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毕业设计外文翻译 原文标题：Proposal for a high efficiency LNG power-generation System utilizing waste heat from the combined cycle 中文标题:一个高效的利用液化天然气联合循环余热的发电系统学院名称： 能源与动力工程学院 专业名称： 热能与动力工程 Proposal for a high efficiency LNG power-generation system utilizing waste heat from the combined cycleY. Hisazumi*, Y. Yamasaki, S. SugiyamaEngineering Department, Osaka Gas Co., 1-2 Hiranomachi 4-chome Chuo-ku, Osaka 541, Japan Accepted 9 September 1998AbstractHigh-efficiency power-generation with an LNG vaporizing system is proposed: it utilizesthe LNGs cold energy to the best potential limit. This system can be applied to LNG vaporizers in gas companies or electric power companies and recovers the LNGs cold energy as electric power. The system consists of a Rankine cycle using a Freon mixture, natural-gas. Rankine cycle and a combined cycle with gas and steam turbines. The heat sources for this system are the latent heat from the steam-turbines condenser and the sensible heat of exhaust gas from the waste-heat recovery boiler. In order to find out the optimal condition of the system, several factors, such as gas turbine combustion pressure, steam pressure, condensing temperature in combined cycle, composition of mixture Freon, and natural gas vaporizing pressure are evaluated by simulation. The results of these studies show that in the total system, about 400 kWh can be generated by vaporizing 1 ton of LNG, including about 60 kWh/LNG ton recovered from the LNG cold energy when supplying NG in 3.6 MPa. About 8.2MWh can be produced by using 1 ton of LNG as fuel, compared with about 7 MWh by the conventional combined system. A net efficiency of over 53%HHV could be achieved by the proposed system. In the case of the LNG terminal receiving 5 million tons of LNG per year, this system can generate 240 MW and reduce the power of the sea water pump by more than 2MW. 1998 Elsevier Science Ltd. All rights reserved.1. IntroductionIn the fiscal year 1994, the amount of LNG imported to Japan reached about 43 million tons; of this 31 million tons were used as fuel for power generation. As shown in Fig. 1, about 20% of the LNG imported was used for power generation 2. Fig. 2 shows the major LNG power generation systems now in operation and their outputs. Several commercial LNG power generation plants have been constructed since 1979, and their total output has reached approximately 73 MW. Among the new power-generation plants without CO2 emission, this value of 73 MW is second to the 450 MW input of geo-thermal power generation plants in Japan, with the exception of power generation by refuse incinerators, and is much larger compared with the 35 MW output of solar-power plants and the 14 MW output of wind-power stations. Table 1 shows the LNG power generation plants constructed in Japan. The economics of LNG power generation became worse as the appreciation of the yen madethe cost of energy kept constant but while raising the construction cost; the adoption of the combined cycle utilizing gas-turbine and steam turbine (hereafter called combined cycle) increased the gas send-out pressure and lowered the power output per ton of LNG. Therefore, no LNG power generation plants were constructed in the 1990s due to lower cost effectiveness of the systems.As for the thermal power plant using natural gas as fuel, the steam turbine produced only about 6 MWh of power output per ton of LNG. But recently, improvement in blade-cooling technology and materials of the gas turbine enabled a 1400 class turbine to be designed and increased the combustion pressure up to 3 MPa. Therefore, as shown in Fig. 3, the heat efficiency of the combined cycle has been improved and the electrical output from 1 ton of LNG has reached about 7MWh.In this paper, a proposal is made for the high-efficiency LNG power