PARTICLE DEVELOPMENT IN A FLUIDIZED BED

BLACK LIQUOR STEAM REFORMER

by

Mauricio Naranjo

A thesis submitted to the faculty of The University of Utah

in partial fulfillment of the requirements for the degree of

Master of Science

Department of Chemical Engineering

The University of Utah

August 2006

Copyright © Mauricio Naranjo 2006

All Right Reserved

ABSTRACT

Low-temperature gasification of black liquor is a new alternative to the conventional

Tomlinson recovery boiler for recovering pulping chemicals and energy from black

liquor in pulp and paper mills. This process can be carried out in a fluidized bed based on

steam reforming at 600 oC using indirect heat supplied via heat exchangers built into the

fluid bed. As black liquor is injected into the bed, bed particles evolve through different

mechanisms, exhibiting variations in size and characteristics. Coating, agglomeration,

attrition and fragmentation are mechanisms that have been identified as contributing to

bed particle evolution.

Four experimental approaches were used to develop fundamental understanding of

these mechanisms, and to qualitatively address particle development in this unit. Visual

analysis of cross-sectional areas of bed solids from two commercial steam reforming

units showed that bed particles grow by deposition of one to two superimposed layers of

residue rich in Na and K, which are more distinctive on bed particles larger than 250 µm.

Lab-scale fluidized bed reactor experiments showed important effects of adding large

liquor droplets on bed particle characteristics, particularly on particle size distribution and

shape, which promotes the formation of droplet-induced agglomerates. Cold fluidization

of bed solids in a Plexiglas column indicated that considerable mechanical attrition

initially occurs with bed particles larger than 300 µm, followed by a rapid attrition decay

as particles are rounded off. Compression, glass cyclone and tube furnace experiments

showed that chemical consumption of the organic matter remaining in the char residue

combined with vigorous solids fluidization presumably leads to significant fragmentation

of agglomerates, which leads to the formation of indirectly coated particles.

It was concluded from this study that particle development in a fluidized bed black

liquor steam reformer is a complex combination of mechanisms and submechanisms that

simultaneously participate in bed particle evolution. Coating and attrition seem to be the

most significant mechanisms, followed by agglomeration which results from poorly

controlled conditions. Once agglomeration has occurred, fragmentation then controls the

excessive particle growth caused by agglomeration.

v

Mi vida, la fuerza de nuestro amor nos brindará una vida llena de mutuos éxitos.

CONTENTS

ABSTRACT iv ACKNOWLEDGMENTS viii 1 INTRODUCTION 1

1.1 World and U.S. Pulp and Paper Industry 1 1.2 Pulping Process 2

1.2.1 Black Liquor 2 1.2.2 Black Liquor Recovery Process 5

1.3 Fluidized-bed Steam Reforming of Black Liquor 10 1.3.1 Particle Mechanisms 11

1.4 Research Objectives 16

2 LITERATURE REVIEW 17

2.1 Platform for the Implementation of Black Liquor Gasification 17 2.2 Black Liquor Gasification Technologies 19 2.3 Fluidized Bed Technology 22

2.3.1 Coating 25 2.3.2 Agglomeration 27 2.3.3 Attrition 29 2.3.4 Fragmentation 31

3 EXPERIMENTAL 34

3.1 Introduction 34 3.2 Experimental Set-up 34

3.2.1 Coating Studies 34 3.2.2 Agglomeration Studies 35 3.2.3 Attrition Studies 36 3.2.4 Fragmentation Studies 41

4 RESULTS AND DISCUSSION 51

4.1 Coating Visual Analysis 51 4.2 Agglomeration Results 60 4.3 Attrition Results 78

4.4 Fragmentation Results 91 4.4.1 Compression Results 91 4.4.2 Cyclone Results 93 4.4.3 Agglomerate Break-up by Chemical Reaction 95

5 CONCLUSIONS 101

5.1 Particle Development in a Fluidized Bed Black Liquor Steam Reformer 101 5.2 Recommendations for Future Work 105

APPENDIX STEAM REFORMER BED MEDIA CHEMICAL

ANALYSIS PROCEDURE 107 REFERENCES 109

vii

ACKNOWLEDGMENTS

I would like to truly thank Dr. Kevin Whitty for his guidance and total support toward

completion of this thesis.

Special thanks to Dr. Eric Eddings for giving me the opportunity to work for the

Institute of Combustion and Energy Studies (ICES), and trusting in my professionalism.

This experimental work would not have been possible without the help of the

technical staff of ICES, particularly, Mr. Dana Overacker.

The financial support for this research was provided by DOE cooperative agreement

DE-FC26-02NT41490. The technical contribution of Georgia-Pacific Corporation,

Norampac Corporation and ThermoChem Recovery International is also acknowledged.

CHAPTER 1

INTRODUCTION

1.1 World and U.S. Pulp and Paper Industry

The pulp and paper industry is quite integrated and plays a very important role in both

world and U.S. economies. More than 300 billion tons per year of finished paper products

are produced globally with annual revenues over 500 billion dollars, about 166 billion

dollars are attributable to U.S. industry.1 Economically, the forest products industry (of

which the pulp and paper industry is a major part) ranks as the ninth largest industry in

the U.S. manufacturing sector,2 contributing nearly 5% to the U.S. gross domestic

product (GDP).3 Pulp, paper and paperboard mills account for 12% of the total

manufacturing energy use in the U.S.4 However, the advent of strong competitors in the

global market, such as Asia and South America, and the modernization of European

counterparts, are forcing the U.S. pulp and paper industry to reevaluate its business

models and processes to remain competitive.5

One key factor in this repositioning process is to make U.S. pulp and paper mills

more energetically efficient, which ultimately would contribute to improve the U.S. pulp

and paper industry’s economy. A U.S. government-industry program, Agenda 2020, has

identified technological advancements the U.S. pulp and paper industry must reach in

order to obtain its economic objectives.6 Among those advancements, black liquor

2

gasification arises as a promising technology to replace current black liquor recovery

technology, potentially adding more than 30 GW of power per year and offering

alternatives for synthesis gas marketability not seen before.7

1.2 Pulping Process

Pulping is the process to separate and treat wood fibers to produce pulp. Chemical,

mechanical, semichemical and semimechanical pulping processes are used depending on

the raw material and the final paper product. The predominant pulping process is Kraft

(sulfate) pulping.4 This process accounts for almost 80% of all the pulp produced in U.S.,

as seen in Table 1. In this chemical process, lignin is dissolved in a digester with a

mixture of sodium hydroxide (NaOH) and sodium sulfide (Na2S) that has a pH of 13 - 14.

An alternative process is sulfide pulping, which uses hydrogen sulfide ion (HSO3-) or

sulfite ion (SO32-) in an aqueous acidic or neutral solution.4

In the Kraft process, pulpwood is debarked, chipped and sieved for size before being

fed into the digester. Wood chips are then cooked in the mixture of pulping chemicals

under pressure at temperatures of 160 – 180 oC for 0.5 - 3 h to dissolve the lignin.8 Wood

fibers are separated from the spent pulping liquor through washing stages and the pulp is

then screened, bleached and pumped to the paper mill4, as presented in Figure 1.

1.2.1 Black Liquor

Black liquor is classified here as a biomass that represents over 40% of the total

energy resource for the pulp and paper industry in the U.S.9 Black liquor is the spent

pulping liquor that results after fiber separation. It contains a complex mixture of organic

matter (dissolved ligno-cellulose structures) and inorganic pulping chemicals.

3

Table 1. Estimated pulp production in United States in 2000.9

Process Thousand Tons Percentage (%)

Mechanical 6501 6.13

Semi-chemical 3976 10.02

Chemical 54375 83.84

- Sulfide 1116 1.72

- Kraft 53259 82.12

- Unbleached 21281 32,81

- Bleached 31978 49.31

Total 64852 100

4

Figure 1. Simplified flow diagram of an integrated chemical pulp and paper mill (Adapted from reference 10).

Woodyard and

chipping

Cooking

Washing

Screening 1

Bleaching

Screening 2

Drying machine

Finishing department

5

Due to its chemical complexity, black liquor presents unique physical and chemical

properties. Among these properties, the percentage of dry solids, viscosity, heating value,

heat capacity, surface tension, inorganic component solubility, density and thermal

conductivity are of most importance.11 Chemical composition and molecular structure are

also important parameters that determine the physical and transport properties of black

liquor. For example, black liquor can behave as a Newtonian or non-Newtonian fluid,

depending on lignin and polysaccharides molecular size, shape and conformation.11

Typically, concentrated black liquor has viscosity, density and heating values of 70 – 120

cP, 1400 kg/m3, and 13.5 – 14 MJ/kg dry basis (HHV), respectively.12

1.2.2 Black Liquor Recovery Process

Black liquor recovery is a crucial component of the chemical pulping process. It

recovers pulping chemicals for reuse, and is a source of energy by conversion of the

remaining organic content. Pulp and paper mills experience considerable financial

benefits as a result of utilizing this process, as costs related to the purchase of chemicals

and energy are reduced. In addition, there are environmental benefits, with black liquor

recovery reducing the amount of waste discharge to the environment.

The weak black liquor (solids content of approximately 15% by weight) obtained

from the fiber separation stage is concentrated by evaporation to a solids content between

60 to 80%. At this point, black liquor is burned or gasified to release its chemical energy

and recover its inorganic content. This inorganic residue undergoes a further

causticization process, in which calcium hydroxide (Ca(OH)2) is reacted with the sodium

carbonate residue (Na2CO3) to regenerate sodium hydroxide (NaOH), one of the starting

6

pulping chemicals (white liquor). A schematic of the recovery process is presented in

Figure 2.

The overall pulping efficiency largely depends on the performance of the recovery

unit,8 in which black liquor conversion undergoes droplet drying, pyrolysis and char

conversion. These stages are shown in Figure 3. Initially, contained moisture is dried

from a black liquor droplet. The droplet then goes through pyrolysis, releasing volatile

matter. This volatile matter mainly contains H2, CO, CH4, CO2, H2O, and some heavier

hydrocarbons (tars). The result is a highly porous particle, called char, which contains

about 25% nonvolatile organic matter and inorganic salts such as Na2CO3, Na2S and

Na2SO4. The char is then finally reacted with gas-phase species to consume its organic

constituents.

1.2.2.1 Tomlinson Recovery Boiler

The Tomlinson recovery boiler has long been the conventional unit to fire black

liquor for energy production and recovery of inorganic chemicals. Utilization of this

recovery boiler supplies approximately 58% of the energy requirements for pulp and

paper mills.13 This recovery boiler is composed of three sections: drying, oxidizing, and

reducing. In the drying section, black liquor is sprayed to aid in the release of its water

and volatile matter content. In the oxidizing section, the resulting char burns leaving

sulfur and sodium-based inorganic compounds that are collected at the reducing section.

In this section, the inorganic compounds are mostly converted to molten Na2S and

Na2CO3, and then spouted into dissolving tanks for further causticization. A set of air-

supply ports located along the boiler wall, provides the necessary air for complete

combustion. Combustion gases transfer heat to tubes filled with water located in several

7

Figure 2. Schematic of the recovery process (Adapted from reference 14).

Figure 3. Conversion stages of black liquor (Adapted from reference 14)

Digestion

Concentrators

Recovery unit

Causticizing

Wood

Pulp

Weak black liquor

Strong black liquor

Gases

Water

Smelt

Green liquor

White liquor

Drying Smelt Char conversion Pyrolysis

H2O

Gasifying agent

Fuel gases Volatile

matter

8

areas: the walls of the recovery boiler in the radiant zone, the boiler tube bank and the

economizer section. A schematic of a Tomlinson recovery boiler is presented in Figure 4.

1.2.2.2 Gasification

Gasification is a process that converts any carbonaceous fuel to a gaseous product

that has a usable heating value.12 Under this definition, gasification refers to the

processing of potential fuels ranging from coal and oil, to biomass and wastes, using

technologies that are usually divided into low-temperature and high-temperature

categories. These technologies may include processes like pyrolysis, partial oxidation and

steam gasification, which generally employ units grouped into fixed-bed, fluid-bed and

entrained-flow gasifiers. Gasification also refers to the heterogeneous chemical reaction

to turn a solid (char) to gas, as seen in the char conversion stage of black liquor

processing in Figure 3.

