INTEGRATED GASIFICATION COMBINED CYCLE
A second alternative to the pulverized coal-fired power plant is the integrated gasification combined cycle (IGCC) plant that is based around the gasification of coal (Figure 3.9). Coal gasification is an old technology. It was widely used to produce town gas for industrial and domestic use in the United States and Europe until natural gas became readily available.
Modern gasifiers convert coal into a mixture of hydrogen (H) and carbon monoxide (CO), both of which are combustible. Gasification normally takes place by heating the coal with a mixture of steam and oxygen (or, in some cases, air). This can be carried out in a variety of gasifier designs including fixed bed, fluidized bed,
and entrained flow gasifiers. The primary reaction for combustible gas production is that of the carbon in coal with water to produce hydrogen and carbon monoxide:
However, there are other reactions that produce carbon dioxide and methane or other hydrocarbons. Both hydrogen and carbon monoxide are combustible.
The process that takes place in the gasifier is a partial combustion of the coal. Consequently, it generates a considerable amount of heat. This heat can be used to generate steam to drive a steam turbine. The gas produced, meanwhile, is cleaned and can be burned in a gas turbine to produce further electricity. Heat from the exhaust of the gas turbine is used to raise additional steam for power generation. This is the basis of the IGCC plant although a variety of configurations exist.
There are a limited number of IGCC plants in operation with capacities of 250–300 MW. They have demonstrated efficiencies of 40%. However, capital costs are higher than for pulverized coal-fired power plants. Costs might improve if hot-gas cleanup technologies can be developed, but current IGCC technology does not offer a viable alternative to a supercritical PC plant under most conditions. However, IGCC might offer an effective means of carbon capture in a coal-fired power plant (see the following section) and this may prove its greatest opportunity for the future.