Biogas in the mix

Output from biogas facilities continues to increase. At the turn of the millennium, an average decentralised unit produced 50kW. By 2005 that had jumped to 300kW, making it more important than ever that the plant design should minimise the facilities’ own energy consumption.
For operators to run biogas units efficiently, they must achieve an optimum relationship between input in the form of material and energy for a given output. For profitability, meanwhile, all process equipment, and particularly agitators, mixers and pumps, must run efficiently, and every valve must be totally reliable.

Output of a biogas plant is defined by the rated electrical power requirement of their combined heat and power (CHP) system. The profitability of the plant depends not only on its initial cost of investment, but also on the energy and material expenditures required to obtain a predetermined useful output.

Internal energy consumption

One of the most important parameters for evaluating the energy input is the internal energy consumption of the plant and equipment required to generate the useful output. An effective way to reduce internal energy consumption is to optimise the mixing process, especially in the case of those biogas plants that form part of a farm complex.

The application of mixing technology is a specialist subject and because of this there is often a lack of know-how when it comes to comparing mixers and their mixing performance.

Maximum gas yield depends heavily on the precise and comprehensive agitation and mixing of the biomass. Whether it is for a mixing tank, main digester, post-digester or final storage tank, efficient low-speed agitators are at the heart of every stage of the biogas process.

Correct mixing is essential to enable operators to prevent floating blankets on the surface of the substrate, thus shifting the chemical balance towards the substrate to avoid sedimentation and increasing the space available for the fermentation reaction. Maintenance requirements are also reduced.

Operators must also homogenise the medium and minimise the “tea cup effect” to enable the organisms to interact effectively and efficiently. The reaction, moreover, requires a steady and uniform temperature distribution.

Low-speed submersible agitators are well suited to biogas facilities as they produce efficient bulk flow in large fluid volumes using minimum energy. Today’s state-of-the-art technology features a totally unbreakable propeller that should provide an optimised hydraulic system, enabling the mixer to produce more thrust and consume less energy.

Whilst it is important that the correct agitators are selected, it is equally important that they are positioned to best effect. One reliable and energy-efficient design, for instance, positions two low-speed submersible agitators, both with a prop diameter of 2.5 metres and with a power rating of 6.5kW, at different levels.

These bigger propellers move the same quantity of substrate at a lower flow velocity, reducing flow losses. This mixing process also treats the bacteria gently. Using this configuration in the digester means the hydraulic power is exactly where it is needed and not introduced from the tank edge.