OPERATING PARAMETERS AFFECTING NOx

 Thermal NOx is the major source of NOx from the combustion of gaseous fuels. The parameters that influence the oxygen concentration in the flame zone or the temperatures achieved in the flame zone will affect thermal NOx emissions. The most important parameters are:

• Excess Air

• Fuel Composition

• Air Preheat Temperature

• Furnace Temperature

Excess Air

Excess air provides for additional oxygen beyond the stoichiometric air requirement and is generally required to minimize the emissions of CO and unburned hydrocarbons. It accomplishes this, however, by increasing the concentration of oxygen in the flame zone, which tends to increase NOx. Excess air also decreases the overall flame temperature and contributes to a loss in thermal efficiency. Figure 5 shows the effect of excess air, expressed as percent excess oxygen, on NOx emissions.

As the excess air is steadily increased the reduction in NOx due to the reduction in flame temperature finally overcomes the increase in NOx due to oxygen concentration and the NOx emissions peak. Further increases in excess air then reduce NOx emissions.

Fuel Composition

Fuel composition influences thermal NOx because of its direct effect on flame temperature. Different fuels are capable of achieving different flame temperatures and the maximum potential flame temperature for a fuel is best defined by the adiabatic flame temperature. The adiabatic flame temperature is the theoretical temperature attained when a fuel/air mixture is burned to completion and all of the sensible and chemical energy of the reactants is transferred to the products of combustion.

The variation in flame temperature with composition is apparent from the table, ranging from 3334oF for methane to 3652oF for hydrogen. Although practical flames transfer heat away from the flame zone, the adiabatic flame temperature provides a good method for evaluating the potential effect of fuel gas composition on flame temperatures and, therefore, the potential effect on thermal NOx emissions.

Air Preheat Temperature

Air preheat affects thermal NOx by its direct effect on flame temperature. Preheating the combustion air adds sensible heat to the flame reactants which increases the heat in the products of combustion and, thus, increases the flame temperature. Note that the NOx essentially follows an exponential increase with increasing air preheat temperature. A reasonably good rule of thumb from industrial experience is that the thermal NOx emissions will double as the combustion air temperature is increased from ambient to about 500 to 600 oF.

Furnace Temperature

Furnace temperature affects thermal NOx emissions by its effect on the rate of heat transfer from the flame and, as a result, it influences the actual temperatures attained within the flame zone. The lower the furnace temperature, the higher the heat transfer rate from the flame and the  lower the actual peak flame temperatures within the flame zone. Lower peak flame temperatures mean lower thermal NOx emissions.