United States
Environmental Protection
Agency
Research and Davelopment
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
EPA/600/S2-91/029 Aug. 1991
Project Summary
Sourcebook: NOX Control
Technology Datax
L. M. Campbell, O.K. Stone, and G.S. Shareef
Available Information on control of
NOx emissions from stationary combus-
tion sources has been compiled to as-
sist new source permitting activities by
regulatory agencies. The sources cov-
ered are combustion turbines, Internal
combustion engines, non-utility boilers
and heaters, and waste Incinerators.
The report discusses the background
of NO, formation in the combustion pro-
cess, major NO sources, and processes
for NOx control. The current status of
NO control technology Is discussed
and applications to meet permitting re-
quirements Is detailed. Permitted NOX
emission levels are summarized by
combustion source, fuel type and con-
trol technology. Documentation in-
cludes references and contacts for fur-
ther Information.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that Is fully
documented In a separate report of the
same title (see Project Report ordering
Information at back).
Introduction
Emission of NOX is an environmental
issue which has been attracting increas-
ing regulatory attention at the local, state,
and Federal level. Concerns over ozone
abatement and control, acid rain, the
growth in stationary source combustion
systems, and implementation of preven-
tion of significant deterioration (PSD) in-
crements for NOX have all contributed to
the increased attention. A number of
states, in addition to California, are now
regulating (or are considering regulating)
NOx emission sources stringently.
A wide variety of new and emerging
NOX control technologies are being mar-
keted in the U.S., as well as in Europe
and Japan. The performance reported by
vendors for many of these systems have
not been well documented in the public
literature. Little primary data are available
to demonstrate the actual applicability and
long-term operating performance of the
current generation of NOX control tech-
nologies.
There is interest among state and local
regulatory agencies as to the level of ,NOX
control that can be achieved with current
technology. State and local officials are
not always familiar with some of the tech-
nologies currently being marketed and of-
ten have no basis for critically reviewing
permit applications for major NOx emis-
sion sources. In some states, the "permit-
ting process has been slowed by the un-
certainty over the availability and perfor-
mance of NOx. This has resulted in many
requests to the U.S. Environmental Pro-
tection Agency (EPA) for assistance in
the NOx control (BACT) area. As part of
an effort to respond to these requests,
EPA's Control Technology Center (CTC)
initiated the current study to develop a
reference book of information on NO con-
trols. This report is that book. "
NOX Formation In Selected
Combustion Sources
Combustion Turbines
Gas turbines are rotary internal com-
bustion engines fueled by natural gas, die-
sel or distillate fuel oils (occasionally re-
Printed on Recycled Paper
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sWual or crude oils). The baste gas tur-
bine consists of a compressor, combus-
tion chambers, and a turbine. The com-
pressor delivers pressurized combustion
air to the combustors at compression ra-
tios of up to 20 to 1. Injectors introduce
fuel into the combustors and the mixture
is burned with exit temperatures up to
1100°C. The hot combustion gases are
rapidly quenched by secondary dilution
air and then expanded through the turbine
which drives the compressor and provides
shaft power. In some applications, ex-
haust gases are also expanded through a
power turbine.
NO, is the primary pollutant produced
by combustion turbines. Factors affecting
NO, formation include turbine design, am-
bient conditions, turbine load, and fuel type.
The design parameters that affect the pro-
duction of NO, in the combustor most
significantly are combustor inlet tempera-
ture and pressure and the firing tempera-
ture. As predicted by the Zeldovich equa-
tions, NO emissions rise rapidly with in-
creasing tiring temperatures. NO, forma-
tion is also directly related to combustor
pressure: increased pressure results in
increased NO, emissions.
Turbine efficiency is determined largely
by the combustor firing temperature and
pressure. Therefore, turbine models with
regenerators, which are used to increase
efficiency byincreasing the combustor in-
let air temperature, have higher NO, emis-
sions than the same models without re-
generation. Many cogeneration/combined-
cycle systems also employ natural gas-
fired duct burners for exhaust tempera-
ture control.
Turbine NO emissions change with
changes in ambient temperature, pressure,
and especially humidity. Increases in hu-
midity have a quenching effect on the
combustor zone peak temperature, thereby
reducing thermal NO, formation. The ef-
fect of temperature on NO, emissions
changes as humidity changes. At low
humidity, NO, levels increase with an in-
crease in temperature. At high humidity,
the effect on NO, formation depends on
the range over which the temperature
changes. Increases in ambient pressure
increase gas compressor outlet pressures
which In turn increase NO, emissions. For
example, NO, emissions from a turbine
installed at an elevation of one mi (1.6km)
above sea level decrease by about 10 %
compared to a similar turbine operating at
sea level.
Stationary Combustion (1C)
Engines
Stationary reciprocating 1C engines use
two methods to ignite the fuel/air mixture
in the combustion chamber: (1) compres-
sion ignition (Cl) fueled by diesel oil or a
combination of natural gas and diesel oil
(dual), or (2) spark ignition (SI) fueled by
natural gas or gasoline.
