# NOx Reduction in Industrial Heat Processing 

# d·**'~I** 

# LltJ!I; ifKI;swlI;£cr Handbook 

### Supplement 2fJ7 

### 7-93 NOx REDUCTION 

# [-------- · 

# . North American Mfg. Co. Cleveland

. OH 44105 USA**~..** . ; ; ..

Table I Factors for NO. ReadIng Corrections for 8 Typical (1000 Btulcf) Natural Gas. form until the thermal reaction gets hot enough to break the N

### z into two N 1 ions

.

Small amounts of NOz (nitrogen dioxide) and 

### NzO (nitrous oxide, or laughing gas) may 

### also be generated in combustion reactions

. Two of the nitrogen-oxygen compounds (NO and 

### NO 

## z ) are collectively called "nitrogen oxides," abbreviated 

### as ''NO x . '' After the NOx leaves a stack, in the presence of sunshine, ozone, and volatile organic compounds, it becomes NO

### z , which (in extreme cases) appears as 

### a reddish-brown plume

. It can cause photochemical smog 

### and/or acid rain. Both may be irritating to humans, agriculture, and wildlife. The first
areas to detect and then legislate against NOs - Los Angeles, Denver, Japan, 

### West Germany - were those with both heavy vehicular traffic to

. generate the NOx and weather inversions to trap it in the locale.

NOx emission concentrations and rates depend greatly on operating conditions; so plant operators should not 

### be misled by laboratory or pilot test data that may not simulate the real world situation

. It is necessary to determine NOx ranges within which one can live before meaningful answers can be obtained from burner manufacturers

. 

### Be wary - do not let someone else pick your numbers for you.

200 volumes NOx 

### x million vols dry poe diluted 125 

### v d

# rD di.)uted

, 115 

### v poe ·iJffi**.----.~** = 200 x the factor 1.09 

## = 217 ppm

.

Table I gives values for the 

### 2nd term ("factor") above for a typical natural gas. North American Handbook Supplement 237 provides infor-, mation that can be used to derive similar data for five other common fuels

.

The above correction procedure modifies the reading only for the effect of dilution, not for the effect of excess air on the NO generating mechanism.

An interesting recent observation occurred as follows

. A burner design
was tested in a laboratory and found 

### MeasuringNO x 

### ConcentratIons Chemiluminescent instruments are most widely accepted for measurement of NO concentration in 

### flues and stacks. The readout is usually in parts per million by volume of NOs in the dry products of combustion (poe)

.

A carefu l traverse with a sampling probe of an appropriate material is necessary to obtain a representative reading. Simultaneous readings to determine the % of excess air (02)
are advisable because most 

### regulating agencies will want the resultant NO reading 

### corrected to 3% O2 (about 15% excess air with 

### most fuels) as a standard basis for comparisons.

Example: 

### If a reading of 200 ppm NOx is measured with 25% excess air, find the standardized 

### base equivalent ppm at 15% excess air (-3% O2 

### dry)

.

Convert 25% and 15% excess 

### air to 125 and 115% 

### air and correct for the extra dilution of 25% excess air, as follows: 0.87 0 .91 0.91 0.92 0.95 0 .96 0.97 1 .00 1 .00 1 .03 1.004 1 .06 1.09 1.10 1.12 1.13 1 .17 1.17 1.18 1.22 1.26 1.30 1.36 1.39 1.47 1.57 1.59 1.74 1.75 5.57 9.&4 , 4.53 

## o

6

' 10 11.7 15 15.1 18 .6 20 21 .04 25 26.2 28.4 30 34.8 35 36 .3 40
« .6 50 55 .9 60 68.7 

