Oxymethylene Ethers Research Articles

Sustainable Production of Dialkoxymethane Ethers and Related Acetals Using CO2, H2 and Alcohols or Diols

AbstractDialkoxymethane (DAM) ethers, so‐called oxymethylene ethers (OMEs), are compounds constituted by a CH2‐unit bound to two identical alkoxy substituents. These compounds have showcased interesting technological applications as fuel additives or fuels, as a consequence of their interesting properties such as their low vapor pressure, high viscosity, cetane number and oxygen content. Moreover, they are also employed as environmentally‐benign solvents and bulk compounds useful in fine‐chemical industry. In addition, DAM ethers are considered formaldehyde surrogates. Industrially, dimethoxymethane (DMM) is still produced via two‐step protocol involving toxic formaldehyde that consists of a first step of methanol oxidation (Formox process). Hence, the development of more sustainable protocols for their synthesis is highly desired. Among the different strategies reported for DAM synthesis, its production from CO2/H2 and alcohols constitutes a practical, green and attractive alternative route. In this concept, a careful discussion of the developed catalytic protocols for synthesizing DAM from CO2/H2 or formic acid/H2 and alcohols/diols will be summarized. A special emphasis will be performed on the reaction mechanisms involved and the catalyst nature design and characterization.

Advancements and Challenges in the Synthesis of Oxymethylene Ethers (OMEs) as Sustainable Transportation Fuels.

The urgent need for sustainable alternatives to fossil fuels in the transportation sector is driving research into novel energy carriers that can meet the high energy density requirements of heavy-duty vehicles without exacerbating the climate change. This concept article examines the synthesis, mechanisms, and challenges associated with oxymethylene ethers (OMEs), a promising class of synthetic fuels potentially derived from carbon dioxide and hydrogen. We highlight the importance of OMEs in the transition towards non-fossil energy sources due to their compatibility with the existing Diesel infrastructure and their cleaner combustion profile. The synthesis mechanisms, including the Schulz-Flory distribution and its implications for OME chain length specificity, and the role of various catalysts and starting materials are discussed in depth. Despite advancements in the field, significant challenges remain, such as overcoming the Schulz-Flory distribution, efficiently managing water as an undesirable byproduct, and improving the overall energy efficiency of the OME synthesis. Addressing these challenges is crucial for OMEs to become a viable alternative fuel, contributing to the reduction of greenhouse gas emissions and the transition to a sustainable energy future in the transportation sector.

Combustion characteristics of diisopropoxymethane, a low-reactivity oxymethylene ether

Oxymethylene ethers (OMEs) have been studied for use as low-sooting diesel fuel additives or substitutes; very little literature discusses OMEs as spark-ignition (SI) fuels due to their typically high cetane numbers. In this work, a lower-reactivity, branched OME, diisopropoxymethane (DIPM), is evaluated to determine its effectiveness as a spark-ignition fuel, as it is the lowest-reactivity OME (as determined by Indicated Cetane Number) thus far evaluated in the literature. DIPM is synthesized in-house via acetalization from isopropanol (iPrOH) and trioxane using standard OME production practices. DIPM was then tested in a rapid compression machine (RCM) for autoignition and spark ignition characteristics, and in a modified CFR engine to determine effective octane numbers. In the RCM, an autoignition temperature sweep was performed at stoichiometric conditions from 1000/T = 1.7 - 1.0, at 5:1 inert ratio (comparable to approximately 25% EGR), where it was found that DIPM has ignition delay times 5–10x faster than isooctane and displays NTC ignition behavior. Blends with iPrOH indicate that reactivity can be matched with isooctane with low blend ratios of iPrOH in DIPM. Flame speeds were tested with a laser spark for ignition in the RCM, where the flame speed of DIPM and isooctane is determined to be comparable at engine relevant conditions. In the CFR engine, effective RON and MON based on pressure trace frequency domain measurements were determined for DIPM and a 15 vol% iPrOH in DIPM blend. Neat DIPM has (R+M)/2 = 59 and a negative sensitivity of S = -18, consistent with its NTC behavior and higher reactivity. The DIPM/iPrOH blend has positive sensitivity and a pump-gasoline range (R+M)/2 = 89.3. DIPM on its own is unlikely to be an effective SI fuel, however, when blended with iPrOH as an ON booster, it may be a promising SI candidate fuel.

