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The ignition of a stream of coal particles under oxy-fuel atmosphere is studied numerically in a laminar flow. Devolatilization of coal particles is modeled with the chemical percolation devolatilization (CPD) method coupled with a detailed chemistry model for homogeneous reactions. The particle and gas phase interactions are modeled with a fully coupled Euler-Lagrange code. A parametric study is performed to investigate the influence of particle number density, gas temperature, and velocity on ignition. We found that increasing particle number density delays the onset of ignition. Delayed ignition in denser streams is due to the lower gas temperature, which is caused by the higher energy required for particle heating. In addition, increasing particle number density leads to a more continuous and narrower flame front. Particle heating and ignition induce velocity variations in the gas phase through the coupling with particles. In the denser stream, velocity variations become significant and compromise the validity of a constant velocity assumption that is usually made in computing ignition delay time from the observed ignition location. It is also found that high particle slip velocities lead to a locally low volatile concentration and low temperature, and consequently increase ignition delay time.

In recent years, significant efforts have been made to develop e-fuels from renewable electricity and carbon sources for enabling highly efficient and advanced propulsion systems. Compared to conventional fuels, such fuels can have very different thermo-physical properties depending on their molecular structure. Particularly, fuels with high vapor pressures are highly susceptible to flash boiling depending on boundary conditions, which can significantly alter the spray formation and mixing behavior. Thus, it becomes imperative to develop a fundamental understanding of the underlying physics associated with the flash boiling of these fuels in a single droplet configuration. In this work, oxymethylene ethers (OME x ) are chosen as a generic example to study the flashing behavior of newly developed e-fuels. This study employs the Lagrangian Particle Tracking (LPT) approach considering both internal and external vaporization of flash boiling single droplets. The internal vaporization model includes several sub-models that compute bubble number density, bubble growth rate, and droplet bursting criterion. External vaporization is modeled considering heat transfer from the droplet interior to the droplet surface and from the surrounding gas to the droplet surface. The study reveals that the formation and subsequent growth of vapor bubble nuclei is the primary source causing the transition of the metastable liquid phase into the stable state. We found that for moderate to high superheating degree, the bubble growth characteristics indicate three distinct growth phases: (1) surface tension-controlled, (2) transition, and (3) inertia-controlled, whereas, for low superheating degree, only two of these were present, namely (1) surface tension-controlled, and (2) transition phase. It is also observed that the chain length of OME x has significant impact on bubble dynamics. OME4 is found to have a larger critical nucleus, a longer time delay in bubble growth, and a slower growth rate compared with dimethyl ether (DME). Furthermore, a quantitative analysis shows that droplets burst earlier with increasing superheating degree. In addition, it is found that the system pressure has a negligible influence on the initiation of the bursting process, except when the superheating degree is very low.

The efficiency of spark-ignition engines is limited by the phenomenon of knock, which is caused by auto-ignition of the fuel-air mixture ahead of the spark-initiated flame front. The resistance of a fuel to knock is quantified by its octane index; therefore, increasing the octane index of a spark-ignition engine fuel increases the efficiency of the respective engine. However, raising the octane index of gasoline increases the refining costs, as well as the energy consumption during production. The use of alternative fuels with synergistic blending effects presents an attractive option for improving octane index. In this work, the octane enhancing potential of 2-methylfuran (2-MF), a next-generation biofuel, has been examined and compared to other high-octane components (i.e., ethanol and toluene). A primary reference fuel with an octane index of 60 (PRF60) was chosen as the base fuel since it closely represents refinery naphtha streams, which are used as gasoline blend stocks. Initial screening of the fuels was done in an ignition quality tester (IQT). The PRF60/2-MF (80/20 v/v%) blend exhibited longer ignition delay times compared to PRF60/ethanol (80/20 v/v%) blend and PRF60/toluene (80/20 v/v%) blend, even though pure 2-MF is more reactive than both ethanol and toluene. The mixtures were also tested in a cooperative fuels research (CFR) engine under research octane number and motor octane number like conditions. The PRF60/2-MF blend again possesses a higher octane index than other blending components. A detailed chemical kinetic analysis was performed to understand the synergetic blending effect of 2-MF, using a well-validated PRF/2-MF kinetic model. Kinetic analysis revealed superior suppression of low-temperature chemistry with the addition of 2-MF. The results from simulations were further confirmed by homogeneous charge compression ignition engine experiments, which established its superior low-temperature heat release (LTHR) suppression compared to ethanol, resulting in better blending octane numbers. This work explores and provides a chemically sound explanation for the potential of 2-MF as an octane enhancer.

