Effect of Particle Impact Velocity On Carryover Deposition€¦ · Effect of Particle Impact...

Effect of Particle Impact Velocity On Carryover Deposition

Mojghan Naseri

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

Graduate Department of Chernical Engineering and Applied Chemistry University of Toronto

O Copyright by Mojghan Naseri 2000

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Effect of Particle Impact Velocity On Carx-yover Deposition

Master of Applied Science 2000

Mojghan Naseri

Graduate Department of Chernical Engineering and Applied Chernistry

University of Toronto

Abstract

Carryover deposition by inertial impact on heat transfer surfaces causes severe

operational problems in kraft recovery boilers. This work investigates within an

entnined flow reactor the effect of particle impact velocity and particle size on

deposition rate and characterizes the dynarnic sticking behavior of synthetic canyover

particles upon impact on a surface. The results show that the deposition of 210 Fm

particles is independent of velocity over the range 1.8 to 12 m/s. however. for larger

particles of 390 Fm an increase in velocity decreases the deposition rate. The efFect

of particle size on adhesion eficiency is negligible at very high velocity for partially

molten particles over the 90-425 pn size and 1.8-12 rn/s velocity range studied. A

portion of partially molten particles rebound off the probe and in some cases the solid

pan of particles rebound while the liquid part freezes on the surface. All molten

particles spread and fieeze on the surface.

Acknowledaements

My very special thanks to Professor David C.S. Kuhn for his excellent supervision.

His advice, guidance, and encouragement during this work are very much appreciated.

Many thanks to Professor Honghi Tran for his suggestions and valuable advice. it is

great l y appreciated.

1 am very thankful to Reyhaneh Shenassa for al1 her great and wise comments. but

mostly 1 am grateful for her fiendship.

My thanks to Sue Mao for helping me d u ~ g my expenments.

1 am gratefui to the memben of the research consortium on Improving Recovery

Boiler Performance. Emissions and Safety. and NSERC for their tinanciai support of

this project.

Many thanks to al1 my fnends for their fnendship and encouragement.

Finally. deepest thanks and appreciation to my family for their support.

iii

Table of Contents

ABSTRACT

ACKNOWLEDGMENTS

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

1. INTRODUCTION

2. LITERATURE REVIEW

2.1 Fireside Deposits 2.2 Deposit Stickiness 2.3 Plugging in Kraft Recovery Boilers 7.4 Particle Transport Mec hanism 2.5 Adhesion ERiciency 2.6 Interna1 Circulation 2.7 Behaviour of Liquid Droplets Upon Impact

2.7. I Single Drop Impacts 2.7.1 Spread 2.7.3 Reborrnd 2.7.4 EIasric Reborrnd 2.7.5 Splash

2.8 Summary

3, METHODOLOGY

3.1 Experimental Setup 3.2 Variation of Flue Gas Velocity 3.3 Experimental Procedure

4. PARTICLE IMPACT VELOCITY

4.1 Experimental Reproducibility 4.2 Particle Distribution 4.3 Effect of Particle Size and Flue Gas Velocity

4.3.1 History of Purticles Imide the Reactor -1.3.2 Flue Gas Velocity Profile 4.3.3 Particle Size and Flue Gus Velocity

4.4 Effect of Synthetic Carryover Composition on Velocity 4.5 Visualization of Particle Impact

4.6 Effect of Particle Velocity on Adhesion Efficiency 4.7 Effect of Particle Size on Adhesion Efficiency 4.8 Effect of Probe Temperature on Adhesion Efficiency 4.9 Solidification and Shape of Deposits

4.9. I Solidijcation 4 -92 Cornparison of Erperimentul olesdrs wiîh Spread

and Solidification Models 9 3 Shape of Deposits

4.1 0 S ummary

5. IMPLICATIONS

6. CONCLUSIONS

7. RECOMMENDATIONS

REFERENCES

IVOMENCLA TUR E

List of Figures Figure 1.1

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.1

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 3.10

Figure 2.1 1

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 4.1

Figure 4.2

Figure 4.3

Schematic diagram of a kraft recovery boiler

Smelt composition 8

Carryover deposits composition 8

Fume composition 9

Effect of chlonde on sticky temperature zone I l

Plugging of superheater platens by carryover impaction 12

Particle trajectories 15

Canyover collision probability 16

Schematic diagram of internai circulation in low-viscosity droplet 18

(a) Interna1 circulation in water droplet. D= 1.77cm. Fully circulating 19 (b) Intemal circulation in water droplet. D4.3 1 cm. stagnant cap at top of droplet due to the presence of contaminant 20

Behavior of a drop following impact; spread rebound. splash 2 1

Schematic diagram of the entlained flow reactor 39

Schematic diagram of optical setup to measure particle velocity 3 1

Superimposed images of 1 O mole% CV(Na+K), particle size 150-420 pn

Schematic diagram of the plate at the exit of the EFR 33

Plate at the exit of the EFR

Deposition reproducibility. gas veiocity=l2 m/s. EFR=800°C. particle size 120 p, 10 mole% CV(Na+K) 37

Particle distribution across the EFR particle median size 120 p. EFR=800°C, gas velocity4.8 m/s, 8 mole% CV(Na+K) 39

Particle distribution across the EFR, particle median size 390 pm, EFR=800°C, gas velocity=1.8 m/s. 8 mole% CV(Na+K) 39

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Figure 4.8

Figure 4.9

Figure 4.10

Figure 4.1 1

Figure 4.12

Figure 4.13

Figure 4.14

Figure 4.15

Figure 4.16

Figure 4.1 7

Figure 4.18

Particle deposition on the probe, 10 mole% Ci/(Na+K), EFR=800°C, particle median size 390 pm

Velocity profile across the EFR, 8 mole% CV(Na+K), EFR=800°C. particle median size 120 pm

Velocity profile in a pipe

Particle velocity. EFR=800°C. gas velocity=1.8m/s, 20 mole% CU(Na+K)

Mrasured particle vèlocity, EFR=800°C. 20 mole% CL1(Na+K)

Measured particle velocity. EFR=800°C. 20 mole% CV(Na+K). particle median size 120 pm

Measured particle velocity, EFR=800°C. 20 mole% CV(Na+K). particle median size 390 pm

Effect of chloride content on the particle velocity. EFR=800°C. particle median size 360 p. gas velocity= 1.8mls

Effect of potassium on the particle velocity. EFR=800°C, particle median size 463 p z , gas velocity= 1.8ds. 5 mole% CI/(Na+K)

Images of 10 mole% CV(Na+K), particle median size 390 Fm. gas velocity=1.8ds, EFR=800°C. probe temp=440°C

Images of 10 mole% CV(Na+K), particle median size 390 Pm. gas velocity=47m/s, EFR=800°C, probe temp=440°C

Images of 10 mole% CV(Na+K). particle median size 390 p. gas velocity=12m/s, EFR=800°C, probe ternp=440°C

Sequence of 10 mole% CV(Na+K) particle rebound, particle median size 390 pn, gas velocity4.7m/s, (a) particle on the surface. (b) particle rebounding off the surface 53

Sequence of solid core rebounding, 1 0 mole% CV(Na+K), particle median size 390 pm, gas velocity4 2 d s : (a) particle impact (b) solid core rebounding and liquid part a d h e ~ g to the surface 53

Images of molten droplets. particle median size 390 pm, gas ~ e l o c i ~ . 7 m l s , EFR=800°C, probe temp=440°C 55

vii

Figure 4.19

Figure 4.20

Figure 4.2 1

Figure 4.22

Figure 4.23

Figure 4.24

Figure 4.25

Figure 4.26

Figure 4.27

Figure 4.28

Figure 4.29

Figure 4.30

Figure 4.3 1

Figure 4.32

Effect of chloride content on adhesion efficiency, particle median size 390 p. gas velocity=1.8m/s, EFR=800°C, probe ternp=440°C

Adhesion efficiency as a function of chloride content, particle median size 120 p, EFR=800°C, probe temp=440°C

Adhesion efficiency as a function of particle velocity, particle median size 120 Pm, EFR=800°C. probe temp=440°C. 10 mole% CV(Na+K)

Ailliesion efficiency as a fimction of chloride content, particle median size 390 pim. EFR=800°C, probe temp=440°C

Adhesion efficiency as a function of particle velocity, particle median size 390 p, EFR=800°C. probe temp=J40°C. 10 mole% CV(Na+K)

Images of 10 mole% CU(Na+K). particle median size 390 p. gas velocity=1.8m/s. EFR=800°C. probe temp=440°C

Spread of rnolten particles, 20 mole% CU(Na+K). particle median size 390 p. gas velocity47m/s, EFR=800°C, probe temp=440°C

Frozen particles on the probe surface. 52 mole% CV(Na+K). particle median size 390 W. gas v e l o c i ~ . 7 m l s , EFR=800°C. probe temp=440°C

Effect of size on adhesion efficiency. gas velocity=- 1.8ds. EFR=800°C. probe temp=440°C

Effect of size on adhesion efficiency. gas velocity==.7m/s. EFR=800°C, probe temp=*IO°C

Effect of size on adhesion eficiency, gas velocity42ds. EFR=800°C' probe temp=440°C

EEect of size on adhesion efficiency, 10 mole% CU(Na+K). EFR=800°C, probe tem+O°C

Effect of probe temperature on adhesion efficiency, particle median size 120 p, gas ve l0c i~ .7m/s , EFR=800°C

Effect of probe temperature on adhesion efficiency, partide median size 3 90 pm, gas velocity--4.7m/s, EFR=800°C

viii

Figure 4.33 Sequence of solidification, 10 mole% CV(Na+K), particle median size 390 p, gas velocity=12m/s, EFR=800°C, probe temp=440°C

Figure 4.34 Sequence of solidification, 20 mole% CV(Na+K), particle median size 390 pm, gas velocity=12rn/s, EFR=800°C, probe temp=440°C

Figure 4.35 [mages of deposits, particle median size 390 p, gas velocity=4.7m/s, EFR=800°C, probe temp=440°C 76

F r 4 . 6 Dep~sits an thc edge of the probe, 52 mole?4 CV(Na-X), particle median size 390 pn, gas velocity=1.8m/s, EFR=800°C 76

Figure 4.37 Effect of impact velocity on adhesion eficiency 78

List o f Tables Table 3-1 Particle size cange

Table 4- 1 Solidification time, particle median size 390 pm, EFR=800°C, probe temp=440°C

Table 4-2 Excess rebound energy, particle median size 390 Fm, EFR=800aC. probe temp=440°C

Table 4-3 Solidification criterion. particle median size 390 W. EFR=800°C. probe temp=440°C

Table 4 4 Maximum spread. particle median size 390 Pm. EFR=800°C. probe temp=440°C

1. Introduction

Kraft recovery boilen burn black liquor. which is by-product of chernical pulping to

generate power and stem. and to recycle the pulping chemicals. Figure 1.1 shows a

schernatic diagram of a recovery boiler. The accumulation of fireside deposits on heat

transfer surfaces in the convection section of kraft recovery boilers is a major problem:

deposits decrease heat transfer efficiency in the boiler. corrode tube surfaces. impede flue

gas flow. and in severe cases. cause complete pluggage of flue gas passages. which leads

to an unscheduled shutdown of the boiler [l, 21. It is essential to have a fundamental

undestanding of the ash deposition mechanism since it plays an important role in

determining the design and operation of a recovery boiler.

