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Effect of sintering temperature in preparation of granular ceramic filter
Abstract
Ceramic filters are quite an efficient component for hot gas filtration due to their high collection efficiency and high thermal durability. Among the various steps to fabricate a ceramic filter, sintering is one of the most important processes. In this study, disk-type ceramic filters were prepared at various sintering temperatures, and the effect of sintering temperature on physical/chemical properties and filtration performance were investigated.
We found that the sintering temperature strongly influenced the physical and chemical properties as well as the filtration performance of a ceramic filter. The adhesive force among powders and the oxidation degree of Si species are more important parameters to determine the mechanical strength of a ceramic filter than other physical properties. Filters prepared at sintering temperatures lower than 1450 °C have low mechanical strength due to insufficient binding energy among powders, and filters prepared at sintering temperatures higher than 1500 °C also have low mechanical strength due to the oxidation of SiC to SiO2. However, ceramic filters sintered at 1350 °C and 1400 °C achieve a better filter quality factor than other filters, even though they have lower mechanical strength. Therefore, both mechanical strength and filter quality factor should be considered to choose a better ceramic filter.
... Ceramic media are available as dense granular and light fiber elements [25,[53][54][55][56][57][58][59][60]. The "rigid sintered dense granular ceramic elements" have been historically used foremost in the power generating industry when using pressurized fluidized bed combustion and gasification combined cycles. ...
... Other mineral fibers, glass and combined products are also currently offered for hot gas filtration. Research assessed the effects of temperature, dust concentration, and face velocity on the loading and dust cake properties of ceramic candle filters manufactured from mullite and a compound of alumina and silicon carbide respectively [57]. The production of fibrous filter media produced by plasma spray from mineral zeolites [54], and the manufacture of ceramic filters for use at 800 • C, and prepared from aluminum silicate fibres, mullite fibers, and nano-silica powder as a high temperature ceramic adhesive were additionally reported. ...
... Choi et al. [57,58] examined how the sintering temperature affected the physical, chemical and filtration performance of ceramic disk-type filters. The specific application in gasification of biomass at 850 • C using porous ceramic filter elements, with special emphasis on corrosion behavior in contact with different biomass ashes, was investigated by Rys-Matejczuk and Müller [59], while Voigt et al. [60] examined the use of silicate-bonded SiC candles in hot gas filtration for pressurized coal-fired power stations. ...
The development of de-dusting hot gases is of increasing importance and growing interest, especially in advanced power generation systems and (petro-)chemical processes. High temperature de-dusting has advantages of exceeding dew points, enhancing energy recovery at higher temperatures, protecting the downstream heat recovery equipment from fouling or erosion, and potentially simplifying the overall process. Additional benefits are a reduced power consumption and the possibility to simultaneously capture gaseous pollutants (SO2, HCl) by the gas-solid reaction with injected alkali or by integrating catalysts to abate NOx, VOC (volatile organic compounds) and PCDD/F (polychlorinated dioxins and furans). Both porous ceramic and porous metal fiber filters have been proposed, with a growing preference for the rigid metal fibre filters. Either surface or depth filters are applied.
The operation parameters of hot gas de-dusting by rigid fibre filters include the applied face velocity, the porosity of the filter fleece, its construction and the pulse jet cleaning performance. These parameters will be integrated into design equations to determine the operational pressure drop and filtration efficiency for a given gas flow rate, operating temperature, dust characteristics and pulse-jet cleaning mode.
The broad range of potential applications of the filters prior to an energy recovery stage will be illustrated for petrochemical/chemical processes and for combustion and gasification. The expected durability and long-term successful operation necessitate a proper design of the filter media (porosity, pore size, thermal, mechanical and corrosion resistance). The appropriate fabrication materials will be evaluated, with stainless steel or high grade metal alloys recommended for use in the extreme conditions at high temperatures and pressures. Finally prospects and challenges are summarized.
... Following the compaction of the cylindrical samples, to remove humidity, the batch of samples was dried in the open air at a temperature of T = 20-23 ºC, for 48 h. Afterwards, the samples were sintered [19] at a temperature of T = 1100 ºC in a Nabertherm type oven according to the following treatment cycle: heating from T = 40 °C to T = 1100 °C for 71 min, holding at T = 1100 °C for 30 min, followed by cooling from T = 1100 ºC to T = 40 °C for 90 min. ...
... The compressive strengths are high enough to preserve the geometry and dimensions of the finished products during processing and use. They are comparable to those of materials presented in similar studies [13,17,19,20]. The average porosity of the composites obtained ranged between 15% and 25%. ...
The paper presents the results of a study of the filtering capacity of some filters from ceramic composite materials made on laboratory installation. The filtration process involved applying a pressure gradient to the filter by means of a centrifugal pump from the filter installation. The ceramic composite materials used to make the filters have a bentonite-based matrix. To achieve the porous structure, the matrix was reinforced with SiC and Al 2 O 3 ceramic powders and to control the pore size, additional Al metal particles were added. The stages of obtaining and characterizing the filters, respectively of testing their filtering capacity of the suspended particles from the rainwater are presented distinctly. In conclusion, the dependencies between a series of structural and geometric characteristics of the filtering capacity were established.
... According to BP's 2016 statistical review of world energy [1], coal makes up the second largest portion of global energy consumption, and will likely dominate the fuel market in the foreseeable future because of its economical attractiveness [2][3][4]. Using coal to generate electricity through various combined cycle power systems, such as pressurized fluidized bed combustion (PFBC) and integrated gasification combined cycles, has been a focus in developing advanced power plants [5,6] because of the high energy conversion efficiency and low emission levels of derivate air pollutants (e.g., particulate matter and greenhouse gases) in such systems. To transform coal or biomass in a reducing environment and produce a fuel gas (syngas) is a so-called ''gasification" thermochemical conversion process. ...
