The current work explores the operation of an industrial gas turbine (25 MWe) unit working under variable NOx emission limits (from 75 to 120 mg/Nm3) and variable fuel composition. The latter is typical for flared streams in oil process industry (refineries/well operation/petrochemicals).
In the current case the nitrous oxides (NOx) emission limits are dependent on the mixing ratio of natural gas (NG, 89%vol. methane) and refinery gas (RG, 40% vol. H2, 56%vol. CH4, C4+) that is used as fuel for the engine. This mixture is process dependent and varies in matter of minutes from 100% NG to 100% RG. Siemens has faced this challenge by using a SGT-600 unit (25MWe) operating in non-premixed combustion mode with a novel water injection strategy, the adaptive water injection for the control of NOx.
The adaptive water injection for NOx control has been developed after evaluating different NOx control strategies. It consists of a closed loop, in which NOx emission measurements are used as the main control parameter, linked to a fixed parametric curve based on the compressor discharge pressure. Fast response times and fail–safe strategies have also been developed and tested under operating conditions of the GT. The water injection method could cope with the variation of load, fuel supply mixture and NOx limit without any flame instability problems. This is done while maintaining the optimal amount of water needed to achieve the emissions target. The robustness of the system has been satisfactory over the tested period and seems a viable solution.