Hydrogen-rich materials as auxiliary reducing agents in the blast furnace

Transcription

1 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

2 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 CO 2 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. 1

3 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. 2

4 Table of contents Abstract... 1 Acknowledgements... 2 List of Figures... 4 List of Tables... 5 Abbreviations and Symbols Introduction Background Auxiliary Reducing Agents Injection State of the art Modelling of auxiliary reducing agents injection through the tuyeres Scope of the current work Literature review Factors influencing injection materials behavior in the raceway Particle size Composition Calorific value Ash properties Injectants reactions in the raceway Devolatilization Combustion Gasification Materials Methods Material Pretreatment Particle size analysis Helium pycnometry Bomb calorimetry Thermogravimetric analysis Devolatilization Combustion Gasification Mass Spectrometry Ash production Heating microscopy Scanning Electron Microscopy Coke reactivity evaluation

5 5 Results Grinding and sieving the materials Particle size analysis of the μm size fraction Density determination Bomb Calorimetry Thermogravimetric Analysis Devolatilization Combustion Gasification Mass Spectrometry Devolatilization Combustion Gasification Ash production Heating microscopy Scanning Electron Microscopy Coke Reactivity Discussion Material characterization Thermal analysis Ash analysis Conclusions Future work References Appendices Helium pycnometry measurements TGA data analysis Devolatilization Combustion Gasification TGA MS 3D graphs Heating microscopy results SEM photomicrographs 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)

6 Figure 1.3. Auxiliary reducing agents injection (Geerdes, et al., 2015)... 9 Figure 1.4. Pulverized coal reaction in the raceway (Ishii, 2000) Figure 4.1. Material pretreatment process scheme Figure 4.2. Temperature profile during devolatilization Figure 4.3. Temperature profile during combustion Figure 4.4. Temperature profile during gasification Figure 5.1. Automated particle size analysis of the μm size fraction Figure 5.3. Comparison between experimental and theoretical dry mass HHV Figure 5.4. TGA graphs during devolatilization Figure 5.5. TGA graph during combustion Figure 5.6. TGA graph during gasification Figure 5.7. Mass spectrometry graphs illustrating the samples' devolatilization Figure 5.8. Mass spectrometry graphs illustrating the samples' combustion Figure 5.9. Mass spectrometry graphs illustrating the samples' gasification Figure Comparison of ash production using different techniques Figure Swelling behavior of PC ash Figure Swelling behavior of PUR ash Figure Ternary phase diagram of CaO-SiO 2-MgO with fixed 10 wt% Al 2O 3 (Process Metallurgy Course, 2017) Figure Isothermal section of the CaO-SiO 2-Al 2O 3 phase diagram at 1800K (MTDATA, 2010) Figure Comparison of SEM photomicrographs of coke before (left) and coke after (right) thermal treatment Figure Photomicrograph of PC ash on coke using SEM (498x) Figure Photomicrograph of PUR ash on coke using SEM (85x) Figure Photomicrograph of Carbon PIMIENTO ash on coke using SEM (999x) Figure Coke reactivity evolution after thermal treatment with the injection materials' ashes Figure Photomicrograph of Carbon PODA ash on coke using SEM (201x) List of Tables Table 2.1. Particle reactions during auxiliary reducing agents injection Table 3.1. Proximate, ultimate and ash analyses Table 4.1. Machine parameters Table 4.2. Stream definition Table 5.1. Helium pycnometry results Table 5.2. Bomb calorimetry results Table 5.3. Higher heating value for dry materials Table 5.4. HHV d calculated using Gaur and Reed formula Table 5.5. Results from graphical evaluation of kinetic parameters for devolatilization Table 5.6. Results from graphical evaluation of kinetic parameters for combustion Table 5.7. Results from graphical evaluation of kinetic parameters for CO 2 gasification Table 5.8. Ash production by oxidation at 950 o C Table 5.9. Summary of heating microscopy results Table Mass loss during heating microscopy experiments Table Reduction of ash composition to four basic components