generation system based on a new concept which fully utilizes the cold energy without discarding it into the sea. The system is composed of the combined cycle and the LNG power-generation plant.2. High-efficiency LNG power-generation system2.1. Basic componentsFig.4 shows the process flow diagram of the high-efficiency LNG power-generation system. This complex system consists of the combined cycle and the LNG power generation cycle. The combined cycle is composed of a gas turbine (GAS-T) and a steam turbine (ST-T) using natural gas (NG) as fuel, while the LNG power generation cycle is composed of a Freon (uorocarbon) mixture turbine (FR-T) and a natural-gas turbine (NG-HT, NG-LT) using the latent heat of condensation from the exhaust steam and the sensible heat of the exhaust gas as heat sources. The plate fin type heat exchanger can be used for the LNG/natural gas (LNG-CON) and LNG/ Freon mixture (FR-CON). The shell-and-tube type can be selected as exchangers for exhaust steam/natural gas (LNG-VAP),exhaust steam/Freon mixture (FR-VAP), and exhaust gas/natural gas (NG-SH) applications according to the operating conditions.Ice thickness on the surface of the heat-exchanger tubes becomes a problem as heat is exchanged between exhaust gas and cold natural gas or Freon mixture. The ice thickness can be estimated by the technology of heat transfer between LNG and sea water, thus enabling one to avoid blockages due to ice inside the tubes.In addition, stable and continuous send-out of gas is made possible by using a bypass system, even if turbines and pumps for the Freon mixture and natural gas circulating systems (FR-RP, LNG-RP) stop.2.2. Features of the systemThe practical use of the following existing technologies in combination shows the high feasibility of the proposed system:. Power generation using Freon or hydrocarbon type Rankine cycle,. Power generation by natural-gas direct expansion,. TRI-EX type vaporizer which vaporizes LNG by using an intermediatemedium or vacuum type LNG vaporizer.The Freon mixture is made up of the HFC type, which is a fluorocarbon consisting of H, F, and C and has no adverse influence on the ozone layer; it enables reduction in exergy loss at the heat exchanger and increases its circulating flow rate to be achieved.The effective recovery of cold exergy and pressure exergy is made possible by the combined system using natural gas and Freon mixture Rankine cycle.Fig. 5 shows the temperature-heat duty relation when vaporizing 1 kg of LNG in the system shown in Fig. 4. Separation of the condensed natural-gas in two sections enables an increase in the heat duty between Freon (FR) and LNG, and a reduction of difference in temperature of LNG and natural gas between the inlet and outlet of the heat exchanger.3. Evaluation of the characteristics of the proposed system3.1. Process simulationThe characteristics of this system were evaluated by using process simulator. The followings are the conditions used for the calculation:Effciencies of rotating machines LNG compositionGas turbine (GAS-T) 88% CH4 89.39%Steam turbine (ST-T) 85% C2H6 8.65%Natural-gas turbine (NG-HT, LT) 88% C3H8 1.55%Freon turbine (FR-T) 88% iC4H10 0.20%Air compressor (AIR-C) 85% nC4H10 0.15%LNG pump (LNG-MP, RP) 70% iC5H12 0.01%Freon pump (FR-RP) 70% N2 0.05%Natural gas gross heat-value: 10,510 kcal/Nm3AIR/NG flow ratio of gas turbine: 323.2. Effects of send-out pressure of the natural gasWhen natural-gas is sent out at 3.5 or 1.8 MPa, evaluations were made of the effects of send-out pressure of the LNG and change in superheating temperature of the natural gas on the total output of the high pressure (NG-HT) and the low pressure (NG-LT) natural-gas expansion-turbines. Fig. 6 shows the results of this calculation, where self consumption of power is calculated from the power, raising the pressure of the LNG up to