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Integrated Gasification Combined Cycle – Post- vs. Pre- Combustion CO2 Capture

The following two models describe an IGCC plant with differing CO₂ capture systems. The IGCC plants consist of a GE 7FB GT with a Type 4 gasification system, a 3-pressure reheat HRSG, and a single reheat steam turbine. Each gasification system includes a Type 4 gasifier (a two-stage, dry-fed, enriched air-blown unit) with a convective syngas cooler, an air separation unit, and a gas cleanup system. The gasifier air supply is a mixture of air bled from the GT’s compressor and oxygen from the ASU.

In Plant A, CO₂ is captured post-combustion. An amine-based chemical absorption process separates 85% of the CO₂ from the flue gas stream. This CO₂ is then compressed to 2200 psia (152 bar) for delivery.

Post-combustion CO₂ Capture

In Plant B, CO₂ is captured pre-combustion. Ninety percent (90%) of CO₂ is removed from the syngas using a physical absorption process. The separated CO₂ is then compressed to 2200 psia (152 bar) for delivery.

Pre-combustion CO₂ Capture

Both plants use the same steam cycle with HP conditions of 1600 psia/1026°F (110 bar/552°C), reheat conditions of 350 psia/1025°F (24 bar/552°C), and a condenser pressure of 0.7 psia (48 mbar). With these parameters, Plant A has a gross output of 292 MW_e, a net output of 233 MW_e, and a net LHV efficiency of 36.7%, while Plant B has a gross output of 319 MW_e, a net output of 260 MW_e, and a net LHV efficiency of 37.9%. Despite removing a lower percentage of the CO₂ produced, a plant utilizing a post-combustion capture method has both lower output and lower efficiency than one capturing CO₂ pre-combustion in an IGCC plant.