Ignition Characteristics of Methane - American Chemical Society

Apr 18, 2018 - Results show that the initial temperature and equivalence ratio have a significant influence on the ID under all research conditions. A...
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Ignition Characteristics of Methane/n‑Heptane Fuel Blends under Engine-like Conditions Haiqiao Wei,* Jiayue Qi, Lei Zhou,* Wanhui Zhao, and Gequn Shu State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China ABSTRACT: There is significant interest in using pilot fuel as a source of ignition for enhancing the performance of natural gas engines. In this work, ignition of methane/n-heptane fuel blends is numerically studied based on the conditions after compression in a dual-fuel engine. Chemkin-Pro is used to model ignition in a closed homogeneous reactor. Compared with the calculated IDs based on four mechanisms (Liu 44, Sk 88, GRI 3.0, Detailed Zhang), the Liu 44 mechanism yields the closest agreement with experimental data both for CH4/air mixtures and C7H16/air mixtures, which is adopted in this investigation. Results show that the initial temperature and equivalence ratio have a significant influence on the ID under all research conditions. Although the effects of pressure and blend ratio depend on the special condition, such effects are large at high equivalence ratio but small at low equivalence ratio. It is interesting that the ignition delay map can be divided into four different zones, which can be derived from the coupling effect of methane concentration and equivalence ratio. The negative temperature coefficient (NTC) in particular can be observed for dual fuel, and the sensitivity analysis indicates that the effect of C7H16 addition on the total reaction rate is high in the NTC regime. The rate of production and consumption analysis shows the main production and consumption path of the important radicals. Such analysis also shows that initial temperature and equivalence ratio have a significant influence on not only the reaction rate but also the reaction temperature region. These studies can provide a theoretical basis for studies on ignition control of a dual-fuel engine.

1. INTRODUCTION With the growing number of vehicles, the shortage of fossil energy, and increasing environmental pollution, there is worldwide interest in the use of renewable and environmentally friendly fuels for automobile engines. In this regard, natural gas can be used for its rich resources, widespread distribution infrastructure, low cost, and clean-burning qualities.1−3 Furthermore, the use of natural gas is not prone to knock due to its high methane number; thus, it can achieve high efficiency with a relatively high compression ratio.4,5 However, it suffers from poor ignition characteristics due to the high autoignition temperature and low cetane number compared with those of diesel fuel.6−9 Previous studies found that the addition of a small amount of higher alkanes such as ethane, propane, and butane can lower the IDs of methane-oxygen mixtures in shock tubes.10,11 Therefore, the dual fuel engine, in which the pilot diesel ignition is used to trigger hightemperature ignition, is applied to ensure reliable combustion initiation. The ignition stability plays an important role in this kind of engine because longer IDs will lead to unacceptable rates of pressure rise followed by knock.9,12 Therefore, there is considerable interest in investigating the ignition characteristics of a methane-diesel fuel blend. The present work has a new contribution for providing fundamental understanding of the ignition features of this dual fuel. In recent years, a number of studies of gas engines with pilot ignition have focused on combustion, emissions, and engine performance characteristics.13,14 However, the studies on ignition characteristics and chemistry for methane/n-heptane mixtures are limited. Some experimental studies have been performed using rapid compression machines, shock tube,15 constant volume configuration, and single cylinder four stroke direct injection engines.16 It was found that the autoignition of © XXXX American Chemical Society

methane is promoted by addition of n-heptane; however, in other words, the ignition of n-heptane is delayed by the presence of methane in ambient gas.17,18 Schlatter et al.18 conducted an experimental study in a rapid compression expansion machine and found that lower ambient temperatures or dilution of the ambient oxidizer caused longer IDs, which resulted in a higher heat release due to more methane/air entrained and mixed with the pilot injection. They also investigated the effect of nozzle geometry, methane equivalence ratio, EGR (exhaust gas recirculation) dilution of the premixed charge, and ambient temperature on the ignition.19,20 Polk et al.16 studied the effect of equivalence ratio, pilot fuel quantity, gaseous fuel percent energy substitution (PES), and brake mean effective pressure (BMEP) at a constant engine speed (1800 rev/min) using a four-cylinder direct injection diesel engine. However, the types of data obtained from the experiments were still limited by the measurement techniques. For simulation studies, a reduced mechanism21 of CH4/C7H16 including 41 species and 109 reactions was developed by Keyvan Bahlouli et al. Maghbouli et al.22 performed a numerical investigation in a natural gas-diesel dual fuel engine using CFD model and found a shorter ID and peak pressure increment caused by increasing pilot fuel amount. Demosthenous et al.17,23 conducted a direct numerical simulation in a constant volume configuration, finding that ignition was localized and occurred at rich mixture fractions, and the methane oxidation behavior followed roughly an autoignition regime and a canonical premixed flame for large and small amounts of nReceived: December 28, 2017 Revised: March 13, 2018 Published: April 18, 2018 A

