旋轉窯焚化爐操作特性及應用於處理事業廢棄物之研究

The Study on the Operation Characteristics of a Rotary Kiln Incinerator and Its Application to Industrial Wastes Treatment

李志萍(Chih-Ping Lee)

指導教授 ： 吳中興

國立臺灣大學/生物資源暨農學院/生物產業機電工程學研究所/博士(2004年)

https://doi.org/10.6342/NTU.2004.01662

摘要

本研究以計算流體力學軟體Fluent，進行旋轉窯焚化爐燃燒流場之模擬研究，以稻殼、廢輪胎、油污泥等三種不同性質之廢棄物做為事業廢棄物的代表。其中，稻殼之低位發熱量為2767kcal/kg，屬於低熱值之廢棄物；油污泥之低位發熱量為5888kcal/kg，屬於具有高熱值之半固態廢棄物；廢輪胎之低位發熱量為8318kcal/kg，屬於具有高熱值之固態廢棄物。本研究中的模擬，以低位發熱量2778kcal/kg(5000BTU/lb)為廢棄物續燃的標準，若廢棄物的低位發熱量低於此值，則必須添加輔助燃料以維持續燃能力，因此在稻殼焚化的質能平衡中，必須將輔助燃料一併考慮。

本研究之計算模擬為3D燃燒流場，數值方法採用有限體積法，以SIMPLE演算法校正壓力與速度。本研究假設流場為穩態不可壓縮流場，紊流模式採用標準k-ε紊流模式，輻射熱傳模式採用P-1模式，燃燒模式分別採用渦流消散模式(EDM)與機率密度函數模式(PDF)。關於旋轉窯的尺寸與操作條件，完全參照實際運轉中的旋轉窯焚化爐，並且於該座旋轉窯焚化爐進行焚化試驗，進行溫度的量測，所得的試驗數據做為模擬計算的比對參考。

本研究討論不同爐體轉速(0.2、0.5與0.8rpm)與不同過剩空氣量(0、50、100與150%)操作下的模擬結果，一次室與二次室過剩空氣量的比例為1：4。在稻殼的焚化模擬中，爐體轉速0.5rpm，過剩空氣量100~150%，可得到99.3%的焚化效率，因此為最適當的操作。在油污泥的焚化模擬中，爐體轉速0.5rpm，可得到99.7%的焚化效率，為最適當的操作，但是過剩空氣量的改變，對於的燃燒效率並沒有明顯的影響。由廢輪胎之模擬得知，具有高熱量之固態廢棄物，在此旋轉窯的設計值下操作，合理的溫度範圍為1088~1142K。

本研究比較預混焰與擴散焰的差異，在預混焰的燃燒模擬中，燃燒溫度隨著過剩空氣量的增加而下降；在擴散焰的燃燒模擬中，燃燒溫度則是隨著過剩空氣量的增加而上昇。這是因為過剩空氣量在預混焰中扮演著吸收熱量的角色；而在擴散焰中則可以增加燃料與空氣接觸的機會。

在渦流消散模式(EDM)與機率密度函數模式(PDF)的比較中，渦流消散模式(EDM)反應速率最高的位置，並不是溫度最高的位置；機率密度函數模式(PDF)stoichiometric反應混合分率 位置即是溫度最高的位置，因此以機率密度函數模式(PDF)預測火焰位置較為準確。

關鍵字

機率密度函數燃燒模式 ； 旋轉窯焚化爐 ； 廢輪胎 ； 渦流消散燃燒模式 ； 計算流體力學 ； 油污泥 ； 稻殼

並列摘要

This study simulated the combustion field of a rotary kiln incinerator using the computational fluid dynamics code FLUENT. Rice hulls, oil sludge and waste tires represent three different kinds of industrial wastes in this study. The lower heating values of rice hulls, oil sludge and waste tires are 2767kcal/kg, 5888kcal/kg and 8318kcal/kg respectively. If the lower heating value is less than 2778kcal/kg (5000BTU/lb), then auxiliary fuel is required to keep combusting. Therefore, the amount of the auxiliary fuel should to be estimated in the case of treating rice hulls.

In this study, the combustion field was regarded as a three-dimensional flow field and the flow was supposed to be a steady state and incompressible. The finite volume method and SIMPLE (Semi-Implicit Method for Pressure Linked Equation) algorithm, which ensures correct linkage between pressure and velocity, were adopted to solve the governing equations. The standard k-ε turbulence model and the P-1 radiation model were included in the simulation of this study. Two modeling approaches, EDM and PDF, were employed for the combustion reactions respectively. The dimensions and operating conditions were determined according to an industrial rotary kiln incinerator. In situ temperature measurement form the rotary kiln incinerator had been finished and was compared with the numerical results.

The revolutionary speed of the rotary kiln incinerator was controlled at 0.2 or 0.5 or 0.8rpm. The excess air was supplied with 0 or 50 or 100 or 150% and the excess air ratio of the rotary kiln to the secondary chamber was 1 to 4. In the case of treating rice hulls, the combustion efficiency reached to 99.3% while the revolutionary speed was set at 0.5rpm and excess air was supplied with 100 or 150%. In the case of treating oil sludge, the combustion efficiency reached to 99.7% while the revolutionary speed was set at 0.5rpm but wasn’t obviously affected by changing the amount of excess air. In the case of treating waste tires, this study predicted that the reasonable range of temperature is 1088~1142K while solid wastes with higher heating value are treated in the rotary kiln incinerator under the designed operating conditions.

The difference between a premixed flame and a diffusion flame was also discussed in this study. In the simulation of the premixed flame, the average temperature dropped as the supply of excess air increased. In the simulation of the diffusion flame, the average temperature rose as the supply of excess air increased. The reason is that the excess air absorbed the heat energy in the premixed flame but helped for the mixture of the fuel and oxidant in the diffusion flame.

In the comparison between the EDM and the PDF combustion model, the PDF combustion model is more proper than the EDM combustion model in the description of a flame. In the EDM modeling, the top temperature didn’t happen in the position which had the most reaction rate; hence the simulation results don’t coincide with physical phenomenon. In the PDF modeling, the top temperature happened near the position which had the stoichiometric mixture fraction ; hence the simulation results coincide with physical phenomenon.

並列關鍵字

waste tires ； Eddy Dissipation Model (EDM) ； Probability Density Function model (PDF model) ； rice hulls ； Computational Fluid Dynamics (CFD) ； Rotary kiln incinerator ； oil sludge

參考文獻

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