Air Process Systems L.L.C.

In hundreds of industrial and utility power plants, coal is burned to produce steam. The steam is used in heating and/or electrical generation, in which the steam drives a turbine/generator unit. However, coal-fired boilers result in high fly ash resistivity, which can cause high opacity, and excess sulfur trioxide (SO3) emissions, which can cause blue or brown plumes. When coal-fired boilers are used, a certain percentage (<10% to >30%) of the coal is ash, mostly silica, alumina and iron oxides. Some of the ash is light enough to be carried out of the boiler and eventually out of the stack. This light ash is called "fly ash", which flies away and doesn't drop out of the flue gas stream, as does "bottom ash". Fly ash emissions are controlled by either bag houses or electrostatic precipitators (ESP's). ESP's are large electrical devices that use high voltages to electrostatically charge the fly ash and cause it to precipitate out of the flue gas stream. The collection efficiency of the ESP partially depends on the electrical resistivity of the fly ash. Ash that is too resistive is difficult to charge and collect, and once collected, is difficult to remove from the ESP collection electrodes. High resistivity ash can also cause "back corona," in which corona is generated on the collection electrodes, further degrading precipitator performance. The temperature of the flue gas affects the resistivity of the fly ash. Resistivity is also dependent on absorbed surface contaminants such as moisture and sulfur trioxide (SO3). Humidification causes the SO3 in the flue gas to condense on the fly ash particles, resulting in improved precipitator collection efficiencies. The Expensive Alternative: Alternative solutions to reduce blue plumes include chemical injection (such as magnesium oxide, calcium carbonate or sodium hydroxide) upstream of the precipitators, installing wet precipitators, or burning low sulfur coal. However, chemical additives have material handling a