Nature Gas Dual-Fuel Mechanism

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Construction of a Reduced PODE3/Nature Gas Dual-Fuel Mechanism under Enginelike Conditions Haozhong Huang,* Yingjie Chen, Jizhen Zhu, Yajuan Chen, Delin Lv, Zhaojun Zhu, Lixia Wei, and Yaopeng Wei College of Mechanical Engineering, Guangxi University, Nanning 530004, China

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S Supporting Information *

ABSTRACT: The focus of the current research is the selection of fuels used in a dual-fuel engine. These engines have received a great deal of attention in recent years because they can offer high thermal efficiency together with reduced emissions. Natural gas (NG) is the first choice for an alternative engine fuel because of its advantages of environmental friendliness, large energy storage capacity, and good economics. Polymethoxy dimethyl ether (PODE) with a high cetane number and low soot emissions is deemed to be one of the promising fuels or additives for direct injection compression ignition engines. Therefore, this study constructs the 124-species and 650-reaction PODE3/NG reduced mechanism for dual-fuel engine application. First, the PODE3 detailed mechanism was reduced under a wide range of enginelike conditions (the initial pressures were 10 and 15 bar, the equivalence ratios were 0.5−1.5, and the initial temperatures were 600−1400 K) by using the reduced methods of direct relation graph (DRG), directed relation graph with error propagation (DRGEP), rate of production (ROP), and sensitivity analysis (SA). Then, the NG reduced mechanism established by our research group was combined with the reduced PODE3 mechanism to develop the final PODE3/NG dual-fuel mechanism. The key chemical kinetic parameters of the combined mechanism were optimized by using SA. This optimized mechanism was evaluated using uncertainty analysis based on polynomial chaos expansions. Finally, the modeled dual-fuel mechanism behavior was subjected to extensive experimental verification for ignition delays, species profiles, and laminar flame speeds. The rate constants of key reactions related to H2O2 and methyl (CH3-) were improved so that the predicted values of laminar flame speed at high-pressure lean-burn conditions agreed well with the experimental data, and the improved mechanism does not affect the species concentration and ignition delay. In addition, since the construction of the reduced mechanism is a relatively compact model, it can be coupled with CFD for numerical simulation of dual-fuel engines.

1. INTRODUCTION

octane number, indicating that it has good detonation resistance. Therefore, natural gas can be applied to engines with large compression ratios to achieve higher thermal efficiency. Compared to diesel, natural gas has weaker ignition performance due to its high autoignition temperature.2 One of the solutions is to inject high-cetane fuel into the cylinder to ignite the natural gas. The high-cetane fuel can be compression-ignited due to the in-cylinder high pressure and temperature at the top dead center (TDC). High-cetane fuel becomes the ignition source after ignition, igniting the mixture of natural gas and air.3,4 In recent years, more and more researchers have conducted a great deal of research on diesel/natural gas (NG) dual-fuel engines. Papagiannakis et al.5 conducted an experimental study of the effect of dual-fuel combustion on the performance and emissions of a modified single-cylinder diesel engine. They found that dual-fuel engines had lower NOx and soot emissions than normal diesel engines, whereas CO2 and HC emissions were much higher than diesel engines. Liu et al.6 conducted experimental studies on CNG/diesel dual-fuel engine emissions using varied injection timings and different pilot fuel quantities. They observed that with the increase in fuel

The intensification of the global energy crisis and increasingly strict emission regulations posed great challenges to internal combustion engine development. To achieve higher thermal efficiency while reducing emissions, many combustion strategies have been proposed. One of these strategies is to use alternative fuels. Natural gas is regarded as one of the most promising alternatives for diesel fuel: its many advantages include clean combustion, large reserves, cost advantages, and wide availability.1 Nowadays, natural gas engines are developed by adding a set of supply systems of natural gas with structures that are the same as traditional diesel ones. Compared with diesel and gasoline (see Tables 1 and 2), natural gas has a large Table 1. Main Physical and Chemical Characteristics of Fuels28 properties

natural gas

diesel

gasoline

vapor density (g/L) octane number density (g/mL) low heating value (MJ/kg) autoignition temperature (°C) sulfur content (ppm) latent heat of vaporization (kJ/kg)

