Numerical investigation of turbulent combustion with hybrid enrichment by hydrogen and oxygen

In this study, the NG + H₂/air + O₂ turbulent flame is numerically investigated using the Computational Fluid Dynamics CFD code. The modulation of combustion and radiation is performed respectively by the Eddy Dissipation Model and the Discrete Ordinate Model. The turbulence modeling is carried out by Shear Stress Transport (SST/k-ω) turbulence model. The H₂ amount in the fuel mixture varies under constant volumetric fuel flow between 0 and 60% and the oxidant is composed by 80% air and 20% pure oxygen. The results obtained show the hydrogen addition to Natural Gas improves the mixing between the reactants, reduces their residence time and reduces the length and thickness of the flame. On the other hand, the hydrogen enrichment minimizes the CO₂ and CO production and increases the NO_x level.     

Study on the extension of lean operation limit through hydrogen enrichment in a natural gas spark-ignition engine

An experimental study aimed at investigating the extension of lean operation limit through hydrogen addition in a SI engine was conducted on a six-cylinder throttle body injection natural gas engine. Four levels of hydrogen enhancement were used for comparison purposes: 0%, 10%, 30% and 50% by volume. The effects of various engine operating conditions on engine's lean burn capability were also examined. Test results were then analyzed from a combustion point of view. The results show that engine's lean operation limit could be extended through adding hydrogen and increasing load level (intake manifold pressure). Effect of engine speed on lean operation limit is smaller. At low load level increase in engine speed is beneficial to extending lean operation limit but this is not true at high load level. The effects of engine speed are even weaker when the engine is switched to hydrogen enriched fuelling. Spark timing also influences on lean operation limit and both over-retarded and over-advanced spark timing are not advisable. It is also observed there existed a limiting value imposed on spark-90% MFB burn duration if lean operation limit is not to be exceeded and interestingly, this limiting value was independent on hydrogen enhancement level and engine operating conditions examined in this study.     

Effect of hydrogen enrichment on swirl/bluff-body lean premixed flame stabilization

This paper reports the mechanism of hydrogen enrichment in stabilizing swirl/bluff-body CH₄/air lean premixed flame. Large Eddy Simulation (LES) coupled with Thickened Flame (TF) model was performed to resolve the turbulent reacting flow. A detailed chemistry was used to describe the oxidization of CH₄/H₂/air mixtures. Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence of OH (OH-PLIF) simultaneous measurements were conducted to obtain the velocity fields and flame structures respectively. The numerical methods were validated by experimental data and showing good agreements. Both the experimental and numerical results show that, the flame brush attachment tends to leave the inner shear layer with increasing hydrogen addition, which will reduce the risk of flame lift-off. The chemical analyses prove that the attachment of CH₄/air flame is inherently weak. On the one hand, the CH₄/air flame is stabilized by the hot products inside the recirculation. On the other hand, the burnt gas suppresses the oxidation of H₂ and CO through H₂ + OH = H + H₂O and CO + OH = CO₂ + H, respectively. Although the proportion of CH₄ decomposition through CH₄ + OH = CH₃ + H₂O will be reduced by hydrogen addition, the path of CH₄ + H = CH₃ + H₂ will be enhanced significantly. Hydrogen addition will not only increase the overall reaction rate, but also change the combustion intensity at the nozzle exit from relatively weak to strong, which is also important for flame stabilization. The robust flame attachment obtained by hydrogen addition can attributed to the enhanced reactions of H₂ + OH = H + H₂O and CH₄ + H = CH₃ + H₂.     

Experimental investigation of the flame characteristics of a fuel mixture with high hydrogen content enriched with oxygen under the externally acoustic enforcement conditions

