Fuel-rich Flames Research Articles

A high-temperature study of 2-pentanone oxidation: experiment and kinetic modeling

Abstract Small methyl ketones are known to have high octane numbers, impressive knock resistance, and show low emissions of soot, NOx, and unburnt hydrocarbons. However, previous studies have focused on the analysis of smaller ketones and 3-pentanone, while the asymmetric 2-pentanone (methyl propyl ketone) has not gained much attention before. Considering ketones as possible fuels or additives, it is of particular importance to fully understand the combustion kinetics and the effect of the functional carbonyl group. Due to the higher energy density in a C5-ketone compared to the potential biofuel 2-butanone, the flame structure and the mole fraction profiles of species formed in 2-pentanone combustion are of high interest, especially to evaluate harmful species formations. In this study, a laminar premixed low-pressure (p = 40 mbar) fuel-rich (ϕ = 1.6) flat flame of 2-pentanone has been analyzed by vacuum-ultraviolet photoionization molecular-beam mass-spectrometry (VUV-PI-MBMS) enabling isomer separation. Quantitative mole fraction profiles of 47 species were obtained and compared to a model consisting of an existing base mechanism and a newly developed high-temperature sub-mechanism for 2-pentanone. High-temperature reactions for 2-pentanone were adapted in analogy to 2-butanone and n-pentane, and the thermochemistry for 2-pentanone and the respective fuel radicals was derived by ab initio calculations. Good agreement was found between experiment and simulation for the first decomposition products, supporting the initial branching reactions of the 2-pentanone sub-mechanism. Also, species indicating low-temperature chemistry in the preheating zone of the flame have been observed. The present measurements of a 2-pentanone flame provide useful validation targets for further kinetic model development.

Extinction Limits of Swirl Nonpremixed Methane Flames and CH4/CxHy/N2 Mixtures: Experimental Evaluation and Thermodynamic Calculations

In this study, we aimed to evaluate the extinction limits of fuel-rich and fuel-lean flames created in non-premixed coaxial swirl burner of combustion substrates. The experiments were performed with three types of fuels: methane and two mixtures of CH4/CxHy/N2. The analyzed fuels reflected varied composition of the natural gas containing methane, ethane, propane, butane, and nitrogen. The examination of the extinction limits of flames isolated from the surroundings with a quartz tube was based on the limit ratios of equivalence (Φ). Results showed that an increase in the stream of methane directed toward the burner improves blowout limits as determined by the equivalence ratios. However, this pattern was not observed for the other studied fuels. A concurrent increase in the content of CxHy and N2 in natural gas implies worsening of extinction limits of fuel-lean flames and improvement of extinction limits for fuel-rich flames. Also performed thermodynamic calculations using the FactSage 6.3 software packa...

Photoelectron Photoion Coincidence Spectroscopy to Unveil Reaction Mechanisms by Isomer-selective Detection of Elusive Molecules: From Combustion to Catalysis.

Elusive and reactive intermediates, such as radicals, play a central role in reaction mechanisms. Photoelectron photoion coincidence spectroscopy with tunable vacuum ultraviolet synchrotron radiation offers a multiplexed, sensitive, mass- and isomer-selective way to identify and, in some cases, determine mole fractions of reactive species. It thus helps to unveil the missing link(s) between reactants and products. After a brief overview of the technique, we review two systems in three different reactive environments. First, the unimolecular decomposition mechanism of ortho-xylyl radicals is revealed in pyrolysis experiments. Second, the insights gained are used to analyze a fuel-rich meta-xylene flame, which suggests that important xylyl isomerization reactions are currently missing in combustion models. Third, photoion mass-selected threshold photoelectron spectra identify the fulvenone ketene as the crucial intermediate in the catalytic fast pyrolysis of a lignin model compound and help map heterogeneous catalysis mechanisms.

