LNG RECEPTION TERMINAL: OPERATING ASPECT OF RE-GASIFICATION RE-GASIFICATION SYSTEM INCLUDING LNG COLD UTILIZATION
TERMINAL DE RECEPTION GNL: ASPECT DE L’EXPLOITATION L’EXPLOITATION DE DIFFERENTS SYSTEMES DE REGAZEIFICATION, Y COMPRIS CELUI D’UTILIZATION DE L’ENERGIE FRIGORIFIQUE DU GNL 1. INTRODUCTION Since the first receipt of Liquefied Natural Gas (LNG) from Alaska in 1969, LNG importation into Japan has grown at a remarkably high rate. In the meantime, technological development and improvements on LNG facilities have continued, accomplishing high operational reliability and increased availability of re-gasification facilities, and reductions reductions in power consumption in re-gasification re-gasification processing. In its first part, this paper briefly introduces the past thirty years’ history of LNG reception terminals and a forecast mainly of their operational aspect. Then, this paper describes features of the operation of re-gasification processing and discusses further reducibility of power consumption and an increase in recovery of cold energy.
2. OUTLOOK FOR LNG RECEPTION TERMINALS IN JAPAN Japan now imports over 55 million tons of LNG per year and accounts for around 50% of the world’s LNG trade. Many LNG reception terminals have been built and operated to supply gas to users. The reception terminals constructed early in the LNG history are located in urban areas, and natural gas has been sent to power plants and/or town gas gates near terminals. Subsequently several new terminals have been built, and gas distribution systems from terminals have been expanded. Construction of gas pipelines has been continuously promoted to expand gas distribution networks with the aim of supplying gas to major users located in wider areas. Natural gas has been distributed within certain districts around central reception terminals. However, the network is currently being still more extended, and in some cases, two or three terminals are linked to one another through pipelines. If a large-scale trunk pipeline, pipeline, running throughout Japan, is constructed in future, it will allow more flexible re-gasification and gas distribution. Concerning send-out pressure, natural gas for power generation has been delivered to plants under low pressure conditions since the early phase of the LNG history. Then, the application of gas turbines for the power generation and expansion of pipeline distribution have come to require higher send-out pressures in the standardized range of 40 - 70 bar. Since 1993, small-scale LNG import terminals have been built to expand natural gas utilization for new districts. As LNG imports have been growing, an increasing number of satellite terminals have been built in local areas. Land transportation using LNG road tankers is commonly applied for local town gas business. Presently, a project of construction of a secondary terminal is being planned. This new option, in which LNG will be transported to secondary terminals by coastal ship, will be put into practice in the near future. Extension of transportation systems from reception terminals is progressing with the aim of promoting LNG utilization. Technologies for operation of re-gasification have been developed, developed, taking into consideration the foregoing developments developments in downstream conditions.
3. FEATURES OF TERMINAL OPERATION 3.1 Gas Demand Patterns and Load Factor
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The first argument concerns demand patterns in re-gasification operation. Since the LNG reception terminal provides natural gas as a public utility, the terminal shall be operated for continuous year-round supply. The re-gasification rate is highly dependent on gas demand patterns. Significant load gap between daytime and night is observed in seasonal and daily demand patterns. A terminal comprises LNG storage tank(s), LNG pumping system, re-gasification facilities, boil-off gas (BOG) handling system, ship-unloading facilities and other support systems. The terminal shall be operated to accommodate requirements of fluctuation of gas demand, and the total capacity of re-gasification facilities shall be large enough to send gas at the peak rate. A reduction of the send-out rate during night requires stable turndown operation. An annual load factor as a ratio of average/peak actually varies over a wide range, depending on users’ operating purposes, namely base-load or peak shaving. Actual requirements for gas send-out patterns at terminal exit depend on gas inventories in downstream pipeline or gasholder. Line packing brings the benefit of steady load in re-gasification. Taking advantage of line packing in gas pipeline, a gap between maximum and minimum gas send-out rates can be reduced by utilizing a pressure swing. 3.2 Availability and Reliability The overall function of a terminal is to receive, store and re-gasify LNG to be distributed to power plants and town gas gate stations via a pipeline, and is performed by various operating components of the terminal. Since unscheduled shutdown results in loss of public energy source, in principle, all facilities shall be operated to maintain 100 % availability. Inspection and repairing of equipment components of the facilities shall be completed while natural gas send-out is being continued. LNG pressurizing and re-gasifying, which are performed by means of LNG pumps, vaporizers and related support facilities, are critical functions for the terminal. Stand-by equipment, which should be ready for quick starting, should be kept available anytime, considering unexpected failure of any running item of equipment. If downstream gas inventory is not sufficient, a trip of the running equipment could cause rapid depressurizing in gas send-out stream, resulting in gas supply shortage. In such a case, re-gasifying by running the standby equipment should be applied to fulfill the availability requirements. 3.3 Maintenance and Sparing To prevent any unexpected shutdown, the terminal should employ preventative and predictive maintenance. Routine on-stream monitoring, inspection and maintenance, which do not affect operation, should be undertaken. While equipment shutdown maintenance and inspection for major deterioration diagnoses are being carried out, operation shall be continued, using spare items of equipment. Legally required inspection and maintenance also involve equipment shutdown. Due to the 100% availability requirements of the terminal, sufficient spare facilities are required to ensure that the terminal operation can continue even during legally required inspection and maintenance work. Failures of running items should be taken into consideration in scheduling shutdown maintenance. Stand-by items should be kept available while scheduled maintenance is underway. Monthly demand pattern is usually taken into consideration in planning an adequate maintenance schedule. Shutdown maintenance and inspection should be undertaken during off-peak season. 3.4 Operating Variables in LNG Vaporizing and Gas Handling Most of operating conditions of main streams and equipment are not in a steady state, but are variable due to periodical ship unloading and fluctuations in gas demand. To achieve safe, effective and reliable operation, the maximum and minimum conditions in flowing rate, pressure and temperature are to be clarified, based on the following two operation modes. All equipment components shall be operated to control operating conditions within their respective acceptable
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ranges.
