Nitrogen oxide chemistry in combustion processes. Based on material originally by Prof. Mikko Hupa

Transcription

1 Nitrogen oxide chemistry in combustion processes Based on material originally by Prof. Mikko Hupa

2 Background - NOx Nitrogen oxides, NO X = NO + NO 2 In combustion flue gases >95% NO and <5% NO 2 NO 2 formed via NO In the atmosphere NO reacts to NO 2, and further to compounds such as HNO 3 and nitrates (acidification) NO 2 + hydrocarbons photochemical smog In some special cases also emissions of N 2 O

3 Background NOx (ii) In some special cases also emissions of N 2 O N 2 O stable in the atmosphere not part of the acidification However, N 2 O is green-house gas and upper atmosphere ozone destructor Nitrogen oxide chemistry in combustion complicated and fascinating!

4 Emissions are expressed in a variety of units be aware! 100 ppm (3 % O 2 ) = 72 ppm (8 % O 2 ) = 83 ppm (8 % O 2, dry gas) = 237 mg NO 2 /m 3 (3 % O 2, dry) = 71 mg NO 2 /MJ (LHV) = gr/dscf (3 % O 2, dry) = gr/btu (LHV)

5 Background -Nitrogen oxide emissions from various combustion processes without reduction measures (Hupa et al.)

6 Nitrogen oxide chemistry in combustion processes Terms and concepts Chemical equilibrium and NO Thermal-NO (Zeldowich NO) and kinetics Fuel-NO Volatile-NO HCN, NH 3 Char-NO Staged Combustion: Low Nox Technologies NO formation tendency characterization (NO chemistry in fluidised bed combustion)

7 Equilibrium concentration Nitrogen species in a typical flue gas under chemical equilibrium conditions Temperature Temperature

8 Nitrogen species in a typical flue gas under chemical equilibrium conditions Most abundant ones λ=1.1: N 2, NO, NO 2, N 2 O λ=0.9: N 2,HCN, NH 3 Equilibrium conc. of NO x decreases with temp.: data from flue gas with λ=1.1: T=1800 K 1200 ppm T=500 K <1ppm

9 NO Chemical equilibrium vs. Kinetics NO concentration Schematic view of NO formation in a boiler Time Equilibrium concentration (λ=1.1) Typical measured concentration Furnace Superheaters Temperature

10 Mole fraction (-) 1.E+00 CO Vs. NO kinetics 1000 C 1.E-02 1.E-04 1.E-06 1.E-08 1.E-10 1.E-12 1.E-14 1.E-16 1.E-18 1.E-20 1.E-22 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 Time (s) O2 CO CO2 H2O NO NO2 N2O H O OH H2 HO2 H2O2 HCO Kinetic calculation, plug flow reactor

11 Mole fraction (-) 1.E+00 1.E-02 1.E-04 1.E-06 CO Vs. NO kinetics 1000 C 1.E-08 1.E-10 1.E-12 1.E-14 1.E-16 O2 CO CO2 H2O NO Equilibrium NO Equilibrium CO 1.E-18 1.E-20 1.E-22 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 Time (s) Kinetic calculation, plug flow reactor

12 Mole fraction (-) 1.E+00 1.E-02 1.E-04 1.E-06 CO Vs. NO kinetics 1500 C 1.E-08 1.E-10 1.E-12 1.E-14 1.E-16 O2 CO CO2 H2O NO Equilibrium CO Equilbrium NO 1.E-18 1.E-20 1.E-22 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 Time (s) Kinetic calculation, plug flow reactor

13 Mole fraction (-) 1.E+00 1.E-02 1.E-04 1.E-06 CO Vs. NO kinetics 2000 C 1.E-08 1.E-10 1.E-12 1.E-14 1.E-16 O2 CO CO2 H2O NO Equilibrium CO Equilibrium NO 1.E-18 1.E-20 1.E-22 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 Time (s) Kinetic calculation, plug flow reactor

14 NO Chemical equilibrium vs. Kinetics NO concentration Schematic view of NO formation in a boiler Time Equilibrium concentration (λ=1.1) Typical measured concentration Furnace Superheaters Temperature

15 Nitrogen chemistry in boilers At equilibrium: high temperatures -> high NO, low temperatures -> no NO Formation of NO determined by kinetics not equilibrium Decomposition at lower temperatures kinetically frozen Great variations in NO emissions. Which factors influence NOformation kinetics? Major research activities to understand mechanisms of NO formation since 1970 s NO emissions brought detailed chemistry into combustion engineering

16 NO formation mechanisms

17 Thermal-NO NO formed from N 2 in combustion air Direct reaction N 2 + O 2 2NO very slow even at high temperatures (N 2 Zeldowich (1946): atomic oxygen needed to react with N 2 : O + N 2 NO + N (1) The N atom reacts further to yield a second NO N + O 2 NO + O (2) d(no)/dt = k 1 (O) (N 2 )

18 Estimated rates of thermal-no formation atm/s ppm/s K Temperature

19 Thermal NOx - Summary Molecular nitrogen in air reacts to form NO Reaction kinetically possible with atomic oxygen Significant only if T > 1400 C Formation steeply dependent on temperature Main contributor to NO formation in gasolin engines and gas combustion systems

