Gas Turbine Engineering Handbook 2 Episode 9 potx

//INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 385 ± [370±408/39] 29.10.2001 4:02PM Advantages of the TBCs are the reduction of metal temperatures of cooled components, by about 8  ±14  F(4  ±9  C) per mil (25.4 microns) of coating, the microstructure, and a coated liner. The primary benefit of the TBCs is to provide an insulating layer that reduces the underlying base material tem- perature and mitigates the effects of hot streaking or uneven gas temperature distributions. These coatings are now standard on most high-performance gas turbines and have demonstrated excellent performance in production machines. The third major change was the introduction of steam cooling of the liners. This concept, especially in combined cycle application, has great potential. Transition Pieces. Although technically not part of the combustor they are an important part of the combustion system. Less complicated than the liners, the transition pieces have probably been more challenging from a materials/processes standpoint. Therefore, new materials have tended to be first introduced on the transition piece. From a design standpoint, significant improvements have been made on advanced models through the use of heavier walls, single-piece aft ends, ribs, floating seal arrangements, and selective cooling. These design changes have been matched by material improvements. Early transition pieces were made from AISI 309 stainless steel. In the early 1960s, nickel base alloys Hastelloy-X and RA-333 were used in the more limiting parts. These alloys became standard for transition pieces by 1970. In the early 1980s, a new material, Nimonic 263, was introduced into service for transition pieces. This material is a precipitation-strengthened, nickel-base alloy with higher strength capability than Hastelloy-X. Since the early 1980s, Thermal Barrier Coatings (TBCs) have been applied to the transition pieces of the higher firing temperature gas turbine models and to uprated machines. Field experience over thousands of hours of service has demonstrated good durability for this coating on transition pieces. Improvement has also been made to increase the wear resistance of some transition pieces in the aft end or picture frame area. Cobalt-base hard coatings applied by thermal spray have been tested in field machines and the best spray has been shown to improve the wear life of sealing compo- nents by more than four times. Reliability of Combustors. The heat from combustion, pressure fluc- tuation, and vibration in the compressor may cause cracks in the liner and nozzle. Also, there are corrosion and distortion problems. The edges of the holes in the liner are great concern because the holes act as stress concen- trators for any mechanical vibrations and, on rapid temperature fluctua- tions, high-temperature gradients are formed in the region of the hole edge, giving rise to a corresponding thermal fatigue. Combustors 385 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 386 ± [370±408/39] 29.10.2001 4:02PM It is necessary to modify the edge of the hole in various ways to reduce these stress concentrations. Some methods of modification are priming, plunging, and standard radiusing and polishing methods. In the Dry Low NO x Combustors, especially in the lean pre-mix chambers, pressure fluctua- tions can set up very high vibrations, which lead to major failures. Typical Combustor Arrangements All gas turbine combustors provide the same function; however, there are different methods to arrange combustors on the gas turbine. Designs fall into three major categories: 1. Tubular (single can) 2. Tubo-annular 3. Annular Figure 10-12a. Top view of a large side combustor with special tiles. (Courtesy Brown Boveri Turbomachinery, Inc.) FPO 386 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 387 ± [370±408/39] 29.10.2001 4:02PM Figure 10-12b. Special tiles for a large side combustor. (Courtesy Brown Boveri Turbomachinery, Inc.) FPO Figure 10-13. Single can combustor. (Courtesy Brown Boveri Turbomachinery, Inc.) FPO Combustors 387 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 388 ± [370±408/39] 29.10.2001 4:02PM Tubular or single-can designs are preferred by many European industrial gas turbine designers. These large single combustors offer the advantage of simplicity of design and long life because of low heat-release rates. These com- bustors are sometimes very large. They can range in size from small units of about 6 inches (15.24 cm) in diameter and a 1 ft (0.3 m) high to combustors, which are over 10feet(3 m)in