Rajesh Dorai and Mark J. Kushner University of Illinois Deparment of Electrical and Computer Engineering Urbana, IL 61801

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1 EFFECT OF MULTIPLE PULSES ON THE HETEROGENEOUS AND HOMOGENEOUS CHEMISTRY DURING THE PLASMA REMEDIATION OF X AND OXIDATION OF SOOT USING DIELECTRIC BARRIER DISCHARGES * Rajesh Dorai and Mark J. Kushner Deparment of Electrical and Computer Engineering Urbana, IL Khaled Hassouni LIMHP, CNRS-UPR1311, Universite Paris Nord, Villetaneuse, France. dorai@uiuc.edu mjk@uiuc.edu hassouni@limhp.univ-paris13.fr * Work supported by Ford Motor Company and NSF (CTS )

2 INTRODUCTION Nitrogen oxides (, 2 ) - X, are one of the six major pollutants identified by the EPA, others being CO, Pb, SO X, volatile matter and particulates. All emissions have decreased except for x (EPA, 1998). Harmful effects of X Acid deposition Formation of ozone Eutrophication of water bodies Industrial/ Commercial/ Residential 19% All Other Sources 5% Motor Vehicles 49% Inhalable fine particles Visibility degradation Utilities 27% Major sources of x (EPA, 1998) AICHE-01-03

3 PLASMA REMEDIATION OF X USING DBDs Dielectric barrier discharges (DBDs) are well suited for generation of gasphase radicals at atmospheric pressures. Electron impact processes in DBDs produce radicals and ions which initiate the plasma chemistry. e + N 2 N + N + e e + O 2 O + O + e Exhaust with x & soot DBD Clean Efflluent e + H 2 O OH + H + e Reduction + N N 2 + O Oxidation + O OH H 3 AICHE-01-04

4 DESCRIPTION OF GLOBAL-KIN GLOBAL-KIN is a spatially homogeneous plasma chemistry simulation coupled with circuit and surface reaction modules. The model uses a lookup table generated by an offline Boltzmann solver to obtain the e-impact reaction rate coefficients. Offline Boltzmann Solver Lookup Table of k vs. Te Circuit Module Gas-Phase Kinetics Surface Kinetics N(t+ t),v,i dsoot LSODE ODE Solver AICHE-01-05

5 OPERATING CONDITIONS Typical diesel exhausts contain N 2, O 2 (excess air); H 2 O, CO 2 (products) and trace amounts of, CO, H 2 and unburned hydrocarbons (UHCs). To simulate actual exhausts, we have used propane (C 3 H 8 ) and propene (C 3 H 6 ) as representative of the UHCs. Inlet gas composition N 2 /O 2 /H 2 O/CO 2 =78/8/6/7 =260 ppm, CO=400 ppm, H 2 =133 ppm C 3 H 6 =500 ppm, C 3 H 8 =175 ppm T=180 o C, P=1atm τ = residence time of exhaust in DBD = 0.2 s AICHE-01-06

6 REACTION MECHANISM: -C 3 H 6 C 3 H 6 reactions are initiated by O and OH. Peroxy radicals formed from OH-initiated reactions with propene, oxidize to 2. x is also converted to other organic nitrates and nitrites, but most of the initial x () is primarily oxidized to 2. C2H5 + HCO C2H5CHO Methyl Oxirane CH2CHO + CH3 CH3CHCH2OH O O O O OH C3H6 OH CH3CH(OH)CH2 CH3CH(OO)CH2OH 2 CH3CH(O)CH2OH Decomposition 2 CH 3CH(OH)CH2OO HCHO + CH2OH + CH3CH(OH)CH2O CH3CHOH + Decomposition CH3CHO H + CH3CHO HCHO H AICHE-01-07

7 REACTION MECHANISM: -C 3 H 8 The initiating reaction with propane is an abstraction by OH and the resulting radicals then consume O 2 to form the peroxy radicals. These peroxy radicals then react with to convert it to 2. (CH3)2CH (CH3)2CH(O)2 (CH3)2CHO C3H8 OH OH i-c3h7 n-c3h7 2 2 CH3C(O)CH3 [ Acetone ] [ Propionaldehyde ] CH3CH2CHO C3H7O nc3h7 C3H7 AICHE

8 SOOT PARTICLES EFFECT ON X REMEDIATION Soot particles found in diesel exhausts are typically 100 nm and containing C/H/O=89/1/10. The radicals produced in the plasma diffuse to the soot surface and react. O + Su O adsorbed CO OH + Su OH adsorbed CO + H Su 2 (adsorbed) + CO Exhaust with x & soot Soot DBD Clean Efflluent CO OH CO,H2 O Soot surface CO, AICHE-01-08

