A survey of greenhouse gases production in central European lignites

Highlights: • The GWP of the gases released by xylitic lignite was the lowest among the samples. • Extraction of the detritic lignite may reduce GHG emissions from the abandoned mines. • Detritic lignites show the potential to serve as a source of coalbed methane. Due to changes in the energy market, it is projected that lignite excavation will be reduced in the near future. Cessation of exploitation is associated with restitution of natural water conditions and flooding of the resources left in the mines. Flooded lignite mines are a potential source of greenhouse gases (GHG) (CH_4, CO₂ and N₂O), which should be monitored due to growing environmental concerns. Here, we aim to recognize GHG release from the lignites collected from the main deposits of Poland, Slovenia and Serbia. GHG production was studied along with a range of physical and chemical parameters that are crucial for microbial growth and activity. The microcosm experiments showed that the main gas emitted from the lignites was carbon dioxide. Daily CO₂ production was highly variable. The highest values were recorded for detroxylitic lignite collected from the Konin deposit (402.05 nmol CO₂ g^-1 day^-1) while the lowest were for the Kolubara xylitic lignite (19.64 nmol CO₂ g^-1 day^-1). Methane production was much lower and ranged from nearly zero to 66.75 nmol g dry mass^-1 d^-1. Nitrous oxide production was not detected. It was found that CO₂ production, being a general measure of microbial activity, was positively affected by NO₃^- concentration and redox potential. With respect to methane formation, the lower atmospheric oxygen exposure of the sample from the Velenje underground mine compared to the samples from the opencast mines has been identified as a possible cause of the high methane production. The overall global warming potential (GWP) of the gases released by xylitic lignite was lowest among the samples. Preferential extraction of the detritic lignites is suggested as a means to reduce GHG emissions from the abandoned lignite mines.