In Cl engines, air is first compression
heated in the cylinder, and the diesel fuel
is injected into the hot air where ignition is
spontaneous. In SI engines, combustion
is spark initiated with the natural gas or
gasoline being introduced either by injec-
tion or premixed with the combustion air
in a carburetted system. Either 2- or 4-
stroke power cycle designs with various
combinations of fuel charging, air charg-
ing, and chamber design are available.
Due to the high flame temperatures and
pressures of 1C engines, most of the NO
formed is thermal. As diesel fuel and
natural gas are the predominate fuels for
this source, little fuel NO, is formed, ex-
cept in engines that fire residual and/or
crude oils. Formation of prompt NO, is
also negligible in Cl engines which oper-
ate with large amounts of excess air.
When fuel is injected into the cylinder, it
undergoes a series of reactions that lead
to ignition. The time between the start of
injection of the fuel and the start of com-
bustion (as measured by the onset of en-
ergy release) is called the ignition delay.
Initial combustion occurs around the pe-
riphery of the fuel jet, where the air/fuel
ratio is close to the stoichiometric ratio.
During ignition delay, some of the fuel
is pre-mixed with air and evaporates. Af-
ter ignition occurs, the pre-mixed charge
burns extremely rapidly, thereby quickly
releasing energy. Most of the burning
takes place as a diffusion flame after the
pre-mixed charge has burned.
NO, emissions are directly affected by
the amount of pre-mixing which, in turn, is
a function of the ignition delay. When the
ignition delay is large, there is more pre-
mixing and a greater energy release rate
at the start of combustion. This generally
leads to higher temperatures and, accord-
ingly, higher NO, emissions.
Non-Utility Boilers and Heaters
Industrial boilers are typically classified
by the type of firing mechanism employed,
the heat transfer mechanism, and the type
of fuel fired. Firing mechanisms include
burners, spreader-fed, or mass-fed. With
burners, the fuel is injected into the boiler
through a nozzle and burns while sus-
pended within the boiler combustion cham-
ber. Mass-fed and spreader-fed boilers
are used for most solid fuel industrial boil-
ers. They combust the fuel on a grate in
the boiler.
Watertube is the most common mecha-
nism used for heat transfer in industrial
boilers. In watertube boilers, the water for
steam generation is contained in banks of
tubes suspended in the boiler combustion
chamber and flue. Firetube boilers invert
this configuration and pass hot flue gas-
ses through tubes suspended in a water
drum.
Industrial boilers are fired with a wide
variety of fossil and nonfossil fuels, in-
cluding natural gas, fuel oil, and coal.
Nonfossil fuels fired in industrial boilers
include wood, bark, agricultural waste,
municipal waste, and industrial waste.
Many designs of fired heaters are avail-
able. All fired heaters have a radiant
section, and most have a convection sec-
tion. The radiant section is in the firebox
and contains the burners and a single row
of tubular coils. The primary heating of
the feedstocks occurs in the radiant sec-
tion. As the name implies, radiation is the
primary method of heat transfer.
Combustion air preheaters are often
used to improve the efficiency of a fired
heater. In the air preheater, heat is trans-
ferred from the flue gas to the combustion
air. Therefore, less heat,is required to
heat the combustion air which allows a
greater proportion of the;total heat re-
leased to be absorbed in the radiant sec-
tion. Also, less fuel is required to reach
the required combustion temperature. In
addition, the preheater raises the adia-
batic flame temperature above that of am-
bient air heaters.
NO, emissions from boilers and fired
heaters depend on such design and oper-
ating parameters as fuel type, burner type,
combustion air preheat, firebox tempera-
ture, draft type, excess air level, and heater
load.
The most important design parameter
affecting NO emissions from a boiler/fired
heater is fuel type (i.e., the nitrogen con-
tent of the fuel). Coal-firing and gas-firing
can be expected to generate higher NO,
emissions per unit of energy input than
comparable oil-fired units. In addition,
fluctuations in fuel composition and heat-
ing value may affect NO, emissions.
Another design factor having a large
effect on NO, emissions is burner type.
Oil-fired burners differ primarily in the meth-
ods used to atomize the oil prior to com-
bustion. Steam atomization is used al-
most exclusively in fired heaters. Steam-
atomized oil burners can be divided into
two categories: conventional and staged
combustion air oil burners. Conventional
oil burners have a single combustion zone
in which all of the air is fed to the atom-
ized fuel oil. Staged combustion air oil
burners make use of at least two air injec-
tion sections. Decreased NO, emissions
are achieved with these burners by oper-
-------
ating a primary air/fuel injection section at
substoichiometric air conditions and in-
jecting secondary air downstream of the
primary section to complete combustion.
