Energy 211 (2020) 118651
Contents lists available at ScienceDirect
Energy
journal homepage: www.elsevier.com/locate/energy
Energy and exergy analyses of SeCO2 coal-fired power plant with
reheating processes
Zhewen Chen a, b, Yanjuan Wang a, b, *, Xiaosong Zhang c
a
The Beijing Key Laboratory of Multiphase Flow and Heat Transfer, North China Electric Power University, Beijing, 102206, China
The Key Laboratory of Power Station Energy Transfer Conversion and System, North China Electric Power University, Ministry of Education, China
c
Hainan University, Hainan, 570228, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 March 2020
Received in revised form
12 August 2020
Accepted 16 August 2020
Available online 26 August 2020
SeCO2 (Supercritical-CO2) coal-fired power plant is a promising technology for efficient and clean utilization of coal for power generation. The conversion and transfer of the energy and exergy in the power
plants with double-reheat and single-reheat processes are studied. With the main gas parameters of 32
MPa/893.15 K, the power generation efficiencies of the SeCO2 coal-fired power plant with double-reheat
and single-reheat processes are 49.06% and 48.72%, respectively. The corresponding exergy efficiencies
are 48.02% and 47.69%, respectively. The origins of exergy destructions in different units are studied using
the Energy Utilization Diagram (EUD) method. The exergy distributions of the power plants are presented. For the power plant with double-reheat process, the work output, the exergy exhaust into the
atmosphere, the exergy destruction in combustion process, the exergy destruction in heat transfer
processes, the exergy destruction caused by pressure loss, and the exergy destructions in turbo systems
account for 48.02%, 9.66%, 20.45%, 17.56%, 1.08%, and 3.23% of the total exergy input of the power plant,
respectively.
© 2020 Elsevier Ltd. All rights reserved.
Keywords:
SeCO2 brayton cycle
Reheat process
Exergy analysis
Coal-fired power plant
1. Introduction
Coal is an important component of the world energy system. In
2012, coal was the second largest source of primary energy in the
world (approximately 29%), and the world’s largest consumed energy source for electricity production (over 40%) [1]. China is
currently the largest consumer of coal and the largest emitter of
energy-related carbon dioxide (CO2). In 2015, 47% of the China’s
coal was used to generate electricity. Approximately 80% of China’s
annual energy-related CO2 emissions were from coal combustion
and coal-derived products [2]. One of the most promising ways to
reduce CO2 emissions in coal combustion processes is reducing the
coal consumption of a certain power plant by improving the net
power generation efficiency [3].
The generation efficiencies of typical double-reheat ultra-supercritical (USC) power plants include Mannheim Power Plant in
Germany, Kawagoe Power Plant in Japan, and Nordjylland Power
Plant in Denmark can reach over 45%-LHV [3]. Until the end of 2016,
* Corresponding author. The Beijing Key Laboratory of Multiphase Flow and Heat
Transfer, North China Electric Power University, Beijing, 102206, China
E-mail address: 90102184@ncepu.edu.cn (Y. Wang).
https://doi.org/10.1016/j.energy.2020.118651
0360-5442/© 2020 Elsevier Ltd. All rights reserved.
there are 6 double-reheat USC power plants in China. The unit 3
and unit 4 of Guodian Taizhou power plant were successfully
operated from September 2015 and January 2016, respectively. The
boiler efficiencies of the two units reach 94.78% and 95.12%, and the
power generation efficiencies reach 47.81% and 47.95%, respectively
[4]. Further improvement on the steam parameter is limited by the
material problem, which becomes the obstacle for further
enhancement on the power generation efficiency of coal-fired power plant.
Supercritical CO2 (SeCO2) cycle has advantages of compactness,
simpler cycle layout, sustainability, enhanced safety and superior
economy over water based power cycles [5e7]. The SeCO2 cycle has
been widely utilized for nuclear power [8,9], solar power [10,11],
gas turbine [12,13], geothermal power [14,15], and so on. The energy structure of China determines that developing SeCO2 coalfired power plants is greatly necessary and promising. Implementing efficient SeCO2 cycle for coal-fired power plant can reduce
the coal consumption while maintaining the same electricity supply of the society, which would be of great benefit to the emissions