0.63 130 0.81 49.1 635−735 7−26 510

20−30 0.82 42.8 260 250 205

5.10 85−95 1.019 44.4 230−470 200 285

© XXXX American Chemical Society

Received: November 11, 2018 Revised: January 23, 2019

A

DOI: 10.1021/acs.energyfuels.8b03926 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 2. Physical and Chemical Properties of Ether29 name

chemical formula

molecular weight (g/mol)

boiling point (°C)

melting point (°C)

density (g/cm3) @25 °C

cetane number

oxygen content (%)

low heating value (MJ/kg)

DME DMM PODE2 PODE3 PODE4 PODE5 PODE6

CH3OCH3 C3H8O2 C4H10O3 C5H12O4 C6H14O5 C7H16O6 C8H18O7

46 76 106 136 166 196 226

−35 42 105 156 202 242 280

−138 −105 −65 −41 −7 18.5 58

0.00197 0.86 0.96 1.02 1.06 1.1 1.13

55 29 63 78 90 100 104

34.8 42.1 45.3 47.1 48.2 49 49.6

28.8 22.4 20.3 19.1 18.4 17.9 17.5

atoms (C-O-C) interlinked alternately. Unlike the carbon− carbon chain (C-C) of linear alkanes, the combustion of PODEn on the engine can effectively reduce the generation of soot precursors (C2H2, C2H4, and C3H3) because they do not contain carbon−carbon bonds and have a high oxygen content. When n > 1, the cetane number of PODEn exceeds the minimum value of 6023 required by the European EN590 standard, but the lower flash point of PODE222 limits its practical application. In addition, the melting point of the PODEn increases as the number of functional group (-CH2O-) units increases; when PODEn (n > 5) is used as the fuel, it has poor fluidity at low temperature and can easily clog the fuel system.24 Therefore, the optimal chain length of PODEn as an alternative fuel for diesel should be n of 3 or 4, whereas PODE3 is the smallest compound that can represent PODEn in actual use. PODE3 has a lower melting point and boiling point, indicating better low-temperature fluidity and volatility in practical applications. Also, the boiling point of PODE3 is only 156 °C, lower than the common hydrocarbons’ boiling point (180−360 °C) in diesel; it can be seen that PODE3 can replace diesel as a dual-fuel ignition agent. At present, there are a few studies on the PODEn mechanism. Sun et al.25 proposed the first chemical ksinetic model of PODEn in combination with simulations and experiments. The premixed flame species concentration and laminar burning velocity of PODEn were obtained through experiments, and the constructed mechanism was used for verification. Sun et al.25 found that the experimental data and predicted values have been in good agreement. However, their detailed mechanism lacks the lowtemperature reactions. Based on the high-temperature mechanism of PODEn proposed by Sun et al.,25 He et al.26 developed a detailed chemical kinetic reaction, a mechanism of PODE3 containing 225 species and 1082 reactions by analyzing the rate constants of the five reaction categories of PODE1 and, using the canonical transition state theory (CTST) to calculate the rate coefficients. They used the detailed mechanisms for ignition delay and engine verification. The results show that the calculation of the detailed mechanism agreed well with their experimental results. Song et al.27 conducted experimental studies on PODEn/ CNG and diesel/CNG under low and high load conditions. After a comparative analysis, PODEn was found to be a better experimental fuel for the dual-fuel combustion mode due to its high oxygen content, high flammability, and high volatility. It can significantly reduce NOx, CO, THC, and soot emissions, and it can achieve higher thermal efficiency. To deeply understand the combustion mechanism, it is necessary to construct a combustion mechanism of PODEn/NG dual fuel. Therefore, the PODE3 detailed mechanism developed by He et al.26 is simplified by using the reduced methods of direct relation graph (DRG), directed relation graph with error

