Gas Pressures and the Resistance of Materials (Biodigester Generates Methane)

[Steven Bolgiano]

TL;DR

I've confused myself on Gas Physics. As to Pressure Equalization and Force.
I designed, built, and for 500 days run a solar assisted anaerobic digester.
I have two questions, one I think I know the general answer to but not the principals. The second question is more of a mystery to me.

Hi Physics Forum Folks! What an amazing resource of knowledge, ... thanks!

The question is on the attached graphic, as its much more clear with an illustration.
(Please read questions on this graphics first before looking at the video below,)

This video is a very quick look at the actual equipment in the questions, just for further illustration:


And this video gives a wider perspective of the project (and hopefully my questions)

 

[Dale]

That is interesting. I have never seen an anaerobic digester on this small of a scale. At this scale, what do you plan to do with the biogas? I don’t think anyone would buy 300 L, so do you have an onsite use in mind?

 

[Dullard]

I'm not 100% certain that I understand your question, but:

For 2 bags "in series" the pressure will be the same.

Pressure and force are related, but different. 0.5 PSI can produce a lot of force over a large area - that pressure should be more than sufficient to 'inflate' your heavy bag. I would expect the indicator bag to show signs of "inflation" first - the lighter material requires less force/area (pressure) to lift it.

I'm not sure which bag is the "weakest link" - that depends on the design of the bags. You can determine that empirically with a simple pressure test.

 

[Steven Bolgiano]

I guess the concern is the smaller bag has a breaking point that's less than the bigger bag, ... but if I don't fill the bigger bag to the point that it is physically stretching and creating a "back pressure" on the smaller bag, how does that affect the pressure on the smaller bag? If I keep the the big bag at slightly less than fully inflated (pumping that gas to a much larger storage bladder that is isolated from this system) does the system exist at just slightly above atmospheric pressure, ..... and maintaining the small bag intact?

 

[Dullard]

Yes. Some small "back-pressure" will exist at any level of inflation of the large bag. I wouldn't expect a problem with the large bag less than "fully inflated,"
Disclaimer:
All of my experience in this area comes from youthful filling/ignition of garbage bags of oxy-acetylene at assorted ratios. My judgement may be suspect.

 

[Steven Bolgiano]

Hi Dale!
I'm 74. The last system I built was 800 cubic meters, using chicken manure, and based on multiple travel to work with systems in Wales and China. The problem we encountered is AD needs heat to operate, and the parasitic load of using the methane for the heat, limits efficiency obviously, but more importantly limits the regions where AD can be feasibly utilized.

This project started out as a prototype that was intended to fit somewhere between laboratory models (whose micro systems to not pick up on real problems in production), and production system (where retro engineering issues not identified in laboratory models being very expensive if even possible depending on the stage of construction. Typically heating of Biodigesters is done by injection or insertion. My hypothesis is based on utilizing mass. And the method is soar heated water and forced air. The adjacent greenhouse is as much a heat resource for the biodigester as it is for growing the plants that are used for the feedstock for the biodigester.

A point that's in another direction, is that I have found that often there is innovation that is helpful, or can even rescue a community, that provides profit to the community in less expenses, ... BUT there's not enough profit in it for an Investor to get involved. And so that innovation that has the potential to rescue a community just remains a concept.

Unexpectedly I ran into a local biochemist from Ghana who wanted to see my biodigester system.

Our discussions led me to realize in remote, nutrient poor soil regions, it would be possible to (as we used to hear on sci-fi TV) ..... to incrementally Terraform communities in Africa or other soil, diet, and waste management challenged regions, using my system to grow food AND use the digestate from the biodigester to slowly enrich soil to produce more food AND manage waste water/health issues.
This is a much longer topic .... But I'll get off my "soap box" now, and encourage young scientists to innovate for the common good of communities, and not just for their own over consumer gain,

 

[Steven Bolgiano]

Dullard: The question is then, do you still have your eyebrows?! I first started studying hydrogen generation using electrolysis .... And I too entertained myself. However when I went to my mentors to understand how I could increase the scale of the systems I was building. They unscientifically replied I would die a very quick death with my remain fitting in a shoebox. So I went to study methane (much safer).
In terms of gas pressure, I guess I'll temporarily bypass the pump, and keep an eye on it!

