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.

## Introduction

Chemical kinetic modeling of hydrocarbon fuel components in practical fuels (e.g., natural gas, kerosene, gasoline, or diesel) relies on the hierarchical nature of fuel decomposition [1] to form C1C3 species. During the oxidation of long chain hydrocarbon fuels, the fuel molecules often undergo unimolecular and bimolecular decomposition to form alkyl radicals (CH3, C2H5, C3H7, etc.) and alkenes (e.g., C2H4, C3H6, etc.) as intermediates. Figure 1 shows the major reaction pathways that involve formation of alkene and alkyl radicals from beta-scission reactions. This was further demonstrated by the shock tube experiments of Davidson et al. [2] in which the time-history profile of n-dodecane oxidation showed that C2H4 was formed as one of the major intermediate species. Moreover, the experimental data showed that there was a significant time lag (an order of magnitude difference) between the consumption of n-dodecane and C2H4 under combustion conditions. Therefore, the primary objective of the current work is to investigate the effect of nitric oxide (NO) on the ignition behavior of low molecular weight hydrocarbons that are relevant to the oxidation of practical fuels.

Fig. 1
Fig. 1
Close modal

The presence of NOx (NO and NO2) in the oxidizer stream occurs in various practical devices where combustor design includes exhaust (or flue) gas recirculation (e.g., internal combustion engines and furnaces), vitiated air (e.g., gas turbine exit streams, and high-speed propulsion test rigs), and internal recirculation zones (e.g., gas turbines). Previous works by the authors [35] showed that NOx plays a significant role in promoting ignition of jet fuels and its surrogate components. Experimental and direct numerical simulation studies of Lee et al. [6] have demonstrated the importance of NO on the reignition process of a vortex-perturbed counter flow flame. In the current work, experiments are carried out to investigate the effect of NO on the ignition of C1C3 hydrocarbons, namely, CH4, C2H4, C2H6, and C3H6. The current study builds upon previous work performed with C3H8 [7]. For the reasons discussed above, it is important to understand the interactions of these fuel molecules with NOx in order to develop a detailed reaction mechanism for the combustion of jet fuels under vitiated conditions.

CH4, C2H6, and C3H8 are primary fuel components in natural gas fuels. The influence of NOx on CH4 oxidation has been studied extensively [817], whereas limited work is reported on C2H4 [10,18,19], C2H6 [10,18,20,21], C3H6 [10], and C3H8 [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. Detailed information from the literature on NOx interaction with C1C3 hydrocarbon fuels is provided in Table 1. The table compares the current experimental conditions with previous work, most of which used heavily diluted fuel/oxidizer mixtures with low carbon content (<1 mole%) except for few studies related to ignition experiments [7,8,20]. Previous work by the authors [7] showed that the reaction pathways of NO in influencing hydrocarbon oxidation are sensitive to the ratio of NO to the total carbon present in the system. It was shown that experimental measurements from heavily diluted oxidizer systems are less influenced by the interaction between NOx and the fuel molecule or its derivative radicals.

Table 1

Literature experimental work on C1C3 hydrocarbon fuels with NOx compared with current experimental conditions

Fuel typeData typeaPress (atm)Temp (K)O2 (mole%)NOx (ppm)ϕbDilution ratiocCarbon (mole%)dReference
IDT2773–97319.22000–38,0000.504.38[8]
CH4Species1773–9731–1425–200< 0.10.50.13[16]
Species1600–11002120< 0.100.01[10]
Species1750–12502.7–3.7186–211< 0.14.680.22[17]
Species1, 10800–11501–52000.1–1.03.190.25[14]
Species20–100600–9000.28–4.5179–2140.04–1.153.450.16[12]
IDT25–50900–105020100, 270107.63[20]
IDT1900, 95015, 210–750107.99Current
C2H6Species1650–13004485< 0.14.250.05[21]
Species1600–11002120< 0.100.01[10]
Species1800–12001.54750112.570.86[9]
Species1700–11500.9–4.40–12000.1, 0.53.80.25[18]
IDT25–50900–105020100, 270108.88[20]
IDT1900, 95015, 210–750109.23Current
C3H8Species1600–11002120< 0.100.01[10]
Species1773–10731667< 0.10.310.13[22]
IDT1850–95014–210–7500.5–1.5013.12[7]
C2H4Species1600–11002120< 0.100.01[10]
Species1700–11500.9–4.20–12000.1, 0.540.30[18]
Species60600–9000.9–4.25000.05–5.02.50.20[19]
IDT1900, 95015, 210–7501010.93Current
C3H6Species1600–11002120< 0.100.01[10]
IDT1900, 95015, 210–7501010.94Current
Fuel typeData typeaPress (atm)Temp (K)O2 (mole%)NOx (ppm)ϕbDilution ratiocCarbon (mole%)dReference
IDT2773–97319.22000–38,0000.504.38[8]
CH4Species1773–9731–1425–200< 0.10.50.13[16]
Species1600–11002120< 0.100.01[10]
Species1750–12502.7–3.7186–211< 0.14.680.22[17]
Species1, 10800–11501–52000.1–1.03.190.25[14]
Species20–100600–9000.28–4.5179–2140.04–1.153.450.16[12]
IDT25–50900–105020100, 270107.63[20]
IDT1900, 95015, 210–750107.99Current
C2H6Species1650–13004485< 0.14.250.05[21]
Species1600–11002120< 0.100.01[10]
Species1800–12001.54750112.570.86[9]
Species1700–11500.9–4.40–12000.1, 0.53.80.25[18]
IDT25–50900–105020100, 270108.88[20]
IDT1900, 95015, 210–750109.23Current
C3H8Species1600–11002120< 0.100.01[10]
Species1773–10731667< 0.10.310.13[22]
IDT1850–95014–210–7500.5–1.5013.12[7]
C2H4Species1600–11002120< 0.100.01[10]
Species1700–11500.9–4.20–12000.1, 0.540.30[18]
Species60600–9000.9–4.25000.05–5.02.50.20[19]
IDT1900, 95015, 210–7501010.93Current
C3H6Species1600–11002120< 0.100.01[10]
IDT1900, 95015, 210–7501010.94Current
a

