Experimental Study of the Effects of CO2 on the Noncatalytic

Energy & Fuels 2009, 23, 2429–2436

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Experimental Study of the Effects of CO2 on the Noncatalytic Reduction Reaction of NO by Carbonaceous Materials Weidong Fan,* Zhengchun Lin, Youyi Li, and Jinguo Kuang School of Mechanical and Power Engineering, Shanghai Jiao Tong UniVersity, No. 800, Dongchuan Road, Minhang District, Shanghai, P.R. China 20024 ReceiVed NoVember 7, 2008. ReVised Manuscript ReceiVed March 11, 2009

In a fixed bed reactor with a quartz tube, the effects of the concentration of CO2 in the feed gas on the uncatalyzed reaction between soot produced in a natural gas diffusion flame and NO were investigated. They were compared with CO2 effects on reactions involving candle soot and bituminous coal char. The presence of CO2 in the feed gas exerted no influence on the reaction of NO with natural gas soot. However, it did result in a lower initial temperature in the reaction of candle soot or coal char with NO, and separated the whole initial reaction process into two stages. At higher CO2 concentrations, more NO reduction occurred in the reaction with candle soot or coal char during the initial reaction process. However, no dramatic changes in the amount of NO reduction were observed for natural gas soot. The addition of CO2 seemed to have no effect on the apparent activation energy of the NO-natural gas soot reaction, while a lowering of the apparent activation energy was observed in the reaction of NO with candle soot or coal char as the CO2 concentration increased. The abundant C(O) complexes formed during sample gasification by CO2 were assumed to play an important role in the enhancement of the reaction.

1. Introduction The reaction of NOx with carbonaceous material exists in various combustion systems extensively. It is necessary to be studied because it is recognized as an important means of reducing NOx emissions during combustion.1 For example, as a carbonaceous material, the dispersion concentration of soot in a flame is nearly the same as the concentration of solid particles in a flame of pulverized coal, but the dispersion effect of the former is better and its particle size (primary structure) is smaller. So, the direct reduction of NO by soot produced in the flame of the combustion cannot be neglected, and research on NO reduction by soot produced during combustion at high temperatures is worth working on. Taking as an example a natural gas combustion device of which the main emission is thermodynamic NO, soot intentionally produced at the initial stage of the combustion can not only increase the radiative conductivity of the flame and enhance radiative heat transfer, but also reduce the NO preformed in the combustion, and this is beneficial for reducing NOx emissions. Taking as another example, for the latest low NOx pulverized coal combustion technique, the technical key is how better the reduction reaction of NO formed in the pulverized coal combustion process by coal char is carried out. As for the char reduction, Tomita2 pointed out that although part of NO generated in the process of coal burning will be quickly reduced by HCN and NH3, which are produced by the pyrolysis of coal, these gaseous products will run out quickly and then char becomes the main reducer * Corresponding author. E-mail: [email protected]. Phone: +86-2134206049. Fax: +86-21-34206115. (1) Yang, J.; Mestl, G.; Herein, D.; Schlogl, R.; Find, J. Carbon 2000, 38, 729–740. (2) Tomita, A. Fuel Process. Technol. 2001, 71, 53–70.

of NO. Goel and Sarofim3 also pointed that the heterogeneous reaction of NO and coal char is the main NO reducing mechanism. Therefore, coal char is very important to the heterogeneous reaction of NO. In a practical combustion process, NO coexists with other gases, such as O2, CO2, etc., so the effect of other gases on a pure NO-carbonaceous material reaction is worth studying. The effect of oxygen on the reduction of NO by carbonaceous material has attracted increasing attention. Many investigators have observed an enhancement in the reduction of NOx by carbonaceous material in the presence of oxygen.1,4,5 As CO2 is usually supposed to be inert during a pure NO-carbon reaction, there has been little systematic study of the effects of CO2 on the reduction of NO by carbonaceous material. However, Suuberg studied the behavior of NO reduction by graphite, Wyodak coal char, and resin coal char at different CO2 concentrations via a TPR experiment carried out in a fixed bed.6 His experiments showed that CO2 is not an inert gas in the reaction system and that the presence of CO2 can strongly inhibit the reduction reaction of NO by these carbonaceous materials. Obviously, it is possible that the CO2 atmosphere has some influence on the reaction of carbonaceous material and NO. And maybe there is the different influence degree on different carbonaceous material. However, CO2 always inhibited the reactions in all cases of Suuberg. Can it say the inhibit function of CO2 is common? Namely, does CO2 inhibit the reduction (3) Goel, S. K.; Morihara, A.; Tullin, C. J.; Sarofim, A. F. Twenty-Fifth Symposium International on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; pp 1051. (4) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. Combust. Flame 1994, 99 (4), 499–507. (5) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1991, 78 (2), L1-L6. (6) Suuberg, E. M.; Aarna, I. Kinetics and mechanism of NOx-char reduction. DOE-PC94218-13; U.S. Department of Energy: Pittsburgh, PA, 1998.

