Using an Opposed Flow Diffusion Flame to Study theOxidation of C 4 Fatty Acid Methyl Esters

Using an Opposed Flow Diffusion Flame to Study the

Oxidation of C4 Fatty Acid Methyl Esters

by

Subram Maniam Sarathy

A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science

Graduate Department of Chemical Engineering and Applied ChemistryUniversity of Toronto

Copyright c© 2006 by Subram Maniam Sarathy

Abstract

Using an Opposed Flow Diffusion Flame to Study the Oxidation of C4 Fatty Acid

Methyl Esters

Subram Maniam Sarathy

Master of Applied Science

Graduate Department of Chemical Engineering and Applied Chemistry

University of Toronto

2006

The oxidation of saturated (i.e methyl butanoate) and unsaturated (i.e. methyl cro-

tonate) C4 fatty acid methyl esters in an opposed flow diffusion flame has been studied

to better understand the role of molecular structure in biodiesel combustion. The results

indicate that the methyl crotonate flame produces higher levels of unsaturated hydrocar-

bons, which are known soot precursors in combustion applications. In addition, higher

levels of acrolein, acetone, methanol, and benzene are observed in the methyl crotonate

flame. The double bond in methyl crotonate is identified as the reason for the observed

differences in species formation.

The experiments are also used to validate an improved detailed chemical kinetic model

for methyl butanoate. The model exhibits good agreement with the experimental data.

The major reaction pathways for methyl butanoate oxidation in the opposed flow diffusion

flame are presented herein.

ii

Sabbo pajjalito loko, sabbo loko pakampito

The entire universe is nothing but combustion and vibration

-Buddha-

Dedication

To all beings.

May you be happy and peaceful.

May you enjoy good health and harmony.

Acknowledgements

Deepest thanks to Professor Murray Thomson for his guidance and support in my aca-

demic endeavors.

Je remercie Monsieur Sandro Gaıl, Docteur en cinetique chimique et combustion, pour

avoir partage ses connaissances en chimie de la combustion et sur les methodes analy-

tiques, et en plus, pour avoir attise mon interet en francais.

Warm thanks to Sajid Syed, my predecessor, who provided the experimental founda-

tion and training I needed to be successful.

My appreciation towards Professor Boocock and Professor Wallace for being members

on my MASc defense committee.

Thanks to AUTO21 and NSERC for providing research funding.

My blessings to my twin brother, my wife, and all my family and friends. Words cannot

express my gratitude towards you, so here is a loving smile. :)

iii

Contents

1 Introduction 1

1.1 Research Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Research Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Research Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Background Research 4

2.1 Basics of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Chemistry of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.1 Feedstocks for Biodiesel . . . . . . . . . . . . . . . . . . . . . . . 6

Surrogates of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.2 Production of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Properties of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4 Biodiesel Use in Diesel Engines . . . . . . . . . . . . . . . . . . . . . . . 11

2.4.1 NOx Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4.2 Biodiesel as a Lubricity Additive . . . . . . . . . . . . . . . . . . 14

2.4.3 Oxygenated Fuels as Soot-Reducing Additives . . . . . . . . . . . 14

2.5 Modeling the Oxidation of Biodiesel . . . . . . . . . . . . . . . . . . . . . 14

2.5.1 Chemical Kinetic Modeling . . . . . . . . . . . . . . . . . . . . . 15

2.5.2 Oxidation of Hydrocarbons . . . . . . . . . . . . . . . . . . . . . 16

Intermediate and Low Temperature Oxidation of Hydrocarbons . 16

High Temperature Oxidation of Alkanes . . . . . . . . . . . . . . 17

Oxidation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.5.3 Oxidation of Methyl Esters . . . . . . . . . . . . . . . . . . . . . 20

Oxidation of Methyl Acetate . . . . . . . . . . . . . . . . . . . . . 20

Oxidation of Dimethyl Carbonate . . . . . . . . . . . . . . . . . . 21

Oxidation of Methyl Butanoate . . . . . . . . . . . . . . . . . . . 22

iv

2.6 Mechanism of Soot Formation in Combustion Processes . . . . . . . . . . 22

3 Experimental Apparatus and Analytical Methodology 26

3.1 Opposed flow diffusion burner setup . . . . . . . . . . . . . . . . . . . . . 28

3.2 Fuel Preparation and Vaporization . . . . . . . . . . . . . . . . . . . . . 30

3.3 Supply of Fuel and Oxidizer Streams . . . . . . . . . . . . . . . . . . . . 32

3.4 Gas Sampling System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.4.1 Sampling Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.4.2 Sampling Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.5 Analytical Tehniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.5.1 Non-Dispersive Infrared Analysis . . . . . . . . . . . . . . . . . . 36

CO and CO2 Measurements . . . . . . . . . . . . . . . . . . . . . 37

3.5.2 Gas Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . 37

GC Carrier Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Injection System . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

GC Measurement Procedures . . . . . . . . . . . . . . . . . . . . 40

Methodology for Analysis of Hydrocarbons . . . . . . . . . . . . . 41

Methodology for Analysis of Oxygenates . . . . . . . . . . . . . . 42

GC Calibration Procedure . . . . . . . . . . . . . . . . . . . . . . 43

3.5.3 High Pressure Liquid Chromatography . . . . . . . . . . . . . . . 43

DNPH Sampling Procedure . . . . . . . . . . . . . . . . . . . . . 45

HPLC Measurement Procedure . . . . . . . . . . . . . . . . . . . 46

3.5.4 Temperature Measurement . . . . . . . . . . . . . . . . . . . . . . 46

Correction for Radiation Losses . . . . . . . . . . . . . . . . . . . 47

4 Modeling 49

4.1 Modeling Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.2 Details of Input Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5 Results and Discussion 52

5.1 The Role of FAME Molecular Structure in Combustion . . . . . . . . . . 52

5.1.1 Major Reaction Pathways for the Oxidation of Methyl Butanoate 52

5.1.2 Major Reaction Pathways for the Oxidation of Methyl Crotonate 54

5.1.3 Comparison of Opposed Flow Diffusion Flame Emissions Profiles . 57

v

5.1.4 Rationale for Differences in Methyl Butanoate and Methyl Croto-

nate Emissions Profiles . . . . . . . . . . . . . . . . . . . . . . . . 64

5.2 Chemical Kinetic Modeling of Methyl Butanoate . . . . . . . . . . . . . . 66

5.2.1 Modification of Reaction Rates - Contributions of this Study . . . 66

5.2.2 Model Validation with Opposed Flow Diffusion Flame Results . . 67

5.2.3 Error Analysis of Major Differences in Modeling and Experimental

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Experimental Errors . . . . . . . . . . . . . . . . . . . . . . . . . 71

Inaccuracies in Model Reaction Rate Coefficients . . . . . . . . . 71

5.2.4 Major Reaction Pathways for the Oxidation of Methyl Butanoate

in the Opposed Flow Diffusion Flame . . . . . . . . . . . . . . . . 74

6 Conclusions and Recommendations 77

6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

6.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Bibliography 80

Appendices 84

A Flow Rates of Fuel and Oxidizer 85

B Structures of Chemical Compounds 88

C Sample Chromatograms 90

D Sample Calculations 94

vi

List of Tables

2.1 Saturated fat composition of different vegetable oil and animal fats . . . 6

2.2 Short chain and long chain fatty acid methyl esters . . . . . . . . . . . . 8

2.3 Properties of biodiesels compared to standard diesel . . . . . . . . . . . . 11

2.4 Relative magnitudes of rate constants for H abstraction from different CH

bonds [38] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.1 Physical properties of the fuels used . . . . . . . . . . . . . . . . . . . . . 31

3.2 Settings for the gas sampling valve and split valve . . . . . . . . . . . . . 41

3.3 Flow rate of GC gases when measuring hydrocarbons . . . . . . . . . . . 41

3.4 Flow rate of GC gases when measuring oxygenates . . . . . . . . . . . . . 42

3.5 Determination of ideal sampling times for carbonyl compounds using the

DNPH technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.1 A comparison of measured peak species mole fractions in methyl butanoate

(MB) and methyl crotonate (MC) flames . . . . . . . . . . . . . . . . . . 63

5.2 Modified reaction rate constants [10, 8, 43]. . . . . . . . . . . . . . . . . . 66

5.3 Decomposition pathways of CH3OC·=O and corresponding reaction rate

constants [43]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.4 A comparison of peak species mole fractions from experimental data and

model-predicted values . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.5 A comparison of reaction rates given in the present model [10] to those

available in literature [67] . . . . . . . . . . . . . . . . . . . . . . . . . . 75

B.1 Structures of relevant chemical species . . . . . . . . . . . . . . . . . . . 89

vii

List of Figures

2.1 Composition of a Triglyceride from [16] . . . . . . . . . . . . . . . . . . . 5

2.2 Biodiesel Production and Blending Flowchart . . . . . . . . . . . . . . . 8

2.3 Transesterification Reaction . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4 Decay of isopropyl radical . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.5 Addition of O radical to ethene from [38] . . . . . . . . . . . . . . . . . . 19

2.6 Structure of Methyl Acetate . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.7 Main reaction pathways for DMC in the opposed flow diffusion flame (per-

centages refer to a reaction pathway’s share of a species consumption) from

[43] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.8 Main reaction pathways for the oxidation of methyl butanoate from Fisher

et al. [8] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.9 General mechanism for soot formation from Glassman [38] . . . . . . . . 25

3.1 Schematic of the experimental setup . . . . . . . . . . . . . . . . . . . . 27

3.2 Diagram of burner port . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.3 Photograph of burner setup . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.4 Photograph of fuel vaporization column . . . . . . . . . . . . . . . . . . . 32

3.5 Schematic of microprobe (not to scale) . . . . . . . . . . . . . . . . . . . 34

3.6 Schematic of microprobe and burner setup (not to scale) . . . . . . . . . 35

3.7 Schematic of NDIR instrument from [55] . . . . . . . . . . . . . . . . . . 37

3.8 Schematic of GC instrument . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.9 Schematic of FID from [57] . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.10 Schematic of injector in the fill position . . . . . . . . . . . . . . . . . . . 40

3.11 Schematic of injector in the load position . . . . . . . . . . . . . . . . . 40

3.12 Oven temperature program for measuring hydrocarbons using HP Al/S

PLOT column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

viii

3.13 Oven temperature program for DB-624 Column . . . . . . . . . . . . . . 43

3.14 Schematic of HPLC Setup from [62] . . . . . . . . . . . . . . . . . . . . . 44

3.15 Thermocouple Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.1 Geometry of opposed-flow diffusion flame from [64] . . . . . . . . . . . . 50

5.1 Hydrogen abstraction from carbons atoms in methyl butanoate . . . . . . 53

5.2 Decomposition reaction pathways for methyl butanoate . . . . . . . . . . 55

5.3 Reaction pathway for forming methyl crotonate from methyl butanoate . 55

5.4 Hydrogen abstraction from carbons atoms in methyl crotonate . . . . . . 56

5.5 Decomposition reaction pathways for methyl crotonate . . . . . . . . . . 56

5.6 Reactions with biradical O for methyl crotonate . . . . . . . . . . . . . . 58

5.7 Diagram of the burner setup to clarify the orientation of experimental

profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.8 Measured temperature profiles in the methyl butanoate (MB - closed sym-

bols with lines) and methyl crotonate (MC - open symbols without lines)

flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.9 Measured concentration profiles for fuel (MB or MC), CO, and CO2 in the

methyl butanoate (MB - closed symbols with lines) and methyl crotonate

(MC - open symbols without lines) flames . . . . . . . . . . . . . . . . . 59

5.10 Measured concentration profiles for CH4, C2H4, and C2H2 in the methyl

butanoate (MB - closed symbols with lines) and methyl crotonate (MC -

open symbols without lines) flames . . . . . . . . . . . . . . . . . . . . . 60

5.11 Measured concentration profiles of C3H6 and C2H6 in the methyl butanoate

(MB - closed symbols with lines) and methyl crotonate (MC - open symbols

without lines) flames, and CH3OH in the methyl crotonate flame . . . . . 60

5.12 Measured concentration profiles of 2-C4H6, C3H8, and CH3CHO in the

methyl butanoate (MB - closed symbols with lines) and methyl crotonate

(MC - open symbols without lines) flames . . . . . . . . . . . . . . . . . 61

5.13 Measured concentration profiles of C4H8, C3H4, 1,3-C4H6, and CH2O in

the methyl butanoate (MB - closed symbols with lines) and methyl croto-

nate (MC - open symbols without lines) flames . . . . . . . . . . . . . . . 61

ix

5.14 Measured concentration profiles of C5H12 and C4H10 in the methyl bu-

tanoate (MB - closed symbols with lines) and methyl crotonate (MC -

open symbols without lines) flames, and C6H6, C3H4O, and C3H6O in the

methyl crotonate flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

5.15 Analogous reaction pathways for methyl butanoate and methyl crotonate

oxidation in the opposed flow diffusion flame . . . . . . . . . . . . . . . . 65

5.16 Modeling predictions (small symbols with lines) and experimental results

(large symbols without lines) for methyl butanoate (MB), CO, and CO2 . 68


(large symbols without lines) for CH4, C2H4, and C2H2 . . . . . . . . . . 68


(large symbols without lines) for C3H6, C2H6, and CH2O . . . . . . . . . 69


(large symbols without lines) for C4H8, C3H8, pC3H4, CH3CHO, and 1,3-

C4H6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

5.20 Primary reaction pathways for methyl butanoate oxidation in the opposed

flow diffusion flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

C.1 Example of HPLC chromatogram of DNPH derivatives . . . . . . . . . . 91

C.2 Example of GC chromatogram for the HP Plot Column . . . . . . . . . . 92

C.3 Example of GC chromatogram for the DB 624 Column . . . . . . . . . . 93

x

Chapter 1

Introduction

There is a need for renewable fuels in North America due to the negative impacts of non-

renewable fuels (petroleum) on the environment and societies. Among renewable fuels,

those derived from biomass sources (biofuels) are gaining importance. The primary

drivers for using biofuels are to reduce greenhouse gas emissions and consumption of

fossil fuels. Reducing harmful emissions to the atmosphere is imperative for mitigating

global warming and sustaining healthy metropolitan areas for human inhabitation. In

addition, the ability for citizens to locally grow and produce their own fuel minimizes the

dependence on nonrenewable energy sources.

The Canadian Greenhouse Gas Inventory [1] reported that the transportation sector

accounts for nearly 25% of all greenhouse gas emissions to the atmosphere. The majority

of these emissions came from gasoline vehicles (53%) and diesel vehicles (35%) [1]. Both

the gasoline engine and the diesel engine are capable of operating on biofuels. The re-

spective biofuels of interest for gasoline and diesel technologies are ethanol and biodiesel.

In North America, ethanol has been blended with gasoline for over a decade; however,

the use of biodiesel has only started gaining momentum in the past few years. Rudolf

Diesel, the inventor of the diesel engine stated in 1912 that, “the use of vegetable oils for

engine fuels may seem insignificant today. But such oils may become in course of time

as important as petroleum and the coal tar products of the present time” [2].

Biodiesel is defined as a mixture of mono-alkyl esters of long chain fatty acids derived

from vegetable oils or animal fats [3]. Biodiesel can be used in its pure form or it can be

blended with petroleum diesel without major modifications to the existing engine and

fuel distribution infrastructure [4]. Engine combustion studies [5, 6, 7] have shown that

the use of biodiesel can significantly reduce hydrocarbon, particulate, carbon monoxide,

1

Chapter 1. Introduction 2

and greenhouse gas emissions while increasing fuel economy; however, a slight increase in

NOx emissions have been observed. These benefits can be optimized by understanding

the fundamentals of biodiesel combustion.

1.1 Research Motivation

In order to design engine systems, designers require information about the fundamental

combustion properties of the fuel. However, detailed studies have not been conducted

for biodiesel. In addition, the properties of each biodiesel variant depend on the feed-

stock used for its preparation, i.e., biodiesel derived from animal fats performs differently

from biodiesel derived from vegetable oils. This lack of information has inhibited the

widespread use of biodiesel in the automotive sector because engine manufacturers are

reluctant to design systems for a poorly understood fuel.

Detailed chemical kinetic models are an essential input into the engine computational

fluid dynamic (CFD) models used by designers. There are currently no validated chemi-

cal kinetic models for the long chain fatty acid methyl esters (FAME) found in biodiesel.

However, Fisher et al. [8] have provided a chemical kinetic mechanism for methyl bu-

tanoate. Methyl butanoate is a fully saturated short chain FAME with a molecular

structure similar to that of the saturated long chain FAMEs found in biodiesel. It was

chosen as a modeling surrogate of biodiesel because it is “large enough to allow fast RO2

isomerization reactions important in the low-temperature chemistry that controls fuels

auto-ignition under conditions found in diesel engines” [8]. The authors were not able

to robustly validate the model due to limited experimental data on methyl butanoate

combustion.

Previously, Syed [9] provided opposed flow diffusion flame data to validate the Fisher

et al. model [8]; however, measurements for higher molecular weight hydrocarbons and

important oxygenated species were not provided. Following the Syed study [9], an im-

proved methyl butanoate oxidation model has been developed by Dr. Sandro Gaıl and

coworkers [10]. However, the existing opposed flow diffusion flame data is not sufficient

to validate the Gaıl et al. model [10].

In addition, methyl butanoate is not a good surrogate fuel for the unsaturated FAMEs

found in some biodiesels. It has been shown [6, 7, 11] that biodiesel combustion perfor-

mance depends on the molecular structure of the fuel (i.e. extent of unsaturation).

Therefore, it would be interesting to understand the role of molecular structure during


combustion by comparing flame emissions and oxidation reaction pathways for saturated

and unsaturated FAMEs.

1.2 Research Objectives

The ultimate goal of our research is to develop detailed chemical kinetic models for the

oxidation of biodiesel, and then validate the models using a number of experimental

platforms. However, for this study the scope has been reduced to the oxidation of C4

fatty acid methyl esters in an opposed flow diffusion flame. The goal to oxidize methyl

butanoate (saturated) and methyl crotonate (unsaturated) in an opposed flow diffusion

flame, and then measure the flame species profiles to:

1. determine the relationship between the FAME molecular structure (i.e. unsatura-

tion) on the flame characteristics;

2. and validate an existing chemical kinetic model for methyl butanoate.

1.3 Research Execution

Laboratory experiments were conducted at the Combustion Research Laboratory1. Flame

studies were carried out at atmospheric pressure in an opposed flow diffusion flame, which

burns the fuel at a stagnation point, so only combustion kinetics, diffusion (i.e. no tur-

bulent mixing), and thermodynamics influences the emissions formed. Methyl butanoate

(saturated) and methyl crotonate (unsaturated) were the two fuels tested. The emissions

were sampled by drawing the gases from within the flame using a quartz microprobe.

The emissions were characterized using a number of analytical techniques. CO and CO2

concentrations were measured using nondispersive infrared (NDIR) spectroscopy. Hy-

drocarbon concentrations were obtained by gas chromatography with a flame ionization

detector (GC/FID). High pressure liquid chromatography (HPLC) was used to measure

concentrations of formaldehyde and other oxygenated species.

The structure of the opposed flow diffusion flame was modeled using a modified version

of the one-dimensional steady-state flame code OPPDIF and the chemical kinetic model

for methyl butanoate oxidation by Dr. Gaıl [10]. The chemical kinetic mechanism was

1University of Toronto Department of Mechanical and Industrial Engineering


then validated against the experimental data. After validation, the model was used to

analyze the major reaction pathways for methyl butanoate combustion. Pathway analysis

was further conducted to explain the role of molecular structure in combustion.

Chapter 2

Background Research

2.1 Basics of Biodiesel

ASTM International defines biodiesel as “a fuel comprised of mono-alkyl esters of long

chain fatty acids derived from vegetable oils or animal fats, designated B100, and meeting

the requirements of ASTM D6751” [3]. Biodiesel refers to the pure fuel before blending

with diesel fuel. Biodiesel blends are denoted as, “BXX” with “XX” representing the

percentage of biodiesel contained in the blend (ie: B20 is 20% biodiesel, 80% petroleum

diesel)” [4].

Biodiesel offers many advantages [12]:

• It is a renewable fuel.