Condensing gas boilers exhibit complex global flow-field patterns, which lead to noteworthy inhomogeneities of the burnt gas temperature distribution and levels of local pollutant formation in the combustion chamber. To enable numerical simulations of realistic heating systems, a modeling framework was developed. Challenges like high computational times and the combined multi-physics aspects that occur in quite complex geometries were addressed. The flamelet progress variable (FPV) model was applied, which tabulates detailed reaction kinetics as a function of progress variable and enthalpy. As the larger timescales of CO and NO formation inhibit a direct lookup of local CO and NO mass fractions from the chemistry table, the so called NOMANI model was utilized for NO and a rescaling model was applied for CO. All models were validated against resolved simulation results and experimental measurements. The multi-hole burner was modeled as a porous medium, because it was computationally unfeasible to resolve each burner hole. While local gas acceleration and preheating was considered, the exact flame structure as well as the recirculation zone behind the flame front could not be represented. To still capture the burnt gas temperature, the missing heat losses from the recirculation zone were modeled and coupled to the conjugate heat transfer (CHT) model. The modeling framework was applied to a commercial condensing gas boiler device at nominal load utilizing a lean methane-air mixture. The simulation shows reasonable agreement with experimentally measured emission levels and critical regions with CO and NO x emissions twice as high as in other regions were identified. Temperature variations of 200 K in the flue gas throughout the combustion chamber and 450 K in the solid region of the multi-hole burner were found. Based on these results, design aspects are correlated to CO and NO formation and suggestions on possible design modifications are discussed to further reduce pollutant emissions of such condensing gas boilers.

Polyoxymethylene dimethyl ethers (OMEn) with the chemical structure of CH 3 O ( CH 2 O ) n CH 3 are a promising synthetic e-fuel class that can be produced from renewable electricity and improves the emission characteristics of internal combustion engines. However, knowledge of their fundamental combustion properties, such as the laminar burning velocity, is limited in the literature, preventing the full assessment of their application potential in combustion devices. Thus, a heat flux burner is applied in this study to measure the laminar burning velocities of OME1 and OME2 over a wide range of equivalence ratios of 0.6–1.9 and at initial temperatures of 383–401 K under atmospheric pressure for the first time. The obtained data are compared with computed results using a recently developed reaction mechanism for OME1–4. Good agreement is observed. The experimental results are also discussed in the context of a comparison to literature data of dimethyl ether and OME3, demonstrating the minor impact of chain length of (OMEn) on their burning velocities, which is further analyzed in terms of sensitivity analysis on reaction kinetics. Furthermore, the exhaust gas composition of NO x and CO is determined experimentally for flames of OME1 and OME2 using a probe method and compared to the numerical results, observing mostly good agreement, where the remaining differences could be explained on the basis of combined experimental and model uncertainties. This provides unique knowledge on the emission performance of the combustion of (OMEn).

The aim of this work is to study spatially and chemically resolved particle combustion cases to understand chemical and laminar transport processes and to support model development. In the present study, the combustion process of a single char particle located in air or oxy-fuel atmosphere composed of oxygen, carbon dioxide, and steam is investigated. Char burnout is represented in highly resolved numerical simulations including a detailed description of the surface and the gas phase chemistry. At the solid-gas interface, heat and mass fluxes due to the surface reactions involving carbon oxidation and gasification are considered. The model is validated based on experimental results for char burnout phase in a flat flame burner. We perform a comprehensive set of fully resolved reactive 2-D simulations by varying particle size, relative velocity, diluent, and oxygen composition in the surrounding gas. The simulation results are discussed regarding the CO 2 and N 2 content of the atmosphere highlighting the effects of oxy-fuel combustion. Furthermore, the impact of the particle flow motion on the flame that forms around the char particle is investigated by varying relative Reynolds number with particle size and relative slip velocity.

The ignition and volatile combustion of single coal particles were investigated under laminar conditions. Relevant physico-chemical processes were analyzed under conventional and oxy-fuel atmospheres with varying O2 contents in experiments and simulations. An optically accessible laminar flow reactor with well-defined boundary conditions measured with PIV and quantitative OH-LIF was employed. Multi-parameter optical diagnostics were conducted, including OH-LIF, luminescence imaging, and backlight illumination. Simultaneously acquired experimental data allowed for the evaluation of particle size, ignition delay time, and volatile combustion duration for individual particles. A statistical analysis revealed the improved accuracy of OH-LIF compared to luminescence imaging regarding ignition detection. Simulations within an Eulerian–Lagrangian framework were introduced and validated against experiments. On this basis, particle temperatures, local gas temperatures, and fuel mass fraction were evaluated, providing insights into the devolatilization. Both experimental and numerical results indicated that increasing particle sizes significantly retarded homogeneous ignition and volatile consumption. When increasing the O2 content, a shorter ignition delay time and volatile combustion duration were observed experimentally, which was more significant for larger particles. High slip velocities accelerated convective transport resulting in an earlier ignition and faster volatile combustion. An atmosphere change from N2 to CO2 showed an earlier ignition and increased volatile combustion duration for larger particles, whereas the differences were insignificant for small particles. Simulation results suggested that the local heat transfer was improved by CO2, mainly due to the lower temperature sink close to particles and hence higher volatile release rates. As the initial ambient temperatures were similar, the introduction of CO2 favored homogeneous ignition and slowed down the volatile consumption.

