|Hydrogen Fuel Cells have their fans and their research projects, and the “Hydrogen Economy” will no doubt have a share in the world’s energy pie, but methane is an altogether more interesting fuel.
Some will dispute this, but methane is easier to store and transport than hydrogen gas, and will play a very important role in balancing electricity supply in networks that have a high proportion of variable and intermittent renewable generation.
|After energy conservation (through efficiency and waste reduction) and renewable electricity generation, the use of renewable gas with a high methane content can act as the stabilising third leg of a sustainable energy tripod – raising the upper limit of zero carbon energy possible.
Natural Gas is generally 80% to 85% methane, and many developed countries already have gas networks, gas appliances and power stations that can make use of renewable gas without modification. This is a pragmatic consideration that should not be ignored when choosing future fuels.
Next, renewable gas can maximise the energy that can be obtained from biomass. There are several decompositional pathways that can make use of biomass to produce biogas with relatively high methane content, mixed with carbon dioxide and other carbon-based components, and some hydrogen. High heat treatment of biomass can produce synthesis gas – or syngas, with a high carbon content. The total output from these two pathways can be put through the process of methanation to increase the methane content of the renewable gas, which improves its usefulness.
To really increase the amount of useful fuel however, there needs to be an additional source of hydrogen, which can optimise the output from the methanation of a mixture of syngas and biogas. This is where electrolysis comes in – using the under-utilised night-time renewable electricity from wind power to “reform” water into hydrogen and oxygen. With this additional hydrogen feedstock, methanation of a wide spectrum of carbon-intensive gases can produce high quality fuel.
If the output gas, with its high methane content, is burned in a standard gas power station, the waste carbon-rich gases can be captured and recycled into the methanation unit, making it fully renewable.
However, if the methanated gas is “washed” to be clean enough to use in vehicle engines, burning the fuel will create carbon-rich gas emissions – albeit part of the normal living carbon cycle since it came from biomass – so carbon neutral, although not fully renewable.
Audi’s e-gas project makes use of wind power to produce hydrogen from water for use in a methanation stage – but the other details of the fuel production are somewhat different :-