7 Abbreviations and Symbols Abbreviations CFD DIA HHV HV-TSD LTU MS PC PSD PUR RAFT SEM TGA Computational fluid dynamics Dynamic image analysis Higher heating value High volatile torrefied saw dust Luleå Tekniska Universitet Mass spectrometry Pulverized coal Particle size distribution Polyurethane Raceway adiabatic flame temperature Scanning electron microscopy Thermogravimetric analyzer Symbols A A p C <S> d 50 D p E E a f s k k G k MV m m c m o M O2 m RC m VM n p P O2 P 80 p g R R C r com R D R K R VM t T V p,o V total,o X X O2 ρ φ Pre-exponential kinetic factor Particle surface area Char 50% passing size Char particle diameter Activation energy Apparent activation energy Mass fraction of reacting solid species in a particle Kinetic rate constant Granular model kinetic rate constant Modified volumetric model kinetic rate constant Total mass remaining Mass of remaining char Initial sample mass Oxygen molecular weight Mass of unreacted coal Mass of remaining volatile matter Number of particles in a sample Oxygen partial pressure 80% passing size Bulk partial pressure of reacting gas Universal gas constant Particle surface reaction rate Combustion rate Diffusion rate coefficient Reaction rate coefficient Rate of devolatilization Time Absolute temperature Initial particle volume Initial sample volume Conversion Oxygen mole fraction in the gas Density Ratio of reacting surface to external area 6

8 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 7

9 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 200 o C. Shaft or stack: in this zone the burden is in the solid state but reacts with the ascending gasses that contain CO and H 2 and gets reduced from the higher iron oxides (hematite- Fe 2O 3, and magnetite-fe 3O 4) into the lower iron oxides (wustite-feo, and iron-fe), while at the same time gets heated to o C. 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 1200 o C. 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). 8

10 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 + H 2 O CO 2 + H 2 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 CO 2 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. 9

11 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 CO 2 and H 2O, while the remaining solid char particles are also ignited and oxidized by the atmosphere containing O 2. In the last step, the remaining char particles reform the generated CO 2 and H 2O into CO and H 2 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 CO 2 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). 10

12 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 CO 2 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 950 o C 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 11

13 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. 12

14 2 Literature review 2.1 Factors influencing injection materials behavior in the raceway Particle size The common practice for injecting materials into the raceway is to grind and pulverize them, with coal being pulverized to P 80=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 CO 2 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) 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 carbonoxygen 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) 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 13

15 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 1100 o C) to casting temperature of 1500 o C (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 H 2. 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 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 (SiO 2) 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: 14

16 Table 2.1. Particle reactions during auxiliary reducing agents injection Devolatilization Volatile Matter (VM) Raw Injection Materials Char (C <S>) + Residue (Ash) Combustion C <S> O CO CO 2 Gasification C <S> + CO 2 2 CO C <S> + H 2O CO + H 2 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 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, R VM, is proportional to the amount of volatile matter remaining, m VM : R VM = dm VM dt = k m VM Eq. 2.1 where dm VM 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 E RT Eq

17 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 k 1 and k 2, that control the devolatilization rate over different temperature ranges: (Raw material) < k2 k1 a 1 (Volatile)+(1 a 1 )(Residue) a 2 (Volatile)+(1 a 2 )(Residue) R. 3 The expression that describes the releasing rate of volatile matter in this case is: dm VM dt = (a 1 k 1 + a 2 k 2 )m RC Eq. 2.3 where a i, k i and m RC 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 k 1 and k 2 are given by Arrhenius type equations and k 2 contains a higher activation energy (Ishii, 2000) 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 + O 2 k CO/CO 2 R. 4 with the corresponding rate law given by: n r com =kp O2 Eq

18 where r com is the combustion rate, k is the kinetic constant and P O2 is 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+O 2 k 1 2C(O) C(O) k 2 CO R. 5 R. 6 with the corresponding reaction rate: r com = k 1k 2 P O2 k 1 P O2 +k 2 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+O 2 k 1 C(O) C(O) + O 2 k 2 CO CO 2 + C(O) C(O) k 3 CO R. 7 R. 8 R. 9 with the corresponding reaction rate law: r com = k 1k 2 P O2 +k 1 k 3 P O2 k 1 P O2 + k ; 3 2 CO CO 2 = k 3 k 2 P O2 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 = πd p 2 ρrt ( X O 2 ) ( ) M O2 R D R K Eq. 2.7 where dm/dt is the rate of char mass loss, D p is the char particle diameter, ρ is the coal density, X O2 is the oxygen mole fraction, M O2 is the molecular weight of oxygen, while R is the universal gas constant. R D and R K are the diffusion and reaction rate coefficients respectively, with R K defined as: 17

