Methane-hydrogen Mixtures Research Articles

A simulation study of methane-hydrogen gas mixture permeation through nanoporous palladium membrane using molecular dynamics

We conducted molecular dynamic (MD) calculations to explore the efficiency of H2/CH4 separation through nanoporous palladium membrane. A palladium membrane with engineered-porosity of 0.1%–2.2% is used in our model of gas component separation from a mixture. We use computations of molecular dynamics to measure many trajectories of the molecules and thereby collect low statistical uncertainty projections of the gas flow rates. Our simulations demonstrate that high porosity palladium membranes are permeable to both gasses. As the porosity decreases, the permeability of larger molecules greatly reduced, which contributes to an exclusion effect of molecular size for a range of porosity that can permit smaller molecules. This implies that the determined porosity can achieve high selectivity in the separation of gas molecules while the desired gas molecules exhibiting high permeability. We also found that external driving force has a good effect on hydrogen permeation mechanism through a membrane. The gas flux of hydrogen levels increases as the pressure difference increases. The real mechanism of hydrogen permeability can also be visualized through our simulation, showing time dependence of flux, selectivity and pressure dependence. This work is expected to provide the framework for the development of an energy-efficient palladium-based gas separation system.

Optical diagnostics of microwave plasma in process of micro/nanocrystalline diamond deposition on hard alloy tools

Diamond coatings on hard alloy tools produced by chemical vapor deposition (CVD) from methane-hydrogen mixtures greatly enhance the functional properties such as wear resistance and quality of the material processing. By varying the gas composition during the diamond film growth it is possible to cyclically change the film structure between microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) to produce MCD/NCD multilayers that improves the mechanical properties of the film and reduces its stress and roughness. As the diamond growth occurs with participation of radicals generated in the plasma thermo-chemical reactions it is important to control in situ the presence of those species and gas temperature Tg. Here we used optical emission spectroscopy (OES) to monitor atomic hydrogen, CN, CH and C2 dimer radicals in course of deposition of MCD/NCD coatings by microwave plasma CVD in CH4-H2 and CH4-N2-H2 mixtures. The gas temperature has been calculated from analysis of rotational components of structure of C2(Δν = 0) emission line. We found that addition of nitrogen (4%) in CH4-H2 plasma, which is needed to form NCD, significantly, by about 500 K reduces Tg (from Tg = 3910 K to Tg = 3330 K). We developed an algorithm tolerable to small shifts of the measured spectral lines which provided fast calculation of Tg, so that the spectrum record and the automatic processing of fine structure of C2(Δν = 0) line totally take the time of about 5 min.

Deposition of diamond films on Si by microwave plasma CVD in varied CH4-H2 mixtures: Reverse nanocrystalline-to-microcrystalline structure transition at very high methane concentrations

Microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films were synthesized on Si substrates by a microwave plasma chemical vapor deposition in methane-hydrogen gas mixtures with a wide range of methane concentrations of 0.5–40%. The samples were investigated with scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. We observed an unusual transition of the produced material structure from MCD to NCD and again to MCD with the increase in CH4 content in the gas. The deposition rate, grain size, and the roughness of the films exhibited extrema in the middle of the methane concentration range ([CH4] = 20%). Further increase of CH4 percentage leads to improvement of the diamond film quality, as confirmed by Raman spectra and SEM images while keeping a high deposition rate. These results may be used for the growth of the wide range of PCD films to be used as multipurpose protective layers, hard coatings for the cutting tools, and the diamond base for composite MCD/NCD structures.

EFFECT OF THE GAS MIXTURE FLOW RATE ON THE PROCESS OF DIAMOND SYNTHESIS FROM A HIGH-VELOCITY MICROWAVE PLASMA

A method of gas-phase deposition of diamond structures with the use of high-velocity jets for transporting gases activated in a microwave plasma to the substrate is developed. The diamond structures are synthesized from a hydrogen–methane mixture with the methane concentration of 1%. The influence of the gas mixture flow rate on the synthesis of the diamond structures with an unchanged composition of the mixture, pressure in the deposition chamber, and substrate temperature is studied. It is found that an increase in the flow rate leads to an increase in the polycrystalline film density and in the size of the diamond crystals forming the film. The composition of the mixture at the discharge chamber exit is numerically analyzed for different flow rates of the gas mixture. A correlation of the mixture composition with the growth rate and quality of the diamond structures is demonstrated.