consumption, PM emissions increased but THC was significantly reduced. Gharehghani et al.7 found that increasing the intake pressure and eddy flow ratio can effectively reduce the cylinder knock of dual-fuel engines and enhance their power. They also found that hydrogen-doped natural gas can be a practical alternative fuel, which not only reduces the emissions of CO, HC, and CO2 but also increases the operating efficiency of engines.8 Besides traditional diesel, other high-cetane number ignition agents have been explored as for dual-fuel engines. Ryu et al.9 studied the emission characteristics of natural gas and biodiesel dual-fuel engines and concluded that injection timing affected engine power and emissions in the dual-fuel mode. As the diesel injection timing was advanced, soot emissions decreased but nitrogen oxide (NOx) emissions increased. Imran et al.10 experimentally compared the performance and emissions of diesel and rapeseed methyl ester (RME) on a natural gas engine and found that the use of RME as an ignition agent can significantly reduce NOx emission compared to diesel. Shoemaker et al.11 conducted an experimental study on the emission and combustion characteristics of a direct injection supercharged engine using methane and propane fuels with biodiesel as an igniting agent. Their results showed that the emissions of total hydrocarbon (THC) and CO increased with the increase of the substitution rate of methane and propane for diesel, whereas the emissions of NOx and soot decreased. To improve the combustion efficiency of diesel engine and further reduce emissions, diesel-alternative fuels must have high cetane number and high oxygen content. At present, more research on oxygen-containing fuels include alcohols,12−14 esters,15−17 and ethers.18−21 Alcohol is a compound containing hydroxyl (OH). As a diesel-alternative fuel, it has advantages of low viscosity, easy atomization, high latent heat of vaporization, a high heat value, and a high cetane number. However, alcohols’ oxygen content is relatively low. Esters are a functional group of ester-based (-COO-) compounds. Although the ester carbon chains are longer and have a higher cetane number, unfortunately, similar to alcohols, the oxygen content of the ester is quite limited. Ethers have shorter molecular chain lengths and lower carbon content, but their cetane number is higher. Most of the ether molecular structures contain multiple CH2O functional groups, and the molecular oxygen content is large. Scholars have found that the molecular structure peculiar to the ether can greatly reduce soot emission.18,19 Therefore, ethers are the best choice among these three categories of alternative fuels. Polyoxymethylene dimethyl ethers (PODEn) are considered to be potential diesel-alternative fuels.22 These are a class of polyether compounds with a chemical molecular structure of CH3O(CH2O)nCH3 (n > 1) (see Table 2). PODEn is composed of chain structures of carbon atoms and oxygen B


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Energy & Fuels

Figure 1. Main reaction pathways of PODE3 at P = 15 bar, ϕ = 1.0, and 20% fuel conversion. Black font, 750 K; red font, 1250 K.

Figure 2. Approach for the formation of the simplified mechanism.

were simulated by coupling the GXU simplified mechanism and the computational fluid dynamics (CFD) for four different natural gas substitution rates. The results showed that the prediction of the reduced mechanism of GXU agreed well with experimental data. This means that the GXU simplified mechanism can accurately predicted the emission and combustion characteristics of actual diesel/natural gas dualfuel engines. Since this mechanism also contains species and reactions related to n-heptane, n-butylbenzene, and PAH, these reactions and species are removed from the GXU reduced mechanism to obtain a single-component reduced mechanism for pure natural gas. The simplified NG submechanism is taken as the base mechanism.

propagation (DRGEP), rate of production (ROP), and sensitivity analysis (SA). Moreover, the simplified mechanism was combined with the simplified mechanism of natural gas developed by our research group1 to develop the simplified mechanism of PODE3/NG dual fuel.

2. MECHANISM DEVELOPMENT 2.1. Base Natural Gas Mechanism. The GXU reduced mechanism1 was developed by our research group, including 143 species and 746 reactions, and has been extensively verified on species concentrations, ignition delay, laminar flame speed, and in HCCI engines. In addition, the combustion and emission performances of diesel/natural gas dual-fuel engines C


