Hot Gas Cleanup Research Articles

Low temperature toluene and phenol abatement as tar model molecules over Ni-based catalysts: Influence of the support and the synthesis method

Ni addition over two ceria-based mixed oxides (Ce0.63Zr0.33Sm0.04O2− δ, and Ce2Y2O7) has been carried out by sol-gel (Ni-CZS and Ni-CY) and wet impregnation (Ni/CZS and Ni/CY) methods. The four catalysts were characterized by BET, XRD, TGA, XPS, H2-TPR and H2-chemisorption. Partial insertion of Ni2+ ions into both mixed oxide’s structure was found in the corresponding sol-gel catalysts. Their catalytic activity was evaluated in tar reforming, with toluene and phenol as model molecules, under isothermal condition at 400 and 550 °C for 6 h, using a typical ex-biomass syngas composition in order to mimic the real conditions of a gasification process. Among the four materials, Ni/CY was the best catalyst in terms of average tar conversion, whereas the best catalytic stability was obtained for Ni-CZS, owing to inserted Ni2+ species. This study demonstrates that Ni/CY can be a promising system for tar abatement at relatively low temperature through catalytic hot gas cleaning of a raw syngas generated by biomass gasification.

Experimental study on the size segregation of binary particles in a moving granular bed

The moving granular filter technology based on size segregation is a promising technology for hot gas cleanup. A moving granular bed that can separate a mixture of particles spontaneously and replace the particles continuously layer by layer was design in this work. We experimentally studied the effects of free slope length (L), particle replacement rate (Q), and inlet large particle mass fraction (α) on the degree of size segregation. Results showed that the degree of size segregation increased with an increase in L and decreased with an increase in α. Q exerted little effect on size segregation. The control of the layer thickness and flow velocity of the particles after segregation was achieved by changing α and the angle of the movable plate (θ). We also captured the size segregation process of the particles with a camera and analyzed the influence mechanisms of L, Q, and α on size segregation.

Size segregation of binary particles in a moving granular bed filter for hot gas clean-up

High-temperature dust removal is one of the important procedures for integrated coal gasification combined cycle. In this work, a moving granular bed filter that can separate the mixture of particles of different sizes spontaneously was designed. We studied the flow characteristics of binary particles in the filter based on the discrete element method (DEM). We gave the formation process of the boundary between the stationary layer and the flowing layer, which should have a reference value for the control of the stationary layer and the design of the filter inclined groove. Typical separation trajectories of large and small particles during size segregation were also given. The degree of size segregation decreased with the increase of the mass flux of particles. Real-time control of the large particle layer thickness in the filter area can be achieved by changing the mass fraction of the inlet large particles.

Catalytic Hot Gas Cleanup of Biomass Gasification Producer Gas via Steam Reforming Using Nickel-Supported Clay Minerals

Amongst the issues associated with the commercialization of biomass gasification, the presence of tars has been one of the most difficult aspects to address. Tars are an impurity generated from the gasifier and upon their condensation cause problems in downstream equipment including plugging, blockages, corrosion, and major catalyst deactivation. These problems lead to losses of efficiency as well as potential maintenance issues resulting from damaged processing units. Therefore, the removal of tars is necessary in order for the effective operation of a biomass gasification facility for the production of high-value fuel gas. The catalytic activity of montmorillonite and montmorillonite-supported nickel as tar removal catalysts will be investigated in this study. Ni-montmorillonite catalyst was prepared, characterized, and tested in a laboratory-scale reactor for its efficiency in reforming tars using naphthalene as a tar model compound. Efficacy of montmorillonite-supported nickel catalyst was tested as a function of nickel content, reaction temperature, steam-to-carbon ratio, and naphthalene loading. The results demonstrate that montmorillonite is catalytically active in removing naphthalene. Ni-montmorillonite had high activity towards naphthalene removal via steam reforming, with removal efficiencies greater than 99%. The activation energy was calculated for Ni-montmorillonite assuming first-order kinetics and was found to be 84.5 kJ/mole in accordance with the literature. Long-term activity tests were also conducted and showed that the catalyst was active with naphthalene removal efficiencies greater than 95% maintained over a 97-h test period. A little loss of activity was observed with a removal decrease from 97% to 95%. To investigate the decrease in catalytic activity, characterization of fresh and used catalyst samples was performed using thermogravimetric analysis, transmission electron microscopy, X-ray diffraction, and surface area analysis. The loss in activity was attributed to a decrease in catalyst surface area caused by nickel sintering and coke formation.