the inlet pressure of the turbine minus the power required for the original send-out pressure. In both cases, the inlet pressure rise for the turbine causes an increase of self consumption power, but brings about a greater out-put. About 7 MPa of the inlet pressure of the turbine is appropriate considering the pressure tolerance of the heat exchangers.When the superheating temperature of the natural gas at the inlet to the turbine becomes high, the recovery of power increases, but the temperature of the exhaust gas from the outlet of the natural-gas super heater (NG-SH) declines, thus indicating that there is a limitation to superheating gas.3.3. Effects of combustion pressure of the gas turbineThe outputs of the gas turbine and the steam turbine, and the efficiency per gross heating value were evaluated by changing the combustion pressure of the gas turbine operating at 1300 turbine-inlet temperature - see Fig. 7.If the combustion pressure of the gas turbine becomes high, the output of the gas turbine increases, but the output of the steam turbine decreases because the rise in combustion pressure causes a lowering of the exhaust-gas temperature at the outlet of the gas turbine and consequently a decline in the steam temperature at the inlet of the steam turbine. However, the overall efficiency of the turbines increases upon increasing the combustion pressure because the increment of gas-turbine output exceeds the decrement of steam turbine output. As a result, taking the pressure loss into account, it is appropriate to set the send-out pressure of the natural gas at the LNG terminal at 3.5 MPa. （FR-vap），3.4. Effects of Inlet pressure of the steam turbine Fig. 8 shows the relations between the steam-turbines output and exhaust gas temperatures by changing the steam pressure in the range of 3-7 MPa. As the steam pressure increases, the output of the steam turbine rises and the temperature of the exhaust gases also increase. Besides, the power required for the water-supply pump increases with a rise in the steam pressure. Therefore, the current combined cycles operate at steam pressure of 7 MPa or more because the increment of the output of steam turbine exceeds the additional power required for the water-supply pump.3.5. Rankine cycle using a Freon-mixture refrigerant.The Freon refrigerant was selected from the HFC refrigerants on the basis of marketability, boiling point and freezing-point. Table 2 shows the physical properties of HFC Freon.When only HFC-23 is used as the medium, because of its low freezing-point it never freezes even if heat is exchanged between the LNG and HFC-23. But if HFC-23 is heated by the exhaust steam of the steam turbine, the pressure rises approximately up to the critical pressure. Therefore, the use of HFC-23 is not cost effective, because it is then necessary to set a high design pressure. To cope with this problem, we evaluated the compound refrigerant composed of HFC-134a (with high boiling point) and HFC-23.Fig. 9 shows saturated vapor pressure at various temperatures, the boiling point and the dew point at atmospheric pressure for mixtures of HFC-23 and HFC-134a of various compositions. The saturated pressure at each temperature rises with the increasing mole ratio of HFC-23: Hence, 40-45% of the mole ratio of HFC-23 is the optimal value considering the design pressure of the equipment.Fig. 10 shows the plots of the output of the Freon turbine versus the condensing temperature of the steam turbine when changing the composition of the HFC-23. In this figure, the turbine outlet pressure is determined in such a way that the difference in temperature between the LNG and Freon mixture is not less than 5 in the Freon condenser (FR-CON). The Freon turbines inlet-pressure is set to the saturated temperature of the Freon mixture, i.e. less than 2 from the steam-condensing