Effects of methane addition on exhaust gas emissions and combustion

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Effects of methane addition on exhaust gas emissions and combustion efficiency of the premixed n-heptane/air combustion Peng Zhang, Jingyu Ran, Changlei Qin, Xuesen Du, Juntian Niu, and Lin Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03469 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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Fig. 1 Schematic of the flow system and burner assembly

Fig. 2 Comparison of the molar fraction of n-heptane and air combustion products from model (dashed lines) and experiment (solid lines) with different equivalence ratios.For the mole fractions of the inlet, XF + XA = 1 and XM = 0.

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Fig. 3 The reaction rate of intermediate product in n-heptane combustion varies along the reaction direction. For the mole fractions of the inlet, XF + XA = 1 and XM = 0, equivalence at 1.0.


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Fig. 4 N-heptane combustion reaction path with equivalence of 1.0


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Fig. 5 The reaction rate of methane varies along the reaction direction. For the mole fractions of the inlet, XF + XA + XM = 1, equivalence at 1.0.

Fig. 6 The variation of molar fraction of CxHy with the different equivalent ratios and


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contents of methane

Fig. 7 The reaction rate of H2 varies along the reaction direction

Fig. 8 The reaction rate of CO varies along the reaction direction


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Fig. 9 The reaction rate of H2O varies along the reaction direction

Fig. 10 The reaction rate of CO2 varies along the reaction direction


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Fig. 11 The variation of molar fraction of CO with different equivalent ratios and the contents of methane

Fig. 12 The variation of molar fraction of CO2 with different equivalent ratios and the contents of methane


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Fig. 13 The variation of CO/CO2 ratio with different equivalent ratios and the content of methane

Fig. 14 Comparison of the peak value of R24 reaction rates in n-heptane/air combustion (solid line) and n-heptane/methane/air combustion (dashed line) with


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different C/O ratios

Fig. 15 N-heptane conversion of different equivalence ratios and methane contents


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Effects of methane addition on exhaust gas emissions and combustion efficiency of the premixed n-heptane/air combustion Peng Zhang 2, Jingyu Ran *1, 2, Changlei Qin 1, 2,Xuesen Du 1, 2, Juntian Niu 2, Lin Yang 3 1

Key Laboratory of Low-grade Energy Utilization Technologies and Systems,

Chongqing University, Ministry of Education, Chongqing 400030, P. R. China; 2

College of Power Engineering, Chongqing University, Chongqing 400030, P. R.

China; 3

College of Computer Science and Information Engineering, Chongqing Technology

and Business University, Chongqing, 400067, China * Email: [email protected] ABSTRACT: Combustion of n-heptane usually yields unburned hydrocarbon and carbon monoxide, which are environmentally harmful. As a result, to improve the combustion efficiency is much more important. In the work, the effect of methane addition on the premixed n-heptane/air combustion (0.1 Mpa and 400K) was experimentally and numerically investigated. The equivalent ratio (the ratio of theoretical volume of air required for complete combustion of per unit volume of methane and n-heptane blended fuel to the actual volume of air supplied) was between 0.8 and 1.2, for which methane content was varied from 0% to 50%. The experimental results showed that with the increasing methane content, the main unburned hydrocarbons (C2H2, C3H8, C2H4 and C4H10) and CO2 decreased, while CO increased. The sensitivity analysis shown that R24: CO + OH = CO2 + H plays a key role in the emissions of unburned hydrocarbon and carbon monoxide. At the same


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C/O ratio of n-heptane/air and n-heptane/methane/air mixtures, the reaction rate of R24 increased with the addition of methane, which in turn reduced the hydrocarbon and carbon monoxide emissions and improved the combustion efficiency. Keywords: N-heptane; Methane; Premixed combustion; Exhaust gas emissions

1. Introduction Energy shortage and environmental pollution are the two major issues facing all over the world. Development and utilization of clean renewable energy and reduction of greenhouse gas emissions have been significant parts of the world’s energy sustainable development strategy.

1–5

In this regard, n-heptane has been considered as

a potential surrogate fuel to address these concerns. N-heptane is an important part of commercial gasoline and one of the primary reference fuels for spark ignition engines. Moreover, n-heptane has low water absorption, good miscibility with current fuels, and good compatibility with conventional engines, low levels of corrosion, low pollutant emission, a mature production process and a good regenerability.

