Feasibility of subsurface storage of hydrochar in the Netherlands as carbon dioxide removal technique

• Evaluates hydrochar-based CDR via subsurface storage as a novel mitigation pathway. • Assesses seven Dutch storage options across technical, environmental, and stability criteria. • Salt caverns and lightweight fills show promise; quarry lakes with appropriate safeguards. • Highlights key knowledge gaps on hydrochar stability under subsurface conditions. Hydrothermal carbonisation enables the conversion of wet biomass into hydrochar, a carbon-rich solid with potential for durable carbon dioxide removal (CDR). While hydrochar has been studied extensively for topics as soil application or wastewater treatment, its role in subsurface storage remains underexplored. This study examines the feasibility of hydrochar-based biomass carbon removal and storage (BiCRS) in the Netherlands, where abundant wet biomass and well-developed subsurface infrastructure offer a promising deployment context. We characterise the chemical and mechanical properties of manure-derived hydrochar and evaluate seven potential storage configurations, from abandoned coal mines to quarry lakes and lightweight fill applications, based on technical feasibility, environmental risk, and long-term containment. Our findings identify two priority pathways: storage in salt caverns and use as lightweight filling material for land elevation. A third pathway, storage in sand quarry lakes, also holds potential, though additional safeguards and site-specific assessments are needed to ensure environmental integrity and carbon retention. Hydrochar’s compatibility with wet, low-value feedstocks and potential for decentralised implementation position it as a flexible addition to the CDR portfolio. However, realising this potential will depend on further field validation, material optimisation, and regulatory alignment. Key uncertainties remain regarding long-term degradation, leachate behaviour, and performance under representative subsurface conditions. This study highlights hydrochar as a scalable, technically viable CDR approach. If supported by robust containment, monitoring, and governance frameworks, it could play a meaningful role in national and regional climate mitigation strategies.