Influence of Ozone on Ignition and Combustion Performance of a Lean

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Influence of ozone on ignition and combustion performance of lean methane/air mixture Shaobo Ji, Xin Lan, Jing Lian, Huaimin Xu, Yanqiu Wang, Yong Cheng, and Yongqi Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02389 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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Influence of ozone on ignition and combustion performance of lean methane/air mixture Shaobo Ji1, Xin Lan1, Jing Lian1, Huaimin Xu1, Yanqiu Wang1, Yong Cheng1, Yongqi Liu2

1.

College of Energy and Power Engineering, Shandong University, Jinan 250061, Shandong

Province, China 2.

College of Transportation and Vehicle Engineering, Shandong University of Technology, Zibo

255000, Shandong Province, China

ABSTRACT: The feasibility of improving ignition and combustion performance of methane by ozone addition has been studied in present work. Combustion of lean methane/air mixture with ozone addition was conducted using constant volume combustion bomb firstly. Results showed that ozone could extend lean combustion limit and accelerate flame speed. Then, chemical reaction kinetic analysis was adopted to obtain ignition delay time and laminar flame speed which were used to analyze influence of ozone on ignition and combustion performance of lean methane/air mixture respectively. Analysis carried out with different equivalence ratio, initial temperature and pressure showed that ignition delay time was shortened obviously with ozone addition. Peak concentration of CH2O increased and appearing time of peak concentration advanced with ozone addition, which indicated that the beginning time of low temperature reaction was advanced with ozone addition and thus improved ignition performance. Laminar flame speed could be accelerated obviously with ozone addition in different equivalence ratio, especially under the condition of research. Concentration of OH and other intermediate products with ozone addition was compared and results showed that ozone could increase 1

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the amount of OH and other products, thus improve combustion performance. The results obtained in this study showed that ozone could shorten ignition delay time and accelerate burning velocity of lean methane/air mixture.

Keywords: ozone, constant volume combustion bomb, combustion pressure, ignition delay time, laminar flame speed, chemical reaction kinetic analysis

1. Introduction In order to reduce environmental pollution, higher emission standards which have been operated in many other countries are being imposed on engines in China. Therefore, alternative fuels are being used to meet the new standards. Methane has been used in the field of heavy duty gas engine due to its advantages, such as availability, low cost and less emissions and so on [1, 2]. For these engines, lean burn combustion has been widely adopted to increase engine thermal efficiency and improve its performance [3]. However, combustion of methane requires higher ignition energy and flame propagation velocity is also slower due to its physicochemical properties. Those problems will be exacerbated in lean burn combustion. To improve ignition and combustion performance, high energy ignition has been widely used in lean burn heavy duty gas engine. One of the most promising methods for lean combustion improvement is to adopt pre-combustion chamber. Combustion initially begins in pre-combustion chamber and its reacting products ignite the main chamber charge through chemical, thermal and turbulence effects, thus producing a distributed ignition system and providing a higher energy ignition source [4,5,6]. Another way to obtain high ignition energy is dual fuel mode which uses methane as primary fuel and diesel or other fuel with low cetane value as pilot fuel. This way can 2


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maintain high compression ratio with the resulting benefits in thermal efficiency due to compression ignition and multi-point ignition [7, 8, 9, 10]. The above two approaches need a higher requirements on engine structure, such as combustion chamber and fuel supply system and so on. There is another way to improve ignition and combustion performance of gas engine and that is to improve the physicochemical properties of methane. Molecular structure of methane contains C-H bond only. Compared to C-C bond energy, 347.3 kJ/mol, C-H bond energy is 415.2 kJ/mol

[11]

.This

shows that more energy is needed to break C-H bond and it is the main reason for ignition and combustion problem of methane. Therefore it is feasible to realize oxidative dehydrogenation of methane by adding special active species into lean methane/air mixture, which will reduce activation energy of the reactants and improve ignition and combustion performance of methane. In view of the strong oxidizing property, ozone is a high potential active substance. Influence of ozone on combustion was conducted with different kinds of engines. Alessandro et al.[12] and Foucher et al.[13] studied the effect of ozone on the ignition and combustion process in homogeneous charge compression ignition (HCCI) engine. They observed that low concentrations of ozone are needed to strongly advance the combustion phasing which was combined with a higher heat release rate on the cool flame. Masurier et al. [14] founded that ozone can strongly enhance and advance combustion when its concentration was lower than 50ppm. Moreover, the results on kinetic computations established that the promoting effect came from the decomposition of ozone into oxygen (O2) and O-atoms followed by rapid oxidation of the fuel. Yamada et al. [15] used ozone as an improver for the combustion and it showed that the use of this oxidizing species lead to an earlier thermal ignition and impacted the cool flame of this fuel. These results showed that ozone acts at the 3


