1. Introduction - Firenze University Press

Fig. 7. Steam extraction scheme of Case 1 Fig. 8. Steam extraction scheme of Case 2
3.4. Capture Case 2: CO2 capture case with consideration of the
constraint of existing power plant
Case 2 is generally similar to Case 1 in the process flow. (see Fig. 6 for details). However, Case 2
makes adequate consideration of the constraint of existing power plant. For example, as shown in
Fig. 8, the steam extraction point of Case 2 is located at the crossover pipe between the intermediate
pressure (IP) and low pressure (LP) cylinders of the steam turbine, which may be the only feasible
point to achieve large amount steam extraction in an existing power plant. Here, the steam pressure
of the crossover pipe between the IP and LP cylinders can reach 9.32bar, much higher than the
required steam pressure for absorbent regeneration (about 2.1bar), which will bring extra power loss
due to steam extraction. Furthermore, the extra pressure loss due to the large amount steam
extraction and the additional power loss resulted from the extraction are also be well considered in
Case 2.
Besides, the steam extraction for amine absorption process can account for approximately 50% of
the total steam flow exhausted from HP turbine cylinder. It may lead to unstable operation
conditions and bring about some safety problems. For example, the pressure at the exhaust of the
existing IP turbine would be dropped to a low level, which results in increased mechanical loading
of the IP blades, especially the last stages of IP cylinder. Besides, because the flow area of the LP
turbine cylinder is not variable, such a large decrease in the steam flow may lead to an unstable
operation condition in the LP turbine.
3.5. Performance analysis
The performance analysis of three cases is listed in Table 3, the three cases include:
Base Case: A typical 600MW supercritical power generation unit, as discussed in Section 3.1;
Case 1: CO2 capture case without considering the constraint of existing power plant, as discussed
in Section 3.3;
Case 2: CO2 capture case with consideration of the constraint of existing power plant, as
discussed in Section 3.4;
Table 3. Performance analysis of Base Case and Case 1-2
26
Base Case Case1 Case2
Coal input rate (kg/s) 46.66 46.66 46.66
CO2 capture amount (kg/hr) - 447423 447423
CO2 capture rate (%) - 90 90
Reboiler heat duty (MW) - 420.83 420.83
Extracted steam flow (kg steam/kg CO2) - 1.489 1.439
CO2 compression work (kWhe/tonne CO2) - 39.91 39.91
Power output of steam turbine (MW) 604 517.09 461.57
Auxiliary work (MW) 30.22 85.66 85.44
Net power output 573.8 431.43 376.13