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High pressure oxidation of C2H4/NO mixtures
Abstract
An experimental and kinetic modeling study of the interaction between C2H4 and NO has been performed under flow reactor conditions in the intermediate temperature range (600–900 K), high pressure (60 bar), and for stoichiometries ranging from reducing to oxidizing conditions. The main reaction pathways of the C2H4/O2/NOx conversion, the capacity of C2H4 to remove NO, and the influence of the presence of NOx on the C2H4 oxidation are analyzed. Compared to the C2H4/O2 system, the presence of NOx shifts the onset of reaction 75–150 K to lower temperatures. The mechanism of sensitization involves the reaction HOCH2CH2OO + NO → CH2OH + CH2O + NO2, which pushes a complex system of partial equilibria towards products. This is a confirmation of the findings of Doughty et al. [3] for a similar system at atmospheric pressure. Under reducing conditions and temperatures above 700 K, a significant fraction of the NOx is removed. This removal is partly explained by the reaction C2H3 + NO → HCN + CH2O. However, a second removal mechanism is active in the 700–850 K range, which is not captured by the chemical kinetic model. With the present thermochemistry and kinetics, neither formation of nitro-hydrocarbons (CH3NO2, C2H3NO2, C2H5NO2, CHOCH2NO2) nor nitroso-compounds (CH3NO, C2H3NO, C2H5NO, ONCH2CHO, CH3C(O)NO, ONCH2CH2OH) contribute to remove NOx.
... The authors emphasized the importance of NO conversion to NO2 by HO2, i.e., NO + HO2 = NO2+ OH, with NO to NO2 conversion enhanced and fuel oxidation also promoted due to the generation of OH radicals. Gimenez-Lopez et al. [29] studied the interaction between C2H4 and NO (500 ppm) in another flow reactor under a high pressure of 60 bar and low-to-intermediate temperatures (600-900 K). It was observed that the presence of NO shifted the onset of the oxidation of C2H4 to lower temperatures, indicating the promoting effect of NO on C2H4 oxidation. ...
... The authors emphasized the importance of NO conversion to NO 2 by HO 2 , i.e., NO + HO 2 = NO 2 + OH, with NO to NO 2 conversion enhanced and fuel oxidation also promoted due to the generation of OH radicals. Gimenez-Lopez et al. [29] studied the interaction between C 2 H 4 and NO (500 ppm) in another flow reactor under a high pressure of 60 bar and low-to-intermediate temperatures (600-900 K). It was observed that the presence of NO shifted the onset of the oxidation of C 2 H 4 to lower temperatures, indicating the promoting effect of NO on C 2 H 4 oxidation. ...
... Comparison of experimental (symbols) and predicted (lines) concentration profiles as a function of the reactor temperature for the stoichiometric experiment with a C2H2/O2/NOx/N2 mixture [54]. The reactor temperature profile was obtained from [29]. Figure 18 shows the simulated and measured IDTs of a C2H4/N2O mixture in a shock tube [77] within a high temperature regime and at three different pressures. ...
Unsaturated hydrocarbons are major components of transportation fuels, combustion intermediates, and unburnt exhaust emissions. Conversely, NOx species are minor species present in the residual and exhaust gases of gasoline-fueled engines and gas turbines. Their co-existence in transportation engines is quite common, particularly with exhaust gas recirculation, which can greatly influence engine combustion characteristics. Therefore, this paper presents a review on the combustion chemistry of unsaturated hydrocarbons and NOx mixtures, with a focus on their chemical kinetic interactions. First, a comprehensive overview of fundamental combustion experiments is provided, covering mixtures of C2–C5 unsaturated/oxygenated species (namely alkenes, alkynes, dienes, alcohols, ethers, ketones, and furans) and three major NOx species (namely NO, NO2, and N2O), as well as reactors including jet-stirred reactors, flow reactors, burners, shock tubes, and rapid compression machines. Then, two widely adopted nitrogen chemistry models are evaluated in conjunction with a core chemistry model (i.e., NUIGMech1.1) via detailed chemical kinetic modeling, and the model similarities and differences across broad temperature ranges are highlighted. Thereafter, the unique interconversions between the three major NOx species are presented. In particular, the controversy regarding the pathways governing NO and NO2 conversion is discussed. Following this, the key direct interaction reactions between unsaturated species and NOx species are overviewed. Finally, the distinguishing features of the combustion chemistry for unsaturated hydrocarbon and NOx mixtures are summarized, and recommendations for future research on this topic are highlighted.
... In order to simulate NO X formation during the combustion of the investigated alcohols and poly-alcohols, thermal and non-thermal NO X chemistry was incorporated into the reaction mechanism. The NO X sub-mechanism includes all relevant N-containing reactions of GRI-Mech 3.0 [191], Dagaut et al. [18], and Giḿenez-López et al. [192], including mechanisms for: thermal NO, prompt NO, N 2 O, NNH, nitro-hydrocarbons, and nitroso-compounds. ...
... CH + N 2 HCN + N (6.1) Therefore reaction 6.1 has been replaced with reaction 6.2, which has been shown to be a more appropriate prompt initiating reaction [18] CH + N 2 N CN + H ( 6.2) Rates for this reaction and the NCN sub-mechanism are from Giḿenez-López et al. [192]. It has been shown that even after replacing the HCN initiating reaction with the NCN reaction, HCN remains a significant contributor to NO formation [18,22]. ...
... These trends are also consistent with those observed for the other HC species. Additionally, the increase in HCN, especially in the rich regions, increases potential for NO removal through the NO→HCN→N 2 mechanism [19, 27, 28,192,194,195]. ...
This work investigates the influence of one or more OH groups present on the fuel molecule and the resultant formation of NOX emissions. Combustion of oxygenated fuels has been increasing globally and such fuels offer significant potential in the reduction of pollutant emissions. One such emission class is the oxides of nitrogen, which typically form through a combination of two regimes: the thermal and non-thermal mechanisms. While thermal NO formation can be reduced by lowering the combustion temperature, non-thermal NO formation is coupled to the fuel chemistry. An experimental and computational investigation of NOX formation in three different burner configurations and under a range of equivalence ratios and temperature regimes explored the differences in NO formation. Measurements of temperature profiles and in-flame species concentrations, utilizing both probed and non-intrusive laser based techniques, allowed for the investigation of NO formation through non-thermal pathways and the differences that exist between fuels with varying numbers of OH groups.
The first burner configuration was composed of a high swirl liquid spray burner with insulted combustion chamber walls designed specifically for the combustion of low energy density fuels. In this system the combustion of alcohols and glycerol (the largest by-product of biodiesel production), along with other fuels with multiple hydroxyl groups, was studied. Measurements of the mean flame temperature and exhaust gas measurements of NOX showed significant reductions in non-thermal NO concentrations with increasing numbers of OH groups. An accompanying modeling study and detailed reaction path analysis showed that fuel decomposition pathways through formaldehyde were shown a preference due to the presence of the OH groups which resulted in reduced contributions to the hydrocarbon radical pools subsequent reductions to the Prompt NO mechanism.
Two burner configurations with reduced dimensionality facilitated measurements in premixed flames for temperature and species in high and low temperature flames. These measurements included probed thermocouple temperature measurements, extractive gas sampling for NO and intermediate hydrocarbon species, and planar Laser Induced Fluorescence (LIF) measurements for 2OH-LIF thermometry, semiquantitative CH2O LIF, and quantitative NO LIF. Additionally, the simplified nature of the burner geometries allowed for the modeling of the flames incorporating detailed reaction kinetics for fuel decomposition and NOX formation. Significant reductions in NO formation were observed in comparisons of alcohol and alkane flames, resulting in up to 50% reductions in the pollutant. Computational analyses and nitrogen flux accounting allowed for the identification of the reduction in NO formation through all the known NOX formation pathways. It was observed that all of the known pathways exhibited reductions in contributions to NO formation in the presence of OH functional groups, indicating a complex coupling of fuel and NOX chemistry.
... The NO x sub-mechanism includes all relevant N-containing reactions of GRI-Mech 3.0 [21], Dagaut et al. [22] and Giménez-López et al. [23], including mechanisms for: thermal NO, prompt NO, N 2 O, NNH, nitro-hydrocarbons, and nitroso-compounds. The prompt-initiating reaction of Fenimore [24,25] used in the GRI-Mech 3.0 shown in reaction (2) has been shown to be spin forbidden by Moskaleva and Lin [26]. ...
... Rates for this reaction and the NCN sub-mechanism are from Giménez-López et al. [23]. It has been shown that even after replacing the HCN initiating reaction with the NCN reaction, HCN remains a significant contributor to NO formation [22,10]. ...
... While the formation of prompt NO is largely controlled through the formation of NCN as discussed in Section 2.2.1, concentrations of NCN are very low as this species quickly converts to HCN [10,22]. These trends are consistent with the NO formation measured in the experiments and reported in Section 3.2.2 and Fig. 9. Additionally, the increase in HCN formation, especially in the rich regions, increases the potential for NO removal through the NO!HCN!N 2 mechanism [35][36][37][38][39]23]. ...
This work investigates the influence of molecular structure in hydroxylated fuels (i.e. fuels with one or more hydroxyl groups), such as alcohols and polyols, on formation. The fuels studied are three lower alcohols (methanol, ethanol, and n-propanol), two diols (1,2-ethanediol and 1,2-propanediol), and one triol (1,2,3-propanetriol); all of which are liquids at room temperature and span a wide range of thermophysical properties. Experimental stack emissions measurements of NO/NO2, CO, and CO2 and flame temperature profiles utilizing a rake of thermocouples were obtained in globally lean, swirling, liquid atomized spray flames inside a refractory-lined combustion chamber as a function of the atomizing air flow rate and swirl number. These experiments show significantly lower formation with increasing fuel oxygen content despite similarities in the flame temperature profiles. By controlling the temperature profiles, the contribution to formation through the thermal mechanism were matched, and variations in the contribution through non-thermal formation pathways are observed. Simulations in a perfectly stirred reactor, at conditions representative of those measured within the combustion region, were conducted as a function of temperature and equivalence ratio. The simulations employed a detailed high temperature chemical kinetic model for formation from hydroxylated fuels developed based on recent alcohol combustion models and extended to include polyol combustion chemistry. These simulations provide a qualitative comparison to the range of temperatures and equivalence ratios observed in complex swirling flows and provide insight into the influence of variations in the fuel decomposition pathways on formation. It is observed that increasing the fuel bound oxygen concentration ultimately reduces the formation of by increasing the proportion of fuel oxidized through formaldehyde, as opposed to acetylene or acetaldehyde. The subsequent oxidation of formaldehyde contributes little to the formation of hydrocarbon (HC) radicals. Ultimately, by reducing the contributions to the HC radical pool, can be effectively reduced in these fuels through suppression of non-thermal formation pathways.
... Because of the fact that NO can be produced in the combustion chamber of the engine, the interaction between MF and NO is also analyzed by adding a given amount of NO (at a constant pressure, 20 bar). The NO evolution at high pressure in the NO x interaction with CO/H 2 /O 2 has been previously studied by Rasmussen et al. 23 and in the C 2 H 4 /NO x interaction by Gimeńez-Loṕez et al. 24 Specifically, the oxidation of MF has been investigated under flow reactor conditions, in a new high-pressure setup, at different pressures (atmospheric, 20, 40, and 60 bar), in the 573−1073 K temperature interval, from reducing to very fuellean conditions, in both the absence and presence of NO. Additionally, the experimental data are interpreted in terms of a detailed kinetic modeling study based on the MF mechanism subset by Dooley et al., 12 updated by Alzueta et al. 17 and revised and completed in the present work. ...
... The full mechanism takes as the starting point an earlier work on MF conversion at atmospheric pressure, 17 which includes the Dooley et al. MF reaction subset, 18 even though it has been revised according to the present high-pressure conditions and the presence of NO, taking into account the considerations by Rasmussen et al. 23,26,27 and Gimeńez-Loṕez et al. 24 The decomposition reaction of MF, with CH 3 OH and CO as main products, constitutes the beginning of the MF oxidation. Methanol formed is rapidly consumed, giving mainly hydroxymethyl radicals and these, formaldehyde. ...
