Particulate fouling is defined as the accumulation of particles on a heat exchanger-surface forming an insulating powdery layer. Particulate fouling in biomass gasifiers is a major problem that may lead to inefficient operation. As observed in a large-scale biomass gasifier the character of the fouling layer is related to the local gas temperature and velocity. At high gas-side temperatures, the fouling layer structure changes from a fragile powdery form to a coherent sintered structure. Particulate fouling of heat exchangers shows an asymptotic growth rate with a levelling off increase of the thickness. The fouling growth rate is determined by the difference between the deposition rate and the removal rate of particles on and from the fouling layer respectively. Till now, most attention has been given to the deposition process. The objective of this research project is to study and model removal of particles from particulate fouling layers as a function of the fouling layer structure. By integrating the removal rate and the depositing rate a complete perspective can be given about the fouling process. Two mechanisms contribute to the removal of particles from fouling layers: the transfer of kinetic energy from an incident particle to bed particles or the forces exerted on the bed of particles by a shear flow. To study the removal of particles from fouling layers by shear flow, fouling experiments in a heat exchanger set-up have been done with particles of different sizes and different materials running under different gas speeds. It is found that the smallest particles in the flow deposit first on the tubes of the heat exchanger at areas of minimum flow velocities. Then the larger particles deposit and the fouling layer starts to build up. The fouling layer thickness and growth over the heat exchanger tube is influenced by the flow speed. As the flow speed in the heat exchanger increases, the thickness and the surface area of the fouling layer deposited over the heat exchanger tube are reduced. There is a limiting flow speed above which fouling is avoided. This limiting speed appears to be related to the critical flow velocity required to roll a particle resting on a flat surface. To prevent fouling, the gas speed in a heat exchanger should be larger than the critical flow velocity that corresponds to the particle size most likely to stick on the heat exchanger tube. It appeared that the removal of particles from fouling layers by shear flow becomes important at high gas speeds. The second mechanism, which is the removal of particles by an incident particle impact, is the dominant removal mechanism at low gas speeds. The removal of particles from powdery fouling layers due to an incident particle impact is investigated numerically and experimentally. A numerical model is developed to simulate the interaction between a particle hitting a bed of particles. The numerical model is based on the discrete element method. The forces during collision are based on the concepts of contact mechanics. The numerical model can predict the particle velocity at which an incident particle starts either to stick, rebound or remove other particles from the impacted bed of particles. The critical sticking and removal velocities of a particle hitting a powdery layer become independent of the layer thickness, if the thickness is larger than a certain limit. This limit is of a thickness of 2 particles in case of a monodisperse particulate layer and when the particles in the layer are arranged in an orthorhombic structure. It is found that the ejection time of particles from a bed of particles due to an incident particle impact is proportional to its diameter and to the square root of the number of bed layers. To validate the numerical model, experiments are carried out in a vacuumed column. In the experiment, incident particles drop on a bed of particles and the sticking, bouncing and removal behaviour is measured as a function of the incident particle impact speed. The numerical model predictions regarding the critical sticking and removal velocities are in agreement with the experimental results. During operation of heat exchangers, particulate fouling layers may sinter due to the high gas-side temperature. The influence of the gas-side temperature on the fouling layer structure and consequently on the removal and deposition of particles are investigated. The fouling layer structure is dependent on the gas-side temperature in relation to the minimum sintering temperature. Impaction experiments are carried out to determine the sticking and removal velocities for an incident particle hitting a bare tube surface, a powdery fouling layer and a sintered layer. The change in the heat exchanger surface from a bare tube to a powdery one increases the critical sticking velocity with at least one order of magnitude, which consequently speeds up the fouling process. The further change in the heat exchanger surface from a powdery form to a sintered one lowers significantly the ability of an incident particle to stick on the fouling layer or to remove particles out of the fouling layer. Particles that are still able to deposit on the sintered fouling layer will not sinter immediately and can therefore be removed, due to an incident particle impact. Sintering reduces the fouling rate of heat exchangers by lowering the deposition rate of new particles and increasing the removal rate of newly deposited particles, such that the fouling process becomes as slow as the formation of a single layer on a bare tube during the initiation period. When the initiation period is longer than the characteristic sintering time, the newly deposited particles become sintered and we revert again to the sintered case, which leads to a very slow fouling process known as the asymptotic behaviour of particulate fouling layers. The numerical model is extended to account for an incident particle hitting a sintered layer. Summarizing, the removal and deposition of particles from particulate fouling layers as a function of the fouling layer structure have been numerically modelled and the model is experimentally validated. By combining the numerical model together with a CFD-particle transport model, the growth rate of the particulate fouling layer around a heat exchanger tube can be quantitatively predicted.
|Qualification||Doctor of Philosophy|
|Award date||15 Sep 2005|
|Place of Publication||Eindhoven|
|Publication status||Published - 2005|