James M. Childress
An alternative to coal combustion is coal gasification. Rather than burning coal directly, a coal gasifier reacts coal with steam and controlled amounts of air or oxygen under high temperatures and pressures. The heat and pressure break apart the chemical bonds in the coal’s molecular structure, setting into motion chemical reactions with the steam and oxygen to form a gaseous mixture, called a synthesis gas (or syngas) made up primarily of carbon monoxide and hydrogen, which is then combusted in a gas turbine to generate electricity.
In gasification-based systems, pollutant-forming impurities can be separated from the gaseous stream before combustion. As much as 99% of sulfur and other pollutants can be removed and processed into commercial products such as chemicals and fertilizers. Solid byproducts formed from the mineral matter in the coal can be collected and used for various purposes, such as road building.
If the syngas is used to produce electricity, it is typically used as a fuel in an Integrated Coal Gasification Combined Cycle (IGCC) system. The IGCC system has two basic components. A high efficiency gas turbine, widely used in power generation today, burns the clean syngas to produce electricity. Exhaust heat from the gas turbine is recovered to produce steam to power traditional high efficiency steam turbines. IGCC power generating systems are presently being developed and operated in Europe and the United States. These systems increase the efficiency of the electricity generation by using the heat from the product gas to produce steam to drive a steam turbine. Existing commercial systems can achieve a thermal efficiency greater than 40%. With further advances in gas turbine technologies these systems are capable of reaching above 50% efficiency. IGCC systems can be designed to produce little solid waste and low emissions of SOx and NOx. Over 99% of the sulfur present in the coal can be recovered for sale as chemically pure sulfur.
Another attractive feature of the IGCC system is that carbon monoxide in the synthesis gas can be reacted with steam to make hydrogen and carbon dioxide. This creates the potential to separate a relatively pure stream of carbon dioxide (a greenhouse gas) for “sequestration,” thus avoiding its emission into the atmosphere. The resulting hydrogen product can be burned in a gas turbine for electricity generation, or used as a fuel in other applications, such as hydrogen-powered vehicles. “Fuel cells” which can convert hydrogen to electricity, are being developed for both applications.
Instead of burning it to generate electricity, the syngas can be processed using commercially available technologies to produce a wide range of fuels (such as diesel oil), chemicals, and fertilizer or industrial gases. Some facilities have the capability to produce both electricity and other products from the syngas, depending on the plant’s configuration as well as site specific technical and market conditions. This process is referred to as “co-production” or “poly-generation.”
IGCC is a relatively new technology and needs further research and development to reach its full technical and economic potential. Over 1500 megawatts (MW) of coal-fired IGCC are operating today. In addition, 1900 MW equivalent of IGCC are operating worldwide using refinery wastes as fuel. An additional 2200 MW are in design. Increasing numbers of coal-fired IGCC systems are expected as the cost of natural gas and the successes of the current demonstrations begin to affect large capital project spending decisions.