19 R K = Aφe E α RT 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 E a 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, R K, and the diffusion rate, R D, according to the formula: R K R D R C = A p f s p g Eq. 2.9 R K + R D where A p is the particle surface area, f s is the mass fraction of reacting solid species in a particle and p g is the bulk partial pressure of reacting gas species (Ölund, et al., 2017) Gasification Gasification of injection material chars with CO 2 starts in the raceway when the CO 2 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 CO 2 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 CO 2-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+CO 2 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 where X is the material conversion, t is the time and k is the kinetic constant. X is given by the formula: 18

20 X = w 0 w t w 0 w f Eq where w 0 is the weight before gasification, w f the weight after gasification and w t 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 with k following the Arrhenius law. k=aexp (- E RT ) Eq 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 =k MV(X)(1-X) Eq and after integration: ln(1 X) = at b Eq where k MV(X) is the model corresponding kinetic constant and a and b empirical constants. The kinetic constant can be expressed through the following formula: k MV (X) = abb[ ln 1 (1 X)] b 1 b Eq 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: 19

21 dx dt =k G(1-X) 2 3 Eq and the integrated form by: 3[1-(1-X)] 1 3=k G t Eq where k G 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, R K, and the diffusion rate, R D, according to the formula: R K R D R C = A p f s p g Eq R K + R D where A p is the particle surface area, f s is the mass fraction of reacting solid species in a particle and p g is the bulk partial pressure of reacting gas species (Ölund, et al., 2017). 20

22 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 Volatile Matter Fixed Carbon Ash Ultimate Analysis (wt% dry basis) C H N < O Cl < S < Ash Ash Analysis (wt% in total ash content) Al Ba Ca Cr Fe K Mg Mn Na P S

23 Si Ti Others 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. 22

24 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 23

25 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 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 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, ). 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. 24

26 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 CO 2 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 Devolatilization 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 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 800 o C. Then the sample was cooled with a cooling rate of 20 K/min up to the starting temperature for the subsequent program, which was 100 o C in case combustion followed or 500 o C in case gasification followed. 25

27 Temperature [ o C] Devolatilization temperature profile Gasification starting point Combustion starting point Time [min] Figure 4.2. Temperature profile during devolatilization 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: dm VM dt = A exp ( E RT ) m VM Eq. 4.1 And by transformation: ln ( dm VM dt 1 ) = ln(a) E m VM 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 -E a/r. The value for dm VM/dt (the devolatilization rate) is acquired by dividing the mass difference by the time difference between two subsequent data points Combustion Experimental procedure 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 %O 2, 79.1 %N 2 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 100 o C before air was injected. Then, under the synthetic air 26

28 Temperature [ o C] flow, the sample was heated to 700 o C with a heating rate of 2 K/min and then cooled to 200 o C with a cooling rate of 20 K/min Combustion temperature profile Combustion time [min] Figure 4.3. Temperature profile during combustion Modeling methodology 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 (R D) was assumed to be much higher than the reaction rate coefficient (R K), so the rate equation (after incorporating the reaction rate coefficient formula) reduces to: dm dt = πd p 2 ρrt ( X O 2 ) (Aφe E a M O2 RT) 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 (m c) to the remnant mass of the sample (combustible + ash, m). After transformation, the equation becomes: ln ( dm dt m m c M O2 X O2 πd p 2 ρrt ) = ln(a) E a RT Eq. 4.4 Regarding the particles diameter, D p, spherical particles with initial diameter equal to the d 50 of the measured Particle Size Distribution were assumed. Thus the initial volume of each particle (V p,o) is equal to: 27

29 V p,o = πd Eq. 4.5 and the initial total volume of the sample: V total,o = n p V p,o Eq. 4.6 where n p is the total number of particles in the sample. The initial number of particles (n p) was calculated by taking into account the initial sample weight (m o) and the true density (ρ) (measured by helium pycnometry), according to the formula: m o πd 50 ρ = n V p total,o 6 = m o ρ 3 n p = 6m o πd 3 50 ρ Eq. 4.7 Thus the number of particles for each sample can be calculated by using the initial sample weight and the d 50 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: D p = ( 6m 1 πn p ρ ) 3 Eq. 4.8 By substituting Eq into Eq. 4.4, the following formula is derived: ln ( dm dt 1 m c M o2 X O2 RT (n p 6 )2 3( m πρ )1 3) = ln(a) E a RT 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 -E a/r Gasification Experimental procedure Gasification was performed by injecting a flow of 200 ml/min pure CO 2 in the TG chamber after devolatilization was completed and the charified samples were cooled down to 500 o C. Under the CO 2 flow, the samples were heated to 1000 o C with a heating rate of 2 K/min and then cooled to 200 o C with a cooling rate of 20 K/min. 28