Comparison between carbon molecular sieve and Pd-Ag membranes in H2-CH4 separation at high pressure

From a permeability and selectivity perspective, supported thin-film Pd–Ag membranes are the best candidates for high-purity hydrogen recovery for methane-hydrogen mixtures from the natural gas grid. However, the high hydrogen flux also results in induced bulk-to-membrane mass transfer limitations (concentration polarization) especially when working at low hydrogen concentration and high pressure, which further reduces the hydrogen permeance in the presence of mixtures. Additionally, Pd is a precious metal and its price is lately increasing dramatically. The use of inexpensive CMSM could become a promising alternative. In this manuscript, a detailed comparison between these two membrane technologies, operating under the same working pressure and mixtures, is presented.First, the permeation properties of CMSM and Pd–Ag membranes are compared in terms of permeance and purity, and subsequently, making use of this experimental investigation, an economic evaluation including capital and variable costs has been performed for a separation system to recover 25 kg/day of hydrogen from a methane-hydrogen mixture. To widen the perspective, also a sensitivity analysis by changing the pressure difference, membrane lifetime, membrane support cost and cost of Pd/Ag membrane recovery has been considered. The results show that at high pressure the use of CMSM is to more economic than the Pd-based membranes at the same recovery and similar purity.

Low-Temperature Steam Conversion of Natural Gas to Methane–Hydrogen Mixtures

A thermodynamic analysis is performed of the patterns of steam conversion of natural gas at temperatures of 300–600°C, pressures of 0.1–4 MPa and H2O : C molar ratios of 0.8‒1.2. Under these conditions, the reaction product is methane–hydrogen mixtures with hydrogen concentrations of 10–30 vol %. A rise in temperature, molar ratio Н2О : С, and a decrease in pressure contribute to an increase in the concentration of hydrogen in the reaction products. The thermodynamic boundaries of the process with no carbonization of the catalyst are determined. Experiments are performed to obtain methane–hydrogen mixtures from methane with an output concentration of 15–35 vol % hydrogen on industrial Ni-CrOx-Al2O3 catalyst at 325–425°C, a H2O : C molar ratio of 0.8–1.0, and atmospheric pressure. It is shown that under these conditions, the process proceeds without the formation of carbon on the catalyst.

Low-temperature Steam Reforming of Natural Gas to Methane-Hydrogen Mixtures

Thermodynamic analysis of the steam reforming of natural gas at a temperature of 300–600 °C, pressure 0.1–4 MPa and Н2О : С molar ratio 0.8–1.2 was carried out. Under these conditions, the reaction products are methane-hydrogen mixtures with the hydrogen concentration 10–30 vol.%. Raising the temperature and Н2О : С molar ratio as well as decreasing the pressure make it possible to increase the hydrogen concentration in the reaction products. Thermodynamic boundaries of the process in the absence of catalyst coking were determined. Experiments on the formation of methane-hydrogen mixtures from methane with the outlet hydrogen concentration 15–35 vol.% were performed on a commercial Ni-CrOx-Al2O3 catalyst at a temperature of 325–425 °С, Н2О : С molar ratio 0.8–1.0, and atmospheric pressure. Under the indicated conditions, the process was not accompanied by the formation of carbon on the catalyst.