Small- to medium-scale deep syngas purification: Biomass-to-liquids multi-contaminant removal demonstration

The complete and economic removal of harmful components from biomass gasification-based syngas is a major challenge. A final gas cleaning concept for syngas purification to catalytic synthesis quality was developed as an alternative to organic-solvent scrubbing technologies. The motivation is to present smaller-scale BTL processes with a potentially lower-capex gas cleaning solution. The purpose of this study was to realize and validate the final gas cleaning concept in a real syngas environment and to study longer-term performance of deep gas cleaning. Two successful PDU-scale campaigns in complete biomass-to-liquids production chain were performed. A total on-stream time of 163 h was realized in syngas generated from residual woody- and agro-biomasses, with coupled gas feeding to Fischer-Tropsch synthesis. For S-species removal the final gas cleaning featured a novel hybrid of activated carbon- and ZnO-based bed materials. NH3 and partial CO2 removal was achieved by pressurized water scrubbing. The campaigns employed extensive continuous and non-continuous analysis techniques for the study of syngas impurities such as H2S, COS, NH3, HCN, HCl, benzene and tar. The final gas cleaning process demonstrated flexible deep removal of syngas contaminants of all tested biomass origins, thus achieving similar or better purification levels as conventional wet-scrubbing technologies. The cleaned gas was therefore suitable for catalytic synthesis purposes, demonstrating the technical feasibility of the new final cleaning process in conjunction with optimized hot gas cleaning.

Performance of hot-gas cleanup technology for clean coal processing

The integration of advanced coal-based power generation systems with hot-gas cleanup technology endows the former with higher efficiency and reduces their environmental impacts. Syngas from coal gasification or flue gas from combustion contains dust particulates that must be eliminated before the raw gas is burned in gas turbines to protect the turbine blade and control particulate emissions. The present work attempts to provide a general understanding of moving granular bed filters (MGBFs), which are regarded as a promising technology for hot-gas cleanup, and proposes a filtration technology for application to near-industrial environments. An experimental method was established to investigate the filtration processing of dust particulates under various operating conditions. The experimental facility included an air fan, two-stage gas heaters, a screw feeder for dust particulates, a measurement system to detect the size distribution of dust particulates, a filter granule supply device, a rotary valve, and an MGBF filled with filter granules. The collection efficiency of the MGBF was assessed at different temperatures ranging from room temperature to 500 °C and granular mass flow rates of 300–1500 g/min under a fixed filtration superficial velocity of 2100 cm/min and dust concentration 7500 ppmw. Variations in the outlet concentration and size distribution of dust particulates were measured to evaluate the dynamic characteristics of the cleanup process. Results revealed that an optimal mass flow rate must exist for high collection efficiency under specific operating temperatures. In addition, important design constraints were identified for the successful operation of the proposed MGBF system. The findings indicated that the proposed design could be useful in different cross-flow filter systems for gas cleanup. This work lays a solid foundation for future research on clean coal processing.

Finding synergy between renewables and coal: Flexible power and hydrogen production from advanced IGCC plants with integrated CO2 capture

Variable renewable energy (VRE) has seen rapid growth in recent years. However, VRE deployment requires a fleet of dispatchable power plants to supply electricity during periods with limited wind and sunlight. These plants will operate at reduced utilization rates that pose serious economic challenges. To address this challenge, this paper presents the techno-economic assessment of flexible power and hydrogen production from integrated gasification combined cycles (IGCC) employing the gas switching combustion (GSC) technology for CO2 capture and membrane assisted water gas shift (MAWGS) reactors for hydrogen production. Three GSC-MAWGS-IGCC plants are evaluated based on different gasification technologies: Shell, High Temperature Winkler, and GE. These advanced plants are compared to two benchmark IGCC plants, one without and one with CO2 capture. All plants utilize state-of-the-art H-class gas turbines and hot gas clean-up for maximum efficiency. Under baseload operation, the GSC plants returned CO2 avoidance costs in the range of 24.9–36.9 €/ton compared to 44.3 €/ton for the benchmark. However, the major advantage of these plants is evident in the more realistic mid-load scenario. Due to the ability to keep operating and sell hydrogen to the market during times of abundant wind and sun, the best GSC plants offer a 6–11%-point higher annual rate of return than the benchmark plant with CO2 capture. This large economic advantage shows that the flexible GSC plants are a promising option for balancing VRE, provided a market for the generated clean hydrogen exists.

