ABSTRACT
Ideally, it would be expected that the presence of excess air with a hydrocarbon fuel at high temperatures and pressures, as in dual-fuel operation, would see, following combustion, the carbon in the fuel appearing as CO2 and the hydrogen appearing as H2O, together with unutilized oxygen and unchanged nitrogen. However, when much excess air is supplied, then combustion will take place at correspondingly lower temperatures and slower rates, which may lead to the incomplete production of combustion products and even ignition failure. In regions where there is an excess of fuel, such as those associated with the pilot fuel and its immediate surroundings, incomplete combustion products will be produced, with some of the fuel remaining unconverted. Much of the hydrogen in the fuel will be oxidized to H2O, and with some appearing as molecular H2. The carbon in the converted fuel, because of the insufficient amount of oxygen locally available, will produce soot, CO, and CO2, with hardly any unutilized oxygen remaining. In practice, depending on the operational conditions and the fuels employed, a departure from this idealized composition will be encountered for a variety of reasons, producing a range of other products, albeit often in small concentrations. Some of these survive to the final product stage, while others are transiently unstable and would not last to the exhaust stage. In reality, they can still play a critical part in the conduct of combustion processes. Some of the main factors that bring about such a behavior are the following:
• Insufficient mixing of the fuel and air and of the time available to fully complete combustion; some of the flame propagation does not extend throughout the whole mixture, such as through a rapid drop in temperatures.
