Low-Temperature Selective Catalytic Reduction of NO with Urea

Jun 22, 2010 - Urea as a reducing agent supported on pitch-based spherical activated carbon (PSAC) was studied for NO reduction at low temperatures ...
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Ind. Eng. Chem. Res. 2010, 49, 6317–6322

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Low-Temperature Selective Catalytic Reduction of NO with Urea Supported on Pitch-Based Spherical Activated Carbon Zhi Wang, Yanli Wang, Dengjun Wang, Qingjun Chen, Wenming Qiao,* Liang Zhan, and Licheng Ling State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China

Urea as a reducing agent supported on pitch-based spherical activated carbon (PSAC) was studied for NO reduction at low temperatures (30-90 °C). The results showed that PSAC with 8 wt % urea loading exhibited high activity in the selective catalytic reduction (SCR) of NO at 30 °C. The SCR activity decreased markedly when urea loading was increased above 8 wt % due to pore plugging, which restricted the adsorption of gas phase reactants on PSAC, although the NOx removal period was extended. A low reaction temperature was favorable for NO reduction on account of the increased NO adsorption on PSAC. It was found that the SCR activity was improved by increasing NO or O2 concentration in the feed gas, owing to the enhanced NO oxidation by O2 to NO2, which was then reduced by urea to form N2. Increasing space velocity not only decreased the SCR activity but also shortened the NOx removal period. More than 85% NOx conversion for 55 h could be achieved over PSAC with 8 wt % urea loading at 30 °C under the conditions of 500 ppmv NO, 21 vol % O2, and a space velocity of 2000 h-1. Furthermore, PSAC showed a superior hydrodynamic property, and the pressure drop ratio of PSAC to a commercial granule activated carbon with the equivalent particle size was about 35% with the apparent air flow velocity in a range of 0.12∼0.51 m/s. 1. Introduction

NO + 0.5O2 f NO2

(1)

NO2 + NO + (NH2)2CO f 2N2 + CO2 + 2H2O

(2)

Nitrogen oxides (NOx) are considered to be among the most common toxic air pollutants released from the combustion of fossil fuels. The control of NOx emissions becomes a worldwide concern because NOx can cause serious environmental problems, such as acid rain, photochemical smog, ozone depletion, and a greenhouse effect.1 Nowadays, selective catalytic reduction (SCR) with NH3 as a reducing agent is the most widely used method to remove NOx from stationary sources due to its high efficiency and selectivity. Generally, the industrial SCR operations are carried out on V2O5/TiO2-based catalysts in an optimum temperature range of 300-400 °C.2-8 However, NH3 does not appear to be an ideal reducing agent when considering its corrosiveness and toxicity. Furthermore, it is very difficult to exactly control an appropriate NH3 input because of the fluctuating NOx concentration in exhaust gas, which is very likely to cause additional environmental problems due to NH3 slip.9,10 Therefore, it is of great significance to develop new SCR technologies with other proper reducing agents as a substitution for NH3. A variety of reducing agents, such as hydrocarbons,11,12 CO,13,14 ethanol,15 and carbon,16,17 have been reported for the SCR application. They show various reducing abilities toward NOx under different conditions. Among them, the use of hydrocarbons, CO, and ethanol, however, inevitably causes leakage. In the case of carbon, the SCR process requires a high reaction temperature. And it suffers from a preferential reaction between carbon and oxygen in an oxidizing atmosphere, resulting in fast carbon consumption and high concentration CO release. Recently, Shirahama et al.18,19 proved that urea supported on activated carbon fibers (ACFs) could efficiently reduce NO to N2 at 30 °C in the presence of O2. The possible reaction steps were proposed as follows:

Here, urea was used as a reducing agent in place of NH3 to avoid the above-mentioned difficulties. However, some potential problems in the urea-supported ACFs system, such as the poor mechanical properties and unstable bed resistance of ACFs, should be taken into consideration in industrial applications. Spherical activated carbon (SAC) is a novel porous carbon material. It possesses properties of common activated carbon, along with some unique advantages, such as high mechanical strength, smooth surface, uniform spherical shape, and good fluidity. Recently, it has attracted much interest and enjoyed wide applications in the field of medical treatment and environmental protection.20-22 Basically, when activated carbon is used as catalyst or catalyst support, the surface area and pore size distribution play the most important role in obtaining high activities. With large surface area and easily controlled pore size distribution,23,24 SAC is expected to serve as one of the most promising catalysts or catalyst supports. However, to our knowledge, so far, few reports have covered the introduction of SAC as a catalyst to the NOx removal field. In this paper, the reactivity of NO with urea supported on pitch-based spherical activated carbon (PSAC) was studied. The research focused on the influence of urea loading and operating conditions (reaction temperature, feed concentration of reactants, and space velocity) on low-temperature NOx removal efficiency, and the hydrodynamic property of PSAC was compared with that of a commercial granule activated carbon (GAC). The experimental data provided rather important information for constituting a profound understanding of the urea-SCR mechanism over PSAC, and especially for industrial applications.