... The Association of German Engineers (VDI) defined hot gas filtration as filtration conducted above 533 K (guideline 3677-3). Ceramic control units at high temperatures because ceramic media possess sufficient thermal shock resistance and high chemical and mechanical stability [5,6,10]. To achieve cost efficiency and reliable performance at high temperatures, ideal dry hot gas cleaning techniques should exhibit high filtration efficiency while simultaneously maintaining pressure drops within acceptable limits. ...
... In addition to ceramic membrane filter cartridges which are characterized by low porosity and high filter performance, there are also ceramic fiber filters existing with a high porosity and lower filter performance. The latter is also suitable for catalytic coating, as the catalyst particle do not clog the huge channels and thus not increase the backpressure [9][10][11][12][13][14][15][16][17][18]. Those ceramic materials can be coated be several procedures like dip coating or incipient wetness coating, e.g. ...
For industrial processes—like waste incineration—it is necessary to reduce solid components (like dust or fly ash) as well as gaseous components (like dioxins, CO and other harmful hydrocarbons) to fulfill legal requirements. Therefore, catalytically functionalized filters based on polymers already exist. However, it is known that such filters are always constructed in multiple layers to prevent the migration of catalyst particles. This study demonstrates that it is possible to prepare a stable catalytic functionalized single-layer filter based on polyester needle felt by using flame spray pyrolysis. The catalyst is a low temperature active Pt/TiO2 with a loading weight of 38 g/l on the filter. Via SEM images the uniform distribution of the catalytic particles even in the deeper regions of the single-layer filter was proven. The structure was confirmed after experiments under realistic conditions—migration could not be obtained. Likewise, it was obtained that the oxidative conversion of carbon monoxide (CO) to carbon dioxide (CO2) is completely even at temperatures below 100 °C. Furthermore, comparative studies with catalysts on a honeycomb and a ceramic foam have shown that the conversion on the polyester needle felt textile catalyst is comparable.
... At this stage, cracks are generated and transmitted through extension or deflection along the grain boundaries, while the corundum grains are broken. Porous ceramics [56], layered ceramics [57,58] and fiber composite ceramics [59] have similar displacement behaviors [60]. Figure 10(b) shows a representative load-displacement curve of fracture toughness. ...
Coal gangue is used to prepare flake nano-mullite with a molten salt method. In order to increase the utilization value, corundum composite ceramics were prepared from the nanomullite and alumina. The effect of particle size of the raw material size and the content of mullite on mechanical properties of the corundum composite ceramics was studied. It is found that the mullite with appropriate sizes can be filled in between the corundum grains, thus increasing the density of the samples. The mechanical properties were enhanced due to the variation in the crack propagation mode. The optimized sample exhibited flexural strength and fracture toughness of 263.7 MPa and 5.0 MPa·m1/2, respectively. Griffith fracture theory analysis results indicated that the critical crack size reached 173.9 μm and the fracture energy was 127.6 J/m². Coal gangue has realized deep resource utilization, and the mechanical properties of composite ceramics are higher than alumina ceramics prepared under the same conditions. Flake nanomullite contributes to mechanical properties.
... In recent years, industrial wastewater has been increasing due to the industrial development, Physical filtrations using membrane filters are widely used for the wastewater treatment [1]. For such purposes, ceramic filters [2,3], organic membrane filters [4], and biological filters [5] have been studied. Ceramic filters are generally more expensive than organic filters, but have superior properties such as corrosion resistance and higher mechanical properties. ...
Ceramic filters can be adapted to various liquids even under adverse conditions. Al2O3-based composites dispersed with 3 mol%-Y2O3-doped ZrO2 (3Y-ZrO2), or Al2O3/3Y-ZrO2, are well-known as extremely strong oxide-based ceramics. In this study, we have focused on the Al2O3/20 wt.% 3Y-ZrO2 composites as ceramic water filters. Disk-shape Al2O3/3Y-ZrO2 filters with different pore structures were prepared by pressureless sintering at 1200–1500 °C in air. The pore-size distribution of the filters was analyzed by mercury porosimetry. Some filtration properties, such as turbidity, were evaluated, and the Herman and Bredée's filtration models were discussed. Among the filters sintered at different temperatures, the Al2O3/3Y-ZrO2 filter sintered at 1300 °C showed the best performance due to the lack of large pores of > ~ 2 µm. The average pore diameter of the A8Z2-1300 sample was 64 nm, the pore volume was 0.157 cm³/g, and the open porosity was 40.2%. SEM observations confirmed the change of the cake-layer thickness after the filtrations, i.e., ~ 27, ~ 17, ~ 15 and ~ 2 µm for the A8Z2-1200, 1300, 1400 and 1500 samples, respectively, which revealed that the actual filtration in this study was the combination of (i) the standard blocking with the larger pores at the initial stage and (ii) the cake-filtration with the smaller pores at the middle and late stages.
... As an alternative low-cost material for adsorption processes and the removal of iron from water, porous ceramic materials can be used as adsorbents owing to their high specific surface areas and surface and structural properties. Porous ceramic filters are efficient in removing various pollutants and eliminating bacteria, as well as having low thermal expansion and conductivity, high permeability, and chemical inertness [26][27][28]. Thus, porous ceramic materials have potential as inexpensive alternative adsorbents for the removal of iron from water. ...
