Forms of potassium in coal and its combustion products

High resolution X-ray absorption spectroscopy utilizing synchrotron radiation as a light source was used to probe the bonding and structure of potassium in coal and coal-derived products. The potassium sites in coals of various ranks and in the products of thermochemical processing were identified by comparison of unknown spectra with those of a wide range of selected potassium model compounds and minerals. For most coals examined, potassium was found to occur in a site in illite, a layered clay related to muscovite. Such coals included subbutuminous, high and low volatile bituminous, cannel and anthracite types. The lower rank subbituminous and lignite coals show other forms of alkali, where potassium appears to be in a noncrystalline environment. Examination of the near-edge spectra fails to reveal similarities between potassium in low-rank coal and in any organically bonded potassium model compounds such as carboxylic, phenolic, benzoic, phthalimide or intercalated groups. It is possible that the exchangeable potassium is still inorganically associated on disordered clay surfaces, though this remains equivocal. Thermal treatment of potassium-bearing phases inherent to coals generally leads to formation of a potassium aluminosilicate glass. Samples include those obtained from flash pyrolysis, gasification, laboratory combustion and a large scale pressurized fluidized-bed combustor including bed material, scrubbing hot cyclones, and quench-cooled pins for vapour phase deposition. No potassium sulphates were observed in the deposits.