Experimental study on secondary air mixing along the bed height in a CFB with a multi-tracer-gas method

A multi-tracer-gas method was proposed to study the secondary air (SA) mixing along the bed height in a circulating fluidized bed (CFB) using carbon monoxide (CO), oxygen (O2), and carbon dioxide (CO2) as tracer gases. Experiments were carried out on a cold CFB test rig with a cross-section of 0.42 m×0.73 m and a height of 5.50 m. The effects of superficial velocity, SA ratio, bed inventory, and particle diameter on the SA mixing were investigated. The results indicate that there are some differences in the measurement results obtained using different tracer gases, wherein the deviation between CO and CO2 ranges from 42% to 66%, and that between O2 and CO2 ranges from 45% to 71% in the lower part of the fluidized bed. However, these differences become less pronounced as the bed height increased. Besides, the high solid concentration and fine particle diameter in the CFB may weaken the difference. The measurement results of different tracer gases show the same trends under the variation of operating parameters. Increasing superficial velocity and SA ratio, and decreasing particle diameter result in better mixing of the SA. The effect of bed inventory on SA mixing is not monotonic.