Pollutant emissions are one of the major problems for the World. In this regard, researchers focus on the studies on emission reduction. Hydrogen is an alternative solution for this problem. Hydrogen produces only water as a result of combustion with oxygen. Therefore, this study examines the combustion stability and emissions of a high hydrogen content fuel mixture. The fuel mixture containing 45% H₂ by volume was supported with 5% CH₄ in order to provide stable combustion. In addition, in order to reduce the instabilities caused by the high laminar burning rate of hydrogen, it was diluted with 50% CO₂ which equal volume with the fuel mixture. After the fuel mixture containing 45% H₂ - 5% CH₄ - 50% CO₂ was burned with air containing 21% O₂, enrichment was applied at the rates of 24% and 27% O₂. The flame that contains different oxygen ratios was acoustically forced through the speakers around the combustion chamber. The stability data, dynamic pressure, and light intensity fluctuation of the flame were recorded under different acoustic resonance frequencies (110 Hz, 190 Hz, 260 Hz). In this way, the oxygen enrichment performance and flame characteristics of hydrogen in a premixed burner, which is promising in zero-emission studies, were investigated. As a result, when the combustion condition of 21% O₂ and 24% O₂ ratios are compared, the instability increased slightly from 801 Pa to 887 Pa, respectively. However, at 27% O₂, the flame could not perform a stable combustion under acoustic enforcement. The flame flashbacked with a dynamic pressure fluctuation of 1577 Pa under an acoustic frequency of 110 Hz. In addition, it was observed that CO emissions have decreased with the increase in oxygen enrichment rate. CO emission measured at 1080 ppm at 21% O₂ decreased to 542 ppm and 276 ppm respectively at 24% and 27% oxygen enrichment levels. While NOx emission was measured at 10 ppm in the case of combustion with air, it was observed that decreased to 4 ppm at the rate of 27% O₂.     

Effect of hydrogen percentage and air jet Reynolds number on fuel lean flame stability of LPG-fired inverse diffusion flame with hydrogen enrichment

The flame type studied in this paper is a circumferential-fuel – jet inverse diffusion flame, and the fuel is liquefied petroleum gas enriched with hydrogen gas. Fuel lean flame stability limit regarding to the volumetric percentage of hydrogen and the air jet Reynolds number was investigated. There were three flame stable-related limits examined: local extinction limit, restore limit, and complete extinction limit. Global Energy Consumption Rate of fuel, fuel jet velocity, and overall equivalence ratio of the air/fuel mixture at the three stable-related limits were presented. Experimental results indicate that with hydrogen addition, the inverse diffusion flame can sustain burning with a lower global energy than without it. The most significant stabilization effect was obtained with 30% hydrogen addition for complete extinction limit and 30%–90% for local extinction limit. The corresponding fuel jet velocity at complete extinction limit also decreases with hydrogen addition. However, fuel jet velocities at local extinction limit and restore limit increase significantly, when hydrogen percentage is larger than 70%. Air jet Reynolds number does not show notable influence on Global Energy Consumption Rate or fuel jet velocity at the three stability limits. In addition, overall equivalence ratio, which is an important parameter of inverse diffusion flame combustion dropping dramatically with air jet Reynolds number when it is less than 2000.     

Extension of the lean limit through hydrogen enrichment of a LFG-fueled spark-ignition engine and emissions reduction

In this experimental investigation the affect of hydrogen addition to a landfill gas-fueled naturally-aspirated spark-ignition engine was explored. Hydrogen concentrations of 0%, 30%, 40%, and 50% by volume were added to simulated landfill gas (60% CH₄ and 40% CO₂). Efficiency, coefficient of variance of indicated mean effective pressure, and CO emissions were measured from near stoichiometric mixtures up to the lean operating limit. Engine-out NOx emissions were compared to predicted future best available control technology targets for NOx emissions in landfill gas-to-energy projects. From this study, it was determined that with 40% hydrogen by volume untreated exhaust NOx emissions can meet the 0.22 g/kWh NOx target while retaining 95% of baseline power and low CO emissions.     

Enhancement of efficiency and power output of hydrogen fuelled proton exchange membrane (PEM) fuel cell using oxygen enriched air

This paper deals with the effects of the oxygen-enriched air (up to 50% oxygen by mass) along with other operating parameters (hydrogen flow rate, temperature, and relative humidity) on the performance of hydrogen-fuelled proton exchange membrane (PEM) fuel cell. The active area of a fuel cell considered was 50 cm² with three cells in series connections. The air was supplied with O₂ enriched from 23% to 50% at the cathode. The voltage obtained with the respective enriched air was 2.52 and 2.80 V respectively. The optimum oxygen enrichment was found as 45%. The stack temperature plays a significant role on performance improvement and the optimum temperature was found as 50 °C. The voltage efficiency and power output were improved by 9% and 33% with 45% oxygen-enriched air. Electrochemical impedance spectroscopy was used to analyze the impedance behavior of the fuel cell with the variable current demand. The bode plot indicates current dominates voltage at low oxygen-enriched air (25%) and vice-versa at high-enriched air. The inductive effect was dominating at the low frequency and overtaken by the capacitive effects at the higher frequency. These results would be useful to develop a dedicated fuel cell with the oxygen-enriched air.     

