Parameter study and optimization on filtration and resistance characteristics of granular bed filter

In this paper, experimental studies were carried out to investigate the filtration and resistance characteristics of a fixed granular bed filter (GBF) during the process of dust particles removal for hot industrial flue gas. The effects of geometric and operating parameters (including filtration superficial velocity, granules size, bed height, dust particles concentration and flue gas temperature) were examined according to the orthogonal array design method. The experimental results show that the filtration superficial velocity has the most significant effect on filtration efficiency, which is followed by granules size, bed height, flue gas temperature and dust particles concentration respectively; for pressure drop, the order of the factors are granules size, filtration superficial velocity, bed height, flue gas temperature and dust particles concentration respectively. Based on the experimental results, correlations of filtration efficiency and pressure drop with these parameters were established, which have good prediction accuracy with most of the deviations within 15%. The optimized parameters combinations of the maximum filtration efficiency and the minimum pressure drop of the GBF were gained which agree well with the experimental results. The correlations are significant for design and evaluation of GBF in practical applications.     

Structure optimization of granular bed filter for industrial flue gas filtration containing coagulative particles: An experimental and numerical study

A numerical model for the flow and filtration characteristics of industrial flue gas in granular bed filter (GBF) was established and the local filtration efficiency for different granule layers was investigated. Numerical validation results show that the GBF structure with large size granules at the inlet region and small size granules at the outlet region can effectively improve the filtration performance of GBF and the underlying mechanism was revealed. Then an experimental system was built to validate the suitability of the optimized GBF structure for the filtration of industrial flue gas with coagulative particles. The experimental results show that the optimized GBF structure is also suitable and its superiority is more significant with the increase of filtration time. The results show that the pressure drop and filtration efficiency of the experimental system increase with the increase of dust particles concentration. The existing of coagulative particles is conducive to the growth of smaller size dust particles, the pressure drop and filtration efficiency increase significantly. In addition, the pressure drop and filtration efficiency decrease with the increase of cooling rate. The results of this study are expected to be useful for the design and optimization of industrial flue gas purification and waste heat recovery.     

Filtration of dust in a circulating granular bed filter with conical louver plates (CGBF-CLPs)

A novel circulating granular bed filter with conical louver plates (CGBF-CLPs) was designed to remove dust particulates from the flue gas stream of a coal power plant. The purpose of this investigation was to evaluate the performance of the CGBF-CLPs. Dust collection efficiency and pressure drop data were analyzed to determine better operating conditions. The effect of solid mass flow rate, collector particle size and dust/collector particles separator types on the dust collection efficiency and pressure drop in the CGBF-CLPs were investigated in this study. The solid mass flow rate (B) varied from 15.59+/-0.44 to 20.36+/-0.68 g s(-1) and the initial average collector particle sizes were 1500 and 795 microm, respectively. Two types of separators, a cyclone and an inertial one, for separating the dust and collector particles were used in the CGBF-CLPs system. An Air Personal Sampler (SKC PCXR8) was used to determine the inlet and outlet dust concentrations. A differential pressure transmitter and data acquisition system were used to measure the pressure drop. Experimental results showed that the highest dust collection efficiency was 99.59% when the solid mass flow rate was 17.08+/-0.48 g s(-1) and the initial average collector particle size was 795 microm with the cyclone type separator. The results showed that the attrition fines of the original collector particles returning to the granular bed filter (GBF) reduced bed voidage. This phenomenon significantly increased the dust collection efficiency in the CGBF-CLPs. As a consequence, a bigger bed voidage creates a lower dust collection efficiency in the GBF.     

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.     

