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I'm looking at building a sustainable energy project that will convert biogas (methane) into electricity. I haven't found any biogas or methane electric generators for sale. It seems that there are some projects that are using generators to convert the methane gas into electricity, however, I can't tell if this is special kind of generator and I just don't know what it's called or if it is a natural gas or propane generator.

So will off the shelf natural gas or propane generators also run off of methane?

Also, how would I go about trying to calculate how much methane it would take to run the generator to produce enough electricity to say charge a cell phone or another small appliance?

Thanks!

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  • $\begingroup$ You'll need to consider the ignition temperature, the compression ratio (if any) and pressure flash point, the oxygen mix ratio, the effect on your exhaust pipe material of the byproducts of each reaction. And that's just for starters. $\endgroup$
    – Carl Witthoft
    Commented Apr 4, 2018 at 17:48
  • $\begingroup$ Standard available generators with internal combustion engines are way too big to be any efficient for your application. (charging a phone etc) I'd recommend looking up the stirling engine. It's a very simple concept, and they can be small; lots of modelbuilders have built them. You just make a little flame with your methane to run the engine, attach an alternator to it, and be done. They're also moderately efficient. $\endgroup$
    – Bart
    Commented Apr 4, 2018 at 17:56
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    $\begingroup$ I have my doubts, based on domestic gas heaters. Heaters designed to burn propane cannot be used safely to burn methane & vice versa. $\endgroup$
    – Fred
    Commented Apr 4, 2018 at 18:16

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natural gas is methane, so no problems there. an internal combustion engine that is equipped to operate on gasoline can be converted easily to run on gaseous fuels and the conversion kits usually contain a little fitment which allows the user to run the engine on their choice of either propane or methane.

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The average low heating value (LHV) of biogas is around 5.5 kWh/m³ for a methane % around 40-70%, and Propane has around 24.2 kWh/m³ of LHV, and the Natural gas (NG) has around 10 kWh/m³.

Gas LHV
Methane 40 - 70% 5.5 kWh/m³
Propane 24.2 kWh/m³
Natural gas 10.0 kWh/m³

As you can see, the Propane can be between 4 to 5 times better than biogas, ans NG is twice better than biogas.

If you want to know the amount of gas to charge a PC of 0.4 kW during 4 hours, you would need 1.6 kWh. Then, if the generator efficiency (eta_gen) is 20%, the equation is:
Energy (kWh) = LHV(kWh/m³) × gas_volume (m³) × 0.20

Gas_volume = Energy/(LHV × 0.2)
=1.6 kWh/(0.2x5.5)
=1.45 ≈ 1.5 m³.

If a cow can produce about 0.35 m³/cow*day, you would need about 4.3 cow ≈ 5 cow 🐮.

The engine (gasoline, propane or NG engine) should be modified to work with biogas and the amount of gas should be calculated according to the LHV of the new fuel.

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    $\begingroup$ Welcome to the EE.SE, Victor. I took the liberty of replicating your data into a table where the relative values can be compared more easily. If you don't like it then you can see the revision history by clicking the "edited xxx time ago" link and roll it back. For paragraph breaks you need 2 x <Enter>. To force a line break add two spaces at the end of the line. $\endgroup$
    – Transistor
    Commented Mar 5 at 22:44
  • $\begingroup$ Nice first answer Victor. Your gonna fit right in here. $\endgroup$
    – Phil Sweet
    Commented Mar 6 at 22:34
  • $\begingroup$ Thanks. I'm here to help. I wish I could have more time. $\endgroup$
    – Victor Sena
    Commented Mar 8 at 9:33
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It depends on the source of the methane. Coal bed methane is relatively sweet. Biomethane has all kinds of crap in it that makes it problematic from a combustion and corrosion standpoint. Engines won't like it and you'll probably have trouble finding tanks and lines and carburetors and such.

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