temperature.This figure indicates that the output of the turbine scarcely correlates with the mole ratio of HFC-23. The higher the steam-condensing temperature becomes, the greater the output per ton of LNG the turbine produces, but in such a case, it is necessary to evaluate the system as a whole because more fuel is required, as described below. The result indicates that the optimal mole composition of HFC-23 and HFC-134a is 40%/60% considering both design pressure and the output of the turbine.3.6. Comprehensive evaluation from the viewpoint of the steam-condensing Temperature.As the dew point of the exhaust gas is 42, it is wise to set the exit temperature of the exhaust gas from the natural-gas super heater (NG-SH) to 80 or more in order to prevent white smoke from the smoke stack. Table 3 shows the effect of the steam-condensing temperature on the generated output of the total system. The lower steam-condensing temperature brings about a higher efficiency of the total system, but also causes a lowering in the inlet temperature of natural-gas turbine. Therefore, it is appropriate to set the steam-condensing temperature at approximately 30.When the condensing temperature is 30C, the generated outputs per ton of LNG of the combined cycle and LNG power generation plant are 342.83 and 67.55 kWh, respectively, resulting in 402.64 kWh of total generated output after subtracting the self-use power. As 48.94 kg of fuel is used for operating the system, the generated outputs of the combined cycle and the total system reach about 7 and 8.2 MWh, per ton of fuel respectively.3.7. Evaluation of exergyNatural-gas is liquefied at an LNG liquefaction terminal, with the consumption of about 380 kWh/LNG-ton: 1 ton of LNG having about 250 kWh of physical exergy as cold exergy and 13.5 MWh of chemical exergy. Fig. 11 shows the result of evaluating the exergy of the system shown in Fig. 4 under the optimal condition. The total output of Freon and natural gas turbines is 67.5 kWh, and the effective recovery percentage of cold exergy is 56%. As 90 kWh out of the pressure exergy can be recovered as output, about 157 kWh of net recovery can be obtained, which indicates the recovery percentage reaches about 63% for 250 kWh of LNG cold exergy. This conversion efficiency is higher than that achieved from chemical exergy to electric power.Most of the exergy loss occurs in the heat exchanger and the turbine, and in mixing with re-condensed LNG. As for the turbines, the loss of energy may be improved by using high-efficiency turbines. On the other hand, modification of the heat exchanger to reduce the energy loss may cause increased complexity of the system and is difficult to be done from the economic viewpoint. Though the recovery.percentage of cold energy in this system is low compared with the 80% in air-separation equipment, this system has the advantage of recovering a large amount of the available cold energy.4. ConclusionThe paper has proposed a high-efficiency LNG power generation system in combination with a combined-cycle power generation system fueled by natural-gas. The system utilizes LNG cold energy and it requires no sea water as a heat source.This system can be applied to LNG vaporization and send-out processes of gas companies or electric-power companies. The system recovers LNG cold energy as an electric-power output without wasting it into sea water. The system consists of Rankine cycle with Freon mixture and a natural-gas Rankine cycle using the latent heat of exhaust steam from the steam turbine and the sensible heat of exhaust gas from the waste-heat recovery boiler. To improve the total efficiency of the system, a simulation was conducted to evaluate several factors, such as the composition of the Freon mixture, natural gas send-out pressure, as well as the combustion pressure steam inlet pressure, and