Kinetic Modeling Study of the Effect of Iron on Ignition and Combustion

Dec 2, 2016 - It is known that the ignition of a n-C7H16/air mixture is characterized by a two-stage ignition phenomenon.(27, 28) Figure 2 shows typic...
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Kinetic Modeling Study of the Effect of Iron on Ignition and Combustion of n‑Heptane in Counter-flow Diffusion Flames Mingming Zhu,* Zhezi Zhang, and Dongke Zhang Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia S Supporting Information *

ABSTRACT: A kinetic modeling study of the effect of iron on the ignition and combustion characteristics of diesel, modeled as n-heptane, in compression ignition engines was carried out using CHEMKIN PRO. The ignition was simulated using the SENKIN code, and combustion was modeled using the OPPDIF code. The kinetic models incorporated n-heptane mechanisms involving 159 species and 1540 reactions and iron reaction mechanisms of 7 iron species and 46 reactions. It was found that small amounts of iron in the fuel significantly reduced the ignition delay time. The ignition delay time decreased with an increasing iron concentration. A reaction pathway analysis showed that the ignition was promoted as a result of an early injection of the OH radicals. It was also showed that the addition of iron increased the peak flame temperature of n-heptane in the counter-flow diffusion flame and reduced the maximum mole fractions of H and O in the peak flame region as a result of the catalytic recombination cycles involving FeO, Fe(OH)2, and FeOH. The reaction rates of H + O2 ⇔ O + OH and CO + OH ⇔ CO2 + H in the peak flame region were found to increase, which is considered to be responsible for the increased peak flame temperature. time by a factor of 2−3 for H2/air flames. Matsuda23 found that Fe(CO)5, in the order of a few 100 ppm, greatly accelerated the consumption of CO, and in recent shock-tube studies of CH4/ O2/Ar mixtures, Park at al.24 found that Fe(CO)5 at dosing ratios of 500, 1000, and 2000 ppm in the mixture shortened the ignition delay time, indicating a promotion effect of the ironbased additive. In the case of the own recent work of the authors using a small diesel engine, the brake-specific fuel consumption was increased by up to 4.2% and the smoke emission was reduced up to 40% with the addition of the ferrous picrate catalyst (FPC) in diesel fuel.7−9 A set of comprehensive laboratory tests, including a phenomenological study of the combustion of single-diesel and biodiesel droplets,9 found that the use of the FPC shortened the burnout time and increased the burning rate and the flame temperature. It was also found that ferrous picrate decomposed and released iron atoms into the flame during the droplet combustion process.9 Therefore, it was proposed that the iron atoms presented in the flame promoted the combustion rate of diesel fuel. However, there is a general lack of literature reports about the detailed mechanisms of the effects of iron atoms in diesel ignition and combustion. Against this backdrop, the present kinetic modeling effort, building on the previous experimental work of the authors, was devoted to investigate the chemical effects of gas-phase iron species in the ignition and combustion of n-heptane as a model fuel for diesel to elucidate the mechanisms of the FPC in promoting diesel combustion in CI engines.

1. INTRODUCTION There has been intensive research over the past few decades aiming to search for new techniques to improve fuel efficiency and abate emissions of compression ignition (CI) engines.1−5 One promising approach is to add organometallic-based homogeneous combustion catalysts in diesel fuel to improve diesel combustion processes within CI engines. Numerous recent studies2,6−17 on the application of organometallic-based homogeneous combustion catalysts in diesel engines to achieve higher fuel efficiency and lower emissions have been reported, including the own work of the authors on a ferrous-picrate-based homogeneous combustion catalyst.6−9,15−17 The improvements in fuel efficiency and reductions in engine emissions are significant, as summarized in Table 1. In addition to the studies on the performance of the catalyst in diesel engines, as shown in Table 1, there have been few studies with an effort to understand working mechanisms of organometallic-based catalysts in the ignition and combustion processes of different fuels recently. Iron pentacarbonyl [Fe(CO)5] was shown to accelerate ignition and combustion of CO, H2, and hydrocarbons.18−24 Staude et al.18 measured the temperature profiles in a premixed lean laminar flame of H2/ O2/Ar with Fe(CO)5 added and found that the addition of Fe(CO)5 increased the flame temperature throughout the combustion zone. Shvartsberg et al.19 numerically modeled hydrogen flames at a low pressure of 3 kPa and found that the addition of iron to the flame resulted in additional heat release and higher flame temperatures. Li et al.20 found that the gaseous iron species promoted the carbon monoxide oxidation during high-temperature off-gas combustion. Contrary to reports that Fe(CO)5 inhibited combustion of hydrocarbon and hydrogen in premixed flames by reducing the flame speed,21 Linteris and Babushok22 found that the addition of iron-containing compounds [Fe(CO)5, Fe, FeO, and FeOH] at dosing ratios less than 150 ppm decreased the ignition delay © XXXX American Chemical Society

