EFFECTS OF PROPENE ON THE REMEDIATION OF NOxFROM DIESEL EXHAUSTS*
SAE_99-Title
Rajesh Dorai and Mark J. KushnerUniversity of Illinois
Department of Electrical and Computer EngineeringUrbana, IL 61801
Email : Mark J. Kushner [email protected] Rajesh Dorai - [email protected]
October 28 1999
* Work supported by Ford Motor Company
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99-Agenda
AGENDA
• Introduction
• Description of the model
• Reaction kinetics in the propene system
• Simulation results of the propene system
• Interesting implications of rate-coefficient data
• Use of multiple pulse inputs and impact on overall reaction chemistry
• Conclusions
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#1
l Nitrogen Oxides, NOx ( NO and NO2 )
l Hazard to human health
l Precursors to urban ozone
l Derivatives of NOx are greenhouse gases.
l Acid rain
l Dielectric Barrier Dishcharge Reactors
l Best suited for the generation of gas-phase radicals with plasmas
l Effective for the operation of the reactor at atmospheric pressure
INTRODUCTION
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
l The basis of the model is to integrate the non-linear ordinary differentialequations describing the reaction chemistry over the residence time with the simultaneous solution of the equations for the circuit parameters.
l The rate coefficients for the electron impact reactions are obtained from a lookup table produced by an offline Boltzmann solver.
DESCRIPTION OF THE MODEL
Offline Boltzmann Solver
Lookup Table of k vs. Te
dN(t) dt
CircuitModule
LSODEODE Solver
N(t+∆t), V, I
SAE_99#2
l A dielectric barrier discharge reactor has been modelled with the followingcharacteristics.
l Dielectric discharge height = 2.5 mml Reactor pressure = 1 atml Single pulse input
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#3
THE EXPERIMENTAL SETUP MODELLED
V
C
Dielectric
L
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#4
l Basic composition of the gas at reactor entrance :
CO2 = 7% O2 = 8% H2O = 6%
CO = 400 ppm NO = 260 ppm H2 = 133 ppm
C3H6 = 0 -1100 ppm
N2 = Balance
l Temperature = 453 K
l Pressure = 1 atm
l Applied voltage = 14 - 25 kV
REACTOR OPERATING CONDITIONS
l The basic electron impact reactions are :
l e + O2 O + O + e
l e + N2 N + N + e
l e + H2O OH + H + e
l OH radicals are also generated by the reactions of O(1D) with water
l O(1D) + H2O OH + OH
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#5
BASIC ELECTRON IMPACT REACTIONS TRIGGERINGTHE PLASMA PROCESSING
TEMPORAL EVOLUTION OF SPECIES
__________________University of Illinois
Optical and Discharge Physicssae_extra
• Electron, O atom and OH radical densities peak around 100s of nano seconds
Time (secs)
Ele
ctro
n D
ensi
ty [
cm -3
]
Time (secs)C
once
ntra
tion
of O
, OH
and
HO
2 [ c
m -3
]
O
HO2
OH
Electron density profile Time evolution of OH, O, HO2
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#6
l The presence of propene affects the reaction mechanism significantly.l One of the major sources for the NO consumption is the peroxy radical
produced by the reaction of O2 with the products of the OH-propene
reaction.
PROPENE AS A CONSTITUENT IN THE INLET GAS
C3H6
CH3CHCH2OH
CH3CH(OH)CH2
OH
OH
O2
O2
CH3CH(OO)CH2OH
CH3CH(OH)CH2OO
O Methyl Oxirane
NO
NONO2
NO2
CH3CH(OH)CH2O
CH3CH(O)CH2OH
CH3CHO
HCHO
l l
l
lllO2
l
l l
C2H5CHO
CH2CHO
+ CH3
C2H5 + HCO
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
SAE_99#7
l In the absence of propene, increasing input energy results in an increasedrate of NO reduction, but a much lesser rate of NOx removal.
l The same trend continues with propene included in the system, except that the NOx removal increases quite a bit.
EFFECTS OF INPUT ENERGY ON PROPENE-NOx SYSTEM
0
50
100
150
200
250
0
5
10
15
0 10 20 30 40 50 60Energy Deposition [ J/L ]
NO
NO2
NOx
HNO2
HNO30
50
100
150
200
250
0
100
200
300
400
500
0 10 20 30 40 50 60Energy Deposition [ J/L ]
NONO2
NOx
Propene
NOx processing in the absence
of propene
NOx processing in the presence
of propene
v
v
v Experiments by Hoard et al. Similar conditions except for addition of propane.
l Higher energy inputs result in increased production of radicals
which help in accelerating thereactions in the reaction network
l Increasing energy in general, results in increased production ofend products, though at a highercost of energy efficiency.
l C3H6 + O Methyl Oxirane
C3H6 + O CH2CHO + CH3
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#8
END PRODUCTS : EFFECTS OF INPUT ENERGY
0
20
40
60
80
100
120
0 10 20 30 40 50 60Energy Deposition [ J/L ]
HCHO
Moxirane
CH3CHO
ONCH2CHO
NO
ONCH2CHO
v
v Experiments by Hoard et al. Similar conditions except for addition of propane.
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#9
l Increasing amounts of the hydrocarbon in the inlet gas results in anincreased conversion of NO to NO2 and hence, the NOx removal doesn’t
get affected.
l More amount of inlet hydrocarbon effectively reduces the concentrationof the radicals available to other species in the reactor. This is evidentfrom the decrease in the concentration of HNOx with propene.
