The effect of using thermocouples on the char particle combustion in a fluidized bed reactor

The char temperature during combustion in a fluidized bed (FB) is often measured by thermocouples due to simplicity, and because it is assumed that the thermocouple effect on the movement of the char is negligible. It is also accepted that the combustion temperature of a char particle fluidized with an embedded thermocouple is similar to a freely fluidized particle. However, few publications have given evidence of this fact. In the present article this question is dealt with by comparing the evolution of conversion and temperature of fluidized char particles with and without an embedded thermocouple. Char from beech wood, sub-bituminous and bituminous coal of an initial diameter of 10 mm is burnt in two laboratory FBs; one 2-dimensional made of quartz with rectangular cross-section allowing visual observation and temperature tracking of the particles, and the other being 3-dimensional with 50 mm internal diameter. The surface temperature of the char particles is followed by a recently developed visual technique, based on pyrometry coupled with a digital camera. The consumption of char is evaluated by sampling particles from the bed at different stages of conversion, analyzing the density and size of the particle. It is found that, due to the greater resistance of the particle-thermocouple to the drag of the bed, the time that the char particle is in the bubble phase is 40% longer when the thermocouple is used, leading to higher combustion rate and temperature, and consequently, shorter burnout time. Moreover, the rotation of the char particle is restricted, provoking a non-homogeneous consumption of char, which enhances the size reduction of the particle along the direction perpendicular to the insertion of the hermocouple. Overall, these findings suggest that conclusions about char conversion by temperature measurements using thermocouples should be made with caution, since significant error might be committed under some operating conditions