SO₂ (Sulfur Dioxide) Air Pollution Control – Acid Rain Prevention – Wet Scrubber Design Using Q-PAC®

Process Description
When sulfur compounds are burned, most of the sulfur is converted to SO₂. Typical examples are coal fired electric power plants, burning of high sulfur diesel or fuel oil, etc. The SO₂ that is formed needs to be removed from the process off gas. If not removed, acid rain downwind of the combustion process will result. The US EPA’s Phase II regulations intended to further reduce SO₂ emissions took effect in 2000. These regulations apply to all power generating stations of 25 megawatt and greater capacity.

SO₂ Emission Control – Packed Bed Wet Scrubber
A very efficient and cost effective method to limit SO₂ air emissions is wet scrubbing.

Using a typical example of an air stream of 10,000 m³/hr at 66 ºC, the design of the scrubber would be:

Diameter = 1100 mm
Inlet CO₂ = 44,000 ppmv
Inlet SO₂ = 300 ppmv
Recirculation Rate = 21 m³/hr
Blow-down Rate = 0.5 m³/hr
Total Dissolved Solids in Blow-down ~ 5%
Scrubbing Liquor = water + caustic (NaOH)
Makeup Caustic Strength = 50%
Sump pH ~ 7
Packing = 3000 mm Q-PAC

Q-PAC Pressure Drop = 4.7 mbar

Mist Capture = 500 mm #2 NUPAC

#2 NUPAC Pressure Drop = 1.8 mbar

SO₂ Scrubbing Efficiency = 98%+

Chemical Losses to CO₂ Absorption = 0

Theoretical Caustic Consumption = 34 Liter/hr

Operational Notes
The sump should initially be filled with about 4% NaOH solution. pH will be high at first, but gradually comes down as SO₂ is absorbed. When the pH falls to ~6, caustic should be added to raise the pH by a few tenths of a unit. At low pH the scrubber will remove SO₂ and other strong acid gases, but not waste NaOH by absorbing CO₂. Combustion exhaust may contain 1~6% CO₂ by volume. The pH won't change very fast, because a sodium bisulphate / sulphite buffer solution is formed in the water.

Blow-down rate is usually adjusted to maintain of 2~5% TDS. This can be done automatically by using a conductivity probe in the sump to monitor the byproduct salt content. When sulfur compounds are burned, most of the S is converted to SO₂, but depending on conditions, there may also be a small amount of SO₃. SO₃ reacts with water vapour to form an aerosol of sulfuric acid droplets, most of which are too fine to be removed by a packed scrubber.

Please consult with S G Plastic Construction regarding the design requirements of your specific project. No warranty is given or implied with this design example. All designs that are reviewed by S G Plastic Construction will carry a full
performance warranty.

EngNet Design Group

SO2 (Sulfur Dioxide) Air Pollution Control – Acid Rain Prevention – Wet Scrubber Design Using Q-PAC®

SO₂ (Sulfur Dioxide) Air Pollution Control – Acid Rain Prevention – Wet Scrubber Design Using Q-PAC®