close

Вход

Забыли?

вход по аккаунту

код для вставкиСкачать
AME 436
Energy and Propulsion
Lecture 5
Pollutant formation and remediation
Outline







Description of pollutants
Emissions standards
CO
Hydrocarbons
Nitrogen oxides
Soot
Remediation (cleanup) of emissions
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 2
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 it’s BROWN who wants to look at a brown sky???
» N2O (nitrous oxide): not poisonous, but a “greenhouse gas”
 UHCs (unburned hydrocarbons): participates in photocatalytic
cycles of the form
NO + O2 + UHC + hn  NO2 + O3 + UHC
(Methane does not participate, hence only Non-Methane Organic
Gases (NMOG) are regulated)
 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)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 3
Description of pollutants
 Formaldehyde (HCHO): irritates eyes, mucous
membranes, lungs
 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 UHCs, but still a greenhouse gas!
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 4
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 don’t emit/absorb) to ≈ 10
µm where CO2 & other gases emit &
absorb strongly
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 5
Tier II emissions standards
 Emissions (in grams per mile) measured using 2 EPA-standard
driving cycles - city & highway
 U.S. “Tier II” emissions standards require a certain fleet average for
each manufacturer - can produce/sell “dirty” Bin 8 vehicles if offset
by enough “clean” lower-number Bin vehicles
 Average “Bin” requirement decreases with time (cleaner, lowernumbered bins)
Pollutant
Mileage
Bin 8
Bin 7
Bin 6
Bin 5
Bin 4
Bin 3
Bin 2
50k
120k
0.100
0.125
0.075
0.090
0.075
0.090
0.075
0.090
0.070
0.055
0.010
50k
120k
3.4
4.2
3.4
4.2
3.4
4.2
3.4
4.2
2.1
2.1
2.1
50k
120k
0.14
0.20
0.11
0.15
0.08
0.10
0.05
0.07
0.04
0.03
0.02
PM
120k
0.02
0.02
0.01
0.01
0.01
0.01
0.01
HCHO
50k
120k
0.015
0.018
0.015
0.018
0.015
0.018
0.015
0.018
0.011
0.011
0.004
NMOG
CO
NOx
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 6
Unburned hydrocarbon reactivity
 UHCs are weighted by reactivity of
hydrocarbon to produce photochemical
smog in a standardized test
 CH4 is almost completely inert with
respect to photochemical smog
 Other paraffins (C2H6, etc.) weakly active
 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. 1,3,5
trimethylbenzene)
1, 3 butadiene
Volatile Organic
Compound (VOC)
carbon monoxide
alkanes
methane
ethane
propane
n-butane
olefins
ethylene
propylene
1,3 butadiene
aromatics
benzene
toluene
meta-xylene
1,3,5trimethylbenzene
oxygenates
methanol
ethanol
MTBE
ETBE
Reactivity (mg
Ozone produced
per mg VOC)
0.054
0.0148
0.25
0.48
1.02
7.29
9.40
10.89
0.42
2.73
8.15
10.12
0.56
1.34
0.62
1.98
1,3,5 trimethyl
benzene
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 7
Description of pollutants
 Emissions are trace amounts in the combustion products




Example: octane-air combustion
C8H18 + 12.5(O2 + 3.77N2)  8 CO2 + 9 H2O + 12.5*3.77 N2
Mole fraction CO2 in exhaust ≈ 8/(8 + 9 + 12.5(3.77)) = 0.125
Allowable CO mole fraction in exhaust typically ≈ 10-3 – i.e. only
1 C atom in ≈ 100 can be emitted as CO instead of CO2
 Other emissions (NO, CH2O, etc.) much lower allowable mole
fractions, e.g. 10-5
