In a fluidized bed boiler, limestone is primarily used as a:

In a fluidized bed boiler, limestone is primarily utilized as a de-sulphuring agent. This is due to its chemical properties that enable it to effectively react with sulfur dioxide (SO₂) released during the combustion of fuels containing sulfur. When limestone is introduced into the fluidized bed, it undergoes a calcination process during which it decomposes to form calcium oxide (CaO). The calcium oxide then reacts with sulfur dioxide to form calcium sulfate (CaSO₄), thus reducing the overall emissions of sulfur compounds into the atmosphere.

The use of limestone in this manner is crucial for meeting environmental regulations and minimizing air pollution generated by power plants. By enhancing the desulfurization process, limestone significantly contributes to cleaner combustion practices, making fluidized bed boilers more environmentally friendly options compared to traditional boilers.

In this context, the other options do not effectively capture the primary role of limestone in fluidized bed boilers. While it might seem plausible to consider limestone a fuel source due to its combustion characteristics, its purpose is not to provide energy but rather to facilitate the removal of sulfur. It does not function as a heat exchanger or contaminant; instead, it supports the overall efficiency and emission control of the system.