Emissions and properties of Bio-oil and Natural Gas Co ......Abstract Emissions and properties of...

Emissions and properties of Bio-oil and Natural Gas Co-combustion in aPilot Stabilised Swirl Burner

by

Dylan Kowalewski

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

Graduate Department of Mechanical and Industrial EngineeringUniversity of Toronto

c© Copyright 2015 by Dylan Kowalewski

Abstract

Emissions and properties of Bio-oil and Natural Gas Co-combustion in a Pilot Stabilised Swirl Burner

Dylan Kowalewski

Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

2015

Fast pyrolysis oil, or bio-oil, has been investigated to replace traditional fossil fuels in industrial burners.

However, flame stability is a challenge due to its high water content. In order to address its instability,

bio-oil was co-fired with natural gas in a lab scale 10kW swirl burner at energy ratios from 0% bio-oil to

80% bio-oil. To evaluate the combustion, flame shape, exhaust and particulate emissions, temperatures,

as well as infrared emission were monitored. As the bio-oil energy fraction increased, NO emissions

increased due to the nitrogen content of bio-oil. CO and particulate emissions increased likely due to

carbonaceous residue exiting the combustion zone. Unburnt Hydrocarbon (UHC) emissions increased

rapidly as combustion became poor at 60-80% bio-oil energy. The temperature and infrared output

decreased with more bio-oil energy. The natural gas proved to be effective at anchoring the bio-oil flame

to the nozzle, decreasing instances of extinction or blowout.

ii

Dedication

For Marianne, without whom this would not have happened

iii

Acknowledgements

I would like to acknowledge Prof. M. J. Thomson and Prof. H. Tran for their guidance and support.

I would like to thank Y. Afarin, S. Zadmajid and V. Sookrah for their assistance in running these

experiments. I would like to thank the Surface Ontario Lab for providing the photomicroscope to

examine the particles contained in the bio-oil. Financial support for this work was provided by NSERC

and BioFuelNet Canada. Finally, I would not have been able to complete this without the support of

my friends and family.

iv

Contents

1 Introduction 1

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

1.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Literature Review 3

2.1 Bio-Oil Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Bio-Oil Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 Bio-Oil Upgrading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3.1 Physical Bio-Oil Upgrading Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3.2 Chemical Bio-Oil Upgrading Methods . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Bio-Oil as a Fossil Fuel Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.5 Bio-Oil Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Apparatus Design 8

3.1 Burner Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Energy Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Variable Swirl Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.4 Fuel Atomizing Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4.1 Natural Gas System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.5 Pilot Flame System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.5.1 Pilot Flame Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Experimental Methodology 13

4.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2 Fuel Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2.1 Bio-Oil - Natural Gas Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2.2 Bio-Oil Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2.2.1 Thermogravimetric Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2.2.2 Photomicroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2.3 Bio-Oil Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.2.4 Natural Gas Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3 Gas Phase Emission Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3.1 Unburnt Hydrocarbon Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3.2 Detailed Exhaust Gas Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

v

4.3.3 Equivalence Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.4 Flame Visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.5 Flame Infrared Emission Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.5.1 Infrared Background Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.6 Burner Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.7 Particulate Measurement and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.7.1 Isokinetic Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.7.2 Particulate Sampling System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.7.3 Particulate Sample Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.8 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.8.1 Burner Start-up and Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.8.2 Gas Phase Emissions Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.8.3 Particulate Emissions Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.8.4 Infrared Emissions Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.8.5 Flame Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5 Results and Discussion 23

5.1 Fuel Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2 Base Point Operation and Repeatability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

5.3 Reaction Zone Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.4 Flame Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.5 Particulate and Gas Phase Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.5.1 Exhaust Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.5.2 Particulate Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.6 Near Infrared Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.7 Effect of Natural Gas on Bio-Oil Combustion . . . . . . . . . . . . . . . . . . . . . . . . . 34

6 Conclusions and Recommendations 36

6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.2 Implications for Industrial Burners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.3 Recommendations and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.3.1 Burner Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.3.2 Experimental Methodology Improvements . . . . . . . . . . . . . . . . . . . . . . . 38

6.3.3 Future Bio-Oil Combustion Research . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Bibliography 39

Appendix A Fuel Flow Rate Calculation 43

Appendix B Peristaltic Pump Flow Rate Calibration 44

vi

Chapter 1

Introduction

1.1 Motivation

There are many industrial processes that require heat that currently derive this heat with fossil fuels.

However, due to liquid fuel prices and environmental considerations, there is an interest to replace a

portion of these fossil fuels with biofuels. Biomass is an important fuel world wide, making up 12% of

the worlds energy[1]. However, for many applications, a solid fuel is inappropriate, either due to storage

or incompatibility. A biomass derived liquid fuel could act as a drop in replacement for fossil fuels.

Biomass can be converted to liquid through the fast pyrolysis process[2]. The biomass is first finely

ground, then heated in the absence of oxygen at a high temperature. The vapours that come off the

biomass are then condensed, yielding fast pyrolysis liquid, or bio-oil. Bio-oil holds many advantages over

raw biomass, which is often used in residential heaters[3]. Primarily, due to being liquid, bio-oil is more

energy dense and is easier to transport and store. Furthermore, bio-oil can be produced using the waste

wood produced in the forestry and pulp and paper industries. Because of this, compared to other crop

based biofuels such as ethanol, the biomass used in bio-oil production does not need to be grown on land

that could otherwise be used for food agriculture[4].

To show how bio-oil can be used in furnaces and burners in industry, previous work has examined its

performance when blended with ethanol[5]. However, for industries in most parts of the world, ethanol is

too expensive to be used in heating applications[6]. However, in these studies, the ethanol was found to

improve combustion due to its low viscosity and ease of ignition[7]. A more viable solution would utilise

bio-oil as-is as a partial replacement for a fossil fuel. In this way, the fossil fuel provides the ignition

energy required to stabilise the bio-oil combustion. However, it is important to understand what effect

the change in fuel will have on the combustion.

Specifically, lime kilns in the pulp and paper industry offer a unique opportunity for the use of bio-oil.

Capable of burning a wide variety of fuels from natural gas to crushed petcoke, lime kilns are the only

part of the pulp production process that uses fossil fuels. Natural gas use in lime kilns has increased

recently as the fuel cost has decreased. Replacing some of this natural gas with bio-oil can help pulp

mills run more carbon neutral and have less impact on global warming. Furthermore, because pulp

mills often have waste wood on site, producing bio-oil at the mill eliminates the need to transport large

amounts of fuel, adding to the appeal.

1

1.2 Objective

The overall objective of this study is to investigate the combustion properties of bio-oil when it is

co-fired with natural gas. To achieve this, a 10 kW swirl burner is used and modified to atomize the

bio-oil with natural gas. The total energy of the system is kept constant and is run with different ratios

of bio-oil and natural gas to evaluate how combustion changes as more bio-oil is used. The combustion is

evaluated by looking at the gaseous (NOx, CO, unburnt hydrocarbons) and particulate emissions as well

as the near infrared heat output, flame images and temperatures. These measurements will be related

to the properties of bio-oil and the physical phenomena observed in the burner. The observations made

in this study will provide insight to how bio-oil and natural gas can be used in industrial burners to

reduce their use of fossil fuels.

2

Chapter 2

Literature Review

2.1 Bio-Oil Production

Bio-oil is produced in a fast pyrolysis process by burning biomass at high temperature in the absence

of oxygen[2]. As the biomass breaks down and thermally cracks, vapours are given off that can be

condensed into a liquid fuel. The process is run at high temperature to maintain a low vapour residence

time to prevent decomposition of the hydrocarbons so the remain condensable[2]. In addition to bio-

oil, char and non-condensable gases are also produced which can provide the energy needed to dry the

biomass as well as run the pyrolysis process[8]. By using these excess products to run the plant, there

are very few waste products in bio-oil production and the need for energy input from fossil fuels can be

minimized. Figure 2.1 shows a typical fast pyrolysis process schematic.

Figure 2.1: Fast pyrolysis plant schematic[8]

3

Chapter 2. Literature Review 4

The process used to produce bio-oil can be controlled to improve the quality of the fuel produced.

Firstly, by drying the feedstock as much as possible, the water content of the produced bio-oil can be

minimized [2]. However, it is important to note that the water contained in bio-oil reduces its viscosity,

making it easier to pump[5]. Secondly, the char that is produced in the pyrolysis process acts as a

catalyst for thermal cracking[9]. If the char is not separated from the bio-oil quickly, this cracking can

increase the amount of non-condensable gases produced, reducing conversion efficiency. Furthermore,

char remaining in the final product increases fuel aging caused by polymerization, introducing storage

challenges[10]. Typical bio-oil production systems can convert up to 70 wt% of the biomass to bio-oil.

