Reduction of the air preheat temperature (RAPH) is another viable technique for cutting NOx emissions. This technique lowers the peak flame temperatures, thereby reducing NOx formation. The thermal efficiency penalty, however, can be substantial.
Post-combustion control techniques such as SCR and SNCR by ammonia or urea injection are described in Section 5.24.
The techniques of OSC, FGR, and RAPH can be effectively combined. The OSC techniques, namely LEA and two-stage combustion, reduce the quantities of combustion gases reacting at maximum temperature, while FGR and RAPH directly influence the maximum level of combustion temperatures. The reductions obtained by combining individual techniques are not additive but multiplicative. However, the combined conditions necessary to achieve such low levels of oxides are not compatible with operational procedures.
Figure 5.19.3 summarizes the NOx control technology choices. An environmental engineer can use this figure and Table 5.19.1 to identify the potential control technologies for boilers and process heaters. After identifying the applicable technologies, the engineer must conduct an economic analysis to rank the technologies according to their cost effectiveness. Management can then select the optimum NOx control technology for a specific unit.
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