Ind. Eng. Chem. Res. 2009, 48, 10541–10550
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Effects of H2O and Particles on the Simultaneous Removal of SO2 and Fly Ash Using a Fluidized-Bed Sorbent/Catalyst Reactor Jui-Yeh Rau,† Jyh-Cherng Chen,‡ Ming-Yen Wey,*,† and Min-Der Lin† Department of EnVironmental Engineering, National Chung Hsing UniVersity, Taichung 402, Taiwan, Republic of China, and Department of Safety, Health and EnVironmental Engineering, HungKuang UniVersity, Taichung 433, Taiwan, Republic of China
This study investigated the potential of a fluidized-bed sorbent/catalyst reactor for the simultaneous removals of SO2 and fly ash from a simulated flue gas containing different H2O and particles. Experimental results showed that the removal efficiency of particles and SO2 was 85%-96% and 5.75-2.97 mg SO2/g, respectively, as the H2O content was 1.5-5.3%. The activities of sorbent/catalysts for simultaneous removals of SO2 and particles were inhibited by H2O and particles, and the inhibition effects increased with the content of H2O. As the H2O content increased, the particle size distribution (PSD) of fine particles shifted to the coarse particles. The results of BET analysis show that the obstruction phenomenon of the sorbent/catalyst caused by the particles was diminished with the increased content of H2O. The results showed this aggregation phenomenon of fine particles shifted to the coarse particles may cause increased water vapor content in fluidized-bed sorbent/catalyst reactor. 1. Introduction Many combustion processes such as waste incineration and thermal power plants produce various air pollutants including acid gases (SO2, NOx, and HCl), organic compounds (VOCs and PAHs), and particulate matters. Among these pollutants, acid gases and fly ash have been studied intensively because they comprise the major causes of acid rain that is considered harmful to human health and the environment. In the conventional desulfurization process, wet and semidry scrubbing are the most prevailing methods that use limestone or lime as the absorbents/sorbents.1-3 With regard to the particulate matters, the mass median diameter (MMD) of the fly ash produced in waste incineration processes was usually at 4 µm, and that produced in the thermal power plants was usually at 4 and 40 µm, respectively.4,5 Previous studies found that the major compositions of fly ash for SiO2, Al2O3, Fe2O3, and CaO were 43%, 22.5%, 7.7%, and 7.5%, respectively.5-7 Among the conventional air pollution control devices (APCDs), the bag filter (BF) and electrostatic precipitator (ESP) are prevalent because of their relatively higher control efficiencies.1-3 However, these devices cannot remove acid gases and organic compounds, not to mention the fact that they require a larger space for installation. Some studies have shown that the removal of SO2 has been enhanced by the granular support (such as activated carbons (AC) and alumina (Al2O3)) coated with transition metals.8-15 Such AC/catalyst reactors are attractive because of their high control efficiency, low reaction temperature, and low energy/ fuel requirements. The effects of a specific surface area, pore size distribution, acidic and basic surface chemical groups, as well as metallic derivatives on the removal of SO2 with AC or carbon-based materials were well investigated.16-18 Our groups have studied the activities of different transition metal catalysts (Cu, Fe, V, Co, and Ni) supported by AC for SO2 removal as * To whom correspondence should be addressed. Tel.: +886-422852455. Fax: +886-4-22862587. E-mail:
[email protected]. edu.tw. † National Chung Hsing University. ‡ HungKuang University.
