OPTIMIZATION OF KAZAKHSTAN COALS GASIFICATION PROCESS IN THE CIRCULATING FLUIDIZED BED GASIFICATION PROCESS Diyar Tokmurzin A thesis submitted in partial fulfilment of the requirement of Nazarbayev University for the degree of Doctor of Philosophy 2020 ii Table of Contents Declaration ...................................................................................................................................... 1 Abstract ........................................................................................................................................... 2 Chapter 1 – Introduction ................................................................................................................. 3 1.1. The novelty of the research .............................................................................................. 7 1.2. Thesis hypotheses and statements .................................................................................... 8 1.3. Research contribution ..................................................................................................... 10 1.4. A brief introduction to the methodology ........................................................................ 12 Chapter 2 – Literature review ....................................................................................................... 15 2.1 Motivation of the study .................................................................................................. 15 2.2 Current semi-coke production processes in industry ..................................................... 17 2.3 Proposed semi-coke production process based on partial gasification .......................... 18 2.4 Gasification of coal in a circulating fluidized bed ......................................................... 20 2.5 Devolatilization .............................................................................................................. 24 2.5.1 Kinetics of coal devolatilization ................................................................................... 25 2.6 Overview of methods for CFD studies of fluidized beds ............................................... 28 2.6.1 Eulerian-Eulerian two fluid model ............................................................................... 28 2.6.1 Eulerian-Lagrangian model .......................................................................................... 30 Chapter 3 – Materials and Methods .............................................................................................. 32 3.1 Investigation of coal devolatilization properties ................................................................. 32 3.1.1 Materials ....................................................................................................................... 32 3.1.2 Thermogravimetric analysis ......................................................................................... 33 3.1.3 Thermobalance reactor ................................................................................................. 33 3.1.4 Wire-mesh reactor ........................................................................................................ 35 3.2.1 Materials ....................................................................................................................... 38 3.2.2 Experimental setup ....................................................................................................... 39 3.2.3 Reactor start-up............................................................................................................. 43 3.2.4 Reactor operation .......................................................................................................... 43 3.2.5 Semi-coke characterization ........................................................................................... 45 iii 3.2.6 Investigation of semi-coke intrinsic properties ............................................................ 48 3.3 Development of a computational fluid dynamics model for circulating fluidized bed coal partial gasification ..................................................................................................................... 49 3.3.1 Governing equations ..................................................................................................... 50 3.3.2 Fuel devolatilization model .......................................................................................... 57 3.3.3 Chemical reaction model .............................................................................................. 58 Chapter 4 – Isothermal and Non-Isothermal Pyrolysis ................................................................. 60 4.1 Proximate and ultimate analysis of coal samples ................................................................ 61 4.2 Thermogravimetric analyses ............................................................................................... 63 4.3 Thermobalance reactor experiments ................................................................................... 65 4.4 Wire-Mesh Reactor experiments ......................................................................................... 70 Chapter 5 – CFB partial gasification experiments ........................................................................ 78 5.1 Standard coal analysis ......................................................................................................... 78 5.2 Effect of air-to-fuel equivalence ratio on bed temperature ................................................. 81 5.3 Effect of bed temperature on semi-coke carbon content ..................................................... 83 5.4 Size distribution of semi-coke ............................................................................................. 88 5.5 Char particles characteristics ............................................................................................... 90 5.6 Effect of temperature on syngas characteristics .................................................................. 93 5.7 Co-generation of electricity ................................................................................................. 95 5.8 Semi-coke intrinsic properties ............................................................................................. 98 Chapter 6 – CFB partial gasification process simulation ............................................................ 103 6.1 Model setup ....................................................................................................................... 103 6.1.1 Reactor description ..................................................................................................... 103 6.1.2 Grid characteristics ..................................................................................................... 104 6.1.3 Materials ..................................................................................................................... 105 6.1.4 Initial and boundary conditions. ................................................................................. 107 6.2 Flow patterns ..................................................................................................................... 108 6.3 Model validation ............................................................................................................... 110 6.4 Gasification case study ...................................................................................................... 111 6.5 Partial gasification case study ........................................................................................... 112 iv Conclusion .................................................................................................................................. 120 Future research ............................................................................................................................ 122 Bibliography ............................................................................................................................... 124 1 Declaration I declare that the research contained in this thesis, unless otherwise formally indicated within the text, is the original work of the author. The Thesis has not been previously submitted to this or any other university for a degree and does not incorporate any material already submitted for a degree. 2 Abstract Coal, coke, and semi-coke are critical feedstock for the production of iron and steel. For over a century coke and semi-coke has been produced from coal using the slow pyrolysis thermal treatment process in fixed bed coke ovens. The coke-oven slow pyrolysis process produces vast quantities of gaseous and liquid emissions associated with coal tar which are not and sometimes cannot be always captured and recycled. Contrary to this, fast pyrolysis associated with gasification processes produces less tar. In this work a novel method incorporating fast pyrolysis to produce semi-coke using circulating fluidized bed partial coal gasification is experimentally studied. The present study includes an investigation of coal fast devolatilization properties, a pilot scale experimental proof of concept, and optimization of the process. Fast pyrolysis characteristics are explored using a wire mesh reactor and a thermobalance reactor experiments, and semi-coke is produced using a high- volatile Shubarkol coal in a custom-built atmospheric lab-scale reactor comprising a riser, a cyclone, a loop seal, and fitted with mechanized systems for semi-coke retrieval. The reactor is operated autothermally, at temperatures varying from 700 to 1000oC. The experimental results indicate the operating conditions for maximum product output. The product characterization revealed that semi-coke gains distinctive characteristics, including lower density, lower volatile matter content, lower ash content, higher porosity, and higher crystallinity of the carbon matrix. In addition, a Computational Fluid Dynamics simulation employing the Eulerian-Lagrangian multiphase particle-in-cell approach reveals fluidization properties and further optimization opportunities. 3 Chapter 1 – Introduction Economic development and rapid urbanization put a strain on infrastructure requiring the expansion of housing, water supply, electricity supply, and transport. This expansion causes increase in demand for steel and its alloys, especially in developing Asia, including India, China and Kazakhstan. Global steel demand has nearly tripled in the last fifty years and expected to increase further [1]. According to the world coal institute, demand in Asia is the main driver, where steel demand has grown four times since the 1970s [2,3]. Currently no substitute is available for coal, coke, and semi-coke as reducing agents in the iron and steel industries and steel demand has driven their consumption. Most of steel worldwide is smelted in conventional metallurgical blast furnaces, where coke serves as a deoxidizing material and a heat source, and also provides mechanical support to descending burden material. In addition coke provides permeability in the layers of raw material for gas and drainage for liquid iron and slag. [4]. Currently, coke and semi- coke carbonaceous materials are produced from scarce and expensive coking coal in coke-oven batteries, which have numerous disadvantages for example, they are economically costly and environmentally harmful. It is proposed here to produce semi-coke from high volatile low ash coal using low air-to-fuel equivalence ratio partial gasification process in a circulating fluidized bed (CFB) as an alternative for coke-oven batteries. Baking in coke ovens takes place in packed beds inside batteries, sealed to isolate coal from oxygen and to remove all volatiles, which are regarded as impurities in iron and steel manufacturing. Coke-ovens are batch process units with cycles lasting from 14 to 36 hours. The remaining char consists of almost pure fixed carbon and ash. 4 Environmental impairment of coke-oven batteries is mostly associated with coke-oven gas containing organic compounds evolved from coal volatile matter. Normally, production of a tonne of coke produces approximately 360 m3 of coke-oven gas (COG) and each year 70 billion Nm3 of COG is produced in China alone [5]. However, commonly only one fifth of the coke-oven gas generated is utilized further [5]. Disposing of COG requires proper recovery and effective utilization, otherwise COG can cause severe environmental pollution and serious waste of energy [5]. COG recovery and utilization is low because of high capital expenditures required for recovery and utilization facilities. The coking process is a significant source of pollution. COG contains approximately 30 wt.