NOX is a pollutant formed in nearly all combustion reactions, including fired equipment such as ovens, heaters, dryers, boilers and furnaces. As NOX 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 NOX. Fortunately, there are many well-established methods for controlling and minimizing NOX.
NOX 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 NOX 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 NOX formation are thermal NOX, prompt NOX and fuel NOX. Thermal NOX is 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.
How Is NOX Controlled?Four basic NOX control strategies (figure 3) may be used in combination to control NOX, depending on the emission limits.1 These include pretreatment, process modification, combustion modification and post-treatment. Table 1 shows a summary of NOX control techniques while table 2 shows some common NOX reduction technologies.
NOX 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 NOX formation.
Process modifications cannot reduce or eliminate NOX emissions in all process applications, however. Some process modifications are radical and expensive and are only used under certain circumstances.
Alternatively, reducing excess air is a good way to reduce NOX and increase thermal efficiency. However, reducing excess air levels too much can increase carbon monoxide emissions (figure 5), which is another regulated pollutant.
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.