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Shock tube study of the auto-ignition of n-butane/hydrogen mixtures Xue Jiang, Youshun Pan, Wuchuan Sun, Yang Liu, and Zuohua Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02423 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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Shock tube study of the auto-ignition of n-butane/hydrogen mixtures Xue Jiang*, Youshun Pan, Wuchuan Sun, Yang Liu, Zuohua Huang*

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

KEYWORDS: hydrogen enrich, n-butane, binary mixture, auto-ignition, chemical kinetic

ABSTRACT: Shock-tube measurements and kinetic study on the auto-ignition of hydrogen/n-butane blends were carried out. The Aramco2.0 model was employed in numerical simulation, this model can well capture the auto-ignitions of the hydrogen/n-butane blends under all test conditions. The pressure dependence, equivalence dependence, and the influence of blending of the auto-ignitions for the pure hydrogen, pure n-butane and hydrogen/n-butane binary mixture have been studied. Negative pressure dependence of auto-ignition delay is obtained at the intermediate and low temperatures for the hydrogen and lean XH2 = 98% mixture. The auto-ignition of n-butane can be non-linearly enhanced by hydrogen addition. The auto-ignition of hydrogen was insensitive to the equivalence ratio, but ignitions of the n-butane and binary blends get longer with the rising fuel concentration. The ignition chemistry of hydrogen and n-butane were interpreted.

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1. INTRODUCTION

As energy and environmental issues become increasingly prominent, the utilization of alternative fuel received much attention. Hydrogen is a promising alternative fuel, it has some unique characteristics, such as clean emissions, broad flammability limit, fast flame speed and low minimum ignition energy 1. However, the pure hydrogen combustion may lead to some problems in the practical application, such as engine knocking, high combustion temperature and the high NOx emissions 2-3

, thus hydrogen has been blended with hydrocarbon fuels as the additive.

Researches on the hydrogen-hydrocarbon dual-fuel IC engine have been widely conducted, early studies can be traced back to the 1980s 4. Previous studies suggested that using the hydrogen/hydrocarbon dual fuel is a practical method to improve the IC engine performance and realize the cleaner combustion. Generally, hydrogen blending will lead to higher thermal efficiency and higher NOx emission on IC engine. With hydrogen addition, the combustion in IC engine becomes more complete and the combustion duration becomes shorter, which also results in the higher combustion pressures and temperatures

5-8

. However, engine knocking occurs if the growth in

pressure-rise rate becomes too remarkable

2, 9

. Compared with normal IC engine, the

hydrogen dual-fuel engine usually has the lower smoke and HC emissions since hydrogen blending can reduce the total carbon amount in the fuel and also lead to more completely premix

4, 10

. Furthermore, the higher combustion temperatures with

hydrogen blending are also helpful to the burn-off of particulate matter formed during the pilot combustion of the dual-fuel engine. Besides, many studies in the gas turbine 11-14

also pointed out that hydrogen can improve the burn stability; moreover, by


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applying some controlling strategies, the NOx emissions can be limited.

Fundamentally, many studies concentrated on the combustion of methane with hydrogen enrichment have been reported. Those fundamental combustion data can be used to guide the engine design, to verify the dual-fuel kinetic models, and to understand the inter-reaction between hydrogen and methane. The auto-ignition delays of hydrogen enriched methane were tested by applying the shock tubes and rapid compression machines 24-26

15-21

22-23

. In addition, by means of the stagnation flame

and spherically expanding flame

27-29

, the laminar flame speed of

hydrogen/methane was measured. Besides, some theoretical and numerical studies on the combustion characteristic of hydrogen/methane mixtures 30-33 were also conducted. Generally, hydrogen addition can enhance the laminar premixed combustion and auto-ignition of methane from low to high temperatures. Although the combustion properties of hydrogen/methane dual fuel have been intensively studied, the fundamental study on the hydrogen/hydrocarbons blends is far from comprehensive. So far, there are not many studies focusing on the combustion characteristic of hydrogen and higher order alkanes blends.

n-Butane, as a high octane number substitute fuel, has wide application prospects. It is the main component of liquid petroleum gas, the ingredient of natural gas, and also be used for gasoline blending. The laminar flame speeds of the hydrogen/n-butane binary fuel ( maximum hydrogen/fuel mass ratio is 6%) have been measured at the pressure of 1 atm, the equivalence ratio from 0.52 - 1.20, and initial temperature of 270 - 410 K by Sher and co-worker

34

using a flat flame burner. It is


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found that even with 5% hydrogen blending, the burning velocity can be significantly increased by about 35%. Sung et. al.

35

computationally studied the laminar burning

velocities of adding hydrogen to butane isomers (p = 1-20 atm, φ =0.7-1.4). They indicated that the hydrogen blending gives more remarkable effects for the fuel mixtures that deviate from stoichiometric conditions. Moreover, lean flammability limit were enhanced with hydrogen addition. More recently, Cheng et. al.