Methane partial oxidation in a two-layer porous media burner with Al2O3 pellets of different diameters

In this study, the fuel-rich combustion of methane in a two-layer porous media burner with Al2O3 pellets of different diameters was experimentally investigated. The upstream layer consisted of 2.5-mm diameter packed Al2O3 pellets, and the downstream layer consisted of 5-mm, 6.5-mm, 7.5-mm, and 9.5-mm diameter packed Al2O3 pellets. The effects of pellet diameter on the temperature distribution, exhaust composition, and the syngas energy conversion efficiencies were studied at a fixed operation condition with an equivalence ratio of 1.6 and a gas velocity of 0.13 m/s. An optimal downstream pellet diameter of 7.5 mm was determined for the partial oxidation of methane corresponding to the highest syngas energy conversion efficiency. Stabilized methane fuel-rich flames were realized in the optimized burner for various gas velocities (0.09 m/s–0.15 m/s) and equivalence ratios (1.2–1.7). The effects of operation conditions (gas velocities and equivalence ratios) on the combustion performance were also tested. We found that 50.0% of the methane was converted to H2 and CO at an equivalence ratio of 1.7 and an inlet gas velocity of 0.15 m/s with burner energy conversion efficiency based on lower heating values.

Detection of carbon monoxide (CO) in sooting hydrocarbon flames using femtosecond two-photon laser-induced fluorescence (fs-TPLIF)

Ultrashort-pulse, femtosecond (fs)-duration, two-photon laser-induced fluorescence (fs-TPLIF) measurements of carbon monoxide (CO) are reported in rich, sooting hydrocarbon flames. CO-TPLIF detection using conventional nanosecond or picosecond lasers are often plagued by photochemical interferences, specifically under fuel-rich flames conditions. In the current study, we investigate the commonly used CO two-photon excitation scheme of the B1Σ+ ← X1Σ+ electronic transition, using approximately 100-fs-duration excitation pulses. Fluorescence emission was observed in the Angstrom band originating from directly populated B1Σ+ upper state, as well as, in the third positive band from collisionally populated b3Σ+ upper state. The current work was focused on the Angstrom band emission. Interference from nascent C2 emissions originating from hot soot particles in the flame could be reduced to a negligible level using a narrower detection gate width. In contrast, avoiding interferences from laser-generated C2 Swan-band emissions required specific narrowband spectral filtering in sooting flame conditions. The observed less than quadratic laser pulse energy dependence of the TPLIF signal suggests the presence of strong three-photon ionization and stimulated emission processes. In a range of CH4/air and C2H4/air premixed flames investigated, the measured CO fluorescence signals agree well with the calculated equilibrium CO number densities. Reduced-interference CO-TPLIF imaging in premixed C2H4/O2/N2 jet flames is also reported.

A laminar flame study on di-n-butyl ether as a potential biofuel candidate

The combustion chemistry of di-n‑butyl ether (DBE, C8H18O), conceivable as a potential biofuel candidate, was studied here for the first time under flame conditions with examining a detailed species dataset. Two flames, i.e., a fuel-rich (ϕ = 1.5) and a fuel-lean (ϕ = 0.8) one, were analyzed at the same pressure (4 kPa), argon dilution (50%), and cold-gas velocity (60 cm s−1) by molecular-beam mass spectrometry (MBMS) using electron ionization (EI). About 50 species including reactants, diluent, combustion-related intermediates, and products, were detected with a special interest in fuel-specific intermediates, harmful species as well as important soot precursors. n-Butanal was detected in high concentration and identified as an important fuel-specific intermediate in the DBE flames, while cyclic soot precursors (e.g., 1,3-cyclopentadiene, benzene, toluene) were detected with low mole fractions (∼10 ppm). The present experimental data were compared to simulations by the kinetic model of Cai et al. [Combust. Flame 161 (2014)] and the very recent model of Thion et al. [Combust. Flame 185 (2017)], with the dual purpose to support the data interpretation and to examine the performance of these models also in comparison to each other. Note that species in the range of C5 to C7 (known as important soot precursors) that are not yet addressed in either of the models have been well detected and are presented in the present work.For further insight into the combustion chemistry of DBE, its fuel-rich flame was also compared to those of successively smaller ethers, namely dimethyl ether (DME) and diethyl ether (DEE). This comparison showed a strong effect of the aliphatic side chains of ethers on the formation of intermediate species. Not unexpectedly, increasing the length of side chains enhances the formation tendency of larger unsaturated hydrocarbons. However, the contribution of highly toxic carbonyl compounds, namely formaldehyde and acetaldehyde, was found to be much lower upon the use of DBE as a fuel than for DME and DEE, respectively. Similarly, DBE was also compared to its half-structurally similar fuels, i.e., n-butane and n-butanol. This comparison generally indicates that the distribution of hydrocarbon species (except for C4H8) in DBE flames is quite similar to those in n-butane and n-butanol flames, whereas the formation of oxygenated species is very different for the three fuels.

