Heat transfer in circulating fluidised

Fundamental Aspects Of Heat Transfer In Circulating Fluidised Bed Combustion Systems... 

Heat transfer in circulating fluidised bed boilers occurs on two types of surfaces. The bulk of heat transfer from the combustion side to the water side occurs on planar membrane wall surfaces which shape the furnace. Heat may also be transferred to horizontal tubes located in external bubbling bed heat exchangers. The latter may be treated using conventional bubbling bed heat transfer correlations. These two surface locations are shown in Figure 15. This discussion focuses entirely upon heat transfer to vertical membrane wall type surfaces. These are identical to the surfaces found in more conventional boilers such as pulverised coal, oil fired and stoker systems. 

Chen, J.C., Cimini, R.J. and Dou, S.-S. (1988) "A theoretical model for simultaneous convective and radiative heat transfer in circulating fluidised beds", in P.Basu and J.F.Large (eds.), Circulating Fluidized Bed Technology II, Pergamon Press, Oxford, pp. 255-262. 

Leckner B. Heat transfer in circulating fluidized bed boilers. Circulating Fluidised Technology 3 (P Basu, M Horio, M Hasatani, eds.). Pergamon Press, Oxford, 1991. 

Zevenhoven R, Kohlmann J, Laukkanen T, Tuominen M, Blomster AM. Near-wall particle velocity and concentration measurements in circulating fluidised beds in relation to heat transfer. Proc 15 Conf Fluidized Bed Combustion, Savannah, Georgia, USA, 1999. 

Reactors with Field tubes should as a rule have a small diameter (400 mm), because a Field tube ensures that the heat is efficiently withdrawn from the whole surface of the apparatus only if the size is small. The Field tube is situated axially in the fluidised layer of contact mass and longitudinally selects the layer from top to bottom. There is cold tap water circulating in the tube (river water can not be tolerated, because it forms scale deposits in the tube and reduces the heat transfer coefficient). 

Figure 15. Possible locations for heat transfer surfaces in industrial circulating fluidised bed boilers and different commercial configurations. (Brereton, 1987).

Initial studies of circulating fluidised bed heat transfer show the importance of the solids in the heat transfer process. Kobro and Brereton (1986) presented results for CFBC sized solids at intermediate and combustion temperatures. These results suggest an approximately linear dependence of the heat transfer coefficient for a short annular calorimetric section upon the local suspension density. These results appear in Figure 16. Simple correlations for heat transfer coefficients may be based upon this type of data, with radiation accounting for most of the difference in heat transfer coefficient at elevated and ambient temperatures (Wu et al, 1989a). Assuming that the gas radiates as a grey cloud accounts for the radiation contribution adequately. 

Hence, while the macroscale fluid mechanics can likely be better described in a circulating fluidised bed than in a bubbling fluidised bed, the opposite may be said of the microscale heat transfer. 

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