Methane Air Research Articles

Investigation of Sub-Grid Scale Turbulence-Radiation Interaction Effects on Turbulence Energy Transport and Varying Thermophysical Properties Using Large Eddy Simulation

Abstract The main objective of this article is to investigate sub-grid scale turbulence–radiation interaction (SGS TRI) effects on SGS turbulence kinetic energy (TKE) fluctuations and varying thermophysical properties in a partially premixed combustion system for a laboratory-piloted methane/air flame. The large eddy simulation approach is employed to simulate the turbulence of the compressible reactive flow. SGS quantities, including turbulent stress and fluxes of enthalpy and species in the sub-grid scale, are computed using the standard Smagorinsky–Lilly model. The radiative transfer equation is modeled by applying the spherical harmonic P1 approximation by considering the radiative heat source related to the SGS TRI contribution. Optically thin fluctuation approximation is utilized to simplify the radiative absorption term. A chemical reaction mechanism comprising 41 steps and 16 species is applied to model methane–air mixture combustion. Diffusion flamelet-generated manifolds are employed to govern the species transport equation. About 87% of TKE is resolved by applying the finest grid consisting of 1,822,580 cells. Impacts of SGS TRI on the spatially filtered density, eddy viscosity, SGS velocity and TKE, overall radiative emission, RMS temperature fluctuations, and nitrogen monoxide (NO) formation are studied. The results reveal that considering SGS TRI in the simulation leads to remarkable discrepancies, particularly in SGS velocity and TKE by 6.70% and 7.40%, respectively. Meanwhile, SGS density and eddy viscosity deviate negligibly in the presence of SGS TRI. Also, the filtered mass fraction of NO reduces up to 17.54% on average by considering TRI.

Performance Characteristics of a Rotating Detonation Combustor Exiting Into a Pressurized Plenum to Simulate Gas Turbine Inlet

Abstract The present study aims to experimentally characterize the performance of a rotating detonation combustion (RDC) system integrated with a pressurized downstream plenum to simulate the high-pressure inlet conditions of power-generating gas turbines. A thorough understanding of the operational behavior including wave mode behavior, static pressure profile along the combustor length, and dynamic features of pressure fluctuations is crucial for successful integration of RDC with the turbine. In this study, two RDC configurations are investigated, RDC with a constant area annulus and RDC with a converging nozzle. In both cases, the RDC flow exited into a plenum chamber kept at pressures varying from 155 kPa to 330 kPa. RDC was operated on methane and oxygen-enriched air to represent reactants used in land-based power generation. Experiments were conducted for the two RDCs configurations operated at three reactant mass flow rates (0.23, 0.32, and 0.46 kg/s). The RDC performance is characterized by time-averaged static pressure measurements, and wave velocity determined by ionization probes. In addition, dynamic pressure measurements were recorded both inside and near the exit of RDC channel to investigate wave interactions between RDC and downstream plenum. Results show that the RDC with the converging nozzle achieved superior performance while minimizing detrimental interactions with the reflected shock and/or acoustic waves from the downstream plenum.

Obtaining accurate non-methane hydrocarbon data for ambient air in urban areas: comparison of non-methane hydrocarbon data between indirect and direct methods

Abstract. Since non-methane hydrocarbons (NMHCs) include numerous species of volatile organic compounds (VOCs), they are the only indicators that can characterize the total amount of VOCs in ambient air. More than 90 % of NMHC instruments in the market use the indirect method for NMHC determination, utilizing a gas chromatograph to determine the concentrations of total hydrocarbons (THCs) and methane in air. However, we found indirect NMHC measurements incorrectly characterized the low NMHC concentrations in the urban air. These measurements were hindered by the uncertain changes in the errors associated with low THC concentrations, humidity and macroscopic substances (oxygen) in the atmosphere. In this study, we conducted intercomparisons between 10 instruments in the market using different operation principles; among these, two used the direct method, and eight used the indirect method. For the indirect method, experiments showed that when the NMHC concentration was 50 ppb C, the errors in the measurement results were 32 % (CH4) and 98 % (THC) when compared to theoretical values. The oxygen content in the sample gas directly affected the response of the THC. The effect of increased oxygen content on the analysis of oxygen-containing substances was more significant than that of the increase in hydrocarbon substances. The response of dry zero was blank, and the response value increased non-linearly with relative humidity (RH). RH had a great impact on low NMHC concentrations of nearly 50 %. For the direct method, although various flow designs were applied for different instruments, the results indicated that they generally showed lower interference and higher accuracy for the ambient NMHC measurements. With the online direct methods, we obtained more reliable data and characteristics for low-concentration NMHCs in downtown Shanghai. The median of hourly data was 163.1 ppb C, and the highest ratio of the maximum and minimum concentrations of NMHCs reached 9.56 in a single day.

