Critical energy for direct initiation of spherical detonations in H2/N2O/Ar mixtures

Although the detonation phenomenon in hydrogen-nitrous oxide mixtures is a significant issue for nuclear waste storage facilities and development of propulsion materials, very limited amount of critical energy data for direct initiation - which provides a direct measure of detonability or sensitivity of an explosive mixture − is available in literature. In this study, the critical energies for direct blast initiation of spherical detonations in hydrogen-nitrous oxide-Ar mixtures obtained from laboratory experiments and theoretical predictions at different initial conditions (i.e., different initial pressure, equivalence ratio and amount of argon dilution) are reported. In the experiments, direct initiation is achieved via a spark discharge from a high voltage and low inductance capacitor and the initiation energy is estimated accordingly from the current output. Characteristic detonation cell sizes of hydrogen-nitrous oxide-Ar mixtures are estimated from chemical kinetics using a recently updated reaction mechanism. A correlation expression is developed as a function of initial pressure, argon dilution and equivalence ratio, which is fitted to provide good estimation of the experimental measured data. The direct link between cell size and critical energy for direct blast initiation is then analyzed. Good agreement is found between experimental results and theoretical predictions, which make use of the cell size estimation correlation and the semi-empirical surface energy model. The effects of the initial pressure, equivalence ratio and the amount of Ar dilution on the critical initiation energy H₂-N₂O-Ar mixtures are investigated. By comparing the critical energies with those of H₂-O₂-Ar mixtures, it is shown that H₂-N₂O mixtures are more detonation sensitive with smaller initiation energies than H₂-O₂ mixtures at the same initial pressure, equivalence ratio and amount of argon dilution, except for higher diluted condition with amount of argon in the mixture above 20%. Attempt is made to explain the critical energy variation and comparison between the two H₂-N₂O-Ar and H₂-O₂-Ar mixtures from the induction length analysis and detonation instability consideration.     

Hydrogen separation from blended natural gas and hydrogen by Pd-based membranes

Hydrogen separation membranes based on a heated metal foil of a palladium alloy, offer excellent permeability for hydrogen as a result of the solution-diffusion mechanism. Here, the possibility to separate hydrogen from the mixture of Natural Gas (NG) and hydrogen (NG+H₂) with various NG concentrations using Pd, PdCu₅₃ and PdAg₂₄ hydrogen purification membranes is demonstrated. Hydrogen concentrations above ∼25% (for Pd and PdCu₅₃) and ∼15% (for PdAg₂₄) were required for the hydrogen separation to proceed at 400 °C and 5 bar pressure differential. Hydrogen permeability of the studied alloys could be almost fully recovered after switching the feed gas to pure hydrogen, indicating no significant interaction between the natural gas components and the membranes surface at the current experimental condition. Hydrogen flux of the membranes at various pressure differential was measured and no changes in the hydrogen permeation mechanism could be noticed under (NG 50%+H₂) mixture. The hydrogen separation capability of the membranes is suggested to be mainly controlled by the operating temperature and the hydrogen partial pressure.     

Hydrogen permeation through the Pd-Nb-Pd composite membrane: Surface effects and thermal degradation

The composite membranes based on Group 5 metals are capable of H₂ separation with the high speed and infinite selectivity. The chemical and thermal stability are critical issues for the application of such membranes in the field of hydrogen energy. In order to understand the degradation mechanisms, the H₂ permeation through composite Pd_2μm-Nb_100μm-Pd_2μm membranes was investigated in a very wide pressure range: (10⁻⁵-10⁴) Pa. At higher pressures the surface contaminations only moderately decreased the permeation. However the permeation experiments at lower pressures demonstrated that an orders of magnitude change in the probability of H₂ molecule dissociative sticking is actually hidden behind this relatively moderate effect. The membranes poisoned by the surface contaminations could be recovered by their exposure to O₂ at (300-400) °C. Heating at temperature higher than 500 °C resulted in the irreversible decrease of permeation and in the pronounced change of permeation behavior at the variation of H₂ pressure. An extremely high permeation was observed at lower pressures at the clean surface of Pd coating. That allows developing an effective membrane pump for hydrogen isotopes.     

