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Publication Name: Power Plant Carbon Capture with CHEMCAD
Original File Name Searched: Power_Plant_Carbon_Capture_with_CHEMCAD.pdf
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rev. 031109 Power Plant Carbon Capture with CHEMCAD
Background 1, L 2
The worldwide focus on clean power generation and carbon capture has increased the importance of the associated
technologies, which involve two distinct approaches, namely pre-combustion and post-combustion carbon capture.
In pre-combustion CO2 capture, fuel is gasified by applying heat under pressure in the presence of steam and air and/or oxygen to form synthetic gas (Syngas). CO2 is then captured from the Syngas, before being mixed with air in a combustion turbine, resulting in the CO2 being relatively concentrated and at a high pressure.
In post-combustion CO2 capture, mainly, pulverized coal is burnt in air to raise steam. CO2 is exhausted in the flue gas at atmospheric pressure and concentrations of 10-15% v/v. This process is more challenging due to the low pressure and dilute CO2 concentration resulting in a high volume of gas having to be treated. Also trace impurities in the flue gas tend to reduce the effectiveness of the CO2 absorbing processes and compressing the captured CO2 from atmospheric pressure to pipeline pressure represents a large parasitic load.
Another post-combustion capture technology, oxy-combustion, involves combustion of the fuel with near pure oxygen resulting in a flue gas stream of higher CO2 concentration. This technology relates more to combustion, and is not discussed further in this paper.
When CO2 is captured, power station generating efficiency is significantly reduced. Therefore a power cycle with a high thermal efficiency is essential to ensure an acceptable outcome. Power cycle efficiency is continually being improved by increasing the steam temperature and pressure; this development is limited by the availability of suitable alloys.
Ultra supercritical (USC), once-through utility (OTU) operation is considered to be the best option for “Clean Coal Technology”. USC operation reduces the CO2 emissions and therefore improves the economics of carbon capture and sequestration (CCS).
Pre-combustion and post-combustion CCS methods are similar, in that both require significant power for blowers, pumps and compressors. Studies are indicating parasitic power in the range 15 to 25% with the technology, plant layout, pressure drop, compressor operation, transportation, and sequestration configurations each having a significant effect. Crucial to CCS economics is the optimization of the heat integration circuits and the minimization of CO2 stripping steam to reduce the impact on power plant parasitic power.
CO2 is present at much higher concentrations in Syngas than in post-combustion flue gas, so CO2 capture should be less expensive for pre-combustion than for post-combustion capture. However, there are few gasification plants in full-scale operation, and capital costs are higher than for conventional pulverized coal plants.
Process simulators are playing an increasingly important role in finding the optimum economic solutions. CHEMCAD is being used in modeling pre-combustion and post-combustion processes and has developed thermodynamics specifically for the pre-combustion and compression technologies.
Historical Trend in Boiler Operation (Note: water PC = 221 bar, TC = 374oC)
1960 - 1970
38 to 42
1970 - 1990
1990 - present
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