Life cycle assessment of producing biocoal from rice straw via torrefaction

目前政府相關單位計畫配合焚化廠屆齡除役期程，將其逐步轉型為生質能源中心，處理生質垃圾及農業廢棄物，以提高國內再生能源比例。而稻稈為我國數量最多的農業生質廢棄物，但其高含水率、低熱值等特性易影響熱化學轉換(燃燒、氣化)效率，致目前在利用率偏低。採用焙燒前處理技術能改善生質物燃燒特性，並減少生質物體積及增加可磨性。故本研究以生命週期評估方法分析國內生質能源中心以焙燒前處理技術產製稻稈生質炭，並應用於混燒發電之環境衝擊與效益。使用生命週期評估軟體SimaPro 7.2對生質能供應鏈各階段的投入產出進行盤查分析後，再經 IMPACT 2002+評估模式量化其潛在衝擊。而環境效益部份則是建立我國目前的進口燃煤發電供應鏈做為參考比較系統，評估生質能供應鏈的衝擊減量或增量效果。結果顯示生質電能供應鏈在水域生態毒性、陸域生態毒性、全球暖化及非再生能源使用四個衝擊類別具有衝擊減量的正面效益，並隨生質炭混燒比增加而減量程度越高，但在致癌性、非致癌性、土地利用、游離輻射、臭氧層耗竭、可吸入性有機物/無機物、土壤酸化、水體酸化、水體優養化及礦物資源等類別則造成更高的衝擊；在溫室氣體減量部份，生質炭混燒發電一度的減碳量隨混燒比上升而增加，最高發電一度可減量0.61 kg CO2-eq (完全使用生質炭)，而每公斤稻稈產製生質炭用做混燒發電之減碳量約為0.49 kg CO2-eq。稻稈產製生質炭混燒發電雖具有減少非再生能源消耗及溫室氣體排放的效益，但可能會造成更高的水體優養化(額外肥料使用所致)、游離輻射等其他環境衝擊，故決策者必須權衡環境衝擊，以評估稻稈生質電能在環境面之可行性。According to the target of increasing proportion of renewable energy and CO2 reduction, Taiwan Environmental Protection Administration draft a promotion plan of the transformations from expired incinerators to bio-energy centers which utilize bio-waste as feedstock. Base on the estimations in this study, crop residue such as rice straw is produced in abundance on Taiwan. However, the high moisture content and low heating value of rice straw affect its biomass utilization efficiency. Furthermore, poor grindability of rice straw decreases rates of co-milling with coal at coal-fired power plants. Torrefaction is a thermal pretreatment technology which improves the properties of biomass in order to deal with the above problems. This study focuses on production chains which utilize torrefied biomass (biocoal) derived from rice straw for co-firing in power plant. The production chains are investigated using a Life Cycle Assessment (LCA) approach, which takes into account all the input and output flows occurring along the production chain. Investigation was carried out using a SimaPro 7.2 LCA software and adopting the IMPACT 2002+ methodology for evaluation of potential environmental impact. Environmental impact reductions were also evaluated by comparing with fossil reference system generating electricity with combustion of imported hardcoal. The comparative results show that the bioelectricity production chain had impact reductions in impact categories of aquatic ecotoxicity, terrestrial ecotoxicity, global warming and non-renewable energy use. Moreover, results indicated that the bioelectricity had higher impact reduction with increasing in biocoal replace ratio. On the other hand, the bioelectricity production chain had higher impact potential than reference system in carcinogens, non-carcinogens, ionizing radiations, ozone layer depletion, respiratory organics, terrestrial acidification, aquatic acidification, aquatic eutrophication, respiratory inorganics, land occupation and mineral extraction. Concerning GHG emission savings, the possible saving of biocoal co-firing increased with increasing in biocoal replace ratio, and had a maximum of 0.61 kg CO2 eq/kWh (100% replace with biocoal). Also, the GHG saving was 0.49 kg CO2-eq per kg of rice straw which was used for generating bioelectricity. Based on the results, the bioelectricity production chain has environmental benefits in GHG savings and non-renewable energy consumption, but cause higher impact in other impact categories than fossil system. Therefore, policy makers should comprehensively consider various impacts in order to determine the feasibility of developing bioenergy in aspect of environment