### eo 82.6 100 102 

### o 0.82 0.00 1 1.66 1.73 22.049 2.51 33.20 3.38 3 .85 44.26 5 .02 5

. 5 .02 5 .16 5.55 6

. 6.50 7.01 7.35 86.77 8.93

. 9.92 10 

## o 1 1.10

1.22 2 2.09

2.41 2.98 3 3.67



### 3.eo 04 04.54

4.71 5 5.22

6.83 5 .65 6 6.43

6 .92 7.048 8 6.38

9.004 9 .83 1011.0

11.1 By RICHARD 

### J. REED and RUSSELL LANG North AmerIcan Manufacturing



### Co. Cleveland. OH 44105

Until now, NOx has been sort of like Mark Twain's comment on the weather

- everybody's been talking about it

. And there 

### seemed to be little that could be done about this intolerable situation

. Some have been st.udying the problem for a long time

. If the size of North American files on generation and control of nitrogen oxides were an indication of cumulative knowledge o n this complex subject, this company would raw an A for "advanced;" but it's experience that counts

.

Although the media and the general public think of NOx control as a single problem, most of the readers of this publication have already discovered that NOx control for industrial heating processes is really 

### a chain of complex problems

. And that indus

try's problems are very different from the much-talked about approaches for automobiles and for boilers and large power plants, each of which far ex- 

# ceeds the total emissions of industrial heating equipment.

NO, nitric oxide, is formed by such diverse sources as earthworms and combustion

.. .the former in tremendous volume, the latter in high 

### concentrations, especially at high ternperature. NO results from the 

### chemical combination of N, atoms or ions

, with 

### 01' These originate from the

fuel or combustion air (21% O2 

### +. 79% N2) . If there is N in the fuel, that is more easily "freed " to combine 

### with 

# ° than is the N in N

### z from air. Both are more readily available and more prone to form NO at higher temperatures

.

Liquid and solid fuels often contain nitrogen; so are likely to produce "fuel NO z ," sometimes termed "chemical NO x." 

### If a gas burner pro-

. duces NO x, it is usually "thermal NO x" (from Nand 

# ° from combustion air) because few gaseous fuels contain N in any form other than N

### z . With gaseous fuels containing no N except as 

### N

### z, NO usually cannotf i

..

NORTH AMERICAN Mig " Co" Cleveland

. OH 44105 USA 

## • Handbook Supplement 257, 7·93 Page 2

Fig

. 1 Comp8Jison o( actual and equ ilibrium coocenua/iolls (or fuel NO and thermal NO. 

# n

Thl'O"tJ.c.aJ Eqvillbrium

o z 

# i

II: 

# -- ",..---

10 ",0 nmt 10..,

to emit only "x" ppm of NOx• When the same burner was installed on an industrial furnace, measurements showed more than 1.5 times as many ppm NO x • The burner was transported back to the laboratory furnace and again developed only 

### x ppm NOx• A thorough investigation of this mysterious chain of events showed that the lab furnace was in good condition, sealed tightly, whereas the industrial furnace had leaks allowing infiltration of considerable "tramp air" (excess air), which made much oxygen available for production of NO

. If the observed ppm NOx had been corrected (as in Table 1) for the higher 

### % oxygen (excess air) in the flue, the ppm NO x would have 

### been even higher.

This incident shows why it is difficult for a burner manufacturer to guarantee the NOx level that a burner will develop in 

### a customer's furnace

. He can "guarantee 

### burner performance, but he cannot guarantee 

### furnace performance, which must be conditioned upon very specific circumstances. Not just burners, but 

## rotal furnace temperature, pressure, and configuration

- design, construction, maintenance

- the whole combustion environment/process affect NO formation

. 

### Parameters In NOx Problems

The results of'thermo-chemical equilibrium studies predict that above 3000°F flame temperatures, equilibriurn NO concentrations will exceed 1000 ppm (by volume), and increase rapidly with temperature. Indications
are that formation of nitrogen oxides is not favored at loui temperature s nor when fuel rich (reducing) conditions exist

. It is the responsibility of indus

trial heating engineers to apply the results of these scientific equilibrium studies to minimize NOx pollution

.