Ship unloading and minimum gas send-out (Maximum boil-off & minimum re-gasification) No ship unloading and maximum gas send-out (Minimum boil-off & maximum re-gasification)
The re-gasifying rate control is achieved by adjusting the flow rate at the inlet of vaporizers in order to control send-out pressure within a normal range. When the flow rate decreases to the minimum set point, one of running vaporizers is stopped. Conversely, when the flow demand increases to the maximum set point of vaporizer, a stand-by vaporizer is switched on. All pumping facilities of re-gasification system are operated by the similar operating methodology. LNG pumps and seawater pumps for vaporizers are allowed to operate within their respective suitable operating ranges to maintain high efficiency. Auto switching on/off equipment, combined with the flow rate adjustment, attains effective operation to meet hourly movements in demand. Boil-off gas (BOG) rate, which varies depending upon weather conditions and operating modes, is separately handled. BOG compression load is adjusted to control the pressure in storage tanks within a normal range. The compressed BOG is sent directly to gas stream or re-condensed into LNG stream before vaporization. 3.5 Control System Since Distributed Control System (DCS) has been applied to the LNG reception terminal, automatic operating system has remarkably developed. The DCS, which is supported by control console with display and engineering workstation, forms the core for the monitoring and control. Through the interface with operators, DCS provides useful information and guidance to support the terminal operation. In addition, several management systems can be linked with DCS system to assist operational work on tank inventory management, maintenance scheduling, guide for stepwise operation and so on. Current sophisticated control, monitoring and support systems allows of full automatic operation, which includes switching on/off in the re-gasification facilities. 3.6 Power Consumption To discuss energy saving in LNG business, energy consumptions throughout LNG transportation process are assessed at first. The conventional system of LNG re-gasification is to vaporize pumped-up LNG, using large quantities of seawater or combustion of fuel gas as heat source. To minimize operating expenditures, seawater, which is a natural heating medium, is usually used to vaporize LNG. Electric power is principal energy source and is consumed, mainly in pumping facilities. Accordingly, consumption in re-gasification is very low in LNG chain. To achieve long distance transportation of LNG from production site to end users, liquefaction processing consumes a large quantity of energy, and the significant part of this energy input is transferred to the reception terminal as cold energy. For further energy saving in the LNG transportation stream, it is essential to recover as much cold energy as possible, as generally understood. Potential quantity of cold energy recovery is quite large, suggesting that effective cold utilization brings more energy saving in the whole of the LNG chain. This theme is discussed in Section 4. Regarding power consumption for terminal operation, re-gasification and BOG handling are major consumers in the terminals. In the long history of the LNG industry, development in operating techniques supported by automatic control system has substantially reduced power consumption for re-gasification.
4. LNG COLD UTILIZATION
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4.1 Current Status of LNG Cold Utilization The most effective way to use LNG cold is to provide cold energy to users, which require refrigeration in their production facilities. It is obvious that refrigeration at a lower temperature level requires larger power consumption. In other words, the utilization of LNG cold energy to replace the conventional electrical refrigeration under cryogenic temperature level accomplishes a large quantity of power saving. According to the typical heating curve of LNG, potential rate in power saving is estimated at around 270 kWh/ton LNG. In parallel with the LNG growth, several cold utilizations have been realized and promoted. A total of approximately 2000 t/h of LNG is used to provide cold energy to utilization systems in Japan, and this quantity accounts for around 20% of total LNG imports. In assessing an extent of power saving in existing systems, overall power recovery rate is estimated at around 8% of the total potential value. This percentage suggests that the remaining 92% of the cold energy is still being wasted and more cold utilization will attain more energy saving. Existing cold utilization facilities in Japan are classified into two types.
