Combustion Kinetics of Light Hydrocarbons in the Presence of

Ind. Eng. Chem. Res. 1998, 37, 4241-4252

4241

Combustion Kinetics of Light Hydrocarbons in the Presence of Nitrogen Oxide Renato Rota,* Massimo Morbidelli,† and Sergio Carra` Politecnico di Milano, Dipartimento di Chimica Fisica Applicata, via Mancinelli 7, I-20131 Milano, Italy

An experimental analysis of the interactions between different hydrocarbons and NO is reported. All the experiments have been carried out in a perfectly stirred reactor, operated isothermally in the temperature range 1050-1250 K, with stoichiometric ratios ranging between 1.0 and 1.3. It has been found that, close to the higher temperature values investigated, the NO conversion as a function of the stoichiometric ratio shows a maximum around 1.15-1.20, both in the case of pure methane and methane-ethane mixtures in the feed. Moreover, the addition of NO significantly enhances the system reactivity at the lower temperatures investigated. The ethane content in the feed plays a different role depending upon the temperature value considered. At the lowest temperatures investigated the larger the amount of ethane, the higher the NO abatement, while at the higher temperatures the methane-ethane mixtures always show a larger NO conversion than that of pure methane. However, when increasing the ethane content in the feed, the NO conversion decreases. Finally, various detailed kinetic models (with particular reference to that developed by Miller and Bowman [Prog. Energy Combust. Sci. 1989, 15, 287]) have been discussed and used to interpret the experimental results. 1. Introduction It is known that the oxidation of light hydrocarbons and the reduction of nitrogen oxide when run simultaneously strongly interact with each other, leading to several unexpected behaviors. For instance, the addition of NO significantly decreases the temperature at which both propane and n-butane oxidation starts, while on the other hand, the presence of hydrocarbons strongly promotes the oxidation of NO to NO2.2-4 Several processes of industrial relevance also involve reactions among NO and hydrocarbons. In particular, the emission of nitrogen oxides from power generation plants has a relevant impact on atmospheric pollution. NO emissions can be decreased through a selective noncatalytic reduction (thermal DeNOx process) by adding ammonia to the hot combustion gases in a suitable temperature range centered at about 1250 K.5 Such a temperature range, where the thermal DeNOx process is effective, can be modified through the addition of light hydrocarbons.6,7 This is due to the interactions between the radical intermediates involved both in NO reduction and hydrocarbon oxidation, whose understanding thus provides the basis for the optimization of the hydrocarbon-promoted thermal DeNOx process. Interactions between hydrocarbon-derived radicals (mainly CH, CH2, and CH3) and NO are also involved in the reburning (or fuel-staging) process.8 This is a staged combustion where about 10-20% of the fuel is added after the main combustion chamber. While lean fuel conditions are maintained in the primary combustion chamber, in the reburning stage fuel-rich conditions * To whom correspondence should be addressed. Tel.: +39 0223993154. Fax: +39 0223993180. E-mail: renato.rota@ polimi.it. † Currently at ETHZ, Labor fur Technische Chemie, Universitastrasse 6, 8092 Zurich, Switzerland.

are attained to promote the reduction of NO to N2 and other nitrogen species. Typical reburning processes are generally carried out using natural gas in the temperature range of 1400-1700 K. However, when more reactive fuels are used, such as methane-ethane mixtures, then the process can be operated at lower temperature values since reactive hydrocarbon fragments are available also at lower temperatures. Due to the hierarchical structure of the oxidation mechanism of hydrocarbons, the most important species are the C1- and C2-derived radicals.9,10 This, in addition to the intrinsic interest of methane and C2 as fuels provides the motivation for elucidating the main reaction patterns involved in NO-light hydrocarbon flames. However, the experimental evidence currently available in the literature is not sufficient to reaching this aim. Usually they refer to the oxidation of single hydrocarbons carried out using flat burners or plug-flow reactors in the high-temperature range.1 Although the combustion of a single hydrocarbon (say, methane) also involves several other hydrocarbons, their concentrations are usually too small to allow for a safe estimation of the interconnecting reaction paths. For this, experiments directly involving hydrocarbon mixtures and NO are clearly best-suited. In this work we have investigated experimentally the interaction between NO and several fuel mixtures, involving methane and ethane, using an isothermal perfectly stirred reactor (PSR), operating in the temperature range 1050-1250 K, with stoichiometric ratios ranging between 1.0 and 1.3. Since the adopted NO stream also contains NO2 as an impurity (about 1-3%), this experimental analysis also yields some qualitative information about the reduction of this species. The main aim of this work is to produce experimental data that are able to evidence the mutual influence of NO and hydrocarbons on their reactivity and to provide further insight into the understanding of the different

10.1021/ie970474f CCC: $15.00 © 1998 American Chemical Society Published on Web 09/25/1998

