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 Nz into two

N1 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

NOz) are collectively called "nitrogen

oxides," abbreviated as ''NOx.'' After

the NOx leaves a stack, in the presence of sunshine, ozone, and volatile

organic compounds, it becomes NOz, 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

MeasuringNOx 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

2

2.049

2.51

3

3.20

3.38

3.85

4

4.26

5.02 5 .

5.02

5.16

5.55

6 .

6.50

7.01

7.35

8

6.77

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

10

11.0

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 on 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 industry'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 Nz 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 NOz ," sometimes termed

"chemical NOx." If a gas burner pro- .

duces NOx, it is usually "thermal

NOx" (from Nand°from combustion air) because few gaseous fuels

contain N in any form other than Nz. With gaseous fuels containing no N

except as Nz, NO usually cannot
f

i

..

NORTH AMERICAN Mig"Co"

Cleveland . OH 44105 USA

•

Handbook Supplement 257, 7·93

Page 2

Fig. 1 Comp8Jison o( actual and

equilibriumcoocenua/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 NOx• 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

NOx 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 industrial heating engineers to apply the

results of these scien tific 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 equilibrium at any point during the process. 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 combustion 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 NOx 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 N1• In tum the N1 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 NOx

is not. Engineers must determine

whether their NOx emission problems

a re from thermal or fuel NO. because

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, NOx 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 2.18Ild2.1201lila NorthAmefi<:anCombustiOn

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 100ppm

NOxin #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 measurement.

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. N1 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 than
NORTH 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 Piping/cOfltrol BJT8ngement

for exh8LISt gas recirculation applied

Fuel Tube to an integral regenerative/burner.

EGR can provide substantial

reduction of /hermsJ NOrTable III Teat Data Showing Progreulve NOx 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) mechanism. By tying up the oxygen atoms

with much faster reactions, such as

~H3 + H2 + 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

Fig. 4 Ctoss-secton of a

fuel·directed burner with

two-steqecombustion

(staged air), and wffh

exhaustgas recirculation.

Table fII shows the NO.

reduction from this

combination.

effect similar to EGR in automobile

engines, but with twin integral burner/

regenerators. the exhaust gas ternperature is lower, and therefore more

effective. H20 in the recycled flue gas

also has a cooling effect on the flame;

and the presence of CO2, H20, 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 NOx

reduction (especially if NOx is from

. fuel N>, use staged air (TSC = twostage 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 originally 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 (exhaust gas recirculation = flue gas

recirculation). If still further NOz reo

duction is required, consider adding

staged air to EGR. .

Exhaust gas recirculation is easily

accomplished with twin integral burner/regenerators as shown in Fig. 3. In

one case, a regenerator cooled the exhaust 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 NOz; and 5.4%

EGR resulted in 37% NOz 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 invisible 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 mixmg 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 pollutant 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 of furnace temperature. (A quick fix. where necessary,

is best achieved by addition of flue

gas 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 will

meld 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 reports 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 provide

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.