steam-condensing temperature of the combined cycle. As a result, not less than 60 kWh/LNG-ton of output was generated even at a high natural-gas send-out pressure of 3.5 MPa. This value is considerably higher than the output generated at a LNG send-out pressure of 3 or 4 MPa, as given in Table 2.The system can produce about 400 kWh of net output when vaporizing 1 ton of LNG. While the conventional combined-cycle system in operation generates about 7 MWh when 1 ton of LNG is used as fuel, the system using the same amount of fuel generates about 8.2 MWh with a high degree of efficiency: a not-less-than 53% conversion efficiency was achieved per gross heat value.In the case of an LNG terminal receiving 5 million tons of LNG per year, this system can generate a power of about 240 MW when 600 t of LNG is used in an hour. With the elimination of about 24,000 tons per hour of sea water, which has been used for vaporizing 600 t/h of LNG in the conventional system, no less than 2 MW of electric power for operating sea water pumps can be saved.The proposed system emits no CO2, and can generate a large amount of electricity with high cost efficiency when incorporated into a combined cycle, with no use of sea water. Therefore, we consider that installation of this system is the one of the most favorable means of investment to put a new energy source or energy-saving equipment to practical use.To realize the full potential of this system, it is necessary to understand the heat characteristics of the Freon mixture, the icing and heat transfer characteristics of exhaust steam, the controllability of total system and the characteristics against partial load.References1 The Center for Promotion of Natural gas Foundation. Research and development report of cold energy utilizing system, 19942 Japans Energy and Economy Research Center. Energy and economy statistical data in 19953 Abe. Operating results and future prospect of a recent combined-cycle power generation plant. Thermal and Nuclear Power 1995;46(6):33-414 Maertens J. Design of Rankine cycles for power generation. Int. Refrig. 1986;9:137-435 Terada, Nakamoto. Power generation utilizing LNG cold. Thermal and Nuclear Power Generation 1986;37(10):66-716 Ooka, Ueda, Akasaka. Advanced LNG vaporizer and power generation utilizing LNG cold. Chemical Engineering 1981;45(3):187-907 Miura. The development of LNG vaporizer using vacuum steam heat (VSV). Journal of Japan Gas Society 1992;45:34-68 Nagai. Software-package and the usage. Chemical Equipment1994;August:31-79 Daikin Co. Ltd. Freon Data Sheet of HFC23一个高效的利用液化天然气联合循环余热的发电系统日本大阪541燃气有限公司工程部1-2平野町4肖梅中央谷，1998年9月概 述本文提出了一个高效液化天然气气化发电系统，它是利用液化天然气冷能的最佳潜能极限。该系统可用于液化天然气汽化器在天然气公司和电力公司来回收液化天然气的冷能发电。该系统由一个朗肯循环使用氟里昂混合天然气。朗肯循环和一个燃气蒸汽轮机联合循环。这个系统的热源来自汽轮机凝汽器和回收锅炉烟气的余热、显热、潜热。为了找出影响系统的最佳条件几个因素，如燃气轮机燃烧压力，蒸汽压力，冷凝联合循环温度，混合氟利昂成分，天然气汽化压力的仿真评价。这些研究结果表明，在整个系统中，约通过蒸发1吨液化天然气可以产生400千瓦时电能，其中约60千瓦时/（吨液化天然气）液化天然气冷能当提供纳克在3.6 MPa被回收.大约8.2mwh可以利用1吨的液化天然气作为生产燃料，与约7兆瓦的系统相比较，一个净效率超过53% HHV可以通过该系统实现。每年可在液化天然气终端接收5000000吨液化天然气，该系统可以产生240兆瓦电能并且使海水泵的功率降低超过2MW。 1简介在1994年度的日本财政中，日本的液化天然气进口量达到约43000000吨；其中31000000吨作为发电燃料。如图1所示，约20%的液化天然气用于发电原燃料。图2显示了主要的液化天然气发电系统的操作和它们的输出。自从1979以来一些商业液化天然气发电厂已建成，其总发电量已达到约73兆瓦。在新的发电厂无CO2的排放，这是第二个在日本输入总量450兆瓦系统的73兆瓦的地热发电厂，通过垃圾焚烧发电的例外，往往是比35兆瓦太阳能发电站和14 MW风电站的输出更大。 （表1显示了液化天然气发电厂建在日本。由于日元升值使液化天然气发电的经济性变得更糟）。提高了施工成本而能源成本保持不变，但采用燃气轮机和蒸汽轮机联合循环（以下简称联合循环）提高了气体发出的压力降低了每吨液化天然气的输出功率。因此，在上世纪90年代,由于系统的效益较低，没有使用天然气发电的电厂。对以天然气为燃料的火电厂，汽轮机只产生大约6兆瓦（每吨液化天然气）的输出功率。但最近，在叶片冷却技术和燃气轮机材料的改进使1400级涡轮提高了燃烧压力高达3兆帕。因此，如图3所示，热联合循环效率得到提高并且电气输出从1吨的液化天然气已达到约7兆瓦时。本文提出了高效的液化天然气发电系统基于一个新的概念，充分利用冷能不丢弃它入大海。该系统由联合循环和液化天然气发电装置组成。2.高效的液化天然气发电系统2.1基本成分图4显示了高效液化天然气发电系统工艺流程图。这个复杂的系统组成的联合循环和液化天然气发电循环。联合循环是由燃气轮机和蒸汽涡轮机用天然气作为燃料，而天然气发电的循环是由一个氟利昂混合机和天然气透平使用排气的显热为热源的冷凝潜热。板翅式换热器可用于液化天然气/天然气和液化天然气（液化天然气-con）/氟里昂混合。管壳式换
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