30 Temperature [ o C] Gasification temperature profile Gasification time [min] Figure 4.4. Temperature profile during gasification Modeling methodology 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 (R D) is much greater than the reaction rate coefficient (R K). Under the aforementioned assumption and after substituting R K with the Arrhenius equation, Eq reduces to: R C = A p f s p g Ae E a RT Eq f s can be substituted by m c/m (in this case m c represents the remaining mass available for gasification). Regarding the particles surface area (A p), spherical particles with initial diameter equal to the d 50 of the measured Particle Size Distribution was assumed. This way an initial number of particles (n p) was calculated by taking into account the initial sample weight (m o) and the true density (ρ) (measured by helium pycnometry), according to the formula: ρ= m 3 o πd 50 n V p total 6 = m o ρ n p= 6m o πd 3 50 ρ Eq 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: 29

31 D p = ( 6m 1 πn p ρ ) 3 Eq while the total surface area of the particles becomes: A p =n p πd 2 =(n p π) 1 3( 6m ρ )2 3 Eq By substituting f s and A p, Eq becomes: dm dt = (n pπ) 1 3( 6m m ρ )2 c 3 m p gae E a RT ln ( dm dt (n pπ) 1 3 ( 6 2 ρ ) 3 m 1 3 ) = ln(a) E a m c p g RT Eq 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 -E a/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 950 o C 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. 30

32 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 15 o C/min to 600 o C and then 10 o C/min to 1550 o C with 2 h dwell time at 1550 o C 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 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 CO 2 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 1000 o C where it stayed for 1h and was afterwards cooled with a rate of -20 K/min to 200 o C. 31

33 Cumulative undersize [wt%] 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 ( 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 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 μm size fraction The results from the automated particle size analysis are shown in the diagram bellow Particle size distribution of the μm sieved samples HV-TSD Carbon PIMIENTO Carbon PODA PUR Size [mm] Figure 5.1. Automated particle size analysis of the μm size fraction Separation of the 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 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 32

34 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/cm 3 ) Stdev (g/cm 3 ) HV-TSD Carbon PIMIENTO Carbon PODA PUR PC 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 g/cm 3. The two food residues (Carbon PIMIENTO and Carbon PODA) have very similar densities (~1.50 g/cm 3 ). 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 Carbon PIMIENTO Carbon PODA PUR PC 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: 33

35 HHV d = HHV 1 M Eq. 5.1 where HHV is the higher heating value determined by the calorimeter, HHV d 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 HHV d (MJ/kg) HV TSD Carbon PIMIENTO Carbon PODA PUR 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): HHV d,th = 0.35X C X H X S 0,02X N 0,10X O 0.02X ash Eq. 5.2 where HHV d,th is the dry basis higher heating value (in MJ/kg), X c, X H, X S, X N, X O, X ash 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 HHV d,th (MJ/kg) HV TSD Carbon PIMIENTO Carbon PODA PUR 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: 34

36 HHV d (MJ/kg) Measured versus calculated HHV d Higher Heating Value dry basis - measured Higher Heating Value dry basis - calculated 0 HV TSD Carbon PIMIENTO Carbon PODA PUR Figure 5.2. Comparison between experimental and theoretical dry mass HHV A good correlation between the measured and calculated HHV d 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 CO 2-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 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: 35

37 Figure 5.3. TGA graphs during devolatilization 36

38 The two torrefied food residues start losing their volatile matter at around 170 o C, followed by PUR and lastly by HV-TSD, which starts devolatilization above 220 o C. 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 400 o C, followed by PUR (devolatilization ends at ~500 o C), while the two food residues continue losing volatile mass above 600 o C, 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 400 o C 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 The results from the graphical evaluation for the activation energies and the corresponding pre-exponential factors are listed in the following table: 37

39 Table 5.5. Results from graphical evaluation of kinetic parameters for devolatilization HV-TSD Carbon PIMIENTO Carbon PODA Primary Devoaltilization Ea (kj/mol) A (s -1 ) Temperature range ( o C) Secondary Devolatilization Primary Devoaltilization Secondary Devolatilization Primary Devoaltilization Secondary Devolatilization d+c* E d+g* E d+c E E d+g E E d+c E E d+g E E d+c E E PUR d+g E E * 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 CO 2. 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 CO 2(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 Combustion Combustion took place by injecting synthetic air (20.9 vol% O 2, 79.1 vol% N 2) 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: 38

