Ceramic filters for high-temperature flue gas filtration and their regeneration: A review of the current state of knowledge

Ceramic filters for high-temperature flue gas filtration and their regeneration: A review of the current state of knowledge Eva Gregoroviˇ cov´ a * , Jiˇ rí Pospíˇ sil Energy Institute, Brno University of Technology, Brno 616 69, Czech Republic ARTICLE INFO Keywords: Ceramic filters High temperature Flue gas filtration Filtration efficiency Pressure drop Regeneration ABSTRACT The combustion of solid fuels in combustion devices produces pollutants that have a negative impact on human health and the environment. One way to reduce the concentration of sub-micron particulate matter (PM) in the flue gas is to filter the flue gas using ceramic filters. Their main advantage, compared to other flue gas cleaning methods, is the possibility of using them at high flue gas temperatures (>400◦C). This review article presents an overview of research articles published mainly between 2020 and 2023 on flue gas filtration using ceramic filters (candles, membranes, foams, and discs) and on their regeneration. Ceramic filters are used in many flue gas cleaning applications where the cleanliness of hot flue gas is required. Their inclusion in the flue gas path is associated with increased energy consumption for forced exhaust and pressure pulse filter regeneration. The review also includes an introduction to the materials and methods used to manufacture the filters and to test their effectiveness. This review provides a comprehensive overview of the current state of knowledge on ceramic filters. Specifically, the focus is on the filtration efficiency, pressure drop, and regeneration of the ceramic filters under consideration. 1. Introduction With increasing economic development and new knowledge about the negative impact of emissions, great emphasis is being placed on reducing the production of particulate matter (PM) in the flue gases produced by the combustion of solid fuels. Particulate matter from combustion sources has a direct negative impact on human health, and the environment (ecosystem), and indirectly contributes to climate change (global warming). Exposure to polluted air shortens life expectancy and has a negative impact on health, especially for newborns and the elderly (Zneˇ ciˇ stˇ en´ e Ovzduˇ sí Zkr´ atí ˇ Zivot Aˇ z o Tˇ ri Roky, December 2023). Submicron particles (PM1) penetrate the human body to the lung chambers and are therefore very dangerous (Pol´ etavý Prach - PM Available online, February 2024). An important measure for improving air quality (reducing particulate matter) is the filtration of fine particles from the flue gas stream. Improved air quality will result in a better quality of life and reduced healthcare costs in the long term. The basic equipment for flue gas cleaning includes cyclones, fabric filters (also known as bag filters or baghouses), electrostatic precipitators, and wet scrubbers (Li et al., 2021). The disadvantages of these filtration devices are limited filtration efficiency, low temperature, and chemical resistance (greater distance from the combustion source is required) or high financial costs. Currently, increased attention is being paid to ceramic filters of various types (e.g., candles, membranes, foams, and discs), which, compared to other filtration methods, exhibit high filtration efficiency of fine particles, low pressure drop; and their main advantage is their chemical stability and the possibility of using them at high flue gas temperatures. Ceramic filter candles are currently used in many applications for high-temperature flue gas cleaning. The disadvantage is their energy consumption for regeneration (or cleaning) with pressure pulsed air, which is currently the most used technology. Ceramic filters are made of different materials and by different methods to produce different types of filters (which vary in shape and properties) that are currently being investigated for their potential use in high-temperature filtration of fine particles in flue gases. This review on ceramic filters for high-temperature flue gas filtration contains a theoretical overview of this issue, an overview of current studies (2020–2023), and commercially available ceramic filters in the Czech Republic. The overview of current studies is with emphasis on properties, filtration efficiency, pressure drop, and regeneration of * Corresponding author. E-mail address: [email protected] (E. Gregoroviˇ cov´ a). Contents lists available at ScienceDirect Process Safety and Environmental Protection journal homepage: www.journals.elsevier.com/process-safety-and-environmental-protection https://doi.org/10.1016/j.psep.2024.07.088 Received 11 March 2024; Received in revised form 1 July 2024; Accepted 22 July 2024 Process Safety and Environmental Protection 190 (2024) 688–703 Available online 22 July 2024 0957-5820/© 2024 The Author(s). Published by Elsevier Ltd on behalf of Institution of Chemical Engineers. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). filters. The current state of the art is important to determine the potential of ceramic filters and for their further development leading to wider use in flue gas filtration. 2. Flue gas filtration Filtration of particulate matter (PM) emitted by the combustion of solid fuels in combustion devices is an important measure to improve air quality, the environment, and human health. In addition, in some applications (e.g. power industry), clean flue gases (purity of the flue gases) are required, e.g. to ensure the safe operation of a combustion turbine (Mai et al., 2003). The basic equipment for flue gas cleaning includes cyclones, bag filters, electrostatic precipitators, and wet scrubbers. In recent decades, porous ceramic and metal filters, woven glass/ceramic filters, and moving granular bed filters have been used (Li et al., 2021). Cyclones are simple and low-cost devices (requiring moderate energy (Peck et al., 2023)). They remove particles larger than 5 μ m in diameter from the flue gas with 90 % efficiency (Rahimpour et al., 2022) and are therefore used as pre-treatment devices. The disadvantage of cyclones is the formation of erosion (Li et al., 2021). The operating temperature of the cyclone corresponds to the construction materials used (Rahimpour et al., 2022). Electrostatic precipitators have high collection efficiency (>99 %) (Lind et al., 2003), but are energy intensive and are used at low flue gas temperatures up to 350 ◦C (Li et al., 2021). Wet scrubber flue gas cleaning has a relatively high efficiency (90 %) for particulates larger than 5 μ m (Rahimpour et al., 2022), but a high pressure drop and moderate energy consumption (Li et al., 2021; Rahimpour et al., 2022). Other disadvantages of wet scrubber are the possibility of fouling and corrosion, and the low operating temperature of up to 250 ◦C (Li et al., 2021). The moving bed filter has a relatively high-pressure drop but allows filtration up to 1000 ◦C (Li et al., 2021). Filter elements for flue gas filtration can be either fabric filters (baghouses), pleated filter cartridges, or ceramic filters (Li et al., 2024). Fabric filters have a temperature limitation of up to 250 ◦C (Li et al., 2021). For high-temperature filtration, ceramic filters are preferred for the above reasons (Sparks and Chase, 2013). Metal porous filters made of sintered metallic powder or fibers are more robust and more expensive compared to ceramic filters, and their disadvantage is that they can be prone to corrosion and erosion during filtration. Ceramic filters are more stable to chemical/extreme conditions, and have higher porosity, but are prone to damage due to pressure pulse cleaning (Li et al., 2021; Peck