Estimating air methane and total hydrocarbon concentrations in Alberta, Canada using machine learning

Fugitive emission sources are significant contributors to methane emissions, and time series data on reported emissions from such sources remain underutilized. The Alberta Energy Regulator (AER) has been collecting air quality data since 1986, including methane and total hydrocarbons concentration data. However, this data has not been thoroughly analyzed to forecast air quality trends. Our analysis of the data shows that average methane concentrations measured at most Alberta airshed stations exceed the global average, and the data exhibits increasing and decreasing trends depending on the station. We compared the predictive performance of three machine learning methods: Long Short-Term Memory (LSTM) recurrent neural network, Fully-Connected Neural Network (FC-NN), and Autoregressive Integrated Moving Average (ARIMA), using the AER methane concentration data. Our results indicate that the LSTM neural network model outperforms the other two methods. Also, our findings suggest that the AER methane concentration data can be effectively analyzed and utilized to forecast air quality trends in the region.

Ultra-lean premixed combustion of a holder-stabilized 80 %hydrogen–20 %methane–air mixture in a preheated mesoscale combustor

To achieve cleaner emissions, the ultra-lean premixed flames of 80 %H2–20 %CH4–air with the coupling effect of flow and heat recirculation were experimentally investigated. The flame behaviors for various Re values are observed and discussed in detail. The conventional stable flame, stable wedge-shaped residual flame, and wedge-shaped residual flame with upward and downward oscillations appeared in sequence as the equivalence ratio decreased. For a stable wedge-shaped residual flame with a decreasing Re value, the operating range broadens first and then narrows. By contrast, the operating range of the wedge-shaped residual flame with oscillations narrowed monotonically and slightly. With a decreasing Re value, the resident location of the conventional stable flame base remained nearly the same. When the equivalence ratio was reduced further, a stable wedge-shaped residual flame appeared. Subsequently, a wedge-shaped residual flame with oscillation occurred when the equivalence ratio was further reduced. Finally, the blow-off mechanisms were revealed.

Relaxational oscillations of burner-stabilized premixed methane–air flames

In this paper, the dynamics of diffusion-thermal oscillations of burner-stabilized methane–air flames are numerically and experimentally investigated. High speed imaging and laser induced fluorescence of OH radicals are used as experimental techniques, while a hierarchy of models is employed in numerical simulations: a system of ordinary differential equations describing dynamics of flame location and temperature, models with one-step and detailed reaction mechanisms. It is demonstrated that for small (near the neutral stability boundary) and moderately relaxational oscillations, all three approaches are able to describe the flame dynamics however with different levels of detailization. This is due to the fact that in these regimes the flame front structure remains similar, although perturbed, to that observed in freely propagating combustion waves. However, this flame structure breaks in the regime of highly relaxational oscillations, which can only be described within the detailed reaction mechanisms. The combustion front splits into the high and low-temperature reaction zones, which are shown to be highly and weakly sensitive to the diffusive-thermal pulsations. The high temperature reaction zone is blown away and extinguishes in the course of flame pulsation, and the low temperature zone remains near the burner surface and causes re-ignition of the combustion front. Prospects of further investigations are discussed.