1. Introduction

Superheaters

Figure 1.1 Schemaiic diagram of a kra8 recovery boiler

Deposits are formed from three different sources: carryover. intermediate size particles.

and Fume. Carryover is smelt andlor unburned black liquor droplets entrained in the flue

gas tlow that is canied to the upper h a c e fiom the lower fumace. In the upper furnace

a carryover particle can be an in-flight buming char particle. a burned-out

rnoltedpartially molten alkali sait particle. or a solidified particle, depending on its

residence time, composition, temperature, and the flue gas chemistry. Carryover particles

range in size between 100 p n to 3 mm diameter, and form hard, fused deposits upon

impact on heat transfer surfaces. Carryover deposition is dominant in the screen tube and

superheater regions [ 5 ] . Intermediate-sized particles have diameters between 1 and 100

pm and are formed by condensation of vaporized alkali salts, hgrnentation of buming in-

flight black liquor particles and possibly the ejecta of molten carryover particles

1. Introduction

following impact on heat transfer surfaces [IO, 1 11. Fume is generated as a result of

condensation of aikali vapoe, which forms micron or submicron sized particles. Fume

deposition is dominant in the generating bank and the economizer.

Carryover accumulation on heat transfer surfaces continues to be of great importance to

boiler perforn~ance and is die focus of this study. Deposit groavth is 3 cornplex function

of boiler operation as it affects carryover and heat transfer surface properties and

carryover transport charactenstics. The state of carryover through the convection section

is a function of its history and composition. As carryover particles travel from the lower

furnace and through the upper fumace, they begin to cool and change from a molten state.

Carryover becomes less sticky and therefore less likely to adhere to heat transfer surfaces

as it solidifies. It is not feasible to conduct experiments and study the sticking behavior

of carryover particles in a recovery boiler due to the high temperature and hostile

environment. Also, many variables affect the stickiness of the particles and it is

impossible to prevent their interaction and study the effect of each variable in an actual

boiler. Therefore. an entrained 80w reactor (EFR) which simulates similar conditions as

in the superheater region of a recovery boiler is used to study the stickiness behavior of

the synthetic carryover particles.

Since the primary transport mechanism of carryover to heat transfer sdaces is inertial

impact. particle impact velocity. energy, and size are ail expected to influence the

collision probahility of the carryover particles. Upon impact, carryover particles may hit,

stick and solidify on tube d a c e s , or they may strike and bounce off the suface and be

re-entrained in the flue gas depending on particle size, temperature, liquid content, and

velocity, and flue gas temperature and velocity. The objectives of this thesis are to

1. Introduction 4

examine the effect of particle impact velocity on deposition rate of synthetic carryover

particles and to charactenze the dynamic sticking behavior of synthetic carryover

particles upon impact on a surface.

The relevant literature related to the objectives of this thesis is reviewed in Chapter 2.

Chapter 5 describes the experimèntal mehodology, which sonsists of novel mcthods for

particle velocity measurement and particle impact visualization. Experimental results are

presented and discussed in Chapter 4. This chapter is divided into two sections: first. the

efTect of flue gas velocity and particle impact velocity on deposition rate is discussed:

second. the particle impact process on a surface is described and the existing rebound and

solidification models are compared with the expenmental data. In Chapter 5 practical

implications to this study are presented. and the conclusions and recomrnendations are

presented in Chapter 6 and 7, respectively.

2. Literature Review

2.1 Fireside Deposits

The kraft pulping chemical recovery cycle is a closed process which has three main

hnctions: to generaie power and steam from the combustion of organic material present

in the hel. to recycle pulping chemicais, and to destroy the toxic compounds from

pulping 13. 41. The Fuel used in a kraft recovery boiler is black liquor. which is the by-

product of chemical pulping and is the sixth most important fuel in the world [3]. Before

delivery into the boiler. the black liquor is concentrated to between 65% and 85% dry

solids content using an evaporation plant [4]. The black liquor is one of the highest ash

containhg hels due to the presence of 40 to 50% inorganic materials [5] as compared

with 547% for pulverized coal [6] . The fly-ash deposition on kat transfer surfaces is

inevitable since the ash has a low melting temperature. Massive deposition reduces the

heat transfer eEciency. whîch results in a lower superheated s t em production rate. The

growth of deposits restricts the flue gas flow in the upper fumace and eventually leads to

an unscheduled shutdown of the boiler due to plugging [SI. The


shutdowns are very troublesome for kraft pulp mills and the loss of production is very

costly .

There are two types of fly ash particles in recovery boilers: canyover and fume. The

diffewnce between these two is the formation mechanism [5] .

Carryover: As black liquor is sprayed into the boiler, it foms droplets ranging From 0.5 to

5 mm in diameter which undergo four stages of physicai changes: drying.

devolatitlization. char burning. and smelt coalescence and reactions [7]. The droplet

swells during the devolatilization or pyrolysis process and the organic material

decomposes into tar and gases. During the swelling process, particle size suddenly

increases while its density decreases. which causes the entrainment of droplets in the flue

gas [8]. Carryover has a composition similar to partially or fully oxidized smelt as shown

in Figure 2.1. It rnainly contains sodium carbonate. sodium sulphide, and smdl fraction

of sodium chloride. potassium salts, and unburned carbon [5. 91. Carryover. however.

changes composition in-flight due to reaction with fûrnace gases and fume, which has

deposited on its nuface. Sodium sulphate is formed due to the oxidization of sodium

sulphide: also, carbonates and some other alkali compounds are converted to sulphates as

a result of their reaction with the sulphur compounds present in the flue gas. A Fraction of

chloride and potassium contents may also vaporize at the elevated temperature. Figure

2.2 iI1usu;ites the typical composition of carryover deposits. Carryover deposits on heat

transfer surfaces by inertial impact and the deposits are d l y pi&, fused and very hard

[SI-


Figure 2.1 Smek Composition [j]

Figure 2.2 Cawyover deposiîs composiîion [j]

Fume: Fume consists of micron and submicron particles with diameter of 0.1 pin to 1 p.

There are four rnechanisms of fume formation: direct vaporization of alkali chlorides and

hydroxides. elemental sodium vaporization under reducing conditions, and reaction-

enhanced sodium vaporization under oxidizing conditions [l O]. Fume is mauily made up

of sodium suiphate with a higher amount of chioride and potassium and a Iower amount

of sodium carbonate compared to carryover, as illustrated in Figure 2.3 [Il] . Char bed


temperature has a significant effect on fume generation, i.e. a higher temperature result in

higher fÙme formation. and the fume contains more carbonate [12]. Fume. alkaii vapon

in the flue gas, condense on heat transfer surfaces to form deposits.

Figure 2.3 Fume composition [5]

2.2 Deposit Stickiness

Deposits have two distinct melting temperatures. The temperature at which the liquid

phase first appears is called the fint melting temperature. while the temperature at which

the deposit is completely molten is called the complete melting temperature. There are

two other temperatures that lie between the first melting and complete melting

temperatures; the sticky temperature (TgK) dehned to be the temperature at which

deposit becomes sticky and the radical deformation temperature (TRD) defined to be the

temperature at which deposits run off due to their own weight [13]. It was observed

during cone slurnping tests under static conditions (Figure 2.4) that TsrK and TRD

occurred at liquid contents of between 15 to 20% and 70% respectively [13].


1600 " 1400

3 t 1200 B

iooo

800

Completî -17 melting

~akca l A deformation

Temperature, OC

Figure 2.4 Appearance of deposit cones at dlfferent temperatures [j]

Deposit stickiness under static conditions has been studied previously by Isaak et al.. and

it was found that the deposit stickiness is a function of liquid phase [14. 151. Both

chloride and potassium lower the deposit melting temperature, however they have

different effects on deposit thermal properties [16]. As chloride concentration exceeds 1

mole% CV(Na+K). it has no effect on the deposit first rnelting temperature. However.

high chloride content increases the deposit liquid content at temperatures higher than the

first melting temperature. Potassium lowers the first melting temperature as well.

However. an increase in potassium concentration has no M e r effect on the deposit

liquid content once the temperature is higher than the first melting point. Therefore. it

was concluded that the deposit stickiness is a primarily function of chloride content and

temperature. The efTect of chloride on the TsX and TRD for a typicd canyover deposit

containing 5 mole% W(Na+K) is shown in Figure 2.5. As the chloride content inc~ases


fiom 1.5 mole% to 9 mole% CV(Na+K), TsTK decreases from 700°C to 560°C. while TRD

decreases but to a lesser extent. Deposit accumuIation occurs if the deposit composition

and temperature are in the sticky region. TsTK and TRD cuves forrn the lower limit and

the upper limit of the sticky region, respectively. Below TsTK deposits do not have

enough iiquid conrent to accumulate and abovc TRD deposits have so much liquid content

that accumulation ceases and further deposit runs off 151.

Slagging

\ Flrst rneiüng temperature

O 2 4 6 8 I O 12 14 16 18 20

CV(Na+K), mde%

Figrtre 2.5 Effect of chloride on sticky temperature zone [5]

Deposit stickiness is an important parameter determining the rate of deposition in the

lower superheater and upstream of the generating bank. Although the above snidies have

contributed significantly to the better understanding of deposit stickiness, the experiments

were conducted under static conditions. In these studies, the effects of variables such as

inertial impact and flue gas aerodynamics were not considered.


2.3 Plugging in Krap Recovery Boilers

PIugging occurs in different parts of kraft recovery boilers depending on how deposits

fonn and grow, and how they are removed by sootblowers. In the lower superheater

section. the flue gas temperature is above 800°C. As carryover particles strike the tubes,

they solidify and form hard deposits. The deposit continues to grow and the surface

temperature increases until it reaches the radical deformation temperature, at which point

the deposit starts to flow. Therefore, plugging does not occur in this region [5.17].

In the upper superheater region, the flue gas temperature ranges from 800°C to 700°C.

which is the sticky temperature zone for deposits as determined from static studies.

Canyover particles fom deposits. they continue to grow and massive accumulation

occun. which results in severe plugging in this section. Figure 2.6 schematicaily shows a

flue gas passage plugged between superheater platens [ 5 ] .

Figure 2.6 Plugging of superheater platenî by carryover impuction [j]


In the generating bank inlet, plugging occurs because of the narrow tube spacing, and

high flue gas temperature caused by severe fouling in the superheater region. In the

regions downstream From the generating bank, the flue gas temperature is as low as

550°C. which cause alkali vapon to condense and deposit as fume. Therefore, in this

section. deposits are mostly fume since most canyover has soiidified or deposited on Lhe

superheater tubes [S. 1 81.

2.4 Particle Transport Mechanisms

The transport of particles to heat transfer surfaces is affected by factors such as particle

shape and density. and flue gas flow charactenstics. Particles are transported to tube

surfaces by several mechanisms such as: molecular diffision, Brownian motion.

thermophoresis. turbulent diffision. and Uiertial impaction [6. 19-22].

Molecular diffision applies to particles considerably smaller than 0.1 Fm. Particle

motion and collision fiequency are controlled by the gas laws based on kinetic theory. the

same way as gas molecuIes. The particles' mean fkee path and collision rate are

dependent on their concentration in the gas Stream. and they move with velocities

approaching that of gas molecules [6].

Larger particles in the range of 0.1 pn to 1 pm undergo Brownian motion. These

particles experience a 'kandom-walk" motion due to collisions with gas molecules, and

their flow pattern depends mainly on the path of the gas sneam [19,20].

2. Literatwe Review

A temperature gradient, near a heat transfer surface, can result in the motion of particles

toward the colder region. This motion is cailed thermophoresis and mostly applies to

particles ranging fiorn 0.1 jm to 5 p. This themal force is a result of greater

momentun transfer from the gas molecules on the hot side of particles compared to the

cold side. whicii causes panicies deposition on a rold surface 1221.

Turbulent diffusion mechanism applies to particles ranging from 1 pm to 10 p.

Particles in the turbulent regime pick up kinetic energy from the gas eddies. and due to

the fluctuation of velocity components normal to the surface, they are able to move

through the laminar sub-layer and deposit on the heat transfer surfaces [6.21].

Inenial impaction is the dominant particle transport mechanism for the large particles

suspended in a gas Stream. These particles have suficient kinetic energy to rnove

independently of local variations in the gas ffow pattern and strike the heat transfer

surfaces. Carryover transport to heat transfer surfaces by inertial impaction is discussed

in detail below.