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beginning of the combustion and can promote combustion process. Most of the studies conducted on engine were mainly focused on HCCI engine and most of the fuels used in the study were n-heptane, alcohol fuels, dimethyl ether and so on. There were few researches about influence of ozone on lean burn heavy duty gas engine. Influence of ozone on combustion has been conducted with different kinds of burners. Halter et al. [16] conducted experiments with a circular nozzle burner and their data showed that adding about 5000ppm ozone increased laminar flame speed (SL) by 5%. Wang et al. [17] using the heat flux burner showed 3.5% increase in SL with 3730ppm ozone addition, and 9% increase for 7000ppm ozone addition. Ombrelloet et al.[18] founded that approximately 4% enhancement of SL with 1260ppm ozone addition based on lifted laminar flame. Liang et al. [19] highlighted that 9% enhancement to SL could be reached with 8500ppm ozone addition. Experiments performed by different researchers have indicated that SL can be enhanced effectively by adding ozone. Most of the study was conducted with minimum equivalence ratio 0.8, while equivalence ratio of heavy duty gas engine was about 0.6 at present. In the study, Constant volume combustion bomb experiment and chemical reaction kinetic analysis were adopted to analyze influence of ozone on ignition performance and combustion characteristic under lean methane/air mixture condition, especially under 0.6 equivalence ratio condition which was widely used in heavy duty gas engine. The results had some specific implementation guidance for performance improvement of lean burn combustion heavy duty gas engine. 2. Experimental setup and kinetic modeling 4


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2.1. Apparatus The apparatus used in the study was shown in Fig.1. The constant volume combustion test equipment was composed of the constant volume combustion bomb body, ignition system, combustion data acquisition system and mixture supply system. The constant volume combustion bomb was the combustion chamber, in which methane and air mixture was burned. The combustion bomb is a cylindrical type with diameter of 130mm and length of 150mm. The mixture was ignited by two centrally located electrodes. Combustion pressure was measured using KISTLER 6055C piezoelectric pressure transducers (maximum calibration error of 0.6%). The sensor was connected to a KISTLER 5011 charge amplifier (maximum calibration error of 0.3%). Output of charge amplifier was recorded using a high speed data acquisition system. Mixture supply system was composed of gas source for methane, nitrogen (N2) and oxygen (O2), ozone generator, ozone analyzer and valves used for flow rate control. The ozone was generated by a dielectric barrier discharge (DBD) ozone generator (Jinan Ruiqing Co., Ltd.), supplied with a pure oxygen flow. The production of ozone was adjusted by changing the voltage of the ozone generator and the concentration of ozone in the O2 was measured with an ozone analyzer (Mini-HiCon high concentration ozone analyzer with a resolution of 0.1 g/m3). At the exit of the analyzer the ozone and O2 mixture was mixed with N2 to receive an air-like mixture (21% O2 and 79% N2 in volume). The equivalence ratio of mixture was adjusted according to the Dalton Partial Pressure Law. And partial pressure of each component in the mixture was calculated according to the initial pressure and equivalence ratio of mixture. Each component of mixture was filled into the constant volume combustion bomb sequentially. The pressure in the bomb was monitored by high-precision pressure sensor (maximum calibration error of 0.5%), which was also used to control equivalence ratio of the mixture.

5


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Fig.1. Sketch of constant volume combustion test equipment 2.2 Initial condition setting and signal processing method for combustion pressure Experiment was conducted by adjusting initial pressure and ozone addition amount, while initial temperature was fixed at 300K. The study was mainly to reveal influence of ozone addition on lean methane/air mixture, so equivalence ratio was set as 0.6, other initial condition setting was listed in Table.1. In order to ensure validity of measurement, combustion pressure was measured five times under each condition. The closest three measurement in each initial condition was averaged and used as the final results. Table.1Initial condition setting in the study Parameter

Parameter setting

equivalence ratio ∅

0.6

initial pressure T (K)

300

initial pressure P (bar) ozone addition (ppm)

2.5 0

3.0

500

1000

Pressure increase rate (PIR) was calculated from measured combustion pressure and PIR can be 6


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expressed in Eq. (1), where n is the number of samples, ( = 1, … , n ) is measured combustion pressure, is calculated pressure increase rate, ∆ is sampling interval which is 0.025ms in the study.