Numerical study of novel OME1−6 combustion mechanism and spray combustion at changed ambient environments

For a climate-neutral future mobility, the so-called e-fuels can play an essential part. Especially, oxygenated e-fuels containing oxygen in their chemical formula have the additional potential to burn with significantly lower soot levels. In particular, polyoxymethylene dimethyl ethers or oxymethylene ethers (PODEs or OMEs) do not contain carbon-carbon bonds, prohibiting the production of soot precursors like acetylene (C2H2). These properties make OMEs a highly interesting candidate for future climate-neutral compression-ignition engines. However, to fully leverage their potential, the auto-ignition process, flame propagation, and mixing regimes of the combustion need to be understood. To achieve this, efficient oxidation mechanisms suitable for computational fluid dynamics (CFD) calculations must be developed and validated. The present work aims to highlight the improvements made by developing an adapted oxidation mechanism for OME1−6 and introducing it into a validated spray combustion CFD model for OMEs. The simulations were conducted for single- and multi-injection patterns, changing ambient temperatures, and oxygen contents. The results were validated against high-pressure and high-temperature constant-pressure chamber experiments. OH*-chemiluminescence measurements accomplished the characterization of the auto-ignition process. Both experiments and simulations were conducted for two different injectors. Significant improvements concerning the prediction of the ignition delay time were accomplished while also retaining an excellent agreement for the flame lift-off length. The spatial zones of high-temperature reaction activity were also affected by the adaption of the reaction kinetics. They showed a greater tendency to form OH* radicals within the center of the spray in accordance with the experiments.

Exploring flow reactor pyrolysis of branched OMEs: Insights into multi-sidechain effects on pyrolysis chemistry of CH3OCH3-n(OCH3)n (n = 1–3)

Oxymethylene ethers (OMEs) have received great attention due to their carbon-neutral nature and advantageous properties as alternative transportation fuels or fuel additives. Previous studies focused on the combustion of linear OMEs with the chemical formula of CH3O(CH2O)mCH3 (m ≥ 0), while a deep understanding of the combustion of branched OMEs with the chemical formula of CH3OCH3-n(OCH3)n (0 ≤ n ≤ 3) is still absent. In this work, the flow reactor pyrolysis of dimethoxymethane (DMM, n = 1), trimethoxymethane (TMM, n = 2), and tetramethoxymethane (TeMM, n = 3) is investigated using gas chromatography, with special attention given to the multi-sidechain effects on the pyrolysis chemistry. A pyrolysis model of branched OMEs is developed and validated against the new pyrolysis data. Modeling analysis is performed to provide insights into the pyrolysis chemistry of the three branched OMEs and the multi-sidechain effects. Unimolecular decomposition and H-abstraction reactions are found to be two major types of fuel decomposition pathways, with the former dominating TMM decomposition, the latter dominating DMM decomposition, and both contributing almost equally to TeMM decomposition. Sensitivity analysis shows that CO bond dissociation is the most sensitive reaction to fuel decomposition in DMM pyrolysis, while methanol elimination is the most sensitive reaction in TMM and TeMM pyrolysis. Among the three fuels, DMM has the lowest pyrolysis reactivity and produces the least abundant methanol. In contrast, TMM is observed with the highest pyrolysis reactivity and produces the most abundant methanol. Theoretical calculations reveal that TMM and DMM have the lowest and highest energy barriers of methanol elimination reactions, which determines the order of their pyrolysis reactivities and methanol formation trends. As a result, DMM pyrolysis occurs at higher temperature regions where the CO bond dissociation and H-abstraction reactions are preferred, while TMM pyrolysis occurs at lower temperature region where methanol elimination dominates.