Kinetic and thermodynamic measurements of the reactions of the positive ions, Mn+ and MnOH+, formed by adding manganese to fuel-rich flames of either H2 + O2 or C2H2 + O2

Mass spectrometric sampling of flat, fuel-rich flames of H2 or C2H2, burning at 1 atm. and at 1800–2650 K, has revealed that the ions MnOH+ and Mn+ are present in such flames, when seeded with amounts of manganese less than 1 p.p.m. Free electrons were the only negatively charged species. The concentrations of these two positive ions were shown to be linked by the reaction MnOH+ + H = Mn+ + H2O being close to equilibrium downstream of the reaction zone of a flame, provided it was hotter than 2100 K. The equilibrium constant of this reaction was measured in flames differing in temperature; the values showed that the reaction's forward step is exothermic by 233 ± 20 kJ mol−1. This magnitude indicates that the ionisation energy of MnOH is 763 ± 35 kJ mol−1. In addition, the rate constant of the forward step was measured and found to have an activation energy of 112 ± 25 kJ mol−1.In a simple flame of H2 + O2 + N2, ions were produced from the major species containing manganese, i.e. MnO and atomic Mn, by chemi-ionisation in: MnO + H → MnOH+ + e− and Mn + OH → MnOH+ + e−. Free atoms of Mn were roughly three times as abundant as molecules of MnO. The kinetics of these chemi-ionisation reactions were measured; the reactions resulted in elevated levels of ionisation because of a flame having concentrations of the free radicals H and OH in excess of those for equilibrium.In flames of C2H2, their significant natural ionisation was transferred to the metallic additive by: H3O+ + MnO → MnOH+ + H2O and also H3O+ + Mn → Mn+•H2O + H, followed by Mn+•H2O → Mn+ + H2O. These processes led to the level of ionisation early in a hydrocarbon flame being well above that for equilibrium, which was approached by ions recombining in MnOH+ + e− → MnO + H or Mn + OH, as well as in the three-body reaction: Mn+ + e− + M → Mn + M. Recombination coefficients were measured for these processes, as well as for MnOH+ + Cl− → Mn + OH + Cl and Mn+ + Cl− + M → Mn + Cl + M.

Role of hydroxyl production and heat release in the two-zone fuel-rich adiabatic dimethyl ether/air flames at atmospheric pressure

Fuel-rich laminar adiabatic flames of premixed dimethyl ether/air mixtures at a high initial temperature and atmospheric pressure have been studied by numerical simulation and sensitivity analysis. These flames, having two heat release zones, are of great interest as an unusual and little-studied subject. We have investigated the chemical processes occurring in the two zones and analysed the mechanism of heat release in the flame. It has been found that the key reactions that have a significant influence on the flame speed are those involving dimethyl ether and the products of its incomplete oxidation. Calculation of the heat release rate confirms the presence of two heat release zones in the flame. A comparison of the reactions making a major contribution to the heat release with those significantly affecting the flame speed indicates that the main factor determining the flame speed is the formation of hydroxyls, rather than heat release. Analysis of the flame speed sensitivity shows that in the case of a two-zone structure of the flame, its speed is mainly determined by the reactions taking place in the low-temperature zone. That is, the cool zone with a higher temperature gradient is the leading one.