et al., 2023). However, with continuous development, these disadvantages are reduced and metal filters e.g. made of Ti-40Al-10 Nb-10Cr porous alloy are suitable for high-temperature filtration (Gui et al., 2022). There are two methods of particulate (solid particles) filtration that can be achieved using ceramic porous filters. These are depth and cake (surface) filtration (Peck et al., 2023; Li et al., 2024; Riedel and Chen, 2013). During depth filtration, particles with a smaller diameter than the pore diameter are captured on the inner wall of a pore (inside the pores). The gradual clogging of the particles forms dendritic structures that expand until they join and form bridges across the pore. In this case, the filter is completely clogged with particles, where the particles cannot pass through the pores and the filter transits to surface filtration (cake filtration), where the captured particles form a filter cake on the surface (particle deposits are formed) (Peck et al., 2023; Li et al., 2024) (see Fig. 1). Surface filtration is the main mechanism for capturing particles (Li et al., 2024). The cake layer on the filter surface increases the filter efficiency and increases the filter life (creates additional filter media). However, the accumulated cake layer increases the pressure drop of the filter, so the deposit must be removed. The filtration properties are significantly affected by the pore size of the porous media. With a smaller pore size than the particle size, faster cake formation occurs, hence more efficient filtration of particles and at the same time increasing the pressure drop. Conversely, with a pore size larger than the particle size there is slower cake growth, less efficient particle filtration, and less pressure drop. If depth filtration predominates, the pores clog rapidly, and the pressure drop increases exponentially. This is followed by cake (surface) filtration, which in turn leads to a linear increase in pressure drop (Peck et al., 2023). It is supposed that the highest filtration efficiency is reached just before cleaning when the filter cake is the thickest (Li et al., 2024; Jeong et al., 2017). Dust particles larger than the pores are captured by sieving (sieving is applied to fabric filter (Cho et al., 2020)) (Karabulut et al., 2021). 2.1. Mechanisms of particle capture The particle capture filtration process on porous ceramic filters is primarily realized by the following particle settling mechanisms: gravitational settling (for very heavy particles (Hinds, 1999)), inertial impaction, interception, Brownian diffusion, and electrostatic attraction (related to the electric charge carried by particles) (Li et al., 2024; Hinds, 1999) (see Fig. 2). The filtering mechanism for the ceramic filters is that the polluted flue gas passes through the tortuous pore channels, and by inertial impaction, the particulates are captured on the element of the filter (i.e. fiber in the case of fibrous filter). The particles deposited on the surface of the element of the filter capture other particles and form a chain-like aggregate. Subsequently, the dispersed agglomerates form bridges in the tunnel to form a dust cake layer, and the particles are captured on the cake layer by interception (Gong et al., 2022; Wang and Otani, 2013). Interception occurs when a particle follows a stream of flue gas that closely surrounds an element of porous ceramic at a distance of one radius of the particle. Interception is more effective when the particles are larger. Particle interception has minimal efficiency and is particularly important for fibrous filters. Interception is one of the particle Fig. 1. Scheme of surface filtration and depth filtration and formation of filter cake (adapted from (Gong et al., 2022)). E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 689 capture mechanisms that does not depend on the face velocity v s . In addition, interception is the only mechanism where the particle does not deviate from the flue gas streamline (Hinds, 1999). The inertial impaction occurs in the case of a particle trajectory directed at the filter element and a long stopping distance when the particle due to its inertia (momentum) does not have time to avoid the element and settles by impact on the element filter. The controlling parameter of the inertial impaction is the dimensionless Stokes number, which is an indicator of the particle’s ability to follow the streamline. The larger the Stokes number, the less well the particle follows the streamline, and the efficiency of impaction capture increases. Inertial impaction is an important mechanism, especially for large particles (Hinds, 1999). Stk = τ U0 do = ρ pd2 pCcU0 18 η do (1) where d o is the characteristic dimension of the obstacle (i.e. fiber), m; τ is the relaxation time of the particle, s; U 0 is the flow velocity of the medium, m/s; ρ p is the density of the particle, kg/m 3 ; d p is the diameter of the particle, m; C c is the Cunningham correction factor; and η is the dynamic viscosity of the medium, Pa•s (kg/m•s). Particle diffusion is related to Brownian motion (causing random motion of particles) and is important for small particles. The control parameter is the dimensionless Peclet number. Brownian forces acting on small particles increase the probability of a particle colliding with a filter element, even if the original streamline bypasses the element. Diffusion is the only mechanism for which efficiency increases with decreasing particle diameter. The smaller the particle and the Peclet number, the more efficient the mechanism. Diffusion is often combined with interception (Hinds, 1999). Pe =doUo D(2) where D is the particle diffusion coefficient, m 2 /s. 3. Ceramic porous filters For high-temperature flue gas filtration, porous matrices are used, most often in the form of ceramic candles, membranes, foams, and disks. In the energy industry (thermal processes), ceramic filters are used in applications such as coal gasification and combustion, waste incineration and biomass gasification and pyrolysis, etc. (Li et al., 2024). Ceramic filters can withstand temperatures up to 1000 ◦C (Sparks and Chase, 2013; Shah, 2017). The selection of the appropriate ceramic filter type depends on the operating temperature and composition of the gas to be cleaned (Shah, 2017). Ceramic filters can also be coated with catalysts (serve as catalyst supports/carriers) (Xu et al., 2023). The catalysts supporting the chemical reaction are mostly used to remove or reduce gaseous emissions of NOx, and dioxins, and to oxidize VOCs (Peck et al., 2023; Sparks and Chase, 2013). In addition to the high-temperature filtration of flue gases from the combustion of solid fuels or waste, ceramic filters are used, e.g., for the exhaust gas filtration for diesel engines (for cars) (Chen et al., 2022; Hwang et al., 2022; Jia et al., 2020; Omeraˇ sevi´ c et al., 2022; Viswanathan et al., 2021), filtration of polluted water (Alymov et al., 2020; Chen et al., 2022; Du et al., 2022; Farrow et al., 2018; Rivera-S´ anchez et al., 2020; Yang et al., 2020; Yüzbasi et al., 2022) and filtration in the foundry industry (filtration of cast iron, non-ferrous metals, steel filtration (Kalisz and Kuglin, 2019) (Keramick´ e Filtry VUKOPOR®, December 2023). 