For the purposes of this research, the term black liquor steam reforming will be

understood as one of the gasification technologies implemented by pulp and paper

industries to replace the Tomlinson recovery boiler. The fluidized-bed steam reforming

process belongs to the low-temperature category, in which black liquor is processed

below the melting temperature of its inorganic material.

1.2.2.3 Other Technologies

Besides the direct firing and gasification of black liquor in pulp and paper mills, other

technologies have been developed for its recovery. In general, these technologies are

comprised of processes in which solid or liquid liquors are pyrolyzed to yield fuel gases

and char. However, pyrolysis (as a stand-alone process) is of less importance in gas

9

Figure 4. Tomlinson recovery boiler.

Primary air supply

Secondary air supply

Tertiary air supply

Black liquor spray nozzles

Reduction section

Drying section

Oxidation section

Char bed

Superheater

Boiler tube bank

Screen tubes

Smelt spout

10

production today.12

1.3 Fluidized-bed Steam Reforming of Black Liquor

Manufacturing & Technology Conversion International, Inc., (MTCI) has developed

a “PulseEnhanced” steam reformer design.12 The process centers on the steam reforming

of black liquor at about 600 oC, using indirect heat provided by pulsed heaters built into a

fluid-bed reactor. Black liquor is sprayed onto the bed solids where it quickly dries,

pyrolyzes and reacts with steam to form a medium heating value syngas. Due to the low-

temperature operation, the inorganic residue remains solid and contributes to the

evolution of the starting bed material. The steam reforming reactions for conversion of

carbon in the remaining char are:

C + H2O ↔ CO + H2 (1)

C + 2H2 ↔ CH4 (2)

CH4 + H2O ↔ CO + 3H2 (3)

CO + H2O ↔ CO2 + H2 (4)

C + CO2 ↔ 2CO (5)

The reactions for sulfur reduction and sulfur – sodium separation are:

Na2SO4 + 4CO ↔ Na2S + 4CO2 (6)

Na2SO4 + 2C ↔ Na2S + 2CO2 (7)

Na2SO4 + 4H2 ↔ Na2S + 4H2O (8)

Na2S + H2O + CO2 ↔ Na2CO3 + H2S (9)

11

1.3.1 Particle Mechanisms

Particle development plays an important role in the performance of fluidized beds. As

black liquor is injected into the fluid-bed reactor, bed solids evolve through different

particle mechanisms, exhibiting variations in particle size distribution and solids

characteristics. The spraying of black liquor on bed solids forms a coating process that

leads to particle growth by layer formation. However, other particle mechanisms occur

depending on the operating conditions. Four basic particle development mechanisms

have been identified. Coating and agglomeration correspond to particle size growth, and

attrition and fragmentation correspond to particle size reduction.

1.3.1.1 Coating

Coating is defined as the particle growth mechanism in which one or several

consecutive layers of a substance accumulate on the particle surface.15 This mechanism

occurs when bed solids, which are usually the inorganic residue of steam reforming, pass

through the black liquor spraying zone. Liquor droplets and bed solids randomly collide,

forming coated solids to undergo drying, pyrolysis and steam reforming steps. A

schematic of the coating mechanism is presented in Figure 5.

1.3.1.2 Agglomeration

Agglomeration is defined as the particle growth mechanism in which two or more

particles adhere uncontrollably, yielding large undesirable agglomerates that can result in

bed defluidization and system shutdown.16 In general, agglomerates can be formed by: 1)

“sticky” particles and 2) droplet-particle collisions. The former occurs when the inorganic

residue of black liquor melts, and creates molten phases on particles that cause them to

12

Figure 5. Schematic of coating mechanism.

Heat

H2O(g) VOC H2O(g)

H2O(g)

H2

CO

CH4

Drying Pyrolysis

Steam reforming

Coating Inorganic residue

Bed solids

Liquor droplets

Collision Spray Nozzle

~Na2CO3 ~Na2CO3 ~Na2CO3 ~Na2CO3 ~Na2CO3

13

stick together. The latter occurs when large droplets capture several particles to create a

“cluster” of individual particles bound by liquor. Such agglomerates nevertheless

undergo drying, pyrolysis and steam reforming. The agglomeration mechanism is

presented in Figure 6.

1.3.1.3 Attrition

Attrition is defined as the particle reduction mechanism in which abrasive wear

between particles leads to the removal of asperities and fines from particle surfaces.17

Attrition includes two categories: 1) mechanical attrition, which involves the purely

physical interaction between particles, and 2) reaction-enhanced attrition, which consists

of the same abrasive wear, but is enhanced by surface weakening as solids react.

Industrially, attrition is seen as a particle degradation phenomenon, usually associated

with problems such as slight changes in particle size distribution (PSD) and the

generation of fines. The former affects bed solids quality and/or operation, whereas the

latter involves loss of material and particulate emissions.

1.3.1.4 Fragmentation

Fragmentation is defined as the particle reduction mechanism in which a rapid

fracturing of particles produces new particles that are all distinctly smaller than the

original ones.17 Fragmentation includes two categories: mechanical fragmentation, which

results from intense particle collisions, and reaction-enhanced fragmentation, which

consists of fracturing enhanced by particle structure weakening as solids react. This

mechanism is also a particle degradation phenomenon but with aggravated changes in

PSD. The attrition and fragmentation mechanisms are presented in Figure 7.

14

Figure 6. Agglomeration mechanism: (a) agglomerate formation due to particle-particle collision and (b) agglomerate formation due to droplet-particle collision.

Drying +

Pyrolysis +

Steam reforming

CapturingBed solids

Liquor droplets

CollisionSpray Nozzle

Agglomerate

Molten phase Agglomerate

Collision(a)

(b)

15

Figure 7. Particle reduction mechanisms: (a) attrition and (b) fragmentation.

Collision

Bed particles Fragments

(a)

(b)

Abrasive contact

Fines Attriting particles

16

1.4 Research Objectives

The research described in this thesis was conducted to achieve the following

objectives:

1. Identify and describe mechanisms that participate in particle size development in

a fluidized bed black liquor steam reformer;

2. Experimentally decouple each particle mechanism to study it separately;

3. Identify the important process variables for each particle mechanism under lab-

scale conditions, and

4. Interpret the experimental data to qualitatively address particle development in a

fluidized bed black liquor steam reformer.

17

CHAPTER 2

LITERATURE REVIEW

2.1 Platform for the Implementation of Black Liquor Gasification

Pulp and paper mills are large and complex facilities that consume significant

amounts of energy. Energy consumption represents the second largest cost among

material inputs and accounts for an average of 17% of the total material and energy costs

to the pulp and paper industry. However, pulp and paper mills generate about half of their

total fuel and electricity use.4

Energy use in integrated pulp and paper mills consists of the production of steam and

electricity. Steam is mainly used in wood digestion, black liquor evaporation, and pulp

and paper drying. Electricity is more uniformly distributed around the mill, but mostly

used in pumping, air-handling and lighting.18 The energy demand in pulp and paper mills

is partially satisfied by Tomlinson recovery boilers that produce high pressure steam to

generate electricity in steam turbines. The low pressure steam exhaust is then used for

process heating.19

This conventional steam cycle technology has certain drawbacks: (1) aging recovery

equipment, (2) low electricity-to-heat ratio, (3) high capital cost and (4) risk of

smelt/water explosions.20 This combination of factors makes this industry a good

candidate to adopt new energy technologies. However, any energy-related investment for

this industry represents a large portion of its economic resources and competes with other

18

capital investments; therefore, promising energy improvement projects must have

additional benefits such as improvements in productivity, chemical recovery,

environmental performance, and/or worker safety.4

Integrated gasification combined cycle (IGCC) technologies (based on biomass

gasification and gas turbine cycle configuration) have been identified as potential

alternatives to substitute for the conventional steam cycle, generating significantly (3 to

4.5 times) more electricity21, 22 and important reductions in carbon emissions.23 The

reason for this increment in electricity output is the high fuel calorific value of product

gases when biomass is gasified rather than combusted,24 whereas the reduction in

emissions is because more electricity is generated from gasification, thereby reducing the

import of fossil-based power to the mill. Thus, there is less net CO2 emitted into the

atmosphere.25 Economic studies focused on black liquor gasification combined cycle

(BLGCC) have shown that capital costs are similar to those for an equivalent Tomlinson

recovery boiler/steam turbine configuration, but that 100 – 175% more electric power is

generated.26, 27 In fact, a technical energy study in the Swedish pulp and paper industry

has manifested the potential of BLGCC technologies to minimize CO2 emissions.28

Black liquor gasification may also facilitate the implementation of autocausticization,

which is the direct regeneration of NaOH from Na2CO3 using a metal oxide like Al2O3,

Fe2O3 or TiO2. The generalized reactions for autocausticization are:

aNa2CO3 + bMxOy → aNa2O•bMxOy + aCO2 (10)

aNa2O•bMxOy + (a+z)H2O → 2aNaOH + bMxOy•zH2O (11)

19

This process may completely eliminate the lime cycle, and thus bring out some extra

energy and cost savings for the whole recovery system.29 A study on TiO2-based direct

causticization during black liquor gasification in a fluidized bed30 showed that this

process is technically possible and can achieve a causticizing efficiency of 97% as long

as temperature, air ratio and Na2O/TiO2 ratio are about 750 oC, 0.3, and 0.35,

respectively. Moreover, low-temperature black liquor gasification, where carbon

conversion is achieved below the melting temperature of the inorganic matter, prevents

smelt formation, thus avoiding smelt/water explosions and increasing worker safety. On-

site availability of biomass fuels, particularly black liquor, in pulp and paper mills makes

the implementation of such technologies less costly.

Despite the fact that some studies have exposed technical difficulties of black liquor

gasification as compared to Tomlinson recovery boilers, for instance costly gas cleaning

systems19 and the need for advanced building materials;31, 32 black liquor gasification is

nevertheless gaining more attention from the pulp and paper industry and has the

potential to attain the additional benefits that this industry seeks for significant capital

investments.

2.2 Black Liquor Gasification Technologies

Although several studies have evaluated the potential benefits of the overall BLGCC

process,20-30 a large portion of the pulp and paper industry’s attention has been centered

on alternative units to the Tomlinson recovery boiler. A comprehensive review33 of these

alternative technologies classified them into: solid-phase nongasification processes,

liquid-phase coking processes, and low and high temperature gasification processes.

Among the reviewed configurations, two process alternatives are on the verge of

20

becoming commercially successful: Chemrec high-temperature, downwards-flowing,

entrained-flow gasifier and MTCI low-temperature, bubbling fluidized bed steam

reformer.

The Chemrec system is a partial oxidation process carried out in a refractory-lined

entrained-flow quench reactor that operates at 950-1000 oC, above the melting point of

the inorganic matter in black liquor. This reactor is divided into a reaction zone and a

quench zone. In the reaction zone, organic material is gasified to yield fuel gases and

smelt droplets composed of sodium and sulfur compounds. In the quench zone, smelt

droplets are then separated from fuel gases and dissolved in a quench bath to form a

green liquor solution.12 A diagram of the Chemrec reactor is presented in Figure 8. One

of the main advantages of this system is its low tar production due to the high

temperature operation. However, molten inorganic compounds can create a severely

corrosive environment for both metals and refractories. Indeed, new phases, e.g. NaAlO2

formed as a result of reaction of the alumina refractory phase with sodium compounds,

can expand to generate significant stresses on the surface refractory.34 In addition, high

temperature processes may generate alkali aerosols (fume), which are known to cause

significant gas turbine blade corrosion.20 This major corrosion problem must be

overcome to be able to operate this unit reliably and economically for sufficient periods

of time.

The MTCI system is a gasification process based on steam reforming of black liquor

and carried out in a fluidized bed reactor that operates at about 600 oC, below the melting

point of the inorganic matter in black liquor. This reactor supplies the necessary heat for

the endothermic reactions by a set of “Pulsed-Enhanced” combustor tubes located within

21

Figure 8. The Chemrec reactor (Adapted from reference 32).

Gasifying agent Black liquor

Reactor

Quench

Syngas

Green liquor

22

the reactor.12 Black liquor is sprayed onto the bed solids where it quickly dries, pyrolyzes

and reacts with steam to form a fuel syngas that is hydrogen rich (63 – 65%)35 and has a

medium heating value of 9 – 11 MJ/Nm3.36 The inorganic residue remains solid and

contributes to the evolution of the starting bed material. As residue is being formed,

batches of bed solids are periodically drained to proceed for further causticization. A

diagram of the MTCI reactor is presented in Figure 9.

