Influence of Ozone on Ignition and Combustion Performance of a Lean

Article Cite This: Energy Fuels 2017, 31, 14191−14200

pubs.acs.org/EF

Influence of Ozone on Ignition and Combustion Performance of a Lean Methane/Air Mixture Shaobo Ji,*,† Xin Lan,† Jing Lian,† Huaimin Xu,† Yanqiu Wang,† Yong Cheng,† and Yongqi Liu‡ †

College of Energy and Power Engineering, Shandong University, Jinan, Shandong Province 250061, China College of Transportation and Vehicle Engineering, Shandong University of Technology, Zibo, Shandong Province 255000, China

‡

ABSTRACT: The feasibility of improving ignition and combustion performance of methane by ozone addition has been studied in the present work. First, combustion of a lean methane/air mixture with ozone addition was conducted by using a constant volume combustion bomb. Results showed that ozone could extend the 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 the influence of ozone on ignition and the combustion performance of the 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. The peak concentration of CH2O increased and the time of peak concentration appearance advanced with ozone addition, which indicated that the beginning time of the 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 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 the lean methane/air mixture.

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 engines. One of the most promising methods for lean combustion improvement is to adopt a precombustion chamber. Combustion initially begins in a precombustion 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.4−6 Another way to obtain high ignition energy is dual fuel mode which uses methane as a primary fuel and diesel or other fuel with low cetane value as a pilot fuel. This way can maintain a high compression ratio with the resulting benefits in thermal efficiency due to compression ignition and multipoint ignition.7−10 The above two approaches need 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 a gas engine and that is to improve the physicochemical properties of methane. The molecular © 2017 American Chemical Society

structure of methane contains a C−H bond only. Compared to the C−C bond energy, 347.3 kJ/mol, the C−H bond energy is 415.2 kJ/mol.11 This shows that more energy is needed to break the C−H bond, and it is the main reason for the ignition and combustion problem of methane. Therefore, it is feasible to realize oxidative dehydrogenation of methane by adding special active species into the 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 highly potential active substance. The influence of ozone on combustion was conducted with different kinds of engines. Schönborn 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 50 ppm. 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 beginning of the combustion and can promote the combustion process. Most of the studies conducted on an engine were Received: August 15, 2017 Revised: November 2, 2017 Published: November 22, 2017 14191

DOI: 10.1021/acs.energyfuels.7b02389 Energy Fuels 2017, 31, 14191−14200

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Energy & Fuels

Figure 1. Sketch of constant volume combustion test equipment. 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 the mixture was adjusted according to the Dalton Partial Pressure Law, and the partial pressure of each component in the mixture was calculated according to the initial pressure and equivalence ratio of the mixture. Each component of mixture was filled into the constant volume combustion bomb sequentially. The pressure in the bomb was monitored by a highprecision pressure sensor (maximum calibration error of 0.5%), which was also used to control the equivalence ratio of the mixture. 2.2. Initial Condition Setting and Signal Processing Method for Combustion Pressure. The experiment was conducted by adjusting the initial pressure and ozone addition amount, while the initial temperature was fixed at 300 K. The study was mainly to reveal the influence of ozone addition on the lean methane/air mixture, so the equivalence ratio was set as 0.6; other initial condition settings are listed in Table 1. To ensure validity of the measurement, the

mainly focused on the HCCI engine, and most of the fuels used in the study were n-heptane, alcohol fuels, dimethyl ether, and so on. There was little research about the influence of ozone on a lean-burn heavy duty gas engine. The 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 5000 ppm ozone increased laminar flame speed (SL) by 5%. Wang et al.17 using the heat flux burner showed 3.5% increase in SL with 3730 ppm ozone addition, and 9% increase for 7000 ppm ozone addition. Ombrello et al.18 founded that approximately 4% enhancement of SL with 1260 ppm ozone addition based on lifted laminar flame. Liang et al.19 highlighted that 9% enhancement to SL could be reached with 8500 ppm 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 a minimum equivalence ratio 0.8, while the equivalence ratio of a heavy duty gas engine was about 0.6 at present. In this study, a constant volume combustion bomb experiment and a chemical reaction kinetic analysis were adopted to analyze the influence of ozone on ignition performance and combustion characteristics under lean methane/air mixture conditions, especially under a 0.6 equivalence ratio condition which is widely used in heavy duty gas engines. The results had some specific implementation guidance for performance improvement of lean-burn combustion heavy duty gas engines.