40 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 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 ( o C) primary secondary primary secondary primary secondary HV TSD E Carbon PIMIENTO E E Carbon PODA E PUR E 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 R 2 values that are unacceptable. 39

41 5.5.3 Gasification Gasification took place by injecting pure CO 2 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 CO 2 at the predetermined temperature profile is illustrated in Figure 5.5: Figure 5.5. TGA graph during gasification Gasification begins for all samples above 600 o C. 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 ~850 o C and a mass gain when it cools down to ~850 o C that provides evidence for reversibility of this process. This phenomenon can be attributed to carbonates (such as CaCO 3) dissociation while heating above 850 o C and reformation when cooling back to 850 o C. 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 ( o C) HV-TSD E Carbon PIMIENTO E Carbon PODA E PUR E

42 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 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. 41

43 Figure 5.6. Mass spectrometry graphs illustrating the samples' devolatilization 42

44 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 320 o C 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 100 o C for all samples, while release of light hydrocarbons occurs for all samples at 300 o C. The CO/CO 2 ratio of the detected peaks can be observed to increase with increasing temperature, while there is a broad temperature region from 500 o C to 800 o C where release of hydrogen can be detected. Detection of CO 2 peaks (m/z=44) for the two food residues (Carbon PIMIENTO and Carbon PODA) at elevated temperatures of >400 o C and >600 o C could correspond to decomposition of organic compounds or to dissociation of carbonates (MgCO 3 and CaCO 3 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 300 o C, where the observed mass loss is most acute. The two food residues exhibit the first and biggest peak at ~300 o C, a second at ~450 o C and a third at ~650 o C. Regarding PUR, there is a switch of the detected gases at 300 o C from the CO 2 gas (m/z=44) to the H 2O gas (m/z=18), while a peak for hydrogen release can be detected at temperatures lower than 500 o C Combustion For combustion with air the intensity of N 2 and O 2 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 CO 2 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: 43

45 Figure 5.7. Mass spectrometry graphs illustrating the samples' combustion 44

46 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 CO 2 detection. In the diagrams presented in Figure 5.7 there is an obvious difference in the peak temperature where the maximum CO 2 emission occurs for the different materials. Thus the two food residues (Carbon PIMIENTO and Carbon PODA) exhibit their peak CO 2 production, at 310 o C and 320 o C respectively, followed by PUR at around 380 o C and lastly by HV-TSD at around 410 o C Gasification Gasification was performed by injecting pure CO 2 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 CO 2 by the small amount of char are negligible. Instead, the resulting diagrams are presented for the m/z=28 ratio: 45

47 Figure 5.8. Mass spectrometry graphs illustrating the samples' gasification 46

48 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+CO 2 2CO), at the elevated temperatures where the mass loss takes place (above 750 o C). Although gasification with CO 2 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 800 o C, while PUR and HV-TSD exhibit the same behavior at around 850 o C. 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 950 o C in a muffle furnace (no heating rate was determined) with dwell time of 3h at 950 o C 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 950 o C Sample Original sample weight Sample after oxidation Ash content (g) (g) (%) HV-TSD Carbon PIMIENTO Carbon PODA PUR PC 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 550 o C, instead of the experimental one of 950 o C used in this project. A compilation of the ash produced by the methods employed in this project is illustrated in the following figure: 47

49 Final residue produced [wt% of m o ] Comparison of the ash produced from the different techniques employed HV-TSD Carbon PIMIENTO Carbon PODA PUR Proximate analysis Devolatilization and combustion Oxidation at 950oC Devolatilization and gasification 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 Deformation temperature ( o C) n.d Sphere temperature ( o C) 1549 n.d Hemisphere temperature ( o C) 1549 n.d Flow temperature ( o C) n.d. n.d ( n.d. = not determined ) PUR PC 48

50 From the results presented in Table 5.9 one can observe that the dwell time at 1550 o C serves as time to acquire the equilibrium state for many materials. The heating rate (10 K/min) used to reach 1550 o C 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 1550 o C. 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 Mass loss during heating microscopy experiments Ash type Before heating microscopy experiment Substrate mass (g) Sample mass (g) Substrate + Sample mass (g) After the experiment Substrate + Sample mass (g) Total mass loss (g) HV TSD Carbon PIMIENTO Carbon PODA PUR n.d. n.d PC ( n.d. = not determined ) The coke mass loss during heat treatment to 1550 o C 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 Swelling behavior of PC ash 49

