Spherical expanding flames in H2–N2O–Ar mixtures: flame speed measurements and kinetic modeling

Although ignition of hydrogen–nitrous oxide mixtures is a serious issue for nuclear waste storage and semi-conductor manufacturing, available flame speed data have not been recently updated and thermodiffusive stability is not known. In order to palliate this, the flame speed of a hydrogen–nitrous oxide mixture diluted in Ar (60% mol) was measured in a spherical bomb as a function of equivalence ratio. The initial pressure and temperature were held constant around ambient conditions. It is shown that the unstretched flame speed of the hydrogen–nitrous oxide mixture is relatively low for a hydrogen-based mixture, with a maximum of 56 cm/s for the stoichiometric condition. Further, hydrogen–nitrous oxide–argon flames appear unstable with respect to thermodiffusive effects at an equivalence ratio of 1. The downward flammability limit of hydrogen–nitrous oxide–argon was observed for hydrogen content of 8 mol%. The modeling of these experimental data has been performed with three recently developed models. All kinetic schemes give satisfactory predictions of the experimentally observed data. Sensitivity and reaction pathway analysis have demonstrated that the dynamic of the system is dominated by the reaction N₂O + H = N₂ + OH which governs the rate of energy release.     

Hydrogen permeation through a Pd-based membrane and RWGS conversion in H2/CO2, H2/N2/CO2 and H2/H2O/CO2 mixtures

This study investigated the effect of gases such as CO₂, N₂, H₂O on hydrogen permeation through a Pd-based membrane −0.012 m² – in a bench-scale reactor. Different mixtures were chosen of H₂/CO₂, H₂/N₂/CO₂ and H₂/H₂O/CO₂ at temperatures of 593–723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H₂ loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H₂ and the H₂/CO₂ mixtures. The best operating conditions to separate hydrogen from the mixtures were identified.     

Determination of laminar burning velocities for lean low calorific H2/N2 and H2/CO/N2 gas mixtures

Combustion of lean and ultra-lean synthetic H₂/CO mixtures that are highly diluted in inert gases is of great importance in several fields of technology, particularly in the field of post combustion for combined heat and power (CHP) systems based on fuel cell technology. In this case H₂/CO mixtures occur via hydrocarbon reforming and their complete conversion requires efficient, compact and low emission combustion systems. In order to design such systems, knowledge of global flame properties like the laminar burning velocity, is essential. Using the heat flux burner method, laminar burning velocities were experimentally determined for highly N₂ diluted synthetic H₂ and H₂/CO mixtures with low calorific value, burning with air, at ambient temperature and atmospheric pressure. Furthermore, numerical 1-D simulations were performed, using a series of different chemical reaction mechanisms. These numerical predictions are analysed and compared with the experimental data.     

Conditional reaction rate in a lifted turbulent H2/N2 flame using direct numerical simulation

In this study, reaction rate sub-models are investigated in the framework of conditional moment closure (CMC) using the direct numerical simulation (DNS) database of a lifted turbulent H₂/N₂ flame. The DNS code solves the fully compressible Navier–Stokes equation system. A 9 species and 19-step mechanism for hydrogen combustion is adopted. The comparison of the DNS results and the measurements shows that, in spite of the under predicted lift-off height, the predictions of the conditional means are satisfactory. Two improved models for the conditionally averaged reaction rate are investigated a-priori. The doubly conditioned reaction rate accounts for the fluctuations with two conditioning variables while the second-order closure is based on the Taylor expansion. It is shown that both of the models give promising results.     

Combustion characteristics of H2/N2 and H2/CO syngas nonpremixed flames

Turbulent nonpremixed H₂/N₂ and H₂/CO syngas flames were simulated using 3D large eddy simulations coupled with a laminar flamelet combustion model. Four different syngas fuel mixtures varying from H₂-rich to CO-rich including N₂ have been modelled. The computations solved the Large Eddy Simulation governing equations on a structured non-uniform Cartesian grid using the finite volume method, where the Smagorinsky eddy viscosity model with the localised dynamic procedure is used to model the sub-grid scale turbulence. Non-premixed combustion has been incorporated using the steady laminar flamelet model. Both instantaneous and time-averaged quantities are analysed and data were also compared to experimental data for one of the four H₂-rich flames. Results show significant differences in both unsteady and steady flame temperature and major combustion products depending on the ratio of H₂/N₂ and H₂/CO in syngas fuel mixture.     

