ARTICLE pubs.acs.org/EF
Exergy Analysis of a 600 MWe Oxy-combustion Pulverized-Coal-Fired Power Plant Jie Xiong, Haibo Zhao,* and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074 Hubei, People’s Republic of China ABSTRACT: In an oxy-combustion pulverized-coal-fired power plant (PC), a high CO2 content flue gas could be obtained, and this allows CO2 sequestration in an efficient and energy-saving way. To better understand the thermodynamic characteristics of the oxycombustion process, a detailed exergy analysis of a 600 MWe oxy-combustion PC was carried out. The whole oxy-combustion PC system was divided into four models: boiler, turbines and feedwater heaters (FWHs), air separation unit (ASU), and flue gas treatment unit (FGU). The exergy (including physical exergy and chemical exergy) of each model was obtained, showing that the oxy-combustion boiler could reach higher exergy efficiency than the conventional combustion boiler. There is a significant difference between the exergy efficiencies of furnaces in the two boiler models, and the combustion exergy efficiency in the oxy-combustion furnace is about 4% higher. In each boiler model, the combustion process contributes nearly 60% exergy destruction. The exergy efficiency of the air heater in the oxy-combustion boiler is 10.3% higher than that in the conventional combustion boiler because of the flue gas recycle in the oxy-combustion boiler. The exergy destruction in the turbine and FWH model is just 9.01% of the total fuel exergy, and the turbines contribute about half. The exergy efficiencies of the ASU and FGU processes are 15.84 and 73.45%, respectively. The exergy efficiency of the oxy-combustion system is 37.13%, which is 4.08% lower than that of the conventional system.
1. INTRODUCTION The emission of CO2 in China has reached about 6.55 gigatons (22.3% of the world’s CO2 emission)1 in 2008. Coal-fired power plants contribute most CO2 emission in China, because over 60% of China’s total energy is supplied by coal-fired power plants. CO2 capture and sequestration (CCS) from power plants is a feasible and effective choice, perhaps the only choice, to control the CO2 emission at the present stage,2 especially for China. Oxycombustion (or oxy-fuel) technology is considered a feasible choice to reduce the CO2 emission from the coal-fired power plant through combining a conventional pulverized-coal-fired power plant (PC) with a cryogenic air separation unit (ASU) and a flue gas treatment unit (FGU) (as shown in Figure 1). High-purity oxygen (greater than 95% by volume3) from the ASU, instead of air, is used as the oxidizer in the oxy-combustion technology. About 7080% of the flue gas4,5 is recycled back to the furnace with the oxygen stream, which could keep the combustion temperature inside the furnace within the conventional range by appropriately adjusting the recycle ratio and other parameters, such as the arrangement of the heat-exchange surface in the furnace. The resulting flue gases from the furnace consist primarily of CO2 and water vapor5 because there is no N2 dilution during the fuel combustion. The flue gas from the boiler is then cleaned, dried, and compressed, followed by separation of noncondensable gases (Ar, O2, and N2) from CO2. Then, a 99 mol % CO2 product could be obtained and finally boosted to pipeline pressure.5,6 In comparison to conventional air-fired combustion flue gases, which contain a high N2 fraction and relatively low CO2 fraction (1315% by volume),7,8 the CO2enriched flue gas from the oxy-combustion process is obviously less energy-demanding. r 2011 American Chemical Society
The addition of ASU and FGU processes in oxy-combustion plants leads to a net power output decrease (2530%)35,9 and an electricity generation cost increase (3050%).3,911 However, the economic feasibility of the oxy-combustion technology still holds if the CO2 tax and CO2 sale can be considered11 or policy rewards can be conducted. Although there are presently some economic barriers for the oxycombustion technology, even for any CO2 emission control technology, the oxy-combustion technology demonstrates a high adaptability for existing coal-fired power plants and actually better economic properties than some other CO2 emission control technologies.4,9,10 System process simulation is an effective tool to help us understand the thermodynamic properties and adjust operation conditions of oxy-combustion systems. Moreover, the simulation results are an indispensable basis to conduct a more sophisticated thermodynamic analysis, such as exergy analysis, and even optimization as well as thermoeconomic cost accounting.12 Aspen Plus is considered to be a proper tool to study the oxycombustion technology, and some works about that have been published.5,13,14 The authors have previously finished a process simulation work of an 800 MWe supercritical oxy-combustion PC.5 In that paper, Aspen Plus was used to simulate the whole system. The results of the previous work show that the CO2 concentration in the flue gas from the boiler can be more than 80%, and the purity of the CO2 product from FGU can reach 99%. In addition, some critical parameters (such as recycle ratio, oxygen concentration from ASU, etc.) and different cases were Received: May 12, 2011 Revised: July 7, 2011 Published: July 08, 2011 3854
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Figure 1. Schematic diagram of the oxy-combustion PC system.
