SO₂ Scrubber

Mechanism

SO₂ scrubber: A mechanism to reduce sulfur dioxide emissions from industrial processes.

What is sulfur dioxide (SO₂)?

Sulfur dioxide is a colorless gas that can be released into the air through various manufacturing processes, particularly through the combustion of coal for power generation.

SO₂ is also released into the air from natural sources, but on average, human activities are the predominant global source.

Exposure limit of SO₂

OSHA PEL:

• Permissible Exposure Limit (PEL): 2 ppm (Time Weighted Average)
• Short-Term Exposure Limit (STEL): 5 ppm

Desulfurization

A SO₂ scrubber system is the informal name for flue gas desulfurization (FGD) technology, which removes or “scrubs” SO₂ emissions from the exhaust of coal-fired power plants.

A scrubber works by spraying a wet slurry of limestone into a large chamber where the calcium in the limestone reacts with the SO₂ in the flue gas.

There are some variations in scrubber designs. For example, some scrubbers may use other chemicals such as lime or magnesium oxide to react with the SO₂ in the flue gas.

Working Principle

Packed-bed scrubbers, also called wet scrubbers or absorbing towers, are pieces of equipment installed in power plants to remove SO₂ gases from combustion fumes to meet emission standards.

The key aspect of the process is the dissolution of the gas from the fumes into a liquid made of water with suitable additives. The outflowing liquid is collected, concentrated, and recycled. Ultimately, the stripped gas ends up as a solid residue.

A scrubber is a vertical tank (tower) in which gas and liquid flow against each other: the liquid solution flows downward while the gas bubbles to the top, each moved under the action of gravity.

To maximize contact between liquid and gas, the tower is also packed with a large number of small objects forcing the liquid to percolate slowly through tortuous paths and the gas to rise in small bubbles.

As the liquid and gas compete for space, very intimate contact takes place between the two, creating a large contact area through which the transfer of chemical species occurs.

The counterflow with vapor moving upward and liquid downward is very practical for two reasons: (1) It happens naturally by gravity as the liquid trickles down while the gas bubbles up through it, and (2) the fumes encounter increasingly less loaded liquid as they get progressively cleaner on their way upward.

The Chemistry of Desulfurization

The liquid used to “wash” the flue gas is a water-based calcium solution.

Limestone (CaCO₃) is cheap and creates an alkaline environment in water:

CaCO₃ + H₂O ⇌ Ca⁺⁺ + HCO₃^- + OH^-

The OH^- radicals bind with SO₂ in solution to form a sulfur ion:

SO₂ + OH^- ⇌ HSO₃^-

While water always dissociates slightly:

H₂O ⇌ H⁺ + OH^-

The bicarbonate HCO₃^- ions bind with H⁺ to create H₂CO₃ (carbonic acid), which creates an equilibrium with water (H₂O) and carbon dioxide (CO₂):

HCO₃^- + H⁺ ⇌ H₂CO₃ ⇌ H₂O + CO₂

The net result is the transformation of CaCO₃ to calcium ions (Ca⁺⁺), the release of carbon dioxide, and the production of HSO₃^-:

CaCO₃ + 2 SO₂ + H₂O ⇌ Ca⁺⁺ + CO₂ + 2 HSO₃^-

The carbon dioxide bubbles up as a gas, while HSO₃^- remains in the water solution.

Once collected at the bottom, the laden solution undergoes further reactions until a more valuable substance can be extracted, such as calcium sulfite (CaSO₃) or calcium sulfate (CaSO₄), which can be turned into plasterboard.