THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y. 10017 9-310 ES The Society shall not be responsible for statements or opinions advanced in papers or 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. Authorization to photocopy for internal or personal use is granted to libraries and other users registered with the Copyright Clearance Center (CCC) provided S3/article or $4/page is paid to CCC, 222 Rosewood Dr., Danvers, MA 01923. Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright 1998 by ASME All Rights Reserved Printed in U.S.A. DRY LOW EMISSIONS COMBUSTOR DEVELOPMENT Narendra D. Joshi, Hukam C. Mongia, Gary Leonard, Jim W. Stegmaier, and Ed. C. Vickers General Electric Aircraft Engines Cincinnati, OH 45215, USA ABSTRACT Lower Emissions have become key characteristics of most new gas turbine engines over the last several years. The `lean premixed' approach has been used in the development of the Dry Low Emissions (DLE) technology. The LM6000 and the LM2500 combustors employ a triple dome design with staging of fuel and air flows to achieve lean-premixed operation from light-off to full power. This technology permits the operator to run with reduced emissions of NOx as well as CO and UHC over a wide load setting. Emissions goals of 25 ppm have been successfully met at site rating conditions for the entire family of LM DLE products. The DLE combustor operates on the mid dome at light-off, the inner and the outer domes are brought on progressively, as the engine is loaded. The combustor utilizes a small quantity of air for dome and liner cooling as most of the combustor air is mixed with fuel in the premixers. Backside cooling enhancements permit the reduction of film cooling, which can cause quenching of CO oxidation reactions. Combustion acoustics are controlled by the use of passive devices on the exterior of the engine as well as by fuel staging within premixers and by the use of a control system which senses and alters the combustor operation to limit acoustics. The DLE technology meets the emissions and reliability needs of the industry with limited package modifications. This paper describes the DLE technology, developed to meet the needs of the industry. Critical design features including the Double Annular Counter-Rotating Swirler (DACRS) premixer, the triple annular dome design, the heat shield design and the staging sequence are discussed, in addition to the field experience gained on the LM2500 and LM6000 DLE models. INTRODUCTION Emissions reduction has provided impetus for significant advances in combustion technology over the last 25 years. Recent advances have stemmed from the needs of the power generation industry to lower emissions (Davis 1992, Leonard et. al., 1993, Joshi et. al.,1994). NOx formation mechanisms have been extensively researched ( Correa 1992). Heavy duty gas turbine engines have adopted lean premixed approach to reduce emissions. (Maughan et. al., 1995). A two step approach has been utilized for the development of Dry Low Emissions technology. A gas fuel only combustion system has been developed while maintaining flexibility for liquid fuel supply system. This paper describes the results of this step. This effort has been followed by the development of a dual fuel (gas and liquid) combustion system for the LM6000 for customer site installation in 1998. The first step includes the triple annular Dry Low Emissions combustors developed for the LM6000, LM2500 and the double annular LM1600 combustors utilizing gas fuels. These multi-dome combustors operate in the lean premixed mode from light-off to full power. Radial and circumferential fuel stagings are used to maintain flame temperatures within narrow limits permitting low NOx operation over a broad operating range in addition to meeting all other design requirements including high combustion efficiency and low levels of combustion dynamics. Leonard and Stegmaier (1993) first described the early phases of the Dry Low Emissions combustor development. Joshi et. Al (1994) have described the premixer developed for this application. DEVELOPMENT STRATEGY The Dry Low Emissions combustor development program evaluated the development needs of the Land and Marine (LM) series of engines, eg., LM1600, LM2500 and LM6000. Since the LM6000 engine model has the most challenging conditions in regard to operating pressure and the attendant impact on NOx formation, the DLE system technology development was first done for this engine model with special attention paid to the technology transition requirements for the other LM engine models. Operating characteristics of the LM family of engines are listed in Table 1. The Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Stockholm, Sweden June 2 June 5, 1998
