AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 17 ppsx

For coal-fired burners, fuel-NOx typically falls in the range of 50 to 70% of the total NOx emissions.2 Nitrogen in the fuel reacts with oxygen regardless of the flame temperature or exc

NO x Control

There are a number of oxides of nitrogen, including nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O3), and nitrogen pentoxide (N2O5), that are referred to collectively as NOx The two oxides of nitrogen that are

of primary concern to air pollution are NO and NO2 NO is a colorless gas that is

a precursor to NO2 and is an active compound in photochemical reactions that produce smog NO2 is a reddish brown gas that gives color to smog and can contribute

to opacity in flue gas plumes from stacks

NO2 is a criteria pollutant with a National Ambient Air Quality Standard of

100 µg/m3, or 053 ppm, annual average It is also a precursor to nitric acid, HNO3,

in the atmosphere and is a major contributor to acid rain, although less important than SO2, which is discussed in the next chapter Nitric acid contributes only one proton per molecule while sulfuric acid has two protons per molecule, and mass emissions of sulfur compounds are larger than oxides of nitrogen Finally, NOx and volatile organic compounds (VOC) react photochemically in a complex series of reactions to produce smog, which includes ozone, NO2, peroxyacetyl nitrate (PAN), peroxybenzoyl nitrate (PBN) and other trace oxidizing agents

By far the largest source of NOx is combustion, although there are other industrial sources such as nitric acid manufacturing Figure 17.1 shows the relative contribution from NOx emission sources The large amount of NOx generated at coal-fired electric power plants is evident, and the very large contribution from motor vehicles and other forms of transportation, including ships, airplanes, and trains, is pronounced

23 tons per year, despite industrial growth and a growing number of vehicles on the road Preventing an increase in total NOx emissions can be attributed to the increased use of NOx controls, especially in automobiles and in industrial fuel consumption

17.1 NO X FROM COMBUSTION

NOx is generated during combustion from three mechanisms: (1) thermal NOx, (2) prompt NOx, and (3) fuel NOx Understanding these mechanisms enables one to utilize control methods for NOx emissions

17.1.1 T HERMAL NO X

The thermal NOx mechanism was first proposed by Zeldovich1 and involves radicals

to produce the overall reaction of combining oxygen and nitrogen:

(17.1)

17

O2↔2O 9588ch17 frame Page 241 Wednesday, September 5, 2001 10:01 PM

(17.2) (17.3) (17.4) The overall reaction that produces NO2 is

(17.5)

FIGURE 17.1 NOx emission sources.

FIGURE 17.2 NOx emission trends in the U.S.

O+N2↔NO+N

N+O2↔NO O+

N2+O2↔2NO

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Both thermodynamics and kinetics are important to the formation of thermal

NOx, so both concentration and temperature influence the amount of NOx produced The thermodynamic equilibrium concentrations for reactions 17.4 and 17.5 are

(17.6)

(17.7)

Equilibrium constants at different temperatures are listed in Table 17.1 Equi-librium concentrations calculated from Equations 17.6 and 17.7 are presented in

Table 17.2 for oxygen and nitrogen concentrations in air and in a typical combustion source flue gas

Based on thermodynamic equilibrium alone, calculated NOx concentrations in flame zones at 3000 to 3600°F would be about 6000 to 10,000 ppm, and the NO to

NO2 ratio would be 500:1 to 1000:1 At typical flue gas exit temperatures of 300 to

TABLE 17.1 Equilibrium Constants for NO and

NO 2 Formation

°F

N 2 + O 2↔ 2NO NO + ½ O 2↔ NO 2

80 1.0 × 10 –30 1.4 × 10 6

1340 7.5 × 10 –9 1.2 × 10 –1

2240 1.1 × 10 –5 1.1 × 10 –2

3500 3.5 × 10 –3 2.6 × 10 –3

TABLE 17.2 Equilibrium Concentrations

Air 21% O 2 , 79% N 2

Flue Gas 3.3% O 2 , 76% N 2

80 3.4 × 10 –10 2.1 × 10 –4 1.1 × 10 –10 3.3 × 10 –5

980 2.3 0.7 0.8 0.1

2060 800 5.6 250 0.9

2912 6100 12 2000 1.8

P NO

1

2

[ ][ ]

P

NO

2

2

2 1

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600°F, the NOx concentration would be very low at less than 1 ppm, and the NO to

