44_NOXC

245

The first priority in designing a strategy to con-

trol nitrogen oxides is to protect human health.

Human health impacts appear to be related to

peak exposures to nitrogen oxides (NO

). In ad-

dition to potentially damaging human health,

nitrogen oxides are precursors to ozone (O

) for-

mation, which can harm human health and veg-

etation. Finally, nitrogen oxides contribute to acid

deposition, which damages vegetation and

aquatic ecosystems.

The extent to which NO

emissions harm hu-

man health depends on ground-level concentra-

tions and the number of people exposed. Source

location can affect these parameters. Gases emit-

ted in areas with meteorological, climatological,

and topographical features that favor dispersion

will be less likely to concentrate near the ground.

However, some meteorological conditions, such

as inversion, may result in significantly higher

ambient levels. Sources away from population

centers will expose fewer people to harmful pol-

lution. Plant siting is a critical feature in any air

pollution management strategy. However, due to

the dispersion of nitrogen oxides that may con-

tribute to ozone formation and acid deposition

far from the source, relying on plant siting alone

is not a recommended strategy. The long-term

objective must be to reduce total emissions.

Effective control of NO

emissions will require

controls on both stationary sources and mobile

transport sources. Each requires different strate-

gies. This guideline focuses on control strategies

for stationary sources (primarily fossil-fuel-fired

electricity-generating plants).

Limiting Emissions from Stationary Sources

Nitrogen oxides are produced in the combustion

process by two different mechanisms: (a) burn-

ing the nitrogen in the fuel, primarily coal or

heavy oil (fuel NO

); and (b) high-temperature

oxidation of the molecular nitrogen in the air used

for combustion (thermal NO

). Formation of fuel

depends on combustion conditions, such as

oxygen concentration and mixing patterns, and

on the nitrogen content of the fuel. Formation of

thermal NO

depends on combustion tempera-

ture. Above 1,538

C (2,800

F), NO

formation rises

exponentially with increasing temperature (Stultz

and Kitto 1992). The relative contributions of fuel

and thermal NO

to emissions from a par-

ticular plant depend on the combustion condi-

tions, the type of burner, and the type of fuel.

Approaches for controlling NO

from station-

ary sources can address fuel NO

, thermal NO

or both. One way of controlling NO

emissions

is to use low-nitrogen fuels. Another is to modify

combustion conditions to generate less NO

. Flue

gas treatment techniques, such as selective cata-

lytic reduction (SCR) processes, can remove NO

Choice of Fuel

Coals and residual fuel oils containing organi-

cally bound nitrogen contribute to over 50% of

total emissions of NO

, according to some esti-

mates. The nitrogen content of U.S. coal ranges

between 0.5% and 2% and that of residual fuel

oil between 0.1% and 0.5%. In many circum-

stances, the most cost-effective means of reduc-

ing NO

emissions will be to use low-nitrogen

fuels such as natural gas. Natural gas used as fuel

can emit 60% less NO

than coal and virtually no

particulate matter or sulfur oxides.

Combustion Control

Combustion control may involve any of three

strategies: (a) reducing peak temperatures in the

combustion zone; (b) reducing the gas residence

Nitrogen Oxides: Pollution Prevention

and Control

Pollution Prevention and Abatement Handbook

WORLD BANK GROUP

Effective July 1998

246

PROJECT GUIDELINES: POLLUTANT CONTROL TECHNOLOGIES

time in the high-temperature zone; and (c) reduc-

ing oxygen concentrations in the combustion

zone. These changes in the combustion process

can be achieved either through process modifi-

cations or by modifying operating conditions on

existing furnaces. Process modifications include

using specially designed low-NO

burners,

reburning, combustion staging, gas recirculation,

reduced air preheat and firing rates, water or

steam injection, and low excess air (LEA) firing.

These modifications are capable of reducing NO

emissions by 50 to 80%. The method of combus-

tion control used depends on the type of boiler

and the method of firing fuel.

