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separating carbon dioxide from biogas


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#1 FutureFarmer

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Posted 22 July 2005 - 04:50 PM

Let me say that I have no wish to go boom. I have (and recommend) a healthy respect for explosive gasses. Reading the Darwin Awards is fine, I have no intention of qualifying for one. I asked this question on another forum and got nowhere because "someone stupid might do something bad" if it was discussed. There are 60,000 biogas cookstoves used in Nepal everyday and I don't see why the technology can't be used intelligently here.

That said, I'm interested in small-medium scale biogas production for electricity production (~1-3 KW) for a remote greenhouse. The issue is that the gas is produced continuously so I can either find a generator that will use it at nearly the production rate or I can store it. A generator that small isn't available so I need to be able to store it and run a larger generator when the batteries run down.

The actual storage isn't an issue, but Biogas contains a significant portion of carbon dioxide (40%). Wasting almost half of the storage capacity on useless CO2 doesn't make much sense to me. I know absorber systems are capable of scrubbing the CO2, but I think the cost of the chemicals and upkeep would offset the savings of not having to run a power line to the greenhouse.

Here's my question: If I can pour CO2 into a box and displace enough air to put out a candle then why can't I put the biogas into a tank, allow the CO2 to settle on the bottom, close a valve near the middle and vent the CO2? The reduced CO2 biogas could be compressed and stored more efficiently.

In chemical terms, how do I figure out how much mixing Brownian motion can cause in gasses of widely differing densities?

Any help you can offer would be appreciated.
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#2 jdurg

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Posted 22 July 2005 - 05:00 PM

The problem is, carbon dioxide isn't that much more dense than most other gases. Therefore, just letting it settle won't really get rid of it. Look at the natural atmosphere. There's a good deal of CO2 in there, but it doesn't all settle down to the earth. If it did, we'd have a hard time living. The only really 'easy' way to get rid of the CO2 from the gas mixture is chemically. This is because the amount of time you'd have to wait for the CO2 to settle to the bottom would be ridiculously long.

Perhaps the easiest, and cheapest, way to remove CO2 is a very concentrated sodium hydroxide solution. NaOH absorbs CO2 from the air to form NaHCO3. If you pass this gas through a solution of sodium hydroxide (NaOH is incredibly inexpensive), it will absorb a great deal of the CO2. You'll just have to replenish the NaOH supply every now and then, but based upon the amount of gas going through you should be able to calculate how much is needed on a weekly basis.
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#3 Silencer

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Posted 22 July 2005 - 05:27 PM

What, exactly, is biogas?
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#4 neo007

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Posted 22 July 2005 - 07:55 PM

What, exactly, is biogas?

Biogas is the gas produced from the fermentation of dead organic matter. Carbon dioxide and methane gas are the main constituents of biogas.

Perhaps the easiest, and cheapest, way to remove CO2 is a very concentrated sodium hydroxide solution

Also wouldn't it be possible to use calcium carbonate to remove the CO2?
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#5 Borek

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Posted 22 July 2005 - 10:26 PM

Ca(OH)2. It should be also possible to freeze it out, although it may be expensive. Perhaps just water will be effective - IIRC CO2 dissolves much better in water then methane does.

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#6 FutureFarmer

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Posted 27 July 2005 - 07:52 PM

The problem is, carbon dioxide isn't that much more dense than most other gases. Therefore, just letting it settle won't really get rid of it. Look at the natural atmosphere. There's a good deal of CO2 in there, but it doesn't all settle down to the earth. If it did, we'd have a hard time living. The only really 'easy' way to get rid of the CO2 from the gas mixture is chemically. This is because the amount of time you'd have to wait for the CO2 to settle to the bottom would be ridiculously long.

Perhaps the easiest, and cheapest, way to remove CO2 is a very concentrated sodium hydroxide solution. NaOH absorbs CO2 from the air to form NaHCO3. If you pass this gas through a solution of sodium hydroxide (NaOH is incredibly inexpensive), it will absorb a great deal of the CO2. You'll just have to replenish the NaOH supply every now and then, but based upon the amount of gas going through you should be able to calculate how much is needed on a weekly basis.