2.2 Bio-Oil Properties

Table 2.1 shows the typical properties of bio-oil from literature as well as the requirements according

to the ASTM D7544 standards. It is important to note that the properties of bio-oil vary based on the

feedstock used for its production. For example, the ash content of a bio-oil made with only core wood

will be lower than that of a bio-oil that is made using bark.

Table 2.1: Typical range and ASTM D7544 standard properties of bio-oil [11, 12, 13, 14, 15, 16]

Parameter Units Typical Range ASTM D7544 Grade GWater Content wt% 15-30 30

Viscosity cSt@40◦C 10-100 125Acidity pH 2-3 ReportDensity kg/L 1.2 1.1-1.3

Solids Content wt% 0.2-1 2.5Ash Content wt% 0-0.3 0.25

Carbon Content wt% 50-60 -Hydrogen Content wt% 5-7 -Nitrogen Content wt% 0-0.3 -

Sulfur Content wt% 0-0.5 0.05Oxygen Content wt% 35-40 -

LHV MJ/kg 13-18 15Flash Point ◦C 60-100 45Pour Point ◦C -40-3 -9

The typical chemical composition of bio-oil influences its combustion emissions. Because it is made

from biomass, there is a significant amount of nitrogen contained in the fuel will result in increased NOx

emissions. However, because there is typically very little sulfur in bio-oil, it does not suffer from SOx

emissions like many petroleum based fuels.

The high water content of bio-oil results in poor ignition and can cause instability during its burning.

This is compounded with the fact that a large portion of the fuel is non-volatile. For this reason, many

previous studies have used blends of bio-oil and ethanol, which adds a volatile fraction to the fuel [5].

However, in a low grade heating application, ethanol blending may be too expensive. For this reason,

using natural gas, which is often used in such applications, can help with ignition and stability[7].

Ash contained in the bio-oil results in higher particulate matter (PM) emissions during combustion.

In many applications, this may not be ideal and can cause clogging of equipment [17].

The acidity must be taken into account when designing the fuel delivery system for bio-oil. While

it will not immediately corrode steel, using stainless steel for all surfaces that come into contact with


liquid bio-oil would greatly increase the life of the system.

2.3 Bio-Oil Upgrading

Because bio-oil contains many compounds such as water and ash that make it difficult to use as a

direct fossil fuel replacement, there is often a desire to upgrade it[18]. These methods can be broken

down into physical upgrading and chemical upgrading.

2.3.1 Physical Bio-Oil Upgrading Methods

Physical methods of upgrading bio-oil attempt to remove specific undesirable properties without

changing the chemical composition of the fuel. Typical methods of physical upgrading include filtering

the solids out of the bio-oil to slow down fuel aging [10] through hot gas filtration. Additionally, adding

solvents can reduce its viscosity to improve spray characteristics. Lastly, distillation can remove the

water from the bio-oil improving its heating value.

Cyclonic separation is a common method of filtration; however because cyclonic separation is not

very efficient at removing 5-10 micron particles, char remains in bio-oil filtered this way [19]. If hot

gas filtration is performed bio-oil after cyclonic separation, the remaining particles can be removed [20].

The result is a very low solids and ash bio-oil that ages more slowly [21]. It has been shown that aging

can greatly affect the combustion quality and emissions when burned at the same conditions [10]. The

drawback of hot gas filtration is that because the filter is heated from 350 ◦C to 400 ◦C, the residence

time in the reactor is increased, resulting in further thermal cracking [21][22]. While the quality of the

bio-oil produced is increased, only 40-50 wt% of the biomass is converted to bio-oil[22]. While this loss

to non-condensable gases can be recovered to run the process and dry the feedstock, if the fuel is more

expensive, it may not be viable for use in industry.

Adding a solvent to the bio-oil can help with fuel flow and improve spray characteristics. Adding

methanol has been observed to greatly decrease the rate of aging when 10 wt% is blended with bio-

oil [23]. Furthermore, the decreased fuel viscosity achieved with solvent blending improves the spray

characteristics of the fuel, improving combustion stability [5]. The main drawback of solvent blending is

the cost associated with methanol and ethanol. While methanol is produced in paper pulp production,

water and impurities must be removed [24]. Ethanol, while in some areas of the world, such as Brazil,

can be produced in large enough quantities to be used as fuel, it is typically reserved for transportation

fuel and not as an alternative to fuel oil [25]. The most viable solution would be to utilize bio-oil in an

unblended form.

Removing the water contained in bio-oil would be useful to maximize its specific heating value.

However, if the water is simply boiled off, the bio-oil will rapidly polymerize [26]. By reducing the

pressure, the temperature at which the water can be boiled off is reduced, thus minimizing the effect of

of polymerization during the distillation [27]. Vacuum distillation is often used when a solution contains

compounds that are sensitive to temperature [27]. However, the water that is contained in the bio-oil

aids in its use by reducing its viscosity. A dewatered bio-oil is very viscous and may be difficult to pump

in a burner, possibly requiring the use of a solvent. As previously discussed, this study is focused on

utilizing bio-oil without the need for solvents.


2.3.2 Chemical Bio-Oil Upgrading Methods

Bio-oil can be chemically upgraded to produce a fuel that is more similar to the fuels that it is replac-

ing. The two main methods of upgrading bio-oil chemically are catalytic cracking and hydrotreating.

Catalytic cracking is a method of using a catalyst and heat to break apart high molecular weight

hydrocarbons to produce a fuel made of lower molecular weight hydrocarbons. In addition, the oxygen

contained in the fuel is removed and released as CO and CO2 [28]. The result is the removal of the water,

which contains most of the oxygen in the bio-oil. While the result is a transportation grade fuel, there

are some significant drawbacks to catalytic cracking. Because the zeolite catalyst requires a temperature

of 450 ◦C, the polymerization of the bio-oil can deactivate the catalyst over time [28]. Furthermore,

carbon released in the deoxygenation means that the conversion rate is typically poor (20-30%) [29].

Hydrotreating, or hydrodeoxygenation, addresses some of the issues with catalytic cracking. Using a

catalyst at high temperatures and pressures in a hydrogen rich environment, the oxygen is removed as

water [29]. Similar to catalytic cracking, the high molecular weight hydrocarbons are also broken down

to produce a higher grade fuel. Because the oxygen isn’t released as CO or CO2, there is a much higher

yield compared to catalytic cracking (up to 60%) [29].

These catalytic methods however both suffer from the same issues. Catalysts are typically expensive,

so chemically upgrading bio-oil is not currently the best solution to broadening its appeal. Furthermore,

this study is focused on the use of bio-oil as a replacement in low grade applications. The fuels produced

with these upgrading methods would be more suited as a replacement for transportation fuels.

2.4 Bio-Oil as a Fossil Fuel Replacement

Due to the impurities contained in bio-oil, it is not directly suitable for a replacement for transporta-

tion fuels like gasoline or diesel. This study is focused on evaluating its potential for use as a replacement

for heavy fuel oil, often used in low grade heat applications. For example, in the pulp and paper industry,

many different fuels can be used to power the lime kiln used to convert calcium carbonate (CaCO3) into

lime (CaO) [30]. There are many reasons that lime kilns are a good candidate for bio-oil use, primarily

due to other types of fuels typically used, the combustion environment and the availability of feedstock.

Rotary lime kilns are very simple combustion devices that are used to produce lime in the pulp

and paper and cement industries. Because of this simplicity, a wide variety of fuels can be used, from

natural gas to fuel oils to petroleum coke [30]. The versatility of these systems lends itself well to

adapting to different fuels as long as they can be burned via spray combustion. Furthermore, due to the

environment inside the lime kiln, the ash content of the bio-oil is not as important as it would be inside

an internal combustion engine or other combustion devices with moving parts that can get clogged with

the ash left on the walls. In a lime kiln, the mass of the ash from the bio-oil is small relative to the

inorganics contained in the lime and would therefore not impact coking inside the burner [31]. Lastly,

as with all biofuels, feedstock availability is always a concern. By using waste wood and implementing

a bio-oil reactor as part of the pulp and paper plant, not only is access to feedstock improved, but

the cost of transporting the fuel can be minimized [32]. Previous research has shown that utilizing

bio-oil in lime kilns is possible, however the heat release compared to natural gas was shown to be lower

[33]. However, this study did not investigate the synergistic effect of co-firing natural gas and bio-oil to

improve combustion.


2.5 Bio-Oil Combustion

The overall combustion phenomena occurring within the spray combustion of bio-oil can be explained

by knowing what happens during the combustion of a single droplet. As the droplet of bio-oil is exposed

to the hot combustion environment, the low molecular weight volatile fraction begins burning. During

this phase of combustion, the higher molecular weight compounds at the surface of the droplet experience

polymerization [34]. As the surface of the droplet polymerizes, the pressure inside the droplet increases

due to water and volatiles boiling. The result is that the volatiles force their way out of the skin, leaving

a hollow cenosphere behind which then burns out [35]. This process is very important to understand

what occurs inside a lab scale burner. Because there is solid char remaining after the volatiles have

burned, the combustion only occurs on the surface of the char particles, so the reaction is much slower.

