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800, Dongchuan Road,. Minhang District, Shanghai, PR China 20024. ReceiVed January 25, 2007. ReVised Manuscript ReceiVed July 19, 2007. In this paper ...
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Energy & Fuels 2007, 21, 3134–3143

Experimental Study on Soot Combustion and Its Noncatalyzed Reaction with NO Guanglu Xie,* Weidong Fan, Zaile Song, Jie Lu, Juan Yu, and Mingchuan Zhang School of Mechanical & Power Engineering, Shanghai Jiao Tong UniVersity, No. 800, Dongchuan Road, Minhang District, Shanghai, PR China 20024 ReceiVed January 25, 2007. ReVised Manuscript ReceiVed July 19, 2007

In this paper, the combustion characteristic of soot was investigated in a quartz tube fixed-bed reactor. The soot was generated from the natural gas diffusion flame with different oxygen concentrations (20%, 15%, 10%, and 5%). Noncatalyzed reaction between the soot and NO was also investigated in different NO concentrations (∼200 ppm to ∼1500 ppm). The candle soot, the butane soot, and the coal coke were chosen as references. The ignition temperature of natural gas soot is lower than that of the coal coke. Oxygen concentration has a little influence on the ignition temperature of different samples but a great influence on the combustion process. Reactions of different samples under different NO concentrations have definite initial reaction temperatures and quite good regularities. Compared with other samples, natural gas soot has the highest NO removal rate and lowest initial reaction temperature. Increasing NO concentration will enhance the initial reaction temperature of the soot. On the other hand, the denser the NO concentration is, the lower the reaction rate and the higher reduction proportion of NO. The moisture content plays an important roll in the natural gas combustion, which can increase the soot combustion rate evidently. Moreover, the higher the moisture is, the lower the initial reaction temperature. However, impact of moisture on maximum NO reducing rate of soot is not obvious. Analysis reveals that although soot needs a higher combustion activation energy than the coal coke, it has lower reduction activation with NO. Transmission electron microscopy pictures demonstrate that natural gas soot has a layer microstructure. Both candle and butane soot have quite similar chain microstructures. Such differences among these microstructures lead to the discrepancy of macro-reaction performances among different samples.

1. Introduction Usually, large-scale combustion equipment, such as some industrial furnaces and boilers, take natural gas as the fuel.1 Under premixed combustion circumstances, the flame is blue or achromatous (weak flame), with quite weak radiative capacity. To strengthen radiative heat transfer, diffusion combustion is adopted to produce the soot. Afterwards, the soot should be oxidized by post-combustion with air. On the other hand, there is a close relationship between soot and NO. Recently, removal of NO from exhaust gas has drawn more attention. Schlogl2 pointed out that the soot itself has a very strong reducibility, which can lessen the NOx in the late phase of combustion. For instance, within the exhaust gas pipes of a gas engine, the soot can reduce the NO in a catalytic atmosphere. Hence, using carbonaceous materials to reduce the NO in exhaust gas becomes a very important method in relevant fields. Compared with the zeolite catalytic technology, one notable advantage of the carbonaceous materials should be the effectiveness under the concurrence of oxygen and moisture content. Normally, there exist both soot and NO in the exhaust gas so that many researchers are engaged in studying the direct- and catalyticreductive mechanism between soot and NO.3,4 Meanwhile, some results showed that the soot can decrease local flame temperature * Corresponding author. E-mail: [email protected]. Tel: 86-2134205696. Fax: 86-21-34206115. (1) Niksa, S.; Cho, S. Energy & Fuels 1996, 10, 463. (2) Schlogl, R. In Handbook of heterogeneous catalysis; Erti, G., Knozinger, H., Weitkamp, J., Eds.; VCH: Weinheim, 1997; p 183. (3) Sui, L.; Yu, L.; Zhang, Y. Energy Fuels 2007, 21, 1420. (4) Chan, T. L.; Cheng, X. B. Energy Fuels 2007, 21, 1483.

