Oxidation of Acetyleneâ Ethanol Mixtures and Their Interaction with NO

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Energy & Fuels 2008, 22, 3814–3823

Oxidation of Acetylene-Ethanol Mixtures and Their Interaction with NO ´ ngela Millera, Rafael Bilbao, and Marıá U. Alzueta Marıá Abiań,* Claudia Esarte, A Aragoń Institute of Engineering Research, Department of Chemical and EnVironmental Engineering, C/Marıá de Luna 3 (Torres QueVedo Building), UniVersity of Zaragoza, Zaragoza 50018, Spain ReceiVed July 11, 2008. ReVised Manuscript ReceiVed September 15, 2008

An experimental and theoretical study of the oxidation of acetylene-ethanol mixtures in the absence and presence of NO has been carried out. The experiments were conducted in an isothermal quartz flow reactor at atmospheric pressure in the 775-1375 K temperature range. The influence of the temperature, stoichiometry (by varying the O2 concentration for given C2H2 and C2H5OH initial concentrations), presence of different amounts of ethanol added to acetylene, and presence of NO on the concentrations of C2H2, C2H5OH, CO, CO2, NO, and HCN has been analyzed. The gas-phase kinetic mechanism used for calculations was that developed by Alzueta et al. (Alzueta, M. U.; Borruey, M.; Callejas, A.; Millera, A.; Bilbao, R. Combust. Flame 2008, 152, 377-386) for acetylene conversion, on the basis of a previous work by Skjøth-Rasmussen et al. (Skjøth-Rasmussen, M. S.; Glarborg, P.; Østberg, M.; Johannessen, J. T.; Livbjerg, H.; Jensen, A. D.; Christensen, T. S. Combust. Flame 2004, 136, 91-128), with reactions added from the ethanol oxidation mechanism of Alzueta and Hernańdez (Alzueta, M. U.; Hernańdez, J. M. Energy Fuels 2002, 16, 166-171), as well as reactions from the mechanism developed by Glarborg et al. (Glarborg, P.; Alzueta, M. U.; DamJohansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1-27) to describe the interactions among C1/C2 hydrocarbons and nitric oxide. The experimental results show that the ethanol presence significantly modifies the acetylene conversion regime, inhibiting soot formation. An increase of the oxygen level and temperature favor acetylene conversion. The presence of NO results in some differences in relation to the oxidation regimes of the acetylene-ethanol blends. The reduction of NO by the mixture is favored at the highest temperatures of the considered range, above 1275 K, and for moderately fuel-rich conditions (λ ) 0.7). In general, the kinetic model satisfactorily simulates the experimental trends. Model predictions indicate that, under the conditions of this study, HCCO + NO is the most important reaction in reducing NO. Moreover, the ethanol presence slightly inhibits the NO reduction in relation to the oxidation of pure acetylene.

1. Introduction During combustion processes, important pollutants can be emitted to the atmosphere, such as soot, the principal component of particulate matter, and NOx. Diesel engines are one of the major contributors to these pollutant emissions. The regulations introduced by government agencies are becoming stricter, and the research area has a great role in this context. Among the different strategies used for reducing these harmful emissions, the addition of oxygenated compounds as additives to conventional diesel fuels appears to be one of the most promising alternatives. Numerous studies have pointed out that using oxygenated compounds (alcohols, esters, ethers, etc.) as additives can considerably minimize the sooting tendency.5-11 In * To whom correspondence should be addressed. Telephone: +34-976761876. Fax: +34-976-761879. E-mail: [email protected]. (1) Alzueta, M. U.; Borruey, M.; Callejas, A.; Millera, A.; Bilbao, R. Combust. Flame 2008, 152, 377–386. (2) Skjøth-Rasmussen, M. S.; Glarborg, P.; Østberg, M.; Johannessen, J. T.; Livbjerg, H.; Jensen, A. D.; Christensen, T. S. Combust. Flame 2004, 136, 91–128. (3) Alzueta, M. U.; Hernańdez, J. M. Energy Fuels 2002, 16, 166–171. (4) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1–27. (5) Frenklach, M.; Yuan, T. 16th International Symposium on Shock Tubes and Waves, 1987; Vol. 16, pp 487-493. (6) Alexiou, A.; Williams, A. Combust. Flame 1996, 104, 51–65. (7) Miyamoto, N.; Ogawa, H.; Nurun, N. A.; Obata, K.; Arima, T. SAE Tech. Pap. 980506, 1998.

these cases, the extent of the soot reduction depends upon the oxygen content as well as the specific structure of the oxygenate compound.6,7,9,11-15 Among the different alcohols studied, ethanol has received considerable attention because it is easily obtained from renewable resources (bioethanol), improving the reduction in net emissions of greenhouse gases if it is produced from biomass.8 Some researchers have pointed out the limitations on the amount of ethanol that should be added in real diesel engines, because the presence of ethanol can modify fuel properties and its adequacy to real combustion systems.8,16-18 Given the advantages that the presence of ethanol shows on the emission (8) He, B. Q.; Shuai, S. J.; Wang, J. X.; He, H. Atmos. EnViron. 2003, 37, 4965–4971. (9) Song, K. H.; Nag, P.; Litzinger, T. A.; Haworth, D. C. Combust. Flame 2003, 135, 341–349. (10) Wu, J.; Song, K. H.; Litzinger, T.; Lee, S. Y.; Santoro, R.; Linevsk, M.; Colket, M.; Liscinsky, D. Combust. Flame 2006, 144, 675–687. (11) Pepiot-Desjardins, P.; Pitsch, H.; Malhotra, R.; Kirby, S. R.; Boehman, A. L. Combust. Flame 2008, 154, 191–205. (12) Ni, T.; Gupta, S.; Santoro, R. J. Proc. Combust. Inst. 1994, 25, 585–592. (13) Beatrice, C.; Bertoli, C.; Giacomo, N. D. Combust. Sci. Technol. 1998, 137, 31–50. (14) Kitamura, T.; Ito, T.; Senda, J.; Fujimoto, H. JSAE ReV. 2001, 22, 139–145. (15) Mueller, C. J.; Martin, G. C. SAE Tech. Pap. 2002-01-1631, 2002. (16) Ahmed, I. SAE Tech. Pap. 2001-01-2475, 2001. (17) Satge´ de Caro, P.; Mouloungui, Z.; Vaitilingom, G.; Berge, J. Ch. Fuel 2001, 80, 565–574.