NO2 ratio would be very low at 1:10 to 1:10,000 Yet typical NOx emissions from

uncontrolled natural-gas fired boilers are 100 to 200 ppm Typical NOx emissions

from uncontrolled fuel-oil-fired and coal-fired boilers, which have higher flame

temperatures, are 200 to 400 ppm and 300 to 1200 ppm, respectively And the typical

NO to NO2 ratio in boiler emissions is 10:1 to 20:1

The reason for the observed NOx concentrations being so different from

equi-librium expectations is the effect of kinetics The reaction rate is a strong function

of temperature Gases reside in the flame zone of a burner for a very short time, less

than 0.5 s The time required to produce 500 ppm NO at 3600°F is only about 0.1 s,

but at 3200°F the required time is 1.0 s Once the gases leave the flame zone, reaction

rates are reduced by orders of magnitude, so NO formation stops quickly Also, the

reversible reactions shown by Equations 17.4 and 17.5 slow to nearly a halt, thereby

“freezing” the NOx concentration and the ratio of NO to NO2

17.1.2 P ROMPT NO X

NOx concentrations near the flame zone for hydrocarbon fuels demonstrate less

temperature dependence than would be expected from the thermodynamic and

kinet-ics considerations of the Zeldovich mechanism discussed above for thermal NOx

Near the flame zone, radicals such as O and OH enhance the rate of NOx formation

Hence, some NOx will form despite aggressive controls on flame temperature and

oxygen concentration

17.1.3 F UEL NO X

Some fuels contain nitrogen, e.g., ammonia or organically bound nitrogen in

hydro-carbon compounds For coal-fired burners, fuel-NOx typically falls in the range of

50 to 70% of the total NOx emissions.2 Nitrogen in the fuel reacts with oxygen

regardless of the flame temperature or excess oxygen concentration in the

combus-tion air Carbon–nitrogen bonds are broken more easily than diatomic nitrogen bonds,

so fuel-NOx formation rates can be higher than thermal-NOx Combustion control

techniques that aim at reducing thermal-NOx formation by reducing flame

temper-ature may not be effective for fuels that have high nitrogen content

17.2 CONTROL TECHNIQUES

Two primary categories of control techniques for NOx emissions are (1) combustion

controls, and (2) flue gas treatment Very often more than one control technique is

used in combination to achieve desired NOx emission levels at optimal cost When

evaluating control technology, it is desirable to quantify the capability for percent

reduction of NOx This can be difficult, however, because the baseline operations

may or may not be established at good combustor operation, and because the

performance of individual technologies is not additive And there are a number of

techniques that can be used in a wide variety of combinations

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17.2.1 C OMBUSTION C ONTROL T ECHNIQUES

A variety of combustion control techniques are used to reduce NOx emissions by

taking advantage of the thermodynamic and kinetic processes described above Some

reduce the peak flame temperature; some reduce the oxygen concentration in the

primary flame zone; and one, reburn, uses the thermodynamic and kinetic balance

to promote reconverting NOx back to nitrogen and oxygen

17.2.1.1 Low-Excess Air Firing

Combustors tend to be easier to operate when there is plenty of oxygen to support

combustion, and operators like to adjust the air-to-fuel ratio to produce a stable and

hot flame By simply cutting back the amount of excess air, the lower oxygen

concentration in the flame zone reduces NOx production In some cases where too

much excess air has become normal practice, thermal efficiency is improved

How-ever, low excess air in the resulting flame may be longer and less stable, and carbon

monoxide emissions may increase Tuning the combustion air requires minimal

capital investment, possibly some instrumentation and fan or damper controls, but

it does require increased operator attention and maintenance to keep the system in

optimal condition Depending on the prior operating conditions, combustion air

tuning can produce NOx reduction of 0 to 25%.3 Tuning ranges from simple

adjust-ments to advanced modeling that incorporates neural networks Applying advanced

optimization systems at four coal-fired power plants resulted in NOx emission

reduc-tions of 15 to 55%.4

17.2.1.2 Overfire Air

The primary flame zone can be operated fuel rich to reduce oxygen concentration,

then additional air can be added downstream This overfire air provides oxygen to

complete combustion of unburned fuel and oxidizes carbon monoxide to carbon

dioxide, creating a second combustion zone Because there is so little fuel in this

overfire zone, the peak flame temperature is low Thus, NOx formation is inhibited

in both the primary and overfire combustion zones

17.2.1.3 Flue Gas Recirculation

In this technique, some of the flue gas, which is depleted in oxygen, is recirculated

to the combustion air This has two effects: (1) the oxygen concentration in the

primary flame zone is decreased, and (2) additional nitrogen absorbs heat, i.e., acts

as a heat sink, and reduces the peak flame temperature NOx reduction as a function

of the amount of recirculated flue gas is plotted in Figure 17.3.5

17.2.1.4 Reduce Air Preheat

Combustion air often is preheated in a recuperator with the heat from the flue gas

