Dual-layer Granular Bed Filter Research Articles

Study on the construction and filtration characteristics of a Triple-Layer granular bed filter

Granular bed filtration is a high-temperature dust removal technology with unique advantages. In this paper, through the fluidization tests, the densities and granule sizes of the triple-layer filter granules were optimally matched, and a triple-layer granular bed filter was constructed, which could be fluidized in layers and meet the requirement for removing ash deposited in the layers by fluidization without destroying the triple-layer structure. The whole-bed filtration tests at room and high temperatures showed that the triple-layer granular bed filter which consisted of cenosphere layer, desert yellow sand layer and 304 stainless steel shot layer had lower total pressure drop and outlet dust concentration, and higher filtration efficiency than the dual-layer granular bed filter which consisted of cenosphere layer and baozhu sand layer. The separated-bed filtration tests at high temperature found that the upper filter layer experienced the deep-bed, transition and surface filtration periods. The middle and lower filter layers didn’t experience the surface filtration period and the grade filtration efficiency curves were “V” shapes, with the lowest efficiency for particles around 1.3 μm.

Study on filtration enhancement of a dual-layer granular bed filter for bipolar-charged dust

To improve the filtration efficiency of the dual-layer granular bed filter, a new method to enhance filtration with dust bipolar-charging was proposed. The characteristics of positive and negative corona discharge and dust charging at different temperatures were measured and analyzed. The filtration experiments of non-charged dust, positive charged dust, negative charged dust, and bipolar charged dust in a dual-layer granular bed filter were carried out. The results confirmed that the best filtration effect was achieved when the dust was bipolar-charged. Furthermore, because of the loose structure of dust cake formed by the charged dust, the filter regeneration cycles were prolonged. At 450 ℃, when the inlet dust concentration was 10 g/m3, and the bi-charging voltages were + 20 kV and - 14 kV, the mean outlet dust concentration in a filtering cycle was 38.1%, 46.6% and 68.2% lower than that of negative-charged, positive charged, and non-charged dust, respectively.

Filtration of submicron dust by a dual-layer granular bed filter with an external electric field.

To improve the filtration efficiency of submicron dust by dual-layer granular bed filters, filtration experiments for micro-silica powder were conducted for removing particles smaller than 1 μm that account for more than 96% (by volume) using a dual-layer granular bed filter with an external electric field. Electrostatic enhancement methods, including dust pre-charging, application of an electric field to the lower filter layer, and that to both the upper and lower filter layers, were examined. Results showed that the average filtration efficiency of a dual-layer granular bed filter for micro-silica powder without electric field was 76.52%, the average outlet dust concentration was 263.53 mg/m3, and the filtration cycle time was 73 min. With pre-charged dust, the average outlet dust concentration dropped to 82.51 mg/m3. A decrease in the thickness of the lower filter layer from 45 to 25 mm with electric field reduced the pressure drop from 2570 to 1770 Pa. Meanwhile, the application of an electric field to the lower/upper filter layer reduced the average outlet dust concentration to 25.98 mg/m3. Increasing the initial face velocity from 0.25 to 0.45 m/s increased the average outlet dust concentration from 25.98 to 30.27 mg/m3 and increased the pressure drop from 2570 to 3500 Pa.

Integrated Dust Removal and Denitration Using a Dual-Layer Granular Bed Filter with SCR Catalyst

To solve the issues of serious wear, blockage, and poisoning of the catalyst in the denitration process of industrial furnaces such as glass furnaces and cement kiln, it was proposed to integrate the high-temperature dual-layer granular bed filter and the selective catalytic reduction (SCR) reactor into a device to realize the denitration of the SCR catalyst in a dust-free state. In the device, the upper part was a dual-layer granular bed filter and the lower part was a V2O5-WO3/TiO2 honeycomb ceramic catalyst. In the self-built dust removal and denitration integrated test equipment with an inner diameter of 100 mm, the effects of flue gas temperature, flue gas flow rate, nitric oxide (NO) concentration, and catalyst height on the denitration efficiency of the catalyst were investigated by orthogonal testing under dust-free conditions. The results show that the flue gas flow rate had the greatest influence on the denitration efficiency of the catalyst. With an increase in flue gas flow rate, the denitration efficiency first increased and then decreased. When the flue gas flow rate was 0.3 m/s, the denitration efficiency reached a maximum. The denitration test was carried out using flue gas with a dust concentration of 15 g/m3. The results show that the denitration efficiency continued to decrease with the deposition of dust on the catalyst surface (from an initial value of 96.51 to 80.59% at the 140th min), and the pressure drop correspondingly increased from 100 to 630 Pa. Additional integration testing of dust removal and denitration showed that the average dust removal efficiency of the integrated device for dust removal and denitration using a dual-layer granular bed filter could reach 99.84%, and the average denitration efficiency could reach 95.07%.

