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Enhancement of APCI cycle efficiency with absorption chillers
Energy 35 (2010) 3877e3882
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Enhancement of APCI cycle efficiency with absorption chillers Amir Mortazavi, Christopher Somers, Abdullah Alabdulkarem, Yunho Hwang*, Reinhard Radermacher Center for Environmental Energy Engineering, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2009 Received in revised form 26 May 2010 Accepted 30 May 2010 Available online 2 July 2010
Liquefied natural gas (LNG) plants consume a great amount of energy. In order to enhance the energy efficiency of the LNG plant, the potential energy efficiency enhancements of various options of utilizing the waste heat powered absorption chillers in the propane pre-cooled mixed refrigerant (APCI) liquefaction cycle were investigated in this study. After developing models of the LNG process, gas turbine and absorption chillers, eight options of gas turbine waste heat utilization were simulated. The simulation results show that by replacing 22 C and 9 C evaporators and cooling the condenser of propane cycle at 14 C and inter-cooling the compressor of mixed refrigerant cycle with absorption chillers which are powered by waste heat from the gas turbine, both the compressor power and fuel consumption reduction can be achieved as much as 21.32%. This enhancement requires recovering at least 97% of gas turbine waste heat. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Natural gas liquefaction Gas turbine Absorption chiller Waste heat utilization
1. Introduction Due to the fact that natural gas is the cleanest fossil fuel, its demand has increased recently [1]. Natural gas is mainly transported either through the pipe lines in gaseous phase or by LNG ships in liquefied natural gas (LNG) [2]. LNG plants consume great amount of energy. Liquefaction of 1 kg of natural gas needs about 1188 kJ of energy [3] depending on the liquefaction cycle and site conditions. The energy efficiency of the LNG plant can be enhanced by improving the liquefaction cycle efficiency, improving the compressor and driver efficiency and utilizing waste heat. Cycle efficiency could be improved by several ways such as optimizing the refrigerant composition, mass flow rate and operating pressures or improving cycle components such as expansion valves and heat exchangers. Lee et al. [4], Vaidyaraman et al. [5], Del Nogal et al. [6] and Paradowski et al. [7] used optimization techniques to enhance the efficiency of mixed (multi-component) refrigerant cycles by optimizing refrigerant composition, mass flow rate and operating pressures. Mixed refrigerant cycles are used in three types of LNG plants. Which are propane pre-cooled mixed refrigerant, single cycle mixed refrigerant and dual cycle mixed refriglu et al. [9] erant natural gas liquefaction cycles [8]. Kanog investigated the benefits of using turbine expander instead of Joule Thomson valve for LNG expansion process. Renaudin et al. [10] examined the effect of replacing LNG and mixed refrigerant expansion valves by liquid turbines. Mortazavi et al. [11] examined
* Corresponding author. Tel.: þ1 301 405 5247; fax: þ1 301 405 2025. E-mail address: [email protected] (Y. Hwang). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.05.043
the effect of replacing expansion valves with liquid turbines and two-phase expanders on the efficiency and capacity of the propane pre-cooled multi-component refrigerant (MCR) natural gas liquefaction cycle licensed by Air Products and Chemicals, Inc. (APCI) [12]. Kalinowski et al. [13] considered replacing the propane cycle with absorption chillers. Mortazavi et al. [14] investigated enhancing the performance of the propane cycle of the APCI cycle with absorption chillers powered by the gas turbine waste heat. None of the previous studies quantify the waste heat from the LNG plant gas turbine driver and investigate different waste heat utilization options based on the driver waste heat. In order to clearly address the waste heat utilization options, this paper theoretically investigates the potential energy efficiency enhancement in the APCI LNG plant by utilizing waste heat from the gas turbine with absorption chillers. For this purpose, the gas turbine, absorption chiller and the LNG plant process were modeled with ASPEN plus software [15], which is one of the preferred software in the oil and gas industry.
2. APCI base cycle modeling 2.1. APCI natural gas liquefaction process About 77% of LNG plants, including one at Abu Dhabi in the U.A. E., are using the APCI cycle for natural gas liquefaction, as illustrated in Fig. 1 [12]. As shown in Fig. 1, the feed gas is processed through the gas sweetening plant for the removal of H2S, CO2, H2O and Hg. As it passes through the pre-cooler and cold box, its temperature
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A. Mortazavi et al. / Energy 35 (2010) 3877e3882
Gas Pre cooler
Separator
Feed Gas Gas Sweetening Expansion Valve
LNG
Cold Box
Propane Cycle
Cryogenic Column
MCR Cycle
Separator
Propane
Gas
Butane
Liquid Fractionation
Pentane Heavier Hydrocarbons Fig. 1. Schematic diagram of APCI LNG production process.
decreases to approximately 30 C and some components are condensed at the same time. In the separator, the remaining gas and condensate are separated. The condensate is being sent to the fractionation unit, where it is separated to propane, butane, pentane, and heavier hydrocarbons. The remaining gas is further cooled in the cryogenic column to below 160 C and liquefied. Its pressure is then reduced to atmospheric pressure by passing through the LNG expansion valve. There are two refrigeration cycles utilized in this whole process: the propane cycle and the MCR cycle. The first cycle provides the required cooling to the pre-cooler, cold box and fractionation unit. The second cycle supplies the cooling needed in the cryogenic column. 2.2. Model development ASPEN Plus, which is steady-state process modeling software, was employed for modeling the APCI LNG production process. For modeling the property of substances, the PengeRobinsoneBostoneMathias equation of state was used for both the liquefaction cycle and the driver (gas turbine) [16]. All ASPEN model convergence tolerances for the relative residuals were set to be 1 104. The gas sweetening process was not modeled for the sake of simplicity. Instead the gas composition after gas sweetening unit were used for modeling the liquefaction cycle. Which is shown in Table 1. Hexane plus was approximated by n-hexane and isohexane with 0.16 and 0.24 for their mole fractions, respectively. Some of the other modeling assumptions used are listed in Table 2. Centrifugal and axial type compressors were used for the propane and MCR cycles respectively. All condensers and inter-coolers were assumed to be cooled by seawater. It was assumed that the propane cycle have five stages of cooling. The MCR is composed of nitrogen, methane, ethane, and propane with mole fractions of 0.09, 0.36, 0.47 and 0.08, respectively. The MCR cycle has a two-stage compressor with an intercooler. The fractionation unit was modeled by using “radfrac” component of ASPEN. The APCI cycle expansion processes were done by expansion valves. Flash gas recovery process is not considered. The APCI cycle without any enhancements is referred as “APCI base cycle” in this paper. The schematic of the APCI base cycle modeled in ASPEN is shown in Fig. 2. 2.3. Base cycle simulation results The simulation results of the APCI base cycle are shown in Table 3.
