Atmospheric pressure fluid bed combustors

Both the atmospheric and pressurized fluid bed combustors bum coal with limestone or dolomite in a fluid bed that, with recent modifications to the system, allows the limestone sorbent to take up about 90% of the sulfur that would normally be emitted as sulfur dioxide. In addition, combustion can achieved at a lower temperatnre than in a conventional combustor thereby reducing the formation of nitrogen oxide(s). 

In preparation for the pilot plant design and testing, numerous tests were conducted in subscale fluid bed combustors to develop feed and gas cleanup hardware and to evaluate process conditions. The majority of these tests were accomplished using a 0.20 m FBC. Early testing of the pilot plant was conducted at atmospheric pressure conditons. Operation of the CPU-400 pilot plant on MSW at its design point of 4.5 atmospheres was short lived. 

Today the circulating fluidized bed (CFB) has become the dominating design for combustors operated at atmospheric pressure. Pressurized circulating fluidized bed combustors are under development for combined power cycle applications, but so far no clear advantages have been revealed yet. For this reason the existing commercial pressurized fluid bed systems are bubbling beds. 

Fluidized-bed combustors can be either atmospheric or pressurized (Yeager and Preston, 1986). The atmospheric type operates at normal atmospheric pressure while the pressurized type operates at pressures 6-16 times higher than normal atmospheric pressure. The pressurized fluid-bed boiler offers a higher efficiency and less waste products than the atmospheric fluid-bed boiler. There is also a circulating (entrained) bed combustor which allows for finer coal feed, better fuel mixing, higher efficiency, as weU as an increased sulfur dioxide capture. 

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