Experimental Study on the Time Evolutions of Methane Reburning and

Jul 28, 2011 - pubs.acs.org/IECR. Experimental Study on the Time Evolutions of Methane Reburning and Combustion Process. Enlu Wang,*. ,†. Xuchang Xu...
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Experimental Study on the Time Evolutions of Methane Reburning and Combustion Process Enlu Wang,*,† Xuchang Xu,†,‡ and Mingchuan Zhang† † ‡

Institute of Thermal Energy Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: An electrical heated test rig with nonintrusive multipoint sampling analysis system was built up. On such a test rig, the experiments of time evolution of methane reburning and combustion process have been carried out. In the methane reburning process, the concentrations of O2, CH4, C2H4, C2H6, and NO decrease during 0.2 1.0 s, while the concentrations of C2H2, CO, H2, and HCN increase within the same time. Through comparison of the time evolutions of methane reburning and combustion process, the changing trends of CH4, C2H2, C2H4, C2H6, H2, O2, and CO are similar, and the concentrations of CH4, C2H4, and C2H6 in the methane reburning process are slightly higher than these in the methane combustion process, while the concentration of C2H2 in the methane reburning process is obviously lower than that in the methane combustion process. The experimental results suggest that C2H2 is the key hydrocarbon in the methane reburnin process, and the NO reduction by methane as the reburning fuel mainly proceeds via the reaction with C2H2. The further validating experimental results show that C2H2 is of the most significant effects on NO reduction when compared with CH4, C2H4, and C2H6 at the same stoichiometric ratio. In the case with C2H2, the NO reduction efficiency is about 90%. But for CH4, C2H4, and C2H6, the NO reduction efficiency is only about 28, 35, and 45%, respectively.

1. INTRODUCTION Gas reburning technology, originally developed by John Zink Co. and Wendt et al., is a three-step combustion process.1 4 NO first forms in the primary combustion zone with excess air and then reacts with hydrocarbons in the reburning zone to form HCN, NH3, the nitrogen intermediate radicals, and N2. The unreacted intermediate components complete combustion in the burnout zone where the additional air is added.1 8 Gas reburning is a chemically complex process. The process involves partial oxidation of the reburning gas under fuel-rich conditions as well as the reactions of hydrocarbon radicals, NO, and other intermediate nitrogenous species. Many researchers have studied the NO reduction effects by using different hydrocarbons, including CH4, C2H4, C2H6, and C2H2, as the reburning fuel in the electrically heated test rigs.9 22 The experimental results show that the NO reduction effects for each hydrocarbon were influenced by the operating conditions in the reburning zone, such as the reaction temperature, the stoichiometric ratio, and the oxygen concentration coming from the primary zone. And the optimum operating conditions were different with the hydrocarbons. The main question is that these experimental results only show the concentrations of the components, including CH4, C2H2, C2H4, C2H6, H2, O2, NO, HCN, NH3, CO, and CO2, at the exit of the reburning zone after a fixed residence time. It is really difficult for using these inlet outlet experimental results to analyze the details of the reburning process and further to find out the key hydrocarbon for reducing NO. For this reason, an electrically heated test rig with a nonintrusive multipoint sampling analysis system was built up, and the concentrations of the components in the whole process could be measured point by point, intrusively. As a sample, the experiments r 2011 American Chemical Society

of time evolution of the methane reburning process and combustion process in the conditions with and without NO were carried out, and the changing trends of the components that appeared in the whole processes were analyzed.

