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ABSTRACT Subscripts m mixed working fluids A type of coupled steam—gas turbine plant is pro- a air posed here. It is composed of a regenerative extraction g gas steam turbine and a steam injected gas turbine. Extracted s steam steam of the regenerative extraction steam cycle is not used t turbine to heat water through the regenerative feed—water heater c compressor as in conventional plant, but injected into a gas turbine to b extraction point or burner augment the output of the gas turbine, while the exhaust f fuel gas of the gas turbine now displaces the extracted steam to GT. ST refers to gas turbine and steam turline heat the feed water of the steam turbine plant. The pro- 1,2,3••• positions as shown in figure 1. posed repowering turbine plant has two merits: the further utilization of extraction steam and the elimination of the Superscripts complicated waste heat recovery boiler of a conventional T3 temperature of gas turbine inlet steam injected gas turbine plant, in favor of a gas—to—wat- Tr fuel inlet temperature er heat exchanger. after refitted into coupled plant
NOMENCLATURE INTRODUCTION
C average specific heat capacity kJ / (kg • K) To increase the energy utilization ratio in a gas turbine f fuel—to—air ratio Brayton cycle , "Cheng cycle heat engine", or "Dual fluids G flowrate kg / s parallel compound cycle gas turbine", or "Steam injected h enthalpy kJ/kg gas turbine" was patented in reference'. In the patents, a pressure Pa heat recovery boiler, which is used to generate steam, is T temperature K added to the exhaust duct of the gas turbine. The steam w specific work kw / kg generated by the boiler is injected into the combustion x injected steam—to—air ratio — chamber or turbine to augment the power output and a extraction steam fraction — thermal efficiency of the gas turbine plant. Many Steam In- 0 low heating value of fuel kJ / kg jected Gas turbine or STIG plants as called by Gereral efficiency — Electric have been applied around the world.
Presented at the International Gas Turbine and Aeroengine Congress and Exposition Houston, Texas - June 5-8,1995 To increase the thermal efficiency of a steam turbine minimum temperature difference in the heat recovery boil- Rankine cycle, steam is extracted through the extraction er restricts the steam injection amount, or x m , the ratio of openings of the steam turbine to preheat the feed water of injected steam to intake air of the gas turbine. For the giv- the steam turbine plant. en pinch point temperature difference OTPP and the hot That is to say, for STIG gas turbine plant, a heat re- end temperature difference ATh of the heat recovery covery boiler is necessary to generate steam. While for boiler, there is a critical injected steam—to—air ratio the regenerative extraction steam turbine plant, steam, which connertroral xT. When x m >xcr, the performance of the still has the ability to generate power, must be extracted to conventional steam injected gas turbine plant begins to de- Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A005/2406586/v004t10a005-95-gt-170.pdf by guest on 29 September 2021 preheat the feed water. teriorate. Therefore, a coupled steam—gas turbine plant is pro- If the injection steam for gas turbine does not come posed here. The idea of the proposed coupled plant is just from its heat recovery boiler, there is no restriction on the the combination of the steam injection of STIG gas turbine ratio of injected steam—to—air. This can be seen from the and the steam extraction of the steam turbine. The coupled following analgses. plant is composed of a regenerative extraction steam tur- bine and one or several gas turbines. As shown in figure 1, the steam which is extracted from the steam turbine is in- jected into the gas turbines and the exhaust gas of the gas turbines is used to preheat the feed water of the steam tur- bine plant to make best use of energy. The high grade ener- gy o the extracted steam of the steam turbine cycle is util- ized further in the steam injection gas turbine. For example, the temperature of the extracted steam of steam turbine S in this paper is 330 C (No.5 regenerator). The extracted steam can be injected into a gas turbine to gener- ate more power. And then, the exhaust gas replaces the ex- tracted steam to heat feed water of the steam turbine. The Fig. I diagram of coupled steam—gas turbine plant exhaust temperature of the two gas turbines of the pro- posed two schemes are respectively 346t , 396t . The exhaust gas can replace steam to heat feed water. A Snecific work gas—to—water heat exchanger takes the place of one or sev- The specific work of a steam injected gas turbine is: eral steam—to—water regenerators. The entropy production w=(1 + + m ( 3 — —( — a in a gas—to—water heat exchanger is slightly more than a hm enthalpy of mixed working fluid. heat recovery boiler or a steam—to—water regenerator. Here, the injected steam comes from the extraction opening There were some gas—to—water heat exchengers which of regenerative extraction steam turbine of the coupled have been used in other kinds of combined turbine plants. plant. For convenience sake, suppose the steam is injected Because of the utilization of the extracted steam of the into the combustion chamber only. steam turbine and the exhaust gas of the gas turbine of the w = [(1 + c a3 + C ]( 3 — coupled plant, the output and efficiency of the plant are —C a 1 ( — im o e . here, T ; —isentropic temperature at position i,
