Magnesium Oxide Wet Scrubbing for Flue Gas Desulferization0 pages
Application Data Sheet Power
Magnesium Oxide Wet Scrubbing System for
Flue Gas Desulfurization
Benefits
■ Reduced sulfur dioxide (S02) emissions
■ Improved corrosion resistance
■ Reduced sorbent consumption
Background
Wet scrubbers are used in utilities, paper mills, and chemical
plants to remove sulfur dioxide (S02) and other pollutants from
gas streams. Undesirable pollutants are removed by contacting
the gases with an aqueous solution or slurry containing a sorbent.
Lime / limestone systems are most common for S02 removal but
magnesium oxide (MgO) slurry is sometimes used as an alternative.
Process
After fly ash removal, the flue gas enters the scrubber (Figure 1)
where it comes in contact with the MgO slurry. The S02 is absorbed
in the scrubber slurry and forms insoluble magnesium sulfite
(MgS03) as in the equation below.
MgO<s> + S02<9>->MgS03<s'
Oxygen injected at the base of the scrubber further oxidizes the
MgS03 to magnesium sulfate (MgS04). A bleed line from the
scrubber recirculation line carries the slurry to a centrifuge where
MgS03 crystals are separated from the scrubber slurry. The MgS03
and MgS04 is used in the acid plant to produce S02 for sulfuric acid
production. The MgO reagent is regenerated and returned to the
scrubber system.
The pH of the slurry must be carefully maintained between 4.5 and
5.0 because the MgS03 has a very limited solubility above pH 5.0.
Proportional control, using the 4-20 mA current output, is desirable
when hold-up time in the recirculating tank is five minutes or less.
When hold-up time is fifteen minutes or more, a less expensive on-
off control by the pH analyzer is adequate.
If the pH of the scrubber slurry is too low, S02 will not be absorbed,
corrosion will result, and S02 regulatory emissions standards will
be violated. S02 will be absorbed if the pH is too high, but reagent
usage will be excessive, and scale will form inside the scrubber
leading to plugging due to formation of calcium carbonate (CaC03)
Instrumentation
Since the MgO slurry has a tendency to coat the pH sensor, the
3400 HT PERpH-X or the 396 TUpH1 pH sensor should be used
in line, with a sample velocity of at least 3 ft / sec. The 3400 HT
features a rebuildable reference while the 396 is a disposable design.
Appropriate analyzers include the dual input 1056, which offers a
large display, intuitive menus, and four optional alarm relays for basic
process control. If more advanced control features are required, the
56 offers four programmable relays with Time Proportional Control
(TPC) and PID control outputs.
' TUpH is a trademark of Rosemount Analytical
ROSEMOUNT EMERSON.
Analytical Process Management
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