KUSHAGRA ABHISHEK 11/CH/25

Selective catalytic reduction

of NOx

WHAT IS NOX

NOX - Family of SEVEN Nitrogen compound (NO,NO2,N2O2,N2O3,N2O,N204,N2O5)Of these NO2 is surrogate of this family of compound as it is most prevalent form of NOx in the atmosphere generated by anthropogenic activities.

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

Sources of Nox

NOX EFFECTS: formation of ozone Acid rain Toxic chemicals Global warming Causes visibility impairment. Deteriorates water quality

NO – Nitric oxide• Colorless and odorless gas• Insoluble in water• Toxic

NO2 – Nitrogen dioxide• Usually exists as a dimer compound

(N2O4) at low 0C• Has distinct reddish-brown color• Moderately soluble in aqueous liquids• Toxic• Contributes to brown haze that occurs

with smog

Characteristics of NOx compounds

NOX CONTROL TECHNOLOGIES

Selective Catalytic Reduction (SCR)Reaction

With several secondary reactions:2SO2 + O2 → 2SO3 2NH3 + SO3 + H2O → (NH4)2SO4 NH3 + SO3 + H2O → NH4HSO4

The reaction for urea instead of either anhydrous oraqueous ammonia is:4NO + 2(NH2)2CO + O2 → 4N2 + 4H2O + 2CO2

OHNONHNO

OHNONHNO

22catalyst supported OVor TiO

232

22catalyst supported OVor TiO

23

6342

6444522

522

Temperature ~ 300 - 400 oC

Selective catalytic reduction (SCR) 1)Beds of NH3 or urea.

2)Catalysts (Ti and V ) are embedded in following design:

• Plate configuration – lower pressure drop, less prone to

Plugging and fouling , much larger and more expensive.

• Honeycomb configurations- smaller than plate, high pressure drop and plug much more easily.

3)Efficiency Up to 95 %.

SCR – THE PROCESS

Catalyst used in SCR for reduction of NOx

Titanium oxideOxide of base metals like vanadium and tungstenZeolitesEtc…

Several reductant are currently used in SCR applications: Anhydrous ammonia Extremely toxic and difficult to safely store, but needs no further

conversion to operate within an SCR. Typically favoured by large industrial SCR operators

Aqueous ammonia must be hydrolyzed in order to be used, but it is substantially safer to

store and transport than anhydrous ammonia.

Urea the safest to store, but requires conversion to ammonia through

thermal decomposition in order to be used as an effective reductant

Ap

plica

tion

Limitations of SCR:The most common limitation of SCR in power plants is the ammonia slipemission of ammonium sulfate and ammonium bisulfateVery high capital costHigh operating costCatalyst bed increases pressure dropPossible water wash RequiredHigh Space requirements

Conclusion: SCR technology is one of most simple clean and reliable technology present today.

It is a flue gas treatment method which is successfully operating in various field like automobile, industry etc.. It is most effective as it is easily operated after proper installation, have better efficiency than other available technology for Nox reduction.

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