Experimental study on oxy-fuel combustion behavior of lignite char and carbon transfer mechanism with isotopic tracing method

In this work, the conventional combustion in O-2/Ar and O-2/CO2 atmospheres at temperatures of 700-1000 degrees C and isotopic tracing experiments at 700-800 degrees C were conducted respectively in a micro fluidized bed reactor to clarify the conversion characteristics of lignite char and influence mechanism of elevated CO2 concentrations. The isotopic method employing (COO)-O-18-O-18 provides an intuitive insight into the CO2 gasification behavior by tracking the evolution of the O-18 atom throughout the oxy-fuel combustion. The results reveal that char conversion rate significantly reduces when the atmosphere is switched from O-2/Ar to O-2/CO2 at low temperature range (e.g. 700-800 degrees C), primarily caused by thermo-physical properties of CO2. However, at higher temperature range (e.g. 950-1000 degrees C), the C-O-2 reaction is limited by the external O-2 diffusion and the C-CO2 reaction contributes to the carbon consumption, resulting in a higher combustion rate, especially at O-2-deficient conditions. The presence of the atomic O-18 in the product (COO)-O-18-O-16 originating from (COO)-O-18-O-18 confirms that CO2 is indeed involved in the whole oxy-fuel combustion process and the overall reaction could be described as C + (COO)-O-18-O-18+(OO)-O-16-O-16 -> 2C(18)O(16)O. Besides, total contributions to carbon conversion by the C-CO2 reaction decrease as O2 concentrations and particle sizes increasing.