Coal Gasification Process



The coal gasification process begins with the gasifier. The coal feedstock is fed into the gasifier. Inside the gasifier, the coal is exposed to intense heat, oxygen and steam. However, the levels of oxygen are tightly controlled inside the gasifier. The levels of oxygen can run as high as 99 percent. These high levels of oxygen help to enable the "partial oxidation" process that occurs. Instead of burning, the coal is broken apart and the result is what is called syngas. Syngas is a usually composed of carbon monoxide and hydrogen, though other gases can occur depending on the coal composition. The portions of coal which are not transformed into syngas may become a slag material that resembles glass, sulfur byproducts or ammonia. There is also some ash formed in the gasifier which is removed later in the process.


Some coal gasification plants use Integrated Gasification Combined-Cycle (IGCC) systems. In IGCC systems the sulfur byproducts and ammonia are put to use. These byproducts from the gasifier are used as fuel for a combustion turbine. This turbine is used as part of an electric generator. The waste heat from this process is used to heat the water for a steam driven turbine. This two turbine system is referred to as a combined cycle. Combined cycle systems like this are used commercially and achieve relatively high efficiency. Typically, the efficiency is around 40 percent. The real advantage of the higher efficiency is that it requires less fuel to produce equivalent or better results than more conventional coal plants.


The syngas which the gasifier produces undergoes a significant amount of processing. The pollutants, such as sulfur, are stripped out of the gas, and in some cases the carbon dioxide is also removed on site. The carbon dioxide may also be removed offsite in hydrogen, ammonia, or fuel production facilities. The remaining hydrogen and byproducts are used for a variety of purposes. Hydrogen can be converted for both transportation and non-transportation fuels. It is can be used for the production of commercial chemicals. Byproduct methanol can also be used in the commercial chemical production process. Hydrogen is used as a feedstock for fertilizer production and it can be utilized for fuel cells.

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