This study proposes the use of an ultrasound-assisted adsorption system coupled to porous ceramic fragments to improve the removal of iron from FeSO4 aqueous solution. Ultrasound was applied using an ultrasound bath at a low frequency (37 kHz, 330 W). The optimized conditions for Fe removal were achieved by 7 g of adsorbent, 40 min of sonication, 20 mg L-1 of initial Fe concentration, and 30 °C of reaction temperature. After optimizing the conditions, the method was applied for the removal of iron in groundwater. A central composite design and response surface methodology were used to evaluate the degree to which different variables had a significant effect on iron removal. The efficiency of iron removal using the selected conditions for FeSO4 solution was near to 100%. However, for groundwater samples, the maximum iron removal efficiencies of the system with and without ultrasound were 80.7% and 51.1%, respectively, indicating that the adsorption with ultrasound was significantly higher than that without ultrasound. It was shown that the proposed ultrasound-assisted adsorption system can be used to enhance the removal of inorganic iron from groundwater.
... These results confirm that the binder content has a significant impact on the density and porosity of the fibrous ceramics [24]. Furthermore, the high porosity (> 80%) and thermal insulation of the matrix materials resulted in an ultra-low thermal conductivity of the fibrous ceramic filter elements, which lead to an excellent thermal shock resistance relative to the traditional sintered powder ceramics [4,25,26]. Besides, the pore size is also an important structural parameter. As shown in Table 1, the mean pore size decreased slightly as the amount of SL added increased from 0.00 to 0.18 wt.% and then remained almost constant upon further addition of SL. ...
Sodium lignosulfonate (SL) and silane coupling agent (SCA) were used to modify the fibrous filter elements to remove the useless binder and improve the strength, and their effects on the microstructures and properties were investigated. SL reduced the slurry viscosity to remove the binder present in the pores. The bulk density of the filter element was decreased after modification by SL, but the flexural strength was hardly affected. SCA enabled more Si–O bonds to be formed to enhance the adhesion between the binder and fibers. After further modification by SCA, the binder content at the crossing points between the fibers and the bonding strength between the binder and fibers were improved so that the flexural strength was significantly increased. Furthermore, owing to the decrease in the surface tension after adding SL, more binder bridges were formed on the upper surface of the filter element, affecting the gas permeability.
... Porous SiC ceramics are also quite widely used as heat-protection elements, catalyst supports for highly exothermic reactions [12][13][14][15][16][17] (owing to relatively high thermal conductivity of SiC), and filtering elements for chemically reactive gases and melts [18][19][20][21][22]. Such materials are produced by a wide diversity of methods: gas-phase and liquid-phase conversion of biomorphic carbon [18,[23][24][25][26], silicon interaction with polymers foaming during thermolysis [27], sol-gel synthesis during the reaction of hydrolysis of tetraethoxysilane with resorcinol and formaldehyde [28,29], vacuum casting of composites based on polysiloxane with silicon dioxide [30], and thermal conversion of allylhydropolycarbosilane or poly-1,3,5-trisilacyclohexane casted in mesoporous silica templates [31]. ...
Porous ceramic materials were produced by hot pressing of a nanocrystalline (19 nm) silicon carbide powder synthesized by a hybrid method that combined the sol–gel processing of a finely divided and chemically reactive SiO2–C system and the carbothermic synthesis at moderate (1400°C) temperature in a vacuum. It was studied how such characteristics as density, porosity, sizes of crystallites and aggregates of SiC particles, specific surface area, and compressive strength depend on pressing temperature (1400, 1500, 1600, and 1700°C).
The paper presents the results of a study on the compressive behaviour of ceramic composite made with bentonite matrix. During the study, samples were made of ceramic composite, of filter type, cylindrical with a diameter of 16 mm and heights between 14-16 mm. To achieve the porous structure, the matrix was reinforced with ceramic powders of Al2O3 and SiC, and to control the pore size, metal powders of Al and Fe were used in percentages between 3-12%. The compaction of the samples thus obtain was performed by mechanical pressing in a metal mould on a universal testing machine with a force F=20 kN. In order to test them in compression, the samples were sintered in an oven at T=1100°C in a heat treatment oven after a predetermined treatment cycle. For each test, the diagram was registered with the help of the software from the machine’s equipment, respectively the behaviour of the different sintered ceramic composite samples. The results thus obtained were analysed in correlation with the composition of the tested samples. In conclusion, dependencies were established between the type, nature and ratio of the constituent samples of ceramic composite and their behaviour at the request of compression.
Oxide-bonded silicon carbide supports of porosity ranging from 33% to 47% were prepared by heating powder compacts (SiC, clay and alumina) at 1400°C in air with graphite acting as a pore former. The supports were spray-coated with an aqueous slurry of fine SiC powder (d50=15 μm), then sintered to produce a filtering layer with thickness ranging from 116 to 200 μm and average pore size ranging from 5 to 20 μm. Airflow tests were performed on both supports and coated filters at temperatures ranging from 25° to 700°C and superficial velocities ranging from 0.02 to 0.9 m.s⁻¹. Experimental permeability coefficients were used to simulate the pressure drop behavior of hypothetical candle filters for industrial combustion/gasification processes (biomass combustion in water-tube steam boilers (BCSB), pressurized fluidized-bed combustion (PFBC) and integrated gasification combined cycle (IGCC)). The simulated permeation properties of the hypothetical candles were compared to those of commercial hot gas filters.
