Lean-burn Conditions Research Articles

Insights into the Construction of Robust Pt Clusters with Satisfactory Stability on CeO2 for the Catalytic Oxidation of CO.

Improving the efficiency of platinum group metals (Pt, Pd, Rh, etc.) in catalytic oxidation reactions remains an urgent topic. The conflict between the low-temperature activity and high-temperature stability of noble metals can hardly reach a consensus. For instance, Pt cluster catalysts supported on CeO2 with high low-temperature activity will suffer from deactivation due to the redispersion under high-temperature lean-burn reaction conditions. Herein, two Pt1/CeO2 prepared by the incipient wetness impregnation method using different Pt precursors possessed varied Pt-O and Pt-O-Ce coordination numbers (CNs). They showed various priorities in CO oxidation versus NH3 selective catalytic oxidation, materials with higher CNPt-O-Ce selectively catalyzing NH3 oxidation to N2 more superior, conversely materials with lower CNPt-O-Ce performing better in CO oxidation. After activation by H2 reduction, both formed massive Pt clusters on the CeO2 surface but showed drastically distinct stability in lean-burn CO oxidation reactions. By summarizing the experimental results of high-angle annular dark-field scanning transmission electron microscopy, X-ray absorption spectroscopy, Raman spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy, etc., it is beyond doubt that the difference in the initial states of Pt1 due to distinct precursors indeed determine the redispersion behavior of the reduced Pt clusters on CeO2. Materials with lower CNPt-O-Ce and higher CNPt-O are more likely to form robust Pt clusters, as they are not conducive to Pt anchoring, thus restricting the reversible structural evolution occurring under lean-burn CO oxidation and reductive conditions. This approach serves as a guide for the convenient and efficient construction and exploration of robust Pt cluster catalysts.

Experimental investigation of the effect of the electromagnetic field on the stability of the hydrogen enriched methane flame under acoustic enforcement

ABSTRACT The current research has investigated the effects of magnetic fields on molecular structures, especially on flame formation, flame behavior and emission gases. This study particularly examines the phenomenon of combustion instability and emission characteristics of CH4 and CH4/H2 fuels under lean combustion conditions and under external acoustic forcing. Conducted within a combustor utilizing premixing and swirl techniques, experiments strategically generated magnetic fields in specific regions: the burner input, pre-mixer, and fuel supply lines. Trials maintained a constant magnetic field intensity of 3500 gauss, with a thermal output of 3 kW, swirl number of 1, and equivalency ratio of 0.7. Evaluation of acoustic resonance was performed at 95 Hz and 175 Hz frequencies within the combustion chamber. Findings suggest that applying a magnetic field positively impacts the combustion process, reducing instabilities in fuels like CH4 and CH4/H2, especially at a 95 Hz frequency. During pure methane combustion, CO emissions initially measured 4785 ppm but decreased to 4143 ppm under the magnetic field’s influence. Introducing the magnetic field to the pre-mixer increased CO emissions to 4233 ppm, while its application to the fuel line reduced emissions to 4104 ppm. For the CH4/H2 fuel mix, CO emissions decreased from 2638 ppm to 1961 ppm with the magnetic field, indicating improved combustion and reduced pollutant gases, including CO and NOx. This study highlights the potential of magnetic fields to improve combustion efficiency and address environmental concerns.

Stable lean co-combustion of ammonia/methane with air in a porous burner

Ammonia is being considered as a promising zero carbon fuel due to its unique properties, while its combustion still poses challenges in term of instability and higher nitrogen oxides (NOx) emissions. In this study, the co-combustion stability of ammonia/methane with air under lean burn condition is experimentally investigated using a silicon carbide (SiC) ceramic foam burner with a two-layer structure stabilizing the flame. The influences of different operating parameters (methane fraction ω, equivalence ratio φ, and flow velocity V) on the co-combustion flame temperature and emissions are analyzed. The results indicate that the lean stable co-combustion of ammonia/methane with air can be achieved in two-layer ceramic foam burner, with its lower limit extending to φ = 0.5, even for the pure ammonia at specific operating conditions. Notably, the nitric oxide (NO) emissions decrease significantly as the equivalence ratio increases, particularly with a rise in methane fraction. The reduction can reach as low as 843 ppm with φ increasing to 0.9, further reducing to an ultra-low level of less than 150 ppm at φ = 1.3, significantly lower than those of traditional free flame burners. Carbon monoxide (CO) emissions remain at ultra-low levels, indicating the excellent co-combustion efficiency of ammonia/methane-air in the porous burner. An optimal co-combustion region with lower NO and CO emissions were conclusively identified. These findings demonstrate the effectiveness of porous media combustion in improving the co-combustion stability of ammonia/methane and reducing NO emissions, thereby helping to develop the porous burners of ammonia-based fuel and promoting its clean utilization for realizing carbon neutrality.

