The influence of CO2 gas concentration on the char temperature and conversion during oxy-fuel combustion in a fluidized bed

In spite of the extensive theoretical and experimental work carried out on coal/char oxy-combustion in a fluidized bed (FB), the effect of changing the atmosphere from O2/N2 to O2/CO2 for a high O2 concentrations is not entirely understood. In this work, experiments with single char particles are conducted in a bi-dimensional FB at 800 and 850 °C, varying the O2 concentration from 11 to 50%v/v in N2 or CO2. The FB reactor has a quartz window for visual observation, allowing the measurement of temperature and tracking the char conversion process by pyrometry with a digital camera. The method is shown to overcome the inherent limitations of other methods used in FB, such as thermocouples or pyrometry with an optical probe. Results indicate that the transfer of O2 from the bulk gas of the bed to the surface of a char particle controls the overall rate of char conversion in O2/N2 and in O2/CO2. In the latter gas mixture, the carbon consumption by gasification is significant even at a relatively low char temperature (850 °C). This additional carbon consumption makes the apparent char consumption rate in both atmospheres roughly equal (at the same O2 concentration) for char temperatures below 925 °C, and higher in O2/CO2 than in O2/N2 for char temperatures above 925 °C. Moreover, during the time in which the char stays in the emulsion phase, its temperature is roughly the same in both atmospheres, but when the char is in the bubble or splash zone its temperature is much higher than that in the emulsion phase. As a result, the difference in char conversion rate, observed in both atmospheres, is mainly controlled by the time in which the char particle is out of the emulsion phase. These results underline the importance of paying attention to the movement of a char particle through the different phases of the bed in order to improve the understanding of the oxy-fuel behavior in FB.