=

∆

(i = 1,…,n - 1)

(1)

2.3 Kinetic model In this study, influence of ozone addition on methane/air mixture ignition and combustion process was investigated through chemical reaction kinetic analysis from two aspects: ignition delay time and SL. The study was conducted based on Chemkin which is a powerful software package for solving the problem of complex chemical reactions. Accurate chemical reaction mechanism is the basis of the study and chemical reaction mechanism was composed of gas-phase kinetics input files, surface kinetics input files, thermodynamic data input file and transport input files. The input files contain elements, components, chemical reaction and corresponding parameter in the reaction process. Chemical reaction mechanism should be established according to reactants, products and chemical reactions. Reaction mechanism with methane, air and ozone should be established since chemical reaction kinetic analysis mainly involved the three substances in this study. GRI-mech3.0 is an optimized reaction mechanism for combustion analysis of methane and other hydrocarbons. In the study, GRI-mesh3.0 was coupled with ozone sub-mechanisms developed by Daguat[16]. Ozone sub-mechanisms were shown in Table.2. Reaction mechanism contained 54 kinds of species and 342 steps of elementary reactions. Coupled model was used for prediction influence of ozone on ignition 7


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delay time and SL of lean methane/air mixture, especially under 0.6 equivalence ratio condition. Table.2 Ozone sub-mechanisms, including decomposition, combination reaction and oxidation reaction of ozone O3+N2=>O2+O+N2 O2+O+N2=>O3+N2 O3+O2=>O2+O+O2 O2+O+O2=>O3+O2 O3+O3=>O2+O+O3 O2+O+O3=>O3+O3 O3+HO2+OH O3+OO2+O2 O3+OHO2+HO2 O3+HO2O2+OH+O2 O3+H2OO2+H2O2 O3+CH3O2+CH3O O3+NOO2+NO2 O3+NO2+NO O3+HO+HO2 O3+H2OH+HO2

8


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O3+CH4CH3O+HO2

Influence of ozone on ignition delay time was analyzed using closed homogeneous batch reactor which presented steady-state combustion simulation of ozone, methane, nitrogen and oxygen in a perfectly-stirred reactor. There was no flow of mass into or out of the reactor and the combustion was conducted in homogeneous condition. There were various ways to define ignition delay time, such as the time that derivative of temperature with respect to time reached specified value, the time that maximum or onset of certain species concentrations is reached and the time that luminous radiant output from the system was first observed and so on. In the study, Ignition delay time was defined as the time when derivative of temperature with respect to time reached peak value. Influence of ozone on SL was analyzed using premixed laminar flame-speed calculation model which could be used to solve flame speed of freely propagating methane/air mixture. In the model, the flame speed was defined as the inlet velocity (velocity of unburned gas moving towards the flame) that allowed the flame to stay in a fixed location, which was an eigenvalue of the solution method. To assist in the solution of the freely propagating flame problem, a method with continuations was used to refine the domain and grid of the solution until a desired accuracy and grid-independence was achieved. This method helped assuring quick convergence to an accurate solution. 2.4 Model validation Kinetic model was validated using experimental data obtained by other researchers. Larsson et al. [20] developed a skeletal kinetic reaction mechanism for methane-air combustion and validated it using experimental data. Their initial conditions were input to the kinetic model to calculate 9


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ignition delay time and the results were compared with Larsson’s data. Fig. 2 showed comparison result of ignition delay time under 10bar initial pressure and stoichiometric condition. It can be seen that the changing trend of the two results were similar. Combining with comparison data under other conditions, it can be concluded that simulation results were agreed well with Larsson’s data and this showed that the kinetic model can meet the needs of ignition delay time analysis.

