The three main components of an IGCC power plant are the gasification with coal processing, including drying and air separation (1), the gas processing and CO2 capture (2) as well as the combined cycle gas turbine power unit for producing energy with the fuel gas (3).
(1) Whereas the coal processing fulfils the same task as with conventional power plants, air separation is necessary in IGCC plants because technical oxygen (O2) is used as a gasification agent for the gasification. The nitrogen (N2) produced as a by-product can be used for diluting the fuel gas in the gas turbine. In the gasifier, the coal (or material being gasified) is converted to a flammable gas using the main components H2 and CO with reduced oxygen levels. Entrained flow gasification processes are mainly used in existing IGCC power plants.
(2) The gasification is followed by a chain of different gas processing stages that principally remove noxious components, such as H2S, using chemical or physical scrubbing agents to prevent, for example, impermissibly high SOx emissions. With IGCC power plants that combine CO2 capture, the gas processing includes additional steps for gas conditioning. Here, all the carbon monoxide contained in the gas is converted to CO2 and H2 by adding water vapour, as part of a CO conversion stage, the so-called CO shift. The CO2 then contained in the gas is present in very high concentrations and can be separated from the gas with comparatively little effort using chemical or physical absorption. This process used in CO2-free coal-fired power plants is known as pre-combustion carbon capture because the CO2 is already separated and captured before the actual combustion of the gas.
(3) After the gas processing, either a cleaned raw gas or a hydrogen-rich gas is therefore available for combustion in the gas turbine. Since hydrogen and synthesis gas (syngas) have considerably different properties to natural gas, it is currently necessary to dilute them with nitrogen to prevent high NOx emissions during combustion. Continuing and new developments of gas turbine combustors should enable the problems associated with hydrogen combustion to be overcome in future. ©RWE
