NitrogenOxides NOx Emissions
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Transcript of NitrogenOxides NOx Emissions
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Nitrogen Oxides (NOx)
Chapter 12Page 147-168
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NOx emissions include:
• Nitric oxide, NO, and Nitrogen dioxide, NO2, are normally categorized as NOx
• Nitrous oxide, N2O, is a green house gas (GHG) and receives special attention
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Smog precursors:
• NOx, SO2, particulate matter (PM2.5) and volatile organic compounds (VOC).
smog calphotochemi O VOCs NOozone level Ground
3Sunlight
x
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“Developing NOx and SOx Emission Limits” – December 2002, Ontario’s Clean Air Plan for Industry
Broad base of sources with close to 50% from the Electricity sector in 1999
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NOx reaction mechanisms:
NO O 21 N
21
22
• highly endothermic with hf = +90.4 kJ/mol
• NO formation favoured by the high temperatures encountered in combustion processes
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Zeldovich mechanism (1946):
N NO O N1-
1
k
k
2
O NO O N2-
2
k
k
2
H NO OH N3-k
3k
k+1 = 1.8 108 exp{-38,370/T}k-1 = 3.8 107 exp{-425/T}
k+2 = 1.8 104 T exp{-4680/T}k-2 = 3.8 103 T exp{-20,820/T}
k+3 = 7.1 107 exp{-450/T}k-3 = 1.7 108 exp{-24,560/T}
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N NO O N1-
1
k
k
2
k+1 = 1.8 108 exp{-38,370/T}k-1 = 3.8 107 exp{-425/T}
Rate-limiting step in the process
K+1 is highly temperature dependent
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Combine Zeldovich mechanism with
H O OH O 24-k
4k
To obtain
]OH[k ]O[k]NO[k
1
]O[k]NO[kk
- ]N[k [O] 2
dtd[NO]
322
1-
22
22-1-
21
]N[ [O]k 2 dt
d[NO]2 1If the initial concentrations of [NO]
and [OH] are low and only the forward reaction rates are significant
Modelling NOx emissions is difficult because of the competition for the [O] species in combustion processes
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“Prompt” NO mechanism (1971):
N HCN N CH 2
H NO OH N
O NO O N 2
N CO NO O HCN 2
This scheme occurs at lower temperature, fuel-rich conditions and short residence times
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Fuel NOx
Organic, fuel bound nitrogen compounds in solid fuels
C-N bond is much weaker than the N-N bond increasing the likelihood of NOx formation
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Example of proposed reaction pathway for fuel-rich hydrocarbon flames
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NOx control strategies:
• Reduce peak temperatures• Reduce residence time in
peak temperature zones• Reduce O2 content in
primary flame zone
• Low excess air• Staged combustion• Flue gas recirculation• Reduce air preheat• Reduce firing rates• Water injection
Combustion Modification Modified Operating Conditions
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Control strategies:
• Reburning – injection of hydrocarbon fuel downstream of the primary combustion zone to provide a fuel-rich region, converting NO to HCN.
• Post-combustion treatment include selective catalytic reduction (SCR) with ammonia injection, or selective noncatalytic reduction (SNCR) with urea or ammonia-based chemical chemical injection to convert NOx to N2.
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SCR process:
4 NO + 4 NH3 + O2 4 N2 + 6 H2O
2 NO2 + 4 NH3 + O2 3 N2 + 6 H2O
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SNCR process:
4 NH3 + 6 NO 5 N2 + 6 H2O
CO(NH3)2 + 2 NO ½ O2 2 N2 + CO2 + 2 H2O
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Low NOX burners:
Dilute combustion technology
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Industrial furnace combustion:• Natural gas is arguably “cleanest” fuel – perhaps not
the cheapest.• Independent injection of fuel and oxidant streams
(“non-premixed”). Industrial furnaces have multi-burner operation.
• Traditional thinking has been that a rapid mixing of fuel and oxidant ensures best operation.
• This approach gives high local temperatures in the flame zone with low HC but high NOx emissions.
• Heat transfer to a load in the furnace (radiatively dominated) must be controlled by adjustment of burners.
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• High intensity combustion with rapid mixing of fuel and oxidant• High temperature flame zones with low HC but high NOx• Furnace efficiency increased by preheating the oxidant stream
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A conventional burner
Lance Air
FuelGas
Combustion Air
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Dilute oxygen combustion:• An extreme case of staged-combustion.• Fuel and oxidant streams supplied as separate
injections to the furnace.• Initial mixing of fuel and oxidant with hot combustion
products within the furnace (fuel-rich/fuel-lean jets).• Lower flame temperature (but same heat release)
and more uniform furnace temperature (good heat transfer).
• Low NOx emissions – “single digit ppm levels”
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Strong-jet/Weak-jet Aerodynamics
•Strong jet = oxidant
•Weak jet = fuel
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Strong-jet/Weak-jet aerodynamics
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CGRI burner
Pilot burner portUV scanner port
Fuel nozzle
Air/oxidant nozzle
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• Dilute oxygen combustion operation with staged mixing of fuel and oxidant• No visible flame (“flameless” combustion)• More uniform temperature throughout flame and furnace• Low HC and NOx emissions
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Queen’s test facility
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Queen’s test facility
2750-362 0 750 1750 54624500 5100
1362z
0
1000
500
3000
3362
y
x
-362
0
B2B1 B3
Water-cooled floor panels
SideView
Plenum Wall
FurnaceExhaust
TopView
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CGRI burner in operation at 1100OC
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CFD rendering of the fuel flow pattern
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CGRI burner performance (1100OC)
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Oxygen-enriched combustion:
• Oxidant stream supplied with high concentrations of oxygen.
• Nitrogen “ballast” component in the oxidant stream is reduced – less energy requirements and less NOx reactant.
• Conventional oxy-fuel combustion leads to high efficiency combustion but high temperatures and high NOx levels.
• Higher efficiency combustion leads to lower fuel requirements and corresponding reduction in CO2 emissions.
• Does this work with dilute oxygen combustion???
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NOx emissions as a function of oxygen enrichment
2
2
2 2
OO
O O A
m 100
m + m
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Firing rate as a function of oxygen-enrichment level required to maintain 1100oC furnace temperature
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Is oxygen-enrichment a NOx reduction strategy?
• NOx emissions are reduced at high oxygen-enrichment levels … but …
• Only at quite significant enrichment levels, and• With no air infiltration (a source of N2).
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NOx emissions as a function of furnace N2 concentration
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Capabilities of oxygen-enriched combustion:
• Dilute oxygen combustion systems can work with oxygen-enriched combustion.
• NOx emissions are comparable to air-oxidant operation and NOx reductions are limited by air infiltration.
• NOx emissions also limited by N2 content of the fuel.• Primary benefit is energy conservation (reduced fuel
consumption) and associated CO2 reduction.
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Limitations of oxygen-enrichment:
• This is not a totally new technology !!!• Cost of oxygen – high purity O2 is expensive, lower
purity is more feasible in some situations.• Infrastructure costs – oxygen supply and handling.• Furnace modifications – burners, gas handling, etc.
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Final Examination• Tuesday, April 22, 1900h• 3rd Floor Ellis Hall• Open book, open notes• Red or gold calculator
CHEE 481 Tutorial Session• Saturday, April 19, 0900h
• Dupuis Hall 217