* To whom correspondence should be addressed. Tel.: +86-2164253730. Fax: +86-21-64252914. E-mail: [email protected].

2.1. Supporting Urea on PSAC. A commercial PSAC supplied by Shanghai Heda Carbon Company was used in this

2. Experimental Section

10.1021/ie901772y  2010 American Chemical Society Published on Web 06/22/2010

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Table 1. Physical and Chemical Properties of PSAC elemental analysis (wt %)

sample

BET surface area (m2/g)

total pore volume (cm3/g)

micropore volume (cm3/g)

ash (wt %)

C

N

Oa

H

S

PSAC

1182

0.47

0.41

0.37

93.00

1.40

3.91

0.89

0.43

a

By difference.

Figure 1. SEM images of (a) PSAC and (b) urea8/PSAC.

work. Its properties were summarized in Table 1. Urea was supported on PSAC by pore volume impregnation using an aqueous solution of urea. After impregnation, the obtained sample was kept at room temperature for 24 h and then was vacuum-dried at 60 °C for 24 h. The sample is designated as ureaM/PSAC, where M represents the weight percentage of urea loading on PSAC, for example, urea8/PSAC. 2.2. Characterization. Nitrogen adsorption-desorption isotherms of PSAC and urea-supported PSAC were measured at 77 K using a Micromeritics ASAP 2020 M analyzer to determine the texture properties. The specific surface area was determined according to the Brunauer-Emmett-Teller (BET) method. The total pore volume, micropore volume, and micropore surface area were calculated according to density functional theory (DFT). Elemental analysis was carried out using a Vario EL elemental analyzer. The morphology of the sample was observed under a JEOL, JSM-6360LV scanning electron microscope (SEM) with a 15 kV beam. 2.3. Activity Test. The reactivity of NO with urea supported on PSAC was measured in a vertical fixed-bed glass reactor with an internal diameter of 10 mm. The feed gas consisted of 100-1000 ppmv NO, 0-21 vol % O2, and a balance of N2. In all tests, the total flow rate was maintained at 100 mL/min. The reaction temperature was controlled from 30 to 90 °C, and the space velocity was in the range of 2000-12 000 h-1. The concentrations of NO, NO2, and NOx were continually measured by an ECO PHYSICS CLD62 chemiluminescence NO/NOx analyzer with a minimum detectable concentration of 0.5 ppmv. NO conversion, percentage yield of NO2 produced, and NOx conversion during SCR tests were defined by the following equations: NO conversion ) (CNO,in - CNO,out)/CNO,in × 100%

(3)

percentage yield of NO2 produced ) CNO2,out/CNO,in × 100% (4) NOx conversion ) (CNO,in - CNOx,out)/CNO,in × 100% (5) 2.4. Hydrodynamic Property Test. The hydrodynamic property test of PSAC was performed in a vertical fixed-bed glass reactor with an internal diameter of 30 mm and a height of 300

