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[6]: 2 Methane is a major source of water vapour in the stratosphere through oxidation; [7] and water vapour adds about 15% to methane's radiative forcing effect. [8] The global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe, and 28 in terms of its impact over a 100-year timeframe. [9] [10]
Hydrogen, being the lightest existing gas (7% the density of air, 0.08988 g/L at STP), seems to be the most appropriate gas for lifting.It can be easily produced in large quantities, for example with the water-gas shift reaction or electrolysis, but hydrogen has several disadvantages:
The heat needed for the reaction can also be GHG emission free, e.g. from concentrated sunlight, renewable electricity, or burning some of the produced hydrogen. If the methane is from biogas then the process can be a carbon sink. Temperatures in excess of 1200 °C are required to break the bonds of methane to produce hydrogen gas and solid carbon.
Methane has a limited atmospheric lifetime, about 10 years, due to substantial methane sinks. The primary methane sink is atmospheric oxidation, from hydroxyl radicals (~90% of the total sink) and chlorine radicals (0-5% of the total sink). The rest is consumed by methanotrophs and other methane-oxidizing bacteria and archaea in soils (~5%). [7]
Hydrogenotrophic methanogenesis – archaea oxidize hydrogen with carbon dioxide to yield methane and water. 4H 2 + CO 2 → CH 4 + 2H 2 O. While acetoclastic methanogenesis and hydrogenotrophic methanogenesis are the two major source reactions for atmospheric methane, other minor biological methane source reactions also occur.
Additionally, if the water table is lowered significantly enough, then the wetland can actually be transformed from a source of methane into a sink that consumes methane. Finally, the actual composition of the original wetland changes how the surrounding environment is affected by the draining and human development.
Sewer gas can contain methane and hydrogen sulfide, both highly flammable and potentially explosive substances. As such, ignition of the gas is possible with flame or sparks. [7] The methane concentration in open sewers is lower (7 to 15 ppmv) than the closed drains (up to 300 ppmv) in samples collected 2 cm (0.8 in) above the level of sewage. [8]
The exact mechanism is still a topic of debate but the most widely accepted theory is that the archaea use the reversed methanogenesis pathway to produce carbon dioxide and another, unknown intermediate, which is then used by the sulfate-reducing bacteria to gain energy from the reduction of sulfate to hydrogen sulfide and water.
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