Air Pollution Control
With environmental regulations becoming more and
more stringent all over the world, company's have to continually monitor, log, and control
what is released into the atmosphere. Many different types of equipment are used,
each dependent on many different factors.
Particulate and SO2 Emissions
A. Cyclone Separators
Typically used to remove particulate from a gas stream, the
gas enters tangentially at the top of a cylinder and is forced downward into a spiral
motion. The particles exit the bottom while the gas turns upward into the vortex and
leaves through the top of the unit. Pressure drops through cyclones are usually from
13 to 17 mm water gauge. Although seldom adequate by themselves, cyclone separators
are often an effective first step in pollution control.
(photo courtesy of RSG Inc., www.airclassify.com)
Separating solids or liquids from a gas is one use of scrubbers.
However, separating a soluble gas from other gases is the application where
scrubbers see the most action. Typically, a gas enters the bottom of a scrubber and
moves upward while a liquid is sprayed from the top. The soluble gas is carried away
by the liquid exiting out the bottom of the unit. The most common application is
flue gas desulfurization using ammonia as the solvent or spray liquid. Pressure
drops through scrubbers are usually low if they're sized properly, and scrubbers are
generally about 50% efficient so multiple units are sometimes required or packing may be
used to increase efficiency. The contaminated liquid exiting the scrubber represents
it's own disposal problem.
(photo courtesy of Sly, Inc., www.slyinc.com)
C. Semidry Scrubbers
dryer is used to atomize the slurry into the gas which evaporates the water in the
droplets. As this takes place, the acid in the gas neutralizes the alkali material
and forms a fine white solid. Most of the white solids are removed at the bottom of
the scrubber while some are carried into the gas stream and have to be removed by a filter
or electrostatic precipitator (discussed later). Although semidry systems cost 5-15%
more than wet systems, when combined with a fabric filter, they can achieve 90-95%
The advantage of semidry scrubbers is in that they remove
contaminates by way of a solid waste that is easier to dispose of (less expensive).
Initially, the scrubbing medium is wet (such as a lime or soda ash slurry) then a spray
Dry scrubbers are sometimes used in a very similar fashion, but
without the help of gas-liquid-solid mass transfer, these systems use much higher amounts
of the solid alkali materials.
Boasting an efficiency in excess of 99%, electrostatic
precipitators are very effective at removing tiny particles from gas streams. Gas
flows through a rectangular duct containing rows of metallic strips. The strips are
negatively charged by way of a small voltage that is applied (about 200 W for every 1000
ft3/min of gas). The efficiency is a result of the precipitators applying
the collecting force to the particles only and not the gas. Periodically, the
precipitators have to be taken offline and cleaned.
(photo courtesy of Wheelabrator, www.wapc.com)
VOC (Volatile Organic Chemicals) Emissions
A. High VOC Concentrations (>500 ppm)
Three types of treatment are generally used for streams with high
concentrations of VOC: Refrigerated Vapor Condensation, Solvent Vapor Adsorption,
and Flaring. The method chosen is dependent on allowable release concentrations and
the value of the solvent. Refrigerated vapor condensation can mean condensation at
temperatures as low as -80 0C. Due to the high cost of refrigeration,
this option is usually reserved for expensive solvents whose recovery can justify the high
operating costs. Solvent vapor adsorption is a more common application where the VOC
containing gas is bubbled through an organic solvent which "accepts" the VOC in
the gas stream. The VOC are then released from the solvent by heat and a
partial vacuum. They can then be condensed at a much higher temperature than the
refrigerated method in the absence of large amounts of inerts. An example of this
may be the popular removal of propane by using MTBE as a solvent.
Flares can be used to handle flow and concentrations surges along
with other recovery methods for high concentrations of VOC. The combustion (and
thereby loss) of VOC's, produce NOx emissions and is typically unacceptable as the only
means of eliminating VOC from highly concentrated streams.
(photo courtesy of AMCEC Inc., www.amcec.com)
B. Moderate VOC Concentrations
For moderate concentrations of VOC, incineration or regenerative
carbon adsorption is utilized. At temperatures between 750-1000 0C, VOC
are typically destroyed at a rate of 99%. Usually, a heat exchanger is used to
preheat the gas stream with the flue gas to save on fuel costs for the incinerator
(pictured left). Worth a mention is that the presence of chlorides may require a
more exotic material for the incinerator and heat exchanger. Catalytic incinerators
can save on fuel costs by destroying VOC on a catalyst's surface at 430 0C.
With similar capital costs, the extra expense is usually in the catalysts.
Also, streams containing chlorides can produce HCl when burned and the acid can attack
some catalysts. Other compability issues also have to be addressed before using
Regenerative carbon adsorption is where a gas stream passes
through a bed of activated carbon. The VOC are adsorbed into the carbon.
Regeneration consists of stripping with low pressure steam followed by air drying.
Water and VOC are recovered by condensation. This method can achieve 99%
effectiveness for VOC concentrations from 10 to several hundred parts per million.
Economics usually favor this process when the VOC are insoluble in water and are liquids
at room temperatures. Carbon adsorption is not a good choice for gaseous organics
(at room temperature), air streams above 38 0C or 50% relative humidity.
Under these conditions, the VOC do not bind well to the carbon. Using carbon
adsorption for ketones is also not recommended due to their tendency to polymerize on the
carbon's surface thereby deactivating the carbon.
(photo courtesy of Ship & Shore, www.shipandshore.com/thermox.htm)
C. Low VOC Concentrations (<100 ppm)
Low air flows containing low concentrations of VOC are typically
passed through "disposable" canisters of activated carbon. These canisters
are purchased from the supplier then the spent canisters are traded for fresh canisters
(ie. the suppliers typically handle the regeneration process). Activated
carbon will typically have a loading capacity of 0.3 lbs of VOC per pound of carbon at 100
ppm and 0.15 lbs of VOC per pound of carbon at 5 ppm. Typical carbon costs are
$2.30-$2.60 per pound. At higher flowrates, carbon adsorption-incineration
combinations may be needed.
Nitrogen oxides are products of all conventional
combustion processes. They are also a target of many environmental regulations, with
A. Selective Catalytic Reduction (SCR)
In short, SCE is a process to reduce NOx to nitrogen and water
with ammonia in the presence of a catalyst between 540-840 0F (282-449 0C).
Ammonia is usually injected at a 1:1 molar ratio with the NOx contaminants.
Ammonia is used due to its tendency to react only with the contaminants and not with the
oxygen in the gas stream. Ammonia is injected by means of compressed gas or steam
carriers. Efficiencies near 90% have been reported with SCR.
B. Exxon Thermal DeNOx
Similar to SCR, the Exxon Thermal DeNOx process utilizes the
NOx/ammonia reaction. However, this process does not use a catalyst to aid the
reaction. Rather, tightly controlled temperatures are used to steer the reactions.
Optimum reaction temperatures are found between 1600 0F (871 0C)
and 1800 0F (981 0C). Below the optimum temperature range,
ammonia does not fully react and can be released in the flue gas. Above the optimum
temperture, the following competing reaction can begin to take place:
NH3 + 5/4 O2 ---> NO + 3/2 H2O
Ammonia is injected in a 2:1 molar ratio in this process.