2. EXPERIMENTAL SYSTEM AND ARRANGEMENT Figure 1 shows the diagram of the experimental system. It is an electrically heated test rig with a multipoint sampling analysis system. The experimental system consists of a gas dosing system, a horizontal electrically heated tube reaction system and a nonintrusive multipoint sampling analysis system. The gas dosing system was used to control the formation of the simulate flue gas. In the system, high-purity gases of N2, CO2, O2, and 2% NO with N2 from different gas cylinders entered a mixing chamber through the mass flow controllers, which were used to control the mass flow rate of each gas flow. In the mixing chamber, the gas flows were mixed to form the homogeneous gas mixture, which was fed into the horizontal tube as the simulated flue gas. In experiments, the water steam component of the simulated flue gas was dosed by a water mass flow control pump. The pure water was pump from the syringe into the water steam feeding pipe, which was a stainless steel pipe of 3 mm in outside diameter and wrapped by the electrically heated belt. In the feeding pipe, the water was heated into steam and then fed into the inlet part of the horizontal tube, and the temperature of the electrically heated Received: November 30, 2010 Accepted: July 28, 2011 Revised: July 20, 2011 Published: July 28, 2011 9834

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Figure 1. Diagram of the experimental system.

Figure 2. Temperature profiles inside the reactor.

belt was chosen as 117 °C for prohibiting the fluctuation of water steam. The reburning fuel, methane, from the gas cylinder was transported into the reburning zone through a ceramic pipe installed inside the horizontal reaction tube, and its mass flow rate was also controlled by a mass flow controller. The ceramic pipe was 6 mm in outside diameter and its exit was 0.4 m apart from the inlet of the horizontal reaction tube. The horizontal electrically heated tube reaction system includes a main ceramic reaction tube and 15 ceramic sample pipes. The reaction tube was 20 mm in inside diameter and 1500 mm in length, and the sample pipes were 6 mm in outside diameter. The sample pipes connected with the main reaction tube along the horizontal axis of the reactor with adhesive. Temperature inside the reactor was controlled by adjusting the three section temperatures of the electrically heated furnace. To determine the location of the reaction zone in the reactor, experiments had been done to investigate the longitudinal temperature profiles inside the reactor. Figure 2 shows the results of the temperature profile inside the reactor at 1100 °C. It was found that there is a central zone (about 700 mm in length), where the temperature profile could be considered as uniform. Therefore, the central zone, which has a length of 700 mm, was considered as the reaction zone and the temperature in this zone as the nominal temperature. The sample gas for analysis was taken from the reaction zone through the sample pipes by using an oil free minipump. The sample point could be changed by switching the valve installed in the sample pipes. Before flowing into the minipump, the hightemperature sample from the reaction tube was first cooled in a quartz double U-type pipe immersed in the water pool with room

temperature, then condensed by a water condenser, and finally dried by a paper filter. Through the minipump, the sample gas was transported into the immediate gas chromatography (GC) and the online continuous analyzers simultaneously. The concentrations of NO, CO, and CO2 were measured by online infrared gas analyzers. The concentration of O2 was measured by online electrochemical gas analyzer. The concentrations of hydrocarbons (CH4, C2H2, C2H4, and C2H6) and hydrogen were measured by the immediate gas chromatography with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The precision for the concentration measurement of NO is better than 1% and for the concentration measurement of CO, CO2, and O2 is better than 5%. The concentration of HCN was measured using a detector tube made by GASTEC Corp. in Kanagawa, Japan. A certain volume of sample gas passes through the detector tube, and HCN contained in the sample gas would react with HgCl2 inside the detector tube, which would cause a change in color. Through measuring the color change length along the detector tube, the concentration of HCN could be determined. The precision for the concentration measurement of HCN is 5%. The measurements of the concentrations of NH3 have not been included here, because NH3 has low significance for the interactions between NO and hydrocarbons.9,10,12 The experiment of the methane reburning process with NO component was carried out first, and the time evolutions of the components that appeared during the process were analyzed. In the experiment, the gas composition of the simulated flue gas was similar to that of the actual flue gas at the exit of the primary combustion zone of a bituminous coal fired boiler, and methane was added as reburning fuel. Another experiment of methane combustion process without NO component was also carried out. By comparing the results of the two experiments, the characteristics of each component appeared in the reburning process and the combustion process could be further investigated. In both cases, the flow rate of the simulating flue gas at the entrance of the reburning zone was fixed at 120 L/h, the concentrations of the components of CO2, O2, and H2O in the simulating flue gas were the same, and the values were kept as 15, 2.6, and 7.8%, respectively. The concentration of NO was 973 ppm in the case of the methane reburning process and 0 ppm in the case of the methane combustion process. The concentrations of methane, which was added into the reactor, were the same in both cases with and without the NO component. In the experiments, the concentration of methane 9835

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Figure 3. Time evolutions of O2 and CO in the conditions with and without NO.

