Gas turbine combustors drive emissions toward nilJune 24, 2008 at 7:06 am | Posted in Environmental, Gas Turbine, Low NOx, New Technology, Power Plant | 2 Comments
|Best available control technology continues to be ratcheted down to achieve ever-lower NOx emissions. Some dry low-NOx combustors can achieve 9 ppm without post-combustion control, while newer catalytic combustors are operating below 5 ppm NOx.
In the late 1980s, gas turbine original equipment manufacturers (OEMs) began to integrate dry low-NOx (DLN)—also referred to as dry low-emission (DLE)—combustion technology into their product lines to eliminate the need for steam or water injection, which had been the traditional method of NOx control. Many DLN technologies were evaluated, and lean premixed combustion emerged as the most promising approach for near-term application.
Throughout the next decade, lean premixed combustors installed in new models and retrofits to existing units dramatically slashed NOx emissions compared with conventional diffusion flame technology (Figure 1). But, like so many things in nature, the benefits of NOx reduction came at a price. Because the optimum flame temperature of a lean premixed combustor is close to the lean flammability limit, combustor performance is characterized by a CO/NOx tradeoff. At the combustor design point, both CO and NOx are below target levels; however, deviations from the design flame temperature cause emissions to increase. This tradeoff becomes particularly important during part-load turbine operation, when the combustor is required to run even leaner (Figure 2, p. 24).
1. NOx limits
The power output of a gas turbine is directly related to the firing temperature, which is directly related to flame temperature and the rate of thermal NOx formation. The formation of NOx increases dramatically when the temperature exceeds 2,900F on natural gas. Because traditional diffusion flame combustor temperatures can exceed 4,000F for brief periods, it is virtually impossible to achieve ultra-low NOx levels when a turbine is fired with a diffusion flame combustor.
Key: Low flame temps
Low emissions of NOx, as well as CO, can be achieved with thorough fuel-air mixing and control of the adiabatic flame temperature below about 3,000F. If, on the other hand, the peak combustion temperature can be limited to about 2,800F, NOx levels can be less than a few parts per million (ppm). Unfortunately, at fuel-air ratios low enough to achieve such low NOx concentrations, flames are highly unstable and are susceptible to flame-out or fluctuations, which can cause severe combustor vibrations and fatigue failure of combustion components.
In contrast to lean premixed combustion, an alternative technology known as “catalytic combustion” does not involve issues of flame stability. The fuel-air ratio entering the catalyst simply needs to be high enough to generate the desired turbine-inlet temperature at full conversion of the fuel.
One of the great difficulties with lean premixed systems is maintaining flame stability in the narrow flame temperature range between high NOx production and lean flame extinction. Aerodynamically stabilized injectors have very narrow ranges of operation, necessitating multiple burner staging (up to four stages in some systems) or piloting. Many turbine manufacturers have refined their DLN systems to sub-25 ppm NOx levels, and some Frame units have achieved single-digit, or sub-10 ppm, NOx levels.
Whereas the U.S. spends millions for increased capital and maintenance costs for the latest DLN technology and postcombustion cleanup equipment to reach ever-lower NOx limits at virtually all powerplants, the European Union has taken a different approach: The EU sets NOx limits based on plant efficiency. For example, the NOx requirement for simple-cycle units with an efficiency greater than 35% is approximately 24 ppm on natural gas and 58 ppm on liquid fuels. Gaseous fuels other than natural gas have a limit of 58 ppm.
NOx limits are less stringent for gas turbines used in combined heat and power systems demonstrating an overall efficiency greater than 75% and for combined-cycle plants demonstrating an annual average electrical efficiency greater than 55%.
Solar does DLE
The first two prototype production DLE systems manufactured by Solar Turbines Inc (San Diego, Calif.) were installed in 1992 for field evaluation. These were a Centaur 50S gas turbine, rated 4.1 MW, at El Paso Natural Gas Co’s Window Rock Station near Window Rock, Ariz., and a Mars 100S gas turbine, rated 10.5 MW, at the Pacific Gas Transmission station near Rosalia, Wash. Initially, high combustion pressure oscillations limited operation to 42 ppm NOx. The oscillations were controlled in the short term by raising pilot fuel flow, but this increased NOx and CO. The pilot fuel injector circuit is used mainly for light-off and low-load operation. During light-off and low-load operation, approximately 30% to 50% of the fuel passes through the pilot injector, providing a rich fuel-air mixture and improved combustion stability. Above 50% load, the pilot fuel is reduced to less than 5% of the total fuel flow to optimize emissions performance (Figure 3, p. 26).
