soot abatement using fuel additives

265 Pages·2004·2.44 MB·English

The Pennsylvania State University

The Graduate School

College of Engineering

SOOT ABATEMENT USING FUEL ADDITIVES

A Thesis in

Mechanical Engineering

by

Juntao Wu

© 2004 Juntao Wu

Submitted in Partial Fulfillment
of the Requirements
for the Degree of

Doctor of Philosophy

December 2004
The thesis of Juntao Wu has been reviewed and approved* by the following:

Thomas A. Litzinger
Professor of Mechanical Engineering
Thesis Advisor
Chair of Committee

Robert J. Santoro
George L. Guillet Professor of Mechanical Engineering

Richard A. Yetter
Professor of Mechanical Engineering

Harold H. Schobert
Professor of Fuel Science

Richard C. Benson
Professor of Mechanical Engineering
Head of Department of Mechanical and Nuclear Engineering

*Signatures are on file in the Graduate School
iii
ABSTRACT

A study of the mechanism of soot abatement by oxygen-containing additives was
conducted by both experimental and modeling methods. The overall technical objective is
to develop a fundamental understanding of the complex roles of oxygen-containing
additives in the processes which lead to particulate matter emissions. Two classes of
compounds were investigated - oxygenates and nitro-alkanes.
Experiments were performed on a premixed ethylene/air flame with two
oxygenated additives - ethanol and dimethyl ether (DME). The experiments were
conducted at two equivalence ratios, φ = 2.34 and φ = 2.64, and two levels of ethanol or
DME, 5% and 10% oxygen in mass of the fuel, were added to the fuel at each
equivalence ratio. The experimental results show that both ethanol and DME reduced
aromatic species and soot, and they were more effective at φ = 2.34 than at φ = 2.64. The
comparison between ethanol and DME shows that, at Ф = 2.34, DME is more effective
than ethanol on PAH and soot reduction; however, at φ = 2.64, there is no detectable
difference between these two additives on the aromatic species and soot suppression.
A chemical model, Howard-DME-Ethanol (HDE) mechanism, was used to
investigate the chemical processes leading to the abatement of aromatic species and soot
by ethanol and DME. The analysis shows that these reduction effects result from the
removal of carbon from the pathway to aromatic species formation. DME is more
iv
effective than ethanol in reducing soot precursors, because more carbon in DME is
removed from the participation of aromatic formation than that in ethanol. This
difference is due to the molecular structure difference between ethanol and DME.
Screening studies of the nitro-alkanes showed them to be effective in reducing
soot. The primary mechanism of soot reduction was hypothesized to be linked to NO , so
2
a modeling study of NO addition to premixed flames was undertaken.
2
A chemical model, Howard-NO (HN) mechanism, was used to investigate the
2
chemical processes leading to the effect of NO on aromatic species and soot reduction.
2
The mechanism analysis shows that the addition of NO increases the level of OH
2
radicals in the flame, through reactions of NO +H⇒NO+OH, HO +NO⇒NO +OH.
2 2 2
Then the increased OH decreases the level of H by reactions H +OH⇒H O+H. The
2 2 2
lower level of H increases the reaction rate of H CCCH+H⇒C H +H , and results a
2 2 3 2 2
lower level of H CCCH. Since all C H comes from H CCCH through reaction
2 6 6 2
2H CCCH⇒C H , low level H CCCH leads to a reduction of C H . As a key element
2 6 6 2 6 6
during the aromatic species growth, low level of C H leads to suppression of overall
6 6
aromatic species formation and growth.
v
TABLE OF CONTENTS

ABSTRACT.......................................................................................................................iii
TABLE OF CONTENTS....................................................................................................v
LIST OF FIGURES.........................................................................................................viii
LIST OF TABLES...........................................................................................................xiii
NOMENCLATURE........................................................................................................xiv
ACKNOWLEDGEMENTS.............................................................................................xvi

Chapter 1
INTRODUCTION..............................................................................................................1
1.1 Motivation...........................................................................................................1
1.2 Objectives...........................................................................................................5
1.3 Soot Formation....................................................................................................8
1.3.1 Soot Particle Precursor and Particle Nucleation.........................................8
1.3.2 Particle Coagulation..................................................................................12
1.3.3 Soot Growth..............................................................................................14
1.3.4 Soot Oxidation..........................................................................................20
1.4 General Soot Physical Properties......................................................................21
1.5 Soot Reduction by Additives............................................................................24
1.5.1 Inert Diluents............................................................................................25
1.5.2 Gaseous Additives....................................................................................26
1.5.3 Metallic Additives.....................................................................................28
1.5.4 Oxygenated Additives...............................................................................30

