Effects of temperature and composition on the laminar burning velocity of CH4 + H2 + O2 + N2 flames

This work summarises available measurements of laminar burning velocities in CH₄ + H₂ + O₂ + N₂ flames at atmospheric pressure performed using a heat flux method. Hydrogen content in the fuel was varied from 0% to 40%, amount of oxygen in the oxidiser was varied from 20.9% down to 16%, and initial temperature of the mixtures was varied from 298 to 418 K. These mixtures could be formed when enrichment by hydrogen is combined with flue gas recirculation. An empirical correlation for the laminar burning velocity covering a complete range of these measurements is derived and compared with experiments and other correlations from the literature.     

Laminar burning velocities of three C3H6O isomers at atmospheric pressure

Laminar flames of three C₃H₆O isomers (propylene oxide, propionaldehyde and acetone), representative of cyclic ether, aldehyde and ketone species important as intermediates in oxygenated fuel combustion, have been studied experimentally and computationally. Most of these flames exhibited a non-linear dependency of flame speed upon stretch rate and two complementary independent techniques were adopted to provide the most reliable burning velocity data. Significant differences in burning velocity were noted for the three isomers: propylene oxide + air mixtures burned fastest, then propionaldehyde + air, with acetone + air flames being the slowest; the latter also required stronger ignition sources. Numerical modelling of these flames was based on the Konnov mechanism, enhanced with reactions specific to these oxygenated fuels. The chemical kinetics mechanism predicted flame velocities in qualitative rather than quantitative agreement with the measurements. Sensitivity analysis suggested that the calculated flame speeds had only a weak dependency upon parent fuel-specific reactions rates; however, consideration of possible break-up routes of the primary fuels has allowed identification of intermediate compounds, the chemistry of which requires a better definition.     

The effect of temperature on the adiabatic laminar burning velocities of CH4−air and H2−air flames

The effect of temperature on the adiabatic laminar burning velocities of CH₄ + air and H₂ + air flames was analyzed. Available measurements were interpreted using correlation S_L = S_L0 (T/T₀)^α. Particular attention was paid to the variation of the power exponent α with equivalence ratio at fixed (atmospheric) pressure. Experimental data and proposed empirical expressions for α as a function of equivalence ratio were summarized. They were compared with predictions of detailed kinetic models in methane + air and hydrogen + air flames. Unexpected non-monotonic behavior of α was found in rich methane + air flames. Modeling results are further examined using sensitivity analysis to elucidate the reason of particular dependences of the power exponent α on equivalence ratio.     

Extension of a compressible lattice model to CO2 + cosolvent + polymer systems

We have recently proposed a compressible lattice model for CO₂ + polymer systems in which CO₂ forms complexes with one or more functional groups in the polymer. Furthermore, we have shown that this model is able to simultaneously correlate phase equilibria, sorption behavior, and glass transition temperatures in such systems. In the present work, we extend the model to ternary CO₂ + cosolvent + polymer systems and demonstrate that cloud point behavior in CO₂ + dimethyl ether + poly (?-caprolactone), CO₂ + dimethyl ether + poly (isopropyl acrylate), and CO₂ + dimethyl ether + poly (isodecyl acrylate) systems can be predicted using parameters obtained from binary data. Our results also suggest that dimethyl ether may form weak complexes with poly (?-caprolactone), poly (isopropyl acrylate), and poly (isodecyl acrylate).     

Laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixtures

Measurements of the adiabatic laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixtures are reported. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss from the flame to the burner is zero. Initial temperatures of the gas mixtures with air were 298 and 338 K. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be better than ±1 cm/s. These new measurements were compared with the literature data when available. Experimental results in lean ethanol + air mixtures are systematically higher than previous measurements under similar conditions. Good agreement for n-heptane + air flames and for iso-octane + air flames was found with the experiments performed in counter-flow twin flames with linear extrapolation to zero stretch.     