EFFECT OF PROPENE CONCENTRATION
0
50
100
150
200
250
200 400 600 800 1000 1200C3H6 [ ppm ]
0
5
10
15
NO
NOx
NO2
HNO2
HNO3
0
20
40
60
80
100
0
10
20
30
40
50
60
200 400 600 800 1000 1200C3H6 [ ppm ]
CONO
NOx
Propene
l There is also a decrease in the propene conversion because the radicalssuch as OH, O are present in limiting quantities.
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#10
l As is expected, morehydrocarbon results in increased production of carbon-end-products.
l High quantities of formaldehydeand acetaldehyde are alsoproduced.
EFFECT OF INLET PROPENE ON END PRODUCTS
0
20
40
60
80
100
120
200 400 600 800 1000 1200
C3H6 [ ppm ]
HCHO
Moxirane
CH3CHO
ONCH2CHO
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#11
l Increasing the gas temperature seems to affect the NOx concentration
minimally.
l There is however an increase in the exit concentration of CO and otherpropene-initiated reaction products with reactor gas temperature.
EFFECT OF GAS TEMPERATURE ON NOx CONVERSION
0
50
100
150
200
250
0
5
10
15
20
300 400 500 600 700Temperature [ K ]
NO
NOx
NO2
HNO2
HNO3
0
20
40
60
80
100
0
40
80
120
160
300 400 500 600 700Temperature [ K ]
CO
NO
NOx
Propene
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
SAE_99#12
l The initial increases in temperaturesresult in increase in end product concentrations.
l The maxima seen in the graph aremainly due to the highly non-linearnature of the inherent reaction ratecoefficients.
k = A (T/300)n exp(-Ea / T )
l The change in temperature alsoaffects the E/N in the plasmathereby causing a change in the rate coefficient values for electron impactreactions.
EFFECT OF TEMPERATURE ON END PRODUCTS
0
20
40
60
80
100
HCHO
Moxirane
CH3CHO
ONCH2CHO
300 400 500 600 700
Temperature [ K ]
l Since reactions of CH2CHO with O2 are so significant, the products need to be
determined
Products 2.6e-13 J.P.R.C.D., 1991, 21411-429.
SAE_99#13
l Rate coefficients for the reaction of O with C3H6
THE ONCH2CHO CONTROVERSY (?) !!!
Reaction Rate Coefficient Source
( cm3/molecule/s) at 298 K
O + C3H6
Products 4.79e-12 J.P.C.R.D., 1991, 20 221-224
Methyl Oxirane 4.81e-12 Gaedtke et.al., Symp. (Int.) Combust., [Proc.]_1973,14,295
CH2CHO + CH3 0.3 x (4.79e-12) Knyazev et.al.,
Int. J. Chem. Kinet., 1992 O2 24,545-561.
HCHO + CO + OH 3.0e-14 -- Do --
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
l There are disparities in the reaction rate coefficients reported in theliterature.
l The reaction of concern is : CH3ONO HCHO + HNO
SAE_99#14
CH3ONO - WHY WAS THIS NOT PREDICTED EARLIER(?)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
Source Rate Coefficient Order Reaction Rate
( cm3/molecule/s ) ( molecule/cm3/s)
Ohmori et.al 7.70e-12 2 = 7.70e-12 x [CH3ONO] x [M]
Bull. Chem. Soc. Jpn.,1993, 66, 51-56.
Batt et. al. 2.067e-08 1 = 2.07e-8 x [CH3ONO]
Int. J. Chem. Kinet.,1975,7,441.
1
2
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#15
l It has been observed experimentally that methyl nitrate (CH3ONO2) is
produced in the plasma processing of the inlet gas with the compositionthat we have used. But, with the single pulse input, less than 1 ppm of itis produced at the maximum energy input.
Why is this difference ?
l The reason for this is the difference in way the plasma is being pulsed. It isimportant to note that the life-zones of the two species, CH3O and NO2
rarely overlap and even if they do, it is for a very short time period. Thusin a single pulse input, one does not see the formation of methyl nitrate.
l However, in the case of multiple pulse inputs, enough NO2 gets formed
for reaction with CH3O to produce methyl nitrate.
MULTIPLE PULSES : EFFECT ON THE REACTION CHEMISTRY
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICSSAE_99#16
l As more energy is deposited in the system, one sees more NOx conversion.
l With the use of multiple pulsing, compounds such as methyl nitrate(CH3ONO2) are produced in significant amounts.
MULTIPLE PULSES : PRELIMINARY RESULTS
0
5
10
15
20
25
30
35
0
10
20
30
40
50
60
70
80
0 5 10 15
Energy Deposition
HNO2
HNO3
CH3ONO
2
0
50
100
150
200
250
0 5 10 15
NO
NOx
NO2
Pulse Number Pulse Number
SAE_99#17
l Important end-products of the HC-NOx include aldehydes, ketones,
oxiranes, CO and NO2.
l Increasing the energy input to the reactor improves the NOx conversion,
but at a high cost because the energy efficiency decreases withincreasing energy deposition.
l Hydrocarbons play a significant role in the plasma processsing of NOx and
hence should not be neglected in any analysis of NOx treatment
processes.
l The inclusion of multiple pulses affects the overall reaction chemistry and care must be taken to include all possible reactions that could possibly
occur under such circumstances.
CONCLUSIONS
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
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