An Experimental Study on the Effect of Rings on Flame ... · Rings can alter the inner surface of the kiln, and can affect the flame pattern or cause impingement, causing further

An Experimental Study on the Effect of Rings on Flame Length

and Stability in Lime Kilns

Girish Mohanan

A thesis submitted in conformity with the requirements for the

degree of Master of Applied Science

Chemical Engineering and Applied Chemistry

University of Toronto

© Copyright by Girish Mohanan 2017

[ii]

An Experimental Study on the Effects of Rings on Flame Length and

Stability in Lime Kilns

Girish Mohanan

Master of Applied Science

Chemical Engineering and Applied Chemistry

University of Toronto

2017

ABSTRACT

Ring formation in lime kilns has been a persistent problem in kraft pulp mills due to the

reversible nature of the calcination reaction that takes place in lime kilns. Rings can form due to

temperature fluctuations in the kiln, which trigger the backward reaction depositing hard calcium

carbonate deposits on the walls of the kiln. Rings can alter the inner surface of the kiln, and can

affect the flame pattern or cause impingement, causing further temperature fluctuations in the

kiln. The effect of ring position, thickness and length on flame length and stability was examined

in this work. Ring position, thickness and length were found to affect the flame shape and

stability greatly. The rings could be altering the recirculation inside the kiln and thereby affecting

the flame.

[iii]

Acknowledgements

“Skepticism is as much a result of knowledge, as knowledge of skepticism”

Alexander Pope

Exhausting has the journey been, sweeter have been the results. Yet, none of that would have

been mine to enjoy without the help and support of a lot of people. First of all, I thank my

supervisor, Professor Honghi Tran, for accepting me into his group. A truly fruitful graduate

experience I had, thanks to him, for he granted me the intellectual freedom to innovate, engaged

me in new and avant-gardist ideas, and ameliorated me by demanding a high-quality work from

all my assignments.

I thank everyone who has been a part of our research group for all their valuable suggestions and

inputs and encouragement. A special thanks to Sue Mao and Anna Ho, for helping me at various

times throughout my program. Professor Bussmann has always been supportive by giving his

suggestions, and for this I thank him. I thank all the industry partners, who funded the project, for

without them, this opportunity may not have existed.

I express my deepest gratitude to my ever-loving mother, Sreelatha, and my inspiring father,

Mattahil Mohanan without whom none of this would have been possible. They have been my

inspiration to dream, confidence to live it, and strength to strive for more. Last, but not the least,

I wish to thank Shruti for all her love.

[iv]

Contents

Acknowledgements ........................................................................................................................ iii

Contents ......................................................................................................................................... iv

List of Figures .............................................................................................................................. viii

Glossary .......................................................................................................................................... 1

Introduction: Kraft Recovery Process ..................................................................................... 2

1.1 Lime Kilns ........................................................................................................................ 2

1.2 Ring formation in kilns .................................................................................................... 4

1.3 Burner operation and ring formation ................................................................................ 5

Hypothesis ............................................................................................................................... 7

Objectives ............................................................................................................................... 8

Literature Review .................................................................................................................... 9

4.1 Ring formation in lime kilns ............................................................................................ 9

4.2 Confined jets and recirculation ...................................................................................... 10

4.3 Ring formation and recirculation ................................................................................... 14

Experimental Apparatus ........................................................................................................ 16

5.1 Apparatus for flame visualization study ......................................................................... 16

5.1.1 Mini-diffusion burner .............................................................................................. 16

5.1.2 Transparent Kiln Shell ............................................................................................ 19

5.1.3 Flow Controllers ..................................................................................................... 20

[v]

5.1.4 Miscellaneous ......................................................................................................... 21

5.2 Apparatus for kiln wall temperature profile study ......................................................... 21

5.3 Data Processing .............................................................................................................. 23

5.4 Summary ........................................................................................................................ 23

Experimental Procedure and Observations ........................................................................... 24

6.1 Estimation of optimum air flow rate .............................................................................. 24

6.1.1 Objective and background ...................................................................................... 24

6.1.2 Procedure ................................................................................................................ 24

6.1.3 Observations ........................................................................................................... 25

6.2 Effect of ring position on flame behavior ...................................................................... 25

6.2.1 Objective and background ...................................................................................... 25

6.2.2 Procedure ................................................................................................................ 27

6.2.3 Observations ........................................................................................................... 28

6.3 Effect of ring position on flame behavior at varying ring thicknesses ........................... 29

6.3.1 Objectives and background ..................................................................................... 29

6.3.2 Procedure ................................................................................................................ 29

6.3.3 Observations ........................................................................................................... 29

6.4 Effect of ring position on flame characteristics at higher primary air flow rates ........... 31


6.4.2 Procedure ................................................................................................................ 31

[vi]

6.4.3 Observations ........................................................................................................... 32

6.5 Effect of ring position on flame length with varying ring length ................................... 34


6.5.2 Procedure ................................................................................................................ 34

6.5.3 Observations ........................................................................................................... 34

6.6 Effect of ring position on kiln wall temperature profile ................................................. 36


6.6.2 Procedure ................................................................................................................ 36

6.6.3 Observations ........................................................................................................... 37

Discussion ............................................................................................................................. 39

7.1 Impact of primary air on flame length ........................................................................... 39

7.2 Impact of ring position on flame length ......................................................................... 42

7.2.1 Zone I ...................................................................................................................... 42

7.2.2 Zone II ..................................................................................................................... 43

7.3 Flame Speed and Flame Instability ................................................................................ 45

7.4 Impact of ring length on flame behavior ........................................................................ 46

7.5 Flame length and temperature profile of kiln ................................................................. 46

Conclusions ........................................................................................................................... 48

Appendix A: Detailed drawings .................................................................................................... 49

A.1 Kiln hood ............................................................................................................................ 49

[vii]

A.2 Nozzle ............................................................................................................................... 49

A.3 Quartz tube kiln .................................................................................................................. 50

A.4 Honeycomb discs ............................................................................................................... 50

Appendix B ................................................................................................................................... 51

B.1 Equipment assembly protocol for flame visualization study ............................................. 51

B.1.1 Assembly of mini-diffusion burner ............................................................................. 51

B.1.2 Assembly for flame visualization ................................................................................ 51

B.1.3 Operating the equipment ............................................................................................. 52

B.1.4 Video recording equipment ......................................................................................... 52

B.2 Equipment assembly protocol for kiln wall temperature profile study .............................. 52

B.3 Image processing protocol ................................................................................................. 53

Appendix C: Calculations ............................................................................................................. 55

C.1 Calculation of total air requirement ................................................................................... 55

References ............................................................................................................................. 56

[viii]

List of Figures

Figure. 1. Kraft recovery process [3] .............................................................................................. 2

Figure 2. Lime kiln showing the direction of flow of mud inside it ............................................... 3

Figure 3. Severe ring formation in lime kiln (on left). .................................................................... 4

Figure 4. Typical lime kiln showing the different regions (not to scale) ........................................ 5

Figure 5. Cross section through a Turbulent Jet Diffusive (TJD) burner ....................................... 6

Figure 6. Ring formation in lime kilns: a vicious cycle in operation? ............................................ 7

Figure 7. Internal and external recirculation [8] ........................................................................... 11

Figure 8. External recirculation in a confined jet ......................................................................... 12

Figure 9. Parameters for calculation of Craya-Curtet number in a lime kiln [15] ........................ 13

Figure 10. Variation in flame shape with changes in operating conditions [18] .......................... 14

Figure 11. (a) Premix burner, (b) Kiln hood and (c) Honeycomb discs used in the experiment .. 18

Figure 12. Cross section through the mini-diffusion burner ......................................................... 18

Figure 13. Distribution of air in the burner ................................................................................... 19

Figure 14. Final assembly of mini-diffusion burner in the kiln shell ........................................... 20

Figure 15. P&ID of fuel and air flow controllers .......................................................................... 20

Figure 16. Refractory rings used in the study (from left to right: thickness of 1.5 cm, 2 cm and

2.5 cm respectively) ...................................................................................................................... 21

Figure 17. Experimental set-up to study kiln wall temperature profile ........................................ 22

Figure 18. Flame characteristics with varying primary air: .......................................................... 25

Figure 19. Ring position – distance of the ring from the burner ................................................... 26

[ix]

Figure 20. (a) Feret diameter of a random object along x-axis and y-axis. (b) Flame length

measured using the concept of Feret diameter .............................................................................. 27

Figure 21. Variation in flame length with ring position (6LPM Fuel at 15% PA) ....................... 28

Figure 22. Effect of ring thickness on flame length (top to bottom: 1.5 cm thick ring, 2 cm thick

ring, and 2.5 cm thick ring) ........................................................................................................... 30

Figure 23. Effect of thick rings on flame stability ....................................................................... 31

Figure 22. Effect of ring on flame length at high primary air ....................................................... 33

Figure 23. Variation in flame length with ring position for varying ring lengths ........................ 35

Figure 24. Effect of long rings on flame shape. For the 7.5 cm long ring, a glass piece was placed

in between the two rings ............................................................................................................... 35

Figure 25. Experimental kiln, after a 38-minute exposure. The ring can be seen present inside the

kiln ................................................................................................................................................ 37

Figure. 26. Effect of rings on temperature profile of kiln wall (ring @ 20 cm from burner) ....... 38

Figure 27. Effect of ring on temperature profile of kiln wall (ring @ 0cm from burner) ............ 38

Figure 28. Variation in flame shape with change in primary air ................................................. 40

Figure 29. Effect of recirculation on flame shape ......................................................................... 41

Figure 30. Zones of a diffusion flame ........................................................................................... 42

Figure 30. Thicker ring blows out flame when placed far away from the flame .......................... 44

Figure 31. Frames immediately before flame blow out when the ring was at 40 cm from burner 45

Figure 32. Cross section through kiln hood .................................................................................. 49

Figure 33. Side view of kiln hood ................................................................................................. 49

Figure 34. National N-2 torch tip – serves as the nozzle for primary jet ...................................... 49

[x]

Figure 35. Front view of the quartz tube ....................................................................................... 50

Figure 36. Honeycomb disc (large) – front view and side view ................................................... 50

Figure 37. Honeycomb disc (small) – front view and side view .................................................. 50

Figure 38. Steps in image data processing using ImageJ .............................................................. 54

1

Glossary

Term Definition

Calcination The process of heating limestone to obtain calcium oxide

Craya-Curtet number Signifies the ratio of momentum between the primary and

secondary jet in a confined jet, defined by Craya and Curtet

Feret’s Diameter (as used in ImageJ software)It is the largest distance between the

parallel lines restricting the object

Flame Length Length of the flame measured along horizontal axis

Honeycomb air-flow straighteners Air-flow straighteners built of ceramic with a honeycomb

structure, was used to straighten the flow of air in the kiln hood

Main fuel

Industrial burners are designed to operate on more than one fuel.

