COMBUSTION AND FUELS
NITROGEN OXIDES FORMATION in combustion processes
COMBUSTION AND FUELS
NITROGEN OXIDES FORMED DURING COMBUSTION
N2O - nitrous oxide
NO - nitric oxide
NO2 - nitrogen dioxide
N = 14, O2 =16, NO = 30, NO2= 46
COMBUSTION AND FUELS
CONTRIBUTION OF PARTICULAR NITROGEN
OXIDES IN TOTAL NOX PRODUCTION
� the biggest contribution has nitric oxide (NO)
� its content in flue gas is in the range of: 100 –1000 mg/m3
� next is nitrogen dioxide NO2 in proportion 5-10% of NO
� its content in flue gas is in the range of: 10 –100 mg/m3
� N2O has the least contribution:
� its content in flue gas is in the range of: 1–10 mg/m3
COMBUSTION AND FUELS
WHAT DOES NOX MEAN ?
COMBUSTION AND FUELS
DEFINITION OF NOX
NOX means the sum of NO and NO2 contents
in flue gas recalculated on NO2
NOx = NO + NO2
(expressed in NO2)
COMBUSTION AND FUELS
UNITS OF NOX
COMBUSTION AND FUELS
Units of NOx content in flue gas
� mg/m3
� ppm (emission)
� µµµµg/m3 (imission)
� g/GJ
Remarks:
1. The NOx content in flue gas is given for a certain content of
oxygen (O2) in flue gas.
2. The NOx content in flue gas is given for normal conditions.
3. ppm - unite – part per million (x10-6)
COMBUSTION AND FUELS
NOX emissions conversion chart
ppm:
NOx [ppm] = NO [ppm] + NO2 [ppm]
ppm →→→→ mg/m3
NO [mg/m3] = 1.3387 NO [ppm]
NOx [mg/m3] = 2.0525 NOx [ppm]
mg/m3 →→→→ GJ (dla NOx)
1 g/GJ = 2.7 [mg/m3] for bituminous coal
1 g/GJ = 2.35 [mg/m3] for lignite
COMBUSTION AND FUELS
NOX emission units which are in use in the power
generation industry
In Poland (and in EU) NOx is expressed as
follows:
[mg NO2/m3]
for 6% O2 in dry flue gas
at normal conditions
COMBUSTION AND FUELS
NOX FORMATION
IN
COMBUSTION PROCESSES
COMBUSTION AND FUELS
COMBUSTION CHEMISTRY OF NITROGEN OXIDES
The chemical mechanism of NOx (NO and NO2)
formation during combustion obeys hundreds of
elementary chemical reactions.
Depending on the temperature range, stoichiometric
ratio and type of nitrous species present in the combustion
zone, it is possible to distinguish predominant groups of
chemical reactions, which are called the mechanisms of
nitrogen oxides formation.
Usually the type of flame determines the conditions
of the predominant mechanism of NOx formation.
COMBUSTION AND FUELS
MAJOR SOURCES OF NOX FORMATION DURING
COMBUSTION
1. Air nitrogen (N2)
� thermal NOx
� prompt NOx
2. Fuel nitrogen (NF)
� fuel NOx
COMBUSTION AND FUELS
MAJOR MECHANISMS OF NITRIC OXIDE (NO)
FORMATION DURING COMBUSTION
�Thermal
�Prompt
�Fuel
COMBUSTION AND FUELS
THERMAL NITRIC OXIDE
MECHANISM
COMBUSTION AND FUELS
ZELDOVICH’s MECHANISM OF NO FORMATION
O2 ++++ M ==== O ++++ O ++++ M (3) (dissociation)
Where M is stable molecule of high energy necessary to break
the bounds of O2 [10].
The „liberated” O atoms can react with N2 through a relatively
slow reaction:
O ++++ N2 →→→→ NO ++++ N, (4)
the N atoms „liberated” in this reaction quickly react with O2
N ++++ O2 →→→→ NO ++++ O (5)
also giving NO.
COMBUSTION AND FUELS
RATE OF NOX FORMATION via ZELDOVICH MECHANISM
It shows that the Zeldovich mechanism becomes important when the temperature reaches the
range of 1600-1800 K.
Rate of NOx formation by thermal mechanism
d[NO]/dt ==== kp[O2]1/2[N2]
where kp ≅ 2K31/2k2, which is in accord with the experimental.
0
10000
20000
30000
40000
50000
1000 1500 2000 2500 3000
T, K
[NO
] r,
ppm
1,0E-05
1,0E-03
1,0E-01
1,0E+01
1,0E+03
1,0E+05
czas
do
0,5
[NO
] r, s
Equilibrium concentration of
NO, [NO]r, and time to reach
0,5[NO]r versus the temperature
COMBUSTION AND FUELS
PROMPT NITRIC OXIDE
MECHANISM
COMBUSTION AND FUELS
REACTIONS OF HYDROCARBON RADICALS
WITH N2
There are many hydrocarbon radicals in flame (CH, CH2, CH3, C2H4, C2H5,
C3H7, C, C2...), which can react with molecular nitrogen (N2).
