What is Flue Gas Humidification and Conditioning for Power Plants?

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 and distribution problems and change the chemistry of the fly ash (both from a land fill and a cement additive prospective), plus there is a large annual fee for proprietary chemicals. Chemical additives can also have adverse effects on wet SO2 scrubber chemistry. Wet precipitators require a large capital outlay and the wet effluent must be processed. Burning low sulfur coal is less expensive, but leads to precipitator collection problems and is counterproductive if an SO2 scrubber has been installed on the boiler.

The Air Process Systems Way: Flue gas humidification reduces fly ash resistivity by condensing the SO3 on the ash, and can also reduce the blue or brown plumes which are caused by SO3 vapor condensing at the outlet of the stack. The keys to successful flue gas humidification are very fine atomization and optimum distribution. The Air Process System is unique because it uses patented nozzles that can atomize down to one micron, has a 50 to one turn-down ratio, and is manufactured for the specific application. Therefore, an Air Process Systems system can reduce particulate and acid plume emissions by condensing sulfur trioxide onto the fly ash, with lower capital, operating and maintenance costs, with the least amount of risk to the existing operation.

Potential Applications: Any industrial or utility power plant burning coal and having problems with high fly ash resistivity and/or blue or brown plumes caused by excess sulfur trioxide emissions is a perfect candidate for an Air Process Systems
flue gas humidification system. The application is particularly suited for boilers that fire alternative fuels with the coal, such as tire chips, RDF (refuse derived fuel), construction debris, wood products, and coal tar soils.

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