Wednesday, February 21, 2007

Cleaning Up Coal Power

Although our concerns about climate change and energy security align neatly with regard to improvements in energy efficiency, coal-fired power plants create a collision between these two issues. Our reliance on coal for power generation is growing, reflecting abundant domestic supplies and high natural gas prices, but the emissions from coal will keep our national greenhouse gas output rising, instead of leveling off. Government, industry and environmental groups all look to "clean coal" technology to resolve this dilemma and provide a way to use our coal reserves without breaking the greenhouse gas bank. An article in today's New York Times highlights the challenges of this approach, based on an upcoming MIT report. It also demonstrates just how much confusion exists about clean coal technology and its capabilities.

The two main competing technologies for turning coal into electric power are "pulverized coal" (PC) and "integrated gasification combined cycle" (IGCC). Nearly all of the coal plants in existence today, and most of the new ones being planned, utilize various forms of PC technology, in which coal is burned to make steam, which in turn drives steam turbines to make electricity. These facilities cost the least to build, even after installing scrubbers to remove pollutants such as sulfates and nitrates from the flue gas. However, PC is less energy efficient than IGCC, which means higher fuel consumption and higher greenhouse gas emissions per kilowatt-hour (kW-hr) produced. And because PC burns coal in air, which is 21% oxygen and 78% nitrogen, the carbon dioxide in its exhaust is diluted. That means that any strategy to capture and sequester it later must first extract the CO2 from this gas stream and concentrate it. That adds to the cost and complexity of carbon sequestration.

But IGCC offers no free lunch in this regard, either. Contrary to the misunderstanding expressed by an opponent of TXU's proposal to build new PC plants in Texas, IGCC does not remove carbon prior to combustion. It works by gasifying, or partially combusting, the coal in pure oxygen. The resulting "syngas", made up of carbon monoxide and hydrogen, is stripped of any impurities (sulfur, mercury, etc.) and then fed into a combined-cycle gas turbine. Because of the higher thermal efficiency of the gas turbine, an IGCC plant produces less CO2 per kW-hr than a PC facility. But because the turbine runs on air, not pure oxygen, the exhaust gas is still diluted by nitrogen, and the CO2 would have to be extracted and concentrated for sequestration, just as at a PC plant. There would just be less of it.

It is possible to build a more extreme version of IGCC, in which the syngas was first reacted with water over a catalyst to yield essentially only hydrogen and a very pure CO2 stream that would be easier to sequester. Perhaps this is what Mr. Smith of Public Citizen was thinking of, though no one has done this on coal, yet. I worked on a unit at Texaco's former Los Angeles refinery that used this process to make hydrogen from fuel oil. It was built in the 1960s, so the basic technology is hardly brand new, though it's expensive, finicky, and expends a lot of energy in the H2 conversion step.

The choice between IGCC and PC for ultimate carbon sequestration comes down to a trade-off between the higher upfront costs of IGCC and what it might save later, once the technology of sequestration is sufficiently perfected to roll out on a wide scale. That creates a serious economic uncertainty around IGCC. From my perspective, however, there is a practical solution to this dilemma. Unless you expect sequestration to be in wide use within a decade, an IGCC plant built today will put significantly less CO2 into the atmosphere over the next ten years. If the economic choice between IGCC and PC is close without paying for the CO2 they emit, then under a national greenhouse gas cap-and-trade system--which seems likelier all the time--the advantage shifts toward IGCC, irrespective of future sequestration options. And the more IGCCs get built, the faster their cost premium over PC will fall. Although I can't say a utility would be wrong to build a new pulverized coal plant, it seems analogous to building a new piston-powered passenger plane after Boeing had flown its 707 prototype.

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