Figure 4. Time evolutions of CH4 and H2 in the conditions with and without NO.

added into the simulating flue gas is 2.0%, and the stoichiometric ratio is 0.93. Here, the stoichiometric ratio (SR) is the ratio of the actual oxygen amount available with the total theoretical air value for the full combustion of CH4. The reaction temperature was fixed at 1100 °C for both cases. Corresponding to the constant flow rate of 120 L/h of the simulated flue gas used in the two cases, the gas residence time in the whole reaction zone was about 1.5 s, and the gas residence time between two sample points was about 0.1 s. For each experiment, the measurements were done from the first point to the last point, and then from the first point to the last point again. The concentrations coming from the same point have a good agreement.

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Characteristic of the Time Evolutions of Methane Reburning Process. The time evolutions of CH4, C2H2,

C2H4, C2H6, H2, O2, NO, HCN, and CO in the reburning zone are shown in Figures 3 7. In the figures, the results from 0.2 to 1.0 s have been presented. As shown in the figures, it was found that, at 0.2 s, the concentrations of CH4 and O2 sharply decrease to about 9500 ppm and 0.5%, while the concentration of NO only decreases to about 750 ppm. For CO, the concentration increases to about 0.56% at 0.2 s. It was suggested that, in the first 0.2 s, the strong oxidation of CH4 by O2 strongly occurs. In Figure 3, it is found that the changing trends of the concentrations of O2 and CO are different with the reaction

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Figure 5. Time evolutions of C2H2 and NO in the conditions with and without NO.

time. The concentration of O2 sharply decreases from 0.5 to 0.2% during 0.2 0.5 s and then is maintained as invariable in the last 0.5 s, while the concentration of CO increases from 0.56 to 1.11% during 0.2 1.0 s. As shown in Figure 4, it is found that the changing trends of the concentrations of CH4 and H2 are also different with the reaction time. As the reaction time increases from 0.2 to 0.5 s, the concentration of CH4 sharply decreases from 9453 to 4419 ppm. In the successive 0.5 s, the concentration of CH4 decreases slowly to 3627 ppm. The concentration of H2 increases gradually from 1573 to 3414 ppm during 0.2 1.0 s. Figure 5 7 shows the time evolution of C2H2, C2H4, and C2H6 during 0.2 1.0 s. It could be seen from the figures that the changing trend of the concentration of C2H2 is totally different with the changing trends of the concentrations of C2H4 and C2H6. The concentration of C2H2 sharply increases from 631 to 1130 ppm during 0.2 0.5 s and then is maintained at about 1200 ppm in the last 0.5 s. The concentrations of C2H4 and C2H6 both slowly decrease, the concentration of C2H4 decreases from 236 to 52 ppm during 0.2 1.0 s, and the concentration of C2H6 decreases from 64 to 0 ppm within the same time. In Figure 4, the concentration of NO decrease from 736 to 363 ppm during 0.2 1.0 s, while HCN increases from about 78 to 288 ppm. It could be calculated that there is about 43% of the NO reduction for converting to HCN during the whole 1.0 s and about 57% for converting directly to N2. Through the observation of Figures 3 7, it could be found that the concentrations of O2, CH4, C2H4, C2H6, and NO decrease during 0.2 1.0 s, the concentrations of C2H2, CO, H2, and HCN increase within the same time. And the concentration of C2H2 sharply increases during 0.2 0.5 s as the concentration of CH4 sharply decreases within the same time. It could be suggested from the above results that the increasing of the concentration of C2H2 is resulted from the decreasing of the concentration of CH4. However, it is hard to distinguish which hydrocarbon is the most effective one for reducing NO. To further investigate the effects of these hydrocarbons on reducing NO in the methane reburning process, the experiment of the time evolution of the methane combustion process without NO component was also carried out. And the experimental results were also shown in Figure 3 7. 3.2. Comparison of Time Evolutions of Methane Reburning and Combustion Process. In Figure 3, it could be seen that, in the condition without NO, the concentration of O2 sharply decreases from 0.4 to 0.1% during 0.2 0.4 s, and maintains invariable during 0.4 0.7 s, then keeps as 0.0% in the last 0.3 s. The concentration of CO in the condition without NO increases 9836

