Particle mass flux in the freeboard of a fluidized bed boiler

The flux of descending particles has beenmeasured in the freeboard of a fluidized bedboiler. The lateral and vertical distributionsof the flux were studied and the influence ofthe superficial gas velocity and the static bedheight analyzed. A tube bundle located in thebed surface region made it possible to investigatethe effects of tubes on the entrainmentof particles from the surface of the bed.The particle flux was relatively constantacross the freeboard except near the wall,where it was about twice as large as the average.In the vertical direction, the flux decreasedexponentially with height exceptclose to the bed, where the rate of decay wasless than at higher elevations. The tube bundlewas found to inhibit the splashing of bedparticles when the bed is below the tubes.A model is used for the estimation of thevertical particle flux and concentrationprofiles in the freeboard. It is based on separatetreatment of small and coarse particlesand calculation of trajectories of ejected bedparticles. The calculated freeboard valuesdescribe the experimental data satisfactorilyexcept close to the bed, where a considerablediscrepancy is observed

Local lateral distribution of heat transfer on tube surface on membrane walls in CFB boilers

The wall to bed heat transfer was investigated in a 12 MWth CFB boiler at different locations in the combustion chamber and under various operating conditions. At high levels in the furnace the lateral distribution of heat flow to the tube and fin was found to be a result of radiation from the hot bed bulk, especially at low bulk densities. The local heat flow to the fin decreased considerably with decreasing height in contrast to the heat flow to the crest of the tube. This is due to the large down-flow of particles in the fin region and the growth of the boundary layer. The average heat transfer coefficient across the membrane wall was found to decrease with decreased bulk density, but at low bulk densities the decrease is very small since radiation dominates heat transfer

Bed-to-wall heat transfer in a 10m2 bubbling fluidized bed

The heat transfer is measured on the membrane tube wall in contact with a bubbling fluidized bed boiler. Measurements are carried out by heat transfer meters through holes in the wall both in the bed and above the bed in the splash zone and freeboard. Heat transfer coefficients are given and compared with previous measurements of material loss at the same wall and under the same conditions (typical for a fluidized bed boiler

Characteristic features of heat transfer in circulating fluidized bed boilers

The characteristic features of heat transfer in circulating fluidized bed boilers are described. First, the conditionsfor heat transfer are listed. Then, examples of measurements from the research boiler at Chalmers Universityof Technology are presented, including thermal and fluid-dynamic boundary layers as well as heat transfercoefficients measured with a heat flow meter and with thermocouples mounted on the tube walls of the boiler.Differences in heat transfer efficiency of the fins and of the tubes of a membrane tube wall are shown. Finally,heat transfer mechanisms are briefly discussed, and the influence of the macroscopic flow pattern caused byexit, secondary air, etc. is indicated

Experimental methods of estimating heat transfer in circulating fluidized bed boilers

Four different experimental methods have been used for the estimation of the bed-to-membranewall heat transfer in a 12 MW,, circulating fluidized bed boiler. The methods are compared for a case ofnormal operating conditions and the measured heat transfer coefficients are presented. In the central partof the combustion chamber where most of the cooling surface is located, the cross-sectional averagesuspension density normally varies in the range of 10-20 kg m -’ and the heat transfer coefficient is around130 W m ’ K-’ with a scatter of + 15% due to the different methods. The methods are critically analyzedand the heat transfer data are compared with relevant literature data

Pressure distribution and particle concentration in bubbling fluidized beds

The expansion of and particle concentration in large-particle (>0.5 mm) bubbling beds, fluidized at high gas velocities (U-Umf>1 m/s) have been studied in a boiler with 10 m2 hot bed and in a cold two-dimensional bed. The significantly different behaviour of a freely bubbling versus a tube-filled bed at high gas velocity is demonstrated

Use of a conductivity heat-flow meter in fluidised-bed boilers

The usefulness of a conductivity type of heat-flow meter for measurements in fluidised-bed boilers is investigated. The design and the calibration of the meter are described. Results from measurements of the local heat tranfer at the walls of fluidised-bed boilers are presented. An error analysis shows that the accuracy of the heat-flow meter is +-9%

Operational results from the 16 MW FBB at the Chalmers University

The 16 MW fluidised bed boiler was first started in 1982 and has since then run for 3325 hours. After only 1200 hours the material wastage on the in-bed heat transfer surfaces was so great that tubes had to be removed. Wear resistant coatings of various kinds have been tried but the result of their use is not quite clear. Measurements of elutriated solids and the corresponding combustion efficiency is discussed. Finally, in contrast to the problems of material wastage and low combustion efficiency, some positive operational characteristics such as start-up and load control are presented