Circulating Fluid Bed Combustors

Manzoori AR, Agarwal PK. The role of inorganic matter in coal in the formation of agglomerates in circulating fluid bed combustors. Fuel 72 1069-1075, 1993. 

Refractory Applications in Refineries and Circulating Fluid Bed Combustors... 

Crowley MS, Bakker WT, Stallings J. Refractory applications in circulating fluid bed combustors. Eleventh International Conference on Fluidized Bed Combustion, ASME, Montreal, Quebec, Canada, April 21-24, 1991. 

CR S is operating a 10 kg/h circulating fluid bed unit with the char combustor integrated into the base of the CFB riser as a bubbling fluid bed (22) as indicated in Figure 5. Liquid yields of 60-70% wt on a dry feed basis have been achieved. 

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. 

An advanced power plant can use FBC to increase its fuel efficiency and lower harmful emissions. Inside the combustor, there is a bed of inert material that will not be burned itself, typically a form of sand. To the solid fuel particles that are added to the bed is mixed a solid sorbent, which absorbs potentially harmful substances released during combustion. Typically, when coal is used as fuel, limestone or dolomite is used as sorbent to absorb sulfur that would otherwise react and be emitted as sulfur oxide, a strong atmospheric pollutant. The solid particles are fluidized by the injection of a fluid, typically heated air. There are two kinds of arrangements for the bed of solids that is hit by the fluid, either a bubbling fluid bed or a circulating fluid bed. 

A system in which defluidization can present a particular problem is the combustion of low-rank coals in a circulating fluid bed (CFB) combustor. Some low-rank coals have a high content of sodium and sulphur, and at operating temperatures in the region of 850°C... 

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. 

Golriz MR, Sunden B. A method for temperature measurements in circulating fluidized bed combustors. Exp Thermal Fluid Science 9 274-282, 1994a. 

Reh L Fluid dynamics of CFB combustors. Circulating fluidized bed technology V, Beijing, 1996, Science Press, 1—15. 

Saroff, L., Zitterbaz, M.T., Ahaz, A.K. and Hooper, H.M. (1989) "A Relationship between Solids Recycling and Sulfur Retention in Fluidized Bed Combustors", in Proc. 10th Int. Conf. on Fluid. Bed Comb., San Francisco, 1003-1006 Shimizu, T., M. Inagaki and T. Furusawa (1990), "Effects of Sulfur Removal and Ammonia Injection on NO Emission From a CFBC", in Proc. of 3rd Int. Conf. on Circulating Fluidized Beds, Oct 15-18, Nagoya, Japan... 

The mechanisms of heat transfer are intimately related to the circulating bed fluid and particle mechanics. Heat is released by reaction throughout the combustor volume. It is carried to the steam-raising surfaces largely by convective flows of solids which have considerably higher thermal capacity than the low density gas phase. Transfer of heat then occurs by contact of the solids with the heat transfer surface in what is generally a downflowing wall layer. 

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