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Pyrolysis of Ethanol: Gas and Soot Products Formed ngela Millera, Rafael Bilbao, and María U. Alzueta* Claudia Esarte, María Peg,† María P. Ruiz,‡ A Aragon Institute of Engineering Research (I3A), University of Zaragoza, Campus Río Ebro, C/Mariano Esquillor s/n, 50018 Zaragoza, Spain ABSTRACT: Ethanol may be used in dieselethanol mixtures in order to decrease soot emissions. Therefore, it is important to evaluate the contribution of ethanol to soot formation. The influence of the temperature on the pyrolysis of ethanol under sooting conditions has been studied. Pyrolysis experiments were carried out in a quartz reactor in the 7001200 C temperature range, for an inlet C2H5OH concentration of 50 000 ppm and a gas residence time in seconds of 1706/(T/K) . The concentrations of the gases analyzed in the experiments are shown in this work together with simulations carried out with a literature kinetic model. Theoretical and experimental results present, generally, a good agreement in spite of the model not taking into account the soot formation. Under the studied conditions, soot formation has been observed above 1050 C. With the soot samples obtained, a study of their reactivity toward O2 and NO has been carried out, and several characterization techniques, such as elemental analysis, BrunauerEmmettTeller (BET) surface area, transmission electron microscopy, and X-ray diffraction have been used to study the structural properties of the collected soot samples and relate them to their reactivity. It has been observed that the soot samples formed at higher temperatures exhibit a lower reactivity toward O2 and NO, and the characterization analyses support these results.
1. INTRODUCTION The use of fuel blends, including oxygenates, is basically motivated by the reduction of harmful emissions, such as soot, coming from diesel engines. A number of studies have dealt in the past with practical experiences related to the diminution of soot emissions when oxygenates are added to the fuel. It is clear that the addition of oxygenated species to diesel fuel produces significant reductions of particulate matter; e.g., see refs 16 The action of oxygenates has been reported to act by three different ways on soot formation,610 namely, (a) affecting the temperature, (b) removing carbon from the typical reaction pathways leading to soot, and favoring instead the formation of CO and CO2, and (c) limiting the formation of soot precursors, since the oxygenate would produce other species different from the typical soot precursors, such as C2H2 and C2H4. While it is not completely demonstrated which of these factors dominates the soot suppression effect, it appears clear that the three of them may have a significant role, and probably the importance of each factor will be dependent on the specific operating conditions. The increasingly favorable economy for ethanol production, together with its renewable nature and its capacity to diminish green house gases, makes ethanol attractive as an alternative fuel and/or a fuel extender in the near future.1,1115 In this context, the determination of ethanol potential for soot generation is important to establish practical conclusions on how the addition of ethanol affects the sooting tendency of fuel mixtures. The present work aims to carry out a study of the decomposition of ethanol under pyrolytic conditions since those are the most favorable for soot formation in real combustion systems. Thus, pyrolysis experiments for a given initial ethanol concentration at different temperatures have been performed. Gas species concentrations at the reactor outlet have been quantified and compared with modeling results. The soot formed in the experiments has been collected and quantified. This way, we intend to analyze the influence of soot reduction paths b and c r 2011 American Chemical Society
on the formation of soot and gas products in the pyrolysis of ethanol and the role they play in the diminution of soot formation. A further study of the interaction of the collected soot samples with O2 and NO has been carried out. Additionally, the soot samples have been characterized through a number of techniques, in order to evaluate the resultant structural properties of the collected soot samples and relate them to their reactivity.
