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High efficiency power plant with liquefied natural gas cold energy utilization
Journal of the Energy Institute xxx (2014) 1–10
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High efficiency power plant with liquefied natural gas cold energy utilization M. Romero Gómez a, *, R. Ferreiro Garcia b, J. Carbia Carril a, J. Romero Gómez a a b
Department of Energy and Marine Propulsion, University of A Coruña, Paseo de Ronda 51, 15 011 A Coruña, Spain Department of Industrial Engineering, University of A Coruña, Paseo de Ronda 51, 15 011 A Coruña, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Accepted 30 April 2013
This article proposes a novel power plant comprising a closed Brayton cycle (CBC) and a Rankine cycle (RC) coupled in series with respect to the flue gases instead of a conventional combined cycle, where the cold energy of the LNG is used to cool the CBC compressor suction. The research study focuses on finding working fluids best suited to the proposed CBC–RC plant and on achieving high efficiency. The proposed working fluids that fulfil the requirements for the CBC are He, N2 and for the RC are CO2, ammonia, ethanol or water. An analysis of the power plant using different working fluids is carried out and it is ascertained that the best efficiency conditions for the CBC are achieved with He and CO2 for the RC. As a result, a thermal efficiency of 67$60%, an overall efficiency of 55$13% and a specific power of 2$465 MW/ (kg s1 LNG) is achieved. Ó 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Closed Brayton cycle LNG Organic working fluids Rankine cycle Series thermal cycles
1. Introduction The objective of this study is to determine the efficiency of a power plant consisting of a closed Brayton cycle (CBC) and a Rankine cycle (RC), innovatively arranged in series, in relation to the power source whilst exploiting the cold energy generated in the regasification process of liquefied natural gas (LNG). The power plant features, as an energy source, a natural gas (NG) flue system where the gases firstly yield heat to the CBC and then to the RC. The CBC can use N2 or He as the working fluid and the RC can either be organic or not. The combination of efficiently power conversion and the regasification of LNG have been under study in recent years [1–5], due to rising fuel prices and environmental restrictions. In power plants, LNG is used complementary as a heat sink to decrease the minimum cycle temperature and thereby increase Carnot efficiency. Studies such as those reflected in Refs. [6–9] show the improvement in the efficiency of organic Rankine cycles (ORCs) by using the LNG vaporisation energy to condense the working fluid, rather than doing it conventionally with water or air. Although the ORCs are a well known option for generating energy associated with LNG regasification, these systems are limited by the physical properties of the working fluid, which should be thermally stable at high temperatures and condensed at low temperatures without issues of freezing. It is for these reasons that ORCs often tend to use low thermal quality heat such as in a waste incineration plant [9]. CBCs are an alternative to ORCs for taking advantage of the cold energy from the LNG [10–13]. In these types of cycles LNG is used to cool the gas to cryogenic temperatures at the compressor inlet. This achieves a decrease in the specific volume of gas and thereby reduces the compression work, implying an increase in the cycle’s net power. By using N2 or He as the working fluid (WF), temperatures of around 1000 C can be reached. Both N2 as well He are stable at very high temperatures. According to Ref. [14], N2 has a range of application of between 210 and 1726 C and He of 271 to 1225 C. Closed loop gas turbines using of He as the working fluid, are very common for generating electricity in nuclear plants. McDonald [15] has been carrying out a study of the evolution of this type of turbine since the early 40’s to date. One of the major advances has been the increase the turbine inlet temperature (TIT). At the beginning it was of around 600 C whereas nowadays can reach 1000 C with the use of turbine blade cooling and high temperature alloys such as titanium, zirconium and molybdenum.
* Corresponding author. Tel.: þ34 981 167000 4226; fax: þ34 981 167100. E-mail addresses: [email protected] (M. Romero Gómez), [email protected] (R.F. Garcia), [email protected] (J. Carbia Carril), [email protected] (J. Romero Gómez). http://dx.doi.org/10.1016/j.joei.2014.02.007 1743-9671/Ó 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
List of symbols h h HX ṁ mass np nr p q r T w x
specific enthalpy, kJ kg1 specific enthalpy, kJ kmol1 heat exchanger flow rate, kg s1 moles of combustion products moles of combustion reactants pressure, bar specific heat, kJ kg1 compressor pressure ratio temperature, C specific work, kJ kg1 excess air ratio, %
y
fraction of steam extracted
b air–fuel ratio h thermal efficiency hov overall efficiency hmec mechanical efficiency halt alternator efficiency hcomb combustion efficiency Subscripts bl blower f fuel g flue gas i inlet o outlet
In the power plant proposed in this paper, comprising a CBC and an RC arranged in series with respect to the combustion gases, it is achieved to unify in one single plant the advantages offered by the high temperature of the CBC’s working fluid as well as its refrigeration to cryogenic temperatures at the compressor suction, with the cooling energy of the LNG. With an RC, (organic or not), the system’s overall efficiency is improved to make the most of the energy available in the flue gases. The present paper is organised as follows: the first section describes and analyses the CBC, obtaining as a result a higher thermal efficiency but low overall efficiency. Next, the proposed CBC–RC power plant is analysed for different working fluids: N2 and He in the case of CBC; and water, ammonia, ethane, and CO2 for the RC, followed by a discussion of results and finally concluding with the outcomes of the conducted study. 2. High temperature CBC and using LNG as a heat sink In conventional closed cycle gas turbine plants, the maximum ratio between the maximum and minimum temperature is of around 3$4 [16]. However, for the proposed high TIT using LNG as a heat sink, the ratio reaches 8$3, which implies a significant decrease of the compression work and a significant increase in efficiency. The flue gases generated in a combustion chamber are used as an energy source. The temperature of the gases is controlled by the air– fuel ratio and is set at 1300 C. Fig. 1 shows a schematic of a regenerative CBC power plant with two turbine stages and a NG based combustion system. The plant data are displayed in Table 1. 2.1. CBC efficiency In any power plant a distinction must be made between the thermal efficiency of the thermodynamic cycle and the overall efficiency of the plant. The thermal efficiency is defined as the ratio of the net power output to the heat input which is computed according to the following expressions: Specific net work:
wnetCBC ¼ h3 h4 þ h5 h6 ðh2 h1 Þ
(1)
Heat input:
qiCBC ¼ h3 h8 þ h5 h4
(2)
Energy balance in the regenerator:
h6 h7 ¼ h8 h2
(3)
In accordance with the T-s diagram shown in Fig. 1(b) and in order to satisfy the first and second law of thermodynamics, it must be fulfilled that T8 < T6 and T7 > T2. As a result of these restrictions, it is of interest to establish a compromise between heat transfer flow and the size of the regenerator. For this, a temperature difference (T7 T2) of 10 C is set to enable an adequate heat flow transfer. Thus, regenerator effectiveness is defined by: 3
¼
h8 h2 qmax
(4)
where maximum transferred heat exchange is given by the following expression:
qmax ¼ h6 h7 assuming T7 ¼ T2 Energy balance of the LNG heat exchanger: Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
(13)
Blower (12)
HX
(15)
(14)
(3)
(4)
3
Fuel
(5) T [°C]
T
C
G
T
3
1000
5
(6)
(8)
8
4
6
(2) (1)
Regenerator 2 -120
LNG (9)
(7)
HX
7
1
NG (10)
(11)
(a)
Entropy
(b)
Fig. 1. CBC power plant using LNG as heat sink: (a) basic plant structure, (b) T–s diagram.
_ WF ðh7 h1 Þ _ LNG ðh11 h10 Þ ¼ m m
(5)
Thermal efficiency:
h¼
wnetCBC qiCBC
(6)
On the other hand, the overall cycle efficiency is the ratio net grid power to the amount of thermal energy really needed to supply the plant power. Therefore, the energy source used and its degree of exploitation will restrict the plant’s efficiency. Assuming the considerations in Table 1, the overall efficiency is defined by equation (7):
hov ¼
_ LNG wLNGpp m _ air wbl _ WF wnetCBC m m hmec halt _ f LHV m
(7)
where the specific work of the LNG pump and the blower are calculated by equations (8) and (9) respectively.
wLNGpp ¼ h10 h9
(8)
wbl ¼ h13 h12
(9)
To determine the mass flow rates of the fuel and the combustion air, it is necessary to know the flow of gases that must be generated to heat the working fluid up to the TIT. An energy balance of the CBC heat exchanger is performed as follows:
_ WF ðh3 h8 þ h5 h4 Þ _ g ðh14 h15 Þ ¼ m m
(10)
Table 1 Main assumptions for the calculations of CBC power plant. Parameters
Value
Temperature combustion gases TIT Compression inlet temperature Compressor isentropic efficiency Turbine isentropic efficiency Blower isentropic efficiency LNG Pump hydraulic efficiency Minimum temperature difference in the regenerator Minimum temperature difference in the Brayton heat exchanger Combustion efficiency Fuel temperature inlet Atmospheric temperature Combustion air pressure Mechanical efficiency Alternator efficiency Pressure drop in the lines Lower heating value CH4 Temperature LNG Pressure distribution NG
1300 C 800–1000 C 120 C 88% 92% 85% 85% 10 C 50 C 99% 15 C 15 C 1.2 bar 98.5% 98% 2% 50 020 kJ kg1 158 C 82 bar
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
The heat rate (HR) is also used for calculating the efficiency of a power plant, which is the amount of supplied heat to generate one kWh of energy and is determined by equation (11).
