Lean Methane Research Articles

AOx–MnOx (A = Ni, Cu, Fe, or Co) Nanocatalysts Fabricated by the Mechanochemical Preparation Method for Lean Methane Catalytic Combustion Assisted by a DBD Plasma Reactor

AO<i>_x</i>–MnO<i>_x</i> (A = Ni, Cu, Fe, or Co) Nanocatalysts Fabricated by the Mechanochemical Preparation Method for Lean Methane Catalytic Combustion Assisted by a DBD Plasma Reactor

Highly active SiO2-supported palladium nanoparticles prepared by a modified ammonia evaporation method for lean methane oxidation

By optimizing the ammonia evaporation (AE) method, highly active palladium nanoparticles were successfully deposited on fumed silica (f-SiO2) and exhibited excellent catalytic activity for the catalytic combustion of methane (CCM). It was manifested that the timing of ammonia introduction was of great importance in determining the particle sizes and electronic states of palladium nanoparticles, the desilication degree of SiO2 support material, and the redox properties and methane affinity of the Pd/f-SiO2-AE catalysts. The optimal Pd/f-SiO2-AE-2 catalyst, which converted 90% of lean methane at 333 °C at GHSV of 30,000 h − 1, performed better than its AE-1- and IM-prepared counterparts. In combination with complementary characterizations, DFT calculations and designed experiments, it was revealed that the uniformly dispersed palladium nanoparticles endowed the Pd/f-SiO2-AE-2 catalyst with more accessible sites. Moreover, the presence of Pd4+ species effectively promoted the methane adsorption capacity of the main active PdO phase, which was another critical reason for the outstanding performance. In addition, the necessity of the ammonia evaporation step was also elucidated by designed experiments. More importantly, taking the universality of this optimized AE method into consideration, our work thus put forward an effective and simple procedure to fabricate highly-active palladium nanoparticles for catalyst designers.

Lean methane combustion over zeolite-supported Pd catalysts: Structure-performance relationship and deactivation mechanism

Zeolites are a promising support for Pd catalysts in lean methane (CH4) combustion. Herein, three types of zeolites (H-MOR, H-ZSM-5 and H-Y) were selected to estimate their structural effects and deactivation mechanisms in CH4 combustion. We show that variations in zeolite structure and surface acidity led to distinct changes in Pd states. Pd/H-MOR with external high-dispersing Pd nanoparticles exhibited the best apparent activity, with activation energy (Ea) at 73 kJ/mol, while Pd/H-ZSM-5 displayed the highest turnover frequency (TOF) at 19.6 × 10−3 sec−1, presumably owing to its large particles with more step sites providing active sites in one particle for CH4 activation. Pd/H-Y with dispersed PdO within pore channels and/or Pd2+ ions on ion-exchange sites yielded the lowest apparent activity and TOF. Furthermore, Pd/H-MOR and Pd/H-ZSM-5 were both stable under a dry condition, but introducing 3 vol.% H2O caused the CH4 conversion rate on Pd/H-MOR drop from 100% to 63% and that on Pd/H-ZSM-5 decreased remarkably from 82% to 36%. The former was shown to originate from zeolite structural dealumination, and the latter principally owed to Pd aggregation and the loss of active PdO.

Surface graft for effective PdO nanoparticles anchoring on Beta zeolite with outstanding lean methane combustion performance

A post-synthetic modification strategy for anchoring PdO nanoparticles on the Beta zeolite surface and obtaining stable and highly dispersed PdO sites with abundant active Pdn+ (0 < n < 2) species was reported. The modified Beta supported Pd catalyst with a 0.94 wt% Pd loading amount shows (>) 99% methane conversion at 375 °C with a space velocity of 30,000 mL h−1 g−1 and a satisfied long-term (>30 h) CH4 oxidation stability at 340 °C. Such a post-synthetic strategy provides a new way to disperse and stabilize noble metal particles on zeolite support for harsh catalytic process.

