Transcript Document 7217249

Nitrogen Oxides (NOx)
Chapter 12
Page 147-168
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
Smog precursors:
• NOx, SO2, particulate matter (PM2.5) and volatile
organic compounds (VOC).
NO x  VOCs

Sunlight
O3
Ground level ozone
 photochemi cal smog
“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
NOx reaction mechanisms:


1
1
N2 
O2
NO


2
2
• highly
• NO
endothermic with Dhf = +90.4 kJ/mol
formation favoured by the high temperatures
encountered in combustion processes
Zeldovich mechanism (1946):
k
N2
1


 O
NO  N

k
k+1 = 1.8  108 exp{-38,370/T}
k-1 = 3.8  107 exp{-425/T}
-1
k
N  O2
2



NO  O

k
k+2 = 1.8  104 T exp{-4680/T}
k-2 = 3.8  103 T exp{-20,820/T}
-2
3



N  OH
NO  H


k
k
-3
k+3 = 7.1  107 exp{-450/T}
k-3 = 1.7  108 exp{-24,560/T}
Rate constant, m3/mol-s
109
k+3
k-1
108
107
k+2
106
105
k-3
k-2
104
103
102
k+1
101
100
10-1
10-2
10-3
10-4
1500
2000
2500
Temperature, K
k
Rate-limiting step in the process 
N2
1


 O
NO  N

k
-1
K+1 is highly temperature dependent
k+1 = 1.8  108 exp{-38,370/T}
k-1 = 3.8  107 exp{-425/T}
Combine Zeldovich mechanism with
4



O  OH
O2  H



k
k
-4
To obtain

k -1 k -2 [ NO] 2 
 k 1 [ N 2 ] 
k  2 [O 2 ] 
d[NO]

 2 [O] 

k
[
NO
]
dt
-1
1 

 k  2 [O 2 ]  k 3 [OH] 
If the initial concentrations of [NO]
and [OH] are low and only the
forward reaction rates are significant
d[NO]
 2 k 1 [O] [ N 2 ]
dt
Modelling NOx emissions is difficult because of the
competition for the [O] species in combustion processes
“Prompt” NO mechanism (1971):


CH  N 2
HCN  N




N  OH
NO  H




N  O2
NO  O




HCN  O 2
NO  CO  N


This scheme occurs at lower temperature, fuel-rich
conditions and short residence times
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
Example of proposed reaction pathway for fuel-rich hydrocarbon flames
NOx control strategies:
Combustion Modification
• Reduce peak temperatures
• Reduce residence time in
peak temperature zones
• Reduce O2 content in
primary flame zone
Modified Operating Conditions
•
•
•
•
•
•
Low excess air
Staged combustion
Flue gas recirculation
Reduce air preheat
Reduce firing rates
Water injection
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 ammoniabased chemical chemical injection to convert NOx to N2.
SCR process:
4 NO + 4 NH3 + O2  4 N2 + 6 H2O
2 NO2 + 4 NH3 + O2  3 N2 + 6 H2O
SNCR process:
4 NH3 + 6 NO  5 N2 + 6 H2O
CO(NH3)2 + 2 NO ½ O2  2 N2 + CO2 + 2 H2O
Low NOX burners:
Dilute combustion technology
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 multiburner 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.
• 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
C om b u s toi n
A ir
L a n ce
A ir
Fue l
G as
A conventional burner
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”
•Strong jet = oxidant
•Weak jet = fuel
Strong-jet/Weak-jet Aerodynamics
Strong-jet/Weak-jet aerodynamics
UV s c a n n e r p o r t
P iol t b u rn e r p o r t
A ir /o x di a n t n o z z el
F u e ln o z z el
CGRI burner
• 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
Queen’s test facility
B2
B1
P el num
W a ll
B3
-362
T op
V ei w
0
x
F u rna ce
E xhau s t
3000
3362
-362 0
750
1750
2750
4500
5100 5462
y
z
1362
S di e
V ei w
1000
500
0
W a et r-coo el d fol o r pane sl
Queen’s test facility
CGRI burner in operation at 1100OC
CFD rendering of the fuel flow pattern
NOX concentration, ppm
15
10
5
0
1
2
3
4
Stack oxygen concentration, % w.b.
5
CGRI burner performance (1100OC)
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???
0.0%< Stack O2 <1.5%
1.5%< Stack O2 <2.0%
2.0%< Stack O2 <3.0%
3.0%< Stack O2 <4.0%
NOx, mg/MJ
15
12
9
6
3
0
0
20
40
60
O , %
2
80
100
O
2

m O2
 
 mO + mO A
 2
2

  100

NOx emissions as a function of oxygen enrichment
400
0 < Stack O2 < 1.5% w.b.
1.5% < Stack O2 < 2.0% w.b.
2.0% < Stack O2 < 3.0% w.b.
Stack O2 > 3.0% w.b.
mf(-Dhc), kW
360
320
280
240
200
0
20
40
60
80
100
O , %
2
Firing rate as a function of oxygen-enrichment level
required to maintain 1100oC furnace temperature
Is oxygen-enrichment a NOx reduction strategy?
• NOx emissions are reduced at high oxygenenrichment levels … but …
• Only at quite significant enrichment levels, and
• With no air infiltration (a source of N2).
0.0%< Stack O2 <1.5%
1.5%< Stack O2 <2.0%
2.0%< Stack O2 <3.0%
3.0%< Stack O2 <4.0%
NOx, mg/MJ
15
12
9
6
3
0
0
20
40
60
80
N2, % w.b.
NOx emissions as a function of furnace N2 concentration
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.
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.
CHEE 481 Tutorial Session
• Saturday, April 19, 0900h
• Dupuis Hall 217
Final Examination
•
•
•
•
Tuesday, April 22, 1900h
3rd Floor Ellis Hall
Open book, open notes
Red or gold calculator