Outer Flow Baffle Diffuser HPT nozzle Premixers Inner Flow Baffle Figure 1. Cross-Section of the LM6000 Dry Low Emissions Combustor development process was divided into a premixer development task(joshi et. al. 1994), a diffuser and combustor development tasks (Leonard and Stegmaier, 1993). The other parallel technology/ product development tasks included a reliable sophisticated control and fuel delivery systems. The combustion technology was systematically developed by using empirical/analytical design process/database, along with the effective use of test rigs including a single cup rig, two-cup and five-cup sector combustor rigs, a full annular combustor rig. The rig tests were followed by final combustion system refinements on an engine test in 1994. The LM2500 and the LM1600 combustors were developed in quick succession to the LM6000 utilizing the technology developed in the process (Patt, 1997). The LM2500+ engine utilizes the LM 2500 combustion system without any change. Table 1: Operational characteristics of the LM family of gas turbine engines (standard day, sea level, no inlet/exhaust losses). LM1600 LM2500 LM2500+ LM6000 Power MW 15 23 29 44 Thermal Efficiency 37% 37.6% 39.1% 42% Air flow KG/Sec 43 69 77 125 Pressure ratio 22 20 22 30 Nominal Firing temp C 1290 1290 1290 1290 Swirl Vane Outer fuel Elbo fuel ELBO Fuel Swirler Injection Ports Inner Swirler Air Swirl vane fuel passage Mixing Duct Plane of Fuel Injection Centerbody ELBO Fuel Figure 2: An LM6000 Middle Dome Premixer Cross - section showing the ELBO Fuel Injection Feature. 2
Turbine Nozzle Baffle Figure 3: Cross-Section of the LM2500 and the LM2500+ Dry Low Emissions Combustors LM6000 COMBUSTOR The LM6000 Combustor is shown in Figure 1. The combustor has three domes arranged radially to permit parallel staging of the three domes. The middle and the outer domes each consist of 30 premixers while the inner dome has 15. This arrangement permits the use of standard premixer sizes in the three domes. In addition, the inner dome is at about half of the radius of the outer dome and thus circumferential spacing can accomodate only 15 standard premixers. The domes of the combustor are protected from the hot combustion gases by segmented heat shields. The 75 heat shields which utilize advanced cooling technology also protect burning domes from the quenching effects of air in non-burning domes during staged operation permitting the combustor to operate lean premixed from light-off to full power. Spent cooling air from the heat shield is directed away from the flame stabilization zones permitting the combustor to operate at leaner fuel-air ratios. Turbine nozzle cooling air is utilized to cool the liners convectively on the backside. The convective air gap between the liners and the casing is controlled by specifically designed baffles. Two cooling nuggets like the ones used on aircraft engine CF6-80C2 (parent engine of the LM6000) are used at the aft end of the liners to trim the temperature profile entering the turbine nozzle. The combustor is forward mounted by 30 pins to the casing. The forward mount is needed to support the three domes of the combustor. This also results in a tighter control of the tolerance stack-up at the premixer-ferrule interface. Tight tolerance stack-up reduces leakage air flows around the premixer. The dome is also supported on the inner casing through the inner liner baffle. The aft end of the liners are supported by leaf-seals to the turbine nozzles. The 75 premixers are arranged on 15 two-cup and 15 three-cup assemblies. The two-cup assemblies do not have the innermost premixer. The removable premixers utilize the DACRS IIIG premixer. The DACRSIIIg premixer comprises of two axial counterrotating coaxial swirlers mounted with a hub separating them followed by a mixing duct. The inner swirler has a centerbody along the premixer and the mixing duct. The centerbody is conical in shape and ends in a point at the exit end of the premixer. A small amount of air is