• It displaces the use of petroleum diesel.

• It is less toxic than table salt and more biodegradable than white sugar.

• It can help reduce greenhouse gas emissions.

• It can reduce particulate matter (PM), carbon monoxide (CO), sulphur oxides

(SOx), and hydrocarbon (HC) emissions.

• It can be made locally from agricultural and/or recycled feedstocks.

Biodiesel can be used in its pure form (i.e. B100), or it can be blended with petroleum

diesel, as is more common in practice. Suppliers can blend 1-2% biodiesel as a lubricity

additive, which will become more important when ultra low sulfur diesel fuels (ULSD)

5

Chapter 2. Background Research 6

are mandated in June, 2006. A higher blend of 20% biodiesel (i.e. B20) is the most

common in the United States because it balances performance, cost, emissions, and

handling with petroleum diesel. Higher blend levels are currently not recommended

because modifications and special care may be required depending on the type of engine

and vehicle being used [12].

It should be noted that raw or refined fats and oils that have not been processed to

meet the ASTM D6751 standards for use in diesel engines are not considered biodiesel

and should be avoided. The main purpose of processing the oil to biodiesel is to change

the fuel’s viscosity. Diesel engines, and in particular the fuel injectors, are designed for

diesel fuel with a viscosity of between 1.3-4.1 mm2/s; however, raw oils tend to be around

40 mm2/s [12]. Research [13, 14, 15] has shown that blends using raw oils and greases can

cause severe long-term damage to the engine system. Therefore, oils must be processed

into biodiesel to reduce the fuel’s viscosity to a value similar to petroleum diesel, usually

4-5 mm2/s [12].

2.2 Chemistry of Biodiesel

As previously mentioned, biodiesel is derived from fats and oils. Oils and fats are made

of compounds termed triglycerides. Triglycerides consist of one molecule of glycerol

combined with three molecules of fatty acid, as shown in Figure 2.1. A fatty acid is a

hydrocarbon chain terminating in a carboxyl group, as shown in Equation 2.1. If the

three fatty acids in the triglyceride are identical, then it is a simple triglyceride. However,

most triglycerides contain a mixture of different fatty acids. Thus, the properties of these

mixed triglycerides depend on the properties of the fatty acids present. Subsequently,

since fats and oils are typically a mixture of different mixed triglycerides, their properties

are ultimately determined by the fatty acid content.

H3C − CH2.............CH2 − CH2 − COOH (2.1)

The most important vegetable oils consist of fatty acids containing 16 and 18 car-

bon atoms [17]. If a double bond exists between two carbon atoms, then the chain is

described as unsaturated because all available carbon valences for hydrogen are not sat-

isfied. Palmitic acid, C15H35COOH, and stearic acid, C15H35COOH, are examples of

saturated fatty acids, while oleic acid, CH3(CH2)7CH=CH(CH2)7COOH, is an unsat-


Figure 2.1: Composition of a Triglyceride from [16]

urated fatty acid.

Unsaturated fatty acids are associated with a lower melting condition, greater sol-

ubility and chemical reactivity. In contrast, saturated fatty acids have higher melting

characteristics, and are less reactive and soluble. Unsaturated fatty acids with two or

more double bonds are called polyunsaturated fatty acids (PUFAs). The extent of unsat-

uration is quantified by the iodine value (IV). It is determined by the amount of iodine

taken up by 100 grams of a fatty acid. Saturated fatty acids take up no iodine; therefore,

their IV is zero [18]. In general, vegetable oils tend to contain more unsaturated fatty

acids while animal fats contain more saturated fatty acids. However, this is not always

true as shown in Table 2.1. It is clear that palm oil and coconut oil do not follow the

general properties of most vegetable oils [19].

2.2.1 Feedstocks for Biodiesel

There are a variety of feedstocks available for biodiesel production, but most can be

classified as vegetable oils, first-use animal fats, or waste greases [19]. The physical

chemical properties and molecular structure of a given biodiesel are directly related to


Oil or Fat Percent of Saturated Fats

Canola 7

Safflower 9

Sunflower 10

Corn 13

Olive 13

Soybean 17

Chicken Fat 30

Beef Tallow 47

Palm 50

Coconut 97

Table 2.1: Saturated fat composition of different vegetable oil and animal fats

the properties of the fat or oil from which it was derived. Biodiesel derived from highly

unsaturated feedstocks, such as canola and soybean oil, will be equally unsaturated.

Similarly, biodiesel derived from animal fats, such as beef tallow, are highly saturated

because the feedstock consists primarily of saturated fatty acids.

Vegetable oils are virgin oils derived from any vegetable source, such as soybeans,

canola, sunflowers, peanuts, etc. Generally they are low in saturated fats, and therefore

the least expensive to process. On the other hand, they are the most expensive feedstock

to purchase. The lack of saturated esters produced from vegetable oils results in a

biodiesel with better cold flow characteristic and lower cetane numbers. The presence of

double bonds in the FAME also makes the fuel more susceptible to oxidation, thereby

reducing its shelf-life [12].

First-use animal fats include any virgin fat or oil derived from an animal source, such

as edible and inedible tallows, grades of lards, poultry fats, etc. These feedstocks are

generally less expensive to purchase than vegetable oils. Biodiesel produced from first-use

animals fats are high in saturated fats; therefore, they are more expensive to produce,

have poorer cold flow characteristics, and higher cetane numbers.

Waste greases include waste vegetable oils (yellow greases) that were used for cooking

and waste trap greases (brown greases). These are the least expensive feedstocks for

biodiesel production, but they require the most intensive processing. Waste greases


display characteristics of both vegetable oils and animal fats [20]

In addition to saturation, the free fatty acid (FFA) content in the feedstock oil or

fat is also an important characteristic. Fatty acid chains become FFAs when they break

off the triglyceride. These FFAs can affect the biodiesel production process, as will be

discussed in Section 2.2.2 . The FFA content of various biodiesel feedstocks is as follows

[20]:

• Refined vegetable oils, such as soybean or canola oil (FFA < 1.5%)

• Low FFA yellow greases (waste vegetable oils)1 and animal fats (FFA < 4%)

• High FFA greases and animal fats (FFA ≥ 20%)

Surrogates of Biodiesel

Surrogates for biodiesel fuel molecules were used in this study to model the combus-

tion characteristics of real biodiesel. The purpose of using surrogates is to simplify the

combustion model by reducing the number of possible chemical reactions, while still rep-

resenting the role of the molecular structure in combustion, i.e. the role of the methyl

ester group and the role of carbon-carbon double bonds. In addition, real biodiesel is a

complex mixture of fatty acid esters with differing chain lengths and degrees of unsatu-

ration, thereby increasing the complexity of the modeling procedure.

In this study, two short chain fatty acid methyl esters, methyl butanoate and methyl

crotonate, were used as surrogates for the saturated and unsaturated long chain fatty

acid methyl esters, respectively. The chemical formula and molecular weight of these

compounds, along with similar long chain fatty acid methyl esters, is show in Table 2.2.

2.2.2 Production of Biodiesel

Producing biodiesel that meets the ASTM D6751 standards requires the feedstock oil

or fat to be processed, so that it’s viscosity is reduced. Several methods by which the

viscosity can be reduced are micro-emulsification, cracking, and transesterification. Ma

and Hanna have reviewed these processes [21]; however, only transesterification produces

a fuel that meets the ASTM’s definition of biodiesel [3], wherein biodiesel is comprised

1Yellow grease is another term for waste vegetable oils usually from restaurants


Table 2.2: Short chain and long chain fatty acid methyl esters

of “long chain mono alkyl esters”. A simplified flowchart for biodiesel production and

blending is shown in Figure 2.2.

Figure 2.2: Biodiesel Production and Blending Flowchart

Transesterification is the process by which a triglyceride (the basic constituent of fats

and oils) is converted to a mono alkyl ester. It involves reacting the fat or oil with an

alcohol, in the presence of a catalyst, to produce glycerol and esters [22]. Methanol is

the most commonly used alcohol, therefore the resulting biodiesel is often referred to as

a mixture of fatty acid methyl esters (FAMEs). From here on, “FAME” will be used to

refer to pure fatty acid methyl ester compounds while “biodiesel” will be used to refer

to a mixture of various fatty acid methyl esters. Ethanol has also been used [23], and in


such cases the resulting biodiesel is called fatty acid ethyl ester. The transesterification

reaction between a typical triglyceride and methanol is shown in Figure 2.3

Figure 2.3: Transesterification Reaction

Three moles of methanol are required to react with each mole of triglyceride in the

oil or fat. The reactions occur sequentially, i.e., the triglyceride reacts to form one mole

of diglyceride and one mole of FAME, followed by the diglyceride reacting to one mole

of monoglyceride plus one mole of FAME, and finally the monoglyceride to a mole of

glycerol and the third mole of FAME [9]. Excess methanol is used to drive the reaction

forward.

The reaction is carried out in the presence of a catalyst to improve the reaction rate.

Ramadhas et al. stated, “enzymes, alkalis, or acids can catalyze the reaction, i.e. li-

pases, NaOH, and sulphuric acid, respectively” [22]. Alkali catalysts such as sodium

methoxide, sodium hydroxide, and potassium hydroxide are the most commonly used

catalysts. Among these, sodium methoxide accounts for 70 percent of the biodiesel pro-

duced in North America [24]. A study by Vicente et al. at the University of Madrid

[25] indicates that the highest yield of biodiesel (wt%) is obtained by sodium methox-

ide (99.33%), when compared to sodium hydroxide (86.71%) and potassium hydroxide

(91.67%). However, sodium methoxide catalysts are the most expensive, and sodium

hydroxide catalysts proved to be the fastest. Currently, only sodium methoxide is used

in commercial applications.

The catalyst, alcohol, and vegetable oil are combined in a batch reactor at approx-


imately 65 ◦C and stirred continuously. The duration of the reaction can range from

1-6 hours, depending on the desired yield. Two immiscible layers are formed once the

reversible reaction reaches equilibrium. The lower layer is glycerol and the upper layer

contains FAMEs and unreacted feedstock. Many processes separate the glycerol and

conduct a second transesterification reaction to increase the yield. Glycerol is a valu-

able by-product that can be sold for a profit. If the biodiesel is to meet ASTM D6751

specifications, it must undergo a series of separation processes for the removal of alcohol,

catalyst, water, soaps, glycerol, and unreacted triglycerides and free fatty acids (FFAs)

[20].

It was previously mentioned that some feedstocks contain FFAs. FFAs react with the

alkali catalyst to form soap and water during the transesterification process. The loss

of catalyst can be compensated for by supplying additional catalyst. However, this is

an expensive option and it does not address the issue of excess water, which can limit

the transesterification. An expensive yet commonly used alternative is to pre-process the

feedstock to remove the small quantities of FFAs. It is not economical to remove FFAs

from high FFA greases and animal fats, and in such cases an acid-catalysis is first used

to convert FFAs to methyl esters, followed by standard base-catalyzed transesterification

[20].

2.3 Properties of Biodiesel

Biodiesel must have physical-chemical properties similar to that of diesel fuel in order

to successfully operate in a compression ignition engines. The properties which most

commonly affect operation are viscosity, heating value, ignition quality (cetane number),

pour point, and cloud point. This section summarizes a study by the NREL [20] on

the important properties of several biodiesel fuels and standard low sulfur diesel, as

quantified by various ASTM standard testing methods. It should be noted that all

engine manufactures design their commercial diesel engine systems to operate of fuel

that meets the ASTM D 975 standards - Standard Specification for Diesel Fuel Oils [26].

The heating value, or heat of combustion, is a measure of the fuel’s energy density.

Diesel engines are capable of accepting a variation in heating values, so there is no

specified heating value in the ASTM D 975 Standard. However, it is beneficial for

biodiesel to have a heating value similar to that of standard diesel [22]. Table 2.3 indicates

that the heating value of biodiesel is slightly lower than that of standard diesel. Therefore,


on a weight basis, biodiesel is slightly less energy efficient than standard diesel.

The cetane number is a measure of the fuel’s ignition delay. Diesel combustion requires

the fuel to self-ignite as it is sprayed into the compressed cylinder gas. The self-ignition

leads to the characteristic diesel ”knock”, wherein an explosion of premixed air and fuel

causes a rapid heat release and a rapid pressure rise. The magnitude of the explosion

can be decreased but shortening the ignition delay time. Higher cetane numbers result

in shorter ignition delay times, and therefore better engine operation. The ASTM D 975

minimum acceptable cetane value is 40. Table 2.3 indicates that all biodiesels have higher

cetane numbers when compared to standard diesel. Therefore, biodiesel is expected to

improve engine operation.

The viscosity measures the fuel’s resistance to flow. Diesel engine fuel injectors are

highly sensitive to fuel viscosity. Higher viscosity results in poorer atomization of the fuel

spray and an increase in engine deposits. The ASTM D 975 maximum acceptable value

for viscosity is 4.1 mm2/sec. Table 2.3 indicates that all biodiesels have a viscosity over

two times greater than the viscosity of conventional diesel. Due to such high viscosities,

pure biodiesel (B100) is not certified for use in most diesel engines. The fuel viscosity can

be reduced by blending the biodiesel with conventional diesel fuel or another viscosity

reducing additive.

The pour point is a measure of the lowest possible temperature at which the fuel is

observed to flow. At temperatures below the pour point, the fuel becomes gelled and can

no longer flow. It is important that the pour point temperature be as low as possible, so

that the fuel can be used in colder climates without the use of heaters. Table 2.3 shows

the pour point temperatures of various fuels. In general, all biodiesels have significantly

higher pour points than conventional diesel. Biodiesel derived from animal fats and

yellow greases have higher pour points due to the higher proportion of saturated FAMEs.

The cloud point measures the temperature at which the fuel becomes hazy due to

crystallization of the fuel particles. Using a product below its cloud point can plug certain

filters in the fuel delivery system. Table 2.3 shows that the cloud point of all biodiesels is

much higher than standard diesel. As expected, biodiesels with more saturated FAMEs

have higher cloud point temperatures. The use of additives can help to improve the cold

flow properties of biodiesel.

ASTM D 975 does not specify a single number for cloud point and pour point tem-

peratures. Instead, values are specified depending on the typical ambient temperature

in the region of use. The specifications indicate that 41 of the 48 states in the conti-


nental US reach temperatures below the lowest cloud point and pour point temperatures

of biodiesel. Therefore, the use of biodiesel in its pure form (B100) is not possible in

most regions; however, property-enhancing additives and petroleum diesel blending can

be used to improve the cold temperature performance of biodiesel.

Physical Chemical Property

Soy ME Canola ME Lard MEEdible

Tallow MEInedible

Tallow ME

Low FFA Yellow

Grease ME

High FFA Yellow

Grease ME

2D Diesel Reference

Fuel

ASTM D975 Specification

Heating Value (Btu/lb)

17153 17241 17165 17144 17061 17215 17154 18600 N/A

Cetane Number

59 53.9 N/A 74.8 54.3 55.2 53.2 47 > 40

Viscosity (mm2/sec)

4.55 4.63 4.85 4.91 4.93 5.62 4.66 2.45 1.9 - 4.1

Cloud Point (°C)

2 -3 14 20 23 42 8 -18 by customer

Pour Point (°C)

-1 -4 11 13 8 12 8 -27 by customer

Table 2.3: Properties of biodiesels compared to standard diesel

2.4 Biodiesel Use in Diesel Engines

There have been a number of studies on the use of biodiesel in diesel engines. The goal

of these studies was to determine the effect of using biodiesel and diesel-biodiesel blends

on the emissions, fuel economy, and operation of the diesel engine. In addition, several

experiments were performed to study the effects of biodiesel feedstock on the resultant

fuel’s combustion properties. Following is a summary of various studies.

Ramadhas et al. [22] conducted an extensive literature review on previous biodiesel

engine studies. The wide variation in some data led them to some rather general conclu-

sions below:

• Cold weather operation with biodiesel is not easy.


• Additives are needed to improve the cold flow properties, material compatibility,

oxidation stability, and viscosity.

• The use of biodiesel results in a thermal efficiency comparable to conventional

diesel.

• Small amounts of power loss (brake horse power) can be expected when using

biodiesel.

• Biodiesel gives performance and emissions characteristics similar to those of con-

ventional diesel, and biodiesel is an appropriate substitute for diesel fuel.

Agarwal and Das [27] conducted performance and emission tests to evaluate perfor-

mance of linseed oil methyl ester blends in diesel engines. Their study concluded, “that

the biodiesel can be used as an alternative, environment friendly fuel in existing diesel

engines without substantial hardware modification.” In addition, the optimal concentra-

tion of biodiesel in the blend was found to be 20%. “It gave net advantage of 2.5 percent

in peak thermal efficiency and there was substantial reduction in smoke opacity values.”

However, the 20% biodiesel blend led to a 5% increase in NOx emissions. NOx is a highly

temperature dependent phenomenon, and it was found that increasing the concentration

of biodiesel led to a increase in exhaust gas temperature and NOx emissions.

The United States Environmental Protection Agency [7] conducted an analysis on

pre-existing biodiesel emissions data to compare emissions profiles for three B20 blends

against standard diesel in heavy-duty applications. The report indicates that a soybean-

based B20 reduces average emissions of PM by 10%, HCs by 20%, CO by 11%, and

increases NOx emissions by 2% and fuel consumption by 1-2%. Rapeseed-based B20

produced similar results. However, it was found that animal fat-based biodiesel emits

lesser quantities of NOx, PM and CO compared to vegetable oil-based fuels.

Durbin and coworkers [6] studied the exhaust emissions of various B20 (2 soybean

based and 1 yellow grease based) relative to standard diesel. Experiments were performed

by chassis dynamometer tests on 7 light-duty diesel vehicles. The yellow grease biodiesel

blend showed significant reductions in HC and CO emissions across the test vehicle fleet,

with HCs reduced by 21-66% and CO reduced by 0-46%, relative to the standard diesel.

HC and CO emissions were comparable for soybean methyl esters and standard diesel.

The PM emissions for yellow grease biodiesel were only lower for the highest emitting

vehicle, and comparable on the remaining vehicles. Soybean B20 had higher PM emission


rates than standard diesel for 4 of the 7 vehicles tested, and comparable PM emissions

on the remaining vehicles. NOx emissions were not significantly different between the

any of the biodiesel blends and standard diesel. No correlation between FAME molecular

structure and PM and NOx emissions could be drawn from this study.

McCormick, Graboski and coworkers [11, 28, 29] have conducted many engine studies

with biodiesel blends. Previous studies [28] showed that a 35% soybean methyl ester

blend reduces PM emissions by 26% and increase NOx emissions by 1%, relative to

standard diesel. B100 was found to increases NOx by 11% and reduce PM by 66%. Their

most recent biodiesel study [11] tested biodiesels from various feedstocks in a heavy-duty

engine to determine the impact of biodiesel chemical structure, i.e. fatty acid chain length

and degree of unsaturation, on NOx and PM emissions. They correlated fuel density and

cetane numbers with NOx and PM emissions: an increase in density correlates with an

increase in NOx and little change in PM; an increase in cetane number correlates to a

decrease in NOx and little change in PM. Saturated esters have higher cetane numbers and

lower densities than unsaturated esters. Thus, NOx emissions are shown to increase with

increasing iodine value. Decreasing fatty acid chain length was also shown to decrease

NOx emissions. PM emissions were found to be unrelated to FAME molecular structure.

Thus, they concluded that modifying the fuel’s molecular structure can alter biodiesel

NOx emissions performance.

2.4.1 NOx Formation

Explanations for the observed increase in NOX in biodiesel are open for speculation. The

three prevailing mechanisms explaining NOx formation in combustion engines are: 1.)

Fuel, 2.) Thermal (Zeldovich mechanism), and 3.) Prompt (Fenimore mechanism). Fuel

NOx is formed when nitrogen compounds fixated in the fuel are oxidized. This is not a

concern for biodiesel since the fuel does not contain any chemically bound nitrogen.

Thermal NOx is a result of high temperature dissociation and chain reaction of elemen-

tal nitrogen and oxygen in the post-combustion regime. The reactions in this mechanism

are shown in Equations 2.2, 2.3, and 2.4. The reactions are highly temperature depen-

dent, such that a decrease in combustion temperature will decrease NOx formation [30].