Special Issue: In Honor of Professor Brian Haynes on the Occasion of His 65th Birthday Received: August 28, 2016 Revised: November 29, 2016 Published: December 2, 2016 A

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Table 1. Summary of Various Literature Studies on the Effects of Various Metal-Based Homogeneous Combustion Catalysts on Fuel Consumption and Emissions of Diesel Enginesa

a

catalyst

fuel saving (%)

platinum ferrocene cerium oxide ferrous picrate ferrous picrate iron chloride

2−9 N/A N/A 6.6−11.7 2−4.2 8.6

UHCb reduction (%)

CO reduction (%)

NOx reduction (%)

smoke reduction (%)

reference

no change 9−17

no change −12 up to 30 from −3 to 6.6 −6 −4.1

N/A up to 37 N/A N/A up to 40 6.9

10 11 12 13 7 and 8 14

27−61 5−35 25−40 from −22 to −24 5−40 26.6

52.6

A negative value in the table means that the emission was increased. bUHC denotes unburned hydrocarbons.

2. KINETIC MODELS AND SIMULATIONS Diesel ignition and combustion in CI engines are characterized by rich premixed ignition and subsequent premixed combustion, followed by diffusion combustion.25 The effect of iron in premixed combustion of hydrocarbon fuels has been studied and reported in the literature.18−24 Therefore, the present kinetic modeling approach included two parts, an idealized premixed ignition model and an opposed counter-flow diffusion flame model. Because of the complex composition of real diesels, n-heptane (n-C7H16) was chosen as a model fuel because it has a similar cetane number to diesel and has also been frequently used as a diesel fuel surrogate.26 Figure 1 shows the schematics of the ignition and combustion models. Figure 2. Schematic of the definition of the ignition delay time as simulated with an initial temperature of 800 K and an equivalence ratio of 1.0 using CHEMKIN PRO. performed for varying different equivalence ratios, defined as the ratio of the n-C7H16/air ratio to the stoichiometric n-C7H16/air ratio. Simulation of the n-heptane diffusion flame was performed using the OPPDIF program within the CHEMKIN PRO software. The model applies a counter-flow configuration in which the counter-flow n-heptane diffusion flame is formed by two coaxial opposing jets of the fuel and air. The simulations were performed at atmospheric pressure as a result of the lack of transport data of iron-related species at high pressures. The distance between fuel and air boundaries was 10 mm. The velocity and temperature of air were set to 0.375 ms−1 and 800 K, respectively, and those of the fuel stream were 0.342 ms−1 and 298 K, respectively. The fuel stream was a mixture of n-heptane vapor and nitrogen with a mole ratio of 15:85. This model was employed by Seiser et al.29,30 to simulate their experimental work, whose reaction mechanism of n-heptane also used in the present model was validated. The reaction mechanism of n-heptane used in the present model includes 159 species and 1540 reactions.29 This mechanism has been validated by experimental n-heptane counter-flow diffusion flames in terms of combustion characteristics as detailed in ref 28. In the present research, we further compared the calculated results of the ignition delay times of stoichiometric n-heptane using this mechanism to the literature experimental data,31,32 as shown in Figure 3. It is evident that the modeling results and experimental data matched each other very well, indicating that the mechanism is also reliable enough to capture the ignition characteristics of n-heptane. A detailed kinetic mechanism proposed by Rumminger et al.33 for gas-phase iron species was also employed in the present simulations. This mechanism has been validated in methane,33 carbon monoxide, 20 and hydrogen22 combustion. The combined mechanism of n-heptane and the gasphase iron species is provided in the Supporting Information.