allowed to pass through the centerbody to eliminate the small recirculation zone that could form at its aft end. Fuel is injected from holes drilled into the hollow outer swirler vane. This premixer is described in detail by Joshi et al. (1994), some features of its performance are also described by Hura et. al. (1998),. The middle and the inner dome premixers are identical while the outer premixers are somewhat larger. This is done to keep the dome reference velocities in the three combustors domes within a narrow band. The middle and the outer dome premixers, mounted on the two cup premixer assemblies, have features to ameliorate combustion dynamics. A small amount of fuel (about 10% of the middle dome flow) is injected into the combustor from holes in the walls of the mixing duct. This feature for Enhancing Lean BlowOut (ELBO) is shown in Figure 2. Fuel injected from the ELBO holes increases the local fuel air ratio in the mixing region between recirculating burnt
gases and fresh incoming mixture. The ELBO feature also provides axial staging of the fuel. The increased fuel air ratio in the mixing region along with axial staging helps to decouple any fuel injection related acoustic coupling mechanisms.. A similar feature also exists on the outer dome premixers A set of 22 damper tubes is provided on the premixers, outside the engine to absorb combustion generated noise. These damper tubes, of three different lengths, are installed on the on the fuel nozzles on the engine as shown in Figures 1, 3 and 4. Each damper tube length is designed for critical operating band dynamic frequencies. The damper tubes open into the diffuser cavity and communicate with the combustor through the premixers. These dampers together with the ELBO feature and the somewhat different operating flame temperatures of the three domes allow stable operation of the combustor from light-off through the maximum rated power settings. Damper Tube Forward Pin Fuel Mount w o c \ "'qouter Diffuser Dome Inner dome Igniter Outer Flow / Baffle LM2500 DLE COMBUSTOR The LM2500 DLE combustor development followed the LM6000 DLE program with about one year lag. The operating conditions of the LM2500 are a subset of the LM6000 and thus the premixer and the combustor technology developed on the LM6000 were utilized for the LM2500 design. The cross section of the LM2500 DLE combustor shown in Figure 3 is identical to the LM6000 DLE combustor (Figure 1), except in small details. The premixers of the LM2500 and the LM2500+ are smaller and the combustion liners are 25% longer than the corresponding LM6000 DLE combustion liners. The LM2500 utilizes the dome and premixer design with minor modifications to accommodate the differences in operating cycles of the two engines. The LM2500 Combustor liner is longer than that of the LM6000 in order to provide larger residence time required for the oxidation of CO at the lower pressures of the LM2500. An advanced technology diffusion system including a fourpassage prediffuser is used for both the LM2500 and LM6000. The combustor liners for the LM2500, like the LM6000, are convectively cooled by the turbine cooling air. The later part of the liners employ two cooling nuggets for film cooling as well as for trimming the temperature profile. The LM2500 DLE combustion system is used in a recently introduced uprated LM2500+ DLE engine. LM1600 DLE Combustor The LM1600 DLE combustor development followed the LM2500 DLE combustor development very closely. This combustor was designed with two domes instead of the three domes used in the LM2500 and LM6000 engines. Because of the higher bleed capabilities of the LM1600 engine, we are able to meet all the design requirements with the two domes instead of the three domes with attendant cost, weight and simplicity benefits. The cross-section of the LM1600 DLE combustor is shown in Figure 4. Premixers Figure 4: LM1600 Dry Low Emissions Combustor Fuel Insertion and Control Systems Nozzle The fuel delivery system consists of individual fuel controls for each dome of the combustion system. The outer dome is fed by its own fuel manifold through five staging valves with each staging valve fueling six premixers. The middle dome is fueled by its own manifold without staging valves. A side branch from the middle manifold feeds the Enhanced Lean Blowout Circuit on the middle dome. The inner dome is fueled from its manifold through five staging valves with each valve fueling three premixers. The fuel flow to each of the three domes is accurately controlled to maintain flame temperatures with sufficient margin above the lean blowout limits to provide reliable operation from no-load to maximum rating. Schematic of the fuel delivery system is shown in Figure 5. 