3.1. Properties of porous ceramics Porous ceramics consist of two phases, a solid ceramic phase and a gas-filled porous phase (Recent Advances in Porous Ceramics; Uthaman et al., 2021). Porous ceramics can have porosity in the range of 20–95 % (Recent Advances in Porous Ceramics). Total porosity is defined as the ratio of all pore volumes to the total volume of the porous body (including pores) (P´ orovitost Available online, December 2023; P´ orovitost –Wikipedie Available online, December 2023). Depending on the structure, ceramic filters for flue gas filtration are divided into granule-bonded filters and ceramic fibrous filters (Gong et al., 2022; Li et al., 2023; Seville, 1993). The main requirements for porous ceramics for filtration purposes are mechanical strength, chemical resistance, heat resistance (Sparks and Chase, 2013), thermal shock resistance, high temperature creep resistance (Skrzypek et al., 2008), narrower pore size distribution (minimum difference between smallest and largest pore size), high porosity (more free spaces for gas passage), high permeability, large specific surface area, low weight, low density and thermal conductivity (Sparks and Chase, 2013; Recent Advances in Porous Ceramics; Wu et al., 2021). According to the definition from the International Union of Pure and Applied Chemistry (IUPAC), porous ceramics are divided according to pore size into microporous (<2 nm), mesoporous (2–50 nm) and macroporous (>50 nm =0.05 μ m) (Uthaman et al., 2021). According to scientific articles, porous ceramics for flue gas filtration have macroporous pores, several times larger than 0.05 μ m. The porosity of ceramic materials can be textured to random or hierarchical. The pore architecture can be, for example, in the form of foam, honeycomb, or fiber networks (SYMPOSIUM 9, February 2024). Ceramic honeycomb filters are used in the automotive industry for exhaust gas filtration (Bao et al., 2018). High temperature can cause volumetric (thermal) expansion of the solid phases of the filters (samples). Depending on the structure and material of the filters, the permeable pores may be narrowed or enlarged (permeability coefficients changed) and the pressure drop increased or decreased. The detection of temporary or irreversible thermally induced changes in the filter structure is determined by the hot gas permeability (HGP), specifically by changes in permeability coefficients at different temperatures (Dey et al., 2022). 3.2. Filter performance The performance of ceramic filters is evaluated according to the particulate capture efficiency and pressure drop of the filter. In publications, the authors evaluate the filtration efficiency R according to Eq. (3) (Zhang et al., 2022). To evaluate the overall filtration performance, the authors use the quality factor Q f , see Eq. (4) (Yang et al., 2023; Wang et al., 2022). A high quality factor means high filtration efficiency while maintaining low pressure drop ΔP (Yang et al., 2020). Fig. 2. Scheme of particle capture mechanisms (adapted from (Bulejko, 2018; Chang et al., 2021)). E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 690 R=(1−n1 n0)×100 (%)(3) where n 0 is the particle concentration before filtration, mg/m 3 ; and n 1 is the particle concentration after filtration, mg/m 3 . Qf=−ln(1−R) ΔP(4) The pressure drop affects the operational energy consumption of the filtration together with the pressure pulse cleaning of the filter candles. A low and stable pressure drop can be achieved by high values of the filter permeability coefficients (Dey et al., 2022), which have a positive effect on separation and permeation properties, power usage, and life span (Xiong et al., 2020; Jiang et al., 2018). The magnitude of the pressure drop is influenced by the properties of the porous filter, i.e., pore size, porosity (high porosity reduces the pressure drop), pore connectivity, filter thickness, face velocity (high face velocities lead to high pressure drop) and filter loading. In the case of fiber filters, the diameter of the fibers and the volume fraction of fibers, i.e. the amount of fibers in the total volume (volume fraction of fibers) (packing density or solidity), affect the pressure drop (Hinds, 1999). Filtration efficiency increases with the gradual settling of particles and therefore with increasing filter cake on the filter surface, however, the pressure drop increases with increasing filter cake thickness (Shah, 2017). The filter pressure drop for the filter candle, for cylindrical coordinates for radial inward flow (through the hollow filter candle) can be estimated based on the Forchheimer’s equation, see Eq. (5) (Das and Kayal, 2020; Innocentini et al., 2012). ΔP= μ o k1(Do 2)ln(Do Di)vso + ρ o k2(Do 2)(Do−Di Di)v2 s(5) where D i , D o is the inner and outer diameter of the candle, ρ o is the density of the gas, μ o is the viscosity of the gas, v s is the face velocity of the gas and k 1,2 is the Darcian and non-Darcian permeability coefficients. The face velocity v s , see Eq. (6) (Das and Kayal, 2020), is the velocity of the flue gas stream arriving at the filter surface and its magnitude affects the filtration efficiency. For particle filtration, it is important to try to keep the face velocity as low as possible to have a longer residence time for the particles and therefore a higher probability of particle contact with the filter. Reduction of the face velocity is achieved according to the continuity equation by decreasing the flow rate or increasing the filter area (Hinds, 1999). The typical face velocity of the flue gas for ceramic filters is 1–5 cm/s (0.01–0.05 m/s) (Peck et al., 2023). vs=Q Sfilter (6) where Q is the volumetric flow rate, m 3 /s; and S filter is the filter area, m 2 . 3.3. Materials and manufacturing methods Generally, ceramic filters are prepared from materials such as silica (SiO 2 ), silicon carbide (SiC), mullite, alumina (Al 2 O 3 ), alternatively cordierite and zirconia (Dong et al., 2022; Liu et al., 2023). Recent studies show that other materials for the production of ceramic filters are clay, graphite, boehmite sol, feldspar, kaolin, silicon nitride (Si 3 N 4 ), Isobam, polymethyl methacrylate (PMMA), diatomite, polyurethane, calcium hexaaluminate (CA 6 ), calcium carbonate (CaCO 3 ), aluminum borate, pectin, and titanium dioxide (TiO 2 ). Ceramic filters are produced by various methods, i.e. ramming process (Das and Kayal, 2020), one-step construction process and rotary spraying method (Wei et al., 2022), polyurethane foaming (Zhang et al., 2022), gel casting with reaction bonding route (Li et al., 2022), direct foaming and gel-tape casting (Wang et al., 2022), foaming sol-gel-tape casting and in situ mullite reaction bonding (Dong et al., 2022), precursor solution dip coating and subsequent firing (Li et al., 2023), isostatic pressing and plasma coating (Yuan et al., 2023), spraying (Zhang et al., 2023), dry pressing process (Gong et al., 2022), fluorine-catalyzed gas-phase process (Yang et al., 2023), freeze casting (Dang et al., 2022), stabilize dual phase sol (Yang et al., 2022), direct foaming (employing bubbles as templates) with additing a pore-forming agent method (Zheng et al., 2021), gel-casting method (Yuan et al., 2022), gel-casting freeze-drying method (Liu et al., 2023). In the manufacture of porous ceramics, it is important to consider many factors that affect pore size, porosity, and pore continuity. These include pore size control, manufacturing technique, the size, shape, and particle distribution of the starting ceramic powder (raw materials), the type of binder used, the concentration and distribution of the binder, and the sintering process (sintering conditions) (Recent Advances in Porous Ceramics; Ohji and Fukushima, 2012; Gopi et al., 2018; Porous Ceramics - an Overview, December 2023). 