Dust Cake Research Articles

Observation of dust release behavior from ceramic filter elements

The dust-releasing behavior from a ceramic candle filter, which is a key technology of the hot gas cleaning system in advanced coal power generation processes such as pressurized fluidized bed combustion and integrated gasification combined cycle, is discussed based on the observation of the dust-releasing process using a high-speed video camera and the measurement of time change of pressure inside the filter. Time changes of dust-released area and geometrical characteristics and motion of released dust cake fragments were investigated by analyzing photo images from the video camera, compared with the time change and distribution of pressure inside the filter. In order to discuss the influence of porosity of the dust layer, a consolidated dust layer was prepared by clean air filtration at an elevated filtration velocity after the dust filtration. The dust-releasing process was found to be a multi-mode process, i.e. release occurred in a short time after the injection of cleaning air, followed by one or two intermittent releases afterwards. The amount of dust released by the first strike increased with tank pressure. The dust-releasing behavior was sensitive to the cake porosity: the mean size and perimeter of cake fragments increased with porosity and tank pressure for the consolidated dust layer. The initial radial velocity of cake fragment released just after the first strike increased with tank pressure and was almost independent of the filter location.

Particle size effect on the filtration drag of fly ash from a coal power plant

In order to investigate the filtration properties of fly ash from a conventional coal power plant, the filtration drag across the dust cake over an absolute fiber- glass filter element was measured. A fluidized bed column was utilized to obtain a well characterized particle stream. The cake resistance coefficient was analyzed by the equation proposed by Endo et al. (1998) in order to observe the effect of particle size and polydispersity. The filtration drag was measured for three kinds of particle stream having the geometric mean particle size of 3.15, 6.07, and 7.83 μm and the geometric standard deviation less than 1.44 in the practical operation conditions for the field applications of face velocity of 0.03- 0.06 m/s and area dust load up to 0.2 kg/m 2 . A dust cake of smaller particle size showed larger pressure drop even though the porosity was higher and presented high compressibility according to the face velocity. The particle polydispersity was also a dominant factor affecting the compressibility of the dust cake.

Evolutions of efficiency and pressure drop of filter media during clogging and cleaning cycles

The collection and cleaning cycles of the filter media used in industrial air cleaning were analysed experimentally. Examination of the simultaneous pressure drop and filtration efficiency changes established that the efficiency of filters increases sharply during the collection phase only to fall back during cleaning. The amplitude of this reduction of efficiency is attained as the cycles progress. In contrast, the residual pressure drop rises. Our study has shown that treating the surface with thermobonding or covering the surface with a fine layer of submicronic fibres promotes detachment of the dust cake and prevents particle penetration inside the initial porous media.

The effect of particle shape on the pressure drop across the dust cake

In order to observe the effect of particle shape of poly-dispersed dusts on filter performance, the pressure drop across the dust cakes of fly ashes from a conventional power plant (PC), fluidized bed combustion (FBC), and paint incinerator (FI) was measured over a metal filter element in the accurate conditions. A fluidized bed column was used to prepare the dust feed stream of uniform particle distribution. The fine particles of FI ash have a tendency to be agglomerated at low transport velocity. The aggregates were broken at high velocity of more than 21 cm/sec. FBC ash composed of jagged type particles and containing high concentration of unburned-carbon showed higher pressure drop than that of PC ash composed mostly of spherical particles. FI ash composed of aggregates of very fine carbon particles presented the highest pressure drop among the fly ashes tested. The shape factors of PC, FBC, and FI ash were estimated as 0.91, 0.76, and 0.65, respectively, by the Ergun equation. The results implied that the irregular particle tends to form a higher pressure drop and to be more compressible than spherical one.

Patchy cleaning of rigid gas filters—transient regeneration phenomena comparison of modelling to experiment

Rigid ceramic filter media, widely used for the removal of particles from gas streams at elevated temperatures tend to show patchy cleaning when the filter regeneration is incomplete [Filtr. Sep. (1989) 187]. In order to investigate the regeneration behaviour and the operational performance of partially regenerated, rigid gas cleaning filter media over many filtration cycles, experiments were performed in a filter test rig. The regeneration behaviour of the filter sample was characterized by the overall regeneration efficiency, the local frequency of regeneration, and the number and size of regenerated filter areas. Using only four adjustable parameters, our modelling results compare favourably with our experimental results, at room temperature. This favourable comparison of the regeneration behaviour between modelling and experiment is achieved only if it is assumed that cohesive and adhesive bonds, which are broken during filter regeneration, do not heal during the next filtration cycle. Assuming otherwise would cause (i) the dust cake to be removed at the same positions during every regeneration and (ii) the patch size to increase from cycle to cycle instead of decreasing as seen in the experiment. Therefore, the model development was guided by our extensive experimental results. This agreement of modelling with experiment indicates that the modelling has real predictive capabilities for operational filter cleaning. Both filter conditioning and dust cake compression significantly influence the operational performance of partially regenerated filter media.