As a particle-laden flue gas fiows around a tube, dependhg on particle size and mass.

tube geometry. gas properties, and the flow field around the tube, the particle may strike

the tube d a c e due to inertia. Very small particles follow the gas streamlines perfectly

since they have negligible inertia. Large and heavy particles on the other hand. do not

follow the flue gas and try to continue in a straight path. The Stokes number (Stk) is

introduced to characterize the cumihear motion of particles which dows a particle to


move in its original direction as the carrier fluid is suddeniy changed 90" and is defined as

[ 19,231:

Stk = P, D I P U

9 4 P.g

where. p, . Dp. L'. 4, and pg are particle density. p ~ t i c l e diameter, Frec gas vclocity. tube

diameter. and gas viscosity, respectively. Figure 2.7 shows particle trajectories around a

circular cylinder.

Gus stream -- .-..- ---.- Particle stream

Figure 2.7 Purticle trajectories [ I 91

The Stokes number is restncted to situations where particle Reynolds

1. and particle Reynolds number is expressed as:

- P A & Re, -

where u, is the particle

number is less than

velocity. In order to take into account the non-Stokesian drag on

the particles, Israel and Rosner dehed an efecrrie Stokes number, S k n [24]. The

effective Stokes number is defined as:

Skn=Stk *


Where y is the non-Stokesian drag correction factor and expressed as [25.26,27]:

y/ = 18.99 ~e;'" - 47.77 tan-' (0.3975 ~ e ' , ' ~ ) / Re, [2-41

Wessel and Righi developed a generalized correlation for the collision probability, q, as a

function of Sthff, and it is vdid for Stokes numbers 2 0.5 [25]:

The collision probability is defined as the ratio of particles that hit the tube surface to the

particles that are in the projected area of the tube [25, 281. Figure 2.8 illustrates collision

probability as a function of effective Stokes nurnber. As is shown, carryover particles

have enough inertia to rnove independently of flue gas and strike heat transfer surfaces

[25.29]:

Figure 2.8 Cmryover collision probability [25]


2.5 Adhesion Eficiency

Carryover strikes heat transfer surfaces by inertid impact. Upon impact, some of the

particles may rebound and not deposit on the tube surfaces. The deposition rate is

dependent on the adhesion efficiency, which is defined as the ratio of particle capture

eficiency to the particle collision probability [26]. Capture efficiency is expressed as the

particle mass deposited on the surface to the particle mass in the projected area [XI .

The sticking behavior of fly-ash particles in coal-fred boilen has been widely studied

and particle viscosity was recognized as the most important parameter affecting ash

de position rate [30-3 51. Recentl y. the adhesion efficienc y of synthetic canyover particles

was investigated using an entrained flow reactor [36]. It was determined that the

adhesion eficiency is mainly a function of chloride content, temperature. and particle

size. In order for particles to start depositing, a critical chioride content is required. This

critical level decreases with higher temperature, however, increases with larger particle

size. A sharp increase in adhesion eficiency is observed up to a critical chloride content,

depending on the entrained 80w reactor temperature, after that, adhesion eficiency

remains constant and at high chloride contents, a slight decrease occurs. A significant

increase in adhesion efficiency is reported with an increase in particle size. It was

suggested that the hcrease is due to the higher kinetic energy of larger particles and

consequently, larger maximum spread upon impact.


2.6 Interna1 Circulation

Intemal circulation develops in a rnoving, low-viscosity liquid droplet due to drag effects

on the surface of the droplet [20, 371. The circulation reduces fiction, and as a

consequence the resistance offered by the medium. Internai circulation is s h o w

schematically in Figure 2.9.

Buoyancy Force Dng Force

Gravitational Force

Figure 2.9 Scherncltic diagram of internal circulation in low-viscosity droplet [20]

The Hadamard-Rybcqnski theory predicts that the terminai velocity of a fluid sphere is

up to 50% higher than that of a solid sphere with the same size and density due to intemal

circulation [38]. Some researchers argued that even though large particles obey the above

theory, small particies tend to follow Stokes' law due to the lack of circulation 139, 401.

Levich [41], however, proved that internai circulation exists in most droplets, no matter

how small. He explained that small droplets tend to follow Stokes' law due to the


presence of surface contaminants. As a droplet moves through a medium. surface

contaminant are swept to the back, which leaves the windward side relatively

uncontaminated. This concentration gradient causes a tangential gradient of surface

tension and consequently tangential stress, which slows d o m the droplet surface motion.

These gradients bave a pronounced effect on smdl particles, ~vhich results in a lower

velocity for these droplets compare to the Hadamard-Rybczynski prediction. Therefore.

the surface contamination theory implies that intemal circulation exists in ail droplets. if

the system is sufficiently free of contaminants. Figure 2.10 illustrates intemal circulation

in a water cirop.

Figure 2.1 O (a) Interna1 circulation in water droplet, D=l . 77cm. fill'y circularing [38]


Figure 2.10 (b) Interna1 circulation in water droplet, D= 1.2 1 cm, stagnant cap ut top of

droplet due tu the presence ufcontaminant[38]

2.7 Behaviov of Liquid Droplets Upon Impact

The impact of liquid drops on a surface is an important process in a variety of technical

applications such as spray coating, spray painting, and injection systems [42. 431.

Understanding the behavior of carryover particles upon impact on heat transfer surfaces is

of particular interest since it affects carryover adhesion efficiency.

2.7.1 Single Drop Impacts

Parameters that have an influence on single drop impacts have been studied extensively

[44-461. The goveming parameten are divided into four main categories [47]:

1) material properties, 2) state of impacting drop such as sphencal, deforrned, etc.

3) impact parameters such as droplet velocity, impact angle, etc. 4) d a c e conditions.

When a droplet strikes a surface, its behavior can be categorised as follows: spread

(possibly with recoil). rebound, and splash, as shown in Figure 2.1 1 [44].


Figure 2.1 1 Behavior of a drop following impact: spread. rebound, and sp fush

Drop impacts have been examined using either synchronized still photographs or hi&

speed cinematography with digital, high resolution equipment. which allows accurate

measwment of velocity, contact angle, and etc. [47, 481. Based on these experiments.

empincally based models. which in most cases neglect fluid mechanics. have been

fomulated. These models provide correlation that are used in technical applications.

Some of these models are discussed beiow in detail.

2.7.2 Spread

The outcome of an

spread. afier which

impact is afTected by the properties of the liquid droplet. Maximum

hrther spreading is prevented by liquid surface tension and viscosity.

is best descnbed by two dimensiodess numbers; the Weber number and the Reynolds

number:

where p, V, D, p, and a are liquid density, impact velocity, droplet diameter, liquid

viscosity. and surface tension, respectively [47,49].


The maximum spread has been modeled in two ways: numencal and analytical.

Numencal models have been developed to study fluid flow and heat transfer during

droplet spread [SO-531. Several analytical models have been developed to predict the

maximum diameter of droplet spread based on an energy balance, which equates initiai

kinetic energy to change in sUTface energy. The changes in surface energy occur due IO

droplet defonation and work done in overcoming viscous forces during impact [42. 43.

541. Recently. Mao et al. developed a serni-empirical isothermal model, which predicted

the mêuimum spread as a h c t i o n of the Reynolds number, the Weber number. and the

static contact angle: [28,55]

where dm is the maximum spread diameter. It was determined that the maximum spread

increases with the Reynolds number and the Weber number. the effect of contact angle

within the range studied is however, insignificant.

Several models have been developed to simulate impact and solidification of molten

droplets on a cold surface [43.52]. These studies showed that droplets spread completely

before solidifjmg. Since solidification has negfigible effect on the maximum spread, the

isothermal spread model, Eq. 2-8, cm be used to predict the non-isothermal maximum

spread [28]. Two dirnensionless numbers that are used to d e t e d e the magnitude of

solidification are: the Stefan number, Ste, and the Peclet number, Pe, [58]


Ste = Cp (Tm - Ts )/ Hf

Pe=VD/a

where C,. Tm, Ts, Hf, and a are specific heat, droplet melting temperature. substrate

temperature. latent heat of fusion, and thermal difi ivity, respectively. A solidification

criterion was proposed based on experirnenrai stuciies of moiten w a x and tin &oplets

impact on cold surfaces [5 7-59]. This criterion indicated that solidification would prevent

droplet spreading when s/D > 0.008 for impact velocity less than 2 d s . where s is the

average thickness of the solid layer that is determined as follows:

1 6 Sie s = D ,/- 3 Pe

Mao et al. tested this critenon for sodium nitrate, NaN03, which bas similar physical and

thermal properties to canyover [28]. It was determined that this criterion can be used to

set an upper limit. at which molten carryover freezes and adheres to tube surfaces upon

impact.

2.7.3 Rebound

In some cases. a liquid droplet may rebound d e r impact with a surface. The ability of a

droplet to rebound is important since it determines whether the droplet remains on the

surface. Mao et al. introduced a rebound model, which predicted for the first tirne the

tendency of a iiquid droplet to rebound upon impact [28]. This mode1 is based on a

rebound criterion; droplet rebound occurs if the maximum energy available for recoil and

rebound is greater than the surface energy. The proposed rebound cntenon is as follows:

2. Literature Review 23

if E L > 0, the droplet rebounds, othewise it remains on the surface. The above mode1

is a funftion of droplet maximum spread and static contact angle. in the case of droplet

impact on a non-wettable surface, the tendency to rebound increases as the maximum

spread and die contact angle increase. Furthemore, it aas concluded that when rapid

solidification occurs. no rebound is possible and droplet fkeezes close to its maximum

spread. The rebound discussed so far is a liquid or plastic rebound. There is. however.

another type of rebound called elastic rebound, which is rebound of a solid particle. This

type of rebound will be discussed in more detail.

2.7.4 Elastic Rebound

Elastic theory originally considered the rebound of elastic spheres, which are oniy subject

to contact forces of elastic deformation. This theory predicts that ngid particles will

always rebound upon impact with their kinetic energy unchanged [60. 621. However.

there is aiways some energy loss and a fraction of the total energy is dissipated. The

kinetic energy of bodies moving towards each other is converted into elastic energy and

vibrational energy afLer their collision [61]. The vibrationai energy is negligible in most

cases and is ignored. The elastic deformation involves compressive stresses between the

colliding bodies, and the elastic energy stored in them is converted into the kinetic energy

of the rebounding bodies.


In a number of physically important cases, a particle impacts a surface with one of them

covered with a thin fluid layer. To account for the efTects of the forces exerted by the

fluid, an elastohydrodynamic theory has been developed [60]. As the fluid is being

squeezed outward fiom the gap between the two solid surfaces. a large pressure develops,

which has two main cffects. First, if the inertia of the paxticles is not very hi$. it slows

down their relative motion. and second, it may cause the surfaces to deform in a small

region around the axis of syrnmetry. In the second case, some of the particle kinetic

energy is stored as elastic strain energy of deformation, while some is dissipated by

viscous forces. If the elastic deformation energy is sufficient, the particle will rebound

d e r coming momentarily to rest. However, the distance of rebound is limited since

further viscous dissipation occurs.

There is also another type of particle impact named elastic-plastic impact. Le. where there

is elastic deformation in both bodies as well as plastic deformation in one of the bodies

[62]. When a sphere with an initial kinetic energy approaches a stationary surface. it

gains energy as a result of the attractive force between two bodies. At the initial contact

of bodies. elastic deformation occurs and the pressure between these two bodies increases

until the peak pressure reaches the elastic yield lirnit of the sofier body. At this point. a

region of plastic deformation occurs, which accounts for some energy loss. If the

available elastic energy, which is the initial kinetic energy minus the energy dissipated in

plastic fiow, is greater than the adhesive energy, the particle rebounds.


