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. 2021 Nov 18;6(47):31499–31512. doi: 10.1021/acsomega.1c03763

Development of a Reduced Methane–Hydrogen–Polyoxymethylene Dimethyl Ether Mechanism under Engine-Relevant Conditions

Weijian Zhou 1, Song Zhou 1, Hongyuan Xi 1,*, Majed Shreka 1, Zhao Zhang 1
PMCID: PMC8637598  PMID: 34869976

Abstract

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Polyoxymethylene dimethyl ethers (PODEn) have a high cetane number and a high oxygen content, which can effectively reduce the soot emission. In this study, PODE3, methane, and hydrogen were used as the characterization fuel. First, the detailed reaction mechanism of PODE3 and GRI-Mech 3.0 was reduced under engine-relevant conditions by using the reduced methods of the direct relation graph, the directed relation graph with error propagation, the sensitivity analysis, and the reaction pathway analysis. Then, the simplified PODE3 and methane–hydrogen mechanism were coupled and optimized. Finally, the simplified chemical kinetics mechanism of methane–hydrogen–PODE3 (67 species, 260 reactions) was developed. After that, the methane–hydrogen–PODE3 mechanism for methane/hydrogen/PODE3 blend combustion was established, and experimental verification was performed against ignition delay times, laminar flame speeds, and premixed flame species profiles, which showed a good agreement between the predicted and experimental data. Finally, the current mechanism was found to have high reliability and can be coupled to computational fluid dynamics.

1. Introduction

In recent years, serious emission problems caused by the excessive use of fossil fuels affects human health, leading to several types of diseases, such as cancer and other diseases.1−3 Therefore, finding clean alternative fuels is the main research trend nowadays.4 So far, many alternative fuels have been developed including vegetable oils, biodiesel, hydrogen, and so forth.5 Natural gas (NG) is an alternative fuel mainly composed of methane, which has the advantages including low pollution, high production, low cost, and broad availability.6 However, the low flame propagation speed and poor lean burn ability are the main disadvantages of NG engines.7 Due to the potential of large reserves and low pollutant emissions and challenges of NG engines, many researchers have done a lot of research on their combustion and emission performance from many aspects.8

Hydrogen is a clean engine alternative fuel because it does not contain carbon and can effectively inhibit the generation of greenhouse gases and soot. When hydrogen is used as an additive to NG as an engine fuel, the storage and transportation problems in the practical application of hydrogen can be effectively solved and the ignition stability and lean burn limit of the NG engine can also be improved.9 Therefore, NG and hydrogen are widely used for commercial transportation as a new alternative fuel. Because NG has a low cetane number and it is not easy to compress ignition, it is necessary to inject fuel with a high cetane number into the cylinder. In practical applications, the diesel/NG dual-fuel combustion mode (using diesel to ignite NG) is usually adopted in order to reasonably apply NG to diesel engines. The dual-fuel combustion mode can effectively reduce the fuel consumption and emissions without major changes to the traditional diesel engine,10−15 so it has become the focus of the industry and has broad application prospects and a strong market demand.

There have been many studies on the possibility of applying additional hydrogen to the NG/diesel engine. Lee et al.16 systematically studied the influence of the hydrogen-enriched compressed NG/diesel engine on combustion and soot emission characteristics. They found that the mixed hydrogen improves the combustion stability by varying the delay ignition time and multi-point ignition. In addition, they reported that adding hydrogen to the fuel could increase the speed of flame propagation and decreased soot emissions. Fu et al.9 used experimental work to study the combustion and emission characteristics of different hydrogen energy fractions and found that the peak of the heat release rate increased greatly with the increase in hydrogen energy fraction, which led to the increase of the maximum pressure. Zhou et al.17 studied the combustion and emission characteristics of dual-fuel and tri-fuel in the diesel engine. They found that the tri-fuel operation with hydrogen addition in methane improved the engine performance, nitrogen oxides (NOx), and particulate matter (PM) emissions. Wang et al.18 systematically investigated the effects of the partially premixed and the addition of hydrogen to NG and reported that the flame propagating speed increased with the decrease of the premixed ratio and the addition of hydrogen. Zhang et al.19 studied the auto-ignition characteristics of hydrogen-enriched methane using numerical simulations and found that the ignition delay times of the methane–hydrogen mixture reduced significantly with the increase of the hydrogen content, but the decrease was not obvious at low temperatures.