Simulation of coal pressurized pyrolysis process in an industrial-scale spout-fluid bed reactor

A three-dimensional Eulerian-Lagrangian model, facilitated with multiphase particle-in-cell (MP-PIC) method, was developed to simulate gas-solid flow and pyrolysis characteristics of coal (with the capacity of 500 thousand tons per year) in an industrial-scale spout-fluid bed reactor (H = 16.6 m and D = 3.1 m), aiming at providing guidance for industrial application of pressurized grade conversion of coal. The performance of the reactor and the effects of operating parameters such as coal feeding rate, semi-coke to coal ratio, and particle sizes were numerically investigated. It was found that the flow pattern in this case is a “jet in the fluidized bed with bubbling”. The raise of pressure has a positive impact on the spouting structure and the flow uniformly. The increase of the semi-coke to coal ratio is beneficial to the coal pyrolysis, but the improvement of the pyrolysis is limited and the number of particles in the reactor will be sharply increased. With the increase of particle sizes, the flow pattern in the pyrolysis reactor tends to be stable while the mixing effectiveness is getting worse. It is suggested that the particle size of the material should range within 0–6 mm.     

INTERACTION BETWEEN PRESSURE DROP OF GAS AND FLOW OF MEDIUM IN A MOVING GRANULAR BED FILTER

Gradually increasing pressure drop, which ultimately leads to a rapid increase, is a problem encountered in moving granular bed filters when gas velocity is elevated during dust collection from the top gas of a blast furnace. The relationship between dust collection and pressure drop was mathematically simulated, and then a half-size test model moving granular bed filter was used to examine the conditions under which the medium can flow. The increase in gas pressure drop proceeds as a circular chain of interrelated events. When gas velocity is increased and the gas pressure gradient exceeds a certain critical value, the medium flow stalls along the gas discharge side of the bed. Dust accumulates in the stalled medium, raising the pressure gradient even further. The growth in pressure drop is thus accelerated, finally resulting in a rapid and undesirable increase that prevents further operation.     

Observation of the process of dust accumulation on a rigid ceramic filter surface and the mechanism of cleaning dust from the filter surface

The accumulation process of captured particles on the surface of a rigid filter element is experimentally studied by measuring the pressure drop. It is then related to the packing density of the dust layer. The process of the release of dust from filter surface is also studied through the changes in the pressure difference across the filter and the movement of the released dust after clean air is injected. As a result, for a given filtration condition, dust forms the densest layer at the initial and the loosest at the middle stage of the filtration, and forms a uniform layer at the final stage. The cleaning efficiency of the dust layer is found to depend upon the layer structure, i.e, it decreases as dust forms a denser layer, even if the accumulated mass per unit filter surface area is the same. The release velocity of dust from the surface is also found to become slower as the porosity of the layer decreases.     

Granular bed filtration of high temperature biomass gasification gas

High temperature cleaning of producer gas from biomass gasification has been investigated with a granular filter. Field tests were performed for several hours on a single filter element at about 550 °C. The results show cake filtration on the granular material and indicate good filtration of the biomass gasification producer gas. The relatively low pressure drop over the filter during filtration is comparable to those of bag filters. The granular filter can operate with high filtration velocities compared to bag filters and maintain high efficiency and a low residual pressure.     

The effect of ash and filter media characteristics on particle filtration efficiency in fluidized bed

The phenomenon of filtering particles by a fluidized bed is complex and the parameters that affect the control efficiency of filtration have not yet been clarified. The major objective of the study focuses on the effect of characteristics of ash and filter media on filtration efficiency in a fluidized bed. The performance of the fluidized bed for removal of particles in flue gas at various fluidized operating conditions, and then the mechanisms of collecting particles were studied. The evaluated parameters included (1) various ashes (coal ash and incinerator ash); (2) bed material size; (3) operating gas velocity; and (4) bed temperature. The results indicate that the removal efficiency of coal ash increases initially with gas velocity, then decreases gradually as velocity exceeds some specific value. Furthermore, the removal of coal ash enhance with silica sand size decreasing. When the fluidized bed is operated at high temperature, diffusion is a more important mechanism than at room temperature especially for small particles. Although the inertial impaction is the main collection mechanism, the "bounce off" effect when the particles collide with the bed material could reduce the removal efficiency significantly. Because of layer inversion in fluidized bed, the removal efficiency of incinerator ash is decreased with increasing of gas velocity.     