2. EXPERIMENTAL SECTION 2.1. Ethanol Pyrolysis and Soot Formation. The experiments of ethanol pyrolysis have been carried out in an experimental installation described in detail elsewhere;6,1618 thus only a brief description is given here. A quartz flow reactor of 45 mm inside diameter and 800 mm length is placed in an electrically heated oven, which allows us to vary the temperature in the 7001200 C range. Ethanol is fed by saturating a nitrogen stream through pure liquid ethanol at room temperature. Additionally, nitrogen is added to reach a total flow rate of 1000 mL(STP)/min, which results in a gas residence time dependent on reaction temperature, tr/s = 1706/(T/K). The fed flow rate introduced in the reactor includes a constant concentration of ethanol of 50 000 ppm (this was tested by total combustion of the feed and analyzing the CO2 amount generated). At the outlet of the reaction zone, the product gas is analyzed online by means of a gas chromatograph with an accuracy of (10 ppm. The gas chromatograph uses helium as carrier gas and is equipped with a TCD detector coupled to HP-Plot MoleSieve and HP-PLOT Q columns and a flame ionization detector (FID) coupled to an HP-PONA column. Soot is collected in quartz fiber filter with a pore diameter lower than 1 μm. Experiments were run Received: November 8, 2010 Accepted: February 23, 2011 Revised: January 9, 2011 Published: March 10, 2011 4412
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until a significant amount of soot was collected (more than 1 g), not exceeding an overpressure limit (50 mbar) above which the functioning of the experimental setup could be disturbed. This results in an about 3 h time experiment. The experiments showed good repeatability through the performance of up to three experiments and uncertainty of (0.01 g as in previous works.6,16 For the studies of soot reactivity and characterization, and in order to eliminate the adsorbed compounds in the soot samples, the raw samples are annealed for 1 h in a N2 atmosphere at 1100 C, following the methodology used by Ruiz et al.17 The soot samples produced at temperatures lower than 1100 C are annealed at their formation temperature to avoid possible structural changes. 2.2. Soot Reactivity. To study the reactivity of the soot samples formed, experiments on the interaction of soot with O2 and with NO have been carried out in a quartz reactor for a temperature of 1000 C, a total flow rate of 1000 mL(STP)/min, and inlet concentrations of O2 and NO of 500 and 2000 ppm, respectively. The experimental setup used for the soot reactivity experiments has been also described in detail elsewhere,1618 and a succinct description is included here. The mixture of reactant gases (N2/O2 or N2/NO) from gas cylinders is prepared by means of mass flow controllers and directed to a quartz reactor of 15 mm inside diameter and 550 mm length, heated by an electrical oven. For every experiment, the amount of soot introduced into the reactor is approximately 10 mg and it is always previously mixed with 350 mg of silica sand (150 μm particle size). The mixture is located on a quartz wool plug placed in a bottleneck in the middle of the reactor, resulting in a thin layer. The sand is necessary to facilitate the introduction of the sample into the reactor and to prevent agglomeration of the soot particles. An inert flow of N2 is fed while the sample is heated to the reaction temperature (1000 C). Once this set point is reached, the reactant gas mixture is fed to the reactor. The reaction temperature is measured by a thermocouple placed 0.5 cm just below the quartz wool plug where the reaction takes place. The reaction products are cooled to room temperature and directed to the analysis equipment. Prior to the analysis systems, a particle filter is placed in order to retain any solid particle. The reaction products are continuously measured by NO and CO/CO2 analyzers. During soot reactivity experiments, carbon is mainly released from the particles in the form of CO and CO2, which are measured. The carbon weight in the reactor at any time (WC) can be calculated from the measured time variation of CO and CO2 concentrations in parts per million (CCO and CCO2, respectively) in the exhaust gas. In this way, the total initial amount of carbon (mg) in the reactor, WC0, is calculated as Z ¥ 3 WC0 ¼ 10 MC FT ðCCO þ CCO2 Þ dt ð1Þ 0
where MC is the atomic weight of carbon and FT is the outing flow expressed in moles per unit time and is given by FT ¼
QP Rg T
ð2Þ
where Q is the feeding flow rate, P is the reactor pressure, Rg is the universal gas constant in appropriate units, and T is the reactor temperature. The amount of carbon (mg) in the reactor at any time is calculated as Z t ð3Þ WC ¼ WC0 103 MC FT ðCCO þ CCO2 Þ dt 0
Figure 1. Soot yields as a function of temperature obtained in the pyrolysis of ethanol and acetylene16 under the same experimental conditions: [C2H5OH] and (C2H2] = 50 000 ppm. tr/s = 1706/(T/K).
