H2-CO2 Mixtures Research Articles

Evaluation of parameters impacting grey H2 storage in coalbed methane formations

Injecting grey hydrogen into coalbed methane formations has the potential to transform coal into a geologically temporary “H2-Battery” and a permanent “CO2-sink”. This study aims to thoroughly evaluate crucial parameters-sorption, diffusion, and matrix swelling/shrinkage-that impact grey hydrogen injection in CH4-enriched coalbed formations. In terms of sorption capacity, CO2 exhibits the highest, followed by CH4 and then H2. Sorption data affirmed that anthracite coal has a certain degree of adsorption capacity for H2, with H2 demonstrating superior diffusive gas deliverability as indicated by its diffusion coefficient, which is approximately six times higher than that of CO2. These characteristic position coal as a promising candidate for temporary H2 storage and cleaning, optimizing injectivity and depletion efficiency. Interestingly, H2 adsorption induced matrix shrinkage in coal, in contrast to other strong sorption gases such as CH4 and CO2, which resulted in matrix swellings. Matrix deformations induced by H2–CO2 mixture (grey hydrogen) injection were conducted and quantified to evaluate grey hydrogen injection in future field trials. The results reveal that the majority of directional strains indicated that when the partial pressure of CO2 in the H2–CO2 mixture is equivalent to pure CO2 pressure, the latter case induces elevated swelling strains. This finding implicitly supports the hypothesis that H2–CO2 mixture injection causes less matrix swelling than pure CO2 injection at equivalent CO2 pressure. This work provides essential parameters to illustrate an enhanced synergism among hydrogen storage, carbon sequestration, and enhance coalbed methane recovery through grey hydrogen injection in coal formations.

Mutual Diffusivities of Mixtures of Carbon Dioxide and Hydrogen and Their Solubilities in Brine: Insight from Molecular Simulations.

H2-CO2 mixtures find wide-ranging applications, including their growing significance as synthetic fuels in the transportation industry, relevance in capture technologies for carbon capture and storage, occurrence in subsurface storage of hydrogen, and hydrogenation of carbon dioxide to form hydrocarbons and alcohols. Here, we focus on the thermodynamic properties of H2-CO2 mixtures pertinent to underground hydrogen storage in depleted gas reservoirs. Molecular dynamics simulations are used to compute mutual (Fick) diffusivities for a wide range of pressures (5 to 50 MPa), temperatures (323.15 to 423.15 K), and mixture compositions (hydrogen mole fraction from 0 to 1). At 5 MPa, the computed mutual diffusivities agree within 5% with the kinetic theory of Chapman and Enskog at 423.15 K, albeit exhibiting deviations of up to 25% between 323.15 and 373.15 K. Even at 50 MPa, kinetic theory predictions match computed diffusivities within 15% for mixtures comprising over 80% H2 due to the ideal-gas-like behavior. In mixtures with higher concentrations of CO2, the Moggridge correlation emerges as a dependable substitute for the kinetic theory. Specifically, when the CO2 content reaches 50%, the Moggridge correlation achieves predictions within 10% of the computed Fick diffusivities. Phase equilibria of ternary mixtures involving CO2-H2-NaCl were explored using Gibbs Ensemble (GE) simulations with the Continuous Fractional Component Monte Carlo (CFCMC) technique. The computed solubilities of CO2 and H2 in NaCl brine increased with the fugacity of the respective component but decreased with NaCl concentration (salting out effect). While the solubility of CO2 in NaCl brine decreased in the ternary system compared to the binary CO2-NaCl brine system, the solubility of H2 in NaCl brine increased less in the ternary system compared to the binary H2-NaCl brine system. The cooperative effect of H2-CO2 enhances the H2 solubility while suppressing the CO2 solubility. The water content in the gas phase was found to be intermediate between H2-NaCl brine and CO2-NaCl brine systems. Our findings have implications for hydrogen storage and chemical technologies dealing with CO2-H2 mixtures, particularly where experimental data are lacking, emphasizing the need for reliable thermodynamic data on H2-CO2 mixtures.