Plasma assisted combustion of different biogas mixtures in low swirl burner

This study explores the impact of plasma driving frequency on diverse biogas compositions, emphasizing flame behaviour by analysing spatial emissions of OH*, C2*, and CH* from plasma-assisted flames. Experimental investigation performed combusting mixtures with 25 % and 60 % CH4 in CO2 under lean combustion conditions with the varying fuel equivalence ratios from 0.71 to 0.83 and plasma discharge frequencies from 60 to 120 kHz resulting in higher energy density.Obtained results indicate that the plasma assistance notably enhances flame stability under lean fuel conditions with increasing frequency from 80 to 120 kHz. The discharge frequency of 60 kHz indicates insufficient energy to enhance combustion of low calorific value mixtures as the flame lift-off height increases approximately by 0.43–1.5 % for the mixture of 25 vol% CH4 in CO2 and by 0.26–7.7 % for the mixture of 60 vol% CH4 in CO2. Changing the plasma frequency from 80 to 120 kHz, the flame position is shifted closer to the burner nozzle for both mixtures and the highest effect is determined at 120 kHz. The flame lift-off height is reduced by 2.43–16.34 % for the mixture with 25 vol% of CH4 and by 3.57–51.28 % for the mixture with 60 % of CH4. The plasma-assisted combustion leads to improved combustion efficiency as CO emissions are reduced from 1550 ppm to 45 ppm for biogas mixture, but in the case of low calorific value gas, the CO emissions increase linearly with the frequency increase even the flame curvature and stability is improved.

Study on ammonia/methanol blends with ammonia cracking for low-carbon combustion and NO reduction

This work aims to study the ammonia/methanol blended fuel with ammonia cracking in lean-burn condition. Ammonia on-line cracking to produce hydrogen production has the potential to realize the high-efficiency combustion. A new ammonia/methanol/hydrogen mechanism was updated and simplified using several reduction methods to optimize the computation time in 3D simulation. Visualization measurements in constant volume combustion chamber (CVCC) for flame propagation and NO generation were conducted, and 3D computational fluid dynamics (CFD) modeling was also established with the simplified mechanism simultaneously to investigate the impact of methanol addition and ammonia cracking in the lean-burn mode. The findings showed that methanol addition and ammonia cracking can significantly improve the combustion rate, and 60%ammonia/40%methanol with ammonia cracking ratio of 90% (N60R90) at equivalence ratio (ER) of 0.7 yields the similar LBV as pure methanol at ER of 1.0 in laminar burning condition. Its ER can be extended to 0.4 to obtain the similar level of LBV as pure ammonia at 1.0. The experiments and 3D simulations revealed that N60R90 at 0.4 yields the equivalent level of combustion rate as N60R0 at 0.8, while obtaining much lower NO emission. In addition, N60R100 at 0.4 without fuel-NO formation during the combustion can get ultra-low NO emission. This study provides the insights on low-carbon combustion strategy for ammonia cracking with low NO.

Multiple species imaging from CFD fused H2O absorption spectral tomography and transfer learning

Laser absorption spectroscopy (LAS) tomography is well-proved in combustion diagnosis but has difficulty especially in the simultaneous imaging of multi-species concentrations. A multiple species imaging method from single species LAS tomography was proposed on the basis of computational fluid dynamics (CFDs) and transfer learning. CFD simulation of the methane/air flat flame was conducted to reveal the relationship among multiple species. A back propagation neural network was pre-trained with the dataset obtained from CFD simulation to predict projection values of OH mole fractions from H2O absorption lines at 7185.6 cm−1 and 7444.4 cm−1. The measurement of flat flame by a single wavelength planar laser-induced fluorescence fused LAS tomography system was conducted for network fine-tuning and experiment verification. Distributions of OH mole fractions in lean-burn conditions and nearly complete combustion conditions were quantitatively reconstructed well, while annulus profiles in fuel-rich conditions were qualitatively retrieved. Reconstructed images with two-fifth experiment data used in the network fine-tuning showed a 31.3% decline in image error compared to those without fine-tuning. This proposed method enables LAS tomography of multiple species via only one species with enough measured projections, and also shows potential in image error reduction by introducing more projections.

