How does a bubbling fluidized bed boiler differ from a circulating fluidized bed boiler in terms of particle size and gas velocity?

Bubbling fluidized bed (BFB) boilers and circulating fluidized bed (CFB) boilers are both types of fluidized bed combustion technologies, but they differ significantly in particle size and gas velocity. In a BFB boiler, the gas velocity is lower, typically around 1 to 3 meters per second. This lower velocity causes a distinct bed of solid particles, such as sand, ash, and fuel, to be suspended by the upward flow of air, creating a bubbling effect similar to a boiling liquid. The particle size in a BFB boiler is generally larger, ranging from approximately 0.5 to 5 millimeters. This larger particle size and lower gas velocity result in a relatively well-defined bed with limited particle carryover from the combustion chamber. In contrast, a CFB boiler operates with a much higher gas velocity, typically in the range of 5 to 10 meters per second. This higher velocity causes the majority of the solid particles to be carried out of the combustion chamber and into a cyclone separator. The particle size in a CFB boiler is smaller, typically ranging from 0.1 to 1 millimeter. The cyclone separator captures the entrained particles and returns them to the combustion chamber, creating a continuous circulating loop. This circulating action promotes better mixing, more uniform temperature distribution, and improved combustion efficiency compared to BFB boilers. The smaller particle size and higher gas velocity in CFB boilers also enable them to handle a wider range of fuel types and sizes, and they tend to produce lower NOx emissions.