
Figure 1. For any
typical fuel, NOX formation is a function of gas
temperature. Thermal NOX is formed by the high
temperature reaction of nitrogen with oxygen and increases exponentially with
temperature.
NOXis a pollutant formed in nearly all combustion reactions, including fired equipment such as ovens, heaters, dryers, boilers and furnaces. As NOXcurrently is or will soon be regulated for all process plants, anyone involved with process heating applications should be familiar with some basic information about NOX. Fortunately, there are many well-established methods for controlling and minimizing NOX.
NOXrefers to oxides of nitrogen. The two most common forms are nitrogen monoxide, also known as nitric oxide (NO), which is colorless and odorless, and nitrogen dioxide (NO2), which is reddish brown and has a suffocating odor. In most high-temperature heating applications such as furnaces, most NOXemissions are in the form of NO, with a significantly lesser amount of NO2. Lower temperature heating applications such as boilers may have comparable amounts of NO and NO2.
The three generally accepted mechanisms for NOXformation are thermal NOX, prompt NOXand fuel NOX. Thermal NOXis formed by the high temperature reaction (hence the name thermal NOX) of nitrogen with oxygen, and it increases exponentially with temperature (figure 1). Above about 2,000°F (1,093°C), it generally is the predominant mechanism in combustion processes, making it especially important in higher temperature heating applications.

Figure 2. For any
typical fuel, NOX formation is a function of the mixture
ratio (combustion air/fuel gas volume).

Figure 3. Four
strategies can be used in combination to control NOX:
pretreatment, process modification, combustion modification and post-treatment.

Figure 4. Reducing
combustion air preheating can significantly reduce NOX.
How Is NOX Controlled?
Four basic NOXcontrol strategies (figure 3) may be used in combination to control NOX, depending on the emission limits.1These include pretreatment, process modification, combustion modification and post-treatment. Table 1 shows a summary of NOXcontrol techniques while table 2 shows some common NOXreduction technologies.
Table 1.
NOX control techniques to minimize
NOX formation include fuel switching or treatment,
additives, oxidizer switching, and product switching or treatment.
NOXcontrol via pretreatment generally is only economically viable for higher temperature applications. Removing organically bound nitrogen that may be present in the feed materials, such as the niter used in making glass, may also reduce NOXformation.

Table 2. Combustion
modification techniques such as using low NOX burners
tend to be the most cost-effective method of reducing
NOX.

Figure 5. Carbon
monoxide formation is a function of the mixture ratio (combustion air/fuel gas
volume).
Process modifications cannot reduce or eliminate NOXemissions in all process applications, however. Some process modifications are radical and expensive and are only used under certain circumstances.

Figure 6. Some
low-NOX burners incorporate air and fuel staging to
minimize NOX formation.
Alternatively, reducing excess air is a good way to reduce NOXand increase thermal efficiency. However, reducing excess air levels too much can increase carbon monoxide emissions (figure 5), which is another regulated pollutant.

Figure 7. Some
combustion systems incorporate internal furnace gas recirculation to minimize
NOX formation.

Figure 8. Some
combustion systems incorporate external furnace gas recirculation to minimize
NOX formation.

Figure 9. The amount of
NOX formed as a function of excess O2
varies by burner design. In the figure, the oldest designs are shown at the top
and the newest are shown at the bottom.
Most post-treatment methods are relatively simple to retrofit to existing processes. However, most are fairly sophisticated and are not trivial to operate and maintain in industrial environments. For example, catalytic reduction techniques require a catalyst that can become plugged or poisoned fairly quickly by dirty flue gases. Post-treatment methods often are capital intensive and usually require halting production if the treatment equipment malfunctions.

Figure 10. Two common
post-treatment methods are selective catalytic reduction (SCR) and selective
non-catalytic reduction (SNCR). SCR is generally used when very low
NOX levels are required.
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