Synthesis and evaluation of layered double hydroxide based sorbent for hot gas cleanup of hydrogen chloride

In this study, we report on the synthesis and evaluation of a sodium, magnesium, and aluminum (Na-Mg-Al) layered double hydroxide (LDH) based sorbent for hydrogen chloride (HCl) removal at concentrations found in lignocellulosic biomass derived syngas. The LDH was synthesized by a spontaneous self-assembly method and further calcined at 700 °C to produce a mixed metal oxide sorbent that we evaluated in the hot gas cleanup of hydrogen chloride at 100 parts per million. The performance of this sorbent was evaluated in a fixed bed reactor from 400 to 600 °C against that of a commercial magnesium and aluminum LDH (ComLDH) material that does not contain sodium in the matrix as well as two other commercial sorbents, sodium carbonate (Na2CO3) and sodium aluminate (NaAlO2).Our Na-Mg-Al LDH is thermally stable in the hot gas cleanup temperature range. During fixed bed experiments, our calcined LDH mixed metal oxide was effective in reducing hydrogen chloride’s concentration below the breakthrough concentration of 1 ppm from 400 to 600 °C for more than 14 h. The better performance of our calcined LDH compared to calcined commercial LDH supported our hypothesis that sodium incorporation in the LDH matrix enhances HCl sorption. Based on comparison against the commercial Na-based sorbents, the following rankings by temperature observed: LDH > NaAlO2 > Na2CO3 at 400 °C; LDH = NaAlO2 > Na2CO3 at 500 °C; and Na2CO3 = NaAlO2 = LDH at 600 °C.

Hydrogen production enhancement using hot gas cleaning system combined with prepared Ni-based catalyst in biomass gasification

This paper investigates the hot gas temperature effect on enhancing hydrogen generation and minimizing tar yield using zeolite and prepared Ni-based catalysts in rice straw gasification. Results obtained from this work have shown that increasing hot gas temperature and applying catalysts can enhance energy yield efficiency. When zeolite catalyst and hot gas temperature were adjusted from 250 °C to 400 °C, H2 and CO increased slightly from 7.31% to 14.57%–8.03% and 17.34%, respectively. The tar removal efficiency varies in the 70%–90% range. When the zeolite was replaced with prepared Ni-based catalysts and hot gas cleaning (HGC) operated at 250 °C, H2 contents were significantly increased from 6.63% to 12.24% resulting in decreasing the hydrocarbon (tar), and methane content. This implied that NiO could promote the water-gas shift reaction and CH4 reforming reaction. Under other conditions in which the hot gas temperature was 400 °C, deactivated effects on prepared Ni-based catalyst were observed for inhibiting syngas and tar reduction in the HGC system. The prepared Ni-based catalyst worked at 250 °C demonstrate higher stability, catalyst activity, and less coke decomposition in dry reforming. In summary, the optimum catalytic performance in syngas production and tar elimination was achieved when the catalytic temperature was 250 °C in the presence of prepared Ni-based catalysts, producing 5.92 MJ/kg of lower heating value (LHV) and 73.9% tar removal efficiency.

Integration of gas switching combustion and membrane reactors for exceeding 50% efficiency in flexible IGCC plants with near-zero CO2 emissions

• Gas switching combustion and membrane reactors are integrated in an IGCC plant. • The proposed plants produce flexible electricity/H 2 with CCS to balance renewables. • IGCC benchmarks with/without CCS incorporate hot gas clean-up and H-class turbines. • Electric/H 2 LHV efficiencies up to 50.3%/62.4% with >98% CO 2 capture are achieved. • The energy penalty is less than 2%-points relative to the unabated IGCC plant. Thermal power plants face substantial challenges to remain competitive in energy systems with high shares of variable renewables, especially inflexible integrated gasification combined cycles (IGCC). This study addresses this challenge through the integration of Gas Switching Combustion (GSC) and Membrane Assisted Water Gas Shift (MAWGS) reactors in an IGCC plant for flexible electricity and/or H 2 production with inherent CO 2 capture. When electricity prices are high, H 2 from the MAWGS reactor is used for added firing after the GSC reactors to reach the high turbine inlet temperature of the H-class gas turbine. In periods of low electricity prices, the turbine operates at 10% of its rated power to satisfy the internal electricity demand, while a large portion of the syngas heating value is extracted as H 2 in the MAWGS reactor and sold to the market. This product flexibility allows the inflexible process units such as gasification, gas treating, air separation unit and CO 2 compression, transport, and storage to operate continuously, while the plant supplies variable power output. Two configurations of the GSC-MAWGS plant are presented. The base configuration achieves 47.2% electric efficiency and 56.6% equivalent hydrogen production efficiency with 94.8–95.6% CO 2 capture. An advanced scheme using the GSC reduction gases for coal-water slurry preheating and pre-gasification reached an electric efficiency of 50.3%, hydrogen efficiency of 62.4%, and CO 2 capture ratio of 98.1–99.5%. The efficiency is 8.4%-points higher than the pre-combustion CO 2 capture benchmark and only 1.9%-points below the unabated IGCC benchmark.

