Flue gas desulphurization

Flue Gas Desulphurization (FGD) refers to a set of technologies designed to remove sulphur dioxide (SO₂) and other acid gases from the exhaust gases (flue gases) produced by fossil fuel-based power plants, industrial boilers, and other combustion systems. This environmental control technology plays a critical role in reducing air pollution, particularly acid rain formation, and in meeting stringent emission standards for sulphur oxides (SOₓ).

Background and Importance

Sulphur dioxide is produced when fuels containing sulphur — primarily coal and oil — are burned. Once released into the atmosphere, SO₂ reacts with water vapour and oxygen to form sulphuric acid, leading to acid rain, respiratory illnesses, and environmental degradation.
To curb these impacts, Flue Gas Desulphurization systems were developed and widely adopted beginning in the 1970s, especially in industrialised nations. The technology has become an essential component of air pollution control systems in coal-fired power plants worldwide.
In India, FGD gained prominence after the Ministry of Environment, Forest and Climate Change (MoEFCC) issued SO₂ emission norms in December 2015, requiring all large thermal power plants to install desulphurization units.

Principle of Operation

The basic principle of FGD involves bringing the flue gas into contact with a sorbent material — typically limestone (CaCO₃) or lime (CaO) — which reacts chemically with sulphur dioxide to form calcium sulphite (CaSO₃) or calcium sulphate (CaSO₄) (gypsum).
The general chemical reaction in a wet limestone system is:
SO2+CaCO3+½O2+2H2O→CaSO4⋅2H2O+CO2SO₂ + CaCO₃ + ½ O₂ + 2H₂O → CaSO₄·2H₂O + CO₂SO2​+CaCO3​+½O2​+2H2​O→CaSO4​⋅2H2​O+CO2​
This process effectively captures sulphur dioxide from the gas stream, converting it into a solid by-product that can be disposed of or commercially utilised.

Types of Flue Gas Desulphurization Systems

FGD systems are broadly classified into two categories based on the state of the sorbent and reaction medium: wet and dry/semi-dry systems.

1. Wet FGD Systems

• The most commonly used and highly efficient method (up to 95–98% SO₂ removal).
• Flue gas is passed through a scrubber tower where it comes in contact with a limestone slurry or other absorbent.
• The sulphur dioxide reacts with the slurry to form gypsum, which can be filtered and reused in the cement and construction industry.

Advantages:

• High efficiency in SO₂ removal.
• Produces usable gypsum as a by-product.
• Suitable for large thermal power plants.

Disadvantages:

• High capital and operating costs.
• Requires significant water usage.
• Sludge handling and disposal can be challenging.

2. Dry and Semi-Dry FGD Systems

• In dry systems, a powdered sorbent such as hydrated lime (Ca(OH)₂) is injected directly into the flue gas stream.
• In semi-dry systems (spray dryer absorbers), the sorbent is sprayed as a fine mist that reacts with SO₂ before drying into solid waste.

Advantages:

• Lower water consumption.
• Simpler operation and lower maintenance.
• Smaller footprint compared to wet systems.

Disadvantages:

• Lower removal efficiency (70–90%).
• Produces dry waste that must be landfilled.

3. Seawater FGD

• Uses alkaline seawater as the absorbent.
• The natural alkalinity (due to bicarbonates) reacts with SO₂, neutralising it into sulphates.
• The treated seawater is aerated and safely discharged back to the sea.

Advantages:

• No chemical sorbents required.
• Produces no solid waste.

Disadvantages:

• Limited to coastal locations with adequate seawater availability.

Components of a Typical FGD System

1. Absorber or Scrubber Tower: The chamber where flue gas and sorbent interact.
2. Gas-Gas Heater (GGH): Reheats the cleaned gas to avoid condensation and corrosion in the stack.
3. Mist Eliminator: Removes fine droplets before the gas is released to the chimney.
4. Slurry Preparation Unit: Mixes water with limestone or lime for the absorption process.
5. Gypsum Dewatering Unit: Separates and dries the by-product for reuse or disposal.
6. Reagent Handling System: Manages storage, feeding, and preparation of sorbent materials.

Efficiency and By-products

• Removal Efficiency:
  • Wet limestone FGD: 95–98%
  • Dry/semi-dry FGD: 70–90%
  • Seawater FGD: 90–95%
• By-products:
  • Gypsum (CaSO₄·2H₂O): Reusable in cement, plaster, and wallboard manufacturing.
  • Calcium Sulphite (CaSO₃): Can be oxidised to gypsum.
  • Solid Waste Residues: Disposed of in controlled landfills.

Application in India

India’s growing reliance on coal-based power generation (accounting for over 70% of total electricity generation) has made SO₂ control through FGD indispensable.

• The Central Electricity Authority (CEA) has directed all thermal power plants above 500 MW capacity to install FGD systems.
• National Thermal Power Corporation (NTPC) and other utilities have begun large-scale FGD retrofits across their power stations.
• Indigenous manufacturing capabilities for FGD equipment are being developed to reduce costs and dependence on imports.

By 2023, over 200 gigawatts of coal-based power capacity in India had been mandated to adopt FGD systems, with phased compliance targets extending to the mid-2020s.

Environmental and Economic Benefits

Environmental Benefits:

• Substantial reduction in SO₂ emissions, mitigating acid rain and particulate pollution.
• Improvement in air quality and reduction of respiratory ailments.
• Prevention of ecosystem acidification in soils and water bodies.

Economic Benefits:

• Generation of commercial-grade gypsum, reducing the need for natural gypsum imports.
• Enhanced corporate compliance with environmental norms, avoiding penalties.
• Potential for green financing and carbon credits under sustainable energy frameworks.

Challenges

1. High Capital Cost: Installation can cost between ₹0.5 to ₹1.5 crore per MW depending on plant size and system type.
2. Water Requirement: Particularly in wet FGD systems, water scarcity poses a significant challenge for inland plants.
3. Operational Complexity: Continuous maintenance and reagent management are required for optimal performance.
4. Space Constraints: Retrofitting older power plants with FGD units can be difficult due to limited space.
5. Waste Disposal: Handling of solid residues or sludge requires proper environmental management systems.

Future Prospects and Innovations

• Hybrid FGD Systems: Combining wet and dry technologies for improved flexibility and efficiency.
• Advanced Sorbents: Development of low-cost, high-reactivity absorbents to reduce operational costs.
• Integration with Carbon Capture Systems: Exploring combined technologies for multi-pollutant control.
• Automation and Digital Monitoring: Use of AI and IoT for real-time monitoring and optimisation of FGD performance.