% of tar as heavy hydrocarbons (HCs) and 70 wt.% in the form of light gases [5], including H2 and CH4. Besides particulate matter, the process emits polycyclic aromatic hydrocarbons, volatile organic compounds (VOCs), ammonia, carbon monoxide, and hydrogen cyanide contained in tar. The process also emits around 100 grams of methane per ton of coke and 50-80 g of hydrogen sulfide per ton of coke [6]. Other emissions, such as 0.7 to 7.4 kg of particulate matter, 0.2 to 6.5 kg of SOx, 1.4 kg of NOx, 0.1 kg of ammonia, and 3 kg of VOCs, can be emitted. Additionally, from 0.3 to 4 m3 of wastewater can be produced per ton of coke. Wastewater can contain 100mg/l of benzene, 1000 mg/l of biochemical oxygen demand (BOD), 1500-1600 mg/l of chemical oxygen demand (COD), 200 mg/l of suspended solids, and 150-2000 mg/l of phenols, and up to 30 mg/l of polycyclic aromatic hydrocarbons (PAHs), ammonia, and cyanides [6]. Long-term exposure to the pollution from coke-ovens can result in dermatitis, conjunctivitis, respiratory and digestive system diseases, including cancers of the lung, trachea, bronchus, kidney, and prostate. Tumors of the lung and skin were found on animals exposed to the inhalation of coal tar [7]. Similarly to carbon dioxide, carbon monoxide, methane and VOCs absorb heat from the earth surface, however their indirect GHG effect is more significant. In the atmosphere they react with 5 hydroxyl radicals (OH-), resulting in formation of tropospheric ozone, water vapor, and methane, which are greenhouse gases. In addition to all the environmental issues given above, economic issues are also associated with coke-ovens, i.e. scarcity and high prices of coke and coking coals, which motivates corporations to search for alternative technologies to conventional blast furnaces in iron production and coke ovens in coke making. For instance, recently developed commercial processes such as iron bath smelting [8,9], blast furnaces with coal injection [10] and ferroalloys production [11,12] allow the fine fraction semi-coke to be substituted for coke as the reducing agent. Semi-coke is essentially a char, thus it is a product of coal pyrolytic devolatilization with a high content of fixed carbon and a low content of volatile matter. In the novel COREX and FINEX steelmaking processes, which were developed by Siemens VAI and Posco, respectively, iron is produced by direct smelting using the non-coking coal. While FINEX does not need coke, COREX still requires some amounts of coke. COREX and FINEX use coal instead of coke, but if the coal does not conform to the requirements, anthracite or semi-coke can be blended with the coal to reduce the bulk volatile content. Semi-coke production in coke-ovens has the same environmental disadvantages as coke production. Substitution of conventional semi-coke production with a method which has less energy demand and emissions can improve the whole steel industry energy consumption (and therefore economics) and environmental performance. In this study, a novel partial coal gasification process using a circulating fluidized bed (CFB) method is suggested for the thermal treatment of coal for production of semi-coke and syngas for its further utilization in a thermal power plant. Very few studies have considered partial gasification in a CFB, with existing studies mainly considering their modelling, fixed bed experiments, or bench scale allothermal circulating fluidized bed experiments [13–15]. Partial gasification reactor operates at an air-to-fuel equivalence ratio below the ratio typical for gasification processes, thus limiting 6 carbon conversion. For efficient production of semi-coke, the partial gasification process needs to be optimized in order to maximize the devolatilization product output and minimize the fixed carbon conversion. This process can also be used for production of charcoal, but special attention needs to be paid to feedstock fixed carbon and ash content. Biomass typically has a high volatile matter content (70- 90%), a low fixed carbon content (10-20%), and low ash content (0.2-6%) [16]. The partial gasification process will yield charcoal with lower fixed carbon compared to charcoal from the pyrolysis process, because part of the fixed carbon will be oxidized in partial gasification. One of the positive examples is woody biomass such as pine chips, which typically has a high volatile matter content and low fixed carbon content, but very low ash content [17]. L. Van de Steene et al[17] pyrolyzed pine chips at 800-1000 oC and the devolatilized charcoal had a fixed carbon content – 93.7%, ash content – 1.4%, volatile matter content – 4.9%. Partial gasification of pine chips will yield charcoal with lower fixed carbon and higher ash content. On the other hand, partial gasification as well as pyrolysis of biomass species such as cassava stalk and sugar cane bagasse will yield charcoal with low carbon content due to their initial low fixed carbon content. In the current work, wire mesh reactor and thermobalance reactor experiments were first conducted in order to determine the isothermal and non-isothermal devolatilization behavior of various coals. This allowed the determination of advanced coal properties required for the development of Eulerian-Lagrangian simulation (EL) model of the circulating fluidized bed using the multiphase particle-in-cell method (MP-PIC). Then it was followed by construction of a pilot scale reactor in which experiments were conducted in order to prove experimentally the feasibility of partial gasification method and to explore process sensitivities necessary for optimization. The operating conditions for optimization included air-to-fuel ratio, temperature, fuel mass flow rate, 7 and air flow rate. The last part of the current work involved the development of the computational fluid dynamics model of the experimental pilot reactor using an Eulerian-Lagrangian simulation (EL) model of the circulating fluidized bed using the multiphase particle-in-cell method (MP-PIC). The fluidized bed model reactor simulation boundary conditions, such as simulation domain, pressure boundary conditions, flow boundary conditions are defined based on the CFB pilot scale reactor design. 1.1. The novelty of the research The literature review on the worldwide development of circulating fluidized bed (CFB) technology demonstrates that very little research has been conducted on CFB partial gasification. To date very few experimental studies have been carried out on the partial gasification of coal in a CFB reactor [13–15,18]. Those that have, have considered production of syngas in CFB under O2- H2O and O2-CO2 and proposed to burn semi-coke as a fuel in boilers, therefore, not considering semi-coke for other uses such as in metallurgy. At the same time, a few studies considered integration of allothermal semi-coke production pyrolizers with a heat source such as boilers and gasifiers based on an Aspen Plus simulation [19–22] without experimental prove of process feasibility and quality of produced semi-coke. The novelty of the present research is summarized as follows: - A novel semi-coke production with power co-generation approach based on partial coal gasification was proposed and experimentally tested; - An advanced framework was proposed for determination of solid fuels devolatilization properties at isothermal and non-isothermal conditions; 8 - Optimum operating conditions, maximizing semi-coke production and minimizing fixed carbon conversion were determined experimentally on a pilot scale CFB reactor; - A Eulerian-Lagrangian multiphase particle-in-cell simulation model was developed, validated, and used for the optimization of a novel partial CFB gasification process; - Optimization unveiled further potential for improvement of the process through structural modifications of the reactor. 1.2. Thesis hypotheses and statements Even though in previous works concerning coal partial gasification or coal pyrolysis one of the outputs was char, these processes were not optimized for maximum devolatilization, maximum semi-coke output, and solid products were not characterized for use in metallurgy. The literature review on carbonation processes and CFB development indicate a potential to use CFB gasification technology for semi-coke production and simultaneous production of synthesis gas using autothermal partial gasification. The syngas evolved from the process and char particles leaving the cyclone can be burned in a boiler for further heat and power production. Thesis hypotheses - Novel partial gasification circulating fluidized bed process can effectively produce high carbon char at optimum conditions, suitable for use in metallurgical processes; - Coal partial gasification is able to produce a novel material, i.e., semi-coke with superior proximate analysis, high structural strength, higher porosity, and lower bulk density; 9 - The CFB partial gasification process can produce high carbon semi-coke autothermally without additional input of energy; - The CFB partial gasification process can effectively be integrated into a power plant; - The CFB partial gasification computational CFB model based on the multiphase particle-in-cell method adequately models devolatilization of coal and change of its chemical composition. Research questions: - What are the optimal values of air-to-fuel ratio, process temperature, and coal mass flow rate for maximizing the semi-coke output? - What is the process’s main product output at optimal conditions? - What are the mass and energy balances between input coal and output products? - What is the potential available power output from integration with a power plant? - Which modifications can be adapted to increase the efficiency of the CFB partial gasification process? Thesis statements: - To conduct a thorough literature review on the devolatilization of coal, operation of CFB partial gasification reactors, and CFD simulations of fluidization processes - Construct a pilot CFB reactor which has a 5.6 m high and 150 mm diameter wide riser, cyclone, and loop seal, for implementation of experiments - Conduct experimentally a sensitivity analysis on the process operating conditions - Produce semi-coke with characteristics satisfactory for use in metallurgy - Characterize the high carbon char produced for use as a semi-coke 10 - Develop and validate a Eulerian Lagrangian MP-PIC model for the simulation of the CFB partial gasification process - Conduct sensitivity analysis using a Eulerian Lagrangian MP-PIC model. 1.3. Research contribution Published Journal papers: 1) Diyar Tokmurzin, Ho Won Ra, Sung Min Yoon, Sang Jun Yoon, Jae Goo Lee, Myung Won Seo, and Desmond Adair. 2019. “Pyrolysis Characteristics of Kazakhstan Coals in Non-Isothermal and Isothermal Conditions.” International Journal of Coal Preparation and Utilization”. doi:10.1080/19392699.2019.1594793. 2) Tokmurzin, D, and D Adair. 2019. “Development of Euler-Lagrangian Simulation of a Circulating Fluidized Bed Reactor for Coal Gasification.” Eurasian Chemico-Technological Journal 21: 45–49. DOI: https://doi.org/10.18321/ectj789. 