Effect of bed material size on gas-solid flow characteristics in a CFB at low solid recirculation rates

The gas-solid flow characteristics greatly influence the heat transfer, combustion and pollutant formation pro-cesses in a circulating fluidized bed (CFB) boiler. The effects of particle size on the gas-solid flow characteristics, including flow structure, axial pressure and bed density profiles were studied in a two-dimensional CFB cold apparatus under low solid circulation rates (Gs's). At the same time, an optical fiber probe was used to assess the particle size effect on the formation of particle clusters and their properties under different operating conditions. Results revealed that when the averaged size of bed materials decreased from-200 mu m to-100 mu m, at the same operating condition, the bottom dense bed became remarkably higher, and the upper dilute zone had stronger pressure fluctuation, more even axial and horizontal particle distributions, and higher solid density. Gs was more sensitive to the variation of the superficial gas velocity than the bed inventory. In addition, the standard devi-ation and power spectrum density of instantaneous pressure decreased in the dense bed and increased in the dilute zone. The particle velocity was slightly lower in the lower section of the riser but noticeably greater in the upper riser section. In addition, the clusters formed more frequently and lasted significantly longer. The study showed that flow characteristics in the bed qualitatively changed from that of Type B particles to that Type A particles in the Geldart's classification, even at a small Gs, and previous studies with Type B particles could very likely not apply in the CFB boilers operating with fine bed materials.