... In the presence of NO in the reactant mixture, it has been observed that, when the pressure is increased from atmospheric to 20 bar, NO is converted almost completely to NO 2 , as reported in previous works. 24,26 The main reaction involved in this conversion is NO + NO + O 2 ⇌ NO 2 + NO 2 , which gains relevance with the pressure. This reaction has already been studied; 30−32 however, the pressure dependence of the kinetic parameters is not well-known presently, and the uncertainty related to it may be significant. ...
An experimental and modeling study of the influence of pressure on the oxidation of methyl formate (MF) has been performed in the 1-60 bar pressure range, in an isothermal tubular quartz flow reactor in the 573-1073 K temperature range. The influence of stoichiometry, temperature, pressure, and presence of NO on the conversion of MF and the formation of the main products (CH2O, CO2, CO, CH4, and H-2) has been analyzed. A detailed kinetic mechanism has been used to interpret the experimental results. The results show that the oxidation regime of MF differs significantly from atmospheric to high-pressure conditions. The impact of the NO presence has been considered, and results indicate that no net reduction of NOx is achieved, even though, at high pressure, the NO-NO2 interconversion results in a slightly increased reactivity of MF.
... The key amine sub-mechanism includes NH 2 + NO/NO 2 /OH, NH 2 /NNH + O 2 reactions and the reactions on the ONNH potential energy surface which were derived from the theoretical work of Klippenstein et al. [23] . The ethanol sub-mechanism of Zhang [31] et al. [22] is based on the AramcoMech 1.3 containing the H 2 /CO/O 2 reactions of Kéromnès et al. [24] , the C 1-C 2 reactions of Metcalfe et al. [25] and some crucial reactions from Mittal et al. [26] . These two sub-mechanisms have been validated with the new IDT data of pure NH 3 and pure C 2 H 5 OH in the present work and have shown the best agreement among other literature mechanisms, as shown in Fig. S4. ...
... The generalization of this method needs to be substantiated through future experimental results or theoretical calculations of these kinetic quantities. Other pathways were drawn from the work of [27][28][29][30][31] using the similar method of R3, which are not sensitive to the prediction results. ...
The auto-ignition properties of ammonia (NH3)/ethanol (C2H5OH) blends close to engine operating conditions were investigated for the first time. Specifically, the ignition delay times (IDT) of ammonia/ethanol blends were measured in a rapid compression machine (RCM) at elevated pressures of 20 and 40 bar, five C2H5OH mole fractions from 0% to 100%, three equivalence ratios (ϕ) of 0.5, 1.0 and 2.0, and intermediate temperatures between 820 and 1120 K. The measurements reveal that ethanol can drastically promote the reactivity of ammonia, e.g., the auto-ignition temperature with merely 1% C2H5OH in fuel decreases accordingly around 110 K at 40 bar as compared to that of neat ammonia. Moreover, the promotion efficiency of ethanol is higher than hydrogen and methane with a factor of 5 and 10 under the same condition. Different dependences of IDT on the equivalence ratio were observed with different ethanol fractions in the blends, i.e., the IDTs of the 5%, 10% and 100% C2H5OH in fuel decrease with an increase of ϕ, but an opposite trend was observed in the mixture with 1% C2H5OH. A new chemical kinetic mechanism for NH3/C2H5OH mixtures was developed and it is highlighted that the addition of cross-reactions between the two fuels is necessary to obtain reasonable simulations. Basically, the newly developed mechanism can reproduce the measurements of IDT very well, whereas it overestimates the reactivity of the stoichiometric and fuel-rich mixture with 1% C2H5OH in fuel. The sensitivity, reaction pathway, as well as rate of production analysis indicated that the ethanol addition to ammonia fuel blends provides key interaction pathways and enriches the O/H radical pool which further promotes the auto-ignition process.
... For the chemistry involving NO x species, the reaction subsets used in the present study have been taken from the work by Colom-Díaz et al. [38], where the oxidation of H 2 and its interaction with NO was studied at high-pressures (10, 20 and 40 bar), in a tubular flow reactor. These subsets are mainly based on the work by Giménez-López et al. [43], who performed experiments of the oxidation of C 2 H 4 /NO mixtures at high pressure (60 bar) and different stoichiometries. Some of the reactions have been updated with the kinetic parameters published in the work by Abián et al. [44], about the formation of NO in N 2 /O 2 mixtures in a flow reactor. ...
... This was observed while the oven was still cool (290 K) [38] and has already been reported in previous works experimenting with NO at high pressures (e.g. [17,[38][39][40][41]43]). For this reason, in the present work, the model was run with temperature profiles in the high-pressure set-up, in order to know at each position of the reactor the concentration of NO and NO 2 due to the oxidation of NO at high pressures occurring at low temperatures. ...
The present study deals with the oxidation of H2S/NO mixtures, in the temperature range of 475–1400 K, at atmospheric pressure and 20 bar of manometric pressure. The experiments have been performed in two different set-ups, using tubular flow reactors, for different air excess ratios (λH2S = 0.3–6). A kinetic model has been updated with recent reactions from the literature. When NO is present, the oxidation of H2S at atmospheric pressure proceeds at slightly higher temperatures (25 K) with respect to neat H2S oxidation. At high pressure (20 bar), the experiments of the oxidation of H2S in the absence and presence of NO have been performed only at oxidizing conditions (λH2S = 2 and λH2S = 6), in order to avoid sulfur formation under reducing conditions. The outcomes of these experiments show that, in presence of NO, at the lowest temperature considered (475 K), at least 50% of H2S conversion for λH2S = 2 and 90% for λH2S = 6 is obtained. In order to further evaluate the influence of the presence of NO in H2S oxidation, additional experiments of neat NO oxidation have been performed. As NO2 formation is favored at high pressures and high O2 concentrations, the NO2-H2S interaction is thought to be responsible for the consumption of H2S, even at low temperatures (475 K). While the kinetic mechanism is able to reproduce the experimental results at atmospheric pressure, discrepancies are more relevant at high pressure (20 bar).
... However, recent studies strongly suggest that current models do not accurately capture these interactions. For example, a flow reactor study by Giménez-López et al. [6] examined C 2 H 4 /O 2 /NO mixtures under high pressure (60 bar) and temperatures of 600 K to 900 K, and significant removal of NO x was found experimentally that was not predicted by the kinetic mechanism. A key first step in closing the nitrogen balance for EHN-doped fuels in LTCI engines is understanding how the initial NO 2 that is formed upon dissociation of EHN is further reduced in the cylinder. ...
... Zhang et al. examined mechanism performance versus experimental data for the combustion of hydrogen and syngas in the presence of NO x [9]. Drawing on the review in ref. [9], the mechanisms of Abian et al. [10], Ahmed et al. [11], Dagaut et al. [12], Glarborg et al. [6,13,14], Konnov [15], and Mathieu et al. [16,17] reactions could be justifiable. If that is the case, however, it also raises the question as to whether HNO 2 contributes anything to the overall kinetics, as implied by the mechanisms of Dagaut et al. and Konnov. ...
This work investigates whether both HONO and HNO2 are essential in describing the reactivity for NO2-doped ignition experiments or if a strategy could be developed that lumps the two isomers into a single species without adversely affecting the model fidelity. First, the possibility of different product branching fractions is considered; temperature- and pressure-dependent rate constants are computed for H and CH3 addition to the N=O bond in both HONO and HNO2. These results suggest that addition of a radical to HONO and HNO2 do indeed have different products, but that the results are not likely to have a significant effect. Next, two different approaches to simplifying the HONO submechanism are considered. In the first, HNO2 is removed from the mechanism. In the second, HNO2 is replaced with HONO. These two strategies are implemented in different literature mechanisms and then used to compute ignition delay times for H2 and CH4. The results show that removing HNO2 has a modest effect on the ignition delay time, whereas systematically replacing HNO2 with HONO decreases the predicted ignition delay by approximately a factor of two. The recommendation is that for larger fuels, both HONO and HNO2 should be included in the mechanism.
... For clarification, we compared the ignition delay times of model predictions and experimental measurements for pure ethane reported in our previous work [1], Fig. 2. It can be seen that the Aramco Mech 2.0 shows an excellent agreement with the experimental data over the studied conditions. Four recently published NO x models, Sivaramakrishnan et al. [21], Giménez-López et al. [33], Mathieu et al. [18] and Faravelli et al. [22] were integrated into the Aramco Mech 2.0 to evaluate the effects of NO x sub-models. The four assembled models are thus named Aramco-S, Aramco-G, Aramco-M and Aramco-F, respectively. ...
... proposed in this study is also used to compare with other four models and the experiments. Specifically, the proposed model consists of reactions involving the H 2 /CO/O 2 /NO x sub-mechanism developed by Zhang et al. [34], the C 1 /NO x sub-mechanism constructed by Mathieu et al. [18], the C 2 /NO x sub-mechanism built by Giménez-López et al. [33], and the C 2 H 5 NO 2 sub-mechanism taken from Zhang et al. [35,36] The rate constants of reactions C 2 H 6 + NO 2 <=> Ċ 2 H 5 + HONO/HNO 2 and CH 4 + NO 2 <=> Ċ H 3 + HONO/ HNO 2 was updated by the latest value calculated by Chai et al. [37]. ...
Nitrogen dioxide (NO2) is a dominant component of NOx pollution in combustion of internal combustion engines and gas turbines. Its sensitization on ignition of ethane which is a main component of natural gas has been investigated in this experimental and kinetic study. Ignition delay times of NO2/C2H6/O2/Ar mixtures, with blending ratios of NO2:C2H6 of 0.3:1 and 1:1, were measured in a shock tube. Experimental conditions cover a range of pressures (1.2–20 atm), temperatures (950–1700 K) and equivalence ratios (0.5–2.0). Similarly to our previous work of CH4/NO2 (Deng et al., 2016) [14], NO2 addition promotes the reactivity of ethane and reduces the global activation energy particularly at higher pressures (p > 5.0 atm) and lower temperatures (T < 1175 K), whereas it only presents a limited effect at low pressure (1.0 atm) and higher temperatures (T > 1175 K). Furthermore, an opposite effect of NO2 addition is observed in both the experiments and the simulations at different temperature regimes. Compared to fuel-rich mixture, NO2 addition exhibits more significantly promoting effect on the ignition of fuel-lean mixture under given NO2 concentration. Four literature kinetic mechanisms and an updated mechanism proposed in this study have been compared to simulate the new ignition delay time data and the literature data. Overall, the proposed model is capable of reproducing the experimental results measured by various facilities over a wide range of conditions. The proposed model is thus used to carry out the sensitivity and flux analyses to clarify the chemistry interaction between NO2 and ethane. Based on the kinetic analyses, the impact of NO2 has been expounded at different conditions.
... The objective of the present study is to develop and evaluate a detailed chemical kinetic model for oxidation of acetylene at high pressure and intermediate temperatures. The model draws on previous work on the high-pressure, medium temperature oxidation of small hydrocarbons and alcohols [34][35][36][37][38][39][40], as well as recent results in atmospheric chemistry. The rate constants for the reactions of acetylene with HO 2 and O 2 are investigated from theory. ...
... The starting mechanism and corresponding thermodynamic properties were drawn from previous mechanisms for high-pressure oxidation of small hydrocarbons [34][35][36][37][38][39]. The thermodynamic properties for selected species are shown in Table I, while Table II lists key reactions in the C 2 H 2 oxidation scheme. ...
