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% 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 minimizes 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.
|Figure 1. A 15 ppm NOx DLE combustor|
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 guarantee of NOx < 15 ppm emissions (at 15% 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% 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.
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.
|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|
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% and 100%.
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 4. Cross-section of pilot fuel burner at the single-digit NOx DLE combustor|
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 5. Fuel-air concentration at the exit plane of the pilot fuel burner visualized through CFD|
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.
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.
The rig test was conducted at an air-to-fuel ratio (AFR) equivalent to the load range of between 50% and 100% 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% to 100%. 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 realized super low emissions (NOx < 9 ppm and CO < 25 ppm at a load range of 50% to 100%) 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.
Two recent cogeneration contracts for Kawasaki
Kawasaki Heavy Industries has delivered a 7.2 MW class gas turbine generation system via Kawasaki Gas Turbine Asia – its gas turbine sales and service subsidiary for South and Southeast Asia, located in Kuala Lumpur, Malaysia – to Indonesian engineering company PT. Euroasiatic.
The gas turbine system has been installed by PT. Euroasiatic as part of the cogeneration system at a plant belonging to P.T. Sumi Rubber Indonesia, the local subsidiary of leading tyre manufacturer Sumitomo Rubber Industries. The cogen system consists of a natural gas-burning GPB80, a gas turbine power generation system powered by the m7A-03 Kawasaki gas turbine, and a waste heat recovery boiler that captures exhaust gas as steam from the power generator. All power and steam generated by the system is supplied to production facilities within the plant, helping to deliver a more stable supply of power. Peripheral equipment for the boiler, as well as local installation and operational testing was undertaken by Euroasiatic. The power generation system came on line in July 2011.
Beginning with the delivery of its 3 MW class gas turbine in 1994, Kawasaki has shipped 21 gas turbines to Indonesia. The current order is the second GPB80-series gas turbine power generation system to be delivered. Because the supply of power in Indonesia tends to be unstable, the use of power generation systems using natural gas is rising in urban and industrial areas, where natural gas pipelines are growing. Demand for gas turbines can be expected to continue to climb.
Meanwhile, Kawasaki Gas Turbine Europe (KGE), Kawasaki’s Frankfurt-based gas turbine sales and service subsidiary for Europe, received an order for a 1.7 MW class GPB17D gas turbine cogeneration system in December 2010. The order came from Industrielle Werke Basel (IWB), a Swiss energy service company based in Basel. This is the first order for the GPB17D and the first Kawasaki gas turbine generator set delivered to Switzerland.
The cogeneration system consists of a natural gas-fuelled turbine power generation system employing Kawasaki’s newly developed m1A-17D gas turbine, as well as a waste heat recovery boiler. Boasting an electrical efficiency of 27%, the system will be used for IWB’s regional thermal energy project to be implemented in Basel. It will supply electricity as well as heat recovered from the gas turbine exhaust system, cutting carbon dioxide emissions while providing a steady supply of power.
Kawasaki will supply the m1A-17D gas turbine generator while KGE will be responsible for procuring the exhaust recovery boiler and auxiliary equipment, as well as for assembly, on-site installation and test runs of the gas turbine power generator system. The cogeneration system is scheduled to go on line in March 2012.
The GPB17D gas turbine power generation system, driven by Kawasaki’s new m1A-17D gas turbine, builds on the technological foundation of the GPB15D gas turbine generator that has earned an outstanding reputation in the market and can show a proven track record. The GPB17D outperforms the GPB15D with 13% more output and 10 percentage points more of thermal efficiency. Kawasaki’s dry low emission (DLE) technology for lower NOx emissions levels cuts NOx emissions to 15 ppm, the lowest in this turbine class.