Solar hydrogen generation with H2O/ZnO/MnFe2O4 system

The hydrogen generation reaction in the H₂O/ZnO/MnFe₂O₄ system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H₂. At 1273 K, the mixture of ZnO and MnFe₂O₄ reacted with water to generate H₂ gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn^II (divalent manganese ion) in the A-site of MnFe₂O₄ was substituted with Zn^II (divalent zinc ion) and that chemical formula of the solid product was estimated to be Zn_0.58Mn^II_0.42Mn^III_0.39Fe_1.61O₄ (Mn^III: trivalent manganese ion). Its lattice constant was smaller than that of the MnFe₂O₄ (one of the two starting materials). From the chemical composition, the reaction mechanism of the H₂ generation with this system was discussed. Since the Mn ions in the product solid after the H₂ generation reaction are oxidized to Mn³⁺, which can readily release the O²⁻ ions as O₂ gas around 1300 K, the two-step of H₂ generation and O₂ releasing seem to be cyclic.     

CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: Thermogravimetric analysis

The thermochemical dissociation of CO₂ and H₂O from reactive SnO nanopowders is studied via thermogravimetry analysis. SnO is first produced by solar thermal dissociation of SnO₂ using concentrated solar radiation as the high-temperature energy source. The process targets the production of CO and H₂ in separate reactions using SnO as the oxygen carrier and the syngas can be further processed to various synthetic liquid fuels. The global process thus converts and upgrades H₂O and captured CO₂ feedstock into solar chemical fuels from high-temperature solar heat only, since the intermediate oxide is not consumed but recycled in the overall process. The objective of the study was the kinetic characterization of the H₂O and CO₂ reduction reactions using reactive SnO nanopowders synthesized in a high-temperature solar chemical reactor. SnO conversion up to 88% was measured during H₂O reduction at 973 K and an activation energy of 51 ± 7 kJ/mol was identified in the temperature range of 798-923 K. Regarding CO₂ reduction, a higher temperature was required to reach similar SnO conversion (88% at 1073 K) and the activation energy was found to be 88 ± 7 kJ/mol in the range of 973-1173 K with a CO₂ reaction order of 0.96. The SnO conversion and the reaction rate were improved when increasing the temperature or the reacting gas mole fraction. Using active SnO nanopowders thus allowed for efficient and rapid fuel production kinetics from H₂O and CO₂.     

Detonation cellular structure in NO2/N2O4-fuel gaseous mixtures

Experimental and numerical studies of the detonation in NO₂-N₂O₄/fuel (H₂, CH₄, and C₂H₆) gaseous mixtures show that for equivalence ratio Φ>0.8-1, (1) the detonation has a double cellular structure, the ratio between the cell size of each net being at least one order of magnitude; (2) inside the detonation reaction zone the chemical energy is released in two successive exothermic steps. Their chemical induction lengths, defined between the leading shock front and each local maximum heat release rate associated with each step, differ by at least one order of magnitude. The chemical reaction NO₂ + H → NO + OH is mainly responsible for the first exothermic step (fast kinetics), NO being the oxidizer on the second one (slow kinetics). Existence of correlations between calculated induction lengths and corresponding cell sizes strengthen the assumption that the cellular structure originates from local strong gradients of chemical heat release inside the detonation reaction zone.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

Ionothermal synthesis of TiO2 nanoparticles for enhanced photocatalytic H2 generation

