Removal of NO and fly ash over a carbon supported catalyst: Effects of fly ash concentration and operating time

Powder Technology 239 (2013) 239–247

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Removal of NO and fly ash over a carbon supported catalyst: Effects of fly ash concentration and operating time Chi-Yuan Lu a, b, Jui-Yeh Rau c, Jyh-Cherng Chen d, Shih-Teng Huang c, Ming-Yen Wey c,⁎ a

School of Public Health, Chung Shan Medical University, Taichung 402, Taiwan, ROC Department of Occupational Medicine, Chung Shan Medical University Hospital, Taichung 402, Taiwan, ROC Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC d Department of Safety, Health and Environmental Engineering, Hungkuang University, Taichung 433, Taiwan, ROC b c

a r t i c l e

i n f o

Article history: Received 27 June 2012 Received in revised form 29 January 2013 Accepted 2 February 2013 Available online 8 February 2013 Keywords: Fluidized-bed catalytic reactor Fly ash NO Catalyst

a b s t r a c t This study investigated the removal efficiency of NO and fly ash from flue gas over a Cu catalyst supported on modified activated carbon (AC) in a pilot-scale fluidized-bed catalytic reactor. The fly ash came from a coal-fired power plant was used in our experiment. The AC support was pretreated by either HNO3 (AC–N), H2O2, H2SO4, or NaOH. The acidic concentrations in the four catalysts followed the order CuO/AC–N (0.174 mol g−1) > CuO/AC–H (0.138 mol g−1) > CuO/AC–S (0.103 mol g−1) > CuO/AC–Na (0.009 mol g −1). Good dispersion of nanoscale CuO particles was observed over the modified supports having a large total number of acidic sites. Moreover, the increase in the number of phenolic and carboxylic groups may increase the NO removal efficiency over modified CuO/AC catalysts. The results was also suggested that the total number of acidic groups decreased in the reduction of NO over the CuO/AC catalysts due to the adsorption of NH3 on NO reduction. Simultaneous removal of NO and fly ash in flue gas under different fly ash concentrations and operating times was studied over the CuO/AC–N catalyst. When fly ash (1406–49,108 mg m −3) was added in the flue gas, the removal efficiencies of NO and fly ash over Cu/AC–N were 58%–61% and 82%–86%, respectively. At an operating time of 240 min, the removal efficiency of fly ash was 76%, and the NO removal efficiency decreased slightly to 55%. The NO removal efficiency was inhibited slightly under high fly ash concentration and long operating time. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Coal-fired power plants typically generate massive amounts of air pollution, with NOx and fly ash concentrations as high as 200–400 ppm and 1000–10,000 mg m −3, respectively [1,2]. The composition of fly ash is 43% SiO2, 22.5% Al2O3, 7.7% Fe2O3, and 7.5% CaO [1,3,4]. This pollution represents an ever-increasing threat to human health and the natural environment. In power plants, effluents such as acid gases and particles are removed by a two-stage process using either an electrostatic precipitator integrated with a wet scrubber or selective catalytic reduction with ammonia or urea. However, conventional multi-stage equipment is expensive and requires large amounts of space. Hence, various catalytic treatment units have been investigated for the simultaneous removal of SO2, NO, HCl, and fly ash with low treatment costs and small space requirements. Numerous catalyst filters have been investigated recently for the removal of pollutants, for example, alumina fiber, sol–gel-derived ⁎ Corresponding author at: Department of Environmental Engineering, National Chung Hsing University, Taichung 40227, Taiwan, ROC. Tel.: +886 4 22852455; fax: +886 4 22862587. E-mail address: [email protected] (M.-Y. Wey). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.02.002

Al2O3 fiber, and traditional filters supported by different metals (i.e., Cu, Ni, V, and Ce) [5–9]. Moreover, complete oxidation of propene to CO2 with a 96% NO removal efficiency has been achieved at 300 °C over a Pt/V2O5 catalyst supported on a TiO2-impregnated filter element [8]. A multifunctional reactor for fly-ash filtration and simultaneous removal of NOx and volatile organic compounds over catalytic foams (MnOx·CeO2–V2O5·WO3·TiO2) have also been successfully investigated [9]. However, catalytic fibers have certain disadvantages. At long operating times, high pressure drops in catalytic filters could increase the maintenance cost. Moreover, the catalytic activities in such filters would decrease easily over time. Although Fino et al. [9] mentioned that a fine filtering membrane with applied backpulse cleaning prevents a strong differential pressure increase in catalytic filters with time and protects the integrated catalyst against particle deposition. The operation is complex and the cost of maintain for the filtering membrane equipment is high. In recent years, fluidized bed removal devices have been developed for use in pollutant control. A fluidized-bed reactor offers the following advantages: (1) easily achieved continuous operation; (2) high mass transfer efficiency; (3) high thermal transfer efficiency; (4) high contact opportunities for gases and solids; (5) renewability; and (6) small space requirements [10]. Therefore, a fluidized bed has