36

numerically investigated laminar premixed combustion of the hydrogen enriched C1 C4 n-alkanes based on one-step reaction assumption. The influence of hydrogen addition was explicated in three respects, including kinetics, Lewis number and adiabatic flame temperature.

For the auto-ignition features of the hydrogen enriched n-butane, up to now, only one available study has been reported with the emphasis on the effect of hydrogen blending

21

. The auto-ignition delays of hydrogen/n-butane blend at the equivalence

ratio of 0.5 and the temperatures from 1000 K to 1600 K were obtained. Although the lean burn technology has some advantages in practice, in this study, we extended the ignition delay research to the stoichiometric and rich conditions, the ignition behaviors of fuel mixtures in a broad regime of equivalence ratios can make guidance for the applications and can provide the targets for mechanism validation. More important, it is found that under the fuel-lean condition21, the ignitions of the neat hydrogen show the non-monotonic pressure dependence, that is to say, under certain conditions, the increase of pressure will inhibit the fuel ignition. Such negative pressure dependence was also observed for the lean XH2 = 98% mixture which is not common. However, it is found in the current study that although the stoichiometric


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and rich hydrogen mixtures still retain the non-monotonic pressure dependencies on the ignition, such phenomenon for the XH2 = 98% mixtures gradually disappeared as the equivalence ratio increases. Thus, kinetic analyses were employed to explore the changing kinetic for the ignition of the hydrogen and XH2 = 98% mixture as the equivalence ratio and pressure increases to obtain a more in-depth understanding of the ignition kinetic on the hydrogen enriched combustion. Furthermore, the influence of hydrogen addition was analyzed. The effects of equivalence ratio at various hydrogen addition levels were also interpreted.

2. EXPERIMENT AND SIMULATION

2.1. Experimental System

Details of the experimental device are presented in Ref.

17

. The test mixtures

were diluted by 80% argon, the XO2/XAr = 21%/79% mixture was considered as “air”. The experimental gas composition are shown in Table 1, the XH2 is the percentage of hydrogen in total fuel.

Briefly speaking, the auto-ignition delays were captured using a shock tube. The shock tube and mixing tank were evacuated below 1 Pa before the test. The driving gas in the experiment are helium and nitrogen. The pressure signal were recorded by four PCB 113B26 pressure transducers installed near the bottom of the driven section. The incident shock velocity were obtained by recording the time intervals of the shock wave to reach each pressure transducer. Gaseq software37 were used to calculate the ignition temperature. The OH* emission signals were captured during the measurement. The auto-ignition is established by the maximum-slope of OH*.


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The uncertainty of the experiment is evaluated by the standard root-sum-squares method 38. To be more specific, the largest temperature uncertainty is 20 K, the error in the auto-ignition delay measurements is 20% 39.

2.2. Chemical Kinetic Simulation

The simulation were conducted using CHEMKIN II

40-41

and zero dimensional

model. The pressure rise of 4.2%/ms17 was taken into account in the simulation42. Aramco2.0 model

43-48

, been updated in 2016 based on the previous version48, was

adopted in this study. This model involves 493 species and 2716 reactions, the C3 sub-model (butene43, 45 and propene

44, 46

) and oxygenated fuel

47

sub-model have

been updated. Aramco2.0 model was tested by a large number of experimental data, however, not yet been validated against the auto-ignitions of the hydrogen enriched n-butane.

3. RESULTS AND DISCUSSION

3.1 Influence of Pressure

Fig.1 (1) - (12) shows the effect of pressure of the auto-ignition of hydrogen/n-butane mixtures under stoichiometric and rich conditions. For the neat hydrogen mixture, the auto-ignition delays were measured in ref.

49

using the device

at the pressures of 1.2 and 10 atm, thus the previous data of hydrogen auto-ignition were also used in the current study. The numerical predictions by the Aramco2.0 model are also shown in the figures, it is found that the simulation results and the suto-ignition delay data of hydrogen/n-butane mixtures agree well. All the measured


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auto-ignition data at various hydrogen addition levels are given in the appendix.

It can be seen from Fig.1 (1) - (2) that, for the XH2 = 100% mixture (neat hydrogen) under the stoichiometric and rich conditions, the auto-ignition delays give the non-monotonic variation tendency with pressure. In particular, pressure rise leads to the faster auto-ignition at high temperatures, indicating that at high temperature condition, the increases of pressure increases the reactivity. However, in the low and intermediate temperature region, the pressure rise will lead to slower auto-ignition, indicating that the increases in pressure decrease the reactivity.

Note that although the ignition delays for the lean XH2 = 98% mixture showed the hydrogen-like non-monotonic pressure dependence 21, such appearance no-longer retained at φ =1.0 and 2.0 conditions, as shown in Fig.1 (3) - (4). Under the φ =1.0 and 2.0 conditions in the current study, for the XH2 = 98% mixture, the pressure rise always promote the reactivity, such promoting effect becomes less pronounced as temperature decreases.

