A combined modeling and experimental study on low- and high-temperature oxidation chemistry of OME3 as novel fuel additive

International audienceThe present research focuses on combined modeling and experimental work on the com-bustion of oxymethylene ethers (OMEs). OMEs are promising synthetic fuels which can beproduced in a carbon-neutral manner starting from captured CO2 and renewable energy.Moreover, blending them with conventional diesel reduces soot emissions because of the ab-sence of carbon-carbon bonds. This results in less harmful emissions and contributes to amore sustainable transport sector as aimed by the Paris climate agreement objectives. Topromote the use of these kind of molecules as fuel additive, it is important to understand theirlow- and high-temperature combustion kinetics. The development of detailed microkineticmodels provides this fundamental insight and enables predictive simulations for combustionapplications.During the last decade, great progress has been made in the construction of reliable kineticmodels for numerous technologically important radical chemistry processes. The resultingmodels typically contain hundreds of species, and several thousands of associated reactions.The manual generation of microkinetic models would be a tedious, error prone and oftenincomplete process. To prevent this, automatic kinetic model generation routines have beendeveloped to systematically develop models, such as Genesys at the Laboratory for Chemi-cal Technology (Ghent University). A kinetic model for both oxidation and pyrolysis hasbeen developed for OME3 based on first principles using Genesys.A prerequisite for the generation of detailed kinetic models is the availability of accuratethermodynamic and kinetic data for species and reactions respectively. Ideally, these pa-rameters are available from experiments or high-level quantum chemical calculations. Sincethese methods are expensive and time-consuming, Genesys instead often relies on approxi-mation methods such as group additivity and rate rules. In this work, thermodynamic andkinetic parameters are obtained from quantum chemical calculations at the CBS-QB3 levelof theory for important reaction pathways for both low- and high- temperature oxidation ofOME3. The results of these calculations are extrapolated to be valid for long-chain OMEsby regression of new group additive values and rate rules.Within Genesys, the possible reactions are generally defined in terms of reaction families,e.g. hydrogen abstraction by molecular oxygen from a secondary carbon atom. Reactionfamilies from earlier studies on smaller oxymethylene ethers such as dimethoxy methaneare taken over and applied for the OME3 model. The outcome is a model containing thechemistry for OME3. To include the chemistry of smaller (oxygenated) hydrocarbons in thefinal model, the Genesys model is merged with the AramcoMech 1.3 base model.Both at ame burner and rapid compression machine experiments have been performedwith OME3 for validation of the combustion model. The ame experiments are performedat 0.053 bara and with a fuel composition of 20 mol% OME3 and 80 mol% CH4. Some mea-sured concentration profiles in function of the height above burner (HAB) of small species(i.e. OME3, CH2O, CH3OH, H2, CO2 and CO) are shown in Figure 1. Other impor-tant species which are observed include ethane, ethylene, dimethyl ether, methyl formate,dimethoxy methane and methoxymethyl formate.Ignition delay times have been measured via rapid compression at 5 bara for and to ad-ditionally validate the low-temperature section of the model. Samples were taken to identifythe reactants and products, including OME3, methyl formate, methoxymethyl formate andmethoxymethoxymethyl formate. Similarly, pyrolysis experiments are performed for OME3in a bench-scale steam cracker setup over a broad range of temperatures (723 K - 1073 K)to validate both the primary and secondary chemistry of the pyrolysis model