Combining mass spectrometry, i2PEPICO, and FTIR spectroscopy: Comprehensive speciation in DMM/NO oxidation

Oxymethylene ethers (OMEs) are potential alternative fuels that can be produced in a sustainable way based on CO2 and electricity. Additionally, they exhibit low tendencies to emit NOx or soot during practical combustion. Dimethoxy methane (DMM, CH3OCH2OCH3) has received increasing attention as the smallest OME with a O-CH2-O moiety. In the face of a possible application in exhaust gas recirculation (EGR) scenarios, understanding the influence of NOx on DMM oxidation is of importance. In this work, eleven mixtures of DMM/NO/O2/Ar were investigated in an atmospheric laminar flow reactor employing an extensive set of diagnostics. Molecular-beam mass spectrometry (MBMS), Fourier-transform infrared (FTIR) spectroscopy, NOx chemiluminescence detection, and double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy were combined to obtain a reliable speciation. All in all, species concentrations from different instruments showed a good agreement and one technique could counterbalance the physical limitations of another technique in many cases. In this manner, accurate mole fraction profiles of intermediates like e.g. CH4, C2H4, NO2, and methyl formate (C2H4O2) were gained. Based on i2PEPICO results, nitromethane (CH3NO2) and trans-HONO could additionally be identified as crucial intermediates in the NO-assisted oxidation of DMM. The present data set therefore provides an excellent basis to enhance future model development.

Insight into premixed diethoxymethane flames: Laminar burning velocities, temperatures, and emissions behaviour

Diethoxymethane ((CH3CH2O)2CH2, DEM) is a promising carbon-neutral fuel. DEM is a diether or acetal with a molecular structure similar to oxymethylene ethers (CH3O–(CH2O)n–CH3, OMEn). Thus, DEM can be expected to have a similar combustion behavior to OMEs, reducing harmful emissions such as NOx and particulate matter (PM) in internal combustion engines. From both experimental and kinetic modeling, fundamental studies on DEM are scarce in the literature. More studies are required to gain a detailed insight into the oxidation kinetics of DEM. Laminar burning velocity (LBV) is a critical property that allows a detailed assessment of the potential application of DEM in combustion devices. Unfortunately, the literature on the LBV of DEM is limited. Therefore, in this study we have investigated the LBV of DEM using two reactors for the first time, namely a heat flux burner and a combustion chamber. The experimental data is reported for equivalence ratio between 0.7 and 1.7, initial temperatures of 368–423 K, and initial pressure of 1–5 bar. In addition, we developed a detailed kinetic model extending our recent work of Shrestha et al. (Combust. Flame. 246 (2022) 112,426) to characterize the combustion behavior of DEM utilizing the new experimental data from this work and the literature data. Our model performs remarkably well in capturing the newly measured LBV experimental data over various experimental conditions. We found that DEM and dimethoxy methane (DMM) have similar values of LBVs (within ±1.5 cm/s) for a given condition, which indicates that intermediate chemistry governs the flame chemistry. Despite DEM being a larger molecule that is expected to have slightly lower LBVs than DMM, its effect on the measured values of LBVs is negligible. Finally, we experimentally measured NOx formation in DEM flame for the first time. The stochiometric flame has the highest NOx formation. The proposed model predicted the equivalence ratio dependence of NOx nicely. However, it overestimates the NOx formation for stoichiometric DEM/air mixtures by ∼30 %. The model suggests that the thermal NO formation route is favored at lean and stochiometric conditions. In contrast, the prompt NO formation route is enhanced for rich mixtures.

Detailed mixing measurements from single-hole converging ECN injectors using Rayleigh scattering

Time resolved liquid and vapor fields of dodecane and oxymethylene ethers are measured from Spray A-3 and Spray D using high speed Rayleigh scattering and diffuse back illumination at the Engine Combustion Network (ECN) Spray A condition of 900 K and 22.8 kg/m3. Global quantities including mixture fraction, vapor and liquid penetration, as well as spreading angle are measured. The mixture fraction fields and vapor penetration profiles are well predicted by the 1-D Musculus-Kattke model. The mixture fraction field and vapor penetration from Spray A-3 are similar to those measured from Spray A in previous works. Spray D exhibits higher mixture fraction fields and vapor penetration due to its larger nozzle diameter. The quasi-steady mixture fraction fields from these injectors scale well when distance from the injector is normalized by the nozzle diameter. The turbulent dissipation structures were also analyzed based on the orientation, thickness, and magnitude of the mixing layers. The orientation and thickness are similar to other measurements in atmospheric gas jets, while the magnitude is lower. The thickness and magnitude are subject to uncertainties due to limitations in the imaging resolution of the system but still provide an order of magnitude as a useful reference for comparison against computational fluid dynamic simulations.