An assessment on the quantification of hydrogen releases through oxygen displacement using oxygen sensors

Gas sensors that respond directly to hydrogen are typically used to detect and quantify unintended hydrogen releases. However, alternative means to quantify or mitigate hydrogen releases are sometimes proposed. One recently explored approach has been to use oxygen sensors. This method is based on the assumption that a hydrogen release will displace oxygen, which can be quantified using oxygen sensors. The use of oxygen sensors to monitor ambient hydrogen concentration has drawbacks, which are explored in the current study. It was shown that this approach may not have adequate accuracy for safety applications and may give misleading results under certain conditions for other applications. Despite its shortcomings, the Global Technical Regulation (GTR) for Hydrogen and Fuel Cell Vehicles has explicitly endorsed this method to verify hydrogen vehicles' fuel system integrity. Experimental evaluations designed to impartially assess the ability of oxygen and hydrogen sensors to reliably measure hydrogen concentration changes are presented. Specific limitations on the use of oxygen sensors for hydrogen measurements are identified and alternative sensor technologies that meet the requirements for several applications, including those of the GTR, are proposed.     

Effect of hydrogen enrichment and electric field on lean CH4/air flame propagation at elevated pressure

Electric assisted combustion for hydrogen enriched hydrocarbons may even extend the lean burn limit and provide the further improvement on combustion stability. This study investigates the effect of hydrogen enrichment and DC electric field on lean CH₄/air flame propagation. Electric field inside the chamber was generated by mesh and needle electrodes. Effect of hydrogen enrichment on the ion mole fraction in the flame was discussed based on reaction mechanism included neutral and ion reactions. The flame propagation images, flame displacement speed were used to evaluate the combined influences of hydrogen enrichment and electric field on propagating flame. Results showed that the hydrogen addition would increase positive ions mole fraction and the peak value is mainly determined by H₃O⁺. This would be due to that CH increases with hydrogen fraction, which is the main species in the initial reaction for the ion reactions. Electric field effect about flame propagation was suppressed with hydrogen addition due to the competition between the increment in ion mole fraction and the decrement in flame time. Electric assisted combustion is more evident at leaner conditions and elevated pressure. The ratio of ionic wind velocity to flow velocity may be the determined factor to predict the electric field effect about propagating flame. The tendency based on this ratio is in accordance with the experimental results for various hydrogen fraction and equivalence ratio at elevated pressure.     

Experimental results of hydrogen enrichment of ethanol in an ultra-lean internal combustion engine

An investigation was made to determine the effects of hydrogen enrichment of ethanol at ultra-lean operating regimes utilizing an experimental method. A 0.745 L 2-cylinder SI engine was modified to operate on both hydrogen and ethanol fuels. The study looked at part throttle, fixed RPM operation of 0%, 15%, and 30% hydrogen fuel mixtures operating in ultra-lean operating regimes. Data was collected to calculate NO and HC emissions, power, exhaust gas temperature, thermal efficiency, volumetric efficiency, brake-specific fuel consumption, and Wiebe burn fraction curves.     