3. Driver selection and modeling To quantify the available amount of waste heat, gas turbines were assumed to be the driver for the compressors of the propane and MCR cycles. To obtain an accurate estimate of gas turbine performance, a gas turbine having a rated capacity of 130 MW was modeled using ASPEN software. The block diagram of the gas turbine cycle modeled with ASPEN is shown in Fig. 3. The compressor and the turbine were modeled using the compressor and turbine blocks of ASPEN software. The combustion chamber was modeled using RGIBBS block, which minimizes the Gibbs free energy of inlet streams to the combustion chamber. The fuel was assumed to be pure methane. To account for air leakages and blade cooling a portion of compressor discharge is diverted directly to the turbine instead of passing through the combustion chamber. To verify the gas turbine model vender’s data [17] at ISO condition, which is 15 C and 1 atm inlet pressure, was used. The comparison of the gas turbine simulation results with vender’s data is shown in Table 4. As it is shown in Table 4, the maximum discrepancy of the simulation results from the data is about 1.16%, which is in acceptable range. 4. Absorption cycle modeling With a heat input, absorption chillers can provide the refrigeration effect. Since waste heat sources and cooling requirements are plentiful in the APCI base cycle, absorption chillers were considered as a useful efficiency improvement option to the APCI base cycle. The absorption chiller was modeled in ASPEN as briefly described
Table 1 Gas composition after gas sweetening. Component
Mole Fraction [%]
Nitrogen Carbon Dioxide Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane Hexane Plus Total
0.100 0.005 85.995 7.500 3.500 1.000 1.000 0.300 0.200 0.400 100
A. Mortazavi et al. / Energy 35 (2010) 3877e3882 Table 2 Modeling assumptions for APCI base cycle.
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Table 3 Simulation results of APCI base cycle.
Axial compressor isentropic efficiency Centrifugal compressor isentropic efficiency Pinch temperature Seawater temperature Refrigerant temperature at condenser or super heater exit LNG temperature at the exit of cryogenic column Degree of superheating in propane cycle LNG expander exit pressure
0.86 0.83 3K 35 C 40 C 160 C 10 K 101.3 kPa
Propane compressor power Mixed refrigerant compressor power Propane cycle cooling capacity Mixed refrigerant cycle cooling capacity Propane cycle COP LNG vapor fraction after the expansion valve LNG production LPG (propane, butane, pentane and heavier hydrocarbons) Flash gas flow rate after LNG expansion valve
below. Comprehensive details of modeling absorption chillers can be found in Somer’s work [18,19].
43.651 MW 66.534 MW 115.469 MW 67.635 MW 2.645 0.014% 98.83 kg/s 11.00 kg/s 1.28 kg/s
cycle evaporators and a premium is placed on COP, water/lithiumebromide was selected as a working fluid pair.
4.1. Design selection 4.2. Model development There are a number of absorption cycles, with varying strengths and applications. Water/lithiumebromide chillers have higher coefficient of performances (COPs) than that of ammonia/water chillers but cannot be used below about 2 C evaporating temperature due to freezing concern on its refrigerant and cannot be used for the high absorption temperature due to crystallization issues of its absorbent. On the other hand, ammonia/water chillers can be used for a wide range of temperatures including below freezing temperature of water. Additionally, each fluid pair can be single or double-effect. Single-effect absorption chillers have the advantage of having less number of components and requiring lower temperature waste heat but have lower COP than those of doubleeffect chillers. After considering high temperature gas turbine exhaust, a double-effect design was selected in this study. Moreover, since ample cooling is required for high temperature propane
Compressor
Vapor Liquid Seperator
As described earlier, the double-effect water/lithiumebromide model was developed in ASPEN. First, a suitable property method was selected for the working fluid properties. For the pure water properties, the steam NBS property method in ASPEN was used [20]. For the water/lithiumebromide solution properties, an electrolyte property method known as ELECNRTL was used [21]. Next, the process was broken into individual components, and individual components were modeled. After these individual models were compiled, the complete cycle model was built. 4.3. Simulation results The parameter values listed in Table 5 were selected for the absorption cycle model inputs. A seawater temperature of 35 C
Heat Exchanger Heat Exchanger
Pump Fractionation Column
Multi Stream Heat Exchanger
Expansion Valve Fig. 2. APCI base cycle modeled with ASPEN.
Mixer, Spliter Natural Gas Stream Propane Cycle Stream Mixed Refrigerant Cycle Stream Propane Butane Pentane Plus
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A. Mortazavi et al. / Energy 35 (2010) 3877e3882 FUEL
Combustion Chamber
Combustion Chamber Outlet
Compressor Discharge
Compressor
Blade Cooling
Turbine W
Compressor Power
Output Power
Inlet Air
Exhaust Fig. 3. Gas turbine cycle modeled with ASPEN.
