# Methane Digester



## SouthWesteader (Jul 21, 2005)

I've read a lot about methane digesters, and how they work, but how would I make one compatible with my existing LP system? I don't have horses, but my neighbor does, and he has a manure disposal problem, and with the prices of LP... Also, how would I remove the CO2 from the methane?


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## JAK (Oct 15, 2005)

I'm not sure you would need to remove the CO2 from the methane. I think you do want to keep air out to avoid an explosive mix, but most oxygen would get digested into CO2 I should think. Not sure. You might have to adjust your burners and fuel to air ratios. The tricky thing might be that the digester gas might not be of consistent composition, and so you might not get ideal combustion all the time without constant attention. Not sure. You could test your system before hand to see how it holds up to various mixes of Methane, Carbon Dioxide, Moisture, Air, and whatever else. I know you can also mix in some hydrogen. You could produce it from a windmill when it produces to much electricity in high winds, allowing you to install an oversized windmill for more consistent power in low to moderate winds.


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## SouthWesteader (Jul 21, 2005)

By "ideal combustion", what do you mean? I run a piloted water heater, so I definitely don't want interrupted combustion. Right now, I'm looking towards a low-priced system, maybe $400-$500. Wind Turbines get pretty pricey, with the steel poles, and such, even for something as simple as hydrogen separation. Thanks for your info!


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## JAK (Oct 15, 2005)

Yes the wind power hydrogen thing isn't really practical, unless you already have an oversized wind turbine that produces too much power on some days. Even then you could just use the extra electricity to heat water or air directly if that is your intention.

The water heater with pilot light was exactly what I was thinking. You wouldn't want it to sputter out or something like that. You would probably want to keep the pilot on clean propane. Then you would probably be safe, as long as the burner was not to temperamental. Do you know if it works the same with either propane or natural gas or if requires adjustment from one to the other? Digester gas would be closer to natural gas, but it also has 20%-50% carbon dioxide. This wouldn't really effect combustion other than to reduce efficiency somewhat. If you kept the fuel-air ratio the same you would have more air than you need to complete the combustion of the methane, and this would reduce the combustion temperature somewhat. I think the methane would still burn completely as it is quite volatile, but efficiency might suffer a little. You certainly wouldn't need to remove all the carbon dioxide. How is it removed?

As far as $500 digesters go, I don't know that much about them price wise. I have seen some interesting sites of them built in the third world for small households, like a family of four plus a cow. Enough methane for cooking and lighting. Interesting stuff.


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## SouthWesteader (Jul 21, 2005)

Natural Gas outputs 1000 BTU, LP outputs 92,000 BTU, which is roughly 600 per cubic foot propane gas, and Digester gas without filtering puts out 600-650 BTU. I guess that means I shouldn't filter it. BTW, I just discovered that it is CO2 filtered with lime-water (I'm assuming limestone, not citrus ). I wonder how? BTW, I was thinking "manure tea" in a mostly airless water heater, connected to my propane tank. The water heater would use methane/propane to heat it when it dipped below 90 degrees F. Would that work? Also, how well does horse manure work in a digester?


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## Al. Countryboy (Oct 2, 2004)

I think that pig manure is supposed to be the best for a digester. Horse way down the list. Years ago I built a digester. I took a 55 gallon drum filled with chcken manure and water. A pipe from the top of this drum went to another drum the same size that had another slightly smaller drum turned upside down into the first drum and was filled with water. The pipe ran from the manure drum and went into the bottom of the second drum filled with water. As gas started making after a few days the inverted smaller barrel in the second drun began to rise out of the water. You could hear making a bloop, bloop sound as it went into the inverted barrel.This drum had a hole with a valve that be cut on and off. The gas in this barrel began to build up and rise out of the water so I turned on the valve a little and stuck a lighter to it. Nothing happened or I thought nothing. I was a sunny day and I could see the flame. Burned the hair off my arm. The gas seemed to burn up pretty fast, but the digester produced gas for over 3 weeks that summer. Never tryed to hook it up to anything. Mostly would just light it up for friends and family to let them see how it worked. The problem with my digester was that you could on add manure to it and it was almost impossible to clean out after it had finished working off. The sludge was wonderful for the garden and flowers though.


