GasTurbine Engineering HandbookSecond Edition phần 6 doc

//INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 395 ± [370±408/39] 29.10.2001 4:02PM zones inside a ``conventional'' combustor. This design deliberately burned all of the fuel in a series of zones going from fuel-rich to fuel-lean to provide good stability and combustion efficiency over the entire power range. The great dependence of NO x formation on temperature reveals the direct effect of water or steam injection on NO x reduction. Recent research showed an 85% reduction of NO x by steam or water injection with optimizing combustor aerodynamics. In a typical combustor as shown in Figure 10-19, the flow entering the primary zone is limited to about 10%. The rest of the flow is used for mixing the combusted air and cooling the combustor can. The Maximum temperature is reached in the primary or stoichiometric zone of about 4040  F (2230  C) and after the mixing of the combustion process with the cooling air the temperature drops down to a low of 2500  F (1370  C). Basis for NO x Prevention. Emissions from turbines are a function of temperature and thus a function of the F/A ratio. Figure 10-20 shows that as the temperature is increased the amount of NO x emissions are increased and the CO, and the unburnt hydrocarbons are decreased. The principal mechanism for NO x formation is the oxidation of nitrogen in air when exposed to high temperatures in the combustion process, the amount of NO x is thus dependent on the temperature of the combustion gases and also, to a lesser amount on the time the nitrogen is exposed to these high temperatures. CO, UHC NO X FUEL/AIR RATIO LEAN RICH TEMPERATURE EMISSIONS Figure 10-20. The effect of flame temperature on emissions. Combustors 395 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 396 ± [370±408/39] 29.10.2001 4:02PM The challenge in these designs is to lower the NO x without degradation in unit stability. In the combustion of fuels that do not contain nitrogen compounds, NO x compounds (primarily NO) are formed by two main mechanisms, thermal mechanism and the prompt mechanism. In the thermal mechanism, NO is formed by the oxidation of molecular nitrogen through the following reactions: NO x is primarily formed through high temperature reaction between Nitrogen and Oxygen from the air. O  N 2 6 NO N 10-8 N  O 2 6 NO O 10-9 N  OH 6 NO  H 10-10 Hydrocarbon radicals, predominantly through the reaction, initiate the prompt mechanism CH  N 2 3 HCN  N 10-11 The HCN and N are converted rapidly to NO by reaction with oxygen and hydrogen atoms in the flame. The prompt mechanism predominates at low temperatures under fuel-rich conditions, whereas the thermal mechanism becomes important at tempera- tures above 2732  F (1500  C). Due to the onset of the thermal mechanism the formation of NO x in the combustion of fuel/air mixtures increases 0 10 20 30 40 50 60 70 80 90 1200 1300 1400 1500 1600 1700 1800 1900 2000 FLAME TEMPERATURE NO PPH X Figure 10-21. Correlation of adiabatic flame temperature with NO x emissions. 396 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 397 ± [370±408/39] 29.10.2001 4:02PM rapidly with temperature above 2732  F (1500  C) and also increases with residence time in the combustor. The production rate of NO can be given as follows: dNO dt  K  T p e 1 T  O 2 p N 2 10-12 The important parameters in the reduction of NO x as seen in the above equation are the temperature of the flame, the nitrogen and oxygen content and the resident time of the gases in the combustor. Figure 10-21 is a correlation between the adiabatic flame temperature and the emission of NO x . Reduction of any and all these parameters will reduce the amount of NO x emitted from the turbine. Dry Low NO x Combustor The gas turbine combustors have seen considerable change in their design as most new turbines have progressed to Dry Low Emission NO x Combus- tors from the wet combustors, which were injected by steam in the primary zone of the combustor. The DLE approach is to burn most (at least 75%) of the fuel at cool, fuel-lean conditions to avoid any significant production of NO x . The principal features of such a combustion system is the premixing of Lean Limit Catalytic Lean Rich FUEL/AIR RATIO Conventional Combustor Lean Pre-mixed Ultra-Lean Pre-mixed FLAME TEMPERATURE NO EMISSIONS X Figure 10-22. Effect of fuel /air ratio on flame temperature and NO x emissions. Combustors 397 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 398 ± [370±408/39] 29.10.2001 4:02PM the fuel and air before the mixture enters the combustion chamber and leanness of the mixture strength in order to lower the flame temperature and