Comparison of the gasification of coals and their chars with CO2 based on the formation kinetics of gaseous products

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The aim of the study was to analyse the kinetics of coal and char gasification in an atmosphere of carbon dioxide. Samples of two Polish coals and their chars were gasified under isothermal conditions at the temperatures of 850, 900 and 950°C and under the pressure of 0.1, 0.6 and 1.1 MPa. Based on the obtained results, curves of the formation rate of gaseous products of gasification such as carbon monoxide, hydrogen and methane were developed and yields of products were evaluated. The influence of temperature, pressure and type of sample on the kinetics of carbon monoxide formation was determined and the reaction rate constant of CO formation for each sample as well as the kinetic parameters (pre-exponential factor and activation energy) were calculated. Firstly, the activation energies both for coal and char gasification with CO2 were calculated based on the isoconversional method. Next, four kinetic models: the n-order volumetric model (NVM), the first order volumetric model (FVM), the grain model (GM) and the random pore model (RPM) were used for the calculation of kinetic parameters. The kinetic curves of gaseous products formation clearly indicate the dominance of the reaction of carbon monoxide formation and its dependence on temperature. The yield of carbon monoxide grows with the increase of the coal rank of the examined sample. The gasification of coals with CO2 is significantly different from the gasification of their chars derived by the ex situ method. Coals are more reactive than chars, and thus, the gasification of coals progresses much faster and, consequently, with a higher formation rate of gaseous products. The analysed coals are characterised by lower values of the parameters A and E compared to their chars. The increasing pressure causes the activation energy decrease, which confirms the positive effect of pressure on reactivity.