Fig.2. Validation of the model for analysis of ignition delay time at P=10bar and ∅=1

The kinetic model was also validated for SL analysis. Wang et al. [17] measured SL of methane with different ozone addition. The experiment conditions of Wang were set as input parameters for the model and simulation results were compared with the experimental results. Fig.3 showed comparison results between two cases and it can be seen that changing trend between simulation data and experimental data was similar in both cases. Relative errors were calculated and results showed that the maximum error was 5.62% at all comparison data. Comparison data in other conditions demonstrated that the model can be used for SL analysis.

10


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(a)

(b)

Fig.3. Validation of the model for SL analysis at P=1bar and T=300K with different equivalence ratio, (a) ozone=0ppm and (b) ozone=2369ppm.

3. Results and discussions 3.1 Influence of ozone on combustion characteristics in constant volume combustion bomb Fig.4 showed comparison results of combustion pressure and PIR with different ozone addition under 2.5bar initial pressure. It can be seen from Fig.4 (a) that no combustion occurred under this initial condition without ozone addition, while combustion occurred with ozone addition which meant that ozone could extend lean combustion limit of methane/air mixture. Peak combustion pressure increased and appearing time of peak combustion pressure advanced with increment of ozone addition. Table.3 showed peak combustion pressure and appearing time were 1.24MPa/147ms and 1.32MPa/99ms respectively. Fig.4 (b) showed peak PIR were 0.014 and 0.024 MPa·ms-1 and this meant combustion of methane/air mixture was more vigorously with ozone addition. Analysis of combustion performance showed that ozone could improve combustion process and accelerate burning velocity of lean methane/air mixture. 11


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（a）

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（b）

Fig.4. Comparison result of combustion performance under 2.5 bar initial pressure, (a) combustion pressure and (b) PIR

Table.3 Contrast with different ozone addition under 250kPa initial pressure Ozone addition (ppm)

0

500

1000

Peak Pressure (MPa)

--

1.24

1.32

Appearance timing of peak pressure (ms) --

147

99

Peak PIR (MPa·ms-1)

-- 0.014 0.024

Fig.5 showed comparison results of combustion pressure and PIR with different ozone addition under 3.0 bar initial pressure and 300K initial temperature. Fig.5 (a) showed that peak combustion pressure increased and appearing time advanced with increment of ozone addition, which was similar with Fig.4. It can be seen from Table.4 that peak combustion pressure and appearing time were 1.50MPa/239ms, 1.56MPa/173ms and 1.61MPa/123ms respectively. Table.4 also showed peak PIR were 0.014, 0.017 and 0.026 MPa·ms-1 respectively which meant that combustion of methane/air mixture was more vigorously with increment of ozone addition. 12


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（a）

（b）

Fig.5. Comparison result of combustion performance under 3.0 bar initial pressure, (a) combustion pressure and (b) PIR

Table.4 Contrast with different ozone addition under 300kPa initial pressure Ozone addition (ppm)

0

500

1000

Peak Pressure (MPa)

1.50

1.56

1.61

Appearance timing of peak pressure (ms)

239

173

123

Peak PIR (MPa·ms-1)

0.014 0.017 0.026

Experiment conducted with constant volume combustion bomb showed that ozone could improve combustion performance of lean methane/air mixture. Lean burn limit was extended and burning velocity was accelerated with ozone addition. To further investigate influence of ozone on ignition and combustion performance of lean methane/air mixture, chemical reaction kinetic analysis was conducted. Ignition delay time and laminar flame speed were obtained and used to analyze influence of ozone on ignition and combustion performance of lean methane/air mixture respectively. 13


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3.2 Influence of ozone on ignition delay time Fig.6 showed influence of ozone on ignition delay time with equivalence ratio varied from 0.5 to 0.8. It can be concluded from Fig.6 (a) that ozone addition could shorten ignition delay time obviously. Ignition delay time decreased with increment of equivalence ratio when ozone addition stayed constant and decreased with increment of ozone addition in the same equivalence ratio. Fig.6 (b) showed relative variation of ignition delay time with different ozone addition compared to none ozone addition situation under different equivalence ratio conditions. It can be seen that the relative variation of ignition delay time was more obvious when ozone addition was less than 4500ppm under the condition of research. For instance, when equivalence ratio was 0.8 and ozone addition was 4500ppm, relative variation of ignition delay time was about 24797μs compared to none ozone addition situation. While the relative variation of ignition delay time was only about 63μs with ozone addition increased from 6500ppm to 8500ppm.

