Analysis of Steady-State Filtration and Backpulse Process in a Hot-Gas Filter Vessel

The need to develop a technology for clean and efficient electric power generation has led to the development of advanced pressurized fluidized bed combustors (PFBC) and integrated gasification combined cycles (IGCC). The effective filtration of hot gases for removal of ash and sulfur sorbent, however, is the key to the success of these advanced coal energy systems. Recently, attention has been given to the use of ceramic candle filters for hot-gas cleaning. The ash cake formation on these filters needs to be removed by the backpusle for their successful operation. In this paper, steady-state filtration as well as the transient gas flow during the backpulse process in the integrated gasification and cleanup facility (IGCF) (located at the National Energy Technology Laboratory, NETL) is studied. The steady-state filtration condition is first evaluated, using a compressible heat-conducting flow analysis. Particle transport patterns are studied, and the deposition patterns of 1-30 w m particles on the ceramic filters and the vessel surfaces are analyzed. To simulate the backpulse process, the pressure at the filter exit is increased sharply in a period of about 0.01 s pressure. The stress transport model of the FLUENT code is used to evaluate the time evolution of the transient gas flow velocity, pressure and thermal fields, as well as turbulence intensities and stresses inside the candle filter and in the IGCF filter vessel. Contour plots of the hot-gas flow conditions from the start of the pressure buildup to its saturation level are presented. The results show the rapidly changing flow conditions during the initial stages of the backpulse. The pressure wave propagates along the length of the filter until a monotonic increase of pressure with time is achieved; that is, the pressure field inside the filter at the initial stages of the backpulse is strongly nonuniform. Therefore the potential for incomplete filter cake removal exists. Motions of particles that enter the vessel and/or are ejected from the candle filter during the backpulse process are also studied, and illustrative particle trajectories are presented.     

多管陶瓷过滤器内耦合效应数值模拟

The Reynolds stress transport model and the Eulerian two-fluid model provided by the FLUENT code were applied to evaluate the gas-particle two-phase flow in the ceramic filter vessel. The ceramic filter vessel contains six candle filters, which are arranged in the form of equilateral hexagon. The variation of the areal density of the filter cake during the filtration and the back-pulse process were analyzed. The coupling effect between filters, gas and solid, filtration and pulse cleaning process were investigated, respectively. The numerical results show a good approach to predict the particle distribution in the vessel and the particle deposition on the filter element. This study provides the base for the intensive study on the analysis of the gas-particle flow in the filter vessel.     

Numerical analysis of flow field in the hot gas filter vessel during the pulse cleaning process

The flow field is simulated for a ceramic filter vessel containing three candle filters which are arranged in the form of an equilateral triangle. Grids generated by GAMBIT are adopted for the simulations. The Reynolds stress model provided by FLUENT code is applied to evaluate gas flow and temperature field in the filter vessel. The temperature profiles in the ceramic candle filter cavity during the pulse cleaning process are analyzed under different operating conditions and for different lengths of candle filter. The evolution of radial velocity in the porous wall of the filters being cleaned and the normal working filters as well as around the filters is discussed. Sharp temperature change takes place in the top of the candle filter which is subject to thermal stress. The phenomenon of temperature increase during the pulse cleaning process has been carefully observed and interpreted based on the effect of gas compression. The simulated results show qualitative agreement with the experimental field observations with the filter vessel.     

CFD and PIV Investigation of Flow past a Single Hot Gas Filter

Ceramic candle filters have been developed for cleaning high‐temperature high‐pressure (HTHP) gas streams. They meet environmental and economical considerations in combined cycle power plants, where gas turbine blades can be protected from the erosion that occurs due to using HTHP exhaust from the fluidized bed. Ceramic candle filters are the most promising hot gas filtration technology, which has demonstrated high collection efficiencies at high‐temperature high‐pressure conditions. This paper reports computational fluid dynamics (CFD) and experimental investigations of a candle filter. Experimentally, 18 and 108 μm particles are tracked in the vicinity of a filter using Particle Imaging Velocimetry (PIV). The images are processed to give the radius of convergence, which defines the critical trajectory for particles just impinging on the filter. In the computational investigation, constant filtration velocity boundary models have been used to investigate the filter in cross flow conditions using the CFD code FLUENT. Different approach (inlet) velocity to filter face velocity ratios and different face velocities (ranging from 2–5 cm/s) are used in the CFD calculation. Particles in the diameter range 1–100 μm are tracked through the domain. The radius of convergence is compared and plotted as a function of many parameters. Validation of the computational study in this work was adequate and the deposition process and the factors that affect the build up of the filter cake have also been studied.     