Catalytic filters including catalytic bag filters and catalytic filter candles, which couple the filters with denitrification catalysts to obtain the ability to simultaneously remove SOx, NOx and dust, have become the promising applied technology for the integrated flue gas treatment because of their huge advantages in reducing the initial investment, floor occupancy and maintenance cost. In this review, we will summarize the recent advances in the development of catalytic filters in terms of their process principles, filter material, denitrification catalysts, structure-function relationships and industrial applications. Moreover, suggestions about the current challenges and future opportunities are also given from the viewpoints of catalysts and filter material design, catalytic filter preparation methods, and their poisoning and regeneration, etc. With the further development of theory and engineering research, the extensive industrial application of catalytic filters in the field of multiple pollutants flue gas treatment is highly anticipated in the future.
Bayesian estimation theory was used in this study to establish a state-space model of the ceramic filter tube degradation process. The model continuously integrates the latest residual pressure drop to update its own parameters, then the change rate of the degradation state, remaining life, and failure probability density distribution of the tube in real time. The residual pressure drop of the Shell Coal Gas Process was analyzed to find that the model results converge to real values as the residual pressure drop increases. The change rate of the ceramic filter tube degradation state calculated by the model gradually decreases over time, which is consistent with the initial rapid increase in residual pressure drop followed by a slower increase in later stages of operation. The amount of particle deposition in the ceramic filter tube wall under different operating times was measured and predicted; the predictions are consistent with the state-space model results. The state-space model also reflects variations in filter tube performance degradation caused by emergent conditions such as leakage or fractures, as it does not make stationarity assumptions for the degradation process.
Abstract Despite the high potential of the freeze-casting technique for production of porous inorganic substrates, there is a lack of studies on tubular geometries and their mechanical behavior under different pressure scenarios. In this work, the mechanical behavior of tubular freeze-cast alumina substrates was assessed by mathematical models from experimental O-ring tests. The stress distributions revealed a concentration of tensile stresses (within 0.2-25.0 MPa) on the plane of the load, causing brittle fracture. Furthermore, the results confirmed that the honeycomb model for brittle material adequately predicted the mechanical strength of the tubular freeze-cast substrates. Finally, fracture criteria from honeycomb model was used to estimate the maximum homogeneously distributed pressures, such as in fluids, that the substrates can withstand. This configuration represents more precisely practical conditions, though is hard to experimentaly replicate. Therefore, the developed procedure is paramount to simulate the mechanical behaviour of the tubular freeze-cast substrates under real operating conditions.
Benefitting from the combined properties of intrinsic ceramic materials and advanced porous configuration, lightweight porous ceramics with porosity ranging from 2.3 to 99% and pore size distribution within 3 nm - 3 mm exhibit low density, large specific surface area, high toughness, strong thermal shock resistance, good thermal insulation capability, excellent high temperature stability, and low dielectric constant, which are barely offered by metal, polymer or even their dense counterpart. This unique set of features endow porous ceramics an indispensable role in the future development of sustainable energy and environmental applications. To be successfully applied in these applications, precise selection of the type of ceramics and creation of their detailed structural features of the pores are the most important stages that require intensive investigations and comprehensive understanding. For a given ceramic, the synthesis process is critical for achieving the desired pore configuration and geometry which eventually determines the final properties of the products, including both the usual mechanical properties and other advanced functionalities. In this review, we will first focus on the fabrication processes that determine the pore structures and geometries. Four mainstream fabrication methods: partial sintering, replica template, sacrificial template, and direct foaming will be discussed, in addition to the additive manufacturing technique which has emerged as a promising process for the direct fabrication of porous ceramics components. Each approach demonstrates its unique suitability for a specific range of materials, porosity, pore size, pore connectivity, and pore distribution. The principles and challenges of each synthesis strategies will be summarised and discussed, the progress that can be made to meet the requirement of advanced applications has been clarified. We then focus on the properties derived from different pore features. The superior damage tolerance and thermal insulation capability of porous ceramics, as compared with their dense counterpart, are presented. Thirdly, the great potentials of these interesting porous ceramics for the energy- and environmental-based applications, including filters, catalyst support, energy storage and conversion components, energy harvesting devices, and insulators are highlighted, in association with the criteria and demands for manufacturing processes. It is envisaged that this review will provide a guidance in the manufacture of advanced porous ceramics with desired pore structures and properties tailored for specific applications. Finally, we will demonstrate how these porous ceramics could contribute to the development of current and future energy and environment technologies.
Aluminum silicate fiber-based ceramic filter elements were prepared using different drying methods and calcination temperatures. The effects of these parameters on the performance, including the microstructure, bulk density, shrinkage, flexural strength, and pressure drop, were investigated. The results indicated that there were differences in the microstructures of the filter elements fabricated via three drying methods and that the microstructure significantly affected the flexural strength and initial pressure drop. Compared with blast and vacuum drying, microwave drying inhibited the migration of the binder, improving the structural uniformity and mechanical strength. The filter element effectively removed particles with a size of >1 μm and exhibited a good regeneration performance. Furthermore, owing to the crystallization of fibers after calcination at 1000 °C, the density and shrinkage of the sample were significantly increased and the initial pressure drop was reduced. The mechanical strength was significantly reduced after calcination at 800 and 1000 °C. As the calcination temperature increased, the filter element gradually became brittle. Therefore, the optimal calcination temperature of 600 °C was important for obtaining an excellent filter element with a high flexural strength and low pressure drop.