4242 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998

paths involved in these complex oxidation reactions. For this, conditions involving various amounts of oxygen as well as various methane-ethane ratios in the feed have been investigated, and detailed kinetic modeling (with particular reference to the mechanism developed by Miller and Bowman1) has been used to discuss the experimental results. 2. Materials and Methods Experimental Apparatus. The experimental apparatus used in this work has been described earlier in detail.11,12 In the following we review only the most relevant aspects as well as some modifications needed for this specific application. All the experiments have been performed in a jetstirred reactor operated at atmospheric pressure up to 1300 K. It consists of a sphere made from silica of about 40 mm in diameter. This kind of reactor has several advantages compared with both the usual plug-flow reactors (PFR) or the flat-flame burners. In particular, it gives direct measurements of the rate of formation or depletion of the involved chemical species, which are not affected by fluid dynamics provided that the characteristic time for mixing is smaller than that for chemical reactions. Moreover, a smaller surface-tovolume ratio with respect to the PFR allows minimization of the influence of surface reactions. This is probably not really important for this study, but it could become significant in other situations, like those prevailing in the thermal DeNOx process.13 The PSR is located inside an oven, designed to maintain the reaction temperature at the desired value. The flow rates of the reactants (fuel, air, nitrogen, carbon dioxide, and nitrogen oxide) are measured and regulated through mass-flow controllers to attain the required feed composition. Nitrogen, carbon dioxide, and nitrogen oxide were fed through a preheating section and heated at the desired reaction temperature before entering the reactor. The fuel, properly diluted with cold nitrogen, was fed through a capillary tube to reduce its residence time in the hot zone outside the reactor, thus minimizing pyrolysis reactions. All reactants are mixed at the entrance of the injectors where the residence time is much smaller than that inside the reactor. Exhausts leave the reactor through four holes in the upper part of the sphere. The temperature is measured by a chromel-alumel thermocouple located inside the reactor. Sampling is realized using a sonic quartz probe jacketed with cooling water, which is able to freeze the composition of the sample.14 After recompression, the sample is fed directly either to a single column (Carboxen-1000) gas chromatographic apparatus equipped with a TCD and FID connected in series or to an instrument (Madur GA60) equipped with electrochemical sensors. The former measures the concentration of O2, CO, CO2, CH4, and C2H6, while the latter measures that of NO and NO2. For the gas chromatographic measurements an error upper bound of about (5% has been estimated. The reliability of the electrochemical sensor for NO measurements is similar to that of the standard chemiluminescence method as long as the NO concentration values are not too low. For concentration values of 250 ppmv, a maximum deviation of 6% has been found between the two methods.15 However, at lower values higher errors have been evidenced. Since the concen-

tration range of interest in this work is about 400-1100 ppmv, measurements based on electrochemical sensors can be considered sufficiently reliable. No information is available in the literature on the reliability of the electrochemical sensors for NO2. However, it can be reasonably assumed to be similar to that of NO. In all cases the reliability of this technique has been verified using standard gas mixtures of known composition. Finally, it is important to stress that the main performance required for the reactor is to provide a good mixing, whose characteristic time must be less than that of the reactions involved. The attainment of good mixing conditions has been verified experimentally in conditions similar to those considered here,16,17 and a validation of the experimental procedure concerning hydrocarbon oxidation has been performed18 by comparing some results previously presented in the literature with those produced in our laboratory in similar conditions. Mathematical Modeling. The behavior of the isothermal perfectly stirred reactor has been modeled using the computer code PSR version 1.8,19 based on the CHEMKIN subroutine library,20 which implements a suitable algorithm for solving the system of NS (which is the number of the chemical species) nonlinear algebraic equations representing the mass balance of the various involved chemical species:

QoutCout - QinCin i i - riV ) 0

(1)

where Q is the volumetric flow rate, Ci the concentration of the ith species and ri its rate of production, and V the reactor volume; the subscripts in and out refer to the inlet and outlet stream, respectively. We used the detailed kinetic model proposed by Miller and Bowman1 (in the following indicated as the MB mechanism) to describe the chemistry of NO-hydrocarbon systems. It involves 235 reversible reactions and 51 species, and it has been calibrated using data relative to mainly methane and acetylene flames, HCN-doped hydrogen flames, ammonia flames, thermal DeNOx, RapreNOx, and reburning processes, as well as nitrogen oxide formation in well-stirred reactors. It is worth noting that most of these data were collected in the hightemperature range, that is, above about 1200 K. In addition to the MB mechanism, other two kinetic schemes have been considered. These include the GriMech2 mechanism21 (referred to in the following as the GRI2 scheme), involving 279 reversible reactions and 49 species, which was developed to reproduce reburning conditions, and a kinetic scheme developed starting from that proposed by Tan et al.22,23 (referred to in the following as the TDCB mechanism) involving 506 reactions (almost all reversible) and 79 species, originally formulated to reproduce the combustion of pure hydrocarbons (and their mixtures) up to C3 in the intermediate- and high-temperature range. The forward reaction rate constants have been computed through the modified Arrhenius expression:

ki ) AiTβi exp(-Ei/RT)

(2)

where ki is the reaction rate constant of the ith reaction, while Ai, βi, and Ei are its kinetic parameters, R the ideal gas constant, and T the temperature. The backward reaction rate constants have been computed from the forward ones using the equilibrium constant values

Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4243 Table 1. Synoptic of the Experimental Runs Carried outa reactants run

