135
15
Thermal Nitrogen
Oxide (NO
x
) Control
Joseph Colannino
John Zink Company, LLC, Tulsa, Oklahoma
15.1 DEVICE TYPE
The control of nitrogen oxides (NO
x
) using thermal methods encompasses a variety
of devices. This chapter focuses on NO
x
and its control using combustion modica-
tions, postcombustion thermal and catalytic methods, and combinations thereof.
15.2 TYPICAL APPLICATIONS AND USES: COMBUSTION SOURCES
Various combustion sources produce NO
x
. Boilers use a burner to combust the fuel
and release heat. The heat boils water and generates steam. Larger boilers usually con-
tain the water and steam inside tubes (water-tube boilers) surrounding a rebox. Some
smaller boilers have a combustion tunnel surrounded by water (re-tube boilers). The
water-tube boiler has an analog in the petroleum renery—the process heater.
The process heater is used to heat or transform a process uid, for example,
crude oil. Analogous to the water-tube boiler, the process uid is pumped through
tubes surrounding a rebox. Most boilers are heated with burners in the horizontal
direction. Process heaters are often red with the burners in the oor. However,
some process heaters are wall red, and some specialty reactors such as reform-
ers are down red from the roof. Process heaters may be tall, round oor-red
units (known as vertical cylindrical [VC] heaters) or rectangular units known as
cabin type, which are often oor red but may also be wall red. Some specialty
heaters, such as ethylene cracking furnaces and reformers, use heat to chemically
transform the process uid.
Gas turbines and reciprocating engines transform heat into mechanical motion.
Hazardous waste incinerators use high temperatures to destroy waste products. All
conventional combustion processes form NO
x
.
15.3 OPERATING PRINCIPLES
Nitrogen oxides are criteria pollutants as classied by the Environmental Protection
Agency (EPA). Accordingly, the EPA has established National Ambient Air Quality
Standards (NAAQS). Local air quality districts translate the NAAQS into local reg-
ulations for various combustion sources. These regulations vary widely from region
136 Air Pollution Control Equipment Selection Guide
to region. The purpose of this chapter is to show how NO
x
is formed and to discuss
some methods for ameliorating it.
NO
x
is generated from combustion systems in three ways. The mechanisms are
referred to as thermal (Zeldovich), fuel bound, and prompt (Fenimore).
15.4 PRIMARY MECHANISMS USED
NO
x
may be reduced at the source (combustion modication) or after the fact (post-
combustion treatment). Combustion modications comprise thermal strategies, stag-
ing strategies, and dilution strategies. Postcombustion methods comprise ue-gas
treatment techniques described in Sections 15.5.2 and 15.6.
15.5 DESIGN BASICS
15.5.1 Different forms of no
x
Nitric oxide (NO) is the most predominant form of NO
x
. Most boilers and process
heaters generate more than 90% of NO
x
as NO. However, gas turbines and other
combustion systems that operate with lots of extra air can generate signicant quan-
tities of visible nitrogen dioxide (NO
2
). NO
2
is a reddish-brown color and responsible
for the brown haze called smog. NO, although odorless, oxidizes slowly to NO
2
in the
atmosphere. Hence most NO
x
requirements are given as NO
2
equivalents.
Hydrocarbons and NO
x
react to ground-level ozone. Ozone at high altitude is good
because it lters out harmful ultraviolet rays. Ozone at ground level is bad because
it interferes with respiration, especially for sensitive individuals such as asthmatics
and the elderly. The complicated chemistry among ozone, NO
x
, and hydrocarbons is
why hydrocarbons and NO
x
are strictly regulated. Carbon monoxide (CO) can also
participate in the chemistry and is also a regulated pollutant.
15.5.2 no
x
measurement units
NO
x
is measured in a variety of differing units depending on the source. For example,
NO
x
from most boilers is regulated as volume concentrations at a reference oxygen con-
dition, for example, 100 parts per million, dry volume, corrected (ppmvdc) to 3% O
2
.
Most NO
x
meters analyze their samples after water is condensed. Failure to condense
the water before measurement in a dry analyzer could damage the analyzer. Such ana-
lyzers are known as extractive analyzers because they must rst extract a sample from
the stack, condense the water, and then send the dry conditioned sample to the analyzer.
In situ analyzers read NO
x
directly in the hot wet stream. Figure15.1 shows an analyzer
designed to measure the NO
x
content in situ and report the result in meaningful NO
x
units. It uses a nondispersive infrared beam and optical measurement techniques.
The most popular type of postcombustion treatment is selective catalytic reduc-
tion (SCR). Ammonia or urea is injected in the ue gas near a catalyst. The net reac-
tion is as follows:
2NO + 0.5 O
2
+ 2NH
3
2N
2
+ 3 H
2
O

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