Numerical and experimental investigations on the particle formation in oxymethylene ethers (OME[formula omitted], n = 2−4)/ethylene premixed flames

Alternative synthetic fuels can be produced by renewable energy sources and represent a potential route for solving long-term energy storage. Among them, oxygenated fuels have the advantage of significantly reducing pollutant emissions and can therefore be used as carbon-neutral substitute fuels for transportation. In this work, the sooting propensity of different oxymethylene ethers (OMEs) is investigated using a combined experimental and numerical study on a series of burner-stabilized premixed flames under mild to severe sooting conditions. Herein, mixtures of ethylene in combination with the three individual oxymethylene ether (OMEn) for n=2,3,4 are compared in terms of soot formation behavior with pure ethylene flames. The kinetic mechanism from Sun et al. (Proc. Combust. Inst. 36 (2017) 1269–1278) for OMEn combustion with n=1,2,3 is extended to include OME4 decomposition and combustion kinetics. It is combined with a detailed quadvariate soot model, which uses the Conditional Quadrature Method of Moments (CQMOM), and the soot simulation results are validated with Laser-Induced Fluorescence (LIF) and Laser-Induced Incandescence (LII) measurements. It is observed that the three investigated OMEn with n=2,3,4 show similar sooting behavior, mainly reducing larger aggregates while not significantly affecting the formation of smaller particles. Furthermore, the extent of soot reduction is comparable among the three OMEn. The trends and overall reduction are captured very well by the model. The modeling results are analyzed through reaction pathway analyses and sensitivity studies that show the importance of OMEn decomposition and the formation of formaldehyde (CH2O) under rich conditions for reducing species relevant for soot precursor formation. These findings are based on the negligible differences in soot formation between the different OMEn fuels observed in this study.

Experimental study of droplet vaporization for conventional and renewable transportation fuels: Effects of physical properties and chemical composition

As part of the global energy transition, an increasing number of sustainable liquid fuels with a wide range of physical and chemical properties becomes available for gas turbine and IC engines. For an optimal design and operation of these engines, an improved understanding of the effects of fuel properties on atomization, vaporization and combustion kinetics is required. The present work provides a comprehensive experimental study of fuel effects on droplet vaporization for numerous conventional and alternative fuels. The study employs a recently developed flow channel, where free-falling droplets with initial diameters of D≈77μm vaporize under technically relevant gas temperatures Tg of up to 1300 K. The evolution of droplet diameters over travel distance and time is measured using microscopic shadow imaging. A total of 13 single fuel components, five bi- and tri-component mixtures, six conventional and alternative jet fuels and mixtures thereof, and three IC engine fuels and mixtures thereof have been tested under well-reproducible ambient conditions. The fuels represent a wide range of chemical classes such as n-alkanes, iso-alkanes, cycloalkanes, aromatics, alcohols and oxymethylene ethers, and most of them were measured for the first time with realistic D and Tg. The results for the single fuel components reveal in detail the temperature-dependent effects of physical properties such as boiling point Tb, liquid density ρl and enthalpy of vaporization Hv. In particular, it was found that for high gas temperatures of about 1250 K, the vaporization rate is almost completely determined by the product of Hv and ρl. By comparison with the results for single n-alkanes, the measurements for bi- and tri-component mixtures accurately characterize the preferential vaporization of individual fuel components over droplet lifetime. The results for various multi-component fuels reveal complex interplays of preferential vaporization and temperature-dependent influences of physical properties. It is shown that the vaporization for conventional and corresponding alternative fuels can differ significantly. Finally, the droplet vaporization is characterized for mixtures of PRF90+ethanol and ethanol+water, which exhibit a non-ideal vapor–liquid equilibrium.

