Hydrogen-rich materials as auxiliary reducing agents in the blast …1263437/... · 2018-11-15 · Hydrogen-rich materials as auxiliary reducing agents in the blast furnace Dimitrios

Hydrogen-rich materials as auxiliary

reducing agents in the blast furnace

Dimitrios Sideris

Chemical Engineering, master's level (120 credits)

2018

Luleå University of Technology

Department of Civil, Environmental and Natural Resources Engineering

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Abstract The blast furnace is an energy intensive and efficient counter current heat exchange apparatus

used in ironmaking. Energy consumption occurs mainly through usage of fossil fuels and an

important effect on the environment is the release of pollutants, with carbon dioxide being its

largest airborne emission.

Counter measures to reduce resource utilization and environmental impact of the blast furnace

are sought through injection of auxiliary reducing agents. These materials are used in

combination with the main reducing agents to increase the overall efficiency of the process

while decreasing CO2 emissions.

This project intends to evaluate the use of four hydrogen rich materials as auxiliary reducing

agents in the blast furnace. The materials tested in this study are carbonaceous materials that

have undergone torrefaction or no preprocessing. The hydrogen content of these materials is

comparatively high (5-6 wt%) and the expectancy to mitigate the carbon dioxide emissions by

substituting part of the pulverized coal (that is the currently used injection material) is

reasonable. At the same time the fact that the materials tested are secondary materials

originating from the recycling chain reduces the carbon footprint of the overall process.

Kinetic parameters of the materials’ reactions (devolatilization, gasification and combustion)

have been determined, along with the materials’ particle size, true density, calorific value and

composition. The interaction of the materials’ ashes with coke substrates has also been

investigated in order to acquire insight about the effect of the materials’ residue on coke

reactivity and consequently its integrity.

Ultimate goal of these investigations is to apply the data and parameters derived to a

Computational Fluid Dynamics model and have a credible estimation about the effect of these

materials when injected into the blast furnace, avoiding costly pilot scale experiments and

industrial trials.

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Acknowledgements I would like to thank my supervisor Hesham Ahmed (LTU) for his help and guidance

throughout the project. I am also most grateful to Caisa Samuelsson (LTU) for giving me the

chance to conduct a thesis in the Processmetallurgy laboratory and for all her support. Britt-

Louise Holmqvist (LTU) contributed the maximum to the realization of my experiments and

for this I thank her. Special thanks to Martin Ölund (Swerea MEFOS) for explaining me basic

theory and answering to all my questions.

Finally, I would like to express my humble gratitude to everyone in my Department who

provided me with means and motivation to complete this task.

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Table of contents Abstract ..................................................................................................................................... 1

Acknowledgements ................................................................................................................... 2

List of Figures .......................................................................................................................... 4

List of Tables ............................................................................................................................. 5

Abbreviations and Symbols ....................................................................................................... 6

1 Introduction ....................................................................................................................... 7

1.1 Background ............................................................................................................... 7

1.2 Auxiliary Reducing Agents Injection ........................................................................ 9

1.3 State of the art ............................................................................................................ 9

1.4 Modelling of auxiliary reducing agents injection through the tuyeres .................... 11

1.5 Scope of the current work ........................................................................................ 11

2 Literature review ............................................................................................................. 13

2.1 Factors influencing injection materials’ behavior in the raceway ........................... 13

2.1.1 Particle size ...................................................................................................... 13

2.1.2 Composition .................................................................................................... 13

2.1.3 Calorific value ................................................................................................. 13

2.1.4 Ash properties .................................................................................................. 14

2.2 Injectants reactions in the raceway .......................................................................... 14

2.2.1 Devolatilization ............................................................................................... 15

2.2.2 Combustion...................................................................................................... 16

2.2.3 Gasification...................................................................................................... 18

3 Materials .......................................................................................................................... 21

4 Methods ........................................................................................................................... 23

4.1 Material Pretreatment .............................................................................................. 23

4.2 Particle size analysis ................................................................................................ 24

4.3 Helium pycnometry ................................................................................................. 24

4.4 Bomb calorimetry .................................................................................................... 24

4.5 Thermogravimetric analysis .................................................................................... 25


4.5.2 Combustion...................................................................................................... 26

4.5.3 Gasification...................................................................................................... 28

4.6 Mass Spectrometry .................................................................................................. 30

4.7 Ash production ........................................................................................................ 30

4.8 Heating microscopy ................................................................................................. 30

4.9 Scanning Electron Microscopy ................................................................................ 31

4.10 Coke reactivity evaluation ....................................................................................... 31

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5 Results ............................................................................................................................. 32

5.1 Grinding and sieving the materials .......................................................................... 32

5.2 Particle size analysis of the 53-106 μm size fraction .............................................. 32

5.3 Density determination ............................................................................................. 33

5.4 Bomb Calorimetry ................................................................................................... 33

5.5 Thermogravimetric Analysis ................................................................................... 35


5.5.2 Combustion...................................................................................................... 38

5.5.3 Gasification...................................................................................................... 40

5.6 Mass Spectrometry .................................................................................................. 41


5.6.2 Combustion...................................................................................................... 43

5.6.3 Gasification...................................................................................................... 45

5.7 Ash production ........................................................................................................ 47

5.8 Heating microscopy ................................................................................................. 48

5.9 Scanning Electron Microscopy ................................................................................ 52

5.10 Coke Reactivity ....................................................................................................... 54

6 Discussion ....................................................................................................................... 56

6.1 Material characterization ......................................................................................... 56

6.2 Thermal analysis ...................................................................................................... 56

6.3 Ash analysis ............................................................................................................. 57

7 Conclusions ..................................................................................................................... 58

8 Future work ..................................................................................................................... 59

9 References ....................................................................................................................... 60

10 Appendices .................................................................................................................. 63

10.1 Helium pycnometry measurements ......................................................................... 63

10.2 TGA data analysis ................................................................................................... 67


10.2.2 Combustion...................................................................................................... 72

10.2.3 Gasification...................................................................................................... 74

10.3 TGA – MS 3D graphs.............................................................................................. 76

10.4 Heating microscopy results ..................................................................................... 79

10.5 SEM photomicrographs ........................................................................................... 82

List of Figures Figure 1.1. Outline of the blast furnace mass balance (Geerdes, et al., 2015) .......................... 7 Figure 1.2. Zones in the blast furnace (Geerdes, et al., 2015) ................................................... 8

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Figure 1.3. Auxiliary reducing agents injection (Geerdes, et al., 2015) .................................... 9 Figure 1.4. Pulverized coal reaction in the raceway (Ishii, 2000) ........................................... 10 Figure 4.1. Material pretreatment process scheme .................................................................. 23 Figure 4.2. Temperature profile during devolatilization ......................................................... 26 Figure 4.3. Temperature profile during combustion ................................................................ 27 Figure 4.4. Temperature profile during gasification ................................................................ 29 Figure 5.1. Automated particle size analysis of the 53-106 μm size fraction .......................... 32 Figure 5.3. Comparison between experimental and theoretical dry mass HHV ..................... 35 Figure 5.4. TGA graphs during devolatilization...................................................................... 36 Figure 5.5. TGA graph during combustion ............................................................................. 39 Figure 5.6. TGA graph during gasification ............................................................................. 40 Figure 5.7. Mass spectrometry graphs illustrating the samples' devolatilization .................... 42 Figure 5.8. Mass spectrometry graphs illustrating the samples' combustion .......................... 44 Figure 5.9. Mass spectrometry graphs illustrating the samples' gasification .......................... 46 Figure 5.10. Comparison of ash production using different techniques .................................. 48 Figure 5.11. Swelling behavior of PC ash ............................................................................... 49 Figure 5.12. Swelling behavior of PUR ash ............................................................................ 50 Figure 5.13. Ternary phase diagram of CaO-SiO2-MgO with fixed 10 wt% Al2O3 (Process

Metallurgy Course, 2017) ........................................................................................................ 51 Figure 5.14. Isothermal section of the CaO-SiO2-Al2O3 phase diagram at 1800K (MTDATA,

2010) ........................................................................................................................................ 52 Figure 5.15. Comparison of SEM photomicrographs of coke before (left) and coke after (right)

thermal treatment ..................................................................................................................... 52 Figure 5.16. Photomicrograph of PC ash on coke using SEM (498x) .................................... 53 Figure 5.17. Photomicrograph of PUR ash on coke using SEM (85x).................................... 53 Figure 5.18. Photomicrograph of Carbon PIMIENTO ash on coke using SEM (999x) .......... 54 Figure 5.19. Coke reactivity evolution after thermal treatment with the injection materials' ashes

................................................................................................................................................. 55 Figure 5.20. Photomicrograph of Carbon PODA ash on coke using SEM (201x) .................. 55

List of Tables Table 2.1. Particle reactions during auxiliary reducing agents injection ................................. 15 Table 3.1. Proximate, ultimate and ash analyses ..................................................................... 21 Table 4.1. Machine parameters ............................................................................................... 23 Table 4.2. Stream definition .................................................................................................... 23 Table 5.1. Helium pycnometry results .................................................................................... 33 Table 5.2. Bomb calorimetry results ....................................................................................... 33 Table 5.3. Higher heating value for dry materials ................................................................... 34 Table 5.4. HHVd calculated using Gaur and Reed formula ..................................................... 34 Table 5.5. Results from graphical evaluation of kinetic parameters for devolatilization ........ 38 Table 5.6. Results from graphical evaluation of kinetic parameters for combustion .............. 39 Table 5.7. Results from graphical evaluation of kinetic parameters for CO2 gasification ...... 40 Table 5.8. Ash production by oxidation at 950oC ................................................................... 47 Table 5.9. Summary of heating microscopy results ................................................................ 48 Table 5.10. Mass loss during heating microscopy experiments .............................................. 49 Table 5.11. Reduction of ash composition to four basic components ..................................... 50

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Abbreviations and Symbols Abbreviations

CFD Computational fluid dynamics

DIA Dynamic image analysis

HHV Higher heating value HV-TSD High volatile torrefied saw dust LTU Luleå Tekniska Universitet MS Mass spectrometry PC Pulverized coal PSD Particle size distribution PUR Polyurethane RAFT Raceway adiabatic flame temperature SEM Scanning electron microscopy TGA Thermogravimetric analyzer

Symbols

A Pre-exponential kinetic factor

Ap Particle surface area

C<S> Char

d50 50% passing size

Dp Char particle diameter

E Activation energy

Ea Apparent activation energy

fs Mass fraction of reacting solid species in a particle

k Kinetic rate constant

kG Granular model kinetic rate constant

kMV Modified volumetric model kinetic rate constant

m Total mass remaining

mc Mass of remaining char

mo Initial sample mass

𝑀𝑂2 Oxygen molecular weight

mRC Mass of unreacted coal

mVM Mass of remaining volatile matter

np Number of particles in a sample

𝑃𝑂2 Oxygen partial pressure

P80 80% passing size

pg Bulk partial pressure of reacting gas

R Universal gas constant

RC Particle surface reaction rate

rcom Combustion rate

RD Diffusion rate coefficient

RK Reaction rate coefficient

RVM Rate of devolatilization

t Time

T Absolute temperature

Vp,o Initial particle volume

Vtotal,o Initial sample volume

X Conversion

𝑋𝑂2 Oxygen mole fraction in the gas

ρ Density

φ Ratio of reacting surface to external area

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1 Introduction

1.1 Background The blast furnace constitutes the most efficient way of producing pig iron. The basic principles

that govern its function originate from the antiquity but it acquired its current form during the

last three centuries.

The main operating principle of the blast furnace is the reduction of iron oxides into metallic

iron. This is accomplished by the effective contact of the iron minerals with reducing agents, a

reaction that is achieved in several ways (regarding the physicochemical state of the reactants)

and under various conditions (temperature, pressure) throughout the furnace’s different zones.

An outline of the blast furnace process is illustrated in the following figure.

Figure 1.1. Outline of the blast furnace mass balance (Geerdes, et al., 2015)

The charge materials or stock consists of iron ore, coke and fluxes. These are solid materials

which are fed at the furnace’s charging system on top and slowly travel downwards, undergoing

several changes during their descent, ending up being collected as hot metal and liquid slag, at

the bottom of the furnace through notches and as gases at the gas uptake on top. The main cause

of these physicochemical transformations of the burden is the oxygen injected as air hot blast

through the tuyeres. The hot blast creates voidage in front of the tuyeres where coke is

consumed by oxidation with oxygen producing CO at elevated temperatures. The resulting gas

which is a mixture of the reducing CO gas and the inert gaseous components of the air blast

ascends through the furnace melting and reducing the burden and ends up at the gas uptake at

the top of the furnace.

Concerning the furnace configuration, it is divided into several zones that can be distinguished

from each other because of the different physical and chemical status of the materials flowing

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through them, their temperature profile and their position. These zones are formed during the

blast furnace operation and are namely the following:

Figure 1.2. Zones in the blast furnace (Geerdes, et al., 2015)

• Throat: this is where the solid materials fall after being fed into the furnace through the

charging system. Ore and coke are charged in discrete layers and in the throat they form

the stockline, where they are first dried by the ascending off gases and get heated to

approximately 200oC.