was selected because it is the maximum temperature found in Abu Dhabi. This is a conservative assumption because the COP decreases with increasing seawater temperature. In Table 5, “Seawater Temperature Difference (DT)” means the temperature difference between the seawater temperature and the condenser (and absorber) outlet temperatures. Finally, a desorber outlet temperature of 180 C was selected as a temperature representing typical chiller operating conditions and is feasible while considering the waste heat temperature. Based on these input parameter values, the modeling results shown in Table 6 were obtained. These two evaporating temperatures were selected for the simulation because these are the two highest temperature stages of the propane cycle. These COP values are used later to estimate the amount of waste heat that is needed to obtain the amount of cooling required. 5. Efficiency enhancements of APCI base cycle by absorption chiller 5.1. Integration of gas turbine model to LNG plant model To investigate the available amount of waste heat for different waste heat utilization options, the gas turbine model was integrated to the APCI base cycle model. To meet the plant power demand, the basic gas turbine model was scaled to provide the plant demand at 45 C ambient temperature under its full load condition. This is not an unreasonable assumption since some gas turbine manufactures scale their gas turbine design to meet for different demands. To investigate the part load effects two cases were considered for each enhancement option. The first case assumed that for each option the gas turbine would be the same size as that of the base cycle. Therefore, there could be some part load degradation effects. This
Table 4 Comparison of gas turbine simulation results with vender’s data at Iso condition. Parameter
ISO rated power [MW]
Efficiency [%]
Exhaust Temperature [ C]
Actual Gas Turbine ASPEN Model Discrepancy
130.100
34.6
540.0
130.103 þ0.003 (0.0%)
35.0 þ0.4 (1.16%)
540.4 þ0.4
case is referred as an “unscaled case.” On the other hand, in the second case it was assumed that for each option a gas turbine is sized to deliver the maximum plant demand at its full load. This case is referred as a “scaled case.” To prevent condensing issues and excessive pressure drop at the gas turbine exhaust it was assumed that the gas turbine exhaust gas could be cooled down to 200 C. To calculate the energy consumption of the gas turbine, pure methane was assumed as the gas turbine fuel and 50.1 MJ/kg was used as the low heating value of methane. To utilize waste heat, double-effect water/lithiumebromide absorption chillers working at 22 C and 9 C evaporating temperatures were used. 5.2. Option 1: Replacing 22 C propane cycle evaporators with absorption chillers The first option considered is replacing the 22 C evaporators of the propane cycle with a double-effect absorption chiller powered by gas turbine waste heat. The modeling results of section 4 were used to calculate the required amount of waste heat to run the absorption chiller. 5.3. Option 2: Replacing 22 C propane cycle evaporators and cooling the inlet of gas turbine with absorption chillers In this option, the 22 C propane cycle evaporator was replaced by an absorption chiller. The gas turbine inlet was cooled to 30 C using the absorption chiller. The evaporator of gas turbine inlet cooler was assumed to be at 22 C. 5.4. Option 3: Replacing 22 C and 9 C propane cycle evaporators with absorption chillers For the third option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers with evaporator
Table 5 Modeling input parameters of absorption cycle. Parameter
Value
Seawater Temperature Difference (DT) Desorber Outlet Temperature
5K 180 C
A. Mortazavi et al. / Energy 35 (2010) 3877e3882 Table 6 Simulation results of absorption cycle. Evaporating Temperature
COP
9 C 22 C
1.284 1.489
temperatures of 22 C and 9 C. Propane is also subcooled to 25 C and 12 C by 22 C and 9 C absorption chillers evaporators, respectively.
5.5. Option 4: Replacing 22 C and 9 C propane cycle evaporators cooling the inlet of gas turbine with absorption chillers In this option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers. Propane was also subcooled to 12 C and gas turbine inlet was cooled down to 17 C by using both the 22 C and 9 C evaporators of absorption chillers.
5.6. Option 5: Replacing 22 C and 9 C evaporators and cooling the condenser of propane cycle at 27 C with absorption chillers In Option 5, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 27 C and subcooled to 12 C by absorption chillers.
5.7. Option 6: Replacing 22 C and 9 C evaporators and cooling the condenser of propane at 27 C cycle and turbine inlet with absorption chillers In this option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 27 C and subcooled to 12 C and the gas turbine inlet was cooled to 17 C by absorption chillers. The gas turbine inlet was first cooled to 30 C by a 22 C evaporator and then cooled down to 17 C by a 9 C evaporator.
5.8. Option 7: Replacing 22 C and 9 C evaporators and cooling the condenser of propane cycle at 14 C with absorption chillers For Option 7, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 14 C and subcooled to 12 C by an absorption chillers.
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5.9. Option 8: Replacing 22 C and 9 C evaporators and cooling the condenser of propane at 14 C cycle and inter-cooling the compressor of mixed refrigerant cycle with absorption chillers For the final option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 14 C and subcooled to 12 C by absorption chillers. Mixed refrigerant was intercooled fist by seawater to 40 C and then it was cooled down to 14 C by a 9 C absorption chiller evaporator.