(a)

(b)

Fig.6. Effect of ozone addition on ignition delay time with different equivalence ratio, P=30bar and T=1000K. (a) 14


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Ignition delay time with different ozone addition and (b) relative variation of ignition delay time with different ozone addition compared to none ozone addition situation. Due to the great effect of ozone on ignition delay time, logarithmic coordinate system has been used in Fig.6 (a)

Fig.7 showed influence of ozone addition on ignition delay time in different initial temperature and pressure when equivalence ratio was set as 0.6. It can be seen that ignition delay time decreased with increasing of temperature and pressure whether there was ozone addition or not. Compared with pressure, temperature had a greater influence on ignition delay time and this was consistent with previous analysis. The variation tendency of ignition delay time with ozone addition remained unchanged in different temperature and pressure situation, however the ignition delay time was shortened obviously from the order of 104μs to102μs.

(a)

(b)

Fig.7. Effect of ozone addition on the ignition delay time in different temperature and pressure situation with ∅=0.6, (a) ozone=0ppm and (b) ozone=4500ppm

In order to clarify influence of ozone addition on ignition delay time in different temperature and pressure situation more clearly, part of data in Fig.7 was compared using the form of histogram 15


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and the results were showed in Fig.8. It can be concluded that ozone addition had more influence on ignition delay time compared to initial temperature and pressure. For the compared data, whether the ozone was added or not, the ignition delay time of mixture under the condition of 20bar pressure and 1000K temperature was longer than that in other situations because of the smaller pressure and lower temperature, and the difference of the ignition delay time was more obvious without ozone addition than that with ozone addition. Therefore, the ignition delay time under the condition of 20bar pressure and 1000K temperature with ozone addition was improved more greatly. So the maximum variation of ignition delay time appeared at 20bar pressure and 1000K temperature situation, in which the ignition delay time with 4500ppm ozone addition was only about one of seventy-ninth of that without ozone addition. The minimum variation of ignition delay time appeared at 30bar pressure and 1150K temperature situation. Even in this case, the ignition delay time with 4500ppm ozone addition was about one of thirty-first of that without ozone addition. Comparison results showed that ozone addition could shorten ignition delay time obviously and could be an effective way to improve ignition performance of lean methane/air mixture.

(a)

(b) 16


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Fig.8. Contrast of ignition delay time with different ozone addition, ∅=0.6. (a) ozone=0ppm and (b) ozone= 4500ppm

CH2O is a product of low temperature oxidation and can be used to reflect status of low temperature reaction

[21]

. Appearing time of OH could be used to reflect beginning time of

combustion. The variation trend of CH2O and OH with different ozone addition was studied and the results were shown in Fig.9. Peak concentration and appearing time of CH2O and OH were shown in Table.5. Compared with none ozone addition situation, peak concentration increased about 870ppm and 290ppm for OH and CH2O respectively when ozone addition was 6500ppm and their relative variations were 18.2% and 8.5% respectively. Appearing time of peak concentration of CH2O and OH was advanced obviously with ozone addition. Earlier appearance of CH2O peak concentration indicated that low temperature reaction was advanced. The increment of OH peak concentration reflected that reaction was accelerated with ozone addition.

(a)

(b)

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(c)

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(d)

Fig.9. Comparison of radicals generation with different ozone addition with P=30bar,T= 1000K and ∅=0.6, (a) ozone=0ppm, (b) ozone=2500ppm,(c) ozone= 4500ppm and (d) ozone=6500ppm

Table.5 Peak concentration and its appearing time with different ozone addition Appearing time of Peak peak

Ozone concentration

concentration

addition （ppm））

）（μs）

（ppm）） OH

CH2O

OH

CH2O

0

4770

3390

24462

24311

2500

5100

3510

675

647

4500

5360

3590

332

304

6500

5640

3680

198

170

3.3 Influence of ozone on SL Influence of ozone on SL of lean methane/air mixture was studied using kinetic modeling. Fig.10 18


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showed comparison of SL with different ozone addition and the variation trend was similar in different situation. It can be seen that SL increased with the increment of equivalence ratio when ozone addition stayed constant. In the same equivalence ratio, SL increased with the increment of ozone addition.

