USE OF LNG FOR WORLD HIGH-EFFICIENCY THERMAL POWER GENERATION

Transcription

1 USE OF LNG FOR WORLD HIGH-EFFICIENCY THERMAL POWER GENERATION UTILISATION DU GNL POUR LA PRODUCTION DE L'ENERGIE THERMIQUE A RENDEMENT MONDIALEMENT ELEVE Motomi Nasu Chubu Electric Power Co., Inc. Nagoya, Japon Nasu.Motomi@chuden.co.jp ABSTRACT Japanese electric power companies have adopted combined cycle power generation which combines gas turbines and steam turbines since the latter half of the1980s to efficiently convert liquefied natural gas (LNG) into electric energy. This is an effective power supply measure for preventing global warming. Chubu Electric Power Company has held first place in the thermal efficiency of thermal power generation in Japan for five years since This is a result of operations that give priority to high-efficiency power generation plants, in addition to the adoption of high-efficiency apparatus for power generation plants. This paper first introduces power generation efficiency-improving measures from the viewpoint of equipment and operation, and then introduces actual examples of efficient power generation using LNG to elucidate why Chubu Electric can maintain high thermal power generation efficiency. As deregulation of the gas business and flexibility of LNG procurement progress throughout the world, the paper provides various highly suggestive findings concerning the efficient use of energy from the viewpoint of equipment and operation. RESUME Les compagnies d'électricité japonaises font appel depuis le milieu des années 1980 à une production d'électricité à cycle combiné qui allie la turbine à gaz avec la turbine à vapeur, afin de transformer plus efficacement le GNL (gaz naturel liquéfié) en énergie électrique. Ce mode d'exploitation d'électricité est favorable du point de vue de la protection contre le réchauffement du globe. La compagnie d'électricité Chubu Electric Power Co., Inc. a été pendant 5 ans depuis 1997 le plus efficace des producteurs thermiques de l'énergie électrique au Japon. Ceci grâce au matériel à grand rendement utilisé dans les centrales thermiques et au mode de fonctionnement privilégiant les centrales les plus efficaces. Dans cette communication seront d'abord présentées les mesures d'amélioration du rendement des centrales thermiques, aussi bien sur le plan matériel que sur le plan du mode de fonctionnement, puis des exemples concrets de production d'énergie électrique à partir du GNL afin d'expliquer pourquoi cette firme japonaise peut maintenir son rendement à un niveau élevé. Enfin, diverses connaissances significatives pour permettre une utilisation plus rentable de l'énergie électrique seront présentées dans le contexte d une libéralisation mondiale de l'industrie de distribution de gaz ainsi que d une fourniture plus souple du GNL. PS7-S.1

2 LNG CONSUMPTION IN JAPAN Japan s LNG import volume is about 55,000,000 tons a year, of which 36,000,000 tons are used for power generation. This means that about two thirds of imported LNG is used for power generation. Of this, Chubu Electric consumes about 9,000,000 tons a year (about 16% of the imported volume). ELECTRIC ENERGY OUTPUT OF CHUBU ELECTRIC The annual electric energy output of Chubu Electric is 120 TWh. The breakdown is 48% from LNG, 25% from coal, 12% from atomic power, 7% from petroleum, and 8% from hydraulic. Consequently, 9,000,000 tons of LNG generates 60 TWh power. In addition, to efficiently convert LNG into electric energy, Chubu Electric has adopted state-of-the-art technologies such as combined cycle power generation. This is also an effective means for preventing global warming. Thermal Efficiency of Thermal Power Generation In Japan, there are nine major electric utility companies. Each electric utility company publicity announces the thermal efficiency of thermal power generation every year, and there is statistical data that set thermal efficiency in third place in the order of efficiency. According to these, Chubu Electric has maintained first place for the overall thermal efficiency of thermal power generation in Japan for 5 years straight from The reason why Chubu Electric has been able to maintain its high place in thermal efficiency is described below. Note: The thermal efficiency indicates a ratio of thermal energy which has effectively become electric power to the thermal energy of fuel consumed. It is a good-rule-of-thumb to measure the thermal energy utilization rate of thermal power plants. Thermal Efficiency of Thermal Power Generation of Chubu Electric Of the nine electric power companies, Chubu Electric has taken the prize for thermal efficiency (generating end) for 5 consecutive years since Expressing thermal efficiency as a numerical value, it has been improved from 39.77% in 1996 to 41.85% in 2001, up 2.08 points in 5 years. POWER GENERATION SYSTEM Equipment and operations to maintain the thermal efficiency of thermal power generation at a high level are introduced in this section. First, among equipment, there are [1] conventional power generation system, [2] same conventional power generation system with steam pressure increased, and [3] combined cycle power generation system. The development period of the generation systems varies and the efficiency of each system has been improved through technological progress. Of these three classes of system, the power generation system with high efficiency is briefly described below. Chubu Electric adopts ultra-super critical pressure power generation system called ultra super critical (USC) which provides the highest efficiency of the conventional power generation systems and the company has been operating the system since In the conventional system, pressure was supplied to the steam turbine at 25 MPa, but the USC system is a power generation system opening at 32 MPa steam pressure. In the conventional system, steam was superheated in two stages, but in the USC system, steam PS7-S.2