HR ¼ 3600=hov
(11)
Another parameter which contributes to the study of the performance of power plants associated with LNG regasification is the Specific Power Performance (SPP). This is defined as the power output to the LNG mass rate (MW/(kg s1 LNG)) and is calculated by equation (12). During the study carried out along the paper it has been considered that the energy analysis is performed per 1 kg s1 of LNG.
SPP ¼
_ LNG wLNGpp m _ air wbl _ WF wnetCBC m m hmec halt _ LNG 1000m
(12)
2.2. Combustion analysis An analysis of the combustion to determine the fuel mass flow rate, depending on the combustion air temperature, is carried out. Aiming at analysis simplification, it is assumed that the NG is pure methane and the flue gases have ideal gas behaviour. The model used for the analysis is based on equation (13), where x represents the percentage of excess air.
CH 4 þ 2ð1 þ x=100ÞðO2 þ 3$76N 2 Þ/CO2 þ 2H 2 O þ ð2x=100ÞO2 þ 7$52ð1 þ x=100ÞN 2
(13)
The mass and energy balance, taking the combustion efficiency into account, is established respectively with equations (14) and (15).
_ air þ m _f ¼ m _ f ðb þ 1Þ _g ¼ m m LHV
P
hcomb ¼
r
nr h
P p
(14) !
np h (15)
LHV
Equation (15) is resolved by the iteration method, for calculating the temperature of the combustion products. It is assumed that at point 14 of Fig. 1(a), the gases have the same temperature as the combustion products. 2.3. Case study: CBC The case study carried out of the power plant shown in Fig. 1, is based on energy analysis using He or N2 as the working fluid. The analysis is performed using the EES (Engineering Equation Solver) [17] which has the advantage of including fluid properties and ready to use optimisation tools. The obtained results are calculated for a TIT of 800 and 1000 C by varying the compression ratio to determine the optimum operating point. The parameters used for CBC computation are those shown in Table 1 and the results are shown in Table 2. Fig. 2 shows the improvement of the CBC thermal efficiency when exploiting the LNG cool energy to cool the working fluid at the compressor inlet, thereby causing a sharp drop in compression specific work. This reduction in work is due to the decrease in specific volume with temperature. The reduction of compression specific work implies an increase in the cycle’s net power and hence an increase in thermal efficiency. It should also be noted that maximum efficiency of the CBC is achieved with low compression ratios. This is primarily because regeneration is most effective at lower compressor pressure ratios. The results obtained demonstrate that, despite having a high thermal efficiency, the overall efficiency is quite low because the working fluid enters the heat exchanger at high temperature, so that the flue gases are released to the atmosphere at higher temperature. This is because the regenerator heated the working fluid. Therefore, there is a need to develop changes in the system in order to achieve a better use of the fuel and thus increase the overall efficiency.
Table 2 Calculation results for the CBC power plant. Bold values indicate results maxima. r
Results for TIT 800 C 2 3 8 16 30 Results for TIT 1000 C 2.2 3 12 40 50
h, %
SPP, MW/kg s1 LNG
hov, %
He
N2
He
N2
He
N2
74.93 72.84 64.60 56.24 46.50
74.95 74.55 70.44 66.16 61.39
31.80 34.68 36.58 34.44 29.79
29.62 32.41 35.72 36.30 35.68
0.998 1.492 1.191 0.836 0.557
0.707 1.215 1.561 1.287 1.046
78.40 77.10 66.32 50.57 46.80
78.86 78.55 73.21 65.63 63.91
25.18 28.13 34.17 30.27 28.47
22.05 24.43 31.24 33.26 33.22
1.473 1.870 1.293 0.662 0.564
0.995 1.511 1.797 1.261 1.169
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
Compression specific work [kJ/kg]
M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
5
2000 T =15 ºC
1600
r =5
T = -20 ºC
r =4
1200
T = -60 ºC T = -120 ºC
r =3
800
r =2 r =1·6
400 0 45
50
55
60 65 70 75 Thermal efficiency [%]
80
85
Fig. 2. Thermal efficiency and compression specific work of the CBC operating with He and TIT 1000 C.
3. CBC and RC in series In the present investigation, effort was placed on the search for alternatives that allow the better use of available energy in the fuel, in which the main contribution is a combination of: - A novel association of cycles in series (CBC and RC) with respect to the power source instead of a conventional combined cycle structure. - Avoiding the combustion system’s heat rejection to the atmosphere, by recovering the heat of the flue gases. The proposed power plant’s structure is shown in Figs. 3 and 4. These schematics depict a CBC and a RC associated in series, with regard to the flow of flue gases, where flue gases first yield heat to the CBC and then to the RC. For the low temperature an ORC can be used. These types of cycles are used for recovering low-grade heat [18]. In recent research, the ORC has also been used in combined cycles, replacing the conventional Rankine steam cycles [19]. In Ref. [20] a combined cycle consisting of a closed Brayton cycle is analysed and an ORC that exploits the Brayton cycle’s rejected heat, obtaining satisfactory results. The ORC posed in Fig. 3 is regenerative and has two turbine stages. Although regeneration is a common means of increasing efficiency in an RC, in this case, the regenerator is placed between the low pressure turbine and condenser, which is not typical in a steam Rankine. This contribution is significant for improving efficiency when coupled with quasicritical condensation for organic fluids such as CO2 and ethane. This enables to reduce the cycle’s output heat and to use back pressure turbines, which is beneficial to the plant’s design compactness.
(12)
Blower (13)
(14) HX
(17)
(18)
(14)
Combustion system envelope HX
(16)
Fuel
(15)
Air preheater (19)
(20)
(21)
T
T
(26)
(3)
T
C
(22)
G
T
(8)
(6)
(2)
(23) Regenerator
(1)
(24)
(7) Regenerator
HX
Condenser (25)
(5)
(4)
LNG
NG (10)
(11)
Fig. 3. CBC–RC series power plant, using LNG as a heat sink.
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
Blower
(12)
(13)
(14) HX
(17)
(18)
(14)
Combustion system envelope HX
(16)
Fuel
(15)
Air preheater (19)
(20) (1-y)
(21)
T
T
(3)
(5)
(4)
T
C
(26) (20) (y)
(8)
(22)
(25)
G
T (6)
(2)
Open feed water heater
(1)
(7) Regenerator
HX
Condenser (24)
(23)
LNG
NG (10)
(11)
Fig. 4. CBC–Steam RC series power plant, using LNG as a heat sink.