The nature of silica-to-alumina ratio effect on lean methane oxidation over Pd/Beta zeolite catalysts

The silica-to-alumina ratio (SAR) effect on lean methane oxidation over Pd/Beta catalysts was investigated. Light-off test, steady-state performance and kinetic analysis demonstrate the activity of Pd/Beta catalysts shows a volcano-shaped dependence on SAR. Pd/Beta402 presents the highest activity among all samples, regardless of the presence of water. By the employment of H2-TPR, CO-chemisorption, STEM-EDS, TGA and NH3-TPD, it is found that the activity of Pd/Beta catalysts is essentially influenced by their acidity, which is the nature of the SAR effect on methane oxidation. Increasing SAR leads to lower acidity, resulting in less formation of inactive Pd2+ and higher hydrophobicity which benefit methane oxidation. However, the larger PdO particles are formed at the same time, leading to a decrease in the amount of exposed PdO which is detrimental to methane oxidation. This work can guide the design of state-of-the-art zeolite-supported Pd-based methane oxidation catalysts.

Investigating mass transfer coefficients in lean methane combustion reaction through the morphological and geometric analysis of structured open cell foam catalysts

In this work, we characterized ceramic open cell foams made of zirconia, silicon carbide and alumina of different nominal pore densities (30–45 ppi) using X-ray computed microtomography. This technique allowed the comprehensive and quantitative extraction of morphological and geometrical characteristics of the structures (pore size, strut diameter and length, node diameter, open porosity and specific geometrical surface area). An empirical model was proposed to determine the specific surface area from parameters easily accessible with standard laboratory equipment. Moreover, lean CH4 combustion tests were performed on ceramic foams coated with PdO/Co3O4 at 3 wt%. Mass transfer coefficients of the foams were measured by monitoring the oxidation reaction under diffusional control conditions. We derived a dimensionless correlation of the type Sh = A∙Rem∙Sc1/3. Finally, the pressure drop across the foams as a function of surface velocity was evaluated and compared with various experimental data and models available in the literature.

Large-eddy simulation of dual-fuel spray ignition at varying levels of methane diluted ambient oxidizer using FGM

In the current work, the Flamelet Generated Manifold (FGM) method is applied with large-eddy simulation (LES) to investigate the effect of methane on dual-fuel (DF) spray ignition. The diesel surrogate n-dodecane is injected as the so-called pilot fuel into selected lean methane–air mixtures, ranging from ϕCH4=0 to ϕCH4=0.75, at engine relevant conditions. The operating conditions are those of the completely characterized Engine Combustion Network (ECN) Spray A configuration, for which the modeling approach adopted in the present study was extensively validated. The specific purpose of this study is to extend and validate the FGM approach for dual-fuel combustion. In order to understand the interplay of chemistry and mixing, the ignition behavior of selected cases is investigated. It is found that both low and high temperature combustion (LTC and HTC, respectively) are increasingly retarded by higher values of ϕCH4, while the induction time between LTC and HTC is relatively insensitive compared to the ignition delay time (IDT). Analysis reveals a more prominent role of mixing for increased ϕCH4. The development of LTC and HTC are quantitatively analyzed for different cases. The transition from LTC to HTC is found to be highly correlated with the evolution of lift-off length (LOL), which on its turn is seriously affected by ϕCH4. The local flame behavior is analyzed via chemical explosive mode analysis (CEMA), suggesting a clear flame propagation due to diffusion towards lean mixtures after the ignition of the pilot fuel. Besides, it is found that diffusion helps to stabilize the flame in leaner mixtures, which is more important in DF combustion. The results show FGM to be a promising tool in modeling the DF sprays.