5.10. Simulation results The simulation results of eight enhancement options for the gas turbine fuel consumption, the power reduction, and the required amount of waste heat to operate the absorption chiller are summarized in Table 7. In Table 7 the options are ranked based on their enhancements where the option 1 has the lowest amount of reduction in fuel consumption which is directly related to the energy efficiency of the plant. Since the production capacity of plant is held constant. The fuel consumption could be reduced by either reducing the compressor power demand and/or increasing the gas turbine power generation efficiency. The compressor power demand will be reduced by lower the propane cycle condenser temperature, replacing the propane evaporators by waste heat run absorption chillers and/or inter-cooling the compressor of the MCR cycle. The gas turbine power generation efficiency could be increased by cooling the inlet air of the gas turbine. As it is shown in Table 7, by applying option 8 which is replacing 22 C and 9 C evaporators and cooling the condenser of propane at 14 C cycle and inter-cooling the compressor of mixed refrigerant cycle with absorption chillers and using a scaled gas turbine the compressor power demand could be reduced by 21.3%. Which also leads to reduction of gas turbine fuel consumption by 21.3%. The fuel consumption of the scaled gas turbine case for the same option is lower than that of the unscaled gas turbine case. Hence the efficiency of the scaled case is higher than unscaled case for the same option. This is due to the fact that in the unscaled case the gas turbine is operated at part load running condition while in the scaled case the gas turbine is operated in full load and at part load running condition the gas turbine firing temperature is lower than the full load condition. Lower firing temperature leads to lower efficiency of the gas turbine. Moreover the unscaled case requires both more amount of waste heat and higher percent of available waste heat than the scaled case for the same option. As shown in Table 7, the better the option the more amount of waste heat is required to operate the absorption chillers. The enchantments options with the scaled gas turbines will result in lower capital cost
Table 7 Enhancement results of different waste heat utilization options. Gas Turbine Sizing
Scaled gas turbine case
Unscaled gas turbine case
Option
Compressor Power [MW]
Power Reduction [MW]
Required Amount of Waste Heat [MW]
Fraction of Available Amount of Waste Heat [%]
Fuel Consumption [MW] (% saving)
Required Amount of Waste Heat [MW]
Fraction of Available Amount of Waste Heat [%]
Fuel Consumption [MW] (% saving)
APCI base cycle Option 1 Option 2 Option 3 Option 4 Option 5 Option 6 Option 7 Option 8
110.185 107.510 107.510 100.334 100.334 94.043 94.043 88.420 86.696
e 2.675 (2.43%) 2.675 (2.43%) 9.851 (8.94%) 9.851 (8.94%) 16.142 (14.65%) 16.142 (14.65%) 21.765 (19.75%) 23.489 (21.32%)
e 8.865 12.572 34.998 41.579 98.306 104.474 105.553 116.391
e 5.948 8.985 25.162 33.537 75.408 89.899 86.112 96.844
329.448 321.444 314.002 299.999 287.624 281.186 269.598 264.378 259.217
e 8.865 12.913 34.998 43.112 98.306 106.420 105.553 116.391
e 5.958 9.341 25.311 36.306 76.133 96.977 87.227 98.195
329.448 322.754 318.175 304.859 296.482 289.482 281.006 275.340 271.086
(2.43) (4.69) (8.94) (12.70) (14.65) (18.17) (19.75) (21.32)
(2.03) (3.42) (7.46) (10.01) (12.21) (14.70) (16.42) (17.72)
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for the gas turbine driver due to their smaller power capacity in comparison to the base line plant gas turbine driver. 6. Conclusion Any improvement in LNG plants efficiency will result in a considerable amount energy consumption reduction due to their huge energy consumption scale. A gas turbine driven APCI LNG plant was modeled to identify the available amount of waste heat. Double-effect water/lithiumebromide absorption chillers were modeled to investigate various waste heat utilization options. Based on the simulation results for eight enhancement options, it was found that utilizing the gas turbine waste heat by absorption chillers could reduce both the gas turbine fuel consumption by 21.3% and also the capital cost of the gas turbine driver due to reduced capacity. The maximum fuel consumption reduction was achieved by implementing option 8 with a scaled gas turbine. References [1] Hubbard B. A fresh approach to LNG. Hydrocarbon Engineering October 2004;9:29e32. [2] Bumagin G, Borodin D. Natural gas liquefier based on an EGD-generatorexpander. Chemical and Petroleum Engineering 2007;43(5e6):17e20. [3] Finn A, Johnson G, Tomlinson T. Developments in natural gas liquefaction. Hydrocarbon Processing April 1999;78:47e59. [4] Lee GC, Smith R, Zhu XX. Optimal synthesis of mixed refrigerant systems for low temperature processes. Industrial and Engineering Chemistry Research 2002;41:5016e28. [5] Vaidyaraman S, Maranas CD. Synthesis of mixed refrigerant cascade cycles. Chemical Engineering Communications 2002;189:1057e78.
[6] Del Nogal F, Kim J, Perry S, Smith R. Optimal design of mixed-refrigerant cycles. Industrial and Engineering Chemistry Research 2008;47(22):8724e40. [7] Paradowski H, Bamba M, Bladanet C. “Propane precooling cycles for increased LNG train capacity”, 14th International Conference and Exhibition on Liquefied Natural Gas, pp. 107e124; 2004. [8] Vink K, Nagelvoort R. Comparison of base load liquefaction processes, Twelfth International Conference on Liquefied Natural Gas, May 4e7; 1998. lu M. Cryogenic turbine efficiencies. Exergy International Journal [9] Kanog 2001;1(3):202e8. [10] Renaudin G. Improvement of natural gas liquefaction processes by using liquid turbines. In: Proceedings of the eleventh international conference on liquefied natural gas. Chicago: Institute of Gas Technology; 1995. [11] Mortazavi A, Somers C, Hwang Y, Radermacher R, Al-Hashimi S and Rodgers P. Performance enhancement of propane precooled mixed refrigerant LNG Plant. Energy 2030 Conference, Abu Dhabi, UAE; November 2008. [12] Barclay M. Selecting offshore LNG processes. LNG Journal; October 2005:34e6. [13] Kalinowski P, Hwang Y, Radermacher R, Al Hashimi S, Rodgers P. Application of waste heat powered absorption refrigeration system to the LNG recovery process. International Journal of Refrigeration June 2009;32(4):687e94. [14] Mortazavi A, Hwang Y, Radermacher R, Al-Hashimi S, and Rodgers P. Enhancement of LNG propane cycle through waste heat powered absorption cooling, Energy 2030 Conference, Abu Dhabi, UAE; November 2008. [15] Aspen plus, version. Burlington, MA, U.S.A: Aspen Technology Inc.; 2006. [16] Boston J, Mathias P. In: Proceedings of the 2nd International Conference on phase equilibria and fluid properties in the chemical process industries, West Berlin, pp. 823e849; March 1980. [17] GE Energy, Gas Turbine Performances Data; 2007. [18] Somers C, Mortazavi A, Hwang Y, Radermacher R, Al-Hashimi S, and Rodgers P, Modeling water/lithium bromide absorption chillers in ASPEN plus, Energy 2030 Conference, Abu Dhabi, UAE; November 2008. [19] Somers C. Simulation of absorption cycles for integration into refining processes, Master’s Thesis, University of Maryland, College Park, MD, USA; 2009 [20] Haar L, Gallagher J, Kell G. NBS/NRC steam tables. Boca Raton: CRC Press; 1984. [21] Reid R, Prausnitz J, Poling B. The properties of gases & liquids. 4th ed. New York, NY: McGraw-Hill; 1988.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Enhancement of APCI cycle efficiency with absorption chillers Amir Mortazavi, Christopher Somers, Abdullah Alabdulkarem, Yunho Hwang*, Reinhard Radermacher Center for Environmental Energy Engineering, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2009 Received in revised form 26 May 2010 Accepted 30 May 2010 Available online 2 July 2010