10.1021/ef800550k CCC: $40.75  2008 American Chemical Society Published on Web 11/07/2008

Oxidation of Acetylene-Ethanol Mixtures

of contaminants9,10,14 and taking into account the technical difficulties on diesel-ethanol blending, it is interesting to carry out research work that allows us to understand the influence of ethanol addition on the behavior of soot precursors, such as small hydrocarbons and aromatic compounds.6,9,10,12,14,19,20 In this work, acetylene was selected as fuel because of its use in other previous research works related to soot formation, e.g.,21-23 because it is recognized as one of the most important soot precursors,24-26 and the availability of a chemical kinetic mechanism for modeling its conversion.1 Different studies concerning acetylene oxidation have been reported until now.1,2,27-30 The oxidation3,31-33 and pyrolysis34 of ethanol have also been studied earlier. However, no work has been found in the literature that specifically investigates the influence of the ethanol addition on the acetylene oxidation. In addition, the reactivity of mixtures and the mechanisms through which the oxidation of a mixture occurs can be different from those corresponding to the sum of the individual component oxidations; hence, it is necessary to study the behavior of specific mixtures in different operating conditions. Furthermore, the capacity of hydrocarbons to reduce NO in reburning-type reactions is well-known.4,35,36 The hydrocarbon radicals generated from the acetylene-ethanol mixture conversion can interact with NO and therefore reduce the final NOx emissions. Several studies were devoted to the establishment of a kinetic scheme for NO reburning using various fuels, e.g.,4,35-39 demonstrating that ketyl radicals (HCCO) play a significant role in the reduction of NO by light hydrocarbons. These radicals are also well-known to be formed through acetylene oxidation.1,2,27-30 The study of the capacity of NO (18) Jia, L. W.; Shen, M. Q.; Wang, J.; Lin, M. Q. J. Hazard. Mater. 2005, 123, 29–34. (19) Kohse-Hoïnghaus, K.; Osswald, P.; Struckmeier, U.; Kasper, T.; Hansen, N.; Taatjes, C. A.; Wang, J.; Cool, T. A.; Gon, S.; Westmoreland, P. R. Proc. Combust. Inst. 2007, 31, 1119–1127. (20) Bo¨hm, H.; Braun-Unkhoff, M. Combust. Flame 2008, 153, 84– 96. (21) Mendiara, T.; Domene, M. P.; Millera, A.; Bilbao, R.; Alzueta, M. U. J. Anal. Appl. Pyrolysis 2005, 74, 486–493. (22) Ruiz, M. P.; Callejas, A.; Millera, A.; Alzueta, M. U.; Bilbao, R. J. Anal. Appl. Pyrolysis 2007, 79, 244–251. (23) Ruiz, M. P.; Guzmań de Villoria, R.; Millera, A.; Alzueta, M. U.; Bilbao, R. Chem. Eng. J. 2007, 127, 1–9. (24) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26, 565–608. (25) Appel, J.; Bockhorn, H.; Frenklanch, M. Combust. Flame 2000, 121, 122–136. (26) Frenklach, M. Phys. Chem. Chem. Phys. 2002, 4, 2028–2037. (27) Hidaka, Y.; Hattori, K.; Okuno, T.; Inami, K.; Abe, T. Combust. Flame 1996, 107, 401–417. (28) Ryu, J. C.; Seo, H.; Kang, J. G.; Oh, K. H. Bull. Korean Chem. Soc. 1996, 18, 1071–1075. (29) Laskin, A.; Wang, H. Chem. Phys. Lett. 1999, 303, 43–49. (30) Varatharajan, B.; Williams, F. A. Combust. Flame 2001, 125, 624– 645. (31) Norton, T. S.; Dryer, F. L. Int. J. Chem. Kinet. 1992, 24, 319–344. (32) Marinov, N. M. Int. J. Chem. Kinet. 1999, 31, 183–220. (33) Kasper, T. S.; Osswald, P.; Kamphus, M.; Kohse-Hoïnghaus, K. Combust. Flame 2007, 150, 220–231. (34) Peg, M.; Esarte, C.; Ruiz, M. P.; Millera, A.; Bilbao, R.; Alzueta, M. U. Proceedings of the European Combustion Meeting, 2007. (35) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1997, 109, 25–36. (36) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M. Fuel 1999, 78, 1245–1252. (37) Bilbao, R.; Millera, A.; Alzueta, M. U.; Prada, L. Fuel 1997, 76, 1401–1407. (38) Prada, L.; Miller, J. A. Combust. Sci. Technol. 1998, 132, 225– 250. (39) Faravelli, T.; Frassoldati, A.; Ranzi, E. Combust. Flame 2003, 132, 188–207.

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reduction by acetylene has already been carried out.4,36,38,40 In this way, the results obtained on the reduction of NO in the presence and absence of ethanol40 will be compared in the present work. In this context, the aim of the present work is to achieve a better understanding of the oxidation process of acetylene-ethanol mixtures, as well as their interactions with NO, analyzing the reaction pathways through which the process occurs. The oxidation of the acetylene-ethanol mixtures has been studied under flow reactor conditions at atmospheric pressure in the 775-1375 K temperature range. For a given acetylene concentration, different amounts of ethanol have been added and the oxygen concentration has been varied to achieve different air excess ratios. The experiments have been carried out in both the absence and presence of nitric oxide, aiming to analyze the interactions between the mixture and NO and its capacity to reduce such a contaminant. Experimental results have been simulated and interpreted in terms of a detailed gas-phase kinetic mechanism built from individual reaction subsets taken from the literature,1-4 for the oxidation of acetylene and ethanol, in both the absence and presence of NO. 2. Experimental Section The experimental installation used in the present work is described in detail elsewhere,3,41,42 and only a brief description is given here. A quartz plug flow reactor, according to the design of Kristensen et al.,43 is placed in a three-zone electrically heated oven, securing a uniform temperature profile throughout the reaction zone within (10 K. The temperature in the reaction zone was measured with a type-K fine-wire thermocouple placed into a thin tube along the reactor without contact with reactants. The reactor tube has a reaction zone of 8.7 mm inside diameter and 200 mm in length. Pure gases from gas cylinders are led to the reactor, through Fisher-Rosemount mass flow controllers, in up to four separate streams. A main flow containing nitrogen and water vapor, which is fed by saturating a nitrogen stream through a bubbling water system at room temperature, and three injector tubes for the rest of reactants (C2H2, C2H5OH, NO, and O2; all of them diluted in nitrogen) and N2 to balance up to obtain a total flow rate of 1000 (STP) mL/min. The configuration of the injection system has been designed following the investigations of Alzueta et al.35 After the product gas passed through the reactor, it is efficiently quenched at the outlet of the reaction zone by means of external refrigeration with cooling air. Prior to the analysis system, a particle filter is placed to retain any impurity. The CO, CO2, and NO concentrations have been measured by means of Uras14/IR analyzers. The HCN is measured with an Ati Mattson Fourier transform infrared (FTIR) spectrometer. The gas stream is also analyzed by an Agilent 6890 gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). A HP-PLOT Q capillary column of 50 m × 0.2 mm × 0.5 µm was used with the FID, and the TCD is accompanied with a HP-PLOT Q capillary column of 30 m × 0.53 mm × 40 µm and a molecular sieve of 15 m × 0.53 mm × 25 µm. Hydrocarbons, ethanol, and permanent gases (such as CO and CO2) have been analyzed. Very good agreement between the CO and CO2 data was obtained by both a CO/CO2 analyzer and GC. Apart from C2H2, C2H5OH, CO, and CO2, which are the majority gases, the GC equipment was calibrated to quantify other species, such as H2, CH4, C2H6, C2H4, C3H4, C3H6, (40) Giner, F. J. Estudio de la oxidacioń de acetileno y su interaccioń con NO. Master’s Thesis, University of Zaragoza, Zaragoza,, Spain, 2006 (in Spanish). (41) Alzueta, M. U.; Bilbao, R.; Finestra, M. Energy Fuels 2001, 15, 724–729. (42) Alzueta, M. U.; Tena, A.; Bilbao, R. Combust. Sci. Technol. 2002, 174, 151–169. (43) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1996, 107, 211–222.

