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.
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?
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.
Where are these technologies tested?
There are two key national ultrasupercritical projects underway, both supported by the Department of Energy and the Ohio Coal Development office (OCDO).
The first is a five-year effort to develop a sufficiently robust boiler, an effort joined by Energy Industries of Ohio, the Oak Ridge National Laboratory and the Electric Power Research Institute.
The other is focused on developing steam turbine materials, which have the support of France's Alstom, Germany's Siemens and General Electric.
Are there any deployments in the field?
There are just four commercial-sized coal-fired IGCC plants in operation. Two are in Europe and two in the US, one each in Florida and Indiana.
There have been significant capital and engineering investments made in IGGC technology in recent years by a small number of industry leaders, including ConocoPhillips , Shell and GE.
The federal government made a significant commitment to advancing this technology through its $2 billion FutureGen project, but support was withdrawn in January 2008 because of cost overruns and concerns the technology might quickly become obsolete. The next administration is expected to revive support.
Despite the compelling need, there are precious few IGCC projects still being pursued. Known plans include one by Duke Energy and a joint project undertaken by Hydrogen Energy and Edison Mission Energy, a subsidiary of Edison International .
Are there other potential solutions?
Anticipation of some form of carbon controls (most likely a market-based cap & trade system) has stimulated investment in other ways to capture carbon. Most are a variation on how other pollutants have been controlled.
For instance, liquids or solids (or static electricity) are injected into the plant’s flue gas exhaust to capture particles. Carbon is currently captured by exposing flue gas to an ammonium carbonate solution, which is then heated under pressure, reversing the absorption process so pure carbon is recovered. Georgia Tech University researchers recently reported using a solid adsorbent called "hyperbranched aminosilica" to capture seven-times more carbon. The substance can be recycled and reused.
Another capture method uses chilled ammonia, with which Alstom has demonstrated (in a lab) a capture rate of more than 90 percent and at a far less cost. The company is running a pilot project at Wisconsin Energy’s Pleasant Prairie Power Plant.
Technologically-based upstart companies — as well as infrastructure firms — offer investors limited entry into this sector, which is for the most part dominated by large firms.