DOI: 10.1021/acs.energyfuels.7b04128 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Measured and simulated ignition delay times for CH4 with different pressures and equivalence ratios: (a) 25 bar, ϕ = 0.3, (b) 25 bar, ϕ = 0.5, (c) 25 bar, ϕ = 1.0, (d) 10 bar, ϕ = 0.5, (e) 10 bar, ϕ = 1.0, and (f) 10 bar, ϕ = 2.0.

heptane, respectively. Wang24 found that the ID was a function of pressure and an exponential function of temperature. The available literature has mostly studied the effects of many factors on the IDs, but the deep mechanism of ignition characteristics for the methane/n-heptane fuel blend needs further study. Because n-heptane (C7H16) is an important surrogate for diesel fuel, its oxidation chemistry has been extensively researched, and the NTC behavior in autoignition has been observed and studied in homogeneous systems such as the shock tube,25 rapid compression machine,26 and computational reactors.27,28 The view that the peroxy-chemistry pathways in the low-to-intermediate temperature regimes lead to the NTC behavior is widely accepted.26,29 In addition, the NTC behavior of the CH4/C7H16 mixture has been found under some conditions,30 whereas fundamental investigations on the effect of temperature, pressure, equivalence ratio, and fuel blend on the NTC behavior of the CH4/C7H16 mixtures are limited. Therefore, the objective of this work is to understand the effects of temperature, pressure, equivalence ratio, and fuel blend on the ignition process. As the effect of one factor varies with the working conditions, the effect sensitivities of various factors on the ignition delay time under different conditions are discussed in detail to provide a theoretical basis for the study on

ignition control of dual-fuel engines. In the present work, a study on the IDs using the closed homogeneous reactor model in CHEMKIN is conducted first. The NTC behavior of the fuel blend with various CH4 mass fractions is analyzed. Then, sensitivity analysis is employed to examine the effect of the main elementary reactions on the ignition process. Finally, the rate of production and consumption of key species, including CH4, C7H15, and C7ket, are performed, explaining the process of CH4/C7H16 fuel blend ignition. The present study is organized as follows. The physicalnumerical model is shown and validated in section 2. The effects of different factors on the ID, sensitivity analysis, and rate of production and consumption are discussed in sections 3.1, 3.2, and 3.3 respectively. Finally, the present work and the main conclusions are summarized in section 4.

2. NUMERICAL MODEL The physical model is based on the 0-D homogeneous, isobaric, adiabatic system with equations of mass, energy, and species. The IDs are calculated using the closed homogeneous reactor model in Chemkin-Pro 18.0 software package.31 The forward and reverse rate constants of the ith reaction are

( ) and k

k fi = Ai T βi exp B

−Ei R cT

ri

=

k fi Kci

, respectively, where Ai is DOI: 10.1021/acs.energyfuels.7b04128 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Measured and simulated ignition delay times for C7H16 with different pressures and equivalence ratios: (a) 13.5 bar, ϕ = 0.5, (b) 42 bar, ϕ = 0.5, (c) 13.5 bar, ϕ = 1.0, and (d) 42 bar, ϕ = 1.0.

pre-exponential factor, βi is the temperature exponent, Ei is the activation energy, Rc is the gas constant, T presents the temperature, and Kci is the equilibrium constant. More details can be found in ref 31. A set of specified initial conditions, including the initial temperature, pressure, equivalence ratio, and reactant mixture composition, have been performed in the present work. The IDs are calculated based on two distinct definitions. One is the time when the mixture temperature increases by 400 K over one time step during simulations, and the other is the time when a max rate of increase of OH radicals occurs. These two types of definitions yield basically the same ID time. For validation, the IDs are computed using four different mechanisms and are compared with experimental ignition data for n-heptane/air mixtures and CH4/air mixtures, respectively. The mechanisms include (i) the Liu 44,32 (ii) Sk 88,33 (iii) GRI 3.0,34 and (iv) Zhang detailed n-C7H16 mechanism.35 Figure 1 shows a comparison of simulated IDs for CH4/air mixtures and the experimental data from Burke et al.36 at different initial pressures and equivalence ratios. The simulated IDs are obtained by three available mechanisms (Liu 44, Sk 88, GRI 3.0). The Liu 44 and Sk 88 mechanisms have good predictions in IDs and are able of capturing the experimentally observed effects of pressure, temperature, and equivalence ratio on the IDs, whereas the GRI mechanism gives a slightly low value at high temperature and a slightly high value at low temperature. A further validation study is performed to predict the IDs for the heptane/air mixture at different initial pressures and equivalence ratios. Figure 2 compares the predicted IDs using three available mechanisms (Liu 44, Sk 88, Detailed Zhang) with the experimental data of Liu et al.32 The IDs calculated by