CH2 ++++ N2 →→→→ HCN ++++ NH
CH ++++ N2 →→→→ HCN ++++ N
C ++++ N2 →→→→ CN ++++ N
general
CHx ++++ N2 →→→→ HCN and other radicals (CN, NH, N...)
As a result: HCN, NH i CN are easily oxidized to NO in flame.
COMBUSTION AND FUELS
FUEL NITRIC OXIDE
MECHANISM
COMBUSTION AND FUELS
WHAT IS A SOURCE OF FUEL NITRIC OXIDE
1. The source of fuel NO are nitric compounds in fuel, often
called fuel nitrogen (denoted NF).
2. The content of fuel nitrogen in fuels is very different!!!!
3. Fuel nitrogen can be a very important source of nitric oxides.
COMBUSTION AND FUELS
FUEL NITROGEN (NF) IN FUELS
COMBUSTION AND FUELS
FUEL NITROGEN IN GAS
Natural gas practically doesn’t havefuel nitrogen.
COMBUSTION AND FUELS
FUEL NITROGEN IN LIQUID FUELS
Crude oil has fuel nitrogen in the range of
0.01 do 0.3% wt.
Only exceptionally NF content excess 0.9%.
Major groups of nitric compounds are: pyridyne, indoles, chinolines,
tetrahydrochinolines, carbazoles i pyroles.
Nitric compounds in oil are relatively stable in the elevated temperature,
therefore during crude oil destillation they are cumulated in heavy
fractions of oil. For example, the content of NF in asphaltes reaches 1.5%.
COMBUSTION AND FUELS
FUEL NITROGEN IN HEATING OILS
FuelContent, %
WdMJ/kg
Va, minm3/kg
Tcomb°C
C H S N O
Gas 85 15 – – – 47 11,57 2263
Heating oil1
86,4 12,5 0,05 0.05 1 45,2 11,18 2258
Heating oil2
85,5 13 0,4 0.3 0,8 43,6 11,96 2210
Heating oil3
87 11,4 0,5 0.6 0,3 43,1 10,74 2262
COMBUSTION AND FUELS
THE INFLUENCE OF NF CONTENT ON NOX EMISSION
COMBUSTION AND FUELS
FUEL NITROGEN IN COAL
The origin of fuel nitrogen in coal is organic material of coal, these
are vegetables, bacteria and fungi containing amines, alkaloid and
chlorophyll being source of nitrogen.
NF in different coals:
Bituminous coal: 0.6 - 2.8% N (85% C)
Anthracite <1% N
Lignite: 0.6 - 2% N.
COMBUSTION AND FUELS
SELECTED NITRIC COMPOUNDS IN COAL
COMBUSTION AND FUELS
Major nitric compounds in coal
COMBUSTION AND FUELS
CONVERSION OF
FUEL NITROGEN
DURING COAL COMBUSTION
COMBUSTION AND FUELS
CONVERSION OF FUEL NITROGEN DURING
COAL PARTICLE BURNING
Pirolysis
Coal particle
Rapid combustion
of v.m.
Slow burning of char
Conversion of v.m. into CO, CO2, H2O, N2,
N2O, NO etc.
Products:HCN, NH3, tar,
aromatic compounds
Gasification of char, releasing of: CO, CO2, N2, N2O, NO etc.; reduction of NO on the
surface and in the pores of char
COMBUSTION AND FUELS
EVALUATION OF FUEL NITROGEN DURING
COAL PYROLYSIS
COMBUSTION AND FUELS
NITRIC OXIDE FORMATION
DURING COAL COMBUSTION
COMBUSTION AND FUELS
FUEL NITRIC OXIDE FORMATION DURING
COAL COMBUSTION
N paliwowy
N, części lotne
NH3
HCN
OH, O, O2
NO
NO2
N koks.
N2
C(s)
C(s)
N poz. koks. NO
N koks.
O2
OH, O, O2
nitrogen in volatile matter
nitrogen in fuel
nitrogen in char residue
nitrogen in char residue
nitrogen in char residue
COMBUSTION AND FUELS
CONVERSION OF GAS-PHASE FUEL–N TO NO
COMBUSTION AND FUELS
CONVERSION OF CHAR FUEL–N TO NO
COMBUSTION AND FUELS
NITRIC OXIDE REDUCTION ON CHAR
COMBUSTION AND FUELS
NITROGEN DIOXIDE (NO2)
FORMATION IN FLAMES
COMBUSTION AND FUELS
CONDITIONS OF NITROGEN DIOXIDE (NO2)
FORMATION IN FLAMES
�NO2 is a secondary product, and is formed by oxidation
of NO in combustion processes.