An experimental and kinetic modeling study on the low-temperature oxidation of oxymethylene ether-2 (OME-2) by means of stabilized cool flames

Oxymethylene ethers have received much attention in recent years as a high-potential alternative for fossil-based fuels. These alternative fuels produced via carbon capture and utilization technologies driven by renewable energy can contribute to the solution of environmental issues in the short term. In this study, the low-temperature oxidation chemistry of oxymethylene ether-2 was investigated by combining experimental and kinetic modeling work. New experimental data were acquired from stabilized, ozone-seeded oxymethylene ether-2/dimethyl ether/oxygen premixed cool flames in a heated stagnation plate burner. Two fuel-lean equivalence ratios were investigated, i.e., ϕ = 0.3 and ϕ = 0.5. The observed and quantified reaction products were methoxymethyl formate, methyl formate, methanol, formaldehyde, CO and CO2. A new detailed kinetic model based on first principles was constructed for the pyrolysis and oxidation of oxymethylene ether-2 with the in-house developed automatic kinetic model generation code Genesys. Compared to an earlier study by De Ras et al. (Combustion and Flame, 2022), additional species and reactions were added to describe the low-temperature oxidation chemistry with more detail, in addition to an update of several thermodynamic and kinetic parameters based on new quantum chemical calculations. The newly developed kinetic model is able to predict the experimental observations of the stabilized cool flames satisfactorily and can reproduce ignition delay times from the literature on average within the experimental uncertainty margin. Rate of production and sensitivity analyses were performed for different reaction conditions to unravel the important decomposition pathways during low-temperature oxidation. It is concluded that oxymethylene ether-2 is a highly reactive fuel, and this without fuel-specific chain branching reactions significantly contributing to the low-temperature oxidation chemistry.

Dimensional analysis of vapor bubble growth considering bubble–bubble interactions in flash boiling microdroplets of highly volatile liquid electrofuels

Electrofuels (e-fuels) produced from renewable electricity and carbon sources have gained significant attention in recent years as promising alternatives to fossil fuels for the transportation sector. However, the highly volatile e-fuels, such as short-chain oxymethylene ethers (OMEx) are prone to flash vaporization phenomena, which is associated with the formation and growth of vapor bubbles, followed by explosive bursting of the liquid jet. This phenomenon is important in many practical applications, for example, superheated liquid sprays in gasoline direct injection engines as well as cryogenic engines. The simulation of a flash boiling spray of such highly volatile liquid fuels is numerically challenging due to several reasons, including (1) the complexity of the bubble growth process in the presence of multiple vapor bubbles and (2) the need to use an extremely small time step size to accurately capture the underlying physics associated with the flash boiling process. In this paper, we first present a bubble growth model in flash boiling microdroplets considering bubble–bubble interactions along with the finite droplet size effects. A dimensional analysis of the newly derived Rayleigh–Plesset equation (RPE) with bubble–bubble interactions is then performed for Reynolds numbers of different orders of magnitude to estimate the relative importance of different forces acting on the bubble surface. Based on this analysis, a simplified nondimensional semi-analytical solution for bubble growth, which also includes the bubble–bubble interactions, is derived to estimate the bubble growth behavior with reasonable accuracy using the larger time step sizes for a wide range of operating conditions. The derived semi-analytical solution is shown to be a good approximation for describing the bubble growth rate over the whole lifetime of the bubble, thus making it useful for simulations of superheated sprays with large numbers of droplets and even more bubbles. The bubble–bubble interactions are found to have a significant impact on the bubble growth dynamics and result in delaying the onset of droplet bursting due to the slower growth of the vapor bubble compared to the bubble growth without bubble–bubble interactions. Furthermore, in a comparison with DNS results, the proposed bubble growth model is shown to reasonably capture the impact of bubble interactions leading to smaller volumetric droplet expansion.

