1 October 1997
(a) Nitric oxide (NO) is the predominant form of NOx produced during combustion
withNO2 produced in smaller quantities.
(b) NOx emissions come from two separate sources during combustion. The first source
is oxidation of nitrogen in the fuel (fuel NOx); the second source is the high temperature oxidation of
atmospheric nitrogen (thermal NOx). The amount of NOx generated is highly dependent upon the
furnace heat release rates and residence time (see figure 3-4). NOx emission rates increase with
increasing furnace temperatures and excess air. The rate of thermal NO2 generated at
temperatures below 2900F is substantially reduced by lowering the temperature. The primary
combustion process in starved-air incinerators typically operates well below 2900F with a S.R.
below 1. As noted, these conditions combine to produce minimal thermal NOx.
(c) Rates of NOx formation are affected by nitrogen availability as well as temperature.
Waste containing a high nitrogen-content, such as leaves and grass clippings, will increase the fuel
(d) An uncontrolled NOx emission rate between 258-327 ppm can be expected from
most incinerators. The starved-air configuration which limits excess air during thermal
decomposition of wastes in the primary chamber, typically produces emissions below or at the low
end of this range. The federal limit on NOx for incinerator unit sizes of 250 tpd or larger is 180 ppm.
(e) NOx can be reduced by the injection of ammonia of urea. Injection is accomplished
using either steam or compressed air. The relationship between NOx removal efficiency and
reagent utilization is known as the Normalized Stoichiometric Ratio, which is the actual molar ratio
of reagant to inlet NOx divided by the stoichiometric molar ratio of reagant to inlet NOx. The
efficiency of the reduction process is temperature dependent which usually requires multiple
injection points, as the temperature at any given point in the system tends to vary in proportion to
the heat content of the fuel source. By monitoring the temperature and injecting the reagant at the
appropriate locations, the effectiveness of the NOx reduction system can be maintained. Reduction
levels as high as 65 percent can be obtained.
d. Carbon Monoxide.
Carbon monoxide (CO) is a product of incomplete oxidation of carbon compounds. The control of
CO is by way of complete combustion. Federal Regulations 40CFR 60, Sect. 129 and 56FR 5488
set CO emission limits for all types of incinerators. Excerpts from the regulations are listed in table
5-2. Existing control systems and standard operating and maintenance practices are generally
sufficient to regulate CO emissions. Maintenance on the natural gas burners and their temperature
controllers, in the primary and secondary chambers, will assure that temperatures in these
chambers are maintained above the 1130-1215 F ignition temperature of CO. Bag house and
scrubber systems have no effect on CO emissions. High CO emissions reflect inefficiency in the
combustion process, incomplete destruction of organic compounds, and loss of energy that should
have been released in the combustion process. Also, there is a strong correlation between the
presence of CO above 50 ppm and the presence of chlorinated hydrocarbons (especially dioxins),
making close control of CO very important to minimize release of toxic chemical compounds.
e. Dioxins and Furans.