Dynamic tracking of exsolved PdPt alloy/perovskite catalyst for efficient lean methane oxidation

Supported Pd based catalysts are considered as the efficient candidates for low-carbon alkane oxidation for their outstanding capability to break C-H bond. Whereas, the irreversible deactivation of Pd based catalysts was still frequently observed. Herein, we reinforced the extruded Pd nanoparticles with quantitive Pt to assemble the evenly distributed PdPt nanoalloy onto ferrite perovskite (PdPt-LCF) matrix with strengthened robustness of metal/oxide support interface. We further co-achieved the enhanced performance, anti-overoxidation as well as resistance of vapor-poisoning in durability measurement. The operando X-ray photoelectron spectroscopy (O-XPS) combined with various morphology characterizations confirms that the accumulation of surface deep-oxidation species of Pd4+ is the culprit for fast activity loss in exsolved Pd system, especially at high temperature of 400 °C. Conversely, it could be completely suppressed by in-situ alloying Pd with equal amount of Pt, which helps maintain the metastable Pd2+/Pd shell and metallic solid-solution core structure. The density function theory (DFT) calculations further buttress that the dissociation of CH was facilitated on alloy/perovskite interface which is, on the contrary, resistant toward O–H bond cleavage, as compared to Pd/perovskite. Our work suggests that the modification of exsolved metal/oxide catalytic interface could further enrich the toolkit of heterogeneous catalyst design.

Ageing Studies of Pt- and Pd-Based Catalysts for the Combustion of Lean Methane Mixtures

This paper presents results obtained for the thermal and hydrothermal ageing of seven commercial precious metals-based catalysts for the combustion of methane. Experiments are performed in a large excess of oxygen representing lean conditions. Temperatures used are those typically found in lean burn compression ignition engines. The precious metals used were platinum, palladium and rhodium, present either singly or in combination. The most active catalyst contains a platinum and palladium mixture, with palladium being dominant. This catalyst was also the least affected by both thermal and hydrothermal ageing. The second most active catalyst contained only palladium, but this catalyst also demonstrated more susceptibility to ageing. The least active catalyst contained only platinum, although this catalyst was also the least affected by hydrothermal ageing. The addition of rhodium to either palladium or platinum–palladium catalysts caused a more rapid loss in activity at higher temperatures, although the loss in activity at lower temperatures was similar in magnitude to those catalysts without rhodium. In some cases, cycling the reactor temperature between high and low restored some activity to the catalyst. In all cases, the catalyst activity was observed to be lower in the presence of water, after both thermal and hydrothermal ageing.

Numerical investigation of extracted position characteristics in porous media with lean methane and air premixtures combustion

Hydrogen production through steam generated from low-grade fuel recovery is considered a prospective method. To maximize the energy recovery, the influence of the heat exchanger position on the heat extraction stability of lean methane-air premixtures combustion in a porous burner has been numerically explored. The extracted position coefficient (EPC) is defined and a heat extraction model is validated through the experimental findings. It is shown that extracted position has a significant influence on the temperature profile, heat transfer capacity, heat loss as well as heat recovery rate. When the EPC is lower than 0.18, the filtration burner shows worse stability with great heat loss, the heat transfer behavior of the heat exchange tubes is severely weakened and heat recovery efficiency is merely about 13%. The optimal heat recovery position is recommended in the EPC range of 0.36–0.55, which has shown excellent heat extraction performance and accelerated the process of acquiring a quasi-steady state. Within this EPC range, the fluctuation of outlet temperature difference can be stabilized within 0.2 K, which maximizes the heat flux of heat exchangers, and heat recovery efficiency of about 70%–89% recovered with a concentration of 0.4%–0.8%.

