Challenges in the production of livestock manure-derived biochar (LMB) for sustainable carbon neutrality: Evaluation of hazardous volatile organic compound (VOC) emission characteristics

Livestock manure-derived biochar presents an environmentally sustainable approach for managing livestock waste and achieving carbon neutrality through carbon sequestration. However, incomplete pyrolysis during biochar production may result in the retention of hazardous organic compounds, which can be emitted into the atmosphere during field application. In this study, biochar was produced at both lab- and pilot-scales under varying conditions, including manure type (cow and chicken), pyrolysis temperature (400°C and 700°C), and residence time (5–12 min). Emissions of volatile organic compounds (VOC) were evaluated across a temperature range of 35–280°C. Lab-scale results revealed that pyrolysis temperature had a more significant influence on VOC emissions than residence time. For example, total VOC concentrations measured at 50°C were 73.4 μg g^–1 for biochar produced at 400°C, compared to 4.96 μg g^–1 at 700°C. In a modeled field scenario, where 1 metric ton of pilot-scale biochar is mixed into soil (density 1.5 g cm^–3; application rate 5 wt%), predicted ambient total VOC concentrations at 2 m height ranged from 8740 to 52,513 μg m^–3. These levels far exceed the WHO's indoor air quality target of 250 μg m^–3, indicating substantial risk. Therefore, pyrolysis at or above 700°C and strict thermal management are essential for safe biochar production.