A steam RC is chosen in Fig. 4 in order to exploit the heat from the flue gases after the CBC. The steam RC is regenerative with an open feed water heater and two turbines, which is a conventional way of retrieving heat from the flue gases. This type of power plant is examined and compared with the power plant shown in Fig. 3 to determine the configuration of the plant with greater efficiency. In order to efficiently exploit the available heat in the flue gases as much as possible, the power plants shown in Figs. 3 and 4 are designed with a combustion system including heat recovery to heat the combustion air. After yielding heat to the RC’s working fluid, the gases are used to preheat the air. At the preheater outlet, the combustion air passes through an envelope towards the combustion chamber. The function of the combustion air preheating is to encourage the decrease of fuel input and thereby increase the overall efficiency of the power plant. 3.1. Efficiency of the CBC–RC in series As the analysis of the CBC has already been carried out in the previous sections, to follow is a review of the RC and the combustion system. To this end, energy and mass balances are established taking into account the different structures implemented for the ORC (Fig. 3) and steam RC (Fig. 4) respectively: The case of the ORC is analysed using the following equations: Specific net work:
wnetRC ¼ h19 h20 þ h21 h22 ðh25 h24 Þ
(16)
Heat input:
qiRC ¼ h19 h26 þ h21 h20
(17)
Energy balance in the regenerator:
h26 h25 ¼ h22 h23 To ensure heat transfer in the regenerator, a temperature difference de 10 the effectiveness of the regeneration by the following expression: 3
¼
h26 h25 qmax
(18) C
is established between points (23) and (25), thus defining
(19)
where the maximum amount of heat transferred is determined by:
qmax ¼ h22 h23 assuming T23 ¼ T25 Energy balance in the heat exchanger: Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
7
_ WF ðh19 h26 þ h21 h20 Þ _ g ðh16 h17 Þ ¼ m m
(20)
Energy balance in the air preheater:
_ g ðh17 h18 Þ _ air ðh14 h13 Þ ¼ m m
(21)
The case of the steam RC is analysed using the following equations: Specific net work:
wnetRC ¼ h19 h20 h26 þ h25 þ ð1 yÞðh21 h22 h24 þ h23 Þ
(22)
Heat input:
qiRC ¼ h19 h26 þ ð1 yÞðh21 h20 Þ
(23)
Energy balance in the open feed water heater:
h25 ¼ yh20 þ ð1 yÞh24
(24)
Energy balance in the heat exchanger:
_ WF ðh19 h26 Þ þ m _ WF ð1 yÞðh21 h20 Þ _ g ðh16 h17 Þ ¼ m m
(25)
Performance analysis and evaluation of the CBC and series (organic and steam) RC are established by: Thermal efficiency:
h¼
_ WF wnetRC _ WF wnetCBC þ m m _ WF qiRC _ WF qiCBC þ m m
(26)
Overall efficiency:
hov ¼
_ WF wnetRC m _ LNG wLNGpp m _ air wbl _ WF wnetCBC þ m m hmec halt _ f LHV m
(27)
Specific power performance:
SPP ¼
_ WF wnetRC m _ LNG wLNGpp m _ air wbl _ WF wnetCBC þ m m hmec halt _ LNG 1000m
(28)
_ f is calculated by the equations (14) and (15), based on the combustion air temperature (T14 in Fig. 3), achieved by the energy where m balance of the air preheater. 3.2. Selection of the working fluid He and N2 are the working fluids which best suit the needs of the CBC, as they are stable at high temperatures, have a critical temperature below the LNG temperature, are not toxic, inflammable nor corrosive, neither do they require high pressures to perform the process efficiently. This benefits the simplicity of the equipment and reduces installation costs. The criterion for selecting the ORC’s working fluid complies primarily with their physical properties of condensation. The objective for the ORC’s working fluid is that it can be condensed at room temperature and can be carried out under quasicritical temperature and pressure conditions, in order to decrease the heat rejected to the heat sink. Few fluids meet these required characteristics and among them, only the three shown in Table 3, are capable of working at high pressures and temperatures. 3.3. Case study: CBC–RC The case study in this section corresponds to the power plants shown in Figs. 3 and 4. The CBC–RC is analysed for the proposed fluids: He and N2 for the CBC and carbon dioxide, ethane, ammonia and steam for the RC. The thermal and overall efficiencies are computed according to the TIT and CBC pressure ratio, in order to determine which working fluids are best coupled to one another and to achieve the maximum technically possible efficiency. The data in Table 1 is assumed for the calculation of the CBC–RC efficiency. For the RC, an isentropic efficiency of 90% is considered for the turbines and 85% for the feed pumps. A temperature difference of 80 C and 50 C respectively is assumed at the high and low temperature sides of the RC heat exchanger. Table 3 Working fluids selected for the ORC. Range of applicable temperatures, C
WF
Critical point P, bar
T, C
Minimum
Maximum
Carbon Dioxide Ethane Ammonia
73.77 48.72 113.30
30.98 32.17 132.25
56.56 182.78 77.65
1726.85 401.85 426.85
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
Table 4 CBC–RC results with TIT of 800 C. SPP, MW/kg s1 LNG
r
h, %
hov, %
He–CO2
N2–CO2
He–CO2
N2–CO2
He–CO2
N2–CO2
1.8 2 3
66.04 65.94 64.71 He–C2H6
65.82 66.00 65.82 N2–C2H6
52.89 53.22 52.93 He–C2H6
52.32 52.88 53.68 N2–C2H6
1.063 1.290 1.845 He–C2H6
0.821 0.944 1.561 N2–C2H6
2 4 6 8
62.03 62.83 61.79 60.41 He–NH3
60.65 62.42 62.57 62.37 N2–NH3
49.76 51.27 50.43 49.23 He–NH3
48.40 50.95 51.26 51.14 N2–NH3
1.303 1.872 1.549 1.337 He–NH3
0.964 1.842 1.973 1.865 N2–NH3
3 4 6 8
60.33 60.26 59.34 58.11 He-Steam
59.46 59.87 60.02 59.84 N2-Steam
49.53 49.60 48.82 47.73 He-Steam
48.80 49.35 49.61 49.40 N2-Steam
1.979 2.015 1.669 1.442 He-Steam
1.699 1.985 2.124 2.008 N2-Steam
2 3 4 8
61.60 62.17 61.93 59.39
60.51 61.54 61.83 61.53
50.17 50.94 50.68 50.63
49.11 50.66 51.01 50.62
1.452 2.021 2.045 1.520
1.081 1.752 2.038 2.186
A fraction of steam extracted “y” is considered for the steam RC in order to feed the open water heater of 0$1. Tables 4 and 5 show the results obtained. They consist of the thermal efficiency, overall efficiency and SPP as function of the pressure ratio for different working fluids and different TIT (800 and 1000 C). 4. Discussion of results From the results obtained in Table 2, it is deduced that the CBC power plant shown in Fig. 1 has a high thermal efficiency due to its wide range of temperatures between the TIT and the heat sink temperature. However, the overall efficiency is relatively low because the flue gases are expelled to the atmosphere at a high temperature. This temperature belongs to point (15) in Fig. 1(a). Its value corresponds to T15 ¼ T8 þ 50. If He is used as the working fluid and a TIT of 800 C, T15 ¼ 477 C for the condition of maximum efficiency. By increasing the TIT to 1000 C, T15 ¼ 558 C. This means that with the increase of TIT, the temperature of the flue gases released to the environment also increases, thereby decreasing the overall efficiency. As shown in Table 2, moving from a TIT of 800–1000 C favours the increase in thermal efficiency but decreases the overall efficiency. In order to use most of the energy from the flue gases at the CBC heat exchanger outlet, an RC in series has been added to the CBC as shown in Figs. 3 and 4. Using this strategy increases the overall efficiency and specific power with respect to the LNG (SPP). Furthermore, the heat from the flue gases is used to heat the combustion air so that the amount of heat transferred to the atmosphere is the lowest technically possible. Fig. 5 represents the flue gas energy distribution in the CBC, RC and the air preheater. These results are obtained with He and CO2 as working fluids, under a compression ratio of 2$6 and TIT 1000 C. As shown, only 34$11% of the flue gas energy is utilised in the CBC. By Table 5 CBC–RC results with TIT of 1000 C. SPP, MW/kg s1 LNG
r
h, %
hov, %
He–CO2
N2–CO2
He–CO2
N2–CO2
He–CO2
N2–CO2
2.6 3 4 8
67.60 67.27 66.33 62.80 He–C2H6
67.90 67.85 67.57 66.12 N2–C2H6
55.13 55.02 54.48 51.66 He–C2H6
55.04 55.23 55.28 54.42 N2–C2H6
2.465 2.592 2.699 1.923 He–C2H6
1.909 2.250 2.623 2.663 N2–C2H6
4 6 12 20
60.71 61.21 59.77 56.70 He–NH3
58.46 59.54 60.42 60.25 N2–NH3
49.54 50.04 48.82 46.11 He–NH3
47.66 48.74 49.56 49.43 N2–NH3
2.707 2.219 1.541 1.147 He–NH3
2.766 2.913 2.378 1.980 N2–NH3
4 6 12 18
58.31 58.78 57.51 55.48 He-Steam
56.27 57.25 58.06 58.00 N2-Steam
48.12 48.54 47.39 45.55 He-Steam
46.48 47.42 48.13 48.05 N2-Steam
2.926 2.396 1.665 1.324 He-Steam
3.001 3.155 2.570 2.223 N2-Steam
3 5 12 16
59.93 60.80 59.10 57.60
58.11 59.40 60.21 60.09
49.55 50.38 48.81 47.45
48.09 49.35 50.06 49.93
3.038 2.710 1.703 1.446
2.743 3.244 2.655 2.391
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
9
Fig. 5. Flow diagram of the of the flue gas energy, operating with He and CO2 as the working fluid, a compression ratio of 2$6 and TIT 1000 C.