The principal advantage of the MTCI system over the Tomlinson recovery boiler or

Chemrec technology is its low-temperature operation. This prevents smelt formation and

involves less heat loss, no risk of smelt/water explosions and a reduced corrosive

environment for both metals and refractories.34 Although low-temperature operation

decreases the rate of carbon conversion, alkali compounds in the remaining char act as

catalysts for the steam reforming reactions.37 In addition, the resulting H2/CO gas

environment contributes to the reduction of sulfur compounds into hydrogen sulfide24

while achieving near complete sulfur-sodium separation from the salts present in the

black liquor,38 as shown by equations 6-9. However, the tendency for bed

agglomeration39 and the efficient removal of tars40 still remain as major technical barriers

for successful implementation of MTCI’s fluidized bed system.

2.3 Fluidized Bed Technology

Industrial applications of fluidized beds range from basic physical operations (drying,

coating, adsorption, etc.) to complicated chemical operations such as synthesis reactions

(Fischer-Tropsch synthesis, polymerization, etc.), hydrocarbon cracking, calcination,

incineration, combustion and gasification.41 Among the many contacting methods

between liquid, gas and solids, fluidized beds provide a fluid-like behavior of solids that

23

Figure 9. The MTCI reactor

(Adapted from reference 42).

Fluidized bed

Syngas

Steam

Black liquor

Bed solids

Tube heaters

24

greatly enhances phase mixing, isothermal operation, and heat and mass transfer rates.41

Such features make fluidized bed technology suitable for steam reforming of black

liquor. Indeed, gasification via steam reforming of straw black liquor using a fluidized

bed reactor achieved 75% carbon conversion to gas, and cold gas efficiency of 90%.43

Moreover, bed solids provide enough surface area for black liquor to dry, pyrolyze and

react with the gasifying agent.

Numerous studies have been carried out on fluidized bed applications in thermo-

chemical conversion of biomass fuels.39, 43-55 In general, it can be said that fluidized bed

performance is strongly dependent on bed particle characteristics, which result from

physical and chemical phenomena inherent in this type of solid-gas-liquid configuration.

Such phenomena will be called particle mechanisms from now on.

Fluidization of particles varies greatly among gas-solid systems and numerous factors

affect it, including type of solids, process, geometry, and PSD.41 PSD is of fundamental

importance to the performance and operation of fluidized beds, because it influences

fluidization behavior, transport and kinetic properties, and process considerations for

operation.56 Effects of PSD in fluidized beds have been extensively investigated. For

example, a wide PSD of a fluid cracking catalyst during ozone decomposition showed to

have high chemical conversion, reactor efficiency and regime transition (from bubbling

to turbulent).57 Kunii et al.41 also noted that with a wide PSD, the presence of fines act as

a lubricant for large particles helping to improve the quality of fluidization. Lv et al.44

observed that smaller particles produced more CH4, CO, C2H4 and less CO2, being more

favorable for gas quality and yield, when gasifying biomass particles in a fluidized bed.

Attempts to measure PSD in fluidized beds56, 58, 59 have been made in order to obtain

25

accurate, on-line measurements of this important parameter to improve plant safety,

product quality, and process performance in many fluidized bed processes.

PSD is influenced by many factors, including gas-liquid injection, solids feed,

discharge, elutriation, and particle growth and shrinkage.56 However, PSD seems to be

mostly dependent on the changes to the size of individual particles.60 In general, typical

mechanisms that change the size of individual particles are particle coating,

agglomeration, attrition and fragmentation. These mechanisms comprise chemical and

physical particle interactions that can have a significant effect on their microscopic and

macroscopic behavior.61

2.3.1 Coating

Coating is defined as the particle growth mechanism in which one or several

consecutive layers of a substance accumulate on a particle surface.15 Beyond thermo-

chemical conversion of biomass fuels, fluidized bed coating and agglomeration are

broadly practiced in areas such as detergent manufacturing, food processing, and

especially in the pharmaceutical industry.16 Some of the reasons for particle size

enlargement by coating are: 1) to improve the flow and physical properties of powders; 2)

to produce a desired PSD or product geometry; 3) to improve appearance, taste, odors of

particles, and 4) to protect particles from oxygen, humidity, light, etc.15 The influence of

this mechanism on bed solids has been investigated, e.g., studies on fluidized bed coating

and agglomeration using sand particles under spraying of an aqueous solution of NaCl

concluded that coating is a size-dependent phenomenon, which occurs principally on

particles larger than 200 µm.15, 16 Calculation of heat-transfer coefficients from fluidizing

particles to immersed metallic spheres reported lower values for the coated particles than

26

those for the uncoated ones.62 Seville et al.61 also found that bed particles in a fluidized

bed can become more cohesive with the addition of thin layers of liquids. These studies

clearly show that coating can have important effects on the particle size as well as on the

thermo-physical properties of the bed material.

The importance of coating in fluidized bed combustors and gasifiers relies on the

direct relation between coating and agglomeration. Fluidizing particles provide surface

area for ash deposition during either combustion or gasification. Cammarota et al.45

showed that co-deposition of sodium compounds and carbon resulted in a compact

coating on sand particles during incineration of slurry waste in a fluidized bed.

The formation of coating layers on different bed materials was studied in the fluidized

bed combustion of various biomasses.25 This study concluded that the composition of the

coating layers depended on both the nature of bed material and fuel. One to three

superimposed layers were observed on bed particles, with the innermost layer rich in

alkali silicates, whereas the outermost layer was rich in calcium and magnesium. Coating

layers containing sodium or potassium may be adhesive and can cause the formation of

agglomerates, while coating layers containing magnesium may protect the bed particles

from agglomeration.25

For high-alkali-containing biomass fuels, direct attack by potassium compounds into

quartz sand results in low-melting potassium silicates.51 Visser et al.63 suggested that

coating layers formed with time on the surface of the bed material might form necks

between coatings of individual particles at certain critical conditions (coating thickness,

temperature, etc.), which initiates agglomeration. Coating of bed solids during steam

reforming of black liquor may follow the formation of superimposed layers driven by

27

physical residue deposition, since bed solids used as fluidization media result from the

same inorganic residue of the black liquor processing. Therefore, any agglomeration

tendency in the MTCI system would derive from factors other than the formation of new

phases. Typically, coating has been studied by using scanning electron microscopy

(SEM) and electron dispersive X-ray spectroscopy (EDS) to observe layer build up and

elemental distribution.15, 25, 50, 52

2.3.2 Agglomeration

Agglomeration is defined as the particle growth mechanism in which several particles

adhere uncontrollably,16 yielding large undesirable agglomerates that might result in bed

defluidization and system shutdown.46-52, 64-67 To avoid early defluidization during

operation of full-scale fluidized bed combustors or gasifiers, frequent bed change is the

common method applied to ensure problem-free operation, but this is associated with

additional costs and is generally not sustainable on a long-term basis.66 Typically,

agglomeration is studied using bench-scale fluidized beds to perform controlled

agglomeration tests46-52, 64, 65 where correlations between ashing temperature, fuel/bed

material chemical composition, flow rates, defluidization time, etc., can be determined.

This mechanism has been also studied by sampling and analyzing (with SEM/EDS

techniques) bed materials collected in commercial fluidized beds.63, 66-71

Agglomeration is a particle phenomenon that occurs in both combustion and

gasification. With regard to these operational modes, Öhman et al.50 showed no

differences in bed agglomeration tendencies or bed particle layer characteristics when

performing controlled agglomeration tests of six biomass fuels. Three different routes for

agglomeration have been suggested in the literature: 1) melt-induced agglomeration,63 for

28

bed material glued together by ash-derived melt phases attributed to local peak

temperatures; 2) coating-induced agglomeration,63 for the formation of consecutive layers

on bed particles that at certain conditions (coating thickness, temperature, chemical

composition, etc.) might form necks between the coated layers of individual particles,

thus initiating agglomeration, and 3) droplet-induced agglomeration,72 whereby several

bed particles are captured by a droplet of liquid fuel.

Characterization studies46-48, 50 of bed materials during combustion and gasification of

biomass fuels have demonstrated the strong chemical role of the inorganic constituents in

both fuels and bed materials and their dependence on temperature. Öhman et al.47

reported that the composition of coatings on bed particles was mostly constricted to the

ternary systems K2O-CaO-SiO2 during combustion of biomass fuel in a bench-scale

fluidized bed. This study also pointed out to the limited participation of sulfur or chlorine

in agglomeration. In addition, Ghaly et al.67 depicted the chemical interaction between

the silica (SiO2) and potassium oxide (K2O) at elevated temperatures (850-1000 oC) when

burning sand-straw ash mixtures using a high-temperature furnace. Studies performed by

Lin et al.48 and Brus et al.52 agreed on the formation of layers of low-melting potassium

silicates as precursors to molten phases directly associated with melt-induced and

coating-induced agglomeration. This chemical activity has also been observed during

coal combustion in fluidized beds.49, 64, 65, 68 Similar conclusions were drawn from these

studies, in which high ash content is linked to coating molten phases and subsequent

particle bonding and loss of fluidization.

Physical interactions between liquid droplets and bed particles in fluidized beds can

initiate droplet-induced agglomeration. Leclère et al.72 stated that agglomeration

29

processes may occur when liquid droplets are injected into a hot fluidized bed following

these paths: 1) when droplets are larger than a critical value and capture several particles,

and 2) when the amount of liquid injected per unit area is larger than a critical value and

the bed is saturated. The second path resembles what Visser et al.63 discussed about

coating formation at certain conditions; that is, a tendency for agglomeration as a result

of increasing coating thickness due to liquid overfeeding. In a complementary manner,

liquid-solid distribution in the spraying region plays another important role in

agglomeration. McMillan et al.73 developed an experimental method to measure the

quality of the liquid-solid mixing when a gas-liquid jet is injected into a fluidized bed of

coke particles. The measuring technique used temperature to characterize the liquid-solid

distribution and to determine the mixing uniformity. House et al.74 also noted that good

and uniform contact of liquid droplets with solid particles enhances the transport

mechanisms between gas, liquid and solids species. On the other hand, droplet-induced

agglomeration may see interparticle bond breakage as a result of increasing fluidization

velocity as was shown by Smith et al.75

Steam reforming of black liquor in a fluidized bed has operational characteristics that

make it susceptible to agglomeration tendency. Care must be taken to prevent the

occurrence of this mechanism to improve performance and reliability.

2.3.3 Attrition

Attrition is defined as the particle reduction mechanism in which an abrasive wear

between particles leads to the removal of asperities and fines from particle surfaces.17

This mechanism is intrinsic of fluidized beds and has been often associated with material

loss and particle degradation.76 This situation is seen in fluidized-bed catalytic cracking

30

units, where losses of catalyst come directly from catalyst attrition.77 Forsythe et al.77

developed an accelerated-attrition test for measuring the relative resistance of a catalyst

to attrit. This study concluded that attrition caused a rounding off of sharp edges of

catalyst particles, which led to their degradation. Gwyn78 expressed the rate of catalyst

attrition as a function of particle size and time and indicated the influence of initial PSD

and mean particle diameter on attrition. However, attrition on shrinking fuel particles in

fluidized beds seems to be more complex. Chirone et al.79 experimentally described

differences between two types of char attrition during combustion of coal particles in a

fluidized bed: 1) mechanical attrition, and 2) reaction-enhanced attrition. The former is

characterized by the rapid decay of the elutriation of attrited carbon from an initial high

rate typical of angular solids to a lower stationary rate when particles are rounded off.

The latter is characterized by the enhanced detachment of asperities due to weakening of

the particle surface as the solid reaction progresses. Results from this study showed that

reaction-enhanced attrition is 10 times larger than mechanical attrition. Ray et al.80

pointed out the importance of the interactions between coal particles and bed particles

(limestone) and suggested that large coal particles and small limestone particles are more

desirable to reduce attrition in a fluidized-bed coal combustor.

With respect to biomass conversion, Scala et al.55 and Leckner et al.54 concluded that

biomass char conversion during combustion is largely due to generation of carbon fines,

followed by postcombustion during their residence time in the bed. Both studies noted

percolative fragmentation,81 or loss of particle connectivity due to enlargement and

overlapping of pores, as the prevailing mechanism of carbon fines generation in the bed.

