Page 303 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

280 10 Post-combustion Air Emission Control

Particle resistivity is also affected by the moisture content in the gas, especially
at temperatures below 200 °C. The moisture content may lower the particle resis-
tivity by several orders of magnitude. This can be explained that water molecules
are very active in removing electric charge from the particles in addition to the
formation of sulfuric acid in the gas.
Sulfur content in the coal also affects the resistivity of the coal-based fly ash. For
low sulfur coal, the resistivity of the corresponding fly ash particles is likely to be
too high (>10 12 ohm-m). As a result lower fuel sulfur is likely lead to charging and
more serious spark problems. Therefore, switching from high sulfur coal to low
sulfur coal could result in unexpected lose in ESP performance. For high sulfur
coal, part of the fuel sulfur is oxidized to SO 2 and then to SO 3 . With the presence of
water vapor in the flue gas, sulfuric acid is produced and water condensates on the
surface of the fly ash particles. This reduces the resistivity and increases the
coagulation of the fly ash particles. Typically, switching from a 1 %-wt sulfur coal
to a 0.6 %-wt sulfur coal will require a 20 % larger ESP collection area in order to
compromise the loss of performance [9].
In addition to sulfur content, many other chemical compounds in the coal used as
fuel also affect the resistivity of the fly ash particles. Iron (Fe 2 O 3 ), sodium (Na 2 O),
and water can decrease the resistivity and hence are favorable to ESPs, whereas
calcium (CaO), magnesium (MgO), silicon (SiO 2 ) and aluminum (Al 2 O 3 ) increase
the resistivity of the particles, thus worsening the performance of ESPs.
Industrial field evaluations have shown that industrial ESPs are effectively for
only large particles, and they have limited capability in capturing fine particles. It is
mainly because of the low precipitation speed of these small particles. These fine
particles have to be captured by filtration in order to meet more and more stringent
fine particulate matter emission standards.

10.2.2 Filtration System Designs

10.2.2.1 Filter Media

In the engineering designs of industrial air filtration systems, one first needs to
consider the materials of filter media according to the gas properties. Typical filter
media include, but are not limited to, bag filters made of fabric fiber materials,
textile, plastics, and ceramics. Rigid barrier filters are made of metal or sintered
ceramic, powder or fibers; Granular filters based on layers of granular solids are
widely employed in liquid-solid separation for water treatment [28], and they can
also be used for air purification.
The selection of the media is determined by many factors such as the operation
temperature, property of the particles, and availability of the media. Fabric filters
are widely used for environment where temperatures are relatively low. Cotton may
be used for the temperatures below 80 °C while Teflon and glass fiber work for up
to 260 °C. For applications up to 450 °C stainless steel can be used under