10.1021/ef8009712 CCC: $40.75  2009 American Chemical Society Published on Web 04/17/2009

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Figure 1. Diagram of the experimental setup.

reaction of any carbonaceous material with NO? But there has been little systematic study on its effects aside from the experiments in the reference.6 So, we selected three kinds of carbonaceous materials including two kinds of soot from hydrocarbon flames and a kind of bituminous coal char. In a fixed bed reactor with a quartz tube, the effects of the concentration of CO2 in the feed gas on the uncatalyzed reaction between the three carbonaceous materials and NO were investigated. And its influence mechanism was also deeply analyzed. Meanwhile, we have summarized the effects of CO2 on the kinetics and mechanism of reduction of NO by the three carbonaceous materials. On the one hand, this work discloses the influence trends of CO2 on the reactions of some carbonaceous materials and NO. On the other hand, it also provides a theoretical foundation for exploring the technique of reducing NO using carbonaceous material produced in the flame under rich CO2 condition during combustion.

Table 1. Characteristics of Shen Hua Coal Proximate Analysisa volatile matter

ash

moisture

fixed carbon

net heating value (kJ/kg)

24.22

10.7

11.5

53.58

24136

Ultimate

Analysisa

carbon

hydrogen

sulfur

nitrogen

oxygen

63.13

3.62

0.41

0.70

9.94

CaO 22.23 TiO2 0.78

MgO 0.86

Ash Content SiO2 39.25 SO3 8.55 a

Al2O3 14.48 K2O 1.16

Analysisb

Fe2O3 9.86 Na2O 1.16

wt % (as air-dried). b wt %.

Table 2. Characteristics of Soots Ultimate Analysisa

2. Materials and Methods The experiment was carried out in a fixed-bed reactor. A diagram of the system is shown in Figure 1. The reactor tube was made from refractory quartz glass and has an inner diameter of 24 mm. The heating device is a rotary tube-type electric resistance furnace with a power-handling capability of 2 kW. Temperature programming can be performed using a temperature controller, and the highest achievable temperature is 1300 °C. Two thermocouples were employed in the experiment. Thermocouple Ι was used to measure the actual temperature of the reactant, while thermocouple Π was used to assist in temperature control of the furnace. In this study, a temperature programmed reaction (TPR) experiment was performed to heat the sample from room temperature to 1000 °C, at a certain heating rate, and to keep the sample at the final temperature long enough to allow it to react completely, using an intelligent temperature controller. This method is useful for determining the initial temperature and studying the mechanism of the reaction. The outlet gas was continuously analyzed using an NO infrared analyzer (Shimazu SOA-7000). One of the samples employed was a kind of soot collected by a stainless steel stick in the Bunsen diffusion flame of lightweight hydrocarbon fuel, natural gas (94% methane, flow rate 100 mL/min), which we called natural gas soot. Another soot is one of the samples that collected by a stainless steel stick in the diffusion flame of heavy hydrocarbon fuel, candle,

natural gas soot candle soot a

oxygen

carbon

11.48 1.06

88.52 98.94

wt % (as air-dried).

which we called candle soot. In addition, a kind of bituminous coal char was employed as comparison samples. This bituminous coal char was made of the pulverized coal called Shen Hua coal, which was kept at 900 °C in the muffle furnace for 7 min without air. Here, Shen Hua coal is used extensively in China as a high-quality power coal. The characteristics shown in Table 1 indicate that it is a bituminous coal with a high volatile content. The coal char particles whose diameters were less than 5 µm by pneumatic screen classification were employed as the experiment sample. Obviously, three kinds of samples come from different fuels. They must be different from their macrostructures. Concerning their differences, the detailed analysis will be carried out in the following text. For understanding the surface chemistry of two kinds of soot, their ultimate analyses were carried on respectively. The characteristics shown in Table 2 indicate that two kinds of soot mainly contain carbon element. It implies a certain oxygen element in the meantime. And it still contains very little ash, hydrogen element, and nitrogen element. In the table, only carbon element and oxygen element are shown, and the very little other elements were neglected.

