Pressure fluid bed combustor

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. 

Verloop W, Boersma D, Van den Akker H, Hein K. The fluid dynamics of particles in the freeboard of a pressurized fluidized bed combustor. Proc n" FBC Conf, San Diego, CA, USA, 1993, pp 53-62. 

If turndown is desired, the grid pressure drop criteria (Eqs. 3 and 4) should apply at the minimum gas flow rate. This can be a problem for circulating fluidized bed combustors, since this means that under full load the grid pressure drop will be unacceptably high. Also, if the grid is curved, i.e., concave, convex, or conical, the criterion must apply with respect to the lowest hole on the grid. Take an example of a fluid bed with curved grid, as shown in Fig. 3. 

In a very recent study by Farzaneh Kaloorazi [42], it was presumed that the mechanical behavior of dense granular flows is not approximated sufficiently accurately by the soil mechanics theory representing the mechanical behavior of the system as a rate independent plastic regime characterized by a constant friction coefficient. It was claimed that somewhat improved model predictions were obtained for bubbling fluidized bed combustors with a somewhat more comprehensive theory proposed by Jop et al. [77]. This alternative frictional pressure tensor closure was derived based on visco-plastic fluid analysis. 

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