Study on gravity flow of granules in beds supported by louver–sublouver system

The moving granular bed filter has been developed for high-temperature gas cleanup. The existence of stagnant zone is an important problem to be solved since the dust particulates and fly ash coming with flue gas may plug the system and the filtration efficiency decreases remarkably.We proposed the idea of placing sublouvers into the convergent channel between louvers. The idea has been demonstrated the possibility to diminish the stagnant zones. This paper tries to study the influence of vertical shift of the sublouver position on the flow pattern. A quasi two-dimensional cross-flow moving granular bed, with systems of louvers and sublouvers, was used as the experimental facility. We used the silica sands as filter granules discharged from an upper hopper and circulated to the bed. An image processing system was used to record the granular flow for analyzing the flow patterns and velocity fields of filter granules. In this study, the optimum design of louver–sublouver moving bed has been demonstrated to almost completely eliminate the stagnant zones.Besides the quasi two-dimensional cross-flow moving bed tests, we also tested a three-dimensional moving bed using the optimum design of louver–sublouver. The results were also very satisfactory.     

Experimental study of the gas flow behavior in the inlet of a granular bed filter

The Moving Granular Bed Filter (MGBF) is an important apparatus being developed for filtration of the hot gas. Our research group demonstrated a good solution to diminish stagnant zones in MGBF during the filtration process. However, there still remain some systematic problems that have to be worked out prior to commercializing a whole facility.The design of the gas inlet component of the granular bed filter is important for achieving a uniform gas distribution and higher usage rate of the filter media. Non-uniformity may lead to a lower usage rate. The new gas inlet component design uses baffle devices in order to achieve a more uniform gas velocity distribution. Fixed bed and moving bed conditions were studied. The uniformity of the gas velocity distribution can be characterized by the standard deviation definitions and the differences in the mean velocities between the two filtration surfaces. The baffle lengths and angles affected the uniformity of gas velocity in inlet and filtration surfaces. The optimal experimental parameters were found by using different baffle lengths, angles and mass flow rate of filter media. The uniform gas velocity distributions were obtained by a series experiments. Furthermore, the results give important information about IGCC system that will be helpful for designing better models of moving granular bed filters in the future.     

静电增强纤维动态过滤的理论研究

为了推导、预测滤料间有外加静电场时过滤器效率和压降的变化并且分析各参数如纤维厚度、颗粒直径、电场强度对效率和阻力的影响,提出了建立在D．Thorns建立的数学模型的基础上的静电增强纤维动态过滤效率和压降的计算方法,首先计算了单纤维的过滤效率和压降,然后计算纤维过滤器稳定状态的过滤效率和压降,从而计算了纤维过滤器动态过滤时的效率和压降,得出了重要结论：粒子直径越小,压力损失越大；电场强度越大,过滤效率越大,压力损失越小；增加纤维厚度能显著提高过滤器的过滤效率。     

Investigation to rectangular flat pleated filter for collecting corn straw particles during pulse cleaning

This work investigated the amounts of dust residual of a rectangular flat pleated filter for collecting corn straw particles during pulse cleaning and attempted to explore the causes of incomplete cleaning. In this study, dust residual, filter’s pressure drops and static peak pressures were obtained across flat pleated filter during the pulse cleaning. The optimum parameters were obtained that the pulse electromagnetic valve size was one inch, the pulse pressure was 0.2–0.3?MPa, and the filtration velocity was 0.6?m/min for the nozzle type with 7 holes with a diameter of 7?mm (7?×?Ф7?mm). Under this condition, the experimental results show that the dust residuals were 198.4 (64%), 52.7 (17%), 58.9 (19%)?g for initial collected dust residual 310?g at top, middle and bottom areas of the filter panel, respectively. The dust residuals were major on the top area of the filter panel, especially on the gap locations between the two-adjacent pulse airflows. Meanwhile, the more pulse interval or dust concentration was increased the dust residuals of the filter panel and the pressure drops of the filter were increased. Moreover, the static peak pressure distribution can give guidance to the dust residual distribution.     