3) Diyar Tokmurzin, Desmond Adair, Timur Dyusekhanov, Kalkaman Suleymenov, Boris Golman, Berik Aiymbetov. “Development of a circulating fluidized bed partial gasification process for co-production of metallurgical semi-coke and syngas and its integration with power plant for electricity production”., https://doi.org/10.1080/19392699.2019.1674842 Conference proceedings: https://doi.org/10.18321/ectj789 https://doi.org/10.1080/19392699.2019.1674842 11 1) Diyar Tokmurzin, Desmond Adair, Myung Won Seo, Berik Aiymbetov, Kalkaman Suleymenov, Yerbol Sarbassov. Shubarkol coal pyrolysis in a wire mesh reactor, production of semi-coke in an atmospheric circulating fluidized bed reactor and its integration with a thermal power plant. 23rd Fluidized bed conversion conference, Seoul, 14-17th of May 2018 2) Diyar Tokmurzin, Kalkaman Suleymenov, Berik Aiymbetov, Yerbol Sarbassov, Myung Won Seo, Desmond Adair. Flash pyrolysis of Shubarkol coal in a CFB reactor for semi-coke production. 9th International symposium Combustion and Plasmochemistry. 13-15 September 2017. Almaty. Kazakhstan. Presentations at international conferences 1) Myung Won Seo, Diyar Tokmurzin, Tae Young Mun, Ho Won Ra, Sang Joon Yoon, Jae Ho Kim, Yong Ku Kim, Jae Goo Lee. Thermal decomposition characteristics of Kazakhstan coals. 5th Asia-Pacific Forum on Renewable Energy. 6th November 2015. Jeju, Korea 2) Diyar Tokmurzin, Desmond Adair. Characterization of selected Kazakhstan coals and simulation of their gasification process using an Eulerian-Lagrangian multiphase particle-in-cell approach. 11th European Conference on Clean Coal Research and its Applications (11th ECCRIA) 5–7 September 2016. Sheffield, United Kingdom. 3) Diyar Tokmurzin, Desmond Adair. Development of Euler-Lagrangian Simulation of a Circulating Fluidized Bed Reactor for Coal Gasification. INESS-2018. 8-10 August. Astana. Kazakhstan 12 1.4. A brief introduction to the methodology In the present study, a variety of approaches are used in order to develop a novel partial gasification process. These approaches can be summarized as: - Experimental characterization of carbonaceous materials pyrolysis properties; - Experimental work, including proof of concept and process sensitivity analysis; - Characterization of produced semi-coke; - Investigation of the process behavior using numerical methods; - Optimization using a simulation model after validation. A combined two-stage experimental method is developed for the determination of devolatilization characteristics of coal and other carbonaceous materials. The characterization is aimed to experimentally determine the pyrolysis kinetics, mass balance of materials, and composition of gas derived from volatile matter during pyrolysis at high temperatures. An elaborate procedure for characterization of fuels has been developed to cover all aspects required for the numerical optimization of the process. The first stage includes thermogravimetric analysis and thermobalance reactor experiments, which reveal the devolatilization kinetics of coal used for partial gasification. The second stage includes wire mesh reactor experiments which reveal the evolved materials mass balance and composition of gases released from volatile matter. The experiments were designed to reflect the conditions typical of industrial CFB gasifiers. As a result, all necessary devolatilization characteristics were obtained for adequate simulation of the partial gasification process using the numerical MP-PIC method. The experimental investigation of developed partial gasification process was conducted in a customized CFB reactor which consisted of a cylindrical riser column, cyclone separator, standpipe, 13 and U-shape loop seal. The experimental study aims to prove the concept, investigate the optimal operating regime of the installation, and produce semi-coke samples for characterization. Experiments were conducted using Shubarkol coal, which is used to produce semi-coke in coke- ovens, with a 40-50 kg/hr fuel feed rate. Gas composition was measured continuously during the experiments. During the experiments semi-coke was retrieved from the riser and standpipe and entrained char particles were captured using cyclones. Semi-coke proximate analysis was carried out using a thermogravimetric method. Entrained char samples were also captured using exhaust cyclones. The entrained char samples proximate analysis was conducted using the thermogravimetric method and a bomb calorimetry. Numerical modelling of the process allows the determination of the fluidization phenomena behavior which is not possible to measure during experiments, such as reactions rates along the height of the reactor, distribution of voidages, particle segregation, and so on. The aim of the numerical modelling is to develop the model, validate it and use it to investigate the effect of process parameters on reactor performance. Multiphase flow modelling can be conducted using the two- fluid model approach or the Eulerian-Lagrangian approach. This study uses the Eulerian- Lagrangian approach with MP-PIC method, which is less demanding on computational capacities compared to other Eulerian-Lagrangian methods. At the same time, the MP-PIC method allows for a better understanding of the change of particle size distribution, particle physical and chemical properties, gas-solid interactions, and gas-solid reactions. Possessing this information enhances process optimization. This dissertation focuses on the development, analysis, and optimization of the partial gasification process for production of semi-coke and its integration with the power plant. 14 The following chapters of this thesis describe the main results: Chapter 2 presents a literature review summary on key aspects of fluidization phenomena, numerical studies of fluidized beds, and a description of conversion processes in numerical studies of fluidized beds. Chapter 3 represents the detailed description of research methodology covering the investigation of isothermal and non-isothermal pyrolysis. Chapter 4 describes results of isothermal and non-isothermal pyrolysis experiments with the aim of determining the devolatilization characteristics. Chapter 5 represents results of the experimental investigation of low equivalence ratio gasification, and semi-coke production in a circulating fluidized bed reactor, with sensitivity analysis of the process. Chapter 6 describes the simulation of the CFB reactor for semi- coke production and its validation with experimental data and investigates flow patterns and segregation phenomena which influence the partial gasification process. 15 Chapter 2 – Literature review 2.1 Motivation of the study Economic development and growth of metropolitan areas are increasing the demand for water supply and sanitation network expansion, energy system development, expansion of transportation, public facilities, and housing, which in turn increase the demand for iron and steel and alloys [1– 3]. Coal, coke, and semi-coke are vital for the production of iron, steel, and ferroalloys, which serve as carbonaceous deoxidizing agents, sources of heat, and provide mechanical support to burden and permeability to gas, and liquid iron and slag in blast furnaces [4]. Coke is produced in fixed bed coke-oven batteries by heating coking coals. Increasing prices for coking coals and their sparsity encourages steel producing companies to search for substitute raw materials, alternative coke and steel producing processes. Recently introduced commercial processes such as blast furnaces with coal injection [10], ferroalloys production [11,12], iron bath smelting [8,9] make it possible to reduce coke consumption or substitute coke to the fine fraction semi-coke as the reducing agent. Recent research demonstrates that semi-coke injection into blast furnace can reduce coke consumption, replace anthracite coal as an injected fuel, and can be used in the sintering process [10]. Semi-coke is essentially a devolatilized coal char having low volatile content and high fixed carbon content, which is achieved through a pyrolytic process. In COREX® and FINEX® steel smelting and reduction processes developed by Siemens VAI with partners [23], coal is used instead of coke as the reducing agent and energy source. Coal is pyrolyzed in a melter-gasifier in both processes. When coal properties do not conform to the COREX and FINEX processes requirements, due to high volatiles content, the fine fraction semi-coke can be blended with additional coal [10]. 16 Another important characteristic of semi-coke is that it is smokeless and can be used for household heating instead of coal. Emissions from coal combustion in households are a threat to public health and pose serious problem across developing and northern countries. Coal is the preferred source of heat due to its affordable price and its ease of storage. For instance, coal plays the role of a major source of heating in Poland, Ireland, Republic of Korea, Czech Republic, Kazakhstan, South Africa, Mongolia and China [24]. Since the volatile matter is considered as a precursor for PM2.5 formation and elemental carbon emissions [25], less PM2.5 emissions are generated during devolatilized semi-coke combustion than coal combustion[26]. The previous study conducted by Li et al demonstrated that use of briquetted semi-coke can reduce PM2.5, elemental carbon, organic carbon, and carbon monoxide emissions by factors 92%, 98%, 91%, and 34%, respectively [25]. 17 2.2 Current semi-coke production processes in industry In industry, semi-coke is currently produced using pyrolytic devolatilization in coke-ovens with no oxygen supply and with the supply of heat from an external source. Coal is fed into fixed bed coke-ovens, compacted and then sealed to ensure the absence of oxygen. Coke-ovens are then heated and devolatilization is carried out at temperatures up to 700-800 oC with heating rates less than 1 oC per minute. Volatiles are driven out of the coal after 14 to 36 hours in the coke-oven and then coke is ejected from coke-oven batteries. In order to avoid the combustion of semi-coke carbon content in the open air, semi-coke is extinguished using water quenching. Semi-coke typically has high fixed carbon content not less than 70% on a dry basis [27] and a low volatiles content of not more than 10% [25]. According to particle size, semi coke is classified as powder (0-6 mm), small lumps (6-13 mm), and middle lumps (13-25 mm) [25,28]. The semi-coke production processes currently in use have substantial disadvantages, including: (i) formation of environmentally harmful phenol compounds and their release with tar to soil and water, (ii) water quenching produces a wet product, which then requires drying before semi-coke can be used in metallurgical processes, and (iii) high capital cost of coke-oven gas treatment and utilization facilities [29]. The desire to overcome these disadvantages partly motivated the development of a novel semi-coke production process. Additionally, conventional semi-coke production process requires coal with size in the range of 20-80 mm and therefore, coal of a smaller size is separated from the coal and considered unsuitable, which leads to economic losses. Consequently, the economic attractiveness of the newly developed process is further improved when fine fraction coal is utilized. 18 Other semi-coke production technologies (besides coke-oven), also known as carbonation processes, can be divided into two distinctive groups according to their operating temperature as low-temperature (up to 700 oC) and high temperature (700 oC and more) [26]. The products of the low temperature carbonation process are not used in industry as semi-coke due to their higher volatile matter content of 10-20%. High-temperature carbonation processes are more suitable for the production of metallurgical grade semi-coke because higher temperatures favour deeper devolatilization [30]. Fluidized bed reactors were also experimentally tested for this purpose, notably CSIRO, CONSOL, and SINGH processes [31]. These processes are either low temperature processes, use sand as bed material that increases the non-combustible content of semi-coke, or are two-three step processes with a separate heat source and a pyrolizer. 