The high activity of Co-Mn-based solid solution catalysts for lean methane combustion

Developing non-noble metal-based catalysts for methane combustion with high catalytic activity and stability is important in the new context of a changing global energy landscape. In this study, we reported an Co5Mn1-O catalyst achieved 90% conversion of methane to CO2 at 305 °C, which was about 60 ℃ lower than the catalysts reported. The synthetic strategy via lattice distortion that inhibits the deep oxidation of Mn2+ with prepared a highly catalytic reactive Co-Mn-based solid solution material. The results show that introduction of Mn atoms into the Co3O4 spinel structure caused lattice distortion, which modulated the oxygen coordination environment and oxygen vacancies of the catalyst, and the Co5Mn1-O (Optimum) catalyst prepared under optimized conditions obtained excellent methane activation capacity and lattice oxygen mobility. The higher lattice oxygen mobility of the Co5Mn1-O catalyst facilitated the adsorption, activation, and oxidation of methane, especially the rate-determining step of methane oxidation, the conversion of CH4 to CH3-. The conversion rate decreases slightly in the presence of 5 vol% or 10 vol% H2O, but it also still maintained good catalytic activity and stability under high temperatures and long-term reactions. In addition, in situ infrared specra was used to explore the reaction mechanism of methane combustion under real reaction conditions.

Experimental investigation on flame stability and emissions of lean premixed methane–air combustion in a developed divergent porous burner

Flame stability and pollutant emissions are issues of the greatest concern for low-calorific-value gas combustion. Although the conventional straight porous burner is conducive to improving the flame stability, it still cannot meet the industrial demand. To improve the flame stability and reduce the pollutant emissions during fuel-lean combustion, a divergent double-layer porous burner was creatively proposed in this study. The influences of the inlet velocity and equivalence ratio on the flame stability, temperature distributions, the emissions were experimentally investigated in detail. The results showed that the flame front gradually moved downstream, the preheating zone length increased as the inlet velocity increased. Compared with the traditional straight porous burner, the divergent porous burner extended the blowout limit by 300%. Since the burner was operated under fuel-lean conditions (the equivalence ratio range was 0.45–0.6), the CO emissions were relatively high when the inlet velocity was less than 0.4 m/s. When the inlet speed was equal to or greater than 0.4 m/s, the maximum CO emissions concentration was 132 ppm. The burner showed superior performances in terms of NO emissions, with emissions concentrations of less than 20 ppm under all test conditions. In summary, the great advantage of the divergent porous burner lies in extending the flame stability limit. For clean combustion it is recommended that the burner be operated at a higher velocity (greater than or equal to 0.4 m/s) to obtain lower CO and NO emission levels.

Reaction gas treatment promoting activity and stability of PdO for lean methane oxidation over phosphorus modified Pd/Al2O3 catalysts

Pretreatment under specific atmosphere is a general strategy to activate catalysts. How physicochemical properties of supports are modulated during activation process and their influence on active sites are still the ongoing topic. Herein, the effect of reaction gas treatment on performance of phosphorus modified Pd/Al2O3 catalysts in lean methane oxidation was studied. The pretreated Pd/P-doped Al2O3 catalyst exhibited a full conversion of CH4 at ∼400 °C, excellent stability under both dry and wet conditions. Characterization results reveal that the uniform and stable P species in Al2O3 enhanced the metal-support interaction and availability of oxygen in support. Under reaction gas condition, PdO-support connection was further promoted to favor generation of oxygen vacancies on support with the help of CH4, leading to weakened Pd–O bond, facile reformation and increased fraction of PdO. These facilitated formation of intermediates and surface dehydroxylation and mitigated the deactivation caused by thermodynamic decomposition of PdO.

Pore-scale Numerical Simulations on the Lean Concentration Methane Combustion in Porous Medium with Gyroid Triply Periodic Minimal Surfaces Structures

ABSTRACT In this work, a series of porous medium with Gyroid Triply Periodic Minimal Surfaces (G-TPMS) structures were established for lean concentration methane combustion. Based on pore-scale numerical simulations, the influence of TPMS parameters on porous media combustions, as well as the shape of flame surface in different TPMS structures, heat transfer characteristics between solids and fluids, the radiation characteristics and heat flux on the solid surface were investigated. Results show that, under the same inlet flow rate and equivalence ratio, the value and axial location of the maximum average fluid temperature will be changed due to the difference in porosity and flow resistance of each structure. For the G-TPMS structures’ porous medium, the change of structure period had a greater impact on the combustion compared with the wall thickness. And, in the G-TPMS structures’ porous medium, the flame surface always shows a finger structure. The flow rate that shows the same increase trend with the fluid temperature in the preheating region, which has a periodic change like the porosity in the axial direction. Based on the results, it can be seen that the region of solid-to-solid surface radiative heat transfer and fluid-solid heat flux on the interface was also changed due to the influence of flame front position.

