The third method of disposing of ammonia discharges is by flaring or burning the ammonia. This method is particularly applicable when the release to be treated will be located at an identifiable point, such as the discharge vent of a relief valve. It is recognized that mixtures of ammonia and air with ammonia concentrations between about 15 percent and 27 percent of ammonia will burn. This combustion can be sustained, however, only through the use of a flame holder, such as an iron screen. This iron screen is maintained at a high temperature by the burning ammonia. Practical flares must provide a support flame from the burning of a hydrocarbon such as methane. Tests on ammonia flares show that the methane concentrations of between 4 and 10 percent in the ammonia/air mixture are effective in facilitating the combustion. Figure 13.13 shows the effectiveness of combustion as a function of the percentages of methane and ammonia in the feed stream. Bracketed within the two straight lines is the flammability region of the mixture, and the contours within this region show the concentration of ammonia in ppm in the combustion products.
The-map shows that with an initial concentration of less than 15 percent ammonia in air, most of the ammonia can be oxidized by providing a methane concentration of between 4 to 10 percent. With initial ammonia concentrations higher than 15 percent, there will still be unburned ammonia in the combustion products, but significantly less than the original 150,000 to 250,000 ppm.
The complete and ideal chemical reaction in the combustion of ammonia is:
When the flame temperatures are especially high, however, some of the NH3 dissociates and along with a small quantity of nitrogen in the air combines to yield NO and NO2, which are the NOx compounds in Fig. 13.14. Figure 13.14 shows contour lines of the NOx concentration indicating that the concentrations can be reduced by tuning the flare to operate toward the upper flammability limit. The region of lower NOx concentrations is that portion of the map associated with reduced flame temperatures in contrast to the high flame temperatures near the lower flammability limit.
One of the conclusions from the curves is that the flaring of pure ammonia is not possible. The ammonia must be diluted with combustion air, preferably with an incoming mixture of less than 15 percent ammonia.