Page 180 - Advanced Gas Turbine Cycles

146 Advanced gas turbine cycles

FUEL (METHANE)
1

\

ABSORPTION,
STRIPPING,
ILIQUEFACTION rAJ HEAT EXCHANGER
HEAT~N
1
LIQUID COz

Fig. 8.8. Cycle A3. Semi-closed recuperative plant with COz removal (after hkdnfrida 141).

8.6. I .2. Mod$cations of the cycles of conventional plants using the semi-closed gas
turbine cycle concept
Fig. 8.7 shows a second example (Cycle A2) of carbon dioxide removal by chemical
absorption from a CCGT plant, but one in which the semi-closed concept is introduced-
exhaust gas leaving the HRSG is partially recirculated. This reduces the flow rate of the
gas to be treated in the removal plant, so that less steam is required in the stripper and the
extra equipment to be installed is smaller and cheaper. This is also due to the better
removal efficiency achievable-for equal reactants flow rate-when the volumetric
fraction of C02 in the exhaust gas is raised from the 4-6% value typical of open cycle gas
turbines to about 12% achievable with semi-closed operation.
Chiesa and Consonni [I] gave another detailed analysis for this plant in comparison
with Cycle AI. They found that the efficiency dropped by 5% from that of the basic CCGT
plant; this is somewhat surprising as the absorption plant is smaller than that for Cycle A1
and it might have been expected that the penalty on efficiency of introducing the
absorption plant would have been much less than that of Cycle Al. With this calculated
efficiency and a detailed estimate of capital cost, the price of electricity was virtually the
same as that of Cycle Al, Le. 40% greater than that of the basic CCGT plant.
Corti and Manfrida [2] have also done detailed calculations of the performance of
plant A2. They drew attention to the need to optimise the amines blend (including
species such as di-ethanolamine and mono-ethanolamine) in the absorption process, if a
removal efficiency of 80% is to be achieved and in order to reduce the heat required
for regenerating the scrubbing solution. Their initial estimates of the penalty on
efficiency are comparable to those of Chiesa and Consonni (about 6% compared
with the basic CCGT plant) but they emphasise that recirculation of water from