Особенности горения метано-водородных смесей в поршневых энергоустановках и двигателях

Особенности горения метано-водородных смесей в поршневых энергоустановках и двигателях

An experimental study of the effect of 2.5% methane addition on self-ignition and flame propagation during high-pressure hydrogen release through a tube

This paper demonstrates experimental investigation on the self-ignition and subsequent flame propagation of high-pressure hydrogen-methane mixture release via a tube. The proportion of methane added to hydrogen is 2.5% (vol.). A transparent rectangular tube (d = 15 mm, L = 400 mm) is used in the experiments. It is shown that the minimum burst pressure required for self-ignition increases 1.57 times for only 2.5% methane addition from 2.89 MPa (pure hydrogen) up to 4.68 MPa (2.5% CH4 addition). This is mainly caused by the following reasons: on the one hand, methane addition can result in the decease of shock intensity inside the tube, thereby lowering the temperature of the combustible mixture; on the other hand, the hydrogen-methane mixture has the higher minimum ignition energy than that of pure hydrogen. Besides, 2.5% methane addition can increase the initial ignition time, weaken the flame intensity and reduce the flame propagation velocity relative to tube wall inside the tube. Moreover, for cases with 2.5% methane addition, the complete flame throughout the tube is formed closer to the back end of the tube. When the self-sustained flame exits from the tube, the maximum overpressure in a confined space increases with 2.5% methane addition.

Effect of Tungsten Carbidization on Gas Activation in Synthesis of Diamond Structures

With the aim of developing a gas-jet technique for depositing diamond structures, the flow of a hydrogen–methane mixture through hot coaxial tungsten cylindrical channels with a tungsten coil inside has been investigated. Emphasis has been placed on tungsten surface carbidization during gas-jet deposition and the influence of carbidization on the deposition rate and morphology of applied coatings. Several experiments on tungsten surface carbidization under different conditions for hydrogen–methane mixture supply have been conducted. Simulation of the mixture flow by direct simulation Monte Carlo method has been performed. Possible ways of how methane and its fragments fall on the surface of the coil has been discussed.

Use of methane-hydrogen mixtures for energy accumulation

Hydrogen-methane mixtures draw attention due to an idea of injection of hydrogen into natural gas network, thus lowering the need of huge investing in development of hydrogen infrastructure. Extraction of hydrogen from dilute mixtures with natural gas differs from traditional separations, since hydrogen concentration is low. Due to selective absorption of hydrogen from the feed, epy metal hydrides purification achieves high values of separation efficiency. We have experimentally demonstrated successful separation of the hydrogen (10%) – methane (90%) mixture at inlet pressure 9.5 bar by the LaNi4.8Mn0.3Fe0.1 intermetallic compound in one step with roundtrip (absorption/desorption) hydrogen recovery 74% and absorption hydrogen recovery 76%.

Lean combustion analysis using a corona discharge igniter in an optical engine fueled with methane and a hydrogen-methane blend

The robustness of combustion initiation is one of the main issues of actual spark-ignition engines, especially for highly-diluted or lean mixtures. In this work, the effects on combustion stabilization obtained by the usage of a radio-frequency corona igniter were evaluated on a single-cylinder optical engine. The comparison with a conventional spark igniter was carried out using pure methane fuel and a blend of hydrogen and methane. For each combination of fuel and igniter, the combustion stability was explored at different air–fuel ratios, from stoichiometric conditions to the lean stable limit (up to λ = 2.0 with the corona igniter and the hydrogen-methane mixture). The combustion analysis was carried out by using the synchronized indicating and imaging data. The latter is essential to estimate the contribution of the corona igniter, which was found to be considerable only before the 5% of mass fraction burned. The corona effect igniter, with respect to a conventional spark igniter, was able to extend the lean stable limit of about 0.15 λ units with methane fuel, and about 0.10 λ units with the hydrogen-methane blend in the tested engine point. Early flame analysis confirmed the capability of corona igniter to improve combustion onset speed and to obtain a more stable and repeatable flame kernel. The findings of this study can help for a better implementation of corona ignition with gaseous low-carbon fuels, and in particular to achieve a higher lean limit extension without the drawback of a performance decay given by a substantial hydrogen enrichment.