A detailed chemical kinetic model for oxidation of acetylene at intermediate temperatures and high pressure has been developed and evaluated experimentally. The rate coefficients for the reactions of C2H2 with HO2 and O2 were investigated, based on the recent analysis of the potential energy diagram for C2H3 + O2 by Goldsmith et al. and on new ab initio calculations, respectively. The C2H2 + HO2 reaction involves nine pressure- and temperature-dependent product channels, with formation of triplet CHCHO being dominant under most conditions. The barrier to reaction for C2H2 + O2 was found to be more than 50 kcal mol−1 and predictions of the initiation temperature were not sensitive to this reaction. Experiments were conducted with C2H2/O2 mixtures highly diluted in N2 in a high-pressure flow reactor at 600–900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions were generally in satisfactory agreement with the experimental data. Under the investigated conditions, the oxidation pathways for C2H2 are more complex than those prevailing at higher temperatures and lower pressures. Acetylene is mostly consumed by recombination with H to form vinyl (reducing conditions) or with OH to form a CHCHOH adduct (stoichiometric to lean conditions). Both C2H3 and CHCHOH then react primarily with O2. The CHCHOH + O2 reaction leads to formation of significant amounts of glyoxal (OCHCHO) and formic acid (HOCHO), and the oxidation chemistry of these intermediates is important for the overall reaction.
... Glarborg et al. [17] has put forward a NO-hydrocarbon reaction mechanism in 1998 which was widely accepted and applied [28][29][30]. In the recent years, the mechanism has been updated several times for a wider applicability and a better accuracy, forming GIM/GLA10 mechanism in 2010 [31]. For a deeper discussion, the reactions between NO and CH i , HCCO (the key radicals in NO-reduction [16][17][18]27]) in the GIM/GLA10 mechanism were listed in Table 4. ...
... It might also explain why no advantage of syngas reburning is found in Figure 6 under fuel-rich conditions, although having a richer hydrocarbon radical pool due to tar cracking. Refer to GIM=GLA10 mechanism [31]. Temperature Figure 7 suggests that the increase in temperature can promote the NO reduction, and the promotion is more significant with a high oxygen flow rate. ...
Experiments of nitric oxide reduction by the tar-included syngas from a small biomass updraft gasifier have been carried out on a tubular reactor under a temperature range of 900 to 1200°C. The initial mole fraction of NO was 1000 ppm and that of the oxygen and gasification syngas were varied. Comparison of the reburning results of the present syngas with other fuels clarified the influence of tar. Under the conditions of this work, the participation of tar compounds in NO reduction mainly begins with cracking into light hydrocarbons which is sensitive to both the oxygen concentration and the temperature. Tar leads to a satisfactory NO-reduction improvement under relative oxygen-rich conditions, supplying extra hydrocarbon radicals and creating a more appropriate atmosphere for reburning reactions. As for a reductive environment, little benefit was gained from the presence of tar. The rise of temperature brings a clear promotion only to NO reduction with a high oxygen flow rate, while polymerization should be concerned for a low oxygen flow rate. It is believed that a substantial portion of tar converts to acetylene through cracking under suitable reburning conditions, resulting in an active NO-reduction reaction pathway. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 602–608, 2014
... For instance, one common method used in gas turbines and internal combustion engines consists of re-circulating the exhaust gases in the combustion chamber, to limit the combustion temperature and minimize NOx formation via the Zeldovich mechanism (so-called exhaust gas recirculation (or EGR) method). However, while NOx reduction is achieved, doing so also introduces some NOx into the fresh charge, which can dramatically change the combustion properties of the mixture, even at low NOx levels, as shown in a large number of fundamental experiments with H 2 - [3][4][5][6][7][8][9], CH 4 - [10][11][12][13][14][15][16], or larger hydrocarbon-based mixtures [17][18][19][20][21][22][23]. ...
Modern gas turbines use combustion chemistry during the design phase to optimize their efficiency and reduce emissions of regulated pollutants such as NOx. The detailed understanding of the interactions during NOx and natural gas during combustion is therefore necessary for this optimization step. To better assess such interactions, NO2 was used as a sole oxidant during the oxidation of CH4 and C2H6 (the main components of natural gas) in a shock tube. The evolution of the CO mole fraction was followed by laser-absorption spectroscopy from dilute mixtures at around 1.2 atm. The experimental CO profiles were compared to several modern detailed kinetics mechanisms from the literature: models tuned to characterize NOx-hydrocarbons interactions, base-chemistry models (C0–C4) that contain a NOx sub-mechanism, and a nitromethane model. The comparison between the models and the experimental profiles showed that most modern NOx-hydrocarbon detailed kinetics mechanisms are not very accurate, while the base chemistry models were lacking accuracy overall as well. The nitromethane model and one hydrocarbon/NOx model were in relatively good agreement with the data over the entire range of conditions investigated, although there is still room for improvement. The numerical analysis of the results showed that while the models considered predict the same reaction pathways from the fuels to CO, they can be very inconsistent in the selection of the reaction rate coefficients. This variation is especially true for ethane, for which a larger disagreement with the data was generally observed.
... The presence of NO x in hydrocarbon combustion is experimentally-observed to have an ignition-promoting (reactivity-enhancing) effect, e.g. [1][2][3][4][5]: NO 2 serves to promote chain-branching by abstracting a hydrogen to form HONO or HNO 2 , both of which may then decompose to OH and NO. NO may recycle to NO 2 particularly through interactions with oxy and peroxy radicals. ...
stuff Introduction The presence of NO x (NO + NO 2) in hydrocarbon combustion has an ignition-promoting (reactivity-enhancing) effect: NO 2 serves to promote chain-branching by abstracting a hydrogen to form HONO or HNO 2 , both of which may then decompose to OH and NO. NO may recycle to NO 2 particularly through interactions with oxy and peroxy radicals. In this work, we examine reactions between NO and a peroxy radical leading to NO 2 and an oxy radical, i.e. NO + RO 2 <=> NO 2 + RO. The exact impact of these reactions on mixture reactivity is of interest as the conversion of peroxy (RO 2) to oxy (RO) radicals is ignition inhibiting, while the conversion of NO to NO 2 feeds a chain-branching pathway initiated by hydrogen abstraction reactions with NO 2. In systems doped with NO, the low-temperature combustion system may be particularly sensitive to the peroxy radical branching ratio between reactions with NO interactions and isomerization to QOOH. Determination of NO x-cycling reaction rates for small, straight-chain and branched alkanes (methyl, ethyl, n-propyl, i-propyl radicals) is intended to be the first in a series of necessary, systematic investigations of reaction classes relevant to combustion in systems containing reactive nitrogen compounds. Results are extended by analogy to butyl and pentyl radical systems. Comparison of the newly-calculated rates is made to literature values, where available , and the impact of the different rates on simulations of ignition delay time and jet-stirred reactor profiles is compared to recent data on n-pentane / NO x systems. Construction of the potential energy surface and solution of the master equation for each system involves connecting the two bimolecular states through a unimolecular adduct (ROONO). Where dissociation to a bimolecular product is barrierless, we use phase space theory to connect the two states; rigid bond scans were made to determine the correct potential shape, with the barrier height rescaled to the results of the single-point energy computations. When the dissociation occurs via a first-order saddle point, the reaction is modeled using transition state theory.
... Recent studies strongly suggest that current models do not accurately capture low-temperature nitrogen chemistry and fuel-NO x interactions: [6] conducted flow reactor studies of C 2 H 4 /O 2 /NO mixtures under high pressure (60 bar) and temperatures of 600 K to 900 K. The experiments found significant removal of NO x ; this was not predicted by the kinetic mechanism. ...
Alexander von Humboldt Stipendium proposal
... The interpretation of the above mentioned high-pressure measurements in terms of a detailed chemical kinetic model agreed reasonably well. The interaction between C2H4 and NO under high-pressure and intermediate temperature flow reactor conditions were investigated both in terms of experimental measurements and a kinetic modeling study by Giménez-López et al. [8]. The high-pressure capability of ethylene to reduce NO and the influence of temperature and oxygen content on C2H6/NO interaction were experimentally observed by those studies. ...
The enhanced ignition of hydrocarbon fuels in presence of trace amounts of NOx (NO and NO2) and the faster NO-NO2 interconversion in the presence of hydrocarbons (HC), known as HC-NOx mutual sensitization have been an active area of research. In order to meet the stricter NOx emission regulations during the combustion of C1-C2 hydrocarbon and their blends for stationary gas turbine power generation, a definitive comprehension of the chemical interplay among the NOx species and the hydrocarbon fuel fragments is important. In light of that, this paper presents experimentally observed reactivity of NOx (NO and NO2) and fuel species, with and without small amounts of ethane present at 800-960 K and 10.0 atm. The experiments were carried out in a variable pressure flow reactor designed to perform well-defined experimental investigations of homogeneous gas phase chemistry. The experimental results have been interpreted in terms of a newly proposed detailed NOx chemical kinetic model for natural gas combustion. The majority of the previous natural gas/NOx kinetic models were validated against homogeneous systems with a rarity of models validated against both homogeneous and transport dependent experimental targets. This has motivated the current investigation to validate the proposed natural gas/NOx model against atmospheric and high-pressure experimental venues of premixed burner-stabilized and freely-propagating methane, ethane and ethylene flames and counter flow premixed, partially-premixed and non-premixed methane flames available in the literature, in addition to the currently conducted homogeneous flow reactor experiments. A recently developed NOx model for synthetic gas combustion (Ahmed et al., Energy & Fuels, 2016) has been extended further to accommodate C1-C2+NOx chemistry with comprehensive validation encompassing a wide range of equivalence ratio (0.5-2.0) and pressure (1-60 atm). Rate constants of several important reactions involving HONO and HNO2 species are updated based on the recent ab-initio calculations. Furthermore, the role of transport dependent validation is critically assessed. The comparison of kinetic model predictions against the experiments conducted in this study shows a significant deviation of NO2 reactivity at temperatures below 900 K primarily due to the NO2 sequestration into CH3NO2, that significantly impacts the predictions of NO2 evolution
... [143] Recent studies strongly suggest that current models do not accurately capture low-temperature nitrogen chemistry and fuel-NO x interactions: Flow reactor studies of C 2 H 4 /O 2 /NO mixtures under high pressure (60 bar) and temperatures of 600 K to 900 K found significant removal of NO x ; this was not predicted by the kinetic mechanism. [144] Experimental studies of low-temperature compression ignition (LTCI) engines with 2-ethylhexyl nitrate (EHN) as the cetane enhancer have also found discrepancies between model and experiment with roughly one-third of the fuel-bound nitrogen found in the exhaust as NO x . [21,77,145] The number of reactions containing HONO and HNO 2 is shown for each mechanism in Figure 7.1. ...
This thesis describes both the design and construction of a new shock tube facility and a series of investigations, both theoretical and experimental, meant to support research into low-temperature combustion chemistry. With recent developments and interest in highly-efficient low-temperature compression ignition (LTCI) engines, a potential enabling concept, reactivity-controlled compression ignition (RCCI) is at the center of the motivation behind much of this work. In RCCI engines, the LTCI concept utilizes a fuel of variable reactivity to ensure consistent ignition of the wide range of load conditions demanded by, in particular, vehicle engines. Alkyl nitrate fuel additives are one possible compound which could be added to existing transportation fuels to adjust the reactivity over the necessary range. The combustion chemistry of these additives, particularly at the lower temperatures where LTCI engines would operate to avoid NOx formation, is not completely understood.
In order to perform relevant experiments, a new shock tube facility was designed and constructed at Brown University. Shock tubes are a class of high-temperature reactor which utilize shock waves to achieve nearly instantaneous changes in the temperature and pressure of a reactant mixture. In addition to utilization of the facility at Brown University, this dissertation includes the results of additional studies carried out at two other shock tube facilities. Laser-schlieren densitometry is utilized to measure dissociation rates in pyrolysis experiments and measurement of ignition delay times is used to assess the overall behavior of fuel and fuel-nitrate blends at engine-relevant conditions.
Complementary to the experimental investigations, modeling work is presented to assess the role of key intermediates HONO and HNO2 , which are not readily accessible via experiments. The theoretical investigations apply transition state theory and master equation solutions to develop pressure-dependent rates for a number of reactions which cannot be probed directly. Finally, some discussion of an effort to develop improved methodologies for estimating collisional energy transfer parameters, a large source of uncertainty in master equation system solutions, is presented.