adding by the RC and the air preheater in series to the CBC, it is possible to recover respectively in each of these 20$73 and 25$23% of the flue gas energy. The rest of the energy is lost in the stack gas. In order to determine the efficiency provided by the CBC on the one hand and by the RC on the other, on the overall efficiency of the plant, equations (29) and (30) are applied. According to the plant’s working conditions as presented in the previous paragraph, the RC provides 16$23% on the overall efficiency and the CBC 38$90%, thereby attaining between the both a total of 55$13%.
hovCBC ¼ hovRc ¼
_ LNG wLNGpp m _ air wbl _ WF wnetCBC m m hmec halt _ mf LHV
(29)
_ WF wnetRC m hmec halt _ f LHV m
(30)
The results, in terms of overall efficiency of the CBC–RC, are very similar when using He or N2 as working fluids. The difference when using one fluid or another is in the compression ratio of the Brayton cycle. As shown in Tables 4 and 5, in the case of N2, the compression ratio of maximum efficiency is greater than that of He. With regard to the RC working fluid, CO2 proves to be the most suitable fluid, obtaining an increase of approximately 3% with a TIT of 800 C and 5% at 1000 C, in relation to other fluids. This is because the maximum applied temperature is higher than that of ammonia, ethane and steam, which respectively correspond to 425, 400 and 580 C. Table 6 shows the results of analysis of the two cycles with greater overall efficiency (He–CO2, N2–CO2), where mass flow rates are calculated per unit of LNG mass. It is observed that the He–CO2 case exhibits a slightly lower overall efficiency and SPP, but has the following advantages over the N2–CO2: -
The Brayton cycle operates at a lower compression ratio. The He mass flow rate per kg s1 of LNG is lower than that of N2, due to its high specific heat. Less flow of combustion gases and fuel is required, thereby reducing emissions. More compact and economical equipment from working with a low compression ratio and lower fluid flow rates.
In accordance with the results obtained and the advantages of He compared with N2 as working fluids, the best option for the working fluids is that of He and CO2 as they are best suited to the CBC–RC arranged in series. With the proposed power plant arrangement, approximately 2$40% more of overall efficiency and 8% more of thermal efficiency is achieved than with conventional combined cycles, making the CBC–RC with LNG energy recovery a good option for converting thermal energy into electrical energy. 5. Conclusions This paper has proposed a power plant comprising a CBC and an RC arranged in series. The installation takes advantage of the cooling energy generated during LNG regasification. The LNG cooling energy is used to decrease the CBC’s gas compressor inlet temperature, thereby increasing its thermal efficiency. From the results of the analysis of the power plant the following conclusions are made:
Table 6 Results obtained for the most efficient CBC–RC at a TIT of 1000 C. CBC–RC
He–CO2 N2–CO2
r
2.6 4
h
hov
HR
SPP
WFCBC
WFRC
Flue gas
Fuel
%
%
kJ kW1 h1
MW/kg s1 LNG
kg s1
kg s1
kg s1
kg s1
b kg air/kg f
67.60 66.33
55.13 55.28
6530 6512
2.465 2.623
1.220 5.999
3.210 3.449
4.205 4.521
0.089 0.095
46.05 46.66
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
- The CBC associated with the LNG regasification, using the NG combustion gases as a heat source, has a maximum thermal efficiency of 78$40% (He as working fluid at TIT of 1000 C) but its overall efficiency drops to 34$17%. The loss of efficiency is due to the combustion gases being expelled to the environment at an elevated temperature which approaches 558 C. - With the novel proposed strategy based on the CBC–RC in series, overall efficiency and specific power is increased per kg s1 of LNG. Furthermore, the design of the combustion system with heat recovery allows the heating of the combustion air so that the amount of heat from the flue gases transferred to the atmosphere is the lowest technically possible. - He is proposed as the CBC working fluid because of its high specific heat and low compression ratio in order to obtain maximum efficiency. - CO2 is proposed as the working fluid for the RC, due to its high maximum application temperature in comparison with the other fluids analysed. It also features the advantage of performing condensation under quasicritical conditions. This allows reducing the cycle’s heat output and employing counter pressure turbines, thus encouraging the design of a more compact plant. - For the CBC–RC power plant of He and CO2 as working fluids, a total efficiency of 55$13% is obtained and a specific power of 2$465 MW/ (kg s1 LNG) which is above the conventional and alternative thermal power plants. References [1] T. Otsuka, Evolution of an LNG terminal: Senboku terminal of Osaka gas, in: 23rd World Gas Conference, 2006. Amsterdam. [2] Arnie Smith, John Y. Mak, LNG regasification and utilization, in: 5th Annual Atlantic Canada Oil and Gas, 2005, pp. 30–31. [3] H.K. Ersoy, S.O. Demirpolat, Using liquefied natural gas cold energy for power generation: case study for Marmara Ereglisi receiving terminal, J. Energy Inst. 82 (1) (2009) 11–18. [4] Xiaojun Shi, Defu Che, Thermodynamic analysis of an LNG fuelled combined cycle power plant with waste heat recovery and utilization system, Int. J. Energy Res. 31 (2007) 975–998. [5] F. BAI, Z. ZHANG, Integration of low-level waste heat recovery and liquefied nature gas cold energy utilization, Chin. J. Chem. Eng. 16 (1) (2008) 95–99. [6] N. Zhang, N. Lior, M. Liu, W. Han, COOLCEP (cool clean efficient power): a novel CO2-capturing oxy-fuel power system with LNG (liquefied natural gas) coldness energy utilization, Energy 35 (2) (2010) 1200–1210. [7] M. Liu, N. Lior, N. Zhang, W. Han, Thermoeconomic analysis of a novel zero-CO2-emission high-efficiency power cycle using LNG coldness, Energy Convers. Manage. 50 (11) (2009) 2768–2781. [8] G. Angelino, C.M. Invernizzi, Carbon dioxide power cycles using liquid natural gas as heat sink, Appl. Therm. Eng. 29 (14–15) (2009) 2935–2941. [9] G. Oliveti, N. Arcuri, R. Bruno, M. De Simone, A rational thermodynamic use of liquefied natural gas in a waste incinerator plant, Appl. Therm. Eng. 35 (1) (2012) 134–144. [10] W.- Liu, N. Zhang, R. Cai, Further analysis of closed brayton cycle with LNG cryogenic exergy and waste heat utilization. Kung Cheng Je Wu Li Hsueh Pao/J. Eng. Thermophys. 25 (5) (2004) 733–736. [11] C.- Chiu, Y. Kikkawa, M. Ohishi, S. Rush, H.E. Kimmel, Power Recovery in LNG regasification plants, in: 10AIChE – 2010 AIChE Spring Meeting and 6th Global Congress on Process Safety, 2010. [12] M.R. Salimpour, M.A. Zahedi, Proposing a novel combined cycle for optimal exergy recovery of liquefied natural gas, Heat. Mass Transfer 48 (8) (2012) 1309–1317. [13] G. Angelino, C.M. Invernizzi, The role of real gas Brayton cycles for the use of liquid natural gas physical exergy, Appl. Therm. Eng. 31 (5) (2011) 827–833. [14] E.W. Lemmon, M.L. Huber, M.O. McLinden, Reference Fluid Thermodynamic and Transport Properties, NIST Standard Database 23, REFPROP Version 8.0, 2007. [15] C.F. McDonald, Helium turbomachinery operating experience from gas turbine power plants and test facilities, Appl. Therm. Eng. 44 (2012) 108–142. [16] C.F. McDonald, K.T. Etzel, Closed brayton cycle – a fuel neutral gas turbine to meet energy user's needs in the pacific rim nations, Proc. Am. Power Conf. 57 (1) (1995) 382–387. [17] S.A. Klein, Engineering Equation Solver (EES), Academic Professional V9.172, 2012. [18] F. Vélez, J.J. Segovia, M.C. Martín, G. Antolín, F. Chejne, A. Quijano, A technical, economical and market review of organic Rankine cycles for the conversion of low-grade heat for power generation, Renewable Sustainable Energy Rev. 16 (6) (2012) 4175–4189. [19] R. Chacartegui, D. Sánchez, J.M. Muñoz, T. Sánchez, Alternative ORC bottoming cycles for combined cycle power plants, Appl. Energy 86 (10) (2009) 2162–2170. [20] R.F. Garcia, Efficiency enhancement of combined cycles by suitable working fluids and operating conditions, Appl. Therm. Eng. 42 (2012) 25–33.