the Sk 88 mechanism are slightly high at high temperature and are lower than experimental data in the NTC area at ϕ = 0.5. The Liu 44 and n-heptane detail mechanisms also have slightly lower predictions in the NTC regime at ϕ = 0.5. Overall, the Liu 44 and n-heptane detail mechanisms provide reasonable agreement with measurements. On the basis of the comparison above, the Liu 44 mechanism is able to reasonably predict both the IDs of C7H16/air mixture and the IDs of CH4/air mixture. Furthermore, the Liu 44 mechanism has successfully been validated and used in the CH4/C7H16 mixture combustion in previous studies.23 Therefore, the Liu 44 mechanism is employed to characterize the effect of different initial conditions on the ignition of the C7H16/CH4 mixture fuel as follows. As shown in Table 1, the calculation conditions are set up based on the dual fuel microignition engine in which a small Table 1. Calculation Settings variable

range

mass fraction of C7H16 mass fraction of CH4 equivalence ratio temperature pressure

0−30% 100−70% 0.2−3.0 800−1200 K 40−80 bar

amount of diesel fuel is injected during the compression stroke to act as a source of ignition for the methane-air mixture, and the amount of pilot fuel is between 10 and 20% or even less of the total fuel.13,37 Therefore, the mass fraction of C7H16 and CH4 is set from 0 to 30% and 100 to 70%, respectively. The initial temperature and pressure are set based on the conditions after compression in the dual-fuel engine.14,38 C

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Figure 3. Simulated ignition delay times with different (a) CH4/C7H16 blends, (b) pressures, and (c) equivalence ratios.

3. RESULTS AND DISCUSSION 3.1. Ignition Process of Methane/n-Heptane-Air Mixtures. 3.1.1. The Effect of Single Variable. Many experiments have shown the existence of autoignition negative temperature coefficient (NTC) behavior for n-heptane. NTC behavior means that the ignition delay time increases with the increase in temperature changing from low to medium. The plot of IDs as a function of temperature for CH4/C7H16 also demonstrates NTC behavior as shown in Figure 3a, and the IDs of dual fuel are between those of CH4 and C7H16. As the amount of C7H16 in the blend increases from 10 to 30%, the ID decreases and the NTC behavior becomes more pronounced. This is because, as the amount of C7H16 increases, the heat released from the C7H16 reaction increases and the ignition delay period is shortened. Figure 3b shows that, with increasing pressure, the IDs decrease and the turning point of the ignition delay moves toward higher temperature, which has also been seen in previous literature.28 Figure 3c shows that, with increasing equivalence ratio, the NTC behavior can be seen at ϕ = 3.0 and IDs decrease. This is because the ignition in the NTC regime of C7H16 consists of two stages and ID is the sum of the first and second stage ignition delays, i.e., τ1 and τ2, respectively.39 The heat release rate at the first stage increases with increasing ϕ, causing shorter τ2 at the second stage and thereby shorter IDs of dual fuel.27 Figure 4 presents the effects of CH4 mass fraction, equivalence ratio, and initial temperature on the ignition of nheptane/methane mixtures at an initial pressure of 50 bar. Different from above, results are shown in terms of the plot of ID as a function of equivalence ratio, and meanwhile, several new trends are discovered. As the temperature increases, the distances between the curves with different blend ratios decrease, indicating that the effect of blend ratio decreases.

Figure 4. Simulated ignition delay times at initial temperatures of 800, 1000, and 1200 K and different CH4/C7H16 blend ratios with 70, 80, and 90% CH4 in the mass fraction.

