Mechanisms of Coke Formation in Gas Turbine Combustion Chambers

New gas turbine combustor designs are developed to reduce pollutant and NO_x emissions. In these new combustors, the formation of carbonaceous deposits, especially in prevaporizers, affects the reliability and effectiveness of operation. To avoid deposits, a detailed knowledge of the origins and mechanisms of formation is required. To obtain a deeper insight, the phenomena were studied systematically. The deposits under consideration show differing characteristics suggesting more than one formation mechanism in the combustor. Consequently, the primary goal was to identify the formation mechanisms and, subsequently, to simulate the mechanisms under well-defined conditions in bench tests for determining the relevant parameters of deposit build-up. The mechanisms of formation were identified based on the properties of the deposits in the combustion chamber. In order to characterize the deposits, physical and chemical analysis techniques were utilized. In summary, tests and numerical predictions identified two major paths of formation: a deposit build-up resulting from flame products such as soot or coked droplets and a deposit build-up resulting from liquid fuel impinging the wall accompanied with chemical reactions at the wall. The deposits caused by fuel droplet impingement were intensively studied in bench tests. In analyzing the processes, the influence of wall temperature, fuel composition, and the oxygen content in the environment is shown in detail. In addition, the importance of thermal instabilities of the fuel, previously studied under fuel supply system conditions, is demonstrated for a deposit formation inside a combustion chamber.