Trimming NOx From Furnaces

7/31/2019 Trimming NOx From Furnaces

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Furnace Improvements Serviceswww.heatflux.com 1

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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Trimming NOx from Furnaces

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

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Trimming NOx From Furnaces