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