多管陶瓷过滤器内瞬态流场的计算

用雷诺应力模型模拟了含有三根滤管的过滤器内过滤和反吹的整个过程的流场。结果表明,滤管外速度分布是不均匀的。反吹时,滤管开口端附近最容易受到热应力的冲击。计算结果与实验值和文献报道吻合很好,可以用该模型来分析预报陶瓷过滤器内的气体流动情况。     

Assessment performance of high-temperature filtering elements

An extensive experimental campaign has been carried out in a hot gas filtration test facility so as to test several filtering elements and configurations, particularly, three different types of bag filters and one ceramic candle. The facility was designed to operate under a wide range of conditions, thus providing an excellent tool for the investigation of hot gas filtration applications for the advanced electrical power generation industry such as IGCC, PFBC or fuel cell technologies.Relevant parameters for the characterisation and optimization of the performance of the filters have been studied for a variety of operating conditions such as filtration velocity, particle concentration, pressure and temperature among others. Pressure drop across the filter, cleaning pulse interval, baseline pressure drop, filtration efficiency and durability of the filter have been investigated for each type considered and dependences on parameters have been established. On top of that, optimal operating conditions and cleaning strategies were determined.The tests results show that bag filters are a suitable alternative for the hot gas filtration due to the better performance and the high efficiency observed, which makes them suitable for industrial applications operating under high temperature high pressure conditions considered within the study (200-370 °C and 4-7.5 barg, respectively).     

高温气体过滤技术及装备发展概况

随着洁净煤发电技术的发展及环保要求的提高,高温气固分离技术取得了重要的进展。本文阐述了在高温过滤材料方面,经过了早期的金属多孔材料、均质陶瓷多孔材料、纤维增强陶瓷复合材料、金属合金及金属间化合物多孔材料等四个阶段的探索,逐渐形成了陶瓷粉末、陶瓷纤维、金属粉末、金属纤维和金属丝网等多种高温气体过滤元件,催化与过滤复合元件近年来也逐渐得到工程应用。此外,在高温过滤元件表面粉尘层结构、脉冲反吹循环再生、高温过滤器结构设计及实时运行优化等方面都取得了重要的技术进展。目前,高温气体过滤技术及装备已在煤气化、催化裂化、冶金及垃圾焚烧处理等方面得到了广泛的应用,其适用的温度范围为260~650℃,压力范围从常压到6.0MPa,过滤效率达到了99.9%以上,可有效除净1μm以上的颗粒,净化后气体中的颗粒物含量低于5mg/m³。指出随着国家经济转型和环保排放要求的提高,高温气体过滤技术在产品质量升级、高温余热利用及颗粒物排放控制等领域具有更加广泛的应用前景。     

SIMULATION OF PARTICLE TRANSPORT AND DEPOSITION IN A COMBUSTOR

A computational model for Lagrangian particle tracking for studying dispersion and deposition of particles in a combustor with swirling flow and chemical reaction is developed. The model accounts for the effect of thermophoretic force, as well as the drag and lift forces acting on particles, in addition to the Brownian motion and gravitational sedimentation effects. The mean turbulent gas flow, temperature fields and chemical species concentration in the combustor are evaluated using the stress transport turbulent model of the FLUENT code. The instantaneous fluctuation velocity field is generated by a Gaussian filtered white noise model.

The simulated axial, radial and tangential mean gas velocities are compared with the existing experimental data. Ensembles of particle trajectories are generated and statistically analyzed. The effects of size and initial distribution on particle dispersion and deposition are studied. The particle concentration at different sections are also evaluated and discussed. The results shows that the turbulence dispersion effect is quite important, while the thermophoresis effect is small.     

陶瓷过滤器脉冲反吹系统的流场的数值模拟

In the commercial utilization of rigid ceramic filters, the performance of pulse cleaning has crucial effects on the long-term stable operation. In order to get a clear insight into the nature of this cleaning process and provide a solid basis for industrial applications, the flow in ceramic candle filter was investigated. The flow in the pulse-jetspace and inside the ceramic candle is regarded as two- dimensional, unsteady, compressible flow, and numerical simulation is carried out by computational fluid dynamics. The numerical predictions of flow field are in good agreement with the experimental measurements. Effects of the candle diameter, the separation distance between the nozzle and the candle injector and the length of the candle on the flowfield have been numerically analyzed to provide the basis for the optimum design of the pulse cleaning system.     