Species data from perfectly stirred reactor or plug-flow reactor, IDT—ignition delay time data from shock tube, rapid compression machine, or flow reactor.

b

ϕ—equivalence ratio.

c

Minimum dilution ratio = (0.21/XO2)—1; XO2—mole fraction of O2; for standard air dilution ratio = 0.

d

Maximum total carbon in the fuel/oxidizer mixture.

It was shown in a previous study [7] that the chemical kinetics of NOx in promoting oxidation at low and intermediate temperatures can be classified into two routes: a fuel-independent pathway and a fuel-dependent pathway. Along the fuel-independent pathway, the interaction between NOx and common radical species, such as HO2, CH3, and CH3O2 formed irrespective of the fuel type, promotes the oxidation at low and intermediate temperatures. This phenomenon is generally referred to as mutually sensitized oxidation [11], in which NOx promotes the oxidation by converting HO2, created from a chain-terminating reaction (R1), into the more reactive, chain-propagating hydroxyl radical (OH) via a catalytic cycle of reactions (R2) and (R3).
$H+O2(+M)= HO2(+M)$
(R1)
$NO+ HO2= NO2+OH$
(R2)
$NO2+H=NO+OH$
(R3)
Furthermore, NO2 and NO react with CH3 and CH3OO to form the more reactive CH3O radical via reactions (R4) and (R5), respectively.
$NO2+ CH3=CH3O+NO$
(R4)
$NO+ CH3OO=CH3O+ NO2$
(R5)

Recent work by the authors [7] also showed that the fuel-dependent reaction pathway—interactions between NOx and a fuel molecule and its radicals—plays a crucial role in promoting oxidation. For example, H-atom abstraction by NO2 from normal alkanes to form alkyl radicals plays a significant role in promoting oxidation. It was also shown [7] that contribution of this fuel-dependent reaction pathway becomes significant at conditions relevant to practical applications where the absolute value of the fuel concentrations is much higher than many reported studies listed in Table 1. Also, it should be noted that there is a lack of experimental data for the ignition of C2H4 and C3H6 in the presence of NOx. Hence, in the current work, experiments were carried out to investigate the effect of NOx on the ignition of CH4, C2H6, C2H4, and C3H6 with varying levels of O2 present in typical vitiated conditions. A detailed kinetic mechanism is used to investigate reaction pathways of NOx in promoting ignition of these fuels.

## Experimental Setup

Experiments were performed using an atmospheric pressure flow reactor to investigate the effect of NO on the ignition delay time (IDT) of C1C3 hydrocarbon fuels. The species examined in this study included common natural gas components (CH4 and C2H6) and olefin intermediates (C2H4 and C3H6). The flow reactor consists of a premixing section and a test section. The premixing section, shown in Fig. 2, consists of radial injectors and a swirler to achieve near-perfect mixing of the fuel and oxidizer at inlet of the flow reactor. The portion of the premixing section located downstream of the swirler is an expanding duct that gradually increases the reactor diameter from 1.3 cm to 5.1 cm. The test section consists of an alumina tube with a nominal diameter of 5.1 cm and an overall length of approximately 240 cm.

Fig. 2
Fig. 2
Close modal

For each test condition, the mass flow rate of the oxidizer stream was maintained nominally at 2 g/s. The fuel flow was independently varied to provide test conditions at the targeted fuel equivalence ratios. The gaseous fuel was premixed with nitrogen (N2 to fuel molar ratio is 5) and preheated up to 675 K prior to radial injection upstream of the swirler. The fuel was premixed with nitrogen in order to improve jet penetration and mixing of the fuel and oxidizer. The oxidizer stream was preheated (up to 1000 K) so that the temperature of fuel/oxidizer mixture reaches the temperature of the test section prior to entering the mixing section. A uniform temperature in the test section was maintained by independently controlled electrical heating. A more detailed description of the flow reactor apparatus can be found in Ref. [7].

Ignition delay time is determined by measuring the time delay between when the fuel/oxidizer mixture enters the diffuser section and the time of ignition. The time at which the fuel/oxidizer mixture enters the diffuser is determined using the activation time of the fuel injection solenoid and infrared laser absorption measurements to determine the presence of hydrocarbons entering the diffuser. Ignition of the fuel/oxidizer mixture is measured by the detection of the chemiluminescence emission of the OH* radical that occurs during ignition. At the downstream end of the flow reactor test section, a photomultiplier tube (PMT) is placed behind a quartz window with direct line of site down the axis of the entire flow reactor test and premixing sections. A 310 nm narrow bandpass filter (±5 nm) is used to filter out light that is not emitted by OH*. The difference between the time at which the fuel/oxidizer mixture enters the diffuser section and the time of initial PMT excitation due to OH* emissions is the measured ignition delay time. An example of the time-history profile of the signals used to determine the ignition delay time is provided in Fig. 3.

