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The Carbon Conundrum

Technology exists to sequester carbon-but will utilities ever buy in?
Fortnightly Magazine - August 2003

coal and power systems at DOE: FutureGen must be located near a geologic formation suitable for sequestration and must also be close to the grid.

The experimental plant is also widely expected to use coal gasification technology.

Coal gasification gets a lot of press, but it has yet to catch on with utilities. There is only one gasification plant operating as baseload today in the United States-Tampa Electric Co.'s Polk plant, a 250-MW integrated gasification combined-cycle plant (IGCC) that has been in operation since 1996.

Gasification plants are among the cleanest coal technology available to produce power. Yet even if a utility considers an IGCC plant, so far pulverized coal technology has won out. That was the case at Wisconsin Public Service Corp., which has proposed a 500-MW, supercritical pulverized coal plant, rather than an IGCC plant that was considered.

There's no question that IGCC plants cost more to build than a comparable pulverized coal facility. But if carbon sequestration costs are figured in, IGCC plants cost considerably less than pulverized coal. For example, Klara says that an IGCC plant that captured carbon would increase electricity costs by 30 percent. In comparison, he says, retrofitting a pulverized coal plant would increase electricity costs anywhere from 58 to 100 percent.

Gasification or Pulverized: Current Costs vs. Future Costs

It's the economics of capturing carbon that has those pushing sequestration worried, because the new coal plants being proposed are all some version of pulverized coal. Pulverized coal plants can do a decent job of meeting current environmental standards, but outfitting them to capture carbon is prohibitively expensive.

So a big part of the conundrum is cost-do you measure cost solely now, or also consider future carbon limitations? If carbon restrictions become reality-and in all likelihood they will sometime in the next 20 years, if not much sooner-it seems foolish to build large amounts of new capacity fitted with pulverized coal technology.

The reason lies in the concentration of CO 2 in the flue gas stream. Natural gas produces around 4 percent CO 2 in the flue gas stream. Pulverized coal, in comparison, yields up to a 15 percent concentration of CO 2. But coal gasification outstrips both, with a whopping 35 to 40 percent CO 2 concentration in the flue gas stream. "Gasifiers are a lot closer to being 100 percent CO 2" than pulverized coal, notes Herzog. And, he says, the carbon sequestration process isn't linear, meaning that it is exponentially easier and cheaper to concentrate and sequester CO 2 from a 40 percent concentrated stream than a 10 percent concentrated stream.

Apart from economics, where it clearly comes out ahead, gasification must still prove its mettle in terms of plant availability. On that score, Tampa Electric says that overall, the Polk gasification plant has availability in the mid-90 percent range. The gasifier portion of the plant is available in the 80 percent range, which Black says is consistent with its design. The combined-cycle portion of the plant can run on distillate oil, as well as its usual feedstock, so that