[Digestion] Biogas Safety

Fri Oct 27 04:35:22 CDT 2006

David/Paul/all

I'm sure I've posted this before but .....

Paul is right that an explosion needs all three of oxygen, fuel and
ignition.

But the golden rule for preventing explosions is that sooner or later there
WILL be a source of ignition.

Perhaps a worker will break one of your site rules, perhaps lightning will
strike, perhaps something will happen that you never even thought of. So
trying to exclude sources of ignition is certainly wise but is not enough on
its own. You must make sure that flammable mixtures cannot form.

Problems arise only in a (quite wide) range of concentrations around the
stiochiometric mixture. There is a flammable range (in which the mixture can
burn but cannot explode) and a narrower explosive range. These vary quite
widely but are well documented for all common flammable gases.

I am sure that you both know this but other readers might not.

Traces of oxygen in biogas (and vice versa) are almost unavoidable but would
not sustain a flame, let alone an explosion, because the mass of cold gases
would absorb the small amount of energy released, even if they did react
with each other. This has the surprising result that several % of most fuel
gases in air or vice versa would be a perfectly safe mixture - if only you
could be sure the % could never rise. If only ....

Bear in mind that a space full of diverse gases may not be well mixed,
especially if one of them is entering from a localized source. So a 'safe'
average concn might not prevent a localized explosion.

Thus, it is not smart to install feed pumps capable of pumping air into your
digester when the feed tank runs dry or to house a large biogas holder
inside a building that would contain any leakage.

Has anyone ever written up a comprehensive list of similar practical tips?

ADKarve might well be right that LPG is more dangerous than biogas - I have
no reference sources to hand. However, that might be largely because we
store it as liquid (that's what the L stands for) so that any leaks are
usually of very cold gas, which tends to form a layer at floor level and is
slow to disperse.

The inverse argument that methane will rise and disperse is common but is
simply not good enough. It might not disperse fast enough unless the leak is
very small. Local concentrations might fall into the explosive range near
the source or near the ceiling - and lighting, fans etc provide plenty of
high-level sources of ignition.

These arguments are NOT contradictory. To say that flammable gases MIGHT
form an explosive mixture due to stratification is very different from
saying that we can be SURE that stratification will separate them safely.

Good digestion!

Duncan

================================
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Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
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================================

----- Original Message ----- 
From: "Paul Harris" <paul.harris at adelaide.edu.au>
To: "Chynoweth,David P" <dchyn at ufl.edu>
Cc: <digestion at listserv.repp.org>
Sent: Friday, October 27, 2006 2:01 AM
Subject: Re: [Digestion] Biogas Safety

> G'day David,
>
> You need fuel(methane), oxygen(air) and heat(ignition source) for an
> explosion/fire - try to eliminate at least one of the three. I favour
> plastic for construction as it won't turn to shrapnel if the unthinkable
> combination happens to occur.
>
> Also use burn back preventers - see
> http://www.ees.adelaide.edu.au/pharris/biogas/safety.html
>
> All the best,
> HOOROO
>
> "Chynoweth,David P" wrote:
>>
>> I'm looking for information (comments and references) on digester design
>> issues related to biogas explosion.
>>
>> -----Original Message-----
>> From: digestion-bounces at listserv.repp.org on behalf of
>> digestion-request at listserv.repp.org
>> Sent: Wed 10/25/2006 1:00 PM
>> To: digestion at listserv.repp.org
>> Subject: Digestion Digest, Vol 4, Issue 4
>>
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>> Today's Topics:
>>
>>    1. Re: Starch & Sugar VIDEO (David Fulford)
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Wed, 25 Oct 2006 13:44:52 +0100
>> From: David Fulford <d.j.fulford at reading.ac.uk>
>> Subject: Re: [Digestion] Starch & Sugar VIDEO
>> To: <tmiles at trmiles.com>,"'Art Krenzel'" <phoenix98604 at msn.com>,
>>         <digestion at listserv.repp.org>,  "'Marcel Lenormand'"
>>         <marcel.lenormand at gmail.com>
>> Message-ID: <5.2.1.1.2.20061025134154.035aa608 at pophost.rdg.ac.uk>
>> Content-Type: text/plain; charset="us-ascii"; format=flowed
>>
>> Tom, Art and all,
>>
>> The link http://www.extra.rdg.ac.uk/eng/energy/video/ will give you both
>> the Ashden Award video of the ARTI work by Dr Karve and also a longer
>> video
>> made by ARTI and Dr Karve themselves. There are various versions of both.
>> If you have a slow connection, the faster versions do work, but give poor
>> quality video.
>>
>> David F
>>
>> At 07:40 23/10/2006 -0700, Tom Miles wrote:
>> >David,
>> >
>> >I'm surprised that MPG4 has stumbled in with so little help for users.
>> >
>> >A real player version would be helpful thanks.
>> >
>> >Tom
>> >
>> >-----Original Message-----
>> >From: digestion-bounces at listserv.repp.org
>> >[mailto:digestion-bounces at listserv.repp.org] On Behalf Of David Fulford
>> >Sent: Monday, October 23, 2006 5:31 AM
>> >To: Art Krenzel; digestion at listserv.repp.org; Marcel Lenormand
>> >Subject: Re: [Digestion] Starch & Sugar VIDEO
>> >
>> >Art,
>> >
>> >It is the movie the Ashden Awards did, based on my earlier visit. Dr
>> >Karve
>> >also has a movie made by ARTI. You need a player that will cope with
>> >mpeg
>> >4. It plays OK with the free VLC player (www.videolan.org). If people
>> >are
>> >interested I can make both available as a real player stream.
>> >
>> >David F
>> >
>> >At 14:48 22/10/2006 -0700, Art Krenzel wrote:
>> > >Marcel,
>> > >
>> > >I tried to download this 81 mb movie but it would not play.
>> > >
>> > >Has anyone else had problems or success in downloading?
>> > >
>> > >Art Krenzel
>> > >
>> > >----- Original Message -----
>> > >From: "Marcel Lenormand" <marcel.lenormand at gmail.com>
>> > >To: <digestion at listserv.repp.org>
>> > >Sent: Friday, October 20, 2006 3:34 AM
>> > >Subject: [Digestion] Starch & Sugar VIDEO
>> > >
>> > >
>> > > > You may like to see...
>> > > > http://video.green.tv/feeding_a_revolution/feeding_a_revolution.mp4
>> > > >
>> > > > [m]
>> > > >
>> > > > _______________________________________________
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>> > >
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>> >
>> >*** Dr David Fulford, MSc Renewable Energy, Engineering Building ***
>> >***     School of Construction Management and Engineering        ***
>> >***      The University of Reading, Whiteknights,                ***
>> >***    Reading RG6 6AY, UK      Tel: +44-(0)118-378 8563,        ***
>> >*** Fax: +44-(0)118-931 3327 E-mail: D.J.Fulford at Reading.ac.uk   ***
>> >
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>>
>> *** Dr David Fulford, MSc Renewable Energy, Engineering Building ***
>> ***     School of Construction Management and Engineering        ***
>> ***      The University of Reading, Whiteknights,                ***
>> ***    Reading RG6 6AY, UK      Tel: +44-(0)118-378 8563,        ***
>> *** Fax: +44-(0)118-931 3327 E-mail: D.J.Fulford at Reading.ac.uk   ***
>>
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>
> -- 
> Mr. Paul Harris
> Faculty of Sciences, DP710
> The  University of Adelaide, Roseworthy Campus, AUSTRALIA 5371
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