τ, s

Φ

MER

CH4

C2H6

NO

I II III IV V VI VII VIII IX X XI XII XIII XIV XV

550/T 550/T 550/T 550/T 550/T 550/T 110/T 110/T 110/T 110/T 110/T 110/T 110/T 110/T 110/T

1.00 1.11 1.13 1.19 1.21 1.26 1.00 1.06 1.15 1.22 1.27 1.14 1.16 1.17 1.16

∞ ∞ ∞ ∞ ∞ ∞ 3 3 3 3 3 1 0.3 0 1

6460 7005 7470 7875 7995 8520 2105 2225 2370 2485 2605 1375 585 0 1330

0 0 0 0 0 0 720 735 810 845 885 1395 1800 2200 1330

1100 1075 1145 1035 1070 1080 760 770 780 770 770 790 770 760 0

a T is the reaction temperature value (K) used in residence time (τ) computation, which varies with temperature (e.g., 550/T means a residence time at 1000 K equal to 550/1000 ) 0.55 s). MER is the methane-to-ethane ratio in the feed that contains also CO2 (about 8.5% for runs I-VI, 6% for runs VII-XIV, and about 100 ppmv for run XV), NO2 as an impurity (about 1-3% of the NO amount), O2 as determined from the Φ ) O2,st/O2 values, and N2 to 100%. Reactant concentrations in ppmv.

obtained from the thermodynamic database CHEMKIN24 for the MB mechanism, while the GRI2 and the TDCB schemes have been used together with their own thermodynamic databases. Figure 1. Experimental results of methane conversion: (A) pure methane feed (runs I-VI in Table 1); (B) methane-ethane feed (runs VII-XI in Table 1).

3. Results and Discussion All the experimental runs carried out are summarized in Table 1. They can be grouped to investigate various aspects of the interactions between NO reduction and light hydrocarbon mixtures oxidation. For instance, runs I-VI in Table 1 elucidate the effect of temperature and stoichiometric ratio on the interactions between methane-derived radicals and NO. The same experiments have been repeated (runs VII-XI in Table 1) to investigate the effect of the presence of a small amount of ethane on the aforementioned interactions. Due to the larger reactivity evidenced, these runs have been performed at lower residence time and temperature values than the previous ones. Then, the influence of the ethane content in the feed, from pure methane to pure ethane, can be discussed for a single value of the stoichiometric ratio through the results of the runs III, IX, and XII-XIV. Finally, the influence of the presence of NO on the oxidation of an equimolar methaneethane mixture can be discussed by comparing the results of runs XII and XV. Methane-NO Mixtures. Let us first discuss the results obtained using methane alone with NO (runs I-VI in Table 1). Figure 1 (part A) shows the conversion values of CH4 as a function of temperature for various stoichiometric ratios, Φ ) (O2,st/O2). Conversion of the generic ith reactant is defined as

ηi )

out xin i - xi

xin i

× 100

(3)

out where xin are the mole fractions of the ith i and xi reactant in the feed and in the outstream, respectively. This allows us to compare the results of different

runs in a systematic way, regardless of the inlet composition. We can see that, as expected, methane conversion increases with temperature. On the other hand, the stoichiometric ratio seems not to affect conversion at T ≈ 850 nor 940 °C, but only at T ≈ 1025 °C. Note that at this temperature the conversion does not decrease with the stoichiometric ratio, but it shows a maximum at about Φ ≈ 1.20. A similar behavior is exhibited by the NO conversion values shown in Figure 2, part A. It is seen that NO is almost unreacted at T ≈ 850 °C (conversion values lower than 10%), and the stoichiometric ratio affects NO conversion only at the largest temperature values i.e., T ≈ 1025 °C. This is better illustrated in Figure 3, where the conversion values of NO at T ≈ 1025 °C are shown as a function of Φ (full symbols), exhibiting a maximum at about Φ ≈ 1.20. It is noteworthy that a similar behavior can be deduced from the experimental data reported by Bilbao et al.25 that examined the influence of oxygen concentration on the effectiveness of the reburning process in the temperature range 1200-1500 °C using a plug-flow reactor. Their results indicate that for the lowest temperatures investigated (1200 and 1300 °C) the conversion of nitrogen oxide as a function of the oxygen content (that is, of the Φ value) shows a maximum. This disappears at the higher temperatures, where NO conversion steadily increases (or, at least, remains constant) as the stoichiometric ratio increases. Thus, this behavior seems to be characteristic, for the pure methane feed, of the temperature range between 1000 and 1300 °C. Figures 1 and 2 (parts A) indicate a correlation between methane and NO conversion. This


Figure 2. Experimental results of nitrogen oxide conversion: (A) pure methane feed (runs I-VI in Table 1); (B) methane-ethane feed (runs VII-XI in Table 1).