This way, the evolution of carbon conversion (XC) as a function of time can be calculated for every experiment. The carbon conversion is defined as the amount of carbon reacted in the experiment at any time, related to the amount of carbon fed into the reactor, as in XC ¼
WC0 WC WC0
ð4Þ
2.3. Soot Characterization. Additionally, different characterization techniques have been used to contribute to the knowledge of the structure and composition of the soot samples formed, as well as to analyze the relationship with their reactivity. These techniques are as follows: elemental analysis, determination of BrunauerEmmettTeller (BET) area, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The elemental analyses have been carried out in a Carlo Erba CHNS-O EA1108 analyzer. A Quantachrome AUTOSORB-6 gas adsorption analyzer is used for the surface area analyses with N2 at 77 K. TEM images have been obtained with a JEOL JEM-2010 microscope, and the XRD analyses, with a Seifert JSO-DEBYEFLEX 2002 model.
3. RESULTS AND DISCUSSION 3.1. Ethanol Pyrolysis and Soot Formation. The pyrolysis of ethanol has been carried out for a given ethanol concentration of 50 000 ppm and temperatures in the range of 7001200 C, for a given total flow rate (including ethanol diluted in nitrogen) of 1000 mL(STP)/min. Figure 1 shows the results of soot yield, as a function of temperature, obtained during the pyrolysis of ethanol. Soot yield is defined as the percentage of the carbon amount in mass in soot related to the carbon amount fed into the reactor. Soot appears to be formed under the studied conditions from 1050 C, and it is seen to increase as the reaction temperature does, reaching a maximum soot yield value, in the considered temperature interval, of approximately 30%. The increase of soot yield with temperature is a fact that has been reported earlier in literature; e.g., see refs 1622 The soot yield values attained in the pyrolysis of ethanol are considerably lower than those obtained in the pyrolysis of acetylene under similar conditions, which reached values of approximately 60% for the highest temperature studied of 1200 C,16 and they are also shown in Figure 1. This is consequent to the fact that acetylene is a key 4413
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Industrial & Engineering Chemistry Research compound in soot formation process via acetylene additionhydrogen abstraction (HACA) mechanism.23 Apart from soot, the outlet gases produced in pyrolysis experiments, H2, CO, CO2, CH4, C2H2, C2H4, C2H6, and C6H6, have also been quantified. The experimental results obtained are plotted in Figure 2 as symbols, together with simulations carried out with a literature kinetic model, represented with lines. Specifically, for calculations the mechanism described by Esarte et al.24 has been used. This mechanism is based on that developed by Abian et al.,25 which includes reaction subsets for C1 and C2 hydrocarbons and ethanol,26,27 and also takes into account polycyclic aromatic hydrocarbon (PAH; up to pyrene) formation and consumption reactions.23 Calculations were performed using Senkin,28 which runs in conjunction with the Chemkin library.29 The reverse rate constants were obtained from the forward rate constants and the thermodynamic data, which were taken from the same sources as the different submechanisms. It should be noted that the mechanism used for simulations only includes the gas-phase reactions and not the formation of soot. Because of this, the predictions of the mechanism have to be cautiously interpreted. CO and CO2 are found in the gas outlet stream (Figure 2a,b), since ethanol contains oxygen in its structure. While the concentration of CO, which presents significant concentration values, appears to be independent of the temperature once the conversion of ethanol has started, the CO2 amount, which concentration is lower, is seen to increase drastically as temperature goes above 1000 C. The impact of the oxygenated compounds, ethanol in this case, on the removal of carbon from the paths leading to soot and directing the reaction toward CO and CO2 can be evaluated, i.e., path b of the three previously mentioned. In the present work, only ethanol is pyrolyzed and part of it is released in the form of CO and CO2, indicating that effectively a fraction (of about 2530%) of the carbon from ethanol is converted into COx species. Therefore, path b is clearly contributing to the removal of carbon, favoring CO and CO2 formation. The concentrations of most of the analyzed hydrocarbons (CH4, C2H4, C2H6) exhibit a maximum with the temperature, at around 800850 C, pointing to the decomposition of ethanol into these hydrocarbons up to such temperature, and their further recombination at higher temperatures to produce larger hydrocarbons and PAH. Meanwhile, acetylene reaches a maximum at 1000 C, matching with soot formation at higher temperatures. It is noticeable that a great fraction (up to 2530%) of the carbon present in ethanol is converted to methane (Figure 2c). Methane is a fairly unreactive hydrocarbon and, at the conditions considered in this work, will remain as a final reaction product, limiting thus the generation of the typical soot precursors, such as acetylene and ethylene. Comparing the amounts of methane produced in the present study with the methane produced under similar conditions from hydrocarbons with a higher tendency to form soot1618 can be helpful in understanding the lower tendency of ethanol to produce soot. Ethanol pyrolysis produces up to 25 times more methane than the other hydrocarbons because its decomposition route takes place through the diagram in eq 5, as previously observed,24 limiting the formation of soot precursors and removing carbon from the typical pathways leading to soot formation.
