A systematic assessment of environmental impacts and techno-economic benefits of solid digestate conversion to biochar: a case study of pig farm digestate in Taiwan

Abstract

Digestate, the solid by‑product of anaerobic digestion, can cause nutrient leaching, greenhouse‑gas release and heavy‑metal accumulation. Slow pyrolysis offers a promising route for upgrading digestate into biochar, yet its high energy demand and uncertain economics must be carefully weighed. This study evaluates the functional unit of 1 t solid digestate, the conversion of digestate to biochar at a pig farm in Pingtung, Taiwan, and compares the results with composting and incineration. Life‑Cycle Assessment and Techno‑Economic Assessment covered five scenarios: composting (S1); solid digestate pyrolysis at 500 °C (S2), 600 °C (S3) and 700 °C (S4); and incineration (S5). Because S2 required the least process energy, it yielded the lowest global-warming potential of 67.7 kg CO₂-eq t⁻¹. In contrast, S4 produced higher-value biochar and showed the strongest economic performance, with a positive Net Present Value of USD 371,480 and a payback period of 11 year under the scenario without carbon fees. Sensitivity tests (± 20% capital expenditure, operating expenditure and benefits) confirmed S4’s economic resilience, while S2–S3 were vulnerable to market shifts. A Technique for Order Preference by Similarity to an Ideal Solution multi-criterion ranking placed S4 first under the stakeholder-derived weighting scheme, mainly because of its stronger economic performance and product value. These results suggest that digestate-to-biochar conversion can improve digestate management, but the preferred pyrolysis temperature depends on the decision priority. In this case, 500 °C showed stronger environmental performance, whereas 700 °C provided greater economic potential when high-value biochar use and energy recovery were assumed.