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NOx

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. Thermal NOx vs Fuel NOx.

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NOx

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  1. 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 Aerosol & Particulate Research Laboratory

  2. 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? Aerosol & Particulate Research Lab

  3. Strategies for Combustion Modification Q: Temperature as a function of equivalent ratio? • Reduce peak temperatures of the flame zone • Reduce gas residence time in the flame zone Aerosol & Particulate Research Lab

  4. Modification of Operating Conditions • 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 Why? http://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket) Aerosol & Particulate Research Lab

  5. 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 Aerosol & Particulate Research Lab

  6. Low-NOx burner: inhibit NOx formation by controlling the mixing of fuel and air; lean excess air and off-stoichiometric combustion Avoid Peak Temperature Aerosol & Particulate Research Lab

  7. Flue Gas Treatment • Selective Catalytic Reduction (SCR) Temperature ~ 300 - 400 oC http://www.lanl.gov/projects/cctc/factsheets/scr/selcatreddemo.html Q: Why is it called “selective”? Q: Should a SCR reactor be installed before or after particle control devices? Also good for Hg emission control!!! Aerosol & Particulate Research Lab

  8. Selective Noncatalytic Reduction (SNR) Temperature ~ 800 - 1000 oC Q: Disadvantages of SCR and SNR? Above 1000 oC • Wet Absorption: generally good for only NO2 because NO is insoluble. Need to oxidize NO first. Aerosol & Particulate Research Lab

  9. 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) Aerosol & Particulate Research Lab

  10. Quick Reflection Aerosol & Particulate Research Lab

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