Advanced Coal Technologies

In the nationwide effort to reduce emissions of carbon dioxide (CO2) and other pollutants when fossil fuels are burned, coal is a prime target: It accounts for more than one-quarter of America’s CO2 emissions. And although coal’s share of all fuels used for electrical generation is expected to decline during the next 25 years, the U.S. Energy Information Administration projects that it will remain the largest source through 2027.

As a result, there is intense interest in new technologies for increasing the efficiency of the electricity generating process and in separating, capturing, and safely storing CO2 (a process called sequestration) before it is discharged from the smokestack. Several approaches are possible, although none has been extensively explored.

To minimize emissions, once CO2 has been captured it must either be sequestered (trapped for storage) or used to improve the efficiency of some process.

The latest generation of pulverized coal plants, incorporating “ultrasupercritical” boilers capable of operating at extremely high temperatures and pressures, can achieve efficiencies—before CO2 is captured and stored—of up to 45%, meaning that 45% of the energy stored in the coal is converted to electrical energy. (By contrast, new natural gas plants achieve about 43% efficiency.) However, when a portion of the energy from burned coal is diverted from making electricity to carbon capture and storage using current technology, efficiency is projected to drop to about 30%, slightly below average for today’s coal-fired plants.

Another advanced coal technology is coal gasification, a process in which coal is converted to a gas (called syngas) before it is burned, making it easier to separate CO2 as a relatively pure gas before power is generated. Alternatively, it is possible to burn coal in oxygen instead of air (as is currently done) in order to reduce the amount of flue gas—essentially exhaust—that must be processed to isolate CO2. These techniques show promise but require more research and development. They also substantially increase the cost of the electricity produced.

To minimize emissions, once CO2 has been captured it must either be sequestered (trapped for storage) or used to improve the efficiency of some process. Current sequestration options include using natural geological formations, unmineable coal seams, and deep saline aquifers, all of which are geologically sealed and, presumably, unlikely to allow injected CO2 to escape. While these technologies are very promising, it still must be proven that large quantities of CO2 can be stored effectively underground and monitored for long periods of time.

The methods also must be acceptable to the public and regulatory agencies. Large-scale field trials of prototypes of coal-fueled, near-zero-emissions power plants are needed to test the viability of several of these new “clean coal” technologies.

Alternatively, CO2 can also be put to use for one type of “enhanced oil recovery.” By injecting the gas under pressure into existing wells, an increased amount of oil can be extracted—30% to 60% of deposits versus 20% to 40% without enhanced recovery.