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Table 1. Matrix of Experimental Conditionsa experiment

λ

[C2H2] (ppm)

set 1 set 2 set 3 set 4 set 5 set 6 set 7 set 8 set 9 set 10 set 11 set 12 set 13 set 14 set 15 set 16 set 17 set 18 set 19 set 20 set 21 set 22

1 1 1 1 0.7 0.7 0.7 0.7 0.2 0.2 0.2 0.2 20 20 20 20 1 0.7 0.2 1 0.7 0.2

500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500

[C2H5OH] (ppm)

[O2] (ppm)

0 50 100 200 0 50 100 200 0 50 100 200 0 50 100 200 100 100 100

1250 1400 1550 1850 875 980 1085 1295 250 280 310 370 25000 28000 31000 37000 1550 1085 310 1250 875 250

[NO] (ppm)

sourceb

500 500 500 500 500 500

pw pw pw pw pw pw pw pw pw pw pw pw pw pw pw pw pw pw pw ref 40 ref 40 ref 40

a All of the experiments are run at a given flow rate of 1000 mL (STP)/min, resulting in a residence time dependent upon the temperature of 195/T (K) seconds. Nitrogen is used to balance. Water vapor was kept constant in all of the experiments with a concentration of 7000 ppm. b “pw” denotes present work.

C3H8, isobutene, n-butane, 1,3-butadiene, C6H6, ethylbenzene, C7H8, and xylenes. Among those, only appreciable amounts of CH4, C2H4, and C2H6 were detected within the uncertainty of the measurements, which is estimated as (5% but not less than 10 ppm. The experiments were carried out at atmospheric pressure in the 775-1375 K temperature range for different stoichiometries, ranging from fuel-rich (λ ) 0.2) to very fuel-lean (λ ) 20) conditions. The total gas flow rate was kept constant during the experiments, 1000 mL (STP)/min, leading to different residence times (tr) depending upon the temperature in the reaction zone, tr (s) ) 195/T (K). Table 1 lists the conditions of the different experiments performed, with all of them carried out under high diluted conditions to ensure an isothermal reaction zone. Additional experimental data were taken from previous work of Giner,40 who, in the same experimental setup, investigated C2H2/NO interactions under the same conditions as the ones in the present work but without adding ethanol. Carbon balance was performed by a comparison of the carbon contained in the exhaust gas and the carbon contained in the reactants fed to the reactor. The carbon balance checked for λ ) 20, 1, and 0.7 was found very good. Nevertheless, the carbon balance sometimes did not close so satisfactorily, especially under very fuel-rich conditions (λ ) 0.2) and low ethanol concentrations (0 and 50 ppm). It can be due to the fact that in these experiments a thin layer of soot was formed on the reactor walls. For higher ethanol concentrations, no soot was formed. Because these operating conditions (λ ) 0.2) are close to pyrolysis, it is thought that soot could be formed at high temperatures.23

3. Reaction Mechanism The experimental results have been analyzed in terms of a detailed gas-phase chemical kinetic model for the oxidation of acetylene-ethanol mixtures in the absence and presence of NO. The mechanism used for the modeling study was that developed by Alzueta et al.1 for acetylene conversion, on the basis of previous work by Skjøth-Rasmussen et al.2 for benzene formation, using methane or methane doped with C2, C3, and C4 hydrocarbons, as the initial hydrocarbon, under fuel-rich conditions. This mechanism1 was previously used for modeling acetylene conversion and was able to reproduce the main

experimental trends and concentration results for C2H2, CO, CO2, and intermediates, such as C2H4. Additional reactions were added from the ethanol oxidation mechanism of Alzueta and Hernańdez,3 as well as reactions from the mechanism developed by Glarborg et al.,4 updated by Glarborg et al.,44 to describe the interactions among C1/C2 hydrocarbons and nitric oxide. Calculations are performed using Senkin,45 which runs in conjunction with the Chemkin library.46 The reverse rate constants were obtained from the forward rate constants, and the thermodynamic data were taken from the same sources as the different submechanisms. The full reaction mechanism includes 78 species and 532 reversible reactions and can be obtained directly from the authors. 4. Results and Discussion A study of the oxidation of acetylene-ethanol mixtures at atmospheric pressure in the temperature range of 775-1375 K has been carried out. In addition to temperature, the influence of the air excess ratio (λ), the amount of ethanol added to the blend, and the NO presence have been analyzed. The main results obtained in this study are shown below. The influence of these variables on this process has been analyzed by measuring the output concentration of the reaction zone of different carbonaceous species (C2H2, C2H5OH, CO, CO2, CH4, C2H4, and C2H6) and nitrogenous species (NO and HCN). Only under very fuel-rich conditions (λ ) 0.2), methane, ethane, and ethylene in small amounts were detected in the chromatograms. These concentrations can be considered negligible compared to the concentrations of the major products quantified (C2H2, C2H5OH, CO, CO2, NO, and HCN), and the results of those species are not shown. 4.1. Influence of Stoichiometry on the Acetylene-Ethanol Mixture Oxidation. To evaluate the influence of the air excess ratio (λ), the results of the experiments performed with an initial concentration of 100 ppm of ethanol and 500 ppm of acetylene, sets 3, 7, 11, and 15 in Table 1, have been selected. Figures 1 and 2 show, respectively, both the experimental and calculated results of acetylene and ethanol concentrations and the formation of CO and CO2 as a function of the temperature for different air excess ratios, ranging from fuelrich (λ ) 0.2) to fuel-lean (λ ) 20) conditions. Symbols denote experimental results, and lines denote model calculations. The mechanism used provides, in general, good agreement between modeling predictions and experimental results, reproducing well the experimental trends obtained, under the conditions of this study. As can be seen in Figure 1, the onset temperature of acetylene and ethanol conversion is similar for the stoichiometries studied, around 900 K. Because the acetylene-ethanol mixture becomes richer (λ < 1), the window for further reaction is shifted to higher temperatures. Independent of the oxygen availability, ethanol presents conversions of 100% (Figure 1). This can be due to the fact that the presence of oxygen in the ethanol molecule favors a major extent of its oxidation,3 even under fuel-rich conditions. In the case of acetylene, it can be observed that, at the leanest conditions studied, the full conversion of acetylene is produced (44) Glarborg, P.; Alzueta, M. U.; Kjærgaard, K.; Dam-Johansen, K. Combust. Flame 2003, 132, 629–638. (45) Lutz, A. E.; Kee, R. J.; Miller, J. A. Senkin: A Fortran Program for Predicting Homogeneous Gas Phase Chemical Kinetics with Sensitivity Analysis. Sandia National Laboratories Report SAND87-8248, 1988. (46) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics. Sandia National Laboratories Report SAND87-8215, 1991.