Figure 3. Nitrogen oxide conversion as a function of the stoichiometric ratio, Φ: (b) pure methane feed (runs I-VI in Table 1), temperature between 1015 and 1030 °C (see Figure 2, part A); (O) methane-ethane feed (runs VII-XI in Table 1), temperature between 940 and 970 °C (see Figure 2, part B).

was expected since the same radicals play a role in the reaction patterns of both these compounds. In the examined conditions, methane reacts mainly through the following reactions:

CH4 + X f CH3 + XH where X ) H, OH, O, while NO disappearance proceeds most likely through reactions of the general type

NO + H (+M) f HNO (+M) CHi + NO f products where i ) 0-3, although the reactions involving C, CH,

Figure 4. Nitrogen oxide versus methane conversion: (A) pure methane feed (runs I-VI in Table 1); (B) methane-ethane feed (runs VII-XI in Table 1).

and CH2 are probably negligible since the concentrations of these species are likely to be rather small at these temperatures. The correlation between NO and methane conversion is well-illustrated in Figure 4 (part A), where all the experimental data of runs I-VI are reported. An almost linear relation between NO and methane conversion is evident. It is worth noting that a significant amount of methane (about 60%) reacts before the NO conversion becomes significant (larger than 10%). This can be explained not only by the higher temperature required to activate the reburning reactions but also by the interaction between methane and NO, as discussed in the next sections. As mentioned earlier, since NO2 is present in the feed NO stream as an impurity, it has been possible to evaluate NO2 conversion, thus obtaining the values shown in Figure 5. Although, as discussed in the Materials and Methods section, these data are affected by measurement uncertainties due to their low values (less than 20 ppmv), a well-defined qualitative trend is evident; that is, NO2 conversion increases with temperature. This is due to the reducing conditions investigated (excess of fuel) that allow for the NO2 reduction to proceed mainly through the following reaction:

NO2 + H f NO + OH Methane-Ethane-NO Mixtures. An environment enriched by hydrocarbon radicals, which is convenient for investigating the interactions between hydrocarbons and NO, can be achieved even at relatively low temperature values by adding to the methane fuel about 25% of the more reactive ethane. This mainly under-

Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4245

Figure 5. Experimental results of nitrogen dioxide conversion for pure methane feed (runs I-VI in Table 1).

goes the following reactions:

C2H6 + X f C2H5 + XH where X ) H, OH, and O, while C2H5 in turn goes partially to CH3: H

C2H5 98 2CH3 O

C2H5 f C2H4 98 CH3 + HCO and it implies also a larger concentration of radicals that contribute to the enhancement of the methane reactivity.18,22 This clearly appears from the results summarized in part B of Figure 1, referring to methane conversion. It can be seen that although the residence time was decreased by a factor of 5, methane conversion is always higher than that measured at the same temperature in the case of methane alone (see the same figure, part A). Almost complete conversion was in fact reached at about 950 °C for all the values of the stoichiometric ratio investigated. Moreover, except for the lowest temperatures (i.e., about 800 °C) where conversion values equal to about 75% were found, ethane was substantially completely consumed in all experimental runs. The same conversion increase at all temperature values was also observed for NO, as shown in Figure 2, part B. It is interesting to note that also in this case a significant scatter was evidenced at T ≈ 950 °C, where the NO conversion does not decrease with the stoichiometric ratio, but rather presents a maximum around Φ ≈ 1.15 as shown in Figure 3 (empty symbols). The experimental data referring to NO2 conversion lead to similar conclusions; that is, the main trends evidenced in the methane experiments were found also in the methane-ethane experiments but with all conversion values shifted toward lower temperatures. Figure 4, part B, illustrates the correlation between NO and methane conversion using all the data collected in the experimental runs VII-XI. Similar to the case where methane alone is considered, this relation is almost linear for methane conversion up to 100%. However, the presence of ethane seems to induce a larger scattering in the experimental results. The results discussed above suggest that NO exhibits qualitatively similar interactions with different hydrocarbons since these all eventually produce similar reactive fragments. This observation could be extended to larger temperature values not covered in this work

Figure 6. Experimental results (O) and model predictions for run IX of Table 1, (s) MB kinetic mechanism, and (- - -) GRI2 kinetic mechanism.

so as to extend the results of the investigations carried out using methane as the reburning fuel25 to other light hydrocarbons. With illustrative purpose, a typical comparison of experimental results (run IX in Table 1) with the predictions of two detailed kinetic mechanisms, MB and GRI2, is shown in Figure 6. Comparisons with other experimental data (runs I-XI in Table 1) show a similar behavior. In particular, the MB mechanism is always more reactive (about 80 K) than the GRI2 one. Apart from this slightly different reactivity, the behavior of the two models is quite similar. They both exhibit a reasonable agreement with experimental data, in particular in the higher temperature range. This is not surprising since both were validated for high-temperature values. Another behavior common to both models is the very sharp transition, a sort of on-off behavior,


