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.