Hot Side

Hot-side ESPs (Figure 1-14) are placed in locations where the flue gas temperature is relatively high. Their collection electrodes can be either tubular or plate. Hot-side ESPs are used in high-temperature applications, such as in the collection of cement kiln dust or utility and industrial boiler fly ash. A hot-side precipitator is located before the combustion air preheater in a boiler. The flue gas temperature for hot-side precipitators is in the range of 320 to 420°C (608 to 790°F).

The use of hot-side precipitators helps reduce corrosion and hopper plugging. However, these units (mainly used on coal-fired boilers) have some disadvantages.

Because the temperature of the flue gas is higher, the gas volume treated in the ESP is larger. Consequently, the overall size of the precipitator is larger making it more costly. Other major disadvantages include structural and mechanical problems that occur in the precipitator shell and support structure as a result of differences in thermal expansion.

For years, cold-side ESPs were used successfully on boilers burning high-sulfur coal. However, during the 1970s when utilities switched to burning low-sulfur coal, cold side ESPs was no longer effective at collecting the fly ash. Fly ash produced from low sulfur coal-fired boilers has high resistivity (discussed in more detail later in the course), making it difficult to collect. As you will learn later, high temperatures can lower resistivity. Consequently, hot-side ESPs became very popular during the 1970s for removing ash from coal-fired boilers burning low sulfur coal. However, many of these units did not operate reliably, and therefore, since the 1980s; operators have generally decided to use cold-side ESPs along with conditioning agents when burning low sulfur coal.

Hot-side ESPs are also used in industrial applications such as cement kilns and steel refining furnaces. In these cases, combustion air pre-heaters are generally not used and hot side just refers to the high flue gas temperature prior to entering the ESP.