Release of Ca during coal pyrolysis and char gasification in H2O, CO2 and their mixtures

The release characteristics of Ca during a Chinese bituminous coal pyrolysis and char gasification in different gasifying agents was studied. Residual chars with different carbon conversion levels were obtained by gasifying in a quartz fixed-bed reactor under H2O, CO2, and H2O/CO2 atmospheres at 800-1000 °C. Sequential chemical extraction and microwave digestion were used to determine the modes of occurrence of Ca in chars. X-ray diffraction and scanning electron microscopy were used to analyze the ash composition and the morphology of minerals. The contents of Ca in chars were measured by atomic absorption spectroscopy. The results indicate that water-soluble form and HCl-soluble form Ca transforms mainly into gas phase during pyrolysis. The release ratio of Ca at 800 °C under different atmospheres is H2O/CO2 >H2O > CO2. The main reason is that minerals are more prone to sintering in CO2 atmosphere. The interaction of H2O and CO2 can expand pores, which precipitate the minerals to the char surface and release into the gas phase. Increasing temperature induces the collapse of the pore structure which changed the release rule of Ca. The composition of the ash obtained from gasification at 1000 °C in CO2, H2O and H2O/CO2 are nearly same.