Gaseous compounds and efflorescences generated in self-heating coal-waste dumps — A case study from the Upper and Lower Silesian Coal Basins (Poland)

• Unexpected differences in gases emitted from self-heating coal waste dumps
• Relationship between gas composition and efflorescences blooming around vents
• Factors affecting gas products and efflorescence composition assessed
• Rare organic minerals found

Gases emitted in coal waste self-heating and efflorescences blooming around fissures of coal waste dumps of the Upper and Lower Silesian Coal Basins were investigated. Gas chromatography (GC-FID) and charcoal tubes were applied for gas emissions, gas chromatography–mass spectrometry (GC–MS) for organic fraction of blooms, and X-ray diffraction and scanning electron microscopy for inorganic efflorescences. The results show that coal waste dump gases contain a wide range of inorganic and organic compounds that are highly variable in their occurrence and concentrations, even in samples from the same dumps or the same coal basin. Chemical composition of self-heating products is related to source organic matter characteristics, composition and amount of coal waste material deposited on individual dumps, time and temperature range of the self-heating, composition and temperature of gas jets and condition variations at different depths. Carbon dioxide and carbon monoxide dominate in all gases, with additional components such as carbonyl sulphide, carbon disulphide, methane, light n-alkanes, n-alkenes, iso-alkanes, cyclic alkanes, chloroform, and benzene and its alkyl derivatives. Efflorescences blooming at vents and fissures at all sampling sites differ greatly in their phases, amounts, and chemical compositions, with organic blooms including phenanthrene (ravatite), phthalimide (kladnoite), and 9,10-antraquinone (hoelite) found only in one of the dumps. They are accompanied by salammoniac and native sulphur which are ubiquitous at a newly formed vents as in coal waste dumps with long time range of self-heating. Self-heating leads to the synthesis of new components such as unsaturated hydrocarbons, sulphides, chloroform, and phthalimide (kladnoite), for which formation mechanism is proposed in the paper.