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Energy & Fuels 2007, 21, 3208–3215
Oxidation of Acetylene Soot: Influence of Oxygen Concentration T. Mendiara,* M. U. Alzueta, A. Millera, and R. Bilbao Aragón Institute of Engineering Research, Department of Chemical and EnVironmental Engineering, C/María de Luna, 3 (Torres QueVedo building), UniVersity of Zaragoza, 50018, Zaragoza, Spain ReceiVed April 11, 2007. ReVised Manuscript ReceiVed July 13, 2007
Soot was produced from acetylene pyrolysis at 1100 °C using 50 000 ppmv acetylene. At 900 and 1100 °C, the influence of the oxygen concentration in a range of 100–30 000 ppmv was analyzed. Applying the shrinking core model with decreasing particle size and chemical reaction control, the reaction order with respect to oxygen was obtained, which turned out to be 1. Experiments at 850 and 1000 °C with an oxygen concentration of 500 ppmv were also done to extend the study of the temperature influence. The activation energy of the soot oxidation process was also obtained and resulted in a value of 42.8 kJ/mol. Additionally, another soot with different structural characteristics was obtained by acetylene pyrolysis at 1100 °C, using 5000 ppmv acetylene. Its oxidation over a range of oxygen concentrations (250–20 000 ppmv) was studied. The shrinking core model with chemical reaction control was also applied, and the reaction order with respect to oxygen was obtained, again being equal to unity.
Introduction The small sized carbonaceous particles known as soot are formed during different combustion processes, mainly in fuelrich zones.1 The amount and structural characteristics of the generated soot depend on different variables of the process, such as the type and concentration of fuel, temperature, and residence time. Soot formation represents a serious problem for the design and operation of combustion systems,2 because it leads to a reduction in the efficiency of the process. Additionally, there are legal restrictions on soot emission values from combustion systems. It is also well-known that soot formation is accompanied by the generation of a great diversity of hydrocarbons. Some of these, in particular some polycyclic aromatic hydrocarbons (PAHs), may be retained inside soot particles and may have carcinogenic effects on humans.3 Given these circumstances, it is necessary to develop and implement techniques that allow the amount of soot produced in a combustion system to be controlled. One possibility may lie in the “in situ” elimination of soot through its oxidation. Soot is composed mainly of carbon. In the literature, the generic carbon–oxygen reaction has been addressed in many works resulting in various different oxidation mechanisms.4 Nevertheless, there is general agreement that CO and CO2 are the most important products of this reaction. It is assumed that both CO and CO2 are produced from desorption of surface carbon–oxygen complexes. The characteristics of these surface complexes play a decisive role in the oxidation reaction.5 * Corresponding author. Phone: +34-976-761880. Fax: +34-976-761879. E-mail address:
[email protected]. (1) Bozzano, G.; Dente, M.; Faravelli, T.; Ranzi, E. Appl. Therm. Eng. 2002, 22, 919–927. (2) Kennedy, I. M. Prog. Energy Combust. Sci. 1997, 23, 95–132. (3) Stanmore, B. R.; Brilhac, J. F.; Gilot, P. Carbon 2001, 39, 2247– 2268. (4) Phillips, R.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1970, 8, 205– 210. (5) Petersen, R. C. The Oxidation Rate of Diesel Particles which Contain Lead; SAE Paper 870628; Society of Automotive Engineers: Warrendale, PA, 1987.