The use of combustion air preheat in-
creases the amount of NOX formed by
virtue of increased flame temperature. In-
creasing the degree of preheat will like-
wise increases NO, emissions.
The firebox temperature required for a
given application influences NO,, emissions
because of the relation between firebox
temperature and flame temperature. High
firebox temperature applications are ex-
pected to have higher NOX emissions. The
fractional use of firebox capacity can re-
duce NOx emissions by lowering firebox
temperature. An increase in the excess
air level of a fired heater under typical
operating conditions results in an increase
in NOX emissions due to the resulting
higher peak flame temperature.
Waste Incinerators
The most common type of refuse incin-
erator consists of a refractory-lined cham-
ber with a grate upon which refuse is
burned. Combustion products are formed
by heating and burning of refuse on the
grate. The most prevalent types of sew-
age sludge incinerators are multiple hearth
and fluidized bed units. In multiple hearth
units, the sludge enters the top of the
furnace where it is first dried by contact
with the hot, rising combustion gases, and
then burned as ft moves slowly down
through the lower hearths. At the bottom
hearth, any residual ash is then removed.
In fluidized bed reactors, the combustion
takes place in a hot, suspended bed of
sand with much of the ash residue being
swept out with the flue gas. Tempera-
tures in a multiple hearth furnace are
300°C in the lower, ash cooling hearth;
750 to 1100°C in the central combustion
hearths, and 550 to 650°C in the upper,
drying hearths. Temperatures in a fluid-
ized bed reactor are fairly uniform, from
675 to 800°C. In both types of furnace,
an auxiliary fuel may be required either
during start-up or when the moisture con-
tent of the sludge is too high to support
combustion.
Influencing the production of fuel NOX
in an incinerator are the distribution of the
combustion air (underfire versus overfire),
the fuel nitrogen content, and the total
excess air rates. Thermal NOX formation
rates increase with temperature, oxygen
availability, heat release rate, and flue gas
residence time at high temperature.
The relative contribution of fuel and ther-
mal NOX to the total NOX emitted from an
incinerator depends on the design and
operation of the furnace and the nitrogen
content of the refuse burned. Testing has
demonstrated a seasonal increase in NOX
emissions during the summer months. It
is theorized that the higher emissions are
due to the higher nitrogen content of the
fuel because the raw refuse contains more
yard waste, which has a high nitrogen
content. At temperatures less than
1100°C1 typical of municipal solid waste
incinerators, NOf emissions are influenced
mainly by oxidation of fuel nitrogen. Thus,
generally, 75-80 % of the total NO emit-
ted from incinerators may be fuel NOX.
Combustion Control of NO
The key parameters controlling the rate
of NOX formation for a given fuel and com-
bustor design are the local oxygen con-
centration, temperature, and time history
of the combustion products. Each of these
parameters, within the temperature range
of NOX formation, is determined by the
system design and operation. The com-
bustion system, therefore, determines the
NOX formed and represents the only con-
trol capability for reducing the rate of NOx
formation. Techniques concerned with re-
ducing NOX formation are applied in this
region and are collectively referred to as
"combustion controls." All other NOX con-
trol techniques applied in downstream
zones work to reduce the NO formed
during combustion and are called post-
combustion controls.
The combustion controls discussed be-
low are grouped according to the follow-
ing source categories: combustion tur-
bines, 1C engines, and other. Combustion
turbine controls include dry control and
wet injection. 1C engine controls include
injection timing retard, pre-ignftton cham-
ber combustion, air to fuel ratio, and wet
injection. Controls applicable to other com-
bustion sources (including boilers, process
heaters, and municipal waste incinerators)
include low excess air, low NOX burners,
overfire air, burners out of service, flue
gas recirculation, reburn, reduced com-
bustion air temperature, and derating/load
reduction.
Combustion Turbine NOX
Control
Extensive progress has been made in
commercializing dry low NO combustors
for natural gas-fired gas turbine applica-
tions. NOx reductions of up to 60 % have
been achieved, and in special instances,
reductions of over 80 % have been re-
ported. Emissions in the range of 25-50
ppm at 15 % O2 have been achieved for
large, natural gas-fired heavy duty tur-
bines. However, additional development
is. needed prior to commercialization of
oil-fired applications.
Although combustors are proprietary,
similarities among designs can be noted.
The technologies are based on reducing
the flame turbulence and intensity, en-
hancing the fuel/air mixing, and establish-
ing fuel-lean zones in the combustor. They
generally rely on staged combustion with
the first stage used as a pilot burner fol-
lowed by a second stage of multiple fuel
injection nozzles. The second stage often
burns a lean, premixed fuel/air mixture to
ensure a uniform mixture and avoid high
temperature regions in the combustor. The
control of the two burner zones and the
preparation and control of premix air com-
plicate the combustor control systems.
The control system can be based on us-
ing variable geometry, and/or variable air
flow scheduling.