2.3 Proposed semi-coke production process based on partial gasification The novel method for production of metallurgical semi-coke involving partial gasification of coal in a CFB is proposed for investigation in the present study. Partial gasification implies simultaneous production of semi-coke and syngas, where syngas and entrained char particles then can be combusted in a thermal power plant with a Rankine cycle for co-production of electricity. Integration of the partial gasification process with a thermal power plant is schematically shown in Figure 2.1. Full gasification of coal using CFB is a mature technology, however partial gasification in CFB is not well developed. Even though the technology has numerous opportunities for integration in metallurgy, energy, and the chemical industry [15,32] it is frequently overseen due to its disadvantages. In metallurgy CFB partial gasification has long been ignored because partial gasification reduces semi-coke yield and overall particle size, and therefore such semi-coke is useful for a limited number of metallurgical processes. Fast pyrolysis co-occurring during partial 19 gasification causes reduction of the tar yield, which is used for production of carbon black, commodity grade phenol, or adhesive material for carbonaceous anode mass. The energy industry has ignored the partial gasification process because normally CFB technology is used in the energy sector to produce heat and power with reduced NOx and SOx emissions. At the same time an introduction of stricter environmental requirements can motivate decision makers to search for alternative technologies for production of semi-coke with less emissions. Such integration makes possible the retrofit of existing industrial facilities, which have on-site power plants. The efficiency of the gasification process is expressed through carbon conversion, which during the partial gasification process, does not reach full conversion and is kept at the desired level in order to produce carbon rich char [13,18]. In order to maximize the output of carbon contained in semi-coke, carbon conversion should be kept close to the carbon content of the volatile matter. Just a small number of studies have reported research on coal partial gasification in a CFB. They mainly focus on aspects such as thermodynamic analysis of partial gasification [33] and the development of integrated systems for the production of hydrogen and power [14,15] or have reported small scale allothermal CFB tests [13]. Ye et al. reported their experimental work with H2O-O2 [13] and CO2-O2 [13] injection using a lab-scale CFB gasifier with external heating using a vertical tube furnace. The CFB had a cylindrical riser with a 120 mm cross-section diameter and a 2.6 m height. Authors conducted the experiments in the relatively limited temperature range of 885 to 980oC using a relatively narrow coal particle size of 0.35-0.9 mm in diameter. None of these studies focused on the characterization of solid products of the partial gasification process. Devolatilization and volatile matter conversion plays a more important role in partial gasification than in coal gasification, therefore a thorough understanding of devolatilization is essential for the design and simulation of CFB reactors and the understanding of the experimental results 20 Figure 2.1 Proposed integration of a partial gasification process with a thermal power plant. 1) circulating fluidized bed partial gasification process 2) boiler 3) steam turbine 2.4 Gasification of coal in a circulating fluidized bed Gasification is a thermo-chemical conversion process of carbonaceous solid fuels, such as coal, to a gas through a combustion process in an oxygen deficiency environment, with some gasifying agents which can be used such as oxygen, air, and steam [34]. Coal gasification is decomposed to three consecutive stages, as is schematically depicted in Figure 2.2, including evaporation (drying), pyrolysis (devolatilization) and a gasification stage including gas-gas reactions and gas-solid char reactions. The gas-gas reactions and gas-solid reactions within a gasification process are listed in Table 2.1. Reactions such as gasification with oxygen, combustion with oxygen, gasification with carbon dioxide are more prevalent. Due to the absence of steam injection, reactions such as gasification with steam, gasification with hydrogen, and water-gas-shift reactions are less prevalent. Due to atmospheric pressure, the methanation reaction is also less prevalent. Methane combustion, carbon hydrogen combustion, and methane thermal decomposition are gas phase reactions that 21 occur in pyrolysis and gasification reactions products. As a result of gasification, a mixture of intermediate combustion gases is formed, which includes synthesis gas, containing CH4, CO2, CO, H2, and CO2, and devolatilization products such as methane homologues and tar. During a gasification process, coal particles undergo drying, devolatilization, and gasification stages. The devolatilization process occurs due to the heating of fuel and reactions when solid long chain hydrocarbon molecules break. As a result, fuel particles emit gaseous and condensable tar products derived from their volatile content. These products then undergo homogeneous reactions. The conversion processes that proceed at high temperature and with oxygen absence, thus limiting the progression to devolatilization, are called pyrolysis. The devolatilized fuel char particles containing mainly fixed carbon further undergo heterogeneous reactions. In industry, the gasification process is typically operated at a 0.3-0.4 oxygen-to-fuel equivalence ratio. Figure 2.2 Schematic diagram of fuel particle gasification stages Table 2.1 Gasification and combustion reactions in the gasification process. 22 Reaction Process Chemical Formula Gasification with Oxygen C (s)+ 0.5 O2 → CO Combustion with Oxygen C (s) + O2 → CO2 Gasification with Carbon Dioxide C (s) + CO2 → 2 CO Gasification with Steam C (s) + H2O → CO + H2 Gasification with Hydrogen C (s)+ 2 H2 → CH4 Water Gas Shift CO + H2O → CO2 + H2 Methanation CO + 3 H2 → CH4 + H2O Methane combustion CH4+O2→CO2+2H2 Carbon monoxide combustion 2CO+O2→2CO2 Hydrogen combustion 2H2+O2→2H2O Methane thermal decomposition CH4→C+2H2 The circulating fluidized bed (CFB) reactor is a widely used technology under rigorous development for use in combustion and gasification of solid fuels, such as biomass, refuse derived fuels, and pet-coke, and is thus not limited to coal [35–37]. CFB reactors have a high overall rate of gasification due to exceptionally high rates of heat and mass transport rates, and, long residence time of char particles owing to char recirculation which results in low char loss [33,35,38]. A circulating fluidized bed reactor is a fluidized bed reactor consisting of the main reactor called a riser, a gas-solid separator, commonly a cyclone, a return standpipe and a solids return system, such as a loop seal. The typical operating temperature of the CFB reactor is between 750oC and 1000oC. Circulating fluidized bed reactors have certain advantages and disadvantages, which are listed in Table 2.2. In terms of producing semi-coke in CFB partial gasification process the most important advantage is CFB reactor adaptability to handle particles with widely varying size, densities, and sphericities. This allows to produce semi-coke with desired PSD and even feed coarse coal particles in order to produce semi-coke with larger particle size distribution. 23 Table 2.2 Advantages and disadvantages of CFB reactors [34]. Advantages Disadvantages - High gas through-put capability - Requires very a high vessel: - Limited reverse mixing of gas - Wall wastage sometimes a serious problem - Long and adjustable particle residence time - Lateral gradients can become significant - Temperature uniformity, without ‘‘hot spots’’ - Suspension-to-surface heat transfer less favourable than for low-velocity fluidization - Adaptability to handle particles with widely varying sizes, densities, and sphericities - Loss of bed material due to entrainment - Effective contacting between gas and particles - Substantial back-mixing of solid particles Internals (e.g., baffles, heat transfer surfaces) not viable because of wear/attrition - Insufficient gas bypassing with minimal limitation of mass transfer - Opportunity for detached and secondary function operation (e.g., catalyst regeneration or particle cooling) in the return loop 24 2.5 Devolatilization Upon introduction into the CFB gasifier, coal particles are subjected to rapid heating up to the operating temperature of the reactor. Fuel particle gasification can be divided to three successive steps, consisting of drying, pyrolysis (devolatilization), and gasification. Devolatilization is a stage when solid fuel volatile matter breaks down and releases gaseous products, such as tar, CO, CO2, H2, CH4, and methane homologues. This stage is especially important for high-volatile fuels pyrolysis, gasification, and partial gasification. The pyrolysis processes can be categorized as slow and fast pyrolysis [39–41]. Pyrolysis in a conventional stationary bed coke-oven has slow heating rates below 1oC/s and this is considered as slow pyrolysis. When a fuel particle is introduced into a circulating fluidized bed gasifier it is heated with heating rates of 1000oC/s, and thus the particle undergoes fast pyrolysis. The yield of main devolatilization products, including tar, gas, char, and devolatilization gases composition vary between fast and slow pyrolysis [42–46]. Devolatilization products mass balance and gaseous products composition also largely vary depending on the type of reactors used such as fixed bed, fluidized bed, or wire mesh [39,41]. Pyrolysis at high temperatures and fast heating rates, such as in a circulating fluidized bed gasifier, demonstrates improved efficiency and lower tar yield [47]. Tar is a source of ammonia (NH3), PAHs (polycyclic aromatic hydrocarbons), and phenols contamination. Therefore, high-temperature processes utilizing CFB reactors are anticipated to produce coke with improved quality coke and at the same time reduced environmental impact. Numerous publications report studies on coal pyrolysis [48– 51]. A considerable number of researchers investigated the effect of coal pyrolysis with biomass and the cumulative effect of co-pyrolysis[51–53]. The focus of all of these studies was on the pyrolysis gas. 25 It is noteworthy to mention that few studies considered the production of semi-coke, but all of them considered the integration of the atmospheric allothermal pyrolizer with a heat source such as a gasifier and combustor [19–22]. All these studies were conducted using ASPEN PLUS software with the aim to simulate the process and explore its integration opportunities. Integration of the partial gasification process provides polygeneration opportunities and in general, allows reduction of the CO2 recovery cost [49]. However, the simulations did not include the characterization of the semi-coke, even though its characteristics will influence process operations downstream. The physical characteristics include particle size distribution, density, porosity, sulfur content, arsenic content, total pore volume, and structural strength. 