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2 Geothermal Resources Council, TRANSACTIONS Vof. 7, October 1983 DESIGN OF A GEOTHERMAL POWER PLANT WITH HIGH NON-CONDENSABLE GAS CONTENT Hiroshi Hamano Toshiba Corporation Tokyo, Japan ABSTRACT Xn the planning stage of a geothermal power plant with high non-condensable gas (herein called N.C.G.) content, the total system engineering, including selection of N.C.G. removal and of condenser vacuum is very important. This report discusses the design of a geothermal power plant with a large quantity of N.C.G. content in the steam. The captioned power plant is a hot water dominated geothermal power plant and the hot water contains a large quantity of N.C.G.. The system and equipment of the plant are designed under consideration of their characteristics and also for the preservation of the scenery and the environment. Therefore, this plant is to provide a large-sized gas compressor coupled directly with the main steam turbine shaft as the N.C.G. removal system and condenser pressure selected has to be relatively high to reduce auxiliary power. The design offers a compact and efficient power plant. as this site is located in a national park, the equipment was designed to discharge pollusive gases in the air after dilution and to reduce the site area by compact facilities for the purpose of preservation of the scenery and enyironment. From the N.C.G. composition shown in Table 1, it is noticed that there is a large quantity of N.C.G. content and that N.C.G. consists primarily of c02. N.C.G. TABLE 1 COMPOSITION (by volume) N.C.G. content 2.4?. 5.4% N.C.G. composition C02: % H2S: 0.6 -J 2.2% H2, CH4, N2 and so on: 0.4 ** 1.3% INTRODUCTION The geothermal power plant discussed in this paper is located in the southern part of Hokkaido Island. In this geothermal area, Toshiba constructed a pilot binary cycle plant of 1 MW, as one of several national research projects called the "Sunshine Project", under the supervision of the Agency of Industrial Science & Technology, Ministry of International Trade & Industry, in 1977, and achieved full power generating capacity the same year. In the same period, Hokkaido Electric Power Company, Ltd., and Donan Geothermal Energy Company? Ltd., were developing geothermal resources jointly in this district. In November, 1982, a SO MW power plant, called the Mori Geothermal Power Station of Hokkaido Electric Power Company, Ltd.? started commercial operation. Toshiba designed the system and equipment for the power generating section and contracted the manufacturing and installation of the equipment. Since this geothermal area has features of water-dominated hydrothermal, with a large quantity N.C.G. content in the steam, a highly efficient and reliable system was designed in careful consideration of these characteristics. Further, HEAT CYCLE CHARACTERISTICS The heat cycle adopts a double-flash system for efficient utilization of geothermal energy in water-dominated hydrothermal fields, supplying the primary steam separated through cyclonic separaters and the secondary steam produced in flashers. In the equipment for the steam generating section, the primary and the secondary steam are fed to the power generating plant through the respective steam mixture headers. For the steam condition, it was decided that the primary steam pressure of 6.7 kg/cm2 abs. and the secondary of 2.0 kg/cm2 abs. attains the optimum point of the steam pressure, after examining the well's characteristics. GAS REMOVAL SYSTEM CHARACTERISTICS The steam jet air ejector is widely used for gas removal systems when the quantity of N.C.G. content in the steam is small. The structure and operation is simple and the steam consumption is not predominant. But this method is uneconomical for a large quantity of N.C.G., due to higher steam consumption. Therefore, a geothermal power plant with a large quantity of N.C.G.? or low pressure driving steam (generally primary steam) 15

3 tends to adopt the vacuum pump or gas compressor for the gas removal system. Furthermore, if there is a large quantity of N.C.G. content, more than 10 percent by weight, the geothermal power plant may adopt a back pressure turbine without a condenser. But the back pressure turbine causes a decrease of the total output or increases the steam consumption, in spite of lower equipment cost. In such situations, either case is not economical. At the Mori Geothermal Power Plant, the N.C.G. content is about 10 percent by weight and rated output is 50 MW. The optimization of a N.C.G. removal system was studied, and the evaluated systems were: 1) the steam jet gas ejectors; 2) centrifugal gas compressors; 3) combined system of steam ejectors and