Process Modifications

New low-NO

burners are effective in reducing

emissions from both new power plants and

existing plants that are being retrofitted. Low-

burners limit the

formation of nitrogen ox-

ides by controlling the mixing of fuel and air, in

effect automating low-excess-air firing or staged

combustion. Compared with older conventional

burners, low-NO

burners reduce emissions of

by 40–60%. Because low-NO

burners are

relatively inexpensive, power utilities have been

quick to accept them; in fact, low-NO

burners

are now a standard part of new designs. Capital

costs for low-NO

burners with overfire air (OFA)

range between US$20 and US$25 per kilowatt

(Bounicore and Davis 1992; Kataoka, personal

communication, 1994).

Unfortunately, low-NO

burners are not suit-

able for reducing NO

emissions from cyclone-

fired boilers, which emit large quantities of NO

due to their high operating temperatures. Be-

cause combustion takes place outside the main

furnace, the use of low-NO

burners is not suit-

able for these applications (Bounicore and Davis

1992). However, reburning technology can reduce

emissions.

Reburning is a technology used to reduce NO

emissions from cyclone furnaces and other se-

lected applications. In reburning, 75–80% of the

furnace fuel input is burned in the furnace with

minimum excess air. The remaining fuel (gas, oil,

or coal) is added to the furnace above the pri-

mary combustion zone. This secondary combus-

tion zone is operated substoichiometrically to

generate hydrocarbon radicals that reduce to ni-

trogen the nitrogen oxides that are formed. The

combustion process is then completed by add-

ing the balance of the combustion air through

overfire air ports in a final burnout zone at the

top of the furnace.

Staged combustion (off-stoichiometric combustion)

burns the fuel in two or more steps. Staged com-

bustion can be accomplished by firing some of

the burners fuel-rich and the rest fuel-lean, by

taking some of the burners out of service and al-

lowing them only to admit air to the furnace, or

by firing all the burners fuel-rich in the primary

combustion zone and admitting the remaining

air over the top of the flame zone (OFA); see

Cooper and Alley 1986). Staged combustion tech-

niques can reduce NO

emissions by 20–50%.

Conventional OFA alone can reduce emissions of

by 30%, and advanced OFA has the poten-

tial of reducing them still further, although po-

tential for corrosion and slagging exists. Capital

costs for conventional and advanced OFA range

between US$5 and $10 per kilowatt (Bounicore

and Davis 1992).

Flue gas recirculation (FGR) is the rerouting of

some of the flue gases back to the furnace. By

using the flue gas from the economizer outlet,

both the furnace air temperature and the furnace

oxygen concentration can be reduced. However,

in retrofits FGR can be very expensive. Flue gas

recirculation is typically applied to oil- and gas-

fired boilers and reduces NO

emissions by 20–

50%. Modifications to the boiler in the form of

ducting and an energy efficiency loss due to the

power requirements of the recirculation fans can

make the cost of this option higher than for some

of the in-furnace NO

control methods.

Reduced air preheat and reduced firing rates lower

peak temperatures in the combustion zone, thus

reducing thermal NO

. This strategy, however,

carries a substantial energy penalty. Emissions

of smoke and carbon monoxide need to be con-

trolled, which reduces operational flexibility.

Water or steam injection reduces flame tempera-

tures and thus thermal NO

. Water injection is

especially effective for gas turbines, reducing NO

emissions by about 80% at a water injection rate

of 2%. For a gas turbine, the energy penalty is

about 1%, but for a utility boiler it can be as high

as 10%. For diesel-fired units, 25–35% reductions

in NO

emissions can be achieved using water-

fuel mixtures.

247

Nitrogen Oxides: Pollution Prevention and Control

Modifications in Operating Conditions

Low-excess-air firing (LEA) is a simple, yet effec-

tive technique. Excess air is the amount of air in

excess of what is theoretically needed to achieve

100% combustion. Before fuel prices rose, it was

not uncommon to see furnaces operating with 50–

100% excess air. Currently, it is possible to achieve

full combustion for coal-fired units with less than

15–30% excess air. Studies have shown that re-

ducing excess air from an average of 20% to an

average of 14% can reduce emissions of NO

an average of 19% (Cooper and Alley 1986).

