60% Efficient Gas Turbine System for Base Load Use

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-145 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Released for general publication upon presentation. Full credit should be given to ASME, the Technical Division, and the author(s). Papers are available from ASME for nine months after the meeting. Printed in USA. Copyright 1985 by ASME 60% Efficient Gas Turbine System for Base Load Use C. E. JAHNIG Consultant Rumson, N.J. ABSTRACT Efforts to improve the performance of combustion gas turbines has emphasized increasing the operating temperature. This paper shows that an alternative approach using higher pressure is very attractive, giving efficiencies of 55-60% together with much more power output at a given air flow. Maximum temperature is 1200 C. at a pressure of atmospheres. The new systems use supplemental combustion within the gas turbine to approach an isothermal expansion, in contrast to the adiabatic expansion normally used with gas turbines. Details for three specific systems are compared in the paper. INTRODUCTION There is a great incentive to increase the efficiency of combustion gas turbine systems in view of the high cost of fuel. An important goal is to achieve much greater net power output at a given air flow so that the investment and cost of electricity is decreased to a point where gas turbines become attractive for base load power generation. Considerable effort has been directed a improving gas turbine systems by increasing the turbine inlet temperature (1, 2). Also, some plants have operated gas turbines at high pressure ratios. Commercial demonstration has been obtained at Huntorf, Germany on gas turbines and combustors operating at high pressure in connection with compressed air energy storage (3). A plant in Japan has operated at high pressure with reheat (4). Advantages for such systems have been evaluated and the effects of operating variables defined (5). In addition, alternative high pressure cycles have been screened, show[ng the need for supplemental heat input in order to make best use of higher pressure (6). Thermodynamics of a gas turbine with multiple reheats has been examined, and the potential advantages shown for combining isothermal expansion with a Brayton cycle (7). A lot of improvement has resulted from this work but further progress is becoming more difficult. Therefore, it is timely to explore other approaches to achieve efficiency ioprovements in combustion gas turbine systems. Some approaches have been identified and examined and are the subject of this paper. Specific areas covered deal with increasing the pressure ratio on expansion while using means to approach an isothermal expansion rather than the conventional adiabatic one. OBJECTIVES Increased pressure ratio is clearly a way to increase power output from a gas turbine. It is interesting to compare the relative effect of this versus an increase in temperature. Thus, an effect comparable to that for raising temperature from 1200 C. to 1370 C. can be obtained by alternatively increasing the expansion pressure ratio by 50% (e.g. from 10/1 to 15/1 ). This is for adiabatic expansion, and for isothermal expansion the increase would be 37% instead of 50%. These results follow from the basic equations below: n-1 n Adiabatic n t' 2 w RT ( 1- Expansion n-1 P1 ( 1 ) Expansion \'I RT ln Pr, "- Increased pressure ratio offers the potential to improve gas turbine systems but has been limited in the conventional combin- ( 2) Presented at the Gas Turbine Conference and Exhibit Houston, Texas - March 18-21, 1985