This is because, as the initial temperature is very high, it provides some energy for the ignition of methane, and the dependence on n-heptane releasing heat is reduced. The distances between the curves are smaller at a low equivalence ratio than those at a high equivalence ratio, which means that the effect of the C7H16 fraction on ignition increases with increasing equivalence ratio. In addition, there is overlap area at 800 and 1000 K, which suggests that, when the equivalence ratio is greater than 2.0, the ID under the conditions of 1000 K and 98% CH4 is greater than that under the conditions of 800 K and 90% CH4. The result also indicates that, in this respect, both the initial temperature and little heptane concentration can control the ignition process of methane fuel with equivalent effect. D

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Energy & Fuels 3.1.2. The Effect of Multivariables. As mentioned above, the effect sensitivity of various factors on the IDs at different working conditions is studied. Then, the effects of different factors on ID and NTC behavior of the fuel blend under different CH4 fractions is discussed below. The X and Y axes represent two variables, and the color represents the IDs. Figure 5 shows the effect of temperature and fuel blend on the IDs. With CH4 mass fractions of 70−80%, the NTC

the C7H16/CH4 mixture even with the very low mass fraction of heptane. However, in the high temperature region, the heptane concentration has little influence on the ignition process because the reaction time of the C7H16 low temperature reaction becomes short at high temperature. Another reason is that the ignition of CH4 can obtain some heat from the high temperature condition and is less restricted by the heat release from heptane combustion. Therefore, the IDs are short, and the influence of blend ratio becomes small at high temperature. As shown in Figure 6, the NTC behavior is clearly observed with 70 and 80% CH4. With the 70% CH4 fraction, the ID is influenced by both pressure and temperature in the temperature range of 800−1000 K. However, it is mainly influenced by temperature when the temperature is above 1100 K as the ignition delay time changes significantly with temperature but slightly with pressure. The result with 80% CH4 is similar to that with 70% CH4, but the critical temperature point shifts to low temperature. As the CH4 mass fraction is up to 90%, the effect of C7H16 decreases, and the NTC behavior gradually disappears. Figure 7 shows that both temperature and equivalence ratio have a significant influence on the IDs. It can be seen that the IDs change significantly with these two factors, which can also be observed in previous studies.16,19 However, the present result shows that both temperature and equivalence ratio have profound effects on the IDs, especially at low temperature and low equivalence ratio. As shown in Figure 7c, at high temperature and rich equivalence ratio, the sensitivities of ignition delay time on temperature and equivalence ratio decline. Furthermore, as the amount of CH4 increases and C7H16 decreases, the total ignition delay time increases because the oxidation of CH4 is slower than that of C7H16. This can be

Figure 5. Simulated ignition delay time versus the CH4 mass fraction and temperature.

behavior of the C7H16/CH4 mixture is obviously observed in the temperature range of 860−950 K. Furthermore, the ID is influenced by both temperature and CH4 mass fraction in the low temperature conditions from 800 to 1000 K, where the heptane concentration has a significant effect on the ignition of

Figure 6. Simulated ignition delay time versus pressure and temperature for (a) 70%, (b) 80%, and (c) 90% CH4 mass fractions. E

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Figure 7. Simulated ignition delay time versus the temperature and equivalence ratio for the (a) 70%, (b) 80%, and (c) 90% CH4 mass fractions.

by equivalence ratio for the CH4 fraction below 86%, and it is influenced by both equivalence ratio and CH4 fraction when the proportion of CH4 is greater than 86%. This indicates that adding a small amount of C7H16 in the lean methane-air mixture can significantly promote methane ignition. As the addition of C7H16 increases, the effect of equivalent ratio is more significant. In the two regions on the right with an equivalence ratio of more than 1.0, the ID is mainly influenced by the CH4 fraction: it varies greatly with the CH4 mass fraction but only slightly with equivalence ratio. Hence, the effect of blend ratio is dominated in the rich fuel mixture. Only in the region with larger heptane concentration and richer fuel is the ignition process not sensitive to the blend and equivalence ratios. Figure 9 shows that the ID is mainly influenced by equivalence ratio for an equivalence ratio below 1.0, and the influence of pressure is obvious with an equivalence ratio of more than 1.0. It can be seen that the evolution of ignition delay time demonstrates similar trends at different blend ratios of methane and heptane. Moreover, the effect of pressure increases with an increasing proportion of CH4. To summarize the results so far, the NTC behavior of the fuel blend becomes stronger and the NTC temperature region expands as the fraction of C7H16 components increases. As for the wide ranges (T = 800−1200 K, P = 40−80 bar, ϕ = 0.2− 3.0, and mass fraction of CH4 from 70 to 98%), the temperature and equivalence ratio exert great influence on the IDs all the time, whereas the effect of pressure and blend ratio on the IDs depends on the condition. The effects of pressure and CH4 mass fraction are small when the temperature

proven by the longer IDs for CH4/air than those for C7H16 under similar conditions as discussed in Figure 3a. Figure 8 shows the ignition delay time versus the methane concentration and equivalence ratio. It was found that the

Figure 8. Simulated ignition delay time versus the CH4 mass fraction and equivalence ratio.