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been considered in order to reduce the treatment cost and save energy. In order to study the pollutant removal by a fluidized bed reactor, a pilot-scaled fluidized bed reactor was build up in our laboratory. Fly ash and nano/micro-particles (SiO2, Al2O3, and Fe2O3) have been successfully filtered by a fluidized bed reactor in our earlier studies, and the removal mechanisms of fly ash, including inertial impaction, direct interception, and diffusive motion [1–3,11–15]. The results showed that the optimum operating condition of the fluidizing velocity, i.e., the operating/minimum fluidizing velocity (Uo/Umf), was less than 2.5, and the aspect ratio (static bed height/diameter) was 1.4. Moreover, Active Carbon (AC) was adopted as a bed material, and AC showed the good filter efficient than silica due to high specific surface area of AC. The filtration was a dynamic process over AC bed material by a pilot-scaled fluidized bed reactor. To increase the operating efficiency, NO and fly ash must be removed simultaneously. Metals such as CuO, V2O5, MnO2, MoO3, Fe2O3, and TiO2 supported on activated carbon (AC) have been investigated for NO removal [16–19]. Among them, CuO/AC exhibited good catalytic activity at low temperatures. Hence, although SiO2 has long been used as a bed material for fluidized-bed reactors, we used a catalyst as the bed material for the reactor in our study. NO removal over a pilot-scaled fluidized-bed CuO/AC catalytic reactor with a reducing agent-NH3 was evaluated that contains [13,15]: (1) the effects of different concentrations and sizes of simulated fly ash (Al2O3 or SiO2) in the flue gas, and (2) the effects of different operating temperatures, different operating velocities (Uo/Umf), different H/D on the simultaneous removals of NO, SO2, and coal ash. The initial results showed that when NO and fly ash coexisted in the flue gas, the fly ash would inhibit the activity of catalyst and inhibition effect increased with the concentration of fly ash. When fly ash contained SiO2, fine SiO2 particles would integrate to form coarse particles easily, and this aggregation phenomenon on the catalyst surface led to the decrease in NO removal efficiency. In our earlier studies, the operating conditions were chosen in a fixed value, i.e., short operating time, concentration of simulated fly ash, and the composition of simulated fly ash. However, the real composition of flue gas in a power plant is varied. Thus, a real fly ash came from a coal-fired power plant (Taichung Coal-Fired Power Plant, Taiwan) that was used in our experiment so that real flue gas conditions could be approximated. In this study, an AC-supported copper catalyst was employed as the bed material in a pilot-scale fluidizedbed catalytic reactor in order to simultaneously remove NO and fly ash. First, AC supports pretreated with various solutions (i.e., HNO3, H2O2, H2SO4, and NaOH) were used for CuO/AC catalyst preparation. We then investigated the NO removal efficiency over different CuO/ AC catalysts in a fluidized-bed catalytic reactor using NH3 as the reducing agent; the concentrations of acidic and groups on various fresh and reacted CuO/AC catalysts were studied. The best CuO/AC catalyst was then chosen and used for the other activity tests. Second, the effects of fly ash concentration (0–49,108 mg m −3) and operating time (60–240 min) were investigated in order to evaluate the simultaneous removal of fly ash and NO from a fluidized-bed catalytic reactor. Moreover, the particle size distribution (PSD) in the exhaust was measured. We also characterized the specific surface area, physical texture, morphology, and crystal shape of the CuO/AC catalyst.

After pretreatment, the modified AC supports were washed with distilled water and air-dried at 110 °C for 24 h. Next, CuO/AC catalysts were prepared by impregnating AC pellets with an aqueous copper (II) nitrate solution. The pellets were calcined at 400 °C for 4 h in a muffle furnace. The catalysts each contained 3.0 wt.% Cu, which was confirmed by an inductively coupled plasma (ICP) analysis. The catalysts were labeled CuO/AC–N, CuO/AC–H, CuO/AC–S, and CuO/AC–Na depending on the pretreatment solution. 2.2. Activity tests A laboratory-scale fluidized-bed catalytic reaction system was set up (Fig. 1). It included systems for (1) NO production, (2) fly ash feeding, (3) fluidized-bed catalytic reaction, and (4) pollutant analysis. NO was generated by combustion of an artificial feedstock at 700 °C in a fluid-bed incinerator. The fluidized-bed catalytic reaction system was fitted with a stainless steel distributor plate (mesh 35). The input air flow rate during the experiment was approximately 45 L/min at room temperature, and the operating temperature of the combustion chamber was set to 700 °C. When the temperature reached a steady state, the artificial feedstock was semi-continuously fed into the combustor at regular intervals of 25 s. A unit of the artificial feedstock contained one packing paper (O: 52.09%; S: 0.30%; H: 6.16%; C: 41.01%; N: 0.22%) and 4 g of urea powder (O: 0.49%; S: 0.00%; H: 6.84%; C: 20.17%; N: 47.68%). Subsequently, the fly ash from a coal-fired power plant (Taichung Coal-Fired Power Plant, Taiwan) was added to the flue gas using a semi-feeder, and the fly ash compositions were also analyzed (SiO2: 42.9%; Al2O3: 22.2%; Fe2O3: 7.7%; CaO: 7.5%; MgO: 2.2%; TiO2: 1.0%; K2O: 0.6%). NO, fly ash, and the reducing agent NH3 (produced by aeration of ammonia water) were introduced into the reactor. The reaction temperature was measured by a K-type thermocouple linked to a proportional integral derivative controller to regulate the reaction temperature. To examine gaseous pollutants, the O2 and NO concentrations in the input and output flue gas were continuously measured during the experiment. A portable multi-gas online analyzer (Horiba, PG-250A) was used to monitor these gases simultaneously. In the tests, Teflon filters were used to collect fly ash. Sampling was performed when the system reached a steady state; the sampling time was 3 min. The removal efficiency of fly ash and NO is defined as follows: Removal efficiencyð% Þ ¼

