Particulate fouling of dry and liquid coated surfaces

Particulate fouling is the process of deposition of extraneous particulate matter on other surfaces. In particular, particulate fouling is a major cause of concern in energy-intensive heat recovery systems like biomass gasifiers, coal fired boiler and waste incinerators. The thermal energy is extracted from the flue gas using a system of heat exchangers. The flue gas is however contaminated with particulate matter, tar, nitrogen, sulphur and alkali compounds. The contaminants are transported by the flue gas and interact with the heat exchanger surface eventually forming a deposit layer. The deposit layers have very low thermal conductivity and leads to drastic loss in thermal efficiency apart from maintenance problems and capital losses. The focus of this research is to understand the process of particulate fouling from a fundamental view point based on particle surface interactions and the global effects associated with process conditions by experiments. A numerical model to capture the deposition and removal of particles over heat exchanger surfaces is aimed at. Particles which arrive at the heat exchanger surface and undergo inertial impaction can stick to the surface, rebound and might remove other previously deposited particles. In order to model the process, a sticking criterion is necessary. The interaction of a particle with other particles on the heat exchanger surface can be either in a dry state or in the presence of a thin liquid film due to condensation of alkali compounds. Detailed experiments were performed to evaluate the sticking criterion for particle impaction over a liquid coated surface under elastic and elastic-plastic deformation conditions. An empirical relation in terms of Stokes number was evaluated to determine the energy loss in the thin interstitial liquid film. A critical Stokes number range between 3 and 8 was observed below which particles do not rebound from the surface. In the Stokes number range of 8 to 20, the particles were observed to rebound but do not overcome the viscous effects of the liquid layer. A high-temperature closed-loop vertical wind tunnel was designed and constructed to perform fouling experiments under controlled conditions. The effect of gas velocity, particle concentration, particle size distribution, gas temperature, heat exchanger tube orientation and geometry was studied. A measurement technique that allowed the evaluation of temporal evolution of the fouling layer thickness was used. The experimental investigations revealed that the shear induced by the gas flowing around the tube has a major effect on the overall deposit growth dynamics. The geometry and orientation of the tube indicated that deposition and removal of particles is strongly coupled to the flow dynamics and particle surface interactions. A numerical model was implemented in a commercial software package to capture the deposition and removal of particles. The deposition model was based on particle-surface interactions including elastic-plastic deformations and the removal model was based on the rolling moment induced by the flow and on the energy transferred by other impacting particles. The fundamental impaction experiments along with the controlled experiments have provided better insight into the process of particulate fouling and resulted in the development of a numerical model which can be used to devise mitigation strategies for particulate fouling

Particulate fouling of dry and liquid coated surfaces

Particulate fouling is the process of deposition of extraneous particulate matter on other surfaces. In particular, particulate fouling is a major cause of concern in energy-intensive heat recovery systems like biomass gasifiers, coal fired boiler and waste incinerators. The thermal energy is extracted from the flue gas using a system of heat exchangers. The flue gas is however contaminated with particulate matter, tar, nitrogen, sulphur and alkali compounds. The contaminants are transported by the flue gas and interact with the heat exchanger surface eventually forming a deposit layer. The deposit layers have very low thermal conductivity and leads to drastic loss in thermal efficiency apart from maintenance problems and capital losses. The focus of this research is to understand the process of particulate fouling from a fundamental view point based on particle surface interactions and the global effects associated with process conditions by experiments. A numerical model to capture the deposition and removal of particles over heat exchanger surfaces is aimed at. Particles which arrive at the heat exchanger surface and undergo inertial impaction can stick to the surface, rebound and might remove other previously deposited particles. In order to model the process, a sticking criterion is necessary. The interaction of a particle with other particles on the heat exchanger surface can be either in a dry state or in the presence of a thin liquid film due to condensation of alkali compounds. Detailed experiments were performed to evaluate the sticking criterion for particle impaction over a liquid coated surface under elastic and elastic-plastic deformation conditions. An empirical relation in terms of Stokes number was evaluated to determine the energy loss in the thin interstitial liquid film. A critical Stokes number range between 3 and 8 was observed below which particles do not rebound from the surface. In the Stokes number range of 8 to 20, the particles were observed to rebound but do not overcome the viscous effects of the liquid layer. A high-temperature closed-loop vertical wind tunnel was designed and constructed to perform fouling experiments under controlled conditions. The effect of gas velocity, particle concentration, particle size distribution, gas temperature, heat exchanger tube orientation and geometry was studied. A measurement technique that allowed the evaluation of temporal evolution of the fouling layer thickness was used. The experimental investigations revealed that the shear induced by the gas flowing around the tube has a major effect on the overall deposit growth dynamics. The geometry and orientation of the tube indicated that deposition and removal of particles is strongly coupled to the flow dynamics and particle surface interactions. A numerical model was implemented in a commercial software package to capture the deposition and removal of particles. The deposition model was based on particle-surface interactions including elastic-plastic deformations and the removal model was based on the rolling moment induced by the flow and on the energy transferred by other impacting particles. The fundamental impaction experiments along with the controlled experiments have provided better insight into the process of particulate fouling and resulted in the development of a numerical model which can be used to devise mitigation strategies for particulate fouling