When further decrease the hydrogen fractions, the ignition delay times of the binary mixture resembles more and more that of n-butane. The stoichiometric and rich XH2 = 95%, 90%, and 70% mixtures show the similar pressure dependence as n-butane, gives in Fig. 1 (5) - (10),

As the Figs.1 (11) - (12) show, for XH2 = 0% mixture (neat n-butane), auto-ignition delays give the Arrhenius relationship with respect to temperature, faster auto-ignition can be obtained as the pressure increases. This phenomenon is common for hydrocarbon fuels.


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3.1.1 Pressure dependence of the ignition delays of H2

The non-monotonic effect of pressure was observed in hydrogen ignition in the current study. The negative pressure dependence in combustion is not unusual, but have been reported by Burke et. al.50 in the measurements and simulations of the mass burning rates of H2 flames and the H2/CO flames, under restricted conditions (when the pressure is high and the low flame temperature is low). Moreover, the global reaction order in the corresponding negative pressure dependence region is found to be negative. Kinetic analyses were undertaken to understand the pressure dependence on the homogeneous auto-ignition of hydrogen.

Fig.2 shows the simulated pressure dependence of ignition delays for hydrogen at the equivalence ratio of 2.0, the temperatures of 1000 K and 1300K, and the pressures from 2 atm to 30 atm using the Aramco2.0 model. For the fuel-lean and fuel-stoichiometric conditions, similar phenomenon was obtained. According to the simulation, at higher temperature of 1300 K, the auto-ignition delay of the hydrogen monotonically decreases as the pressure increase from 2 atm to 30 atm. However, at the lower temperature of 1000 K, when rising the pressure from 2 to 15 atm, the auto-ignition delays of hydrogen become obviously longer as the pressure increases, that is to say, the pressure dependence on the ignition is found to be negative in this region. When further rising the pressure from 15 to 30 atm, the ignition delay times gradually decreases with the elevated pressures, indicating that the pressure dependence turned into positive.

The brute force sensitivity analyses were carried out


21

to find out the reactions

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that influence the ignition, shown in Fig. 3. The positive coefficient indicating the inhibiting effect on auto-ignition and vice versa. Besides, Figs. 4 - 6 show the H, HO2 and H2O2 consumption rates normalized through the total rates of consumption of hydrogen ignition.

It is found in the sensitivity analysis in Fig.3 that for the rich hydrogen mixture, the auto-ignition is dominated by H + O2 O + OH (R5) under all pressures. Especially, at 2 atm, the sensitivity index of (R5) is significantly higher than other reactions. This indicates that the auto-ignition is more sensitive to (R5) at 2 atm relative to that at 10 atm and 20 atm. Meanwhile, at 2 atm, the reaction H + O2 (+M) HO2 (+M) (R34) is the sensitive ignition inhibiting reaction. (R34) is the competing reaction of (R5). Nevertheless, at 10 and 20 atm, the HO2 relevant reactions HO2 + H H2 + O2 (R28) and HO2 + HO2H2O2 + O2 (R32) give the high positive sensitivity indexes thus inhibiting the ignition.

According to the H rate of consumption analysis in Fig. 4, at 2 atm and 1000 K, during the induction period before the ignition, the H radicals are mainly consumed from the comparable fluxes of (R5) and (R34). When the pressure increases to 10 atm, the reaction (R34) becomes more frequent and replaced the reaction (R5) as the leading reaction consuming the H and O2. When further increase the pressure to 20 atm, the fraction of H consumption by the reaction (R34) becomes even higher than that at 10 atm.

Therefore, the negative pressure dependence on hydrogen ignition is closely concerned with the competing between (R5) and (R34). The reaction (R5), showing


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strong promotion on auto-ignition, is of a high value of activation energy and exhibit strong temperature dependence. Thus, at the high temperature of 1300 K, the reaction (R5) is always dominating the consumption of H radical, thus the auto-ignition delay of hydrogen monotonically decrease with the rising pressure. However, at 1000 K, the reaction (R5) becomes less pronounced because of the decrease of temperature. Note that (R34) is the third-body involved chain propagating reaction showing the inhibiting effect on the ignition, the yield HO2 from the reaction (R34) will further decrease the global reactivity through the reactions (R28) and (R32). Therefore, at the intermediate temperatures and high pressures, where the chain-branching of the reaction (R5) is suppressed while the chain-propagating of the reaction (R34) is promoted, the negative pressure dependence on the auto-ignition of hydrogen appears.

In the same way, Burke et. al.50 also found in the experiments of the hydrogen flame that when using the CO2 (higher collider efficiency) as dilution gas instead of Ar (lower collider efficiency), the negative pressure dependence on the burning rate can be strengthened. This indicates that the negative pressure dependence of hydrogen combustion is closely related to the third-body involved reaction, which is consistent with the findings of this study.