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Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
High efficiency power plant with liquefied natural gas cold energy utilization M. Romero Gómez a, *, R. Ferreiro Garcia b, J. Carbia Carril a, J. Romero Gómez a a b
Department of Energy and Marine Propulsion, University of A Coruña, Paseo de Ronda 51, 15 011 A Coruña, Spain Department of Industrial Engineering, University of A Coruña, Paseo de Ronda 51, 15 011 A Coruña, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Accepted 30 April 2013
This article proposes a novel power plant comprising a closed Brayton cycle (CBC) and a Rankine cycle (RC) coupled in series with respect to the flue gases instead of a conventional combined cycle, where the cold energy of the LNG is used to cool the CBC compressor suction. The research study focuses on finding working fluids best suited to the proposed CBC–RC plant and on achieving high efficiency. The proposed working fluids that fulfil the requirements for the CBC are He, N2 and for the RC are CO2, ammonia, ethanol or water. An analysis of the power plant using different working fluids is carried out and it is ascertained that the best efficiency conditions for the CBC are achieved with He and CO2 for the RC. As a result, a thermal efficiency of 67$60%, an overall efficiency of 55$13% and a specific power of 2$465 MW/ (kg s1 LNG) is achieved. Ó 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Closed Brayton cycle LNG Organic working fluids Rankine cycle Series thermal cycles
1. Introduction The objective of this study is to determine the efficiency of a power plant consisting of a closed Brayton cycle (CBC) and a Rankine cycle (RC), innovatively arranged in series, in relation to the power source whilst exploiting the cold energy generated in the regasification process of liquefied natural gas (LNG). The power plant features, as an energy source, a natural gas (NG) flue system where the gases firstly yield heat to the CBC and then to the RC. The CBC can use N2 or He as the working fluid and the RC can either be organic or not. The combination of efficiently power conversion and the regasification of LNG have been under study in recent years [1–5], due to rising fuel prices and environmental restrictions. In power plants, LNG is used complementary as a heat sink to decrease the minimum cycle temperature and thereby increase Carnot efficiency. Studies such as those reflected in Refs. [6–9] show the improvement in the efficiency of organic Rankine cycles (ORCs) by using the LNG vaporisation energy to condense the working fluid, rather than doing it conventionally with water or air. Although the ORCs are a well known option for generating energy associated with LNG regasification, these systems are limited by the physical properties of the working fluid, which should be thermally stable at high temperatures and condensed at low temperatures without issues of freezing. It is for these reasons that ORCs often tend to use low thermal quality heat such as in a waste incineration plant [9]. CBCs are an alternative to ORCs for taking advantage of the cold energy from the LNG [10–13]. In these types of cycles LNG is used to cool the gas to cryogenic temperatures at the compressor inlet. This achieves a decrease in the specific volume of gas and thereby reduces the compression work, implying an increase in the cycle’s net power. By using N2 or He as the working fluid (WF), temperatures of around 1000 C can be reached. Both N2 as well He are stable at very high temperatures. According to Ref. [14], N2 has a range of application of between 210 and 1726 C and He of 271 to 1225 C. Closed loop gas turbines using of He as the working fluid, are very common for generating electricity in nuclear plants. McDonald [15] has been carrying out a study of the evolution of this type of turbine since the early 40’s to date. One of the major advances has been the increase the turbine inlet temperature (TIT). At the beginning it was of around 600 C whereas nowadays can reach 1000 C with the use of turbine blade cooling and high temperature alloys such as titanium, zirconium and molybdenum.
* Corresponding author. Tel.: þ34 981 167000 4226; fax: þ34 981 167100. E-mail addresses: [email protected] (M. Romero Gómez), [email protected] (R.F. Garcia), [email protected] (J. Carbia Carril), [email protected] (J. Romero Gómez). http://dx.doi.org/10.1016/j.joei.2014.02.007 1743-9671/Ó 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
List of symbols h h HX ṁ mass np nr p q r T w x
specific enthalpy, kJ kg1 specific enthalpy, kJ kmol1 heat exchanger flow rate, kg s1 moles of combustion products moles of combustion reactants pressure, bar specific heat, kJ kg1 compressor pressure ratio temperature, C specific work, kJ kg1 excess air ratio, %
y
fraction of steam extracted
b air–fuel ratio h thermal efficiency hov overall efficiency hmec mechanical efficiency halt alternator efficiency hcomb combustion efficiency Subscripts bl blower f fuel g flue gas i inlet o outlet
In the power plant proposed in this paper, comprising a CBC and an RC arranged in series with respect to the combustion gases, it is achieved to unify in one single plant the advantages offered by the high temperature of the CBC’s working fluid as well as its refrigeration to cryogenic temperatures at the compressor suction, with the cooling energy of the LNG. With an RC, (organic or not), the system’s overall efficiency is improved to make the most of the energy available in the flue gases. The present paper is organised as follows: the first section describes and analyses the CBC, obtaining as a result a higher thermal efficiency but low overall efficiency. Next, the proposed CBC–RC power plant is analysed for different working fluids: N2 and He in the case of CBC; and water, ammonia, ethane, and CO2 for the RC, followed by a discussion of results and finally concluding with the outcomes of the conducted study. 2. High temperature CBC and using LNG as a heat sink In conventional closed cycle gas turbine plants, the maximum ratio between the maximum and minimum temperature is of around 3$4 [16]. However, for the proposed high TIT using LNG as a heat sink, the ratio reaches 8$3, which implies a significant decrease of the compression work and a significant increase in efficiency. The flue gases generated in a combustion chamber are used as an energy source. The temperature of the gases is controlled by the air– fuel ratio and is set at 1300 C. Fig. 1 shows a schematic of a regenerative CBC power plant with two turbine stages and a NG based combustion system. The plant data are displayed in Table 1. 2.1. CBC efficiency In any power plant a distinction must be made between the thermal efficiency of the thermodynamic cycle and the overall efficiency of the plant. The thermal efficiency is defined as the ratio of the net power output to the heat input which is computed according to the following expressions: Specific net work:
wnetCBC ¼ h3 h4 þ h5 h6 ðh2 h1 Þ
(1)
Heat input:
qiCBC ¼ h3 h8 þ h5 h4
(2)
Energy balance in the regenerator:
h6 h7 ¼ h8 h2
(3)
In accordance with the T-s diagram shown in Fig. 1(b) and in order to satisfy the first and second law of thermodynamics, it must be fulfilled that T8 < T6 and T7 > T2. As a result of these restrictions, it is of interest to establish a compromise between heat transfer flow and the size of the regenerator. For this, a temperature difference (T7 T2) of 10 C is set to enable an adequate heat flow transfer. Thus, regenerator effectiveness is defined by: 3
¼
h8 h2 qmax
(4)
where maximum transferred heat exchange is given by the following expression:
qmax ¼ h6 h7 assuming T7 ¼ T2 Energy balance of the LNG heat exchanger: Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
(13)
Blower (12)
HX
(15)
(14)
(3)
(4)
3
Fuel
(5) T [°C]
T
C
G
T
3
1000
5
(6)
(8)
8
4
6
(2) (1)
Regenerator 2 -120
LNG (9)
(7)
HX
7
1
NG (10)
(11)
(a)
Entropy
(b)
Fig. 1. CBC power plant using LNG as heat sink: (a) basic plant structure, (b) T–s diagram.
_ WF ðh7 h1 Þ _ LNG ðh11 h10 Þ ¼ m m
(5)
Thermal efficiency:
h¼
wnetCBC qiCBC
(6)
On the other hand, the overall cycle efficiency is the ratio net grid power to the amount of thermal energy really needed to supply the plant power. Therefore, the energy source used and its degree of exploitation will restrict the plant’s efficiency. Assuming the considerations in Table 1, the overall efficiency is defined by equation (7):
hov ¼
_ LNG wLNGpp m _ air wbl _ WF wnetCBC m m hmec halt _ f LHV m
(7)
where the specific work of the LNG pump and the blower are calculated by equations (8) and (9) respectively.
wLNGpp ¼ h10 h9
(8)
wbl ¼ h13 h12
(9)
To determine the mass flow rates of the fuel and the combustion air, it is necessary to know the flow of gases that must be generated to heat the working fluid up to the TIT. An energy balance of the CBC heat exchanger is performed as follows:
_ WF ðh3 h8 þ h5 h4 Þ _ g ðh14 h15 Þ ¼ m m
(10)
Table 1 Main assumptions for the calculations of CBC power plant. Parameters
Value
Temperature combustion gases TIT Compression inlet temperature Compressor isentropic efficiency Turbine isentropic efficiency Blower isentropic efficiency LNG Pump hydraulic efficiency Minimum temperature difference in the regenerator Minimum temperature difference in the Brayton heat exchanger Combustion efficiency Fuel temperature inlet Atmospheric temperature Combustion air pressure Mechanical efficiency Alternator efficiency Pressure drop in the lines Lower heating value CH4 Temperature LNG Pressure distribution NG
1300 C 800–1000 C 120 C 88% 92% 85% 85% 10 C 50 C 99% 15 C 15 C 1.2 bar 98.5% 98% 2% 50 020 kJ kg1 158 C 82 bar
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The heat rate (HR) is also used for calculating the efficiency of a power plant, which is the amount of supplied heat to generate one kWh of energy and is determined by equation (11).