A numerical study on the laminar diffusion and premixed flames for methane−air and methane−ethanol with ozone at atmospheric and elevated pressures

This work elucidated the laminar diffusion and premixed flames for methane–air and methane–ethanol (20 % and 40 %) with ozone concentrations of 100 ppm and 5000 ppm from 1 bar to 10 bar. The predicted diffusion flames were sensitive to ozone but depended on the fuels and pressures. At 1 bar, ozone induced higher peak flame temperatures and acetylene (C2H2) but with lower flame heights and soot. At higher pressures, the soot yields predicted for methane were increased by ozone whereas those for methane–ethanol were varied non-linearly. Meanwhile, C2H2 and soot formation were raised with ethanol while the maximum flame temperatures behaved non-monotonically. Besides, ethanol extended the flames with ozone at 1 bar but shortened these flames from 2 bar onwards whereas this trend was opposite for the flames without ozone. Owing to the higher pressures, the diffusion flame heights, temperatures and soot were raised. C2H2 for methane was unchanged while those for methane–ethanol flames were declined by 30 %. For the laminar premixed flames, ozone encouraged fuel decomposition and hence increased the laminar burning velocities for methane–air and methane–ethanol flames at all pressures by 11 % (maximum deviation). At 1 bar, the consumption of intermediate species concentrations i.e. formaldehyde (CH2O), hydroxyl (OH) and the formation of benzene (A1) and naphthalene (A2) were raised by ozone. At 2 bar and above, ozone generally produced lower intermediate species whereas inconsistent trends were found for the soot/polycyclic aromatic hydrocarbon (PAH) species. Separately, the effect of ethanol was only predicted at 1 bar, where the intermediate and soot/PAH species were promoted. As pressure increased from 1 bar to 10 bar, the laminar burning velocities and intermediate species for methane and methane–ethanol were decreased but with higher soot/PAH species.

Mitigation of ventilation air methane (VAM) using novel methanotrophic coating materials: a technical analysis

Ventilation air methane (VAM) is a potent greenhouse gas source originating from geological wells, current and extinct mineshafts and other terrestrial conduits venting methane to the atmosphere, contributing to global methane emissions and disproportionate warming potential. Herein, we introduce the concept of the methanotrophic material as an engineering solution. Such materials should be capable of converting methane at ambient temperatures and pressures to a binder product, capturing and permanently sequestering the methane while simultaneously restricting its further emission. While such materials are currently under research development, this goal is supported and facilities by the mathematical framework, introduced and used herein, to evaluate the ability to convert methane, using currently published activity data. We include a case study of the conversion of a characteristic stream of VAM (0.6% methane in air, 1.7 × 108 l hr−1 equivalent to 100 000 standard cubic feet per minute). We show that when appropriately designed, such systems require a surface coverage of less than 1000 m of mine tunnel length (equivalent to 20 000 m2 areal coverage) in order to reduce the methane emission from this stream by over 99%. Finally, we highlight formaldehyde as a reactive intermediate of methane oxidation which may itself be incorporated into these coating materials. As a component of binders and polymers already used ubiquitously in commercial products, this intermediate ultimately allows these systems to sequester the carbon from methane in a stable and solid form. The results presented here are easily extended to the treatment of other methane streams—either more concentrated or dilute—and the results herein will guide the design and development of a new class of carbon-negative materials.

Estimation of total landfill surface methane emissions using geospatial approach combined with measured surface ambient air methane concentrations

