Originally appeared in: January 1994 issue, pgs 46-49
CHEMICAL ENGINEERING PROGRESS Reprinted with publisher’s permission.
AIR POLLUTION CONTROL
Low NOx Burners
temperature. Whether for a new furnace Staged-air burners lend themselves very
Air staging, fuel staging, and or a retrofit application, these burners well to external flue gas recirculation
must meet five major requirements. (FGR). In such designs, flue gas is
internal flue gas recirculation Operation with lower NOx formation: generally introduced into the primary
are among the design fea-
A flame pattern compatible with
tures that help reduce NOx furnace geometry; Easy maintenance and Staged Fuel Burners. The fuel gas is
accessibility; A stable flame at turndown injected into the combustion zone in two
emissions conditions; and the ability to handle a stages, thus creating a fuel-lean zone and
wide range of fuels. delaying completion of the combustion
process. The fuel supply is divided into
Burner Types primary fuel and secondary fuel in a ratio
Ashutosh Garg, that depends on the NOx level required.
Kinetics Technology Table 1 lists the types of burners The flame length of this type of burner is
International Corp. currently in use in chemical process about 50% longer than that of a standard
industries (CPI) plants and petroleum gas burner.
refineries. Figure 1 compares staged- Staged-fuel burners are ideal for
combustion burners with standard gas fuel gas fired natural draft applications.
Specifying the right requirements for low and oil burners.
NOx burners can significantly reduce Low Excess Air Burners. These burners
nitrogen oxides (NOx) emissions from a Staged-Air Burners. Combustion air is reduce NOx emissions by completing
furnace. Ultra Low NOx burners that can split and directed into primary and combustion with the lowest amount of
meet even the most stringent emission secondary zones, thus creating fuel-rich excess air possible, usually no more than
control limits imposed by some states, and fuel-lean zones. 5-8%. Increases in excess air result in
are now available and offer a very These burners are most suitable increases in NOx formation (Figure 2a).
attractive route to NOx reduction. for forced-draft
However, burner selection and liquid-fuel-fired
specification should be done very applications.
carefully, because burner operation has a Combustion air
direct effect on furnace performance. pressure energy
This article describes the various types of lends itself to
low NOx burners and outlines the main better control of
design parameters that must be the staging air
considered when selecting a burner flows. It ensures
system. a high enough air
Low NOx burners generally modify velocity to
the means of introducing air and fuel to produce good air-
delay the mixing, reduce the availability fuel mixing and a
of oxygen, and reduce the peak flame good flame.
Furnace Improvements Services
Most forced-draft burners have the
ability to operate at very low levels of
excess air. In a multiple-burner
installation, it is essential that all burners
receive equal amounts of air. This can be
achieved by simulating the flow profiles
in the ducts and burners. Flow
deficiencies and other irregularities can
then be detected and corrected using
splitters and vanes, ensuring equal air
distribution within +1%.
Flue Gas Recirculation Burners. In
these burners, 15-25% of the hot (300-
500oF) flue gas is circulated along with
combustion air. The flue gas acts as a
diluents, reducing flame temperature and
suppressing the partial pressure of
oxygen, thus reducing NOx formation.
Flue gas can be injected into burners
through a separate scroll into the primary
zone or mixed with incoming air.
External FGR can be used with natural
draft burners, although it is mostly used
with forced-draft preheated air burners.
In some new burner designs, flue gas
is internally re-circulated using the
pressure energy of fuel gas, combustion
air, or steam. This makes the operation of
burners simple and eliminates the FGR
fan and its controls, although burner size
Ultra Low NOx Burners. Several
designs are available today that combine
two NOx reduction steps into one burner
without any external equipment. These Figure 1: Off-stoichiometric
burners typically inappropriate staged air combustion can be achieved
with internal FGR or staged fuel with by air staging or fuel staging.