Miscibility in systems containing (poly(oxymethylene) ethers (OME) + hydrocarbons + water)

Poly(oxymethylene) dimethyl ethers (OME) are interesting synthetic fuels: they have better combustion properties than fossil diesel fuel and can be obtained from renewable resources. Furthermore, they can be blended with hydrogenated vegetable oils (HVO) to obtain a new class of environmentally benign fuels. However, it has been observed in technical processes that initially homogeneous mixtures of (OME + HVO) split into two liquid phases after some time, which is unacceptable for use as fuels. It is assumed that this de-mixing is a consequence of the uptake of small amounts of water from the air by the mixtures. However, up to now, only qualitative information on this phenomenon was available. We have, therefore, carried out systematic experimental studies of liquid–liquid equilibria (LLE) and liquid–liquid–liquid equilibria (LLLE) in ternary model systems of the type (OME + n-alkane + water) at temperatures between 265 and 293 K. The n-alkanes were (n-dodecane, n-hexadecane); two different OME were studied, OME2 and OME4, respectively. Additionally, also multicomponent mixtures containing different OME and mixtures containing additionally the aromatic hydrocarbon toluene were studied. The results were modeled using UNIFAC, a group-contribution model of the Gibbs excess energy, and a good correlation of the observed complex phase behavior was obtained. The results from the present work confirm that the uptake of small amounts of water is the reason for the phase split observed in technical (OME + HVO) mixtures. Measures to circumvent this problem are discussed.

Characterization of the oxymethylene ether fuels flame structure for ECN Spray A and Spray D nozzles

Synthetic fuels will play a major role on the reduction of pollutant emissions and carbon footprint of ICE engines. The use of renewable energy and CO2 for its production maps out a promising route to achieve ICE carbon neutrality. Among these fuels, oxymethylene ethers are also interesting for their potential to drastically reduce soot formation. However, a proper characterization of their properties and behaviour is mandatory to reach full implementation in commercial engines. For this reason, this work presents a detailed characterization of the flame structure of two types of oxymethylene ethers (OMEX and OME1) using high-speed chemiluminescence imaging and Planar Laser-Induced Fluorescence (PLIF). Test were performed in a constant-pressure combustion vessel, with the operating conditions and two different injector nozzles from the Engine Combustion Network (Spray A and Spray D). Regions associated with low-temperature chemical reactions are observed thanks to the formaldehyde PLIF while evolution of the high-temperature reactions has been analysed based on hydroxyl excited state (OH*) chemiluminescence and hydroxyl PLIF. On-resonant and off-resonant measurements were performed for OH PLIF with a dye laser in order to remove other radiation sources not linked to OH fluorescence, whilst 355 nm radiation from the third harmonic of a Nd:YAG laser are used to excite CH2O molecules. On the one hand, the results show that the combustion of OME1 with Spray A is characterized by a flame structure very different to that of a diffusion flame. It is characterized by large cool flame region followed by a short high-temperature zone. On the other hand, the combustion of OMEX with both nozzles and OME1 with Spray D show more similarities with a diffusion flame structure. The stoichiometry of the fuel and the equivalence ratio fields achieved strongly affect the structure and latter evolution of the flames.

Role of CH2O moiety on laminar burning velocities of oxymethylene ethers (OMEn): A case study of dimethyl ether, OME1 and OME2

Oxymethylene ethers (OMEn) are an important family of e-fuels that can be produced sustainably from carbon dioxide and hydrogen via renewable electricity. In this work, laminar flame propagation of dimethyl ether (DME, which can be deemed as OME0), dimethoxymethane (OME1) and methoxy(methoxymethoxy)methane (OME2) was investigated in a constant-volume cylindrical combustion vessel. Laminar burning velocities (LBVs) of the three fuels were derived at 423 K, 1–10 atm and equivalence ratios of 0.7–1.5. A kinetic model for the high-temperature oxidation of the three fuels was developed with the isomerization and decomposition reactions of OME2 radicals theoretically calculated. Reasonable predictions can be achieved by the present model during the validation against the new data in this work and previous data in literature. Based on the modeling analysis, fuel-specific flame chemistry of the three fuels was analyzed, especially for the key formation pathways of major intermediates including formaldehyde, methyl formate and CH3. Special attentions were paid on the role of CH2O moiety, which is demonstrated by the variation of LBV and flame chemistry with the ratio (α) of CH2O moiety to the rest moiety in the fuel molecule (α = 1, 2 and 3 for DME, OME1 and OME2). It is observed from the experimental and simulated results that as α increases, the LBV profile has close peak values and peaks towards rich conditions, which results in the crossings of profiles and ascending LBV values under the richest conditions. Reactions involving fuel-specific radicals HCO and CH3 result in the peak shift of H profile and different LBV values, especially under the richest conditions. Furthermore, extended α values at 0 and ∞ by using methane and formaldehyde respectively were also explored with kinetic modeling to provide more insight into the effects of fuel molecular structures.