reduction in coal utilization. The power generation efficiency of the
SeCO2 coal-fired power plant can easily reach over 48% from
literature data [3,16]. The efficiency of the system reached 48%
2
Z. Chen et al. / Energy 211 (2020) 118651
Nomenclature
A
AP
ar
C
CON
Cp
DEA
e
EUD
ef,ph
ef,ch
FC
H
h
HRH
HT
HTR
HV
i
j
k
L
LHV
LRH
LT
LTR
M
Energy level
Air preheater
As-received
Carbon
Condenser
Specific heat capacity
Deaerator
Exergy
Energy Utilization Diagram
Physical exergy of different flows
Chemical exergy of different flows
Fixed carbon
Hydrogen
Enthalpy
High-pressure regenerative heater
High-pressure turbine
High temperature recuperator
Heating value
Unit i
Component j
Composition k
Latent heat of vaporization
Lower heating value
Low-pressure regenerative heater
Low-pressure turbine
Low temperature recuperator
Moisture
under the condition of 30MPa/893.15 K and a double reheat single
recompression cycle [3]. Xu et al. [16] emphasized two key issues of
large boiler pressure drops and residual flue gas heat extraction of
SeCO2 coal-fired power plant. The power generation efficiency of
the power plant reaches 48.37% under the condition of 30 MPa/
893.15 K and a double-reheat recompression cycle.
Within all possible configurations of SeCO2 cycles, the recompression cycle has been proved to be the most efficient [17,18].
Another traditional way to improve the cycle efficiency is reheating.
Dostal [9] showed that the single-stage reheat introduces approximately 1.2% efficiency enhancement, while the double-reheat
yields only 0.46% additional benefit. Sarkar [7] compared the
SeCO2 recompression cycle with and without reheating, and found
that the maximum efficiency improvement using reheating is 3.5%
at optimum conditions. Mecheri and Moullec [3] investigated
different configurations of SeCO2 cycles for coal power plant applications. The results showed that single reheat is an effective
configuration with 1.5%pt efficiency increases compared to noreheat cycle, and a second reheat increases the efficiency by 0.5%
pt at the optimal pressure ratio and for an 873.15 K turbine inlet
temperature.
Zhou et al. [22] conducted the exergy analysis of a single reheat
SeCO2 Brayton cycle coal-fired power plant. The results showed
that the exergy loss ratios of the SeCO2 boiler system and fuel
combustion process were 82.2% and 53.5%, respectively. The former
is close to that of the steam boiler, but the latter is about 5.3% higher
than that in the traditional steam boiler. Chen et al. [23] revealed
m
MC
MT
N
O
Q
R
RC
S
s
SeCO2
T0
V
VHT
W
w
x
Mass flow rate
Main compressor
Mid-pressure turbine
Nitrogen
Oxygen
Heat
Universal gas constant
Re-compressor
Sulfur
Entropy
Supercritical carbon dioxide
Environment temperature
Volatile matter
Very high-pressure turbine
work
Mass fraction
Mole fraction
Greek symbols
Exergy change
Exergy destruction
Enthalpy change
Boiler efficiency
Cycle efficiency
Exergy efficiency of the boiler system
Exergy efficiency of the power cycle
Exergy efficiency
Power generation efficiency
De
DEXL
DH
hb
hc
heb
hec
hex
hp
the energy-saving mechanism of SeCO2 coal-fired power plant
compared to coal-fired power plant with steam Rankine cycle. The
exergy balance analysis is conducted out for the SeCO2 coal-fired
power plant. However, the exergy balance is analyzed through
exergy distributions in different units. The exergy distribution for
certain processes including work output, combustion, heat transfer,
turbo systems, pressure losses, and exhaust flows is not clarified.
Viewing from the literature review, many works have been done
in the aspects of energy and exergy analyses on the SeCO2 coalfired power plant with reheating processes. However, the exergy
destructions caused by pressure losses and heat transfer processes
are calculated by the difference between the exergies of the inlet
flows and the exergies of the outlet flows for particular heat exchangers. In this study, the exergy destructions caused by the
pressure losses are separated from the total exergy destructions for
heat exchange processes in certain units. In this way, the influences
of mass transfer and heat transfer processes on the exergy destructions can be clearly presented.
2. Proposal of the power plants
Supercritical-CO2 coal-fired power plant is a power generation
technology with integrated coal combustion process and SeCO2
cycle. The operation parameters of CO2 are beyond its critical point
(7.38 MPa/304.13 K). Thermodynamic calculations in the modelling
processes are based on mass balances, energy balances, and exergy
balances in particular units and the entire power systems. The
Z. Chen et al. / Energy 211 (2020) 118651
processes simulations have been performed with Aspen Plus software. Among property methods such as PENG-ROB, PR-BM, RKSOAVE, SRK, BWRS and LK-PLOCK, the LK-PLOCK property
method was proved to exhibit satisfactory results and revealed the
best trends near the critical point compared to REFPROP [3]. LKPLOCK is also more accurate at high pressure and temperature
[3]. Thus, LK-PLOCK property method is selected to perform the
simulations in this paper.