reduce NO x emission. This action brings the full load operating point down on the flame temperature curve as seen in Figure 10-22 and closer to the lean limit. Controlling CO emissions thus can be difficult and rapid engine off-loads bring the problem of avoiding flame extinction, which if it occurs cannot be safely reestablished without bringing the engine to rest and going through the restart procedure. Figure 10-23 shows a schematic comparison of a typical dry low emission NO x combustor and conventional combustors. In both cases, a swirler is used to create the required flow conditions in the combustion chamber to stabilize the flame. The DLE fuel injector is much larger because it contains the fuel/air premixing chamber and the quantity of air being mixed is large, approximately 50  ±60% of the combustion air flow. PILOT LP STAGE 1 LP STAGE 2 RICH STABLE LEAN, COOL LOW NOX LEAN, COOL LOW NOX Main Fuel Swirless DRY LOW EMISSIONS COMBUSTOR CONVENTIONAL COMBUSTOR Main Fuel Pre-mix Zone Figure 10-23. A schematic comparison of a typical dry low emission NO x combustor and a conventional combustors. 398 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 399 ± [370±408/39] 29.10.2001 4:02PM The DLE injector has two fuel circuits. The main fuel, approximately 97% of the total, is injected into the air stream immediately downstream of the swirler at the inlet to the pre-mixing chamber. The pilot fuel is injected directly into the combustion chamber with little if any premixing. With the flame temperature being much closer to the lean limit than in a conventional combustion system, some action has to be taken when the engine load is reduced to prevent flame out. If no action were taken flame-out would occur since the mixture strength would become too lean to burn. A small propor- tion of the fuel is always burned richer to provide a stable ``piloting'' zone, while the remainder is burned lean. In both cases, a swirler is used to create the required flow conditions in the combustion chamber to stabilize the flame. The LP fuel injector is much larger because it contains the fuel/air pre-mixing chamber and the quantity of air being mixed is large, approxi- mately 50  ±60% of the combustion air flow. Figure 10-24 shows a schematic of an actual dry low emission NO x combustor used by ALSTOM in their large turbines. With the flame tem- perature being much closer to the lean limit than in a conventional combus- tion system, some action has to be taken when the engine load is reduced to prevent flame out. If no action were taken flame-out would occur since the mixture strength would become too lean to burn. COMPRESSOR AIR Figure 10-24. Schematic of a dry low emission NO x combustor (Courtesy ALSTOM.) Combustors 399 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 400 ± [370±408/39] 29.10.2001 4:02PM One method is to close the compressor inlet guide vanes progressively as the load is lowered. This reduces the engine airflow and hence reduces the change in mixture strength that occurs in the combustion chamber. This method, on a single shaft engine, generally provides sufficient control to allow low emission operation to be maintained down to 50% engine load. Another method is to deliberately dump air overboard prior to or directly from the combustion section of the engine. This reduces the airflow and also increases the fuel flow required (for any given load) and hence the combus- tion fuel/air ratio can be held approximately constant at the full load value. This latter method causes the part load thermal efficiency of the engine to fall off by as much as 20%. Even with these air management systems lack of combustion stability range can be encountered particularly when load is rapidly reduced. If the combustor does not feature variable geometry, then it is necessary to turn on the fuel in stages as the engine power is increased. The expected operating range of the engine will determine the number of stages, but typically at least 2 or 3 stages are used as seen in Figure 10-25. Some units have very complex staging as the units are started or operated at off-design conditions. Gas turbines often experience problems with these DLE combustors, some of the common problems experienced are: . auto-ignition and flash-back . combustion instability These problems can result in sudden loss of power because a fault is sensed by the engine control system and the engine is shutdown. Auto-ignition is the spontaneous self-ignition of a combustible mixture. For a given fuel mixture at a particular temperature and pressure, there is a finite time before self-ignition will occur. Diesel engines (knocking) rely on it to work, but