112/04/18Aerosol & Particulate Research Laboratory

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NOx

• 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

Reading: Chap 16

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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

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

Q: Can absorption, adsorption or incineration be used to control NOx?

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Strategies for Combustion Modification

• Reduce peak temperatures of the flame zone

• Reduce gas residence time in the flame zone

Q: Temperature as a function of equivalent ratio?

tricStoichiomeair

fuel/

air

fuel:)( ratio Equivalent

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• Off-Stoichiometric Combustion/Staged combustion: combusting the fuel in two or more steps. Fuel rich then fuel lean.

• 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

Modification of Operating Conditions

Why?

http://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket)

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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.

http://www.lanl.gov/projects/cctc/factsheets/eerco/gasreburndemo.html

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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)

OHNONHNO

OHNONHNO

22catalyst supported OVor TiO

232

22catalyst supported OVor TiO

23

6342

6444522

522

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

Q: Why is it called “selective”?

Temperature ~ 300 - 400 oChttp://www.lanl.gov/projects/cctc/factsheets/scr/selcatreddemo.html

Also good for Hg emission control!!!

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OHNOONH

OHNONONH

223

2223

6454

6444

Temperature ~ 800 - 1000 oC

Q: Disadvantages of SCR and SNR?

Above 1000 oC

• Selective Noncatalytic Reduction (SNR)

222

32

2222

32

22 2

OOHOHHO

HNONOHO

OHHOOHOH

HNONOOH

HONONOOH

OHOH

• Wet Absorption: generally good for only NO2 because NO is insoluble. Need to oxidize NO first.

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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 H2SO4

– Shell Flue Gas Treating System (~ 400 oC)

– Alkali Metal and Alkali Earth Metal based sorbents: form metal nitrates (e.g. NaNO3, Mg(NO3)2)

CuOOCu

OHSOCuHCuSO

OHNONHNO

CuSOSOOCuO

2

2224

22catalysts as CuSOor CuO

23

422

5.0

22

6444

5.04

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Quick Reflection