Advanced fundamental topics

1
Advanced fundamental topics

• Emissions formation and remediation
• Description of pollutants
• CO
• Unburned hydrocarbons (UHC)
• NOx
• Soot
• Reference Turns Ch. 15, Glassman Ch. 8

2
Combustion science for 9/11

• 767 aircraft can carry up to 160,000 lb
  72,600 kg of fuel
• Hydrocarbon fuels QR 4.5x107 J/kg (by
  comparison, nitrogylcerin QR 6.2 x106 J/kg)
• If a 767 enters a building half-full of fuel, and
  half the fuel burns inside the building, energy
  release 1.6x1012 J
• Steel has Cp 450 J/kgK and melts at 1809K, thus
  1.6x1012 J would melt 2.4x106 kg 2650 tons of
  steel (or maybe weaken twice this much steel)
• Weight of towers 500,000 tons each
• 2650 tons 0.58 floors melted or 1 floor
  severely weakened
• If all energy were concentrated at 1 floor,
  damage would be sufficient to destroy 1 floor and
  start a collapse
• but since most of fire was spread over 10 - 20
  floors, it seems unlikely that the energy of the
  burning fuel itself would have been sufficient to
  weaken the towers enough to cause the collapse
• ? Burning of material (paper, wood, plastics,)
  inside the towers (which is gtgtgt 40 tons) must
  have been responsible for the collapse
• ? The towers probably would have stood if the
  fire extinguisher system had not failed due to
  massive damage from the aircraft impacts

3
Description of pollutants

• Photochemical smog - soup of O3, NOx, and
  various hydrocarbons / nitrates / sulfates etc.
• Nitrogen oxides - collectively NOx (pronounced
  knocks)
• NO (nitric oxide) poisonous, but concentrations
  are low - main problem is that it is the main NOx
  emission from most combustion processes -
  feedstock for atmospheric NOx
• NO2 (nitrogen dioxide) some produced during
  combustion, most in atmosphere powerful oxidant
  main problem it that its BROWN - who wants to
  look at a brown sky???
• N2O (nitrous oxide) not poisonous, but a
  greenhouse gas
• UHC (unburned hydrocarbons) participates in
  catalytic cycles of the form
• NO 2O2 UHC hn ? NO2 O3 UHC
• O3 (ozone) - not produced by combustion (produced
  by atmospheric reactions above) powerful
  oxidant, highly irritating to lungs excellent
  disinfectant (i.e. it kills everything in its
  path)

4
Description of pollutants

• CO (carbon monoxide) poisonous in large
  concentrations, otherwise not much of a problem
• Soot (mostly carbon, fine particles) causes
  respiratory problems, obscures sky, excellent
  substrate for all kinds of atmospheric chemical
  reactions
• CO2 - the carbon has to go somewhere, CO2 is
  better than CO or UHC, but still a greenhouse gas!

5
Greenhouse effect (http//www.ucar.edu/learn/1_3_1
.htm)

• Peak of Planck function shifts from visible (
  0.5 µm) at solar T (where most gases dont
  emit/absorb) to 10 µm where CO2 other gases
  emit absorb strongly

6
Description of pollutants

• Mantra - emissions are a NON-EQUILIBRIUM
  PROCESS
• If we follow two simple rules
• Use lean or stoichiometric mixtures
• Allow enough time for chemical equilibrium to
  occur as the products cool down
• then NO, CO, UHC and C(s) (soot) are
  practically zero
• So the problem is that we are not patient enough
  (or unable to allow the products to cool down
  slowly enough)!
• Check this out using a chemical equilibrium
  program, e.g. GASEQ by Chris Morley
  (http//www.gaseq.co.uk)

7
Methane-air equilibrium products (1 atm)

• Relatively high NO CO at adiabatic flame
  temperature, practically none if we cool this
  mixture down to 700K

8
CO and Unburned HydroCarbons (UHCs)

• Wont discuss at length here - covered in AME 513
• Key steps in oxidation
• Fuel O2 ? CO H2 (fuel breakdown in flames is
  relatively fast OO bond can be broken at
  relatively low temperatures in the presence of
  hydrocarbons see next page)
• H2 O2 ? H2O
• CO O2 ? CO2 (CO is last thing to oxidize if
  insufficient time for combustion, CO is emitted
  from flame) (need OH radicals to obtain CO OH ?
  CO2 H, so need high enough temperatures for H
  O2 ? OH O chain branching to occur, otherwise
  CO cant get oxidized)
• UHCs are weighted by the reactivity of the
  hydrocarbon to produce photochemical smog in a
  standardized test
• CH4 is almost completely inert with respect to
  photochemical smog
• Other paraffins (C2H6, etc.) inactive also
• 2, 3 butadiene is the mother of all photochemical
  agents (not a common component of fuels, but
  produced in flames (also an important precursor
  to soot)
• Some aromatics bad also (e.g. meta-xylene)