It is noticeable the satisfactory agreement between experimental results and calculations for most of the modeled species, even
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though the kinetic model used does not include the formation of soot and exclusively models the gas-phase processes. Precisely, this fact, the nonmodeling of soot formation through the used mechanism, may allow also explaining the differences found between experiments and calculations for the concentrations of C2H2 and H2 (Figure 2e,f). For these two species, the model provides a good description of experimental concentrations for temperatures up to 1000 C, temperature beyond which the formation of soot is observed. Thus, above 1000 C, calculations underpredict the levels of H2, which is the main gas product, and instead overpredict the levels of C2H2. However, the model is able to predict the experimental trends observed. Experimentally, the C2H2 profile exhibits a maximum at the temperature of 1000 C; this maximum is shifted to approximately 1100 C for modeling results (Figure 2e) and above this temperature is seen to decrease monotonically. As can be observed, an increase in the temperature leads to an increase in H2 concentration (Figure 2f). Acetylene decrease is explained in terms of soot formation from acetylene accompanied of the release of H2. It can also be observed the disagreement between experimental and modeling data for benzene (Figure 2g), which is the first aromatic ring and therefore it plays an important role into the growth of larger aromatic molecules and soot. This fact is due, once more, to the noninclusion of soot formation reactions; therefore only the increase of benzene is predicted, and its consumption into soot formation processes is not reproduced by the model. Taking into account that PAHs are regarded as the main soot precursors, the comparison of the amount of soot collected in each experiment with the sum of the theoretically calculated PAH amounts (up to pyrene) may help to qualitatively analyze the experimental observations (Figure 2h). This way, it is detected that modeling results fit well the experimental trends, indicating that, although further model improvement has to be implemented, this attempt to model the production of soot is a good approach toward a complete model including soot formation. 3.2. Soot Reactivity. The reactivity toward O2 and NO of the soot samples formed in ethanol pyrolysis has been also analyzed. To carry out this reactivity study, the procedure of Ruiz et al.17 has been followed, where a mixture of gases either of N2/O2 (500 ppm O2) or N2/NO (2000 ppm NO) is prepared and made it to interact with the soot samples produced at different temperatures. Figures 3 and 4 show the evolution of carbon conversion as a function of time obtained in the soot/O2 and soot/NO interactions, respectively. The figures include the results of the interaction experiments carried out with the soot samples formed at 1050, 1150, and 1200 C. Besides, the evolution of the amount of reduced NO with carbon weight (WC) in soot/NO experiments is shown in Figure 5. Results on carbon conversion indicate that soot samples obtained at a lower temperature are more reactive toward both O2 and NO, compared to the samples obtained at the highest temperatures studied. It can be also observed that the soot samples formed at lower temperatures lead to slightly higher reductions of the concentration of NO. The experimental data for carbon conversion have been fitted with a simple reaction model, the “shrinking core model”17,30,31 that can describe the present noncatalytic gassolid reaction. This model has been chosen because of its simplicity and because it has turned out to provide significantly good results. Furthermore, taking into account the experimental system, a constant 4414
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Figure 2. Experimental and theoretical results at the reactor outlet of (a) carbon monoxide, (b) carbon dioxide, (c) methane, (d) ethylene, (e) acetylene, (f) hydrogen, (g) benzene, and (h) sootPAH.
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Figure 3. Evolution of carbon conversion as a function of time in soot/ O2 interaction experiments.
Figure 5. Evolution of reduced NO amount as a function of carbon weight in soot/NO interaction experiments.
