Gas-side particulate fouling in biomass gasifiers

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

OPTIMIZATION OF FLOW DIRECTION TO MINIMIZE PARTICULATE FOULING OF HEAT EXCHANGERS

The influence of flow direction with respect to gravity on particulate fouling of heat exchangers is investigated experimentally to determine the optimal flow direction to minimize fouling. Four orientations of flow have been investigated, horizontal flow, upward flow, downward flow and a flow under an angle of 45°. It is observed that fouling starts at the point of stagnation irrespective of the flow direction, and also at the top of the heat exchanger tubes. Particulate fouling grows from these two points till they meet and the fouling layer covers the whole surface of the heat exchanger tube. Fouling at the upper half of the tubes is much faster than the lower half of the tubes, and the fouling rate is faster at the bottom tubes of the heat exchanger section than at the upper tubes. The best orientation for lingering particulate fouling is the downward flow, where the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer only starts from one point

Improving the performance of evacuated tube heat pipe collectors using oil and foamed metals

The objective of this research is to improve the heating capability of evacuated tubes that comprises heat pipes. Thermal oil is inserted in the evacuated tube in order to improve the rate of heat transfer, such that the mode of heat transfer from the inner surface of the evacuated tube to the heat pipe becomes convection via the oil, as well as conduction through the installed fin. The finned surface has been replaced by a foamed-copper. An experimental setup has been developed to study the influence of oil and foamed metals on the performance of evacuated tubes with heat pipes. It has been found that the bulb temperature as well as the heating efficiency of the evacuated tube heat pipe has increased in case of inserting oil in the evacuated tube and replacing the finned surface with foamed copper. Also, the thermal oil acts as a heat storage. Keywords: Evacuated tube, Heat pipe, Energy storag

Removal of dust particles from the surface of solar cells and solar collectors using surfactants

One of the challenges that face the operation of solar cells and solar collectors in sandy areas is the deposition of sand particles on the glass-surface of the cells and collectors. The objective of this research is to remove the deposited particles using a surfactant with the minimum amount of water utilization. Three types of surfactants have been examined; anionic, cationic and zwitterionic. The influence of a surfactant on removal of sand particles from a fouled surface has been examined under the microscope. It is found that the most effective surfactant is the anionic, and the least effective is the cationic in case of purely sand particles. This could be due to the repulsive forces between the negatively charged sand particles and the negatively charged molecules of the anionic surfactant, which repel and remove the deposited sand particles. Cationic surfactants are positively charged, which causes adhesion to the sand particles, and no removal takes place. Another set of experiments is performed utilizing sand particles covered with carbon particles. It found that the most effective surfactant is the cationic, and the least effective is the anionic, which is opposite to the behaviour of the surfactant with purely sand particles, and that is due to the positive charge on the carbon particles. It can be concluded that the surfactant behaviour is dependent on the electrical charge of the deposited particles. The influence of the zwitterionic is between the influence of the anionic and the cationic surfactants and depends on the ph value of the water used

Contact time of an incident particle hitting a 2D bed of particles

Removal of particles from fouling layers due to an incident particle impact is affected bythe fluid fluctuations in industrial applications if the contact time is larger than the fluctuationstime scales. The contact time is an important parameter when analysing the influence of the fluidstructure interaction on a fouling process. The contact time for a particle hitting a bed of particlesis defined as the time it takes for the incident particle to bounce off the bed. The contact time fora particle hitting a bed of particles arranged in a rectangular and a hexagonal array is measuredexperimentally and calculated numerically based on the discrete element method. The incidentparticle and the bed particles are of the same size and material. It is found that the contact time isproportional to the number of bed layers in case of a rectangular bed array and independent of thenumber of bed layers in case of a hexagonal bed of particles. The contact time is inverselyproportional to the impact speed. The rebound speed of the incident particle is independent of thenumber of bed layers in case of a hexagonal arrangement of particles and is exponentiallydependent on the number of bed layers in case of a rectangular arrangement. A hexagonal bed ofparticles acts as a massive particle due to its large co-ordination number compared to arectangular bed of particles. The force propagation speed in granular matter could be calculatedby plotting the path of the force as a function of the contact time and finding the gradient of thisgraph