discussed. However, to better understand the thermodynamic characteristics of the oxy-combustion technology and find out its source of inefficiency for further optimization, an exergy analysis on the oxy-combustion system is necessary, which activates this work. Exergy is an important concept in the thermodynamics. It is defined as “the maximum theoretical useful work (shaft work or electrical work) obtainable as the systems interact to equilibrium, heat transfer occurring with the environment only”.15,16 Exergy analysis focuses on the quality of energy rather than the quantity of energy. Therefore, the exergy analysis is usually carried out to determine the magnitudes, locations, and types of exergy losses occurring in the system,17 and in this situation, there will be a guideline for reducing the inefficiencies, saving energy consumptions, and optimizing the system. Not surprisingly, the exergy analysis has been widely used to study different thermodynamic systems. Lots of processes, such as coal-fired steam power plants,18 combustors and combustion processes,19 combined cycle power plants,20 fluidized beds,21 and some CO2 emission control processes, such as integrated coal gasification combined cycle (IGCC),22 chemical looping,23 CO2 capture by amine or chilled ammonia scrubbing,24,25 as well as oxy-fuel, have been studied from the viewpoint of the exergy analysis methodology. With respect to the oxy-fuel systems analyzed, there are two different types: firing gaseous fuels, such as natural gas,26
methane, and propane,27 as well as syngas,28 and firing coal.29 Actually, the exergy analysis of an oxy-combustion PC is still very limited. Seepana and Jayanti proposed a “flue gas conditioner” addition to the “conventional oxy-combustion” for high ash coal combustion.29 An energy analysis of these two oxy-combustion systems was performed, and the results were compared. Nevertheless, the exergy analysis was processed in a low disaggregation level [such as ASU, CO2 compression, and high pressure (HP)]. Furthermore, only physical exergies (almost all are power productions and consumptions) were calculated. As known, flue gas from oxy-combustion PC and conventional PC differs greatly in gas composition and flow rate, and chemical components of flue gas are obviously different from these in an atmospheric environment. It is thus necessary to calculate definitely chemical exergy, which takes into consideration the composition difference between the system analyzed and the environment. Therefore, a detailed exergy analysis of a complete oxy-combustion system, especially the boiler model, which was not analyzed in detail before, is required. The simulation study on an 800 MWe supercritical oxy-combustion PC carried out before5 was based on a U.S. Department of Energy (DOE) report,30 which also provides some benchmark results for validating our simulation results from Aspen Plus. In this paper, we focused on a 600 MWe supercritical PC, which is typical in China. First, Aspen Plus, version 7.1, was used to simulate a 3855
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Table 1. Proximate and Ultimate Analyses of Coal (ar Basis) proximate analysis (wt %) 13.8
C
60.51
volatile matter
26.2
H
3.62
ash
11
O
9.94
fixed carbon
49
N
0.7
S
0.43
22768
conventional
ultimate analysis (wt %)
moisture
LHV (kJ/kg)
Table 2. Simulation Results of Two Plants item
In this paper, a 600 MWe supercritical oxy-combustion PC fueled by Shenhua coal was simulated using Aspen Plus. This simulation work is similar to that presented in ref 5, and the simulation results are the basis to perform the exergy analysis, which is the objective of this paper; thus, a brief introduction of the system simulation is given as follows. This system also has four sections: boiler, turbines and feedwater heaters (FWHs), ASU, and FGU. The schematic diagram of this oxy-combustion system is shown in Figure 1. Some initial inputs required in this simulation work can be found in ref 5. The turbine and FWH model in this simulation work is in proportion to that in the 800 MWe system simulated in ref 5. It should be pointed out that the preferred flue gas recycle ratio5 in this boiler model was adjusted to be 0.695, which is a little different to the value given in ref 5 (0.705) because of the different coal samples. With the preferred flue gas recycle ratio, the combustion temperature inside the furnace could be within the conventional range and the coal combustion properties in the oxy-combustion case could be fine.4,31 A detailed sensitivity analysis and discussion of the recycle ratio can also be found in ref 5. The proximate and ultimate analyses of the Shenhua coal are listed in Table 1. In the table, all data are on the as-received (ar) basis, C, H, O, N, and S mean carbon, hydrogen, oxygen, nitrogen, and sulfur in the coal, respectively, and LHV is the lower heating value of the raw coal. Some important simulation results of the oxy-combustion system are listed in Table 2, and more detailed simulation results and discussions can be found in ref 5. Process simulation on a 600 MWe supercritical conventional PC firing Shenhua coal was also carried out (also shown in Table 2).
3. EXERGY ANALYSIS Exergy is a measure of the departure of the system state from the environment state and also the potential of a stream to cause change.16 Exergy is also the measure that coordinates quality with quantity of energy. The total exergy (E) can be divided into four parts: physical exergy (EPH), kinetic exergy (EKN), potential exergy (EPT), and chemical exergy (ECH), which can be described as15,32
PT
ð1Þ
E and E are relating to velocity and elevation, respectively; therefore, in a thermodynamic system, such as the coal-fired
gross system
599.43
599.43
HP (22)
184.49
184.49
IP (23)
175.66
175.66
LP (24)
239.27
239.27
Power Consumed (MW) ASU (1)
102.24
FGU (15) circulating pump (27)
47.13 4.26
4.26
condenser pump (28)
0.75
0.75
total
154.33
5.01
coal feed rate (kg/s)
60.52
61.58
oxygen stoichiometric amount
3.42
3.48
(kmol/s)
2. SYSTEM SIMULATION
KN
combustion
Power Generated (MW)
600 MWe supercritical oxy-combustion PC fueled by Shenhua coal (shown in section 2). Then, a detailed exergy analysis on the system (shown in section 3) was carried out based on the simulation results and exergy calculation methods. At the same time, an exergy analysis of a conventional PC with the same gross power output was also conducted as a comparison case. In the exergy calculation process, physical exergy and chemical exergy of each stream in both systems were calculated and, moreover, material streams were divided to different phases for more accurate results.