For the high pressure ignition of hydrogen, owing to the enlarged flux of the reaction (R34), the productions of HO2 and H2O2 significant increased, the corresponding kinetics become gradually important. Therefore, the rate of consumption of HO2 and H2O2 were also analyzed in Fig. 5 and 6. Note that at 1000 K and 2 atm, Fig. 5 (1) , the HO2 radicals were mainly reacted with the H radical


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through the competing channels of HO2 + H H2 + O2 (R28), ignition promoting, and HO2 + H OH + OH (R27), ignition inhibiting. Through the reaction (R27), the relatively inactive HO2 radical was consumed, while two active OH radicals were produced therefore the ignition was promoted. The reaction (R28) is the dominating ignition inhibiting reaction at 10 atm and 20 atm, but becomes much insensitive at 2 atm, gives in Fig. 3. As the pressure increases, another reaction path is introduced, the HO2 radicals were also consumed through HO2 + HO2H2O2 + O2 (R32) at 10 atm and 20 atm. Since (R32) is a chain propagating in essence, increase its reaction rate will result in the increase in ignition delay. As shown in Fig. 3, (R32) is showing the gradually increased positive sensitivity index as the pressure rising from 10 to 20 atm, indicating the more pronounced ignition inhibiting effects form (R32) as pressure increases.

Note that for the ignition of hydrogen at 10 atm and 20 atm, in addition to the reaction (R5), the reaction H2 + HO2 H2O2 + H (R23) is recognized as the second important reaction that promotes the ignition, Fig. 3 (2) and (3). Through this reaction channel, the hydrogen and relative inactive HO2 was consumed to produce the active H radical and H2O2. The produced H2O2 from the reactions (R23) are then proceed by the third-body reaction H2O2 (+M) OH + OH (+M) (R21) and the chain-propagating reaction H2O2 + HH2O+OH (R22), among which the (R21) plays a leading role, shown in Fig. 6. The (R21) is the third-body involved chain-branching reaction which becomes more frequent at high pressures. When rise the pressure from 10 to 20 atm, the sensitivity of (R23) and (R21) all increase with pressure, lead to the increased ignition promoting tendency, this explained the reason


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why the ignition of hydrogen at 20 atm again becomes slightly faster than that at 10 atm, shown in Fig. 2.

3.1.2 Pressure dependence of the ignition delays of XH2=98% mixture

The auto-ignition of hydrocarbons usually becomes faster with the increase of pressure. However, in the current study, it is found that for the ignition of the XH2 = 98% mixture, the non-monotonic pressure dependence was observed at the fuel-lean condition, but gradually disappeared as the equivalence ratio increases, Fig. 1 and Fig. 7. In the above pressure dependence analysis of hydrogen ignition, is found that the competing of H radical between the reactions (R5) and (R34) results in the negative pressure dependence of hydrogen. To further understand the pressure dependence of hydrogen enriched combustion under various equivalence ratios, the sensitivity analysis and rate of consumption analysis for the XH2 = 98% mixture was conducted.

Sensitivity analysis showed that for the XH2= 98% mixture at 2 atm, gives in Fig. 8 (1) and (3), the reaction (R5) is the most remarkable ignition promoting reaction while the reaction (R978) is the most inhibiting one, indicating that the auto-ignition of the hydrogen/n-butane binary mixtures is controlled by the kinetic of both hydrogen and n-butane. While at the higher pressure of 10 atm, Fig. 8 (2) and (4), the ignition inhibiting effect from the reaction (R28) becomes significant, similar as that of the hydrogen ignition at 10 atm.

It can be see clearly from the H rate of consumption in Fig. 9 (1) and (3) that, at 2 atm and 1000 K, the H radical rate of consumption of during the ignition induction time is dominated by the reaction (R5), (R34), and (R978). At 10 atm, however, note


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that the reaction (R34) is the dominating reaction under the fuel lean condition, Fig. 9 (2), the governing kinetic of which leads to the negative pressure dependence on the auto-ignition of XH2 = 98% mixture. As the equivalence ratio increases from 0.5 to 2.0, the n-butane relevant kinetic becomes important due to the increased fuel concentration. Therefore, under the fuel-rich condition, Fig. 9 (4), the large proportion of the H radical was consumed by the (R978). At the same time, the consumption rate by the (R34) obvious decreases, thus, for the XH2 = 98% mixture, the negative pressure dependence no longer existed under the rich condition.