several hundreds of degrees, which brings a great benefit for the natural gas combustion equipment, where thermal NOx acts as a main pollutant. As a kind of carbonaceous material, soot comes into being as solid granules in the high temperature region during combustion. However, seldom can publications be found to investigate how the soot directly reduces NO in the flames, which earns profound theoretical and applied meaning. On the other hand, the size of soot granules belongs is nano-phase. If these fine particles were discharged in the air directly, they would pollute the environment horribly. Simultaneously, the resultant soot will lower the combustion efficiency and contaminate the convective heat exchange system of boilers weakening the convective heat exchange rate. To ensure the combustion’s stability, appropriate amounts of soot should be generated to form a radiation flame at the early stage of the natural gas combustion. The residual soot must be burned out before it leaves the reaction region, bringing higher energy efficiency. To realize the purpose mentioned above, generation of the soot and its post-oxidation must be manipulated through certain technical approaches. Therefore, through studying the combustion characteristics (e.g., ignition and burn-out characteristics) of soot under noncatalytic conditions and reductive reaction between the soot and NO, the present paper attempts to figure out how the soot intensifies the radiation heat transfer in the flame and to investigate mechanisms of direct reductive reaction between the soot and NO. 2. Experimental Methods In the practical experimental process, thermal gravimetric analysis (TGA) is always employed, and the change in soot mass can be

10.1021/ef070046e CCC: $37.00  2007 American Chemical Society Published on Web 09/27/2007

Soot Combustion and Its Reaction with NO switched to its conversion rate. But for a conventional TGA, the heat transfer loss and the limitation of the external mass transfer are two fatal problems to be overcome.5,6 In his early study, Yezerets found that the aforementioned problems can be counteracted when the oxidation rate is low enough, and the experimental data of TGA and a fixed-bed reactor could match with each other well.7 Compared with TGA setup, the reacting gas can contact with soot particles better in a fixed-bed reactor. Furthermore, mixtures of high ratio of inertial diluent (R-Al2O3) to soot can avoid the thermal loss in the reaction process approvingly. Consequently, a fixedbed reactor was employed in this paper. When the inhomogeneity of oxygen and NO concentration in actual flames or gases was considered, different concentrations of soot were tested. 2.1. Samples. The soot samples were collected from a diffusion torch by the combustion of a kind of natural gas used in a power plant (methane content 94%) under laboratory conditions. A series of combusting oxidation and NO reduction experiments were made to simulate the noncatalytic combusting process and reduction of NO in the flames. Some other kinds of soot and coal coke were also chosen as references. As a comparison, the candle soot and the butane soot were collected from diffusion torches of Bunsen burner type, which represent the soot generated from the combustion of organic substances with complex molecule structures. At the same time, a kind of coal coke (Shenmu Coal) was chosen as the reference sample, produced within a muffle furnace according to standard procedures. Before the experiment, coke was ground fully. R-Al2O3 served as the inert substances to dilute the soot or the coke samples, which is taken as a common treatment to investigate reaction between soot and NO in fixed-bed reactors. Yang et al.8 took the composition with the ratio 40:3 of the soot to the silicon carbide as a soot bed. And, Ciambelli9 used 100:1 excessive quartz beads to dilute the samples. Before the experiments, blank experiments of R-Al2O3 were done. Moreover, Yang et al.8 argued that the NO would be inevitably decomposed in the gas phase under such circumstances. Accordingly, the tested soot should be treated under inert atmosphere at high temperature, ahead of the experiment. Therefore, all the tested soots were pretreated for 1 h in helium flow at 1000 °C, to get rid of the impurity on the sample’s surface. 2.2. Procedure. As mentioned above, combustion and reduction reactions were carried out in a fixed-bed reactor, that is, a quartz tube with 24 mm inner diameter is put into the furnace, to simulate the combustion temperature and atmosphere inside the furnace. The soot sample (0.008 g) and R-Al2O3 (2 g) were well mixed and ground in an agate mortar for several minutes. Prepared samples were set into the quartz tube, filling the whole tube and forming a cylindrical plug. Both of the two ends of the tube were blocked out with asbestos. As a result, the soot homogeneously disperses in the reaction chamber approximately. The reaction gas goes through the tube and acts on the soot in suspended status. The dispersing concentration of the soot is 1.59 kg/m3, the mass concentration of the soot is 0.4%, the flow rate of the reaction gas is 1 L/min, and the average residence time in the reaction region is 0.077 s. The whole experimental process was carried out in a temperatureprogrammed reaction (TPR) system (Figure 1). Such intelligent temperature controlling equipment heats the samples from the ambient temperature to 1000 °C at a given temperature rising rate and then is maintained at the high temperature for a period of time for thorough reactions. All these steps are crucial in investigating the reaction mechanism and the initial reaction temperature. The discharged gases were continuously monitored with an infrared NO (5) Gilot, P.; Brillard, A.; Stanmore, B. R. Combust. Flame 1995, 102 (4), 471. (6) Neeft, J. P. A.; Hoornaert, F.; Makkee, M.; Moulijn, J. A. Thermochim. Acta 1996, 287 (2), 261. (7) Yezerets, A. Catal. Today 2003, 88 (1–2), 17. (8) Yang, J.; Mestl, G.; Herein, D.; Schlogl, R.; Find, J. Carbon 2000, 38, 715. (9) Ciambelli, P.; Corbo, P.; Parrella, P.; Scialo, M.; Vaccaro, S. Thermochimica Acta 1990, 162, 83.

