FutureFarmer

The problem is' date=' 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.[/quote']

 

I don’t know if the atmosphere is a good analogy for this. The atmosphere receives 10^22 BTUs of energy from the sun per day and it’s 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 don’t 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.

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.