H2 permeance and surface characterization of a thin (2 μm) Pd-Ni composite membrane prepared by electroless plating

The high cost of palladium membranes limits their widespread use in industrial applications. We report a novel strategy for the preparation of economic hydrogen separation membranes. In this study, a Pd88-Ni12 composite membrane was prepared with a considerable nickel content (>10 wt%) for the first time using the organic–inorganic activation method in the ELP approach. The H2 permeance of the Pd88-Ni12 composite membrane was 4.34 × 10-7 mol m−2 s−1 Pa−1 at 500 °C with infinite selectivity (H2/N2). The membrane exhibited suitable performance when exposed to binary mixtures under various operating conditions. At 500 °C and 100 kPa, with the same concentration of impurity gases, the H2 permeance of the Pd88-Ni12 composite membrane was 3.33 × 10 -7 mol m−2 s−1 Pa−1 for the H2-Ar mixture, 3.12 × 10 -7 mol m−2 s−1 Pa−1 for the H2-N2 mixture, 2.56 × 10 -7 mol m−2 s−1 Pa−1 for the H2-CO2 mixture, and 2.54 × 10 -7 mol m−2 s−1 Pa−1 for the H2-CO mixture. The effect of operating parameters and the amount of impurity concentration in the gas feed on the stage cut in binary gas mixtures was investigated. Increasing operating temperature and pressure enhanced the stage cut for all mixtures while increasing the impurity concentration in the gas feed led to a decrease in the stage cut, independent of operating conditions. The Pd88-Ni12 composite membrane indicated suitable long-time permeance stability for 150 h at a transmembrane pressure range of 100–300 kPa and 500 °C under an H2/N2 gas atmosphere. Finally, the cost investigation demonstrated that the Pd88-Ni12 composite membrane had a 38 % lower specific cost than the pure palladium composite membrane.

Manipulating hydrogen oxy-combustion through carbon dioxide addition

Hydrogen (H2) is increasingly viewed as an attractive carbon-free fuel due to its potential compatibility with the existing transportation and conversion infrastructure. However, one of the major challenges facing large-scale deployment is the fundamentally different combustion properties it has in comparison to commonplace hydrocarbon fuels such as natural gas. For example, the laminar burning velocity (LBV) of a combustible mixture has a direct impact on how it can be used in internal combustion engine or gas turbines; the LBV of hydrogen is over 7 times greater than that of natural gas when combusted in air. Carbon dioxide (CO2) can be used as a working fluid, as opposed to nitrogen (in air), to reduce the flame speed of hydrogen combustion. A mixture of hydrogen, oxygen, and carbon dioxide as a working fluid can provide LBVs comparable to natural gas in air, which potentially enables existing conversion architectures. Furthermore, by replacing nitrogen (N2) with CO2 in the mixture, NOx emissions are avoided and opportunities for carbon sequestration or closed-cycle processes are possible. This study experimentally explores fundamental premixed oxy-combustion properties of H2-CO2 mixtures in a constant volume combustion chamber (CVCC) across a range of initial pressures (1, 1.5, and 2bar) and equivalence ratios (0.4, 0.6, 0.8, and 1). The spherically expanding flames are examined to determine the flame speed, LBV, and lower flammability limits (LFL) with respect to different CO2 concentrations (40%, 60%, 65%). Furthermore, at an initial pressure of 1bar and stoichiometric conditions, it was found that the flame speed of the 65% CO2 case was lower than that of CH4-air combustion.

Energy separation and CO2 nonequilibrium condensation effects in the pressure recovery process of hydrogen-rich fuel purification and decarburization

Supersonic condensation separation is a potential environmentally friendly technology for hydrogen purification and CO2 capture. The nozzle plays a crucial role in achieving CO2 capture in practical applications. It requires specific pressure recovery abilities to deliver pre-purified hydrogen-rich fuel through downstream pipelines efficiently. However, excessive pressure recovery can quickly induce shock waves. Therefore, it is necessary to clarify the mechanisms involved in this process. This paper establishes a mathematical model for H2–CO2 mixture gas condensation. It examines the energy separation effect and the interaction mechanism of boundary layer/shock wave/CO2 phase change condensation in the pressure recovery process. The results indicate that the interaction between the boundary layer and shock wave is essential for inducing the energy separation effect. As pressure recovery increases, shock waves appear and move toward the throat, which hinders carbon capture. The maximum carbon capture efficiency decreases from 33.1% to 0%. Therefore, maintaining a pressure recovery efficiency between 33.3% and 50% is more conducive to considering nozzle pressure recovery and CO2 condensation separation.