Table 1. Carbon Complete Conversion Times (τ) Obtained in Soot/O2 and Soot/NO Interaction Experiments, Carried out with the Soot Samples Produced in the Pyrolysis of 50 000 ppm Ethanol τ (s)
Figure 4. Evolution of carbon conversion as a function of time in soot/ NO interaction experiments.
gas concentration on the soot particle surface can be assumed. The equation that results of the application of the shrinking core model for particles decreasing in size and under chemical reaction control conditions is 1 ð1 XC Þ1=3 ¼
t τ
ð6Þ
τ being the so-called in the model carbon complete conversion time, and it can be also related to soot reactivity. The values of τ are calculated using eq 6 from the fitting of the experimental data obtained (1 (1 XC)1/3) versus time. Table 1 shows the τ values calculated for soot samples obtained in the pyrolysis of ethanol at different temperatures in the interactions soot/O2 and soot/NO. Comparing reactivity toward O2 and NO, it is clearly seen that the soot samples are significantly more reactive toward O2, presenting lower τ values in relation to the interaction experiments with NO. Moreover, it can be seen that the soot samples produced at lower temperatures present, as a general trend, lower τ values and, thus, are more reactive compared to those produced at higher temperatures. 3.3. Soot Characterization. The soot collected during the ethanol pyrolysis experiments is characterized through a number of different techniques: analysis elemental, determination of the BET surface area, transmission electron , and X-ray diffraction. The main
soot formation T (C)
in oxidation
in interaction with NO
1050
3780
11 130
1100
5220
14 850
1150
4500
12 540
1200
6990
14 940
results of these analyses are shown in Table 2. The reactivity of a material can be related to its composition and structure,3234 and the results obtained through the mentioned techniques increment the database on soot properties; e.g., see refs 1618 The obtained soot is a heterogeneous material, which makes it difficult to relate each of the studied properties to the reactivity results. The elemental analysis and the C/H molar ratio of the soot samples obtained in the pyrolysis of ethanol are shown in Table 2. The soot samples contain more than 90% carbon in its composition in all cases, with this carbon percentage increasing as the pyrolysis temperature is increased. On the contrary, the percentage of hydrogen in the soot samples is found to decrease as temperature increases. Both effects result in a C/H molar ratio that increases with temperature. The trends found are similar to those obtained when soot is produced in the pyrolysis of other hydrocarbons.16,17 The hydrogen content and C/H ratio values are directly related to the material reactivity.16,3537 This is in agreement with the reactivity results obtained, since, in general, the most reactive soot samples (the samples formed at lower temperatures) are the solids with higher hydrogen contents and lower C/H ratio values. The surface area (Sg) can be related to the reactivity of a given material. The surface area values, determined by nitrogen adsorption at 77 K, are also shown in Table 2. Surface area values are low and comparable to the external surface area, indicating the nonporous character of the soot particles formed from ethanol under the conditions studied, which is in agreement with previous studies addressing the limited porosity of soot particles.17,38 Surface area values are very low, and given the 4416
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Table 2. Elemental Analysis, C/H Ratio, Surface Area, Particle Size, and Aromaticity of the Soot Samples Formed from Ethanol Pyrolysis at Different Temperatures elemental analysis (wt %, dry basis)
a
Tformation (C)
C
H
C/H (molar basis)
Sg a(m2/g)
f
1050
90.91
0.21
36.61
16.72
0.56
1100
97.20
0.20
41.47
34.48
0.72
1150 1200
99.51 98.82
0.18 0.17
45.01 49.02
45.76 24.91
0.70 0.78
Sg, surface area.
Figure 6. TEM images of soot formed at 1050 C.
