Coal-Fired Power Plants

Integrated gasification combined cycle (IGCC) electric power plant. From left to right, railroad cars or barges transport coal from the mines to the plant, and a conveyor belt carries it into the plant and fills a hopper. Water mixes with the coal before

Published: November 9, 2010, 12:00 am

Updated: July 9, 2012, 9:34 pm

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Coal (main) Energy (main) Business & The Environment (main) Chemical Engineering (main)

Electric power plants commonly use a pulverized coal steam-generating system, in which metal spheres or cylinders crush pieces of coal into a fine powder. Hot air blows the coal powder into a furnace, where it combusts at high temperatures and converts water to steam. At present coal power plants are critical in supplying a base load to the electric grid, since wind and solar plants are not reliable in all times of day and weather conditions. The steam passes through a series of high-, mid-, and low-pressure turbines connected to a common shaft that spins a generator. This type of power plant requires high-grade (low ash content) coal.

Another type of coal-fueled power plant utilizes “fluidized bed combustion,” a method that involves burning coal in a layer (bed) of heated particles suspended in flowing air. [1] At sufficiently high air velocity, the bed acts as a fluid that thoroughly mixes coal particles with air, completely combusting the coal at relatively low temperatures. The advantage of fluidized bed combustion is that it can use almost any grade of coal, including coal rejected by power plants that use pulverized coal systems.

One more type of power generation from coal is an integrated gasification combined cycle (IGCC) system. This method does not combust coal directly, but first converts it to a gas called “syngas” composed of carbon monoxide (CO) and hydrogen (H₂). This process involves some of the same reactions as hydrogen production from fossil fuels and Fischer-Tropsch synthesis.

Essentially, a gasification reactor introduces a limited amount of oxygen to burn some of the organic carbon material in the coal and release carbon monoxide (CO) and energy. This drives a second reaction that converts additional organic carbon material in the coal to CO and H₂ and a third reaction, in what is known as a water-gas shift converter, changes CO and steam to CO₂ and H₂. The H₂ combusts directly in a gas turbine that spins an electrical generator. Heat from this combustion also generates steam that drives a steam turbine and another electrical generator.

IGCC power plants have several advantages over other types of coal-fired power plants. They release less sulfur dioxide (SO₂) and nitrogen oxides (NO_x) pollution because filters remove these compounds from the syngas before it combusts. They achieve high fuel efficiencies, from 38% to 56% [1], [2] They can capture CO₂ more easily and more cheaply than other types of coal-fired plants because they generate it at high concentrations in the water-gas shift converter before combustion rather than emit it, diluted, in a large volume of flue gas after combustion.

The main problem with IGCC plants is their high capital costs. Construction of an IGCC plant costs from 15% to 20% more than construction of a conventional pulverized coal-fired plant. [2], [3] Currently, IGCC plants generate only 0.1% of the world’s electricity, [4] and only two IGCC plants are operating in the United States.

Water at temperatures above 374°C and at pressures above 22 Mega Pascales (MPa) transforms into a subcritical fluid that has properties of both a liquid and a gas. Supercritical water has a density that changes with temperature and pressure in a continuous manner, and so power plants designed to operate under supercritical conditions can scale to higher temperatures and pressures without having to accommodate two phases of water (liquid and gas) in various locations.

The downside of supercritical operation is that higher temperatures and pressures demand stronger, more corrosive-resistant materials, tighter tolerances, and more complex control systems, which add about 7% to the cost of construction and maintenance. [2] Nevertheless, supercritical plants are more efficient, and their lower fuel costs eventually compensate for higher costs. Currently, more than 400 supercritical power plants are in service worldwide, [1] and a few ultra-supercritical plants, which operate at even higher temperatures and pressures, are beginning operations in Europe and Japan. [2]

[1] World Coal Institute (2004) Clean Coal - Building a Future through Technology, Richmond, UK, http://www.worldcoal.org/assets_cm/files/PDF/clean_coal_building_a_future_thro_tech. pdf.

[2] MIT Study Group (2007) The Future of Coal, Massachusetts Institute of Technology, Cambridge, MA, http://web.mit.edu/coal/The_Future_of_Coal.pdf.

[3] Moore, T. (2005) Coal-based generation at the crossroads. EPRI Journal, Summer, 2005:6-15.

[4] IPCC (2005) IPCC Special Report on Carbon Dioxide Capture and Storage prepared by Working Group III of the Intergovernmental Panel on Climate Change, Metz, B., O. H. Davidson, C. de Coninck, M. Loos, and L. A. Meyer, eds. Cambridge University Press, New York.

This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.

©2010 Sinauer Associates and UC Regents

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Citation

Bloom, A. (2012). Coal-Fired Power Plants. Retrieved from http://editors.eol.org/eoearth/wiki/Coal-Fired_Power_Plants