Extracting And Storing Carbon Dioxide

To slow climate change, the authors urge power providers to build integrated gasification combined cycle (IGCC) coal power plants with carbon dioxide capture and storage (CCS) capabilities (below) rather than conventional steam-electric facilities. Conventional coal plants burn the fuel to transform water into steam to turn a turbine-generator. If CCS technology were applied to a steam plant, CO2 would be extracted from the flue exhaust. An IGCC plant, in contrast, employs a partial oxidation reaction using limited oxygen to convert the coal into a so-called synthesis gas, or syngas (mostly hydrogen and carbon monoxide). It is much easier and less costly to remove CO2 from syngas than from the flue gases of a steam plant. The hydrogen-rich syngas remaining after CO2 extraction is then burned to run both gas and steam turbine-generators. The world's first commercial IGCC project that will sequester CO2 underground is being planned near Long Beach, Calif.

CO2 EXTRACTION

GASIFICATION

2 The syngas is reacted with steam to produce a gaseous mixture of mostly carbon dioxide and hydrogen (H2) from which CO2 is extracted for burial (yellow pathways at bottom)

3 Hydrogen-rich syngas is burned, and the combustion products drive a gas turbine-generator

4 The hot gas turbine exhaust passes to a heat-recovery steam generator, which converts water to steam that turns a steam turbine-generator

CO2 EXTRACTION

GASIFICATION

2 The syngas is reacted with steam to produce a gaseous mixture of mostly carbon dioxide and hydrogen (H2) from which CO2 is extracted for burial (yellow pathways at bottom)

3 Hydrogen-rich syngas is burned, and the combustion products drive a gas turbine-generator

4 The hot gas turbine exhaust passes to a heat-recovery steam generator, which converts water to steam that turns a steam turbine-generator can meet the objectives of climate change mitigation at the least cost. Fundamentally different approaches to CCS would be pursued for power plants using the conventional pulverized-coal steam cycle and the newer integrated gasification combined cycle (IGCC). Although today's coal IGCC power (with CO2 venting) is slightly more expensive than coal steam-electric power, it looks like IGCC is the most effective and least expensive option for CCS.

Standard plants burn coal in a boiler at atmospheric pressure. The heat generated in coal combustion transforms water into steam, which turns a steam turbine, whose mechanical energy is converted to electricity by a generator. In modern plants the gases produced by combustion (flue gases) then pass through devices that remove particu-lates and oxides of sulfur and nitrogen before being exhausted via smokestacks into the air.

Carbon dioxide could be extracted from the flue gases of such steam-electric plants after the removal of conventional pollutants. Because the flue gases contain substantial amounts of nitrogen (the result of burning coal in air, which is about 80 percent nitrogen), the carbon dioxide would be recovered at low concentration and pressure—which implies that the CO2 would have to be removed from large volumes of gas using processes that are both energy-intensive and expensive. The captured CO2 would then be compressed and piped to an appropriate storage site.

In an IGCC system coal is not burned but rather partially oxidized (reacted with limited quantities of oxygen from

▲ Commercial power plants using IGCC technology, such as this one in Italy, have been operating since 1994. Together they generate 3,600 megawatts of electricity.

an air separation plant, and with steam) at high pressure in a gasifier. The product of gasification is so-called synthesis gas, or syngas, which is composed mostly of carbon monoxide and hydrogen, undiluted with nitrogen. In current practice, IGCC operations remove most conventional pollutants from the syngas and then burn it to turn both gas and steam turbine-generators in what is called a combined cycle.

In an IGCC plant designed to capture CO2, the syngas exiting the gasifier, after being cooled and cleaned of particles, would be reacted with steam to produce a gaseous mixture made up mainly of carbon dioxide and hydrogen. The CO2 would then be extracted, dried, compressed and transported to a storage site. The remaining hydrogen-rich gas would be burned in a combined cycle plant to generate power [see box on preceding page].

Analyses indicate that carbon dioxide capture at IGCC plants consuming high-quality bituminous coals would entail significantly smaller energy and cost penalties and lower total generation costs than what could be achieved in conventional coal plants that captured and stored CO2. Gasification systems recover CO2 from a gaseous stream at high concentration and pressure, a feature that makes the process much easier than it would be in conventional steam facilities. (The extent of the benefits is less clear for lower-grade subbi-tuminous coals and lignites, which have received much less study.) Precombus-tion removal of conventional pollutants, including mercury, makes it feasible to realize very low levels of emissions at much reduced costs and with much smaller energy penalties than with cleanup systems for flue gases in conventional plants.

