NITROGEN OXIDES FORMATION in combustion processes COMBUSTION AND FUELS

Transcription

1 NITROGEN OXIDES FORMATION in combustion processes

2 NITROGEN OXIDES FORMED DURING COMBUSTION N 2 O - nitrous oxide NO - nitric oxide NO 2 - nitrogen dioxide N = 14, O 2 =16, NO = 30, NO 2 = 46

3 CONTRIBUTION OF PARTICULAR NITROGEN OXIDES IN TOTAL NO X PRODUCTION the biggest contribution has nitric oxide (NO) its content in flue gas is in the range of: mg/m 3 next is nitrogen dioxide NO 2 in proportion 5-10% of NO its content in flue gas is in the range of: mg/m3 N 2 O has the least contribution: its content in flue gas is in the range of: 1 10 mg/m 3

4 WHAT DOES NO X MEAN?

5 DEFINITION OF NO X NO X means the sum of NO and NO 2 contents in flue gas recalculated on NO 2 NO x = NO + NO 2 (expressed in NO 2 )

6 UNITS OF NO X

7 Remarks: Units of NOx content in flue gas mg/m 3 ppm (emission) µg/m 3 (imission) g/gj 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 )

8 NO X emissions conversion chart ppm: NO x [ppm] = NO [ppm] + NO 2 [ppm] ppm mg/m 3 NO [mg/m 3 ] = NO [ppm] NO x [mg/m 3 ] = NO x [ppm] mg/m 3 GJ (dla NOx) 1 g/gj = 2.7 [mg/m 3 ] for bituminous coal 1 g/gj = 2.35 [mg/m 3 ] for lignite

9 NO X emission units which are in use in the power generation industry In Poland (and in EU) NO x is expressed as follows: [mg NO 2 /m 3 ] for 6% O 2 in dry flue gas at normal conditions

10 NO X FORMATION IN COMBUSTION PROCESSES

11 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.

12 MAJOR SOURCES OF NO X FORMATION DURING COMBUSTION 1. Air nitrogen (N 2 ) thermal NO x prompt NO x 2. Fuel nitrogen (N F ) fuel NO x

13 MAJOR MECHANISMS OF NITRIC OXIDE (NO) FORMATION DURING COMBUSTION Thermal Prompt Fuel

14 THERMAL NITRIC OXIDE MECHANISM

15 ZELDOVICH s MECHANISM OF NO FORMATION O 2 + M = O + O + M (3) (dissociation) Where M is stable molecule of high energy necessary to break the bounds of O 2 [10]. The liberated O atoms can react with N 2 through a relatively slow reaction: O + N 2 NO + N, (4) the N atoms liberated in this reaction quickly react with O 2 also giving NO. N + O 2 NO + O (5)

16 RATE OF NO X FORMATION via ZELDOVICH MECHANISM Rate of NO x formation by thermal mechanism d[no]/dt = k p [O 2 ] 1/2 [N 2 ] where k p 2K 3 1/2 k 2, which is in accord with the experimental ,0E+05 [NO]r, ppm ,0E+03 1,0E+01 1,0E-01 1,0E-03 czas do 0,5 [NO]r, s Equilibrium concentration of NO, [NO] r, and time to reach 0,5[NO] r versus the temperature 0 1,0E T, K It shows that the Zeldovich mechanism becomes important when the temperature reaches the range of K.

17 PROMPT NITRIC OXIDE MECHANISM

18 REACTIONS OF HYDROCARBON RADICALS WITH N 2 There are many hydrocarbon radicals in flame (CH, CH 2, CH 3, C 2 H 4, C 2 H 5, C 3 H 7, C, C 2...), which can react with molecular nitrogen (N 2 ). general CH 2 + N 2 HCN + NH CH + N 2 HCN + N C + N 2 CN + N CH x + N 2 HCN and other radicals (CN, NH, N...) As a result: HCN, NH i CN are easily oxidized to NO in flame.

19 FUEL NITRIC OXIDE MECHANISM

20 WHAT IS A SOURCE OF FUEL NITRIC OXIDE 1. The source of fuel NO are nitric compounds in fuel, often called fuel nitrogen (denoted N F ). 2. The content of fuel nitrogen in fuels is very different!!!! 3. Fuel nitrogen can be a very important source of nitric oxides.

21 FUEL NITROGEN (NF) IN FUELS

22 FUEL NITROGEN IN GAS Natural gas practically doesn t have fuel nitrogen.

23 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 N F in asphaltes reaches 1.5%.

24 FUEL NITROGEN IN HEATING OILS Fuel Content, % C H S N O W d MJ/kg V a, min m 3 /kg T comb C Gas , Heating oil 1 Heating oil 2 Heating oil 3 86,4 12,5 0, ,2 11, ,5 13 0, ,8 43,6 11, ,4 0, ,3 43,1 10,

25 THE INFLUENCE OF NF CONTENT ON NO X EMISSION

26 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. N F in different coals: Bituminous coal: % N (85% C) Anthracite <1% N Lignite: 0.6-2% N.