ð1Þ

5 12 11 4

13 10

3 9 1

2. Experimental

Cin −Cout  100 Cin

2

8 7 6

2.1. Catalyst preparation Coconut-shell-based AC, which was purchased from China Activated Carbon Industries Ltd., Taiwan, was used as the support. Before catalyst preparation, AC supports were immersed in different solutions (HNO3, H2O2, H2SO4, and NaOH) at room temperature for 48 h as a pretreatment, producing AC–N, AC–H, AC–S, and AC–Na, respectively. The pretreatment method is described in detail elsewhere [20,21].

1. Air compressor ; 2. Flow meter; 3. Combustion chamber; 4. Cooler; 5. Feedstock feeder; 6.Fly ash feeder; 7. Flue gas online analyzer (input); 8. NH3 feeder; 9. Heating tape; 10. Fluidized-bed catalytic reactor; 11. Particle sampling; 12. Flue gas online analyzer (output); 13. Induced fan. Fig. 1. Schematic of experimental apparatus.

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Here, Cin and Cout are the inlet and outlet concentrations of the pollutants, respectively. The major operation parameters are summarized in Table 1. 2.3. Characterization The fly ash from the coal-fired power plant was analyzed by elemental analysis. The PSDs of the original fly ash and that collected during activity tests were determined using a laser-diffraction particle size analyzer (Fritsch Particle Size Analysette 22 COMPACT; analysis range: 0.31–300.74 μm). An X-ray powder diffractometer (XRPD) equipped with a Cu tube was used as the X-ray source (MAC Science, MXP 18); this instrument was used to determine the active site phase. The CuO/AC catalysts were pressed into suitable holders and scanned within a 2θ range of 20°–60° in steps of 0.04°. The scanning speed was 4° min −1. The dispersion of active sites on the supports was observed by transmission electron microscopy (TEM; Philips 400 T) at 120 keV. The morphology of the CuO/AC catalyst was investigated using field emission scanning electron microscopy (FESEM; Model JSM-6700F, JEOL, Tokyo, Japan) at an accelerating voltage of 3 kV. This equipment included an X-ray energy dispersive spectrometer facility. The surface area of the CuO/AC catalyst was measured at 77 K using gravimetric methods with a vacuum microbalance (BET-201-AEL, Porous Materials Inc., New York, USA). A N2 adsorption–desorption surface was used for its textural properties. The surface area was calculated from the adsorption isotherms using the Brunauer–Emmett–Teller (BET) method. An STA 6000 simultaneous thermal analyzer (PerkinElmer, USA), controlling thermo gravimetric analysis (TGA) and differential scanning calorimeter (DSC), was used to identify the thermal stability of the materials. For TGA/DSC measurements, 20.0± 0.5 mg samples were placed in a ceramic boat. Data were recorded upon heating up to 600 °C at a rate of 10 °C min−1 and in a stream (30 mL min−1) of air gas. 2.4. Titration of samples The concentrations of acidic and basic sites on the surface of CuO/ AC catalyst were determined by titration method [22,23]. A 1 g sample was placed in a 50 mL vial with 0.2 N aqueous solutions of NaOH, Na2CO3, NaHCO3, and HCl. The vials were sealed, ultrasonically vibrated for 1 h, and shaken for 24 h. The samples were then filtered, and 10 mL of the filtrate was extracted with a pipette for analysis. The acidic and basic sites on the catalysts were determined by titration with NaOH, Na2CO3, NaHCO3, and HCl. The concentrations of acidic sites were calculated under the assumption that NaHCO3 neutralizes Table 1 The major operation parameters in the experiment. Catalyst Density Reactor dimension Reaction Temp. Gas velocity (U0) Minimum fluidization velocity (Umf) NO-producing system Combustion temp. Combustion of artificial waste Feed rate of urea powder Conc. of input NO

Cu loading weight 3% 1.36 g cm−3 Size I.S.: 5.5 cm Height 250 °C 0.51 m s−1 0.32 m s−1