Splashing occurs if the veelocity of the liquid front is greater than that of the liquid-solid

contact edge. Splashing is part of the deformation process and can also be explained as

follows. When a droplet strikes a surface, a liquid film spreads outwards and a corona is

formed around the deforming droplet, whch expands in the radial direction as the droplet

fluid continues to feed the film. When the lower half of the droplet has deformed, the

total volume flow rate into the wall film starts decreasing. As a consequence. the corona

becomes thimer and instability develops, which leads to formation of secondary droplets

[63]. If the kinetic energy of a droplet is relatively low, the droplet deforms without any

splashing and the droplet deposits on the surface. Furthemore, it was concluded that the

momentum of the prirnary droplet, i.e. droplet impact velocity and diameter has an

important influence on splashing. Another parameter affecting droplet splash. is the

substrate temperature [28]. When a molten droplet hits a cold surface. rapid solidification

occurs. which restricts the outward velocity of the contact edge while the top liquid layer

spreads with higher velocity. Thus more splashing occurs at a lower substrate

temperature.

2.8 Summary

Deposition of carryover particles on heat transfer surfaces of kraft recovery boilers is

inevitable since blac k liquor contains low melting temperature inorganic materials.

Carryover particles are transported to convective heat transfer d a c e s by inertial impact.

Previous studies have examined the effects of mornenturn and inertia on the adhesion


eficiency of molten and partially molten synthetic carryover particles, however, these

studies were performed under a gas strearn of low velocity. The inertial impaction

process of a liquid droplet on a cooled substrate surface has been studied extensively.

Most of the studies, however, used aqueous solutions and liquid metals to investigate the

impact process. Mac, [28] perhmed fundamental studies on sprcad. recoif!rebound of

liquid droplets analogous to molten canyover droplets without considering the inertial

impaction of partially molten carryover particles on a substrate surface. A molten

particle's impact velocity affects particle rebound and satellite droplet ejection. The

deposition rate of partially molten particles is afYected by impact velocity since the

rebound of the solid portion of the particle is dependent on its kinetic energy. Therefore.

the present work focuses on: (i) examining the rffect of particle impact velocity at high

flue gas velocity on the deposition rate of synthetic carryover particles. and (ii)

characterïzing the dynamic sticking behavior of synthetic carryover particles upon impact

on a surface.

3. Methodology

In the present study, the effect of particle impact velocity on carryover deposition was

studied by examining the deposition of synthetic canyover particles at different flue gas

velocities. The behavior of synthetic particles upon impact on a temperature-controlled

probe was visudized using a hi&-speed carnera. To study the effect of velocity and to

visualize the particle impaction process. the University of Toronto entrained flow reactor

(EFR) was used. which simulates the conditions in the upper section of recovery boilen.

In this chapter. the experirnental setup consisting of the EFR and optical equipment is

described. then the method used to Vary flue gas velocity is discussed, and finally. the

experirnental procedure is fully described.

3.1 Experimental Setup

The experimentai setup of this study consists of the following equipment: the EFR, a fast

shutter-speed device (CCD) camera, a visible HeNe laser, an optical trigger, a high speed

digital imaging system, and a haiogen light. The detailed description of the EFR is given

elsewhere [3 61 and is reviewed briefly here.

3. Methodolow 28

EFR is a down-flow laminar reactor consisting of a gas combustion section, a heated

section and an unheated section. The gas combustion unit, which burns naturai gas as a

fuel and produces a flue gas up to a temperature of 1200°C, is placed at the top of the

heated section. The heated section is made up of five tubular fumaces that can be

operated independently to temperatures as high as 1350°C. Below the Iowest fumace. a

non-heated sampling section exists, which accomrnodates the sarnpiing probe. The

temperature of the probe is controlled intemally by cooling air and the probe is connected

to an electromagnetic force compensation weight cell, which detemines the weight of

particles sticking to the probe. Optical visualization and measurements are conducted in

the non-heated section. A simple schematic diagram of the EFR is shown in Figure 3.1.

- Pürticie Fcedcr n

Pro bs

Figure 3.1 Schematic diagram of the enîrainedflow reactor

3. Methodolow 29

In the present study, an opticai measurement method was used to determine the particle

velocity prior to impact on the probe surface. A laser/detector system was designed to

trigger multiple image capture of particles about to strike the probe. Visible class IIIB

HeNe laser beam (Red HeNe Laser Head, Melles Griot, 632.8 am and 2 mW) was located

2 to 3 cm above the probe and was focused on a detector of an optical trigger (MAZOF

VIS ii Yrigger Systrrn). The detrctor waj set to tigger a signal when o particle pssed

through the beam. A 5V pulse that was produced by the trigger was sent to a

timerlcounter board in a personal computer which, after a preset tirne delay. activated the

shutter of a fast shutter-speed charge coupled device (CCD) camera (FlashCam, Optikon

Corporation). Images of 768 pixels (H) by 493 pixels (V) resolution taken by the camera

were stored in a digital format. The camera has a multiple exposure feature that rnakes it

possible to superimpose up to 10 images on a single h e with a time delay of lps to

Ims between consecutive exposures. The shutter speed was set between 140 to 170 ps

and a 210-mm C-mount macro lens was used to collect sharply magnified (X2) images.

A thermocouple with known diameter of 1.59 mm was used to calibrate the images. A

500 W halogen light was used to backlight the irnaging area. The distance between the

optical equipmenr and the outside surface of the h a c e was set to approximately 80 cm.

Figures 3.2 and 3.3 show the schematic diagram of the opticai setup and a typical stored

frarne of superimposed images of 10 mole% CV(Na+K) particles, respectively.

3. Methodolow

EFR

He-Ne laser - Optical trigger

Halogen CCD Camera Probe light

9 Monitor PC with a

timedcounter board

Figirre 3.2 Schematic diagram of opticai setup to rneaswe particle velociiy

Figure 3.3 Superimposed images of I O mole% CI/(Na+K), particle sire I jO-42Ojm

A high-speed digital imaging system ( MotionScope PCI 1000 S, Redlake Imaging

Corporation) was employed to study the particle Mpaction process on the probe d a c e .

The MotionScope PCI system can record a sequence of digital images of the impact

process at a Frame rate of 60 to 1000 fiames per second. The system stores these Mages

in image memory on the controller unit and the desired images can be subsequently saved

to a personal compter. The images were viewed frame-by-frame to analyze the motion

and deformation of particles during impact. The h e rate chosen in the present study

was 500 fiames per second. The record time for the above frame rate was 2 seconds and

the image resolution was 740x21 0 pixels. A 2 10-mm lens was ussd to iake s h q images

of particles impacting the surface and a thermocouple with known diameter was used to

calibrate the system. To illuminate the imaging area, a 1000-watt lighting system (Lowe1

DP System. Lowel-Light Manufactunng, Inc.) was used. The DP light had a reflector

with large focusing range; the light intensity was controlled by changing the reflector.

3.2 Variation ofFlue Gus Velociîy

The gas combustion unit used to produce hot gas. burns natural gas at a flow rate of 1 to

2.5 standard m3/h producing a maximum flue gas velocity of 2.8-3 m/s. There are two

ways of varying flue gas velocity in the EFR; changing the flue gas flow rate by

increasing the naturai gas flow, or modi@ing the exit of the EFR to increase the flue gas

velocity and consequently particle velocity before impact on the probe surface. In

previous studies where the former method was used, the effect of residence time in the

EFR and impact velocity on adhesion eficiency could not be decoupled. In the present

study, the latter method was chosen since it allows variation of the particle velocity

without decreasing the particle's residence time in the heated section of EFR. An orifice

shaped plate with smooth edges was placed flush to the exit of the EFR so there would be

no gap between the plate and the exit of the EFR. Two stainless steel flat plates were

made with diRerent diameters of 6 cm and 10 cm to produce two different exit velocities.

The use of exit plates had the beneficid effect of keeping the particle retention time

constant in the heated section. It is essential for particles to not only have the same

residence time but also sufficient residence time in the reactor since it is one of the factors

that determines the particle phase; solid, parîially molten, or completely molten. The

schematic diagram and pictures of the plate at the exit of the EFR are illustrated in

Figurés 3.4 and 3.5.

EFR Exit

Flue Gas Svcvnline

Plate

F

Figure 3.4 Schernatic diagram of the plate ai the exit cf the EFR

Figure 3.5 Plate at the exit of the EFR

3.3 Experimental Procedure

Synthetic carryover particles with compositions of 1, 2, 2.5, 5, 10, 20, and 52 mole%

Cl/(Na+K) were prepared by rnixing Na2S04 and NaCl. Also, particles with a fixed

chloride composition of 5 mole% CI/(Na+K) and different potassium contents, 5. 10, 15

and 20 mole% K/(Na+K), were prepared by rnixing Na2S04, NaCl, and K2S04. The

mixtures were melted in a muffle hiniace at 1000°C. cooled, ground. and sieved into

different size ranges of 90- 150 p, 150- 180 p, 180-2 12 Pm, 2 12-250 p, 250-300 Pm.

300-355 Pm. 353-425 Pm. 425-500 p, and 500-600 p using a RO-Tap Testing Sieve

Shaker. Table 3.1 sumrnarizes particle size ranges with their medians.

Table 3.1 Particle size range

1 Particle size range (pm) Median (p)

50 g of synthetic particles was introduced into the reactor at a feed rate of 2 g/min. n i e

particles were heated by the fumaces as they travelled down the reactor, which were

controlled at 800 OC. The flue gas velocity inside the EFR was kept constant at 1.8 mls.

The particles deposited on a temperature-controlled probe, which was controlled at 440 k

15 O C . and the deposit mass was recorded by the electromagnetic weight cell. Plates were

positioned at the exit of the reactor to increase the the gas velocity in the non-heated

section to 4.7 m/s and 12 m/s. Since a portion of the particles deposited on the plate. the

plate was weighed before and after each expenment to later determine the particle mass in

the projected area of the probe. Particle impact velocity was detennined using the optical

measurement method described in the previous section and the particle impact process

was visualized using the MotionScope PCI system. In dl experiments, the velocity of the

particles was measured with the camera placed perpendicular (normal position) to the

probe. In order to determine the accuracy of velocity measurement. experiments were

conducted with the camera positioned at 80° and dong the axis of the probe. These

measurements were perfonned to determine the velocity measurement error due to the

capture of out plane particle images. The experiments were conducted for two different

flue gas velocities of 1.8 and 2.8 m/s, at EFR temperature of 800°C and for particles with

a composition of 10 mole% CV(Na+K). The velocity measurement error was determined

to be less than 6%.

4. Particle Impact Velocity

The dominant transport mechanism

surfaces in the upper h a c e is inertial

that cames canyover particles to heat transfer

impact. Therefore, the impact velocity of particles

is expected to play an important role in the deposition of particles. However. there are

other factors such as particle composition, size and flue gas velocity, which might interact

with particle velocity to affect deposition. Therefore, several expenments were

conducted to isolate the effect of particle impact velocity on the deposition rate. The

results are presented and discussed in this section.

4.1 Experimental Reproducibility

The reproducibility of experiments ushg the plates placed at the exit of the EFR was

determined by repeating deposition growth rate experiment three times for a single size

range of 90-1 50 p; for example, the reproducibility of experiments for Bue gas velocity

of 12 m/s is illustrated in Figure 4.1. The calculated coefficient of variations for 4.7 m/s

and 12 m/s gas velocities are 8% and I l%, respectively. This variation is consistent with

an earlier study that determined a coefficient of variation of 7% for a 1.8 mls gas velocity.

4. Particle Impact Velocity 36

The coefficients of variation values are used to represent errors sssociated with each

experiment.

O 5 10 15 20 25 Time (min)

Figure 4. 1 Deposition reproducibility, gus velociv= 12 d s , EFR =800 C

particle size 220 pm, IO mole% CV(Na+K), O mole% KI/(Nu+K)

4.2 Particle Distribution

To calculate adhesion efficiency it has been assumed that particies are distributed

uniformly in the cross section of the entrained flow reactor. Previously, the validity of

this assumption was verified by measuring the mass distribution of deposits on non-

cooled bars [36]. In the present study, a more accurate rnethod is employed for

verification of this assumption. This rnethod takes into account the number of particles

that exit the reactor per unit time.

