Lime, in various forms such as limestone, quicklime, and hydrated lime, is primarily used in coal-fired power plants to remove sulfur oxides from flue gas. This process is known as flue gas desulfurization (FGD).
Flue Gas Desulfurization (FGD)
The combustion of coal releases sulfur oxides (primarily sulfur dioxide - SO2 and sulfur trioxide - SO3) into the flue gas. These sulfur oxides contribute to acid rain and respiratory problems. Lime-based FGD systems effectively capture these pollutants.
Here's a breakdown of how lime is used:
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Reaction with Sulfur Oxides: Lime (calcium oxide - CaO) or limestone (calcium carbonate - CaCO3) reacts with the sulfur oxides in the flue gas to form calcium sulfite (CaSO3) and calcium sulfate (CaSO4), also known as gypsum.
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Wet Scrubbing: In wet scrubbing systems, a slurry of lime or limestone is sprayed into the flue gas stream. The sulfur oxides dissolve in the slurry and react with the lime, forming solid calcium sulfite and calcium sulfate.
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Dry Scrubbing: In dry scrubbing systems, hydrated lime (calcium hydroxide - Ca(OH)2) is injected into the flue gas duct. The hydrated lime reacts with the sulfur oxides to form solid calcium sulfite and calcium sulfate, which are then collected as dry powder.
Types of Lime Used
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Limestone (CaCO3): A naturally occurring sedimentary rock composed mainly of calcium carbonate. It is relatively inexpensive but less reactive than quicklime.
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Quicklime (CaO): Produced by heating limestone in a kiln. It is more reactive than limestone and requires hydration before use in some FGD systems.
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Hydrated Lime (Ca(OH)2): Produced by reacting quicklime with water. It is easier to handle and more reactive than quicklime, making it suitable for dry scrubbing systems.
Benefits of Using Lime in FGD
- Effective Removal of Sulfur Oxides: Lime-based FGD systems can remove up to 95% or more of the sulfur oxides from flue gas.
- Relatively Inexpensive: Lime and limestone are relatively inexpensive compared to other FGD technologies.
- Readily Available: Lime and limestone are abundant natural resources.
- Gypsum Production: The calcium sulfate produced as a byproduct can be sold for use in construction materials like drywall.
In summary, lime plays a crucial role in mitigating air pollution from coal-fired power plants by effectively removing sulfur oxides from flue gas. This helps to reduce acid rain and improve air quality.