Assessment of CO2 capture options from various points in steam methane reforming for hydrogen production

Steam methane reforming (SMR) is currently the main hydrogen production process in industry, but it has high emissions of CO₂, at almost 7 kg CO₂/kg H₂ on average, and is responsible for about 3% of global industrial sector CO₂ emissions. Here, the results are reported of an investigation of the effect of steam-to-carbon ratio (S/C) on CO₂ capture criteria from various locations in the process, i.e. synthesis gas stream (location 1), pressure swing adsorber (PSA) tail gas (location 2), and furnace flue gases (location 3). The CO₂ capture criteria considered in this study are CO₂ partial pressure, CO₂ concentration, and CO₂ mass ratio compared to the final exhaust stream, which is furnace flue gases. The CO₂ capture number (N_cc) is proposed as measure of capture favourability, defined as the product of the three above capture criteria. A weighting of unity is used for each criterion. The best S/C ratio, in terms of providing better capture option, is determined. CO₂ removal from synthesis gas after the shift unit is found to be the best location for CO₂ capture due to its high partial pressure of CO₂. However, furnace flue gases, containing almost 50% of the CO₂ in produced in the process, are of great significance environmentally. Consequently, the effects of oxygen enrichment of the furnace feed are investigated, and it is found that this measure improves the CO₂ capture conditions for lower S/C ratios. Consequently, for an S/C ratio of 2.5, CO₂ capture from a flue gas stream is competitive with two other locations provided higher weighting factors are considered for the full presence of CO₂ in the flue gases stream. Considering carbon removal from flue gases, the ratio of hydrogen production rate and N_cc increases with rising reformer temperature.     

Combustion performances of premixed ammonia/hydrogen/air laminar and swirling flames for a wide range of equivalence ratios

Ammonia, a carbon-free source of hydrogen has recently gained considerable attention as energy solution towards a green future. Previous works have shown that adding 30_VOL.% hydrogen with ammonia can eradicate the drawbacks of pure ammonia combustion but no study in the literature has investigated this blend across a wide range of equivalence ratios. The present work investigates 70/30_VOL.% NH₃/H₂ blend from 0.55 ≤ Φ ≤ 1.4 for both premixed laminar spherically expanding flames and turbulent swirling flames at atmospheric conditions. A detailed chemistry analysis has been conducted in Ansys CHEMKIN-PRO platform using a chemical reactor network (CRN) model to simulate the swirling turbulent flames. NO and NO₂ emissions have followed similar bell-shaped trends, peaking at around Φ = 0.8, while N₂O emission rises at lean conditions (Φ ≤ 0.7). The results indicate that Φ = 1.2 is the optimum equivalence ratio with reduced NO_X emissions and some ammonia slip.     

Experimental study of using biogas in Pulse Detonation Engine with hydrogen enrichment

Pulse Detonation Engine (PDE) is one of the pressure gain engines that provide prominent features over conventional engines such as higher thermodynamic efficiency, higher thrust, and less design complexity. Biogas is one of the promising alternative energy sources. The aim of this paper is to study the influence of hydrogen addition in biogas on detonation characteristics performance of PDE. The ideal detonation combustion characteristics of biogas are estimated using NASA Chemical Equilibrium with Applications (CEA). The equivalence ratio range is varied from 0.7 to 1.4. It was found that the higher the percentage of methane concentration in biogas increased the detonation characteristics of biogas such as pressure, temperature, and Mach number. However, a reduction in the flame speed, temperature, and pressure of biogas compared to pure methane was stipulated to be due to the presence of CO2 that acts as a dilutant in biogas. It was found also that, the optimal percentage of hydrogen addition into the biogas fuel mixture is 15%. At this percentage, the detonation pressure improved by 23%. The performance of PDE fuelled by biogas is presented experimentally at a frequency of 10 Hz. The results show a successful and stable detonation initiation in PDE by using biogas with 15% and 20% addition of hydrogen.     

Combustion characteristics of a swirling inverse diffusion flame upon oxygen content variation