Measurement of phase equilibria of the systems CO2 + styrene and CO2 + vinyl acetate using different experimental methods

The experimental determination of high-pressure phase equilibria is often the only suitable method to obtain reliable data because high-pressure phase behavior is complex and difficult to predict. This contribution gives a brief classification of applied experimental methods. A new high-pressure apparatus is described, which can be used for phase-equilibrium measurements with different experimental methods, namely the analytical-isothermal method, the synthetic-isothermal method as well as the non-visual- and the visual-synthetic method. The different techniques have been tested for the measurement of the phase behavior of systems containing CO₂ + styrene and CO₂ + vinyl acetate. The measured data were compared with data from literature and discussed in terms of accuracy, advantages and drawbacks of the applied methods.     

Oxygen and chlorine evolution on RuO2 + TiO2 + CeO2 + Nb2O5 mixed oxide electrodes

A systematic investigation was conducted on the mechanism and electrocatalytic properties of O₂ and Cl₂ evolution on mixed oxide electrodes of nominal composition: Ti/[Ru_(0.3)Ti_(0.6)Ce_(0.1−x)]O₂[Nb₂O₅]_(x) (0 ≤ x ≤ 0.1). For the oxygen evolution, a 30 mV Tafel slope is obtained in the presence of CeO₂, while in its absence a 40 mV coefficient is observed. The intrinsic electrocatalytic activity is mainly due to electronic factors, as result of the synergism between Ru and Ce oxides. For chlorine evolution, the Tafel slope (30 mV) is independent on oxide composition. The best global electrocatalytic activity for ClER was observed in the absence of Nb₂O₅ additive. Variation of the voltammetric charge throughout the experiments confirms high CeO₂ content compositions are fragile, due mainly to the porosity caused by CeO₂ presence. On the other hand, Nb₂O₅ addition decreases considerably this instability.     

The effects of enrichment by H2 on propagation speeds in adiabatic flat and cellular premixed flames of CH4 + O2 + CO2

Experimental studies of adiabatic flat and cellular premixed flames of (CH₄ + H₂) + (O₂ + CO₂) are presented. The hydrogen content in the fuel was varied from 0% to 35% and the oxygen content in the oxidizer was 31.55%. These mixtures could be formed when oxy-fuel combustion technology is combined with hydrogen enrichment. Non-stretched flames were stabilized at atmospheric pressure on a perforated plate burner. A heat flux method was used to determine propagation speeds under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + carbon dioxide + oxygen mixtures were found in satisfactory agreement with the detailed kinetic modeling employing the Konnov mechanism. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Visual and photographic observations of the flames were performed to quantify their cellular structure. The results obtained in the present work in (CH₄ + H₂) + O₂ + CO₂ mixtures are in good accordance with the previous observations for different fuels, CH₄, C₂H₆ and C₃H₈. The enrichment by hydrogen leads to: the increase of the laminar burning velocities; the increase of the number of cells observed; the decrease of the mean cell diameter. The flame acceleration due to cellularity was not affected by the hydrogen enrichment.     

Phase behavior of the poly(neopentyl methacrylate) + supercritical fluid solvents + neopentyl methacrylate system and CO2 + neopentyl methacrylate mixtures at high pressure

High-pressure phase behaviors are measured for the CO₂ + neopentyl methacrylate (NPMA) system at 40, 60, 80, 100, and 120 °C and pressure up to 160 bar. This system exhibits type-I phase behavior with a continuous mixture-critical curve. The experimental results for the CO₂ + NPMA system are modeled using the Peng-Robinson equation of state. Experimental cloud-point data up to the temperature of 180 °C and the pressure of 2000 bar are presented for ternary mixtures of poly(neopentyl methacrylate) [poly(NPMA)] + supercritical solvents + NPMA systems. Cloud-point pressures of poly(NPMA) + CO₂ + NPMA system are measured in the temperature range of 60-180 °C and to pressures as high as 2000 bar with NPMA concentration of 0.0, 5.2, 19.0, 28.1 and 40.2 wt%. It appears that adding 51.2 wt% NPMA to the poly(NPMA) + CO₂ mixture does significantly change the phase behavior. Cloud-point curves are obtained for the binary mixtures of poly(NPMA) in supercritical propane, propylene, butane, 1-butene, and dimethyl ether (DME). The impact of dimethyl ether concentration on the phase behavior of the poly(NPMA) + CO₂ + x wt% DME system is also measured at temperature of 180 °C and pressure range of 36-2000 bar. This system changes the pressure-temperature (P-T) slope of the phase behavior curves from upper critical solution temperature (UCST) region to lower critical solution temperature (LCST) region as the NPMA concentration increases.     

