Mechanisms of Coke Formation in Gas Turbine Combustion Chambers

Abstract

New gas turbine combustor designs are developed to reduce pollutant and NOx-emissions. In these new combustors, the formation of carbonaceous deposits, especially in prevaporizers, affects the reliablility 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.

Jet Fuel Oxidation and Deposition

“…For comparison, the figure also shows a retention time reference sample of linear even-carbon hydrocarbons between C-8 and C-I6. This agrees with earlier reports by Brandauer et at (1995) whose results also implicate high boiling point species in deposit formation for No. 2 diesel.…”

Deposit Formation From No. 2 Distillate at Gas Turbine Conditions

“…The conversion of fuel to carbon deposits occurs in a narrow range of surface temperatures with a maximum It a surface temperature of 330 C. Such temperature behavior has been observed in experiments ranging from heated fuel tubes (e.g. Taylor, (1974)) to horizontal plates (Brandauer et al, (1995)) for a wide range of fuels. The low temperature conversion of fuel to deposits is due to auto-oxidation of some component in the fuel and hence there is an increase in deposits with temperature.…”

“…This result was unanticipated because the high-boiling fractions of the No.2 fuel have been identified by Brandauer et al (1995) as the dominant source of precursors to deposits. In their tests, the 18% of No.…”

“…Recent work by Brandauer et al (1995) investigated the formation of deposits using No. 2 fuel on a horizontal plate.…”

“…The conversion of fuel to deposits is lower for a vertical plate than for a horizontal plate. Figure 4 shows data from horizontal and vertical configurations including data from Brandauer et al (1995). The plot shows the fraction of fuel converted to solid deposit (on a mass basis) as a function of surface temperature.…”