台灣林木生質碳生產的生命週期比較評估

Abstract

生物質經由慢速熱裂解可得到一種固態產物-生質碳（Biochar），經過此技術可以將大氣中的碳長期封存在生質碳中，並可以產生替代能源如合成氣、生質焦油，更可藉由將生質碳施放於土壤當中來提升土壤的肥沃度。本研究將以台灣發展生質碳系統的觀點，並以生命週期評估方法來分析生質碳的固碳貢獻度。研究中選出三種台灣生長期較短的能源樹種：銀合歡（三年生）、桉樹（六年生）、相思樹（十年生）作為慢速熱裂解的生質原料，此外也將漂流木之廢棄木材列入原料類別作評估。能源樹種從平地造林開始進行生命週期分析，漂流木則從蒐集木材開始分析。結果顯示將四種原料製成生質碳之生命週期過程中，淨溫室氣體排放量皆為負值，表示製造生質碳中所產生的能源和降低的溫室氣體皆可有效補償過程中的能源消耗和污染排放，達到正面的效益。其中平地造林能源樹種又以桉樹在固碳力和產生能源的表現最佳，以30年為輪作單位，種植桉樹每公頃可降低112公噸的等量二氧化碳排放並產生3,100 GJ的替代能源，其次為銀合歡的82公噸、2,500 GJ和相思樹的48公噸、1,300 GJ。而漂流木因數量龐大且估算過程不包含其生長過程，結果顯示每10萬公噸的漂流木可降低高達5,500公噸的溫室氣體並產生484 TJ的替代能源。其中也包含評估木材運送對能源的耗用和環境污染的計算。整體而言，生質原料製成生質碳將可提供一個有效的碳庫，並減少化石燃料的製造和使用，而又以使用廢棄生物質進行熱裂解對減緩環境衝擊的能力最為顯著。

Slow pyrolysis offers an energetic efficiency for bio-energy production, and the soil application of biochar reduces greenhouse gas （GHG） emissions to a greater extent than when the biochar is used as fuel to offset fossil fuel emissions. Scenarios for biochar production were examined using a life-cycle assessment which is known as a “cradle to grave” approach. The purposes of this study are assessing the capability of GHG reduction in biochar system and the total energy generation during biochar production. The life cycle environmental performances of three different Taiwan trees （Leucaena leucocephala, Eucalyptus spp. and Acacia confusa） and the waste driftwoods were estimated. And the functional units of each system are 30-year period/ha and 10,000 tons of driftwoods. The effects of impact assessment were calculated by using LCA software – SimaPro program. All of the feedstock provides a positive energy generation and negative GHG emission. The cultivation of Eucalyptus spp. performs a better way to decrease GHG emission (112 ton CO2 eq) and fossil fuel consumption (3,100 GJ eq) due to its high generation of biomass. The driftwood system could reduce 5,500 ton CO2 eq and provide 484 TJ of substitution energy. The result shows that the bio-energy system with bio-energy and biochar production rather than solely for bio-energy production can efficiently mitigate the climate change by a great amount of GHG sequestration.