The main fuel could vary with mills, but this fuel port would

always be at the center of the burner

Mini-Diffusion Burner The diffusion burner assembled in the lab

Primary air Air supplied through the air-port of the premix burner

Ring Length Length of the ring measured along the axial direction of the kiln

Ring Thickness Length of the ring measured along the radial direction of the kiln

Secondary air Air supplied through the air-ports in the kiln hood

Swirl air Air supplied at a helical angle to the center fuel port, it helps in

improving internal recirculation

Total air The sum of stoichiometric air and excess air

Unstable flame A flame that blew out in less than a time span of one minute

during the experiments

2

Introduction: Kraft Recovery Process

The kraft recovery process uses cooking chemicals to de-lignify wood or separate pulp from

wood in a digester. The spent chemicals form a liquid stream called weak black liquor and are

separated while the pulp is sent for further processing.

Figure. 1. Kraft recovery process [3]

The weak black liquor is concentrated in evaporators to produce heavy black liquor and burned

in a recovery boiler to generate steam, which is used to power the plant. The chemicals are then

dissolved in water to produce green liquor which is then treated with lime to recover the cooking

chemical or white liquor. To complete the recovery process, the lime which gets converted into

lime mud (mostly calcium carbonate) is calcined in a long, cylindrical, rotary kiln to regenerate

lime as per the following reaction. [3]

𝐶𝑎𝐶𝑂3 → 𝐶𝑎𝑂 + 𝐶𝑂2

1.1 Lime Kilns

Lime kilns are rotary kilns, used in calcination of lime. Rotary kilns are also used in other

industries like cement manufacturing and drying of materials. However, owing to their

chemistry, lime kilns are different from the others. They are large steel tubes, lined with

refractory bricks on the inside. These kilns are slightly inclined from the horizontal and are

rotated at a set speed about their axis. Typical rotary kilns vary in size from 7ft (2.1m) in

3

diameter by 175ft (53m) long to as large as 15ft (4.6m) in diameter by 450ft (137.2m) long,

producing anywhere between 45 – 600 metric tons per day of lime. The massive weight of the

kiln is supported on riding rings/tires that encircle the kiln, which also provide the rotary motion

to allow movement of particles inside the kiln [4]. Figure 2 shows a lime kiln, and indicates the

direction of lime mud flow in it.

Figure 2. Lime kiln showing the direction of flow of mud inside it

Lime kilns operate as counter-current heat exchangers, by accepting the wet mud feed at the

higher end. The process of calcination is endothermic and the heat required is supplied by a

burner at the other end of the kiln. As the wet mud tumbles through the kiln due to its rotary

action and inclination, it gets heated gradually, dehydrated, finally undergoes calcination reaction

and is discharged from the lower end of the kiln as lime. The process gasses move through the

kiln, carrying with it the products of combustion and a significant amount of thermal energy.

While the capacity of a kiln is a function of its internal volume, the product quality would

depend upon the firing, feeding rate, operating temperature, and kiln speed or retention time [5].

4

1.2 Ring formation in kilns

As lime mud traverses the length of the kiln, it gradually gets converted to lime. Since this is a

dynamic process, a layer of lime dust is always present along the walls of the kiln. Whenever a

second layer of mud or lime is deposited on top of this layer, the bottom layer of lime is shielded

from the heat and undergoes recarbonation or a reverse reaction to form calcium carbonate that

sticks on to the refractory. With time, depending on the adhesiveness of the lime and lime mud,

more particles adhere and the formation grows into a ring. Often, these rings are very thin, and

the tumbling action of the mud destroys them. However, in severe cases (like the one shown in

Fig.3), rings can grow rapidly and lead to an unscheduled shut-down of the kiln in less than a

month [6].

Figure 3. Severe ring formation in lime kiln (on left). Pneumatic jack hammers have to be used to

remove these (as seen on right)

Based on their position inside the kiln, there are three types of rings that can form in lime kilns.

Mud rings, which are formed within 30 meters from the feed end are observed to be formed with

a high moisture content in the mud and when the feed temperature is low. They are usually soft,

but can grow rapidly. Mid-kiln rings occur in the middle of the kiln, and therefore, are the most

difficult to tackle. They are formed in the region constituting the calcination zone and up to about

30m from the front end. Figure 4 shows the regions inside the kiln where the different types of

rings can form. Since carbon dioxide and calcium oxide are always present in the calcination

zone, a temperature variation can lead to the recarbonation reaction:

𝐶𝑎𝑂 + 𝐶𝑂2 → 𝐶𝑎𝐶𝑂3

5

Even in an ideal kiln with no temperature fluctuations, the reversible nature of the calcination

reaction signifies that some amount of calcium oxide will undergo carbonation and form calcium

carbonate along the refractory layer. Deposition of dust on this surface will shield this layer from

the heat source and thereby prevent this layer from undergoing calcination. Thus, this layer of

carbonate thickens along the radius of the kiln. However, for it to grow along the axis and

withstand the constant attrition faced by the ever-tumbling lime mud, temperature fluctuations

are necessary. Temperature fluctuations inside the kiln will lead to formation of more zones

within the kiln which would favor recarbonation, thus resulting in ring growth along the axis of

the kiln [7].

Figure 4. Typical lime kiln showing the different regions (not to scale)

Front end rings are formed closest to the burner. Sulphation of calcium oxide, besides

recarbonation, can be a mechanism responsible for ring growth close to the burner. The

concentration of sulphur dioxide at the high temperature close to the burner is the driving force

for this sulphation reaction [6] [7].

The lime kiln is demanding in terms of its thermal energy requirements. Specific energy

consumption in a typical lime kiln is about 8.4 MMBtu/ ton of Calcium oxide [4].

1.3 Burner operation and ring formation

Burners are usually of two categories: premix burners and diffusion burners. In premix burners,

the fuel and air are first mixed in a chamber or a venture mixer, and then ignited once outside the

chamber. The mixing chamber is designed to efficiently mix fuel and air so that the mixture

ejecting from the mixing chamber may have a fuel-air ratio within the flammability range. This

may be ensured by providing the air and fuel at a constant designed flow rate and pressure [8].

These burners are usually used in small scale or at a laboratory scale.

6

Industrial burners are usually diffusion burners. In these burners, the mixing of fuel and air

happens outside the burner. The fuel stream is ejected into a slow-moving stream of air or

oxidizer. The difference in velocities of the fuel and air streams cause the air to entrain into the

fuel stream, thereby mixing with it, and increasing the air-fuel ratio to the flammability range. A

steady change in the concentrations of gasses may also be observed in such burners [8].

A distinctive character of a diffusion flame which favors its use in the industry is that the rate of

combustion is determined by the rate at which air and fuel are brought together in proportion for

combustion. This reduces the process control required to produce a steady combustion when

compared to that when using a premix burner [9].

Newer designs of burners tend to improve the efficiency of combustion by various mechanisms.

Figure 5. Cross section through a Turbulent Jet Diffusive (TJD) burner

Most of them are designed to operate using different fuel sources, giving the flexibility for the

mills to change their fuel according to the economic situation at hand. Figure 5 shows a

Turbulent Jet Diffusion burner (POLFLAME rotary kiln burner by Thyssenkrupp 2016).

The main fuel port supplies the main fuel, while the air is supplied through two different

channels. A primary air channel supplies about 6-20% of the total air, while the rest of the air is

supplied as secondary air. The primary air exits the burner as swirl air which helps to improve

the air-fuel mixing and the secondary air enters the kiln through the kiln hood.

Fuel ports

Swirl/ Primary Air

ports

7

Hypothesis

As discussed before, combustion happens outside the burner in a burner and happens in two

steps: first, bringing the fuel and air into contact, and second is maintaining the air to fuel ratio

within the explosive limit to sustain the combustion. The first step, of bringing the air and fuel

together is determined by recirculation, which again depends on many factors, as discussed later

in this document. Any obstruction to the air or fuel flow patterns could therefore affect the

combustion and therefore the performance of these burners.

Rings formed close to the burner (front end rings or mid-kiln rings) could possibly disrupt the

aerodynamics of burner operation, which could lead to further temperature fluctuations.

Following this, the initial layer of ring already formed on the kiln wall could grow by the

recarbonation mechanism. Moreover, rings could lead to flame impingement, because of which

the surface temperature of the rings may rise significantly.

Once the ring grows thicker, it could lead to even more flame impingement and temperature

fluctuation, and lead to ring growth again. Thus, a cycle could be in play during ring formation in

lime kilns (Figure 6).

Figure 6. Ring formation in lime kilns: a vicious cycle in operation?

?

Ring formation

Affect flame shape

Temperature fluctuations

8

Objectives

The objectives of this study are as follows:

• Determine if rings can induce changes in flame length and stability

• Observe the effects of ring position, thickness and length on flame length and stability

• Examine the effect of a ring on the temperature profile on the inner surface of a kiln

9

Literature Review

4.1 Ring formation in lime kilns

Ring formation in lime kilns is a very common problem in most pulp mills. Depending on the

severity of ringing, maintenance labor and make-up lime required, the expenses can increase to a

significant amount. Understanding the theory behind ring formation would help operators to

make modifications to standard operating procedures, and thereby, prevent or reduce the rate at

which rings form in kilns. Based on the type of ring formed, their composition can vary from the

initial mud composition to the final product composition. Mud rings are formed at lower

temperatures, close to the feed end of the kiln, thereby resulting in a composition like that of lime

mud. Mid-kiln rings and front-end rings have more calcium oxide (CaO) and calcium sulphate

(CaSO4) than mud rings. It has also been observed that mills that burn Non-Condensable Gasses

(NCG) or other fuels which have a high sulphur content, tend to have rings higher in sulphur

content (usually present as CaSO4). Another interesting observation is that rings formed in the

front end and mid-kiln sections of the kiln, where the gas temperatures are higher than the

calcining temperature, still contain some CaCO3 [6]. This observation supports the recarbonation

mechanism of ring formation.