Dr. Jost O. L. Wendt of the University of Arizona, a world authority
on this subject, warns : "Although equilibria provide insight

...at the end of a process, it is erroneous to consider equilibrium concentrations a limit during the course of a process. 

### Combustion is a very non-equilibrium process, and it is possible to be below, above, or far removed from equi

librium at any point during the proc

ess. Indeed, in multi- reaction systems, it is possible to start 

### off below equilibrium, then to exceed and surpass equilibrium, and only finally tend to approach it after some long time has passed."

Fig. 1 compares theoretical and actual NO levels, and their change with time

. As the products of com

bustion cool, NO reduction is negligible; 80 the NO generated in the primary flame zone is, in effect, "fro: zen," with the result that the emission considerably exceeds the predicted equilibrium level based on stack temperature.

.



### Thermal 

### NO x is most often formed with " gaseous fuels and results from temperature causing the breakdown of O2 to 0h which reacts with N2 to form NO and N 1• In tum the N 1 

### combines with O2 

### to make more NO and 

### 0 1 , OH, "prompt NO ," and HCN (formed when operating fuel rich,
even with gaseous fuels) aggrevate the situation. All three of these 

### are prolific at high temperature.

The mechanism for forming thermal NO is strongly temperaturedependent; whereas that for fuel NO x is not. Engineers must determine whether their NO x emission problems
are from thermal or f uel NO

. bec ause abatement techniques that utilize flame temperature are very effective for thermal NO, but have little effect on fuel NO

. The effects are similar with preheated air and with oxygen enrichment, both of which raise flame temperatures

.

For liquid or solid fuels containing no N except N2 , NO x emission is thermal NO, just as with gaseous fuels

. Small droplets and resultant rapid evaporation from good atomization can drive peak flame temperatures 

### to high enough levels 

### to pro

- duce as much thermal NO with liquid fuels as with gaseous fuels.

Flame temperature control can be effective in minimizing NOx• This is
accompli shed by varying the rates with which fuel and air molecules make contact, i.e

. mixing rates. This is achieved through control of the flow passage configurations, velocity jets, and spin or swirl. Other variables that might have secondary effects on fuel and air 

### contact rates are turbulence, luminosity, and flue gas recirculation. Because ofall these fluidynamic variables, control is very complex , and to some extent, still an art because of the need to accomplish the same effect over a wide range of flow rates (wide turndown ratio).

No swirl results in the common type A flame. At higher swirl (culminating in the compact types C flame), mixing 

### is more prompt, the flame is shortened and more intense, 

### with internal recirculation; so the re is less opportunity for heat loss, producing a higher flame temperature, and increased**f> al** **~()** Further inrrease .'•Formorelnfomallononthese fuels.seeTables 2.1 8Ild2

. 1201lila NorthAmefi<:an CombustiOn Handbook. Vol

. I, 3m Ed .; and HandbookSuppklmenI237. Table II SpreadahMt of Equivalent. for 7 Fuel••
V N P
9060 0.110 909

9900 0.120 832

9939 0.121 829

10140 0.123 812

9401 0.114 876 10740 0.130 767 10652 0.129 n3 T

9.939 10.22

25.35 Handbook Supplement 257 , 7·93 Page 3

607 .6 1472 1631

1.214 X 10- 7 x P X V. 530 H

1032

1000

2499

64630 137080 153120

heating value, 

### L, in which case 

### L should be substituted for H. When comparing #NO"/million Btu, 

### be sure both are based on 

### H, or both are based on 

### L. (If these figures are not available, ask the fuel supplier for 

### a volumetric analysis, if the fuel is a gas, or an analysis by weight, and Btu/#; then North American's STOIC computer program can determine the required information for the specific

case.)