High temperature H2/CO2 separation using cobalt oxide silica membranes

In this work high quality cobalt oxide silica membranes were synthesized on alumina supports using a sol–gel, dip coating method. The membranes were subsequently connected into a steel module using a graphite based proprietary sealing method. The sealed membranes were tested for single gas permeance of He, H₂, N₂ and CO₂ at temperatures up to 600 °C and feed pressures up to 600 kPa. Pressure tests confirmed that the sealing system was effective as no gas leaks were observed during testing. A H₂ permeance of 1.9 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ was measured in conjunction with a H₂/CO₂ permselectivity of more than 1500, suggesting that the membranes had a very narrow pore size distribution and an average pore diameter of approximately 3 Å. The high temperature testing demonstrated that the incorporation of cobalt oxide into the silica matrix produced a structure with a higher thermal stability, able to resist thermally induced densification up to at least 600 °C. Furthermore, the membranes were tested for H₂/CO₂ binary feed mixtures between 400 and 600 °C. At these conditions, the reverse of the water gas shift reaction occurred, inadvertently generating CO and water which increased as a function of CO₂ feed concentration. The purity of H₂ in the permeate stream significantly decreased for CO₂ feed concentrations in excess of 50 vol%. However, the gas mixtures (H₂, CO₂, CO and water) had a more profound effect on the H₂ permeate flow rates which significantly decreased, almost exponentially as the CO₂ feed concentration increased.     

High performance composite hollow fiber membranes for CO2/H2 and CO2/N2 separation

The search for a clean energy source as well as the reduction of CO₂ emissions to the atmosphere are important strategies to resolve the current energy shortage and global warming issues. We have demonstrated, for the first time, a Pebax/poly(dimethylsiloxane)/polyacrylonitrile (Pebax/PDMS/PAN) composite hollow fiber membrane not only can be used for flue gas treatment but also for hydrogen purification. The composite membranes display attractive gas separation performance with a CO₂ permeance of 481.5 GPU, CO₂/H₂ and CO₂/N₂ selectivity of 8.1 and 42.0, respectively. Minimizing the solution intrusion using the PDMS gutter layer is the key to achieving the high gas permeance while the interaction between poly(ethylene oxide) (PEO) and CO₂ accounts for the high selectivity. Effects of coating solution concentration and coating time on gas separation performance have been investigated and the results have been optimized. To the best of our knowledge, this is the first polymeric composite hollow fiber membrane for hydrogen purification. The attractive gas separation performance of the newly developed membranes may indicate good potential for industrial applications.     

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.     

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

In this work, the explosion behavior of stoichiometric CH₄/O₂/N₂/CO₂ and H₂/O₂/N₂/CO₂ mixtures has been studied both experimentally and theoretically at different CO₂ contents and oxygen air enrichment factors. Peak pressure, maximum rate of pressure rise and laminar burning velocity were measured from pressure time records of explosions occurring in a closed cylindrical vessel. The laminar burning velocity was also computed through CHEMKIN–PREMIX simulations.     

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.     

Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases

The hydrogen-based economy is one of the possible approaches toward to eliminate the problem of global warming, which are increases because of the gathering of greenhouse gases. Palladium (Pd) is well-known material having a strong affinity to the hydrogen absorbing property and thus appropriate material to embed in the membrane for the improvement of selective permeation of hydrogen gas. In present work, we have functionalized polycarbonate (PC) membranes with the help of UV irradiation to embed the Pd nanoparticles in pores as well as on the surface of the PC membrane. Use of Pd Nanoparticles is helpful to enhance the H₂ selectivity over other gases (CO₂, N_2, etc.). Also, the UV based modification of membrane increases the attachment of Pd Nanoparticles. Further to enhance the Pd nanoparticles attachment, we used PVP binder with Pd nanoparticles solution. Gas permeability measurements of functionalized PC membranes have been carried out, and better selectivity of hydrogen has been found in the functionalized and Pd nanoparticle binded membrane. PC membrane with 48 h UV irradiated and Pd NPs with PVP have been found to have maximum selectivity and permeability for H₂ gas. All the samples being characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and UV–Vis spectroscopy for their morphological and structural investigation.     

