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Everything You Need to Know About NOxControlling and minimizing pollutant emissions is critical formeeting air quality regulations.By Charles Baukal, Director of R&D, John Zinc Co. LLC, Tulsa, Okla.

This article provides a basic primer on animportant pollutant emission referred to asNOx. This regulated contaminant is formed in

nearly all combustion reactions. This includes firedequipment such as heaters, dryers, and furnaces.Metal processors should be familiar with some basicinformation about NOx as it will likely be regulatedin their plants in the very near future, if it is notalready. Fortunately, there are many well-estab-lished methods for controlling and minimizing NOx,which are briefly discussed here.

WHAT IS NOx?Most of the world’s nitrogen occurs naturally in theatmosphere as an inert gas contained in air, whichconsists of approximately 78% N2 by volume. NOxrefers to oxides of nitrogen. These generally includenitrogen monoxide, also known as nitric oxide (NO),and nitrogen dioxide (NO2). They may also includenitrous oxide (N2O), also known as laughing gas, aswell as other less common combinations of nitrogenand oxygen, such as nitrogen tetroxide (N2O4) andnitrogen pentoxide (N2O5).

The EPA defines nitrogen oxides as “all oxides ofnitrogen except nitrous oxide.1” In most high-tem-perature heating applications, the majority of theNOx exiting the exhaust stack is in the form ofnitric oxide (NO).

WHAT’S WRONG WITH NOx?Health hazards: NO is poisonous to humans andcan cause irritation of the eyes and throat, tightnessof the chest, nausea, headache, and gradual loss of

strength. Prolonged exposure to NO can cause vio-lent coughing, difficulty in breathing, and cyanosis;it could be fatal. NO2 is a reddish-brown, highlyreactive gas that has a suffocating odor and is astrong oxidizing agent. It is highly toxic and haz-ardous because of its ability to cause delayed chem-ical pneumonitis and pulmonary edema. NO2 vaporsare a strong irritant to the pulmonary tract.

Ground level ozone: Ozone (O3) is present in theupper atmosphere and is desirable as it shields theearth against high-intensity ultraviolet rays fromthe sun. However, ozone in the lower atmosphere isundesirable. There, NO reacts with oxygen to formozone, in addition to NO2:

NO + HC + O2 + sunlight → NO2 + O3Ozone is also a health hazard that can cause res-

piratory problems in humans. It is an irritant to theeye, nose, and throat. Ozone can cause damage toplants including crops, and deterioration of textilesand other materials.

Acid rain: Rain is effective at removing NO2 fromthe atmosphere. However, NO2 decomposes on con-tact with water to produce nitrous acid (HNO2) andnitric acid (HNO3), which are highly corrosive (seeFigure 1). NO is the predominant form of NOx pro-duced in industrial combustion processes.

It reacts with O2 in the atmosphere to form NO2.When NO2 forms in the atmosphere and comes intocontact with water in the form of rain, nitric acid isformed. Acid rain is destructive to anything it con-

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Figure 1: Acid rain formation. Figure 2: Smog formation.

tacts, including plants, trees, andman-made structures like build-ings, bridges, and the like.

Smog: Another problem withNO2 is its contribution to smog.Smog is the combination ofsmoke and fog and occurs wherethere are high concentrations ofpollutants combined with fog.Smog impairs visibility throughthe atmosphere.

When sunlight contacts a mix-ture of NO2 and unburnedhydrocarbons in the atmosphere,photochemical smog is produced(see Figure 2).

HOW DOES NOx FORM?There are three generally accept-ed mechanisms for NOx forma-tion: thermal NOx, prompt NOx,and fuel NOx. Thermal NOx isformed by the high-temperaturereaction (hence, the name ther-mal NOx) of nitrogen with oxy-gen, by the well-known Zeldovichmechanism:

N2 + O2 → NO, NO2Thermal NOx increases expo-

nentially with temperature.Above about 2,000°F (1,100°C), itis generally the predominantmechanism in combustionprocesses, making it importantin most high-temperature heat-ing applications. It means thismechanism becomes moreimportant when air preheatingof the combustion air is used,which normally increases the

flame temperature that leads toincreased NOx.

Prompt NOx is formed by therelatively fast reaction (hence,the name prompt NOx) betweennitrogen, oxygen, and hydrocar-bon radicals:CH4 + O2 + N2 → NO, NO2, CO2,H2O, trace species

In reality, this very complicat-ed process consists of hundredsof reactions and dozens ofspecies. Prompt NOx is generally

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an important mechanism in lower- temperaturecombustion processes, but is generally much lessimportant compared to thermal NOx formation atthe higher temperatures found in many industrialcombustion processes.

Prompt NOx becomes more important under fuelrich conditions.

Fuel NOx is formed by the direct oxidation of organo-nitrogen compounds contained in the fuel (hence, thename fuel NOx) and is given by the overall reaction:

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Figure 3: Fuel gas staging.

Figure 4: Internal furnace gas recirculation.

Figure 5: External flue gas recirculation.

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RxN + O2 → NO, NO2, CO2, H2O, trace speciesFuel NOx is not a concern for high-quality

gaseous fuels like natural gas or propane, whichnormally have no organically-bound nitrogen.However, fuel NOx may be important when oil(e.g., residual fuel oil), coal, or waste fuels are used,

which may contain significant amounts of organi-cally bound nitrogen.

HOW IS NOx CONTROLLED?Before air quality regulations, the flue gases fromcombustion processes were vented directly into theatmosphere. As air quality laws tightened and publicawareness increased, the industry began looking for

Figure 6: NOx as a function of excess O2 for different burner withthe oldest designs at the top and the newest at the bottom.

Figure 7: Selective noncatalytic reduction system.