• Shaft or stack: in this zone the burden is in the solid state but reacts with the ascending

gasses that contain CO and H2 and gets reduced from the higher iron oxides (hematite-

Fe2O3, and magnetite-Fe3O4) into the lower iron oxides (wustite-FeO, and iron-Fe),

while at the same time gets heated to 1100-1200oC.

• Belly: this region is occupied by alternate layers of permeable solid coke and

impervious, semifused mass of iron and primary slag, through which the ascending

gases are unable to flow. It is also called the cohesive zone and the gases diffuse in the

burden volume through the coke slits and cause further reduction. The gangue in

admixture with the flux starts to fuse in this region at temperatures above 1200oC.

• Bosh: here the reduction is completed and the ores are melted down. The sectional area

of the furnace is reduced by about 20-25% in harmony with the resultant decrease in

the apparent volume of the charge. It is at the lower part of this zone where the air blast

is introduced through tuyeres, creating a raceway in front of each tuyere where

combustion of the coke takes place.

• Hearth: The unburnt coke from the tuyere region descends into the hearth, forming the

‘deadman’ coke layer which saturates with carbon the down coming molten metal. The

metal and slag stratify into separate layers in the hearth, from where they are tapped

periodically (Geerdes, et al., 2015).

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1.2 Auxiliary Reducing Agents Injection Injection of auxiliary reducing agents in the blast furnace has been practiced for some decades,

in order to substitute part of the coke in the process. Auxiliary reducing agents are introduced

into the blast furnace through injection with the air blast in the tuyeres, while coke is fed along

with ore and fluxes through the top charging system at the blast furnace’s stockline.

Auxiliary reducing agents serve two major purposes: the short term temperature control in the

furnace and the reduction of the burden material. In the course of their itinerary in the blast

furnace, these materials are injected along with hot air in the tuyeres where they first lose their

volatile content through the rapid heating they are subjected to, while the released volatiles

react with the atmosphere and combust, thus producing the raceway flame. At the same time

the remaining charified solid material undergoes combustion with oxygen while further in the

process the char particles gasify with the carbon dioxide formed. The final residue of these

processes represents the ash content of the original material and reaches the stagnant coke layer

or ascends through the blast furnace along with the high flow of gases interacting with the coke

either in the deadman zone or the descending which may alter its properties.

Figure 1.3. Auxiliary reducing agents injection (Geerdes, et al., 2015)

Hydrogen rich materials when co-injected along with the blast generate moisture which

provokes the water gas shift reaction in the middle zone of the furnace:

𝐶𝑂 + 𝐻2𝑂 → 𝐶𝑂2 +𝐻2 R. 1

The hydrogen produced by the water gas shift reaction is more reactive than CO and its reaction

with the iron oxides in the middle and upper zones of the furnace produces water, which exits

the blast furnace, reducing the final CO2 release (Lundgren, 2013).

1.3 State of the art The basic types of injection materials at the tuyere level are natural gas, oil and pulverized coal.

Injection of auxiliary reducing agents started in the 1960’s with natural gas in Ukraine, but

nowadays the use of pulverized coal is more common, mainly due to price and availability,

which are to a large extent influenced by regional factors.

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In order to inject pulverized coal in the tuyeres, a plant for processing raw coal has to be

installed. This installation has to perform the following processes for preparing coal to be mixed

with the air blast in the raceway:

• Grinding

• Drying

• Transportation through the pipelines

• Injection through lances in the blast

When pulverized coal is injected via lances into the tuyeres, it immediately undergoes

devolatilization caused by the elevated temperature of the air hot blast. The volatiles that are

released ignite and combust by the oxygen in the air blast producing CO2 and H2O, while the

remaining solid char particles are also ignited and oxidized by the atmosphere containing O2.

In the last step, the remaining char particles reform the generated CO2 and H2O into CO and H2

gas by the carbon solution loss reaction. These steps can be illustrated schematically in Figure

1.4:

Figure 1.4. Pulverized coal reaction in the raceway (Ishii, 2000)

The unburnt charified coal that passes through the raceway boundary enters the coke bed and

is consumed along with coke fines in high temperature regions by reaction with CO2 in gas and

FeO in slag. As char is more reactive than coke, accumulation of coke fines may occur, causing

permeability problems, channeling and low gas efficiency. This is the reason why a high char

burnout in the raceway is needed (Ölund, et al., 2017).

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1.4 Modelling of auxiliary reducing agents injection through the tuyeres Injection through the tuyeres is a complex phenomenon since it involves reaction kinetics, mass

transfer, heat transfer and momentum transfer. The raceway has to come to a steady or quasi

steady state in order for the blast furnace to operate continuously. To be able to combine all

these physical and chemical processes in a simulation model that can be used to predict the

response of the system to input changes, the first step is to divide the individual phenomena.

The components that constitute the total model are:

• Fluid mechanics: turbulence, particle dispersion

• Particle reactions: devolatilization, char reaction

• Gaseous reactions: homogeneous reactions, turbulent combustion

• Heat transfer: convection, radiation, reaction heat

• Others: pollutant formation, particle deformation, fragmentation, etc. (Ishii, 2000).

1.5 Scope of the current work The purpose of this project is to test several hydrogen rich carbonaceous materials originating

from neutral, renewable carbon sources and/or the recycling chain, in order to assess their

suitability of being injected as auxiliary reducing agents into the blast furnace. By using

hydrogen rich materials in the process the CO2 emissions of the blast furnace may be reduced

while at the same time recycled materials will be used in a profitable manner, thus mitigating

the overall carbon footprint of the process.

The way to perform this evaluation is by:

• studying their comminution characteristics and particle size distribution

• studying the composition of the materials (proximate and ultimate analyses)

• performing helium pycnometry in order to derive their true density

• determine the calorific value of the materials by bomb calorimetry

• performing thermogravimetric analysis in order to derive their corresponding reactions

kinetic constants and consequently predict their behavior in the raceway.

It has to be noted that the kinetic constants derived in this study are apparent kinetic constants

and cannot be compared with reference values for pure compounds, but serve well the purpose

of characterizing the materials under investigation in terms of their simulated behavior when

injected into the raceway, so as to come to a conclusion which one is more appropriate to

improve the performance of the blast furnace.

Another major part of this project is the study of the ash content of the materials under

investigation and its interaction with coke when the ash reaches the coke layer after char is

gasified. In order to achieve the ash evaluation, a number of experiments were performed,

namely:

• Analysis of the ash composition of the materials

• Ash production from the materials by burning the materials in a furnace at 950oC for 3

hours

• Heating microscopy of ash briquettes on coke substrate to monitor the softening

temperature, melting temperature, wettability of molten ash on coke

• Separation of the coke substrate and examination with Scanning Electron Microscopy

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• Examination of the reactivity of the evolved coke after contact with the ashes by

Thermogravimetric Analysis

The data produced as outcome of these investigations will be later used as input to a

Computational Fluid Dynamics simulation software in order to model the behavior of the

materials in the blast furnace raceway.

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2 Literature review

2.1 Factors influencing injection materials’ behavior in the raceway

2.1.1 Particle size The common practice for injecting materials into the raceway is to grind and pulverize them,

with coal being pulverized to P80=75 μm. By reducing the particle size of the materials, a larger

specific surface area is achieved, which facilitates and accelerates devolatilization. This way

higher amounts of volatile mater are released from the material, a fact that has an effect on the

subsequent CO2 gasification which can be negatively affected (reduced char combustibility and

furnace permeability) by the presence of remnant volatile matter in the charified material

(Carpenter, 2010).

2.1.2 Composition Composition of the injection materials in the blast furnace is one of the most decisive factors

for their suitability as auxiliary reducing agents. Materials containing high amounts of hydrogen

generate less heat in the raceway than materials with higher fixed carbon content but have a

high replacement ratio since hydrogen is very efficient in the indirect reduction reaction of iron

oxides. The moisture content causes a cooling effect in the raceway due to the endothermic

solution loss reaction and injection of moisture increases the reductant rate. The oxygen

percentage of the injectants is a material characteristic that lowers the heating value of the

injectants since oxidation of the carbonaceous materials cannot take place in case carbon-

oxygen bonds have already been formed.

Volatile matter content increases gaseous homogeneous combustion in the raceway since blast

oxygen primarily reacts with the injected particles volatile content and can then penetrate into

the solid particle’s porous structure to oxidize the solid carbon content. Thus, in case high

amounts of VM exist they preferentially consume oxygen leaving the remaining amount for

char oxidation, while the overall replacement ratio of the material is low. Volatile matter also

has an effect on RAFT, since devolatilization is endothermic.

Sulfur and phosphorus are elements that can degrade the hot metal quality while their removal

results in additional costs associated with increased slag volume generation and basicity

requirements for sulfur removal or hot metal treatment for phosphorus removal. The sulfur

content in coal is preferably below 0.8% while that of phosphorus below 0.05%.

Alkalis can contribute to coke degradation and sinter disintegration while they attack the

refractory lining. The way for these effects to take place is by catalyzing the coke gasification

reaction and decreasing the coke strength in the lower part of the blast furnace. Furthermore

alkali condensation on the lining causes the formation of scaffolds which affects the burden

descent and reduces lining life (Lundgren, 2013). The combined upper limit for sodium and

potassium oxides is usually 0.1% for coal.

Chlorine is another undesirable element in the injectants composition and if present it exits the

blast furnace either through the off gas or the slag. Although generation of dioxins in the blast

furnace offgas is not detected, chlorine forms hydrochloric acid which corrodes metal

components and in particular steel in the blast furnace gas cleaning system. The limit for coal

chlorine is typically 0.05% (Carpenter, 2010).

2.1.3 Calorific value Calorific value is the heat released during complete combustion of the materials. One of the

most important properties of the injectants is the amount of heat generated when oxidized by

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the air blast immediately after entering the raceway. This heat is used at the lower part of the

furnace to heat up and melt the burden material from where it starts softening (about 1100oC)

to casting temperature of 1500oC (Geerdes, et al., 2015).

The calorific value of the materials determines the amount of heat that can be supplied to the

furnace and does not correspond to the actual heat release of the materials in the raceway, since

the overall process in the raceway includes gasification of char thus producing CO and H2. The

calorific value provides though an indication about the heat potential of each material and its

ability to reduce coke consumption. High calorific value injection materials are expected to

increase the heat flux in the raceway and consequently the RAFT.

2.1.4 Ash properties The ash content of the injection materials along with the composition of the ash play a decisive

role in the evaluation of the suitability of a material for injection in the blast furnace. A high

ash content of the material can cause lance blockage while it consumes energy to remain in the

molten phase and increases the slag volume. At the same time it may contribute to blockage of

the raceway through the formation of a ‘bird’s nest’, while its deposition on the stagnant coke

layer may alter the reactivity of coke and cause permeability problems in the ‘deadman’ zone.

The composition of the ash content is the major factor influencing its fusion characteristics and

an excess of acidic (SiO2) or basic (CaO) oxides may give ash deposition problems due to

increased deformation and melting temperatures.

The effect of the injectant’s residue when it comes into contact with coke is of outmost

importance because it influences coke reactivity. Most coke weakening by the solution loss

reaction takes place in the active coke zone and ashes with high alkali, iron oxides, CaO and

MgO content can catalyze the endothermic solution loss reaction in case of effective contact

with coke (Björkman, 2017).

2.2 Injectants reactions in the raceway Reactions between the solid particles and the gaseous atmosphere take place as soon as

auxiliary reducing agents are introduced into the gaseous stream. Devolatilization is a process

which occurs throughout the solid particle’s volume, while combustion and gasification are

surface reactions that take place at the boundary between solid and gas.

Heterogeneous reactions of injected particles with the gaseous atmosphere are highly dependent

on temperature, which defines whether the rate limiting step of the overall process is chemical

reaction or diffusion of the gaseous reactants and products. In the low temperature range

chemical reaction is the rate limiting step, in the middle temperature range the rate is controlled

by both chemical reaction and diffusion, while at high temperatures diffusion of reactants and

products in the boundary layer limits the rate (Ishii, 2000).

Devolatilization, combustion and gasification are heterogeneous reactions intimately connected

to the injected material’s behavior in the raceway. The basic formulas that can describe these

phenomena are listed in Table 2.1:

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Table 2.1. Particle reactions during auxiliary reducing agents injection

Devolatilization

Raw Injection Materials → Volatile Matter (VM)

→ Char (C<S>) + Residue (Ash)

Combustion

C<S> + 0.75 O2 → 0.5 CO + 0.5 CO2

Gasification

C<S> + CO2 → 2 CO

C<S> + H2O → CO + H2

These reactions cannot describe the process by themselves since they constitute only a part of

the overall phenomenon. Moreover, they cannot be separated completely since they overlap in

the actual process. However, by studying them separately, insight in the process can be obtained

and an injection material’s beneficial and detrimental characteristics can be determined.

2.2.1 Devolatilization When an auxiliary reducing agent is injected in the blast furnace raceway, it is heated up by

convection from the hot blast and radiation from the furnace walls, flame and other burning

particles. This causes the material to release gaseous and liquid products which create a burning

atmosphere around the particles and further provoke the particles’ devolatilization.