The principal requirements in using wet
injection for NOX reduction are to inject
sufficient water or steam at the proper
location in the flame envelope and, with
appropriate dispersion, to reduce the peak
flame temperature without degrading com-
bustion efficiency. Although it is easier to
ensure uniform mixing if steam is used,
water can be effective as well.
The major factor affecting NO reduc-
tion is the water/steam-to-fuel ratio. The
NOx reduction achievable for a particular
unit is directly related to the amount of
water or steam which can be injected be-
fore serious impacting combustor perfor-
mance. The impacts include flameouts,
reduced thermal efficiency expressed as
a heat rate penalty, large increases in
carbon monoxide (CO) and hydrocarbon
(HC) emissions, and pressure pulsations
which result in significantly reduced com-
bustor reliability. Higher corrosion rates
may also be experienced. The highest
ratio sustainable will vary depending on
the tradeoff between NOX, emissions and
CO, and HC emissions and combustor
design characteristics.
Water/steam can be injected in a vari-
ety of ways ranging from premixing with
the intake air or fuel to injecting it directly
into the combustor. The effectiveness of
the method is a function of the atomiza-
tion and mixing of the water/steam within
the combustion charge. Wet injection is
applicable to gas or liquid fuels.
Stationary 1C Engine Controls
Ignition in a normally adjusted 1C en-
gine is set to occur shortly before the
piston reaches its uppermost position (top
dead center, or TDC). At TDC, the air or
air/fuel mixture is at maximum compres-
sion. The timing of the start of injection or
of the spark is given in terms of the num-
ber of degrees that the crankshaft must
still rotate between this event and the
arrival of the piston at TDC.
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Retarding the timing beyond TDC. the
point of optimum power and fuel consump-
tion, reduces the rate of NOX production.
Retarding causes more of the combustion
to occur during the expansion stroke, thus
lowering peak temperatures, pressures,
and residence times. Injection timing re-
tard is an applicable control with all 1C
engine fuels.
The use of a pre-ignition chamber can
improve fuel efficiency and reduce NOX
emissbns. The system is designed to
burn lean fuel/air mixtures. The fuel charge
is introduced into the pre-chamber as a
rich mixture and ignited by a spark plug.
Since it burns in the absence of excess
oxygen, NOS formation is inhibited. This
"torch" of burning fuel expands into the
power cylinder where it thoroughly ignites
a lean mixture at reduced temperatures.
Therefore, combustion is completed in an
overall lean mixture at temperatures which
are adequate for combustion but below
those where NO forms. This NOX control
is currently applied to gas-fired engines
only.
In injection type engines, which include
all diesel and many dual fuel and gas
varieties, the air to fuel ratio for each
cylinder can be adjusted by controlling the
amount of fuel that enters each cylinder.
These engines are therefore operated lean
where combustion is most efficient and
fuel consumption is optimum.
The most practical use of air-to-fuel ra-
tio adjustment as a control technique is to
change the setting toward leaner opera-
tion. The oxygen availability will increase
but so will the capability of the air and
combustion products to absorb heat. Con-
sequently, the peak temperature will fall,
resulting in lower NO, formation rates. The
limiting factor for lean operation is the
increased emissions of HC at the lower
temperatures.
Carbureted engines are beset by large
variations in cylinder air-to-fuel ratios.
Therefore, they must operate near the stoi-
chfometric ratio to ensure that no indi-
vidual cylinder receives a charge which is
too lean to ignite.
As with combustion turbines, wet injec-
tion can reduce NO emissions from 1C
engines. Wet control effectiveness corre-
lates inversely with excess air levels.
Since wet controls reduce peak tempera-
ture by increasing the charge mass, the
technique is more effective in a tow ex-
cess air system than in one with much
excess air and, hence, much thermal
mass. Presumably, systems with high
excess air absorb all the heat that can be
transferred to a fluid in the short time
between combustion and peak tempera-
ture. Because of difficulty of water injec-
tion, use of water/fuel emulsions is under
development for 1C engine applications,
but not currently available.
NOX Control for Other
Combustion Sources
There are two fundamental ways of con-
trolling flame stoichiometry: regulating the
overall fuel/air ratio supplied (low excess
air), or gross staging of combustion (low
NOX burners, overfire air ports, removing
burners from service, derating, reburn).
In addition to being influenced by burner
design and fuel/air stoichiometry, the flame
temperature can be reduced from its peak
value by the introduction of heat absorb-
ing inerts (recirculated flue gas) or by re-
duction in the temperature of the combus-
tion air supplied to the burners.
The NOX combustion controls discussed
below are applicable to all of the combus-
tion sources grouped collectively as "other"
including non-utility boilers, process heat-
ers, and waste incinerators.
