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Originally appeared in: CHEMICAL ENGINEERING November 1992 Issue, pgs 122-128. Reprinted with publishers permission.
Each passing year seems to bring aboutincreasingly stringent pollutant-emissionlaws governing combustion equipment. In
the future, one can expect even-stricteremissions limits to be imposed on thechemical process industries (CPI).
As regulations tighten, the necessity to
consider environmental concerns in theoperation of furnaces is also mounting. Ofthe various environmental laws now
affecting the CPI, laws covering nitrogenoxides (NOx) are among the mostsweeping.
For example, in the U.S., the SouthCoast Air Quality Management District, or
SCAQMD, which covers the Los Angelesbasin, already has one of the strictest
standards. Under SCAQMDs standard,furnaces with capacities of less than 40million Btu/h must release less than 40
ppm of NOx by September 1991. Forfurnaces larger than 40 million Btu/h, thelimit is less than 25 ppm by December1995 (NOx emissions from 36 % of the
units greater than 40 million Btu/h must becut down to 25 ppm by September, 1992).
Finding the means for limiting NOx
from fired heaters has become a majorthrust of many sectors in the CPI. Theutility industry the first industrial sectorin the U.S. to be affected by NOx controls
has been the spawning ground for many
of the new technologies now being used tostem NOx from CPI furnaces. Other
TRIMMING NOx FROM FURNACES
Emissions-control
technologies need
not limit a fired
heaters perform-
ance
Ashutosh Garg,Furnace Improvements
FIGURE 1. CPI furnaces, available in
box or cylindrical designs, can be fitted
with a number of coil configurations.
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sectors that have also been stronglyaffected by NOx standards are petroleumrefining and petrochemicals.
The stricter NOx limits means that it isincreasingly important to understand both
the capabilities of these new emission-control technologies, and how they affect a
fired heaters overall performance,reliability and operating flexibility. This isespecially significant when retrofittingcombustion equipment with new emission
controls.
The NOx Dilemma
Vertical heaters used in the CPI fall
broadly into two categories: cylindricaland box heaters. In both types, the tubesare laid out on the walls of the radiantsection.
In cylindrical heaters, the tubes are
installed vertically, while in box heaters,the tubes are arranged horizontally. In bothdesigns, the burners are installed on the
floor, and fire vertically upwards.Most of burners employ a natural-
draft design, in which the stack providesthe draft for drawing air into the furnace
for combustion. Newer units are equippedwith forced-draft firing systems and air
preheaters to improve fuel efficiency. The
convection section consists of bare andextended-surface tubes to recover heatfrom the flue gases before they exit from
the stack. Figure 1 shows the typical heatertypes used in the CPI.
The pollutants generated by burningfuel fall into three primary categories:
carbon monoxide, unburned hydrocarbons,and partially oxidized organic materialsand soot that result from incompletecombustion; sulfur oxides and ash directly
attributable to fuel composition; andnitrogen oxides formed at fireboxtemperatures by the reaction of the oxygenand nitrogen present in the air and fuel.
Incomplete combustion products canusually be held to tolerable minimums bythe proper operation of modern burnerequipment, while sulfur oxide and ash
emissions can be cut by using the right
fuel. However, nitrogen oxideconcentrations are primarily functions offuel composition, burner design and
firebox temperature, and so have to becontrolled by choosing the right operatingconditions.
There are several ways that NOx is
formed in a furnace. Thermal fixation ofatmospheric nitrogen and oxygen in thecombustion air produces thermal NOx
while the conversion of chemically boundnitrogen in the fuel produces fuel NOx.
For natural-gas and light-distillate-oil
firing, nearly all NOx emissions resultfrom thermal fixation. With residual fuel
oil, the contribution from fuel-boundnitrogen can be significant and, in certain
cases, predominant. This is because thenitrogen content in residual fuel oil can beas high as 0.3% N2, and conversion to
NOx may be 50-60%.
The formation rate of thermal NOx isdependent on the reaction temperature, thelocal stoichiometry, and the residencetime. The fuel-NOx formation mechanism
is more complex, depending upon fuelpyrolysis and subsequent reaction betweenmany intermediate nitrogenous species andthe oxidant species.