Besides using diesel to ignite NG, other ignited NGs with a high cetane number as fuels were also studied. Ryu20 studied the effects of the biodiesel injection pressure on the performance of NG and biodiesel dual-fuel engines. The results showed that in the dual-fuel mode, increasing the injection pressure of biodiesel reduces PM emissions but increases NOx emissions. Ghaffarzadeh et al.21 studied the dual-fuel engine performance using diesel/NG and biodiesel/NG. The results showed that the thermal efficiency and NOx emissions increased when the direct injection timing was advanced, while the carbon monoxide and unburned hydrocarbons were reduced due to the stratified premixed fuel and the increase of temperature and in-cylinder pressure rise rate. Imran et al.22 studied the NG/diesel and NG/rapeseed methyl ester compression ignition engines. They found that the rapeseed methyl ester as the ignition agent decreased the NOx emission as compared to diesel.

The use of high oxygen content fuels can solve the problems of combustion efficiency and soot emissions, which have been of wide concern. Therefore, the development of oxygenated fuels such as biodiesels and alcohols has been subject to a lot of research.23,24 Alcohols are organic compounds containing hydroxyl groups bonded to carbons on the side chains of hydrocarbons or benzene rings. Alcohol fuels are frequently utilized in engines and generally contain25 ethanol.26 In addition, alcohols have a high latent heat of vaporization and a high oxygen content, which have a significant impact on reducing emissions. However, methanol and ethanol have a low calorific value, low miscibility with diesel, and high volatility. The alcohol fuel is often used as a gasoline alternative fuel in spark-ignition engines and has a low cetane number, so it is difficult to apply in diesel engines.27

Polyoxymethylene dimethyl ethers (PODEn) are a potential ether fuel for engines because they are composed of C–O bonds and have a high cetane number. Therefore, soot emission and polycyclic aromatic hydrocarbon (PAH) precursors (C2H2, C2H4, and C3H3) can be reduced. As shown in Table 1, their chemical structural formula can be represented by CH3O(CH2O)nCH3 (n ≥ 2). When n = 2, the flashpoint is low, and when n > 5, it is not easy to mix with diesel. When n = 3–5, the fuel is liquid at room temperature and miscible with diesel under certain conditions.28 Therefore, it is considered as a potential fuel. PODEn can be synthesized by methanol as a fuel through polymerization. The melting point of PODE3 is low, the boiling point is only 15 °C, and its volatility is in the distillation range of diesel and gasoline. Besides, it has a cetane number that is relatively high, and it can be dissolved in gasoline and diesel in any proportion. Burger et al.29 reported that PODE3 could be blended directly with diesel and applied to the engine without any changes to the engine.

Table 1. Fuel Properties of Polyoxymethylene Dimethyl Ethers30.

name chemical formula mole weight density (g/cm3) cetane number lower heating value (MJ/kg)
PODE1 C3H8O2 76 0.86 29 22.4
PODE2 C4H10O3 103 0.96 63 20.3
PODE3 C5H12O4 136 1.02 78 19.1
PODE4 C6H14O5 166 1.06 90 18.4
PODE5 C7H16O6 196 1.10 100 17.8
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Due to the special molecular structure of PODEn, the reaction path of its combustion process is different from that of the traditional hydrocarbon fuel, and the development of a related chemical reaction kinetics mechanism is still in its infancy. In 2017, Sun et al.31 developed the first high-temperature reaction mechanism of PODE3 and verified the mechanism by using the data of the laminar premixed flame and laminar flame velocity. Subsequently, He et al.32 developed a reaction path in the middle and low-temperature regions on the basis of the high-temperature reaction mechanism, thus forming the first detailed mechanism of PODE3. He et al.33 further integrated the detailed mechanism with a multi-component reaction mechanism,34 developed a multi-component characterization fuel kinetics model suitable for the gasoline/diesel/PODEn blended fuel, and realized the computational fluid dynamics (CFD) simulation of a gasoline/diesel/PODEn-blended fuel engine combustion for the first time. However, the mechanism scale of PODE3 constructed by He et al.32 was too large (225 species and 1082 reactions), the calculation efficiency was not high, and only the data of ignition delay time were verified.