Pre-vaporized ignition behavior of ethyl- and propyl-terminated oxymethylene ethers

Oxymethylene ethers (OMEs) have been studied in recent years for use as compression ignition fuel blendstocks, but the methyl-terminated OMEs commonly studied exhibit properties that are poorly optimized for engine use and distribution. Recent work has shown that OMEs with larger (ethyl, propyl, or butyl) end groups may have superior properties for fuel usage/storage. In this work, we consider ignition of four OMEs - diethoxymethane (E-1-E), dipropoxymethane (P-1-P), ethoxy-(methoxy)2-ethane (E-2-E), and diisopropoxymethane (iP-1-iP) - as representatives of the possible effects of changes to OME structures. To our knowledge, ignition behaviors of the latter three fuels have not been studied prior to this work. We find that all of the tested linear OMEs (E-1-E, P-1-P, and E-2-E) show two-stage ignition at low temperatures and nonlinear ignition behavior, consistent with literature on methyl-terminated OMEs and E-1-E. The nonlinear, branched OME (iP-1-iP) required higher pressure and temperature to ignite than the linear OMEs; further, this fuel experienced only single stage ignition and a linear ignition delay curve. By analogy to existing kinetic mechanisms for ethers and higher alcohols, the chemical basis for the observed trends are hypothesized. Faster ignition of E-2-E results from the additional oxymethylene group providing additional sites for ROO formation and more possible QOOH structures. Slower low temperature ignition of P-1-P is driven by lower H abstraction rates in comparison to E-1-E, however at high temperatures P-1-P ignites faster, driven by increasing abstraction from the additional H site on the propyl group that opens up additional QOOH formation pathways. iP-1-iP ignition is slowed significantly by preferential H abstraction from the central carbon of the isopropyl group, which is crowded and unlikely to bond with O2, however at high temperatures, abstraction from H sites on the methyl groups allows for the ROO cascade initiation and subsequent rapid ignition.

Improved Energy Efficiency of Power‐to‐X Processes Using Heat Pumps Towards Mobility Sector Defossilization

AbstractCombining Power‐to‐X (PtX) processes with high temperature heat pumps (HTHP) can significantly increase their energy efficiency. Evaluating the example of oxymethylene ethers (OME) production from H2 via H2O electrolysis and captured CO2 from air shows that by upgrading waste heat streams using HTHP, a process overall energy efficiency of higher than 61 % can be achieved compared to 30 % in conventional integrated processes. Thereby, the waste heat stream from H2O electrolysis already covers the low temperature heat demand for CO2 capture via direct air capture, not only for OME but also for various PtX products. Importantly, a significant lever for the energy efficiency enhancement is the consideration of other low temperature heat‐demanding sectors. High overall process energy efficiencies and the electrification of the industry are key aspects towards a sustainable mobility sector.