 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, UHCs 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 via chemical equilibrium, e.g. with GASEQ
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 8
Methane-air equilibrium products (1 atm)
 Relatively high NO &
CO at adiabatic flame
temperature, practically
none if we cool this
mixture down to
equilibrium at 700K
Species
N2
H2O
CO2
CO
O2
OH
H
O
H2
NO
HCO
CH2O
CH4
CH3
HO2
NO2
NH3
NH2
N
HCN
CN
N2O
C2
CH
At AFT
(2226K)
0.70864
0.18336
0.08536
0.00896
4.561e-03
2.922e-03
3.898e-04
2.130e-04
3.621e-03
1.975e-03
7.688e-10
2.002e-11
2.712e-17
7.107e-17
5.585e-07
3.306e-07
2.740e-09
9.167e-10
1.416e-08
1.547e-11
8.234e-14
9.383e-08
2.205e-26
4.128e-18
At 1500K
At 700K
0.71488
0.18997
0.09495
6.698e-05
4.675e-05
1.350e-05
1.264e-07
2.745e-08
5.208e-05
1.883e-05
1.292e-14
2.853e-15
2.170e-22
1.275e-23
5.826e-10
1.439e-09
2.017e-11
1.079e-13
5.112e-14
2.045e-16
3.688e-21
9.708e-10
9.109e-41
3.336e-28
0.71493
0.19005
0.09502
9.168e-13
8.094e-12
9.757e-14
1.049e-19
1.008e-21
1.736e-11
1.971e-12
9.577e-32
4.096e-29
9.773e-41
0.00000
3.034e-20
1.320e-17
5.795e-19
1.474e-27
5.604e-33
6.612e-34
0.00000
1.934e-16
0.00000
0.00000
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 9
CO, formaldehyde, UHCs
 Won’t discuss hydrocarbon oxidation chemistry at length
here - covered in AME 513 (Fundamentals of Combustion) &
AME 579 (Combustion Chemistry & Physics); also a bit in
Lecture 10 (in context of engine knock)
 Key steps in oxidation
Fuel + O2  CO + H2 (fuel breakdown in flames is relatively fast)
H 2 + O2  H 2O
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 can’t get oxidized)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 10
CO
BMEP = Brake Mean Effective
Pressure (measure of work output)
BSCO = Brake Specific CO (measure
of CO emissions per unit work
produced)
BSCO emission (g/kW-hr)
 If mixture is rich, CO is unavoidable since there is not enough O2 to
burn all the C to form CO2 (but we don’t want to go rich anyway,
since fuel efficiency will decrease also)
 For lean conditions, CO is still formed, and actually gets worse as 
decreases (TPCE), or as engine is throttled (P decreased), or as
more EGR is added - decreases Tad, slower reaction, not enough
time for CO to CO2 conversion
 Thus, CO is minimum at
Throttle
stoichiometric or slightly
Best TPCE
lean conditions, with high
Other TPCE
Tad and excess O2 available
EGR
10
Ronney et al. (1994)
1
0
1
2
3
BMEP (bar)
4
5
6
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 11
Unburned hydrocarbons (UHCs)
 If fuel decomposes quickly, why are UHCs still emitted?
 In the engine, emissions of UHCs come from
 Raw unburned fuel (see next slide)
 Fuel that didn’t burn all the way to CO2 and H2O
 Lubricating oil (especially in 2-stroke engines using fuel + oil
mixtures)
 Other than tailpipe, UHCs may come from
 Crankcase fumes (older engines without crankcase gas
recycling)
 Fuel tank (older cars without evaporative emission controls)
 Filling station (in regions without 2nd hose to recover gas tank
vapors)
 Tires (!!!)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 12
Unburned hydrocarbons (UHC)
 Why didn’t fuel burn in engine?
 Bad mixing (especially Diesels at high load, near  = 1); last
molecule of fuel can’t find last molecule of air in time available
 Misfire - too small ST (low , high EGR, etc.), bad spark, etc.
