning facility and storing it at a long-term storage site, generally impermeable geologic formations. By preventing the CO2 from entering the atmosphere, CCS offers a way to nearly eliminate the greenhouse gas emissions from large point sources, such as coal-fired power plants (hence the term “clean coal”). From the viewpoint of short-term emissions reductions, CCS seems to have potential. But looking deeper, we can see that it’s riddled with risk and may actually perpetuate other pressing environmental issues.
The first step in CCS is to “capture” the CO2 emissions before they are released into the atmosphere. This can be done on-site (at the power plant) by a number of systems, separating and purifying the gas either before, during or after combustion. Next, the CO2 is transported to the storage site, most commonly by pipeline. Geologic structures, such as depleted oil and gas wells, are receiving the most attention for long-term storage. Once at the storage site, the CO2 is injected underground at extremely high pressure. In many cases, CO2 is already being pumped into partially depleted oil wells to force out the remaining petroleum, a process known as Enhanced Oil Recovery. If a coal-fired power plant’s emissions can be successfully captured, transported and stored, then its contribution to climate change can be dramatically reduced.
But CCS is plagued by uncertainty, and the implications of a potential failure are cause for reconsideration. One major concern with geologic storage is the possibility of leakage. If a leak occurs at a storage site containing several years of CO2, for example, the result could be equivalent to emitting years’ worth of CO2 into the atmosphere over a much shorter time period. Even now, when the vast majority of existing CCS projects are only pilot scale, we’re already seeing anecdotal evidence of leaks. While such leaks might be kept to a minimum with the appropriate technology, even rare occurrences would be likely to mitigate many of the benefits to the climate. And in addition to reentering the atmosphere, the impact of leaked CO2 on drinking water has been well documented.
Another critique of CCS has to do with its limited scope. While storing the gas underground could theoretically reduce the impact of coal combustion on the climate system, it does nothing to address a number of other environmental issues associated with coal extraction and use. Coal mining ravages large swaths of land, most dramatically when employing mountain top removal. Once the mines are open, acidic water often flows out, polluting nearby surface water. And the exhaust from coal-fired power plants has consistently been linked to poor public health and respiratory ailments. A technology that allows these and other impacts to persist while encouraging increased use of coal doesn’t fit any reasonable definition of “clean”.
It turns out that the process of capturing CO2 on-site at power plants is also a substantial parasitic load – this means that it consumes a significant amount of the electricity generated by the facility, requiring even more production to meet normal demand. Current CCS technology is estimated to require 15-30% of a power plant’s output. Not only does this increase the amount of CO2 to be sequestered, it also requires more coal, perpetuating the impacts associated with extraction. All in all, CCS seems to have the potential to address one problem alongside the certainty of exacerbating a number of others.
The Verdict: The main technology relied on by clean coal, CCS, is plagued by risk and uncertainty, and only deals with a portion of the impacts that make current coal use dirty. Although the effectiveness of burying the emissions from coal combustion is still debatable, evidence of coal’s effects on water, air and human health is rock solid. Even if CCS were a proven technology, the concept of clean coal would remain a fantasy.