Courtesy of John Zink Co
internal FGR. In the former design, fuel
is mixed with part of the combustion air,
creating a fuel-rich zone. High pressure viscosity, and heating value.
atomization of liquid fuel or fuel gas Fuel Specification. Correct and accurate
creates FGR. The secondary air is routed fuel specifications are essential for Atomization Medium. For low NOx
by means of pipes or ports in the burner predicting NOx emissions. burners, steam is preferred as the
block to complete combustion and For gaseous fuels, the complete atomization medium over compressed
optimize the flame profile. analysis listing all the constituents is air, because higher quantities of steam
In staged-fuel gas burners with required, as well as any possible decrease the amount of NOx in the flue
internal FGR, fuel gas pressure induces variations in gas composition. Major gas. Increases in steam temperature
recirculation of flue gas, creating a fuel components affecting NOx emissions are increase NOx emissions.
lean zone and a reduction in oxygen hydrogen and hydrocarbons in the C3-C4
partial pressure. range. Other physical properties, such as Fuel Filters. Staged fuel gas burners
The former design can be used with pressure, temperature, and heating value have more gas tips and rises than
the liquid fuels, whereas the latter design are required for burner design. standard burners, and the fuel gas flow
is used mostly for fuel gas applications. For liquid fuels, the most important per tip is reduced to as low as one-fourth.
parameter is the fuel’s nitrogen content – It is important that these burners be used
Design Parameters about 40-90% of the fuel nitrogen shows with clean fuel gases. To accomplish this,
The following parameters require up as NOx in the flue gas. Other liquid installation of fuel gas filters and
attention during system design. fuel parameters required by burner knockout pots to remove particulates and
vendors are pressure, temperature, condensate is recommended.
Furnace Improvements Services
Some plants have opted for low fuel
gas pressures or double orifice designs
for the gas tips to keep the tip size large
enough to avoid plugging when firing
dirty gases. These options generally do
not give good results, and they also
produce longer flames (flame length is
Heat Release And Turndown. Plant
engineers have typically specified a
margin on the heat release rate as high as
30-50% over the design heat release.
Furthermore, on most standard burners,
the turndown for gas fuel is generally 5:1
and for oil it is 3:1. These two parameters
offered virtually unlimited flexibility to
overfire and underfire the furnaces.
However, to ensure optimum
performance of low NOx burners, it is
important to limit the overdesign margin
to only 10%. In most cases, the turndown
should be limited to 3:1 for gas and 2:1
for oil. This will require more attention
from the operators and minimization of
burner outages. The result, though, will
be better performance from the low NO x
Heater Draft. The available heater draft
is a very important design parameter,
especially for natural-draft burners,
because it directly indicates the air
pressure energy available for air/fuel
mixing. It is, therefore, important that
available draft be specified correctly.
In some cases, it may be
advantageous to increase the available
draft by increasing the stack height or Figure 2. NOx emissions are
diameter. Increased draft availability can a function of various
reduce the size of the burners. However, furnace parameters.
increased draft at the hearth also
increases the likelihood of air leakage Combustion Air Temperature. prevent flame impingement damage to
into the furnace, so the furnace should be Combustion air temperature has a direct the furnace tubes. The key to getting a
made leak-tight to prevent such air bearing on flame temperature, and the short flame has been to increase excess
infiltration. higher the flame temperature, the more air until the flames are blue sand short.
thermal NOx is formed, as shown in This practice has been curbed to some
Firebox Temperature- In the past, Figure 2c. If the heater is already extent by the installation of oxygen
standard burners specified independent of equipped with an air preheater, then analyzers.
the furnace design parameters. However, burners utilizing flue gas recirculation The basic design principle of low
the performance of low NOx burners is offer a good degree of NOx reduction. In NOx burners calls for staged combustion
closely linked with furnace design and new heaters, alternative methods of waste and cooler flames. This is in direct
firing arrangements. NOx formation is heat recovery should be investigated. conflict with the good mixing of air and
dependent on firing density and firebox fuel required for efficient combustion.