6, 7

Therefore, n-heptane combustion has been the subject of many experimental and numerical studies. The combustion process of n-heptane has been carried out in many reactors, such as shock tubes,

8, 9

, micro channels, jet stirred reactors

the characteristics of premixed combustion

12-15

10, 11

, etc. Also,

and diffusion combustion

16, 17

of

n-heptane have been studied in experiment. And the experiments have been performed which were focused on polycyclic aromatic hydrocarbons (PAH) emissions, laminar burning velocities, Markstein lengths and soot formations of n-heptane premixed combustion flames.12, 14, 17 Additionally, Curran et al. ACS Paragon Plus Environment

18

and Hakka

19

have

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proposed detailed chemical kinetics mechanism of n-heptane combustion process, and Lu

20

carried on simplification processing to the mechanism. Furthermore, the

understanding of n-heptane combustion has attracted more and more interest, and the combustion chemistry of n-heptane has been widely investigated in detail. Some other mechanisms have been developed such as NIST mechanism, 21 San Diego mechanism, 22

Lawrence Livermore National Laboratory (LLNL) mechanisms,

mechanisms,

25

Ranzi mechanisms,

Chalmers University.

27

26

23, 24

Dryer

and a skeleton mechanism developed at the

Besides, experimental and simulation methods have been

combined to study the oxidation of n-heptane. 28, 29 Also, the predictions of the models were almost in satisfactory agreement with the obtained experimental results.

Natural gas is known to have high combustion efficiency and low pollution emissions,

30, 31

and can be an alternative fuel for coal, diesel gasoline and other

petroleum fuels. The selection of methane addition into n-heptane fuels is based on the consideration that there is a significant interest in using it to improve the stability of combustion and reduce the pollutant emission of n-heptane, especially through a dual-fuel mode.

32

Therefore, several researches have studied the use of methane in

liquid fuel combustion systems, whether pure natural gas or mixed natural gas. 33-36

Previous studies of methane/n-heptane mixtures have mostly focused on the analysis of ignition delay time and laminar burning velocities. Aggarwal

37

has

studied the ignition delay time of methane/n-heptane mixture in a closed homogenous reactor by CHEMKIN 4.1. The result showed that the ignition delay time was decreased first and then increased with the increase of the concentration of methane in ACS Paragon Plus Environment

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the methane/n-heptane/air mixtures. The sensitivity analysis indicated that the hydroxyl and heptyl radicals played a key role in determining the ignition behaviour of n-heptane/methane mixtures. Additionally, Li et al.

38

has studied the laminar

burning velocities and Markstein lengths of methane/n-heptane/air flame and a combustion chamber with central ignition was used. The results shown that with the increase of methane content in the mixtures, both laminar burning velocities and Markstein lengths was decreased. However, with the increase of methane in the mixtures, the parameters did not vary linearly. Based on the difference of laminar burning velocities and Markstein lengths of the variation of methane/n-heptane mixtures, the ratio of 0.75 is considered as the critical methane concentration that laminar burning velocity and Markstein length of methane/n-heptane/air flames start to change significantly.

38, 39

However, few studies on the premixed laminar flame

characteristics and the conversion rate of combustion of methane/n-heptane mixtures are available in the literatures.

In this study, the first aim is to investigate the effect of the methane addition on the exhaust gas emissions of methane/n-heptane/air laminar flame. The second objective is to obtain the influence of methane on n-heptane combustion conversion efficiency. The gas chromatography has been used to investigate the emissions of the methane/n-heptane/air flame. Then, the numerical simulation and ROP (rate of production) analysis have been carried out by CHEMKIN-PRO and the results have been compared with the experimental data.

This study will be of benefit in helping the development of fundamental technology ACS Paragon Plus Environment

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with respond to reduce pollutant emissions and improve n-heptane combustion rate. Additionally, the present paper is expected to assist in developing a sustainable economy and provide more guidance for industrial applications.

