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This work introduces a newly developed reaction mechanism to predict fuel/NOx interaction in freely propagating, burner stabilized premixed flames, shock tubes, jet stirred reactors and plug flow reactors experiments. The study focuses on pure ammonia as fuel as well as H₂,H₂/CO, CH₄ doped with NO, NO₂ and N₂O. The kinetic scheme also focuses on the formation and consumption of nitrogen oxides at different experimental conditions. It is found that the doping with N₂O has almost no sensitivity on the H₂ chemistry, while small amount of NO₂ is enough to change the pressure dependence of H₂ auto ignition. The experiments with N₂O still show the significant influence of the chain breaking reaction H+O₂(+M)⇋HO₂(+M) on the H₂ ignition delay times, resulting in the crossing lines for the ignition delay times, which are moved at higher pressure to higher temperature. The concentration of HO₂ is decisive for the active reaction pathway in shock tube oxidation as well as in jet stirred and flow reactor. Reaction NO+HO₂⇋NO₂+OH and NO₂+H⇋NO+OH are very important because they strongly influence the inter conversion process of NO and NO₂ in jet stirred reactor and flow reactor for cases studied herein. The formation of NO in burner stabilized premixed flames is shown to demonstrate the capability of the mechanism to be integrated into mechanisms for hydrocarbon oxidation.
Ammonia (NH3) has recently received much attention as a promising future fuel for mobility and power generation. The use of ammonia as a fueling vector can help curb global warming by cutting CO2
emissions because it is a carbon-free fuel and a hydrogen carrier with a high percentage of hydrogen atoms per unit volume. Liquid ammonia contains a higher volumetric density of hydrogen than liquid
hydrogen. The low reactivity of ammonia, however, hinders its direct usage as a combustible fuel. One feasible way to boost the reactivity of ammonia is to target a dual-fuel system comprising of ammonia
and a suitable combustion promoter. In this work, combustion properties of ammonia were investigated by blending it with various proportions of dimethyl ether (DME) using a rapid compression machine
(RCM) and a constant volume spherical reactor (CVSR) over a wide range of experimental conditions. DME is a highly reactive fuel that may be produced in a sustainable carbon cycle with a net zero-carbon
emission. Ignition delay times (IDTs) of NH3/DME blends were measured over a temperature (T) range of 649e950 K, pressures (P) of 20 and 40 bar, equivalence ratios (F) of 0.5 and 1 for a range of DME mole
fractions (cDME) of 0.05e0.5 in the blends. In addition, the laminar burning velocities of NH3/DME blends were measured at P ¼ 1, 3 and 5 bar, F ¼ 0.8e1.3 and T ¼ 300 K for cDME ranging from 0.18 to 0.47. Our
results suggest that DME is a good ignition promoter, resulting in a significant shortening of IDTs and an increase of flame speeds of NH3. A detailed chemical model has been developed and validated against the
experimental data. Overall, our kinetic model offered reasonable predictive capabilities capturing the experimental trends over a wide range of conditions. In the worst-case scenario, our model underpredicted IDTs by a factor of ~2.5 while overpredicting laminar flame speed by ~20%.
In this work, we investigated the combustion characteristics of ammonia (NH3) by blending it with various proportions of diethyl ether (DEE). We measured laminar flame speed of various NH3/DEE blends (DEE, 10–40% by mole) using a constant volume spherical vessel at Ti = 298 K and Pi = 3 and 5 bar and Φ = 0.8–1.3. We developed a detailed kinetic model to describe the trends of the current and previously published experimental data. For the robustness of the model, we first developed a comprehensive diethyl ether kinetic mechanism to accurately characterize neat DEE oxidation behavior. We validated the kinetic model using a large pool of experimental data comprising shock tube, rapid compression machine, jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames. The developed kinetic model performs remarkably in capturing the combustion behavior of pure DEE and NH3. Importantly, our model captures the experimental data of laminar flame speed and ignition delay times of various NH3/DEE blends over a wide range of conditions. We found that DEE is a promising candidate to promote the combustion characteristics of NH3. A small portion of DEE (10%) enhances the laminar flame speed of NH3 by a factor of 2 at Pi = 1 bar, Ti = 298 K, and Φ = 1.0. A further doubling of the DEE mole fraction to 20% did not enhance the laminar flame speed of NH3 with the same propensity. At low temperatures, adding 5% DEE in NH3 blend has significantly expedited the system reactivity by lowering the autoignition temperature. A further 5% increment of DEE (i.e., 10% DEE in NH3) lowers the autoignition temperature by ∼120 K to achieve the same ignition delay time. The “NOsingle bondNO2” looping mechanism predominantly drives such reactivity accelerating effect. Here, the reactions, NO + HO2 = NO2 + OH and NO2 + H = NO + OH, appear to enhance the reactive radical pool by generating OH radicals. We observed that the HNO path is favored more with increasing DEE content which eventually liberates NO. Other key reactions in “NOsingle bondNO2” looping mechanism are: CH3 + NO2 = CH3O + NO, CH3O2 + NO = CH3O + NO2, C2H5 + NO2 = C2H5O + NO, C2H5O2 + NO = C2H5O + NO2. In addition, CH3 + NH2(+M) = CH3NH2(+M) reaction is also one of the important cross-reactions which leads to the formation of HCN. Therefore, cross-reactions between the nitrogen and carbon family are crucial in accurately predicting autoignition timing. This work provides a detailed chemical insight into the NH3 and DEE interaction, which could be applied to other fuel blends of NH3. The kinetic model is also validated for several C1single bondC3 fuels including their interaction with NOx.
