Biogas Digester Pressure

There is no structure on this Earth that has an unlimited structural pressure rating! Well, not made with real-world materials, anyway. What is this thing made of - unobtainium?

Just hypothetical I want to know what pressures can be reached though fermentation.

The maximum pressure that can be reached is the design pressure of the structure within which the fermentation is taking place.

Therefore assuming the digester is capable of pressures to say 100 psi the biogas could be compressed into a storage tank without a compresser. Does higher pressures have an effect on the fermentation process?

Why would they?

Ok, how many m3 of biogas would be compressed into a 25m3 receiver at 100psi?

Starting from where?

What are the starting and end temperatures, for example?

In modern sewage works, the gas is usually collected in a bag, where pressures will be low, typically a few inches water gauge. The bag acts as an output smoothing device that regulates the supply of gas to burners and engines.

So, why would 100psi be needed?

Start approx. 30deg. C maybe 10deg C at storage as it will displace head of water in storage tank.

That would be unusual.

Thanks i'll keep the lime comment in mind.

There are many variables. I know you're not talking about propane, but temperature comes into play.

assuming the digester's structural pressure limit is unlimited?

Assuming that the structural pressure is unlimited, then the maximum pressure of the sealed biogas is unlimited. It will continue building pressure until fermentation ceases. Again, this is not a static number, many variables come into play.

Assuming that you want to use the biogas as fuel, this number does not matter. The only number that matters is maintaining the proper vessel pressure to successfully bleed off the biogas to be burned as fuel. This would also vary according to fuel use. Just off hand I'm thinking that for cooking, you would want about 6PSI.

<...this number does not matter...>

Quite.

It is the structural design of the vessel that determines how and for how long fermentation takes place.

True, but we're assuming that the vessel can hold unlimited pressure, which is impossible, so it doesn't matter.

The OP should be searching for some real numbers.

From memory of the fermentation process, if the resultant gas is not drawn off it results in poisoning of the liquor. So with your sealed presure system I would sugest it would get to the stage that the fermentation stops.

The gas would be drawn off at approx. 100psi or less so am I correct in assuming because the gas is drawn off this would be acceptable and not retard production of the gas?

Thanks for the effort you have put in. I will explain fuller seeing how the discussion has progressed to this stage.

I am considering setting up a farm biogas production. Although we run beef cattle it would be time consuming to collect enough manure to feed a continuous batch biogas digester, so was considering a single batch digester using more fibrous material such as lucerne and some manure on the first layer to start the process.

I was hoping to keep labour input down by avoiding hands on operation as much as possible hence my thoughts on the initial design which I am researching as to its viability

Concept:- A sealed single batch digester fed with manure and lucerne, every 4-6 weeks, the gas feed to a 25m3 storage tank which is full of water supplied by a stock water tank (greater volume than the storage tank) on a hill 60m+ higher than the storage tank (60m of head gives approx. 86psi ) the water pipe is situated at the bottom of the storage tank. The digester feed line is also fed into the bottom of the storage tank and as the pressure builds up it would displace the water back up in to the stock water tank, this would also act as a pressure release. As the gas is bubbled through the water it would scrub the gas, also keeping the gas as it was removed at a constant pressure and avoid air getting into the storage tank. The stock water is constantly being changed due to stock usage and automatically replenished by fresh water from the river. The methane would be drawn from the top of the storage tank at a constant 100psi?

I haven't included non-return valves, bleed off's etc. in the above description as I need to know approx. how many m3 of biogas is compressed into a 25m storage at 100psi. Also is the used water harmful to stock and is fermentation effected at that pressure?

The toxins in the biogas would be scrubbed by the water. I can't imagine it would be good for the cattle...but only experiment and a year or two of testing would tell if the water would end up being pure enough to even dump back into the aquifer. It would be good for the gas though...what would come out would be able to be distributed and burned just as any natural gas would be.

I like your idea though. So you dont' want to "bottle" the stuff...just store it in a high tank.

Great!

That is helpful. The pressure would not be that high in such a tank, so a lot of the problems would be avoided. The hydrogen embrittlement problems (if any) would be confined to the digester tank itself.

A couple of the links in my previous post are to sites by people much more knowledgable than myself. I courteously refer to them. I suspect though that you will go with the tractor tire inner tube method. (I remember some great pictures of Prague during the '50s that had big bags containing gas on top of the busses. Ugly as heck, but beautiful in a renewable energy sort of way!)