Fly ash resistivity

A second effect of high fly ash resistivity is observed when the difference in potential across the collected dust layer rises to too high a value. A back corona can form as the EMF differential breaks down due to sparking across the dust layer. High currents flow between the wire cathode and plate anode thereby destroying the ability of the ESP to charge particulates. 

High fly ash resistivity may be overcome by introducing polar chemicals which adsorb themselves on the fly ash surface and reduce surface resistivity. Howard ( .) had reported in 1918 that particulate capturability was excellent in those flue gases containing high concentrations of sulfur trioxide. This polar molecule was proven to be the species responsible for reducing surface resistivity. The unusual characteristic of western coal is its low sulfur content (approx. 0.5% S). When burned, sulfur is converted to sulfur dioxide and a portion of the dioxide is fully oxidized to the trioxide. 

Figure 2. Effects of temperature and sulfur content on fly ash resistivity (13)...

The tests were performed with the plates spaced at 300, 400 and 500 mm at gas flow rate of 8000 Nm /h and gas temperatirre of 165°C, the 35 MWe unit was burning Soirth African/TCOA coal imder these conditions the fly ash resistivity was approximately 2.5-10 ohm.cm. 

FIG. 17-68 Trends in resistivity of fly ash with variations in fliie-gas temperature and coal sulfur content. °C = (°F — 32) X %. (Ogleshij and Nichols, A Manual of Electrostatic Precipitator Technology, pait 11, Southein Research Institute, Binningham, Ala., Z.970.)... 

Fabric filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitators. Fabric filters therefore may be good candidates for collecting fly ash from low-sulfur coals or fly ash containing high unburned carbon levels, which respectively have high and low resistivities, and thus are relatively difficult to collect with electrostatic precipitators. 

Electrostatic precipitators are satisfactory devices for removing small particles from moving gas streams at high collection efficiencies. They have been used almost universally in power plants for removing fly ash from the gases prior to discharge. Electrostatic precipitators have the capability of fine particulate control. Resistivity plays an important role in determining whether a particle can be readily collected in this device. 

Corrosion-resistant cements have been developed for use in wells used to inject supercritical carbon dioxide for enhanced oil recovery (32). These are based on Portland cement and high levels (as much as 40% wt.) of additives such as fly ash. Epoxy resins have been successfully used as cements in corrosive environments... 

Korfmacher, W.A., Wehry, E.L., Mamantov, G., and Natusch, D.F.S. Resistance to photochemical decomposition of polycyclic aromatic hydrocarbons vapor-adsorbed on coal fly ash. Environ. Sci. Technol, 14(9) 1094-1099, 1980. 

The SCR catalysts are used in the form of honeycomb monoliths or plates to guarantee low pressure drops in view oflarge frontal area with parallel channels, high external surface area per unit volume of catalyst, high attrition resistance and low tendency for fly ash plugging. The SCR monoliths and plates are assembled into standard modules and inserted into the reactor to form catalyst layers. 

Just as with organic combustion aerosols, the chemical and physical nature of inorganic solid substrates can have a dramatic impact on the photoreactivity of adsorbed PAH. In 1980, Korfmacher and co-workers reported that BaP, pyrene, and anthracene all pho-tolyzed efficiently in liquid solution but were resistant to photodegradation when adsorbed on coal fly ash. Subsequent studies confirmed this observation and revealed that the carbon content of the ash (and the associated darkening of color) is a key factor in establishing the photostability of these PAHs. Indeed, they were stabilized at relatively small percentages of carbon, e.g., 5% or less (Behymer and Hites, 1985, 1988 Yokley et al., 1986 Dunstan et al., 1989 Miller et al., 1990). 

Korfmacher, W. A., E. L. Wehry, G. Mamantov, and D. F. S. Natusch, Resistance to Photochemical Decomposition of Polycyclic Aromatic Hydrocarbons Vapor-Adsorbed on Coal Fly Ash, Environ. Sci. Technol., 14, 1094-1099 (1980). 

Dunstan, E. R. 1980. A possible method for identifying fly ashes that will improve the sulfate resistance of concrete. Cement, Concrete, Aggregates, 2, 22-30. 

Tikalsky, P. J. 1989. The Effects of Fly Ash on the Sulfate Resistance of Concrete. PhD dissertation, University of Texas at Austin, August. 

With the knowledge that the concentration of sulfur trioxide acts as a control on resistivity of fly ash, it is not surprising that precipitators not meeting design requirements made use of SO3 addition to the flue gas. The injection takes place after the air heater, prior to entering the ESP unit for a cold side precipitator. In some cases, improvement in ESP performance was significant. In others, because of the nature of the fly ash, the improvement was marginal. 

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