However, a corresponding increase in PM will occur; thus the so-called NOx/PM tradeoff.

Agarwal and Das [27] provide engine studies supporting the thermal NOx theory. Tat

and Van Gerpen [31] have shown that biodiesel has a higher isentropic bulk modulus


of compressibility than conventional diesel, which causes an inadvertent advance in fuel

injection time in a pump-line-nozzle fuel injector. The advance in fuel injection timing

increases the ignition delay, and therefore increases thermal NOx formation. Boehman

and coworkers [32] have shown that the bulk modulus of compressibility increases with

increasing iodine value. This explains why fuels with higher iodine values produce more

NOx.

However, engine studies by McCormick and coworkers [11] have shown that NOx emis-

sions increase in unsaturated FAMEs while PM emissions remain unchanged. Therefore,

the increased formation of NOx in biodiesel combustion cannot be entirely attributed to

thermal NOx. This leads to the possibility that NOx is formed by the prompt NOx mech-

anism. Prompt NOx forms from a complex set of reactions between elemental nitrogen,

hydrocarbon radicals, and oxygen under fuel-rich conditions [30]. It is proposed that

combustion of unsaturated FAMEs leads to an increase of certain hydrocarbon radicals

in the fuel rich zone of the diesel spray, and results in prompt NOx formation. Hess and

coworkers [30] have shown that the use of antioxidants can terminate radical reactions

and lead to reduction in NOx emissions.

O· + N2 → NO + NO2 (2.2)

N · + O2 → NO + O· (2.3)

N · + HO· → NO + H · (2.4)

2.4.2 Biodiesel as a Lubricity Additive

Conventional diesel fuel contains small amounts of sulphur containing molecules which

serves as a lubricity agent, but at the same time increases SOx emissions. The mandate

for ultra low sulfur diesel (ULSD) poses a concern since a reduction in diesel fuel lubricity

can be damaging to the engine and fuel injection system. Biodiesel has been explored as

an ULSD lubricity additive for extending the life of diesel engine components.

The NREL [20] studied the properties, including lubricity, of biodiesel derived from

various feedstocks. They reported that even small additions of biodiesel to diesel can

significantly improve lubricity. Lubricity was shown to improve by 10% with 0.25%

biodiesel present, and by 30% with 0.5% biodiesel. Biodiesel derived from animal fat and

yellow grease were found to increase lubricity better than biodiesel derived from vegetable


oils. However, the decrease in lubricity found in vegetable oils cannot be attributed to

lower saturated FAME content, as is shown in the following study.

Geller and Goodrum [33] examined the effect of FAME chain length and saturation on

the fuel’s lubricity properties. They chose to study individual FAMEs rather than actual

biodiesel fuels, which contain a mixture of FAMEs. No correlation between FAME chain

length and lubricity enhancing properties was reported. However, saturated FAMEs were

found to be less effective as a lubricity enhancer than unsaturated FAMEs. It was also

found that hydroxylated FAMEs are the best at increasing lubricity.

The author of this thesis hypothesizes that higher concentrations of hydroxylated

FAMEs may explain the increased lubricity of biodiesel derived from animal fat, as seen

in the aforementioned NREL study [20]. Hydroxylated FAMEs are usually derived from

hydroxylated fatty acids which are found in small quantities in some oils and fats. An-

other possibility is that FFAs are converted to hydroxylated FAMEs during the trans-

esterification process. Thus, the higher FFA content of animal fat and yellow grease

would result in the formation of greater amounts of hydroxylated FAMEs. However,

experimental data is not available to test this hypothesis.

2.4.3 Oxygenated Fuels as Soot-Reducing Additives

The use of oxygenated fuels has been shown to be an effective way of reducing soot

emissions in diesel engines [34]. Oxygenated fuels prevent carbon atoms from forming soot

by creating carbon-oxygen bonds. Ideally, each fuel bound O atom should pair up with a

C atom to form CO. This prevents the formation of C-C bonds, which ultimately lead to

soot formation. In addition, the oxidation of oxygenated species forms OH radicals, which

readily attack unsaturated hydrocarbons and prevent their participation in soot growth

reactions. It is for these reasons that the oxygenated methyl ester portion of FAMEs has

made biodiesel an attractive blending agent for reducing diesel soot emissions. Several

insights into the use of biodiesel for this purpose are presented in this study.

2.5 Modeling the Oxidation of Biodiesel

The combustion of fuel in the diesel engine is an oxidation reaction. Fuel is combined

with hot compressed oxygen (air) at a ratio such that the fuel oxidizes rapidly. Chemical

kinetic phenomena play an important role in understanding the initiation of the oxidation


process and the emissions formed during, and after, the combustion. There have not been

any detailed studies on the chemical kinetic pathways for the oxidation of long chain

FAMEs. However, numerous studies have been conducted for hydrocarbon fuels and

oxygenated compounds. The development of chemical kinetic models for the oxidation of

the FAMEs found in biodiesel can greatly benefit from existing oxidation pathway studies

for aliphatic hydrocarbons. FAMEs consist of a long hydrocarbon chain terminated by

a carboxylate ester group. Therefore, it is likely that the oxidation of these compounds

resemble the oxidation of alkanes and alkenes.

2.5.1 Chemical Kinetic Modeling

Detailed chemical kinetic mechanisms are used to model the molecular level transforma-

tion of reactants into products [35]. Take for example the oxidation of methane, as shown

in Equation 2.5. Although the reaction looks simple, it actually proceeds through a num-

ber of elementary reactions. The rates of formation and consumption in the elementary

reactions are described by a set of differential equations, which are numerically inte-

grated to determine the concentrations of reactants, intermediates, and products. The

modeling results are then compared against a set of experimental data for validation,

and appropriate changes are made, if necessary.

Reaction rates are expressed by the modified Arrhenius form, as shown in Equa-

tion 2.6. The modified Arrhenius form with temperature dependence is used because

non-Arrhenius behavior is often encountered in the temperature ranges encountered in

combustion processes [36]. The specific reaction rate constants are best obtained from

direct experimental evaluations within defined temperature ranges. A list of elementary

combustion reactions and their reactions rates are given by Westbrook and Dryer [36].

CH4 + 2O2 → CO2 + 2H2O (2.5)

kA = A · (T/Tref )n exp

Ea

R · T(2.6)

where

k is the reaction rate

A is the Arrhenius constant

T is temperature


Tref is the reference temperature (usually 298 K or 25 ◦C)

n is the pre-exponential factor

Ea is the activation energy

R is the ideal gas constant

The process for developing and validating chemical kinetic models was outlined by

Frenklach et al. [35]:

1. Generate a complete list of elementary reactions.

2. Determine reaction rate constants (see Equation 2.6) for each reaction using litera-

ture sources or estimation, paying attention to temperature and pressure dependen-

cies. Provide thermodynamic data to calculate equilibrium reverse rate constants.

3. Conduct controlled experiments that can be used to validate the reactions and rate

parameters given in the model.

4. Solve the reaction mechanism kinetics and transport equations using a computer

simulation of the experimental configuration. Conduct a sensitivity analysis to

determine the impact of specified rate constants on the final result.

5. Compare the experimental data to the model predicted values. Optimize reac-

tion rate parameters that have the greatest impact on fitting desired experimental

values.

In this study, the user-defined chemical kinetic model was validated against an op-

posed flow diffusion flame setup. The model was numerically solved using the computer

application CHEMKIN 4.0 [37]. More information on the experimental configuration and

modeling procedure is given in Sections 3 and 4.

2.5.2 Oxidation of Hydrocarbons

A clear and simple explanation of the oxidation of hydrocarbons is given by Glassman

[38]. Oxidation reactions are driven by the formation of highly reactive O, OH, and

H radicals. During combustion, fuels are oxidized by a series of chain reactions which

can be categorized as one of following: 1.) chain initiating, 2.) chain propagating and

chain branching, or 3.) chain terminating. Chain initiating occurs when radical species


are produced by dissociation of the reactants. The chain is propagated and branched as

radicals react with stable compounds to form additional radical species. Finally, the chain

terminates when two radicals recombine to form stable species. The following subsections

describe the major reaction pathways for the oxidation of alkanes and alkenes under low

and high temperature conditions. Very complete mechanisms would contain a number

of minor reactions; however, for the sake of simplicity, they have not been included here.

Intermediate and Low Temperature Oxidation of Hydrocarbons

The oxidation of hydrocarbons is different at low temperatures than high temperatures.

The general mechanism for the low temperature oxidation of hydrocarbons was first

developed by Semenov [39]. Benson [40] introduced the isomerization reaction of large

hydrocarbons (Equation 2.10) to the Semenov mechanism. The following is a simplified

form of the Semenov mechanism including isomerization of large hydrocarbons:

RH + O2 → R ·+HO2 · (2.7)

R ·+O2 → alkene + HO2 · (2.8)

R ·+O2 → RO2 · (2.9)

RO2· → ROOH · (2.10)

ROOH· → RO ·+OH · (2.11)

HO2 ·+RH → H2O2 + R · (2.12)

H2O2 + M → OH ·+OH ·+M (2.13)

The chain is initiated by low temperature oxidation of the hydrocarbon to form an

alkyl radical and a hydroperoxy radical (Equation 2.7). Next, the chain is propagated by

one of two parallel reactions between alkyl radicals and oxygen to form alkenes, HO2·, and

RO2· (Equations 2.8 and 2.9). These reactions compete with each other depending on


the temperature. At temperatures above 500 K, Equation 2.8 predominates, wherein the

oxygen abstracts a hydrogen from the alkyl radical to form an alkene and a hydroperoxy

radical. At temperatures below 500 K, Equation 2.9 is favored, wherein the oxygen adds

to the alkyl radical to form an alkylperoxy radical.

At low temperatures (below 500 K), propagation continues by the isomerization of

RO2· to produce hydroperoxyalkyl radicals (·ROOH) (Equation 2.10). The radical pool

then builds up by degenerate branching of ·ROOH to form RO· and OH· radicals 2.11.

Further developments on this low temperature mechanism have been published by Zhao

et al. [41].

At intermediate temperatures (above 500 K), the HO2· radical is more abundant,

so the reaction is propagated by hydrogen abstraction on the hydrocarbon by HO2· to

form hydrogen peroxide (H2O2) and an alkyl radical (Equation 2.12). As the temperature

increase, hydrogen peroxide to decompose to form two hydroxyl radicals (Equation 2.13).

The fuel-air mixture explodes once the radical pool builds up, and then high temperature

oxidation predominates.

High Temperature Oxidation of Alkanes

The high temperature oxidation of alkanes larger than methane is initiated by the break-

ing of CC bonds to form hydrocarbon radicals, as shown in Equation 2.14.

RH + (M) → R′ ·+R′′ ·+(M) (2.14)

where

RH is an alkane molecule

R′· and R′′· are alkyl radicals such as CH3, C2H5, etc.

(M) is a non-reacting collision partner

This step dominates because CC bonds are weaker than CH bonds in the molecule.

However, CH bonds can also be broken at higher temperatures, as shown in Equation

2.15. In addition, the low temperature abstraction initiation step can occur, as shown in

Equation 2.16.

RH + (M) → R ·+H ·+(M) (2.15)


RH + O2 → R ·+HO2 · (2.16)

The production of H radicals in the chain initiating steps leads to chain propagation,

wherein H· radicals react with O2 to form OH· and O· radicals, as shown in Equation

2.17. Chain propagation can also occur by Equations 2.18 and 2.19. The disappearance

of fuel occurs after the reservoir of H·, OH·, and O· radicals has been built up, as shown

in Equation 2.20.

H ·+O2 → O ·+OH · (2.17)

O ·+H2 → OH ·+H · (2.18)

O ·+H2O → OH ·+OH · (2.19)

RH + X· → R ·+XH (2.20)

where

X is any radical specie, usually O·, OH·, H·, and CH3·

Relative rate coefficients for H abstraction by radicals from tertiary, secondary, and

primary CH bonds are given in Table 2.4 [38]. Tertiary CH bonds are those on a carbon

atom connected to three other carbon atoms. Secondary CH bonds are on a carbon

atom connected to two other carbons. A primary CH bond is one on a carbon connected

only to one other carbon, such as the carbon at the end of a hydrocarbon chain. The

table indicates that tertiary CH bonds are the weakest and primary CH bonds are the

strongest. If the fuel is an alkene then radicals will abstract H from carbon atoms that

are singly bonded because CH bonds on doubly bonded carbon atoms are very strong.

The alkane radical then decays to form an alkene and a radical specie, as shown

in Equation 2.21. For example, the isopropyl radical, obtained from H abstraction of

the secondary CH bond in propane, will decay to propene and a H atom, as shown in

Figure 2.4. The process by which the alkyl radical decomposes is called β-scission. In

β-scission, the bond once removed from the radical site will break to form an alkene

without a hydrogen shift. Furthermore, CC bonds are more likely break than CH bonds

since CC bonds are weaker.


Table 2.4: Relative magnitudes of rate constants for H abstraction from different CH

bonds [38]

R + (X) → alkene + R′ ·+(M) (2.21)

where

R′ is a hydrocarbon radical or H atom

Figure 2.4: Decay of isopropyl radical

Oxidation of Alkenes

Alkane oxidation ends with the formation of alkenes and a pool of radical species, so the

oxidation of these alkene compounds will now be discussed, taking ethene as an example.

First, the C=C double bond is attacked primarily by the biradical O· , which forms an

intermediate species that subsequently decays, as shown in Figure 2.5. Thus, the two

primary addition reactions are shown in Equations 2.22 and 2.23. Some minor reactions

involving H abstraction by OH· and H· radicals also play a role, as shown in Equations

2.24 and 2.25, respectively.


C2H4 + O· → CH3 ·+HCO · (2.22)

C2H4 + O· → CH2 ·+CH2O (2.23)

Figure 2.5: Addition of O radical to ethene from [38]

C2H4 + OH· → C2H3 ·+H2O (2.24)

C2H4 + H· → C2H3 ·+H2 (2.25)

Then the vinyl radical (C2H3) decays to acetylene, as shown in Equation 2.26. The

acetylene is consumed by a reaction with the biradical O· to form a methylene radical and

carbon monoxide, as shown in Equation 2.27. The fate of CH3, CH2O (formaldehyde),

CH2, and CO are described in the mechanism for methane [38].

C2H3 ·+M → C2H2 + H ·+M (2.26)

C2H2 + O· → CH2 ·+CO (2.27)

2.5.3 Oxidation of Methyl Esters

Oxidation of Methyl Acetate

Dagaut et. al [42] developed a chemical for the oxidation of methyl acetate and validated

it against experiments conducted in a jet-stirred reactor at 800-1230 K and 10 atm.


Methyl acetate is a methyl ester with two carbon atoms in the hydrocarbon chain, as

shown in Figure 2.6. The study indicates that methyl acetate is primarily consumed by

H abstraction by H, O, and OH radicals, as shown in Equations 2.28 and 2.29.

Figure 2.6: Structure of Methyl Acetate

CH3C(= O)OCH3 + X → CH3C(= O)OCH2 + XH (2.28)

CH3C(= O)OCH3 + X → CH2C(= O)OCH3 + XH (2.29)

where

X is an H·, OH·, or O· radical

The resulting methyl acetate radicals are then decomposed by β-scission, as shown

in Equations 2.30 and 2.31.

CH3C(= O)OCH2 → CH3C(= O) + H2C(= O) (2.30)

CH2C(= O)OCH3 → CH2C(= O) + CH3O (2.31)

H abstraction on CH2C(=O) produces HCCO, which further reacts with O2 to form

CO2. Thermal decomposition reduces CH3C=O to CH3 and CO, and CH3O to CH2O

(formaldehyde) and H. Further decomposition of formaldehyde was found to lead to the

formation of the main active species in the mechanism: HCO, H, OH, HO2, and O.

Oxidation of Dimethyl Carbonate

A chemical kinetic model for dimethyl carbonate in an opposed flow diffusion flame

was studied by Glaude and coworkers [43]. Dimethyl carbonate ((CH3O)2C=O) has a

chemical structure similar to that of the FAMEs found in biodiesel. In addition, the


oxidation of DMC leads to the formation of the methoxy formal radical (CH3OC·=O),

which is structurally similar to the key moiety (ROC·=O) found in all oxygenated fuels,

including FAMEs.

Figure 2.7 shows the major reaction pathways for the consumption of DMC in their

model. Initially, DMC is consumed by reaction with H and OH to produce the DMC

radical. This intermediate quickly decomposes to CH2O (formaldehyde) and CH3OC·=O

(methoxycarbonyl radical). Finally, 78% of the CH3OC·=O radical leads to CO2, while

the remaining 22% forms CO.

Figure 2.7: Main reaction pathways for DMC in the opposed flow diffusion flame (per-

centages refer to a reaction pathway’s share of a species consumption) from [43]

A key contribution of this paper is new reaction rate constants for the decomposition

of CH3OC·=O. The authors state that these reaction rate rules can be applied for any

ROC·=O radical, including the ones found in FAMEs. The study also concludes that

the formation of the (ROC·=O) radical leads to more CO2 than CO by decomposition.

From a soot formation standpoint, the formation of CO2 is not desirable because two

oxygen atoms are bonded to one carbon atom. Ideally, each fuel-bound oxygen atom


should bond with one carbon atom to suppress the formation of carbon-carbon bonds,

which lead to soot. Other studies on the oxidation of esters have provided additional

evidence supporting this theory [44].

Oxidation of Methyl Butanoate

Methyl butanoate (nC3H7C(=O)OCH3) is a short chain FAME with a structure very

similar to the long chain FAMEs found in biodiesel, which have the general formula

RC(=O)CH3 (where R is an alkyl or alkenyl radical). Fisher et al. [8] have studied the

oxidation of methyl butanoate in an effort to better understand the oxidation of the long

chain FAMEs. The authors state, ”methyl butanoate was chosen as a surrogate molecule

for the larger methyl esters, in order to obtain a reaction mechanism of manageable size.

Yet methyl butanoate is large enough to allow fast RO2 isomerization reactions important

in the low-temperature chemistry that controls fuel autoignition under conditions found

in diesel engines.”

Figure 2.8 displays the key low temperature and high temperature pathways for the

oxidation of methyl butanoate. Initially, the fuel molecule undergoes H abstraction to

form one of four possible radicals. In this case, H is abstracted from the carbon alpha to

the carbonyl group (reaction 1). The resulting radical species can then follow one of two

pathways depending on the temperature. At high temperatures the radical decomposes

by β-scission to produce one stable molecule and a CH3 radical (reaction 2). On the

other hand, at low temperatures the radical reacts with O2 to produce a complex radical

(reaction 3). An isomerization reaction then occurs to shift a hydrogen to the O2 addition

products (reaction 4). This species can then follow one of two reaction pathways. In this

first instance, the molecule decomposes unimolecularly to form an OH radical, ethylene,

and an epoxide-like species (reaction 5). In the second instance, O2 is added to the

molecule (reaction 6), which leads to a series of chain branching reactions (reactions 7

and 8).

An attempt to validate the mechanism was made by testing it against existing com-

bustion data, consisting of pressure measurements. However, the experimental data was

not well characterized for model validation. Therefore, the authors urged researchers

to obtain more complete experimental data so that the mechanism could be rigorously

tested.


Figure 2.8: Main reaction pathways for the oxidation of methyl butanoate from Fisher

et al. [8]

2.6 Mechanism of Soot Formation in Combustion

Processes

The term soot refers to tiny amorphous carbon particles produced from the combustion

of hydrocarbon fuels. Soot emissions have become an environmental concern due to the

negative impacts of particulate matter on the human respiratory system. In addition, soot

particles in the atmosphere can contribute to global warming by altering the radiative

balance of the atmosphere [45, 46].

The presence of soot particles in hydrocarbon flames is identifiable by their char-

acteristic yellow-orange appearance. This color is generated by photons emitted from

the solid carbon particulates. On the other hand, flames that do not contain soot are


blue in color. The formation of soot particles in a combustion environment is a highly

complex mechanism and depends on a number of factors, e.g. fuel composition, temper-

ature, fuel-oxygen ratio, flame configuration, etc.. However, the formation of soot has

been shown to be highly controlled by chemistry related phenomenon [47]. Glassman

[38] has summarized the work of other researchers to describe the dominant route of soot

formation.

