Post-Combustion

How is the CO2 Captured?

When capturing CO2 from a typical air-fired combustion unit after the burning process has taken place, it is referred to as post-combustion capture.

Any industry that generates thermal electricity as part of its process (either by using fossil fuels or biomass) is a primary opportunity for post-combustion. More than 90% of industrial facilities today use conventional process heaters and industrial utility boilers in which post-combustion systems could be tagged-on to existing facilities. The disadvantage of these systems is that typical flue gas streams have CO2 concentrations of 20% or less. Although the CO2 can be separated using membranes or cryogenics, these are costly endeavours, and only absorption (using chemical solvents like amines) is commercially viable today.

The challenge for post-combustion capture systems is to develop new designs for commercial-scale applications in large industrial facilities. Specifically, there is a need for improved solvents which could significantly reduce both the high energy penalty and capital cost of post-combustion capture. Amine scrubbing capture processes require lots of heat for solvent regeneration which contributes to the energy penalty. Because the process operates at atmospheric pressure, a lot of energy is needed to compress the CO2 for transportation. Parasitic losses for thermal power plants that use amine scrubbing ranges between 10 and 30% of the total power the plant would generate if CO2 capture were not included (IEA, 2004). This energy penalty translates into a noticeable impact on electricity prices.

Other needed technologies for post-combustion systems are energy efficiency and integrated pollutant controls, waste management processes and CO2 separation technologies (both for retrofits and for new facilities). The control technologies include combined CO2/sulphur oxide (SOx) removal systems for multi-pollutant capture. Additional work is needed to improve instrumentation and controls, new process integration methods and tools to conserve in-plant energy use. A final area of focus is on the co-production of other useful industrial by-products, such as fertilizer, ash and gypsum.

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