diameter and 30  ±40feet(3  ±12 m) high. Theselarge combustors use special tiles as liners. Any liner damage can be easily corrected by replacing the damaged tiles. Figure 10-12 shows such a liner. The tubular combustors can be designed as ``straight-through'' or ``reverse-flow designs.'' Most large single-can combustors are of the reverse-flow design. In this design, the air enters the turbine through the annulus between the combustor can and the hot gas pipe asseen in Figure 10-13.The air then passes between the liner and the combustor can and enters the combustion region at various points of entry. About 10% of the air enters the combustion zone, about 30  ±40% of the air is used for cooling purposes, the rest is used in the dilution zone. Reverse-flow designs are much shorter than the straight-through flow designs. The tubular, or single-can, for large units usually has more than one nozzle. In many cases a ring of nozzles is placed in the primary zone area. The radial and circumferential distribution of the temperature to the turbine nozzles is not as even as in tubo-annular combustors. Figure 10-14. Tubo-annular or can-annular combustor for a heavy-duty gas turbine. (Courtesy of General Electric Company.) 388 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 389 ± [370±408/39] 29.10.2001 4:02PM Tubo-annular combustors are the most common type of combustors used in gas turbines. The industrial gas turbines designed by U.S. companies use the tubo-annular or can-annular type seen in Figure 10-14. The advantage to these types of combustors are the ease of maintenance. They also have a better temperature distribution than the side single-can combustor and can be of the straight-through or reverse-flow design. As with the single-can combustor, most of these combustors are of the reverse-flow design in indus- trial turbines. In most aircraft engines the tubo-annular combustors are of the straight- through flow type seen in Figure 10-15. The straight-through flow type Figure 10-15. ``Straight-through'' flow-type can-annular combustors. ( # Rolls- Royce Limited.) Combustors 389 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 390 ± [370±408/39] 29.10.2001 4:02PM tubo-annular combustor requires a much smaller frontal area than the reverse-flow type tubo-annular combustor. The tubo-annular combustor also requires more cooling air flow than a single or annular combustor because the surface area of the tubo-annular combustor is much greater. The amount of cooling air is not much of a problem in turbines using high- Btu gas, but for low-Btu gas turbines, the amount of air required in the primary zone can be as high as 35% of the total air needed, thus reducing the amount of air available for cooling purposes. Figure 10-16. Aircraft-type annular combustion chamber. ( # Rolls-Royce Limited.) 390 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 391 ± [370±408/39] 29.10.2001 4:02PM Higher temperatures also require more cooling and, as temperatures increase, the single can or annular combustor design becomes more attrac- tive. The tubo-annular combustor has a more even combustion because each can has its own nozzle and a smaller combustion zone, resulting in a much more even flow. Development of a tubo-annular combustor is usually less expensive, since only one can needs to be tested instead of an entire unit as in an annular or single-can combustor. Therefore, the fuel and air requirements can be as low as 8  ±10% of the total requirements. Annular combustors are used mainly in aircraft-type gas turbines where frontal area is important. This type of combustor is usually a straight- through flow type. The combustor outside radius is the same as the com- pressor casing, thus producing the streamline design seen in Figure 10-16. The annular combustor mentioned earlier requires less cooling air than the tubo-annular combustor, and so it is growing in importance for high- temperature application. On the other hand, the annular combustor is much harder to get to for maintenance and tends to produce a less favorable Figure 10-17. Industrial-type can-annular combustor. (Courtesy of Solar Turbines Incorporated.) FPO Combustors 391 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 392 ± [370±408/39] 29.10.2001 4:02PM radial and circumferential profile as compared to the tubo-annular combus- tors. The annular combustors are also used in some newer industrial gas turbine applications as seen in Figure 10-17. The higher temperatures and low-Btu gases will foster more use of annular-type combustors in the future. Air Pollution Problems Smoke In general, it has