840 to 1190 μm 120 cm

Conc. of input O2

700 °C Flow rate 45 L min−1 Urea powder 4.0 g min−1 513 ± 52 ppm (NO) 411 ± 43 ppm(NO + 10,143 ± 884 mg m−3 fly ash) 453 ± 107 ppm(NO + various conc. fly ash) 4.2–6.3%

Particle feeder system Conc. of input fly ash

0, 1406, 5580, 10,737 and 49,108 mg m−3

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only carboxylic groups in basic solutions, Na2CO3 neutralizes carboxylic and lactonic groups, and NaOH neutralizes phenolic, carboxylic, and lactonic groups. The concentrations of phenolic, lactonic, and carboxylic groups can be calculated from the differences in the titrated amounts of alkaline solutions. 3. Results and discussion 3.1. Basic analysis before activity test In this study, CuO/AC catalyst was employed as the bed material in a pilot-scale fluidized-bed catalytic reactor for the removal of NO and fly ash. Some basic analyses were evaluated if CuO/AC catalyst was suitable for this experiment or not. The elutriation tests were carried out to study the amounts of CuO/AC catalyst elutriated from the fluidized bed reactor before the experiments. The results indicated the elutriated ratio of CuO/AC is smaller and unconsidered. The TGA and DSC curves show the thermal properties of various CuO/AC catalysts over the range of 50 °C–600 °C in an air flow. The TGA curves for different CuO/AC catalysts are compared and shown in Fig. 2. The TGA curves show that the main decomposition process occurs in two temperature regions. The TGA results indicate that the weight loss attributed to the decomposition of impurities (at high temperature) and moisture (at low temperature) on the AC surface. Fig. 2(a) shows that the decreases of Td point followed the order of CuO/AC–Na (451 °C) > CuO/AC (448 °C) > CuO/AC–H (391 °C) > CuO/ AC–S (386 °C) > CuO/AC–N (371 °C). Obviously, the thermal stability of the catalyst was reduced due to the acidic pretreatment of support before the catalyst preparation. As to the DSC curves of fresh catalysts, the results revealed that these signals were the exothermic peaks between 372 and 468 °C (Fig. 2(b)). The exothermic signal of degradation is correlated with the mass loss of catalyst due to thermooxidation, as shown in Fig. 2(a). Lu et al. [24] indicated that the reasons for the phenomenon may be due to the burning off AC supports. With temperature increasing, there are heat releasing from exothermic reaction of CuO and O2, therefore, under this situation the heat capacity of AC support was decreased and then burned off. Comparing with TEM images (Fig. 3), the results revealed that the good dispersion of nanoscaled CuO particles on the catalyst, such as CuO/AC–H, CuO/AC–S, and CuO/AC–N, would decrease the reaction temperature of CuO and O2, and then the thermal stability of catalyst was reduced, which was similar to the results in other studies [25–28]. Therefore, the thermal stability of the catalyst was reduced due to the CuO particle supported on the support pretreated by the acidic solution. The concentrations of oxygen groups on the surfaces of the fresh CuO/AC catalysts can be determined from the titration results listed in Table 2. The concentrations of lactonic, carboxylic, phenolic, total acidic, and total basic groups on the original AC supports are 0.067, 0.021, 0.004, 0.092, and 0.106 mol g −1, respectively. After pretreatment of AC support and catalyst preparation, the acidic concentrations in the four catalysts followed the order CuO/ AC–N (0.174 mol g −1) > CuO/AC–H (0.138 mol g −1) > CuO/AC–S (0.103 mol g −1) > CuO/AC–Na (0.009 mol g −1). Miyazaki et al. [29] indicated that the dispersion of active sites may be greater when the support contains a large number of acidic sites. This was confirmed by the TEM images (Fig. 3). Good dispersion of nanoscale CuO particles was observed over the three AC-supported Cu catalysts (CuO/AC–N, CuO/AC–H, and CuO/AC–S) that had a large total number of acidic sites. In this study, analysis results indicated that all CuO/AC catalysts would not be burned off in a pilot-scale fluidized-bed catalytic reactor while operating at 250 °C. 3.2. NO removal over different CuO/AC catalysts The NO removal efficiency over different CuO/AC catalysts was evaluated, as shown in Fig. 4. The NO removal efficiency was in the

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C.-Y. Lu et al. / Powder Technology 239 (2013) 239–247 Table 2 Concentrations of acidic and basic groups on CuO/AC catalysts obtained by titration method. Sample

Fig. 2. The TGA (a) and DSC (b) patterns of fresh CuO/AC catalysts.