 Solution / dissolution of fuel into oil or engine deposits
 Quenching near walls and in crevice volumes - if ratio of crevice
thickness (d) to flame thickness  ≈ /SL < 40,
flame will not be able to propagate
into crevice, mixture will not be burned,
UHCs will be formed
 Of course, some of the UHCs formed
in these ways will be burned before
leaving engine
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 13
Unburned hydrocarbons (UHC)
 Net result - similar to CO, higher for leaner mixtures (TPCE), or
as throttling (lower P), or as more EGR is added - decreases
Tad, slower reaction, not enough time for CO to CO2 conversion
 Much less UHCs when using throttling rather than lean
mixtures or EGR to reduce BMEP - with throttling, still  = 1, Tad
≈ constant, fuel gets broken down quickly
BSHC emission (g/kW-hr)
100
Throttle
Best TPCE
Other TPCE
EGR
10
Ronney et al. (1994)
1
0
1
2
3
BMEP (bar)
4
5
6
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 14
Nitrogen oxides
Typical experimental result
 Peak NO slightly lean of
stoichiometric ( ≈ 0.9) since N2 is
plentiful at all , but surplus O2 is
present only for lean mixtures
 Very sensitive to temperature (high
activation energy) so peak still
close to  = 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
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 15
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)
(
d
d
n
n
{n A [A]} = {n B [B]} = -Z [ A] A [ B] B T n exp -E ÂT
dt
dt
d[N 2 ] d[O]
1 n
æ-E1
ö
1
1
ç
÷
Þ
=
= -Z1[N 2 ] [O] T exp
ÂT
è
ø
dt
dt
d[N] d[O2 ]
æ-E 2
ö
1 n2
1
ç
÷
Þ
=
= -Z 2 [N] [O2 ] T exp
ÂT
è
ø
dt
dt
Generic :
)
 Reaction (1) is usually limiting; Z1exp(-E1/T) < Z2exp(-E2/T) for T < 3394K
 1 NO molecule formed from (1) yields 2 NO molecules if (2) is fast
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 16
Zeldovich mechanism
 Where do O atoms come from? From inside the flame (often superequilibrium O concentration) or equilibrium dissociation of O2 in
products
d[NO]
= 2kO+N [N 2 ][O] = 2kO+N [N 2 ]Keq (OÛ0.5O ) [O2 ]1/2
2
2
2
dt
 EO+N2 = 76.5 kcal/mole, Keq(O.5O2) ≈ 60 kcal/mole, overall > 135
kcal/mole
 Heywood (1988): characteristic time t = [NO]equil/(d[NO]/dt)[NO]=0 for
initial formation rate of NO in lean combustion products, assuming
equilibrium [O]
t NO
æ116,000 cal/mole ö -1/ 2
= 8x10 T expç
÷P (T in K, P in atm, t in sec)
è
ø
ÂT
-16
 T = 2200K, P = 1 atm: tNO = 0.59 second
 By comparison, time scale for chemical reactions in flame front
tflame ~ /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
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 17
Zeldovich mechanism
 Physical interpretation of tNO - infinite time required to reach
equilibrium, but tNO is the the time constant in the asymptotic
approach to equilbrium, e.g. [NO](t) = [NO]equil{1 - exp(t/tNO)}
NO concentration
Equilibrium NO concentration
tNO
Time
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 18
Prompt mechanism
 …but this doesn’t 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)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 19
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  ≈
1.4 - suggests a CH or C2-based mechanism - but changing 
changes both chemistry AND Tad
 Eberius and Just (1973)
 Propane-O2-N2 mixtures used to adjust  and Tad independently
 Shows two types of prompt NO
» T < 2400K: more prompt NO for rich mixtures, E ≈ 15 kcal/mole
» T > 2400K: more prompt NO for lean mixtures, E ≈ 75 kcal/mole
(close to E for N2 + O  NO + O), probably due to superequilibrium concentrations of O
 Since maximum Tad for HC-air mixtures ≈ 2200K, CH/C2
mechanism dominates “real” flames at 1 atm, but for constant-V
combustion with 10:1 compression, Tad ≈ 2890K, so O-atom
based NO mechanism dominates)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 20
Prompt NO experiments
Dominated
by CH + N2
Eberius and Just (1973)
Dominated
by O + N2
Bachmeier et al. (1973)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 21
Factors affecting NO formation in engines
 Equivalence ratio or FAR - already discussed
 Exhaust residual - dilutes fuel-air mixture, reduces T (assuming
exhaust is cooler than adiabatic T) (diluting a cold fuel-air mixture
with adiabatic exhaust has no effect on flame temperature!)
