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I have an interesting problem. We have an off-grid house that uses mostly solar + microhydro + batteries for most of energy needs (which include heatpump-based heating).

However, we do need to run a diesel generator for about a month a year during the winter, when the stream is frozen and there isn't enough sunlight. Ideally we would like to fill up biodiesel tanks once every couple of years and forget about them.

The problem is that biodiesel goes rapidly bad (pulls in water from the atmosphere, then various microbial life sets in and creates gunk). We tried a couple of different inhibitors and they allow the fuel to survive for about 1 year, but not more.

So my question is, has anybody tried making an inerting system for biodiesel tanks? I'm thinking about flushing the tank with CO2 from a cylinder and then keeping the tank pressurized to something like 5kPa. The mechanical stress would be negligible and 5kPa should be enough to keep away water vapor from seeping in (it's far greater than the osmotic pressure of water vapor). Pressure can be maintained stable by a simple automated system.

I tried to search for similar systems on the Internet and in the patent database, but so far found nothing interesting.

Thoughts?

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  • $\begingroup$ Why so complicated? Just run the air vent through a dessicator and dry it every so often. We needed to make on to dry a failed window - plastic tupperware box and those sachets of granules that come with electronics: worked well. $\endgroup$
    – Solar Mike
    Commented Jan 5, 2022 at 9:44
  • $\begingroup$ An interesting idea. Just add a dessicator to the vent? One problem might be that it'll just keep pulling the moisture until it gets saturated. I guess I can just seal it completely and add a one-way valve to let in air as fuel gets used up. $\endgroup$
    – Cyberax
    Commented Jan 5, 2022 at 10:22
  • $\begingroup$ I would make sure nothing can grow; use argon from a weld supply shop. Assuming a sealed tank , the only use would be initial flush, fill, and small additions to replace oil as it is used. Still need poison as there are anaerobic bacterial, sort of a problem in the oil business. $\endgroup$
    – blacksmith37
    Commented Jan 5, 2022 at 16:50
  • $\begingroup$ I don't know what you are calling biodiesel , but petroleum oils dissolve a tiny bit of water. It comes out on steel tanks and pipe walls causing another sort of problem in the oil business ( there may be ppm level water in your biodiesel supporting bacteria.) $\endgroup$
    – blacksmith37
    Commented Jan 5, 2022 at 16:56
  • $\begingroup$ For future readers - Trying to maintain an inerted atmosphere inside a tank that has a vent constantly open to the atmosphere is going to consume a lot of gas. As a general warning, it's not advised to increase the internal pressure of tanks that were designed as atmospheric tanks. $\endgroup$
    – J. Ari
    Commented Jan 6, 2022 at 15:56

2 Answers 2

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A very nice PhD guy contacted me privately! Apparently what I'm doing is going to mostly work. It's going to "mostly" work because commercially available biodisel already has a fair amount of dissolved water and oxygen, enough to produce a fair bit of gunk. That's why biocides don't really work well with biodiesel.

Regular diesel is much better in this regard, it's produced entirely by dry processes so it contains very little dissolved water.

For longer-term storage there are commercial systems that circulate diesel fuel over a bed of desiccant to dry it. They can easily extend the lifetime to 5-7 years. They are not very expensive (it's just a pump, a tank of pellets and a filter) so I'm going to look into that.

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  • $\begingroup$ Close, but refined petroleum products do contain a very small amount of water. That produces a small water phase on the bottom of the tank to support anaerobic bacteria. A desiccant will probably remove the water. $\endgroup$
    – blacksmith37
    Commented Jan 6, 2022 at 16:25
  • $\begingroup$ @blacksmith37 You can't cook out the bacteria out can you? Or do they find their way down through the diesel into the water pocket? $\endgroup$
    – DKNguyen
    Commented Jan 6, 2022 at 20:37
  • $\begingroup$ You'll need to sterilize everything in a tank, by cooking it at more than 120C to kill spores. And you'll get bacteria back the moment you open the tank to refill it. $\endgroup$
    – Cyberax
    Commented Jan 6, 2022 at 21:10
  • $\begingroup$ They are versatile . Many oil wells are sour ( hydrogen sulfide) because anaerobic bacteria get in with drilling mud and eat sulfate producing H2S. Same problem in some home water wells. $\endgroup$
    – blacksmith37
    Commented Jan 7, 2022 at 1:04
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You will probably discover that there are anaerobic bacteria that will grow in biodiesel and produce highly corrosive byproducts which will eat the tank walls. Sorry- I do not know how to kill those bugs!

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  • $\begingroup$ Seem to remember a suggestion (or paper, article) about the use of silver anodes to reduce / kill bacteria. $\endgroup$
    – Solar Mike
    Commented Jan 6, 2022 at 7:43
  • $\begingroup$ Biodiesel is not very hospitable to anaerobic bugs, they just don't have much to digest. And adding a bit of biocide is not a big deal. It's the aerobic bugs that kill the fun. $\endgroup$
    – Cyberax
    Commented Jan 6, 2022 at 13:49

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