The most important parameter for bio-oil combustion is droplet size. While in any spray, there is

a distribution of droplet diameters, the Sauter Mean Diameter, or SMD, describes the diameter of the

droplets if they all had the same volume to surface area ratio [36]. For practical devices, this parameter

can be estimated using a variety of empirical equations, and one for air blast nozzles has been selected

for this study, shown in Equation 2.1[36], while Table 2.2 describes the variables in this equation.

SMD = 0.95[(σmL)33

ρ0.37L ρ0.30A UR](1 +

mL

mA)1.70 + 0.13(

µ2LdoσρL

)(1 +mL

mA)1.70 (2.1)

Table 2.2: Variables in SMD calculation

Variable Parameterσ Surface tensionmL Liquid mass flow ratemL Air mass flow rateρL Liquid densityρA Air densityUR Air-liquid relative velocityµL Dynamic viscositydo Discharge orifice diameter

In order to keep the bio-oil combustion the same in each test, the atomizing gas flow rate is adjusted

to keep a constant SMD. Taking the nozzle capabilities into account, an SMD of 100 µm was selected.

By keeping the droplet size constant, the size of the droplet will not influence the results.

Chapter 3

Apparatus Design

3.1 Burner Design

The apparatus used in this study is a 10 kW pilot stabilised spray burner. The burner consists of

a swirl generator, fuel nozzle, pilot flame, and the main burner section and its overall configuration is

shown in 3.1.

Figure 3.1: Overall burner configuration [5]

8

Chapter 3. Apparatus Design 9

The burner is constructed from 3.2 mm thick 316 stainless steel to prevent corrosion that may occur

if liquid bio-oil collects on the walls. The overall size of the burner is 1.2m in length and a 221 mm inner

diameter at its widest point. The nozzle is mounted such that the fuel is sprayed vertically downward.

The swirl generator is mounted to the top of the main burner section and is constructed from aluminium

and mild steel. There was no need to construct the swirl generator from stainless steel because its

components do not come into contact with the bio-oil.

In order to connect the components of the burner, flange connections are used to make maintenance

and upgrading easier. The flanges are sealed with silicone rubber gaskets rated for 260 ◦C in lower

temperature areas and with compressible graphite gaskets rated for 450 ◦C in higher temperature areas,

such as near the flame. The components of the burner are described in more detail below.

3.2 Energy Throughput

The burner is designed for a 10 kW input energy. While this is significantly lower than the typical

input energy for a lime kiln or other industrial burner, it allows for the combustion properties of bio-oil

to be studied in a lab setting without needing to store huge amounts of fuel. Furthermore, there are

currently few producers that make bio-oil in large quantities. However, by using a smaller burner, it is

possible to have custom bio-oils made to investigate the ideal properties that a bio-oil should have to be

best suited for an industrial burner.

To ensure each test run is comparable with each other, the overall input energy is kept at a constant

10 kW. So, for each test, the input energy is divided between bio-oil and natural gas by energy using the

Lower Heating Value (LHV) of each fuel. For example, in the case where 80% of the input energy comes

from bio-oil, the flow rate of the two fuels is set so that 8 kW is input from bio-oil and the remaining

2 kW is input from natural gas.

3.3 Variable Swirl Generator

At the top of the burner, the primary combustion air enters through a variable swirl generator as

shown in figure 3.2.


Figure 3.2: Variable Swirl Generator Design

The swirl generator guides the heated primary combustion air entering the burner to impart a degree

of angular momentum, the amount of which is determined by the position of the movable blocks. Previous

research on this burner has shown the effects the degree of primary air swirl has on bio-oil combustion, so

these effects are not investigated in this study. Instead, the swirl number is set to the maximum value of

5.41, which is estimated using Equation 3.1. This value was chosen because it has been shown to produce

the most stable flames[5]. The value of the swirl produced depends on the physical characteristics of the

swirl generator which are provided in Table 3.1

S ≈ 2π

nξmsinα

cosα[1 + tanα tan(ξ/2)](ξ/ξm)

{1− [1− cosα(1 + tanα tan(ξ/2))]ξξm}2R

2B

[1− (

Rh

R)2]

(3.1)

Table 3.1: Geometric parameters for movable block type swirl generator

Parameter Description Design Valuen Number of swirl blocks 8R Swirl generator exit radius 76.2 mmRh Swirl generator inner radius 9.53 mmB Depth of swirl blocks 38.1 mmα Fixed swirl block angle 60◦

ξ Adjustable swirl block angle -ξm Maximum opening angle 12◦

By giving the primary air swirl, as it enters the main burner section, the flow expands outward

reducing the pressure in front of the nozzle, producing a recirculation zone as shown in Figure 3.3. This

recirculation improves combustion stability by bringing hot combustion products back toward the nozzle,

increasing the residence time of fuel gases and droplets as well as greatly improves mixing.


Figure 3.3: Recirculation zone formed in a swirling flow[37]

3.4 Fuel Atomizing Nozzle

The burner uses an internal-mix air blast atomizing nozzle (BEX Engineering: model 1/4” JX6BPL11

with a 152 mm long extension tube and 2X2JPL back-connect body, a JPG60 air cap and JPL40100

liquid cap) to atomize the fuel. All components are made from 316 stainless steel to prevent corrosion

due to the acidic nature of the fuel. The overall configuration of the nozzle is shown in Figure 3.4.

Figure 3.4: Burner nozzle assembly[38]

The bio-oil enters the nozzle through the central 1.0 mm diameter orifice in the liquid cap. The liquid

fuel is then forced through six 0.89 mm orifices in the air cap. The spray pattern that is produced is six

equally spaced jets around a 65◦ hollow cone. This spray pattern was selected due to its compatibility


with the central recirculation zone. That is, the spray does not introduce enough axial momentum to

penetrate the entire recirculation zone the way a single axial fuel jet would. The recirculation zone is

then able to form in the hollow cone between the fuel jets, improving flame stability [5].

3.4.1 Natural Gas System

In previous studies using this burner, the atomizing gas was compressed air. For this experiment

however, it was necessary to burn both bio-oil and natural gas together. The atomizing gas was changed

by replacing the air input with a tee connected to natural gas and nitrogen cylinders. By using nitrogen

as a balance gas, the atomizing flow rate can be adjusted to provide adequate atomization for all natural

gas energy flow rates. Furthermore, by using nitrogen, the gas mixture inside the tube is non-flammable

for safer operation.

3.5 Pilot Flame System

The flame is stabilised by a oxygen-natural gas pilot flame (Hoke model No. 110-406) which is run

for the duration of each test to prevent any extinguishing of the flame. The nozzle tip is a 1.2 mm orifice

with a hexagonal ”rosebud” pattern. This nozzle pattern produces a wider flame than a single orifice

nozzle, which is more stable in the turbulent environment inside the burner. The torch is mounted to

the burner with a 1/4” bore-through compression fitting such that only 5 mm of the tip is inside the

burner. This reduces the impact the pilot flame has on the air flow of the burner. The flow rate of

natural gas through the nozzle is set to 0.95 SLPM to achieve 0.5 kW pilot flame energy. The energy

from the pilot flame is not part of the 10 kW burner energy.

3.5.1 Pilot Flame Alignment

It is important for the pilot flame to be properly aligned relative to the nozzle jets to achieve good

combustion. While the pilot flame port is stationary, the fuel atomizing nozzle can be rotated in its

mounting collar. While burning pure ethanol, the fuel atomizing nozzle is rotated until the pilot flame

is in a good position between two fuel jets as shown in Figure 3.5.

(a) Good pilot alignment (b) Poor pilot alignment

Figure 3.5: Pilot flame alignment quality [5]

Chapter 4

Experimental Methodology

4.1 Experimental Setup

Figure 4.1 shows an outline of how the overall burner system and analysis equipment are connected.

Figure 4.1: Overall burner setup schematic

The bio-oil is pumped to the nozzle using peristaltic pumps through Teflon tubing connected with

316 stainless steel Swagelok fittings to prevent the bio-oil from corroding the materials. Two peristaltic

pumps are used in parallel to reduce the intermittent flow that was observed when using only one pump,

especially at low flow rates. The pumps are set to an rpm that is calibrated to the desired flow rate in

mL/min. This calibration method is shown in Appendix B. A pressure relief valve is installed between

the pumps and the nozzle to prevent any issues that may arise if the nozzle becomes clogged. To prepare

the burner for bio-oil combustion, it is first heated up with ethanol. By switching to bio-oil after the

burner is up to temperature, the occurrence of bio-oil droplets hitting the walls and forming char deposits

13

Chapter 4. Experimental Methodology 14

is greatly reduced.

