Interactions between CaCO3 and low-rank coals were examined, and the steam gasification of the resulting Ca-loaded coals was carried out at 973 K with a thermobalance. Chemical analysis and FT-IR spectra show that CaCO3 can react readily with COOH groups to form ion-exchanged Ca and CO2 when mixed with brown coal in water at room temperature. The extent of the exchange is dependent on the crystalline form of CaCO3, and higher for aragonite naturally present in seashells and coral reef than for calcite from limestone. The FT-IR spectra reveal that ion-exchange reactions also proceed during kneading CaCO3 with low-rank coals. The exchanged Ca promotes gasification and achieves 40-60-fold rate enhancement for brown coal with a lower content of inherent minerals. Catalyst effectiveness of kneaded CaCO3 depends on the coal type, in other words, the extent of ion exchange.