2.5.1 Kinetics of coal devolatilization The coal devolatilization stage can be divided into two stages, non-isothermal and isothermal devolatilization. Fast heating of coal particles upon introduction to the reactor is considered a non- isothermal stage and further devolatilization at constant temperature is considered an isothermal stage [54]. Feedstock properties considerably influence the performance of thermal processes, including pyrolysis, gasification and combustion, thus their specification is important for the design and optimization of the process. One such property is devolatilization kinetics in non-isothermal and isothermal conditions at the conditions specific to thermal process. Basic fuel properties include proximate analysis, ultimate analysis, particle size distribution, coal ash interphase change characteristics, and coal microstructure. However, advanced pyrolysis properties such as pyrolysis kinetics, mass balance of pyrolysis products, and composition of evolved gases are not known. The advanced devolatilization properties are essential for Eulerian-Lagrangian CFD simulation of 26 partial gasification, such as the composition of materials evolved from fast pyrolysis. Therefore, a combination of thermobalance reactor and wire mesh reactor experiments is a useful technique that allows the determination of the properties required for simulation. This is even more important for partial gasification process due to the higher share of heat from devolatilization products reactions. Properties considered basic include proximate analysis, ultimate analysis, particle size distribution, and coal ash fusion temperature, while detailed properties include thermogravimetric characteristics, devolatilization products mass balance, evolved gases composition, fuel devolatilization kinetics at non-isothermal and isothermal conditions. Also important to take into account is that when the aim of the Eulerian-Lagrangian CFD simulation is to investigate devolatilization of fuel particles, and their segregation in the bed, advanced pyrolysis properties, such as pyrolysis kinetics and the composition of materials evolved from fast pyrolysis are crucial. Therefore, a combination of thermobalance reactor and wire mesh reactor experiments have been used as the technique that allows the determination of the properties required for simulation. The mass balance and the composition of gases evolved from the coal volatile matter are crucial properties that need to be determined. Recent developments within the computational fluid dynamics field allow the simulation of the fluidized bed reactor at an industrial scale and investigate phenomena that was not previously possible at available computational cost and time [55]. Multiple studies have been published on the CFD simulation of fluidized bed thermal processes such as pyrolysis, gasification, combustion, and chemical looping cycle [56–65], whereas commonly reported fuel properties are limited to proximate and ultimate analysis. Simulation studies have coupled two-phase fluid dynamics with chemical reactions and specified fuel devolatilization, gas- solid, and gas-gas reactions kinetics using the Arrhenius expression. 27 Specification of the devolatilization gases composition is necessary for the simulations utilizing the multiphase particle-in-cell approach [66]. However, this data is not always available and it is frequently left behind in publications, and not reported [61,67–70]. However, devolatilization gas composition significantly influences reactor output gas composition and heat produced by gas-to- gas homogeneous reactions. In the case of partial gasification, where the process is kept lean on oxygen in order to limit the carbon conversion and exploit mainly devolatilization gases combustion heat, devolatilization gases composition significantly influences simulation of the process. Chemical reaction rates, including combustion and gasification reactions, are a function of the reacting gas species concentrations and temperature [67,71]. Gas species concentration is largely dependent on the devolatilization gases composition, which in turn is similar qualitatively but may vary widely quantitatively [30,41,72]. Consequently, the composition of devolatilization gases is a significant feature that needs to be determined in order to improve the accuracy of the combustion/gasification processes calculations [73]. Few experimental studies report isothermal thermobalance reactor experiments [72,74–76] and wire mesh reactor experiments [30,40,72,77–86], and very few studies have been reported where isothermal and non-isothermal pyrolysis characteristics including pyrolysis kinetics, mass balance of pyrolysis products, and evolved gases composition are investigated [46,72]. Most of the publications report either isothermal pyrolysis kinetics [54,72,87–90], non-isothermal pyrolysis mass balance [40,41,77–86,91,92], or evolved gas composition [46,78,93]. As a result, finding the complete set of properties for a specific fuel is not trivial. 28 2.6 Overview of methods for CFD studies of fluidized beds Gas-solid flows in circulating fluidized beds have been numerically studied using various multiphase CFD models, but none of these are capable of simulating every aspect and investigating every issue encountered in industry [94]. At the same time, multiphase models have specific advantages and disadvantages that make them suitable for the investigation of certain aspects of circulating fluidized bed operation. Therefore, a clear understanding of these models is crucial for development of adequate simulations. Figure 2.3 Classification of CFD models for two-phase flows and used for fluidized bed simulations Figure 2.3 illustrates schematically the types of CFD models used for fluidized beds and their relationship to each other. The two most frequently used approaches for the simulation of a fluidized bed system are the Eulerian-Eulerian and Eulerian-Lagrangian methods. 2.6.1 Eulerian-Eulerian two fluid model 29 The Eulerian-Eulerian method considers the gas and solid phases as interpenetrating fluid continua and are also commonly known as the two-fluid model (TFM) [95–97]. In this approach, both gas and solid phases occupy the same volume and are distinguished by the volume fraction. Unlike the Eulerian-Lagrangian approach, where all particles are tracked, the Eulerian-Eulerian approach is less demanding on computational resources, making it possible to use the approach for large scale applications. At the same time, continuum treatment of solids enables a finer computational grid, which makes TFM models efficient for the simulation of small scale fluidized beds [98,99]. The equations for conservation of mass, conservation of momentum, and conservation of energy are determined using appropriate averaging, and, the constitutive relations for the solid phase are typically closed using the kinetic theory of granular flow (KTGF) [94]. One of the complications is that special care has to be taken to correctly implement the momentum exchange between gas and solid phases [100–102]. TFM is not capable of simulating the particle size distribution and changes in physical and chemical properties of particles. Eulerian-Eulerian TFM is the most widely used approach for dense gas-solid flows [100–110]. Due to the complexity of the fluidization process numerous factors has to be taken into account, and consequently no universally applicable simulation setup exists [99]. Recent research publications considered the influence of parameters governing the hydrodynamic behavior in different gas-solid fluidized bed applications, such as wall-boundary conditions [104–108] and drag models [103,109,110]. Most of the studies considered the Eulerian-Eulerian TFM approach with single particle diameter, which is a mean diameter of particle-size distribution (PSD). Thus, the accuracy when using this approach of particles representation is limited to processes with narrow PSD. However, most of the industrial applications have the wide PSD, which requires approximation of solid phase to two or more granular phases and mean size diameters respectively 30 with an extension of the Eulerian-Eulerian approach to multi-fluid model (MFM) [99]. Strongly polydisperse systems will require numerous granular phases making the MFM computationally costly. When one of the goals of fluidized bed system simulation is investigation of particle physical and chemical properties alongside with particle size distribution, the Eulerian-Eulerian approach is not suitable. 2.6.1 Eulerian-Lagrangian model The Eulerian-Lagrangian approach enables the tracking of particles individually or in parcels according to Newton laws of motion. Unlike the TFM model, the Discrete Element Method (DEM) considers solids as a discrete phase with detailed descriptions of the particle–particle and particle– wall collisions based on the soft-sphere approach [98]. Use of DEM, however, has a limitation due to its high computational cost. The dense discrete phase model (DDPM) [111], coarse grain CFD-DEM, and multiphase- particle-in-cell (MP-PIC) [68,71,112] methods are developed to overcome computational cost limitations of DEM. DDPM and MP-PIC as they do not track explicitly the details of particle- particle and particle-to-wall collisions but rather replace them with forces representing the details of collisions. The particle properties are interpolated to and from a Eulerian grid. Additionally, DDPM and MP-PIC use particle parcels allowing a reduction of the computational cost and so enabling their application to large systems [113]. 31 MP-PIC is considered suitable for the simulation of large scale gas-solid flow applications [114]. MP-PIC has been widely used for the simulation of bubbling fluidized beds [115–117], fast fluidized beds [118–121] and other fluidized bed systems. 32 Chapter 3 – Materials and Methods 3.1 Investigation of coal devolatilization properties Properties of coals vary largely depending on their origin. These properties influence coal behaviour and final product characteristics in thermal treatment processes. Devolatilization is an important stage of coal thermal treatment. Coals devolatilization properties can be revealed using experiments, such as thermogravimetric analysis (TGA), thermobalance reactor (TBR) experiments, and wire-mesh reactor (WMR) experiments. Particularly, thermogravimetric analysis allows the investigation of non-isothermal devolatilization kinetics with slow heating rates. TBR reactor and WMR experiments simulate the fast heating rate conditions that occur during fluidized bed partial gasification and reveal devolatilization properties that are necessary for answering questions, such as, ‘what coal is more suitable for gasification or partial gasification?’; ‘what devolatilization products and their amount can be expected?’, and ‘what are the devolatilization kinetics of coals?’. Additionally, devolatilization properties are vital for the simulation of the partial gasification process in a circulating fluidized bed. 3.1.1 Materials In this chapter isothermal and non-isothermal pyrolysis characteristics of six coal samples from five Kazakhstan coal fields are investigated. The six coal samples include Turgay lignite coal, Karagandy A cleaned coal, Karagandy B mid coal, Ekibastuz high as steam coal, Maikuben high- volatile coal, and Shubarkol high-volatile coal. The samples preparation procedure was to first dry them in an oven for 7 hours at 105°C. Then the coal samples were ground in a mill (RM 200 series, Retsch, Germany) and screened through a 150 μm sieve using a mechanical sieving machine 33 (AS200, Retsch, Germany). A PSD below 150 µm improves the representative quality of the results by minimizing the internal temperature gradients. 