Maize now has competition as the most important energy crop for German biogas plants: beets, particularly sugar beets and specially-bred energy beets, are becoming more and more popular. Firstly, they achieve a very high dry matter content, which is quickly transformed into gas thanks to high sugar content and secondly, they enable a greater diversity of crops and can therefore prevent crop rotation diseases and pest infestations.
However, the storage of the beets in silos or silage tubes is sometimes complicated and expensive. Collecting beet pulp in foil-lined reservoirs represents a comparatively simple and cost-effective alternative to this. So as to be able to convey the substrate to the biogas plant from these lagoons, Netzsch Pumpen & Systeme GmbH has developed an immersible pump, tailoring its materials for use with the abrasive pulp.
In a specific application, around 10 m³ per hour of finely diced beet was to be transported to the fermenter. However, as a semi-solid oxidation layer forms on the pulp in the lagoon, the high-viscosity substrate must be drawn off underneath this layer of scum. For this application, a Nemo BT immersible pump was therefore installed. These are also used elsewhere for emptying tough to high-viscosity media such as crude oil out of tanks and pits.
Progressing cavity technology provides high suction power
The system is based on the progressing cavity principle where a helix rotor rotates within a stator that has a correspondingly reversed helix. This means that chambers of the same shape are formed between the rotor and stator and these move the medium from the suction to the discharge side smoothly and without pulsation. In the process, the volume conveyed is directly proportional to the speed, which enables accurate dosing. Since the clearances between the two components are very tight and the geometries are precisely attuned to one another, the displacement pump can create a sufficiently large vacuum to enable suction in a vertical position. The special pump technology makes suction possible with any substance, just as long as this is freeflowing.
On the other hand, the challenges were the low pH value of the sugar beet pulp and the high, abrasive sand content of up to 10 percent of the substrate volume. For this application, Netzsch s developers therefore fitted a stator made of highly-resistant elastomer and a wear-resistant rotor. In addition, the pump is operated at an appropriate speed which increases its service life. Material wear could therefore be kept low in spite of high stresses.
Hoist system for adjusting immersion depth to the reservoir fill level
Since the fill level of the lagoon falls when it is being emptied, it was essential to find a way of adapting the immersion pump to the fill level. Due to the existing wall design on site, the customer constructed a special rail frame which was fitted directly onto the wall. The pump runs along these rails on wheels and its height is adjusted according to the fill level of the reservoir using a hoist. With this pump design, the drive is directly flanged onto the pump s lantern, which means the system can be kept compact and weight can be saved. A coupling rod with two cardan joints transmits the torque from there to the rotor. The beet pulp is conveyed at 10 bar. To keep any possible pressure losses to a minimum, the discharge side was generously dimensioned, which enables any pressure drop to be limited to a maximum of 0.1 bar/m of discharge pipe.
Nemo BT immersion pumps are generally suited to differential pressures of up to 24 bar and pumping capacities of up to 140 m³/h, depending on the size. There are also various options for the materials, seals and rotor-stator geometry to adapt the conveying system to the medium and the required performance.
Picture: The high suction performance of the progressing cavity pump also enables reliable conveyance in a vertical position. In this case, the immersion depth for the process is adjusted to the fill level using rails and a hoist. (Source: Netzsch Pumpen & Systeme GmbH)