ABSTRACT The concentration of surface air methane (CH4) measured in parts per million by volume (ppmv) near the soil/atmosphere interface should, in theory, have a positive correlation with surface methane emissions fluxes, measured in grams per square meter per day (gm−2d−1). Some researchers suggest that CH4 flux can be reasonably inferred from simple measurements of CH4 concentrations near the landfill surface. Ground-based and drone-based surface emissions monitoring (SEMs) were performed at several municipal solid waste landfills as tracer correlation method (TCM) testing was being used to measure total methane emissions from the same landfills. The TCM data and SEM data were used to establish a new simple correlation to convert surface methane concentrations in ppmv to localized surface methane emission flux in gm−2d−1. The SEM data obtained from ten ground and drone monitoring campaigns were log-transformed and geospatially treated using inverse distance weighting to the power of 2 to predict methane surface concentrations in the entire footprint of the SEM measurements area. The developed new correlation equation was then used to convert every predicted surface methane concentration to an emissions flux. The total estimate of surface emissions from the entire landfill was obtained by integrating the predicted fluxes over the area of the footprint of the SEM measurement area. The use of the new developed correlation resulted in higher total emissions estimates than other correlations reported in the literature and should be considered more conservative. Not including other factors, the proposed approach provides estimate of total methane emissions with a coefficient of variation of 20%. This study introduces a novel approach that utilizes a developed correlation between surface methane concentrations and surface emissions fluxes to estimate total methane emissions from municipal solid waste landfills or from a specified area. This study provides an additional use of the quarterly SEM data. Implications: The proposed approach provides an occasion for additional use of the easily obtainable quarterly SEMs data that can be performed by most landfills. The SEMs data are the most abundant landfill methane concentrations data. This approach gives them more benefit for the user. It is intended to convert ambient air concentrations to some estimates of surface emissions that can help landfill owners with decision making such as remediation activities or adjustments of their gas collection a systems.

How pulse energy affects ignition efficiency of DBD plasma-assisted combustion

This work aims to find how coupled energy per pulse influences the ability of a pulsed dielectric barrier discharge (DBD) plasma to ignite fuel-lean methane–air flow. For that, experiments are performed on a custom-built DBD flow reactor with a variable dielectric thickness and the discharge is operated by bursts of 10 ns duration pulses at 3 kHz repetition rate. With an increase in dielectric thickness, we observe that the coupled energy per pulse decreases even though applied voltage conditions are similar and so more pulses are required to ignite the lean mixture. Interestingly, we observe a significant increase in the minimum ignition energy (MIE) with an increase in the thickness beyond 3 mm. Moreover, the ignition kernel growth rate is much slower in the thicker dielectric cases even though total energy coupling per burst is similar. This phenomenon is investigated further by evaluating plasma parameters using electrical and optical diagnostics. Effective dielectric capacitance, discharge current, and voltage drop across the gas gap are derived from an equivalent circuit analysis, whereas plasma gas temperature and effective reduced electric field ( E/N ) are estimated from optical emission spectroscopy. From these analyses, we conclude that a thicker dielectric limits the discharge current and so the plasma filament temperature. For more than 3 mm thick dielectric cases, the filament heating per pulse is too low to achieve strong enough plasma pulse-to-pulse coupling which eventually leads to higher MIE and slower ignition kernel growth rate or the inability to ignite at all.

Oxide Scale Formation of Stainless Steels with Different Heating Methods – Effect of Hydrogen as Fuel

The evolution from natural gas usage to new technologies, such as the use of hydrogen as fuel or electricity‐based heating, strongly influences the oxidation of the stainless steel surface in the reheating furnace. Thermogravimetric tests using different simulated combustion and induction reheating conditions are performed for austenitic AISI 301, AISI 304, and ferritic AISI 444 steel grades. Simulated furnace atmospheres in combustion methods are based on methane–air, methane–oxygen, hydrogen–oxygen, and methane–hydrogen–oxygen combinations. For induction simulations, air and nitrogen are used as furnace atmospheres. The results indicate that changes in heating conditions to H2‐fueled combustion or induction only have a minor influence on the oxidation of the ferritic grade; whereas, their effects on the austenitic grades are more pronounced. The transition from a methane–air to H2–oxyfuel combustion increases the total oxidation by 1.7 and 4 times for steel grades 304 and 301, respectively; therefore, grade 304 can be considered better suited for transition for H2–oxyfuel use. The shorter induction heating considerably decreases the amount of oxide scale for austenitic grades, but the nitrogen atmosphere produces a subscale inside the steel matrix, which can hinder the descaling process.