well as the effects of different metal loadings.14,15,19,20 The results showed that only 3 wt % Cu had optimum activity. Lower loadings offered few active sites, thereby demonstrating lower activities. On the other hand, higher loadings resulted in active site aggregation and the subsequent reduction of catalytic activity. The AC or carbon-based materials can be pretreated by sulfuric acid (H2SO4), nitric acid (HNO3), or hydrogen peroxide (H2O2) to increase the surface area, oxygen deposition, and acidity of the catalysts.21-24 The fluidized-bed reactor used for filtration is usually the granular-bed type, and it has the advantages of easy continuous operation and regeneration. In terms of simultaneous removal of acid gases and particles, the fluidized-bed sorbent/catalyst reactor offers the following advantages: (1) fly ash can pass through the reactor without the plugging phenomenon that may occur in a fixed-bed reactor; (2) higher thermal/mass transfer efficiency and higher gas/solid or solid/solid contact areas; (3) higher tolerance to the variation of gas flow rate; and (4) the reactor does not need to shut down to replace the catalysts.4-7,25-28 The filtration efficiency of fly ash by a fluidized-bed reactor was already examined in our previous studies.4-7 The results recognized the mechanisms of particle filtration in the fluidizedbed reactor involving the equilibrium of collection, accumulation, and elutriation of particles. Furthermore, the inter forces existing between particles such as the van der Waals force, liquid-bridging, and electrostatics, all of which determine the elutriation of particles, were affected by the variances in parameters (the species of particles, temperature, gas flow rate, and humidity). The results also indicated the effects of fluidization velocity on particle filtration, and the optimum condition occurred at an operation fluidizing velocity/minimum fluidizing velocity (Uo/Umf) value of 5%) or because of the good dispersion on the support surface. This suggests that the particles (SiO2 and Al2O3) filtered using CuO/ AC-N are highly dispersed on the AC surface. This is caused by the fact that the maximum value might have increased the contact between the particles and the interparticle forces such as van der Waals, liquid-bridging, and electrostatic force.4-6 In addition, the filtration of particles by a fluidized bed reactor can be considered a dynamic process because the removal of
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particles involves a balance between the collection and the elutriation of particles, both processes being time-dependent. On the other hand, the FESEM analyses showed that the sorbent/catalyst was actually tiny crystalline grains of copper metals and high aggregations of particles (SiO2 and Al2O3) over the AC-N, as seen from the small regions analyzed by EDS. Figure 9A-G shows the EDS patterns of CuO/AC-N of fresh and reacted sample surface morphologies. As seen in Figure 9A, the results of the analysis indicated lower metal coating and the existence of carbon, oxygen, and copper element. Thus, the XRPD, FESEM, and EDS results confirmed that the sorbent/ catalyst prepared by the HNO3 treatment process shows tiny crystalline grains and highly dispersed copper metal characteristics. From the reacted sample case (Figure 9B-G), we can see the energy dispersive spectrometer of filtered of particles (SiO2 or Al2O3) and adsorption/catalysis of SO2 in 1.5%, 3.4%, and 5.3% H2O content conditions, respectively. From Figure 9B to E, we can see the results clearly confirming that the aggregations of particles (Al2O3 or SiO2) that occurred for the sorbent/catalyst surface dominated were a reacted sample with 1.5% and 3.4% H2O content; on the other hand, the sorbent/ catalyst of the surface demonstrated higher energy dispersive spectrometer of particles (Al2O3 or SiO2) in the fluidized bed reactor during the experimental period. Therefore, the results of the analysis indicated higher concentrations of particle aggregations for the sorbent/catalyst surface with low H2O content condition. Finally, Figure 9F and G shows the energy dispersive spectrometer of the simultaneous oxidation of SO2 and the filtration of particles (Al2O3 and SiO2) using CuO/AC-N sorbent/ catalyst with 5.3% H2O content through the use of a fluidized bed reactor. The results clearly indicate that the energy of particles (Al2O3 and SiO2) decreased when H2O content was 5.3%. The results may show that the particles lessen the aggregations of the sorbent/catalyst surface. Thus, the particles may increase the accumulation phenomenon in the fluidizedbed reactor with the added H2O content. This phenomenon results in higher filtration efficiency, although the results can decrease the sorbent efficiency of SO2. 