Effects of CO2 on Reaction of NO

Energy & Fuels, Vol. 23, 2009 2431

The past research announces that the oxygen content in soot mainly exists in the form of surface complexes, such as carboxy, quinonyl, phenolic group, inner ester, and so on.7 Therefore, there are abundant surface complexes containing oxygen on the surface of natural gas soot. Each sample (8 mg) was well mixed with R-Al2O3 (2 g). Then, the mixture was put into the middle part of the quartz tube to form a plunger, where it was held in place with two quartz wool plugs. This reaction is equivalent to a suspension reaction in which the soot is dispersed in the space and the reactant gas moves through this space to react with the soot. R-Al2O3 was used to dilute the sample mainly because it is inert under the reaction conditions. It is often used in studying NO-carbon reactions in fixed beds. For example, in experiments carried out by Yang et al., the sample adopted was a mixture of soot with SiC at a mixing rate of 40:3.1 CO2 is one of the products of combustion of carbonaceous materials. For example, in the exhaust gas of the combustion of CH4 with air at chemically equivalent proportions, CO2 accounts for 9.1%. Under the same condition, for the combustion of various coals, CO2 accounts for ∼10-20%. In this experiment, CO2 volume concentrations being close to that of the exhaust gas of the real combustion are employed as experiment parameters. Obviously, it is more valuable for engineering application that some results are obtained from this experiment. In order to consider the difference of CO2 concentration in the different combustion zone, five kinds of CO2 with volume concentrations of 0%, 5%, 10%, 15%, and 20% in the modeling gas were chosen. Only NO volume concentrations of 550 ppm in the modeling gas were chosen for all cases. Highly purified N2 was employed as a carrier gas while other gases such as O2 were of concentration zero. 3. Results 3.1. Influence of CO2 Concentration on the Reduction of NO with Samples. Figure 2 presents the absorption and reaction curves we obtained for the reduction of NO by natural gas soot, candle soot, and coal char in the presence of CO2 at different concentrations for a set temperature change rate (30 °C/min). In Figure 2, the curves corresponding to CO2 concentrations of zero can be divided into four sections: (1) desorption section, (2) initial and accelerating section, (3) saturation section, and (4) the final section, in which the concentration of NO reverts. Obviously, the behavior of NO during the desorption section was due to the strong absorption characteristics of samples at the beginning stage of the reaction. Overall, the CO2 concentration hardly affected the NO-natural gas soot reaction, but it had a great influence on the reduction of NO with candle soot or coal char. In each case, the same experiment was repeated several times, and the same results were obtained. As shown in Figure 3, the initial temperatures for the reactions of NO with candle soot or coal char decreased rapidly with increasing CO2 concentration. The presence of CO2 reduced the initial temperatures of these two reactions, but had very little effect on the initial temperature of the NO reduction reaction with natural gas soot. Taking coal char as an example, the initial temperature of the reaction, when the CO2 concentration was 10%, was about 350 °C lower than that without CO2. In these reactions, the initial and accelerating sections could be separated into two stages. The higher the CO2 concentration, the more NO was reduced in the first stage. These characteristics were highly regular, and changes in the CO2 concentration did not (7) Donnet, J. B. Carbon 1982, 20, 266–277.

Figure 2. Reactivity curves of various samples with NO as a function of temperature at different inlet CO2 concentrations.

Figure 3. Starting reaction temperature curves of various samples with NO at different inlet CO2 concentrations.

affect the saturation section greatly. Figures 4-6 present the influence of the CO2 concentration on the reduction of NO in the initial section, in the saturation section, and in both sections together, respectively. It can been seen that, in the initial and accelerating sections, the amount of NO reduced by candle soot or coal char increased with increasing CO2 concentration, while that reduced by natural gas soot remained the same; in the saturation section, variations in the reduction of NO by the three samples were quite small, while NO reduction by natural gas soot was higher compared to that of the other two samples. This

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Figure 4. Effect of different inlet CO2 concentrations on the reduction amounts by various samples of NO at the initial and accelerating stage of reaction.