Fig. 3
Fig. 3
Close modal

The experimental data reported in this paper are the average of at least three measurements. The experimental uncertainty based on two standard deviations (95% confidence interval) of the measured values are provided in the included figures.

The scope of the current experimental work is comprised of the following:

• Fuels: CH4, C2H4, C2H6, and C3H6

• O2 in the oxidizer stream: 15 and 21 mole%

• Reactor temperature: 900 and 950 K

• Equivalence Ratio: 1.0

• NOx in the oxidizer stream: 50, 100, 250, 500, and 750 ppm

Standard gas cylinders with 0.1% and 0.5% (nominally) NO in N2 were used in the experiments to make desired concentrations of NO in the oxidizer (O2/N2) using rotameters. Testing has shown that mixing of NO with the oxidizer converts only a very small amount of NO into NO2 depending on the NO concentration. Measurements at room temperature showed that mixing of NO with the oxidizer produced 0 ppm NO2 with 50 ppm NO, whereas only 5% of the NO was converted to NO2 for the mixture with 750 ppm NO. Measurements of NO and NO2 in the oxidizer were made using a thermo environmental chemiluminescence NOx analyzer.

## Chemical Kinetic Modeling

The detailed chemical kinetic mechanism used in the current work is based on the natural gas mechanism developed by the authors [7,2325] that was validated for CH4, C2H4, C2H6, C3H6, and C3H8 oxidation chemistry over a wide range of conditions. This mechanism also includes a detailed reaction subset for NOx chemistry that has been validated for high-temperature NOx emissions [23,24] as well as low-temperature vitiated kinetics [7]. In the current work, the mechanism is validated for the kinetics of NOx interactions with CH4, C2H4, C2H6, and C3H6 under vitiated conditions. Most of the rate parameters for the reactions of hydrocarbon species with NOx were adopted from the chemical kinetic mechanism developed for the influence of NOx on C3H8 oxidation [7] as well as the work of Mendiara and Glarborg [26]. The rate parameters of some of the important reactions are listed in Table 2. Most of the reaction rate parameters for C2 and C3 hydrocarbon species with NOx are based on the values recommended for C1 species. Therefore, there is a considerable uncertainty in the reaction rates due to the lack of experimental or theoretical work in the literature for the rate estimation of these reactions [20]. The reaction rate parameters for H-atom abstraction of C2H6 and C2H5 molecules by NO2 were reduced by a factor of 2 (within the uncertainty limits of the published values) in order to accurately predict the effect of NOx on the ignition delay time of C2H6 obtained in the present work. The current model was able to capture the effect of NOx on the ignition of C2 and C3 hydrocarbon fuels as discussed below.

Table 2

Rate parameters of important reactions used in the current work

ReactionAnERef.
NO + HO2 = NO2 + OH2.10 × 10120−497[26]
CH3 + NO2 = CH3O + NO1.00 × 101300[26]
CH3O2 + NO = NO2 + CH3O1.40 × 10120−715[26]
CH4 + NO2 = HONO + CH36.50 × 1014045,800[26]
C2H6 + NO2 = HONO + C2H56.50 × 1014041,400[26]
C3H8 + NO2 = HONO + nC3H73.00 × 1014032,000[7]
C3H8 + NO2 = HONO + iC3H72.00 × 1013028,500[7]
CH4 + NO2 = HNO2 + CH36.00 × 1014037,600[26]
C2H6 + NO2 = HNO2 + C2H53.00 × 1014033,200Currenta
C3H8 + NO2 = HNO2 + nC3H79.60 × 1014033,800[7]
C3H8 + NO2 = HNO2 + iC3H76.00 × 1013030,300[7]
C2H4 + NO2 = C2H3 + HONO6.50 × 1014041,400[26]
C2H4 + NO2 = C2H3 + HNO26.00 × 1014033,200[26]
C3H6 + NO2 = aC3H5 + HONO6.50 × 1014041,400Currenta
C3H6 + NO2 = aC3H5 + HNO26.00 × 1014033,200Currentb
C2H5 + NO2 = C2H5O + NO5.00 × 101200Currentc
aC3H5 + NO = aC3H4 + HNO1.00 × 101201000Currentd
aC3H5 + NO2 = C3H5O + NO7.70 × 1014−0.60Currentd
ReactionAnERef.
NO + HO2 = NO2 + OH2.10 × 10120−497[26]
CH3 + NO2 = CH3O + NO1.00 × 101300[26]
CH3O2 + NO = NO2 + CH3O1.40 × 10120−715[26]
CH4 + NO2 = HONO + CH36.50 × 1014045,800[26]
C2H6 + NO2 = HONO + C2H56.50 × 1014041,400[26]
C3H8 + NO2 = HONO + nC3H73.00 × 1014032,000[7]
C3H8 + NO2 = HONO + iC3H72.00 × 1013028,500[7]
CH4 + NO2 = HNO2 + CH36.00 × 1014037,600[26]
C2H6 + NO2 = HNO2 + C2H53.00 × 1014033,200Currenta
C3H8 + NO2 = HNO2 + nC3H79.60 × 1014033,800[7]
C3H8 + NO2 = HNO2 + iC3H76.00 × 1013030,300[7]
C2H4 + NO2 = C2H3 + HONO6.50 × 1014041,400[26]
C2H4 + NO2 = C2H3 + HNO26.00 × 1014033,200[26]
C3H6 + NO2 = aC3H5 + HONO6.50 × 1014041,400Currenta
C3H6 + NO2 = aC3H5 + HNO26.00 × 1014033,200Currentb
C2H5 + NO2 = C2H5O + NO5.00 × 101200Currentc
aC3H5 + NO = aC3H4 + HNO1.00 × 101201000Currentd
aC3H5 + NO2 = C3H5O + NO7.70 × 1014−0.60Currentd

