# Methane leaks from biogas

Lately, I have been thinking about biogas and the impact leaks might have on climate. Biogas is mostly methane, and we know that methane has a much higher global warming potential than $\sf{CO_2}$. There will inevitably be leaks from biogas production plants, vehicles running on biogas, and other infrastructure.

In Norway, sorting of food waste has been introduced in many areas. This food waste is taken to a treatment facility where biogas is produced, and the leftovers from the biogas production are used as bio-fertiliser. The biogas is used to power buses and dustcarts in and around Oslo. The climate effect is then positive, since you are using a resource that would otherwise be wasted, potentially turning into methane at a landfill.

My question is: how large would the leaks have to be to counteract the positive effects?

• Wouldn't a lot of the biogas feedstock produce methane if allowed to decompose naturally? Mar 14 '17 at 20:23
• i dont think this have any positive or negative effect as no co2 is removed or added,lets say you have one ton of potatoes and convert this to methane,and use this methane to power a bus this releases the co2 that was in the ton of potatoes we started with.we now uses the byproduct from methane production as fertilizer to grow one ton of potatoes this removes the same amount of co2 as the bus released,so the cyklus can start again,if the methane is released to the atmosphere it will be converted to the same amount of co2 as if it was burned it only takes a little longer. Mar 15 '17 at 6:57
• @jamesqf Yes, and it has to be taken into account. My point is that: if you are replacing buses running on diesel, leaks will have an effect on whether biogas is better, neutral, or worse in terms of climate effect. Then again, some of the feedstock would decompose into methane if you were to dump it on a landfill. Many landfills attempt to deal with this problem by flaring the methane. It sort of muddies the picture, but I would think that the leaks from the biogas infrastructure would be lower than those from landfills. Mar 15 '17 at 13:44

how large would the leaks have to be to counteract the positive effects?

Trying a ballpark estimate ... biogas replaces natural gas. For simplicity let's assume the replacement is 1:1, one mole of methane from biogas* replaces one mole of natural gas. This also means that one mole of biomethane "saves" one mole of CO2.

Methane is about 25 times more intensive in terms of cliamte change than CO2. So if 100/25 = 4% leaks, the positive effect would be offset.

The factors influencing gas leakage, from biggest to smallest influence (this is mostly from memory):

• Retention time of substrate in gas tight system - depending on retention time in the digesters, the digestate could still have 10% or so of it's total biogas potential, when the digestate storage is open and the HRT is short. One of the later iterations of the German rennewable energy laws specifies 150 days retention time in the gas tight system for this reason & I think this is reasonable for many substrates.

• Leaking foil roofs - foil roofs are common on large tanks, especially the connection foil roof/digester is a common point for leakage. This study found leakage rates from plants between 0 and 20 Nm³/h (median 5-10 Nm³/h), size of plants was not given but probably around 500 Nm³/h biogas production.

• Methane slip - I don't have numbers to go by, but I think 0.5-1% of gas slip (unburnt gas in the exhaust) in IC motors is common. This regardless of wether you burn natural gas or biogas, AFAIK.

• Manure storage - some wastes will ferment on their own, esp. manure.

• Everything else - IMO the art of building gas pipes etc. is pretty well developed, if proper procedures are followed and proper materials used leakages from pipes should be neglible.

In summary, the methane loss of biogas plants may well be above the 4% cutoff point I mentioned above, or well below, depending on plant design and quality of building.

However, as you pointed out in comments to your question, you need to also look at what would happen to the feedstock without biogasification. Manure would ferment (but not as completely) in an open pit and emit gasses. Municipal wastes would be either incinerated (likely with additional fuel because of the high moisture content), composted or landfilled (landfilled wastes also ferment and produce biogas, if this goes to the atmosphere, is flared or used productivly depends on the site). I have worked in landfill supervision and design of biogas plants.

At this point, I lack good numbers to really compare biogasification to other disposal paths. Having worked in landfill supervision and design of biogas plants, I have more trust in a properly designed biogas plant to keep methane out of the atmosphere than in a typically, well maintained landfill. But I'm sure you can find shoddy biogas plants that leak, for the same amount of input, worse than some landfills!

Likewise, I'm still looking for hard numbers of fuel usage for incineration of biowastes, so I can't offer a good comparison here.

So IMO, while biogas from energy crops is highly questionable, biogasification of wastes is a net benefit if done properly.

*biogas is anything between 50 and 70% methane, the rest CO2 and traces.

• CAveat: If the biogas replaces Diesel (in a bus, common in sweden among other places), the balance may be diffrent and far more in favor of biogas - need to check carbon intensity for these fuels in vehicles
– mart
Mar 23 '17 at 6:55