Experimental studies on poisoning of La0.9Ce0.1Ni5 based hydrogen purification system with CO2 as impurity

In the present study, bed poisoning characteristics of CO2 as gaseous impurity in the La0.9Ce0.1Ni5 based, “Metal Hydride Hydrogen Purification System (MHHPS)” and regeneration of poisoned bed are reported. The study was conducted by varying impurity content of CO2 gas in the H2–CO2 mixture, in the range of 10–50% by weight and the cyclic stability of alloy was tested with 10% CO2 in H2–CO2 mixture. The study was conducted on a lab scale reactor with 6 embedded cooling tube (ECT) filled with 1.2 kg of La0.9Ce0.1Ni5. According to the experimental outcomes, the MHHPS was capable of delivery 99.99% pure hydrogen for CO2 impurity up to 20%. However, for higher impurities level (20–50%), the purity level was in the range of 97.3–99.8%. With CO2 as impurity, significant drop in the absorption capacity of the MHHPS was observed, which was in the range of 0.92 wt% to 0.67 wt% for impurity of 10–50%. However, the bed regeneration was performed, wherein the bed was desorbed and evacuated at 95 °C and 10−2 mbar, followed by absorption of pure hydrogen at 20 bar and 25 °C. Within 2-3 regeneration cycle, the alloy got regenerated and achieved storage capacity of 1.27 wt% in 200 s.

Thermodynamic modeling of hydrogen–water systems with gas impurity at various conditions using cubic and PC-SAFT equations of state

Hydrogen (H2) has emerged as a viable solution for energy storage of renewable sources, supplying off-seasonal demand. Hydrogen contamination due to undesired mixing with other fluids during operations is a significant problem. Water contamination is a regular occurrence; therefore, an accurate prediction of H2-water thermodynamics is crucial for the design of efficient storage and water removal processes. In thermodynamic modeling, the Peng–Robinson (PR) and Soave Redlich–Kwong (SRK) equations of state (EoSs) are widely applied. However, both EoSs fail to predict the vapor-liquid equilibrium (VLE) accurately for H2-blend mixtures with or without fine-tuning binary interaction parameters due to the polarity of the components. This work investigates the accuracy of two advanced EoSs: the Schwartzentruber and Renon modified Redlich–Kwong cubic EoS (SR-RK) and perturbed-chain statistical associating fluid theory (SAFT) in predicting VLE and solubility properties of H2 and water. The SR-RK involves the introduction of polar parameters and a volume translation term. The proposed workflow is based on optimizing the binary interaction coefficients using regression against experimental data that cover a wide range of pressure (0.34 to 101.23 MPa), temperature (273.2 to 588.7 K), and H2 mole fraction (0.0004 to 0.9670) values. A flash liberation model is developed to calculate the H2 solubility and water vaporization at different temperature and pressure conditions. The model captures the influence of H2-gas (CO2) impurity on VLE. The results agreed well with the experimental data, demonstrating the model’s capability of predicting the VLE of hydrogen-water mixtures for a broad range of pressures and temperatures. Optimized coefficients of binary interaction parameters for both EoSs are provided. The sensitivity analysis indicates an increase in H2 solubility with temperature and pressure and a decrease in water vaporization. Moreover, the work demonstrates the capability of SR-RK in modeling the influence of gas impurity (i.e., H2–CO2 mixture) on the H2 solubility and water vaporization, indicating a significant influence over a wide range of H2–CO2 mixtures. Increasing the CO2 ratio from 20% to 80% exhibited almost the opposite behavior of H2 solubility compared to the pure hydrogen feed solubility. Finally, the work emphasizes the critical selection of proper EoSs for calculating thermodynamic properties and the solubility of gaseous H2 and water vaporization for the efficient design of H2 storage and fuel cells.