Figure 1. Concentrations of C2H2 and C2H5OH as function of the temperature for different air excess ratios. A comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 3, 7, 11, and 15 in Table 1. Inlet conditions: 500 ppm C2H2 and 100 ppm C2H5OH.

Figure 2. Concentrations of CO and CO2 as function of the temperature for different air excess ratios. A comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 3, 7, 11, and 15 in Table 1.


at approximately 100 K below compared to the λ ) 0.7 and stoichiometric conditions. The acetylene conversion increases with an increasing temperature for all of the blends studied. For the richest fuel conditions considered (λ ) 0.2), acetylene is not totally consumed, even at the higher temperatures studied, as can be seen in Figure 1. Once ethanol is totally consumed, the acetylene concentration shows a slightly sharper decay. The onset of CO formation (Figure 2) happens in parallel with the decrease in acetylene and ethanol concentrations, increasing when temperature increases up to reach a maximum around 1025 K, and subsequently, its concentration decreases monotonically. The availability of oxygen affects the width of the CO peak and thereby the CO2 concentration profiles. In this way, as the mixture becomes leaner, the CO concentration profile is shifted to lower temperatures as it becomes sharper, keeping the same tendency for all of the studied conditions, with the exception of very fuel-rich conditions (λ ) 0.2). In this latter case, the CO concentration increases as the temperature increases in all of the temperature range studied. The CO2 concentration shows a sharper formation as the oxygen availability is increased, coinciding with the peak in CO. At higher temperatures and/or leaner conditions, the oxidation of CO to CO2 is favored. In general, modeling predictions are in reasonably good agreement with the experimental observations. Nevertheless, as the acetylene-ethanol mixture becomes richer (λ ) 0.7 and lower), the theoretical results separate from the experimental ones, especially regarding the CO and CO2 concentrations. This can be due to the fact that the model exclusively includes the gas-phase reactions; thus, it does not consider the pathways for polycyclic aromatic hydrocarbon (PAH) formation and soot nucleation that may be important as the mixture becomes richer. The main routes for ethanol and acetylene consumption are discussed below. These routes include mainly reactions with O2 and different radicals (OH, H, O, CH3, and HO2). In general, alcohols may undergo dehydration (loss of OH or H2O), decomposition reactions (bond cleavage), and dehydrogenation (loss of H by abstraction), leaving the O atom of the original molecule intact and leading directly to the production of an oxygenated intermediate (aldehyde or ketone) under conditions of intermediate temperature.47 Frenklach and Yuan5 determined that the source of OH radicals, which can oxidize soot precursors, during ethanol pyrolysis at high temperature, was obeying to the following sequence of reactions: C2H5OH (+M) a C2H4 + H2O (+M)

(1)

H2O + H a OH + H2

(2)

In this work, reaction rate analysis was performed to determine the ethanol consumption routes for the many reaction intermediates formed. The ethanol dehydrogenation step was found to be dominant for the initiation reactions of ethanol conversion. These results are in agreement with the remarks of Norton and Dryer,47 who pointed out that dehydrogenation of ethanol is significantly faster than dehydration under flow reactor conditions at atmospheric pressure and initial temperatures near 1100 K. The reaction rate analysis showed that the main reaction pathways achieved for ethanol conversion are basically the same as those proposed by Alzueta and Hernańdez.3 The oxidation of ethanol is initiated by reaction with the radical pool, mainly with OH radicals, even though the interaction with H and CH3 (47) Norton, T. S.; Dryer, F. L. Proc. Combust. Inst. 1991, 23, 179– 185.


Abiań et al.

radicals is also significant. The CH3 radicals are formed, to a major extent, from both the acetylene and ethanol consumption routes, and their formation reactions, reactions 4, 6, 22, and 23, will be seen later on. Independent of the stoichiometry conditions considered, ethanol reacts with these radical species through H abstraction, producing three different isomeric forms of the C2H5O radical CH3CH2OH + /OH/H/O/CH3/HO2 a CH3CHOH + /H2O/H2/OH/CH4/H2O2 CH3CH2OH + /OH/H/O/CH3/HO2 a CH3CH2O + /H2O/H2/OH/CH4/H2O2 CH3CH2OH + /OH/H/O/CH3/HO2 a CH2CH2OH + /H2O/H2/OH/CH4/H2O2 The important roles in the reaction mechanism of these three isomeric radicals for ethanol oxidation have been demonstrated earlier.3,31,32,47 The CH3CHOH radical, being dominant under all of the conditions of the present work, reacts almost exclusively with molecular oxygen CH3CHOH + O2 a CH3HCO + HO2

(3)

giving acetaldehyde, which in turns reacts with the radical pool, suffering hydrogen abstraction and producing the acetyl radical CH3HCO + /OH/H/O/ a CH3CO + /H2O/H2/OH which subsequently decomposes into CH3 radicals and CO CH3CO (+M) a CH3 + CO (+M)

(4)

The CH3CH2O radical formed is converted into either CH3HCO, reaction 5, or decomposed to formaldehyde and methyl radicals, reaction 6. CH3CH2O + M a CH3HCO + H + M

(5)

CH3CH2O + M a CH3 + CH2O + M

(6)

The formaldehyde formed by reaction with the radical pool follows the CH2O f HCO f CO f CO2 sequence. Formaldehyde is also the main product coming from the reaction of the third isomer formed, CH2CH2OH, with O2 CH2CH2OH + O2 a CH2O + CH2O + OH

(7)

The reaction rate analysis performed for the oxidation of different acetylene-ethanol mixtures allowed for the identification of the reactions that significantly contribute to acetylene consumption. Because of the strong C-H bond in acetylene,1,29 this species has difficulty in undergoing abstraction reactions with radicals, as many hydrocarbons do. Acetylene thus undergoes addition reactions generating intermediate adducts, followed by either reverse reaction or reaction into new different products.1 The initiation reactions for acetylene conversion, under the conditions of this work, include its interaction with O2 and the H, O, and OH radical pool. The addition reaction of C2H2 and H radicals to produce vinyl radicals, reaction 8, appears to be the main acetylene consumption reaction under fuel-rich and stoichiometric conditions. Varatharajan and Williams30 identified the formation of vinyl radical as a key step during acetylene consumption at low temperatures. The vinyl radical oxidation produces, on the one hand, HCO and CH2O radicals, reaction 9, which are subsequently oxidized to CO and CO2. On the other hand, the oxidation of vinyl radicals also produces CH2HCO, reaction 10, which, in most cases, derives to the generation of CH2O, directly through reaction 11 and, subsequently, by reactions 12-14.