mm. The sample was packed into the reactor with a height of 200 mm. The flow of air passed through the carbon bed from the top to the bottom of the reactor with a U-shaped differential pressure meter to measure the pressure drop. The flow rate of air was controlled by a rotameter, and the apparent flow velocity was calculated according to the internal diameter of the reactor. In each test, the pressure drop of the empty reactor was first measured and was then subtracted from that of the reactor with carbon loaded. For comparison, GAC supplied by Shanghai Activated Carbon Company with an equivalent diameter of 0.8 mm was also tested. 3. Results and Discussion 3.1. SEM Images of PSAC and Urea8/PSAC. Figure 1 shows the SEM images of PSAC and urea8/PSAC. It was observed that PSAC was characterized by the uniform spherical shape, smooth surface, and a size distribution of 0.5-0.8 mm, which could provide a low bed resistance and an even gas flow in the reactor. After 8 wt % urea was supported on PSAC, the outer surface of PSAC was still smooth, and no urea crystal particles were observed, suggesting that all supported urea had deposited in the internal pore structure of PSAC. 3.2. Reactivity of NO with Urea over Urea8/PSAC. Figure 2 shows the reactivity of NO with urea over urea8/PSAC at 30 °C and with a space velocity of 6000 h-1. The NO conversion decreased from an initial 88% to about 80% in the first 2 h and maintained this level for 9 h, which was defined as the steadystate NOx removal period in this paper. After that, it decreased to the minimum value of 75% at 19 h and then increased to reach a stable value of 86%. It is important to note that no NO2 was detected at the outlet of the reactor in the initial 12.6 h, suggesting that the produced NO2 was quickly reduced by supported urea on the PSAC surface, and NO2 could not release until urea was completely consumed. After the start of NO2 emission, the outlet NO2 concentration quickly increased with time on stream and finally stabilized at 430 ppmv, while the outlet NO concentration kept the value of 70 ppmv. At the end of the experiment, although the NO conversion achieved a stable high value of 86%, the sample lost its ability to remove NOx

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Figure 2. Reactivity of NO with urea over urea8/PSAC. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, reaction temperature ) 30 °C, space velocity ) 6000 h-1.

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Figure 4. Nitrogen adsorption-desorption isotherms of PSACs with different urea loadings. Table 2. Pore Parameters of PSACs with Different Urea Loadings

Figure 3. Effect of urea loading on NOx conversion over urea-supported PSACs. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, reaction temperature ) 30 °C, space velocity ) 6000 h-1.

due to the complete consumption of urea, and all consumed NO was catalytically oxidized to NO2 by O2 in the presence of PSAC. Figure 2 also shows NOx conversion vs reaction time over urea8/PSAC. It was observed that, at the early stage of the reaction, the NOx conversion was equal to the NO conversion, and then it gradually decreased to 10% at 25 h and finally to zero at 44 h. Since the NOx conversion directly reflects the SCR activity of the sample, for convenience, only this parameter is discussed below. 3.3. Effect of Urea Loading on NOx Conversion over Urea-Supported PSACs. Figure 3 shows the effect of urea loading on NOx conversion over urea-supported PSACs at 30 °C and with a space velocity of 6000 h-1. For PSAC without urea, the NOx conversion first sharply decreased from an initial 99% to 83% in 1.3 h, followed by a gradual decrease to reach zero at 18 h. The NO removal process over PSAC without urea mainly included the oxidation of NO to NO2 and the adsorption of NO and NO2 on the carbon surface. Until the adsorption saturation was reached, PSAC lost its ability to remove NOx. It was found that urea-supported PSACs showed high SCR activities. A larger amount of urea loading prolonged the NOx removal period. With increasing urea loading from 8 wt % to 16 wt % and 30 wt %, the NOx removal period increased from 45 to 55 h and 118 h, respectively. However, a larger amount of urea loading decreased the SCR activity. For urea8/PSAC,

samples

BET surface area (m2/g)

PSAC urea8/PSAC urea16/PSAC urea30/PSAC

1182 909 683 456

total pore micropore volume volume micropore surface (cm3/g) (cm3/g) area (m2/g) 0.47 0.39 0.27 0.16