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Energy & Fuels 2.2. PODE3 Submechanism. In this study, the PODE3 submechanism was simplified based on the detailed mechanisms with 225 species and 1082 reactions developed by He et al.26 They referred to the relatively mature reaction mechanism of dimethyl ether (DME), dimethoxymethane (DMM), dimethyl carbonate (DMC), and diethyl ether (DEE) and combined those mechanisms with the high-temperature mechanism of PODE3 developed by Sun et al.25 For the first time, a detailed mechanism of PODE3 containing both hightemperature and low-temperature reactions was proposed. Based on the detailed mechanism of PODE3 proposed by He et al.,26 this study constructed the simplified mechanism of PODE3 by using the reduced methods of DRG, DRGEP, ROP, and sensitivity analysis (SA). First, DRG and DRGEP were used to eliminate the nonessential species and reactions. Then, sensitivity analysis and ROP were used to further reduce the mechanism. By repeating the above process, the mechanism was reduced to the target size. To ensure the reliability and accuracy of the reduced mechanism, a wide operating range was selected in the process of using the reduced methods of DRG and DRGEP: pressures were 10 and 15 bar, the equivalent ratios were 0.5−1.5, the temperatures were 600− 1400 K, and PODE3, CH2O, and other important species were used as the initial retention species. Figure 1 shows the main reaction path of PODE3 at P = 15 bar, ϕ = 1.0, and 20% fuel conversion. The reaction pathways of PODE1‑3 and n-alkanes have certain similarities; unlike n-alkanes, the reactions of PODE3 will not form an olefin but will form a carbonyl (-C O-) compound due to the lack of a C−C bond in the molecular structure of PODEn. Olefins have always been an important soot precursor in the soot formation mechanism. Through analysis of the reaction path of PODE3, it was found that the reaction process did not produce olefins, which is an important reason for the low emission of soot. It can be seen from Figure 1 that PODE3 has five main reaction pathways. The low-temperature stage mainly involves the dehydrogenation and oxidation reactions. These reactions promote the CH2O molecules, which accumulate in the lowtemperature exothermic stage. The dehydrogenation reactions that promote the cold flame stage generate a large set of strongly oxidative radicals (such as OH). The high-temperature reactions mainly add the β cleavage reaction of PODE3 based on the three dehydrogenation reactions, and CH2O is generated through a series of (-CH2O-) chain breaking reactions. 2.3. Formation and Optimization of the PODE3/NG Reduced Mechanism. By combining the above submechanisms and removing the repeated reactions of PODE3 from the combined mechanism, a PODE3/NG reduced mechanism containing 124 species and 650 reactions was obtained. The specific steps are shown in Figure 2. The preliminary verification of the combined mechanism showed that the predicted values of the ignition delay of natural gas and its detailed mechanism were in good agreement, but the predicted value of the ignition delay of PODE3 deviated greatly from the experimental values. Therefore, the reduced mechanism needs to be optimized. The sensitivity analysis was performed on the ignition delay of PODE3/air mixtures with P = 15 bar, T = 600, 900, and 1200 K, and ϕ = 1.0. The sensitivity coefficients of the reactions are calculated by eq 130

S=

log τ+ − log τ − log 2 − log 0.5

(1)

where τ+ represents the calculated ignition delay of the nonoptimized mechanism for the pre-exponential factor multiplied by 2, whereas τ− represents the calculated ignition delay of the nonoptimized mechanism for the pre-exponential factor multiplied by 0.5. A positive sensitivity coefficient indicates that the reaction is inhibited (ignition delay increases as the rate constant increases). Conversely, a negative sensitivity coefficient indicates that the reaction is promoted (ignition delay decreases as the rate constant increases). Figure 3 shows the normalized results from the sensitivity analysis for

Figure 3. Normalized sensitivity coefficients of the ignition delay for PODE3 at low temperature, NTC region, and high temperature.

PODE3 in the low temperature (T = 600 K), negative temperature coefficient (NTC) (T = 900 K) region, and high temperature (T = 1200 K) for ignition delay. Finally, ten of the most sensitive reactions in this analysis of PODE3 were selected. Therefore, by optimizing the pre-exponential factor of the reactions with a large sensitivity coefficient, the ignition delay predicted by the optimized mechanism agrees well with the experimental data, and the NTC phenomenon can be reproduced effectively (see Figure 5). The reaction of CH3OCHOOHOCHOCH3OCHO + OCHO + OH (R111) has a large negative sensitivity coefficient only at low temperature, whereas the reactions of DMM3 + CH3 = DMM3B + CH4 (R159) and DMM3 + HO2 = DMM3B + H2O2 (R161) have a large negative sensitivity coefficient for both the NTC region and high temperatures. The reaction of DMM3 + OH = DMM3B + H2O (R155) has a large positive sensitivity coefficient at low temperatures. Therefore, at high temperatures, R155 competes with R161 and R159. To summarize, the rate constant of the reaction is changed by adjusting the pre-exponential factors of the corresponding sensitive reaction to improve the accuracy of the mechanism for the prediction of ignition delay. The final adjusted results are shown in Table 3. In addition, the transport parameters and thermodynamic parameters of the mechanism are derived from their detailed mechanisms. 2.4. Uncertainty Analysis. Uncertainty analysis is very effective in evaluating the performance of the mechanism. The purpose of the uncertainty analysis is to determine the error range of the mechanism prediction, that is, the boundary of the reaction rate constant k, thereby greatly reducing the range of D