This result coincides with Chirone’s work on coal combustion; therefore, it can be said

31

that percolative fragmentation refers to reaction-enhanced attrition. In general, models

and experimental observation have found attrition to be proportional to fluidization

velocity, as well as mass inventory and particle characteristics (size, strength, etc.).17, 82, 83

Despite the fact that attrition has been mostly conceptualized on coal processing, it is

valuable to include it into the general particle description for steam reforming. Although

black liquor steam reforming is not based on injection of liquor particles, it makes sense

to recognize that liquor-coated particles may suffer some size reduction due to attrition

and may possibly offset particle growth by reducing coating thickness at the cost of fines

generation.

2.3.4 Fragmentation

Fragmentation is defined as the particle reduction mechanism in which a rapid

fracturing of particles produces new particles that are all distinctly smaller than the

original ones.17 This mechanism has been coupled with attrition in some experimental

and modeling studies of coal combustion.84-87 Walsh et al.87 measured the formation of

small char particles to determine the parallel occurrence of attrition and fragmentation

during combustion of large coal char particles. The conclusion of this study resembles the

work of Chirone et al.79 and Salatino et al.81 Brown et al.84 also reported that percolative

fragmentation makes coal particle surfaces susceptible to mechanical abrasion.

Additionally, Zhang et al.85 and Stubington et al.86 suggested that fragmentation occurs

early in the devolatilization stages and that the content of volatile matter can drastically

influence the degree of fragmentation of coal particles. Stubington et al.86 also defined

two types of fragmentation: 1) primary fragmentation, occurring during devolatilization

32

and 2) secondary fragmentation, occurring during conversion of the char by consumption

of carbon bridges connecting parts of the particles.

In terms of biomass processing, Miccio et al.53 confirmed that percolative

fragmentation takes place in parallel with carbon postconversion during bubbling

fluidized bed gasification of a biomass fuel. The results of these experiments showed that

fines generation is relevant during gasification, and postconversion of particles takes

place after their release from the bed. Fragmentation is conceptualized on the framework

of coal conversion; however, results from fluidized-bed biomass studies indicate that the

same conceptualization is valid for such systems. The configuration of the liquor feeding

in the black liquor steam reforming system points toward fragmentation as a mechanism

that might not be expected to occur at all.

In summary, fluidized bed steam reforming of black liquor has operational

characteristics that may involve the participation of the various particle mechanisms

described above. This fluidized bed system is intended to convert black liquor while

taking advantage of the available surface area of the bed material, through the process of

coating. However, operational parameters such as temperature, liquor feed rate and

spraying, and fluidizing velocity make this system susceptible to agglomeration, attrition

and possibly fragmentation. A compilation of these mechanisms is shown in Figure 10.

From this diagram, the relevance of each mechanism with respect to each other, as well

as overall particle size development, can help to qualitatively address particle

development in this process.

33

Figure 10. Schematic map of particle mechanisms.

Coating

Agglomeration Attrition

Fragmentation

Bed solids

Layer formation

Mechanical

Reaction-enhanced

Mechanical

Melt-induced

Coating-induced

Droplet-induced Neck formation

Reaction-enhanced

34

CHAPTER 3

EXPERIMENTAL

3.1 Introduction

The different experimental apparatus and procedures used to study mechanisms that

contribute to particle size evolution in this fluidized bed system are described in this

chapter. Experimental decoupling of these mechanisms was necessary in order to provide

basic understanding of each one under specific laboratory conditions.

Samples of bed solids from two commercial steam reforming units (Georgia-Pacific

Corporation and Norampac Inc.) were used for different testing campaigns. These

commercial units are based on the MTCI design. All the bed solids samples used in this

study were within a size range of 75 – 1300 µm and had a chemical composition of 75 –

80% wt. Na2CO3, 10 – 20% wt. C and 0 – 5% wt. CaCO3. Samples of black liquor from

Georgia-Pacific’s Big Island mill with 40% solids content were used for all tests that

required liquor feed.

3.2 Experimental Set-up

3.2.1 Coating Studies

Coating was studied by visual inspection of cross-sectional areas of samples of bed

solids. Layer formation was then addressed by characterization of layers observed around

cores of cross-sectioned particles. Scanning electron microscopy (SEM) and Energy

35

dispersive X-ray spectroscopy (EDS) were the analytical techniques used to characterize

the cross-sections. In SEM, an electron beam is scanned across the samples surface and

when the electrons strike the sample, a variety of signals are generated. The detection of

such signals is what produces images for analysis. SEM can provide high-quality imaging

of samples at large magnifications. In EDS, interaction of the primary electron with

atoms in the sample causes shell transitions, which result in the emission of an X-ray.

The emitted X-ray has an energy characteristic of the parent element. The detection and

measurement of such energy is what permits elemental analysis. EDS can provide rapid

qualitative and quantitative analysis of elemental composition as well as maps or line

profiles for elemental distribution in a sample surface.

Samples of bed solids from Georgia-Pacific were sieved using meshes No. 30, 40, 50,

70, 100, 140, and 200. Samples were mounted in epoxy discs and polished for

approximately 10 h to obtain cross-sectional areas. Three kinds of sandpaper (240, 400

and 600 grit), diamond paste and kerosene were used as polishing agents. After polishing,

the epoxy discs were coated with a very thin film of gold for imaging and elemental

distribution analyses.

3.2.2 Agglomeration Studies

Agglomeration was studied by identifying the effect of liquor droplets on PSD and

solids characteristics. A lab-scale fluidized bed reactor was used to pyrolyze black liquor

droplets in a hot bubbling fluidized bed. Liquor droplets were added from the open top of

the reactor and limestone, glass beads and Norampac bed solids were used as bed

materials. Nitrogen (N2) was used as fluidizing gas. The experimental apparatus was

comprised of three sections: the reactor, the heating system and the pumping system. The

36

reactor was a stainless steel (SS) tube of 0.047 m ID and 0.254 m height placed vertically

on a SS-machined cup. This cup held inside two perforated SS discs compressing a fine

mesh used for even distribution of the fluidizing gas. The heating system was composed

of an 1100 W heating rod placed inside of a 1900 W tube furnace. The pumping system

used a peristaltic tubing pump that transported black liquor from a hot reservoir to the top

of the reactor. A schematic of the experimental set-up is presented in Figure 12.

Samples of bed materials (approx. 200 g) were loaded into the reactor and hot air was

circulated through the system. At the desired bed temperature, air was switched to N2 and

heated black liquor was pumped from the reservoir. Experiments were carried out at bed

temperatures between 450 – 600 oC with flowing N2 at 15 scfh and liquor flow rates of

0.5 and 1 ml/min over periods of 8 – 12 h. A summary of the experimental conditions of

the agglomeration tests is presented in Table 2. Bed samples were generally taken at 45

min, 1.5, 3, 6 and 12 h. It is important to note that samples of 40%-solids black liquor

from Georgia-Pacific were diluted to 12% solids in order to facilitate liquor pumping at

lab scale.

3.2.3 Attrition Studies

Mechanical attrition was studied by tracking the generation of fines and the bed’s

PSD evolution during cold fluidization of samples of bed solids from Georgia-Pacific and

Norampac. A cold flow model of the University of Utah fluidized bed steam reformer

was built and used for visualization of reactor flows and interpretation of data. It was

designed to simulate the bed behavior of the real system such that the dimensionless

scaling parameters (Reynolds number, density ratio, Froude number and geometric

similarity of distributor, bed, particles) were the same as the real unit. For this reason, the

37

Figure 12. Schematic of the agglomeration apparatus.

N2

Black liquor reservoir

Heating section

Pumping section

SS tube Air

Rod

Furnace

SS-machined cup

Reaction section

Flow meter

Peristalticpump

T

T

T

T

38

Table 2. Summary of experimental conditions for agglomeration tests.

Note:

• Harmonic mean particle diameter (HMDp) is the reciprocal of the sum of the

reciprocals divided by the total.

• Standard cubic feet per hour (scfh).

Experiment Parameter

1 2 3 4 5

Pyrolysis time (h) 8 12 12 10 8.5

Bed material: Limestone Glass beads Norampac Norampac Norampac

Weight (g) 150 200 200 200 200

Size range (µm) 180 – 351 0 – 2800 0 – 2800 180 – 351 180 – 351

HMDp (µm): 265.5 231.31 176.36 265.5 265.5

Temperature(oC): 547 547 600 525 450

Fluidizing gas: N2 N2 N2 N2 N2

Flow rate (scfh): 15 15 15 15 15 Fluidizing velocity

(m/s) 0.21 0.21 0.22 0.19 0.17

Black liquor:

Solids (%) 3 12 12 12 12

Flow rate (ml/min) ~1 ~1 ~ 1 ~ 0.5 ~0.5

39

study of purely mechanical attrition could reliably be accomplished in this unit. This cold

flow model was made of a Plexiglas column of 0.165 m ID and 1.52 m height with four

tube bundles that were transversely located along the height of the bed. Each bundle

contained 20 glass tubes of 0.011 m OD. The distributor was made of two plates with

forty-two 0.014 m holes evenly distributed over the area. Two layers of tight-weave

fabric were sandwiched between these plates to create high-pressure drop across the

distributor to ensure even distribution of the fluidizing gas. A 90-degree elbow was

placed at the top of the unit to hold a fine filter that collected elutriated material. A

photograph of the cold flow model is shown in Figure 13.

Samples of bed solids were loaded to approximately 0.31 m height and fluidized with

air at room temperature. The fluidization velocity (Uf) was set to 0.24, 0.32 and 0.43 m/s,

which correspond to 3, 4.5 and 6 times the calculated minimum fluidization velocity

(Umf), respectively. The Umf is given by Kunni et al.41:

( ) ( )

μ

ρ−ρρ=⎟⎟

⎠

⎞⎜⎜⎝

⎛μ

ρ

φεε−

+⎟⎟⎠

⎞⎜⎜⎝

⎛μ

ρ

φε

gdUd1150Ud75.1 gsg3pgmfp

2S

3mf

mf

2gmfp

S3mf

(12)

where

dp is the mean particle diameter (µm),

g is the acceleration of gravity (9.8 m/s2),

Umf is the minimum fluidization velocity (m/s),

εmf is the voidage at minimum fluidization conditions,

µ is the gas viscosity (kg/m-s),

ρg is the gas density (kg/m3),

ρs is the particle density (kg/m3), and

40

Figure 13. Photograph of the University of Utah cold flow model of the fluidized bed steam reformer unit.

41

Sφ is the sphericity.

Each test was carried out for 9 h and bed samples were collected at different lengths

of time (0, 4, and 9 h) through a sampling port located at the bottom of the cold flow

model. Elutriated fines were collected hourly from the top filter for weighing and sieving.

A summary of the operational parameters used in these mechanical attrition tests is

presented in Table 3.

3.2.4 Fragmentation Studies

Fragmentation was studied by two experimental approaches, physical and chemical.

The physical approach included compression tests and cyclone tests. Compression tests

were performed to determine the load required to break individual particles of bed solids

into fragments. Cyclone tests were performed to determine the superficial velocity

required to break samples of bed solids into fragments. The chemical approach included

gasification tests. These tests were performed to determine reaction conditions at which

samples of bed solids broke into fragments, because the carbon-based bridges between

particles reacted away. Samples of bed solids from Georgia-Pacific and Norampac were

only used in the physical tests. Agglomerate samples generated during agglomeration

tests were used in both physical and chemical tests.

3.2.4.1 Compression Tests

The department of Mechanical Engineering at the University of Utah has an

INSTRON 4303 load cell for material testing. It consists of a vertical movable load cell

that compresses samples placed between two parallel plates. A lab telescope located in

front of the fixed-bottom plate is used to follow particle breakage as load is exerted. A

42

Table 3. Summary of experimental conditions for mechanical attrition tests.

Note:

• * Sample without fine particles of size below 106 µm.

• Minimum fluidization velocity (Umf).

Test #

Sample Weight (kg)

Time (h)

Gas flow rate (scfh)

Uf (m/s)

1 Norampac 8 9 500 3 x Umf = 0.2313

2 Norampac* 8 9 515 3 x Umf = 0.2406

3 Georgia-Pacific 8 9 910 6 x Umf = 0.426

4 Georgia-Pacific 8 9 680 4.5 x Umf = 0.3195

43

picture of this equipment is shown in Figure 14.