For rings to form, it is necessary that materials first adhere to the surface of the refractory lining,

and then gain sufficient mechanical strength to resist the constant attrition caused by the

tumbling materials inside the kiln. Addition of sodium compounds (Na2CO3) to lime mud more

than 2% by weight resulted in increased adhesion, as observed by Tran et. al. in their

experimental kiln. These observations were made at temperatures of 9000C and 10000C [7].

The process by which rings gain strength and grow to disrupt operations may be explained using

the phenomena of recarbonation. When calcium carbonate is calcined in the lime kiln, the

products – lime and carbon dioxide, remain inside the environment of the kiln. A fall in

temperature can result in the backward reaction happening, forming calcium carbonate. Calcium

carbonate has more mechanical strength than calcium oxide. When a layer of calcium carbonate

is formed on the surface of the refractory lining (due to the reversible calcination reaction), and a

layer of calcium oxide gets deposited above this layer, the calcium oxide layer insulates the

calcium carbonate, and thereby prevents it from undergoing further calcination. A second coat of

calcium oxide would now shield the first layer of calcium oxide from high temperatures, and the

10

presence of carbon dioxide in this environment could lead to carbonation of the first layer of

calcium oxide. This process gets repeated and subsequently leads to an increase in thickness of

the ring [7].

This mechanism of ring growth is probable even in a kiln that is operated perfectly, the only

causes being the reversible nature of the reaction, and mechanical strength of calcium carbonate.

However, the rings formed in this case would thin and formed at the point where the temperature

inside the kiln is the same as the calcination temperature. The ring would be thin and weak, and

the high mass flow rate of material through the kiln would destroy this ring, and prevent it from

being catastrophic [7].

If a fluctuating temperature profile on the kiln wall is added to this ideal condition, it would

result in the ring growing in length along the axis of the kiln, since now, the calcination

temperature may be achieved in other cross sections along the length of the kiln. Therefore, even

though ring growth happens through recarbonation, fluctuation in temperature profile within the

kiln is necessary for the ring to grow in length along the axis and make it stronger. A primary

source of temperature fluctuation in the kiln is flame impingement. A ring of a significant size

can cause a local hot spot [10].

A second source of temperature fluctuation is the result of modifications in fluid flow patterns

caused by the ring. Since air-fuel mixing happens outside the burner, the ring can affect the fluid

flow patterns and thereby the air-fuel mixing. Understanding the fundamentals of a TJD burner

used in kilns is therefore necessary, to evaluate the situations that might lead to temperature

fluctuations within the kiln.

4.2 Confined jets and recirculation

As discussed before, the burners used in kilns are diffusion type. Recirculation is an important

concept that helps to sustain the combustion in a TJD burner. Once a flame has been established

in a diffusion burner, it would continue to burn only if it receives a feedback of heat energy from

the combustion zone to sustain the process of combustion. The two major mechanisms for this

process are re-radiation and recirculation. Of these, re-radiation happens due to the heat radiation

from the combustion zone and depends, to a great extent, on the fuel burnt and on the air-fuel

11

ratio. Recirculation, however, depends on the fuel and air flow patterns, which can be controlled

by the design of the burner and the kiln.

Recirculation happens when some of the flow in or around the burner is caused to move in a

direction opposite to that of the jet. It transports the hot combustion products back to the nozzle

or close to it. These hot combustion products raise the temperature of the fuel-air mixture to the

ignition temperature and sustain the combustion process. The process of recirculation may be

internal or external, as shown in figure 7. Furnaces with smaller volume tend to show little or no

external recirculation. Rotary kilns, however, have a high volume and therefore, have significant

external recirculation. This not only helps to confine the flame to a medium length and keep it

compact, but also protects the refractory walls from the extreme heat [8].

Figure 7. Internal and external recirculation [8]

To achieve recirculation in burners in lime kilns, the burners are designed so that the fuel ejects

out of the central port in the burner and the air flows around this – creating a confined jet. Due to

the small radius of the fuel port, the velocity of the fuel jet is higher than the air that flows

around it creating a local acceleration at point of contact between the fuel and air streams,

thereby causing an entrainment of air into the fuel jet. Since the entrainment of air into the fuel

jet happens inside a confined space (of the kiln), diffusion of air into the primary jet creates a

vacuum along the wall of the kiln, as shown in Figure 7. This creates zones of eddies, and results

in external recirculation [11]. External recirculation improves air-fuel mixing by creating

turbulence around the flame front [11, 8].

Internal recirculation is achieved in burners using swirl air. A small portion of the air required for

combustion is supplied as primary air via the swirl air-ports (as seen in figure 5). They are

introduced at a higher velocity than the surrounding air stream and at an angle so as to form a

12

helix around the fuel stream. The swirl air increases the air-fuel mixing, thereby increasing the

efficiency of combustion. However, this increase in efficiency is not without a cost since the

swirl air or the primary air is injected at a higher velocity than the air surrounding the jet, which

requires a higher fan power [8].

Figure 8. External recirculation in a confined jet

Figure 8 shows the recirculation patterns in a confined jet stream as explained by Craya and

Curtet. They developed a mathematical approach [11] and treats the difference in momenta

between the primary and the secondary jets to estimate a similarity parameter, 𝑚, which

quantifies the strength of recirculation. A number of studies have examined the criteria for

recirculation, both numerically and experimentally [12, 13, 14]. The criteria of recirculation for a

constant density system may be obtained by applying the momentum and continuity equations.

Assuming:

• shear stress at wall is negligible

• density is uniform

• flows of primary and secondary jets are uniform

• radial pressure gradient is neglected

Craya and Curtet defined the similarity parameter, 𝑚, as follows:

𝑚 = −1.5𝑅2 + 𝑅 + 𝐾𝑅𝑑1

𝑑0 (1)

Kiln Wall

Secondary Air Recirculation Zone

Entrainment of Secondary Air Burner

Mix of primary air and fuel

13

𝑅 = (𝑢0−𝑢1)𝜌0(

𝑑02

)2

𝑢1𝜌1(𝑑12

− 𝛿)2+(𝑢0−𝑢1)𝜌0(𝑑02

)2 (2)

where,

𝐾 = Jet shape factor, has a value of 1 for a round jet

𝑢0 = primary jet velocity

𝑢1 = secondary jet velocity

0 = density of primary jet

1 = density of secondary jet

𝑑0 = diameter of primary jet

𝑑1 = diameter of secondary jet

𝛿 = boundary layer thickness

In a lime kiln system, the above parameters may be represented as shown in Figure 9.

Figure 9. Parameters for calculation of Craya-Curtet number in a lime kiln [15]

Recirculation and the conditions necessary for recirculation have been extensively studied [12,

14, 13, 16]. While some studies were experimental, others used computational methods to

estimate the causes and effects of recirculation. It was noted that beyond a Reynolds number of

5000, a further increase in Reynolds number did not have an impact on the similarity parameter,

14

while below this value, a reduction in Reynold’s number reduced the value of the similarity

parameter. It was also estimated that when the similarity parameter is 1.5, the primary has

enough momentum in excess to that of the secondary jet to entrain the air required for complete

combustion of the fuel in the primary jet. Another important observation due to a reduction in

similarity parameter is the increase in flame length. At lower values of similarity parameter, the

flame requires more time to mix and undergo combustion, thereby increasing the flame length.

Figure 9 shows the different kinds of flame shapes possible in a kiln based on recirculation.

Increasing flame length leads to long flame that is buoyant and would therefore result in

lowering the heat flux available at the kiln wall.

4.3 Ring formation and recirculation

Figure 10. Variation in flame shape with changes in operating conditions [18]

Most lime kilns operate at a similarity parameter between 1.5 and 2 [17] to have sufficient

recirculation for a steady flame, but not too much recirculation to blow out the flame. The swirl

air, even though unaccounted for in the Craya Curtet similarity parameter, significantly increases

the air-fuel mixing by improving the internal recirculation [8]. However, if we neglect the effects

of the swirl air, and consider the similarity parameter alone, theoretically, a ring reduces

recirculation by reducing the inner diameter of the kiln shell. Subsequently, air-fuel mixing

reduces, thereby reducing the heat flux from the flame. It could also lead to a long, lazy and

buoyant flame which could impinge on the refractory causing significant refractory damage.

15

Both these cases could lead to a fall in temperature, which causes recarbonation leading to ring

growth and/or ring strengthening. Thus, ring growth could be accelerated over its course due to

the interaction between the flame and the ring.

As is evident from the literature review, a significant knowledge gap exists in the understanding

of the effects that a ring could have on the kiln by affecting the burner. Collection of data from

operating lime kilns is very difficult due to the extreme operating conditions of the kiln. The

complex heat and mass transfer mechanisms makes dynamic modelling of the system extremely

difficult. A solution to both these problems is to set up a laboratory model, which may be used to

study these effects. The next sections describe the experiment and discuss the results obtained.

16

Experimental Apparatus

The experiments conducted in this work may be broadly classified into two types: flame

visualization studies which focused on understanding the impact of rings on the visual

characteristics of the flame, and a temperature profile study, which focused on understanding the

effects of rings on the temperature profile of the kiln wall.

In the first part of the study, the visual appearance of the flame was studied extensively.

Characteristics of the flame like its length and stability were observed under different conditions,

like varying flow rates, and presence of rings of varying thickness and length at different

locations inside the kiln. The second part of the study was to understand the variations in

temperatures inside the kiln caused by the presence of the ring.

The apparatus for each of these studies are as follows.

5.1 Apparatus for flame visualization study

The apparatus for flame visualization study had three important components: a mini-diffusion

burner, a transparent kiln shell and flow controllers.

The mini-diffusion burner was the most important part of the apparatus. It was set-up so as to

mimic the operation of an industrial TJD burner. This mini-diffusion burner was used to fire into

a transparent kiln made of quartz tube, which facilitated an unobstructed view of the flame. A

ring of certain thickness and length was placed inside the quartz kiln at predetermined locations.

The burner was connected to air and fuel supplies via separate flow controllers to have a control

over the operating conditions of the burner.

Thus, the effect of different operating conditions of the burner, as well as the effect of variations

in properties of the ring, could be simulated in this apparatus. The following sections explain in

detail the construction of each piece of this apparatus.

5.1.1 Mini-diffusion burner

Configuring a laboratory scale turbulent jet diffusive burner was challenging for the following

reasons:

17

1. scale and complexity: increasing the scale would reduce the complexity in construction of

the burner, but it would make the burner too big and too dangerous to use in a laboratory

environment. Reducing the size, however, would increase the complexity of the burner.