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Energy & Fuels Table 3. Optimization of the PODE3 Submechanism reaction

nonoptimized pre-exponential factor

optimized pre-exponential factor

CH3OCHOOHOCHOCH3OCHO + OCHO + OH DMM3 + OH = DMM3B + H2O DMM3 + CH3 = DMM3B + CH4 DMM3 + HO2 = DMM3B + H2O2

5.0 × 1015 37 849.65162 1.0 × 1013 2.0 × 1013

2.5 × 1015 76 666.3032 2.0 × 1013 1.0 × 1014

Figure 4. Comparison between the predicted value and experimental value of ignition delay for optimization and nonoptimized mechanism under P = 10 and 15 bar at ϕ = 1.0. Symbols represent the experimental values from He et al.26 Light bands represent the 2σ uncertainty of the model prediction.

the uncertainty can be obtained by solving for the coefficients of the PCE. As shown in Figure 4, the uncertainties of the ignition delay for the nonoptimized and optimized mechanisms were calculated using PCE and compared with the experimental values. It can be clearly seen that the uncertainty is greater in the NTC region than it is in both the low-temperature and the high-temperature region irrespective of whether the mechanism is optimized or not. This is because the sensitivity coefficients in the NTC regions of the PODE3 submechanism are greater than the sensitivity coefficients in the lowtemperature and the high-temperature region. In addition, the uncertainty of the nonoptimized mechanism’s prediction is greater than the uncertainty of the optimized mechanism’s prediction because the uncertainty of the mechanism can be

the optimization. Since prediction of the ignition delay is related to the macromolecular mechanism, and only the preexponential factor A is optimized in this study, the uncertainty analysis of the pre-exponential factor of each macromolecular reaction of PODE 3 is performed. The corresponding uncertainty factors are taken from the detailed mechanism.25 The range of the pre-exponential factor is determined by eq 2 Ak /fk ≤ Ak ≤ Ak fk

(2)

where Ak is the pre-exponential Arrhenius factor of the kth reaction, and f k represents an uncertainty factor for the rate constant of the kth reaction. The uncertainty of the predicted value of the reaction mechanism is calculated using polynomial chaos expansions (PCEs).31 The uncertainty of the input parameters is transferred to the polynomial’s coefficients, and E


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Figure 5. Comparison between the simulations and experiments26 of ignition delay for PODE3 under three different equivalence ratios.

Figure 5 shows the comparison between the predicted and experimental data of the ignition delay of PODE3 under the conditions of equivalence ratios of 0.5, 1.0, and 1.5 and pressures of 10 and 15 bar. The ignition delay time is defined as the time required when the temperature of the mixture is higher than the initial temperature of 400 K. It can be concluded from Figure 5 that the predicted values of the reduced mechanism agree well with the experimental values, especially in the low-temperature region (T < 750 K), and that the negative temperature coefficient phenomenon of PODE3 is reproduced well. The main reason for the NTC phenomenon is that the reaction R + O2 = RO2 is inhibited due to the increase of temperature, and the reverse reaction rate is greater than the positive reaction rate. The R-base concentration increases, but the reaction RH + O2 = R + HO2 is greatly affected by the temperature, so it proceeds to the positive reaction direction and causes a large accumulation of HO2. However, the accumulation of HO2 will lead to the occurrence of the reaction HO2 + HO2H2O2 + O2. Since the combustion reaction is a process involving free radicals, and

reduced by optimizing. Supporting Information contains the complete mechanism.

