1 October 1997
effect of introducing moderate levels of excess air in this stage has the least effect on the total
c. Effect of Air Control On Secondary Combustion.
(1) Since this is the last of the two processes, the secondary combustion process is
designed to provide the final destruction and "polishing" of the combustion gases to achieve as
benign a discharge gas as possible. This requires that the amount of air and the points of
introduction to the secondary combustion process be closely controlled.
(2) Large water-wall incinerator systems may achieve the highest destruction rates, produce
the lowest CO concentrations (below 30-50 ppm), and have the highest combustion efficiency, with
exit gas S.R.s of 1.4 to 1.5 and secondary gas temperature maintained above 1,6000F for 1.0 to
2.0 s. (see figure 3-3).
(3) The smaller modular and packaged incinerator units achieve their best performance with
exit gas S.R.s of 1.5-2.0 and exit gas temperature in the 1800-20000F range. To attain these
temperatures with the higher air dilution, auxiliary burners must be used.
d. Effect of Air Control On Emissions.
(1) Control of the stoichiometry, temperature and time relationship in the secondary
combustion process have been shown to be the primary factors in meeting emissions requirements.
(a) Analysis of operating waste to energy facilities in the United States and Canada
have shown that the lowest levels of chlorinated hydrocarbons and benzopyrenes are achieved
when the CO levels are below 50 ppm (see figure 3-3).
(b) The achievement of low NOx is a function of what happens to the air as it passes
through both the primary and secondary chamber. Since the kinetics of NOx formation is a function
of both temperature and the stoichiometric ratio of the gas (i.e., NOx decomposes in
substoichiometric gases), systems that use the starved-air mode of operation in the primary
combustor have lower levels of NOx than do excess-air systems, even though they operate their
secondary combustion chambers under essentially the same temperature, time, and S.R.
conditions. Figure 3-4 shows the equilibrium concentration of NOx.