木質素水熱碳化產製生物炭: 嚴謹建模、生命週期評估與技術經濟分析

Abstract

在全球暖化背景下，生物炭近年來被視為最具潛力的負碳方案之一。而水熱碳化（Hydrothermal Carbonization, HTC）技術能夠將濕生物質直接轉化為生物炭，特別適合處理木質素。目前已經有許多實驗研究，然而反應機制過於複雜，導致既有動力學模型仍為總集模型，無預測生物炭性質之能力。因此，本研究透過模型化合物方法，建立了木質素HTC的嚴謹模型，不僅能預測生物炭的產率與元素組成，也可以同時描述液相與氣相產物組成。此外，本研究也將此模型整合至製程設計與優化，結合生命週期評估（Life Cycle Assessment, LCA）及技術經濟分析（Techno-Economic Analysis, TEA），全方位評估木質素 HTC 之可行性。 LCA指出，生物炭的負碳效益為 -2.05 CO2 eq/kg biochar，具有顯著負碳效益。然而，原料木質素的分配情境將會大幅度地影響生物炭的整體效益: 若是增益分配情境下，生物炭的全球暖化為 -2.49 kg CO₂ eq/kg biochar；如果在主產品分配情境時，生物炭的全球暖化則是 +1.87 kg CO₂/kg biochar，反而成為排放源。 TEA表明，生物炭最低售價（Minimum Selling Price, MSP）為 1.42 USD kg⁻¹，而製程對原料價格與規模經濟高度敏感。納入碳權收益後，MSP 可降至 1.10 USD kg⁻¹；若碳價提高至極端情境，則MSP理論上可能降至0；但在主產品分配情境下，反而會使成本上升。因此，碳價與分配情境皆是木質素HTC經濟性的關鍵要素。 綜上所述，本研究首度以嚴謹模型、程序模擬、 LCA以及TEA，指出木質素 HTC 在適當分配法與碳價條件下兼具環境與經濟可行性，為木質素HTC領域奠定堅實的基礎。

Biochar has emerged as one of the most promising carbon removal strategy climate change mitigation. Hydrothermal carbonization （HTC） can convert wet biomass directly into biochar and is particularly suitable for wet feedstocks such as lignin. However, the complexity of HTC mechanism has confined existing kinetic studies to lumped models that cannot predict detailed product properties. To bridge the gap, this work develops a rigorous model for lignin HTC using a model-compound approach. The model can predict the yields and elemental composition of biochar as well as the composition of liquid and gaseous products. It is further integrated with process design and optimization, providing the foundation for a comprehensive life cycle assessment （LCA） and techno-economic analysis （TEA） of the lignin HTC process. The LCA shows that the global warming of the biochar is –2.05 kg CO₂ eq per kg biochar. However, the result is highly sensitive to the lignin allocation scenario. If treating lignin extraction as an added benefit to existing process （“benefit allocation”） deepens the advantage to –2.49 kg CO₂ eq per kg. By contrast, when all upstream impacts of extraction are attributed to lignin because lignin is considered the main product （“main product allocation”）, the result swings to +1.87 kg CO₂ eq per kg, turning the biochar into a net emission source. The TEA indicates a minimum selling price （MSP） of 1.42 USD per kg, driven primarily by feedstock cost and plant scale. Monetizing carbon credits at 0.155 USD per kg CO₂ lowers the MSP to 1.10 USD per kg, and an extreme carbon price could theoretically drive the MSP to zero; nevertheless, under the “main product” allocation, the carbon credits raise rather than reduce costs, highlighting the coupled influence of carbon price and allocation scenario on economic viability. Overall, this work is the first to integrate a rigorous model with process simulation, LCA, and TEA, demonstrating that lignin HTC can achieve negative carbon emissions and competitive economics under appropriate allocation and carbon-price conditions, thereby providing a solid foundation for future research and development in lignin HTC.