Table 1. Initial Condition Setting in the Study parameter

parameter setting

equivalence ratio φ initial pressure T (K) initial pressure P (bar) ozone addition (ppm)

0.6 300 2.5, 3.0 0, 500, 1000

combustion pressure was measured five times under each condition. The closest three measurements in each initial condition was averaged and used as the final results. The pressure increase rate (PIR) was calculated from the measured combustion pressure, and PIR can be expressed in eq 1, where n is the number of samples, pi(i = 1, ..., n) is the measured combustion pressure, PIRi is the calculated pressure increase rate, Δt is sampling interval which is 0.025 ms in the study. p − pi PIR i = i + 1 (i = 1, ..., n − 1) (1) Δt

2. EXPERIMENTAL SETUP AND KINETIC MODELING 2.1. Apparatus. The apparatus used in the study was shown in Figure 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 the methane and air mixture was burned. The combustion bomb is a cylindrical type with a diameter of 130 mm and length of 150 mm. The mixture was ignited by two centrally located electrodes. The 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. A mixture supply system was composed of a 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

2.3. Kinetic Model. In this study, the influence of ozone addition on methane/air mixture ignition and combustion process was investigated through the 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. An accurate chemical reaction mechanism is the basis of the study and the 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 the corresponding parameter in the reaction process. The 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 14192


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Energy & Fuels 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, GRImesh3.0 was coupled with ozone submechanisms developed by Daguat.16 Ozone submechanisms were shown in Table 2. The reaction

Table 2. Ozone Submechanisms, Including Decomposition, Combination Reaction, and Oxidation Reaction of Ozone O3 O2 O3 O2 O3 O2 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3

+ + + + + + + + + + + + + + + + +

N2 → O2 + O + N2 O + N2 → O3 + N2 O2 → O2 + O + O2 O + O2 → O3 + O2 O3 → O2 + O + O3 O + O3 → O3 + O3 H ↔ O2 + OH O ↔ O2 + O2 OH ↔ O2 + HO2 HO2 ↔ O2 + OH + O2 H2O ↔ O2 + H2O2 CH3 ↔ O2 + CH3O NO ↔ O2 + NO2 N ↔ O2 + NO H ↔ O + HO2 H2 ↔ OH + HO2 CH4 ↔ CH3O + HO2

Figure 2. Validation of the model for analysis of ignition delay time at P = 10 bar and φ = 1. experiment conditions of Wang were set as input parameters for the model, and simulation results were compared with the experimental results. Figure 3 shows the comparison results between two cases, and it can be seen that the trend of change 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.

3. RESULTS AND DISCUSSIONS 3.1. Influence of Ozone on Combustion Characteristics in Constant Volume Combustion Bomb. Figure 4 showed comparison results of combustion pressure and PIR with different ozone addition under 2.5 bar initial pressure. It can be seen from Figure 4a that no combustion occurred under this initial condition without ozone addition, while combustion occurred with ozone addition, which meant that ozone could extend the lean combustion limit of the methane/air mixture. Peak combustion pressure increased, and time of appearance of peak combustion pressure advanced with the increment of ozone addition. Table 3 shows peak combustion pressure and appearance time were 1.24 MPa/147 ms and 1.32 MPa/99 ms, respectively. Figure 4b showed peak PIR were 0.014 and 0.024 MPa·ms−1, and this meant that the combustion of methane/air mixture was more vigorous with the ozone addition. Analysis of the combustion performance showed that ozone could improve the combustion process and accelerate the burning velocity of the lean methane/air mixture. Figure 5 showed comparison results of combustion pressure and PIR with different ozone addition under 3.0 bar initial pressure and 300 K initial temperature. Figure 5a showed that peak combustion pressure increased and appearance time advanced with the increment of ozone addition, which was similar to that seen in Figure 4. It can be seen from Table 4 that peak combustion pressure and appearing time were 1.50 MPa/ 239 ms, 1.56 MPa/173 ms, and 1.61 MPa/123 ms, respectively. Table 4 also showed peak PIR were 0.014, 0.017, and 0.026 MPa·ms−1, respectively, which meant that combustion of the methane/air mixture was more vigorous with the increment of ozone addition. An experiment conducted with a constant volume combustion bomb showed that ozone could improve the 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