across the ignition temperature, where the conversion values suddenly change from about zero to a definitely larger value. This leads to the underestimation of the experimental conversions in the low-temperature range since the experimental findings show a smoother transition from nonreactive to reactive conditions. A rather interesting feature, exhibited again by both models, is the hysteresis in the conversion-temperature plane shown in Figure 6. In other words, in this region two different steady-state solutions can be computed. The two branches of the hysteresis cycle have been computed numerically by starting from low (lower conversion branch) or high (higher conversion branch) temperatures, and then slowly increasing or decreasing the temperature while computing at each step the steadystate solution (using as a first trial solution the solution obtained at the previous step). Unfortunately, the presence of such a hysteresis cycle is not clearly identified experimentally. However, this does not allow us to draw final conclusions since the experiments were not designed to analyze such phenomena. Despite complex phenomena (such as hysteresis) arising in a PSR that have been identified for the oxidation of other hydrocarbons in different operating conditions,26 in our knowledge they have not been previously reported for the conditions considered here. Moreover, hysteresis cycles similar to that reported in Figure 6 have been found also in the absence of NO in the feed. Then, they should probably be mainly ascribed to the C/H/O chemistry involved in the kinetic model. It is worth noting that the above hysteresis was found using both MB and GRI2 kinetic mechanisms for all the experimental conditions considered up to now. However, no hysteresis cycles were evidenced for feed streams containing more than 25% of ethane as fuel (runs XII-XIV in Table 1), at least in the investigated temperature range (1000-1400 K). This is probably due to the increasing reactivity of mixtures containing a larger fraction of ethane, which shifts the ignition conditions (where the hysteresis cycle can be evidenced) toward lower temperature values. The presence of complex kinetic behaviors, such as those typically corresponding to hysteresis cycles, indicates that several reaction paths become simultaneously important in determining the process behavior. Therefore, these are usually the most interesting conditions for analyzing the relative importance of the different reaction paths in a complex kinetic mechanism and probably the most severe conditions for their validation. A rate of production analysis carried out using the MB mechanism in a region where the hysteresis cycle was identified (that is, run IX in Table 1 at T ) 1150 K) evidenced that methane is converted to CH3 radicals which in turn can follow the dimerization path toward C2H6 or the oxidation pattern toward CO and CO2 through the following channels:

{

fCH O CH3 fCH3 2

}

f CH2O f HCO f CO f CO2

Ethane formed by CH3 radicals is converted to C2H5 and then to ethylene, which in turn goes toward CO2 through the channels

C2H4

{

}

fC2H3 f C2H2 f HCCO f CO f CO2 fHCO

These reaction patterns are common to both branches

Figure 7. NO conversion values versus temperature for different ethane content in the feed and Φ ≈ 1.15. Note the different residence time for the pure methane feed (full squares; τ ) 550/T s) and the other runs (τ ) 110/T s).

of the hysteresis cycle, with an obvious higher specific rate for the higher conversion branch. However, in the higher conversion branch vinyl radicals also react with O2, leading to HCO and CH2O that produce CO2 through the path

CH2O f HCO f CO f CO2 This clearly enhances the overall reactivity of the system and is probably responsible for the existence of the high conversion branch. In this context, it is worth mentioning that the MB model considers only the reaction of vinyl radicals with oxygen leading to HCO and CH2O. However, recent studies27 demonstrated that the channel leading to C2H3O+O from the vinyl radicals is very important in the low-temperature oxidation of ethylene and probably contributes also in the conditions investigated here. Effect of the Methane-Ethane Content in the Feed. In Figure 7 are reported, as a function of temperature, the NO conversion values measured in various experiments conducted with different methaneethane concentration ratios in the feed stream, but equal values of the stoichiometric ratio and of the NO inlet flow rate (runs IX and XII-XIV in Table 1). As a comparison, the data obtained using pure methane, and with a 5 times larger residence time in the reactor, are also shown. From the obtained results it appears that the percentage of ethane in the feed mixture plays a different role, depending upon the temperature value. Namely, at the lowest temperatures investigated, the NO abatement increases continuously with the ethaneto-methane ratio in the feed. In particular, the NO conversion increases from zero, corresponding to the pure methane experiments, to about 25% in the case where ethane is the only hydrocarbon in the feed. On the other hand, at the higher temperature values ethane has an opposite effect on NO conversion which in fact increases for a decreasing ethane-to-methane ratio. An exception is the case where no ethane is present in the feed, where NO conversion remains at the lowest value. A qualitative explanation of such experimental behavior can be obtained by considering the concentration level of the CHi radicals, which are primary responsible, in these experiments, for the NO abatement. On one side, ethane is less stable than methane and therefore enhances the production of radicals. On the other side, ethane oxidation proceeds mainly via ethylene and vinyl radicals without involving a large amount of CHi radi-


Figure 8. Experimental results of methane conversion for runs IX, XII, and XIII in Table 1. MER is the methane-to-ethane ratio in the feed.

cals. This means that increasing the ethane content in the feed (and thus reducing correspondingly the amount of methane) results in a decrease of the amount of CHi radicals available for NO reduction. Now, at the lowest temperature values used in the experiments, the first effect prevails. In these weak conditions pure methane does not react while it does in the presence of ethane since this decomposes and produces radicals which then ignite both reactants, thus leading to a radical-enriched environment which also favors NO conversion. The same mechanism operates also at higher temperatures. However, in this case the role of ethane in producing the first radicals by decomposition is now less important and the second effect discussed above prevails. Thus, for increasing values of the ethane-tomethane ratio, the fraction of CHi radicals in the system decreases, and then also the NO conversion. However, if no ethane is present, the ignition of all oxidation reactions becomes difficult, and then the smallest NO conversion values are obtained. Following these qualitative considerations, it is expected that, for temperature values larger than those considered in this work, where the oxidation process can be initiated directly by methane decomposition, a small amount of ethane would not significantly influence the NO reduction rate. This is coherent with the results reported by Kilpinen et al.28 based on a model analysis of a reburning unit operating at temperature values larger than about 1300 K. In a previous study of the combustion of methaneethane mixtures without NO added18 it was found that the presence of ethane increases methane reactivity up to a given concentration value, beyond which higher ethane concentrations do not significantly affect methane conversion. This conclusion has been confirmed in this work by a set of independent experiments in the case where also NO is present in the feed. In particular, in Figure 8 the methane conversion values for the experimental runs IX, XII, and XIII (see Table 1) are shown as a function of temperature. It can be seen that methane conversion is lowest in the case where 25% ethane is present in the feed, while it remains substantially unchanged for feeds with higher ethane content. Effect of NO on the Reactivity of Hydrocarbons. The effect of NO on methane and ethane reactivity can be best investigated by comparing experimental runs with and without NO in the feed. This is done in Figures 9 and 10 where the experimental results of run XII are compared with those of run XV in terms of

