[Digestion] Pressure

[David Fulford]

David and listers, Hello,

The microbes do not seem to be affected significantly by pressure, at 
least up to the 1 atmosphere found in small dome plants as used in 
India, Nepal and China. In the 1970s there were reports of some earlier 
work in India that suggested less gas was generated at higher pressures; 
but gas analysis demonstrated that the reduction in volume at higher 
pressures was mainly due to carbon dioxide dissolving in the slurry. It 
is suggested that slurry from a dome plant will release carbon dioxide 
when it enters the reservoir chamber from the bottom of the digestion 
chamber, thus reducing its proportion in the gas chamber.

This early work in India led to attempts to create negative relative 
pressure by using floating gas drums suspended from cables with 
counter-weights. This occasionally allowed air to get in and mix with 
the gas. Many years ago, a colleague (long since dead, unfortunately) 
told me the story of a farmer in India trying to demonstrate biogas 
production to some visitors by lighting the gas from the outlet. The 
explosion made the gas drum jump upwards, spraying slurry all over the 
visitors.

A quick look at the chemistry of how carbon dioxide dissolves in slurry 
suggests that the process is fairly complex. Carbon dioxide forms 
carbonic acid with water, which then forms carbonates and bicarbonates 
with alkalis in solution. This is affected by the volatile fatty acids 
produced by the digestion process. A biogas plant is self-buffering; 
i.e. it controls its own pH as carbon dioxide is absorbed or released. 
It is possible that the pH is slightly decreased at higher pressures, as 
more carbon dioxide is dissolved, which might reduce biogas production, 
but I have never seen this reported. So, the answer is that the 
relationship between pressure and biogas production seems rather 
complicated.

Regards,

David

On 19/11/2010 22:31, David wrote:
>
> Dear friends,
>
> Excellent discussion lately. I was particularly impressed by the 
> article by Dr. Fulford about biogas burners. Best information I've 
> seen on this subject.
>
>
> I wanted to ask two questions just now, and I'll therefore be sending 
> another message to the list quite soon. Separating the questions will 
> allow the development of two different threads, if there is much 
> discussion about either question.
>
> The first question concerns pressure. Obviously AD is able to generate 
> biogas at pressure. Any vessel 33 feet deep will have two atmospheres 
> of pressure at the bottom, and there are a number of digesters which 
> are at least that deep. The gas produced in deeper vessels-- if they 
> are not stirred-- tends to have a bit less carbon dioxide, because 
> that gas dissolves rather easily as compared with methane.
>
> But I am not familiar with any work that has been done regarding the 
> relationship between pressure and the rate of production. That is, one 
> would expect that at increasing pressure, less biogas (measured, of 
> course, at STP) would be produced. But what is the relationship? How 
> dramatic is the reduction in the rate of production with deeper vessels?
>
>
> 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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>

-- 

********************************************************************
Dr David Fulford CEnv MEI, 15, Brandon Ave, Woodley, Reading RG5 4PU
d.j.fulford at btinternet.com <mailto:d.j.fulford at btinternet.com>, Tel: 
+44(0)118 326 9779 Mob: +44(0)7746 806401
Kingdom Bioenergy Ltd, www.kingdombio.com <http://www.kingdombio.com>, 
davidf at kindombio.com <mailto:davidf at kindombio.com>

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