20 Fuel-NO: Gradual Exploration of the Mechanisms

21 NO concentration (λ=1.0) Fuel NOx Fuel Oil with Organic Nitrogen Addition Oil + pyridine (0.5% N) Pure Oil (<0.01% N) Air factor λ Based on Martin and Berkan, Air pollution and its control, AICHE symp. Series 68, Nr 126, 45, 1972

22 Fuel-NO NO formed from N in fuel Fuel-N organically bound - reactive Until the 1970 s fuel-n was not considered a source of NO Amines Pyridine Pyrrole Quinoline (1 (2 (3 1) Calculated as CaO 2) Calculated as Na 3) Not organic-n, but molecular-n, N 2

23 NO formation Fuel Oil Containing N N 2 Thermal NO N fuel N vol N tar N 2 HCN NH 3 NO N 2

24 NO formation N 2 Thermal NO N vol N 2 HCN, NH 3 NO N fuel N 2 N 2 N char NO

25 NO (ppm) NO formation volatiles and char Time (s)

26 How to Characterize Fuels with Respect to their NO Formation Tendency? N 2 Thermal NO N vol N 2 HCN, NH 3 NO N fuel N 2 N 2 N char NO

27 Experimental procedure #1 2-step burning of fuel in 5 vol-% O 2 +Ar - Pyrolysis: Ar through bed + secondary O 2 - Char ox: Ar + O 2 through bed Release curves integrated to obtain cumulative CO 2 and NO (NO I + NO II) European bark T bed =850 C

28 Volatile NO and Char NO Laboratory Tests (NO results given as weight-% N per original fuel) Fuel-N Vol NO (NO I) Total NO (NO I + NO II)

29 Volatile NO and Char NO Laboratory Tests (NO results given as weight-% N per original fuel) Fuel-N Vol-NO (NO I) Total NO (NO I + NO II)

30 Fuel NOx Summary Fuel-N very reactive: % conversion to NO Not very sensitive to temperature (cf thermal NO) Main contributor to NO in solid fuel combustion Different fuels have different fuel-n bahavior

31 NO x -emissions, mg NO 2 /MJ Schematic view of NO emission distribution by source Thermal-NO (Combustion air) (Volatiles) (Char) Fuel-NO Prompt-NO o C

32 Fuel NOx How to Control? Traditional efficient combustion: Maximize the three T s: Temperature, Turbulence, Time Gives low CO and CxHy...but effective conversion of Fuel-N to NO! Low NOx concept: staged combustion to convert Fuel-N to N 2 A variety of novel burner and furnace technologies with Low-NOx

33 Conventional Burner for Solid Fuels Fuel powder + Transport air Combustion air Devolatilization and gas combustion Char combustion Ash

34 Low-NO x Burner for Solid Fuels Secondary air Fuel powder + Transport air Primary air Secondary air Primary zone l < 1 Secondary zone, Burn-out zone l > 1

35 Examples of NO x chemistry in a BFB and in a Kraft recovery boiler

36 Bubbling fluidized bed (BFB) boiler MW th, 118 bar, 535 C Valmet Power

37 Boiler parameters during the campaign Air distribution Fluidizing air: 50% Secondary air: 25% Tertiary air: 25% λ = 1.3 Bark / sludge / SRF 74 / 16 / 10 wt-% Temperatures Bed: 800ºC Peak temp. 1100ºC At bull nose level 950ºC In-furnace gas measurements

38 Boiler parameters during the campaign Air distribution Fluidizing air: 50% Secondary air: 25% Tertiary air: 25% λ = 1.3 Bark / sludge / SRF 74 / 16 / 10 wt-% Temperatures Bed: 800ºC Peak temp. 1100ºC At bull nose level 950ºC Gas composition from bed NH ppm HCN 162 ppm NO 216 ppm H 2 O 31.4 Vol-% N Vol-% CO Vol-% CO 2.5 Vol-% CH Vol-% C 2 H Vol-% C 2 H Vol-%

39 BFB main nitrogen species 10% 27% 95%

40 BFB simulations to understand NO chemistry Prim 50%, sec 25%, tert 25% of total air Furnace temperature 1000 C Furnace gas below sec. air as input λ = 1.3 Air λ < 1 Air Fuel Gas composition NH ppm HCN 162 ppm NO 216 ppm H 2 O 31.4 Vol-% N Vol-% CO Vol-% CO 2.5 Vol-% CH Vol-% C 2 H Vol-% C 2 H Vol-%

41 BFB simulations to understand NO chemistry Prim 50%, sec 25%, tert 25% of total air Furnace temperature 1000 C Furnace gas below sec. air as input λ = 1.3 Air λ < 1 Air Fuel Gas composition NH ppm HCN 162 ppm NO 216 ppm H 2 O 31.4 Vol-% N Vol-% CO Vol-% CO 2.5 Vol-% CH Vol-% C 2 H Vol-% C 2 H Vol-%

42 BFB simulations Secondary air jet TFN TFN = all nitrogen species except N 2 Temp. Temp. NH 3 TFN NO NO NO 2 N 2 O HCN Entrainment, no chemistry

