Primer: Clean Coal
TOPICS:Alternative Energy | Energy
COMPANIES:McDermott International, Inc. | Wisconsin Energy Corporation | General Electric | ROYAL DUTCH SHELL PLC | Conocophillips | Siemens AG | Duke Energy Corp
By Kenneth Stier, Features Writer | 20 Jun 2008 | 03:53 PM ET Font size:
What is it?
Clean coal refers to an array of technologies that sharply reduce pollutant emissions from coal-burning power plants, which supply roughly one-half of all electricity in the US. In the 1980s and 1990s, efforts focused on reducing sulfur, nitrogen oxides and soot — which cause acid rain, damaging forest and watersheds.
Nati Harnik / AP
The latest and perhaps biggest concern is greenhouse gases, especially carbon dioxide, and that has presented the industry with its greatest engineering challenge yet.
Traditional coal combustion emits far more carbon than other fossil fuels. Thus, maintaining coal as an option for power generation (electricity), will require dramatically reducing these emissions. A breakthrough is critical to the long-term energy needs of the US, which is considered the "Saudi Arabia of coal." Coal represents some 90 percent of the nation's recoverable fossil fuels, with reserves sufficient for 200 years, at current rates of use.
What are the leading emerging technologies?
There are two leading approaches to meeting the challenge. The first is an advanced steam cycle technology, known as ultrasupercritical (USC) cycles. The other is integrated gasification combined cycle (IGCC) technology.
USC promises significant efficiency gains, which could reduce carbon emissions by about a third. The US, long a leader in advanced coal combustion technology, has 170 supercritical units in operation.
IGCC, which is still a few years from commercial deployment, promises a potential quantum leap, approaching zero-emissions. Full-deployment, however, depends on overcoming another technological challenge — not just the ability to capture carbon but also to safely dispose of it indefinitely underground, in a process known as sequestration. This is not expected to be commercially deployable until 2020.
The choice of technology is hardly academic. In planning for new base-load (constant) power plants, utility companies must choose plants with carbon capture capabilities or face steep future costs under anticipated new laws establishing a cost to carbon.
How do these technologies work?
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Ultrasupercritical plants operate at significantly higher steam temperature and pressure to achieve a significant improvement in efficiency. The current aging fleet of conventional plants typically runs at roughly 35-percent efficiency; ultrasupercritical plants can operate at 47-percent efficiency (at 1100 degrees Fahrenheit), reducing the fuel needed and carbon emissions by roughly one-third. (Every one percent improvement in efficiency yields roughly two percent reduction in carbon emissions.)
IGCC involves two steps. First, coal is converted by gasification into a synthetic gas ("syngas") from which pollutants and particulates are chemically scrubbed, or removed, before the clean syngas is burned in a combustion turbine to generate electricity. In both steps waste heat, from the gasifier and the combustion turbine, is recovered and used to generate steam for additional electrical generation. IGCC uses less water and produces less waste, and its emissions are almost as low as natural gas combustion.
One critical drawback to current carbon capture methods is that they reduce plant efficiency by 25 to 34 percent. They are also costly — roughly $40 per ton of carbon. Added to the cost of transportation and current estimates of sequestration brings this more environmentally-friendly coal combustion in rough parity with the cost of other forms of renewable energy. For carbon capture and sequestration (CCS) to be economical, the International Energy Agency has calculated there needs to be a long-term minimum carbon cost of $53 per ton.
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