Impact of using thermocouples to measure char particle temperature in a fluidized bed combustor

Circulating fluidized bed can be applied in several methods of CO₂ capture. In oxy-combustion the fuel particle temperature can peak in the regions near the entrance with high oxygen concentration, being a critical factor. To study this aspect, techniques to measure fuel particle temperature have been developed mainly based of thermocouples, where it is usually assumed that the thermocouple does not affect the movement of the fuel particle, although there is no rigorous evidence. In the present paper this aspect is studied by comparing the temperature of char particles with and without an embedded thermocouple (with 0.25 and 0.5 mm sheath diameters). Char particles (10 mm) are burnt in a laboratory fluidized bed made in quartz, allowing visual observation of the particles. The surface temperature is measured by pyrometry coupled to a digital camera at the same time as the particle's temperature is recorded by a thermocouple. The temperature of a char particle fluidized with an embedded thermocouple is shown to be higher than that of a freely fluidized char particle, and its burnout time is shorter. This is because the fluidization with a thermocouple makes the char particle sink in the bed, increasing its residence time in the bubble phase compared to the freely fluidized particle. Besides, this work shows how the pyrometry technique is able to track the size and movement of a particle and its surface temperature gradients during conversion, improving the data collected with this method as compared to other measurement techniques.