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 NO 2 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) µµµg/m3 (imission) g/GJ Remarks:
1. The NO x content in flue gas is given for a certain content of oxygen (O 2) in flue gas.
2. The NO x 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:
NO x [ppm] = NO [ppm] + NO 2 [ppm]
ppm → mg/m3 NO [mg/m3] = 1.3387 NO [ppm]
3 NO x [mg/m ] = 2.0525 NO x [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:
3 [mg NO2/m ]
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 NO x (NO and NO 2) 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 NO x formation.
COMBUSTION AND FUELS MAJOR SOURCES OF NOX FORMATION DURING COMBUSTION
1. Air nitrogen (N 2)
thermal NO x
prompt NO x
2. Fuel nitrogen (N F)
fuel NO x
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
Rate of NO x formation by thermal mechanism === 1/2 d[NO]/d t kp[O 2] [N 2] ≅ 1/2 where kp 2K3 k2, which is in accord with the experimental.
50000 1,0E+05 , s , 40000 1,0E+03 r
30000 1,0E+01 , ppm r Equilibrium concentration of 20000 1,0E-01 NO, [NO] r, an d time to reach [NO] 0,5[NO] versus the temperature 10000 1,0E-03 r czas do 0,5 [NO] 0,5 do czas 0 1,0E-05 1000 1500 2000 2500 3000 T, K
It show s that the Zeldovich mechanism become s important when the temperature reach es the range of 1600-1800 K. COMBUSTION AND FUELS PROMPT NITRIC OXIDE MECHANISM
COMBUSTION AND FUELS REACTIONS OF HYDROCARBON RADICALS
WITH N2
There are many hydrocarbon radicals in flame (CH, CH 2, CH 3, C 2H4, C 2H5, C3H7, C, C 2...), which can react with molecular nitrogen (N 2). +++ → +++ CH2 N2 HCN NH +++ → +++ CH N2 HCN N +++ → +++ C N2 CN N general +++ → CH x 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 have fuel 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 N F 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 Content, % W V T Fuel d a, min comb MJ/kg m3/kg °C C H S N O
Gas 85 15 – – – 47 11,57 2263
Heating oil 86,4 12,5 0,05 0.05 1 45,2 11,18 2258 1 Heating oil 85,5 13 0,4 0.3 0,8 43,6 11,96 2210 2 Heating oil 87 11,4 0,5 0.6 0,3 43,1 10,74 2262 3
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 Rapid Slow burning combustion of char of v.m.
Coal particle
Products: Conversion of v.m. into Gasification of char, releasing of: CO, CO 2, HCN, NH 3, tar, CO, CO 2, H 2O, N 2, N2, N 2O, NO etc.; reduction of NO on the aromatic compounds N2O, NO etc. 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
OH, O, O 2 C(s) NO N nitrogen in NH 3 2 volatile matter nitrogen in C(s) N, cz ęś ci lotne char residue N koks.
OH, O, O 2 nitrogen in fuel HCN NO2 N paliwowy nitrogen in char residue N koks.
O2 N poz. koks. NO 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
NO 2 is a secondary product, and is formed by oxidation of NO in combustion processes.
NO 2 is formed in cooler regions of flame, in the temperature range of: T < 800 °°°C
NO 2 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 NO 2 formation is with hydroperoxide radical HO 2 : → NO + HO2 NO2 + OH → (where from HO 2: H + O 2 + M HO 2 + M) The temperature range of this reaction: T < 1000 K
COMBUSTION AND FUELS ADDITIONAL MECHANISM OF NO2 FORMATION
Hydrocarbonperoxides RO 2 plays a considerable part in NO 2 formation: → NO + RO2 NO2 + RO → (where from RO 2: R + O 2 + 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 +M
NO 2 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 NO 2 contribution to the total NO x formation during pulverized coal combustion is minor: ∗ ≈ [NO2]/[NOx] 100% 5% More important contribution to the total production of
NO x NO 2 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 (NH 3, 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 N 2O is formed mainly in fluidized beds (N 2O 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 NO x is dominating NO x
l NO tota
l NO rma the ° Bituminous coal (Tpł = 1650 C)
volatile matter NO
° Lignite (Tflame = 1250 C): approx.95% of NO x is fuel NO x NO from char COMBUSTION AND FUELS NOx EMISSION vs. FURNACE TYPE (PF BOILERS)
Power NOx Furnace type Coal type 3 MWe mg/m
Tangencial 650–750 bituminous 710 ±60 Tangencial 465–490 bituminous 750–1000 Tangencial 200–360 bituminous 500–900 Wall fired 425–460 bituminous 1360 ±110 FBB 110 bituminous 400 ±80 FBB 160 bituminous 520 ±80 FBB 230 lignite 280 ±50
COMBUSTION AND FUELS OIL FLAMES
Heavy heating oil has much of fuel nitrogen NF > 0,5%.
NO x emission from oil burners operating on heating oils nr 2 and 3 is in the range: 300-700 mg/m3 (mainly fuel NOx).
Fuel nitrogen N F is a dominating factor.
Light heating oil has less fuel nitrogen NF < 0,05%
NO x emission form oil burners operating on light heating oil (nr 1) is in the range: 180-220 mg/m3 (mainly thermal NO ). x 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%)
COMBUSTION AND FUELS