ANS:D - all 'a' , 'b' & 'c'.

The correct statement among the options provided is: Presence of sulphur in the fuel fired in a furnace corrodes ducts & air preheater, if the flue gas temperature is low. While sulfur in the fuel can contribute to its heating value, and it can also aid in the electrostatic precipitation of ash from flue gases, it is primarily known for its corrosive effects on ducts and air preheaters, particularly when the flue gas temperature is low. Sulfur compounds in the fuel, when burned, can form acidic compounds such as sulfur dioxide (SO2) and sulfur trioxide (SO3). These acidic compounds can combine with moisture in the flue gases to form sulfuric acid (H2SO4), which is highly corrosive. This corrosion can lead to damage to ducts, air preheaters, and other components of the flue gas system.

1. Contribution to Heating Value: Sulfur can indeed contribute to the heating value of the fuel. This is because when sulfur-containing compounds are burned, they release energy during combustion, which contributes to the overall heat output of the fuel. However, it's important to note that sulfur's contribution to the heating value depends on various factors, including the type and concentration of sulfur compounds in the fuel, combustion efficiency, and the specific application.
2. Aid in Electrostatic Precipitation: Sulfur compounds in the fuel can aid in the electrostatic precipitation of ash from flue gases. During combustion, sulfur compounds can form sulfur dioxide (SO2) and sulfur trioxide (SO3), which are acidic in nature. These acidic compounds can react with alkaline ash particles in the flue gases, forming sulfates. These sulfates can have a higher electrical resistivity compared to the original ash particles, making them easier to capture using electrostatic precipitation methods. This process helps in reducing particulate emissions from the flue gases.
3. Corrosion of Ducts and Air Preheaters: Sulfur in the fuel can lead to corrosion of ducts and air preheaters, especially when the flue gas temperature is low. When sulfur-containing compounds are burned, they produce acidic gases such as sulfur dioxide (SO2) and sulfur trioxide (SO3). These acidic gases can react with moisture in the flue gases to form sulfuric acid (H2SO4), which is highly corrosive. This sulfuric acid can corrode metal surfaces, including ducts and air preheaters, leading to degradation and potential failure of these components over time. Low flue gas temperatures exacerbate this corrosion process by providing favorable conditions for acid condensation and corrosion.