Deep seam and minesoil carbon sequestration potential of the South Wales Coalfield, UK.

Combustion of coal for energy generation has been a significant contributor to increased concentrations of atmospheric carbon dioxide. It is of interest to evaluate the potential of former coalfields for mitigating these increases by carbon sequestration and to compare different options to achieving this end. Here, carbon sequestration in residual coal seams and through reclamation of spoil tips is compared, and their carbon dioxide storage potential in the South Wales Coalfield estimated. Coal seam sequestration estimates come from an established methodology and consider the total unmined coal resource below 500 m deep with potential for carbon sequestration. The most likely effective deep seam storage capacity is 104.9 Mt carbon dioxide, taking account of reservoir conditions and engineering factors. Whilst many spoil tips in South Wales have been reclaimed, the focus has not been on carbon sequestration potential. Estimates of minesoil restoration sequestration capacity were based on a survey of restored minesoil and vegetation carbon stocks, mainly on sites 20-30 years after restoration; data from this survey were then extrapolated to the coalfield as a whole. Minesoil storage is estimated at 1.5 or 2.5 Mt (+2.2 Mt in tree biomass) carbon dioxide based on average grassland or woodland measurements, respectively; modelled data predicted equilibrium values of 2.9 and 2.6 Mt carbon dioxide respectively in grassland or woodland minesoils. If all sites achieved close to the maximum capacity in their land use class, minesoil storage capacity would increase to 2.1 or 3.9 Mt carbon dioxide, respectively. Combining the best woodland minesoil and standing biomass values, sequestration capacity increases to 7.2 Mt carbon dioxide. The wider social, economic, environmental and regulatory constraints to achieving this sequestration for each approach are discussed. Coal seam sequestration has a much higher capacity but sequestration in mine sites is less costly and has fewer regulatory constraints. Findings indicate a significant combined potential for carbon sequestration in the South Wales Coalfield and highlight challenges in achieving this potential. On a global scale, ex-coalfield sequestration could contribute to broader efforts to mitigate emissions.