Pumps for Biogas Plants
Pumps are required to bridge differences in height between the levels of slurry-flow through
the biogas unit. They can also be required to mix the substrate or to speed up slow flowing
substrates. If substrates have a high solids content and do not flow at all, but cannot be
diluted, pumps or transport belts are essential.
Pumps are driven by engines, are exposed to wear and tear and can be damaged. They are
costly, consume energy and can disrupt the filling process. For these reasons, pumps should
be avoided where possible and methods of dilution and use of the natural gradient be utilized
instead.
If pumps cannot be avoided, they can be installed in two ways:
• Dry installation: the pump is connected in line with the pipe. The substrate flows freely
up to the pump and is accelerated while passing through the pump.
• Wet installation: the pump is installed with an electric engine inside the substrate. The
electric engine is sealed in a watertight container. Alternatively, the pump in the
substrate is driven by a shaft, the engine is outside the substrate.
Types of pumps
Rotary pumps
Rotary pumps operate with a rotor which presses the liquid against the outside wall of the
rotor chamber. Due to the geometry of the chamber the liquid is pushed into the outlet pipe.
Rotary pumps are very common in liquid manure technology. They are simple and robust
and used mainly for substrates of less than 8% solids content. The quantity conveyed per
time unit depends largely on the height of lift or the conveying pressure. The maximum
conveying pressure is between 0,8 and 3.5 bar. The quantity that can be conveyed varies
from 2 to 6 m3per min. at a power input of 3 - 15 kW. Rotary pumps cannot, usually, be used
as a sucking device. As a special form of rotary pumps, the chopper pump deserves
mentioning. It’s rotor is equipped with blades to chop substrates with long fibers like straw
and other fodder parts before pumping them up. Both wet and dry installation is possible with
rotary pumps.
Positive displacement pumps
Positive displacement pumps are normally used for substrates with higher solids content.
They pump and suck at the same time. Their potential quantity conveyed is less dependent
on the conveying pressure than with rotary pumps. The direction of pumping / sucking can be
changed into the opposite direction by changing the sense of rotation. In biogas units, mainly
the eccentric spiral pump and the rotary piston pump (both positive displacement pumps) are
used. For better access, a dry installation is the preferred option.
Eccentric spiral pump
This pump has a stainless steel rotor, similar to a cork screw, which turns in an elastic
casing. Eccentric spiral pumps can suck from a depth of up to 8.5m and can produce a
pressure of up to 24 bar. The are, however, more susceptible to obstructive, alien elements
than rotary pumps. Of disadvantage is further the danger of fibrous material wrapping round
the spiral.
Rotary piston pump
Rotary piston pumps operate on counter-rotating winged pistons in an oval casing. They can
pump and suck as well and achieve pressures of up to 10 bar. The potential quantity
conveyed ranges from 0.5 to 4 m3/min. The allow for larger alien objects and more fibrous
material than eccentric spiral pumps.
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