Effect of highly dispersed Co3O4 on the catalytic performance of LaCoO3 perovskite in the combustion of lean methane

In this work, a series of nano LaCoO3 perovskite catalysts were effectively synthesized by a sol-gel method through modulating the La/Co molar ratio. These catalysts were characterized by ICP, XRD, N2 sorption, H2-TPR, O2-TPD, and XPS, and their catalytic performance in the lean methane combustion were then investigated. The results indicate that highly dispersed Co3O4 nanoparticles on the LaCoO3 perovskite catalysts are beneficial to the activation of CH4 at a low temperature, while the La-Co-perovskite bulk phase can provide a large amount of lattice oxygen, which can enhance the reaction rate of methane combustion and the catalytic stability at a high temperature. Through altering the La/Co molar ratio, the dispersion of Co3O4 nanoparticles in the La-Co-perovskite catalyst can be effectively modulated, to achieve the concurrence of low-temperature activity and high-temperature stability in the lean methane combustion. In particular, the La0.9CoO3 perovskite catalyst with a La/Co molar ratio of 0.9 exhibits excellent performance in lean methane combustion, with a light-off temperature of 382 °C at a space velocity of 30000 mL/(g·h); in addition, the methane conversion holds above 95% even after 72 h on stream. These results should be helpful for the development of low-cost, high-activity and high-stability catalysts for lean methane combustion.

Effect of hydrogen addition on the consumption speed of lean premixed laminar methane flames exposed to combined strain and heat loss

This study presents a numerical analysis of the impact of hydrogen addition on the consumption speed of premixed lean methane-air laminar flames exposed to combined strain and heat loss. Equivalence ratios of 0.9, 0.7, and 0.5 with fuel mixture composition ranging from pure methane to pure hydrogen are considered to cover a wide range of conditions in the lean region. The 1-D asymmetric counter-flow premixed laminar flame (aCFPF) with heat loss on the product side is considered as a flamelet configuration that represents an elementary unit of a turbulent flame and the consumption speed is used to characterise the effect of strain and heat loss. Due to the ambiguity in the definition of the consumption speed of multi-component mixtures, two definitions are compared. The first definition is based on a weighted combination of the consumption rate of the fuel species and the second one is based in the global heat release rate. The definition of the consumption speed based on the heat release results in lower values of the stretched flame speed and even an opposite response to strain rate for some methane-hydrogen-air mixtures compared to the definition based on the fuel consumption. Strain rate leads to an increase of the flame speed for the lean methane-hydrogen mixtures, reaching a maximum value after which the flame speed decreases with strain rate. Heat loss decreases the stretched flame speed and leads to a sooner extinction of the flamelet due to combined strain and heat loss. Hydrogen addition and equivalence ratio significantly impact the maximum consumption speed and the flame response to combined strain rate and heat loss. The effect of hydrogen on the thermo-diffusive properties of the mixture, characterised by the Zeldovich number and the effective Lewis number, are also analyzed and related to the effect on the consumption speed. Two definitions of the Lewis number of the multi-component fuel mixture are evaluated against the results from the aCFPF.