E ¼ EPH þ EKN þ EPT þ ECH
oxy-combustion
raw air flowing into the ASU
16.62
(kmol/s) net fuel input (MW)
1377.92 (LHV)
1402.05 (LHV)
gross efficiency (%)
43.50 (LHV)
42.75 (LHV)
net efficiency (%)
32.30 (LHV)
42.40 (LHV)
unit power consumption
0.247
for oxygen production (kWh/kg of O2) unit power consumption for CO2
0.319
emission control (kWh/kg of CO2) CO2 emission control efficiency (%) 96.12
power plants analyzed in this paper, they could not be taken into consideration because the velocity and elevation have negligible changes.32 Just EPH and ECH are left for calculation. EPH is defined as the maximum theoretical useful work obtained as a system passes from its initial state (T and P) to the environment state (T0 and P0);15,32 therefore, it arises from the temperature and pressure differences between the system analyzed and the environment. ECH arises from the departure of the chemical composition of a system from the environment.32 The calculation methods and equations about EPH and ECH are introduced as follows. 3.1. Exergy Calculation Methodology. First of all, an environment state should be defined for the exergy calculation. The state contains not only the temperature and the pressure but also the chemical components of the environment. An appropriate environment model33 is given in Table 3. With the definition of the environment model, each kind of exergy could be calculated by the equations presented below. For the unit physical exergy15 (ePH, kJ/kmol) ePH ¼ Δh T0 Δs ¼ ðh h0 Þ T0 ðs s0 Þ
ð2Þ
Here, h and s mean unit enthalpy (kJ/kmol) and unit entropy (kJ kmol1 K1), respectively. The subscript “0” means the reference state. For the ECH calculation, there are different cases depending upon stream materials. First, stream materials can be differentiated between components in the reference environment model and components not in the reference environment. For a gaseous 3856
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Table 3. Definition of the Environment Model temperature, T0
Table 4. Unit Enthalpies, Unit Entropies, Unit Gibbs Free Energies, and Unit Chemical Exergies of Some Components
298.15 K h0
pressure, P0
component
1 atm
N2 O2
component gaseous phase
mole fraction
N2 O2
condensed phases (T0 and P0)
0.7567 0.2035
state
H2O CaCO3
liquid solid
CaSO4 3 2H2O
solid
Ar
(kJ/kmol)
(kJ kmol
0 0
0 0
0
1
K )
g0
eCH
(kJ/kmol)
(kJ/kmol)
0 0
691.07 3946.50
0
11649.16
CO2
393510
2.88
394370
20107.51
H2O
241810
44.35
228590
8667.46
0
235284.21
H2
0
0
0
0.0303
Ar
0.0091
NO
90250
12.34
86570
88888.78
0.0003
CO
110530
89.28
137150
275354.26
NO2 SO2
33180 296840
60.87 11
51328 300120
55620.03 306269.34
83.08
component in the reference environment model, its unit chemical exergy15 (eCH, kJ/kmol) can be calculated by eCH ¼ RT0 ln x0k k
ð3Þ
in which R is the gas constant, 8.314, and x0k means the mole fraction of the gaseous component in the environment model. For example, for O2, x0k is 0.2035. It should be declared that all of the gaseous components are treated as ideal gases in this paper. For the condensed phase in the environment model, its unit chemical exergy equals 0. On the other hand, for a component not in the environment model, there are also two different cases: elementary substance and compound. In this situation, to calculate the chemical exergy of an elementary substance Y, the most stable compound that contains this element, YyAaBbCc, should be found and used. Because eCH of this most stable compound equals 0, eCH of this elementary substance can be calculated by15 1 eCH ðYÞ ¼ ðg0 ðY y A a Bb Cc Þ þ aeCH ðAÞ y þ beCH ðBÞ þ ceCH ðCÞÞ
ð4Þ
in which g is unit Gibbs free energy (kJ/kmol). Furthermore, for the chemical exergy calculation of a compound, the unit chemical exergies of all of the elements contained in the compound should be already known. The chemical exergy of the compound AaBbCcDd is then calculated using eqs 3 and 4 as follows: e
1
H2O CO2
CH
s0
ðA a Bb Cc Dd Þ ¼ g0 ðA a Bb Cc Dd Þ þ ae þ ce
CH
ðCÞ þ de
CH
CH
ðDÞ
ðAÞ þ be
CH
ðBÞ
eCH ¼
∑xk eCH k
∑
þ RT0 xk ln xk
395720
370950
237412.59
0
0
0
410531.01
S(s)
0
0
0
602442.84
in which xk means the mole fraction of the gaseous component in the mixture stream. For a non-gaseous mixture, its eCH is the weighted sum of unit chemical exergies of all components in the mixture. There is another special material, coal. The chemical exergy calculation of coal is quite complex, and some empirical equations exist. The detailed chemical exergy calculation process of coal can be found in refs 15 and 34, which was adopted in this paper. An introduction of the calculation method is given below. The coal combustion process could be expressed as ðcC þ hH þ oO þ nN þ sSÞ þ vO2 O2 f vCO2 CO2 þ vH2 O H2 OðlÞ þ vSO2 SO2 þ vN2 N2