The flow sheet of SeCO2 coal-fired power plant with doublereheat recompression cycle is illustrated in Fig. 1, which is
composed of [19,20]:
- [1e6] turbine {total flow}
- [6,7] high temperature recuperator (HTR) hot side {total flows
at hot and cold sides}
- [7,8] low temperature recuperator (LTR) hot side {total flow at
the hot side, part flow at the cold side}
- [9,10] heat sink {part flow}
- [8e13] re-compressor {part flow}
- [10,11] main compressor {part flow}
The fuel coal is combusted with the pre-heated air in the boiler
to produce high-temperature flue gas. Half of the sensible heat
carried by the flue gas is transferred by radiation at the maximum
combustion temperature, and the other half is delivered by convection [21]. The remaining sensible heat is discharged into the
atmosphere with a temperature of 393.15 K. The maximum temperature of the SeCO2 cycle is 893.15 K. The parameters of CO2 at
the outlet of the LT are 7.9 MPa/834.9 K. The CO2 flows into the HTR
and LTR for two-stage heat recovery. After the heat recovery process, the CO2 flow is split into two separate flows. The first flow
(68.3% of the total flow) is cooled in the condenser and compressed
to 33.45 MPa in the main compressor (MC). Then this flow is heated
to 503.15 K in the LTR by the recovered heat. The second flow (31.7%
of the total flow) is compressed to 33.4 MPa in the re-compressor
(RC), and mixes with the first flow at the outlet of the LTR. The
mixed total flow is heated to 788.65 K in the HTR by the recovered
heat.
The following assumptions are made for calculation:
- The pinch points are set to 5 K for recuperators and 30 K (±1 K)
for air-gas heat exchangers [22].
- The pressure drops of the HTR and LTR are 0.05 MPa [24], and
the pressure drop for the SeCO2 boiler is 1.5 MPa.
3
Fig. 2. The flow sheet of SeCO2 coal-fired power plant with single-reheat recompression cycle.
- Neglect the shaft seal loss, mechanical loss, the pressure loss of
pipelines and separators [22].
- The isentropic efficiencies of the turbomachinery and compressors are set to 93% [25] and 90% [24,25], respectively. And
the compressors are assumed to be driven by turbine shaft and
not by electrical motors.
The flow sheet of the SeCO2 coal-fired power plant with singlereheat recompression cycle is illustrated in Fig. 2. The only difference in the system structure is that two turbines are included in the
power plant, and one-stage reheat is implemented. Other parts of
the power plant are similar to that in the power plant with doublereheat process.
3. Methodology
The energy and exergy analyses are conducted out for
comprehensive comparison of SeCO2 coal-fired power plant with
double-reheat and single-reheat SeCO2 cycles. As mentioned
above, thermodynamic calculations in the modelling processes are
based on mass balances, energy balances, and exergy balances in
particular units and the entire power systems.
3.1. Mass, energy, and exergy balances
The mass balance, energy balance, and exergy balance for
certain unit i can be expressed as follows [23]:
X
X
m_ ij ¼
m_ ij
in
(1)
out
X
X
mij hij mij hij ¼ Wi þ Qi
in
DEXLi þ Wi þ eQi ¼
X
X
mij eij mij eij
in
Fig. 1. The flow sheet of SeCO2 coal-fired power plant with double-reheat recompression cycle.
(2)
out
(3)
out
While m_ ij , hij , and eij are the mass flow rate, specific enthalpy,
and specific exergy of component j at the inlet or outlet of unit i. Wi
is the work output of unit i. Wi is positive for turbines, negative for
compressors and pumps, and is zero for other units. Qi is the heat
released from unit i. Qi is positive for condensers. DEXLi is the
exergy destruction of unit i; eQi is the exergy of Qi .
4
Z. Chen et al. / Energy 211 (2020) 118651
The energy balances of the power plants are based on the energy balances of each unit. Particularly, for the boiler, the enthalpy
of the flue gas consists of the sensible heat of the flue gas, latent
heat of the H2O in the flue gas, and the heating value of the CO in
the flue gas.
The sensible heat of the flue gas is calculated by: h0fluegas þ
ðCp Þfluegas ðTfluegas T0 Þ;
The latent heat of the H2O in the flue gas is calculated by:
ðm_ H2 O m_ coal Mar ÞLH2 O ; Because the lower heating value of the coal
is used in the energy input of the power plant, and the coal contains
H2O. When calculating the latent heat of the H2O in the flue gas, the
latent heat of the amount of the H2O in the coal should not be
contained.
The heating value of the CO in the flue gas is calculated by: HVCO .
Thus, the enthalpy of the flue gas exhausted into the atmosphere can be calculated by the following equation when the lower
heating value of the fuel coal is adopted [23]:
hfluegas ¼ h0fluegas þ Cp fluegas Tfluegas T0
m_ H2 O m_ coal Mar LH2 O þHVCO
(4)
where hfluegas is the enthalpy of the flue gas; h0fluegas is the enthalpy
of the flue gas at standard status; (Cp)fluegas is the average specific
heat capacity of the flue gas between T0 to Tfluegas; Tfluegas is the
_ H2 O is the mass flow rate of H2O in the
temperature of the flue gas; m
flue gas; Mar is the mass fraction of H2O in the as-received fuel coal;
L H2O is the latent heat of vaporization for H2O; HVCO is the heating
value of CO.