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Figure 1. Reaction system.

and oxygen analyzing instrument, SOA-7000, made by Shimazu Corporation, Japan.

3. Results and Discussion 3.1. Noncatalytic Combustion Reaction. 3.1.1. Oxygen Concentration CurVes at the Outlet. Many researchers concentrated on the generation and change of products during the combustion. However, reactions 1 and 2 occur synchronously during the soot combustion. The CO/CO2 ratio in the reaction products depends on the oxygen supply and reaction temperature. At low temperature with excessive oxygen, the CO/CO2 ratio is less than 0.12.10 If the temperature is over 600 °C, such a ratio will be enhanced with the increase of the temperature. Nevertheless, the ratio will decrease due to the intervention of the moisture content, which is caused by the shift reaction between the water and the flue gas.11 Ahlström et al.12 put forward that, in the case of the appearance of the moisture content, the CO product is supposed to accept the selectivity of the water and gas balance. At a higher temperature, CO is the primary product. But when the temperature is over 700 °C, CO will be converted to CO2 homogeneously. In fact, such a conversion can be completed at 800 °C.13 a aC + O2 f aCO 2 bC+bO2 f bCO2

(1) (2)

In current experiments, the proportion of CO/CO2 varies continuously with the increasing temperature. Besides, many other factors make the combustion process even more complicated, such as secondary oxidization of CO, reactions among CO, the moisture content, and the flue gas, and so forth. However, judging from eq 3 (eq 3 ) eq 1 + eq 2), by only considering the change of reactant oxygen concentration can the combustion characteristics of the soot be significantly simplified. (a + b)C +

( 2a + b)O f aCO + bCO 2

2

(3)

Figure 2 shows oxygen concentration curves at the reactor outlet for the natural gas soot, the candle soot, the butane soot, (10) Neri, G.; Bonaccorsi, L.; Donato, A.; Milone, C.; Musolino, M. G.; Visco, A. M. Appl. Catal. B 1997, 11, 217. (11) Neeft, J. P. A.; Nijhuis, X.; Smakman, E.; Makkee, M.; Moulijn, J. A. Fuel 1997, 76 (12), 1129. (12) Ahlström, A. F.; Odenbrand, CUIngemar. Carbon 1989, 27, 75. (13) Marcucilli, F.; Gilot, P.; Stanmore, B. R.; Prodo, G. Proceedings of the Twenty-fifth international symposium on combustion; The Combustion Institute: Philadelphia, 1994; p 619.

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Figure 2. Reactor outlet oxygen concentrations with various oxygen supplying for combustion.