2. Emissions compromises
3. Dry low-emission (DLE) fuel injectors
Design of Solar’s fuel injector premixing section was optimized by the mid-1990s. This reduced combustor pressure oscillations and, when combined with injector design improvements, enabled emissions warranties of 25 ppm NOx on natural gas and 96 ppm NOx on liquid fuels. This guarantee was extended to all of Solar’s models.
In 2001, an augmented backside-cooled combustor liner was incorporated into the OEM’s Centaur 50S and Taurus 60S models, allowing a further emissions reduction to 15 ppm NOx and 25 ppm CO from 50 to 100% load, following more than 22,000 hrs of field testing.
The most significant difference in Solar Turbines’ new lean premixed combustor liner is the increased combustor volume required for low CO and unburned hydrocarbon (UHC) emissions at the lower overall flame temperature. Because combustor length was constrained by field retrofit opportunities, the only option was to increase the diameter of the combustor liner. A second difference is the absence of large air injection ports in the combustor primary zone. All air used in the combustion process is introduced through the air swirler of the fuel injectors (Figure 4, p. 26).
4. Larger-diameter combustor liners
The new DLE combustor incorporates the capability to regulate the combustor airflow and pilot fuel flow over the entire engine-operating map. The requirement to control these two parameters has added a degree of complexity to the control system that is not required in a conventional gas turbine. During start-up and low-load operation, the pilot flow rate has been optimized to achieve maximum flame stability to handle load transients. Below 50% load the combustor airflow is managed in the same way as in a conventional engine. Above 50% rated load, Solar’s DLE engine enters the “low emissions mode,” modulating either the bleed valves or inlet guide vanes to keep the combustion primary zone temperature within a specified range. Solar’s DLE gas turbine controls use the power turbine inlet temperature as an indirect measurement of the primary zone temperature to control the bleed valve or inlet guide vane position as a function of turbine load (Figure 5).
5. SoLoNOx system
Local ambient conditions also affect emissions. NOx is observed to increase with increasing ambient temperature, decreasing relative humidity, and increasing barometric pressure, whereas CO emissions trend in the opposite way. However, field experience has shown that relative humidity and barometric pressure have only a small effect on the combustor primary zone temperature and, therefore, a small influence on emissions from DLE gas turbines. In practice, emissions from DLE packages vary less than 5 ppm from 0F to 120F. Many DLE packages are configured to increase pilot fuel flow at temperatures below 68F to augment flame stability. Below 0F, NOx and CO emissions increase.
Future emissions reductions mean developing new technologies to take up where DLE leaves off. One area being explored by Solar Turbines is a surface-stabilized fuel injector for use with lean, premixed combustors. Developed by Alzeta Corp (Santa Clara, Calif.), surface-stabilized fuel injectors provide extended turndown and ultra-low NOx emission performance.
The new injectors form interacting radiant and blue-flame zones immediately above a selectively perforated porous metal surface. This allows stable operation at low reaction temperatures. A proof-of-concept injector in a full-pressure test rig at the U.S. DOE’s National Energy Technology Laboratory in Morgantown, W.Va., achieved sub-3 ppm NOx emissions with concurrent single-digit CO emissions. Operating conditions ranged between inlet pressures of 26.5 psia and 180 psia, inlet temperatures between 186F and 850F, and calculated adiabatic flame temperatures between 2,670F and 2,900F. Testing with prototype fuel injectors in test rigs at Solar Turbines yielded similar results. In May 2001, the OEM’s 1-MW Saturn gas turbine was operated to 95% load with a surface-stabilized injector. Currently, Alzeta and Solar Turbines are completing a multi-burner annular combustor test, which will then proceed to an engine test later this year. In addition, the companies have initiated a project to apply Alzeta’s nanoSTAR technology to the Titan 130 gas turbine.