To sum up, it is necessary to improve the chemical kinetics mechanism of methane–hydrogen–PODE3. It is urgent to develop a smaller scale and a more comprehensive mechanism of PODEn chemical reaction kinetics to promote the development of numerical simulation.

2. Mechanism Development

2.1. Base Methane–Hydrogen Mechanism

The main component of NG is methane. Methane is the simplest of alkanes, which is composed of one carbon atom and four hydrogen atoms. Hydrogen is a combustible material with the smallest relative molecular weight, which is composed of two hydrogen atoms. Nowadays, the detailed chemical reaction mechanism of NG is relatively mature. As shown in Table 2, there are five kinds of detailed hydrogen and methane chemical reaction mechanisms commonly used in the numerical simulation to construct a PODE3 sub-mechanism. An appropriate mixture of methane–hydrogen mechanisms must be developed to represent the oxidation of NG and hydrogen. Khan et al.35 conducted a systematic study of the hydrogen enrichment on laminar flame speed and combustion stability and compared the San Diego, GRI-Mech 3.0, and USC-Mech 2.0 mechanisms. The research results showed that the GRI-Mech 3.0 mechanism has the best prediction effect on the laminar flame speed of the methane hydrogen blend. Gimeno-Escobedo et al.36 studied the reduced methane–hydrogen mixture mechanism from the GRI-Mech 3.0 mechanism. The results showed that the validation for zero-dimensional and one-dimensional calculations has a small error between the predicted and original mechanisms. Besides, the mechanism was applicable to a variety of methane–hydrogen mixtures. As shown in Table 2, the AramcoMech 1.3 mechanism has the most reactions and species and coupling it with CFD for numerical calculation is prohibitively costly. Therefore, it was not selected in this work. The Princeton mechanism mainly studies CO, CH2O, and CH3OH combustion. It cannot accurately reflect the chemical reaction process of NG, so it was not selected. In this work, the constructed methane–hydrogen mechanism is developed from published mechanisms: GRI-Mech 3.0 NG mechanism. Although the reaction path of methane and hydrogen should be included in the PODEn mechanism, not all reactions and rate constants are the same for smaller hydrocarbons in the GRI-Mech 3.0 NG mechanism and the PODEn mechanism. In the absence of strict verification of the PODEn mechanism based on the experimental data of NG and hydrogen, the two mechanisms are each selected to be reduced and then combined with the reduced mechanism. This methane–hydrogen mechanism will also be the basis for constructing a PODE3 sub-mechanism.

Table 2. Overview of the Chemical Kinetics Mechanisms for Methane–Hydrogen Kinetics.

mechanism year species reaction reference
Princeton 2017 92 642 (37)
San Diego 2016 57 268 (38)
AramcoMech 1.3 2013 253 1542 (39)
USC-Mech 2.0 2007 111 784 (40)
GRI-Mech 3.0 2000 53 325 (41)
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In this work, the reduced methane–hydrogen mechanism was applied using the directed relation graph (DRG),42 DRG with error propagation (DRGEP),43 and sensitivity analysis (SA).44,45 These simplified methods have been further applied to obtain reduced mechanisms, and satisfactory results were achieved.28,46,47 The GRI-Mech 3.0 mechanism contained 53 species and 325 reactions, and ANSYS CHEMKIN Reaction Workbench software was used to conduct the reduced mechanism.48 To guarantee the rationality of the simplified methane–hydrogen mechanism, 990 batch reactors were used in the reduced methane–hydrogen mechanism. In addition, in the process of simplifying the mechanism, the following wide range of working conditions were selected: initial pressure (Pin) = 1–60 bar, initial temperature (Tin) = 1000–2000 K, hydrogen blending ratio = 0–0.8, and equivalence ratio (φ) = 0.5–1.5. Finally, the new methane–hydrogen mechanism based on the above simplification methods consisted of 20 species and 80 reactions.

