A SOPHISTICATED, LOW-COST, ENERGY-EFFICIENT, SMALL-CAPACITY LNG VAPORIZER AND ITS PRACTICAL USE VAPORISATEUR DE LNG SOPHISTIQUE A FAIBLE CAPACITE FONCTIONNANT A COUT REDUIT AVEC HAUT RENDEMENT ENERGETIQUE ET SON APPLICATION PRATIQUE Masaru Sekiguchi Tokyo Gas Co., Ltd. Tokyo, Japan www.tokyo-gas.co.jp/index_e.html Hirokazu Mori Tokyo Gas Engineering Co., Ltd. Tokyo, Japan ABSTRACT Tokyo Gas in collaboration with Tokyo Gas Engineering has succeeded in developing a completely new kind of compact LNG vaporizer for small-capacity LNG terminals. It makes optimum use of ambient heat to vaporize LNG for the supply of town gas above the minimum allowable temperature requirement, even in cold regions. The new vaporizer costs 20% less to build than previous systems, and enables a massive 70% reduction in operating costs through considerably reducing reliance on fuel as a heat source. LNG vaporizer systems in small-capacity LNG terminals have up to now had hot water vaporizers for use during winter fitted alongside air fin vaporizers. The requirement for additional equipment to provide a heat source for the hot water vaporizers also made each system complex and costly. The new system combines an air fin vaporizer with a hot air source in one unit. The resulting system is simple, can be used throughout the year even in cold climates, and is very energy-efficient, and accordingly environment-friendly. The very first unit was installed at the Tokyo Gas Kofu satellite terminal, where it demonstrated excellent vaporizer performance. The first unit to be made commercially available in the Japanese market was operated for over a year by Chubu Gas at its Hamamatsu satellite terminal, and the units have been adopted for base load operation at a large number of terminals, including some in cold regions. RESUME Tokyo Gas, en collaboration avec Tokyo Gas Engineering, a réussi à développer un vaporisateur de GNL compact de type complètement nouveau pour les terminaux de GNL de faible capacité, ce qui permet l utilisation optimum de chaleur ambiante pour la vaporisation de GNL en vue de la fourniture de gaz de ville à une température inférieure à la valeur minimum admissible ou dans des régions froides. Le coût de construction du nouveau vaporisateur est inférieur de 20% à celui des systèmes précédents, ce qui réalise une forte réduction du coût d exploitation de 70% en réduisant considérablement la dépendance du combustible comme source de chaleur. PO-14.1
Jusqu à présent, les systèmes de vaporisateur de GNL des terminaux de GNL de faible capacité utilisaient des vaporisateurs à eau chaude pendant l hiver, en les installant juste à côté des vaporisateurs à air à ailettes. La nécessité d un équipement supplémentaire destiné à fournir une source de chaleur pour les vaporisateurs à eau chaude rend ainsi chaque système complexe et coûteux. Le nouveau système combine le vaporisateur à air à ailettes avec la source d air chaud dans un seul bloc. Le système ainsi obtenu est simple, capable d utilisation à longueur d année et très efficace sur le plan énergétique, d où le respect de l environnement. L appareil initial a été installé au terminal satellite de Kofu de Tokyo Gas, où il a fait preuve d excellentes performances de vaporisation. Le premier appareil rendu commercialement disponible sur le marché japonais a été mis en service durant plus d un an par Chubu Gas à son terminal satellite de Hamamatsu, et les appareils ont été adoptés pour la demande de base dans un grand nombre de terminaux, y compris certains situés dans les régions froides. 1. INTRODUCTION In areas of Japan where the supply of town gas through pipelines is not economically feasible, LNG is transported by road tankers and other means from receiving terminals to small-capacity LNG terminals, where the LNG is converted to town gas. There are currently about 40 small-capacity LNG terminals in Japan, and more are expected to be built in the future. At small-capacity LNG terminals, the most common means of vaporizing LNG is to use natural draft air fin vaporizers. These utilize ambient heat, and thus cost very little to operate. However due to accumulation of frost on the heat exchange surface, the duration of continuous operation is limited, making changeover vaporizers essential. Moreover in colder regions, hot water vaporizers and accompanying hot water generation equipment are also needed to counter drops in the gas temperature at the exit from the air fin vaporizer. As a result, the overall vaporizer system flow becomes very complex, making for high construction costs, and the large amount of fuel required as a heat source for hot water vaporizers in winter also makes operation very expensive. To overcome these drawbacks, Tokyo Gas and Tokyo Gas Engineering developed a new low-cost, energy-efficient vaporizer, known as an HAV (Hot air draft superheater with Air fin Vaporizer), that incorporates a hot air source into a single unit based on a conventional air fin vaporizer. This report introduces HAVs, including the results of test operation of the first unit at the Tokyo Gas Kofu satellite terminal, and also reports on the operating performance of the first commercial unit at the Chubu Gas Hamamatsu satellite terminal. 2. CONVENTIONAL LNG VAPORIZERS SYSTEM 2.1. Natural Draft Air Fin Vaporizers Air fin LNG vaporizers that made the atmosphere as the heat source are mainly being used in Japan for small-capacity LNG terminals, where the amount of LNG handled is relatively small. PO-14.2