Study of CH4-H2-CO-Air premixed combustion characteristics based on fractal dimension and flame crack method

The laminar combustion characteristics of CH4-H2-CO-Air are studied in a constant volume combustion chamber with different hydrogen content ratios and the initial conditions of 300 K and 0.2 MPa by schlieren technology. The influence laws of flame instability are analyzed through the influence laws of hydrogen content ratio and equivalence ratio (Φ) on the fractal dimensions of the flame contour and the ratio of the total flame face cracks' lengths to the flame radius (L/R), as well as the influence laws of hydrodynamic instability and thermal-diffusive instabilities on the flames. The results show that L/R is more sensitive to flame instabilities than flame front fractal dimensions due to a wider sampling range. After R exceeds a certain value, for lean and rich combustion conditions with the same distance from the stoichiometric ratio, the L/R is greater during lean combustion, and as the equivalence ratio gradually increases from 0.6 to 1.4, the effect of increasing hydrogen content (Hydrogen mole fraction ranges from 60% to 66.67%) on flame instability gradually shifts from promotion to suppression. This study has certain reference value for the combustion application of biomass synthesis gas.

Modeling and chemical kinetic analysis of methanol and reformed gas (H2/CO2) blending with ammonia under lean-burn condition

Hydrogen can be easily produced by methanol steam reforming. This work aims to investigate the effects of methanol addition and its reformed gas of H2/CO2 on ammonia combustion under lean-burn operation. A newly-developed reaction mechanism for NH3, H2, CH3OH, CO and their blends was established based on the highly-precise validations of laminar burning velocity (LBV), nitrogen monoxide (NO) and ignition delay time (IDT). The premixed flame analysis showed that methanol blends and reforming with hydrogen addition can greatly enhance the combustion rate, resulted from the chemical effect. The equivalence ratio (ER) can be extended to 0.45 for a methanol blending ratio of 60 % and reforming ratio of 90 % (M60R90) to achieve the similar level of LBV at pure ammonia at an ER of 1.0 under ambient condition. The methanol blends and reforming lead to the increase of NO, while the lean-burn operation can help to reduce the NO emission.

An experimental investigation on the lean-burn characteristics of a novel hydrogen fueled spark ignition engine: Hydrogen injection via a micro-hole on the spark plug

Hydrogen addition in SI engines has been a subject of extensive research over the years, primarily owing to its environmentally friendly attributes and its potential to optimize combustion, especially in lean-burn conditions. However, the use of hydrogen as a gaseous fuel presents challenges due to its high mass-to-volume ratio, which can substantially reduce engine power density, whether in direct injection (DI) or port injection PI conditions. Thus, how to maximum hydrogen's benefits on combustion while minimizing its consumption has become a topic worthy to be studied. In this research, a novel concept where hydrogen direct injected through an aperture on edge of the spark plug (SHDI) was proposed and experimentally investigated on a 1.5 L turbocharged gasoline direct injection (GDI) engine under lean-burn condition. Experiments were conducted at 1600 rpm and medium load conditions with hydrogen injection ratios φH2 varying from 0 % to 5.5 % and excess air coefficients (λ) up to 1.5. Hydrogen port injection (HPI) strategy was also employed as a reference to gauge the performance enhancement achieved through SHDI. The experimental results reveal that, in comparison to HPI, SHDI exhibited more pronounced effects in enhancing initial flame propagation and assisting to formation of stable flame nucleus which, in turn, extended the lean-burn limit substantially. Moreover, SHDI strategy also resulted in a more substantial improvement in mean effective pressure (IMEP) and brake thermal efficiency (BTE) compared to the HPI. However, emissions of NOX, HC and CO were higher in the SHDI mode as opposed to the HPI mode This disparity may be attributed to the elevated local temperatures in the vicinity of the spark plug and the interference caused by the hydrogen jet flow, which disrupted the in-cylinder flow pattern, consequently leading to increased levels of unburnt mixture.