One single particle or agglomerate was selected and placed on the fixed-bottom plate.

The top plate was lowered enough to barely touch the sample. The starting distance

between plates was reset and load was exerted on the sample until breakage. Ten

compression tests were performed for each selected size range. Readings of exerted load

versus plate displacement were collected for each sample. A summary the experimental

conditions of the compression tests is presented in Table 4.

3.2.4.2 Cyclone Tests

A glass cyclone of 0.33 m height with a reducing body diameter of 0.063 m to 0.012

m was used for these tests. This cyclone has an inlet of 0.02 m ID and 0.063 m length and

a top gas outlet of 0.025 m ID and 0.09 m length. A nozzle was inserted in the inlet to

generate a circulating stream of air and solids. Air flow was regulated by a flow meter

located upstream. A fine filter was placed at the top outlet in order to retain any elutriated

material. A picture of the glass cyclone is shown in Figure 15.

Samples of bed solids as single particles and agglomerates were circulated through

the cyclone at different superficial velocities for 1 h. Material carried away with the

exiting air was recovered in a fine filter. After circulation, the samples were weighed and

sieved to determine the percentage of fragmented material. This percentage was

calculated by taking the ratio between the weight of the final sample that passed through

the screen corresponding to the lower limit size of the initial sample, to the weight of the

initial sample.88 A list of the experimental conditions used in these tests is presented in

Table 5.

44

Figure 14. The University of Utah INSTRON 4304 load cell.

45

Table 4. Experimental conditions of compression tests.

Note:

• Harmonic mean particle diameter (HMDp).

Test #

Sample Size fraction (µm)

HMDp (µm)

Compression speed

(mm/min) 1 Norampac 180 – 350 265 1

2 Norampac 350 – 700 525 1

3 Georgia-Pacific 150 – 212 181 1

4 Georgia-Pacific 300 – 425 362 1

5 Norampac agglomerates 350 – 700 525 1

6 Norampac agglomerates 700 – 1400 1050 1

7 Glass-bead agglomerates 350 – 700 525 1

8 Glass-bead agglomerates 700 – 1400 1050 1

46

Figure 15. Glass cyclone set-up.

47

Table 5. Summary of experimental conditions of cyclone tests.

Note:

• Norampac agglomerates refer to samples of Norampac bed solids used during

agglomeration studies.

Test #

Sample Size range (µm)

Initial weight

(g)

Air flow rate

(scfh)

Air superficial

velocity (m/s)

1 180 – 350 10 75 8.8

2 180 – 350 10 100 11.7

3 180 – 350 10 125 14.6

4 180 – 350 10 150 17.6

5

Norampac single particles

180 – 350 10 175 20.5

6 350 – 700 10 75 8.8

7 350 – 700 10 100 11.7

8 350 – 700 10 125 14.6

9 350 – 700 10 150 17.6

10

Norampac agglomerates

350 – 700 10 175 20.5

48

3.2.4.3 Agglomerate Break-up by Chemical Reaction

This experimental set-up consisted of a quartz tube of 0.025 m ID and 0.5 m length

placed horizontally along a rapid-heating tube furnace. A type-K thermocouple was

radially centered into the quartz tube, which held a ceramic boat of 0.065 m length. Gases

were regulated by two flow meters located upstream of the furnace. A picture of the

experimental apparatus is shown in Figure 16.

Samples of glass-bead agglomerates were placed into the ceramic boat and heated up

to temperatures between 600 – 800 oC. A gasifying agent (CO2) was circulated through

the quartz tube to react with the sample according to reaction 5. Experiments were carried

out for times ranging from 30 min to 24 h. Final samples were then weighed and sieved

to determine the percentage of fragmented material. This percentage was calculated by

taking the ratio between the weight of the final sample that passed through the screen

corresponding to the lower limit size of the initial sample, to the weight of the initial

sample.88 The experimental conditions of gasification tests are presented in Table 6.

49

Figure 16. Tube furnace set-up.

50

Table 6. Summary of experimental conditions of gasification tests.

Note:

• Glass-bead agglomerates refer to samples of glass beads used during

agglomeration studies.

Test #

Sample Size range (µm)

Initial weight (g)

Temperature (oC)

Time (h)

1 350 – 700 1 600 0.5

2 350 – 700 1 650 24

3 350 – 700 1 700 1.75

4 350 – 700 1 720 2

5 700 – 1400 1 730 1.25

6

Glass-bead agglomerates

350 – 700 1 800 2

51

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Coating Visual Analysis

Mounted samples of bed solids from Georgia-Pacific were visualized using SEM at

different levels of magnification. Some bulk images of selected size ranges at 15 kV and

x35 magnification are shown in Figure 17. Important characteristics are observed when

comparing this set of images. Individual particles are present in all samples regardless of

their particle size. No agglomerates are seen and certain fragments are observed in

samples within 150 – 212 and 300 – 425 µm. It is difficult to say whether these fragments

derive from some kind of fragmentation mechanism. Perhaps, these fragments are the

result of sections of mounted particles not taken through the center of the particle, or the

result of incomplete sieving. Cracking on some cross-sectional areas is seen in most of

the size ranges. Some cracks are located in the center of particles while others are

forming a type of round groove around cores. Analysis of carbon content in a sample of

bed solids yields about 18% wt. carbon, as shown in Figure 18. More details about the

analysis of carbon content are presented in the Appendix. Possibly, this cracking results

from removal of the portion of localized nonvolatile organic matter of the sample during

the polishing process. This localized nonvolatile organic matter does not mean it is not

well distributed across the particle matrix, but such round grooves would indicate the

place of initial contact between a bed particle and a coating layer. It is also possible that

52

Figure 17. SEM images of selected size fractions of bed solids from Georgia-Pacific at 15kV and x35. Size: (a) 75 – 106 µm, (b) 150 – 212 µm, (c) 300 – 425 µm, and (d) 425 – 600 µm.

(a) (b)

(c) (d)

53

Figure 18. Chemical composition of a sample of bed solids from Georgia-Pacific.

CaCO35%

Na2CO377%

C 18%

54

cracks result simply from thermal shock of particles as they are heated and cooled for

multiple testing campaigns. Another general characteristic of these bed particles is

roundness due to solids agitation and inherent attrition of fluidized beds, helping shape

particles in this way.

A closer visualization of individual particles for different sizes is shown in Figure 19.

Interesting details of these cross-sectional areas are depicted by these images. The

formation of distinct layers is not characteristic for particles below 212 µm. On the other

hand, layer formation is distinguishable on particles over 250 µm as a result of the pattern

given by these round grooves. This result agrees with the work of Saleh et al.16 on

particle coating. Coating of these bed particles seems to consist of one or two

superimposed layers, and more than two layers are not observed on any single particle.

Previous research25, 50 has also reported the formation of one to three superimposed layers

during combustion or gasification of biomass fuel in lab-scale, pilot-scale and full-scale

fluidized bed units. However, it is instructive to note that those studies used solid fuel

feed compared to the liquid fuel feed of this fluidized bed system. Comparing the

structure of cross-sectional areas of bed particles below and over 250 µm, it can be said

that development of bed particles over 250 µm mainly results from coating, while the

development of bed particles below 250 µm might result from another mechanism.

According to Figure 19, the structure of cross sections of bed particles below 250 µm

suggests that their origin could be attributed to atomization. Perhaps, the liquor

atomization yields a range of droplet sizes that might include droplets that dry and

pyrolyze before contacting any bed particle, thus forming new solids within 0 – 250 µm.

Visualization of the edges of bed particles confirms the absence of defined layers on bed

55

Figure 19. SEM images of individual bed particles. Size range and magnification: (a) 75 – 106 µm and x900, (b) 150 – 212 µm and x400, (c) 300 – 425 µm and x200, and (d) over 600 µm and x90.

(a) (b)

(c) (d)

56

particles smaller than 250 µm, as shown in Figure 20.

Estimated measures of layers for large bed particles show thickness to increases with

particle size, as shown in Figure 21. For particle sizes within 300 – 425, 425 – 600 and

over 600 µm, layer measures were about 67, 87 and 181 µm, respectively. Although,

similar layer thicknesses were reported by Nuutinen et al.,25 typically within a range of 50

– 70 µm, with a maximum thickness of 100 µm, the cores of coated particles analyzed in

that study yielded sizes over 100 µm. Coating occurs on smaller particles because of fuel

feeding. In that study, solid fuel particles react to yield an ash, which deposits on bed

particles to form layers of residue. In this fluidized bed system, liquid liquor directly

coats bed particles to react and form layers of residue. This direct coating process may

become self-inducing if coated particles reach sizes that make their Umf larger than the

Uf. This difference in velocity might contribute to localize relatively large particles lower

in the bed, which in turn would make them more susceptible to reach again the spraying

zone. Moreover, Georgia-Pacific’s samples have about 85% of bed particles over 250 µm

(see Figure 22), which makes them more likely to grow by coating. Although it is

important to recognize that there exists a growth pattern on bed particles over 250 µm, it

is very difficult to determine the exact correlation between particle size and layer

thickness from these samples, since these have undergone multiple testing campaigns in

the Georgia-Pacific’s Big Island unit.

EDS analysis on cross-sectional areas shows that major elements are: Na, Ca, K, Cl

and Mg. Some traces of S were detected but in very low proportions compared to the

other components. This relative absence of S in the solid inorganic residue is caused by

the reduction of Na2S to H2S as shown by equation 9.

57

Figure 20. SEM images of edges of small bed particles. Size, magnification and thickness: (a) 75 – 106 µm, 15kV x1.8k, (b) 106 – 150 µm, 15kV x1.2k, and (c) 150 – 212 µm, 15kV x1.2k.

(a)

(b)

(c)

58

Figure 21. SEM images of edges of large bed particles. Size, magnification and thickness: (a) 300 – 425 µm, 15kV x700, 66.8 µm, (b) 425 – 600 µm, 15kV x150, 87 µm, and (c) over 600 µm, 15kV x90, 263 µm and 181 µm.

(a)

(b)

(c)

59

Figure 22. PSD of bed solids from Georgia-Pacific’s Big Island steam reforming unit

0

0.1

0.2

0.3

0.4

0.5

0-75 75-106 106-150 150-212 212-300 300-425 425-600 >600

Size range (micron)

Mas

s fra

ctio

n

60

EDS analyses across some bed solids are shown in Figures 23 to 25. Bed particles in

Figures 23 and 24 show uniform distribution of Na across their cross sections, suggesting

that these particles are mainly composed of Na2CO3. On the other hand, bed particles in

Figures 25a and 25b show a decrease of Ca from cores to edges, which suggests that

starting bed solids (CaCO3) evolve in time by deposition of layers of Na2CO3 residue.

Sanchez et al.43 also showed CaCO3 substitution by char and inorganic matter from black

liquor fed in a fluidized bed steam reformer.

4.2 Agglomeration Results

Large changes in PSD were observed after addition of black liquor droplets into the

hot bubbling fluidizing bed. Comparative graphs of initial and final PSD for each

experiment are shown in Figures 26 to 30. The excessive particle growth obtained after

each test is evident, particularly at high solids content and liquor flow rate, where

HMDps doubled and tripled their initial values as shown by Figures 27 and 28,

respectively. Similar distributions were obtained after pyrolyzing 3% and 12%-solids

liquor droplets at 0.5 and 1 ml/min as shown by Figures 26, 29 and 30. Starting with bed

samples within 180 – 350 µm, final distributions were spread across larger fractions

making the HMDp larger. It was necessary to adjust the fluidizing gas flow rate in order

to maintain fluidization.

Some important observations can be made from comparisons between these

experiments. At similar temperatures and black liquor volumetric flow rates, pyrolysis of

12%-solids liquor droplets yielded a final distribution with a HMDp about 200 µm larger

than 3%-solids liquor droplets as shown by Figures 27 and 26, respectively. This suggests

that the percentage of black liquor solids is a factor that highly influences particle growth.

61

Figure 23. EDS line analysis of a Georgia-Pacific bed particle. Size: 75 – 106 µm.

Cl

0

0.5

1

Inte

nsity

Distance (μm) 0 45

Ca

0

0.5

1In

tens

ity

Na

0

0.5

1

Inte

nsity

K

0

0.5

1

Inte

nsity

0 45Distance (μm)

62

Figure 24. EDS line analysis of a Georgia-Pacific bed particle. Size: 212 – 300 µm.