2. buoyancy: The flame produced is always buoyant. The Froude number (Fr) is a

dimensionless number used to estimate the effects of buoyancy in a flame [8]. It may be

calculated as follows.

𝐹𝑟 ≡ (𝑣𝑜

2

𝑔𝑦𝑓) (3)

where,

𝑣𝑜 = axial velocity of the stream at the exit of the nozzle

𝑔 = acceleration due to gravity

𝑦𝑓 = flame height

The higher the Froude number, the less is the effect of buoyancy on the flame and vice-versa. To

reduce the buoyancy, therefore, a high velocity jet is required. However, increasing the

volumetric flow rate of the fuel would lead to a high heat flux, and would make the equipment

difficult to operate on a bench scale. Therefore, the only solution is to reduce the diameter of the

nozzle, to increase the velocity of the primary jet, but keep the volumetric flow rate of the fuel to

a minimum.

Due to the above-mentioned reasons, a mini-diffusion burner was set-up instead of a full-scale

TJD burner. The mini-diffusion burner was configured using a premix burner (Figure 11 a), a

kiln hood (Figure 11 b) and honeycomb discs (Figure 11 c). The premix burner was a standard

3A hand torch, supplied by National Torch with a 330-16S extension and an N-2 nozzle. It was

designed to accept fuel and air through the two ports. The burner was designed with air and fuel

lines opening into a common chamber, which ends in a burner tip, so that by the time the gasses

exit the burner, they are thoroughly mixed. This burner was placed co-axially inside a kiln hood,

using the honeycomb-discs as support. A cross section of the final assembly of the mini-diffusion

burner is as shown in Figure 12. A detailed drawing of the mini-diffusion burner assembly is

attached in Appendix A.

18

Figure 11. (a) Premix burner, (b) Kiln hood and (c) Honeycomb discs used in the experiment

Figure 12. Cross section through the mini-diffusion burner

The total air required for combustion was calculated based on the stoichiometry of combustion

reaction of methane, as shown in Appendix C. A small fraction of this air, the primary air, was

supplied to the system via the air-port of the premix burner. The rest of the air was supplied via

the secondary air-ports of the kiln hood. The following infographic clarifies the air distribution to

the burner.

Thus, a primary air stream was introduced into the system, which behaves like the swirl air in the

TJD burner, and a secondary air stream, which moves relatively slower than the primary jet. The

fuel, methane, was supplied via the fuel-port of the premix burner. The honeycomb discs serve

(a)

(b)

Burner Port Secondary Air-port

Secondary Air-port

Honeycomb disc

(c)

Premix

Burner

Secondary Air-port Kiln hood Honeycomb discs Nozzle

19

dual purposes: supporting the premix burner by helping it to align co-axially, and creating a

uniform flow of secondary air.

Figure 13. Distribution of air in the burner

A major difference in operation of the diffusion burner in the lime kiln and the experiment was

that while the burners in kilns are operated at a turbulent flow regime, the burner in the

experiment was operated in a laminar flow regime.

5.1.2 Transparent Kiln Shell

A major challenge faced in this study was the material of construction for the kiln shell – it has to

be a material that can withstand temperatures up to 14000C while being transparent at the same

time, so that the flame maybe observed. An operational kiln shell is made of iron and has a

coating of refractory materials on its inside, to reduce heat losses to minimum, which, however,

prevents optical exposure of the processes happening inside. Therefore, in this study, a

transparent kiln shell, made of quartz, was used to visually observe the changes happening to the

flame shape under different conditions. Quartz was an ideal material since it has a high

temperature resistance while being transparent at the same time.

The inner diameter of the tube was such that the kiln hood fits perfectly into the kiln shell. The

apparatus was configured by inserting the nozzle-side of the mini-diffusion burner into the kiln

shell. Different conditions were simulated inside the kiln by placing rings made of refractory

material inside the kiln. The whole setup was supported on steel blocks, after checking their

horizontality using a spirit level. Figure 14 illustrates the final burner-kiln shell assembly.

Stoichiometric Air

Excess Air

Total Air

Primary Air

Secondary Air

20

Figure 14. Final assembly of mini-diffusion burner in the kiln shell

The apparatus was placed on a laboratory bench, in such a position that video recordings of

experiments could be recorded with the least parallax error.

5.1.3 Flow Controllers

Accurate flow control was a requirement of this study due to the dependence of recirculation and

air-fuel mixing on the flow rates. While a manual flow meter was used to control the fuel flow

rate, computer-controlled Alicat (Model MCR 150) flow meters were used to control the primary

Figure 15. P&ID of fuel and air flow controllers

Met

han

e A

ir

21

and the secondary air flow rates. The P&ID of the setup is as shown in figure 15.

5.1.4 Miscellaneous

Besides the above-mentioned pieces of equipment, a video camera and refractory rings were

used in the study.

The camera was a Sony design, and used to record the runs. It was mounted on a tripod, and set

to capture videos at the maximum quality possible. Due to the extreme heat generated from the

experiment, it was impossible to carry out live measurements of the flame characteristics. The

video recording facilitated analysis at a later point in a safer environment.

Refractory rings were used to simulate the rings inside lime kilns. These rings were made of K-

26 refractory bricks. They were made in three sizes: 1.5 cm thick, 2 cm thick and 2.5 cm thick, as

shown in Figure 16. All of them had the same outer diameter, of 8.9 cm, so that they could be

placed inside the quartz kiln and the refractory kiln, which is described below.

Figure 16. Refractory rings used in the study (from left to right: thickness of 1.5 cm, 2 cm and

2.5 cm respectively)

5.2 Apparatus for kiln wall temperature profile study

Temperature is an important factor that determines the rate of ring formation. A modification of

flame shape will therefore, not affect the dynamics of ring growth, unless accompanied by

temperature fluctuation.

The apparatus to study the kiln wall temperature profile is similar to that used for the flame

visualization study. It uses the same mini-diffusion burner and the flow controllers. However, a

kiln made of refractory bricks is used in place of the kiln made of quartz.

22

Figure 17. Experimental set-up to study kiln wall temperature profile

A disadvantage of using the quartz glass tube as the kiln is that accurate temperature

measurements are difficult from the curved surface of the glass. To overcome this difficulty, a

kiln was made by drilling a hole through an assembly of 12 refractory bricks as shown in the

figure. These bricks had a temperature rating of 2600oF (1425oC). A solid block was made by

joining 12 refractory bricks along the long face using mortar. The hole drilled had a diameter of

90mm, so that the mini-diffusion burner can perfectly slide into this. Small holes, 2mm in

diameter, 2.5 cm apart, were drilled through the top face of the block, all the way up to the

central kiln. These holes would later be used to insert thermocouples into the kiln, to measure the

Burner Thermocouples

Thermocouples

23

temperature profile of the kiln wall. A 3-D model of the final assembly is as illustrated in Figure

17.

5.3 Data Processing

Characterizing the physical properties of a flame was quite challenging due to its dynamism. The

flame, therefore, had to be analyzed at each instant. The video was broken down into frames, and

each frame was analyzed for flame length to provide useful information. ImageJ (a free and open

source software) was used to analyze a series of 320 random frames, processing each to give the

feret diameter of the flame along the horizontal axis. Due to the large number of frames

considered, error was reduced significantly in the final readings.

5.4 Summary

To summarize, two series of experiments were conducted. While one was focused on analyzing

the changes in visual patterns of the flame under different conditions, the other was aimed at

understanding the variations in temperature profile of the kiln wall surface under different

process conditions. Studying the temperature profile of the kiln was necessary since temperature

is the driving force in ring formation and ring growth.

The main apparatus used in the study included a mini-diffusion burner, to simulate flow patterns

of a TJD burner, control valves for accurate flow control, a kiln made of quartz tube to visualize

the flame, and a kiln made of refractory bricks, to measure the temperature profile of the kiln

wall. Other apparatus included refractory bricks which were used to simulate rings inside the

kilns, and honeycomb air-flow straighteners, to aid in developing a fully developed flow of

secondary air in the burner.

24

Experimental Procedure and Observations

The steps for assembling the equipment for the flame visualization study and the kiln wall

temperature profile study are discussed in Appendix B. The study was conducted in steps as

explained in the following sections.

6.1 Estimation of optimum air flow rate

6.1.1 Objective and background

Since the mechanism of combustion and aerodynamics of the mini-diffusion burner plays a

major role in determining the flame shape and thereby the effects on rings, it was necessary that

a consistent and stable operating condition was first identified for the burner. The flame

produced by the burner depends on recirculation which can be varied by changing the primary

and secondary air flow rates.

Due to the small size of the kiln, high similarity parameters – similar to those in operating lime

kilns – cannot be used as they would blow out the flame. The mathematical estimation of these

values would require in-depth analysis of fundamentals, which would be extravagant for this

study. Therefore, a trial and error method was employed to estimate the ideal flow conditions to

operate the burner.

This study would also help us understand how recirculation, signified by the value of similarity

parameter, can affect the flame. Increasing the primary air flow rate increases the similarity

parameter and vice-versa.

6.1.2 Procedure

A step-by-step description for setting up the apparatus is given in Appendix B. The general

protocol was as follows: for a fixed flow rate of methane, primary air was varied from 0% of the

total air to 25% of total air.

Runs were now conducted at each of these flow conditions. Each run lasted for 1 minute and was

recorded using the video recorder. The experiment was then repeated to minimize error. The

flow conditions were calculated as given in Appendix C.

25

6.1.3 Observations

It was observed that as the primary air increased from 0% of total air to 15% of total air, the

flame length reduced and the flame became less buoyant. Its tint changed from yellowish to

bluish and became less sooty. The length of the flame that had a bluish tint increased gradually

with increase in primary air. Increasing the primary air further, to 25% of the total air, did not

reduce the flame length significantly. However, it resulted in an unstable flame which blew out

immediately. The observations from this run are presented in Figure 18 as snapshots from the

video recordings.

Figure 18. Flame characteristics with varying primary air: (a) PA 0% (b) PA 5% (c) PA 10% (d)

PA 15% (e) PA 20% (f) PA 25% (all values expressed in percentage of total air

6.2 Effect of ring position on flame behavior

6.2.1 Objective and background

Rings can form at different locations within the kiln. If the ring were to form at a location very

close to the hot end of the kiln (practically, this is less probable due to the ease of access to this

section of the kiln and any build up can be treated quickly), the only factor it may affect would

be the secondary air flow. However, if a ring forms further away, where the flame is expanding,

it would reduce the area available inside the kiln, thereby affecting the flow of secondary air.