Techniques involving low-excess-air firing,

staged-combustion, and flue gas recirculation

are effective in controlling both fuel NO

and

thermal NO

. The techniques of reduced air

preheat and reduced firing rates (from normal

operation) and water or steam injection are ef-

fective only in controlling thermal NO

These will

therefore not be as effective for coal-fired units,

since about 80% of the NO

emitted from these

units is fuel NO

Flue Gas Treatment

Flue gas treatment (FGT) is more effective in re-

ducing NO

emissions than are combustion con-

trols, although at higher cost. FGT is also useful

where combustion controls are not applicable.

Pollution prevention measures, such as using a

high-pressure process in nitric acid plants, is more

cost-effective in controlling NO

emissions. FGT

technologies have been primarily developed and

are most widely used in Japan and other OECD

countries. The techniques can be classified as se-

lective catalytic reduction, selective noncatalytic

reduction, and adsorption.

Selective catalytic reduction (SCR) is currently

the most developed and widely applied FGT

technology. In the SCR process, ammonia is used

as a reducing agent to convert NO

to nitrogen

in the presence of a catalyst in a converter up-

stream of the air heater. The catalyst is usually a

mixture of titanium dioxide, vanadium pentox-

ide, and tungsten trioxide (Bounicore and Davis

1992). SCR can remove 60–90% of NO

from flue

gases. Unfortunately, the process is very expen-

sive (US$40–$80/kilowatt), and the associated

ammonia injection results in an ammonia slip-

stream in the exhaust. In addition, there are some

concerns associated with anhydrous ammonia

storage.

Selective noncatalytic reduction (SNCR) using

ammonia- or urea-based compounds is still in the

developmental stage. Early results indicate that

SNCR systems can reduce NO

emissions by 30–

70%. Capital costs for SNCR are expected to be

much lower than for SCR processes, ranging be-

tween US$10 and US$20 per kilowatt (Bounicore

and Davis 1992; Kataoka, 1992). Several dry ad-

sorption techniques are available for simultaneous

control of NO

and sulfur oxides (SO

). One type

of system uses activated carbon with ammonia

(NH

)

injection to simultaneously reduce the NO

to nitrogen (N

) and oxidize the SO

to sulfuric

acid (H

). If there is no sulfur in the fuel, the

carbon acts as a catalyst for NO

reduction only.

Another adsorption system uses a copper oxide

catalyst that adsorbs sulfur dioxide to form cop-

per sulfate. Both copper oxide and copper sul-

fate are reasonably good catalysts for the selective

reduction of NO

with NH

. This process, which

has been installed on a 40-megawatt oil-fired

boiler in Japan, can remove about 70% of NO

and 90% of SO

from flue gases (Cooper and

Alley 1986).

Applications of NO

Control Systems

For coal-fired boilers (which accounted for a major

portion of all utility NO

emissions), the most

widely applied control technologies involve com-

bustion modifications, including low-excess-air

firing, staged combustion, and use of low-NO

burners. For oil-fired boilers, the most widely ap-

plied techniques include flue gas recirculation,

in addition to the techniques used for coal-fired

units. For gas-fired units, which in any case emit

60% less NO

than coal-fired units, the primary

control technologies include flue gas recircula-

tion and combustion modifications. Finally, for

diesel plants, the common technologies are water-

steam injection, and SCR technology.

Table 1 summarizes the NO

reduction rates

that are normally achieved through combustion

modifications and flue gas treatment systems.

