Capturing CO2 by Chemical Absorption

The flue gas from a coal fired power plant contains about 12% CO2 with the rest being N2 and Oxygen. A 1000 MW Coal fired unit produces around 2500 tons of gas per hour, which requires the separation of almost 300 tons per hour of CO2. The task is enormous.

How to cost effectively separate the CO2?

The simple method is to pass the gas through a chemical solvent that selectively absorbs only the CO2 and keeps it in a weak chemical bond. The O2 and N2 then release to the atmosphere. The CO2 releases from the chemical solvent during regeneration by heat addition that breaks the bond of the CO2. The almost pure CO2 is collected, compressed and sent to storage. The retrieved solvent goes for re-use.

The most common chemical solvents used are amines. Amine capture is a proven system in the natural gas cleaning process. The scale of operations is much larger for the removal of CO2 from power plants. Even though many pilot plants are in service, a commercially viable operation on a large scale is yet to take place.

Certain factors makes the system very costly.

• The flue gas has to be devoid of SO2 or NOx. This requires a very good FGD (flue gas desulfurization). Another option is to go with a Circulating Fluidised bed combustion technology that can cost effectively remove these pollutants.
• The gas has to cool to around 40 ºC for the CO2 absorption to take place. This requires additional cooling water.
• Steam heat is required to heat the solvent to release the CO2 during regeneration.
• Parasitic power is required for pumping the fluids.
• Heat exchangers, scrubber towers, absorption towers, and heaters are required for the process.
• Replacement cost of the chemical solvent is high because regeneration is only for few cycles.

All these add to the cost of investment and operating the system. This also have an effect on the plant heat rate. This is what makes plant operators delay investment. However, penalties due to CO2 emissions may make this viable.

This system can capture around 85% of the CO2 generated.

To overcome the issue of cost, other processes and materials are under development. Some, which are promising and are in and advanced stage, are using special membrane filters and hyperbranched aluminosilica (HAS).