ASH VAPORIZATION IN CIRCULATING FLUIDIZED-BED COAL COMBUSTION

In this work, the vaporization of the ash forming constituents in circ ulating fluidized bed combustion (CFBC) in a full-scale 80 MW(th) unit was studied. Ash vaporization in CFBC was studied by measuring the fl y ash aerosols in a full-scale boiler upstream of the electrostatic pr ecipitator (ESP) at the flue gas temperature of 125 degrees C. The fue l was a Venezuelan bituminous coal, and a limestone sorbent was used d uring the measurements. The fly ash number size distributions showed t wo distinct modes in the submicrometer size range, at particle diamete rs 0.02 and 0.3 mu m. The concentration of the ultrafine 0.02-mu m mod e showed a large variation with time and it decreased as the measureme nts advanced. The concentration of the 0.02-mu m mode was two orders o f magnitude lower than in the submicrometer mode observed earlier in t he bubbling FBC and up to three orders of magnitude lower than in the pulverized coal combustion. Scanning electron micrographs showed few u ltrafine particles. The intermediate mode at 0.3 mu m consisted of par ticles irregular in shape, and hence in this mode the particles had no t been formed via a gas to particle route. We propose that the 0.3-mu m mode had been formed from the partial melting of the very fine miner al particles in the coal. The mass size distribution in the size range 0.01-70 mu m was unimodal with maximum at 20 mu m. Less than 1% of th e fly ash particles was found in the submicrometer size range. Ninety percent of Mg in coal was organically bound, and it was found to react with quartz and aluminosilicate minerals inside the coal particle. No Mg was found to be released to the gas phase and Mg mass fraction siz e distribution was size independent. A fraction of halogens Cl, Br and I were found to be in the gas phase after the combustion.