 Intake pressure - tNO ~ P-1/2 - weak effect
 Engine RPM (N): higher N  less time for NO to form, but less time
to shift to equilibrium, so no clear winner
 Spark timing - see lecture 10 - more advance improves th up to a
point, but yields higher maximum T, more NOx
0.3
1300
1200
Brake thermal efficiency
1100
0.26
T
max
0.24
1000
/3
900
T
0.22
0.2
max,end gas
0
0.05
0.1
Advance
Temperature, K
Brake efficiency
0.28
800
700
0.15
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 22
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  = 1 mixtures and minimize temperature
with Exhaust Gas Recirculation (EGR)
  = 1 mixtures have less available O atoms
  ≈ 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
w(1900) ~ exp(-100000/(1.987*1900)) = 3.14 x 10-12
w(2100) ~ exp(-100000/(1.987*2100)) = 3.91 x 10-11
Average = 2.11 x 10-11
whereas w(2000) = 1.18 x 10-11, nearly 2x smaller
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 23
How to reduce NO during combustion?
 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!
 Recall for premixed flames, every parcel of gas has same peak
temperature - lean mixtures will have much lower NO
0.0035
Premixed: Sakai et al. (1973)
Nonpremixed: Vioculescu &
Borman (1978)
NO mole fraction
0.0030
Premixed
Non-premixed
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Equivalence ratio
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 24
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
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 25
Soot photographs
http://www.asn.u-bordeaux.fr/images/soot.jpg
L: laser soot absorption;
R: direct photo
(R. Axelbaum, Washington Univ.)
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
A. Boehman, Penn State
Soot “particle”
3.5 Å
105-106 atoms
http://www.atmos.umd.edu/~pedro/soot2.jpg
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 26
Soot formation mechanisms
 Ring structures form soot because most other large molecules
won’t survive at flame temperatures (even if no O2 present)
 Formation of 1st ring typically slowest - growth & merging of rings
relatively rapid
 Formation limited by rate of fuel breakdown to form key species:
acetylene, aromatics, butadiene (H2C=CH-CH=CH2), etc.
 Mechanism of soot formation seems to be related to Hydrogen
Abstraction C2H2 Addition (HACA) (next slide) (original paper:
Frenklach & Wang, 1991) - captures three important factors of
molecular weight growth
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 27
HACA mechanism - Frenklach, 2002
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 28
Soot formation - premixed flames
 For fixed experimental conditions, soot formation occurs for
mixtures richer than a critical equivalence ratio (c) - higher c,
less sooting tendency
Aromatics > alkanes > alkenes > alkynes
e.g. C6H6 > H3C-CH3 > H2C=CH2 > HCCH
 …but changing  changes both chemistry AND Tad
 Tad doesn’t 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 > alkynes > alkenes > 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)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 29
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
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 30
Soot formation - premixed flames
 Note fuel structure doesn’t matter except in terms of number of CC 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)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 31
Soot - nonpremixed flames
 c irrelevant parameter for nonpremixed flames - always have full
range of  from 0 to ∞
 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 > alkynes > alkenes > alkanes
e.g. C6H6 > HCCH > H2C=CH2 > H3C-CH3
(don’t 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)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 32
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,
where all fuel molecules are destroyed before carbons re-assemble
in the combustion products)
 Side note: methanol doesn’t soot at all - Indy 500 race cars use
methanol fuel & add aromatic compounds so that fires are visible on
sunny days!