Liquefied natural gas (LNG) plants consume a great amount of energy. In order to enhance the energy efficiency of the LNG plant, the potential energy efficiency enhancements of various options of utilizing the waste heat powered absorption chillers in the propane pre-cooled mixed refrigerant (APCI) liquefaction cycle were investigated in this study. After developing models of the LNG process, gas turbine and absorption chillers, eight options of gas turbine waste heat utilization were simulated. The simulation results show that by replacing 22 C and 9 C evaporators and cooling the condenser of propane cycle at 14 C and inter-cooling the compressor of mixed refrigerant cycle with absorption chillers which are powered by waste heat from the gas turbine, both the compressor power and fuel consumption reduction can be achieved as much as 21.32%. This enhancement requires recovering at least 97% of gas turbine waste heat. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Natural gas liquefaction Gas turbine Absorption chiller Waste heat utilization
1. Introduction Due to the fact that natural gas is the cleanest fossil fuel, its demand has increased recently [1]. Natural gas is mainly transported either through the pipe lines in gaseous phase or by LNG ships in liquefied natural gas (LNG) [2]. LNG plants consume great amount of energy. Liquefaction of 1 kg of natural gas needs about 1188 kJ of energy [3] depending on the liquefaction cycle and site conditions. The energy efficiency of the LNG plant can be enhanced by improving the liquefaction cycle efficiency, improving the compressor and driver efficiency and utilizing waste heat. Cycle efficiency could be improved by several ways such as optimizing the refrigerant composition, mass flow rate and operating pressures or improving cycle components such as expansion valves and heat exchangers. Lee et al. [4], Vaidyaraman et al. [5], Del Nogal et al. [6] and Paradowski et al. [7] used optimization techniques to enhance the efficiency of mixed (multi-component) refrigerant cycles by optimizing refrigerant composition, mass flow rate and operating pressures. Mixed refrigerant cycles are used in three types of LNG plants. Which are propane pre-cooled mixed refrigerant, single cycle mixed refrigerant and dual cycle mixed refriglu et al. [9] erant natural gas liquefaction cycles [8]. Kanog investigated the benefits of using turbine expander instead of Joule Thomson valve for LNG expansion process. Renaudin et al. [10] examined the effect of replacing LNG and mixed refrigerant expansion valves by liquid turbines. Mortazavi et al. [11] examined
* Corresponding author. Tel.: þ1 301 405 5247; fax: þ1 301 405 2025. E-mail address: [email protected] (Y. Hwang). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.05.043
the effect of replacing expansion valves with liquid turbines and two-phase expanders on the efficiency and capacity of the propane pre-cooled multi-component refrigerant (MCR) natural gas liquefaction cycle licensed by Air Products and Chemicals, Inc. (APCI) [12]. Kalinowski et al. [13] considered replacing the propane cycle with absorption chillers. Mortazavi et al. [14] investigated enhancing the performance of the propane cycle of the APCI cycle with absorption chillers powered by the gas turbine waste heat. None of the previous studies quantify the waste heat from the LNG plant gas turbine driver and investigate different waste heat utilization options based on the driver waste heat. In order to clearly address the waste heat utilization options, this paper theoretically investigates the potential energy efficiency enhancement in the APCI LNG plant by utilizing waste heat from the gas turbine with absorption chillers. For this purpose, the gas turbine, absorption chiller and the LNG plant process were modeled with ASPEN plus software [15], which is one of the preferred software in the oil and gas industry.
2. APCI base cycle modeling 2.1. APCI natural gas liquefaction process About 77% of LNG plants, including one at Abu Dhabi in the U.A. E., are using the APCI cycle for natural gas liquefaction, as illustrated in Fig. 1 [12]. As shown in Fig. 1, the feed gas is processed through the gas sweetening plant for the removal of H2S, CO2, H2O and Hg. As it passes through the pre-cooler and cold box, its temperature
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A. Mortazavi et al. / Energy 35 (2010) 3877e3882
Gas Pre cooler
Separator
Feed Gas Gas Sweetening Expansion Valve
LNG
Cold Box
Propane Cycle
Cryogenic Column
MCR Cycle
Separator
Propane
Gas
Butane
Liquid Fractionation
Pentane Heavier Hydrocarbons Fig. 1. Schematic diagram of APCI LNG production process.
decreases to approximately 30 C and some components are condensed at the same time. In the separator, the remaining gas and condensate are separated. The condensate is being sent to the fractionation unit, where it is separated to propane, butane, pentane, and heavier hydrocarbons. The remaining gas is further cooled in the cryogenic column to below 160 C and liquefied. Its pressure is then reduced to atmospheric pressure by passing through the LNG expansion valve. There are two refrigeration cycles utilized in this whole process: the propane cycle and the MCR cycle. The first cycle provides the required cooling to the pre-cooler, cold box and fractionation unit. The second cycle supplies the cooling needed in the cryogenic column. 2.2. Model development ASPEN Plus, which is steady-state process modeling software, was employed for modeling the APCI LNG production process. For modeling the property of substances, the PengeRobinsoneBostoneMathias equation of state was used for both the liquefaction cycle and the driver (gas turbine) [16]. All ASPEN model convergence tolerances for the relative residuals were set to be 1 104. The gas sweetening process was not modeled for the sake of simplicity. Instead the gas composition after gas sweetening unit were used for modeling the liquefaction cycle. Which is shown in Table 1. Hexane plus was approximated by n-hexane and isohexane with 0.16 and 0.24 for their mole fractions, respectively. Some of the other modeling assumptions used are listed in Table 2. Centrifugal and axial type compressors were used for the propane and MCR cycles respectively. All condensers and inter-coolers were assumed to be cooled by seawater. It was assumed that the propane cycle have five stages of cooling. The MCR is composed of nitrogen, methane, ethane, and propane with mole fractions of 0.09, 0.36, 0.47 and 0.08, respectively. The MCR cycle has a two-stage compressor with an intercooler. The fractionation unit was modeled by using “radfrac” component of ASPEN. The APCI cycle expansion processes were done by expansion valves. Flash gas recovery process is not considered. The APCI cycle without any enhancements is referred as “APCI base cycle” in this paper. The schematic of the APCI base cycle modeled in ASPEN is shown in Fig. 2. 2.3. Base cycle simulation results The simulation results of the APCI base cycle are shown in Table 3.
