Ways to Control Dispersion of Fly Ash

When industrial and utility power plants burn coal to produce electricity, the process of combustion produces a fine, powdery particulate called fly ash. This particulate must be cleaned from the gas stream before it escapes from the stack and pollutes the environment. Depending on the method used, the captured ash can be put to practical use--in landfill, as soil modification and as a component of road bases, concrete products and asphalt--or it must be disposed of in settling ponds or sludge clarifiers.

Fabric Filters

• Dust collectors that use filtration to capture particulate from the gas stream are commonly called baghouses. The filter medium--woven or felted cotton, synthetic or glass fiber--captures ash as the exhaust stream passes through it, forming a dust cake that must be removed to maintain efficiency.

Electrostatic Precipitators

• When the gas stream from a furnace passes through an electrostatic precipitator, the ash particles are given a negative charge that causes them to be attracted to positively charged collection plates. As the ash collects on the plates, rappers periodically knock it off into hoppers, where it's removed by conveyors. Very large utility furnaces often require several precipitators to handle the dust load.

Wet Scrubbers

• Liquid--usually water--in combination with various chemicals can capture pollutant gases, as well as extremely fine particulate, from the exhaust stream. Although they can handle extreme temperatures and neutralize corrosive gases, these scrubbers produce waste sludge that requires considerable treatment and from which it is difficult and expensive to recover and recycle fly ash. Scrubbers that use water alone to capture particulate produce a dust-water mixture that can be cleaned by using a clarification tank, which enables the solids to settle to the bottom.