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 33
Soot formation - nonpremixed - Gomez et al. (1984)
-log10(Fuel mass flow (g/s) at smoke point)
More tendency to soot
Higher temperature
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 34
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  = 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
 Can’t use  = 1 in diesels - massive sooting would result!
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 35
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)
Kummer (1981)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 36
NOx cleanup - non-premixed-charge engines
 NOx a major issue for non-premixed charge engines
 Can use EGR to reduce Tad, thus reduce NOx, but can’t 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
 Until recently there were different emission standards for Diesels!
 With Tier II system, clean small gasoline vehicles can offset dirty large
diesels
 Larger vehicles, > 8500 lb gvw, have more lenient standards on a
g/mile basis
 “Thermal DeNox” & “Selective Catalytic Reduction” is currently
used for stationary applications and might be used for vehicles
(but need urea {(NH2)2CO} supply!) (now called “Diesel Exhaust
Fluid” (DEF) because “urea” has a bad connotation!)
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 37
Emissions cleanup - non-premixed-charge engines
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 38
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 can’t 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
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 39
Summary - most important points
 Emissions are a non-equilibrium effect - depends on rates of reactions
 CO & UHC - form due to flame quenching or incomplete combustion - go
lean (extra O2) and hot (high reaction rate) to oxidize to CO2 & H2O
 NOx formation very high activation energy - temperature dependent - small
decrease in T causes large decrease in NOx; also need O - go rich and cool
 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; fuel structure unimportant
 Nonpremixed - higher T leads to more soot since formation on rich side
of flame front (no O2 present, no oxidation); fuel structure important
 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!
» Particulate traps for soot?
» SCR for NOx?
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 40
Example
Planet X is exactly the same as earth except that, due to a disturbance in The Force, all chemical reaction
rates are a factor of 2 lower than on earth. How would each of the following be affected, i.e., state
whether the property would increase, decrease or remain the same?
(a)
(b)
(c)
(d)
(e)
(f)
Amount of NO in the combustion products of a premixed-gas flame, far downstream of the
flame front. Would not change since this corresponds to equilibrium, and in equilibrium, the
forward and reverse rates are equal, thus decreasing both rates by a factor of 2 would have no effect
on the balance between N2, O2 and NO at equilibrium.
Rate of formation of Zeldovich (thermal) NO. Would increase (probably by a factor of 2.)
Amount of CO emission from a premixed-charge engine. Since combustion would be slower,
more CO would be emitted (i.e. less of the CO to CO2 conversion would occur).
Amount of unburned hydrocarbon emission from a premixed-charge engine. Similar to CO,
since combustion would be slower, more unburned hydrocarbons would be emitted would be emitted
(i.e. less of the hydrocarbon conversion to CO2 and H2O would occur).
Amount of soot emission from a premixed flame. Both soot formation and oxidations rates
would decrease by the same factor, so probably not much change in the amount of soot emitted.
Amount of soot emission from a non-premixed flame. Would decrease by a factor of 2 since in
this case there is no competition between formation and oxidation.
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 41
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. AGARDCP-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, 2028–2037.
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.
Kummer, J. T., Prog. Energy Comb. Sci. 6, 177 (1981)
Ronney, P. D. Shoda, M., Waida, S., Durbin, E. (1994). J. Auto. Eng., (Proc. Instit. Mech. Eng., Part
D), Vol. 208, pp. 13-24.
Sakai, Y., Miyazaki, H., Mukai, K. (1973). SAE paper 730154.
Takahashi, F., Glassman, I. (1984). Combust. Sci. Technol. Vol. 37, p. 1.
Vioculescu, L. A., Borman, G. L. (1978). SAE paper 780228.
Wang, H., Frenklach, M. (1997). “A detailed kinetic modeling study of aromatics formation in
laminar premixed acetylene and ethylene flames.”. Combustion and Flame, Vol. 110, 173-221
AME 436 - Spring 2013 - Lecture 5 - Emissions formation & remediation 42
1/--страниц
Пожаловаться на содержимое документа