Methane Combustion over the Porous Oxides and Supported Noble Metal Catalysts

Methane is the most stable hydrocarbon with a regular tetrahedral structure, which can be activated and oxidized above 1000 °C in conventional combustion. Catalytic oxidation is an effective way to eliminate lean methane under mild conditions, and the key issue is to develop the catalysts with high efficiencies, good stability, and high selectivities. Catalytic combustion of low-concentration methane can realize the light-off and deep conversion at low temperatures, thus achieving complete combustion with fewer byproducts below 500 °C. This review article summarizes the recent advances in preparation of ordered porous oxides and supported noble metal catalysts and their methane combustion applications. The results reveal that the superior performance (good hydrothermal stability and excellent moisture- or sulfur-resistant behavior) is associated with the well-ordered and developed three-dimensional porous structure, large surface area, ultrahigh component dispersion, fast mass transfer, low-temperature reducibility, reactant activation ability, and strong interaction between metal and support. In addition, the development trend of porous oxides for industrial applications in the future is also proposed.

Intensified reactor for lean methane emissions treatment

AbstractThe Global Methane Pledge declared at the 2021 United Nations climate change conference (COP26) marked the world's commitment to eradicate methane emissions. Regardless of the source, methane emissions are typically generated in a lean composition (e.g., &lt;1% vol), remote and scattered. This work explores the use of an intensified reactor that implements the chemical looping principle to handle lean methane emissions. A model‐based framework is used to showcase the baseline performance of the proposed reactor in converting methane emissions using nickel‐based oxygen carriers. Sensitivity analysis of the reactor showed that the Ni percentage in the oxygen carrier, the feed air temperature, feed air to flared gas ratio, and oxidation to reduction duration ratio are the most deciding variables for reactor performance. The reactor is subsequently optimized to minimize the methane emitted, using a dynamic program with safety and operability constraints for the alternating redox process. With the optimal cycle strategy, we demonstrate that near‐complete methane conversion (&gt;98% methane conversion) can be achieved by the reactor without external heating.

Effect of Two Mechanisms Contributing to the Cyclic Variability of a Methane–Hydrogen-Fueled Spark-Ignition Engine by Using a Fast CFD Methodology

The effect of two mechanisms that contribute to the cyclic variability of a spark-ignition (SI) engine fueled with lean methane–hydrogen blends was examined. This assessment was performed using a research computational fluid dynamics (CFD) code, employing a fast methodology that allowed the simulation of several cases (three per mechanism and per case) and thus greatly reduced the computational time. The first mechanism is related to the local gas turbulent properties at the spark plug, and the second mechanism is related to the fuel mass variation per cycle. The engine performance and emissions results were processed using a linear regression approach, and then using a Monte Carlo–based approach to extract the main indicators such as the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) for each individual mechanism as well as for their combination. It was found that the contribution of the first mechanism is significant and it mainly affects the initial flame propagation process. On the other hand, the second mechanism affects the equivalence ratio and subsequently the whole combustion process with a high variability of CO emissions, and its uncertainty was accounted for by expanding its range.

Pd and Pd-Pt catalysts supported on SnO2 and γ-Al2O3: Kinetic studies of wet lean methane combustion

Kinetic investigation of methane combustion on hydrothermally aged SnO2 and Al2O3 supported 1 wt% Pd and 1 wt% Pd-Pt(1:1) catalysts is reported in the presence of 5 and 10 vol% H2O. First-order to CH4, activation energy of 143 kJ/mol and −1 order to H2O were observed for Pd/Al2O3. Both Pd-Pt catalysts demonstrated −1 order to water and activation energies of 128 kJ/mol, proposing the choice of support is inconsequential with Pt addition from the viewpoint of the activation energy. Pd/SnO2 illustrated −0.2 order to H2O. A rate law was suggested with two active sites, one affected by H2O with −1 order and activation energy of 140 kJ/mol, and the other unaffected by water with activation energy of 88 kJ/mol. Turnover frequencies at wet conditions at 425 degC were found in the order of Pd/Al2O3(“x”) < Pd/SnO2(24x) < Pd-Pt/ Al2O3(102x) < Pd-Pt/SnO2(275x). The Pd-Pt/SnO2 catalyst also demonstrated the least deactivation during hydrothermal ageing.