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Paper PS3-7 1 TECHNICAL ISSUES RELATED TO SEND OUT GAS CALORIFIC VALUE CONTROL AT LNG RECEIVING TERMINALS IN JAPAN Yoshi
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Paper PS3-7
TECHNICAL ISSUES RELATED TO SEND OUT GAS CALORIFIC VALUE CONTROL AT LNG RECEIVING TERMINALS IN JAPAN Yoshifumi Numata Technical Adviser Process Engineering Center Gas and LNG Daisuke Wada Process Engineering Center Gas and LNG Chiyoda Corporation Yokohama, Japan [email protected]
ABSTRACT Most domestic LNG receiving terminals in Japan import LNG from various sources including the LNG spot market and, consequently, they are required to handle gas with differing calorific values such as rich LNG and lean LNG. Furthermore, the calorific value of LNG increases if stored long-term in LNG storage tanks due to the generation of boil off gas (BOG). Since the domestic LNG receiving terminals are mainly owned by gas or electric power companies, the following requirements have to be considered on the quality of the send out gas. For city gas, the send out gas should be maintained within a specified narrow calorific value range. For fuel gas exported to relatively newly-advanced combined cycle power plants, it is not only the narrow calorific value range limitation that needs to be considered but also the limitation on the change rate of gas calorific values. As the receiving terminals import LNG from various sources, they need to adopt the following measures so as to render the LNG suitable for use as send out gas: •
Calorific value is adjusted by adding LPG in LNG or vaporized gas.
•
Calorific value increase in the LNG concentration, due to long-term storage, is suppressed by returning re-liquefied BOG to the LNG storage tank.
•
A large gas mixing vessel is installed to limit the rate of the change of the gas calorific value.
This paper initially introduces the situation as it relates to LNG in Japan, and then goes on to describe the technologies that are effective in controlling the calorific value and its change rate of sent out gas.
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2. PROCUREMENT OF LNG It is 40 years since Japan started to import LNG in 1969. Up to now, the main buyers of LNG have been East Asian countries such as Japan, Korea and Taiwan, where LNG has been procured by tying in to long-term contracts with LNG producers to secure supplies of LNG with stable calorific values. As shown in Table-1, the East Asian countries import LNG with relatively high calorific values (“rich LNG”). The volume of LNG imported into Europe and the United States has recently been increasing as well as there being an increase in the number of countries importing LNG. These changes have resulted in an increase in the number of countries exporting LNG to satisfy the demands associated with the increase in consumption. In these circumstances, the LNG market, which was known as a closed market in the past, has become more flexible and the number of spot contracts has proliferated as shown in Figure-1. Even in Japan, when the lean LNG spot price is much more attractive than rich LNG spot price, lean LNG is imported by spot contract. The LNG receiving terminals therefore need to operate under the condition that both lean LNG and rich LNG should be treated. Table-1 LNG calorie and LNG importing / exporting countries Export country / region
Calorie
Import country
3
Btu/scf
MJ/Nm
Libya
1375
54.15
Spain
Oman
1160
45.68
Japan, Korea, Spain, France
Abu Dhabi
1141
44.93
Japan
Brunei
1134
44.66
Japan, Korea
Australia
1132
44.58
Japan, Korea
Malaysia
1122
44.18
Japan, Korea, Taiwan
Indonesia
1118
44.03
Japan, Korea, Taiwan
Algeria (Azrew)
1116
43.95
France, Spain, Belgium, Italy, USA, Turkey
Qatar
1114
43.87
Japan, Korea, India
Nigeria
1110
43.71
France, Spain, Italy, Turkey, Portugal
Algeria (Skikda)
1082
42.61
France, Spain, Greece
Trinidad Tobago
1041
40.99
USA, Spain, Puerto Rico
Alaska
1011
39.81
Japan
Source: JOGMEC website Petrol and Natural Gas Review 2005.9 VOL.39 No.5
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(100 million m 3)
3,000
19.7% Non-spot contract
Spot contract 15.1%
2,500
10.7%
2,000
12.1%
8.7% 1,500
1,000
0.6%
1.8% 1.6%
2.8%
2.1%
1.5% 2.0%
3.8%
5.5%
7.5%
7.6%
500
0 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 (Year)
Figure-1 Spot contract share in global LNG trade Source: Statistics by BP, Petrostrategies (Note) The “Spot contract” refers to the trade with less than 1 year contract period.
