Specific Features of Solid Fuels Combustion in Oxygen Atmosphere with Recirculation of CO2

Abstract

Aspects of coal combustion have been experimentally studied under oxyfuel conditions, one of the promising technologies for carbon capture and storage (CCS). Here, the thermogravimetric analysis (TGA) method was chosen as an experimental technique. Coal pyrolysis tests performed under an O₂/CO₂ atmosphere were compared with a conventional O₂/N₂ environment in terms of reaction rate and total volatile yield. Combustion of the resulting chars in the corresponding atmospheres revealed somewhat different combustion rates with a less vigorous reaction in the O₂/CO₂ medium. The two manipulated factors – namely, the inherently different char reactivities due to the different atmospheres they were obtained in and the different atmospheres of the actual combustion process – were distinguished by performing another series of tests with chars pyrolysed under identical conditions using a standard routine. These chars also showed a weaker reaction in O₂/CO₂ atmosphere, which was attributed to the lower binary diffusion coefficient of the O₂/CO₂ pair. The activity of the char – CO₂ gasification reaction in an O₂/CO₂ environment was also investigated and revealed some contribution of this reaction to the conversion process. This was particularly noticeable at temperatures above 750 °C and under an internal diffusional controlled regime (zone II), implying displacement of oxygen out of the char particle pore volume, which allowed free reaction of CO₂ on the developed pore surface. Non-isothermal kinetic analysis of the intrinsic kinetics of the oxidation reaction in O₂/CO₂ revealed no particular difference compared to the O₂/N₂ medium, at least when the char-CO₂ reaction was inhibited. The obtained data were used to develop a coal combustion model under O₂/CO₂ conditions, which was then incorporated as a combustion module into circulating fluidized bed (CFB) computation software.