Filtration and dust cake experiment by ceramic candle filter in high temperature conditions

Particulate collection at high temperature and high pressure (HTHP) is important in an advanced coal power generation system not only to improve the thermal efficiency of the system, but also to prevent the gas turbine from erosion and to meet the emission limits of the effluent gas. The specifications for particulate collection in those systems such as Integrated Gasification Combined Cycle (IGCC) and Pressurized Fluidized Bed Combustion (PFBC) require absolutely high collection efficiency and reliability. Advanced cyclone, granular bed filter, electrostatic precipitator, and ceramic filter have been developed for particulate collection in the advanced coal power generation system. However, rigid ceramic filters and granular bed filters among them show the best potential. The problems experienced of these systems on performance, materials, and mechanical design were investigated. Ceramic candle filters have the best potential for IGCC at this moment because they have nearly the highest efficiency compared with other filtering systems and have accumulated many reliable design data from many field experiences. The purpose of this study was to investigate the efficiency of ceramic filters and stability of material against high temperature and longterm operation condition by applying fly ash on the surface of the filter and relation of pressure drop and dust cake thickness. Experimental conditions were 50 hours at 450 °C, 650 °C and 850 °C.     

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.     

陶瓷过滤器脉冲反吹全过程的瞬变流场计算

采用二维轴对称非稳态流动模型对陶瓷过滤器脉冲反吹全过程中滤管内外的瞬变流场进行了数值模拟,所计算的滤管外径向速度波形与实验测定结果基本吻合,给出了从正常过滤过程到脉冲反吹结束的全过程中引射区域和滤管内速度场及温度场的变化, 分析了反吹气体温度对滤管内温度分布的影响.模拟结果表明,过滤器的操作温度与反吹气体温度差提高,会加剧滤管轴向方向脉冲清灰不均匀性.     

Effects of a Shroud Tube on Flow Field and Particle Behavior Inside a Bag-Filter Vessel

A shroud tube was used to decrease the amount of particles toward the bag filters from whole particles entering a filter vessel. The effects of the shroud tube on the flow field and particle behavior inside the vessel were studied. The air mixed with dust particles enters the vessel through a tangential inlet duct. Some of the particles are deposited on the inside wall of the vessel and the surface of the shroud tube. The other ones are collected on the filter surface or passed through it. The particles deposited on the wall surfaces fall into a hopper by gravity, and those collected on filters are removed by back pulse-jet flow. Computational simulation was performed to know the prereduction rate of particles by deposition on the wall surfaces for the different shroud tubes. The experiment was accomplished with some shroud tubes suggested by the results of computational simulation, and the experimental results were compared qualitatively with the computational results. The shroud tube blocked the direct transport of particles toward the bag filters and reduced the particle loading onto the filters. The particle loading was reduced when the upper region of the vessel was not blocked by the shroud tube more than when the vessel was blocked wholly with the filters from the upper end wall. However, the re-entrainment of the particles removed from the filters by the back pulse cleaning increased when the upper region of the vessel was not blocked by the shroud tube more than when the vessel was blocked wholly with the filters from the upper end wall.     

Numerical simulation of thermophoretic particle decontamination in clean rooms

In order to develop strategies for minimizing deposition of contaminant particles of diameters ranging from 0.1 to 1.0 μm on a wafer, the effect of thermophoresis on a particle deposition velocity was numerically studied. The angle between wafer surface and direction of free-stream flow was introduced as a system parameter. Convection, diffusion, sedimentation, and thermophoresis were included as particle transport mechanisms. Similarity transform was applied to the model equations and obtained equations with dimensions reduced by one. The results suggest that it is possible to enhance the removal of particles of diameter ranging from 0.1 to 1.0 μm by heating with a temperature difference of 10–30°C between wafer surface and the air stream. If the filter of a clean room removes well around 0.1 μm sized particles, the free-slream velocity or flow angle should be increased for the effective removal of particle by thermophoresis, but, if the filter is efficient in removing particles around 1 μm, the free-stream velocity or flow angle should be decreased.     

