Inside Hydrochar-Based Carbon Storage with Recoal

During Climate Week Zurich, our member, Recoal organized a first visit to their carbon removal storage site in the underground research facility at VersuchsStollen Hagerbach in Flums (VSH).  The excursion offered a rare opportunity to see what permanent carbon storage, specifically of hydrochar, can look like in practice. The visit focused on the HyLock project, a pilot system for storing carbon-rich hydrochar produced from wet biomass residues through hydrothermal carbonization (HTC).

What is Recoal?

Recoal is a Swiss startup founded in 2023 developing a Biomass Carbon Removal and Storage (BiCRS) pathway that transforms wet biomass residues into a stable, coal-like material called hydrochar, which is then stored permanently underground. Unlike many carbon removal approaches that require dry feedstocks or energy-intensive preprocessing, Recoal’s system is designed specifically for wet-process biomass, meaning there is no need to dry input material. This is a key advantage, as wet biomass is often underutilised waste from biogas and agricultural systems and has limited alternative uses.

How does the technology work?

Recoal’s process is based on three main steps:

1. Wet biomass as feedstock
   The system uses wet biomass residues (often with >30% moisture content), including by-products from biogas fermentation processes. These streams are typically difficult to use elsewhere, which reduces competition for resources.

Importantly, this creates a cascade use of biomass - first energy recovery via biogas, followed by carbon storage as a final sink.

1. Hydrothermal carbonization (HTC)
   Through heat and pressure, the wet biomass is converted into hydrochar, a carbon-rich, stable material similar in structure to natural coal formation processes. In contrast to hydrochar, biochar is a carbon-rich material produced by pyrolysis of dry biomass under oxygen-free, high-temperature conditions. 
2. Long-term underground storage
   The hydrochar is then sealed and stored underground in engineered caverns. Each batch is individually packaged and protected through engineered barrier systems before emplacement. At VSH, storage takes place in a ~1,500 m² cavern system (“Caterpillar cavern”), which will gradually be filled with Recoal’s batches over the duration of the pilot.

What does the Hylock Pilot look like?

Once the hydrochar is produced, how is it safely stored underground? The HyLock storage system is designed as a multi-barrier approach combining geological and engineered safety layers to make sure the hydrochar remains sealed underground. The cavern itself already provides natural protection due to limited groundwater flow and low ventilation, which is further reinforced by a low-permeability hydraulic barrier. In addition, each hydrochar batch is individually sealed in airtight packaging before deposition, and the site is equipped with continuous monitoring systems tracking groundwater conditions and gas diffusion to enable early detection of any anomalies.

As a research facility, VSH plays a key role in iteratively testing and refining the storage design under real subsurface conditions.

The HyLock pilot is planned as a minimum of 650tCO₂eq gross storage demonstration, representing the total physically stored carbon material before life-cycle deductions such as transport and production emissions. As it is a pilot, the final packing configuration and system boundaries may still be refined during implementation.

Indicatively, 1 m³ of compressed hydrochar corresponds to ~1 tCO₂eq (gross stored CO₂eq), with roughly 500 kg CO₂eq per large storage bag. 

The VersuchStollen Hagerbach site is a research and demonstration tunnel rather than a commercial storage facility. This makes it ideal for early-stage deployment:

• Controlled underground environment
• Strong subsurface engineering expertise
• Flexibility for experimental storage setups
• Ability to test monitoring systems under real conditions

For Recoal, it provides the opportunity to validate long-term storage concepts before scaling. Risks are considered manageable and include potential failure of individual packages or localised barrier issues. These are addressed through compartmentalised storage design, a minimum 30-year monitoring programme covering both the cavern and surrounding aquifers, and the possibility of retrieval and reprocessing in a worst-case scenario.

From Idea to Start up: 

Recoal’s progress from concept to pilot has been enabled by a combination of funding, science, and ecosystem support:

• Funding and institutional support
  Early-stage and pilot development has been supported by the Migros Pioneer Fund, Swiss Climate Foundation and the City of Zurich (KlimUp program), as well as Innosuisse funding for the HyLock pilot. These contributions have been essential to move from concept development to real-world demonstration. 
• Strong scientific ecosystem
  Close collaboration with ETH Zurich, TU Delft, ZHAW, FHNW, OST, and Empa provides both technical depth and applied research capacity, ensuring the system is grounded in robust scientific validation. VersuchsStollen Hagerbach is also recognized as Research Infrastructure of National Importance.
• Verification and integrity frameworks
  Partnerships such as Rainbow Standard support methodology development and MRV design, helping ensure credibility and transparency of the carbon removal approach. 
• Swiss ecosystem advantages
  Switzerland combines clear net-zero policy signals, strong corporate CDR demand, and proximity to financial and sustainability decision-makers, creating a supportive environment for early-scale deployment. 

Looking ahead: next steps

While already in the initiation phase of the Hylock-pilot, recoal will start their hydrochar storage in mid-2026. The storage cavern is expected to reach full capacity by the end of 2027 or early 2028. Following the pilot phase, the focus will shift to commercial-scale plants with a target capacity of around 5,000 tCO₂ per year per facility, and a long-term ambition of scaling up to 1 Mt CO₂ per year across multiple sites. Future development will also include larger single-deposit storage systems and diversification of storage locations as the technology moves from demonstration to industrial deployment.