The influence of conditioning and regeneration on the separation behaviour of rigid surface filters for the separation of particles from gases

Rigid ceramic filter media can be used for the separation of particles from gas streams at elevated temperatures. In order to characterize comparatively the separation behaviour of differently structured filter media over a multitude of filtration cycles, experiments were performed in a filter test rig. The filter test rig used is built in accordance with VDI guideline 3926 and equipped with a special type of optical particle counter, which has the advantage of measuring both the particle size and the particle concentration simultaneously and in situ on the clean gas side. It is demonstrated that by far largest share of the particles reaches the clean gas as a result of the filter regeneration process. During the subsequent formation of the dust cake, the particle penetration is almost zero. The regeneration parameters, actually in essence only the tank pressure, possess the decisive influence on the separation behaviour. In the course of a filtration experiment, the number of particles reaching the clean gas reduces with the increasing number of filtration cycles due to the filter conditioning. The particles arriving in the clean gas during regeneration are extremely fine. The mean particle size is almost entirely independent of the regeneration conditions and the filter cycle number. The fine particulate emissions measured here are an anthropogenic source of particulate matter in ambient air. They have to be considered as relevant with respect to the new standards of ambient air quality (PM 10 and PM 2.5). It is shown that membrane-coated ceramic filter media at identical operating conditions exhibit, as expected, a better separation and regeneration behaviour than fibrous ceramic, open-pored filter media. However, the advantages are at the expense of a 10 times higher pressure loss, which is essentially attributed to the membrane layer.

Air and water permeation resistance across dust cakes on filters—effects of particle polydispersity and shape factor

The permeability of dust cakes on a filter for both air and water was studied theoretically and experimentally. The study investigated, in particular, the effects of particle polydispersity and shape factor on the permeability. In the experimental study, both air and water permeability tests were carried out on six different types of powders including monodisperse spheres and various polydisperse non-spherical particles. Using the parameters derived from a theory that took into account the effects of the particle polydispersity and particle shape, all the collected data were found to correlate well with the dust cake porosity ranging from 0.3 to 0.8 for both air and water permeation. The results suggest that the well-known Kozeny–Carman equation is not applicable for fluid permeation through particle layers having wide powder size distributions or irregular shape. With the proposed correlation, the permeation resistance of gas and liquid across powder layers consisting of non-spherical and polydisperse particles can be quantified. The correlation is useful for the design of filtration devices and the determination of operating conditions in filtration processes for both dust aerosols and colloidal suspensions.

Industrial separation of fine particles with difficult dust properties

This paper describes possibilities to separate particles with difficult dust properties from gases. Difficult dust properties are related to extreme values of particle size and shape and to the flowability, the adhesion properties or the reactivity of the particles. Special emphasis is given to submicron particles. In cyclones, conductive particles such as diesel soot can be removed by means of additional electrostatic forces. Experimental investigations into wet tubular electrostatic precipitators show that the measured separation efficiencies are much higher than theoretically anticipated. This result is explained by higher particle charges than predicted by the existing charging models. For small flow rates where electrostatic precipitators are economically not feasible, a new type of wet scrubber may be an alternative. The critical issue of surface filters is the adhesion of the dust cake at the surface of the filter medium. Regeneration of the dust cake as well as trends for the separation efficiency can be determined by small coupon testers which can be used for lab investigations and for field tests. Surface filters are widely used in industry for the separation of nanoparticles. Even extremely sticky particles such as tar particles can be removed by surface filters if a precoat layer protects the filter medium. It is shown that by looking at the physical fundamentals of particle separation, new and innovative solutions can be discovered. Guidelines for the separation of particles with difficult dust properties are given.