Initially, the fuel breaks down to acetylene, as mentioned in the previous discussion

on hydrocarbon oxidation. In the high-temperature post-flame regime, soot formation is

initiated by the growth of small straight-chain alkenes (acetylene) to small aromatic com-

pounds (e.g. benzene). The aromatic hydrocarbons then react sequentially with smaller

hydrocarbons (acetylene, in particular) to form larger polyaromatic hydrocarbon (PAH)

species. Gaseous PAH molecules continue to nucleate until the smallest identifiable soot

particles appear, with diameters of a few nanometers.

Figure 2.9 [38] shows the mechanism for soot formation in more detail. Initially, the

acetylene (C2H2) undergoes H abstraction to form the vinyl radical. The vinyl radical

then reacts with another acetylene molecule to form the 1,3-butadienyl radical. The

1,3-butadienyl radical can also be readily formed from C4 hydrocarbons via hydrogen

abstraction then β-scission.

In diffusion flames, the alternate route A is then followed, wherein the 1,3-butadienyl

radical reacts again with acetylene to form the cyclic phenyl (C6H5) radical, following

ring closure. The phenyl radical is essentially a benzene molecule missing one hydrogen

atom. The phenyl radical is also produced by alternate route C, wherein methyl acetylene

(propyne C3H4) pyrolyzes rapidly to form the aromatic. The phenyl radical can proceed

to grow into a larger aromatic via the two-step hydrogen abstraction-carbon addition

(HACA) mechanism. In the HACA mechanism, the aromatic molecule is converted to

a radical by hydrogen abstraction, and then grows in size by the (carbon) addition of

an acetylene molecule. The HACA mechanism continues until larger PAH molecules

appear. As the concentration of gaseous PAH species increases, nucleation occurs and

soot particles begin to appear.

This thesis study does not directly measure the soot levels generated in biodiesel

diffusion flames. However, the concentrations of many soot precursors are measured,

specifically, acetylene, C4 hydrocarbons, 1,3-butadiene, and benzene. Analyzing the con-

centrations of these species in the diffusion flame can offer an insight into the sooting

potential of various biodiesel fuels.


Fig

ure

2.9:

Gen

eral

mec

han

ism

for

soot

form

atio

nfr

omG

lass

man

[38]

Chapter 3

Experimental Apparatus and

Analytical Methodology

The details of the experimental setup and the analytical techniques employed are dis-

cussed in this chapter. Figure 3.1 is a schematic of the setup. The purpose of the setup

was to generate an opposed flow diffusion flame from the liquid fuel feed stock (i.e. methyl

butyrate or methyl crotonate). The concentrations of stable species and the temperature

profile in the flame were then obtained. The setup consists of the fuel delivery system,

the opposed flow diffusion flame burner, the sample collection apparatus, and a number

of analytical instruments. The fuel delivery system pumped the liquid fuel into a vaporiz-

ing unit that produced a gaseous fuel mixture. The gaseous fuel stream and the oxidizer

stream then flowed into an opposed flow burner to produce a planar flame. Samples

were extracted from the flame region using a quartz micro-probe connected to a vacuum

pump. The hydrocarbon species and several oxygenated compounds were analyzed by a

gas chromatograph (GC) coupled with a flame ionization detector (FID). Carbon diox-

ide and carbon monoxide were quantified by a non-dispersive infra-red (NDIR) analyzer.

Carbonyl compounds were measured by a high pressure liquid chromatograph (HPLC)

coupled with a ultraviolet-visible (UV-Vis) spectrophotometer. The flame temperature

was measured by an R-type thermocouple.

32

Chapter 3. Experimental Apparatus and Analytical Methodology 33

Fig

ure

3.1:

Sch

emat

icof

the

exper

imen

talse

tup


3.1 Opposed flow diffusion burner setup

Opposed flow diffusion flames offer greater experimental and modeling advantages than

co-flow flames, although their stability is more sensitive to flow conditions [38]. The

opposed flow configuration results in a flat flame which simplifies flame analysis to a one-

dimensional system. Therefore, the species concentration and temperature are a function

of axial distance only. Furthermore, the modeling of fluid mixing is simplified in laminar

flames because fuel and oxidizer mixing is limited to diffusional processes (i.e. turbulent

mixing patterns are not considered).

The experiments utilized two identical flat flame burners1 with circular burner ports.

The two burner ports are placed opposite each other in the same vertical plane. Fuel was

fed through the bottom port while oxygen was fed through the top. The two opposing

streams flow into each other to create a stagnation plane between the two ports. The

vertical location of the stagnation plane depends on the momentum of the two streams.

The stagnation plane prevents any non-diffusional mixing between the fuel and oxidizer.

The mixture was ignited to create flat stoichiometric flame lying slightly above the stag-

nation plane. The exact location of the flame front depends on the mass fraction of fuel

and oxidizer in the fuel stream and oxidizer stream.

A diagram of the burner port is shown in Figure 3.2. Each burner port consists of a

stainless steel housing enclosing a porous sintered bronze matrix. The porous material

is divided into inner and outer coaxial cylinders of diameter 25.4 mm and 38.1 mm,

respectively. The inner cylinder directs the fuel or oxidizer streams towards each other,

and the outer annulus can be used to create a nitrogen shroud around the flame for

minimizing external flow disturbances. The annulus’ shrouding feature was not exploited

in this study. The temperature of the gases flowing through the ports is controlled by

circulating water through porous plugs packed between two co-axial cylinders. This

ensures that the gases have a uniform laminar flow and flat velocity profile at the port’s

surface. The water is maintained at 70 ◦C and circulated by a heating recirculator2.

Condensation of fuel vapor in the bottom port is prevented by maintaining the port

temperature near 80 ◦C with heating tapes3.

1Purchased from Holthuis & Associates McKenna Flat Flame Burners2PolyScience Heating Recirculator Model 2103OmegaR© FGS Standard Insulated High Temperature Heating Tapes


Figure 3.2: Diagram of burner port

A photograph of the actual burner setup is shown Figure 3.3. The two burners

are coaxially mounted 20 mm apart facing each other. A custom-built aluminum holder

provides support to the burners, and a clear quartz shroud 22 cm in diameter protects the

flame from airflow disturbances from the surrounding environment. The entire assembly

is mounted atop a Newport translation stage which moves along the vertical axis with

the rotation of a micrometer knob. This made it possible to obtain a vertical profile of

the flame temperature and emissions between the two burner ports.


Figure 3.3: Photograph of burner setup

3.2 Fuel Preparation and Vaporization

The fuel stream fed to the burner was a mixture of the biodiesel surrogate fuel (i.e.

methyl butanoate or methyl crotonate) and nitrogen gas. Properties of each surrogate

fuel are shown in the following Table 3.1, which indicates that both fuels are liquids at

room temperature. Excess dissolved gases were eliminated by degassing the fuel in an

ultrasonic bath4. The fuel was heated in the water bath to 70 ◦C, and then ultrasonic

sound waves were applied until tiny bubbles no longer rose to the surface of the liquid5.

The fuel was then pumped from a bottle to an ultrasonic atomizer. The atomizer probe

sprays the fuel into a stainless steel mixing chamber where it mixes with preheated

nitrogen gas. The mixture of fuel and nitrogen was then fed to the bottom burner port.

The fuel was pumped from its stock bottle by a peristaltic pump head driven by

a motor6. The pumping system was calibrated for each fuel at the beginning of each

4Cole-Parmer 8890 Ultrasonic Cleaner5Approximately 60 minutes6Cole-Parmer MasterflexR© L/S Microprocessor Pump Drive


Fuel Compound Boiling point range

(degrees C)

Purity

(%)

Specific gravity

Methyl Butanoate 100-103 99 0.898

Methyl Crotonate 116-120 98 0.945

Table 3.1: Physical properties of the fuels used

experiment. This involved using a graduated cylinder to measure the volume of liquid

pumped, and then setting the pump to correspond with the measured volume. Initially,

a polycarbonate pump head housing and VitonR© tubing were used to dispense the fuel;

however, it was found that methyl butanoate was reacting with the materials and causing

component failures. This problem was overcome by replacing the damaged components

with parts made of PTFE7. PTFE is inert to methyl butanoate and methyl crotonate.

The pumped fuel was delivered to a ultrasonic atomizer8 unit that breaks the fuel

into micro-droplets. The unit’s ultrasonic power supply converts 60 Hz energy to a high

frequency electrical energy at 40 kHz. Then, a piezoelectric transducer converts the

electrical energy to mechanical vibrations. The vibrations are intensified in a probe and

focused at its tip. The liquid fuel is dispensed in the probe, where it spreads out as a

thin film on the tip. The oscillations at the tip atomizes the liquid into micro-droplets

to form a gentle, low viscosity mist. The median droplet size is 45 microns [48].

A picture of the fuel vaporization unit is shown in Figure 3.4. The atomized fuel is

sprayed into a mixing chamber, where it mixes with nitrogen gas9 . The nitrogen was

preheated by an inline electric process heater10, and then delivered to the mixing chamber

via 1/4 inch copper tubing. The temperature in the chamber and all copper tubing was

maintained at 85 ◦C using heating tapes11. The high temperature in the mixing chamber

served to vaporize the fuel droplets. The temperature inside the chamber is measured

using a stainless steel sheathed K-type thermocouple inserted at the top of the chamber.

7Polytetrafluoroethylene or TeflonR©

8Sonics VibracellTM VC 134 AT9Grade 4.8

10OmegaR© process heater11OmegaR© FGS Standard Insulated High Temperature Heating Tapes


The mixing chamber is a custom-made stainless steel column12 30 cm in length and 6

cm in outer diameter. The lower half of the column is packed with a 15 cm long bed of

glass marbles. The bed increases the path length traversed by gaseous fuel and nitrogen

molecules; thereby, increasing mixing of the two streams.

Figure 3.4: Photograph of fuel vaporization column

Table 3.1 indicates that neither fuel used in these experiments is a pure component. In

fact, each fuel contains 1-2% of viscous substances which would not vaporize in the mixing

chamber. These greasy viscous substances were found to clog the burner’s sintered porous

matrix. Therefore, an inline coalescing filter13 was installed at the exit of the mixing

chamber to remove these impurities. The filter assembly consists of a filter element, an

aluminum housing, and an EPDM14 o-ring. The filter element is made of borosilicate

glass micro-fibers bound by a silica inorganic resin. The filter coalesces and removes

99.5% of 0.1 micron sized particles. The purified gaseous mixture of fuel and nitrogen

flows to the bottom port of the burner apparatus via a heated15 1/4 copper tube.

12Designed and constructed at Dept. of Mechanical and Industrial Engineering Machine Tool Lab13Manufactured by United Filtration Systems14Ethylene Propylene Diene Monomor15OmegaR© FGS Standard Insulated High Temperature Heating Tapes


3.3 Supply of Fuel and Oxidizer Streams

The flow rates of fuel and oxidizer streams through the burner ports were key parameters

in these experiments. It is important that the momentums of the two streams be nearly

equal, so that a stagnation plane is created where the streams meet. The molar concen-

trations of fuel and oxidizer in each stream needs to be sufficient enough to light a flame;

however, high sooting flames are not desired. The flow rates of nitrogen, oxygen and

air were controlled using rotameters, while the flow rates of liquid fuel were controlled

by a peristaltic pump. The inlet oxidizer and fuel stream concentrations were selected

based on the following criteria: i.) a low sooting flame is preferred to prevent clogging

of sampling probe; ii.) a very hot flame that will damage the probe is unwanted; iii.) a

balanced momentum of the two streams is required to form a stagnation plane at their

intersection; iv.) excessive unburned fuel is not desired; v.) at the flame plane, an N2/O2

ratio near that of air is desired to make the study relevant to actual flames.

The molar composition of the fuel stream was 4.72% fuel (methyl butanoate or methyl

crotonate) and 95.28% nitrogen. Additional flow parameters are given in Appendix A.

The volumetric flow of fuel to the vaporizing column was chosen to obtain an identical

molar flow rate for both fuels. Since the two fuels have different liquid densities, the vol-

umetric flow rates were different. Methyl crotonate was dispensed at 0.67 ml/min, while

methyl butanoate was dispensed at 0.72 ml/min. In our experiments, the temperature

of the fuel stream exiting the bottom burner port was 80 ◦C.

The molar composition of the oxidizer stream was 57.75% nitrogen and 42.45% oxy-

gen16. Unexpectedly, the temperature of the unheated oxidizer stream exiting the top

burner was near 141 ◦C. The cause of this increased temperature was heat convected

towards the top burner port from rising combustion product gases.

3.4 Gas Sampling System

The previous sections discussed the procedure for producing an opposed flow diffusion

flame. Once the flame was generated, a sampling system was used to obtain qualitative

and quantitative information about the flame’s characteristics. Specifically, the species

concentrations at various points between the two burner ports was measured to obtain

characteristic profiles. The following sections discuss the gas sampling system, as well as

16Grade 4.3


the sampling procedure.

3.4.1 Sampling Apparatus

Microprobes, due to their small perturbation of flow fields, are commonly used in flame

studies to acquire the concentration of stable species [49, 50, 51, 52]. The gas sampling

system in these experiments consists of a quartz microprobe connected to a dual-stage

pump17 via 1/4 inch PTFE tubing, a vacuum pressure gauge, and an inline filter. The

microprobe is mounted on a sliding stage, allowing it to move into and out of the flame

region easily. A schematic of the microprobe is shown in Figure 3.5. The first stage

(vacuum) of the pump creates a suction in the sampling line to withdraw gas samples

from the flame. An analytical instrument was connected downstream of the second stage

(compressor) of the pump to study the gases flowing through the line. The compressor

head on the pump pushes samples into the analytical instrument. A two-way valve,

installed downstream of the pump, controls gas flow to the instrument.

OD: 585 µm

ID : 202

3.75 6.35

300 mm (approx.)

Figure 3.5: Schematic of microprobe (not to scale)

Previous studies have identified precautions to take when using the microprobe sam-

pling technique. The primary objective is to eliminate chemical reactions within the

probe and sampling lines. A combination of rapidly reducing temperature and pressure

in the probe helps meet this objective.

Kassem and coworkers [53] studied the effect of microprobe cooling on fuel-rich, lam-

inar flat flames of chlorinated hydrocarbons. Their results indicate that cooled probes,

17KNF oil-free dual-staged pump Model UN035.3 ST11 with heated heads


as opposed to uncooled probes, provided more accurate profiles of species concentra-

tion. Schoenung and Hanson [49] showed that carbon monoxide (CO) measurements

in the post-flame region of a premixed methane/air flame were affected by the pressure

within the probe and sampling lines. Their results indicate that the CO concentration

increases as the pressure in the probe decreases, with the concentration reaching equi-

librium around 50 mm Hg. This finding suggests that CO is converted to CO2 in the

probe region unless the pressure is 50 mm Hg or below. Therefore, low temperatures and

pressures in the probe are required to quench the reactions.

Fristrom and coworkers [54] argue that it is not rapid temperature drop that is re-

quired for successful flame sampling. Instead, a combination of rapid pressure drop and

the destruction of radicals on the probe walls is responsible for quenching reactions at

the probe tip. They explain that 2nd order molecule-radical reaction rates vary with the

square of the gas density, which varies linearly with pressure; therefore, the reaction rate

decreases with a decrease pressure. Furthermore, they argue that if a rapid temperature

drop is induced while keeping pressure constant, then reactions with activation ener-

gies of 20 kJ/mol or lower will increase in rate. These reaction rates vary quadratically

with density, which varies inversely with temperature at constant pressure; therefore,

decreasing the temperature alone would not quench all flame reactions.

The experiments in this study used a quartz microprobe fabricated in the Department

of Chemistry Glass Blowing Shop. The inner diameters (ID) of the probe’s tip could not

be precisely controlled; therefore, a number of probes were made and measurements were

taken to select the best one. The probe tip ID was measured using a travelling microscope

mounted on a metric scale. A previous study by Syed [9] determined the appropriate

probe tip size. The study measured CO2 concentrations in a propane-air flame using

probe tips with various IDs. Syed suggests that a probe tip with an ID of approximately

180-220µm is ideal. Probes with larger IDs do not successfully quench reactions at the

probe tip. In addition, probes with smaller IDs restrict gas flow through the sampling

line, such that it is not possible to take measurements.

As mentioned previously, the microprobe was connected to a doubled-headed pump

via 1/4 inch PTFE tubing. The vacuum pump head creates a vacuum pressure of 710-730

mm Hg18, which is sufficient to quench most reactions. The probe was not cooled since

previous studies have shown that a rapid pressure drop alone provides accurate sampling.

18Gauge vacuum pressure measured by a vacuum pressure gauge


Extra precautions were taken to eliminate leakage of gases from the surrounding

environment into the sampling line. Leaks into the sampling system dilute the sample

gas and lead to incorrect measurements. For this reason, SwagelokR© fittings are used for

all connections. Leaks into the sampling line were detected by using a container filled

with dry ice (i.e solidified carbon dioxide). The container was placed near a suspected

point of leakage, and carbon dioxide gas fills the surrounding area. The NDIR analyzer

(see section 3.5.1) was connected to the sampling line and the dual-stage pump was

turned on. If a spike in CO2 concentration was observed on the analyzer, then there

was a leak present in the sampling line. The necessary steps were then carried out to

eliminate the leak (e.g. the fittings were changed, the tube was replaced, etc.).

3.4.2 Sampling Procedure

The sampling objective was to obtain flame measurements at various points along the

vertical axis separating the two burner ports. Thus, the sliding stage, on which the

microprobe is mounted, was inserted between the two burner ports. A window was cut

into the quartz shroud enclosing the burner setup to permit insertion of the sampling

probe. Figure 3.6 is a schematic of the probe and burner setup. The tip of the probe

was placed approximately 1.5 mm behind the central vertical axis separating the burner

ports. This allows samples to be withdrawn from the middle points of the flame region.

After insertion, the probe was held stationary, while the burner assembly was moved

along the vertical axis with the turn of a micrometer knob on the translation stage. One

complete turn of the micrometer knob corresponds with a 0.5 mm change in distance.

Clockwise rotation moves the burner assembly up.

A measuring system was used to define the exact position of the microprobe between

the two burner ports. For this purpose, the bottom (fuel) port is taken as the zero height,

while the top (oxidizer) port is taken as the maximum height. The position of the probe

was zeroed by touching its tip to the bottom port. The reading on the micrometer knob

was thus noted as the zero distance. Each counterclockwise turn of the micrometer knob

moved the burner assembly down; thus, increasing the distance between the probe and

the fuel port, i.e., moving the probe upwards. The exact height at which the probe

withdraws samples is equal to the total distance travelled plus the outer radius of the

probe.

Once the probe was positioned at the desired height, the dual-stage pump was turned


Fuel Port

Oxidizer Port

Sliding Stage

Microprobe

Burner Platformwith rotating micrometer

Microprobe moves Horizontally

Burner moves Vertically

Probe tip behind central vertical axis

Figure 3.6: Schematic of microprobe and burner setup (not to scale)

on and gases withdrawn from the flame filled the sampling line. The sampling lines were

purged before any analytical measurements are taken. The purge time varies depending

on the location of the probe in the flame. Sampling points away from the center of

the flame require short purge times (e.g. 2-3 minutes), since the low temperature, high

density gases permit high flowrates. In contrast, sampling points near the center of the

flame require longer purge times (e.g. 15 minutes), since gases in this high temperature

region have an extremely low density and permit low flowrates. The sampling line filled

with gases from the specified flame region were then ready for analysis.

3.5 Analytical Tehniques

The hydrocarbon species and several oxygenated compounds (e.g. methanol, methyl

butanoate, acetone, etc.) were analyzed by gas chromatography coupled with a flame

ionization detector (GC/FID). Carbon dioxide and carbon monoxide were quantified by

non-dispersive infra-red (NDIR) analysis. Carbonyl compounds (e.g. formaldehyde, ac-

etaldehyde, and acrolein) were measured by high pressure liquid chromatography coupled

with an ultraviolet-visible spectrophotometer (HPLC/UV-Vis). The flame temperature

was measured by an R-type thermocouple.