HR ¼ 3600=hov
(11)
Another parameter which contributes to the study of the performance of power plants associated with LNG regasification is the Specific Power Performance (SPP). This is defined as the power output to the LNG mass rate (MW/(kg s1 LNG)) and is calculated by equation (12). During the study carried out along the paper it has been considered that the energy analysis is performed per 1 kg s1 of LNG.
SPP ¼
_ LNG wLNGpp m _ air wbl _ WF wnetCBC m m hmec halt _ LNG 1000m
(12)
2.2. Combustion analysis An analysis of the combustion to determine the fuel mass flow rate, depending on the combustion air temperature, is carried out. Aiming at analysis simplification, it is assumed that the NG is pure methane and the flue gases have ideal gas behaviour. The model used for the analysis is based on equation (13), where x represents the percentage of excess air.
CH 4 þ 2ð1 þ x=100ÞðO2 þ 3$76N 2 Þ/CO2 þ 2H 2 O þ ð2x=100ÞO2 þ 7$52ð1 þ x=100ÞN 2
(13)
The mass and energy balance, taking the combustion efficiency into account, is established respectively with equations (14) and (15).
_ air þ m _f ¼ m _ f ðb þ 1Þ _g ¼ m m LHV
P
hcomb ¼
r
nr h
P p
(14) !
np h (15)
LHV
Equation (15) is resolved by the iteration method, for calculating the temperature of the combustion products. It is assumed that at point 14 of Fig. 1(a), the gases have the same temperature as the combustion products. 2.3. Case study: CBC The case study carried out of the power plant shown in Fig. 1, is based on energy analysis using He or N2 as the working fluid. The analysis is performed using the EES (Engineering Equation Solver) [17] which has the advantage of including fluid properties and ready to use optimisation tools. The obtained results are calculated for a TIT of 800 and 1000 C by varying the compression ratio to determine the optimum operating point. The parameters used for CBC computation are those shown in Table 1 and the results are shown in Table 2. Fig. 2 shows the improvement of the CBC thermal efficiency when exploiting the LNG cool energy to cool the working fluid at the compressor inlet, thereby causing a sharp drop in compression specific work. This reduction in work is due to the decrease in specific volume with temperature. The reduction of compression specific work implies an increase in the cycle’s net power and hence an increase in thermal efficiency. It should also be noted that maximum efficiency of the CBC is achieved with low compression ratios. This is primarily because regeneration is most effective at lower compressor pressure ratios. The results obtained demonstrate that, despite having a high thermal efficiency, the overall efficiency is quite low because the working fluid enters the heat exchanger at high temperature, so that the flue gases are released to the atmosphere at higher temperature. This is because the regenerator heated the working fluid. Therefore, there is a need to develop changes in the system in order to achieve a better use of the fuel and thus increase the overall efficiency.
Table 2 Calculation results for the CBC power plant. Bold values indicate results maxima. r
Results for TIT 800 C 2 3 8 16 30 Results for TIT 1000 C 2.2 3 12 40 50
h, %
SPP, MW/kg s1 LNG
hov, %
He
N2
He
N2
He
N2
74.93 72.84 64.60 56.24 46.50
74.95 74.55 70.44 66.16 61.39
31.80 34.68 36.58 34.44 29.79
29.62 32.41 35.72 36.30 35.68
0.998 1.492 1.191 0.836 0.557
0.707 1.215 1.561 1.287 1.046
78.40 77.10 66.32 50.57 46.80
78.86 78.55 73.21 65.63 63.91
25.18 28.13 34.17 30.27 28.47
22.05 24.43 31.24 33.26 33.22
1.473 1.870 1.293 0.662 0.564
0.995 1.511 1.797 1.261 1.169
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
Compression specific work [kJ/kg]
M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
5
2000 T =15 ºC
1600
r =5
T = -20 ºC
r =4
1200
T = -60 ºC T = -120 ºC
r =3
800
r =2 r =1·6
400 0 45
50
55
60 65 70 75 Thermal efficiency [%]
80
85
Fig. 2. Thermal efficiency and compression specific work of the CBC operating with He and TIT 1000 C.
3. CBC and RC in series In the present investigation, effort was placed on the search for alternatives that allow the better use of available energy in the fuel, in which the main contribution is a combination of: - A novel association of cycles in series (CBC and RC) with respect to the power source instead of a conventional combined cycle structure. - Avoiding the combustion system’s heat rejection to the atmosphere, by recovering the heat of the flue gases. The proposed power plant’s structure is shown in Figs. 3 and 4. These schematics depict a CBC and a RC associated in series, with regard to the flow of flue gases, where flue gases first yield heat to the CBC and then to the RC. For the low temperature an ORC can be used. These types of cycles are used for recovering low-grade heat [18]. In recent research, the ORC has also been used in combined cycles, replacing the conventional Rankine steam cycles [19]. In Ref. [20] a combined cycle consisting of a closed Brayton cycle is analysed and an ORC that exploits the Brayton cycle’s rejected heat, obtaining satisfactory results. The ORC posed in Fig. 3 is regenerative and has two turbine stages. Although regeneration is a common means of increasing efficiency in an RC, in this case, the regenerator is placed between the low pressure turbine and condenser, which is not typical in a steam Rankine. This contribution is significant for improving efficiency when coupled with quasicritical condensation for organic fluids such as CO2 and ethane. This enables to reduce the cycle’s output heat and to use back pressure turbines, which is beneficial to the plant’s design compactness.
(12)
Blower (13)
(14) HX
(17)
(18)
(14)
Combustion system envelope HX
(16)
Fuel
(15)
Air preheater (19)
(20)
(21)
T
T
(26)
(3)
T
C
(22)
G
T
(8)
(6)
(2)
(23) Regenerator
(1)
(24)
(7) Regenerator
HX
Condenser (25)
(5)
(4)
LNG
NG (10)
(11)
Fig. 3. CBC–RC series power plant, using LNG as a heat sink.
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
Blower
(12)
(13)
(14) HX
(17)
(18)
(14)
Combustion system envelope HX
(16)
Fuel
(15)
Air preheater (19)
(20) (1-y)
(21)
T
T
(3)
(5)
(4)
T
C
(26) (20) (y)
(8)
(22)
(25)
G
T (6)
(2)
Open feed water heater
(1)
(7) Regenerator
HX
Condenser (24)
(23)
LNG
NG (10)
(11)
Fig. 4. CBC–Steam RC series power plant, using LNG as a heat sink.