Experimental and chemical kinetic modeling study of trimethoxy methane combustion

The use of oxymethylene ethers (OMEs) as alternative fuels or fuel-additives has motivated the present study on the combustion behavior of trimethoxy methane (TMM), which is a branched version of OMEs. A detailed chemical kinetic model for TMM combustion is provided, which is based on a recent model for the structurally similar dimethoxy methane (DMM) and recent elementary reaction kinetics studies. This model is validated against TMM ignition delay times measured in a shock tube at 20 and 40 bar, under fuel lean, stoichiometric, and fuel rich conditions. Further, the TMM model is validated against laminar burning velocities, which were measured in a heat flux setup for a variation of fuel/air ratios. A good agreement between the TMM model predictions and the measured ignition delay times and laminar burning velocities is observed. At fuel rich conditions in the shock tube, however, the model underestimates the reactivity of TMM by up to 60%, which could not be compensated by physically meaningful modifications of the TMM model. The analysis of the TMM model shows that at low temperatures, TMM is primarily consumed via H-atom abstraction by ȮH radicals, followed by classical β-scission and low-temperature chemistry. At higher temperatures, a significant fraction of TMM is consumed via unimolecular decomposition, leading to a higher reactivity compared to OME2. The present model and experiments lay the foundation for future kinetic modeling of TMM and other branched OME-like compounds.

A comprehensive kinetic modeling of oxymethylene ethers (OMEn, n=1–3) oxidation - laminar flame speed and ignition delay time measurements

This work reports on the development and experimental validation of a detailed reaction mechanism for the oxidation of polyoxymethylene dimethyl ethers (OMEn, n = 1–3). The validation is done by constant-volume chamber laminar flame speeds (393 K and 443 K, 1 to 5 bar, and equivalence ratio 0.8 to 1.6) and Rapid Compression Machine ignition delay times (550–680 K, 10 and 15 bar, equivalence ratios of 0.5–2.0) in OMEn/air mixtures. Using our new experimental and published data, the validation basis for the new kinetic model comprises the pyrolysis and oxidation of OMEn (n = 1–3) in freely propagating flames, auto-ignition in rapid compression machines and shock tubes, and speciation in jet-stirred and flow reactors as well as burner-stabilized premixed flames. The model provides a reasonable agreement with the experimental data for a broad range of conditions investigated. The performance of the developed model is compared against the recent literature models. OMEn (n = 1–3) all have the same laminar flame speed. The model suggests that the chemistry of OME0 (DME) and CH3OCHO (methyl formate) is the one that dictates the flame chemistry. Under the same pressure and equivalence ratio conditions, ignition delay times of OME2 and OME3 are similar for the investigated temperature range. This work helps to improve the understanding of OMEs chemistry. The model developed in this work will serve as the base mechanism for higher chain length OMEs (n>3).

Unraveling the carbene chemistry of oxymethylene ethers: Experimental investigation and kinetic modeling of the high-temperature pyrolysis of OME-2

Oxymethylene ethers (OMEs) form an interesting family of synthetic compounds to replace fossil fuels. This alternative liquid energy carrier can contribute to a circular carbon economy when produced via carbon capture and utilization technology using renewable electricity. Despite the potential to reduce greenhouse gas and particulate matter emissions and their ideal ignition characteristics, little is known about the thermal decomposition behavior of OMEs. In this work, new insights are obtained in the pyrolysis chemistry of oxymethylene ether-2 (OME-2) and the role of carbenes by performing experiments at high temperatures (> 850 K) in a tubular quartz reactor. The used continuous bench-scale pyrolysis unit has a dedicated on-line analysis section including comprehensive two-dimensional gas chromatography (GC × GC) coupled with flame ionization detection (FID) and mass spectroscopy (MS) to identify and quantify the full product spectrum over the complete temperature range. The reactor temperature was varied between 850 and 1150 K at a fixed pressure of 0.15 MPa and residence times of 400 to 850 ms. The major products are dimethoxymethane, formaldehyde, methyl formate, methane, CO2, CO and H2. Minor intermediate compounds comprise dimethyl ether, formic anhydride, formic acid, methoxymethyl formate and methoxymethanol. The yields of compounds with carbon-carbon bonds are low since no such bonds originally occur in OME-2. Precursors of aromatic compounds and soot particles are absent in the reactor effluent. The experimental results are simulated with a new first principles-based kinetic model for pyrolysis and combustion of OME-2. This model can predict the experimental trends of major products on average within the experimental uncertainty margin of ± 10% relative for major product species. A reaction pathway and sensitivity analysis are presented to highlight the importance of the carbenes for initiation of the radical chemistry under pyrolysis conditions.