Direct Soot Formation

Transcription

1 Direct Soot Formation Claus Wahl 14 th ET Conference on Combustion Generated Particles August 1 st 4 th, 2010 Deutsches Zentrum für Luft und Raumfahrt. (DLR-Stuttgart) Mailing address DLR-VT Pfaffenwaldring 38-40, D Stuttgart Phone / Fax. +49-(0) claus.wahl@dlr.de Measurements show, that inside an aircraft gas turbine combustor, the soot concentration can be much higher than at combustor exit. The high soot concentrations inside the combustor can not be explained by ACA soot formation mechanism (ydrogen Abstraction Carbon Addition). Therefore a new soot formation mechanism is suggested and discussed. I call it Direct Soot Mechanism. Jet fuel contains aliphatic molecules from C8 to C16. In a gas turbine combustor, the fuel spray is injected into hot compressed air. Fuel molecules vaporize from the spray droplets and heat up by heat transfer and flame radiation, before reaching the flame zone. Some fuel molecules crack by pyrolysis and built radicals and ions. These radicals and ions can recombine and built first clusters. If this happens, soot formation can start before the fuel molecules reach the flame region! But the main part of soot formation takes place inside the flame, in fuel rich zones. Fuel molecules are cracked by pyrolysis and oxidation. This results in high concentrations of fuel-radicals and fuel-ions. The recombination of this radicals and ions built first clusters (fig. 1). These first clusters have very low density. The formation of the first soot particles in the flame results from further cluster growth, cluster agglomeration and cluster densification. The densification is a result of Fig. 1: Soot Cluster

2 the formation of more and more C=C double bonds and the formation of new C-C bonds inside the cluster. The aliphatic side branches of the particles are burned. At the end, the density and C/ ratio of the soot particles is a function of residence time, temperature and phi history. Densification and graphitization takes place on the way through the combustor. At combustor exit we see the well known soot particles. In the direct soot formation mechanism, it is not necessary to crack the fuel down to acetylene to built the first aromatic rings. Aromatic rings can be formed directly via aliphatic rings (fig. 2).. + ydrogen Abstraction + first aliphatic ring dehydrogenation double bonds further dehydrogenation double bonds first aromatic ring no need to crack the fuel down to small molecules (acetylene)!! Fig. 2 Formation of first aromatic ring via aliphatic ring Direct Soot is a complex, low density, three dimensional polymer of fuel molecules. During the way through the combustor, direct soot is transformed to soot particles. With the Direct Soot Mechanism, it is easy to explain fast soot formation from fuel molecules to soot particles.

3 Direct Soot Formation in kerosene and diesel combustion Claus Wahl 14th ET Conference on Combustion Generated Nanoparticles Folie

4 Gas Turbines show no visible soot, even at Take Off no aftertreatment possible! Folie 2

5 GT-Combustor tubular or annular nozzle, swirl, airblast air fuel droplets probe compressor hot air up to 45bar air combustor diffusion flame with local fuel rich flame zones turbine Folie 3

6 2000 igh CO Emissions hot spots igh Smoke Emissions Flame Temp [degc] evaporated 1000 fuel + air aircraft engine overall EQR NOx Temperature around droplets local EQR 0 100% air Lean Equivalence Ratio Rich 100% fuel EQR= fuel/air Folie 4

7 Diffusion Flame 2000 igh CO Emissions igh Smoke Emissions Flame Temp [degc] LPP NOx Temperature 1000 evaporated fuel + air around droplets local EQR 0 100% air Lean Equivalence Ratio Rich 100% fuel EQR= fuel/air Folie 5

8 Soot is a Mixing Problem! - liquid fuel is injected as droplet into the combustor - only the evaporated and air mixed fuel can be ignited - soot is formed in local fuel rich flame zones around the droplets - particles move on the coldest trajectory through the combustor - in the flame there is: soot formation and surface growth soot oxidation soot agglomeration, aggregation soot densification pollutant formation and fuel oxidation all in parallel Folie 6

9 Pollutants as a Function of Combustor Design high temperature: -Soot, CO and UC thermal NOx formation long residence time (long combustor = lower efficiency) Soot, CO and UC NOx formation (ppm/msec) combustor concepts: tubular, annular, dual annular, staged. LPP, RQL,. Folie 7

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11 Candle / probe in the flame zone dn/dlogdp (#/cm³)[e7] 6,0 5,5 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 Impactor D Diameter (nm) Folie 9

