Assessing the sustainability of bioethanol crops of Taiwan

從國際間已提出的生質燃料永續性準則進行分析，其中能源投入、溫室氣體減量、農藥與肥料使用量及廢棄物減量等，可使用生命週期評估法進行評估；至於用水量、農藥與肥料使用量及廢水處理等，利用水足跡分析則可清楚描繪出對水資源環境影響；在能源作物與生質燃料的經濟性與生產力部分，透過成本與效益分析將可有效的進行不同作物間經濟性比較。因此，為探討適合台灣用於推廣之生質酒精能源作物，本研究透過以目前國內廣為探討之第一代與第二代生質酒精潛在料源，包含甘蔗、玉米、甘藷、甜高粱與稻稈等五種料源為例，利用多年期田野調查資料，分別採用生命週期評估法、水足跡及成本效益分析法，探討我國生質酒精能源作物在能源、環境、經濟與水資源之永續性。 在生命週期評估法研究結果顯示，所有能源作物中以甘蔗在能源平衡、溫室氣體減量及成本經濟性表現最好；在政府補貼每期作每公頃新台幣4.5萬元、農民淨收益維持與稻農相當約5~6.5萬元下，蔗農甘蔗酒精出廠價格每公升約27.72~33.00元，與進口酒精價格約37.2元相較已具備競爭力。以台灣目前纖維酒精技術發展水準，酒精轉換率僅200公升/公噸，使得其能源平衡、溫室氣體減量與生產成本等方面，表現皆不如第一代生質酒精，與國外文獻結論有所不同。本研究同時發現，既使稻稈酒精轉換率可達理論值270公升/公噸，其能源平衡率也僅達1.30~1.32，仍比台灣第一代料源中最具發展潛力的甘蔗（蔗農種蔗約2.92，台糖種蔗約2.29）為差；稻稈纖維酒精在生產效率提升至理論值時，加上5%利潤率後纖維酒精出廠價格約為30.77元，方可下降至與政府補貼4.5萬元下蔗農甘蔗酒精價格相當；但此時稻稈酒精之生命週期碳排放量每公升約557~583公克CO2e，將低於甘蔗酒精排放量約26.45%，減碳效果顯著。 至於水足跡研究結果顯示台灣以甘藷總水足跡最低，甘蔗與甜高粱次之，玉米最高，而水稻耗水量更高於玉米，與全球相關作物水足跡的平均值研究結果有一致性的結論。且除玉米外，所有能源作物之綠色水足跡比重達50%以上，顯示這些作物生長過程使用自然的雨水較多，符合能源作物以低投入為篩選準則之要求。屬於糧食作物的水稻水足跡約為甘藷13倍、甘蔗8.8~10.4倍，顯示台灣種植水稻相較能源作物對水資源的消耗較高。台灣甘蔗、玉米與甘藷之水足跡表現皆優於全球主要生產國，其中玉米水足跡更只有美國52~62%，台灣利用甜高粱莖桿榨汁產製酒精，也比國際上利用甜高粱穀粒產製酒精之水足跡明顯較低。在溫帶、亞熱帶和熱帶不同氣候條件下能源作物水足跡比較分析顯示，甘蔗在熱帶與亞熱帶地區水足跡相對溫帶地區為低，且以仰賴自然界雨水的綠色水足跡結構為主。玉米在溫帶、甜高粱在溫帶與亞熱帶地區則有較好表現。 透過能源作物永續性之探討，利用能源平衡率、溫室氣體減量效益及生產成本經濟性等建立篩選能源作物之指標，將可使得能源作物發展不但滿足環境永續性之需求，同時具備經濟性發展誘因。綜合生命週期評估法、水足跡及成本效益分析結果，顯示甘蔗為各種能源作物中最適合台灣發展之能源作物；並且從水足跡的表現分析，甘蔗水足跡明顯低於水稻，適合於休耕地活化及水資源匱乏地區推廣。An analysis of internationally-proposed biofuel sustainability criteria indicates that certain criteria including energy input, greenhouse gas (GHG) emissions, pesticide and fertilizer usage and waste reduction, can be assessed using life cycle analysis (LCA) tools, while water footprints (WFs) analysis can be used to describe the environmental impact of water, pesticide and fertilizer usage, along with wastewater treatment. Cost-benefit analysis can provide effective comparisons of the economics and productivity of different energy crops and biofuels. Therefore, to investigate the suitability of cultivating bio-ethanol en-ergy crops in Taiwan, this research conducts a broad investigation of first and second generation bioethanol feedstocks, including sugar cane, corn, sweet potato, sorghum and rice straw. Multi-year field survey data were analyzed us-ing LCA, water footprint and cost-benefit analysis methods to explore the po-tential for bioethanol energy crops as a source for sustainable energy, and their impact on the environment, economy and water resources. LCA analysis results show that, of all energy crops, sugar cane provides the best performance in terms of energy balance, GHG emissions and costs. Given current government subsidies of NT$45,000 per hectare of energy crops per crop period, and a net profit between NT$50,000 to NT$65,000 per crop peri-od, the market price of the resulting bioethanol ranges from NT$27.72~33.00 per liter, making it competitive with imports which are sold for NT$37.2/liter. Given current technology for the production of cellulosic ethanol in Taiwan, the bioethanol conversion rate