4. Tested filters and achieved filtration parameters 4.1. Ceramic candles Ceramic filter candles have high filtration efficiency, high permeability, low coefficient of thermal expansion (Das and Kayal, 2020), temperature resistance above 1000 ◦C, erosion and corrosion resistance, high chemical inertness, mechanical strength, and resistance to thermal shock (Sparks and Chase, 2013). The surface of ceramic candles, especially the fibrous structure, is suitable for catalyst coating e.g. for selective catalytic reduction (SCR) to reduce NO x or dioxin emissions (Sparks and Chase, 2013). Ceramic rigid candle filters have a typical hollow cylindrical shape, which is closed on one side (bottom) (e.g. T-shaped collar or conical collar (Dey et al., 2022), on the other side is open (upper) (flat or hemispherical cap (Dey et al., 2022). Commercial filter candles have a length of 1000–3000 mm, an outer diameter between 60–150 mm, and a wall thickness between 10–20 mm. The filter area of the candle ranges from 0.19–1.42 m 2 (Das and Kayal, 2020; Stringer and Leitch, 1992; Judkins et al., 1996). (The candle filter with a length of 1500 mm, an outer diameter of 60 mm, a thickness of 10 mm, and a filter area of 0.27 m 2 weighs 4.5 kg (Riedel and Chen, 2013)). Commercial ceramic filter candles are either high-density (30–40 % porosity, sintered grains of silicon carbide, alumina, mullite, or cordierite) or low-density (80–95 % porosity, made of aluminosilicate (Al 2 O 3 - SiO 2 ) and other inorganic (non-carbon) bonded fibers) (Dey et al., 2022; Heidenreich, 2013; Purchas and Sutherland, 2002). Ceramic candles are often self-supporting, so they do not need any supporting metal constructions (Sparks and Chase, 2013; Tenmat, February 2024). SiC ceramic candles generally have an asymmetric structure consisting of solid support and a thin separation layer (membrane) (Wei et al., 2022; Zou et al., 2021). A ceramic candle consists of a filter flap (used for fixing and supporting the candle), pipe body, and sealing end. The filter flap must withstand high pressure and vibration during backflush, therefore the strength of the filter flap affects the lifetime of the filter (Wei et al., 2022). The direction of the flue gas flow is from the sides of the ceramic candle towards the inside of the filter (radial inward). The filter cake is formed on the surface of the candle (see Fig. 3). Das et al. (Das and Kayal, 2020) prepared porous oxide-bonded ceramic candles from silicon carbide (SiC =support (coating) material) by ramming process using the main additives, i.e., SiC powder, alumina (even without alumina), and a small amount of clay (as the binder phase additives). The dimensions of the of the ceramic candle were (OD ×ID ×H) (85 ×75 ×650) mm 3 . The porosity of the fabricated ceramic filters ranged from 36–40 vol% and the strength of the filter sample (ceramic candles were cut into so-called C-rings) ranged from E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 691 15–23 MPa. The average pore size of the filter was 90 µm. By examining the microstructure of the ceramic candles, the formation of interconnected pores was detected. The ceramic candle made with alumina additive showed better mechanical properties than the candle without alumina due to the presence of mullite in the oxide bonds. Using a laboratory-made test setup, the authors investigated the air permeability and filtration efficiency of the ceramic filter candles at room temperature. The permeability coefficients (Darcian k 1 1.9–2.2 ×10 –12 m 2 and non-Darcian k 2 5.4–9.72 ×10 −8 m) were determined from experimental data from airflow tests at room temperature (least squares method), with a working inlet pressure of 3–7 bar. The face velocity of the air was between 0.01 and 0.1 m/s. (The air flow was generated by an air compressor). As the face velocity increased, the pressure drop of the filter increased from 1500 to 3500 Pa. Ceramic candles filtered coal airborne fly ash that was crushed and sieved from the thermal power plant to achieve a uniform particle size distribution of 0.5–20 micrometers. The crushed dried powder was fed into the filter chamber at a rate of 0.8 gm/min at room temperature. The fractional efficiency before and after filtration was determined using an Aerodynamic Particle Size Analyzer (APSA). Particle sampling for identification was performed before and after the filter twice, and the particles were passed through the filter for five minutes before measurement. The process time of filtration was 10 min, so no filter cake formation occurred. For a face velocity of 0.1 m/s, a maximum filtration efficiency of ~99 % was achieved and the pressure drop was 4439 Pa. The fabricated ceramic candle filters have potential applications in hot gas filtration areas. Dey et al. (Dey et al., 2022) investigated air permeation behavior at high temperatures on uncoated and coated SiC filters with different porosity levels (with different porosity and pore size). The authors fabricated oxide-bonded circular carbide supports from powder compacts (SiC, clay, and alumina), and graphite was used as a pore former. The supports were then coated with an aqueous suspension of fine SiC powder and subsequently sintered to produce a filter layer 116–200 µm thick, with an average pore size of 5–20 µm. The resulting filter had a diameter of 40 mm and a thickness of 7–8 mm. The porosity of the filter thus produced was 33–47 %. In this work, air permeability tests were carried out on a laboratory apparatus. The airflow tests were performed on uncoated filter (support without water suspension spray) and coated filter (composite) (support with water suspension spray) respectively at temperatures of 25–700 ◦C and superficial velocities of 0.02–0.9 m/s. The permeability parameters of the filter were determined using Forchheimer’s equation. For the permeability tests, the circular filters were tightly fixed (to prevent leakage) in a stainless-steel sample holder using heat-resistant "O-rings". The circular flow area of the filter was 1.77 cm 2 . The air velocity was in the range of 0–0.58 m/s. It was found that the presence of a coating layer on the substrate did not significantly increase the pressure drop. (Minor changes in the k 1 and k 2 permeability coefficients of the uncoated and coated filters were observed with increasing temperature). It was also found that as the room temperature (26–30 ◦C) increased to higher temperatures (650–693 ◦C), the gas became more viscous and had a lower density, due to which the pressure drop increased. The actual effect of the permeability coefficients k 1 and k 2 on the total pressure drop of the filter is due to the characteristics of the test flow conditions (temperature, pressure, viscosity, density, and face velocity). Using an empirical model, the pressure drop of hypothetical filter candles whose material corresponds to the investigated SiC circular filters (composites) is predicted. The pressure drop parameter was estimated using the differential form of Forchheimer’s equation. The pressure drop parameter (permeability curves) of the hypothetical filter candles were compared with the real permeability curves of commercial filter candles used in industrial combustion and gasification processes (under typical conditions of BCSB, PFBC, and IGCC). The permeability coefficients of the hypothetical filter candles were found to be consistent with those given in the catalogs of commercial ceramic filters. It was also found that in addition to viscosity, inertia also has a large effect on pressure drop depending on gas conditions. In general, the denser the gases are (the higher the gas pressure and the lower the temperature), they have a larger inertia contribution. This effect increases with a higher filtration velocity. For the simulated conditions, inertial effects (specifically for the IGCC application) were found to contribute significantly to the pressure drop, which means that knowledge of the Darcian permeability coefficient k 1 alone is insufficient to reliably predict the relationship between pressure drop and face velocity. The results