Therefore, based on its position, the effect a ring can have on the flame may vary.

The main objective of the following experiment was to understand how rings can affect flames

based on their position. This may be understood by placing a ring at different locations inside the

kiln, which is operated at the same conditions. The primary variables in this study include ring

PA 0% PA 5% PA 10%

PA 15% PA 20% PA 25%

26

position and flame length. Ring position is defined as the distance of the ring from the burner,

inside the kiln. This is a set length, which is one of the defining conditions at which the

experiment is performed. Figure 19 illustrates the definition of ring position.

Figure 19. Ring position – distance of the ring from the burner

A primary variable that is studied in these series of experiments is the flame length. The flame

length can give us a lot of useful information about the flame. For example, a longer flame is an

indication that the air and fuel have been mixed slowly, which results in a longer time for

completion of combustion. However, measuring the length of the flame, which is dynamic, is a

very tricky proposition. A looming problem here is how to define the length of the flame. Some

authors previously used temperature profile to visualize flame [19, 20]. However, in this study

we are more interested in a comparison, rather than an absolute measurement of flame lengths.

As such, flame length is defined as the Feret diameter of the flame along the horizontal. The

Feret diameter is defined as the maximum distance or the caliper distance along a particular axis

for the given figure. Figure 20 gives an understanding of the Feret’s diameter and definition of

flame length. To estimate this, each run is recorded on a video recorder and analyzed frame by

frame using an image processing software. Flame length in each of these frames was analyzed,

and then an average flame length for each run was calculated. Details on image data processing

for calculating the flame length is given in Appendix B, section 9.3.

27

As evident from here, a drawback in this method is the use of a visual flame to measure its

length. Some portions of the flame may be barely visible to the naked eye, due to the chromatic

aberrations of the lens of the video recorder. To standardize the process, all the frames were

subject to the same processing techniques, to keep the results as uniform as possible.

Figure 20. (a) Feret diameter of a random object along x-axis and y-axis. (b) Flame length

measured using the concept of Feret diameter

6.2.2 Procedure

The equipment was set up by inserting the mini-diffusion burner into the experimental kiln made

of quartz. Gas connections were made as shown in P&ID in figure 14 and a ring, 1.5cm thick

was placed directly in front of the burner. The burner was ignited and fuel flow rate increased in

increments of 1.5 LPM to reach 6 LPM. Total air to be supplied was calculated as in Appendix

C, with an addition of 10% excess air. Primary air was set at a constant flow rate of 9.43 LPM

(15% of total air) and the run was recorded for a period of 1 minute. The ring was then moved

further away from the burner so that it is now 3 cm from the face of the burner. The burner was

operated and resulting flame video recorded, as in the previous case. The procedure was repeated

Feret diameter along x-axis

Feret

diameter

along y-axis

(a)

Flame Length

(b)

28

by changing the position of the ring with respect to the burner as indicated in the table below.

Beyond 40 cm, the ring seemed to have no effect on the flame and these runs were therefore

neglected.

The video recordings were separated into frames, and each frame analyzed individually to

measure the flame length. A detailed procedure followed has been explained in Appendix B.

6.2.3 Observations

Image analysis using ImageJ yielded the following curve between flame length and ring position

(Figure 21). A series of three experiments were conducted and the images from all the three were

randomly chosen to give the following result. Standard error was less than 2 mm and therefore,

error bars have been omitted from the figures.

Figure 21. Variation in flame length with ring position (6LPM Fuel at 15% PA)

The maximum flame length was observed when the ring was placed right in front of the burner.

As the ring is moved away from the burner, to a distance of 15 cm from the burner, the flame

length gradually decreases. Beyond this point, the effect of ring position is negligible on the

flame length. However, the flame is significantly longer than when there is no ring.

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Fla

me

Len

gth

(cm

)

Ring Position (cm)

PAF 0.15

No Ring 0.15

With RingWith ring

Without Ring

29

6.3 Effect of ring position on flame behavior at varying ring thicknesses

6.3.1 Objectives and background

Ring thickness is the thickness of the ring measured along its diameter. In lime kilns, rings

formed can vary in thickness. Rings, when thin, do not pose much of a problem in operation of a

lime kiln [6], while thicker rings have to be removed and might require a kiln shut-down –

leading to additional cost. The thickness of rings change the available area for air flow inside the

kiln, which might affect recirculation patterns and thereby affecting the burner operation.

However, as observed in the previous section, the position of a ring can also affect the flame.

Therefore, it is necessary to include the position of ring with respect to the burner while studying

the effects of its thickness. The methodology described in the following section is aimed at

collecting information that can shed light on the effect of ring thickness on flame behavior.

6.3.2 Procedure

The equipment assembly was similar to that used for studying effects of flame on ring position

(as described in section 5.2.2). The primary air flow rate was set to be 15% of the total air to be

supplied. Total air to be supplied was calculated as given in Appendix C, corresponding to 6

LPM of methane flow. An excess air of 10% of the primary air was supplied.

Initially, the 1.5 cm thick ring was used. Next, the run was repeated by replacing the ring with

the 2 cm thick ring and later, with the 2.5 cm thick ring. Runs were conducted by placing the

rings, one after the other, at varying locations from the burner as in the previous experiment.

Each run was conducted for one minute, or until the flame blew out, whichever happened earlier.

Video recordings of the experiments were captured, and processed as per the image processing

protocol as explained in Appendix B, section 7.3.

6.3.3 Observations

Flame length versus ring positions for the three different rings is as shown in figure 22. During

this series of runs, sometimes the flame blew out, and these have been represented in the figure

with a flame length of 0 cm. A plot of flame length versus ring position is as follows.

30

Figure 22. Effect of ring thickness on flame length (top to bottom: 1.5 cm thick ring, 2 cm thick

ring, and 2.5 cm thick ring)

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Fla

me

Len

gth

(cm

)

Ring Position (cm)

No Ring

2 cm Ring

Without Ring

With Ring

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Fla

me

Len

gth

(cm

)

Ring Position (cm)

PAF 0.15

No Ring 0.15Without Ring

With Ring

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Fla

me

Len

gth

(cm

)

Ring Position (cm)

No Ring

2.5 cm Ring

Without Ring

With Ring

1.5

cm

thick

ring

2 c

m th

ick rin

g

2.5

cm

thick

ring

31

Figure 23. Effect of thick rings on flame stability

Increasing the ring thickness from 1.5 cm to 2.5 cm for a ring placed right in front of the burner

leads to flame blow out. The thicker rings blew out the flame when placed close to the burner. It

may also be noted that thicker rings, when kept within 15 cm of the burner, reduced the flame

length significantly, as compared to the flame length while using a 1.5 cm ring. The flame

lengths while using 2 cm and 2.5 cm thick rings, at distances less than 10 cm from the burner,

were equal to or less than that while not using any rings.

6.4 Effect of ring position on flame characteristics at higher primary air flow rates


Increasing the primary air flow rate increases the similarity parameter, and thereby the

recirculation. We may calculate similarity parameter, 𝑚, using equations 1 & 2. At a fuel flow

rate of 6 LPM, total air flow rate of 62.86 LPM, and when 20% of the total air is supplied as

primary air, the similarity parameter is calculated to be 0.6. When primary air is increased to

22% of total air, the similarity parameter increases to 0.66, while decreasing the primary air to

17% of total air reduces the similarity parameter to 0.51.

6.4.2 Procedure

For this experiment, the burner was operated at a constant fuel flow rate, but varying primary air

flow rates. Initially, it was run at a primary air flow rate equal to 22% of the total air supplied,

Ring at 0 cm Ring at 15 cm Ring at 30 cm

32

followed by 20%, and 17% respectively. At each of these flow rates, the 1.5 cm thick ring is

placed at different distances from the burner: 0 cm, 3 cm, 6 cm, 9 cm, 12 cm, 15 cm, 20 cm, 25

cm, 30 cm, and 40 cm.

Once the apparatus has been set up as described in the previous sections, the primary air flow

rate is set to the 13.83 LPM and secondary air flow rate to 49.03 LPM, which corresponds to

22% of total air supplied as primary air. The 1.5 cm thick ring is placed right in front of the

burner. A one-minute video recording of the run, after the fuel flow rate reaches its final value

(6LPM) is captured. Next, the ring was moved to its next location (3 cm) and the run is repeated.

Once, runs have been completed at all the ring positions, primary air flow rate is changed to

12.57 LPM and secondary air flow rate to 50.29 LPM. The runs are then repeated by varying the

ring positions. Next, primary and secondary air flow rates were set to 10.69 LPM and 52.17 LPM

respectively, and the runs were repeated by varying the ring positions.

6.4.3 Observations

The effects of rings on flames while changing the primary air flow rates were interesting. At a

primary air flow rate of 13.83 LPM, which corresponds to 22% of the total air, the flame blows

out in the absence of a ring. Placing a ring right in front of the burner or at 3 cm from it did not

change this behaviour. However, when the ring was placed at positions 6 cm, 9 cm, and 12 cm

away from the burner, the flame did not blow out.

A flame that blew out is represented by a flame length of 0 cm. At a primary air flow rate of

12.57 LPM (20% of total air), the effect of rings on the flame characteristics is different from

that in the above case. In the absence of a ring, the flame is unstable in these process conditions

and blows out. However, placing the ring between 3 cm and 20 cm from the burner, prevents the

flame from blowing out.

When the primary air flow rate was reduced to 10.69 LPM, (17% of total air), the flame behavior

was affected by the ring in a manner similar to that when primary air was 15% of total air. The

flame remained steady and was not blown out.

Figure 22 shows the results from these observations.

33

Figure 22. Effect of ring on flame length at high primary air (from top to bottom: 17%, 20% and

22% primary air). X-axis represents ring position in all the cases.

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Fla

me

Len

gth

(cm

)PAF 0.22


With Ring

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40

Fla

me

Len

gth

(cm

)

PAF 0.2


With Ring

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40

Fla

me

Len

gth

(cm

)

Ring Position (cm)

PAF 0.17


With Ring

34

6.5 Effect of ring position on flame length with varying ring length


The length of a ring, in this context, is defined as the length of the ring along the axis of the kiln.