Primary combustion air is pulled through the burner through the two stack fans downstream of the

burner with its flow rate monitored with a flow meter and is adjustable with variable voltage transformers.

This maintains a negative pressure of approximately 150 Pa inside the combustion region of the burner.

The main benefit of this compared to pushing air through the burner is that the negative pressure ensures

that no gases leak out of the burner. The primary air is heated with a 1.5 kW air heater controlled with a

variable voltage transformer to preheat the combustion air to 230 ◦C to 250 ◦C. Heating the combustion

air also helps in atomization by warming up the bio-oil as it enters the fuel nozzle, reducing its viscosity.

The exhaust stream is tapped at two locations for analysis. First, for the isokinetic particulate

matter sampling system and second for the gas phase sampling systems. The isokinetic particulate

matter sampling system is used to collect particulates on filters for gravimetric analysis. For the gas

phase sampling system, the exhaust is transported with a 1/4” heated sampling line at 190 ◦C to 195 ◦C to

prevent condensation of water or hydrocarbons. The exhaust is then passed through a heated glass/Teflon

filter to remove particulates before the gas composition is measured. The unsampled exhaust is passed

through a heat exchanger and condenser to reduce the temperature of the gas going through the stack

fan and remove as much moisture as possible.

4.2 Fuel Analysis

4.2.1 Bio-Oil - Natural Gas Mixtures

In this study, two fuels is co-fired with natural gas. It has been observed that 100% bio-oil will

not ignite in the burner, likely due to the lack of low molecular weight volatile hydrocarbon content.

By atomizing with natural gas, the initial energy for bio-oil combustion is provided and the flame is

”anchored” to the nozzle. Mixtures of 20%, 40%, 60% and 80% were selected to be compared against a

pure natural gas flame to determine how the combustion behaves.

4.2.2 Bio-Oil Analysis

4.2.2.1 Thermogravimetric Analysis

Thermogravimetric Analysis (TGA) is a method of determining the volatilization properties of a fuel

[39]. A small sample around 50 mg is heated at a constant rate up to 700 ◦C while nitrogen is passed

over the sample at a rate of 100 mL/min to carry away vapours and prevent the fuel from oxidizing.

These samples are taken from a larger bottle of bio-oil which has been thoroughly blended. The weight

change of the sample is measured throughout the test. This test provides a detailed description of how

much of the fuel evaporates, as well as what the rate of devolatilisation with respect to temperature.

For this analysis, a Texas Instruments Q50 TGA was is used at SAPL.

4.2.2.2 Photomicroscopy

A common issue with the burner used for this study in its current configuration, is the diameter

of the nozzle orifices. The 0.89 mm holes in the air cap can become blocked due to the solid particles

contained in the bio-oil. In order to evaluate the chance of a blockage occurring, the bio-oil is inspected

using a Leica EZ4D photomicroscope at a magnification of 35x to measure the size of the particles.


The bio-oil used for this study is a low solids, filtered bio-oil selected specifically to reduce the risk of

blockage. This analysis is performed at the Surface Interface Ontario Lab.

4.2.3 Bio-Oil Properties

The bio-oil was provided from a commercial supplier with an analysis certificate. These values were

validated by sending the bio-oil to Alberta Innovates Technology Futures for independent evaluation.

4.2.4 Natural Gas Properties

The natural gas used in this study is Linde Gas Methane Grade 1.3 (Natural Gas). The only

composition data Linde provides for this product is that it is 93% methane. The balance is considered

inert for this study so that the heating value of the fuel by volume is 93% that of pure methane.

4.3 Gas Phase Emission Measurement

4.3.1 Unburnt Hydrocarbon Emissions

A California Analytical Instruments model 600 Flame Ionization Detector (FID) is used to measure

the UHC in the exhaust. By passing the exhaust through a hydrogen flame, a current proportional

to the number of carbons in the exhaust is produced [40]. Exhaust is sampled at a rate of 1.5 SLPM

controlled by the built in sampling pump and is transferred with heated lines at 190 ◦C to 195 ◦C to

prevent condensation of water or hydrocarbons. The FID is calibrated using dry air as the zero gas and

a mixture of 90 parts per million (ppm) methane in nitrogen as the span gas. The FID uses these two

points provide a linear range from 0 to 300 ppm CH4 with an uncertainty of ±3 ppm. It is important

to note that the output does not take into account the actual composition of the UHC contained in the

exhaust.

4.3.2 Detailed Exhaust Gas Composition

A Nicolet 380 Fourier transform infrared spectrometer (FTIR) is used to determine a more detailed

composition of the gas phase emissions. Specifically, the concentrations of CO2, CO, H2O and NO are

measured. The FTIR is fitted with a gas cell with a 2 m path length and a volume of 0.19 l. The gas

is sampled with 24 scans over 1 minute with a wave number resolution of 1 cm-1. The samples are then

compared to the mid infrared (500 to 4000 cm-1) absorption spectra of known gas compositions.

The FTIR is calibrated using a partial least squares model. Software is used to randomly generate

an array of combinations of the gases that are to be measured that covers the full range of the desired

detection limits. These mixtures are then manually input to the gas cell to generate spectra for each.

While collecting a spectrum during a test, the exhaust is drawn through the gas cell at a rate to

keep the pressure at 86.3 kPa. Similar to all the transfer lines, the gas cell is heated to 120 ◦C to prevent

water from condensing. By passing the exhaust through the gas cell constantly while collecting the

spectrum, a time averaged gas composition is measured. At each test condition, five spectra are taken

consecutively and are averaged arithmetically.


4.3.3 Equivalence Ratio

The %O2 in the exhaust is measured using a Zirconia (ZrO2) oxygen sensor (Engine Control and

Monitoring, model OXY6200). The sensor produces a 0 to 5 VDC signal which is linearly proportional

to %O2. Using room air supplied with a vacuum pump at 1.8 SLPM, the sensor is calibrated to 21%

O2. The O2 sensor raises the temperature of the gas, which may oxidize the hydrocarbons and other

gas species. To prevent affecting the measurements of the FID and FTIR, the O2 sensor is not in-line

with the rest of the gas phase sampling system as shown in Figure 4.1. The output of the oxygen sensor

is used to determine the equivalence ratio using the composition of the fuels and assuming complete

combustion.

4.4 Flame Visualization

A 4 mm Lennox Instruments Co. borescope with a 90◦ mirror tube is used to visually evaluate the

flame. The borescope is inserted in the burner as shown in Figure 3.1 and provides an axial view towards

the nozzle. The borescope can also be connected to a camera (Kodak Z1012S) to take photographs and

video of the flame. A detailed setup for how the borescope is inserted into the burner is shown in Figure

4.2.

Figure 4.2: Borescope insertion into burner

The borescope is inserted into the burner through a 9.5 mm tube with a connection to compressed

air which flows at 250 SLPM to cool the borescope as well as prevent fuel droplets or PM from settling

on the mirror. The camera is attached to the end of the borescope and is set up on a rail which allows

for the rapid insertion of the camera. By minimizing the time the borescope is inside the burner, there

is less chance particles coming to rest on the mirror. The camera is therefore set up with maximum

zoom, an aperture of f=5.0 and a shutter speed of 1/6s. These settings are kept constant for all pictures

so that the relative luminosity of the flames is reflected in the photographs. By setting up the camera


before insertion, the borescope only needs to be in the burner for 5-10s to take a photo, at which point

it is removed and the borescope port is immediately capped.

4.5 Flame Infrared Emission Measurement

In addition to visible light measurements, the borescope allows for infrared light measurements using

an adapter to connect to an optical fibre. An Edmund Optics InGas Near Infrared (NIR) spectrometer

(model BTC261E-512) is used for these measurements. The principle for these measurements is the

same as for taking photographs; the borescope is briefly inserted to take a spectrum looking up at the

flame. For each test, 5 spectra are taken and averaged together to take into account any fluctuations

in the flame. Using these spectra and integrating over all wavelengths, a relative value for how much

infrared light each flame emits is obtained.

4.5.1 Infrared Background Measurement

In early tests, it was noted that the walls of the burner emit infrared light and affect the results. To

investigate the effect the walls have on the NIR spectra, two successive spectra were obtained: one with

the flame on, and one immediately after extinguishing the flame. These spectra are shown in Figure 4.3.