Assessment of uncertainties of laminar flame speed of premixed flames as determined using a Bunsen burner at varying pressures

Laminar methane/air premixed flames at different pressures in a newly developed high-pressure Bunsen flame rig are studied using detailed numerical simulations and laser diagnostics. In the numerical simulations, one-dimensional and two-dimensional axisymmetric configurations were considered employing detailed transport properties and chemical kinetic mechanisms. In the measurements, OH PLIF was employed. The aims are to improved the understanding of the structures of the flames at varying pressures, to measure the laminar flame speed at different pressures, and to quantify the accuracy of the Bunsen flame method for measurement of laminar flame speed at different pressures. The stoichiometric and fuel-rich flames were found to exhibit a two-reaction-zone structure: an inner premixed flame in which the fuel was converted to CO and H2, and an outer diffusion flame in which CO and H2 were oxidized further to form combustion products. With increasing pressure, the inner premixed flame becomes thinner and the flame as a whole has the tendency to become unstable. Using the numerical and the experimental data, the methods of flame-cone-angle and flame-area were used to extract the laminar flame speed for different equivalence ratios and pressures. The flame-cone-angle method showed slightly better accuracy than the flame-area method did. The accuracy of both methods became lower under high pressure conditions. The inlet velocity of the burner was shown to affect the accuracy of the extracted laminar flame speed. For a stoichiometric atmospheric flame it was found that the most suitable inlet velocity for the fuel/air mixture was about 6 times the laminar flame speed, yielding a flame length about 7 times the radius of the burner. With appropriate flame length, the mid-height of the flame showed a rather low flame stretch rate, the laminar flame speed being in close agreement with the unstretched laminar flame speed, the error being less than 6% for the flames that were studied.

Biogas-fueled flame fuel cell for micro-combined heat and power system

A biogas-fueled flame fuel cell (FFC) unit is proposed for the micro-combined heat and power (CHP) system using indigenous energy source. With fuel-rich flame as both heater and reformer, FFC is advantageous for quick start-up, no sealing, and simple thermal management. In this study, a porous media burner with non-catalytic fuel-rich combustion was utilized to provide both stable high temperature environment and reformed syngas for the solid oxide fuel cell (SOFC). The porous media burner and a micro-tubular SOFC were integrated directly in the biogas FFC reactor. The performance of fuel-rich combustion and electrochemical characteristic was studied for various equivalence ratios from 1.2 to 1.4. Experimental results showed that the reforming efficiency reached 42.3% with the porous media burner, using model biogas of 60% CH4 and 40% CO2 as fuel. The maximum power for a single tubular fuel cell reached 1.4W when fed with model biogas at an equivalence ratio of 1.4. Furthermore, performance degradation caused by carbon deposition at the anode was investigated.

Morphology and growth of carbon nanotubes catalytically synthesised by premixed hydrocarbon-rich flames

Synthesis of carbon nanotubes (CNTs) was performed by using a laminar premixed flame burner at open atmospheric condition. The growth of CNTs on the substrate was supported catalytically by a transition metal under high temperature, hydrocarbon-rich environment. Analysis of the CNTs using high resolution electron microscope reveals the structure of synthesised nano-materials in disarray, clustered and tubular form. The graphitic structure of the CNTs are rather similar for all fuel-rich equivalence ratios tested, with an average diameter of ∼11–13 nm. Removal of the amorphous carbon and catalyst in the CNTs was performed via purification treatment using H2O2 and HCl solutions. Detail characterisation indicates the oxidation temperature of purified CNTs ranges between 497 and 529 °C. Deconvolution of the Raman spectra in the range of 900–1800 cm−1 shows the distinct characteristic bands of CNTs with IG/ID ratio of 0.66–0.72 for all the samples tested. In addition, the high level carbon concentration and sp2 CC bond in the CNTs is shown by X-ray photoelectron spectroscopy analysis. The present study demonstrates that CNTs can be effectively synthesised from fuel-rich hydrocarbon flames at ϕ = 1.8–2.0 supported by nickel-based substrate.

OH and CH2O Laser-Induced Fluorescence Measurements for Hydrogen Flames and Methane, n-Butane, and Dimethyl Ether Weak Flames in a Micro Flow Reactor with a Controlled Temperature Profile

Laser-induced fluorescence (LIF) measurements were applied for the first time to a micro flow reactor with a controlled temperature profile to investigate general combustion and ignition characteristics of hydrogen (H2/O2/N2 mixture at O2/N2 = 1:9), methane, n-butane, and dimethyl ether (DME) (fuel/air mixtures). For the hydrogen case, overall flame responses of the H2/O2/N2 mixture against inlet flow velocity were investigated on the basis of the OH-LIF measurement. The existence of the three kinds of flame responses, such as normal flames in the high inlet flow velocity, flames with repetitive extinction and ignition (FREI) in the intermediate inlet flow velocity, and weak flames in the low inlet flow velocity were confirmed at ϕ = 0.6, 1.0, and 1.2. Experimental identification of the hydrogen weak flame was established for the first time. However, the OH-LIF signal level from fuel-rich hydrogen weak flames are quite low, and that at ϕ = 3.0 could not be detected. The reason for the low-level OH-LIF sig...