Na0

0.5

1

Inte

nsity

Ca

0

0.5

1In

tens

ity

Cl

0

0.5

1

Inte

nsity

0 85Distance (μm)

K

0

0.5

1

Inte

nsity

0 85Distance (μm)

63

Figure 25. EDS spot analysis of Georgia-Pacific bed solids. Size: (a) 425 – 600 µm, and (b) 0ver 600 µm

0

0.25

0.5

0.75

1

Na Ca K Cl

Inte

nsity

Core Inner layer Outer layer

0

0.25

0.5

0.75

1

Na Ca K Cl Mg

Inte

nsity

Core Layer

Core

Outer layer

Inner layer

(b)

Core

Layer

(a)

64

Figure 26. Initial and final PSD of limestone particles during pyrolysis of black liquor droplets at 1 ml/min, 3% solids, and 547 oC for 8 h. Initial HMDp = 265 µm and final HMDp = 325 µm.

0

0.2

0.4

0.6

0.8

1

0-43 43-88 88-180 180-351 351-701 701-1397 >1397Size range (micron)

Mas

s fra

ctio

n

Initial weight = 200 gFinal weight = 224 g

65

Figure 27. Initial and final PSD of glass beads during pyrolysis of black liquor droplets at 1 ml/min, 12% solids, and 547 oC for 12 h. Initial HMDp = 231 µm and final HMDp = 525 µm.

0

0.2

0.4

0.6

0.8

1

0-43 43-88 88-180 180-351 351-701 701-1397 >1397Size range (micron)

Mas

s fra

ctio

n

Initial weight = 200 gFinal weight = 264 g

66

Figure 28. Initial and final PSD of Norampac bed solids during pyrolysis of black liquor droplets at 1 ml/min, 12% solids, and 600 oC for 12 h. Initial HMDp = 176 µm and final HMDp = 567 µm.

0

0.2

0.4

0.6

0.8

1

0-43 43-88 88-180 180-351 351-701 701-1397 >1397Size range (micron)

Mas

s fra

ctio

n

Initial weight = 200 gFinal weight = 220 g

67

Figure 29. Initial and final PSD of Norampac bed solids during pyrolysis of black liquor droplets at 0.5 ml/min, 12% solids, and 525 oC for 10 h. Initial HMDp = 265 µm and final HMDp = 345 µm.

0

0.2

0.4

0.6

0.8

1

0-43 43-88 88-180 180-351 351-701 701-1397 >1397

Size range (micron)

Mas

s fr

actio

n

Initial weight = 200 gFinal weight = 220 g

68

Figure 30. Initial and final PSD of Norampac bed solids during pyrolysis of black liquor droplets at 0.5 ml/min, 12% solids, and 450 oC for 8.5 h. Initial HMDp = 265 µm and final HMDp = 395 µm.

.

0

0.2

0.4

0.6

0.8

1

0-43 43-88 88-180 180-351 351-701 701-1397 >1397Size range (micron)

Mas

s fra

ctio

n

Initial weight = 200 gFinal weight = 276 g

69

When bed particles grow very rapidly, as seen in all of these tests, the particle shape

drastically changes (see Table 7), which in turn affects fluidization.41 The formation of

agglomerates that are within large size fractions is the result of capturing several bed

particles with large liquor droplets during liquor injection, which clearly refers to droplet-

induced agglomeration. Leclère et al.72 noted that droplet diameter was found to be a

dominant factor for particle growth, and that the relative size of the droplets compared to

the solid particles is very important. Large droplets, as in this study (approx. 2 mm in

diameter), are then the reason for the formation of agglomerates.

Pyrolysis of liquor droplets at different temperatures shows little influence on final

PSDs of glass beads and Norampac solids when 12%-solids liquor droplets are pyrolyzed

at 0.5 and 1 ml/min as shown by Figures 27 to 30. On the other hand, the effect of black

liquor flow rate on HMDp is shown in Figure 31; where doubling the liquor feed rate

makes bed particles grow faster. This is the result of uneven deposition of residue on bed

particles; that is, the formation of droplet-induced agglomerates. This uneven deposition

of residue can be depicted by comparing the experimental results with the expected

particle growth from coating alone for both liquor feed rates. The experimental HMDp at

1 ml/min grows faster than the predicted by coating alone because more and larger

agglomerates are formed (see Figure 28) and counted in the calculation of HMDp. On the

other hand, the experimental HMDp at 0.5 ml/min grows slower that the predicted by

coating alone because less and smaller agglomerates are formed (see Figure 29) and

counted in the calculation of HMDp. This result clearly shows that the way in which

black liquor was added to the system is what most affects particle growth. In this system,

the addition of liquor by large droplets localizes the residue on some bed particles, which

70

Table 7. Comparative optical-microscopy images of agglomerates

Sample

Glass beads 1 ml/min

12% solids 547 oC 12 h

Norampac bed solids 1 ml/min

12% solids 600 oC

12 h

Norampac bed solids 1 ml/min

12% solids 525 oC 10 h

Initial

~ 200 µm 0 – 2800 µm 180 – 351 µm

Final

701 – 1397 µm 351 – 701 µm 180 – 1397 µm

71

Figure 31. Evolution of HMDp for two different liquor flow rates. (▲) Norampac bed solids within 0 – 2800 µm during pyrolysis of 12%-solids liquor droplets at 600 oC for 12 h. (♦) Norampac bed solids within 180 – 351 µm during pyrolysis of 12%-solids liquor droplets at 525 oC for 10 h. (Δ) Assuming uniform char residue deposition (specific volume 25 cc/g89) on mono-size bed particles of 176 µm. (◊) Assuming uniform char residue deposition (specific volume 25 cc/g89) on mono-size bed particles of 265 µm.

0

100

200

300

400

500

600

0 2 4 6 8 10 12

Pyrolysis time (h)

HM

Dp

(mic

ron)

Experimental growth at 1 ml/minExperimental growth at 0.5 ml/minExpected growth by coating at 1 ml/minExpected growth by coating at 0.5 ml/min

72

in turn cools them down to reduce their capacity to completely pyrolyze such droplets.

Since this particle cooling is localized, the average bed temperature did not change.

Selected samples were analyzed using optical microscopy and SEM in order to

characterize final agglomerates. A comparison of optical images of initial and final glass

beads and Norampac particles is shown in Table 7. In general, bulk images of final

samples show a similar agglomeration tendency: agglomerates formed of “clusters” made

up of single particles. This particle appearance is not surprising due to the nature of black

liquor feeding in this system that leads to these final shapes.

SEM images of cross-sections of selected samples are shown in Figure 32.

Agglomerates formed of small bed particles rather than large ones are shown in Figure

32a. Small bed particles seem more prone to get captured by large liquor droplets

(~2mm), thus forming “clusters” as shown in Figure 32b. Large particles may be part of

agglomerates, too. However, it is possible that they have smaller and fewer connecting

points within the agglomerate such that it is easier for them to be released or broken off

the “cluster.” After breakage, large particles might gain a layer of deposit that ultimately

contributes to their growth, as shown in Figure 32c. Closer inspection of binding areas is

shown in Figure 32d. Remaining residue is responsible for building up those “necks” that

hold particles together. Since liquor droplets collide, dry and pyrolyze on bed particles,

the presence of residue in these areas suggests that large liquor droplets do not

completely pyrolyze before clusters are formed. Estimated measures of agglomerates are

also shown in Figures 32b and 32c. Layer formation can add up to 70 µm to the diameter

of individual particles while agglomerates can reach dimensions of 1000 µm. In

accordance with this observation, some bed particles can get indirectly coated if they are

73

Figure 32. SEM images of cross-section areas: (a) bulk image of the final Norampac bed solids during pyrolysis of 12%-solids liquor droplets at 600 oC for 12 h, (b) agglomerates in the range of 701 – 1397 µm from final Norampac bed solids during pyrolysis of black liquor droplets at 0.5 ml/min, 12%-solids, and 450 oC for 8.5 h, (c) coated glass beads within 180 – 351 µm, and (d) small agglomerate of glass beads.

“necks”

237μm

307μm

(a) (b)

(c) (d)

176 µm

74

released off a droplet-induced agglomerate. Therefore, it is possible to conclude that this

experimental configuration lacked vigorous fluidization and complete liquor conversion

to balance the excessive particle growth given by these large droplets.

A general elemental scanning of coated limestone particles is shown in Figure 33.

The largest peak on the spectrum corresponds to Na, whereas the second largest

corresponds to Ca. Very small peaks of K, S, Al and Mg elements also appear on the

spectrum. Mapping of some elements such as Na on limestone particles and glass beads

shows no intra-particle deposition of the inorganic fuel constituents into the structure of

the bed particles, as shown in Figure 34. Contrary to what Brus et al. 52 have reported

during formation of layers by direct attack of potassium compounds on quartz bed

particles at high temperatures, here the observed deposition seems to be a physical

phenomenon instead of a chemical one.

A line scan over an agglomerate from experiment three is shown in Figure 35. Both

inorganic and organic elements are scanned to observe their distribution across the drawn

line. The line crosses layers, cores and “necks” of those two particles. S is uniform across

the line, except for a peak on the “neck” area. This peak indicates that Na2S is not

reduced according to equations 6 to 9. The line scan of C is fairly uniform. However, it is

relevant to note that C is contained in Na2CO3 as well as in the remaining organic matter.

Since these line scan data collect relative intensity of an element with respect to previous

scanned points on the same line, larger peaks of C indicate that more C exists (in any

form) in a region relative to another, i.e. the core (mainly Na2CO3). On the other hand,

Na is highly intensive in the core relative to the “neck”; therefore, it can be said non-

reacted C (nonvolatile matter) might compose a large portion of these binding areas.

75

Figure 33. General elemental scanning of coated limestone particles during pyrolysis of black liquor droplets at 1 ml/min, 3% solids, and 547 oC for 8 h.

1 1.5 2 2.5 3 3.5 4 4.5 keV

76

Figure 34. Elemental mapping of cross-section areas of: (a) Ca on a coated limestone particle, (b) Na on a coated limestone particle, (c) Si on a coated glass bead, and (d) Na on a coated glass bead.

(a) (b)

(c) (d)

77

Figure 35. Line scan of S, Na, C and O across a Norampac agglomerate

S

0

0.5

1

Inte

nsity

Na

0

0.5

1

Inte

nsity

C

0

0.5

1

0 550Distance (micron)

Inte

nsity

neck

neck

neck

neck

78

4.3 Attrition Results

Attriting particles suffer a gradual size reduction due to the inherent abrasive wear

between particles, and particles and vessel components in a fluidized bed. This reduction

can be determined by tracking changes of PSD with time. Figures 36 to 39 show the PSD

evolution for four different attrition tests using samples of bed solids from Norampac and

Georgia-Pacific steam reforming units. The main difference between these starting

samples is the concentration of the Norampac’s bed solids within 212 – 300 µm

compared to Georgia-Pacific’s bed solids.

Without considering the source of material, initial PSD or fluidization velocity, a

similar PSD transition is observed for these experiments. This trend shows a reduction of

material for size fractions over 300 µm and a respective accumulation of material for size

fractions below that size. This result suggests that bed particles over 300 µm seem to

contribute more to attrition, providing fines and attrited particles to the size fractions

below 300 µm. It is important to recognize the complexity associated with the attrition

phenomenon on bed particles below 300 µm, because there could be overlap between

small attriting particles and migrating material from larger size fractions. However,

Wether et al.76 stated that attrition depends on factors related to material properties and

process conditions; often related to the availability of the external surface area and

dynamic condition of attriting particles. Hence, attrition likely decreases as particle size

decreases due to less available external surface areas and lower particle momentum. For

that reason, it is believed that attrition is also a size-dependent phenomenon, occurring

here principally on bed particles over 300 µm. Alternatively, the fact that attrition occurs

on large bed particles is important to establish the existence of a natural offset between

79

Figure 36. PSD evolution of Norampac bed solids fluidizing at 0.231 m/s (3xUmf) and 25 oC.

0

0.1

0.2

0.3

0.4

0.5

0-75 75-106 106-150 150-212 212-300 300-425 425-600 >600

Size range (micron)

Mas

s Fra

ctio

n

0 h 4 h 9 h

80

Figure 37. PSD evolution of Norampac bed solids, without initial particles below 106 µm, fluidizing at 0.241 m/s (3xUmf) and 25 oC.