Effect of condensable species on particulate fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprises particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger like superheater, evaporator etc. If the temperature of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles which are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a simple modeling approach for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with the detailed impaction experiments

Preliminary study of particulate fouling in a high temperature controlled experimental facility

Fouling is a highly complex process and numerical modeling of fouling has been an evasive task. One of the reasons for this is attributed to the lack of detailed experimental data. In-situ experiments performed at the power plants give a global picture of the overall deposition process in a qualitative manner. However, detailed understanding of the underlying mechanisms becomes difficult. This is due to the fact that too many parameters like varying particle composition and size, gas phase dynamics, chemical reactions etc. are lumped together in such experiments. On the other hand, controlled lab-scale experiments that have been reported are meager and those that have been published are performed either at low temperatures or at very high temperatures (>1000 °C). In order to understand the underlying mechanisms of particulate fouling and to provide experimental data for validation, a high temperature controlled fouling experimental facility has been built. The facility is a vertically oriented closed loop wind tunnel with which parameters like gas phase temperature, velocity and particle concentration can be controlled. The setup was tested for proper operation and preliminary experiments were performed on particulate fouling over a circular cylinder as function of gas phase velocity and temperature. It was found that the gas phase velocity and temperature has a major influence on particulate fouling. This is a preliminary study and will be extended in future

Particle impaction over a substrate coated with a liquid film

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Effect of condensable species on particulate fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprise particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger, such as the superheater, evaporator, and so on. If the temperatures of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles that are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness, and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a suitable model for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with detailed impaction experiments

Growth rates and morphology of dry particulate fouling under variable process conditions

Particulate Fouling is a complex phenomenon and is governed by various process conditions. It is imperative to elucidate the effects of individual parameters governing the fouling process for better understanding and to aid in numerical modeling. Most of the experimental studies on fouling involve in-situ measurements in the process plants where the individual effects of process conditions are difficult to evaluate. Controlled lab scale experiments that have been reported are limited to a few. In this direction, a controlled high temperature experimental facility has been built to study dry particulate fouling under varying process conditions. Experiments have been conducted for a range of process parameters like: gas phase velocity, particle size distribution, particle mixtures, deposition probe materials and geometries. An optical technique is developed to measure the evolution of fouling layer growth with time. The particle deposition and fouling layer growth is measured for a single cylinder to avoid the complexities associated with the flow dynamics of multiple tube arrays. It is found that the gas phase velocity plays a vital role in the overall process. Experiments with a square tube with its sides inclined at various angles to the mainstream flow direction indicated a reduction in deposition. A cylindrical tube oriented at an angle to the flow also indicated reduced fouling tendency. The geometry of the heat exchanger tube influences the deposition process and by modifying the geometry dry particulate fouling can be reduced

Effect of condensable species on particulate fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprises particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger like superheater, evaporator etc. If the temperature of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles which are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a simple modeling approach for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with the detailed impaction experiments