For all conventional combustion pro-
cesses, some excess air is required to
ensure that all fuel molecules can find
and react with oxygen. In the low excess
air (LEA) approach to NO^ control, less
excess air (oxygen) is supplied to the com-
bustor than normal. The lower O2 con-
centration in the burner zone reduces the
flame temperature and the formation of
thermal NOX. In the starved-air flame zone,
fuel bound nitrogen is converted to nitro-
gen thus reducing formation of fuel NOX.
The limiting criteria which define minimum
acceptable excess air conditions are in-
creased emissions of CO and visible
plume, and a reduction in flarne stability.
New designs and most existing com-
bustion operations will incorporate LEA
firing as standard practice. LEA operation
has an economic incentive since it results
in increased fuel efficiency. The reduced
airflow decreases the volume of combus-
tion air to be heated, allowing more heat
of combustion to be transferred, thus low-
ering fuel requirements for a given-output.
LEA operations may be used as the pri-
mary NOX control method or in combina-
tion with other NOX controls such as low
NO burners, overfire air, or flue gas
recifculation. LEA operations are appli-
cable to all combustion sources/fuels.
Low NOX burners control NOX formation
by carrying out the combustion in stages.
These burners control the combustion
staging at and within the burner rather
than in the firebox. Low NOX burners are
designed to control both the stoichiomet-
ric and temperature histories of the fuel
and air locally within each individual burner
flame envelope. This control is achieved
through design features which regulate
the aerodynamic distribution and mixing
of the fuel and air.
As with overfire air, the burner staging
delays combustion and reduces the peak
flame temperature, thus reducing thermal
NOX formation. The sub-stoichiometric
oxygen introduced with the primary com-
bustion air into the high temperature, fuel
nitrogen evolution zone of the flame core
reduces fuel NOX formation. Thus, low
NO burners are effective for reducing NOX
emissions independent of fuel. There are
two distinct types of designs for low NO
burners: staged air burners and staged
fuel burners.
Staged air burners have been in ser-
vice since the early 1970s and were the
first type of burner designed to specifically
reduce NO emissions. They are designed
to reduce flame turbulence, delay fuel/air
mixing, and establish fuel-rich zones for
initial combustion. The reduced availabil-
ity of oxygen in the initial combustion zone
inhibits fuel NOX conversion. Radiation of
heat from the primary combustion zone
results in reduced temperature as the fi-
nal unburned fuel gases mix with excess
air to complete the combustion process.
The longer, less intense flames resulting
from the staged stoichiometry lower flame
temperatures and reduce thermal NOX for-
mation. Staged air burners are effective
with all fuels.
A more effective concept for NO reduc-
tion is staging of the fuel rather than the
air. Staged fuel burners mix a portion of
the fuel and all of the air in the primary
combustion zone. The high level of ex-
cess air greatly lowers the peak flame
temperature achieved in the primary com-
bustion zone, thereby reducing formation
of thermal NOX. The secondary fuel is
injected at high pressure into the combus-
tion zone through a series of nozzles which
are positioned around the perimeter of the
burner. Because of its high velocity, the
fuel gas entrains furnace gases and pro-
motes rapid mixing with first stage com-
bustion products. The entrained furnace
gases simulate flue gas recirculation. Un-
like the staged air burner, staged fuel burn-
ers are designed only for gas firing.
An overfire air (OFA) system uses con-
ventional burners to introduce the fuel and
sub-stoichiometric quantities of combus-
tion air (primary air). The remaining com-
bustion air (secondary air) is introduced
approximately 1-3 of the distance down
the firebox through overfire air ports. The
overfire air system reduces NOX formation
by two mechanisms: (1) Staging the com-
bustion air partially delays the combustion
process, resulting in a cooler flame and
suppressed thermal NOx formation: and
(2)staging of the combustion air also al-
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lows deprivation of oxygen and less mix-
ing of fuel and air in the combustion re-
gion where fuel nitrogen evolves, thereby
reducing fuel NOX formation. This method
is effective with all fuels.
Burners out of service is a variation of
the staged combustion technique for re-
duction of NOX formation. It is a low cost
retrofit NOX control measure for existing
fireboxes. Ideally, all of the fuel flow is
diverted from a selected number of burn-
ers to the remaining firing burners. Since
airflow is maintained unchanged among
all the burners, a staged combustion ef-
fort is obtained. The fuel-admitting burn-
ers fire more fuel-rich than normal, with
the remaining air required for combustion
being admitted through the inactive burn-
ers. NOX is reduced by decreasing the
excess air available in the active burner
zone. This reduces both fuel and thermal
NO formation and, thus, is applicable to
all fuels.
Reburn, also referred to as in-furnace
NOX reduction or staged fuel injection, is
the only NOX control approach imple-
mented in the furnace zone - defined as
post-combustion, preconvectton section.