Figure 3. Reactor outlet oxygen concentrations for natural gas soot with various vapor supplies.

and Shenmu coal coke with different oxygen concentrations (5%, 10%, 15%, 20%), respectively. It can be observed that the quite regular combustion curves of different samples share similar tendencies. There exists an obvious trough in a certain temperature zone for each curve, which manifests that the combustion reaction occurs in this zone. Different reaction temperature zones correspond to different concentration curves: the higher the oxygen concentration is, the narrower the reaction zone and then the faster the combustion will be. Up to now, few investigations on the noncatalytic soot–O2 reaction with a moist background have been conducted. Because according to the stoichiometry the humidity of flue gas after the natural gas (main component CH4) complete combustion should be 0.1184 mg/mg, it is worth of studying whether H2O in the reaction gas influence the soot combustion performance. Figure 3 illustrates how moisture content affects the combustion process of the natural gas soot, adopting a constant temperature water bath method. It can be observed that several reaction

gases, which have different moisture capacities, have very similar combustion curves. It is known from the diagram that with the increase of the moisture content, combustion curves shift to the temperature ascent direction. 3.1.2. Reaction Rate CurVes. Figure 4 gives combustion rate curves of self-made natural gas soot, candle soot, butane soot, and Shenmu coal coke at different oxygen concentrations. It can be observed that reaction rates have great discrepancies at different oxygen concentrations, even for the same sample. Higher oxygen concentration results in a larger combustion rate and narrower combustion rate peak zone. Influences of moisture capacity on the combustion rate of the natural gas soot are shown in Figure 5, with the oxygen concentration same as that of Figure 3. It is obvious that change in moisture content of the gas has a prominent influence on the combustion rate of the gas soot. Increase of the humidity enhances the peak value of the combustion rate, with the combustion rate peak zone becoming narrower. Also when the moisture gets denser, the curves move in the temperature ascent direction, which is consistent with the results of some other researchers. Neeft et al.11 compared the noncatalytic combustion between a flame soot and a diesel exhaust gas. They found that, for the case of the flame soot, 10% moisture volume concentration in the reaction gas can increase the combustion rate dramatically in comparison with the gas without moisture. As for the diesel soot, there is almost no change with or without moisture content in the reaction gas. However, Ahlström et al.12 drew an opposite conclusion: if 2–10% volume concentration of vapor is added into the O2–N2 reaction gas, the combustion rate of the diesel soot can be increased remarkably. They explained their results as following: the reaction between soot and water can be accelerated as a result of the hydrogen atoms escaping from the adsorption by the oxygen, and the gasification

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Figure 4. Combustion speed of different samples with various oxygen supplies.

Figure 5. Combustion rate of natural gas soot under various moisture concentrations.

Figure 6. Ignition point.

reaction between soot and water leads to the increase of soot surface area evidently. Neeft et al.11 also gave two explanations: first, the gasification reaction between the soot and water brings on the increase of the soot surface area, similar to that of Ahlström et al.;12 second, the wet vapor alters the stability of the oxygen group on the soot granule surface, and the oxygen group has a significant influence on the soot oxidation reaction. 3.1.3. Ignition Temperature. Different researchers gave different definitions of the ignition process due to the different experiment conditions. For the present programmable temperature rising experiment in a fixed-bed reactor, a TGA–differential thermal gravimetry tangential method is used to determine the ignition temperature, as shown in Figure 6. Comparisons of ignition temperatures among different samples are shown in Figure 7. It is evident that ignition temperatures of all samples are higher than that of the ordinary pulverized coal. And, with the exception of the coal coke, there exist almost no great differences of ignition temperatures between the natural gas soot and others, between 480 °C and 540 °C. The butane soot has the lowest ignition temperature, with its minimum value equal to 480 °C. Coal coke earns the highest one, 584 °C. It is