Figure 9. Influence of NO on methane conversion. Experimental data (b), MB model predictions (s), and TM model predictions (- - -). (A) Run XII in Table 1 (with NO in the feed). (B) Run XV in Table 1 (without NO in the feed).

Figure 10. Influence of NO on ethane conversion. Legend as in Figure 9.

methane and ethane conversion. It should be noted that in these last experiments no CO2 was added to the feed.


NO + H (+M) f HNO (+M) HNO + OH f NO + H2O H + OH f H2O

Figure 11. Experimental results and MB model predictions of NO conversion for run XII of Table 1.

This, however, since carbon dioxide is almost inert in these conditions, should not significantly affect the system behavior. From Figures 9 and 10 it is seen that the addition of NO significantly decreases (by approximately 100 °C) the temperature at which both methane and ethane oxidation are completed. A similar behavior was evidenced by Nelson and Haynes3 for the case of propane oxidation with large oxygen excess. The same figures also show the predictions of the MB mechanism. In a previous work11 it was found that this mechanism fails to represent the experimental oxidation kinetics of hydrocarbon mixtures in the intermediate temperature range. From Figures 9 and 10 we see that the agreement between experiments and model predictions is better in the case where NO is present in the feed stream, probably because the MB mechanism was mainly tuned on the flame of hydrocarbons doped with nitrogen compounds. However, it is also evident that the comparison between model predictions and experimental results is still unsatisfactory. The most serious disagreement between experimental results and model predictions refers to the qualitative trend evidenced at the lowest temperature: experiments indicate that the addition of NO increases the hydrocarbon reactivity, while the MB mechanism predicts the opposite behavior. This is probably due to the scavenging effect of NO predicted by the model, which is able to reduce significantly the concentration of the active radicals H and OH. To better understand this point, let us analyze more in detail the behavior of the involved elementary reactions as predicted by the MB mechanism. It is first convenient to compare the NO conversion values predicted by the model with the experimental ones, as shown in Figure 11. It appears that the agreement is generally quite reasonable, although the MB model always underestimates the NO abatement rate, and actually predicts rather low conversion for NO at the lowest temperatures. However, this does not mean that NO is inert, and through a proper rate of production analysis it has been in fact evidenced that NO is involved in two closed sequences, namely

NO + HO2 f NO2 + OH NO2 + H f NO + OH H + HO2 f 2OH and

In both the sequences NO is first depleted and then produced so that each of them can repeat itself through many cycles without affecting the overall production rate of NO. However, the net result of the first cycle is that a less reactive radical (HO2) is transformed into a more reactive one (OH), thus enhancing the reactivity of the system. On the other hand, the second cycle strongly depresses such a reactivity since two radicals terminate in a stable molecule of water. Actually, the two reactions in each cycle are not perfectly balanced. In particular, in the first cycle the second reaction evolves at a slightly larger rate than the first one, thus leading to a depletion of NO2 in the system. In the second cycle the first reaction is faster than the second one, allowing for HNO accumulation in the system. With respect to NO, these two effects tend to balance each other, thus leading to the apparently nonreactive behavior for NO mentioned above. On the other hand, since the second sequence above is much faster than the first one, the presence of NO implies a significant decrease of the system reactivity. When compared with the experimental findings, the model representation of the elementary reactions involved in the two cycles above appears to lead both to the underestimation of the NO abatement rate and to an erroneous prediction of a decrease of the system reactivity in the presence of NO. It is now interesting to understand whether these problems of the MB mechanism arise from a poor description of the C2 mechanism (in the intermediate temperatures and hydrocarbon-rich mixtures considered in this work) or they are indeed associated with the nitrogen chemistry. For this we have carried out a few simulations using a kinetic mechanism obtained by merging the TDCB mechanism, which is able to reproduce well the C1-C2 chemistry in conditions close to those investigated here,22 with the subset of reactions of the MB mechanism which describes the nitrogen chemistry (in the following referred to as the TM model). Note that this is an arbitrary procedure since the MB kinetic scheme is a highly balanced mechanism between hydrocarbon and nitrogen chemistry. However, this can give an idea on the relative importance of the C1-C2 mechanism versus the nitrogen one to explain the discrepancies between experimental data and model predictions. The predictions of the TM model for methane and ethane conversion are compared with the experimental data in Figures 9 and 10. As expected, the TM model represents correctly (and much better than the MB model) the oxidation of hydrocarbon mixtures without NO in the range of temperatures considered. However, the effect of NO addition is again wrong even in qualitative terms. It is in fact evident in Figures 9 and 10 that while the experimental values of methane and ethane conversion increase (at the lower temperatures investigated) in the presence of NO, the values predicted by the TM model exhibit a rather small change, which actually goes in the wrong direction, although to a much smaller extent than those for the MB model (see the same figures). By repeating the rate of production analysis along the same lines discussed above for the


