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  2. 2. CONTENTSCONTENTS• Specific sources of NOx• NOx Effects• Characteristics of NOx compounds• NOx Regulation•NOx Formation• Control Technologies & Techniques• Combustion Modification• Add-On Controls (Flue Gas Treatment )2
  3. 3. SpecificSpecificsources ofsources ofNONOxx3
  4. 4. 1. Combustion sources• Automobiles• Boilers• Incinerators2. High-temperature industrial operations• Metallurgical furnaces• Blast furnaces• Plasma furnaces• Kilns3. Other sources• Nitric acid plants• Industrial processes that use nitricacid4
  5. 5. 5NONOxxEffectsEffects
  6. 6.  It is one of the main ingredients involved in theformation of ground-level ozone, which cantrigger serious respiratory problems contributes to nutrient overload that deteriorateswater quality contributes to atmospheric particles that causevisibility impairment most noticeable in nationalparks reacts to form nitrate particles, acid aerosols, aswell as NO2, which also cause respiratoryproblems contributes to formation of acid rain reacts to form toxic chemicals contributes to global warming6
  7. 7. 7CharacteristicsCharacteristicsof NOxof NOxcompoundscompounds
  8. 8. NO – Nitric oxide• Colorless and odorless gas• Insoluble in water (Remember this for later on!)• ToxicNO2 – Nitrogen dioxide• Usually exists as a dimer compound (N2O4) at low0C• Has distinct reddish-brown color• Moderately soluble in aqueous liquids• Toxic• Contributes to brown haze that occurs with smog8
  9. 9. 9NOxNOxRegulationRegulation
  10. 10. NOx concentrations are relatively lowin the atmosphere, so why are theyregulated?NO and NO2 react rapidly with othercompounds, creating ozone and otherundesirable compounds. The NO and NO2may never reach high concentrations, but arecreating other air pollutants.10
  11. 11. 11NONOxxFormationFormation
  12. 12. • 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 reactsin a variety of photochemical and thermalreactions to form NO2.12
  13. 13. • Thermal NOx:Thermal NOx: formed by reactionbetweenN2 and O2 in the air; sensitive totemperature• Fuel (or Prompt) NOx:Fuel (or Prompt) NOx: formed fromcombustion of fuel containing organicnitrogen; dependent on local combustionconditions and nitrogen content in the fuel.Not all of the fuel nitrogen compounds arereleased during combustion. Unlike sulfur,a significant fraction of the fuel nitrogenremains in the bottom ash or in the fly ash.13
  14. 14. Location of NoLocation of Noxx formation in a Boilerformation in a Boiler 14
  15. 15. 15ControlControlTechnologiesTechnologies&&TechniquesTechniques
  16. 16. NOx control technologiesNOx control technologies16
  17. 17. 1. Modify combustion to suppress NOx formation• Low excess air operation• Off-stoichiometric combustion• Flue gas recirculation• Natural gas reburning2. Reduce Nox to molecular nitrogen throughcontrols (also known as flue gas treatment)• Selective Non-Catalytic Reduction (SNCR)• Selective Catalytic Reduction (SCR)• Dry Sorption17NOx control TechniquesNOx control Techniques
  18. 18. Strategies for Combustion ModificationStrategies for Combustion ModificationReduce peaktemperaturesof the flamezoneReduce gasresidence timein the flamezone18
  19. 19. Would wet scrubbers be a good controltechnique for NOxemissions?Wet scrubbers would not be a goodcontrol technique for NOxemissions.ReasonNO is mainly formed during thecombustion process and NO2 isformed in the atmosphere.Since NO is insoluble in water, wetscrubbing would not work very well!19
  20. 20. 20CombustionCombustionModificationsModifications
  21. 21. Low excess air operation: Involves a reduction in thetotal quantity of air used in the combustion process. Byusing less oxygen, the amount of NOx produced is not asgreat.21
  22. 22. Off-stoichiometric combustion: Involves the mixing ofthe fuel and air in a way that reduces the peakgas temperatures and peak oxygen concentrations. Low NOx burners: Keeps temperaturesdown and dissipates heat quickly Overfire air (OFA): Keeps mixture fuel richand completes combustion process using airinjection nozzles Burners out of service (BOOS): Operatesalternate burners in combustion zone asfuel rich, air rich, and air only22
  23. 23. Off-Stoichiometric Combustion in BurnerOff-Stoichiometric Combustion in Burner(Low No(Low Noxx Burner)Burner)23
  24. 24. Off-Stoichiometric CombustionOff-Stoichiometric Combustion(Overfire Air)(Overfire Air)24
  25. 25. Off-Stoichiometric CombustionOff-Stoichiometric Combustion(Burners-Out-of-Service)(Burners-Out-of-Service) 25
  26. 26. Flue gas recirculationFlue gas recirculationInvolves the return of cooledcombustion gases to the burner areaof the boiler. Reduced temperaturesproduce less NOx. The processrequires a recirculation fan and ductsystem.26
  27. 27. Involves the operation of the main burners in aboiler at very low excess air (fuel richconditions).A series of overfire air ports are used in thisupper region to provide all of the air needed forcomplete combustion.Fuel ReburningFuel Reburning27
  28. 28. Fuel Reburning:28
  29. 29. 29Add-OnAdd-OnControlsControls(Flue Gas(Flue GasTreatmentTreatment ))
  30. 30. Selective non-catalytic reduction systems(SNCR)Involves the injection of ammonia (NH3) or ureainto the hot gas zone where reactions leading toreduction of nitrogen oxides can occur. Thereactions are completed within the boiler, andno waste products are generated. There is arisk of ammonia (NH3) being emitted into theatmosphere if temperatures are too low,however. SCNR systems are capable ofreducing nitrogen oxides from 20 to 60%.30
  31. 31. Selective Noncatalytic Reduction (SNCR) Reactions:OHNOONHOHNONONH223222364546444+→++→++Above 1000 oC31
  32. 32. Example SNCR System for NoExample SNCR System for Noxx Control in a BoilerControl in a Boiler 32
  33. 33. Selective catalytic reduction (SCR)Involves using beds containing ammonia or urea toreduce nitrogen oxides to molecular nitrogen and water.Two or three catalysts (usually tungsten and vanadium)are arranged in honeycomb shapes in the beds so aircan flow through. NOx reduction efficiencies rangingfrom 75 to 90% are possible when the amount ofcatalyst is sufficient, the catalyst is in good condition,the ammonia reagent flow is sufficient, and theammonia is adequately distributed across the gasstream.33
  34. 34. Selective Catalytic Reduction (SCR) ReactionsOHNONHNOOHNONHNO22catalystsupportedOVorTiO23222catalystsupportedOVorTiO2363426444522522+ →+++ →++Temperature ~ 300 - 400 oC34
  35. 35. Exapmle SCR System for NoExapmle SCR System for Noxx Control in a BoilerControl in a Boiler 35
  36. 36. Dry SorptionDry Sorption◦ Activated carbon (220 ~ 230 oC)◦ Shell Flue Gas Treating System (~ 400 oC)CuOOCuOHSOCuHCuSOOHNONHNOCuSOSOOCuO→+++→++ →++→++2222422catalystsasCuSOorCuO234225.02264445.0436