3. MECHANISM VALIDATION 3.1. Ignition Delays. Autoignition characteristics of the fuel−air mixture are extremely important to internal combustion engines. In particular, the autoignition strategies of advanced internal combustion engines are only controlled by the chemical kinetics of the fuel, such as homogeneous charge compression ignition (HCCI), reactivity controlled compression ignition (RCCI), and premixed-charge compression ignition (PCCI). Autoignition determines the engine’s emissions and combustion characteristics. Therefore, it is necessary to provide reliable ignition delay in internal combustion engines for a wide range of operating conditions. Over the entire temperature range, the transient closed homogeneous batch reactor of the SENKIN program32 was used for simulation under constant volume, adiabatic, and homogeneous conditions. The experimental data for the ignition delays of PODE3 in RCM were verified. F


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Figure 6. Comparison of ignition delays for NG simplified and detailed mechanisms.

to occur, generating a large amount of OH and releasing heat. Due to the increase in the free radicals’ concentration and the temperature rise, the NTC phenomenon is broken and the reaction rate increases with increasing temperature. At low temperatures, it is interesting that differences in ignition delay

this reaction causes the concentration of free radicals to decrease rapidly, the reaction rate decreases as the temperature rises. However, as the reaction continues, and the temperature rises, finally the temperature climbs high enough to decompose H2O2. This causes the reaction of H2O2 + M = OH + OH + M G


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Figure 7. Comparison of simulated and experimental results for species concentrations for PODE3 flame. Figure 10. Comparison of the predicted and experimental values of laminar flame speeds for CH4 under the pressures of 10 and 20 atm. Symbols represent experimental data,34 solid lines represent calculations with the reduced mechanism, and dashed lines represent calculations with the NUIG detailed mechanism.

Figure 8. Comparison of predicted and experimental values25 of PODE3 laminar flame speed.

Figure 11. Sensitivity coefficients of methane laminar flame speeds.

Figure 6 shows a comparison of the natural gas reduced mechanism and the detailed mechanism for the ignition delay prediction under different conditions. Among them, 90/6.7/ 3.3 represents that natural gas is composed of 90% CH4, 6.7% C2H6, and 3.3% C3H8, whereas 70/20/10 represents that the composition of natural gas is 70% CH4, 20% C2H6, and 10% C3H8. It can be seen from Figure 6 that the ignition-delaypredicted values of the simplified mechanism almost coincide with the predicted values of the detailed mechanism under the different temperatures, pressures, and equivalence ratios. 3.2. Species Concentrations. Sun et al.25 studied the oxidation characteristics of PODE3 under low- and hightemperature conditions. They measured the evolution of reactants, final products, and stable intermediates. This section further evaluates the performance of the mechanism based on these data. The premixed laminar flame was simulated using the PREMIX code,32 and the experimental temperatures (T =

Figure 9. Comparisons of experiments34 and simulations of laminar flame speeds for methane. Symbols are the experimental data; solid lines are the calculations for the reduced mechanism; dashed lines are the calculations with the NUIG detailed mechanism.

between different pressure calculations disappear as the temperature decreases. H


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Figure 14. Comparison of simulated and experimental results34 for methane flame speed under low pressure after improving the smallmolecule mechanism. Symbols represent experimental data, solid lines represent calculation with the reduced mechanism, and dashed lines represent calculation with NUIG detail mechanism.

Figure 12. Comparison of laminar flame speed simulations and experiments25 of the PODE3/air mixture after improving the mechanism for small molecules.

3.3. Laminar Flame Speeds. Laminar flame speed is the fundamental property of a strictly defined combustible mixture, reflecting the net effect of its reactivity, exothermicity, and diffusivity; therefore, it is commonly used to verify the chemical kinetic mechanisms. In this section, the laminar flame speed is simulated by the PREMIX code,32 and the model is verified with the existing experimental data. In this study, the laminar flame speeds of natural gas and PODE3 were verified. Sun et al.21 tested the laminar flame speeds of PODE3/air mixtures under equivalence ratios of 0.7−1.6, unburned gas temperature of 408 K, and standard atmospheric pressure. Rozenchan et al.34 measured the laminar flame speeds of methane under different pressures and equivalence ratios in a constant-volume combustion bomb. The laminar flame speeds of natural gas diluted with inert gas (helium gas) were measured at different pressures (10, 20, 40, and 60 atm) and an unburned gas temperature of 298 K. They also measured the laminar flame speeds of natural gas and air mixtures under the condition of dilute inert gas (helium). The resulting comparisons between the experimental data and the predicted values for the laminar flame speeds of PODE3 and natural gas are shown in Figures 7, 8, and 9, respectively. It shows that the flame speed predicted by the mechanism is larger than the experimental data. To find the source of the error, the laminar flame speed of methane was used to verify the NUIG detailed mechanism.35 Comparing the predicted value simulated by the natural gas detailed mechanism with the experimental value, it was found that there is also a large deviation (see Figures 9 and 10), so the constructed reduced mechanism must be improved. Figure 11 shows the sensitivity coefficients of natural gas flame propagation speed under the conditions of dilution of helium, for the case of T = 298 K, P = 60 atm, and equivalence ratios of 0.7, 1.0, and 1.3. A positive sensitivity coefficient indicates that the reaction promotes flame propagation, whereas a negative sensitivity coefficient indicates that the reaction inhibits flame propagation. As shown in Figure 11, the sensitive reactions to the propagation of natural gas flame are related to C0-C1 reactions. Therefore, further optimizing the

