Flue gas desulfurization (FGD) is a crucial process in thermal power plants for removing sulfur dioxide (SO₂) from the exhaust flue gases. SO₂ is a significant pollutant produced by burning fossil fuels such as coal and oil. Here's an overview of the FGD process, its methods, and its significance:

Overview of Flue Gas Desulfurization

1. Purpose: The primary purpose of FGD is to reduce SO₂ emissions, which are harmful to the environment and human health. SO₂ can lead to acid rain, respiratory problems, and other environmental issues.

2. Process: FGD systems treat the flue gases before they are released into the atmosphere, capturing and converting SO₂ into a less harmful substance.

Common Methods of Flue Gas Desulfurization

1. Wet Scrubbing:

• Reaction: CaCO3+SO2→CaSO3+CO2CaCO3+SO2→CaSO3+CO2

• Oxidation: CaSO3+12O2+2H2O→CaSO4⋅2H2OCaSO3+21O2+2H2O→CaSO4⋅2H2O

• Limestone-Gypsum Process: This is the most commonly used method. Flue gas is passed through a slurry of limestone (calcium carbonate), which reacts with SO₂ to form calcium sulfite. The calcium sulfite is then oxidized to form gypsum (calcium sulfate), which can be used in the construction industry.

• Seawater Scrubbing: Uses natural alkalinity of seawater to absorb SO₂. The absorbed SO₂ is neutralized and then returned to the sea after treatment to ensure it meets environmental standards.

2. Dry and Semi-Dry Scrubbing:

• Reaction: Ca(OH)2+SO2→CaSO3⋅12H2OCa(OH)2+SO2→CaSO3⋅21H2O

• Spray Dryer Absorbers (SDA): Flue gas is sprayed with a fine mist of lime (calcium hydroxide) slurry. The SO₂ reacts with the lime to form a dry powdery residue that can be collected.

• Dry Sorbent Injection (DSI): A dry alkaline material, such as hydrated lime or sodium bicarbonate, is injected into the flue gas. The SO₂ reacts with the sorbent to form a dry, particulate matter that can be captured by downstream particulate control devices.

3. Regenerable Processes:

• Activated Carbon Process: SO₂ is absorbed onto activated carbon and then released in a concentrated form when heated. This allows for the recovery of sulfur.

Benefits of Flue Gas Desulfurization

1. Environmental Protection: Reduces SO₂ emissions, thereby minimizing acid rain and its harmful effects on water bodies, forests, and soils.

2. Health Benefits: Lowering SO₂ emissions improves air quality, reducing respiratory problems and other health issues among the population.

3. By-product Utilization: Gypsum produced in wet scrubbing processes can be used in the construction industry, particularly for manufacturing wallboard.

Challenges and Considerations

1. Cost: FGD systems can be expensive to install and operate, especially wet scrubbing systems which require significant water usage and produce a waste stream that must be managed.

2. Waste Management: The by-products, such as sludge or spent sorbents, must be handled and disposed of properly to avoid secondary pollution.

3. Energy Consumption: FGD processes can increase the energy consumption of power plants, slightly reducing their overall efficiency.

Conclusion

Flue gas desulfurization is a vital technology for reducing sulfur dioxide emissions from thermal power plants. It plays a significant role in protecting the environment and public health by mitigating the harmful effects of SO₂. Various methods are available, each with its own advantages and challenges, allowing power plants to choose the most appropriate technology based on their specific requirements and constraints.