The combustion characteristics of a swirling inverse diffusion flame (IDF) upon variation of the oxygen content in the oxidizer were experimentally studied. The oxidizer jet was a mixture mainly composed of oxygen and nitrogen gases, with a volumetric oxygen fraction of 20%, 21% and 26%, and liquefied petroleum gas (LPG) was used as the fuel. Each set of experiment was conducted with constant oxygen content in the oxidizer. When the oxygen was varied, the changes in flame appearance, flame temperature, overall pollutant emission and heating behaviors of the swirling IDF were investigated. The swirling IDFs with different oxygen content in the oxidizer have similar flame structure involving a large-size and high-temperature internal recirculation zone (IRZ) which favors for thermal NO formation, and the thermal mechanism dominates the NO production for the swirling IDFs. The use of nitrogen-diluted air (with 20% oxygen) allowed the IDFs to operate at lower temperature with reduced NO_x formation, compared to the case of air/LPG combustion (with 21% oxygen). Meanwhile, an increase in CO emission is observed. With oxygen-enriched air (26% oxygen), the increase in temperature and EINO_x under lean conditions is more significant than under rich conditions. With 26% oxygen in the oxidizer stream, the IDF produces: (1) a shorter and narrowed navy-blue flame ring located closer to the burner exit, (2) highly luminous yellow flame extending into the central IRZ and above the blue flame ring, (3) a low CO emission, especially under lean conditions, (4) an increase in temperature at low Ф while a decrease in temperature at high Ф, and (5) an increase in EINO_x at all Ф. The heating test using the swirling IDFs in flame impingement heat transfer reveals that the heating rate can be monotonically increased as oxygen content in the oxidizer jet increases under the lean condition (Ф = 1.0). The oxygen enrichment does not contribute to the heating rate under the rich condition (Ф = 2.0), because for the non-premixed combustion of an IDF, the enrichment in oxygen means a lower oxidizer jet Reynolds number and thus less complete combustion occurs as a result of reduced amount of entrained ambient air.     

Experimental evaluation of hydrogen enrichment in a dual-fueled CRDI diesel engine

The influence of hydrogen enrichment on the dieselengine fueled with diesel and palm biodiesel blend (P20) is investigated in this study. The hydrogen is injected into the intake manifold at different flow rates of 7 lpm and 10 lpm for each loading condition of 30%, 60%, 80%, 90%, and 100%, respectively. Hydrogen enrichment improves the BTE and BSEC due to its high calorific value and decreases emissions like HC, CO, and CO₂ due to its carbon-free structure. However, due to a rise in EGT, NO_x emission has increased. With the addition of hydrogen, combustion properties such as in-cylinder pressure (ICP), heat release rate (HRR), and ignition delay (ID) improve while the combustion duration (CD) drops. Compared to P20 fuel,P20 + 10H₂ has a 28% increase in BTE and a 20% decrease in BSEC at 90% load. Similarly, HC, CO, and CO₂ emissions decrease by 16%, 35%, and 12%, while NO_x emission increases by 13% compared to P20. At full load, P20 + 10H₂increasesin-cylinder pressureand heat release ratebyupto 1–5%, while CD decreases by 12.5% compared to the P20 blend.     

Combustion and emission characteristics of an Acetone-Butanol-Ethanol (ABE) spark ignition engine with hydrogen direct injection

Butanol could reduce emissions and alleviate the energy crisis as a bio-fuel used on engines, but the production cost problem limits the application of butanol. During the butanol production, ABE (Acetone-Butanol-Ethanol) is a critical intermediate product. Many studies researched the direct application of ABE on engines instead of butanol to solve the production cost problem of butanol. ABE has the defects of large ignition energy and vaporization heat. Hydrogen is a gaseous fuel with small ignition energy and high flame temperature. In this research, ABE port injection combines with hydrogen direct injection, forming a stratified state of the hydrogen-rich mixture around the spark plug. The engine speed is 1500 rpm, and λ is 1. Five αH₂ (hydrogen blending fractions: 0, 5%, 10%, 15%, 20%) and five spark timings (5°, 10°, 15°, 20°, 25° CA BTDC) are studied to observe the effects of them on combustion and emissions of the test engine. The results show that hydrogen addition increases the maximum cylinder pressure and maximum heat release rate, increases the maximum cylinder temperature and IMEP, but the exhaust temperature decreases. The flame development period and flame propagation period shorten after adding hydrogen. Hydrogen addition improves HC and CO emissions but increases NO_x emissions. Particle emissions decrease distinctly after hydrogen addition. Hydrogen changes the combustion properties of ABE and improves the test engine's power and emissions. The combustion in the cylinder becomes better with the increase of αH₂, but a further increase in αH₂ beyond 5% brings minor improvements on combustion.     