Note: Rate constant, k = ATnexp(−E/RT); units: cm3 mole−1 s−1; E – activation energy in cal/mol; T—temperature in Kelvin; R—universal gas constant.

a

Rate constant of Mendiara and Glarborg [26] is reduced by a factor of 2 (see text).

b

Same as C2H4 + NO2 from Ref. [26].

c

Rate constant of nC3H7 + NO2 from Ref. [7] is reduced by a factor of 2 (see text).

d

Same as C2H3 + NO/NO2 from Ref. [26].

In the current work, the flow reactor experimental facility was modeled as a constant-pressure, ideal plug-flow reactor using Cantera [27] to compute the ignition delay time. For the purpose of modeling, the NOx concentrations used in the experiments were assumed to be completely composed of NO since small amounts of NO2 present in the mixture (up to 5%) did not make significant difference in the model predictions for the current experimental conditions. A sensitivity analysis of the reactions on ignition delay time was performed to understand the chemical kinetic effects of NOx on fuel oxidation. The sensitivity coefficient for each reaction (Sj) with respect to ignition delay time (τidt) is computed using Eq. (1) by perturbing the reaction rate constant (kj) in the detailed chemical kinetic mechanism [7,25] by ±10%
$Sj=ln(τidt+τidt−)ln(kj+kj−)$
(1)

A positive sensitivity coefficient indicates that the reaction inhibits ignition, whereas a negative value means that the reaction promotes ignition.

Figures 4 and 5 compare the current model predictions with the experimental data of Dagaut et al. [18] for the effect of NO on the oxidation of C2H4 and C2H6. These experiments were conducted in an atmospheric pressure jet-stirred reactor (JSR) with fuel/O2 in ∼99 mole% N2 at 0.5 equivalence ratio with and without the NO addition. The reactor residence time was maintained at 120 ms. The JSR experiments were modeled using perfectly stirred reactor simulations in Cantera [27]. The experimental data and the model predictions show that the effect of NO in reducing the temperature of the onset of fuel decomposition is more pronounced for C2H4 (from 950 K to 800 K) than C2H6 (from 975 K to 875 K). A similar trend was observed for reducing the ignition delay time with NO addition in the current work as discussed in the Results and Discussion section. However, the reaction pathways for the effect of NO in promoting the oxidation have significant differences for the conditions of Dagaut et al. [18] (Figs. 4 and 5) and the current work.

Fig. 4
Fig. 4
Close modal
Fig. 5
Fig. 5
Close modal

Figure 6 shows the reaction sensitivity coefficients for important reactions computed with respect to C2H6 concentration for JSR experimental conditions (ϕ = 0.5; 0.875% O2) in Fig. 5 at 950 K with 220 ppm NO. Figure 6 also shows the reaction sensitivity coefficients with respect to ignition delay time computed for C2H6/air (ϕ = 0.5) at 950 K with 220 ppm NO. Because of the differences in the dilution, the carbon to NO molar ratio is approximately 11 for the experimental conditions in Fig. 5 and 210 for the current work, at an equivalence ratio of 0.5.

Fig. 6
Fig. 6
Close modal
Sensitivity analysis in Fig. 6 shows that the chain-branching reaction (R6) is the main source of OH radicals for the conditions of Dagaut et al. [18] whereas H2O2 decomposition reaction (R7) is the main source of OH production for the current experimental conditions.
$H+O2=O+OH$
(R6)
$H2O2(+M)=OH+OH (+M)$
(R7)
Also, H-atom abstraction of C2H6 by OH is an important reaction for the JSR conditions of Dagaut et al. [18] whereas H-atom abstraction by HO2(R8) and NO2(R9) are important for the current ignition experiments.
$C2H6+HO2=C2H5+H2O2$
(R8)
$C2H6+NO2=C2H5+HNO2$
(R9)
$C2H5+NO2= C2H5O+NO$
(R10)

It should also be noted that reactions (R9) and (R10) have the third and fifth largest negative sensitivity coefficients for the ignition of C2H6, respectively. However, reaction (R9) does not have any significant influence on C2H6 decomposition at JSR conditions while a positive sensitivity coefficient for reaction (R10) indicates that it hinders C2H6 oxidation at these conditions. Nevertheless, fuel-independent reactions (R2) and (R4) are the two most important NOx reactions that promote the oxidation at JSR conditions.

## Results and Discussion

Figure 7 shows the effect of NOx on ignition delay time of a stoichiometric CH4/air mixture at 900 K and 950 K. It should be noted that ignition was not recorded in the absence of NOx as the ignition delay time is longer than the maximum reactor residence time at these temperatures. The detailed chemical kinetic mechanism predicts an ignition delay time of around 2 s at 950 K for the CH4/air mixture, whereas the reactor residence time is approximately 1 s at these conditions. Figure 7 shows that an addition of 100 ppm NOx reduced the ignition delay time significantly (more than 75% based on the model predictions) as compared to 0 ppm NO addition. The effect of NOx in promoting the ignition becomes marginal for NOx addition above 500 ppm. It is also interesting to note that the addition of NOx reduces the effect of temperature on ignition delay time. For example, the ignition delay time at 900 K is almost three times that of 950 K with 100 ppm NOx, whereas it is only twice as long with 750 ppm NOx addition.