The migration, transformation and control of trace metals during the gasification of rice straw

This research investigates the trace metals speciation, partitioning and removal in rice straw gasification equipped with an integrated hot gas cleaning (HGC) system. The experiments were conducted by fluidized bed gasifier and controlled at 800 °C with equivalence ratio (ER) varied between 0.2 and 0.4. The experimental results indicated that the concerned trace metals Zn, Cr, Cd, and Pb partitioning in the gas phase were increased significantly with an increase in ER. This is because the exothermic reaction could enhance the trace metals reacted with chlorine and/or sulfur as well as correspondingly formed highly volatile metals compounds. However, other tested metals Cu, Na, K, Ca, Mg partitioning was obviously decreased in the gas phase with ER increasing. These tested metals tend to form oxides speciation leading the variation in their partitioning characteristics. The XRD identification and thermodynamic equilibrium simulation results were also confirmed the tested metals speciation and partitioning characteristics. The dominant gaseous species produced from rice straw gasification, such as KCl(g), NaCl(g), KO(g), K2O(g), ZnCl2(g), CrO2Cl2(g), CuCl2(g), PbCl2(g), PbO(g), and Cd(g), were predicted by thermodynamic equilibrium model. The tested metals removal by adsorbents of hot gas cleaning system was found to be adsorbed in decreasing order as: K > Cr > Ca > Pb > Mg > Cd > Na > Zn > Cu. Activated carbon was used in hot gas cleaning system and showed a good performance for adsorbing tested metals, especially for Pb, Cd, Cr, Ca, K, and Mg. In summary, HGC system is proposed as an effective way for improving the syngas quality and reducing trace contaminants emission in rice straw gasification.

Filtration of dust in an electrostatically enhanced granular bed filter for high temperature gas cleanup

High temperature gas cleanup is an important technology. In order to develop an efficient filter for hot gas cleanup, three kinds of filters were built: 1) Rare earth tungsten (RE-W) cathode electrostatic precipitator (ESP), 2) classical granular bed filter (CGBF), and 3) electrostatically enhanced granular bed filter (EGBF). The effect of gas temperature, voltage, granule size, and bed height on the dust collection efficiency of the filters were investigated. It is found that the efficiency of the EGBF is the highest among the three filters. The efficiency of the double-zone RE-W cathode ESP ranks second, though decreasing collection area reduces the efficiency significantly. The efficiency of the CGBF is not high compared to the above two filters. On the whole, the EGBF is a very promising technology for hot gas cleanup, especially when cooperated with the RE-W thermionic emission cathode which is very suitable for particle charging at high temperature.

Influence of removal efficiency on a moving granular bed filter

High power generation efficiency and environmental performance are obtained in the integrated coal gasification combined cycle (IGCC) power plants. Hot gas clean-up is a key technology in IGCC systems. In this paper, a moving granular bed filter was constructed to examine the effect on collection efficiency when variations in the working parameters, namely, the mass flow rate, the filtration superficial velocity, and the amount of dust particulates at the inlet were introduced. A method to enhance particulate removal was proposed and tested in a lab-scale experiment. In addition, the study analyses the motion of gas flow in the inlet gas device of filter system. The results indicated a low amount of dust in the outlet of filter system when a mass flow rate of 0.008 kg/s and a filtration superficial velocity of 0.42 m/s were used. Furthermore, this new method is suitable for IGCC and other industrial areas involving high-temperature environments.