3.1.2 Thermogravimetric analysis Thermogravimetric analysis in a nitrogen environment is a useful technique for characterizing fuel non-isothermal devolatilization properties with a slow heating rate. In this study, TGA analysis was carried out to detect the starting and ending temperatures for the devolatilization and the differential thermogravimetric curve revealed the intensity peaks of coal devolatilization and also the temperature at which the peak occurs. TGA analysis in an air atmosphere was used to determine when the coal devolatilization and the fixed carbon oxidation occurred. Thermogravimetric properties of coal samples were investigated using an automatic thermogravimetric analyzer (Q500, TA instruments, USA). TGA analyses were conducted under nitrogen and air atmosphere with 10oC/min heating rate and a maximum temperature of 900 oC. 3.1.3 Thermobalance reactor Thermobalance reactor experiments were conducted to investigate coal isothermal pyrolysis characteristics at a temperature range typical for fluidized bed gasification (500-1000oC). The main goal of the TBR experiments was to investigate the devolatilization kinetics of the solid fuels isothermally at specific temperatures, thereby simulating the conditions the solid fuel is subjected to upon introduction into a reactor. The kinetic data investigated using TBR experiment include reaction order, reaction rate constant, frequency factor, and activation energy. The TBR illustrated in Figure 3.1 and previously described by Yun et al. [72] was used in this study. The reactor chamber was made of a 1 m long and 0.055 m in diameter stainless-steel pipe. The chamber was heated using a 5 kV Kanthal electric heater surrounding it. Each sample (approximately 0.2 g) was 34 located in a stainless-steel wire-mesh basket (5). The wire-mesh basket was hanging on a wire attached to an electronic balance (10). The basket was raised and lowered into the chamber high temperature zone surrounded with heating elements using a windlass assembly (11) through the access door (8). The air was heated using an electric preheater (1) before being injected into the chamber at the reactor bottom. The chamber was preheated using hot air and when the temperature reached the required temperature, nitrogen (13) was purged into the chamber in order to form a neutral environment in the chamber. Once air was displaced from the chamber, the sample basket was lowered into the high temperature chamber [122]. Figure 3.1. Schematic diagram describing the structure of the thermobalance Reactor installation: (1) air/N2 preheater, (2) thermocouple inside the reactor chamber, (3) temperature controller, (4) Kanthal electric heater, (5) basket for the sample, (6) exhaust gas cooler, (7) vacuum pump, (8) access door, (9) N2 supply, (10) electronic balance, (11) windlass assembly, (12) gas flowmeters, (13)supply of gas. 35 3.1.4 Wire-mesh reactor Pyrolysis experiments can also be conducted using a wire mesh reactor installation for the experimental study of particle devolatilization with high heating rates from 100 K/s up to 1000 K/s). The main aim of WMR experimental work is to obtain data on the mass balance of the pyrolysis process products such as tar, char, and gas, and to determine the composition of the gas produced from the coal volatile matter during fast pyrolysis with varied peak temperature. Generally, coal pyrolysis reactions have a two-stage mechanism, where the primary step comprises initial transitional pyrolysis reactions and the following stage gas-to-gas reactions. During the primary stage, reactions are characterized as very rapid and are comprised of radicals formation, radicals recombination, polymerization-condensation, and hydrogen addition reactions. During the secondary stage, reactions include decomposition of the volatile products evolved during the initial primary stage reactions [123]. It is nearly impossible to quantify the products from the pyrolysis reactions of the primary stage, so therefore the WMR reactor experiments were developed for the measurement of the products from the second-stage reaction. The secondary stage products are regarded as the final pyrolysis products. The formation mechanisms of each pyrolysis product including tar, gas species, and char from first stage reaction products are described by Zhan et al. [124]. The WMR used for the experiments in this study is shown in Figure 3.2. The WMR contains the following main units: the electric power supply, the mass flow controller, the reaction chamber with a gas outlet with built-in cooling jacket, sample holder clamps for a wire mesh wrap with the coal sample inside, and a ceramic wool trap located at the reaction chamber outlet for the collection of produced gases and tars. Rectangular wire mesh made of SS 316L stainless steel with 75 μm mesh openings and an overall size of 0.076 m × 0.02 m was used to wrap the sample. The coal sample 36 and the steel wire mesh were weighed separately and after wrapping before being deployed into the reactor. The ceramic wool tar trap was also weighed before being deployed into the gas outlet of the reaction chamber. Approximately 20 mg of the coal sample was wrapped inside the wire mesh and the wrap was then fixed on a copper electric clamp/sample holder. After assembling the reactor chamber, nitrogen purge gas was continuously blown into the chamber at a rate of 3 L/min. The mesh was heated by applying an electric current of about 48 A and 6.2 V. The operating temperature of the reactor was set to 400 ºC, 600 ºC, 800 ºC, and 1000 ºC, with the holding time at the operating temperature set to 10 s, where 10 s is sufficient to minimize the effect of temperature gradients occurring because of the volume of fuel particles [91]. The nitrogen injection into the chamber assisted in the reduction of the temperature of the coal sample and at the same time served to carry the evolved pyrolysis gas to the outlet tube. The outlet tube was fitted with ceramic wool inside and a cooling jacket around the tube, which was filled with liquid nitrogen. At the reactor outlet, gas went through the ceramic wool tar trap and the gas sample was collected using a gas sampling tedlar bag. The ceramic wool with trapped tar and the wire mesh with char were weighed separately using a scale with 0.1 mg resolution (XP205, Mettler Toledo, USA). Gas samples were analysed using a gas chromatograph (GC7890, Agilent, USA) with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The experiments were repeated in triplets for each coal sample at each set temperature. 37 Figure 3.2. Schematic of the wire mesh reactor apparatus [30]. 38 3.2 Partial gasification experiments 3.2.1 Materials Partial gasification experiments were conducted with high-volatile Shubarkol coal that is mined in the Karagandy region of the Central Kazakhstan from open pit mines. The CFB reactor tailored for the experiments was fed with Shubarkol coal. The coal particle size was in the range of 0-20 mm. The feedstock characteristics include proximate analysis, ultimate analysis, particle size distribution (PSD), and calorific value [37,125]. These feedstock characteristics are also required for quantitative estimation of the process operation characteristics and determining opportunities to improve the process efficiency. Operation characteristics, including syngas quality and escaping char characteristics, helped to identify opportunities to combine a semi-coke production plant with a downstream power plant fueled with syngas and escaping char. The total amount of materials used in the CFB experiments was screened using a sieve with a 20 mm mesh size. Coning and quartering methods was used to extract three representative samples from the sieved material. Further proximate analysis, CHNS analysis, ash fusion temperature measurement, and PSD analysis were completed. The PSD analysis was conducted using an automatic sieving machine where the three samples were sieved using sieves with 10, 8, 6, 4, 2, 1, and 0.5 mm mesh openings. For the PSD analysis, three samples were taken from the bulk coal and screened using an automatic sieving machine. Before conducting the CHNS analysis, proximate analysis, and ash fusion temperature analysis the coal samples were ground to powder and sieved using a 200 µm sieve. A thermogravimetric analyser (TGA Q500, TA instruments) was used for the proximate analysis. Coal was heated at 20oC/min to a maximum temperature of 900 oC in the TGA with 20 minutes holding time. Before conducting the ultimate analysis using the CHNS analyser (Variocube, Elementar) coal samples were dried at 105oC for 12 hours to minimize discrepancies 39 associated with the moisture content. The coal heating value was analyzed using a bomb calorimeter (B-08МАК, Etalon). Agglomeration of coal particles due to ash coalescence is directly linked to ash melting characteristics. The ash fusion temperature analysis was analysed according to the ASTM D1857 standard with a pyramid cone method in an ash fusion determinator (5E-AF400, CKIC). 3.2.2 Experimental setup The feasibility of the proposed partial gasification process for the production of semi-coke was validated experimentally using a tailor-made CFB reactor. Figure 3.3 shows a diagram schematically describing the experimental CFB setup. The experimental CFB reactor consisted of a cylindrical riser, a cyclone separator, a standpipe, a loop seal, and auxiliary systems including a coal supply line, sample withdrawal devices, and a syngas treatment subsystem. The cylindrical riser (1) had a height of 5.6 m and an inner diameter of 150 mm. The reactor body was made of heat resistant stainless steel. The air stream (I) was blown by the air blower (15) into the riser air distribution chamber located at the bottom of the riser. Then air was injected from the air distribution chamber into the riser chamber through a series of bubble caps located on the air distribution plate. Coal from a bunker (4) was supplied using one of two 70 mm inner diameter hoses to two coal inlet ports (13) which introduced coal into the riser. Coal inlet ports (13) were situated at heights of 350 mm and 700 mm above the air distribution plate. During the experiments only one of two ports, the port at the height of 700 mm, was used. The coal was fed from the bunker to the riser using a screw feeder (5), which rotationally adjusted the feed rate. 40 Figure 3.3 Schematic diagram of experimental setup. The gas-particle stream from the riser containing syngas and entrained char particles was conveyed to a cyclone separator (2), where the particles were separated and gravitationally fell through the standpipe (14) to a loop seal (3). The main purpose of the loop seal is to return char particles with unburnt carbon back into the riser. The loop seal is essentially a pneumatic device with a rectangle 600 mm high chamber divided with a divider plate beginning from the top and not reaching the bottom of the loop seal, thus making two chambers interconnected with each other. The first, receiving chamber which had an 80 mm × 80 mm cross-section is located under the standpipe. The second, conveying chamber with a 120 mm × 80 mm cross-section is connected to the riser with a tube. Air for the loop seal chambers (III) is injected into both chambers through 41 bubble caps in order to form a bubbling fluidized bed. Rigorous mixing of particles in the loop seal causes an exchange of particles between two chambers. At the same time accumulation of the particles falling from the standpipe displaced the particles from the receiving chamber to the conveying chamber, and in turn from the conveying chamber to the riser (1). After separation of particles from the syngas, the syngas was streamed into two exhaust cyclones (6.1 and 6.2) deployed in series. After separation of the particles the syngas composition was streamed further to a combustor (8). The combustion products from the combustor were passed through a heat exchanger (9) and a fabric filter (10) to an exhaust chimney (12). Each cyclone dipleg had an attached bunker for accumulation of the char particles (7.1 and 7.2). Outer surfaces of the riser (1), loop cyclone (2), two exhaust cyclones (6.1 and 6.2) were fully covered with a 100 mm thick ceramic wool for insulation. Air and gas streams included: I – primary air, II – secondary air, III – air to loop seal chambers, IV – air blown to L-valve, V – an air stream for aeration of the standpipe, VI – an air stream for the pneumatic transport of coal, VII – syngas from the reactor, VIII – an air stream for the syngas afterburner. K-type thermocouples were used for measuring the temperature along the reactor riser (1), in the loop seal (3), and in the recirculation cyclone (2). Thermocouples were connected to a multichannel pressure-temperature data logger (RMT59, Elemer). Pressure transducers (PD100, Owen) were used for measuring the pressure along the riser. 