Organic compounds pose a risk for thermal storage in abandoned coal mines

The development of renewable energies and the sustainable utilisation of geo-resources is evident in the increasing interest in mine water utilisation. In the densely populated regions of former coal mining areas, flooded mine structures present a promising opportunity for seasonal heat storage called mine thermal energy storage MTES. In addition to the general risks associated with post-mining utilisation, it is essential to assess the potential hazards posed by contaminants that may be remobilised through this geotechnology. Hard coal naturally contains contaminants such as polycyclic aromatic hydrocarbons (PAHs) and NSO-heterocycles, which have been detected in mine water. The utilisation of coal mines as thermal energy storage facilities leads to significant heating of the mine water (up to 80°C), which can enhance the solubility and mobilisation of contaminants into the water. However, to date, no comprehensive understanding exists regarding the mobilisation potential of these contaminants from coal mines at varying temperatures.

In this contribution, we present initial systematic flow-through experiments using columns filled with different coal types at various temperatures demonstrating that contaminant mobilisation, after an initial first flush, is primarily dominated by diffusion processes at the phase interface. Differences in the mobilisation of PAHs between the various coal types and at various temperatures are discussed.

Using numerical simulations, we demonstrate that the compound concentrations grow exponentially over the runtime of the MTES system due to the growing mass of coal being thermally stimulated. High temperature storage can lead to a short production time until the regulatory limit for PAH is reached. Without regulatory action an MTES in coal mines might not be economically.  We highlight that depending on mine-specific factors countermeasures need to be installed to contain the potential risk to the economic feasibility of such a storage system.  A reduction of the pollutants trough remediation techniques might be possible to enhance the lifetime of the MTES system, if natural attenuation through micro-biological activity is not sufficient.