The exergy of the coal and particular flow is calculated by
Refs. [23]:
ecoal ¼ LHVcoal
w
w
w
1:0064 þ 0:1519 H þ 0:0616 O þ 0:0429 N
wC
wC
wC
defined as:
A¼
.
De
¼ 1 T0 DS DH
DH
(9)
where A is the energy level of the process; De, DS and DH are the
exergy change, entropy change and enthalpy change during the
process, respectively. T0 is the environmental temperature. For an
energy-transformation process, there exists an energy donor (Aed)
and an energy acceptor (Aea):
Aed ¼
Deed
DHed
(10)
Aea ¼
Deea
DHea
(11)
And DHea þ DHed ¼ 0, (12).
Thus, the total exergy destruction during the energytransformation process is:
Deea þ Deed ¼ Aed DHed þ Aea DHea
¼ ðAed Aea Þ DHed
(13)
For a continuous energy-transformation process, the exergy
destruction can be obtained by the integral form:
ð
De ¼ ðAed Aea Þ dH
(14)
In the energy-utilization diagram (EUD diagram), the x-coordinate is energy change, and the y-coordinate is energy level A, which
is a dimensionless criterion. So the exergy destruction is illustrated
by the shaded areas between the curves of the energy donor and
(5)
X
X
ef ¼ ef ;ph þ ef ;ch ¼ ðh h0 Þ T0 ðs s0 Þ þ
xk ef ;k þ RT0
xk ln xk
X
X
X
¼
xk ½ððh h0 Þ T0 ðs s0 ÞÞk þ
xk ef ;k þ RT0
xk ln xk
(6)
In the equations, wH,wO, wN,wC are the mass fractions of elements H, O, N, C in the coal; ecoal is the specific exergy of the fuel
coal, kJ/kg; ef is the specific exergy of different flows in the system;
ef,ph and ef,ch are the physical exergy and chemical exergy of
different flows in the system; ef,k and xk are the standard chemical
exergy and the mole fraction of the composition k in the flow.
In accordance with the energy balance and exergy balance
equations, the power generation efficiency and exergy efficiency of
the overall system can be expressed as follows:
hp ¼
Woutput
m_ coal LHVcoal
hex ¼
Woutput
ecoal
(7)
(8)
3.2. Energy utilization diagram (EUD methodology)
The energy utilization diagram (EUD methodology) method was
firstly proposed by Ishida [26], and is implemented to analyze the
origins of the exergy destructions. The energy level of a process is
Fig. 3. Computation scheme for SeCO2 power plant considering pressure drops.
Z. Chen et al. / Energy 211 (2020) 118651
5
Table 1
The values of key parameters in the calculation.
Parameters
values
Parameters
values
Pressure/temperature of the fresh steam and SeCO2/MPa/K
Pinch points of the boilers/K
Pinch points of the heat exchangers/K
Excess air ratio in the boiler
Pressure drop in SeCO2 boiler/MPa
Pressure drop in HTR and LTR/MPa
32/893.15
30 [22]
5 [22]
1.3
1.5
0.05 [24]
Isentropic efficiency of turbines/%
Double-reheat temperatures/K
Isentropic efficiency of compressors/%
Pump mechanical efficiency/%
Pressure drop in water boiler/MPa
93 [25]
893.15
90 [24,25]
99
5.9
All other parameters are determined by process constraints after the main parameters are set. The main parameters used in the simulation are concluded in Table 1.
Table 2
Ultimate analysis and proximate analysis of the fuel coal.
Ultimate analysis, wt%
Car
Har
Oar
Nar
Sar
68.55
3.96
6.85
0.74
1.08
Proximate analysis, wt%
Mar
Aar
Var
FCar
LHVcoal, MJ/kg
8.84
9.98
49.52
31.66
26.51
energy acceptor.
The computation flowchart of the SeCO2 power plant is illustrated in Fig. 3.
The ultimate and proximate analyses of the fuel coal are listed in
Table
2.
For
the
ultimate
analysis,
Car þ Har þ Oar þ Nar þ Sar þ Mar þ Aar ¼ 100; for the proximate
analysis, Mar þ Aar þ Var þ FCar ¼ 100.
The parameters of key flows in the power plants are illustrated
in Table 3. As can be seen in Table 3, in the SeCO2 coal-fired power
plant with double-reheat and single-reheat processes, the temperature of the exhaust CO2 at the outlet of LT are 834.9 K and
780.95 K, respectively. Thus, the heat recovery processes should be
implemented to maintain high energy efficiency.
4. Results and discussions
The optimal intermediate pressure for the single-reheat SeCO2
cycle can be represented by the following correlation while the
effect of component performance on optimization is neglected [5]:
Popt ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Tmax 0:15
Pmin Pmax
Tmin
(15)
As a result, the optimal reheat pressure in this paper (18.8 MPa)
is slightly larger than that calculated by equation (15) (18.6 MPa),
which is in accordance with the result presented in Ref. [5].