Collection efficiency in a three-dimensional randomly arranged dual-layer granular bed filter

The dual-layer granular bed filter packed with randomly arranged granules was simulated to study the effects of bed depth of the lower layer of fine granules and the inlet gas velocity on the collection mechanism. The computational results show that the collection efficiency is much better from this granular bed than a single-layer granular bed, especially for particle diameters of 1–10 μm. The inlet gas velocity has less effect on the grade collection efficiency of the dual-layer granular bed than of the single-layer granular bed. The dual-layer granular bed provides a high collection efficiency and low pressure drop. The relationship between the grade collection efficiency and the Stokes number (St) based on the inlet gas velocity is obtained. If St is below a threshold, the grade collection efficiency remains stable; if St is in value above threshold, the grade collection efficiency increases linearly with lg(St). As the bed depth of the lower layer of fine granules increases, the threshold for St shifts forward.

Semi-coke powder filtration experiments using a dual layer granular bed filter

The separation of semi-coke powders from tars and gases during coal pyrolysis is of crucial importance to the coal chemical industry. This work describes an experimental study on semi-coke powder filtration using an experimental dual layer granular bed filter with an inner diameter of 100 mm. With an initial filtration velocity of 0.2 m/s, the dual layer granular bed filter had a filtration efficiency of 99.943% and a pressure drop across the filter of only 1456 Pa. When the initial filtration velocity was increased to 0.25 m/s, the filtration efficiency was 99.937% and the pressure drop was 1834 Pa. Our results indicated that the dual layer granular bed filter we developed and tested showed significant promise as a high-temperature dust collector in low-temperature coal pyrolysis.

Effect of filter layer thickness on the filtration characteristics of dual layer granular beds

We herein report our investigation into the effect of the filter layer thickness on the filtration performance of dual layer granular bed filters using an experimental granular bed filter with an inner diameter of 100 mm and employing fly ash as the example dust. It was found that increasing the thickness of the upper filter layer from 180 to 280 mm reduced the average outlet dust concentration of the dual layer granular bed from 8.69 to 6.57 mg/m3, in addition to prolonging the time between regeneration cycles from 43.3 to 56 min, and increasing the pressure drop across the bed from 1873 to 1978 Pa. Furthermore, increasing the thickness of the lower filter layer from 45 to 85 mm reduced the average outlet dust concentration of the dual layer granular bed from 8.69 to 3.94 mg/m3 and extended the time between regeneration cycles from 43.3 to 59 min. However, this also resulted in an increased pressure drop across the filter from 1935 to 3077 Pa. These results indicated that an increase in the thickness of the upper filter layer reduced outlet dust concentrations and extended the regeneration cycles, without having a significant impact on the total pressure drop across the dual layer granular bed. Although an increase in the thickness of the lower filter layer significantly reduced outlet dust concentrations, the pressure drop across the dual layer granular bed filter was greatly increased in this case.

Cascade filtration properties of a dual-layer granular bed filter

Dual-layer granular beds consist of a lower layer of fine granules and an upper layer of coarse granules. In order to study the individual functions of the two granular layers with respect to filtration, a small granular bed filtering device (inner diameter of 100mm) was employed. The two granular layers were separated using a wire mesh such that the upper layer was laid over the wire mesh. A welas® 3000 aerosol spectrometer was used to determine the dust concentrations and size distributions of the inlets and outlets of the upper and lower layers. According to the results, the upper layer was able to remove 96.08–98.78% of the dust. Moreover, owing to the features of the upper bed, including the coarse nature of its granules, its large voidage, and its deep-bed filtration ability, the pressure drop through the granular layer increased slowly with the amount of dust deposited. The lower layer, which had fine granules, was able to effectively remove the fine particulates that passed through the upper layer, reducing the dust concentration at the outlet to 3–7.5mg/m3. The experimental results confirmed that the dual-layer granular bed filter exhibits unique synergistic properties, namely, a low pressure drop because of the upper layer and a high filtration efficiency because of the lower layer.