9
Unburned hydrocarbon reactivity
10
Oxidation of typical hydrocarbon

• Start with fuel molecule RH, where R is an
  organic radical, e.g. propane without an H
• Abstract an H atom from RH
• RH O2 ? R? HOO?
• Add an O2 to R?
• R? O2 ? ROO?
• Produce peroxides with O-O single bond (half as
  strong as OO double bond (120 kcal/mole vs. 60
  kcal/mole) much easier to break)
• ROO? RH ? R? ROOH, HOO? RH ? R? HOOH
• Break O-O single bond, create chain branching
  process
• ROOH M ? RO? ?OH, HOOH M ? HO? ?OH
• Newly created radicals generate more organic
  radicals
• RH ?OH ? R? HOH, RH RO? ? R? ROH
• Note that rate of reaction will be sensitive to
  rates of H atom abstraction from fuel molecule RH

11
Nitrogen oxides

• Typical experimental result
• Peak NO slightly lean of stoichiometric (f 0.9)
  since N2 is plentiful at all f, but surplus O2 is
  present only for lean mixtures
• Very sensitive to temperature (high activation
  energy) so peak still close to f 1 where T is
  highest (thermal NO)
• Slower decrease on rich side than lean side due
  to prompt NO formation
• Two flavors of NO
• Thermal or Zeldovich
• Prompt or Fenimore (actually 2 sub-flavors)
• Due to O atoms in flame front
• Due to CH C2 molecules in flame front

Heywood (1988)
12
Zeldovich mechanism

• Extremely high activation energy due to enormous
  strength of N?N bond ( 220 kcal/mole)
• (1) O N2 ? NO N (E1 76,500 cal/mole
  Z1 2 x 1014, n1 0)
• (2) N O2 ? NO O (E2 6,300 cal/mole
  Z2 6 x 109, n2 0)
• -------------------------
• N2 O2 ? 2 NO
• Recall reaction rate expressions (Lecture 1)
• Reaction (1) is usually limiting Z1exp(-E1/?T) lt
  Z2exp(-E2/?T) for T lt 3394K
• 1 NO molecule formed from (1) yields 2 NO
  molecules if (2) is fast

13
Zeldovich mechanism

• Where do O atoms come from? From inside the
  flame (often super-equilbrium O concentration) or
  equilbrium dissociation of O2 in products
• EON2 76.5 kcal/mole, Keq(O?.5O2) 60
  kcal/mole, overall gt 135 kcal/mole
• Heywood (1988) estimate of characteristic time
  t NOequil/(dNO/dt)NO0 for initial
  formation rate of NO in lean combustion products,
  assuming equilibrium O
• T 2200K, P 1 atm ?NO 0.59 second
• By comparison, time scale for chemical reactions
  in flame front ?flame ?/SL2 0.0006 second for
  stoichiometric hydrocarbon-air (see lecture 4) -
  WAY shorter
• Thus, Zeldovich NO occurs in the burned gases
  downstream of the flame front, not in the flame
  front itself

14
Prompt mechanism

• but this doesnt tell the whole story -
  experiments show that some NO forms inside the
  flame (Prompt NO)
• Plot NO vs. distance from flame, extrapolate
  back to flame front location, NO there is
  defined as prompt NO
• Experiments show that prompt NO is more prevalent
  in hydrocarbon flames (not CO, H2), and for
  fuel-rich flames (even though less O in rich
  mixtures, thus Zeldovich less important)

15
Prompt mechanism

• Fenimore (1971) proposed either
• CH N2 ? HCN N followed by (e.g.) N O2 ? NO
  O
• (Z 3.12 x 109, n 0.9, E 20,130 cal/mole
  much faster than N2 O due to lower E, even
  though Z is much lower also) (CH is a much more
  active radical than O, but is present only in the
  flame front, not in the burned gases like O, so
  only affects prompt NO)
• C2 N2 ? 2CN followed by CN O2 ? CO NO
• Bachmeier et al. (1973) in fuel-air mixtures,
  prompt NO peaks at f 1.4 - suggests a CH or
  C2-based mechanism - but changing f changes both
  chemistry AND Tad
• Eberius and Just (1973)
• Propane-O2-N2 mixtures used to adjust f and Tad
  independently
• Shows two types of prompt NO
• T lt 2400K more prompt NO for rich mixtures, E
  15 kcal/mole
• T gt 2400K more prompt NO for lean mixtures, E
  75 kcal/mole (close to E for N2 O ? NO O),
  probably due to super-equilibrium concentrations
  of O
• Since maximum Tad for fuel-air mixtures 2200K,
  hydrocarbon-based prompt NO mechanism more
  important for real flames at ambient pressure
  (but for constant volume combustion after 101
  compression, Tad 2890K, so O-atom based NO
  mechanism more important)