Force propagation speed in a bed of particles due to an incident particle impact

The force propagation speed in granular matter is a very difficult property to be measured. A new technique has been developed to calculate the force propagation speed in granular matter based on measuring experimentally the contact time. The contact time for a particle hitting a bed of particles is estimated as the time taken for a particle to strike a bed of particles till the time of its ejection, and it is calculated using the discrete element method. The speed of force propagation in a bed of particles is estimated by plotting the dependence of the path length of the contact force on the contact time and finding the gradient of such dependence. Such approach leads to accurate results if the impact speed is below the yield velocity, i.e. no plastic deformations. It is found that the force propagation speed in spherical granular matter is proportional to the impact speed of the incident particle, which is different from force propagation in continuum matter. It is also found that the propagation speed is dependent on the material and diameters ratio of the interacting particles, but it is not dependent on the number of bed layers. The propagation speed in granular matter is normalized by dividing it by a reference propagation speed, i.e. the propagation speed at an impact speed of 1 m/s. It is found that the normalized propagation speed is independent of the material and diameter of the interacting particles, but it is logarithmically proportional to the impact speed. The proportionality constant is equal to 0.16, which can be taken as a universal constant for force propagation in spherical granular matter

Removal of gas-side particulate fouling layers by foreign particles as a function of flow direction

Removal of particulate fouling layers by externally injected particles as a function of flow direction with respect to gravity is investigated experimentally. Three orientations of flow have been investigated, horizontal flow, upward flow and a downward flow. It is found that fouling starts at the point of stagnation irrespective of the flow direction, and also starts at the top point of the heat exchanger tubes. Particulate fouling grows from these two points except for the downward flow, were the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer starts only from one point. It was not possible to remove the fouling layer in case of a horizontal and an upward flow by the externally injected particles, however in case of a downward flow most of the fouling layers were removed by the external particles. It can be concluded that the downward flow is the best flow orientation to linger particulate fouling and for removal of fouling layers by externally injected particles

Influence of the apex angle of cone shaped tubes on particulate fouling of heat exchangers

A two-dimensional (2D) cone shape has been added to the normal circular tubes of heat exchangers to minimize the area of stagnation and to streamline the air flow around the heat exchanger tubes. An experimental setup has been developed to study the influence of the apex angle of the cone-shaped tubes on particulate fouling of heat exchangers. Fouling experiments have been performed in which calcium carbonate particles are injected during the experiments and the deposition of particles on the tubes of the heat exchanger is monitored. Four sets of experiments have been performed, in which normal cylindrical tubes and coned tubes with an apex angle of 60°, 90°, and 120° are examined. It was found that particulate fouling ceased if the apex angle of the cone-shaped tubes is smaller than 90°. The attached cones enhance the flow around the tubes of the heat exchanger, by minimizing the stagnation area and keeping the flow attached to the tubes starting from the tip of the attached cone until separation, such that particles that deposit on the top of the tubes of the heat exchanger can be removed by the air flow

Removal of gas-side particulate fouling layers by foreign particles as a function of flow direction

Removal of particulate fouling layers by externally injected particles as a function of flow direction with respect to gravity is investigated experimentally. Three orientations of flow have been investigated, horizontal flow, upward flow and a downward flow. It is found that fouling starts at the point of stagnation irrespective of the flow direction, and also starts at the top point of the heat exchanger tubes. Particulate fouling grows from these two points except for the downward flow, were the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer starts only from one point. It was not possible to remove the fouling layer in case of a horizontal and an upward flow by the externally injected particles, however in case of a downward flow most of the fouling layers were removed by the external particles. It can be concluded that the downward flow is the best flow orientation to linger particulate fouling and for removal of fouling layers by externally injected particles

Influence of the apex angle of cone shaped tubes on particulate fouling of heat exchangers

A two-dimensional (2D) cone shape has been added to the normal circular tubes of heat exchangers to minimize the area of stagnation and to streamline the air flow around the heat exchanger tubes. An experimental setup has been developed to study the influence of the apex angle of the cone-shaped tubes on particulate fouling of heat exchangers. Fouling experiments have been performed in which calcium carbonate particles are injected during the experiments and the deposition of particles on the tubes of the heat exchanger is monitored. Four sets of experiments have been performed, in which normal cylindrical tubes and coned tubes with an apex angle of 60°, 90°, and 120° are examined. It was found that particulate fouling ceased if the apex angle of the cone-shaped tubes is smaller than 90°. The attached cones enhance the flow around the tubes of the heat exchanger, by minimizing the stagnation area and keeping the flow attached to the tubes starting from the tip of the attached cone until separation, such that particles that deposit on the top of the tubes of the heat exchanger can be removed by the air flow