2. Experimental and modelling

2.1. Experimental method. As shown in Fig. 1, the system is designed for the ability to perform experiments with both gaseous and liquid fuels. The system composed of a gas/liquid delivery system, a premixed Bunsen burner, and an emission measurement system. A tabular stainless steel tube (ID = 6 mm, L = 500 mm) is employed as the Bunsen flame burner with a fully developed flow profile. The flame is covered by a quartz tube to ensure the sealing of the system and the stability of the combustion. Each gas component is purged by a delicate mass flow controller (Alicat Company America) with an accuracy of 0.1 mL/min. The liquid fuel is injected into the experimental system by an injection pump (Smiths micro-infusion pump WZ-50C6). The line flowing liquid is covered with the heating belt of 400 K to ensure the liquid can be gasified in the system. Reverse flow is prevented by one-way valves along each gas and liquid supplying line. The gases and liquid from different lines are mixed in a mixing chamber (400 ml stainless steel) prior to entering the burner. The total flow rate is adjusted to ensure a laminar flow (Reynolds number, Re < 2300). The average velocity is at least six times as high as that of the laminar flame speed to obtain a stabilized flame.

It has been mentioned that a quartz tube is located outside the flame. The emissions


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of the combustion have been entered into a gas chromatograph. Then, the components of the combustion product will be obtained. Finally, the conversion rate of the fuels will be obtained.

All computations are carried out based on a constant pressure. The flow rate of the inlet mixture of n-heptane, methane and air is calculated to make sure it enters the system in a perfect homogeneous gas phase. The Reynolds number is maintained at 1000. The mole fraction of the inlet charge is written as:

XF + XA + XM = 1 where XF, XA and XM are mole fractions of n-heptane, air and methane, respectively. In this study, the equivalent ratio of design is achieved by adjusting the volume flow of n-heptane, air and methane, which can be defined as: φ =（XFuel : XA）real / (XFuel : XA) stio XFuel = XF + XM The equivalent ratio is the ratio of theoretical volume of air required for complete combustion of per unit volume of methane and n-heptane blended fuel to the actual volume of air supplied.

2.2 Numerical method. In present study, the simulations were performed using the PREMIX

40

model of CHEMKIN-PRO. The physical model is on the basis of the

transient, spatially homogeneous form of the conservation equations for mass, energy, and species in a given adiabatic system. In the simulations, the influence of thermal


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diffusion has been considered, and the multi component method has been chosen to calculate the transport parameters. The specified initial condition including initial temperature, pressure, and reactant mixture composition were selected for computation. The detailed reaction mechanism of n-heptane put forward by LLNL (Lawrence Livermore National Laboratory)

23, 24, 41

has been used in this paper. A

total of 654 components and 2827 elementary reactions have been included in the mechanism. In addition, this mechanism has been widely tested for several hydrocarbon fuels, including n-heptane

3. Results and discussion

3.1 Model validation. The numerical model accuracy has been validated by comparing the molar fraction of n-heptane and air combustion products at different equivalence ratios in numerical simulation and experimental results, as shown in Fig. 2. With equivalence ratio increasing, n-heptane content increases and oxygen content decreases during combustion. Therefore, the oxygen content in reaction products decreases, while the incomplete combustion products increase. Because the content of each component of incomplete combustion products is relatively rare, CxHy is used to represent their total content including CH4, C2H2, C3H8 and C4H10. Moreover, the equivalence ratio, and it reaches the peak value with the equivalence ratio near 1.0. Fikret

42

measured the molar fraction of each combustion product in the premixed

flame of n-heptane/oxygen/argon, and the combustion products included Ar, H2, CO, H2O, C2H2, CO2, O2, CH4, C3H6, C4H6, etc. Compared with data reported in this paper, the content of the same product is basically in the same order of magnitude. However, ACS Paragon Plus Environment

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its reactions were carried out at equivalence ratios of 1.97 and 2.01 in the literature, under severe incomplete reaction conditions. The values of molar fraction of incomplete combustion products are higher than the values of this paper (equivalent ratio of 0.8-1.2). As shown in Fig. 2, the molar fraction of products of simulation and experiment show the tendency trend at different equivalence ratios. Also, the simulation results of this paper are in good agreement with the experimental. As a result, the reaction mechanism and calculation method applied in this work are reasonable.