Blowoff mechanism of airburst blast extinguishment of a methane air jet diffusion flame with micro explosive

In the study, experiments of airburst blast extinguishment of a methane-air jet diffusion flame have been conducted with 20 mg silver azide pellets, and the extinguishing processes have been observed using a high-speed camera and schlieren method. From the experimental results, we have examined the extinguishing mechanisms. When the blast wave interacts with the diffusion flame, vortex pair is generated in the flame due to the baroclinic effect which is caused by the misalignment between the pressure gradient of the blast wave and the density gradient in the combustion region. The vortical motion leads to the local extinction, and finally, the whole flame is blown off. In airburst, a reflected shock wave occurs on the ground surface. Depending on the height of burst and the distance between the explosion location and the extinguishing target, the reflected blast wave merges with the incident blast wave and the Mach stem is formed before the blast wave interferes with the diffusion flame. As a result, it is found that the extinguishing effectiveness is increased by the presence of the Mach Stem. Moreover, the optimal height of burst in the airburst blast extinguishment is clarified.

Flames with Alternating-Sign Velocity in Methane–Air and Methane–Air–Coal Dust Mixtures in a Closed Vertical Tube

Flames with Alternating-Sign Velocity in Methane–Air and Methane–Air–Coal Dust Mixtures in a Closed Vertical Tube

Parametric instability in propagating flames and the impact of hydrogen enrichment on the onset of secondary instability

A self-excited acoustic instability of laminar premixed flames propagating in an open-ended tube with a length of 700 mm and a radius of 10 mm was simulated by solving the reacting unsteady compressible Navier–Stokes equations, to understand the way of massive acoustic generation and its onset behaviors. Four fuel–air mixtures with an equivalence ratio of 1.2 were considered, namely, methane–air and methane–hydrogen–air mixtures, to identify the role of hydrogen in rich methane–air mixture. Parametric instability, which generated huge acoustic disturbance and violent flame pulsations, was observed only for a particular methane–hydrogen–air mixture with RH = 0.2, consistent with previously reported experimental observations. For the investigation of the reinforcement mechanism of acoustic instability under parametric instability, the flame surface area modulation was examined. It was found that violent subharmonic flame front pulsations could strongly modulate the flame surface area in the fundamental mode, resulting in a fluctuating heat release rate and increased thermoacoustic coupling. When hydrogen addition was small, attaining a higher level of primary instability, which is the precursor of the parametric instability, was more dominant than increasing the threshold level for the onset of the parametric instability. With larger hydrogen addition, the increase in the threshold level was more dominant than attaining a higher level of the primary instability. In particular, as the flame propagation time decreased, the level of the primary instability was saturated in larger hydrogen addition. This study elucidates the mechanism for the acoustic generation of propagating flames under the parametric instability, and the effects of hydrogen enrichment within rich methane–air mixtures.

Effect of the Ignition Position and Obstacle on Vented Methane–Air Deflagration

Effect of the Ignition Position and Obstacle on Vented Methane–Air Deflagration

Subgrid-Scale Models for Predicting Premixed Methane–Air Flame Propagating in a Chamber with a Rectangular Obstacle

Subgrid-Scale Models for Predicting Premixed Methane–Air Flame Propagating in a Chamber with a Rectangular Obstacle

Role of condensate bubbles in enhancing oil droplets and organic acid flotation

Poor adhesion of oil droplet-bubble and the inability to manage dissolved organic matter have hindered flotation efficiency. X-ray photoelectron spectroscopy (XPS) was used for this research to describe the surface chemistry of oil droplets. Multi-component condensate droplet vaporization experiments in produced water (PW). PW at different temperatures was used to investigate droplet vaporization behavior, and the interfacial energy theory and vapor boiling theory were employed to explain the vaporization nucleation mechanism. Contact angle, induction time measurements, and flotation analyses were used to assess the surface chemistry of condensate bubbles and the oil removal effect. The results revealed that when multi-component condensate was vaporized in PW at different temperatures, the light components nucleated inside the condensate droplets and generated bubbles enveloped by the condensate film, and the diameter of the bubbles rose as the temperature increased. X-ray photoelectron spectrometry (XPS) investigation showed the existence of hydrophilic oxygen-containing functional groups on the crude oil's surface. The contact angle between the condensate bubbles and the mask oil surface in PW is >150° and spreads faster, while the contact angle between the methane bubbles, the air bubbles and the oil surface are both <90°. The contact angle and the induction time decrease as the organic acid content of the water increases. Condensate bubble flotation has a greater oil removal efficiency than methane bubble and air bubble, which can achieve the extraction and flotation of emulsified oil droplets (83 %) and dissolved acid (63 %).