Fig. 7
Fig. 7
Close modal
Figure 8 shows the sensitivity coefficients for the most important reactions responsible for CH4 ignition with 100 ppm NO addition at 950 K. The figure also shows the sensitivity coefficients without the NO addition where reactions (R11) and (R12) have the largest positive and negative sensitivity coefficients, respectively.
$CH3+CH3(+M)= C2H6(+M)$
(R11)
$CH3+HO2= CH3O+OH$
(R12)
$CH3+HO2= CH4+O2$
(R13)
Fig. 8
Fig. 8
Close modal

Methane initiation reactions with OH, O, and H species generate CH3 radicals with the consumption of approximately 90% of the fuel molecules. However, the chain-branching reaction (R12) has to compete with the termination reactions (R11) and (R13) to convert relatively less reactive CH3 into CH3O and OH. This results in a longer chemical induction period for the ignition of CH4 at these conditions in the absence of NO.

When NO is present, the reaction between CH3 and NO2 via (R4) shifts the balance toward CH3O formation. This is further enhanced by reaction (R2) in which the NO is converted to NO2 while HO2 is converted to the more reactive OH radical. In the process, the chain branching reaction (R6) becomes the most sensitive reaction for the ignition of CH4 with NOx addition as shown in Fig. 8. Species flux analysis for the conditions in Fig. 8 shows that reactions (R2) and (R5) are responsible for 92% and 8% of the NO2 generated during the induction period, respectively. Meanwhile, 88% of the NO2 is consumed by reaction (R4) to convert CH3 radical into CH3O, whereas around 10% of the NO2 takes part in reaction (R3).

Figure 9 shows the influence of NOx addition on the ignition delay time of stoichiometric C2H6/air mixture at 900 and 950 K. Figure 10 shows the effect of NOx on C2H6 ignition at 950 K with 15 mole% and 21 mole% O2. The presence of NOx in the inlet stream reduces the ignition delay time at these conditions. However, the effect of NOx in promoting the ignition of C2H6 is not as pronounced as in CH4 cases shown in Fig. 7. For example, 100 ppm of NOx addition reduces the ignition delay by less than 25% based on the experimental data in Figs. 9 and 10. As expected, ignition delay time of mixtures with 21% O2 is shorter than that of 15% O2 level in the absence of NOx. However, as the NOx level is increased above 250 ppm, the ignition delay times are almost the same for 21% O2 and 15% O2 levels. A similar trend was observed for C3H8 in a previous study by the authors [7].

Fig. 9
Fig. 9
Close modal
Fig. 10
Fig. 10
Close modal

Figure 11 shows the sensitivity coefficients for some of the most important reactions for the ignition of C2H6 with and without NO addition. The sensitivity coefficients were computed for stoichiometric C2H6/air mixture at 950 K with 0 and 100 ppm NO using the detailed kinetic mechanism. Reactions (R7) and (R8) have the two largest negative sensitivity coefficients. Species flux analysis shows that the initiation reaction (R8) is responsible for approximately 50% of the H2O2 produced while chain branching reaction (R7) generates approximately 80% of the OH radical produced at 950 K with 0 ppm NO.

Fig. 11
Fig. 11
Close modal

With the addition of 100 ppm NO, reaction (R9) becomes an important initiation step for C2H6 ignition with the third largest negative sensitivity coefficient. H-atom abstraction of C2H6 by NO2, which was generated from reaction (R2), produces C2H5 radical and HNO2. Also, the C2H5 radical reacts with NO2 to regenerate NO while producing C2H5O via reactions (R10). In order to demonstrate the importance of reactions (R9) and (R10) for C2H6 ignition, Fig. 9 also shows the model predictions without these reactions at 900 K.

Species flux analysis for the conditions in Fig. 11 shows that 99% of the NO2 is generated from reaction (R2) during the induction period. Then, 40% of the NO2 is converted back to NO via reaction (R4) while about 5% and 15% of the NO2 are consumed by reactions (R9) and (R10), respectively. HNO2 generated in reaction (R9) is converted to HONO intermediate via reaction (R14), which is then decomposed into NO and OH via reaction (R15).
$HNO2(+M)=HONO (+M)$
(R14)
$HONO (+M)=NO+OH (+M)$
(R15)

Figure 12 shows the ignition delay time of stoichiometric C2H4/air mixture as a function of NOx addition at 900 K and 950 K. Figure 13 shows the effect of NOx on C2H4 ignition at 950 K with 15 mole% and 21 mole% O2 levels. The presence of NOx in the inlet reduces the ignition delay time of C2H4 at these conditions. For example, 100 ppm of NOx reduced the ignition delay time by approximately 40%. This puts the effectiveness of NOx in promoting C2H4 ignition between CH4 and C2H6: CH4 > C2H4 > C2H6. It should also be noted that the effect of NOx in promoting ignition appears to be less sensitive to O2 level for C2H4 than C2H6 based on the experimental data. For example, 100 ppm NOx reduced the ignition delay times of stoichiometric C2H6 mixtures by approximately 25% and 10% at 15 mole% and 21 mole% O2 levels, respectively, as shown in Fig. 10. However, 100 ppm of NOx addition reduced the ignition delay time of C2H4 by approximately 40% at 15 mole% and 21 mole% O2 levels as shown in Fig. 13.