16
Prompt NO experiments

• Eberius and Just (1973) Bachmeier et al.
  (1973)

17
How to reduce NO during combustion?

• Premixed flames - every parcel of gas experiences
  same peak temperature - lean mixtures (good idea)
  or rich mixtures (bad idea)with lower Tad will
  have much lower NO (but then have
  flammability/stability limit problems)
• Better idea use f 1 mixtures and minimize
  temperature with Exhaust Gas Recirculation (EGR)
• f 1 mixtures have less available O atoms
• f 1 mixtures needed for 3-way catalyst
  operation (next slide)
• Improve mixing - if poor mixing, get hot spots
  with much more NOx
• Example 2 equal volumes of combustible gas with
  E 100 kcal/mole, 1 volume at 1900K, another at
  2100K
• ?(1900) exp(-100000/(1.9871900)) 3.14 x
  10-12
• ?(2100) exp(-100000/(1.9872100)) 3.91 x
  10-11
• Average 2.11 x 10-11
• whereas ?(2000) 1.18 x 10-11, nearly 2x smaller
• Non-premixed flames
• Always have hot stoichiometric surfaces with T
  Tad,stoich - even when overall ? is very low ?
  thermal NO NO fuel used
• Always have fuel-rich, warm regions - Fenimore
  NO
• ? Hard to control NO in Diesel (non-premixed
  charge) engines!

18
How to reduce NO during combustion?
Premixed-charge Heywood (1988)
Non-premixed-charge Heywood (1988)
19
Catalytic converters for premixed-charge engines

• 3-way catalyst - since 1975
• Reduce NO to N2 O2, oxidize CO UHC to CO2
  H2O
• Can only get simultaneous reduction oxidation
  very close to ? 1 - need good fuel control
  system with sensor to monitor O2 level in
  exhaust, adjust fuel to maintain ? 1
• Use EGR with ? 1 to lower Tad, thus lower
  in-cylinder NO
• Poisoned by lead - have to remove antiknock agent
  Pb(C2H5)4 from gasoline (good idea anyway)

Heywood (1988)
20
NOx cleanup - non-premixed-charge engines

• Can use EGR to reduce Tad, thus reduce NOx, but
  cant use catalytic converter to reduce NOx
  further, since mixtures are always lean
• As a result, diesels produce less CO UHC (lean
  and hot), but more NO - so we have different
  emission standards for Diesels!
• NOx a major issue for non-premixed charge engines
  - if future regulations are enforced, light-duty
  Diesel engines may become extinct in the U.S.
• Thermal DeNox Selective Catalytic Reduction
  is currently used for stationary applications and
  might be used for vehicles (but need urea
  (NH2)2CO supply!)
• Pulsed corona discharges (sidebar topic)

21
Emissions cleanup - non-premixed-charge engines
22
Soot formation - what is soot?

• Soot is good and bad news
• Good increases radiation in furnaces
• Bad radiation abrasion in gas turbines,
  particles in atmosphere
• Typically C8H1 (not a misprint - mostly C)
• Structure mostly independent of fuel
  environment
• Quasi-spherical particles, 105 - 106 atoms (100 -
  500 Å), strung together like a fractal pearl
  necklace
• Each quasi-spherical particle composed of many
  (104) slabs of graphite (chicken wire) carbon
  sheets, randomly oriented
• Quantity of soot produced highly dependent on
  fuel environment
• Does not form at all in lean or stoichiometric
  premixed flames
• Forms in rich premixed flames and nonpremixed
  flames, where high T and carbon are present, with
  a deficiency of oxygen
• Formation dependent on
• Pyrolysis vs. oxidation of fuel
• Formation of gas-phase soot precursors
• Nucleation of particles
• Growth of particles
• Agglomeration of particles
• Oxidation of final particles

23
Soot photographs
L laser soot absorption R direct photo
Candle flame
Soot particle
Nonpremixed flames, e.g. candle soot is formed,
gives off blackbody radiation (thus light), but
soot is oxidized to CO2, so soot is not emitted
from the flame

• Left center courtesy Prof. R. Axelbaum,
  Washington Univ.