3.5.1 Non-Dispersive Infrared Analysis

Non-dispersive infrared (NDIR) analysis is a technique used to measure gas concentra-

tions based on the energy absorption characteristics of a gas in the infrared red region.

Figure 3.7 is a schematic of a typical NDIR instrument [55]. The NDIR instrument passes

infrared (IR) light through two identical tubes, in parallel, and then onto a detector. The

first tube is filled with nitrogen, which does not absorb IR light and serves as a reference

cell. The second tube contains the sample gas, which absorbs IR energy. The detector

measures the difference in energy between the two streams of IR light [55]. This differ-

ence is the absorption, which is proportional to the concentration of sample gas by the

Beer-Lambert Law in Equation 3.1:

A = ε · b · c (3.1)

where

A is the gas absorbance.

ε is the molar extinction coefficient (concentration−1 · length−1).

b is the path length that the beam travels in the sampling tube.

c is the gas concentration.

Figure 3.7: Schematic of NDIR instrument from [55]


CO and CO2 Measurements

The NDIR instrument19 was used to quantify levels of CO and CO2 in the flame samples.

The instrument is capable of measuring concentrations from 0% to 10.5%. It contains

an upper and lower screen displaying the concentration of CO and CO2, respectively.

Initially, the instrument was zeroed and calibrated. To zero the instrument, the built-in

pump was started to allow air flow through the unit; then the zero adjustment knob

was turned to set the display to zero. Next, the unit was calibrated with a gas mixture

containing 8.9% CO and 9.1% CO2. The gas mixture was passed through the detector,

and the span adjustment knob was turned to match the calibration gas concentration.

Flame measurements were taken by using the dual-stage pump to withdraw samples

from the flame and then pushing them into the NDIR analyzer. The analyzer’s built-

in pump was not used during sampling. The concentrations were recorded once the

displayed reading remained constant for 5 minutes. As mentioned previously, samples

withdrawn from points away from the flame center have higher flowrates than samples

from within the middle of the flame. Therefore, the analyzer’s display reading stabilizes

much quicker for the former (e.g. 1-3 minutes) than the latter (e.g. 15-20 minutes).

3.5.2 Gas Chromatography

Gas chromatography (GC) is a technique used for separating volatile organic compounds

based on their differences in partitioning between a flowing mobile phase and a stationary

phase. The GC method was used to measure C1 - C5 alkanes, alkenes, and alkynes,

benzene, methyl butanoate, methyl crotonate, methanol, and acetone. A GC instrument

consists of a flowing mobile phase (carrier gas), a stationary phase (separation column),

an injection port, an oven, and a detector [56]. A schematic of the GC instrument is

shown in Figure 3.8.

The carrier gas carries the sample gas through the separation column, and compounds

are separated due to partitioning between the two phases. Since partitioning behavior is

a strong function of temperature, the separation column is placed inside a temperature-

controlled oven. Separation of compounds with a range of boiling points is achieved by

starting at low temperatures and then increasing the temperature until high boiling point

compounds are eluted. The injection port is always maintained at a temperature higher

19NOVA NDIR Analyzer Model 7800P2A


Data Acquisition System

Separation Column(stationery phase)

Column OvenCarrier Gas

(mobile phase)

Injector Detector

Figure 3.8: Schematic of GC instrument

than the boiling point of the least volatile compound in the mixture [56]. A 10% sorbitol

pre-column is installed prior to the separation column to remove moisture and oxygen

contaminants from the sample gas.

The amount of time a given component spends in the separation column is called the

retention time. The retention time of a given component remains the same provided the

mobile phase, stationary phase, temperature control, and gas flowrates remain constant.

As each component of the separated sample falls onto the detector a quantitative response

in the form of a peak is generated. A series of peaks with the retention time on the x-axis

and the detector (e.g. voltage) on the y-axis is called a chromatogram (see Appendix C

for an example). The peak retention time is used to identify each compound, and the

peak area is used to determine the quantity of the compound.

The instrument used in these experiments was a Varian 3800 GC20 with a 1079 injector

and a flame ionization detector (FID). Figure 3.9 shows a schematic of an FID [57]. The

FID consists of a hydrogen/air flame and a collector plate. As gases flow from the

separation column, the flame burns organic molecules to produce ions. The ions are

attracted to the collector plate, which generates a voltage depending on the quantity

of ions collected [57]. Additional details of the GC setup are discussed in the follow

subsections.

20Remotely controlled by a PC using STAR Chromatography Worstation 4.5


Figure 3.9: Schematic of FID from [57]

GC Carrier Gas

The carrier gas (mobile phase) used for the GC was 99.997 % helium. Hydrocarbon and

oxygen traps were placed before the column to filter out any contaminants. The flowrate

of helium through the separation column was varied depending on the compounds being

studied. Details are available in subsection “GC Measurement Procedures” below.

Injection System

The purpose of the injection system is to load the sample gas onto the separation column.

The GC has a 1079 universal capillary injector, which can be run in several modes based

on the type of injector insert used. The injector temperature was set at 200 ◦C to

prevent condensation of sample components. An unpacked 3.4 mm ID insert for split

mode operation was used. Split mode operation is discussed below. Flame samples are

pushed into the GC sampling line by the dual-stage pump. A valve downstream of the GC

sampling line was initially closed to increase the sample pressure in the line. The sample

pressure, measured by an in-line pressure gauge, was allowed to reach approximately 8.7

psia. At this point the dual-stage pump was shut off and the GC sampling line was closed

to isolate the sample for injection to the GC column. The flow of sample gas and carrier

gas into the column is then controlled by the gas sampling valve (GSV) and the split


valve.

The GSV is a 10 port rotary valve which directs the sample and carrier gases into

the injector. Schematics of the GSV are shown in Figures 3.10 and 3.11. The GSV has

two positions; the fill position and the load position. In the fill position, the sample

gas flows through the 0.25 ml sample loop, while carrier gas flows through the sorbitol

pre-column, injector, and separation column. As the GSV turns to the load position, the

sample trapped in the loop comes in-line with the carrier gas flow. The sample is carried

through the sorbitol pre-column and into the injector. From the injector, it is directed

into the column. The GSV’s duration in the load position is a user controlled parameter,

usually less than a minute. As the GSV returns to the fill position, the normal flow

pattern resumes.

Helium To injector

1109

2

345

6

7

8

Sorbitol

precolumn

GSV ventHelium

Sample in

Sample out

Sample

loop

Figure 3.10: Schematic of injector in the fill position

1109

2

3

45

6

7

8

Sorbitol

precolumn

GSV vent

Helium To injector

Helium

Sample in

Sample out

Sample

loop

Figure 3.11: Schematic of injector in the load position


The sample gas in the injector can be introduced to the column via split mode or

splitless mode. In splitless mode, the entire sample is loaded onto the column. This

mode is advantageous for detecting trace compounds in the sample gas. However, highly

concentrated samples can damage the separation column. Therefore, split mode opera-

tion was used here, wherein only a portion of the sample entered the column. A special

3.4 mm ID glass insert for split mode operation was used in these experiments. In split

injection, a solenoid is activated such that only a portion of the sample gas flows into the

column while the remainder is vented by the carrier gas flowing through the split vent.

The split ratio is calculated as the ratio of the carrier gas flow through the column to the

carrier gas flow through the split vent (see Tables 3.3 and 3.4 for split ratio values). The

flow of carrier gas through the split vent and separation column is manually adjusted

by the split ratio flow controller and the back pressure regulator, respectively. Table 3.2

indicates the settings used for the split valve and GSV.

Time Interval

(min)

Gas Sampling Valve

Setting

Split Valve

Setting

0 Fill Split

0.1 Inject Split

1 Fill Split

Table 3.2: Settings for the gas sampling valve and split valve

GC Measurement Procedures

Two different GC measurement procedures were developed to analyze the flame sam-

ples. The first method was designed to measure C1-C5 hydrocarbons using a HP-Al/S

PLOT capillary column21. The second method measured methyl butanoate, methyl cro-

tonate, methanol, and acetone using a DB-624 column22. Two different columns were

used because neither is capable of separating both hydrocarbon and oxygenate species.

The PLOT column is ideal for separating hydrocarbon compounds that are gases are

room temperature. The DB-624 column is suitable for the separation of oxygenated

compounds. The two columns were not mounted in parallel. Instead, first hydrocarbons

21Agilent Technologies HP-Al/S 50 m x 0.53 mm (L x ID)22Agilent Technologies DB-624 30 m x 0.32 mm (L x ID)


were measured, and then the columns were switched for sampling of oxygenates.

Methodology for Analysis of Hydrocarbons

The PLOT column has a stationary phase comprised of alumina deactivated with sodium

sulfate. The column’s length, inner diameter, and film thickness are 50 m, 0.53 mm, and

15 µm, respectively. Additional separation characteristics of the column are shown in

Appendix C.

A column oven temperature program was designed to obtain quick separation of low

molecular weight compounds (e.g. methane, ethane, etc.) and high molecular weight

compounds (e.g. benzene). The column oven temperature program is shown in Figure

3.12. The initial temperature is held at 50 ◦C to separate the more volatile species. A

gradual increase in temperature (15 ◦C/min) then forces less volatile compounds out the

column. Finally, the temperature is held at 200 ◦C to elute the remaining components

with very high boiling points. A sample chromatogram using this column and oven

temperature program is shown in Appendix C.

Figure 3.12: Oven temperature program for measuring hydrocarbons using HP Al/S

PLOT column

The inlet pressure of hydrogen, air, and helium were 40, 60, and 80 psig, respectively.

The flow of gases in the GC and FID are shown in Table 3.3.


Split Vent Flow (He)

Carrier Gas Flow

(He)

Carrier + Make up

(He)

Hydrogen for FID

Air for FID

Flowrate (mL/min)

18.0 5.0 30 30 300

Split ratio = 1 : 3.6

Table 3.3: Flow rate of GC gases when measuring hydrocarbons

Methodology for Analysis of Oxygenates

The DB-624 column was chosen for measuring oxygenates because the mid-polarity sta-

tionary phase provides good separation of alcohols, ketones, and aldehydes. The column’s

length, inner diameter, and film thickness are 30 m, 0.53 mm, and 1.8 µm, respectively.

Additional properties and separation characteristics of the column are shown in Appendix

C.

The oven temperature program was designed to separate oxygenated compounds.

The column oven temperature program is shown in Figure 3.13. The initial tempera-

ture is held at 70 ◦C to separate the lighter, less polar species. A gradual increase in

temperature (15 ◦C/min) then forces heavier, more polar compounds out the column.

Finally, the temperature is held at 200 ◦C to elute the remaining components. A sample

chromatogram using this column is shown in Appendix C

The inlet pressure of hydrogen, air, and helium were 40, 60, and 80 psig, respectively.

The flow of gases in the GC and FID are shown in Table 3.4.

Split Vent Flow (He)

Carrier Gas Flow

(He)

Carrier + Make up

(He)

Hydrogen for FID

Air for FID

Flowrate (mL/min)

100 2.0 30 30 300

Split ratio = 1 : 50

Table 3.4: Flow rate of GC gases when measuring oxygenates


Figure 3.13: Oven temperature program for DB-624 Column

GC Calibration Procedure

The GC was calibrated for hydrocarbons using four different ScottyR© calibration gas

mixtures: 100 ppm C1-C6 alkanes in nitrogen; 100 ppm C2-C6 alkenes in nitrogen; 15 ppm

C2-C4 alkynes in nitrogen; and 100 ppm benzene in air. The calibration was performed

by flowing each gas mixture directly into the GC sampling line. The operating conditions

for the GC were identical to those aforementioned.

GC calibration for the oxygenated species involved using the fuel delivery and burner

setup because gas cylinders were not available. The liquid oxygenate was pumped into

the vaporization and diluted with nitrogen gas. The exact concentration of the nitrogen-

oxygenate mixture was determined from the nitrogen gas flowrate and the liquid flow rate

into the mixing column. This “calibration gas” was flowed to the bottom burner port, and

samples were obtained using the microprobe sampling technique. The aforementioned

GC sampling method was used to calibrate the system.


3.5.3 High Pressure Liquid Chromatography

High pressure liquid chromatography (HPLC) was used to measure concentrations of

formaldehyde, acetaldehyde, and acrolein in the flame. This technique was selected be-

cause quantification of formaldehyde by GC/FID is not recommended[58]. Using HPLC

for analysis of the aforementioned carbonyl compounds is mentioned in the literature[59,

60]. The procedure for analysis is outlined in the ASTM Standard Test Method for De-

termination of Formaldehyde and Other Carbonyl Compounds in Air[61]. The method

involves drawing sample gases through a cartridge coated with 2,4-dinitrophenylhydrazine

(DNPH) reagent. The carbonyl species form stable derivatives with the DNPH reagent,

and are subsequently eluted from the cartridge. The derivatized liquid sample is analyzed

for the parent carbonyls using HPLC.

A schematic of the HPLC instrument is shown in Figure 3.14. The instruments

consists of a reservoir of mobile phase, a pump, an injector, a separation column, and

a detector. The mobile phase is a mixture of polar and non-polar solvents. It carries

the injected sample through the stationary column and compounds are separated due to

partitioning between the two phases. The partitioning behavior is a strong function of

polarity, so separation of compounds with a range of polarities is obtained by changing

the mobile phase mixture.

Figure 3.14: Schematic of HPLC Setup from [62]


The amount of time a given component spends in the separation column is called

the retention time. The retention time of a given component remains the same provided

the mobile phase, stationary phase, temperature control, and liquid flowrates remain

constant. As each component falls onto the detector a quantitative response in the form

of a peak is generated. A series of peaks with the retention time on the x-axis and the

detector ouptut (e.g. voltage) on the y-axis is called a chromatogram (see Appendix C

for an example). The peak retention time is used to identify each compound, and the

peak area is used to determine the quantity of the compound.

The HPLC system used in this study was a Perkin Elmer series 200 quatenary LC

pump coupled with a Perkin Elmer 785A UV/Vis detector. The instrument was located

in the Department of Chemistry ANALEST Laboratory. Samples are injected by a Perkin

Elmer auto-sampler through a Rheoudyne injector. The TurboChrome client/server soft-

ware package was used to remotely control the HPLC equipment. Additional details of

the HPLC technique are discussed in the following subsections.

DNPH Sampling Procedure

Samples of carbonyl compounds were obtained by passing the sample gas through DNPH

coated sampling cartridges23. Two cartridges (i.e. main and backup) were connected,

in series, downstream of the dual-stage pump, and the pump was started. The flowrate

of the sample was measured with a bubble meter downstream of the cartridges. The

total amount of gas passing through the cartridges is the product of the flowrate and the

sampling time. The second cartridge (i.e. backup) was used to identify breakthrough of

carbonyl compounds from the first cartridge.

A liquid sample was then obtained by elutriating the cartridge with 5 mL of 99% pure

acetonitrile. The eluted sample was transferred to a 25 mL vial and stored at 4 ◦C. Vials

with septum caps were used to facilitate auto-sampling. The liquid solutions containing

acetonitrile and carbonyl derivatives were then analyzed using the HPLC technique.

Each DNPH cartridge is capable of holding approximately 75 mg of total carbonyls.

Tests were conducted to determine the ideal gas sampling time required to prevent ex-

ceeding the cartridge capacity. The experiments involved lighting a methyl butanoate

flame and varying the sampling duration. Samples were obtained from two points be-

tween the burner ports; one point is near the fuel port (2 mm from the fuel port) and one

23SUPELCO LpDNPH S10 cartridges


point is near the flame front (7.5 mm form the fuel port). The point near the fuel port

represents a region of high flowrate and low concentration of carbonyls, while the point

near the flame represents a point of low flowrate and high concentration of carbonyls.

The main and backup cartridges were then tested using the HPLC technique. The pres-

ence of species on the backup cartridge indicates a failure condition, with breakthrough

of samples from the first cartridge. Table 3.5 summarizes the results.

Distance from fuel port(mm)

Duration(min:sec)

Breakthrough(Yes/No)

2 2:42 No

2 3:22 No

2 5:02 No

2 7:12 Yes

2 10:30 Yes

7.5 2:02 No

7.5 3:45 No

7.5 4:33 Yes

7.5 6:29 Yes

7.5 10:12 Yes

Table 3.5: Determination of ideal sampling times for carbonyl compounds using the

DNPH technique

It was determined that the ideal sampling duration is between 2 and 3 minutes for

all points in the flame. All subsequent experiments used this sampling duration. Backup

cartridges were always used during gas sampling. They were then stored at 4◦C, and a

few were randomly selected for testing on the HPLC. Backup cartridges were also selected

for HPLC testing if the mass of total carbonyl compounds on the corresponding main

cartridge was greater than 60 mg. However, the failure condition was never observed for

sampling times between 2 and 3 minutes. Following these procedures ensured accurate

sampling of carbonyl compounds using the DNPH cartridges.

HPLC Measurement Procedure

Before sampling, the mobile phase solvents were degassed by bubbling helium for 5-6

minutes. The flow lines were then purged with water and acetonitrile, separately. After

purging, the mobile phase flow was brought in-line with the separation column. A 0.5


µm frit and a guard cartridge system24 were used to prevent damage to the separation

column. The system was allowed to equilibrate by pumping 60% acetonitrile and 40%

water at 1 mL/min for 20 minutes. The UV/Vis detector was operated at 360 nm with

a range of 0.1.

The experiments used an LC-18 reverse phase column25 that operates on the basis of

hydrophilicity and lipophilicity. The stationary phase consists of an ocatdecyl ligand of

covalently bonded n-alkyl chains in a silica based packing. The less polar the analyte,

the longer its retention. The length, ID, and particle packing size are 25 cm, 4.6 mm,

and 5 µm, respectively.

The system was calibrated before each set of experiments using standard carbonyl-

DNPH26 mixes with known concentrations of various carbonyl compounds. The samples

were then placed in the HPLC auto-sampler and ready for injection. The HPLC pump

draws a set ratio of water and acetonitrile at 1.2 mL/min, as prescribed in the HPLC

method. After injection of 20 µL of sample, a gradient elution method was used to

separate the components. Initially, the mobile phase mixture was 50% acetonitrile and

50% water. After 4 minutes, the pump increased the acetonitrile concentration to 100%

over the next 25 minutes, via a linear gradient. The mobile phase flow then returned to

the initial 50:50 ratio and was held there for an additional 5 minutes.

The concentrations obtained from the HPLC/UV-Vis technique were for the liquid

solutions consisting of acetonitrile and carbonyl-derivatives. The concentration of the

parent carbonyl in the gas samples was obtained via a series of calculations. These

calculations are shown in Appendix D.

3.5.4 Temperature Measurement

The flame temperature profile was obtained by measuring the local temperature at various

regions in the flame. The measurements were obtained by the most direct method:

inserting a thermocouple into the flame. The thermocouple was small compared to the

thickness of the flame front so as to not disturb the flame. The primary drawbacks

of using a thermocouple are the aerodynamic wake behind left the flame front and the

possible catalytic activity of the thermocouple material[54].

The thermocouple measures the temperature by employing the difference in ther-

24Phenomenex SecurityGuardTM

25SUPELCOSILTM LC-18, 5 µm, 25 cm26SUPELCO Carbonyl-DNPH Mix 1


moelectrical properties of different metals. When two dissimilar conductors are welded

together, an electric potential is generated which is proportional to the difference in tem-

perature conducted by each metal. Thermocouples made of thin noble metal wires such

as platinum and rhodium are advantageous for the following reasons: they allow a high

resolution to be obtained; the aerodynamic disturbance of the flame front is minimized;

and the materials can withstand high temperature environments [54].

A schematic of the thermocouple apparatus is shown in Figure 3.15. The flame

temperature was measured by inserting the apparatus between the two burners. The

two R-type thermocouple wires27 (legs) are made of dissimilar metals butt-welded at a

junction. The positive leg is made of pure platinum while the negative leg is comprised of

87% platinum and 13% rhodium. Each wire’s diameter is 254 µm while the butt-welded

junction is approximately 500 µm. The wires are housed in ceramic tubes which provide

support while spreading the wires apart. The junction, at which the temperature is

measured, lies between the ceramic tubes. At the opposite end, the ceramic tubes are

fixed to an aluminum plate which is mounted on a sliding rail. A spring is used to pull

the ceramic tubes together and keep the wires taught at the other end. The positive

and negative legs are connected to an R-type extension wire which carries the measured

signal to a digital thermometer28.