New types of synthetic fuels are introduced in internal combustion engine applications to achieve carbon-neutral and ultra-low emission combustion. Dimethyl Ether (DME) and Polyoxymethylene Dimethyl Ethers (OMEn) belong to such kind of synthetic fuels. Recently, Shrestha et al. (2022) have developed a novel detailed chemistry model for OMEn (n=1-3) to predict the ignition delay time, laminar flame speed and species formation for various thermodynamic conditions. The detailed chemistry model is applied in the zero dimensional (0D) stochastic reactor model (DI-SRM) to investigate the non-premixed combustion in a 2-liter diesel engine. Further insights in the formation of unburned hydrocarbons (HC), carbon monoxide and nitrogen oxides during the combustion of OMEn fuels are obtained in this work. The combustion and emission formation of DME and OMEn (n=1-3) are investigated and compared to conventional Diesel combustion. The mixture formation is governed by an earlier vaporization of the DME and OMEn fuels, faster homogenization of the respective air-fuel mixture and higher reactivity. At the same injection pressure, the OMEn fuels obtain higher NOx but lower CO and HC emissions. High amounts of aromatics, ethene, methane formaldehyde and formic acid are found within the Diesel exhaust gas. The DME and OMEn exhaust gas contains higher fractions of formaldehyde and formic acid, and fractions of methane, methyl formate and nitromethane.
This work reports the laminar burning velocities (LBV) for the liquid fuels methanol, and polyoxymethylene dimethyl ethers (OMEn, n = 1-3) in mixtures with air utilizing the heat flux burner and constant volume chamber at temperature 393 to 443 K, pressure 1 to 10 bar, and equivalence ratio 0.6 to 1.9. Laminar burning velocities for OME2 and OME3 higher than 1 bar are reported for the first time. A detailed chemical kinetic model for OME2 and OME3 was developed based on our previous work utilizing experimental data from this work. Overall, model predictions are in good agreement with the experimental data. It is previously shown that with increases in unburned gas temperature laminar burning velocity increases and shows a linear trend with respect to temperature. Further, laminar burning velocity decreases with an increase of initial pressure. The dependence of burning velocity is not linear for all the fuels investigated. For comparable temperature and pressure conditions, it was found that with an additional CH2O group the laminar burning velocities increase marginally and there is no shift of maximum laminar burning velocity with respect to equivalence ratio.
This work reports the laminar burning velocities for CH4/H2 blends in mixtures with air utilizing the constant volume chamber at temperatures from 300 to 423 K, pressures from 2 to 10 bar and equivalence ratios from 0.7 to 2.4. A detailed chemical kinetic model based on our previous work is used to reproduce the experimental data. The chemical kinetic model can reproduce the experimental data better at lean conditions compared to rich conditions. It is observed that with an increase in H2 fraction in fuel blend, laminar burning velocities increase. Model reveals that with increasing H2 fraction in the fuel blend, formation of key radicals H, OH and O are promoted leading to higher laminar burning velocities. The most sensitive reactions are O2+H=OH+O, CO+OH=CO2+H, CH4+H=CH3+H2 and CH4+OH=CH3+H2O. It is found that as initial mixture temperature increases, the laminar burning velocity increases and shows a linear trend whereas this trend is reversed as the initial pressure increases.
A Kinetic Modeling Study for the Effect of NOx on Oxymethylene ethers (OMEn, n = 0 and 1) oxidation
(2021)
We present a detailed kinetic model for the oxidation of dimethyl ether (OME0) and dimethoxymethane (OME1) in presence of NOx. We further explored the effect of NOx chemistry on the oxidation kinetics of the two OMEs. Our kinetic model is validated against the recent flow reactor data from Zhang et al. (Combust. Flame. 224 (2021) 94– 107). The results indicated that NO doping severely alters the oxidation kinetics of both fuels. The onset temperature for total fuel consumption is significantly shifted to lower temperatures for both fuels, which is in line with the experimental observation. We found that the addition of NO significantly inhibited the NTC behaviour of dimethyl ether. This inhibiting effect appears to stem from the competition between CH3OCH2O2 radical consumption by NO directly and the isomerization/dissociation reactions of CH3OCH2O2. Unlike dimethyl ether, dimethoxymethane does not exhibit a strong NTC behavior, and NO addition completely inhibited its weak NTC behavior.
