Flue Gas Desulfurization (FGD): Effective Solutions for SO₂ Emissions

Sulfur Dioxide Emissions and Their Effects
Sulfur dioxide (SO₂) is primarily released through the combustion of fossil fuels or other sulfur-containing materials. This compound poses a significant threat to both the environment and human health. It can harm trees and plants, hinder plant growth, and damage sensitive ecosystems and waterways. Furthermore, SO₂ contributes to respiratory diseases and worsens pre-existing heart and lung conditions.

• Environmental Acidification: SO₂ damages vegetation and ecosystems through acidification.
• Formation of Fine Particulate Matter: SO₂ plays a critical role in generating secondary fine particulate matter, which impacts air quality.
• High-Sulfur Fuels: Emissions are largely attributed to the combustion of fuels with a high sulfur content.

Health Impacts of Sulfur Dioxide Exposure
Sulfur dioxide is a respiratory irritant that can cause serious health issues when inhaled at high concentrations. Individuals with asthma, chronic lung diseases, or breathing difficulties are particularly at risk. Elevated levels of SO₂ in the air can trigger asthma attacks and lead to respiratory problems such as bronchitis. It can also irritate the nose, throat, and lungs, causing symptoms like coughing, wheezing, and increased mucus production. Physical activity during exposure may worsen these effects.

Additionally, sulfur dioxide has been linked to cardiovascular diseases, further highlighting its impact on overall health.

Wet Gas Scrubbers: A Key Solution for SO₂ Removal
The most widely used technique for removing sulfur dioxide (SO₂) from emissions is the use of wet gas scrubbers. These systems employ chemical scrubbers to treat contaminated gas through one or more stages of scrubbing, which can be arranged either horizontally or vertically for optimal efficiency. During this process, the gas flows through a packed bed or an open spray system, where it interacts vigorously with a washing liquid containing chemical reagents. Similar to other absorption processes, odor components and chemical particles dissolve in the washing liquid before undergoing reactions with the additives.

The Role of Washing Liquid in Wet Gas Scrubbers
The washing liquid is circulated using a centrifugal pump connected to the suction side of the liquid buffer, where the liquid is stored. In regions with hard water, a water softener may be used to ensure the efficiency of the cleaning process, as water hardness significantly impacts scrubber performance. The washing liquid is recirculated and periodically sprayed based on specific parameters, with the liquid level replenished using fresh water.

Addressing Water Hardness and Efficiency
If the fresh water used for replenishment contains high levels of magnesium and calcium, the hardness of the liquid may increase due to salt accumulation in the liquid buffer. This can compromise scrubber efficiency. To address this issue, facilities may employ water de-ironing systems or discharge a portion of the washing liquid. However, frequent discharge can lead to excessive water consumption, requiring a balanced approach to maintain efficiency while conserving water resources.

Additives in Washing Liquids for Sulfur Dioxide Removal
The application of additives in washing liquids is crucial for initiating chemical reactions with contaminants present in the gas. Specifically for sulfur dioxide (SO₂) emissions, it is essential to use additives that react swiftly and effectively with this compound. The choice of chemicals depends on the properties of SO₂ and the specific capabilities and requirements of the customer. In some cases, it is not permissible to simply supply chemicals, use them, and dispose of the residual products. Alternative cleaning agents or modifications to the scrubber’s configuration may be necessary to ensure an effective and sustainable solution.

Common Scrubbing Liquids for SO₂ Removal
The removal of sulfur dioxide from flue gases can be achieved by washing with a scrubbing liquid. The choice of absorbent depends on several factors, including process conditions, environmental regulations, and the client's specific needs. The most commonly used scrubbing liquids in wet gas scrubbers include:

• Sodium Hydroxide (NaOH)
  Sodium hydroxide reacts with sulfur dioxide to produce sodium sulfite and water. Sodium sulfite (Na₂SO₃) is a highly water-soluble salt derived from sodium and sulfurous acid. Upon contact with oxygen, it can convert into sodium sulfate (Na₂SO₄), which is also soluble. Regular draining of the liquid is required to maintain the scrubber’s efficiency.
• Calcium Hydroxide [Ca(OH)₂]
  Also known as lime, calcium hydroxide is an inorganic compound that forms a suspension called lime milk when mixed with excess water. After filtration, this produces lime water, a transparent solution. Calcium hydroxide is less aggressive than sodium hydroxide due to its limited solubility in water. Its use leads to the formation of calcium sulfite, a crystalline solid that is poorly soluble in water, making it easy to separate through filtration. When calcium sulfite reacts with oxygen, it forms calcium sulfate (gypsum), which can be extracted and repurposed, such as in the construction of gypsum boards and blocks.
• Ammonia (NH₄⁺)
  Ammonia solutions in water act as strong alkalis. These solutions are particularly useful when generating byproducts such as ammonium sulfate and ammonium sulfite is desired.
• Magnesium Hydroxide [Mg(OH)₂]
  Magnesium hydroxide appears as a white, water-insoluble solid. It is considered a safer alternative to sodium hydroxide for similar applications. However, its slower reaction time may necessitate larger scrubber columns and liquid buffers. Despite this limitation, it enables the safe capture of harmful substances, preventing their release into the environment.