Acidic sites (mol g−1)

Basic sites (mol g−1)

Lactonic

Carboxylic

Phenolic

Total

AC

0.067

0.021

0.004

0.092

0.106

Fresh catalyst CuO/AC–N CuO/AC–S CuO/AC–H CuO/AC–Na

0.143 0.094 0.117 0.009

0.025 0.007 0.014 –

0.006 0.002 0.004 –

0.174 0.103 0.138 0.009

0.034 0.037 0.059 0.134

Reacted catalyst CuO/AC–N CuO/AC–S CuO/AC–H CuO/AC–Na

0.065 0.079 0.068 0.017

0.014 0.005 0.008 –

0.002 – – –

0.081 0.084 0.076 0.017

0.225 0.216 0.231 0.210

order CuO/AC–N (65.7%) > Cu/AC–S (60.0%) > Cu/AC–H (57.6%) > Cu/ AC–Na (47.4%). In a pilot-scale fluidized-bed catalytic reactor, an AC support pretreated with HNO3 exhibited good catalytic activity for NO removal with NH3; this is in accordance with the analysis results. The concentrations of the acidic and basic groups on the fresh catalysts are presented in Table 2. In the earlier studies, Xue et al. [30,31] indicated that the basic sites have good affinity towards weakly NO due to the surface basic sites helps to the NO adsorption, and product Oad would be accepted by acidic sites and then released. By comparing with our studies in Table 2, high concentration of acidic sites and moderate basic sites on the AC supported catalysts may increase the catalytic activity of NO reduction, and high concentration of basic sites (CuO/AC–Na–0.134 mol g −1) may inhibit the release of Oad by acidic sites and reduce the NO removal efficiency. Moreover, the spent AC supported Cu catalysts were also analyzed by titration, as shown in Table 2. The results show that the total number of acidic groups decreased in the reduction of NO over the CuO/AC catalysts, whereas the number of basic groups increased. The ratios of the basic group contents in reacted/fresh catalyst for the CuO/AC–N, CuO/AC–S, CuO/

Fig. 3. TEM images of Cu particles on the different AC supports: (a) CuO/AC–N, (b) CuO/AC–H, (c) CuO/AC–S, (d) CuO/AC–Na.

C.-Y. Lu et al. / Powder Technology 239 (2013) 239–247

70 60

NO removal (%)

50 40 30 20 10 0 CuO/AC-N

CuO/AC-H

CuO/AC-S

CuO/AC-Na

Catalyst Fig. 4. Effect of different CuO/AC catalysts on the removal efficiency of NO by a fluidized bed catalytic reactor (reaction conditions: 513 ± 52 ppm NO, NH3/NO: 1.5; U0/Umf: 1.59 at 250 °C).

AC–H, and CuO/AC–Na catalysts were 6.6, 5.8, 3.9, and 1.2, respectively. The increase of basic sites was due to the adsorption of NH3 on NO reduction, and the results suggested that the absorbed amount of NH3 on CuO/AC–N catalyst was more than the other catalysts. Boyano et al. [23] indicated that the number of acidic groups (carboxylic acids and phenols) also increased with the adsorption of NH3 on the surface of pretreated AC supports; thus, the increase in the number of phenolic and carboxylic groups may increase the NO removal efficiency over modified CuO/AC catalysts. In the following experiment, CuO/AC–N was chosen to evaluate the NO removal efficiencies under different operating conditions. 3.3. Simultaneous removal of NO and fly ash: Effects of fly ash concentration 3.3.1. NO removal in the flue gas NO + fly ash Fly ash from a coal-fired power plant was used to study the effects of various parameters on particle filtration. The PSD pattern of the original fly ash is shown in Fig. 5(a); the diameters of the fly ash were primarily 1–10 μm and 40 μm. In this study, the simultaneous removal of NO and fly ash over the CuO/AC–N catalyst was evaluated using a simulated flue gas in a fluidized-bed catalytic reactor. Fig. 5(b) shows a FESEM image of the fresh CuO/AC–N catalyst; CuO particles with sizes of approximately 30–50 nm were well dispersed on the surface of the AC support. The effects of different fly ash concentrations in the flue gas (1406 mg m−3, 5580 mg m−3, 10,737 mg m−3, and 49,108 mg m−3) on the simultaneous removal of NO and fly ash were investigated. Fig. 6 shows the removal efficiencies after the fluidized-bed catalytic reactor was operated for 30 min. The removal efficiency of NO without added fly ash was 66%. When fly ash was added, the removal efficiency of NO was 58%–61% for fly ash concentrations of 1406 mg m − 3, 5,580 mg m − 3, and 10,737 mg m − 3. The removal efficiency of NO decreased sharply to 52% when the fly ash concentration was increased to 49,108 mg m − 3. This decrease in NO conversion above a certain value is probably due to the increasing physical deactivation of the AC-supported catalyst surfaces by fly ash [2]. The active sites of the catalysts became covered by fly ash as the concentration increased (Fig. 5 (c)–(f)). When the fly ash concentration was increased to 49,108 mg m − 3, the activity of the bed material of CuO/AC–N reached a tolerance condition during 30 min of operation. The active sites of the catalyst were obviously covered by fly ash at this concentration (Fig. 5(f)). The NO conversion