In order to simulate the devolatilization process in mathematical terms, several models can be

used. The most primitive is the first order reaction model:

(𝑅𝑎𝑤 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙)𝑘→(𝑉𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑀𝑎𝑡𝑡𝑒𝑟) + (𝑅𝑒𝑠𝑖𝑑𝑢𝑒) R. 2

where k is the kinetic rate constant.

This model postulates that the rate of devolatilization, 𝑅𝑉𝑀, is proportional to the amount of

volatile matter remaining, 𝑚𝑉𝑀:

𝑅𝑉𝑀 =

𝑑𝑚𝑉𝑀𝑑𝑡

= 𝑘 ∙ 𝑚𝑉𝑀 Eq. 2.1

where 𝑑𝑚𝑉𝑀 is the mass change of volatile matter, 𝑑𝑡 is the change in time. The kinetic constant

k is defined by the Arrhenius law:

𝑘 = 𝐴 ∙ 𝑒−

𝐸𝑅𝑇 Eq. 2.2

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where A is the pre-exponential factor, E is the activation energy, R is the universal gas constant

and T is the absolute temperature.

The competing rate model assumes that devolatilization can be described by a pair of competing

first order reactions with corresponding kinetic rates k1 and k2, that control the devolatilization

rate over different temperature ranges:

(𝑅𝑎𝑤 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙) <

𝑘2→ 𝑎2(𝑉𝑜𝑙𝑎𝑡𝑖𝑙𝑒)+(1−𝑎2)(𝑅𝑒𝑠𝑖𝑑𝑢𝑒)

𝑘1→ 𝑎1(𝑉𝑜𝑙𝑎𝑡𝑖𝑙𝑒)+(1−𝑎1)(𝑅𝑒𝑠𝑖𝑑𝑢𝑒) R. 3

The expression that describes the releasing rate of volatile matter in this case is:

𝑑𝑚𝑉𝑀𝑑𝑡

= (𝑎1𝑘1 + 𝑎2𝑘2)𝑚𝑅𝐶 Eq. 2.3

where ai, ki and mRC are the stoichiometric coefficient, reaction rate constant and mass of

unreacted coal in a coal particle (in case injection material is pulverized coal) respectively. The

rate constants k1 and k2 are given by Arrhenius type equations and k2 contains a higher

activation energy (Ishii, 2000).

2.2.2 Combustion Combustion takes place at the charified materials surface, in combination with the evaporation

of the remnant volatile mater and its combustion in the gas phase. Combustion of the solid

material takes place at higher temperatures than devolatilization, so combustion is preceded by

devolatilization.

Efforts to model the char combustion near or at atmospheric pressure have produced several

results which establish the temperature and oxygen concentration dependence of the process

considering single step or multi step reactions, with the corresponding number of kinetic

constants. All models presented below assume surface reaction, so C stands for active carbon

site.

The Global Power- Law Kinetics model considers the following reaction between the active

carbon site and oxygen:

C + O2

𝑘→ CO/CO2

R. 4

with the corresponding rate law given by:

rcom=kPO2n Eq. 2.4

17

where rcom is the combustion rate, k is the kinetic constant and 𝑃𝑂2is the oxygen partial pressure.

The Langmuir-Hinshelwood-form model considers the intermediate complex C(O) generation

between an active site and an absorbed oxygen atom, which influences the overall kinetic rate

according to the reaction mechanism:

2C+O2

k1→ 2C(O)

R. 5

C(O)

k2→CO

R. 6

with the corresponding reaction rate:

rcom=

k1k2PO2k1PO2+k2

Eq. 2.5

The Three-Steps Semiglobal Kinetics model includes two intermediate reactions whose rate

depends on their corresponding kinetic constants according to the reaction mechanism:

C+O2

k1→ C(O)

R. 7

C(O) + O2

k2→ CO − CO2 + C(O)

R. 8

C(O)

k3→CO

R. 9

with the corresponding reaction rate law:

rcom=

k1k2PO2+k1k3PO2

k1PO2+k32

; CO

CO2=

k3k2PO2

Eq. 2.6

The Baum and Street model assumes that the char particles are spherical and that the reaction

rate is determined by the chemical and/or diffusion kinetics. This model is expressed by the

following equation:

𝑑𝑚

𝑑𝑡= −𝜋𝐷𝑝

2𝜌𝑅𝑇(𝑋𝑂2𝑀𝑂2

)(1

𝑅𝐷+1

𝑅𝐾)−1

Eq. 2.7

where dm/dt is the rate of char mass loss, Dp is the char particle diameter, ρ is the coal density,

XO2 is the oxygen mole fraction, 𝑀𝑂2 is the molecular weight of oxygen, while R is the universal

gas constant. RD and RK are the diffusion and reaction rate coefficients respectively, with RK

defined as:

18

𝑅𝐾 = 𝐴𝜑𝑒−𝐸𝛼𝑅𝑇 Eq. 2.8

where A is the pre-exponential factor, φ is the ratio of reacting surface to external (equivalent

sphere) area of the particle and Ea is the chemical reaction activation energy (Barranco, et al.,

2009).

Similar to the Baum and Street model is the Multiple Surface Reaction model, where the

particle surface reaction rate is controlled by the kinetic rate, RK, and the diffusion rate, RD,

according to the formula:

𝑅𝐶 = 𝐴𝑝𝑓𝑠𝑝𝑔

𝑅𝐾𝑅𝐷𝑅𝐾 + 𝑅𝐷

Eq. 2.9

where Ap is the particle surface area, fs is the mass fraction of reacting solid species in a particle

and pg is the bulk partial pressure of reacting gas species (Ölund, et al., 2017).

2.2.3 Gasification Gasification of injection material chars with CO2 starts in the raceway when the CO2 content

of the gaseous atmosphere and the prevailing temperature are adequate for the reaction to occur.

The gasification reaction continues to take place outside the raceway boundaries, where unburnt

char fines are entrained into the gas flow. In general the reaction of char carbon with CO2 is

slower than combustion and this is reflected in the comparison between the combustion and

gasification kinetic parameters.

Several models have been proposed to describe the char CO2-gasification, with the most

appropriate to fit the TGA data those that consider a single step reaction. This reaction is the

solution loss or Boudouard reaction given by the formula:

C+CO2

k→ 2CO R. 10

The simplest model is the Volumetric model, which assumes homogeneous reaction of the char

by uniform diffusion of the gas in the entire particle volume. This model can be represented by

the formula:

dX

dt=k(1-X) Eq. 2.10

where X is the material conversion, t is the time and k is the kinetic constant. X is given by the

formula:

19

𝑋 =

𝑤0 −𝑤𝑡𝑤0 −𝑤𝑓

Eq. 2.11

where w0 is the weight before gasification, wf the weight after gasification and wt the weight at

time t. By integrating the formula in Eq. 2.10, the following expression for the conversion

degree is derived:

ln (1 − X) = kt Eq. 2.12

with k following the Arrhenius law.

k=Aexp (-

E

RT)

Eq. 2.13

where A is the pre-exponential factor and E is the activation energy.

The Modified Volumetric model is a variation of the Volumetric model, with the addition of

the assumption that the kinetic constant (k) is changing with conversion (X) as the reaction

proceeds. The reaction rate and the conversion degree correspond to the following equations:

dX

dt=kMV(X)(1-X) Eq. 2.14

and after integration:

− ln(1 − X) = atb Eq. 2.15

where kMV(X) is the model corresponding kinetic constant and a and b empirical constants. The

kinetic constant can be expressed through the following formula:

kMV(X) = a

1bb[−ln (1 − X)]

b−1b

Eq. 2.16

The Granular model assumes that the reaction occurs at the external surface of the spherical

particle and as the reaction moves towards smaller particle diameters, only the ash layer

remains. This model is given by the formula:

20

dX

dt=kG(1-X)

23 Eq. 2.17

and the integrated form by:

3[1-(1-X)]

13=kGt

Eq. 2.18

where kG is given by the Arrhenius law (Irfan, et al., 2011).

The multiple surface reaction model also applies for gasification, where the particle surface

reaction rate is controlled by the kinetic rate, RK, and the diffusion rate, RD, according to the

formula:

𝑅𝐶 = 𝐴𝑝𝑓𝑠𝑝𝑔

𝑅𝐾𝑅𝐷𝑅𝐾 + 𝑅𝐷

Eq. 2.19

where Ap is the particle surface area, fs is the mass fraction of reacting solid species in a particle

and pg is the bulk partial pressure of reacting gas species (Ölund, et al., 2017).

21

3 Materials

The materials under investigation in this project were four hydrogen rich carbonaceous

materials:

• High volatile torrefied saw dust (HV-TSD)

• Torrefied food residue with code name Carbon PIMIENTO

• Torrefied food residue with code name Carbon PODA

• Recycled foam of Polyurethane (PUR)

The initial shape of the materials was irregular while they contained some coarse particles

greater than 1 cm in size, with the exception of PUR that was already fine in size and granular.

The materials were characterized by means of proximate, ultimate and ash analyses. These

analyses were conducted in ALS Scandinavia AB laboratories in Luleå, while the reference PC

used in this project has been characterized previously by (Ölund, et al., 2017) and the results

are given in Table 3.1:

Table 3.1. Proximate, ultimate and ash analyses

HV-TSD Carbon

PIMIENTO Carbon PODA

PUR PC

Proximate Analysis (wt%)

Moisture 2.5 2.6 1.8 14.3 1.2

Volatile Matter 72.3 62.7 67.5 66.4 18.4

Fixed Carbon 24.7 12.8 14.7 6.5 69.6

Ash 0.5 21.9 16.0 12.9 10.8

Ultimate Analysis (wt% dry basis)

C 55 48.8 49.7 63.2 79.12

H 5.8 4.7 5.4 6 3.93

N <0.10 2.66 1.2 5.81 1.96

O 38.7 20.5 27 9.5 4.02

Cl <0.01 0.32 0.18 0.43 0.00

S <0.012 0.465 0.212 0.051 0.27

Ash 0.5 22.6 16.3 15.0 10.70

Ash Analysis (wt% in total ash content)

Al 0.24 0.64 2.25 2.28 13.08

Ba 0.44 0.02 0.03 0.95 0.00

Ca 22.60 27.52 19.39 5.30 5.14

Cr 0.01 0.00 0.01 0.15 -

Fe 1.14 0.74 1.57 31.53 5.21

K 9.12 2.99 3.23 0.36 1.30

Mg 2.42 2.61 1.52 1.26 1.74

Mn 2.68 0.06 0.04 0.12 -

Na 0.33 0.84 0.62 0.55 0.29

P 0.96 1.43 0.98 0.12 0.34

S 0.00 3.08 1.69 0.00 -

22

Si 0.86 2.97 10.86 0.00 24.56

Ti 0.02 0.06 0.17 1.26 -

Others 59.18 57.04 57.65 56.12 48.33

All materials contain a high amount of volatile matter that varies between 63 and 72%, except

PC. HV-TSD is the material that contains the lowest amount of ash (0.5%) while it contains the

greatest percentage of volatile matter (72.3%). Carbon PIMIENTO and Carbon PODA contain

large amounts of calcium, magnesium and silicon, but this is expected since they are food

residues. Polyurethane foam is the only material that was not subjected to thermal pretreatment

and contains the highest percentage of moisture and the lowest percentage of fixed carbon,

although its dry basis carbon content is the highest (63.2%). Polyurethane foam also contains

the highest amount of chlorine (0.43% in total solids, mainly due to flame retardant additives)

which could pose a problem for utilization through combustion (chloride content can cause

corrosion of the steel in the blast furnace gas cleaning system) while a large amount of iron is

detected in its ash content that is connected to its origin which for the present project remains

unknown. The possibility that PUR could act as a credit material for hot metal production is

reasonable since a previous experience with injecting in-plant fines has shown that injected iron

oxides are quite early reduced to a state between wustite (FeO) and metallic iron (Björkman,

2017), although the quantity of iron contained in PUR can only have a negligible contribution

to the total metallic iron production.

23

4 Methods

4.1 Material Pretreatment The materials provided had to undergo comminution and stratification, in order to be suitable

for use in the subsequent analytical methods. Comminution was performed using a mortar mill

(PULVERISETTE 2), while stratification was done by means of a stack consisting of sieves

with nominal aperture sizes 106 and 53 μm and a bottom container. A simplified representation

of the pretreatment flowsheet using MODSIM software is illustrated in the following figure:

Figure 4.1. Material pretreatment process scheme

The corresponding machine definitions and parameters are listed in Table 4.1:

Table 4.1. Machine parameters

Machine number Definition

1 Mortar mill

2 Screen (nominal aperture: 106 μm)

3 Screen (nominal aperture: 53 μm)

4 Mixer

Mortar mill is represented by machine number 1 due to availability of shapes in MODSIM,

while the sieve stack is analyzed in machines 2 and 3. The materials were processed in the

mortar mill for 5 min (except from PUR which was already fine in size) and then sieved in a

sieve shaker for 5 min using the configuration described in Figure 4.1.