An example of the structure of an air fin LNG vaporizer is shown in Fig. 1. This vaporizer has vertical heat exchanger tubes made from an aluminum alloy, with the fin and the pipe being formed as a single unit by extrusion molding. The heat exchanger tubes are connected to the header pipe or the bend pipe. The flow of an air fin LNG vaporizer is shown in Fig. 2. The vaporizer consists of an evaporation part and a superheating part. There are two types of natural draft air fin vaporizer. In one, the evaporation part consists of parallel-connected heat exchanger tubes connected with the header pipe, and the superheating part consists of serial heat exchanger tubes connected with the bend pipe. In the other, both the evaporation part and the superheating part consist of serial-connected heat exchanger tubes connected with the bend pipe. A natural draft system is a system in which air convection occurs automatically from top to bottom due to density change of the air cooled by the heat exchange with LNG. This system is currently used in small-capacity LNG terminals because there is no need for power to provide a heat source, but its continuous operation time is as short as about 4 hours. Moreover, when the air exchanges heat with the LNG, and its temperature drops, the moisture in the atmosphere condenses, producing clouds of mist, meaning that consideration must be given to the surrounding environment. Cross-section of a tube Plane view Heat exchanger tube Header pipe Bend pipe Elevation view Edge view Fig. 1 An example of the structure of an air fin LNG vaporizer Header pipe Bend pipe Bend pipe Heat exchanger tube Heat exchanger tube Vaporizer Superheater Vaporizer Superheater Fig. 2 The flow of an air fin LNG vaporizer PO-14.3
2.2. An Example of a System Configuration The exit gas temperature of an air fin vaporizer does not exceed ambient air temperature, and usually, because the moisture in the atmosphere freezes on the surface of heat exchanger tubes and acts as heat resistance, exit gas temperature falls as operation progresses. Therefore, when exit gas temperature becomes lower than the minimum use temperature of the piping material, the taking into account the relation between design air temperature and design continuation operation time, either a hot water bath vaporizer is installed alongside the air fin vaporizer, or an afterheater is installed downstream of the air fin vaporizer. Especially, in cold districts where the lowest temperature of a day becomes 0 C or less, either the hot water bath vaporizer is installed alongside an air fin vaporizer and used in winter, or only the hot water bath vaporizer is installed and is used year-round. The general example of a system configuration with a hot water bath vaporizer installed alongside an air fin vaporizer is shown in Table 1. When supply capacity 1 t/h is made into 100%, two air fin vaporizers of 100% capacity are installed, and it operates, taking turns to stop for defrosting at regular intervals, in order to restrict the exit gas temperature fall due to frosting on the heat exchanger tube surface. Moreover, the hot water bath vaporizer of 100% capacity is installed for winter season operation. In order to supply hot water to the hot water bath vaporizer, incidental facilities such as a hot water boiler and a hot water pump are also installed. Table 1 An example of system configuration Supply capacity Design pressure Production gas temperature Configuration Flow 1 t/h 0.97 MPa over 0 C Air fin vap.: 1 t/h x 2 units Hot water vap.: 1 t/h x 1 unit Hot water boiler: 400kW x 1 unit Hot water pump: 7.5kW x 1 unit Capacity:100% LNG Air fin 100% Air fin 100% Hot water Gas Boiler PO-14.4
