Coal combustion efficiency

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Subcritical, Supercritical, Ultra-supercritical, and Circulating Fluidized Bed (CFB)

Virtually all coal plants in current operation employ pulverized coal combustion technology, which involves grinding coal into talcum-powder fineness and then burning it to heat water into high-pressure steam to drive an electrical generator. The difference between subcritical, supercritical, and ultra-supercritical versions of pulverized coal combustion technology has to do with the steam pressure within the boiler. In a subcritical plant, steam pressure is below 3200 pounds per square inch and temperature is below 1025 degrees Fahrenheit (550 degrees Celsius). Subcritical units have efficiencies of between 33% and 37%; i.e. between 33% and 37% of the energy in the coal is converted into electricity. In the late 1960s, supercritical combustion technology was commercialized, after advances in materials technology made it possible to build boilers that could operate at higher pressures. In supercritical units, the pressure of the boiler is about 3530 pounds per square inch and temperatures are 1050 degrees Fahrenheit (565 degrees Celsius). At this higher pressure and temperature, water can be maintained as a fluid despite being above the atmospheric boiling plant, allowing greater efficiency. Efficiency ratings for supercritical coal plants range from 37% to 40%. In ultra-supercritical units, pressures are at 4640 pounds per square inch and temperatures of 1112-1130 degrees Fahrenheit (600-610 degrees Celsius), and current research and development is targeting pressures of 5300-5600 pounds per square inch and temperatures of 1290-1330 degrees Fahrenheit (700-720 degrees Celsius), with the possibility of raising generating efficiency to the 44-46% rangeal.

In fluidized bed plants, coal is burned with air in a circulating bed, typically made of crushed coal combined with limestone The advantage of this technology is that it is suited to a wide variety of coals and no-coal fuels such as biomass. An advantage of CFB technology is that it favors low NO_x formation and capture of SO₂. Efficiencies are comparable to subcritical and supercritical plants.

Studies

Future of Coal 2007

MIT's "Future of Coal" study estimated the following representative efficiencies for plants burning Illinois #6 coal, a bituminous grade of coal with 25,350 kJ/kg heat rate:

• Subcritical: 34.3%
• Supercritical: 38.5%
• Ultra-supercritical: 43.3%
• Subcritical fluidized bed: 34.8%

Hardisty et al. 2012

Noting there is a range of efficiencies for coal plants, a study in Energies by Hardisty et al. assumed the following efficiencies for coal plants burning Australia black coal, to compare the emissions to gas plants:

• Subcritical: 33%
• Supercritical: 41%
• Ultra-supercritical: 43%

Plants

Turk, Arkansas

According to Power, the steam cycle efficiency of the USC John W. Turk Jr. Power Plant in Arkansas is 39 to 40 percent, compared with 35 percent for a boiler operating at subcritical steam conditions.

Integrated gasification combined cycle (IGCC)

For more details, see Integrated Gasification Combined Cycle (IGCC)

Integrated gasification combined cycle (IGCC) plants use a two-step process to create electricity. In the first step, coal is converted into synthetic gas or syngas. In the second step, the gas is used to power a steam turbine, with waste heat from the turbine being recovered to provide additional power (hence the term "combined cycle"). Such plants are considered "integrated" because the two steps occur at the same facility in tandem. IGCC plants have theoretical advantages in controlling emissions as well as in separating carbon dioxide gas; however, the technology has been deployed at only a handful of plants worldwide.

Articles and Resources

Related SourceWatch Articles

• Existing U.S. Coal Plants

Other resources

• Michael Lazarus and Chelsea Chandler, "Coal Power in the CDM: Issues and Options," Stockholm Environment Institute, 2011
• "Power Generation from Coal: Measuring and Reporting Efficiency Performance and CO₂ Emissions," International Energy Agency, 2010