Rings formed in lime kilns, which can disrupt operations, are often long and thick [7]. A long

ring can reduce the volume inside the kiln area by a significant amount. The following

experiment was set-up to study the effects a long ring would have on flame shapes when placed

at different locations within the kiln, while other conditions like primary air remained constant.

6.5.2 Procedure

To study the effect of long rings, runs were conducted by placing three rings of varying length

(2.5 cm, 5 cm, and 7.5 cm) at different positions inside the kiln. All the rings had the same

thickness (2 cm) and the operating conditions of the burner were maintained the same through

these runs. A 2.5 cm long ring was the same ring used previously. Two of these rings were

placed together to simulate a ring that is 5 cm long. A 7.5 cm long ring was configured by

placing two rings, supporting a glass piece between them.

6.5.3 Observations

Figure 23 shows the observations from the experiment. As the ring length increased from 2.5 cm

to 5 cm, the flame length increased significantly. The flame still blew out when the ring was

placed right in front of the burner or when placed more than 30 cm away from it and increasing

the ring length from 5 cm to 7.5 cm had no significant difference. These observations suggest

that ring thickness plays a major role in determining the flame shape. Increasing the ring length

can increase the flame length, presumably due to the reduction in volume inside the kiln.

Figure 24 shows the images from the experiments conducted to study the effect of ring length on

flame length. The 7.5 cm rings were constructed by placing a glass piece in between the two

rings.

35

Figure 23. Variation in flame length with ring position for varying ring lengths (primary air was

15% of total air, fuel flow rate was 6 LPM)

Figure 24. Effect of long rings on flame shape. For the 7.5 cm long ring, a glass piece was

placed in between the two rings

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30 35 40 45

Fla

me

Len

gth

(cm

)

Ring Position (cm)

Ring length (cm)

7.5

5

2.5

0

(no ring)

36

6.6 Effect of ring position on kiln wall temperature profile


As discussed before, temperature plays an important role in the recarbonation mechanism. Any

changes to flame behavior, therefore, if not associated with a temperature change, may not be

sufficient to promote ring growth.

The results obtained from the above tests suggest that having a ring inside the kiln affects the

flame length. Flame length is a measure of the combustion reaction zone in the flame [8]. Having

a longer flame, therefore, corresponds to having a longer reaction zone, which would lead to a

reduced heat flux from the flame. Thus, if the ring were to affect the combustion reaction itself,

this change in flame length should have an impact on the temperature profile of the kiln wall.

Experiments were carried out to examine if a change in flame length produced by a ring can

cause a temperature fluctuation on the kiln wall.

6.6.2 Procedure

The apparatus to study the kiln wall temperature profile was set up by inserting the burner into a

kiln made from refractory bricks. The kiln was designed by drilling a hole through an assembly

of refractory bricks, which are as shown in Figure 17. Thermocouples were inserted into this

channel so as to measure the kiln wall temperature. Rings were introduced inside the assembly

and temperatures were measured.

Since the heat transfer is unsteady, an approximation was employed to heat the kiln to a constant

temperature. Steady state was assumed when the temperature variation was less than 1 0C/ min.

Correspondingly, each run had to be 40 minutes long, so that a steady state temperature

measurement could be made. After each 40-minute long run, the apparatus was allowed to cool.

The first run was conducted without a ring in the kiln, at fuel flow rate of 6 LPM and primary air

flow rate corresponding to 15% of total air (9.43 LPM). Following this, the run was repeated

with the 1.5 cm thick ring placed right in front of the burner at the same flow rate. Next, the run

was repeated without the ring and later with the ring placed at 20 cm from the burner.

37

6.6.3 Observations

Figure 25 is a photograph of the experimental setup from its side at the end of a run. The kiln

became red hot uniformly on the inside.

Figure 25. Experimental kiln, after a 38-minute exposure. The ring can be seen present inside

the kiln

The temperature profile of the kiln after the two runs is as shown in figure 26 and figure 27. As

discussed in section 5.2, flame length when a ring is placed inside the kiln was longer than that

when there was no ring in the kiln. The brick kiln deteriorated under the thermal stresses it

underwent. This resulted in heat loss from the kiln, leading to a lower kiln wall temperature

profile in the second run (figure 27).

38

Figure. 26. Effect of rings on temperature profile of kiln wall (1.5cm thick ring @ 20 cm from

burner)

Figure 27. Effect of ring on temperature profile of kiln wall (1.5cm thick ring @ 0cm from

burner)

250

350

450

550

650

750

850

950

1050

0 10 20 30 40 50 60 70

Kil

n W

all

Tem

p. (0

C)

Distance from Burner (cm)

No Ring

Ring @ 20 cm

250

350

450

550

650

750

850

950

1050

0 10 20 30 40 50 60 70

Tem

per

ature

(0C

)

Distance from burner (cm)

Ring @ 0 No Ring

39

x

Discussion

7.1 Impact of primary air on flame length

As observed in section 5.1.3, flame length had a high correlation with primary air flow rate.

Examining the similarity parameters associated with these primary air flow rates helps to analyze

this observation better.

Figure 28 shows the flame obtained when all the air required is supplied as secondary air via the

kiln hood. The theory of recirculation in confined jets as explained by R Curtet [11] can explain

this observation. Using equation 1 & 2 and the dimensions of the burner from Appendix A, the

similarity parameter in this run is 0.09, which indicates very little mixing [11]. Thus, the majority

of the methane exiting from the nozzle is diffusing at a slow rate into the slow moving secondary

air around it. Once this mixture comes in contact with an ignition source, it undergoes

combustion. Since the rate of diffusion of methane is slow, it takes a long time to mix with air,

by which time it would have travelled a longer distance than when the similarity parameter is

higher. Since the longer flame indicates a larger combustion zone, its density would be lesser and

therefore, becomes buoyant.

Increasing the primary air flow rate, increases the similarity parameter, which signifies better air-

fuel mixing. The primary jet, which is now at a higher velocity, undergoes more rapid

combustion. However, the majority of the fuel still escapes from this zone without getting mixed

with air. This fuel, then gradually comes in contact with the slow moving secondary air, leading

to a slow rate of combustion, elongating the flame and making it buoyant. As may be observed

from Figure 28, the flame would be less buoyant close to the burner while away from it, it

becomes buoyant, similar to the flame in the previous case.

Increasing the primary air flow rate to 10% and 15% of the total air improves the similarity

parameter (to 0.32 and 0.45 respectively) and thereby the mixing, which is evident from the

shorter flames as shown in figure 30. Thus, it may be concluded that primary air plays an

important role in mixing of fuel with air, and thereby, the flame length and its behavior.

40

Figure 28. Variation in flame shape with change in primary air

However, it was also observed that primary air flow rate, (20% and 25% of total air) beyond a

certain value, blew out the flame (figure 24 & 25). To understand this occurrence, it is important

to understand the similarity parameter as a measure of the strength of recirculation eddies.

Increasing values indicate stronger recirculation. Figure 29 illustrates how the streamlines may

be formed around the flame front. As illustrated in figure 29, when the recirculation eddies get

stronger, they push more of the secondary air into the combustion zone. This increases the air-

fuel ratio in the combustion region and thereby starves the flame of methane.

0% primary

air

5% primary

air

10% primary

air

15% primary

air

41

Figure 29. Effect of recirculation on flame shape: (a)When recirculation is present, but not

strong enough to blow out a flame; (b) High recirculation deflects more secondary air into the

combustion zone

These results are similar to the results obtained by Alyaser [21] where a pilot kiln was used to

study the effects of primary air on flame shape and temperature profile. It was observed that

decreasing the primary air flow rate increases the flame length and flattens the flame temperature

profile. However, the primary air flow rates used were different than those of this study.

Extending these results to the lime kiln, it means that very strict control of primary and

secondary air flow rates must be maintained in order to maintain a constant temperature profile.

Premix Burner

Secondary Air Recirculation eddies

Quartz Tube

Entrainment of secondary air

(a)

Combustion

products

Quartz Tube

Flame front

Secondary Air Strong recirculation eddies push the flame front

inwards which can blow out the flame

Premix Burner

Entrainment of secondary air

(b)

Combustion zone

42

Any change in primary air flow rate leads to a change in the flame shape and can affect the

temperature profile inside the kiln.

7.2 Impact of ring position on flame length

As seen in sections 5.2, 5.3, and 5.4, ring position can significantly affect flame length. Rings

may affect flames via different mechanisms, as explained below. Since the concept of

recirculation in enclosed jet flames has not been extended to consider obstructions in the path of

the secondary jet, an approach that includes, but is not limited to recirculation, is taken here.

Figure 30. Zones of a diffusion flame

Based on the behavior of a flame in the presence of a ring, three different zones may be

identified here: Zone I – a zone of mixing, where majority of air-fuel mixing happens and is in

front of the burner; Zone II – a zone of combustion where majority of combustion occurs, which

is cone-shaped with its axis at the center of the nozzle; Zone III – a zone of expanding

combustion products (figure 30).

Placing the ring in different zones affects recirculation in different ways.

7.2.1 Zone I

This region may be divided into two different zones, based on the kind of air-fuel mixing

happening – a zone where entrainment of secondary air is predominant, and a zone where

recirculation eddies are predominant.

Premix Burner

Secondary Air Recirculation eddies

Quartz Tube

I - Mixing Zone

III - Zone of expanding products

II - Combustion

Zone

Flame front

43

A ring in the zone where entrainment of secondary air is occurring would deflect the secondary

air into the combustion zone, and thereby improve the mixing. Thus, if a ring placed in this zone

were to be replaced by a thicker ring, it would deflect more air into the combustion zone,

improving the mixing resulting in a shorter flame, as evident from figure 22. The flame gets

shorter with the use of thicker ring, when they are placed close to the flame.

Away from the burner, the recirculation eddies become stronger and become the primary

mechanism of air-fuel mixing. A ring placed in this zone of recirculation, reduces the

recirculation by breaking down the recirculation eddies. As discussed before, higher primary air

flow rates are associated with higher similarity parameters and stronger recirculation. As

illustrated in the observations of section 5.4, at high primary air flow rates, it was observed that

the introduction of a ring at a specific region within the kiln stabilized the flame, and it no longer

blew out.

Thus, a ring in the mixing zone closer to the burner, where mixing happens by entrainment of

secondary air, improves mixing and increases the efficiency of combustion, while rings placed

further away in the mixing zone, where mixing happens by recirculation, reduces recirculation

and decreases the efficiency of combustion.