Fig.10. Effect of ozone addition on SL with P=3bar and T=300K

Fig.11 showed comparison of SL when equivalence ratios increased from 0.60 to 0.80 with 2500, 4500, 6500 and 8500ppm ozone addition respectively. It can be seen that SL increased with the increment of ozone addition when equivalence ratio kept constant. Compared with none ozone addition situation, SL with 8500ppm ozone addition increased 5.76cm/s, 6.60cm/s, 7.38cm/s, 7.98cm/s and 8.44cm/s and relative increment were 50.3%, 43.1%, 38.2%, 34.2% and 31.1% respectively for different equivalence ratio. It can be concluded that ozone played important role on SL under lean methane/air mixture condition. So it is feasible to improve performance of lean methane/air mixture.

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Fig.11. Comparison of SL in different equivalence ratio and ozone addition with P=3bar and T=300K

Fig.12 (a) showed influence of initial temperature and pressure on SL with 0ppm and 4500ppm ozone addition. It can be seen that SL increased with the increment of temperature whether there was ozone addition or not. This was because the number of activated molecules would increase and molecular thermal motion would intensify when initial temperature was increased. Due to the above reasons, effective collision frequency between methane and oxygen molecules would increase, thus accelerated the reaction rate [22].It can also be seen that SL decreased with the increment of pressure. This was because mixture density would increase with increment of initial pressure, which would slow down SL

[23]

. Fig. 12(b) also indicated that the variation tendency of SL with ozone addition

remained unchanged in different temperature and pressure situation, while SL increased obviously with ozone addition.

20


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(a)

(b)

Fig.12. Effect of ozone addition on SL at ∅=0.6 with different initial temperature and pressure, (a) ozone=0ppm and (b) ozone= 4500ppm

Fig.13 showed influence of ozone addition on SL under different initial temperature and pressure condition with equivalence ratio was 0.6. It can be seen that laminar flame velocity increased with ozone addition. Compared to the condition of none ozone addition, the maximum relative increment of SL was about 36% at 5.4 bar and 300K condition and the minimum relative increment of SL was 19% at 3bar and 600K condition with 4500ppm ozone addition.

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(a)

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(b)

Fig.13. Effect of ozone addition on SL at ∅=0.6, (a) ozone=0ppm and (b) ozone= 4500ppm

Because flame and reaction of mixture is not ‘visible’, in order to further analyze influence of ozone on SL, concentration of OH was applied to explain flame behavior of mixture with ozone addition [24]. Concentration of OH was compared at 3bar pressure, 300K temperature and 0.6 equivalence ratio with different ozone addition and the results were shown in Fig.14. It can be seen that concentration of OH increased with the increment of ozone addition. Compared to none ozone addition situation, concentration of OH increased 646ppm and relative increment was about 28% with 6500ppm ozone addition. So it could be concluded that combustion can be enhanced with ozone addition and thus increased concentration of OH.

Fig.14. Influence of ozone on concentration of OH, P=3bar，T=300K，∅=0.6

Except for OH，combustion process would produce other intermediate products such as O, H, CH2O, H2O2, HO2 and so on. These products were originated from some reactions initiated by ozone decomposition and they had great influence on combustion process. To explain the enhancement of ozone on combustion performance and the formation of those intermediate products, kinetic analysis 22


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was conducted at 3bar pressure, 300K temperature and 0.6 equivalence ratio with 0ppm and 4500ppm ozone addition. It can be seen from Fig.15 that compared with none ozone addition situation, concentration of those intermediate products increased with 4500ppm ozone addition. In methane-air flames, O atoms were mainly derives from reactions of H atoms with O2 and HO2. H+O2O+OH

(R38)

H+HO2H+H2O

(R46)

While in ozone-methane-air flames, O-atom was mainly produced by O3 decomposition and reaction of O3 with O2. O3+N2O2+O+N2

(R326)

O3+O22O2+O

(R328)

As a result, the production rate of O atom was strongly increased with ozone addition. This conclusion can also be seen in Fig. 15, with 4500ppm ozone addition, the peak concentration of O increased 310ppm compared with none ozone addition situation. The variation of concentration of other active species was also caused by ozone addition. As shown in Fig. 15, with 4500ppm ozone addition, the concentration of H, CH2O, H2O2 and HO2 increased 190, 199, 55 and 32 ppm respectively. Following reactions produced these active species: O+CH4OH+CH3

(R11)

O3+CH3O2+CH3O

(R337)

In turn, these species enhanced decomposition of methane: O+CH3H+CH2O HO2+CH3OH+CH3O 23


(R10) (R119)

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CH3O(+M)H+CH2O(+M)

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(R57)

With ozone addition, augmentation of these reactions increased the concentration of OH and other active species while the sensitivity factors of termination reactions decrease, which improves the ignition and combustion performance. In addition, the peak temperature also increased with ozone addition, which meant an improvement of ozone on some exothermic reactions, such as: O+CH3H+CH2O

(R10)

O+CH2OOH+HCO

(R15)

For these reasons, it can be concluded that ozone could increase the generation quantity of the intermediate products, thus it could enhance combustion process.