Figure 4.3: NIR background spectrum test

The spectrum with no flame was then observed over time as the walls cooled and it was noted that

the shape remains the same, but the magnitude decreased. Furthermore, the spectrum with no flame

closely matches the smooth part of the spectrum with the flame, indicating that the flame is represented

by the part of the spectrum that deviates from this line. So for analysis, the background was subtracted

from each spectrum to get a measure of the NIR emission solely from the flame.


It is important to note that the output from the spectrometer is a non-dimensional, relative measure-

ment. The results are therefore reported as normalized values relative to the NIR emission of a flame

consisting of 100% natural gas.

4.6 Burner Temperatures

Several thermocouples are placed around the burner to provide real time monitoring of key temper-

atures of the burner. The exact location of these thermocouples is shown in Figure 4.4.

Figure 4.4: Thermocouple placement for burner monitoring

The intake air and nozzle sheath temperatures are measured with exposed bead J-Type thermocou-

ples. The intake air is heated with an electric heater that is controlled with a variable voltage power

supply. The voltage is tuned so that the temperature of the air is 230 ◦C to 260 ◦C as it enters the


combustion region. To ensure the burner has reached steady state, another J-Type thermocouple is

installed on the outer wall of the burner. This temperature is continuously monitored during operation

and measurements are only taken once the wall temperature is no longer increasing.

To evaluate the quality of the combustion, an exposed bead sheathed K-Type thermocouple is inserted

into the thermocouple insertion port shown in Figure 4.4. The thermocouple is inserted 11 cm until the

bead is located along the centreline of the burner. The thermocouple is left in position for 30s until

it reaches an equilibrium temperature. Due to the turbulent nature of the combustion region, this is

not considered the flame temperature, rather the average temperature of the combustion gases. This

temperature provides insight to the conditions just after the flame that helps explain exhaust composition

measurements. However, due to losses from radiation, conduction along the thermocouple sheath as well

as cool room air leaking in from the insertion port, these measurements should be considered relative

[41].

4.7 Particulate Measurement and Analysis

4.7.1 Isokinetic Sampling

To sample the particulates contained in the exhaust stream, it is important to sample isokinetically,

that is, to sample the gas without changing its velocity [42]. By keeping the velocity constant at the

sampling probe, the sampled gas will contain a representative quantity of particles [43]. By measuring the

pressure difference between the exhaust stream and sampling probe inlet, isokinetic sampling happens

when the pressure difference is zero.

4.7.2 Particulate Sampling System

The particulate sampling system is shown in Figure 4.5. A flow straightener is located at the exit of

the burner to eliminate any angular momentum that may remain. The flow straightener is made from

0.25 mm stainless steel in a checkerboard pattern. This ensures that in the particulate sampling system,

there is only axial flow with a uniform particulate distribution. A detailed description of the design of

the particulate sampling system can be found elsewhere [5].

There are two taps for pressure measurement, one in the main exhaust stream and another in the

sampling probe. These are connected to a manometer so the flow rate through the sampling probe can

be adjusted with a needle valve until the pressure difference is zero. The pressure tap is calibrated to

measure the pressure at the centreline of the exhaust stream. To achieve this, the sampling probe is

offset from the pressure tap 12.7 mm [5]. A 47 mm Pall Life Sciences Tissuquartz filter (model 7202) is

placed in the filter holder to collect PM during the tests. The filters are made out of borosilicate glass

and are able to sustain very high temperatures (up to 1100 ◦C) and have a very low air resistance. They

retain 99.9% of aerosols at 0.3 micron.

The filter temperature is monitored during each test using a J-Type thermocouple to ensure water will

not condense on the filter. While the filters do not absorb water due to humidity, it has been observed

that because they do not contain a binder, moisture causes them to break down. The particulate

sampling system is heated up with heating tape to 120 ◦C to 140 ◦C before taking any data to make sure

condensation does not become an issue.


Figure 4.5: Overall particulate sampling system [5]

4.7.3 Particulate Sample Measurement

During particulate collection, a dummy filter is used to run exhaust through the system to heat all

the components to 120 ◦C to 140 ◦C. Once this is done, a second dummy filter is placed in the filter

holder to adjust the sampling pressure to achieve isokinetic conditions (zero pressure difference between

the sampling probe and exhaust line). Once the system is set up, the first test filter is placed in the

filter holder and the exhaust is run through it for 3 minutes. The filter is then placed in a petri dish to

prevent any contamination and the process is repeated with 4 subsequent filters.

The collected particulate samples are weighed with a Scientech SM-128D Microbalance to determine

the total amount of particulates in the exhaust stream. The filters are weighed after resting in ambient

conditions for 24 hours. The filters are then placed in a Thermo Scientific Thermolyne oven at 640 ◦C for

1 hour to burn off all the carbonaceous residue (CR). The samples are then weighed again to determine

the organic/inorganic composition of the particles.

After each test, it was observed that significant amounts of ash were left on the walls, especially on

the horizontal surfaces in front of the viewports. In order to avoid misrepresenting the particulate mea-


surements, the data is presented as a value relative to the largest value obtained. While the magnitude

of the particulates in the exhaust stream is likely underestimated, due to the consistent measurement

procedure across all tests, the trend is retained.

4.8 Test Procedure

For all tests, one batch of bio-oil was used. The bio-oil was received in a large bucket that was then

thoroughly mixed and distributed into 2 l bottles which were then kept in a refrigerator to fuel aging. By

dividing the bio-oil into smaller bottles immediately after mixing, there is less chance that the properties

of the bio-oil change from test to test. The following sections outline the procedure that was carried

out to collect each set of data. Table 4.1 shows the fuel and nitrogen flow rates as well as the %O2 in

the exaust measured at each test points. The procedure for calculating the fuel flow rates is shown in

Appendix A.

Table 4.1: Fuel flow rates for each burner test

Bio-oil EnergyFraction

Bio-oil FlowRate (mL/min)

Natural GasFlow Rate(L/min)

Nitrogen FlowRate (L/min)

Oxygen inExhaust (%)

0% 0 18.29 6.70 7.0620% 5.85 14.63 10.37 6.4840% 11.70 10.97 14.02 6.5360% 17.54 7.31 17.68 5.8980% 23.38 3.66 21.34 5.59

To ensure the SMD in each test is approximately the same, nitrogen is added to the natural gas to

control the overall atomizing gas flow rate. The empirical relation in Equation 2.1 is used as a starting

point so the SMD is 100µm. However, it was found in preliminary tests that combustion fluctuations

became large at low bio-oil flow rates. At 20% bio-oil energy especially, carbonaceous residue built up

on the nozzle and partially clogged the fuel jets. To remedy this, in each test, the nitrogen was varied to

achieve the lowest CO emissions possible. By doing this, the optimal atomization is achieved with the

minimal amount of flame lift off.

4.8.1 Burner Start-up and Shutdown

The burner is first warmed up on ethanol with air atomization before running bio-oil through the

system. The warm-up period lasts for 15-20 minutes sets up the burner so that there are no extinction

issues when bio-oil is introduced. Once the burner wall temperature has reached 425 ◦C to 450 ◦C, the

fuel is switched to bio-oil with the appropriate flow rate, and the atomizing air is switched to the natural

gas and nitrogen mixture required for the test. At the end of each test, the bio-oil is switched back to

ethanol and the atomization gas is switched back to air. This shutdown procedure ensures that bio-oil

does not remain inside the hot nozzle, which would cause polymerization and clogging. The operating

conditions used for each of the tests is shown in Table 4.2.


Table 4.2: Operating condition of burner variables for each test point

Parameter ValuePrimary Air Flow (SLPM) 250

Equivalence Ratio 0.66Pilot Methane (SLPM) 0.88Pilot Oxygen (SLPM) 2.3

Primary Air Preheat (◦C) 240-260Energy Throughput 10 kW

4.8.2 Gas Phase Emissions Tests

After switching to the bio-oil, the burner is allowed to run for another 15-20 minutes to allow the

burner to reach steady state. While this is happening, the FID is calibrated and the FTIR gas cell is

purged with nitrogen. The FID and FTIR measurements are carried out in succession. A valve controls

the flow selection to either the FID or FTIR. Once the FTIR spectra are collected, the gas cell shut off

from the exhaust, evacuated and purged with nitrogen to limit its exposure to the exhaust.

4.8.3 Particulate Emissions Tests

Before the test has begun, 5 filters are weighed and placed in individual petri dishes to prevent

contamination. After the gas phase measurements are taken, the particulate sampling system is heated

up. Dummy filters are placed in the test and bypass filter holders, and exhaust is pulled through the

system while the flow rate is adjusted to reach isokinetic flow. Once the filter holder has reached 120 ◦C

to 140 ◦C, the first test filter is placed in the test filter holder while the exhaust passes through the

bypass filter. The bypass valve is then closed and the valve to the test filter is opened. After 2 minutes

of sampling, the gas flow is switched back to bypass and the test filter is removed and placed back in its

petri dish. This procedure is then repeated with the remaining test filters.

