Low NOx Burners
In recent years, tightening environmental restrictions have led to the manufacture of low NOx burners. The formation of nitrogen oxides from the nitrogen contained in air depends on the reaction rate in the combustion zone and is, therefore, strongly influenced by the flame temperature as shown in Figure 500-3.
The flame temperature peaks sharply near the stoichiometric mixture of fuel and air (Figure 500-3). If combustion could be carried out in two stages, first rich and then lean or the other way round, then the flame temperature and hence the reaction rate in each stage would be lower, thus producing less NOx.
Since two things can be staged (air and fuel), there are several ways to design low NOx burners.For gas firing, fuel-staged low NOx burners (as shown in Figure 500-4) achieve lower emissions than their air-staged predecessors. For nitrogenous liquid fuels, deep air-staging is the technique of choice. For an expanded discussion, see “Reduce Heater NOx in the Burner,” Hydrocarbon Processing, November 1982. (See Appendix A.)
There are currently two types of ultra-low NOx burners. The first uses flue gas recirculation in addition to fuel staging to lower the flame temperature. In this design, some of the gas jets are configured to entrain cool flue gasses near the floor or wall into the flame, usually through holes in the burner tile. The relatively inert flue gasses dilute the flame and lower its temperature further.
The second type of ultra-low NOx burner is the fiber matrix burner. These burners pass premixed gas and air through a ceramic fiber matrix, producing surface combustion at temperatures substantially lower than can be obtained by fuel
staging, so the reaction rate for nitrogen oxides is much lower. There are none currently (1997) installed in the Company’s process units, but they are being considered.
Typical values for NOx are as follows: Normal gas burners above 100 ppm NOx; fuel staged burners, 40 to 60 ppm NOx; flue gas re-circulating ultra-low NOx burners, 20 to 30 ppm; and fiber matrix ultra-low NOx burners, 10 to 20 ppm NOx.