Figure 5. Effect of different inlet CO2 concentrations on reduction amounts of NO by various samples at the saturation stage of reaction.

Fan et al.

on the surface of the samples could be expected to produce CO that can accelerate the reduction. However, our study on the effects of CO2 on the reduction of NO by samples does not show any inhibitory effect. As shown in Figure 6, CO2 exerts little influence on the reaction of natural gas soot and NO. On the contrary, it improves the reaction of candle soot or coal char and NO. Therefore, the CO2 gasification reaction is the mechanism of CO2 atmosphere improvement or influence on the reduction of NO by samples. However, a problem is put forward whether CO from the gasification reaction of CO2 and samples influences the reduction reaction. Our analysis can refer to the mechanism analysis cited by Suuberg as following eqs 1-6. A mechanism for the CO2 gasification was recently proposed, involving a two-site adsorption and dissociation of CO2:8,9 CO2 + 2C* T C*

(1)

C** T C(O) + C(CO)

(2)

C(CO) T C* + CO

(3)

C(O) f CO

(4)

C* and C** are a vacant single active surface site and a twosite surface complex, respectively, and C(O) and C(CO) are surface complexes. Reactions 1 and 2 represent the formation of the surface complexes; reactions 3 and 4 are the desorption processes. Formation of CO and the surface complexes would have a significant effect on the reduction of NO by samples. CO can significantly accelerate the reduction of NO by coal char, probably in the two ways shown below. One is through the reaction taking place on the surface of the coal char:10 CO + C(O) f CO2 + C*

(5)

This reaction implies the formation of a vacant active site C*, which will be reduced with increasing temperature. Another way is through the direct catalytic reduction of NO by CO on the surface of the coal char, where coal char is the catalyst:10,11 coal char

NO + CO 98 CO2 +

Figure 6. Effect of different inlet CO2 concentrations on the total reduction amount of NO by various samples during the whole stage of reaction.

is because the saturation section began at a lower temperature in the natural gas soot reaction, so the saturation section lasted longer. The addition curves shown in Figure 6 display the same behavior as that in the initial and accelerating sections. 3.2. Reaction Mechanism Underlying the Effects of CO2 Concentration on the Reduction of NO with Samples. As mentioned in the Introduction, Suuberg systematically studied the effects of CO2 on NO reduction by graphite, Wyodak coal char, and resin coal char, and analyzed mechanisms of the NOx-char reduction.6 His experiments showed that the presence of CO2 could inhibit the reduction reaction of NO by these carbonaceous materials. For this result, with further analysis, Suuberg proposed that, on the one hand, CO2 could be expected to inhibit the reduction due to occupation of the active sites on the surface of the samples. On the other hand, CO2 gasification

1 N 2 2

(6)

The surface complex C(O) produced during CO2 gasification on the surface of the samples could also accelerate the NO reduction, and the production of C(O) is also regarded as a mechanism for the enhancement of NO reduction in the presence of oxygen.1,12,13 Therefore, the effect of CO2 on the reduction of NO by samples depends on the degree of CO2 gasification on the surface, that is, the amount of CO and surface complex C(O) produced during CO2 gasification. But the degree of effect of CO2 on the reduction of NO by samples is very different because of the difference of their amount under the different conditions. We think that this condition is exactly temperature condition. Reactions 1 and 2 can take place under the lower temperature (8) Koenig, P. C.; Squires, R. G.; Laurendeau, N. M. Carbon 1985, 23 (5), 531–536. (9) Koenig, P. C.; Squires, R. G.; Laurendeau, N. M. Fuel 1986, 65 (3), 12–416. (10) Levy, J. M.; Chan, L. K.; Sarofim, A. F.; Beer, J. M. 18th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; pp 111-120. (11) Takehiko, F.; Mikio, T.; Motoki, T.; Tadafumi, A. Fuel 1985, 64 (9), 1306–1309. (12) Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. Fuel 2005, 84, 2275– 2279. (13) Chambrion, P.; Orikasa, H.; Suzuki, T.; Kyotani, T.; Tomita, A. Fuel 1997, 76 (6), 493–498.