0.41 0.35 0.24 0.13

805 748 423 177

the steady-state NOx conversion was 82%. The increase in urea loading significantly decreased the steady-state NOx conversion, 73% for urea16/PSAC and 67% for urea30/PSAC, respectively. In addition, the initial NOx conversion decreased from 89% for urea8/PSAC to 69% for urea30/PSAC. Urea loading plugged part of the pore structure of PSAC and thus decreased the SCR activity. In order to illuminate the effect of urea loading on the pore structure of PSAC, nitrogen adsorption was used to determine the change of pore structure after urea loading. Figure 4 shows the nitrogen adsorptiondesorption isotherms of PSACs with different urea loadings. The pore parameters of the samples are summarized in Table 2. PSAC gave a micropore surface area of 805 m2/g and a micropore volume of 0.41 cm3/g. When 8 wt % urea was supported, the micropore surface area and micropore volume decreased to 748 m2/g and 0.35 cm3/g, respectively. This suggested that 8 wt % urea loading on PSAC caused a mild extent of micropore plugging, and therefore the adsorption capacity of PSAC was not seriously affected. However, with increasing urea loading to 16 wt % and 30 wt %, both the micropore surface area and micropore volume markedly decreased, 427 m2/g and 0.24 cm3/g for urea16/PSAC and 177 m2/g and 0.13 cm3/g for urea30/PSAC, respectively. These data indicated that urea loading above 8 wt % could cause serious micropore plugging, which retarded the adsorption of gas molecules and hence decreased the initial NOx conversion and the SCR activity. For PSAC used in this work, the optimum urea loading was considered to be 8 wt %. 3.4. Effect of Reaction Temperature on NOx Conversion over Urea8/PSAC. Figure 5 shows the effect of reaction temperature on NOx conversion over urea8/PSAC. It is clear that the SCR activity decreased with increasing reaction temperature. The initial and steady-state NOx conversions were 90% and 82% at 30 °C, 65% and 56% at 50 °C, 41% and 39% at 70 °C, and 31% and 27% at 90 °C, respectively. Generally, a high reaction temperature benefits the activation of reactants and the desorption of products but inhibits the adsorption of

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Figure 5. Effect of reaction temperature on NOx conversion over urea8/ PSAC. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, space velocity ) 6000 h-1.

reactants on the surface of catalysts. Therefore, the data found in this work suggested that the NO removal process over ureasupported PSAC was controlled by the adsorption of NO. Similar results were previously reported. Zhang et al.25 investigated NO adsorption and oxidation on activated carbons at temperatures below 100 °C and found that both the NO adsorption capacity and the reaction rate of NO oxidation to NO2 decreased with increasing temperature. Mochida et al.26 observed that, in NH3-SCR over pitch-based ACFs in the low temperature window of 25-70 °C, NO adsorption was mainly dependent on the reaction temperature, and the increased reaction temperature was unfavorable for NO adsorption and oxidation. It was concluded that the decrease in NH3-SCR activity with increasing reaction temperature was attributed to poor NO adsorption. Thus, high urea-SCR activity achieved at 30 °C in the present research was due to high NO adsorption on PSAC. However, it is worth noting that a high reaction temperature prolonged the NOx removal period over urea8/ PSAC. This was because the high reaction temperature inhibited the adsorption of NO and thus decreased the reaction rate. On the basis of the above data, the optimum reaction temperature for NO removal over urea8/PSAC was 30 °C. 3.5. Effect of NO Feed Concentration on NOx Conversion over Urea8/PSAC. Figure 6 shows the effect of NO feed concentration on NOx conversion over urea8/PSAC at 30 °C. High SCR activity was achieved with increasing NO feed concentration while the NOx removal period was shortened. The steady-state NOx conversions and the NOx removal periods were about 66% and 191 h for 100 ppmv NO, about 72% and 65 h for 250 ppmv NO, about 82% and 44 h for 500 ppmv NO, and about 87% and 22 h for 1000 ppmv NO, respectively. This result suggested that a high NO feed concentration accelerated the production of NO2 and had a beneficial effect on the NOx conversion. However, it should be pointed out that all samples were supported with the same urea loading (8 wt %) in this study. Therefore, a high reaction rate resulted in fast urea consumption and a short NOx removal period. 3.6. Effect of O2 Feed Concentration on NOx Conversion over Urea8/PSAC. Figure 7 shows the effect of O2 feed concentration on NOx conversion at 30 °C. In the absence of O2, the initial NOx conversion was merely 26%, and it decreased rapidly to zero within 3 h. During this process, no NO2 was detected at the outlet of the reactor, implying that only NO adsorption occurred on the surface of PSAC and the urea-SCR reaction could not take place without the participation of O2.

Figure 6. Effect of NO feed concentration on NOx conversion over urea8/ PSAC. Reaction conditions: 21 vol % O2, balance N2, urea loading ) 8 wt %, reaction temperature ) 30 °C, space velocity ) 6000 h-1.

Figure 7. Effect of O2 feed concentration on NOx conversion over urea8/ PSAC. Reaction conditions: 500 ppmv NO, balance N2, urea loading ) 8 wt %, reaction temperature ) 30 °C, space velocity ) 6000 h-1.