Table 1. Reaction Order with Respect to Oxygen for the Oxidation of Different Kinds of Soot According to Different Authors material
order 1
fractional order
(0.5) Petersen 19875 Otto et al. 19816 Miyamoto et al. 19887 (0.65) Ahlström and Odenbrand 19898 (0.76–0.80) Neeft et al. 19979 (0.61) Yezerets et al. 200510 flame soot (0.83) Du et al. 199112 commercial Gilot et al. 199311 (0.8) Ciambelli et al. 199413 carbon black (0.85–0.94) Neeft et al. 19979 diesel soot
Several studies have been reported for diesel soot oxidation5–10a, but studies relating to the oxidation of soot formed from hydrocarbons are scarce. Oxidation results depend on the origin of the soot and the experimental conditions. Many of the kinetic models employed to describe soot oxidation are based on power kinetic equations, and different reaction orders with respect to oxygen have been proposed, as is shown in Table 1. In this context, the objective of the present work has been to carry out an experimental and kinetic investigation into the oxidation of the soot formed from acetylene pyrolysis. Acetylene was chosen as a reactant because it is known to be one of the most important precursor species in soot formation from various (6) Otto, K.; Sieg, M. H.; Zinbo, M.; Bartosiewicz, L. The Oxidation of Soot Deposits from Diesel Engines; SAE Paper 800336; Society of Automotive Engineers: Warrendale, PA, 1981. (7) Miyamoto, N.; Hou, Z.; Ogawa, H. Catalytic Effects of Metallic Fuel Additives on Oxidation Characteristics of Trapped Diesel Soot; SAE Paper 881224; Society of Automotive Engineers: Warrendale, PA, 1988. (8) Ahlström, A. F.; Odenbrand, C. U. I. Carbon 1989, 27, 475–483. (9) Neeft, J. P. A.; Nijhuis, T. X.; Smakman, E.; Makkee, M.; Moulijn, J. A. Fuel 1997, 76, 1129–1136. (10) Yezerets, A.; Currier, N. W.; Kim, D. H.; Eadler, H. A.; Epling, W. S.; Peden, C. H. F. Appl. Catal. B 2005, 61, 134–143. (11) Gilot, P.; Bonnefoy, F.; Marcuccilli, F.; Prado, G. Combust. Flame 1993, 95, 87–100. (12) Du, Z.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1991, 5, 214– 221. (13) Ciambelli, P.; d’Amore, M.; Palma, V.; Vaccaro, S. Combust. Flame 1994, 99, 269–279.
10.1021/ef070182j CCC: $37.00 2007 American Chemical Society Published on Web 10/02/2007
Oxidation of Acetylene Soot
Figure 1. Experimental facility: (1) reactant gases; (2) mass flow meters and control unit; (3) flow measurement; (4) electrical furnace; (5) reactor; (6) particle filter; (7) CO/CO2 analyzer.
fuels. A study was made of the oxidation at different temperatures and over a wide range of oxygen concentrations of one soot produced from acetylene pyrolysis. As it is known that the structural characteristics of soot can affect its oxidation behaviour,15 it was decided to perform a second oxidation study on another soot formed under different acetylene pyrolysis conditions. Experimental Section The experimental setup and the methodology used for the study of soot formation have been described before,16 and only a brief explanation is given here. Soot was produced by pyrolysis at 1100 °C of acetylene diluted in nitrogen, with a total flow rate of 1000 mL/min (STP). The first soot studied was produced by pyrolysis of 50 000 ppmv acetylene. The second soot analyzed in this work was obtained by pyrolysis of 5000 ppmv acetylene. These will hereafter be referred to as soot A and soot B, respectively. The pyrolysis reaction took place in a quartz tube reactor placed in an electrically heated three zone furnace. Gases exiting the reactor were quantified by means of an FTIR spectrometer. Soot formed in the reactor was collected in quartz fiber filters and analyzed. The BET area with nitrogen at 77 K was quantified, and an elemental analysis was conducted in order to determine the molar C/H ratio. Scanning electron microscopy (SEM), high-resolution tunneling electron microscopy (HRTEM), and X-ray diffraction (XRD) analyses of the soot samples were also carried out. For the study of the soot reaction with oxygen, a new experimental facility was used, as shown in Figure 1. Nitrogen and oxygen were supplied at 4 bar pressure by gas cylinders and then conveyed to the mass flow meters. The reaction zone included a quartz reactor of 15 mm internal diameter. This was heated by an electrical furnace of 5 kW allowing temperatures to be reached up to 1600 °C. The quartz reactor used in these experiments included a bottleneck in the middle, where a quartz wool plug was placed. Soot was mixed with silica sand particles with a size less than or equal to 150 µm, in a soot/sand weight ratio of 1/30, to facilitate the introduction of the soot sample into the reactor and to prevent soot particle agglomeration. The mixture was (14) Haynes, B. S.; Wagner, H. G. Prog. Energy Combust. Sci. 1981, 7, 229–273. (15) Vander Wal, R. L.; Tomasek, A. J. Combust. Flame 2003, 134, 1–9. (16) Mendiara, T.; Domene, M. P.; Millera, A.; Bilbao, R.; Alzueta, M. U. J. Anal. Appl. Pyrolysis 2005, 74, 486–493.