Fabrication of nanoporous gold-islands via hydrogen bubble template: An efficient electrocatalyst for oxygen reduction and hydrogen evolution reactions

In this report, the fabrication of a high surface area nanoporous gold-island (NPG-islands) onto a glassy carbon (GC) surface by a simple one-step electrodeposition procedure based on a dynamic hydrogen bubble template method is described. The surface morphology, purity and crystalline structure of the porous NPG-islands were analyzed by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD) techniques. Cyclic voltammetry and linear sweep voltammetry methods were used for electrochemical studies and the electrocatalytic activity of the NPG-islands surface was investigated towards the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Electrochemical results revealed exceptional ORR activity of the NPG-islands evaluated by the shift of the onset potential towards less negative values compared to bare GC (0.55 V) and Au (0.25 V) electrodes, respectively, with a 3-fold increased current density in neutral PBS solution (pH 7). Rotating-disk measurements indicate a direct conversion of oxygen to water via a four-electron reduction pathway. The electrocatalytic activity was also evaluated for HER in 0.5 mol L⁻¹ H₂SO₄ solution and a benchmark current density of 10 mA cm⁻² at a very low overpotential of −0.075 V was obtained, which is similar to bulk Pt performance. The plausible mechanism of the HER was realized from the Tafel plot and the obtained slope of 46 mV dec⁻¹ suggests the Volmer-Heyrovsky mechanism takes place in such electrochemical process. Furthermore, the durability of the catalyst was also studied and exceptional stability was observed in cyclic voltammetry (up to 2000 cycles) and chronopotentiometry (at 10 mA cm⁻² for 19 h).     

Thermodynamics applied to oxygen enrichment of combustion air

In order to reduce overall fuel consumption, or to partially substitute a “valuable” fuel with a poor one, in industrial heating, oxygen enrichment of combustion air can be very effective. For the second option, a general criterion is stated in this paper for examining the suitability of oxygen enrichment in single cases. The topic is particularly interesting, as for the first time, it is now feasible to produce oxygen enriched air using permeable membranes on a commercial scale and with costs that are remarkably lower than those of other existing techniques. In this paper, the subject is investigated after some remarks about the definition of the “usable exergy” parameter, which was already proposed in previous papers by one of the authors and is here utilized for the above criterion.     

Effect of hydrogen enrichment on combustion characteristics of methane swirling and non-swirling inverse diffusion flame

Influence of hydrogen addition on appearance of swirling and non-swirling inverse diffusion flame (IDF) along with emissions characteristics are investigated experimentally. The combustion characteristics including flame length, axial and radial temperature variation, and noise level are analysed for hydrogen addition in methane by mass basis for constant energy input and by volume basis for constant volumetric fuel flow rate. Hydrogen addition in methane IDF produces shorter flame by compressing entrainment zone, mixing zone, reaction zone, and post-combustion zone. Hydrogen addition shift these zones towards fuel and air exit from the burner. Enrichment of methane with hydrogen on a mass basis up to 6% reduces CO emission considerably and increases NO_x emission moderately. Effect of H₂ addition on combustion and emission characteristics is more prominent in non-swirling IDF. Combustion noise is augmented with the hydrogen addition and the magnitude of sound level depends on the hydrogen concentration.     

Analysis of performance parameters of an ethanol fueled spark ignition engine operating with hydrogen enrichment

Concerns with the environment and energy security have increased interest in phasing out fossil fuels in the automotive industry, as it transitions from conventional internal combustion engines (ICE) to electric and fuel cell powertrains. During this transition, ethanol is of particular interest as a renewable fuel option in ICE, despite drawbacks compared to gasoline. Adding hydrogen to ethanol could remedy the disadvantages associated with ethanol, while maintaining the benefits of using renewable fuels. There is a gap in the literature of both experimental and numerical studies considering hydrogen addition in turbocharged ethanol engines. Therefore, this paper presents an experimental and numerical study of a turbocharged ethanol engine operating with hydrogen enrichment at stoichiometric conditions under boosted conditions. It was concluded that hydrogen addition allowed spark ignition engines to achieve lower brake specific energy consumption, better performance, and lower emissions. Thus, after proper calibration, a simulation model was created and shown to be a suitable tool to predict engine performance of a spark ignition engine operating with hydrogen enrichment and reduce the overall number of experimental tests needed to tune engines operating with this fuel blend. Finally, some operating strategies are recommended based on these findings.