ð7Þ
in which c, h, o, n, and s (kmol/kg) mean the mole amount of elements C, H, O, N, and S in unit mass coal under the dry and ash-free (DAF) basis, respectively. On the basis of the equation equilibrium rule, we can obtain 1 1 vCO2 ¼ c, vH2 O ¼ h, vSO2 ¼ s, vN2 ¼ n, 2 2 1 1 v O2 ¼ c þ h þ s o 4 2
ð8Þ
Thus, the unit chemical exergy of coal [kJ/kg (DAF)] can be calculated by eCH DAF ¼ HHV DAF T0 ðsDAF þ vO2 sO2 vCO2 sCO2
ð5Þ
Therefore, eCH of any component can be obtained using the methods described above. Table 4 shows the calculated unit chemical exergies of some components relating to the systems analyzed in this paper. In addition, some basic data about h0, s0, and g0 from the Aspen Plus database are also given in Table 4. However, in an exergy calculation process, streams in the system often contain many different components; that is to say, they are mixtures. For the chemical exergy calculation of a mixture material stream, the gaseous mixture and non-gaseous mixture are treated differently. In detail, for a gaseous mixture, its eCH can be calculated by15
SO3 C(s)
vH2 O sH2 O vSO2 sSO2 vN2 sN2 Þ þ ðvCO2 eCH CO2 CH CH CH þ vH2 O eCH H2 O þ vSO2 eSO2 þ vN2 eN2 vO2 eO2 Þ
ð9Þ
in which HHVDAF [kJ/kg (DAF)] is the higher heating value of coal under the DAF basis and sDAF is the unit standard entropy of coal [kJ kg1 (DAF) K1] under the DAF basis, which can be calculated by15,35 h sDAF ¼ c 37:1653 31:4767 exp 0:564682 c þ n o n s þ 54:3111 þ 44:6712 þ 20:1145 c þ n c þ n c þ n
ð6Þ
ð10Þ 3857
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Figure 2. Schematic diagrams of boiler models.
If the HHVDAF value in eq 9 is not given, it can be estimated by the following empirical equation:15,35
Table 5. Exergy Analysis Results of Boiler Models oxy-combustion
HHV DAF ¼ ð152:19H þ 98:767ÞðC=3 þ H ðO SÞ=8Þ
EPH (kW)
ð11Þ In which C, H, O, and S are the mass fractions of elements C, H, O, and S in the coal under the DAF basis. Therefore, eCH of the Shenhua coal can be calculated to be 24 686.6 kJ/kg (ar). It is worth noting that eCH of a coal sample nearly equals its HHV.15,34 3.2. Exergy Analysis Results. To obtain detailed exergy analysis results, the whole oxy-combustion system was divided into four different models: boiler, turbines and FWHs, ASU, and FGU. The oxy-combustion system includes all four models, whereas the conventional combustion system just includes the former two. However, there is almost no difference between the turbine and FWH models for the oxy-combustion system and the conventional combustion system; therefore, a comparison will only be given to the two boiler models. After the exergy analysis of each model was conducted, an exergy analysis was also carried out for the two whole systems. The exergy analysis results for each model are shown as follows. 3.2.1. Boiler Model. In the oxy-combustion system, high-purity oxygen, instead of air, is used in the coal combustion process. Also, about 70% flue gas is recycled back to the furnace. The schematic diagrams of the two boiler models are shown in Figure 2. Because coal is combusted with oxygen in the furnace (FUR) and the ECH of coal is converted to be EPH as radiation heat and convection heat, then the heat is absorbed by the feedwater or the reheat steam in several heat exchangers (HEXs). They are two different processes; thus, each boiler model in this section was divided into FUR and HEXs. Moreover, the HEXs include a convective superheater (CSH), a radiation superheater (RSH), a reheater (RH), an economizer (ECO), a water wall (WW), and an air heater (AH). On the basis of the simulation results and exergy calculation methods introduced above, a detailed exergy analysis work was performed on the two boiler models and the results are all listed in Table 5. The items in the table correspond to names defined in Figure 2. The results in Table 5 provide a basis to calculate exergy destruction (ED) and exergy loss (EL). Exergy destruction arises
item C1 G1 G2
conventional combustion EPH (kW)
gaseous
solid
ECH
gaseous
solid
ECH
part
part
(kW)
part
part
(kW)
1494033.03 12861.07
0 30537.42
130749.82
1520126.77 1398.47
0 28601.96
1398.47
G3
398002.71 6501.81
195669.38 431057.67 6532.55
51514.14
G4
304787.51 5141.59
195669.38 311405.84 4886.98
51514.14
G5
130424.25 2593.15
195669.38 139110.64 2543.46
51514.14
G6
60372.72 1484.79
195669.38
69318.79 1527.08
51514.14
G7
17288.39
195669.38
19054.15
51514.14
G8 G9
17288.39 539.25
195669.38 58490.19
19054.15 3474.69
G11
4817.03
193353.19
G12
1228.78
133280.94
G13
1433.95
130749.82
630.79
G10
641.96
51514.14 49174.34
2305.99
Q1
242083.71
Q2
378643.76
231153.90 361548.41
Rad ECOA
647940.99 59707.84
