生質煤與粉煤混燒應用於切線式鍋爐之數值模擬

Abstract

中鋼公司動力工場所使用之鍋爐為切線式鍋爐，主要燃料為阿達羅煤(Adaro Coal)，溫室氣體排放量每年約為70~80萬噸，占中鋼公司總排放量百分比3.4%，為有效降低二氧化碳排放量兼具環境友善、潔淨能源及分散能源供應，使用生質燃料取代或部分取代燃煤以達前述目標。本研究採用商業套裝軟體ANSYS/FLUENT模擬生質煤(Torrefied Biomass)與燃煤進行混合燃燒之現象及研發相關之控制技術。本研究之目的是建立切線式鍋爐之數值模型，計算不同比例(0%(純煤)、20%、40%、60%、80、100%)之生質煤與燃煤混合燃燒之結果，供實廠操作或後續研究評估生質煤取代燃煤之可行性參考。本研究完成建立鍋爐數值模擬模型，以純煤燃燒進行格點測試且與中鋼公司提供之實驗數據比對。經模擬結果得到最佳的格點數為130萬，其中，過熱水管群1出口溫度之誤差為3.3%、過熱水管群2出口溫度之誤差為2.7%、節煤器入口量測點溫度之誤差為6.7%以及出口氧含量誤差6.3%，由上述比較結果顯示模擬的確能呈現鍋爐內的燃燒行為。由各組混燒比例的模擬結果發現，混燒比例每提升20%，燃料的總熱值雖減少3.6%，但煙氣質量流率減少7.4%，混燒比例提高後，燃燒區域之溫度會有提升的現象，但也因為煙氣(flue gas)質量流率降低導致熱容量降低，熱交換後煙氣溫度下降幅度變大，其結果會嚴重地影響後續煙氣預熱空氣和熱水的效果。故未來應考量煙氣質量流率來決定降載的程度，以符合邊界條件對熱水及過熱蒸氣入口的設定情況。

The Adaro coal is used by the tangentially coal-fired boiler of China Steel Corporation (CSC) for power generation. This boiler produces about 700,000 to 800,000 tons of greenhouse gas emission every year. CSC is trying to burn the biomass fuel to replace (or partially replace) the coal for the reduction of¬ carbon dioxide emission¬, which is apparently beneficial for company reputation. This research employs the Computational Fluid Dynamics (CFD) tool - ANSYS/FLUENT - to analyze the two-phase combustion flow fields of mixing torrefied biomass and Adaro coal inside the boiler. The main purpose is to build the corresponding numerical model and calculate the performance of combustion with different mixing rates of torrefied biomass (0%(pure coal), 20%, 40%, 60%, 80%, 100%). The assessment of replacement ratio can be used as a reference for real operation and future research. From the results of grid tests with the case of pure coal combustion, the grid number 1,300,000 is considered to be the optimal choice, which is the trade-off between the computational time and acceptable accuracy. Comparing with the numerical results with the experiment data from CSC, the error of temperature at superheated steam tube group 1 outlet is 3.3%, the one at superheated steam tube group 2 is 2.7%, the one at economizer inlet is 6.7% and the error of concentration of O2 at economizer inlet is 6.3%. According to the prediction results with different mixing rates of torrefied biomass, the total heating value will reduce 3.6% and the mass flow rate of flue gas will reduce 7.4% when the replacement ratio increases every 20%. Increasing the replacement ratio will raise the temperature of the combustion chamber and increase the temperature gradient of the flue gas after heat exchange. It may extremely affect the quality of preheating air and water. From the previous discussion, consideration of the loading of the water to satisfy the boundary conditions of hot water and superheated steam is recommended for the near future work.