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It is basically possible to upgrade Biogas to something very close to natural gas, and then use it to replace said gas. Biogas to grid is a mature technology now, with about 100 active plants in Germany alone. It is also often discussed to use the upgraded biogas as a vehicle fuel, replacing compressed natural gas (CNG).

If the fueling station is taking the gas from the grid, it does not make a difference if it is natural gas or biomethane. I'm looking for installations where the gas is produced, stored, upgraded and filled into vehicles on site. As we are talking about gas pressures upwards of 200 bar, energy efficient handling of the gas is not trivial. Any pointer to reference material that goes into the gas management would be greatly appreciated.

Edit to add (after Energy Numbers Answer):
Gas upgrading to grid-qualioty is to me an understood problem. The headaches arise with the vehicle fuel application. While a gas to grid installation can basically feed gs into the grid as it is produced, an installation for vehicle fuel would face a very uneven demand (the production is always very steady).
Depending on gas upgrading technology, the gas is delvered at practically air pressure to 5-15 bar. Vehicle fuel needs 200-250 bar. Compressed Natural Gas fueling stations do an interesting juggling act with 3 banks of gas tanks (hih-, mid-, ow pressure) to minimise the energy expenditure in gas compression - and they have the gas grid that delivers on demand.
With a constant supply (that can't be switched off), you'd need to store that gas between the tanking either at low pressure (and we may be talking 1000 m³/h), or compress it and store it at high pressure - and then minimize the loss of energy when the compressed gas is expanded into a vehicle tank. Also, high pressure storage costs serious money.

So what is a cost- and energy-effcient way to match supply and demand here?

  • The question may be borderline on topic here, someone once claimed that environmental engineering questions should be on topic here too - Let's see it as a testing baloon how far from everyday life, and into engineering we want to go here. – mart Aug 26 '13 at 14:33
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    I'm voting for on-topic. Let's see if anyone can help with an answer. – Laizer Aug 26 '13 at 20:43
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exploring the problem

I've asked around on the Claverton Energy Group for some help on this. Thanks to John Baldwin, I learnt that biogas tends to be about 65% methane, with a lot of carbon dioxide, and some oxygen and hydrogen sulphide. Whereas gas as supplied for industrial, commercial and residential use is 90%+ methane, plus gases such as propane and butane to boost the energy density, and very very low levels of oxygen and hydrogen sulphide,

To replace CNG, biomethane needs to go through purification, to bring it up to similar levels of quality. That means raising the methane quantity to 90%+, removing almost all of the oxygen and hydrogen sulphide.

It may also be necessary to add a tracer scent, as methane is odorless and colourless.

Landfill gas has to undergo similar treatment.


towards finding the technical solution

The keywords for searches in the technical literature seem to be: H2S CO2 methane purification

From that, the most accessible documents I've found so far are these:

This draft document from British Columbia's Environmental Protection Division (in the Ministry of the Environment) (pdf) deals with the cleaning (purification) of biogas produces from on-farm anaerobic digestion; the next chapter in the same document deals with biogas upgrading (pdf), to boost the energy density and raise its quality to the specification required for CNG and for injection into the gas grid.

If you've questions on the specifics of the chemistry, please do ask over on the Chemistry Stack Exchange


important caveat

Do bear in mind that methane is a very powerful greenhouse gas, and that leakages, even at a small percentage rate, can make biogas a damaging global pollutant.

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    I find this caveat hugely troubling, at least in the context of "natural gas" even being touted as better than coal or oil. Mention at Environmental Defense Fund website. – Nate Sep 6 '13 at 4:57
  • Thanks for the detailed answer, I'll look into the linked documents. I work in the field and I'm aware of the challenges of purification. One additional headache with the use as Vehicle fuel (opposed to gas to grid) is that you have a constant supply and a very varyign demand, so you need lots of storage at high pressures (expensive!) plus compression is energy intensive so you have an interesting juggling act at the best of times. I'll think about how I can improve my question. – mart Sep 24 '13 at 7:19
  • Oh, and re. your caveat: with all purification processes except amine scrubbing you get a certain percentage (1-5%) of the input methane in the CO2 rich off-gas. Depending on where you are you will have to burn it (difficult, low CH4, requires RTO) or may be allowed to simply vent it. – mart Sep 24 '13 at 7:22

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