4.8.4 Infrared Emissions Tests

Once the emission testing is done, the borescope is set up for NIR emission testing. After taking

a dark scan (NIR scan of the ambient conditions in the lab), the borescope is inserted into the burner

and 5 successive spectra are obtained. Between each test, the borescope is removed to prevent it from

getting too hot or contaminating the mirror. The borescope tube is marked to ensure the each spectra

is taken at the same depth.

4.8.5 Flame Imaging

To ensure the photographs of the flames are all taken with the same conditions, the photos of all test

points were taken during a separate test. After heating up on ethanol, the fuel was switched to 100%

natural gas and photos were taken. The fuel flow rates were then changed to run through tests with

20%, 40%, 60% and 80% bio-oil. The burner was then flushed with ethanol in the same way as the other

tests.

Chapter 5

Results and Discussion

5.1 Fuel Analysis

The properties of the two fuels used in this study are provided in Table 5.1. For the bio-oil, the

properties were provided by the manufacturer, and the testing methods are listed. The natural gas

properties are calculated based on a purity of 93% by volume methane provided by Linde Canada, and

assuming the remaining 7% is inert.

Table 5.1: Bio-oil and natural gas properties [44]

Parameter Test MethodBio-Oil

Manufacturer DataBio-Oil

Lab TestNatural Gas

Water Content, wt% as is* ASTM E203 25.1% - -Viscosity 25 ◦C, cSt ASTM D445 93.7 - -Viscosity 40 ◦C, cSt ASTM D445 35.4 - -Viscosity 60 ◦C, cSt ASTM D445 13.3 mm - -

Solids Content, wt% as is ASTM D7579 0.05% - -Ash Content, wt% as is EN 055 0.17% - -

Density 20 ◦C, kg/L EN 064 1.22 - 0.000 656Carbon Content, wt% as is ASTM D5291 42.4% 41.45 -

Hydrogen Content, wt% as is ASTM D5291 7.59% 7.2 -Nitrogen Content, wt% as is ASTM D5291 0.13% 0.13 -

Sulfur Content, wt% as is ASTM D5453 0.01% - -Oxygen Content, wt% as is Difference 49.71% 40.54 -

HHV, MJ/kg as is ASTM D240 17.1 - -LHV, MJ/kg as is Calculated 15.12 - 46.5

*Included in the hydrogen and oxygen content

23

Chapter 5. Results and Discussion 24

There is a very clear contrast between the two fuels used in this study. While natural gas has a high

LHV and does not contain any undesirable compounds for combustion, bio-oil has several properties

that pose a challenge for its use as a fuel. Bio-oil is liquid and it has a low LHV, typically around half

that of Diesel, or Number 2 Fuel Oil. Combined with its high water content, bio-oil combustion suffers

by requiring a high ignition energy. Its low volatility therefore can cause difficulties with ignition and

extinction. The volatility curve produced from the TGA is shown in Figure 5.1.

Figure 5.1: Bio-oil TGA curve

The TGA curve shows that there is a significant portion of the total mass that evaporates at around

100 ◦C. This is mainly the water as well as the low molecular weight hydrocarbons and acids contained

in the bio-oil. The peaks and fluctuations that are shown in this area signify the polymerization that

bio-oil goes through at high temperatures. As the temperature increases, a skin forms on the surface of

the bio-oil which then bursts as the water boils [45]. Fuel polymerization poses a clogging threat to the

fuel delivery system, so the fuel line is water-cooled as it enters the nozzle. Once the test is complete,

approximately 17 wt% of the original mass remains as solid char. The majority of the remaining char

is carbon, so nearly half of the carbon in the bio-oil (42.4 wt%) is not volatile. This has been shown in

a previous study to increase the particulate emissions [46]. Because there is no oxygen available to the

bio-oil during the TGA test, there is no char oxidation, so the only mass lost is due to evaporation. In

a combustion application, this char forms particles which react much slower than vaporised fuel. These

organic particles may remain in the exhaust stream if the residence time or temperature too low.

The solids content of bio-oil also presents a challenge for the use of bio-oil, especially for a small,

lab-scale burner. To investigate the risk of nozzle clogging due to particle accumulation, the size of the


particles was examined with an optical photomicroscope, shown in Figure 5.2.


Figure 5.2: Typical solid particles contained in this bio-oil

It is important to note that the bio-oil used in this study was specially designed for use in the

burner. That is, that it was filtered to remove potentially problematic particles. The solids content

varies significantly between bio-oils based on feedstock and processing, so while one bio-oil may not

cause clogging, another may. Measuring the particles contained in the bio-oil used in this study yielded

an average particle size of about 100µm. Also shown is a larger particle, around 250 µm long, which

demonstrates the variability in particle size in the bio-oil. However, because the bio-oil has been filtered,

the few large particles left in the bio-oil do not pose a clogging risk. The nozzle opening diameter is

1 mm, much larger than the largest particles in the bio-oil.

5.2 Base Point Operation and Repeatability

During each test, the burner is allowed to heat up on ethanol for 30 minutes before switching to

the bio-oil and natural gas. Once the fuel is switched, the burner is allowed to run for an additional

30 minutes to ensure it is running at steady state before taking any measurements. To ensure steady

state operation, and that the exhaust composition is stable during each test, a test was performed at

60% bio-oil energy. After heating up using the same method, the exhaust composition was measured

five times over the course of an hour. The results of this test are shown in Table 5.2.


Table 5.2: Exhaust composition repeatability at 60% bio-oil energy

Species Average Value (ppm) Average Deviation (ppm) Percent DeviationCO 653.2 49.1 7.5%NO 108.6 2.5 2.3%

5.3 Reaction Zone Temperature

The temperature in the reaction zone is a very important measurement to determine what is occur-

ring in the combustion reaction and helps to explain other measurements. Figure 5.3 shows how the

temperature changes with respect to bio-oil input energy.

Figure 5.3: Temperature vs bio-oil input energy percent

It is important to note that this is not the peak temperature inside the burner, but more of an average

temperature within the turbulently mixed recirculation zone. Because the exhaust temperature at the

burner exit, as shown in Figure 4.4 remains relatively constant in all tests, around 250 ◦C to 300 ◦C due

to the heat losses to the walls, the temperature of the reaction zone will greatly affect how effectively the

combustion products convert to CO2 and water. The temperature after the flame region drops quickly,

so the combustion reaction does not continue further down in the burner. It is important to note that the

temperature only 10 cm below the nozzle is significantly lower than the adiabatic flame temperature of

approximately 1930 ◦C of methane [44]. This shows that the heat loss to the walls introduces a significant


temperature gradient in the burner. While this would not be the case in a lime kiln or industrial burner,

the heat loss will exaggerate the effects that bio-oil has on combustion.

As shown in Figure 5.3, as the fraction of the total energy input from bio-oil increases, there is a

decrease in reaction zone temperature. The pure natural gas flame burns very quickly and releases its

energy very high up in the burner, resulting in a higher temperature in the reaction zone. However,

as bio-oil is added, more time is required for complete combustion due to the water content and char

formation of the bio-oil. So, the same amount of energy will be released over a larger volume inside the

burner, resulting in a lower average temperature. Furthermore, if the solid char left over after the volatile

compounds in the bio-oil evaporate (about 17 wt%) exits the hot reaction zone too quickly, it may not

oxidize. The energy in these particles will not release their energy, further contributing to the decrease

in temperature (Discussed further in Section 5.5.2). The longer flame seen as the bio-oil energy fraction

increases indicates that bio-oil has a longer burnout time than natural gas which will be proportional to

the CO and UHC in the exhaust.

A decrease in reaction zone temperature also has implications for incomplete combustion products.

If the temperature inside the burner decreases too quickly, there will be increased CO emissions simply

because the temperature is too low to convert the CO to CO2[44]. These effects are discussed further in

Section 5.5.1.

5.4 Flame Images

Figure 5.4 shows the images captured using the borescope looking axially toward the nozzle.


(a) 0% bio-oil energy (b) 20% bio-oil energy

(c) 40% bio-oil energy (d) 60% bio-oil energy

(e) 80% bio-oil energy

Figure 5.4: Flame images viewing axially towards the nozzle

The images of the flames provide a way to evaluate the flames qualitatively. Figure 5.4a shows the

clean burning flame characteristic of natural gas. Because the fuel is quickly mixed with the air in

the recirculation zone, there is no soot to produce light. Furthermore, the natural gas flame does not

propagate far down the burner due to its relatively rapid combustion [44]. Because there is a smaller

volume heated by the natural gas flame, the average temperature of the gas in this case is increased as

discussed in Section 5.3. In the remaining four tests, there are two different regimes that are visible.