Removal of hydrogen chloride gas using honeycomb-supported natural soda ash

In this study, the HCl absorption performance of cheap natural soda ash loaded on a honeycomb support as a hot gas cleanup method was investigated by using flow-type fixed-bed reactor. The honeycomb-supported soda ash improved the HCl absorption extent significantly at the 1-ppm breakthrough time compared to the result without the support; thus, the use of the support was justified. The HCl removal performance of the honeycomb-supported soda ash depended significantly on temperature (300–600°C). Under the present conditions, the absorption extent at 1-ppm breakthrough time was maximized at 500°C. This absorption extent increased as the HCl concentration in the supplied gas decreased, and it was assumed that this absorbent was suited to actual coal gasification processes. No change in the HCl absorption extent (60–65 %) was observed for Na2CO3 loading between 33 and 65 mass%. The natural soda ash (Na2CO3) loaded on a honeycomb support changed to NaCl after HCl absorption examination. Regenerated HCl absorbent from spent honeycomb showed similar HCl absorption performance with fresh absorbent at 300–500°C.

Evaluation of sorbents for high temperature removal of tars, hydrogen sulphide, hydrogen chloride and ammonia from biomass-derived syngas by using Aspen Plus

Biomass gasification is a promising technology to produce secondary fuels or heat and power, offering considerable advantages over fossil fuels. An important aspect in the usage of producer gas is the removal of harmful contaminants from the raw syngas. Thus, the object of this study is the development of a simulation model for a gasifier including gas clean-up, for which a fluidized-bed gasifier for biomass-derived syngas production was considered, based on a quasi-equilibrium approach through Gibbs free energy minimisation, and including an innovative hot gas cleaning, constituted by a combination of catalyst sorbents inside the gasification reactor, catalysts in the freeboard and subsequent sorbent reactors, by using Aspen Plus software. The gas cleaning chain simulates the raw syngas clean-up for several organic and inorganic contaminants, i.e. toluene, benzene, naphthalene, hydrogen sulphide, hydrogen chloride and ammonia. The tar and inorganic contaminants final values achieved are under 1 g/Nm3 and 1 ppm respectively.

Determining kinetic constants for reactions of zinc oxide sorbents with syngas components

Hot gas clean-up (HGC) technology uses special zinc oxide based sorbents. Sorbent attrition is one of the negative factors that should be improved. The current study has determined that one of the main reasons of sorbent reduction is interaction with syngas components, namely, carbon monoxide, hydrogen, methane and carbon (side reactions). In this study, kinetics of main reaction (h2s adsorption by zinc oxide) and side reactions were considered and compared. Models of chemical processes and mechanisms were considered as well. Complicated nature of HGC process was researched. It is stated that the process can run in three different phases (gas, solid, dust). Methods of TGA data calculation are presented and compared with literature. In syngas where hydrogen sulphide concentration is much lower than concentration of hydrogen and carbon monoxide, main reaction is dominant at temperatures near 300–500 °C. At 650 °C main reaction takes the second place after H2 reduction, and at 850°C it is the third after CO reduction (with diffusion resistance of product shell). Hydrogen reduction is more active than carbon monoxide reduction, that is why CO/H2 ratio in syngas should be increased for rising the HGC operation temperature. A wide range of processes (main and side reactions) was considered with the use of the same materials, equipment and methods.

Effect of steam supply to the air-blown gasifier on hot syngas desulphurization

The IGCC technology serves to efficiently produce thermal and electrical energy with minimal impact on the environment. In operating IGCC, wet desulphurization is used at temperatures below 200°C. The use of hot desulphurization at temperatures around 500°C will significantly improve IGCC efficiency. The preferred sorbent for hot gas cleaning is ZnO. At temperature of 450-500°C, ZnO begins decomposing because of reactions with syngas components (primarily hydrogen). Steam impedes reaction of ZnO with H2 and increases ZnO thermal stability. Syngas H2/H2O ratio is determined by gasifier operation mode. The purpose of this work is to determine maximum temperature of hot gas cleaning depending on condition of ZnO-sorbent thermal stability and steam-air-blown mechanically activated coal gasifier operation mode. To determine the effect of steam supply to syngas composition, experiments were performed on entrained-flow gasifier (1 MW). Experimental results were processed using thermodynamic analysis to determine idealized syngas composition and CFD-modeling to determine real experiment process parameters. Syngas H2O content was determined by CFD-modeling results. Study of ZnO-sorbent thermal stability depending on H2 concentration and syngas H2/H2O ratio was performed by TGA. As a result of experimentally confirmed thermodynamic calculations, ZnO-sorbent thermal stability was found to increase to 815°C due to steam dilution.