618687.17 59795.66
WWA
263001.91
250951.20
RSHA
181736.96
171866.61
CSHA
75083.94
97004.99
RHA
149677.76
149676.27
AHA
29103.47
28601.96
ECOS
71159.89
70808.24
WWS RSHS
378643.76 242083.71
361548.41 231153.90
CSHS
94575.42
121297.40
RHS
176911.70
174638.72
AHS
43938.33
51149.75
from irreversibility occurring in a system or units.15,16 The exergy loss for a boiler includes three parts: exergy in the flue gas flowing 3858
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Table 6. Results about Exergy Destruction and Exergy Loss for Boiler Models (kW) oxy-combustion ED,i(EL,i)
ED,i/ED,T (%)a
ED,i(EL,i)/EF,B (%)b
FUR
407205.38
58.89
27.02
WW
115641.85
16.72
7.67
RSH
60346.76
8.73
4.00
CSH
19491.48
2.82
RH
27233.94
ECO
ED,i/ED,T (%)
ED,i(EL,i)/EF,B (%)
442335.67
61.99
29.07
110597.21
15.50
7.27
59287.30
8.31
3.90
1.29
24292.41
3.40
1.60
3.94
1.81
24962.45
3.50
1.64
11452.04
1.66
0.76
11012.58
1.54
0.72
AH
14834.86
2.15
0.98
22547.79
3.16
1.48
ESP FGD
630.79 14787.55
0.09 2.14
0.04 0.98
641.96 17919.26
0.09 2.51
0.04 1.18
DRY
4631.06
0.67
0.31
MIX
15187.02
2.20
1.01
ED,T
691442.74
100.00
45.89
713596.63
100.00
46.90
ED,T,HEX
249000.94
36.01
16.52
252699.73
35.41
16.61
unit
ED,i(EL,i)
EL,FG
59029.44
3.92
52649.03
EL,Rad
27213.52
1.81
25984.86
ED,T + EL,T EF,B a
conventional combustion
777685.71
51.61
3.46 1.71 792230.51
1506894.10
52.07 1521525.24
Ratio of ED to ED,T, named as yD,a. b Ratio of ED or EL to EF provided, named as yD,b or yL.
Table 7. FP Definition of the Boiler Model
Exergy efficiency (ηex) of each unit or even the whole system can also be calculated. ηex is also called second-law efficiency, effectiveness, or rational efficiency and is usually defined as used exergy divided by provided exergy.15,36 Because there are many kinds of definitions of the “used exergy” and “provided exergy”, there are lots of different exergy efficiency definitions.36 In this paper, ηex is defined as the ratio of product (P) to fuel (F), which is illustrated as eq 1237 ηex ¼ EP =EF
EP
(E6 E5) + (E8 E7)
EF
(E1 + E2) (E3 + E4 + E9)
into the environment, exergy in the ash, and radiation exergy loss. The calculation results about ED and EL are given in Table 6, which show that the yD,b of FUR in the oxy-combustion system is 2.05% lower than that in the conventional system; the sum of the yD,b of all HEXs (WW, RSH, CSH, RH, ECO, and AH) in the oxy-combustion system is 0.09% lower (almost equal) than those in the conventional system. Moreover, the yD,b,T decreases 1.01%, and the sum of yD,b,T and yL,T decreases 0.46% in the oxycombustion boiler model in comparison to that in the conventional boiler model. Furthermore, the two boiler models have a similar ED distribution rule. FUR is the main exergy destruction unit, taking about 60%, that is because the combustion process is high irreversible. HEXs take about 36%, and others take just a little. It is worth noting that the yD,b of the MIX unit is about 1% because of the mixture process of oxygen and recycled flue gas; therefore, reducing this ED should be kept in mind during the operation.
ð12Þ
where EP and EF are exergies from product and fuel, respectively. Usually, each device has its own productive purpose, such as steam for the boiler and power for the generator. The productive purpose of a process device measured in terms of exergy is named “product”, and the consumed exergy flow to create the product is “fuel”.38,39 Therefore, it is important to define the F and P for each unit in the system, and FP definitions about some important thermodynamic devices can be found in refs 15 and 40. For a compressor, pump, or fan, the power consumption is the fuel, but for a turbine or expander, the power generated is the product. For a HEX, there are two cases. If the HEX is used for heating, then the exergy decrease of the hot steam is the fuel and the exergy increase of the cold steam is the product, whereas if the HEX is used for cooling, the definition is opposite.15 The FP definition of a boiler is more complicated, which is given in Table 7.15 Some ηex definitions about units in this paper are described as follows: FUR:
CH ηex, FUR ¼ ðEPH Rad þ EG3 Þ=ðEG2 þ EC1 Þ
ð13Þ
HEX:
PH ηex, HEX ¼ EPH A =ES
ð14Þ
CH boiler: ηex, B ¼ EPH A, FW =ðEC1 þ EG1 EL Þ
3859
ð15Þ
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Table 9. Results about Exergy Destruction and Exergy Loss for the Turbine and FWH Model (kW) ED,i
ED,i/ED,T (%)
ED,i/EF,tur (%)
turbine
64915.03
48.16
8.90
FWH condenser
14897.79 33826.14
11.05 25.09
2.04 4.64
generator
8634.48
6.41
1.18
deaerator
2009.83
1.49
0.28
BFPT
5435.60
4.03
0.75
Else
5075.52
3.77
0.70
ED,T
134794.40
100.00
18.49
unit
EF,tur
Figure 3. Exergy calculation results for two boiler models.