Heat release rate variations in high hydrogen content premixed syngas flames at elevated pressures: Effect of equivalence ratio

Three-dimensional direct numerical simulations with detailed chemistry were performed to investigate the effect of equivalence ratio on spatial variations of the heat release rate and flame markers of hydrogen/carbon monoxide syngas expanding spherical premixed flames under turbulent conditions at elevated pressures. The flame structures and the heat release rate were analysed and compared between fuel-lean, stoichiometric and fuel-rich centrally ignited spherical flames. The equivalence ratio changes the balance among thermo-diffusive effects, Darrieus–Landau instability and turbulence, leading to different flame dynamics and the heat release rate distribution, despite exhibiting similar cellular and wrinkling flames. The Darrieus–Landau instability is relatively insensitive to the equivalence ratio while the thermo-diffusive process is strongly affected by the equivalence ratio. As the thermo-diffusive effect increases as the equivalence ratio decreases, the fuel-lean flame is more unstable than the fuel-rich flame with the stoichiometric flame in between, under the joint effects of the thermo-diffusive instability and the Darrieus–Landau instability. The local heat release rate and curvature display a positive correlation for the lean flame, no correlation for the stoichiometric flame, and negative correlation for the rich flame. Furthermore, for the fuel-lean flame, the low and high heat release rate values are found in the negative and positive curvature zones, respectively, while for the fuel-rich flame, the opposite trends are found. It is found that heat release rate markers based on species concentrations vary strongly with changing equivalence ratio. The results suggest that the HCO, HO2 concentrations and product of OH and CH2O concentrations show good correlation with the local heat release rate for H2/CO premixed syngas-air stoichiometric flame under turbulent conditions at elevated pressures.

Mathematical Modeling of a Porous Media Burner Based Methane Flame Fuel Cell

A detailed two-dimensional axisymmetric computational model of a flame fuel cell (FFC) unit was developed and presented. The FFC unit is based on the integration of a fuel-rich methane flame in a porous media burner and a micro-tubular solid oxide fuel cell (SOFC). The model considered the coupling effects of the chemical reactions and electrochemical reactions and the heat-transport, mass-transport and charge -transport processes in the FFC. The simulated temperature distribution and electrochemical characteristics showed good agreement with experimental data. The coupling mechanism of the fuel-rich flame and the SOFC anode were clarified. The Ni catalyst in the anode and the electrochemical reactions promoted the conversion of CH4 in porous media fuel-rich combustion.

Di-tertiary-butyl Peroxide Decomposition and Combustion with Air: Reaction Mechanism, Ignition, Flame Structures, and Laminar Flame Velocities

This study investigates the ignition/combustion of di-tertiary-butyl peroxide (DTBP) over a wide range of fuel/air ratios using numerical flame calculations to gain insight into species profiles and ignition/combustion characteristics of DTBP/air mixtures. The results are flame zone and reaction zone structures as well as the corresponding laminar flame speeds and propagation speeds of reaction zones. Several detailed reaction mechanisms are evaluated in the flame simulations, and sensitivity of the computed results to important reaction steps was evaluated. The ignition of DTBP in the absence of oxygen belongs in the category of thermal runaway because there is no chain branching associated with the early stages of the reaction; i.e., decomposition steps lead to acetone and ethane. In the presence of oxygen, the development of a flame is more complex and exhibits a two-stage ignition, where the first stage, at lower temperatures, involves the thermal decomposition of DTBP to acetone, which further oxidizes to the final combustion products. This second high-temperature ignition process involves chain-branching reactions for the combustion of acetone and ethane and the usual chain-branching reactions of the H2/O2 system. The calculations reproduce this two-stage ignition behavior for fuel-rich flames, and the calculated flame structures reflect the interaction of decomposition of DTBP to acetone and ethane and the subsequent combustion. DTBP has a relatively weak RO–OR bond and readily undergoes dissociation at higher temperatures to two RO• radicals, and it is widely used as a radical source, reaction initiator. Five reaction mechanisms (M1–M5) were employed in this study for analysis of the reaction system, which differ mainly in the reaction rates for the initial DTBE decomposition reaction. When using relative high reaction rates for the initial decomposition reactions (M1 and M2), the flame velocities of DTBP/air mixtures exhibit two maxima over the entire range of the mixture fraction. The first maximum is located at stoichiometric conditions, while the second prevails under fuel-rich conditions. Decelerating the primary decomposition reactions (M3–M5) in favor of the combustion reactions of the DTBP decomposition products shows the second maximum disappearing as result of the relative low propagation speed of the reaction zone. The first maximum is only slightly altered by varying the reaction rates, because the decomposition step of DTBP and the combustion of the primary decomposition products merge at stoichiometric conditions and temperatures are high enough to result in high decomposition rates.