12 Candle with long combustor (long residence time) Folie 10

13 Candle with long combustor (long residence time) dn/dlogdp (#/cm³)[e7] 6,0 5,5 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 Impactor D Diameter (nm) Folie 11

14 Particle Size Distribution / Approach Folie 12

15 SEM picture / Approach all SEM pictures are done by Roland Borath, DLR, WB-WF Folie 13

16 Bockhorn, time consuming steps soot formation gap growth by ACA mechanism formation of first aromatic ring cracking of fuel to small molecules and radicals Folie 14

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18 Existing Soot Formation Mechanisms -all soot formation mechanisms cut fuel molecules down to small molecules like acetylene - Soot formation starts with first aromatic rings made from acetylene this may be true, but only for acetylene flames Folie 16

19 Combustor tubular or annular nozzle, swirl, airblast air fuel droplets probe air compressor combustor turbine diffusion flame with local fuel rich flame zones Folie 17

20 Probe inside the Combustor Soot concentration inside the combustor can be more than 1000 times higher than at the combustor exit Folie 18

21 Probe inside Combuster big Particles high soot conc. low density Folie 19

22 first clusters are formed by recombination of fuel-radicals and charged molecules (pyrolysis of fuel molecules) fuel-radical and fuel-ion formation Direct Soot Formation (C. Wahl) ESM Improved ACA ACA Combustor Fuel nozzle 600 C + flame radiation Exit Folie 20

23 Formation of Fuel-Radicals and Fuel-Ions 600 C + flame radiation Pyrolysis: + can happen before ignition! - - O - O + C 3 + O Folie 21

24 Recombination of Fuel-Ions and -Radicals First Cluster - + cluster dimerisation, radical and ion formation and dehydrogenation all in parallel Folie 22

25 Further Cluster Growth in the Flame + O + _ O cluster dimerisation, radical and ion formation and dehydrogenation all in parallel, more and more double bonds and aromatic rings graphene structures Folie 23

26 O O O + O O + Folie 24

27 First Clusters have very low density Folie 25

28 - radical formation - oligomerization and aliphatic ring formation - cluster growth - agglomeration -. - dehydrogenation O O 2 - densification more and more C-C bonds inside the cluster - oxidation (preferably side branches spherical) Folie 26

29 Cluster densification, more and more double bonds and new C-C bonds Folie 27

30 Soot Particle Properties at Exit depend on: - residence time - temperature history - fuel composition - residual aliphatic parts (sp3) inside particles (not UC!) - densification - functional groups - formation of graphene structures - at the end the well known soot particles can appear Folie 28

31 Soot structure, developed from res TEM pictures + - -O - - graphene layers + - -C3 Radicals, pos. and neg. charges and functional groups on the surface Folie 29

32 n - undecane JET A1 Standard JET -A1 n - dodecane n - decane n - tridecane n-nonane n - tetradecane n - pentadecane Folie 30

33 Main Compounds of Jet-A1 ( C8 to C15 ) C n 2n+2 Folie 31

34 Folie 32 C C C C C C C C C C C C C C C C C Direct Formation of first aliphatic Ring dehydrogenation

35 . + ydrogen Abstraction + first aliphatic ring dehydrogenation double bonds further dehydrogenation double bonds first aromatic ring no need to crack the fuel down to small molecules (acetylene)!! Folie 33

36 Flash pyrolysis experiment can support aromatic ring formation Folie 34

37 Flash Pyrolysis 650 C Lignite C n 2n Folie 35

38 Flash Pyrolysis 650 C Lignite main compounds C n 2n all from C 8 to C 22 Folie 36

39 Flash Pyrolysis 750 C Lignite Folie 37

40 Flash Pyrolysis 750 C Lignite Folie 38

41 Flash Pyrolysis 1100 C Lignite Folie 39

42 Flash Pyrolysis 1100 C Lignite Folie 40

43 Conclusion Direct Soot Formation is a possibility to explain high soot concentrations Fuel molecules must not be broken down to acetylene Direct Soot Formation is a fast pathway to soot, in addition to ACA Flash pyrolysis experiments support the theory Ion reaction may also play a role More basic research is necessary (shock tube and flow reactor experiments) Folie 41

44 Soot is a mixing problem Lean, Premixed, Prevaporized Combustors (LPP) produce no soot GE-nx Thank you for your attention Folie 42

45 EI s for existing Engines: NOx CO 1 9 g/kgfuel 1 67 g/kgfuel Soot mg/kgfuel # ~ E16 / kgfuel UC Aldehyde 0-30 g/kgfuel 0-6,2 g/kgfuel Folie 43

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