Table 4.2. Stream definition

Stream number Stream definition

1 Circuit feed

2 Mortar mill feed

3 Mortar mill product

4 Screen 2 oversize

5 Screen 2 undersize

6 Screen 3 oversize

7 Screen 3 undersize

24

The pretreatment process for each material was carried out until more than 5 g in stream number

6 were collected, so as to have sufficient quantity for the subsequent experiments. Stream

number 6 (size fraction 53-106 μm) was used for thermogravimetric analysis and particle size

analysis, stream number 7 (size fraction <53μm) was used for density determination while the

bulk samples were used for proximate and ultimate analysis, calorific value determination and

ash production.

4.2 Particle size analysis The particle size distribution of a material affects its physicochemical properties, such as the

flow characteristics, heat transfer and reactivity. In order for the particle size analysis to be

reliable and accurate, the sample analyzed has to be representative of the bulk material. Particle

size analysis is usually performed by sieving, but automatic analysis devices based on

technologies such as high definition image processing are becoming most common.

Dynamic Image Analysis (DIA) is a method used to automatically measure the particle size

distribution of a sample. The operating principle of the DIA method is that the particles of the

sample under investigation pass in front of two bright, pulsed led light sources, where their

shadows are captured with two digital cameras and analyzed to produce their size distribution

curves in real time.

A Retsch CAMSIZER X2 was used to automatically analyze the 53-106 μm samples and

produce their Particle Size Distribution graphs employing the Dynamic Image Analysis

technology. This apparatus is optimized for fine samples analysis (from 0.8 μm to 8 mm) and

the particular samples fall into this category and are hence suitable for analysis with the specific

equipment (HORIBA, 1996-2018).

4.3 Helium pycnometry A helium pycnometer calculates the true volume of a solid from the measured drop in pressure

when a known amount of gas is allowed to expand into a chamber containing the sample. This

volume, combined with the mass of the sample under investigation, gives the true density of

the sample.

An AccuPyc II 1340 helium pycnometer was used to measure the true density of the <53μm

sieved samples. The true density of the sieved fine fraction is equal to the true density of the

other size fractions, since the true density should not be affected by milling or sieving of the

material.

4.4 Bomb calorimetry A bomb calorimeter consists of a steel container (bomb) where a weighted mass of the sample

under investigation is loaded and then the whole inner chamber is pressurized with excess pure

oxygen at 30 bar. The bomb is submerged under a known volume of water and the weighted

reactant is ignited. The energy released by the combustion as heat crosses the stainless steel

wall raising the temperature of the surrounding water jacket. The temperature change in the

water is then accurately measured and used to calculate the energy given out by the sample

burn.

An IKA C200 bomb calorimeter was used to assess the higher heating value of the samples

provided. Approximately 0.5 g of each bulk sample was used for each test, while each material

was tested three times (with the exception of PUR that was tested twice) in order to get an

average value for each material that would be more credible.

25

4.5 Thermogravimetric analysis Thermogravimetric analysis (TGA) is a technique in which the mass of a substance is monitored

as a function of temperature or time as the sample is subjected to a controlled temperature

program in a controlled atmosphere.

During a thermogravimetric analysis, the sample under investigation is put in a crucible which

is supported by a precision balance. This crucible resides in a water cooled furnace and is heated

or cooled during the experiment. The mass of the sample is monitored throughout the

experiment while a purge gas controls the sample’s environment. This gas may be inert or

reactive, flows over the sample and exits through an exhaust (PerkinElmer, Inc., 2010).

Themogravimetric analysis of carbon containing substances can indicate mainly four material

characteristics:

• Drying: occurs when moisture and other solvents are removed from the material

through evaporation at temperatures around the water boiling point.

• Devolatilization: occurs when loosely bonded hydrocarbon compounds are liberated

from the material through heating in an inert atmosphere forming charified residue.

• Combustion under oxygen rich atmosphere: occurs when char undergoes oxidation

(complete or partial) with oxygen.

• Gasification under CO2 atmosphere: occurs when char reacts with available carbon

dioxide according to the Boudouard reaction and forms gaseous products.

The employed device was a Netzsch STA 409 instrument with simultaneous thermogravimetric

measurement (TGA) with sensitivity ±1 μg and differential thermal analysis (DTA) coupled

with a quadruple mass spectrometer.

4.5.1 Devolatilization

4.5.1.1 Experimental procedure

Devolatilization was carried out twice for each material, once followed by combustion and once

followed by gasification. A weighted amount of ~50 mg of the 53-106 μm sieved fraction of

material was used in each experiment. The material was placed into an alumina crucible inside

the TG chamber and heated under an argon stream of 100 ml/min with a heating rate of 5 K/min

from ambient temperature up to 800oC. Then the sample was cooled with a cooling rate of 20

K/min up to the starting temperature for the subsequent program, which was 100oC in case

combustion followed or 500oC in case gasification followed.

26

Figure 4.2. Temperature profile during devolatilization

4.5.1.2 Modeling methodology

In order to extract the kinetic constants for the devolatilization process, a first order reaction

model was selected. By combining the rate equation for volatile matter (Eq. 2.1) and the

Arrhenius equation for the kinetic constant (Eq. 2.2), the following formula is derived:

𝑑𝑚𝑉𝑀

𝑑𝑡= A · exp (−

𝐸

𝑅𝑇) ∙ 𝑚𝑉𝑀 Eq. 4.1

And by transformation:

ln (dmVMdt

1

mVM) = ln(𝐴) −

E

RT

Eq. 4.2

Consequently by using the mass loss data derived from the experiments and by plotting the first

part of the equation against 1/T, a straight line is derived whose extrapolation to the y-axis gives

the ln(𝐴) value, while its slope equals to -Ea/R. The value for dmVM/dt (the devolatilization

rate) is acquired by dividing the mass difference by the time difference between two subsequent

data points.

4.5.2 Combustion


Combustion was carried out once for each material, each time preceded by devolatilization,

thus it was implemented on char. Combustion was accomplished by injecting a flow of 200

ml/min of synthetic air (20.9 %O2, 79.1 %N2 by vol.) into the TG chamber where the sample

lingered after devolatilization. In order for combustion not to start immediately with the

injection of synthetic air and to derive a mass loss curve amenable to analysis, the charified

sample was first cooled down to 100oC before air was injected. Then, under the synthetic air

Combustion starting point

Gasification starting point

0

100

200

300

400

500

600

700

800

900

0 50 100 150 200 250

Tem

per

atu

re [

oC

]

Time [min]

Devolatilization temperature profile

27

flow, the sample was heated to 700oC with a heating rate of 2 K/min and then cooled to 200oC

with a cooling rate of 20 K/min.

Figure 4.3. Temperature profile during combustion


In order to model combustion a surface reaction model had to be chosen, where the mass loss

depends on particle density and diameter. Thus the Baum and Street model was deemed

appropriate, a model customized to spherical char particles combustion. In the specific

conditions under which the experiments were performed, the diffusion rate coefficient (RD) was

assumed to be much higher than the reaction rate coefficient (RK), so the rate equation (after

incorporating the reaction rate coefficient formula) reduces to:

dm

dt= −πDp

2ρRT(XO2MO2

)(𝐴φe−EaRT)

Eq. 4.3

Assuming homogeneous composition in the particle, the ratio of reacting surface to external

surface of the particle (φ) can be approximated by the ratio between the remnant combustible

mass of the char (mc) to the remnant mass of the sample (combustible + ash, m). After

transformation, the equation becomes:

ln (

dm

dt

m

mc

−MO2XO2πDp

2ρRT) = ln(𝐴) −

EaRT

Eq. 4.4

Regarding the particles diameter, Dp, spherical particles with initial diameter equal to the d50 of

the measured Particle Size Distribution were assumed. Thus the initial volume of each particle

(Vp,o) is equal to:

0

100

200

300

400

500

600

700

800

0 50 100 150 200 250 300 350

Tem

per

atu

re [

oC

]

Combustion time [min]

Combustion temperature profile

28

𝑉𝑝,𝑜 =

𝜋𝑑503

6

Eq. 4.5

and the initial total volume of the sample:

𝑉𝑡𝑜𝑡𝑎𝑙,𝑜 = 𝑛𝑝𝑉𝑝,𝑜 Eq. 4.6

where np is the total number of particles in the sample. The initial number of particles (np) was

calculated by taking into account the initial sample weight (mo) and the true density (ρ)

(measured by helium pycnometry), according to the formula:

𝜌 =

𝑚𝑜𝑉𝑡𝑜𝑡𝑎𝑙,𝑜

↔ 𝑛𝑝𝜋𝑑50

3

6=𝑚𝑜𝜌 ↔ 𝑛𝑝 =

6𝑚𝑜

𝜋𝑑503 𝜌

Eq. 4.7

Thus the number of particles for each sample can be calculated by using the initial sample

weight and the d50 derived by DIA. The number of particles is assumed to remain constant

throughout the devolatilization and combustion process (no particle fragmentation is supposed

to occur) and so does the true density. For this reason, the particle diameter can be calculated

at any time using the sample mass (m) according to the formula:

𝐷𝑝 = (6𝑚

𝜋𝑛𝑝𝜌)

13

Eq. 4.8

By substituting Eq. 4.11 into Eq. 4.4, the following formula is derived:

𝑙𝑛 (−

𝑑𝑚

𝑑𝑡

1

𝑚𝑐

𝑀𝑜2XO2𝑅𝑇

(𝑛𝑝6)23(𝑚

𝜋𝜌)13) = ln(𝐴) −

EaRT

Eq. 4.9

Using the mass loss data and plotting the first part of the equation against 1/T, one gets a straight

line whose extrapolation to the y-axis gives the ln (𝐴) value, while its slope equals to -Ea/R.

4.5.3 Gasification


Gasification was performed by injecting a flow of 200 ml/min pure CO2 in the TG chamber

after devolatilization was completed and the charified samples were cooled down to 500oC.

Under the CO2 flow, the samples were heated to 1000oC with a heating rate of 2 K/min and

then cooled to 200oC with a cooling rate of 20 K/min.

29

Figure 4.4. Temperature profile during gasification


A heterogeneous surface reaction model where the reaction rate depends on the particle surface,

the fraction of reacting solid species and the bulk partial pressure of the reacting gas species

was chosen to represent the gasification process. Thus, the gasification TG results were

analyzed using the surface particle reaction model. In the temperature range the experiments

were conducted, diffusion is much quicker than reaction so the diffusion rate coefficient (RD)

is much greater than the reaction rate coefficient (RK). Under the aforementioned assumption

and after substituting RK with the Arrhenius equation, Eq. 2.19 reduces to:

𝑅𝐶 = 𝐴𝑝𝑓𝑠𝑝𝑔𝐴𝑒

−𝐸𝑎𝑅𝑇 Eq. 4.10

fs can be substituted by mc/m (in this case mc represents the remaining mass available for

gasification). Regarding the particles’ surface area (Ap), spherical particles with initial diameter

equal to the d50 of the measured Particle Size Distribution was assumed. This way an initial

number of particles (np) was calculated by taking into account the initial sample weight (mo)

and the true density (ρ) (measured by helium pycnometry), according to the formula:

ρ=

moVtotal

↔ npπd50

3

6=moρ ↔ np=

6mo

πd503 ρ

Eq. 4.11

The number of particles is assumed to remain constant throughout the devolatilization and

gasification process (no particle fragmentation is supposed to occur) and so does the true

density. For this reason, the particle diameter can be calculated at any time using the sample

mass (m) according to the formula:

0

200

400

600

800

1000

1200

0 50 100 150 200 250 300 350

Tem

per

atu

re [

oC

]

Gasification time [min]

Gasification temperature profile

30

𝐷𝑝 = (6𝑚

𝜋𝑛𝑝𝜌)

13

Eq. 4.12

while the total surface area of the particles becomes:

Ap=npπd

2=(npπ)13(6m

ρ)23 Eq. 4.13

By substituting fs and Ap, Eq. 4.10 becomes:

−𝑑𝑚

𝑑𝑡= (npπ)

13(6m

ρ)23𝑚𝑐𝑚𝑝𝑔𝐴𝑒

−𝐸𝑎𝑅𝑇 ↔

ln(−𝑑𝑚

𝑑𝑡(𝑛𝑝𝜋)

13 (6

𝜌)−23 𝑚

13

𝑚𝑐𝑝𝑔) = 𝑙𝑛(𝐴) −

𝐸𝑎𝑅𝑇

Eq. 4.14

By plotting the left side of the equation against 1/T one gets a straight line whose extrapolation

to the y-axis gives the ln (𝐴) value, while its slope equals to -Ea/R.

4.6 Mass Spectrometry Mass spectrometry is an analytical technique that detects the substances that compose a sample

by separating them according to their molecular mass. The way to achieve this separation is by

ionizing a small amount of the material and uniformly accelerating them through a pair of

oppositely charged plates. Then a vertical magnetic field deflects the accelerated ions and

causes them to follow different trajectories depending on the inertia of each ion which is

proportional to its mass-to-charge ratio (Reusch, 2013).

A Quadruple Mass Spectrometer was integrated in the Netzsch STA 409 off gas port in order

to monitor the composition of the evolved gases in the Thermogravimetric chamber during the

mass loss cycles of the materials under investigation. This way additional information about

the evolved gas composition under thermal treatment of the sample would be provided.

