利用焙燒程序將紙漿污泥轉製固態生質燃料之研究

Abstract

臺灣紙漿工廠每年產生大量的廢水處理衍生有機污泥及廢棄木材。這些生質廢棄物通常被轉製為有機肥料或土壤改質劑供農業使用。此生質廢棄物也可以用其他再利用的方式來彌補能源短缺。本研究探討將紙漿工廠有機廢棄物之紙漿污泥利用焙燒程序轉製為固態生質燃料之可行性。研究中的案例紙漿污泥來源為原生紙漿廠的木屑廢棄物及二級污泥的混合物(wood with pulp sludge, WPS)，包含兩個批次，分別為WPS1(乾基可燃分約為95 wt.%)與WPS2(乾基可燃分約為80 wt.%)。 這些生質廢棄物含有高百分比的有機成分，且其乾基熱值(high heating value in dry basis, HHVD)可達3,757至4,374 kcal/kg，適合做為固態生質燃料之原料，然而其水分(高於百分之五十)及揮發分甚高。實驗結果顯示，WPS1經300 °C，20 min焙燒處理後，其HHVD可提升至6,570 kcal/kg以上並有較佳的燃燒表現。WPS2經320 °C，20 min焙燒處理後，HHVD可提升至4,718 kcal/kg。故WPS經焙燒程序後，固態生質燃料的能量緻密比可達原始生質物之1.26至1.50倍。再者，本研究也檢測焙燒過程中氣體的排放情形。氣態副產物可以經由冷凝成有機酸液(木醋酸液)具有高的經濟價值，其中WPS1所得有機酸液含有短碳鏈之酯類及醯胺類，而WPS2所得則含有高碳數烷類及醇類，純化後亦可做為液態燃料。由研究中可知不同時期的紙漿污泥因操作及收集處理過程不同會有不同的性質差異，故於混合初期時應對其特性進行分析，決定紙漿污泥與木屑的混合比例。焙燒後的固態生質燃料其性質類似於煤炭，可直接應用於發電廠使用並減少化石燃料的使用。

The pulp industry in Taiwan produce tons of organic wastewater sludge and wood waste per year. These biowastes were then converted to organic fertilizers for agriculture uses or soil conditioners. The biowaste can also be used in different ways to mitigate the energy shortage. This study investigates a biowaste-to-energy (BTE) method to convert pulp industry waste into solid biofuel via torrefaction. The pulp waste examined is the mixture of wood waste and secondary sludge obtained from a virgin pulp factory in eastern Taiwan (denoted as wood with pulp sludge, WPS).Two batches of WPS were received and tested, namely, WPS1 and WPS2, which contain about 95 and 80 wt.% combustibles in dry basis. The biowaste WPS has a high content of organic compounds with the dry basis high heating value of 3,757 to 4,374 kcal/kg and can be used as raw materials for producing primary solid biofuel. However, the wet WPS bears high water (>50 wt.%) and volatiles contents and needs de-watering, drying and upgrading. The results of this study show that after 20 min torrefaction at 300 °C, the HHVD of WPS1 can be promoted to 6,570 kcal/kg, while after 20 min torrefaction at 320 °C, the HHVD of WPS2 increases to 4,718 kcal/kg. Both cases also have better combustion performance. The results also indicate that the energy densification of torrefied solid biofuel can rise to 1.26-1.50 time than the raw biowaste. The condensable gaseous by-products portions of were condensed and collected as liquid products which is essentially an organic acid liquid (wood vinegar) having highly economic value. Both condensable liquid and non-condensable gaseous by-products were analyzed. The liquid product of case WPS1 include some esters and amides, while those of case WPS2 consist of some alkanes and alcohols. The carbon numbers of alkanes in liquid of case WPS2 are above 10, which indicate that the liquid products can use for liquid fuels after purification. From this study, WPS1 and WPS have different characteristics due to different operation, collection and handling conditions when producing pulp sludge at different times. Therefore, some characteristics analysis should be conducted before mixing the pulp sludge and wood waste in order to provide information for choosing better torrefaction conditions. Moreover, because the combustion performance of torrefied WPS is similar to that coal, the torrefied solid biofuel can be mixed with coal and used in power plant directly. This thus reduces the usage of fossil fuels.