... [25] and [26]; the model from Ref. [27]; and the recent paper from Ref. [15]. This latest model is built upon the recent H 2 /CO/NO x model of Zhang et al. [28], the CH 4 /NO x model from Mathieu et al. [11], the C 2 /NO x chemistry from Gim enez-L opez et al. [29] and Zhang et al. [30] along with recent updates from Chai et al. [31]. The H 2 /hydrocarbon part was adopted directly from AramcoMech 2.0, which is available to download. ...
One method frequently used to reduce NOx emissions is exhaust gas recirculation, where a portion of the exhaust gases, including NOx, is reintroduced into the combustion chamber. While a significant amount of research has been performed to understand the important fuel/NOx chemistry, more work is still necessary to improve the current understanding on this chemistry and to refine detailed kinetics models. To validate models beyond global kinetics data, such as ignition delay time or flame speed, the formation of H2O was recorded using a laser absorption diagnostic during the oxidation of a mixture representing a simplistic natural gas (90% CH4/10% C2H6 (mol)). This mixture was studied at a fuel lean condition (equivalence ratio=0.5) and at atmospheric pressure. Unlike in conventional fuel-air experiments, NO2 was used as the oxidant to better elucidate the important, fundamental chemical kinetics by exaggerating the interaction between NOx and hydrocarbon-based species. Results showed a peculiar water formation profile, compared to a former study performed in similar conditions with O2 as oxidant. In the presence of NO2, the formation of water occurs almost immediately before it reaches more or less rapidly (depending on the temperature) a plateau. Modern, detailed kinetics models predict the data with fair to good accuracy overall, while the GRI 3.0 mechanism is proven inadequate for reproducing CH4/C2H6 and NO2 interactions.
... [25] and [26]; the model from Ref. [27]; and the recent paper from Ref. [15]. This latest model is built upon the recent H 2 /CO/NO x model of Zhang et al. [28], the CH 4 /NO x model from Mathieu et al. [11], the C 2 /NO x chemistry from Gim enez-L opez et al. [29] and Zhang et al. [30] along with recent updates from Chai et al. [31]. The H 2 /hydrocarbon part was adopted directly from AramcoMech 2.0, which is available to download. ...
One method frequently used to reduce NOx emissions is exhaust gas recirculation (EGR), where a portion of the exhaust gases, including NOx, is reintroduced into the combustion chamber. While a significant amount of research has been performed to understand the important fuel/NOx chemistry, more work is still necessary to improve the current understanding on this chemistry and to refine detailed kinetics models. To validate models beyond global kinetics data such as ignition delay time or flame speed, the formation of H2O was recorded using a laser absorption diagnostic during the oxidation of a mixture representing a simplistic natural gas (90% CH4 /10% C2H6 (mol.)). This mixture was studied at a fuel lean condition (equiv. ratio = 0.5) and at atmospheric pressure. Unlike in conventional fuel-air experiments, NO2 was used as the oxidant to better elucidate the important, fundamental chemical kinetics by exaggerating the interaction between NOx and hydrocarbon-based species. Results showed a peculiar water formation profile, compared to a former study performed in similar conditions with O2 as oxidant. In the presence of NO2, the formation of water occurs almost immediately before it reaches more or less rapidly (depending on the temperature) a plateau. Modern, detailed kinetics models predict the data with fair to good accuracy overall, while the GRI 3.0 mechanism is proven inadequate for reproducing CH4 / C2H6 and NO2 interactions.
... In addition, to study the influence of the NO x submodels is the focus in this study. Therefore, five literature NO x submodels (from Zhang et al., 41 Sivaramakrishnan et al., 20 Gimeńez-Loṕezet et al., 46 Mathieu et al., 16 and Konnov et al. 47 ) are assembled with Aramco Mech 2.0 to eliminate the impact of ethylene chemistry. ...
To explore the effects of N2O and O2 on C2H4 ignition, ignition delay times of stoichiometric C2H4/O2/N2O/Ar mixtures with mole blending ratios of N2O:(N2O + O2) = 0%, 50%, 80% and 100% were measured in a high-pressure shock tube. Reflected shock conditions cover a range of pressures from 1.2 to10 atm and temperatures from 1090 to 1760 K. In addition, ignition delay times of C2H4/N2O/Ar mixtures are measured at pressures of 1.2 – 10 atm, equivalence ratios of 0.5– 2.0 and temperatures of 1214 – 1817 K. The results indicate that, in the studied conditions, the ignition delay times of C2H4 greatly increase as N2O concentration increases at a given pressure and temperature. Five recent literature models are tested against the new measured ignition delay times, and show very small discrepancies among each other for the C2H4/N2O/Ar mixtures, but exhibit significant discrepancies for the C2H4/N2O/O2/Ar mixtures. Moreover, the kinetic analysis are performed to reveal the reason for the discrepancies among the five models and to investigate the different effects of N2O and O2 on the C2H4 ignition.
... CH 4 , C 2 H 6, and C 3 H 8 are primary fuel components in natural gas fuels. The influence of NO x on CH 4 oxidation has been studied extensively [8][9][10][11][12][13][14][15][16][17], whereas limited work is reported on C 2 H 4 [10,18,19], C 2 H 6 [10,18,20,21], C 3 H 6 [10], and C 3 H 8 [7,10,22]. However, most of these studies focused on oxidation with heavily diluted fuel/oxidizer mixtures, which do not represent the scenario in practical combustion systems. ...
Nitric oxide (NO) produced during combustion will be present in vitiated air used in many devices. An experimental and modeling investigation of the effect of NO on the ignition of C1-C3 hydrocarbon fuels, namely, CH4, C2H4, C2H6, and C3H6, is presented. These molecules are important intermediate species generated during the decomposition of long-chain hydrocarbon fuel components typically present in jet fuels. Moreover, CH4 and C2H6 are major components of natural gas fuels. Although the interaction between NOx and CH4 has been studied extensively, limited experimental work is reported on C2H4, C2H6, and C3H6. As a continuation of previous work with C3H8, ignition delay time (IDT) measurements were obtained using a flow reactor facility with the alkanes (CH4 and C2H6) and olefins (C2H4 and C3H6) at 900 K and 950 K temperatures with 15 mole% and 21 mole% O2. Based on the experimental data, the overall effectiveness of NO in promoting ignition is found to be: CH4 > C3H6 > C3H8 > C2H6 > C2H4. A detailed kinetic mechanism is used for model predictions as well as for reaction path analysis. The reaction between HO2 and NO plays a critical role in promoting the ignition by generating the OH radical. In addition, various important fuel-dependent reaction pathways also promote the ignition. H-atom abstraction by NO2 has significant contribution to the ignition of C2H4 and C2H6, whereas the reaction between NO2 and allyl radical (aC3H5) is an important route for the ignition of C3H6.
... To this end, the use of chemical kinetic models has been invaluable, and in recent years there have been quite a few proposed mechanisms concerned with NO x chemistry, and with interactions of nitrogen oxides with hydrocarbons. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] However, as will be discussed, there can be significant disparity between published models in terms of the thermochemical properties of nitrogen-containing species therein. This is often coupled with further disparity in terms of kinetic rate parameters within the models. ...
In order to simulate emissions of nitrogen-containing compounds in practical combustion environments, it is necessary to have accurate values for their thermochemical parameters, as well as accurate kinetic values to describe the rates of their formation and decomposition. Significant disparity is observed in the literature for the former, and we therefore present herein high-accuracy ab initio gas-phase thermochemistry for 60 nitrogenous compounds, many of which are important in the formation and consumption chemistry of NOx species. Several quantum-chemical composite methods (CBS-APNO, G3, and G4) were utilised in order to derive enthalpies of formation via the atomisation method. Entropies and heat capacities were calculated from traditional statistical thermodynamics, with oscillators treated as anharmonic based on ro-vibrational property analyses carried out at the B3LYP/cc-pVTZ level of theory. The use of quantum chemical methods, along with the treatments of anharmonicities and hindered rotors, ensures accurate enthalpy of formation, entropy, and heat capacity values across a temperature range of 298.15-3000 K. The implications of these results for atmospheric and combustion modelling are discussed.
Calorimetric monitoring of the autoclave reaction N2O4 + C2H4 at –85 to +10 °C under argon pressure 10–30 bar revealed that the exothermic chemical reaction started at temperatures above –52 °C at 10 bar, whereas an intensive exothermic reaction started at –85 °C and pressure of 30 bar. IR study showed that oligo/polynitroethylene was formed at 30 bar, while carbonyl and hydroxy compound as well as nitrate R–ONO2 formation occurred upon processing at 10 bar.
This study reports new ignition delay time (IDT) measurements of ethane (C2H6)/‘air’ mixtures with NOx (nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O)) addition in the range 0 – 1000 ppm at stoichiometric fuel to air (φ) ratios, at compressed temperatures (TC) of 851 – 1390 K and at compressed pressures (pC) of 20 – 30 bar. In addition, new IDT measurements of three highly diluted C2H6/NO2 mixtures at φ = 0.5, TC = 805 – 1038 K, and pC = 20 – 30 bar are also studied. These new experimental data, together with data already available in the literature, are used to validate NUIGMech1.2 with an updated NOx sub-mechanism. Although the addition of 200 ppm of NO or NO2 to ethane shows a minimal promoting effect, the addition of 1000 ppm significantly promotes its reactivity. The similarity of the effect of the addition of both NO and NO2 addition is due to the fast conversion of NO into NO2 in the presence of molecular oxygen. However, the 1000 ppm NO doped ethane mixtures exhibit ∼20% shorter IDTs compared to the NO2 blended ones. The addition of 1000 ppm of N2O exhibits no effect on ethane oxidation at the conditions studied. The NUIGMech1.2 predictions can reproduce the sensitisation effect of NOx on ethane with good agreement over a wide range of pressure, temperature, equivalence ratio, and percentage dilution. Sensitivity and flux analyses of C2H6/NOx are performed to highlight the key reactions controlling ignition over the different temperature regimes studied. The analyses show that there is a competition between the reactions Ṙ + NO2 ↔ RȮ + NO and Ṙ + NO2 (+M) ↔ RNO2 (+M). This governs NOx sensitization on C2H6 ignition.
NOx in the exhaust gas can influence the auto-ignition properties, thus are important for the design and operation of HCCI engines and predicting engine knock in spark ignition engines. More work is clearly required to improve the understanding on fuel/NOx chemistry for both reaction mechanism construction and engine application. In this study, n-C4H10, as a representative hydrocarbon fuel with the negative temperature coefficient (NTC) behavior, was selected for test. Ignition delay times of n-C4H10/air mixtures with varying NO2 addition (0, 500 ppm, 1%, and 5%) have been measured behind reflected shock waves. The sensitization effect of NO2 on n-C4H10 auto-ignition was investigated at engine relevant conditions covering temperatures from 700 to 1200 K, pressure of 20 atm, and equivalence ratios from 1.0 to 2.0. Besides, the mixtures with excessive amounts of NO2 (5% and 10%) were also studied at relatively high temperatures and 10 atm, aiming at highlighting the interactions between NO2 and n-C4H10. Results indicated the effect of NO2 on n-C4H10 oxidation exhibits prominent temperature and NO2 concentration dependences. Trace NO2 addition (500 ppm) promotes low-temperature reactivity of n-C4H10 and reduces the ignition delay times. While higher levels of NO2 no longer promote ignition, instead, it turns gradually to be an inhibiting effect. At high temperatures, NO2 dramatically accelerates the n-C4H10 oxidation, and the promoting effect peaks at around 5% NO2 then saturates with further NO2 addition. In the negative temperature coefficient region, the NTC behavior of n-C4H10 is weakened and even disappears by the introduction of NO2. A preliminary detailed kinetic model was proposed to describe the n-C4H10/NO2 chemistry, and in general, is capable of capturing the NO2 sensitization behaviors under different conditions. Kinetic analyses were then performed to clarify the sensitization mechanisms of NO2 and the chemical interactions between NO2 and n-C4H10.
Modern engine concepts present several opportunities for nitrogen combustion chemistry, particularly the interaction of NO x (NO + NO 2 ) with fuel fragments and products of partial combustion.
