Transformation of alkali and alkaline earth metals in low rank coal during gasification

Transformation of alkali and alkaline earth metals (AAEM) in low rank coals during gasification was examined by combining computer-controlled scanning electron microscopy (CCSEM) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Two sub-bituminous coals were pyrolyzed at 1500 °C using a drop tube furnace, and the resultant chars were then gasified in CO₂ atmosphere at the same temperature. Total amounts of AAEM species in the raw coals and the chars were determined by ICP-AES. Minerals in the raw coals and ash particles in the chars were analyzed by CCSEM.AAEM species were mainly present in the raw coals as dispersed species, organically associated cations or fine mineral particles (<1 μm), which cannot be quantified by CCSEM. It was found that the dispersed Ca species were first converted into fine ash particles upon the devolatilization and then most of the particles interacted with inherent clay minerals to form complex aluminosilicates. In the case of Na and K, the dispersed species mostly vaporized and the interaction with inherent minerals was not observed.