Chapter 2
PRINCIPLES OF DIAGNOSTIC METHODS................................................................36
2.1 Laser-induced Incandescence...........................................................................37
2.1.1 Theoretical Analysis.................................................................................38
2.1.2 Experimental Considerations....................................................................41
2.1.2.1 Laser Excitation: Intensity Profile, Energy, and Wavelength...............43
2.1.2.2 Spectral Detection Region....................................................................46
2.1.2.3 Detection Gate Width and Timing........................................................47
2.1.2.4 Calibration.............................................................................................49
2.1.2.4.1 Calibration via Laser Extinction.....................................................49
2.1.2.4.2 Calibration via Cavity Ringdown...................................................50
vi
2.1.2.4.3 Calibration via Gravimetric Techniques.........................................51
2.1.2.4.4 Calibration via 3-Color Pyrometry.................................................52
2.1.2.4.5 General Concerns in Calibrating LII...............................................53
2.2 Laser Extinction................................................................................................54
2.3 Laser-induced Fluorescence..............................................................................59
2.3.1 Theory of Laser-induced Fluorescence.....................................................60
2.3.2 LIF Calibration..........................................................................................65
2.4 Temperature Measurement...............................................................................66

Chapter 3
EXPERIMENTAL APPROACH......................................................................................69
3.1 Materials...........................................................................................................69
3.2 Experimental Approach....................................................................................71

Chapter 4
EXPERIMENTAL RESULTS AND MODELING ANALYSIS FOR ETHANOL AND
DME..................................................................................................................................83
4.1 Experimental Results........................................................................................84
4.1.1 Temperature Profiles.................................................................................84
4.1.2 Spectra Scanning and PAH Concentration Profiles..................................91
4.1.3 Soot Volume Fraction Profiles..................................................................95
4.1.4 Effect of Additives on PAH and Soot Volume Fraction Profiles.............97
4.2 Modeling Analysis..........................................................................................106
4.2.1 DME Consumption.................................................................................109
4.2.2 Ethanol Consumption..............................................................................114
4.2.3 Effect of Molecular Structure of Additives............................................120
4.2.4 Effect of Temperature.............................................................................121

Chapter 5
MODELING STUDY OF NO ON SOOT REDUCTION............................................123
2
5.1 Modeling Conditions......................................................................................123
5.2 Chemical Mechanisms....................................................................................124
5.3 Validation of Combined Mechanisms............................................................125
5.4 Reaction Pathways..........................................................................................133
5.4.1 Fuel Destruction to Benzene...................................................................134
5.4.2.1 Ethylene Consumption........................................................................134
5.4.2.2 Benzene Formation.............................................................................137
5.4.2 PAH Growth...........................................................................................141
5.4.2.1 Naphthalene (C H ) Formation.........................................................141
10 8
5.4.2.2 Phenanthrene (A3) Formation.............................................................144
vii
5.4.2.3 Pyrene Formation................................................................................148
5.5 Mechanisms of Soot Reduction......................................................................153
5.5.1 Overview.................................................................................................153
5.5.2 Chemical Effect of NO ..........................................................................153
2
5.5.2.1 NO Reactions.....................................................................................153
2
5.5.2.2 Effect of NO on Benzene Formation.................................................160
2
5.5.2.3 Effect of NO on PAH Growth...........................................................174
2
5.5.3 Effect of Temperature.............................................................................183
5.5.3.1 Temperature Effect on Benzene Formation........................................187
5.5.3.2 Temperature Effect on C H Formation.............................................198
8 6

Chapter 6
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK................................210
6.1 Conclusions...........................................................................................................210
6.2 Suggestions for Future Work................................................................................212

REFERENCES...............................................................................................................214