of this work support the potential of ceramic candle filters in hot gas filtration. Wei et al. (Wei et al., 2022) presented a robust SiC ceramic membrane filter (filter candles) with an optimized flap for industrial coal-fired flue gas filtration. The high bending strength of the flap is important for the longer lifespan of candles and their stability. The production of the SiC filter candle was divided into two parts: SiC support and SiC membrane (250 μ m thick). The support was fabricated by using a one-step construction process and the membrane was fabricated by rotary spraying method. The dimensions of the ceramic candle were (OD ×ID ×H) (60 ×40 ×1520) mm 3 and the filtration area was 2830 cm 2 . The bending strength of the filter flap was increased from 18 MPa to 37 MPa (due to more neck connections). The porosity of the SiC filter candle was 43.5 %, and the average pore size of SiC support and membrane was 45 μ m and 5.7 μ m. Coal-fired flue gas at 300–450 ◦C passed through the SiC membrane (SiC candle). Spherical fly ash particles were 0.6–115 µm in size, mean diameter 35 µm. At a surface velocity of flue gas of 0.8 m/min (0.013 m/s) and a back flushing pressure of nitrogen of 0.6 MPa (6 bar) with pulse duration of 100 ms, the pressure drop was relatively stable 2420–2700 Pa. Maximum allowable pressure drop was 4000 Pa. The filtration (retention) efficiency was 99.98 % and the concentration at the outlet was less than 5 mg/m 3 . Such a robust filter is suitable for cleaning flue gas containing submicron-sized solid particles. 4.2. Ceramic membranes Porous ceramic membranes have excellent properties such as low Fig. 3. Ceramic candle clogged with particles and its attachment (Tenmat, February 2024). E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 692 density, high thermal and chemical durability, and low thermal conductivity, but the disadvantages are low porosity, high pressure drop, and high self-weight (Pal, 2020). However, with development, the disadvantages of ceramic membranes are reduced. According to studies, the use of composite membranes is an effective solution for increasing gas permeance without compromising the mechanical strength of the membrane (Dey et al., 2022). To improve the properties a composite filter membrane is used, which consists of a support layer (support body) with a larger pore size (to ensure the strength of the filter) and membrane layers (separation membrane) with a small pore size (to ensure surface filtration) (Yuan et al., 2023; Benfer et al., 2004). Ceramic membranes can be used not only for air separation (Xiong et al., 2020; Koukou et al., 1999) but also for hot flue-gas filtration (Xiong et al., 2020). The ceramic membrane filter has an asymmetric structure consisting of a rigid support layer, a thin separation layer (membrane), and a dust cake (Xiong et al., 2020; Zhang et al., 2023). Ceramic membranes exhibit higher stability than polymer or metal membranes (Liu et al., 2023). Xiong et al. (Xiong et al., 2020) investigated the prediction of pressure drop of ceramic membrane filter tube at high flue gas temperature using a model (equations, formula, and experimental data). SiC support had an average pore size of 80 μ m and the porosity was 38 %, the mullite with SiC support had an average pore size of 10 μ m and the same porosity. The dimensions of ceramic filters were (OD ×ID ×H) (60 ×40 ×1500) mm 3 . It was found that high temperature leads to high flue gas viscosity and low filtration efficiency. Furthermore, high temperature leads to the sintering of dust cake (as already mentioned), which leads to fluctuations in pressure drop (increases at the beginning and then decreases) and to a decrease in the regeneration efficiency of the membrane. It was found that up to 1050 ◦C, the pressure drop increased with filtration time, but the growth rate of pressure drop decreased with increasing temperature. For filtration tests, air and petroleum gas were burned in the burner, and dust produced by the argon-oxygen decarburization (AOD) furnace was added to the flue gas. The particle size of the dust particles (containing f.e. SiO 2 ) was 0.04–8.93 μ m. The test temperatures ranged from 800 to 1050 ◦C. The superficial velocity of the flow through the ceramic membrane was 0.65 m/min to 1.5 m/min. The pressure drop across the ceramic membrane was 500–2500 Pa. Outlet concentration of dust was below 10 mg/m 3 . Wang et al. (Wang et al., 2022) prepared highly porous foam mullite-bonded SiC ceramic membranes (MSCM) with 3D interconnecting-pore network for high-temperature particulate matter (PM) filtration. The ceramic membranes were fabricated using a combination of direct foaming and gel-tape casting. The membranes have large interconnecting pore sizes of 60–150 µm in diameter, which are connected by smaller functional (for filtration) pores of less than 10 µm. With the increase in the amount of foaming agent, the filtration (removal) efficiency decreases as it narrows the pore walls and increases the size and number of interconnecting pores. By increasing the amount of foaming agent, the membrane porosity increased from 69.2 % to 84.1 %, the bending strength decreased from 10.3 MPa to 4 MPa, the average pore size increased from 6.9 to 8.8 μ m, and the N 2 permeation increased at 0.5 bar pressure from 17×10 6 to 36×10 6 L/m 2 h. The higher the membrane porosity and the larger the pore size, the lower the pressure drop and filtration efficiency. The experimental filtration test was carried out in a quartz tube in which filter samples of size ∅20 mm ×0.95 mm were inserted. The polluted air produced by incense burning was passed through the samples through the quartz tube, which was maintained at 750 ◦C by combustion. The flow rate of the flowing air was 8 cm/s. Particle size and concentration of PM was measured by laser PM sensor counter and pressure drop was measured by pressure-drop instrument. The highest quality factor of the sample was 0.097 Pa −1 . The best-measured membrane sample showed a filtration efficiency of 70.2 %, 90.1 %, and 94.6 % for PM0.3, PM2.5, and PM10, respectively, and a very low pressure drop of 27 Pa. When the membrane thickness was increased from 0.5 to 2 mm, the filtration efficiency increased from 80.3 % to 97.5 %, and the pressure drop increased from 17 to 52 Pa. Furthermore, the cycling performance of MSCMs was evaluated when the sample was cleaned by alcohol and sonication after 60 min. The sample showed a slight decrease in filtration efficiency (removal efficiency) of 87.6 % and 90.9 % for PM2.5 and PM10 after three cycles. Membranes produced in this way have the potential for highly efficient high-temperature filtration of PM. Wang et al. (Wang et al., 2022) simulated the deposition characteristics of particles in the pores of ceramic membranes using the computational fluid dynamics –discrete element method (CFD-DEM) bidirectional coupling method. Simulations were performed for understanding of particle bridging mechanism (cake formation) during filtration. In surface filtration with pores of ceramic membrane larger than the particle size, the formation of particle bridges affects the cake structure and filtration efficiency. To simulate the bridging process, two ceramic particle balls (two ceramic particle balls) with sizes of 20 μ m were used to form the pores of the ceramic membrane. The filtration process is divided into (A) clean filtration stage, (B-C) transition stage, and (C-D) cake filtration stage (Tao et al., 2020). The bridging function of particles takes place in the clean filtration stage and the transition stage. In the clean filtration stage, the particles are uniformly deposited on the surface of the ceramic membrane. Subsequently, in the transition