The U.S

. EPA compares all NO" readings at 3% oxygen, which is very close to 15% excess air for almost all fuels. (See section on Measuring NOx Concentrations .) The actual total poe (products of cornbustionl/unit of fuel at 3% oxygen will therefore be



### T 

## = B 

### + (0.15 

### x 

### A>.
To figure #NOx/million Btu, it is first necessary 

### to know 

### V, the cf total 

### poe/million Btu:

V T cfpoclunit fuel 

### x 1000000 Btu H gross 

### Btu! "" million Btu

. For a specified ppm NOx, 

### P

( hi 

### h P cfNOx ) w 

### c means 1 000 000 cf 

### poe ' N _ '#NOx

- million Btu

PcfNO" 

### x V 

### cf'poc x 1 000 000 cf poe million Btu 46 #NO,,/mol NOx_ 379 cf " I II II

-

Table 

### II is a spreadsheet for seven typical fuels, listing

"N", the equivalent of 

### 100 ppm NOx in #NOx/mHlion Btu, and 

### ''P'', the equivalent of 

### 1 #NOxl million Btu in ppm

. NO"Control**T~hnlques**



### If firing burners with 

### cold air (ambient, 60°F), the first choice for a NO" B
4.118

8 .523 8.756

21.78

524 1269 1408 4.56

9.44 9 .74 A

23 .8

560 1354 1484 unit
cf

cf

cf

cf gal gal gal CoI<e (),'en GuIR) Natutal OM, N.Sea Nalufa! Gas, AL Commercial Propane (nail) Methanol #2 FuelOil #6 FuelOil

once per year

.

Relating ambient standards to "source" (in stack) values is 

### cornplex, Source review programs include:

(a) new source performance standards

- generally, 0.14 #NOx/ million Btu 

### {l75 to 

### 193 ppm}; (b) prevention of significant deterioration in attainment areas

- no more than 40 tons NOx per year; and (c) non-attainment area bubbles, offsets, trading, or banking

. Note that the PSD 40 tons/year x 2000 lb/ton x

But if a burner that emits only 

### 1/4 lb NOx/million Btu (207 ppm) becomes available, then burners totaling 4 

### x 9.524 or 38 million Btulhr could be added.

.

These rulings and allowable exceptions thereto may change at any time. 

### It is the equipment owner's reo sponsibility to keep abreast of applicable regulations. Section B, on NO" detection and monitoring, gives details on EPA standards for 

### measure

ment.

To relate # NO,,/million Btu to ppm, you must know: A = the stoichiometric cf air/unit of fuel, B :: the stoichiometric products of combustion (poe) per unit of fuel, 

### H = the higher of gross Btu/unit of fuel. In some parts of the world, # NOxt million Btu is based on 

### net or 

### lower NORTH-AMERICAN Mig. Co

. Cleveland, OH 44105 USA in swirl C8Wle8 the type C compact 

### ball of flame to flare out like 

### a flower bud opening into a trumpet or disc (type 

### E flame). Then the sheet of flame is so thin that it loses heat
more rapidly 

### to the wall behind it and to the 

### load in front of it, and therefore 

### has less internal temperature build-up. 

### In otber words, immediate heat absorption by the sur- .rounding heat sinks depresses the flame temperature; so that thermal NO drops dramatically

.

No swirl, combined with delayed mixing produces long axial flames with no internal recirculation (type F
or 

### G). The 

### slower- mixing of such diffusion type flames results in heat dispersal before combustion is complete; so the maximum flame temperature is lower than with fast mixing, and thermal NO development is low. (If care is not exercised in this slow mixing arrangement, carbon monoxide, CO, may 

### be formed; and that is
more harmful than NO.)

A high velocity type H flame, even with little 

### or no swirl, induces its
own external recirculation, a cooling phenomenon similar 

### to flue gas recirculation, that lowers thermal NO production.



### Fuel NOs comes from chemically. bound nitrogen particularly in residual fuel oils , bottoms, crudes, shale oils, hazardous wastes, coals, and other solids. N 1 atoms are more easily released from C-H" C.O-, or C·Scompounds or combinations thereof, 