3. CURRENT SITUATION ON THE LNG CUSTOMERS’ SIDE Japan imports approximately 67million ton per year (based on 2007 data) from various LNG plants throughout the world, approximately 70% of which is allocated to thermal power plants and the remainder allocated to city gas. Most LNG receiving terminals are owned by electric power or gas companies (some terminals are owned by oil companies) where, basically, individual companies install and operate their own pipelines for send out gas. The current situation of the send out gas for city gas and thermal power plant is as follows. 3.1 City gas application Almost 90% of city gas originates from imported LNG (2008) and is used not only for household applications but is also supplied for industrial and commercial purposes. To ensure optimum combustion conditions when being used in gas appliances, city gas needs to satisfy some criteria regarding combustion characteristics. For instance, “WI (Wobbe Index)” for “13A” should be within the range of 52.7 - 57.8 (13A is one of the standard gas specifications used widely in Japan). Wobbe Index WI = H/√¯s (MJ/Nm3) H: Gross calorific value (MJ/Nm3) s: Specific gravity (air =1) 3
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While calorific values differ depending on the gas company that supplies city gas, it is normally controlled at approximately 46MJ/Nm3 (WI= approx. 57.5) and the allowable range of fluctuation is less than ±1% of the standard value. Therefore, LNG receiving terminals have to carry out strict calorific value adjustment to ensure this requirement is maintained for users. Since city gas is required to have a slightly higher calorific value than conventional rich LNG, it is essentially calibrated by adding LPG. However, because the price of LPG fluctuates so much and the price is uptrend, gas companies have started to reduce the standard calorific value in recent years so as to save on the amount of LPG that needs to be injected. [1] 3.2 Thermal power plant application As the distance from the LNG receiving terminal to the power plant is relatively short, and no gas holder is provided in most cases, the rate of gas flow is adjusted at the LNG receiving terminal side in response to the fluctuation in consumer demand for the gas. Because conventional power generation facilities consist of boilers and steam turbines, the allowable range of calorific value of fuel gas is wide which, unlike the case with city gas, eliminates the necessity for calorific value adjustment. However, in recent years relatively new advanced combined cycle (“ACC”) power generation (GT 1,300 - 1,500°C class) facilities have been introduced into power plants, or have replaced conventional boilers and steam turbines completely. The ACC allows only a narrow range of calorific values for fuel gas compared with conventional power generation facilities and there are stringent restrictions in its use, particularly for the change rate of calorific value.
4. TECHNOLOGIES FOR CALORIFIC VALUE ADJUSTMENT IN LNG RECEIVING TERMINALS To address the situations mentioned in Section 2 & 3, and LNG concentration issues, Chiyoda Corporation has carried out design development and studies under the EPC or FEED contracts with owners for the following technologies related to the calorific value adjustment. 4.1 Calorific value adjustment by adding LPG As stated in Paragraph 3.1, because the calorific value of received LNG is slightly lower than that required for city gas, LPG is added to the LNG as an agent to increase its calorific value to convert it into city gas (product gas) before sending out. Domestic terminals use either propane or butane to increase the calorific value, depending on the procurement environment of the calorie increase agent, the send out gas pressure and so on. For example, Table-2 indicates the volume of LPG necessary to increase the calorific value of LNG from 43 MJ/Nm3 to 45 MJ/Nm3 as well as the dew point of the product gas at a pressure of 3MPaG. If butane is used to increase the calorific value, the dew point of the product gas is higher than that treated with propane. Therefore when the pressure of the send out gas is high, butane cannot be used as an agent to increase the calorific value since a part of the product gas may cause condensation.
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Table-2 Comparison of propane and butane as calorie increase agent Propane
n-Butane
45.955 MJ/kg
45.342 MJ/kg
Volume necessary to increase LNG calorific value of LNG from 43 MJ/Nm3 to 45 MJ/Nm3 to make 100ton product gas.
