Decomposition of key minerals in coal gangues during combustion in O2/N2 and O2/CO2 atmospheres. (5th February 2019) - Zhang, Yuanyuan
Highlights: Coal gangues rich in kaolinite or dolomite differed in their mass loss behaviour. Kaolinite decomposition and coal gangue combustion took place concurrently. Dolomite decomposition occurred after completion of coal gangue combustion. O2 and CO2 strongly affected dolomite decomposition behaviour. Kaolinite decomposition behaviour was not influenced by the gas atmosphere. Abstract: Thermal decomposition characteristics of kaolinite and dolomite during combustion of coal gangues in different O2 /N2 and O2 /CO2 mixtures, simulating normal air and oxy-fuel combustion atmospheres, were examined using thermogravimetric analysis (TGA), with a focus on the effect of O2 and CO2 concentrations. A kaolinite-rich coal gangue and a dolomite-rich coal gangue were studied along with their low-temperature ashes prepared by oxidising the organic matter in an oxygen plasma asher thus leaving the minerals in their original form. The TGA experiments were carried out in O2 /N2 and O2 /CO2 atmospheres with varying O2 concentration and from room temperature to 950 °C at a heating rate of 10 °C min −1 . The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of these samples were examined and compared so as to reveal key thermal events and the on-set temperatures of their occurrence. The results showed that in both pyrolysis and O2 /N2 atmospheres, the mass loss of the kaolinite-rich coal gangue occurred in the temperature range of 300–600 °C in one single stage, but the mass loss of the dolomite-rich coal gangue occurred at 300–730 °C in two distinct stages. In the O2 /CO2 atmospheres, the mass loss of the kaolinite-rich coal gangue retained the one stage behaviour, however, the dolomite-rich coal gangue featured a three-stage mass loss event. Compared to their low-temperature ashes, the temperature range of mass loss of the kaolinite-rich coal gangue was consistent with that of its low-temperature ash in N2, O2 /N2 and O2 /CO2 atmospheres. The temperature range of mass loss of the dolomite-rich coal gangue in the second stage corresponded to that of the low-temperature ash in N2 and O2 /N2 atmospheres and the temperature range of mass losses in the second and third stages corresponded to that of the low-temperature ash in O2 /CO2 atmospheres. These observations indicate that kaolinite decomposed concurrently with the host coal gangue combustion while dolomite decomposed after the combustion. In addition, the decomposition of dolomite changed from a one-step reaction to a two-step reaction while kaolinite decomposition remained unchanged as the gas environment was changed from O2 /N2 atmospheres to O2 /CO2 atmospheres. When oxygen concentration increased from 2% to 21% in the O2 /N2 atmosphere and 20% to 80% in the O2 /CO2 atmosphere, the mass loss curves of the two coal gangues moved towards lower temperatures and the ignition temperature decreased. However, the mass loss of the kaolinite-rich coal gangue always showed a single peak characteristic regardless of the oxygen concentration while the mass loss of the dolomite-rich coal gangue was affected significantly by decreasing the O2 concentration in O2 /N2 atmospheres and increasing the CO2 concentration in O2 /CO2 atmospheres. This was attributed to the partial overlap of carbon combustion and dolomite decomposition in the O2 /N2 atmosphere with low O2 concentrations and the suppression of dolomite decomposition in the O2 /CO2 atmosphere.