Bengt-Åke Andersson

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-membrane wall heat transfer in a 12 MW,, circulating fluidized bed boiler. The methods are compared for a case of normal operating conditions and the measured heat transfer coefficients are presented. In the central part of the combustion chamber where most of the cooling surface is located, the cross-sectional average suspension density normally varies in the range of 10-20 kg m -’ and the heat transfer coefficient is around 130 W m ’ K-’ with a scatter of + 15% due to the different methods. The methods are critically analyzed and the heat transfer data are compared with relevant literature data.

Effects of bed particle size on heat transfer in circulating fluidized bed boilers

Local values of heat transfer to membrane walls of a circulating fluidized bed (CFB) boiler were measured for three sizes of the same silica sand, with mean diameters of 0.22, 0.34 and 0.44 mm. A change from 0.44 to 0.22 mm sand at constant fluidization velocity led to a considerable increase in particle concentration and, hence, heat transfer. However, at a given cross-sectional average bulk density, the average heat transfer coefficient across the membrane wall was insensitive to the changes of bed particle size. Also, the lateral distribution of heat flow, to the crest and side of the tube and to the fin, was independent of particle size when the bulk densities were similar. This was achieved by keeping constant the ratio of fluidization velocity to terminal velocity of a single average size particle. It was possible to estimate the vertical distribution of the heat transfer coefficient in the CFB furnace with an accuracy of ± 20% by a simple semi-empirical method.

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 conditions for heat transfer are listed. Then, examples of measurements from the research boiler at Chalmers University of Technology are presented, including thermal and fluid-dynamic boundary layers as well as heat transfer coefficients 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 by exit, secondary air, etc. is indicated.

Particle mass flux in the freeboard of a fluidized bed boiler

The flux of descending particles has been measured in the freeboard of a fluidized bed boiler. The lateral and vertical distributions of the flux were studied and the influence of the superficial gas velocity and the static bed height analyzed. A tube bundle located in the bed surface region made it possible to investigate the effects of tubes on the entrainment of particles from the surface of the bed. The particle flux was relatively constant across the freeboard except near the wall, where it was about twice as large as the average. In the vertical direction, the flux decreased exponentially with height except close to the bed, where the rate of decay was less than at higher elevations. The tube bundle was found to inhibit the splashing of bed particles when the bed is below the tubes. A model is used for the estimation of the vertical particle flux and concentration profiles in the freeboard. It is based on separate treatment of small and coarse particles and calculation of trajectories of ejected bed particles. The calculated freeboard values describe the experimental data satisfactorily except close to the bed, where a considerable discrepancy is observed.

Leaching of phosphorous from ashes of co-combustion of sewage sludge and wood

Municipal sewage sludge has a high concentration of phosphorus, which should be recovered because phosphorus is a limited natural resource. In this work, sewage sludge was co-fired with wood in a FBC boiler. The aim of the investigating was to study the solubility of phosphorus in the ashes, by leaching as an alternative to the phosphorous recovery method of using the ashes directly on farmlands. The fly ashes from the boilers secondary cyclone and bag filter were leached at various pH-values and the release of phosphorus was measured. Only acidic leaching was applied. The ashes precipitated with Al2(SO4)3 released nearly all phosphorus at a pH-value of 1, whereas the ashes precipitated with Fe2(SO4)3 did not release all phosphorus even at the very low pH of 0.5. The concentrations of phosphorous in the leachate must be compared with liquid phosphorus sources such as human urine or liquid animal manure used as fertilisers. This may result in that the leachate has to be processed further. A continuation of the work to investigate to what extent the leachate is contami-nated with toxic trace elements is necessary.

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%.

Phosphorous in Ash From Co-Combustion of Municipal Sewage Sludge With Wood in a CFB Boiler: A Comparison of Experimental Data With Predictions by a Thermodynamic Equilibrium Model

Combustion tests have been carried out in a circulating fluidised bed (CFB) boiler co-firing wood and municipal, digested and mechanical dewatered sewage sludge from two Swedish wastewater treatmen ...