3. Driver selection and modeling To quantify the available amount of waste heat, gas turbines were assumed to be the driver for the compressors of the propane and MCR cycles. To obtain an accurate estimate of gas turbine performance, a gas turbine having a rated capacity of 130 MW was modeled using ASPEN software. The block diagram of the gas turbine cycle modeled with ASPEN is shown in Fig. 3. The compressor and the turbine were modeled using the compressor and turbine blocks of ASPEN software. The combustion chamber was modeled using RGIBBS block, which minimizes the Gibbs free energy of inlet streams to the combustion chamber. The fuel was assumed to be pure methane. To account for air leakages and blade cooling a portion of compressor discharge is diverted directly to the turbine instead of passing through the combustion chamber. To verify the gas turbine model vender’s data [17] at ISO condition, which is 15 C and 1 atm inlet pressure, was used. The comparison of the gas turbine simulation results with vender’s data is shown in Table 4. As it is shown in Table 4, the maximum discrepancy of the simulation results from the data is about 1.16%, which is in acceptable range. 4. Absorption cycle modeling With a heat input, absorption chillers can provide the refrigeration effect. Since waste heat sources and cooling requirements are plentiful in the APCI base cycle, absorption chillers were considered as a useful efficiency improvement option to the APCI base cycle. The absorption chiller was modeled in ASPEN as briefly described
Table 1 Gas composition after gas sweetening. Component
Mole Fraction [%]
Nitrogen Carbon Dioxide Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane Hexane Plus Total
0.100 0.005 85.995 7.500 3.500 1.000 1.000 0.300 0.200 0.400 100
A. Mortazavi et al. / Energy 35 (2010) 3877e3882 Table 2 Modeling assumptions for APCI base cycle.
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Table 3 Simulation results of APCI base cycle.
Axial compressor isentropic efficiency Centrifugal compressor isentropic efficiency Pinch temperature Seawater temperature Refrigerant temperature at condenser or super heater exit LNG temperature at the exit of cryogenic column Degree of superheating in propane cycle LNG expander exit pressure
0.86 0.83 3K 35 C 40 C 160 C 10 K 101.3 kPa
Propane compressor power Mixed refrigerant compressor power Propane cycle cooling capacity Mixed refrigerant cycle cooling capacity Propane cycle COP LNG vapor fraction after the expansion valve LNG production LPG (propane, butane, pentane and heavier hydrocarbons) Flash gas flow rate after LNG expansion valve
below. Comprehensive details of modeling absorption chillers can be found in Somer’s work [18,19].
43.651 MW 66.534 MW 115.469 MW 67.635 MW 2.645 0.014% 98.83 kg/s 11.00 kg/s 1.28 kg/s
cycle evaporators and a premium is placed on COP, water/lithiumebromide was selected as a working fluid pair.
4.1. Design selection 4.2. Model development There are a number of absorption cycles, with varying strengths and applications. Water/lithiumebromide chillers have higher coefficient of performances (COPs) than that of ammonia/water chillers but cannot be used below about 2 C evaporating temperature due to freezing concern on its refrigerant and cannot be used for the high absorption temperature due to crystallization issues of its absorbent. On the other hand, ammonia/water chillers can be used for a wide range of temperatures including below freezing temperature of water. Additionally, each fluid pair can be single or double-effect. Single-effect absorption chillers have the advantage of having less number of components and requiring lower temperature waste heat but have lower COP than those of doubleeffect chillers. After considering high temperature gas turbine exhaust, a double-effect design was selected in this study. Moreover, since ample cooling is required for high temperature propane
Compressor
Vapor Liquid Seperator
As described earlier, the double-effect water/lithiumebromide model was developed in ASPEN. First, a suitable property method was selected for the working fluid properties. For the pure water properties, the steam NBS property method in ASPEN was used [20]. For the water/lithiumebromide solution properties, an electrolyte property method known as ELECNRTL was used [21]. Next, the process was broken into individual components, and individual components were modeled. After these individual models were compiled, the complete cycle model was built. 4.3. Simulation results The parameter values listed in Table 5 were selected for the absorption cycle model inputs. A seawater temperature of 35 C
Heat Exchanger Heat Exchanger
Pump Fractionation Column
Multi Stream Heat Exchanger
Expansion Valve Fig. 2. APCI base cycle modeled with ASPEN.
Mixer, Spliter Natural Gas Stream Propane Cycle Stream Mixed Refrigerant Cycle Stream Propane Butane Pentane Plus
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Combustion Chamber
Combustion Chamber Outlet
Compressor Discharge
Compressor
Blade Cooling
Turbine W
Compressor Power
Output Power
Inlet Air
Exhaust Fig. 3. Gas turbine cycle modeled with ASPEN.