MB model, the same elementary reactions are found to be responsible for the wrong predictions of this mechanism. However, this predicts lower radical concentrations which are affected less by the presence of NO than those predicted by the MB mechanism. Other mechanisms for the nitrogen chemistry7 showed similar behaviors when merged with the TDCB model. These observations lead to the conclusion that the incomplete description of the C1-C2 chemistry is not solely responsible for the qualitative disagreement evidenced between model predictions and experimental findings, and that modifications and further reconsideration of models for nitrogen chemistry are required (at least at the intermediate temperature values examined here). This agrees with the conclusions of Nelson and Haynes3 drawn from experimental evidence collected under completely different conditions. Further Comparisons between Experimental Trends and Model Predictions. Through the experimental analysis described above a few distinct trends have been identified. The one relative to the effect of NO on hydrocarbon reactivity has already been discussed in connection with appropriate kinetic models. This allowed us to conclude that at low temperature values the kinetic models currently available for nitrogen chemistry need to be reconsidered and improved. In the following we intend to extend the comparison of some of these experimental trends with appropriate kinetic models, particularly in the upper part of the temperature interval investigated. In the experimental part it was shown that, both for pure methane and methane-ethane mixtures (with the methane-to-ethane ratio, in the following referred to as MER, equal to 3), the NO conversion exhibits a maximum value as a function of the stoichiometric ratio. In Figures 12 and 13 the predictions of the MB and TM models are compared with the experimental data reported earlier in Figure 3. Note that the model results are shown for different temperature values. This facilitates the interpretation of the results reported in the following. Let us consider first the pure methane data. From Figure 12 it is seen that the qualitative behavior of these models is quite similar, thus confirming that the nitrogen chemistry is more important that the hydrocarbon one in determining the model predictions. Moreover, both models represent reasonably well the experimental behavior in the first part of the stoichiometric range investigated, that is, up to the occurrence of the NO conversion maximum. However, they both fail to reproduce the following descending portion of the experimental curve, at least for temperature values close to the experimental ones (1288-1303 K). It is in fact necessary to reduce the temperature of about 100 K to find a slight maximum in the model predictions, more evident for the TM model. To understand this behavior, we applied the rate of production analysis for Φ ) 1.3, T ) 1200-1300 K, and a pure methane feed. It has been found that the MB mechanism involves the production and consumption of NO through the following closed sequences:

NO2 + H f NO + OH NO + HO2 f NO2 + OH and

Figure 12. Nitrogen oxide conversion as a function of the stoichiometric ratio, Φ, for the case of the pure methane feed (runs I-VI in Table 1): (b) represents the experimental data measured at temperatures between 1015 and 1030 °C (see Figure 2, part A). (A) Curves are the MB model predictions at different temperature values. (B) Curves are the TM model predictions at different temperature values.

NO + H (+M) f HNO (+M)

{

HNO + H f NO + H2 HNO + OH f NO + H2O

These sequences are almost completely balanced both at 1200 and 1300 K, although the rate of each individual reaction changes significantly. On the other hand, the closed sequence

NO + HCCO f HCNO + CO HCNO + H f NO + CH2 is highly unbalanced, the rate of the second reaction being much larger than that of the first one, thus leading to a net production of NO. Moreover, for decreasing temperature values the HCNO + H reaction is unfavored, and the NO conversion increases. Similar conclusions arise from the analysis of the TM model results, where the aforementioned mechanisms are more pronounced, leading to even larger differences in the NO conversion at different temperature values. The same comparison is shown in Figure 13 for the case where a mixture of methane and ethane is fed into the reactor. It can be seen that the qualitative behaviors discussed above for the case of pure methane remain unchanged. However, the larger complexity of the reacting system due to the presence of ethane leads to an even poorer agreement between model predictions


Figure 13. Nitrogen oxide conversion as a function of the stoichiometric ratio, Φ, for the case of the methane-ethane mixture feed (runs VII-XI in Table 1): (b) represents the experimental data measured at temperatures between 940 and 970 °C (see Figure 2, part B). (A) Curves are the MB model predictions at different temperature values. (B) Curves are the TM model predictions at different temperature values.

Figure 14. Nitrogen oxide conversion as a function of the content of ethane in the feed stream (runs III, IX, and XII-XIV in Table 1). (b) represents the experimental data measured at temperatures between 940 and 970 °C (see Figure 7). Curves are the model predictions at different temperature values. (A) MB detailed kinetic mechanism. (B) TM detailed kinetic mechanism.