Photocatalytic hydrogen generation is one of the most promising solutions to convert light energy into green chemical energy. In the present work, methoxy ethyl methyl imidazolium methyl sulphonate ionic liquid is used for the synthesis of i-TiO₂ nanoparticles via ionothermal method at 120 °C. The obtained products were characterized by various spectroscopic techniques like XRD, FTIR, Raman, UV–visible, DRS, TEM and TG-DSC analysis. XRD pattern confirmed the anatase phase with minor rutile phase having average crystallite size of 5 nm. From the FTIR spectrum, the band appeared at ～547 cm^?1 confirmed the Ti–O–Ti stretching and also few bands of ionic liquid. UV–vis spectrum clearly reveals the blue shift due to size effect of TiO₂. The spherical surface structure and particle size (15–30 nm) have been studied in detail using TEM images. Finally, the practical applicability of the as synthesized i-TiO₂ nanoparticles is shown by using it as a photocatalyst towards the generation of H₂ through water splitting reaction and it is found to be 462 μmol h^?1g^?1.     

Shock tube and kinetic study of C2H6/H2/O2/Ar mixtures at elevated pressures

Using a high-pressure shock tube facility, the ignition delay times of stoichiometric C₂H₆/H₂/O₂ diluted in argon were obtained behind reflected shock wave at elevated pressures (p = 1.2, 4.0 and 16.0 atm) with ethane blending ratios from 0 to 100%. The measured ignition delay times were compared to the previous correlations, and the results show that the ignition delay times of ethane from different studies exhibit an obvious difference. Meanwhile, numerical studies were conducted with three generally accepted kinetic mechanisms and the results show that only NUIG Aramco Mech 1.3 agrees well with the measurements under all test conditions. Sensitivity analysis was made to interpret the poor prediction of the other two mechanisms. Furthermore, the effect of ethane blending ratio on the ignition delay times of the mixtures was analyzed and the results show that ethane blending ratio gives a non-linear effect on the auto-ignition of hydrogen. Finally, chemical interpretations on this non-linear effect were made from the reaction pathway analysis and normalized H radical consumption analysis.     

Assessment of H2-CH4-air mixtures oxidation kinetic models used in combustion

Because of a wide number of applications, the potential hazards of H₂-CH₄-air mixtures have to be characterised. For hazard evaluation, an important element is a reliable detailed kinetic scheme. In the present study, three modern kinetic models, those of Konnov, of Dagaut and the GRI-mech 03, have been evaluated with respect to a large set of experimental data, including species profiles obtained in jet-stirred reactor, laminar flame speed, ignition delay time and detonation cell size, for hydrogen-methane-air mixtures. For jet-stirred reactor data, the model of Dagaut provides significantly better results. For flame speed data modeling, the three models are as reliable. For ignition delay times, the model of Dagaut seems the most reliable. For detonation cell size predictions, the model of Konnov is the best. Important chemical reactions are underlined through sensitivity and reaction pathway analysis and are discussed in the frame of rate constant values recommended by Baulch et al.     

H2O chemisorption and H2 oxidation on yttria-stabilized zirconia: Density functional theory and temperature-programmed desorption studies

The mechanism of H₂O dissociation as well as the adsorption and oxidation reaction of H₂ on yttria-stabilized zirconia (YSZ), commonly used as part of solid oxide fuel cell (SOFC) anodes, was investigated employing temperature-programmed desorption (TPD) spectroscopy and density functional theory (DFT). In agreement with theory the experimental results show that interaction of gaseous H₂O with YSZ results in dissociative adsorption leading to strongly bound OH surface species. In the interaction of gaseous H₂ with an oxygen-enriched YSZ surface (YSZ + O) similar OH surface species are formed as reaction intermediates in the H₂ oxidation. Our experiments showed that in both the H₂O/YSZ and the H₂/YSZ + O heterogeneous reaction systems noticeable amounts of H₂O are “dissolved” in the bulk as interstitial hydrogen and hydroxyl species. The experimental H₂O desorption data is used to access the accuracy of the H₂/H₂O/YSZ adsorption/desorption and surface reaction kinetics data, employed in previous modeling studies of the electrochemical H₂ oxidation on Ni-pattern/YSZ model anodes by Vogler et al. [J. Electrochem. Soc., 156 (2009) B663] and Goodwin et al. [J. Electrochem. Soc., 156 (2009) B1004]. Finally a refined experimentally validated H₂/H₂O/YSZ adsorption/desorption and surface reaction kinetics data set is presented.     

