AME514-F06-lecture3

AME514-F06-lecture3 - Advanced fundamental topics Emissions...

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Unformatted text preview: Advanced fundamental topics Emissions formation and remediation Description of pollutants CO Unburned hydrocarbons (UHC) NOx Soot Reference: Turns Ch. 15, Glassman Ch. 8 AME 514 - Fall 2006 - Lecture 3 1 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 >>> 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 AME 514 - Fall 2006 - Lecture 3 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 feedstock" 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 it' brown sky??? N2O (nitrous oxide): not poisonous, but a "greenhouse gas" gas" UHC (unburned hydrocarbons): participates in catalytic cycles of the form 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) NO + 2O2 +UHC + h NO2 + O3 + UHC 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! AME 514 - Fall 2006 - Lecture 3 3 Greenhouse effect (http://www.ucar.edu/learn/1_3_1.htm) Peak of Planck function shifts from visible ( 0.5 m) at solar T (where most gases don't emit/absorb) to 10 m where CO2 & other gases emit & absorb strongly -1 ) 10000 1000 100 10 1 0.1 0.01 1 10 Abs. coeff. (m -1 atm C O2 300K Wavelength (m) AME 514 - Fall 2006 - Lecture 3 4 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.c.morley.ukgateway.net) AME 514 - Fall 2006 - Lecture 3 5 Methane-air equilibrium products (1 atm) Relatively high NO & CO at adiabatic flame temperature, practically none if we cool this mixture down to 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 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 At 700K 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 514 - Fall 2006 - Lecture 3 6 CO and Unburned HydroCarbons (UHCs) Won't discuss at length here - covered in AME 513 Key steps in oxidation Fuel + O2 CO + H2 (fuel breakdown in flames is relatively fast; O=O 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 can't 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) AME 514 - Fall 2006 - Lecture 3 7 Unburned hydrocarbon reactivity Volatile Organic Compound (VOC) carbon monoxide alkanes methane ethane propane n-butane olefins ethylene propylene 1,3 butadiene aromatics benzene toluene meta-xylene 1,3,5-trimethylbenzene 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 H C H C H C H H C H 1, 3 butadiene H H H C H H H C C C C C H C H H C H meta-xylene AME 514 - Fall 2006 - Lecture 3 8 Oxidation of typical hydrocarbon Start with fuel molecule RH, where R is an "organic radical", e.g. propane without an H H H H H H H H C H C H C H + H = H C H C H propane C H H propyl radical Abstract an H atom from RH Add an O2 to R RH + O2 R + HOO R + O2 ROO Produce peroxides with O-O single bond (half as strong as O=O 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 AME 514 - Fall 2006 - Lecture 3 9 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 NO or Tad NO Tad Lean limit Stoichiometric Rich limit Fuel % 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) AME 514 - Fall 2006 - Lecture 3 10 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) d d " " {" A [A]} = {" B [B]} = #Z [ A] A [B] B T n exp #E $T dt dt d[N 2 ] d[O] 1 &#E ) % = = #Z1[N 2 ]1 [O] T n1 exp( 1 $T + ' * dt dt d[N] d[O2 ] &#E ) 1 % = = #Z 2 [N]1 [O2 ] T n 2 exp( 2 $T + ' * 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 514 - Fall 2006 - Lecture 3 11 Zeldovich mechanism Where do O atoms come from? From inside the flame (often super-equilbrium O concentration) or equilbrium dissociation of O2 in products ! EO+N2 = 76.5 kcal/mole, Keq(O.5O2) 60 kcal/mole, overall > 135 eq(O kcal/mole Heywood (1988): estimate of characteristic time = [NO]equil/(d[NO]/dt)[NO]=0 for initial formation rate of NO in lean combustion products: d[NO] = 2kO +N 2 [N 2 ][O] = 2kO +N 2 [N 2 ]K [O2 ]1/ 2 1 eq (O" O2 ) dt 2 " NO %116,000 cal/mole ( #1/ 2 = 8x10 T exp' (T in K, P in atm, " in sec) *P & ) $T #16 ! 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 AME 514 - Fall 2006 - Lecture 3 12 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) NO Equilibrium NO Thermal NO Prompt NO 0 Distance (or time) from flame front AME 514 - Fall 2006 - Lecture 3 13 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 super-equilibrium concentrations of O Since maximum Tad for fuel-air mixtures 2200K, hydrocarbonbased prompt NO mechanism more important for "real" flames at ambient pressure (but for constant volume combustion after 10:1 compression, Tad 2890K, so O-atom based NO mechanism more important) 14 AME 514 - Fall 2006 - Lecture 3 Prompt NO experiments Dominated by CH + N2 Dominated by O + N2 Eberius and Just (1973) Bachmeier et al. (1973) AME 514 - Fall 2006 - Lecture 3 15 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 Non-premixed flames Example: 2 equal volumes of combustible gas with E = 100 kcal/mole, 1 volume at 1900K, another at 2100K (1900) ~ exp(-100000/(1.987*1900)) = 3.14 x 10-12 (2100) ~ exp(-100000/(1.987*2100)) = 3.91 x 