3.3.3. BET Analysis of Fresh and Reacted Sorbents/ Catalysts. Table 3 presents a summary of the textural characterization of the bed material of fresh and reacted simultaneous oxidations of SO2 and filtration of particles with varying H2O content and particle conditions. The fresh sorbent/catalyst of CuO/AC-N shows that the texture for the surface area, Vmicro, Vmeso, and Vmacro were 889 m2 g-1, 0.4425 m3 g-1, 0.0409 m3 g-1, and 0.0046 m3 g-1, respectively. In the case of the simultaneous oxidation of SO2 and the filtration of Al2O3 particles, the micropore was the most abundant, constituting 89.2-91.3% of the total pore volumes (0.3367-0.4732 m3 g-1) with 1.5-5.3% H2O content. This mesopore/total pore ratio was followed by 1.5%, 3.4%, and 5.3% of H2O content at 9.9%, 8.2%, and 9.8%, respectively. This was followed by 1.5% H2O content (0.39%) > 5.3% H2O content (0.36%) > 3.4% H2O content (0.19%) for the macropore/total pore ratio. In the case of the simultaneous oxidation of SO2 and the filtration of SiO2 particles, the micropore was again found to be the most abundant texture among the reacted samples, with an H2O content of 1.5%, 3.4%, and 5.3% and volume of 0.3631, 0.3923, and 0.4236 cm3 g-1, respectively. In adding the H2O content condition, the results clearly confirmed that the mesopore/total pore ratio of the reacted sample was followed by 1.5%, 3.4%, and 5.3% H2O content at 5.9%, 7.4%, and 10.2%, respectively, and followed by 1.5% H2O content (0.17%), 3.4%
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H2O content (0.0.26%), and 5.3% H2O content (0.31%) for the macropore/total pore ratio. These results clearly show that the SiO2 particles lessen the obstruction phenomenon of the sorbent/ catalyst volume and that this obstruction effect increases with decreasing H2O content. However, these results do not clearly show the simultaneous oxidation of SO2 and filtration of Al2O3 particles with the content of H2O condition. In addition, a similar study1,4 indicated that the pore volume increases with the burnoff during the gas-phase oxidations, but that there will be a decrease first in the micropore and mesopore volume due to the collapse of the pore. 4. Conclusions The removal characteristics of SO2 and fly ash (SiO2 and Al2O3) by CuO/AC-N adsorption/filtration were determined through the simultaneous removal of the SO2 and fly ash using a fluidized bed reactor in varying H2O and fly ash conditions. In the content of condition of fly ash and H2O, the results indicate that H2O and fly ash inhibited the adsorption/catalysis activity and that this inhibition effect may increase with the increasing H2O content. Nevertheless, the results indicate that the SO2 adsorption/catalysis may be restored when H2O and fly ash are removed from the flue gas. In addition, the simultaneous removal results of particles and SO2 were maintained at 85-96% and 5.75-2.97 mg SO2/g catalyst, respectively, with an H2O content of 1.5-5.3%. Moreover, the accumulation phenomenon occurred for the sorbent/catalyst surface could be attributed to the high concentration of fly ash (614-886 mg m-3) condition in flue gases. When the contents of H2O were increased, the fine particles (4-10 µm) may be able to increasingly shift to the coarse particles (near 100 µm). Several experimental techniques (FE-SEM/EDS, XRPD, and BET) were also used to characterize the sorbent/catalyst of fresh and reacted with content fly ash and H2O. BET results show that the particles may lessen the obstruction of the sorbent/ catalyst volume with increasing content of H2O. This aggregation phenomenon of fine particles shifted to the coarse particles may cause increasing water vapor content, and this result could increase the removal efficiency of fly ash. Therefore, the results demonstrate the high potential of a fluidized-bed sorbent/catalyst reactor for the simultaneous removal of SO2 and fly ash from high concentrations of particle flue gases. Literature Cited (1) Chiang, B. C.; Wey, M. Y.; Liu, K. Y. Filtration of Fly Ash by a Fluidized Bed sorbent. J. Air Waste Manage. Assoc. 2005, 55, 181. (2) Ergut, A.; Levendis, Y. A.; Simons, G. A. High-Temperature Injection of Sorbent-Coal Blends Upstream of a Ceramic Filter for SO2, NOx, and Particulate Pollutant Reductions. Combust. Sci. Technol. 2003, 175, 579. (3) Shemwell, B.; Atal, A.; Levendis, Y. A.; Simons, G. A. A Laboratory Investigation on Combined In-Furnace Sorbent Injection and Hot FlueGas Filtration to Ssimultaneously Capture SO2, NO(x), HCl, and Particulate Emissions. EnViron. Sci. Technol. 2000, 34, 4855. (4) Liu, K. Y.; Wey, M. Y. Filtration of Fly Ash Using Fluidized Bed at 300-500 °C. Fuel 2007, 86, 161. (5) Liu, K. Y.; Wey, M. Y. Dynamic Purification of Coal Ash by a Gas Solid Fluidized Bed. Chemosphere 2005, 60, 1341. (6) Liu, K. Y.; Wey, M. Y. Filtration of Nano Particle by a Gas-Solid Fluidized Bed. J. Hazard. Mater. 2007, 147, 618. (7) Wey, M. Y.; Chen, K. H.; Liu, K. Y. The Effect of Ash and Filter Media Characteristics on Particle Filtration Efficiency in Fluidized Bed. J. Hazard. Mater. 2005, B121, 175. (8) Loı`pez, D.; Buitrago, R.; Sepuı`lveda-Escribano, A.; Rodriı`guezReinoso, F.; Mondragoı`n, F. Surface Complexes Formed During Simultaneous Catalytic Adsorption of NO and SO2 on Activated Carbons at Low Temperatures. J. Phys. Chem. 2007, C111, 1417.
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ReceiVed for reView May 22, 2009 ReVised manuscript receiVed August 26, 2009 Accepted September 23, 2009 IE900843F