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placed over the plug, resulting in a thin layer. For every experiment, an amount of 5–10 mg of soot was introduced into the reactor. The sample was heated up to the reaction temperature in an inert flow of 1000 mL/min (STP) N2, the same as the total gas flow rate to be used during the oxidation reaction. Once the desired temperature was reached, the soot was exposed to the oxygen and nitrogen mixture. A blind test was performed to ensure that the quartz wool and silica sand particles were inert under the reaction conditions employed. A study of the oxidation of soot A was performed at 900 and 1100 °C, in the presence of different oxygen concentrations ranging from 100 to 30 000 ppmv. In addition, experiments at 850 and 1000 °C with an oxygen concentration of 500 ppmv were also done to extend the study of the temperature influence. For soot B, a study was carried out of its oxidation at 1100 °C and oxygen concentrations ranging from 250 to 20 000 ppmv. The reaction products, mainly CO and CO2, were cooled down to room temperature. Prior to the analyzing system, a particle filter was arranged in order to retain any solid particles that may have escaped from the reactor. The CO and CO2 concentrations were measured using a continuous infrared gas analyser. When the CO and CO2 values reached a value of approximately 10 ppmv, the experiment was considered to be finished. For both soot A and B, the carbon weight percentage determined through elemental chemical composition analysis turned to be greater than 96.0 % and the soot reactivity was quantified by the amount of carbon consumed. The initial moles of carbon (NC0) introduced into the reactor were calculated from the measured time evolution of CO and CO2 concentrations in ppmv (CCO and CCO2, respectively) of the exhaust gas by means of the following equation: ∞
NC0 ) F · 10-6
∫ (C
CO + CCO2)dt
(1)
0
where F is the outgoing molar flow. The moles of carbon remaining at any time in the reactor, NC, can be calculated through the following equation: t
NC ) NC0 - F · 10-6
∫ (C
CO + CCO2)dt
(2)
0
The corresponding milligrams of carbon, WC, can be calculated as follows: WC ) NC · MC
(3)
where MC is the atomic weight of carbon. Results and Discussion Soot A was oxidized at temperatures of 900 and 1100 °C and at different oxygen levels, ranging from 100 to 30 000 ppmv. The oxidation results are analyzed separately for low oxygen concentrations (100–1000 ppmv) and high oxygen concentrations (5000–30 000 ppmv). The experiments with low oxygen concentrations (100–1000 ppmv) are analyzed considering a constant temperature in the sample. Figure 2 shows the results of soot A oxidation for low oxygen concentrations at 900 and 1100 °C. Specifically, the oxidation of acetylene soot is shown by means of a representation of the carbon consumption rate (–dWC/dt), defined as milligrams of carbon consumed per second, versus the remaining carbon weight at different oxidation times.
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Figure 2. Evolution of the carbon consumption rate versus remaining carbon weight in the oxidation of soot A with O2 concentrations up to 1000 ppmv at T ) (a) 900 and (b) 1100 °C.
Figure 3. Evolution of CO/CO2 ratio versus remaining carbon weight in the oxidation of soot A with O2 concentrations up to 1000 ppmv at T ) (a) 900 and (b) 1100 °C.
