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AFBC Boiler Technology

Atmospheric Fluidized Bed Combustion (AFBC) technology has the potential to use alternative fuel sources such as coal, wood, or waste, and is able to reduce and control nitrogen oxide (NOx) and sulfur dioxide (SO2) emissions. AFBC involves burning sulfur-containing fuel particles suspended in an air stream, which causes them to behave like a fluid. The bed of particles is normally only about 10 percent fuel; the remainder is inert materials and sorbent (dolomite or limestone), which is used to capture up to 90 percent of the sulfur. This sorbent is continually injected into the bed while a gravity drain system withdraws spent material and ash particles. Combustion also occurs at relatively low temperatures (14(X) to 15(X) 'F), which maximizes sulfur capture. This low bed temperature also reduces NOx emission while minimizing clinker (a hard mass of fused furnace refuse) formation. Fly ash and spent sorbent are removed from the stack gas by particulate collectors. This technology is very insensitive to the fuel quality, allowing solids with a widely varying calorific value to be burned. The steam produced by AFBC ranges from low-pressure process steam to superheated high-pressure steam. AFBC has also been applied to cogeneration. where high-pressure steam drives turbines to generate electricity. Low pressure steam from the steam turbines is then used for process applications.

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Atmospheric fluidized bed boilers consist of a chamber in which fuel is burned "hilc being suspended in a gaseous mixture with inert material and sorbent. The sorbent (most commonly liniestolic) reacts with SO2 released during combustion to form a solid sulfate material. The fluidized bed is maintained at 1400 to 1500 'F to maximize sulfur capture. This low temperature also reduces NOx emission while minimizing clinker formation. Although these characteristics are common to all fluidized bed boilers, the fuel and solvent feed systems, ash recycle/removal methods, and heat transfer surface vary depending on the type of AFBC boiler. And there are mainly three types AFBC boilers, and they are Bubbling Bed, Circulating Bed, Multiple Bed.

AFBC Boiler Salient Features:

* Low fluidizing velocity for lower wear of bed tubes.
* Deep bed operation for better combustion.
* High pressure drop bed nozzles which ensures even air distribution even at low loads. No clicker formation in our AFBC boilers.
* Optimized stud pattern for long life.
* Flexibility to choose from Charcoal start-up, Oil assisted Charcoal start-up or pure Oil start-up.
* Flexibility to choose from either Underfeed, Overbed feeding or even a combination of both.
* Normally, superheaters are in convection and bed superheaters avoided for improved reliability.
* The smallest of the boilers will have proper drum internals and cyclones for effective steam separation.
* The ever reliable spreaders & feeders employed in overbed feeding.
* For abrasive fuels, special design considerations and provision of sacrificial shields.
* Continuous improvement & refinement ongoing.

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