Flue Gas Desulfurization

Flue gas desulfurization(FGD) is the technology used for removing sulfur dioxide (SO₂) from the exhaust flue gases in power plants that burn coal or oil to produce steam for the steam turbines that drive their electricity generators.

SO2 is responsible for acid rain formation. Tall flue gas stacks disperse the emissions by diluting the pollutants in ambient air and transporting them to other regions.

As a result of stringent environmental protection regulations regarding SO₂ emissions that have been enacted in many countries, it is now being removed from flue gases by many methods, with the following being the most common:

§  Wet scrubbing using a slurry of alkaline sorbent, usually limestone or lime, or seawater to scrub the gases.

§  Spray-dry scrubbing using similar sorbent slurries.

§  Dry sorbent injection systems.

For a typical coal-fired power station, FGD will remove 95 percent or more of the SO₂ in the flue gases.

Sulfuric acid mist formation

Fossil fuels contain significant amounts of sulfur. When burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide (SO₂). This happens under normal conditions of temperature and of oxygen present in the flue gas. However, there are some exceptional cases.

For example, when the flue gas has too much oxygen and the SO₂ is further oxidized to sulfur trioxide (SO₃), which is only one of the ways of forming SO₃. Gas temperature is also an important factor. At about 800 °C, formation of SO₃ is favoured. Another way that SO₃ can be formed is through catalysis by metals in the fuel. This is particularly true for heavy fuel oil, where significant amounts of vanadium are present. No matter what, it does not behave like SO₂ in that it forms a liquid aerosol known as sulfuric acid (H₂SO₄) mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to SO₃. Sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates. Increasingly, this problem is being addressed by the use of wet electrostatic precipitators.

Basic principles

Most FGD systems employ two stages: one for fly ash removal and the other for SO₂ removal. Attempts have been made to remove both the fly ash and SO₂ in one scrubbing vessel. However, these systems experienced severe maintenance problems and low simultaneous removal efficiencies. In wet scrubbing systems the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a wet scrubber, and then into the SO₂ absorber. However, in dry injection or spray drying operations, the SO₂ is first reacted with the sorbent and then the flue gas passes through a particulate control device.

Another important design consideration associated with wet FGD systems is that the flue gas exiting the absorber is saturated with water and still contains some SO₂. (No system is 100% efficient.) Therefore, these gases are highly corrosive to any downstream equipment - i.e., fans, ducts, and stacks. Two methods that minimize corrosion are:

(1) Reheating the gases to above their dew point and

(2) Choosing construction materials and design conditions that allow equipment to withstand the corrosive conditions. The selection of a reheating method or the decision not to reheat (thereby requiring the use of special construction materials) is very controversial topics connected with FGD design. Both alternatives are expensive and must be considered on a by-site basis.

Scrubbing with a basic solid or solution

SO₂ is an acid gas and thus the typical sorbent slurries or other materials used to remove the SO₂ from the flue gases are alkaline. The reaction taking place in wet scrubbing using a CaCO₃ (limestone) slurry produces CaSO₃ (calcium sulfite) and can be expressed as:

CaCO₃ (solid) + SO₂ (gas) → CaSO₃ (solid) + CO₂ (gas)

When wet scrubbing with a Ca(OH)₂ (lime) slurry, the reaction also produces CaSO₃ (calcium sulfite) and can be expressed as:

Ca(OH)₂ (solid) + SO₂ (gas) → CaSO₃ (solid) + H₂O (liquid)

When wet scrubbing with a Mg(OH)₂ (magnesium hydroxide) slurry, the reaction produces MgSO₃ (magnesium sulfite) and can be expressed as:

Mg(OH)₂ (solid) + SO₂ (gas) → MgSO₃ (solid) + H₂O (liquid)

To partially offset the cost of the FGD installation, in some designs, the CaSO₃ (calcium sulfite) is further oxidized to produce marketable CaSO₄ · 2H₂O (gypsum). This technique is also known as forced oxidation:

CaSO₃ (solid) + H₂O (liquid) + ½O₂ (gas) → CaSO₄ (solid) + H₂O

A natural alkaline usable to absorb SO₂ is seawater. The SO₂ is absorbed in the water, and when oxygen is added reacts to form sulfate ions SO₄- and free H⁺. The surplus of H⁺ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO₂ gas:

SO₂ (gas) + H₂O + ½O₂ (gas)→ SO₄^2- (solid) + 2H⁺

HCO₃^- + H⁺ → H₂O + CO₂ (gas)