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2 Geotherml Resources Council, TRANSACTIONS Vol. 6, October 1982 OPTIMIZATION OF NON-CONDENSABLE GAS REMOVAL SYSTEM IN GEOTHERMAL POER PLANT Shigeru Tjim nd Mitsuo Nomur Toshib Corportion Tokyo, Jpn ABSTRACT Optimition of non-condensble gs (hereinfter clled N.C.G.) removl system in geotherml power sttion, in specil cse tht the geotherml stem contins lrge mount of noncondensble gs, is subject to be discussed. Four different lterntive N.C.G. removl systems re studied, which re 1) Stem jet gs ejectors, 2) Centrifugl gs compressors, 3) Combined systems of stem ejectors nd centrifugl compressors nd 4) Bck pressure turbine-without N.C.G removl system. This report summries the results of study, nd give recommendtions s to the most suitble gs removl system, nd lso s to optimum condenser *pressure, in cse of lrge quntity N.C.G. content in geotherml stem. INTRODUCTION Rted output t genertor terminl 5, k Turbine inlet stem pressure 4.5 kg/cm2 bs Turbine inlet stem temperture 147 OC NCG content 7.25% by volume NCG composition C2 : 98.1 % H2S : 1.2 % :.2 % :2 :.42 % C#4 :.26 % Condenser pressure 75 mmhg bs (.lo2 kg/cm2 bs), 1 mmhg bs(.136 kg/cm2 bs), 125 mmhg bs, ((.17 kg/cm2 bs),.3 kg/cm2 bs,.5 kg/cm2 bs (vried s prmeter) Cooling wter inlet temperture 22 OC Stem price Cpcity fctor Fixed chrge rte.6 yen/kg 9 %- 18 % Bsed on bove mentioned conditions four(4) Geotherml stem contins non-condensble lterntive N.C.G extrction systems re evlute.d. gses in lrge mount compred with tht of conventionl therml power plnt. For tht reson, (Fig. 1) 1. Stem jet gs ejectors (single stge nd geotherml power plnt requires lrge cpcity nontwo stge), condensble gs removl system nd it plys very 2. Centrifugl gs compressors. importnt role in geotherml power genertion, nd 3. Combined system of stem ejectors nd lso occupies lrge portion in its totl plnt cost nd totl uxiliry power consumption. n----i centrifugl compressors. So, 4, Bck pressure turbine. it becomes mjor concern to be studied t plnning nd bsic design stge of geotherml power sttion to select the type of N.C.G. removl system, together with determintion of rted turbine exhust pressure (=condenser pressure). ---j I :D--b I f Mny fctors ffects on optimition of N.C.G. j &--A L-.l I removl system, nd the most importnt one is N.C.G. content in geotherml stem. Generlly, IA) SINGLE STXE STEAM EJECTOR IB) TO STAGE STEAM EJECTOR N.C.G. content in geotherml stem lies between.3 nd 2% by volume in most geotherml resource re, but in the cse treted in this report, it reches round 8% by volume. The min objective of this report is to select the optimum N.C.G. removl system in such lrge N.C.G. content of geotherml stem. The optimition is mde bsed 2j CENTRIFUGAL COMPRESSOR 3) COM~INEG SYSTEM STEAM EJECTORS on economic evlution (Comprison of electricl genertion cost for ech lterntives). BASIC CONDITION FOR OPTIMIZATION AND CENTRIFUGAL COMPRESSOR Following conditions re ssumed s bse 4) 8ACK PRESSURE TURBINE for optimition of N.C.G removl system In this cse study. Figure 1. Alterntive N.C.G. removl system 397

3 Tjim et l. ALTERNATIVE 1: STEAM EJECTORS Fig. 2 shows the turbine inlet stem flow, N.C.G. flow extrcted from condenser, driving stem flow of stem ejector V.S. condenser pressure, in cse of stem ejector system. As condenser pressure becomes high, turbine requires more stem flow to produce rted power output becuse of decresed vilble energy, nd N.C.G. flow extrcted from condenser increses together with turbine exhust flow. hile, driving stem flow of stem ejector decreses s condenser pressure becomes high becuse of decresed pressure rtio. Two kinds of stem ejector system, which re single stge stem ejector system nd two stge stem ejector system, re studied. In cse tht the condenser pressure is reltively low, two stge stem ejector system is indispensble, with inter condenser to reduce the cpcity of second stge ejector. In cse tht the condenser pressure is reltively high, it cn be possible to extrct N.C.G. from condenser by single stge stem ejector, nd no inter condenser nd cooling wter is required. The study shows tht single stge stem ejector is enough to extrct N.C.G. from condenser for condenser pressure bove.22 kg/cm2 bs, nd below.22 kg/cm2 bs of condenser pressure two stge ejector is needed, Driving stem flow of two stge stem ejector system decreses up to.25 kg/cm2 bs of condenser pressure when condenser pressure increses. Above condenser pressure of.25 kg/cm2 bs, driving stem flow increses gin, becuse of the design limit of flow rtio (=extrcted N.C.G. flow/driving stem flow). For optimition of stem ejector system, it is importnt to minimie the driving stem flow. In this view-point, single stge gs ejector system is better choice thn two stge gs ejector system when the condenser pressure is higher thn.3 kg/cm2 bs. And two stge stem ejector system is prefered for condenser pressure less thn 3 kg/cm2 bs. ALTERNATIVE 2: CENTRIFUGAL GAS COMPRESSOR Stem ejector mentioned bove hs mny dvntges such s simple construction, esy opertion nd mintennce nd low cpitl cost, nd is commonly used s N.C.G. removl system in geotherml power plnt. But when N.C.G. content in geotherml stem is lrge, s in the cse treted in this pper, driving stem consumption of ejector becomes considerbly lrge mount, nd centrifugl compressor comes into spotlight s n effective lterntive becuse of its reltively high efficiency. Fig. 3 shows N.C.G quntity extrcted from condenser nd required power for driving gs compressor. In cse tht the condenser pressure is low, turbine inlet stem flow is smll nd N.C.G. flow extrcted is lso smll ccordingly. hile sie or number of gs compressor instlled becomes lrge becuse of lrge specific volume of N.C.G. in such low suction pressure condition nd cpitl cost lso becomes high. As condenser pressure increses, required power for gs compressor decreses becuse the pressure rtio decreses. ci U U I \ -1.3 v u U I LL - 1. x w I I I I ' Figure 3. CONDENSER PRESSURE (KG/CM~ ABS I Effect of condenser pressure on required power for gs compressor I I I I I I I I CONDENSEF! DRESSURE ( KG/CM* ABS. 1 ALTERNATIVE 3: COMBINED SYSTEM OF STEAM EJECTOR AND CENTRIFUGAL COMPRESSOR Figure 2. Effect of condenser pressure consumption of stem ejector on stem system To Improve the chrcteristics of stem ejector system (low cpitl cost nd esy mintennce, but require lrge driving stem consumption) nd centrifugl compressor system(re1tively high efficiency but high cpitl cost), combined 398