51 Figure Swelling behavior of PUR ash This phenomenon is observed after the temperature has risen above 1430 o C for PUR and at 1550 o C 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 1550 o C. 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: 2613 o C) 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 1550 o C for 16 min. and melts after 29 min. The principal constituents of both materials are CaO, SiO 2, MgO and Al 2O 3. Albeit availability of quaternary phase diagrams, the ternary phase diagram of CaO-SiO 2-MgO with fixed 10 wt% Al 2O 3 and the isothermal section of the CaO-SiO 2-Al 2O 3 diagram at 1800K ( o C) were chosen. In case one reduces the constituents to only CaO, SiO 2, MgO and Al 2O 3, the materials ash composition becomes: Table Reduction of ash composition to four basic components Carbon PIMIENTO Carbon PODA Component composition (wt%) composition (wt%) CaO SiO MgO Al 2O

52 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-SiO 2- MgO with fixed 10 wt% Al 2O 3 diagram: Figure 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 ~2200 o C and solidus temperature at less than 1300 o C, while Carbon PODA ash is found at the Pseudowollastonite primary crystallization field with liquidus temperature at ~1400 o C and solidus temperature at less than 1300 o C. At the isothermal section of the CaO-SiO 2-Al 2O 3 diagram at 1800K ( o C) the observations extracted from the ternary diagram can be verified: 51

53 Figure Isothermal section of the CaO-SiO2-Al2O3 phase diagram at 1800K (MTDATA, 2010) This diagram shows that at 1526 o C, 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 1100 o C and leaves about 1% volatile matter in the produced coke. By further heating the coke plates in the heating microscope to 1550 o C 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 Comparison of SEM photomicrographs of coke before (left) and coke after (right) thermal treatment 52

54 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 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 Photomicrograph of PUR ash on coke using SEM (85x) 53

55 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 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 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 CO 2 at C for 1h: 54

56 Mass loss [wt% of m o ] Mass loss during gasification of the coke substrates Coke thermally treated PC ash on coke HV-TSD ash on coke Carbon PIMIENTO ash on coke Carbon PODA ash on coke PUR ash on coke Figure 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 1201 o C, while it finally melted at 1547 o C and flowed over the coke substrate. Figure 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. 55

57 6 Discussion 6.1 Material characterization All materials tested in this project contain increased amounts of hydrogen that can contribute to CO 2 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 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/cm 3, 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/cm 3 ) 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 200 o C, followed by PUR and lastly by HV-TSD, which starts devolatilization at ~222 o C. 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 56

58 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 CO 2 obeys the Boudouard reaction s norms and produces CO at temperatures above 700 o C. 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 1550 o C while Carbon PIMIENTO ash does not even deform at 1550 o C, 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 1550 o C 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. 57

59 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 CO 2 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 H 2O as a gaseous effluent, which will replace part of the emitted CO 2. 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. 58

60 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. 59

61 9 References Asanuma, M., Ariyama, T., Sato, M., Murai, R., Nonaka, T., Okochi, I., Tsukiji, H., Nemoto, K., Development of Waste Plastics Injection Process in Blast Furnace. ISIJ international, 40(3), pp Asanuma, M., Terada, K., Inoguchi, T. & Takashima, N., Development of Waste Plastics Pulverization for Blast Furnace Injection, s.l.: JFE TECHNICAL REPORT. Babich, A., Senk, D., Knepper, M. & Benkert, S., Conversion of injected waste plastics in blast furnace. Ironmaking and Steelmaking, 43(1), pp Barranco, R., Rojas, A., Barraza, J. & Lester, E., A new char combustion kinetic model 1. Formulation. Fuel, 88(12), pp Björkman, B., IRON AND STEELMAKING, Section 1: Ironmaking. Luleå: Luleå Tekniska Universitet. Carpenter, A. M., Injection of coal and waste plastics in blast furnaces. s.l.:iea Clean Coal Centre. Engineering Toolbox, Fuels - Higher and Lower Calorific Values. [Online] Available at: [Accessed ]. Geerdes, M., Lingiardi, O., Ricketts, J., Chaigneau, R., Kurunov, I., Modern Blast Furnace Ironmaking : An introduction. 3rd edition ed. Amsterdam: IOS Press BV. Gray, R. & Devanney, K., COKE CARBON FORMS: MICROSCOPIC CLASSIFICATION AND INDUSTRIAL APPLICATIONS. International Journal of Coal Geology, 6(3), pp He, R., Sato, J., Chen, Q. & Chen, C., Thermogravimetric analysis of char combustion. Combustion Science and Technology, 174(4), pp Hesse-instruments, Heating microscope. [Online] Available at: [Accessed 09 June 2018]. HORIBA, Ltd., CAMSIZER X2 - Overview. [Online] Available at: [Accessed ]. IKA -Werke GmbH & Co. KG, Calorimeter System C 200. [Online] Available at: [Accessed ]. Irfan, M. F., Usman, M. R. & Kusakabe, K., Coal gasification in CO 2 atmosphere and its kinetics since 1948: A brief review. Energy, 36(1), pp Ishii, K., Advanced Pulverized Coal Injection Technology and Blast Furnace Operation. 1st ed. Oxford: Pergamon. Jernkontoret, Environmental impact of the processes. [Online] Available at: 60