Pressure Drop and Gas Holdup Studies in a Spout-Fluid Bed

Spout-fluid beds are used for a variety of processes involving particulate solids. They are employed where the particle agglomeration, dead zones, and sticking of particles to the vessel are the common problems in conventional spouted beds. Applications involved are granulation, coating, drying, combustion, and gasification. In this study, experimental studies have been carried out in a cylindrical Perspex column (0.094 m internal diameter and 1.217 m height) using glass beads and air. The effects of initial bed loading, spout velocity, and background (fluidization) velocity on pressure drop and gas holdup have been investigated. It is found that the minimum spout-fluidizing velocity increases with increase in initial bed loading. The pressure drop and gas holdup increase with increasing bed loading. In spout-fluid bed condition, at a constant spout velocity, as the background gas velocity increases, the gas holdup increases, and it is found to be high for smaller bed loading and is low for larger bed loading at higher velocities. The fountain height increases as spouting velocity increases and it decreases with initial bed loading. The total velocity required to fluidize the particles in spout fluidization is lower in comparison to spouted beds and fluidized beds.     

Increase in processing flue gas flow rate while maintaining the fluidization state and filtration performance in a low-temperature continuous regeneration filter using a fluidized bed

To increase the processing gas flow rate in a fluidized bed filter, the effects of superficial velocity and fluidization state on PM filtration and combustion were examined by experiments using large bed particles (710 μm). The fluidization state at 710 μm was measured by image analysis and recurrence plot, and the superficial velocities as experimental conditions were determined to obtain almost the same fluidization state and filtration efficiency as those for small bed particles (420 μm) in previous studies. The BET-surface area of 710 μm is slightly larger than that of 420 μm, and the amount of potassium catalyst doped on large bed particles is comparable to that at 420 μm. The gas phase velocity is increased by increasing the processing gas flow rate, and the contact probability between PM and oxidizer increases. The PM combustion reaction is significantly promoted owing to the effects of the potassium catalyst and the increase in the gas phase velocity, and the minimum continuous regeneration temperature is 30 °C lower than that at 420 μm. As a result, fluidized bed filters using large bed particles can be operated in continuous regeneration mode at a bed temperature of 320 °C while maintaining a filtration efficiency of 100%.     

Development of Venturi negative-pressure secondary dedust device and application of local spray closure technique

In order to enhance the spray for dust suppression performance in a fully-mechanized mining face, based on orthogonal test, comparison test, numerical simulation and field application, the Venturi negative-pressure secondary dedust device was developed. Meanwhile, the local spray closure technique was proposed, which can not only effectively prevent the escape of coal dust from entering the footway space, but also remove coal dust around the respiratory zone from the footway space successfully. The results demonstrated that, under spray pressure of 6?MPa, the Sauter mean droplet size, negative pressure suction flow rate and effective spray distance of the novel dedust device with a throat-to-nozzle distance and throat diameter of 20?mm and 65?mm were 47.37?µm, 11.21?m³/min and 5.4?m respectively. Moreover, the novel dedust device produced droplets to a smaller scale than a single-nozzle. Additionally, the negative pressure and airflow velocity in the negative pressure suction inlet of novel dedust device were within the range from ?0.97?Pa to ?541.04?Pa and from 2.96?m/s to 27.29?m/s respectively. Finally, compared with the original dust suppression measures, the local spray closure technique can enhance the removal ratio of respiratory dust by an average of 44.3%.     