figure can be divided into four regions: (1) significant effect by only equivalence ratio, (2) effect by both methane mass fraction and equivalence ratio, (3) significant effect by only the methane mass fraction, and (4) less effect by both methane mass fraction and equivalence ratio. In detail, in the two regions on the left with equivalence ratio of less than 1.0, the ID is mainly affected F

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Figure 9. Simulated ignition delay time versus the pressure and equivalence ratios for (a) 70%, (b) 80%, and (c) 90% CH4 mass fractions.

is higher than 1100 K, and the effects are great when the equivalence ratio is greater than 1.0 but small with an equivalence ratio of lower than 1.0. As for the large CH4 fraction and lean fuel conditions (70−98% CH4 mass fraction, 0.2−1.0 fraction) relevant to the natural gas engine with diesel micro pilot method, the effect of pressure and CH4 mass fraction decreases with the proportion of CH4 increasing because the NTC behavior is weakened. 3.2. Sensitivity Analysis. A sensitivity study is performed to identify the dominate reactions associated with the ignition of the CH4/C7h16 mixture under the conditions of 70 and 90% CH4 concentration, ϕ = 0.5 and 1.0, at six different initial temperatures of 850, 900, and 950 K, which correspond to the temperature of the NTC regime, and 800, 1000, and 1200 K, which are set to investigate the effect of initial temperature. The normalized sensitivity coefficient is defined as15 S=

Table 2. Important Reactions for the Mixture Ignition Based on Previous Studies26,28,40

τ(2ki) − τ(0.5ki) 1.5τ(ki)

#

reaction

13 14 36 37 41 42 43 45 53 91 92 93 100

HO2 + OH ⇌ H2O + O2 2HO2 ⇌ H2O2 + O2 CH2O + OH ⇌ HCO + H2O CH2O + HO2 ⇌ HCO + H2O2 CH3 + O2 → CH2O + OH CH3 + HO2 ⇌ CH3O + OH CH3 + HO2 ⇌ CH4 + O2 2CH3 (+m) ⇌ C2H6 (+m) CH4 + OH ⇌ H2O + CH3 PXC7H15 → PXC5H11 + C2H4 SXC7H15 → PXC4H9 + C3H6 SXC7H15 → PXC6H12 + CH3 C7H16 + OH → SXC7H15 + H2O C7H16 + HO2 → SXC7H15 + H2O2 PXC7H15 + O2 ⇌ PC7H15O2 SXC7H15 + O2 ⇌ PC7H15O2 PC7H15O2 → PHEOOHX2 PHEOOHX2 + O2 → SOO7OOH1

102

where τ is ignition delay of the fuel blend and ki is the specific rate coefficient. A positive value indicates an inhibition effect on the total reaction and a negative value indicates a promotion effect. As the temperature change can directly reflect the ignition process, the top 18 greatest sensitivity value reactions based on the temperature sensitivity analysis have been chosen, which are also important in previous sensitivity analyses.26,28,40 These 18 reactions along with their kinetic parameters are listed in Table 2. The reactions can be divided into three parts:

105 106 107 108

A

b

E

+13

6.000e 2.500e+11 3.400e+09 3.000e+12 3.300e+11 1.800e+13 3.600e+12 1.813e+13 1.600e+07 2.500e+13 1.600e+13 4.000e+13 5.200e+09

0.000 0.000 1.200 0.000 0.000 0.000 0.000 0.000 1.830 0.000 0.000 0.000 1.300

0.00 −1242.83 −454.11 13073.61 8938.81 0.00 0.00 0.00 2772.47 28824.09 28322.18 33030.59 693.12

1.340e+13

0.000

17017.21

2.000e+12 2.000e+12 6.000e+11 5.000e+11

0.000 0.000 0.000 0.000

0.00 0.00 20458.89 0.00

reactions 13 and 14 are important for CH4; reactions 36−53 are important for both the combustion of CH4 and C7H16, and reactions 91−108 are relevant to the combustion of C7H16. G

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Figure 10. Normalized sensitivity of ignition delay at (a) 800 K, ϕ = 0.5, (b) 1000 K, ϕ = 0.5, (c) 1200 K, ϕ = 0.5, (d) 800 K, ϕ = 1.0, (e) 1000 K, ϕ = 1.0, (f) 1200 K, ϕ = 1.0 (g) 850 K, ϕ = 1.0, (h) 900 K, ϕ = 1.0, and (i) 950 K, ϕ = 1.0.

