Characteristics of Fly

Hart et al. recently performed a comprehensive investigation of the composition and mineralogy of fly ash from three utility boilers. Using instrumental neutron activation analysis and X-ray fluorescence spectroscopy, they found Si, Al, Fe and Ca to account for more than 90% of major elements from all three boilers. Scanning electron microscopy with energy dispersive spectroscopy revealed successive ESP ashes to be composed mainly of spherical particles which decrease in average diameter with increasing distance from the boiler. Concentrations of As, Co, Cr, Ni, Mo and Sb increased from bottom ash through the sequence of ESP ashes. These trace elements are volatilized and transported to cooler regions, where they condense or are adsorbed onto fly ash particles. The major fly ash mineral phase found by these and other researchers is quartz (SiO2) with magnetite (Fe3O4), anhydrite (CaSO4), and mullite (Al6Si2O13) among other minerals commonly present.

Resistivity (p) is the most important property of material to be collected by an ESP. The optimum range of resistivity is 104-1011 Q-cm. On collection, low resistivity particles (p < 103 Q-cm) release charge to the collector plate and may be re-entrained. Particles with p > 1011 Q-cm insulate the collector plate, ultimately producing a sufficiently large electric field within the dust layer to cause a counterproductive 'back corona'.

Two types of resistivity may be important in particle collection in an ESP. Ions collected at the surface of particles control 'surface resistivity', which dominates at temperatures below 250°C. As indicated in Figure 2, particle resistivity first increases with temperature, then decreases. Removal of the surface film (adsorbed water) by heating in vacuo at 360°C eliminates this initial increase in resistivity. Above 200°C, removal of adsorbed material no longer affects resistivity, and at higher temperatures resistivity is attributed to ions in the bulk of the particles,

'volume resistivity'. Both types of resistivity are primarily functions of Na + and Li + ion concentrations, and in some cases K+ and I" ions.

Two aspects of the p-T relationship affect ESP collection efficiency. First, the maximum resistivity of fly ash occurs within the range of temperature at which ESPs are commonly operated, 130-180°C. Second, SO3, produced from sulfur in coal, adsorbs onto the fly ash particles and has traditionally been responsible for lowering the resistivity of the particles to the optimum range for collection. Present use of low sulfur coals (< 1% S) leads to inadequate collection.

Figure 2 Effect of surface film resistivity on flue-dust resistivity. (Reproduced with permission from Busby HGT and Darby K (1963) Journalofthe Institute ofFuel36(268): 184. Copyright The Institute of Fuel).

Operation at non-optimal temperature can be avoided by lowering the temperature, but this requires energy input to a cooling device, and also can lead to difficulties with corrosion due to condensation. The temperature of the exhaust is normally cooled by heat exchange in an air pre-heater prior to injection into the precipitator, hence the name 'cold-side' precipitator. 'Hot-side' precipitators operate at temperatures as high as 370°C; resistivity is often reduced to a desirable level of 2 x 1010 Q-cm at above 315°C. However, difficulties are encountered with the greater volume of the hot gas, and these units require more careful construction.

More commonly, resistivity of high-resistance ash is lowered by chemical conditioning.

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Solar Panel Basics

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