Flue gas contains gas pollutants such as CO2 and SOx and non-gaseous aerosols. Aerosols comprise fly ash, soot,
condensible vapors, mist, and dust.
These fractions are airborne because they are very small and tend to
float around in the air rather than drop out.
Scrubbing of aerosols agglomerates these fractions so they can be
separated more easily downstream.
However, approximately 6% by mass of particle emissions from pulverized
bituminous and sub-bituminous coal combustion is in the form of aerosols too
small to separate by known processes or devices.
Fly ash is fine inorganic
(principally silicon dioxide) particulate matter formed during coal
combustion. The most troublesome fly ash
is in the form of minute glassy dust less than 2.5 millionths of a meter
(micron) in diameter (PM-2.5). Collected
fly ash is valuable as a concrete additive and as a material for making durable
and impervious bricks which require no firing.
Fly ash can therefore be seen as both a problem and an unexploited
resource.
Soot, another particulate
emission, is uncombusted fuel, which is usually not a problem in power plants
where combustion is complete. Combustion
is frequently not complete, and in gaseous emission streams from ships and
vehicles, soot is a serious problem.
Other aerosols include vapors,
mist, dust, and trace metals. Mercury
and VOCs (volatile organic compounds) are condensible vapors which are
regulated emissions because of their known harmful effect. Mist is tiny liquid droplets, including
sulfuric acid droplets, water droplets, and droplets from condensed condensible
vapors. Dust is airborne fragments of
inorganic material. Trace metals in flue
gas include uranium, arsenic, lead, cobalt, chromium, and thorium.
Dry electrostatic precipitators
(ESPs) are the principal means used for collecting aerosols from coal-fired
flue gas. Other industries using ESPs
for emission control are cement (dust), pulp and paper (salt cake and lime
dust), petrochemicals (sulfuric acid mist), and steel (dust and fumes). A
cathode in the flow path of a gaseous emission stream imparts a negative
charge to the entrained particles. A
positively charged collector plate (anode) downstream in the flow path attracts
the negative charges. Charged aerosols
adhere to the collector plate and agglomerate in a coating. The coating is dislodged by rapping into a
hopper.
ESPs, when working properly and
with the right fuel, may have an overall collection efficiency as high as
99.2%. Where ESPs fail is in collecting
fly ash under 2.5 microns and other fine particulates. The size limit for effective aerosol
collection in ESPs is approximately 10 microns.
Even lower collection efficiency
for fine particulates is found in the performance of inertial collectors, such
as cyclones. Estimated overall control
efficiency for a cascade of multiple cyclones is 94%, but fine particulates
mostly escape collection. Cyclones are
often used as a precollector upstream of an ESP, fabric filter, or wet scrubber
so that these devices can be specified for lower particle loadings to reduce
capital and/or operating costs.
Wet scrubbing of aerosols is
done by injecting water, which contacts the particles and agglomerates them
into a sludge. The 94% overall
collection efficiency for particulates in wet scrubbers is inferior to the
maximum collection efficiency of ESPs. A
major unsolved problem is the volume of wastewater produced by wet
scrubbing. Small particles remain in
suspension, and don’t settle out by gravity.
The wastewater contains sulfuric acid and nitric acid, along with
mercury, so it can’t just be dumped.
Mechanically aided wet scrubbers
known to the art spray liquid onto centrifugal fan blades as waste gas flows
through the fan. The advantage of
mechanical assistance is less water usage and a smaller footprint. Collection takes place in the spray and in
the film that forms on the fan blades.
Venturi scrubbers, the most
turbulent of wet scrubbers, have the highest collection efficiency. Venturis are pressure driven devices which
jet a combined stream of waste gas and liquid through a nozzle into a
tank. However, venturis have the
disadvantage that turbulence quickly dissipates into pressure. The time during which turbulent mixing occurs
is short.
Dewatering the scrubbing sludge
is another solution from Vorsana.
Our turbulent McCutchen Scrubber not only very efficiently sweeps aerosols out of
flue gas, but also concentrates them into an easily disposable sludge.
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