}]]}]}]}] 30:T4d81,

### Fluidized Bed Combustion (FBC) Principle Fluidized Bed Combustion (FBC) is a combustion technology used in boilers to burn solid fuels. The key principle is to suspend solid fuel particles in an upward-flowing stream of air. - **Fluidization:** Air is blown through a bed of inert material (like sand or limestone) and fuel particles. At a certain velocity, the solid particles become suspended and behave like a fluid, creating a "fluidized bed." - **Combustion:** Fuel is continuously fed into this fluidized bed. The intimate mixing of fuel, air, and inert material ensures efficient and uniform combustion at lower temperatures ($800-900^\circ C$) compared to conventional boilers. - **Pollution Control:** Limestone in the bed reacts with sulfur dioxide ($SO_2$) released from burning coal, capturing it *in-situ* and reducing sulfur emissions. Lower combustion temperatures also reduce nitrogen oxide ($NO_x$) formation. - **Fuel Flexibility:** FBC boilers can efficiently burn a wide variety of fuels, including low-grade coals, biomass, and industrial wastes, due to the excellent mixing and heat transfer characteristics. ### Circulating Fluidized Bed (CFB) Boiler A Circulating Fluidized Bed (CFB) boiler is an advanced type of FBC boiler characterized by a higher fluidization velocity and continuous circulation of bed material. - **Operation:** Unlike bubbling FBC (BFBC) where particles remain largely within the bed, in CFB, the fluidization velocity is high enough to entrain a significant portion of the bed material and unburnt fuel particles out of the combustion chamber. - **Cyclone Separator:** These entrained solids are separated from the flue gas by a cyclone separator and then returned to the combustion chamber. This continuous circulation improves combustion efficiency, allows for longer residence times for fuel particles, and enhances pollutant capture. - **Advantages:** - Higher combustion efficiency due to longer residence time of fuel. - Better sulfur capture efficiency due to increased contact time between limestone and $SO_2$. - Even greater fuel flexibility. - Reduced $NO_x$ emissions due to staged combustion and lower temperatures. - Compact design for larger capacities. ### Compounding of Steam Turbine Compounding in steam turbines refers to the method of reducing the rotational speed of the turbine shaft to a practical limit. High-speed steam exiting the nozzles can reach speeds of over 1000 m/s, which would lead to extremely high turbine rotor speeds (e.g., 30,000 rpm). Such high speeds cause: 1. High stresses in turbine blading. 2. High vibrational losses. 3. High reduction gear ratio requirements for power generation (typically 3000 rpm). Compounding is achieved by absorbing the kinetic energy of the steam in stages, allowing the rotor to rotate at a lower, more manageable speed. The main types are: - **Velocity Compounding:** Steam expands in a single nozzle, gaining high velocity, which is then absorbed in multiple rows of moving and fixed blades. - **Pressure Compounding:** Steam is expanded in multiple stages, with each stage consisting of a nozzle and a moving blade row. - **Pressure-Velocity Compounding:** A combination of both methods. #### Pressure Compound Method In the pressure compound method (also known as Rateau turbine), the total pressure drop of the steam is divided into a number of stages. Each stage consists of a set of fixed nozzles followed by a row of moving blade