On the effect of ionic wind on structure and temperature of laminar premixed flames influenced by electric fields

The general influence of electric fields on flames has been known for many years, the underlying mechanisms, however, are not finally identified yet. The changes observable in the flame structure and the pollutant emission when a flame is exposed to an electric field are mainly attributed to the ionic wind. Yet, especially the resulting flame temperature under the influence of electric fields is still an open research topic. Thus, in the present study the temperature distributions in a laminar premixed Bunsen type flame were measured for different air–fuel-mixtures using point-wise vibrational coherent-anti-Stokes Raman spectroscopy (vibrational-CARS) when a static electric field is activated. Additionally, CH2O- and OH-PLIF (planar laser-induced fluorescence) and particle image velocimetry (PIV) were applied to identify the best-suited locations for the temperature measurements and to visualize changes in the flame structure induced by the electric field for clarifying the underlying mechanisms.The results show that the electric field leads to increased fresh gas and maximum flame temperatures and that this effect is most distinct for rich premixed flames. Both PIV/PLIF and CARS experiments confirm that the flame is constricted and pushed towards the burner due to the ionic wind, which explains the increased temperature of the fresh gas and of the exhaust gas. The increased burner and fresh gas temperatures due to the ionic wind were verified by additional thermocouple measurements. The ionic wind is maximal for the fuel-rich flame, which is explained by the lowest electric resistance due to high charge carrier concentration, and larger flame area, which reduces the gap between the flame and the positively charged electrode. Furthermore, a clear broadening of the hot exhaust gas region was measured, which increases the burn out of UHC and CO reported in the literature. Furthermore, this effect increases the residence time that could also contribute to changed pollutant emission for flames under the influence of electric fields. It can be concluded that the change of the flame temperature is mainly the result of the modified velocity field induced by the ionic wind. Consequently, the ionic wind is the dominant mechanism for many effects observed in flames when applying weak electric fields.

Spatiotemporal Distribution of Soot Temperature for Flames Using Optical Pyrometry Under Unsteady Inlet Airflow Conditions

Two pyrometric tools for measuring soot temperature response in fuel-rich flames under unsteady inlet airflow conditions are developed. High-speed pyrometry using a high-speed color camera is used in producing soot temperature distributions, with its results compared with those of global soot temperature response measured using a multiwavelength pyrometer. For the former, the pixel red, green, and blue (RGB) values pertaining to respective bandwidths of red, green, and blue filters are used to calculate temperature and for the latter, the emission from whole flame at 660 nm, 730 nm, and 800 nm is used to measure temperature. The combustor, running on jet-A fuel, achieves unsteady inlet airflow using a siren running at frequencies of 150 and 250 Hz and with modulation levels (root mean square (RMS)) 20–50% of mean velocity. Spatiotemporal response of flame temperature measured by the high-speed camera is presented by phase-averaged with average subtracted images and by fast Fourier transform (FFT) at the modulation frequencies of inlet velocity. Simultaneous measurement of combustor inlet air velocity and flame soot temperature using the multiwavelength pyrometer is used in calculating the flame transfer function (FTF) of flame temperature response to unsteady inlet airflow. The results of global temperature and temperature fluctuation from the three-color pyrometer show qualitative agreement with the local temperature response measured by the high-speed camera. Over the range of operating conditions employed, the overall flame temperature fluctuation increases linearly with respect to the inlet velocity fluctuation. The two-dimensional map of flame temperature under unsteady combustion determined using a high-speed digital color camera shows that the local temperature fluctuation during unsteady combustion occurs over relatively small region of flame and its level is greater (∼10% to 20%) than that of overall temperature fluctuation (∼1%).

