都市廢棄物機械熱處理程序之生命週期分析

Abstract

自從人類聚落都市化以來，垃圾(即都市廢棄物) (municipal solid waste, MSW)處理就成為都市問題中非常重要的課題。人們對於MSW的厭惡，不僅在於它的外觀，更會因為MSW中所包含的有機物質，使得MSW中的細菌大量滋生，並發出腐敗的臭味，讓人產生不舒服感。因此尋找有效又經濟的MSW處理技術，就成為一個非常受關切的議題。近年來由於化石燃料的短缺，使能源問題也受到高度的重視。本研究主要的目的正是為了同時解決這兩個問題；將都市廢棄物MSW進行蒸煮-機械分選處理，總稱機械熱處理(簡稱蒸煮)，從產物中分離出有用的生質纖維(bio-fiber, BF)，再把這些生質纖維進行乾燥及造粒之後，製造成生質纖維衍生燃料碇(bio-fiber derived fuel, BFDF)；甚至再進行焙燒(torrefaction)製成生質纖維衍生生質炭(簡稱生質炭) (bio-fiber derived biochar, BFBC)。使用MSW製成的生質纖維衍生燃料BFDF，不但可以解決目前MSW直接焚化處理所產生的污染問題，同時利用混燒的方式，可以減少目前對於化石燃料的依賴性與使用量。 在實驗室的紙類蒸煮試驗中發現，半纖維素比纖維素更容易因蒸煮而水解斷裂，且隨著溫度的升高水解的效果越好，讓整個產物更容易進行後續的加工或研磨。在實驗室的竹筷蒸煮試驗中，隨著溫度越高，產物的能源緻密度(energy desification)會提高，但是熱值也會隨著時間越長而降低。在模廠規模的蒸煮試驗也發現有相似的結果。 經濟評估的結果顯示，當MSW蒸煮處理規模過小時，處理費過高；但當處理達一定規模後即具有降低MSW處理費之經濟效益。例如:若蒸煮產物生質纖維BF未出售且不計處理費，則當蒸煮處理規模為142.5公噸MSW /日(25% MSW處理量)時，MSW蒸煮處理費用約為1,275元/公噸；如果規模達到285公噸MSW /日(50% MSW處理量)，則MSW蒸煮處理費用為1,196元/公噸；如果規模達到427.5公噸MSW /日(75% MSW處理量)，則MSW蒸煮處理費用可降至1,165元/公噸。若再將產物BF製成BFDF出售，則可將前述規模的MSW蒸煮至製成BFDF之處理費(淨成本)分別降至881元/公噸、802元/公噸與772元/公噸，遠低於MSW焚化處理費用1,667元/公噸。但是如果再進行焙燒處理製成生質纖維衍生生質炭BFBC並出售，則處理每公噸MSW由蒸煮至製成BFBC之淨成本會變成3166元、3087元及3057元，費用幾乎是直接焚化處理的兩倍左右。因此如非必要，將產物處理至BFDF為最具有經濟效益的。若未來此BFDF之使用者可以扣抵碳稅，甚至取得碳交易權，必可同時解決MSW處理與能源問題，又有其附加的經濟效益。 蒸煮產物產製BFDF及BFBC的生命週期盤查分析結果顯示，當MSW蒸煮溫度為155℃，時間為60 min，產製BFBC的焙燒過程(由BFDF至BFBC)所消耗的能量為6.16 MJ/kg MSW，較產製BFDF (由BF至BFDF)的1.76 MJ/kg MSW及BF (由MSW至BF)的0.39 MJ/kg MSW高，導致製作BFBC對環境的各項影響衝擊皆大於BFDF。 生命週期分析探討了蒸煮溫度為155℃，時間為60 min之四種MSW蒸煮(S1-S4)情境及焚化(S9)並加以比較。S1及S2產製BFDF，但分別使用再生能源(ER)及電力公司(EP)之電力；S3及S4則產製BFBC，使用之電力分別來自ER及EP。結果顯示，單一化得點點數排序由低至高為S1、S9、S3、S2及S4，其點數分別為6.94E-05、9.72E-05、1.52E-04、1.89E-04及3.83E-04 Pt (points)。無論產品為前面任何一項，以再生能源做為製程電力來源所造成的環境衝擊，皆會小於使用市電(台灣電力公司，台電)。所以MSW機械熱處理的最適方式為將MSW進行蒸煮-機械分選處理，製成BFDF，過程中使用的電力為再生能源所產生的。

