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Biogas seems to be a really great idea but I'm wondering how much energy you get out vs. what you need to put in. I know natural gas is around 10.0 and I was wondering how biogas stacks up by this metric.
So if you have any source with the EROI for biogas or other related information or similar measurements, that would be awesome, thanks!
First of all, EROI depends on the calculation method (what is included and excluded), on the production method, and on the used resources. This means that there isn't one uniform EROI for biogas. It can vary from researcher to researcher and from site to site. This also means that it's often possible to find very positive and very negative numbers, depending on who did the research (as you can also see in the varying numbers in this answer on EROI of solar panels for example)
Now some numbers for biogas.
This 2013 paper mentions an EROI of 3.5 for a biogas power plant. Sadly the paper lacks details on the production process and used materials.
This paper from 2014 covers biogas production from different combinations of pig manure and jatropha press-cake and they estimate the EROI to be between -0.16 and 0.32, depending on the usage rate of both resources. The researchers do mention that they didn't take into account any EROI from by-products of the biogas production.
Another 2014 paper estimates an EROI of biogas from micro-algal biomass to be between 2.3 and 3.4
This is in contrast with this 2015 paper that mentions an EROI of 0.6 in an algal biomass production plant in Italy.
