[Digestion] Dry fermentation in developing countries

David david at h4c.org
Thu Oct 28 15:44:51 PDT 2010


On 10/28/2010 3:03 PM, Markus Schlattmann wrote:
> I don't know if I have understood this 100% correctly, but if you 
> have inlet air you will get a potentially explosive atmosphere 
> inside the digester. 

If the digester needs to be refilled then it must be opened. And if it 
has methane in it, there is no question that at some point, when air 
is introduced, there will be a potentially explosive mix. That will be 
the case whether one opens the door on the digester without previous 
evacuation, whether one introduces air rapidly or slowly, whether the 
digester is flexible and allows [only most] of the biogas to be 
"squeezed out" before opening, etc. Whatever process is used, at some 
point between operating the digester and opening it, air will have to 
mix with biogas and a potentially explosive mix will result.

But an actual explosion will require not merely an explosive mix, but 
as well a spark of some sort. Methane is not self-igniting. As such, 
if air can be introduced into the digester in a way that insures that 
no spark or flame is introduced, and as well that if any of the 
biogas/air mix that is being evacuated does ignite, it cannot in turn 
ignite the biogas/air mix in the digester, then there will not nor 
cannot be an explosion in the digester. A water trap is one of the 
simplest ways of accomplishing the task, since flames cannot travel 
down a column of bubbles.

That was the principle to which I was referring in suggesting to 
Yvonne that two flame traps be used, one to force air in, and one to 
allow the biogas/air mix out. As long as the mix being forced out is 
evacuated into moving air, outside, then in the presence of a spark it 
may ignite, but it will not explode because it is not contained. 
Likewise, if it ignites, the flame front will not be able to travel 
down the column of bubbles into the digester. Sparks and flames would 
likewise be excluded from the incoming air by the same means, since 
incoming air would likewise be introduced to the digester via a column 
of bubbles.

This is a very simple, and if well designed, very reliable means of 
insuring that enough methane can be flushed out of the digester so 
that any traces which remain cannot ignite or explode, and at that 
point the digester can be safely opened. Using simple measurements-- 
properly made, i.e. at the pressures experienced when using both flame 
traps-- one can find out how much air is forced into the digester 
during a given period, and one can then choose a duration for the 
evacuation process which will insure a non-flammable mix in the digester.

My apologies if this was not clear from my short explanation.

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)

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