Figure 7. Gibbs function increment of the reactions relation with this experiment vs the reaction temperature.

condition, but reactions 3 and 4 take place under the higher temperature condition. The temperature in this experiment is below 1000 °C, but the initial temperature of reduction of NO by the samples is below 800 °C. This belongs to the lower temperature condition. The effect of CO2 atmosphere on the reduction of NO by samples depends on the gasification reactions 1 and 2 of the CO2 and the samples. This analysis is proven as follows. As shown in Figure 2, in the presence of CO2, the initial and accelerating section of the NO-candle soot or NO-coal char reaction can be divided into two stages: a high temperature stage and a low temperature stage. The effect of CO2 concentration during the low temperature stage was very strong, while the CO2 concentration hardly affected the reaction during the high temperature stage, where there is an independent temperature window of range 400-800 °C. However, on the basis of chemical thermodynamics, CO2 gasification that produces CO


occurs to a limited extent during the low temperature stage but is much stronger during the high temperature stage, when the CO-NO reaction is heavily weakened. Figure 7 shows that the increment of Gibbs function of the reaction is related with the variety of temperature. As shown in the figure, even theoretically the reaction never carries on below 700 °C. Therefore, it can be concluded that, during the low temperature stage, CO2 gasification with candle soot or coal char goes through steps 1 and 2, and plenty of C(O) will be produced that will accelerate the reduction of NO. Moulijin and Kapteijin proposed that the decomposition of NO is mainly accelerated by the complexes C(O), produced on the surface of carbon that directly reacts with NO, but not by CO produced during CO2 decomposition at high temperature.14 In the study by Yang et al.,1 it was proven that the presence of CO is not beneficial for the enhancement of the reaction between NO and carbon black, at least at temperatures below 500 °C. This mechanism explains the effect of CO2 on the stepwise reduction reaction of NO by candle soot or coal char. On the other hand, the mechanism of CO2 gasification reaction on the surface of sample can be still analyzed through the analysis of mesoporous structure of the sample. Figure 8 shows the pore structure analysis of the three kinds of experimental sample using the BJH method from the adsorption branch of the N2 isotherm, including the variety of the volume and surface area of inner and exterior pores with pore diameter and total variety. After figure contrast, it is thus clear that two kinds of soot have plenty of micropores, and micropores of natural gas soot are most abundant. Figure 8c shows that the pores with 2 and 40 nm diameter are most abundant for the two kinds of soot. Figure 8c,d also shows that there are differences for the specific surface area values of the three carbonaceous materials. According to their test results, we can

Figure 8. Pore size distribution of samples determined by the BJH method from the adsorption branch of the N2 isotherm.

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Figure 9. TEM images of two types of soot (scale 100 nm).

know that the total specific surface area values of natural gas soot, candle soot, and coal char are about 164.0, 53.2, and 6.9 m2/g, respectively. It is clear that the specific surface area value of natural gas soot is much bigger than those of the other two samples. Therefore, if reactions 3 and 4 can carry on, the gasification reaction of CO2 and natural gas soot will produce the most CO. Then, it results in more reduction of NO by natural gas soot. But Figure 2 shows that CO2 atmosphere has hardly any influence on the reduction of NO by natural gas soot. It explains that the CO production of reactions 3 and 4 is weak. So, plenty of C(O) through reactions 1 and 2 is the most major mechanism of influence of CO2 atmosphere on the reduction of NO by the samples employed by this experiment. However, Figure 2 indicates CO2 atmosphere exerts an obvious influence on the reduction of NO by candle soot or coal char. This is because reaction 1 is through an occurrence on the vacant active surface site. If there were not vacant active surface sites, the gasification reaction of CO2 and sample would be quite faint. The active sites on the surface of natural gas soot have been occupied, and have no vacant active surface sites. This conjecture derives from Table 2. The containing amount of oxygen of natural gas soot is up to 11.48%, and is 11 times larger than that of candle soot, but total specific surface area of pores of natural gas soot is only 3 times larger than that of candle soot. The oxygen exists with the form of surface C(O).15 So it explains that there are not any vacant active sites on the surface of natural gas soot. Although the total specific surface area of pores of candle soot or coal char is smaller, the active sites on its surface are empty. So, reaction 1 of CO2 and the active sites easily takes place in the low temperature stage to produce C(O). Thereby these C(O) species promote the decomposition of NO on their surface. Certainly, natural gas soot having abundant surface C(O) is also why it results in the decomposition of NO under the much lower temperature. As shown in Figure 3, the initial temperature of reduction of NO by natural gas soot is lowest without CO2 atmosphere. The increase of CO2 concentration can lower the initial temperature of reduction of NO by candle soot or coal char, exactly resulting from a great deal of C(O) producing on its surface. For coal char, there is a high calcium content that has proven to be a good catalyst for CO2 gasification.16 The catalytic role of calcium was found to be analogous to the role it has in carbon gasification, that of (14) Moulijn, J. A.; Kapteijn, F. Carbon 1995, 33 (6), 1155–1165. (15) Zhang, W. X. Ph.D. Thesis, Si Chuan University, China, 2000. (16) Muralidhara, H. S.; Sears, J. T. Coal Process. Technol. 1978, 4, 22–25.