CO2 and H2O gas conversion into CO and H2 using highly exothermic reactions induced by mixed industrial slags

This communication reports conversion phenomena in which CO₂ and H₂O gases are transformed into CO and H₂, respectively, when exposed to a mixture of molten CaO-rich metallurgical slag and V₂O₃-rich gasifier slag. On reaction, CO₂ and H₂O are thermodynamically driven to become CO and H₂, respectively, by giving up oxygen over the formation of calcium orthovanadate in the slag. The concept was experimentally investigated with a synthetic slag heated to 1500 °C (an assumed slag tap-out temperature in the metallurgical industry) in a CO₂ saturated atmosphere. On heating, a rapid drop in oxygen partial pressure occurred between 1405 °C and 1460 °C, where 97% of CO₂ transformed to CO. Potential industrial applications with the H₂O-to-H₂ conversion are then explored using detailed process computations. If the process is made economically viable, CO₂ and H₂O could be converted into products that are environmentally and industrially attractive and that have the potential for energy savings and greenhouse gas reduction in a process.     

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.     

Evaluation of emissivity correlations for H2OCO2N2/air mixtures and coupling with solution methods of the radiative transfer equation

This study is directed towards the limitations of applying total emissivity correlations in computational fluid dynamics (CFD) computer codes for flame modeling. The predictions of nine widely applied total emissivity models for H₂O---CO₂ homogeneous mixtures are compared with the exponential wide band model (EWBM) calculations. The comparison covers a range of total pressures, temperatures and path lengths which are suitable for the use of fine numerical grids in CFD simulations of atmospheric and high pressure combustors.

Attention is paid to coupling of the property models with the radiative transfer equation (RTE) and their performance in non-homogeneous applications. In this respect both the total transmittance non-homogeneous (TTNH) model and the spectral group model (SGM) are used. The latter model is combined with five weighted sum of gray gases models (WSGGM), the single line based sum of gray gases model (SLW) and the k-distribution model. The non-homogeneous validation tests used in situ total radiance measurements in two non-luminous natural gas flames representing two industrial situations, a water cooled furnace and a refractory lined furnace. The main conclusions are as follows. The spectral group model provides an elegant and accurate method of coupling WSGGM, k-distribution and SLW property models to the equation of radiative transfer. Both homogeneous and non-homogeneous tests indicate the advantage of using the Smith, Shen and Friedman weighted sum of gray gases model over polynomial correlations and the SLW model. It has been shown that in the near burner region of a natural gas diffusion flame, the water vapor to carbon dioxide partial pressure ratio departs significantly from the value expected for the complete combustion of methane in air. This finding emphasizes the limitation of existing WSGGM to H₂O to CO₂ partial pressure ratios of one and two only.     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

Characterizations of polybenzimidazole based electrochemical hydrogen pumps with various Pt loadings for H2/CO2 gas separation

Carbon capture and storage (CCS) technologies have been intensively researched and developed to cope with climate change, by reducing atmospheric CO₂ concentration. The electrochemical hydrogen pumps with phosphoric acid doped polybenzimidazole (PBI) membrane are evaluated as a process to concentrate CO₂ and produce pure H₂ from anode outlet gases (H₂/CO₂ mixture) of molten carbonate fuel cells (MCFC). The PBI-based hydrogen pump without humidification (160 °C) can provide higher hydrogen separation performances than the cells with perfluorosulfonic-acid membranes at a relative humidity of 43% (80 °C), suggesting that the pre-treatment steps can be decreased for PBI-based systems. With the H₂/CO₂ mixture feed, the current efficiency for the hydrogen separation is very high, but the cell voltage increase, compared to the pure hydrogen operation, mainly due to the larger polarization resistance at electrodes, as confirmed by electrochemical impedance spectroscopy (EIS). The performance evaluation with various Pt loadings indicates that the hydrogen oxidation reaction at anodes is rate determining, and therefore the Pt loading at cathodes can be decreased from 1.1 mg/cm² to 0.2 mg/cm² without significant performance decay. The EIS analysis also confirms that the polarization resistances are largely dependent on the Pt loading in anodes.     

