Page 184 - Wastewater Solids Incineration Systems

Emission Control and Monitoring 151

ious size particles ranging from 0 to 100 μm. The three efficiency curves reflect three
different sized cyclones. Note that the curves shown in Figures 7.5 are for a low effi-
ciency cyclone. Significantly higher efficiency cyclones are available. The predomi-
nant factor in determining collection efficiency is the inlet velocity into the cyclone
and pressure drop through the unit.

2.3.2 Advantages and Disadvantages
A cyclone should be used where the particulate loading in the exhaust gas stream
consists of large size particles. Cyclones are relatively inexpensive, and removal of
particulate matter upstream of other processes will reduce loading on downstream
equipment and control devices. One of the significant advantages of a dry cyclone in
a fluid bed incinerator system is that it removes particulate matter in dry form, which
may be a desired objective. Note that dry collection of particulate matter from a fluid
bed incinerator typically will require a waste heat boiler to cool the exhaust gas
before a dry ESP or fabric filter. Another significant advantage of the cyclone is that
there are no operating parts so operation and maintenance requirements are mini-
mized. It also has a relatively low pressure drop compared with wet scrubbers. A sig-
nificant disadvantage of the cyclone is that it is best suited for removing larger size
particles, typically greater than 10 μm.

2.3.3 Operation and Maintenance
Gas velocity through the unit is an important parameter regarding sizing and control
efficiency of the cyclone. Unit sizing and operation should, therefore, minimize the
variability of the exhaust gas flow rate. Removal efficiency of a cyclone will deterio-
rate significantly if there are appreciable fluctuations in the gas flow rate or if the gas
velocities are significantly lower than the design velocity. Cyclones have no opera-
tional components except for a bottom air lock to allow removal of the collected par-
ticulate under negative pressure conditions. Maintenance includes periodic inspec-
tion inside the cyclone to determine if significant erosion has occurred because of
high-velocity gases passing through the unit. Also, in some cases, particulate matter
can collect on the walls of the cyclone and require periodic cleaning.

2.4 Venturi Scrubbers
Venturi scrubbers are the predominant particulate control devices used. In a Venturi
scrubber, the exhaust gas is accelerated to a high velocity, typically 61 to 183 m/s (200
to 600 ft/sec) at the Venturi throat, where the gas comes in contact with the scrubbing