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Dealing with carbon from power plants
NARUC report overviews the current status of carbon capture, usage and sequestration technologies development in the United States
Alan Bailey Petroleum News
The National Association of Regulatory Utility Commissioners has published a report on the current status of the development and use of carbon capture, usage and sequestration technology. CCUS, as it is termed, reduces the emission of carbon dioxide into the atmosphere from power plants that burn fossil fuels by capturing the gas and either sequestering it underground or finding some commercial use for it. The NARUC report particularly focuses on applications for coal-fired power plants, although the approach can also be applied to gas fired systems.
While providing a means of dealing with carbon dioxide, a greenhouse gas blamed for rising global temperatures, CCUS faces both technical and economic challenges. From a technical perspective, the carbon dioxide must be separated from other exhaust products, in particular nitrogen. From an economic perspective, the use of CCUS adds significant cost to electricity production, unless some market value can be found for the captured carbon dioxide.
On the other hand, the cost of adding CCUS capability to an existing coal-fired plant, for example, may be comparable to replacing that plant with some equivalent capacity of wind generation, the NARUC report says. And traditional coal and gas fired power plants can provide stable and predictable amounts of power, perhaps counterbalancing the variable power outputs from wind and solar systems.
Not originally regulated The NARUC report also points out that, although traditional pollutants from coal-fired power plants such as particulates, nitrogen oxides and sulfur dioxide have been regulated under the terms of the Clean Air Act since 1970, carbon dioxide was not regulated under the act. Until, that is, a Supreme Court decision in 2007 required the Environmental Protection Agency to regulate greenhouses gases, including carbon dioxide, as air pollutants.
Those pollutants that have been regulated for some time have been cut significantly. However, although significant progress has been made in research into CCUS techniques for dealing with carbon emissions from traditional coal-fired power plants, commercial applications of the techniques have been slow in coming. In the United States the emergence of abundant and relatively cheap natural gas from shale gas development has brought a halt to the development of new coal-fired power plants. On the other hand, developing countries tend to depend heavily on coal-fired generation, the NARUC report says.
A variety of capture techniques In terms of technology, there are a variety of ways in which to implement CCUS at a coal-fired plant. Carbon dioxide may be extracted from exhaust gas using solvents, absorbents, membranes or some combination of these techniques. Another approach involves separating nitrogen from the air inflow to the power plant combustion system, so that the exhaust gas consists essentially of water and carbon dioxide, with the water then condensing out. Another technique, called chemical looping, involves generating heat by oxidizing a chemical, reducing the oxidized chemical back again using coal fuel, and then continuously cycling the process - again, the exhaust consists essentially of carbon dioxide and water. In another type of process, coal gasification, coal is converted to synthetic gas, which is then burned in a gas-fired power plant: Carbon dioxide can be removed from the synthetic gas, before the gas is burned.
Dealing with the gas Whatever technique is employed, the end result is a stream of carbon dioxide that has to be either stored or used. But given the cost and energy consumption of operating the carbon dioxide extraction process, a major challenge is to deal with the carbon dioxide without making the generated electricity excessively expensive. For example, although there are plenty of subsurface locations where carbon dioxide could be sequestered, the cost of sequestration deters people from taking this approach. Much research has, however, been conducted into carbon dioxide sequestration, and there has been success in sequestration in a variety of different types of underground site around the world.
And in the U.S., there are tax credits available for implementing CCUS systems.
It would be preferable to find some productive use for the carbon dioxide, thus giving the gas some economic value. It can be possible to use carbon dioxide in the manufacture of plastics, fuels, fertilizer or chemicals. But manufactured fuels, for example, when used, can end up cycling the carbon dioxide into the atmosphere. And the volume of carbon dioxide obtained from power plants may be much greater than is required for manufacturing other products. Overall, finding uses for the gas remains a minor part of any climate change mitigation strategy, the NARUC report says.
Enhanced oil production However, one practical and viable use of captured carbon dioxide that has seen significant success is in enhanced oil production from oil fields, the report says. The economic value of carbon dioxide used in this way can exceed the carbon dioxide capture cost, although that value is closely tied to the volatile price of oil.
The NARUC report cites the Petra Nova carbon capture and storage project near Houston, Texas, as a successful example of cycling waste carbon dioxide into an oil field for enhanced oil production. This project captures 90 percent of the carbon dioxide from a coal-fired power plant, delivering and storing about 1.4 million metric tons of the gas per year for enhanced oil recovery in the West Ranch oil field. Use of the carbon dioxide is expected to increase oil production from the field from around 300 barrels per day to perhaps 15,000 barrels per day.
However, any large-scale, regional implementation of carbon dioxide capture and storage would require a pipeline infrastructure for transporting the gas under pressure, the report cautions.
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