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A NEW COMBUSTOR REDUCES NOX BELOW 10 PPM IN AN 8 MW, M7A-O3 GAS TURBINE
Kawasaki Heavy Industries (KHI) introduced its first industrial gas turbine in 1974 for stand-by use sets, followed by a baseload gas turbine in 1984. Since then, the company has delivered more than 10,000 GTs in the 0.2 to 18MWrange.
In keeping with regulatory requirements, KHI released a Dry Low Emissions (DLE) combustion system in 2009, which guaranteed <15 ppm NOx (O² = 15%) for the 8 MW Kawasaki M7A-03 and the 18 MW Kawasaki L20A. Most recently, the company developed a single-digit NOx Dry Low Emissions (DLE) combustor for the M7A- 03 turbine. It achieves <9 ppm NOx and <25ppm CO emissions. Specifications and general layout of the M7A-03 are shown in Table 1 and Figure 1, respectively.
The new combustor was the result of incremental improvements made to the 15ppm NOx DLE combustor. That previous generation combustor’s burner systemconsists of pilot, main and supplemental fuel burners.
The pilot fuel burner is the diffusion type, and themain and supplemental fuel burners are premixed. The supplemental fuel burner has fuel injection holes between its slits and a longer mixing area than the diffusion type. By applying these technologies to enhance the mixing between the air and fuel to the supplemental fuel burner, the guarantee of <15 ppm NOx had been achieved at the load range of 50 - 100%. The pilot fuel burner of the 15ppm NOx DLE combustor is a diffusion type and has six injection holes at the center and tip corner of the burner. A fuel-rich zone exists at the exit plane of the pilot fuel burner. This area is effective in flame stabilization, but has a negative effect on the reduction of NOx emissions.
For the new combustor, a premixing pilot burner was applied for the first time. The fuel-air concentration was investigated using Computational Fluid Dynamics (CFD). The emissions, ignition and flame stability were evaluated using rig and engine tests.
The premixed pilot fuel burner of the single-digit NOx DLE combustor has a series of air slits and fuel injection holes between the slits. The fuel is perpendicularly injected into the air-flow direction and the shearing force of the air enhances mixing of air and fuel. After that, the mixed gas is deflected by 90 degrees. The blending between the air and fuel finally completes through the longer mixing pass and is injected from the injection holes at the nozzle tip ((Figure 2).
There is a small fuel-rich area at the center of the burner-end to ensure flame stability. A tiny quantity of pilot fuel is led directly to the center of the burnerend. The fuel-air concentration is uniform at the inside of the burner. At the exit plane of the burner, the fuel-air concentration is uniform except for a small zone around the center of the burner.
The fuel for each burner can be separately and manually controlled and fed to each burner of the test combustor. There is a duct to measure the combustor outlet temperature and exhaust gas emissions at the downstream of the combustor.Multi-probes (R-type thermocouples) measure the combustor outlet temperature distribution.
A part of exhaust gas is led to the emissions gas analyzer through a gas sample probe.A sight glass downstream of the test section is used to observe the behavior of the combustion flame. The maximum air temperature and pressure of the supplying air are 600°C (1,112°F) and 0.3 MPa (43.5 psi), respectively. By adjusting the combustor inlet/outlet temperatures and the air velocity to meet actual engine conditions, the basic data of any type of gas turbine combustors can be measured.
The pilot fuel burnerwas switched to the premixed type in the single-digit NOx DLE combustor, while the main and supplemental burners remained the same as those on the 15pm NOx DLE combustor. The diameter of the dilution holes was changed to adjust the combustion airflow rate.
The series of rig tests for the Singledigit NOx DLE combustor were conducted at different air fuel ratios (AFRs), which were equivalent to the load range of 50-100%. NOx emissions remained at a low level with all tested AFRs.
With the newly tested combustor installed in the M7A-03 engine, complete engine tests were conducted. The fuel is controlled by the fuel governor, and it rises as the load increases. The control valves divide the fuel flow rates to the three groups of fuel burners. The open rates of the control valves are set in accordance with the load condition.
During ignition, acceleration and up to 40% load, the fuel was supplied only to the pilot fuel burner in order to stabilize its combustion. Within the 40-50% load range, the AFR of the pilot and main fuel burner achieves optimum condition.
Fuel was then shifted from the pilot fuel burner to main fuel burner. After this changeover, a small part of the fuel continued to be supplied to the pilot burner. After that, the control valve of the supplemental fuel burner opened.
Between 50% to 100% load, the AFR of the pilot and main fuel burner were kept constant by adjusting the supplemental fuel flow rate. The combustion airflow rate was modulated by the variable stator vane to enlarge the DLE operational load range. The DLE bleed system was not used on the single- digit NOx DLE combustion system, thus it was possible to maintain thermal efficiency at part load range.
During engine tests, the combustor achieved its target emissions level (<9 ppm NOx, <25 ppm CO) between the load ranges of 50% to 100%.At full load, NOx dropped to 4 ppm. Flame stability tested well during the load rejection tests.