电气石颗粒/聚苯硫醚针刺毡复合过滤材料的过滤性能

为提高工业用过滤材料对细颗粒物的捕集效率,以袋式除尘用聚苯硫醚（PPS）针刺毡为基材,聚氨酯热熔胶膜为黏合层,通过溶液沉淀法将具有自发极化特性的电气石（TM）颗粒覆于基材表面,经热压处理制备了含不同纯度、不同含量、不同颗粒粒度的TM颗粒/PPS针刺毡复合过滤材料;利用SEM研究了TM对微细粒子的吸附情况,利用滤料性能测试装置研究了TM颗粒/PPS针刺毡的过滤性能,结果表明：附着TM颗粒后,TM颗粒/PPS针刺毡对亚微米粉尘过滤效率明显提高,TM纯度越高效果越好,纯度为87.16%时,滤料对0.3~1 μm粒子过滤效率提高幅度≥13.35%;最优附着浓度为5 mg·cm^-2时,用于综合评价滤料过滤效率与阻力的滤料品质因数Q_F值最高;TM颗粒粒径越小,过滤效率提升效果越明显,TM颗粒粒径18~38 μm时,对0.3~1 μm粒子过滤效率提高幅度≥7.25%。TM颗粒/PPS针刺毡复合滤料较传统针刺毡滤料过滤性能明显增强。     

Modeling filtration performance of elliptical fibers with random distributions

The fibrous media with elliptical cross sections may improve the filtration performance, however, current researches mainly focus on the capture mechanisms of a single elliptical fiber, and the fibrous media with randomly distributed fibers are rarely involved. In this work, a 2D numerical model was developed to predict the pressure drop and particle penetration for the fibrous filter composed of randomly distributed elliptical fibers. The results show that a big solid volume fraction of filter increases the effective collision area, and enhances the capture at a low face velocity. The particle penetrations through the fibers with the diameter of 5 μm are conspicuously weaker than those with the diameter of 10 μm, especially at big solid volume fractions and high face velocities. The blunt elliptical fibers restrain the penetration more effectively than the circle ones when the solid volume fraction is high. Though the blunt fibers lead to a large drag force, the increased pressure drop cannot improve the filtration performance at low solid volume fractions. In most cases, the slim elliptical fibers can enhance the filtration performance. A bigger aspect ratio of elliptical fibers leads to a low quality factor, showing the capture efficiency increases with the penalty of a high pressure drop.     

Development of a double-layered ceramic filter for aerosol filtration at high-temperatures: the filter collection efficiency

The performance of double-layered ceramic filters for aerosol filtration at high temperatures was evaluated in this work. The filtering structure was composed of two layers: a thin granular membrane deposited on a reticulate ceramic support of high porosity. The goal was to minimize the high pressure drop inherent of granular structures, without decreasing their high collection efficiency for small particles. The reticulate support was developed using the technique of ceramic replication of polyurethane foam substrates of 45 and 75 pores per inch (ppi). The filtering membrane was prepared by depositing a thin layer of granular alumina-clay paste on one face of the support. Filters had their permeability and fractional collection efficiency analyzed for filtration of an airborne suspension of phosphatic rock in temperatures ranging from ambient to 700 degrees C. Results revealed that collection efficiency decreased with gas temperature and was enhanced with filtration time. Also, the support layer influenced the collection efficiency: the 75 ppi support was more effective than the 45 ppi. Particle collection efficiency dropped considerably for particles below 2 microm in diameter. The maximum collection occurred for particle diameters of approximately 3 microm, and decreased again for diameters between 4 and 8 microm. Such trend was successfully represented by the proposed correlation, which is based on the classical mechanisms acting on particle collection. Inertial impaction seems to be the predominant collection mechanism, with particle bouncing/re-entrainment acting as detachment mechanisms.     

Approach the powder contact force,voidage, tensile stress,wall frictional stress and state diagram of powder bed by simple pressure drop monitoring

The evaluation of the mechanical properties and the state of a powder bed are essential for industrial powder operations. We assume that the bed incipient yield is approximately the bed incipient fluidization, and the particle contact force, the bed voidage, the bed tensile stress and the bed-wall frictional stress can be determined by simple pressure drop monitoring when gradually increasing the superficial gas velocity from zero. A two-dimensional powder bed voidage-tensile stress state diagram at zero shear stress under anisotropic consolidation is initially prepared. For the sample powder bed, we show that the isotropic tensile stress estimated by the powder yield locus extrapolation, 340 Pa−770 Pa, from a shear tester is different from the anisotropic tensile stress evaluated, 120 Pa–180 Pa, by the pressure drop overshoot approximation.