Figure 3.15: Thermocouple Schematic

27OmegaR© R-type butt-welded unsheathed fine-gauge thermocouple wires28DigisenseR© DualLogRR© Thermocouple Thermometer


Correction for Radiation Losses

The temperature measured by the thermocouple differs from the true flame temperature

due to aerodynamic, thermal, and/or chemical perturbations. The methods of minimizing

the effect of these perturbations are discussed in detail by Fristrom and Westenberg [50].

However, even if all disturbances are minimzed, the thermocouple will register a different

temperature than the true stream temperature due to radiation losses. Correcting for

these losses is estimated by equating the heat transferred to the thermocouple from the

gas to the heat lost by radiation from the wires. The equation given for a spherical device

is:

Tg − Tc =ε · σ · d · (T 4

c − T 4w)

2 · k(3.2)

where

Tg is the true gas temperature (K)

Tc is the measured gas temperature (K)

ε is the emissivity of the thermocouple element (dimensionless)

σ is the Stefan-Boltzmann Constant = 5.67x10−08 ( Wm2·K4 )

d is the wire diameter (m)

k is the thermal conductivity of gas ( Wm·K )

Tw is the wall (ambient) temp. to which heat is radiated = 300 (K)

The thermal conductivity, k, of the gases was estimated as the thermal conductivity

of air at the measured temperature. The thermal conductivity of air at various temper-

atures was obtained from the CRC Handbook (CRC online). Linear interpolation and

extrapolation was used to determine thermal conductivity at temperatures not listed in

the handbook.

The emissivity, ε, of the thermocouple element was obtained from the study by

Bradley and Entwistle [63].

Chapter 4

Modeling

The experimental flame setup was modeled using the CHEMKIN 4.0 software package1

[37]. CHEMKIN is a set of software tools useful when dealing with chemical kinetic

problems. The package contains a number of reactor models that can be used to represent

the different real-world systems via a graphical user interface (GUI).

Our opposed-flow diffusion flame was modeled using the OPPDIF code. As previously

mentioned, the opposed-flow diffusion flame configuration has an axisymmetric geometry

consisting of two concentric, circular ports facing each other. A simplified representation

of the opposed-flow geometry is shown in Figure 4.1. Fuel is fed through the bottom

port while oxidizer is fed through the top. The opposing ports produce an axisymmetric

flow field with a stagnation plane lying in the middle. A flat diffusion flame lying on

the oxidizer side of the stagnation plane is generated, since most fuels require more air

then fuel by mass. The two-dimensinal flow field is mathematically simplified to one

dimension by assuming fluid properties are a function of axial distance only. The one-

dimensional model predicts the temperature and species profiles between the two burner

ports. The sensitivity of species concentration and temperature to chemical kinetic and

thermodynamic data can also be calculated [64].

4.1 Modeling Procedure

Initially, the CHEMKIN 4.0 GUI was used to set up a diagram of the opposed-flow

diffusion flame with two inlet streams. The next step was to generate the linking files

1Distributed by Reaction Design www.ReactionDesign.com

59

Chapter 4. Modeling 60

Figure 4.1: Geometry of opposed-flow diffusion flame from [64]

for the OPPDIF code. This required using the pre-processors to access three important

information files: i.) the gas-phase kinetics file; ii.) the thermodynamic data file; and iii.)

the gas phase transport data file. More information on these files is given below. The

CHEMKIN Gas-Phase Interpreter reads the first two files and generates the CHEMKIN

linking file. The third file is used by the TRANSPORT Preprocessor to generate the

Transport Linking File.

Next, the characteristics of the reactor and inlet flows were inputed. This includes

the velocity of each inlet stream, initial concentrations, pressure, burner configuration,

temperature, and a number of solution method options. An input file was then created

from these user-defined parameters, and the OPPDIF model was run. The OPPDIF

code outputs a text file containing the solution. The Solution Export Utility was used

to convert this text file into a comma separated values file format that was readable by

Microsoft Excel.

4.2 Details of Input Files

The gas-phase kinetics file identifies all the gaseous species present, and it provides a user-

defined chemical kinetic mechanism for the production and consumption of these species.

The chemical kinetic mechanism details each reaction taking place and the appropriate

Chapter 4. Modeling 61

reaction rate parameters in the Arrhenius form, as shown in Equation 4.1. The gas phase

kinetic file conforms to the CHEMKIN input format.

kA = A · (T/Tref )n exp

Ea

R · T(4.1)

where

k is the reaction rate

A is the Arrhenius constant

T is temperature

Tref is the reference temperature (usually 298 K or 25 ◦C)

n is the pre-exponential factor

Ea is the activation energy

R is the ideal gas constant

The thermodynamic data file contains all the thermodynamic data for the species.

Contained within this file are the species’ name, elemental composition, electronic charge,

and phase. In addition, fourteen polynomials fitting coefficients are provided to calculate

enthalpy and entropy at any temperature. The thermodynamic data file conforms to the

NASA formatting requirements.

The gas transport data file contains molecular parameters for each molecule. In-

cluded is this file are the Lennard-Jones well depth in Kelvin, the polarizibility in cubic

angstroms, the rotational relaxation and collision number, the dipole moment in Debyes,

the Lennard-Jones collision diameter in angstroms, and the geometrical configuration of

the molecule. The gas transport data conforms to the CHEMKIN formatting require-

ments.

This study uses the methyl butanoate chemical kinetic mechanism developed by Dr.

Sandro Gaıl 2. Dr. Sandro Gaıl also provided the gas transport data file. The thermo-

dynamic data file was obtained from the website of the Lawrence Livermore National

Laboratory Combustion Chemistry Group [65].

2University of Toronto Department of Mechanical and Industrial Engineering Combustion ResearchGroup

Chapter 5

Results and Discussion

The primary goal of this study is to understand the effect of molecular structure (i.e.

effect of unsaturation) on the combustion of FAMEs. This is accomplished by comparing

opposed flow diffusion flame species profiles of methyl butanoate and methyl crotonate,

which are surrogates for saturated and unsaturated long chain FAMEs, respectively.

Any observed differences between the flame species profile will be explained by oxidation

reaction pathways analysis.

The secondary goal of this study is to validate a chemical kinetic model for methyl

butanoate, a short chain FAME used as a modeling surrogate for the long chain FAMEs

found in biodiesel. The mechanism was developed by a team of individuals in our Com-

bustion Research Group. The mechanism was validated against experimental data ob-

tained from an opposed flow diffusion flame, jet-stirred reactor, and plug flow reactor.

The opposed flow diffusion flame studies are the contribution of this thesis. The flame

measurements are compared against modeling predictions obtained from Chemkin 4.0.

5.1 The Role of FAME Molecular Structure in Com-

bustion

It was previously mentioned that FAME molecular structure can alter the emissions pro-

files from engines operating on biodiesel. However, there is little understanding of the

role of unsaturation (carbon-carbon double bonds) on the fundamental combustion prop-

erties of the fuel. The following sections present opposed flow diffusion flame emissions

profiles generated by a saturated FAME (i.e. methyl butanoate) and an unsaturated

62

Chapter 5. Results and Discussion 63

FAME (i.e. methyl crotonate), and the differences between the two profiles are ratio-

nalized by oxidation pathway analysis. Before discussing the actual results, the general

pathways for fuel consumption will be discussed.

5.1.1 Major Reaction Pathways for the Oxidation of Methyl

Butanoate

The original methyl butanoate mechanism proposed by Fisher et al. [8] was analyzed to

identify the initial reaction pathways responsible for fuel consumption. The mechanism

identifies hydrogen abstraction and decomposition as the key mechanisms by which the

fuel is consumed. In Figure 5.1, it is evident that hydrogen can be abstracted from any of

the carbon atoms to form one of four possible radical species. As previously mentioned,

the abstraction of H occurs preferentially on secondary carbon atoms, followed by primary

carbon atoms. Note that the pathways presented here are all the possible reactions.

The most important pathways for fuel oxidation in the opposed flow diffusion flame are

presented later.

At higher temperatures, the decomposition of methyl crotonate becomes increasingly

important. The fuel consumption is driven either by unimolecular thermal decomposi-

tion or by decomposition via collision with a third body that causes fragmentation of

the parent fuel. Figure 5.2 displays all the possible decomposition reactions for methyl

butanoate. As previously mentioned, all these reactions are not necessarily important in

the opposed flow diffusion flame chemistry.


H3CCH2

CH2

C

O

OCH3

- H

- H- H

H3CCH

CH2

C

O

OCH3

or

H3CCH2

CH

C

O

OCH3

H2CCH2

CH2

C

O

OCH3 H3C

CH2

CH2

C

O

OCH2

methyl butanoate

Figure 5.1: Hydrogen abstraction from carbons atoms in methyl butanoate

H3CCH2

CH2

C

O

OCH3

CH2

CH2

C

O

OCH3

CH3

+

C2H5

CH2

C

O

OCH3

+C

O

OCH3

C3H7

+

H3CCH2

CH2

C

O

CH3O

+

H3CCH2

CH2

C

O

O

CH3

+

methyl butanoate

Figure 5.2: Decomposition reaction pathways for methyl butanoate



Crotonate

Currently there is no detailed chemical kinetic mechanism for the oxidation of methyl

crotonate. Dr. Gaıl and coworkers, including the author of this paper, are in the process

of developing such a mechanism. The methyl butanoate mechanism proposed by Fisher

et al. [8] contains some reactions for methyl crotonate, since methyl crotonate is a

possible intermediate in methyl butanoate combustion. One pathway for forming methyl

crotonate from methyl butanoate is by a series of hydrogen abstraction reactions, as

shown in Figure 5.3.

The major reaction pathways for the oxidation of methyl crotonate can be predicted

from the aforementioned analysis of methyl butanoate oxidation. At low and intermediate

temperatures methyl butanoate is primarily consumed by hydrogen abstraction from

the carbon atoms. Therefore in the same temperature range, hydrogen abstraction is

the primary method of consuming methyl crotonate, as shown in Figure 5.4. Hydrogen

abstraction can occur on any of the carbon atoms in the fatty acid portion of the molecule.

This is because the radicals A, B, and C in the figure are in equilibrium due to internal

isomerization. The radicals C and D are equivalent due to resonance.

The decomposition of methyl crotonate is an important consumption pathway at

higher temperatures. Figure 5.5 shows the possible C-O and C-C bonds that can be

broken to form radical species. These decomposition pathways are more significant at

high temperatures.

In the oxidation of alkenes, the biradical O was shown to attack the C=C double

bond to form intermediate species. The double bond in methyl crotonate is susceptible

to the same attack, as shown in Figure 5.6. The biradical O reacts with methyl crotonate

to form aldehyde radicals and simpler ester radicals.


H3C

C

C

H2

H2

C

O

CH3

O

+ X

methyl butanoate

H3C

C

C

H

H2

C

O

CH3

O

+ XH

H3C

C

C

H

H2

C

O

CH3

O

radical

+ X

radical

H3C

C

C

H

H

C

O

CH3

O

+ XH

methyl crotonate

Figure 5.3: Reaction pathway for forming methyl crotonate from methyl butanoate

H3C

C

C

H

H

C

O

CH3

O

methyl crotonate

-H -H

- H

H2C

C

C

H

H

C

O

CH3

O

H3C

C

C

HC

O

CH3

O

or

H3C

C

CH

C

O

CH3

O

H3C

C

C

H

H

C

O

CH2

O

A

B

C

H2C

C

H

C

C

O

O

CH3

D

internal

isomerization

resonance

Figure 5.4: Hydrogen abstraction from carbons atoms in methyl crotonate


H3C

C

C

H

H

C

O

CH3

O

methyl crotonate

C

O

CH3

O

+

H3C

C

CH

H3C

C

C

H

H

C

O

O

+

CH3

O

CH3

H3C

C

C

H

H

C

O

+

C

C

H

H

C

O

CH3

O

+

CH3

Figure 5.5: Decomposition reaction pathways for methyl crotonate

H3C

C

C

H

H

C

O

CH3

O

methyl crotonate

+ O

+ O

+ C

O

CH3

O

H3C

C

C

H

H

O

H2C

C

O

CH3

O

+H3C

CO

Figure 5.6: Reactions with biradical O for methyl crotonate


5.1.3 Comparison of Opposed Flow Diffusion Flame Emissions

Profiles

Methyl crotonate and methyl butanoate were used to generate opposed flow diffusion

flames as described in Section 3. Gas samples were withdrawn from a series of locations

by moving the sampling probe from the fuel port to the oxidation port in a stepwise

manner. The luminous blue flame front was located at approximately 8.5 mm above

the fuel port. The temperature and emission profiles of methyl butanoate and methyl

crotonate are now compared to determine the role of FAME molecular structure (i.e. the

double bond in methyl crotonate) in combustion. Figure 5.7 shows a simplified diagram

of the burner setup rotated clockwise by 90 degrees. The species concentration and

temperature profiles in the following sections are oriented with respect to this diagram;

the distance from the fuel port is plotted on the X-axis, while the measured parameter

is plotted on the Y-axis. Figures 5.8 to 5.14 display the profiles for the two fuels.

Figure 5.7: Diagram of the burner setup to clarify the orientation of experimental profiles


0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20DISTANCE FROM FUEL PORT (mm)

TE

MP

ER

AT

UR

E (

K)

TEMP- MC

TEMP- MB

Figure 5.8: Measured temperature profiles in the methyl butanoate (MB - closed symbols

with lines) and methyl crotonate (MC - open symbols without lines) flames

0

2

4

6

8

10

12


MO

L F

RA

CT

ION

(%

)

CO- MB

CO2- MB

CO- MC

CO2- MC

MB

MC

Figure 5.9: Measured concentration profiles for fuel (MB or MC), CO, and CO2 in the

methyl butanoate (MB - closed symbols with lines) and methyl crotonate (MC - open

symbols without lines) flames


0

1000

2000

3000

4000

5000

6000

2 3 4 5 6 7 8 9 10DISTANCE FROM FUEL PORT, mm

MO

L F

RA

CT

ION

(P

PM

)

C2H4- MB

C2H4- MC

C2H2- MB

C2H2- MC

CH4- MB

CH4- MC

Figure 5.10: Measured concentration profiles for CH4, C2H4, and C2H2 in the methyl

butanoate (MB - closed symbols with lines) and methyl crotonate (MC - open symbols

without lines) flames

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2 4 6 8 10

DISTANCE FROM FUEL PORT (mm)

MO

L F

RA

CT

ION

(P

PM

)

C3H6- MB

C2H6- MB

C2H6- MC

C3H6- MC

CH3OH- MC

Figure 5.11: Measured concentration profiles of C3H6 and C2H6 in the methyl butanoate

(MB - closed symbols with lines) and methyl crotonate (MC - open symbols without

lines) flames, and CH3OH in the methyl crotonate flame


0

10

20

30

40

50

60

70

2 3 4 5 6 7 8 9 10

DISTANCE FROM FUEL PORT, mm

MO

L F

RA

CT

ION

(P

PM

)

2-C4H6- MB

2-C4H6- MC

C3H8- MB

C3H8- MC

CH3CHO- MB

CH3CHO- MC

Figure 5.12: Measured concentration profiles of 2-C4H6, C3H8, and CH3CHO in the



0

100

200

300

400

500

600

2 3 4 5 6 7 8 9 10

DISTANCE FROM FUEL PORT, mm

MO

L F

RA

CT

ION

(P

PM

)

1-C4H8- MB

1-C4H8- MC

C3H4- MB

C3H4- MC

1,3-C4H6- MB

1,3-C4H6- MC

CH2O- MC

CH2O- MB

Figure 5.13: Measured concentration profiles of C4H8, C3H4, 1,3-C4H6, and CH2O in the




0

10

20

30

40

50

60

70

2 3 4 5 6 7 8 9 10DISTANCE FROM FUEL PORT, mm

MO

L F

RA

CT

ION

(P

PM

)

C5H12- MB

C5H12- MC

C6H6- MC

C3H4O- MC

C3H6O- MC

C4H10- MB

C4H10- MC

Figure 5.14: Measured concentration profiles of C5H12 and C4H10 in the methyl butanoate

(MB - closed symbols with lines) and methyl crotonate (MC - open symbols without lines)

flames, and C6H6, C3H4O, and C3H6O in the methyl crotonate flame

Table 5.1 shows the peak temperatures obtained while burning methyl butanoate and

methyl crotonate are 1758 K and 1750 K, respectively. In addition, the flame tempera-

ture profiles for the two fuels are very similar (Figure 5.8). The similarity in measured

temperatures is expected because the fuels have similar heating values. The heating value

is not available for the two fuels, but Demirbas [66] has shown that a fuel’s heating value

can be directly correlated to the density and viscosity of the fuel. Since the two fuels are

structurally similar, the physical chemical properties, and therefore, the heating values

would be comparable. However, methyl crotonate would have slightly lower heating value

because the C=C double bond lowers the heat content of the fuel [66].

Concentration profiles were obtained for a number of hydrocarbon and oxygenated

species (Figures 5.9 to 5.14). The fuel reactivity, CO, CO2, CH4, C2H6, CH2O, and

CH3CHO profiles for the two fuels are very similar. The methyl crotonate flames shows

consistently higher levels of C2H2, C3H4, C3H4, 1-C4H8, 2-C4H6, C4H10, C5H12, 1,3-

C4H6. In addition, methyl crotonate profiles are available for CH3OH, C3H4O, C3H6O,

and C6H6, but the same species were not detected in methyl butanoate. Consistently


lower levels of C2H4 and C3H8 were detected in the methyl crotonate flame1.

Table 5.1 summarizes the measured peak mole fractions in the two flames. Methyl

butanoate and methyl crotonate produce similar peak mole fractions for carbon monoxide

(CO), carbon dioxide (CO2), methane (CH4), ethane (C2H6), formaldehyde (CH2O), and

acetaldehyde (CH3CHO). However, the methyl crotonate flame produces approximately

twice the mole fraction of propene (C3H6) and acetylene (C2H2); half the mole frac-

tion of ethylene (C2H4) and propane (C3H8); five to seven times more propyne (C3H4),

1-butene (1-C4H8), 2-butyne (2-C4H6), butane (C4H10), and n-pentane (C5H12); and

twelve times more 1,3-butadiene (1,3-C4H6). In addition, the methyl crotonate flame

produces methanol (CH3OH), acrolein (C3H4O), acetone (C3H6O), and benzene (C6H6)

in detectable quantities, while these species were below the detection limit in methyl

butanoate flames.

Opposed Flow Diffusion Flame

Measured ParameterExperimental

Results (MC)

Experimental

Results (MB)

Ratio

(MC/MB)

Temperature (K) 1758 1750 1.0

Carbon Dioxide CO2 (%) 10 10.1 1.0

Carbon Monoxide CO (%) 3 3 1.0

Methane CH4 (ppm) 2185 1957 1.1

Formaldehyde CH2O (ppm) 280 222 1.3

Ethane C2H6 (ppm) 1802 1405 1.3

Ethylene C2H4 (ppm) 2567 4842 0.5

Acetylene C2H2 (ppm) 4781 1951 2.5

Propene C3H6 (ppm) 1394 790 1.8

Propyne C3H4 (ppm) 434 62 7.0

Acetaldehyde CH3CHO (ppm) 40 43 0.9

1-Butene 1-C4H8 (ppm) 353 58 6.1

1,3-Butadiene 1,3-C4H6 (ppm) 568 48 11.8

Acrolein C3H4O (ppm) 50 < 3

Propane C3H8 (ppm) 31 62 0.5

n-pentane C5H12 (ppm) 13 2 6.5

2-Butyne 2-C4H6 (ppm) 34 6 5.7

Butane C4H10 (ppm) 61 12 5.1

Acetone C3H6O (ppm) 21 < 3

Benzene C6H6 (ppm) 16 < 5

Methanol CH3OH (ppm) 617 < 5

Table 5.1: A comparison of measured peak species mole fractions in methyl butanoate

(MB) and methyl crotonate (MC) flames

1See Appendix B for structures of measured chemical species


5.1.4 Rationale for Differences in Methyl Butanoate and Methyl

Crotonate Emissions Profiles

The aforementioned experimental results indicate noticeably different levels of many

species. In general, the unsaturated methyl crotonate leads to the formation of more

unsaturated species. Analogous reaction pathways for the oxidation of methyl butanoate

and methyl crotonate can be used to explain these observed differences. Decomposition

and hydrogen abstraction reactions were shown to be the main pathways during com-

bustion. These reactions can lead to the production of C3H7 and C3H5 from methyl

butanoate and methyl crotonate, respectively.