Protonic ceramic cells with thin BaZr0.8Y0.2O3-δ electrolytes for stable separation of H2 from H2–CO2 mixtures

Here we report selective separation of high-purity H 2 from a CO 2 -containing stream using protonic ceramic cells with thick Ni–BaZr 0.1 Ce 0.7 Y 0.2 O 3-δ cathode substrates, thin BaZr 0.8 Y 0.2 O 3-δ electrolytes and thin Ni–BaZr 0.8 Y 0.2 O 3-δ anodes, as fabricated by the tape casting and tape lamination methods. Scanning electron microscopy observations show an overall micron porous structure for both electrodes with some nano-scale porosities inside Ni particles. The measured H 2 flux out of 3%-humidified 10% H 2 – 90% CO 2 at 700°C was 3.2 ml•cm −2 •min −1 at 0.90 V with a Faraday's efficiency of 97.8%. A preliminary 50-hour measurement in 10% H 2 – 90% CO 2 shows a slight increase in the pumping voltage from 0.96 to 1.03 V with a constant H 2 flux of 6.5 ml•cm −2 •min −1 and Faraday's efficiencies continuously above 97%, as enabled by excellent stability of BZY against CO 2 . • Protonic ceramic cells with thin BaZr 0.8 Y 0.2 O 3-δ electrolytes are fabricated. • Ni–BaZr 0.8 Y 0.2 O 3-δ anodes show promise for stable operation in 10%H 2 –90%CO 2 . • The H 2 recovery rate from 10%H 2 –90%CO 2 is 3.2 ml•cm −2 •min −1 at 1.5 V.

Effect of the coexistence of CO2 and H2 on the kinetics of cerium hydriding

The reactions of cerium in hydrogen mixed with different concentrations of CO2 were conducted by using a pressure-volume-temperature (PVT) method combined with an in situ hot-stage microscope. CO2 has a great suppression effect on the hydriding reaction of cerium. The existence of CO2 leads to longer induction time, less pitting, and much slower hydriding rate. It is found that the induction time of hydriding reaction grows exponentially as a function of CO2 concentrations. Besides, the nucleation and growth of hydride spots are also suppressed with increase of CO2 concentrations. Scanning electron microscopy (SEM) and in situ optical microscope measurements reveal that the hydride spots formed in H2-CO2 mixture have a prolate hemispherical morphology while the normal hydride spots formed in pure H2 show an oblate hemispherical morphology. Thermal desorption results indicate CO2 preferentially adsorbs on the active sites of CeO2 surface resulting in block of hydrogen dissociation, which is accountable for the observed hydriding inhibition phenomenon. Our studies provide new insights into the CO2 effect on the hydriding process of active metals.

Effect of CO2 on the performance of an electrochemical hydrogen compressor

Abstract It is well known that CO2 has a negative effect on the catalyst of the fuel cell causing inhibition in presence of H2-CO2 mixture. According to literature, CO formation is the responsible of catalyst inhibition. Since an electrochemical hydrogen compressor (EHC) has a working principle similar to a fuel cell, the presence of CO2 can also be detrimental for the behavior of the compressor. In this work, the effect of CO2 in binary H2-CO2 mixtures on the performance of an EHC has been investigated in detail. Both Reverse Water Gas Shift (RWGS) and electrochemical reduction of CO2 may occur on the catalyst of an EHC. The main aim of the manuscript is to show RWGS is the most probable cause of catalyst inhibition. For a better understanding of the parameters involved in the catalyst inhibition in presence of a H2-CO2 mixture, different working conditions, viz. voltage and H2 concentration, are applied to the compressor. These experiments show a faster catalyst inhibition for lower H2 concentrations and lower voltages supplied. The temperature also plays an important role on the performance of the compressor. Indeed, since the RWGS is an endothermic reaction, the pollution of the catalyst is faster at higher temperatures. Recovery methods are also evaluated, and it was observed that air bleeding is most convenient because of its effectivity and recovery rate.

Study on the Separation of H2 from CO2 Using a ZIF-8 Membrane by Molecular Simulation and Maxwell-Stefan Model.

