Air Pollution Control Equipment Selection Guide, 2nd Edition

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