3.2 Effects of methane addition on CxHy content in n-heptane combustion products. The ROP (Rate of Production) analysis of n-heptane/air combustion with equivalence of 1.0 was carried out by CHEMKIN-PRO. As shown in Fig.3, the ROP value of a molecule more than zero indicates the rate of formation of the molecule, and the value less than zero indicates the rate of consumption of the molecule. In the process of n-heptane combustion, oxygen has the maximum reaction rate, and it participates in the through reaction. At the same time, n-heptane is mainly decomposed in the initial stage of the reaction, and carbon dioxide is mainly produced in the middle-late stages. Moreover, for intermediate products, carbon monoxide, hydrogen and ethane have a higher reaction rate, while methane, propane and butane have a less rate of reaction. These indicate that carbon monoxide, hydrogen and ethylene have much higher contents in the reaction process as intermediate products, while methane, propane and butane have fewer contents, respectively.

According to the sensitivity analysis and ROP analysis of the model, the main ACS Paragon Plus Environment

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reaction path of n-heptane/air combustion is given with equivalence ratio of 1.0. As shown in Fig.4, n-heptane first reacts with H and OH radicals to produce the isomers of C7H15. Then, the reaction process is dominated by cracking reactions and the reactions generate a large amount of ethylene. Finally, ethylene reacts with oxygen, OH and H radicals to produce carbon dioxide and water. As shown in the reaction path, it can be found that the combustion process of n-heptane can be divided into three parts, dehydrogenation stage, cracking stage and combustion stage. Meanwhile, ethylene is the main product of the cracking stage and the main starting compound of the combustion stage. Therefore, ethylene plays a crucial role in the oxidation of n-heptane.

As shown in Fig.3 and Fig.4, methane is produced in the reaction of n-heptane/air combustion and the formation of methane is mainly through the reaction of ethylene with CH3 radicals. As shown in the reaction path, the main products of ethylene are HCO, CH3, C2H3 and CH4. In these products only methane is a stable compound, the others are unstable compounds. Evidently, the unstable compounds are easier to combine with the active groups such as H, O and OH radicals in the reaction process to promote the overall reaction. Also, methane is a component measured in combustion products in experiment. Thus, it can be found that methane, as a stable intermediate product of n-heptane combustion process, is not only unfavourable to react with the active groups in the reaction process, but also a part of methane could dissipated into the air during the combustion process. Then, this leads to an increase in the incompleteness of the n-heptane combustion. So, the formation of intermediate


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product methane is not the optimal path for ethylene oxidation.

To further analyse the addition of methane on the combustion of n-heptane, the change of reaction rate of methane under different methane contents with equivalence of 1.0 was studied. The methane content is defined as: NM =

As shown in Fig. 5, with the increase of methane content, methane creation rate decreases, while methane consumption rate increases. So, the addition of methane to n-heptane can reduce the conversion of n-heptane to methane during the reaction.

In order to study the effect of methane addition on n-heptane combustion reaction path optimization, the variation of molar fraction of CxHy with the different equivalent ratios and contents of methane are measured by experiment. CxHy represents the total amount of incomplete product of n-heptane combustion, and CxHy includes CH4, C2H2, C3H8 and C4H10 in this study. As shown in Fig. 6, the oxygen content is more than that required molar ratio for complete reaction in the combustion process of n-heptane/methane/air at the equivalence ratio less than 1.0, and the incomplete combustion products are seldom produced. The change of methane content in fuel has little effect on CxHy emissions. However, with the equivalence ratio increasing, the oxygen content decreases and the emissions of incomplete combustion products increase. It can be obviously found that with the methane content increasing, the content of CxHy in combustion products decreases. The results indicate that in the combustion process of n-heptane/methane/air,


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increase of the methane content can reduce the conversion of ethylene to methane and promote the conversion of ethylene to other active radicals such as HCO, CH3 and C2H3. Also the active radicals are easier to combine with H, O and OH radicals in the reaction process to produce CO, CO2 and H2O. As a result, the reaction path can be optimized and the emissions of incomplete combustion products can be decreased. In addition, the effect of methane on CxHy emission is more obvious for equivalence ratio more than 1.0.

