1 October 1997
(b) The other device is called a packed tower and is a chamber with baffles or other
materials that cause the gas to flow through numerous small passages which constantly change
(2) The gas normally flows up through the scrubber as the scrubber solution is sprayed
down from the top. The solution mixes with the gas as both flow through the small passages. In
most cases, the gas flows through a venturi then through a packed tower.
(3) This system will remove particulates as well as acid gases. The scrubber solution needs
to be removed and replaced frequently, as it becomes contaminated with the particulates and salts
from reaction with the acid. Before exiting to the atmosphere, the gases pass through a demister
(usually chevron type) in order to remove the scrubber solution liquid entrained in the gas. If the
demister does not function with a high degree of efficiency, the chemical mist in the gas will be
detected as particulate and the incinerator will fail the environmental test.
(4) The used scrubber solution is a liquid and may have to be dewatered before disposal.
e. Dry Scrubber.
(1) A spray dry scrubber involves a tall reaction tower that sprays a fine mist of scrubber
solution into the flue gas. The acid/scrubber solution reacts in the flue gas and produces a soluble
salt which when dried becomes a fine particulate. The temperature of the ambient flue gas into
which the scrubber solution is sprayed enhances the drying process and causes the reacted
solution (salts) to dry into fine powder. The warm gases, with powder entrained, go into either an
ESP or a bag house for removal of the ash particulate and dry salt powder.
(2) Units that utilize a bag house generally remove more pollutants than those utilizing an
ESP because of the higher fine particle collection efficiency of the bag house and the additional
acid gas reaction that occurs as the gases pass through the powder cake on the surface of the
bags. All of the material to be discarded is a dry powder.
(3) This equipment is commonly used in large incinerator plants, but is prohibitively
expensive for use in small plants. The spray dry scrubber followed by a bag house is generally
regarded as the standard system against which all other systems must compete in regard to
5-3. TYPICAL STACK EMISSIONS AND CONTROL STRATEGIES.
(1) Particulate is composed of fine ash (usually inert) and unburned carbon particles
(smoke). The amount of particulate in the gas is directly related to the velocity and turbulence of
(2) Reported values for particulate emissions from uncontrolled starved-air, staged-
combustion incinerators vary from 0.012 to 0.212 grains/dscf at 12% CO2. Starved-air incinerators
have the ability to operate with particulate emission rates consistently below 0.08 grains/dscf at
12% CO2 for IIA Waste Types O, 1, and 2 (see table 3-1). Other technologies have much higher