Table 8. Energy Quality Coefficient Calculation Results for Boiler Models energy quality coefficient (λ) oxy-combustion HEX unit
conventional combustion
hot side
cold side
hot side
cold side
ECO WW
0.588 0.783
0.493 0.544
0.584 0.783
0.493 0.543
RSH
0.783
0.588
0.783
0.582
CSH
0.766
0.608
0.764
0.611
RH
0.704
0.596
0.695
0.596
AH
0.442
0.293
0.445
0.249
In eq 15, EL includes exergy losses of flue gas, ash, and radiation and subscript “FW” means feedwater. On the basis of the data shown in Table 5 and ηex calculation equations defined above, the ηex calculation results for the two boiler models are given in Figure 3. The results in Figure 3 show that ηex,FUR in the oxy-combustion system is about 4% higher than that in the conventional combustion system, which should be the primary reason why ηex of the oxy-combustion boiler is 0.8% higher, because exergy efficiencies of the two FGFW heatexchange processes are nearly equivalent. The FGFW heat-exchange process relates five HEXs: WW, RSH, CSH, RH, and ECO, and WW has the lowest exergy efficiency among the five HEXs. Moreover, the exergy efficiency of AH is even lower than that of WW. The heat-exchange process in AH occurs between flue gas and inlet gas. ηex,AH in the oxycombustion system is 10.3% higher than that in the conventional combustion system. To further understand how the differences about the exergy efficiencies happen, a calculation about energy quality coefficients was also performed in this paper, and the results are shown in Table 8. The definition of energy quality coefficient (λ) is described as eq 16, in which H means enthalpy (kJ). λ ¼ ΔE=ΔH
ð16Þ
The results in Table 8 show that, for the FGFW heat-exchange process in each system, λECO is the lowest on the cold side (feedwater) because the cold feedwater enters the ECO first. On
729208.80
the other hand, λECO is also the lowest on the hot side (flue gas) because the hot flue gas enters the ECO last; thus, the ratio of λECO, cold to λECO,hot is not the lowest. It is worth mentioning that, the lower the ratio of λcold to λhot, the stronger the irreversibility in that device and the lower its ηex. Therefore, although λECO is lower than λWW on the cold side, ηex,ECO is not the lowest because λECO is much lower than λWW on the hot side. However, there is a significant difference between the results of the two AHs. λcold,AH in the oxy-combustion boiler model is 0.044 higher than that in the conventional combustion boiler model. This is because there is about 70% flue gas recycled in the oxy-combustion system; thus, much heat in the recycled flue gas is used, and λcold,AH is improved. However, in general, the convective heat-exchange properties in the two boiler models are similar because the biggest difference between the two boiler models is the coal combustion property, the flue gas composition5 (chemical exergy), as well as the flue gas recycle, but only the flue gas recycle affects the heat exchange in AH. 3.2.2. Turbine and FWH Model. An exergy analysis on the turbine and FWH model was conducted in this section. The structure of the turbine and FWH model can be found in Figure 1. The turbine and FWH model is a Rankine cycle, and heat is converted into work in this cycle. The high pressure and temperature steam expands through the turbines to generate power, and then the wet steam enters the condenser, where it is condensed to become a saturated water. The water is pumped from low to high pressure and heated by the steam extracted from the turbines. Finally, the feedwater enters the boiler to become the dry saturated steam and completes one cycle. Because there are so many material and energy steams in this model, instead of listing all of the exergy calculation results, only the results of exergy destructions and exergy losses are given in Table 9. It should be mentioned that water or steam in this model is just recycled in the pipeline; therefore, only physical exergies about the water/steam were calculated. Furthermore, as mentioned above, because there is almost no difference between the turbine and FWH models for the oxy-combustion system and the conventional combustion system, the results in Table 9 are for both systems. The results in Table 9 show that ηex of the turbine and FWH model could reach up to 81.51%. Because the expansion process occurring in the turbine is high irreversible, in the turbine and FWH model, turbines contribute most ED, about a half of the total ED, and the condenser and FWHs follow (in that order). Exergy destructions of other units are slight. 3.2.3. ASU Model. The ASU model analyzed in this section and the FGU model analyzed in the next section are important units 3860
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Table 11. Results about Exergy Destruction and Exergy Loss for the ASU Model (kW) ED,i(EL,i)
ED,i/ED,T (%)
ED,i(EL,i)/EF,ASU (%)
MCOM
27042.79
40.72
26.12
HEX DC
11768.54 26618.95
17.72 40.08
11.37 25.71
unit
Figure 4. Schematic diagram of the ASU model.