The oxidation of 1-butene and i-butene with and without addition of 1000 ppm NO was experimentally and numerically studied primarily at fuel-rich (ϕ = 2.0) conditions under high dilution (96% Ar) in a flow reactor operated at atmospheric pressure in the low temperature range of approximately 600-1200 K. Numerous intermediate species were detected and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). An elementary-step reaction mechanism consisting of 3996 reactions among 682 species, based on literature and this work, was established to describe the reactions and interaction kinetics of the butene isomers with oxygen and nitrogenous components. Model predictions were compared with the experimental results to gain insight into the low- and high-temperature fuel consumption without and with NO addition and thus the respective interaction chemistry. This investigation firstly contributes a consistent set of temperature-dependent concentration profiles for these two butene isomers under conditions relevant for engine exhaust gases. Secondly, the observed oxidation kinetics is significantly altered with the addition of NO. Specifically, NO promotes fuel consumption and introduces for i-butene a low-temperature behavior featuring a negative temperature coefficient (NTC) region. The present model shows reasonable agreement with the experimental results for major products and intermediate species, and it is capable to explain the promoting effect of NO that is initiated by its contribution to the radical pool. Further, it can describe the observed NTC region for the i-butene/NO mixture as a result of the competition of chain propagation and chain terminating reactions that were identified by reaction flow and sensitivity analyses.
Ignition delay times of C2H6/O2/Ar mixtures with NO2 concentration changing from 0 to 2100 ppm were measured at T = 772–980 K, φ = 0.5–2.0 and p = 15–30 bar in a rapid compression machine. In general, the results show that NO2 addition significantly promotes ethane ignition and reduces its global activation energy. And NO2 promoting effect is more remarkable at fuel-lean and low-pressure condition at given NO2 concentration. The results also demonstrate that pressure shows smaller effect on the ignition of mixtures doped with NO2 than those on neat ethane ignition. Three literature NOx models are selected to validate against the new measured data set to evaluate their performances. Finally, sensitivity and flux analyses are conducted to identify the key reaction classes controlling the ignition of C2H6-NO2 mixtures.
This work aims to provide insight into the interaction of propene with NOx from both experimental and kinetic modeling perspectives. The oxidation of propene at fuel-lean (ϕ=0.23) condition and the oxidation of propene doped with NOx at fuel-lean (ϕ=0.23) and fuel-rich (ϕ=1.35) conditions have been investigated in a laminar flow reactor at atmospheric pressure in the temperature range of 725-1250 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to achieve comprehensive, isomer-resolved identification of major products and critical nitrogenous, carbonyl and hydrocarbon intermediates. To complement the experiments, a detailed kinetic model, starting from widely used core mechanisms, was developed. Rate of production analyses and sensitivity analyses were performed to interpret the experimental observations. The results show that the promoting effects of NOx on the oxidation reactivity of propene are initiated by the reactions of allyl radical with NO2 at low temperature, i.e. C3H5A+NO2C3H5O+NO. For the oxidation of the fuel-rich propene/NOx mixture, temperature-dependent mole fraction profiles of propene, O2 and products show several distinct regions reflecting a competition between chain propagation via C3H5A+NO2C3H5O+NO and chain termination via C3H5A+NOC3H5NO. The formation and consumption chemistry of carbonyl and hydrocarbon intermediates in the presence of NOx was also analyzed and discussed.
Nitrogen oxides (NOx: NO2 and NO) are the main pollutants produced in combustion. Previous studies focus on NOx forming mechanism and control strategies, but pay less attention to the interaction between NOx and hydrocarbon fuels; while even a small amount of NOx can significantly affect the ignitions of hydrocarbons. Ignition characteristic is of importance for the optimization design and operation control of engines such as diesel engines, which are affected by the self-ignition characteristics of fuels. In this study, a rapid compression machine was used to measure the ignition delay times of seven fuel-lean C2H6/O2/Ar mixtures with [NO2]/[C2H6] ranging from 0.0 to 50% at Tc = 760 – 970 K and Pc = 30 bar. The results exhibit no negative temperature coefficient phenomenon in the ignition of ethane with or without NO2. A great reduction in the ignition delay time and the global activation energy is observed at the presence of NO2, which become more noticeable with increasing the NO2 concentration. Subsequently, four literature models involving the C2H6/NO2 chemistry were validated against the new measurements. Moreover, the key elementary reactions and important reaction channels controlling the ignition of C2H6/NO2 mixtures were then identified by the detailed chemical kinetic analysis.
Direct dynamics trajectory simulations were carried out for the NO2 oxidation of EMIM⁺DCA⁻, which were aimed at probing the nature of the primary and secondary reactions in the system. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for NO2 with EMIM⁺DCA⁻ as well as with its analogues BMIM⁺DCA⁻ and AMIM⁺DCA⁻. Reactions of the dialkylimidazolium-DCA ILs are all initiated by proton transfer and/or alkyl abstraction between 1,3-dialkylimidazolium cations and DCA⁻ anion, of which two exoergic pathways are particularly relevant to their oxidation activities. One pathway is the transfer of a Hβ-proton from the ethyl, butyl or allyl group of the dialkylimidazolium cation to DCA⁻ that results in concomitant elimination of the corresponding alkyl as a neutral alkene, and the other pathway is SN2-alkyl abstraction by DCA⁻. The intra-ion-pair reaction products, including [dialkylimidazolium – HC2⁺], alkylimidazole, alkene, alkyl-DCA, HDCA and DCA⁻, react with NO2 and favor formation of nitrite (-ONO) complexes over nitro (-NO2) complexes, albeit the two complex structures have similar formation energies. The exoergic intra-ion-pair reactions in the dialkylimidazolium-DCA ILs account for their significantly higher oxidation activities over the previously reported 1-methyl-4-amino-1,2,4-triazolium dicyanamide [Liu, J.; et al. J. Phys. Chem. B, 2019, 123, 2956], and for the relatively higher reactivity of BMIM⁺DCA⁻ versus AMIM⁺DCA⁻ as BMIM⁺ has a higher reaction path degeneracy for intra-ion-pair Hβ-proton transfer and its Hβ-transfer is more energetically favorable. To validate and directly compare our computational results with spectral measurements in the ILs, infrared and Raman spectra of BMIM⁺DCA⁻ and AMIM⁺DCA⁻ and their products with NO2 were calculated using an ionic liquid solvation model. The simulated spectra reproduced all of the vibrational frequencies detected in the reactions of BMIM⁺DCA⁻ and AMIM⁺DCA⁻ IL droplets with NO2 [as reported by Brotton, S. J.; et al. J Phys. Chem. A, 2018, 122, 7351; and Lucas, M.; et al. J. Phys. Chem. A, 2019, 123, 400].
The kinetic study of the reaction of 1-hydroxyethyl radical (CH3CHOH) with nitric oxide (NO) was performed over the temperature range of 200 – 1100 K and the pressure range of 1.0 × 10-5 – 10.0 bar. The geometries of all of the stationary points were optimized at the B3LYP/6–311++G(df, pd) and the energetics were refined at the CCSD(T)/cc-pVTZ level of theory. Eight reaction pathways were explored and they all consisted of a common first step involving the formation of a deep potential well. Three favorable pathways were confirmed and they were the channels producing the adducts of CH3CO(NHOH) and CH3NOHCHO, and the products of H2O and CH3CNO. The RRKM - CVT method with Eckart tunneling correction was used to calculate the rate coefficients of the system. The predicted total rate coefficients agree well with the available literature data, and show negative temperature dependence and positive pressure dependence. The reaction producing adduct of CH3CHOHNO in the entrance channel is dominant at 1.0 bar and its branching ratio is almost 100% at a temperature less than 670 K. At 3.0 Torr, it is only dominant at a temperature less than 600 K.
The present study deals with the oxidation of H2 at high pressure and its interaction with NO. The high pressure behavior of the H2/NOx/O2 system has been tested over a wide range of temperatures (500–1100 K) and different air excess ratios (λ = 0.5–6.4). The experiments have been carried out in a tubular flow reactor at 10, 20 and 40 bar NO has been found to promote H2 oxidation under oxidizing conditions, reacting with HO2 radicals to form the more active OH radical, which enhances the conversion of hydrogen. The onset temperature for hydrogen oxidation, when doped with NO, was approximately the same at all stoichiometries at high pressures (40 bar), and shifted to higher temperatures as the pressure decreases. The experimental results have been analyzed with an updated kinetic model. The reaction NO+NO+O2⇌NO2+NO2 has been found to be important at all conditions studied and its kinetic parameters have been modified, according to its activation energy uncertainty. Furthermore, the kinetic parameters of reactionHNO+H2⇌NH+H2O have been estimated, in order to obtain a good prediction of the oxidation behavior of H2 and NO conversion under reducing conditions. The kinetic model shows a good agreement between experimental results and model predictions over a wide range of conditions.
A computational investigation into the kinetics of the NO + CH2CCH reaction is presented. The stationary points on the C3H3N1O1 potential energy surface are analyzed using the compound method ANL0, with key regions of the potential energy surface computed using multi-reference methods. The temperature- and pressure-dependent rate constants are computed using the RRKM/Master Equation. The dominant bimolecular products are HCN + CH2CO, CH2CNH + CO, and CH3CN + CO. Additional calculations for the thermal decomposition of an unimolecular intermediate, isoxazole, are in excellent agreement with the available experimental data. The new rate constants are implemented in a detailed chemical kinetic mechanism for the oxidation of C2H4 by O2 + NO. Analysis of a constant temperature, constant pressure batch reaction suggests that NO + CH2CCH could be an important pathway for both NO reduction and CH2CCH oxidation in reburn chemistry.
The reaction of C2H2 with NO2 has been studied theoretically. It is a complex overall reaction with multiple wells and multiple product channels. The calculated rate constant for the preferred channel, formation of a CHOCHON adduct, is compatible with the only experimental determination. The CHOCHON adduct is assumed to dissociate rapidly to form the triplet carbene CHCHO and NO. An experimental and kinetic modeling study of the interaction between C2H2, O2 and NOx was performed under flow reactor conditions in the intermediate temperature range (600-900 K), high pressure (50-60 bar), and for stoichiometries ranging from reducing to strongly oxidizing. The results show that presence of NOx serves both to sensitize and inhibit oxidation of C2H2. Calculations with a detailed chemical kinetic model, partly established in the present work, confirm that C2H2 + NO2 is the major initiation step, as well as the major sensitizing reaction. This reaction converts NO2 to NO, which is then partly converted to HCN by reaction with C2H3 and CHCHOH. The latter reactions are both chain terminating and serve as the major inhibiting steps.
The pyrolysis of nitroethane has been investigated over the temperature range of 682–1423 K in a plug flow reactor at a low pressure. The major species in the pyrolysis process have been identified and quantified using tunable synchrotron vacuum ultraviolet photoionization mass spectrometry and molecular beam sampling techniques. The rate constants for the primary pyrolysis of nitroethane as well as those for the decomposition of the secondary product CH3CHNO2 have been obtained via ab initio calculations. These results have been adopted in a detailed chemical kinetic model, which contains 95 species and 737 reactions. The model was validated against the experimental results with satisfactory agreement for most of the identified and quantified species. Further analysis on the results indicates that both the concerted molecular elimination and C–N bond rupture are significant in the primary pyrolysis of nitroethane, with the latter channel being more important at high temperatures. The adoption of new decomposition pathways of CH3CHNO2 has resulted in reasonable predictions for relevant intermediates.
The thermal decomposition of cyanogen azide (NCN3) and the subsequent collision-induced intersystem crossing (CIISC) process of cyanonitrene (NCN) have been investigated by monitoring excited electronic state 1NCN and ground state 3NCN radicals. NCN was generated by the pyrolysis of NCN3 behind shock waves and by the photolysis of NCN3 at room temperature. Falloff rate constants of the thermal unimolecular decomposition of NCN3 in argon have been extracted from 1NCN concentration–time profiles in the temperature range 617 K
An experimental and kinetic modeling study is reported on three premixed nitroethane/oxygen/argon flames at low pressure (4.655 kPa) with the equivalence ratios (Φ) of 1.0, 1.5 and 2.0. Over 30 flame species were identified with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry, with their mole fractions quantified as the function of the height above burner. The flame temperature profiles were measured with a Pt–6%Rh/Pt–30%Rh thermocouple. A detailed kinetic mechanism with 115 species and 730 reactions was proposed and validated against experimental results. The computed predictions have shown satisfactory agreement with the experimental results. Basing on the rate-of-production analysis, the reaction pathways that feature the combustion of nitroethane were revealed, including the primary decomposition of C–N bond fission, the oxidation of C2 and C1 hydrocarbons and the formation of nitrogenous species. The presence of NO2 and NO has been proved to be important for these processes.