5 staging valves on outer dome fuel manifold Pressurized I Fuel Supply Each staging valve fuels 6 outer i_p remixers.430 middle dome premixers 15 middle dome ELBO circuits Each staging valve fuels 3 inner dome premixers Individual metering valves for each dome 5 staging valves on the inner dome fuel manifold Figure 5: Schematic of the LM2500, LM2500+ and the LM6000 Fuel Delivery System. 4
System Operation The LM6000/2500 combustor is lit by fueling the middle dome, and a set of six outer dome premixers in the vicinity of the igniter. The fuel to the outer dome premixers is switched off as soon as the full flame propagation around the combustor is detected. The engine accelerates on the middle dome to core idle speed. As load is increased further, the inner dome is fueled in two steps with the first step bringing fuel to nine of the inner dome premixers, followed by the remaining six mixers in the second step. On further addition of load, the outer dome is lit while the inner dome is switched off. On further addition of load the inner dome is fueled again. All domes are fueled for engine power greater than 50% of rated power on a standard day. The staging of the combustor is accompanied by airflow modulation effected through the eighth stage and Compressor Discharge Port (CDP) bleed valves. The staging scheme of the combustor is shown in Figure 6. Premixers are switched on/off during staging in groups of three (inner) or six (outer) by turning appropriate staging valves located on the manifolds on/off. Precise flame temperature control is obtained by the air modulation and fuel flow control to the three combustor domes of the LM2500 and the LM6000 engines. Combustor operation in each mode is limited by available bleed modulation that controls combustor air flow and flame temperature limits with lean blowout, liner/dome metal temperatures, and combustion dynamics limits as illustrated in Figure 6. The staging sequence for the LM1600 is simpler since it has only two domes. The outer dome is the pilot dome and is lit at ignition. When the load on the engine is increased the air flow to the combustor is increased by closing bleed flow down and then fueling the inner dome. The flame temperatures are held in a narrow operating band by air flow modulation along with precise fuel flow control. Line of max. flame temperature Middle+ Line of Max. Inner + Permissible Bleed i Middle + Outer Outer Middle + Inner Line of min. flame Middle + temperature pan er Middle Line of Zero Bleed Compressor Discharge Temperature Figure 6: Staging Scheme for the LM2500, LM2500+ and the LM6000 Combustors. Acoustics and Blowout Avoidance Logic (ABAL) The combustor operating regions are limited by lean blowout, combustion dynamics and available air flow modulation (CDP and eighth stage bleed). These limits have been correlated to flame temperatures. The combustor flame temperatures are scheduled consistent with these limits to maintain low emissions and stable operation of the gas turbine engine over its entire operating range. The combustor operating windows are affected by fuel variations and load changes and thus unstable operation of the combustor could develop from changes in operating conditions. Combustion dynamic pressures are measured continuously and monitored by the Acoustics and Blowout Avoidance Logic within the control system. If the monitored dynamic pressures exceed factory set limits, for more than a set period, the control system takes action to alter flame temperatures based on algorithms developed in factory testing of the engines, to reduce dynamic pressures to acceptable levels. If the control system is unable to effect a reduction in dynamic pressures then it commands the gas turbine to step to idle as a precautionary measure. The ABAL logic within the control system can also detect incipient lean blowouts by comparing measured and calculated fuel flows for the operating conditions based on a cycle model calibrated for each engine. The ABAL control system increases the