In thermal power plants, ESP hoppers are used to collect fly ash which is generated from the burning of coal. Since coal burns continuously in these plants, the exhausting gases contain high amounts of fly ash. The fly ash is, hence, collected by the boilers. These boilers contain economizers that work on the principle of gravity. Yet, a large amount of fly ash still manages to escape, and hence additional plates are placed to collect the fly ash. These plates are placed horizontally and are known as Electrostatic Precipitator (ESP) Hoppers.
The ash handling system in a thermal power plant can be either dry, wet, or semi-automatic wet. ESP hoppers are installed to control the pollutants in the flue gas. Hence, they are placed between the furnace and chimney. ESP hoppers consist of a collection surface for fly ash and electrodes that are placed in the center. The collection surface and the flow of flue gas are parallel to each other so that the fly ash passes through them. A high voltage direct current is used to charge the electrodes negatively. The freely suspended fly ash particles are negatively charged and are attracted towards the positively charged plates. This collected ash is periodically hammered and dusted off the plates and collected in the hoppers.
The process of fly ash collection may sound simple enough to be implemented, however, it involves a range of external factors that drop the functioning, life, and operational cost of ESP hoppers.
The flue gases that flow in between the plates have a relatively low temperature, hence, the temperature of the ESP hoppers tends to drop. The drop in temperature leads to condensation of water and acid in the gases which can crystalize the ash. Moreover, the electrodes present at the base of the ESP Hoppers should not come in contact with the fly ash particles. Once the collected ash starts to settle down, there is a risk of fly ash coming in contact with the electrodes that leads to a short circuit. Hence, ESP Hoppers have to be emptied periodically. Usually, this activity is carried out at a fixed period irrespective of the level of ash present in the hoppers.
Another reason why ESP hoppers have a significant rise in maintenance cost is due to the power required to maintain the temperature in ESP hoppers. A feasible solution to the problem of high-cost maintenance of ESP hoppers is to place an RF level switch for ESP hopper level measurement of fly ash.