analyzed that self-made soot almost contains no ash and a small quantity of volatile content, which facilitates the ignition. The other way around, coal coke is nearly free of volatile but contains some ashes, which block the combustion and make it difficult to ignite. From the figure, how oxygen concentration impacts the ignition temperature can be observed. The ignition temperature of the natural gas soot, the butane soot, and coal coke is affected by oxygen concentration infirmly, changing in a range of 50 °C. But for the candle soot, the increase of oxygen concentration tends to heighten the ignition temperature. These results are coincident with those of the thermal balance experiments.14 Moreover, Figure 7 indicates influences of moisture content on the ignition temperature of the natural gas soot, with the oxygen concentration identical with the condition of Figure 3. It is clear that enhancing moisture content increases the ignition temperature, and the changing scope is larger than the influence of the oxygen concentration on the ignition temperature of the natural gas soot. (14) Fan, W. D.; Xie, G. L.; Xu, B.; Yu, J.; Zhang, M. C. Journal of Fuel Chemistry and Technology (Chinese) 2005, 33 (5), 550.

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Figure 7. Ignition temperatures of soot samples with various oxygen supplies.

Figure 8. Absorption of natural gas soot to NO with temperature rise under various NO concentrations.

3.2. Noncatalytic Reductive Reaction between Soot and NO. Hitherto, the soot–NO noncatalytic reaction has been rarely investigated. It is still not very clear about the mechanism of the soot–NO reaction, and there are some disagreements about the reaction process according to different gasification conditions. Fundamental research on such a process is supposed to provide valuable instructions for NOx removal. Focusing on the macro-reaction regularities, the complex and disputed group reactions can be neglected. According to the general reaction (eq 4), it is theoretically known that the main component of soot can reduce NO without a strict reaction restriction. Under the noncatalytic condition and with the temperature over 500 °C, the soot will react with NO:15 C + 2NO f N2 + CO2

(4)

At even higher temperatures, the soot will react with NO further: 2C + 2NO f N2 + 2CO

(5)

It is distinct that except for different products, eq 4 has a larger reductive mass rate of NO than eq 5. 3.2.1. Adsorption, Desorption and Reaction Characteristics of NO. First, a blank carrier was used to make a programmed temperature rising reaction. Figure 8 shows the reaction curve of pure R-Al2O3 to NO (concentration 900 cm3 · m-3). It can be seen that there is no adsorption, desorption, and reaction reactivity between the pure R-Al2O3 and NO. Such (15) Illan-Gomez, M. J.; Linares-Solano, A.; Salinas-Martnez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7 (1), 146.

phenomena demonstrate that R-Al2O3 just acts as an inactive and adsorptive filling. Other curves in Figure 8 present the original adsorption reaction of the natural gas soot to NO under different NO concentrations (200–1500 cm3 · m-3), with the temperature increasing at the rate of 30 °C/min. High purity nitrogen is the balance gas, and there is no oxygen or any other content in the reaction gas. In the test, there happens to be the physical adsorption of the tested gas by the soot. When the temperature increases from 20 °C to 700 °C in each curve, there exists adsorption at lower temperatures and desorption of NO at higher temperatures. At low temperatures, concentration of NO is lower than the initial concentration, because NO is adsorbed by the system. This process is very short and can not be observed very clearly. With the temperature rising, NO is desorbed and concentrated gradually. The peak value of NO concentration locates at almost 400 °C. Over 600 °C, the gas in the reaction system enjoys a balance of adsorption and desorption, with the NO concentration coming back to the initial value. Obviously, it is the natural gas soot that causes the NO concentration diversification in the adsorption and desorption curves, which indicates the soot has a strong adsorption ability due to its porous structure. After desorption of NO from the soot, the reaction starts visibly, runs fast, and approaches to the saturation point gradually. Afterwards, with the consumption of the soot, the reaction is weakened and NO concentration comes back. Therefore, curves shown in Figure 8 can be analyzed and divided into the following different phases: desorption phase, reaction starting and acceleration phase, reaction saturation phase, and reaction close and concentration returning phase. It is found that the soot–NO reaction has a definite initial temperature and remarkable reaction regularity at the reaction starting and acceleration phase. The higher the NO concentration is, the larger the curve slope will be, which is coincident with the reaction regularity very well: the higher the NO concentration, the larger the reaction speed will be. 3.2.2. ReductiVe Characteristics of NO. Figure 9 shows original curves of the adsorption reaction of the candle soot, the butane soot, and the coal coke under the same conditions as Figure 8. Similarly, these samples take on comparable adsorption and desorption characteristics. The curves can be also divided into desorption phase, reaction starting and acceleration phase, reaction saturation phase, and reaction close and concentration returning phase. Figure 10 reveals how the desorption quantity changes with the initial concentration of NO in the desorption phase. With the initial NO concentration becoming denser, the desorption quantity of different samples tends to rise. In other words, the higher NO concentration induces more adsorption quantity in the starting phase. As compared with the candle soot, the butane soot, and the coal coke, the natural gas soot has a weaker desorption potential, under the condition that the initial NO concentration is quite high. This hints that natural gas soot has a smaller porous surface area. The comparability of the reaction curves of different samples demonstrates that the reaction processes are alike: different samples react with NO at a definite initial reaction temperature and an obvious reaction regularity. The reaction curve of the same soot illustrates that the higher the NO concentration is, the larger the curve slope will be. Accordingly, the denser NO concentration brings the faster reaction speed. Comparing the curve slopes of different soot samples, it can be discovered that the reaction rate of the natural gas soot is evidently slower than that of the candle soot and the butane