Figure 1. Schematics of the n-heptane (a) ignition and (b) combustion models. The ignition was simulated using the SENKIN program within the CHEMKIN PRO software. The temporal evolution of mole fractions of various species involved in the ignition phase for a homogeneous, adiabatic, and gaseous mixture in a closed reactor under a constant pressure was calculated.26 The ignition delay time was used to describe the ignition characteristics of a n-C7H16/air mixture. It is known that the ignition of a n-C7H16/air mixture is characterized by a two-stage ignition phenomenon.27,28 Figure 2 shows typical profiles of the OH concentration and temperature during the ignition process of a nC7H16/air mixture, and the two-stage ignition behavior is clearly evident. In the present study, the first-stage ignition was too weak under the conditions studied and the ignition was said to take place when the second-stage ignition was achieved. The ignition delay time was, therefore, defined as the time required for the mixture to achieve the maximum concentration of OH, as shown in Figure 2. The ignition delay time was calculated under the constant pressure of 50 bar with various initial temperatures, the characteristic condition of the diesel engine at the top of the piston stoke.6 The simulations were also

3. RESULTS AND DISCUSSION The kinetic mechanism was first validated against some limited experimental data involving iron in hydrocarbon combustion available in the literature. Staude et al.34 studied the effect of the iron atom on the flame temperatures of laminar propene/ oxygen/argon flames using laser-induced fluorescence. Iron was B

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Figure 3. Comparison of calculated results and experimental data31,32 of ignition delay times of n-heptane/air mixtures.

Figure 5. Effect of the iron concentration in the fuel on the ignition delay time.

added in the form of ferrocene at a concentration of 95 ppm in the flame. In the present study, the flame temperatures of the propene/oxygen/argon flames with and without iron addition were simulated using the mechanism adopted in the current work. Note that the current mechanism was slighted modified to include the ferrocene decomposition mechanism taken from ref 30. The simulation was performed using the burnerstabilized flame program within the CHEMKIN PRO software. The calculated results are compared to the measured temperatures34 in Figure 4. The results clearly indicate that,

identified and the relative rates of n-heptane consumption as a result of these reactions are presented in Figure 6, sorted in the

Figure 6. Relative rates, as indicated by the horizontal bars, of consumption of n-heptane by various elementary reactions at a residence time of 0.02 ms, an initial temperature of 800 K, and an equivalence ratio of 1.0.

order of decreasing reaction rate. Figure 6 shows that it is the OH radical rather than HO2 that dominated the n-heptane consumption during the early stage of ignition. It has been reported that n-heptane ignition occurs when OH radicals accumulate in the reaction mixture.27,28,35 During the early stage of ignition, alkyl hydroperoxy radicals are formed, which can also react and lead to the formation of ketohydroperoxide and OH radicals.27,28 Ketohydroperoxide is highly unstable and can quickly decompose to form OH.27,28 Most OH radicals then react with fuel molecules, producing water and heat, increasing the mixture temperature, and accelerating the total nheptane oxidation rate. The present simulation confirmed that OH is the most critical species for initiating the ignition of nheptane. The role of Fe addition in reducing the ignition delay time can be explained with the aid of Figure 7, which shows the mole fractions of iron-related species (Figure 7a) and OH (Figure 7b) as a function of time during the ignition of n-heptane with 50 ppm of iron in the fuel. Before the ignition occurred, as characterized by the instantaneous rapid rise of the OH concentration, most iron atoms reacted with oxygen to form FeO2, which was not significantly consumed during the buildup of the radical pool. On the other hand, a small portion of iron reacted with oxygen to form FeO and radical O; the latter, in turn, was consumed primarily via n-heptane (n-C7H16) to