### 1 year = than from 1'2' Even 

## o.ss N by weight 24 x 7x50 in a fuel can double NOx emissions. 9 .524 lh NOxlhr

. For a heater that

Minimization of fuel NOx forma- emits, for example, 1 lb NOx/million tion can 

### be accomplished by control- Btu, the maximum input that could ling the availability of 

### 0 1 atoms at 

### be added equals the critical conditions that would 

### otherwise produce NO. Variables that· 

### 9524 lb NO Ihr affect 0 1 availability are (2) excess lIb NO /miilion Btu 

## = air (keep it low), (b) rich atmosphere x9.524 million BtuJhr

. (keep it

- 

### use staged air addition), and (c) two-stage combustion with first stage scavenging of 

### 0 1 atoms using highly reactive radicals from polymerization (cracking) of fuel molecules. These methods will be discussed 

### in the section on Control Techniques. 

### In incineration of wastes containing 

### N, the "starved air" procedure in the first phase of incineration is therefore a step in the right direction

. Enforcement, Specifications, Units

In the U.S., enforcement to date has been left to the states or local agencies, but they must set up standards that are within U .S. federal guidelines. The basic specification is 

### "ambient"

- i.e

. on the sidewalk, where people are exposed - at 0.05 ppm (by volume) NCh, or 100 micrograms per cubic meter of air. This is
an allowed average over a 24-hour period, not to 

### be exceeded more thanNORTH AMERICAN Mlq

. Co. Cleveland, OH 44105 USA Handbook Supplement 257, 7 ·93 Page 4



### EGR**-} ~~** **,.: st1**  AI,

Cooling

, ,,,'.--Alr 

### r

- - S 11444-03-1 I I I I I I I I I

1442 1807 PSL

Fig

. 3 P iping/cOfltrol BJT8ngement for exh8LISt gas recirculation applied Fuel Tube to an integral regenerative/burner. EGR can provide substantial reduction of/hermsJ NO r
Table III Teat Data Showing Progreulve NO x Reduction

. 

### STAGED COMBUSTION

TSC 

### + FGR, INOx/ml1fion Btu TSC + FGR, ppm 168·175 237

·· 248 370·418

107 · 109 148·153 195·204 TSC,ppm 

### The 'fuel-rich zone is followed by a fuel-lean zone (air staging). In the

. early stage, fuel-cracking 

### produces highly reactive radicals that 

### scavenge available 

### 0 1 

### atoms, thereby inhibiting No production by the Zeldavich (N2 

## + 0 = 

### NO 

## + N) mecha

### nism

. 

### By tying up the oxygen atoms with much faster reactions, such as**~H3** 

## + H 2 

### + 0 

### and CO 

### + 0, 

### very httle NO is 

### formed. 

### In the 2nd stage, the radicals are oxidized ' 

### to poc as

0.14 - 0.15 0.18·0.21 0.27 ·0.30 TSC, #N<>.fm/Uion Btu
0.23- 0.24 0.32· 0.34 0.50·0.57 1800F 2000F 2200F

1800F 2000F 2200F Furnace F ig

. 4 C toss-secton of a fuel·directed burne r w ith two-steqecombustion (staged air), and wffh e xhaustgas reci rculation

. Table fII shows the NO. reduction from this comb ination.



### effect similar to 

### EGR in automobile engines, but with twin integral burner/ regenerators. the exhaust gas tern

### perature is lower, and therefore more effective

. H 20 

### in the recycled flue gas also 

### has a cooling effect on the flame; and the presence of CO2, 

### H 20, 

### or NO has a kinetic effect that is helpful.



### Staged air addition. Fig. 4 

### shows one of many possible configurations for twa-stage combustion (TSC burners), this with a fuel-directed burner

. . control method would be to switch 

### to type E,F,G, or H flames, all of which naturally reduce NOx 

### emissions. If that does not provide sufficient NO x 

### reduction (especially 

### if NOx 

### is from

. fuel N>, use staged air (TSC = two

### stage combustor)

.

