How is NOx formed?
There are several ways that NOx is formed in a furnace. The thermal fixation of atmospheric nitrogen and oxygen in the combustion air produces “thermal NOx; while the conversion of the chemically bound nitrogen in the fuel produces “fuel NOx”.

For natural gas and light-distillate-oil firing, nearly all NOx emissions result from thermal fixation.

The formation rate of thermal NOx is dependent on the reaction tempera­ture, the local stoichiometric, and the residence time. The fuel NOx formation mechanism is more complex depending upon fuel pyrolysis and subsequent reaction between many intermediate nitrogenous species and the oxidant species.

Thermal NOx is most readily influenced by the combustion system.

“PROMPT NOx is produced by the breakdown of CH portions of methane and other hydrocarbons in the fuel and their subsequent combination with nitrogen in the air.

The rates for formation of both thermal NOx and fuel NOx are kinetically or limited, with the amount of NOx formed being much less than the equilibrium value. The rate of formation of NOx is dominated by combustion conditions and can be suppressed by modifying the combustion process. Both thermal and fuel NOx are promoted by rapid mixing of oxygen with the fuel Thermal NOx is greatly increased by long residence time at high temperature.

In the next part of this article, we are going to review NOx reduction through burner in fired heaters and boilers.

Free "Reducing NOx from fired heaters and boilers" paper presented at Chemical Engineering Expo 2000 at Houston.

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