2 ations with adiabatic compression because the power for compression increases rapidly with pressure ratio and the penalty soon offsets the gain realized on the expansion step. To some extent this penalty can be overcome by adding cooling during compression, to move toward the ideal of isothermal compression. expansion gives much more power than adiabatic expansion at the same pressure ratio and turbine inlet temperature. At 1200 C. the increase is 36% at 10/1 pressure ratio, or 66% at 50/1 pressure ratio. Of course, isothermal expansion requires that heat be added during the expansion step, and ways to accomplish this heat input will be described later. An inherent feature of isothermal expansion is that the gases leave the expansion step at such a high temperature that conventional metal heat exchangers are costly and may not be practical to give efficient heat recovery and utilization. New developments on heat exchange could provide solutions to this problem but other ways to recover the heat effectively have been identified. To summarize, the objective is to define a gas turbine system that will allow approaching the ideal of isothermal expansion and compression, with efficient use of the heat in the hot gases leaving expansion by recuperation or by other means. Several promising cases will now be described. BASIS FOR COMPARISON Three cases have been defined based on using isothermal expansion. All of the cases use C. turbine inlet temperature at about atmospheres pressure, using a combustion gas turbine that is modified to provide for catalytic combustion of supplemental fuel within the turbine (8). Figure 1 illustrates the modified turbine. Combustion Catalyst Fuel (or oxygen ) Inlet,,_ Gas Hot gases at high pressure from a precombustor enter the first stage of expansion. Supplemental fuel is burned within the turbine and some fuel is added at the inlet to the first stage but there is not a lot of combustion until the fuel enters the catalytic zone on the second stage of expansion. More fuel is then added before the catalytic zone of the third _ stage. Good distribution and mixing of the fuel is needed to avoid hot spots. The overall result is to approach an isothermal expansion by offsetting the drop in temperature that would occur in a simple adiabatic expansion. Although ideal isothermal expansion may not be achieved in practice a reasonably close approach to it appears practical. Gases leave the turbine at high temperature and flow to a recuperator for recovery of heat. The recovered heat is used to preheat air going to the high pressure turbine but it is not possible to use all of the heat available because the flow rates are out of balance. For maximum overall efficiency it is important to make effective use of all available heat, and several attractive alternatives have been developed.?ir compr ssion uses a number of stages with intercooling to 40 C. at an efficiency of 90%. The calculated power is increased 15% when air for cooling is included. Cooling may not be needed, depending on new developments such as ceramic blades and components. Calculations are based on an ideal gas to assure consistent reltionships between power output, sensible heat loads etc. in making these screening type comparisons. Specific cases were selected for evaluation representing various ways to use the sensible heat in the hot gases leaving the isothermal turbine. These cases were compared as to net power and fuel efficiency. DESCRIPTION OF CASES 8ase 1 The first case uses an adiabatic expansion at the outlet of the isothermal turbine to drop the temperature to 650 C. and thereby allow using conventional metal heat exchangers for recuperation. Figure 2 shows the system for.;at>e 1 Hot gas from the adiabatic expander is cooled to 204 C. while the compressed air is preheated to 650 C. Pressure ratio is 50/1 for the isothermal turbine and 6/1 for the adiabatic turbine. Net power output for Case 1 corresponds to an efficiency of 57.1% on total fuel fired when allowing for air cooling of the turbine blades. If cooling were not needed, as with ceramic blades, the efficiency would be 61.7% based on low heating value of the fuel. Net power output is much higher than for conventional gas turbine systems at the same air flow, therefore more fuel is reauired and the fraction of oxygen in the air that is used up is correspondingly higher. The conditions that give 57.1% efficiency result in using up 64.2% of the available oxygen. Figure 1 Provision for Catalytic Combustion in Turbine. -2-

3 Fuel Gas Turbine Adiabatic Gas Turbine ::tecuperator.flue Gas atm 6.0 atm 1.3 atm 733 c. 1.o atm?04 c. Fuel 650 c. 350 Precombustor Compressor Figure 2. Case 1 With Adiabatic Gas Turbine. Case 2 uses recuperation to give 980 C. air preheat assuming that suitable heat exchange equipment will be available. No adiabatic turbine is used since recuperation provides equivalent heat recovery. The system is shown in Figure 3. Again, pressure at the high pressure turbine inlet is atmospheres. Efficiency for Case 3 is 54.6% including a debit for cooling air, or 58.8% if cooling air is not needed. Fuel and oxygen consumption are somewhat higher than for Case 1. Case 3 This case uses a different approach to recover sensible heat in the exhaust gases leaving the isothermal turbine. As shown in igure 4 the hot gases are passed through a conventional heat exchanger to indirectly heat a separate air stream to 650 C. at a pressure of 15 atmospheres. This air is further heated to 815 C. by direct combustion and then goes to an auxilliary expander to make additional power. Gas Turbine Fuel Recuperator atm 1. 5 atm 1 atm 204 c. Flue Gas Fuel 980 c. 350 atm Precombustor Compressor Figure 3. Case 2 With Recuperation Only. -3-

4 Fuel Com bus tor i<:xpander Recuperator Ur 1 atm 204 c. Gas Turbine Fuel /\uxilliary Compressor Precombustor atm 1.4 atm 1?04 c. Recuperator Flue Gas 1 atm 204 c. Figure 4. Case 3 With Auxilliary to Expander. Although the auxilliary air system is not highly efficient it is simple and straie;htforward and avoids the hie;h investment that would result if steam bottoming were used instead. One advantage for Case 3 is the high net power output for a given air flow rate to the isothermal gas turbine. Efficiency for this case is 55.7% with a debit for cooling, or 58.9% without the debit for air cooling. The fraction of oxye;en in air that is used up is high, and approaches 100% when no air cooling is used. This sets a limit on the ultimate performance for these gas turbine systems. To decrease oxygen consumption, turbine inlet pressure can be decreased to say 250 atmospheres, with only a small drop in efficiency. COMPARISON OF CASES The cases are compared in Table 1 with regard to efficiency, power output and oxygen consumption. The debit for air cooling is approximated by adding 15% to the power for air compression. As shown, efficiency for all three cases is about 55% when including the debit for cooling, or about 60% with no debit. Net power output per unit of air flow is much higher for Case 3, resulting in lower projected investment in $/kw. In all three cases equipment development is needed to realise the improvements shown. Oxygen consumption becomes a limiting factor for the conditions used in Case 3 and sets a limit on the maximum pressure ratio in expansion. This limit is affected by air preheat temperature and increases with higher air preheat. Use of refractory components has been proposed as a way to avoid the debit for cooling the equipment. Future developments in this area will affect the choice between various options. A lower turbine operating temperature of perhaps 800 C. is an alternative to consider. Detailed cost estimates are not available at this time due to uncertainty in the cost of non-conventional equipment; however, rough projections show an economic advantage for Case 3 over Cases 1 and 2, al though all three cases are quite attractive compared to conventional power generation systems. The lower investment for Case 3 can be expected because of its hie;her power output, which is 37% greater than Case 1 In setting up the comparisons the effects of operating variables were screened in order to roue;hly optimize each case. Some of the results are summarized in Tables 2, 3 and