Oxygenated E-fuel decomposition : kinetic modeling based on first principles

Meer dan een kwart van de uitstoot van broeikasgassen is afkomstig van de transportsector en daarom is een ingrijpende aanpassing noodzakelijk om te voldoen aan de doelstellingen van het Klimaatakkoord van Parijs. Om de uitstoot van broeikasgassen te verminderen en een duurzame samenleving te creëren, wordt onderzoek uitgevoerd naar hernieuwbare brandstoffen. In dit proefschrift wordt de decompositie van oxymethyleenethers bestudeerd. Dergelijke zuurstofhoudende moleculen vormen een veelbelovende klasse van synthetische e-brandstoffen vanwege hun gunstige verbrandingseigenschappen. E-brandstoffen kunnen worden geproduceerd op basis van koolstofdioxide en hernieuwbare elektriciteit. Oxymethyleenethers bestaan uit afwisselend koolstof- en zuurstofatomen en dus niet enkel koolstofatomen zoals het geval is bij fossiele brandstoffen. Door dit structurele verschil zijn de reacties die optreden anders. Om het gebruik van deze brandstoffen te stimuleren, is het echter belangrijk om de verbrandingschemie te begrijpen. Dit kan door kinetische modellen te ontwikkelen op basis van fundamentele principes. Kwantumchemische berekeningen worden uitgevoerd om nieuwe reactiepaden te ontdekken en om thermochemische data te verkrijgen. Nieuwe experimentele datasets worden verkregen van verschillende opstellingen om de kinetische modellen te valideren. Deze modellen maken voorspellende simulaties mogelijk om verbrandingstoepassingen te optimaliseren en het gebruik van alternatieve brandstoffen in zwaar transport, waarvoor elektrificatie niet haalbaar is, mogelijk te maken

Oxymethylene ethers as fossil fuel alternative to reach carbon neutrality

More than a quarter of the global greenhouse gas emissions are related to the transportation sector and, therefore, a major revamp of this sector is required to comply with the Paris Climate Agreement objectives. Our research focuses on combined modeling and experimental work on combustion of oxymethylene ethers (OMEs). These molecules form a promising family of synthetic fuels, which can be produced starting from CO2 via carbon capture and utilization technology with renewable electricity, so-called e-fuels. OMEs consist of alternating carbon and oxygen bonds in the backbone saturated with hydrogen (structural formula CH3O(CH2O)nCH3), instead of alternating carbon-carbon bonds in the case of fossil-based fuels. OMEs are compatible with the current generation of diesel engines and blending them with conventional diesel fuels reduces the soot emissions significantly because of the absence of carbon-carbon bonds. As such, OMEs and OME-diesel blends result in less harmful emissions and can contribute to a more sustainable transportation sector on the short term due to the transition towards a circular carbon economy. To promote the use of these molecules, it is important to understand their low- and high-temperature combustion chemistry. Detailed microkinetic models have been developed systematically with in-house developed automatic kinetic model generation code `Genesys' to gain fundamental insight into the decomposition chemistry. Quantum chemical calculations are performed on the HPC to unravel the OME decomposition pathways, and to obtain both accurate thermodynamic and kinetic parameters for species and reactions, respectively. New experimental data is obtained from a variety of experimental units to validate the constructed models under a broad set of conditions. Our kinetic models will enable predictive simulations to optimize the combustion and allow global deployment in heavy-duty transportation for which electrification is unfeasible

The secondary chemistry of synthetic fuel oxymethylene ethers unraveled : theoretical and kinetic modeling of methoxymethyl formate and formic anhydride decomposition

Replacing fossil fuels by oxymethylene ethers (OMEs) produced from renewable resources could help to reduce the rising CO2 levels. In this work, the thermal decomposition chemistry of methoxymethyl formate (CH3O-CH2OCHO) and formic anhydride (OCHOCHO) is investigated by means of a combination of quantum chemical calculations and kinetic modeling. The latter compounds are two important intermediates formed during the thermal decomposition chemistry of synthetic fuel OMEs. Two detailed kinetic models are developed based on first principles to describe the radical decomposition chemistry of methoxymethyl formate and formic anhydride, which are ultimately incorporated into the OME-2 model from De Ras et al. (Combustion and Flame, 2022). This newly obtained kinetic model describes experimental measurements for pyrolysis from literature significantly better than the model from the original study, without any fitting of thermodynamic or kinetic parameters. More particularly, some minor compounds are now satisfactorily reproduced within the experimental uncertainty margin of 10 mol% relative. Methoxymethyl formate and formic anhydride are found to be more reactive compared to OMEs. Both a reaction pathway analysis and sensitivity analysis reveal the important decomposition pathways under pyrolysis conditions