The phenomena that permit types E,F,G, 

### and H 

### burners to operate with less NOx 

### are described in the section
on Parameters in NOx 

### Problems

. When the type H 

### burners were used with a recirculating "venturi" such as origi

### nally prescribed for firing through thick walls, their NOx 

### emission was reduced from 55 ppm to 40 ppm. Burners that develop flame types E 

### and H 

### can be used with moderate amounts of air preheat; types 

### F and G 

### with high air preheat

.

If 

### preheated air is used. 

### and if the NOz 

### is from thermal N, try EGR (ex

### haust gas recirculation = 

### flue gas recirculation). If 

### still further NO z reo 

### duction is required, consider adding staged air to EGR.

.



### Exhaust gas recirculation is easily accomplished with twin integral burn

### er/regenerators as shown in Fig

. 3. 

### In one case, a 

### regenerator cooled the ex

### haust gas to such a low temperature that NOz 

### was lowered from 136-138 

### ppm to 82-86 

### ppm (by volume, dry, 

### at 3% 

### O2) 

### when using 1000°Fpreheated air. 

### In another case , 2.7% 

### EGR resulted in a 20% reduction of NO z ; 

### and 5.4% EGR resulted in 37% 

### NO z 

### reduction. The effectiveness of recirculated flue gas is enhanced by mixing it with the fuel, or by using it as a separating shroud between annular streams of fuel and air, or between primary air and secondary air

. Addition of EGR usually increases a flame length, but causes a 

### high percentage of the flame to become nearly invisi

### ble so that it may appear to be shortened.

EGR in industrial burners has an. . -

. " " "" 

### . .

_ , ; 

## - Clevelend , OH 44105 USA



# . , the secondary 

### air is added.

Designing the fluid dynamics of the delayed mixing in the fuel-rich**~ne.** 

### and the 

### subsequent prompt mix

### mg m the 

### fuel-lean zone requires 

### exacting care plus thorough testing to be 

### sure that carbon monoxide (CO) is not emitted in the process. CO is not just 

### a by-product, but 

### a harmful pol

### lutant that control authorities will not 

### permit.

Studiously engineered staged air addition has 

### been very successfully used for years on oil field boilers.This retrofit for the combustion chamber has now been compacted into an integral burner unit.

EGR and 

### TSC 

### combine the best of both NO. control methods, as shown in Table 

## m, all with air preheated to within about 300°F offurnace temperature

. (A quick fix. where necessary, is best achieved by addition of fluegas recirculation combined with lowered air preheat.) Continued development, new techniques, and improved tuning procedures change .the situation (for the better) monthly. Much more attention is being paid to the 

### specific 

### situ

- ations of individual applications at 

### the moment. Hopefully, these willmeld into a general solution applicable to a wide variety of industries in 

### the future

.

Thermal de-NO. is 

### possible as an after-fix

. Ammonia is the most commonly used, but it is applicable only within certain temperature windows

. Other materials are being investigated

. Conclusions



### Cold air burners are less of a problem, NO.-wise

. A number of modern fuel-directed and other burners have been retrofitted or supplied new with low-NO. equipment and have performed well

. Hot air burners, particu

- larly the regenerative types (which • • 1

. . . . ... "

. .. l

... ... , " ,

_ .' • • "

-. • • _ • • Page 5 

### develop very high air preheat temperatures) are more of a problem

.

The original outline for this article included a number of case history re

### ports on NO.-lowered installations with regenerative type burners, in 

### aluminum and glass melting and other industrial heating processes. These 

### ranged from 36 

### to 44% 

### reduction in NO•

. 

### However, they have already been surpassed with even better suppression of NOx, 

### and further improvements are being instituted every week; so it is hopeless to prov ide a case history report that will be 

### up to date by the time this article is printed. For the latest developments,
users are advised to consult vendors in this field, and ask for substantiated 

### data on actual operating installations similar 

### to the application being 

### considered.