4 /- Tjim et l. system of stem ejector nd centrifugl compressor is considered. The system comprises stem ejector s first stge, intercondenser nd centrifugl compressor s second stge, s shown in Fig. 1. Fig. 4 shows N.C.G extrcted, electricl power required, driving stem of gs ejector nd turbine inlet stem flow. Driving stem flow of stem ejector nd required power for gs compressor decreses s condenser pressure increses. cost, stem consumption of turbine, driving stem flow of ejector nd electricl power for compressor into considertion. (Electricl Genertion Cost) = (Stem Cost)* + (Cpitl cost) *Stem cost mens cost of geotherml stem required to produce unit electricl power (yen/kwh 1 The result is shown in Fig. 4, nd the conclusion is summeried below. u. c3 1%- I- U' y p14- &.E 1.2 -?3 " ILL 14' & = 51- g &12- I I I JI CONDENSER PRESSURE I mmhg obs ELECTRICAL GENERATION COST _---- STEAM COST --- CAPITAL COST BT Figure 4. Effect of condenser pressure on driving stem flow of stem ejector nd required power of gs compressor ALTERNATIVE 4: BACK PRESSURE TURBINE Bck pressure turbine is considered to be nother effective lterntive in the cse of lrge N.C.G. content in geotherml stem, Mdjor specifictions of bck pressure turbine studied here re s follows. TYPE Tndem compound, four flow, two csing, bck pressure turbine Rted output 5, k Exhust pressure 1.11 kg/cm2 bs Number of stges 2 stges x 4 flow (2 csing) The cpitl cost of bck pressure turbine system is the lowest mong other lterntives. becuse no N.C.G. removl system is necessry, Furthermore uxiliry stem nd electricl power is not needed. These dvntges re emphsied when N.C.G. content in geotherml stem is considerbly lrge. On the other hnd, stem consumption of bck pressure turbine is the lrgest mong other lterntives. So effectiveness of bck pressure turbine system depends on stem price s well s N.C.G. content in geotherml stem. EVALUATION I- v) e 1 U J U u_ 5 J CL X n C '. ' \ \ _----_ #-- I- //-- I I I I O. 5 ;' I, ( KG/CM2 A X (rnmhg bs.1 EJ CMP EJ+CMP BT CON DENSER PRESS LEE : Stem ejector : Centrifugl compressor : Combied system of stem ejector nd compressor : Bck pressure turbine Figure 5. Effect of type of N.C.G removl system nd condenser pressure on electricl genertion cost ) Evlution is mde by comprison of electricl power generting cost for bove mentioned four lterntive N.C.G. removl systems. Electricl generting cost is clculted tking equipment 399

5 Tjim et l. CONCLUSIONS In cse N.C.G content is 7.25% by volume, nd under the given condition s mentioned bove: Centrifugl compressor system, nd condenser pressure between 125 mmhg bs(o.17 kg/cm2 bs)nd.3 kglcm2 bs, is the optimum selection mong ll lterntives. Centrifugl compressor is the most economicl selection when condenser vcuum is higher thn.12 kg/cm2 bs. Below.12 kg/cm2 bs, combined system of stem ejector nd centrifugl compressor becomes the most economicl selection. Centrifugl compressor gives the highest cpitl cost nd the lowest stem cost, Bck pressure turbine gives the lowest cpitl cost nd highest stem cost. Optimum condenser vcuum tht gives the lowest cpitl cost lies round.3 kg/cm2 bs for ll lterntives except for bck pressure turbine. Bck pressure turbine is not recommended in the cse of reltively lrge cpcity unit such s 5M, even in the cse the N.C.G. content is lrge. Optimum vcuum for stem ejector system lies round 125 mmhg bs(o.17 kg/cm2 bs) 4