Fig. 12
Fig. 12
Close modal
Fig. 13
Fig. 13
Close modal
Figure 14 shows the sensitivity coefficients of the most important reactions for the ignition of C2H4 with and without NO at 950 K. Reactions (R16), (R17), (R7), and (R18) are the four most important reactions with the largest negative sensitivity coefficients.
$C2H3+ O2= CH2CHO+O$
(R16)
$C2H4+ HO2= C2H4O+OH$
(R17)
$C2H4+OH= C2H3+ H2O$
(R18)
Fig. 14
Fig. 14
Close modal

Species flux analysis for the conditions in Fig. 14 shows that approximately 70% of the C2H3 radical is produced via initiation reaction (R18) while approximately 50% of it is consumed by reaction (R16). Production of O and OH species via reactions (R16) and (R17) makes C2H4 a more reactive fuel with shorter ignition delay times than C2H6.

With the addition of 100 ppm NO, H-atom abstraction from CH2CHO (R19) and C2H4(R20) by NO2 becomes important in addition to reaction (R2) in promoting ignition.
$CH2CHO+NO2=CH2CO+HONO$
(R19)
$C2H4+NO2= C2H3+HNO2$
(R20)

A portion of the NO2 is regenerated from HONO produced in reaction (R19) via reaction (R15). It is also interesting to note that the consumption of HO2 by NO to form NO2 and OH via reaction (R2) reduces the importance of reaction (R17) while increasing for the importance of reaction (R18). The sensitivity coefficient for reaction (R17) is reduced by about 65% in the presence of NO while the value for reaction (R18) is increased by about 50% as shown in Fig. 14.

Species flux analysis shows that 98% of NO2 is produced by reaction (R2) in the induction period while about 35% of it is converted to HONO via reaction (R19). Most of the HONO generated in reaction (R19) is decomposed into NO and OH via reaction (R15). Meanwhile, reactions (R3) and (R4) convert about 10% and 35% of NO2 into NO, respectively, while generating OH and CH3O radicals in the process.

Figure 15 shows the ignition delay times for stoichiometric C3H6/air as a function of NOx at 900 K and 950 K. Figure 16 shows the effect of NOx on C3H6 ignition delay time at 950 K with 15 mole% and 21 mole% O2 levels. The addition of NOx reduces the ignition delay time C3H6 significantly. For example, 100 ppm NOx reduced the ignition delay time by about 60%, whereas 500 ppm NOx reduced it by about 90% at 950 K.

Fig. 15
Fig. 15
Close modal
Fig. 16
Fig. 16
Close modal
Figure 17 shows the sensitivity coefficients of the most important reactions for the ignition of C3H6 at 950 K with and without NO addition. Though both are olefins, C3H6 is a less reactive fuel than C2H4 with a longer ignition delay time. This can partly be prescribed to the fact that the largest positive sensitivity coefficient belongs to reaction (R21) in which C3H6 is consumed to produce ally radical (aC3H5) by H-atom abstraction reaction via OH.
$C3H6+OH= aC3H5+H2O$
(R21)
Fig. 17
Fig. 17
Close modal
The subsequent competition between the propagation reaction (R22) and the termination reaction (R23) for the aC3H5 radical slows the induction process for ignition at the current experimental conditions. A detailed discussion on this can be found in a recent publication [25].
$aC3H5+ HO2= C3H5O+OH$
(R22)
$aC3H5+ HO2= C3H6+O2$
(R23)
When NO is present, it reacts with HO2 to form OH and NO2 via reaction (R2). Then, NO2 reacts with aC3H5 to generate C3H5O and NO via reaction (R24),
$aC3H5+ NO2= C3H5O+NO$
(R24)

It should also be noted that reaction (R2) has the largest negative sensitive coefficient in the presence of NOx as shown in Fig. 17. Species flux analysis for the conditions in Fig. 17 shows that reactions (R2) and (R4) generate 95% and 5% of the NO2, respectively, in the induction period while around 80% of it is consumed by reaction (R24). It is also interesting to note that reaction (R24) becomes an important pathway for allyl radical consumption with 23% compared to 19% for reaction (R22).

Figure 18 compares the effectiveness of NOx in promoting the ignition of the fuels investigated in the current work. The change in ignition delay time relative to the change in NOx addition (i.e., ΔτidtXNOx) is provided in Fig. 18 as a function of NOx in the inlet. The effectiveness of NOx addition in promoting the ignition is ranked based on the overall average percent reduction in the ignition delay time observed for each fuel: CH4 > C3H6 > C3H8 > C2H6 > C2H4. It should be noted that the negative value for C2H6 at 50 ppm is due to a small increase in the ignition delay time, shown in Fig. 9, which lies within the experimental uncertainty of the measurement.

Fig. 18
Fig. 18
Close modal

## Conclusions

An experimental and modeling study was performed to investigate the effect of nitric oxide on the ignition of C1C3 fuels, which are important intermediate species generated during the decomposition of long chain hydrocarbon fuel components typically present in jet fuels. CH4 and C2H6 are also the major components of natural gas fuels. Ignition delay time measurements were carried out using an atmospheric pressure flow reactor facility with 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 ranking of the effectiveness of NOx in promoting the ignition is: CH4 > C3H6 > C3H8 > C2H6 > C2H4. For example, 500 ppm of NOx reduced the ignition delay time of CH4 by approximately 90%.