Recommendations

The most cost-effective methods of reducing

emissions of NO

are the use of low-NO

burners

248

PROJECT GUIDELINES: POLLUTANT CONTROL TECHNOLOGIES

and the use of low nitrogen fuels such as natural

gas. Natural gas has the added advantage of

emitting almost no particulate matter or sulfur

dioxide when used as fuel. Other cost-effective

approaches to emissions control include combus-

tion modifications. These can reduce NO

emis-

sions by up to 50% at reasonable cost. Flue gas

treatment systems can achieve greater emissions

reductions, but at a much higher cost.

Table 2 shows applications of NO

abatement

technologies.

References and Sources

Bounicore, Anthony J., and Wayne T. Davis, eds. 1992.

Air Pollution Engineering Manual. New York: Van

Nostrand Reinhold.

Cooper, C. David, and F. C. Alley. 1986. Air Pollution

Control: A Design Approach. Prospect Heights, Ill.:

Waveland Press.

Godish, Thad. 1991. Air Quality. Chelsea, Mich.: Lewis

Publishers.

Jechoutek, Karl G., S. Chattopadhya, R. Khan, F. Hill,

and C. Wardell. 1992. “Steam Coal for Power and

Industry.” Industry and Energy Department Work-

ing Paper. Energy Series Paper 58. World Bank,

Washington, D.C.

Kataoka, S. 1992. “Coal Burning Plant and Emission

Control Technologies.” Technical Note. World Bank,

China Country Department, Washington, D.C.

————. 1994. Personal communication.

OECD (Organisation for Economic Co-operation

and Development). 1983. Control Technology for

Nitrogen Oxide Emissions from Stationary Sources.

Paris.

Stern, C., R. Boubel, D. Turner, and D. Fox. 1984. Fun-

damentals of Air Pollution. Orlando, Fla.: Academic

Press.

Stultz, S. C., and John B. Kitto, eds. 1992. Steam: Its

Generation and Use. 40th ed. Barberton, Ohio: The

Babcock & Wilcox Co.

USEPA (United States Environmental Protection

Agency). 1992. Evaluation and Costing of NO

Con-

trols for Existing Utility Boilers in the NESCAUM Re-

gion. Washington, D.C.

————. 1986. “Compilation of Air Pollution Emis-

sion Factors. AP-42. (October 1992 version).” Wash-

ington, D.C.

Vatavuk, W. M. 1990. Estimating Costs of Air Pollution

Control. Chelsea, Mich.: Lewis Publishers.

Table 1. NO

Removal Efficiencies for Combustion Modifications and Flue Gas Treatment

(percentage reduction in NO

)

reduction technique Coal Oil Gas

Combustion modification

Low-excess-air firing 10–30 10–30 10–30

Staged combustion 20–50 20–50 20–50

Flue gas recirculation n.a. 20–50 20–50

Water/steam injection n.a. 10–50 n.a.

Low-NO

burners 30–40 30–40 30–40

Flue gas treatment

Selective catalytic reduction 60–90 60–90 60–90

Selective noncatalytic reduction n.a. 30–70 30–70

n.a. Not applicable.

Table 2. Applicability of NO

Abatement Technologies, by Type of Facility and by Technique

Petro- Cement Waste Nitric Internal

Boiler Metal leum Sinter- calci- Glass inciner- acid combus-

Large/ heating heating ing nation melting Coke ation manu- tion Gas

Technique medium Small furnace furnace furnace furnace furnace oven furnace facture engine turbine Diesel

Low excess air ••••U••M

Two-stage combustion

(including off-stoichio-

metric combustion) • M • •

Flue gas recirculation • M M U M M M

Water/steam injection

(including emulsion fuel) M U U M M M M

Low-NO

burners • • M • U

Selective catalytic

reduction M U M U U U M M U U M

Nonselective catalytic

reduction U U M M

Noncatalytic reduction M U U

Wet-chemical scrubbing M U U U U M

Other: change of temperature M • (Use pre- • (Use high-

profile; nonsuspension heaters pressure

preheater kiln and pre- process)

calciners)

Notes:

• indicates high reliability; M, some points must be taken into account in the case of actual application; U, under study in a test plant.

Source:

Adapted from OECD 1983 (verified as current).

249