been found that much visible smoke is formed in small, local fuel-rich regions. The general approach to eliminating smoke is to develop leaner primary zones with an equivalence ratio between 0.9 and 1.5. Another supplementary way to eliminate smoke is to supply relatively small quantities of air to those exact, local, over-rich zones. Unburnt Hydrocarbons and Carbon Monoxide Unburnt hydrocarbon (UHC) and carbon monoxide (CO) are only produced in incomplete combustion typical of idle conditions. It appears probable that idling efficiency can be improved by detailed design to pro- vide better atomization and higher local temperatures. CO 2 production is a direct function of the fuel burnt (3.14 times the fuel burnt) it is not possible to control the production of CO 2 in fossil fuel combustion, the best control is the increasing of the turbine efficiency, thus requiring less fuel to be burnt for the same power produced. Oxides of Nitrogen The main oxides of nitrogen produced in combustion are NO, with the remaining 10% as NO 2 . These products are of great concern because of their poisonous character and abundance, especially at full-load conditions. The formation mechanism of NO can be explained as follows: 1. Fixation of atmospheric oxygen and nitrogen at high-flame tempera- ture. 2. Attack of carbon or hydrocarbon radicals of fuel on nitrogen molecules, resulting in NO formation. 3. Oxidation of the chemically bound nitrogen in fuel. 392 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 393 ± [370±408/39] 29.10.2001 4:02PM In 1977, the Environmental Protection Agency (EPA) in the U.S. issued proposed rules that limited the emissions of new, modified and reconstructed gas turbines to: . 75 vppm NO x at 15% oxygen (dry basis) . 150 vppm SO x at 15% oxygen (dry basis), controlled by limiting fuel sulfur content to less than 0.8% wt. These standards applied to simple and regenerative cycle gas turbines, and to the gas turbine portion of combined cycle steam/electric generating sys- tems. The 15% oxygen level was specified to prevent the NO x ppm level being achieved by dilution of the exhaust with air. Figure 10-18 shows how in the past 30 years the reduction of NO x by first the use of steam (Wet Combustors) injection in the combustors, and then in the 1990s, the Dry Low NO x Combustors have greatly reduced the NO x output. New units under development have goals, which would reduce NO x levels below 9 ppm. In 1977 it was recognized that there were a number of ways to control oxides of nitrogen:- 1. Use of a rich primary zone in which little NO formed, followed by rapid dilution in the secondary zone. 2. Use of a very lean primary zone to minimize peak flame temperature by dilution. 3. Use of water or steam admitted with the fuel for cooling the small zone downstream from the fuel nozzle. 4. Use of inert exhaust gas recirculated into the reaction zone. 5. Catalytic exhaust cleanup. 0 20 40 60 80 100 120 140 160 180 200 1970 1975 1980 1985 1990 1995 2000 2005 2010 Years Dry Low NOx Combustor Catalytic Combustor Water injection NOx Emissions (ppm) Figure 10-18. Control of gas turbine NO x emissions over the years. Combustors 393 //INTEGRA/B&H/GTE/FINAL (26-10-01)/CHAPTER 10.3D ± 394 ± [370±408/39] 29.10.2001 4:02PM ``Wet'' control became the preferred method in the 1980s and most of 1990s since ``dry'' controls and catalytic cleanup were both at very early stages of development. The catalytic converters were used in the 1980s and are still being widely used; however the cost of rejuvenating the catalyst is very high. There has been a gradual tightening of the NO x limits over the years from 75 ppm down to 25 ppm, and now the new turbine goals are 9 ppm. Advances in combustion technology now make it possible to control the levels of NO x production at source, removing the need for ``wet'' controls. This of course opened up the market for the gas turbine to operate in areas with limited supplies of suitable quality water, e.g., deserts or marine platforms. Although water injection is still used, ``dry'' control combustion technology has become the preferred method for the major players in the industrial power generation market. DLN (Dry Low NO x ) was the first acronym to be coined, but with the requirement to control NO x without increasing carbon monoxide and unburned hydrocarbons this has now become DLE (Dry Low Emissions). The majority of the NO x produced in the combustion chamber is called ``thermal NO x .'' It is produced by a series of chemical reactions between the nitrogen (N 2 ) and the oxygen (O 2 ) in the air that occur at the elevated temperatures and pressures in gas turbine combustors. The reaction rates are highly temperature dependent, and the NO x production rate becomes sig- nificant above flame temperatures of about 3300  F (1815  C). Figure 10-19 shows schematically, flame temperatures and therefore NO x production 2500 K 4040°F 1697°F 1200 K NOx Production Zone Figure 10-19. A typical combustor showing the NO x production zone. 394 Gas Turbine Engineering Handbook [...]