Managing Atmospheric Pollution » Flue-gas Desulfurisation

What you'll learn this session

Study time: 30 minutes

What flue-gas desulfurisation (FGD) is and why it's important
Different methods of removing sulfur dioxide from power plant emissions
How wet scrubber systems work in detail
The environmental benefits and challenges of FGD
Real-world examples of FGD implementation
Economic considerations and efficiency of FGD systems

Introduction to Flue-gas Desulfurisation

When we burn fossil fuels like coal and oil in power stations, they release harmful gases into the atmosphere. One of the most problematic is sulfur dioxide (SO₂), which causes acid rain, damages buildings, harms plants and can make breathing difficult for people. Flue-gas desulfurisation (FGD) is a technology that removes sulfur dioxide from the exhaust gases (flue gases) of power plants before they're released into the air.

Key Definitions:

Flue-gas desulfurisation (FGD): A set of technologies used to remove sulfur dioxide from exhaust flue gases of fossil-fuel power plants.
Scrubber: A pollution control device that uses a liquid to wash unwanted pollutants from a gas stream.
Sorbent: A material used to absorb or adsorb liquids or gases.
Slurry: A semi-liquid mixture, typically of fine particles suspended in water.

💡 Why Remove Sulfur Dioxide?

Sulfur dioxide contributes to several environmental problems:

Forms acid rain when it combines with water in the atmosphere
Damages forests, crops and aquatic ecosystems
Corrodes buildings and monuments
Causes respiratory problems in humans
Reduces visibility (creates haze)

🌎 Global Impact

Before FGD technologies became widespread, power plants were major contributors to acid rain. In the 1970s and 1980s, acid rain damaged forests across Europe and North America. Today, countries with strict emissions controls have seen dramatic reductions in sulfur dioxide emissions and acid rain damage.

How Flue-gas Desulfurisation Works

There are several methods to remove sulfur dioxide from flue gases, but they fall into two main categories: wet scrubbers and dry scrubbers. Let's look at how each works.

Wet Scrubber Systems

Wet scrubbers are the most common type of FGD system. They use a liquid slurry to remove sulfur dioxide from flue gases.

📈 Efficiency

Wet scrubbers can remove 90-99% of sulfur dioxide from flue gases, making them highly effective.

➕ Advantages

High removal efficiency, can handle high sulfur content fuels and produces useful by-products like gypsum.

➖ Disadvantages

Requires significant water use, creates wastewater that needs treatment and has high installation costs.

The Wet Scrubbing Process

Here's how a typical limestone wet scrubber works:

Preparation: Limestone (CaCO₃) is crushed and mixed with water to form a slurry.
Contact: Flue gas enters the scrubber and comes into contact with the limestone slurry, which is sprayed from nozzles.
Chemical Reaction: The sulfur dioxide in the flue gas reacts with the limestone slurry to form calcium sulfite (CaSO₃).
Oxidation: Air is blown into the mixture, which converts calcium sulfite to calcium sulfate (CaSO₄), also known as gypsum.
Collection: The gypsum is separated from the water, dried and can be sold for use in making plasterboard and cement.
Release: The cleaned flue gas, now with much less sulfur dioxide, is released through the stack.

The chemical reactions involved are:

CaCO₃ + SO₂ → CaSO₃ + CO₂

CaSO₃ + ½O₂ + 2H₂O → CaSO₄·2H₂O (gypsum)

Dry Scrubber Systems

Dry scrubbers use a dry sorbent or a slurry with minimal water to remove sulfur dioxide.

💡 Spray Dry Scrubbers

In this system, a slurry of lime (CaO) is sprayed into the hot flue gas. The water in the slurry evaporates, leaving dry particles that react with SO₂. The resulting dry waste is collected in a fabric filter or electrostatic precipitator.

Efficiency: 80-90% SO₂ removal

💡 Dry Sorbent Injection

Dry powdered sorbents like lime or sodium bicarbonate are injected directly into the flue gas stream. They react with SO₂ and are collected as dry waste.

Efficiency: 50-70% SO₂ removal

Environmental Benefits and Challenges

✅ Benefits

Significant reduction in acid rain
Improved air quality and visibility
Protection of ecosystems and buildings
Creation of useful by-products (gypsum)
Reduction in respiratory illnesses

⚠ Challenges

High water consumption (for wet systems)
Wastewater treatment requirements
Energy consumption (parasitic load)
Disposal of waste if not sold as by-product
High installation and operating costs

Case Study Focus: Drax Power Station, UK

Drax Power Station in North Yorkshire is one of the UK's largest power stations. In the 1990s, it was also one of the largest sources of sulfur dioxide emissions in Europe. Between 2007 and 2010, Drax installed FGD equipment on all six of its generating units.

Results:

SO₂ emissions reduced by over 90%
The FGD process produces about 15,000 tonnes of gypsum each week
This gypsum is sold to the construction industry for plasterboard manufacturing
The project cost approximately £300 million but has significantly reduced environmental impact

Economic Considerations

Installing FGD systems is expensive, but there are economic benefits too:

💰 Costs

High capital costs for installation
Ongoing operational costs
Energy consumption reduces plant efficiency

💲 Benefits

Sale of gypsum by-product
Avoided environmental damage costs
Avoided health costs

📈 Policy Drivers

Environmental regulations
Carbon taxes and emissions trading
Public pressure for cleaner energy

Future Developments

While FGD technology is mature, research continues to improve efficiency and reduce costs:

Advanced sorbents: New materials that can capture more SO₂ with less material
Multi-pollutant control: Systems that remove SO₂, NOx and mercury in a single process
Reduced water use: New designs that require less water or recycle water more efficiently
Energy recovery: Systems that capture waste heat from the FGD process

Summary

Flue-gas desulfurisation is a crucial technology for reducing the environmental impact of burning fossil fuels. While it adds cost to electricity generation, the environmental and health benefits are substantial. As countries work to reduce their carbon emissions, many power plants are switching from coal to cleaner fuels, but FGD systems will remain important for those fossil fuel plants that continue to operate during the transition to renewable energy.

Quick Facts

A typical 500 MW coal-fired power plant without FGD might emit 10,000 tonnes of SO₂ per year
With FGD, this can be reduced to less than 1,000 tonnes
The UK has reduced its SO₂ emissions by over 95% since the 1970s, largely due to FGD and switching to cleaner fuels
Modern FGD systems can achieve 99% SO₂ removal efficiency
The global market for FGD systems is worth billions of pounds

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