4. Conclusions and experimental data. This effect is of course in addition to the low temperature considered, which lies outside the reliability range of the MB model. Finally, let us consider the NO conversion values shown in Figure 14 as a function of the ethane content in the feed at T ) 1220 K. It appears that in the case where the methane-ethane mixture is fed to the reactor the NO conversion values predicted by the MB and TM models slightly increase with the content of ethane, while the experimental values slightly decrease. However, the more evident discrepancy between model and experimental results refers to the effect of the ethane addition to a pure methane feed. While it is found experimentally that the addition of ethane leads to a sharp increase of the NO abatement rate, both models predict the opposite effect. This is mainly due to the larger rate attained by some reactions in the case of the pure methane feed with respect to the case where a methane-ethane mixture is fed to the reactor, since almost all the reactions involved in NO consumption and production do not change when feeding a methaneethane mixture instead of pure methane. However, in the case of pure methane, the rates of both the production and consumption reactions are about 2 times larger than their values in the case of the methane-ethane mixture, thus leading to a higher abatement of NO. This suggests that the MB model is not able to account for slowly reacting systems, such as the one where pure methane is fed to the reactor, which is coherent with the conclusion of previous discussions.

In this work several experimental data have been reported in operating conditions not previously investigated in the literature, particularly with respect to the reactions between NO and mixtures of fuels. This allows a better estimation of the interconnecting reaction paths among C1-C2 hydrocarbons and NO, which exhibit a rather different relative importance at different reactant concentrations. Several interesting trends have been evidenced experimentally, which provide a rather severe test for assessing the reliability of detailed kinetic mechanisms. A few of these trends were reported earlier in the literature for experimental conditions completely different from those considered in this work. This supports the reliability of the experimental results presented and provides insights for the improvement of detailed kinetic models. However, since the detailed kinetic mechanisms here considered were developed for temperature values higher than those investigated in this work, the disagreement found between experimental and model results is not entirely surprising. This also means that the need for better NO kinetics is mainly in the low-intermediate range of temperature. Moreover, it should be stressed that we were not interested in examining all the models available in the literature since the aim of this work was to investigate experimentally the mutual influence of NO and hydrocarbons on their reactivity, while detailed modeling was used only to support the discussion of the experimental results. Summarizing, the main findings of this work are as follows:


(1) Close to the higher temperatures investigated, the conversion of NO as a function of the stoichiometric ratio shows a maximum around Φ ≈ 1.15-1.20 for both the pure methane and the methane-ethane feeds considered. Both the MB and the TM kinetic models represent reasonably well this trend up to the maximum. However, neither of them is able to reproduce the descending trend at the temperature values used in the experiments. Only at temperature values lower by about 100 K as the model predictions exhibit a similar behavior. Moreover, the qualitative behavior of the two models is quite similar, thus confirming that in the conditions investigated the nitrogen chemistry is more important than the hydrocarbon one in determining the model predictions. (2) In both cases where pure methane and methaneethane mixtures are fed to the reactor there exists an almost linear correlation (for methane conversion up to 100%) between methane and NO conversion. This suggests that also at low temperature values the same radicals play a fundamental role in the reaction mechanism of both these compounds. However, the poorest correlation found in the case of methane-ethane mixtures suggests also that other reaction patterns play a role in these conditions. (3) The addition of NO significantly decreases (by approximately 100 °C) the temperature at which both methane and ethane oxidation are completed. At the lower temperatures investigated the presence of NO significantly enhances the system reactivity. On the contrary, the MB mechanism predicts a significant decrease in the hydrocarbon conversion due to the presence of NO. This is mainly due to the scavenging effect of NO that, despite its apparent low reactivity, is able to reduce significantly the concentration of H and OH through a closed sequence of elementary reactions. The poor description of both C1 and C2 combustion and nitrogen chemistry are responsible for this qualitative disagreement. While more detailed descriptions of C1C2 chemistry are available, modifications and further reconsideration of kinetic models for nitrogen chemistry at intermediate temperatures are needed. (4) The amount of ethane in the feed plays a different role at different temperatures. At the lowest temperature values investigated, the larger the amount of ethane, the higher the NO abatement. At the higher temperatures, ethane-containing feeds always show a larger NO conversion than that in the case of pure methane. However, increasing the ethane content decreases the NO conversion. (5) The presence of ethane increases the methane reactivity up to a given concentration value. Above this value, higher ethane concentrations do not affect significantly methane conversion. This qualitative behavior is similar to that evidenced earlier for the oxidation of methane-ethane mixtures without NO. Acknowledgment The contribution to this work by F. Bonini, A. Correngia, and C. Pagano, as well as the financial support by ENEL-Polo Termico, Pisa (Italy), is gratefully acknowledged. We also wish to acknowledge Dr. P. Dagaut for making available the 1996 version of the TDCB detailed kinetic mechanism. Nomenclature A ) preexponential factor C ) concentration

E ) activation energy k ) reaction rate constant MER ) methane-to-ethane ratio in the feed NS ) number of species Q ) volumetric flow rate r ) production rate R ) ideal gas constant T ) temperature V ) reactor volume x ) mole fraction Greek Letters β ) temperature exponent η ) conversion τ ) residence time Φ ) stoichiometric ratio (O2,st/O2) Superscripts in ) in flow out ) out flow Subscripts i ) ith species st ) stoichiometric

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Received for review July 8, 1997 Revised manuscript received July 22, 1998 Accepted July 28, 1998 IE970474F

Combustion Kinetics of Light Hydrocarbons in the Presence of

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