As can be seen in Fig. 4, the optimal power generation efficiency
of the SeCO2 coal-fired power plant with single-reheat process is
48.722% in this paper.
4.1. T-Q curves of the SeCO2 cycles
The T-Q curves of the SeCO2 coal-fired power plant with
double-reheat and single-reheat processes are illustrated in Fig. 5
and Fig. 6, respectively. The average endothermic temperature T
can be calculated by the following correlation:
Table 3
The key parameters in the power plants.
Coal
Air
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Flue gas
The temperatures of pre-heated air enters the boilers are
779.68 K and 536.79 K, respectively. In current conditions, the hightemperature characteristic of the pre-heated air is a challenge faced
by the SeCO2 coal-fired power plant.
Temperature/K
Pressure/bar
Mass flow/kg/s
298.15
298.15
893.15
823.92
893.15
833.73
893.15
834.9
508.15
359.73
359.73
305.65
353.89
503.15
502.81
788.63
816.63
779.68
893.15
820.97
893.15
780.95
508.15
359.73
353.89
738.06
724.24
393.15
1.0
1.0
320.0
192.3
192.3
123.3
123.3
79.0
79.0
79.0
79.0
79.0
334.5
334.0
334.0
333.5
1.0
1.0
320.0
188.0
186.5
79.0
79.0
79.0
334.5
333.5
1
1.0
0.479
5.534
39.565
39.565
39.565
39.565
39.565
39.565
39.565
39.565
27.023
27.023
27.023
27.023
12.542
39.565
5.965
5.534
40.502
40.502
40.502
40.502
40.502
40.502
27.663
40.502
5.534
5.965
T¼
Qe
DSe
(16)
Qe is the total heat absorbed by the SeCO2 in the heating and
Fig. 4. The influence of reheat pressure on the power efficiency in the power plant
with singe-reheat process.
6
Z. Chen et al. / Energy 211 (2020) 118651
4.2. Energy balances of the power plants
Fig. 5. The T-Q curve of the double-reheat SeCO2 cycle.
Fig. 6. The T-Q curve of the single-reheat SeCO2 cycle.
reheating processes. DSe is the entropy increase in the heating and
reheating processes.
The average endothermic temperatures Td and Ts of the doublereheat and single-reheat cycles can be calculated to be 827.03 K and
802.60 K, respectively. Thus, the double-reheat cycle has advantages over the single-reheat cycle from viewpoint of cycle thermal
efficiency. However, The SeCO2 coal-fired power plant consists of
two parts: the boiler system, and the SeCO2 cycle. Different cycle
configurations (reheating stages) not only influence the cycle efficiency, but also affect the boiler system.
As can be seen in Figs. 5 and 6, the temperatures of the feed
SeCO2 to the boiler in the double-reheat and single-reheat processes are 788.63 K and 738.06 K, respectively. Under the condition
of same coal input and excess air ratio in the boiler, higher temperature of the feed SeCO2 would lead to higher combustion
temperature. More CO exists in the flue gas of the boiler when the
combustion temperature is higher according to the chemical
equilibrium. Thus, the enthalpy of the flue gas exhausted into the
atmosphere is higher in the power plant with double-reheat process, which leads to lower boiler efficiency of the boiler system. In
the following part, detailed analyses would be carried out.
The energy balance diagram of the SeCO2 coal-fired power plant
with double-reheat process is illustrated in Fig. 7 [23]. In the figure,
the widths of the blocks and lines represent the amount of energy.
The only energy input of the power plant is the enthalpy of the fuel
coal, 12.70 MW. The electricity generated by the HT, MT and LT is
8.63 MW. However, the main compressor and the re-compressor
consume 1.07 MW and 1.33 MW, respectively. Thus, the work
output is 6.23 MW, leading to a net power generation efficiency of
49.06%. The exhaust energy of the boiler is 1.11 MW, including the
enthalpy of the high-temperature ash and enthalpy of the flue gas.
Thus, the boiler efficiency can be calculated by the ratio of heat
absorbed by the working fluids in the power cycles and the
enthalpy of the coal, which is 91.26%. The exhaust energy of the
condenser in the SeCO2 cycle is 5.36 MW.
The energy balance diagram of the SeCO2 coal-fired power plant
with single-reheat process is illustrated in Fig. 8. The electricity
generated by the HT, MT and LT is 8.65 MW. However, the main
compressor and the re-compressor consume 1.10 MW and
1.36 MW, respectively. Thus, the work output is 6.19 MW, leading to
a net power generation efficiency of 48.72%.
The exhaust energy of the boiler is 1.07 MW, including the
enthalpy of the high-temperature ash and enthalpy of the flue gas.
Thus, the boiler efficiency is 91.61%. The exhaust energy of the
condenser in the SeCO2 cycle is 5.44 MW.
