NO_X is a pollutant formed in nearly all combustion reactions, including fired equipment such as ovens, heaters, dryers, boilers and furnaces. As NO_X currently is or will soon be regulated for all process plants, anyone involved with process heating applications should be familiar with some basic information about NO_X. Fortunately, there are many well-established methods for controlling and minimizing NO_X.

NO_X refers 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 NO_X emissions 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 NO_X formation are thermal NO_X, prompt NO_X and fuel NO_X. Thermal NO_X is formed by the high temperature reaction (hence the name thermal NO_X) 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.

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

Prompt NO_X is formed by the relatively fast reaction between nitrogen, oxygen and hydrocarbon radicals (hence the name prompt NO_X). Prompt NO_X generally is an important mechanism in lower temperature combustion processes and also becomes more important under fuel rich conditions (figure 2).

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

Fuel NO_X is formed by the direct oxidation of organo-nitrogen compounds contained in the fuel (hence the name fuel NO_X). Ammonia (NH₃) is an example of a chemical that could be present in a waste stream being combusted that would produce fuel NO_X. Fuel NO_X is not a concern for high-quality gaseous fuels like natural gas, which normally have no organically bound nitrogen. However, fuel NO_X may be important when oil (e.g., residual fuel oil), coal, or waste fuels are used, which can contain significant amounts of organically bound nitrogen.

How Is NO_X Controlled?

Enlarge this picture Figure 4. Reducing combustion air preheating can significantly reduce NOX.

Four basic NO_X control strategies (figure 3) may be used in combination to control NO_X, depending on the emission limits.¹ These include pretreatment, process modification, combustion modification and post-treatment. Table 1 shows a summary of NO_X control techniques while table 2 shows some common NO_X reduction technologies.

Enlarge this picture Table 1. NOX control techniques to minimize NOX formation include fuel switching or treatment, additives, oxidizer switching, and product switching or treatment.

Pretreatment. This preventive technique is used to minimize NO_X where the incoming feed materials (fuel, oxidizer and/or the material being heated) are treated or substituted to reduce NO_X. Some examples include fuel switching or treatment, additives, oxidizer switching, and product switching or treatment. For example, partially or completely substituting natural gas for fuel oil often can significantly reduce NO_X emissions by reducing or eliminating fuel-bound nitrogen. Switching from air to pure oxygen for combustion eliminates most, if not all, of the nitrogen from the process, so NO_X is minimized or eliminated.

NO_X control 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 NO_X formation.

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

Process Modification. These techniques are employed to change the existing production process to reduce NO_X emissions. For example, reducing the firing rate reduces NO_X, where the reduction in NO_X is proportional to the reduction in firing rate: As less fuel is burned; therefore, less NO_X is produced. However, production is reduced as well. Another example is to replace some or all of the gas-fired equipment with electrically heated units that do not produce any NO_X emissions at the point of use. NO_X is produced at the power station instead of at the process plant. However, operating costs often increase as electricity usually is more expensive than fossil fuels in heating applications.

Enlarge this picture Figure 5. Carbon monoxide formation is a function of the mixture ratio (combustion air/fuel gas volume).

Another method is to improve the thermal efficiency of the process so less fuel is consumed per unit of production. This approach reduces both pollution emissions and operating costs. In special cases, it may be possible to switch the material being heated to one that requires less energy to process.

Process modifications cannot reduce or eliminate NO_X emissions in all process applications, however. Some process modifications are radical and expensive and are only used under certain circumstances.

Enlarge this picture Figure 6. Some low-NOX burners incorporate air and fuel staging to minimize NOX formation.

Combustion Modification. Overall, combustion modification techniques such as using low NO_X burners tend to be the most cost-effective method of reducing NO_X. In this strategy, NO_X formation is minimized by changing the combustion process. Numerous methods have been used to accomplish this. For example, reducing combustion air preheating, if present, can significantly reduce NO_X (figure 4). However, this also reduces thermal efficiency and productivity.

Alternatively, reducing excess air is a good way to reduce NO_X and increase thermal efficiency. However, reducing excess air levels too much can increase carbon monoxide emissions (figure 5), which is another regulated pollutant.

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

Another popular method is to replace existing burners with low NO_X designs.² These incorporate many techniques for reducing NO_X such as air and fuel staging (figure 6), internal furnace gas recirculation (figure 7), water or steam injection, and ultra-lean premixing. External flue gas recirculation is another technique for reducing NO_X (figure 8). Most of these techniques involve reducing the peak flame temperatures that produce high NO_X levels. Figure 9 shows one example of how new generations of burner designs continue to reduce NO_X emissions.

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

Post-Treatment. In this strategy, NO_X is removed from the exhaust gases after it has already been formed in the combustor. The general strategy is to use a reducing agent such as CO, CH₄, other hydrocarbons or ammonia to remove the oxygen from the NO and convert it into N₂ and O₂. Often, some type of catalyst is required for the reactions. (A catalyst is a substance that causes or speeds up a chemical reaction without undergoing a chemical change itself.)

Enlarge this picture 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.

Two common post-treatment methods are selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). SCR is generally used instead of SNCR when very low NO_X levels are required (figure 10). One advantage of post-treatment NO_X-reduction methods is that multiple exhaust streams can be treated simultaneously, thus achieving economies of scale.

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.

Enlarge this picture 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.

In conclusion, NO_X, which is formed in nearly all industrial combustion processes, is a regulated pollutant that has some serious health and environmental effects. Generally, it can be controlled using one or more proven strategies. The most cost-effective technique tends to be combustion modification such as using low NO_X burners. In virtually all cases, proper care must be taken to carefully operate and maintain the combustion equipment to keep it within the specified range for low emissions. Suitable instrumentation such as gas analyzers for measuring O₂ and NO_X in the exhaust products is recommended to ensure equipment is operating according to specifications. This will help those using process heating equipment continue to be environmentally friendly and within compliance of their air permits.