Internal use of energy such as part of terminal functions: BOG re-condensing and cold power generation Integration with external production plants or other refrigeration systems: Air separation, liquid carbon dioxide production, and refrigerated warehouse
Cold power generation can be easily applied to re-gasification processing, because the power generated can be used internally for the terminal operation and no operational problem with outside arises. Many power generation plants are actually running in the terminal sites. There are three types of plants, namely simple direct expansion, Rankine cycle, and combined system. The combined system, in which both direct expansion and Rankin cycle are integrated on a re-gasification stream, produces high power output. The direct expansion system, however, is applicable when send-out pressure is low. Therefore, Rankine cycle will be applied mainly to future projects. Power production rate is not high, as compared with other cases, because the cycle system converts thermal energy into work for power output. Concerning BOG re-condensing, saving power can be evaluated as a reduction of power for boosting compression. Since re-condensing process requires a significant quantity of LNG due to low boiling point of the BOG, the rate of energy saving is low. The above re-condensing system, however, has been applied as part of re-gasification facilities, because elimination of boosting-up provides benefit in reduction of investment cost. Re-condensing, which is a very simple process, does not aff ect any operational requirement of re-gasifying function. Regarding integrated systems, the air separation business has been widely applying this type of cold utilization system. Since cold energy is utilized in a deeper low temperature range, this type accomplishes a high rate of power saving, namely around 250 kWh/ton LNG. As for the other categories of business, cold energy is utilized to replace electric refrigeration in carbon dioxide production and refrigerated warehouses. Power saving rate in these businesses is moderate, because LNG cold energy in a low temperature range is transferred to users at high service temperature level. However, required quantities of refrigeration for commercial scaled plants are comparatively small. Accordingly, even if continuous re-gasification load is limited, export of cold energy may realize feasible businesses if terminal sites are conveniently located. 4.2 Integration System Expansion of recovery of cold energy requires a larger continuous re-gasification rate. This depends on further extension of downstream network. Sufficient line packing would create favourable situation. Such circumstance will facilitate the scaling up of cold power generation. Although internal cold use is easily applied, efficiency in power saving is limited. For cold utilization with higher efficiency, the integration of a large-scale system with the
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outside should be studied. Users of cold energy should be located, in principle, close to terminals. In addition, since cold utilization requires additional investments to supply cold energy, high load factor is necessary for feasible integration. Possible approaches are: Integration with power plant or refinery/petrochemical complex, which requires significant refrigeration capacity Integration with user, which requires significant quantities of nitrogen and/or oxygen
With regard to the above integrations, one typical approach is integration with ethylene plant. The ethylene plant requires significant amount of refrigeration in a wide temperature range, where LNG heating curve is adequately matched with requirements for refrigeration in the ethylene plant. Although such integration requires consideration of several operational interfaces with the partner, it is essential to tackle the challenge of how to realize effective cold utilization.
5. Observations Through the overlooking of the development of LNG handling technologies, it has been found high-level operating technique has been established and long-term operating experience has proven the achievement of safe, reliable and cost-effective operation in the reception terminals. A further expansion of downstream distribution systems is foreseeable, and advanced design and operating technologies will be developed, with requirements on interface with downstream network incorporated. Further technological development in equipment components of terminals will continue to be promoted. However, since energy consumption for re-gasification processing is at a low level, substantial further reduction of energy consumption seems to be difficult. Another direction of power saving will be to increase the cold utilization. Expansion of the existing cold utilization businesses or integration with the outside to provide high recovery rate will contribute to saving in power consumption in the LNG chain. To realize the integration of terminals with the outside plants on a large scale, adequate conditions in locality, interface, load factor, matching in process variables and so on are essential. Cold utilization projects should be promoted based on circumspect planning. References 1. Fereidun Fesharaki, “Asia-Pacific Energy Outlook” LNG-13, 2001 2. M. Oka, T. Matsutani, M. Torihara, T. Sano, “National Gas Transmission Pipeline” LNG 10, 1992 3. M. Sugiyama, M. Ishida, T. Sasaki, K. Ichimura, “The Operation Technology of LNG Terminals of Tokyo Electric Power Company” LNG 12, 1998 4. Y. Hirayama, “Small Scale LNG Project” Gastech 1994 5. J. Jensen, “The Emerging Competition between Pipelines and LNG” Gastech 2000 6. Hyun Cho, Sbhash N. Shah, “Maximized Synergy Effects by Integration of an LNG Terminal and an Adjacent Power Plant” LNG 13, 2001
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