Fig.15. Effect of ozone addition on generation of intermediate materials at P=3bar，T=300K and φ=0.6, 4500ppm ozone introduced(solid line) and no ozone introduced(dashed line)

4 Conclusion Constant volume combustion bomb experiment and chemical reaction kinetic analysis were conducted to study influence of ozone on ignition and combustion performance of lean methane/air mixture, especially concerned 0.6 equivalence ratio which was frequently used in heavy duty gas 24


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engine. Combustion pressure and PIR were used to study influence of ozone on combustion performance of lean methane/air mixture. Kinetic model was validated by experimental data and then adopted to obtain ignition delay time and SL which were used to reflect influence of ozone on ignition and combustion performance of lean methane/air mixture respectively. From the study, the conclusions were drawn as follows： 1) Experiment conducted with constant volume combustion bomb showed that ozone could extend lean burn limit and improve combustion performance of lean methane/air mixture. Peak combustion pressure increased and appearing time of peak combustion pressure advanced with increment of ozone addition. Analysis of peak PIR showed that combustion of methane/air mixture was more vigorously with increment of ozone addition. 2) Ignition delay time could be reduced from the order of 104μs to 102μs with ozone addition under the conditions used in the study. For all equivalence ratio used in the study, ignition delay time reduced obviously when concentration of ozone was less than 4500ppm, while ignition delay time did not change significantly when ozone addition was more than 4500ppm. 3) Influence of ozone on ignition delay time was studied under different initial temperature and pressure conditions with 4500ppm ozone addition. Results showed that the ignition delay time with ozone addition was only about one of seventy-ninth of that without ozone addition under the most obvious condition in the study, which was about one of thirty-first under the least obvious condition. Ozone had greater influence on ignition delay time compared with initial temperature. Ozone addition was an effective way to improve ignition performance of lean methane/air mixture. 25


Energy & Fuels

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4) Peak concentration of CH2O increased 290ppm with 6500ppm ozone addition and corresponding relative variation was 8.5%. The appearing time of peak concentration of CH2O was advanced significantly with ozone addition. 5) Influence of ozone on SL was studied under different equivalence ratio. It can be concluded that SL increased with the increment of ozone under all equivalence ratio used in the study. The maximum relative variation was 50.3% with 8500ppm ozone addition when equivalence ratio was 0.6 under the condition in the study. Ozone addition had obvious positive impact on combustion improvement of lean methane/air mixture. 6) Influence of ozone on SL was studied under different initial temperature and pressure conditions with 4500ppm ozone addition. Results showed that the maximum increment of SL was 36% and the minimum of that was 19% under the initial condition used in the study, which indicated that it was feasible to use ozone to improve combustion performance of lean methane/air mixture. 7) Influence of ozone addition on concentration of OH and other intermediate materials was studied under lean methane/air mixture conditions and results showed that concentration of OH and other intermediate materials increased with the increment of ozone. The concentration of OH increased 646ppm and corresponding relative variation was 28% with 6500ppm ozone addition. This study is financially supported by State Key Laboratory of Engines, Tianjin University (Grant No. K2017-09), Natural Science Foundation of Shandong Province (Grant No. ZR2013EEQ026) and China Postdoctoral Science Foundation (Grant No. 2015M572029). 26


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Abbreviations: SL, laminar flame speed; HCCI, homogeneous charge compression ignition; N2, nitrogen;O2, oxygen; DBD, dielectric barrier discharge; P, pressure; T, temperature; ∅, equivalence ratio; PIR, Pressure increase rate; O, O-atom; H, H-atom; CH2O, formaldehyde; OH, hydroxyl; H2O2 ,hydrogen peroxide; HO2 hydroperoxyl References: （1）

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Influence of Ozone on Ignition and Combustion Performance of a Lean

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