1. Introduction - Firenze University Press

solvent regeneration mainly comes from the condensation of the steam extracted from steam
turbine.
However, retrofitting of the existing power plant for CO2 capture would encounter many constrains
and be more complex. Compared with the virtual plant or redesigned newly-built plant, the
steam/water cycle of an existing power plant can not be made great changes due to the restriction of
process and devices.
2.1. Restrictions of steam extraction parameters
In a chemical absorption process for CO2 capture, the solvent desorption temperature would vary
with different absorbents. However, most chemical absorption methods need to provide thermal
energy with temperature in the range of 100 o C and 150 o C for stripping process. Take MEA for
example, the CO2-rich amine stream, leaving from the absorber bottom, is regenerated by thermal
treatment at 100 o C up to 140 o C in the stripper, releasing CO2 [17,24-25,28]. The stripper makes use
of steam extracted from the steam/water cycle of the power plant. As economic consideration, the
extracted steam at 2.1-3.4bar is suitable to provide the solvent regeneration heat.
Besides, the amount of extraction steam is enormous, which can be half of the total steam flow of
LP turbine cylinders[24-25], due to the extensive heat demand of solvent desorption. However, in
the existing power plant, it is impossible to extract too much steam within low-pressure turbines
(LPT) due to the constraint of the structure of turbines. The only feasible steam extraction point for
an existing power plant may be located at the crossover pipe between the intermediate pressure (IP)
and low pressure (LP) cylinders of the steam turbine, [23,25]. And the crossover pipe is also the
quite place to extract a large amount of steam for heat supply in many combined heat-and-power
units[29-32].
In most supercritical or ultra supercritical units, the pressure of steam extracted from the IP/LP
steam turbine can be as high as 9-12 bar [23,33-34], this is far higher than the required parameters
of stripper for absorbent regeneration, which will bring extra power loss due to steam extraction.
Figure1 shows the relationship of power loss per kg extracted steam with its pressure. As is shown
in Fig. 1, the higher the extracted pressure, the higher the specific power loss. When the pressure of
extracted steam reaches 9-12 bar, its power loss will be almost twice as much as that of the steam
extracted at 2.1bar.
Power loss kwh/kg
0.24
0.20
0.16
0.237 kWh/kg
0.20 kWh/kg
0.146 kWh/kg
2.1 bar
0.12
0 2 4 6 8 10 12
Steam extraction pressure bar
20
9 bar
12 bar
Fig. 1. Relationship between power loss and steam extraction pressure
2.2. Off-design conditions of LP turbine due to huge extraction steam
As is mentioned above, a large amount of heating steam will be extracted from the crossover pipe
between IP and LP turbines in CO2 capture retrofitting of existing power plants, which leads to LP
cylinders operating under off-design conditions. In this situation, the steam mass flow rate of LP
cylinders will drop deeply, which leads to the substantial deviation of steam parameters from the
rated values.