3.3 Effects of methane addition on CO and CO2 content in n-heptane combustion products. Results of the reaction rate of intermediate products in n-heptane combustion show that carbon monoxide and hydrogen are very active in the process of n-heptane combustion. In this section, the effect of methane addition on carbon monoxide and hydrogen contents in n-heptane combustion products is analysed. As shown in Fig. 3, in the n-heptane combustion process, hydrogen appears earlier than carbon monoxide. In the final stage of the reaction, hydrogen is transformed first, and finally carbon monoxide is converted to carbon dioxide. Additionally, ROP analyses of hydrogen, carbon monoxide, water and carbon dioxide have been carried out. As shown in Fig. 7, the main reactants for generating hydrogen includes CH2O, HCCO, C2H4 and NC7H16, and the major reactions that consume hydrogen are OH + H2 H + H2O and O + H2 H + OH. Fig. 8 shows that HCO and HCCO are the main reactants to generate carbon monoxide and the major reaction to consume carbon monoxide is CO + OH CO2 + H. At the same time, Fig. 9 and Fig. 10 represent the main reaction rates to generate water and carbon


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dioxide. Same as the results of Fig. 7 and Fig. 8, the main consumption paths of hydrogen and carbon monoxide are OH + H2 H + H2O and CO + OH CO2 + H, respectively. Furthermore, by comparing the locations where the maximum reaction rates of the two reactions occur, it can be found that hydrogen is easier to combine with hydroxyl groups to form water and then, carbon monoxide reacts with hydroxyl groups to form carbon dioxide.

It is well known that with the increase of methane content in the combustion of methane/n-heptane/air mixture, the C/O ratio decreases while the H/O ratio increases under the condition of constant equivalence ratio for n-heptane/methane/air combustion. As a result, the H2O/CO2 ratio increases in combustion product. Liang 43 studied the remixed laminar combustion characteristics of methane/n-heptane mixtures under 0.1 MPa at 393 K. Variations of the concentrations of the main radicals (H, OH and O) and the reaction rates of the main elementary reactions (R1 H ＋ O2 ＝ O ＋ OH and R24 CO ＋ OH ＝ CO2 ＋ H) in the planar flames of methane/n-heptane mixtures were analyzed, and it was found that the peaks of mole fractions of the main radicals and reaction rates of the main elementary reactions decreases with methane content increases. Thus, it can be concluded that the increase of methane content is unfavorable for the reaction CO＋OH＝CO2＋H. Carbon monoxide and carbon dioxide are the main products of premixed combustion of methane/n-heptane/air. The molar fraction of carbon monoxide and carbon dioxide in combustion products were measured under different equivalence ratios and methane contents by experiment. As shown in Fig. 11, with the increase of ACS Paragon Plus Environment

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equivalence ratio, the oxygen content decreases and the carbon monoxide content in combustion products increases. Furthermore, the content of carbon monoxide in the product increases with the increase of methane content under the same equivalence ratio. The increase of CO content is more significant with equivalence ratio more than 1.0. It indicates that increasing methane content is beneficial to the combustion of n-heptane, and the effect is more obvious while the equivalence ratio more than 1.0.

As shown in Fig.12, the molar fraction of carbon dioxide in combustion products is measured under different equivalence ratios and methane contents. With the increase of the equivalence ratio, the mole fraction of carbon dioxide in combustion products increases first and then decreases. At the same time, the molar fraction of carbon dioxide reaches its maximum when the equivalence ratio is near 1.0. At that point, the fuel and oxygen can react sufficiently and the combustion effect reaches to the best item.

4, 44

For the same equivalence ratio, the molar fraction of carbon dioxide

decreases with the increase of methane content, and this phenomenon is more obvious when the equivalence ratio is more than 1.0. It is well known that the C/O ratio decreased with the increase of methane content in the combustion process. As a result, the increase of methane content is not conducive to the conversion of carbon monoxide to carbon dioxide.

To further analyze the relationship between carbon monoxide and carbon dioxide mole fraction in combustion process, the molar fraction ratio of carbon monoxide to carbon dioxide is studied. As shown in Fig. 13, the combustion products are mainly carbon dioxide, and carbon monoxide content is very little, when the equivalence ratio ACS Paragon Plus Environment

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is less than 1.0 and the oxygen content in the combustion process is sufficient. With the increase of equivalence ratio, the oxygen content in the reaction process is decreased, and the fuel content is increased. Moreover, the incomplete combustion and carbon monoxide content are increased in the combustion products, while carbon dioxide content is decreased. When the equivalence ratio is about 1.15, the content of carbon monoxide and carbon dioxide is almost the same. Then, the carbon monoxide content is higher than that of carbon dioxide as the equivalence ratio continues to increase. It can be seen that as the methane content increases, the CO/CO2 ratio is increased. The results further indicate that the increase of methane is conductive to produce more carbon monoxide, while it is not conducive to the formation of carbon dioxide.