4. Particle Impact Velocitv 37

The CCD camera was used to capture images of 120 pm and 390 p median particles

with 8 mole% CV(Na+K) composition. The camera was focused above the probe surface

in order to capture and consequently, count the number of particles exiting the reactor and

impacting the probe. It is assumed that there is no angular variation in particle

distribution. For measurement the probe surface was divided equally into six sections of

length 3 cm with the first section close to the balance auid the last section close io the tip

of the probe. Images at each section were captured over one second intervals. and the

nurnber of particles was counted. Figures 4.2 and 4.3 illustrate the particle distribution

across EFR for the 120 pm median and 390 p phcles . The standard deviations shown

in these two figures are calculated for each section individually. The average number of

particles exiting the reactor for 120 pn and 390 pm are 74 _+ 17 particles/sec and 49 t 9

particles/sec where 17 and 9 represent the confidence intervals based on 95% confidence

level. respectively. Therefore, it can be concluded that there is no signiticant difference

between the number of particles at different locations. Thus, adhesion efficiency can be

used to represent the percentage of particles sticking to a surface upon impact. In the

cases of 4.7 m/s and 12 m/s flue gas velocities, the particle distribution across EFR was

not determined experimentally. However, it was assumed that particles have a uniform

distribution since particles were observed to deposit uniformly on the probe surface as

was observed for the 1.8 m/s flue gas velocity case. Deposits formed on the probe for the

two cases of 4.7 m/s and 12 ds flue gas velocities are shown in Figure 4.4.


0 3 3 6 6 3 9,12 12,15 1518

Distance From the Balance(cm)

Figure 4.2 Particle distribution across the EFR, partide mediun size 120 p.

EFR=800 S'. gas veloci&= 1.8 rnk, 8 mole% Clifla+ K). O mole% K;i'(iVuf K)

Distance From the Balance (cm)

Figure 4.3 Particle distribution across the EFR, particle median size 390 p,

EFR =8UO Y, gus velocity =l.8 d s , 8 mole% CU(Na+ K), O mole% IV(NatK)

Flue gas velocity=-l. 7 m/s Flue gus velocity=12 rn/s

Figure 4.4 Particle deposition on the probe. 20 mole% Cl/(Na+Q, O mole% W(Na+K).

EFR =800 'C, particle median size 390 p

4.3 EfSect of Particle Size and Flue Gus Velociv

4.3.1 History of Particles Inside the Reactor

Solid particies at different size ranges are introduced into the reactor at the room

temperature. As particles enter the EFR, they are accelerated downwards by gravity and

the drag force. The drag force acts downwards for particles that have lower velocity than

the flue gas. However. the drag force acts in the opposite direction of gravity for particles

with higher velocity than the flue gas. Particles falling through the EFR may reach their

terminal velocity, Vrr, pnor to impact on the temperaturetontrolled probe [19]. Because

the flue gas in the EFR is a carrier gas, the final velocity of particles is the sum of gas

velocity and particle terminal velocity. The terminal velocity is reached when the drag

force acting on the particle is balanced by the force due to gravity and can be detemiined

from the equation:


where p,, d,, CD, p, are particle density, particle diameter, coefficient of drag, and gas

density, respectively. The coefficient of drag for sphencal particles with Re< 3* 10' is

given by:

To caiculate Vsr, tliè conêct value of Co must bc detennined, however to obtain Co the

particle Reynolds Number is needed, which in turn requires the value of VSI. One way

around this dilemma is a trial-and-error solution, which c m be solved numencally. An

alternative approach is to calculate c ~ R ~ ' by rearranging equation 1-1 as:

c,, ~ e ' = 4 D 3 ~ p ~ K g 3p ' .

where y is gas viscosity. Since al1 the terms on the right hand side of equation 4-3 are

known and CD is only a function of Re, Re and thereby Vsr can be calculated. In this

study. equation 4-3 is used to solve VsT and for ease of computation of Reynolds number.

tabulated data fiom literature are used; the tabulated data are listed in Appendix 1. A

particle velocity and hence residence time strongly affects its phase upon exit fiom the

EFR heated section.

As particles travel d o m the reactor, they are heated by radiation and convection from the

surroundhg walls and the flue gas. Particles change phase from solid to liquid as they

move d o m the reactor depending on their composition, temperature and size, and flue

gas velocity. When they exit the reactor and before their irnpact on the probe surface,

particles travel a distance of 15 cm in a non-heated section. in the non-heated section,

particles are cooled since the gas has a lower temperature due to entrainment of the


surrounding air and due to radiation from colder walls. Therefore, depending on their

size. particles may be solid, partially or completely molten as they impact the probe.

4.3.2 Flue Gas Velocity Profie

Before examining the effect of particle size and flue gas velocity on particle impact

velocity, it is beneficial to have an understanding of the velocity profile in the EFR. The

entrained flow reactor is designed to provide a laminar flow at a Reynolds number of

approxirnately 2000. A senes of erperiments were conducted to determine the velocity

profile across the EFR. Small particles of median size 120 p were seeded in the flow

and their velocity $vas measured across the probe surface. Particles of this size have a

very low Stokes number and move with the flow to within f 5%. The particle velocity

was measured 2 cm above the probe surface and the resulting velocity profile is shown in

Figure 4.5. The velocity profile is pmbolic with 8 to 9% differences between the highest

and the lowest velocity measured. The relatively Bat velocity profile is desirable but was

initiaily surprising.

In general. as fluid moves through a Long tube as shown in Figure 4.6. a boundary layer is

produced, which causes the velocity profile to change with distance, until the fluid

reaches the end of the entrance length where the velocity profile does not change

anymore. In this section. the flow is termed Mly developed and the velocity profile

remains constant. To deterrnine the entrance length for laminar flow. the following

correlation is used: L - =0.05 Re, D

w here 1, and D are entrance length and tube diameter.

respectively. The calculated entrance length for the EFR is 18m where the total length of

the EFR is 6m. The measured centre velocity is only moderately above average velocity.


The moduiar design of EFR makes it impossible to have a Mly developed flow. Since

the five split-shell tube fumaces that form the heated section of the EFR do not aiign

perfectly, the boundary layer is disturbed at each fumace junction. The numencal work

of Vafa [65] supports this conclusion showing that the mis-aiigned h a c e cm lead to

disruption of boundary layer growth and the acceleration of the inner invisid flue gas

core.

5 10 15 Distance From Balance (cm)

Figure 4.5 Velociv profile across the EFR, 8 mole% CI/(Na+K), O mole% K/(Na+ K).

EFR =a00 T. particle sire 120 jm, gus velocity= 1.8m/s


Fully DeveIopsd Flow

Figure 4.6 Velocity profile in a pipe [66]

4.3.3 Particle Size and Flue Gus Veloci~,

Now that the velocity profile in the EFR has been determined, the effect of particle size

and gas velocity on particle impact velocity can be determined. Particle velocity results

are presented in Figures 7 to 12 and a particle velocity data is listed in Table 1-1 of

Appendix 1. Particle impact velocity as a fiction of particle size is shown in Figure 4.7.

An increase in particle size and therefore, mass results in an increase in particle impact

velocity since the gravitational force is proportional to the diameter cubed whereas the

drag force is proportionai to the square of particle diameter. The cuve is expected to be

linear and as illustrated in Figure 4.7 particle velocity increases linearly for particles up to

400 p. Large particles. however, deviate from the linear path since they have not

reached their terminal velocity pnor to impact on the probe surface.

In order to examine particle velocity and particle size independently, a series of

experiments were carried out with identical sire range particles at different flue gas

velocities. Flue gas velocity was varied fiom 1.8 mis to 12 m/s using the plates as


described in the previous section. Figure 4.8 illustrates particle velocity as a function of

size and gas velocity. Srnall particles have a tendency to follow the gas streamline and

have the same velocity as the gas. Therefore, increasing flue gas velocity significantly

increases particle impact velocity of smdl particles in the size range of 90-150 pm (120

pm rnedian). Large particles, on the other hand, have sufficient inertia to deviate fiom the

path and velocity of the Bue gas and travel with velocities higher than ùiat of the flue gas.

However. when the flue gas velocity is as high as 12 m/s. large particles have lower

velocity than the flue gas since these particles take longer time to accelerate and reach the

flue gas velocity. Figures 4.9 and 4.10 contain the same data as Figure 4.8. but it is re-

ploned to show particle impact velocity as a function of gas velocity for different particle

size diameten. Srnall particles have a linear relationship with the flue gas velocity since

they are able to adjust to changes in the flow very fast. Large particles. on the other hand.

have a shallower slope compared to srnall particles as shown in Figure 4.10 since their

relaxation time is long and they can not reach their terminal velocity over the Iength of

the EFR.


O O. 1 0.2 O. 3 0.4 0.5 0.6 0.7 Particle Diameter (mm)

Figttre 4.7 Particle v e h i t y , EFR=800LL, gas velocity=1.8m/s, 22 mole% CI/(Na+K),

O 100 200 300 400 500

Particle Diarneter

Figure 4.8 Measured particle velocity, EFR=800 T, 20 mole% CI/(Na+ K),

O mole% W(Na+K)

4. Particle Impact Velocitv

Figure 4.9 n/Ieasured particle velocity, EFR =8OO CL. 2 O mole% C l m a + K),

O moL% &(Na + K). particle median size 220 pm

14

12

10

8 -

6 -

4 -

2 -

O

O 2 4 6 8 10 12 14

Gas Velocity (mls)

- -

1 t 1 * I I

Figure 4.10 Memured particle velocity, EFR=BOO r, 20 mole% CU(Na+ K).

O mole% X/(Na+K), particle median size 390 jcm

O 2 4 6 8 10 12 14

Gas Velocity (mls)


4.4 Effect of Synthetic Carryover Composition on Velocity

Previous studies have s h o w that particle composition, particularly chloride, has an effect

on the deposition rate. Under given boiler conditions a particle's composition and history

determines its liquid content and shape. Both particle shape and liquid content may affect

a particle's impact velocity. In order to examine the effect of composition on particle

impact velocity. the composition of particles was varied from 1 to 20 mole% CV(Na+K)

at O mole% W(Na+K), and fiom 5 to 20 mole% W(Na+K) at a fixed chloride

composition of 5 mole% CV(Na+K), The velocity of particies was determined using the

optical measurement method described in the previous section.

Figure 4.1 1 shows particle velocity as a function of chloride content for a particle size

range of 300-425 prn with a median of 360 p. The particles with low chloride content

have the lowest velocity since they are mostly solid and have an irregular shape. An

irregular shaped particle causes increased Fnction between die particle surface and the

flue gas. resulting in a higher drag force acting on the particle. and a lower particle

velocity. Since the EFR temperature is greater than the first melting temperature.

increasing the chloride content significantly increases the amount of liquid phase.

Particles containhg 5 to I O mole% CV(Na+K) have a srnooth surface since their outer

layer is molten and have formed sphencal shape particles. As a consequence, they

experience a low drag force and have a higher velocity than the particles with lower

chloride content. Particles with a very high chloride concentration such as 20 mole%

CV(NatK) are completely molten and have formed liquid droplets. It is possible that

intemal circulation has developed in these low-viscosity liquid droplets due to drag

effects. The circulation reduces fiction, and thereby the resistanfe offered by the flue gas

[20].

Figwe Xi 1 Effect of chloride contenr on the particle v e l o c i ~

particle median size 360 p, EFR =8UO T, gus velociv= 1.8m/s. O mole% mat K)

The effect of potassium on particle impact velocity is illustrated in Figure 4.12. Particles

with potassium have a higher velocity than the ones without potassium. Potassium lowers

the first melting temperature of particles; the addition of 5 mole% Ki(Na+K) to particle

composition iowers the first melting temperature of particies fiom approxirnately 700°C

to 580°C. Particles containing potassium begin to melt sooner than the ones without and

therefore. they have smoother surfaces and are more sphencal. As a consequence, the

drag force applied to these particles is srnail, which results in higher particle velocity.