decreases appreciably with increasing equivalence ratio, thus weakening the promotion effect. This is because CH4 is mainly consumed by reaction 53 at low equivalence ratio as discussed in the following section. The effect of reaction 36 (CH2O + OH ⇌ HCO + H2O) changes from inhibition to promotion as the equivalence ratio is increased from 0.5 to 1.0 at T = 1200 K as shown in Figure 10c,f. The effects also change from inhibition to promotion as the temperature increases from 800 to 1200 K as shown in Figure 10d−f. As shown in Figure 10g−i, these three different initial temperatures of 850, 900, and 950 K correspond approximately to the beginning, middle, and end temperatures of the NTC regime, respectively. It can be noted that the sensitivity coefficients of the reactions associated with C7H16 ignition increase significantly when the initial temperatures are in the NTC regime, indicating that the effect of C7H16 reactions on the total reaction rate is increased. In the NTC regime, however, the sensitivity coefficients to these reactions change slightly with the initial temperature, i.e., the effect of initial temperature on the sensitivity coefficient is small. This is consistent with the results that the ID is changed little in the NTC regime under these conditions. 3.3. Rate of Production (ROP) Analysis. The reaction rate of production (ROP) is the important information to understand the mechanism of mixture fuel ignition and oxidation. Radicals like CH4 and CH2O are the main

Figure 10 shows the normalized sensitivity of ignition delay under various conditions. It can be seen in Figure 10a−c that the sensitivity coefficients to reactions 91−108 (reactions associated with C7H16 ignition) decrease significantly as the temperature increases from 800 to 1200 K, which indicates that both inhibition and promotion effects are weakened. Therefore, the effects of relevant reactions of C7H16 on the total reaction rate decrease with increasing temperature. The sensitivity coefficients of reactions important for both CH4 and C7H16 are greater than those of reactions associated with C7H16, so the ignition of mixture fuel is mainly dominated by these reactions. Reactions 42 (CH3 + HO2 ⇌ CH3O + OH) and 43 (CH3 + HO2 ⇌ CH4 + O2) are a pair of reactions competing for the HO2 radical, the sensitivity coefficient of reaction 43 is larger than that of 42 at T = 800 K, which leads to an inhibition contribution to the total reaction, and the sensitivity coefficient of R43 decreases to approximately that of 42 at T = 1200 K, leading to a comparable contribution to the total reaction. The sensitivity of reactions associated with C7H16 ignition decreases with an increasing proportion of CH4, meaning that the effect of n-C7H16 reactions on the total reaction rate decreases. Panels a−f in Figure 10 depict the effect of equivalence ratio on the sensitivity coefficient at 800, 1000, and 1200 K. It can be noted that the sensitivity coefficient to most of the reactions increases as the equivalence ratio is increased, but the sensitivity coefficient to reaction 53 (CH4 + OH ⇌ H2O + CH3) H

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Figure 11. Rates of production and consumption of (a) CH4, (b) CH2O, (c) HO2 and (d) OH with various CH4 mass fractions.

Figure 12. Rates of production and consumption of (a) SXC7H15, (b) PC7H15O2, (c) PHEOOHX2, and (d) OC7OOH with various CH4 mass fractions at 800 K and ϕ = 1.0.

intermediate products for the ignition of methane,15 and SXC7H15, PC7H15O2, PHEOOHX2, and OC7OOH are main

intermediate radicals for the reaction of n-heptane.28 In addition, HO2 and OH are the most important radicals that I

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Figure 13. Rates of production and consumption of (a) CH4, (b) SXC7H15, (c) CH2O, and (d) OC7OOH with various CH4 mass fractions at 800 K and ϕ = 0.5.

connect the reaction paths of n-heptane and methane.41 In Figure 11, as the CH4 mass fraction varies from 70 to 90%, the reaction rates of intermediate species of CH4, CH2O, HO2, and OH have no apparent change in the high temperature region (1500−2500 K), whereas they change significantly in the low temperature region (800−1200 K). Therefore, the increase in the C7H16 mass fraction has a significant influence on the CH4 low temperature reaction rates, whereas such an increase on the CH4 high temperature reaction rates is small. In addition, the consumption of methane occurs mainly in the high temperature region. In Figure 12, the ROPs of SXC7H15, PC7H15O2, PHEOOHX2, and OC7OOH (or C7KET) change appreciably with the CH4 mass fraction, and the reactions occur mainly in the low temperature region (800−1100 K), which means that the low temperature reaction of CH4 and C7H16 occurs first to release a lot of heat, and then triggers the CH4. Figure 12 shows that reaction rates of SXC7H15, PC7H15O2, and PHEOOHX2 increase significantly as the proportion of C7H16 increases, whereas the total reaction rates of SXC7H15, PC7H15O2, and PHEOOHX2 are always small, indicating that the rates of production and consumption are basically consistent. With the increase in the proportion of C7H16, the heat release rate of C7H16 combustion increases, which leads to the earlier ignition of methane and then the shorter ignition delay. The total reaction rate of OC7OOH increases noticeably as the proportion of C7H16 increases, and it produces OC7OOH at 800−830 K and consumes OC7OOH at 830− 880 K under these conditions. For the effect of the equivalence ratio on the ROP to be clarified, the rates of production and consumption of four important species are calculated at ϕ = 0.5: CH4 and CH2O,