spark-ignition engines must avoid it DLE combustors have pre-mix modules on the head of the combustor to mix the fuel uniformly with air. To avoid auto-ignition, the residence time of the fuel in the premix tube must be less than the auto-ignition delay time of the fuel. If auto-ignition does occur in the pre-mix module then it is probable that the resulting damage will require repair and/or replacement of parts before the engine is run again at full load. Some operators are experiencing engine shutdowns because of auto- ignition problems. The response of the engine suppliers to rectify the situa- tion has not been encouraging, but the operators feel that the reduced reliability cannot be accepted as the ``norm.'' 400 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 401 ± [370±408/39] 29.10.2001 4:02PM If auto-ignitions occur, then the design does not have sufficient safety margin between the auto-ignition delay time for the fuel and the residence time of the fuel in the pre-mix duct. Auto-ignition delay times for fuels do exist, but a literature search will reveal that there is considerable variability for a given fuel. Reasons for auto-ignition could be classified as follows: . long fuel auto-ignition delay time assumed . variations in fuel composition reducing auto-ignition delay time . fuel residence time incorrectly calculated . auto-ignition triggered ``early'' by ingestion of combustible particles Flashback into a pre-mix duct occurs when the local flame speed is faster than the velocity of the fuel/air mixture leaving the duct. Flashback usually happens during unexpected engine transients, e.g., compressor surge. The resultant change of air velocity would almost certainly result in flashback. Unfortunately, as soon as the flame-front approaches Pilot Main Fuel Stage 1 Stages 1+2 RICH LEAN POWER Figure 10-25. Shows the staging of dry low emissions combustor as the turbine is brought to full power. Combustors 401 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 402 ± [370±408/39] 29.10.2001 4:02PM the exit of the pre-mix duct, the flame-front pressure drop will cause a reduction in the velocity of the mixture through the duct. This amplifies the effect of the original disturbance, thus prolonging the occurrence of the flashback. Advanced cooling techniques could be offered to provide some degree of protection during a flashback event caused by engine surge. Flame detection systems coupled with fast-acting fuel control valves could also be designed to minimize the impact of a flashback. The new combustors also have steam cooling being provided. High pressure burners for gas turbines use pre-mixing to enable combus- tion of lean mixtures. The stoichiometric mixture of air and fuel varies between 1.4 and 3.0 for gas turbines. The flames become unstable when the mixture exceeds a factor of 3.0 and below 1.4 the flame is too hot and NO x emissions will rise rapidly. The new combustors are therefore shortened to reduce the time the gases are in the combustor. The number of nozzles is increased to give better atomization and better mixing of the gases in the combustor. The number of nozzles in most cases increases by a factor of 5  ±10, which does lead to a more complex control system. The trend now is to an evolution towards the can-annular burners. For example, ABB GT9 turbine had one combustion chamber with one burner, the new ABB 13 E2 has 12 can-annular combustors and 72 burners. Combustion instability only used to be a problem with conventional combustors at very low engine powers. The phenomenon was called ``rumble.'' It was associated with the fuel-lean zones of a combustor, where the conditions for burning are less attractive. The complex 3D-flow structure that exists in a combustor will always have some zones that are susceptible to the oscillatory burning. In a conventional combustor, the heat release from these ``oscillating'' zones was only a significant percentage of the total combustor heat release at low power conditions. With DLE combustors, the aim is to burn most of the fuel very lean to avoid the high combustion temperature zones that produce NO x . So these lean zones that are prone to oscillatory burning are now present from idle to 100% power. Resonance can occur (usually) within the combustor. The pressure amplitude at any given resonant frequency can rapidly build up and cause failure of the combustor. The modes of oscillation can be axial, radial or circumferential, or all three at the same time. The use of dynamic pressure transducer in the combustor section, especially in the low NO x combustors ensures that each combustor can is burning evenly. This is achieved by controlling the flow in each combustor can till the spectrums obtained from each combustor can match. This technique has been used and found to be very effective and ensures combustor stability. 