A reduced mechanism for the prediction of methane-hydrogen flames in cooktop burners

In order to reduce computing costs in the calculation of combustion in cooktops burning methane-hydrogen mixtures, automatic reduction techniques have been used to develop a reduced mechanism for atmospheric, methane-hydrogen-air flames involving 26 species and 143 reactions starting from GRI-Mech 3.0. Validation for 0-D and 1-D calculations shows good agreement with the original mechanism for gas mixtures ranging from pure methane up to a 1:1 methane-hydrogen ratio, showing that it can be used for a variety of methane-hydrogen mixtures. An improved, customized OpenFOAM solver has been developed and employed to perform 2-D and 3-D calculations. Results from a computational experiment for an axisymmetric, partially premixed flame are in good agreement with the original mechanism and with published experimental results. Flame stability has been studied in realistic 3-D methane-hydrogen cooktop calculations. Using the reduced mechanism, light back phenomena are reproduced accurately while providing a fourfold speed-up over the original mechanism.

Detonation limits in methane-hydrogen-oxygen mixtures: Dominant effect of induction length

New-concept detonation combustors, for example, rotating detonation engines (RDEs), motivate investigations of the phenomena and theory relevant to detonation limits. Because CH4–H2 binary fuel mixtures have excellent combustion performance in engines, it is interesting to investigate the application possibilities of such mixtures in advanced detonation engines. Therefore, the detonation characteristics (e.g., initiation, propagation and failure) and their mechanisms for methane-hydrogen mixtures under different thermodynamic conditions need further study. In this work, the physical connection between detonation limits and induction length (ΔI) of detonation structures for CH4–H2–O2 mixtures is investigated; the dominant effect of induction length on the detonation limits is examined. The results show that a single-headed spinning structure is a unique feature of detonation limits; hence, the detonation limits can be qualitatively estimated by this phenomenon. The relation between λ and ΔI is proportional, and the proportionality factor is 34.62; this relation is applicable in methane/hydrogen and oxygen mixtures with various fuel contents. By scaling the critical pressure pc with ΔI near the detonation limits, the relation between them is shown to be an exponential function: ΔI = 201.2·(1+pc)−2.1.

A skeletal mechanism for prediction of ignition delay times and laminar premixed flame velocities of hydrogen-methane mixtures under gas turbine conditions

The aim of this study is to eliminate unimportant steps from a detailed chemical-kinetic mechanism in order to identify a skeletal kinetic mechanism that can predict with sufficient accuracy ignition delay times and laminar premixed-flame velocities for H2-CH4 mixtures under conditions of practical interest in gas-turbine applications, which pertain to high pressure, high reactant temperature, and primarily lean-to-stoichiometric mixture compositions (although somewhat rich conditions also are considered for completeness). The accuracy of selected detailed chemical-kinetic mechanisms that are suited to represent combustion of hydrogen-methane mixtures in air was evaluated through comparison of computed and measured ignition delay times and laminar flame velocities, and because of its relative simplicity and sufficient accuracy, the San Diego mechanism was selected for the needed chemical-kinetic reduction. Under the pressure and temperature conditions of the mixture composition addressed, thirty nine reversible elementary steps involving eighteen species were found to suffice to describe with acceptable accuracy both the ignition delay time and the laminar burning velocities. The skeletal mechanism is given here, along with discussion of its derivation and characteristics, as well as comparison of its predictions with those of the detailed mechanism and, where possible, with experiment.

Experiment investigation of the effects of hydrogen content on the combustion instability of methane/hydrogen lean premixed swirl flames under different acoustic frequency ranges

Experimental study on combustion instability of lean premixed swirl flame was conducted for methane-hydrogen mixtures, and the influence mechanism of hydrogen content (0%, 10%, 20%, and 40%) on the combustion instability under different acoustic frequencies (110–240 Hz) was investigated. A forced acoustic field created by a loud speaker was to simulate the complex and variable acoustic environment of unstable combustion. The flame transfer function was used to characterize the combustion instability. The morphologies of the flame, the evolution of vortex in the flame flow field, intensity of OH* chemiluminescence of flame front structure, and the degree of local thermo-acoustic oscillation of flame were also investigated to facilitate this study. The results show that the increase in hydrogen content weakens the combustion instability of lean premixed swirl flame in the acoustic frequency ranges of 170–240 Hz, while the combustion instability is enhanced for the acoustic frequency ranges of 110–160 Hz. The main reason for this phenomenon is the increase in the hydrogen content changes development of the vortex in the flame flow field, the evolution of the flame surface and the intensity of OH* chemiluminescence of flame front structure. This study provides an important reference for mastering the mechanism of the combustion instability of mixed gas flames and for developing technologies to inhibit combustion instability.