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## SouthWesteader (Jul 21, 2005)

Thanks for sharing, Al! My neighbors are raising pigs for 4-H, but the pigs somehow mix their manure thoroughly into their mud. To get dairy manure, I would have to drive 20+ miles, but my backfence-neighbor has trouble getting rid of his manure, hence the choice. Thanks again for your info!


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## WisJim (Jan 14, 2004)

Big problem with methane digesters in cool or cold climates is keeping the whole thing at the right temp for the bacteria. This is where bigger is better up to a point, as a larger mass of digesting manure is easier to maintain at a constant temp while occassionally adding new material. A small digester of just a few hundred gallons needs lots of insulation and additional heat to keep it going well. On the other hand, I have somewhere in my library and old booklet about making an experimental methan digester using an inner tube for storage. Don't recall many details as it has probably been over 20 years since I read it.


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## SouthWesteader (Jul 21, 2005)

Thanks for the advice, Jim. I was thinking more like 50 gallons. But then I live under the Great Southern California Sun. In summer, outdoor temperatures reach 110 degrees F daily and rarely go below 65 degrees, even in early morning. In winter, thats a different story, but even then it doesn't go below 18 degrees F. I _am _planning on using some insulation, because I think 110 is a little too hot, and 18 degrees is definitely too hot. My idea is to use a horizontal water heater, possibly with some extra insulation.


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## mikee22712 (Sep 29, 2005)

Hmmm well I just did some reading and there is a picture that shows the methane digester and it doesnt look that hard but looks can be decieving. I opened a book that is titled "self-sufficient life and how to live it" By John Seymour. it tells about all kinds of things and how to do it.The list of things to get manure from goes from the bottom to the top.chicken human cow pig this measures the amount of gas produced by the waste in a day.The book does say that the process does need to be about body temp to work properly.


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## JAK (Oct 15, 2005)

I understand John Seymour passed away in 2004 at the age of 90. He lived in Britain and wrote 41 books on simple living and lived what he wrote. Hs wife was a potter, which is always a good start I think.

Here is a good site on Methane Digesters by that famous South African Hog Farmer, John Fry. He did a lot of research, perhaps second only to that Indian gentleman, Ram Bux Singh, whom he references a lot.

http://journeytoforever.org/biofuel_library/MethaneDigesters/MD1.html


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## WisJim (Jan 14, 2004)

I feel that the late Al Rutan was the authority on methane generation in colder climates (he lived in Iowa) I first met him in the late 1970s and saw him a few times over the years until he passed away. He had lots of first had knowledge of working systems in the upper Midwest, and had good advice on dealing with cold climates and homestead sized systems.
http://www.builditsolar.com/Projects/BioFuel/biofuels.htm#Methane

Both John Fry and Ram Bux Singh have done a lot of methane work, but they worked in warmer climates, so Al's work is a useful supplement to what they have done if you are in a cooler area.


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## Cabin Fever (May 10, 2002)

When I was a grad student (about 30 years ago) I worked in the Agricultural Engineering Dept. at the Univ. of Minn. One of the projects I worked on was a biogas production study using swine manure. We used a recycled fuel tank from a gas station for a digester (about 20 feet high and 12 feet in diameter). We insultated the outside of the tank with spray foam. To heat the digester we used a hot water heater with a coiled type heat exchanger in the digester. The water heater was fueled by biogas. A small pump constantly circulated antifreeze from the water heater to the heat exhanger and back again.

Every few days we'd replace about 1/20th of the digesters contents by pumping fresh liquid swine manure into the digester. 

The biogas was stored in a large rubber bag...real large....about the size of a one car garage. That would seem like a lot of gas...but it was only equivalent to about one gallon of gasoline in energy terms.

What I learned:
1. It is difficult to store large quantities of biogas....it cannot be econoically compressed.
2. Because of the storage problem it's best to use the biogass while it's being produced.
3. Biogas is extememly dirty and corrosive. The pilot light and burner of the water heater didn't even last a year.
4. CO2 will not be produced in the digester if you keep air out of the digester.

That's about all I know, but I do work with a lot of wastewater treatment plant operators who use sludge digesters to produce gas, as well as landfill operators who deal with biogas at their landfills.


