Effects of Ca/Na compounds on coal gasification reactivity and char characteristics in H2O/CO2 mixtures

Experiments were conducted to investigate the coal gasification performance and char characteristics affected by Ca/Na compounds. Firstly, the gasification reactivities of Ca-loaded coal, Na-loaded coal, and Ca/Na-loaded coal were compared by TGA in different gasifying agents including 100% H2O, 100% CO2, and 66.7% H2O + 33.3% CO2. Then, partially-gasified chars of Ca/Na-loaded coal with carbon conversion of 50% were prepared in different gasifying agents and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen adsorption measurement. It was found that Ca together with Na showed a combined effect towards the gasification reactivity which was the most obvious in H2O/CO2 mixtures, and Na could promote the synergistic effect caused by Ca catalysis at higher temperature (900 °C). Na can inhibit the agglomeration of CaO due to its excellent mobility and the formation of a eutectic mixture Na2Ca(CO3)2. The marked improvement of CaO dispersion on the partially-gasified char in H2O/CO2 mixtures results in the significant combined effect between Ca and Na and improves the synergistic effect at higher temperature. Moreover, the combined effect between Ca and Na during catalytic gasification can not only facilitate the gasification reactivity but also promote the development of pore structures for the gasified char.