操作参数对陶瓷过滤管脉冲反吹清灰过程的影响

在由单根陶瓷过滤管组成的实验装置上,利用U形管压差计及压阻式压力传感器分别测定了过滤含粉煤灰气体时滤管内外压差和脉冲反吹时滤管内的瞬态压力. 结果表明,在进入连续稳定循环过程后,单根滤管在各个循环的清灰效率仍存在较大的波动.在过滤参数不变而仅改变反吹参数的情况下,可依据反吹压力波形正压峰值来判断清灰效率的优劣. 同时分析了重要操作参数对反吹压力波形及清灰效率的影响. 指出脉冲宽度对清灰效率几乎没有影响;在满足清灰要求的前提下,再提高反吹压力对清灰并没有明显的改善效果;而过高的过滤速度对清灰效率极为不利,会导致过滤循环操作无法正常进行.     

Axial and transverse characterizations of ceramic candle filters

Ceramic candle filters are stiff composite tubes used in particle-removal during hot-gas filtration in coal energy generation. These filters are designed to withstand high temperature and pressure gradients. To determine the consistency in manufactured qualities of these filters, dynamic characterization method is recommended as a nondestructive evaluation technique. Six filters of the same manufactured batch were tested dynamically to establish the baseline properties of these filters. The test results are compared to theoretical values and are used to identify the variation in the manufactured products. This paper reports the frequencies in both transverse and axial directions indicating acceptable variations between all filters that are compatible to variations in measured physical parameters including density, stiffness and elastic modulus.     

Particle deposition in turbulent duct flows—comparisons of different model predictions

Numerical studies of transport and deposition of nano- and micro-particles in turbulence flow field have been studied in the past few decades. In most current industrial applications, Reynolds averaged turbulence models were used due to its relative simplicity and computational efficiency. In this work, a series of numerical simulations were conducted to study the transport and deposition of nano- and micro-particles in a turbulent duct flow using different turbulence models. Commercial software (FLUENT^TM 6.1.22) was used for turbulence mean flow simulation. Simulations of the instantaneous turbulence fluctuation with and without turbulence near wall correction, and particle trajectory analysis were performed with the in-house PARTICLE (object-oriented C++) code, as well as with FLUENT^TM code with and the use of user's defined subroutines. The simulation results for different cases were compared with the available experimental data, and the accuracy of various approaches was evaluated. In addition, the importance of turbulence model, boundary conditions, and turbulence fluctuation particularly near wall on particle transport and deposition were carefully evaluated. It was shown that when sufficient care was given to the modeling effort, the particle deposition rates could be predicted with reasonable accuracy. The presented results could provide guidelines for selecting appropriate procedure for simulating nano- and micro-particle transport and deposition in various applications.     

Simultaneous Filtration and Catalytic Oxidation of Carbonaceous Particulates

Two catalytic filters prepared by the deposition of Cu–V–K–Cl catalyst on different ceramic supports, foam and sintered aluminosilicate monolith, were employed for the removal of soot from the exhaust of a gas-oil burner. The foam filter was more suitable than the sintered filter for this application allowing better soot–catalyst contact, lower gas pressure drop and easier spontaneous filter regeneration.     

Experimental and analytical modeling of the filtration mechanisms of a paper stack candle filter

A candle filter consisting of a paper stack is an adequate device to clarify the grinding oil employed in the industrial post-processing of hard metal because of its cleaning efficiency and its regenerative capability by means of backwashing. The complex particle deposition mechanisms occurring in this device have not yet been investigated. Filtration experiments with a suitable particle-oil suspension were performed in a pilot filter and in a laboratory filter in order to investigate and model the mechanisms taking place in the process. The effect of operating parameters such as particle concentration and pressure difference on filter performance was evaluated. The results show that more than one filtration mechanism takes place simultaneously. While some depth filtration occurs at the beginning of the lifetime of a candle filter, blocking and cake filtration are the major mechanisms responsible for the filter clogging. Although blocking and cake filtration occur, to some extent, simultaneously, the cake filtration is the mechanism relevant for the modeling and scale-up of the long term filtration process.     

陶瓷过滤管管壁内气体流动的LBM并行模拟

采用格子Boltzmann方法,编制并行程序,计算了陶瓷过滤管管壁内的流体流动,从微观角度对滤管多孔介质结构内的流动进行分析。以陶瓷过滤管管壁扫描电镜图片为基础,根据实际滤管的厚度,确定计算中的多孔介质结构。分析了无膜滤管微细通道内的速度及压力随入口参数的变化情况以及多孔介质结构对速度的影响,给出了压力沿滤管厚度方向的变化曲线;分析了有膜滤管内的流动情况,给出了压力变化曲线,由计算结果可知,滤膜压降占滤管压降的比例较大。对过滤管微孔结构内流动的研究,可为陶瓷过滤管的性能优化提供理论指导。