In the 20% and 40% bio-oil input energy cases, shown in Figure 5.4b and Figure 5.4c respectively,

the flame structure is dominated by the natural gas that comprises the majority of the flame energy.

In these tests, there are fewer droplets (assuming a constant average droplet diameter), and the natural

gas anchors the flame to the nozzle, preventing extinction. However, it is clear that the flame continues

further down in the burner, shown by the increased curvature of the fuel jets. Because the air in the

burner is swirling as it travels through the burner, fuel jet curvature is an indicator of vertical distance

travelled.


In the 60% and 80% bio-oil input energy cases, shown in Figure 5.4d and Figure 5.4e respectively,

the flame has become dimmer, with more of the flame composed of burning droplets. During these tests,

there was a significant increase in fluctuations, likely caused by the decrease in anchoring provided by

the natural gas. In these cases, the flame length grew long enough to be visible in the viewport, shown

in Figure 3.1. In these cases, significantly more air is being heated by the flame compared to the natural

gas case, contributing to the trend seen in the average reaction zone temperature in Section 5.3.

Whereas the natural gas flame is nearly invisible, the bio-oil flames are very luminous with an orange

colour. This is due to the char particles formed in the flame heating up and emitting light. It is

important to note that because bio-oil is a heavily oxygenated fuel, the orange colour is not a product

of soot formation in the fuel [47]. The volatile components of the bio-oil evaporate until all that is left

is the char as was shown in the TGA test [48]. The light emission increases heat transfer to the wall,

which due to the burner design (non-refractory lined) results in increased heat loss. The light emission

therefore contributes to the lower reaction zone temperature. Furthermore, char particles are more likely

to travel further from the nozzle where the burner is much cooler. The flame volume is therefore larger

in the tests with more bio-oil energy.

5.5 Particulate and Gas Phase Emissions

5.5.1 Exhaust Composition

As more of the total input energy is replaced with bio-oil, significant changes occur in the combustion

region that are reflected in the exhaust composition. Figure 5.5 shows the amount of CO and NO and

Figure 5.6 shows the UHC in the exhaust as more of the input energy comes from bio-oil.

Figure 5.5: CO and NO concentration in exhaust with respect to bio-oil input energy


Figure 5.6: UHC concentration in exhaust with respect to bio-oil input energy

The NO produced in the combustion increases linearly as more energy comes from bio-oil. This

corresponds to the nitrogen content of the bio-oil being the source of the nitrogen oxides formed in the

combustion. Due to the high swirl number in each test, the recirculation zone effectively acts as exhaust

gas recirculation, helping to limit the thermal NOx produced in the burner[49]. However, previous

research has been done with this burner to investigate the effect of swirl number on NOx formation and

found that increasing the swirl number yielded no change in NOx, indicating that the NOx formed in

the burner is generally not produced thermally[5]. The increase in NOx therefore is due to the nitrogen

content of the bio-oil.

The UHC content in the exhaust increases as more bio-oil is input to the burner. Due to the

low volatility of the bio-oil and the decreased temperature of the gas inside the burner, not all of the

hydrocarbons in the combustion region will ignite. Furthermore, due to the heat lost to the walls, the gas

downstream of the flame quickly becomes too cool to allow the UHC to oxidize. However, the increase in

UHC is not linear as bio-oil input is increased. Significantly more UHC is produced when the majority

of energy is bio-oil. During these tests, there were significantly more fluctuations in flame stability,

likely due to the lack of natural gas providing the energy for ignition. These instabilities result in the

flame locally extinguishing, allowing unburnt fuel to exit the reaction zone. Because the temperature

downstream is too low, this results in a greater increase in UHC emission when there is more extinction.

This is also reflected in the flame images shown in Section 5.4 where at 60% and 80% bio-oil energy, the

flame appears to be made up of mostly burning fuel droplets, rather than at 20% and 40% where the

burning droplets are contained in a turbulent gaseous flame.

The trend in CO emissions further demonstrates the requirements needed to burn bio-oil. As the

bio-oil energy percentage increases, there is a significant increase in CO concentration in the exhaust,


indicating that combustion is not as complete. There are two sources of this CO. First, due to the

decreased temperature in the reaction region, CO that exits the hot reaction zone, or quenches on

the wall, is not exposed to a high enough temperature to oxidize to CO2, as discussed in Section 5.3.

Secondly, the char particles that exit the recirculation zone quickly cool down and are similarly unable

to convert to CO2, but are still hot enough to slowly release CO. Examining the carbon input into

the burner and calculating the amount of that carbon that exits as CO, it is found that approximately

96% of that carbon is being converted to CO2 in the worst case. Considering the UHC, which in the

worst case is 300ppm CH4, the vast majority of the carbon is being converted to CO2, indicating that

it is likely the recirculation maintains the combustion products at a high enough temperature and the

majority of the CO is being released by char particles further away from the flame. The reaction rate

from solid carbon to CO2 is much slower, and these particles do not experience a high temperature for

very long. The measurement of these particles is discussed in more detail in Section 5.5.2.

5.5.2 Particulate Emissions

The collected particulate emission of each test is shown in Figure 5.7. Due to the design of the

burner, significant amounts of particulates deposit on the walls and on horizontal surfaces inside the

burner. However, the linear trend seen in the ash indicates a constant percentage loss in each test.

Figure 5.7: Relative particulate emission

As more bio-oil energy is used, there is a larger amount of fuel that has the potential to form

particulates. Natural gas is very clean burning and does not produce any particulates, shown visually

by its dim, blue flame. However, when bio-oil burns, once the volatile compounds evaporate, there is a

solid char particle that is left over. These particles oxidize at a much slower rate simply because they


are solid and there is less surface area available to react. These particles give the flame a bright orange

colour, however, they also allow heat to radiate to the walls more efficiently. Because the walls are not

well insulated, there is a large heat loss to the walls, which reduces the temperature of the gas inside

the burner.

In order to examine why there is a large increase in particulates between 0% and 20% and then a

subsequent linear increase, the organic component of each particulate sample was burnt off. Figure 5.8a

shows what a typical filter looks like with particulates immediately after sampling, while Figure 5.8b

shows a filter after the carbon has been burned off. The resulting filter mass provides the mass of the

inorganic ash component of the particulates, shown in Figure 5.7.

(a) Filter with particulates as collectedfrom the burner

(b) Filter after carbon burn off in ovenat 640 ◦C for 1 hour

Figure 5.8: Particulates collected on a quartz filter before and after carbon burn off

There is no ash produced in a natural gas flame, so any ash produced in the flame comes from the

bio-oil. The ash weight percentage of the bio-oil is shown in Table 5.1, allowing the expected mass of

ash per kilogram of fuel to be calculated. Due to particulates coming to rest on horizontal surfaces in

the burner, only approximately 20% of the total ash input from the bio-oil has been captured. However,

because the loss is relatively constant, the linear trend that is expected is captured with the correct

slope.

Once the ash is subtracted from the total mass of the particles, the mass of the total char is deter-

mined. There is a relatively flat relation between bio-oil energy fraction and char particulates collected

on the filter. This is in contrast with the ash, which increases linearly. This indicates that with even a

small amount of bio-oil a significant amount of char escapes the reaction zone. A test was run at 10%

bio-oil energy to validate the trend between 0% and 20% bio-oil energy. However, the nozzle became

clogged due to char build up approximately 15 minutes after switching from ethanol. It is likely that

the elevated char mass seen in 5.7 at 0% and 20% is due to the nozzle not operating at its design liquid

flow rate.

5.6 Near Infrared Emissions

The near infrared emission measurements reflected what was seen in all other measurements. The

quality of the combustion is poorer and less complete in tests with higher bio-oil energy input. These


measurements are shown in Figure 5.9

Figure 5.9: NIR emission vs bio-oil input energy

The trend observed in these NIR measurements is mirrored in the reaction zone temperature data

shown in Figure 5.3. While the natural gas flame is very dim, it is very hot, and the CO2 and H2O

produced in the flame are good at transmitting in the infrared. Despite the bio-oil containing flames

being much brighter and emitting more light in the visible spectrum, in the near infrared, there is less

measured radiation due to the decrease in temperature. It has previously been noted that in lightly

sooting flames, the infrared radiation due to the effect of soot is not as important as the effect of

temperature [44]. Bio-oil is generally not considered a highly sooting flame because of its oxygen content

[44]. The temperature is therefore the determining factor for the magnitude of the infrared emission

for bio-oil and natural gas co-combustion. Further investigation should be done with a refractory lined

burner to eliminate heat loss to the walls.

