Fluidized bed combustion (FBC) is an efficient technology used to burn solid fuels, such as coal, biomass, or waste. The process suspends fuel particles within a bed of hot, granular inert material, often sand or ash, by injecting an upward stream of air. This suspension causes the mixture of solids and gas to behave like a fluid, creating a highly turbulent and controlled burning environment. FBC offers distinct advantages over conventional combustion by integrating pollution control directly into the process.
The Fluidization Process
FBC relies on transforming a bed of solid particles from a static state into a dynamic, fluid-like state. This occurs when an upward flow of air is introduced through a perforated plate, known as the distributor plate, at the base of the combustion chamber. As the air velocity increases, it counteracts the gravitational force on the solid particles, causing them to lift and become suspended. The minimum velocity required to achieve this suspension is termed the “minimum fluidization velocity.”
Once this velocity is surpassed, the entire bed of material begins to vigorously mix and bubble, resembling a boiling liquid. This turbulent movement ensures rapid and intimate contact between the solid fuel, the sorbent material, and the combustion air. The constant, intense mixing of the bed material and fuel promotes highly effective heat transfer, which is a significant factor in the system’s overall efficiency.
The fluidization mechanism also distributes heat throughout the bed with high uniformity, preventing the formation of localized hot spots. This uniform temperature distribution is a consequence of the intense particle movement and gas mixing. The high rate of heat transfer allows heat-exchange tubes to be submerged directly within the bed material. This enhances the transfer of thermal energy to the working fluid, such as water or steam, creating a steady environment for controlled combustion.
Environmental Advantages of Fluidized Bed Combustion
The fluidization process results in two significant environmental benefits: the reduction of sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxide (\(\text{NO}_x\)) emissions. The design inherently allows for the capture of \(\text{SO}_2\) during the combustion phase itself, eliminating the need for expensive, external flue gas treatment systems. This is achieved by adding a sorbent, typically limestone or dolomite, directly into the combustion bed.
The calcium carbonate in the limestone reacts chemically with the sulfur released from the fuel, a process known as in situ sulfur capture. This reaction converts the sulfur compounds into solid calcium sulfate, an inert, non-gaseous product easily removed with the ash. FBC systems achieve a substantial reduction in \(\text{SO}_2\) emissions, often capturing 90% to 95% of the sulfur content.
FBC technology limits the formation of thermal \(\text{NO}_x\) emissions by operating at lower temperatures than conventional boilers. Traditional combustion processes often exceed \(1370^\circ\text{C}\) (\(2500^\circ\text{F}\)), a temperature where nitrogen and oxygen in the air readily combine to form \(\text{NO}_x\) pollutants. Fluidized beds operate within a moderate range of \(760^\circ\text{C}\) to \(930^\circ\text{C}\) (\(1400^\circ\text{F}\) to \(1700^\circ\text{F}\)). This lower thermal environment significantly inhibits the chemical reactions responsible for forming thermal \(\text{NO}_x\), resulting in inherently lower nitrogen oxide output.
Operational Types of Fluidized Bed Combustors
Fluidized bed combustors are categorized into three operational configurations, defined by the velocity of the fluidizing air and the movement of the bed material. The Bubbling Fluidized Bed (BFB) uses a relatively low air velocity, around 2 meters per second. In this design, the bed material remains contained within the combustion chamber, and distinct gas bubbles rise through the dense bed, resembling a boiling pot. BFB systems are structurally simpler and often used for smaller-scale applications.
The Circulating Fluidized Bed (CFB) employs a much higher air velocity, often reaching 4.5 to 6 meters per second. This increased velocity carries most of the bed material, including unburnt fuel and sorbent, out of the chamber with the exhaust gases. These entrained solids are captured by a cyclone separator and cycled back into the combustor, forming a continuous loop. This constant circulation provides a longer residence time for the fuel and sorbent, enhancing combustion efficiency and pollutant capture.
The third type is Pressurized Fluidized Bed Combustion (PFBC), which operates under high pressure, whether bubbling or circulating. Operating at elevated pressures, often up to 16 times atmospheric pressure, allows the equipment to be significantly smaller for the same power output. The high-pressure, hot exhaust gases drive a gas turbine, while heat recovered from the bed generates steam for a separate steam turbine. This combined-cycle operation results in a higher overall electrical efficiency compared to atmospheric FBC systems.