Physical and chemical properties of the dust

Such as dust type, size of the dust particles, and average and maximum concentrations in the gas stream are important ESP design considerations. The type of dust to be collected in the ESP refers to the chemical characteristics of the dust such as explosiveness. For example, a dry ESP should not be used to collect explosive dust. In this case, it might be a better idea to use a bag house or scrubber. Particle size is important; small particles are more difficult to collect and become reentrained more easily than larger particles. Additional fields may be required to meet regulatory limits. The dust loading can affect the operating performance. If the dust concentration is too high, the automatic voltage controller may respond by totally suppressing the current in the inlet fields. Suppressed current flow drives the voltage up, which can cause sparking. For this reason, it might be a good idea to install a cyclone or multicyclone to remove larger particles and reduce the dust concentration from the flue gas before it enters the ESP. The facility could install a larger ESP (with more plate area), however, this technique would be more costly.

Resistivity is a function of the chemical composition of the dust, the flue gas temperature and moisture concentration. For fly ash generated from coal-fired boilers, the resistivity depends on the temperature and moisture content of the flue gas and on the sulfur content of the coal burned; the lower the sulfur content, the higher the resistivity, and vice versa. If a boiler burns low-sulfur coal, the ESP must be designed to deal with potential resistivity problems. High resistivity can be reduced by spraying water, SO3 or some other conditioning agent into the flue gas before it enters the ESP.

Predicting the gas flow rate and gas stream properties is essential for proper ESP design. The average and maximum gas flow rates through the ESP, the temperature, moisture content, chemical properties such as dew point, corrosiveness, and combustibility of the gas should be identified prior to final design. If the ESP is going to be installed on an existing source, a stack test should be performed to determine the process gas stream properties. If the ESP is being installed on a new source, data from a similar plant or operation may be used, but the ESP should be designed conservatively (with a large SCA, a high aspect ratio, and high corona power). Once the actual gas stream properties are known, the designers can determine if the precipitator will require extras such as shell insulation for hot-side ESPs, corrosion-proof coatings, and installation of heaters in hoppers or duct work leading into and out of the unit.