flame temperature in the appropriate dome when an incipient lean blowout is detected. Table 2: NOx / CO (ppmv corrected to 15% 02 dry in exhaust) emissions from the family of DLE engines on a standard day (sea level, no inlet/exhaust losses). Power setting LM1600 LM2500 LM2500 + LM6000 Synchronous idle 35/300 35/300 40/200 25% power 40/50 40/50 40/50 50% power <50/25 <25/20 <25/20 <25/20 100 % power <25/20 <25/20 <25/20 <25/20 Emissions Emissions signature of the three DLE engines is shown in Table 2. The NOx emissions stay low as the combustor operate lean premixed over the entire operating envelop. CO emissions are within the 25 ppm guarantee levels at 50% power on up. UHC emissions are typically less,than 10 ppm everywhere except during start-up. Higher emissions are encountered transiently during staging when additional domes are either being lit or extinguished. The NOx and CO emissions are shown as a function of power in Figure 7 for the LM2500 DLE combustor. The LM1600, LM2500 and the LM6000 are two shaft engines. Unlike in single shaft engines, the compressor discharge temperatures and pressures increase as the load increases. Higher combustor inlet air temperatures result in a decrease in flame quenching and thus the lean blowout limit moves to leaner fuel/air ratios and lower flame temperatures. The combustor can operate with lower flame temperatures as the load is increased. This is clearly observed in the average NOx emissions signature for the LM6000, shown in Figure 8. The NOx emissions decrease slightly as the load on the engine is increased in the region where the flame temperatures are held constant with air flow control. Once the load increased past the point of air flow control, additional power is produced by simply increasing the flame temperature. In this region NOx emissions increase with increasing load. CO emissions are strongly dependent on the quenching of the CO oxidation process in the neighborhood of the combustor liners. Leakage air from the heat shields also contributes to the early quenching of the CO oxidation reactions. In order to reduce quenching of the CO oxidation reactions, in the forward regions of the primary combustion zone, cooling air and
leakage air flows in this region have been kept at minimum by the use of advanced cooling technology and materials. In the LM2500, 2500+ and the LM6000 lowest NOx emissions are obtained between 90% and 95% of rated power. The engine to engine variation in NOx emissions is dependant on performance of the engines as well as specific combustion system parameters. The 25 ppm NOx (corrected to 15% 02 dry in exhaust) emissions guarantee is met by all LM2500, 2500+ and LM6000 DLE combustors. Emissions, reported in Figure 8, for the LM6000 DLE, include the impact of additional ELBO circuits added in the outer dome for improve operability. These engines do not exhibit pressure dependence of NOx emissions confirming earlier single premixer and two premixer measurements reported by Joshi et al.(1994) and Leonard et. al. (1993). E a x 0 Z E a O U 50 40 30 20 10 50 40 30 20 10...Middle + Inner _Middle + Outer -*_Middle + Inner + Outer 10 15 20 25 _Middle + inner *_Middle + Outer & Middle + Inner + Outer 0 5 10 15 20 25 Figure 7: The LM2500 DLE Emissions Signature, (ppmv corrected to 15% 02 dry in exhaust). Field Experience The LM6000 DLE engines started field operation in december of 1994 and since then have logged over 100,000 hours at a variety of sites in US and in Europe. The availability of the fleet is >96%. The introduction of the new technology has been successful based on the market acceptance of the DLE products. 90 80 70 > 60 a50 a k 4o o 30 z 20 10 0 E a 0 U 100 80 60 40 20...Middle + Inner Middle + Outer..^ _Middle + Inner + Outer 10 20 30 40 50.