of rice straw is only 200 liters per ton, thus un-derperforming 1st generation bioethanol in terms of energy balance, GHG emissions and production costs, results which conflict with conclusions in previous studies. WFs analysis results indicate that sweet potatoes have the smallest WFs, fol-lowed by sugar cane and sweet sorghum, and corn has the largest WFs of en-ergy crops, though the WFs of rice is much larger, which results are consistent with the average global water footprints for these crops. The WFgreen accounts for more than 50% of the overall WF for all energy crops except of corn, indi-cating that the cultivation processes of these energy crops relies more on rain-water, thus, fulfilling low input selection criteria. The WFs of rice is about 13 times larger than that of sweet potatoes and 8.8-10.4 times that of sugar cane, indicating the relative resource inefficiency of cultivating rice as a food crop. WFs for Taiwan-grown sugar cane, corn, and sweet potatoes compare favour-ably to those in the top 3 producing countries (Brazil, United States and Chi-na), as the WFs for Taiwan-cultivated corn is only 52-62% of that for corn planted in the United States. A comparison of energy crop WFs in temperate, subtropical, and tropical climate zones indicates that sugar cane cultivation is relatively more efficient in tropical and subtropical regions, due to its heavy reliance on rainwater, while corn and sweet sorghum perform better in tem-perate and subtropical regions. An assessment of energy crop sustainability shows that energy balance, GHG reduction benefits and production costs can be used as indicators for the selection of potential energy crops, and that the cultivation of energy crops would not only satisfy environmental sustainability requirements, but would also spur economic development. Overall LCA, WFs and cost-benefit analysis results show that sugar cane is the most suitable energy crop for cultivation in Taiwan. Furthermore, water footprint analysis shows that the water footprint of sugar cane is significantly lower than that of rice, making it suitable for cul-tivation on fallow land and in water-deficient areas.口試委員會審定書 I 誌謝 II 中文摘要 III 英文摘要 V 目錄 VII 圖目錄 IX 表目錄 XI 第一章 緒論 1 1.1 生質能與能源作物 1 1.2 國外生質燃料政策措施 5 1.3 生質燃料產製技術 11 1.4 生質燃料永續性指標 14 1.5 生命週期評估法與水足跡簡介 17 1.6 我國生質燃料政策規劃與產業發展現況 20 1.6.1 我國生質燃料政策規劃 20 1.6.2 台灣能源作物推動歷程與發展概況 21 1.6.3 我國生質燃料產業發展現況 23 1.7 研究目的 24 1.8 參考文獻 27 1.9 圖與表格 38 第二章 生質酒精能源作物能源經濟與環境效益評估 49 2.1 前言 49 2.2 材料與方法 52 2.2.1 研究材料 52 2.2.2 生命週期評估法 54 2.2.3 生產成本分析 58 2.3 結果與討論 61 2..1 生質酒精能源平衡分析 61 2.3.2 生質酒精溫室氣體減量效益分析 65 2.3.3 生質酒精生產成本分析 69 2.4 結論 75 2.5 參考文獻 77 2.6 圖與表格 84 第三章 生質酒精能源作物水足跡分析 105 3.1 前言 105 3.2 材料與方法 107 3.2.1 材料 107 3.2.2 方法 109 3.3 結果與討論 111 3.3.1 各項水足跡分析 111 3.3.2 整體水足跡分析 113 3.3.3 台灣能源作物水足跡與主要生產國比較 115 3.3.4 不同氣候條件之能源作物水足跡比較 116 3.4 結論 118 3.5 參考文獻 120 3.6 圖與表 124 第四章 結論 137 4.1 台灣生質酒精永續性指標探討 139 4.2 我國生質酒精產業化發展潛力分析 140 4.3 總結 145 4.4 建議 147 4.4.1台灣發展生質酒精可行推動策略建議 147 4.4.2後續研究建議 149 4.5 參考文獻 150 4.6 圖與表格 15