mechanism contained 54 kinds of species and 342 steps of elementary reactions. The coupled model was used for the prediction influence of ozone on ignition delay time and SL of lean methane/air mixture, especially under a 0.6 equivalence ratio condition. The influence of ozone on ignition delay time was analyzed using a 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 the derivative of temperature with respect to time reached a 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 the derivative of temperature with respect to time reached a peak value. The influence of ozone on SL was analyzed using a premixed laminar flame-speed calculation model which could be used to solve the flame speed of a freely propagating methane/air mixture. In the model, the flame speed was defined as the inlet velocity (velocity of unburned gas moving toward 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. The 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 ignition delay time, and the results were compared with Larsson’s data. Figure 2 shows the comparison result of ignition delay time under 10 bar initial pressure and stoichiometric conditions. It can be seen that the changing trend of the two results were similar. When combined with comparison data under other conditions, the simulation results agreed well with Larsson’s data, and this showed that the kinetic model can meet the needs of ignition delay time analysis. The kinetic model was also validated for SL analysis. Wang et al.17 measured the SL of methane with different ozone additions. The 14193


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

Figure 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 250 kPa Initial Pressure ozone addition (ppm) peak pressure (MPa) appearance timing of peak pressure (ms) peak PIR (MPa·ms−1)

0

Table 4. Contrast with Different Ozone Addition under 300 kPa Initial Pressure

500

1000

ozone addition (ppm)

0

500

1000

1.24 147 0.014

1.32 99 0.024

peak pressure (MPa) appearance timing of peak pressure (ms) peak PIR (MPa·ms−1)

1.50 239 0.014

1.56 173 0.017

1.61 123 0.026

3.2. Influence of Ozone on Ignition Delay Time. Figure 6 showed influence of ozone on ignition delay time with equivalence ratio varied from 0.5 to 0.8. It can be concluded from Figure 6a that ozone addition could shorten ignition delay time obviously. Ignition delay time decreased with increment of

mixture, chemical reaction kinetic analysis was conducted. Ignition delay time and laminar flame speed were obtained and used to analyze the influence of ozone on ignition and the combustion performance of the lean methane/air mixture, respectively.

Figure 5. Comparison result of combustion performance under 3.0 bar initial pressure: (a) combustion pressure and (b) PIR. 14194


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Figure 6. The effect of ozone addition on ignition delay time with different equivalence ratio, P = 30 bar and T = 1000 K. (a) Ignition delay time with different ozone addition and (b) relative variation of ignition delay time with different ozone addition compared to the no ozone addition situation. Owing to the great effect of ozone on ignition delay time, the logarithmic coordinate system has been used in panel a.

Figure 7. Effect of ozone addition on the ignition delay time in different temperature and pressure situations with φ = 0.6: (a) ozone = 0 ppm and (b) ozone = 4500 ppm.

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

Figure 7 shows the influence of ozone addition on ignition delay time in different initial temperatures and pressures when the equivalence ratio was set as 0.6. It can be seen that ignition delay time decreased with increasing 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 analyses. The variation tendency of ignition delay time with ozone addition remained unchanged in different temperature and pressure situations; however, the ignition delay time was shortened obviously from the order of 104 μs to 102 μs. To clarify the influence of ozone addition on ignition delay time in different temperature and pressure situations more clearly, part of data in Figure 7 was compared in the form of

equivalence ratio when ozone addition stayed constant and decreased with increment of ozone addition in the same equivalence ratio. Figure 6b showed relative variation of ignition delay time with different ozone additions compared to a no 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 4500 ppm under the condition of research. For instance, when the equivalence ratio was 0.8 and ozone addition was 4500 ppm, the relative variation of ignition delay time was about 24797 μs compared to the no ozone addition situation, whereas the relative variation of ignition delay time was only about 63 μs with an ozone addition increase from 6500 to 8500 ppm. 14195


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

histograms, and the results were shown in Figure 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 the mixture under the conditions of 20 bar pressure and 1000 K 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 20 bar pressure and 1000 K temperature with ozone addition was greatly improved. So the maximum variation of ignition delay time appeared at the 20 bar pressure and 1000 K temperature situation, in which the ignition delay time with 4500 ppm ozone addition was only about 1/79 of that without ozone addition. The minimum variation of ignition delay time appeared at the 30 bar pressure and 1150 K temperature situation. Even in this case, the ignition delay time with 4500 ppm ozone addition was about 1/31 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 a lean methane/air mixture. CH2O is a product of low temperature oxidation and can be used to reflect the status of a low temperature reaction.21 The appearance time of OH could be used to reflect the beginning time of combustion. The variation trend of CH2O and OH with different ozone additions was studied, and the results were shown in Figure 9. Peak concentration and appearance time of CH2O and OH were shown in Table 5. Compared with the no

