K2CO3 catalyzed CO2 gasification of ash-free coal. Interactions of the catalyst with carbon in N2 and CO2 atmosphere

The interactions of K2CO3 with ash-free coal in N2 or CO2 atmospheres were studied at 700 °C by thermogravimetry and in situ X-ray diffraction. Upon heating in a N2 atmosphere, K2CO3 influenced the pyrolysis of the ash-free coal by reducing the temperature from 320 to 240 °C at which the maximum rate of devolatilization occurred. At 650 °C, the devolatilization of ash-free coal was finished. However, ash-free coal mixed with K2CO3 exhibited a further weight decrease due to the release of CO from the carbonate while heating in N2 at 700 °C. In this step, the potassium catalyst was reduced to a –COK and/or –CK surface complex, which might be the active species in the catalytic gasification process. In situ X-ray diffraction analysis also suggested that K2CO3 does not exist in inert gas (N2) and gasification (CO2) atmospheres at 700 °C. At this temperature, the evaporation of potassium is negligible. A faster CO2 gasification rate was achieved by holding the sample (ash-free coal mixed with K2CO3) for 150 min at 700 °C in N2 prior to switching to the reaction gas, compared with no holding time. This result indicates that catalyst reduction is necessary for a fast char conversion and also shows that the reaction gas (i.e., CO2) itself inhibits the reduction process.