402 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 403 ± [370±408/39] 29.10.2001 4:02PM The calculation of the fuel residence time in the combustor or the pre- mixing tube is not easy. The mixing of the fuel and the air to produce a uniform fuel/air ratio at the exit of the mixing tube is often achieved by the interaction of flows. These flows are composed of swirl, shear layers, and vortex. CFD modeling of the mixing tube aerodynamics is required to ensure the success of the mixing process and to establish that there is a sufficient safety margin for auto-ignition. By limiting the flame temperature to a maximum of 2650  F (1454  C) single digit NO x emissions can be achieved. To operate at a maximum flame temperature of 2650  F (1454  C), which is up to 250  F (139  C) lower than the LP system previously described, requires pre-mixing 60  ±70% of the air flow with the fuel prior to admittance into the combustion chamber. With such a high amount of the available combustion air flow required for flame temperature control, insufficient air remains to be allocated solely for cool- ing the chamber wall or diluting the hot gases down to the turbine inlet temperature. Consequently some of the air available has to do double duty, being used for both cooling and dilution. In engines using high turbine inlet temperatures, 2400  ±2600  F (1316  ±1427  C), although dilution is hardly necessary there is not enough air left over to cool the chamber walls. In this case, the air used in the combustion process itself has to do double duty and be used to cool the chamber walls before entering the injectors for pre- mixing with the fuel. This double duty requirement means that film or effusion cooling cannot be used for the major portion of the chamber walls. Some units are looking into steam cooling. Walls are also coated with thermal barrier coating (TBC), which has a low thermal conductivity and hence insulates the metal. This is a ceramic material that is plasma sprayed on during combustion chamber manufacture. The temperature drop across the TBC, typically by 300  F (149  C), means the combustion gases are in contact with a surface that is operating at approximately 2000  F (1094  C), which also helps to prevent the quenching of the CO oxidation. Catalytic Combustion Catalytic combustion is a process in which a combustible compound and oxygen react on the surface of a catalyst, leading to complete oxidation of the compound. This process takes place without a flame and at much lower temperatures than those associated with conventional flame combustion. Due partly to the lower operating temperature, catalytic combustion pro- duces lower emissions of nitrogen oxides (NO x ) than conventional combus- tion. Catalytic combustion is now widely used to remove pollutants from Combustors 403 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 404 ± [370±408/39] 29.10.2001 4:02PM exhaust gases, and there is growing interest in applications in power genera- tion, particularly in gas turbine combustors. In catalytic combustion of a fuel/air mixture the fuel reacts on the surface of the catalyst by a heterogeneous mechanism. The catalyst can stabilize the combustion of ultra-lean fuel/air mixtures with adiabatic combustion temperatures below 1500  C. Thus, the gas temperature will remain below 1500  C and very little thermal NO x will be formed, as can be seen in Figure 10-21. However, the observed reduction in NO x in catalytic combustors is much greater than that expected from the lower combustion temperature. The reaction on the catalytic surface apparently produces no NO x directly, although some NO x may be produced by homogeneous reactions in the gas phase initiated by the catalyst. Features of Catalytic Combustion Surface Temperatures. At low temperatures, the oxidation reactions on the catalyst are kinetically controlled, and the catalyst activity is an important parameter. As the temperature increases, the build-up of heat on the catalyst surface due to the exothermic surface reactions produces ignition and the catalyst surface temperature jumps rapidly to the adiabatic flame temperature of the fuel/air mixture on ignition. Figure 10-26 shows a 2400°F 1316° c Substrate GAS CH /Air 4 Mixture 1500°F 816°C 600°F 316°C Figure 10-26. Schematic temperature profiles for catalyst (substrate) and bulk gas in a traditional catalytic combustor. 404 Gas Turbine Engineering Handbook [...]