temperature. The burner vendor needs the Flame Length- This parameter has the Thus, a balance needs to be struck
firebox temperature and geometry to most important effect on the operation of between the two requirements so as to
predict NOx emissions correctly. Higher the furnace. achieve acceptable NOx levels and flame
firebox temperature leads to higher NOx Traditionally, furnace operators are dimensions.
formation, as depicted in Figure 2b. accustomed to short, crisp flames, which A typical low NOx burner has a
Furnace Improvements Services
The author is thankful to KTI manage-
ment for permission to publish this arti-
cle. Thanks are also due to Rose Wil-
liams for repeatedly typing the manu-
Bell, C.T., and S. Warren, “Experience
with Burner NOx Reduction,” Hydrocar-
bon Processing, 62(9), pp.145-147
Garg A., Trimming NOx from Fur-
naces”, Chem. Eng. 99 (11), pp.122-
130 (Nov. 1992)
Johnson, W.M., and R.R. Martin,
“Staged Fuel Burners for NOx Control
in Fired Heaters”, presented at the
1984 Winter National Meeting of
AIChE, Atlanta, GA (Mar. 1984)
Kunz, R.G., et al., “Control NOx from
Gas Fired Hydrogen Reformer Fur-
naces” presented at the National Petro-
leum Refiners Association.
Waibel, R., et al., “Fuel Staging Burn-
ers for NOx Control”, presented at the
1986 Symposium on Industrial Com-
flame that is about 50-100% longer than burner throats. bustion Technology, sponsored by Gas
the flame in a standard burner (when Research Institute, U.S. Dept. of En-
operated design conditions). Any Internal flue gas recirculation calls for ergy, American Flame Research Com-
variation in operating condition tends to larger burner tiles and re-circulation flue mittee, and American Society for Met-
increase the flame length in low NOx gas ports. als (now ASM International), Chicago,
IL (Apr. 1986)
burners, thereby increasing the chance of
Thus, it is becoming very difficult to fit the
flame impingement. new low NOx burner in an existing heater
The expected flame length must be floor without sacrificing some degree of
kept in mind when specifying the heat operational and maintenance flexibility. It is
release rate and the total number of essential that the engineering contractor be
burners. It is also recommended that the given drawings of the general arrangement of
maximum heat release rate per burner be the heater and the steelwork to work out the
limited to 10 MMBtu/h. Furthermore, the installation details.
burner flame length should be kept to a
third of the firebox height for low-roof Burner Testing
cabin heaters. Typical clearances for low Burner design is mostly empirical and
NOx burners are recommended in Table 2. predicted design and operating conditions can
only be verified through performance tests.
Burner Size. Today’s low NOx burners are Thus, burner testing is strongly recommended A. Garg is manager of thermal
much larger than standard burners for several for all new low NOx burners. engineering at Kinetics Technology
reasons: Testing of these burners should be International Corp. (KTI), Houston,
handled with care. The flue gas flow and the TX (713/974 5581; 713/974 6691).
Air staging has led to the use of expected temperature profile in the furnace He has more than 19 years of ex-
secondary and tertiary air controls. usually cannot be reproduced exactly in the perience in process design, sales,
test furnace. For this reason, emission test and commissioning of fired heaters
Fuel staging had led to the segregation of results should be considered estimates, and and combustion systems. Previ-
gas tips and, thus, larger diameter actual emission calculations should ously, he worked for Engineers
burners. It also requires more gas piping incorporate a margin to account for this. India Ltd., and for KTI in India. He
and separate gas controls. Low NOx burners have been installed in a received a B.Tech in chemical engi-
variety of applications in both new facilities neering from the Indian Institute of
and in revamped plants. Table 3 summarizes Technology. He is a registered pro-
Recirculation of flue gases requires
fessional engineer in California and
separate gas tubes, and the increased several installations.
a member of AIChE.
volumes of gas and air require larger
Furnace Improvements Services *Reproduced with the permission of Chemical Engineering Progress.