2.2. PODE3 Sub-mechanism

Although there are a lot of experimental studies on PODE3 in diesel engines, there are few studies on the chemical kinetics mechanism of PODE3. Based on the determination of the reduced mechanism of basic methane–hydrogen, the PODE3 sub-mechanism combined with the reduced mechanism of basic methane–hydrogen was established. Besides, a sub-mechanism describing the oxidation of PODE3 was developed based on the recently published kinetics mechanisms developed by He et al.32 The chemical kinetics mechanism of PODE3 was first proposed by Sun et al.31 Subsequently, He et al.32 proposed the reaction path in the middle- and low-temperature regions on the basis of the high-temperature reaction mechanism, thus forming the first detailed mechanism of PODE3.

The PODE3 reduced mechanism was performed using ANSYS CHEMKIN Reaction Workbench software.48 The main steps for simplifying the mechanism in this work are as follows: (a) the DRG method was used to eliminate unimportant species and reactions from the detailed mechanism of He et al.,32 225 species of 1082 reactions were simplified to 94 species of 434 reactions, and the eliminated species and reactions belonged to some irrelevant species and reactions in the PODE3 reaction pathway; (b) DRGEP combined with the SA method was used for deep simplification to identify the most important species/reactions and effectively remove redundant species and related reactions; (c) the number of species and reactions was further reduced by using reaction pathway analysis (RPA); the simplified mechanism was verified by using the experimental data of the laminar flame speed and the ignition delay time; and (d) the above steps were repeated to further reduce the mechanism until the simulated data deviated greatly from the experimental values. The reduced mechanism can still be further adjusted and simplified to combine it with the methane–hydrogen sub-mechanism. In order to reflect the oxidation process of the fuel as accurately as possible, the evaluation criterion was specified as the PODE3 ignition delay. The reduction was performed at Pin = 0.5–1.0 MPa, Tin = 650–1250 K, and φ = 0.5–1.5. The reduced PODE3 mechanism based on the above simplification methods consisted of 64 species and 249 reactions. According to He et al.,32 the species reduced by 71.5% and the reactions decreased by 77%.

The main reaction path of PODE3 in the He et al.32 mechanism was determined by analyzing different conditions, as shown in Figure 1. Because of the complexity of the chemical reaction flow, only the main reaction pathways are given. PODEn can usually be replaced by DMMn.49 Initially, the H atom is abstracted from DMM3 to produce isomers DMM3X (including DMM3A, DMM3B, and DMM3C). The DMM3X undergo a series of β-scission reactions and unimolecular decomposition reactions to form DMM2A, dimethoxymethyl (CH3OCH2OCH2), and CH3OCH2, respectively. Then, DMM3X undergo first and second oxygen addition processes to form DMM3XO2 and DMM3_OOH_O2, respectively. Subsequently, DMM3_OOH_O2 products decompose into ketohydroperoxide (KET), oxygenated, and hydroxyl radicals. Finally, all pathways end with a C1 radical. Because of the lack of C–C bonds, PODEn will not form olefins in the reaction process. Therefore, it is difficult to form soot precursors such as C2H2 and C2H4.

Figure 1.

Figure 1

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Major reaction pathways of PODE3.

2.3. Formation of the Methane–Hydrogen–PODE3 Mechanism

In this work, the simplified multi-component mechanism of the methane–hydrogen mechanism was used as the basic mechanism. Besides, application of the reduced PODE3 sub-mechanism through reduction techniques was performed in this study and incorporated into the methane–hydrogen mechanism. By removing the repeated elements, species, and reactions, the new mechanism of methane–hydrogen–PODE3 consisting of 67 species and 260 reactions was finally developed. Figure 2 illustrates the process of simplifying the methane–hydrogen–PODE3 mechanism constructed. Therefore, the newly generated methane–hydrogen–PODE3 mechanism consists of 67 species and 260 reactions. Finally, the SA of the ignition delay time and the laminar flame speed and the adjustment of the pre-exponential factors of sensitivity reactions are used to optimize the coupled mechanism.

Figure 2.

Figure 2

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Procedure used to generate the skeletal methane–hydrogen–PODE3 mechanism.

2.4. Optimization of the Methane–Hydrogen–PODE3 Mechanism

In order to verify the compactness and reliability of the methane–hydrogen–PODE3 mechanism under engine-relevant conditions, the new mechanism was compared with the original mechanism. There is a certain error between the prediction results of the methane–hydrogen–PODE3 mechanism and the original mechanism ignition delay time. Therefore, adjusting the reduced methane–hydrogen–PODE3 mechanism using the optimization method is necessary.