A steam RC is chosen in Fig. 4 in order to exploit the heat from the flue gases after the CBC. The steam RC is regenerative with an open feed water heater and two turbines, which is a conventional way of retrieving heat from the flue gases. This type of power plant is examined and compared with the power plant shown in Fig. 3 to determine the configuration of the plant with greater efficiency. In order to efficiently exploit the available heat in the flue gases as much as possible, the power plants shown in Figs. 3 and 4 are designed with a combustion system including heat recovery to heat the combustion air. After yielding heat to the RC’s working fluid, the gases are used to preheat the air. At the preheater outlet, the combustion air passes through an envelope towards the combustion chamber. The function of the combustion air preheating is to encourage the decrease of fuel input and thereby increase the overall efficiency of the power plant. 3.1. Efficiency of the CBC–RC in series As the analysis of the CBC has already been carried out in the previous sections, to follow is a review of the RC and the combustion system. To this end, energy and mass balances are established taking into account the different structures implemented for the ORC (Fig. 3) and steam RC (Fig. 4) respectively: The case of the ORC is analysed using the following equations: Specific net work:
wnetRC ¼ h19 h20 þ h21 h22 ðh25 h24 Þ
(16)
Heat input:
qiRC ¼ h19 h26 þ h21 h20
(17)
Energy balance in the regenerator:
h26 h25 ¼ h22 h23 To ensure heat transfer in the regenerator, a temperature difference de 10 the effectiveness of the regeneration by the following expression: 3
¼
h26 h25 qmax
(18) C
is established between points (23) and (25), thus defining
(19)
where the maximum amount of heat transferred is determined by:
qmax ¼ h22 h23 assuming T23 ¼ T25 Energy balance in the heat exchanger: Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
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_ WF ðh19 h26 þ h21 h20 Þ _ g ðh16 h17 Þ ¼ m m
(20)
Energy balance in the air preheater:
_ g ðh17 h18 Þ _ air ðh14 h13 Þ ¼ m m
(21)
The case of the steam RC is analysed using the following equations: Specific net work:
wnetRC ¼ h19 h20 h26 þ h25 þ ð1 yÞðh21 h22 h24 þ h23 Þ
(22)
Heat input:
qiRC ¼ h19 h26 þ ð1 yÞðh21 h20 Þ
(23)
Energy balance in the open feed water heater:
h25 ¼ yh20 þ ð1 yÞh24
(24)
Energy balance in the heat exchanger:
_ WF ðh19 h26 Þ þ m _ WF ð1 yÞðh21 h20 Þ _ g ðh16 h17 Þ ¼ m m
(25)
Performance analysis and evaluation of the CBC and series (organic and steam) RC are established by: Thermal efficiency:
h¼
_ WF wnetRC _ WF wnetCBC þ m m _ WF qiRC _ WF qiCBC þ m m
(26)
Overall efficiency:
hov ¼
_ WF wnetRC m _ LNG wLNGpp m _ air wbl _ WF wnetCBC þ m m hmec halt _ f LHV m
(27)
Specific power performance:
SPP ¼
_ WF wnetRC m _ LNG wLNGpp m _ air wbl _ WF wnetCBC þ m m hmec halt _ LNG 1000m
(28)
_ f is calculated by the equations (14) and (15), based on the combustion air temperature (T14 in Fig. 3), achieved by the energy where m balance of the air preheater. 3.2. Selection of the working fluid He and N2 are the working fluids which best suit the needs of the CBC, as they are stable at high temperatures, have a critical temperature below the LNG temperature, are not toxic, inflammable nor corrosive, neither do they require high pressures to perform the process efficiently. This benefits the simplicity of the equipment and reduces installation costs. The criterion for selecting the ORC’s working fluid complies primarily with their physical properties of condensation. The objective for the ORC’s working fluid is that it can be condensed at room temperature and can be carried out under quasicritical temperature and pressure conditions, in order to decrease the heat rejected to the heat sink. Few fluids meet these required characteristics and among them, only the three shown in Table 3, are capable of working at high pressures and temperatures. 3.3. Case study: CBC–RC The case study in this section corresponds to the power plants shown in Figs. 3 and 4. The CBC–RC is analysed for the proposed fluids: He and N2 for the CBC and carbon dioxide, ethane, ammonia and steam for the RC. The thermal and overall efficiencies are computed according to the TIT and CBC pressure ratio, in order to determine which working fluids are best coupled to one another and to achieve the maximum technically possible efficiency. The data in Table 1 is assumed for the calculation of the CBC–RC efficiency. For the RC, an isentropic efficiency of 90% is considered for the turbines and 85% for the feed pumps. A temperature difference of 80 C and 50 C respectively is assumed at the high and low temperature sides of the RC heat exchanger. Table 3 Working fluids selected for the ORC. Range of applicable temperatures, C
WF
Critical point P, bar
T, C
Minimum
Maximum
Carbon Dioxide Ethane Ammonia
73.77 48.72 113.30
30.98 32.17 132.25
56.56 182.78 77.65
1726.85 401.85 426.85
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
Table 4 CBC–RC results with TIT of 800 C. SPP, MW/kg s1 LNG
r
h, %
hov, %
He–CO2
N2–CO2
He–CO2
N2–CO2
He–CO2
N2–CO2
1.8 2 3
66.04 65.94 64.71 He–C2H6
65.82 66.00 65.82 N2–C2H6
52.89 53.22 52.93 He–C2H6
52.32 52.88 53.68 N2–C2H6
1.063 1.290 1.845 He–C2H6
0.821 0.944 1.561 N2–C2H6
2 4 6 8
62.03 62.83 61.79 60.41 He–NH3
60.65 62.42 62.57 62.37 N2–NH3
49.76 51.27 50.43 49.23 He–NH3
48.40 50.95 51.26 51.14 N2–NH3
1.303 1.872 1.549 1.337 He–NH3
0.964 1.842 1.973 1.865 N2–NH3
3 4 6 8
60.33 60.26 59.34 58.11 He-Steam
59.46 59.87 60.02 59.84 N2-Steam
49.53 49.60 48.82 47.73 He-Steam
48.80 49.35 49.61 49.40 N2-Steam
1.979 2.015 1.669 1.442 He-Steam
1.699 1.985 2.124 2.008 N2-Steam
2 3 4 8
61.60 62.17 61.93 59.39
60.51 61.54 61.83 61.53
50.17 50.94 50.68 50.63
49.11 50.66 51.01 50.62
1.452 2.021 2.045 1.520
1.081 1.752 2.038 2.186
A fraction of steam extracted “y” is considered for the steam RC in order to feed the open water heater of 0$1. Tables 4 and 5 show the results obtained. They consist of the thermal efficiency, overall efficiency and SPP as function of the pressure ratio for different working fluids and different TIT (800 and 1000 C). 4. Discussion of results From the results obtained in Table 2, it is deduced that the CBC power plant shown in Fig. 1 has a high thermal efficiency due to its wide range of temperatures between the TIT and the heat sink temperature. However, the overall efficiency is relatively low because the flue gases are expelled to the atmosphere at a high temperature. This temperature belongs to point (15) in Fig. 1(a). Its value corresponds to T15 ¼ T8 þ 50. If He is used as the working fluid and a TIT of 800 C, T15 ¼ 477 C for the condition of maximum efficiency. By increasing the TIT to 1000 C, T15 ¼ 558 C. This means that with the increase of TIT, the temperature of the flue gases released to the environment also increases, thereby decreasing the overall efficiency. As shown in Table 2, moving from a TIT of 800–1000 C favours the increase in thermal efficiency but decreases the overall efficiency. In order to use most of the energy from the flue gases at the CBC heat exchanger outlet, an RC in series has been added to the CBC as shown in Figs. 3 and 4. Using this strategy increases the overall efficiency and specific power with respect to the LNG (SPP). Furthermore, the heat from the flue gases is used to heat the combustion air so that the amount of heat transferred to the atmosphere is the lowest technically possible. Fig. 5 represents the flue gas energy distribution in the CBC, RC and the air preheater. These results are obtained with He and CO2 as working fluids, under a compression ratio of 2$6 and TIT 1000 C. As shown, only 34$11% of the flue gas energy is utilised in the CBC. By Table 5 CBC–RC results with TIT of 1000 C. SPP, MW/kg s1 LNG
r
h, %
hov, %
He–CO2
N2–CO2
He–CO2
N2–CO2
He–CO2
N2–CO2
2.6 3 4 8
67.60 67.27 66.33 62.80 He–C2H6
67.90 67.85 67.57 66.12 N2–C2H6
55.13 55.02 54.48 51.66 He–C2H6
55.04 55.23 55.28 54.42 N2–C2H6
2.465 2.592 2.699 1.923 He–C2H6
1.909 2.250 2.623 2.663 N2–C2H6
4 6 12 20
60.71 61.21 59.77 56.70 He–NH3
58.46 59.54 60.42 60.25 N2–NH3
49.54 50.04 48.82 46.11 He–NH3
47.66 48.74 49.56 49.43 N2–NH3
2.707 2.219 1.541 1.147 He–NH3
2.766 2.913 2.378 1.980 N2–NH3
4 6 12 18
58.31 58.78 57.51 55.48 He-Steam
56.27 57.25 58.06 58.00 N2-Steam
48.12 48.54 47.39 45.55 He-Steam
46.48 47.42 48.13 48.05 N2-Steam
2.926 2.396 1.665 1.324 He-Steam
3.001 3.155 2.570 2.223 N2-Steam
3 5 12 16
59.93 60.80 59.10 57.60
58.11 59.40 60.21 60.09
49.55 50.38 48.81 47.45
48.09 49.35 50.06 49.93
3.038 2.710 1.703 1.446
2.743 3.244 2.655 2.391
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
9
Fig. 5. Flow diagram of the of the flue gas energy, operating with He and CO2 as the working fluid, a compression ratio of 2$6 and TIT 1000 C.