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new strategies to curb NOx emissions. The generalstrategies for reducing NOx are discussed next2.A summary of NOx control techniques developed for

the U.S. Environmental Protection Agency (EPA) canbe found in EPA Report 450/1-78-0013. Table I shows asummary of common NOx control technologies,including combustion modifications, process modifica-tions, and post-treatment4.The four basic controlstrategies are:

Pretreatment: The first NOx reduction strategy isreferred to as pretreatment, which is a preventativetechnique to minimize NOx generation. In pretreat-ment, the incoming feed materials (fuel, oxidizer,and/or material being heated) are treated in such away as to reduce NOx.

Some examples include fuel switching or treatment,using additives, oxidizer switching, and productswitching or treatment.

For example, partially or completely substitutingnatural gas for fuel oil can often significantly reduceNOx emissions by reducing the amount of nitrogen inthe fuel. Switching from air to pure oxygen for com-bustion eliminates most, if not all, of the nitrogenfrom the process so NOx is minimized or eliminated.

Removing nitrogen that may be present in the feedmaterials may also reduce NOx formation.

Process modification: There are a number oftechniques that may be employed to change theexisting process in such a way as to reduce NOxemissions. For example, if the mass of NOx emittedfrom a plant is too high, one alternative is to reducethe firing rate. The reduction in NOx is proportion-al to the reduction in firing rate as less fuel isburned and, therefore, less NOx is formed. However,production is also reduced as well.

Another example is to replace some or all of the gas-fired equipment with electrically heated units that donot produce any NOx emissions at the point of use.However, the operating costs often increase as elec-tricity is usually more expensive than fossil fuels inheating applications.

Another method is to improve the thermal efficiencyof the process so less fuel is consumed per unit of pro-duction. This not only reduces pollution emissions, butoperating costs as well. In some special cases, it maybe possible to switch the material being heated to onethat requires less energy to process.

Some of these process modifications are radical and

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expensive and not often used exceptunder certain circumstances.

Combustion modification:Another strategy for reducingNOx is known as combustionmodification, where NOx forma-tion is minimized by changing thecombustion process. There arenumerous methods that havebeen used to accomplish this.

Reducing the level of air pre-heat, if present, can significantlyreduce NOx. However, this alsoreduces thermal efficiency andproductivity. Reducing theamount of excess air is a goodway to reduce NOx and increasethermal efficiency. This includesreducing air infiltration into thecombustion process.

However, reducing the excessair level too much can increasecarbon monoxide, which is anoth-er regulated pollutant.

A popular method is to replaceexisting burners with low NOxdesigns5. These incorporate avariety of techniques for reducingNOx such as air and fuel staging(see Figure 3), internal furnacegas recirculation (see Figure 4),water or steam injection, andultra-lean premixing.

External flue gas recirculationis another common technique forreducing NOx (see Figure 5).Most of these techniques involvereducing the peak flame tempera-tures that produce high levels ofNOx. Figure 6 shows an exampleof how new generations of burnerdesigns continue to reduce NOxemissions.

Post-treatment: Another strat-egy for minimizing NOx is knownas post-treatment where NOx isremoved from the exhaust gases after it has alreadybeen formed in the combustion chamber.

The general strategy is to use a reducing agent, suchas CO, CH4, other hydrocarbons, or ammonia, toremove the oxygen from the NO and convert it into N2and O2. Often, some type of catalyst is required for thereactions.

Two of the most common methods of post-treat-

ment are selective catalytic reduction (SCR) andselective noncatalytic reduction (SNCR). SCR isgenerally used instead of SNCR (see Figure 7) whenhigher NOx reduction is required.

A catalyst is a substance that causes or speeds upa chemical reaction without undergoing a chemicalchange itself. One of the advantages of post-treat-ment is that multiple exhaust streams can be treat-

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ed simultaneously, thus achieving economies ofscale.

Most of the post-treatment methods are relativelysimple to retrofit to existing processes. However,most are also fairly sophisticated and are not trivialto operate and maintain in industrial environments.For example, the catalytic reduction techniquesrequire a catalyst that may become plugged or poi-soned fairly quickly by dirty flue gases. Post-treat-ment methods are often capital intensive, and theyusually require halting production if there is a mal-function of the treatment equipment.

CONCLUSIONNOx is a regulated pollutant formed in nearly allindustrial combustion processes. It can generally beeasily controlled using a variety of proven strategies.The most cost-effective technique tends to be com-bustion modification, such as using low NOx burners.In virtually all cases, proper care must be taken tocarefully operate and maintain the equipment tokeep it within the desired range for low emissions.Suitable instrumentation, such as gas analyzers for

measuring O2 and NOx in the exhaust products, isrecommended to ensure the equipment is operatingaccording to specifications.

This will help metal processors continue to be envi-ronmentally friendly and within compliance of theirair permits.

REFERENCES1. Office of the Federal Register, 60.2 Definitions, U.S. Code of

Federal Regulations Title 40 Part 60, Washington, D.C.: U.S.Government Printing Office: 2001.

2. Baukal, C.E., Industrial Combustion Pollution and Control,Marcel Dekker, New York: 2004.

3. U.S. EPA, Control Techniques for Nitrogen Oxides Emissionsfrom Stationary Sources, EPA Report 450/1-78-001,Washington, D.C., U.S. Environmental Protection Agency:1978.

4. Bradford, M., Grover, R., and Paul, P., “Controlling NOxEmissions– Part 1,” CEP Magazine, 98(3):42-46: 2002.

5. Baukal, C.E. (ed.), Handbook of Industrial Burners, CRCPress, Boca Raton, Fla.: 2004.

For more information, contact Charles Baukal at (e-mail) charles.baukal@johnzink.com. mf

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