Reburning involves passing the burner
zone products through a secondary flame
or fuel-rich combustion process. This pro-
cess is designed to reduce NOX formation
without generating CO emissions. This
NOX control approach diverts a fraction of
the fuel to create a secondary flame or
fuel-rich zone downstream of the burner
(primary combustion zone). Sufficient air
is then supplied to complete the oxidation
process. These reactions are NOX form-
ing as well as NOX reducing. Laboratory
results indicate that a maximum reduction
in NOX is achieved when the reburning
zone stoichiometry is approximately 0.9
(90% theoretical air).
The burner pattern plus overf ire air ports
provides an existing, potential capability
to implement the reburning NOX control
approach. In fact, the burners out of ser-
vice mode of operation implemented on
some units to achieve fuel-rich primary
combustion may also result in partial
reburning. LEA and flue gas recirculation
(FGR) controls are combustion modifica-
tion techniques often combined with
reburning.
FGR is based on recycling a portion of
cooled flue gas back to the primary com-
bustion zone. The FGR system reduces
NOX formation by two mechanisms: (1)
the recycled flue gas is made up of com-
bustion products which act as inerts dur-
ing combustion of the fuel air mixture (this
additional mass is heated and lowers the
peak flame temperature, thereby reducing
the amount of thermal NO formation); and
(2) to a lesser extent, FGR also reduces
thermal NOx formation by lowering the oxy-
gen concentration in the primary flame
zone (the decrease in flame temperature
alters the distribution of heat and bwers
the fuel efficiency).
The recycled flue gas may be pre-mixed
with combustion air or injected directly
into the flame zone. Because FGR benefi-
cial effects are limited to reduction of ther-
mal NOX, the technique is applied prima-
rily to natural gas or distillate oil combus-
tion, in these applications, the thermal
NO component is virtually 100% of the
total NOX. The amount of recirculation is
limited by flame stability. Typically, 15-
20% is employed. FGR is more adapt-
able to new designs than as a retrofit
application.
Other techniques discussed in the re-
port include reduced combustion air tem-
perature, derating, and load reduction.
Post-Combustion NOX Control
This section describes NOX reduction
techniques applied downstream of the
combustion zone to reduce the NO formed
during the combustion process. Tne three
post-combustion NOX control technologies
described include selective catalytic re-
duction (SCR), non-selective catalytic re-
duction (NSCR), and selective noncatalytic
reduction (SNCR).
In the SCR process, ammonia (NH3),
usually diluted with air or steam, is in-
jected through a grid system into the flue/
exhaust gas stream upstream of a cata-
lyst bed. On the catalyst surface, the NH3
reacts with NOx to form molecular nitro-
gen and water. Depending on system
design, NOX removal of 80-90% and higher
is achievable. The reaction of NH3 and
NOX is favored by the presence of excess
oxygen (air-rich conditions). The primary
variable affecting NO reduction is tem-
perature. Optimum NOX reduction occurs
at catalyst bed temperatures between 300
and 400°C for conventional (vanadium or
titanium-based) catalyst, and 240 and 265°
for platinum catalysts. The catalyst se-
lected depends largely on the tempera-
ture of the flue gas being treated. A given
catalyst exhibits optimum performance
within a temperature range of ±30°C. Be-
low this optimum temperature range, the
catalyst activity is greatly reduced, allow-
ing unreacted NH3 to slip through. Above
450°C, NH3 begins to be oxidized to form
additional NO . The NH3 oxidation to NOX
increases with increasing temperature?
Depending on the catalyst substrate ma-
terial, the catalyst may be quickly dam-
aged due to thermal stress at tempera-
tures in excess of 450°C. It is important,
therefore, to have stable operations and
thus uniform flue gas temperatures for
this process to be an effective NO con-
trol.
A new family of zeolite catalysts has
been developed and is in use in the U.S.
These can function at higher tempera-
tures than conventional catalysts. Zeo-
lites are claimed to be effective over the
range of 300 to 600°C, with the optimum
temperature range states as 360 to 580°C.
Ammonia oxidation to NOX begins at
around 450°° and is predominant at tem-
peratures in excess of 500°C. Zeolites
suffer the same performance and poten-
tial damage problems as conventional
catalysts when used outside their opti-
mum temperature range. In particular, at
around 550°C, the zeolite structure may
be irreversibly degraded due to loss of
pore density. Zeolite catalysts have not
been continuously operated commercially
at temperatures above 510°C due to NH3
oxidation to NOx and potential damage to
the catalyst.
With zeolite catalysts, the NOX reduc-
tion reaction takes place inside a molecu-
lar sieve ceramic body rather than on the
surface of a metallic catalyst. This differ-
ence is reported to reduce the effect of
particulates/soot, SO /SO, conversion,
heavy metals, etc. which poison, plug, and
mask metal catalysts. These catalysts
have been in use in Europe since the
mid-1980s, with approximately 100 instal-
lations on-stream. Process applications
range from gas to coal fuel. Typically,
NO levels are reduced 80-90%.
