Fluidized beds offer several advantages over traditional energy generation methods from solid
fuels, notably low NOX emission, in-process capture of SO2 and the ability to efficiently burn a wide
range of low-grade, unequal quality, and potentially problematic fuels (including waste and
biomass), as well as mixed fuels.
The phenomenon "fluidization" of a solid powder-like material (e.g., sand) takes place when a fluid (e.g., the combustion air) is passed up through the powder-like material. If the gas velocity is increased above a certain limit, the powder-like material starts expanding and behaving like a fluid. This is the "fluidized bed". Solid fuel particles can now be added, which float in the bed, and chemical conversion processes (e.g., combustion) may take place.
Due to the fluidization of the bed material and the fuel particles the three-dimensional motion of the solids ensures good mixing conditions in horizontal and vertical directions. This excellent mixing behavior leads to even temperature and fuel distributions within the combustion chamber, which is a significant advantage compared to grate furnaces and pulverized fuel combustors.
Floating in the bed allows sufficient time for complete combustion of the solid particles, which allows bigger fuel particle sizes and lower temperature. This is also the reason for the well-known flexibility against fuel composition and quality.
Due to the even temperature distribution and lower temperature levels (typically 850°C) no thermal NOX is produced, which results in low NOX emission.
In-situ desulfurization (SO2 removal) is possible with direct limestone addition into the fluidized bed, which results in low SO2 emission without external equipment for desulfurization.
The history of this environment-friendly and highly economical technology started in 1922 already, but its further development sped up in the mid 60s only, and the first commercial FBC units were put into operation in the early 80s. The history since then indicates a convincing success story for fuels ranging from top quality coals to the most problematic solid wastes and biomasses. Nowadays rapid growth can be observed in both unit sizes and number of installed units worldwide, which necessitates intensive research activities on technology developments, optimal fuel mix adaptations, and scale-up.