January 1, 2012

Level Switches Keep Electrostatic Precipitators Online

By Ravi Jethra and Keith Riley

Measuring the level of dust and fly ash collected in electrostatic precipitators (ESPs) is a very difficult technical problem. At one utility, level switches were so unreliable that operators could not trust their readings because failures were so frequent. When a switch did fail, the precipitator would often clog up, costing the utility up to $100,000 in downtime and repair costs.

Recycling Coal Combustion Products

Electric power is the primary industry that burns coal. Coal combustion to produce electricity usually yields quantities of noncombustible minerals that require disposal. The coal combustion products that remain after combustion, such as fly ash, bottom ash, and flue gas desulfurization gypsum, have proven to be economical, high-quality raw material feedstock for several other industries.

Fly ash is a good example of a potentially valuable and profitable by-product. Fly ash is a powdery material that would “fly” out of power plant stacks if it were not captured. Today, most modern power plants collect their fly ash to meet environmental regulations, but that fly ash can be profitably recycled.

Fly ash is composed primarily of inorganic noncombustible minerals such as alumina, silica, and iron. It exists as a very finely divided, silt-sized particulate that is removed from combustion gases by using various types of collection equipment, such as ESPs and baghouses. Fly ash is also very abrasive. Bottom ash has a similar chemical composition to fly ash, although the bottom ash is larger, generally ranging in size from sand to gravel.

Fly ash can be recycled to produce many materials. For example, it can be used in structural fill and in land development, building materials, and highway construction. Fly ash is also used in metals recovery, the manufacture of agricultural fertilizers, and as plastics filler. Many coal-fired power plants find it profitable to sell coal combustion products to other companies, as it adds to their bottom line, and ESPs are the best way to capture and collect fly ash and related products.