3 is superheated in three stages with one stage increased by improving steam conditions to improve efficiency. Using this system, power generation efficiency is 42%, which is 4 percent greater than the 38% efficiency of the previous conventional system which was constructed in older days. COMBINED CYCLE POWER GENERATION Chubu Electric began to adopt the combined cycle power generation system in At present, it possesses 31 units, with more than 6,000 MW output. This accounts for one fourth of total power generation. Combined cycle power generation is a system to generate steam using exhaust heat generated simultaneously when natural gas is combusted in the gas turbine and a generator is rotated to drive a steam turbine to generate power. Using this system, efficiency is about 50% by reusing exhaust heat. The features of combined cycle power generation include the following: Short starting time Because the combined cycle power generation system is formed by combining a gas turbine, which is easy to warm up, with a small steam turbine of a simple construction, starting and stopping can be carried out in a short time. High partial load efficiency A unit in which a gas turbine, a steam turbine, and a generator are assembled is called a power train, and a plurality of power trains are installed to control output. Consequently, efficiency at a partial load is high. Good load response Because the gas turbine directly combusts fuel to convert it into rotating energy, a better load response is achieved than a steam turbine, which thermally converts fuel into steam and drives a generator. Based on these features, the combined cycle power generation is the system that is best suited to Japan s daily load change characteristics. EFFICIENT OPERATION OF POWER GENERATING FACILITIES While the combined cycle generation system provides about 50% efficiency, the conventional power generation system provides about 40%, and the LNG consumption rate differs by the difference in efficiency. An electric power company that possesses combined cycle power generation and conventional power generation for the thermal power generation system can achieve overall economy by operating high-efficiency combined cycle power generation system and stopping operation of the conventional power generation system, which is old and expensive to maintain. HIGH-PRESSURE BOG COMPRESSOR The third aspect of equipment, discussed is pressurization boil-off gas (BOG) using a high-pressure compressor. PS7-S.3

4 When LNG tanks arrive at LNG terminals and unload LNG, BOG is generated. This BOG is at a low pressure and has been consumed in conventional power generating facilities in the vicinity of LNG terminals. Because the conventional power generation efficiency is about 40%, it is better to generate power using the combined cycle system which achieves a higher efficiency. Consequently, Chubu Electric investigated a system to pressurize the BOG that is generated upon the arrival of LNG tanks by a high-pressure compressor and is consumed by high-efficiency combined cycle power generation, and concluded that investment could obtain positive effects. INSTALLATION OF BOG COMPRESSOR As one item of control equipment for efficiently consuming LNG, a BOG compressor was installed in the Kawagoe Thermal Power Plant. The reason is that existing facilities of the Kawagoe Thermal Power Plant comprise LNG receiving terminal, USC conventional power generation, and combined cycle power generation, and it was able to rationally combine the BOG compressor with them. In addition, these facilities are monitored and controlled from the same control room. LOAD CHANGE IN ONE DAY A power plant varies its output in accordance with electricity demand, which varies every moment, and when changes of electricity demand in one day are observed for thermal power generation, they can be classified into three stages: base, middle, and peak. A thermal power plant is operated in accordance with these stages: Coal-fired power plant with inexpensive fuel cost is operated at base load, Combined cycle generation which needs short start/stop time and operates at high efficiency, even at partial load is operated for middle load changes, and Oil-fired power plant with a high fuel unit price is operated at the peak load. When electricity demand is comparatively low, combined cycle power generation may be carried out in response to peak load. On the other hand, in the case of Chubu Electric, the oil-fired thermal generation system is old having been operated for about 30 years since its inauguration, and power generation efficiency is relatively low. Consequently, it is used for peak load. SEASONAL CHANGES OF ELECTRICITY DEMAND IN JAPAN Next, we discuss yearly electricity demand characteristics in Japan. Japan has four clearly defined seasons and changes in temperature in spring, summer, fall, and winter are conspicuous, so seasonal fluctuations of electricity demand are remarkable. Electricity demand in the summertime comes from air-conditioning, while that in the wintertime comes from space heating. To respond to electricity demand, which varies widely throughout a year, highly efficient combined cycle power generation is preferentially operated in thermal power plants. PS7-S.4

5 OPERATION IN RESPONSE TO CHANGES IN ELECTRICITY DEMAND For light electricity demand such as at night, and on Saturdays, and Sundays, thermal power plants are operated at a partial load. This section introduces a subtle operating method which has been carried out since the 1970s. In day-to-day operation, the thermal plant is started in the morning and stopped tothe night, and for week-by-week operation, the thermal plant is started on Monday morning and stopped on Friday night. The former is called daily start and stop (DSS) and the latter weekly start and stop (WSS). The thermal power generation plant Chubu Electric possesses is designed to carry out this DSS and WSS operation. Inputting the start/stop time from the operation panel enables the computer to automatically map out a start/stop schedule. SUMMARY Chubu Electric adopts high-efficiency facilities and carries out efficient operation as follows. Facilities USC is adopted for the conventional power generation system, 31 power plants of the combined cycle power generation system have been constructed, which account for one fourth the total thermal power generation output, and BOG generated while LNG is being received is pressurized by a compressor and is used by combined cycle. Efficient operation Combined cycle power generation is preferentially operated, and Operation in accordance with load changes is carried out. EFFECTS BY IMPROVED THERMAL EFFICIENCY The increased efficiency of facilities and the increased efficiency of operation described above have improved thermal efficiency by 2.08 points in 5 years. As a result, the following effects have been obtained. For economic effects, fuel can be reduced by about 180,000 kl a year in terms of heavy oil, and For environmental effects, emissions of carbon dioxide can be reduced by about 440,000 tons a year. Chubu Electric will make efforts to positively operate high-efficiency power generation facilities using natural gas and continue supplying electric power with an excellent cost and environmental load in the years to come. PS7-S.5