was selected because it is the maximum temperature found in Abu Dhabi. This is a conservative assumption because the COP decreases with increasing seawater temperature. In Table 5, “Seawater Temperature Difference (DT)” means the temperature difference between the seawater temperature and the condenser (and absorber) outlet temperatures. Finally, a desorber outlet temperature of 180 C was selected as a temperature representing typical chiller operating conditions and is feasible while considering the waste heat temperature. Based on these input parameter values, the modeling results shown in Table 6 were obtained. These two evaporating temperatures were selected for the simulation because these are the two highest temperature stages of the propane cycle. These COP values are used later to estimate the amount of waste heat that is needed to obtain the amount of cooling required. 5. Efficiency enhancements of APCI base cycle by absorption chiller 5.1. Integration of gas turbine model to LNG plant model To investigate the available amount of waste heat for different waste heat utilization options, the gas turbine model was integrated to the APCI base cycle model. To meet the plant power demand, the basic gas turbine model was scaled to provide the plant demand at 45 C ambient temperature under its full load condition. This is not an unreasonable assumption since some gas turbine manufactures scale their gas turbine design to meet for different demands. To investigate the part load effects two cases were considered for each enhancement option. The first case assumed that for each option the gas turbine would be the same size as that of the base cycle. Therefore, there could be some part load degradation effects. This
Table 4 Comparison of gas turbine simulation results with vender’s data at Iso condition. Parameter
ISO rated power [MW]
Efficiency [%]
Exhaust Temperature [ C]
Actual Gas Turbine ASPEN Model Discrepancy
130.100
34.6
540.0
130.103 þ0.003 (0.0%)
35.0 þ0.4 (1.16%)
540.4 þ0.4
case is referred as an “unscaled case.” On the other hand, in the second case it was assumed that for each option a gas turbine is sized to deliver the maximum plant demand at its full load. This case is referred as a “scaled case.” To prevent condensing issues and excessive pressure drop at the gas turbine exhaust it was assumed that the gas turbine exhaust gas could be cooled down to 200 C. To calculate the energy consumption of the gas turbine, pure methane was assumed as the gas turbine fuel and 50.1 MJ/kg was used as the low heating value of methane. To utilize waste heat, double-effect water/lithiumebromide absorption chillers working at 22 C and 9 C evaporating temperatures were used. 5.2. Option 1: Replacing 22 C propane cycle evaporators with absorption chillers The first option considered is replacing the 22 C evaporators of the propane cycle with a double-effect absorption chiller powered by gas turbine waste heat. The modeling results of section 4 were used to calculate the required amount of waste heat to run the absorption chiller. 5.3. Option 2: Replacing 22 C propane cycle evaporators and cooling the inlet of gas turbine with absorption chillers In this option, the 22 C propane cycle evaporator was replaced by an absorption chiller. The gas turbine inlet was cooled to 30 C using the absorption chiller. The evaporator of gas turbine inlet cooler was assumed to be at 22 C. 5.4. Option 3: Replacing 22 C and 9 C propane cycle evaporators with absorption chillers For the third option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers with evaporator
Table 5 Modeling input parameters of absorption cycle. Parameter
Value
Seawater Temperature Difference (DT) Desorber Outlet Temperature
5K 180 C
A. Mortazavi et al. / Energy 35 (2010) 3877e3882 Table 6 Simulation results of absorption cycle. Evaporating Temperature
COP
9 C 22 C
1.284 1.489
temperatures of 22 C and 9 C. Propane is also subcooled to 25 C and 12 C by 22 C and 9 C absorption chillers evaporators, respectively.
5.5. Option 4: Replacing 22 C and 9 C propane cycle evaporators cooling the inlet of gas turbine with absorption chillers In this option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers. Propane was also subcooled to 12 C and gas turbine inlet was cooled down to 17 C by using both the 22 C and 9 C evaporators of absorption chillers.
5.6. Option 5: Replacing 22 C and 9 C evaporators and cooling the condenser of propane cycle at 27 C with absorption chillers In Option 5, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 27 C and subcooled to 12 C by absorption chillers.
5.7. Option 6: Replacing 22 C and 9 C evaporators and cooling the condenser of propane at 27 C cycle and turbine inlet with absorption chillers In this option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 27 C and subcooled to 12 C and the gas turbine inlet was cooled to 17 C by absorption chillers. The gas turbine inlet was first cooled to 30 C by a 22 C evaporator and then cooled down to 17 C by a 9 C evaporator.
5.8. Option 7: Replacing 22 C and 9 C evaporators and cooling the condenser of propane cycle at 14 C with absorption chillers For Option 7, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 14 C and subcooled to 12 C by an absorption chillers.
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5.9. Option 8: Replacing 22 C and 9 C evaporators and cooling the condenser of propane at 14 C cycle and inter-cooling the compressor of mixed refrigerant cycle with absorption chillers For the final option, the 22 C and 9 C propane cycle evaporators were replaced with absorption chillers and propane was condensed at 14 C and subcooled to 12 C by absorption chillers. Mixed refrigerant was intercooled fist by seawater to 40 C and then it was cooled down to 14 C by a 9 C absorption chiller evaporator.
