Nox.ppt 6z432j

NOx Reading: Chap 16

• Thermal NOx vs Fuel NOx • Strategies for Combustion Modifications – Off stoichiometric combustion, flue gas recirculation, water injection, gas reburning, low NOx burner

• Flue Gas Treatment – SCR, SNR, Absorption, Adsorption

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Thermal NOx vs Fuel NOx • Thermal NOx: formed by reaction between N2 and O2 in the air; sensitive to temperature – Fast formation rate at high temperature – Fast cooling rate freezes formed NOx

• Fuel NOx: formed from combustion of fuels containing organic nitrogen in the fuel; dependent on local combustion conditions and nitrogen content in the fuel Q: Can absorption, adsorption or incineration be used to control NOx?

• NOx Control: – combustion modification --> prevent formation – flue gas treatment --> treat formed NOx 09/04/15

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Strategies for Combustion Modification Q: Temperature as a function of equivalent ratio?  fuel   fuel   /  air air    

Equivalent ratio () : 

Stoichiometric

• Reduce peak temperatures of the flame zone • Reduce gas residence time in the flame zone

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Modification of Operating Conditions • Off-Stoichiometric Combustion/Staged combustion: combusting the fuel in two or more steps. Fuel rich then fuel lean. Why? • Flue gas recirculation: reroute some of the flue gas back to the furnace; lower O2 and allow NOx to proceed the “frozen” reactions • Water injection: reduce flame temperature; energy penalty http://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket) 09/04/15

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• Gas reburning: injection of natural gas into the boiler above the main burner to create a fuel-rich reburn zone; hydrocarbon radicals react with NOx to reduce NOx to N2.

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http://www.lanl.gov/projects/cctc/factsheets/eerco/gasreburndemo.html Aerosol & Particulate Research Lab

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• Low-NOx burner: inhibit NOx formation by controlling the mixing of fuel and air; lean excess air and off-stoichiometric combustion

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Flue Gas Treatment • Selective Catalytic Reduction (SCR) 4 NO  4 NH 3  O2  TiO  2 or V 2O5 ed   catalyst    4 N 2  6 H 2O 2 NO2  4 NH 3  O2  TiO  2 or V 2O5 ed   catalyst    3 N 2  6 H 2O Temperature ~ 300 - 400 oC

Q: Why is it called “selective”?

http://www.lanl.gov/projects/cctc/f actsheets/scr/selcatreddemo.html

Q: Should a SCR reactor be installed before or after particle control devices?

Also good for Hg emission control!!! 09/04/15

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• Selective Noncatalytic Reduction (SNR) 4 NH 3  4 NO  O2  4 N 2  6 H 2O Temperature ~ 800 - 1000 oC

4 NH 3  5O2  4 NO  6 H 2O Above 1000 oC

Q: Disadvantages of SCR and SNR?

• Wet Absorption: generally good for only NO2 because NO is insoluble. Need to oxidize NO first. H 2O2  2OH OH  NO  HONO OH  NO2  HNO3 OH  H 2O2  HO2  H 2O HO2  NO  HNO3 HO2  OH  H 2O  O2 09/04/15

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• Dry Sorption – Activated carbon (220 ~ 230 oC): reduce NOx to N2; oxidize SO2 to H2SO4 if NH3 is injected, and carbon is thermally regenerated to remove concentrated H 2SO4 – Shell Flue Gas Treating System (~ 400 oC)

CuO  0.5O2  SO2  CuSO4 4 NO  4 NH 3  O2  CuO   or CuSO   4 as catalysts   4 N 2  6 H 2O CuSO4  2 H 2  Cu  SO2  2 H 2O Cu  0.5O2  CuO – Alkali Metal and Alkali Earth Metal based sorbents: form metal nitrates (e.g. NaNO3, Mg(NO3)2) 09/04/15

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Quick Reflection

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