Mitsubishi hits 15
Mitsubishi Power Systems Inc’s (Lake Mary, Fla.) first DLN combustor was introduced in 1984 on a 50-Hz M701D gas turbine. By 1992, 25-ppm NOx levels were being commercially achieved in the OEM’s F-class turbine operating at full load, with a turbine inlet temperature of 2,460F.
Mitsubishi’s F-class fleet experience with DLN combustors now exceeds 1.5 million hours of meeting the 25 ppm NOx limit. The OEM reports that recent tests have achieved 15 ppm NOx.
The pilot nozzle and air bypass valve in this combustor make it possible to sustain a stable flame and accurately control the fuel-air ratio. The main nozzles are composed of air swirlers together with a fuel nozzle. The swirlers introduce highly turbulent air, causing complete mixing of gas and air, while the pilot nozzle allows stable combustion of the premixed gas by diffusion firing. A cone attached at the tip makes flame holding stable by recirculating combustion gases. The air bypass valve, infusing air directly into the transition piece, is installed at the transition piece, enabling an almost constant fuel-air ratio at various operating conditions, such as ignition, acceleration, and partial-load operation (Figure 6).
6. Contrasting cooling schemes
In 1996, a steam-cooled DLN combustor was developed for Mitsubishi’s G-class gas turbines. The M501G and M701G models feature a turbine inlet temperature of 2,730F and are designed to achieve a combined-cycle plant efficiency of greater than 58%. Prototype testing has shown the G series to be capable of 25 ppm NOx.
To increase the turbine inlet temperature to 2,730F, almost all of the compressed air is needed for combustion, and steam is introduced to perform the cooling. (In conventional gas turbines, much of the compressed air is diverted to cool hot-gas-path components.) Overall, Mitsubishi’s G-class combustor has nearly the same configuration as its F class, except at the combustor basket, where the transition piece integrates the basket part and the liner. The cooling steam, supplied from a required bottoming steam cycle, cools that structure over the entire surface area of the combustor.
The M701G1 combustion chamber and first- and second-stage blades and vanes are similar to those used in the prototype 501G design, but the size has been increased to give a higher output. The 701G1 is a single-shaft machine with the generator at the compressor end of the turbine to allow the turbine exhaust to be directed into the heat-recovery steam generator. The turbine combustion chamber uses steam cooling to maintain temperature in the range 2,730F to 2,910F to keep NOx emissions at 25 ppm or less. Mitsubishi’s G-class fleet experience exceeds 75,000 hours on seven units.
GE Power Systems’ (Atlanta, Ga.) DLE combustors have been developed for GE’s aeroderivative gas turbines—the gas-fueled 44-MW LM6000, the 23-MW LM2500, and the double annular 15-MW LM1600. Twenty-five ppm has been successfully demonstrated for the entire family of LM DLE products.
Factory testing of components and engine assembly on the first 41-MW DLE LM6000 gas turbine was completed in 1994 and demonstrated less than 15 ppm NOx, 10 ppm CO, and 2 ppm UHC at a firing temperature of 2,350F. In 1995, the 43-MW Ghent power station in Belgium became the first commercial operator to use the LM6000 fitted with the DLE combustor system, demonstrating 16 ppm NOx, 6 ppm CO, and 1 ppm UHC. The high-time LM6000 engine has accumulated more than 34,000 hours. As of the end of 2002, GE had more than 200 DLE LM engines in operation with almost 3.5 million operating hours.
The LM6000 DLE combustor employs a triple-dome design with staging of fuel and air flow to achieve lean premixed operation from light-off to full power—using the mid-dome at light-off, with the inner and outer domes brought on progressively as the engine is loaded. The middle and outer domes each consist of 30 premixers; the inner dome has 15. The domes of the combustor are protected from the hot combustion gases by segmented heat shields.
The middle and the inner dome premixers are identical, but the outer premixers are somewhat larger to manage the dome reference velocities in the three combustor domes within a narrow band. 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. Combustion acoustics are controlled by the use of passive devices on the exterior of the engine, fuel staging within premixers, and by the use of a control system that senses and alters the combustor operation to limit combustion-generated noise. 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. Another feature, Enhancing Lean Blow Out holes, increase the local fuel-air ratio in the mixing region between recirculating burned gases and fresh, incoming mixture and also provide 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 (Figure 7).
