Risk of congestion averted: Robust grinder prevents shutdown of system due to coarse material in the biomass or sludge

In order to be economically viable, biogas plants today must be able to be operated using a variety of energy crops and bio waste. However, changing sources can rapidly become a problem for the operating company The consistency and composition of the biomass is difficult to predict due to natural fluctuations in their origin, coarse material continues to block lines and pumps – a phenomenon that the operating company of sewage plants must regularly combat. From this experience, NETZSCH Pumpen & Systeme GmbH has developed different series of grinders in order to protect the units installed downstream and, thus, increase the reliability of the process. Even media with a high ratio of dry matter content as well as solid and fibre parts can then be pumped.

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Silo maize, chicken manure with straw, liquid cow manure – practically everything that is produced in agriculture can be utilized for generating biogas. However, many of these media present a challenge when planning the plants. On the one hand, the mixtures cannot be conveyed very easily with parts that are too dry. On the other hand, in particular long fibres, such as in grass and straw, can also block the conveying pumps. The results extend from serious damage to the units through to operating shutdown during the period of maintenance that could result in "toppling“ the complete biogas plant. The pumps must be specifically designed for certain sizes of impurities and dry matter content, however, this is not always possible. It is not easy to predict the quality of the biomass, thus, the only solution is overdimensioning, to the detriment of efficiency.

Robust construction for high reliability of process

An economical alternative here is presented by reliable preliminary grinding of the medium. Large hazardous parts in the material mixture are destroyed and the medium is homogenized overall, whereby, it is made more fluid and can be reliably pumped. For this, NETZSCH has developed two designs of the cutting plate grinder M-Ovas® that attain throughput rates of up to 300 m³/h. Even at twelve percent dry matter content, up to 80 m³/h can be achieved. The basis for this is its hydrodynamic interior and, in particular, the cutting mechanism integrated in the housing cover: This combines an edge made of wear-resistant, hardened steel that can be used on both sides and a knife head with hard metal blades. A flywheel assists the grinding and, thus, saves energy so that only low drive power is required. Due to the product inlet and outlet being at the same height, the grinder can be directly integrated in a horizontal line, or connected to an inlet port rotated through 90°.

The fixed cutting unit leads to high cutting efficiency that prevents blocking the blades. The four impeller design of the edge support also increases the cutting frequency. Simultaneously, the edge can also be installed very close to the shear plate, whereby, the overall wear is reduced. If the cutting blade or plate must be replaced or cleaned, this can be very easily carried out through the large housing cover without laborious removal of the pipework. The housing itself is galvanized and anti-corrosive. For special cases a stainless steel deign is also possible. In order to prevent leakage of the product in the bearing, a single-operating hard metal mechanical shaft sealing with oil trap is used. Due to the different design measures, the overall plant is very easy to maintain, or only requires low maintenance.

Individual adaptation to the application requirements

In addition, a heavy matter collection separator is integrated in the M-Ovas® grinder. Thus, heavy parts such as stones, are separated from the actual biomass when passing through the plant, before it can damage the machines installed downstream. The sedimented materials can be removed without complication by a separate cleaning and drainage aperture.

The grinders can each be adapted for specific applications, thus, amongst other things, there are different geometries of perforated plate depending on the application. This flexibility can contribute, for example, in a small biogas plant in which leftovers of food and grass cuttings should also be recycled in addition to manure, liquid manure and maize. The dry matter content was approximately twelve percent. The cutters, which were already built in were not suitable for this substrate. The pieces were still too coarse after processing and blocked the pumps. In contrast, after initial test operation on a 4 kW drive and perforated plate with approximately 18 to 20 mm sized holes, it was converted to the M-Ovas®, whereupon the mixture can be transferred with no problem at 8 to 10 m³/h to the connected NETZSCH progressing cavity pumps. Since then the plant operates without further malfunctions.

Cartridge cutting elements in the twin shaft grinder makes maintenance easier

Where the cutting plate grinder is no longer sufficient, due to very large or solid impurities, the pump specialists provide the twin shaft grinder. Here, two cutting elements, each comprising of six blades and six spacing rings and operating in the opposite direction, reliably separate coarse parts in the medium. Depending on the application, there are different designs of blade available so that throughput rates of up to 300 m³/h can be attained, at a dry matter content of up to ten percent. Thereby, because only low drive power is required for this throughput, the operating costs are low.

This grinder is designed to be very robust and easy to service. For this, amongst other things, the cutting blade is designed as one piece, because as one unit, it is very easy to maintain and replace. The shaft seals that protect the bearing are also constructed as a cartridge. Robust ball bearings on both ends of the shaft ensure smooth operation and prevent deformation of the shafts for hard external materials. Furthermore, self-cleaning of the device is possible due to the different, very low speed of the shafts. The twin shaft grinder can be connected to every conventional IEC or gear motor by a flexible coupling with attachment flange. The unit – as the M-Ovas® – is intended for installation in a horizontal line, but is also available as a duct unit if required.

Can be used for heavy inhomogeneous biomasses or sludge

This type of grinder is used, for example, for very high inhomogeneous media that a shear plate system can no longer manage, for example, in a biomass recycling plant that processes material that flows well, but greatly differs in size and consistency. The medium is already preground here to a maximum of 40 mm using a submersible grinding pump, however, before entry into the progressing cavity pump, must be further refined and homogenized. For this, a twin shaft grinder with rigid cutting unit is installed that processes 25 m³ of the difficult biomass per hour and, thereby, ensures stable process sequences.

A sewage plant in Germany, near Essen also had similar problems: The external sludge produced there featured greatly fluctuating consistencies with dry matter content of between eight and eleven percent. In addition, the sizes and type of fibre and solid impurities in the sludge could never be predicted. A NETZSCH twin shaft grinder is also used here, in order to protect the displacement pumps installed downstream from damage and blockage. The machine installed, with its 8 mm thick blades, also grinds hard materials without interruption at high torque and low speed of 40 to 50 rpm. For this model, the complete cutting cartridge is 30 cm long, the blade has five or eleven teeth. The material used is 1.7218 chromium-molybdenum steel and tungsten carbide for the sliding ring pair of the seal. The result is a very robust unit that, has been in service for many years independent of the composition of the external sludge.