Investigating the Ignition and Stability Limits of Premixed Methane/Air Combustion in Micro-Channels

Understanding and improving the performance of miniature devices powered by micro-combustion have been the focus of continued attention of researchers recently. The goal of the present work is to investigate the behavior of premixed methane–air combustion in a quartz microreactor with an externally controlled wall temperature. Specifically, the impacts of the flow inlet velocity, the equivalence ratio, and the microreactor channel size are examined. This study is conducted by means of computational simulations, and the results are validated against prior experimental data, as well as by other similar studies in the literature. Utilizing simulation results with detailed chemistry, the present work provides more in-depth insight into a variety of phenomena, such as ignition, flame propagation, flames with repetitive extinctions and ignitions (FREI), and flame stabilization. In particular, the ignition, the flame span, and the FREI-related characteristics are scrutinized to understand the underlying physics of the flame stability/instability modes. It is shown that the flames appear stable at higher inlet velocities, while the FREI mode is detected at a lower inlet velocity, depending on the equivalence ratio and the channel size. The findings also explain how different operating conditions impact the flame characteristics in both stability modes.

Effect of a single nanosecond pulsed discharge on a flat methane–air flame

Successful implementation of plasma-assisted combustion in applied thermal processes heavily relies on how the plasma can be formed as it interacts with the reactive flow and what the effects are of such a plasma on the combustion process. The current study is an experimental investigation of a plasma-assisted lifted flat methane–air flame by a nanosecond pulsed discharge at atmospheric pressure. The nanosecond pulsed discharge, with a pulse duration of 4 ns and an amplitude of 30 kV to 50 kV, is used to stimulate the flame with a repetition rate of 1 Hz. The flame/plasma interactions are investigated with electrical and optical/laser diagnostics to study plasma-formation and its effect on the temperatures and formaldehyde formation. The flame speed seems to be accelerated for tens of milliseconds after the plasma stimulation, without noticeable gas temperature increase at the flame front and in the post-flame region. Formaldehyde is formed in the unburnt region while there is a slight increase in formaldehyde signal in the preheat zone. These results show that a volumetric effect of plasma-assisted combustion can be achieved with a short nanosecond plasma from a single excitation.

Characteristics of Explosion Hazards in Methane–Air Mixtures Diluted by Hydrogen

The combustion efficiency of methane can be effectively enhanced with the occurrence of hydrogen. However, the combustion characteristic of premixed methane/hydrogen/air is not fully understood. In this study, the effect of the amount of hydrogen addition on the explosion risk of premixed CH4/air combustion was fully investigated through experiments and simulations. The explosion overpressure of premixed CH4/air combustion with various hydrogen additions was measured in a standard 20 L spherical closed vessel. Meanwhile, the microscopic flame structures for the same cases were simulated using 2022 Chemkin-Pro software. The results showed that hydrogen could increase the explosion risks of premixed CH4/air combustion. The rate of key elementary reactions R38: H + O2 &lt;=&gt; O + OH and R84: OH + H2 &lt;=&gt; H + H2O in the system could be accelerated by hydrogen. The peak explosion overpressure in the closed chamber is boosted and the arrival time of peak overpressure rise rate is shortened, which raises the danger. Especially under lean and rich combustion conditions, hydrogen could potentially lead to more dangerous situations. With the increase in hydrogen concentration, the reaction rate of key elementary reactions accelerates faster, the peak explosion overpressure increases more, and the peak overpressure rise rate arrives earlier.