The rates for formation of both
thermal NOx and fuel NOx are kineticallyor aerodynamically limited, with theamount of NOx formed being much less
than the equilibrium value. The rate offormation of NOx is dominated bycombustion conditions and can besuppressed by modifying the combustion
process. Both thermal and fuel NOx arepromoted by rapid mixing of oxygen withthe fuel. Thermal NOx is greatly increased
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by long residence time at hightemperature.
Emission limits are usually specified
in terms of pounds of NOx per millionBtu of gross heat released, or pounds perhour. NOx concentrations, however, aremeasured in terms of ppm (volume)
basis. Since operating conditions varyamong various furnaces, the NOxmeasurements are converted to standardconditions at 3% oxygen.
By calculating the dry combustionproduction per million Btu and the heatrelease rate, R, it is possible to convertfrom ppm to lb/million Btu or lb/h. NOx
emission calculations are made on thebasis of NO2 (molecular weight of 46),
although NO2 is only 10-15% of the totalNOx.
ppm vol. (at 3% O2) = ppm vol.measured x (21-3)/21 - % O2)
where, % O2 = vol. % O2, dry basis
NOx, in lb/million Btu = (ppm NOx) x(DSCF/ million Btu) x 46/ (1,000,000 x
379.3)
For quick NOx level estimations, thefollowing DSCFdry volume of flue-gasin standard cubic feet/ million Btu at 3%
O2 concentration values arerecommended:
Natural Gas 10,127
Propane 10,127Butane 10,127Fuel Oil 10,684
Post Combustion NOx Treatment
The concentration of NOx in combustionflue gases can be cut by:
Modifying combustion conditions toinhibit the mechanisms for formation of
NOxLowering NOx generated duringcombustion by either catalytic or non-
catalytic reduction.The NOx-control processes discussed
below utilize one or a combination of theabove techniques.
Flue gas recirculation (FGR) extracts aportion of the flue gas from the stack andreturns it to the furnace along withcombustion air (Figure 3). This lowers
the peak flame temperature, and cutsthermal-NOx formation. The addition offluegas also reduces the oxygen availableto react with the nitrogen. A comparison
of the two heat duties for a furnace withand without flue gas recirculation is
shown in the table.Increasing the recirculation rate
generally corresponds to a decrease inthermal NOx, but flame instability and a
decrease in the net thermal output limitsthe recirculation rate. Recirculation rates
for gas-fired units are limited to about15% to 20%, resulting in maximumthermal-NOx reductions on the order of50%. It is useful where low nitrogen
fuels, such as natural gas, are used.Recirculating flue gas temperature shouldnot be more than 600oF.
Flue gas recirculation has beenmostly applied to forced-draft burners.
Installation requires additional duct work,a flue gas recirculation fan, a flow controldamper, special burners and combustioncontrol instrumentation (such as
continuous oxygen and carbon monoxideanalyzers in the stack). If the heat contentof the fuel is highly variable, a flamesafeguard system is required to monitor
the flames continuously. The technique issuitable for heaters with a few burners,such as vertical, cylindrical heaters.
Flue gas recirculation
does not affect the overallefficiency of fired heaters
if the temperature offluegas leaving theconvection zone is the
same as that of the flue gasbeing recirculated.However, the split of
radiant heat andconvection heat duty willchange, since therecirculating flue gas acts
as a diluent, reducing the
FIGURE 2. Actualmeasurements (in
ppm) of nitrogenoxides in fluegasescan be convertedto the more com-mon way of repre-senting emissions,in lb NOx (as NO2)/million Btu ofgross heat re-leased
TABLE. Fluegas recirculation changes the splitof convection -and radiant - heat duties.
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uptake of heat in the radiant section andincreasing it in the convection section.
Selective catalytic reduction (SCR)involves injecting ammonia into the flue
gas upstream of a catalyst bed. The
chemical reaction involved is:
O2 + 4NO + 4NH3 4N2 + 6H2O
NOx and NH3 combine on the catalysts
surface, forming an ammonium saltintermediate that subsequentlydecomposes to produce elementalnitrogen and water. The catalyst lowers
the activation energy of the NOxdecomposition reaction, thereby enablinguse of this technology at lower flue gastemperatures. The optimum temperature
range for SCR is 600oF to 700oF.SCR removes 70 to 90% of the NOx,
using between 0.9 to 1.0 mole NH3 forevery mole of NOx; this leaves behind 5
to 10 ppm of unreacted NH3. The majorcomponents of an SCR system are acatalyst bed reactor, an ammonia injectiongrid, and an ammonia storage unit.