7. Individual fuel controls
The NOx emissions stay low as the combustor operates lean premixed over the entire operating envelope. CO emissions are less than 25 ppm 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 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. NOx emissions decrease slightly as the load on the engine is increased in the region where the flame temperatures are held constant with airflow control. Once the load increases past the point of airflow control, additional power is produced by simply increasing the flame temperature.
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 airflows in this region have been kept at minimum by the use of advanced cooling technology and materials.
The only utility-scale gas turbine currently marketed with single-digit NOx guarantees is GE Power Systems’ Frame 7FA outfitted with DLN-2.6. The first of these units was placed in service in March 1996 by Public Service Co of Colorado. The newer model reduced NOx levels from the OEM’s previous 25-ppm guarantee on the DLN-2 combustor down to 9 ppm. This required 6% more air to pass through the premixers in the combustor due to reductions in cap- and liner-cooling airflows and increased cooling effectiveness.
However, without changes in the operation of the DLN-2 system, certain penalties would have been incurred for achieving 9 ppm baseload performance. The turndown of a DLN-2 combustor tuned to 9/9 operation (9 ppm NOx/9 ppm CO) was estimated to be about 70% load, compared with 40% load for the 25/15 system. A new combustor configuration was developed based on the DLN-2 burner because of its excellent flame-stabilization characteristics and operating experience. The key feature of the newer configuration is the addition of a sixth burner located in the center of the five existing DLN-2 burners. The presence of the center nozzle enables the DLN-2.6 to extend its 9/9 turndown well beyond the five-nozzle DLN-2.
By fueling the center nozzle separately from the outer nozzles, the fuel-air ratio can be modulated relative to the outer nozzles leading to approximately 200F of turndown from baseload with 9 ppm NOx. Burners were turned and a fourth premixed manifold for injecting quaternary fuel was added to reduce combustion dynamics. The first three premixed manifolds, designated PM1, PM2, and PM3, are configured such that any number of burners can be operated at any time. The PM1 manifold fuels the center nozzle, the PM2 manifold fuels the two outer nozzles located at the cross-fire tubes, and the PM3 manifold fuels the remaining three outer nozzles. The five outer nozzles are identical to those used for the DLN-2, while the center nozzle is similar but with simplified geometry to fit within the available space. Emissions performance of the DLN-2.6 is 9 ppm NOx and CO over a 50% load range.
GE recently announced a series of power and efficiency uprates, as well as NOx reductions, to its Frame units. This includes the PG6111FA (the uprated version of the Frame 6FA), which generates 75.9 MW with 15 ppm NOx at a firing temperature of 2,420F. The Frame 6FA machines are manufactured in GE’s facility in Belfort, France, and are scheduled to start shipping in the second quarter of 2003.
GE also introduced the Frame 9FB, a 50-Hz version of the 7FB, which was introduced in 1999. The first 7FB was installed at a Reliant Energy project in Hunterstown, Pa., where it is being tested and is scheduled to enter commercial service in 2003. The Frame 9FB benefits from what GE calls an “advanced technology flowback” from the company’s planned steam-cooled H-class gas turbine. The 9FB will be equipped with GE’s advanced DLN-2+ combustion system, and NOx emissions will be less than 25 ppm. The Frame 9FB will be available for shipment in early 2004.
Catalytica cleans up
A technology called catalytic combustion promises to be an alternative to DLN technologies, with even lower NOx levels—below 5 ppm. Catalytica Energy Systems Inc (Mountain View, Calif.) took an early lead in this field in 2000, when it installed its Xonon system on a 1.4-MW Kawasaki gas turbine at Silicon Valley Power’s (Santa Clara, Calif.) Gianera station. Catalytica recognized that it’s tough to break into the gas-turbine business with a potentially revolutionary new product, especially when the product lies directly in the gas path. “Show me an operating turbine and then we’ll talk,” was the message.
