CFB含有金属丝的物料流化及循环特性冷态试验研究

在主床截面为600mm×400mm、高6000mm的大型循环流化床(CFB)冷态试验台上进行了含有金属丝的物料流化及循环特性试验。将不同长度不同直径的钢丝和铜丝加入炉膛,观察且对取样进行分拣,分析金属丝在CFB中的流化和循环特性。试验结果表明,加入长度20mm和50mm的金属丝会造成循环回路的堵塞,而加入长度5mm和10mm金属丝能够正常流化且循环回路正常运行,粗金属丝极易沉积在返料器中。

Heat Transfer in a Liquid-Solid Circulating Fluidized Bed Reactor with Low Surface Tension Media

Heat transfer characteristics between the immersed heater and the bed content were studied in the riser of a liquid-solid circulating fluidized bed, whose diameter and height were 0.102 m (ID) and 2.5 m, respectively. Effects of liquid velocity, particle size, surface tension of liquid phase and solid circulation rate on the overall heat transfer coefficient were examined. The heat transfer coefficient increased with increasing particle size or solid circulation rate due to the higher potential of particles to contact with the heater surface and promote turbulence near the heater surface. The value of heat transfer coefficient increased gradually with increase in the surface tension of liquid phase, due to the slight increase of solid holdup. The heat transfer coefficient increased with the liquid velocity even in the higher range, due to the solid circulation prevented the decrease in solid holdup, in contrast to that in the conventional liquid-solid fluidized beds. The values of heat transfer coefficient were well correlated in terms of dimensionless groups as well as operating variables.

Model prediction of the operating behavior of a circulating fluidized bed boiler

An improved mathematical model for a circulating fluidized bed (CFB) boiler based on the model developed earlier by the authors was applied to simulate the operation of a 12 MW CFB boiler. The influences of the excess air ratio, primary air ratio, coal particle size distribution, coal properties (ash content and volatile content) and Ca/S ratio on the boiler operation were analyzed. The results showed that the model simulation may be applied to the optimum design and economic operation of the CFB boiler.

Effect of Scale-up on Heat Transfer Characteristics in Upper Splash Region of Circulating Fluidized Bed Risers

In the present work, steady state heat transfer experiments were conducted in the upper splash region of three cold circulating fluidized beds （CFB）, B 1, B2 and B3, with height of each 2.85 m and bed cross sections of 0.15 m × 0.15 m, 0.20 m × 0.20 m and 0.25 m × 0.25 m, respectively. Experiments were conducted under similar operating conditions on all the three CFt3 setups for two different non-dimensional air velocities （U＊ = 5 and 8） and two different sand inventories with average particles size of 460 μm. Effect of cross section of riser on heat transfer characteristics was studied. Bed temperature distribution across the heater placed in the upper splash region of riser was measured at two sections, at a height of 1.96 m and 2.24 m above the distributor plate. Axial distribution of local heat transfer coefficients along the height of heater were evaluated and compared for different bed cross sections. Results obtained were compared with the available literatures.

The Impact of Bed Temperature on Heat Transfer Characteristic between Fluidized Bed and Vertical Rifled Tubes

In the present work,the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes(38mm-O.D.) in a commercial circulating fluidized bed(CFB) boiler.Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246 mm.The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler.A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach.The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.

Gas-solid Flow Characteristics in High-density CFB

The gas-solid flow characteristics in the riser of a high density CFB of square (0.27 m×0.27 m×10.4 m) or circu-lar (? 0.187m×10.4 m) cross section, using Geldart B particles (quartz sand), was investigated experimentally. The influence of riser structure on the hydrodynamic behaviors of a high-density circulating fluidized bed was investigated. The solid circulation rate was up to 321 kg/(m2s) with the circular cross-section under the operating conditions of the main bed air velocity 12.1 m/s and loosen wind and back-feed wind flow 25.1 m3/h. Different operating conditions on realizing high density circulation was analyzed, while both solids circulation rate and particle holdup depended highly on operating conditions. The circulating gas-solid flow was accompanied by an evidently-dense character in the riser's bottom zone and became fully developed in the middle and upper zones.

Experimental Investigation on Heat Transfer Characteristics of Water in Inclined Downward Tube of a Supercritical Pressure CFB Boiler

In this study, experiments have been performed for an investigation on heat transfer of water in an inclined downward tube with an inner diameter of 20 mm and an inclined angle of 45° from the horizon, with the range of pressure from 11.5 to 28 MPa, mass flux from 450 to 1550 kg/(m2 s), and heat flux from 50 to 585 k W/m2. Based on the experimental data, the temperature distribution in the tube wall was derived. The heat transfer characteristics of inclined downward flow were compared with that of vertical downward flow. The effects of heat flux on wall temperature were analyzed and the corresponding empirical correlations were presented. The results show that heat transfer characteristics of water in the inclined downward tube are not uniform along the circumference from the top surface to the bottom surface. An increase in heat flux exacerbates the non-uniformity. At subcritical pressures, both dry-out and departure from nucleate boiling(DNB) occur at the top surface of the inclined downward tube; inversely, only dry-out takes place on the bottom surface of the inclined downward tube and in the vertical downward tube. At near-critical pressures, DNB and dry-out occur in the comparing tubes with greater possibility. At supercritical pressures, heat transfer gets enhanced in the pseudo-critical enthalpy region; in the high enthalpy region, the top surface temperature of the inclined downward tube decreases obviously.