Ash Vaporization in Circulating Fluidized Bed Coal Combustion

Abstract

ABSTRACT. In this work, the vaporization of the ash forming constituents in circulating fluidized bed combustion (CFBC) in a full-scale 80 MW,, unit was studied. Ash vaporization in CFBC was studied by measuring the fly ash aerosols in a full-scale boiler upstream of the electrostatic precipitator (ESP) at the flue gas temperature of 125°C. The fuel was a Venezuelan bituminous coal, and a limestone sorbent was used during the measurements. The fly ash number size distributions showed two distinct modes in the submicrometer size range, at particle diameters 0.02 and 0.3 pm. The concentration of the ultrafine 0.02-pm mode showed a large variation with time and it decreased as the measurements advanced. The concentration of the 0.02-pm mode was two orders of magnitude lower than in the submicrometer mode observed earlier in the bubbling FBC and up to three orders of magnitude lower than in the pulverized coal combustion. Scanning electron micrographs showed few ultrafine particles. The intermediate mode at 0.3 p m consisted of particles irregular in shape, and hence in this mode the particles had not been formed via a gas to particle route. We propose that the 0.3-pm mode had been formed from the partial melting of the very fine mineral particles in the coal. The mass size distribution in the size range 0.01-70 p m was unimodal with maximum at 20 pm. Less than 1% of the 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 size distribution was size independent. A fraction of halogens C1, Br and I were found to be in the gas phase after the combustion. AEROSOL SCIENCE AND TECHNOLOGY 24:135-150 (1996)

The Properties of Cenospheres and the Mechanism of Their Formation During High-Temperature Coal Combustion at Thermal Power Plans

“…The existing overall combustion models are combinations of physical-mathematical models and empirical correlations 21,[43][44][45][46][47][48][49][50][51][52][53] . The combustion usually starts in typically middlesize coal particles of 20 to 100 m and includes the following characteristic stages: a) fast heating and de-volatilization (emission of water and volatile combustible gases) with a residence time less than 1 s, b) the burning of char with a residence time of no more than 1-2 s, c) cooling, the residence time is several seconds [48][49][50][51][52] . The combustion of de-volatilised gases outside the coal particle occurs during stage "a" , and some of the oxygen can penetrate through the flame towards the par ticle surface.…”

Control of PM1 by kaolin or limestone during O2/CO2 pulverized coal combustion

“…From these viewpoints, it is imperative to reduce/control the emission of PM 1 during coal combustion. The behavior of alkali metal compounds [7][8][9][10][11], formation of sub-micron particulate matter [12][13][14][15] and control of semi-volatile metals [16][17][18][19][20][21][22][23][24][25] during coal combustion in air have been extensively reported.…”

Combustion Aerosols: Factors Governing Their Size and Composition and Implications to Human Health

“…125,[190][191][192] Elemental balances on power plants show that ~1-4% of most metals in the fuel are emitted in the stack PM. 143,148,152,178,193 The major exceptions are Se and Hg, which escape as vapors. Rapid quenching from high temperature results in the formation of glass-phase species of indeterminate composition rather than the crystalline minerals with similar elemental composition.…”

“…Full-scale data show that an ultrafine particle mode can be detected for both circulating and bubbling fluidized bed coal combustion, but the ultrafine concentration is several orders of magnitude smaller than the ultrafine PM concentration produced by pulverized coal combustion. 177,178 Many of the field studies of coal-fired power plants were aimed at obtaining the information needed for regulatory purposes, so the measurements have focused on the total mass of the emissions. Selected field studies have determined the fractions of the trace elements entering a boiler that enter the flue gases and pass out of the unit through the stack.…”