However. an increase in potassium content beyond 5 mole% CV(Na+K) has no M e r

significant effect on particle impact velocity. Although increasing potassium


concentration lowers first melting temperature further, the effect is less significant.

Iowering the temperature fiom 5 80°C to 520°C.

'r A

1

Calculated final velocity N+Vg)

Figure 4.112 Effecr of poiassium on the particle velocity, EFR =BO0 OC.

particle median sire 463 jm, gar velocity = l.8d.s. 5 rn&% CU(Na + K)

4.5 Visualization of Purticle Impact

Tne impact of particles on probe surface was visualized using the high-speed camera as

mentioned in the methodology section. The images were captured at the tirne of impact

for 390 p.m particles with different &onde contents of 10,20, and 52 mole% CV(Na+K)

at three different flue gas velocities of 1.8. 4.7. and 12 m/s. In this section. the behavior

of 10 mole% CV(Na+K) particles upon impact will be discussed separately fiom 20 or 52

mole% CV(Na+K) since these particles are partially molten and their behavior is

somewhat different fkom the molten particles. The solid and liquid portions of particles

are differentiated by their color; the solid portion is opaque, reflects lights and appears

white whereas the liquid portion is semi-transparent, reflects little light and appears black.


Particle containing 10 mole% CU(Na+K) is partially molten with a solid core as shown in

Figure 4.1 3.

Figure 4.13 Images of 10 mole% CU(Na+K), O mole% U(iVa+K). gas velocity 1.8 m/s.

particle median size 390 p, EFR=BOO OC, probe temp. =44O LT

Upon impact. these particles deform but they do not spread completely due to the

presence of the solid core. Upon impact, the liquid spreads outwards while the solid core

moderately flattens acting like a deformable pseudo-plastic material. As particle impact

velocity increases. particles defonn M e r and cover a greater probe surface area as

shown in Figures 4.14 and 4.15. The average diameter of deposits on the probe surface

for the cases of 1.8m/s, 4.7mls, and 12 m/s flue gas velocities is 6 4 2 p t 0.066.

70 1 p f 0.093, and 8 6 9 p f 0.027. respectively.


Figare 4. 14 Images of I O mole% Cl/(iVa+K), O mole% X/(Nu+K), gas velociry 4.7m/s.

particle median size 3 9 4 m . EFR=800 T, probe remp. =#O LL

Figure 4. l j Images of IO mole% CU(Na+K), O mole% H(Na+K). gas velocityli d s .

particle median size 390,i~n, EFR =a00 93, probe remp. =440 T

Due to the presence of a solid core, a portion of 10 mo1eY0 CCU(Na+K) particles bounces

off the surface pseudo-elastically. Figure 4.16 illustrates the sequence of particle rebound

for 10 mole% CV(Na+K) at flue gas velocity of 4.7 mis. The particle impact velocity is

6.0 mis while the rebound velocity off the d a c e is approximately 0.88 mfs. in some

cases. the solid core separates from the Iiquid part; the solid core bounces off while the

liquid part adheres to the surface. The solid core has high enough kinetic energy to

rebound while the liquid part of the particle solidifies on the surface. Figure 4.1 7 shows

the sequences of this separation for the case of 12 m/s flue gas velocity.


(a) (b)

Figure 4.16 Sequence of 10 mole% CV(Na+ K) particle rebound, O mole% W(lya+ K).

part icle rnedian size 3 9Oprn. gus velocity 47m/s: (a)particle on the surface. t = 0.0 ms.

(b)purticle rebounding, t =8. O ms

. .. * .. -* - k>X. - " .. . _ - t

(a) f i) Figure 4.1 7 Sequence of solid core re bounding, 1 O mole% CU(Na + K).

O mole% W(h/a+ K), partide rnedian size 3 9 0 p , gas velocity 12 mk: (a)particle impact,

@)particle separures into two parts, solid cure reboundr while liquid part adheres

tu the surface

Particles with hi& chloride content such as 20 or 52 mole% CU(Na+K) are completely

molten and they spread on the surface upon impact. Figure 4.18 shows images of molten

particles after impact on the probe d a c e . Upon impact, these particles deform and

spread in the radial direction since the pressure increases at the point of impact [67]. The

kinetic energy of particles is dissipated in overcoming viscous flow and in creating new


surface area. The average spread diameter for 390 p median molten particles is

approximately 1.3 mm& 0.12 regardless of particle impact velocity, which may indicate

that particles have reached their maximum spread. Particles solidi& rapidly and adhere to

the probe surface. therefore, they do not rebound off the surface. The solidification of

particles on the surface will be discussed in more detail in the next section. Particles that

hit the side of probe are dragged on the surFdcr due to the influence of gravis- and high

liquid content. However, they do not run off the surface since they solidi@ rapidly. As

illustrated in Figure 4.18, molten deposits have a smooth edge and no splatters are

obsewed. which indicates that particles do not splash upon impact even at the highest

impact velocity (8 mls). Splashing cm occur if the velocity of the liquid front is higher

than that of the liquid-solid contact edge, which results in a jetting action [63]. When a

molten droplet strikes a cold surface, splashing occurs if the spreading of the liquid layer

exceeds the outward velocity of the contact edge, which has become restncted by rapid

solidification. In the present snidy however, the combination of a hi& enough probe

surface temperature of 440°C and low enough particle impact velocity prevents splashing.

20 mole% C[/(Na+K) 52 mole% CV(Na+K)

Figure 418 Images of molten droplets, gas velocity 4.7 mis, ppariicle median size 3 9 0 ~

O mole% K;/(Na+K,, EFR=800 T, probe temp. = f i 0 T


4.6 Eflect of Particle Velocity on Adhesion Eficiency

Carryover particles deposit on heat transfer surfaces by inertial impact. Therefore.

particle impact velocity is expected to play an important role in the deposition of

particles.

The effect of composition on adhesion efficiency has been studied enensively elsewhere

136. 641 and is briefly reviewed here with replicate expenment of previous experiments.

The adhesion efficiency as a Function of chloride content is shown in Figure 4.19 for

3 9 0 p median particles. Carryover particles must have suficient liquid content to

adhere to a surface. Since particle liquid content increases with an increase in chlonde

concentration. particles with higher chloride content have higher adhesion rficiency. In

previous studies it was concluded that the main factor affecthg adhesion eficiency is

particle liquid content; particles with higher liquid content have higher adhesion

efficiency. However. there is a slight decrease in adhesion eficiency at a very hi&

chloride content of 20 mole% C V(Na+K), this discrepancy will be discussed later.


Figure 4.19 Efect of c.hloride content on adhesion eflciency, O mole% U(Na+K).

Figures 4.20 and 4.21 show the effect of particle velocity on adhesion efficiency for

particles in the size range of 90-150 W. The flue gas cools rapidly in the non-heated

section and temperature Calls below 800°C. Due to rapid heat transfer between particles

and flue gas, most particles solidify prior their impact on the probe and rebound pseudo-

elastically fiom the surface. Particles in the 4.7 and 12 m/s flue gas strearns have slightly

lower adhesion efficiency than particles in the 1.8 m/s flue gas Stream, but there is no

clear trend with velocity. Increasing particle impact velocity has two opposing effects on

adhesion efficiency: reduced residence time in the non-heated section and higher kinetic

energy. Particles with higher velocity and shorter residence tirne in the non-heated

section will have in general, higher liquid content, but also have higher kinetic energy

upon impact on the probe. Therefore, the effect of velocity on adhesion efficiency is a

complex compromise between solidification and rebound.

4. Particle Lmpact Velocitv

Figure 4 20 Adhesion eflciency as a finction of chloride content. O mole% W(Na+ K).

EFR=800 @ particle median size 120 pn , probe temp. =-/-/O

O 2 4 6 8 10 12 14

Impact Velocity (rnls)

Figure 4.21 Adhesion eficiency as a function ofpartide velocity, EFR=800 93, 10 mole% CU(Na+K), O moie% K;l(;iva+K), particle median ske 120 jm,

probe temp. =&O 97


Figures 4.22 and 4.23 illustrate the effect of gas velocity and particle impact velocity on

adhesion efficiency for particle size range between 355 to 425 pm with a median of 390

p As s h o w adhesion efficiency decreases fiom 84% to 24% for particles with 10

mole% CV(Na+K) as particle velocity increases. Particles containing 5 and 10 mole%

CV@Ja+K) are partiaily molten and they have a solid core as illustrated in Figure 4.24.

As velocity increases. particles have higher kinetic energy and they rebound pseudo-

elastically in a greater quantity. Therefore, paxticles with higher velocity need higher

liquid content to adhere to the probe surface upon impact.

Gas Velocity , m/s

//c-----;

Figure d22 Adhesion efficiency as afinction of chloride content, O mole% K;/(Nn+ K).

EFR=800 93. purticle medicm size 390 p, probe temp. =44O 57


2 3 4 5 6 7 8 9

Impact Velocity (Ws)

Figure 4.23 Adhesion eflciency as a funcf ion of particle veloci& EFR =8OU Y'.

10 mole% CL@Ja+K). O mole% W(Na+K), particle rnedian size 3 9 0 ~ .

probe temp. = G O CT

Figure 4.24 Image of 10 mole% CI/(Na+K), O mole% Wfla+K). patticle median size

390 jnn, gas velociîy 1.8& EFR=BOO X', probe temp. =-MO T


As illustrated in Figure 4.22, the adhesion efficiency of particles containing high chloride

content, which are completely molten, decreases with an increase in gas velocity.

Particles widi 20 mole% CV(Na+K) also have lower adhesion efficiency than 10 mole%

CV(Na+K). This behavior was observed previously and a nurnber of hypotheses have

been proposed to account for the decrease in adhesion efficiency with liquid content. It

has been argueil that as the liquid content incrcases uith chloride content, liquid droplets

are formed, which rebound pseudo-plastically following impact on the probe, and are

swept away [36]. Another hypothesis is that molten particles splash upon impact and

generate splatters that are entrained with the flue gas. As particle impact velocity

increases. more splatters are generated. therefore, adhesion eficiency is decreased. A

M e r suggestion was that particles containing high chlonde content slag and run off the

probe surface due to their hi& liquid content. To investigate these three hypotheses.

particles behavior upon impact on a probe was visualized. As illustrated in Figures 4.25

and 4.26. no molten particles were observed to rebound or splash upon impact and no

slagging occurred since particles were fiozen on the probe surface.


Figure -1.25 Spread of molten portides, 20 mole% CU(No+K). O mole% W@a+ K).

gas velocity 4.7ds, purticle median size 390 pn, EFR=100 CL. probe temp. =-/-/O P:

Figrire 4.26 Frozen particles on the probe surfce, 52 mole% CU(Na+ K,.

O mole% W(Na+K), gas velocity 4 7 d s . particle median size 390 p. EFR=800 93

In contrast to visual observation, experimental resuits indicate that adhesion eficiency of

high chloride particles, Le., particle with close to 100% liquid content, is much less than

100%. The adhesion efficiency of 390 pm particles conr;tining 20 mole% CV(Na+K) at

three different velocities of 1.8 ds, 4.7 mis, and 12 m/s is 66%, 28%, and 17%,

respectively. As was discussed in chapter 3,50g of particles are fed into the reactor at a

flue gas velocity of 1.8 d s . The adhesion efficiency calculations are based on the

assurnption that al1 50g of particles exit the reactor Le., mass in is equai to mass out. In

4. Particle Impact Velocif,

other words, there is no particle loss at the open section and no adhesion of particles to

fumace walls. When particles move in a cylindncal tube, there is always particle-wall

collision. It has been previously observed that sphere particles migrate toward the wall if

they have velocity higher than flue gas, and move toward the centre line if their velocity

lags the fluid [38]. The reason for this migration is not well undentood, but is believed to

'oz the resuli of either the lifi force or inertial efccts. In the present case, avhehen partic!es

containing hi& chloride content collide with the EFR wall, due to their high liquid

content. they adhere to the surface. Near the exit of EFR, particles are observed to be

sticking to the walls, also large pieces of deposits are seen to fa11 d o m the reactor which

indicates that some portion of particles have adhered to the walls. Therefore. the

assurnption, that d l 50g of particles exit the reactor. is not accurate and the calculated

adhesion eficiency is underestimated. The flue gas velocity was increased by placing an

orifice shape plate at the exit of the EFR as discussed in Chapter 3. The obstacle in front

of the flue gas streamline disturbs the flue gas causing recirculation in the last zone of the

EFR. It is possible that this disturbance causes more particles to collide with the wall

surface. resulting in lower adhesion efficiency for particles with higher velocity: the

higher chloride containing particles will begin sticking higher up in the EFR.