3.2 Influence of Hydrogen Enrichment

It is observed in Fig 10. (1) - (4) that the auto-ignition of the hydrogen/n-butane mixtures become faster as the hydrogen fraction rises at both the stoichiometric and rich conditions at 2 atm and 10 atm. Fig. 11 shows the relationship between the auto-ignition delay and XH2. It is found in Fig. 11 that in all conditions, as XH2 increases, the ignition delay time shows the gradual decrease initially, the auto-ignition delays of the XH2 = 70% blends only show the moderately decrease relative to n-butane. Once continue increasing the XH2, the influence of hydrogen addition becomes gradually significant. The auto-ignition delay gives the rapid decrease when the hydrogen blending ratio XH2 > 90%.

Sensitivity analysis and the H rate of consumption analysis were conducted to understand the non-linear promoting effect of hydrogen addition. Fig. 12 shows the sensitivity analysis for the XH2 =0%, 70%, 98% and 100% mixtures. Fig. 13 shows the main consumption flux of H radicals ( (R5) and (R978)) during the ignition


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induction time at 1300 K, 10 atm, and φ = 2.0.

Gives in Fig. 12 , as expected, the (R5) is always the dominating ignition promoting reaction for the ignition of all the mixtures. Especially, for the hydrogen (XH2 = 100%), Fig. 12 (1), the (R5) occupied the absolutely leading position, since the absolute value of sensitivity coefficient of which is obviously larger than that of other reactions. Besides, other hydrogen molecule relevant reaction H2 + M H + H+ M (R1), H2 + O H + OH (R2) and H2 + OH H + H2O (R3) are also highly sensitivity to the ignition delays. In addition, for the hydrogen system, the sensitivity coefficients of the ignition inhibiting reactions are much smaller relative to the promoting ones, indicating the weak ignition inhibiting effects. Therefore, hydrogen is highly reactive at high temperature.

When a small amount of n-butane adds to the hydrogen system, such as XH2 = 98%, (R5) is still dominating the system reactivity. While the reaction CH3 + H2 CH4 + H (R44) becomes the second important ignition promoting reaction. Note that the reaction (R44) is usually been deemed to be the H consumption path in the hydrogen/ hydrocarbon system 51 and lead to reduced reactivity, however, it is found in current study that when the concentration of hydrogen is high enough, the equilibrium of (R44) changes and lead to the promoted reactivity. Through this reaction, the active H radical can be yield and promote the ignition. Besides, it inhibits the termination efficiency of the CH3 + H (+M) CH4 (+M) (R43), CH3 + HO2 CH4 + O2 (R49), and CH3 + CH3 (+M) C2H6 (+M) (R194), by competing for the CH3 radicals thus promote the ignition. The reaction (R44) becomes less sensitive in the


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n-butane ignition where the concentration of hydrogen is not high enough. Note that even with 2% n-butane addition, the reaction (R978) becomes the most inhibiting reaction for the XH2 = 98% system, this reaction is very competitive in competing for the H radical with (R5). This is mainly due to that the C - H bond in n-butane have lower bond dissociation energy (101.5 kcal/mol for the primary and 98.5 kcal/mol for the secondary) relative to that of the O = O bond (118 kcal/mol) in oxygen. In addition, there are ten C - H bond in the molecular structure of n-butane, which has a higher collision efficiency with the H atom relative to the oxygen, which only has one O = O bone in the structure. Therefore, with a small amount of n-butane addition, saying that the XH2 = 98% mixture, although the hydrogen kinetic is still governing, the ignition was influenced by the n-butane chemistry.

When 70% hydrogen was added to the n-butane system, shown in Fig. 12 (3), although the proportion of hydrogen still accounts for 70% of the total fuel, the variation trend of sensitivity coefficients become similar to that of n-butane. Same as that of the neat n-butane, Fig. 12 (4), the reactions (R978) and CH3 + HO2 CH4 + O2 (R49) are the dominating ignition inhibiting reactions. Although (R5) is still the most sensitive reaction, the dominance is no longer as obvious as that of the hydrogen system, the sensitivity coefficients of other ignition promoting and/or inhibiting reactions become comparable to (R5).

For the H radical consumption, shown in the Fig. 13, generally, the rate (R5) reduces non-linearly with the rising n-butane fraction, while that through the reaction (R978) increases non-linearly correspondingly. When adding 70% hydrogen into the


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n-butane, the H radical consumption rate by the reactions (R5) and (R978) do not show obvious changes relative to the n-butane. When further increase the hydrogen addition level, the growth of H consumption from the reaction (R5) become apparent. Correspondingly, the H consumption from the reaction (R978) decreases. Finally, the auto-ignition delays gives the non-linearly dependence on the hydrogen addition level.