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Figure 9. Absorption of soot samples to NO with temperature rise under various NO concentrations.

Figure 10. Desorption of soot samples to NO under various NO concentrations.

Figure 11. Start temperature of soot samples to NO under various NO concentrations.

soot. This means that the gas soot needs a longer time to reach its saturation phase. Figure 11 displays initial reaction temperatures of different samples reacting with NO. The same method as that of Figure 6 is used to determine the temperature value. It can be observed that the natural gas soot has the lowest initial reaction temper-

Figure 12. NO reducing rates of soot samples under various NO concentrations.

ature (615–675 °C). But for the coal coke, the temperature is over 850 °C, while the candle soot and the butane soot have almost the same initial reaction temperature. Such discrepancy may be caused by different microstructures of the samples and will be analyzed later. Moreover, the initial reaction temperature of the soot rises with the increasing NO concentration. In industrial applications, the NO concentration is normally quite low (lower than 200 cm3 · m-3) during the actual natural gas combustion process. So, it can be known from the aforementioned results that due to the difficulties in complete oxidation of soot in the late stage, a certain quantity of soot rests in the flame, which facilitates the soot–NO reaction at a quite low temperature and brings quite a high reduction rate. Reduction rates of NO by different samples at different NO concentrations are given in Figure 12. Here, the reduction rate of NO ) (inlet NO concentration – outlet NO concentration)/ inlet NO concentration. Each curve shows that with the increasing NO concentration, the maximum reduction rate is decreased. It can be inferred that, at the same soot concentration in the gas flame, NO can be reduced to a greater extent for the case of the thinner atmosphere of NO. Otherwise, if the NO

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Figure 13. NO reducing amount of soot samples under various NO concentrations.

concentration is given, a greater quantity of soot in the gas should be generated to strengthen the degree of reduction reaction in the flame. For different samples, all of them have considerably high maximum reduction rates, over 40% at low NO concentrations. The maximum reduction rate of the natural gas soot is larger than that of others. For a given sample, the maximum reduction rate does not necessarily represent the reduction quantity of NO within the whole process. Hence, by integrating the original reaction curve in various phases, the NO reduction quantity of samples within the initial and acceleration phase, the saturation and close phase, and the whole process are obtained as shown in Figure 13, respectively. During the initial and acceleration phase (Figure 13a), the initial NO concentration has no great influence on its reduction quantity. Meanwhile, some fluctuations are found in the curves, whose tendency slightly appears to be more visible with the increasing NO initial concentration. In the case of coal coke, initial NO concentration affects the reduction quantity of the coal coke to NO more evidently: the latter will be enhanced with the increasing former. But, generally speaking, the reduction quantity of the coal coke to NO is still less than that of the soot. During the saturation and close phase as shown in Figure 13b, reduction quantity of NO by the soot is much more than that in the initial and acceleration phase and increases significantly with the increasing initial NO concentration. On the other hand, the reduction quantity of the coal coke to NO is quite little and does not vary too much with the initial NO concentration. If these two parts are accumulated, the whole reductive mass of the sample to NO can be obtained. As shown in Figure 13c, the reduction amount of NO by all samples augments distinctly with the rising initial reaction concentration. Remarkably, such a conclusion is the reverse of that regularity reflected in Figure 12. Figure 12 illustrates the maximum reduction degree of different samples to NO, that is, the deepness of the reaction,