As can be seen in Figure 2, the evolution of the carbon consumption rate with the carbon weight is different at 900 and 1100 °C. At 900 °C, there is a maximum in the carbon consumption rate for all the oxygen concentrations considered. At 1100 °C, there is no maximum observed. This could indicate that, at 900 °C, an initial development of soot surface area could be produced. At both temperatures, the increasing presence of oxygen leads to a higher carbon consumption rate. Thus, higher oxygen concentration implies faster oxidation. This trend was also observed in other studies with soot obtained from acetylene.15 From previous studies concerning soot oxidation, it is known that the CO/CO2 ratio depends principally on the oxygen concentration and temperature. Figure 3 shows the evolution of the CO/CO2 ratio versus the carbon weight, for the experiments of soot A oxidation at 900 and 1100 °C with low oxygen concentrations up to 1000 ppmv. In Figure 3a, at 900 °C, a maximum in the CO/CO2 ratio can be observed. In Figure 3b, at 1100 °C, the ratio value can be considered as roughly constant for a significant interval of carbon weight values in all the cases. This is in agreement with previous results for flame soot12 and also diesel soot.8,9 The value of the CO/CO2 ratio diminishes with the increase in the inlet oxygen concentration. This trend has also been observed before for different kinds of soot, i.e. diesel soot, carbon black and ethylene flame soot,8,9,12 and also other carbonaceous materials.4,17–19 At 1100 °C, in the oxidation with
oxygen concentrations lower than 1000 ppmv, the ratio value is greater than or close to unity. From 1000 ppmv oxygen upwards, this value decreases to below 1. These results confirm that at high oxygen concentration, CO2 formation is favored. The values of the CO/CO2 ratio obtained for the oxidation with similar oxygen concentrations at 900 and 1100 °C are greater for the lowest temperature. The oxidation results attained are analyzed with the aid of a kinetic model applied to soot oxidation. Soot A has a BET specific surface area of 13.1 m2/g. Although there is no defined BET area limit value which allows differentiating between porous and nonporous materials, the former value can be considered low enough to be characteristic of nonporous materials. Moreover, soot A can be considered as an ash-free material. Taking all of the above into consideration, the model known as the shrinking core model for decreasing size particles with chemical reaction control20,21 is used in this work. The global oxidation process can be represented by
(17) Tognotti, L.; Longwell, J. P.; Sarofim, A. F. Proc. Combust. Inst. 1990, 23, 1207–1213. (18) Phillips, R.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1969, 7, 479– 485. (19) Du, Z.; Sarofim, A. F.; Longwell, J. P.; Tognotti, L. Fundamental issues in control of carbon gasification reactiVity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; pp 91–106.
O2(g) + bC(s) f cCO(g) + dCO2(g)
(4)
where b, c, and d are stoichiometric coefficients. According to the shrinking core model, the carbon consumption rate is referred to as -
1 dNC ) bksCOn 2 · Sext dt
(5)
(20) Levenspiel, O. Chemical reaction engineering; John Wiley & Sons. Inc.: New York, 1999. (21) Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas-solid reactions; Academic Press: New York, 1976. (22) Higgins, K. J.; Jung, H.; Kittelson, D. B.; Roberts, J. T.; Zachariah, M. R. J. Phys. Chem. A 2002, 106, 96–103. (23) Higgins, K. J.; Jung, H.; Kittelson, D. B.; Roberts, J. T.; Zachariah, M. R. EnViron. Sci. Technol. 2003, 37, 1949–1954.
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Energy & Fuels, Vol. 21, No. 6, 2007 3211
Table 2. Values of b and τ for the Experiments of Soot A Oxidation with Different Oxygen Concentrations up to 1000 ppmva T (°C) 850 900
1000 1100
a
O2 (ppmv)
b
τ (s)
R2
500 350 500 750 1000 500 100 250 350 500 700 1000
1.74 1.83 1.81 1.79 1.76 1.45 1.86 1.65 1.54 1.27 1.31 1.18
8929 8850 8130 5587 4292 7692 28571 12500 8929 6757 5051 4525
0.9985 0.9992 0.9997 0.9989 0.9901 0.9957 0.9994 0.9960 0.9970 0.9911 0.9964 0.9904
R2 corresponds to the fitting of experimental data to eq 10.
