The fluidized bed combustion of a medium volatile bituminous coal from British Columbia

Abstract

Byron Creek, a bituminous coal from British Columbia, was burnt in a pilot scale atmospheric fluid bed combustor (with a cross section of 0.155 m 2 ). Some of the surface seams of this coal are partially oxidized and coal from an oxidized portion of such a seam was used in this study. A test matrix requiring three bed temperatures (800°, 900 0 , and 1000°C); three fluidizing velocities (1.8, 2.4 and 3 m/s) and one excess oxygen level of 4% was employed. Ten of the tests were carried out with fly ash recycle and the remaining nine tests were carried out without recycle. Without recycle the combustion efficiencies varied from 70 to 90%. With recycle they rose to 93 to 98%. Attempts to correlate carbon carryover, fcc, with fluidizing velocity (expressed as a dimensionless fluidizing velocity p = gL/u2 where g is the acceleration due to gravity; L, a characteristic length, in thiS cale 406 mm, the length of one side of the bed and U is the fluidizing velocity) and bed temperature gave the following result: fcc = 0.139 p - 0.348 exp (5740) Varying the number of cooling tubes in the bed frod 2 to 20 for these tests did not affect the correlation. No such correlation could be developed for tests with recycle. * Trials with recycle had higher combustion efficiencies when the bed temperatures were high and the fluidizing velocities were low. However, attempts to determine the dependence of the combustion efficiency on the recycle rate, either expressed directly or as a ratio of the feed rate were unsuccessful. The bedside heat transfer coefficients H w for the inbed cooling tubes were determined. The majority of these ranged from 300 to 400 W/m 2 K. The emissions from the bed were within acceptable limits. NOx levels varied from 20 to 350 ppm; SO2 from 200 to 370 ppm and CO concentrations from 100 to 1500 ppm.