Viewing from system level, the power plants are divided into
two parts: the boiler system and the power cycle, as can be seen in
Fig. 9. In the boiler system, the chemical energy in the coal is
converted to the sensible and latent heat of the flue gas through
combustion. In the power cycle, the sensible and latent heat of the
substance is converted to electricity though turbines. Between the
boiler system and the power cycle, the energy is transferred from
the combustion product to the SeCO2 in the cooling wall, superheaters, and re-heaters throughout the boiler. The overall power
generation efficiency is the product of the boiler efficiency and the
cycle efficiency.
As can be seen in Table 4, the boiler efficiency of the SeCO2 coalfired power plant with double-reheat process is slightly lower than
that of the power plant with single-reheat process. In the other
hand, the cycle efficiency of the double-reheat SeCO2 Brayton cycle
is higher than that of the single-reheat SeCO2 Brayton cycle. As a
result, the net power generation efficiency of the SeCO2 coal-fired
power plant with double-reheat process is larger than that of the
power plant with single-reheat process.
In our research, the boiler efficiencies of the SeCO2 coal-fired
power plant with double-reheat and single-reheat processes are
91.26% and 91.61%, respectively. The lower boiler efficiency of the
power plant with double-reheat process is mainly caused by the
larger incomplete combustion loss of CO in the boiler, as can be
seen in Fig. 10.
Due to the higher temperatures of CO2 entering the boiler
(788.63 K) and the pre-heated air (779.68 K) in the SeCO2 coal-fired
power plant with double-reheat process, the average temperature
in the SeCO2 boiler is higher than that in the power plant with
single-reheat process. According to the mass balance and the
chemical reaction (R1), more CO would exist in the flue gas of the
SeCO2 boiler with double-reheat process, which causes larger
incomplete combustion loss. To avoid this and improve the SeCO2
boiler efficiency, new boiler configuration design should be
adopted.
2CO2
Endothermal
0
2CO þ O2
(R1)
Z. Chen et al. / Energy 211 (2020) 118651
7
Fig. 7. The energy balance diagram of SeCO2 coal-fired power plant with double-reheat process [23].
Fig. 8. The energy balance diagram of SeCO2 coal-fired power plant with single-reheat process.
4.3. Exergy balances of the power plants
The exergy of the flows in the flow sheets of the technologies,
exergy destructions in different processes or units, and the exergy
efficiencies of the power plants are calculated by equations (3), (5),
(6) and (8). According to the equations, the exergy balances of the
SeCO2 coal-fired power plants in Table 5.
The exergy inputs of the power plants are 12.975 MW. The
exergy outputs are considered to only be the work output of the
power plants. The exergy of the flue gas and ash is handled to be the
exergy loss of the power plants. As can be seen in Table 5, the
exergy outputs of the SeCO2 coal-fired power plants with doublereheat and single-reheat processes are 6.231 MW and 6.188 MW,
respectively. Thus, the exergy efficiencies of the power plants are
48.02% and 47.69%, respectively. The largest exergy destructions
exist in the boilers in both power plants, which account for
approximately 34.41% and 35.50% of total exergy inputs. For the
condensers, although the energy losses of the SeCO2 coal-fired
power plants with double-reheat and single-reheat processes are
separately 5.36 MW and 5.44 MW, the exergy destructions are only
0.356 MW and 0.363 MW, respectively.
The exergy destructions of the boiler systems (include the
exergy destruction in the boiler, the air-preheater, and the exergy
loss of the flue gas) of the SeCO2 coal-fired power plant with
double-reheat and single-reheat processes are 5.582 MW and
5.649 MW, respectively. Thus, the exergy efficiencies of the boiler
8
Z. Chen et al. / Energy 211 (2020) 118651
Table 5
The exergy balances of the power plants with reheating processes.
Fig. 9. The energy conversion and transfer routes in the power plants.
Table 4
The comparison on relative efficiencies of the power plants.
Power plant with
Power plant with
double-reheat SeCO2 cycle single-reheat SeCO2 cycle
Boiler efficiency/%
91.26
Cycle efficiency/%
53.76
Net power efficiency/% 49.06
91.61
53.18
48.72
systems are 56.98% and 56.46%, respectively.
As can be seen in Fig. 11, the exergy destruction distributions of
the boiler systems are illustrated. The largest exergy destructions
exist in the coal combustion processes, which are 2.653 and
2.74 MW for SeCO2 boilers with double-reheat and single-reheat
processes, respectively. Due to the higher average combustion
temperature in the SeCO2 boiler with double-reheat process, the
energy level difference between the coal combustion reaction and
the high-temperature flue gas is smaller than that in the SeCO2
boiler with single-reheat process. Thus, the exergy destruction of
the coal combustion process in the SeCO2 boiler with doublereheat process is smaller. The exergy loss of the flue gas in the
SeCO2 boiler system with double-reheat process is larger due to the
existence of more CO in the flue gas. For the air-preheater, the heat
exchange capacity of the AP in the SeCO2 boiler system with
double-reheat process is much larger than that in the SeCO2 boiler
with single-reheat process, which leads to larger exergy destruction in the air-preheating process.
