Dieter Köneke

Modeling of Gasification of Wood in a Circulating Fluidized Bed

An one-dimensional mathematical model for describing the fluidization and the gasification reactions of wood in a circulating fluidized bed combustor (CFBC) has been developed. The mathematical model consists of modules to calculate the fluid-dynamic parameters, the kinetics of the gasification reactions and the temperature in the CFBC. It is shown that in the pilot plant measured pressure drop and the temperatures correspond very well with the calculation. The calculation of the product gas composition shows qualitatively correct tendencies. Further comparison with measurements has to be done in the future.

Heat transfer and flow of bulk solids in a moving bed

Abstract The indirect heat transfer of steam-heated tube bundles in a moving bed has been examined in an experimental apparatus. Heat transfer in single tubes is typified by a characteristic flow of the bulk solids along the outer tube wall surface. A cuneiform rest zone is created at the upper tube wall (stagnation point), in which the particles remain for a long time. An ‘insulating’ effect is exhibited by the dammed bulk zone and is responsible for the poor heat transfer in this area. Near the sides of the lateral tubes heat transfer is good and icreases with increasing mass flux and bulk solids velocity. Bubbling occurs at the lower tube wall and the heat transfer again decreases due to the small number of wall-particle contacts. The experimentally confirmed ‘trace theory’ describes the temperature profile at the outlet of a moving bed heat exchanger, being characterized by very good cross-mixing of the bulk solids which allows the intergral heat transition to be calculated. A modelling approach to the heat transfer and bulk solids movement in the moving bed provides a physical model which describes the dependence of the heat transfer at a single tube on the flow profile between two neighbouring tubes. In order to determine the flow profile, the continuity equation is solved vectorially, allowing an analytical relationship of the velocity profile between two tubes to be obtained via the coaxiality of stress and deformation. To allow such a calculation, the heat-transfer model makes use of the residence and contact time behaviour resulting from the velocity profile, with the different components of heat transfer at a tube being determined from the friction properties of the specific bulk material. Calculation of the integral heat transfer in the moving bed may be achieved via heat transfer at a single tube. By using the theory of ‘extended contact time’, the total residence time of the bulk at the first tubes may be considered as a case history for the other tubes. The integral overall heat-transfer coefficients of moving bed heat exchangers thereby determined have been verified experimentally.

Heat transfer to trickling granular materials

Abstract Heat transfer to trickling granular materials, falling on single horizontal tubes, was investigated. The tubes could be heated by steam or by electricity. Local or total heat transfer coefficients were determined. The experimental results show that the maximum of the local heat transfer coefficients at low solids mass fluxes occurs at the stagnation point. With an increasing solids mass flux two maxima occur, each at an angle of about 30°–45° to the vertical. The total heat transfer coefficient increases with increasing solids mass flux or temperature and with decreasing tube or particle size. A model of heat exchange to trickling solids falling on single tubes was developed. It is based on a serial connection of the particle-to-surface heat transfer and the heat conduction in bulk solids packets. The heat transfer coefficient depends on the contact time of a packet with the wall. This contact time can be determined considering the forces affecting a packet. An energy balance is developed taking the influence of these forces into consideration in order to derive a set of dimensionless groups. A suitable simplification of this set of groups leads to an easy equation which allows contact times to be calculated as a function of all important properties.

Radiation heat transfer in dust-loaded systems

Abstract The design of radiation heat exchangers in the high-temperature range is influenced by the solid loading of the participating media. In order to obtain data and criteria for a more sophisticated design, emission measurements of different gas-dispersed solids (quartz sand, ash from fluidized bed combustion, corundum) have been carried out in an experimental plant specially designed for this purpose. An equation is derived to describe the particle emission coefficient in a geometry with surrounding grey walls. The experimental data fit this model well. An examination of the composition of the solids has shown the considerable influence of the quartz content on the radiation behaviour.