Hello list,<br>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.<br><br>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) :<br>
- start-up or recommissioning of digester<br>- repairs in explosive zones<br><br>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).<br>
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.<br>
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.<br>
There is also some different opinions for first filling procedure for the digester tank to minimize further issues:<br>- 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.<br>
- 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.<br>
<br>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.<br>
<br>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).<br><br>Please get your hands on the biogas safety rules from the German Agricultural Occupational Health and Safety Agency (<a href="http://www.lsv.de">www.lsv.de</a> - 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.<br>
<br>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.<br>
<br>Do not handle this topic lightly and rely on expert opinion. Accidents happen quickly and without warning.<br><br>Cheers,<br>Terrence<br><br><div class="gmail_quote">On Sat, Aug 4, 2012 at 3:00 PM, <span dir="ltr"><<a href="mailto:digestion-request@lists.bioenergylists.org" target="_blank">digestion-request@lists.bioenergylists.org</a>></span> wrote:<br>
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<br>Today's Topics:<br>
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1. Re: Biogas Flashback, does it really happen? (William Childs)<br>
<br><br>---------- Forwarded message ----------<br>From: William Childs <<a href="mailto:wbchilds@gmail.com">wbchilds@gmail.com</a>><br>To: <a href="mailto:david@h4c.org">david@h4c.org</a>, For Discussion of Anaerobic Digestion <<a href="mailto:digestion@lists.bioenergylists.org">digestion@lists.bioenergylists.org</a>><br>
Cc: <br>Date: Sat, 4 Aug 2012 12:56:30 +1000<br>Subject: Re: [Digestion] Biogas Flashback, does it really happen?<br><div>Hello all,</div><div><br></div>Here are some reasons why biogas is pretty safe and flashback is unlikely:<div>
<br></div><div>-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.</div>
<div><br></div><div>-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).</div>
<div><br></div><div>-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.</div>
<div><br></div><div>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.</div>
<div><br></div><div>Source: I'm a chemical engineering undergrad.</div><div><br></div><div>Will</div><div>Chemical Engineering/Science IV, Sydney University</div><div><br></div><div><br><div class="gmail_quote">On 4 August 2012 03:40, David <span dir="ltr"><<a href="mailto:david@h4c.org" target="_blank">david@h4c.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<font face="Georgia"><br>
<br>
Mark,<br>
<br>
</font><div>
<div>On 8/2/2012 3:29 PM, Mark Wells wrote:<br>
</div>
<blockquote type="cite">Hi folks,
<br>
I have been wondering if it is really possible to blow up a
digester, has any one actually ever seen this happen?
<br>
There is very little info on the web about this subject except
boring hype from flame and flashback arrestor manufactures.
<br>
</blockquote>
<br></div>
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.<br>
<br>
In response to the most general interpretation of the question,
then, yes: it is possible to blow up a digester.<span><font color="#888888"><br>
<br>
<br>
<br>
d.<br>
<div>-- <br>
<div style="font:Georgia"><span style="font-size:110%">David
William House<br>
</span>
<div style="padding-left:3em;font-size:80%">"The Complete
Biogas Handbook" <code><a href="http://www.completebiogas.com" target="_blank">www.completebiogas.com</a></code><br>
<i>Vahid Biogas</i>, an alternative energy consultancy <code><a href="http://www.vahidbiogas.com" target="_blank">www.vahidbiogas.com</a><br>
<br>
</code></div>
<code>
<div style="padding-left:2em">"Make no search for water.
But find thirst,<br>
And water from the very ground will burst."
<div style="padding-left:2em;font-size:80%">(Rumi, a
Persian mystic poet, quoted in <i>Delight of Hearts</i>,
p. 77) <br>
<br>
<a href="http://bahai.us/" target="_blank">http://bahai.us/</a></div>
</div>
</code></div>
</div>
</font></span></div>
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for more information about digestion, see<br>
Beginner's Guide to Biogas<br>
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for more information about digestion, see<br>
Beginner's Guide to Biogas<br>
<a href="http://www.adelaide.edu.au/biogas/" target="_blank">http://www.adelaide.edu.au/biogas/</a><br>
and the Biogas Wiki <a href="http://biogas.wikispaces.com/" target="_blank">http://biogas.wikispaces.com/</a><br>
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<br></blockquote></div><br>