Clean Coal Gasification

One solution that is gaining more attention is coal gasification. Clean coal gasification is the concept of changing the coal to a gas before burning and electricity generation. There are three types of gasification processes currently: the moving bed, fluidized bed and the entrained flow. The most popular gasification method is used in Integrated Gasification Combine Cycle (IGCC) coal plants (entrained flow method).

The process begins with the feedstock, coal going into a gasifier. The gasifier converts the coal into a gas through heat and high pressure mixing with air or pure oxygen. This process is tightly controlled so as not to burn off too much of the fuel as a synthetic gas,“syngas”, is produced. The syngas is a mixture composed primarily of hydrogen (H2),carbon monoxide (CO) and other gaseous constituents.

Clean coal gasification process also facilitates a cleaner separation of the solid materials from the coal instead of the traditional ash that results. The syngas is then treated for the removal of hydrogen sulphide (separates to hydrogen and sulphur), ammonia and particulate matter. The cleaned syngas is then burned as a fuel in a combustion turbine, similar to that of a natural gas turbine. The resulting hot air is used to generate electricity through the turbine and the exhaust heat is recovered into a heat exchanger that creates steam and generates electricity through a second turbine. This duel system is commonly known as a combined cycle, which allows for a more efficient process of generation.

This combined cycle can currently operate with 45% efficiency (heat rates of 8400 to 8600 BTU/KWh range) and future growth to 60% efficiency or higher, well above current coal plants but below natural gas plants. This higher efficiency not only saves money for the plant but it also reduces the waste of coal resources and allows for extension of resource reserves.The efficiency also lowers the emissions of carbon dioxide (CO2) by as much as 40% compared to conventional coal plants. The nature of the clean coal gasification process also reduce sulphur oxides (SOx) and even eliminates nitrogen oxides (NOx), therefore making great strides in almost near zero emissions for actors in acid rain and smog. The use of pure oxygen in the gasifier also allows for easier separation of carbon dioxide (CO2). Although the emissions of carbon dioxide are reduced, they are still a major problem as a greenhouse gas.