Clearing the air about “clean coal”

By Matthew Bates
CG Science

Throughout Barack Obama’s campaign for the presidency, voters —­ especially those in Ohio — were bombarded with the buzzwords “clean coal.” Beyond the hopeful ideal touted by politicians, however, are more questions than answers. What exactly is clean coal? Is the term “clean coal” misleading? Is this only a temporary solution?  Is clean coal, in fact, sustainable?

Gavin coal-fired power plant

File photo

The simple explanation is that “clean coal” is a blanket term used to describe any measure being taken to reduce the environmental impact of the coal industry. This includes, most famously, technologies used to capture or divert the release of carbon dioxide and minimize the carbon footprint of coal burning power plants. Clean coal also refers to actions taken to reduce the release of sulfur dioxide emissions and neutralize the threat of acid rain. To get a more complete explanation, it is necessary to look at the individual strategies being used by different coal companies and researchers.

The most basic method is scrubbing, which involves a physical interaction with the flue gases of power plants. Wet scrubbing uses a solvent to remove pollutants, such as sulfur dioxide, from the gases. Dry scrubbing uses a sorbent, such as a mixture of lime and limestone, to absorb certain gases from the flue composite. While scrubbing can be effective for the removal of gases that lead to environmental problems like acid rain, the scrubbing of carbon dioxide and other greenhouse gases is a little more complex and costly.

Another popular “clean” method is coal gasification. Coal, which is mostly carbon, is combined with water and heat to form syngas, a mixture of hydrogen gas and carbon monoxide. The contents are then separated, and the carbon monoxide can be burnt to create energy or converted into a synthetic diesel fuel. Through the gasification process, nitrogen and sulfur can be extracted from the coal, resulting in a “more clean” fuel that reduces the amount of acid rain and smog. The problem with this method, however, is that neither the resulting syngas nor the carbon-to-liquid diesel fuel are very efficient power sources, both giving off significant levels of carbon dioxide.

The Mountaineer Power Plant in New Haven, W.Va., is experimenting with a new strategy to curb emissions. The plant is sequestering carbon from the exhaust smoke and storing that carbon deep below the surface of the plant, where it is hoped it will remain permanently.

This idea is not new. The science behind capturing carbon has been developed for some time, but the method of storage is a controversial topic.

For years underwater storage was the hot topic in carbon sequestration, but eventually the idea of shoving huge amounts of gas to the bottom of the ocean raised some big questions. According to Ben Stuart, an assistant professor of civil engineering at Ohio University, the Massachusetts Institute of Technology was a proponent of storing carbon underwater for over 10 years before deciding that the escalating level of risk involved was not worth the benefits.

The problem with this strategy is that not only will the increased carbon dioxide pose a threat to organisms living in the deep water where it is stored, but that the carbon would eventually escape back into the atmosphere. Although the enhanced pressure at the ocean depths would stop the gas from rising, it would not stop the gas from dissolving, creating a carbonation not unlike that in the sodas we drink. This in itself is not such a big problem, but, as with soda, the carbon dioxide will rise to the surface of the water where it is free to enter the atmosphere again, defeating the whole purpose of the sequestration.

In response, researchers have developed a different strategy to prevent sequestered carbon from re-entering the atmosphere. The current thinking of American Electric Power, owner of the Mountaineer plant, is that carbon should be stored deep underground in saline reservoirs 8,000 feet beneath the power plant. The carbon dioxide, pressurized and converted to a liquid-like state, is injected into porous rock, where the increased pressure and the physical geological barrier contain the carbon. This process is reminiscent of the ways oil and gas were stored in the earth for millions of years until humans began extracting them.

Sequestering carbon deep in the earth sounds good in theory, but the natural world is not theoretical. The carbon should stay mostly lodged in the rock layers, unable to circumvent its confinement; however, the fear is that any major disturbance, such as a fracture or earthquake, would cause the carbon to be released back into the atmosphere. This could happen rapidly, which could lead to a sharp spike in the amount of greenhouse gases in the atmosphere, or it could happen gradually, slowly releasing carbon dioxide into the surrounding area.

Carbon dioxide is denser than the gas mixture comprising our atmosphere. This concentrated gas could create an asphyxiation environment that would blanket the ground, choking not only animal and plant life, but risking human life as well. On top of that, the possible leakage of carbon dioxide into aquifers adds to the dangers associated with carbon sequestration. Although this is largely a worst case scenario, the potential danger of this storage system is frightening.

A similar fear ultimately shut down the Yucca Mountain nuclear waste repository after risk assessment specialists could not guarantee that a seismic event would not lead to nuclear contamination leaking out from beneath the ground, Stuart said.

Stuart and David Bayless, director of the Ohio Coal Research Center and Loehr Professor of mechanical engineering at Ohio University, are working on a different idea for the method of capture, taking coal sequestration in a completely different direction. While the Mountaineer plant uses a physical separation situation, a process in which carbon dioxide is extracted from the mixture of flue gases and injected into the earth, Stuart’s plan involves a biological separation.

Using an extremophile algae found in Yellowstone National Park, which can withstand the harsh temperatures and gases found in the flues of the coal power plants, carbon can be sequestered directly into biomass through natural, photosynthetic reactions. The flues are basically void of light, but one of Bayless’s students suggested the use of solar collecting and fiber optic cables to direct sunlight to the algae for photosynthesis. The algae can then be used for a variety of purposes, including use as a fertilizer, conversion to methane, or the creation of a biodiesel fuel. This research is responsible for starting the biofuel department at Ohio University, Stuart said, and could lead, at least partially, to better fuels and energy independence.

“There is no magic bullet that fixes everything,” Stuart pointed out.

Cleaner coal alternatives can only realistically be part of the solution. The future image of energy will likely need to draw extensively on other sources, such as wind and solar power, before we can hope to have a fully functional carbon-neutral energy system.

This brings us back to the concept of “clean coal.” The methods mentioned above are only a sample of the techniques being employed and perfected by energy companies and independent researchers. No method alone is perfect. Even in combination, the different tactics let other gases not even included in this discussion escape into the atmosphere, along with causing other environmental complications.

Richard Conniff, a magazine writer who specializes in environmental reporting, wrote in an article published in Yale Environment 360 that the term “clean coal” was not developed by scientific researchers or environmental activists, but by the same advertising agency that developed the Las Vegas tagline “Whatever happens here, stays here.” His assertion is that “clean coal” is something invented for politicians and power companies.

So while the idea of a cleaner version of coal is certainly appealing, and might receive a lot of political attention, it may actually be serving to misdirect and misinform the public in terms of actual environmental advantage. At best, “clean coal” is a partial solution, still relying on a limited resource, and research money might be better spent on developing technologies that will take advantage of renewable energy sources. At worst, the notion of “clean coal” stops people from looking for better solutions.

In the rush to find a solution for energy problems in America and around the globe, it is easy to overlook some of the dangers that could spin out of these solutions, and with “clean coal” those dangers include carbonation of water, pollution of aquifers, and potential asphyxiation on a large scale, not to mention the risk of futility – the carbon could just end up back into the atmosphere in a few years, anyway.

Clearly, “clean coal” is a murky issue that scientists and decision makers must wade through carefully.

2 thoughts on “Clearing the air about “clean coal”

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