24
Soot formation mechanisms

• Ring structures form soot because most other
  large molecules wont survive at flame
  temperatures (even if no O2 present)
• Mechanism seems to be related to Hydrogen
  Abstraction C2H2 Addition (HACA) (next slide)
  (original paper Frenklach Wang, 1991)
• Formation of 1st ring typically slowest - growth
  merging of rings relatively rapid
• Formation limited by rate of fuel pyrolysis to
  form key species acetylene, aromatics, butadiene
  (H2CCH-CHCH2), etc.

25
Soot mechanisms - Frenklach, 2002
26
Soot formation - premixed flames

• For fixed experimental conditions, soot formation
  occurs for mixtures richer than a critical
  equivalence ratio (?c) - higher fc, less sooting
  tendency
• Aromatics gt alkanes gt alkenes gt alkynes
• e.g. C6H6 gt H3C-CH3 gt H2CCH2 gt HC?CH
• but changing ? changes both chemistry AND Tad
• Tad doesnt change much with fuel, but soot
  formation has high activation energy steps, so
  these small differences matter!
• Experiments controlling ? and Tad independently
  (using fuel-O2-N2 mixtures) show, at fixed Tad,
• Aromatics gt alkynes gt alkenes gt alkanes
• which can be related to the number of C-C bonds
  in the fuel molecule (makes sense - more C-C
  bonds already made, easier to make soot (many C-C
  bonds, few C-H bonds) (consistent with HACA
  mechanism)

27
Soot formation - premixed flames

• Note fuel structure doesnt matter except in
  terms of number of C-C bonds
• Most important point in premixed flames, there
  is less soot tendency (higher ?c) at higher Tad
  because soot formation has high activation
  energy, but oxidation has higher activation
  energy since fuel and air are premixed, both
  soot formation and oxidation occur simultaneously
  (a horse race formation wins at low T, oxidation
  at high T)

28
Soot formation - premixed - Takahashi Glassman
(1984)

• Critical ? vs. Tad Critical ? at Tad 2200K
• Note ? (called ? in these plots) is referenced
  to
• CO H2O, not CO2 H2O, as products

29
Soot - nonpremixed flames

• ?c irrelevant parameter for nonpremixed flames -
  always have full range of ? from 0 to 8
• For fixed experimental conditions, soot emission
  from flame (black smoke) occurs at a flow rate
  higher than a critical value, corresponding to
  critical flame height residence time
• Aromatics gt alkynes gt alkenes gt alkanes
• e.g. C6H6 gt HC?CH gt H2CCH2 gt H3C-CH3
• (dont confuse soot emission with formation,
  i.e. yellow flame color, which occurs even for
  lower flow rates)
• Note this smoke height criterion refers to soot
  emission (black smoke), whereas criterion used
  for premixed flames (?c) refers just to formation
  (yellow flame color)

30
Soot - nonpremixed flames

• Note different ordering than for premixed flames
• but changing fuel type changes both chemistry
  AND Tad
• Experiments with fuel dilution to control Tad
  show less soot tendency (higher flow rate at
  onset of soot) at lower Tad (different from
  premixed flames!) because soot forms on rich side
  of stoichiometric where no O2 is present (no
  competition between soot oxidation growth)
• Note fuel structure matters in this case (unlike
  premixed flames)
• Side note methanol doesnt soot at all - Indy
  500 race cars use methanol fuel add aromatic
  compounds so that fires are visible on sunny days!

31
Soot formation - nonpremixed - Gomez et al. (1984)
Higher temperature
More tendency to soot
-log10(Fuel mass flow (g/s) at smoke point)
32
Emissions cleanup in premixed-charge engines

• Conflicting needs
• For NOx control, go rich and cool
• For CO UHC, want lean (but still near ? 1)
  mixtures to provide good oxidizing environment
  (lean and hot)
• Soot formation is not an issue for
  premixed-charge engines (since lean or
  stoichiometric premixed)
• Early methods (late 1960s - 1975)
• Lean out mixture, blow air into exhaust manifold
  (reduces CO, UHC)
• Retard spark to reduce peak temperature (reduces
  NO, but not much)
• Since 1975 use f 1 mixtures and minimize Tad
  with Exhaust Gas Recirculation (EGR)
• ? 1 mixtures have less available O atoms
• ? 1 mixtures needed for 3-way catalyst
  operation - simultaneous reduction of NO to N2
  O2, oxidation of CO and UHCs to CO2 H2O

