Fluidized bed combustion boilers

Fluidized bed combustion boilers are often conventional boilers but with unconventional, specially designed furnace beds. Fluidized bed combustion boilers are used to burn solid fuels, such as low-grade, high-sulfur crushed coal. The benefit of this process is its ability to provide 95 to 100% combustion (depending on the reactivity of the fuel) combined with reduced emissions of sulfur dioxide (S02) and nitrogen oxides (NOx). 

Northern States Power Co. Alternative Fuel Firing in Atmosphere Fluidized-Bed Combustion Boiler. EPRI CS-4023. Final Report. June 1985. 

Cao, B. Internal Separator for Circulating Fluidized Bed Combustion Boiler, in Circulating Fluidized Bed Technology (Basu, P-, ed.), p. 385. Pergamon Press, Canada (1985). 

Na, Y., Lu, Q., and Wanf, T. The Model Test of Staged Circulating Fluidized Bed Combustion Boiler with Louver Type Separators, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 613-617. Somerset, Pennsylvania (1993). 

Fluidized-Bed Combustion The principles of gas-solid fluidization and their application to the chemical process industry are treated in Sec tion 17. Their general application to combustion is reviewed briefly here, and their more specific application to fluidized-bed boilers is discussed later in this section. 

Chen, G. Fang, M. Luo, Z. Li, X. Shi, Z. Cen, K. Ni, M. Preto, I., Experimental research on rice husk combustion in CFB boiler and the design of a 35 t/h rice husk-fired CFB Boiler. 14th International Conference on Fluidized Bed Combustion, ASME, Vancouver, 1997. 

Fluidized-bed combustion (FBC) is a different technology from conventional pulverized-coal-fired boilers. A mixture of powdered rather than pulverized coal and a sorbent (usually limestone) are injected directly into the furnace and burned while suspended on a bed of high-velocity air. The turbulent mixing of coal and sorbent causes the solids to behave as a pseudo-fluid, enhancing heat generation and transfer at much lower temperatures (800-900 °C) compared to conventional pulverized-coal-fired furnaces (1500 °C) (US Department of Energy 1992). 

A fluidized bed combustion system (FBC) is one that has a high temperature (1500 F to 1600 F) inert material, such as sand, ash, or limestone, occupying the bottom of the chamber.14 Figure 2-3 illustrates a typical fluidized bed boiler. Limestone, either as primary bed material, or as an addition, provides the additional advantage of S02 scrubbing.14,15 The advantage of fluidized bed combustion over the other 3 boiler types is that the fluidization of the inert bed material allows fuels with higher moisture and ash content to be burned, and still yield nearly complete combustion. Further, SOx control is easily and efficiently accomplished. The bed material is fluidized by one of two methods as described below. 

Pope, Kent M. Tires to Energy in a Fluidized Bed Combustion System. Presented at EPRI Conference Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991. 

There are several approaches available to a utility to construct a boiler that will meet New Source Performance Standards. These approaches can be classified according to the position in the combustion system at which pollutant control technology is applied. Precombustion control involves removal of sulfur, nitrogen, and ash compounds from the fuel before it is burned. For coal combustion this approach involves the application of coal-cleaning technology. Combustion control relies on modifications to the combustion process itself or the addition of material to the combustion process to reduce pollutant formation or capture the pollutants formed in the combustion chamber. Examples of combustion control include staged combustion, boiler limestone injection, and fluidized-bed combustion with limestone addition. Post-combustion control involves removal of pollutants after they have been formed but before they are released into the atmosphere. Traditionally, flue gas desulfurization has meant the application of postcombustion control either alone or in conjunction with another... 

Figure 12 Theoretical and experimental NO emissions at coal combustion that were calculated by model (90)—(95) (curve 7) and presented in the work by Gorban (2001,2006) equilibrium (1), maximum (2) actual (3-6) fluidized bed combustion (3), low-temperature combustion of brown coals (4), high-temperature combustion of hard coals (5), averaged for coal-fired boilers (6) A— prompt NO, B— fuel NO, C— thermal NO.

Fast or circulating fluidized bed combustion systems have become popular since the late 1970s and now represent the current level of activity in the area. It is clearly recognized that this technology has a stable future in the boiler market. In competition with conventional bubbling beds, the FAB/CFB boiler demonstrates the following advantages ... 

Duerrfels, H., and Failing, K. H. Circulating Fluidized-Bed Combustion in Boiler Firing with Low-Grade Fuels and Coal, Comm. Eur. Communities, [Rep.] EUR, EUR 9404,77(1985). 

Highley, J., and Kaye, W. G. Fluidized Bed Industrial Boiler and Furnaces, in Fluidized Beds Combustion and Applications (Howard, J. R., ed.), p. 127. Applied Science, London-New York (1983). 

Herstad, S., and Olofsson, J. Operating Experience of Circulating Fluidized Bed Boiler, The 8th Int. Conf. on Fluidized Bed Combustion, Houston, Texas (1985). 

In the past 10 years, the circulating fluidized bed (CFB) boiler as an efficient and clean combustion technology has rapidly developed and is now widely accepted throughout the world (Kullendorif and Anderson, 1986 Yerushalmi, 1986 Engstrom and Lee, 1991 Xu and Zhang, 1991). Its pronounced characteristics are... 

Lee, Y.. and Hyppanen, T. A coal combustion model for circulating fluidized bed boilers, International Conference on Fluidized Bed Combustion, San Francisco, California, April 30-May 3, Vol. 2, pp. 753-764 (1989). 

Finally, the low-sulfur char is used as a clean boiler fuel for electric power generation. The hot char can be used directly, at pressure, with obvious advantages in thermal efficiency by using a pressurized, fluidized-bed combustion system (PFBC). The size consist of the char should ensure good performance when fluidized in the PFBC. Because the char has been partially desulfurized, the flue gas can easily meet a S02 emission specification of less than 0.8 lb per 106 Btu without the need to add limestone. This is in contrast to burning the Wyodak feed coal in a... 

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