Heat transfer from a circulating fluidized bed to membrane waterwall surfaces

Abstract

Abstract Average and local heat transfer coefficients were measured for transfer from circulating fluidized beds of sand particles (mean size 188 and 356 μm) to two water‐cooled membrane wall surfaces located on one face of a 152 mm square by 7.3 m tall column. The measurements cover a range of superficial gas velocities from about 4 to 7 m/s, suspension densities from about 8 to 130 kg/m 3 , suspension temperatures from 150 to 400°C, and secondary‐to‐primary air ratios of 0 to 1.5. Heat transfer coefficients, averaged over a 1.53 m length of the membrane waterwall surfaces, depend strongly on suspension density, but show almost no separate dependence on gas velocity, bed temperature, or secondary‐to‐primary air ratio for the conditions studied. For the surface nearest the top of the unit, the coefficient decreases with distance measured downward from the top, suggesting that particles travel downward along the surface. As a result, averaged coefficients are lower and the influence of particle size is less than for previously reported circulating fluidized bed heat transfer measurements where miniature heat transfer surfaces were employed.