Energy

Picking a Winner in Clean-Coal Technology

A new MIT study says that no single technology is the solution to economically cutting carbon-dioxide emissions from coal.

Technologies for cleaning up one of the cheapest and dirtiest sources of electricity–coal–are promising. But an MIT study released last week suggests that no single technology will do the trick. (See “The Precarious Future of Coal.”)

Coal’s crystal ball: Ernie Moniz, professor of physics at MIT, announces a new road map for reducing carbon emissions from coal.

According to the MIT report, a clean-coal solution will likely lie in a combination of several new technologies for capturing carbon dioxide and storing it to keep it out of the atmosphere. “The world is going to have to do something to adopt serious constraints on the emission of greenhouse gases, and carbon dioxide in particular,” says John Deutch, professor of chemistry at MIT and one of the authors of the study. “All of these approaches are promising. All these technologies are amenable, at some cost, to carbon capture and sequestration. We do not see that there is any reason to pick a technology winner today. There are several different avenues that should be pursued.”

Indeed, the MIT report reached the surprising conclusion that an acclaimed new type of coal-fired power plant, called integrated gasification combined cycle (IGCC), may not provide the best solution for reducing carbon emissions. So far no commercial-scale coal plants have been designed to capture carbon dioxide–and without a price on the greenhouse gas, there has been no economic reason to do so. But IGCC has long been lauded as a type of plant that would make it less expensive to capture carbon dioxide in the future because it produces more concentrated carbon dioxide than is emitted from conventional coal plants.

Capturing carbon dioxide from an IGCC could be, in theory, relatively cheap and easy to implement. IGCC plants use a process called gasification, in which coal is heated to produce syngas, a combination of carbon monoxide and hydrogen. The carbon monoxide can be converted into carbon dioxide using high-pressure steam. Because the carbon dioxide is highly concentrated, it’s possible to separate it from the hydrogen using weakly binding solvent. The hydrogen can then be burned to turn a turbine, or it can be run through a fuel cell to generate electricity. The carbon dioxide would be released from the solvent when engineers allowed the pressure to drop.

These and other advantages, including easier capture of pollutants such as sulfates, have led many environmentalists and policy makers to favor IGCC. But the MIT researchers say that things aren’t so simple. The key issue is that not all coal is the same. “There are many different types of coal, not only in the United States, but around the world,” Deutch says. “Different coals will suggest different carbon-capture schemes and different technologies.”

Coal from certain areas of the United States, for example, might contain twice the amount of energy as coal in parts of India. The amount of water, ash, carbon, and sulfur varies markedly, and all have an impact on the efficiency and economics of coal plants. And the impact of different coals can be significantly greater for IGCC than for more-conventional types of coal plants.

For example, designing an IGCC plant to run on Texas lignite, a lower-quality coal, adds 37 percent to the cost of the plant, compared with designing it to run on a high-quality coal called Pittsburgh #8. And the resulting plant is 24 percent less efficient. Designing a conventional plant to run on low-quality coal also costs more, but the increase is only 24 percent–less than the 37 percent with IGCC. The hit to efficiency is also less–10 percent versus 24 percent with IGCC. While the added costs for capturing carbon dioxide are greater for conventional coal plants, these can be largely offset if the plant is being designed for use with lower-quality coal.

As a result, it’s less clear which technology would really make the most economic sense. For some parts of the world, where high-quality coal is easily accessible, IGCC will probably be the clear winner. But areas using low-quality coal could be better served by pulverized coal plants, especially the new ultra-supercritical coal plants that power turbines with very high temperature steam. Such plants are about 13 percent more efficient than IGCC, and higher efficiency translates into less coal needed to generate a certain amount of electricity, and hence less emission of carbon dioxide.

Given so many uncertainties, the MIT report recommends that, rather than picking one technology to support, the government fund large-scale, carbon-dioxide-capturing demonstration plants using various technologies. These plants would be run as commercial projects to reveal the real-world costs.

The report also says that the Department of Energy should increase funding for research on a new generation of technologies for coal power plants. The MIT researchers singled out a process called chemical looping, which Deutch says “offers a completely different process for getting the energy out of coal.” In one version, pulverized coal reacts with particles of metal oxides, such as rust. The reaction converts the oxide into a metal, such as iron, and produces carbon dioxide. The carbon dioxide can be compressed for storage. The iron is then exposed to steam. The reaction between the water and the iron produces heat, converts the iron back into rust, and releases hydrogen gas that can power a fuel cell to make electricity.

“Nobody can tell you what the economics are–it’s just being explored now,” Deutch says. “But it’s an example of a different approach to getting energy out of coal to avoid the emissions of greenhouse gases. We think those kinds of completely different approaches ought to be explored.”

Hear more from MIT at EmTech 2016.
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