Table 5. Peak Concentration and Its Time of Appearance with Different Ozone Addition ozone addition

peak concentration (ppm)

appearing time of peak concentration (μs)

(ppm)

OH

CH2O

OH

CH2O

0 2500 4500 6500

4770 5100 5360 5640

3390 3510 3590 3680

24462 675 332 198

24311 647 304 170

ozone addition situation, peak concentration increased about 870 and 290 ppm for OH and CH2O, respectively, when ozone addition was 6500 ppm, and their relative variations were 18.2% and 8.5%, respectively. The appearance time of peak concentration of CH2O and OH was advanced obviously with ozone addition. The earlier appearance of the CH2O peak concentration indicated that low temperature reaction was advanced. The increment of OH peak concentration reflected that the reaction was accelerated with ozone addition. 3.3. Influence of Ozone on SL. The influence of ozone on the SL of lean methane/air mixture was studied using kinetic modeling. Figure 10 showed comparison of SL with different ozone additions and the variation trend was similar in different situations. It can be seen that SL increased with the increment of equivalence ratio when the ozone addition remained constant. In the same equivalence ratio, SL increased with the increment of ozone addition. Figure 11 showed comparison of SL when equivalence ratios increased from 0.60 to 0.80 with 2500, 4500, 6500, and 8500 14196


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Figure 12a showed the influence of initial temperature and pressure on SL with 0 and 4500 ppm 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 the initial temperature was increased. Because of the above reasons, the effective collision frequency between methane and oxygen molecules would increase, thus accelerating 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 Figure 12b also indicated that the variation tendency of SL with ozone addition remained unchanged in different temperature and pressure situations, while SL increased obviously with the ozone addition. Figure 13 showed the influence of ozone addition on SL under different initial temperatures and pressure conditions when the 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 300 K conditions and the minimum relative increment of SL was 19% at 3 bar and 600 K conditions with 4500 ppm ozone addition. Because the flame and reaction of the mixture is not “visible”, in order to further analyze the influence of ozone on SL, the concentration of OH was applied to explain flame behavior of the mixture with ozone addition.24 The concentration of OH was compared at 3 bar pressure, 300 K temperature and 0.6 equivalence ratio with different ozone additions and the results were shown in Figure 14. It can be seen that the concentration of OH increased with the increment of ozone addition. Compared to the no ozone addition situation, the concentration of OH increased to 646 ppm and relative increment was about 28% with 6500 ppm ozone addition. So it could be concluded that combustion can be enhanced with the ozone addition and thus the increased concentration of OH. Except for OH, the 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 the combustion process. To explain the enhancement of ozone on the combustion performance and the formation of those intermediate products, kinetic analysis was conducted at 3 bar pressure, 300 K temperature, and 0.6 equivalence ratio with 0 and 4500 ppm ozone addition. It can be seen from

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

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

ppm ozone addition, respectively. It can be seen that the SL increased with the increment of ozone addition when the equivalence ratio remained constant. Compared with no ozone addition situation, SL with 8500 ppm ozone addition increased 5.76, 6.60, 7.38, 7.98, and 8.44 cm/s, and relative increments were 50.3% 43.1% 38.2% 34.2% and 31.1%, respectively, for different equivalence ratio. It can be concluded that ozone played an important role on SL under lean methane/air mixture conditions. So it is feasible to improve performance of lean methane/air mixture.

Figure 12. Effect of ozone addition on SL at φ = 0.6 with different initial temperature and pressure: (a) ozone = 0 ppm and (b) ozone= 4500 ppm. 14197


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Figure 13. Effect of ozone addition on SL at φ = 0.6: (a) ozone = 0 ppm and (b) ozone = 4500 ppm.

