Méthane atmosphérique | EPFL Graph Search

vignette|redresse=1.2|Augmentation annuelle de la moyenne mondiale du méthane atmosphérique depuis 2004. Le méthane atmosphérique désigne la quantité de méthane présent dans l'atmosphère terrestre. Sa concentration est intéressante car il s'agit de l'un des gaz à effet de serre les plus puissants et qu'elle est en hausse depuis plusieurs décennies. Le potentiel de réchauffement planétaire du méthane sur 20 ans est de 84, c'est-à-dire que sur une période de 20 ans, il piège 84 fois plus de chaleur par unité de masse que le dioxyde de carbone () et 105 fois plus si l'on tient compte des interactions avec les aérosols. Les concentrations mondiales de méthane sont passées de 722 parties par milliard (ppb) à l'époque préindustrielle à en 2021, une augmentation d'un facteur de 2,6 et la valeur la plus élevée depuis au moins . Sa concentration est plus élevée dans l'hémisphère nord car la plupart des sources (naturelles et humaines) sont situées sur terre et l'hémisphère nord a plus

Methane oxidation pathways in Lake La Cruz

Corinne Jegge

Even though lakes only occupy a small percentage of Earth's surface, they represent a natural source of methane, contributing substantially to methane emissions and ultimately to global warming. There is extensive evidence that methane emissions from lakes are mitigated by its oxidation. However, many uncertainties concerning biotic controls on methane release from lakes remain, especially in zones where oxygen is depleted and other electron acceptors might be important. In this study, biological methane oxidation was investigated in the water column of Lake La Cruz, a small karstic lake located in Central Eastern Spain near the city of Cuenca. Permanent stratification and unusually high concentrations of dissolved iron(II) make this lake ideal for studying methane oxidation in the absence of oxygen, particularly the possibility of this process coupled to iron reduction. The physical and chemical properties of Lake La Cruz were investigated, including the presence of methane and possible electron acceptors. Incubation experiments with 13C-methane were carried out to measure methane oxidation rates, test possible electron acceptors and other coupled processes. Highest methane oxidation rates were measured at the oxic-anoxic boundary, where aerobic oxidation dominated. Moreover, the aerobic pathway also seemed relevant below the oxycline, likely triggered by in-situ production of oxygen via photosynthesis. Evidence suggests that nitrate, nitrite and iron also function as electron acceptors for methane oxidation, whereas manganese and sulfate are unlikely oxidants. Methane oxidation coupled to denitrification seemed to proceed in close vicinity to aerobic methane oxidation and was likely limited by nitrate/nitrite availability and competition with denitrifiers. Iron mediated methane oxidation only appeared relevant in lower parts of the lake where both oxygen and nitrate were depleted. Though known aerobic methanotrophs were present and appeared to mediate methane oxidation to some degree, the involvement of unknown groups of aerobic and also anaerobic methane-oxidizers is probable. Methane emissions to the atmosphere are effectively mitigated by a seemingly complex interplay of aerobic and anaerobic processes in Lake La Cruz.

2015

Methane accumulation and oxidation under-ice in various lakes in Quebec

Amélie Marie Corinne Séchaud

Les lacs, en tant que grands émetteurs de méthane (CH4) dans l'atmosphère, sont d'importants composants du cycle global du carbone. Malgré le comportement saisonnier marquée du méthane, peu de choses sont connues sur les processus hivernaux régulant le méthane dans les lacs couverts de glace. Ainsi, nous évaluons l'étendue de l'accumulation et l'oxydation du méthane dans 10 lacs gelées au Sud-Est du Québec, chaque mois entre janvier et mars 2018. Pour cela, nous examinons à la fois temporellement et verticalement les variations de concentration et signature isotopique du méthane, ainsi que les caractéristiques physiques et chimiques des lacs. Nous avons observé des concentrations de méthane relativement faibles (

2018

Emission reduction by targeted transient operations in three-way catalysts for heavy-duty natural gas applications

Moyu Wang

To curb the severe effects of climate change, our society needs to radically reduce its CO2 footprint. For the heavy-duty sector, where electrification is difficult, alternative fuels can be the solution. Methane-fueled engines have lower energy-specific CO2 emissions than conventional Diesel-fueled engines. Furthermore, they have the potential of becoming carbon-neutral, when combined with bio-methane/synthetic methane. However, oxidation of unburnt methane in the exhaust gas poses a challenge for aftertreatment systems. The aim of this thesis is to investigate the mechanism of methane abatement and to reveal methods to reduce methane and other hazardous gas emissions. The majority of the experiments were conducted directly on an engine test bench, which is rarely seen in studies focusing on catalytic reaction pathways. Initial investigations focused on methane abatement under steady state and Î»-step transitions. Under steady state, the presence of oxygen was identified as a prerequisite for methane conversion. Reacting with oxygen is the only methane conversion pathway. However, after step transition from oxygen excess conditions (slightly lean) to oxygen-poor conditions (slightly rich), high methane conversion was observed under rich conditions with no oxygen available. This high conversion was attributed to steam reforming (SR), which was activated by the step transition. The SR reaction rate decreased over time when staying at rich conditions, until full deactivation. Investigations in the lab-scale model gas reactor confirmed this analysis. In addition, the reason for SR deactivation was identified by DRIFTS measurements as the accumulation of carbonates on the catalytic surface, blocking the active sites. Based on the identified importance of the SR reaction, targeted Î» oscillations across stoichiometry were introduced, in order to repeatably activate SR and achieve sustainable high methane conversion. During the rich parts of the oscillations, methane was converted via SR, while, during lean parts, the carbonates were periodically removed from the catalyst surface. With these oscillations, methane conversion has been significantly improved, in comparison to steady state. In parallel, a numerical model has been developed in order to simulate the catalyst behavior under oscillating conditions. The model provided insights on the reaction pathways and their distribution along the catalyst axis. The catalytic activity of the different Platinum-group metals has been investigated for the identified reactions. Various catalysts of different compositions were tested under cold start, Î» oscillations and quasi-steady state conditions. Both Pt and Pd activated SR reactions, however SR attenuation was faster in Pt catalysts. In lean conditions, Pt exhibited higher methane oxidation. Rh was identified as important for enhancing NOx reduction and lowering NH3 emissions. The combination of all three metals has improved the overall catalyst performances. In the final part of the thesis, a special aftertreatment system was investigated. It combines a Pd/Rh catalyst subject to stoichiometric conditions with a Pt oxidation catalyst subject to lean conditions. In the Pd/Rh catalyst, methane was removed via Î» oscillations. In the Pt catalyst, the remaining CO, H2 and NH3 were oxidized. The setup provided a novel perspective in reducing the overall environmental impacts.

EPFL2022