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Effect of Phosphorus Compounds in the Fouling Behavior of Hydrothermal Carbonization Sewage Sludge via Drop Tube Furnace Experiments

  • 박주창 (아주대학교 일반대학원)

초록/요약

Phosphorus is a very important factor that greatly affects the ash polluted sediments generated during the combustion of sewage sludge solid fuel. Existing studies have mainly focused on the increase of slag and sinter due to the presence of phosphorus. Therefore, few studies have been conducted on the effect on the temperature reduction of the heat exchanger surface due to fouling and fouling fusion that may occur in the heat exchanger in the combustion boiler. The fouling fusion phenomenon is an important factor to consider for power generation using solid fuel from sewage sludge because the ash generated in the combustion boiler moves and settles on the surface of the heat exchanger to interfere with heat exchange efficiency. In particular, the decrease in the temperature of the heat exchanger surface caused by heat transfer obstruction due to fouling fusion is directly related to the efficiency of the heat exchanger, so it should be considered together with the fouling fusion. Synthetic ash, phosphorus evaporation, and drip pipe experiments were conducted to investigate the mechanism of fouling formation when phosphorus is burned and how fouling fusion affects the heat exchanger surface temperature reduction. Because phosphorus is highly reactive, it preferentially reacts with alkali metals and alkaline earth metals to lower the melting point of ash or promote fouling fusion. On the contrary, when the fouling fused to the surface of the heat exchanger is melted, a large hollow structure is formed, and the decrease in the surface temperature of the heat exchanger is rather alleviated due to the empty space created inside the fouling. The phosphorus content had a major effect on the fouling fusion and also had an effect on the reduction of the surface temperature of the heat exchanger. During the thermochemical transformation process, deposition phenomena cause operational problems in the plant. In the deposited material, inorganic particulates, especially alkaline earths and alkali metals, are mainly responsible for the accumulation of contamination. Alkali metals such as Na and K are vaporized during combustion of coal particles. Contamination is first initiated by a vaporized material that provides a sticky surface that traps colliding particles. On the other hand, as the charcoal particles are decomposed, alkaline earth metals (Mg, Ca) and Fe compounds attached to the precipitation surface are exposed by the inertial force. They can fuse and agglomerate, after which large ash particles can form. Many researchers have found that base to acid ratio, total alkali content, B/A(%Na2O), iron to calcium ratio, and %Na2O ratio in ash. Existing indicators agree well with the experimental results when coal is used, but do not match the experimental results of sewage sludge solid fuel because the phosphorus content is limited to less than 1%. Therefore, it is important to identify the types of inorganic constituents that influence pollution and the growth of particle agglomerates in order to develop suitable pollution indexes for sewage sludge solid fuels and to predict pollution trends. An improved fouling index was developed by introducing the effect of P to the previously known fouling formation mechanism. Based on the results of the previous study, it was found that the formation of phosphates in P2O5, CaO, and MgO can be a major factor in fouling in samples with a high P content. Several other parameters such as the ash deposit temperature, flue gas temperature, initial deformation temperature of ash, %(P2O5+CaO+MgO)/%(SiO2+Al2O3), and ash weight ratio as a correction coefficient are considered in the newly developed fouling index. Based on these results, the existing fouling index was improved. Although additional research is needed on the fouling phenomenon in the actual combustion environment, the newly developed fouling index can be applied to predict the fouling tendency in sewage sludge solid fuel and biomass pre-fired power generation and co-fired power generation.

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목차

Chapter 1. Introduction 1
1.1 Background and Motive 1
1.2 Scope and Objective 4
Chapter 2. Literature Review 7
2.1 Introduction 7
2.2 Composition of ash 8
2.2.1 Composition of coal ash 8
2.2.2 Trend of Inorganic matters in Coal 10
2.2.3 Thermal properties of coal ash 15
2.3 Fouling growth mechanism 21
2.3.1 Physical Factors Affecting Fouling depostion 26
2.3.2 Chemical Factors Affecting Fouling depostion 29
2.4 Previous Biomass Fouling Studies 32
2.4.1 Reactivity of Phosphorus 33
2.4.2 CaO-MgO-P2O5 terneary system research for biomass combustion 34
2.5 Previous Fouling Index for Predicting the deposition Growth of Ash 36
Chapter 3. Experimental Method and Apparatus 41
3.1 Experimental Method and Apparatus 41
3.1.1 Sample Preparation Used in Drop Tube Furnace Experiment 41
3.1.2 Synthetic Ash Preparation 44
3.1.3 Phosphorus evaporation experiment using a muffle furnace 45
3.1.4 Drop Tube Furnace Ash Deposition Experiment 46
3.1.5 Drop Tube Furnace temperature measurement system 47
3.2 Instrument and Analytical Method for Ashes 50
3.2.1 Laser Flash Apparatus (LFA) 50
3.3 Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX) 53
Chapter 4. Results and discussion 53
4.1 Synthetic ash experiment result 53
4.1.1 Physical change of synthetic ash 53
4.1.2 XRD Analysis 57
4.1.3 Weight reduction measurement of synthetic ash 63
4.1.4 Heating temperature and P mixing ratio chang experiment 65
4.1.5 Weight reduction measurement of changing P mixing ratio synthetic ash 67
4.1.6 Agglomeration test of synthetic ash 68
4.2 Phosphorus evaporation experiment using a muffle furnace 69
4.3 DTF temperature reduction experimental results 71
4.3.1 Fouling rate 71
4.3.2 Temperature variation by fouling deposition 74
4.3.3 Relationship between chemical component and fouling deposition 79
4.3.4 Thermal conductivity of deposited fouling 83
4.4 Prediction model of ash fouling deposition trend integrated with P2O5 influence 84
4.4.1 Fouling deposition experiment according to deposit probe temperature 84
4.4.2 Fouling deposition experiment according to combustion temperature 90
4.4.3 Fouling deposition experiment according to mineral component 91
4.4.4 Fouling deposition experiment according to kaolin blend 93
4.4.5 Fouling deposition experiment using various sludge solid fuel 96
4.4.6 Fouling index using fouling deposition experiment results 100
Chapter 5. Conclusion 102
References 104

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