8.6 ton
7.4 ton
Dew point when pressure of the above product gas with 45MJ/Nm3 is 3MPaG
-23°C
-8°C
Calorie increase agent Gross calorific value (GPSA)
Table-3 indicates the methods of adding LPG. The appropriate method will be selected considering LPG composition, availability of the thermal source, such as steam or hot water, plot plan and facility investment costs. Table-3 Comparison of calorific value adjustment methods Liquid / Gas calorific value adjustment
LPG (Liquid)
Product gas
Gas / Gas calorific value adjustment
Natural gas
Mixer
Natural gas
Concept chart
Product gas
LPG (Gas)
Gas heater
LPG (Liquid)
Drum
Natural gas and LPG are mixed by spraying LPG inside a venturi type mixer or a drum.
Both natural gas and LPG are mixed in the gas phase.
Both LNG and LPG are mixed in the liquid phase. Vaporization is conducted by using an LNG vaporizer after calorific value adjustment.
Basic concept Sensible heat of heated natural gas is used as thermal source to evaporate LPG.
Characteristics
Liquid / Liquid calorific value adjustment
• A natural gas heater is necessary.
• An LPG vaporizer is necessary.
• Thermal source of a lower temperature may be used.
• Thermal source of a high temperature, such as steam, is necessary.
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• A natural gas heater and an LPG vaporizer are unnecessary. • A different facility would be needed for calorific value adjustment of BOG that is generated in a storage tank.
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In general, a combination of feedforward control and feedback control are used to regulate the calorific value adjustment within the specified range, as shown in Figure-2. Feedforward control:
Flow rate control is conducted by calculating the necessary LPG flow rate based on the flow rate and calorific value of natural gas, calorific value of LPG (fixed value) and calorific value of target product gas.
Feedback control:
Correction is made on the LPG flow rate control to eliminate the gap between the calorific value based on actual measurement and the targeted calorific value for product gas.
Heat adjustment processing
Natural gas
QX
FX
QX
Product gas
Gas heater FX
LPG (Liquid) Figure-2 Typical calorific value control for LPG liquid / gas calorific value adjustment method. 4.2 BOG Re-liquefaction in case of LNG concentration Since LNG unloading does not happens that often, perhaps up to 20 times per year in some LNG receiving terminals in Japan, consideration should be given to the LNG concentration issue. If LNG is stored long-term in an LNG storage tank, the light components (i.e. methane) evaporate earlier as BOG than the heavier components, resulting in an increase in the calorific value of the remaining fraction of LNG. BOG generated in an LNG storage tank is normally pressurized by BOG compressors and sent out together with vaporized gas from the LNG vaporizers. The concentration of the LNG is not so significant when stored for short periods but when stored for long periods, due to the prolonged receiving intervals, the calorific value of LNG may exceed the level agreed upon with gas customers. As a countermeasure to prevent such an LNG concentration issue, the BOG is re-liquefied and returned to the LNG storage tanks. As indicated in Figure-3, BOG pressurized by the BOG compressor is heat-exchanged with LNG at the BOG re-liquefaction condenser and the re-liquefied BOG is returned to the LNG storage tank.
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Figure-3 Flow chart for returning re-liquefied BOG to LNG storage tank The following points need to be considered for BOG re-liquefaction technology. •
Availability of LNG flow rate
The sensible heat of LNG supplied to send out gas is used to re-liquefy BOG at the BOG re-liquefaction condenser. Because the flow rate of send out gas follows the consumption level on the demand side, the available flow rate of LNG for cooling will depend on the flow rate of the send out gas. Therefore, when demand for send out gas is low, all the BOG may not be reliquefied due to there being insufficient cooling capacity. •
Returning re-liquefied BOG to an LNG storage tank
When the re-liquefied BOG is returned to an LNG storage tank, a part of it is flashed and approximately 10% becomes gas again. Because this flashed gas contains a high nitrogen content, the concentration of nitrogen components in the BOG from the LNG storage tank will increase, resulting in increasing difficulty in the re-liquefaction of the BOG. Therefore, it is required to return the re-liquefied BOG to liquid phase section of the LNG storage tank as shown in Figure-3. 4.3 Calorific value adjustment by blending rich LNG and lean LNG Due to its cost advantage, lean LNG with a calorific value lower than the lowest allowable level agreed upon with gas customers is occasionally purchased. In such a case, rich LNG and lean LNG will need to be stored in different storage tanks to avoid the occurrence of LNG stratification in the tank (a contributory factor to LNG rollover). In LNG receiving terminals where such a strict calorific value adjustment is not required, the blending method indicated in Figure-4 may be used. In this method, separate lines for lean LNG and for rich LNG each lead to an LNG vaporizer, and conventional rich LNG is added to the lean LNG at the inlet of the vaporizer. This approach will ensure that the calorific value of send out gas is adjusted to a level higher than the lowest allowable level agreed upon with the customer side.
