Biogas Energy Storage

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I am always on the lookout for energy storage solutions, so I am very interested in this aspect of Mich Hein’s (Nidus) “Renewable Natural Gas” lecture (Coursera (MOOC): Our Energy Future, U of San Diego, Oct 2014).

Biogas contains approximately 50% methane and 50% carbon dioxide.  Biogas is a natural product of organic material decomposition.  Commercially biogas is produced in anaerobic digesters (food, human and animal waste) and landfill gas collection.

Methane is a very potent greenhouse gas, about 20 times more so than CO2, but less abundant or long lived in the atmosphere.

Harvesting biogas has the dual benefit of preventing it vent to atmosphere and it’s use as a green fuel.  Since the feedstocks are not fossil fuels, but derived from recent plant growth, it is not adding carbon to the atmosphere, so long as the source plant matter is replaced by replanting.

Upgrading of the biogas CO2 component to Methane (CH4) which requires energy input can bring the total methane content up to 98%, which is comparable to that of natural gas (fossil fuel).  This seems to have great potential in several ways:

  • Use excess renewable energy from wind, tidal and solar (possibly nuclear) when demand is low to upgrade CO2 to methane, thus storing energy in high quality biogas.
  • Add value to raw low grade biogas.
  • Bring quality of biogas up to conventional natural gas levels for direct substitution into existing infrastructure.
  • Displacement of fossil fuel natural gas.
  • No need to alter existing gas fuel infrastructure or equipment.

I am most enthusiastic about the storage capability, which on its own could be used to store the excess energy from renewables and nuclear electricity production, and used in gas turbine plant to provide electricity on demand.

Prof Mayfield, in the follow-up lecture, proposed that instead of upgrading the CO2 component of biogas, other sources of CO2, conventional power-plant stacks for example, could be upgraded to methane in a similar manner.

It seems possible that CO2 from power stations could be continuously collected, upgraded as an energy storage solution in tandem with renewables, and used to generate electricity.  Some number crunching and flow models would help to determine the viability of such a scheme.

As always, the economics of these technologies would be greatly improved with a realistic tax on carbon emissions.

Whether upgraded biogas can significantly displace natural gas in sufficient quantities to effectively reduce fossil carbon emissions on a significant scale is less clear.  Are there enough sources of biogas raw materials to feed demand?

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