A detailed kinetic mechanism, developed for natural gas fuels ranging from C1 to C3, was used for model predictions as well as for the sensitivity and species flux analyses. The reaction between HO2 and NO plays a critical role in promoting the ignition by generating OH radical while converting NO into NO2. Furthermore, fuel-dependent reaction pathways also play important role in promoting the ignition of these fuels. NO2 reacts with C2H6 to enhance the ignition of C2H6, whereas its reactions with CH2CHO and aC3H5 radicals are important rate-limiting steps for the ignition of C2H4 and C3H6, respectively. In addition, NO2 reactions with C2H4 and C2H5 also play important roles in promoting the ignition of C2H4 and C2H6, respectively. Reaction between NO2 and the relatively unreactive CH3 radical to produce CH3O plays an important role in reducing the ignition delay time of CH4 compared to the other fuels examined.

## Acknowledgment

The material was cleared for public release by the U.S. Air Force (Case No. 88ABW‐2016‐6489).

## Funding Data

• United States Air Force (Grant No. FA8650-15-C-2531).

## Nomenclature

• IDT =

ignition delay time

•
• JSR =

jet-stirred reactor

•
• k =

reaction rate constant

•
• Tign =

ignition delay time

•
• X =

mole fraction

•
• ϕ =

equivalence ratio

## References

1.
Westbrook
,
C. K.
, and
Dryer
,
F. L.
,
1984
, “
Chemical Kinetic Modeling of Hydrocarbon Combustion
,”
Prog. Energy Combust. Sci.
,
10
(
1
), pp.
1
57
.
2.
Davidson
,
D. F.
,
Hong
,
Z.
,
Pilla
,
G. L.
,
Farooq
,
A.
,
Cook
,
R. D.
, and
Hanson
,
R. K.
,
2011
, “
Multi-Species Time-History Measurements During n-Dodecane Oxidation Behind Reflected Shock Waves
,”
Proc. Combust. Inst.
,
33
(
1
), pp.
151
157
.
3.
Fuller
,
C. C.
,
Gokulakrishnan
,
P.
,
Klassen
,
M. S.
,
Roby
,
R. J.
, and
Kiel
,
B. V.
,
2009
, “
Investigation of the Effects of Vitiated Conditions on the Autoignition of JP-8
,”
AIAA
Paper No. 2009-4925.
4.
Fuller
,
C. C.
,
Gokulakrishnan
,
P.
,
Klassen
,
M. S.
,
Roby
,
R. J.
, and
Kiel
,
B. V.
,
2011
, “
Investigation of the Effect of Nitric Oxide on the Autoignition of JP-8 at Low Pressure Vitiated Conditions
,”
AIAA
Paper No. 2011-96.
5.
Gokulakrishnan
,
P.
,
Fuller
,
C. C.
,
Klassen
,
M. S.
, and
Kiel
,
B. V.
,
2014
, “
Ignition Characteristics of Alternative JP-8 and Surrogate Fuels Under Vitiated Conditions
,”
AIAA
Paper No. 2014-3664.
6.
Lee
,
U. D.
,
Yoo
,
C. S.
,
Chen
,
J. H.
, and
Frank
,
J. H.
,
2010
, “
Effect of NO on Extinction and Re-Ignition of Vortex-Perturbed Hydrogen Flames
,”
Combust. Flame
,
157
(
2
), pp.
217
229
.
7.
Gokulakrishnan
,
P.
,
Fuller
,
C. C.
,
Klassen
,
M. S.
,
Joklik
,
R. G.
,
Kochar
,
Y. N.
,
,
S. N.
,
Lieuwen
,
T. C.
, and
Seitzman
,
J. M.
,
2014
, “
Experiments and Modeling of Propane Combustion With Vitiation
,”
Combust. Flame
,
161
(
8
), pp.
2038
2053
.
8.
Slack
,
M. W.
, and
Grillo
,
A. R.
,
1981
, “
Shock Tube Investigation of Methane-Oxygen Ignition Sensitized by NO2
,”
Combust. Flame
,
40
, pp.
155
172
.
9.
Dagaut
,
P.
,
Lecomte
,
F.
,
Chevailler
,
S.
, and
Cathonnet
,
M.
,
1999
, “
Mutual Sensitization of the Oxidation of Nitric Oxide and Simple Fuels Over an Extended Temperature Range: Experimental and Detailed Kinetic Modeling
,”
Combust. Sci. Technol.
,
148
(
1–6
), pp.
27
57
.
10.
Hori
,
M.
,
Matsunaga
,
N.
,
Marinov
,
N.
,
Pitz
,
W.
, and
Westbrook
,
C.
,
1998
, “
An Experimental and Kinetic Calculation of the Promotion Effect of Hydrocarbons on the NO-NO2 Conversion in a Flow Reactor
,”
Proc. Combust. Inst.
,
27
(
1
), pp.
389
396
.
11.
Gokulakrishnan
,
P.
,
McLellan
,
P. J.
,
Lawrence
,
A. D.
, and
Grandmaison
