應用蒸煮程序處理都市廢棄物及其生質物回收再利用之研究

Abstract

本研究利用蒸煮程序處理都市廢棄物(municipal solid waste, MSW)，將其中之有機纖維(organic fibre material, OFM)成分回收再進行其後之轉製應用。高溫高壓的蒸汽可對OFM成分行水解反應，將半纖維素及纖維素中之長鍊結構斷鍵；另外亦可使塑膠類蜷縮成顆粒。可大幅減少MSW的體積；且經由蒸煮過後的MSW (稱為AMSW)仍保有一定程度的熱值，使得其能源密度獲得提升。最後由於OFM與塑膠類及金屬無機類顆粒大小之差異，可利用震動篩使其分離，獲得較純之OFM成分。 本研究進行135、155、165 ℃及15、30、60分鐘的蒸煮實驗，探討臺灣MSW之最適蒸煮條件。結果顯示，於135 ℃之蒸煮溫度，其蒸汽能量仍不足以使一般廢棄物之體積充分縮減，然而熱值卻降低，造成其能源密度不升反降。而155 ℃的溫度，使MSW的體積獲得顯著的縮減，並可有效提升其能源密度。165 ℃之條件，亦可使一般廢棄物體積縮減有如155 ℃之條件，但已無法使能源密度有更進一步的提升。鑒於165 ℃會花費更多的能源，故165 ℃之溫度條件進行蒸煮並不適宜。而蒸煮時間須達60分鐘方可達到較大的體積縮減效果。故155 ℃蒸煮60分鐘為最適的蒸煮條件。另外蒸煮過程必須搭配7 rpm的翻轉攪拌，使蒸煮物得以充分破碎，並且有助於篩分程序的進行。 蒸煮後之MSW (AMSW)水分仍有72 ~ 77 wt.%，而最適的篩分水分條件介於50 ~ 60 wt.%，故須進行脫水程序降低AMSW之水分。經大型離心脫水機脫水後，脫水AMSW (DAMSW)之水分降為56 wt.%。其後將DAMSW以1 cm x 1 cm篩網進行篩分，約有46 wt.%之DAMSW為OFM可被篩分出來。此篩分出的OFM經造粒壓碇，可進一步提升其能源密度。而最適的OFM造粒水分約為20 ~ 25 wt.%，可得到密度較高較緻密的造粒成品。經由實驗測試發現MSW蒸煮並篩分後之OFM不易利用焙燒處理再提升其能源密度，故此OFM以逕行利用為宜。最終50 kg的MSW中之乾基MSW (21 kg)約有38.6 wt.%乾基OFM (8.1 kg = 21 kg x 0.39)可被分離出來製成燃料碇，其乾基發熱量為3,891 kcal kg-1，密度為1,346 kg m-3。

Autoclaving treatment of municipal solid waste (MSW) for the recovery of organic fibre material (OFM) and its reutilization was investigated in this study. High-temperature and high-pressure saturated steam was used to hydrolyze the OFM of MSW, and break the long chain structure of hemicllulose and cellulose into small fragments. Meanwhile, the plastics were softened and shrunken into small lumps by the heating of steam. Therefore, autoclaving process can effectively reduce the volume of MSW. Besides, the heating value of autoclaved MSW (noted as AMSW) only decreased slightly. Thus the energy density of AMSW per volume increased. After autoclaving, the OFM can be easily separated from the AMSW by a vibratory screen according to the size diference between OFM, plastics and the other inorganic contents, obtaining the homogenous OFM. The autoclaving experiments were conducted at 135, 155 and 165 ℃ with various operating times of 15, 30 and 60 minutes. The results indicated that the energy of steam at 135 ℃ was not enough to reduce the volume of MSW sufficiently, while slightly decreased the heating value of AMSW, therefore reducing the energy density. At 155 ℃, the volume of MSW reduced significantly, so the energy density increased effectively. Comparing with 155 ℃, 165 ℃ did not further increase the energy density of AMSW. Hence, autoclaving at a higher temperature of 165 ℃ was not suitable because it consumed more energy without further improvement. In order to achieve an effective volume reduction, the operating time must be at least 60 minutes incorporation with rotational shredding at proper rotating speed, say, 7 rpm. The rotational shredding can provide a shear stress to break down the OFM into small size. Therefore, the suitable operating conditions of autocalving of MSW is 155 ℃, 60 minutes with 7 rpm rotation. The moisture content of the AMSW was 72 ~ 77%, while the proper moisture content of materials for screening is 50 ~ 60%. Thus, a decrease of the moisture content of AMSW before screening is required. After the dehydration using a centrifuge, the moisture content of dewatered AMSW (DAMSW) decreased to 56%, suitable for screening. About 46 wt.% of DAMSW, which is OFM can be separated from the DAMSW employing 1 cm x 1 cm sieve. The separated OFM obtained was further pelleted to reduce the volume, while increase the energy density. The proper moisture content of OFM for pelleting was found to be about 20 ~ 25 wt.%. The pelleted OFM with dry-basis heating value of 3,891 kcal kg-1 and density of 1,346 kg m-3 can be used as fuel for the combustion device, coal-fired boiler and co-firing cement furnace. Further torrefaction of the OFM obtained did not increase its energy density, suggesting no need of subsequent torrefaction. The information obtained is useful for the proper design and operation of autoclaving of MSW and the subsequent treatments for the better recovery and reutilization of the biomass of OFM from MSW.