C2H2 (+M) + H a C2H3 (+M)

(8)

C2H3 + O2 a CH2O + HCO

(9)

C2H3 + O2 a CH2HCO + O

(10)

CH2HCO + O2 a CH2O + CO + OH

(11)

CH2HCO a CH3 + CO

(12)

CH3 + HO2 a CH3O + OH

(13)

CH3O (+M) a CH2O + H (+M)

(14)

The reactions of C2H2 with H radicals are in competition with the O radical interactions, giving two important product channels, namely, C2H2 + O a CO + CH2

(15)

C2H2 + O a HCCO + H

(16)

with the formation of HCCO, reaction 16, as the dominant product channel under fuel-rich conditions. The HCCO formed is directly oxidized to CO and CO2 by interaction with O2. For temperatures up to 1000 K, reactions 15 and 16 are not very fast and therefore the concentration of radicals present is a determinant of whether or not they occur. As the temperature increases, the radical pool is significantly increased and the conversion of acetylene becomes faster, as observed in Figure 1, accompanied by the formation of CO and CO2. Apart from the interactions with H and O radicals, which are the main consumption routes under all of the stoichiometries considered, acetylene is also consumed by a number of reactions on a comparatively minor scale depending upon the oxygen level. The interactions with O2 and OH radicals gain importance as the reaction environment becomes leaner, through C2H2 + O2 a HCO + HCO

(17)

C2H2 (+M) + OH a C2H2OH (+M)

(18)

C2H2 + OH a C2H + H2O

(19)

C2H2 + OH a CH2CO + H

(20)

C2H2 + OH a HCCOH + H

(21)

It is important to notice in Figure 1 that both model calculations and experimental results show a sharper decay of C2H2 concentration with a sharper formation of CO and CO2 (Figure 2), as the oxygen availability is increased. This is attributed to the fact that, after the initiation of the reaction process, the main reactions for acetylene conversion at lean conditions are the interactions with O and O2 radicals, reactions 16 and 17, followed by the formation of CO and CO2 from the HCCO and the fast conversion of HCO into CO. Taking into account the reaction rate analysis performed, it can be said that under very fuel-rich conditions (λ ) 0.2), when the available oxygen and ethanol have been consumed, acetylene follows reacting (Figure 1) and the concentrations of CO and CO2 increase in all of the ranges of temperatures studied (Figure 2). These routes for acetylene conversion can be both the thermal decomposition of acetylene (promoted by temperature) and its oxidation, reactions 15, 16, 20, and 21, favored by the oxygen content in the ethanol molecule, which converts ethanol into a source of O and OH radicals. Because it has already been mentioned, under fuel-rich conditions (λ ) 0.2), small amounts of methane, ethane, and ethylene were detected in the chromatograms. Through the reaction rate analysis, it has been possible to determine how these species can be generated.



Model calculations show that ethylene detected is directly produced from ethanol decomposition, reaction 1. The methyl radical, CH3, is a necessary precursor species to methane and ethane formation and evolves primarily from the decomposition of both CH3CO, reaction 4, and CH3CH2O, reaction 6, and to a smaller extent from CH2HCO a CH3 + CO

(22)

CH2(s) + H2 a CH3 + H

(23)

Ethane is formed exclusively by methyl radical recombination, reaction 24 CH3 + CH3 (+M) a CH3CH3 (+M)

(24)

Ethane and ethylene, which is a stable form of vinyl radical, may participate in reactions with benzene or with the C2 derivatives to produce PAHs,48 which are precursors of soot,24,25,49,50 which would contribute to the soot formed in these conditions (λ ) 0.2). Finally, methane formation is primary controlled by the following reactions, with a very low generation rate at low temperatures: CH3 + H (+M) a CH4 (+M)

(25)

CH4 + O2 a CH3 + HO2

(26)

CH2O + CH3 a HCO + CH4

(27)

whereas at intermediate temperatures, around 1075 K, only the hydrogenation of the methyl radical, reaction 25, contributes to methane formation. 4.2. Influence of the Amount of Ethanol Added on the Oxidation of Acetylene-Ethanol Mixtures. To analyze the influence of the amount of ethanol added to the mixture on the main products obtained (C2H2, C2H5OH, CO, and CO2), the experiments performed under stoichiometric conditions (λ ) 1) have been selected, sets 1-4 in Table 1. Figure 3 shows the concentrations of acetylene and ethanol, for different amounts of ethanol added to the mixture, as a function of the temperature for stoichiometric conditions (λ ) 1). In comparison to the evolution of acetylene and ethanol concentrations, it can be said that ethanol is more reactive than acetylene, being consumed at lower temperatures. Notwithstanding, once ethanol is consumed, the acetylene concentration shows a sharper decay. The major differences between alcohols and hydrocarbons are the presence of an oxygen atom and the reduced R-C-H bond strength in an alcohol molecule (those attached to the same carbon atom as the OH group).6,47 Moreover, ethanol, as a primary alcohol, is more susceptible to dehydrogenation than to dehydration, because of the weakness of the R-C-H bond,47 which is in agreement with the results obtained in this study. Therefore, the direct production of aldehydes from ethanol causes this fuel to have a shorter reaction time than acetylene does as hydrocarbon. In this way, the faster conversion of ethanol can be explained. From Figure 3, it can also be pointed out that the presence of ethanol produces a variation in the temperature regime for acetylene conversion, shifting the acetylene conversion at higher temperatures as the amount of ethanol added is increased. (48) Ledesma, E. B.; Marsh, N. D.; Sandrowitz, A. K.; Wornat, M. J. Proc. Combust. Inst. 2002, 29, 2299–2306. (49) Thomas, S.; Ledesma, E. B.; Wornat, M. J. Fuel 2007, 86, 2581– 2595. (50) Haynes, B. S.; Wagner, H. G. Prog. Energy Combust. Sci. 1981, 7, 229–273.

Figure 3. Concentrations of C2H2 and C2H5OH as a function of the temperature depending upon the amount of C2H5OH added to the blend for stoichiometric conditions (λ ) 1). A comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1-4 in Table 1.

The reaction rate analyses performed show that the main reaction pathways for acetylene conversion in the presence of ethanol are equal to those obtained in the absence of ethanol. In such a way, the effect of ethanol addition comes from its capacity of modifying the composition of the radical pool, thus limiting benzene and soot precursor growth. Because of the oxygen content in ethanol, the oxidation of acetylene and intermediates toward CO and CO2 seems to be favored, preventing the formation of soot precursors. Under the studied conditions, the initiation reactions for acetylene conversion present little dependence with the ethanol concentration in the blend. Nevertheless, the favorite consumption route depends upon the amount of ethanol added. In such a way, as the inlet ethanol concentration is increased, the reaction C2H2 + O f HCCO + H (reaction 16), appears to be favored. To elucidate the influence of the amount of ethanol added on the CO and CO2 yields, the COoutlet/(2[C2H2 + C2H5OH]inlet) and CO2outlet/(2[C2H2 + C2H5OH]inlet) ratios have been defined as the percentage of the amount of carbon present in CO and CO2, respectively, related to the amount of carbon fed into the reactor from the acetylene and ethanol molecules. Figure 4 displays the COoutlet/(2[C2H2 + C2H5OH]inlet) ratio and CO2outlet/(2[C2H2 + C2H5OH]inlet) ratio values, for different amounts of ethanol added to the mixture, as a function of the temperature for stoichiometric conditions. The above-cited ratios keep the same tendency, despite the amount of ethanol present in the mixture. When the amount of ethanol added increases, the peak decay in the ratio involving CO is shifted toward higher temperatures, which is in correlation with the shift of acetylene conversion toward upper temperatures. This is more evident when 200 ppm ethanol is added. In this case, the complete