 

[Steven Bolgiano]

Dale: Apologies. Typical with old farts, I totally ran past your question.
The methane here will heat the greenhouse you see in the second video.
The methane in Ghana will run a well pump.

 

[Dale]

Hi Dale!
I'm 74. The last system I built was 800 cubic meters, using chicken manure, and based on multiple travel to work with systems in Wales and China. The problem we encountered is AD needs heat to operate, and the parasitic load of using the methane for the heat, limits efficiency obviously, but more importantly limits the regions where AD can be feasibly utilized. This project started out as a prototype that was intended to fit somewhere between laboratory models (whose micro systems to not pick up on real problems in production), and production system (where retro engineering issues not identified in laboratory models being very expensive if even possible depending on the stage of construction. Typically heating of Biodigesters is done by injection or insertion. My hypothesis is based on utilizing mass. And the method is soar heated water and forced air. The adjacent greenhouse is as much a heat resource for the biodigester as it is for growing the plants that are used for the feedstock for the biodigester.
A point that's in another direction, is that I have found that often there is innovation that is helpful, or can even rescue a community, that provides profit to the community in less expenses, ... BUT there's not enough profit in it for an Investor to get involved. And so that innovation that has the potential to rescue a community just remains a concept.
Unexpectedly I ran into a local biochemist from Ghana who wanted to see my biodigester system.
Our discussions led me to realize in remote, nutrient poor soil regions, it would be possible to (as we used to hear on sci-fi TV) ..... to incrementally Terraform communities in Africa or other soil, diet, and waste management challenged regions, using my system to grow food AND use the digestate from the biodigester to slowly enrich soil to produce more food AND manage waste water/health issues.
This is a much longer topic .... But I'll get off my "soap box" now, and encourage young scientists to innovate for the common good of communities, and not just for their own over consumer gain,

That is cool. I am a small-scale farmer that has been interested in anaerobic digesters for quite some time, but the usual sizes are too big.

But again, my question is what are you planning to do with the biogas? If you are planning to use it on-site then you can probably leave it uncompressed so you wouldn't need a pump, just a valve or two would be sufficient. But if you plan on taking it offsite then you are surely going to need to pressurize it anyway. So getting rid of the pump here doesn't seem helpful.

 

[Dale]

Dale: Apologies. Typical with old farts, I totally ran past your question.
The methane here will heat the greenhouse you see in the second video.
The methane in Ghana will run a well pump.

Sorry, we were cross-posting. For those use cases then I think a pump is not needed unless the well pump requires a certain fuel pressure. Pressurizing the gas would represent an energy inefficiency in the system.

 

[Steven Bolgiano]

Dale: You nailed the main point, ... Methane requires an on-site system, and primarily because the methane costs almost more energy to compress than its energy value. There is kind of an "inverse" present for agricultural biodigestion. A poultry farm or dairy farm's energy value of it's manure, exceeds the electrical demands of the farm, that it could produce from methane powered electrical generators. So it requires a utility company willing to purchase the electrical over production. I supplement my crop/kitchen food waste system with a beef farmers manure. Which means I have to strain out the straw and fodder, because it would clog a small tank system. So it means either hand labor or a mechanism that subtracts the energy value of the system through parasitic load. Mid-sized systems for mid-sized green houses, restaurants, farms, remote villages can provide a significant heat resource. Larger systems are required for electrical generation.

I use mine to heat my greenhouse, but more for the digestate (plant nutrient byproduct) that is created.
The results using it for growing vegetable crops are mind blowing.
But so much of the process is applied physics ... Thank You Physics Forum!

The construction of my system isn't difficult or even that complex but requires a wide range of technology disciplines, of which most of them I am marginally proficient! The colony of methanogens living in the digester, would be similar to keeping goldfish in a bowl, ... feed them the right things at the right time, in the right amounts, at the right temperatures, ... using the right remedies when they get "sick".

I'll be happy to tell you anything about this system.

I would really like to create an open-source collective of individuals whose interests are focused on one or more of the disciplines.....enviro-biologists, hydrologists, thermodynamics, photovoltaics, materials engineers, plumbers, etc.
Its not exactly a forum, but close enough, its poor form to say more in consideration of this terrific format here we are so fortunate to have been here for so long.

