AME 436 Energy and Propulsion Lecture 5 Pollutant formation and remediation

AME 436

Energy and Propulsion

Lecture 5Pollutant formation and remediation

2AME 436 - Spring 2015 - Lecture 5 - Emissions formation and remediation

Outline

Description of pollutants Emissions standards CO Hydrocarbons Nitrogen oxides Soot Remediation (cleanup) of emissions


“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 formNO + 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)

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!


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


Pollutant Mileage Bin 8 Bin 7 Bin 6 Bin 5 Bin 4 Bin 3 Bin 2

NMOG 50k120k

0.1000.125

0.0750.090

0.0750.090

0.0750.090 0.070 0.055 0.010

CO 50k120k

3.44.2

3.44.2

3.44.2

3.44.2 2.1 2.1 2.1

NOx 50k120k

0.140.20

0.110.15

0.080.10

0.050.07 0.04 0.03 0.02

PM 120k 0.02 0.02 0.01 0.01 0.01 0.01 0.01

HCHO 50k120k

0.0150.018

0.0150.018

0.0150.018

0.0150.018 0.011 0.011 0.004

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, lower-numbered bins)

Tier II emissions standards


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)



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


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


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)

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 can’t get oxidized)


CO 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 stoichiometric or slightly lean conditions, with high Tad and excess O2 available

Ronney et al. (1994)

BMEP = Brake Mean Effective Pressure (measure of work output)

BSCO = Brake Specific CO (measure of CO emissions per unit work produced)


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 (!!!)


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 propagateinto 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


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

Ronney et al. (1994)


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


Zeldovich mechanism Extremely high activation energy due to enormous strength of NN 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) < Z2exp(-E2/T) for T < 3394K 1 NO molecule formed from (1) yields 2 NO molecules if (2) is fast


Zeldovich mechanism Where do O atoms come from? From inside the flame (often super-

equilibrium O concentration) or equilibrium dissociation of O2 in products

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 = 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


Zeldovich mechanism

Physical interpretation of NO - infinite time required to reach equilibrium,

but NO is the the time constant in the asymptotic approach to equilbrium, e.g. [NO](t) = [NO]equil{1 - exp(-t/NO)}

NO

co

nce

ntr

atio

n

Time

Equilibrium NO concentration

NO


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)


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 < 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 super-equilibrium 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)


Prompt NO experiments

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

Dominated by CH + N2

Dominated by O + N2


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 - NO ~ 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


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

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


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

Premixed: Sakai et al. (1973)Nonpremixed: Vioculescu &

Borman (1978)


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


Soot photographs

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

Soot “particle”L: laser soot absorption; R: direct photo (R. Axelbaum, Washington Univ.)

http://www.asn.u-bordeaux.fr/images/soot.jpg

http://www.atmos.umd.edu/~pedro/soot2.jpg

3.5 Å

105-106 atoms

A. Boehman, Penn State


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


HACA mechanism - Frenklach, 2002


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 > alkanes > alkenes > alkynese.g. C6H6 > H3C-CH3 > H2C=CH2 > HCCH

…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 > alkaneswhich 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)


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


Soot formation - premixed flames Note fuel structure doesn’t 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)


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 > alkanese.g. C6H6 > HCCH > 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)


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!


Soot formation - nonpremixed - Gomez et al. (1984)

-lo

g10

(Fu

el m

ass

flo

w (

g/s

) at

sm

oke

po

int)

Mo

re t

end

ency

to

so

ot

Higher temperature


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 Can’t use = 1 in diesels - massive sooting would result!


Catalytic converters for premixed-charge engines

3-way catalyst - since 1975Reduce NO to N2 & O2, oxidize CO & UHC to CO2 & H2OCan 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 NOPoisoned by lead - have to remove antiknock agent Pb(C2H5)4 from

gasoline (good idea anyway)

Kummer (1981)


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!)


Emissions cleanup - non-premixed-charge engines


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 state 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

Designs use extra fuel periodically to burn off particles accumulated in traps


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?


ExamplePlanet 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) 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.

(b) Rate of formation of Zeldovich (thermal) NO. Would increase (probably by a factor of 2.)

(c) 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).

(d) 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).

(e) 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.

(f) 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.


ReferencesBachmeier, 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, 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

Documents

AME 436 Energy and Propulsion Lecture 5 Pollutant formation and remediation