MINIMIZING NOX EMISSIONS FROM STATIONARY COMBUSTION - REACTION-ENGINEERING METHODOLOGY

The chemical pathways of the formation of nitrogen oxides and destruction in flames provide the basis for the development of design strategies for low emission combustion sources. The design protocol varies according to the fuel type (gaseous, liquid or solid, organically-bound nitrogen bearing or not) and the combustion equipment (boilers, industrial furnaces, gas turbines, etc.). Research and Development has to rely on a combination of modeling and experiment. In the modeling, engineering insight is required concerning acceptable simplifications in the predictions of flow, the progress of combustion reactions, the gas temperature distribution, and the nitrogen compound interconversions. Examples are shown of NOx emissions reduction via fuel-rich/fuel-lean sequencing of the combustion, either by the use of physically separated fuel-rich and fuel-lean combustion chamber sections, or by the use of staged air or staged fuel injection combined with the recirculation of burned flue gas through the burner, and the avoidance of premature burner air admiring with the fuel by the suppression of turbulent mixing at their interface. Staged combustion is especially effective in reducing NOx emissions for fuels which contain organically bound nitrogen. However, in natural gas burning gas turbines, because of the special limitations on the temperature at the entry to the gas turbine, the prevailing developmental trend is towards lean premixed combustion. While such systems promise very low NOx emissions, there is a challenge in flame holding at mixture ratios close to the lean flammibility limit of combustion. Combustion process modification is a proven and cost effective method of NOx reduction, but for the cleanest exhaust it will have to be combined with post combustion clean-up, catalytic or noncatalytic. In all these cases modeling of the different degrees of sophistication is used to aid experiment.