Autoignition delay times of stoichiometric methane, ethane and methane/ethane mixtures doped with 100 and 270ppm of NO2 have been measured in a RCM in the temperature range 900–1050K and pressures from 25 to 50bar. The measurements show that addition of NO2 to CH4/O2/N2/Ar and CH4/C2H6/O2/N2/Ar mixtures results in a significant reduction in the autoignition delay time and that the ignition-promoting effect of NO2 increases substantially with increasing temperature, from ∼20% to more than a factor of two over the range of temperature studied. Addition of NO2 to C2H6/O2/N2/Ar mixtures results in only a modest reduction in ignition delay time over the range of pressure and temperature measured. Computations with an updated chemical mechanism show good agreement with the measurements for undoped methane, but overpredict the delay times for undoped ethane and underestimate the effects of replacing 10% methane by ethane. For NO2-containing mixtures, the model predicts the observed trend in decreasing delay time with increasing NO2 fraction. However, the computations tend to overestimate the effect of NO2 addition on ignition, particularly for C2H6 mixtures. Analysis of the reaction mechanism for the effects of NO2 addition to methane mixtures indicates that the ignition-promoting effect of NO2 is related to the appearance of new conversion channels for CH3 and CH3OO, i.e., NO2+CH3→NO+CH3O and NO+CH3OO→NO2+CH3O, generation of chain-initiating OH radicals through NO/NO2 interconversion, i.e., NO2+H→NO+OH and NO+HO2→NO2+OH, and to the direct initiation step CH4+NO2→CH3+HNO2. Analyses further show that the formation of CH3NO2 via CH3+NO2(+M)↔CH3NO2(+M) essentially inactivates NO2. This reaction limits the promoting effect of NO2 at lower temperatures and higher pressures, where stabilization of CH3NO2 is favored, explaining the experimentally observed trends.
The high-temperature rate constants of the reactions NCN + NO and NCN + NO(2) have been directly measured behind shock waves under pseudo-first-order conditions. NCN has been generated by the pyrolysis of cyanogen azide (NCN(3)) and quantitatively detected by sensitive difference amplification laser absorption spectroscopy at a wavelength of 329.1302 nm. The NCN(3) decomposition initially yields electronically excited (1)NCN radicals, which are subsequently transformed to the triplet ground state by collision-induced intersystem crossing (CIISC). CIISC efficiencies were found to increase in the order of Ar < NO(2) < NO as the collision gases. The rate constants of the NCN + NO/NO(2) reactions can be expressed as k(NCN+NO)/(cm(3) mol(-1)s(-1)) = 1.9 × 10(12) exp[-26.3 (kJ/mol)/RT] (±7%,ΔE(a) = ± 1.6 kJ/mol, 764 K < T < 1944 K) and k(NCN+NO(2))/(cm(3) mol(-1)s(-1)) = 4.7 × 10(12) exp[-38.0(kJ/mol)/RT] (±19%,ΔE(a) = ± 3.8 kJ/mol, 704 K < T < 1659 K). In striking contrast to reported low-temperature measurements, which are dominated by recombination processes, both reaction rates show a positive temperature dependence and are independent of the total density (1.7 × 10(-6) mol/cm(3) < ρ < 7.6 × 10(-6) mol/cm(3)). For both reactions, the minima of the total rate constants occur at temperatures below 700 K, showing that, at combustion-relevant temperatures, the overall reactions are dominated by direct or indirect abstraction pathways according to NCN + NO → CN + N(2)O and NCN + NO(2) → NCNO + NO.
The rate constant of the reaction NCN + O has been directly measured for the first time. According to the revised Fenimore mechanism, which is initiated by the NCN forming reaction CH + N(2)→ NCN + H, this reaction plays a key role for prompt NO(x) formation in flames. NCN radicals and O atoms have been quantitatively generated by the pyrolysis of NCN(3) and N(2)O, respectively. NCN concentration-time profiles have been monitored behind shock waves using narrow-bandwidth laser absorption at a wavelength of λ = 329.1302 nm. Whereas no pressure dependence was discernible at pressures between 709 mbar < p < 1861 mbar, a barely significant temperature dependence corresponding to an activation energy of 5.8 ± 6.0 kJ mol(-1) was found. Overall, at temperatures of 1826 K < T < 2783 K, the rate constant can be expressed as k(NCN + O) = 9.6 × 10(13)× exp(-5.8 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (±40%). As a requirement for accurate high temperature rate constant measurements, a consistent NCN background mechanism has been derived from pyrolysis experiments of pure NCN(3)/Ar gas mixtures, beforehand. Presumably, the bimolecular secondary reaction NCN + NCN yields CN radicals hence triggering a chain reaction cycle that efficiently removes NCN. A temperature independent value of k(NCN + NCN) = (3.7 ± 1.5) × 10(12) cm(3) mol(-1) s(-1) has been determined from measurements at pressures ranging from 143 mbar to 1884 mbar and temperatures ranging from 966 K to 1900 K. At higher temperatures, the unimolecular decomposition of NCN, NCN + M → C + N(2) + M, prevails. Measurements at temperatures of 2012 K < T < 3248 K and at total pressures of 703 mbar < p < 2204 mbar reveal a unimolecular decomposition close to its low pressure limit. The corresponding rate constants can be expressed as k(NCN + M) = 8.9 × 10(14)× exp(-260 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(±20%).
The potential of the thermal decomposition of cyanogen azide (NCN3) as a high-temperature cyanonitrene (NCN) source has been investigated in shock tube experiments. Electronic ground-state NCN(3Σ) radicals have been detected by narrow-bandwidth laser absorption at overlapping transitions belonging to the Q1 branch of the vibronic 3Σ+−3Π subband of the vibrationally hot 3Πu(010)−3Σg−(010) system at = 30383.11 cm(-1) (329.1302 nm). High-temperature absorption cross sections σ have been directly measured at total pressures of 0.2−2.5 bar, log[σ/(cm2 mol(-1))] = 8.9−8.3 × 10(-4) × T/K (±25%, 750 < T < 2250 K). At these high temperatures, NCN(3Σ) formation is limited by a slow electronic relaxation of the initially formed excited NCN(1Δ) radical rather than thermal decomposition of NCN3. Measured temperature-dependent collision-induced intersystem crossing (CIISC) rate constants are best represented by kCIISC/(cm3 mol(-1) s(-1)) = (1.3 ± 0.5) × 1011 exp[−(21 ± 4) kJ/mol/RT] (740 < T < 1260 K). Nevertheless, stable NCN concentration plateaus have been observed, showing that NCN3 is an ideal precursor for NCN kinetic experiments behind shock waves.
New experimental results were obtained for the mutual sensitization of the oxidation of NO and ethane and NO and ethylene in fuel-lean conditions. An atmospheric fused-silica jet-stirred reactor operating over the temperature range 700–1150 K was used. The initial carbon mole fraction was 2500 ppm whereas that of NO varied from 0 to 1200 ppm. Sonic quartz probe sampling followed by on-line Fourier transform infrared analyses and off-line gas chromatography-thermal conductivity detection flame ionization detection analyses were used to measure the concentration profiles of the reactants, stable intermediates, and the final products. A detailed chemical kinetic modeling of the present experiments was performed (147 species, 1085 reversible reactions). An overall good agreement between the present data and modeling was obtained. Furthermore, the proposed model was able to simulate, better than in previous modeling efforts, plug-flow reactor experimental results available in the literature. According to the proposed model, the mutual sensitization of the oxidation of ethane or ethylene and NO proceeds mostly through the conversion of NO to NO2 by HO2 radicals. The NO-to-NO2 conversion is enhanced by the production of HO2 radicals from the oxidation of the fuel. The production of OH resulting from the oxidation of NO by the hydroperoxy radical promotes the oxidation of the fuel: NO + HO2 ⇒ OH + NO2 is followed by OH + C2H4 ⇒ C2H3 + H2O and OH + C2H6 ⇒ C2H5 + H2O. In the case of ethane, at low temperature, the reaction further proceeds via CH3 + O2 ⇒ CH3O2; CH3O2 + NO ⇒ CH3O + NO2; C2H5O2 + NO ⇒ C2H5O + NO2; C2H5 + O2 ⇒ C2H4 + HO2. At higher temperature, the sequence is followed by CH3O ⇒ CH2O + H; C2H5O ⇒ CH3CHO + H; C2H5O ⇒ CH3 + CH2O; CH2O + OH ⇒ HCO + H2O; HCO + O2 ⇒ HO2 + CO; and H + O2 ⇒ HO2. In the case of ethylene, the reaction further proceeds via C2H3 + O2 ⇒ CH2O + HCO; CH2O + OH ⇒ HCO + H2O; HCO + O2 ⇒ HO2 + CO; and H + O2 + M ⇒ HO2 + M. The main chemical kinetic differences between the two fuels in presence of NO were analyzed.
The thermochemical database of species involved in combustion processes is and has been available for free use for over 25 years. It was first published in print in 1984, approximately 8 years after it was first assembled, and contained 215 species at the time. This is the 7th printed edition and most likely will be the last one in print in the present format, which involves substantial manual labor. The database currently contains more than 1300 species, specifically organic molecules and radicals, but also inorganic species connected to combustion and air pollution. Since 1991 this database is freely available on the internet, at the Technion-IIT ftp server, and it is continuously expanded and corrected. The database is mirrored daily at an official mirror site, and at random at about a dozen unofficial mirror and 'finger' sites. The present edition contains numerous corrections and many recalculations of data of provisory type by the G3//B3LYP method, a high-accuracy composite ab initio calculation. About 300 species are newly calculated and are not yet published elsewhere. In anticipation of the full coupling, which is under development, the database started incorporating the available (as yet unpublished) values from Active Thermochemical Tables. The electronic version now also contains an XML file of the main database to allow transfer to other formats and ease finding specific information of interest. The database is used by scientists, educators, engineers and students at all levels, dealing primarily with combustion and air pollution, jet engines, rocket propulsion, fireworks, but also by researchers involved in upper atmosphere kinetics, astrophysics, abrasion metallurgy, etc. This introductory article contains explanations of the database and the means to use it, its sources, ways of calculation, and assessments of the accuracy of data.
Calculations by the B3LYP density functional method with various basis sets and by the QCISD(T)/6-31G(d) ab initio method showed that the main pathway of monomolecular gas-phase decomposition of nitroethylene is that involving a cyclic intermediate, 4H-1,2-oxazete 2-oxide; the barrier of its formation (201.9, 203.9, and 216.5 kJ mol–1, as estimated by various methods) reasonably agrees with the experimental value (191.9 kJ mol–1). The barriers of alternative pathways of gas-phase decomposition of nitroethylene are considerably higher. The barriers of reactions involving radical cations are considerably lower than those of the similar reactions involving molecules. Among all the considered pathways of nitroethylene decomposition, bimolecular pathways are the most favorable energetically.
