Chemical Engineering August 2016 - 68

John Zink
Durr Environmental
Independent burners
Clean
process air
Purge air
Heat
exchange
media
Solvent-laden process air
FIGURE 8. Shown here (above left and top) is a
small, elevated enclosed flare that operates in a
" smokeless, " air- assisted type
ters for PM removal, combined with
urea or ammonia injection for NOx
control and 1-in. thick catalytically
treated filter walls. Temperatures of
400°F are preferred for dry-reagent
injection for SO2 removal (Figure 10).
Yet another firm sells filter bags with
a coating that removes dioxins. Activated
carbon can be added for mercury
and dioxin removal.
PAC producers have made major
expansions over the past five years
to keep up with demand. PAC is effective
at temperatures up to 350°F.
Higher than that, heat desorbs the
mercury. Typical carbon-injection
rates are 2 to 5 lb/million acf (actual
cubic foot) of stack gas, varying with
the amount of mercury in the coal
and type of particulate filter.
Wet scrubbers can be used to
remove elemental mercury if they
are equipped with cooling towers
to reduce stack exit temperatures
to the required level. Meanwhile,
dioxins
are formed
from organic
chlorine in the feedstock, and even
inorganic chlorine forms some HCl
and Cl2 in high-temperature combustion
processes. The solution
here is twofold - keep baghouse
or ESP temperatures below 450°F
to limit " de novo " formation from
the chlorine in the stack gas, and
use PAC or catalyst-coated filters to
remove it.
Multi-pollutant control
While individual APC components
can be placed in series to get the job
done, single units with multiple capabilities
also exist. An example are fil68
Airflow
Ceramic
filter tubes in the
ultratemp filtration system
for hot gas filtration
Small ultratemp module
incorporating new
ceramic filter tubes
NOx and ammonia react with
catalyst to destroy NOx
Sorbent
injection
for SO2/HCI
control
Control of PM, SO2, NOx in one system
Urea/
ammonia
injection for
NOx control
Airflow
Auxiliary systems
Fans and blowers. These components
are of prime importance because
they move both the air into a
combustor and the fluegas or process
vent streams out of it. Combustion
air is supplied by either a
forced-draft fan, an induced-draft
fan or both.
Fans may be driven by electric
motors or in large sizes, steam turbines.
The speed of the fan may be
variable or fixed. Fixed-speed fans
Tri-Mer
High temp for PM removal
FIGURE 9. This three-chamber RTO, with 95%
heat recuperation, has the ability to capture and
treat fumes when the valve change over occurs to
reverse the direction of flow
and blowers control air flow via a
mechanical damper on the inlet to
the fan, while variable-speed systems
control air flow by changing
the shaft speed. Variable-speed
drives (VFDs) are
popular, due to
decreased power costs when compared
to damper flow control. Gas
temperature, fan alloy, resistance to
corrosion, and sound level must be
figured into the fan specifications.
Power required for the induced
draft (ID) fan is equal to flow times
pressure drop. In most applications,
the flowrate of fume is fixed, however,
pressure drop - and power
consumption - varies widely with
the control device. Table 1 provides
some typical pressure drop values.
As can be seen, venturis are not a
good choice at high pressure drops,
and even then, may struggle to remove
very fine particles.
Mist and aerosol eliminators.
PM and NOx with reagent
Nano-catalysts embedded
in the walls of the filter
Particulate
captured
on filter
surface
PM does not
penetrate walls
of the filter
Ceramic filters with embedded
catalyst for NOx and dioxin
control, PM capture
FIGURE 10. This example of an all-in-one, multi-pollutant-removal system uses reagent injection and
catalytic bag filters
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