First, SA of the delayed ignition time is carried out, and the contribution of the reaction to ignition delays of methane–hydrogen and PODE3 is determined. Li et al.50 showed that the temperature is the most sensitive factor to the ignition delay of PODE3. Therefore, the SA of ignition delay for PODE3 at temperatures of 650, 950, and 1200 K was chosen. The sensitivity coefficient of ignition delay of each reaction is calculated by the following formula51

2.4. 1

where τ represents the ignition delay and ki represents the pre-exponential factor (A). The sensitivity coefficient Si is negative, which indicates that the reaction can promote the ignition of the fuel, whereas Si is positive, indicating that the reaction inhibited the ignition of the fuel.

Figure 3a shows the four reaction-sensitive coefficients of the methane–hydrogen–PODE3 mechanism to the ignition delay of methane–hydrogen when φ = 1; Pin = 10 bar; Tin = 1200, 1500, and 1800 K; and 20% H2. It can be seen from Figure 3a that R65, R66, and R68 exhibit high sensitivity to ignition delay at all temperatures. Figure 3b shows the sensitive coefficients of eight reactions in the methane–hydrogen–PODE3 mechanism for the ignition delay of PODE3 when Pin = 1.0 MPa; φ = 1.0; and Tin = 650, 950, and 1200 K. From Figure 3b, the reaction of R213 and R227 shows high sensitivity to the ignition delay at 650 K. Besides, at 950 and 1200 K, the reaction of R230 and R221 is most sensitive to the ignition delay, but the importance of some key reactions in different temperature ranges is quite different. This means that a sensitive reaction A can be modified, which will affect the ignition delay time in an initial temperature range.

Figure 3.

Figure 3

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SA of the ignition delay time for (a) methane–hydrogen (φ = 1, Pin = 10 bar, Tin = 1200, 1500, and 1800 K and 20% H2) and (b) PODE3 (φ = 1, Pin = 1.0 MPa and Tin = 650, 950, and 1200 K).

Subsequently, Figure 3a,b shows some sensitive reactions, which are selected for optimization by adjusting A to reduce the error of the methane–hydrogen–PODE3 mechanism. In this work, the relative error Ei is defined as

2.4. 2

where τc,i represents the ignition delay simulated by the methane–hydrogen–PODE3 mechanism with different adjustments of the pre-exponential factors A = kAbase (k = 2.0, 1.0, and 0.5), while τe represents the ignition delay of the detailed mechanism.

Equation 2 illustrates the difference between the original mechanism and the methane–hydrogen–PODE3 mechanism. Figure 4 shows the relative error of the sensitive reactions. There is a larger deviation in the ignition delay time by comparing the methane–hydrogen–PODE3 mechanism and the GRI-Mech 3.0 mechanism. 2CH3(+M) = C2H6(+M) has a great effect in the whole temperature range, while CH3 + O2 = O + CH3O and CH3 + O2 = OH + CH2O have a great effect in the medium initial temperature region. In addition, CH3 + H2O2 = HO2 + CH4 has a great effect in the low initial temperature region.

Figure 4.

Figure 4

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Relative error of ignition delay time for the main reaction of methane–hydrogen (Pin = 10 bar; φ = 1.0 and 20% H2). (a) 2CH3(+M) = C2H6(+M), (b) CH3 + H2O2 = HO2 + CH4, (c) CH3 + O2 = O + CH3O, and (d) CH3 + O2 = OH + CH2O.

For PODE3, Figure 5 shows that DMM3 + CH3 = DMM3B + CH4 and DMM3 = DMM2A + CH3O strongly influence the ignition delay time in the high initial temperature range, while DMM3 + HO2 = DMM3A + H2O2, DMM3 + HO2 = DMM3B + H2O2, and DMM3 + OH = DMM3B + H2O have a great impact in the whole temperature range. Other reactions also have some effect at the low-temperature region.

Figure 5.