Numerical evaluation of ignition timing influences on performance of a stratified-charge H2/methanol dual-injection automobile engine under lean-burn condition

Influences of ignition timing (IT) on combustion, carbon monoxide (CO) and nitric oxide (NO) regulated emissions and formaldehyde (HCHO) and unburned methanol (CH3OH) unregulated emissions in a hydrogen (H2)/methanol dual-injection automobile engine under lean-burn condition were studied numerically. The results demonstrate that at lean-burn (excess air ratio = 2.0) condition, the peak cylinder pressure (PCP), peak heat release rate (PHRR) and peak cylinder temperature (PCT) all decrease with delayed IT, and their corresponding crank angles are also late; ignition delay (ID) and combustion duration (CD) decrease with delayed IT; CO, HCHO and CH3OH emissions increase with delayed IT, while NO emission decreases with delayed IT. At a fixed IT, PCP, PHRR and PCT for with adding 5 % H2 (RH2 = 5 %) are higher than those for without adding H2 (RH2 = 0 %), and their corresponding crank angles for RH2 = 5 % are earlier than those for RH2 = 0 %; ID and CD for RH2 = 5 % are lower than those for RH2 = 0 %. CO, HCHO and unburned CH3OH emissions for RH2 = 5 % are also much lower than those for RH2 = 0 %, while NO emission for RH2 = 5 % is also higher than that for RH2 = 0 %. The influence order of adding H2 to reduce emissions of engine is: CO > HCHO > unburned CH3OH.

Study on lean combustion of ammonia-hydrogen mixtures in a pre-chamber engine

Ammonia holds promise for achieving zero carbon emissions from internal combustion engines. The pre-chamber turbulent jet ignition, characterized by multipoint distributed ignition and rapid combustion, could be applied to ammonia combustion to extend combustion limits and stability. This study investigates the impacts of hydrogen energy fraction (Hfrac), equivalence ratio, and ignition timing on the combustion and emission characteristics of ammonia-hydrogen mixture at lean combustion conditions in a pre-chamber (PC) marine engine by CFD Simulation. In H2-fuel active PC mode, increasing the hydrogen content or achieving stoichiometric combustion within the PC will shorten the cold jet phase and accelerate the formation of a more intense hot jet. As the Hfrac and the equivalence ratio in the main-chamber increase, the combustion pressure, temperature, and heat release rate (HRR) rise, while the combustion duration shortens due to enhanced reactivity, potentially leading to end gas auto-ignition and knock due to lower auto-ignition energy of the mixture. NO primarily forms near the flame front, and both NO concentration and the generation area increase with an increase in hydrogen content and equivalence ratio. Fuel-NO is likely to be the dominant factor in NO generation with lean combustion conditions. N2O, as a product of NH3 low-temperature combustion, is mainly concentrated near the flame front. An increase in unburned NH3 emissions corresponds to an increase in N2O emissions. Delaying the ignition timing to reduce NOx emissions is primarily achieved by controlling NO emissions, even though there is a slight increase in N2O.

A novel Fe2O3/Al2O3 porous media catalyst prepared by ultrasonic-assisted impregnation for low-concentration methane catalytic combustion

Porous media (PM) catalytic combustion offers a potential strategy for stable and efficient combustion of low-concentration methane (LCM). Herein, a porous media catalyst loaded with Fe2O3 as the active component (Fe2O3/Al2O3) was developed by ultrasonic-assisted impregnation for LCM catalytic combustion in a four-layer porous media burner. The influences of the gas flows, velocities, equivalence ratios, and PM arrangement patterns on the temperature distribution, combustion stability, methane conversion, and emissions were investigated in detail. The results indicated that the Fe2O3/Al2O3 catalyst possessed exceptional catalytic activity and thermal stability at medium-to-high temperatures, which broadened the limiting equivalence ratio for CH4 stationary combustion to 0.43 with the CH4 conversion exceeding 99 %. LCM enabled higher combustion stability in the PM burner with a gradually varied configuration, achieving steady combustion for more than 120 min under the lean combustion condition (0.43 equivalence ratio and 50 L/min). The high-quality flue gas from LCM catalytic combustion with an average temperature over 600 ℃, lower CO (< 150 ppm) and NOx emissions (< 10 ppm) could serve for the comprehensive utilization of power generation, heating, and cooling. The catalytic oxidation of LCM on the Fe2O3 surface was primarily divided into four steps with the second-step CH4 dehydrogenation in CH4 dissociation being the dominant rate-limiting step. This work proves that the catalyst fabricated by ultrasonic-assisted impregnation can effectively break through the lean combustion threshold, which provides a valiant reference for the clean and efficient utilization of LCM.

