生命週期評估三種料源生產生質酒精與其廢渣再利用

Abstract

近年來木質材料利用於生質酒精生產，以減少能源作物與糧食來源競爭的爭議，頗受囑目。木質材料來源廣、組成複雜，需要有一個能有效地從生態、經濟、能源消耗等不同角度來評估的策略，生命週期評估可分析產品從原物料的取得、生產、消費到最終處置的生命週期，能資源投入與產出對環境之影響。 本研究就稻桿、狼尾草以及短輪伐期的桉樹三種不同料源在臺灣的休耕地種植情境，先進行生產生質酒精的生命週期評估；再進一步討論生質酒精之廢棄物的再利用，分別為顆粒燃料與再生紙模，使成為整體生產的附加產物，並以生命週期評估與能源利用、經濟考量整體發展的可能性。生質酒精的生產過程與整體過程中的能源消耗，與不同栽植情境的差異即為評估的重點。分析結果顯示，以生質酒精生產代替燃料，可有效的減少化石燃料的使用。三種不同的料源由於不同的施作條件，有著不同的環境衝擊，若以最高能源產出為考量，由於狼尾草相對於其他料源可以提供較高的產量，因此以栽植狼尾草為最佳的選擇；若以同等酒精產量來看，桉樹由於其生態特性，對環境的衝擊最低；另一方面，相同面積土地使用為考量，則為稻桿較佳。將發酵的廢棄物當作原料再利用，顆粒燃料與再生紙模與傳統製造方法的比較，兩種利用方式皆可減少環境的衝擊，尤其以資源消耗方面特別顯著，顯示整體資源的運用將更完善。然而就能源效率的角度來看，顆粒燃料可以提供相對更高的能源。總體而言，狼尾草的發酵廢渣再利用為顆粒燃料，是能源生產面考量下本研究各方案中的最佳選擇。

Bioethanol made from lignocellulosic materials is the most promising alternative devoid of competing with land source for food production. This study employed life cycle assessment to investigate scenarios for plantation of three different energy crops in fallow paddy fields in Taiwan: rice straw as agricultural waste, napier grass and eucalypt species as short rotation coppice, for bioethanol production. Utilization schemes for aforementioned wastes for fuel pellets as secondary energy source and recycled molded pulp as secondary products are also evaluated by life cycle assessment considered energy consumptions and environmental emissions for processes of each utilization scenario. Analysis shows alternative fuels made by bioethanols can effectively reduce the usage of fossil fuels. Different plantation practices for individual feedstock cause different environmental impacts. With regards to optimal energy production, napier grass plantation is the best choice due to its highest overall biomass production. With regards to optimal energy production, napier grass plantation is the best choice due to its highest biomass production. With regards to optimal bioethanol production, eucalypt plantation is the best choice due to its minimal ecological impacts. With regards to unit areal land productivity, rice straw plantation is the best choice. Reutilization schemes of fermentation wastes for pellet fuel and recycled molded pulp are compared: pellet fuel production can achieve better energy efficiency. This study found the combination of napier grass plantation and pellet fuel made by bioethanol fermentation wastes is the optimal alternative among other options in the present study.