Experimental Study on the Loading Characteristics of Needlefelt Filters with Micrometer-Sized Monodisperse Aerosols

In this work, three types of needlefelt filters, made of Polyester (PE), Ryton Sulfar (RS), and Polyaramid (PA), were tested to in- vestigate the aerosol loading characteristics of fabric filters when challenged with micrometer-sized monodisperse potassium sodium tartrate (PST) particles. A fibrous filter with packing density of 9%, thickness of 0.38 mm, and fiber diameter of 5.1 θ m was included for comparison. A vibrating orifice monodisperse aerosol generator was used to produce three different sizes (5, 10, and 20 θ m) of PST particles for aerosol loading experiment. An ultrasonic atomizing nozzle and a TSI constant output nebulizer were used to generate polydisperse PST particles for the aerosol penetration test. The aerosol penetration of submicrometer-sized particles through the filters was measured by using a Scanning Mobility Particle Sizer. An Aerodynamic Particle Sizer was used to measure the penetration fraction of aerosol particles larger than 0.8 θ m. The pressure drop across the filter was monitored by using pressure transducers, which were calibrated against an inclined manometer. Airflows of 5, 10 , 20, and 30 cm/s were used to study the flow dependency. The aerosol penetration results showed that the particles larger than 3 θ m did not penetrate the clean fabric filters tested in the present study. The loading curves (plots of pressure drop against sampling time) displayed three regions: an initial region of fast increase, a transition region, and a final linear region after the dust formation point. After the formation point of the dust cake, both fabric and fibrous filters shared the same slope (of the loading curves). The slope of different regions of the loading curves was determined by many factors, such as size of challenge aerosol, face found to be critical to the performance of the fabric filters. In order lower porosity, which caused an extra rise in pressure drop across velocity, surface treatment, and the compressibility of the dust cake forming on the filter. The method of final surface treatment was to avoid the unnecessary rise in air resistance, the melting clumps formed during final surface treatment should be as thin and narrow as possible, just enough to support the filter bag cleaning. From the standpoint of filter quality and energy consumption, the low filtration velocity has to be adopted whenever possible, because high filtration velocity not only led to lower filter quality (in particular for submicrometer-sized particles) but also created dust cake of lower porosity, which caused an extra rise in pressure drop across thet dust cake.

Simulation of operational behaviour of patchily regenerated, rigid gas cleaning filter media

Particle removal from hot process gases is frequently accomplished with regenerable ceramic filters. When regenerating such media periodically, the dust cake may be detached from parts of the filter surface while other regions remain intact (‘patchy cleaning’). The filtration process depends on how these patterns of incomplete regeneration evolve over a number of cycles, how they change the build-up of the new cake, and how they affect the pressure drop. A two-dimensional quasi stationary flow model is used to predict pressure drops as a function of regeneration efficiencies and regeneration patterns, taking into account the finite thickness and flow resistance of the medium itself. The effect of non-uniform cake build-up on the pressure rise during a filter cycle is also modelled for a partially regenerated filter. The calculations prove that the pressure drop rises faster for lower regeneration efficiencies and that also cycle times become briefer with lower regeneration efficiency. It can also be shown, that the regeneration pattern only influences the pressure drop curve at the very beginning of the filtration cycle but does not influence the filtration cycle times.

Optical in situ measurement of dust cake thickness distributions on rigid filter media for gas cleaning

Rigid barrier filters are one of the options for particulate removal from gas streams by means of surface filtration. Non-uniform gas flow through the filter, possibly caused by inhomogeneities from the manufacturing process, affects the filtration behavior of such media, is suspected of causing an inhomogeneous dust cake build-up, and may influence regeneration adversely, resulting in an instable operating cycle. So far little is quantitatively known about the distribution of cake properties (e.g., porosity, thickness, etc.) across the filter surface, possible variations of this distribution over a number of filtration cycles, and how such variations may be connected with the distribution of residual cake patches after regeneration. This paper introduces an optical in situ technique to measure dust cake height distributions across a flat area of filter surface of 14 cm in diameter. Distributions are recorded by an image acquisition process with a lateral resolution of about 280 μm (1 pixel) and a vertical resolution of about 50 μm. Sample data of a partially regenerated ceramic filter medium are presented to illustrate the capabilities of the technique. The experiments were performed with quartz dust at ambient temperature and pressure on a filter test rig according to VDI Standard 3926 [VDI Richtlinie 3926, Prüfung von Filtermedien für Abreinigungsfilter, VDI Handbuch Reinhaltung der Luft, Band 6, 1991]. The data can be used to determine point-by-point growth rates of the filter cake during filtration and residual cake thickness after pressure pulse or reverse gas flow regeneration.