10-11 Average = 2.11 x 10-11 whereas (2000) = 1.18 x 10-11, nearly 2x smaller Always have hot stoichiometric surfaces with T Tad,stoich - even when ad,stoich 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! AME 514 - Fall 2006 - Lecture 3 16 How to reduce NO during combustion? Premixed-charge Heywood (1988) Non-premixed-charge Heywood (1988) AME 514 - Fall 2006 - Lecture 3 17 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) AME 514 - Fall 2006 - Lecture 3 18 NOx cleanup - 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 - so we have different emission standards for Diesels! NOx a major issue for non-premixed engines - depending on if currently planned 2007 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) AME 514 - Fall 2006 - Lecture 3 19 Emissions cleanup - non-premixed-charge engines AME 514 - Fall 2006 - Lecture 3 20 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 ), 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 514 - Fall 2006 - Lecture 3 21 Soot photographs Soot "particle" particle" L: laser soot absorption; R: direct photo Candle flame 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. Axelbaum, Univ. AME 514 - Fall 2006 - Lecture 3 22 Soot formation mechanisms Ring structures form soot because most other large molecules won't 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 (H2C=CH-CH=CH2), etc. AME 514 - Fall 2006 - Lecture 3 23 Soot mechanisms - Frenklach, 2002 AME 514 - Fall 2006 - Lecture 3 24 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 > 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 > 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) 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) 25 AME 514 - Fall 2006 - Lecture 3 Soot formation - premixed - Takahashi & Glassman (1984) Critical vs. Tad vs. Note: (called in these plots) is referenced to CO + H2O, not CO2 + H2O, as products Critical at Tad = 2200K AME 514 - Fall 2006 - Lecture 3 26 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 > 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) 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 doesn't soot at all - Indy 500 race cars use methanol fuel & add aromatic compounds so that fires are visible on sunny days! AME 514 - Fall 2006 - Lecture 3 27 Soot formation - nonpremixed - Gomez et al. (1984) Higher temperature -log10(Fuel mass flow (g/s) at smoke point) More tendency to soot AME 514 - Fall 2006 - Lecture 3 28 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 1960's - 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 AME 514 - Fall 2006 - Lecture 3 29 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 can' soot (e.g. like a vacuum cleaner bag) periodically Proposed designs use extra fuel periodically to burn off particles accumulated in traps AME 514 - Fall 2006 - Lecture 3 30 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 - so 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 everything 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 ahead! AME 514 - Fall 2006 - Lecture 3 31 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, 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 Sidebotham, diffusion flames," Combustion and Flame, Vol. 58, 45-57 flames," Flame, Vol. 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. AME 514 - Fall 2006 - Lecture 3 32 Pulsed corona discharges Reference: Pucharev and Gundersen (1997), Roth & Gundersen (1999) Characteristics Initial phase of spark discharge (< 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 (>50%) - far greater than arc or laser sources AME 514 - Fall 2006 - Lecture 3 33 Corona vs. arc discharge Plasma Zone Corona Streamers High voltage pulse Corona dies out in pulsed mode Coaxial ground electrode no dielectric barrier needed Corona phase (0 - 100 ns) Arc channel High voltage pulse Arc phase (> 500 ns) AME 514 - Fall 2006 - Lecture 3 34 Experimental apparatus AME 514 - Fall 2006 - Lecture 3 35 Images of corona discharge & flame Axial (left) and radial (right) views of discharge Axial view of discharge & flame (6.5% CH4-air, 33 ms between images) AME 514 - Fall 2006 - Lecture 3 36 Characteristics of corona discharge Voltage (kV) or power (MW) Voltage (kV) or power (MW) 400 300 Voltage Current 50 40 30 20 10 0 0 -100 400 Current 300 Voltage Start of arc Power -10 -20 50 40 30 200 100 0 -100 -50 0 50 100 150 Time (ns) 200 250 Power 20 10 0 -10 -20 300 Corona only Current (amps) Current (amps) 200 100 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 AME 514 - Fall 2006 - Lecture 3 37 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 AME 514 - Fall 2006 - Lecture 3 38 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%) 0 0 10 20 3 0 40 50 2 4 6 G a s F lo w 8 22 6 n m la s e r s he e t 10 . 0 12 0 2 4 6 8 0 10 mm 1 2 14 16 18 2 2 100 4 4 80 6 6 60 8 8 40 10 10 20 12 0 2 4 6 8 10 mm 12 14 16 18 12 0 2 4 6 8 10 mm 12 14 16 18 0 Roth & Gundersen, 1999 AME 514 - Fall 2006 - Lecture 3 39 ...
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