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Figure 6. Time evolutions of C2H4 and HCN in the conditions with and without NO.

Figure 7. Time evolutions of C2H6 in the conditions with and without NO.

from 0.71 to 1.29% during 0.2 1.0 s. Through comparison of the time evolutions of O2 and CO in the two conditions, it was found that the concentrations of O2 and CO have similar changing trends, respectively. The differences are that the concentration of O2 in the condition with NO is always higher than that in the condition without NO, while CO is always lower. It could be suggested from the results that the component of NO in the reburning process promotes the decreasing of the production of CO. In Figure 4, it could be seen that, in the condition without NO, the concentration of CH4 sharply decreases from 8359 to 4415 ppm during 0.2 0.5 s and then decreases slowly to about 3611 ppm in the successive 0.5 s. The concentration of H2 in the condition without NO increases gradually from 1760 to 3683 ppm during the whole reaction time. Through comparing of the time evolutions of CH4 and H2 in the two conditions, it was found that the changing trends of the concentrations of CH4 and H2 are similar during 0.2 1.0 s, respectively. The differences are that the concentration of CH4 in the condition without NO is lower than that in the condition with NO at the first 0.5 s, while H2 are always higher than that in the condition with NO. In Figure 5, it is shown that, in the condition without NO, the concentration of C2H2 sharply increases from about 761 to 1236 ppm during 0.2 0.5 s and then is maintained at about 1265 ppm in the last 0.5 s. The concentration of NO is certainly kept as 0 ppm during 0.2 1.0 s in the condition without NO. Through comparison of the time evolutions of C2H2 in the two conditions, it could be found that the changing trends of the concentrations of C2H2 are similar. The differences are that the concentration of C2H2 in the condition without NO is higher than that in the condition with NO during 0.2 1.0 s.

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Figure 8. NO and HCN results of validation experiments.

In Figure 6, it is shown that the concentration of C2H4 in the condition without NO decreases from 237 to 45 ppm during 0.2 1.0 s, and the concentration of HCN remains at 0 ppm during 0.2 1.0 s. Through comparison of the time evolutions of HCN and C2H4 in the two conditions, it is found from the figure that the concentration of C2H4 in the condition without NO is slightly lower than that in the condition with NO during 0.2 1.0 s. For HCN, the concentration in the condition without NO is kept at 0.0 ppm during the whole process, while it increased in the condition with NO. Therefore, it could be concluded that HCN which appeared in the reburning process should be a result from NO. In Figure 7, it could be seen that the concentration of C2H6 in the condition without NO decreases from 55 to 3 ppm during 0.2 1.0 s. Through comparison of the time evolutions of C2H6 in the two conditions, it is found that the concentrations of C2H6 in the condition without NO are always lower than those in the condition with NO during 0.2 1.0 s. From the comparison of the figures, it could be seen that the changing trends of the concentrations of O2, CH4, C2H4, C2H6, C2H2, CO, and H2 in the condition without NO have similar changing trends with those in the condition with NO, respectively. The concentrations of O2, CH4, C2H4, and C2H6 in the condition without NO also decrease during 0.2 1.0 s, while the concentrations of C2H2, CO, and H2 increase with the reaction time. By comparison of the concentrations of CH4, C2H2, C2H4, and C2H6 in the two conditions with and without NO, respectively, it could be found that the concentrations of CH4, C2H4, and C2H6 in the condition with NO are slightly higher than these in the condition without NO. However, the concentration of C2H2 in the condition with NO is obviously lower than that in the condition without NO. It could be suggested that the component of NO could promote the decreasing of the concentration of C2H2 and the increasing of the concentrations of CH4, C2H4, and C2H6. Therefore, it could be concluded from the comparison of the experimental results that C2H2 is the key hydrocarbon in the methane reburning process, and the NO reduction by methane as the reburning fuel mainly proceeds via the reaction with C2H2. The experimental results prove the simulated results in the literature,22 which shows that the first step of methane as the reburning fuel to reduce NO emission is through converting methane to C2H2. 3.3. Further Validating of the Effect of C2H2 on NO Reduction. For further validating that the C2H2 is the key hydrocarbon for reducing NO, the experiments were done by using CH4, C2H4, C2H6, and C2H2 as the reburning fuel, respectively. The concentrations of the components in the simulating flue gas are the same as the prior experiments, and the temperature was fixed at 1100 °C. 9837