In order to develop a flexible composite material that could play a good cushion role for high temperature ceramic membrane cleaning, we herein report a flexible composite material which could achieve effective buffering to relieve the stress generated by rigid contact and play a certain sealing effect. The coating method is applied to fabricate the composite material, which can be produced by multi-layer superposition using high silica fiber fabric and waterborne polyurethane. The results show that waterborne polyurethane (WPU) as a binder coating plays a key role in improving the strength by six times compared to raw materials and maintaining the elasticity of materials. Meanwhile, the thermal degradation mechanism of the composites calcined at different temperatures (700–1200 °C) in air atmosphere is investigated. The as-made composite materials still have good flexibility, and the tensile strength is about 2.4 MPa after calcination at 1000 °C for 80 h. This provides a method to prepare flexible cushion composite materials for ceramic membrane filtration.
Macroporous cordierite ceramics, comprising hierarchical microstructures, are produced by a method of particle-stabilized emulsions, combined with a followed reactive thermal treatment. The microstructure is tailored by altering sintering temperature and solid content in the emulsion templates. Pore throats generate in-situ by introducing magnesite in templates, in contrast to conventional methods, which use either surfactants or depending on thin film contact. Moreover, microstructural evolution of samples is studied by DTA/TG, XRD, and SEM analyses. The results of analyses show that the formation of much more pore throats is closely related to the release of gas from the raw materials and volume shrinkage. The optimal process conditions are a temperature of 1300°C and a solid content of 30vol.%. The as-prepared sample displays a nitrogen permeability of ∼1.8×10⁻¹¹ m². The method shows great promise for producing many other highly permeable ceramics using pore former agents in the emulsion templates.
Porous silicon carbide ceramics with high open porosity and large pore size are usually applied as filters for the cleaning of hot dust gases. However, the large porosity and large pore size will decrease the mechanical properties. In the present study, porous SiC ceramics were prepared using bentonite as bonding phase. The effects of sintering temperature on the microstructure and the compressive strength were studied. Bentonite could melt and spread on the surface of silicon carbide particles at all the sintering temperatures. However, the bonding effects were very different at different temperatures.
A well-defined porous ceramic having effective pore size less than 1 µm was fabricated by slip casting technique which provides a low-cost and single-stage filtration process. This single-stage filtration process removed suspended solids and pathogenic organism. In this study, diatomaceous earth was used as pore forming agent in clay based ceramic body. Varied amount of diatomaceous earth was used with fixed proportion of china clay, ball clay and lime stone. X-ray fluorescence spectroscopy (XRF) was used to determine the composition of raw materials and final sintered filter body. In terms of porosity and flow rate, 25% diatomaceous earth shows better filtration property. It was found that total porosity increased with decreasing milling time. As the firing temperature increased, porosity decreased rapidly after 1100 °C. However, the change of porosity in firing range of 1000 °C–1100 °C was found very small. Mercury intrusion porosimetry was used to measure the pore size and its distribution. An effective intra-particle pores in a range of 0.2-0.5 μm was identified. Inter- and intra-particle pores were observed using scanning electron microscope (SEM). Microbial removalefficiency of 99.99% was measured, while water flow rate was found in a range of 200-700 ml/hour.
A tubular ceramic support intended for use in transesterification process was prepared using the polymeric foam replication method. Pieces of foam with pores 120 ppi in size were completely immersed in a 25wt% solid-loading of porcelain slurry. The dried samples were sintered for 2 hours to temperatures ranging from 1200°C to 1265°C, respectively. It was found that the optimum sintering temperature was 1250°C as the following values were obtained for each characteristic: 1.98 MPa compressive strength, 47.3% porosity, 2.773gcm 3 density and 0.45mm pore size. XRD results show that the sintered ceramic consists of mullite and quartz.
In this study, the reaction-bonded SiC (RBSC) hot gas filter was newly developed using Si melt infiltration technique. Si melt was infiltrated into the green body through the carbon source coated surface of SiC powder forming strong neck phases between SiC powders. The maximum flexural strength of porous RBSC support at room temperature and at 900 °C were about 80 MPa and 63 MPa, respectively, which were much higher than those of commercially available porous clay-bonded SiC with similar pore size and porosity.
To analyze the phenomenon of increasing pressure drop during dust removal using a ceramic filter system under high temperature conditions, the pressure drop in an ash powder layer on a ceramic filter during the gas permeation and dust detachment processes was directly measured with a small-scale testing apparatus. Dust detachment behavior of the ash layer on the ceramic filter was evaluated by using both quasi-static and pulse reverse flow methods. Under conditions of relatively low temperatures (below 1050 K), the pressure drop in the gas permeation test was almost fixed, and the maximum pressure drop (Δpmax) at the first detachment of a dust layer by quasi-static reverse flow increased gradually in proportion to temperature. At temperatures higher than 1123 K, a time-dependent increase of pressure difference during gas permeation occurred before dust detachment and a rapid increase of Δpmax during dust detachment was observed. The time-dependent densification of ash layers and a rapid increase of adhesion between ash particles were observed by thermo-mechanical analysis (TMA) and a diametral compression test of ash powder pellets. In this paper, we suggest that the pressure drop increase is controlled by a decrease of pore volume and an increase of fracture strength within the ash layer.
A simple processing route for manufacturing highly porous, silica-bonded SiC ceramics with spherical pores has been developed.