Figure 4. Comparison of calculated results and experimental data34 of maximum flame temperatures of propene/oxygen/argon flames with and without iron addition.

although there were some differences between experimental data and simulations, the effect of iron on the flame temperature was consistent in both measurements and simulations; namely, the iron addition increased the flame temperature by 50−100 K. This suggests that the current mechanism is capable of predicting the role of iron in the ignition and combustion processes of hydrocarbon fuels. Figure 5 shows the ignition delay time of a n-heptane/air mixture as a function of the iron concentration at an initial temperature of 800 K and an equivalence ratio of 1.0. Clearly, the addition of iron in the fuel reduced the ignition delay time, suggesting that iron promoted ignition. The ignition delay time decreased rapidly with an increasing iron concentration from 0 to ca. 30 ppm, and this decrease slowed as the concentration further increased and, finally, leveled off when the concentration was >500 ppm. The addition of 100 ppm of iron reduced the ignition delay time by >10%. To understand the reasons for the effect of iron on the ignition process, major reactions contributing to n-heptane consumption at an early stage (20 ms) of ignition were C

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Figure 8. Effect of iron on ignition delay time as a function of the initial temperature.

Figure 7. (a) Mole fractions of various iron-related species with the addition of iron in the fuel and (b) mole fractions of OH with and without the addition of iron in the fuel.

Figure 9. Effect of iron on the ignition delay time as a function of the equivalence ratio.

O was higher than in the stoichiometric or richer mixtures. As a result, O was formed much earlier, and its subsequent branching reaction with n-heptane led to a richer radical pool, mainly OH, in accordance with reaction 5. Typical flame temperature profiles as a function of the distance from the fuel side with and without iron in the fuel and the peak flame temperature as a function of the iron concentration are compared in Figure 10. It is apparent from Figure 10a that iron addition only affected the peak flame temperatures. The simulated peak flame temperatures, occurring at ca. 6.7 mm away from the fuel side, were 2016 and 1982 K for the cases with and without Fe addition, respectively. Figure 10b shows that the peak flame temperature logarithmically increased with increasing the iron concentration and became invariant when the concentration was greater than 400 ppm. To gain insight into the mechanism of iron in promoting the combustion of the fuel, how iron atoms chemically affected the major species, various radicals, and iron-bearing species present in the flame was analyzed. The results are presented in Figure 11. The concentrations of three major species, CO2, H2O, and CO, during n-heptane combustion, with and without iron, are compared in Figure 11a, which shows that, with the addition of iron in the fuel, the mole fraction of H2O increased, while the mole fraction of CO decreased significantly on both sides of the peak flame region. It is also interesting to note that the mole fraction of CO2 decreased slightly on the fuel side. However, the mole fraction of CO2 is slightly higher on the air side of the flame for the case with Fe addition than that without Fe, suggesting that the combustion was more efficient as more CO was converted to CO2 as a result of iron addition.

produce radical OH, and the former (FeO) was reduced by H to Fe. Because OH was primarily responsible for the ignition of n-heptane, the role of iron was to add OH radicals to the system in the very early stage by participating in the following reactions: Fe + O2 ⇒ FeO2

(1)

Fe + O2 ⇒ FeO + O

(2)

FeO2 + H ⇒ FeO + OH

(3)

FeO + H ⇒ Fe + OH

(4)

O + n‐C7H16 ⇒ C7H15 + OH

(5)

The effect of the initial temperature on the ignition delay time (ms) is shown in Figure 8 for an equivalence ratio of 1.0 with Fe concentrations of 0, 50, and 100 ppm. It is seen that the ignition delay time decreased with increasing both the temperature from 700 to 900 K and Fe addition. Note that Fe addition to reduce the ignition delay time was more effective at low initial temperatures. Hence, with everything else being equal, the addition of Fe was also expected to make the nheptane ignition occur at a lower temperature. Figure 9 shows the effect of the equivalence ratio on the ignition delay time at Fe concentrations of 0, 50, and 100 ppm. The ignition delay time decreased with increasing the equivalence ratio, and the decrease was greater at low equivalence ratios. The Fe addition also monotonically reduced the ignition delay time, being more significant at high equivalence ratios. A reaction pathway analysis indicated that, under leaner conditions, the rate of reaction Fe + O2 ⇒ FeO + D