The exergy destruction in the heat exchange process of the flue
Fig. 10. The energy loss distribution in boiler of the power plants.
Items
SeCO2 power plant with
double-reheat process
SeCO2 power plant with
single-reheat process
Exergy input
Coal
Total
Exergy output
Work output
Exergy Destruction
Boiler
Air-preheater
HT
MT
LT
HTR
LTR
Condenser
MC
RC
Flue gas
Total
Exergy efficiency/%
Values/MW
12.975
12.975
Proportion/%
100
100
Values/MW
12.975
12.975
Proportion/%
100
100
6.231
48.02
6.188
47.69
4.465
0.223
0.098
0.074
0.077
0.296
0.088
0.356
0.09
0.08
0.897
6.744
48.02
34.41
1.72
0.76
0.57
0.59
2.28
0.68
2.74
0.69
0.62
6.91
51.98
4.606
0.216
0.103
35.50
1.67
0.79
0.160
0.248
0.090
0.363
0.092ss
0.082
0.827
6.787
47.69
1.23
1.91
0.69
2.80
0.71
0.63
6.38
52.31
gas and the working medium is comprised of the exergy destructions caused by the heat transfer between the flue gas and the
working medium, and the pressure loss of the CO2 flow in the
cooling wall. The former can be analyzed through Fig. 12. The heat
transfer process between the flue gas and the working medium is
divided into 30 units. The heat transfer process adopts countercurrent flow heat exchange. The heat exchange between the flue
gas (the hot flow in Fig. 11) and the working medium (the cold flow
in Fig. 11) in the unit u (u ¼ 1, 2, 3 … 30) can be seen in Fig. 11.
(Thin)u, (Thout)u, (Tcin)u, (Tcout)u are the temperatures of the inlet hot
flow, outlet hot flow, inlet cold flow, and the outlet cold flow,
respectively. Qu is the heat exchange capacity in the unit u.
The temperature of the energy donor is calculated by: ðThÞu ¼
ðThin Þu þðThout Þu
,
2
The temperature of the energy acceptor is calculated by: ðTcÞu ¼
ðTcin Þu þðTcout Þu
,
2
T0
,
ðThÞu
T0
1
.
ðTcÞu
The energy level of the energy donor is: Aed ¼ 1 The energy level of the energy acceptor is: Aea ¼
The exergy destruction caused by the heat transfer in the unit u
can be calculated by Ref. [26]:
DEXLu ¼ ðAed Aea Þ Qu
The exergy destruction caused by the heat transfer process between the flue gas and the working medium in the whole cooling
P
wall of the boiler is: DEXL ¼ 30
u¼1 DEXLu , which forms the shaded
areas in Fig. 13 and Fig. 14.
As can be seen in Figs. 13 and 14, the energy donor is the flue gas,
and the energy acceptor is the working medium. The exergy destructions caused by the heat transfer processes between the flue
gas and the working medium in the whole cooling wall of the boiler
are 1.686 MW and 1.743 MW for the power plants with doublereheat and single-reheat processes, respectively. In the other
hand, as can be seen in Fig. 11, the total exergy destructions in the
heat exchange processes with the working medium are 1.812 MW
and 1.866 MW for the power plants with double-reheat and singlereheat processes, respectively. Thus, the exergy destructions caused
by the pressure losses in the cooling wall of the boiler are 0.126 MW
and 0.123 MW for the power plants with double-reheat and singlereheat processes, respectively. The exergy destructions caused by
heat transfer and pressure losses in the HTRs and LTRs in the power
plants are calculated using the previous method.
Z. Chen et al. / Energy 211 (2020) 118651
Fig. 11. The exergy destructions distribution in boiler systems.
Fig. 12. The exergy destruction caused by heat transfer in the cooling wall of the boiler.
Fig. 13. The exergy destruction caused by heat transfer in the cooling wall of the boiler
in the power plant with double-reheat process.
9
Fig. 14. The exergy destruction caused by heat transfer in the cooling wall of the boiler
in the power plant with single-reheat process.
According to the exergy balances of the power plants, the exergy
distributions of certain processes are presented in Fig. 15 and Fig. 16
for the power plants with double-reheat and single-reheat processes, respectively. The exergy output of the power plants is
comprised of work output, the exergy exhaust, the exergy
destruction in the combustion process, the exergy destruction in
heat transfer process, the exergy destruction caused by pressure
loss, and the exergy destruction in turbo system. The exergy
exhaust includes the exergy of the flue gas at the outlet of the AP,
and the exergy loss in the condenser. The exergy destructions in
heat transfer processes are the sum of the exergy destructions of
the heat transfer processes in the cooling wall of the boiler, the AP,
the HTR, and the LTR. The exergy destructions caused by the
pressure losses exist in the cooling wall of the boiler, the HTR, and
the LTR. The exergy destructions in turbo systems include the
exergy destructions in the turbines and compressors.