valve
982.43
1.48
0.95
ED,T
66412.71
100.00
64.15
EL
20718.74
ED,T + EL EF,ASU
20.01 87131.45
84.16 103527.41
Table 10. Exergy Analysis Result of the ASU Model stream air C air airincol
EPH (kW)
ECH (kW)
0.00
1284.91
75199.71 188643.73
1284.91 1284.91
N2
79073.45
7784.69
O2
63590.49
12861.07
N2 out
12933.98
7784.76
O2 out
3534.97
12861.07
62608.06
12861.07
O2 V W
102242.50
in the oxy-combustion system. However, they are not included in the conventional combustion system. A detailed exergy analysis about the ASU model, which is shown in Figure 4, was performed in this section, and the mole fraction of the O2 product from the ASU is designed to be 95%, which is suggested by many researchers3 and also discussed in ref 5. In the ASU model, there are four units: multi-stage compressor (MCOM), heat exchanger (HEX), distillation column (DC), and expansion valve (valve). The exergy analysis results of the ASU model are given in Tables 10 and 11. The results in Table 11 show that MCOM and DC have high ED values, each about 40% of the ED,T value. ED,T can reach 64.15% of EF,ASU. The power consumption is the major part of the ASU EF,ASU value. In addition, yL is about 20%, but this value is affected by the definition of EL in the ASU model. In this paper, EL of the ASU model is defined to be the exergy contained in the “N2 out” stream and the exergy contained in the “O2 out” stream is defined to be P of the ASU. In this case, ηex,ASU is very low, just 15.84%. 3.2.4. FGU Model. In this section, the exergy analysis was performed on the FGU model, which is shown in Figure 5. There are five units in the FGU model: clean, compression, and cooling process (CCC), HEX, DC, and two valves. In the oxy-combustion system, the CO2 content in the flue gas is very high; therefore, a high-purity CO2 product could be easily obtained by some physical processes. In this FGU model, the flue gas is cleaned, compressed, dried, and distilled and, finally, a 99 mol % purity CO2 product is obtained. The exergy analysis results about the FGU model are given in Tables 12 and 13. During the ED and EL calculation, the clean process, the compression process, and the cooling process were combined (CCC). The results in Tables 11 and 13 indicate that there is a little difference between the results of the ASU model and the FGU model. In the FGU model, the CCC process contributes most ED, about 73%, but ED,DC is much lower. That is because
Figure 5. Schematic diagram of the FGU model.
Table 12. Exergy Analysis Result of the FGU Model EPH (kW)
ECH (kW)
flue gas
539.25
58490.19
dry gas
28783.25
58487.44
airincol
36464.94
58487.44
4869.01
2305.99
stream
gas gas V gas out
835.62
2305.99
0.00
2305.99
CO2
28373.91
58789.93
CO2 V CO2 out
28222.03 19183.82
58789.93 58789.93
W
47127.35
the CO2 content in the flue gas is already very high (above 80 mol %)5 and the distillation process of CO2 to other impurities is much easier than that in the air separation process. ED of valve2 is also high because of the intense expansion process. In this exergy analysis, EPH contained in “gas out” is nearly zero; therefore, it is ignored. ECH contained in “gas out” is defined to be EL. Also, the power consumption is the major part of EF,FGU. On the other hand, because there is a high CO2 content of flue gas flowing into the FGU model, although the EF,FGU and the EF, ASU are almost equivalent, the power fraction in EF,FGU is much lower. Moreover, the power is the primary energy consumption for ASU or FGU; therefore, ηex,FGU (73.45%) is much higher than that of the ASU model. Moreover, this result also indicates that this FGU process is proper for flue gas treating. 3.2.5. Whole System. In this section, an exergy analysis from the viewpoint of the whole system was conducted. First of all, on the basis of the data given above and eqs 17 and 18, ηex of the oxycombustion system is calculated to be 37.13%, which is 4.08% 3861
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Table 13. Results about Exergy Destruction and Exergy Loss for the FGU Model (kW) ED,i(EL,i)
ED,i/ED,T (%)
ED,i(EL,i)/EF,FGU (%)
CCC
18886.10
72.98
17.79
HEX DC
2192.15 613.55
8.47 2.37
2.07 0.58 0.14
unit
valve1
151.87
0.59
valve2
4033.39
15.59
ED,T EL ED,T + EL
25877.06
3.80
100
24.38
2305.99
2.17 28183.05
EF,FGU
26.55 106156.79
Figure 6. Distributions of exergy destructions and exergy losses in two systems.