Therefore, hydrogen addition perturbs the n-butane ignition process and leading to a non-linear promoting effect, this is mainly due to the alternating from the n-butane dominated kinetic to the hydrogen dominated kinetic with the increasing hydrogen concentration. More specifically, this is mainly due to the competing between the reactions (R5) and (R978). When 70% hydrogen adds the n-butane, the system is still governing by the n-butane kinetic, the consumption flux of the reactions (R5) and (R978) does not change much relative to that of the n-butane ignition, thus the promotion of hydrogen enrichment is not significant. When further increase the hydrogen addition level, the hydrogen chemistry took over and become dominating, (R5) becomes more frequent thus the ignition was significantly promoted at higher hydrogen addition level. Fig. 14 illustrates the high temperature ignition of methane17, ethane18 , propane19 , and n-butane as the function of hydrogen addition level from the literature and the present study. Although those comparisons were conducted under different experimental conditions, the influence tendency of hydrogen enrichment can be illustrated. Note that for the ethane, propane and n-butane , as XH2 increases,


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auto-ignitions were promoted non-linear. However, the influence of hydrogen addition on methane auto-ignition is approximate to linear, such linear tendency was also observed in the low temperature RCM measurement 22 .

As observed in the current study, for n-butane auto-ignition, the non-linear enhancement of hydrogen addition is closely related to the competition of H radicals between (R5) and the H-atom abstraction of n-butane from (R978). Similarly, the previous researches18-19 indicated that when the ethane and propane were added to hydrogen, the fuel molecule H-atom abstraction reactions, namely the C2H6 + H C2H5 + H2 and C3H8+H H2 + nC3H7 become the most sensitive ignition inhibiting reactions, respectively. Pan et. al.

18

also found that during the auto-ignition of

ethane/hydrogen, with the increase of hydrogen fraction, (R5) and C2H6 + H C2H5 + H2 are showing a non-linear competitive relationship on the H rate of consumption. Therefore, it can be seen that the non-linear influences of hydrogen enrichment on ethane, propane, and n-butane ignition are with close links to the non-linear competing of H radical between the chain-branching of (R5) and the H-atom abstraction of fuel molecule. However, for the hydrogen/methane system, the chain-termination reaction CH3 + CH3 (+M) C2H6 (+M) is playing the governing inhibitory role17, 22, the rate of which is largely relevant to the methane concentration. Therefore, the influence on auto-ignition of hydrogen blending to methane is almost linear. 3.3 Influence of Equivalence Ratio

The equivalence ratio dependencies on hydrogen/n-butane auto-ignition at 10


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atm are given in Fig. 15 (1) - (6), the similar tendencies were also observed at 2 atm. Note that for the neat hydrogen mixtures (XH2 = 100%), the lean, stoichiometric, and rich hydrogen mixtures show almost identical auto-ignition delays. For the XH2 = 0% mixtures, the auto-ignitions show the Arrhenius dependence on temperature which become longer as the equivalence ratio increases. With n-butane addition, the XH2 = 98%, 95%, 90% and 70% mixtures all exhibit the n-butane-like equivalence ratio dependence.

Fig. 16 (1) - (3) show the top 12 most sensitive reactions of the auto-ignition of the XH2 = 100%, 98% and 0% mixtures. Note that the auto-ignition of hydrogen is mainly governed by the consumption and production of the H radical since the ignition delays are highly sensitivity to the reactions (R5), (R2) and (R3) , shown in Fig. 16 (1). For the hydrogen and XH2 = 98% mixture, the sensitivity index of (R5) grows with the rising equivalence ratios due to the increased hydrogen concentration. As shown in the rates of consumption of H radical in Fig. 17, at 1300 K and 10 atm, H radicals are predominantly consumed by the (R5) at all the equivalence ratios. In addition, the reaction (R34) is another major consumption channels of the H radical in current conditions, although not shown in the top 12 sensitive reactions. It will lead to the reduced reactivity because it is the chain propagating reaction competing with (R5) and produced the relative inactivity HO2. Note the H radical consumption rates by the reaction (R5) and (R34) are showing no significant changes with the equivalence ratio, especially during the initial stage, this can explain the negligible influence of equivalence on hydrogen auto-ignition.


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For the XH2 = 0% mixture, gives in Fig. 16 (3), the most important reactions that giving the acceleration and restrain effect on the ignition are the reactions (R5) and (R978), respectively. Note that the reaction (R5) becomes more favored with the reduced the equivalence ratio because of the increased oxygen concentration. According to the H rate of consumption in Fig.18, during the ignition inducing time, (R978) initially dominates the consumption of H radical. Meanwhile, the contribution of the reaction (R5) is relatively small at the beginning, but show a dramatically increased flux just before the ignition. The ignition promoting reaction (R5) becomes more pronounced under higher oxygen concentration. Meanwhile, the ignition inhibiting reaction (R978) becomes more pronounced under higher n-butane concentration. Therefore, the auto-ignition delay s of n-butane are increased with increasing equivalence ratios.

