Short Answer:

Flue Gas Desulfurization (FGD) is a process used to remove sulfur dioxide (SO₂) gas from the exhaust of power plants and industrial processes that burn fossil fuels like coal or oil. The main purpose of FGD is to control air pollution and prevent the formation of acid rain caused by SO₂ emissions.

The process works by passing flue gases through a scrubber where they come into contact with a liquid or solid absorbent, such as limestone or lime, which reacts chemically with sulfur dioxide to form gypsum or other harmless compounds. This helps in maintaining cleaner air and a healthier environment.

Detailed Explanation :

Flue Gas Desulfurization (FGD)

Flue Gas Desulfurization (FGD) is a pollution control technology used to remove sulfur dioxide (SO₂) from the exhaust gases of fossil-fuel-based industries and power plants. The burning of coal, oil, and other sulfur-containing fuels releases SO₂ into the atmosphere, which is one of the major causes of acid rain. FGD systems are designed to minimize these emissions and protect the environment.

The process involves chemical reactions between the sulfur dioxide present in the flue gas and an alkaline substance (like lime or limestone) that absorbs and neutralizes the gas. As a result, sulfur dioxide is converted into solid or liquid byproducts, such as gypsum (CaSO₄·2H₂O), which can be safely disposed of or used in other industries.

1. Purpose of FGD:
   The main purpose of installing a flue gas desulfurization system is to reduce sulfur dioxide emissionsfrom power plants and other industrial units. Sulfur dioxide is a harmful gas that contributes to:

• Formation of acid rain.
• Damage to crops, soil, and aquatic life.
• Respiratory and heart diseases in humans.
• Corrosion of buildings and monuments.

By removing SO₂, FGD systems help industries meet environmental regulations and ensure cleaner air quality.

2. Principle of FGD Operation:
   The working principle of FGD is based on a chemical absorption process. In this method, flue gases containing SO₂ pass through a contact chamber or scrubber where they are treated with an alkaline absorbentlike lime (CaO), limestone (CaCO₃), or sometimes magnesium hydroxide (Mg(OH)₂).

The chemical reaction between sulfur dioxide and the absorbent forms a stable compound such as calcium sulfite (CaSO₃) or calcium sulfate (CaSO₄). In some systems, water is also used to help dissolve the gases and aid in the chemical reaction.

A simple reaction example is:
SO₂ + CaCO₃ + ½O₂ + 2H₂O → CaSO₄·2H₂O + CO₂
Here, sulfur dioxide reacts with limestone and oxygen to produce gypsum and carbon dioxide.

3. Types of Flue Gas Desulfurization Systems:
   There are mainly two types of FGD systems used in industries depending on the process used:

• Wet FGD System:
  In a wet FGD system, the flue gas comes into contact with a liquid absorbent. Limestone or lime slurry is sprayed into the gas stream in a scrubber tower. The SO₂ is absorbed by the slurry and chemically converted into gypsum. This method is widely used due to its high efficiency (up to 95–98%).
• Dry or Semi-Dry FGD System:
  In this process, a dry powder such as lime or sodium bicarbonate is injected into the flue gas stream. The gas reacts with the dry absorbent to form solid byproducts that are later collected using filters or electrostatic precipitators.
  Dry systems are simpler, require less water, and are suitable for small and medium-sized plants.

4. Components of a Typical FGD System:
   An FGD system mainly consists of the following components:

• Absorber or Scrubber Tower: Where the flue gas and absorbent come into contact for the desulfurization reaction.
• Reagent Preparation Unit: Used to prepare the absorbent solution (limestone or lime slurry).
• Mist Eliminator: Removes moisture droplets from the treated flue gas before releasing it into the atmosphere.
• Gypsum Dewatering System: Used to remove water from the gypsum formed during the reaction.
• Waste Handling Unit: Collects and manages the solid waste produced in the process.

These components work together to ensure maximum SO₂ removal and clean gas discharge.

5. Advantages of Flue Gas Desulfurization:

• Removes up to 95–98% of sulfur dioxide from flue gases.
• Reduces air pollution and prevents acid rain.
• Produces useful byproducts like gypsum, which can be used in the cement and construction industries.
• Helps industries meet strict environmental emission standards.
• Improves public health and reduces ecological damage.

6. Limitations of Flue Gas Desulfurization:

• High initial installation and maintenance costs.
• Requires a large space for equipment like scrubbers and slurry tanks.
• Generates solid waste that must be properly handled and disposed of.
• Wet systems consume significant amounts of water, which may not be suitable in dry regions.
  Despite these limitations, FGD remains one of the most effective methods for controlling sulfur dioxide emissions globally.

7. Applications of FGD Systems:
   Flue Gas Desulfurization systems are commonly used in:

• Thermal power plants using coal and oil as fuel.
• Cement plants and steel industries.
• Petroleum refineries and chemical industries.
• Waste incineration plants, where burning waste releases sulfur compounds.

These applications ensure that industries maintain environmentally safe emission levels.

Conclusion:

Flue Gas Desulfurization (FGD) is a vital pollution control process used to remove sulfur dioxide from industrial exhaust gases. By reacting SO₂ with alkaline materials like lime or limestone, harmful gases are converted into safe compounds like gypsum. This process helps prevent air pollution, reduces acid rain, and supports environmental sustainability. Although it involves high costs and maintenance, the long-term environmental benefits make FGD an essential system for modern power plants and industries aiming for cleaner production.