 

[Steven Bolgiano]

Dale: Two references to "Pumps"
One type is used to transfer gas to larger storage bladders
The other reference to a pump Is the "Terraforming" Village project in Ghana I'm designing, where a methane powered well pump is used to deliver water to crops.

 

[Dale]

Methane requires an on-site system, and primarily because the methane costs almost more energy to compress than its energy value.

Yes. Onsite use is the only realistic approach for most scenarios, I believe. The only exception would be if there were an existing gas pipeline going by the property. So I think a small scale generator that could run directly off the biogas would be ideal for more farms, but that in itself is a decent engineering challenge.

I use mine to heat my greenhouse, but more for the digestate (plant nutrient byproduct) that is created.

Yes, my interest was primarily for the digestate also. It is surprising how much fossil fuels are consumed in the production of commercial fertilizers. I have been very interested in the "Biogas done right" approach by the Italian biogas consortium. But the scale didn't match.

Dale: Two references to "Pumps"
One type is used to transfer gas to larger storage bladders
The other reference to a pump Is the "Terraforming" Village project in Ghana I'm designing, where a methane powered well pump is used to deliver water to crops.

Yes, I understood. It seems like the transfer-to-the-bladder pump could be limited to just the pressure needed for input to the well pump. So you could use a pretty small, low power, low flow, low pressure pump there.

 

[Lnewqban]

Are inverted bucket generators or storage containers an option to be considered?
I like those, because are capable of maintaining a constant internal pressure and flow in a wide range.

 

[Steven Bolgiano]

YES. When I was in Shenzhen working with the guys from the Puxin company (a GREAT supply resource especially for smaller "home scale" digesters) , they used a technique similar to yours, but kind of reversed from your illustration.... they have an inverted fixed dome in a cylinder of water, and the water goes up and down, not the cap. But its an eloquent piece of engineering, by using the hydrostatic pressure, they don't use any pumps for gas, but can still modulate pressure (I went there in 2009 I'll see if I can find a video of he actual component. Now the ones we got from them were 4 ft in diameter and 5 ft high and concrete. I got the steel molds for them and the 200 cu. meter tanks from them as well as the fiberglass caps for the gas "bells". The drawback was you had to replace your moisture traps for the gas quite often because of the contact with water, the water level in "bell jars" had to be controlled by auto fills, or monitored because of evaporation, and for those who were new to it, we wanted to be able to see the gas produced in real time. its still a great idea though. Every engineering advantage comes with a price.
Puxin builds and supplies for large systems mostly in thirds world regions. But we shipped $110k worth of equipment here for our 800 cu.m. chicken poop to energy plant. And yeah, some of the stuff didn't work (mostly what they had to convert from 50 KHZ equipment to 60 KHZ. But in the end it was still a win.

 

[Steven Bolgiano]

Hi Lnewqban,
If someone recreates your digester model illustration, my suggestions are:
1) Suspend your heater a little higher. The sludge especially with a vertical geometry (my preference is horizontal tanks) .... will gather quickly at the bottom covering up your heater. Also to choose the right heating element (I shopped aquarium equipment) .... decide if you are gong to cultivate methanogen microorganisms at "mesophilic" temperatures (20C - 45C ) or "thermophilic" temps (45C 1222 C), in order to set the heater controls correctly. Quick changes can kill the tank.
2) Here's the one that I learned the hard way: Put an anti-siphon hole in the outlet port at the top of the bend. Without it the first time you load the digester a little too quickly, you'll siphon your slurry out of your tank to the lowest level of your interior exit pipe, with a 50/50 chance of sucking oxygen into your tank ............... like I did.
This why I'd like to get a group of interested people together and make a simple project like mine open-source, no secrets, all collaboration... just in this simple project excluding the actual anaerobic biodigestion, are so many other applied physics principals and calculations. Thank for letting me babble on.

 

[Steven Bolgiano]

Oh! And just to push the solar heating as the best medium ...... calculate (A) the energy created by your electric heater, THEN calculate (B) the energy value of the methane that the little tank creates, ... and subtract A from B and hope for a positive number!