0

0.1

0.2

0.3

0.4

0.5

0-75 75-106 106-150 150-212 212-300 300-425 425-600 >600Size range (micron)

Mas

s Fra

ctio

n

0 h 4 h 9 h

81

Figure 38. PSD evolution of Georgia-Pacific bed solids fluidizing at 0.319 m/s (4.5xUmf) and 25 oC.

0

0.1

0.2

0.3

0.4

0.5

0-75 75-106 106-150 150-212 212-300 300-425 425-600 >600

Size range (micron)

Mas

s Fra

ctio

n

0 h 4 h 9 h

82

Figure 39. PSD evolution of Georgia-Pacific bed solids fluidizing at 0.426 m/s (6xUmf) and 25 oC.

0

0.1

0.2

0.3

0.4

0.5

0-75 75-106 106-150 150-212 212-300 300-425 425-600 >600

Size range (micron)

Mas

s Fra

ctio

n

0 h 4 h 9 h

83

coating and attrition during this steam reforming process. As bed particles gain

consecutive layers of deposited residue, they might simultaneously suffer size reduction

due to attrition.

Figures 40 to 43 show PSD of the material collected in the top filter. For each test, the

6-9 h sample refers to the compilation of four consecutive samples. These graphs show a

shifting of the PSD to larger size fractions as the fluidization velocity is increased, except

for the cold fluidization of Norampac bed solids without initial material below 106 µm,

which reports 70% of material within 75 – 106 µm after 9 h. The reason for finding more

and larger particles when fluidizing solids at different velocities is the critical particle

diameter (dpc). This critical size is the particle diameter at which the terminal velocity

(Ut) equals the fluidization velocity (Uf). The critical particle diameter determines the

limit between fines and coarse particles to define the size of solids that are potentially

elutriable at specific experimental conditions. Based on the Haider et al.90 approximation

for the direct evaluation of the terminal velocity of particles (assuming spherical

particles):

( ) 31

2g

gs*tt

gUU

⎥⎥⎦

⎤

⎢⎢⎣

⎡

ρ

ρ−ρμ= (13)

( ) ( )

1

5.0*p

2*p

*t

d591.0

d18U

−

⎥⎥⎦

⎤

⎢⎢⎣

⎡+= (14)

( ) 3

1

2gsg

PC*p

gdd ⎥

⎦

⎤⎢⎣

⎡μ

ρ−ρρ= (15)

84

Figure 40. PSD evolution of the material collected in top filter during fluidization of Norampac bed solids at 0.231 m/s (3xUmf) for 9 h.

0

0.2

0.4

0.6

0.8

1

0-75 75-106 106-150 150-212

Size range (micron)

Mas

s Fra

ctio

n

Material collected at 1 hMaterial collected at 6-9 h

85

Figure 41. PSD evolution of the material collected in top filter during fluidization of Norampac bed solids, without initial material below 106 µm, at 0.241 m/s (3xUmf) for 9 h.

0

0.2

0.4

0.6

0.8

1

0-75 75-106 106-150 150-212

Size range (micron)

Mas

s Fra

ctio

n

Material collected at 6-9 h

86

Figure 42. PSD evolution of the material collected in top filter during fluidization of Georgia-Pacific bed solids at 0.319 m/s (4.5xUmf)for 9 h.

0

0.2

0.4

0.6

0.8

1

0-75 75-106 106-150 150-212 212-300 300-425

Size range (micron)

Mas

s Fra

ctio

n

Material collected at 1 hMaterial collected at 6-9 h

87

Figure 43. PSD evolution of the material collected in top filter during fluidization of Georgia-Pacific bed solids at 0.426 m/s (6xUmf) for 9 h.

0

0.2

0.4

0.6

0.8

1

0-75 75-106 106-150 150-212 212-300 300-425

Size range (micron)

Mas

s Fra

ctio

n

Material collected at 1 hMaterial collected at 6-9 h

88

where

dPC is the critical particle diameter (µm),

dP* is the dimensionless critical particle diameter,

g is the acceleration of gravity (9.8 m/s2),

Ut is the terminal velocity (m/s),

Ut* is the dimensionless terminal velocity,

µ is the gas viscosity (kg/m-s),

ρg is the gas density (kg/m3), and

ρs is the particle density (kg/m3).

The dpc for tests one to four are: 59, 60, 84 and 71 µm, respectively. One would expect to

find only particles of size smaller than dpc in the top filter. However, particles of sizes

greater than these calculated diameters were also carried over. The reason for such

elutriation is based on the relation between the freeboard height (Hf =1.21 m), which is

the section of the vessel between the surface of the dense phase and the exit of the gas

stream, and the transport disengaging height (TDH), which is the height above the

surface of the dense phase required for the solids to disengage and fall back into the bed.

Above the TDH, only fines are found. Using the empirical correlation proposed by

Amitin et al.91:

( )f10

2.1f Ulog2.133.7U85.0TDH −= (16)

the TDHs for tests one to four are: 1.19, 1.24, 2.37 and 1.71 m, respectively. Clearly, the

available Hf is below TDH, except for test one, and particles larger than each critical size

(59, 60, 84 and 71 µm) are carried over as well. Based on the assumption that elutriated

89

fines are the product of mechanical attrition, this result shows that the practice of direct

collection of elutriated material using the cold flow model can be misleading for the

correct assessment of attrition on this type of bed solids. Hence, to make a better

estimation of this mechanism, it was necessary to calculate the amount of fines produced

in each experiment by using the mass fractions shown in Figures 36 to 39 and 40 to 43,

and the amount of collected material at each sampling time. For this calculation, two

aspects are considered: (1) according to Figures 36 to 39, the amount of initial fines, 0 –

75 µm, of starting samples is negligible compared to other size fractions; which means

that fines collected in the top filter are in fact the result of attrition, and (2) assuming that

the distribution of 75 – 106 µm of elutriated fines from Georgia-Pacific bed solids

fluidized at 0.426 m/s is linear, about half of the elutriated mass within this size range is

also fines resulting from attrition, since its calculated dpc is within that size range. The

results of this calculation are shown in Figure 44.

These results highlight the following aspects of attrition on these bed solids. First,

when increasing the fluidization velocity the amount of elutriated material increases, as

shown in Figure 44. Because the size of that material is below each corresponding dpc,

this material is called fines and assumed to be the product of attrition. The fluidization of

Norampac bed solids at 0.231 m/s reports more elutriated fines than fluidization of

Georgia-Pacific solids at 0.319 m/s. This may be due differences between Norampac and

Georgia-Pacific samples. The fluidization of Norampac bed solids at 0.241 m/s reports no

elutriated fines at 1 h because initial fines below 106 µm were pre-sieved. Second,

comparing the amount of total elutriated fines after 1 h and 6 – 9 h, a sharp decay in

attrition can be seen, being almost equivalent for cold fluidization at 0.231, 0.319, and

90

Figure 44. Estimation of fines produced by mechanical attrition during cold fluidization of Norampac and Georgia-Pacific bed solids.

0

0.2

0.4

0.6

0.8

1

0.231 0.241 0.319 0.426

Uf (m/s)

Tot

al m

ass o

f fin

es p

er in

itial

be

d w

eigh

t (kg

/kg

x 10

2 ) 1 h 6-9 h

No

ram

pa

c

Ge

org

ia-P

aci

fic

GP

No

ram

pa

c

0

0.2

0.4

0.6

0.8

1

0.231 0.241 0.319 0.426

Uf (m/s)

Tot

al m

ass o

f fin

es p

er in

itial

be

d w

eigh

t (kg

/kg

x 10

2 ) 1 h 6-9 h

No

ram

pa

c

Ge

org

ia-P

aci

fic

GP

No

ram

pa

c

91

0.426 m/s. At this point, attriting particles have lost a large portion of their asperities and

attached fines such that the rate of attrition decreases considerably. Third, it is important

to consider the influence of liquor atomization and vigorous particle dynamics due to

high temperatures on fines generation when evaluating attrition in the real system.

4.4 Fragmentation Results

4.4.1 Compression Results

Typical curves for compression tests on both a single particle and an agglomerate are

shown in Figure 45. The x-axis represents the cell displacement (µm) and the y-axis

represents the cell load (kg-f). These curves follow a similar pattern with different load

magnitudes. The first plateau seen at the origin of each curve corresponds to the small

gap between the top plate and the particle at the beginning of the test. The first load slope

represents the initial contact between the top plate and the sample. Once the exerted load

reaches the second plateau, it is assumed that the particle is reorganizing its internal

structure to withstand the exerted load. After passing the third plateau, the particle no

longer withstands the exerted load and ends up breaking into fragments.

The dashed circles on Figure 45 show the point of fragmentation while the dotted

circles show the point of complete crushing for each sample. This first point of

fragmentation was visually followed using a lab telescope placed in front of the bottom

plate. Points of crushing are not considered in this characterization because it is believed

that this fluidized bed system would not see extreme forces necessary for particle

crushing. Despite the fact that compression curves in Figure 45 are presented only for

descriptive purposes and do not represent accurate values of the tested samples, the same

behavior is seen for all the tested particles regardless their size or shape (either single

92

Figure 45. Typical curve of a compression test.

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200 250 300

Displacement (micron)

Loa

d (k

gf)

Single particle HMDp = 265micronAgglomerate HMDp = 525micron

Points of main particle fragmentation

Points of fragment crushing

93

particle or agglomerate). A comparative graph of the results from compression tests on

single particles and agglomerates is shown in Figure 46. Single particles show a trend for

the load to increase with particle size. Because these particles are not all of the exact

same size as indicated in the plot, it is hard to quantitatively estimate the correlation

between load and particle size. However, it is qualitatively possible to state that coating

makes particles larger and consequently stronger, as shown by points one to four in

Figure 46. Moreover, particles below the size of test three (180 µm) were not tested due

to the difficulty in properly handling them.

On the other hand, the load required to break agglomerates into single particles barely

changes with respect to size, as shown by points five to eight. In these cases, the resulting

fragmentation load corresponds to the load required to break the connecting regions

(“necks”) within the agglomerate. It is not surprising that this load does not vary much

with type of material or agglomerate size because their pyrolysis conditions were about

the same (see Table 2). Finally, samples of single particles within the 350 – 700 µm

range (HMDp = 525 µm) seem to withstand higher loads than agglomerates of the same

size. This is an interesting observation because sufficient solids agitation in this fluidized

bed system may help control excessive particle growth by disintegrating such undesirable

agglomerates without fragmenting single particles.

4.4.2 Cyclone Results

Based on the same experimental scheme used by Davuluri et al.,88 cyclone tests were

performed to determine the fragility of circulating single particles and agglomerates with

respect to superficial gas velocity into a glass cyclone. The percentage of fragmentation

for Norampac agglomerates circulated at superficial gas velocities between 8 and 20 m/s

94

Figure 46. Compression load versus HMDp for samples of: (1) and (2) Norampac single particles; (3) and (4) Georgia-Pacific single particles; (5) and (6) agglomerates made of Norampac single particles; and (7) and (8) agglomerates made of glass beads.

4

2

7

3 1

5

6

8Mean

0.0

0.1

0.2

0.3

0.4

0 200 400 600 800 1000 1200

HMDp (micron)

Loa

d (k

gf)

■ Single particles▲ Agglomerates

95

is shown in Figure 47. This percentage is defined as the ratio between the weight of the

final sample that passed through the screen corresponding to the lower limit size of the

initial sample, to the weight of the initial sample. The resulting plot linearly correlates

this percentage with superficial gas velocity for agglomerates of HMDp of 525 µm.

Clearly, high superficial gas velocities are required to break these agglomerates into

single particles. Linear extrapolation of these velocity points estimates that 100%

fragmentation is achieved at about 29 m/s of superficial gas velocity. This means that

these connecting or bridging “necks” are sufficiently strong that they are not easy to

break at superficial gas velocities found within the bulk of the dense phase, which would

be very detrimental to the particle development in this system once agglomerates are

formed. However, high superficial gas velocities can be found at the distributor and

internal cyclones of the fluidized bed steam reformer, which can promote fragmentation.

Following the same experimental procedure, single particles were circulated at

different superficial gas velocities as well. Results show that no fragmentation occurs on

these particles but slight attrition does occur, as shown in Figure 48. This percentage of

attrition is also defined as the ratio between the weight of the final fines collected at the

exit gas filter, to the weight of the initial sample. This result shows that single particles

are definitely not fragile, which agrees with the results of compression tests.

