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Development of Prediction Model of Coal Ash Fouling Behavior in the Entrained-Flow Gasification Condition through the DTF Experiments

초록/요약

Clean coal technologies, are urgently required because global air pollution caused by coal utilization has become a serious problem. The gasification technology, in which coal is used, is one of the clean coal technologies; it is a promising technology for producing electricity and chemical energy. The role of the gasifier, of which there are three types (entrained flow, fluidized bed, and fixed bed), is important in coal gasification technology. Entrained flow gasifiers offer major advantages such as treating of the high coal capacity and carbon conversion and production of pure syngas. Consequently, many entrained flow gasifiers have been installed and are in operation. All gasification plants using coal resources show the fly ash deposition phenomenon. Addressing the problem of ash deposition in the gasification process is critical for continuous plant operation, because it decreases the thermal efficiency and sometimes forces plants to shut down. The deposition of coal ash is extremely complex and occurs in numerous ways during mineral transformation under the gasification process. My PhD study is an attempt to understand the major factors responsible for the fouling phenomenon. Ash deposition trends differ according to the type of coal, which is determined by different mineral components. The sticky characteristic is a major parameter affecting ash deposition, which is influenced by the presence of alkali and alkaline earth metals at a given temperature. The alkali and alkaline earth metals, which influence the fouling growth, lower the melting temperature of ashes and hence increase the particle sintering/agglomeration. The sintering characteristics of the deposited ash particles can influence the fouling rate. Dropped ash particles behave differently on the deposition surface (attachment, rebounding and removal from deposit surface) because of the physical characteristics of the deposited layer. The key mineral matter in fouling growth were investigated by conducting experiment using a drop tube furnace (DTF) attached to various accessory equipment. The DTF experimental results showed that dropped ash particles behaved differently on the deposition surface in that they could attach to the surface, rebound, or be removed. Deposited ash samples were analyzed by performing mineral mapping with scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) for the verification of the mineral distribution in fouling agglomeration. The Na and K components were found to have no clear effect on the agglomeration phenomenon and were widely scattered on the deposited ash sample. On the other hand, Si and Al were detected to often coexist with Fe and Ca. The results obtained for the mineral mapping of coal ash were almost similar to the experimental results obtained for synthetic ash. The SEM analysis of synthetic ash indicated that the particle size increased for high concentrations of alkaline earth and alkali metals and the particles had smooth shapes. FeCl2, CaCl2 and MgCl2 influenced particle agglomeration more intensely than NaCl and KCl. In particular, FeCl2 content was found to influence particle agglomeration. Previously, the deposited ash was weighed after the gasification experiment. However, this method complicated the understanding of the relationship between minerals and fouling growth at any given moment. Therefore, a real-time measurement system, capable of recording the amount of deposited ash every second, was specifically developed for this work. The ash deposition rate was related to the presence of Fe and Ca components, whose high concentrations were found to increase the deposition rate. On the other hand, the ash deposition trend can be varied by maintaining or decreasing the Fe and Ca components, which are responsible for fouling growth. Therefore, the mineral content influences the viscosity and thermal conductivity of coal ash as well as the amount of fouling deposited. The viscosity of ash with high alkaline earth and alkali metal content is lower than that of other coal. Low viscosity leads to increase of capability of particle adhesion and particle agglomeration. The viscosity of deposited coal ash was calculated by using Watt & Fereday’s viscosity model to analyze the relationship between the particle size of deposited ash and viscosity. The deposited particle size was found to increase with decreasing ash viscosity. The thermal conductivity of ash was analyzed by laser flash apparatus (LFA). High concentrations of alkaline earth and alkali metals lead to an increase in thermal conductivity because compounds of these metals lower the melting temperature, which suggested reduction in the porosity of the sample ashes. Therefore, the thermal conductivity of ash increases with increasing amounts of alkaline earth and alkali metals. The physical length variation of ash samples was analyzed by using a dilatometer technique while measuring the sintering behavior. The analysis was conducted under both non-isothermal and isothermal conditions. A higher heating rate was found to increase the sintering rate and lowers the sintering temperature. Furthermore, a high %(Fe2O3+CaO+MgO)/%(SiO2+Al2O3) ratio increases the sintering rate and decreases the sintering temperature. The sintering behavior was measured at different temperatures, i.e., 700, 800 and 900 oC under isothermal conditions, and the sintering rate was found to increase with a high %(Fe2O3+CaO+MgO)/%(SiO2+Al2O3) ratio and increasing reaction temperature. The most well-known sintering model is that developed by Frenkel for predicting the sintering rate, and was adopted by modifying selected parameters for simulating the data measured with the dilatometer. The modified sintering model was able to successfully simulate the dilatometer measurements at 800 oC and 900 oC. Prediction of the fouling amount is the most important aspect of coal ash research. Previously, fouling indices mainly considered the characteristics of dropped particles and operating conditions. This work attempts to address these shortcomings by developing a new fouling index, which considered the sintering behavior of the deposited particle layer as well as the characteristics of dropped particles and the operating conditions. Sintering model is used as a sub-model in fouling index for simulating the characteristics of the deposited fouling layer. Several other parameters such as the deposit temperature, flue gas temperature, initial deformation temperature of ash, %(Fe2O3+CaO+MgO)/%(SiO2+Al2O3) ratio, and ash weight ratio as a correction coefficient were also incorporated into the fouling index. The newly developed fouling index proved capable of more accurately simulating the experimental results (deposit fouling/input ash) compared to a representative fouling index such as the base-to-acid ratio.