I donít know if the atmosphere is a good analogy for this. The atmosphere receives 10^22 BTUs of energy from the sun per day and itís always imbalanced due to the day/night cycle, different latitudes and absorption media. Both the settling due to density and mixing due to Brownian motion are swamped out by our weather.

In a drum with fairly uniform temperature most of the convection would be eliminated and Brownian motion would be the dominant mixing agent. The density of carbon dioxide is 3 times the density of methane. Still not much, but if I remember correctly ambient air is only 20% more dense than hot air in a baloon and there is no problem making use of that differential. I think the original question is valid. It probably would take too long, but how would I go about calculating the settling rate?

I donít actually want to get rid of all of the CO2. I just want to reduce the volume of the gas for storage. NaOH would be OK, except the gas will be saturated with moisture, which I guess I can deal with. Is there any way to calculate first pass efficiency of CO2 absorption without actually building one? Could the NaOH solution be sprayed as a mist through the gas? It would be easy to recharge the solution and the pump would be isolated from the methane.
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#7 jdurg

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Posted 28 July 2005 - 01:55 AM

After bubbling through the NaOH, you could always blow the gas through a dessicant to soak up all of the water. There shouldn't be a heck of a lot of moisture in there unless you have a high temperature.
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#8 Thomas Kirby

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Posted 28 July 2005 - 03:27 PM

Disclaimer: I am not an expert, just an enthusiastic amateur.

I hope I'm not speaking out of turn, and I realize that you know chemistry better than I do, Jdurg. I have a couple of questions, though. Why does the sodium hydroxide have to be concentrated? I would think, and this may just be me, and I may be all wet, but maybe a more watery solution would be more suitable for the formation of small bubbles with a larger ratio of surface area to volume. Increase the viscosity of the solution, you get large bubbles for that "blurp-blurp" swampgas effect. For a source of lye and potash, wood ash can be had for free. That's a price that's hard to beat.

CO2 would never settle to the bottom significantly within a holding tank, not ever. Convection currents in the gas would keep it mixed. Water will condense out and gather at the bottom of the storage tank. It needs to be drained somehow.

A simple plastic sheet can help separate out a lot of the water in the gas, like a solar still. Water condenses on the sheet which is arranged to drip it back into the lye solution. Just cover the tank with the solution in it with sturdy transparent plastic, not particular about how transparent, and put a weight in the center. I would love to see someone invent a way to automatically clear water from the storage tank for the gas, but it's probably already on Google somewhere. Water traps can also be placed on the line between the tank and the heater.

As always, it is good to do a Google search for these things. There is a lot of info out there. Search for the terms "biogas" and "water trap." One thing I've already picked up is that iron shavings can be used to take hydrogen sulfide out of the mix, which seems like a good idea. They also talk about the use of sprays of just plain water to remove carbon dioxide. This means you don't have to buy, handle, or dispose of any chemicals at all to remove CO2.

This is actually getting to be pretty well established technology and there is a lot of prior art out on the web. The best, I would think, would be those that do a job like scrubbing CO2, H2O, H2S, and other undesirables without using any consumables other than water and very little energy. Second best are consumables that are cheap and easy to obtain, do not have to be changed often, and are not hazards to the environment when disposed of.


The problem is, carbon dioxide isn't that much more dense than most other gases. Therefore, just letting it settle won't really get rid of it. Look at the natural atmosphere. There's a good deal of CO2 in there, but it doesn't all settle down to the earth. If it did, we'd have a hard time living. The only really 'easy' way to get rid of the CO2 from the gas mixture is chemically. This is because the amount of time you'd have to wait for the CO2 to settle to the bottom would be ridiculously long.

Perhaps the easiest, and cheapest, way to remove CO2 is a very concentrated sodium hydroxide solution. NaOH absorbs CO2 from the air to form NaHCO3. If you pass this gas through a solution of sodium hydroxide (NaOH is incredibly inexpensive), it will absorb a great deal of the CO2. You'll just have to replenish the NaOH supply every now and then, but based upon the amount of gas going through you should be able to calculate how much is needed on a weekly basis.



