Flue Gas Desulfurization (FGD) Technology

Flue Gas Desulfurization (FGD) Technology

Why in News? 

A recent expert panel led by Principal Scientific Advisor Ajay Sood has suggested scrapping India’s decade-old rule mandating FGD units in coal power stations.

Flue Gas Desulfurization (FGD) Technology

• Introduction

• • • Flue Gas Desulfurization (FGD) is a technological process used to eliminate sulfur dioxide (SO₂) from gases released by the combustion of coal and other fossil fuels.
    • FGD systems are extensively used across thermal plants, waste burners, petroleum refineries, and cement or lime processing industries.
    • The core aim of this technique is to regulate excess sulfur oxide emissions into the environment.

• Purpose

• • • To control the release of SO₂ generated during the combustion of coal.
    • To prevent sulfuric acid formation in the atmosphere, which results in acid rain.
    • To protect crops that are vulnerable to damage from acidic precipitation.
    • To safeguard buildings and infrastructure from deterioration caused by corrosive pollutants.
    • To preserve aquatic ecosystems and water bodies from harmful contamination.
    • To ensure cleaner air by purifying emissions from thermal power stations.

• Special

• • This technology was first experimented with in the 1850s in England.
  • The landmark use of FGD technology began in 1931 when London’s Battersea Power Station adopted it on a large scale.
  • Subsequently, Swansea Power Station (1935) and Fulham Power Station (1938) adopted this technology.
  • The International Maritime Organization (IMO) has also endorsed the installation of FGD-based exhaust cleaning systems on ships under MARPOL Annex VI norms.

Working Mechanism and Types of FGD

• Process Functionality

• • • The FGD system begins by directing the flue gas emitted from power plants into a specialized chamber.
    • In this chamber, it undergoes absorption using an alkaline sorbent such as limestone (CaCO₃), lime (CaO), or ammonia (NH₃).
    • During this chemical reaction, SO₂ is transformed into sulfates or sulfites, which are then collected in solid form.
  • Types

• Wet Scrubbing System: The most commonly applied method, it uses a liquid slurry made from lime or seawater. The flue gas passes through this slurry, allowing SO₂ to dissolve and form compounds. This method can remove over 90% of SO₂ emissions.

• • • • The byproduct, gypsum (CaSO₄·2H₂O), is widely used in the cement industry.

• Dry Sorbent Injection (DSI): This process involves injecting hydrated lime (Ca(OH)₂) directly into the gas stream. The sorbent reacts with SO₂, forming solid compounds that are later filtered. It is cost-effective and simpler than wet systems, making it ideal for small-scale plants.

• Semi-Dry Scrubbing (Spray Dry): Combines features of both wet and dry systems. Flue gases are passed into a reactor chamber, where semi-liquid slurry is sprayed. The slurry’s chemicals react with SO₂, producing solid particles. Final filtration is done using a baghouse system to separate the residual solids.

• • Seawater-Based FGD: Implemented mostly in coastal regions, this technique uses the natural alkalinity of seawater. The SO₂ reacts with seawater, converting into sulfates. While low-cost, its efficiency is limited, making it suitable for regions with mild emission norms.

Advantages and Challenges of FGD Technology

• Advantages

• Significant Reduction in Air Pollution: Using FGD units can result in over 90% reduction in SO₂ emissions. This pollutant is a leading cause of acid rain, which damages crops, water bodies, and infrastructure.

• • • • Since 2015, India’s pollution control norms have required FGD installation in many plants, leading to AQI improvement in urban areas.
    • Useful Byproducts for Industry: FGD systems convert sulfur dioxide into gypsum (CaSO₄·2H₂O), a valuable material in cement manufacturing.
      • As per the Central Electricity Authority (CEA), India can generate over 5 million tonnes of gypsum annually from FGD units—adding economic value to pollution control.
    • Positive Public Health Outcomes: According to the World Health Organization (WHO), prolonged exposure to SO₂ can cause respiratory issues, asthma, and cardiovascular problems. By significantly reducing sulfur dioxide emissions, FGD systems play a critical role in improving air quality.
      • WHO estimates suggest 7 million premature deaths annually due to air pollution—many of which can be prevented through such technologies.
    • Supports Environmental Compliance: FGD usage enables thermal plants to meet national and international environmental norms, including those under MARPOL and CPCB guidelines, helping avoid penalties and legal action.

• Challenges

• High Initial Investment: The installation of an FGD system requires significant capital investment. According to estimates, the installation cost per megawatt ranges from ₹30 to ₹40 lakh.

• • • • In developing countries like India, where many thermal power plants operate under financial constraints, this cost becomes a policy challenge.

• Complexity in Maintenance and Operation: The chemical equipment and scrubbers used in FGD systems demand regular cleaning and inspections. In wet systems, the moisture and residues that accompany the flue gas can lead to corrosion of components. This results in frequent replacements.

• Additional Power Consumption: To run FGD systems, plants need to spend extra electricity. The process involves the use of pumps, scrubbers, heaters, and other supporting equipment, which collectively reduce the plant’s energy efficiency by 2–3%.

• Environmental Impact: Solid and liquid waste generated by FGD systems can become environmental hazards if not managed properly. Particularly in wet FGD systems, chemical slurries and metallic residues are formed, which must not be discharged into natural water sources.

FGD Policy in India: Current Status and Developments

• FGD Policy

• • • In a significant step towards air pollution control, the Ministry of Environment, Forest and Climate Change (MoEF&CC) issued a directive in 2015 mandating the installation of FGD systems in all 537 coal-based thermal power plants in India.
    • As per the 2015 notification, each plant was required to install FGD units within the stipulated time frame.
    • The revised 2022 notification categorized plants into Class A, B, and C, with deadlines as follows:
      • Class A: December 2024
      • Class B: December 2025
      • Class C: December 2026

• Current Status

• • • Only 39 plants (19,430 MW) have completed FGD installation so far.
    • Work is ongoing at 238 plants (1,05,200 MW).
    • 139 plants (42,847 MW) are still in the tendering stage.
    • 121 plants (36,683 MW) remain in the pre-tender phase.
    • Breakdown by category of installed capacity: Class A: 11 units (4,390 MW), Class B: 2 units (1,160 MW), Class C: 26 units (13,880 MW)
    • Installing an FGD unit typically costs around ₹1.2 crore per megawatt of generation capacity.
    • India’s total coal-based capacity stands at 2,18,000 MW, projected to grow to 2,83,000 MW by 2032.

• Challenges

• • • The technology is relatively new in India, and local suppliers have limited capacity.
    • Currently, India can only install FGD systems for 16–20 GW annually (about 33–39 plants).
    • Due to variations in design, layout, and space, standardization is lacking, forcing plants to opt for customized engineering solutions.
    • Around 92% of Indian coal contains only 0.3%–0.5% sulfur, which is low by international standards.
    • Given India’s tropical climate and the use of high chimneys (up to 220 meters), SO₂ disperses significantly in the atmosphere, resulting in limited local air quality impact.
    • India predominantly burns high-ash coal, making particulate matter (PM) pollution a greater concern.

• Solutions

• • Technologies like Electrostatic Precipitators (ESP) can control 99% of particulate matter at a cost of ₹25 lakh per MW, offering a cost-effective alternative.
  • Some experts suggest applying FGD systems only in plants that use imported coal or coal with sulfur content >0.5%.