3. HOT AIR DRAFT SUPERHEATER WITH AIR FIN VAPORIZER (HAV) 3.1 HAV Outline A hot air draft superheater with air fin vaporizer (HAV) system was newly devised and developed. The HAV system consists of a vaporizer, superheater, and hot air draft generator as shown in Fig. 3. After the forced-draft vaporizer makes maximum use of the ambient atmospheric heat to vaporize the LNG (-162 C in its liquid form), an auxiliary hot air draft is produced in the superheater to boost the temperature of the gas. [Vaporizer] [Fan] [Superheater] LNG NG Air Combustor Air [Hot air draft generator] Fig. 3 HAV Outline 3.2 Example of a Design for a New System The changes during a day of the gas supply requirements determine the system configuration of the HAV system. In a standard setup, two separate HAV setups are installed, with each line having capacity equivalent to 50% of the maximum supply. Fig.4 shows the pattern of operation with two lines operated according to the supply required. An example of how a HAV standard system could be configured is shown in Table 2. The elevation view and plane view of the HAV system is shown in Fig. 5. It is basically possible to operate a single line continuously for 24 hours, and even when weather conditions are severe (ambient temperature below 5 C, snow or rain) it can be run continuously for more than 13 hours. By installing two lines, gas can be produced continuously according to the amount of supply required. Supply 100% 75% 50% Time 9 101112131415161718192021222324 1 2 3 4 5 6 7 8 Line A operation Line B operation Fig. 4 Pattern of supply and operation PO-14.5
Table 2 Example of configuration of a standard HAV system Supply capacity Design pressure Production gas temperature Configuration Flow 1 t/h 0.97 MPa over 0 C Vaporizer: 0.5 t/h x 2 units Superheater: 600Nm3/h x 2 units Hot air generator: 2 units LNG Capacity:50% A line Gas B line 50% Bypass piping (dotted line) connects the inlet piping from each unit to the outlet piping of the other unit. Fan 2.8m Vaporizer 7.4m 5.5m 4.0m A line 5.5m Superheater Hot air draft generator B line Fig. 5 Elevation view and plane view (1 t/h) 3.3 Features of the New Vaporizer System (1) Low cost Compared with systems that combine natural-draft air-fin vaporizers with hot water vaporizers, the HAV is a simple system that makes the maximum use of ambient atmospheric heat. Trial calculations were made for the installation of system with 1 t/h supply capacity in a cold district. This achieves a 20% reduction in construction costs. The design also produces a remarkable 70% reduction in operating costs, due to requiring considerably less fuel as a heat source. Moreover, the installation space required for the PO-14.6
vaporizer can be 25% smaller, and there is no need for space to install incidental facilities such as hot water boilers. (2) Use in cold regions - Vaporizer: The fan located at the top of the vaporizer draws in large volumes of air, curbing the accumulation of frost and the resulting reduction in performance this causes. The vaporizer also features newly redesigned air fin tubes with large surface areas for heat exchange. - Superheater: To ensure that the gas absorbs the heat of the hot air draft with minimum waste, the gas and hot air flow in opposite directions, and tubes with a large number of circular fins are used. - Hot air draft generator: When the ambient temperature drops, the temperature of the hot air draft can be raised to maintain the vaporized town gas above the minimum allowable temperature requirements for the downstream piping material. - Defrosting: Defrosting can normally be carried out by forced ventilation using fans, doing away with the need for sprinkler-based defrosting systems. In mid-winter, heat from the hot air draft can be used to augment normal defrosting measures. - Thanks to the above features, continuous operation where output matches town gas demand is feasible without incidental facilities such as hot water boilers. Moreover the HAV system can be used in cold regions where the ambient temperature drops below 0 C in winter. (3) Calorific value stability of production gas - Startup: When one line is stopped due to a change in gas demand during today, overheated gas from the operating line is connected to the stopped line. This setup prevents calorific value fluctuation on starting because it purges LNG that remains inside the vaporizer. - Operation: The superheater structure is arranged so that flow is on the level or descending in order to prevent fluctuation in calorific value due to retention of fluid in the system and consequent evaporation of high calorific value components if non-evaporated LNG enters the superheater. (4) Mist prevention - Problems with clouds of mist, arising in conventional air fin vaporizers from condensation of atmospheric water vapor, are resolved in the HAV by the fan on top of the vaporizer, which disperses the water vapor before it can condense. (5) High reliability - The HAV uses aluminum alloy, well known for its excellent performance and durability at low temperatures, as the heat exchange material. - The fan and hot air draft generator are based on general-purpose products, the durability of which has been proven by performance. We have also established a speedy maintenance service. PO-14.7