Thermodynamic and Technical Issues of Hydrogen and Methane-Hydrogen Mixtures Pipeline Transmission

The use of hydrogen as a non-emission energy carrier is important for the innovative development of the power-generation industry. Transmission pipelines are the most efficient and economic method of transporting large quantities of hydrogen in a number of variants. A comprehensive hydraulic analysis of hydrogen transmission at a mass flow rate of 0.3 to 3.0 kg/s (volume flow rates from 12,000 Nm3/h to 120,000 Nm3/h) was performed. The methodology was based on flow simulation in a pipeline for assumed boundary conditions as well as modeling of fluid thermodynamic parameters for pure hydrogen and its mixtures with methane. The assumed outlet pressure was 24 bar (g). The pipeline diameter and required inlet pressure were calculated for these parameters. The change in temperature was analyzed as a function of the pipeline length for a given real heat transfer model; the assumed temperatures were 5 and 25 ∘ C. The impact of hydrogen on natural gas transmission is another important issue. The performed analysis revealed that the maximum participation of hydrogen in natural gas should not exceed 15%–20%, or it has a negative impact on natural gas quality. In the case of a mixture of 85% methane and 15% hydrogen, the required outlet pressure is 10% lower than for pure methane. The obtained results present various possibilities of pipeline transmission of hydrogen at large distances. Moreover, the changes in basic thermodynamic parameters have been presented as a function of pipeline length for the adopted assumptions.

Contraction of Microwave Discharge in the Reactor for Chemical Vapor Deposition of Diamond

It was found that in a hydrogen-methane mixture in microwave plasma reactor for diamond deposition, there is a threshold pressure after which the contraction of microwave discharge occurs. The results of measurements of gas temperature and spatial distributions of optical emission intensity of discharge are presented. The mechanism of discharge contraction is discussed.

Контракция микроволнового разряда в реакторе для газофазного осаждения алмаза

AbstractIt was found that in a hydrogen-methane mixture in microwave plasma reactor for diamond deposition, there is a threshold pressure after which the contraction of microwave discharge occurs. The results of measurements of gas temperature and spatial distributions of optical emission intensity of discharge are presented. The mechanism of discharge contraction is discussed.

Experimental study of hydrogen explosion in repeated pipe congestion – Part 2: Effects of increase in hydrogen concentration in hydrogen-methane-air mixture

Hydrogen is seen as an important energy carrier for the future which offers carbon free emissions. At present it is mainly used in refueling hydrogen fuel cell cars. However, it can also be used together with natural gas in existing gas fired equipment with the benefit of lower carbon emissions. This can be achieved by introducing hydrogen into existing natural gas pipelines. These pipelines are designed, constructed and operated to safely transport natural gas, which is mostly methane. Because hydrogen has significantly different physical and chemical properties than natural gas, any addition of hydrogen my adversely affect the integrity of the pipeline network, increasing the likelihood and consequences of an accidental leak. Since it increases the likelihood and consequences of an accidental leak, it increases the risk of explosion. In order to address various safety issues related to addition of hydrogen in to a natural gas pipeline a EU project NATURALHY was introduced. A major objective of the NATURALHY project was to identify how much hydrogen could be introduced into the natural gas pipeline network. Such that it does not adversely impact the safety of the pipeline network and significantly increase the risk to the public. This paper reports experimental work conducted to measure the explosion overpressure generated by ignition of hydrogen-methane-air mixture in a highly congested region consisting of interconnected pipes. The composition of the methane/hydrogen mixture used was varied from 0% hydrogen (100% methane) to 100% hydrogen (0% methane) to understand its effect on generated explosion overpressure. It was observed that the maximum overpressures generated by methane-hydrogen mixtures with 25% (by volume) or less hydrogen content are not likely to be significantly greater than those generated by methane alone. Therefore, it can be concluded that the addition of less than 25% by volume of hydrogen into pipeline networks would not significantly increase the risk of explosion.