Figure 4. COoutlet/(2[C2H2 + C2H5OH]inlet) and CO2outlet/(2[C2H2 + C2H5OH]inlet) ratios as a function of the temperature depending upon the amount of C2H5OH added for stoichiometric conditions (λ ) 1). A comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1-4 in Table 1.

oxidation to CO2 is favored, which might be due to the oxygen content in ethanol. A sensitivity analysis to the kinetic parameters included in the mechanism used for simulations has been performed. To do this, the impact on the CO concentration has been selected. Thus, Table 2 shows a first-order sensitivity analysis for CO. The temperatures chosen for carrying out the sensitivity analysis correspond to the initiation conditions of the conversion of the acetylene-ethanol mixtures, i.e., when CO has reached a value of approximately 20 ppm, in the experiments corresponding to sets 1-3, 7, 11, 15, and 17-19 in Table 1. Sensitivity coefficients for acetylene-ethanol blend oxidation in the absence of NO are those corresponding to sets 1-3, 7, 11, and 15, while coefficients in the presence of NO correspond to sets 17-19. Only reactions that exhibit an absolute linear sensitivity coefficient for CO g 0.1 in any of the experimental sets selected are shown. Overall, the sensitivity analysis indicates that acetylene conversion in the presence of ethanol is sensitive to almost the same reactions as in the absence of this additive. Results are sensitive to a number of reactions involving HCO and CH2O, both as reactants or products, because these reactions determine the consumption of C2H2 and C2H5OH to give the final oxidation products. Also, it is interesting to note that the onset of CO formation is very sensitive to the ethanol-radical reactions, in particular those involving the OH radical, making us to notice the importance of the formation of the three isomeric forms of the C2H5O radical (mentioned above) for ethanol oxidation, when it is added to the blend. In addition, the predicted onset of CO formation (in the absence of NO) is very sensitive to the H + O2 + N2 f HO2 + N2 reaction, with a negative sensitivity coefficient. This reaction under low-temperature conditions acts as a radical sink, thus limiting the increase of H and O radicals.

Abiań et al.

4.3. Interactions between Acetylene-Ethanol Mixtures and NO. Nitrogen oxides (NOx), responsible for acid rain among other harmful effects, are produced in combustion processes mainly through the thermal and fuel NO formation mechanism. Nitrogen oxides may interact with acetylene-ethanol mixtures or their derivates, achieving their reduction. NO may be reduced in reburning-type reactions under fuel-rich conditions35,36,43,51 or may favor the oxidation of the mixture in a mutually sensitized oxidation process,39,52 as well as it was mentioned to happen with ethanol3,53 or methane oxidation52 To obtain better knowledge on the interactions between the acetylene-ethanol mixtures and nitric oxide, experiments in the presence of NO, under fuel-rich (λ ) 0.7 and 0.2) and stoichiometric conditions (λ ) 1), have been carried out (sets 17-19 in Table 1). The present experimental results and those of Giner40 for the reduction of NO with C2H2 (sets 20-22 in Table 1) are compared to elucidate the impact of the ethanol presence in the C2H2-C2H5OH mixture for the NO reduction. The corresponding C2H2 and NO concentrations obtained are shown in Figure 5 as a function of the temperature for different air excess ratios. The general shape of the C2H2 and NO profiles does not change with the addition of ethanol. The onset for acetylene conversion as well as NO reduction is produced at lower temperatures when ethanol is not added. The observed decrease in the NO concentration occurs coinciding with the sharp decay in acetylene concentration. Independent of the ethanol presence, in those experiments in which acetylene has been completely consumed, the NO concentration remains approximately constant. As can be observed in Figure 5, significant NO reductions, intherangeof35-38%,havebeenachievedbytheacetylene-ethanol mixtures under stoichiometric (λ ) 1) and λ ) 0.7 conditions. Experiments with only acetylene show a somewhat slightly larger NO reduction than the mixtures, reaching reduction levels around 40% for λ ) 0.7. It is important to notice that, under the conditions studied, pure ethanol oxidation does not show any NO reduction by reburn-type reactions.3 Therefore, it can be concluded that the ethanol presence modifies the composition of the radical pool because of the oxygen content in the ethanol molecule, promoting the oxidation of hydrocarbon radicals to CO and CO2, generating comparatively less active hydrocarbon radicals able to interact with NO. Once established that the acetylene-ethanol mixtures are able to significantly reduce NO, a more detailed study on the interactions between the mixture and NO has been carried out, to analyze the influence of the NO presence in the oxidation of the mixtures. Figure 6 displays the results of acetylene and ethanol concentrations obtained during the oxidation of the acetyleneethanol blend, in the presence and absence of NO, as a function of the temperature for different air excess ratios (λ), corresponding to the experiments of sets 1-3 and 17-19 in Table 1. It can be observed that the shape of the C2H2 and C2H5OH concentration profiles does not change with the addition of NO. The results of the ethanol concentration show that, for λ ) 0.7 and stoichiometric conditions, the presence of NO enhances the onset of the ethanol conversion, shifting it to lower temperatures. As the temperature increases, the enhancing effect of the NO presence on the conversion of ethanol becomes less effective, and for temperatures above (51) Miller, J. A.; Klippenstein, S. J.; Glarborg, P. Combust. Flame 2003, 135, 357–362. (52) Dagaut, P.; Nicolle, A. Combust. Flame 2005, 140, 161–171. (53) Taylor, P. H.; Cheng, L.; Dellinger, B. Combust. Flame 1998, 115, 561–567.



Table 2. Linear Sensitivity Coefficients for CO at Selected Temperatures for the Selected Setsa reaction C2H2 + O f HCCO + H C2H2 + O f CH2 + CO C2H2 + O2 f HCO + HCO H + C2H2 (+M) f C2H3 (+M) C2H3 + O2 f C2H2 + HO2 C2H3 + O2 f CH2O + HCO C2H3 + O2 f CH2HCO + O CH3 + CH3 (+M) f C2H6 (+M) CH3 + HO2 f CH3O + OH CH2 + O2 f CO + H2O CH2O + OH f HCO + H2O HCO + M f H + CO + M HCO + O2 f HO2 + CO O + OH f O2 + H H + O2 + N2 f HO2 + N2 H + HO2 f H2 + O2 H + HO2 f 2OH OH + HO2 f H2O + O2 H2O2 + M f OH + OH + M C2H5OH (+M) f CH2OH + CH3 (+M) C2H5OH + OH f C2H4OH + H2O C2H5OH + OH f CH3CHOH + H2O C2H5OH + OH f CH3CH2O + H2O C2H5OH + H f C2H4OH + H2 C2H5OH + H f CH3CHOH + H2 CH3CH2O + M f CH3HCO + H + M CH3CH2O + M f CH3 + CH2O + M CH3 + NO2 f CH3O + NO NO2 + H f NO + OH C2H3 + NO f C2H2 + HNO

set 1 925 K

set 2 875 K

set 3 900 K

set 7 900 K

set 11 1050 K

set 15 875 K

set 17 875 K

set 18 900 K

set 19 1050 K

0.416 -0.395 1.042 0.051 -0.436 0.258 0.279 -0.020 0.252 -0.206 0.671 1.904 -1.894 2.205 -2.137 -0.547 0.663 -1.008 0.134