stage, the deposits penetrate the membrane pores and form dendritic structures that continuously expand until they join and form a bridge in the ceramic membrane pores. At this stage, the particles cannot pass through the membrane pores because they are closed. A filter cake is formed, which increases the pressure drop but is important for particle capture. Only 4.2 % of the total particles captured were captured by the membrane alone, so the formation of particle bridges is very important for particle capture. Particle concentration and inlet flow rate were found to influence the formation of particle bridges and filtration efficiency. Higher particle concentration causes particles to flow through the pores before bridges are formed (reducing the bridging time), which reduces filtration efficiency. At a constant particle generation rate, the inertial particle collision and scouring effect of the fluid influence bridging time and filtration efficiency. For example, the authors report that at an inlet velocity of 0.1–0.6 m/s, the inertial collision of particles dominates, so bridging time and filtration efficiency increase with increasing inlet velocity. It was also found that the interference resistance increases with increasing inlet velocity as the particles pack more tightly onto the ceramic membrane. Dong et al. (Dong et al., 2022) prepared porous mullite-bonded SiC filters for high-temperature PM filtration. The SiC ceramic membranes are fabricated by foaming sol-gel-tape casting and in situ mullite reaction bonding. The membranes have a three-dimensional network structure with many interconnected large pores. The pore size is 50–150 μ m and the size of the connecting channels for filtration is less than 10 μ m. The membrane has a high porosity in the range of 64.8–80.5 % and a high N 2 permeation of 4.82 ×10 7 to 7.44 ×10 7 L/m 2 hbar (was achieved by an optimal amount of foaming agent). The bending strength of the membrane was 12.1–4.2 MPa. Hot flue gas filtration using a membrane was carried out in self-made filtration equipment. The flue gas produced by burning incense was passed through the filter at a constant temperature of 650 ◦C. Particle size and concentration were measured by a laser PM sensor counter. The flow rate of the polluted air was 8 cm/s. The best measured sample showed filtration efficiencies of 69.2 %, 88.2 %, and 95.1 % for PM0.3, PM2.5, and PM10, respectively. A high filtration efficiency of >89.6 % for PM2.5 and a very low pressure drop of 25 Pa were recorded. The sample showed an increase in pressure drop from 19 to 30 Pa after three cycles of use. The cycle performance of the sample after 60 min exposure to PM was removed for ultrasonic cleaning in alcohol for 30 min. Ceramic membranes fabricated in this way have the potential for hot flue gas filtration. Yuan et al. (Yuan et al., 2023) fabricated calcium hexaaluminate porous ceramic (CA 6 ) for high-temperature flue gas filtration using an E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 693 aqueous gel-casting method using CaCO 3 and ρ -Al 2 O 3 powders as raw materials. The produced ceramics had a high open porosity of 68.1 %, high air permeability (Darcy permeability k 1 5.19×10 −7 m 2 – 2.01×10 −6 m 2 ), low thermal conductivity 0.48 W/(m/K), high compressive strength 7.9–62 MPa. The pores were homogeneous, and the maximum average pore size was 2.028 µm. The porous ceramic thus produced has the potential for high-temperature flue gas filtration as a filtration ceramic or filter membrane. Li et al. (Li et al., 2023) prepared a ceramic whisker membrane for the filtration of hot gas or corrosive liquid. The ceramic membrane has aluminum borate whiskers on the aluminum support. The membrane with a thickness of approximately 50 µm was formed via precursor solution dip coating and subsequent firing. The resulting support had a porosity of ~50 %, a bending strength of 37 MPa, and a Darcy permeability k 1 of 1.54 ×10 −12 m 2 . The filtration area of the membrane was 1.8 m 2 . The filtration performance of the ceramic membrane was measured on a house-made test apparatus. PM particles were produced by burning mosquito coil. Face velocity was 5 cm/s. Removal efficiency was 95 % when filtering PM sizes of 0.3–1µm. The filtration pressure drop increased to approximately 3000 Pa after three cycles of back blow regeneration (30 % more than the pressure drop). Regeneration was performed after every 30 min of filtration, by back blow using compressed air at 300 kPa. Liu et al. (Liu et al., 2023) prepared mullite porous ceramic membranes with directional channels. The membranes were prepared by the gel-casting lyophilization method. Pectin was used as the gel and alumina and silica were used as raw materials. For example, the membrane had a porosity of 56.04–75.34 %, a high gas permeability coefficient of 1.11×10 −10 –4.73×10 −11 m 2 , and a gas permeability 2.18×10 −2 –9.32×10 −3 mol/(m 2 ⋅s⋅Pa). These properties are suitable for high-temperature flue gas filtration. Yuan et al. (Yuan et al., 2023) fabricated an asymmetric mullite/SiC composite filter tube membrane for high-temperature flue gas filtration. The mullite layer as the membrane layer (175 µm thick) was on the SiC support. The support structure was formed by isostatic pressing of SiC powder and the membrane structure was formed by plasma spraying of mullite. The porosity and the pore size of the membrane and support were 9.9 % and 1–10 µm and 19 % and 20–150 µm. The mullite/SiC composite filter tube membrane had a pore size of 45.2–36.4 µm (pore size reduction occurred after 360 days). The dust particle size (from metallurgy) was 0.1–100 µm. The average removal efficiency for PM2.5 was 98.4 % and the maximum removal efficiency was 99.6 %. The ceramic membrane prepared in this way is suitable for filtration capacity for dust particles. Zhang et al. (Zhang et al., 2023) fabricated highly permeable mullite/SiC-TiO 2 fiber membranes for gas/solid filtration at high temperatures (via a spray coating). A mullite fiber membrane layer was used on the SiC support. For larger interfacial strength (contact points) between support and mullite fibers, TiO 2 nanocords were placed on the SiC support. The membrane thickness was 200 µm, the average pore size was 21.9 µm, and the gas permeance of 715 m 3 /m 2 hkPa. The diameter of the mullite fibers was approximately 20 µm and the length was approximately 200–300 µm. The fabricated Mullite/SiC-TiO 2 membrane was used to filter dust-laden gas containing fly ash particles. Fly ash was recycled from an electrical plant. The fly ash had an average particle size of 15.1 µm (had a loose and spherical structure). Its filtration velocity was 2 m/min and its filtration temperature was 500 ◦C. After three filtration cleaning cycles, an efficiency of 99.9 % was achieved. The pressure drop was less than 1 kPa or 0.7 kPa =700 Pa. The ceramic membrane thus fabricated has the potential for high-temperature filtration. This study continues a previous study (Zhou et al., 2023). 