The purification of H2-rich streams using membranes represents an important separation process, particularly important in the viewpoint of pre-combustion CO2 capture. In this study, the separation of H2 from a mixture containing H2 and CO2 using a zeolitic imidazolate framework (ZIF)-8 membrane is proposed from a theoretical point of view. For this purpose, the adsorption and diffusion coefficients of H2 and CO2 were considered by molecular simulation. The adsorption of these gases followed the Langmuir model, and the diffusion coefficient of H2 was much higher than that of CO2. Then, using the Maxwell–Stefan model, the H2 and CO2 permeances and H2/CO2 permselectivities in the H2–CO2 mixtures were evaluated. Despite the fact that adsorption of CO2 was higher than H2, owing to the simultaneous interference of adsorption and diffusion processes in the membrane, H2 permeation was more pronounced than CO2. The modeling results showed that, for a ZIF-8 membrane, the H2/CO2 permselectivity for the H2–CO2 binary mixture 80/20 ranges between 28 and 32 at ambient temperature.

Synthesis of NaX zeolite-graphite amine fiber composite membrane: Role of graphite amine in membrane formation for H2/CO2 separation

Pore surface modification of zeolite membrane is a challenging area to combat the trade-off between the high flux and high selectivity of the gas separation membrane. Herein we report facile method of pore along with defects modification of NaX zeolite membrane by incorporating graphite amine (Ga) into the membrane structure. Permeation properties of different membranes were evaluated by single gas permeance of H2, CO2 and separation factors for mixed gas H2-CO2 with different mixture ratio. These studies were done at room temperature and 100–300 kPa feed pressure. The results show that Ga is not only active for defect removal of NaX membrane but also enhanced the formation of NaX zeolite from mother sol. In addition, presence of graphite amine into NaX zeolite membrane increase the permeance of H2 from 5.1 × 10−7 mol m−2 s−1 kPa−1 to 36.5 × 10−7 moll m−2 s−1 kPa−1 with increase of feed pressure 100–300 kPa feed pressure due to its hydrophobic properties and separation factor of H2-CO2 enhanced up to 31 for H2-CO2 mixture gas compared to literature reported value 6.5 for FAU membrane over 3-aminopropyltriethoxysilane functionalized alumina. Herein for the first time we report that graphite amine catalyzes for the formation of NaX zeolite in addition to pore modification in zeolite membrane.

Separation of Hydrogen from Carbon Dioxide through Porous Ceramics.

The gas permeability of α-alumina, yttria-stabilized zirconia (YSZ), and silicon carbide porous ceramics toward H2, CO2, and H2–CO2 mixtures were investigated at room temperature. The permeation of H2 and CO2 single gases occurred above a critical pressure gradient, which was smaller for H2 gas than for CO2 gas. When the Knudsen number (λ/r ratio, λ: molecular mean free path, r: pore radius) of a single gas was larger than unity, Knudsen flow became the dominant gas transportation process. The H2 fraction for the mixed gas of (20%–80%) H2–(80%–20%) CO2 through porous Al2O3, YSZ, and SiC approached unity with decreasing pressure gradient. The high fraction of H2 gas was closely related to the difference in the critical pressure gradient values of H2 and CO2 single gas, the inlet mixed gas composition, and the gas flow mechanism of the mixed gas. Moisture in the atmosphere adsorbed easily on the porous ceramics and affected the critical pressure gradient, leading to the increased selectivity of H2 gas.

Influence of Reduction Condition on the Morphology of Newly Formed Metallic Iron During the Fluidized Bed Reduction of Fine Iron Ores and its Corresponding Agglomeration Behavior

