Specify Better Low NOx Burners for Fired Heaters by YogitaDoda


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									 Originally appeared in:                                                         January 1994 issue, pgs 46-49
 CHEMICAL ENGINEERING PROGRESS                                                   Reprinted with publisher’s permission.


 Specify Better
 Low NOx Burners
 For Furnaces
                                           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
                                                                                        combustion zone.
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.

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  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
becomes large.

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.

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     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
discussed later).

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

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                                                                                                    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-
                                                                                                              Further Reading

                                                                                                    Bell, C.T., and S. Warren, “Experience
                                                                                                    with Burner NOx Reduction,” Hydrocar-
                                                                                                    bon Processing, 62(9), pp.145-147
                                                                                                    (Sept. 1983).

                                                                                                    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

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