[Digestion] Biogas Flashback, does it really happen?

[William Childs]

Hello all,

Here are some reasons why biogas is pretty safe and flashback is unlikely:

-Flashback generally only occurs where there is an oxygen feed at the site
of the flashback. The most common example is the oxy-acetylene welder,
where the rubber acetylene hose is bound to the rubber oxygen hose. If the
supply line to the welding torch gets too hot the acetylene can autoignite
and melt through the oxygen hose. The result is a hot flame with a good
supply of oxygen that develops a high flame speed as the hose burns away.
Note that the strategy for tackling flashback is to first turn off oxygen,
reducing the flame heat and velocity.

-Methane is very volatile and buoyant. This means that any leaks rapidly
disperse and travel upwards fast, making ignition difficult. This is why
relief valves on large nat gas supply lines often vent to atmosphere, even
in residential areas (for example pressure step down stations on the
outskirts of cities).

-The contents of a digester and attached gas storage usually contains an
extremely small fraction of oxygen. An ignition site within the digester
probably isn't going to ignite the gas. An ignition site on the surface
will, but the flame velocity will probably be relatively slow.

In summary, explosions are possible but unlikely. If you design your
digester and storage vessel properly then you're probably safe. If you want
to be extra cautious, bubble any gas lines through water. That should
arrest any flashback.

Source: I'm a chemical engineering undergrad.

Will
Chemical Engineering/Science IV, Sydney University


On 4 August 2012 03:40, David <david at h4c.org> wrote:

>
>
> Mark,
>
>  On 8/2/2012 3:29 PM, Mark Wells wrote:
>
> Hi folks,
> I have been wondering if it is really possible to blow up a digester, has
> any one actually ever seen this happen?
> There is very little info on the web about this subject except boring hype
> from flame and flashback arrestor manufactures.
>
>
> You have to be more specific. There are a modest number of reports from
> the 20s through, say, the 40s of catastrophic explosive accidents involving
> municipal digesters. (See copies of the venerable "Sewage Works Journal".)
> It is harder to find information about explosive failures of low-tech and
> smaller digesters, but some may argue that part of the reason there are so
> few reports is that these incidents are bound to affect few people, in
> areas where accidental death is not uncommon (i.e. it may not be
> newsworthy), and where such incidents might be reported by those who
> installed the digester(s), they have a vested interest in not so doing....
> In any case, it seems to be a fairly rare consequence.
>
> In response to the most general interpretation of the question, then, yes:
> it is possible to blow up a digester.
>
>
>
> d.
> --
> David William House
>  "The Complete Biogas Handbook" www.completebiogas.com
> *Vahid Biogas*, an alternative energy consultancy www.vahidbiogas.com
>
>  "Make no search for water.       But find thirst,
> And water from the very ground will burst."
> (Rumi, a Persian mystic poet, quoted in *Delight of Hearts*, p. 77)
>
> http://bahai.us/
>
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>
>
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