In contrast, in the presence of O2, the urea-SCR reaction occurred. It was found that increasing the O2 feed concentration achieved high SCR activity. When 4 vol % O2 was added, the steady-state NOx conversion was only 34%. However, it could achieve 64% with 15 vol % O2 and 82% with 21 vol % O2. This revealed that O2 played a decisive role in the urea-SCR reaction, especially at low temperatures. Zhang et al.25 and Zhu et al.27 studied the adsorption of NO as well as the coadsorption of NO and O2 on activated carbons. Their temperatureprogrammed desorption (TPD) results showed that the presence of O2 significantly enhanced the adsorption of NO and hence promoted the production of NO2. Thus, the increased urea-SCR activity with a higher O2 feed concentration was ascribed to the enhancement of NO adsorption and oxidation, which led to more NO2 participating in the urea-SCR reaction. On the basis of the above discussion, an O2 feed concentration of 21 vol % was preferred. 3.7. Effect of Space Velocity on NOx Conversion over Urea8/PSAC. Figure 8 shows the effect of space velocity on NOx conversion over urea8/PSAC at 30 °C. The NOx conversion in the whole time span decreased with increasing space velocity. The initial and steady-state NOx conversions decreased from 100% and 90% at 2000 h-1 to 72% and 62% at 12 000 h-1. The NOx removal period was also shortened with increasing space velocity, from 111 h at 2000 h-1 to 20 h at 12 000 h-1.

Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010

Figure 8. Effect of space velocity on NOx conversion over urea8/PSAC. Reaction conditions: 500 ppmv NO, 21 vol % O2, balance N2, urea loading ) 8 wt %, reaction temperature ) 30 °C.

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NOx conversion in the presence of gaseous O2. Significant findings were as follows: (1) A high urea loading prolonged the NOx removal period at the cost of lowering the SCR activity. Urea loading above 8 wt % led to an excessive decrease in the micropore surface area and micropore volume of PSAC, which impaired the adsorption and oxidation of NO and finally decreased the SCR activity. For PSAC used in the current research, the optimum urea loading was 8 wt %. (2) Reaction temperature, feed concentrations of NO and O2, and space velocity had potent impacts on the SCR activity of urea-supported PSAC. A low reaction temperature was favorable for NO adsorption and thus had a positive effect. Increasing the NO or O2 concentration in the feed gas improved the oxidation of NO to NO2 and therefore resulted in the increase in the SCR activity. A low space velocity also contributed to high SCR activity. More than 85% NOx conversion for 55 h could be achieved over PSAC with 8 wt % urea loading at 30 °C under the conditions of 500 ppmv NO, 21 vol % O2, and a space velocity of 2000 h-1. (3) PSAC showed a superior hydrodynamic property to that of GAC due to its spherical shape and smooth surface. The pressure drop ratio of PSAC to GAC was about 35% with the apparent air flow velocity in a range of 0.12∼0.51 m/s. Acknowledgment The authors gratefully acknowledge the financial support from the National High-Tech Research and Development Program (No. 2007AA05Z311), the Natural Science Foundation of China (No. 20806024 and No. 20977028), and the Research Fund for the Doctoral Program of Higher Education (No. 20090074110009). The authors gratefully acknowledge Xiaojun Liu for assisting with the English language used in this manuscript during its preparation.

Figure 9. Hydrodynamic property comparison of PSAC with GAC.

Literature Cited

These results showed that, in the studied range, the space velocity markedly affected the SCR activity. At a low space velocity, reactants and PSAC had sufficient contact time, and the micropore availability was increased in PSAC, which resulted in the increase in urea-SCR activity. 3.8. Hydrodynamic Property Comparison of PSAC with GAC. Figure 9 shows the hydrodynamic property comparison of PSAC with GAC. It was obvious that the hydrodynamic property of PSAC was much superior to that of GAC. With the apparent air flow velocity increasing from 0.12 to 0.51 m/s, the pressure drop of the PSAC bed gradually increased from 6.42 kPa/m to 31.84 kPa/m, while that of the GAC bed significantly increased from 17.59 kPa/m to 89.14 kPa/m. The pressure drop ratio of PSAC to GAC was about 35% in the experimental range. The much lower pressure drop of PSAC was largely due to much smaller friction between air flow and the carbon surface produced by its spherical shape and smooth surface. Since a superior hydrodynamic property is of great concern for a commercial catalyst, urea-supported PSAC with low and stable bed resistance has promising prospects for NOx removal in industrial applications.

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ReceiVed for reView November 9, 2009 ReVised manuscript receiVed April 30, 2010 Accepted May 15, 2010 IE901772Y