微波共焙燒下水污泥與稻稈產製生質炭研究

Processing Sewage Sludge and Rice Straw by Microwave-Induced Co-Torrefaction

石峻豪(Chun-Hao Shih)

指導教授 ： 闕蓓德

國立臺灣大學/工學院/環境工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.03045

摘要

焙燒程序為生質物熱處理程序中較低溫之輕微裂解程序，能保留較多固態生質物以便後續利用，有利生質能使用過程的改善。而引入微波加熱技術於焙燒製程時，則可改善加熱過程中之效率和產能，加強產物之品質並減少能量消耗。本研究主要以下水污泥再利用之角度出發，使用單模聚焦式微波爐，於氮氣無氧環境下探討污泥焙燒製造生質炭再利用的特性，以及污泥加入稻稈混焙燒後改善生質炭性質之可能。實驗過程主要之操作參數則包含微波功率(污泥為100 W-400 W；污泥稻稈混合分為150 W和250 W)及混合比(分為升溫測試和以污泥為主之產物試製)等，並進行焙燒產物性質之探討，包含重量損失、能量變化、熱值變化、元素組成、近似組成、灰分及表面特性等，最後研究污泥再利用製程中的能量平衡模擬並估算其費用。
本研究結果發現：於下水污泥餅20分鐘微波加熱過程中，最高末溫和加溫速率隨微波功率的加強而增加，但升溫穩定性相較較差，重量損失的部分則隨功率和溫度提升而加速分解，功率超過300 W時污泥重量損失已超過一半，加上污泥之熱值會於250 W以上開始大幅降低，因此污泥單獨焙燒程序建議之使用功率為250 W。當污泥添加稻稈後，150 W微波測試之結果於污泥重量比20 wt.%處和80-90 wt.%處左右，微波吸收效果明顯加強使反應更為劇烈，但重量損失亦會進一步增加。250 W下之混合樣本試產製時，於污泥重量比60%-90 %處之混合焙燒產物，則以90 %時的熱值和重量保留效果較佳，但溫度表現則以80 wt. %處最佳，能量產率部分，250 W下之混合焙燒則以90 wt. %處有較好之表現。
近似分析結果之部分，焙燒功率越高會逐漸提升固定碳之成分，但於較高溫下反而會使污泥中之固定碳成分下降，同時產物灰分也會有過高之狀況，污泥添加稻稈混合之後此一現象依然無法達到理論上的改善作用。元素分析部分，無論是污泥或是稻稈，焙燒後的元素含量中C元素含量均有上升情形，且相較於純污泥焙燒炭之性質，污泥添加稻稈後的產物元素含量分布特性會較佳。
質能平衡部分：本研究以每1kg濕污泥餅為進料基準，計算再製生質炭之能源效率，污泥重量比60 %-100 %之間以60 %處有最好的能源效率，而以100 %純污泥之費用最少，代表以目前設備進行焙燒較難兼顧廢棄物處理和能源效率兩項指標。

關鍵字

微波誘發焙燒 ； 下水污泥餅 ； 污泥再利用 ； 焙燒炭

並列摘要

Torrefaction is a mild pyrolysis treatment that involves applying low temperature heating for improving the properties of solid biomass fuel. Compared with traditional heating, microwave heating demonstrates superior efficiency, production quality, and energy consumption. In this study, microwave heating was applied to the torrefaction treatment by using a single-mode microwave device. Sewage sludge cake was used as the initial material because of its recyclable possibility, and rice straw was used because it acts as an additive for enhancing the character of sludge biochar. Reaction conditions, including microwave power and mixing ratios, were varied in the experiments to optimize the reaction situation in a nitrogen environment. Weight loss, calorific value, ultimate analysis, approximate analysis, and scanning electron microscopy (SEM) were applied to determine the performance of torrefaction production. The microwave power of sludge torrefaction is approximately 100 W to 400 W, and the power output of mixing biomass is approximately 150 W to 250 W. Energy balancing and cost analysis were also conducted in this study.
The maximum temperature and weight loss of sludge during microwave torrefaction increase according to the rising of microwave power level, but the heating curve becomes more unstable. Because the weight loss is more than 50% at 300W and the calorific values start to descend obviously at 250W, the microwave power level of sludge is suggested at 250W. After rice straw is added to the sludge when the sludge weight ratio is at 20wt. % and between 80 wt. % and 90wt. %, the reaction becomes more violent. This means that the absorption of microwave is higher at these ratios, but the weight loss also increases.
At a power level of 250 W and the mixture of 60wt. % to 90wt. % sludge, the calorific value and the mass yield of the torrefaction char at 80 wt. % were higher than before. The biochar performance of the energy yield decreased noticeably greater than the mass yield at an increased power level, specifically after the microwave power was increased to a level above 250 W for sewage sludge. In addition the energy yields of the 250W sludge-straw co-torrefaction have the same situations when the sludge ratio was between 70 wt. % and 80 wt. %. The highest energy yield appeared at sludge ratio of 90wt. %.
According to the results derived from approximate analysis, the increase in the power level can increase the content of fixed carbon. However, higher power levels would not only reduce the content of fixed carbon but also raise the ash content when the temperature is too high. In addition, the products resulting from adding rice straw to sludge do not contain less ash, and it would bring a shortcoming as solid fuel. In ultimate analysis, the C content of sludge is higher after the torrefaction process and also better after rice straw is added to sludge.
In energy balance analysis, 1 kilogram of wet sewage sludge is set as the input unit for simulation. When the weight ratios of sewage sludge are examined at a level of 60 wt. % to 100 wt. % at 250W, energy efficiency is the highest at 60%, and the cost is the lowest at 100% (pure sludge). This means that acquiring a balance between waste treatment and fuel regeneration is difficult in the lab-scale study.

並列關鍵字

microwave-induced torrefaction ； sewage sludge ； sludge reuse ； biochar

參考文獻

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