Figure 5

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Relative error of ignition delay time for the main reaction of PODE3 (Pin = 0.1 MPa and φ = 1.0). (a) DMM3 + CH3 = DMM3B + CH4, (b) DMM3 + H = DMM3A + H2, (c) DMM3 + HO2 = DMM3A + H2O2, (d) DMM3 + HO2 = DMM3B + H2O2, (e) DMM3 + O2 = DMM3B + HO2, (f) DMM3 + OH = DMM3A + H2O, (g) DMM3 + OH = DMM3B + H2O, and (h) DMM3 = DMM3A + CH3O.

According to the above-mentioned relative error analysis, by adjusting the A factor of the corresponding sensitive reaction to change the reaction rate constant, the accuracy of the predicting ignition delay mechanism is improved. In this work, the average error ε is defined as52

2.4. 3

Table 3 shows the final adjustment results of key reactions for the original A factors and the adjusted A factors. Figure 6 displays the average errors ε = 0.00469 and 0.02725 for methane–hydrogen and PODE3, respectively, after optimization. For the methane–hydrogen–PODE3 mechanism, the transport data of PODE3 were derived from Sun’s31 work and the Lawrence Livermore National Laboratory (LLNL) database, while the transport data of other mechanism were taken from their parental mechanisms.

Table 3. Modified Pre-Exponential Factors of Sensitivity Reactions.

reaction no. reactions original A factors adjusted A factors
R66 CH3 + O2 = OH + CH2O 2.31 × 1012 4.60 × 1012
R67 CH3 + H2O2 = HO2 + CH4 2.45 × 104 1.45 × 104
R84 2HCO = CH2O + CO 1.80 × 1013 1.80 × 1015
R213 DMM3 = DMM2A + CH3O 1.24 × 1025 2.60 × 1024
R227 DMM3 + CH3 = DMM3B + CH4 1.00 × 1013 1.00 × 1012
R229 DMM3 + HO2 = DMM3A + H2O2 2.00 × 1013 5.00 × 1013
R230 DMM3 + HO2 = DMM3B + H2O2 4.00 × 1013 3.00 × 1013
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Figure 6.

Figure 6

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Relative error of the methane–hydrogen–PODE3 mechanism before and after optimization for (a) methane–hydrogen and (b) PODE3.

The calculation of the premix laminar flame speed (see Figure 11) shows that it is within the whole range of the equivalent ratio study. The methane–hydrogen–PODE3 mechanism overestimates the laminar flame speed of PODE3 while φ = 0.7–1.6. Therefore, the laminar flame speed of the current mechanism cannot be accurately calculated and needs to be further optimized. In order to deeply study the influence of the equivalence ratio on the laminar flame speed, the SA method was used to study the reactions of each reaction of the methane–hydrogen–PODE3 mechanism, that is, to study the effect of the change of a single reaction rate constant on the laminar flame speed of the whole fuel. Figure 7 shows the eight reactions with the highest sensitivity of laminar flame speeds for PODE3 at φ = 0.6, 1.0, and 1.4. A positive value indicates that the reaction can promote the flame propagation of the fuel, while a negative value indicates that the reaction is inhibited. In chemical reaction kinetics, the reaction of small molecules such as H2/C0/C1 has a great influence on the flame propagation speeds. Figure 7 shows that the sensitive reactions of PODE3 is related to H2/C0/C1 small-molecule reactions. Therefore, the sub-mechanism adjustment of H2/C0/C1 reactions will affect the prediction ability of the current mechanism of the laminar flame speed of PODE3 flames.

Figure 11.

Figure 11

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Comparison of the calculated value of the laminar flame speed of methane–hydrogen–PODE3 and the experimental value. (a) Methane–hydrogen (20% H2 and 40% H2), (b) methane–hydrogen (60% H2 and 80% H2), and (c) PODE3.

Figure 7.

Figure 7

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SA of laminar flame speeds for PODE3.

The key sensitivity reactions including R66, R67, R84, R213, R227, R229, and R230 and their A before and after the adjustment are shown in Table 3. In conclusion, the accuracy of the prediction mechanism of ignition delay time and the laminar flame speed can be improved by adjusting A of the key reaction to modify the rate constant. Since the focus of this study is on compactness of the methane–hydrogen–PODE3 mechanism under engine-relevant conditions, the prediction of PAH and soot emissions is not included in this study. Therefore, the formation mechanism of PAH and soot is ignored in the current methane–hydrogen–PODE3 mechanism, which will be taken into consideration in future research. The reduced methane–hydrogen–PODE3 mechanism is given in the Supporting Information.