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However, because the difference in the calorific values of lean LNG and rich LNG is not as large as the gap between the calorific values of LPG and LNG, a substantial increase in the calorific value of lean LNG cannot be achieved.
PC
Rich LNG
LNG vaporizer
P FT
Rich LNG Line
Lean LNG
P
FT
Lean LNG Line
Figure-4 Flow chart of calorific value adjustment by LNG blending 4.4 Countermeasure for the fluctuation in the calorific value of send out gas The BOG generated in an LNG storage tank is pressurized by BOG compressors and sent out together with vaporized gas in LNG vaporizers. If the BOG compressor trips for some reason, the mixing of the BOG in LNG vapor gas will not happen. In such circumstances, the send out gas supplied to the consumers will change almost immediately from a mixture of LNG vapor gas and BOG to LNG vapor gas only. If the gas user is an electric power plant using around 1,300°C - 1,500°C class ACC, such a severe change in the gas composition (namely, the sudden increase in its calorific value) may lead to damage of the facility and emergency shutdown. To prevent this from happening, a gas mixing vessel is installed on the pipeline between the point of BOG mixing and the ACC so as to maintain the rate of change of calorific value within permitted limits for the ACC (Figure-5). When used in overseas LNG plants, gas mixing vessels have successfully demonstrated their value as a countermeasure against any calorific value fluctuation of fuel gas for gas turbines in power generation or refrigerant compressor driver applications. Figure-6 shows the result of a simulation conducted for the rate of change of calorific value fluctuation (WI fluctuation) of send out gas when the BOG supply is suddenly stopped while a mixture of LNG vapor gas and BOG is being supplied. Without a gas mixing vessel being installed, the gas changes almost immediately after the BOG supply is stopped, from a mixture gas to an LNG vapor gas only, resulting in a drastic change rate of WI. On the other hand, if a mixing vessel is installed, the change rate of WI is mitigated by the mixing effect of the vessel. This result shows that the mixing vessel is an effective system to prevent such a significant change rate of WI.
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LNG vaporizer
Power Plant (ACC)
LNG Gas mixing vessel
BOG BOG compressor
Figure-5 Flow chart of gas mixing vessel arrangement 0.30 Without gas mixing vessel (piping only) With gas mixing vessel
9.00 8.00
0.27 0.24
[Condition] Initial status: LNG vapor gas 30t/h, BOG10t/h Stop of BOG supply: LNG vapor gas 40t/h, BOG 0t/h Time = 0sec refers to the time when BOG compressure stopped
7.00 6.00
0.21 0.18
5.00
0.15
4.00
0.12
3.00
0.09
2.00
0.06
1.00
0.03
0.00
With gas mixing vessel: Change rate of WI [%/s]
Without gas mixing vessel (piping only): Change rate of WI [%/s]
10.00
0.00 0
2
3
4
6
7
9
11 12 14 15 17 18 20 21 23 24 26 27 29 30 Time [s]
Figure-6 Simulation result for time-dependent changes in the change rate of WI due to the arrest of the BOG supply
5. CONCLUSION In this report, the technologies used in the domestic LNG receiving terminals for calorific value adjustment have been discussed and the environment surrounding the domestic LNG market and the gas demand sides’ requirements related to calorific value have been considered. LNG receiving terminals will need to cope more often in the future with LNG with a wide range of calorific values and the requirements of gas users will be more stringent. In this situation, the technology for calorific value adjustment in LNG receiving terminals will increasingly become more relevant and important.
REFERENCE CITED [1] JOGMEC website: Petrol and Natural Gas Review 2005.9 VOL.39 No.5. 9