Since urbanization of human settlements, the problem of disposal of municipal solid waste (MSW) has been one of the important issues of city. People detest MSW not only because of its appearance but also its decayed smell which make people feel uncomfortable. The reason why MSW has decayed smell is that it contains organics producing a large number of becteria. Therefore, finding an efficient and economic MSW treatment method has been deeply concerned. In addition, the energy problem has also been paid much attention to these years due to the shortage of fossil fuel. The objective of the study is to solve these two problems at the same time. By autoclaving-mechanical separation (noted as mechanical heat treatment or briefly as autoclaving) of MSW, the available bio-fiber (BF) can be separated from the waste. Then the BF can be made into bio-fiber refuse derived fuel (BFDF) after drying and granulating. The BFDF can even become bio-fiber derived bio-char (BFBC) through torrefaction. Utilizing BFDF and BFBC can not only solve the pollution problem of incinerating MSW directly but also reduce the dependence on fossil fuel and the amount of it uesd. In the experiment of autoclaving paper in the laboratory, hemicellulose hydrolyses and breaks more easily than cellulose does through autoclaving. The effect of hydrolysis is better with the higher temperature, which can make the whole product proceed manufacturing or grinding more easily. As for the experiments of autoclaving bamboo chopsticks in the laboratory, the densification of energy of the product rises with the higher temperature, but the heat value lowers with the longer processing time. The results are similar to the pilot-scale autoclaving experiments. The calculation of economic assessment shows the results that the treatment fee is higher when the scale of the MSW autoclaved is smaller. However, there are economic benefits in terms of reducing the treatment fee of MSW when the scale gets a certain level. For example, if the product of BF is not sold and its treatment fee is not counted, the fee of autoclaving MSW is about 1,275 NT dollars per ton MSW when the scale is 142.5 tons MSW per day (25% of scale of total MSW). The fee of autoclaving MSW reduces to about 1,196 NT dollars per ton MSW when the scale is 285 tons MSW per day (50% of scale of total MSW). The fee of autoclaving MSW further decreases to about 1,165 NT dollars per ton MSW when the scale is 427.5 tons MSW per day (75% of scale of total MSW). If the products are sold in the form of BFDF, the fee of autoclaving-to-producing BFDF from MSW are respectively reduced to about 881, 802 and 772 NT dollars per ton of MSW, which are far lower than the fee of incineration of MSW of 1,667 NT dollars per ton MSW. But if the BFDF is further torrefied to products of BFBC and sold, the net costs are respectively 3,166, 3,087 and 3,075 NT dollars per ton of MSW, which are about twice as much as the fee of incinerating MSW directly. In consequence, if there is no need, it has the most economic benefits to make the products become BFDF. It can slove the problems of dealing with MSW and energy shortage simultaneously. Further more, it would give extra economic benefits, if people who use BFDF in the future can have deductible carbon tax or even can gain the carbon trade right. From the life cycle inventory of three treatment processes, namely, (1) from MSW to BF (Process 1 or P1), (2) MSW to BF then to BFDF (Process 2 or P2) and (3), MSW to BF to BFDF and then to BFBC (Process 3 or P3), the results reveal that the consumption of energy in making BFBC from BFDF (Stage 3 or SG3) is higher than those in making BFDF from BF (Stage 2 or SG2) and BF from MSW (Stage 1 or SG1). For example, as for the case of autoclaving at 155℃ and 60 min, the respective energy consumptions are 6.16, 1.76 and 0.39 MJ per kg MSW, for Stages 3, 2 and 1. Further, the life cycle assessments of four scenarios of autoclaving of MSW at 155℃ for 60 min (S1 to S4) are compared to that of incireration (S9). S1 and S2 produce BFDF using electricities from renewable energy (ER) and power company (EP), while S3 and S4 manufacture BFBC employing electricities from ER and EP, respectively. The results indicate that the single sores from the lowest to highest are S1, S9, S3, S2 and S4 of 6.94E-05, 9.72E-05, 1.52E-04, 1.89E-04 and 3.83E-04 Pt (points), respectively. In the above assessments, the impact on the environment using renewable energy electricity from incineration plant is lower than that using home electricity from power company as an energy resource. Moreover, S1 induces the lowest environmental impact. Hence, the proper choice of mechanical heat treatment of MSW is autoclaving the MSW to make BFDF using renewable energy.