This conserves energy by recovering the heat in the flue gas However, it also raises

the peak flame temperature because the combustion air absorbs less heat from the

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combustor prior to reacting with the fuel Reducing air preheat lowers the flame temperature to reduce the formation of thermal NOx

17.2.1.5 Reduce Firing Rate

Peak flame temperature is determined by the complete heat balance in the combus-tion chamber, including radiant heat losses to the walls of the chamber Reducing both air and fuel proportionately would result in the same flame temperature if only fuel, air, and combustion products were considered However, reducing fuel and air

in a fixed size chamber results in a proportionately larger heat loss to the chamber walls and peak flame temperature is reduced

17.2.1.6 Water/Steam Injection

Injecting water or steam into the combustion chamber provides a heat sink that reduces peak flame temperature However, a greater effect is believed to result from the increased concentration of reducing agents within the flame zone as steam dissociates into hydrogen and oxygen Compared to standard natural draft, in natural gas-fired burners, up to 50% NOx reduction can be achieved by injecting steam at

a rate up to 20 to 30% of the fuel weight.6

17.2.1.7 Burners out of Service (BOOS)

In a large, multiburner furnace, selected burners can be taken out of service by cutting their fuel The fuel is redistributed to the remaining active burners, and the total fuel rate is not changed Meanwhile, combustion air is unchanged to all burners This becomes an inexpensive way to stage the combustion air The primary flames

FIGURE 17.3 Effectiveness of flue gas recirculation (Reproduced with permission of the

American Institute of Chemical Engineers, Copyright © 1994 AIChE All rights reserved.)

operate fuel rich and depleted in oxygen, reducing NOx formation Outside the hot flame zone, but still inside the combustion chamber where combustion goes to completion, the additional combustion air burns the remaining fuel at a reduced flame temperature Test results on a coal-fired boiler demonstrated NOx emission reduction of 15 to 30%.7

17.2.1.8 Reburn

A second combustion zone after the primary flame zone can be established by adding additional hydrocarbon fuel outside of the primary flame zone NOx is reduced by reaction with hydrocarbon radicals in this zone.8 Overfire air is added after reburn

to complete the combustion process at a low temperature flame Results from five coal-fired boilers from 33 to 158 MW net capacity show NOx reduction from baseline levels of 58 to 77%.9

17.2.1.9 Low-NO x Burners

Low-NOx burners are designed to stage either the air or the fuel within the burner tip The principle is similar to overfire air (staged air) or reburn (staged fuel) in a furnace With staged-air burners, the primary flame is burned fuel rich and the low oxygen concentration minimizes NOx formation Additional air is introduced outside

of the primary flame where the temperature is lower, thereby keeping the thermo-dynamic equilibrium NOx concentration low, but hot enough to complete combus-tion The concept of a staged-air burner is illustrated in Figure 17.4

Staged-fuel burners introduce fuel in two locations A portion of the fuel is mixed with all of the combustion air in the first zone, forming a hot primary flame with abundant excess air NOx formation is high in this zone Then additional fuel

is introduced outside of the primary flame zone, forming a low-oxygen zone that is still hot enough for kinetics to bring the NOx concentration to equilibrium in a short period of time In this zone, NOx formed in the primary flame zone reverts back to nitrogen and oxygen A staged fuel burner is illustrated in Figure 17.5 Low NOx burners can reduce NOx emissions by 40 to 65% from emissions produced by conventional burners

Because low-NOx burners stage either the air or the fuel, the flame zone is lengthened The typical flame length of low-NOx burners is about 50 to 100% longer than that of standard burners This can cause a problem in some retrofit applications

if the longer flame impinges on the walls of the combustion chamber Flame impinge-ment can cause the chamber walls to erode and fail While burner replaceimpinge-ment may

be an easy retrofit technique for NOx control for many furnaces, it cannot be used

in all situations It is recommended that the flame length should be kept to a third

of the firebox height for long vertical cylindrical heaters, and to no more than two thirds of the firebox height for low-roof cabin heaters.10

Low-NOx burners also have been developed for coal-fired applications, where

NOx concentrations are significantly higher than produced in liquid and gaseous fuel applications Using low-NOx burners alone, a NOx emission level of 90 to 140 ppm

at 3% O2 can be achieved.11

FIGURE 17.4 Staged air low-NOx burner (Courtesy of John Zink Company, LLC.)

