Fluidized Bed

 The basic principle behind a fluidized-bed combustor is the distribution of the air homogeneously across a combustion area where the bed of the fuel and inert mineral particles are fluidized, that is, the air velocity achieves the mass suspension of the bed particles. This is made possible by blowing the air at relatively high pressures (around 0.15 bar concerning grate firing in which five times lower pressures are considered) through a distribution plate on top of which the bed of fuel and inert is positioned. A fluidized-bed burner, therefore, is a high cylindrical or square sectioned vessel that rests on the bottom part of a perforated plate, positioned to separate the furnace from a pressurized air-box. The inert material represents up to 98% of the bed material. It acts as a grate because it supports the fuel which floats in it and causes the abrasion of the fuel particles. It also acts as a refractor by radiating heat and providing conduction due to the continuous turbulent collisions with fuel particles. The high turbulence optimizes combustion and hence, low excess air is required (from 10% to 30%). The mixing effect can be used to reduce emissions that are formed by injecting reactants such as urea for NOx control and limestone for reducing acid gases, directly into the bed .   

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