The fine iron ores from Chile are reduced in a laboratory fluidized bed at 973–1173 K with CO–H2–CO2 mixtures to investigate the influence of reduction condition on the morphology of newly formed metallic iron and its corresponding agglomeration behavior. The results reveal that the addition of H2 in CO accelerates the moving rate of the Fe/Fe1–xO interface and increases the amount of the iron nucleus formed during the initial reduction period, which induce the transformation of iron morphology from fibrous to dense. The presence of CO2 in CO makes the fibrous iron shorter and sparser, especially when the content of CO2 is 30% (by volume), the iron appears as “cactus‐like.” The increase of reduction temperature makes the fibrous iron stronger and more active. The sticking index indicates that the long and strong iron whiskers are prone to form stable agglomerates even at a low metallization degree, but the dense iron trends to behave separate. Furthermore, based on the precipitation mechanism of iron proposed by the authors, it is manifested that the agglomeration behavior of the fine iron ores could be controlled by pre‐reduction in H2‐rich reducing gas (H2/(H2 + CO) ≥ 30%, by volume).

Sonication mediated hydrothermal process – an efficient method for the rapid synthesis of DDR zeolite membranes

The formation and growth of a DDR zeolite membrane was developed on the low cost indigenous clay–alumina substrate for separation of H2 from H2–CO2 mixture by selective deposition of oriented seed crystals, followed by secondary growth method with sonication mediated hydrothermal technique. The formation of free radicals by ultrasonic irradiation in the sonochemical method enhances the rate of nucleation which ultimately reduces the DDR zeolite crystallization time. Surface seeding not only accelerates the zeolite crystallization on the support surface but also enhances the formation of an homogenous zeolite membrane layer. The DDR seeds were synthesized by a sonication mediated hydrothermal technique within a short crystallization time i.e. 2 days and used to provide nucleation for the membrane growth. Accordingly DDR zeolite membranes were synthesized on seeded substrate within 5 days. The membrane thickness was found to be ∼26 μm. The synthesized membranes along with seed crystals were characterized by XRD, FTIR, FESEM and EDAX analysis. The performance of the membrane formed was evaluated by single gas as well as mixture gas permeation measurement for H2 and CO2. The H2–CO2 separation selectivity of the membrane increased up to 3.7 at room temperature which is more than the reported values. To the best of our knowledge, there is no report on the synthesis of a DDR zeolite membrane within 7 (2 days for seed crystal and 5 days for membrane synthesis) days by a secondary growth technique.

Stiff metal–organic framework–polyacrylonitrile hollow fiber composite membranes with high gas permeability

Metal–organic framework (MOF) membranes used for gas separation have attracted considerable attention recently. Although there are studies focusing on their continuous growth on inorganic substrates, MOF–polymer membranes are in demand because of their low cost, high processing ability and large membrane area. To achieve this purpose, the flexibility of the polymer must be reduced and the MOF-to-substrate adhesion strength should be greatly enhanced. Herein, a continuous and well inter-grown Cu3(BTC)2–PAN composite membrane has been successfully fabricated by directly growing via a covalent linker. Dehydrogenation, cyclization and crosslinking reactions of the PAN hollow fiber by solvothermal treatment can greatly improve the stiffness and compression strength of the composite membrane. To increase the adhesion strength effectively, a covalent linker between the MOF layer and PAN substrate is provided by chemical modification. The prepared membrane achieves a high H2 permeance of 7.05 × 10−5 mol (m−2 s−1 Pa−1) and a good separation factor of 7.14 for binary H2–CO2 mixtures with high thermal and pressure stability. Our strategy has also been developed for preparing a continuous and well inter-grown ZIF-8–PAN membrane. A very thin ZIF-8 layer is finally synthesized due to the large number of nucleation sites on the substrate. All of these distinguished properties suggest that MOF–PAN composite membranes fabricated by chemical modification are promising candidates for gas separation.

Accelerated Corrosion of Fe–xCr–10Al Alloys Containing 0–20 at.% Cr Induced by Sulfur and Chlorine in a Reducing Atmosphere at 600 °C

The corrosion behavior of five Fe–xCr–Al alloys with a constant Al content of 10 at.% and Cr contents ranging from 0 at.% to 20 at.% was examined at 600 °C in a H2–HCl–H2S–CO2 gas mixture providing 3.7 × 10−22 atm O2, 2.4 × 10−14 atm Cl2 and 3.9 × 10−9 atm S2. All the alloys formed duplex scales containing an outermost layer of iron oxide plus an inner layer composed of mixtures of the oxides of all the alloy components. Besides, a region of internal attack of Al or Al + Cr, whose depth decreased with increasing Cr content, formed in all the alloys. The simultaneous presence of chlorine and sulfur in the gas mixture significantly accelerated the corrosion of all the alloys with respect to their oxidation in a simpler H2–CO2 mixture providing the same oxygen pressure, by forming thick and cracked scales. The effect was particularly large for the high-Cr alloys due to their inability to form external protective alumina scales in the present gas mixture.