Laminar burning velocities and flame stability analysis of H2/CO/air mixtures with dilution of N2 and CO2

Experimental measurement of the laminar burning velocities of H₂/CO/air mixtures and equimolar H₂/CO mixtures diluted with N₂ and CO₂ up to 60% and 20% by volume, respectively, were conducted at different equivalence ratios and conditions near to the sea level, 0.95 atm and 303 ± 2 K. Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. Numerical calculations were also conducted using the most recent detailed reaction mechanisms for comparison with the present experimental results. Additionally, a study was conducted to analyze the flame stability phenomenology that was found in the present experiments. The increase in the N₂ and CO₂ dilution fractions considerably reduced the laminar burning velocity due to the decrease in heat release and increase in heat capacity. At the same dilution fractions this effect was higher for the case of CO₂ due to its higher heat capacity and dissociation effects during combustion. Flame instabilities were observed at lean conditions. While the presence of CO in the fuel mixture tends to stabilize the flame, H₂ has a destabilizing effect which is the most dominant. A higher N₂ and CO₂ dilution fraction increased the range of equivalence ratios where unstable flames were obtained due to the increase in the thermal-diffusive instabilities.     

A molecular dynamics simulation study of PVT properties for H2O/H2/CO2 mixtures in near-critical and supercritical regions of water

Supercritical water gasification technology is an efficient and clean way to use the coal. This technology can convert carbon and hydrogen elements of coal into the mixtures of H₂O/H₂/CO₂ that can be used for electricity generation. The acquisition of PVT properties of H₂O/H₂/CO₂ mixtures is one of the most critical issues in realizing the design and operation of thermal power generation system using this technology. However, no experimental, theoretical and simulation studies exist regarding the PVT properties of H₂O/H₂/CO₂ in the near-critical and supercritical regions of water. In this paper, the molecular dynamics simulations of the PVT properties of H₂O/CO₂ mixtures are carried out and the theoretical calculations are conducted based on the equation of state, and the results are compared with the experimental values. Moreover, the PVT properties for H₂O/H₂ mixtures and H₂O/H₂/CO₂ mixtures in the near-critical and supercritical regions of water are predicted using molecular dynamics simulation and compared with the calculation results of the equation of state. The results of this paper are of great significance to the development of supercritical water gasification of coal, and could offer the reference for the application of H₂O/H₂/CO₂ mixtures in practical production.     

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.     

二氧化碳-氮气混合气体在微粉硅胶中生成水合物的 实验研究

空气中CO₂含量的增加导致了全球气候变暖问题。气体水合物能够有效分离出电厂尾气中的CO₂,对改善环境具有重要意义。考察了微粉硅胶（silica gel）中80mol% N₂与20mol% CO₂混合气体水合物形成特性,选取压力范围为6.0 ~ 8.0 MPa,温度范围为 -20 ~ -5℃。研究发现,N₂与CO₂混合气进入反应釜后,直接生成水合物,诱导时间小于1 min。压力越高、温度越低,生成水合物的相对气体消耗量越大,最大的相对气体消耗量为0.115 (mol/mol),水的转化率最大为77.02mol%,前30 min水合物生成速率与压力无关。水合物气体消耗量越大,反应釜中剩余N₂组分的含量越大,最大为90.95mol%。水合物生成驱动力越低,水合物中CO₂ 组分越高。在6.0 MPa、-5℃下,水合物中CO₂组分最大为65.70mol%。     

Differential diffusion effects in numerical simulations of laminar,axi-symmetric H2/N2–air diffusion flames

The accuracy of a new methodology to include differential diffusion in numerical simulations of reactive flows is illustrated for a set of laminar, axi-symmetric H₂/N₂–air diffusion flames. From the comparison of the calculations with the experimental data it is observed that when differential diffusion effects are properly taken into account in the transport equations in physical space, simulation results agree well with the experimental data. Ignoring differential diffusion effects is not acceptable, due to lack of H₂ diffusion close to the jet inlet. Overall, differential diffusion has a strong influence on the stabilization of the flame as well as on the chemical species concentrations.