27 SELECTED NITRIC COMPOUNDS IN COAL

28 Major nitric compounds in coal

29 CONVERSION OF FUEL NITROGEN DURING COAL COMBUSTION

30 CONVERSION OF FUEL NITROGEN DURING COAL PARTICLE BURNING Pirolysis Rapid combustion of v.m. Slow burning of char Coal particle Products: HCN, NH 3, tar, aromatic compounds Conversion of v.m. into CO, CO 2, H 2 O, N 2, N 2 O, NO etc. Gasification of char, releasing of: CO, CO 2, N 2, N 2 O, NO etc.; reduction of NO on the surface and in the pores of char

31 EVALUATION OF FUEL NITROGEN DURING COAL PYROLYSIS

32 NITRIC OXIDE FORMATION DURING COAL COMBUSTION

33 FUEL NITRIC OXIDE FORMATION DURING COAL COMBUSTION nitrogen in volatile matter N, części lotne NH 3 OH, O, O 2 nitrogen in char residue N koks. NO C(s) C(s) N 2 nitrogen in fuel HCN OH, O, O 2 NO 2 N paliwowy nitrogen in char residue N koks. N poz. koks. nitrogen in char residue O 2 NO

34 CONVERSION OF GAS-PHASE FUEL N TO NO

35 CONVERSION OF CHAR FUEL N TO NO

36 NITRIC OXIDE REDUCTION ON CHAR

37 NITROGEN DIOXIDE (NO 2 ) FORMATION IN FLAMES

38 CONDITIONS OF NITROGEN DIOXIDE (NO 2 ) 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

39 BASIC MECHANISM OF NITROGEN DIOXIDE (NO 2 ) FORMATION IN FLAMES The major reaction of NO 2 formation is with hydroperoxide radical HO 2 : NO + HO 2 NO 2 + OH (where from HO 2 : H + O 2 + M HO 2 + M) The temperature range of this reaction: T < 1000 K

40 ADDITIONAL MECHANISM OF NO 2 FORMATION Hydrocarbonperoxides RO 2 plays a considerable part in NO 2 formation: NO + RO 2 NO 2 + RO where R are alkil radicals: (where from RO 2 : R + O 2 + M ROO + M) CH 3, C 2 H 5 and higher

41 NITROGEN DIOXIDE FORMATION IN REACTION WITH ATOMIC OXYGEN The three body reaction: NO + O + M NO 2 +M NO 2 also is generated, but the contribution of this reaction is not important. The temperature range: T < 800 K.

42 CONTRIBUTION OF NITROGEN DIOXIDE NO 2 TO THE TOTAL NO X The NO 2 contribution to the total NO x formation during pulverized coal combustion is minor: [NO 2 ]/[NO x ] 100% 5% More important contribution to the total production of NO x NO 2 introduces during combustion in gas turbines: [NO 2 ]/[NO x ] 100% 10-15%

43 NITROUS OXIDE (N 2 O) FORMATION IN FLAMES

44 CONDITIONS OF NITROUS OXIDE (N 2 O) FORMATION NO 2 is produced in flames due to: 1. Oxidation of amine radicals (mainly Nhand less significant CN 2 ). 2. In lean regions of gas flames. 3. In fluidized bed furnaces (T approx. 850 C)

45 MAJOR SOURCES OF NIROUS OXIDE N 2 O Major reaction IN FLAMES NH + NO N 2 O + H NH amine radical Where is from NH? Mainly, from decomposition of ammonia (NH 3, perhaps also hydrogen cyjanide (HCN).

46 NIROUS OXIDE N 2 O FORMATION IN LEAN FLAMES (λ<1) O + N 2 + M N 2 O + M The temperature range of the chemical reaction: T < 1500 C

47 NIROUS OXIDE N 2 O FORMATION IN In catalytic reaction: FLUIDIZED BEDS Catalysts in fluidized bed: 1. char 2. Limestone 2NO N 2 O + 0.5O 2

48 HOW IMPORTANT IS NITROUS OXIDE N 2 O? N 2 O is known is the as laughing gas. N 2 O has contribution to the stratospheric ozone destruction In combustion processes N 2 O is formed mainly in fluidized beds (N 2 O content in flue gas up to 50 ppm). N 2 O has also some contribution to NO emission, by chemical reactions with radicals O i OH: N 2 O + O NO + NO

49 DOMINATING MECHANISMS OF NOx IN FLAMES

50 MAJOR FACTORS INFLUENCING NO x FORMATION fuel nitrogen N F flame temperature air excess (λ) residence time in flame

51 NO X FORMATION IN PULVERIZED COAL FLAME

52 SOURCES OF NO X EMISSION IN PULVERIZED COAL FLAME

53 CONTRIBUTION OF PARTICULAR NITRIC OXIDES IN TOTAL NOX EMISSION FROM PF Fuel NO x is dominating NO x total NO thermal NO Bituminous coal (T pł = 1650 C) volatile matter NO NO from char Lignite (T flame = 1250 C): approx.95% of NO x is fuel NO x

54 NOx EMISSION vs. FURNACE TYPE (PF BOILERS) Furnace type Power MW e Coal type NO x mg/m 3 Tangencial Tangencial Tangencial Wall fired FBB FBB FBB bituminous bituminous bituminous bituminous bituminous bituminous lignite 710± ± ±80 520± ±50

55 OIL FLAMES Heavy heating oil has much of fuel nitrogen N F > 0,5%. NO x emission from oil burners operating on heating oils nr 2 and 3 is in the range: (mainly fuel NOx). Fuel nitrogen N F is a dominating factor mg/m 3 Light heating oil has less fuel nitrogen N F < 0,05% NO x emission form oil burners operating on light heating oil (nr 1) is in the range: (mainly thermal NO x ) mg/m 3

56 GAS FLAMES No fuel nitrogen N F in gas. Industrial gas burners (flames). Range of NOx emission: mg/m 3 Dominating mechanism: thermal Small gas burners (flames): Range of NOx emission: mg/m 3 Dominating mechanism: thermal + prompt (20%)