,
E. W.
,
2005
, “
Kinetic Analysis of NO-Sensitized Methane Oxidation
,”
Chem. Eng. Sci.
,
60
(
13
), pp.
3683
3692
.
12.
Rasmussen
,
C. L.
,
Rasmussen
,
A. E.
, and
Glarborg
,
P.
,
2008
, “
Sensitizing Effect of NOx on CH4 Oxidation at High Pressure
,”
Combust. Flame
,
154
(
3
), pp.
529
545
.
13.
Dagaut
,
P.
,
Lecomte
,
F.
,
Chevailler
,
S.
, and
Cathonnet
,
M.
,
1998
, “
Experimental and Detailed Kinetic Modeling of Nitric Oxide Reduction by a Natural Gas Blend in Simulated Reburning Conditions
,”
Combust. Sci. Technol.
,
139
(
1
), pp.
329
363
.
14.
Dagaut
,
P.
, and
Nicolle
,
A.
,
2005
, “
Experimental Study and Detailed Kinetic Modeling of the Effect of Exhaust Gas on Fuel Combustion: Mutual Sensitization of the Oxidation of Nitric Oxide and Methane Over Extended Temperature and Pressure Ranges
,”
Combust. Flame
,
140
(
3
), pp.
161
171
.
15.
Dagaut
,
P.
, and
Dayma
,
G.
,
2006
, “
Mutual Sensitization of the Oxidation of Nitric Oxide and a Natural Gas Blend in a JSR at Elevated Pressure—Experimental and Detailed Kinetic Modeling Study
,”
J. Phys. Chem. A
,
110
(
21
), pp.
6608
6616
.
16.
Bromly
,
J. H.
,
Barnes
,
F. J.
,
Muris
,
S.
,
You
,
X.
, and
Haynes
,
B. S.
,
1996
, “
Kinetic and Thermodynamic Sensitivity Analysis of the NO-Sensitized Oxidation of Methane
,”
Combust. Sci. Technol.
,
115
(
4–6
), pp.
259
296
.
17.
Bendtsen
,
A. B.
,
Glarborg
,
P.
, and
Dam–Johansen
,
K.
,
2000
, “
Low Temperature Oxidation of Methane: The Influence of Nitrogen Oxides
,”
Combust. Sci. Technol.
,
151
(
1
), pp.
31
71
.
18.
Dagaut
,
P.
,
Mathieu
,
O.
,
Nicolle
,
A.
, and
Dayma
,
G.
,
2005
, “
Experimental Study and Detailed Kinetic Modeling of the Mutual Sensitization of the Oxidation of Nitric Oxide, Ethylene and Ethane
,”
Combust. Sci. Technol.
,
177
(
9
), pp.
1767
1791
.
19.
Gimenez-Lopez
,
J.
,
Alzueta
,
M. U.
,
Rasmussen
,
C. T.
,
Marshall
,
P.
, and
Glarborg
,
P.
,
2011
, “
High Pressure Oxidation of C2H4/NO Mixtures
,”
Proc. Combust. Inst.
,
33
(
1
), pp.
449
457
.
20.
Gersen
,
S.
,
Mokhov
,
A. V.
,
Darmeveil
,
J. H.
,
Levinsky
,
H. B.
, and
Glarborg
,
P.
,
2011
, “
Ignition-Promoting Effect of NO2 on Methane, Ethane and Methane/Ethane Mixtures
,”
Proc. Combust. Inst.
,
33
(
1
), pp.
433
440
.
21.
Hjuler
,
K.
,
Glarborg
,
P.
, and
Dam-Johansen
,
K.
,
1995
, “
Mutually Promoted Thermal Oxidation of Nitric Oxide and Organic Compounds
,”
Ind. Eng. Chem. Res.
,
34
(
5
), pp.
1882
1888
.
22.
Nelson
,
P. F.
, and
Haynes
,
B. S.
,
1994
, “
Hydrocarbon-NOx Interactions at Low Temperatures
,”
Proc. Combust. Inst.
,
25
(
1
), pp.
1003
1010
.
23.
Gokulakrishnan
,
P.
, and
Klassen
,
M. S.
,
2013
, “
NOx and CO Formation and Control
,”
Gas Turbine Emissions
,
T.
Lieuwen
and
V.
Yang.
, eds.,
Cambridge University Press
,
New York
, pp.
175
208
.
24.
Gokulakrishnan
,
P.
,
Fuller
,
C. C.
,
Joklik
,
R. G.
, and
Klassen
,
M. S.
,
2012
, “
Chemical Kinetic Modeling of Ignition and Emissions From Natural Gas and LNG Fueled Gas Turbines
,”
ASME
Paper No. GT2012-69902.
25.
Gokulakrishnan
,
P.
,
Fuller
,
C.
,
Klassen
,
M.
,
Zhu
,
Y.
,
Davidson
,
D.
,
Hanson
,
R.
, and
Kiel
,
B.
,
2016
, “
Ignition of Light Hydrocarbon Mixtures Relevant to Thermal Cracking of Jet Fuels
,”
AIAA
Paper No. 2016-0661.
26.
Mendiara
,
T.
, and
Glarborg
,
P.
,
2009
, “
Ammonia Chemistry in Oxy-Fuel Combustion of Methane
,”
Combust. Flame
,
156
(
10
), pp.
1937
1949
.
27.
Goodwin, D. G., Moffat, H. K., and Speth, R. L., 2015, “
Cantera: An Object- Oriented Software Toolkit for Chemical Kinetics, Thermodynamics, and Transport Processes
,” Version 2.2, Cantera Developers, accessed Oct. 13, 2017, http://www.cantera.org