Decomposition of key minerals in coal gangues during combustion in O2/N2 and O2/CO2 atmospheres

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.