Figure 13. Comparison of simulated and experimental34 results for methane flame speed after improving the small-molecule mechanism. Symbols are experimental data, solid lines are the calculation results for the reduced mechanism, and dashed lines are the calculations with the NUIG detail mechanism.

500−1633 K), pressure (P = 25 Torr), and equivalence ratio (ϕ = 1.0)25 were used as initial conditions for the simulation. The temperature file measured during the experiment is used as an input file. Figure 7 shows the comparison of simulated and experimental results for species concentrations for PODE3 flame. As shown in Figure 7, the predicted values of O2 were slightly higher than the experimental values. For hightemperature conditions, the deviation between the predicted values and the experimental data of small-molecule species’ concentrations is mainly caused by the simplified C2-C3 submechanism and the deficiency of the H 2 /CO/C 1 mechanism.33 The H2O concentration is lower in the area near the center of the premixed flame (HAB < 5 mm). As the PODE3 and O2 are gradually consumed, H2O concentration increases to stable values near HAB = 10 mm. Therefore, the mechanism can accurately predict the concentration of key species. I


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Figure 15. Comparison of the simulated and experimental26 results for the ignition delay of PODE3 after improving the small-molecule mechanism.

mostly comes from the mechanism of Li et al.36 However, the mechanism of Burke et al.37 has better performance, especially in high-pressure, lean-burn conditions; the mechanism of Burke et al.37 is used to replace the mechanism of Li et al.34 in this study. Methyl (CH3-) is critical for predicting hydrocarbon flame characteristics,38 and previous studies have shown that the reactions involving CH3 significantly affect the flame propagation speed and the ignition delay times.38−40 Therefore, the CH3 submechanism is optimized, and the main optimization work includes the following: (1) the reaction rate constant recommended by Aul et al.41 was used for the reaction of CH3 + O2CH3O + O; and (2) the reaction rate constant recommended by Jasper et al.42 was used for CH3 + HO2CH4 + O2 and CH3 + HO2CH3O + OH. Figure 12 shows the comparison between the experiments and the simulations of laminar flame speeds for the PODE3/air mixture under the pressure of 1 atm and the unburned temperature of 408 K. It can be seen from the figure that as the equivalence ratio increases, the flame propagation speed first increases, then decreases, and reaches a maximum at an equivalence ratio is 1.2. After improving the small-molecule mechanism, the simulated values of the laminar flame speed of

H2/CO/C1 reactions can improve the accuracy of predicting the propagation speed of natural gas flames. In chemical reaction kinetics, the hydrogen-oxygen reaction system plays a vital role. The intermediate products of the hydrogen-oxygen reaction system are also important intermediates in the oxidation process of all hydrocarbon fuels and oxygen-containing fuels. The H2/O2 mechanism not only forms the basis of the oxidation mechanisms of hydrocarbon fuel and oxygen-containing fuel but also is an important and sensitive reaction system in the oxidation mechanisms of all hydrocarbon fuel and oxygen-containing fuel. When the temperature exceeds 1200 K, the high activation energy barrier of the β-cleavage reaction (H + O2O + OH) breaks down, making it a decisive chain branching reaction. Another important reaction in the high-temperature region is the dissociation of H2O2 into two hydroxyl groups, namely, H2O2 (+M) = 2OH (+M). The chain branching reaction takes place at a temperature of about 1000 K, which is important because it is a sign of ignition. The hydroxyl groups produced by this reaction react rapidly with the fuel, causing the system’s temperature to jump rapidly, which is the onset of ignition. Therefore, the rate constants of the two reactions are optimized. In the previous mechanisms, the H2/O2 mechanism J


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Figure 16. Comparison of the reduced and detailed mechanisms for ignition delay of NG after improvement of the small-molecule mechanism. Dashed lines are for the reduced mechanism, and solid lines are for the detailed NG mechanism.