Figure 5.15 shows the decomposition (#1, #3, #4) and hydrogen abstraction (#2)

reactions responsible for forming the n-propyl radical (C3H7) from methyl butanoate.

C3H7 further reacts to form propane (C3H8) and ethylene (C2H4) by H addition (#6)

and β-scission (#5), respectively. This explains why concentrations of these species are

greater in the methyl butanoate flame.

The analogous decomposition (#7, #10, #11) and hydrogen abstraction (#8) reac-

tions for methyl crotonate lead to the formation of the 1-propenyl radical (C3H5), as

shown in Figure 5.15. In addition, methyl crotonate can decompose (#9) directly to

C3H5 and the methoxycarbonyl radical (CH3OCO) by cleavage between hydrocarbon

chain and the methyl ester group. The possibility of this cleavage is greater in methyl

crotonate than methyl butanoate due to the greater electronegativity of the C=C double

bond near the ester group. The double bond attracts electrons away from the adjacent

single bond, thereby making it weaker and easier to break.

C3H5 then reacts to form propene (C3H6) by H addition (#14), and propyne (C3H4)

and acetylene (C2H2) by β-scission (#12a, #12b). C2H2 then undergoes pyrolysis (#15)

in the presence of smaller hydrocarbon to form 1-butene (1-C4H8), 2-butyne (2-C4H6),

1,3-butadiene (1,3-C4H6) and benzene (C6H6). Figure 5.15 also shows that CH3OCO

decomposes (#16) to CH3O and CO, and then CH3O undergoes H addition (#17) to

form CH3OH. This explains the higher concentrations of all the aforementioned stable

species in the methyl crotonate.

Acrolein (2-propenal C3H4O) was measured in the methyl crotonate flame but was

not detectable in the methyl butanoate flame. Figure 5.15 shows that acrolein can be

derived from methyl crotonate by a series of decomposition and H addition reactions

(#18, #19, #20, #22). In the case of methyl butanoate, it is evident that the analogous


pathways (#22, #23, #24, #25) would lead to propenal (C3H6O), instead of acrolein.

Propanal concentrations were below the detection limit for both fuels.

Benzene (C6H6) was measured in the methyl crotonate flame but not in the methyl

butanoate flame. This can be attributed to the higher levels of acetylene, propyne, and

C4 hydrocarbons present in the methyl crotonate flame. It was previously shown that,

under richer conditions, these species can form complex aromatic compounds, PAHs,

and ultimately soot. The double bond in methyl crotonate is responsible for the higher

levels of acetylene, propyne, 1,3-butadiene, 1-butene, 2-butyne, and benzene. It can

be inferred that unsaturated long chain FAMEs will have a similar tendency to create

the aforementioned soot precursors in greater concentrations than saturated long chain

FAMEs. Therefore, an important observation of this study is that unsaturated FAMEs

have a greater tendency to form soot precursors.


H3C

CCH

C

O

O

CH3

HMethyl Crotonate

- CH3

+ X

- XH

H3C

CCH

C

O

O

H

H3C

C

CH

C

O

O

CH2

H

H3C

CCH

Hpropenyl radical

- CO2

- C

H3O

CO

!-s

ciss

ion

+ H

HC CH acetylene

H3C

C

CH2

H

propene

H3C C CH

pyrolysis

1-C4H8 1-butene

1,3 - C4H6 1,3 - butadiene

2-C4H6 2-butyne

C6H6 benzene

C

O

O

CH3

- CO

+ H

HO CH3

methanol

H3C

CCH2

C

O

O

CH3

H2Methyl Butanoate

H3C

CCH2

H2

n-propyl radical

H3C

CCH2

C

O

O

H2

- CH3

H3C

CCH2

C

O

O

CH2

H2

- CO2

+ X

- XH

- CO, - CH2O

!-scission+ H

CH3 +

ethylene

propane

#1 #2

#3 #4

#5 #6

#7 #8#9

#10 #11

#12a

#14

#15

#16

#17

propyne

H3C

CCH

C

O

H

+ H

H3C

CC

+ H

CH

O

H

- CH3

CC

CH

O

H

H2C

CCH

O

#18

acrolein (2-propenal)

#19

#20

#21

- CH3O

H3C

CC

C

O

+ H

H3C

CC

+ H

CH

O

- CH3

CC

CH

O

H2

H3C

CCH

O

propanal

#22

#23

#24

#25

H2

H2

H2

H

H2

H

H

H2

H2

or#12b

- CO, - CH2O

- CH3O

CH3 +

H

+

H3C

CCH3

H2

O CH3

H2C CH2

Figure 5.15: Analogous reaction pathways for methyl butanoate and methyl crotonate

oxidation in the opposed flow diffusion flame


5.2 Chemical Kinetic Modeling of Methyl Butanoate

The detailed chemical kinetic mechanism used in these calculations was developed by

Dr. Gaıl and coworkers, one of them being the author of this thesis [10]. The original

Fisher et al. mechanism [8] was modified to better reproduce jet-stirred reactor data

obtained at the CNRS, Laboratoire de Combustion et Systemes Reactifs. The modified

mechanism was then tested against experimental data from a plug flow reactor and

opposed flow diffusion flame; the latter being the contribution of this thesis study. A C4

sub-mechanism was also added to the overall mechanism to model the formation of C4

hydrocarbon species. The revised mechanism [10] consists of 1498 reversible reactions

involving 295 species. Several reaction rate parameters from the original mechanism [8]

were modified, as shown in Table 5.2. Some of these reaction rate modifications were

performed by the author, as part of this thesis study.

ReactionFisher et al. rate

constants(A / n / Ea)

Modified rate constants(A / n / Ea)

CH3CH2CH2(C=O)OCH3 + H CH3CH2C’H(C=O)OCH3 + H2 2.52E+14 / 0.00 / 300 1.00E+14 / 0.00 / 300

CH3O (+M) CH2O + H (+M) 5.45E+13 / 0.00 / 13500 1.38E+21 / -6.65 / 33190

C2H3 + O2 CH2O + HCO 1.70E+29 / -5.31 / 6500 1.66E+13 / -1.39 / 1013

CO + CH3O CH3OCO 1.50E+11 / 0 / 3000 1.50E+6 / 2.02 / 5730

CO2 + CH3 CH3OCO 1.50E+11 / 0 / 36730 4.76E+7 / 1.54 / 34700

Table 5.2: Modified reaction rate constants [10, 8, 43].


5.2.1 Modification of Reaction Rates - Contributions of this

Study

A chemical kinetic study on the oxidation of dimethyl carbonate [43] was previously

discussed in Section 2.5.3. The study by Glaude and coworkers used quantum mechan-

ical estimates to provided new reaction rate constants for decomposition reactions of

CH3OC·=O. The decomposition reactions and their respective rate constants are shown

in Figure 5.3.

C

O

O

CH3CO2 + CH3 CO + CH3O

A = 4.76 E 7

n = 1.54

Ea = 34,700

A = 1.55 E 6

n = 2.02

Ea = 5,730

Table 5.3: Decomposition pathways of CH3OC·=O and corresponding reaction rate con-

stants [43].

The author of this thesis was responsible for the testing and implementation of these

rate constants in the chemical kinetic mechanism for methyl butanoate. These modi-

fied reaction rate constants were not included in the mechanism published by Gaıl and

coworkers [10]. The following section displays plots of the model predicted values and the

experimental data. Modeling results are shown for the published Gaıl et al. mechanism

[10] (solid lines, ’old’), as well as the revised model, which includes the aforementioned

reaction rate constants from Glaude et al. [43] (dashed lines, ’new’).

5.2.2 Model Validation with Opposed Flow Diffusion Flame Re-

sults

The methyl butanoate opposed flow diffusion flame was generated as described above.

Gas samples were withdrawn from a series of locations by moving the sampling probe

from the fuel port to the oxidation port in a stepwise manner. The luminous blue flame

front was located at approximately 8.5 mm above the fuel port.

The experimental data was compared against model generated values using Chemkin

4.0 running on the methyl butanoate mechanism. Molecular species concentration profiles


were obtained from the oxidation of methyl butanoate in an opposed flow diffusion flame

for the following species: methyl butanoate, CO, CO2, CH4, C2H4, C2H2, C3H6, C2H6,

CH2O, C4H8, C3H8, pC3H4, CH3CHO, and 1,3-C4H62. Figures 5.16 to 5.19 show the

experimental and modeling results for the aforementioned species. The published Gaıl et

al. model [10] is shown in solid lines and marked ’old’, while model with rate constants

from the study by Glaude et al. [43] is shown as dashed lines and marked ’new’.

0

2

4

6

8

10

12

2 4 6 8 10 12 14 16 18

DISTANCE FROM FUEL PORT (mm)

MO

L F

RA

CT

ION

(%

)

CO old new

CO2 old new

MB old new

Figure 5.16: Modeling predictions (small symbols with lines) and experimental results

(large symbols without lines) for methyl butanoate (MB), CO, and CO2



0

1000

2000

3000

4000

5000

6000


MO

L F

RA

CT

ION

(P

PM

)

CH4 old new

C2H4 old new

C2H2 old new


(large symbols without lines) for CH4, C2H4, and C2H2

0

200

400

600

800

1000

1200

1400

1600

1800


MO

L F

RA

CT

ION

(P

PM

)

C3H6 old new

C2H6 old new

CH2O old new


(large symbols without lines) for C3H6, C2H6, and CH2O


0

20

40

60

80

100

120

140

160

180

200


MO

L F

RA

CT

ION

(P

PM

)

C4H8 old new

C3H8 old new

pC3H4 old new

CH3CHO old new

C4H6 old new


(large symbols without lines) for C4H8, C3H8, pC3H4, CH3CHO, and 1,3-C4H6

Species Name

(measurement units)

Experimental

Results

Model by

Gail et al.

Ratio

(Gail/Exp)

Model in this

Thesis

Ratio

(Thesis/Exp)

Model by

Fisher et al.

Ratio

(Fisher/Exp)

Carbon Dioxide CO2 (%) 10.1 10.6 1.05 10.5 1.04 10.6 1.05

Carbon Monoxide CO (%) 3.0 3.4 1.13 3.0 1.00 3.6 1.20

Ethylene C2H4 (ppm) 4842 4841 1.00 5257 1.08 4089 0.84

Methane CH4 (ppm) 1957 2578 1.32 3114 1.59 2590 1.32

Propane C3H8 (ppm) 62 49 0.79 51 0.82 49 0.79

Ethane C2H6 (ppm) 1405 1077 0.77 1442 1.03 1295 0.92

Propene C3H6 (ppm) 790 1630 2.06 1630 2.06 1452 1.84

Propyne C3H4 (ppm) 62 100 1.61 128 2.06 108 1.74

Acetylene C2H2 (ppm) 1952 4572 2.34 4777 2.44 4321 2.21

1-Butene 1-C4H8 (ppm) 58 187 3.22 180 3.10 --- ---

Formaldehyde CH2O (ppm) 222 866 3.90 593 2.67 1658 7.47

1,3-Butadiene 1,3-C4H6 (ppm) 48 22 0.46 24 0.5 --- ---

Acetaldehyde CH3CHO (ppm) 43 22 0.51 19 0.44 25 0.58

Table 5.4: A comparison of peak species mole fractions from experimental data and

model-predicted values


The results show fairly good agreement between the model predicted values and the

experimental data. However, the consumption of methyl butanoate is under-predicted

by the model (Figure 5.16). This is evident by the spatial delay in methyl butanoate

consumption displayed by the model generated profile, when compared to the experi-

mental data. The maximum species concentrations predicted by the model are shifted

away from the fuel port (Figures 5.17 to 5.19). This is due to the under-prediction of

methyl butanoate consumption; since the fuel consumption is delayed, the production of

species is also delayed. It should be noted that the reactivity of methyl butanoate in the

Fisher et al. model [8] was improved upon by modifying the Arrhenius rate constant of

the hydrogen abstraction reaction forming the 2-methyl butanoate radical, as shown pre-

viously in Table 5.2. The modification of CH3OCO decomposition rate constants further

improved the methyl butanoate reactivity, as is evident by comparing the ’old’ and ’new’

curves.

Table 5.4 compares the peak concentrations from the experimental data to the model-

predicted values. The peak concentrations of carbon monoxide (CO), carbon dioxide

(CO2), methane (CH4), ethylene (C2H4), propane (C3H8), and ethane (C2H6) are well

predicted by the model. However, the model over-predicts the peak concentration of

acetylene (C2H2), 1-butene (C4H8), propene (C3H6), propyne (pC3H4), and formaldehyde

(CH2O) by 2-4 times. The model under-predicts the peak concentrations of 1,3-butadiene

(1,3-C4H6) and acetaldehyde (CH3CHO) by approximately one-half. 3.

A comparison of the models by Fisher et al [8], Gaıl et al. [10], and this thesis study

is also presented in Table 5.4. The gray-highlighted rows indicate which model produces

better agreement with the experimental data. The three models perform similarly, with

this study’s model being marginally better for the majority of species. This study’s model

performs much better for formaldehyde due to the modification of two sets of reaction

rate constants involving formaldehyde and the changes to CH3OCO decomposition re-

action rates, as shown in Table 5.2. The Gaıl et al. model enables the prediction of C4

hydrocarbons, while the Fisher model does not. This is due to the C4 sub-mechanism

included in the mechanism by Gaıl.

The modifications to the CH3OCO decomposition reaction rates have interesting ef-

fects on the mechanism. First, the rate of production of CO and CH3O is reduced,

thereby reducing the CO and CH3O concentrations in the flame. This, in turn, reduces



CH2O concentrations since CH2O is formed by hydrogen abstraction from CH3O. Many

species concentrations were changed by this modification; thus, indicating the sensitivity

of the mechanism to these reaction rate parameters.

5.2.3 Error Analysis of Major Differences in Modeling and Ex-

perimental Results

The previous section showed that the model over-predicts the peak concentration of acety-

lene (C2H2), propene (C3H6), propyne (pC3H4), formaldehyde (CH2O), and 1-butene

(C4H8) by 2-4 times. The model under-predicts the peak concentrations of 1,3-butadiene

(1,3-C4H6) and acetaldehyde (CH3CHO) by approximately one-half. These differences

can be attributed to either experimental error or inaccuracies in the mechanism’s reaction

rate coefficients.

Experimental Errors

The GC/FID sampling method for hydrocarbons is unlikely to be a source of error

because the results show good agreement between modeling and experimental results for

methane, ethylene, ethane, and propane. The presence of an error in the sampling of

hydrocarbon species would be evident in all the species profiles; however, this is not the

case. In addition, extra precautions were taken to ensure that the sampling system’s

credibility was not compromised, such as: i.) detecting and minimizing leakage into the

sampling line, ii.) freezing reactions at the probe tip by creating a vacuum pressure

of 710 - 730 mm Hg, iii.) minimizing aerodynamic disturbances by using the smallest

possible sampling probe, iv.) passing calibration gases through the burner ports and

sampling them before every experiment, and v.) adequately heating all the sampling

lines to prevent condensation of sample gases.

The accuracy of the measured concentrations of carbonyl compounds, such as formalde-

hyde and acetaldehyde, may have been compromised by the sampling method. These

compounds were measured by passing the samples through cartridges coated with DNPH.

The carbonyl compounds form stable derivatives with the DNPH reagent, are are subse-

quently eluted from the cartridge. The derivatized liquid samples were analyzed for their

parent carbonyls using an HPLC/UV-vis setup in another laboratory. This sampling

method required significant human interaction to prepare and transport the samples be-

tween laboratories, thereby creating possible sources of error. The precautions mentioned


above were followed when sampling these compounds, except the calibration method

was different. Calibration for these compounds was not as thorough as the calibration

of hydrocarbons because calibration gases were not available for formaldehyde and ac-

etaldehyde. Therefore, it was not possible to pass a calibration gas through the burner

ports and test the accuracy of this sampling method. Instead, only the HPLC/UV-vis

instrument was calibrated using known concentrations of DNPH derivatives. It is recom-

mended that future experiments either obtain calibration gases or use a GC/FID coupled

with a methanizer to measure these compounds. The results from the GC/FID can then

be compared to those obtained by the DNPH sampling method.

Inaccuracies in Model Reaction Rate Coefficients

Accurate reaction rates coefficients are arguably the most important input into a chemical

kinetic model. Such values are obtained by either experimental or theoretical methods,

and they can be found in the literature. Of these, experimental methods are more

accurate, but more difficult to obtain. Theoretical reaction rate coefficients for a given

reaction may vary significantly depending the correlations used for their calculation. In

addition, reaction rate coefficient in chemical kinetic model are often times guessed. The

guessing of these coefficients is performed by selecting a value that best reproduces a

given set of experimental data. Thus, there is a possible source of error present in the

mechanism’s reaction rate coefficients.

In order to identify possible sources of error, an A-factor sensitivity analysis and a

rate of production analysis were performed for CH2O, C2H2, pC3H4, C4H8, and C3H6.

An A-factor sensitivity analysis provides first-order sensitivity coefficients for the species.

The first-order sensitivity coefficient is defined as the normalized derivative of the species

concentration with respect to the A-factor of an individual reaction. The rate of pro-

duction analysis provides complementary information on the contributions of individual

reactions on the net production rate of a species [64]. Both these analyses were conducted

at the point of maximum concentration of each species.

The reactions with highest first order sensitivity coefficients and rate of production

values are the most likely to impact the predicted concentration of a species. Therefore,

the data from the two aforementioned methods were analyzed to select the reactions which

would have the highest effect on the species of interest. Table 5.5 shows the reactions

that are most likely to have an impact of the maximum concentration of a particular


species. The table also compares the reaction rate coefficients given in the present model

to those found on the National Institute of Standards and Technology (NIST) Scientific

and Technical Databases - Chemical Kinetics [67].

Table 5.5 compares reaction rate values given in the present model [10] to those avail-

able in literature [67]. The first column shows the species sensitivity and the correlation

between species concentration and reaction rate; a plus (+) sign indicates a positive cor-

relation, and increasing the reaction rate will increase the species concentration, while

a minus (-) sign indicates a negative correlation, and increasing the reaction rate will

decrease the species concentration. The second column gives the reaction of interest.

The third and fourth columns show the Arrhenius equation constants (i.e. A, n, and Ea)

and the calculated reaction rate (cm6 · mol−1 · s−1) at the temperature corresponding

with maximum species concentration.

The first row shows a reaction (#1) in which C2H2 reacts with an H radical to form

C2H3. The model calculated reaction rate for this reaction is less than that determined

by Tsang and Hampson [68]. Increasing this reaction rate would increase the forward

reaction rate, thereby decreasing the C2H2 concentration.

The second row shows a reaction (#2) in which C2H2 reacts with a methyl radical

(CH3) to form pC3H4 and an H radical. Again, the model calculated reaction rate is less

than the reaction rate determined in a study by Davis and coworkers [69]. Increasing

this reaction rate would increase the conversion of C2H2 to pC3H4. This would reduce

the predicted C2H2 concentration, but simultaneously increase the predicted pC3H4 con-

centration. The effects of this trade-off can only be quantified by testing in the model.

The reaction (#3) in the third row displays C3H6 decomposing to C2H3 and CH3.

The reaction rate provided in the model is less than that determined in a study by

Hidaka and coworkers [70]. Reducing the reaction rate to the literature values would

decrease the forward reaction rate, thereby increasing the concentration of C3H6. This is

not desired since C3H6 is already over-predicted by the model. However, decreasing the

forward reaction rate would cause C2H3 to decrease. Lower amounts of C2H3 in the gas

phase would drive reaction #1 forward due to Le Chatelier’s principle, thereby decreasing

C2H2 levels. Again, the trade-off between an increase in C3H6 and a decrease in C2H2

concentration would have to be quantified by running the model with the new values.