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exhibited slight variations when the fly ash concentrations were lower than 10,737 mg m − 3 within the 30-min period; further, the bed material (CuO/AC–N catalyst) did not reach decay conditions for NO removal. Obviously, fly ash is an effective factor on the NO removal in the flue gas. In our previous studies [14], the potential of a fluidized-bed catalyst reactor for the simultaneous removals of SO2 and fly ash with the simulated flue gas containing different H2O and particles was investigated. Experimental results showed that the activity of catalyst for the removal of SO2 was inhibited by H2O and particles, and the inhibition effects increased with the content of H2O. As the H2O content increased, the particle size distribution of fine particles shifted to the coarse particles. The aggregation phenomenon of fine particles shifted to the coarse particles maybe caused by the increased water vapor content in fluidized-bed catalyst reactor. Thus, NO conversion would be affected with different pollutants in a real flue gas, which might include water vapor, SOx and heavy metal (Hg). The influences of NO removal with the contents of other pollutants in the flue gas will be studied in the future work. 3.3.2. Fly ash removal in the flue gas NO + fly ash The removal efficiency of fly ash over a 30-min period for the concentrations mentioned above was 82%–86% (Fig. 6); the removal efficiency increased slightly as the fly ash concentrations increased from 1406 to 10,737 mg m −3. This result is primarily due to the enhanced contact efficiency between the fly ash and the bed material (CuO/ AC–N catalyst). The removal efficiency of fly ash was stable within the 30-min operation period for all the fly ash concentrations. Fig. 5(c)–(f) shows the surface morphologies of the catalysts after reaction with different fly ash concentrations. As the concentration increased, fly ash aggregated on the catalyst surface. The FESEM results revealed that fly ash particles 1–2 μm in size (Fig. 5(a)) were easily attached to the uneven surface and pores of the catalyst. Similar filtration studies [2,14,32] indicated that more particles accumulate on the bed material surface or become a part of the bed material of the granular bed filter. This is due to the strong inertial impacts that occur when the surface is replaced by particles from the top to the bottom of the granular bed filter (CuO/AC–N in this study) [2]. The increase may also be due to increases in the contact efficiency between fly ash particles and interparticle forces as well as inertial impacts and interceptions [33,34]. A higher filtration efficiency of fly ash is thereby obtained. However, the CuO/AC–N catalytic reaction efficiency for NO might also decrease when more fly ash particles cover the active sites of the catalyst. 3.4. Simultaneous removal of NO and fly ash: Effects of operating time 3.4.1. NO removal in the flue gas NO + fly ash The effect of the operating time on the simultaneous removal of NO and fly ash from a fluidized-bed catalytic reactor was investigated using a fly ash concentration of 10,143 ± 765 mg m −3. Fig. 7 plots the removal efficiencies of pollutants at different operating times. The removal efficiency of NO was approximately 63% when the operating time was 60 or 120 min. It decreased slightly to 60% at the operating time of 180 min. When the operating time increased to 240 min, the NO removal efficiency decreased to 55%. Thus, the inhibition effect increased with the operating time. This phenomenon, which occurs on the CuO/AC–N catalyst surface, can be attributed to the high concentrations of fly ash accumulating in the fluidized-bed catalytic reactor. The surface morphologies of the CuO/AC–N catalyst at different operating times are shown in Fig. 8(a)–(d). The FESEM images revealed that the fly ash aggregations increased with the operating time. 3.4.2. Fly ash removal in the flue gas NO + fly ash The results in Fig. 7 show that the removal efficiency of fly ash was 86% for a 60-min period. At an operating time of 120 min, it was 80%.

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60

a

50

80

40 60 30 40 20 20

10

0 0.1

1

10

100

b

Differential mass ratio (%)

Cumulative mass ratio (%)

100

100 nm

0 1000

particle size (µm)

c

d

e

f

Fig. 5. (a): The PSD analysis of fly ash of coal-fired power plant; (b): FE-SEM image of CuO/AC–N fresh catalyst; reacted catalysts with different concentrations of fly ash: (c) 1406 mg m−3, (d) 5580 mg m−3, (e) 10,737 mg m−3, (f) 49,108 mg m−3 (reaction conditions: 453 ± 107 ppm NO; NH3/NO: 1.5; U0/Umf: 1.59 at 250 °C).

90

80

70

70

60

60

Removal (%)

Removal (%)

80

NO ash

90

NO ash

50 40 30

50 40 30

20

20

10

10

0

0 0

1406

5580

10737

49108

fly ash conc. (mg m–3) Fig. 6. Effect of different concentrations of fly ash on the removal efficiency of NO and fly ash by a fluidized bed catalytic reactor (reaction conditions: 453 ± 107 ppm NO; NH3/NO: 1.5; U0/Umf: 1.59 at 250 °C).