where Sext represents the external surface of a particle, ks is the rate constant of the oxidation reaction, CO2 is the oxygen concentration, and n is the reaction order with respect to oxygen. Considering the particle as spherical, Sext can be expressed as Sext ) 4πR2
(6)
where R is the particle radius at any time which can be calculated as a function of the carbon weight of the particle and its molar density (FC): R)
(
3WC 4πFCMC
)
1⁄3
(7)
The following equation can be deduced: 1 dWC ) CbksCO2n - 2⁄3 · dt W
(8)
C
where C represents a constant. For each experiment, ks and CO2 can be considered as constant, too. The value of the stoichiometric coefficient b can be calculated for each of the experiments of soot A oxidation. According to the global reaction scheme represented by eq 4, b is calculated as follows: CO +1 CO2 b) 1 CO +1 2 CO2
(9)
For soot A oxidation experiments at 900 °C, the value of the CO/CO2 ratio presents a maximum for all the oxygen concentrations tested. Nevertheless, for each oxygen concentration, the value of the CO/CO2 ratio is so high that its variation with the carbon weight is not translated into an important variation with the carbon weight of the stoichiometric coefficient b value calculated according to eq 9. In soot A oxidation at 1100 °C, the CO/CO2 ratio can be considered as roughly constant during the greater part of the experiment. Therefore, an average b value can be determined for each experiment according to eq 9. The values of b for each oxygen concentration analyzed at 900 and 1100 °C are shown in Table 2. With the aim of testing experimentally the constant character of eq 8 for each oxygen concentration, the values of the expression (–1/WC2/3)(dWC/dt) have been calculated from the experimental results of soot A oxidation at 900 and 1100 °C with low oxygen concentrations. Figure 4 represents these values versus carbon weight for all the experiments.
Figure 4. Evolution of carbon consumption rate referring to external particle surface versus remaining carbon weight in the oxidation of soot A with O2 concentrations up to 1000 ppmv at T ) (a) 900 and (b) 1100 °C.
At 900 °C, the carbon weight interval, for which the expression (–1/WC2/3)(dWC/dt) can be considered as constant, is 1–5 mg. At 1100 °C, this expression can be considered as valid throughout the experiment. The shrinking core model is then used in these weight intervals where eq 8 is considered as constant. The application of the shrinking core model, for decreasing size particles with chemical reaction control, implies the fitting of the soot A oxidation data to the following equation connecting time (t) and carbon conversion (XC): t ) 1 - (1 - XC)1⁄3 τ where carbon conversion is defined as XC )
WC0 - WC WC0
(10)
(11)
In eq 11, WC0 represents the initial weight of carbon at the beginning of the experiment. The parameter τ, which represents the time needed for the complete combustion of carbon, is defined as τ)
FCR0 bksCO2n
(12)
where R0 is the initial radius of the soot particles. Table 2 summarizes the values of τ obtained in the fitting to eq 10 of the experimental data, with the regression coefficient (R2), for the different experiments of soot A oxidation up to 1000 ppmv. These values are obtained for the weight interval in which both expressions of eq 8 can be considered as constant.
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Figure 5. Determination of reaction order with respect to oxygen according to eq 13 in the oxidation of soot A with O2 concentrations up to 1000 ppmv at 900 and 1100 °C.
Figure 6. Comparison of the experimental carbon conversion values in soot A oxidation at 1100 °C for oxygen concentrations between 5000 and 30 000 ppmv with those predicted by a reaction order with respect to oxygen of 1. The experimental conversion values are plotted as scatters, and the calculated ones, as lines.
On the basis of eq 12, the following equation allows the calculation of the reaction order for soot A oxidation at 900 and 1100 °C.
( τb1 ) ) log( F · R ) + n log C
log
ks
C
0
O2
(13)
Figure 5 shows the good fitting of the data at both temperatures to eq 13. An order of 1 with respect to oxygen is obtained. A reaction order of unity has been postulated for the oxidation of other types of soot by several authors, as can be seen in Table 1. The kinetics obtained for low oxygen concentrations is applied to the soot A oxidation results that correspond to higher oxygen concentrations (5000–30 000 ppmv). According to eq 13, using the n and (ks/FCR0) values previously obtained and the CO2 values corresponding to each experiment, the (τb) values can be predicted for the experiments with 5000–30 000 ppmv oxygen. For high oxygen concentrations, the stoichiometric coefficient b can be considered as 1, because the CO/CO2 ratio is lower than 1 in all cases. With these calculated values of τ, XC can be determined for different reaction times by means of eq 10. Calculated and experimental results for carbon conversion at high oxygen concentrations are shown in Figure 6. It can be seen that there is good agreement between the experimental and the calculated XC values. The activation energy value for soot A oxidation is determined in the light of experiments carried out with 500 ppmv oxygen at temperatures in the range of 850–1100 °C. The values of b
Figure 7. Arrhenius plot for soot A oxidation in the 850–1100 °C temperature range (oxygen concentration of 500 ppmv).