4.7 Ash production Ash was prepared by heating the samples for 3 hours at 950oC in a muffle furnace where air

was allowed to circulate, i.e. the atmosphere was ambient. Approximately 10 g of each of the

4 samples and one pulverized coal reference sample were put in separate crucibles and heated

in a muffle furnace under air in order for the volatile and carbon content to oxidize and

evaporate. The products of this process (the solid residues) represented the ash content of each

material and after cooling were collected in separate bottles.

4.8 Heating microscopy When a solid material is heated under inert atmosphere, it undergoes phase transitions such as

melting, where the ordering forces in the solid lattice disappear and the molecules start to move

freely. Transition from the solid to the liquid phase can be observed through a change in external

area and form when a test object of the material under investigation is subjected to an

appropriate temperature program.

31

Heating microscopy is a thermo-optical analysis experimental technique where a sample of the

material under investigation is subjected to thermal treatment in order to monitor its contour

and silhouette changes which are correlated to the materials characteristic temperatures.

A Hesse Instruments heating microscope was used to perform this analysis. The ash samples

were packed in a mold to create small cylindrical briquettes with diameter 2 mm and height 3

mm, where approximately 20 mg of each material was compacted. These briquettes were put

on coke horizontal substrates and the whole assembly was positioned in the tube furnace of the

heating microscope.

A heating program of 15oC/min to 600oC and then 10oC/min to 1550oC with 2 h dwell time at

1550oC under 200 ml/min Ar flow was implemented. The deformation, sphere, hemisphere,

flow temperatures were monitored.

4.9 Scanning Electron Microscopy Scanning Electron Microscopy is based on focusing a fine probe of electrons with energies up

to 40 keV at the surface of a specimen and scanning across it in a pattern of parallel lines.

Several phenomena occur at the surface of the specimen under electron impact, with most

important the emission of secondary electrons with energies of a few tens eV. Only the

secondary electrons produced within a very short distance from the sample are able to escape,

making this type of detection mode appropriate for high resolution topographical images.

The coke substrates after interaction with the ash briquettes in the heating microscopy

experiments were examined with the Scanning Electron Microscopy secondary electron

detection technique. Magnifications used varied from 69x to 3040x, in order to assess the effect

of the injection materials’ residues on coke under the simulated conditions of the blast furnace

stagnant coke layer.

The morphological study with Scanning Electron Microscopy was performed in comparison to

the coke pieces that had not undergone any processing or that have been treated as the other

coke substrates but was not covered with ash.

4.10 Coke reactivity evaluation The reactivity of coke after heating with the investigated materials’ ashes was examined by

thermogravimetric analysis.

The coke substrates that had undergone interaction with the ashes of the four injection materials

and that of the pulverized coal reference material were compared with coke that had not

undergone any interaction with ashes but was pretreated under the same thermal conditions.

The method to examine coke reactivity was thermogravimetric analysis during gasification

under carbon dioxide atmosphere. Each coke substrate after being subjected to the heating

microscopy experiment where it interacted with the ash briquettes was put in the TGA chamber

under 200ml/min CO2 flow and subjected to a heating program. This program consisted of

heating the coke substrate with a rate of 20 K/min from ambient temperature to 1000oC where

it stayed for 1h and was afterwards cooled with a rate of -20 K/min to 200oC.

32

5 Results

5.1 Grinding and sieving the materials Grinding was performed using a mortar mill where the samples were prepared for the

subsequent automated Particle Size and Thermogravimetric Analyses. HV-TSD has a fibrous

texture due to its origin and was not homogeneously ground. The two torrefied food residues

exhibited a better grindability, while PUR was already in granular form.

Sieving was performed on a sieve shaker, using a sieve stack configuration consisting of a 106

μm sieve, a 53 μm sieve and a bottom plate.

Grinding and sieving of each material was performed until more than 5 g of each material was

collected on the 53 μm sieve (53-106 μm size fraction). This size fraction was appropriate for

use in the Thermogravimetric Analyzer, where the materials to be tested have to be finely

ground and approximately 10-100 mg of material (depending on the sample’s specific gravity)

can be fitted in the alumina crucible for analysis.

5.2 Particle size analysis of the 53-106 μm size fraction The results from the automated particle size analysis are shown in the diagram bellow.

Figure 5.1. Automated particle size analysis of the 53-106 μm size fraction

Separation of the 53-106 μm size fraction through sieving seems to be imperfect and all samples

contain particles that do not belong to the pursued size range. PUR is the material that contains

the majority of particles in the 53-106 μm size range, mainly due to its particles’ regular shape

and non-sticking character. Carbon PIMIENTO contains the greatest amount of fine particles

below 53 μm, while HV-TSD and Carbon PODA contain the greatest amounts of coarse

particles above 106 μm. Stratification of the samples in such fine size range by means of

conventional laboratory sieves is proven inefficient.

Agglomeration of the particles during storage is a factor that may have contributed to a large

extent for the detected out of range particles. Dispersion of the particles while feeding them to

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1

Cu

mu

lati

ve u

nd

ersi

ze [

wt%

]

Size [mm]

Particle size distribution of the 53-106 μm sieved samples

HV-TSD

Carbon PIMIENTO

Carbon PODA

PUR

33

the Dynamic Image Analysis device is most important in order to minimize the error of the

analysis, since the dimensions of single and not agglomerated particles should be measured.

Chemical reactivity during combustion and gasification is influenced by the particle size of the

material but due to the other differences between the materials (carbon content, composition

etc.) correlation between particle size and reactivity for the materials under investigation cannot

be made.

5.3 Density determination The results from the helium pycnometer tests of the materials are illustrated in the following

table:

Table 5.1. Helium pycnometry results

Sample True density (g/cm3)

Stdev (g/cm3)

HV-TSD 1.413 0.002

Carbon PIMIENTO 1.503 0.005

Carbon PODA 1.503 0.001

PUR 1.455 -

PC 1.470 -

Measurements for all materials were taken twice (with the exception of PUR, whose true

density was measured once) and an average value was calculated for each material.

The true density of all samples lies in the range between 1.4-1.5 g/cm3. The two food residues

(Carbon PIMIENTO and Carbon PODA) have very similar densities (~1.50 g/cm3).

5.4 Bomb Calorimetry The results obtained by conducting the bomb calorimetry experiments for the materials under

investigation are listed in Table 5.2:

Table 5.2. Bomb calorimetry results

Sample Higher Heating Value

(MJ/kg) Stdev

(MJ/kg)

HV TSD 22.006 0.326

Carbon PIMIENTO 19.411 0.241

Carbon PODA 20.826 0.806

PUR 22.436 0.861

PC 28.810 -

A correlation between the calorific value and the composition of each substance can be

observed. Carbon is the dominant element in all materials with a calorific value of 32.8 MJ/kg

(Engineering Toolbox, 2003). This is the base value which is lowered by the presence of oxygen

and ash.

The measured higher heating value can be expressed based on the dry mass content of the

material by applying the following formula:

34

𝐻𝐻𝑉𝑑 =

𝐻𝐻𝑉

1 −𝑀 Eq. 5.1

where HHV is the higher heating value determined by the calorimeter, HHVd is the higher

heating value of the dry sample and M the moisture content of the sample. Using the moisture

content values obtained by the proximate analysis of the samples, the higher heating value of

the dry samples is:

Table 5.3. Higher heating value for dry materials

Sample HHVd

(MJ/kg)

HV TSD 22.594

Carbon PIMIENTO 19.529

Carbon PODA 20.973

PUR 26.180

The higher heating value obtained by the experiments can be validated by comparing it to the

one theoretically calculated by using the formula by Gaur and Reed (Sokhansanj, 2011):

HHVd,th = 0.35𝑋𝐶 + 1.18𝑋𝐻 + 0.10𝑋𝑆 − 0,02𝑋𝑁 − 0,10𝑋𝑂 − 0.02𝑋ash Eq. 5.2

where HHVd,th is the dry basis higher heating value (in MJ/kg), Xc, XH, XS, XN, XO, Xash the

carbon, hydrogen, sulfur, nitrogen, oxygen and ash content respectively of the dry materials

(derived by the ultimate analysis). The results for the higher heating value for the dry materials

calculated this way are the following:

Table 5.4. HHVd calculated using Gaur and Reed formula

Sample HHVd,th (MJ/kg)

HV TSD 22.171

Carbon PIMIENTO 20.557

Carbon PODA 21.050

PUR 27.853

A comparison between the measured (by bomb calorimetry) and theoretical (by Gaur and Reed

formula) higher heating values can be graphically illustrated in the following figure:

35

Figure 5.2. Comparison between experimental and theoretical dry mass HHV

A good correlation between the measured and calculated HHVd can be observed, while the

biggest difference exists for the PUR material, where the calorimetric value was the most

inaccurate, since it contained the highest standard deviation.

5.5 Thermogravimetric Analysis The samples were tested in a Thermogravimetric Analyzer to monitor their devolatilization,

combustion and CO2-gasification behavior and derive the kinetic constants associated with the

reactions that describe these phenomena.

Thermogravimetric analysis was coupled with Mass Spectrometry in order to acquire a more

credible estimation about what happens to the materials when undergoing the prescribed

treatment in the thermogravimetric chamber.

Devolatilization was performed in all experiments, so that combustion or gasification would be

subsequently implemented on the charified samples.

5.5.1 Devolatilization Devolatilization was monitored twice for each of the four samples in the TG analyzer, once

followed by combustion and once followed by gasification. The results for each material can

be graphically represented in the following figures:

0

5

10

15

20

25

30

HV TSD CarbonPIMIENTO

CarbonPODA

PUR

HH

Vd

(MJ/

kg)

Measured versus calculated HHVd

Higher Heating Value drybasis - measured

Higher Heating Value drybasis - calculated

36

Figure 5.3. TGA graphs during devolatilization

37

The two torrefied food residues start losing their volatile matter at around 170oC, followed by

PUR and lastly by HV-TSD, which starts devolatilization above 220oC.

Carbon PIMIENTO is the material which contains the smallest amount of volatile matter, which

according to its mass loss curve and after subtraction of the moisture content is 55.5 wt%. This

value is inferior to the proximate analysis VM content, which is determined as 62,7 wt%.

Carbon PODA loses 59.4 wt% VM during TGA devolatilization, while its corresponding

proximate analysis value is 67.5 wt%.

The highest amount of VM is contained in HV-TSD, which according to its proximate analysis

is 72.3 wt%, while during TGA devolatilization a value of 61.7 wt% was obtained.

PUR exhibits the greatest mass loss which according to the proximate analysis is due to both

volatile mater and moisture but cannot be readily discriminated in the mass loss curves

produced. The total mass loss for PUR devolatilization is 73.5 wt%, while the proximate

analysis determines 14.3 wt% for moisture and 66.4 wt% for VM, which sum up to 80.7 wt%,

which means that in the TGA devolatilization not all VM is released.

Mass loss is much steeper for HV-TSD, with the mass loss being terminated before 400oC,

followed by PUR (devolatilization ends at ~500oC), while the two food residues continue losing

volatile mass above 600oC, thus having the smoothest mass loss with time.

From the mass loss curves one can distinguish the multi step devolatilization that takes place

for the two food residues, while PUR also exhibits a secondary devolatilization step at around

400oC where there is a second plunge in its mass loss curve.

A graphical evaluation of the devolatilization kinetic parameters was performed by means of

the method described in chapter 4.5.1.2. The results from the graphical evaluation for the

activation energies and the corresponding pre-exponential factors are listed in the following

table:

38

Table 5.5. Results from graphical evaluation of kinetic parameters for devolatilization

Ea (kJ/mol) A (s-1) Temperature range (oC) Primary

Devoaltilization Secondary

Devolatilization Primary


Devolatilization Primary


Devolatilization

HV-TSD d+c* 110 - 8.261E+06 - 222-325 -

d+g* 113 - 1.395E+07 - 222-326 -

Carbon PIMIENTO

d+c 59 182 1.581E+02 2.801E+07 188-301 578-680

d+g 64 229 5.367E+02 3.681E+10 188-301 578-655

Carbon PODA

d+c 57 186 1.008E+02 8.182E+07 179-301 578-629

d+g 54 152 5.781E+01 9.488E+05 164-301 604-685

PUR d+c 84 71 4.378E+04 4.236E+02 197-306 375-426

d+g 90 75 1.565E+05 9.470E+02 212-301 376-426 * d+c: devolatilization followed by combustion, d+g: devolatilization followed by gasification

The secondary devolatilization step activation energies are much higher than those for the first

ones, meaning that the energy barrier that has to be overcome in order for secondary

devolatilization to occur is much greater than the primary one. This can be explained by the

fact that primary devolatilization includes the release and evaporation of tars, while at the

second devolatilization step chemically bonded species are released (Björkman, 2017).

A feature that has got to be taken into account is the mass gain during the change in the gaseous

stream from Ar to synthetic air or CO2. This mass gain can be observed at end of the

devolatilization and the start of the combustion or gasification process but is more pronounced

during switch from Ar to CO2(gasification). This mass gain is attributed to the sensitivity of the

precision balance, the low sample mass and the change in buoyancy during switch between

gases of different molecular weight and injection rate.

