How NOx Is Formed in a Boiler
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. In addition, minor amounts
of NOx, referred to as prompt 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
then oxidized to 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,
the ratio is different, and thermal NOx can be considerably higher
than fuel NOx.
The quantity of thermal NOx formed depends primarily on the
“three t’s” of combustion: temperature, time, and turbulence. In other
words, 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 delay the mixing
of fuel and air, thereby reducing temperature and initial turbulence,
which minimizes NOx formation.