4. THE TEST OPERATION RESULT OF THE FIRST HAV The first HAV was installed in Tokyo Gas Kofu satellite terminal (Fig. 6), and test operation for 15 months proved its outstanding evaporation performance. The results of testing are compared with the development goals are shown in Table 2. The tests were carried out under severe weather conditions, such as snowfall, average temperature 0.4 C, and 90% or more average relative humidity. Despite the severe weather conditions, the plant was successfully operated continuously for 13 hours, and the fuel gas consumption rate became 0.2%. In addition, the temperature of the production gas at the end of the test end was 11 C, sufficiently exceeding the target value of 0 C, the test was stopped as the fuel gas rate had reached the target value. Here, fuel gas rate is defined as the amount of fuel gas consumed by the hot air generator to produce normal temperature gas, divided by the amount of production gas. For the HAV, with fuel needed in order to superheat the LNG evaporated with atmospheric heat in order to produce gas of normal temperature, the target value for fuel gas rate was set at 0.2% (1/10 of the rate for a warm water bath vaporizer). Outline specification Design pressure 0.97 MPa Capacity 0.3 t/h Vaporizer 1 unit Superheater 1 unit Hot air generator 1 unit Fig. 6 HAV at Kofu satellite terminal Table 2 Development goals and test results Parameter Development goal Test result Weather conditions Ambient temperature 0 C Relative humidity 90% min. Ambient temperature 0.4 C Relative humidity 90% min. Run length 12 hours 13 hours Production gas temperature over 0 C 11 C Fuel gas rate 0.2% 0.2% 5. OPERATING PERFORMANCE OF COMMERCIAL UNITS The first commercial unit in Japan (Figure 7) was installed in November 2002 at the Hamamatsu satellite terminal run by Chubu Gas at Hamamatsu in Shizuoka Prefecture. The HAV is evaluated on the basis of actual operating data, verifying that the performance targets have been met. PO-14.8
More than a year has now passed since installation, but the unit is still operating reliably. The issue of clouds of mist produced when water vapor in the air condenses has been resolved by fans installed at the top of the vaporizers to disperse the water vapor into the atmosphere. The calorific value of the manufactured gas is extremely stable, and operations such as starting up, shutting down, and changing the load are all easy to accomplish. Outline specification Design pressure 0.97 MPa Capacity 2.0 t/h Vaporizer 1.0 t/h x 2 units Superheater 2.0 t/h x 1 units Hot air generator 1 unit Fig. 7 HAV at Chubu Gas Hamamatsu satellite terminal The operating data shown in Figure 8 is an example of energy-saving operation implemented in December 2002, when air temperature was relatively low, but humidity was relatively high. Apart from in mid-winter, the HAV system does not use fuel for superheating. The vaporizer fans and the blower in the hot air draft generator are operated instead, enabling the vaporizer to be operated in an energy-saving mode. The weather at the time this data was recorded was rainy, (although the rain stopped in the early evening), with an average air temperature of 6.3 C, and a relative humidity of 80%. This shows continuous operation for about 14 hours with an LNG flow of 2.0t/h. After operation, the vaporizer outlet gas temperature had dropped to -9.7 C, but the temperature of the manufactured gas was 4.4 C, with air from the blower keeping it above 0 C by superheating. This demonstrates that there is no need at all for fuel for the purposes of superheating. PO-14.9
20 Temperature( 10 0-10 Vaporizer exist gas tem p. Ambient tem p. Production gas tem p. -20 8:00 12:00 16:00 20:00 0:00 Time Fig. 8 Data from a commercial unit in operation 6. CLOSING COMMENTS LNG vaporizers are important components of satellite terminals. By making maximum use of ambient heat, the newly developed HAV system achieves a considerable reduction in energy consumption and a corresponding reduction in environmental impact compared with conventional systems. Features such as low cost, compactness, the non-generation of mist, and the ability to function well in cold regions make the HAV an ideal LNG vaporizer for small-capacity LNG terminals. Finally, the authors would like to record their thanks to the Chubu Gas and the staff of its Hamamatsu satellite terminal for making available the precious operating data resulting from the first use of the HAV system in Japan. REFERENCES 1) Masaru Sekiguchi, Hirokazu Mori: Current Air Fin LNG Vaporizer, Piping technology extra number, Sep. 2001, p.62-66. 2) Japan Gas Association: Recommended Practice for LNG Facilities in small terminals, Aug. 2002, p.99-103. PO-14.10