centrifugal compressors; and 4) back pressure turbine without a N.C.G. removal system. The centrifugal gas compressors system was selected. In the above-mentioned studies, the condenser vacuum was also optimized, taking the system selection into consideration. Such optimization was done on the Mori Geothermal Power Plant. Further, four different alternative driving methods of the gas compressor were evaluated, which are: 1) main turbine shaft coupling: 2) exclusive steam turbine shaft coupling; 3) variable speed motor coupling, and: 4) fluid coupling with motor. The parameters are: steam consumption, auxiliary power, equipment cost, and building space. As the driving method of the gas compressor needs about 6 percent of output at the generator terminal, the net output decreases greatly. In this case, the main turbine shaft coupling method *was adopted, in consideration of the starting torque of the gas compressor, reduction of building space and decreasing net output. Conditions in this case study are assumed as follows. The results are shown in Figures 1, 2 and 3. CASE STUDY CONDITION Case : 1) Main turbine shaft coupling 2) Exclusive steam turbine shaft coupling 3) Variable speed motor coupling 4) Fluid coupling with motor N.C.G. content 5.25% by volume (11.85% by wt.) Cooling water inlet temperature 25 OC Steam price 0.6 yen/kg Capac i ty factor 90% Fixed charge rate 18% Output at generator terminal 50 MW (constant) All comparisons are based on the main turbine shaft coupling method. Figure 4 shows the outline of the power generating system. The gas removal system is summarized as follows: N.C.G. in the condenser is drawn from the condenser to the gas compressor at the gas inlet section installed in the lower-half casing at the front side (compressor side) of the gas compressor and is compressed up to about 0.5 kg/cm2 abs. i the gas temperature rises to I- 3 E -1 i3 I- W 2 la -2 0 x n w 9: SEPARATE (31 MOTOR TUR8I NE ORIVl NG Figure 1. Comparison of net output between (based on (1) main turbine shaft Figure 2. +o. 10 M.08 +O.O M.02 0 Comparison of steam cost between (based on (1) main turbine shaft about 150OC. The compressed gas, which becomes hot, is sent out from the lower-half casing to an intermediate cooler, in order to improve compressing efficiency. After the gas is cooled, the gas is sent again from the lower-half casing to the high pressure section. In the high pressure section, the gas pressure is raised to a little higher than atmospheric pressure, and is exhausted from the upper-half casing. At this exhaust, the gas contains pollusive gases, such as H2S and so 16

4 on; it is sent to a fan-stack and is discharged as a mixture with a large amount of air from the cooling tower. The air-gas mixture is discharged into the atmosphere by natural draft. As mist-separaters are installed at respective outlets of the condenser and the intermediate cool- er, the blown-off steam is reduced as much as possible. The flow path section of the gas compressor is protected from corrosion by corrosive gas in saturated water. As the gas compressor is coupled with the turbine shaft, it is operated at constant speed, which is 3,000 rpm from no load to full. Therefore, its surging zone exists up to a relatively high load. The air suction valve, for protection against surging, is installed at the gas exhaust line from the condenser. Further, as the gas compressor cannot lower the condenser pressure before the steam turbine starts up, and when the steam turbine is raising the speed, the plant is started up by starting ejectors connected with the gas exhaust line from the condenser. TURBINE OR I v I NG OR IV I NG (~riable Speed f MAIN EQUIPMENT CHARACTERISTICS The steam turbine consists of five stages and is a double flow type. The primary and secondary flashed steam is supplied, respectively, from the right and left sides of the lower-half casing. The secondary steam is sent into the inlet of the third stage nozzle and then is mixed there with the primary exhausted steam and flows toward the downstream stages. It is exhausted from the exhaust hoods in both the front en3 and the rear end of the turbine. Comparison of equipment cost between (Based on (1) main turbine shaft The design N.C.G. content is about 10 percent by weight. Compared with othergeothermal plants, this N.C.G, content is very high. Therefore, the steam path is to be suitable for a steam-gas mixture. Also, the rated turbine exhaust pressure (= condenser pressure) selected is 0.18 kg/cm2 SILENCER - STEAM GENERATM I COoLlffi TOWER SECONMAY STET - STEAM PRODUCTION KCT?UN AIR SUCTION i VALVE I OONoENSER 1 I CIRCULATING WATER WMP Figure 4. Outline of Power Generating System 17

