Methods to control NOx emissions Air pollution and control Control of gaseous pollutants Environmental engineering

1. Methods to control NOx
emissions
Ravi Kumar GARRE

2. Contents covered
• Introduction
• Absorption methods
• Adsorption methods
• Combustion methods

3. Introduction
• There are three types of sources of NOX:
mobile sources, stationary sources and
chemical factories.
• There are seven types of NOX forms in
atmosphere: NO, N2O, NO2, NO3, N2O3, N2O4
and N2O5.
• Among all these oxides of Nitrogen, NO and
NO2 are primary pollutants.

4. Introduction
• NOX corrodes metal surfaces, damage building
surfaces, reduces visibility due to smog
formation and reduce the life of insulating
materials
• Also causes bronchitis, respiratory diseases,
irritation of eyes, reduce visibility
• Also cause damage to leaf structure of plants
• NOX takes a part in photochemical reactions in
the atmosphere.

5. Introduction
• The methods are available to control NOX
emissions are:
 Absorption in solutions
 Adsorption on solids
 Catalytic combustion
• Improved design of combustion engines/
furnaces, as well as their proper maintenance
and operation also help to reduce pollution
due to NOX.

6. Absorption methods
a) Absorption by slurry of MgOH-MgCO3
b) Absorption by aluminium sulphite and bi
sulphite solutions
c) Absorption by CaOH solution
d) Absorption by FeSO4 (ferrous sulphate
solution)
e) Absorption in other media

7. Absorption by slurry of MgOH-MgCO3
• The effluent gases containing NOx are
absorbed by Magnesium hydroxide-
Magnesium carbonate slurry at 30˚C to 40˚C.
• Magnesium hydroxide can be recovered from
the used-up slurry by heating to 200˚C using
steam.
• This absorption process is can be conducted in
packed tower/ venturi scrubber/ spray
tower.

8. Absorption by slurry of MgOH-MgCO3
• The resultant solution contains magnesium
nitrite, magnesium nitrate, magnesium
hydroxide and other NOX emissions be
produced.
• When we recover Nitric acid from the
resultant solution in Nitric acid plant, the NOX
can be removed/ collected separately.

9. Absorption by aluminium sulphite and bi
sulphite solutions
• This type of absorption also conducted in
packed towers.
• Because in packed towers area for transfer is
high and pressure loss is low.
• The flow of solutions and gases are counter-
current.
• The solution contains the ratio of sulphite and
bisulphite is between 0.1 to 0.4.

10. ABSORPTION BY ALUMINIUM SULPHITE
AND BI SULPHITE SOLUTIONS
• The reactions occurring are
• The absorbent is finally converted to ammonium
sulphate and processed for fertilizer use.
• This method is ideal when the gases containing
NOX are the exhaust from a fertilizer plant.

11. Absorption by CaOH solution
• The gases containing nitrogen oxides are
absorbed in calcium hydroxide solution obtained
from the slaking of lime.
• The cost of the slaking unit adds to the cost of the
process.
• The Calcium nitrite is produce in the reaction and
then it will be converted to Calcium sulphate.
• This Calcium sulphate would be used as fertiliser
after processing.

12. • The Calcium nitrite is produce in the reaction and then it
will be converted to Calcium sulphate.
• This Calcium sulphate would be used as fertiliser after
processing.

13. ABSORPTION BY FERROUS SULPHATE
SOLUTION
• The effluent gases containing nitrogen oxides are treated with
aqueous ferrous sulphate (FeSO4) solution. The reactions occurring
are:
• In another reaction, a mixture of Fe-EDTA and sulphite solution can
be used for absorption of nitrogen oxides from the effluent gases.
• The absorption of nitrogen oxide (NO) can be further increased by
the addition of EDTA to the reaction mixture, resulting in the
formation of Fe-EDTA-NO complex.

14. Absorption in Other media
• Nitrogen oxide can be absorbed by the
absorption media containing alkalized alumina
(Na2O.Al2O3) or a mixture of NaclO2+NaOH or
H2O2, resulting in the formation of Sodium
nitrate.
• The effluent containing NOX passed through Urea
solution. The final products formed in this
chemical reaction are CO2, H2O and N2.
• The absorption of NOX can also be done in
Na2SO3 and NaHSO3 solutions.

15. Advantages of absorption process
• The main advantage of the above mentioned
processes is that the NOX emissions are
considerably reduced.
• The nitrogen oxides evolved can be used to
produce nitric acid, and the nitrate salts can
be further processed for use as a fertilizer.

16. Adsorption methods
• Adsorption is a type of separation process, where the
gaseous pollutants adheres to the adsorbent due to
force of attraction at the interface of two phases.
• Some solids such as, silica gel, activated carbon, ion
exchange resins, manganese oxide, alkalized ferric
oxide, etc. act as adsorbents and oxidise NO and NO2.
• For adsorption of NOX, NO is first oxidised to NO2 which
can be adsorbed by silica gel, activated carbon, etc.
• The adsorbent is regenerated by heating and NO2
evolved is reused for HNO3 manufacture.

17. Adsorption methods
Activated carbon:
• Activated carbon adsorbs NOX quite efficiently
when the carbon bed is fresh.
• Repeated regeneration of the activated
carbon reduces its adsorption capacity to as
little as 50 – 60% due to the presence of
oxygen which is invariably present in the
exhaust gases.

18. Adsorption methods
Silica gel:
• Silica gel has practically no adsorption capacity
for NO. dry silica gel is reported to catalyze the
oxidation of NO to NO2 but moist or saturated
silica gel has no effect on oxidation.
• As silica gel has a high capacity for adsorbing
moisture, it should be used selectively when
the gases contain appreciable amounts of
moisture.

19. Adsorption methods
Zeolite:
• Natural zeolites can be employed to remove
NOX levels up to 200ppm.
• However their capacity is limited to
2.2kg/100kg of zeolite.
• The zeolite bed is regenerated by hot air
stream.

20. Combustion methods
• NOX emission has been reported to be elevated
with increase in the temperature of the reaction
mixture. As the temperature of the reaction
mixture lowers, the NOX emissions are reduced.
• The low temperature of the combustion chamber
(reaction mixture) can be accomplished by the
following ways:
 Slow burning of the fuel
 Multi stage combustion

21. Combustion methods
• NOX reduction can also be achieved through
combustion modification techniques. They are:
 Low excess air combustion
 Multi stage combustion
 Flue gas recirculation
• The oxygen from NOX compounds can be
removed by adding a reducing agent to
combustion gas stream, such process includes
selective catalytic reduction and selective non-
catalytic reduction.

22. Combustion modification techniques
Low excess air operation:
• Involves a reduction in the total quantity of air used in the
combustion process.
• By using less oxygen, the amount of NOX produced is not as
great.

23. Combustion modification techniques
Multi stage combustion:
• Involves the operation
of the main burners in a
boiler at very low excess
air (fuel rich
conditions).
• A series of over fire air
ports are used in this
upper region to provide
all of the air needed for
complete combustion.

24. Combustion modification techniques
• Involves the return of cooled combustion
gases to the burner area of the boiler.
Reduced temperatures produce less NOX .
• The process requires a recirculation fan and
duct system.

25. Add-on controls (flue gas treatment)

26. Add-on controls (flue gas treatment)

27. Add-on controls (flue gas treatment)

28. Add-on controls (flue gas treatment)

29. Add-on controls (flue gas treatment)

30. Add-on controls (flue gas treatment)

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