5.7 Effect of Natural Gas on Bio-Oil Combustion

The trends seen in each set of data demonstrate the effect natural gas has on bio oil combustion. The

bio-oil flame is prone to lift off from the nozzle. This produces many undesirable effects. As the flame

lifts off, the chance of it extinguishing increases either locally or globally. When the fuel extinguishes,

unburned fuel does not fully oxidize and gives rise to the increased UHC measurements observed in the

60% and 80% bio-oil energy tests. With a larger energy percentage of the flame coming from natural

gas, the anchoring greatly improves, significantly reducing the UHC emissions.

With more bio-oil, the temperature inside the burner decreases due to the flame occupying a larger

volume. This decrease in temperature contributes to the poorer flame quality measurements observed.


As the temperature decreases, not only does local extinction become more likely, but the oxidation from

fuel to complete combustion products becomes limited. This gives rise to the increased CO emissions

observed in the tests with higher bio-oil energy percentages. The decreased temperature also contributes

to the reduced near infrared emission, decreasing radiative heat transfer.

The increased NO emission as more bio-oil energy is utilized is due to the nitrogen content of the

bio-oil, rather than the lack of natural gas anchoring the flame. Similarly, the overall PM emission

increase is also primarily due to the increase in bio-oil entering the burner. This is demonstrated by the

linear increase in ash measured in the samples. The carbonaceous residue however did not change much

between 20% and 80% bio-oil energy. Observations during the tests determined this is primarily due to

the nozzle operating at a much lower liquid flow rate than it was designed for.

Chapter 6

Conclusions and Recommendations

6.1 Conclusions

As the bio-oil input energy was increased, there were many changes in the quality of the combustion.

First, the flame changed from a jet that quickly burned in a small, intense volume when only natural gas

was used, to longer, slower flame that burned in a larger volume and was affected more by the airflow in

the burner. This change had an effect on the average temperature in the area just after the flame, which

greatly affected the exhaust composition. This lower average temperature prevented the bio-oil from

fully oxidize, resulting in UHC emissions increasing from 10ppm in the natural gas flame to 300ppm at

80% bio-oil input energy. In addition to the fuel not oxidizing, the reduction of temperature reduces the

time at which exhaust products and char particles are above the CO to CO2 conversion temperature.

This resulted in a dramatic increase in CO emissions from 16ppm in the natural gas flame to 850ppm in

the 80% bio-oil input energy case. Finally, the NO emissions were mainly due to the nitrogen content of

the fuel, seeing a linear increase from 20ppm to 160ppm from the natural gas flame to the 80% bio-oil

input energy case, respectively.

Particulates increased as bio-oil energy was increased, with the ash component increasing proportion-

ately with the ash contained in the bio-oil. However, in each case, the char component of the particulates

was relatively constant. The organic component of the particulates contributes to the elevated CO levels

seen in higher bio-oil energy input tests as well as indicates that there is poorer energy conversion of the

fuel.

The infrared heat output of the flame as bio-oil input energy was increased followed the trend seen in

the average temperature inside the burner. The heat radiated by the particles in the flame therefore do

not compensate for the decrease in temperature that is seen. While the emission in the visible spectrum

because of the broadband emission of the char particles gives a very bright flame, the NIR emission of

the water and mathrmCO2 is more efficient at transferring heat by radiation. This result suggests that

if used in a full scale burner, the input energy would need to be increased compared to only natural gas

in order to achieve the same heat transfer.

36

Chapter 6. Conclusions and Recommendations 37

6.2 Implications for Industrial Burners

Co-combustion of natural gas and bio-oil allows for the utilization of a low grade biofuel that has

a low energy cost to produce. Natural gas is a fuel often used in industrial burners and by co-firing it

with bio-oil, the issues with bio-oil combustion can be minimized. Furthermore, the operator does not

need to fully rely on only bio-oil to provide heat. By showing that bio-oil combustion is more stable

without the need for chemical upgrading or ethanol blending, it becomes a more appealing biofuel to use

in industry. The measurements in this study suggest that using natural gas energy percentages as low

as 40% greatly improve flame stability by anchoring the flame to the nozzle. In an industrial setting,

it is important be sure that changing the fuel will not result in an expensive shut down. However, the

reduction in heat transfer shown in the NIR emission data suggests that the input energy may need to

be increased in order to achieve the same heat delivery.

Specifically for lime kilns in the pulp and paper industry, the char produced in the bio-oil flame may

be a concern. While the lime kilns have very long residence times, giving plenty of time for the fuel to

fully burn, if the char escapes the flame and ends up in the lime, it may work its way into the pulp.

The char may then appear on the finished paper as black dots, and may ruin a batch of paper. Careful

monitoring of the lime exiting the kiln can avoid this issue.

6.3 Recommendations and Future Work

6.3.1 Burner Modifications

In order to better examine the combustion of bio-oil, the burner should be modified to have refractory

lining to reduce heat losses. Heat losses limit the operating points that can be studied with the current

burner. For example, 100% bio-oil will not ignite, likely due to the heat lost to the walls that would

otherwise be available to ignite the bio-oil. By adding refractory lining to the current burner, it may be

possible to investigate how bio-oil burns without any need of ethanol or natural gas.

In addition, a new nozzle design can help improve combustion in the burner at all fuel fuel flow

rates. In cases with very little bio-oil flow (from 10% to 20% bio-oil energy input) there was difficulty

maintaining combustion throughout the full test. After running for an extended period, the nozzle

became clogged due to char build-up on the end of air cap. As shown in Figure ??, tests with low bio-oil

energy input showed an elevated carbonaceous PM output. This was likely due to the nozzle not being

specifically designed for these very low liquid flow rates. A new nozzle designed custom for the burner

can help in these cases, as well as be designed to more realistically resemble industrial burner nozzles.

In many tests, there were fluctuations caused by unsteady primary air intake due to the unstable fan

performance. These fluctuations could be reduced by replacing the current exhaust duct with a rigid

material. The current exhaust duct is made with flexible ducting that, while easy to route, changes

shape when the exhaust fans are turned on. This flexibility is likely contributing to these fluctuations.

By minimizing fluctuations, there will be much less chance for the flame to encounter a condition where

it will extinguish.

Chapter 6. Conclusions and Recommendations 38

6.3.2 Experimental Methodology Improvements

Improvements to the experimental methodology may reduce the variability in the measurements

observed in this study. The FID data was recorded manually on a data sheet for each test. It would be

best to use the FID’s data output capability to record the UHC data with LabVIEW. This could allow

a more thorough statistical analysis of this data set.

When the borescope is used to photograph the flame, a port in the wall must be opened. When the

borescope is inserted, there is not a leak proof seal around the tube, so air is able to enter through this

port. Because the primary air is pulled through the burner, any leak downstream of the main intake

reduces the air flow through the main primary air intake. This may change the shape of the flame

when photos are taken. By ensuring there is a good seal around the borescope, the photos can be more

representative of the flame.

6.3.3 Future Bio-Oil Combustion Research

Currently, the SMD of the bio-oil droplets is calculated with an empirical formula[36]. The droplet

size is a very important parameter for bio-oil combustion quality. Measuring the droplet size in situ

and determining its relationship with flame quality will provide insight for designing optimal nozzles

specifically for bio-oil. Furthermore, the experimental results of this study can inform modelling studies.

In some industrial settings, it may not be viable to use natural gas and bio-oil because of the handling

requirements for both gaseous and liquid fuels. However, there may be interest in examining the effect

of burning mixtures of bio-oil with number 2 fuel oil. These two fuels however are not soluble in each

other, so either a dual fuel nozzle could be tested or an emulsion would need to be prepared.

Bio-oil has a very complicated composition containing many different compounds. The carbonaceous

residue left at the end of the TGA testing was assumed to be pure carbon, however measuring the

chemical composition of fhis residue would add more understanding of bio-oils volatility.

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Appendix A

Fuel Flow Rate Calculation

The fuel flow rates were calculated using the LHV of each fuel and the percentage of the total power

required from each fuel. The LHV for bio oil was provided as kJ/L and the LHV for natural gas was

provided in kJ/L. The flow rates, mL/min for bio-oil and L/min for natural gas, were calculated with

the following equations.

Vbio−oil = %bio−oil ∗10kW

LHVbio−oil∗ 60000 (A.1)

Vnaturalgas = (1−%bio−oil) ∗10kW

LHVnaturalgas∗ 60 (A.2)

43

Appendix B

Peristaltic Pump Flow Rate

Calibration

To calibrate the peristaltic pumps, bio-oil was pumped at various pump RPMs to determine the

relationship between RPM and flow rate. These tests are summarised below.

Table B.1: pumpcal

RPM Flow Rate83 2790 29100 32

Using these points, a line was fit to calculate the RPM for any flow rate.

RPM = 3.1037 ∗ Vbio−oil (B.1)

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