Hydrochar and biochar from dairy manure as stable carbon amendments for greenhouse gas mitigation and carbon sequestration in concentrated animal feeding operations

Abstract Conventional manure management in concentrated animal feeding operations (CAFOs) poses a significant environmental challenge due to substantial greenhouse gas (GHG) emissions, particularly methane from anaerobic storage systems. Thermochemical conversion of dairy manure into hydrochar and biochar via hydrothermal carbonization (HTC) and pyrolysis, respectively, offers a promising alternative to reduce these emissions while improving carbon sequestration. In this study, dairy manure was treated using HTC (180, 220, and 260 °C) and pyrolysis (400, 600, and 800 °C) to produce hydrochars and biochars, respectively. Results show that despite increased carbon stability at higher treatment temperatures, as indicated by increasing fixed carbon content and decreasing H/C and O/C atomic ratios, the carbon sequestration potential decreased, largely due to reduced char yields. However, hydrochars had higher C/N ratios, suggesting reduced susceptibility to initial decomposition. Carbon sequestration potential peaked for biochar produced at 400 °C (32.79% ± 1.28%) and for hydrochar at 180 °C (22.11% ± 1.17%). When integrated into screening-level, model-based environmental impact assessment focusing on GHG emissions and carbon sequestration potential for two CAFOs, HTC and pyrolysis systems reduced net GHG emissions and produced modeled net carbon sequestration under the assumptions used. Compared to conventional manure management systems, which included lagoon storage, land application, or anaerobic digester, HTC at 180 °C to 260 °C resulted in net sequestration of −25 543 to −20 641 MT CO 2 -eq/year for, while pyrolysis at 400 °C to 800 °C achieved −38 553 to −24 723 MT CO 2 -eq/year for a 1500-head CAFO scenario.