4.4.3 Agglomerate Break-up by Chemical Reaction

This experiment was meant to determine the percentage of fragmentation of samples

of agglomerates created by droplet-induced agglomeration. The char residue holding the

glass beads together was reacted away according to equation 5:

96

Figure 47. Percentage of fragmentation versus superficial gas velocity for the rapid circulation of agglomerates made of Norampac single particles using a glass cyclone.

0

20

40

60

80

100

8 10 12 14 16 18 20 22

Superficial gas velocity (m/s)

Frag

men

tatio

n (%

)

97

Figure 48. Percentage of attrition versus superficial gas velocity for the rapid circulation of Norampac single particles using a glass cyclone.

0

1

2

3

8 10 12 14 16 18 20 22

Superficial gas velocity (m/s)

Attr

ition

(%)

98

C + CO2 ↔ 2CO (5)

Based on the results of the agglomeration tests, a mass balance on test 2 shows that

64 g of char residue was deposited on the bed material (200 g of starting glass beads)

after a 12-h pyrolysis test with a feed of 1 ml/min of 12%-solids black liquor droplets.

This calculation means that 24% wt. corresponds to char residue and 76% wt.

corresponds to glass beads. Using these percentages, the consumption of organic matter

via CO2 gasification was then determined as the ratio of the difference of the weight loss

of gasified samples and the initial char weight in the samples of agglomerates, to that

initial char weight. Glass-bead agglomerates were selected for all tests in order to direct

the gasifying agent only to the organic matter in the holding char residue. Similarly, the

degree of fragmentation is defined as the ratio between the weight of the final sample that

passed through the screen corresponding to the lower limit size of the initial sample, to

the weight of the initial sample.

A comparative graph of the percentages of gasification and fragmentation for tests

one to six during CO2 gasification of glass-bead agglomerates at different temperatures

and times is shown in Figure 49. The x-axis refers to the amount of time that samples

were exposed to the gasifying agent at the temperature of interest. This plot clearly shows

the percentage of gasification to increase with respect to temperature and time. This is not

surprising due to the nature of this reaction, as described by Li et al.37 The percent of

fragmentation was observed to be most significant between temperatures 700 oC and 800

oC. However, as the system temperature approaches 800 oC, the inorganic constituents in

the liquor char (mostly Na2CO3) melt and agglomerate the whole sample. This is what

happened during test six. The percentage of gasification is very high (almost 80%), but

99

Figure 49. Percentage of gasification and fragmentation of glass-bead agglomerates due to chemical reaction.

10-1

100

101

102

600

650

700

750

8000

10

20

30

40

50

60

70

80

Time (h)Temperature (C)

Perc

enta

ge (%

)

1

2

3

4

5

6

▲ Gasification ○ Fragmentation

100

the final gasified sample ended up agglomerated. There is some consumption of organic

matter in these agglomerates when reacting samples below 700 oC, but too little to induce

fragmentation. Considerable fragmentation is seen at temperatures between 700 and 730

oC for 1 to 2 h. This might be the result of the consumption of organic matter located in

the connecting regions. It is very important to point out that it was previously said that

non-reacted C (nonvolatile matter) might compose a large portion of these binding areas.

Therefore, consumption of such organic matter might help reduce the strength of such

“necks,” thus leading to fragmentation. However, it would be important to consider that

the chemical consumption of the binding areas, combined with physical interactions,

should lead to higher percentages of fragmentation.

101

CHAPTER 5

CONCLUSIONS

5.1 Particle Development in a Fluidized Bed Black Liquor Steam Reformer

The work presented in this thesis contributes to the understanding of mechanisms

responsible for particle development in a fluidized bed black liquor steam reformer.

Coating, agglomeration and attrition seem to be important mechanisms for this system.

Based on the characterization of bed solids from two commercial steam reformer units, it

was possible to identify the mechanisms that most influence particle size in these units.

Additionally, experimental decoupling of four mechanisms showed important aspects of

coating, droplet-induced agglomeration, mechanical attrition and fragmentation.

A visual analysis of cross-sectional areas of bed solids from the commercial steam

reforming units showed that coating, which results from liquor atomization into the

fluidizing bed, is a very important mechanism for particle development in theses systems.

This mechanism mainly occurs on bed particles larger than 250 µm, as one to two

superimposed layers of inorganic residue rich in Na and K are seen on their external

surfaces. According to the visualization and EDS analysis of bed particles smaller than

250 µm, the coating seems not to participate in their evolution, which suggests that their

development could be attributed to the atomization of liquor droplets that dry and

pyrolyze before contacting with any bed particles. Therefore, liquor atomization appears

to be influential parameter to promote particle coating in this fluidized bed system.

102

Lab-scale fluidized bed reactor experiments showed that excessive particle growth

occurs when large liquor droplets capture several particles upon injection, and this

phenomenon is referred as droplet-induced agglomeration. Results showed that liquor

feed rate and percentage of solids content have a greater influence on the occurrence of

this submechanism than temperature. In addition, the injection of large liquor droplets

might promote particle cooling, which in turn might reduce the capacity of particles to

fully convert such liquor droplets. This observation thus highlights liquor atomization as

a key factor in particle agglomeration. On the other hand, the vigorousness of fluidization

may contribute to the successful fragmentation of droplet-induced agglomerates to

subsequently form indirect-coated particles.

Cold fluidization experiments of samples of bed solids from two commercial steam

reforming units showed that the fluidization velocity has strong influence on both the

mechanical attrition of these particles and the elutriation of material. The generation of

fines that results from mechanical attrition increases with increasing fluidization velocity.

This observation suggests that the fluidization velocity promotes particle size reduction

due to the inherent particle abrasion during fluidization. On the other hand, the elutriation

of material increases with increasing fluidization velocity. That is, more and larger

particles are carried over at higher fluidization velocities. Therefore, for a given fluidized

bed gasifier geometry, it is necessary to consider the fluidization velocity as a factor that

might deplete the amount of material that is potentially elutriable at specific fluidization

conditions; particularly, material that might derive from fine liquor atomization or

reaction-enhanced attrition.

103

Compression tests showed that the force to fragment single particles increases with

increasing particle size, but remains constant with increasing size of droplet-induced

agglomerate. This result is particularly beneficial because the fluidization velocity can

provide vigorous solids agitation to control the excessive particle growth by

disintegrating undesirable agglomerates without fragmenting single particles. On the

other hand, the rapid circulation of single particles and droplet-induced agglomerates in a

glass cyclone unit showed that agglomerates do not fragment at superficial gas velocities

found within the bulk of a bubbling fluidized bed, which might be very detrimental to

particle development in this system once agglomerates are formed.

Chemical consumption of the organic matter within the char residue of droplet-

induced agglomerates showed that this process of fragmentation is very sensitive to the

reaction temperature because it strongly affects the rate of the reaction as well as the

physical state of the inorganic content. Therefore, chemical consumption of the organic

matter, combined with a vigorous fluidization, presumably minimizes the excessive

particle growth caused by agglomeration.

Based on these experimental results, particle development in a fluidized bed black

liquor steam reformer is qualitatively addressed as shown in Figure 50. This figure

combines the most important aspects of each mechanism obtained in this study. The

width of each arrow estimates the relative participation of each mechanism in overall

particle development. This figure can be interpreted as follows:

1) Initial bed solids and liquor droplets interact to form coated particles.

2) Liquor droplets that do no reach bed solids upon injection may form new solids.

3) Initial bed solids and coated solids attrit to generate fines.

104

6a

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2

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8

9

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77

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105

4) Fines from attrition and liquor atomization are elutriated.

5) Elutriated fines may be returned to the bed to contribute to the bed evolution.

6) Three types of agglomerates may appear under the following circumstances:

a. Poor atomization may form either droplet-induced or coating-induced

agglomerates.

b. Local temperature peaks may form melt-induced agglomerates.

7) Droplet-induced and coating-induced agglomerates may fragment to form

indirect-coated particles.

8) Excessive amounts of agglomerates may lead to defluidization and system

shutdown.

9) Bed solids may continuously evolve to reach a steady-state condition.

5.2 Recommendations for Future Work

Experimental work should continue using the recently built University of Utah

fluidized bed black liquor steam reformer. As previously mentioned, coating is an

essential mechanism for particle development in this system. Therefore, it is very

important to analyze the layer build-up characteristics as a function of the different

process variables, specifically; liquor feed rate and droplet size distribution. However,

addition of a significant portion of inert material to the bed, chemically different from the

black liquor residue, is recommended to facilitate the analysis of layer formation on

coated particles. In addition, the characterization of fines generated and elutriated under

steam reforming conditions due to attrition demands further study, since an inherent

offset should exist between coating and attrition as particle growth and reduction

mechanisms.

106

The occurrence of particle agglomeration during steam reforming at different

operating conditions deserves special attention. The observed operating conditions that

lead to agglomeration should be reported and compared with the result of SEM/EDS

analysis of samples of agglomerates in order to establish safe operating limits to assure a

reliable operation of fluidized bed steam reformers. Once the type of agglomerates and

the operating conditions leading to their formation have been experimentally identified, it

will be necessary to study the factors that might contribute to the fragmentation of such

agglomerates. One approach to promoting this fragmentation would be to vary the

fluidization velocity and measure the superficial velocities across different heights of the

bed. As a result, velocity profiles would be obtained and used to determine whether

fragmentation occurs to control the excessive particle growth caused by agglomeration.

Another suggestion for investigating particle development in a fluidized bed black

liquor steam reformer is to model the particle size distribution. Based on the experimental

observations during testing campaigns using the pilot unit, each mechanism can be

modeled by including the most important operating aspects that promote its participation.

This modeling work should help predict the evolution of the bed material for a wide set

of experimental conditions.

107

APPENDIX

STEAM REFORMER BED MEDIA CHEMICAL

ANALYSIS PROCEDURE

This procedure involves the determination of both water insoluble and acid insoluble

content of the media. From these analyses, the water-soluble media content is

“interpreted” as sodium carbonate (Na2CO3), the acid soluble as carbon content and the

difference as limestone (CaCO3). The carbon content determined by this method

overstates to some extent the actual carbon content due to the presence of other

compounds that are also insoluble in acid. The sample handling and procedure consists of

grinding approximately 10 g of sample in a mortar and pestle to add it into two tared

glass beakers (one for acid soluble determination and one for water soluble

determination). The exact weight of sample added to each beaker is recorded.

For the water soluble content, 100 ml of distilled water is added to the beaker

containing the pre-weighed ground sample for soluble content determination. A magnetic

stirrer bar is placed into the beaker, which is then placed on a hot plate stirrer. The

contents are brought to approximately 180 oF and held at this temperature for 30 min

while stirring. After 30 min, the contents are filtered through a pre-tared paper filter using

a vacuum apparatus. Any residual material from the beaker is washed onto the filter using

additional distilled water. Approximately 50 ml more of distilled water is used to

108

wash the filter cake to remove any residual dissolved solids from the filter and cake. The

filter is removed from the Buckner funnel and placed in a drying oven at 120 oC for 1 h.

Once dried, the filter and cake are placed into a desiccator to cool before final weighing.

The percentage (wt. %) of water soluble content is then reported as 100% minus the ratio

of the difference between the dry weight of filter and cake and the filter tare weight, to

the weight of the ground sample.

For the acid soluble content, 40 ml of distilled water and 60 ml of hydrochloric acid

(HCl) are added to the beaker containing the pre-weighed ground sample for insoluble

content determination. A magnetic stirrer bar is placed into the beaker, which is then

placed on a hot plate stirrer. The contents are brought to approximately 180 oF and held at

this temperature for 30 min while stirring. After 30 min, the contents are filtered through

a pre-tared paper filter using a vacuum apparatus. Any residual material from the beaker

is washed onto the filter using additional distilled water. Approximately 50 ml more of

distilled water is used to wash the filter cake to remove any residual dissolved solids from

the filter and cake. The filter is removed from the Buckner funnel and placed in a drying

oven at 120 oC for 1 h. Once dried, the filter and cake are placed into a desiccator to cool

before final weighing. The percentage (carbon wt. %) of acid insoluble content is then

reported as the ratio of the difference between the dry weight of filter and cake and the

filter tare weight, to the weight of the ground sample. The percentage of limestone is then

reported as 100% minus the percentages of sodium carbonate and carbon.

109

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