62 recycling/environmental-impact-of-the-processes/ [Accessed 02 June 2018]. Kwon, T.-W., Kim, S. D. & Fung, D. P., Reaction kinetics of char-co 2 gasification. Fuel, 67(4), pp Liu, X., Qin, X., Chen, L. & Sun, F., CO 2 emission optimization for a blast furnace considering plastic injection. INTERNATIONAL JOURNAL OF ENERGY AND ENVIRONMENT, 6(2), pp Lundgren, M., Development of Coke Properties during the Descent in the Blast Furnace, Luleå: Luleå Tekniska Universitet - Doctoral Thesis. Morcel, A., Reaction Mechanism of C-containing Materials for Blast Furnace Injection - Student thesis, Luleå: Luleå Tekniska Universitet. Morin, M., Pécate, S. & Hémati, M., Kinetic study of biomass char combustion in a low temperature fluidized bed reactor. Chemical Engineering Journal, Volume 331, pp MTDATA, Phase Diagram Software from the National Physical Laboratory - CaO- SiO2-Al2O3 phase diagram. [Online] Available at: [Accessed ]. Ölund, M., Reaction Behaviour of Pulverized Reducing Agents Used in the Blast Furnace - P7010K Project Work, Luleå: Luleå Tekniska Universitet. Ölund, M., Sundqvist-Ökvist, L., From, L.-E., Sandström, D., Alatalo, J., Modelling combustion of pulverized coal and alternative carbon materials in the blast furnace raceway. Trondheim, Norway, 12th International Conference on CFD in Oil and Gas, Matallurgical and Process Industries. PerkinElmer, Inc., Thermogravimetric Analysis (TGA) - A Beginner's Guide. [Online] Available at: [Accessed ]. Process Metallurgy Course, Iron Making - Formation of Hot Metal and Slag, Luleå: Luleå Tekniska Universitet. Reusch, W., Mass Spectrometry. [Online] Available at: m [Accessed ]. Smith, I., THE COMBUSTION RATES OF COAL CHARS: A REVIEW. Haifa, Israel, Nineteenth Symposium (International) on Combustion/The Combustion Institute. Sokhansanj, S., Biomass energy data book. [Online] Available at: [Accessed ]. 61

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64 10 Appendices 10.1 Helium pycnometry measurements HV-TSD helium pycnometry 1st test HV-TSD helium pycnometry 2nd test 63

65 Carbon PIMIENTO helium pycnometry 1st test Carbon PIMIENTO helium pycnometry 2nd test 64

66 Carbon PODA helium pycnometry 1st test Carbon PODA helium pycnometry 2nd test 65

67 I. PUR helium pycnometry test 66

68 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 Devolatilization 67

69 ln ( Mass loss perc. dt ln ( Mass loss perc. dt Temperature ( ο C) 1 m VM dm VM s 1 ) 1/T [K -1 ] y = x R² = HV-TSD devolatilization (devolatilization + combustion experiment) TG data analysis for HV-TSD devolatilization (devolatilization+combustion experiment) Temperature ( ο C) 1 m VM dm VM s 1 ) 1/T [K -1 ] y = x R² = HV-TSD devolatilization (devolatilization + gasification experiment) TG data analysis for HV-TSD devolatilization (devolatilization+gasification experiment) 68

70 ln ( Mass loss perc. dt ln ( Mass loss perc. dt /T [K -1 ] Temperature ( ο C) Carbon PIMIENTO devolatilization (devolatilization + combustion experiment) 1 m VM dm VM s 1 ) y = x R² = y = x R² = TG data analysis for Carbon PIMIENTO devolatilization (devolatilization+combustion experiment) /T [K -1 ] Temperature ( ο C) Carbon PIMIENTO devolatilization (devolatilization + gasification experiment) 1 m VM dm VM s 1 ) y = x R² = y = x R² = TG data analysis for Carbon PIMIENTO devolatilization (devolatilization+gasification experiment) 69