60

120

180

240

Reaction time (min) Fig. 7. Effect of different operating times on the removal efficiency of NO and fly ash by a fluidized bed catalytic reactor (reaction conditions: 411 ± 43 ppm NO, 10,143 ± 884 mg m−3 fly ash; NH3/NO: 1.5; U0/Umf: 1.59 at 250 °C).

C.-Y. Lu et al. / Powder Technology 239 (2013) 239–247

operating times were 60 and 120 min, respectively. The major fraction percentages of fine (1–10 μm) and coarse (75–117 μm) particles for a 120-min operating time were 8.9% and 90.3%, respectively; the PSD was considered polydisperse. Furthermore, the PSD was also polydisperse at operating times of 180 and 240 min, and the major fraction percentages for 1–10, 22–48, and 67–105 μm particles were 9.7%, 7.1%, and 72.3%, respectively. The PSD results showed that particles 60

Cumulative mass ratio (%)

100

a

e

50

80

40 60 30 40 20 20

10

0 0.1

1

10

100

Differential mass ratio (%)

When the operating time was increased to 180 min, the fly ash concentration in the fluidized-bed reactor increased slightly. As a result, the tolerance for fly ash in the fluidized-bed reactor decreased. Moreover, the removal efficiency of fly ash was maintained at approximately 76% as the operating time increased from 180 to 240 min. Fig. 8(e)–(h) shows the PSDs of fly ash filtered in the exhaust gas at different operating times. Fig. 8(e) and (f) shows the PSDs when the

245

0 1000

particle size (µm) Cumulative mass ratio (%)

f

50

80

40 60 30 40 20 20

10

0 0.1

1

10

100

Differential mass ratio (%)

60

100

b

0 1000

particle size (µm)

g

50

80

40 60 30 40

20

20

10

0 0.1

1

10

100

Differential mass ratio (%)

60

100

Cumulative mass ratio (%)

c

0 1000

particle size (µm) 60

h

50

80

40 60 30 40 20 20 0 0.1

10

1

10

100

Differential mass ratio (%)

Cumulative mass ratio (%)

100

d

0 1000

particle size (µm) Fig. 8. FESEM images and Particle size distributions of the filtered fly ash in the exhaust gas at different operating times ((a) and (e): 60 min; (b) and (f): 120 min; (c) and (g): 180 min; (d) and (h): 240 min) (reaction conditions: 411 ± 43 ppm NO, 10,143 ± 884 mg m−3 fly ash; NH3/NO: 1.5; U0/Umf: 1.59 at 250 °C).

C.-Y. Lu et al. / Powder Technology 239 (2013) 239–247

larger than 67 μm could be produced from catalyst erosion and elutriation. Furthermore, a high tolerance for fly ash was observed when the fluidized-bed catalytic reactor was operated for 120 min. As the operating time increased from 180 to 240 min, the fly ash on the catalyst in the fluidized-bed reactor was easily elutriated. As shown in Fig. 8(g) and (h), the fly ash particle size increased with the operating time; as a result, the removal efficiency of fly ash decreased with elutriation. Moreover, fly ash filtration using a fluidized-bed reactor can be considered a dynamic process because particle removal involves balancing the collection and elutriation of particles, and both processes are time dependent.

a

CuO:35.4o;38.6o C:44.3o SiO2:26.6o;34.3o Al2O3:33.1o;43.3o

Intensity (a.u)

246

49108 mg m-3 10737 mg m-3 5580 mg m-3

3.5. Characterization of reacted CuO/AC–N catalysts

3.5.2. BET surface area Table 3 summarizes the textural characterization of the bed material consisting of fresh catalysts and that reacted during the simultaneous removal of NO and fly ash under different conditions. For the fresh CuO/ AC–N catalyst, the specific surface area (BET area), Vmicro, Vmeso, and Vmacro were 1044.1 m2 g−1, 0.4856 cm3 g−1, 0.1033 cm3 g−1, and 0.0068 cm 3 g−1, respectively. The BET areas of the reacted catalysts were 1120, 1073.5, 1035.4, 1111.2, and 1078.4 m 2 g−1 at fly ash concentrations of 0, 1406, 5580, 10,737, and 49,108 mg m−3, respectively. The micropores were most abundant, constituting 81.4%–82.3% of the total pore volume (i.e., 0.4917 m3 g−1–0.5150 m3 g−1) at all the fly ash concentrations after 30 min of operation. Increasing fly ash concentrations did not obviously reduce the catalyst surface area and pore volumes in 30 min. These results were also confirmed in Section 3.1. The mesopore volumes of the reacted samples were 0.0974, 0.0966, 0.0957, and 0.0898 cm3 g−1 for operating times of 60, 120, 180, and 240 min, respectively, whereas the macropore volumes were 0.0068, 0.0071, 0.0070, and 0.0058 cm3 g−1. Fine particles (1–10 μm) of fly ash (Fig. 8) plugged the pore volume of the catalyst, and this effect increased when the operating time was longer than 120 min. Therefore, the removal efficiency of fly ash decreased as the operating time increased (Fig. 7). Moreover, an increase in the operating time decreased the CuO/AC–N activity during NO reduction. The effects of fine fly ash particles on the simultaneous removal of NO and particles using a fluidizedbed catalytic reactor were significant: the particles plugged the surface volume of the catalyst and reduced the catalytic activity of NO. 4. Conclusions In this study, we investigated the removal efficiency of NO and fly ash from flue gas over CuO/AC catalysts in a pilot-scale fluidized-bed

0 mg m-3 20

30

40

50

60

2 theta (o) CuO:35.4o;38.6o C:44.3o SiO2:26.6o;34.3o Al2O3:33.1o;43.3o

b

Intensity (a.u.)