The strategy adopted for making porous silica-bonded SiC ceramics entails the following steps: (i) fabricating a formed body
through a combination of SiC and polymer microbeads (employed as sacrificial templates) and (ii) sintering the formed body
in air. SiC particles are bonded to each other by oxidation-derived SiO2 glass. By controlling the microbead content and the sintering temperature, it was possible to adjust the porosity such that
it ranged from 19 to 77%. The flexural and compressive strengths of the porous silica-bonded SiC ceramics with ≈40% porosity
were ≈65 MPa and ≈200 MPa, respectively. The superior strengths were attributed to the homogeneous distribution of small (≤30
μm), spherical pores with dense struts in the porous silica-bonded SiC ceramics.
The purpose of this study was to investigate the filtration and loading characteristics of granular bed filters. Stainless steel holders (diameter 71.6 mm, height 70 mm) were fabricated to accommodate 500 g of zirconium oxide (ZrO2) beads, as the packed media of granular bed. Monodisperse ZrO2 granules (0.3, 0.8, 2 and 4 mm in diameter) were used to demonstrate the effect of the granule size and packing geometry on both pressure drop and aerosol penetration. From the filter quality perspective, the selection of the ‘best” filter is complicated. Assuming a low face velocity (e.g., 0.58 cm/s), large granule size is more cost-effective because of the higher filter quality factor. The phenomenon implies that the gain in filtration efficiency due to larger surface area (of small granules in the filter) did not compensate for the increase in air resistance. After the cake formation point, the dust cake on glass fiber filter became compressed. This dust cake compaction caused the pressure to drop precipitously and intermittently. In contrast, the rate of increase in pressure drop of the dust cake formed on the granular bed filters decreased with time probably due to the pinhole channels in the increasing mass load. The size and density of the pinholes are determined by the granule size, the face velocity and the size of the challenge aerosols.
Cordierite-mullite bonded porous SiC ceramics were prepared by an in situ reaction bonding technique using a silicon carbide, aluminum hydroxide, MgO, and graphite as starting materials. The starting materials were mixed in different ratios and heated at different temperatures (1300-1450 °C) to obtain good mechanical properties. The pores in the ceramics were formed by burning graphite and by stacking particles of SiC and Al(OH)3. The surface of SiC was oxidized to SiO2 at high temperature, and fine Al2O3 grains produced by the decomposition of aluminum hydroxide. With further increasing the temperature, SiO2 reacted with Al(OH)3 and MgO to form cordierite-mullite. SiC particles were bonded by the oxidation-derived SiO2 and cordierite-mullite. The reaction bonding behavior, phase composition, open porosity, microstructure and mechanical strength as well as pore size distribution of porous SiC ceramics were investigated. The results show that when 4.5 wt% MgO was added, in the absence of graphite, the flexural strength and the open porosity of the porous SiC ceramics were 78.71 MPa and 21.63%, respectively. In addition, when 30% graphite was added to the above composition, a flexural strength of 41.29 MPa was achieved at an open porosity of 42.11%.
The silicon carbide (SiC) ceramic filter is the most favorable component to remove particulate matter from hot flue gas due to its high filtration efficiency and high thermal durability. The effect of SiC powder size on the physical properties and filtration performance to prepare high-performance granular ceramic filter media was investigated in this study. Disk-type filter media were prepared by mixing ceramic components followed by physical compression and sintering. The porosity and average pore diameter in the filter media increased with increasing powder size. However, the mechanical strength decreased with increasing powder size, while it increased with increasing physical compressive force. The filter performance factor, qFM was introduced to evaluate the ceramic filter properties, and the SiC50 filter was the best of the ceramic filters prepared in this study. We also found that diffusion was a dominant collection mechanism for particles smaller than 0.7 μm, and direct interception and inertia were dominant collection mechanisms for particles larger than 0.7 μm in the SiC50 filter based on a single collector efficiency model. In addition, the predicted collection efficiencies showed reasonably good agreement with the experimental ones.Copyright 2014 American Association for Aerosol Research
Porous mullite-bonded SiC (MBSC) ceramics were fabricated at temperatures ranging from 1400 to for 2 h using silicon carbide (SiC), alumina (), strontium oxide (SrO), and poly (methyl methacrylate-coethylene glycol dimethacrylate) (PMMA) microbeads. The effect of template content on porosity, pore morphology, and flexural strength were investigated. The porosity increased with increasing the template content at the same sintering temperature. The flexural strength showed maximum after sintering at /2 h for all specimens due to small pores and dense strut. By controlling the template content and sintering temperature, it was possible to produce porous MBSC ceramics with porosities ranging from 30% to 54%. A maximum flexural strength of ~51MPa was obtained at 30% porosity when no template were used and specimens sintered at /2 h.
The influences of molding pressures, bonding phase contents, and SiC particle sizes on the flexural strength of SiC-based porous ceramics were investigated based on their microstructure of fracture surface. The SEM morphologies and EDS element analysis results of fracture surface showed that there were two different kinds of fracture points: SiC particle fracture points and bonding phase fracture points. It is found that molding pressures, bonding phase contents, and SiC particle sizes affect the SiC particle fracture point area in the fracture surface, and the fraction of the SiC particle fracture point area in the minimum solid area of fracture surface is a determined influence factor for the flexural strength of SiC-based porous ceramics used for hot gas filter support.
Porous SiC candle filter preforms were fabricated by extrusion and ramming process. To fabricate SiC candle filter preform, commercially available 85 -SiC powder and 44 mullite, CaC powder were used as the starting materials. The candle type preforms were fabricated by vacuum extrusion and ramming process, and sintered at 2 h in air atmosphere. The effect of forming method on porosity, density, strength (flexural and compressive strength) and microstructure was investigated. Also, corrosion test of the sintered candle filter specimens as forming method was performed at in IGCC syngas atmosphere. The sintered SiC filter which was formed by ramming process has more higher density and exhibit higher strength than extruded filter. Its maximum density and 3-point bending strength were 2.00 g/ and 45 MPa, respectively.