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목차

Chapter 1. Introduction 1
1.1 Background and Motive 1
1.2 Scope and Objective 2
1.3 Thesis Outline 3
Chapter 2. Literature Review 6
2.1 Introduction 6
2.2 Generation Mechanism of Ash Minerals in Coal Particles during Thermo-chemical Conversion Reactions 7
2.2.1 Ash Particle Generation from Coal 7
2.2.2 Transformation of Mineral Matter during Thermo-chemical Reaction 11
2.2.3 Mineral Networking of Ash at High Temperature 17
2.3 Ash Particle Behavior on Deposit Target 20
2.3.1 Fouling Growth Mechanisms 21
2.3.2 Particle Movements on Deposition Target 24
2.4 Factors Affecting Fouling Growth 26
2.4.1 Physical and Chemical Factors of Fouling Growth 26
2.4.2 Effect of Properties of Deposited Ash Layer 27
2.5 Sintering Behavior of Particles 30
2.5.1 Sintering Fundamentals in-between Particles 30
2.5.2 Sintering Model of Particles 33
2.6 Previous Fouling Indices for Predicting the Deposition Growth of Ash 38
Chapter 3. Experimental and Analytical Method 43
3.1 Introduction 43
3.2 Experimental Method and Apparatus 44
3.2.1 Coal Sample Preparation Used in Experiment 44
3.2.2 Synthetic Ash Preparation 47
3.2.3 Ash Sample Preparation for Dilatometer Analysis 47
3.2.4 Drop Tube Furnace and Accessories for Ash Deposition Experiment 48
3.3 Instrument and Analytical Method for Ashes 56
3.3.1 Laser Flash Apparatus (LFA) 56
3.3.2 Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM - EDX) 58
3.3.3 Dilatometer (DIL) 58
3.3.4 Particle Size Analyzer 59
Chapter 4. Effect of Ash Particle Transformation and Ash Deposition Trend by Minerals 60
4.1 Measurement of Impacting Behavior of Ash Particles on the Deposition Target through the View Port 60
4.2 Sintering Behavior with Mineral Components by Using Coal Ash and Synthetic Ash under Low-Temperature Fouling Conditions 62
4.3 Effect of Mineral Components on Fouling Deposition in Real-Time 66
4.4 Relationship between Ash Viscosity and Fouling Deposition 82
4.5 Relationship between Thermal Conductivity and Mineral Components of Ash 87
Chapter 5. Sintering Behavior of Ash Particles under Non-isothermal and Isothermal
Conditions 97
5.1 Sintering Behaviors under Non-isothermal Conditions 97
5.1.1 Dilatometer Measurements under Non-isothermal Conditions 97
5.1.2 Effect of Heating Rates for the Sintering 106
5.2 Sintering Behaviors under Isothermal Conditions 109
5.2.1 Dilatometer Measurements under Isothermal Conditions 109
5.2.2 Prediction of Sintering Behaviors under Isothermal Conditions 114
Chpater 6. Prediction Model of Ash Deposition Trend Integrated with the Particle Sintering Model 125
6.1 Development of New Fouling Index 125
6.2 Evaluation of Prediction Performance of New Fouling Index 128
Chapter 7. Conclusions 134
References 136
Abstract in Korean 144

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