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Figure 10. Effect of iron addition in the fuel on the flame temperatures: (a) comparison of temperature profiles in the flame zone with and without iron in the fuel and (b) peak flame temperature of a counterflow diffusion flame as a function of the iron concentration in the fuel.

Mole fractions of O, OH, and H radicals in the flame region are compared in Figure 11b. The Fe addition in the fuel resulted in a higher peak mole fraction of OH but lower peak mole fractions of O and H, being more significant for radical H. In comparison to the result without Fe addition, the peak mole fraction of H was reduced by almost 10% as a consequence of 100 ppm of Fe addition. However, the concentration of OH was slightly reduced immediately on the fuel side of the peak flame region as a result of the effect of Fe. Figure 11c shows the mole fractions of key iron-related species in the flame with 100 ppm of iron in the fuel. It is evident that most iron atoms in the reaction zone were converted to iron species FeO2, FeO, Fe(OH)2, and FeOH. As iron diffused from the fuel to the flame zone, it encountered oxygen and reactions occurred. Most iron was converted to FeO2, which reached a peak at a distance of 0.55 cm from the fuel side, then decreased sharply, and finally became negligible in the peak flame temperature region. Interestingly, at 6.4 mm on the fuel side of the peak flame temperature region, Fe(OH)2 was depressed, with sharp increases in FeO and FeOH. Both FeO and FeOH reached their peaks in the peak flame temperature region. The region of high mole fractions of FeO and FeOH corresponded to a high mole fraction of radical H, as seen in Figure 11b. A detailed examination of the key reactions involving iron species and their rates would reveal the underlying mechanisms of iron atoms in the diffusion flame. As shown in Figures 10 and 11, key reactions involving major iron species were examined. Figure 12 illustrates the rates of important reactions of the major iron species in the diffusion flame, which were useful in elucidating the main reaction paths in the n-heptane diffusion flame as affected by the presence of

Figure 11. Comparisons of mole fraction profiles of (a) CO2, H2O, and CO, (b) O, OH, and H radicals, and (c) various iron-related species in the flame zone with and without iron in the fuel.

iron atoms. Iron mainly reacted with oxygen to form FeO2, which then reacted with O to form FeO in the peak flame region. The bulk of FeO reacted with water to begin a catalytic cycle that converted H into less reactive H2 molecules,35 as described by FeO + H 2O ⇒ Fe(OH)2

(6)

Fe(OH)2 + H ⇒ FeOH + H 2O

(7)

FeOH + H ⇒ FeO + H 2

(8)

This is consistent with the trend of decreasing the mole fraction of H with the addition of iron, as seen in Figure 11b. Furthermore, the peak mole fraction of O was similar to that of H in the peak flame region, which increased the importance of the reaction between iron species and radical O. It follows that reaction 9 below was one of the most significant iron reactions. FeOH + O ⇒ FeO + OH E

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Figure 12. Rates of various reactions involving iron species in flame.

Reaction 9 also resulted in a lower peak mole fraction of O in the peak flame region, which is consistent with observations in Figure 11b. How the above iron mechanism affected the flame temperature was further examined. The sensitivity of the flame temperature to the rates of major reactions was analyzed and found that the dominant reactions determining the peak flame temperature in the n-heptane diffusion flame are CO + OH ⇔ CO2 + H

(10)

H + O2 ⇔ O + OH

(11)

OH + H ⇔ H 2 + O

(12)

H + OH ⇔ H 2O

(13)

H + O2 ⇔ HO2

(14)

C2H3 ⇔ C2H 2 + H

(15)

C3H6 + H 2 ⇔ C3H 7 + H

(16)

n‐C7H16 + H ⇔ C7H15 + H 2

(17)