Multilayer ceramic filter was fabricated by fine particle migration occurring during the slip cast compaction of a particulate mix composed of a fine powder and coarse granules. The materials were prepared from the mixture of quartz, natural zeolite and lead borosilicate glass frit. The casting procedure involves the introduction of the powder slurry into a cylindrical shaped mold, allowing it to form a thin filtering layer onto the mold walls. The granules are then added into the mold without settlement of the previously filled slurry. The new casting method leads to a specific interlayer between the granular material and the filtering layer by a fine particle migration phenomenon. Casting, drying and sintering processes of the filters were discussed with respect to two different particle sizes of the powder slurry (δ50=0.98μm and δ50=1.82μm) and with variable solid concentration (5–40% solids by weight). Dilute slurry with relatively coarse size has appropriate condition for the filter fabrication.
An alumina foam with two kinds of pores was prepared by combining the sponge method and the pore-former method. The large pores were provided by the sponge method, and their sizes were in the range of 1–2mm. The small pores were produced by the pore-former method with size in the micrometer range. The large pores offered a high porosity while the small pores offered a large surface area. The strength of samples sintered at different temperatures was measured, and the effect of sintering temperature on foam strength was analyzed by discussing porosity and grain bonding area. The sample sintered at 1550°C has a compressive strength of 1.3MPa and a porosity of 86%.
A reaction bonding technique was used for the preparation of cordierite-bonded porous SiC ceramics in air from α-SiC, α-Al2O3 and MgO, using graphite as the pore-forming agent. Graphite was burned out to produce pores and the surface of SiC was oxidized to SiO2 at high temperature. With further increasing the temperature, SiO2 reacted with α-Al2O3 and MgO to form cordierite. SiC particles were bonded by the cordierite and oxidation-derived SiO2. The reaction bonding characteristics, phase composition, open porosity, pore size distribution and mechanical strength as well as microstructure of porous SiC ceramics were investigated. The pore size and porosity were strongly dependent, respectively, on graphite particle size and volume fraction. The porous SiC ceramics sintered at 1350°C for 2h exhibited excellent combination properties, the flexural strength of 26.0MPa was achieved at an open porosity of 44.51%.
First-generation monolithic porous ceramic filter materials have experienced thermal fatigue, high-temperature creep, and a loss of material strength when operated for extended periods of time in advanced coal-fired combustion and gasification systems. Fiber-reinforced and advanced ceramic composites potentially provide a means to mitigate the degradation mechanisms encountered by monolithic filter matrices and ultimately extend operating life. In this paper we will review advancements which have recently been made during the development of the second-generation porous ceramic filter materials and provide insight into the performance of the filter elements during bench-scale qualification testing and operation in demonstration plant test facilities.
The effect of the operating temperature on the behavior of coal ash on ceramic filters in a hot gas filtration system was studied. It was found that when the operating temperature exceeds the sintering temperature of the ash generated during combustion, the chance of forming ash-bridges between these filters is high. Calcium and potassium aluminosilicate were found to initiate the sintering of these samples, which results in an increase in the compression strength of these samples by an order of magnitude.
The aggressive process environment in advanced coal-fired power generation systems causes microstructural changes in the ceramic hot gas filters used to clean the fuel gas. Changes in microstructure and their effect on strength were studied for commercial SiC-based clay bonded hot gas filters exposed to high temperature, water vapour and gaseous sodium compound. Exposures caused significant crystallization of the binder phase and oxidation of SiC. Loss in strength also occurred. The microstructural changes and their effect on strength are discussed.
Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO2 glass. Because of the crystallization of amorphous SiO2 glass into cristobalite during sintering, the fracture strength of oxidation-bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 μm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation-bonded SiC ceramics were observed to exhibit an excellent oxidation resistance.
Highly ordered mesoporous silicon carbide ceramics have been successfully synthesized with yields higher than 75% via a one-step nanocasting process using commercial polycarbosilane (PCS) as a precursor and mesoporous silica as hard templates. Mesoporous SiC nanowires in two-dimensional (2D) hexagonal arrays (p6m) can be easily replicated from a mesoporous silica SBA-15 template. Small-angle X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images show that the SiC nanowires have long-range regularity over large areas because of the interwire pillar connections. A three-dimensional (3D) bicontinuous cubic mesoporous SiC structure (Ia3d) can be fabricated using mesoporous silica KIT-6 as the mother template. The structure shows higher thermal stability than the 2D hexagonal mesoporous SiC, mostly because of the 3D network connections. The major constituent of the products is SiC, with 12% excess carbon and 14% oxygen measured by elemental analysis. The obtained mesoporous SiC ceramics are amorphous below 1200°C and are mainly composed of randomly oriented β-SiC crystallites after treatment at 1400°C. N2-sorption isotherms reveal that these ordered mesoporous SiC ceramics have high Brunauer-Emmett-Teller (BET) specific surface areas (up to 720 m2 g-1), large pore volumes (∼0.8 cm3 g-1), and narrow pore-size distributions (mean values of 2.0-3.7 nm), even upon calcination at temperatures as high as 1400°C. The rough surface and high order of the nanowire arrays result from the strong interconnections of the SiC products and are the main reasons for such high surface areas. XRD, N2-sorption, and TEM measurements show that the mesoporous SiC ceramics have ultrahigh stability even after re-treatment at 1400°C under a N2 atmosphere. Compared with 2D hexagonal SiC nanowire arrays, 3D cubic mesoporous SiC shows superior thermal stability, as well as higher surface areas (590 m2 g-1) and larger pore volumes (∼0.71 cm3 g-1).
