Repurposing grain silos as biogas digesters: Advantages and Disadvantages

In our quest to find ways to enable local people to use local materials, recycle available resources and re-purpose found items into functional renewable energy systems we are now curious to explore the advantages and disadvantages of turning grain silos into biogas digesters.

While we were touring His Excellency former President Obasanjo's farms in Abeokuta and Ogun State in Nigeria, and discussing with him and his people how we might work together to transform the Obasanjo farm wastes (particularly chicken manure and chicken slaughter waste) into value added products (among them fuel and fertilizer), we took these pictures of the steel grain silos scattered about his broiler farm in between the hedgerows of teak (Tectona grandis) trees, papaya trees and cassava.

Interestingly, if you look at the top of the grain silo, above the conical bottom, it bears a nice similarity to the aspect of some typical commercial biogas facilities, such as these from the Alten Group (a German-Ukranian company) in Azerbajian.

Of course we know from our experiences touring commercial digesters here in NRW in Germany (such as those built by PlanET biogas) that a convex or cylindrical or dome shaped top is the right geometry for build up and release of the biogas itself, so if the grain silo can be made gas tight it will work perfectly well from that regard.

The question is about the conical bottom with the apex pointing downward. What advantages or disadvantages would that pose to the formation of proper bacterial sludge granules? The design for a grain silo is obviously for gravity funneling of the grain out. In the case of a biogas digester we are not interested in releasing anything from the bottom (except during the rare cleaning times). The question is really what effect the cone will have on the bacteria.

Interestingly, however, when we visited Blue Marble Energy in Seattle to see their biogas operation, we learned that they are now using stainless steel for all their digestors and that they have the same shape as Obasanjo's grain silos. The key (though I have yet to confirm this) may be in the use of a technique called the "Upflow Anaerobic Sludge Blanket"

More and more it appears that a modified upflow anaerobic sludge blanket (UASB) reactor, called an "induced blanket reactor (IBR)" may be the way to go for our Botswana project with the Jouberts (who want to build out of stainless steel) and with Obsanjo in Nigeria as well as elsewhere .
These quotes from a 2010 article in BioCycle magazine (the information source on composting and recycling wastes) called Anaerobic Digestion For Smaller Dairies :: BioCycle, Advancing Composting, Organics Recycling Renewable Energy make a case for IBR and UASB:

"The IBR naturally forms a thickened area of sludge in the lower
portion of the tank that contains high concentrations of bacteria.
Bacteria in this sludge blanket digest the manure (or other feedstock -- TH ), forming biogas.
Biogas attaches to the solids, causing the material to float up through the sludge blanket in the tank.

Near the top of the tank is a submerged septum or partition that
provides a means of separating the gas from the solids. An opening in
the center of the cone shaped septum allows the biogas to rise and exit
out the top of the vessel. The solids tend to sink back down the tank
after the gas is knocked off. Effluent also passes through the opening
in the septum and exits the tank via a pipe located above the septum.

The system does not employ any mechanical or hydraulic mixing. If the tank is mixed, the bacteria gets flushed out with the effluent, Watts explains. Not mixing the tank results in more bacteria per cubic foot."


This "Up-flow Sludge Blanket Filtration" system bears investigation. I don't fully understand it yet, but it seems to make sense, allowing aerobic and anaerobic processes to occur in the same unit at lower cost and with lower space requirements. The Covido system also makes use of Upflow Sludge Blanket Filtration (Ecofluid USBF process), and describes an innovative way of designing with conical shapes to allow both aerobic and anaerobic processes to occur (similar to Dr. Martin Denecke's patented 3A three-phase system for winning biogas from all substrates, including ligno-cellulose, but occurring in one small chamber).

My hunch, though I'm not an engineer (my masters and doctorate degrees are in Urban Planning) is that the geometry of the downward pointing cone may provide some benefits. I'm imagining, for one thing, that the fact that an inverted cone has most of its surface area on top and very little on the bottom will provide a temperature advantage for the bacteria -- the coldest water will sink to the apex. The top of the cone, where most of the exposed sludge granules will be relative to the food input, will be in a warmer zone.

Furthermore, the apex should be highly anoxic and somewhat protected from changes in water chemistry above, perhaps acting as a reservoir for bacteria during times of stress. By piping the feedstock slurry with a pipe buried half-way into the apex, whenever feeding occurs it could "thrust" the granules up so that they mix with the food, but then settle again by gravity into the apex.

Another advantage of the grain silo apex is that it offers a much smaller surface area to the outside in the coldest areas (the bottom) but can simultaneously be heated from that very apex without having to use a lot of heating coils. In fact there are a lot of ways such a digester could be heated:

1) heating coils wrapped around the apex of the cone and the cone itself
2) Painting the cone and the entire silo black and placing with best solar exposure
3) Focusing a concentrated solar thermal dish on the apex of the cone (see Gadhia Solar's large parabolic solar cookers, used all over India, for an idea of what I'm talking about)
4) Building a fire right under the apex of the cone

In all cases the concentrated heat would accrue on the smaller surface area of the cone and, since heat travels upward by convection, more efficiently heat the entire vessel.

The most exciting possibility that using a grain silo as a biodigester permits is the use of the Jean Pain method of Compost Heat:

One could pack compost (leaves, grass, wood-chips, rice straw etc.) around the cone from the apex to the part where the cylinder begins, and use compost heat a la the "Jean Pain Method" to heat up the biogas digestor. Because of the conical shape it would be much easier to stack and then to replace that compost when its calorific value is spent.

The grain silo's metal construction would still allow the cylindrical and top conical portions to be painted black and used to help heat the chamber via solar radiation.

From a first glance, then, retrofitting a grain silo to serve as a biogas digester may turn out to be a wise idea. We have yet to determine the costs (used steel grain silos in France seem to cost about 600 Euro according to Agrifaires.de ; we need to work out the possible plumbing and sealing and access difficulties, but it will be worth while finding out.



If anybody reading this has experience or suggestions, please do let us know!