42 Figure 3.4 Mechanisms for retrieving semi-coke: a) from the lower part of the riser and(b) from the bottom of the loop seal and lower part of the standpipe. Mechanisms for retrieving the circulating material from the reactor are illustrated in Figure 3.4. Two motorized installations for retrieving the material from the riser were installed at the lower part of the reactor. The first (16) retrieves material from the bottom of the riser, with the outlet just above the air distribution plate. The second (17) is intended for retrieving the material from the top of the dense phase zone. The loop seal is also fitted with two mechanisms for retrieving the material. First is (19) to retrieve the material from the bottom of the loop seal receiving chamber, with the outlet just above the air distribution plate. The second (14) retrieves the material from the lower part of the standpipe. It was intended to take the material that falls into the reactor. All four have screw transporters which help to retrieve the material without gas leaks and also to adjust the mass flow rate. The riser additionally had a manual mechanism for retrieving the material (18) from the riser lower part, which is basically a hole with a plug. 43 3.2.3 Reactor start-up CFB gasifiers and combustors normally use high density inert materials as a bed material in the riser and loop seal. Inert materials such as sand also serve as heat transfer media, ensuring uniform temperature. However, in this study, unlike previous partial gasification research [13–15,33] the goal is to retrieve the devolatilized char from the riser bed and from the loop-seal. Sand in the fluidized bed inevitably will mix with char and the char retrieved from the bed will include significant portions of sand, which is not desirable for semi-coke quality. Therefore, it was decided not to use sand and instead form the bed in the riser and the bed in loop seal using only char. Low air-to-fuel equivalence ratio (ER) favours accumulation of char. The dense phase zone of the bed in the riser will entirely be formed from large char particles and particles leaving the dense phase zone to contain only small particles. Therefore, we expect loop seal particles to be only small char particles. Before starting the reactor 2.5-3.0 kg of semi-coke with particle size in the range 0-8 mm was put into the riser, and 2.0-2.5 kg of semi-coke with particle size in the range 0-2 mm was put into the loop seal receiving chamber. The primary air and air for the loop seal were preheated to 600-650oC using the electric heater (13). When the riser-bed temperature reached 300oC the motor of the coal screw conveyor (5) was switched on and the coal supply started. When bed temperature reached ~400 oC a sharp temperature increase occurred. From this point on the CFB reactor was operated autothermally and the heater was turned off. When the bed temperature reached 800-900 oC the coal feed rate and air flow rate were set to the required values. 3.2.4 Reactor operation The experiments were conducted with ER in the range 0.19-0.33 and the bed temperature in the range 700oC - 1000oC. The main parameters measured during the experiments included bed 44 temperature fuel feed rate, air flow rate, differential pressure in the riser and in the loop seal, and syngas composition. The air flow rate was varied to ensure a 4-5 m/s superficial velocity and maintain fast fluidized bed conditions in the riser. The height of the dense phase zone was estimated based on the differential pressure. Once the dense phase zone height exceeded the outlet for bed material retrieval (17) in the riser the screw transporter was started to collect the samples. Similarly, when the dense phase zone height exceeded the outlet for bed material in the standpipe (14) the second screw transporter was started. Char samples were retrieved from the top of the dense phase zone in the riser and from the standpipe. Char samples were also retrieved from cyclone bunkers. All solid samples, including semi-coke from the riser and from the loop seal and char samples from bunkers, were analysed for the PSD and for proximate analysis. Operating conditions of eleven experiments are listed in Table 3.1, and these operating conditions are also listed in tables with corresponding results. Table 3.1. Operating conditions of the experiments Experiment No 1 2 3 4 5 6 7 8 9 10 11 Вcoal kg/h 58 44 65 45 45 55 48 48 40 50 50 Vair total Nm3/h 110 94 94 112 112 112 65 80 94 107 72 Тbed оС 930 950 930 810 880 990 700 850 780 865 890 Вcoal – coal feed rate, Vair total – total air flow rate, Тbed – riser dense-phase-zone temperature. 45 3.2.5 Semi-coke characterization Samples were retrieved during every experiment described in Table 3.1. First, a representative sample was taken from retrieved semi-coke using the quartering and coning approach. Then, the sample PSD was determined using the sieving machine and the sample thermogravimetric analysis was carried out by heating the sample at 20oC/min to 900 oC. Moisture content, ash content, volatiles content, and the fixed carbon content were determined using the thermogravimetric analyzer. The sulphur content was determined using the CHNS analyzer. Semi-coke characteristics from this study were compared with the semi-coke produced in a rotary drum type furnace and in a conventional coke oven [126,127]. An automated gas sorption analyser (Autosorb, Quantochrome instruments) which utilizes the nitrogen absorption technique was used to determine the samples’ porosity, specific surface area, and total pore volume. A semi-coke sample weighing 200-300 mg was ground and sieved through a 150 µm sieve and then degassed at 350oC for 8 hours. The Brunauer-Emmett-Teller (BET) method [128] was used to interpret the adsorption and desorption isotherms. The bulk density of the semi-coke samples was determined according to the GOST 54251-2010 standard which corresponds to ISO 567:1995 [128]. A cubic shaped container made of steel with an internal volume of 0.2 m3 was used for the measurements. The container was weighed before and after it was filled with semi-coke. The bulk density (BD) of the semi-coke was determined on a dry basis as BD = 𝑚𝑚2 −𝑚𝑚1 𝑉𝑉 ∙ 100 −𝑊𝑊𝑟𝑟 100 , (3.1) 46 where 𝑚𝑚1 is the weight of the empty container, 𝑚𝑚2 is the weight of the container with semi-coke, 𝑉𝑉 is the container volume, and 𝑊𝑊𝑟𝑟 is the moisture content of the semi-coke. The GOST 9521-2017 standard was selected for structural strength analysis of the semi-coke and for comparison of the obtained results with literature data [126,127]. According to this standard, semi-coke was sieved and sized between 3-6 mm, and taken as a sample for structural strength analysis. The sample was dried in an oven at 105oC for 3 hours. Next, the sample was poured onto a pan and dispensed uniformly on the pan making a 9-10 mm semi-coke layer. The layer was then divided into 20 squares, making 20 portions. A 50 cm3 measuring glass cup was filled with semi- coke taken from each of the 20 squares. The semi-coke in the glass cup was compacted on a vibrating table, and additional semi-coke was added in order to fill the cup completely. Similarly, a second sample was prepared. The device for testing semi-coke strength is shown in Figure 3.6. Two steel cylinders (4) with 25 mm inner diameter and 310 mm inner length were fixed in brackets which make a cross-head (5) connected to a shaft with a revolution counter (3), gearbox (2), and electric motor (1). Each cylinder was first filled with half of the sample, then loaded with five 15 mm diameter steel balls, and followed with the second half of the sample. The cylinders were sealed and firmly fixed in brackets. The cross-head with cylinders was rotated at 25 revolutions per minute for 40 minutes. Both samples were sieved through a 1 mm sieve and weighed. The structural strength was determined as 𝑆𝑆 = 𝑚𝑚2 𝑚𝑚1 ∙ 100%, (3.2) where 𝑚𝑚1 is the total weight of the samples before loading into the cylinders, and 𝑚𝑚2 is the weight of samples left on the 1 mm sieve. The procedure was repeated four times and the results were averaged. 47 Figure 3.6. Device for semi-coke structural strength testing Chlorine content was determined according to GOST 9326-2002 standard [129] using a method of burning the sample in a calorimetric bomb. At first, 1 g of semi-coke sample was placed in a crucible. Then, 1 ml of distilled water was added in the bomb and crucible was fixed in the bomb. After closing the bomb, it was filled with oxygen until the pressure reached 3 MPa. After burning, the bomb content was transferred to a 300 ml vessel. The bomb itself was washed using 150 ml of water, and water was also transferred to 300 ml vessel. The water was evaporated to reduce the total volume down to 100 ml, and the contents were then filtered through a grade 2 filter paper to an Erlenmeyer flask. The glass cup and filter were washed with 150 ml of hot distilled water. 1 ml of 1:10 HNO3 solution (Sigma-Aldrich) and 0.5 ml of diphenylcarbazone (Sigma-Aldrich) were added to the cooled filtrate and chlorine ions were titrated with a solution of mercurous nitrate (Sigma- Aldrich) until the filtrate was coloured violet. In parallel with semi-coke analysis a control analysis was conducted with the same equipment and reagents and using the same procedure, but without burning the semi-coke in the calorimetric bomb. The chlorine content Cl was calculated as Cl = (𝑉𝑉1 − 𝑉𝑉2)𝐶𝐶 ∙ 2 ∙ 34.45 1000 ∙ 100 𝑚𝑚 ∙ (𝑉𝑉1 − 𝑉𝑉2)𝐶𝐶 ∙ 2 ∙ 7.09 𝑚𝑚 , (3.3) 48 where 𝑉𝑉1 is the volume of mercurous nitrate spent for titration, 𝑉𝑉2 is the volume of mercurous nitrate spent for titration during control analysis, C is the concentration of mercurous nitrate, and 𝑚𝑚 is the mass of the burnt semi-coke sample. 3.2.6 Investigation of semi-coke intrinsic properties X-ray diffraction method was used to investigate the crystallite structure of carbon in the semi- coke. The method possesses a significant advantage, it requires substantially larger amount of solid sample per analysis compared to alternative methods. This allows to apply a simple quartering and coning method in order to obtain a representative sample, therefore an average characteristic of a larger samples was analysed rather than local characteristics of separate solid particles. Such approach is advantageous when heterogeneous materials such as coal, coke, or semi-coke are analyzed. X-ray diffraction analysis allows determining the crystalline parameters such as crystallite size and its distribution (crystallite diameter – La, crystallite stacking height – Lc, interlayer spacing – d002). Semi-coke samples were milled, sieved through a 0.075 mm sieve, and then analyzed using a Rigaku SmartLab® X-ray diffractometer (XRD) with Cu-Kα radiation source (35 kV, 28.5 mA). Scanning of samples was conducted with 2θ in the range from 10 to 100 degree at scanning rate of 1 deg/min. The shape of the carbon peak was used for determining the crystalline size. 49 3.3 Development of a computational fluid dynamics model for circulating fluidized bed coal partial gasification Computational fluid dynamics (CFD) simulation coupled with chemical conversion models can predict the output of thermal processes and give insight to the thermo-chemical, fluid dynamics, and heat-mass transfer processes occurring inside a reactor. Consequently, CFD can enhance the design and optimization of the thermal conversion processes. During the last decade, computational fluid dynamics models have been used for the simulation of coal, biomass, and refuse derived fuel thermal conversion processes, including fluidized bed systems. Two different approaches are in wide use for the simulation of fluidized bed systems: the two-fluid (Eulerian-Eulerian) models and the discrete element method (Eulerian-Lagrangian) models. Eulerian-Lagrangian models calculate the trajectories of each solid particle in the system including particles with different physico- chemical characteristics. If an Euleraian-Lagrangian model is integrated with a chemical reactions kinetic model, the model will be able to calculate the variance in the physico-chemical characteristics of the fuel particle during devolatilization and the subsequent char conversion processes [67]. However, such models have the disadvantage of demand for large computational power. The compromise, to avoid this disadvantage and calculate with reasonable accuracy the circulating fluidized bed characteristics, is the multiphase particle-in-cell (MP-PIC) approach. The objective of this section is to investigate the segregation of particles due to devolatilization and develop for this a comprehensive three-dimensional numerical multiphase particle-in-cell (MP- PIC) model for the circulating fluidized bed coal partial gasification process coupled with a chemical conversion model. The chemical conversion model estimates fixed carbon, volatiles, and ash content of the circulating material. Homogenous and heterogeneous chemical reactions are 50 described by reaction kinetics models and the reaction rates are solved numerically using an Eulerian grid [130]. 