Figure 13. Effect of iron on the reaction rates of H + O2 ⇔ O + OH and CO + OH ⇔ CO2 + H.

in Figure 14. During the ignition phase, iron advances the injection of radicals O, OH, and H, particularly OH, into the

It was found that reactions 12−17 are many orders of magnitude slower than reactions 10 and 11. Therefore, reactions 10 and 11 are considered to be primarily responsible for the peak flame temperature. Increasing the forward rate or reducing the backward rate of these two reactions increased the peak flame temperature. Figure 13 shows the rates of these two reactions with the addition of 100 ppm of iron in the fuel. The forward rates of both reactions increased in the peak flame regions, which, in turn, increased the peak flame temperature (Figure 10). Although the mole fractions of H, OH, and O were lowered with iron addition, the overall rates of reactions 10 and 11 increased. The mechanisms of iron during ignition and combustion of diesel as modeled by n-heptane may be proposed, as illustrated

Figure 14. Schematic of the proposed mechanisms of iron in diesel ignition and combustion.

radical pool, resulting in a faster ignition and shorter ignition delay time. In the diffusion flame, iron leads to lower mole fractions of radical H and O in the peak flame region. Nevertheless, the rates of CO + OH ⇔ CO2 + H and H + O2 ⇔ O + OH, which are responsible for the peak flame temperature, increase. Consequently, the peak flame temperature increases with an increasing iron concentration. This is F

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Energy & Fuels consistent with the findings from single-droplet combustion experiments.9

(4) Shi, L.; Cui, Y.; Deng, K. Y.; Peng, H. Y.; Chen, Y. Y. Study of low emission homogeneous charge compression ignition (HCCI) engine using combined internal and external exhaust gas recirculation (EGR). Energy 2006, 31 (114), 2665−2676. (5) Zhu, M. M.; Ma, Y.; Zhang, Z. Z.; Chan, Y. L.; Zhang, D. K. Effect of oxygenates addition on the characteristics and soot formation during combustion of single droplets of a petroleum diesel in air. Fuel 2015, 150, 88−95. (6) Zhang, D. K.; Ma, Y.; Zhu, M. M. Nanostructure and oxidative properties of soot from a compression ignition engine: The effect of a homogeneous combustion catalyst. Proc. Combust. Inst. 2013, 34, 1869−1876. (7) Zhu, M. M.; Ma, Y.; Zhang, D. K. An experimental study of the effect of a homogeneous combustion catalyst on fuel consumption and smoke emission in a diesel engine. Energy 2011, 36, 6004−6009. (8) Zhu, M. M.; Ma, Y.; Zhang, D. K. Effect of a homogeneous combustion catalyst on the combustion characteristics and fuel efficiency in a diesel engine. Appl. Energy 2012, 91, 166−172. (9) Zhu, M. M.; Ma, Y.; Zhang, D. K. Effect of a homogeneous combustion catalyst on combustion characteristics of single droplets of diesel and biodiesel. Proc. Combust. Inst. 2013, 34, 1537−1544. (10) Caton, J. A.; Ruemmele, W. P.; Kelso, D. T.; Epperly, W. R. Performance and fuel consumption of a single-cylinder, directinjection diesel engine using a platinum fuel additive. SAE Tech. Pap. Ser. 1991, DOI: 10.4271/910229. (11) Zeller, H. W.; Westphal, T. E. Effectiveness of Iron Based Fuel Additives for Diesel Soot Control; National Technical Information Service (NTIS): Washington, D.C., 1992; NTIS Report BUMINESIR-9438. (12) Sajith, V.; Sobhan, S. B.; Peterson, G. P. Experimental investigation on the effects of cerium oxide nanoparticles additive on biodiesel. Adv. Mech. Eng. 2010, 2, 581407. (13) Parsons, J. B.; Germane, G. J. The effects of an iron based catalysts upon diesel fleet operation. SAE Tech. Pap. Ser. 1983, DOI: 10.4271/831204. (14) Kannan, G. R.; Karvembu, R.; Anand, R. Effect of metal based additive on performance emission and combustion characteristics of diesel engine fuelled with biodiesel. Appl. Energy 2011, 88 (11), 3694− 3703. (15) Ma, Y.; Zhu, M. M.; Zhang, D. K. The effect of a homogeneous combustion catalyst on exhaust emissions from a single cylinder diesel engine. Appl. Energy 2013, 102, 556−562. (16) Ma, Y.; Zhu, M. M.; Zhang, D. K. Effect of a homogeneous combustion catalyst on the characteristics of diesel soot emitted from a compression ignition engine. Appl. Energy 2014, 113, 751−757. (17) Ma, Y.; Zhu, M. M.; Zhang, Z. Z.; Zhang, D. K. Effect of a homogeneous combustion catalyst on the nanostructure and oxidative properties of soot from biodiesel combustion in a compression ignition engine. Proc. Combust. Inst. 2015, 35 (2), 1947−1954. (18) Staude, S.; Hecht, C.; Wlokas, I.; Schulz, C.; Atakan, B. Z. Experimental and numerical investigation of Fe(CO)5 addition to a laminar premixed hydrogen/oxgen/argon flame. Z. Phys. Chem. 2009, 223, 639−649. (19) Shvartsberg, V. M.; Bolshova, T. A.; Korobeinichev, O. P. Effect of iron and organophosphorus flame inhibitors on the heat release rate in hydrogen/oxgen flames at low pressure. Energy Fuels 2011, 25, 596−601. (20) Li, S.; Wei, X. L. Promotion of CO oxidation and inhibition of NO formation by gaseous iron species during high-temperature off-gas combustion. Energy Fuels 2011, 25, 967−974. (21) Babushok, V.; Tsang, W. Inhibitor ranking for alkane combustion. Combust. Flame 2000, 123 (4), 488−506. (22) Linteris, G. T.; Babushok, V. I. Promotion or inhibition of hydrogen−air ignition by iron-containing compounds. Proc. Combust. Inst. 2009, 32, 2535−2542. (23) Matsuda, S. Gas-phase homogeneous catalysis in shock waves. II: Oxidation of carbon monoxide by oxygen in the presence of iron pentacarbonyl. J. Chem. Phys. 1972, 57 (2), 807−812.

