Regulations on the emission of oxides of nitrogen (NOx) from stationary gas turbines are being tightened around the world. In response, Kawasaki has developed a single-digit NOx DLE combustion system for use with one of its 8 MW gas turbines.
|Kawasaki's 8 MW m7A-03D engine, designed to cut NOx emissions|
Kawasaki has brought NOx emissions to a new low. The guaranteed maximum NOx emissions level of the dry low emissions (DLE) combustion system used in the company's 8 MW class m7A-03 gas turbines has been reduced from 15 parts per million (ppm) to 9 ppm (converted with 15 per cent O2). Kawasaki says it is the first in the world to achieve a single-digit guaranteed maximum NOx emissions level in DLE combustion systems of this class.
DLE combustors employ a premixed combustion method to generate the high-temperature gas needed for high-speed turbine operation. While NOx is generated during the gas turbine fuel combustion process, the amount produced largely depends on the combustion temperature. The premixed combustion technique minimises the combustion temperature without the use of injected water or steam by premixing the fuel with air to significantly reduce NOx emissions. This feature has made DLE combustors the most widely used combustion systems employed in gas turbines.
But the big problem with premixed combustion, especially when used to lower NOx emissions, has always been stability. Now Kawasaki has overcome this perennial technological stumbling block to lowering DLE combustor NOx emissions. The driving force behind the successful development of this new DLE system, boasting outstanding combustion stability, is an innovative proprietary combustion mechanism.
Kawasaki's combustion mechanism features a multistage burner process employing a pilot burner, as well as main and supplemental burners. This latest technological breakthrough in lowering NOx emissions is built on a solid foundation of ongoing DLE combustor research and development. Kawasaki has leveraged the technological expertise the company has gained over the years to replace all its burners with low-emission premix burners.
Since stationary gas turbines emit NOx gases that can result in photochemical smog and acid rain, they are subject to strict regulations across the globe. While NOx emissions standards are set at 25 ppm or below in most countries, an increasing number of local governments in the US and Europe, where public awareness about air quality is high, are implementing even stricter NOx emissions standards that require a maximum emissions level of 15 ppm, or even 10 ppm in some areas.
To meet these increasingly tough NOx emissions standards, Kawasaki is working to enhance overall gas turbine efficiency and to develop innovative combustion technologies that will lower gas turbine emissions. Kawasaki is looking at new ways to put technology to work for the global environment with more efficient products that leave a smaller environmental footprint.
ACHIEVING SINGLE-DIGIT EMISSIONS
Against a < 15 ppm NOx emissions requirement in many parts of the world, Kawasaki established its DLE combustion system with a guarantee of NOx < 15 ppm emissions (at 15 per cent oxygen) for the m7A-03 (8 MW class) in 2009 and for the L20A (18 MW class) in 2010.
In order to meet the new needs of customers, the company has completed development of the single-digit NOx DLE combustion system. Its target is NOx < 9 ppm and carbon monoxide (CO) < 25 ppm emissions (at 15 per cent oxygen). Kawasaki has adopted its single-digit NOx DLE combustion system in the m7A-03 gas turbine, one of Kawasaki's best-selling engines.
This single-digit NOx DLE combustor was developed by enhancing the third-generation 15 ppm NOx DLE combustor. The burner system of the 15 ppm combustor consists of pilot, main and supplemental fuel burners as shown in Figure 1.
|Figure 1: A 15 ppm NOx DLE combustor|
The system's pilot fuel burner is of the diffusion type, while main and supplemental fuel burners are of the premixed type. The premixing supplemental fuel burner was applied for the first time in this combustor unit. It has fuel injection holes between slits, and a longer mixing length than the supplemental fuel burner of the diffusion type. By applying these technologies to enhance the mixing between air and fuel for the supplemental fuel burner, Kawasaki had achieved a guarantee of less than 15 ppm NOx at the load range of between 50 per cent and 100 per cent.
For the development of the single-digit NOx DLE combustor, a premix pilot fuel burner was applied for the first time. Even if the premixing combustion is applied to the pilot burner, the performance of ignition and flame stability is very important and has to equal that of the diffusion burner. Therefore, the fuel-air concentration was investigated using computational fluid dynamics (CFD), and the performance of emissions, ignition and flame stability was investigated in rig and engine tests.
Figure 2 shows a cross-section of a diffusion type of the pilot fuel burner of the 15 ppm NOx DLE combustor and Figure 3 shows its fuel-air concentration at the exit plane of the pilot fuel burner obtained by means of CFD.
|Figure 2: Cross-section of the pilot fuel burner of the 15 ppm NOx DLE combustor|
|Figure 3: Fuel-air concentration at the exit plane of the pilot fuel burner|
The purpose of employing the premix combustion for the pilot fuel burner is to reduce NOx emissions. Figure 4 shows a cross-section of the pilot fuel of the single-digit NOx DLE combustor. This pilot burner has a series of air slits and fuel injection holes between the slits.
|Figure 4: Cross-section of pilot fuel burner at the single-digit NOx DLE combustor|
Fuel is perpendicularly injected into the air flow direction and a shearing force is exerted by the air, which enhances the mixing effect between air and fuel. The mixed gas is deflected by 90°, which means that the mixed gas is thoroughly stirred by strong turbulence.
Figure 5 shows the fuel-air concentration at the exit plane of the pilot fuel burner obtained by means of CFD. The fuel-air concentration is uniform in the inside of the burner and also at the exit plane of the burner – except for the very small area around the centre of the burner.
|Figure 5: Fuel-air concentration at the exit plane of the pilot fuel burner visualised through CFD|
The rig test was conducted at an air-to-fuel ratio (AFR) equivalent to the load range of between 50 per cent and 100 per cent at the actual engine conditions. Figure 6 shows NOx emissions performance at this test. In addition to the pilot and main fuel burner the supplemental fuel burner is used. Through this test, it was proved that NOx emissions were kept at a low level at all ranges of this AFR.
|Figure 6: NOx emissions performance in the rig test|
Figure 7 shows the emission performance at the engine tests. Through these tests the NOx and CO emissions achieved the targeted level (NOx < 9 ppm, CO < 25 ppm) at the load range of 50 per cent to 100 per cent. At the full load, the lowest NOx figure was about 4 ppm. It was confirmed that the flame stability had a good performance in the load rejection tests.
|Figure 7: Emissions performance in the engine test|
To conclude, Kawasaki has realised super low emissions (NOx < 9 ppm and CO < 25 ppm at a load range of 50 per cent to 100 per cent) with the m7A-03 gas turbine engine and started its sales promotion. Kawasaki has also announced that it is going to apply this technology subsequently to its other engine fleets for the market, which is becoming more interested in environmentally friendly products and more eager to reduce its environmental burden.
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