In 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. 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 made the 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°C 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 7 MWh.

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.

Fig. 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 (fluorocarbon) 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 by-pass system, even if turbines and pumps for the freon mixture and natural gas circulating systems (FR-RP, LNG-RP) stop.

The 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 intermediate medium 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.

When 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 output. 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.

The 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°C turbine-inlet temperature.

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.

Fig. 8 shows the relation 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.