#9 YT2095

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Posted 28 July 2005 - 03:37 PM

IIRC methyl amine will lock up CO2 religiously! and then release it upon heating, this can then be pumped BACK into the cavities left from oil/gas extraction and disposed of that way. trials are indeed Currently taking place as I type :)
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#10 Thomas Kirby

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Posted 28 July 2005 - 03:43 PM

That doesn't seem quite as effective as converting it to biomass using hemp and then piling the hemp on the ground, creating an artificial peat bog as a carbon sink.

#11 jdurg

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Posted 28 July 2005 - 04:29 PM

The whole reason for the concentrated NaOH solution is that it provides more NaOH in a smaller amount of liquid. This way you don't have to change out the solution as often. Also, the higher the concentration, the greater the frequency with which the CO2 molecules will interact with the OH- ions in solution to give you the HCO3- ions that should be produced. In fact, I think on the space shuttles they use honeycombs made of KOH to absorb the CO2 that the astronauts are breathing out.
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#12 YT2095

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Posted 28 July 2005 - 04:32 PM

actualy IIRC, they used a peroxide of potassium, that in turn liberated Oxygen after reaction, as they did (maybe even still do) on certain Submarines.
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#13 Thomas Kirby

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Posted 28 July 2005 - 04:47 PM

Jdurg: I see. Of course, if you lose the ability to bring the gas into contact with the solution, the amount you have in a small volume doesn't do you much good. If you bubble it, you need small bubbles for a favorable ratio of surface area to volume. Anyone who has a little space can use a tank that contains a few thousand pounds of solution. A filter that contains a lot of water-based crystals of a metallic hydroxide will also bring a lot of surface area in contact with the gas. My favorite way to build something like that would be to crystallize the material, like the crystals you can get in some preparations from the store, and work it into some kind of paper or cloth filter something like a furnace filter or a water filter, and not even try to form it into something super high tech. Actually, just dip the filters in the chemical. To recharge them, wash them in clean water and dip them again. It might even work well in a wet environment if the fibers that the chemical is embedded in are absorbent, able to wick any dissolved chemical back up where it belongs.

I like the idea of spraying plain water through the gas and warming that water in another place to force it to release the CO2. Cold soda holds a lot more fizz than soda that is at room temperature. I can't take personal credit for this idea.

Also, this situation sounds about perfect for a Stirling engine and a linear alternator. Just burn the gas to heat water to power the Stirling engine to charge the batteries.

This might also be a good time to think about the use of LED grow lights. They are extremely easy to build. You don't need as much ultraviolet as you might think, and some cheap LEDs emit enough to be usable, plus ultraviolet LEDS don't cost a lot either. There are actually three sets of wavelengths across the visible spectrum that chlorophyll uses. This has got to be a huge improvement over the use of fluorescents or halogens. No high voltage is needed. No lamp ballasts are needed. They generate extremely little RF noise. They generate very little heat. If you use a 12 volt source you can wire them in series in sets of three or four depending on your voltage drop. LEDs are much sturdier than any incandescent or fluorescent lamps. They last much longer, and do a slow fade instead of suddenly burning out. They start when cold without damaging themselves. They are very small and lightweight. Fixtures are already on the market but it's just as easy to make panels of them yourself.

#14 Wilmot McCutchen

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Posted 24 July 2011 - 04:11 PM

The boosters of biogas don't have much to say about the difficulties of separating the methane from the CO2. Membranes and sorbents are the conventional prescriptions, but this is not necessarily a chemistry problem. Stovepiped science prevents the consideration of other approaches.

CO2 has a molar mass (molecular weight) of 44 g/mol, water is 18 and CH4 is 16, so centrifugal gas separation in a high g device could separate the CO2 from the methane and water, and then condensation could separate out the water so the biogas could become suitable for pipeline delivery to power plants. Gravity settlement (1 g device) won't be enough. Conventional gas centrifuges (rapidly spinning narrow cylinders) won't work either. Radial counterflow gas separation might be the answer. The idea is that many tiny vortices perform high g separation, and a fractal vortex network collects the effects in a continuous process.