5.5.2 Combustion Combustion took place by injecting synthetic air (20.9 vol% O2, 79.1 vol% N2) into the TG

chamber with the already charified samples (the samples that had been subjected to the

devolatilization program). In the blast furnace raceway part of the injected material (the volatile

content) instantly volatilizes into the gas phase, ignites and burns homogeneously, while part

of the remnant material (the charified carbonaceous content) combusts less vigorously while

still in the solid phase. This later part of the material is the subject of the combustion

experiments in the investigations conducted.

The mass loss curves for the charified samples under these predetermined conditions are

illustrated in Figure 5.4:

39

Figure 5.4. TGA graph during combustion

The starting point for the combustion is the end of the devolatilization process, where the mass

loss corresponds to the volatile matter of the original materials.

The chemical composition of the two food residues exhibits the highest complexity, since their

mass losses during combustion occur in multiple steps, while they take place during a prolonged

period of time compared to the other materials and consequently in a wider temperature range.

HV-TSD exhibits the lowest ash content (final residue after combustion), a fact that is in good

agreement with its proximate analysis.

The results from the Thermogravimetric Analysis for the evaluation of the combustion kinetic

parameters of the four charified samples by means of the method described in chapter 4.5.2.2

are summarized in the following table:

Table 5.6. Results from graphical evaluation of kinetic parameters for combustion

Ea (kJ/mol) A (kg.m-2.s-1.Pa-1) Temperature range (oC)

primary secondary primary secondary primary secondary

HV TSD 116 - 1.723E+00 - 314-452 -

Carbon PIMIENTO 111 148 2.783E+01 4.853E+02 264-314 383-412

Carbon PODA 135 - 7.424E+02 - 285-345 -

PUR 119 - 2.040E+01 - 294-383 -

Secondary combustion for Carbon PODA is distinguishable in its mass loss curve but the

acquisition of credible kinetic parameters for this process is not feasible, since the graphical

representation of the transformed variables produces linear fittings of scattered points with low

R2 values that are unacceptable.

40

5.5.3 Gasification Gasification took place by injecting pure CO2 gas into the TG chamber, after the samples had

been charified through the devolatilization process. This procedure replicates the conditions

that the charified samples encounter at the end of the raceway, where the gaseous atmosphere

consists mainly of carbon dioxide.

The mass loss of the charified samples under gasification with CO2 at the predetermined

temperature profile is illustrated in Figure 5.5:

Figure 5.5. TGA graph during gasification

Gasification begins for all samples above 600oC. PUR is the material whose char starts to gasify

last and this could be attributed to the flame retardants it contains. The two food residues

(CarbonPODA and Carbon PIMIENTO) commence their gasification at lower temperatures

than HV-TSD and PUR, while their gasification mass loss is greater than that of PUR and lower

than that of HV-TSD.

The two food residues exhibit a multi stage gasification while HV-TSD and PUR gasify in a

single step. Carbon PIMIENTO char exhibits a mass loss when it heats up to more than ~850oC

and a mass gain when it cools down to ~850oC that provides evidence for reversibility of this

process. This phenomenon can be attributed to carbonates (such as CaCO3) dissociation while

heating above 850oC and reformation when cooling back to 850oC.

The estimated kinetic parameters given in Table 5.7:

Table 5.7. Results from graphical evaluation of kinetic parameters for CO2 gasification

Ea (kJ/mol) A (kg.m-2.s-1.Pa-1) Temperature range (oC)

HV-TSD 232 1.531E+05 720-840

Carbon PIMIENTO 229 7.057E+05 655-785

Carbon PODA 185 2.104E+03 656-805

PUR 337 5.329E+10 725-826

41

The fact that gasification is the slowest step of the reactions investigated (devolatilization,

combustion and gasification) can be reflected in the activation energy values experimentally

determined. Thus the apparent activation energy for gasification is higher for all materials

compared to combustion and devolatilization. The temperature where gasification takes place

corresponds to the temperature above which the endothermic Boudouard reaction becomes

feasible.

5.6 Mass Spectrometry

5.6.1 Devolatilization The released gases during the devolatilization step were further identified by a Mass

Spectrometer coupled with the TGA, that provided correlation of the mass loss of the material

under investigation to the composition of the detected released volatiles.

There seems to be a good agreement in the results of the MS investigations between the two

devolatilization tests (the one that was followed by combustion and the other that was followed

by gasification) for each material. This fact redounds to the credibility of the results since they

appear to be reproducible. For this reason, only the MS graphs for the second test are analyzed

in this section, since it is redundant to present a replicate test for each material.

42

Figure 5.6. Mass spectrometry graphs illustrating the samples' devolatilization

43

The MS graphs illustrate a correlation between the mass loss curves and the detected

devolatilization effluents. Thus the steepness of the mass loss curves corresponds to the

intensity of the MS peaks. For instance, HV-TSD exhibits a much steeper mass loss than Carbon

PODA at around 320oC and this is reflected to their corresponding MS peaks, which for HV-

TSD are more intense and sharp, while for Carbon PODA they are less intense and wider.

It is obvious from the results that a peak for a certain mass-to-charge ratio can represent

summation of the intensities of two or more different compounds with the same mass-to-charge

ratio and consequently the same molar mass (approximately) but with totally different

composition. That is the case for m/z 28 where nitrogen is detected but also carbon monoxide.

In that case a split up of the contribution of each compound to the total intensity has to be

attempted.

The noise at the start of the MS intensity measurements in some of the experiments is due to

the remnant gases in the pipelines from previous experiments or to incomplete cleaning of the

TG chamber from atmospheric air entrained during setting up the new TG experiments.

Water (m/z=18) peak is detected when the temperature in the TG chamber reaches around

100oC for all samples, while release of light hydrocarbons occurs for all samples at 300oC. The

CO/CO2 ratio of the detected peaks can be observed to increase with increasing temperature,

while there is a broad temperature region from 500oC to 800oC where release of hydrogen can

be detected.

Detection of CO2 peaks (m/z=44) for the two food residues (Carbon PIMIENTO and Carbon

PODA) at elevated temperatures of >400oC and >600oC could correspond to decomposition of

organic compounds or to dissociation of carbonates (MgCO3 and CaCO3 respectively).

The detected peaks correspond to mass loss of the materials, while the intensity of the peaks

corresponds to the steepness of the mass loss curves. The highest intensity peaks exist for HV-

TSD at around 300oC, where the observed mass loss is most acute. The two food residues

exhibit the first and biggest peak at ~300oC, a second at ~450oC and a third at ~650oC.

Regarding PUR, there is a switch of the detected gases at 300oC from the CO2 gas (m/z=44) to

the H2O gas (m/z=18), while a peak for hydrogen release can be detected at temperatures lower

than 500oC.

5.6.2 Combustion For combustion with air the intensity of N2 and O2 is orders of magnitude greater than the other

compounds so the curves for m/z=28 and m/z=32 have no practical value since they do not

indicate products of the mass loss process. The mass to charge ratio that was detected by the

mass spectrometer and its intensity had some correlation to the mass loss curves was the 44

one.

Mass to charge ratio 44 in the case of combustion can be directly associated with CO2 since

hydrocarbons with this mass to charge ratio are not expected to be generated in this process.

This certainty comes from the fact that the material has undergone devolatilization and is

already charified before the combustion process begins.

The mass spectrometry charts for the combustion products of the charified materials are

illustrated in Figure 5.7:

44

Figure 5.7. Mass spectrometry graphs illustrating the samples' combustion

45

The detected off gases correspond to mass loss of the materials. The food residues exhibit more

complexity in their combustion, with Carbon PODA having a sharp peak within the main peak

in the CO2 detection. In the diagrams presented in Figure 5.7 there is an obvious difference in

the peak temperature where the maximum CO2 emission occurs for the different materials. Thus

the two food residues (Carbon PIMIENTO and Carbon PODA) exhibit their peak CO2

production, at 310oC and 320oC respectively, followed by PUR at around 380oC and lastly by

HV-TSD at around 410oC.

5.6.3 Gasification Gasification was performed by injecting pure CO2 gas in the TG chamber, that is why the

m/z=44 curve in the MS diagrams is of very high intensity and no fluctuations can be observed

as the consumption of CO2 by the small amount of char are negligible. Instead, the resulting

diagrams are presented for the m/z=28 ratio:

46

Figure 5.8. Mass spectrometry graphs illustrating the samples' gasification

47

The peaks in the m/z=28 curve correspond to mass loss of the solid char. This is an indication

of occurrence of the solution loss reaction (C+CO2→2CO), at the elevated temperatures

where the mass loss takes place (above 750oC).

Although gasification with CO2 takes place at temperatures higher than combustion with air,

the same trend with combustion is obeyed regarding the peaks where the maximum production

of effluents is detected and consequently where the maximum gasification rates correspond to.

Thus the two food residues exhibit maximum gasification rates at temperatures below 800oC,

while PUR and HV-TSD exhibit the same behavior at around 850oC.

5.7 Ash production Ash production from the four samples plus a reference PC sample was performed by heating

approximately 10 g of each material to 950oC in a muffle furnace (no heating rate was

determined) with dwell time of 3h at 950oC under ambient atmosphere. The product of this

process was the residual ash of each material and the results are listed in Table 5.8:

Table 5.8. Ash production by oxidation at 950oC

Sample Original sample weight

(g) Sample after oxidation

(g) Ash content

(%)

HV-TSD 8.73 0.05 0.7

Carbon PIMIENTO 10.28 1.67 16.2

Carbon PODA 8.96 1.30 14.5

PUR 11.93 1.51 12.7

PC 8.53 0.79 9.3

The values obtained for the ash content of the samples are smaller than those of the proximate

analysis, except for HV-TSD where the deviation from the proximate analysis is within the

scaling accuracy margins. The main difference is that the proximate analysis temperature for

ash production is 550oC, instead of the experimental one of 950oC used in this project. A

compilation of the ash produced by the methods employed in this project is illustrated in the

following figure:

48

Figure 5.9. Comparison of ash production using different techniques

Sampling is a key factor to the fluctuations observed among the different techniques and the

amount of material used in TGA is two orders of magnitude less than the amount used in the

conventional furnace oxidation. There is also a difference in the treatment method with different

maximum temperature, heating rate, holding time and gaseous atmosphere in each technique.

The ash produced from all samples was fine in size and able to form briquettes for the

subsequent heating microscopy tests. Its particle size could not be measured neither by sieving

nor by automatic analysis, due to the low amount produced that was for some materials much

lower than 1 g.

5.8 Heating microscopy The substrate where the ash briquettes were placed for the heating microscopy experiments

were not the alumina plates commonly used, but coke plates, so that apart from testing the

characteristic temperatures of the ash samples, one could also examine the interaction between

the ash and coke at the elevated temperatures that resemble the ones that prevail in the stagnant

coke layer in the blast furnace. The coke plates used in the heating microscopy experiments

were later tested for their surface morphology using Scanning Electron Microscopy and for

their reactivity using Thermogravimetric Analysis.

The results of the heating microscopy experiments showed a direct dependency of the melting

behavior of the different kind of ashes on their composition. These results can be summarized

in the following table:

Table 5.9. Summary of heating microscopy results

Characteristic temperature HV-TSD Carbon

PIMIENTO Carbon PODA

PUR PC

Deformation temperature (oC) n.d. 1547 1201 1120 1268

Sphere temperature (oC) 1549 n.d. 1547 1333 1547

Hemisphere temperature (oC) 1549 n.d. 1547 1430 1547

Flow temperature (oC) n.d. n.d. 1547 1547 1547

(‘n.d.’= ‘not determined’)

0

5

10

15

20

25

HV-TSD Carbon PIMIENTO Carbon PODA PUR

Fin

al r

esid

ue

pro

du

ced

[wt%

of

mo]

Comparison of the ash produced from the different techniques employed

Proximate analysis Oxidation at 950oC

Devolatilization and combustion Devolatilization and gasification

49

From the results presented in Table 5.9 one can observe that the dwell time at 1550oC serves as

time to acquire the equilibrium state for many materials. The heating rate (10 K/min) used to

reach 1550oC causes overheating of the ashes and during their holding time they undergo phase

transformations which are evidenced by the coincidence of their characteristic temperatures

(sphere, hemisphere and flow) at 1550oC.

All coke substrates were corroded during heating microscopy while there was a mass loss

during the treatment that primarily corresponded to the coke plate’s mass loss (ash could not be

separated from the substrate after melting, so they could not be separately weighed). Optical

examination of the treated samples revealed holes formed on the coke areas where the Carbon

PODA and PUR ash briquettes were based on, which were less obvious for PC ash, while not

distinguishable for HV-TSD and Carbon PIMIENTO ash.

Table 5.10. Mass loss during heating microscopy experiments

Before heating microscopy experiment After the experiment

Ash type Substrate mass (g)

Sample mass (g)

Substrate + Sample mass (g)

Substrate + Sample mass (g)

Total mass loss (g)

HV TSD 0.2461 0.0273 0.2734 0.2126 0.0608

Carbon PIMIENTO 0.4968 0.0227 0.5195 0.4468 0.0727

Carbon PODA 0.2285 0.0222 0.2507 0.1885 0.0622

PUR n.d. n.d. 0.3285 0.2657 0.0628

PC 0.3555 0.0217 0.3772 0.3219 0.0553

(‘n.d.’= ‘not determined’)

The coke mass loss during heat treatment to 1550oC can be partly explained by the

devolatilization of the remnant volatile matter of the coking coal and generally the loss of

volatiles due to the increased temperatures that the heating microscopy experiments were

conducted.

