1. Introduction - Firenze University Press

28.67%. In addition, through heat integration of steam turbine cycle with CO2 capture process, the
net efficiency of Case 4 can be further increased by 0.74%-points compared with Case 2, rising
from 28.67% to 29.41%. Furthermore, if one of LP cylinder of steam turbine can be throttled to let
most of steam entering the other LP cylinder, the huge pressure drop of the LP cylinder can be
avoided, which can make system net efficiency further increase by 1.01%-points, rising from
29.41% to 30.42%. And through adopting all of three system integration measures, Case 5 presents
the best performance. Compared with Base Case, the efficiency penalty of Case 5 is only 9.86%points,
even less than that of Case 2.
5. Exergy analysis
To reveal the internal phenomena of the new integration system, an exergy analysis is performed for
both Case 5 and Case 2 with CO2 capture. The results are listed in Table 5. The exergy analysis is
also based on the assumption that the same quantity of coal was consumed in all of the Cases.
As shown in Table 5, the exergy efficiency of Case 5 is 30.42%, which is 4.02% points higher than
that of Case 2, 9.86% lower than that of Base Case. Comparing the exergy distributions of Case 5
and Case 2, we find that the exergy of the net electricity has increased by 57.18 MW and the total
exergy loss of the CO2 capture unit is obviously decreased. Otherwise, the exergy loss of power
generation system is also reduced. And the detailed distribution of exergy loss of these two units is
given in Table 5.
Compared with the Case 2, the exergy loss of the CO2 capture unit of Case 5 is decreased
remarkably 30.25MW. The reason lies in thermal utilization of CO2 capture, it makes heat exergy
utilization rate improve greatly. In comparison, the exergy loss of power generation system of Case
2 is 24.88 MW higher than Case 5.
From the above analysis, we conclude that through thermodynamic system integration of
steam/water system and CO2 recovery and cascade utilization of energy, the key problem of high
energy penalty for CO2 capture may be improved in the Case 5 and favorable thermal and
environment performances can be achieved. However, some challenges still exist, such as the
complexity of the system, as well as the possible high investment of the system, which will be
further studied in our following work.
Table 5. Exergy analysis of Case 2-5 and Base Case
BaseCase Case2 Case3 Case4 Case5
MW MW MW MW MW
Exergy input of coal 1424.53 1424.53 1424.53 1424.53 1424.53
Exergy output
Net electricity 573.80 40.28% 376.13 26.40% 408.36 28.67% 418.97 29.41% 433.31 30.42%
Separated CO2 84.89 5.96% 84.89 5.96% 84.89 5.96% 84.89 5.96%
Exergy loss
CO2 recovery unit:
CO2 separation 71.60 5.03% 71.60 5.03% 71.60 5.03% 71.60 5.03%
CO2 compression 5.16 0.36% 5.16 0.36% 5.16 0.36% 5.16 0.36%
Heat exergy 36.85 2.59% 36.85 2.59% 6.61 0.46% 6.61 0.46%
Subtotal 113.61 7.98% 113.61 7.98% 83.36 5.85% 83.36 5.85%
Power generation system:
Boiler(Fuel combustion) 749.97 52.65% 749.97 52.65% 749.97 52.65% 749.97 52.65% 749.97 52.65%
HTP 14.31 1.00% 14.31 1.00% 14.31 1.00% 14.31 1.00% 14.31 1.00%
IPT 8.46 0.59% 8.27 0.58% 8.27 0.58% 8.27 0.58% 8.27 0.58%
LPT 25.12 1.76% 12.16 0.85% 8.87 0.62% 12.17 0.85% 10.99 0.77%
High temperature heater 5.23 0.37% 5.27 0.37% 5.27 0.37% 5.27 0.37% 5.27 0.37%
Low temperature heater 6.00 0.42% 13.23 0.93% 4.82 0.34% 3.93 0.28% 4.36 0.31%
Condenser 30.08 2.11% 20.93 1.47% 17.05 1.20% 21.94 1.54% 19.80 1.39%
Throttling 15.82 1.11% 11.29 0.79% 16.29 1.14% 4.51 0.32%
Other equipments 7.57 0.53% 5.60 0.39% 3.11 0.22% 3.20 0.22% 3.20 0.22%
Subtotal 846.74 59.44% 845.56 59.36% 822.98 57.77% 835.37 58.64% 820.68 57.61%
Exergy efficiency 40.28% 26.40% 28.67% 29.41% 30.42%
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