Most of the NOx formed during the combustion process is the result of two oxidation mechanisms: (1) reaction of nitrogen in the combustion air with excess oxygen at elevated temperatures, referred to as thermal NOx; and (2) oxidation of nitrogen that is chemically bound in the coal, referred to as fuel NOx. For most coal-fired units, thermal NOx typically represents about 25% and fuel NOx about 75% of the total NOx formed. However, for cyclones and other boilers that operate at very high temperatures, thermal NOx can be considerably higher than fuel NOx. In addition, minor amounts of NOx are formed early in the combustion process through complex interactions of molecular nitrogen with hydrocarbon free radicals to form reduced nitrogen species that are later oxidized to NOx, referred to as prompt NOx. The quantity of thermal NOx formed depends primarily on the "three t ’s " of combustion: temperature, time, and turbulence. Thus flame temperature, the residence time at temperature, and the degree of fuel/air mixing, along with the nitrogen content of the coal and the quantity of excess air used for combustion, determine NOx levels in the flue gas. Combustion modifications manage the mixing of fuel and air, thereby reducing temperature and initial turbulence, which minimizes NOx formation.