Progress continuesin gas-turbine NOx control
By John C. Zink, PhD, PE, Managing Editor
The chemical reaction that produces thermal NOx during combustion occurs at temperatures greater than approximately 2,800 F, and the rate of NOx production accelerates exponentially with increasing temperature. New gas-turbine combustor NOx-control strategies are focused on reducing NOx emissions by reducing the combustion temperature to close to the 2,800 F mark.
The oldest technique for combustion temperature reduction is to provide a heat sink by injecting water or steam into the combustion zone. Depending upon the injection arrangement and the water-to-fuel ratio, levels of 25 to 42 parts per million by volume (ppmv) NOx in the exhaust are typical with steam or water injection in gas-fired turbines, and emissions for distillate-fired engines are on the order of 42 to 75 ppmv.
For many applications, emissions levels achievable by water or steam injection are still too high. Other methods have been devised to reduce the combustion flame temperature, and most of these can be used either by themselves or in combination with water or steam injection. These dry low-NOx (DLN) combustors achieve a low flame temperature by maintaining a low fuel-to-air ratio in the main combustion zone.
Three main types of DLN combustors have been developed. These are lean premixed combustion, rich/quench/lean combustion and catalytic combustion.
Lean premixed combustion
Lean premixed combustion is the most developed of the three DLN types. The concept of lean premixed combustion is to have a uniform, lean fuel-air mixture throughout the combustion zone, with no fuel-rich pockets where high flame temperatures would cause NOx formation. The challenges to making this process work are threefold: maintaining a uniform, lean mixture, yet having enough fuel to have a stable flame; achieving the uniform fuel-air mixture over a wide range of power outputs; and providing adequate residence time for oxidation of the CO to CO2 and for burnout of the unburned hydrocarbons (UHC).
The flame stability issue has been approached in several ways, including swirl stabilization and use of a continuous pilot flame.1 The latter approach is the most popular, with ignition of the premixed lean mixture typically provided by a small pilot flame which creates only a small amount of NOx.
In order to maintain careful control of the lean fuel-air mixture over a practical range of power operations, designers have had to find ways to vary the geometry of the combustion chamber. Siemens has developed variable guide vanes to control the fuel-air mixture. Others, such as General Electric (GE), have opted to make the combustion chamber into a series of small, lean premixed chambers that can be staged in some manner to achieve low-emissions operation over an extended power range. That is, all of the chambers operate at full power, but at lower power levels combustion takes place only in selected chambers. Commercial lean premix combustors can reach NOx levels as low as 15 or even 9 ppmv. The lower of these numbers is adequate to meet the most stringent regulations now in place.
Murphy`s law says that reducing NOx emissions will increase other emissions. In the case of lean premix combustors the low flame temperature necessary to preclude NOx formation also slows down the oxidation of CO to CO2 and the complete combustion of the hydrocarbon fuel. In order to meet regulations on CO and UHC emissions, these species must spend more time in the combustion zone. The straightforward way to accomplish this is to design a combustor which has a large volume downstream of the main combustion zone. A can or silo combustor can usually be enlarged to increase this downstream residence time, but fitting a low-NOx combustor to an engine with an annular combustor is a bit more challenging.
In the rich/quench/lean combustion process a stable flame, which is so fuel rich that there is not enough oxygen to form NOx , is established in the first region of the combustor. The fuel-rich mixture is carried downstream where air is added to cool the flame and dilute the fuel to a lean mixture. Then combustion is completed within the mixture that is so lean the flame temperature remains low enough to prevent significant NOx production. This method is unique in that it reduces the formation of NOx from fuel-bound nitrogen as well as thermal NOx in gaseous fuels. There has been much promising laboratory work on this concept, most notably with the Westinghouse Multi Annular Swirl Burner.
Catalytic combustion resembles lean premixed combustion. In the ideal case of catalytic combustion, however, no pilot flame is necessary. Instead, combustion is initiated by the catalyst interacting with a fuel-air mixture that is, otherwise, too lean to burn. Because even low combustion temperatures are greater than metals can withstand, early attempts at catalytic combustion used a ceramic-based catalyst placed in the combustion zone. This subjected the catalysts to thermal sintering and vaporization, and the ceramic substrate, while able to withstand the high temperatures, could not stand the thermal cycling of repeated hot and cold conditions. This led to cracking and the unacceptable risk of sending abrasive particles through the turbine after a few startup and shutdown cycles.
At least one catalyst technology company, Catalytica Inc., has addressed the problem by designing a catalyst that carefully limits the combustiontemperature in the catalyst zone, with combustion subsequently going to completion downstream. This requires very exacting control of the flow conditions and the fuel-air mixture, but does allow the use of a catalyst with a metallic substrate. Laboratory and test cell data are encouraging, indicating NOx emissions as low as 1 ppmv can be attained with this technology, with adequate catalyst life expected.2
Over the past several years the gas-turbine industry has focused extensive development efforts on DLN technologies, and many of these combustors are commercially available now. Some examples follow.