Another important factor that has an influence on the mass loss during the heating microscopy

experiments is the interaction between the ash and coke. This interaction can be verified apart

from the total mass loss, by the formation of craters at the areas where the briquettes were based,

but also by a swelling behavior that some ash samples exhibited after their corresponding

hemisphere temperature. This swelling behavior is obvious in the sequence of photographs

taken for PC and PUR:

Figure 5.10. Swelling behavior of PC ash

50

Figure 5.11. Swelling behavior of PUR ash

This phenomenon is observed after the temperature has risen above 1430oC for PUR and at

1550oC for PC, in their corresponding flow ranges. After its hemisphere temperature ash starts

flowing over coke and covers an area in an airtight manner. At the interface between ash and

coke, CO gas is formed due to the reduction of metallic oxides by carbon. Oxides that can react

at the specific temperature range are Fe-oxides contained in PUR ash or Si-oxides contained in

PC ash (Björkman, 2017). The gases formed because of these reactions are trapped because of

the high viscosity of molten ash and create blisters which swell and eventually burst. This

phenomenon might cease after the temperature rises further and the ash oxide content

diminishes while its viscosity falls.

Carbon PIMIENTO ash is the material that did not even deform under thermal treatment at

1550oC. This would pose a problem in case Carbon PIMIENTO was injected through the

tuyeres into the blast furnace since the high ash content of the material (~20 wt%) could

aggregate at the end of the raceway contributing to the formation of a ‘bird’s nest’ or blockage

of the lower furnace. The other material whose ash did not finally melt under the specified

thermal treatment was HV-TSD. It has to be taken under consideration that the conditions under

which the ashes of the material were formed and treated do not represent the ones which would

prevail in case these materials were injected in the blast furnace raceway, since the later are

adjustable and reversibly dependable to the nature of the injected materials.

What is obvious (if one takes into account the ash analysis) is that the CaO (melting point:

2613oC) content of the ashes is a factor that to a large extent determines the melting behavior

of each of them and the greater the CaO content, the higher the melting temperature becomes.

In order to elucidate the influence of the composition on the temperature characteristics of the

materials, Carbon PIMIENTO and Carbon PODA ash were compared. Carbon PIMIENTO

does not flow in the designated temperature range while Carbon PODA starts flowing after

staying at 1550oC for 16 min. and melts after 29 min. The principal constituents of both

materials are CaO, SiO2, MgO and Al2O3. Albeit availability of quaternary phase diagrams, the

ternary phase diagram of CaO-SiO2-MgO with fixed 10 wt% Al2O3 and the isothermal section

of the CaO-SiO2-Al2O3 diagram at 1800K (1526.85oC) were chosen.

In case one reduces the constituents to only CaO, SiO2, MgO and Al2O3, the materials ash

composition becomes:

Table 5.11. Reduction of ash composition to four basic components

Component Carbon PIMIENTO composition (wt%)

Carbon PODA composition (wt%)

CaO 76.4 47.5

SiO2 12.6 40.7

MgO 8.6 4.4

Al2O3 2.4 7.4

51

It has to be noted that the compositions can only be approximated and do not correspond to the

actual ones while the phase diagrams presented correspond to simpler systems than the actual

ones.

The regions where the materials would be situated are highlighted in the ternary of CaO-SiO2-

MgO with fixed 10 wt% Al2O3 diagram:

Figure 5.12. Ternary phase diagram of CaO-SiO2-MgO with fixed 10 wt% Al2O3 (Process Metallurgy Course, 2017)

Carbon PIMIENTO ash is found in the lime primary crystallization field with liquidus

temperature at ~2200oC and solidus temperature at less than 1300oC, while Carbon PODA ash

is found at the Pseudowollastonite primary crystallization field with liquidus temperature at

~1400oC and solidus temperature at less than 1300oC.

At the isothermal section of the CaO-SiO2-Al2O3 diagram at 1800K (1526.85oC) the

observations extracted from the ternary diagram can be verified:

52

Figure 5.13. Isothermal section of the CaO-SiO2-Al2O3 phase diagram at 1800K (MTDATA, 2010)

This diagram shows that at 1526oC, Carbon PIMIENTO ash is a mixture of solid Hatrurite,

Lime and liquid, while Carbon PODA ash is in the liquid state.

5.9 Scanning Electron Microscopy The first observation that can be derived as a result from comparing the images of the coke

sample that has not undergone any thermal treatment to the coke that has been treated under the

heating microscopy experiments is that the latter’s porosity was enhanced.

The reason for this phenomenon is probably the fact that the cokemaking process reaches a

maximum temperature of 1100oC and leaves about 1% volatile matter in the produced coke. By

further heating the coke plates in the heating microscope to 1550oC this remnant volatile matter

might evaporate, creating further porosity. At the same time mineral compounds in the coke

ash can be decomposed and altered due to the high temperature.

Figure 5.14. Comparison of SEM photomicrographs of coke before (left) and coke after (right) thermal treatment

53

From the SEM images obtained for the coke substrates where the ash briquettes melted it seems

that most of the mineral mater is gathered on the periphery of the coke pores and not in the

pores. This is an indication of poor wettability of the molten ashes on coke and can be attributed

to the high surface tension of the molten ashes.

Figure 5.15. Photomicrograph of PC ash on coke using SEM (498x)

A comparatively large quantity of material forms the ash briquettes and when it melts due to its

high surface tension it seems unable to penetrate the coke pores. This could be a difference

between the experimental configuration used and the actual injection where the injected

material’s residues would be more susceptible to melting and easier dispersed on the coke

surface.

Figure 5.16. Photomicrograph of PUR ash on coke using SEM (85x)

54

Moreover, this phenomenon can also be observed with the ash briquettes that didn’t eventually

melt, where there is remaining ash material on the pore periphery but not in the coke craters.

Figure 5.17. Photomicrograph of Carbon PIMIENTO ash on coke using SEM (999x)

The importance of the wettability of ash on coke and the dispersion of the mineral mater in the

pores is that the pores constitute the major part of the coke surface where the gases react.

Reactivity of coke might increase or decrease in contact with ash, depending on the nature of

the ash. The presence of alkali, iron oxides, CaO and MgO catalyzes the solution loss reaction

(Björkman, 2017), while occupancy of vacant active sites might inhibit the reaction.

Another remark that has to be stated is that the SEM pictures are taken at room temperature

after the samples have been cooled down, thus the actual phenomenon might differ since the

ash in contact with coke will always be in the high temperature state in the blast furnace.

5.10 Coke Reactivity Thermogravimetric Analysis of the coke substrates that had undergone interaction with the ash

briquettes produced the following results regarding the mass loss of the substrates under 200

ml/min pure CO2 at 10000C for 1h:

55

Figure 5.18. Coke reactivity evolution after thermal treatment with the injection materials' ashes

PC ash seems to inhibit the coke solution loss reaction to some extent, while HV-TSD, Carbon

PIMIENTO and PUR ash enhance the rate of the reaction but to a limited degree. The greatest

positive catalytic effect is attained by Carbon PODA ash, where the mass loss is 2.5 times the

mass loss of plain coke thermally treated.

During the heating microscopy experiment, Carbon PODA ash started deforming at 1201oC,

while it finally melted at 1547oC and flowed over the coke substrate.

Figure 5.19. Photomicrograph of Carbon PODA ash on coke using SEM (201x)

The SEM image seems to confirm the fact that Carbon PODA ash covered the coke substrate

and mineral crystals can be traced within coke macropores. These physical properties along

with its chemical analysis (where CaO, MgO and alkalis are included) could provide adequate

explanation for Carbon PODA’s catalytic effect on coke gasification.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

Cokethermallytreated

PC ash oncoke

HV-TSD ash oncoke

CarbonPIMIENTO ash

on coke

CarbonPODA ash on

coke

PUR ash oncoke

Mas

s lo

ss [

wt%

of

mo]

Mass loss during gasification of the coke substrates

56

6 Discussion

6.1 Material characterization All materials tested in this project contain increased amounts of hydrogen that can contribute

to CO2 emissions mitigation, but the overall carbon footprint of the process might be negatively

influenced in case further preprocessing of the materials before injection is necessary or the

productivity of the blast furnace or the hot metal quality is altered. The common feature is the

high amount of volatile matter (>62 wt%) which is a major difference compared to the reference

PC which contains 18.4 wt% VM. This fact suggests that gas combustion may be more

important than char combustion in the raceway for these materials.

HV-TSD is a carbonaceous material of high VM with low ash content but although its ash

content is very little, its fixed carbon is limited by its high volatile matter content. Carbon

PIMIENTO is the material with the lowest carbon and hydrogen content and the highest ash

content among the ones tested in this project while it contains a considerable amount of S which

could result in extra costs for sulfur removal. PUR is the only material that was not preprocessed

by torrefaction and contains a high amount of moisture, the highest amount of VM and the

lowest fixed carbon content while its Cl content could cause refractory deterioration and

corrosion of the gas cleaning system metallic parts.

Examination by DIA revealed a great number of out-of-range particles entrained in the 53-106

μm sieved size fraction, indicating that fractioning the materials into a narrow size range might

be difficult to achieve. This phenomenon was more pronounced for HV-TSD and Carbon

PODA and can be attributed to agglomeration during storage in combination with the elongated

nature of the materials’ particles and indicates that further preprocessing is necessary before

industrial use in order for the materials not to cause problems to the free flow through the

pneumatic transport system.

The true density of all four materials lies in the range between 1.4 and 1.5 g/cm3, with the two

food residues having the highest values due to their high ash content. These values are

comparable to the corresponding value for the reference PC (1.47 g/cm3) but determination of

porosity, surface area and hardness of the materials might be necessary in order to correlate the

materials’ physical properties to particle dispersion in the raceway and combustion efficiency.

The calorific value of the materials is limited by their oxygen, ash and moisture content. As a

result, combined with their high VM content, decrease in the RAFT can be expected in case

they are injected into the raceway.

6.2 Thermal analysis The basic assumptions made in order to process the results derived by TG analysis of the

materials were that devolatilization occurs uniformly throughout the particles’ mass, while for

combustion and gasification surface reaction on spherical particles with no porosity and with

fixed density throughout the experiment was assumed.

During the devolatilization experiment the materials lost most of the volatile matter determined

by their proximate analysis. The two food residues (Carbon PIMIENTO and Carbon PODA)

start losing their volatile matter at temperatures lower than 200oC, followed by PUR and lastly

by HV-TSD, which starts devolatilization at ~222oC. The apparent activation energies for the

food residues’ primary devolatilization are also lower compared to the other two materials. This

fact constitutes an indication that injection of the food residues might shift the position of the

57

raceway temperature maximum closer to the tuyere tip compared to the other two materials

tested.

Combustion of the charified materials occurs in temperature ranges that overlap with the

devolatilization temperatures and heterogeneous combustion is expected to occur before

devolatilization and homogeneous combustion is complete. Whatsoever, the kinetics defined

for char combustion show that it is a slower process compared to the materials’ devolatilization.

Gasification of the charified materials with CO2 obeys the Boudouard reaction’s norms and

produces CO at temperatures above 700oC. Gasification kinetic parameters derived by analysis

of the experimental results produced the highest values for the apparent activation energies

(compared to devolatilization and combustion), with the highest value obtained for PUR.

Oxygen enrichment of the blast and oxygen deficiency in the raceway are expected to have a

major influence on the extent of the gasification of the charified materials and on coke

consumption.

6.3 Ash analysis The ash content of most of the materials tested was detected to be above the value for the

reference PC. High ash content in the injected materials might reduce coke replacement ratio

and increase slag production. Especially for the materials that contain ashes with high melting

points permeability in the lower part of the furnace will be an issue.

The ash composition of the 4 materials tested is dominated by lime, which is a major difference

with the reference PC whose ash consists mainly of silica along with lime and iron oxides. This

difference is reflected in the melting behavior of the materials’ ashes. The materials whose ash

consists almost exclusively of lime exhibit increased melting temperature. Thus HV-TSD ash

does not melt at 1550oC while Carbon PIMIENTO ash does not even deform at 1550oC, a

behavior that indicates that by using Carbon PIMIENTO as an injection material, permeability

problems in the lower part of the furnace may occur. The silica content of Carbon PODA ash

lowers its melting point and the material exhibits adequate wettability on coke. PUR ash reacted

with the coke substrate while heating to 1550oC during the heating microscopy experiment and

formed a crater on the coke plate but did not cause any considerable alteration of coke

gasification reactivity.

Carbon PODA ash is the material that exhibits the most pronounced effect on coke reactivity

and it is expected to contribute to coke disintegration if used as an injection material. The high

alkali content of HV-TSD ash did not cause considerable enhancement of coke reactivity but

its detrimental effects in the long term on coke disintegration and deterioration of the furnace

lining should be thoroughly investigated prior to application in an industrial scale.