APPENDIX Reactions for HN mechanism...................................................................229
viii
LIST OF FIGURES
Figure 1-1 Jet-Propulsion Fuel 8. .....................................................................................3
Figure 1-2 Laboratory flames and reactors with associated chemical and physical
processes. ...................................................................................................................6
Figure 1-3 TEM photograph of typical soot aggregates emitted form long residence time
turbulent acetylene/air diffusion flames where emitted soot properties were
independent of residence time.16...............................................................................13
Figure 1-4 Total surface area of soot particles per cm3 = π <d2>N, as a function of age
for flames of 3 stoichiometries:◇, C/O=0.79; □,0.76; △, 0.7. ............................15
Figure 1-5 Upper portion: volume fraction of soot as a function of time for flames with
six different stoichiometries, as shown. The approximately 15 data points for each
curve are omitted for clearity. Lower portion: specific surface growth rates as a
function of time. .......................................................................................................15
Figure 1-6 Effect of the relative reactivity of soot and ΣPAH. C/O=0.79. (□) 10mm,(Δ)
17mm, (●) 25mm. ..................................................................................................19
Figure 1-7 Collision efficiencies at different distances from burner. γ (×104) :
acetylene-soot
(□) C/O = 0.70, (○) C/O = 0.79; γ : (■)C/O = 0.70,(●) C/O = 0.79. .....19
PAH-soot
Figure 1-8 Sketch of the structure of soot emitted from a diesel engine. .......................23
Figure 1-9 Influence of various gaseous additives on the critical C/O-ratio for the onset
of sooting in an ethylene/air flame. ..........................................................................26
Figure 1-10 Influence of gaseous additives on the downstream ( t ≈ 30 ms ) soot yield
in an ethylene/air flame. C/O ~ 0.76, fv~ 1.5 x 10-7 ...............................................26
Figure 1-11 Smoke emissions from diesel engines with oxygenated additives, from
Miyamoto et al. ........................................................................................................31
Figure 1-12 Acetylene concentration evolution of fuels with about same centane number.
In the figure, intervals of confidence of 80% are indicated. Test condition: engine
speed = 1250 rpm, 4.5 bar of i.m.e.p., start of combustion = TDC. ........................33

Figure 2-1 Optical setup for 2-D LII measurements. LDF – laminar diffusion flame, BB
– beam block, DM – dichroic mirror, ½-λ - half-wave plate, GL – Glan-Laser prism,
A – aperture, P – prism, CL – cylindrical lens, SL – spherical lens, and ICCD –
intensified CCD. ......................................................................................................42
Figure 2-2 Fluence dependence of LII measured in steady laminar diffusion flames. Data
were collected at H = 20 mm in the ethylenec/air flame for detection gate durations
of 19 ns (+) and 85 ns (◊) – both gates opening coincident with the arrival of the ≈
5 ns laser pulse. Data are also shown for the methane/air flame at H = 50 mm with
85 ns gate (●). Raw signals for each condition have been normalized to a value of
1.0 at a fluence of 0.6 J/cm2. The solid line shown is the least-squares power-law fit
of the methane data for fluences greater than 0.03 J/cm2; the fit follows the
expression signal ∝ fluence 0.34. reference [89].......................................................44
ix
Figure 2-3 Temporal profile of a LII signal obtained in the ethene-air laminar diffusion
flame at heights of 10 and 30 mm above the fuel tube exit and at the radial locations
corresponding to peak soot volume fraction for these heights.................................48
Figure 2-4 Extinction by a collection of particles. ..........................................................54
Figure 2-5 Schematic diagram of two-level model of induced fluorescence..................61

Figure 3-1 Chemical structure of ethanol and DME........................................................70
Figure 3-2 Schematic diagram of experimental setup.....................................................72
Figure 3-3 Schematic diagram of fuel, additive, and air supply systems........................72
Figure 3-4 The size of PAH and their spectral properties. .............................................76
Figure 3-5 Diagram of the Thermocouple Arrangement.................................................80