0.133 -0.104 1.097 0.128 -0.076 0.065 0.073 -0.108 0.403 -0.049 0.259 0.351 -0.349 0.625 -0.406 -0.166 -0.106 -1.398 0.072 0.006 -0.218 -0.117 0.157 -0.082 -0.125 0.146 -0.146

0.142 -0.095 1.014 0.120 -0.050 0.068 0.049 -0.129 0.442 -0.045 0.249 0.214 -0.2117 0.685 -0.271 -1.828 -0.156 -0.111 0.045 0.017 -0.239 -0.075 0.147 -0.136 -0.173 0.184 -0.184

0.137 -0.092 0.938 0.116 -0.049 0.121 0.043 -0.130 0.430 -0.036 0.259 0.126 -0.124 0.693 -0.187 -0.194 -0.166 -0.096 0.021 0.040 -0.216 -0.066 0.099 -0.165 -0.199 0.191 -0.191

0.064 -0.041 0.400 0.017 -0.006 0.195 0.016 -0.113 0.283 -0.006 0.180 0.006 -0.005 0.398 -0.012 -0.150 -0.086 -0.059 0.002 0.239 -0.089 -0.004 -0.027 -0.123 -0.072 0.096 -0.096

0.062 -0.033 0.960 0.000 0.005 -0.002 0.006 -0.041 0.168 -0.019 0.041 0.118 -0.118 0.360 -0.328 -0.020 0.012 -0.169 0.315

0.014 -0.023 0.949 0.126 -0.042 0.126 0.056 -0.115 0.001 -0.024 0.266 0.153 -0.086 0.477 -0.111 -0.001 -0.001 -0.001 0.003 0.012 -0.457 0.088 0.178 -0.139 -0.054 0.142 -0.142 0.369 -0.293 0.129

0.001 -0.012 0.882 0.115 -0.037 0.189 0.046 -0.109 0.001 -0.017 0.240 0.090 -0.042 0.431 -0.063 -0.001 -0.001 -0.001

-0.024 0.011 0.314 0.011 0.004 0.211 0.010 -0.099 0.001 -0.002 0.152 0.008 0.006 0.241 0.021 -0.002 -0.002 -0.001

0.029 -0.355 0.089 0.093 -0.159 -0.051 0.144 -0.144 0.339 -0.275 0.095

0.223 -0.101 0.105 -0.093 -0.104 0.086 0.066 -0.066 0.211 -0.190

-0.127 -0.069 0.189 -0.003 -0.005 0.017 -0.017

a The sensitivity coefficients are given as A δY /Y δA , where A is the pre-exponential constant for reaction i and Y is the mass fraction of the jth i j j i i j species. Therefore, the sensitivity coefficients listed can be interpreted as the relative change in predicted concentration for the species j caused by increasing the rate constant for reaction i by a factor of 2.

Figure 5. Experimental concentrations of C2H2 and NO as a function of the temperature for different air excess ratios (λ) in the presence (empty symbols) and absence40 (solid symbols) of ethanol. The inlet conditions correspond to sets 17-21 in Table 1.

1025 K, the presence of NO shifts the ethanol concentration profile 200 K to higher temperatures compared to the results attained without NO present.

Figure 6. Concentrations of C2H2 and C2H5OH as a function of the temperature for different air excess ratios in the presence (empty symbols) and absence (solid symbols) of NO. A comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1-3 and 17-19 in Table 1.

In the case of acetylene (upper part of Figure 6), at temperatures above the onset for conversion, the presence of


Figure 7. Concentrations of NO and HCN as a function of the temperature for different air excess ratios (λ). A comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 17-19 in Table 1.

NO shifts C2H2 conversion to higher temperatures for all of the stoichiometries considered. In both the presence and absence of NO, the acetylene concentration shows a sharper decay as the oxygen availability is increased because of a shift in the radical pool toward O and OH radicals. Figure 7 shows the results of NO and HCN concentrations as a function of the temperature for different air excess ratios (λ), corresponding to the experiments of sets 17-19 in Table 1. The sharp decay in the acetylene concentration observed in Figure 6 occurs approximately at the same temperature as the NO concentration presents a pronounced descent (Figure 7). As the temperature increases, the NO concentration decreases gradually, reaching a reduction minimum around 1225 K for stoichiometric conditions and λ ) 0.7. The best conditions observed in Figure 7 for NO reduction by the acetylene-ethanol mixture correspond to high temperatures, above 1225 K, and moderately fuel-rich conditions (λ ) 0.7), as observed previously using simple hydrocarbons as reburn fuel.4,35,36 The formation of HCN (Figure 7) coincides in all of the cases studied with the observed reduction in the NO concentration. This is in agreement with the general understanding of the reburn chemistry, in which HCN is an intermediate product in the reduction of NO by hydrocarbon radicals.4,35,54 As the temperature increases, the HCN concentration increases, reaching values between 100 and 140 ppm for an initial NO concentration of around 500 ppm. However, for the highest temperature range studied, the concentration of HCN diminishes again, because the oxidation reactions of HCN become faster at those temperatures.54 At very fuel-rich conditions (λ ) 0.2), a significant concentration of HCN is observed, even at high temperatures. The shift from NO to HCN as the stoichiometry becomes very (54) Dagaut, P.; Glarborg, P.; Alzueta, M. U. Prog. Energy Combust. Sci. 2008, 34, 1–46.

Abiań et al.