5 abs. higher than a conventional one, which is between 0.1 to 0.14 kg/cm2 abs. in general. The pressure selected here reduces the gas compressor power consumption for the evacuating systemagainst a large amount of N.C.G. content. Furthermore, the gas compressor maintains a minimum volume flow of suction air in low load operation for protection against surging. Therefore, as air of lighter specific weight than the N.C.G. increases in no load operation and the compression ratio becomes low, the turbine exhaust pressure rises to 0.3 kg/cm2 abs.. In order to operate in high exhaust pressure, the last stage blades (20") for high exhaust pressure are used. This 20" last stage blade is not only of shorter length than 23" last stage blades used in conventional units of 50 MW, but is also designed to widen the bucket width to more than the conventional 20" last stage bucket. The gas compressor is coupled with the steam turbine at the opposite side of the generator. The turbine and the generator are coupled by a coupling machined integrally from the rotor row stock. The axial location is fixed by a thrust bearing installed in the front of the turbine. Another thrust bearing is also installed on the gas compressor. Therefore, a flexible diaphragm coupling is used, in order to prevent interference caused by relative axial movement of the tur bine rotor and the compressor rotor. In most cases, the control equipment is set in the front standard. But, at the Mori Geothermal Plant, there is no space for the control equipment since the turbine is coupled with the gas compressor in the front section, as mentioned above. An EHC (Electric Hydraulic Control System) is used for the turbine control system to eliminate the mechanical parts, such as a governor, as much as possible. Only an electric speed detector is installed at the bearing in the front section of the turbine. An emergency governor and a main oil pump are installed in parallel with the turbine shaft, connected by a gear. The EHC system not only reduces the space compactly, it also has a fine control function. This unit was designed to be highly reliable for a double flash type unit. In this system, the control oil is used at low pressure of about 14 kg/cm2g and in common with the lubricating oil. The condenser is a direct contact type, low level and tray type jet condenser, installed under the turbine exhaust hoods. It is easier to remove solid particles accumulating in the cooling water than a tray of the spray-nozzle type. The tray type condenser has features that are more efficient, and also more compact, than the spray-nozzle type The gas compressor is a single-suction centrifugal type with 2,850 kw driving shaft power and is of compact structure consisting of three stages in the low pressure section, two stages in the high pressure section and a casing. The compressor is designed to be added to by one stage if the TABLE 2 SUMMARIZED DESIGN SPECIFICATION OF MAIN EQUIPMENT Rated output at generator terminal: 50, 000 kw Steam condition: (primary / secondary) Pressure ( kg/cm2 abs. ) 6.7 / 2.0 Temperature (OZ) / Flow (t/hr) 356 / 136 Gas flow (t/hr) 40 / 0 Steam turbine: Number of stages 5 x 2 flow Exhaust pressure 0.18 kg/cm2 abs. Gas compressor: Discharge pressure atmospheric pressure Removal gas flow 44.1 t/hr Driving shaft power 2, 850 kw Number of stages 5 Intermediate cooler: Cooling flow 1,100 t/hr Starting gas removal equipment: Type single stage and steam jet ejector Removal gas flow 10.3 t/hr Driving steam flow 29.0 t/hr Cooling tower : Cooling water flow 9, 100 t/hr Cooling method counter flow Number of cooling fans 4 sets amount of N.C.G. decreases for the future. The cooling tower is a wet, mechanical forced draft and collective type which consists of four cells. It is collected and reduced, of octagonal shape and compact, so that the stable efficiency of the cooling is maintained in every direction of the wind. Furthermore, as it is designed to be collective, the effect is caused where the drift is diffused far away. The summarized design specification of the main equipment is shown in Table 2. CONCLUSION In the case of a geothermal power plant utilizing steam which contains a high non-condensable gas content of more than 10 percent by weight, a sufficiently economical plant can be supplied, even if a condensing steam turbine is used, if the system is optimized. ACKNOWLEDGEMENT We would like to express our thanks to the Hokkaido Electric Power Company, Ltd., for their cooperation, assistance and offering of valuable data. REFERENCES Tajima, S., and Nomura, M., October, 1982, "Optimization of Non-Condensable Gas Removal System in Geothermal Power Plant", Geothermal Resources Council Transaction, Vol. 6, pp