Ceramic multilayer micro-filtration membranes with smooth surfaces and cracks-free have been fabricated on tubular porous supports by dip-coating using natural zeolite mineral as the starting materials. The preparation process, including the powder classification, the forming and sintering of membranes, was systematically studied. The membrane thickness was determined by dipping time and solid loading of the suspension. XRD reveals that the phase compositions of the membranes were related to the sintering temperature and the final major phases were almost quartz and albite. SEM studies subsequently indicate that solids in the membrane begun to sinter at about 850 °C with alkali metal oxides as the aid fluxes. The interlayers with average pore size in the range of 0.69–1.10 μm were obtained and the optimum firing temperature was between 850 and 950 °C for 1 h. Then, the top-layer membrane with average pore size 0.54 μm could be prepared on the above support. Nitrogen gas permeation flux and pure water permeation flux of the resulting membrane are 1.96 × 105 and 3.20 × 103 l m−2 h−1 ×10−5 Pa−1 (1.96 × 105 and 3.20 × 103 l m−2 h−1 bar−1) with the trans-membrane pressure of 0.1 MPa at room temperature, respectively.
This is a review of gas cleaning methods to be used for particulate removal from the gases emerging from pressurized coal gasifiers or fluid bed combustors at high temperature and high pressure. References are given (collected in tables) for the emissions of these processes, the inlet requirements if the hot gas is to pass a turbine and all cleaning methods that have been proposed in this context, including the removal of gaseous alkalis.The design feasibility and available performance data for high temperature, high pressure applications of cyclones, electrostatic precipitators, barrier filters (using fibre, felt and sintered filter elements) and various types of granular bed filters, are discussed in some detail and suggestions are made for the further research and development. The theory of capture mechanisms is reviewed in so far as relevant in the present context. Turbulent flow, electrostatic forces, and re-entrainment are the phenomena that require special attention in almost all applications.
Contemporary research in ceramic composites for membrane applications targets the fabrication of stable low cost micro-filtration range membranes. This work reports the fabrication of stable low cost mesoporous membranes deploying subsequent formulations based on kaolin and other suitable low cost materials such as quartz, sodium carbonate, calcium carbonate, boric acid and sodium metasilicate. Incidentally, the processing temperature in this work is about 850–1000 °C instead of 1100 °C which is the usual sintering temperature for inorganic membrane fabrication. The membranes casted as circular disks (52.5 mm diameter and 4.5 mm thickness) were subjected for characterization studies using TGA, XRD, SEM analysis, to evaluate the effect of maximum sintering temperature on membrane structure, porosity and mechanical integrity. Pore size distribution, porosity, average pore size of the membrane along with permeation experiments with both water and air is carried out to study the membrane performance. The average pore size of the membrane is observed to increase from 550 nm to 810 nm when sintering temperature increases from 850 °C to 1000 °C. However, with the increase in temperature from 850 °C to 1000 °C, the membrane porosity is reduced (from 42% to 33%) and flexural strength is increased (from 3 to 8 MPa). The membrane corrosion resistance is found to be unaltered with processing temperature. Based on raw-material prices, the membrane cost is estimated to be $130/m2. Henceforth, these low cost membranes with good membrane properties are suggested for cheaper application in chemical and biochemical processes using micro-filtration and ultra-filtration techniques.
Low-cost porous ceramics with controllable pore size were prepared by using natural clay and silica as starting materials and organic polymer sponge as template. The properties of the resulting ceramic material, such as pore size, porosity and mechanical strength, were characterized using different techniques. The results show that this novel preparation procedure provides an easy pathway to produce porous ceramics according to specification for a range of applications. (c) 2007 Elsevier Inc. All rights reserved.
Ultra-thin films of silicon oxides supported on a Mo(100) surface have been studied using X-ray photoelectron spectroscopy (XPS). The films were synthesized by evaporating Si onto the Mo surface in oxygen ambient and were subsequently characterized using XPS with respect to the chemical states of silicon and the composition of the film. It has been found that the silicon oxide, prepared at room temperature with a silicon deposition rate of ∼ 1.2 Å/min and an oxygen pressure of 2 × 10−5 Torr, consisted of predominantly silicon dioxide with a small fraction of suboxides. Annealing to ∼ 1300 K yielded a stoichiometric film of SiO2. The suboxides are believed to further react with oxygen forming SiO2 at an elevated temperature.
A single-step processing method has been previously established to prepare porous alumina microstructures by a controlled sedimentation technique whereby fine powder from an aqueous suspension consolidates over a casting slab. Metastable surface chemical control of the suspension properties was able to produce a highly porous flat disc structure with a continuously increasing mean pore size from top to bottom. Formation of this gradient structure was facilitated by using a relatively broad particle size distribution. Top layer pore sizes less than 50nm have been achieved. Without modification, these structures are suitable for use as ultrafiltration media.The present work presents a comparison of properties and performance data for samples made with the above mentioned functionally gradient characteristics, to samples made with a more uniform microstructure. The effects of sintering time and temperature were analysed in view of overall porosity, pore size distribution and the extent of densification from the green state. These results are presented along with permeation measurements from a filtration test module.
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.