As can be seen in Figs. 15 and 16, the work outputs are the
largest exergy outputs, which account for 48.02% and 47.69% of the
total exergy input of the power plants with double-reheat and
single-reheat processes, respectively. The combustion processes
bring the largest exergy destructions in the power plants. For the
SeCO2 coal-fired power plant with double-reheat process, the
exergy destructions in the combustion process, the heat transfer
process, the turbo system, and the exergy destruction caused by
pressure loss account for 20.45%, 17.56%, 3.23%, and 1.08% of the
total exergy input of the power plant. The exergy exhaust accounts
for 9.66% of the total exergy input of the power plant.
For the SeCO2 coal-fired power plant with single-reheat process, the exergy destructions in the combustion process, the heat
Fig. 15. The exergy distribution in the power plant with double-reheat process.
10
Z. Chen et al. / Energy 211 (2020) 118651
Fig. 16. The exergy distribution in the power plant with single-reheat process.
transfer process, the turbo system, and the exergy destruction
caused by pressure loss account for 21.12%, 17.61%, 3.37%, and 1.04%
of the total exergy input of the power plant. The exergy exhaust
accounts for 9.17% of the total exergy input of the power plant.
According to the results about the energy and exergy balances of
the SeCO2 coal-fired power plants, and adopting the EUD methodology, the energy and exergy conversion and transfer routes in
the power plants are illustrated in Fig. 16 hb, heb, hc, and hec are the
boiler efficiency, exergy efficiency of the boiler system, cycle efficiency and exergy efficiency of the power cycle, respectively; Acoal is
the energy level of the fuel coal; hcoal is the enthalpy of the fuel coal.
Acom pro is the energy level of the combustion product; At is the
energy level of the energy being transferred between the boiler
system and the power cycle:
At ¼
heb ecoal
hb hcoal
(17)
In the coal combustion process, the energy donor is the coal
combustion reaction, and the energy acceptor is the combustion
product. Thus, the exergy destruction of the combustion process
can be expressed as follow:
DEXLcom ¼ Acoal Acompro hcoal
(18)
As can be seen in Fig. 17, the energy levels of the coal in both
power plants are 1.02. The energy level of the combustion product
in the SeCO2 boiler with double-reheat process is 0.813, which is
higher than that in the SeCO2 boiler with double-reheat process
(0.806). This is mainly caused by the higher combustion temperature in the SeCO2 boiler with double-reheat process, which leads to
smaller exergy destruction of combustion process according to (18).
After the energy is transferred from the coal combustion reaction to the combustion product, most of the energy (hbhcoal) is
absorbed by the SeCO2 in the cooling wall, super-heaters, and reheaters. The energy level of the energy being transferred between
the boiler system and the power cycle in the power plant with
double-reheat process is 0.64, which is higher than that in the
power plant with single-reheat process (0.63). And the amount of
energy being transferred is close in the two power plants. The
energy with higher energy level in the SeCO2 coal-fired power
plant with double-reheat process produces more electricity
(6.231 MW) than that in the power plant with single-reheat process
(6.188 MW), which results in the higher entire power generation
efficiency of the SeCO2 coal-fired power plant with double-reheat
process (49.06%) than the power plant with single-reheat process
(48.72%).
5. Conclusion
Implementing supercritical CO2 cycle as the bottom cycle for the
Fig. 17. The energy conversion and transfer route in SeCO2 coal-fired power plant.
coal-fired power plant is a promising technology for efficient and
clean utilization of coal. The researches on the SeCO2 coal-fired
power plant with double-reheat and single-reheat processes are
conducted out from aspects of mass, energy and exergy balances.
The SeCO2 coal-fired power plant with double-reheat process has
advantage in power generation efficiency over the power plant
with single-reheat process. However, the advantage is not significant. The power generation efficiency of the power plant with
double-reheat process is 49.06%, which is only 0.34% higher than
that of the power plant with single-reheat process. The exergy
distributions of the power plants are presented. The exergy destructions and losses of the power plants are classified into the
exergy exhaust, the exergy destruction in the combustion process,
the exergy destruction in heat transfer process, the exergy
destruction caused by pressure loss, and the exergy destruction in
turbo system. The combustion processes cause the largest exergy
destructions in the power plants, which account for 20.45% and
21.12% of the total exergy inputs of the power plants with doublereheat and single-reheat processes.
Author contribution
Zhewen Chen: Conceptualization; Data curation; Formal analysis; Funding acquisition; Investigation; Methodology; Project
administration; Resources; Software; Supervision; Validation;
Visualization; Writing - original draft; Writing - review & editing.
Yanjuan Wang: Conceptualization; Software; Investigation. Xiaosong Zhang: Resources; Investigation.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
This study is supported by the National Key Research and
Development Program of China (2017YFB0601801), and the Project
funded by China Postdoctoral Science Foundation.
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