Figure 10. SEM image of coal char.

increasing the concentration of carbon-oxygen complexes on the carbon surface.17 The calcium content can be determined from the presence of CaO with a mass percentage of about 20% in the ash, as shown in Figure 1. So, the effect of CO2 on the NO-coal char reaction is much more significant. In this research, the porous structures of the three samples have been observed in order to disclose the variations in the reduction reactions of NO with the three samples. Microanalysis of the soot samples was carried out using TEM. Figure 9 presents the microstructures of natural gas soot and candle soot for comparison. TEM images visually represent that natural gas soot is of laminar structure (which can be taken as its grouping body structure). The layers are of distinct rough peripheral outlines with sharp edge angles on the surface. The edge angles are propitious for the reaction, as they make the soot more accessible so that the initial stage will start at a lower temperature. This is another thing that can be seen in the lowest initial temperature in the reaction without CO2, shown in Figure 3. Candle soot is composed of small spherical carbon particles in a chain, and is very regular. The carbon particles have smooth surfaces, which have completely different structures compared to the laminar structure of natural gas soot. Because they have closer structure, light transmission of the coal char particles for TEM is worse. As a result, the effect of their TEM pictures is not good. Microanalysis of the coal char was carried out using SEM. As shown in Figure 10, the coal char particles are really closely combined, but some micropores still exist among the particles. Also, the shape of particle is also very irregular. (17) IllBn-G6mez, M. J.; Linares-Solano, A.; Radovic, L. R.; de Lecea, C. S. Energy Fuels 1995, 9, 112–118.



3.3. Analysis of the Kinetics and Mechanism of the Reaction. For a fixed bed reactor, the reaction rate can be determined from the changes in the concentration of the outlet gas:18 dCNO n ) kCNO (7) dt Most of the literature agrees that the reaction order of an NO-carbon reaction is approximately 1.6,19 So, the apparent reaction order is supposed to be 1 in our case. If eq 7 is integrated, it becomes wNO ) -

CNO ) - kτ C NO0

(8)

CNO ) exp(-kτ) CNO0

(9)

fNO ) 1 - exp(-kτ)

(10)

ln

( RTE )

k ) k0 exp -

(11)

so that ln(1 - fNO) E ) k0 exp (12) τ RT ln(1 - fNO) E (13) ) ln k0 ln τ RT in which τ ) (AεL)/(F) · (PM)/(RgT), and where fNO is the NO transformation rate; T is the residence time of the reactant gas in the bed; F is the mole flow of the reactant gas; A and L are the cross-sectional area and length of the bed, respectively; ε, P, T, and M are bed porosity, bed pressure, bed temperature, and average gas molar weight, respectively; and Rg is the molar gas constant. According to eq 13, the Arrhenius plot of the reaction can be obtained by plotting the log function against 1/T. The Arrhenius plots for the NO reduction reaction by the different samples in the presence of CO2 at different concentrations are shown in Figure 11. It can be concluded from Figure 11 that the kinetics of the reactions with the different samples at different CO2 concentrations can be represented as first-order reactions. The kinetics of the NO reduction by natural gas soot is characterized by a single temperature region, while that for candle soot and coal char is of a multitemperature region character, which is related to their multiple-stage reactions at the initial and accelerating section. Although multiple-stage activation energy can represent a stepwise reaction at different temperatures, it cannot characterize the whole reaction. Cumming proposed the concept of a weighted-average apparent activation energy, Em ) f1E1 + f2E2 +... + fnEn, where f1-fn are the reaction percentages in each reaction region.20 A comparison of the average apparent activation energies of candle soot and coal char to that of natural gas soot calculated from the data in Figure 11 is shown in Figure 12. Natural gas soot has the lowest average apparent activation energy, approximately 150 kJ mol-1, in the absence of CO2, and the effect of CO2 on its activation energy is small. The effects of CO2 on the average apparent activation energy of candle soot and coal char are quite large. Their average apparent activation energies decrease -

