Flue Gas Desulfurization (FGD) - TNPSC - Environmental Scientist
Flue Gas Desulfurization (FGD) - Study Notes
Introduction
Flue Gas Desulfurization (FGD) is a critical
technology used to remove sulfur dioxide (SO₂) from the exhaust gases of
fossil-fuel combustion, especially from power plants and industrial facilities.
Sulphur dioxide is a major air pollutant that contributes to acid rain, smog
formation, and respiratory problems in humans. As environmental awareness and
regulatory standards increase worldwide, FGD systems have become a standard
requirement in industries that burn sulphur-containing fuels like coal and oil.
Why is FGD
Necessary?
Fossil fuels like coal and oil often contain
sulfur. When these fuels are burned for energy, the sulfur in them is oxidized
to form sulfur dioxide (SO₂), a harmful gas. When SO₂ is released into the atmosphere,
it reacts with water vapor to form sulfuric acid (H₂SO₄), leading to acid rain.
Acid rain harms forests, aquatic ecosystems, buildings, monuments, and also has
direct health impacts on humans.
Moreover, regulations such as the Clean Air
Act (U.S.), EU’s Large Combustion Plant Directive, and India’s MoEFCC emission
norms mandate significant SO₂ emission reductions. Hence, FGD systems are vital
for industries to comply with these emission standards.
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Basic
Principle of FGD
FGD involves bringing the SO₂-laden flue gas
into contact with an absorbent or sorbent material, which chemically reacts
with the SO₂ to remove it from the gas stream. The most common reagents used
are:
·
Limestone
(CaCO₃)
·
Lime
(Ca(OH)₂)
·
Ammonia
(NH₃)
·
Sodium-based
compounds
Types
of FGD Systems
FGD systems can be broadly classified into two
main types based on the phase of the absorbent:
·
Wet FGD
·
Dry or
Semi-dry FGD
1. Wet FGD Systems
Wet systems use a liquid absorbent, typically
an aqueous solution or slurry. These systems are the most widely used due to
their high efficiency.
Wet Limestone-Gypsum Process
(Most Common)
·
The flue gas is passed through a scrubber where
it comes in contact with a limestone (CaCO₃) slurry.
·
The sulfur dioxide reacts with limestone and
oxygen to form calcium sulfite, which is then oxidized to form gypsum
(CaSO₄·2H₂O).
·
Gypsum is removed and can be used in the cement
and construction industries.
Key
Reaction:
Advantages:
·
High SO₂ removal efficiency (90–99%)
·
Gypsum by-product is commercially useful
·
Effective for large-scale plants
Disadvantages:
·
High capital and operational costs
·
Large water consumption
·
Requires sludge and waste handling
2. Dry
and Semi-Dry FGD Systems
Dry and semi-dry systems are used where water
availability is limited or for smaller plants.
Spray Dry Absorber (SDA) or
Semi-Dry Scrubbing
·
Lime slurry is sprayed into the hot flue gas.
·
The water evaporates and the lime reacts with
SO₂ to form dry calcium sulfite and sulfate.
·
The dry particles are collected using a bag
filter or electrostatic precipitator (ESP).
Reaction:
Advantages:
·
Lower water usage
·
Lower cost than wet systems
·
Smaller footprint
Disadvantages:
·
Slightly lower SO₂ removal efficiency (70–90%)
·
Dry waste requires landfilling
Dry Sorbent Injection (DSI)
·
Dry powdered lime or sodium-based sorbents are
injected directly into the flue gas duct.
·
The reaction is fast but less efficient compared
to wet systems.
3.
Alternative FGD Methods
Ammonia-Based FGD
·
Uses ammonia (NH₃) as the sorbent.
·
Produces ammonium sulfate ((NH₄)₂SO₄), a useful
fertilizer.
·
Less corrosive than limestone or lime systems.
Advantages:
·
Valuable by-product (fertilizer)
·
High removal efficiency
·
No scaling or plugging issues
Disadvantages:
·
Ammonia handling requires caution
·
Higher reagent cost
Seawater FGD
·
Used in coastal power plants.
·
Seawater absorbs SO₂ due to its natural
alkalinity.
·
Treated water is returned to the sea after
neutralization.
Advantages:
·
No solid waste
·
Environmentally sound when properly managed
Disadvantages:
·
Requires large volumes of seawater
·
Limited to coastal regions
Components
of an FGD System
Regardless of the type, most FGD systems
include the following components:
1.
Absorber/Scrubber
– Where the flue gas and absorbent come into contact.
2.
Reagent
Preparation Unit – Mixes water and sorbent to form slurry (in wet
systems).
3.
Gas-Gas Heater
(GGH) – Reheats flue gas before it exits to the stack to prevent
condensation and corrosion.
4.
Mist Eliminators
– Remove water droplets from treated flue gas.
5.
Sludge Dewatering
Unit – Removes water from solid by-products.
By-Products
of FGD
·
Gypsum
(CaSO₄·2H₂O) – Used in construction, drywall, cement.
·
Calcium
sulfite – Often landfilled unless oxidized.
·
Ammonium
sulfate – Used as fertilizer.
·
Dry waste
– Needs secure landfilling or disposal.
Environmental
and Economic Implications
Environmental Benefits:
·
Reduction in acid rain and related environmental
damage
·
Improved air quality
·
Less health impact on nearby populations
Economic Considerations:
·
High initial investment, but long-term savings
due to by-product utilization
·
Helps industries avoid heavy fines and shutdowns
·
Potential for revenue from gypsum or fertilizers
Challenges
in FGD Implementation
·
High capital and operating costs
·
Maintenance and scaling issues in wet systems
·
Disposal of solid waste in dry systems
·
Need for skilled operation and monitoring
·
Environmental concerns over wastewater discharge
Recent
Trends and Innovations
·
Hybrid
Systems: Combining wet and dry technologies for better flexibility
·
Advanced
Sorbents: Research into more efficient, cost-effective materials
·
Modular
FGD units: For small and medium industries
·
Automation
and IoT-based Monitoring: For optimizing reagent usage and reducing
operational costs
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