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New mixing system defeats biogas digester crust at leading pig producer

A thick floating layer of crust that was hampering the biogas operation of a forward-thinking Danish pig producer has been defeated by Landia’s GasMix digester mixing system.

The Roslev-based business (established 1933), which includes pig production, plant breeding and green energy, first encountered problems with its existing propeller stirrers when a steel wire broke, as Ny Rybjerggaard’s owner, Jens Henry Christensen explains:

In one of our reactor tanks (3300 m³), it transpired that a stirrer that was meant to be operating at the top of the digester had only been running at the bottom, because the wire to raise the propeller stirrer was broken”.

He added: “In no time at all, a floating layer had formed at the top – and although we had the stirrer repaired, the problem remained. It seriously hampered agitation and circulation of the straw and slurry feedstock that we use for our biogas operation. This situation highlighted the limitations to us of propeller-type equipment for this application, which can’t really cope with dry matter – so we knew it was time to find a superior, long-lasting, low-maintenance mixing system”.

Top quality biosolids at Ny Rybjerggaard

To alleviate the problem, Landia (coincidentally, also established in 1933) first lent Ny Rybjerggaard a 30 kW GasMix digester mixing system as a temporary look-see solution.

Steen Buhl Larsen, Landia’s Managing Director, said: “Initially, this larger motor was needed to ensure that the thick floating crust could be dissolved – and to bring about comprehensive mixing of the digester”.

Jens Henry Christensen from Ny Rybjerggaard, continued: “In just a very short time we could see the improvement with the Landia GasMix. We dug a hole through the thick floating layer, where we mounted the Landia GasMix on a rail bracket one meter above one of the old agitators. This was on the opposite side to where one of the old agitators had burnt out. This was quite an achievement because the floating layer had become so thick that you could have stood on it. The retrofit was simple – and slowly but surely, the half-meter thick floating layer was completely gone”.

The digester now has trouble-free circulation – with Jens Henry Christensen also able to save energy by switching off the Landia GasMix during the daytime. This fits perfectly with Ny Rybjerggaard’s modern agricultural technologies at its 870-hectare site, as the business works towards increasing its self-sufficiency with renewable energy, including the sale of gas to Denmark’s national network.

Jens Henry Christensen from Ny Rybjerggaard (right) with Landia’s Steen Buhl Larsen.JPG

When you have a tank with such big problems, you need a bigger motor to begin with”, added Landia’s Managing Director, Steen Buhl Larsen.

Now that the floating layer has gone at Ny Rybjerggaard, we can incorporate an 18 kW Landia GasMix (supplemented by the two 18 kW propeller mixers) that can easily handle the agitation. With GasMix at the top of the digester, together with two propeller mixers at the bottom of the tank, you do not have to keep adjusting to the delicate balance that is needed for propeller systems. The small gas bubbles from the Landia GasMix are enough to prevent a floating layer”.

With no moving parts inside the tank, the Landia digester mixing system also provides considerable maintenance benefits – eliminating the need to empty the tank and halt production.

Mixing is now at an optimum level. There is no doubt about that”, added Jens Henry Christensen. “I can clearly see the circulation and fermentation with bubbles inside the tank – and at the same time there is barely a difference of one degree in temperature between the bottom, the middle and the top of the digester. This is an extremely good sign.We can also see a major improvement in the final digestate, which is thick and uniformed, like a good porridge.

Getting rid of that floating layer is a big relief”, he concluded. “Overall, we are now experiencing a 15-20 percent faster turnover with Landia GasMix, so we can push the plant harder and get much more from it”.

Photos and copy by Jesper Ernlund Lassen

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