Applying these results to an operational lime kiln a-priori would mean that ring growth in the

mixing zone would increase the efficiency of air-fuel mixing. This would lead to an increase in

flame temperature and thereby prevent further ring formation. However, a ring formed further

downstream, in the recirculation zone, would lead to a reduction in flame temperature by

reducing the air-fuel mixing. This could lead to a reduction in temperature and thereby,

accelerate ring formation.

7.2.2 Zone II

Rings, placed in Zone II are observed to increase recirculation. A probable reason for this could

be due to the reflection of fast moving combustion products against the ring surface. The

combustion reaction increases the kinetic energy of the air fuel mixture. The faster, heavier

stream of combustion products, now move through the kiln at a higher velocity than the primary

or the secondary jets.

44

In the absence of a ring, the secondary air gets entrained into the primary jet due to the negative

pressure created by the velocity difference between the primary jet and the secondary jet.

However, in the presence of a ring at 40 cm, the fast moving combustion products collide against

the ring surface, and reflect. This reflected stream would now have to turn back and push the

recirculation eddies, thereby increasing the strength of recirculation. The increased strength of

recirculation increases the concentration of air in the primary jet and blows out the flame.

Figure 30. Thicker ring blows out flame when placed far away from the flame

The thicker the ring, the greater the reflected combustion products, and the more effective would

be the ring in blowing out the flame. This could be a possible reason for the observations in

section 5.2 and 5.3, where the thicker rings blew out the flame when they were placed towards

the end of the flame (30 cm and 40 cm).

A closer look at the video recording of this run can throw more light on this. Figure 31 shows a

few snapshots just before the flame blew out in a run where the ring was place 40 cm away from

the ring.

Kiln wall

Hot expanding

combustion products

Thick Ring

Thin Ring

45

Figure 31. Frames immediately before flame blow out when the ring was at 40 cm from burner.

Imaginary streamlines show how recirculation eddies might be causing flame blow out

As can be seen here, there appears to be a draft blowing around the flame, at some distance,

about 3-10 cm away from the burner, which initially causes flame lift-off and later blows out the

flame. This could be an effect of recirculation.

Applying these results to an operational lime kiln suggests that rings formed ahead of the

combustion zone would increase recirculation. The effectiveness of the ring in blowing out the

burner flame or making it unstable would depend on its thickness. However, the ability of this

increase in recirculation to blow out the flame must be further studied, since an operational kiln

would have a greater diameter to length ratio than the experimental kiln. This could dilute the

effect observed in the experiment.

7.3 Flame Speed and Flame Instability

Flame speed, also called burning velocity or flame velocity is defined as the velocity at which

unburned particles move through the combustion zone in direction normal to the wave surface.

The major theories for predicting flame velocity for a premixed flame consider the diffusion of

air and fuel particles through the flame medium, and the propagation of heat backwards through

the gas layers. Flame velocities for hydrocarbon-air flames are about 40cm/s while for hydrogen-

air flames are about 2m/s, due to the higher diffusivity of hydrogen. An increase in pressure

reduces the flame velocity while a decrease in pressure increases it [9].

t = 1/60 s

t = 6/60s t = 5/60 s

t = 4/60 s t = 3/60 s

t = 2/60s

46

However, for a diffusion flame, the concept of flame velocity is a bit different. The combustion

process is assumed to happen in smaller flamelets, which is one-dimensional. The flamelet has

the air-fuel mixture in a stoichiometric ratio and is ready to undergo the combustion reaction.

The flame propagates in a direction perpendicular to the flame front in this flamelet [22].

In this study, the jet speed of the primary air and methane mixture ejects out of the burner at

speeds ranging from 2 m/s to 5.9 m/s, which is five to fourteen times the typical laminar flame

velocity of a hydrocarbon-air flame for a premixed flame. Thus, if the flame speed were to affect

the flame stability, it would have affected all the observations equally.

7.4 Impact of ring length on flame behavior

The behavior of a flame in the presence of a long ring is consistent with the theory discussed so

far. With the ring position and ring thickness being the most important variables, a long ring

merely forces the flame to elongate, thereby increasing the flame length. This could be because

of the reduction in volume inside the kiln, which increases the flow rates of the fuel and air. A

faster moving fuel now undergoes combustion, thereby elongating the flame. However, placing

this ring before the zone of entrainment can blow out the flame, possibly by reducing the volume

available for air-fuel mixing. In summary, long rings can affect a flame only when placed in

regions where air-fuel mixing is strongest. Placed at other positions, they affect the flame based

only on their thickness and position.

7.5 Flame length and temperature profile of kiln

Any phenomenon that affects flame length, unless accompanied by a corresponding change in

the temperature profile of the kiln, will be ineffective in causing ring growth or ring formation.

This is because temperature is one of the basic driving forces in ring formation [7]. The

observations from section 5.3 make it clear that changes in flame length are associated with a

change in temperature profile within the kiln. Assuming equal completion of combustion, the

heat flux from a longer flame will be less than that from a shorter flame. As discussed in

previous sections, introducing a ring at 20 cm from the burner increases the flame length. The

temperature profile of the flame with a ring at 20 cm from the burner results in a lower

temperature profile than without a ring. The point of highest temperature also shifts away from

the burner. This suggests an extension in the length of the flame and the temperature profile

47

associated it. Thus, it may be concluded that a change in length a of flame is also associated with

a change in temperature profile.

In an operational lime kiln, these results imply that a ring present inside the kiln at a specific

location could lower the temperature profile of the kiln wall. Recall that a reduction in

temperature can increase the rate of growth of rings by the recarbonation mechanism.

48

Conclusions

A systematic study was conducted to examine how ring formation in a lime kiln affects the

burner flame. The study was aimed at gaining a fundamental understanding of the interaction

between ring formation and the operation of a turbulent jet diffusion burner in a lime kilns. Main

conclusions drawn from this study are:

1. Primary air is important for flame stability. Increasing primary air flow rate up to a

certain value, improves fuel mixing and produces a compact and stable flame. Increasing

the primary air flow rate beyond a certain value blows out the flame, which could be due

to very strong recirculation currents, while decreasing it would reduce the amount of

recirculation, and makes the flame buoyant and long. The primary air flow rate which can

blow out a flame may vary from kiln to kiln, depending on its size.

2. Ring position can affect flame shape and stability. When rings are very close to a burner,

they help improve the air-fuel mixing and thereby make the flame shorter and more

compact. Rings that are farther from the burner reduce mixing, possibly by breaking the

recirculation eddies present in this region. When rings are outside the combustion zone,

they have a tendency to blow out the flame, depending on their thickness.

3. Ring thickness can affect flame stability and shape greatly. Thicker rings have a greater

tendency to blow out flames than thin rings.

4. The length of rings affects the flame shape only when placed within the combustion zone

of the flame. Longer rings elongate flames by reducing the volume inside the kiln. When

placed outside the region of the flame, the flame behavior is affected only by the

thickness and position of the ring.

5. Variation in flame length alters the temperature profile of the kiln wall. This could be due

to the change in heat flux associated with change in flame length. As a result, a longer

flame would produce a temperature profile that is lower than a shorter flame.

Thus, rings can affect flame shapes, and in some of these cases, even lead to a decrease in

temperature, by creating unstable flames, which can cause more ring formation in lime kilns.

49

Appendix A: Detailed drawings

A.1 Kiln hood

Figure 32. Cross section through kiln hood

Figure 33. Side view of kiln hood

A.2 Nozzle

Dia. Of each hole: 1.5 mm

Number of holes: 28

Figure 34. National N-2 torch tip – serves as the nozzle for primary jet

11.4

cm

38.5 cm

11.4

cm

8.9

cm

Port for Secondary Air

50

A.3 Quartz tube kiln

Figure 35. Front view of the quartz tube

A.4 Honeycomb discs

Figure 36. Honeycomb disc (large) – front view and side view

Figure 37. Honeycomb disc (small) – front view and side view

9 c

m

150 cm

11.4 cm

1 cm

2.5 cm

8.9 cm

1 cm

2.5 cm

51

Appendix B

B.1 Equipment assembly protocol for flame visualization study

B.1.1 Assembly of mini-diffusion burner

First, two large honeycomb discs, and one small honeycomb disc is placed inside the wide

section and narrow section of the kin hood respectively. Next, the tip extension of the premix

burner is attached to the burner and is inserted into the kiln hood, from the broader end, ensuring

it passes through the central holes of the honeycomb discs as well. To seal the gap between the

burner extension and the hole in the kiln hood base, a resin based gasket sealant is applied.

Proper support to the torch extension has to be ensured so that the premix burner may be parallel

to the axis of the kiln hood. Once the sealant is hardened, the tip maybe attached to the torch

extension.

Primary air connection from the pipeline or from a compressed air cylinder is connected to the

inlet of the flow controller, via a ball valve and a pressure indicator and the outlet of the flow

controller is connected to the air supply port of the premix burner. The same procedure is

repeated for secondary air, except that the outlet is connected via a t-connector to the two

secondary air ports of the kiln hood. Next, the methane line is connected to the flow meter, and

from its outlet to the fuel port of the premix burner.

B.1.2 Assembly for flame visualization

The mini-diffusion burner is now ready to be used in the flame visualization study. To set this

up, we first place the quartz kiln on the metal supports, after ensuring its horizontality using

spirit level. Next, the mini-diffusion burner is inserted into the quartz kiln. Metal supports are

now placed under the burner, ensuring that they take the complete load of the burner. Any stress

on the quartz tube can break the glass. Once the supports are in place and the setup is stable, a

duct tape is wound around the joint between the burner and the kiln to prevent any sort of

leakages. If a ring has to be used in the study, the ring has to placed inside the kiln first, and then

the burner inserted and sealed. Next, a 900 bend of GI pipe is inserted into the other end of the

quartz kiln, so as to direct the exhaust gasses into the chimney, present right above the set-up. A

carbon monoxide detector has to be placed close to the setup for safety reasons.

52

Before beginning the experiment, any leakages in the setup has to be determined. To do this, the

connection from primary and secondary air cylinders are connected to nitrogen cylinders. An

oxygen analyzer is connected to the exhaust stream, and the system is flushed with nitrogen. Any

leakages in the system would be detected by oxygen in the exhaust gasses.

