What kind of digester design should you choose?

Which is the best primary digester? A covered slurry tank with side-mounted, fully submerged agitators - or a purpose built tank with 1:1 dimensions and a central paddle stirrer installed at the top? Xergi News has spoken to a number of biogas producers about their experiences.

A biogas plant not only represents a significant investment. It is also a plant that has to work hard for many years. It is therefore important to consider not only the size of your investment here and now, but also the long-term operating and maintenance tasks and costs required by the different types of plant design.

We spoke to a group of biogas producers to listen to their experiences of different digester designs.

Two types of digester design

Currently, there are two basic digester designs available.

The first is basically a covered slurry tank. The slurry tank commonly seen on dairy and pig farms is round and usually four times as wide as it is tall. In order to optimise the bacterial processes that produce gas in this digester, three or more agitators are placed inside with the motors submerged in the biomass.

With the second option, the digester tank is usually as tall as it is wide. This makes it possible to install a single, centrally mounted paddle stirrer positioned at the top of the digester tank. The motor running the agitator is placed on the top of the digester on the outside and is therefore not submerged in the biomass. The centre shaft of the paddle stirrer runs down from the motor into the digester. This has a number of special rotor blades that circulate the biomass in the digester tank.

Several of the biogas producers we spoke to have biogas plants of the second type with a top-mounted, central agitator - supplied by Xergi. However, several also have experience of the other design with its totally submerged agitators.

Complicated maintenance and unpleasant odours

The Danish biogas producer Linkogas has operated its biogas plant since 1990. At the beginning, totally submerged agitators were used, but since then the company has changed to top-mounted paddle stirrers. Linkogas currently has three main digesters with centrally positioned, top-mounted agitators.

Linkogas operates thermophilic processes, which means that the biomass is digested at a temperature of 50 degrees Celsius.

"Previously, we had four agitators in each digester - but that caused problems. Firstly, electrical installations submerged in warm liquid is not a good idea. It is just very difficult to cool something that is submerged in a liquid with a temperature of 50 degrees Celsius," explains Operations Manager Martin Fey from Linkogas.

When we needed to carry out maintenance on the agitators, we had to open the digester tanks.

"Opening the tanks meant loss of gas and created unpleasant odours. The neighbours weren't very happy about that," says Mr Fey.

The odour problem is confirmed by Dutchman Henk van Oosten, for whom Xergi built a biogas digester in 2007.

He also has other digesters, which are traditional slurry tanks with fully submerged agitators.

"The digester tanks with the submerged agitators produce a lot of unpleasant odours and the biomass is not mixed very well," says Mr van Oosten.

"The only correct solution is a steel tank that is as tall as it is wide - with a top-mounted agitator," comes the unequivocal conclusion from the Netherlands.

Lower energy costs

Martin Fey from Linkogas also points out that the top-mounted agitators make much more sense financially.

"Previously, we had four 11 kW agitators in each of our three main digesters. Now we have one 18 kW agitator in each of the three tanks. That means that we have more than halved our energy consumption for agitation."

What about the agitation?

At Linkogas, the top-mounted agitators have resulted in improved agitation. This means that you no longer have to interrupt the biogas process to empty the digesters of deposited sand.

At Danish-owned Bånlev Biogas, they are also satisfied with the 1:1 digester design with the top-mounted agitators.

Bånlev annually processes 150,000 tons of biomass, primarily pig manure supplemented by organic waste.

"We are thinking about the possibility of using energy crops and, of course, we have to consider whether the system with the top-mounted agitators will be able to handle a more solid kind of biomass," says Arne Jensen, Managing Director of Bånlev Biogas.

Capacity to handle energy crops

Xergi News has therefore spoken to more customers who have experience of energy crops and their response is universally positive.

In Germany, Quarnbek Biogas near Kiel has used a combination of animal manures and energy crops since 2006.

"The operation has been good. We haven't experienced any problems," says owner of the plant Christoph Schoeller.

At Staples Vegetables, one of the UK's largest vegetable producers, Vernon Read - one of the two brothers who own the company - is also very satisfied with their investment in a biogas plant with a central, top-mounted agitator. The plant operates on vegetable waste with a dry matter content of 11% and maize silage with a dry matter content of approx. 30%.

"We are very happy with the plant which provides very good agitation of the biomass. The plant has been in operation for a year and so far the agitation has been maintenance-free. When the time comes, we will be able to carry out maintenance without interrupting the biogas process in the digester because we have direct access to the motor, gearbox and electrical installations at the top of the digester," says Vernon Read. Xergi has just started the construction of an upgrade of Staples' plant which will see capacity increase to almost 3.0 MW.

The investment pays for itself

At Xergi, Managing Director Jørgen Ballermann is very happy with the feedback he has received from biogas producers.

"The challenge is for us to convince our customers to invest a little more initially, as we know this investment will be paid back with many dividends. We can see that the additional investment quickly pays for itself in the shape of lower energy costs and enhanced production because opening the digester tanks is no longer necessary. The good news is that this is not only based on Xergi's calculations, a range of biogas producers in various countries also confirm that this is the case - and they digest widely differing kinds of biomass," says Jørgen Ballermann.

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