the main intermediate products for the ignition of methane;15 SXC7H15, which is a radical after n-heptane dehydrogenation and important for both low and high temperature reactions; and OC7OOH, a key radical of the C7H16 low temperature combustion path.42 Comparing Figures 12 and 13, it can be noted that the reaction rates of these radicals decrease with decreasing equivalence ratio due to the lean fuel. The ROP profile of reaction 53 (CH4 + OH ⇌ H2O + CH3) exhibits a transition to that of the total as the equivalence ratio decreases, implying that CH4 is mainly consumed by reaction 53 for the lean fuel condition. CH2O is also mainly consumed by reacting with OH radical for the lean fuel condition. Furthermore, under such conditions, the reactions of CH4, CH2O, SXC7H15, and OC7OOH take place in a relative low temperature region compared with those in the rich fuel condition due to the lower heat release rate. Figure 14 shows the effect of initial temperature on the ROP of CH4, CH2O, SXC7H15, and OC7OOH. It can be seen that temperature has a significant influence on the ROP of these species as all the reactions have noticeable changes with increasing temperature. The reaction rates of CH4 and CH2O increase with increasing temperature, and the maximum reaction rates occur at a higher temperature. The reaction rate of OC7OOH, a key radical of the C7H16 low temperature combustion path, decreases significantly, which implies that the reaction rates of the n-heptane low temperature reactions decrease as the temperature increases. The reaction rates of reactions 92 and 93 are significantly increased, exceeding that of 106. Therefore, SXC7H15 is mainly consumed by pyrolysis (reactions 92 and 93) at high temperature. This conclusion has been discussed by Zhang et al.26 using C7H16, and it is still J

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Figure 14. Rates of production and consumption of (a) CH4, (b) SXC7H15, (c) CH2O, and (d) OC7OOH at various temperatures with 90% CH4 and ϕ = 1.0.

Figure 15. Rates of production and consumption of (a) CH4, (b) SXC7H15, (c) CH2O, and (d) OC7OOH in the NTC regime. K

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Figure 16. Rates of production and consumption of (a) CH4, (b) CH2O, (c) SXC7H15, and (d) OC7OOH with various CH4 mole fractions at 800 K and ϕ = 1.0.

4. CONCLUSIONS A numerical study has been conducted to examine the effects of temperature, pressure, equivalence ratio, and blend ratio on IDs of methane/n-heptane mixtures. Furthermore, the sensitivity analyses of various factors on the IDs under different conditions are studied to provide a theoretical basis for the study on ignition control of a dual-fuel engine. Moreover, the simulations are carried out based on the software CHEMKIN-PRO with a closed homogeneous reactor. The parameters include temperatures in the range of 800−1200 K, pressures in the range of 40−80 bar, and equivalence ratios in the range of 0.2−3.0. The main conclusions are summarized as follows: 1. NTC behavior occurs for the CH4/C7H16 fuel mixture, and it becomes stronger as the fraction of C7H16 components increases and the equivalence ratio increases. In the NTC regime, the sensitivity coefficients of reactions associated with C7H16 increase, indicating that the effect of C7H16 reactions on the total reaction rate is relatively high in that regime. Furthermore, the reaction rates of SXC7H15 and OC7OOH decrease with increasing temperature, resulting in decreasing IDs with increasing temperature in the NTC regime. 2. Temperature and equivalence ratio have a significant influence on the IDs over the wide range of conditions presented. The present result shows that both temperature and equivalence ratio have a profound effect on the IDs, especially at low temperature and low equivalence ratio. However, the effect of pressure and blend ratio depends on the condition. The effects of pressure and CH4 mass fraction are small when the temperature is higher than 1100 K, and such effects are significant when the equivalence ratio is greater than 1.0 but small with equivalence ratio is lower than 1.0. It is interesting

applicable for the CH4/C7H16 mixture. In addition, the initial temperature has an obvious influence on the reaction temperature regions of CH4, SXC7H15, and OC7OOH. SXC7H15 → PXC4 H 9 + C3H6

(92)

SXC7H15 → PXC6H12 + CH3

(93)

SXC7H15 + O2 ⇌ PC7H15O2

(106)