Numerical Study on the Effects of Partial Oxidation Fuel Reforming (POFR) on the Performance of a Natural Gas Engine

Due to the issues of low flame speed and high CH4 emissions for a natural gas engine, investigations into the partial oxidation fuel reforming (POFR) method used in natural gas engines to blend H2 have become increasingly valuable. In this paper, the combustion process, engine performance, and emissions of a natural gas engine with fuel-reforming gases blended together have been numerically studied. The results show that a higher fuel-reforming ratio can effectively improve the engine combustion performance, especially at lean-burn conditions. Combustion with reformed gases can increase the thermal efficiency by almost 2% at the full-load condition, whereas fuel reforming significantly affects the natural gas engine’s power performance. Furthermore, CH4 and NOX emissions decrease significantly with increasing fuel-reforming ratio. In conclusion, fuel reforming for a natural gas engine has a promising future in reducing greenhouse gas emissions and improving economic performance.

Acoustic and Optical Investigations of the Flame Dynamics of Rich and Lean Kerosene Flames in the Primary Zone of an Air-Staged Combustion Test-Rig

Abstract In this study, the thermoacoustic behavior of nonpremixed kerosene flames from rich to lean combustion conditions is investigated. Flame-transfer-functions (FTF) measured purely acoustically are compared with results based on flame chemiluminescence. OH*, CH*, C2*, and CO2* were selected as potential measures for representing steady and fluctuating heat release when burning nonpremixed kerosene. In addition, their ability for the quantification of equivalence ratio fluctuations will be highlighted. The measurements were performed in the primary zone of an atmospheric rich-quench-lean (RQL) combustion test-rig. The new experimental approach allows a characterization of the primary zone independent of the secondary zone. Rich and lean operating points were analyzed by fueling an aero-engine prototype injector with and without acoustic excitation. To improve the quality of the acoustic wavefield reconstruction a thermocouple correction method was implemented. The flame dynamics determined with the multimicrophone method (MMM) exhibit a frequency and equivalence ratio depending effect of rich combustion conditions. The results for the steady behavior of the chosen radicals by altering equivalence ratio and thermal power indicate proportionality of the chemiluminescence to thermal power. Furthermore, the CH*/C2* ratio is found to be a promising indicator for the global equivalence ratio in the combustion chamber. The flame-transfer-functions based on chemiluminescence show a good qualitative agreement with the multimicrophone method. Based on the experimental findings a calibration curve for the different radicals to obtain quantitatively correct flame-transfer-functions from chemiluminescence is presented.

Effect of direct injection of small amounts of ethanol on port-injected hydrogen internal combustion engines

Hydrogen is a renewable fuel with excellent combustion characteristics. However, the direct use of hydrogen in existing engines faces obstacles such as abnormal combustion and high NOx emissions. This study proposes a strategy for controlling the combustion and emissions performance of a port-injection (PI) hydrogen internal combustion engine using ethanol direct injection (DI). The test conditions are 1000 rpm, 1500 rpm, and 2000 rpm respectively, the excess air coefficient is 1, 1.5, and 2, and 3%, 6%, and 9% ethanol is added. The results showed that the addition of ethanol can significantly reduce the pressure rise rate of the hydrogen engine and prolong CA0-10 and CA10-90. The addition of ethanol can promote the BMEP and BTE of the hydrogen engine. The addition of ethanol can reduce NOx emissions under lean burn conditions by 18%. The disadvantage is that the addition of ethanol increases the hydrocarbon emissions of hydrogen engines by about 50%, but the total amount is less. After the addition of ethanol, the flashback phenomenon of the hydrogen engine was significantly improved.

Chemical kinetic and behavior study of the cracked gas of H2/N2 and DME addition on ammonia combustion in lean-burn condition

The comparisons between the cracked gas and DME addition with a certain proportion on combustion behaviors are studied. A mechanism for the prediction of ammonia/H2/DME is updated based on the previously developed one. Since it is important to use a simplified mechanism in 3D numerical simulations, mechanism simplification is conducted, resulting in one with 72 species and 459 reactions. The premixed laminar flame simulations indicate that both the fuel cracking with hydrogen production and DME addition with a small amount can enhance laminar burning velocity (LBV) and extend lean-burn limit. NO is increased with hydrogen or DME addition due to the chemical and thermal effects, and comparatively, adding cracked gas leads to lower NO generation for the same blending ratio. The 3D simulated results shows that a sudden increase of HNO for DME addition during the combustion induces a higher NO formation relatively to that for hydrogen addition.

