NOx - Air Pollution - Lecture Slides, Slides of Ecology and Environment

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NOx

Sources and Control Methods

Specific sources of NO

x

Combustion sources

• Automobiles• Boilers• Incinerators

High-temperature industrial operations

• Metallurgical furnaces• Blast furnaces• Plasma furnaces• Kilns

Other sources

• Nitric acid plants• Industrial processes that use nitric acid

• NOx

is a main ingredient in the formation of ground-level ozone,

triggering serious respiratory problems •^

reacts to form nitrate particles, acid aerosols, as well as NO

which also cause respiratory problems •^

contributes to formation of acid rain

-^

contributes to nutrient overload that deteriorates water quality

-^

contributes to atmospheric particles that cause visibility impairment most noticeable in national parks •^

reacts to form toxic chemicals

-^

contributes to global warming

NOx effects

(taken from EPA)

NOx characteristicsNO – Nitric oxide

•^

Colorless and odorless gas

-^

Insoluble in water (Remember this for later on!)

-^

Toxic

NO

2

• Nitrogen dioxide

Usually a dimer compound (N

O 2

4 ) at low

(^0)

C

Distinct reddish-brown color

Moderately soluble in aqueous liquids

Toxic

Contributes to brown haze that occurs with smog

NOx Formation

Formed at elevated temperatures duringcombustion of fuel in the presence of air.

Approximately 90 to 95% of the nitrogenoxides generated in combustionprocesses are in the form of nitric oxide(NO). (Remember this for later on!)

Once in the atmosphere, the NO reacts ina variety of photochemical and thermalreactions to form NO

2

•^

Thermal NOx: formed by reaction between N

2

and O

2

in the air; sensitive to temperature

Fuel (or Prompt) NOx: formed from combustion of fuel containing organic nitrogen; dependent onlocal combustion conditions and nitrogen contentin the fuel. •

Not all of the fuel nitrogen compounds are

released during combustion. Unlike sulfur, asignificant fraction of the fuel nitrogen remains inthe bottom ash or in the fly ash.

NOx Formation

NOx control technologies

See also:^ http://www.netl.doe.gov/technologies/coalpower/ewr/nox/control.html

Control Techniques

1. Modify combustion to suppress NOx formation

Low excess air operation

Off-stoichiometric combustion

Flue gas recirculation

Natural gas reburning

2. Reduce Nox to molecular nitrogen through

controls (also known as flue gas treatment) •

Selective Non-Catalytic Reduction (SNCR)

Selective Catalytic Reduction (SCR)

Dry Sorption

Combustion Modifications

Low excess air operation:

Involves a reduction in the total

quantity of air used in the combustion process. By usingless oxygen, the amount of NOx produced is not as great.

Combustion Modifications

Off-stoichiometric combustion: involves mixing offuel and air in a way to reduce peak gas temperaturesand peak oxygen concentrations. 

Low NOx burners: Keeps temperatures down and dissipates heat quickly 

Overfire air (OFA): Keeps mixture fuel rich and completes combustion process using airinjection nozzles 

Burners out of service (BOOS): Operates

alternate burners in combustion zone asfuel rich, air rich, and air only

Combustion Modifications

Fuel reburning:

Involves the operation of the main

burners in a boiler at very low excess air (fuel richconditions). A series of overfire air ports are used inthis upper region to provide all of the air needed forcomplete combustion.

Add-On Controls

(Flue Gas Treatment

)

Selective non-catalytic reduction systems (SNCR) Involves the injection of ammonia (NH

3

) or urea

into the hot gas zone where reactions leading toreduction of nitrogen oxides can occur.

The

reactions are completed within the boiler, and nowaste products are generated. There is a risk ofammonia (NH

3

) being emitted into the atmosphere

if temperatures are too low, however.

SCNR

systems are capable of reducing nitrogen oxidesfrom 20 to 60%.