Non-selective catalytic reduction sys-
tems are often referred to as "three-way
conversion" catalyst systems since they
reduce NOX, unburned hydrocarbon, and
CO simultaneously. To operate properly,
the combustion process must be with an
air-to-fuel ratio slightly fuel-rich of stoi-
chiometric. Under this condition, in the
presence of the catalyst, the NOX are re-
duced by the CO, resulting in nitrogen(N2)
and carbon dioxide (CO2).
The catalyst used to promote this reac-
tion is generally a mixture of platinum and
rhodium. The catalyst operating tempera-
ture limits are 350 to 800°C, with 425 to
650°C being the most desirable. Tem-
peratures above 800°C result in catalyst
sintering.
Typical NOX conversion ranges from 80-
95 % with corresponding decreases in
CO and HC. Potential problems associ-
ated with NSCR applications include cata-
lyst poisoning by oil additives (e.g., phos-
phorous, zinc) and inadequate control sys-
tems.
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There are two commercially available
selective non-catalytic reduction pro-
cesses. Thermal DeNO ™ (TON) devel-
oped by Exxon is an add-on NO control
technique which reduces NOX to N, with-
out the use of a catalyst. The TON pro-
cess injects gaseous NH3 to react with
NO, in the air rich flue gas. The NH, and
NO, react according to:
2 NO + 4 NH,
202— >3N2 + 6H20
(1)
4NH3 + 50a— >4NO + 6H20 (2)
Temperature is the primary variable for
controlling the selective reaction. In the
temperature range of 870 to 1200°C, Re-
action (1) dominates, reducting NOx.
Above1200°C, Reaction (2) dominates, in-
creasing NO production. Below 870°C.
neither reaction is of sufficient activity to
either produce or destroy NO^. NOX re-
duction performance is maximized in the
relatively narrow temperature range of
870 -1040°C, with an optimum tempera-
ture of about 950°C. The 170°C reaction
window can be lowered to a range of 700
- 800 °C by introducing hydrogen, a readily
oxidizable gas.
Without using a catalyst to increase the
reaction rates, adequate time at optimum
temperatures must be available for the
NO, reduction reaction to occur. Design
considerations should allow ample resi-
dence time and good mixing in the re-
quired temperature range. Long residence
times (>1 sec) at optimum temperatures
will tend to promote relatively high perfor-
mance even with less than optimum initial
mixing or temperature/velocity gradients.
However, when the NH3 injection zone is
characterized by tow temperatures and/or
steep temperature declines, a loss of pro-
cess efficiency results.
The initial ratio of NH3 injected to NOX
concentrations is another parameter to
control the process. Maximum NOX re-
ductions (40-60%) require 1.5-2.0 NHJNOx
injection ratios. At these ratios, significant
concentrations of NH3 can exit the con-
vective zone, creating corrosive ammo-
nium sulfates and/or a visible NH3 or NH4CI
stack plume. (Unreacted ammonia emis-
sions from a Thermal DeNOx™ system
are usually higher than from SCR sys-
tems.)
Selection of the optimum NH3 injection
location also affects NOX reduction perfor-
mance and NH slip. In most Thermal
DeNOx applications today, the injection
grids are being replaced by wall injectors.
The temperatures and velocity profiles will
change significantly with load, requiring
multiple NH injection points to achieve
the desired NOX reduction for a range of
operating loads.
In the NOxOUT™ process, a urea type
(or amine salt) compound is injected into
the oxygen-rich upper furnace and/or high
temperature convection section of a boiler
to promote NOx reduction. The exact
chemical mechanism is not fully under-
stood, but involves the decomposition of
urea - C(NH2)2O. The likely decomposi-
tion products include the amino(NH2)
groups. The reaction takes place at tem-
peratures of 950 to 1650°C.
Originally developed for the Electric
Power Research Institute (EPRI) in the
late 1970s, the process is currently li-
censed by Fuel Tech where proprietary
additives (that allow NOX reduction capa-
bility over a temperature range of 425-
1150°C) have been developed. As with
the other post-combustion NOx control sys-
tems, temperature is the primary control
variable for the selective reactions. NOX
reductions up to 80 % are achievable with
this technology. The performance of the
urea injection process is limited by the
time/temperature/flow characteristics of the
flue gas. Residence time in the tempera-
ture window and the urea-to-NOx ratio im-
pact the performance in a manner similar
to Thermal DeNOx. The NOX reduction
capability is limited by mixing because the
reaction times are relatively fast.
The capital costs associated with urea
injection tend to be less than those of NH3
injection. Urea is injected in liquid form,
eliminating the need for a compressor.
The hazards of NH3 storage are also alle-
viated. The lower capital costs are offset,
however, by higher operating costs; urea
is more expensive than NH3. The urea
injection process may better accommo-
date changing conditions due to varying
loads by altering the solution formulation,
in addition to multiple injection points for
varying temperature/load requirements.