GE developed dry low-NOx combustors for both its heavy-duty gas turbines and its aeroderivative engines. The DLN-1 combustor GE uses for its heavy-duty machines uses a staged premix approach. Fuel is fed to the primary zone of the combustor for startup and acceleration to a preselected combustor temperature. Then a transition is made through lean burning in both the primary and secondary zones, to the final configuration of combustion in the secondary zone only, with the primary zone used to create the premixed fuel-air mixture. The unit can operate in a range of about 50 to 100 percent full power in the fully premixed mode. Experience with these combustors on eight "F" series machines in Korea shows regular full-power operation at 30 to 40 ppmv NOx on gas fuel. Testing on "E" class machines shows these units to be capable of levels less than 10 ppmv NOx with natural gas. A second generation combustor, the DLN-2, was developed for machines with higher firing temperatures. This is a single-stage device that operates in two distinct modes. For startup and at low loads, the combustion takes place in a diffusion flame, which is not a low-NOx mode. At about 50-percent power it transitions to a lean premixed mode for low-NOx operation. Twelve DLN-2 machines were operating at the end of 1995, and all achieved their emissions goals of less than 25 ppmv NOx and 15 ppmv CO.3 An advanced version of DLN-2 to meet levels less than 10 ppmv is under development. GE is also working on a catalytic combustor with a metallic catalyst substrate. Initial tests of a full-size catalytic combustor segment have yielded the expected results and hold the promise of NOx emissions well below the current 9 ppmv standards.
The low-NOx combustors for GE`s aeroderivative product line, referred to as DLE (dry, low emissions) combustors, utilize a series of annular rings with a total of 75 premixers. Fuel is staged to various combinations of these premixers during part-power operations, maintaining nearly constant flame temperature over the entire operating range. The first LM6000 units equipped with DLE combustors have been in service since early 1995. NOx levels from these units have averaged 17 ppmv NOx with 5 ppmv CO and 2 ppmv UHC. Early on-site testing of the first LM2500 DLE units yielded results slightly better than those for the LM6000, and a DLE system will also be available on the LM1600 series by late 1996 or early 1997.4
Siemens has a Hybrid Burner that operates in the diffusion mode from startup to approximately 45 percent of full load, then changes over to a lean premixed mode. The Siemens model V84.2 has six of these burners in each of two offboard silo combustors. All burners are ignited simultaneously and all operate in unison at all loads. A central pilot flame in each combustor maintains flame stability but is small enough that its contribution to total NOx emissions is small. Guide vanes are used to help maintain lean premix conditions while extending the power range of low-NOx operations. Burnout of CO and UHC is achieved in these combustors due to the long residence time and the ceramic lining of the flame-tube that permits higher surface temperatures and does not require introducing large quantities of wall cooling air that would quench the burnout process. Operating experience burning natural gas at New York Power Authority`s Richard M. Flynn power station has yielded NOx emissions of 9 ppmv or less from the change-over point up to 100-percent load, with CO emissions below 5 ppmv and UHC essentially zero. Siemens also has developed a modification of the Hybrid Burner that operates on fuel oil by the incorporation of a unique fuel atomization and injection technology. Testing of this burner on a Siemens Model V94.2 at the Halmstad power plant in Sweden has yielded NOx emissions below 70 ppmv with CO less than 10 ppmv when burning fuel oil.5
Westinghouse has been working on DLN systems since the 1970s, and its second generation of dry low-NOx combustors went in service in 1992. These units have can-annular combustors and, thanks to the commonality of components across the Westinghouse product line, they now have more than 20,000 hours of operational experience. Westinghouse has a new ultra low-NOx combustor with two axial stages, called the piloted ring combustor. The primary stage utilizes two counter rotating swirlers to premix the fuel and air mixture, and the secondary stage has a long premixing annulus that provides the proper mix of gas and air to the secondary combustion zone. This design has demonstrated single-digit NOx levels over a wide range of operating temperatures and is focused on the high-temperature units. Another ultra low-NOx design, called a MultiSwirl combustor, uses parallel stages of premixing and is scheduled for commercial operation this year.6
Clearly, the next few years will bring about an interesting array of commercial combustor technologies all focused on making gas-turbine units more environmentally friendly. z
1 Sood, V. M., and J. R. Shekleton, "Ongoing Development of a Low-emission Industrial Gas-turbine Combustion Chamber," ASME 79-GT-203, 1979.
2 Dalla Betta, R. A., J. C. Schlatter, S. G. Nickolas, D. K. Yee and T. Shoji, "New Catalytic Combustion Technology for Very Low Emissions Gas Turbines," ASME 94-GT-260, 1994.
3 Davis, L. B., "Dry Low-NOx Combustion Systems for GE Heavy-duty Gas Turbines," POWER-GEN(TM) Americas `95, December 1995, p. 57.
4 Westerkamp, D. F., "LM6000 Update Program," POWER-GEN(TM) Americas `95, December 1995, p. 445.
5 Medvec, M. D., and V. Rosen, "First Year Operation at New York Power Authority`s 150-MW Richard M. Flynn Combined-cycle Power Station," POWER-GEN(TM) Americas `95, December 1995, p. 69.
6 Antos, R. J., "Westinghouse Combustion Development 1996 Technology Update," POWER-GEN(TM) Americas `95, December 1995.
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For turbines burning natural gas, the important source of NOx is thermal dissociation of nitrogen and oxygen in the combustion air. When nitrogen-containing fuels such as distillate are burned, additional NOx is formed from the nitrogen in the fuel. Typical uncontrolled NOx emissions are in the range of 90 to 500 ppmv for natural gas and from 150 to 700 ppmv for distillate fuels. Because emissions rates are expressed in volume units, it would be possible to dilute the exhaust gases with air to obtain lower readings. To eliminate any confusion this would create, measurements are usually corrected to the concentration of pollutants that would be present in a stream that is 15 percent oxygen by volume.
NOx rules (as of 1996)