Ketohydroperoxides (KHPs) are oxygenated hydrocarbons that include carbonyl and peroxide functional groups. The decomposition pathways of KHPs are chain branching pathways in low temperature chemistry of alkanes and oxygenated biofuels, which makes KHPs critical intermediates in ignition processes in advanced combustion engines. In this work, KHP formation was investigated in the low temperature oxidation of a primary reference fuel (PRF 70), a binary mixture of 30 vol% n-heptane and 70 vol% iso-octane to represent a low octane number gasoline surrogate. Species were detected and measured in a jet-stirred reactor (JSR), coupled with time-of-flight molecular beam mass spectrometer using synchrotron vacuum-ultraviolet radiation as photon ionization source (SVUV-PI-TOF-MBMS) which provides in-situ measurement of unstable KHPs. Simultaneous KHPs production from higher reactivity n-heptane and lower reactivity iso-octane at similar temperatures was observed. Kinetic modeling was used to study the reactivity across a wide temperature range and to examine KHP formation chemistry. Specifically, signals vs. temperature profiles in experiments and mole fraction vs. temperature profiles in simulations, were compared to indicate the most possible saturated KHP isomers. Possible formation pathways for olefinic KHPs and di-olefinic KHPs are discussed. This work reports simultaneous formation of KHPs from two fuel molecules in a gasoline surrogate mixture, discusses important reaction pathways in low temperature oxidation, and explains simultaneous KHP production in n-heptane/iso-octane mixtures, i.e., low temperature oxidation of less reactive iso-octane is initiated by OH radicals produced from high reactivity n-heptane oxidation.

Measures for fuel auto-ignition behavior in form of research octane number (RON), motor octane number (MON), and octane sensitivity (OS) are heavily used for evaluation of fuel knock resistance. In order to allow their rapid assessment for novel fuel candidates, a number of quantitative structure-property relationship (QSPR) models have been proposed. While reasonable prediction accuracies have been achieved for predefined sets of fuel types and molecular groups as input features, the detailed effects of the structural groups and additional descriptors for the positions of side chains, double bonds, or oxygenated functionalities deserve further investigation. The tailored definition of such descriptors based on prior knowledge of the fuel ignition chemistry is expected to be particularly important due to the relative sparsity of available training data. The present work targets these aspects by systematically defining and incorporating such input features in QSPR models to predict RON, MON, and OS based on artificial neural networks. A set of experimental RON and MON data for 260 pure fuel components, including non-oxygenated and oxygenated hydrocarbons, serves as training basis. A backward feature elimination method is employed to reduce the numbers of predefined features of each model, finding that the OS model requires less input features for maximum prediction accuracy than the RON and MON models, while this direct modeling approach still yields lower prediction errors than retrieving the OS as difference of predicted RON and MON values. Finally, a sensitivity analysis on the input features provides insight into their effects on the predicted target quantities.

Carbon-neutral synthetic fuels can be produced from renewable electricity by the hydrogenation of carbon dioxide captured from air or exhaust gas. A promising class of these synthetic fuels are long-chain oxymethylene ethers (OMEs), which exhibit good auto-ignition characteristics for compression-ignition engine application. This study aims to investigate the auto-ignition of three oxymethylene ethers (OMEn, n = 2–4) numerically and experimentally. A shock tube is applied to measure ignition delay times over a range of initial conditions and the obtained results serve as the validation and optimization targets for a chemical mechanism of OME 2 - 4 developed in this work. This model is derived first using an automatic reaction class-based mechanism generator. To ensure the chemical validity of the mechanism, the automatic generator applies reaction classes and rate rules consistently for OME 2 - 4 , which are adopted from a recently published OME 1 mechanism. For improved model prediction accuracy of ignition delay times, the mechanism is then optimized automatically by calibrating these rate rules within their uncertainties using data for all OMEn fuels. It is shown that this highly automated model development process is able to provide accurate chemical mechanisms for large fuel components in a very efficient manner, if accurate prior kinetic knowledge exists for their short-chain counterparts.

Due to their increasing share, the combustion of alternative fuels and in particular oxygenated, bio-derived fuel components need to be characterised. The laminar burning velocity is one key parameter for the characterisation of fuels, and it also serves as an important quantity to validate chemical kinetic models. Methanol, ethanol, n-propanol, and n-butanol laminar burning velocities experiments were conducted in a spherical combustion vessel at an unburnt temperature of 373 K and a pressure of 10 bar. Measured burning velocities from this study and from the published literature are compared with numerical simulation data from published chemical mechanisms. The models tend to underpredict the experimentally measured values. A sensitivity analysis suggests further investigation of the pressure dependence for the fuel specific reactions with hydrogen and hydroxyl radicals.