43 NH 3 + O 2 NO NO + NH N BFB simulations Secondary air jet Temp. NH 3 NO 2 NO TFN N 2 O HCN

44 BFB simulations Secondary air jet Temp. TFN NO 2 N 2 O NO NH 3 HCN Entrainment, no O 2, no chemistry

45 BFB simulations Measured gas composition NH ppm HCN 162 ppm NO 216 ppm H 2 O 31.4 Vol-% N Vol-% CO Vol-% CO 2.5 Vol-% CH Vol-% C 2 H Vol-% C 2 H Vol-%

46 BFB simulations Air Air Fuel Furnace section λ = 1.3 λ < 1

47 BFB simulations Air Air Fuel

48 BFB simulations Tertiary air jet TFN Temp. NO NH 3 HCN N 2 O NO 2

49 BFB simulations λ = 1.3 Air λ < 1 Air Fuel

50 BFB simulations 10% 9% 27% 30% 95% 95%

51 BFB NOx chemistry Efficient NOx reduction NH 3 in furnace gas up to tert air Air jet NO, NH 3, O 2, T

52 Kraft recovery boiler 3000 tds/day DS 80% Air distribution Primary 32 % Lower secondary 32 % Upper secondary 22 % Tertiary 14 % λ =1.2 (Valmet)

53 Kraft recovery boiler To next air jet / exit Fuel particle release profiles - Moisture - Volatiles - Char-C - Char-N Air jet Combustion air jet Entrainment of furnace gas into air jet Furnace section Furnace gas below first jet

54 Furnace height (m) Main difference to BFB is that fuel-n release is not confined to bed Kraft recovery boiler CFD for profiles Temperature, moisture, volatiles, char-c, char-no T ( C) Volume-weighted Droplet NH 3 +NO release average temperature Temperature ( C) (gn/s/m)

55 Kraft recovery boiler N-release λ > 1 Fuel-N release is not confined to bed and there is oxidizing conditions in the furnace

56 Kraft recovery boiler Tertiary Upper secondary Lower secondary

57 Kraft recovery boiler NH 3 reacts between air levels Temp. TFN NO NH 3, NO 2, and N 2 O

58 Kraft recovery boiler Tertiary Upper secondary Lower secondary

59 Kraft recovery boiler Upper secondary air jet Temp. TFN NO NO 2 NH 3

60 Kraft recovery boiler 26% 25% Predicted final NO emission (TFN 100% NO) This would be the NO profile if all released fuel-n was fully oxidized to NO

61 BFB and Kraft recovery boiler NOx chemistry BFB - NOx chemistry takes place in the air jets (more generally, where furnace gas and combustion air mix) Recovery boiler - NOx chemistry between air levels Compared to BFB, fuel nitrogen released in a larger volume of the furnace, not confined to the bed, and in-furnace conditions are more oxidizing Efficient in-furnace NOx reduction: NH 3, NO, and O 2 simultaneously present at temperatures favoring reduction Air staging

62 NO chemistry in CFBC Coal vs Wood

63 Nitrogen Oxides with Coal and Wood (Leckner et al.) Coal Wood Volatile, % db N(fuel), % db CFBC NO, ppm 50 60

64 NO x ppm NO Profiles in CFBC (Leckner at al.) Secondary air Wood Reactor height, m

65 NO x ppm NO Profiles in CFBC (Leckner et al.) Secondary air Wood 100 Coal Reactor height, m

66 NO formation for solid fuels N 2 Thermal NO N vol N 2 HCN, NH 3 NO N fuel N 2 N 2 N char NO

67 NO formation for solid fuels in CFBC Thermal ( N 2 NO ) N 2 N vol N 2 HCN, NH 3 NO N fuel N 2 N 2 N char NO

68 NO x ppm NO Profiles in CFBC (Leckner et al.) Modelling Attempts (Goel et al.) Secondary air Wood 100 Coal Reactor height, m

69 NO Emission with Wood-Coal (Leckner et al.) 150 Nitric oxide ppm (6% O 2 ) Wood 50% (Energy) Coal

70 NO Emission with Wood-Coal (Leckner et al.) 150 Nitric oxide ppm (6% O 2 ) Wood 50% (Energy) Coal

71 NO Emission with W&C (Leckner et al.) 150 Nitric oxide ppm (6% O 2 ) Wood 50% (Energy) Coal With increasing share of coal as fuel, the NO emission initially increases, but with higher share coal, the NO reduction by char carbon becomes more effective

72 NO Emission with W&C (Leckner et al.) and Modelling Attempts (Engblom et al.) 150 Nitric oxide ppm (6% O 2 ) Case 2 Case 1 Measured 0 Wood 50% (Energy) Coal

73 Nitrogen oxide chemistry in combustion processes Terms and concepts Chemical equilibrium and NO Thermal-NO (Zeldowich NO) and kinetics Fuel-NO Volatile-NO HCN, NH3 Char-NO Staged Combustion: Low Nox Technologies NO formation tendency characterization (NO chemistry in fluidised bed combustion)