C C g C S —average specific heat of air, gas and steam. THEORETICAL CONSIDERATIONS OF Therefore, STEAM AND GAS TURBINES WITH STEAM Ow _ o EXTRACTION AND INJECTION O — C S3 o + s3 3 — The ratio of fuel—to—air after steam injection can be For the coupled plant, the main connection of steam determined by the heat balance in the combustion and gas turbines is the extraction or injection steam. the ef- chamber: fect of steam extraction and injection on the performance — O =C — of steam and gas turbines, the appropriate amount of ex- ( 3 + ( + m C 31 traction and injection steam should take preconsiderations. ( 3 —
here, T b —steam temperature at extraction point. Hence,
1. Gas Turbine Performance with Steam Injection 3 — + — C a3 ( C ( 3 For the conventional steam injected gas turbine, the = n48+h11—h13 injected steam is generated by the heat recovery boiler. The
hence, fST> fGT in any case. 3 — — Therefore, the specific work and efficiency of the = C a3 ( + C S3 3 steam injected gas turbine of the coupled plant will increase n b B+hff —hf ^nb0+hf —hf' as the increase of steam—to—air ratio. = + m
f0 , f,--fuel—to—air ratio used for the heating of air and 2. Steam Turbine Performance after Steam Extraction s eam es ec i e y. As shown in figure 2, suppose the original steam tur-
bine has three extaction points a, b, and c, corresponding Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A005/2406586/v004t10a005-95-gt-170.pdf by guest on 29 September 2021 o = , we have, x extraction steam fractions are aa, a b , and a,. The ex- aw traction point for steam injection of the gas turbine is de- a m =17 (C a3 +C S3a ( 3 — termined by the requirement of steam injection of the gas The inequality indicates that the specific work of 'a turbine. The parameters of the selected extraction point are s eam i ec e gas u i e i c eases as e a io o Tb , Pb' extraction steam fraction steam—to—air increases. is a'b, a'b x.G,, / G. Assume the other factors are u c a ge . See ig.2. Efficiency The injection steam has an effect on the efficiency of •6 the gas turbine plant. For the conventional steam injected gas turbine, when x m
an/ax m =(aw/a m —o /a m w / Fig.2 T—S chart of extraction steam turbine cycle The specific work of the original extraction steam tur- 3 — — [ C g3 ( 3 — —w _ u C s3 ( bine is.
— aa b)+(1—aa ab)(hb ws =(hb ha)+(l )(ha h —hC) 0 C s ( 3 3 +(1—aab aC(hC h 7) if f , — { C o 3a ( — — w ] when refitted into the coupled plant, specific work of the steam turbine is modified to, efficiency i will increases as the x m increases. The inequality is valid and can be seen from the fol- w', = (hti hl) —a'b(hb h7) s a lowing: Aw = a (ha h7)+ab(hti h7)+x,(h^ h7) — a'b(he h,) The inequality is when, a bGlaa(ha h7)+ab(hb hl)+ac(hc h7)] / (hb h7) (* ) 3 — / 11 3 4 Cps3a C ,, (T — T ) Aw,>O, —w ]/fa means that the steam, whose amount m b o ( 3 — i C og3 ( 3 — — w is x = a' G, / G a, is injected into the gas turbine, and the or i C output of the gas turbine increases, while the output of the — 3 3— C p,,23 ` 3 steam turbine remains the same as a minimum. If the ex- which corresponds to the assumed efficiency of the steam haust gas heats the feed water as the steam has done, the turbine 17 ST being greater than the assumed efficiency of efficiency of the steam turbine remains unchanged. gas turbine G of a steam injected gas turbine cycle. If xm increases further, a' b increases as well. There is if Tb > T4, then 1 is correct obviously. 7S »1 GT a value at which the output and efficiency of steam turbine Here, Tb—temperature of extraction steam will decrease, but if a'b satisfies, If Tb 3 The inequality of (* *) can be obtained through Aw g steam to heat the feed water. The regenerative steam heater >Aw,. That is to say, if only ab meets the requirement is substituted by gas—to—water heat exchanger, the gas exit of (* * ), the amount of steam extraction can be in- temperature is 200C and the gas has energy to preheat the creased further, and the overall output of coupled plant supplementary water. The schematic will increase further. However, the amount of steam extrac- diagram of the coupled plant I is as shown in fig.3. tion and injection would be restricted by other factors of The coupled results are that the steam turbine output the steam and gas turbines. remains the same 52910kw, and the gas turbines output increase to 3270kw. Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A005/2406586/v004t10a005-95-gt-170.pdf by guest on 29 September 2021 EXAMPLES Scheme II The coupled plant II is composed of steam This type of coupled plant can be applied while taking turbine S and gas turbine GB. According to the steam in- into accout the practical conditions, which include: the feed jection requirements of the gas turbine, steam is extracted water of steam turbine plant can be heated by exhaust gas from two points: one at the No.5 regenerator (9.5kg / s), alone, or be heated by gas and steam in parallell, the ex- the other in front of No.6 regenerator(5.97kg / s). traction and injection points may be one or more. The heat balance ealculations indicate that the exhaust We analyse two coupled plants, which consist of one gas of gas turbine GB can meet the heating demand of the extraction steam turbine and two type of gas turbines. feed water of steam tarbine S (39t - 220t ). The six ❑ Regenerative extraction steam turbine S steam—to—water regenerative feed—water heater are substi- Steam turbine S is a regenerative extraction con- tuted by a gas—to—water heat excharger. The exit tempera- densing steam turbine. The steam turbine has 22 stages ture of gas from the gas—to—water heat exchanger and one control stage. There are seven extraction openings is 173t. The schematic diagram of the coupled plant II at the position of No.6, 9, 11, 14, 16, 18, 20 stage. It is as shown in fig.4. has six regenerative feed—water heater and one deaerator. The rating power is 53MW, The steam inlet pressure: 90 bar, inlet temperature: 535t , the final feed—water temperature: 222t , the heat rate: 2261 kcal / kw • h. We select two steam injected gas turbines. ❑ Gas turbine GA (WJ-5 gas turbine) Rating power: 1300kw; Turbine inlet temperature: 965k; Compressor ratio: 7.2; Air flowrate:14.8kg / s. ❑ Gas turbine GB (LM5000 steam injected gas turbine) When steam injected, the rating power is 51000kw, GA yn — w y the heat rate is 8527KJ/ kw • h. Exhaust gas rate:148.5 - ----- — - - - '.;&,s •} t"M kg/ s; Exhaust temperature:396 C . It has two injection w-- S a - — a A I points: one at the combustion chamber, the other at the low pressure turbine. Fig.3 Schematic diagram of the coupled plant I Based on the above steam and gas turbine, we have done two conceptual designs: Scheme I and Scheme II Scheme T The coupled plant I is composed of the steam tur- bine S and two gas turbine GAs. The injection steam amount of the two gas turbines is 2.96kg / s, almost just equals the original extraction steam amount which is ex- tracted for the feed—water heating in No.5 regenerative feed—water heater of steam turbine S, and the steam pres- sure at the position satisfies the requirement of the steam injection of the gas turbines. The feed water inlet temperature of No.5 regenerator is 145t , outlet temperature is 193C . Through heat bal- Fig.4 Schematic diagram of the coupled plant II ance calculations of the regenerator, the two gas turbine The final results are that the steam turbine output GAs' exhaust gas could displace the original extraction drops to 48398kw as the more steam is extracted, and the 4 gas turbine output grows from 33MW to 51MW. The in- gas of gas turbine, the efficiency of the coupled plant im- crease of the gas turbine output conpensates the decrease proved also. The efficiency of the gas turbine circuit in- of the steam turbine output, with the result that the coup- creases more for the steam injection (scheme II : led plant output is 13488kw more than the original total from 36% to 43%). The energy of exhaust gas is utilized output (85910kw). to heat the feed water of the steam turbine through a Table 1 Calculating results gas—to—water heat exchanger. A gas—to—water heat ex- changer is not as complicated as and costs less than a heat scheme I scheme II recovery boiler of the conventional steam injected gas tur- Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A005/2406586/v004t10a005-95-gt-170.pdf by guest on 29 September 2021 item unit original coupled original coupled bine. The high grade heat energy of the extracted steam is output of the steam turbine KW 52910 52910 52910 48398 utilized further. Although the energy of the gas turbine ex- haust gas is still of high grade, it is waste heat for gas tur- output of the gas turbine K W 2600 3270 33000 51000 bine. Because the gas turbines and steam turbine of the two total KW 55510 56180 85910 99398 examples are based on information at hand, the two exam- ples may be not the best combinations. If the gas and steam turbines match approriately, the performance of the coup- CONCLUSIONS led plant will be improved. The proposed coupled plant is of special value for the repowering of existing regenerative The feature of the coupled plant proposed above is the extraction steam turbine plants with steam injected gas combination of the steam injection of the steam injected turbines. gas turbine and the team extraction of the regenearative ex- traction steam turbine. When gas turbine and steam tur- bine are combined in this way, the total output power of REFERENCES the plant increases, as the extracted steam of the steam tur- bine is not just used to heat the feed water in a Cheng, D. Y. (1981), U. S. Patent4,248,039, steam—to—water heater, but injected into the gas turbine Fob.3 1981,4,128,994, Dcc.12,1978. 4,297.841. Nov.3, 1981 and mixes with gas to generate power. As the utilization of Chen, D. X. (1987) Gas Turbine Cycle Analysis, China Mechanical extracted steam of extraction steam turbine and exhaust Press, pp. 152-158 5