Cell formation in non-premixed, axisymmetric jet flames near extinction

Systematic experiments with CO2 diluted H2–O2 circular jet diffusion flames have been undertaken to study the formation of cellular flames, which occur for relatively low reactant Lewis numbers and near the extinction limit. The jet Reynolds number for all experiments was about 500, based on the centreline velocity, jet diameter and ambient fuel properties. The Lewis numbers, based on the initial mixture strength φ and ambient conditions of the investigated near-extinction mixtures, vary in the range 1.1–1.3 for oxygen and 0.25–0.29 for hydrogen (φ is defined here as the fuel-to-oxygen molar ratio normalized by the stoichiometric value). Various conditions near the extinction limit were investigated by fixing the fuel composition (H2–CO2 mixture), and systematically lowering the oxygen concentration in the co-flowing oxidizer stream past the point where cellular structures formed, until extinction occurred. The observed different instability states were correlated with the initial mixture strength and the proximity to the extinction limit. The parameter space for cellularity was found to increase with decreasing initial mixture strength. For a given initial mixture strength, several cellular states were found to co-exist near the extinction limit, and the preferred number of cells (the azimuthal wave number) was observed to decrease with decreasing oxygen concentration (Damköhler number). These trends are consistent with previous theoretical work and our own stability analysis that will be reported elsewhere.

Equilibrium phase relations and thermodynamics of the Cr-0 system in the temperature range of 1500 °C to 1825 °C

Phase relations and thermodynamic properties of the Cr-O system were studied at temperatures from 1500 °C to 1825 °C. In addition to Cr and Cr2O2, a third crystalline phase was found to be stable in the temperature range from 1650 °C to 1705 °C. The atomic ratio of oxygen to chromium of this phase, which decomposes upon cooling to form Cr and Cr2O3, was determined as 1.33 + 0.02, in good agreement with the formula Cr3O4. Temperatures and phase assem blages for invariant equilibria of the Cr-O system were determined as follows: Cr2O3 + Cr + Cr3O4, 1650 °C ± 2 °C; Cr3O4 + Cr + liquid oxide, 1665 °C ± 2 °C; and Cr3O4 + Cr2O3 + liquid oxide, 1705 °C ± 3 °C. The composition of the liquid oxide phase at the eutectic temperature of 1665 °C was found to be close to CrO. Relations between oxygen pressure and temperature for the univariant equilibria of the Cr-O system were established by equilibrating Cr and/or Cr2O3 starting materials in H2-CO2 mixtures of known oxygen potentials at temper atures from 1500 ΔC to 1825 °C. From this information, the standard free-energy changes (ΔGΔ) for various reactions were calculated as follows: 2Cr (s) + 3/2O2 = Cr2O3 (s): ΔG ° = -1,092,442 + 237.94T Joules, 1773 to 1923 K; 3Cr (s) + 2O2 = Cr2O4 (s): ΔG ° =-1,355,198 + 264.64T Joules, 1923 to 1938 K; and Cr (s) + l/2O2 = CrO (1): ΔG ° =-334,218 + 63.81T Joules, 1938 to 2023 K.

Thermal conductivity of hydrogen-nitrogen and hydrogen-carbon-dioxide gas mixtures

The thermal conductivity of H2-N2 and H2-CO2 mixtures has been measured from -15 to 200°C by using the thick-wire variant of the hot-wire method. The experimental values have been compared with those calculated from the Hirschfelder-Eucken formula. Although fairly satisfactory agreement between theory and experiment is obtained it appears that for a precise interpretation of the thermal conductivity of polyatomic gas mixtures a better representation of the cross-relaxation phenomena is necessary.