4.7 Efect of Particle Size on Adhesion Elfficiency

The effect of particle size on adhesion efficiency was studied by Shenassa [36] at EFR

conditions of 800°C and a flue gas velocity of 1.8 m/s. It was found that larger particles

have higher adhesion efficiency. These experiments were repeated but over narrower size

ranges with the similar results as shown in Figure 4.27. Small particles have lower

adhesion efficiency since most of them soli- pnor to impact and consequently bounce

off the probe surface pseudo-elastically . Larger particles, however, are still molten and

due to their greater rnass, they do not solidie as rapidly. Therefore, they have higher

adhesion efficiency than smaller particles.

Figure 4.2 7 Ejfect of sise on adhesion eflciency. O mole% KI/(Na+ K). EFR =8OO 91

gus velocity=1.8m/s. probe ternp. = M O 'Z:

The eeect of particle size on adhesion efficiency is exarnined at higher flue gas velocities

as illustmted in Figures 4.28 and 4.29. As shown, increasing velocity reduces the

difference between results at different size ranges to zero. At the highest flue gas velocity

of 12 d s . small particles containhg 5 mole% CU(Na+K) have a slightly higher adhesion

efficiency than the large ones. At higher chloride content, particle size has no effect on

adhesion efficiency. As discussed previously, particles with higher kinetic energy need

higher liquid content to adhere to the surface. Therefore at high velocity, particles, which

are partially molten and have a solid core, rebound pseudo-elastically in greater quantity.

At a high chloride content such as 20 mole% CVO\la+K), particles have either high liquid

content or are completely molte* thus a large percentage of particles may adhere to the

EFR walls at a velocity of 12 m/s since the flue gas is recirculating and is highly

disturbed. Large particles appear more sensitive as expected since they are less able to

follow the flow.

Figure 4.28 Effect of size on ndhesion eficiency, O mole% K;/(na+ K., EFR =8OO 0C.

gas velocity=-l. 7mh, probe temp. =440 T


Figure 4.29 Efject ofsize on adhesion efficienq, O mole% K;/(Na+ K). EFR =8OU 9C.

gus velocity = 1 2 mis, probe remp. =&O 93

Figure 4.30 summhzes the effect of particle size on adhesion efficiency at different flue

gas velocities for 10 mole% CV(Na+K). It clearly shows that an increase in flue gas

velocity and consequently particle impact velocity, reduces the effect of particle size on

adhesion efficiency. Although sorne portion of particles may have adhered to the EFR

walls, it may be concluded that the effect of particle size on adhesion efficiency is

negiigible at very high velocity for paaially molten particles over the size and velocity

ranges studied.


O 2 4 6 8 10 12 14

Gas Velocity (mis)

Figure 4.30 Eflct of particle size on adhesion eficiency, EFR =8OO T,

10 mole% CU(Na+K). O mole% KI/(Na+K), probe temp. =./ - /OS-

4.8 Effect of Probe Temperature on Adhesion Enciency

The effect of probe surface temperature on deposition rate was previously studied at a

single flue gas velocity of 1.8 m/s. It was found that a probe temperature between 300°C

to 500°C had a negligible effect on the deposition rate [68]. in this study, the effect of

probe temperature on adhesion efficiency at a higher velocity is examined. As show in

Figures 4.3 1 and 4.32. particles have higher adhesion efficiency at a higher probe surface

temperature. Further studies are needed to better understand the reason behind the higher

adhesion efficiency of panicles at higher surface temperature.


*

Probe Temp.

Figure 4.3 1 Eflect of probe temperature on adhesion eftiriency, O mole% W(Nu + K).

EFR=800 T, gus velocity=-l. 7 d s , particle median size 120 pm

1 Probe Temp.

Figure 1.32 Effect ofprobe temperature on adhesion efficiency, O mole% lW(Na+K),

EFR =BO0 T', gas velocity=-l. 7 ds, partide median size 3 90 p

4. Particle Impact Velociîy

4.9 Solidzj?cation and Shape of Deposits

In kraft recovery boilen, carryover particles impact on a tube surface with a temperature

higher than that of the probe. Therefore, it is important to study the solidification of

carryover particles upon impact. Previous studies have examined the effect of

solidification on maximum spread and rebound of molten wax and NaN03 [28, 591. In

the present study. solidification is examined using synthetic carryover particles in the

EFR. which simulates gas flow and deposition conditions in the upper superheater section

of recovery boilers. An existing spread mode1 and a solidification cntenon are compared

with the experirnental data to determine whether they are applicable to molten carryover.

Figures 4.33 and 4.34 show images of 10 and 20 mole% CY(Na+K) deposits on the bare

probe surface. The deposit is semi-transparent when liquid. and turns white and opaque

as it solidifies. As particle chlonde content increases, the solidification time decreases.

Particles with high chloride content have hi& liquid content and spread further and faster

on the probe surface due to lower viscosity and surface tension. Particles with greater

spread diameter solidi@ more quickly than particles of smaller spread diameter since

there is greater surface area for heat transfer. As particle impact velocity increases, the

splat diameter increases and as before the t h e required for the deposit to solidify

decreases. Deposits formed on the probe have an influence on the time required for

particles to solidi@. Longer solidification tirne is requûed for particles irnpacting on the

deposit as compared to paaicles striking a bare surface. As deposits build up on the

probe, the surface temperature increases; thus particles do not solid* as fast. The

tabulated data for the solidification time at dïf5erent conditions are presented in Table 4-1.


(b)Purticle impacts, t =O. 0ms

(c) Particle solid$es, t=38.Om

Figure 4.33 Sequence of solidifcation, 10 mole% CU(Na+ K), O mo[e % K/(Na + K),

gas velocity 12 d s , particle median size 390 pm, EFR=800 T, probe temp. =#O f-

4. Particle Im~act Velocity

(a) Bare surface (ô) Partide impacts. t =O. Oms

(c) Partide solidif es, t = 1 7. I4ms

Figure 4.34 Sequence of solidification, 20 mole% CP(Na+K), O mole% K/(Na+K),

gas velociîy 22m/s. particle median size 390p1, EFR=800 Y', probe temp. =440 T


Table CI SoZid~jication time, particle median sise 390 p. EFR=800 @

Cas Velocity, (Jw

Soiidification time the bare surface,

(ms) 66.3

SoIidification time the layer of deposit,

(ms) 74

4.9.2 Cornparison of Experimental Results with Spread and

Sokitj?cution Models

A dimensiodess excess rebound energy, EL, was developed by Mao [XI:

where a molten particle will deposit on the surface if E , 5 O . In the present study, the

rebound energy is calcdated for molten particles assuming a contact angle between 60° to

90°. A 60° contact angle was assumed since molten particles cornpletely wet the surface.

also previous studies have shown that contact angle of molten particles on a stainless steel

surface is approximately 67O. The 90' contact angle was chosen as an extreme upper

iimit for molten particles. As shown in Table 4-2, EL, for al1 conditions is either less

4. Particle hpact Velocity 71

than or close to zero, which indicates that rnolten particles deposit on the probe upon

impact. The rebound mode1 can not be used for partially molten particles since it does

not consider elastic rebound.

Table C d Ercess rebound energy, particle size 390 pm, EFR =8OO 93

Bhola and Chandra proposed a solidification criterion to determine when the spread of a

droplet is restricted by solidification [58]. The cnterion is based on the hypothesis that

the solidified layer is thick enough to restrict the liquid from m e r spread and to restrain

the droplet fi-om recoil. They estimated that solidification would restrict droplet

spreading if s/D > 0.008 at an impact velocity lower than 3 d s , where s is the thickness

of solidified layer and D is the droplet diameter. This cnterion is a function of the Stefan

number and the Peclet number and is deterrnined as follows:

7

, where te = c,(T, -T , ) /A , and Pe = Y D l a $=\jx

E*ERE

8=90°

-0.08

-0.05

0.015

Cas velocity

( mfs)

1.8

4.7

12

and C, . Tm TT, A, a are specific heat, particle melting temperature, probe temperature,

latent heat of fusion, and thermal diffusivity, respectively. Mao [28] confirmed the rapid

solidification criterion, s/D>O.OOS, where there was sufficient solidification of the droplet

at maximum spread to elirninate rebound.

E*ERE

0=60°

-0.6 1

-0.62

-0.63


In present study, this criterion is examined for synthetic carryover particles, which have

sirnilar chemistry and thermal properties as carryover. Images of molten particles during

impact show that particles solidi@ without any rebond. The calculated slD values for

completely molten particles at different velocities are presented in Table 4-3. The values

are greater than 0.008, which indicates that molten particles are predicted to fkeeze and

stick to ùie probe surface upon impact with no chance of rsbound. Thercfore, this

criterion is applicable to completely molten carryover particles. However. the

solidification criterion cm not be used for partially molten particles. The calculated s/D

values are greater than 0.008 for partially molten particles, which suggests that al1

particles adhere io the surface with no rebound. Experimental data however. indicate

otherwise. a portion of partially molten particle bounce off the surface due to the presence

of solid core.

Table 4-3 Solidifcution criterion particle size 390 jma, EFR=800 93. probe temp=440 9-

Gas Velocity, mis

4. Particle Impact Velocit, 73

A maximum spread model for a droplet that is solidifying during impact, is proposed by

Bhola and Chandra [69]. The Weber number, the Reynolds number, the Stefan number,

the Peclet number, and the Prandtl number are the relevant dimensioniess nurnbers in

determining the maximum spread factor:

The above can be sirnplified to the following f o m if We >s a and We ~ e » 12 :

J R e where Pr = Pe/Re.

It was found that the effect of solidification on the droplet deformation is negligible if

4- << 1. in the present study, equation 4-5 is used to detemine the maximum

spread factor. The calculated and experimental data are tabulated in Table 4-4 for

particles with different chlonde content at three different flue gas velocities. The

calculated maximum spread factor is in the range of the expenmental data. which

indicates that particles have reached their maximum spread and solidification has

negligible effect on the maximum spread of particles. Therefore, the model of Bhola and

Chandra can be used for partially and completely molten particles to detemine maximum

spread of carryover particles upon impact on a surface.


Table 4-4 Mminium spreud , particle size 390 pm, EFR=800 r, probe temp=440 T

Gas velocity, mis

4.9.3 Shape of Deposits

It is beneficial to have an understanding of deposits shape since it is one of the factors

that has an influence on deposit removai. Figure 4.35 illustrates the difference between

deposits of partially and completely molten particles. Since partially molten particles do

not deform completeiy, they have higher contact angle. Molten particles. on the other

hand, spread and wet the surface completely, i.e. have a low contact angle. These

particles cover a greater d a c e area and they fuse together to form hard deposits.

Particles that strike the edge of the probe are dragged down until they fieeze on the

surface. The deposit formed by the impact of these particles cover the edge of the probe

as shown in Figure 4.36.


10 mole% CU(na+K) 20 mole% CV(Na+K)

Figure 4.33 images of deposia O mole% K/(Na+ K), gus velocity 4.7 m/s.

particle median size 390p, EFR=100 Y, probe temp. =./JO eC

Figure 4.36 Deposits on the edge of the probe. 52 mole% CU(Na+K).