4.3. Ceramic foams Ceramic foam is a fragile and lightweight porous material with a cellular structure that is formed by a three-dimensional network of struts (Recent Advances in Porous Ceramics; Luyten et al., 2009). Ceramic foam consists of a cellular structure, struts, and windows (Luyten et al., 2003). Windows distributed over the pore wall indicate the dimension of the flow path (Ciria et al., 2022). Ceramic foams (cellular structures) can have closed, open, or partially interconnected porosity (Recent Advances in Porous Ceramics; Ahmad et al., 2013). Open porosity is accessible from the outside and is divided into open dead-end pores and open pore channels (Uthaman et al., 2021; Recent Advances in Porous Ceramics). Tortuosity foam is characterized by pore diameter or pore density per inch (PPI) (Sharafat et al., 2006). Ceramic foams have a pore size of 4−50 μ m (Yang et al., 2022). Open porosity ceramic foams are mainly used for molten metal filtration, exhaust gas filtration in diesel engines, and hot gas filtration (Ahmad et al., 2013; Sharafat et al., 2006). Furthermore, they can be applied as fluid filters, catalyst supports, and separation membranes (Yang et al., 2022). Another function of open pores is, in addition to filtration, adsorption and absorption. Closed pores connected by cell walls are suitable, for example, as heat insulators (better thermal insulation properties) (Wu et al., 2021; Liu and Chen, 2014). Open porosity ceramic foam has high porosity (70–90 %) with irregularly spherical cells connected by struts (Sharafat et al., 2006). Due to the three-dimensional reticulated structure with many connecting pores and capillary holes, the ceramic foam has a great specific surface area, low pressure drop, and high specific surface energy on the inner side, so they exhibit high filtration precision (Wu et al., 2021; Liu and Chen, 2014). Ceramic foams can be placed in the flue gas path (only as filters) or inside the combustion chamber (region) (Ciria et al., 2022; Jesús Rico et al., 2020), where they directly interact with the flame and porous combustion occurs (Ciria et al., 2022). Zheng et al. (Zheng et al., 2021) produced three-dimensional porous ceramic foams with open channels. The foams were produced based on zirconium dioxide (ZrO 2 ) and based on alumina (Al 2 O 3 ), by the direct foaming method (employing bubbles as templates) with adding a pore-forming agent method. Ceramic foams have open pores and a three-dimensional hierarchical structure. The diameter of the foam pores is 100–300 µm and the diameter of the open windows on the cell wall is 20–80 µm (derived from silica hollow spheres). Foams based on zirconium dioxide had a porosity of 86.5–95.1 % and compressive strength of 2.05–5.67 MPa. Foams based on alumina had a porosity of 86.2–91 % and a compressive strength of 6.8–13.2 MPa. Ceramic foams prepared in this way are characterized by high porosity, low density, and uniform pore size, which results in high mechanical strength and easy penetration of gas or liquids. The production of ceramic foam is simple, cheap, and eco-friendly thanks to the low sintering temperature. Due to their properties, the ceramic foams with open channels produced in this way are suitable, for example, for filtration. Yang et al. (Yang et al., 2022) produced mullite (3Al 2 O 3 ⋅2SiO 2 ) ceramic foams with a hierarchical pore structure. Ceramic mullite foams were created using dual-phase sol nanoparticles, i.e. boehmite sol (aluminum hydroxide mineral) and silica sol (silicon oxide). The produced ceramic foams have a compressive strength of 6.6–40.4 MPa, an open porosity in the range of 64–87 %, and a thermal conductivity of 0.1–0.58 W/(m•K). The produced ceramic foams have the potential, for example, for filtration. Li et al. (Li et al., 2022) produced porous Si 3 N 4 ceramics by (Isobam) gel casting method with a reaction bonding route using monodispersed PMMA micro balls, which is a pore-forming agent (pore former) and Isobam was used as a gel agent. PMMA addition increased the permeability of the ceramic filter. The Si 3 N 4 ceramic porous filter had a uniformly distributed and uniform size and spherical shape, therefore it had high permeability and strength. The pore size of the PMMA sample was 2µm and the pore size formed by the reacted shell of Si particles was 5µm. With PMMA addition increasing apparent porosity increased from 32.8 % to 53.9 %, Darcy permeability k 1 from 1.59×10 −15 – E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 694 1.91×10 −14 m 2 but bending strength decreased from 94 to 39.1 MPa. The experiment of filtration of samples with a thickness of 2 mm was carried out by combusting a mosquito coil. The measurement revealed a zero concentration of submicron particulates (≥3µm) in the filtered air. With a longer filtration time, the filtration flux decreased, which was caused by the clogging of the pores with particulates. In the study, they assessed four empirical models of pore blocking, i.e. complete, standard, intermediate, and cake filtration (Mouratib et al., 2020). Since most of the solid particles were larger than the pore size of the filter, cake filtration took place due to the effect of clogged particles on the (external) surface of the sample, with particles smaller than the pore size also clogging the surface. The filter cake mechanism (external) is more advantageous from the point of view of reuse after back-blowing than other (internal) clogging mechanisms. Porous Si 3 N 4 produced in this way has great potential for cheap and highly efficient filters, which are high-temperature resistance. Zhang et al. (Zhang et al., 2022) presented hierarchical porous ceramic foams (HPCF) for high-temperature flue gas filtration. Ceramic foams were produced by polyurethane foaming (PU) using diatomite and kaolin powder as the raw (starting) materials. The foams had micron-sized window pores on the sub-micron bubble’s cell. The diameter of evenly arranged submicron pores on the cell wall was 0.19 µm. The pores in the porous ceramic foams were of different sizes and formed a hierarchical structure that could capture PM particles larger than 1 µm in diameter. With diatomite contents apparent porosity of ceramic foams decreased from 95.3 % to 90.5 % and compressive strength increased from 0.77 MPa to 7.39 MPa. The experiment took place in a sealed box, in which burning incense took place, which was connected to the experimental filter device. The tested samples with a thickness of 10 mm and an effective area of 100 m 2 were placed in the center of a glass tube, which was heated to a temperature of 500 ◦C. Airflow with particulates passed through the samples at a speed of 8 cm/s. A laser PM sensor measured the number and concentration of PM. The pressure drop was measured on both sides of the filter using a pressure gauge. HPCF ceramic filters had a high filtration efficiency of 96.3 % and a pressure drop of 33 Pa for PM1~2.5 filtration. The samples were cleaned by sonication in ethanol. Removal efficiency after three cycles of filter use was reduced to 95.6 % and the pressure drop increased to approximately 39 Pa. The ceramic filters produced in this way have good recyclability and have potential applications for high-temperature PM filtration. The authors also tested ceramic foams in small combustion devices, where foams act as particle filters. It was conducted by Rico et al. (Jesús Rico et al., 2020). They dealt with the influence of ceramic porous material on the combustion process (porous inert material (PIM)) in an experimental biomass combustion device (wood pellet) (10–15 kWth). Ceramic porous foam was made from zirconium oxide ceramic foam. The (120 ×120 ×30) mm 3 foam used had a porosity of 10 pores per inch (PPI). The authors tested different total airflow and positions of the PIM (38 and 54 cm above the bed) (always above the secondary air supply). There was a reduction of PM emissions by up to 50 % of the original values. The best results were achieved in the case of the smallest distance of the PIM from the combustion bed. The PIM caused a higher rate of radiation (reflected back to the bed) and an increase in temperature below the PIM (lower than the PIM), which improved the combustion process and led to a reduction in emissions, especially PM. There was no clogging. The authors hypothesized that organic PM and soot were immediately burned due to the high temperature and oxidant atmosphere. In