3.5.1. XRPD analysis The XRPD patterns of the CuO/AC–N catalysts reacted at different fly ash concentrations are shown in Fig. 9(a). A broad diffraction peak can be observed at 2θ = 44.3° (graphitic shape 111) for all the samples, indicating that the AC samples had a low graphitization degree. For all the reacted samples, the major diffraction peaks of CuO can be observed at 2θ = 35.4° (002) and 38.6° (111). Furthermore, for the reacted samples, catalysts with certain crystal morphologies did not show any clear diffraction signals indicating the fly ash composition [i.e., Al2O3 (33.1° and 43.3°) and SiO2 (26.6° and 34.3°)]. This implies inferior detection of the composition of the fly ash particles (i.e., SiO2 and Al2O3) at the concentration limits in the CuO/AC–N catalyst. Related studies [20,35] have shown that the peak intensity decreases because of lower metal loading (b3%) on the support surface. For the CuO/AC–N catalysts reacted for different operating times (Fig. 9(b)), some crystal morphologies of the catalysts clearly showed diffraction signals indicating Al2O3 (33.1° and 43.3°) and SiO2 (26.6° and 34.3°) when the operating time was 120 min. These results clearly demonstrate the fly ash accumulation behavior in the fluidized-bed catalytic reactor as the operating time increased.

1406 mg m-3

240 min

180 min 120 min

60 min 20

30

40

50

60

2 theta (o) Fig. 9. XRPD patterns of reacted CuO/AC–N catalysts ((a): filtrated of different concentrations of fly ash and (b): filtrated of different operating times, fly ash: 10,143 ± 884 mg m−3).

catalytic reactor. The surface chemical properties of an AC support can be modified by chemical solutions with HNO3, H2SO4, H2O2, and NaOH, all of which affect the catalytic reduction of NO with NH3. Combining the FESEM, TEM, TGA/DSC, and titration results confirms that catalysts prepared through the acidic solution pretreatment processes present as tiny crystalline grains and as highly dispersed Cu metals when phenolic or/and carboxylic acid groups are increased, and the concentration of basic groups is decreased. High concentration of acidic sites and moderate basic sites on the AC supported catalysts may increase the catalytic activity of NO reduction, and high concentration of basic sites (CuO/AC–Na–0.134 mol g −1) may inhibit and reduce the NO removal efficiency. An AC support pretreated with HNO3 during CuO/AC–N catalyst preparation showed good NO removal efficiency. CuO particles 30–50 nm in size were well dispersed on the catalyst. Simultaneous removal of NO and fly ash in flue gas under different fly ash concentrations and operating times were studied. For fly ash concentrations ranging from 1406–49,108 mg m −3, CuO/AC–N demonstrated good removal efficiencies for NO and fly ash while the concentration of fly ash was less than (49,108 mg m −3). As the operating time increased from 60 min to 240 min, the NO removal efficiency decreased slightly from 63% to 55% because the catalyst surface was covered with fly ash.

C.-Y. Lu et al. / Powder Technology 239 (2013) 239–247

247

Table 3 Physical characteristics of the CuO/AC–N catalysts of fresh and reacted under different operation parameters. Samples

Fresh catalyst CuO/AC–N

BET area

Vmicro

Vmeso

Vmacro

Vtotal

(m2 g−1)

(cm3 g−1)

(cm3 g−1)

(cm3 g−1)

(cm3 g−1)

1044.1

0.4856

0.1033

0.0068

0.5957

0.5027 0.5007 0.4917 0.5150 0.5014

0.1009 0.1032 0.1033 0.1111 0.1064

0.0069 0.0067 0.0061 0.0063 0.0063

0.6105 0.6106 0.6011 0.6324 0.6141

0.4794 0.4942 0.4713 0.4707

0.0974 0.0966 0.0957 0.0898

0.0068 0.0071 0.0070 0.0058

0.5836 0.5979 0.5740 0.5663

Reacted catalysts Content of coal ash 0 mg m−3 1120.0 1406 mg m−3 1073.5 1035.4 5580 mg m−3 10,737 mg m−3 1111.2 −3 49,108 mg m 1078.4 −3 Operating period (10,143 ± 884 mg m ) 60 min 1055.2 120 min 1100.1 80 min 1008.9 240 min 1036.5

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