Impact of calcium on the synergistic effect for the reactivity of coal char gasification in H2O/CO2 mixtures

- Mechanism of synergistic effect during co-gasification was revealed.
- Catalytic effect of CaO facilitates the synergistic effect.
- H2O can make CaO smaller in size and higher in dispersity.
- Interaction of H2O and CO2 with Ca participation promotes the pore development.
- Sintering of CaO at higher temperature causes the weak synergistic effect.

In this paper, the effect of calcium on coal gasification in mixture agents containing both H2O and CO2 was investigated by comparing the gasification behaviors of the demineralized coal and calcium-loaded coal using Wucaiwan (WCW), a calcium-enriched low rank coal, as the raw material. TG analysis of pure CaCO3 was conducted at different conditions to reveal the catalytic mechanism of calcium. The properties of the chars as well as partially-gasified chars were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Moreover, pore structures of partially-gasified chars were obtained by BET analysis. It was found that under calcium catalysis, the normalized gasification rate K increased more than 20 times in H2O/CO2 mixtures, and this value was much higher than that in pure H2O and pure CO2. The results suggest that under the influence of calcium, there is a synergistic effect between H2O and CO2 during co-gasification when temperature is lower than 900 °C. The reason for this enhanced gasification is the fact that CaO, the main form of calcium mineral during coal catalytic gasification, can catalyze coal gasification in H2O/CO2 mixtures much more effectively and create a strong synergistic effect. H2O can decrease the size of the CaO particles and increase their dispersity, thus facilitating CO2 adsorption on CaO. Furthermore, with CaO participation, interaction between H2O and CO2 can allow both of them to enter the interstices or pores of the coal char more rapidly, which accelerates the reaction rate and results in the synergistic effect. The synergistic effect becomes weaker as the temperature rises, which can be attributed to reduced adsorption of CO2 on CaO and the sintering of CaO at higher temperature.