FutureFarmer

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.