Fabrication of B4C from Na2B4O7 + Mg + C by SHS method

This paper deals with the formation of boron carbide (B₄C) powders from Na₂B₄O₇ + Mg + C system by self-propagating high-temperature synthesis (SHS) method. B₄C without impurities could be obtained after the acid enrichment and distilled water washing. The reaction mechanism of SHS of B₄C was proposed: the synthesis of B₄C is a process involving the decomposition of Na₂B₄O₇ into the intermediate phase B₂O₃, which reacts with Mg and carbon to form B₄C.     

Open-circuit-potentials of gas/electrode/YSZ boundary versus molten carbonate reference electrode at medium temperatures: II. Potential response of Au, Pt and Ni-cermet electrodes in CH4 + O2 and H2 + O2 gas mixtures

Values of open-circuit-potentials (OCP) have been determined for pairs of electrodes: Au and Pt, Ni-Ce_0.8Sm_0.2O_1.9 cermet and Au, Pt and Sm_0.5Sr_0.5CoO₃ composite at the YSZ electrolyte, in the uniform atmospheres of xCH₄ + yO₂ + (1 − x − y)Ar gas mixtures with variable x and y coefficients, at 600 °C. The determined dependencies of OCP values on the initial gas mixture compositions have been compared with the respective dependencies calculated for equilibrium or quasi-equilibrium compositions of these gas mixtures. The OCP values for the pair of Pt and Au electrodes have been measured also in the xH₂ + yO₂ + (1 − x − y)Ar uniform gas mixtures but no distinct difference of the OCP values has been observed in this atmosphere. For some pairs of electrodes investigated in xCH₄ + yO₂ + (1 − x − y)Ar atmospheres the measured OCP values have shown differences up to ca 0.9-1.0 V. These differences were stable within large range of compositions of this gas mixture. Within this gas composition range one of the electrodes conserves the potential of oxygen electrode determined by oxygen partial pressure in the initial gas mixture and is insensitive to reaction occurring in the gas phase. These results are discussed on the basis of equilibria or some quasi-equilibria, that establish in the C-H-O gas mixture and the solid carbon deposition is considered. For a given pair of dissimilar electrodes, their selective sensibility to the electrochemical process of oxygen electrode has been confirmed. Within large range of gas mixture concentrations, in the Pt-Au electrode pair Au has shown behavior of the oxygen electrode, whereas the OCP values of the Pt electrode are within the range of hydrogen electrode, also at gas compositions corresponding to the solid carbon stability. With this pair the OCP differences of ca. 600 mV have been obtained. Among three electrodes studied the cermet Ni-Ce_0.8Sm_0.2O_1.9 electrode shows the best electrocatalytic properties resulting in the OCP values following exactly the respective equilibrium dependence. In the pair Ni-Ce_0.8Sm_0.2O_1.9 and Au a stable potential difference of ca. 900 mV have been established. Unexpectedly, Pt electrode in the pair with the Sm_0.5Sr_0.5CoO₃ composite electrode plays role of the oxygen electrode quite insensitive to other components of the equilibrated initial gas mixture. This surprising fact seems indicate that in conditions of the experiments performed the electrocatalytic behavior of the electrode depends not only of the material of this electrode but also on the properties of the second electrode in the given pairs of electrodes.     

Oxidation behavior and kinetics of in situ (TiB2 + TiC)/Ti3SiC2 composites in air

The isothermal oxidation behavior of in situ (TiB₂ + TiC)/Ti₃SiC₂ composite ceramics with different TiB₂ content has been investigated at 900-1200 °C in air for exposure times up to 20 h by means of X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The oxidation of (TiB₂ + TiC)/Ti₃SiC₂ composites follows a parabolic rate law. With the increase in TiB₂ content, the oxidation weight gain, thickness of the oxidation scale, and parabolic rate constant decrease dramatically, which suggests that the incorporation of TiB₂ greatly improves the oxidation resistance of the composites. With the increase in oxidation temperature, the enhancement effect becomes more pronounced. Due to the incorporation of TiB₂, the oxidation scale of (TiB₂ + TiC)/Ti₃SiC₂ composites is generally composed of an outer layer of coarse-grained TiO₂ and an inner layer of amorphous boron silicate and fine-grained TiO₂. Only the dense inner layer formed on the surface acts as a diffusion barrier, retarding the inward diffusion of O, and consequently contributing to the improved oxidation resistance of the (TiB₂ + TiC)/Ti₃SiC₂ composites.     