�NO2 is formed in cooler regions of flame, in the
temperature range of:
T < 800 °°°°C�NO2 undergoes destruction on tat higher temperatures:
T > 1200 °°°°C
COMBUSTION AND FUELS
BASIC MECHANISM OF NITROGEN DIOXIDE
(NO2) FORMATION IN FLAMES
� The major reaction of NO2 formation is with
hydroperoxide radical HO2 :
NO + HO2 → NO2 + OH(where from HO2: H + O2 + M → HO2 + M)
�The temperature range of this reaction:
T < 1000 K
COMBUSTION AND FUELS
ADDITIONAL MECHANISM OF NO2
FORMATION � Hydrocarbonperoxides RO2 plays a considerable part in
NO2 formation:
NO + RO2 → NO2 + RO(where from RO2: R + O2 + M → ROO + M)
where R are alkil radicals:
CH3, C2H5 and higher
COMBUSTION AND FUELS
NITROGEN DIOXIDE FORMATION IN
REACTION WITH ATOMIC OXYGEN
�The three body reaction:
NO + O + M → NO2 +MNO2 also is generated, but the contribution of this
reaction is not important.
The temperature range: T < 800 K.
COMBUSTION AND FUELS
CONTRIBUTION OF NITROGEN DIOXIDE NO2
TO THE TOTAL NOX
�The NO2 contribution to the total NOx formation during
pulverized coal combustion is minor:
[NO2]/[NOx]∗100% ≈ 5%�More important contribution to the total production of
NOx NO2 introduces during combustion in gas turbines:
[NO2]/[NOx]∗100% ≈ 10-15%
COMBUSTION AND FUELS
NITROUS OXIDE (N2O)
FORMATION IN FLAMES
COMBUSTION AND FUELS
CONDITIONS OF NITROUS OXIDE (N2O)
FORMATION
� NO2 is produced in flames due to:
1. Oxidation of amine radicals (mainly Nhand
less significant CN2).
2. In lean regions of gas flames.
3. In fluidized bed furnaces (T approx. 850 °C)
COMBUSTION AND FUELS
MAJOR SOURCES OF NIROUS OXIDE N2O
IN FLAMES
Major reaction
NH + NO → N2O + H
� NH – amine radical
� Where is from NH? Mainly, from decomposition of
ammonia (NH3, perhaps also hydrogen cyjanide (HCN).
COMBUSTION AND FUELS
NIROUS OXIDE N2O FORMATION IN LEAN
FLAMES (λ<1)
O + N2 + M → N2O + M
The temperature range of the chemical reaction:
T < 1500 °C
COMBUSTION AND FUELS
NIROUS OXIDE N2O FORMATION IN
FLUIDIZED BEDS
In catalytic reaction:
2NO → N2O + 0.5O2
Catalysts in fluidized bed:
1. char
2. Limestone
COMBUSTION AND FUELS
HOW IMPORTANT IS NITROUS OXIDE N2O?
� N2O is known is the as laughing gas.
� N2O has contribution to the stratospheric ozone destruction
� In combustion processes N2O is formed mainly in fluidized
beds (N2O content in flue gas up to 50 ppm).
� N2O has also some contribution to NO emission, by chemical
reactions with radicals O i OH:
N2O + O → NO + NO
COMBUSTION AND FUELS
DOMINATING MECHANISMS
OF NOx IN FLAMES
COMBUSTION AND FUELS
MAJOR FACTORS INFLUENCING NOx
FORMATION
� fuel nitrogen NF
� flame temperature
� air excess (λ)
� residence time in flame
COMBUSTION AND FUELS
NOX FORMATION IN PULVERIZED COAL
FLAME
COMBUSTION AND FUELS
SOURCES OF NOX EMISSION IN PULVERIZED
COAL FLAME
COMBUSTION AND FUELS
CONTRIBUTION OF PARTICULAR NITRIC
OXIDES IN TOTAL NOX EMISSION FROM PF
Fuel NOx is dominating NOx
Lignite (Tflame = 1250 °C): approx.95% of NOx is fuel NOx
Bituminous coal (Tpł = 1650 °C)
total NO
thermal N
O
volatile matter NO
NO from char
COMBUSTION AND FUELS
NOx EMISSION vs. FURNACE TYPE (PF BOILERS)
Furnace typePowerMWe
Coal typeNOx
mg/m3
TangencialTangencialTangencialWall firedFBBFBBFBB
650–750465–490200–360425–460
110160230
bituminousbituminousbituminousbituminousbituminousbituminous
lignite
710±60750–1000500–9001360±110400±80520±80280 ±50
COMBUSTION AND FUELS
OIL FLAMESHeavy heating oil has much of fuel nitrogen NF > 0,5%.
NOx emission from oil burners operating on heating oils nr 2 and 3 is in the range:
300-700 mg/m3
(mainly fuel NOx).
Light heating oil has less fuel nitrogen NF < 0,05%
NOx emission form oil burners operating on light heating oil (nr 1) is in
the range:
180-220 mg/m3
(mainly thermal NOx).
Fuel nitrogen NF is a dominating factor.
COMBUSTION AND FUELS
GAS FLAMES
No fuel nitrogen NF in gas.
Industrial gas burners (flames).
Range of NOx emission: 100-300 mg/m3
Dominating mechanism: thermal
Small gas burners (flames):
Range of NOx emission: 10-100 mg/m3
Dominating mechanism: thermal + prompt (20%)
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