the long term, there could be a problem with ash emissions. The other authors also tested ceramic foams in small combustion devices, but not as filters, but they interacted with the flame in the combustion chamber, and porous combustion occurred. Ciria et al. (Ciria et al., 2022) investigated the effect of inert open ceramic foams on the reduction of solid particles from flue gas generated in a small biomass combustion device (wood pellets). Ceramic foams were made of silicon carbide (SiC), with alumina (Al 2 O 3 ) and silica (SiO 2 ) as sintering additives. Ceramic foams had different pore densities from 10 to 60 PPI (different pore sizes). The dimensions of the ceramic foam in the combustion device were (100 ×100 ×20) mm 3 . The tested ceramic foams were held by a height-adjustable support bracket in the combustion chamber, where they interacted with the flame (4 and 8 cm above the combustion bed). Flame confinement with ceramic foam placed close to the flame reduces the amount of solid (PM) and gas emissions, because the temperature distribution in the combustion chamber changes (according to the size of the airflow rate and ceramic foam position over the grill), better mixing of air with fuel (air-fuel mixture) occurs and the pellets break down more intensively. By placing ceramic foam above the combustion region, PM concentrations were reduced by more than 60 %. The higher pore density of the ceramic foam led to an increase in temperature in the combustion region and a subsequent decrease in PM emissions by up to one order of magnitude, but gaseous emissions of NO x and CO 2 did not change with pore size or foam position. With the densest foam (60 PPI) there was a threefold increase in CO emissions compared to open flame (no foam), due to reduced heat recirculation. The result of the study is that by inducing microporous combustion and confined combustion with porous ceramic foam, emissions can be reduced. 4.4. Ceramic discs Gong et al. (Gong et al., 2022) fabricated a mullite fibrous ceramic filter (MFCF) for dust-laden gas filtration. The MFCF filters (discs) were prepared via the dry pressing process by using a composite sintering aid of kaolin and feldspar. The discs had a porosity of 61.8 %, a bending strength of 13.1 MPa, an average pore size of 14.8 µm, and a nitrogen permeance of above 673 m 3 /m 2 hkPa. The removal (retention) performance of the MFCF was measured on a home-made device for filtration of dust-laden gas with an average particle size of 2.5 µm. The gas velocity was 1.2 m/min. These filters have a filtration (removal) efficiency above 99.9 % for PM2.5. The initial pressure drop was 0.65 kPa. After two hours, the pressure drop was much higher, 1.94–2.22 kPa for a dust concentration of 150–450 mg/cm 3 . The MFCF filters thus produced have potential for industrial hot gas filtration. Yang et al. (2023) produced hierarchical structural alumina/mullite composite discs consisting of alumina platelets and mullite whiskers produced based on the fluorine-catalyzed gas-phase process. Mullite whiskers (length of ~1 µm) grew on the alumina platelets (cactus-like structures). Alumina/mullite composites had a high porosity of 71.4 %, compression strength of 14.2 MPa, low bulk density (1.1 g/cm 3 ), and air permeability of 4.611×10 11 cm 3 / (m 2 ⋅24 h⋅0.1 MPa). The average pore size was 832 nm and the effective surface of the disk was 18.08 cm 2 . A homemade testing system was used for the filtration performance of the samples. The particles were produced by burning a mosquito coil. The airflow speed was constant at 0.2 m/s, and the pressure drop was 360 Pa (between the burning zone and filtering zone). The average filtration efficiencies were for PM2.5 and PM10 78 % and 76 %. The produced material has a potential use for hot gas filtration. 4.5. Summary of ceramic filters Table 1 shows the ceramic filters that the authors tested in terms of filtration efficiency. The articles are sorted according to the individual types (shapes) of ceramic filters according to the year 2020–2023. To compare the filtration efficiency of ceramic filters, it is necessary to take into account the conditions under which the individual filters were tested. 5. Regeneration of filters 5.1. Regeneration of ceramic candles During filtration using ceramic candles, particles settle, and the E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 695 Table 1 Summary of tested ceramic filters in terms of filtration efficiency. Filter type Dimensions (OD£ID£H) (mm 3 ) / filtration area (cm 2 ) Combustion source/ product Production method Porosity (%) Pore size (µm) Strength (MPa) Darcy permeability k1 £10 ¡13 (m 2 ) / permeation Flue gas temperature (◦C) Particle size (µm) Velocity (m/s) Pressure drop(Pa) Filtration efficiency (%) Das et al. 2020 SiC Candle ( Das and Kayal, 2020) (85 ×75 ×660) (mm 3 ) Coal fly ash Ramming process 36–39 (support) ~90 15–23 (mechanical) 19–22 RT 0.5–20 0.1 (face) 4439 ~99 Wei et al. 2022 SiC Candle ( Wei et al., 2022) (60 ×40 ×1520) (mm 3 ) 2830 (cm 2 ) Coal fly ash One-step construction process and rotary spraying method 43.5 (pipe body) 45 and 5.7 (support and membrane) 18 (bending) 300–450 0.6–115 0.013 (surface) 2420–2700 99.98 Xiong et al. 2020 Membrane tube (Xiong et al., 2020) (60 ×40 ×1500) (mm 3 ) Dust-laden flue gas 38 80 and 10 (supports) 800–1050 0.04–8.93 0.01–0.025 (superficial) 500–2500 <10 mg/m 3 Wang et al. 2022 SiC membrane MSCM (Wang et al., 2022) 0.5–2 (thickness) Incense Direct foaming and gel-tape casting 69.2–84.1 60–150 For filtering: <10 10.3–4 (bending) 17–36×10 6 L/ m 2 h0.5 bar 750 PM0.3 27 70.2 PM2.5 90.1 PM10 94.6 Dong et al. 2022 SiC membrane (Dong et al., 2022) Incense Foaming sol-geltape casting and in situ mullite reaction bonding 64.8–80.5 50–150 For filtering: <10 12.1–4.2 (bending) 4.82–7.44×10 7 L/m 2 hbar 650 PM0.3 0.08 (airflow) 25 69.2 PM2.5 88.2 PM10 95.1 Li et al. 2023 Whisker membrane (Li et al., 2023) 1.8 (cm 2 ) Mosquito coil Precursor solution dip coating and subsequent firing ~50 37 15.4 m 2 0.3–1 0.05 3000 (final) >95 Filter type Dimensions (OD£ID£H) (mm 3 ) / filtration area (cm 2 ) Combustion source/ product Production method Porosity (%) Pore size (µm) Strength (MPa) Darcy permeability k1 £ 10 ¡13 (m 2 ) / permeation Flue gas temperature (◦C) Particle size (µm) Velocity (m/s) Pressure drop(Pa) Filtration efficiency (%) Yuan et al. 2023 Mullite/SiC tube membrane ( Yuan et al., 2023) Metallurgical flue gas Isostatic pressing and plasma coating 9.9 and 19 (Membrane and support) 1–10 and 20–150 (45.2–36.4) (Membrane and support) PM2.5 99.6 (max) Zhang et al. 2023 Mullite/SiCTiO2 fiber membranes ( Zhang et al., 2023) Fly ash Spraying 21.9 715 m 3 /m 2 hkPa 500 15.1 0.033 (filtration) 700 99.9 Li et al. 2022 Filter Si 3 N 4 with PMMA (Li et al., 2022) 2 mm (thickness) Mosquito coil Gel casting with reaction bonding route 53.9 (apparent) 2 and 5 µm (formed by PMMA and Si) 39.1 (bending) 0.0159–0.191 ≥.3 100 (continued on next page) E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 696 filtration. J. Memb. Sci. 685, 121918 https://doi.org/10.1016/j. memsci.2023.121918. Zneˇ ciˇ stˇ en´ e Ovzduˇ sí Zkr´ atí ˇ Zivot Aˇ z o Tˇ ri Roky, Tvrdí Nov´ a Studie | Obnovitelnˇ e Available online: 〈https://www.obnovitelne.cz/clanek/1122/znecistene-ovzdusizkrati-zivot-az-o-tri-roky-tvrdi-nova-studie〉(Accessed on 5 December 2023). Zou, D., Fan, W., Xu, J., Drioli, E., Chen, X., Qiu, M., Fan, Y., 2021. One-step engineering of low-cost kaolin/fly ash ceramic membranes for efficient separation of oil-water emulsions. J. Memb. Sci. 621, 118954 https://doi.org/10.1016/j. memsci.2020.118954. E. Gregoroviˇ cov´ a and J. Pospíˇ sil Process Safety and Environmental Protection 190 (2024) 688–703 703