adding by the RC and the air preheater in series to the CBC, it is possible to recover respectively in each of these 20$73 and 25$23% of the flue gas energy. The rest of the energy is lost in the stack gas. In order to determine the efficiency provided by the CBC on the one hand and by the RC on the other, on the overall efficiency of the plant, equations (29) and (30) are applied. According to the plant’s working conditions as presented in the previous paragraph, the RC provides 16$23% on the overall efficiency and the CBC 38$90%, thereby attaining between the both a total of 55$13%.
hovCBC ¼ hovRc ¼
_ LNG wLNGpp m _ air wbl _ WF wnetCBC m m hmec halt _ mf LHV
(29)
_ WF wnetRC m hmec halt _ f LHV m
(30)
The results, in terms of overall efficiency of the CBC–RC, are very similar when using He or N2 as working fluids. The difference when using one fluid or another is in the compression ratio of the Brayton cycle. As shown in Tables 4 and 5, in the case of N2, the compression ratio of maximum efficiency is greater than that of He. With regard to the RC working fluid, CO2 proves to be the most suitable fluid, obtaining an increase of approximately 3% with a TIT of 800 C and 5% at 1000 C, in relation to other fluids. This is because the maximum applied temperature is higher than that of ammonia, ethane and steam, which respectively correspond to 425, 400 and 580 C. Table 6 shows the results of analysis of the two cycles with greater overall efficiency (He–CO2, N2–CO2), where mass flow rates are calculated per unit of LNG mass. It is observed that the He–CO2 case exhibits a slightly lower overall efficiency and SPP, but has the following advantages over the N2–CO2: -
The Brayton cycle operates at a lower compression ratio. The He mass flow rate per kg s1 of LNG is lower than that of N2, due to its high specific heat. Less flow of combustion gases and fuel is required, thereby reducing emissions. More compact and economical equipment from working with a low compression ratio and lower fluid flow rates.
In accordance with the results obtained and the advantages of He compared with N2 as working fluids, the best option for the working fluids is that of He and CO2 as they are best suited to the CBC–RC arranged in series. With the proposed power plant arrangement, approximately 2$40% more of overall efficiency and 8% more of thermal efficiency is achieved than with conventional combined cycles, making the CBC–RC with LNG energy recovery a good option for converting thermal energy into electrical energy. 5. Conclusions This paper has proposed a power plant comprising a CBC and an RC arranged in series. The installation takes advantage of the cooling energy generated during LNG regasification. The LNG cooling energy is used to decrease the CBC’s gas compressor inlet temperature, thereby increasing its thermal efficiency. From the results of the analysis of the power plant the following conclusions are made:
Table 6 Results obtained for the most efficient CBC–RC at a TIT of 1000 C. CBC–RC
He–CO2 N2–CO2
r
2.6 4
h
hov
HR
SPP
WFCBC
WFRC
Flue gas
Fuel
%
%
kJ kW1 h1
MW/kg s1 LNG
kg s1
kg s1
kg s1
kg s1
b kg air/kg f
67.60 66.33
55.13 55.28
6530 6512
2.465 2.623
1.220 5.999
3.210 3.449
4.205 4.521
0.089 0.095
46.05 46.66
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007
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M. Romero Gómez et al. / Journal of the Energy Institute xxx (2014) 1–10
- The CBC associated with the LNG regasification, using the NG combustion gases as a heat source, has a maximum thermal efficiency of 78$40% (He as working fluid at TIT of 1000 C) but its overall efficiency drops to 34$17%. The loss of efficiency is due to the combustion gases being expelled to the environment at an elevated temperature which approaches 558 C. - With the novel proposed strategy based on the CBC–RC in series, overall efficiency and specific power is increased per kg s1 of LNG. Furthermore, the design of the combustion system with heat recovery allows the heating of the combustion air so that the amount of heat from the flue gases transferred to the atmosphere is the lowest technically possible. - He is proposed as the CBC working fluid because of its high specific heat and low compression ratio in order to obtain maximum efficiency. - CO2 is proposed as the working fluid for the RC, due to its high maximum application temperature in comparison with the other fluids analysed. It also features the advantage of performing condensation under quasicritical conditions. This allows reducing the cycle’s heat output and employing counter pressure turbines, thus encouraging the design of a more compact plant. - For the CBC–RC power plant of He and CO2 as working fluids, a total efficiency of 55$13% is obtained and a specific power of 2$465 MW/ (kg s1 LNG) which is above the conventional and alternative thermal power plants. References [1] T. Otsuka, Evolution of an LNG terminal: Senboku terminal of Osaka gas, in: 23rd World Gas Conference, 2006. Amsterdam. [2] Arnie Smith, John Y. Mak, LNG regasification and utilization, in: 5th Annual Atlantic Canada Oil and Gas, 2005, pp. 30–31. [3] H.K. Ersoy, S.O. Demirpolat, Using liquefied natural gas cold energy for power generation: case study for Marmara Ereglisi receiving terminal, J. Energy Inst. 82 (1) (2009) 11–18. [4] Xiaojun Shi, Defu Che, Thermodynamic analysis of an LNG fuelled combined cycle power plant with waste heat recovery and utilization system, Int. J. Energy Res. 31 (2007) 975–998. [5] F. BAI, Z. ZHANG, Integration of low-level waste heat recovery and liquefied nature gas cold energy utilization, Chin. J. Chem. Eng. 16 (1) (2008) 95–99. [6] N. Zhang, N. Lior, M. Liu, W. Han, COOLCEP (cool clean efficient power): a novel CO2-capturing oxy-fuel power system with LNG (liquefied natural gas) coldness energy utilization, Energy 35 (2) (2010) 1200–1210. [7] M. Liu, N. Lior, N. Zhang, W. Han, Thermoeconomic analysis of a novel zero-CO2-emission high-efficiency power cycle using LNG coldness, Energy Convers. Manage. 50 (11) (2009) 2768–2781. [8] G. Angelino, C.M. Invernizzi, Carbon dioxide power cycles using liquid natural gas as heat sink, Appl. Therm. Eng. 29 (14–15) (2009) 2935–2941. [9] G. Oliveti, N. Arcuri, R. Bruno, M. De Simone, A rational thermodynamic use of liquefied natural gas in a waste incinerator plant, Appl. Therm. Eng. 35 (1) (2012) 134–144. [10] W.- Liu, N. Zhang, R. Cai, Further analysis of closed brayton cycle with LNG cryogenic exergy and waste heat utilization. Kung Cheng Je Wu Li Hsueh Pao/J. Eng. Thermophys. 25 (5) (2004) 733–736. [11] C.- Chiu, Y. Kikkawa, M. Ohishi, S. Rush, H.E. Kimmel, Power Recovery in LNG regasification plants, in: 10AIChE – 2010 AIChE Spring Meeting and 6th Global Congress on Process Safety, 2010. [12] M.R. Salimpour, M.A. Zahedi, Proposing a novel combined cycle for optimal exergy recovery of liquefied natural gas, Heat. Mass Transfer 48 (8) (2012) 1309–1317. [13] G. Angelino, C.M. Invernizzi, The role of real gas Brayton cycles for the use of liquid natural gas physical exergy, Appl. Therm. Eng. 31 (5) (2011) 827–833. [14] E.W. Lemmon, M.L. Huber, M.O. McLinden, Reference Fluid Thermodynamic and Transport Properties, NIST Standard Database 23, REFPROP Version 8.0, 2007. [15] C.F. McDonald, Helium turbomachinery operating experience from gas turbine power plants and test facilities, Appl. Therm. Eng. 44 (2012) 108–142. [16] C.F. McDonald, K.T. Etzel, Closed brayton cycle – a fuel neutral gas turbine to meet energy user's needs in the pacific rim nations, Proc. Am. Power Conf. 57 (1) (1995) 382–387. [17] S.A. Klein, Engineering Equation Solver (EES), Academic Professional V9.172, 2012. [18] F. Vélez, J.J. Segovia, M.C. Martín, G. Antolín, F. Chejne, A. Quijano, A technical, economical and market review of organic Rankine cycles for the conversion of low-grade heat for power generation, Renewable Sustainable Energy Rev. 16 (6) (2012) 4175–4189. [19] R. Chacartegui, D. Sánchez, J.M. Muñoz, T. Sánchez, Alternative ORC bottoming cycles for combined cycle power plants, Appl. Energy 86 (10) (2009) 2162–2170. [20] R.F. Garcia, Efficiency enhancement of combined cycles by suitable working fluids and operating conditions, Appl. Therm. Eng. 42 (2012) 25–33.
Please cite this article in press as: M. Romero Gómez, et al., High efficiency power plant with liquefied natural gas cold energy utilization, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.02.007