-...Middle + Inner _Middle + Outer...Middle + Inner + Outer 0 10 20 30 40 50 Figure 8: The LM6000 DLE Emissions Signature (ppmv, corrected to 15% 02 dry in exhaust) The development testing of the combustor had identified several issues of durability with the heat shield which were addressed by material change before the first engines started commercial operation in the field. The flame temperatures in the middle dome in field operation were found to be significantly higher due to the need to control combustion dynamics. Several modifications have been introduced to the field to help improve the ability of the control system to reduced dynamics. Enhanced Lean blowout circuits have been added to the outer dome premixers in all DLE product lines. The two cup premixer assemblies of the LM2500 and the LM6000 combustors have the ELBO circuits in both the outer and the middle dome premixers. The fuel injection directly into the combustor significantly increased the damping of the dynamic pressures in the outer dome, permitting higher temperatures operation of the outer domes and improved stability of the entire combustion system as a whole. The addition of the ELBO circuits to the outer and middle domes of the combustors has increased NOx emissions by approximately two to six ppm. This has, however, not impacted the ability of the fleet to meet 25 ppm NOx (at 15% oxygen dry) guarantees. Additional cooling has been incorporated in the heat-shields of the combustor. Advanced cooling techniques have been utilized to judiciously cool key locations on the heat shields. Improved metal temperatures have resulted in lower distortion of the heat shield resulting in a reduction of leakage of cooling flows from the heat shield cavities. This reduction has compensated for the increased cooling flows. A material change has also been effected to increase the hot strength of the heat shield to improve life further. 6
Factory tests have been performed to alter the staging schemes of the combustor to eliminate the need for higher middle dome flame temperatures needed to avoid acoustics near staging points. An intermediate staging mode has been identified and validated by instrumented engine testing. This mode will be utilized to avoid high dynamic pressures and high middle dome flame temperatures in the staging from middle + inner to middle + outer domes fueled eliminating the zone avoidance logic as well as high middle dome flame temperatures. Heat-shield durability is also improved as a result of this staging mode. Heat shield and other operational improvements first developed on the LM6000 program have been proactively implemented on the LM2500 and LM2500+ programs. Field operating statistics of the LM6000 and the LM2500 engines are shown in Table 3. The LM1600 DLE product has recently been introduced to the field. Table 3: Field operating statistics of the LM2500 and the LM6000 DLE engines as of August 31, 1997. LM2500 LM6000 Units in field 10 12 High time engine, 12,000 20,000 hours Total fired, hours 36,000 93,000 3. Hura, H., Joshi, N., Mongia, H., and Tonouchi, J., "Dry Low Emissions Premixer CCD Modeling and Validation", Paper prepared for presentation at the 43rd ASME Turbo Expo, Stockholm, Sweden, June 1998. 4. Joshi, N., Epstein, M., Durlak, S., Marakovits, S., and Sabla, P., "Development Of A Fuel Air Premixer For Aero-Derivative Dry Low Emissions Combustors," ASME 94-GT-253, Paper presented at the IGTI Conference in The Hague, June 1994. 5. Leonard, G., and Stegmaier, J., "Development of An Aero- Derivative Gas Turbine Dry Low Emissions Combustion System," ASME 93-GT-288, Paper presented at the IGTI Conference in Cincinnati, June 1993. 6. Maughan, J. R. Elward,. K. M.. De Pietro S. M, and Bautista P. J. `Field Test Results of a Dry Low NOx Combustion System for the MS3002J Regenerative Cycle Gas Turbine' ASME 95-GT-47, 1995. 7. Patt, R. "Development & Operating Experience of DLE Combustion Systems", 12th Symposium On Industrial Applications of Gas Turbines, Oct. 15-17, 1997 (97-IAGT- 501) 1997 Summary The Dry Low Emissions technology, first introduced to the commercial operating environment in December 1994, has been successfully accepted by the power-generation and co-generation markets. The LM series of gas turbines have demonstrated with over 100,000 hours of field experience that sub-25 ppm NOx emissions can be obtained with aero-derivative engines despite their inherent disadvantages of high compressor discharge pressures and temperatures. Acknowledgements The authors would like to acknowledge 1) the LM6000, 2) the LM2500 and the LM2500+, and 3) the LM1600 Dry Low Emissions design teams for the work described in this paper. References Correa, S.M., "A Review of NOx Formation Under Gas Turbine Combustion Conditions," Combustion Science and Technology, Vol 87, pp 329-362, 1992. 2. Davis, L.B.,"Dry Low NOx combustion system for GE Heavy- Duty Gas Turbines," ASME Cogen-Turbo Conference, IGTI Vol 7, pp 255-270, Sept. 1992. 7