71 ln ( Mass loss perc. dt ln ( Mass loss perc. dt /T [K -1 ] Temperature ( ο C) Carbon PODA devolatilization (devolatilization + combustion experiment) 1 m VM dm VM s 1 ) y = x R² = y = x R² = TG data analysis for Carbon PODA devolatilization (devolatilization+combustion experiment) m VM dm VM s 1 ) y = x R² = /T [K -1 ] y = x R² = Temperature ( ο C) Carbon PODA devolatilization (devolatilization + gasification experiment) TG data analysis for Carbon PODA devolatilization (devolatilization+gasification experiment) 70

72 ln ( Mass loss perc. dt ln ( Mass loss perc. dt /T [K -1 ] m VM dm VM s 1 ) y = x R² = y = x R² = Temperature ( ο C) PUR devolatilization (devolatilization + combustion experiment) TG data analysis for PUR devolatilization (devolatilization+combustion experiment) /T [K -1 ] Temperature ( ο C) PUR devolatilization (devolatilization + gasification experiment) 1 m VM dm VM s 1 ) y = -9041x R² = TG data analysis for PUR devolatilization (devolatilization+gasification experiment) y = x R² =

73 ln ( ( dm dt Mass loss perc. 1 M o2 m c X O2 RT (n p 6 )2 3( m πρ )1 3) ln ( ( dm dt Mass loss perc. 1 M o2 m c X O2 RT (n p 6 )2 3( m πρ )1 3) Combustion g cm 2 s 1 Pa 1 ) 1/T [K -1 ] y = x R² = Temperature ( ο C) HV-TSD combustion TG data analysis for HV-TSD combustion g cm 2 s 1 Pa 1 ) 1/T [K -1 ] y = x R² = y = x R² = Temperature ( ο C) Carbon PIMIENTO combustion TG data analysis for Carbon PIMIENTO combustion 72

74 ln ( g cm 2 s 1 Pa 1 ) ( dm dt Mass loss perc. 1 M o2 m c X O2 RT (n p 6 )2 3( m πρ )1 3) ln ( ( dm dt Mass loss perc. 1 M o2 m c X O2 RT (n p 6 )2 3( m πρ )1 3) /T [K -1 ] y = x R² = Temperature ( ο C) Carbon PODA combustion TG data analysis for Carbon PODA combustion 100 1/T [K -1 ] g cm 2 s 1 Pa 1 ) y = x R² = Temperature ( ο C) PUR combustion TG data analysis for PUR combustion 73

75 ln [ Mass loss perc. ln [ Mass loss perc Gasification Temperature ( ο C) HV-TSD gasification m 1 3 m c p g dm dt (n pπ) 1 3 ( 6 ρ ) g s 1 cm 2 Pa 1 ] 1/T [K -1 ] TG data analysis for HV-TSD gasification 2 3 m 1 3 m c p g dm dt (n pπ) 1 3 ( 6 ρ ) g s 1 cm 2 Pa 1 ] y = x R² = /T [K -1 ] y = x R² = Temperature ( ο C) Carbon PIMIENTO gasification TG data analysis for Carbon PIMIENTO gasification 74

76 ln [ Mass loss perc. ln [ Mass loss perc Temperature ( ο C) Carbon PODA gasification Temperature ( ο C) PUR gasification 2 3 m 1 3 m c p g dm dt (n pπ) 1 3 ( 6 ρ ) g s 1 cm 2 Pa 1 ] 1/T [K -1 ] TG data analysis for Carbon PODA gasification 2 3 m 1 3 m c p g dm dt (n pπ) 1 3 ( 6 ρ ) g s 1 cm 2 Pa 1 ] TG data analysis for PUR gasification y = x R² = /T [K -1 ] y = x R² =

77 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 76

78 Combustion Mass spectrometry 3-D graphs 77

79 Gasification Mass spectrometry 3-D graphs 78

80 10.4 Heating microscopy results PC heating microscopy results HV-TSD heating microscopy results 79

81 Carbon PIMIENTO heating microscopy results Carbon PODA heating microscopy results 80

82 PUR heating microscopy results 81

83 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) 82

84 Carbon PODA ash on coke (201x) Carbon PODA ash on coke (1010x) PUR ash on coke (85x) PUR ash on coke (747x) 83