Processes for metal-to-char encapsulation

The research project deals with the conversion of biomass from plants to biochar. Thereby heavy metals which were taken up from the soil by the plants and are therefore contained in the plant parts shall be transferred to the biochar. The biochar will finally be subjected to treatment in order to reduce the reactivity, which is the tendency of biochar and heavy metals to react with other chemical compounds. This so-called encapsulation allows for a storage of the final product in underground repositories, that is safe from environmental impacts Thus, two important environmental issues can be addressed at once. First, soils with a locally high concentration of heavy metals can be purified. This contamination with heavy metals might be due to historical exploitation of these metals nearby. In their polluted state, these grounds are not suitable for agriculture, but they can be decontaminated through the cultivation of plants, that take up the heavy metals along with nutrients. In order to avoid further pollution of the environment it has to be made sure that with the disposal of the plants after harvesting no heavy metals end up in the environment anew. For that purpose, biomass is to be converted to biochar which should still contain most of the heavy metals and the chars is then prepared for long-term underground storage through subsequent encapsulation. This leads to the second big impact, which is carbon capture and sequestration or CCS. CCS is a broad term for techniques, that take CO2 out of the atmosphere and store it underground. This leads to a net negative CO2 -balance, which is an important step towards fighting climate change. The CO2 , which is consumed by the plants via photosynthesis, is not released to the atmosphere, but kept in solid state in the biochar. Thereby the long-term storage of the biochar combines heavy metal and carbon reduction above ground. Hence this project will also contribute to the fight against the climate change.

PROMETHEIA investigated how biomass grown on contaminated land can be converted into stable biochar for long-term carbon storage and heavy-metal immobilization. The project addressed an important environmental challenge: contaminated land is often unsuitable for food production, but it can potentially be used to grow biomass that removes carbon dioxide from the atmosphere while also helping to recover polluted areas. The project studied a three-step thermochemical concept. First, contaminated biomass is converted by pyrolysis into biochar and tar-containing gases. Second, the char is partially gasified to create an activated porous structure while keeping heavy-metal compounds mainly in the solid phase. Third, tar compounds are converted on the surface of the activated char to form a coke-derived layer. This layer is intended to reduce the reactivity of the biochar and improve its environmental stability during storage. The research combined literature analysis, experimental work, reactor development, advanced material characterization, and collaboration between TU Wien, Silesian University of Technology, and the University of Ljubljana. Fixed-bed experiments were used first to screen key process conditions. Later, the BIOTA fluidized-bed reactor at TU Wien was commissioned and used to perform more representative pyrolysis and gasification experiments. The project examined the influence of feedstock type, temperature, gasification atmosphere, reaction time, and reactor type on tar formation, syngas composition, pore development, and heavy-metal retention. The resulting biochars and activated biochars were analyzed using methods such as elemental analysis, BET surface area and porosity measurements, SEM, FTIR, XPS, Raman spectroscopy, TGA, and ICP-OES. The results improved the understanding of how contaminated biomass can be converted into functional carbon materials and how process conditions affect metal retention, carbon stability, tar conversion, and adsorption or catalytic properties. PROMETHEIA generated several journal publications, submitted manuscripts and conference presentations that support future environmental assessment of the process chain. Overall, the project provided a strong scientific basis for further research on negative-emission technologies, circular carbon management, contaminated biomass valorization, and the safe storage of carbon and heavy metals in passivated biochar.