The influence mechanism of pre-combustion chamber orifice structure on natural gas engines: Combustion, emissions, and thermofluid analysis

The pre-combustion chamber (PCC) can enhance ignition stability, promote flame propagation and thus improve thermal efficiency. However, the mechanism by which the structure of the PCC orifice affects the combustion characteristics of engines is still unclear. In this study, thermal fluid simulation was used to investigate the effects of different volume ratios (ratio of PCC to main combustion chamber), apertures, and orifice structures (under stoichiometric and lean-burn conditions) on a PCC-type natural gas engine. The results showed that as the volume ratio and pore size increased, the indicated thermal efficiency (ITE) of the engine first increased and then decreased. Furthermore, the ITE of the engine reached its highest value when a PCC structure with a volume ratio of 1.5% and a pore diameter of 1.6 mm was adopted. As the orifice taper increased, the flame propagation velocity in the W-shaped orifice (Din > Dout) and N-shaped orifice (Din < Dout) decreased. The flame produced by the direct orifice (Din = Dout) had the longest penetration distance and the highest propagation velocity, which increased the cylinder pressure. In addition, under lean-burn conditions, in-cylinder combustion was more complete, with the lowest CO and THC emissions.

Visualization and simulation study of ammonia blending with hydrogen as combustion application in lean-burn condition

Ammonia (NH3), as one of the two zero-carbon fuels, has attracted lots of attention. However, its poor properties, including slow laminar burning velocity (LBV), high ignition energy, and low flammability, make it unfavorable for use in engines. On the other hand, hydrogen (H2), another zero-carbon fuel, has a much higher LBV and flammability. Adding a certain amount of hydrogen to ammonia mixture has the potential to improve the combustion process and extend the lean-burn limit of ammonia. This paper investigates the combustion and emission improvements of ammonia blended with a small amount of hydrogen under lean-burn conditions, both experimentally and numerically. A new NH3/H2 chemical reaction mechanism was developed to accurately predict ignition delay time (IDT), LBV and NO generation under a wide range of initial conditions. The flame propagation schlieren and NO chemiluminescence measurements with a homogenous NH3/H2 mixture ignited by a spark plug in constant volume combustion chamber (CVCC) were conducted. The 3D numerical simulations in CVCC, coupled with the new mechanism, were validated with the experimental results. The hydrogen energy fraction up to 30 % (X30) and ER down to 0.4 (ER0.4) were studied in experiments and simulations. The results showed that, in comparison to pure ammonia (X0) at ER1.0, lean-burn conditions can be extended to ER0.6 and ER0.4 for X10 and X30, respectively, while still achieving a shorter combustion duration. NO is initially increased and then decreased with decreasing ER, regardless of hydrogen addition. Comparatively, X30ER0.4 can achieve relatively low NOx emissions, and maintaining an equivalent combustion rate with a low combustion temperature.

Poisoning of automotive methane oxidation catalysts by silicon compounds

Biomethane production offers a viable option to meet the growing demands of renewable energy sources in the transportation sector. However, biomethane contains trace amounts of impurities, such as siloxanes, that can potentially hinder the methane oxidation catalyst performance and lead to excessive greenhouse gas emissions. To investigate this potential problem, the commercial methane oxidation catalysts were poisoned in a laboratory setup with two common siloxanes found in biomethane and with their combustion product, silica. The performances of the clean and poisoned methane oxidation catalysts were measured under realistic lean-burn conditions, after which the catalyst structural changes were determined using XRD, BET and CO chemisorption. The results revealed two different possible poisoning mechanisms. First, both tested siloxanes poisoned the catalysts by migrating selectively on top of the active metal sites and became combusted to form amorphous silica. This caused the methane oxidation performance to drop rapidly with relatively small amounts of siloxane used. In contrast, a significantly higher dose of silica was required to produce a similar poisoning effect because its poisoning mechanism is based on nonselective deposition and physical blocking of the catalyst surface.

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.