It is known that with the increase of methane content in the combustion of methane/n-heptane/air mixture, the C/O ratio is decreased with a constant equivalence ratio. In this section, the effect of methane addition on methane/n-heptane/air combustion is studied under the same C/O ratio. As reported in the literature 43, R24 (CO + OH CO2 + H) has a greater sensitivity coefficient in combustion of n-heptane and the higher the reaction rate of R24, the higher the combustion stability and efficiency, and vice versa. The peak values of R24 reaction rates in n-heptane/air combustion and n-heptane/methane/air combustion with different C/O ratios are shown in Fig. 14. For the combustion of n-heptane/methane/air, the content of methane is zero when the C/O ratio is 0.318. The results show that the peak value of R24 reaction rate decreases with the decrease of C/O ratio and the peak value of R24


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reaction rate of n-heptane/methane/air combustion is more than that of n-heptane/air combustion. Therefore, the addition of methane is beneficial to the combustion of n-heptane with a same C/O ratio. However, when the equivalence ratio is constant, the increase of methane content in fuel leads to the decrease of the C/O ratio in the reaction process and carbon monoxide conversion rate to carbon dioxide.

3.4 Effects of methane addition on n-heptane combustion efficiency. In this work, the effects of methane addition on CxHy, carbon monoxide and carbon dioxide contents of methane/n-heptane/air combustion products are investigated, respectively. The ratio of n-heptane combustion conversion rate is defined as the ratio of the total content of carbon dioxide and carbon monoxide in experiment to the content of carbon monoxide in theory calculation. As shown in Fig. 15, with the equivalence ratio increasing, n-heptane conversion increases first and then decreases, and the equivalence ratio reaches the maximum value at equivalence near1.0. Moreover, n-heptane conversion increases with the increase of methane content, and the maximum conversion of n-heptane can reach 95.7% for equivalence ratio of 1.0 and methane content of 0.5. This can be attributed to the increase of methane content optimizing the reaction path of n-heptane, which reduces the formation of CxHy and promotes the formation of carbon monoxide.

4. Conclusion In the study, the effects of the addition of methane on the emission characteristics of methane/n-heptane/air premixed laminar flames were investigated experimentally and


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Energy & Fuels

numerically. The methane fraction was in the range of 0 to 50% and the equivalence varied from0.8 to 1.2, while the Reynolds number of inlet flow was kept constant at 1000. The conclusions are summarized as follows:

(1) The main reaction path of n-heptane/air combustion was obtained. The combustion process of n-heptane can be divided into three parts, dehydrogenation stage, cracking stage and combustion stage. And, the formation and consumption of ethylene plays a crucial role in the n-heptane combustion, and the addition of methane is conducive to optimize the reaction path of n-heptane combustion and promote the transformation of carbon monoxide.

(2) The content of CxHy in combustion products increases with the increase of the equivalence ratio. With an identical equivalence ratio, increasing the methane content can decrease the content of CxHy in combustion products and improve the combustion efficiency, this effect is more obvious with the equivalence more than 1.0.

(3) The ROP analyses show that the hydroxyl group plays a key role for hydrogen to water and carbon monoxide to carbon dioxide. Also, hydrogen is easier to combine with hydroxyl groups than carbon monoxide. With the increase of methane content in the combustion of methane/n-heptane/air mixture, the C/O ratio decreases while the H/O ratio increases under the condition of constant equivalence ratio. Thus, the increase of methane content is not conducive to the conversion of carbon monoxide to carbon dioxide.


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

(4) The addition of methane is beneficial to the conversion of CO to CO2 under the same C/O ratio.

(5) The addition of methane in n-heptane/methane/air combustion can optimize the reaction path of n-heptane, which reduces the formation of CxHy and promotes the formation of carbon monoxide and improves the conversion efficiency of n-heptane.

Acknowledgments We gratefully acknowledge financial support from Graduate Scientific Research and Innovation Foundation of Chongqing, China (Grant No.CYB16022) and National Natural Science Foundation of China (Project No.51276207).

Nomenclature C x Hy

The total content of incomplete combustion products

NM

Methane content

XA

Mole fractions of air

XF

Mole fractions of n-heptane

XFuel

Mole fractions of fuel

XM

Mole fractions of methane

φ

Equivalent ratio

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Effects of methane addition on exhaust gas emissions and combustion

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