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.”)
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.