FIGURE 17.5 Staged fuel low-NOx burner (Courtesy of John Zink Company, LLC.)

17.2.1.10 Ultra Low-NO x Burners

Ultra low-NOx burners have been developed that incorporate mechanisms beyond simply staging air or fuel as designed in low-NOx burners They may incorporate flue gas recirculation within the furnace that is induced by gas flow and mixing patterns, and use additional levels of air and/or fuel staging High fuel gas pressure

or high liquid fuel atomization pressure is used to induce recirculation Also, ultra low-NOx burners may use inserts to promote mixing to improve combustion despite low oxygen concentrations in the flame Remember the “three Ts” of good combus-tion discussed in Chapter 13 — time, temperature, and turbulence Good combuscombus-tion

at low oxygen concentration is essential to balance low NOx formation while avoid-ing soot and excess CO emissions

Ultra low-NOx burners for gas-fired industrial boilers and furnaces have dem-onstrated the capability of achieving 10 to 15 ppm NOx on a dry basis corrected to 3% oxygen.12

17.2.2 F LUE G AS T REATMENT T ECHNIQUES

17.2.2.1 Selective Noncatalytic Reduction (SNCR)

Selective noncatalytic reduction uses ammonia (NH3) or urea (H2NCONH2) to reduce

NOx to nitrogen and water The overall reactions using ammonia as the reagent are

(17.8) (17.9)

The intermediate steps involve amine (NHi) and cyanuric nitrogen (HNCO) radicals When urea is used, it first dissociates to the primary reactants of ammonia and isocyanic acid.13

No catalyst is required for this process; just good mixing of the reactants at the right temperature and some residence time The key to this process is operating within the narrow temperature window Sufficient temperature is required to promote the reaction The presence of hydrogen in the flue gas, if there is a source of it such

as dissociation of steam, increases the operable temperature range at the cooler end

At higher temperatures, ammonia oxidizes to form more NO, thereby wasting ammo-nia reagent and creating the pollutant that was intended to be removed Above 1900°F, this reaction dominates

(17.10)

The effect of temperature on SNCR performance is illustrated in Figure 17.6 The critical dependence of temperature requires excellent knowledge of the temper-ature profile within the furnace for placement of reagent injection nozzles Compu-tational fluid dynamic (CFD) models often are used to gain this required knowledge

2NH3+2NO O+ 2+H2↔2N2+4H O2 1300 1900– °F

4NH3+5O2↔4NO+6H O2

A significant complication for a practical system is designing the system for the variable temperature profiles with turndown of a boiler Operating a furnace at half load obviously will impact the temperature profile Nozzles may be installed at multiple locations, then reagent is injected at only the locations appropriate for the load conditions A second complicating factor is the availability of residence time

at the proper temperature The available residence time also may change with load conditions due to the flue gas flow and the boiler configuration

In a typical application, SNCR produces about 30 to 50% NOx reduction Some facilities that require higher levels of NOx reduction take advantage of the low capital cost of the SNCR system, then follow the SNCR section with an SCR system (discussed in the following section) Capital costs may be lower than an SCR system alone because the catalyst bed for the SCR can be smaller due to the lower NOx removal requirement for SCR after the SNCR system has removed a significant portion of the NOx

17.2.2.2 Selective Catalytic Reduction (SCR)

A catalyst bed can be used with ammonia as a reducing agent to promote the reduction reaction and to lower the effective temperature An SCR system consists primarily of an ammonia injection grid and a reactor that contains the catalyst bed

A simplified sketch of the system is shown in Figure 17.7

The following reactions result in reducing NOx in an SCR system Reaction 17.11

is dominant Since the NO2 concentration in the flue gas from combustion systems usually is low, then reactions 17.13, 17.14, and 17.15 are not particularly significant

to the overall NOx reduction or to the reagent requirement

FIGURE 17.6 SNCR temperature window.