Applicability of the SOFC technology for coupling with biomass-gasifier systems: Short- and long-term experimental study on SOFC performance and degradation behaviour

Coupling biomass gasification with high temperature Solid Oxide Fuel Cells (SOFCs) is a promising solution to increase the share of renewables and reduce emissions. The quality of the producer gas used can, however, significantly impact the SOFC durability and reliability. The great challenge is to ensure undisturbed operation of such system and to find a trade-off between optimal SOFC operating temperature and system thermal integration, which may limit the overall efficiency. Thus, this study focuses on experimental investigation of commercial SOFC single cells of industrial size fueled with different representative producer gas compositions of industrial relevance at two relevant operating temperatures. The extensive experimental and numerical analyses performed showed that feeding SOFC with a producer gas from a downdraft gasifier, with hot gas cleaning, at an operating temperature of 750 °C represents the most favorable setting, considering system integration and the highest fuel utilization. Additionally, a 120 h long-term test was carried out, showing that a long-term operation is possible under stated operating conditions. Local degradation took place, which can be detected at an early stage using appropriate online-monitoring tools.

Generation and selection of Pareto-optimal solution for the sorption enhanced steam biomass gasification system with solid oxide fuel cell

The biomass gasification coupled with a solid oxide fuel cell (SOFC) system is one of the most efficient and environmentally friendly technologies for combined heat and power generations. For the development and improvement of the integrated process systems, the optimization problem has more than one conflicting objective functions to be optimized (i.e., thermodynamic performance, environmental impacts, annual profit, capital and operating costs) simultaneously. Multi-objective optimization (MOO) methods are used to find a set of optimal (or non-dominated) solutions. In this work, MOO of a sorption enhanced steam biomass gasification (SEG) integrated with an SOFC and gas turbine (GT) system, for combined heat and power production from Eucalyptus wood chips as biomass feedstock, is investigated. Firstly, the model of this integrated plant is developed in Aspen Plus that can be divided into five parts: (1) SEG, (2) hot gas cleaning and steam reforming, (3) SOFC, (4) catalytic burning, GT and CO2 compression, and (5) Portland cement production. As the annual profit demonstrates the economic viability of the plant and annualized capital cost (ACC) indicates availability of investments, the MOO of the integrated plant is performed to obtain Pareto-optimal solutions based on the minimization of ACC and maximization of annual profit with five important decision variables. After that, ten selection methods are used to recommend practical solutions for implementing in the integrated plant. In order to explore the effect of decision variables uncertainty on obtained Pareto-optimal solutions, random variations in decision variables are used to quantify deviations in objective functions. The Pareto-optimal solutions are ranked based on the normalized variations for decision variables uncertainty. At the end of this study, robust MOO of the integrated plant is performed, with respect to uncertainties in the market and operating parameters.

Multi-objective optimization of a dual-layer granular filter for hot gas clean-up by using genetic algorithm

Granular filters are one of the most promising methods for hot gas clean-up in coal-based power generation systems. In order to further improve the filtration performance, a dual-layer granular filter is proposed, and multi-objective optimization via genetic algorithm is applied to optimize the geometric parameters. The optimization result is a Pareto front which includes a set of optimal solutions. Users can select each solution based on trade-offs between filtration efficiency, pressure drop, dust holding capacity and project limitations. An optimal case with relatively high efficiency, high deposition uniformity and low pressure drop is selected from the Pareto front. Numerical simulations are performed, and comparison of filtration performance between the optimal case and a single layer filter are conducted, to verify the superiority of the optimum structure. The optimum dual-layer granular filter has higher filtration efficiency than the single layer filter but with the expense of a small amount of pressure drop. The dual-layer granular filter also has higher deposition uniformity and higher dust holding capacity. Cleaning intervals for the dual-layer filter can be increased approximately 0.4 h, which gives the dual-layer filter better economic performance. The larger granular size in the first layer of dual-layer granular filter can let particles pass-through and deposit more uniformly than in the single layer filter, thus improving the dust holding capacity. The smaller granular size in the second layer also gives it better performance when filtering small particles.