NITROGEN OXIDES FORMATION in combustion processes

1 combustion AND FUELSNITROGEN OXIDES FORMATION in combustionprocessesCOMBUSTION AND FUELSNITROGEN OXIDES FORMED DURING COMBUSTIONN2O - nitrous oxideNO - nitric oxideNO2- NITROGEN dioxideN = 14, O2=16, NO = 30, NO2= 46 combustion AND FUELSCONTRIBUTION OF PARTICULAR NITROGEN OXIDES IN TOTAL NOXPRODUCTION the biggest contribution has nitric oxide (NO) its content in flue gas is in the range of: 100 1000 mg/m3 next is NITROGEN dioxideNO2in 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 FUELSWHAT DOES NOXMEAN ? combustion AND FUELSDEFINITION OF NOXNOX means the sum of NO and NO2contents in flue gas recalculated on NO2 NOx= NO + NO2(expressed in NO2) combustion AND FUELSUNITS OF NOXCOMBUSTION AND FUELSU nits of NOxcontent in flue gas mg/m3 ppm (emission) g/m3 (imission) g/GJRemarks: 1.

2 The NOxcontent in flue gas is given for a certain content of oxygen (O2) in flue The NOxcontent in flue gas is given for normal ppm- unite part per million (x10-6) combustion AND FUELSNOX emissions conversion chartppm:NOx[ppm] = NO [ppm] + NO2[ppm]ppm mg/m3NO [mg/m3] = NO [ppm]NOx[mg/m3] = NOx[ppm]mg/m3 GJ (dla NOx)1 g/GJ = [mg/m3] for bituminous coal1 g/GJ = [mg/m3] for ligniteCOMBUSTION AND FUELSNOX emission units which are in use inthe power generation industryIn Poland (and in EU) NOxis expressed as follows:[mg NO2/m3]for 6% O2in dry flue gasat normal conditionsCOMBUSTION AND FUELSNOXFORMATION IN combustion PROCESSESCOMBUSTION AND FUELSCOMBUSTION CHEMISTRY OF NITROGEN OXIDES The chemical mechanismof NOx(NO and NO2) FORMATION during combustion obeys hundreds ofelementary chemical on the temperature range, stoichiometricratio 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 .

3 Usually the type of flame determines the conditionsof the predominant mechanism of NOxformation. combustion AND FUELSMAJOR SOURCES OF NOXFORMATION DURING NITROGEN (N2) thermal NOx prompt NOx2. Fuel NITROGEN (NF) fuel NOxCOMBUSTION AND FUELSMAJOR MECHANISMS OF NITRIC OXIDE (NO) FORMATION DURING combustion Thermal Prompt FuelCOMBUSTION AND FUELSTHERMAL NITRIC OXIDE MECHANISM combustion AND FUELSZELDOVICH s MECHANISM OF NO FORMATIONO2++++M====O ++++O ++++M(3) (dissociation)Where Mis stable molecule of high energy necessary to break the bounds of O2[10].The liberated O atoms can react with N2through a relativelyslow reaction:O ++++N2 NO ++++N,(4)the N atoms liberated in this reaction quickly react with O2N ++++O2 NO ++++O(5)

4 Also giving AND FUELSRATE OF NOXFORMATION via ZELDOVICH MECHANISMIt shows that the Zeldovich mechanism becomesimportant when the temperature reachesthe range of 1600-1800 of NOxformation by thermal mechanismd[NO]/dt====kp[O2]1/2[N2]wherek p 2K31/2k2, which is in accord with the , K[NO]r, ppm1,0E-051,0E-031,0E-011,0E+011,0E+031, 0E+05czas do 0,5 [NO]r, sEquilibrium concentration of NO, [NO]r, andtime to reach 0,5[NO]rversus the temperatureCOMBUSTION AND FUELSPROMPT NITRIC OXIDE MECHANISMCOMBUSTION AND FUELSREACTIONS OF HYDROCARBON RADICALS WITH N2 There are many hydrocarbon radicals in flame (CH, CH2, CH3, C2H4, C2H5, C3H7, C, ), which can react with molecular NITROGEN (N2).CH2++++N2 HCN ++++NHCH++++N2 HCN ++++NC++++N2 CN ++++NgeneralCHx++++N2 HCNand other radicals (CN, NH, )As a result: HCN, NHi CNare easily oxidized to NO in AND FUELSFUEL NITRIC OXIDE MECHANISM combustion AND FUELSWHAT IS A SOURCE OF FUEL NITRIC OXIDE1.

5 The source of fuel NO are nitric compounds in fuel, often called fuel NITROGEN (denotedNF).2. The content of fuel nitrogenin fuels is very different!!!!3. Fuel nitrogencan be a very important source of nitric AND FUELSFUEL NITROGEN (NF) IN FUELSCOMBUSTION AND FUELSFUEL NITROGEN IN GASN atural gas practically doesn t havefuel AND FUELSFUEL NITROGEN IN LIQUID FUELSC rude oil has fuel nitrogenin the range of do wt. Only exceptionally NFcontent excess Major groups of nitric compounds are: pyridyne, indoles, chinolines, tetrahydrochinolines, carbazoles i compounds in oil are relatively stable in the elevated temperature, therefore during crude oil destillation they are cumulated in heavy fractions of oil.

