Low-NOx Emissions Combustion Research

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Low-NOx Emissions Combustion Research
Shigeru HAYASHI, Aeropropulsion Research Center, E-mail: hayashi@nal.go.jp Keywords: Ultra-low NOx emissions, SST transport engines, Stationary Gas Turbines 1. Introduction We are conducting research on ultra low-NOx combustor technologies for propulsion engines of the next generation supersonic transport and liquid-fueled small stationary gas turbines. In addition, the application of a new low NOx combustion concept, flameless combustion, to aero and industrial gas turbines is investigated. 2. SST propulsion engine combustion It is anticipated that the NOx emissions from the next generation supersonic transport may deplete stratosphere ozone concentrations. Research programs for the development of basic technologies required for the ext eneration SST have been conducted worldwide. In Japan e are involved in a program named the Research and Development of Environmentally Compatible Propulsion System for the Next Generation of Supersonic Transport, sponsored by METI [1]. Cruising NOx emissions less than 5 gNO2/kg-fuel is the reduction target in all programs. In cooperation with Kawasaki Heavy Industry Co. Ltd and Rolls-Royce Plc., we are developing a dual annular combustor with lean premixed and prevaporized main burners and pilot burners for a scaled high temperature core engine. They were tested in single sector combustion liner at major operating conditions including idle, approach, take-off and cruise, by using NAL’s high-temperature and high-pressure combustor test rig, shown in Fig. 1. The combustor inlet pressures and temperatures at these operating conditions are summarized in Table 1. Figure 2 shows a photo and a schematic drawing of a single 1/24-sector combustor liner equipped with an LPP duct for the main burner. The main and pilot burners of the selected configurations will be tested in a full-annular combustor test at Rolls-Royce’s Derby plant, which is scheduled for late 2003. Table 1. Combustor Inlet Conditions of Targeted Engine Mode P3, MPa T3, K Idle .38 466 Take-off 1.88 M2.2 Cruise 1.13 Approach 1.02

790 915 618

The measured EINOx are plotted in Fig. 3 as a function of overall equivalence ratio. The EINOx increases very steeply as the overall fuel-air ratio approaches the design air-fuel ratio of 31 at cruise. Observation of the behavior of the liquid film on the prefilmer of the fuel nozzle revealed that the stretch of the film was not sufficient in the axial direction to achieve good atomization. An improvement in the introduction of the atomizing air has been made in the new model by replacing radial air slots with axial swirler vanes. The emissions tests of the LPP main burner with the new fuel atomizer is scheduled for mid June 2002. 3. Stationary Gas Turbine Energy efficient gas turbine co-generation systems are very attractive from the point of view of the reduction of CO2 emissions. Although single-digit ppm NOx emissions have already been demonstrated in some commercial natural gas-fueled gas turbines, the NOx levels of gas turbines burning liquid are generally higher than the al Ministry of Environment’s regulation of 70 ppm (16% O2). Kerosene and light-A fuel are gas turbine fuels easily available in inland areas, though the natural gas pipeline is restricted to the bay areas in big cities in Japan. In order to enhance the spread of a nationwide co-generation system, we started the development of ultra-low NOx emissions combustor technology for liquid-fueled gas turbines with a sponsor ship by the Environment Agency. The target of NOx reduction is a half of the regulations. A variable geometry combustor with butterfly valves for modulating the air split between main and dilution zones was fabricated. After acquiring the data for optimum air split, it was finally tested on a 200 kW simple cycle gas turbine. The results of the engine testing are shown in Fig. 4, and suggest that a proper control

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of air split can reduce NOx emissions to the 20-30 ppm at high power conditions. 4. New Combustion Concept for Ultra-low NOx Emissions The application of a new low-NOx combustion concept, flameless combustion, is being investigated. In this concept, the thermal reaction of the secondary lean mixtures injected into and mixed with the hot burned gas from the primary stage combustion is used to attaining complete combustion while suppressing NOx formation [2]. In conventional flame combustion, complete combustion of a very lean mixture is generally very difficult to achieve, since flame is extinguished when the mixture approaches the lower limit of flammability. Figure 5 shows an illustration of a typical combustor employing the new low-NOx combustion concept. Conventional flames are stabilized on pilot burners and the secondary fuel is injected into the air flowing through the mixture injection tubes. The secondary mixture jets are directed toward the hot burned gas produced by the pilot burners to promote mixing. The key for the optimal operation is the temperature of the reaction zone where the secondary mixture mixes with the hot burned gas from the first stage. The NOx formation by the reaction of the secondary mixture is negligible. Thus, the range of complete combustion and low NOx emissions extends over a range until the secondary mixture is rich enough to stabilize its own flame. Single digit NOx emissions have been shown at typical temperatures and pressures of stationary gas turbines. References [1] S. Hayashi, et. al, LPP Burner Development and Optical Combustion Zone Diagnostics for Low-NOx Emissions, Proc First Inter. Symp. Environmentally Compatible Propulsion System for Next-Generation Supersonic Transport, Paper No. N-3, 2002. [2] S. Hayashi and H. Yamada, NOx Emissions in Combustion of Lean Premixed Mixtures Injected into Hot Burned Gas, Proc. Combust. Inst. 28,, 2000, 2443-23449.

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Fig.1. High-temperature and high-pressure combustor test rig. Fig.2. Photo of a 1/24-sector combustor liner and fuel nozzle. Fig.3. EINOx as a function of overall equivalence

ratio. Fig.4. Typical NOx emissions and combustion efficiency variation with power. Fig.5. Tubular combustor design based on a new ultra-low NOx combustion concept.

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P,bar T in,K U,m/s 1.4 710 790 780 10.2 10.5 11.0

EINOx (gNO 2 /Fuel kg)

1.3 2.0

10

0 25

30

AFR

35

40

120 100 NOx(16%O2),ppm 80 60 40 20 0 0 20 40 60 80 100 Power,% NOx C.E

100 Combustion Efficiency,%

90

80

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