lower than that of the conventional system mainly because of the exergy destructions occurring in the ASU and FGU models. It should be emphasized that the exergies contained in the CO2 product from FGU, ECO2 out, is also defined as a part of the product of the oxy-combustion plant. ηex, oxy ¼ ðWnet, oxy þ ECO2
CH out Þ=ðEC1, oxy
þ Eair EL, oxy Þ
ð17Þ ηex, con ¼ Wnet, con =ðECH C1, con þ EG1, con EL, con Þ
ð18Þ
To describe the distributions of exergy destructions and exergy losses in the two systems, two pie pictures are shown in Figure 6. The values in the pictures mean the ratios of exergy destructions or exergy losses in some important units to the total fuel exergy of each system, respectively. Moreover, the “product” in each picture means the product of that system, and the “others in boiler” item in the oxy-combustion system does not include the exergy loss of flue gas from the boiler anymore because it is a part of the fuel of the FGU model and does not loss from the viewpoint of the whole system. Thus, in this situation, the percent of the “others in boiler” item in the oxy-combustion system is a little lower than that in the conventional system. ED of the turbine and FWH model is not very high, just about 9% of the total EF. In the oxy-combustion system, ASU and FGU systems contribute much exergy destructions/losses, which are 7.71% of the total EF. It is worth mentioning that the condenser contributes most energy loss in a coal-fired power plant from the viewpoint of the first law of thermodynamics. However, ED of the condenser is just about 2.22% of the total EF if the energy quality is considered, viz., from the viewpoint of the second law of thermodynamics. This also shows the importance of the exergy analysis. Rosen and Tang41 performed an exergy analysis on a steam power plant, and the results show that the boiler is the most inefficient device and the combustion process and heat-transfer process in the boiler account for nearly 43 and 40% of the plant exergy consumption, respectively, while the remainder occurs in turbines (about 10%), condenser (about 6%), and FWHs (about 2%). Aijundi42 also carried out an exergy analysis on the Al-Hussein steam power plant in Jordan, and the results show that the boiler system accounts for 77% of the total exergy destruction, the turbine accounts for 13%, and the condenser accounts for 9%. Moreover, another exergy analysis work conducted on a steam power plant43 presented a similar result that the exergy loss and exergy destruction occurring in the boiler system account for 83% of the total exergy destruction (loss) in the plant and 52% of the chemical exergy entering the plant. The results obtained in this paper coincide with these results.
4. CONCLUSION A detailed exergy analysis of a 600 MWe oxy-combustion PC and a conventional PC was performed, and the exergy analysis results for the two boiler models were compared. The results for the boiler models show that the furnace exergy efficiency in the oxy-combustion system is about 4% higher than that in the conventional combustion system, which should be the primary reason why the exergy efficiency of the oxy-combustion boiler is 0.8% higher. For each boiler model, the combustion process contributes the most exergy destruction, about 60%, and followed by the water wall, about 16%. Total exergy destructions of heat exchangers in two models are nearly equivalent to each other (about 36%). Moreover, the water wall and air heater in each boiler model have very low exergy efficiencies, but the exergy efficiency of the air heater in the oxy-combustion system is 10.3% higher than that in the conventional combustion system. The results for the turbine and FWH model indicate that this process is not a high-exergy-destructing process. The exergy destruction of this model is just 9.01% of the total fuel exergy of the oxy-combustion system, and its exergy efficiency could reach up to 81.51%. Turbines contribute about 50% exergy destruction, followed by condenser and feedwater heaters. The results for the 3862
dx.doi.org/10.1021/ef200702k |Energy Fuels 2011, 25, 3854–3864
Energy & Fuels ASU model and FGU model reveal that multi-stage compressors are the highest exergy-destructing units in both models. In the ASU model, the distillation column contributes similar exergy destruction (about 40%) as the multi-stage compressor. However, in the FGU distillation column, the exergy destruction ratio is much lower compared to that in the ASU model. Finally, the ASU model has very low exergy efficiency (15.84%), but the exergy efficiency of the FGU model is much higher (73.45%) because of the easier distillation process and the higher chemical exergy contained in the inlet gas. The exergy efficiency of the oxy-combustion system is 37.13%, which is 4.08% lower than that of the conventional system. The exergy analysis results obtained in this paper provide a basis to carry out a thermoeconomic cost account or optimization of a complete oxy-combustion PC.
ARTICLE
PH = physical PT = potential Rad = total radiation RH = reheater RSH = radiation superheater SH = superheater T = total WW = water wall Scalars
’ AUTHOR INFORMATION
H and h = enthalpy (kJ) and unit enthalpy (kJ/kmol) E and e = exergy (kJ) and unit exergy (kJ/kmol) F = fuel g = unit Gibbs free energy (kJ/kmol) P = product s = unit entropy (kJ kmol1 K1) T = temperature
Corresponding Author
Greek Letters
*Telephone: +86-27-8754-4779. Fax: +86-27-8754-5526. E-mail:
[email protected].
η = efficiency λ = energy quality coefficient Subscripts
’ NOMENCLATURE
0 = reference state A = absorbed ar = as-received basis B = boiler con = conventional combustion plant D = destruction DAF = dry and ash free ex = exergy L = loss oxy = oxy-combustion plant S = supplied
Abbreviations
Superscripts
AH = air heater ASU = air separation unit BFPT = feed water pump turbine CH = chemical CCC = clean, compression, and cooling process CCS = CO2 capture and sequestration CSH = convective superheater DC = distillation column DRY = dryer ECO = economizer ESP = electrostatic precipitator FG = flue gas FGD = flue gas desulfurization FGU = flue gas treatment unit FUR = furnace FW = feedwater FWH = feedwater heater HEX = heat exchanger HHV = higher heating value HP, IP, and LP = high pressure, intermediate pressure, and low pressure IGCC = integrated coal gasification combined cycle KN = kinetic LHV = lower heating value MCOM = multi-stage compressor MIX = mixer PC = pulverized-coal-fired power plant
0 = reference state
’ ACKNOWLEDGMENT The authors were supported by the National Key Basic Research and Development Program (Grant 2011CB707300), the National Natural Science Foundation (Grants 50936001 and 50721005), and the New Century Excellent Talents in University (Grant NECT-10-0395) for funds. Special thanks should be given to Alstom Power Boiler R&D Execution (Windsor, CT) for providing a 1 year internship and the Aspen Plus software.
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