What distinguishes circulating fluid bed boilers from bubbling fluid bed boilers?

Circulating fluidized bed (CFB) boilers are characterized by their design and operational principles, which allow for a longer fuel residence time compared to bubbling fluidized bed boilers. In CFB boilers, the bed material is aerated and kept in suspension with the help of high-velocity fluidizing gas. This results in a highly efficient system where the material is circulated between the furnace and the combustion chamber, which allows for optimal combustion and heat transfer.

The longer residence time in CFB systems enhances combustion efficiency and ensures that the fuel has adequate time to burn completely, contributing to lower emissions of unburned carbon and other pollutants. This extended contact between the fuel and the heat transfer surfaces leads to improved thermal efficiency.

Bubbling fluidized bed boilers, on the other hand, operate with a lower fluidization velocity, which causes the particles to form a distinct bed and bubble through the material. As a result, the residence time of the fuel is shorter, and therefore, the operational dynamics differ significantly between the two types of systems.

The other options, such as higher temperature ranges or defined bed levels, do not specifically capture the fundamental operational differences highlighted by the residence time. Similarly, while erosion rates may differ in practice due to different operating conditions