For the XH2 = 98% mixture, as shown in Fig. 16 (2), although the reactions (R2), (R3) and (R5) still show the high sensitivity, just like hydrogen, the H-atom abstraction reaction of n-butane molecule, C4H10 + H SC4H9 +H2 (R978) becomes the remarkable inhibiting reaction under all equivalence ratios. For the H radical rate of consumption of the XH2 = 98% mixture, Fig. 19, the reaction (R5) and (R978) are still the dominating reaction, similar equivalence ratio dependence of (R5) and (R978) as n-butane were observed for the XH2 = 98% mixture. Note that for the lean XH2 = 98% mixture (φ = 0.5), the H racial consumption rate from (R5) is initially higher than that of (R978) and is more effective than (R978) during the whole ignition inducing process. However, under the stoichiometric and rich conditions (φ = 1.0 and 2.0), the H racial rates of consumption from (R978) are higher than that of the


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reaction (R5) at the very beginning, but the consumption flux of reaction (R5) then overtakes that from reaction (R978) as time proceeds. Therefore, the governance of reaction (R5) under the lean condition will lead to shorter ignition delay time while the governance of (R978) with the increasing fuel concentration will lead to longer ignition delay. It is also found that even with a small amount of n-butane addition to the hydrogen (the XH2 = 98% mixture), the H radical consumption rate of reaction (R34) becomes much smaller relative to that of reaction (R978) and (R5), this reaction becomes less effective with the rising equivalence ratio.

Generally, the insensitive equivalence ratio dependence of hydrogen ignition is controlled by the hydrogen chemistry through (R5) which is insensitive to the equivalence ratio. Meanwhile, the ignitions of n-butane and hydrogen/n-butane binary mixture at different equivalence ratio are governing by the competition of the n-butane kinetic through the fuel-molecule H-atom abstraction (R978) which lead to reduced reactivity, and the chain-branching of (R5) which leads to increased reactivity.

5. CONCLUSIONS

The findings and conclusions are as follows.

(1) The measured auto-ignition delays of hydrogen, n-butane and their binary blends agree well with the numerical predictions from the Aramco2.0 model under all test conditions.

(2) The pressure dependence of hydrogen, n-butane and hydrogen/n-butane


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auto-ignition have been studied. At high temperatures, the auto-ignition delay of hydrogen is experimentally observed to reduce with the increasing pressure. While at the intermediate and low temperatures, negative pressure dependence of auto-ignition is obtained from lean to rich conditions, where the ignition of hydrogen becomes slower with the rising pressure. A small amount of n-butane adds to the hydrogen, saying that the XH2= 98% mixture, did not obviously influence the negative pressure dependence only under the lean-condition. However, such negative pressure dependence of ignition no-longer retained for the XH2= 98% mixture under the stoichiometric and rich conditions. When further increase the n-butane blending ratio, the pressure dependence of the binary mixture shows the n-butane like behavior, where the auto-ignition delays become faster with the rising pressure. Sensitivity analysis and rate of consumption analysis indicate that the competing channels of the (R5) and (R34), are responsible for the negative pressure dependence of the hydrogen ignition, and hydrogen dominated ignition. The influence of n-butane chemistry through the enlarged reaction path of (R978) with the increase of fuel concentration, which compete for the fate of H radical with (R5) and (R34) thus the negative pressure dependence disappeared.

(3) The non-linear promoting effects of hydrogen blending on n-butane auto-ignition were experimentally observed. Similar non-linear promoting effects were also observed in the high temperature ignitions of ethane and propane. Indeed, according to the kinetic analysis, such phenomenon can be attributed to the alternating from the hydrocarbon dominated kinetic to the hydrogen dominated kinetic with the increasing of hydrogen fraction. More specifically, the non-linear competing of H


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radical between the (R5) and the H-atom abstraction of the ethane, propane, and n-butane fuel molecule, respectively. However, for the hydrogen/methane system, the promoting effect of hydrogen addition is almost linear. This owing to the governing of chain-termination reaction CH3 + CH3 (+M) C2H6 (+M), the rate of which is relevant to the methane concentration.

(4) The equivalence dependence of the auto-ignition for the hydrogen, n-butane and their mixtures have been studied experimentally and numerically. Although the ignition of hydrogen did not remarkably vary with the changing equivalence ratio, the auto-ignition for the n-butane and hydrogen/n-butane binary mixture becomes slower with the increasing equivalence ratio. During the auto-ignition of hydrogen, the H radical was mainly consumed through the reaction (R5), the rate of which basically unaffected by the equivalence ratio, thus lead to identical ignition delays. For the n-butane and hydrogen/n-butane binary mixture, the ignition is governed by the n-butane kinetic through (R978), which leads to the slower auto-ignition delay times as the fuel concentration increase.


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FIGURES


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Auto-ignition delay data and simulations of n-butane/hydrogen mixtures at different hydrogen addition levels.


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Figure 2. Simulated pressure dependence of ignition delays for hydrogen at the equivalence ratio of 2.0 and temperatures of 1000 K and 1300K using the Aramco2.0 model.


