Kinetics of CO2 gasification for coals of different ranks under oxy-combustion conditions

It is widely recognized that char-CO2 gasification reactions present in oxy-fuel processes may considerably alter the overall reaction rates in comparison to those attained in conventional combustion. In this paper, the results of an experimental study on the gasification kinetics of three coals of different ranks (from anthracite to lignite) under realistic conditions in an entrained flow reactor are presented; the work included also devolatilization and char oxidation tests in the same experimental facility. A particle combustion model, based on apparent kinetics, was used to derive kinetic parameters for each reaction with coal chars. A clear gradation of the char reactivity towards both O2 and CO2 with the coal rank was found. The same models were used to assess the relevance of gasification in oxy-combustion conditions; gasification may account for as much as 20% of the overall char consumption rate in the combustion of a low rank coal at low O2, high CO2 concentrations and offset (and exceed) the loss in the oxidation rate due to the change in diffusivity. These results clearly indicate the need to carefully consider the inclusion of gasification in the simulation of coal-fired boilers depending on the operating conditions.