5 Case TABLE 1 COMPARISON Temperature in, C. Pressure in, atm. Pressure out, atm. Adiabatic Turbine Temperature in, C. Pressure in, atm. Pressure out, atm. OF HIGH EFFICIENCY GAS TURBINE CASES _ 3 _ CONCLUSIONS Theoretical considerations have shown that large improvements in gas turbine performance are possible by going to higher pressure ratio with isothermal expansion rather than adiabatic expansion. System designs have been proposed to accomplish the objectives, giving high fuel efficiency to minimize fuel consumption together with high power output to minimize investment. The systems merit further consideration for development in advanced energy programs. Net Power Out, kj turbine Adiabatic turbine , , 72 39, Total Fuel, kj Efficiency, % With air cooling No air cooling , Oxygen Used Up, % With air cooling No air cooling Basis: - One gram mole of air to precombustor. - 90% efficient compression and expansion. - Calculations based on ideal gas with heat capacity of at constant volume and kj/kg K at constant pressure. TABLE 2. EF ECT OF OPERATING VARIABLES IN CASE 2. Temperature in, Pressure in, atm Power out, kj c. Adiabatic Turbine Pressure in, atm Fower out, ? kj Com resso=: Pressure out, atm Fower, no cooling, kj Net Power Out2 kj Total Fuel, kj Efficiency1 % 'Iii th air cooling , 1 No air cooling '.o Basis as in Table

6 TABLE 3. :t;ffect OF OPERATING VARIABLES IN CASE 2. Temperature in, c Pressure in, atm Power out, kj ComEressor Pressure out, atm Power, kj Net Power Out1 kj Total Fuel1 kj Efficienc.z1 o' 7o With air cooling No sir cooling Basis as in Table 1 TABLE 4. EFFECT OF OPnRATING V AlUABLES IN CASE 3. Temperature in, c Pressure in, atm. 275 Power out, kj Adiabatic Turbine Temperature in, c PressurP in, atm Power out, kj H.P.!\ir ComEressor Pressure out, atm Power, no cooling, kj L.P. Com-oressor Pressure out, atm. Power, kj Net Power Out1 kj Total :Fuel1 kj Efficienc.z1 % With air cooling No air cooling Basis as in Table 1-6-

7 RE?ERENCi< S 1. Hottel,H.C. and Howard,J.B. "New Energy Technology- Some Facts and Assessment" M.I.T. Press Nov NASA Lewis Research Center "Comparative Evaluation of Phase I Results from the Energ Conversion Alternatives Study (ECAS) NASA TM X Feb Also Phase II Results TM X April Herbst,H.C. and Stys,Z.S. "Huntorf 290 MW- The World's First Storage Energy Transfer Plant" American Power Conference Chicago, Ill. April Jeffs,E. "Japan's 124 MW High-temp Pilot Plant on Test" Gas Turbine World -13, 4 Sept p Kartsounes,G.T. "Evaluation of Turbine Systems for Compressed Energy Storage Plants" Argonne National Laboratory Report ANL/ES - 59 Oct Stevens,W.A. and Central Electricity Generating Board, England. "Technical and Economic Assessment of Advanced Compressed Storage Concepts" EPRI Report 2M-1289 Dec El-Masri,M.A. and Magnusson,J.H. "Thermodynamics of an Gas Turbine Combined Cycle" ASME Paper 84-GT Jahnig,C.E. "30% r'uel Saving by Adding Combustion Within Gas Turbine" ASME Paper 81-JPGT-GT-4 St. Louis, Mo. Oct