For separating the solids from the water, to thicken the slurry until it is suitable for a digester, a high shear crossflow filter would be appropriate.
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#15 John Cuthber

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Posted 24 July 2011 - 06:11 PM

actualy IIRC, they used a peroxide of potassium, that in turn liberated Oxygen after reaction, as they did (maybe even still do) on certain Submarines.

I take it that, unlike the OP, you are trying to win a Darwin award by mixing up lots of methane and oxygen.

You can scrub the CO2 out of the gas with NaOH, but doing so costs you in raw material and gives an annoyingly alkaline waste product. If you use a different base you can recycle it by heating it.
http://en.wikipedia....tream_scrubbing
This will also strip out some hydrogen sulphide which may prove difficult to dispose of, that problem has been addressed in the past too.
http://en.wikipedia....with_iron_oxide
It used to be commercially viable to sell the spent iron sulphide/ oxide/ sulphur mix to the people who make sulphuric acid.

If you scrub the H2S out of the CO2 then you can feed the CO2 back to the plants.
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#16 skwiff

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Posted 24 July 2011 - 09:04 PM

lime water...
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#17 John Cuthber

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Posted 24 July 2011 - 09:06 PM

lime water...

About a million gallons of it.
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#18 skwiff

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Posted 24 July 2011 - 09:09 PM

About a million gallons of it.



love it or hate it, it will help :)

Edited by skwiff, 24 July 2011 - 09:13 PM.

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#19 John Cuthber

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Posted 25 July 2011 - 12:52 PM

love it or hate it, it will help :)

A litre of lime water contains about 1.75 g of Ca(OH)2
That's about 0.024 moles which will react with 0.024 moles of CO2
which is about 0.6 litres of gas.
So rather than having to store the CO2, Future Farmer will have to make, store, and dispose of nearly twice as much volume of lime water.

If that's your idea of "help" then perhaps you should keep quiet.
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#20 CaptainPanic

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Posted 25 July 2011 - 01:39 PM

Although FutureFarmer has not visited this site for over 5 years, I'll answer anyway. The easiest and most popular solution for the CO2 in biogas is to leave it in the gas. No separation.

Why not to separate
The engine will run just fine with the little extra CO2 (just google for Biogas Engine, although probably regular natural gas engines will run as well - please doublecheck before trying this). I would be surprised if engine-generators aren't simply available on the market today. A small Google search showed me that at least 5 kW engine-generators for natural gas exist (commercial link - I have no ties with it, it's not spam!). Your best bet is to ask the professionals. At some point, you should stop hobbying, and just go to a supplier.

I think you're better off with a larger storage (if you cannot run the engine continuously) than with a separation step. The separation will use material and energy, and will probably require an investment similar or larger than the increased storage.

Separation only makes sense if you wish to sell biogas as fuel grade methane (substitute natural gas). But with such small quantities (1-3 kW of power), I wouldn't bother. Apart from the construction and installation, the permits are probably more expensive than the profits of the next 20 years.

The idea of FutureFarmer is expensive. Such small systems (1-3 kW) are not economically interesting, unless you're not on the grid, and your only options are solar power or small scale wind. With things like these, the rule is simple: larger is better. FutureFarmer is talking about an engine that is 1.3 to 4 horsepower (i.e. 10-20 times smaller than the smallest car on the market today). It doesn't get much smaller than that, so it doesn't get much more expensive than that.

If you insist on a separation
If you insist on a separation, again I would suggest to go with the commercial solutions: absorption, pressure swing adsorption, membranes, cryogenic separation or pressure swing absorption. To my knowledge, the absorber is the most popular. It's simple: water absorbs CO2 far better than methane.
Oh, and btw, nobody uses any system to chemically react CO2 to form carbonates. Lime water is a silly idea, as John Cuthber already explained.

FutureFarmer had a point when he said that he doesn't want to go 'boom' in his 1st sentence. Building your own process equipment for stuff like this will ultimately make you go 'boom'... I hope that's not the reason that FutureFarmer made only 2 posts in 2005 on this forum.
Trying to clean up biogas is no joke, and shouldn't be seen as a hobby project. If you want an installation on your premises, contact the professional suppliers. If you just like to discuss, please don't post any silly ideas.
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