9 SOOT OXIDATION MODEL Region surrounding soot is divided into two zones. Diffusion regime Homogeneous Bulk Plasma Diffusion Regime Homogeneous Bulk Plasma Species that react on the soot surface diffuse through the boundary layer. dsp Individual spherules SOOT ns-s ns-g ds Boundary layer thickness, δ, is obtained from the Reynolds number. For low Re, δ d s /2. CO OH, O, 2 δ Boundary layer thickness The diffusing species have a linear profile in the diffusion regime. AICHE-01-09

10 PLASMA CONDITIONS : n e, T e, [N], [OH], [O] Peak n e cm -3 and T e 3 ev with E dep 38 J/L. Electron impact dissociation of N 2, O 2 and H 2 O produce N, O and OH respectively. ne ( cm -3 ) Te ne Te ( ev ) O, OH, N ( cm -3 ) N OH O Time ( 10-7 s ) Time ( s ) AICHE-01-10

11 X REMEDIATION : SINGLE PULSE With a single pulse, exit densities are high because of the depletion of O 3 and peroxy radicals by the time of desorption of 2(ads). +O 3 (or peroxies) 2 2(ads), 2 (cm -3 ) (ads) Time (s) (adsorbed) (cm -2 ) Density (10 14 cm -3 ) Peroxy radicals O Time (s) AICHE-01-11

12 X REMEDIATION : MULTIPLE PULSE With multiple pulses, is converted to 2 by O 3 and peroxy radicals produced during each pulse. +O O 2 The rate of adsorption of 2 being higher than the rate of desorption, the x remains adsorbed on the surface of soot., 2 (10 15 cm -3 ) (adsorbed) Time (s) 2 (adsorbed) (cm -2 ) Density (10 14 cm -3 ) O Peroxy radicals Time (s) AICHE-01-12

13 EFFECT OF ENERGY DEPOSITION For a single pulse, exit densities are higher. Most of the adsorbed 2 desorbs back as. This does not happen in the case of multiple pulses because of the shorter interpulse time for 2 desorption. With a single pulse, the peroxy radicals which consume are lost by the time is regenerated from 2 and so, exit x is mainly. Density (ppm) SP - Single pulse MP - Multiple pulse - MP - SP 2 - SP 2 - MP Energy Deposition (J/L) 2 (adsorbed) (cm -2 ) Multiple pulse Single pulse Energy Deposition (J/L) AICHE-01-13

14 EFFECT OF ENERGY DEPOSITION: H 2 SP vs. MP H 2 is produced by the reaction of with OH and by HO 2 with 2. + OH + M H 2 + M (1) 2 + HO 2 H 2 + O 2 (2) With a single pulse, most of the HO 2 is consumed by the time 2 is formed and hence H 2 is mainly produced by reaction (1). H2 (ppm) MP SP 2 densities are higher with multiple pulses. HO 2 radicals are produced during each pulse. Hence, reaction (2) also contributes to H 2 production Energy Deposition (J/L) AICHE-01-13_5

15 SOOT OXIDATION With increasing energy deposition, the diameter of the soot decreases due to the oxidation by CO At higher energies, the final diameter of soot increases because the density of 2 decreases due to the increased conversion to products such as H 2, CH 3 O HO 2 H 2 + O CH 3 O + M CH 3 O 2 + M 2 (ppm) Diameter Energy Deposition (J/L) Diameter of soot (nm) Note that the oxidation of soot is partial and results in CO and not CO 2. CO poisonous CO 2 greenhouse gas AICHE-01-14

16 MODIFICATION OF PLASMA CHEMISTRY BY SOOT Soot affects the overall plasma chemistry by affecting the densities of OH, HO 2, and 2. H2 OH CO β HAP radicals β HA radicals OH C3H6 OH 2(ads) HCHO + H CH3CHO 2 SOOT β H alkoxy radicals AICHE-01-14_5 Decomposition and other reactions β-hap - β-hydroxy alkyl peroxy β-ha - β-hydroxy alkyl β-h - β-hydroxy

17 CONCLUDING REMARKS Plasma remediation of x, by itself is not sufficient to completely remove x. Soot chemistry significantly affects the x composition in plasma remediation of x. Soot can be oxidized by plasma and as high as 30% soot removal can be achieved at 60 J/L. Multiple pulse input results in apparent x removal because of the increased adsorption onto the soot surface. With single pulse energy deposition, the exit- x is primarily because of the reconversion of 2 to on soot surface. AICHE-01-15

EFFECTS OF PROPENE ON THE REMEDIATION OF NOx FROM DIESEL EXHAUSTS*

EFFECTS OF PROPENE ON THE REMEDIATION OF NOx FROM DIESEL EXHAUSTS* Rajesh Dorai and Mark J. Kushner University of Illinois Department of Electrical and Computer Engineering Urbana, IL 6181 Email : Mark