The final row in the table shows a reaction (#4) involving the iC3H7 radical under-

going H abstraction to form C3H6 and H2. The sensitivity analysis revealed that CH2O,

C2H2, pC3H4, C4H8, and C3H6 are all sensitive to this reaction. This reaction has not


been studied in the past, so it is likely that the authors of the original methyl butanoate

mechanism [8] guessed the reaction rate coefficients. This leads to the possibility of one

set of reaction rate parameters causing the over-prediction of a number of species. It is

recommended that lower reaction rate values be tested to quantify the effects on decreas-

ing CH2O, C2H2, C4H8, and C3H6 concentrations. However, care must be taken not to

increase pC3H4 concentrations by too much, since pC3H4 has a negative correlation with

the reaction rate.

The aforementioned changes to reaction rates were attempted in this study. However,

due to computational limitations, the effects of the changes could not be quantified. Any

changes made to the existing model would not lead to a converging solution. The problem

was identified to be the C4 sub-mechanism consisting of an additional 279 reactions, which

causes a significant increase in computational load. It is recommended that the model

be reduced to minimize the computations, and then the effects of changes to specific

reaction rate constants can be determined.

Species Sensitivity

(+/-)Reaction

Present reaction rate(A / n / Ea)

reaction rate (cm6.mol-2.s-1) calculated at @ T (K)

NIST reaction rate(A / n / Ea)

reaction rate (cm6.mol-2.s-1) calculated at @ T (K)

C2H2 (-)#1

C2H2 + H (+M) = C2H3 (+M)

3.11E+11 / 0.6 / 2589 2.44E+13@1400 K

3.81E+40 / -7.27 / 72104.16E+17@1400 K

C2H2 (-)pC3H4 (+)

#2

C2H2 + CH3 = pC3H4 + H

1.21E+17 / -1.2 / 16680 2.44E+13@1200 K

1.35E+12 / 1.1 / -27110 3.29E+15@1200 K

C3H6 (-)C2H2 (-)

#3

C3H6 = C2H3 + CH3

2.73E+62 / -13.3 / 123200 3.05E+21@1200 K

8.00E+14 / 0 / 88030 8.01E+14@1200 K

C4H8 (+)CH2O (+)C2H2 (+)C3H6 (+)pC3H4 (-)

#4

iC3H7 + H = C3H6 + H2

2.00E+12 / 0 / 0 2.00E+13@1200 K

NONE

Table 5.5: A comparison of reaction rates given in the present model [10] to those available

in literature [67]



Butanoate in the Opposed Flow Diffusion Flame

The major reaction pathways for methyl butanoate oxidation in the opposed flow diffusion

flame were determined via rate of production and consumption analysis. This involved

studying the Chemkin output file to determine which reactions had the highest rates for

methyl butanoate consumption. The rates of consumption of subsequent species was also

studied until typical combustion intermediates were formed, such as CH3, CH2O, CH3O,

etc..

The rate of production analysis was conducted at a distance of 8.3 mm from the

fuel port, which corresponds to a flame temperature of 1220 K. It should be noted that

this temperature is lower than the maximum flame temperature of 1750 K. Figure 5.20

exhibits the major reaction pathways for methyl butanoate oxidation in the opposed

flow diffusion flame. The red arrows signify the most important reaction pathways. The

weight of the blue arrows signifies the relative rate of reaction, i.e., reactions with heavy

arrows are more important that reactions with lighter arrows. The diagram displays

reaction pathways for species until the point where the functional methyl ester group is

consumed.

Initially, the fuel is consumed primarily by H-abstraction reactions by H atoms due

to the intermediate temperature range. The reactions proceed to form one of four C5H9O2

radical species. Of these, the formation of 2-methyl butanoate (CH3CH2CH·(C=O)OCH3)

and 3-methyl butanoate (CH3CH·CH2(C=O)OCH3) radicals are favored because sec-

ondary C-H bonds are weaker than primary C-H bonds.

The 2-methyl butanoate radical undergoes β-scission to remove a methyl radical

(CH3) and form methyl propenoate (CH2CH(C=O)OCH3). Methyl propenoate then

undergoes hydrogen addition to create formaldehyde (CH2O) and propenoyl radical

(C2H3CO).

The 3-methyl butanoate radical undergoes thermal decomposition to remove propene

(C3H6) and form a methoxycarbonyl radical (·(C=O)OCH3). Subsequently, the methoxy-

carbonyl radical decays to form either a methoxy radical (CH3O) and carbon monoxide

(CO) or a methyl radical (CH3) and carbon dioxide (CO2). Of these two reactions, the

one forming CO2 was found to be more significant than the one producing CO. From a

soot reduction standpoint this is not an efficient use of fuel-bound oxygen because two

oxygen atoms are bonded to one carbon atom thereby wasting an oxygen atom in the


functional methyl ester group. This conclusion is consistent with the findings of other

researchers [43, 44].

Alternative pathways for methyl butanoate consumption include thermal decom-

position to form radicals of methyl propanoate (·CH2CH2(C=O)OCH3), butanoyloxy

(CH3CH2CH·(C=O)O·), methyl acetate (·CH2(C=O)OCH3), methoxycarbonyl (·(C=O)OCH3),

methyl (·CH3), ethyl (·C2H5), n-propyl (CH3CH2CH2·), butanoyl (CH3CH2CH2(C=O)·),

and methoxy (CH3O·). Among these species, the butanoyloxy radical is the most favor-

able because the C-O bond is weaker than the C-C bond. These thermal decomposition

reactions become increasingly important and higher temperatures.


H3C

CC H

2

CO

OC

H3

H2

Met

hyl B

uta

noat

e

H3C

CH2

CCO

O

n-C

3H

7+

H2

- C

H3

CO

2

H2C

CC H

2

CO

CH

3

H2

O

H2C

CO

CH

3

O

CH

2C

O +

C

H3O

H3C

CC

CO

OC

H2

H2

CH

2O

+

n-C

3H

7O

H3C

CC H

CO

OC

H3

H2

H3C

CC

CO

OC

H3

H

H2

- C

3H

6

CO

OC

H3

CH

3O

+

C

O

CO

2 +

C

H3

or

- C

3H

7

CC

CO

OC

H3

H2

H2

- C

H3

- C

2H

4

- C

2H

5

- C

H3

CC

CO

OC

H3

H2

H

H3C

CC H

CO

OC

H3

H

CH

2O

+

C

2H

3C

O

H2C

CC H

CO

OC

H3

H-

CH

3O

H2C

CC H

COH

HC

CO

+

C

H3C

HO

CH

3C

HC

HO

+

C

O

or

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ H

+ X

, - X

H

+ X

, - X

H

H2 +

H3C

CC H

2

CO

OC

H3

H2

Met

hyl B

uta

noat

e

H3C

CH2

CCO

O

n-C

3H

7+

H2

- C

H3

CO

2

H2C

CC H

2

CO

CH

3

H2

O

H2C

CO

CH

3

O

CH

2C

O +

C

H3O

H3C

CC

CO

OC

H2

H2

CH

2O

+

n-C

3H

7O

H3C

CC H

CO

OC

H3

H2

H3C

CC

CO

OC

H3

H

H2

- C

3H

6

CO

OC

H3

CH

3O

+

C

O

CO

2 +

C

H3

or

- C

3H

7

CC

CO

OC

H3

H2

H2

- C

H3

- C

2H

4

- C

2H

5

- C

H3

CC

CO

OC

H3

H2

H

H3C

CC H

CO

OC

H3

H

CH

2O

+

C

2H

3C

O

H2C

CC H

CO

OC

H3

H-

CH

3O

H2C

CC H

COH

HC

CO

+

C

H3C

HO

CH

3C

HC

HO

+

C

O

or

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ X

, - X

H

+ H

+ X

, - X

H

+ X

, - X

H

H2 +

Fig

ure

5.20

:P

rim

ary

reac

tion

pat

hw

ays

for

met

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idat

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inth

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onflam

e

Chapter 6

Conclusions and Recommendations

6.1 Conclusions

This thesis examined the oxidation of two short chain FAMEs in an opposed flow diffusion

flame configuration. The primary goal was to compare the emission profiles for methyl

butanoate and methyl crotonate to determine the role of FAME molecular structure

(i.e. unsaturation) in combustion. The experimental results were also used to validate

a chemical kinetic model for methyl butanoate, in hopes of better understanding the

oxidation of the saturated long chain FAMEs found in biodiesel.

Our results indicate that methyl butanoate and methyl crotonate produce similar

peak mole fractions for carbon monoxide (CO), carbon dioxide (CO2), methane (CH4),

ethane (C2H6), formaldehyde (CH2O), and acetaldehyde (CH3CHO). The methyl bu-

tanoate flame produces higher levels of ethylene (C2H4) and propane (C3H8). However,

the methyl crotonate flame produces higher concentrations of propene (C3H6), acety-

lene (C2H2), propyne (C3H4), 1-butene (1-C4H8), 2-butyne (2-C4H6), butane (C4H10),

n-pentane (C5H12), and 1,3-butadiene (1,3-C4H6). In addition, the methyl crotonate

flame produces methanol (CH3OH), acrolein (C3H4O), acetone (C3H6O), and benzene

(C6H6) in detectable quantities, while these species were below the detection limit in

methyl butanoate flames.

The differences between methyl butanoate and methyl crotonate emissions profiles

were explained by reaction pathway analysis. Higher concentrations of ethylene (C2H4)

and propane (C3H8) in the methyl butanoate flame are attributed to the intermediate

n-propyl radical (C3H7), which is derived from the fuel by a series of H abstraction and de-

composition reactions. However, the analogous H abstraction and thermal decomposition

90

Chapter 6. Conclusions and Recommendations 91

reactions on methyl crotonate lead to the intermediate 1-propenyl radical (C3H5), which

reacts to form propene (C3H6) by H addition and acetylene (C2H2) and propyne (C3H4)

by β-scission. Acetylene pyrolizes to form higher molecular weight alkenes, alkynes,

and aromatic species, thereby rationalizing higher levels of 1-butene (1-C4H8), 2-butyne

(2-C4H6), 1,3-butadiene (1,3-C4H6), and benzene in the methyl crotonate flame.

The higher levels of methanol (CH3OH) in methyl crotonate are attributed to re-

actions involving the intermediate methoxycarbonyl radical (CH3OCO). This radical is

more readily formed in methyl crotonate due to the induction effect of the double bond

adjacent to the carbonyl group. Higher levels of acrolein (C3H4O) in the methyl cro-

tonate flame are attributed to a series of fuel decomposition and H addition reactions;

whereas, the analogous reactions on methyl butanoate lead to the formation of propenal

(C3H6O).

Acetylene, propyne, C4 hydrocarbons, and benzene are known precursors and inter-

mediates in the formation of soot. These compounds were all found in higher levels in

the methyl crotonate flame, and it was shown that the double bond in responsible for

the formation of these compounds. Therefore, it can be concluded that unsaturated long

chain FAMEs will also display a greater tendency to form these soot precursors. Un-

der rich conditions, it can be expected that unsaturated FAMEs will lead to more soot

formation than saturated FAMEs.

The experimental and modeling results exhibit the benefits of the modifications per-

formed by members of our Combustion Research Group on the original methyl butanoate

oxidation mechanism [8]. The modified mechanism improved upon the original by: i.)

including the prediction of C4 hydrocarbons via addition of a C4 sub-mechanism, ii.)

improving the prediction of methyl butanoate reactivity via modification of specific re-

action rate constants, and iii.) improving the prediction of formaldehyde and carbon

monoxide concentration via modification of specific reaction rate constants. The peak

concentrations of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethylene

(C2H4), propane (C3H8), and ethane (C2H6) are well predicted by the modified model.

However, the model over-predicts the peak concentration of acetylene (C2H2), 1-butene

(C4H8), propene C3H6, propyne (pC3H4), and formaldehyde (CH2O), and under-predicts

the peak concentrations of 1,3-butadiene (1,3-C4H6) and acetaldehyde (CH3CHO).

The reaction pathways of methyl butanoate oxidation in the opposed flow diffusion

flame were studied. It is concluded that H abstraction from carbon atoms and unimolec-

ular thermal decomposition are the key consumption reactions, with the former being


more important at low and intermediate temperatures. The primary intermediates are

the 2-methyl butanoate and 3-methyl butanoate radicals, which further react to form

methyl propenoate and methoxycarbonyl radical, respectively. Unimolecular decompo-

sition of the methoxycarbonyl radical (CH3CO·) leads primarily to CO2. From a soot

suppression standpoint, this is undesirable because two fuel-bound oxygen atoms are

bonded to one carbon atom. Ideally, each oxygen atom should bond with a single carbon

atom, thereby prevent the carbon atom from being involved in a soot formation reaction.

Thus, it can be concluded that the ester moiety will not be highly effective at reducing

soot emissions.

The initial reaction pathways of methyl crotonate were predicted from the methyl

butanoate oxidation mechanism. H abstraction from carbon atoms and unimolecular

decomposition are expected to be the major routes of methyl crotonate consumption.

In addition, the double bond in methyl crotonate would be susceptible to attack by the

biradical O. A validated detailed chemical kinetic model for methyl crotonate will be

published in 2006 by our Combustion Research Group.

6.2 Recommendations

1. The methyl butanoate model well-predicts the concentrations of many species in

the opposed flow diffusion flame. However, the model over-predicts the peak con-

centrations of propene, propyne, acetylene, 1-butene, and formaldehyde. It also

under-predicts the concentrations of 1,3-butadiene and acetaldehyde. It is rec-

ommended that the model be further refined to provide better agreement for the

aforementioned species.

2. Currently, a detailed chemical kinetic model for the oxidation of methyl crotonate

does not exist. Such a model would provide a better understanding of the oxidation

of unsaturated long chain FAMEs. Our Combustion Research Group is in the

process of developing this model.

3. Detailed chemical kinetic mechanisms for long chain FAMEs of varying molecular

weight and degree of saturation would also be useful. It is recommended that such

mechanisms be developed and experimentally validated across a number of well-

defined combustion platforms. Such a study will be pursued by the author as a

PhD student in The Combustion Group.


4. The measurement of higher molecular weight PAH species should be measured in

flames of higher molecular weight fuels. The measurement of these species can

be accomplished by sequestering the gaseous species on a resin bed, followed by

thermal desorption of the gaseous species and analysis using a GC/FID. Such a

study will be pursued by the author as a PhD student in The Combustion Group.

5. This study was conducted under non-sooting flame conditions. It is recommended

that a study be conducted under sooting conditions to directly quantify the level of

soot formation by saturated and unsaturated fuels. Measurements can be obtained

using the light extinction technique. Such a study will be pursued by the author

as a PhD student in The Combustion Group.

6. It is recommended that the experimental and analytical equipment in The Combus-

tion Research laboratory be upgraded for future studies. The range and accuracy of

measured species can be greatly improved by purchasing a new GC/FID integrated

with the following:

• a methanizer for measuring CO2 and CH2O;

• a molecular sieve for separating CO and O2 from N2;

• a thermal conductivity detector (TCD) for measuring CO, O2, and H2;

• and electronically controlled flow controls.

7. The opposed flow diffusion flame conditions can be controlled better with the im-

plementation of mass flow meters for measurement of fuel and oxidizer gas flows

to the burner. The health and safety of laboratory personnel can be improved by

installing an improved ventilation system.

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Appendix A

Flow Rates of Fuel and Oxidizer

101

Appendix A. Flow Rates of Fuel and Oxidizer 102

Flow rates of fuel and oxidizer

Methyl butanaote (mb)

Fuel or oxidizer

streamFlow rate

Rotameter

reading

Nitrogen (gas) Fuel 3.11 slpm 65 SS

Methyl butanaote (liq.) Fuel 0.72 ml/min NA

Air (gas) Oxidizer 2.24 41 SS

Oxygen (gas) Oxidizer 0.826 65 SS

Density of mb = 0.8984 g/ml; Molecular weight of mb = 102.13 g/mol

Therefore, molar flow rate of mb = 6.32 E-3 mol/min

Molar flow rate of N2 in fuel stream = 0.127747 mol/min

Mole fraction of N2 in fuel stream = 95.28 %

Mole fraction of mb in fuel stream = 4.72%

Similarly, for the oxidizer stream the mole fractions of nitrogen and oxygen are obtained.

Mole fraction of N2 in oxidizer stream = 57.75%

Mole fraction of O2 in oxidizer stream = 42.25%

Appendix A. Flow Rates of Fuel and Oxidizer 103

Methyl crotonate (mc)

Fuel or oxidizer

streamFlow rate

Rotameter

reading

Nitrogen (gas) Fuel 3.11 slpm 65 SS

Methyl crotonate (liq.) Fuel 0.67 ml/min NA

Air (gas) Oxidizer 2.24 41 SS

Oxygen (gas) Oxidizer 0.826 65 SS

Density of mc = 0.945 g/ml; Molecular weight of mc = 100.12 g/mol

Therefore, molar flow rate of mc = 6.32 E-3 mol/min, same as that of mb

Molar flow rate of N2 in fuel stream = 0.127747 mol/min

Mole fraction of N2 in fuel stream = 95.28 %

Mole fraction of mc in fuel stream = 4.72%

Similarly, for the oxidizer stream the mole fractions of nitrogen and oxygen are obtained.

Mole fraction of N2 in oxidizer stream = 57.75%

Mole fraction of O2 in oxidizer stream = 42.25%

Appendix B

Structures of Chemical Compounds

104

Appendix B. Structures of Chemical Compounds 105

propene

C

allene

CH3•

methyl radical

CH2:

methylene radical

1,3-butadiene

H•C

iso-propyl radical

CH•

propenyl radical

acetylene

O

O

methyl crotonate

O

O

methyl butyrate

C O

carbon monoxide

CO O

carbon dioxide

O

formaldehyde

CH4

methane

ethane

ethylene

acetylene

propene

propyne

propane

1-butene

2-butyne

butane

CH2•

n-propyl radical

O

acetaldehyde

O

acrolein

O

acetone

benzene

OH

methanol

n-pentane

C•

OO

methoxy carbonyl

Table B.1: Structures of relevant chemical species

Appendix C

Sample Chromatograms

106

Appendix C. Sample Chromatograms 107

Figure C.1: Example of HPLC chromatogram of DNPH derivatives


Figure C.2: Example of GC chromatogram for the HP Plot Column


1,3-ButadieneColumn: DB-624

25 m x 0.20 mm I.D., 1.12 µmP/N: 128-1324Carrier: Helium at 1.0 mL/min

Oven: -20°C for 3 min

-20 - 20°C at 4°/min

20 - 200°C at 8°/min

200°C for 10 min

Injector: Split 1:150, 250°C

0.5 µL injection

Detector: FID, 250°C

Agilent Technologies wishes tothank DCG Industries (Pearland, TX)for providing this chromatogram.

Refined Butadiene StandardComponent Gravimetric concentration (PPM)1. Acetylene 20.7

2. Propane 19.8

3. Propylene 296

4. Propadiene (allene) 21.1

5. Propyne (methylacetylene) 21.0

6. Cyclopropane 20.0

7. Isobutane 506

8. Butene-1 999

9. Isobutylene 495

10. n-Butane 494

11. 1,3-Butadiene balance

12. trans-2-Butene 442

13. cis-2-Butene 1946

14. 1-Butyne (ethylacetylene) 20.2

15. 1,2-Butadiene 28.9

16. 3-Methyl-1-butene 19.8

17. Isopentane 50.1

18. Pentene-1 29.8

19. n-Pentane 50.1

20. 2-Butyne (dimethylacetylene) 150

21. trans-2-Pentene 5.57

22. Isoprene 20.0

23. cis-2-Pentene 13.9

24. trans-1,3-Pentadiene 13.8

25. cis-1,3-Pentadiene 7.73

26. Benzene 20.3

27. Toluene 20.2

28. Dimer (4-vinylcyclohexene-1)

C7770 5 10 15 20

Time (min)

1

2, 3

4

56

7 10

8, 9

11

1213

1415

16

1718

19

20

21

2223 24 25

26 27

28

Figure C.3: Example of GC chromatogram for the DB 624 Column

Appendix D

Sample Calculations

110

Documents

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