B.1.3 Operating the equipment

Once the system has been set-up and ensured that it is leak-proof, the equipment is ready for

operation. Firstly, the primary and secondary air flow rates have to be entered into the flow

controller using a laptop. To control the flow controller using a laptop, a USB to RS-232 driver

has to be installed. It converts the USB signals from the laptop to the RS 232 connection of the

flow controller. Once the air flow rates are fixed, the methane flow to the flow controller is

switched on using the ball valve on the methane cylinder. The mini-burner should be placed such

that its nozzle is accessible. Using a lighter, the burner is lighted by increasing the methane flow

rate in flow controller. Once a small stable flame has been established, the mini-diffusion burner

is fitted back into the quartz kiln. A layer of sealant tape is applied and the fuel flow rate is

gradually increased, till it reaches full flow.

B.1.4 Video recording equipment

The video recorder has to be placed at a safe distance from the whole setup, so as to minimize

exposure to the high heat. Also, this helps it to have a wider field of vision, so as to capture the

entire flame length. The recorder has to be mounted on a stand, and the zoom has to be set so that

about half of the kiln is visible.

Once the experiment is ready to be accept methane, the recorder is turned on. This is because, in

some runs, the flame may get extinguished even before reaching the full value of fuel flow.

B.2 Equipment assembly protocol for kiln wall temperature profile study

The equipment for analyzing the kiln wall temperature profile is similar to the equipment used

for flame visualization, except that the kiln used in this case is made of refractory bricks. To set

up this experiment, first the kiln is wound in glass wool to reduce the heat loss to the

surroundings. Next, thermocouples are inserted into the kiln through the small holes in the top of

the kiln. Care has to be taken to ensure that the thermocouples are flushed with the wall of the

53

kiln and do not extend into the volume of the kiln. This is because a thermocouple flushed with

the surface of the kiln would measure the kiln wall temperature, while a thermocouple in the

volume of the kiln would measure the flame temperature, which would be higher. The

thermocouples are then connected to RTD signal converters, which are connected to laptop to

record the values.

Once the kiln has been set up with the temperature measuring equipment in place, the burner is

introduced into the system. The burner, once its assembled, can easily be slid into the opening of

the kiln. Therefore, the burner is first ignited at a low fuel flow rate, and then slid into the kiln.

However, unlike in flame visualization study, the fuel flow rate has to be increased very

gradually. This is because the refractory bricks are less resistant to thermal shock and may give

way if the heat is cranked up at a quick rate. To ensure least thermal shock as possible, the fuel

flow rate was increased by 10% when the rate of temperature rise in the kiln fell to less than 20C

per second.

For a subsequent run, the kiln was allowed to cool down to room temperature. A ring was placed

inside the kiln, if required and the experiment repeated. For each run with a ring, a blank run

without a ring was performed before it. This was because the refractory brick breaks down after

each run, and cracks develop on it which change the heat loss from the kiln. A test run without a

ring can help us negate this.

B.3 Image processing protocol

The flame visualization study gives us a lot of information about the variations in flame with

changes in conditions of operation inside the kiln. However, to capture this vast information

manually is almost impossible, due to the size of data. Thus, the use of an image processing

software.

The first step in image processing is data acquisition. Data is acquired in the form of video from

the video recorder. Each run is recorded for a time of one minute at a frame rate of 60 frames per

second. This results in 3600 frames. The flame is a dynamic object undergoing changes instantly,

and the video recorder captures 3600 of these instances in a minute. A statistical analysis of the

situation results in the conclusion that to have a confidence interval of 95%, with a confidence of

±5 mm, we require processing of 348 images from the 3600. Therefore, the first step in data

54

acquisition would be to extract 348 frames within a time interval of one minute, and can

represent the data contained in the video with a confidence of 95% and an accuracy of ± 5 mm

(since we are measuring the flame length in mm). A video player like KMPlayer (a free

software), has the ability to capture the frames, as and when they are played on screen. The

number of frames to be captured is set to 348 within a time limit of 1 minute and the images are

extracted and stored.

Next, the images have to be processed to obtain the data within them. ImageJ software (a free

and open source software, based on Java) is used for this purpose. The frames are first imported

into the environment of ImageJ. The software bases all its calculation on pixel units by default.

To convert this into an mm scale, a known distance on the image is first mapped to pixel units.

Parallax error reduction was achieved by using two different objects at two different positions.

Next, the imported images are processed in four steps to estimate the length of the flame (figure

38). Since all the images are from the same video, the brightness and color spectrum of every

frame would be identical. The frame is first cropped to show include only the flame in view.

Next, this image is converted to an 8-bit grayscale image. The image is then subject to

thresholding, where every pixel having more than a specified value of grey is highlighted. This

helps to demarcate the areas of the image that are to be processed by the software. The final step

is a particle identifying step, where the software analyses each particle highlighted in the

thresholding step. The software can calculate the Feret’s diameter of each of these particles.

Feret’s diameter estimates the distance between the ends of the image on a specified axis. Since

the flame would be the longest single particle in this group of particles, a simple two stage sort of

the result using MS Excel can give the length of the flame in each frame in millimeters.

Figure 38. Steps in image data processing using ImageJ

Image cropped and

converted to 8-bit

Image thresholded

Particles analyzed

55

Appendix C: Calculations

C.1 Calculation of total air requirement

The total air supplied to the burner was the sum of the stoichiometric air requirement and excess

air. It was calculated based on the methane-air combustion reaction.

𝐶𝐻4 + 2𝑂2 → 𝐶𝑂2 + 2𝐻2𝑂

This implies, 22.414L of methane would require 44.818L of oxygen to undergo complete

combustion.

Therefore, 1L of methane would require 2L of oxygen to undergo complete combustion.

Assuming air is 20.95% oxygen by volume, 1L of methane would require 9.55L of air to undergo

complete combustion.

Adding to this, an excess air of 10%, the total air required for 1L of methane would be 10.505L.

The following table gives a list air requirement for various fuel flow rates used in the study.

Table 1. List of methane flow rates and corresponding total air flow rates (with 10% excess air)

Fuel flow rate (LPM) Total air flow rate (LPM)

4 42.02

4.5 47.27

5 52.53

5.5 57.78

6 63.03

56

References

[1] Natural Resources Canada, [Online]. Available:

http://www.nrcan.gc.ca/forests/industry/overview/13311. [Accessed 19 12 2016].

[2] Candian Industry Program for Energy Conservation, "Benchmarking Energy Use in

Candian Pulp and Paper Mills," 2008.

[3] H. Tran and E. K. Vakilainnen, "The Kraft Chemical Recovery Process," in TAPPI Short

Course, 2012.

[4] G. Hanson, "Lime Kiln Principles and Operation: Part I - Design and Operation," in TAPPI

Short Course, 2012.

[5] D. J. Kramm, W. L. Schulz and G. Hough, "Lime Reburning: The Rotary Lime Kiln," in

Chemical Recovery in the Alkaline Pulping Processes, TAPPI, 1985, pp. 269--297.

[6] H. Tran and D. Barham, "An overview of ring formation in lime kilns," TAPPI Journal, pp.

131-136, 1991.

[7] H. Tran, X. Mao and D. Barham, "Mechanism of Ring Formation in Lime Kilns," Journal

of Pulp and Paper Science, vol. 19, no. 4, pp. 167-174, 1993.

[8] P. Mullinger and B. Jenkins, "Chapter 5 - Flames and burners for industrial processes," in

Industrial and Process Furnaces, Oxford, Butterworth - Heinemann, 2008, pp. 141-207.

[9] I. Glassman, R. A. Yetter and N. G. Glumac, Combustion, Elsevier, 2015.

[10] C. E. Baukal and B. Gebhart, "Heat-transfer from Oxygen enhanced/ natural gas flames

impinging normal to a plane surface," Experimental Thermal and Fluid Science, vol. 16,

no. 3, pp. 247-259, 1998.

57

[11] R. Curtet, "Confined Jets and Recirculation Phenomena with Cold Air," Combustion and

Flame, vol. 2, no. 4, pp. 384-411, 1958.

[12] L. Saeys, M. Zumer and J. Dealy, "Criteria for recirculation in confined jet flames,"

Combustion and Flame, vol. 17, no. 3, pp. 367-377, 1971.

[13] P. Foster and et al., "Approximate similarity of confined turbulent coaxial jets," Journal of

Fluids Engineering, vol. 123, no. 3, p. 707, 2001.

[14] A. Benaissa and et al., "Recirculation of confined variable density turbulent jets,"

TRansactions of the Canadian Society for Mechanical Engineering, vol. 27, no. 4, p. 279,

2004.

[15] A. Aloqaily, A study of aerodynamics in rotary kilns with two burners, Toronto: University

of Toronto, 2008, p. 223.

[16] B. Jenkins and F. Moles, "Modelling of heat transfer from a large enclosed flame in a

rotary kiln," Transactions of the Institution of Chemical Engineers, vol. 59, no. 1, pp. 17-

25, 1981.

[17] H. Abdelmonem, Fluid flow and combustion in rotary kilns, Vancouver: University of

British Colombia, 1998.

[18] T. Adams, "TAPPI Kraft Recovery Course," 2006.

[19] M. Ohbuko, Y. Nakagawa, T. Yamagata and N. Fujisawa, "Quantitative Visualization of

Temperature Field in Non-Luminous Flame by Flame Reaction Technique," Journal of

Visualization, vol. 2, no. 15, pp. 101-108, 2012.

[20] D. Wisman, B. Ganguly and S. D. Marcum, "High-Speed Visualization of a Flame

Reaction Zone Using a Positive Point-to-Plane Corona Discharge," IEEE Transactions on

Plasma Science, Vols. 4,1, no. 36, pp. 928-929, 2008.

58

[21] A. H. Abdolmonem, "Fluid flow and combustion in rotary kilns," University of British

Columbia, 1998.

[22] C. M. Muller, H. Breitbach and N. Peters, "Partially premixed turbulent propagation in jet

flames," Combustion/ The Combustion Institute, pp. 1099-1106, 1994.

[23] National Institute of Standards and Technology, 2016. [Online]. Available:

webbook.nist.gov/chemistry/name-ser.html. [Accessed 29 December 2016].

[24] J. D. Kramm, W. L. Schultz and G. W. Hough, "Lime Reburning," in Chemical Recovery

in the Alkaline Pulping Processes, 1992.

Documents

An Experimental Study on the Effect of Rings on Flame ... · Rings can alter the inner surface of the kiln, and can affect the flame pattern or cause impingement, causing further