The climate protection plan of the European Union requires a significant reduction of CO2 emissions from the transportation sector by 2030. Today ethanol is already blended by 10vol-% in gasoline and further increase of the ethanol content to 20vol-% is discussed. During the ethanol production process, distillation and molecular sieving is required to remove the water concentration to achieve high-purity ethanol. However, hydrous ethanol can be beneficial to suppress knock of spark ignition engines. The hygroscopic nature of ethanol can allow to increase the water content in gasoline – water emulsions even more, without adding additional surfactants, and improve the thermal efficiency by optimized combustion phasing, while keeping the system complexity low. Hence, the effect of gasoline – ethanol – water mixtures on the auto-ignition in a single-cylinder spark ignition engine is investigated by using multi-dimensional simulation and detailed chemistry. The gasoline – ethanol mixtures are defined to keep the Research Octane Number constant, while the Motored Octane Number is decreasing. In total five surrogates are defined and investigated: E10 (10vol-% ethanol-in-gasoline), E20, E30, E70 and E100. The water content is determined according to experimentally defined ternary diagrams that evaluated stable gasoline – ethanol – water emulsion at different gasoline – ethanol blending ratios. The auto-ignition modes of the surrogates are analyzed using the diagram, which determines if hotspots are within harmless deflagration or harmful developing detonation regime. The strongest auto-ignition is observed for the E10 surrogate, while increasing ethanol content reduces the surrogate reactivity and increases the resonance parameter. No auto-ignition of the unburnt mixture is observed for the E70 and E100 surrogates. The addition of hydrous ethanol decreased the excitation time of the surrogates, especially at low ethanol content, wherefor the reactivity parameter is significantly increased. The hotspots for E10, E20 and E30 surrogates with hydrous ethanol are found within the developing detonation regime, while hotspots of the E70 surrogate with hydrous ethanol are found in the transition regime. For the hydrous E100 surrogate no auto-ignition is predicted because of reduced temperature of the unburnt mixture due to water vaporization, which outweighs the increased reactivity due to water vapor addition.
In this study, an engine and fuel co-optimization is performed to improve the efficiency and emissions of a spark ignition engine utilizing detailed reaction mechanisms and stochastic combustion modelling. The reaction mechanism for gasoline surrogates (Seidel 2017), ethanol, and methanol (Shrestha et al. 2019) is validated for experiments at different thermodynamic conditions. Liquid thermophysical properties of the RON95E10 surrogate (iso-octane, n-heptane, toluene, and ethanol mixture), ethanol, and methanol are determined using the NIST standard reference database (NIST 2018) and Yaws database (Yaws 2014). The combustion chemistry, laminar flame speed, and thermophysical data are pre-compiled in look-up tables to speed up the simulations (tabulated chemistry).
The auto-ignition in the stochastic reactor model is predicted by the detailed chemistry and subsequently evaluated using the Bradley Detonation Diagram (Bradley et al. 2002, Gu et al. 2003, Neter 2019), which assigns two dimensionless parameters (resonance parameter and reactivity parameter). According to the defined developing detonation limits, the auto-ignition is either in deflagration, sub-sonic auto-ignition, or developing detonation mode. Ethanol and methanol show a knock-reducing characteristic, which is mainly due to the high heat of vaporization.
The multi-objective optimization process includes mathematical algorithms for design space exploration with Uniform Latin Hypercube, pareto front convergence with Non-dominated Sorting Genetic Algorithm II (NSGA-II), and multi-criteria decision making (Deb et al. 2002). The optimization input parameter ranges are selected according to the previous sensitivity analysis, and the objectives are to minimize specific CO2 and specific CO and maximize indicated efficiency. The performance study of different optimization algorithms shows that the incorporation of metamodels is beneficial to improve the design space exploration, while keeping the optimization duration low. The comparison of different reaction mechanisms, which are applied in the optimization process, shows a strong impact on the pareto front solutions. This is due to differences in the emission formation and auto-ignition between the different reaction schemes. Overall, the engine efficiency is increased by 3.5 % points, and specific CO2 emissions are reduced by 99 g/kWh for ethanol and 142 g/kWh for methanol combustion compared to the base case. This is achieved by advanced spark timing, lean combustion, and reduced C:H ratio of ethanol and methanol in relation to RON95E10.