Influence of the biofuel isomers diethyl ether and n-butanol on flame structure and pollutant formation in premixed n-butane flames

Diethyl ether (DEE) and its isomer n-butanol are both considered as promising fuel additives or neat biofuels. While some effects of their addition to hydrocarbon fuels under engine conditions have been reported, fundamental studies that aim at understanding the joint reaction pathways of such fuel mixtures remain quite scarce. Here, we have chosen n-butane as a well-studied hydrocarbon base fuel, and we have added these oxygenated isomers individually under identical conditions in premixed low-pressure flames. Different combustion behavior of the respective alkane-biofuel mixtures must then be related to the fuel structure. Analyses were performed in five fuel-rich flames including flames of n-butane, DEE, and n-butanol as well as two flames of n-butane doped with 50% DEE or 50% n-butanol. In this series, the carbon-to-oxygen ratio, argon dilution, pressure, and gas velocity were kept constant. More than 40 species in the range of C0–C8 were identified and quantified in each flame by electron ionization (EI) molecular-beam mass spectrometry (MBMS) coupled with gas chromatography (GC). The experiments were partially complemented by tunable synchrotron vacuum ultraviolet (SVUV) photoionization (PI)-MBMS. To assist in the interpretation of the data, a kinetic model was established by combining different sub-mechanisms for these fuels available in the literature. As expected, the formation of toxic carbonyls, such as formaldehyde and acetaldehyde, increased significantly upon addition of both oxygenated fuels to n-butane. Blending n-butane with DEE noticeably reduces the formation of soot precursors, because primary reactions of DEE mainly release C1–C2 hydrocarbon species to the system. n-Butanol addition, however, shows no significant reduction effects or even higher formation of soot precursors. These trends were observed both in the experiments and model predictions, and the higher ability of n-butanol to form soot precursors compared to DEE indeed results mainly from the differences in the fuel structure.

On the effects of hydrogen addition in premixed formaldehyde flames

The influences of hydrogen addition on laminar premixed fuel-rich formaldehyde flames at the atmospheric pressure are studied by the detailed kinetic analysis. The chemical effects of hydrogen addition on flame structures, mole fractions of major species and free radicals are distinguished clearly from the dilution and thermal effects. The results show that the H2 chemical effects can promote the oxidation of formaldehyde and make the mole fraction profile of formaldehyde move toward the upstream side. The increase of H radical concentration is mainly due to the chemical effects of hydrogen addition. The reaction OH + H2 = H + H2O plays a major role in the productions of H and OH radicals. The productions of O, HCO and HO2 radicals are enhanced by the H2 chemical effects. However, the hydrogen dilution and thermal effects on decreasing these radicals' concentrations are more dominant. The mole fractions of CO and CO2 decrease with H2 addition. The H2 chemical effects facilitate the formation of CO, but suppress the production of CO2.

Non-intrusive detection of combustion intermediates by photoionization via Rydberg states and microwave backscattering

A non-intrusive, in-situ detection technique for combustion intermediates in flame environments yields highly resolved spectra that are largely insensitive to molecular vibrations and thus temperature. The technique is based on laser photoionization of target compounds via Rydberg states, followed by detection of the laser-induced plasma with microwave radiation. The feasibility of this approach is tested on methyl radicals that are detected in methane, propane, and hexane fuel-rich flames. The methyl radicals are prepared in the 3s Rydberg state using photoexcitation at 216.4 nm. Using tunable VIS/IR radiation, the molecules are promoted from 3s to the 3pxy and 3pz Rydberg states. Photoionization out of the 3p states with 355 nm photons results in a small-volume plasma that is detected by microwave backscattering. The previously unobserved 3pxy state is found to have a binding energy of 2.57 eV.