which is very useful to control the NO emission. Actually, Figure 13 reveals the utilization rate of the samples. Not only does the initial NO concentration have a great influence on the quantity of reduction reaction between samples and NO, but also the kind of samples impacts the reduction quantity of NO. The largest is by the natural gas soot, the candle soot and the butane soot share almost the same tendency, and the smallest is by the coal coke. For the complicated heterogeneous reaction between the samples and NO, there are also many impact factors for the reduction quantity of NO: the microstructure of the samples; the carbon content of the samples; the temperature range when the reaction is occurring, different reductive reaction modes; and so on. Impact of the microstructure will be discussed later. The carbon contents in different samples are different from each other. The carbon content in the soot is quite high, almost over 90%, but it is very low in the coal coke, only 65% in the test; the rest is ash. Obviously, the lower carbon content causes a lower reduction quantity. In addition, the ash will counteract the reductive reaction to NO. Furthermore, reactions in the low temperature region are prone to taking place as eq 4, and reactions in the high temperature region tend to be in form of eq 5. Of course, the two reactions carry through parallel. The temperature being low, reaction 4 will dominate the process, and the reduction percentage of NO will be larger. Among all samples, reaction between the natural gas soot and NO enjoys the lowest initial temperature, as shown in Figure 11. The coal coke and NO can not react with each other when the temperature is lower than 850 °C. The candle soot and the butane soot have almost the same reaction temperature zone. Therefore, by comprehensively considering various factors, a conclusion can be drawn that the natural gas soot has the largest reduction quantity of NO, the coal coke is the least, and the candle soot and the butane soot belong to the middle, with no visible differences between the two.

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fx ) 1 - exp(-kτ)

(

k ) k0 exp -

E RT

(9)

)

(10)

So -

(

ln(1 - fx) E ) k0 exp τ RT

ln Figure 14. Reaction of soot with NO under various vapor concentrations with temperature rise.

3.2.3. Influence of Moisture on ReductiVe Reaction between Soot and NO. The study on the influence of the moisture content on the noncatalytic reductive reaction of the soot to NO has been scarcely reported. According to the stoichiometry, the humidity of flue gas after the natural gas (main component CH4) complete combustion should be 0.1184 mg/mg. Thus, it should be investigated whether the moisture content of the gas has an influence on the reductive reaction of the soot to NO. The key point of such an experiment is how to accurately control the humidity in the reaction gas. After many attempts, the constant temperature water bath method was adopted and gave a quite good result. Figure 14 shows the NO adsorption curves by the natural gas soot, varying with the increasing temperature under conditions of different moisture contents. It is found that the moisture content has no great influence on the reduction quantity of NO, but it has a significant influence on the initial reaction temperature of the soot to NO, as shown in Figure 15; the moister the gas is, the lower the initial reaction temperature will be. 3.3. Dynamics Analysis. For fixed-bed reactor, the reaction rate can be determined through calculating the change rate of reacting gas concentration at the outlet:16 dCx ) kCnx (6) dt Here, subscript x stands for O2 or NO, used to describe the combustion of the sample or the reduction of NO, respectively. In eq 6, k and n are the reaction rate constant and reaction order, respectively. Generally, the value of n calculated according experimental data varies in a notably wide range. As for the reaction between soot and oxygen at low temperature (