As a result, the production rate of the O atom was strongly increased with the ozone addition. This conclusion can also be seen in Figure 15, with 4500 ppm ozone addition, the peak concentration of O increased 310 ppm compared with the no ozone addition situation. The variation of concentration of other active species was also caused by the ozone addition. As shown in Figure 15, with 4500 ppm ozone addition, the concentration of H, CH2O, H2O2, and HO2 increased 190, 199, 55, and 32 ppm, respectively. The following reactions produced these active species: Figure 14. Influence of ozone on concentration of OH, P = 3 bar, T = 300 K, φ = 0.6.

O + CH4 ⇔ OH + CH3

(R5)

O3 + CH3 ⇔ O2 + CH3O

(R6)

In turn, these species enhanced the decomposition of methane: Figure 15 that compared with the no ozone addition situation, the concentration of those intermediate products increased with 4500 ppm ozone addition. In methane−air flames, O atoms were mainly derived from the reactions of H atoms with O2 and HO2. H + O2 ⇔ O + OH

(R1)

H + HO2 ⇔ H + H 2O

(R2)

(R3)

O3 + O2 ⇔ 2O2 + O

(R4)

(R7)

HO2 + CH3 ⇔ OH + CH3O

(R8)

CH3O( + M) ⇔ H + CH 2O( +M)

(R9)

With the 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

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

O + CH3 ⇔ H + CH 2O

O + CH3 ⇔ H + CH 2O

(R10)

Figure 15. The effect of ozone addition on the generation of intermediate materials at P = 3 bar, T = 300 K, and φ = 0.6: 4500 ppm ozone introduced (solid line) and no ozone introduced (dashed line). 14198


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Energy & Fuels O + CH 2O ⇔ OH + HCO

(R11)

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

4. CONCLUSION A 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 concerning the 0.6 equivalence ratio which is frequently used in heavy duty gas engines. Combustion pressure and PIR were used to study the influence of ozone on combustion performance of the lean methane/air mixture. The kinetic model was validated by experimental data and then adopted to obtain ignition delay time and SL, which were used to reflect the influence of ozone on ignition and combustion performance of lean methane/air mixtures, respectively. From the study, the conclusions were drawn as follows: 1. The experiment conducted with constant volume combustion bomb showed that ozone could extend a lean burn limit and improve combustion performance of a lean methane/air mixture. The peak combustion pressure increased and the appearance time of peak combustion pressure advanced with increment of ozone addition. The analysis of peak PIR showed that the combustion of the methane/air mixture was more vigorous with increment of ozone addition. 2. Ignition delay time could be reduced from the order of 104 μs to 102 μs with ozone addition under the conditions used in the study. For all the equivalence ratio used in the study, ignition delay time reduced obviously when the concentration of ozone was less than 4500 ppm, while ignition delay time did not change significantly when ozone addition was more than 4500 ppm. 3. Influence of ozone on ignition delay time was studied under different initial temperature and pressure conditions with 4500 ppm ozone addition. Results showed that the ignition delay time with ozone addition was only about 1/79 of that without ozone addition under the most obvious conditions in the study, which was about 1/31 under the least obvious condition. Ozone had a greater influence on ignition delay time compared with initial temperature. Ozone addition was an effective way to improve ignition performance of the lean methane/air mixture. 4. Peak concentration of CH2O increased 290 ppm with 6500 ppm ozone addition and corresponding relative variation was 8.5%. The appearance time of peak concentration of CH2O was advanced significantly with the ozone addition. 5. Influence of ozone on SL was studied under different equivalence ratios. It can be concluded that SL increased with the increment of ozone under all equivalence ratios used in the study. The maximum relative variation was 50.3% with 8500 ppm ozone addition when the equivalence ratio was 0.6 under the condition in the study. Ozone addition had an obvious positive impact on the combustion improvement of the lean methane/air mixture.

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6. The influence of ozone on SL was studied under different initial temperature and pressure conditions with 4500 ppm 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 the lean methane/air mixture. 7. The influence of ozone addition on concentration of OH and other intermediate materials was studied under lean methane/air mixture conditions and results showed that the concentration of OH and other intermediate materials increased with the increment of ozone. The concentration of OH increased 646 ppm and the corresponding relative variation was 28% with 6500 ppm ozone addition.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shaobo Ji: 0000-0001-8335-8910 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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).

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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 φ = equivalence ratio PIR = Pressure increase rate O = O atom H = H atom CH2O = formaldehyde OH = hydroxyl H2O2 = hydrogen peroxide HO2 = hydroperoxyl REFERENCES

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Influence of Ozone on Ignition and Combustion Performance of a Lean

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