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Industrial & Engineering Chemistry Research Figure 8 shows the experimental results for every case with the same reaction time of 0.2 s. It could be seen that C2H2 actually has the most significant effect on reducing NO. The concentration of NO decreased from 973 to 100 ppm as the stoichiometric ratio is only about 0.99, and the NO reduction efficiency is about 90%. But for CH4, C2H4, and C2H6, the NO reduction efficiency is only about 28, 35, and 45% at the same stoichiometric ratio, respectively. It could also be seen from the figure that the amount of HCN in the case of C2H2 is higher than that in the other cases. It was suggested that HCN is more easily produced from the NO reduction with C2H2 . Even considering the production of HCN in all cases, the concentration of NO + HCN in the case of C2H2 is also significantly lower than that in other cases. The reduction efficiency of NO + HCN still achieves about 80%.

4. CONCLUSIONS In this paper, the methane reburning and combustion process was investigated on an electrically heated test rig with a nonintrusive multipoint sampling analysis system. The time evolutions of all of the components that appeared in the processes had been analyzed. Conclusions were made as follows. (1) In the methane reburning process, the concentrations of O2, CH4, C2H4, C2H6, and NO decrease during 0.2 1.0 s, while the concentrations of C2H2, CO, H2, and HCN increase within the same time. And the concentration of C2H2 sharply increases during 0.2 0.5 s as the concentrations of CH4 sharply decrease within the same time. It could be suggested that the increasing of the concentration of C2H2 is resulted from the decreasing of the concentration of CH4. (2) Through comparing the experimental results in the conditions with and without NO, the changing trends of CH4, C2H2, C2H4, C2H6, H2, O2, and CO are similar. The concentrations of CH4, C2H4, and C2H6 in the condition with NO are slightly higher than these in the condition without NO, while the concentration of C2H2 in the condition with NO is obviously lower than that in the condition without NO. This experimental result proves that C2H2 is the key hydrocarbon in the methane reburning process, and the NO reduction by methane as the reburning fuel mainly proceeds via the reaction with C2H2. (3) Through comparing the experimental results in the two conditions, it also suggests that the increasing of HCN in the reburning process should be a result from the decreasing of NO, and the decreasing of NO in the reburning process could promote the decreasing of the production of CO. (4) In the further validating experiments, C2H2 shows the most significant effects on NO reduction. In the case with C2H2, the NO reduction efficiency is about 90%. But for CH4, C2H4, and C2H6, the NO reduction efficiency is only about 28, 35, and 45% at the same stoichiometric ratio, respectively. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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’ ACKNOWLEDGMENT Support by the National Basic Research Program of China under Grant 2006CB200300 is gratefully acknowledged. 9838

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