HEAT TRANSFER TO HORIZONTAL TUBES IN A PILOT-SCALE FLUIDIZED-BED COMBUSTOR BURNING LOW-RANK COALS

Abstract

Experimental data are obtained for the heat transfer coefficient between immersed horiziontal tube bundles and an atmospheric-fluidized-bed combustor burning low-rank coals. Silica sand (d_p = 888 to 1484 μm) and limestone (d_p 7= 716 to 1895 μm) are used as bed material. The tests are conducted, with and without limestone addition and ash recycle, at average bed temperatures ranging from 1047 to 1125 K, superficial fluidizing velocity of 1.66 to 2.04 m/s, and excess air levels of 15 to 40 percent. The horiziontal heat transfer tubes (D_T = 25.4 mm) in a bundle are arranged in a staggered array. The bed-side heat transfer coefficient is found to vary between 193 to 348 W/m²K under the present experimental conditions. The experimental data are examined in the light of the existing correlations for the heat transfer coefficient. Among the existing correlations for small particles (d^p < 1 mm) those proposed by Grewal (1981), Bansal et al. (1980) are found to predict the data within ±25 percent, when the contribution due to radiation is included and estimated following Baskakov et al. (1973). Also, the correlations for large particles (d^p,<1 mm) proposed by Glicksman and Decker (1980), Catipovic et al. (1980), Xavier and Davidson (1981), and Zabrodsky et al. (1980), are found to predict the data quite well.

KEYWORDS:

• Heat transfer
• Fluidized-bed coal combustor
• Immersed boiler tubes