The correct option is **B. Flue gas desulfurization**. ### Detailed Explanation **Flue Gas Desulfurization (FGD)** is a set of technologies used to remove sulfur dioxide (SO₂) from the exhaust flue gases of fossil-fuel power plants and other industrial processes. The primary goal of FGD is to reduce the amount of SO₂ released into the atmosphere, which is a significant contributor to air pollution and acid rain. #### How Flue Gas Desulfurization Works: 1. **Capture of Flue Gas**: After combustion, the flue gases, which contain SO₂, are directed through a scrubber system. This system is designed to treat the gases before they are released into the atmosphere. 2. **Chemical Reaction**: In the scrubber, the flue gas is typically passed through a slurry of limestone (calcium carbonate, CaCO₃) or lime (calcium oxide, CaO). The SO₂ reacts with the calcium compounds to form calcium sulfite (CaSO₃) or calcium sulfate (CaSO₄), which can then be removed from the gas stream. - **Reaction Example**: \[ \text{CaCO}_3 + \text{SO}_2 \rightarrow \text{CaSO}_3 + \text{CO}_2 \] 3. **Removal of Byproducts**: The resulting calcium sulfite can be further oxidized to form gypsum (CaSO₄·2H₂O), which is a useful byproduct that can be used in construction materials. 4. **Emission Reduction**: By converting SO₂ into solid compounds, FGD systems can reduce SO₂ emissions by up to 90% or more, significantly improving air quality. ### Why Other Options Are Incorrect: - **A. Catalytic Cracking**: This process is primarily used in the petroleum industry to break down large hydrocarbon molecules into gasoline and other products. It does not address sulfur dioxide emissions directly and is not designed for air quality improvement. - **C. Electrolysis**: This is a chemical process that uses electrical energy to drive a non-spontaneous reaction, often used in the production of metals or the decomposition of compounds. It does not relate to the reduction of sulfur dioxide emissions in industrial settings. - **D. Photocatalysis**: This process involves the acceleration of a photoreaction in the presence of a catalyst and light. While it can be used for various environmental applications, including the degradation of pollutants, it is not primarily focused on the removal of sulfur dioxide from industrial emissions. ### Common Pitfalls: - **Misunderstanding the Purpose of Each Process**: Students may confuse processes like catalytic cracking and electrolysis with those that specifically target air pollutants. It's essential to understand the primary function of each process. - **Overlooking the Role of Byproducts**: In FGD, the formation of useful byproducts (like gypsum) is a significant advantage, but students might overlook this aspect when considering the effectiveness of emission reduction technologies. ### Revision Summary: - **Flue Gas Desulfurization (FGD)** is the primary method for reducing sulfur dioxide emissions in industrial settings. - FGD works by chemically reacting SO₂ with calcium compounds to form solid byproducts, which can be removed from flue gases. - Other options like catalytic cracking, electrolysis, and photocatalysis do not primarily address sulfur dioxide emissions. - Understanding the specific applications and functions of different chemical processes is crucial for exam success.