... alloy are shown in Figure 11 -6 G:/GTE/FINAL ( 26- 10-01)/CHAPTER 11.3D ± 4 26 ± [409±435/27] 1.11.2001 3:51PM Gas Turbine Engineering Handbook 60 6. 0 IN-7 06 A-2 86 4.0 2 KSI 100 Stress 14.0 –3 200 Kg/cm x 10 4 26 M-152 Cr-Mo-V 2.0 20 0 0 450 Temp 100,000 Hr Life 500 550 60 0 °C 800 900 1000 1100 1200 Figure 11 -6 Turbine Wheel Alloys stress rupture comparison 10.0 IN-7 06 A-2 86 2 120 STRESS KG/CM X 10 0.2%... total elongation or the elongation at fracture G:/GTE/FINAL ( 26- 10-01)/CHAPTER 11.3D ± 4 16 ± [409±435/27] 1.11.2001 3:51PM 4 16 Gas Turbine Engineering Handbook Stress 4.0 3.0 FSX-414 IN-738 GTD-222 2.0 3 Kg/cm × 10 –3 40 KSI 60 U-500 N-155 1.0 10 Temp, 100,000 Hrs Life GTD-111 Blades Nozzles 500 60 0 1000 800 700 °C 1400 1200 160 0 °F 40 42 46 48 50 44 Larson - Miller Parameter –3 PLM = T (20 + log t)... temperatures all G:/GTE/FINAL ( 26- 10-01)/CHAPTER 11.3D ± 414 ± [409±435/27] 1.11.2001 3:50PM 414 Gas Turbine Engineering Handbook (1538°C) 2800 Firing Temperature °F (°C) 260 0 (13 16 C) 2400 Steam Cooling (1204°C) 2200 2000 Advanced Air Cooling (982°C) 1800 Convential Air Cooling 160 0 Firing Temperature ( 760 °C) 1400 1200 U 500 Blade Metal Temperature (538°C) 1400 1950 RENE 77 1 960 IN 733 1970 GTD111 1980... 0.12 ±  ±  ±  0.0 06 ±  ±  ±  ±  ±  ±  ±  ±  6. 3 ±  ±  ±  BAL BAL BAL ±  ±  ±  ±  ±  0.8 ±  ±  ±  ±  ±  ±  ±  0.2 ±  ±  ±  ±  0.11 0.15 0.03 ±  ±  ±  ±  ±  ±  3 2.3 9 6 423 18.5 15 16 14 12 12 15.5 18.5 17 8.3 9.5 Fe Materials Turbine Blades U500 RENE 77 (U700) IN738 GTD111 Turbine Nozzles X40 X45 FSX414 N155 GTD-222 Combustors SS309 HAST X N- 263 HA-188 Turbine Wheels... and A2 86 alloy (Figure 11 -6) , and has higher tensile strength than either one These features, together with its favorable coefficient of expansion and good fracture toughness, make the alloy attractive for use in gas turbine applications A2 86 Alloy A2 86 is an austenitic iron base alloy that has been used for years in aircraft engine applications Its use for industrial gas turbines started about 1 965 ,... is a highly active combustion catalyst, whereas palladium metal is much less active Palladium oxide is formed under oxidizing conditions //INTEGRA/B&H/GTE/FINAL ( 26- 10-01)/CHAPTER 10.3D ± 4 06 ± [370±408/39] 29.10.2001 4:02PM 4 06 Gas Turbine Engineering Handbook Temperature Fuel + Air Inlet Stage Surface Outlet Stage Homogeneous Combustion Gas Figure 10-27 Schematic temperature profiles for catalytica... GTD111 Turbine Nozzles X40 X45 FSX414 N155 GTD-222 Combustors SS309 HAST X N- 263 HA-188 Turbine Wheels Alloy 718 Alloy 7 06 Cr-Mo-V A2 86 M152 Compressor Blades AISI 403 AISI 403 + Cb GTD-450 Cr G:/GTE/FINAL ( 26- 10-01)/CHAPTER 11.3D ± 424 ± [409±435/27] 1.11.2001 3:51PM 424 Gas Turbine Engineering Handbook nozzle and blade castings are made by using the conventional equiaxed investment casting process In... 30,000 7500 22,000 60 00 3,500 ‡ 60 ,000 42,000 45,000 35,000 20,000 ‡ 100,000 72,000 72,000 48,000 28,000 15,000 3,750 11,250 3,000 2,500 25,000 20,000 22,000 13,500 10,000 35,000 25,000 30,000 18,000 15,000 SYSTEM PEAKING Normal max load of short duration and daily starts Nat gas Nat gas Distillate Distillate 1/10 1/5 1/10 1/5 7,500 3,800 6, 000 3,000 34,000 28,000 27,200 22,400 60 ,000 40,000 53,500... 60 ,000 40,000 53,500 32,000 5,000 3,000 4,000 2,500 15,000 12,500 12,500 10,000 24,000 18,000 19,000 16, 000 TURBINE PEAKING Operating Above   50  F±100  F (28  C± 56  C) Firing Temperature Nat gas Nat gas Distillate Distillate 1/5 1/1 1/5 1/1 2,000 400 1 ,60 0 400 12,000 9,000 10,000 7,300 20,000 15,000 16, 000 12,000 2,000 400 1,700 400 12,500 10,000 11,000 8,500 18,000 15,000 15,000 12,000 419 Starts/hr... Industrial Gas Turbine with a Low Emissions Catalytic Combustion System,'' ASME 2000-GT-88 G:/GTE/FINAL ( 26- 10-01)/CHAPTER 11.3D ± 409 ± [409±435/27] 1.11.2001 3:50PM Part III Materials, Fuel Technology, and Fuel Systems G:/GTE/FINAL ( 26- 10-01)/CHAPTER 11.3D ± 410 ± [409±435/27] 1.11.2001 3:50PM G:/GTE/FINAL ( 26- 10-01)/CHAPTER 11.3D ± 411 ± [409±435/27] 1.11.2001 3:50PM 11 Materials Temperature limitations . temperature of the fuel/air mixture on ignition. Figure 10- 26 shows a 2400°F 13 16 c Substrate GAS CH /Air 4 Mixture 1500°F 8 16 C 60 0°F 3 16 C Figure 10- 26. Schematic temperature profiles for catalyst (substrate). increases 0 10 20 30 40 50 60 70 80 90 1200 1300 1400 1500 160 0 1700 1800 1900 2000 FLAME TEMPERATURE NO PPH X Figure 10-21. Correlation of adiabatic flame temperature with NO x emissions. 3 96 Gas Turbine Engineering. 405 //INTEGRA/B&H/GTE/FINAL ( 26- 10-01)/CHAPTER 10.3D ± 4 06 ± [370±408/39] 29.10.2001 4:02PM at temperatures higher than 400  F (200  C), but decomposes to the metal at temperatures between 14 36  F (780  C) and 169 0  F