The pulsed laser photolysis/cw laser absorption technique is used to investigate the reaction of vinyl (C2H3) with NO in the temperature range from 295 to 700 K and pressures from 10 to 320 Torr (1.33 to 42.6 kPa). Vinyl radicals are generated by photolysis of vinyl iodide at 266 nm and detected by visible laser absorption in a vibronic band of the (Ã←) transition near 403 nm. The potential energy surface is explored with both quadratic configuration interaction and multi-reference configuration interaction ab initio calculations. These ab initio predictions are employed in RRKM theory based master equation simulations of the temperature and pressure dependent kinetics. At room temperature, the overall rate constant for removal of vinyl radical by NO is measured to be 1.6±0.4×10−11 cm3 molecule−1 s−1, with negligible pressure dependence from 10 Torr (1.33 kPa) to 160 Torr (21.3 kPa) of helium. At constant pressure the rate constant decreases rapidly with temperature. At higher temperatures, a falloff of the rate constant to lower pressure is observed. The ab initio characterizations suggest a significant contribution from HCN+CH2O formation, with both isomerization transition states for the pathway leading to this product lying ∼15 kcal mol−1 (63 kJ mol−1) below the entrance channel. The master equation analysis provides a reasonably satisfactory reproduction of the observed kinetic data. The HCN+CH2O bimolecular channel, which proceeds from the addition complex through tight ring forming and opening transition states, has a negative temperature dependence and is the dominant channel for pressures of about 50 Torr (6.7 kPa) and lower. The theoretically predicted zero pressure rate coefficient is reproduced by the modified Arrhenius expression 5.02×10−11(T/298)−3.382exp(−516.3/T) cm3 molecule−1 s−1 (with T in K).
Gas reburning is a NOx reduction technique that has been demonstrated to be efficient in different combustion systems. An experimental study of gas reburning performance in the low temperature range (at and under 1100°C) has been carried out. An evaluation of the use of different hydrocarbon fuels, such as natural gas, methane, ethane, ethylene and acetylene was performed and the influence of the temperature and stoichiometry is considered. The results show that the reburning process is effective under appropriate conditions at the low temperatures used in this work. However, as the temperature diminishes, the influence of the reburn fuel becomes more marked and the use of acetylene or ethane and ethylene leads to better performance than natural gas or methane, the classical reburn fuels for high temperature applications.
The kinetics of the reduction of nitric oxide (NO) by ethylene have been studied in a fused silica jet-stirred reactor at 1 atm and a temperatures from 900 to 1,400 K to simulate conditions in a reburning zone. The initial mole fraction of NO was 1,000 ppm, that or ethylene was 4,400 ppm. The equivalence ratio was varied from 0.75 to 2. It was found that the reduction of NO varies with temperature and that for a given temperature, the maximum reduction of NO occurs slightly fuel-rich of stoichiometric conditions. Thus, operating under optimal NO-reburning conditions is possible for particular combinations of equivalence ratio and temperature. The results generally agree with previous studies involving simple hydrocarbons or natural gas as reburn fuel. Detailed chemical kinetic modeling of the experiments was performed using an updated and improved kinetic scheme (877 reversible reactions and 122 species). Overall, reasonable agreement was obtained between the present measurements and the modeling although improvements of the model are still necessary. Also, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethane, acetylene, a natural gas blend (methane-ethane 10:1) and HCN, as well as the low temperature interactions between NO and simple alkanes. According to this study, the main way of reducing NO by ethylene involves the ketenyl radical, HCCO.
A detailed chemical kinetic model for oxidation of C2H4 in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C2H3+O2 reaction was used to obtain rate coefficients over a wide range of conditions (0.003–100bar, 200–3000K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH2O and HCO, is the dominant product. The experiments, involving C2H4/O2 mixtures diluted in N2, were carried out in a high pressure flow reactor at 600–900K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C2H4 are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH2CH2OH) and 2-hydroperoxyethyl (CH2CH2OOH) radicals, formed from addition of OH and HO2 to C2H4, and vinyl peroxide, formed from C2H3+O2. Hydroxyethyl is oxidized through the peroxide HOCH2CH2OO (lean conditions) or through ethenol (low O2 concentration), while 2-hydroperoxyethyl is converted through oxirane.
Autoignition delay times of stoichiometric methane, ethane and methane/ethane mixtures doped with 100 and 270ppm of NO2 have been measured in a RCM in the temperature range 900–1050K and pressures from 25 to 50bar. The measurements show that addition of NO2 to CH4/O2/N2/Ar and CH4/C2H6/O2/N2/Ar mixtures results in a significant reduction in the autoignition delay time and that the ignition-promoting effect of NO2 increases substantially with increasing temperature, from ∼20% to more than a factor of two over the range of temperature studied. Addition of NO2 to C2H6/O2/N2/Ar mixtures results in only a modest reduction in ignition delay time over the range of pressure and temperature measured. Computations with an updated chemical mechanism show good agreement with the measurements for undoped methane, but overpredict the delay times for undoped ethane and underestimate the effects of replacing 10% methane by ethane. For NO2-containing mixtures, the model predicts the observed trend in decreasing delay time with increasing NO2 fraction. However, the computations tend to overestimate the effect of NO2 addition on ignition, particularly for C2H6 mixtures. Analysis of the reaction mechanism for the effects of NO2 addition to methane mixtures indicates that the ignition-promoting effect of NO2 is related to the appearance of new conversion channels for CH3 and CH3OO, i.e., NO2+CH3→NO+CH3O and NO+CH3OO→NO2+CH3O, generation of chain-initiating OH radicals through NO/NO2 interconversion, i.e., NO2+H→NO+OH and NO+HO2→NO2+OH, and to the direct initiation step CH4+NO2→CH3+HNO2. Analyses further show that the formation of CH3NO2 via CH3+NO2(+M)↔CH3NO2(+M) essentially inactivates NO2. This reaction limits the promoting effect of NO2 at lower temperatures and higher pressures, where stabilization of CH3NO2 is favored, explaining the experimentally observed trends.
The pyrolysis of acetylene in the presence of nitrogen oxide has been investigated by use of a single-pulse shock tube in the temperature range 1100-1650 K. The hydrocarbons produced in the presence of nitrogen oxide were quite the same as those obtained from the pyrolysis acetylene by itself. The yield-temperature plots of 1-buten-3-yne showed that at temperatures below 1350 K the formation rate of 1-buten-3-yne formation was interpreted in terms of the reactions of nitrogen oxide with 1-ethynylvinyl and 2-ethynylvinyl radicals to yield pro-pynenitrile and hydrogen cyanide, respectively. An illustrative radical chain mechanism is presented for the pyrolysis of acetylene in line with the experimental results obtained from the pyrolysis inhibited by nitrogen oxide.
The kinetics of the reactions of vinyl (C2H3) and propargyl (C3H3) radicals with NO2 have been studied in direct measurements at temperatures between 220 and 340 K, using a tubular flow reactor coupled to a photoionization mass spectrometer. The vinyl and propargyl radicals have been homogeneously generated at 193 nm by the pulsed laser photolysis of methyl vinyl ketone (vinyl bromide) and propargyl chloride, respectively. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of NO2 being in large excess over radical concentrations. The bimolecular rate coefficients of both reactions are independent of the bath gas (He or N2) and pressure within the experimental range (1−7 Torr) and are found to depend on temperature as follows: k(C2H3 + NO2) = [(4.19 ± 0.05) × 10-11](T/300 K)-0.60 ± 0.07 cm3 molecule-1 s-1 and k(C3H3 + NO2) = [(2.55 ± 0.05) × 10-11](T/300 K)-1.06 ± 0.10 cm3 molecule-1 s-1, with the uncertainties given as 1 standard deviation. The photolysis of propargyl chloride has also been observed to produce C3H3Cl2 radicals rapidly under the experimental conditions, thus enabling us to measure the bimolecular reaction rate coefficient of the C3H3Cl2 radical with NO2 at room temperature: k(C3H3Cl2 + NO2) = (2.37 ± 0.05) × 10-11 cm3 molecule-1 s-1. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are about ±20%. The only reaction product observed for the vinyl radical reaction with NO2 is NO. The experimental findings have been compared with the results of ab initio calculations, which give insight into possible reaction pathways.
Rate constants for the reactions of 1- and 2-hydroxyethyl radicals with O2 and NO were measured at room temperature by laser flash photolysis-photoionization mass spectrometry method. The results are as follows (in units of cm3 molecule−1 s−1): CH3CHOH+O2 ((2.8 ± 0.2) × 10−11); CH3CHOH+NO ((2.4±0.6)× 10−11); CH2CH2OH + O2 ((3.0± 0.4)x 10−12); CH2CH2OH+NO ((2.6 ±0.7)× 10−11). The reaction of 1-hydroxyethyl radical with O2 was found to be approximately one order of magnitude faster than 2-hydroxyethyl radical. The reaction mechanism seems to be different for the two hydroxyethyl radicals.
We have used the technique of laser-induced fluorescence to study the addition reactions of the 2-oxoethyl (vinoxy) radical with O2 and NO as a function of temperature and pressure. The bimolecular rate constants for the reaction with NO are observed over the major portion of the transition region from the low- to high-pressure limits and are well fit at room temperature and M = N2 by the expression of Troe14 with coefficients k0 = 6.53 (±0.85) × 10-29 cm6 molecule-2 s-1, k∞, = 2.51 (±0.30) × 10-11 cm3 molecule-1 s-1, and Fc = 0.54 (±0.08). A much smaller pressure dependence is observed for the reaction of vinoxy with O2. Similarities between the reactions of vinoxy radicals and alkyl radicals are discussed.
The reaction of hydrogen atoms with ethylene in excess helium at a total pressure of 8 Torr (1 Torr = 133 N m–2) has been studied in a fast flow discharge system at five temperatures between 321 and 521 K. Mass spectrometric analysis of the reaction products has provided data, numerical analysis of which has given the temperature variation of the rate constants of the reactions H + C2H˙5 [graphic omitted] 2CH˙3, H + CH˙3+ M [grephic omitted] CH4+ M,
log10(k3/dm3 mol–1 s–1)= 10.81 ± 0.05 –(223 ± 70)/4.5757(T/K), log10(k4/dm6 mol–2 s–1)= 13.84 ± 0.7–(0.33 ± 0.27) log10(T/K), for M = He.
The most important reactions in the reduction of NO in reburn zones have been identified for several reburning fuels at low temperatures. This has been accomplished through the analysis of reaction rates and sensitivity in kinetic plug flow calculations simulating the gas reburning process. The kinetic model developed is evaluated through comparisons with experimental data. Acetylene, ethylene, ethane, methane, and natural gas have been studied at various temperatures between 973 and 1373 K., The major NO removal pathway is found to be the HCCO + NO reaction in all cases. Critical to the modeling are the branching fraction of this reaction and the competition for vinyl between dissociation and reaction with molecular oxygen. These points are discussed in detail.
The reduction of nitric oxide by reaction with C1 and C2 hydrocarbons under reducing conditions in a flow reactor has been analyzed in terms of a detailed chemical kinetic model. The experimental data were partly adopted from previous work and partly obtained in the present study; they cover the temperature range 800–1500 K and the reburn fuels CH4, C2H2, C2H4, C2H6, and natural gas. Modeling predictions indicate that, under the conditions investigated, HCCO + NO and CH3 + NO are the reactions most important in reducing NO. The HCCO + NO reaction is the dominant reaction when using natural gas or C2 hydrocarbons as reburn fuels. This reaction leads partly to HCNO, which is recycled to NO, and partly to HCN, which is converted to N2 or NO. When methane or natural gas are used as reburn fuel, the CH3 + NO reaction contributes significantly to remove NO. Modeling predictions are in reasonably good agreement with the experimental observations for the fuels investigated, even though the NO reduction potential is overpredicted for methane and underpredicted for ethane. Modeling predictions for NO are very sensitive to the formation of HCCO and to the product branching ratio for the HCCO + NO reaction. Furthermore, the present analysis indicates that more work is needed on critical steps in the hydrocarbon oxidation scheme.
The recently introduced complete basis set, CBS-Q, model chemistry is modified to use B3LYP hybrid density functional geometries and frequencies, which give both improved reliability (maximum error for the G2 test set reduced from 3.9 to 2.8 kcal/mol) and increased accuracy (mean absolute error reduced from 0.98 to 0.87 kcal/mol), with little penalty in computational speed. The use of a common method for geometries and frequencies makes the modified model applicable to transition states for chemical reactions. © 1999 American Institute of Physics.