4. CONCLUSION A kinetic modeling study of ignition and combustion characteristics of diesel represented by n-heptane and the effect of iron has been carried out. Simulations for a range of equivalence ratios, air temperatures, and iron concentrations in the fuel showed that a small amount of iron (up to 100 ppm) in n-heptane can significantly reduce the ignition delay time, which decreases with increasing the iron concentration. The reaction pathway analysis showed that the promotion of n-heptane ignition by iron atoms is due to the reactions of iron with O2, which lead to an early injection of radicals of H and O. These radicals, in turn, accelerate the subsequent buildup of radical OH, which reacts with the fuel molecules and initiates the ignition. The simulation results also showed that the addition of iron in n-heptane increases the peak flame temperature of the counter-flow diffusion flame of n-heptane, and the peak flame temperature increases with increasing the iron concentration. The maximum mole fractions of H and O in the peak flame region are decreased by the addition of iron in the fuel as a result of the catalytic recombination cycles involving FeO, Fe(OH)2, and FeOH; however, the reaction rates of H + O2 ⇔ O + OH and CO + OH ⇔ CO2 + H in the peak flame region are increased. These two reactions are principally responsible for the increased flame temperature as a result of iron addition in the fuel.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02179. Detailed reaction mechanism of n-heptane and the gasphase iron species (TXT)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +61-8-6488-5528. Fax: +61-8-6488-7622. E-mail: [email protected]. ORCID

Mingming Zhu: 0000-0002-3643-1799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the Australia Research Council (ARC) under the ARC Linkage Projects Scheme (Project LP0989368) in partnership with Fuel Technology Pty Ltd and BHP Billiton Iron Ore Pty Ltd.

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REFERENCES

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H

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