3. Results and Discussion

The newly constructed methane–hydrogen–PODE3 mechanism must be verified to ensure that the zero-dimensional configurations can mimic the test values. At present, the experimental study on the flame species concentration, the ignition delay, and the laminar flame speeds of methane–hydrogen–PODE3 tri-fuel has not been reported. Besides, the research on the ignition delay and the laminar flame speeds of methane, hydrogen, and PODE3 pure fuels has been reported in many literature studies.31−34 Therefore, the verification of the flame species concentration, the ignition delay time, and the laminar flame speeds of the tri-fuel mechanism adopts the method of comparing with the experimental data of the pure fuel, which is widely used in the development stage of a new fuel mechanism.

3.1. Ignition Delay

The ignition delay time was predicted using the CHEMKIN package simulation data.53 The problem type was used to constrain the volume and solve the energy equation with the closed homogeneous model that calculates the ignition delay time of the fuel. The ignition delay time is defined as the time when the initial temperature of the fuel increases by 400 K.28,46,47

Figure 8 shows the comparison between the measured and predicted ignition delay times of the methane–hydrogen–argon mixture based on GRI-Mech 3.0. The reduced methane–hydrogen–PODE3 mechanism and the high-pressure shock tube experiment were conducted by Zhang54 at a hydrogen mixing ratio of 20 and 80% and various pressures and initial temperatures with φ = 1.0. Figure 8a shows that the methane–hydrogen mechanism is in good agreement with the experimental data. From Figure 8b, the methane–hydrogen mechanism and the experimental data have a large error in the low-temperature range, especially in the range of 1000–1100 K. This may be due to the fact that the simulation cannot well reflect the low-temperature ignition characteristics under the ideal constant volume condition. Zhang54 and Pang et al.55 indicated that the ideal constant volume consumption would make the experimental ignition delay times shorter than the predicted ignition delay times.

Figure 8.

Figure 8

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Ignition delay time validation for methane–hydrogen, (a) φ = 1.0, 20% H2 and (b) φ = 1.0, 80% H2.

Figure 9 compares the ignition delay time based on GRI-Mech 3.0 and the methane–hydrogen–PODE3 mechanism at three different equivalence ratios and a hydrogen mixing ratio of 20–80% at higher pressures (20 and 40 bar). It can be seen that compared with the original mechanism, the optimized mechanism is in good agreement with the ignition delay time data measured at different initial temperatures, initial pressures, and equivalence ratios.

Figure 9.

Figure 9

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Comparison of the ignition delay time for the methane–hydrogen–PODE3 mechanism and the GRI-Mech 3.0 mechanism. (a) φ = 0.5, Pin = 20 bar, (b) φ = 0.5, Pin = 40 bar, (c) φ = 1.0, Pin = 20 bar, (d) φ = 1.0, Pin = 40 bar, (e) φ = 1.5, Pin = 20 bar, and (f) φ = 1.5, Pin = 40 bar.

Figure 10 compares the ignition delay times of the methane–hydrogen–PODE3 mechanism with the rapid compression machine (RCM)-measured value by He et al.32 at initial pressures of 1.0 and 1.5 MPa. The calculation results of the ignition delay time of PODE3 in the temperature range of 600–1100 K and the equivalence ratio range of 0.5–1.5 by the improved methane–hydrogen–PODE3 reaction mechanism are shown in Figure 10. It can be seen that compared with the original PODE3 mechanism, the optimized mechanism is in good agreement with the ignition delay time data measured by RCM, but with the increase of the equivalence ratio, the ignition delay time in the low-temperature region is slightly higher than that of the original mechanism. Overall, the prediction accuracy of the methane–hydrogen–PODE3 mechanism can be improved by adjusting the rate constants of some C5 and C1 components.

Figure 10.

Figure 10

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Ignition delay time validation for PODE3. The equivalent ratios were (a) 0.5, (b) 1.0, and (c) 1.5.

