Page 178 - Advanced Gas Turbine Cycles

144 Advanced gas turbina cycles

the temperature T and pressure p are prescribed the mole fractions may be determined
from the equilibrium constants, as described in the last section. The overall process is
endothermic.

8.5.5. Combustion with recycledjue gas as a cam‘er

To complete the set of possible chemical reactions, consider the combustion of a fuel
such as methane with a recirculated flue gas containing m moles of carbon dioxide, but
assuming that water vapour has been removed from the recycling flue gas. If the additional
air supply (n moles) is assumed to be sufficient for complete combustion, then
CH4 + mC02 + no2 + 3.76nN2 3 (m + 1)C02 + 2H20 + (n - 2)02 + 3.76nNz.

From the products of combustion, C02 and 2H20 may be removed subsequently within the
recirculation cycle before the remaining mCOz, reinforced with additional oxygen within
the air supply, are fed back to the combustion chamber. Essentially, the complete
combustion process described in Section 8.5.1 remains undisturbed by the ‘carrying’
recirculating flue gas.

8.6. Descriptions of cycles

With this background of how combustion may be modified we now study in some detail
a number of novel cycles previously listed.

8.6.1. Cycles A with additional removal equipment for carbon dioxide sequestration

We consider first Cycles A of Table 8.lA and the associated Figs. 8.6-8.8. These are
cycles in which the major objective is to separate or sequestrate some or all of the carbon
dioxide produced, and to store or dispose it. This can be achieved either by direct removal
of the C02 from the combustion gases with little or no modification to the existing plant; or
by modest restructuring or alteration of the conventional power cycle so that the carbon
dioxide can be removed more easily.

8.6.1.1. Direct removal of COz from an existing plant
Fig. 8.6 shows an example of the first type of plant having an ‘end of pipe’ solution in
which the C02 is removed from the exhaust of a standard CCGT plant, in an additional
chemical absorption plant (Cycle AI). The products of combustion downstream of the
HRSG (usually oxygen rich) are scrubbed by aqueous or organic based mixtures of
amines. C02 in the exhaust gases is first absorbed and rich Cop liquid is then pumped to
the stripper. The exhaust from the stripper is separated into water and gaseous Cop, which
is then compressed, intercooled and aftercooled before disposal as liquid COz at high
pressure and atmospheric temperature. A reasonably COz free stream is passed to the stack
and hence to the atmosphere.
Chiesa and Consonni [ 11 presented a detailed analysis of this type of plant. They found
that the ner efficiency of the plant dropped by about 5.5% below that of a basic CCCT plant