Experimental study on catalytic cracking of model tar compounds in a dual layer granular bed filter

To simplify the purification process of hot effluent gases produced by biomass and/or coal gasification, an integrated technique employing dual layer granular beds for the simultaneous removal of particulates and tar at high temperature (750–850°C) has been proposed. Compared to the well studied process of particulate removal, investigations for the catalytic cracking of tar are inadequate. The present work conducted an experimental study of the catalytic cracking of tar based on two model compounds: toluene and methylnaphthalene. The experiments were conducted on a granular bed filter with an inner diameter of 40mm and a height of 1000mm. The catalytic cracking performances on an expanded perlite granule layer, an olivine granule layer, and a dual layer granular bed comprised of an upper layer of expanded perlite granules and a lower layer of olivine granules were investigated individually. It was found that the expanded perlite granule layer alone was effective for toluene cracking, and the efficiency was increased with increasing reaction temperature and gas residence time. The olivine granule layer provided excellent catalytic activity for toluene cracking. Furthermore, olivine granules exhibited significantly improved catalytic activity when calcinated at temperatures greater than 900°C, or impregnated with NiO. The conversion efficiencies of toluene and methylnaphthalene attained values as great as 98.89% and 79.33%, respectively, at a temperature of 800°C over a dual layer granular bed comprised of a 150mm thick upper layer of expanded perlite granules and a 50mm thick lower layer of 3wt% NiO impregnated olivine granules.

Study on filtering characteristics of aerosol particulates in a powder-grain dual-layer granular bed

Here we investigated the filtering characteristics of a powder-grain dual-layer granular bed filter to improve its filtration efficiency in an experimental granular bed filter apparatus (inner diameter, 100mm) with aerosol particles generated by firing a smoke agent. The concentration and size distribution of the aerosol particles upstream and downstream of the granular bed filter were detected using a Welas 3000 aerosol particle size spectrometer. Filtration experiments were performed in a powder layer bed, an expanded perlite particle layer bed, and their combined layer bed consisting of a lower layer of heavy powder and an upper layer of expanded perlite particles, respectively. At the superficial filtration velocity of 0.1m/s, the filtration efficiency of a 3.0-mm-thick, 0.074–0.10mm powder layer bed was 96.240%, while that of a 150-mm-thick, 0.3–1.00-mm expanded perlite particle layer bed was 89.905%. At the same superficial filtration velocity of 0.1m/s, the powder-grain dual-layer granular bed filter achieved a filtration efficiency of 99.835%, and the growth rate of the pressure drop with filtration time was similar to that of the single expanded perlite particle layer bed, and much lower than that of the single powder layer bed. These results fully demonstrate the filtering characteristics of high efficiency and low pressure drop increasing rate of cascade filtration in the powder-grain dual-layer granular bed filter.

Laboratory study on high-temperature adsorption of HCl by dry-injection of Ca(OH)2 in a dual-layer granular bed filter

Combustion-generated hydrogen chloride (HCl) is considered to be a very hazardous acid gaseous pollutant. This paper presents a laboratory study on the dry adsorption of HCl. The experiments were conducted in a dual-layer granular bed filter, at gas temperatures of 500°C–700°C and n(Ca)/n(Cl)molar ratios of 1.0–5.0 using the silver nitrate titration method by dry adsorbent powders Ca(OH)2. Mainly, the adsorption efficiency of HCl and utilization efficiency of Calcium were studied, by varying relevant factors including n(Ca)/n(Cl), temperature, feeding method, water vapor and CO2. With a relatively higher HCl concentration of 1000 ppm, the experimental results revealed that 600°C may be the optimum temperature for HCl adsorption when optimum n (Ca)/n(Cl) was 2.5 in our tests. The results also demonstrated that the feeding at a constant pressure was more effective, and the HCl adsorption efficiency could rapidly reach over 90% with n(Ca)/n(Cl) = 2.5 at 600°C. Furthermore, the HCl adsorption efficiency was found to be slightly promoted by water vapor, while could be impeded by CO2, and the utilization efficiency of calcium could be up to 74.4% without CO2, while was only 36.8% with CO2 when n(Ca)/n(Cl) was 2.5 at 600°C.

Experimental Study on a New Dual-Layer Granular Bed Filter for Removing Particulates

A new dual-layer granular bed filter for hot gas cleanup was invented and studied experimentally. Fine sand, 0.5-1 mm grain size and about 1350kg/m^3 bulk density, was used as the lower layer of the filter. Expanded perlite particles, 2-5 mm grain size and about 70kg/m^3 bulk density, was used for the upper layer of the filter in this study. It was confirmed that the sizes and densities of these two media matched well; the binary media remained in complete segregation during regeneration by fluidization. Test results show that the filtration of the expanded perlite particle layer was characterized as ”deep bed filtration.” Filtration of the fine sand layer was ”surface cake filtration.” The expanded perlite particle layer contributed about 90% to the bed dust capacity, but only about 20% to the total bed pressure drop, which increased the bed dust capacity ten fold compared to a single-layer bed of the same sand and the same total bed pressure drop. The dust cake on the surface of the fine sand layer raised the collection efficiencies to over 99.99%.