[Stoves] Compact biogas plant

Sat Jun 23 12:53:30 EDT 2007

Dear AD,

I  just came across this earlier response of yours, concerning the  
issue of added weight reducing production in a biogas genmerator and  
noticed a possible gap in the inferences made.
IE., You state that the former (cow manure-based) tanks were made of  
metal and quite heavy--as opposed to your system using factory made  
plastic tanks which were much lighter. Fine in and of itself, but you  
then go on to state that you have noticed that adding weight  
diminishes production of gas. Were the feedstocks consistent in both  
cases  and if not,  how much is your increased production due to the  
different feedstocks you are using  (ie.,your system with table  
scraps and agro residues  VS., the conventional system (with larger  
heavier tank, using cow manure) and how much is it due to pressure  
differences ?

As concerns tilting and jamming at full stage and the need for the  
upper frame, I noticed that another gentleman from northern India had  
earlier shown his system with  a clever solution which required  
neither weights (of either the top or  bottom ballast variety), or  
your upper guide pipe framework : He simply uses a taller bottom tank  
and a hole/ port for the overflow–located  in the side wall down  
about 30 cm from the upper edge of said tank (versus your duct right  
out the upper edge of your lower tank. This effectively leaves some  
contact area between the upper and lower tank (even at full gas  
production) and prevents its tilting and jamming. This is so  because  
the upper tank could not, by nature of the function of the  
apparatus,  rise above the outlet port.
While he is working with homemade sheet metal tanks, you (and  
probably most of the rest of us) would be limiting yourself to the  
factory made tanks. so the hols in the side of a taller bottom tank  
is probably not an option.

However the question about weight VS production still remains...IE.,  
how much is your higher production due to lower tank weight and how  
much is due to different feed stocks ?

Finally, as in the matter of designing pipelines for water  
distribution, one would need to simply use larger diameter pipe for  
greater distances, starting, particularly, at the tank outlet end and  
reducing diameters, only as you near the appliance in the kitchen. I  
have seen many such systems where one uses a garden hose of say 15mm  
I.D., to span a distance of more than 3 meters (from tank to kitchen  
appliance) only to have to add considerable weight to the tank top,  
with all your mentioned production losses. However this may also be  
due to pipe size !

I have found (although only through experience) that anything beyond  
3 meters one would want to jump up to 20 even 25 mm id pipe of same  
garden hose variety, again starting at the tank and reducing diameter  
only after the water trap just outside the kitchen. There are  
standard nomographs for calculating pipe size per desired delivery  
rate and distance travelled for various kinds of pipe (ie., with  
different coefficients of friction or " Reynolds numbers") ---for  
water but the same or similar standards must apply to gas pipe  
dynamics and hence Biogas pipe sizing as well.

Thanks in advance, for your elucidation . anyone else with gas pipe  
line design as relevent to this please add your comments. Its  
something no one else in the biogas world seems to have analytically  
covered--or at least published widely.

Richard Stanley

On May 6, 2007, at 09:26, adkarve wrote:

> Dear Kevin,
> In the traditional models of biogas plants the movable dome, or gas  
> holder,
> is made of iron plates, either welded or riveted together. The gas  
> holder is
> therefore quite heavy.  In our case, the gas holder is made of moulded
> plastic. It is quite light. There were instances, when the gas  
> holder got
> dislodged by strong wind when it had telescoped fully out of the  
> digester.
> In order to avoid such a mishap, we fitted our biogas plant with a  
> framework
> that holds the gas holder in place. We do not put any weights on  
> top of the
> gas holder, if  the biogas plant is just outside the kitchen.  What  
> Richard
> had suggested was that if we put weights at the bottom of the gas  
> holder,
> the weights would stabilise the gas holder and that we then would  
> not need
> the framework that held the gas holder in place.
> Now I come back to your question. Normally, the weight of the gas  
> holder is
> enough to deliver the gas to the burner in the kitchen. But if the  
> biogas
> plant is more than 6 meters away from the burner, one has to put  
> weights on
> top of the gas holder so that the system develops enough pressure to
> overcome the resistance of the 10 meter long rubber tubing.  In all  
> such
> cases, we found that the amount of biogas produced under pressure  
> was less
> than the amount produced without the weight.
> Yours
> A.D.Karve
>
> ----- Original Message -----
> From: Kevin Chisholm <kchisholm at ca.inter.net>
> To: <adkarve at pn2.vsnl.net.in>
> Sent: Sunday, May 06, 2007 5:14 PM
> Subject: Re: [Stoves] Compact biogas plant
>
>
>> Dear AD
>>
>> I am surprised at this...
>>
>> Certainly, if one tried to make biomethane at "high pressure", say  
>> at 2
>> ATM pressure, one would expect a fall-off, but at the small increased
>> pressure required to deliver gas to the burner, there should only  
>> be a
>> small retardation in methane generation rate.
>>
>> This should certainly be the case, in the case of an "inorganic
>> reaction", where the Law of Mass Action, and its Pressure Equivalent
>> were at play.
>>
>> Is it perhaps a case that in a biological system, the rules for how
>> pressure governs a reaction are different?
>>
>> If so, in nature, would the rate at which methane is generated be
>> different in a "shallow" pond, of say 1" depth, versus a "deep"  
>> pond of
>> say 4 feet depth?
>>
>> If so, then a "deep" biogas reactor (4') would have a slower  
>> production
>> rate than would an agitated biogas reactor that would have an  
>> effective
>> depth of say 1".
>>
>> Note also, that basically, the pressure in the biogas reactor with  
>> the
>> system Richard proposes would not be changed... what he is  
>> proposing is
>> basically to support the same total weight of gas collector weights
>> inside the collector, at its base, rather than outside the  
>> collector on
>> its top. The advantage of his proposed system is that the  
>> collector is
>> inherently stable, and side supports are not necessary.
>>
>> Best wishes,
>>
>> Kevin.
>>
>
>
>
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