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Figure 3. Sensitivity index of the auto-ignition of the XH2 =100% mixtures.


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Figure 4. H radical consumption rate of the XH2 =100% mixtures.


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Figure 5. HO2 radical consumption rates of the XH2 =100% mixtures.


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Figure 6. H2O2 radical consumption rates of the XH2 =100% mixtures.


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Figure7. Simulated pressure dependence of ignition delays for XH2=98% mixture using the Aramco2.0 model.


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Figure 8. Sensitivity index of the auto-ignition of the XH2=98% mixtures.


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Figure 9. H radical consumption rates of the XH2 =98% mixtures.

Figure 10. Influence of hydrogen blending (lines: Aramco2.0 model).

Figure 11. Simulated relationship between the auto-ignition delay and XH2


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Figure 12. Sensitivity index of the auto-ignition of the XH2=100%, 98%, 70% and 0% mixtures.


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Figure 13. Reactions with the highest H radical consumption rates of the XH2 = 0%, 70%, 90%, 95% and 98% mixtures. The dash dot lines represent the moment of ignition.

Figure 14. Influences of hydrogen blending on the auto-ignition of C1-C4 alkanes. (methane/hydrogen17 : 1250 K, 10 atm, and φ =0.5; ethane/hydrogen18 : 1100 K,16 atm, and φ = 1.0; propane/hydrogen19 : 1180 K,10 atm, and φ = 1.0; n-butane//hydrogen: 1200 K, 10 atm, and φ = 1.0 )


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Figure 15. Influence of equivalence ratio on auto-ignition.


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Figure 16. Sensitivity index of the auto-ignition of the XH2=0%, 98% and 100% mixtures.


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Figure 17. H radical consumption rates of the XH2 =100% mixtures.


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Figure 18. H radical consumption rates of the XH2 = 0% mixtures.

Figure 19. H radical consumption rates of the XH2 = 98% mixtures.


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Energy & Fuels

TABLES Table 1. Composition of fuel mixture Mixtures

XC4H10 (%)

XH2 (%)

XO2 (%)

XAr (%)

0

5.915

2.957

91.128

0.101

4.959

3.137

91.803

0.208

3.95

3.326

92.516

4. 90% H2 10% C4H10

0.321

2.885

3.526

93.268

5. 70% H2 30% C4H10

0.502

1.171

3.848

94.479

6. 100% C4H10

0.626

0

4.069

95.305

0

9.13

2.282

88.588

0.162

7.915

2.504

89.419

0.344

6.541

2.754

90.361

10. 90% H2 10% C4H10

0.553

4.973

3.039

91.435

11. 70% H2 30% C4H10

0.926

2.162

3.551

93.361

12. 100% C4H10

1.213

0

3.942

94.845

φ

1. 100% H2 2. 98% H2 2% C4H10 3. 95% H2 5% C4H10

1.0

7. 100% H2 8. 98% H2 2% C4H10 9. 95% H2 5% C4H10

2.0

AUTHOR INFORMATION Corresponding Author * xuejiang1128@ xjtu.edu.cn (X. Jiang). * zhhuang@ xjtu.edu.cn (Z. Huang).

Tel: 0086-29-82665075


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Funding Sources This work is supported by National Natural Science Foundation of China [grant numbers 51506164 and 91441203] and China Postdoctoral Science Foundation [grant number 2015M570831]. REFERENCES (1) Karim, G. A. Hydrogen as a spark ignition engine fuel, International Journal of Hydrogen Energy 2003, 28, (5), 569-577. (2) Das, L. M.; Gulati, R.; Gupta, P. K. A comparative evaluation of the performance characteristics of a spark ignition engine using hydrogen and compressed natural gas as alternative fuels, International Journal of Hydrogen Energy 2000, 25, (8), 783-793. (3) Roy, M. M.; Tomita, E.; Kawahara, N.; Harada, Y.; Sakane, A. Performance and emission comparison of a supercharged dual-fuel engine fueled by producer gases with varying hydrogen content, International Journal of Hydrogen Energy 2009, 34, (18), 7811-7822. (4) Rao, B. H.; Shrivastava, K. N.; Bhakta, H. N. Hydrogen for dual fuel engine operation, International Journal of Hydrogen Energy 1983, 8, (5), 381-384. (5) Santoso, W. B.; Bakar, R. A.; Nur, A. Combustion Characteristics of Diesel-Hydrogen Dual Fuel Engine at Low Load, Energy Procedia 2013, 32, (1), 3-10. (6) Lata, D. B.; Misra, A. Experimental investigations on the performance of a dual fuel diesel engine with hydrogen and LPG as secondary fuels, International Journal of Hydrogen Energy 2010, 35, (21), 11918-11931. (7) Ma, F.; Wang, Y.; Liu, H.; Li, Y.; Wang, J.; Zhao, S. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas


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