Captured carbon dioxide can be transported by pipeline up to several hundred kilometers to suitable geologic storage sites and subsequent subterranean storage with the pressure produced during capture. Longer distances may, however, require recompression to compensate for friction losses during pipeline transfer.

Overall, pursuing CCS for coal power facilities requires the consumption of more coal to generate a kilowatt-hour of electricity than when CO2 is vented— about 30 percent extra in the case of coal steam-electric plants and less than 20 percent more for IGCC plants. But overall coal use would not necessarily increase, because the higher price of coal-based electricity resulting from adding CCS equipment would dampen demand for coal-based electricity, making renewable energy sources and energy-efficient products more desirable to consumers.

The cost of CCS will depend on the type of power plant, the distance to the storage site, the properties of the storage

^ DAVID G. HAWKINS, DANIEL A. LASHOF and ROBERT H. WILLIAMS have endeavored to help cd o stave off climate change problems for decades. Hawkins is director of the Climate Center FE at the Natural Resources Defense Council (NRDC), where he has worked on air, energy ^ and climate issues for 35 years. Hawkins serves on the boards of many bodies that advise government on environmental and energy subjects. Lashof is science director and dep-I uty director of the NRDC's Climate Center, at which he has focused on national energy IH policy, climate science and solutions to global warming since 1989. Before arriving at the NRDC, Lashof developed policy options for stabilizing global climate at the U.S. Environmental Protection Agency. Williams is a senior research scientist at Princeton University, which he joined in 1975. At the university's Princeton Environmental Institute, he heads the Energy Systems/Policy Analysis Group and the Carbon Capture Group under the institute's Carbon Mitigation Initiative (which is supported by BP and Ford).

reservoir and the availability of opportunities (such as enhanced oil recovery) for selling the captured CO2. A recent study co-authored by one of us (Williams) estimated the incremental electric generation costs of two alternative CCS options for coal IGCC plants under typical production, transport and storage conditions. For CO2 sequestration in a saline formation 100 kilometers from a power plant, the study calculated that the incremental cost of CCS would be 1.9 cents per kilowatt-hour (beyond the generation cost of 4.7 cents per kilowatt-hour for a coal IGCC plant that vents CO2—a 40 percent premium). For CCS pursued in conjunction with enhanced oil recovery at a distance of 100 kilometers from the conversion plant, the analysis finds no increase in net generation cost would occur as long as the oil price is at least $35 per barrel, which is much lower than current prices.

CCS Now or Later?

many electricity producers in the industrial world recognize that environmental concerns will at some point force them to implement CCS if they are to continue to employ coal. But rather than building plants that actually capture and store carbon dioxide, most plan to construct conventional steam facilities they claim will be "CO2 capture ready"—convertible when CCS is mandated.

Power providers often defend those decisions by noting that the U.S. and most other countries with coal-intensive energy economies have not yet institut ed policies for climate change mitigation that would make CCS cost-effective for uses not associated with enhanced oil recovery. Absent revenues from sales to oil field operators, applying CCS to new coal plants using current technology would be the least-cost path only if the cost of emitting CO2 were at least $25 to $30 per metric ton. Many current policy proposals for climate change mitigation in the U.S. envision significantly lower cost penalties to power providers for releasing CO2 (or similarly, payments for CO2 emissions-reduction credits).

Yet delaying CCS at coal power plants until economy-wide carbon dioxide control costs are greater than CCS costs is shortsighted. For several reasons, the coal and power industries and

COAL'S TOLL

Despite the current popularity of the term "clean coal," coal is, in fact, dirty. Although carbon capture and storage could prevent much carbon dioxide from entering the atmosphere, coal production and consumption is still one of the most destructive industrial processes. As long as the world consumes coal, more must be done to mitigate the harm it causes.

MINING DANGERS

Coal mining is among the most dangerous occupations. Official reports for 2005 indicate that roughly 6,000 people died (16 a day) in China from coal mine floods, cave-ins, fires and explosions. Unofficial estimates are closer to 10,000. Some 600,000 Chinese coal miners suffer from black lung disease.

The U.S. has better safety practices than China and achieved an all-time low of 22 domestic fatalities in 2005. U.S. mines are far from perfect, however, as evidenced by a series of fatalities in early 2006.

▲ Acid runoff from a Pennsylvania coal mine stains this creek bed orange.

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