Figure 8. SEM pictures of (A) soot A and (B) soot B: zoom (a) 10 000× and (b) 20 000×.
and τ were obtained for each temperature using eqs 9 and 10, respectively, and they are shown in Table 2. Using eq 12 and considering a reaction order equal to unity, the (ks/FCR0) values were obtained at each temperature and treated by the Arrhenius plot shown in Figure 7. The resulting activation energy was 42.8 kJ/mol. To complete this oxidation study and considering that the characteristics of the soot structure may influence its oxidation behaviour,15 another soot with different structural characteristics from soot A was produced. The same experimental methodology was employed as for soot A,16 but in this case, the pyrolysis was done at 1100 °C and 5000 ppmv acetylene. An oxidation study of the resulting soot B was carried out. The shrinking core model is again applied, and the reaction order with respect to oxygen is obtained and compared with that obtained for soot A. Characterization of both soot A and soot B was done to help explain the possible different behaviour of the different soot samples. Figure 8 shows the SEM pictures of soot A and B, respectively, giving an idea of their different structure. There are differences in the particle diameter distribution. For soot A, this is in the 0.6–0.2 µm range while for soot B it is in the range of 0.2–0.1 µm. Furthermore, soot A has a 13.1 m2/g BET area while the BET area for soot B is 30.2 m2/g. The C/H molar ratio is also different, being 13.5 for soot A and 10.0 for soot B.
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Energy & Fuels, Vol. 21, No. 6, 2007 3213
Figure 10. XRD diffraction patterns for soots A and B.
Figure 9. HRTEM images of (A) soot A and (B) soot B: zoom (a) 185 000× and (b) 490 000×.
Figure 9 shows HRTEM images of both soot samples. The graphitic layers can be seen close together in crystallite stacks. In Figure 9A, corresponding to soot A, the graphitic layers are large, parallel to each other and proximately parallel to the particle perimeter. In contrast, Figure 9B, corresponding to soot B, shows shorter graphitic layers with no apparent defined orientation in relation to each other. There are also some regions in the soot B particles in which the graphitic layers show a certain degree of curvature. Different studies with other materials have revealed that the material structure determines the number and accessibility of border active sites, which are considered as potentially reactive, not only in the HACA surface growth process, but also in the reaction with O2.15 Therefore, the ratio between border sites and basal sites in the carbonaceous materials can describe their reactivity. The longer the graphitic layers, the lower the number of border active sites or the lower their reactivity, since active sites are fewer and maybe less accessible. This effect is increased if the graphitic layers are concentrically oriented. On the other hand, if the graphitic layers are short, the number of border active sites is expected to increase as, therefore, is the reactivity of the material. Layer curvature is also relevant to reactivity. Among other reasons, the curvature may be due to the presence of fivemembered rings inside the graphitic layers. Curvature imposes stress in the bonds between atoms and diminishes the electronic resonance stabilization. Due to this weakness of the C–C bonds, the oxygen attack is easier. The nonaligned short graphitic layers of soot B can be translated into a high percentage of border active sites. The curvature observed in some particle zones contributes to structural disorder. Soot A possesses longer graphitic layers, and thus a lower ratio of border to basal sites. Figure 10 shows the XRD diffraction patterns obtained for both soot samples. The XRD technique can be used to estimate the order degree in a solid structure. In Figure 10, the most prominent peak is the (002) Bragg reflex, found at approximately 24° on the 2θ scale. The (100) reflex is found at about 44°. As
can be seen in Figure 10, the positions of these peaks are approximately the same for both soot samples and also for graphite.24 This means that the primary structure of both soots is similar. Both have carbon atom groups disposed in hexagons similarly to graphite, although the crystallites in soot are smaller, as indicated by the broad Bragg reflexes. Besides, soot A shows a higher intensity value at the (002) Bragg reflex, which suggests that it presents a more ordered structure than soot B. The average thickness of the crystallites of soot A and B can be estimated using Scherrer′s formula:25 Lc )