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## Beef11 (Feb 2, 2006)

I looked heavily into digesters on a commercial scale as my old man is an enviromental engineer. In my opinion the best thing to do with it is to burn it as produced by a tuned down generator it also seems that quantity is key, smaller systems are much more temper-mental and higher maintenance. As far as what type of manure look at what the animal has been eating, if it is high energy.. you get the picture. I would look for a situation where you can get a large quanity (like the neighbor empties his spreader into your hopper which pumps directly into your digester. Its easy and everybody is happy, it seems like horse pucky is a tough way to do this. If anybody is building one for real let me know i would love to come look at it and lend a helping hand (slave labor). I'm only a few states away from anywhere.


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## seedspreader (Oct 18, 2004)

ok, crazy question, what does it take to turn methane into a liquid (methanol) for storage?


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## SouthWesteader (Jul 21, 2005)

Cabin Fever said:


> When I was a grad student (about 30 years ago) I worked in the Agricultural Engineering Dept. at the Univ. of Minn. One of the projects I worked on was a biogas production study using swine manure. We used a recycled fuel tank from a gas station for a digester (about 20 feet high and 12 feet in diameter). We insultated the outside of the tank with spray foam. To heat the digester we used a hot water heater with a coiled type heat exchanger in the digester. The water heater was fueled by biogas. A small pump constantly circulated antifreeze from the water heater to the heat exhanger and back again.


You had the heat exchanger inside of the digester? How did you get the pipes down in the tank for the exchanger, while keeping the tank sealed? 



Cabin Fever said:


> What I learned:
> 1. It is difficult to store large quantities of biogas....it cannot be econoically compressed.
> 2. Because of the storage problem it's best to use the biogass while it's being produced.


I would probably use a generator, as well as the water heater and stove.


Cabin Fever said:


> 3. Biogas is extememly dirty and corrosive. The pilot light and burner of the water heater didn't even last a year.


Would I be in danger of corroding my propane tank? Would there be any way of "cleaning" it up a bit?



Cabin Fever said:


> 4. CO2 will not be produced in the digester if you keep air out of the digester.


I will be using it propane, so I will need it to be half carbon dioxide. What amount of air will I need to let in?


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## SouthWesteader (Jul 21, 2005)

ZYG, it takes 5000 psi pressure to liquidize methane. :shrug:


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## Steve L. (Feb 23, 2004)

ZealYouthGuy said:


> ok, crazy question, what does it take to turn methane into a liquid (methanol) for storage?


Uhm, ZYG, methanol (CH40) is an alcohol, _not_ LNG. 
This site, though, does descrbe the process of making methanol from methane. 

```
http://en.wikipedia.org/wiki/Methanol
```



> Today, synthesis gas is most commonly produced from the methane component in natural gas rather than from coal. Three processes are commercially practiced. At moderate pressures of 1 to 2 MPa (10â20 atm) and high temperatures (around 850 Â°C), methane reacts with steam on a nickel catalyst to produce syngas according to the chemical equation...


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## Cabin Fever (May 10, 2002)

SouthWesteader said:


> You had the heat exchanger inside of the digester? How did you get the pipes down in the tank for the exchanger, while keeping the tank sealed?
> We drilled holes in the side of the tank, used flanges that sealed around the holes, and plumbed in from these.
> 
> I would probably use a generator, as well as the water heater and stove.
> ...


Good luck


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## SouthWesteader (Jul 21, 2005)

Cabin Fever said:


> Good luck


Thanks!

I did specific research on the corrosive aspect. You can run it through iron filings. I wonder how? I''m not actually planning to store it iin my propane tank, but make it go through the propane tank, as sort of a duel fuel system. It would actually be stored in a different tank, and it would be released slowly through a reverse pressure switch. Thnaks for your info, BTW, Cabin Fever.


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## Cabin Fever (May 10, 2002)

SouthWesteader said:


> Thanks!
> 
> I did specific research on the corrosive aspect. You can run it through iron filings. I wonder how? I''m not actually planning to store it iin my propane tank, but make it go through the propane tank, as sort of a duel fuel system. It would actually be stored in a different tank, and it would be released slowly through a reverse pressure switch. Thnaks for your info, BTW, Cabin Fever.