Effects of particle polydispersity and shape factor during dust cake loading on air filters

The formation and structure of the dust cake on the surface of air filters have been investigated theoretically and experimentally. A novel experimental setup was developed to perform in situ measurement and monitoring of the dust cake height, pressure drop, and the challenge aerosol concentration and size distribution. Test aerosols included the Arizona Road Dust (AC fine), alumina particles, and tale particles. A CCD camera was used to observe and measure the increase of cake thickness with time. The cake mass at a specific time was inferred from the measurement of the final cake mass and the total collection time. A new analytical equation, including the effects of polydispersity and shape-factor of the particles, was derived to correlate all the parameters involved. The equation gives dust cake structure (porosity) as a function of operating conditions (pressure drop and filtration velocity), particle characteristics (particle size distribution and particle shape), and the cake layer thickness. Results for different particle types used in the experiments show a good correlation using the derived equation. The study finds significant effect of polydispersity and shape-factor of dust particles on filter loading.

Bimodal aerosol loading and dust cake formation on air filters

Dust cake formation and structures on filters under the challenge of bimodal aerosol have been studied theoretically and experimentally. A theoretical model has first been derived based on bimodal aerosol loading with known particle mean sizes and geometrical standard deviations. The dust cake structure (average packing density or porosity) is expressed as a function of the operating conditions (pressure drop and filtration flow rate), the number ratio of two component bimodal particles, the particle characteristics of the respective component ( e.g. particle size distribution and particle shape), and the total cake layer thickness. An experiment was then performed to verify the theoretical model. In the experiment, the dust cake height and pressure drop were measured in real time, and the loaded mass was inferred from the total collection time and the final mass measurement. The bimodal aerosol was generated by mixing both the alumina particles (0.7 μm mean diameter) and the Arizona Road Dust (2.0 μm) in the aerosol phase. Both types of particles were dispersed using a TSI Fluidized Bed Aerosol Generator. The experimental data correlate well with the theoretical model.

98/01550 Possible improvements to the back-pulse cleaning of ceramic candle filters: Christ, A. et al. Proc. Int. Conf. Fluid. Bed Combust., 1995, 13, (1), 241–250

For the successful application of hot gas filters with ceramic filter elements in coal fired power generation processes a stable cleaning of the filter elements must be guaranteed. The most frequent strategy of cleaning candle filters is back-pulse cleaning. To optimize the cleaning event a profound understanding of the physical phenomena involved in the build-up and break up of dust cakes is necessary. To determine the cohesive forces in a filter cake the strength of flat filter cakes was tested, by measuring the pressure difference driving the reversed gas flow through the filter that causes cake failure. Based on a model by Rumpf the measured strength of filter dust cakes is evaluated and discussed. Furthermore video observations of the pulse cleaning event and investigations on the particle aggregation immediately following the cleaning event are analyzed and suggestions towards an improved efficiency of the pulse cleaning are made. For the filtration process itself the permeability of dust cakes is of paramount importance. For the range of porosities above 75% most approaches to calculate the permeability on the basis of porosity and dust properties render poor agreement with experimental data. An extension to the Free-Surface-Model of Happel is proposed that shows amore » strongly improved accuracy in the predicted permeability when compared to the original model or the Carman-Kozeny correlation.« less

Simulation of the filtration behaviour of dust filters

Periodically regenerable cake-forming filters have assumed a commercially dominant role in air purification due to their excellent dust collection capabilities. These filters are used to separate particles from gases with high dust concentrations, whereby the separation arises as the dust-gas mixture passes through the filter medium and the particles are retained. As soon as a certain maximum pressure drop over the filter medium is reached the filter medium is regenerated. In order to optimally design such filters, there are three important parameters for the economical evaluation of the filter: the correlating pressure drop increase of the filter during filtration which is mainly influenced by the compressibility of the dust cake, the dust separation efficiency especially at the beginning of the filtration and the rise of the residual pressure drop of the filter after several cycles of filtration and regeneration. In this work a 2-dimensional computer simulation model for the compressible dust cake build-up on widely used non-woven fabrics (needle-felts) is introduced. First this model is used to investigate the mechanism causing compression inside dust filter cakes and to clarify different compression phenomena. Second it is shown that this simulation program is capable of describing the particle penetration inside and through the filter medium, the clogging of the filter medium and the growing of the dust filter cake out of the filter medium, whereby the corresponding pressure drop and the number of particles in the clean gas as an function of the filtration time show good qualitative agreement with known experimental data.