Kλ B cos θ
(14)
where Lc represents the average thickness of the crystallite, K is a constant whose value is taken as 0.89, τ is the wavelength of the X-ray used, B is the half-height width of the (002) diffraction peak given in radians and θ is the corresponding Bragg diffraction angle. Accordingly, and considering that the λ value used is 1.5418 Å, a crystallite thickness of 11.20 Å is obtained for soot A and 10.52 Å is obtained for soot B. From the (002) Bragg reflex and according to Bragg′s law, the interlayer spacing (d002) for both soots can be calculated. The value of (d002) for soot A was 3.64 Å and for soot B, slightly lower, was 3.58 Å. In both cases, the interlayer spacing was higher than the 3.35 Å registered as the (d002) value for graphite. All the previous analyses underline the differences between soots A and B in terms of composition and structure. These differences in C/H molar ratio, BET surface area, and internal structure may contribute to explaining the differences in reactivity towards oxygen of both soots. The oxidation of soot B was studied at 1100 °C and for oxygen concentrations in the 250–20 000 ppmv oxygen range. The results with oxygen concentrations up to 1000 ppmv were chosen for comparison with soot A oxidation. In these conditions, the oxidation of soot B presents similarities with respect to soot A oxidation, as can be seen in Figures 2b and 11. The increasing presence of oxygen leads to a higher carbon consumption rate for both soots. However, the values of the carbon consumption rate of soot B are higher than the corresponding values of soot A. With respect to the CO/CO2 ratio values determined for different oxygen concentrations, soot B exhibits a similar dependence on the oxygen concentration as soot A. Figure 12 (24) Yang, J.; Sánchez-Cortezon, E.; Pfänder, N.; Wild, U.; Mestl, G.; Find, J.; Schlögl, R. Carbon 2000, 38, 2029–2039. (25) Fukuda, K.; Kikuya, K.; Isono, K.; Yoshio, M. J. Power Sources 1997, 69, 165–168.
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Figure 11. Evolution of the carbon consumption rate versus remaining carbon weight in the oxidation of soot B with O2 concentrations up to 1000 ppmv at T ) 1100 °C.
Figure 12. Evolution of CO/CO2 ratio versus remaining carbon weight in the oxidation of soot B with O2 concentrations up to 1000 ppmv at T ) 1100 °C. Table 3. Values of b and τ for the Experiments of Soot B Oxidation at 1100 °C and 250–20 000 O2 ppmva T (°C) 1100
a
O2 (ppmv)
b
τ (s)
R2
250 500 1000 5000 10000 20000
1.87 1.91 1.46 1.11 1.40 1.09
5714 2801 1453 441 256 119
0.9990 0.9985 0.9986 0.9987 0.9916 0.9990
R2 corresponds to the fitting of experimental data to eq 10.
illustrates the evolution of the CO/CO2 ratio for soot B and different oxygen concentrations up to 1000 ppmv. Comparing Figures 3b and 12, it can be seen that, for a fixed oxygen concentration, the CO/CO2 ratio for soot A tends to be constant during almost all the time of the experiment. This is not the case for soot B, where a progressive diminution in the values of the CO/CO2 ratio with carbon weight is clearly observed in experiments with 250 and 500 ppmv oxygen. Nevertheless, as in the case of soot A oxidation experiments at 900 °C, the value of the CO/CO2 is so high that its diminution with carbon weight does not result in a significant variation with carbon weight of the stoichiometric coefficient b value calculated according to eq 9. Table 3 shows the b values obtained for each oxygen concentration. Moreover, for each oxygen concentration, the value of the CO/CO2 ratio for soot B is greater than that for soot A, as is shown in Figures 3b and 12. As in soot A oxidation, Figure 12 shows a decrease in the CO/CO2 ratio observed in soot B oxidation as the oxygen concentration increases. The shrinking core model for decreasing size particles with chemical reaction control is also considered in the analysis of
Figure 13. Evolution of carbon consumption rate referring to external particle surface versus remaining carbon weight in the oxidation of soot B with 250–20 000 O2 ppmv at T ) 1100 °C.
