[Digestion] Flashback question and other comments

Tue Aug 7 09:39:00 CDT 2012

Hello list,
I can't resist to provide my 2 cents on this topic... Please rely on expert
opinion (and not this message only). This information is provided in good
faith.

Obviously, a well designed and operated biogas system shouldn't cause
explosions related to a mixture of air/oxygen and methane. Fires are more
frequent. Flashbacks are possible in certain situations, such as (there are
other situations, but these are common examples) :
- start-up or recommissioning of digester
- repairs in explosive zones

In normal operating state, the contents of the gasholder should not acheive
explosive range. Situations arise in startup or repairs where air could be
inside the gasholder. When the content of methane is above 30% v/v and the
oxygen content is below 3% v/v, explosion potential is minimal. (Source:
Technische Information 4 Sicherheitsregeln fur Biogasanlagen, Appendix 1
Operating Instructions for Inital Startup/Restart of a Biogas System).
A flashback arresting device would prevent the migration of the flame if an
ignition source would be present near the under-over pressure relief valve
of the gasholder. Certain valves manufactured in Germany maintain a water
column (such as valves manufactured by Biogaskontor) that also act as a
flashback arresting.
Other situation is the operation of a non-explosive proof agitator/mixer in
the digester vessel during start-up repair. The operator should activate
the mixer when the liquid level is above the mixer in the digester vessel.
There is also some different opinions for first filling procedure for the
digester tank to minimize further issues:
- first fill the digester vessel with water and heat the water. Then add
the daily amount of manure you would add normally to the digester. This has
the advantage of troubleshooting heating, sealing and time to tweak other
equipment on the digester before biogas generating materials are in the
tank. Downside, its much slower than the following method. Its safer
because you are doing repairs on a water tank, not a tank full of hot
manure releasing H2S and other hazardous gases.
- fill within a short period of time the digester tank with manure and or
active digestate until all inlets and oulets are sealed with substrate.
Stop feeding materials. Heat the digester. The gas will be vented to the
atmosphere (little note here, lots of carbon dioxide is produced during
this step which would displace the oxygen present in the gasholder - to
what amount I'm not sure). Upside its quicker. The example of this
procedure is in the TI4 biogas safety rules in Appendix 1. Again discuss
with your engineer for the correct procedure.

Fires are also possible in the mechanical room where the biogas blower and
or the engine is present (leaks, parts damage, etc...). Luckily, electrical
codes in your jurisdiction required %LEL monitoring and should alarm and or
shutdown equipment that could generate any ignition source before the fire
occurs.

In general, be careful if you are injecting air in the gasholder to provide
desulfurization and follow manufacturers directions (adjust flow and or
shutoff air injection during repairs).

Please get your hands on the biogas safety rules from the German
Agricultural Occupational Health and Safety Agency (www.lsv.de - they have
an unofficial English version also). There are great examples of operating
procedures and its good for operator training as well. Club Biogaz, ATEE
have the manual in French. CSA standard Code for digester gas and landfill
gas installations is also a great resource (B149.6-11) for design and other
safety requirements.

Other thing that should also be great to have for everyone working around a
biogas system is to wear the appropriate PPE (protective personal
equipment) such as a personal alarm detector that monitor the ambient air
for hazardous gases such as methane (and other explosive gases) and
hydrogen sulfide. Their low cost shouldn't be an excuse not to have one.

Do not handle this topic lightly and rely on expert opinion. Accidents
happen quickly and without warning.

Cheers,
Terrence

On Sat, Aug 4, 2012 at 3:00 PM,
<digestion-request at lists.bioenergylists.org>wrote:

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>    1. Re: Biogas Flashback, does it really happen? (William Childs)
>
>
> ---------- Forwarded message ----------
> From: William Childs <wbchilds at gmail.com>
> To: david at h4c.org, For Discussion of Anaerobic Digestion <
> digestion at lists.bioenergylists.org>
> Cc:
> Date: Sat, 4 Aug 2012 12:56:30 +1000
> Subject: Re: [Digestion] Biogas Flashback, does it really happen?
> 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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> http://www.adelaide.edu.au/biogas/
> and the Biogas Wiki http://biogas.wikispaces.com/
>
>
>
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