6 For example, the content of NFin asphaltes reaches combustion AND FUELSFUEL NITROGEN IN HEATING OILS FuelContent, %WdMJ/kgVa, minm3/kgTcomb CCHSNOGas8515 4711,572263 Heating oil186,412,50, ,211,182258 Heating oil285,5130, ,843,611,962210 Heating oil38711,40, ,343,110,742262 combustion AND FUELSTHE INFLUENCE OF NF CONTENT ON NOXEMISSIONCOMBUSTION AND FUELSFUEL NITROGEN IN COALThe origin of fuel nitrogenin coal is organic material ofcoal, these are vegetables, bacteria and fungi containing amines, alkaloid and chlorophyll being source of different coals:Bituminous - N (85% C)Anthracite<1% N - 2% AND FUELSSELECTED NITRIC COMPOUNDS IN COALCOMBUSTION AND FUELSM ajor nitric compounds in coalCOMBUSTION AND FUELSCONVERSION OFFUEL NITROGENDURINGCOAL COMBUSTIONCOMBUSTION AND FUELSCONVERSION OF FUEL NITROGEN DURING COAL PARTICLE BURNINGP irolysisCoal particleRapid combustion of burning of charConversion of into CO, CO2, H2O, N2, N2O, NO :HCN, NH3, tar, aromatic compoundsGasification of char, releasing of: CO, CO2, N2, N2O, NO etc.

7 ; reduction of NO on the surface and in the pores of char combustion AND FUELSEVALUATION OF FUEL NITROGEN DURING COAL PYROLYSISCOMBUSTION AND FUELSNITRIC OXIDE FORMATION DURING COAL COMBUSTIONCOMBUSTION AND FUELSFUEL NITRIC OXIDE FORMATION DURING COAL COMBUSTIONN paliwowyN, cz ci lotneNH3 HCNOH, O, O2 NONO2N (s)C(s)N poz. , O, O2nitrogen in volatile matternitrogen in fuelnitrogen in char residuenitrogen in char residuenitrogen in char residueCOMBUSTION AND FUELSCONVERSION OF GAS-PHASE FUEL NTO NOCOMBUSTION AND FUELSCONVERSION OF CHAR FUEL NTO NO combustion AND FUELSNITRIC OXIDE REDUCTION ON CHARCOMBUSTION AND FUELSNITROGENDIOXIDE(NO2) FORMATION IN FLAMESCOMBUSTION AND FUELSCONDITIONS OF NITROGEN DIOXIDE (NO2) FORMATIONINFLAMES NO2 is a secondary product, and is formed by oxidation of NO in combustion processes .

8 NO2 is formed in cooler regions of flame, in thetemperature range of: T < 800 C NO2 undergoes destruction on tat higher temperatures: T > 1200 CCOMBUSTION AND FUELSBASIC MECHANISM OF NITROGEN DIOXIDE (NO2) FORMATION IN FLAMES The major reaction of NO2formation 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 FUELSADDITIONAL MECHANISM OF NO2 FORMATION Hydrocarbonperoxides RO2plays a considerable part in NO2formation:NO + RO2 NO2+ RO(where from RO2: R + O2 + M ROO + M)where R are alkil radicals:CH3, C2H5and higherCOMBUSTION AND FUELSNITROGEN DIOXIDE FORMATION IN REACTION WITH ATOMIC OXYGEN The three body reaction:NO + O + M NO2+MNO2also is generated, but the contribution of this reaction is not important.

9 The temperature range: T < 800 AND FUELSCONTRIBUTION OF NITROGEN DIOXIDE NO2TO THE TOTAL NOX The NO2contribution to the total NOxformation during pulverized coal combustion is minor: [NO2]/[NOx] 100% 5% More important contribution to the total production of NOxNO2introduces during combustion in gas turbines:[NO2]/[NOx] 100% 10 -15% combustion AND FUELSNITROUS OXIDE (N2O) FORMATION IN FLAMESCOMBUSTION AND FUELSCONDITIONS OF NITROUS OXIDE (N2O) FORMATION NO2 is produced in flames due to:1. Oxidation of amine radicals (mainly Nhandless significant CN2).2. In lean regions of gas flames. 3. In fluidized bed furnaces (T approx. 850 C) combustion AND FUELSMAJOR SOURCES OF NIROUS OXIDE N2 OIN FLAMESM ajor reactionNH + NO N2O + H NH amine radical Where is from NH?

10 Mainly, from decomposition of ammonia (NH3, perhaps also hydrogen cyjanide (HCN). combustion AND FUELSNIROUS OXIDE N2O FORMATION IN LEAN FLAMES ( <1)O + N2+ M N2O + MThe temperature range of the chemical reaction: T < 1500 CCOMBUSTION AND FUELSNIROUS OXIDE N2O FORMATION IN FLUIDIZED BEDSIn catalytic reaction: 2NO N2O + in fluidized bed:1. char2. LimestoneCOMBUSTION AND FUELSHOW 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 + NOCOMBUSTION AND FUELSDOMINATING MECHANISMSOF NOxIN FLAMESCOMBUSTION AND FUELSMAJOR FACTORS INFLUENCING NOxFORMATION fuel NITROGEN NF flame temperature air excess ( ) residence time in flameCOMBUSTION AND FUELSNOXFORMATION IN PULVERIZED COAL FLAMECOMBUSTION AND FUELSSOURCES OF NOXEMISSION IN PULVERIZED COAL FLAMECOMBUSTION AND FUELSCONTRIBUTION OF PARTICULAR NITRIC OXIDES IN TOTAL NOX EMISSION FROM PFFuel NOxis dominating NOxLignite (Tflame= 1250 C).)