Experimental measurements in conjunction with kinetic modeling have been used to study the NO-accelerated oxidation of ethylene. The reaction between ethylene, O2, and NO was investigated experimentally over a wide range of reactant gas compositions for the temperature range 650–1000 K. The product distribution at low temperatures was found to be quite different from that observed for unpromoted ethylene oxidation, specifically the presence of formaldehyde in similar yields to CO. At higher temperatures, CO and CO2 become the dominant products. The kinetic model developed here allows semiquantitative prediction of the products and extent of reaction. Through kinetic modeling, considerable insight has been gained into the mechanism of NO-promoted ethylene oxidation. Under conditions of low oxygen concentration or at temperatures above 850 K, the mechanism was found to have much in common with unpromoted ethylene oxidation. However, below 850 K or at high oxygen concentrations, the major reaction pathway is through OH addition to ethylene and the subsequent reactions of hydroxyethyl radical. Without consideration of OH addition to ethylene, the observed concentrations of formaldehyde are grossly underestimated. Flux and sensitivity analysis has shown that the NO-promoted oxidation of ethylene involves a complex set of reactions in partial equilibria. The effect of reactant gas composition on the equilibrium concentrations of important intermediate species allows many of the complexities of NO/O2/C2H4 reaction kinetics to be understood.
Four distinct pathways of unimolecular decomposition of nitroethylene, the C−NO2 bond breaking, nitro-to-nitrite rearrangement, 1,2-elimination reaction and 1,1-elimination reaction, have been computationally investigated with ab initio, MP2, MP4, and G2 methods as well as with DFT methods. The nitro-to-nitrite rearrangement and 1,2-elimination reaction are found to give the lowest energy decomposition pathways for this molecule, about 15 kcal/mol lower than the cleavage of the nitro group.
Arrhenius parameters have been determined for the NO2 catalyzed geometric isomerization of the 2-butenes and the 2-pentenes. For cis-2-butene, trans-2-butene, cw-2-pentene, and trans-2-pentene respectively E, kcal mol-1 = 11.8,12.2,11.2, and 12.5 and log A, 1. mol-1 sec-1 = 7.86, 7.65, 7.49, and 7.72 over the temperature ranges 298-366, 297-370, 298-381, and 298-382 °K. Combination of these results with the results of thermochemical calculations permits calculation of 298 °K rate constants for NO2 addition to the double bond often simple olefins. These rate constants suggest that this reaction is unimportant in the bulk consumption of atmospheric olefin pollutants.
The mercury-photosensitized decomposition of acetylene in the presence of NO was studied in a fast-flow apparatus. The main products are HCN, formaldehyde, and propynal. The effects of variation in flow rate, NO concentration, and cell pressure were studied. The results are interpreted in terms of a primary dissociation of the acetylene molecule into an H atom and an ethynyl radical. The H atom adds to acetylene to form vinyl radicals which in turn, with NO, give HCN and formaldehyde. The ethynyl radical also adds to acetylene forming 2-ethynylvinyl radicals, HC≡C-CH=ĊH, which react with NO to give HCN and propynal. These results are discussed in relation to the mechanism of the mercury-photosensitized polymerization of acetylene.
Using a pulse-radiolysis transient UV–VIS absorption system, rate constants for the reactions of F atoms with CH3CHO (1) and CH3CO radicals with O2 (2) and NO (3) at 295 K and 1000 mbar total pressure of SF6 was determined to be k1=(1.4±0.2)×10−10, k2=(4.4±0.7)×10−12, and k3=(2.4±0.7)×10−11 cm3 molecule−1 s−1. By monitoring the formation of CH3C(O)O2 radicals (λ>250nm) and NO2 (λ=400.5nm) following radiolysis of SF6/CH3CHO/O2 and SF6/CH3CHO/O2/NO mixtures, respectively, it was deduced that reaction of F atoms with CH3CHO gives (65±9)% CH3CO and (35±9)% HC(O)CH2 radicals. Finally, the data obtained here suggest that decomposition of HC(O)CH2O radicals via CC bond scission occurs at a rate of <4.7×105 s−1. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 913–921, 1998
The shock tube data of Ogura [5] on the pyrolysis of C2H2/NO mixtures (1100–1500 K) is shown to be consistent with a simple mechanism whereby radicals are both initiated and terminated by NO (Scheme I). The scheme accounts for the rate of formation of the main product, vinylacetylene (VA), the lesser products CO and HCN and a very minor product, propionitrile. It is also shown to be consistent with other studies below 900 K and observation at 300 K on the reactions of vinyl radicals with NO. The substantial inhibition of vinyl acetylene formation by 5% NO makes untenable any substantial role of vinylidene in the C2H2 pyrolysis above 1000°K.
The reaction of NO with acetylene is an efficient source of HCN. It appears to be a general reaction of NO with substituted acetylenes and below 900 K a mechanism is presented to account for the production of acrylonitrile (AN) from the reaction of NO with VA.
Thermochemical data are estimated on ΔfH°298 and S°298 for some alkyl-NO, vinyl NO, and acetylene NO compounds and radicals and some new and some revised group values are estimated for estimating ΔfH°298 of derivatives of hydroxyl amines, imines, and isoxazolines.
A detailed chemical kinetic model for homogeneous combustion of the light hydrocarbon fuels CH4 and C2H6 in the intermediate temperature range roughly 500–1100 K, and pressures up to 100 bar has been developed and validated experimentally. Rate constants have been obtained from critical evaluation of data for individual elementary reactions reported in the literature with particular emphasis on the conditions relevant to the present work. The experiments, involving CH4/O2 and CH4/C2H6/O2 mixtures diluted in N2, have been carried out in a high-pressure flow reactor at 600–900 K, 50–100 bar, and reaction stoichiometries ranging from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Finally, the mechanism was extended with a number of reactions important at high temperature and tested against data from shock tubes, laminar flames, and flow reactors. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 778–807, 2008
This paper presents results from lean CO/H2/O2/NOx oxidation experiments conducted at 20–100 bar and 600–900 K. The experiments were carried out in a new high-pressure laminar flow reactor designed to conduct well-defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H2/O2, CO/CO2, and NOx chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS-QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO2 ⇋ H2O + O2, CO + OH ⇋ [HOCO] ⇋ CO2 + H, HOCO + OH ⇋ CO + H2O2, NO2 + H2 ⇋ HNO2 + H, NO2 + HO2 ⇋ HONO/HNO2 + O2, and HNO2(+M) ⇋ HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H2/O2, H2/O2/NO2, and CO/H2O/O2 at 780–1100 K and 1–10 bar. Moreover, introduction of the reaction CO + H2O2 → HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454–480, 2008
The purpose of this chapter is to examine reactions of nitrogen-containing species that are important in high-temperature gas-phase systems so as to provide the best set of rate coefficients presently available for use in combustion chemistry modeling. Since the 1984 review of N/H/O rate coefficients by Hanson and Salimian 1984 there has been a major review of nitrogen chemistry under combustion conditions by Miller and Bowman (1989). Several compilations of evaluated rate coefficients have also appeared. We update these discussions where appropriate and then analyze a number of chemically activated reactions that are relevant to understanding nitrogen chemistry.
Concern about pollutant formation and emissions continues to be a driving force for research in combustion chemistry. Important pollutants include nitrogen oxides (NOx), sulfur oxides (SOx), chlorine species, unburned or partly burned fuel components (e.g., UHC, aldehydes, CO), aromatic and polycyclic aromatic compounds, and aerosols (soot, alkaline aerosols). In this review, it is discussed how N, S, Cl, and K/Na species, typically present in small quantities, may affect the overall combustion process, as well as the formation or transformation of each other. Of special interest is their ability to sensitize or inhibit oxidation of fuel and CO, depending on the reaction conditions; the impact of S, Cl and K/Na on formation of NOx, PAH, and soot; and the interaction of sulfur, chlorine and alkali species, which may have significant implications for emissions of SO2, HCl, and aerosols.
Experimental and detailed chemical kinetic modeling work has been performed to investigate the role of hydrocarbon oxidation in NO-NO2 conversion. An atmospheric pressure., quartz flow reactor was used to examine the dependence of NO oxidation to NO2 by hydrocarbon type, reaction temperature, and residence time. The five hydrocarbons examined were methane, ethylene, ethane, propene, and propane. In the experiment, probe measurement of the species concentrations was performed in the flow reactor using a mixture of NO(20 ppm)/air/hydrocarbon(50 ppm) at residence times from 0.16 to 1.46 s and temperatures from 600 to 1100 K. In the chemical kinetic calculation, the time evolution of NO, NO2, hydrocarbons, and reaction intermediates were evaluated for a series of the hydrocarbons and the temperatures. The chemical mechanism consisted of 639 reversible reactions and 126 species.Experimental results indicate that, in general, ethylene and propane effectively oxidize NO to NO2 while methane is less effective. The calculation indicates the important chemical kinetic features that control NO-NO2 conversion for each hydrocarbon type. The dependence of NO-NO2 conversion with hydrocarbon type and temperature is qualitatively reproduced by the calculation. The calculation indicates that all five hydrocarbons oxidize NO to NO2 predominantly through NO+HO2 ahNO2+OH and that the contribution of oxidation by RO2 and HORO2 is minor. Highest effectiveness comes from hydrocarbons that produce reactive radicals (i.e., OH, O atom) that promote hydrocarbon oxidation and lead to additional HO2 production. On the other hand, if hydrocarbons produce radicals, such as methyl and allyl, which resist oxidation by O2, then these radicals tend to reduce NO2 to NO. Experimental results show that the effectiveness of hydrocarbons varies appreciably with temperature and only within the low-temperature range. Propane shows the greatest NO-NO2 conversion for the lowest temperatures. This ability is primarily due to the hydroperoxy-propyl plus O2 reactions as indicated by the sensitivity analysis results.
The CH4/O2/NOx system is investigated in a laboratory-scale high pressure laminar flow reactor with the purpose of elucidating the sensitizing effects of NOx on CH4 oxidation at high pressures and medium temperatures. Experiments are conducted at 100, 50, and 20 bar, 600–900 K, and stoichiometric ratios ranging from highly reducing to oxidizing conditions. The experimental results are interpreted in terms of a detailed kinetic model drawn from previous work of the authors, including an updated reaction subset for the direct interactions of NOx and C1–2 hydrocarbon species relevant to the investigated conditions. The results reveal a significant decrease in the initiation temperature upon addition of NOx. A similar effect is observed with increasing pressure. The sensitizing effect of NOx is related to the hydrocarbon chain-propagating NO/NO2 cycle operated by NO2+CH3⇋NO+CH3O and NO+CH3OO⇋NO2+CH3O as well as the formation of chain-initiating OH radicals from interactions between NO/NO2 and the H/O radical pool. At low temperatures, reactions between NO/NO2 and CH3O/CH2O also gain importance. The results indicate a considerable intermediate formation of nitromethane (CH3NO2) as a characteristic high-pressure phenomenon. The formation of CH3NO2 represents an inactivation of NOx, which may result in a temporary reduction of the overall hydrocarbon conversion rate.
We have studied the vinyl + NO reaction using time-resolved Fourier transform emission spectroscopy, complemented by electronic structure and microcanonical RRKM rate coefficient calculations. To unambiguously determine the reaction products, three precursors are used to produce the vinyl radical by laser photolysis: vinyl bromide, methyl vinyl ketone, and vinyl iodide. The emission spectra and theoretical calculations indicate that HCN + CH2O is the only significant product channel for the C2H3 + NO reaction near room temperature, in contradiction to several reports in the literature. Although CO emission is observed when vinyl bromide is used as the precursor, it arises from the reaction of NO with photofragments other than vinyl. This conclusion is supported by the absence of CO emission when vinyl iodide or methyl vinyl ketone is used. Prompt emission from vibrationally excited NO is evidence of the competition between back dissociation and isomerization of the initially formed nitrosoethylene adduct, consistent with previous work on the pressure dependence of this reaction. Our calculations indicate that production of products is dominated by the low energy portion of the energy distribution. The calculation also predicts an upper bound of 0.19% for the branching ratio of the H2CNH + CO channel, which is consistent with our experimental results.
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