Figure 4-1 Temperature profiles of premixed ethylene/air flame, with and without
ethanol and DME addition........................................................................................85
Figure 4-2 Flame temperature profiles from Intal and Senkan.144...................................88
Figure 4-3 Pictures of the ethylene/air flames with and without additives. (Digital
Camera Setting: F=2.8, Time=1/800 s)....................................................................90
Figure 4-4 Fluorescence spectra. (a) φ = 2.34; (b) φ = 2.64............................................91
Figure 4-5 Fluorescence spectra of laminar premixed ethylene/air flat flame at different
heights above the burner surface.145..........................................................................92
Figure 4-6 Normalized fluorescence intensity profiles of small PAH.............................94
Figure 4-7 Normalized fluorescence intensity profiles of large PAH.............................94
Figure 4-8 Comparison of soot volume fraction measurement results............................95
Figure 4-9 The effect of ethanol on small PAH profiles.................................................98
Figure 4-10 The effect of ethanol on large PAH profiles................................................98
Figure 4-11 The effect of ethanol on soot profiles..........................................................99
Figure 4-12 Comparison of ethanol effects on soot volume fraction profiles at Ф=2.34.
.................................................................................................................................101
Figure 4-13 The effect of DME on small PAH profiles................................................102
Figure 4-14 The effect of DME on large PAH profiles.................................................102
Figure 4-15 The effect of DME on soot profiles...........................................................103
Figure 4-16 Comparison of the effects of ethanol and DME on small PAH profiles....104
Figure 4-17 Comparison of the effects of ethanol and DME on large PAH profiles....105
Figure 4-18 Comparison of the effects of ethanol and DME on soot profiles...............105
Figure 4-19 Comparison of the effects of ethanol and DME on predicted small PAH
profiles....................................................................................................................107
Figure 4-20 Comparison of the effects of ethanol and DME on predicted large PAH
profiles....................................................................................................................108
Figure 4-21 Comparison of the effects of ethanol and DME on predicted soot profiles108
Figure 4-22 Reaction flux diagram of DME consumption............................................109
Figure 4-23 Reaction flux diagram of CH consumption with 5% oxygen in the fuel by
3
the addition of DME at Ф=2.34..............................................................................111
x
Figure 4-24 Reaction flux diagram of CH consumption with 5% oxygen in the fuel by
3
the addition of DME at Ф=2.64..............................................................................113
Figure 4-25 Reaction flux diagram of ethanol consumption with 5% oxygen in the fuel
by the addition of ethanol at Ф=2.34......................................................................115
Figure 4-26 Reaction flux diagram of CH consumption with 5% oxygen in the fuel by
3
the addition of ethanol at Ф=2.34...........................................................................117
Figure 4-27 Reaction flux diagram of ethanol consumption with 5% oxygen in the fuel
by the addition of ethanol at Ф=2.64......................................................................118
Figure 4-28 Reaction flux diagram of CH consumption with 5% oxygen in the fuel by
3
the addition of ethanol at Ф=2.64...........................................................................119
Figure 4-29 Mole concentration profiles of pyrene calculated using original and adjusted
temperature profiles................................................................................................122

Figure 5-1 Comparison between Howard and HN mechanisms at Φ =2.34..................126
Figure 5-2 Comparison between Howard and HN mechanisms at Φ =2.64..................126
Figure 5-3 Small PAH profiles......................................................................................129
Figure 5-4 Large PAH profiles......................................................................................129
Figure 5-5 Soot volume fraction with and without NO at two equivalence ratios.......132
2
Figure 5-6 Reaction flux diagram of C H consumption at Ф=2.34.............................135
2 4
Figure 5-7 Reaction flux diagram of C H consumption at Ф=2.64.............................136
2 4
Figure 5-8 Reaction flux diagram of benzene production and destruction at Ф=2.34..138
Figure 5-9 Net production rate of benzene at Ф = 2.34.................................................139
Figure 5-10 Net production rate of benzene at Ф = 2.64...............................................140
Figure 5-11 Reaction flux diagram of C H production and destruction at Ф=2.34....142
10 8
Figure 5-12 Reaction flux diagram of C H production and destruction at Ф=2.64....142
10 8
Figure 5-13 Reaction flux diagram of A3 production and destruction at Ф=2.34.........145
Figure 5-14 Reaction flux diagram of A3 production and destruction at Ф=2.64.........146
Figure 5-15 Reaction flux diagram of pyrene (C H ) production and destruction at
16 10
Ф=2.34....................................................................................................................149
Figure 5-16 Reaction flux diagram of pyrene (C H ) production and destruction at
16 10
Ф=2.64....................................................................................................................150
Figure 5-17 Carbon flux among PAHs at Ф=2.34.........................................................152
Figure 5-18 Carbon flux among PAHs at Ф=2.64.........................................................152
Figure 5-19 Flux diagram of nitrogen atoms introduced by NO at Ф=2.34.................154
2
Figure 5-20 Flux diagram of NO consumption at Ф=2.34...........................................156
2
Figure 5-21 Flux diagram of NO consumption at Ф=2.64...........................................157
2
Figure 5-22 Mole concentration profiles of OH at Ф=2.34...........................................159
Figure 5-23 Mole concentration profiles of OH at Ф=2.64...........................................159
Figure 5-24 Mole concentration profiles of benzene.....................................................160
Figure 5-25 Net production rate of C H at Ф=2.34......................................................161
6 6
Figure 5-26 Net production rate of H CCCH at Ф=2.34...............................................162
2
Figure 5-27 Net production of H CCCH at Ф=2.34......................................................164
2
Figure 5-28 Total production and consumption of H CCCH at Ф=2.34......................165
2