Preliminary study on suspension plasma sprayed ZrO2 + 8 wt.% Y2O3 coatings

ZrO₂ + 8 wt.% Y₂O₃ powder of a mean diameter d_VS = 38 μm was milled to obtain fine particles having mean size of d_VS = 1 μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2 mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60 mm and (ii) torch linear speed varying from 300 to 500 mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9-11 N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95 × 10⁻⁷ to 3.79 × 10⁻⁷ m²/s what corresponds to thermal conductivities of 0.69 W/(mK) and 0.97 W/(mK) respectively.     

Dependencies of the open-circuit voltage of Au|YSZ|Pt single-chamber fuel cell on the composition of the uniform CH4 + O2 gas mixture with solid carbon deposition

Single-chamber solid oxide fuel cell is a device where two electrodes of different materials contacting a solid oxide ionic conductor, may generate a considerable potential difference and electrical power, when supplied by a common fuel + oxidant gas mixture. The Au|YSZ|Pt system in the CH₄ + O₂ gas mixture is one of the simplest examples of such a cell. In this article the open-circuit voltage (OCV) of this cell, supplied with the gas mixture xO₂ + aCH₄ + (1 − x − a)Ar (where a = 0.01, 0.1 or 0.5), is investigated. On the basis of the obtained results, as well as those for the xCH₄ + (1 − x)(0.2O₂ + 0.8Ar) (0 ≤ x ≤ 1) gas mixture, reported in our previous work [Electrochim. Acta, 50 (2005) 2771], we postulate that the OCV of the above system arises as a result of electrode modification resulting from solid carbon deposition in the cell. After oxidation of the carbon deposit, the system, once treated by the gas mixture enabling the formation of the carbon phase, shows more and more tendency to generate the OCV. The open-circuit potential of the Au electrode depends only on the O₂ concentration in the initial gas mixture, whereas in the case of the Pt electrode it becomes dependent on chemical equilibria determining the O₂ content in the converted gas mixture. Our results reveal that the OCV achieves a reproducible limiting value of ∼650 mV, which is lower by ∼400 mV than the calculated equilibrium value.     

Synthesis and characterization of A2 + B3-type hyperbranched aromatic polyesters with phenolic end groups

Hyperbranched (hb) aromatic polyesters with phenolic end groups were synthesized according to the A₂ + B₃ approach both, by solution polymerization and by melt (bulk) polymerization with different monomer ratios (A₂:B₃). The hb polyesters produced from solution polymerization exhibited higher yields, molar masses and glass transition temperature (T_g) compared to the products prepared in the melt. The resulting hb aromatic polyesters from the A₂ + B₃ approach were also compared with their properties to hb aromatic polymers produced from the well known AB₂ monomer 3,5-bis(trimethylsiloxy)benzoyl chloride. Both types of hb aromatic polyesters possess high T_g, high thermal stability and good solubility in common organic solvents. A typical melt viscosity behavior with shear thinning effect was also observed for both. Thus similar polymer properties compared to melt-condensed products were obtained with the A₂ + B₃ approach by solution polycondensation possessing the advantage of easy monomer availability and much milder polymerization conditions (at room temperature in solution) compared to the AB₂ approach.     

Electrochemical characterization of YBaCo3ZnO7 + Gd0.2Ce0.8O1.9 composite cathodes for intermediate temperature solid oxide fuel cells