Ammonia can be injected in anhydrousform or as an aqueous solution. Typicallya residence time of 0.5 to 1.0s allows foradequate mixing of the ammonia and NOx
before the catalyst bed.Several factors in addition to
operating temperature influence SCRperformance. These include the catalyst
composition and configuration, sulfur andmetals content of the fuel, and the designof the ammonia-injection system.
Catalysts are commerciallyavailable in a wide variety of materials.
These include such metals (such astitanium, vanadium and platinum),zeolites and ceramics. Catalyst shapesinclude honeycomb plates, parallel-ridged
plates, rings and pellets.Each combination presents
advantages and disadvantages in terms ofallowable operating temperatures, catalyst
fouling and pressure drop. Typical gasvelocities over the catalyst are around
50ft/s, and the pressure drop is 3-4 in.(water column.)
The early applications of SCR hadbeen prone to a number of problems.These include: catalyst plugging by fine-
particle dust; catalyst poisoning by SO2;
conversion of SO2 to SO3; formation ofammonium bisulfate; and the depositionof ammonium bisulfate on the catalyst attemperatures below 518oF.
All these factors lead to catalyst
deactivation. However, this can beavoided if care is taken during the designstage. For example, if SO2 is present in
the flue gas, then a minimum temperatureof 608oF is recommended for SCR
operation. A catalysts life depends on its
type, the application and other factors,with numbers of three to six years beingreported in oil and gas applications.
SCR systems have the highestinstallation costs and requires the greatest
amount of space of all NOx-controlmethods. They can be easily retrofitted infired heaters with air-preheating systems,since all this involves is re-rating of the
fan and re-routing the duct to the airpreheater via an SCR unit. The leftportion of Figure 3 shows a typical SCRunit retrofitted in a furnace with an air
preheating system.
Selective noncatalytic reduction (SNCR)is a post combustion-control method that
reduces NOx to N2 and H2O. Ammonia isinjected into the upper part of combustionchamber or into a thermally favorable
location downstream. The variousreactions are:
6NO + 4NH3 5N2 + 6H2O
6NO2 + 8NH3 7N2 + 12H2O
Recently, a urea-based regent isincreasingly being used in place of NH3
because it is safer and easier to handle.
Urea decomposes into NH3 and carbondioxide inside the firebox.
The flue gas temperature is critical tothe successful reduction of NOx. For
convectional combustion, the optimalrange for NH3 injection is 1600
o to1,750oF; for urea, 1,000o to 1,900oF. Asthe temperature increases, the NH3 reacts
Figure 3: Popular post-combustion methods of removing NOxinclude selective catalytic reduction (left part of figure) andfluegas recirculation.
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more with oxygen than with NO, forming
more NOx. At flue gas temperaturesbelow the optimal range, the rate ofreaction declines, resulting in reduced
NOx control and greater amounts of
unreacted NH3 slipping into the
effluent.The NOx reduction achieved is of
the order of 50-60%, with the NH3 slip
in the 20 to 30 ppm range. Thetechnique is effective in the presence ofcarbon monoxide, and with oxygencontents of up to 1% (calling for veryclose control of excess air).
The disadvantages of SNCR aresimilar to those of SCR: Ammoniumsalts, namely ammonium sulfate and
bisulfate, may form if excess NH3
reacts with sulfuric acid, form a priorreaction between SO3 and water.Ammonium bisulfate can contribute tofouling and corrosion in low
temperature heat-recovery equipment.Ammonium chloride can also be
formed, which is undesirable since itcauses visible plumes. High levels of
NH3 slip, up to 50 to 100 ppm, canoccur if the NH3-to-NOx ratio is notoptimized. Overall, the method has not
become widely popular with process
fired heaters since it cannot meetNOx-reduction requirements byitself, and needs
to be used along with a second NOx-
reduction technique.
