Retrofit capture to existing fossil power plants in parallel with new built zero emission power installations is necessary to achieve substantial reduction in greenhouse gas emissions.
Figure 1 illustrates post combustion capture by amine absorption is an option for retrofit capture. Extracting CO2 from atmospheric flue gas is relative energy demanding. Nebb has developed CESAm, an oxyfuel process that supply heat and power to an amine absorption unit.
In an amine absorption unit CO2 is chemical bounded to a solvent at relative low temperatures in an absorber tower and released from the bounding under relative high temperature in a stripper tower. CO2 capture requires heat to warm the solvent in a reboiler and power to drive the circulating fan and solvent pumps.
For an atmospheric absorption process it is normal to extract between 80 and 90 % of the CO2 in the inlet flue gas stream.
CESAm is an oxyfuel process that supply heat and power to the absorption process and is illustrated in Figure 2.
CESAm can be applied to any retrofit absorption unit. Figure 2 shows a conventional combined power cycle with an absorption unit and an oxyfuel process that supplies heat to the stripper tower.
An alternative to CESAm is to extract steam from the gas turbine Cycle to supply heat to the stripper tower. By extracting steam from the gas cycle, the cycle will no longer operate according to the design specification.
When heat is supplied by CESAm the cycle will operate under its design conditions and the power plant is completely independent of the CO2 capture.
When the power plant is unaffected of the retrofit capture, the plant does not experience any efficiency loss due to off design operations.
If steam is extracted from the Rankine cycle to supply heat to the reboiler, the power output of the steam turbines will decrease. The absorption unit also requires power to drive the circulating fan and solvent pumps. Together this will decrease the total power output.
With CESAm the power produced by steam turbines is unaffected and the oxyfuel process will supply power for the circulating fan and solvent pumps. In addition the oxygen fired gas turbine produces electricity.
A CESAm installation is a complete power plant in its own right with approximately 100% CO2 capture. Hence, by applying CESAm technology, the overall CO2 capture ratio will increase together with the total power output of the plant. If the plant is designed for less than the maximum CO2 capture ratio, the potential benefit may be realized through decreasing the height of the absorber tower.