Figure 14. Determination of reaction order with respect to oxygen according to eq 13 in the oxidation of soot B with 250–20 000 O2 ppmv at T ) 1100 °C.
soot B oxidation. Equation 8 was used to previously determine the carbon weight intervals in which the values of the parameter τ can be obtained from eq 10. Since good agreement was found in soot A oxidation, when the reaction order with respect to oxygen obtained at low oxygen concentrations was used in the prediction of results at higher oxygen concentrations, in the following calculation of the reaction order with respect to oxygen, eq 8 is applied to all the oxygen concentrations used in the soot B oxidation study. Figure 13 shows the values of the expression (–1/WC2/3)(dWC/ dt) for soot B oxidation with different oxygen concentrations. From the plot, it can be estimated that, at 1100 °C, the expression (–1/WC2/3)(dWC/dt) is constant during all the experiment. By means of eq 10, the τ values are obtained. Table 3 summarizes the average values of b and the values of τ with the regression coefficient (R2) obtained in the fitting of the experimental data to eq 10 for the experiments of soot B oxidation at 1100 °C with different oxygen concentrations. As with the soot A analysis, the reaction order for the oxidation at 1100 °C of soot B was determined according to eq 13. An order with respect to oxygen of 1 was again obtained, as is shown in Figure 14. As well as being equal to that obtained for soot A, this order is also in agreement with the reaction order determined previously for the study of soot oxidation by several authors, as shown in Table 1. Therefore, the oxidation of these two structurally different soots, formed by acetylene pyrolysis at 1100 °C and using different initial acetylene
Oxidation of Acetylene Soot
concentrations, can be described with a reaction order with respect to oxygen equal to unity. Conclusions An experimental and kinetic study was made of the oxidation of two soot samples with different structural characteristics. These were named soot A and soot B, respectively. For soot A, oxidation was analyzed for different temperatures (850–1100 °C) and oxygen concentrations (100–30 000 ppmv oxygen). For soot B, oxidation was carried out at 1100 °C and the range of oxygen concentrations studied was 250–20 000 ppmv. The main conclusions are the following: In the oxidation experiments of soots A and B, the values of the expression (–1/WC2/3)(dWC/dt) and the stoichiometric coefficient with respect to carbon, b, can be considered as constant for every experiment over a wide carbon weight interval. Applying the shrinking core model with chemical reaction control to the results of soot A oxidation under low oxygen concentrations, a reaction order of 1 was obtained. From the experiments performed at different temperatures, the resulting activation energy was 42.8 kJ/mol.
Energy & Fuels, Vol. 21, No. 6, 2007 3215
The application of the kinetic parameters obtained to the results of soot A oxidation for higher oxygen concentrations showed good agreement between calculated and experimental carbon conversion values. Soots A and B presented significant composition and structural differences. The C/H molar ratio was 13.5 for soot A and 10.0 for soot B. For soot A, the particle diameter distribution was in the 0.6–0.2 µm range. For soot B, it was in the range of 0.2–0.1 µm. Furthermore, soot A had a 13.1 m2/g BET area, while the BET area for soot B was 30.2 m2/g. XRD and HRTEM analyses also showed differences in their internal structure. Soot A possesses longer graphitic layers and, thus, a lower ratio of border to basal sites. Soot B showed a higher percentage of border active sites due to its unaligned short graphitic layers. Despite these differences, a reaction order of 1 was also obtained for soot B oxidation at 1100 °C, even though the reactivity to oxygen of soot B differed from that of soot A. Acknowledgment. The authors express their gratitude to MCYT Project PPQ2003-02394 for financial support. T.M. acknowledges MECD for the predoctoral grant awarded. EF070182J