33
Emissions cleanup - non-premixed-charge engines

• Soot is the other major problem for diesels
• Formed at high fuel loads (close to but still
  less than stoichiometric)
• Everyone seems to have given up on the
  possibility of eliminating soot formation in the
  engine, and instead use particulate traps to
  capture emitted soot
• Regulations for passenger vehicles states that
  the emissions system must be zero maintenance -
  you cant require the driver to remove
  accumulated soot (e.g. like a vacuum cleaner bag)
  periodically
• Proposed designs use extra fuel periodically to
  burn off particles accumulated in traps

34
Summary - most important points

• Emissions are a non-equilibrium effect - depends
  on rates of reactions
• NOx formation very high activation energy -
  temperature dependent - small decrease in T
  causes large decrease in NOx also need O - go
  rich and cool
• CO UHC - form due to flame quenching or
  incomplete combustion - go lean (extra O2) and
  hot (high reaction rate) to oxidize to CO2 H2O
• Soot
• Premixed - lower T leads to more soot since
  formation is always competing with oxidation (O2
  always present), and oxidation rates increase
  faster with T than formation rates
• Nonpremixed - higher T leads to more soot since
  formation on rich side of flame front (no O2
  present, no oxidation)
• Either way, lean and hot means less soot
• Emissions cleanup
• Conflicting requirements - rich cool for NOx,
  lean hot for all else
• Catalytic converter can do both jobs only very
  close to stoichiometric use EGR (no excess O2)
  rather than lean mixture to reduce Tf for NOx
  reduction
• Works well for premixed charge, but for
  nonpremixed (Diesels) - many troubles!

35
References

• Bachmeier, F., Eberius, K. H., Just, T. (1973).
  Combust. Sci. Technol. 7, 77.
• Eberius, K. H., Just, T. (1973). Atmospheric
  pollution by jet engines, AGARD Conf. Proc.
  AGARD-CP-125, p. 16.
• Fenimore, C. P. (1971) Proceedings of the
  Combustion Institute, Vol. 13, p. 373.
• Frenklach, M. (2002). Reaction mechanism of soot
  formation in flames, Phys. Chem. Chem. Phys.,
  vol. 4, 20282037.
• Frenklach, M., Wang, H. (1991). Proceedings of
  the Combustion Institute, Vol. 23, 1559.
• Gomez, A., Sidebotham, G., Glassman, I. (1984).
  Sooting behavior in temperature-controlled
  laminar diffusion flames, Combustion and Flame,
  Vol. 58, 45-57
• Heywood, J. B. (1988). Internal Combustion
  Engine Fundamentals, McGraw-Hill.
• Puchkarev, V., Gundersen, M. (1997). "Energy
  efficient plasma processing of gaseous emission
  using short pulses," Appl. Phys. Lett. 71 (23),
  3364.
• Roth, G. J., Gundersen, M. A. (1999).
  Laser-induced fluorescence images of NO
  distribution after needle-plane pulsed negative
  corona discharge, IEEE Trans. Plasma Sci. 27,
  28.
• Takahashi, F., Glassman, I. (1984). Combust.
  Sci. Technol. Vol. 37, p. 1.

36
Pulsed corona discharges

• Reference Pucharev and Gundersen (1997), Roth
  Gundersen (1999)
• Characteristics
• Initial phase of spark discharge (lt 100 ns) -
  highly conductive (arc) channel not yet formed
• Multiple streamers of electrons
• High energy (10s of eV) electrons - couple
  efficiently with cross-section for ionization,
  electron attachment, dissociation
• More efficient use of energy deposited into gas
• Enabling technology USC-built discharge
  generators having high wall-plug efficiency
  (gt50) - far greater than arc or laser sources

37
Corona vs. arc discharge
Corona phase (0 - 100 ns) Arc phase (gt 500
ns)
38
Experimental apparatus
39
Images of corona discharge flame

• Axial (left) and radial (right) views of
  discharge
• Axial view of discharge flame

40
Characteristics of corona discharge
Corona only
Corona arc
 

• Arc leads to much higher energy consumption with
  little increase in energy deposited in gas
• Corona has very low noise light emission
  compared to arc with same energy deposition

41
NO removal by corona discharges

• Energy efficient 10 eV/molecule ( 200
  kcal/mole) or less possible
• Transient plasma provides dramatically improved
  energy efficiency - by 100x compared to prior
  approaches employing quasi-steady discharges
• 10 eV/molecule corresponds to 0.2 of fuel energy
  input per 100 ppm NO destroyed
• Applicable to propulsion systems, unlike
  catalytic post-combustion treatments

42
NO removal by corona discharges

• Diesel engine exhaust
• Needle/plane corona discharge (20 kV, 30 nsec
  pulse)
• Lower left before pulse
• Lower right 10 ms after pulse
• Upper difference, showing single-pulse
  destruction of NO ( 40)

Roth Gundersen, 1999