Pulverized coal combustion: Subcritical, Supercritical, and Ultra-supercritical steam cycles
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 that spins a turbine and drives an electrical generator.
The difference between subcritical, supercritical, and ultra-supercritical versions of pulverized coal combustion technology has to do with the steam pressure and temperature within the boiler.
In a subcritical plant, steam pressure is below 3200 pounds per square inch (psi) (22 MPa) and temperature is below 1025° F (550° C). 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.[1]
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 psi (24.3 MPa) and temperatures are 1050° Fahrenheit (565° C). At this higher pressure and temperature, water can be maintained as a fluid despite being above the atmospheric boiling point, allowing for greater efficiency. Efficiency ratings for supercritical coal plants range from 37% to 40%.[1]
In ultra-supercritical units, pressures are at 4640 psi (32 MPa) and temperatures at 1112-1130° F (600-610° C). Current research and development is targeting pressures of 5300-5600 psi (36.5–38.5 MPa) and temperatures of 1290-1330° F (700-720° C), with the possibility of raising generating efficiency to the 44-46% range.[1] According to the Global Coal Plant Tracker, as of July 2025, more than 600 ultra-supercritical units were operating at coal plants worldwide, with the majority in China, followed by Japan, South Korea, and Germany.
Fluidized-bed combustion
In a coal plant using fluidized bed combustion, crushed coal is combined with an inert material (e.g., sand) and limestone in the boiler. Air is blown into the boiler and keeps the mixture of coal particles, bed material, and limestone in suspension in a fluid-like state. This fluidized bed is where combustion takes place.[2]
In a circulating fluidized bed (CFB), the particles of coal ash, partially-burned coal, and limestone that reach the top of the boiler are separated from flue gases and recirculated into the boiler, increasing combustion efficiency. One advantage of CFB technology is that it can be used for a wide variety of coals as well as non-coal fuels such as biomass. CFB combustion also uses lower temperatures (800° F or 427° C) that favor low NOx formation, while the limestone captures SO2. Most CFB units use subcritical steam cycles, and efficiencies are comparable to subcritical and supercritical pulverized coal combustion plants.[1][3]
Efficiencies
See also Coal combustion efficiency
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:[1]
- Subcritical: 34.3%
- Supercritical: 38.5%
- Ultra-supercritical: 43.3%
- Subcritical fluidized bed: 34.8%
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 lowering emissions as well as in separating carbon dioxide gas to make it easier for carbon capture; however, IGCC technology has been deployed at only a handful of plants worldwide.[4]
Articles and Resources
Sources
- ↑ 1.0 1.1 1.2 1.3 1.4 "The Future of Coal," Massachusetts Institute of Technology, 2007, pages 20-22.
- ↑ Liukkonen M., Heikkinen M., Hälikkä E., Hiltunen T., and Hiltunen Y., “Analysis of Flue Gas Emission Data from Fluidized Bed Combustion Using Self-Organizing Maps,” Applied Computational Intelligence and Soft Computing, September 16, 2010
- ↑ “Developments in circulating fluidised bed combustion,” IEA Clean Coal Centre, April 2013
- ↑ Integrated Gasification Combined Cycle (IGCC)
Related GEM.wiki articles
Other resources
- Michael Lazarus and Chelsea Chandler, "Coal Power in the CDM: Issues and Options," Stockholm Environment Institute, 2011
- Xiaomei Tan et al, "Supercritical and ultrasupercritical coal-fired power generation," Business and Public Administration Studies, 2012
- Fang Rong and David G. Victor, "What does it cost to build a power plant?" ILAR Working Paper, September 2012
- Jens Horbach, Qian Chen, Klaus Rennings, and Stefan Vögele, "Lead markets for clean coal technologies: a case study of China, Germany, Japan, and the US," Center for European Economic Research, undated
- János Beér, "High Efficiency Electric Power Generation; The Environmental Role," Massachusetts Institute of Technology, undated
- Feng Weizhong, "Challenging Efficiency Limitations for Coal-Fired Power Plants," Cornerstone, 2015
- Setting the Benchmark: The World's Most Efficient Coal-Fired Power Plants," Cornerstone, 2015
- https://www.iea.org/publications/freepublications/publication/TechnologyRoadmapHighEfficiencyLowEmissionsCoalFiredPowerGeneration_WEB_Updated_March2013.pdf "Technology Roadmap: High-Efficiency, Low-Emissions Coal-Fired Power Generation,"] International Energy Agency, 2012
- "The Future of Coal: Appendices," MIT, 2007
- "Challenges for Diffusion of Japan's Clean Coal Technologies," Koichi Mogi, IEEJ, May 2012
