Bed-To-Wall Heat Transfer in a Supercritical Circulating Fluidised Bed Boiler

Abstract

The purpose of this work is to find a correlation for heat transfer to walls in a 1296 t/h supercritical circulating fluidised bed (CFB) boiler. The effect of bed-to-wall heat transfer coefficient in a long active heat transfer surface was discussed, excluding the radiation component. Experiments for four different unit loads (i.e. 100% MCR, 80% MCR, 60% MCR and 40% MCR) were conducted at a constant excess air ratio and high level of bed pressure (ca. 6 kPa) in each test run. The empirical correlation of the heat transfer coefficient in a large-scale CFB boiler was mainly determined by two key operating parameters, suspension density and bed temperature. Furthermore, data processing was used in order to develop empirical correlation ranges between 3.05 to 5.35 m·s -1 for gas superficial velocity, 0.25 to 0.51 for the ratio of the secondary to the primary air, 1028 to 1137K for bed temperature inside the furnace chamber of a commercial CFB boiler, and 1.20 to 553 kg·m -3 for suspension density. The suspension density was specified on the base of pressure measurements inside the boiler's combustion chamber using pressure sensors. Pressure measurements were collected at the measuring ports situated on the front wall of the combustion chamber. The obtained correlation of the heat transfer coefficient is in agreement with the data obtained from typical industrial CFB boilers.

A comparison of fuzzy logic and cluster renewal approaches for heat transfer modeling in a 1296 t/h CFB boiler with low level of flue gas recirculation

“…The solid suspension density profile confirmed that above the secondary air injection levels bed particles transport occurred without a splash zone. This trend was also confirmed by Blaszczuk et al [7] and Błaszczuk [57] in several previous studies of solids flow structure.…”

“…This suggests that the furnace temperature has a greater influence on the heat transfer coefficient than the suspension density. A similar trend in heat transfer coefficient variation is observed both for laboratory/pilot units [20,58] and CFB boilers operating with low suspension densities (less than 20 kg/m 3 ) [7,8,[60][61][62][63]. The heat transfer data for both fuzzy logic and cluster renewal approach are close to each other.…”

“…It is well-known fact that the shorter tube length exhibits higher bed-to-wall heat transfer coefficient than the longer one, as indicated by Basu et al [22] and Han et al [64]. A number of researchers, for example: Blaszczuk et al [7,9], Dutta et al [8], Andersson et al [9], Cheng [14], Han et al [64], and Breitholtz et al [65] found that the bed-to-wall heat transfer coefficient increased with the suspension density in large-scale CFB coal combustors. This is expected because the thermal capacity of solids is much higher than that of gas.…”

“…During the test, the volumetric flow of flue gas recirculation into the furnace chamber was kept at the level of 20.5 m 3 n /s. The analyzed CFB boiler is described in detail in the works by Blaszczuk et al [7,8,31,38,56], particularly presenting the construction data, cross-sectional area, geometry of membrane wall, arrangement of measuring points and also data acquisition system. Besides, more detailed information on the flue gas recirculation system, flue gas analysis system and properties of such flue gases fed to the combustion chamber are described by Blaszczuk [57].…”

“…In the recent years, several research works concerning the fundamental analysis of the heat transfer process between the fluidizing medium and water tubes have been carried out both in laboratory and pilot-scale CFB facilities, for example: Chinsuwan et al [1], Nag et al [2], Lockhart et al [3], and Luan [4]. A few information is available on heat transfer data in large-scale circulating fluidized beds in the published literature were focused on: (i) application of artificial neural network approach to predict the bed-to-wall heat transfer coefficient, as discussed by Krzywanski et al [5]; (ii) utilization of cluster renewal approach in detailed heat transfer analysis inside CFB furnace, Basu et al [6], Blaszczuk et al [7][8][9], and Dutta et al [10,11]; (iii) heat transfer experimental methods, as described by Andersson et al [12]; and also (iv) heat transfer performance in CFB boilers with different configurations and loads, for instance in Wedermann et al [13] and Cheng et al [14]. To authors knowledge, there are no literature data about the heat absorption from bed particles to the vertical membrane walls during flue gas recirculation into CFB furnaces in large-scale.…”