Page 168 - Advanced Gas Turbine Cycles

136 Advanced gas turbine cycles

8.3. C02 removal equipment

There are two main schemes proposed for sequestration of carbon dioxide. The first
(referred to as a chemical absorption process), suitable for use at low pressures and tem-
peratures, is usually adopted where the COZ is to be removed from exhaust flue gases. The
second (usually referred to as aphysical absorption process), for use at higher pressures, is
recommended for separation of the COz in syngas obtained from conversion of fuel.

8.3.1. The chemical absorption process
Fig. 8.1 shows a diagram of a chemical absorption process described by Chiesa and
Consonni [l], for removal of COz from the exhaust of a natural gas-fired combined cycle
plant (in open or semi-closed versions). The process is favoured by low temperature which
increases the C02 solubility, and ensures that the gas is free of contaminants which would
impair the solvent properties.
Exhaust gas is fed to an absorber where the solvent (a blend of ethanol amines, mono-
ethanolamine and diethanolamine) absorbs the carbon dioxide, and a COZ free stream is
discharged to the atmosphere from the top of the absorption tower. Condensate is fed via a
heat exchanger to a stripper from which the solvent is drained into a re-boiler (heat is
supplied by steam fed from the HRSG of the combined cycle). Carbon dioxide and water
leave the top of the stripper, passing through a cooler and separator, from which water is
drained. Gaseous C02 leaves the top of the separator to enter an intercooled compressor;
the compressed COz is also aftercooled, and liquid carbon dioxide is discharged ready for
disposal.
The negative aspects of the system on the combined cycle efficiency lie in the steam
consumption for the stripping process, and the extra work inputs, to the C02 compressor
and to the fans required to circulate the gases, through a system with non-negligible
pressure losses. Corti and Manfrida [2] have considered in some detail the losses involved
and argue that by careful optimisation of the composition of the amines blend in the
solvent (50% di-ethanolamine in the aqueous solution containing the amine blends), the
heat required for regeneration of the scrubbing solution can be limited. They have also
drawn attention to the advantages of recovering combustion generated water into the lower
steam cycle.

8.3.2. The physical absorption process

Fig. 8.2 shows a diagram of the physical absorption process suggested by Chiesa and
Consonni [3] for an IGCC plant, with the absorption taking place from the syngas after its
discharge at high pressure from the gasification and HZS cleansing process. The C02 fed to
the absorber is of a high concentration and flows upward, counter current to the GO2 lean
solvent (Selexol is proposed, which is soluble in COz but not in nitrogen).
The COz rich solvent is drained from the bottom of the tower, and led first to a hydraulic
turbo-expander and then to four flash drums connected in series, where COz is de-absorbed
as the pressure is lowered. Lean solvent is pumped back to the top of the absorber tower