different chemical composition of the samples, it is difficult to assess the individual effect of this variable in the reactivity of soot. The TEM technique has been extensively used and has been referred as appropriate for characterizing chemical and morphologically the particles of soot.17,33,3942 As an example, the TEM images obtained for the ethanol soot obtained at 1050 C are shown in Figure 6, for two different scales. Similar TEM pictures are found for the soot samples obtained at the rest of the temperatures. As can be observed in the figure, soot has the appearance of chainlike aggregates composed of several tens or hundreds of subunits, known as monomers or spherules. The soot particles appear to be encapsulated aggregates of highly defective carbon “onions”, presenting a graphitic structure. These onionlike structures are made from parallel graphene sheets arranged with their basal planes perpendicular to the radii of the structures. The XRD technique can be used for the study of the size and structure of molecules and clusters.17,43 Figure 7 shows an example of the diffractograms obtained for the soot generated from ethanol at 1050 C. The diffractograms were collected in the 590 range. Similar plots are obtained for the soot samples produced at other temperatures. The most important peak is the (002) Bragg reflex found at 25 in 2θ scale. Following the discussion of Lu et al.,44 who quantitatively applied XRD analysis to different coals, it can be said that the asymmetric feature of the (002) band suggests the existence of another band, the γ-band, in its left-hand side. The γ-band, which has been observed by many authors, is thought to represent the saturated structure such as aliphatic side chains, while the (002) band is representative of a graphite-like structure. In this work, the (002) Bragg reflex and its γ-band side chain have been fitted with two Gaussian functions in
Figure 7. Diffractogram of soot formed at 1050 C.
order to determine the aromaticity of the soot samples taking into account their relative scattering contributions. To do this, a procedure taken from the literature is used.43,44 Theoretically, the areas under the (002) and γ peaks should be equal to respectively the number of aromatic atoms and saturated atoms. Therefore, the aromaticity, f, can be defined as f ¼
Að002Þ Að002Þ þ Aγ
ð7Þ
where A(002) and Aγ are the areas under each Gaussian curve. The calculated f values are shown in Table 2. Results indicate that roughly the aromaticity increases as the pyrolysis temperature is 4417
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Industrial & Engineering Chemistry Research increased. The aromaticity is directly related to the crystalline carbon content and to the C/H ratio in a carbon material. The trend found in aromaticity coincides thus with the higher C/H ratio found with increasing pyrolysis temperature as shown in Table 2. This aromaticity is also related to the reactivity of the material, supporting the reactivity results, since the more aromatic, the less reactive is the material. The soot samples formed at higher temperatures exhibit lower reactivities toward O2 and NO and have more aromatic structures. It is interesting to notice the general correspondence of the evolution of the aromaticity values with the τ values (Table 1). This correspondence is an evidence of the relationship between the aromaticity of the material and its reactivity.
4. CONCLUSIONS Pyrolysis experiments of 50 000 ppm of ethanol have been carried out at atmospheric pressure in a quartz flow reactor in the 7001200 C temperature range. All the experiments have been performed for a total flow rate of 1000 mL(STP)/min, resulting in a gas residence time, in seconds, of 1706/(T/K). Soot formation is observed from 1050 C. Gas-phase experimental data have been compared against modeling results from a gas-phase detailed chemical kinetic mechanism obtained from literature. Simulation results fit satisfactorily well the evolution of the concentration of most gas species with temperature, especially for temperatures lower than 1050 C when soot formation is not observed. Model overpredicts acetylene and benzene formation and underpredicts hydrogen final concentration when soot formation is observed. This fact may be attributed to the noninclusion of soot formation reactions, which involve acetylene consumption and hydrogen release. The comparison of the amount of collected soot with the sum of the theoretically calculated PAH amounts, main soot precursors, has been performed revealing that, even though further model development is still needed, a satisfactory approach toward a comprehensive model for soot formation has been achieved. Soot samples were collected at different temperatures in order to perform reactivity and characterization studies. Soot obtained from ethanol pyrolysis has been demonstrated to be more reactive toward O2 than toward NO. Besides, soot samples formed from ethanol at higher temperatures showed lower reactivity toward O2 and NO than those samples obtained at lower temperatures. Characterization analyses partially support these results due to the heterogeneous character of the material. The C/H ratio and aromaticity of the material are the characterization parameters which present a clearer correspondence with its reactivity. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ34976761876. Fax: þ34976761879. E-mail: uxue@ unizar.es. Present Addresses †
Centre for Hydrographical Studies (CEDEX), P.O. Bajo Virgen de Puerto 3, 28005 Madrid, Spain. ‡ School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, OK 73019, USA.
’ ACKNOWLEDGMENT We express our gratitude to the Spanish Ministry of Education and Science (MEC), Project CTQ2009-12205, for financial
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support. C.E. acknowledges the MEC for the predoctoral grant awarded (BES-2007-15333).
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