3.2. Laminar Flame Speeds

The laminar flame speed is widely used to verify the chemical mechanism. It is an important combustion characteristic, which also contains the fundamental information about the physical and chemical characteristics. The premixed laminar flame speed simulation by the CHEMKIN package,53 and the experimental data were compared with the calculated results. Hu et al.56 measured the laminar flame speed of methane–hydrogen at Pin = 1 atm and Tin = 303 K. Figure 11a,b compares the laminar flame speed predictions of experimental data and validation results for methane–hydrogen–air. The various mole fractions of hydrogen with a blending ratio in the fuel mixture from 20 to 80% are illustrated. The results show that the prediction accuracy of the new mechanism is very good at different hydrogen blending ratios. Figure 11c shows the laminar flame speed predictions of experimental data and validation results for PODE3. The experimental value of laminar flame speeds for PODE3 was tested under the conditions of Pin = 0.1 MPa, Tin = 408 K, and φ = 0.7–1.6. From Figure 11c, compared with the original mechanism, the optimized mechanism is in good agreement with the laminar flame speed measured by Sun et al.31 Since the original PODE3 mechanism previously proposed by He et al.32 did not include the transport data, the predicted value was slightly higher than the measured laminar flame speed. Figure 11c shows that the laminar flame speed has good accuracy mainly due to the improved reduced C0–C1 kinetics mechanism. Considering that the laminar flame speed of the predicted and experimental data was not included in He et al.32’s work, the prediction results with the mechanism of methane–hydrogen–PODE3 constructed in this work are reliable under engine-relevant conditions.

3.3. Premixed Flame Species Profiles

Because the species concentration is widely used to verify the chemical kinetics mechanism, the current work can also be verified by the species concentration measurement data. The CHEMKIN package53 was utilized to conduct the premixed flame simulation. Figure 12 shows the concentration of key species in the φ = 1 PODE3 premixed flame at a constant pressure of 25.0 Torr. Figure 12 shows that the optimized reaction mechanism can accurately reflect the concentration changes with the experimental data from Sun et al.31 for the key species such as DMM3, products CO2 and H2O, and other species such as Ar, CO, and H2 in the premixed flame of PODE3. Considering that the probe disturbance and the temperature measurement error will cause the measurement deviation of the concentration of key species in the test, the calculation error of the concentration of each important species in the mechanism can be ignored in the flame region of less than 3 mm from the premix flame center in Figure 12. In conclusion, the present methane–hydrogen–PODE3 mechanism can accurately describe the concentration variation of important substances in the oxidation process of PODE3 under certain conditions, which verifies the reliability of the mechanism.

Figure 12.

Figure 12

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Comparison of the calculated results and experimental data of the concentration of key species for PODE3.

4. Conclusions

A reduced chemical mechanism for methane–hydrogen–PODE3 was constructed based on GRI-Mech 3.041 and He et al.32 and was simplified by using DRGEP, DRG, RPA, and SA methods. The reduced methane–hydrogen–PODE3 mechanism contained 67 species and 260 reactions. The reduced methane–hydrogen–PODE3 mechanism has been widely validated by experimental data. First, the ignition delay time and the laminar flame speed were optimized by SA. The ignition delay times and the laminar flame speed of the new mechanism were validated by comparing the experimental data and the simulation. The ignition delay times and the laminar flame speed predicted values of the reduced methane–hydrogen–PODE3 mechanism were in excellent agreement with the experimental data. Second, the species concentration profiles were validated with the new mechanism. The results showed that the species concentrations of PODE3 premixed flame had good agreement with the predicted and the experimental data. Finally, the constructed methane–hydrogen–PODE3 mechanism can be used for CFD numerical calculations because of its compact structure and the moderate number of its reactions and species. In future studies, it can be applied to multi-component fuel engines fueled with methane/hydrogen/PODE3 blends with different combustion modes using numerical investigations based on the new mechanism in this work. Since NG also contains other components, and in order to better understand and improve the combustion and emission performances of the methane/hydrogen/PODE3 mixed fuel and more importantly NG (ethane, propane, etc.) species, the soot and NOx formation mechanism can be added to the existing mechanism in the future.

Acknowledgments

This work was supported by the Marine Power Research and Development Program (grant number DE0302).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03763.

  • Simplified chemical kinetics mechanism of methane–hydrogen–PODE3 (67 species and 260 reactions) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c03763_si_001.pdf (97.2KB, pdf)

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