O mole% K;/(IVa+K), gas velociiy 1.8m/s, particle median size 390 pm, EFR=800 P7

The effect of particle impact velocity on adhesion efficiency combined with particle

impact visaulization are discussed here to better understand the underlying fundarnentals

of dynamic carryover deposition. Synthetic carryover particles at the time of impact are:

solid, partially molten, or completely molten. Solid particles bounce off the probe surface

while only a portion of partiaiiy molten particle rebound depending on their liquid content

and kinetic energy. Molten particles on the other hand, adhere and solidify on the probe

surface upon impact without rebounding.

Particle impact velocity increases with particle diameter. Small particles follow the flue

gas strearnline while large particles have sufEcient inertia to deviate fiom the flue gas

path and have velocities higher than the flue gas. Most particle s ix mges used in +fis

study have reached th& terminal velocity while in the heated section pnor to impact on

the probe surface. Particles containhg high chforide content or potassium have a higher

velocity since they are molten and have formed spherical shaped particles.

The smallest particles of median size 120 pm have Iow adhesion eficiency since they

solidi& and have a low liquid content pnor to impact on the probe. As particle impact

velocity increases, the adhesion eficiency for these particles slightly decreases since they

have higher kinetic energy. In the case of particles with median size 390 p. particle

impact velocity has a s m g e effect on partially molten particles. As particle impact

velocity increases. particles need higher liquid content to adhere to the surface since the

solid core can rebound off the surface. Also, the solid core of a particle cm separate from

the liquid part and be re-entrained in the flue gas. The visualization of molten particles

indicates that these particles solidify at their maximum spread with no rebound.

Particles may splash upon impact at very hi& impact velocity, however, that cntical

velocity was not obtained in the present study. Based on the present experimental data

and observation, the adhesion efficiency graph in Figure 4.37 is proposed. Particle

impact velocity affects the adhesion efficiency of partially molten particles, however has

negligible effect on molten particles. Particle size affects impact velocity and they both

can influence adhesion efficiency. Therefore, their effect on particle deposition was

studied separately. In the case of partiaily molten particles, it was found that as particle

impact velocity increases, the effect of size becomes less important. At very high impact

velocity, the major parameter, which influences adhesion efficiency is particle liquid

content not its size.

Liquid Content %

Figure 4.3 7 Effect of impact velocity on adhesion eflciency for the same size particies

Particle impact on the probe surface was visualized and the effect of solidification on

maximum spread was studied. The maximum spread of molten particles was measured

and was in good agreement with predicted values using the mode1 of Bhola and Chandra

[59]. Over the velocity range studied, there is only a moderate change in the spread of

molten particles of approximately 10%. However, the maximum spread for partially

molten particle increases with an increase in particle impact velocity. Solidification has a

negligible effect on the maximum spread of particles. Particles with high chlonde content

take less time to solidfi; they fieeze on the surface upon impact with no rebound. The

solidification time of particles impacthg on a layer of deposit is longer. An existing

4. Particle Impact Velocitv 7s

solidification cntenoo of Bhola and Chandra [58] cm be applied to molten particles.

Using this criterion, it can be detennined at what surface temperature and particle impact

velocity, particles freeze on the surface, i.e., a 100% adhesion efficiency. Also, the

excess rebound energy of molten particles was calculated using Mao's rebound mode1

[ B I . which indicated that these particles deposit on the surface. The solidification

crirerion and the rebound modd are not fully applicable ~o partially molten particles since

they do not consider the influence of solid core on particle spread and rebound.

3 . Implications

[t should be noted thai while the present study provides some insight into the dynamics of

carryover particle deposition. the results might not be directly applicable to specific cases

since there are some differences between a recovery boiler and the EFR.

Although this work suggests that chloride and potassium have an effect on particle

velocity. it may not be true in recovery boilers. When the black liquor is fired into

recovery h a c e s . the droplet begins swelling slightly in the drying stage and

significantly in the devolatilization stage. After the devolatilization stage. the fomed

particles are swollen. porous, and inegular shaped char particles. At the char burning

stage. the particles start burning and they shrink in size as carbon is burned and form

molten spherical shaped carryover particle. Therefore, the velocity of molten carryover

particles is unlikely to be affected by particle composition.

Visualization of the particle impact process in this study indicated that particles

containing hi& chloride content completely spread and Eeeze on the probe sdace .

Therefore, mills with a very hi& chloride content may have to undertake some preventive

measures such as reducing chloride content through precipitator catch purging. Deposit

5. Implications 80

slagging might occur in the recovery boiler despite the fact that slagging was not

obsemed in the present study due to differences between the EFR and the recovery boiler

environment.

The present study implies that adhesion eficiency of partiaily molten particle decreases

with incrrasing flué pas wlocity and consequently partuticle impact velocity. In the upper

section of a kraft recovery boiler. the flue gas cools down and carryover particles may

partially solidify. Therefore. a higher flue gas velocity may increase the quantity of

carryover particles bouncing off the tube surfaces. However. it shouid be noted that hi&

flue gas velocities in recovery boilers cause entrainment of larger carryover particles.

Larger particles do not solidi@ as fast as small particles, which in tuni may result in an

increase in carryover deposition on the superheater tubes. Thus. an optimum flue gas

velocity should be found as a compromise between these two effects.

The visuaiization of particle impact process indicated that splashing did not occur over

the parametric range studied. It may dso be the case in recovery boilen since particle

impact velocity is not significantly higher than the velocities exarnined in this study, and

the tube surface temperature is approximately the sarne as the probe surface temperature

used in the present study.

6. Conclusions

The effect of carryover particle impact velocity on adhesion eficiency was studied using

an entrained flow reactor. A high-speed imaging digital system was used tu visualize

particle impact on a probe surface. The main findings under EFR conditions are

sumrnarized as fol iows:

Particles with higher chloride content or particles containing potassium have a higher

velocity since they are molten and foim sphencal shaped droplets. These particles

experience less drag force and consequently have higher velocity.

Partially molten particles deform m e r and cover a greater probe surface area at

higher velocities. The average diameter of deposits on the probe surface for the cases

of 1.8 d s . 4.7 m/s, and 1 2 d s flue gas velocities is 0.642 mm. 0.70 1 mm, and 0.869

mm. respectively. Upon impact, these particles deform as the liquid content spreads

dong the probe surface but do not spread as far as molten particles due to the

presence of the solid core.

6. Conchsions 82

An increase in liquid content and particle impact velocity reduces particle

solidification time upon its impact on probe surface. It takes approximately 66 ms for

10 mole% CI/(Na+K) with 390 pm particle size to solidi@ whereas 13 ms for 52

mole% CI/(Na+K). Also. the solidification time for 10 mole% CV(Na+K) is

approxirnately 38 ms at the flue gas velocity of 12 m/s. Particles with high chloride

content such as 20 mo1eY0 Ci/(Na+K) are cornpietely moiren and they spread on the

probe surface upon impact. Due to the resulting large surface contact area. particles

solidie upon impact with no rebound. slagging. or splashing. Also. longer

solidification time is required for particles impacting on a layer of deposits: for

example. the solidification time is 74 ms for 10 mole% CV(Na+K) at 1.8 m/s flue gas

velocity.

The existing maximum spread model. solidification criterion. and rebound model c m

be applied to molten carryover particles. In agreement with our experimental data.

the maximum spread mode1 indicates that solidification has negligible effect on the

maximum spread. The solidification criterion and the rebound model support the

observation that molten particles k z e and adhere to the probe surface with no

rebound.

0 Partially molten particles fom deposits with high contact angle. Completely molten

particles. on the other hand, form fused and hard deposits. Particles stnking the edge

of the probe are dragged down until they fieeze on the probe.

6. Conclusions 83

Particle adhesion to EFR walls was observed however. it was not accounted for in

adhesion efficiency calculations. In general, calcdated adhesion efficiencies were under

estimated.

The adhesion efficiency of small particles is independent of particle impact velocity

over the velocity range O F 1.8 to 12 m i s . The adhesion efficiency of large partially

rnolten particles such as 10 mole% Cl/(Na+K) with median of 390 pm decreases as

particle impact velocity increases from 1.8 d s to 12 mls. A ponion of partially

molten particles bounces off the surface due to presence of the solid core. In some

cases. the solid core separates from the liquid part and is re-entrained in the flue gas.

Particle size has a negligible effect on adhesion efficiency at the flue gas velocity of

12 d s .

f l Particles have higher adhesion eficiency at higher probe surface temperature at a high

flue gas velocity of 4.7mls.

7. Recommendations

Based on the results obtained from the present study. the following recommendations can

be made for hiture investigation.

Since some portion of particles might collide with the EFR walls and adhere to the

surface. the total mass of particles exiting the EFR should be rneasured using a

cyclone or a sieve with a very fine mesh in order to determine the correct adhesion

eficiency.

Most canyover particles in the upper fumace of recovery boilen impact the tube

surface at a strike angle other than 90'. Further investigation should be done using

inclined probes to examine the effect of impact velocity on adhesion efficiency. Also.

the behavior of particles impacting inclined probe can be visualized to characterize

the deposition process.

Higher impact velocity should be considered to detemine if adhesion efficiency for

partially molten particles decreases M e r . Also, visuaikation of particles impacting

at higher velocity might help to determine the criticai velocity that splashing occurs.

A solidification cnterion and a rebound mode1 should be developed as a function of

composition or liquici conrent of panicies hat art: applicable io boih partially and

completely molten particles.

The visualization of impact process on the probe surface should be extended to

particles with different compositions such as particles containing potassium or

carbonate. It is also beneficial to have a fundamental understanding of char or

carryover particle impact process by visuaiizing their behavior.

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33. Abbon. M.F.. Conn. R.E.. and Austin, L.G. *-Studies on Slag Deposit Formation in Pulverized-Coal Cornbuston. 5. Effect of Flarne Temperature. Thermal Cycling of the Steel Substrate and Time on the Adhesion of Slag Drops to Oxidized Boiler Steels". Ftrel. Vol. 64. June (1985).

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Nomenclature

Collision Probability

Excess Rebound Energy

Gas Density (Kg/mJ)

Gas Viscosity (Ns/m2)

Maximum Spread Factor

Non-Stokesian Drag Correction Factor

Particle Density (Q/rn3)

Particle Viscosity (Ns/rn2)

Surface Tension (Nlm)

Thermal Di fhivity (m'ls)

Coefficient of Drag

Specific Heat (J/Kg.C)

Maximum Spread Diameter (m)

Particle Diameter (m)

Tube Diameter (m)

Latent Heat of Fusion (JiKg)

Entrance Length (m)

Nomenc Iature

Pe

Ste

Stk

V D Peclet Number (= - ) a

f randtl Number (=Pe/Re)

Particle Reynolds Number (= ~ K ~ P ~ P ) 4

Average Thickness of Solid Layer (m)

Stokes Number (=

Effective Stokes Number (=Stk* y, )

Particle Melting Temperature ( O C )

Substrate Temperature (OC)

Free Gas Velocity (rn/s)

Particle Velocity ( d s )

Impact Velocity (m/s)

Terminal Velocity (m/s)

V'D Weber Number (=-

6 1

Appendices

Appendix 1 The measured velocity of synthetic carryover particles with

different size ranges and compositions at EFR temperature of 800 O C and flue gas

velocity of 1.8 mls is iisted below.

Table 1-1

Partilce size (pm)

Particle Composition size rnedian CI/(Na+K) hm)

Composition Panicle Standard K/(Na+K) veiocity Deviat ion

Appendices

Particle Particle Composition Composition Particle Standard size (p) size median CV(Na+K) K/(Na+K) velocity Deviation

(pm) (W

Appendices

Particle 1 Particle 1 Composition Composition Particle size (pm) size median C W(Na+K) K./(Na+K) 1 velocity 1 Deviation 1

Particle impact velocity of two different size ranges at different flue gas velocity

Appendices

Table 1-2. Reynolds number for settling spheres [19]

Appendix II Adhesion Efficiency Values

Adhesion Efficiency 940

Particie size rnedian (w)

L

Composition CI/(Na+K)

Flue gas veiocity ( m m

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