Redesigning The Equipment
While SCR and SNCR maintain control
over NOx after is has been formed inthe combustion reaction, modificationsof the combustion equipment or the
burners can also significantly reduceNOx formation. There are a number of
advantages in using such modifiedburners, the major ones beingsimplicity and low cost. At the sametime, since burners form the heart of afurnace. The process of implementing
new ones should always be triedcautiously.
Staged air burner systems divide the
incoming combustion air into primaryand secondary paths: All of the fuel isinjected into the throat of the burnerand is combined with the primary air,
which floss through the venturi andburns (Figure 4).
In this fuel-rich zone, the fuelpartially burns and the nitrogen is
converted into reducing agents.These nitrogenous compounds aresubsequently oxidized to elementalnitrogen, thereby minimizing the
generation of fuel NOx.Also, the peak flame temperature is
lowered in the fuel-rich primarycombustion zone, since the generated heat
dissipates rapidly. Recirculation of
combustion products within the burnerfurther cuts the flame temperature andoxygen concentration, reducing NOx (in
this case the thermal NOx even more. Inthe secondary-combustion zone,additional air is injected throughrefractory ports to complete combustionand optimize the flame profiles.
Staged air burners are simple andinexpensive, and NOx reductions as highas 20 to 35% have been demonstrated.The main disadvantage of the burners is
the long flames, which need to becontrolled. Further, staged-air burnershave proven to be quite successful informed-draft applications, and have even
been used with flue gas-recirculationsystems.
Staged fuel burnersinject a portion of the
fuel gas into the combustion air, and theresulting combustion is very lean (i.e. airrich). This lean combustion reducesthermal NOx. The remainder of the fuel
gas is injected into a secondarycombustion zone through secondarynozzles (Figure 4).
Figure 4: Two ways of modifying the fuel air stoichiometry during com-
bustion is to use a staged fuel burner (left) or a staged air burner.
Figure 5. NOx levels can be cut very low
levels by combining staged fuel burners with
internal fluegas recirculation
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The combustion products and inert
gas from the primary zone reduce the peaktemperatures and oxygen concentration in
the secondary zone, further the inhibitingNOx formation. Some of the NOx formed
in the first stage combustion zone isreduced by the hydrogen and carbonmonoxide that is formed in stagedcombustion.
Staged fuel burners can reduce NOxemissions by as much as 50-60%. Thistype of burner can operate with a small
flame length, and at lower excess-air levels
than can staged air burners. The flames instaged fuel burners are about one and a
half times longer than those in standardburners. Staged fuel burners have been
found more effective in reducing NOx ingas-fired heaters and, so, the majority ofthe applications are gas fired.
Ultra Low NOx Burners, a combination ofstaged fuel burning and internal flue gascirculation (Figure 5), have recently been
used to reduce NOx to very low levels. In
this design, the fuel gass pressure orexternal agents, such as medium-pressuresteam or compressed air, are used to
induce flue gas recirculation within theburner.
Low excess-air burners works on theprinciple that low levels of excess airsuppress NOx formation. Typically, excessair levels are maintained at 5%. The
burners are often of a forced-draft design,and employ a self-recirculating techniqueto produce a multi-stage combustioneffect. A NOx level of 0.06 to 0.08 lb/
million Btu is typically encountered.Generally, it has been found that reducingexcess air from 30% to 10% cuts NOxemissions by 30%.
References
Air Pollution Engineering Manual, AP-
42, U.S. Environmental ProtectionAgency.
NOx Control In fired heaters, Martin,
R.R. and W.M. John Zink Co.
Cleaning Up NOx Emissions, McInnes,R., and M.B. Van Wormer, Chem.
Eng., Sept 1990
Reduce Heater NOx In the Burners,Seabold, J.G., Hydrocarbon Process,
Nov. 1982.
The author
Asutosh Garg is Manager of ThermalEngineering at Kinetics Technology
International Corp. He has more than 18years of experience in process design,sales and troubleshooting of allcombustion systems. He graduated in
chemical engineering in 1974 from IndianInstitute of Technology, Kapur. He isregistered professional engineer in
*Reproduced with the permission of Chemical engineerin