fuel-rich is characteristic of the reburn zone chemistry.35 In this case, the oxygen level is low; consequently, the concentrations of the O and OH radicals remain low, and the rate of the destruction of HCN is significantly decreased.54 The kinetic model has been used for simulating the experimental results in the presence of NO. It has been able to capture the experimental trends obtained in Figures 6 and 7. However, significant quantitative differences can be remarked related to nitrogen-containing species, as shown in Figure 7. The model predicts higher NO reductions than those obtained experimentally. Furthermore, the formation of HCN is somewhat overpredicted at low temperatures, whereas at higher temperatures, the model matches the trends better. For very fuel-rich conditions, the model predictions do not fit the experimental data in such a satisfactory way. The concentrations for CO and CO2 (not shown) keep the same tendency as those obtained in the absence of NO, even though the CO peak level is found at temperatures comparatively higher. This peak coincides with the sharp decrease in the NO concentration, which remains approximately constant at the highest temperatures considered. However, CO by itself is not responsible for the destruction of NO, as was shown in reburning experiments carried out under similar conditions as the present ones.35 According to the literature, the interaction between hydrocarbons and nitrogen species under reburning conditions occurs primarily through the reaction of NO with CHi and HCCO radicals. Dependent upon the hydrocarbon fuel and the temperature, either HCCO or CH3 is the radical most active in reducing NO, with CH2 making a smaller contribution.4 The reaction between CH3 and NO is an important step when methane or natural gas are used as reburning fuel, whereas when acetylene is the fuel, the main route to NO reduction involves the HCCO radical and to a minor extent the CH2 radical, because acetylene is a direct source of these radicals.4,36 In this context, reaction rate analyses were performed to identify the reactions and species that significantly contribute to NO reduction during the oxidation of different acetylene-ethanol mixtures in the presence of NO. The reaction rate analyses show that, for fuel-rich and stoichiometric conditions, apart from the conversion of NO into NO2 by the reaction with the radical pool, mainly with HO2 radicals, reaction 28, there is another important pathway, which implies the conversion of NO into HNO by reaction with H radicals, reaction 29. NO + HO2 a NO2 + OH

(28)

NO + H + M a HNO + M

(29)

In both cases, NO2 and HNO are recycled back to NO by reaction with the radical pool reactions 30-32. Therefore, reactions forming these species do not contribute to NO removal.4 NO2 + H a NO + OH

(30)

HNO + H a NO + H2

(31)

HNO + OH a NO + H2O

(32)

At low temperatures, the NO/NO2 interconversion, reactions 28 and 30, activate the ethanol oxidation (Figure 6) because it is associated with the net effect of transforming the less reactive HO2 radicals into more reactive OH radicals, HO2 + H f 2OH, and with the direct formation of OH radicals, reaction 30. The suppression of the enhancing effect of the NO presence as the temperature is increased can be attributed to reaction 29 and reactions 31-34 under stoichiometric conditions and reactions



29, 31, and 32 under fuel-rich conditions. These reactions limit the growth of the radical pool, inhibiting ethanol oxidation and leading to the scavenging of reactive OH radicals, reaction 32. NO2 + O a NO + O2

(33)

NO + O (+M) a NO2 (+M)

(34)

Together to the NO/NO2 and NO/HNO interconversions, in the initial stages of the reaction, NO is reduced largely by the reaction with HCCO, reactions 35 and 36, and to a smaller extent with the CH2 radical, reaction 37. Both CH2 and HCCO radicals are produced by the reaction of acetylene with O, reactions 15 and 16, which are also the dominant consumption channel for C2H2 under stoichiometric and fuel-rich conditions. NO + HCCO a HCN + CO2

(35)

NO + HCCO a HCNO + CO NO + CH2 a HCN + OH

(36) (37)

At low temperatures, where the formation of CH3 from acetylene through the C2H2 f C2H3 f CH2HCO f CH3 sequence and from ethanol through reactions 4 and 6 is most pronounced, the CH3 + NO reaction is too slow to be competitive. The O2 competes with NO for HCCO and acts to limit NO removal by HCCO, reactions 35 and 36. Moreover, the recombination of C2H2 with H atoms, reaction 8, followed by reaction of C2H3 with O2, reactions 9 and 10, restrains the formation of HCCO and thereby the removal of NO. The HCNO is largely recycled back to NO by reaction with the radical pool HCNO + O a NO + HCO HCNO + OH a NO + CH2O

(38) (39)

The HCN formed through reactions 35 and 37 reacts with the radical pool through a number of reactions, which basically derivates in NCO formation. The NCO radical is mainly consumed by reaction with NO to produce either N2, reaction 40, or N2O, reaction 41. NCO + NO a N2 + CO2

(40)

NCO + NO a N2O + CO

(41)

The sensitivity analysis shown in Table 2 indicates that the oxidation of the acetylene-ethanol mixture in the presence of NO is sensitive to almost the same reactions as in the absence of this compound. Also, the influence of NOx, both NO2 and NO, is noticeable with the appearance of significant sensitivity coefficients for CO when NO is added to the experiments. 5. Conclusions A study of the oxidation of acetylene-ethanol mixtures in a quartz flow reactor at atmospheric pressure, covering the temperature range of 775-1375 K and different air excess ratios, ranging from fuel-rich to fuel-lean environments, has been carried out. The initial concentration of acetylene in every experiment has been 500 ppm, while the ethanol concentration added, as an additive, has been varied in the range of 0-200

ppm. Additionally, the effect of the addition of nitric oxide (500 ppm) in the blend oxidation has been investigated. Experimental results have been simulated and interpreted in terms of a detailed gas-phase kinetic mechanism built from different reaction subsets of the literature. From this study, it can be concluded that ethanol is more reactive than acetylene, being consumed at lower temperatures. The addition of ethanol affects the oxidation regime of acetylene, shifting it to higher temperatures. Once all of the ethanol has been consumed, the acetylene concentration decreases significantly. The onset for the blend oxidation process is roughly independent of the air excess ratio, around 900 K under all of the conditions studied. As the mixture becomes richer, the window for acetylene conversion turns to be wider and, for given conditions, an increased presence of ethanol in the mixture results in an enhancement of the acetylene oxidation. The same fact occurs when the temperature and the oxygen level increase. The main reaction pathways observed for acetylene conversion in the presence of ethanol are basically the same as those in the absence of this additive and agree with those obtained from the references. In a such way, the influence of the ethanol addition comes about its capacity of modifying the composition of the radical pool, promoting the acetylene and intermediate oxidation by action of O and OH radicals from ethanol. The acetylene-ethanol mixture, as well as the intermediate products formed during its oxidation, are able to interact with NO, reaching different levels of NO reduction depending upon the operating conditions considered. The study shows that the reduction of NO is favored at the highest temperatures studied, above 1225 K, and moderately fuel-rich conditions (λ ) 0.7). Because acetylene provides a plentiful source of HCCO, under the investigated conditions, the HCCO + NO reaction is the most important in the reduction of NO according to model predictions. The destruction of NO is accompanied by the formation of large amounts of HCN. As the environment becomes more fuel-rich, the selectivity to HCN increases. The addition of NO in the acetylene-ethanol mixture oxidation experiments results in a slight inhibition of both the acetylene and ethanol conversion. This can be attributed to radical recombination catalyzed by NO through the NO/NO2 and NO/ HNO interconversions, which act as a radical sink. The chemical kinetic model predictions are, in general, in good agreement with the experimental data for the studied blends. The model has been able to reproduce the main experimental trends for C2H2, C2H5OH, CO, CO2, NO, and HCN. However, improvements of the kinetic model are still needed, especially under very fuel-rich conditions and particularly related to the nitrogen-containing species. Acknowledgment. The authors express their gratitude to the MEC (Project CTQ-2006-09963) for financial support. Ms. C. Esarte acknowledges the Spanish Ministry of Science and Education (MEC) for the predoctoral grant awarded (BES-2007-15333). This project is included as a part of the work of the GPT Research Group. EF800550K

Oxidation of Acetyleneâ Ethanol Mixtures and Their Interaction with NO

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