5.10. Simulation results The simulation results of eight enhancement options for the gas turbine fuel consumption, the power reduction, and the required amount of waste heat to operate the absorption chiller are summarized in Table 7. In Table 7 the options are ranked based on their enhancements where the option 1 has the lowest amount of reduction in fuel consumption which is directly related to the energy efficiency of the plant. Since the production capacity of plant is held constant. The fuel consumption could be reduced by either reducing the compressor power demand and/or increasing the gas turbine power generation efficiency. The compressor power demand will be reduced by lower the propane cycle condenser temperature, replacing the propane evaporators by waste heat run absorption chillers and/or inter-cooling the compressor of the MCR cycle. The gas turbine power generation efficiency could be increased by cooling the inlet air of the gas turbine. As it is shown in Table 7, by applying option 8 which is replacing 22 C and 9 C evaporators and cooling the condenser of propane at 14 C cycle and inter-cooling the compressor of mixed refrigerant cycle with absorption chillers and using a scaled gas turbine the compressor power demand could be reduced by 21.3%. Which also leads to reduction of gas turbine fuel consumption by 21.3%. The fuel consumption of the scaled gas turbine case for the same option is lower than that of the unscaled gas turbine case. Hence the efficiency of the scaled case is higher than unscaled case for the same option. This is due to the fact that in the unscaled case the gas turbine is operated at part load running condition while in the scaled case the gas turbine is operated in full load and at part load running condition the gas turbine firing temperature is lower than the full load condition. Lower firing temperature leads to lower efficiency of the gas turbine. Moreover the unscaled case requires both more amount of waste heat and higher percent of available waste heat than the scaled case for the same option. As shown in Table 7, the better the option the more amount of waste heat is required to operate the absorption chillers. The enchantments options with the scaled gas turbines will result in lower capital cost
Table 7 Enhancement results of different waste heat utilization options. Gas Turbine Sizing
Scaled gas turbine case
Unscaled gas turbine case
Option
Compressor Power [MW]
Power Reduction [MW]
Required Amount of Waste Heat [MW]
Fraction of Available Amount of Waste Heat [%]
Fuel Consumption [MW] (% saving)
Required Amount of Waste Heat [MW]
Fraction of Available Amount of Waste Heat [%]
Fuel Consumption [MW] (% saving)
APCI base cycle Option 1 Option 2 Option 3 Option 4 Option 5 Option 6 Option 7 Option 8
110.185 107.510 107.510 100.334 100.334 94.043 94.043 88.420 86.696
e 2.675 (2.43%) 2.675 (2.43%) 9.851 (8.94%) 9.851 (8.94%) 16.142 (14.65%) 16.142 (14.65%) 21.765 (19.75%) 23.489 (21.32%)
e 8.865 12.572 34.998 41.579 98.306 104.474 105.553 116.391
e 5.948 8.985 25.162 33.537 75.408 89.899 86.112 96.844
329.448 321.444 314.002 299.999 287.624 281.186 269.598 264.378 259.217
e 8.865 12.913 34.998 43.112 98.306 106.420 105.553 116.391
e 5.958 9.341 25.311 36.306 76.133 96.977 87.227 98.195
329.448 322.754 318.175 304.859 296.482 289.482 281.006 275.340 271.086
(2.43) (4.69) (8.94) (12.70) (14.65) (18.17) (19.75) (21.32)
(2.03) (3.42) (7.46) (10.01) (12.21) (14.70) (16.42) (17.72)
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for the gas turbine driver due to their smaller power capacity in comparison to the base line plant gas turbine driver. 6. Conclusion Any improvement in LNG plants efficiency will result in a considerable amount energy consumption reduction due to their huge energy consumption scale. A gas turbine driven APCI LNG plant was modeled to identify the available amount of waste heat. Double-effect water/lithiumebromide absorption chillers were modeled to investigate various waste heat utilization options. Based on the simulation results for eight enhancement options, it was found that utilizing the gas turbine waste heat by absorption chillers could reduce both the gas turbine fuel consumption by 21.3% and also the capital cost of the gas turbine driver due to reduced capacity. The maximum fuel consumption reduction was achieved by implementing option 8 with a scaled gas turbine. References [1] Hubbard B. A fresh approach to LNG. Hydrocarbon Engineering October 2004;9:29e32. [2] Bumagin G, Borodin D. Natural gas liquefier based on an EGD-generatorexpander. Chemical and Petroleum Engineering 2007;43(5e6):17e20. [3] Finn A, Johnson G, Tomlinson T. Developments in natural gas liquefaction. Hydrocarbon Processing April 1999;78:47e59. [4] Lee GC, Smith R, Zhu XX. Optimal synthesis of mixed refrigerant systems for low temperature processes. Industrial and Engineering Chemistry Research 2002;41:5016e28. [5] Vaidyaraman S, Maranas CD. Synthesis of mixed refrigerant cascade cycles. Chemical Engineering Communications 2002;189:1057e78.
[6] Del Nogal F, Kim J, Perry S, Smith R. Optimal design of mixed-refrigerant cycles. Industrial and Engineering Chemistry Research 2008;47(22):8724e40. [7] Paradowski H, Bamba M, Bladanet C. “Propane precooling cycles for increased LNG train capacity”, 14th International Conference and Exhibition on Liquefied Natural Gas, pp. 107e124; 2004. [8] Vink K, Nagelvoort R. Comparison of base load liquefaction processes, Twelfth International Conference on Liquefied Natural Gas, May 4e7; 1998. lu M. Cryogenic turbine efficiencies. Exergy International Journal [9] Kanog 2001;1(3):202e8. [10] Renaudin G. Improvement of natural gas liquefaction processes by using liquid turbines. In: Proceedings of the eleventh international conference on liquefied natural gas. Chicago: Institute of Gas Technology; 1995. [11] Mortazavi A, Somers C, Hwang Y, Radermacher R, Al-Hashimi S and Rodgers P. Performance enhancement of propane precooled mixed refrigerant LNG Plant. Energy 2030 Conference, Abu Dhabi, UAE; November 2008. [12] Barclay M. Selecting offshore LNG processes. LNG Journal; October 2005:34e6. [13] Kalinowski P, Hwang Y, Radermacher R, Al Hashimi S, Rodgers P. Application of waste heat powered absorption refrigeration system to the LNG recovery process. International Journal of Refrigeration June 2009;32(4):687e94. [14] Mortazavi A, Hwang Y, Radermacher R, Al-Hashimi S, and Rodgers P. Enhancement of LNG propane cycle through waste heat powered absorption cooling, Energy 2030 Conference, Abu Dhabi, UAE; November 2008. [15] Aspen plus, version. Burlington, MA, U.S.A: Aspen Technology Inc.; 2006. [16] Boston J, Mathias P. In: Proceedings of the 2nd International Conference on phase equilibria and fluid properties in the chemical process industries, West Berlin, pp. 823e849; March 1980. [17] GE Energy, Gas Turbine Performances Data; 2007. [18] Somers C, Mortazavi A, Hwang Y, Radermacher R, Al-Hashimi S, and Rodgers P, Modeling water/lithium bromide absorption chillers in ASPEN plus, Energy 2030 Conference, Abu Dhabi, UAE; November 2008. [19] Somers C. Simulation of absorption cycles for integration into refining processes, Master’s Thesis, University of Maryland, College Park, MD, USA; 2009 [20] Haar L, Gallagher J, Kell G. NBS/NRC steam tables. Boca Raton: CRC Press; 1984. [21] Reid R, Prausnitz J, Poling B. The properties of gases & liquids. 4th ed. New York, NY: McGraw-Hill; 1988.