3.3.1 Governing equations A three-dimensional CFD model using the Computational Particle Fluid Dynamics (CPFD) Barracuda software with multiphase particle-in-cell (MP-PIC) approach was developed for the calculation of fluidized bed processes. This model was used for the simulation of Shubarkol coal partial gasification in a CFB reactor. The CPFD Barracuda software uses Eulerian equations for the continuous carrier fluid phase and a stochastic particle method for the particle phase [131]. The CPFD simulation approach solves the continuous fluid and discrete particle equations in three dimensions. The discrete particle phase is modelled using the Liouville equation, which takes into account the particle size distribution. The model also takes into account collisions between solid phase particles and particle - wall collisions. Fluid-phase mass, momentum, and energy equations conserve mass, momentum, and energy of the two-phase mixture by exchange terms [71]. The equations that are solved for the fluid phase are described, and the inter-phase mass, momentum, and energy exchange terms are introduced. Finally, the equations that are solved for the particle phase are presented. The governing equations of the gas phase are given by by Eqs. (3.4) and (3.5): The fluid mass conservation equation is given as 𝜕𝜕(𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔) 𝜕𝜕𝜕𝜕 + ∇�𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔𝑢𝑢𝑔𝑔� = 𝛿𝛿𝑚𝑚𝑝𝑝̇ , (3.4) The fluid momentum equation is given as 51 𝜕𝜕(𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔𝑢𝑢𝑔𝑔) 𝜕𝜕𝜕𝜕 + ∇�𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔𝑢𝑢𝑔𝑔𝑢𝑢𝑔𝑔� = −∇𝑃𝑃 + 𝐹𝐹 + 𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔𝑔𝑔 + ∇ ∙ 𝜃𝜃𝑔𝑔𝜏𝜏𝑔𝑔 , (3.5) where 𝜃𝜃𝑔𝑔 is the gas volume fraction, 𝜌𝜌𝑔𝑔 is the gas density, 𝑢𝑢𝑔𝑔 is the gas velocity vector, 𝑃𝑃 is the gas pressure, 𝜇𝜇𝑔𝑔 is the gas viscosity, 𝜏𝜏𝑔𝑔 is is the fluid stress tensor, and 𝑔𝑔 is the gravitational acceleration. Here, 𝛿𝛿𝑚𝑚𝑝𝑝̇ defines the amount of gas produced from the particle-gas chemical interaction. The gas mass source is 𝛿𝛿�̇�𝑚𝑝𝑝 = −∭𝑓𝑓 𝑑𝑑𝑚𝑚𝑝𝑝 𝑑𝑑𝜕𝜕 𝑑𝑑𝑚𝑚𝑝𝑝𝑑𝑑𝑢𝑢𝑝𝑝𝑑𝑑𝑇𝑇𝑝𝑝, (3.6) where the time-rate-of-change of particle mass 𝑑𝑑𝑚𝑚𝑝𝑝 𝑑𝑑𝜕𝜕 is the rate of change of the particle mass due to particle-gas chemical interaction and directly linked to particle devolatilization model described in Section 3.3.2 and particle-gas reactions described in Section 3.3.3. Here, 𝑓𝑓 is the probability distribution function of the particles. The term 𝐹𝐹 in Eq. (3.5) represents the rate of momentum exchange per unit volume between the gas and particle phases, and is given as 𝐹𝐹 = ∭𝑓𝑓𝑉𝑉𝑝𝑝𝜌𝜌𝑝𝑝 �𝐷𝐷�𝑢𝑢𝑔𝑔 − 𝑢𝑢𝑝𝑝� − 1 𝜌𝜌𝑝𝑝 𝛻𝛻𝑃𝑃� 𝑑𝑑𝑉𝑉𝑝𝑝𝑑𝑑𝜌𝜌𝑝𝑝𝑑𝑑𝑢𝑢𝑝𝑝, (3.7) 𝑑𝑑𝑢𝑢𝑝𝑝 𝑑𝑑𝑑𝑑 = 𝐷𝐷�𝑢𝑢𝑔𝑔 − 𝑢𝑢𝑝𝑝� − 1 𝜌𝜌𝑝𝑝 ∇𝑃𝑃 + 𝑔𝑔 − 1 𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔 ∇𝜏𝜏𝑝𝑝 (3.8) where 𝑉𝑉𝑝𝑝 is the particle volume, 𝐷𝐷 is the interphase drag force, 𝑢𝑢𝑝𝑝 is the particle velocity, 𝜌𝜌𝑝𝑝 is the particle density, and 𝜏𝜏𝑝𝑝 is the particle normal stress. 52 The gas stress tensor is defined as 𝜏𝜏𝑔𝑔,𝑖𝑖,𝑗𝑗 = 𝜇𝜇 �𝜕𝜕𝑢𝑢𝑖𝑖 𝜕𝜕𝑥𝑥𝑗𝑗 + 𝜕𝜕𝑢𝑢𝑗𝑗 𝜕𝜕𝑥𝑥𝑖𝑖 � + 2 3 𝜇𝜇𝛿𝛿𝑖𝑖𝑗𝑗 𝜕𝜕𝑢𝑢𝑘𝑘 𝜕𝜕𝑥𝑥𝑘𝑘 , (3.9) where 𝜇𝜇 is the shear viscosity, which is the sum of the laminar shear viscosity and a turbulence viscosity from Smagorinsky LES turbulence model [132]. The particle volume fraction, 𝜃𝜃𝑝𝑝, is calculated using Eq. (3.10) and relates to the gas volume fraction in Eq. (3.11) 𝜃𝜃𝑝𝑝 = ∭𝑓𝑓𝑉𝑉𝑝𝑝𝑑𝑑𝜌𝜌𝑝𝑝𝑑𝑑𝑢𝑢𝑝𝑝, (3.10) 𝜃𝜃𝑔𝑔 = 1 − 𝜃𝜃𝑝𝑝, (3.11) Furthermore, the particle normal-stress model is described as 𝜏𝜏𝑝𝑝 = 𝑃𝑃𝑆𝑆𝜃𝜃𝑝𝑝 𝛽𝛽 𝑚𝑚𝑚𝑚𝑥𝑥�(𝜃𝜃𝑐𝑐𝑝𝑝−𝜃𝜃𝑝𝑝)𝜀𝜀(1−𝜃𝜃𝑝𝑝)� , (3.12) where 𝑃𝑃𝑆𝑆 is the positive constant with units of pressure, 𝛽𝛽 is the constant (2 < 𝛽𝛽 < 5 is recommended) [133], and 𝜀𝜀 is the constant having value of the order of 10-7 [131]. 𝜃𝜃𝑐𝑐𝑝𝑝 is the particle close-pack limit volume fraction which depends on the size, shape, and ordering of the particles. Large eddies are calculated directly for the large-scale turbulence simulations. The subgrid turbulence is simulated using Smagorinsky model: 𝜇𝜇𝜕𝜕 = 𝐶𝐶𝜌𝜌𝑔𝑔∆2�(𝜕𝜕𝑢𝑢𝑖𝑖 𝜕𝜕𝑥𝑥𝑗𝑗 + 𝜕𝜕𝑢𝑢𝑗𝑗 𝜕𝜕𝑥𝑥𝑖𝑖 )2, (3.13) where ∆ is the subgrid length scale which is the cube root of the sum of the product of distances across a calculation cell in the three orthogonal directions: 53 ∆= (𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕)1/3 (3.14) 𝐶𝐶 is sub-grid eddy coefficient, also known as the Smagorinsky coefficient, which default value is constant 𝐶𝐶=0.01. The transport equation is solved for each gas species, and the total fluid phase properties are calculated from the mass fractions 𝑌𝑌𝑔𝑔,𝑖𝑖 of the gas species making up the fluid mixture. Through the breaking and forming of chemical bonds, mass is transferred between gas species, which is represented as chemical source terms 𝛿𝛿�̇�𝑚𝑖𝑖.𝑐𝑐ℎ𝑒𝑒𝑚𝑚 in the individual gas species transport equations 𝜕𝜕(𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔𝑌𝑌𝑔𝑔,𝑖𝑖) 𝜕𝜕𝜕𝜕 + ∇�𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔𝑌𝑌𝑔𝑔,𝑖𝑖𝑢𝑢𝑔𝑔� = −∇ ∙ �𝐷𝐷𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔∇𝑌𝑌𝑔𝑔,𝑖𝑖� + 𝛿𝛿�̇�𝑚𝑖𝑖.𝑐𝑐ℎ𝑒𝑒𝑚𝑚 , (3.15) The coefficient 𝐷𝐷 is the turbulent mass diffusivity and is related to the viscosity by the Shmidt number correlation: 𝑆𝑆𝑆𝑆 = 𝜇𝜇 𝜌𝜌𝑓𝑓𝐷𝐷 , (3.16) The typical value of the turbulent Shmidt number is 0.9. The energy balance equation is given as 𝜕𝜕 𝜕𝜕𝜕𝜕 �𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔ℎ𝑔𝑔� + ∇ ∙(𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔ℎ𝑔𝑔𝑢𝑢𝑔𝑔) = 𝜃𝜃𝑔𝑔 � 𝜕𝜕𝑝𝑝 𝜕𝜕𝜕𝜕 + 𝑢𝑢𝑔𝑔 ∙ ∇p� + Ф− ∇ ∙ �𝜃𝜃𝑔𝑔𝑞𝑞� + �̇�𝑄 + 𝑆𝑆ℎ + �̇�𝑞𝑑𝑑, (3.17) where ℎ𝑔𝑔 is the gas enthalpy, 𝑆𝑆ℎ is the conservative energy exchange from the particle phase to the gas phase, �̇�𝑞𝑑𝑑 is the enthalpy diffusion term and q is the gas heat flux. Energy source per volume �̇�𝑄 and the viscous dissipation Ф were ignored. The gas heat flux is defined as 54 𝑞𝑞 = −λ𝑔𝑔∇T𝑔𝑔, (3.18) where λ𝑔𝑔 is the fluid thermal conductivity which is the sum of a molecular conductivity and an eddy-conductivity from Reynolds stress mixing. The eddy-conductivity is obtained from a turbulent Prandtl number correlation 𝑃𝑃𝑃𝑃𝜕𝜕 = 𝐶𝐶𝑝𝑝𝜇𝜇𝑡𝑡 λ𝑡𝑡 , (3.19) using a standard value of the turbulent Prandtl number of 0.9. The enthalpy diffusion term is given by �̇�𝑞𝐷𝐷 = ∑ ∇ ∙ �ℎ𝑖𝑖𝜃𝜃𝑔𝑔𝜌𝜌𝑔𝑔𝐷𝐷∇𝑌𝑌𝑔𝑔,𝑖𝑖� 𝑁𝑁𝑆𝑆 𝑖𝑖=1 , (3.20) where the summation is over all gas species Ns, and the enthalpy of gas-phase species 𝑖𝑖, ℎ𝑓𝑓,𝑖𝑖, is defined below. The mass, momentum, and energy balance Eqs.(3.4), (3.5), and (3.16), respectively, are written for the gas mixture, where the gas mixture properties are evaluated on the basis of the mass fractions of the gas species, solved for using Eq. (3.14). The flow is compressible, and the gas phase pressure, enthalpy, temperature, density, and mass fractions are related through equations of state. In the CPFD software, an ideal gas equation of state is used, so that the partial pressure of gas species 𝑖𝑖 is 𝑃𝑃𝑖𝑖 = 𝜌𝜌𝑔𝑔𝑌𝑌𝑔𝑔,𝑖𝑖𝑅𝑅𝑇𝑇𝑔𝑔 𝑀𝑀𝑤𝑤𝑖𝑖 , (3.21) and the total mean flow gas thermodynamic pressure is 𝑃𝑃 = ∑ 𝑃𝑃𝑖𝑖𝑁𝑁 𝑖𝑖 , (3.22) where R is the universal gas constant, 𝑇𝑇𝑔𝑔 is the gas mixture temperature, and 𝑀𝑀𝑤𝑤𝑖𝑖 is the molecular weight of gas species 𝑖𝑖. 𝑁𝑁 is the total number of gas species The mixture enthalpy is related to the species enthalpies by 55 ℎ𝑔𝑔 = ∑ 𝑌𝑌𝑔𝑔,𝑖𝑖 𝑁𝑁 𝑖𝑖=1 ℎ𝑖𝑖, (3.23) The mixture specific heat at constant pressure, 𝐶𝐶𝑃𝑃, is given by 𝐶𝐶𝑝𝑝 = ∑ 𝑌𝑌𝑔𝑔,𝑖𝑖 𝑁𝑁 𝑖𝑖 𝐶𝐶𝑃𝑃,𝑖𝑖, (3.24) where 𝐶𝐶𝑃𝑃,𝑖𝑖 is the specific heat of species 𝑖𝑖. The species enthalpies are related to the fluid temperature 𝑇𝑇𝑔𝑔 by ℎ𝑖𝑖 = ∫ 𝐶𝐶𝑃𝑃,𝑖𝑖𝑑𝑑𝑇𝑇 + ∆ℎ𝑔𝑔,𝑖𝑖 𝑇𝑇𝑔𝑔 𝑇𝑇𝑟𝑟𝑟𝑟𝑓𝑓 , (3.25) The quantity ∆ℎ𝑔𝑔,𝑖𝑖 is the heat of formation of species 𝑖𝑖 at the reference temperature 𝑇𝑇𝑟𝑟𝑒𝑒𝑓𝑓. To calculate the temperature from given mass fractions and mixture enthalpies using Eqs. (3.22) and (3.24) piecewise linear approximations of the enthalpy curves is used. The momentum equation of the particle phase is expressed as 𝑑𝑑𝑢𝑢𝑝𝑝 𝑑𝑑𝜕𝜕 = 𝐷𝐷�𝑢𝑢𝑔𝑔 − 𝑢𝑢𝑝𝑝� − 1 𝜌𝜌𝑝𝑝 ∇𝑃𝑃 + 𝑔𝑔 − 1 𝜃𝜃𝑝𝑝𝜌𝜌𝑔𝑔 ∇𝜏𝜏𝑝𝑝, (3.26) where 𝑉𝑉𝑝𝑝 is the particle volume, 𝜌𝜌𝑝𝑝 is the particle density, 𝑢𝑢𝑔𝑔 is the gas velocity, 𝑢𝑢𝑝𝑝 is the particle velocity, 𝐷𝐷 is the inter-phase drag force, and 𝜏𝜏𝑝𝑝 is the particle normal stress. For the particle energy equation, it is assumed that the particle temperatures are uniform within the particles, that there is no heat release due to chemical reactions within the particles, and that heat release due to reactions occurring on the surfaces of particles contributes negligibly to the surface energy balance. 𝐶𝐶𝑉𝑉 𝑑𝑑𝑇𝑇𝑝𝑝 𝑑𝑑𝜕𝜕 = 1 𝑚𝑚𝑝𝑝 𝜆𝜆𝑔𝑔𝑁𝑁𝑢𝑢𝑔𝑔,𝑝𝑝 2𝑟𝑟𝑝𝑝 𝐴𝐴𝑝𝑝(𝑇𝑇𝑔𝑔 − 𝑇𝑇𝑝𝑝), (3.27) 56 where 𝐶𝐶𝑉𝑉 is the specific heat of the particle material, 𝑇𝑇𝑝𝑝 is the particle temperature, 𝑇𝑇𝑔𝑔 is the gas temperature, 𝑁𝑁𝑢𝑢𝑔𝑔,𝑝𝑝 is the Nusselt number for heat transfer from the fluid to the particle, 𝜆𝜆𝑓𝑓 is the fluid thermal conductivity, 𝑚𝑚𝑝𝑝 is the particle mass, and 𝐴𝐴𝑝𝑝 is the particle acceleration. The particle acceleration is given as, 𝐴𝐴𝑝𝑝 = 𝑑𝑑𝑢𝑢𝑝𝑝 𝑑𝑑𝜕𝜕 = 𝐷𝐷𝑝𝑝�𝑢𝑢𝑔𝑔 − 𝑢𝑢𝑝𝑝� − 1 𝐴𝐴𝑝𝑝 ∇𝑃𝑃𝑔𝑔 + 𝑔𝑔 − 1 𝜃𝜃𝑝𝑝𝜌𝜌𝑝𝑝 ∇𝜏𝜏𝑝𝑝 + 𝑔𝑔 + 𝑢𝑢𝑝𝑝−𝑢𝑢𝑔𝑔 𝜏𝜏𝑝𝑝 , (3.28) 𝑆𝑆ℎ = ∭𝑓𝑓{𝑚𝑚𝑝𝑝[𝐷𝐷𝑝𝑝(𝑢𝑢𝑝𝑝 − 𝑢𝑢𝑔𝑔)2 − 𝐶𝐶𝑉𝑉 𝑑𝑑𝑇𝑇𝑝𝑝 𝑑𝑑𝜕𝜕 ] − 𝑑𝑑𝑚𝑚𝑝𝑝 𝑑𝑑𝜕𝜕 [ℎ𝑝𝑝 + 1 2 �𝑢𝑢𝑝𝑝 − 𝑢𝑢𝑔𝑔� 2]}𝑑𝑑𝑚𝑚𝑝𝑝𝑑𝑑𝑢𝑢𝑝𝑝𝑑𝑑𝑇𝑇𝑝𝑝, (3.29) where ℎ𝑝𝑝 is the particle enthalpy. The values 𝐷𝐷𝑝𝑝(𝑢𝑢𝑝𝑝 − 𝑢𝑢𝑓𝑓)2 and �𝑢𝑢𝑝𝑝 − 𝑢𝑢𝑔𝑔� 2 are negligible in flow with low Mach number. Heat transfer in the fluidized bed is characterized through gas-solid and gas-wall heat tranfer. Simulating radiative heat transfer with fundamentals oriented model is associated with high computational cost [134–136]. For radiation, particle radiation largely overwhelms gas rdiation in combustion. Due to high computational cost of simulating radiative heat transfer with fundamentals oriented models radiative heat tranfer is represented in a simplified form. Radiative heat transfer emission from particles is defined through particle emissivity. Emission of radiative heat is defined through particle emissivity. Particle emissivity is defined as the ratio of the radiation emitted by its surface to the radiation emitted by a blackbody at the same temperature. The emissivity value for particles in the model is 0.75. The convective heat transfer coefficient between the particles and the gas is determined by ℎ𝑝𝑝𝑑𝑑𝑝𝑝 𝑘𝑘𝑓𝑓 = 𝑁𝑁𝑢𝑢𝑝𝑝 = 0.37𝑅𝑅𝑅𝑅𝑝𝑝0.6 + 0.1, (3.30) where the particle Reynolds number is 57 𝑅𝑅𝑅𝑅𝑝𝑝 = 𝜌𝜌𝑓𝑓𝑈𝑈𝑓𝑓𝑑𝑑𝑝𝑝 𝜇𝜇𝑓𝑓 , (3.31) The drag model is formulated using the combination of Wen-Yu [137] and Ergun [138] drag models. In combination these models are suitable for CFB simulation [68]. Wen-