(

)

( )

(18) Aarna, I.; Suuberg, E. M. Fuel 1997, 76 (6), 475–491. (19) Wongtanakitcharoen, S.; Tatiyakiatisakun, T.; Rirksomboon, T.; Long, R. Q.; Osuwan, S.; Malakul, P.; Yang, R. T. Energy Fuels 2001, 15, 1341–1346. (20) Cumming, J. W. Fuel 1984, 63 (10), 1436–1440.

Figure 11. Arrhenius graphs for various samples reacting with NO at different inlet CO2 concentrations.

Figure 12. Effect of different inlet CO2 concentrations on the activation energy of the reduction of NO by various samples.

dramatically with increasing CO2 concentrations, especially for coal char, where the average activation energy decreases the most before stabilization while the concentration of CO2 increases from 0% to 10%. The decrease in activation energy makes reaction at a lower temperature possible and is the reason the initial temperature of the NO reduction by candle soot and coal char decreases with increasing CO2 concentration.

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There have been very few studies of the effects of CO2 on the kinetics of NO reduction by carbonaceous materials. We drew a conclusion through our experiments and determined that CO2 affects the reduction mainly by producing large numbers of C(O) complexes during low temperature gasification. Usually C(O) will strongly affect the kinetics of the reaction by accelerating the reaction and reducing the activation energy, which can be evidenced by the work carried out by Suuberg on the effect of O2 on NO reduction by coal char.6 He obtained the activation energy for NO reduction by a kind of pine wood coal char through a TPR experiment carried out in a fixed bed at a temperature range 660-1100 K. In the absence of O2, the activation energy is of double-temperature region character, being 168 kJ mol in the high temperature region and 27 kJ mol in the low temperature region, while that for reaction with an O2 concentration of 2.02% is of a single-temperature region character and decreases to 53 kJ mol. The decrease in activation energy is mainly due to large quantities of C(O) produced on the surface of the coal char in the presence of O2. In our study, the decrease in the average activation energy for candle soot and coal char is also evidence of the production of C(O) during low temperature gasification, through which CO2 affects the reduction reaction. 4. Conclusions The CO2 concentration hardly affects the reduction of NO with natural gas carbon, while it has a significant influence on that with candle soot or coal char. The higher the CO2

Fan et al.

concentration, the lower the initial temperatures of the reduction of NO with candle soot or coal char. The initial and accelerating section of the reduction of NO with candle soot or coal char can be divided into two stages; the higher the CO2 concentration, the higher the amount of NO reduction in the first stage, while CO2 has a very small effect on the saturation stage. With increasing CO2 concentrations, the amount of NO reduced by candle soot or coal char increases greatly during the initial and accelerating stage, while that for natural gas soot remains the same. Formation of a complex, C(O), during CO2 gasification at low temperatures is the mechanism of accelerating the NO reduction. CO2 hardly affects the activation energy of NO reduction with natural gas soot. With increasing CO2 concentration, the average apparent activation energy of the reduction of CO2 with candle soot or coal char decreases dramatically, which also leads to decreases in the initial temperature of the reaction. The decrease in average activation energy for candle soot or coal char is also evidence of the production of C(O) during low temperature gasification, through which CO2 affects the reduction reaction. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant 50876061). We thank the Testing Center of Shanghai Jiao Tong University for providing TEM tests and services. EF8009712

Experimental Study of the Effects of CO2 on the Noncatalytic

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