58

7 Conclusions Introduction of auxiliary reducing agents in the blast furnace occurs by injection of

carbonaceous materials through the tuyeres. These auxiliary reducing agents can replace part

of the coke in the process and mitigate the CO2 emissions but at the same time alter the operating

conditions in the furnace. In order to assess the suitability of new materials for use as auxiliary

reducing agents a series of tests has to be conducted.

Four such materials were evaluated in the context of this project. The tools used to model their

behavior in the blast furnace were analysis of their composition, particle size analysis, density

determination, calorific value determination, reactions kinetic analysis and analysis of their

interaction with coke.

The materials tested exhibited diverse characteristics in the experiments conducted. Their

common feature was the high hydrogen content that will lead to generation of hydrogen gas in

the furnace and of H2O as a gaseous effluent, which will replace part of the emitted CO2. All

materials contained large amounts of VM and ash, a fact that may have an immediate impact

on the lower part of the furnace by reducing the temperature and increasing the slag volume.

The two food residues contained the highest amounts of ash and this could be reflected in their

calorific value, which was inferior to the other materials and much lower than that of pulverized

coal.

Their grinding and stratification characteristics were poor (except from PUR) and further

preprocessing might be necessary prior to injection.

Their charified residues exhibited normal combustion and gasification characteristics but the

release of pollutants and alkali metal oxides has to be controlled to avoid deterioration of the

blast furnace components.

There was a difference in the temperature range where combustion and gasification takes place

for the different materials. The two food residues combust and gasify at lower temperatures,

followed by PUR and lastly by HV-TSD. This behavior will have an influence on the

combustion efficiency of the materials in the raceway, where the food residue chars are

expected to be consumed more readily than the other two materials. Their final residue (ash)

increased coke reactivity, compared with PC ash, and this could have an impact on coke

consumption and the CO utilization factor.

59

8 Future work Application of a CFD model for the prediction of the behavior of each material in the raceway

is the next step for acquiring a spherical view about the materials suitability as auxiliary

reducing agents.

For those materials that will be considered appropriate for injection, pilot scale campaigns can

be conducted in order to define the quantity and correct blending that will be used.

60

9 References Asanuma, M., Ariyama, T., Sato, M., Murai, R., Nonaka, T., Okochi, I., Tsukiji, H., Nemoto,

K., 2000. Development of Waste Plastics Injection Process in Blast Furnace. ISIJ

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Asanuma, M., Terada, K., Inoguchi, T. & Takashima, N., 2014. Development of Waste

Plastics Pulverization for Blast Furnace Injection, s.l.: JFE TECHNICAL REPORT.

Babich, A., Senk, D., Knepper, M. & Benkert, S., 2016. Conversion of injected waste plastics

in blast furnace. Ironmaking and Steelmaking, 43(1), pp. 11-22.

Barranco, R., Rojas, A., Barraza, J. & Lester, E., 2009. A new char combustion kinetic model

1. Formulation. Fuel, 88(12), pp. 2335–2339.

Björkman, B., 2017. IRON AND STEELMAKING, Section 1: Ironmaking. Luleå: Luleå

Tekniska Universitet.

Carpenter, A. M., 2010. Injection of coal and waste plastics in blast furnaces. s.l.:IEA Clean

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Geerdes, M., Lingiardi, O., Ricketts, J., Chaigneau, R., Kurunov, I., 2015. Modern Blast

Furnace Ironmaking : An introduction. 3rd edition ed. Amsterdam: IOS Press BV.

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CLASSIFICATION AND INDUSTRIAL APPLICATIONS. International Journal of Coal

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He, R., Sato, J., Chen, Q. & Chen, C., 2002. Thermogravimetric analysis of char combustion.

Combustion Science and Technology, 174(4), pp. 1-18.

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its kinetics since 1948: A brief review. Energy, 36(1), pp. 12-40.

Ishii, K., 2000. Advanced Pulverized Coal Injection Technology and Blast Furnace

Operation. 1st ed. Oxford: Pergamon.

Jernkontoret, 2017. Environmental impact of the processes. [Online]

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considering plastic injection. INTERNATIONAL JOURNAL OF ENERGY AND

ENVIRONMENT, 6(2), pp. 175-190.

Lundgren, M., 2013. Development of Coke Properties during the Descent in the Blast

Furnace, Luleå: Luleå Tekniska Universitet - Doctoral Thesis.

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- Student thesis, Luleå: Luleå Tekniska Universitet.

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temperature fluidized bed reactor. Chemical Engineering Journal, Volume 331, pp. 265-277.

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SiO2-Al2O3 phase diagram. [Online]

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Furnace - P7010K Project Work, Luleå: Luleå Tekniska Universitet.

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combustion of pulverized coal and alternative carbon materials in the blast furnace raceway.

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63

10 Appendices

10.1 Helium pycnometry measurements

HV-TSD helium pycnometry 1st test

HV-TSD helium pycnometry 2nd test

64

Carbon PIMIENTO helium pycnometry 1st test

Carbon PIMIENTO helium pycnometry 2nd test

65

Carbon PODA helium pycnometry 1st test

Carbon PODA helium pycnometry 2nd test

66

I. PUR helium pycnometry test

67

10.2 TGA data analysis The TGA data obtained by the mass loss measurements are transformed according to the

methods described in part 4.5. Then the evolved data are plotted against 1/T and the linear parts

of the scatter plot correspond to mass loss reactions of the materials with the kinetic constants

being extracted from the slope and extrapolation to the y-axis of the lines. The criterion for

estimating the optimum data region for fitting a linear trendline on the scatter plot is the

coefficient of determination or R-squared value, which has to be as close to unity as possible.

10.2.1 Devolatilization

68

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

HV-TSD devolatilization (devolatilization + combustion experiment) TG data analysis for HV-TSD devolatilization (devolatilization+combustion experiment)

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

HV-TSD devolatilization (devolatilization + gasification experiment) TG data analysis for HV-TSD devolatilization (devolatilization+gasification experiment)

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -13248x + 15.927R² = 0.9931

-16

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -13542x + 16.451R² = 0.9931

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

69

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

Carbon PIMIENTO devolatilization (devolatilization + combustion experiment) TG data analysis for Carbon PIMIENTO devolatilization (devolatilization+combustion experiment)

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

Carbon PIMIENTO devolatilization (devolatilization + gasification experiment) TG data analysis for Carbon PIMIENTO devolatilization (devolatilization+gasification experiment)

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -7079.9x + 5.0632R² = 0.9875

y = -21927x + 17.148R² = 0.9964

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -7688.8x + 6.2855R² = 0.9865

y = -27587x + 24.329R² = 0.9581

-16

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

70

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

Carbon PODA devolatilization (devolatilization + combustion experiment) TG data analysis for Carbon PODA devolatilization (devolatilization+combustion experiment)

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

Carbon PODA devolatilization (devolatilization + gasification experiment) TG data analysis for Carbon PODA devolatilization (devolatilization+gasification experiment)

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -6850.4x + 4.6132R² = 0.9596

y = -22431x + 18.22R² = 0.9157

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -6552.2x + 4.0572R² = 0.9697

y = -18265x + 13.763R² = 0.9755

-16

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

71

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

PUR devolatilization (devolatilization + combustion experiment) TG data analysis for PUR devolatilization (devolatilization+combustion experiment)

1/T [K-1]

𝐥𝐧(

𝐝𝐦𝐕𝐌

𝐝𝐭

𝟏𝐦𝐕𝐌

𝐬−𝟏

)

PUR devolatilization (devolatilization + gasification experiment) TG data analysis for PUR devolatilization (devolatilization+gasification experiment)

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -10120x + 10.687R² = 0.9736

y = -8483.5x + 6.0488R² = 0.9803

-16

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -10823x + 11.961R² = 0.9642

y = -9041x + 6.8533R² = 0.9756

-14

-12

-10

-8

-6

-4

-2

0

0.0007 0.0012 0.0017 0.0022

72

10.2.2 Combustion

1/T [K-1]

𝑙𝑛

( (−𝑑𝑚 𝑑𝑡1 𝑚𝑐

𝑀𝑜2

XO2𝑅𝑇(𝑛𝑝 6)2 3(𝑚 𝜋𝜌)1 3)

g cm

−2s−1𝑃𝑎−1

)

HV-TSD combustion TG data analysis for HV-TSD combustion

1/T [K-1]

𝑙𝑛


𝑀𝑜2

XO2𝑅𝑇(𝑛𝑝 6)2 3(𝑚 𝜋𝜌)1 3)

g cm

−2s−1𝑃𝑎−1

)

Carbon PIMIENTO combustion TG data analysis for Carbon PIMIENTO combustion

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -13940x +0.5443R² = 0.991

-30

-25

-20

-15

-10

-5

0

0.001 0.0012 0.0014 0.0016 0.0018 0.002

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -13323x + 3.326R² = 0.99

y = -17813x + 6.1847R² = 0.9409

-35

-30

-25

-20

-15

-10

-5

0

0.001 0.0012 0.0014 0.0016 0.0018 0.002

73

1/T [K-1]

𝑙𝑛


𝑀𝑜2

XO2𝑅𝑇(𝑛𝑝 6)2 3(𝑚 𝜋𝜌)1 3)

g cm

−2s−1𝑃𝑎−1

)

Carbon PODA combustion TG data analysis for Carbon PODA combustion

1/T [K-1]

𝑙𝑛


𝑀𝑜2

XO2𝑅𝑇(𝑛𝑝 6)2 3(𝑚 𝜋𝜌)1 3)

g cm

−2s−1𝑃𝑎−1

)

PUR combustion TG data analysis for PUR combustion

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -16206x + 6.6099R² = 0.9591

-30

-25

-20

-15

-10

-5

0

0.001 0.0012 0.0014 0.0016 0.0018 0.002

0

20

40

60

80

100

0 200 400 600 800

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -14335x + 3.0153R² = 0.958

-30

-25

-20

-15

-10

-5

0

0.001 0.0012 0.0014 0.0016 0.0018 0.002

74

10.2.3 Gasification

1/T [K-1]

ln

[ −𝑑𝑚 𝑑𝑡(𝑛𝑝𝜋)1 3(6 𝜌)−2 3𝑚1 3

𝑚𝑐𝑝𝑔

𝑔 𝑠−1𝑐𝑚

−2𝑃𝑎−1

]

HV-TSD gasification TG data analysis for HV-TSD gasification

1/T [K-1]

ln

[ −𝑑𝑚 𝑑𝑡(𝑛𝑝𝜋)1 3(6 𝜌)−2 3𝑚1 3

𝑚𝑐𝑝𝑔


−2𝑃𝑎−1

]

Carbon PIMIENTO gasification TG data analysis for Carbon PIMIENTO gasification

0

20

40

60

80

100

500 600 700 800 900 1000

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -27952x + 11.939R² = 0.9754

-20

-15

-10

-5

0

0.0008 0.00085 0.0009 0.00095 0.001 0.00105 0.0011 0.00115 0.0012

0

20

40

60

80

100

500 600 700 800 900 1000

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -27537x + 13.467R² = 0.9579

-25

-20

-15

-10

-5

0

0.0008 0.0009 0.001 0.0011 0.0012

75

1/T [K-1]

ln

[ −𝑑𝑚 𝑑𝑡(𝑛𝑝𝜋)1 3(6 𝜌)−2 3𝑚1 3

𝑚𝑐𝑝𝑔


−2𝑃𝑎−1

]

Carbon PODA gasification TG data analysis for Carbon PODA gasification

1/T [K-1]

ln

[ −𝑑𝑚 𝑑𝑡(𝑛𝑝𝜋)1 3(6 𝜌)−2 3𝑚1 3

𝑚𝑐𝑝𝑔


−2𝑃𝑎−1

]

PUR gasification TG data analysis for PUR gasification

0

20

40

60

80

100

500 600 700 800 900 1000

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -22219x + 7.6517R² = 0.9656

-20

-15

-10

-5

0

0.0008 0.00085 0.0009 0.00095 0.001 0.00105 0.0011 0.00115 0.0012

0

20

40

60

80

100

500 600 700 800 900 1000

Mas

s lo

ss p

erc

.

Temperature (οC)

y = -40562x + 24.699R² = 0.9765

-20

-15

-10

-5

0

0.0007 0.00075 0.0008 0.00085 0.0009 0.00095 0.001 0.00105

76

10.3 TGA – MS 3D graphs Three dimensional graphs for the TGA effluents detection provide a better overview of the processes and their temperature dependence.

Devolatilization Mass spectrometry 3-D graphs

77

Combustion Mass spectrometry 3-D graphs

78

Gasification Mass spectrometry 3-D graphs

79

10.4 Heating microscopy results

PC heating microscopy results

HV-TSD heating microscopy results

80

Carbon PIMIENTO heating microscopy results

Carbon PODA heating microscopy results

81

PUR heating microscopy results

82

10.5 SEM photomicrographs

Coke without treatment (201x) Coke thermally treated (100x)

PC ash on coke (498x) PC ash on coke (2030x)

HV-TSD ash on coke (69x) HV-TSD ash on coke (1030x)

Carbon PIMIENTO ash on coke (108x) Carbon PIMIENTO ash on coke (999x)

83

Carbon PODA ash on coke (201x) Carbon PODA ash on coke (1010x)

PUR ash on coke (85x) PUR ash on coke (747x)

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