How to Secure Digestion Process and Maximize Gas Production in Your Biogas Digester

A biogas power plant is a facility where organic wastes are converted into different kind of energy. The heart of biogas plant is the digester where wastes are decomposed by microorganisms during the digestion process. The final product of this process is biogas with methane which then is transformed into heat, electricity or natural gas.

In order to secure biogas production, digester has to be equipped with an efficiently working mixing system. The mixing system shall fulfill functions which are important for maximizing the gas production:

• The mixing system shall enable efficient use of the total digester volume,
• The mixers shall ensure homogeneity of the substrate,
• The mixers shall ensure an even temperature in the digester.
• The mixing system should help in pushing gas bubbles up to the surface,
• The mixing system shall prevent the sedimentation at the digester bottom; and prevents crust and foam formation at the liquid surface.

A mixing system plays a critical role in every biogas digester and thus it has to be designed in a proper way, below are the key requirements:

• The mixer design has to be tailor-made for digester environment,
• The mixer size has to be properly selected,
• The positioning of the mixers has to ensure proper flow in the digester.

Mixer operating conditions in the biogas digesters are mostly very difficult. Temperature usually above 40 C, corrosive and abrasive substrates together with aggressive substances in bio-gas require a robust machine design. Flygt’s range of biogas mixers with our own design of the most exposed components guarantees reliability and safety in biogas conditions. Mixers equipped with stainless steel ASTM 316L, Duplex steel, Hard-Iron™ or polyurethane propellers ensure long life of trouble-free operation.

There are more than 11 000 biogas plants worldwide and almost each plant use different combination of substrates. Most common are: energy crops, liquid or solid manure, domestic (municipal) organic wastes, industrial wastes, food wastes or municipal sewage sludge. Since each substrate has different properties the mixing system has to be designed in a unique way. Taking under consideration shape of the tank, substrate properties, pretreatment method (preparation of input material) we are able to calculate the amount of thrust which has to be generated by the mixing system to ensure homogeneity of the liquid and prevent the occurrence of dead zones in the total digester volume. Having broad choice of mixing devices (Banana, MidSize or Compact submersible mixers, Top Entry Agitators, Jet mixers) we are able to meet unique design or customer requirements.

Energy crop can result in floating material at the liquid surface. This floating layer can dry out, forming a crust that takes up digester volume and prevent gas bubbles from getting to the surface. With insufficient mixing, crust can present a serious headache to digester operators. To prevent crust building (or sedimentation at the tank bottom) positioning of the mixers plays significant role. A high intensity mixer (Compact range) located just under the liquid surface creates turbulences which effectively prevents crust formation. Similarly, a mixer located at the tank bottom prevents sedimentation. Xylem has supplied mixers to 5000 – 6000 biogas plants worldwide thus our application engineers have a deep application knowledge which ensures optimal mixer selection and positioning for every digester.