Life Cycle Assessment and Energy Potential of Residual Biomass

台灣為一個能源缺乏的國家，99%的能源仰賴進口，2010年能源自給率僅達0.7%，政府制定新的能源政策時亦將生質能源之發展應用作為重點項目，以期提高國內再生能源之比例。然而針對刻意栽種之生質作物有與糧食作物爭地之疑慮，故本研究以殘餘生質物作為研究對象，包括稻稈、玉米稈、廚餘、豬糞尿，統計2010年上述廢棄物在台灣之數量，計算其能源潛能。利用生命週期方法評估五個生質能案例之環境衝擊與效益，以及計算其能源投資報酬(r_E)值(稻稈焙燒生質炭燃燒發電、玉米稈酵素水解產製纖維酒精、玉米稈混豬糞尿產製生質沼氣、廚餘混豬糞尿產製生質沼氣、沼氣發電)；此外亦評估了化石能系統之燃煤發電供應鏈、95無鉛汽油供應鏈、天然氣供應鏈三個案例，用於比較生質能源取代化石能源之衝擊減量效益。評估工具為生命週期評估軟體SimaPro 7.2，盤查分析則使用文獻回顧及軟體內建Ecoinvent資料庫，再經IMPACT 2002+評估模式量化其潛在衝擊。結果顯示，稻稈生質炭燃燒發電案例對於全球暖化類別的衝擊最高；玉米稈纖維酒精案例對於非再生能源類別的衝擊最高；玉米稈混豬糞尿產製生質沼氣案例、廚餘混豬糞尿產製生質沼氣案以及沼氣發電案例均以吸入性無機物類別的衝擊最高。溫室氣體減量部分，生質沼氣三個案例皆具減量效益，纖維酒精及生質炭生產過程雖不具減量效益，然而纖維酒精納入汽車使用階段、生質炭僅計算燃燒發電階段亦具有減量效益。能源投資報酬部分各案例之r_E值皆>1，表示皆具有能源效益。生質能系統在非再生能源損耗項目顯著優於化石能源系統，整體之環境衝擊亦較化石能源輕微，然而在其他衝擊類別(如臭氧耗竭)卻可能造成更高的衝擊，雖然本研究取得之資料與評估方式存在諸多不確定因子，仍建議決策者必須考量各種環境衝擊適當權重，以評估生質能的淨環境效益。Taiwan is a country with an energy shortage. Ninety-nine percent of Taiwanese energy is imported, and the energy self-sufficiency rate in 2010 was merely 0.7%. Therefore, when the government develops new energy policies, the development of biomass energy should be a crucial project to increase the proportion of domestic renewable energy.While the first generation biomass may threatening food supplies, this study only explores the issues of residual biomass, including straw, corn stover, kitchen waste, pig manure. The amount of waste mentioned above in 2010 is calculated to estimate its energy potential. Furthermore, a life cycle assessment (LCA) approach is used to evaluate the impacts and benefits of five biomass cases on the environment and to calculate the energy return on investment rate (r_E). ((1) generating electricity by torrified biocoal (2) enzymatic hydrolysis of corn stover cellulosic ethanol (3) biogas production from kitchen waste mix pig manure (4) biogas production from corn stover mix pig manure (5) generating electricity by biogas.) Environmental benefits were also evaluated by comparing with 3 fossil reference system: generating electricity by imported hardcoal, unleaded 95 petrol supply chain, and natural gas supply chain for domestic heating. Inventories came from Ecoinvent database and literature review. Environmental analyses were carried out using a SimaPro 7.2 LCA software and adopting the IMPACT 2002+ methodology for evaluation of potential environmental impact. Results showed that, the case of terrified biocoal had the highest environmental impact in categories of global warming, the case of cellulosic ethanol had the highest environmental impact in categories of non-renewable energy, other three cases of biogas related had the highest environmental impact in categories of respiratory inorganics. Comparing to reference system about greenhouse gas emission reduction, all biogas cases, firing stage in biocoal case and car-use stage included in cellulosic ethanol case were beneficial to reduction. Regarding to energy return on investment, r_E value for all cases are > 1, that means they at least capture some renewable energy value with our nonrenewable investment. Although bioenergy system had less environmental impact than fossil energy system, especially on non-renewable energy consumption; however causes higher impact on other impact categories. Besides, there were many uncertain factors in the data acquired by this study and evaluative methodology. Policy makers should consider the appropriate weights for the different types of environmental impacts to evaluate the net environmental benefit of biomass energy