So the company went it alone by purchasing a used M1A Kawasaki engine, refurbishing it, and retrofitting its new combustion system designed from scratch. Catalytica did this before Kawasaki Motors Corp USA (Grand Rapids, Mich.) decided to come on board, license the technology, and develop what is now the commercial product. After 14,000 grid-connected hours and a yearlong 8,128-hr testing program, the Silicon Valley Power plant performed better than design. According to Catalytica, the results were:
Kawasaki Heavy Industries Limited (Hyogo, Japan) produces the 1.4-MW M1A-13A gas turbine (referred to as the M1A-13X when Xonon is installed). The model has an unimpressive thermal efficiency of just 25.5%—equating to a 13,400 Btu/kWh heat rate. But it is convenient for a Xonon retrofit because it has a readily accessible, single, external can-type combustor. The M1A-13A has a pressure ratio of approximately 9.3 to 1, which is comparable to that found in many other industrial gas turbines in the 1- to 6-MW range. Also, the model’s low mass flowrate (18 lb/sec) allows the catalytic combustor to be maintained as a small system typical of large rig test units, thus reducing the cost of the total system. Finally, its low firing temperature of 1,839F makes the engine ideal for catalytic combustion, because the modest temperatures in the gas-phase burnout zone allow the use of existing liner-cooling technologies.
In the Xonon combustion system, compressor discharge air enters an annular plenum prior to entering the preburner. The preburner is a small DLN-type combustor that preheats the combustor air up to the catalyst operating temperature. Fuel is then injected into the warm air and thoroughly mixed before entering the catalyst module. In the catalyst module, some of the fuel-air mixture is combusted through a flameless catalytic process. The combustion process continues in the burnout zone until all of the remaining uncombusted fuel is reacted (Figure 8).
8. Catalytic combustor
According to Catalytica, its technology is superior if your target is 5 ppm NOx or less, meaning the life-cycle cost of a Xonon-equipped gas turbine is less than that for the same gas turbine equipped with a selective catalytic reduction system. The manufacturer points out that spent Xonon catalysts, which are expected to be depleted annually, are nontoxic and can be recycled to recover precious metals.
The OEM successfully completed an Equipment and Process Precertification Program administered by the California Air Resources Board (CARB), which verified the air quality–related performance claims of its Xonon Cool Combustion system last August. The Equipment and Process Precertification Program is a voluntary statewide program administered by CARB through which manufacturers of products that have a beneficial impact on air quality can have their product’s performance claims independently verified. The objective of the program is to simplify the permitting process by providing advanced independent evaluation of a technology, which can then be used as the basis for BACT (best available control technology) determination.
Last year, Xonon shipped catalysts for what was scheduled to be the first three commercial Xonon-equipped Kawasaki gas turbines for a government healthcare facility in Jamaica Plain, Mass. However, the plant was being developed by an Enron subsidiary, and the project was shelved after the turbines were shipped. After the three turbines had sat idle for over a year, Kawasaki repurchased them at an auction of Enron assets.
The first commercial application of Catalytica’s Xonon technology now is the Sonoma Developmental Center in Eldridge, Calif., where an existing Kawasaki M1A-13D gas turbine was retrofitted with a Kawasaki-designed Xonon-based combustor (Figure 9). The 12-yr-old unit provides supplemental heat and power for a staff of 2,200 and 850 residents across 120 buildings.
9. First commercial catalytic combustor installation
The six-week retrofit involved removing the current M1A-13D engine operating with a DLE combustor configuration, and replacing it with an advanced M1A-13X engine incorporating Xonon Cool Combustion. The reconfigured unit returned to service in November 2002. Post-startup testing confirms the 1.3-MW plant is operating at less than 3 ppm NOx—an overall 90% reduction. Additionally, the Xonon Cool Combustion system makes it possible for Kawasaki to guarantee CO levels of less than 10 ppm and VOC levels of less than 2 ppm.
Catalytica announced in December the purchase of a Kawasaki 1.4-MW GPB15X equipped with Xonon technology. The unit is part of a heating and cooling plant for the Reader’s Digest 650,000-square-foot global headquarters in Pleasantville, N.Y., servicing approximately 1,000 employees. The plant is expected to be in service by this July.
Catalytica reports that it also has demonstrated sub-5 ppm NOx levels on gas turbines manufactured by Rolls-Royce Allison (Indianapolis, Ind.). GE Power Systems plans to begin shipment of a Xonon-equipped 11-MW GE10 gas turbine in 2004. String testing is scheduled in Florence, Italy, this year with orders beginning this fall. Emissions targets for the GE10 are 3 ppm NOx, 100 ppm CO, and 10 ppm VOC.
By Dr. Robert Peltier PE, Sr. Editor