During the last few years OPRA has been working intensively on the development of an ultra low emissions combustor for the OP16 gas turbine. The main focus has been on the combustion of liquid fuels (diesel fuel #2), but a natural gas and a dual fuel system has also been developed. The most important aspect of the development has been the patented Controlled Fuel Air Ratio (COFAR) system incorporating the venturi premixer, the air valve and the fuel injection nozzle. The original diesel fuel injection nozzle of the OP16 was a hybrid design, comprising a pressure swirl central injector surrounded by a classic air-blast atomizer. While the emissions with this fuel nozzle were quite good (30 ppm up to 85% load), subsequent natural gas tests demonstrating single digit emissions, while running at a higher average flame temperature indicated that there was scope for improvement of the fuel preparation system. It was clear that atomization, evaporation and mixing of the diesel fuel could be further improved. For better understanding of the combustion of diesel fuel, an atomization and mixing model was developed, to study the quality of the fuel/air mixture leaving the pre-mixer. Based on the results of this study, a fuel nozzle system, using multipoint injection with small pressure swirl nozzles was selected. Three different sets of atomizers have been evaluated and a nozzle arrangement comprising five identical pressure swirl nozzles showed the best results. The emissions on diesel fuel with the new injector proved very satisfactory. The NOx concentration was kept below 25 ppm from 50% load up to 90% load and below 30 ppm at full load. CO and UHC were well below 10 ppm. These low emissions were achieved by running at a low flame temperature (below 1820K). Furthermore, no combustion dynamics or flame instability was observed.

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