I believe that we passed the biogas thru a tube filled with fine steel wool. For some reason, I also remember passing it thru copper wool. Hey, my grad school days were 30 years ago, so I've forgotten many of the details long ago. One thing I do remember is how little energy you got from such a large volume of biogas.


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## Allen15 (Apr 18, 2013)

I know that passing it through the iron filings/scrap is to scrub the hydrogen sulphide out, and the iron is sacrificial. I don't recall any plans using copper for scrubbing, but the CO2 can also be scrubbed out if you have roughly 15 feet of vertical space to rig up a standing water pipe because you can bubble it through a valve at the bottom, and collect it at the top, with the CO2 getting adsorbed into the water due to the water pressure and relative vapor pressure of CO2 in water. You will have to change the water on occasion, because it will become quite acidic, but you don't have to throw it away - letting it sit out in the open will allow the CO2 to escape to the air over time too.

I had plans for such somewhere around here, but can't put my finger on them right now. Probably something I got from Mother Earth News years ago?


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## Ross (May 9, 2002)

I know you can not burn methane in a propane appliance, the orifice is completely different. Very different btus. My neighbour with 1000 cows tried having a digester built..... I think they're a million into it..... and it doesn't work yet.


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## MichaelK! (Oct 22, 2010)

On another forum I did the math for holding gas in a 2500 gallon irrigation tank. That volume of gas had the energy equivelent of approximately 9 lbs of propane. That amount of gas would run my generator at an 2.0kw load for about 4 hours. In the 55 gallon storage described above, that amount of gas would be the energy equivient of ~0.33 lb of propane, about 1/3 of a standard green Coleman gas cylinder.

Although there are some technically correct solutions to problems such as Hydrogen sulfide remove, I suspect that a detailed cost/benefit analysis would show that you'd spend more money cleaning up the gas than you would producing the energy.

That is, lets say you make 70 cents worth of gas in your tank, but spend three dollars at Home Depot buying steel wool and powdered lime to clean it well enough to use. The cost ratio will really nosedive if you end up spending 29.99$ for a new burner if it corrodes as quickly as Cabin Fever suggests.


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## fishhead (Jul 19, 2006)

There's a dairy farm in Elk River MN that digests the cow manure and sells the electricity it produces in a generator. I think they have 1,600 cows.

Jean Pain in France had a digester in the middle of his wood chip compost pile. The pile also produced hot water and I think heat for the house. A pipe from the digester went into a manifold with truck tires for storage. The methane went to power his little truck and cook their food. 

Here you go. http://journeytoforever.org/biofuel_library/methane_pain.html


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## Ross (May 9, 2002)

There is another million dollar plus digester about 15 miles from me again on a big dairy farm, and yeah it powers the place 90% of the time. Saves a hundred grand or so per year


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## Harry Chickpea (Dec 19, 2008)

JAK said:


> I understand John Seymour passed away in 2004 at the age of 90. He lived in Britain and wrote 41 books on simple living and lived what he wrote. Hs wife was a potter, which is always a good start I think.
> 
> Here is a good site on Methane Digesters by that famous South African Hog Farmer, John Fry. He did a lot of research, perhaps second only to that Indian gentleman, Ram Bux Singh, whom he references a lot.
> 
> http://journeytoforever.org/biofuel_library/MethaneDigesters/MD1.html


Back in the 1970s, I did a rewrite of Ram Bux Singh's book for him. About the time it was near completion, he got involved with the Ral-Jim farm and TMEN - which turned out to be a disaster all around.

The unslaked lime in lime water removes the CO2 - it is bubbled through it. That also helps with the hydrogen sulfide, but iron will do the job. The copper - brillo pad - is part of a method to make methanol - which I would NOT make, as it is much more dangerous and poisonous.

There are a couple of temperature points where digestion is maximized, but the key is the ratio of available carbon and nitrogen. "Hot" manures do well, the others are slower. Just like in septic tanks, crusts can form that impede gas generation, and a method of stirring is important.

As pointed out, storage is a problem. A gas holder or weighted large plastic sausage are possibilities. The positive pressure is a requirement to prevent accidental explosion.


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## fishhead (Jul 19, 2006)

You should be able to get iron shavings from a machine shop that does threading. Remove the oil and then put it in a column and let the gas flow through it.


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