The comparisons between simulated and experimental results for the laminar flame speed of natural gas are shown in Figures 13 and 14. It can be seen from these figures that the

PODE3 are greatly reduced in the rich-fuel region (ϕ > 1). The trend in the predicted value of the mechanism is the same as the experimental value. K


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Figure 17. Comparison of the simulations and experiments25 for important species concentrations for PODE3 after improving the small-molecule mechanism.

reduced mechanism can reproduce the dependence of flame speed on equivalence ratios, pressures, and the temperatures for the full range of working conditions. As can be seen from both Figures 13 and 14, both the simulations and the experiments show that the laminar flame speed decreases as the pressure increases. It can be seen from Figure 14 that the flame speed of the methane/oxygen/helium mixture is much higher than that of the methane/air mixture because of the lower heat capacity and higher transport rate of helium. In the case of high pressures (40 and 60 atm) and inert gas (helium) dilution, the predicted values simulated by the simplified mechanism agreed well with the experimental data. Under low pressures (10 and 20 atm) and helium dilution conditions, the predicted value simulated by the simplified mechanism for the flame propagation velocity of natural gas is closer to the experimental value than the predicted value of the NUIG detailed mechanism, whereas for the mixture of natural gas and air under the low-pressure condition, the predicted value simulated by the simplified mechanism and the detailed mechanism is not much different, and they all agreed well with the experimental values. In conclusion, the improved simplified mechanism can predict the laminar flame speed of natural gas more accurately. Figures 15 and 16, respectively, show the comparison of simulations and experiments of the improved mechanism for modeling PODE3 and natural gas ignition delay. It can be seen that the improved mechanism has no negative effect on the simulated results for ignition delay. Figure 17 shows the comparison between the simulations and experiments of the improved mechanism on the species concentration of PODE3. It can be seen that the improved mechanism has little effect on the species concentration.

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1. Ignition delay data were used to verify the present mechanism, and it was found that the mechanism could reproduce the NTC phenomena of PODE3. Compared with the experimental values for natural gas and PODE3 ignition delays, the predicted values simulated of the reduced mechanism show that the simulations and the experiments agreed well. 2. The species concentrations were used to verify the present mechanism. The results show that the model does a good job at predicting the main species concentrations. The C2-C3 submechanism and the H2/ CO/C1 mechanism can be optimized to improve the accuracy of the species concentration prediction. 3. Experimental data for laminar flame speeds were used to verify the mechanism. Preliminary verification showed that the reduced mechanism was significantly different from the experimental data. Therefore, part of the H2/ CO/C1 submechanism was optimized, and the predicted values modeled by the optimized mechanism in the high-pressure lean-burn conditions were in good agreement with the experimental values. The ignition delays of PODE3 and natural gas were recalculated, and it was found that the improved small-molecule mechanism would not have a negative impact on the ignition delay. 4. The constructed PODE3/NG mechanism does a good job at modeling combustion characteristics, and the number of reactions is moderate. It can be coupled with CFD software to calculate the emissions characteristics of PODE3/NG dual-fuel engines.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b03926. The mech.inp file is the mechanism file, the therm.dat file is the thermal property parameter of each component, and the trans.dat file is the transport parameter of each component; both the thermo-physical parameters and the transport parameters are derived from detailed mechanisms (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 771 3232294. ORCID

Haozhong Huang: 0000-0001-7181-3840 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was sponsored by projects of Guangxi Science and Technology Development Plan (1598007-44) and Natural Science Foundation of Guangxi Province (2014GXNSFGA118005, 2017GXNSFAA0198376).

4. CONCLUSIONS In this study, the detailed mechanism of PODE3 proposed by He et al.26 was simplified using different reduced methods. Then, a natural gas reduced mechanism developed by our research group1 was used as the base mechanism to combine with the PODE3 reduced mechanism, the repeated reactions in the PODE3 submechanism during the merging process were removed, and a PODE3/NG reduced mechanism containing 124 species and 650 reactions was developed. The main conclusions are as follows.

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