YBaCo₃ZnO₇ + Gd_0.2Ce_0.8O_1.9 (GDC) composites with various GDC contents (0-70 wt.%) have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (SOFC). The effect of GDC incorporation on the microstructure, electrochemical properties, and thermal expansion behavior of the YBaCo₃ZnO₇ + GDC composites has been studied. The composite cathodes consist of smaller particles with larger surface area compared to the pure YBaCo₃ZnO₇ cathode, which is beneficial for providing extended triple-phase boundary (TPB) where the oxygen reduction reaction (ORR) occurs. Among the various compositions investigated, the YBaCo₃ZnO₇ + GDC (50:50 wt.%) composite is found to be optimum with the lowest polarization resistance (0.28 Ω cm² at 600 °C) compared to that of pure YBaCo₃ZnO₇ (0.62 Ω cm² at 600 °C). Anode-supported single cell SOFC fabricated with the YBaCo₃ZnO₇ + GDC (50:50 wt.%) composite cathode also exhibits excellent performance with a maximum power density of 743 mW/cm² at 750 °C. Additionally, the YBaCo₃ZnO₇ + GDC (50:50 wt.%) composite shows a low thermal expansion coefficient (TEC) of 10.7 × 10⁻⁶ °C⁻¹, which provides good compatibility with those of standard SOFC electrolytes.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.     

Effects of NO2 ions on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 mol L⁻¹ NaHCO₃ + 0.02 mol L⁻¹ NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with NO₂⁻ concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [NO₂⁻] = 0 up to 400 ± 50 mV/SCE for [NO₂⁻] = 0.1 mol L⁻¹. During anodic polarisation at potentials comprised between E_b([NO₂⁻] = 0) and E_b([NO₂⁻] ≠ 0), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via scanning vibrating electrode technique (SVET). Addition of a NaNO₂ solution after the beginning of the polarisation led to a rapid repassivation of pre-existing well-grown pits. In situ micro-Raman spectroscopy was then used to identify the corrosion products forming inside the pits. The first species to be detected in the presence of NO₂⁻ were mainly dissolved Fe(III) species, more likely [Fe^III(H₂O)₆]³⁺ complexes. Iron(II) carbonate FeCO₃, siderite, and carbonated green rust GR(CO₃²⁻) were also detected in the active pits, as in the absence of nitrite. But they were accompanied by maghemite γ-Fe₂O₃, a phase structurally similar to the passive film, that forms from the Fe(III) complexes. The Raman analyses then correlate with the SVET observations and confirm that the main effect of nitrite ions is to oxidize iron(II) into iron(III). The passive film would then form from the Fe(III) species still bound to the steel surface.     

Flame length scaling in a non-premixed turbulent diluted hydrogen jet with coaxial air

The effect of fuel composition on flame length was studied in a non-premixed turbulent diluted hydrogen jet with coaxial air. Because coaxial air entrained in a fuel stream enhances the mixing rate of fuel and air, it substantially reduces flame length. The observed flame length was expressed as a function of the ratio of coaxial air to fuel jet velocity and compared with a theoretical prediction based on the velocity ratio. Four cases of fuel mixed by volume were determined: 100% H₂, 80% H₂/20% N₂, 80% H₂/20% CO₂, and 80% H₂/20% CH₄. In addition, fuel jet air velocity and coaxial air velocity were varied in an attached flame region as u_F = 86-309 m/s and u_A = 7-14 m/s. In this study, we derived a scaling correlation for predicting the flame length in a simple jet with coaxial air using the effective jet diameter in a near-field concept. The experimental results showed that the visible flame length was in good relation to the theoretical prediction. The scaling analysis is also valid for diluted hydrogen jet flames with varied fuel composition, which affects flame length by varying the density of the fuel.     

Ethanol steam reforming reactions over Al2O3 · SiO2-supported Ni-La catalysts

Nickel-based catalysts supported on Al₂O₃ · SiO₂ were prepared with modification of the second metal involving La, Co, Cu, Zr or Y, of which the catalytic behaviors were assessed in the ethanol steam reforming reaction. Activity test indicated that addition of La resulted in higher selectivity of hydrogen and lower selectivity of carbon monoxide, compared with Co-doped nickel catalyst. Influences of lanthanum amounts on catalytic performance were studied over 30NixLa/Al₂O₃ · SiO₂ (x = 5, 10, 15), and characterizations by XRD, TPR and XPS indicated that low amount of lanthanum additives (5%) was superior to inhibit the crystal growth of nickel as well as beneficial to the reduction of nickel oxide. Finally 100 h test for the optimal catalyst 30Ni5La/Al₂O₃ · SiO₂ indicated its good long-term stability to provide high hydrogen selectivity and low carbon monoxide formation, as well as good resistance to coke deposition at low temperature.