Factors Influencing Pressure Drop through a Dust Cake during Filtration

ABSTRACT Three kinds of dust materials, fly ash (MMAD : 6.28 μm), limestone (MMAD : 2.53 μm) and SAE fine dust (MMAD : 3.88 μm), have been used to test the dust cake compaction and its influence on the specific dust cake resistance coefficient at filtration velocity ranging from 1 to 9 cm/sec. An accurate laser dispacement system was used to determine dust cake thickness and the amount of dust cake compaction during filtration. In the range of filtration velocity tested, dust cake compaction has been found to occur. Higher filtration velocity results in larger amount of irreversible dust cake compaction and lesser amount of elastic dust cake compaction. The experimental specific dust cake resistance coefficient, K2, has also been found to increase with an increasing filtration velocity. For the three dusts tested, K2 relates to filtration velocity Vf as K2 ∞ Vn f, where n is 0.52, 0.38 and 0.43, for fly ash, limestone and SAE fine dust respectively. The theoretical K2 values predicted by the Rudnick-Happe...

Analysis of Operational Filtration Data Part III: Re-entrainment and Incomplete Cleaning of Dust Cake

ABSTRACT Operational data for the hot-gas filtration system of the Integrated Gasification and Cleanup Facility (IGCF) at the Federal Energy Technology Center are carefully analyzed. A model that includes re-entrainment of filter-cake fragments following cleaning of the candle filters, as well as incomplete removal of cake from the filters by the cleaning backpulses of compressed air, is developed and used to interpret the data. The parameters of the model are evaluated with a least-square error fit procedure. It is shown that inclusion of the re-entrainment model significantly improves the fit to the data during the short interval (∼50 s after each backpulse) during which re-entrainment is predicted to occur by particle-transport modeling. The re-entrainment/incomplete-cleaning model is used to evaluate the buildup of filter-cake thickness and the filter-cake permeability. Both effects were significant for the data analyzed.

Particle precharging and fabric filtration - experimental results of a corona precharger

A corona precharger has been installed in front of a pilot-sized, pulse jet cassette filter. Experiments under ambient conditions with re-dispersed fly ash as well as experiments at higher gas temperatures (up to 130°C) have been performed. The net electrostatic charge on the particles was measured directly behind the precharger. The effect of the corona pre-charge, the triboelectric charge and discharging the particles on the filtration behaviour of the fly ashes was evaluated. The pressure loss of the dust cake as well as the cycle time between back-pulse cleaning was improved under certain conditions. Collection efficiency has been enhanced by charging of the fly ash particles. It has been observed that the beneficial effect of the particle precharging depends strongly on the gas velocity in the system, the dust concentration and the tribocharge on the fly ash. Back-corona effects in the precharger unit had been detected. It was tried to suppress these negative effects on performance by intermittent energization of the high-voltage power supply connected to the corona precharger. The results of these experiments will be presented and discussed.

Theoretical investigations into the compression of dust cakes deposited on filter media

Periodically regenerable cake-forming surface filters are widely used in particulate control technology. The particles collected on the filter medium surface build up a so-called ‘dust cake’. The operational characteristics of such filter apparatus are strongly influence by the structure of the dust cake formed. The structure itself depends (among other factors) on the particle size distribution, particle charge, adhesive and cohesive properties, and on the pressure drop. The dust cake mass and therefore its flow resistance with respect to pressure drop increase with filtration time. This rise in pressure drop is in many cases not linear. Local compressions of the dust cake are considered to be responsible for the progressive increase of pressure loss with time. This theory was confirmed by quantitative analyses of dust cake structures in an earlier paper. The present paper reports on a simulation program based on a simple model, by which local transient porosities inside the dust cake and the pressure drop as a function of time can be calculated.

Permeability effects of the ceramic candle filter on back-pulse cleaning system

The flow characteristics inside the ceramic candle filter system used in hot gas cleanup was investigated. The filter system admits high pressure back-pulse air to blow out the dust cake. The pressure drop across the filter barrier plays a critical role in dust removal. The filter medium and dust cakes were considered as multi-layered porous media and were then modeled using Darcy’s effective permeability model. Theκ-e model was adopted to study the turbulent flow in the filter system using FLUENT. A parametric study of the permeability of the filter media was performed to investigate the effects of dust cakes on the flow phenomena, in conjunction with the non-Darcian effect of the porous media flow and non-homogeneous boundary conditions in the inlet and the outlet.