Flammability limits,limiting oxygen concentration and minimum inert gas/combustible ratio of H2/CO/N2/air mixtures

The flammability limits, the limiting oxygen concentration (LOC) and the inert gas/combustible ratio (ICR) of hydrogen/carbon monoxide/nitrogen/air mixtures are determined for hydrogen fuel molar fractions of 0.44, 0.62 and 0.71, at atmospheric pressure and initial temperatures up to 200 °C. The experiments are performed in a glass cylindrical tube with an internal diameter of 80 mm. The mixtures are ignited by a spark discharge between two electrodes placed at the bottom of the tube. Flame propagation is said to have occurred if the flame propagates a distance of at least 100 mm. The experimental procedure is based upon EN 1839 and EN 14756. Le Chatelier's law is used to estimate the flammability limits of the hydrogen/carbon monoxide mixtures, while the LOC and ICR are estimated based upon the lower flammability limit. The estimates are compared with the experimental data.     

Catalytic transformation of H2S for H2 production

Hydrogen sulfide (H₂S) gas is a by-product from natural gas refining, hydrodesulfurization of various fossil fuels, and syngas cleaning from pyrolysis and gasification. Catalytic pyrolysis of H₂S provides an alternative and effective pathway to recover both H₂ and sulfur. Catalysts from hydrotalcite of ZnAl, ZnNiAl, and ZnFeAl were employed for H₂S pyrolysis and compared with TiO₂ and MoS₂ at atmospheric pressure and temperatures in the range of 923–1123 K. Kinetic analysis was carried out in a packed bed reactor which revealed the effect of H₂S partial pressures to be of the order of 0.8–1 with respect to H₂S. The developed novel catalysts showed improved performance with significantly reduced activation energy compared to TiO₂ by 30 kJ/mol as well as higher H₂S conversion during pyrolysis (17% at 1173 K) than with MoS₂ catalyst, even at high H₂S partial pressure which is necessary for viable hydrogen production. The new approach showed an alternate economical and efficient pathway of catalyst design to obtain high activity and stability for simultaneous H₂ energy and pure sulfur recovery from unwanted H₂S resources.     

H2 PSA purifier for CO removal from hydrogen mixtures

A two-bed PSA purifier was developed to produce high purity hydrogen for fuel cell applications. Two types of hydrogen-rich mixtures produced from coal off-gas were used. Feed 1 consisted of a 99% H₂ mixture (H₂:CO:CO₂:N₂ = 99:0.1:0.05:0.85 vol.%) containing 0.1% CO while Feed 2 was a 95% H₂ mixture (H₂:CO:CO₂:CH₄:N₂ = 95:0.3:0.1:0.05:4.55 vol.%) containing 0.3% CO. An increase in the P/F ratio and adsorption pressure led to an almost linear decrease in H₂ recovery with increasing purity. However, a sharp drop in CO concentration occurred at a specific operating range in both feeds. The feed was purified to 1.1 ppm CO with 99.99+% H₂ purity and 80.0% recovery under 6.5 bar and 0.15 P/F ratio while CO in Feed 2 could be reduced to 6.7 ppm with 99.96% H₂ purity and 78.4% recovery. The PVSA process, which combined vacuum and purge steps, could improve recovery by about 10% compared to the PSA process.     

Electrochemical behaviour of carbon supported Pt electrocatalysts for H2O2 reduction

Carbon supported bimetallic Pt-alloys (Pt_0.75M_0.25/C, with M = Ni or Co) are investigated as novel electrode materials for H₂O₂ reduction in acid solution. The alloy electrocatalysts, Pt_0.75Ni_0.25/C and Pt_0.75Co_0.25/C, as well as carbon supported Pt (Pt/C) are characterised using cyclic voltammetry. The electrocatalytic activity of the materials is studied using a rotating disc electrode system with a combination of linear scan voltammetry and chronoamperometry. It is found that the activity of Pt_0.75M_0.25/C electrocatalysts for H₂O₂ reduction is comparable to the activity of Pt/C electrocatalyst, with Pt_0.75Co_0.25/C exhibiting the best performance.