1MW高壓流體化床富氧燃燒之數值模擬分析

Abstract

摘要 本研究採數值模擬軟體ANSYS FLUNET分析富氧燃燒、CO2循環燃燒及熱傳水管系統於高壓流體化床之一系列燃燒行為。其中，固態燃煤的成分及熱裂解機制由中國鋼鐵公司(CSC)量測提供。Chang and Chen [30]於高壓流體化床模擬各種不同比例之氧氣濃度，考慮燃燒強度及噴嘴熱負載之影響，認為氧氣濃度26%為最佳操作條件。因此，本研究採26%之氧濃度比例進行兩部分模擬，第一部分為探討富氧燃燒與CO2循環於鍋爐中的影響，CO2循環需等待鍋爐燃燒一段時間後，才可回收導入鍋爐中混燒，因此模擬採暫態模式進行運算，首先於第一進口由助燃空氣將燃煤燃燒至480秒達穩定狀態，接著於第二進口吹入純CO2進行360秒之混燒模擬，而Chang and Chen [30]的模擬中，此時水/蒸汽管周圍的溫度為720 K，火燄溫度為0.7 m。由結果顯示，吹入純CO2後，燃燒強度只下降了一點，工作區域的溫度約為710 K，鍋爐燃燒產出的CO2為0.0506 kg/s，較吹入的流量更多(0.0285 kg/s)。第二部分主要分析水/蒸汽管對鍋爐的主要影響，當加入直徑為50 mm之水/蒸汽�ue to the effect of the secondary inlet flow with pure CO2 that the proposed working area temperature is about 710K. The produced CO2 mass rate at outlet is 0.0506 kg/s, which is greater than the required amount, 0.0285kg/s. The second part is to investigate the effect by install the water/vapor tube system. The 50mm-diameter tubes are installed in the boiler with combustion gas of 26% O2 for the burning, the amount of coal blend and combustion gas are also increased to generate the desired results. The resultant flame height is 0.63 m, and it moves back toward the inlet. The heat transfer rate to the water/vapor tubes is 3,059.37J/s. Some reference cases are also carried out for comparison purpose.

Key words: pressurized fluidized bed combustor, oxy-fuel combustion, devolatilization mechanism

The research used a commercial package software ANSYS FLUENT to simulate the burning behaviours of the pressurized fluidized bed combustor (PFBC), subjected to heat transfer to water tube system, with the oxy-fuel combustion by using oxygen-enriched air and recirculating flue gas. The fuel used was the coal blend, whose devolatilization mechanism is obtained from the measurements of China Steel Corporation (CSC). It was an extensive work of Chang and Chen [30]. There were two parts of simulation. Considering the burning intensity and heat damage to the nozzle, the previous study recommended the combustion air use 26% of O2 for the oxy-fuel combustion. In the first part, simulation of CO2 recirculation process was carried out. By using the transient analysis, the burning with ER 2 15 MATHEMATICAL MODEL 15 2.1 DOMAIN DESCRIPTION 15 2.2 GOVERNING EQUATIONS 16 2.2.1 The Continuity and Momentum Equation 17 2.2.2 The Energy Conservation Equation 18 2.2.3 The Species Transport Equation 20 2.2.4 The Standard Model 21 2.2.4.1 TRANSPORT EQUATIONS FOR THE STANDARD MODEL 26 2.2.4.1.1 Modeling Turbulent Production in the Models 27 2.2.4.1.2 Effects of Buoyancy on Turbulence in the Models 28 2.2.4.1.3 Effects of Compressibility on Turbulence in the Models 29 2.2.5 Modeling radiation 30 2.2.5.1Radiative transfer equations 30 2.2.5.2 Radiation in combusting flows 30 2.2.5.2.1The Weighted-Sum-of-Gray-Gases Model (WSGGM) 30 2.2.5.2.2 The effect of soot on the absorption coefficient 31 2.2.5.2.3 The effect of particles on the absorption coefficient 31 2.2.5.3 Overview and limitations of DO model 32 2.2.6 Advantages and Limitations of the DO Model 32 2.3 CHEMICAL MECHANISM, COMBUSTION MECHANISM, INITIAL CONDITIONS AND BOUNDARY CONDITION 33 2.3.1 Chemical mechanism and combustion mechanism 33 2.3.2 Boundary conditions 35 CHAPTER 3 38 INTRODUCTION TO NUMERICAL ALGORITHM 38 3.1 INTRODUCTION TO FLUENT SOFTWARE 38 3.1 NUMERICAL METHOD FOR FLUENT 39 3.2.1 Segregated Solution Method 39 3.2.2 Linearization: Implicit form 40 3.2.3 Discretization 41 3.2.4 SIMPLE Algorithm 43 3.3 COMPUTATIONAL PROCEDURE OF SIMULATION 44 3.3.1 Model Geometry 44 3.3.2Grid Generation 45 3.3.3FLUENT Calculation 45 CHAPTER 4 50 RESULTS AND DISCUSSION 50 4.1 THE EFFECT OF FLUE GAS RECIRCULATION PROCESS 50 4.2 THE EFFECT OF WORKING TUBES 57 CHAPTER 5 74 CONCLUSIONS AND RECOMMENDATIONS 74 REFERENCE 76