1 October 1997
(2) High-efficiency cyclones improve the collection of particulates by using additional gas
injected along the sides at high velocity or mechanical impellers to increase the velocity of the
gases inside the cyclone.
(3) A multiclone is a housing containing many small cyclones that is used to improve the
collection efficiency for the smaller particulates.
(4) Typically, high-efficiency cyclones and multiclones cannot meet state particulate control
regulations; however, they may be used in conjunction with other pollution control equipment.
Cyclones have been used to reduce the incidences of fires in bag houses. A refractory-lined
cyclone may be used before the heat recovery boiler to reduce plugging and erosion and lower the
required frequency of cleaning ash from the boiler.
b. Bag house.
(1) A bag house is a housing that contains multiple fabric filters. Depending upon the filter
material selected, flow rate can vary from 3 - 50 scfm per square foot of filtration surface. Generally,
the flue gases are maintained between 300F and 500F. Bags may be damaged or remaining
carbon in the ash may ignite at temperatures exceeding 500F. Below 300F condensed acids
attack the bags and other equipment. Table 5-1 shows a comparison of the intrinsic properties of
various commercial bag materials.
(2) A bag house has a high collection efficiency and is especially good for smaller
(3) Bag houses are generally located upstream of the induced draft fan, operating under
negative pressure. This minimizes the escape of unfiltered flue gases and fugitive emissions.
c. Electrostatic Precipitator.
(1) An electrostatic precipitator (ESP) uses an electrically-charged field between a series of
plates and wires to attract and collect particulates. In general, fly ash from municipal waste is more
difficult to collect and remove than fly ash from coal because its characteristics constantly change
with the composition of waste. Some designs use a variable electrical field to compensate for
changes in the ash characteristics. The collection efficiency decreases as the particle size
decreases. While ESP's can attain 99% removal and an emission limit of 0.012 grains/dscf, they
have problems doing so on a consistent basis.
d. Wet Scrubber.
(1) A wet scrubber is either one of the following two devices, or both devices working
(a) One such device is called a venturi and involves the gases going through a
passageway that first narrows and then expands. An alkaline solution (frequently calcium-based) is
injected into the venturi in order to atomize it, cause it to mix with the gas, and react with the acids in