... 72, 000 72, 000 48,000 28 ,000 15,000 3,750 11 ,25 0 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 ,20 0 22 ,400 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... Gas Turbines Creep and Rupture The melting point of different metals varies considerably, and their strengths at various temperatures are different At low temperatures all G:/GTE/FINAL (26 -10-01)/CHAPTER 11.3D ± 414 ± [4 09 435 /27 ] 1.11 .20 01 3:50PM 414 Gas Turbine Engineering Handbook (1538°C) 28 00 Firing Temperature °F (°C) 26 00 (1316°C) 24 00 Steam Cooling ( 120 4°C) 22 00 20 00 Advanced Air Cooling (9 82 C)... MS9001E gas turbine The MS9001E combustor operates with a full load firing temperature of 20 20  F (1105  C) and a combustor exit temperature of about 21 70  F (1 190  C) The key components of the test stand at the GE Power Generation Engineering Laboratories in Schenectady, New York, are shown in Figure 10 -28 //INTEGRA/B&H/GTE/FINAL (26 -10-01)/CHAPTER 10.3D ± 407 ± [370±408/ 39] 29 .10 .20 01 4:02PM... technology The required 411 G:/GTE/FINAL (26 -10-01)/CHAPTER 11.3D ± 4 12 ± [4 09 435 /27 ] 1.11 .20 01 3:50PM 4 12 Gas Turbine Engineering Handbook Figure 11-1a Specific air versus pressure ratio and turbine inlet temperatures Figure 11-1b Specific fuel consumption versus pressure ratio and turbine inlet temperature G:/GTE/FINAL (26 -10-01)/CHAPTER 11.3D ± 413 ± [4 09 435 /27 ] 1.11 .20 01 3:50PM Materials 413 Figure 11-1c... elongation at fracture G:/GTE/FINAL (26 -10-01)/CHAPTER 11.3D ± 416 ± [4 09 435 /27 ] 1.11 .20 01 3:51PM 416 Gas Turbine Engineering Handbook Stress 4.0 3.0 FSX-414 IN-738 GTD -22 2 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 500 600 1000 800 700 °C 1400 120 0 1600 °F 40 42 46 48 50 44 Larson - Miller Parameter –3 PLM = T (20 + log t) × 10 Figure 11-4 Larson-Miller... above 27 32  F (1500  C) Due to the onset of the thermal mechanism the formation of NOx in the combustion of fuel/air mixtures increases 90 80 NOXPPH 70 60 50 40 30 20 10 0 120 0 1300 1400 1500 1600 1700 FLAME TEMPERATURE 1800 190 0 20 00 Figure 10 -21 Correlation of adiabatic flame temperature with NOx emissions //INTEGRA/B&H/GTE/FINAL (26 -10-01)/CHAPTER 10.3D ± 397 ± [370±408/ 39] 29 .10 .20 01 4:02PM Combustors... 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 1,600 400 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 4 19 Starts/hr 1/1000 1/10 1/1000 1/10 1/1000 1/10 Materials BASE LOAD Nat gas Nat gas Distillate... ASME 97 -GT-57 Yee, D.K., Lundberg, K., and Weakley, C.K., ``Field Demonstration of a 1.5 MW Industrial Gas Turbine with a Low Emissions Catalytic Combustion System,'' ASME 20 00-GT-88 G:/GTE/FINAL (26 -10-01)/CHAPTER 11.3D ± 4 09 ± [4 09 435 /27 ] 1.11 .20 01 3:50PM Part III Materials, Fuel Technology, and Fuel Systems G:/GTE/FINAL (26 -10-01)/CHAPTER 11.3D ± 410 ± [4 09 435 /27 ] 1.11 .20 01 3:50PM G:/GTE/FINAL (26 -10-01)/CHAPTER... the premixing of Lean Limit Lean Pre-mixed Ultra-Lean Pre-mixed Catalytic Lean Rich FUEL/AIR RATIO Figure 10 -22 Effect of fuel /air ratio on flame temperature and NOx emissions //INTEGRA/B&H/GTE/FINAL (26 -10-01)/CHAPTER 10.3D ± 398 ± [370±408/ 39] 29 .10 .20 01 4:02PM 398 Gas Turbine Engineering Handbook the fuel and air before the mixture enters the combustion chamber and leanness of the mixture strength... 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 EMISSIONS NOX TEMPERATURE LEAN FUEL/AIR RATIO RICH Figure 10 -20 The effect of flame temperature on emissions //INTEGRA/B&H/GTE/FINAL (26 -10-01)/CHAPTER 10.3D ± 396 ± [370±408/ 39] 29 .10 .20 01 4:02PM 396 Gas Turbine Engineering Handbook The challenge in these designs . Use of inert exhaust gas recirculated into the reaction zone. 5. Catalytic exhaust cleanup. 0 20 40 60 80 100 120 140 160 180 20 0 197 0 197 5 198 0 198 5 199 0 199 5 20 00 20 05 20 10 Years Dry Low NOx Combustor Catalytic Combustor Water. 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 .20 01 4:02PM The DLE injector has two fuel. chamber. ( # Rolls-Royce Limited.) 390 Gas Turbine Engineering Handbook //INTEGRA/B&H/GTE/FINAL (26 -10-01)/CHAPTER 10.3D ± 391 ± [370±408/ 39] 29 .10 .20 01 4:02PM Higher temperatures also require