NOX Control Applicability and
Effectiveness
Table 1 summarizes the applicability of
NOX controls discussed above for the four
source categories considered. As shown
inTable 1, combustion modifications and
selective catalytic reduction have been
applied universally . The report gives an
extensive breakdown of technologies ap-
plied by combustion source and fuel and
lists U.S., Japanese, and West German
installations.
Npx permit limits for turbines are sum-
marized in Table 2, by fuel type and con-
trol technology. Tables 3, 4, and 5 pro-
vide similar summaries for 1C engines,
boilers and heaters, and waste incinera-
tors, respectively.
The full report is intended to be a handy
desk reference for those involved in per-
mitting of stationary NOX sources. An
extensive vendor listing should provide
easy access to updated information.
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Table 1. NOX Control Technology Applicability '•"
Control Technology
Combustion Controls0
Post-combustion Controls
Selective catalytic
reduction
Combustion
Turbines
X
X
Stationary
Combustion
Engines
X
X
Boilers/
Heaters
X
X
Waste
Incinerators
X
X
Non-selective catalytic
reduction
Selective non-catalytic
reduction
• Refer to full report for facility-specific data.
b Commercial applications in U.S., Germany, and Japan.
c Includes a variety of combustion techniques to reduce NO, formation in the combustion zone depending on the source: wet injection, timing retard,
staged combustion, low excess air, burners out of service, flue gas recirculation, overfire air, dry combustion controls, reduced air preheat, dean bum,
and derating.
Table 2. No, Control Levels - Combustion Turbines **
Control Technology
Post-combustion Controls
Selective catalytic reduction
NO, Permits Limits (ppm NO, at 15% O.,, dry basis)
Natural Gas Oil Dual Fuel
Combustion Controls
Wet injection
Dry tow NOX
25-195
32-188
25-79 (gas)/40-129 (oil)
5-25°
10-25"
8-10 (gasyi2-18(oil)e
* Refer to full report for facility-specific data.
b Commercial applications in U.S. and Japan.
0 In combination with combustion controls.
d Units in Japan.
• Based on two units in Eastern U.S.
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Tablo 3. WO, Control Levels— Stationary Internal Combustion Engines **
NO, Permit Limits (g/hp-hr)
Control Technology
Natural Gas/
Dual Fuel
Oil
Other*
Combustion Controls
Injection timing retard
Pro-ignition chamber combustion
(clean bum)
Air to fuel ratio
Stratified charge
Turbocharging
Wot injection
Derating
Post-combustion Controls
Selective catalytic reduction
Non-selective catalytic
reduction
0.75-16.5
(-80-1,760 ppm)d
70-90% (U.S.)
96-190 ppmd
1.0-1.5
(110 to 160ppm)d
520-1,158 ppmd
1.5-2(160to210ppm)d
90-150 ppmd
(Japan)
1 Refer to full report for facility-specific data.
11 Commercial applications in U.S., West Germany, and Japan.
0 Landfill gas, refinery gas, process gas, digester gas.
d At 15% Oj. dry basis.
Table 4. NO, Control Levels — Boilers and Heaters '*
NO Emission Limits (ng/J)
Control Technology
Combustion Controls0
Coal Wood Gas
Conventional Fluid Bed
73-258 86-129 7.7-43
(40-145 ppm)d (50-75 ppm)d (5-30 ppm)d
Oil
52-164
(30-100 ppm)d
Low excess air
Overfire air
Rue gas ^circulation
Low NO, burners
Post-combustion Controls
Selective catalytic reduction
Selective non-catalytic
reduction
60-250 ppm 213 ppm
(Japan) (Germany site visit)
(25-65 ppm)d
13-21.5
(10-15ppm)d
15-30
(Japan)
21.5
(15 pom)"
25-50 ppm
(Japan)
64.5
(40 ppm)d
* Refer to full report for facility-specific data.
* Commercial applications in U.S., West Germany, and Japan.
e May involve concurrent application of more than one technology.
« At 15% Oa. dry basis.
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Table S. NO. Control Levels-Waste Incineration ••*
Control Technology
NO Emission Limit
(ng/J)
Combustion Controls
Boiler design
Overfire air
Post-combustion Control
Selective catalytic reduction
Selective non-catalytic reduction
34-344
< 21.5-43°
32-146
* Refer to full report for facility-specific information.
b Commercial applications in U.S. and Japan.
0 Japanese applications.
•frU.S. GOVERNMENT PRINTING OFFICE: IM1 - 548-028/40078
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L Campbell, D. Stone, and G. Shareef are with Radian Corp., Research Triangle
Park, NC 27709.
Charles B. Sedman is the EPA Project Officer (see below).
The complete report, entitled "Sourcebook: NOX Control Technology Data," (Order
No. PB91-217364/AS;Cost: $23.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487^4650
The EPA Project Off her can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/029
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