In general, the initial temperature and equivalence ratio have a significant influence on not only the reaction rate but also the reaction temperature region of all of these species. The effect of ϕ on the reaction temperature region of CH4 is greater than that of the initial temperature, whereas the initial temperature has more influence on the SXC7H15 and OC7OOH reaction area than ϕ, which indicates that the initial temperature and equivalence ratio have relatively large influence on the ignition of C7H16 and the combustion of CH4, respectively. Figure 15 shows ROP characteristics in the NTC regime. Three different initial temperatures of 850, 900, and 950 K correspond to the NTC regime as discussed above. The reaction rates of SXC7H15 and OC7OOH decrease with increasing temperature, resulting in increasing IDs with increasing temperature in the NTC regime, and the reaction rates of CH4 and CH2O increase with increasing temperature. The reaction rate of reaction 106 (SXC7H15 + O2 ⇌ PC7H15O2) decreases but is still larger than that of reaction 92 (SXC7H15 → PXC4H9 + C3H6), indicating that SXC7H15 is mainly consumed by reacting with oxygen under this condition. L

DOI: 10.1021/acs.energyfuels.7b04128 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels ORCID

that the ignition delay region can be divided into four different zones, as derived from the coupling effect of methane concentration and equivalence ratio. From the perspective of a dual-fuel engine, these results can be used as a reference to improve the ignition and combustion of natural gas engines using C7H16 (or diesel) as the pilot fuel. 3. A sensitivity study is performed to examine the effect of the main elementary reactions on ignition. At high temperature, the sensitivity coefficients of reactions 91−108 (associated with C7H16 ignition) are small; thus, the effects of reactions associated with C7H16 are small due to its fast reaction. The ignition is mainly controlled by the reactions associated with CH4 as their sensitivity coefficients are high because CH4 takes up most of the fuel. The sensitivity to reaction 53 (CH4 + OH ⇌ H2O + CH3) increases as the equivalence ratio decreases because CH4 is mainly consumed by this reaction at low equivalence ratio. 4. The study on rates of production and consumption indicates that the methane is ignited by the combustion of C7H16. The initial temperature and equivalence ratio have a significant influence on not only the production and consumption rates but also the reaction temperature region, which explains well the result above that the temperature and equivalence ratio have the greatest influence on the IDs.

Lei Zhou: 0000-0002-2686-569X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 91741119, 91641203, 51606133) and the Marine Low-Speed Engine Project (Phase I).

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Notation

P = pressure T = temperature ϕ = equivalence ratio τ1 = first stage ignition delay of C7H16 τ2 = second stage ignition delay of C7H16 Abbreviations

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APPENDIX: EFFECT OF N-HEPTANE WITH LARGE MOLE FRACTION ON THE ROP With 70, 80, and 90% CH4 mass fractions, the ROPs of CH4 and CH2O have no apparent change based on the CH4 mass fraction. This is because the amount of C7H16 addition is too little to significantly affect the oxidation of CH4 in the high temperature region. The molar mass of C7H16 is large, and that of CH4 is small; thus, by the calculation method of the mass fraction, the amount of C7H16 addition is too little. This indicates that, under the condition of C7H16 microignition (the mass fraction of pilot fuel is between 10 and 20% or even less), the effect of C7H16 quantity on combustion of CH4 in the high temperature region is small. For the effect of n-heptane addition on methane oxidation to be further elucidated, the ROPs of CH4, CH2O, SXC7H15, and OC7OOH were investigated at 70, 80, and 90% CH4 mole fractions. In Figure 1c, reactions 110 (PX2HEOOH → OC7OOH + OH) and 111 (OC7OOH → PX2HEPOO + OH) are promoted with the increase in n-heptane addition, producing more OH radicals. Subsequently, the more OH increases the reaction rates of reactions 53 (CH4 + OH ⇌ H2O + CH3) and 36 (CH2O + OH ⇌ HCO + H2O) in the temperature range of 800−1300 K as shown in Figure 16a,b, promoting methane oxidation. Interestingly, at high temperature, the reaction rates of methane decrease with increasing nheptane, meaning that the addition of n-heptane does not promote the high temperature reactions of methane. Therefore, the enhanced oxidation of CH4 by C7H16 addition is realized by the increase in OH radical production, which is supported by Zang et al.41 However, it only affects the low temperature reactions of methane.

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NOMENCLATURE

IDs = ignition delay times NTC = negative temperature coefficient ROP = reaction rate of production EGR = exhaust gas recirculation

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DOI: 10.1021/acs.energyfuels.7b04128 Energy Fuels XXXX, XXX, XXX−XXX