Look behind the scenes
The reason for the slowdown of the large biogas boom in Germany were the reductions in subsidies and above all changes in aid made by the amendment to the German Renewable Energies Act (EEG). For farmers, who are particularly targeted by the current reduction of subsidies for small plants, this is a strong reason to use their operational waste sensibly and to produce environmentally friendly energy from it in the most efficient way possible. Moreover they recycle their own waste, in contrast to purely commercial biogas plant businesses. However, efficiency is questionable when it comes to these small amounts of energy, with the bio-reactors needing some power and heat themselves. The following systems tailored for biogas production enable very high plant efficiency, while at the same time ensuring low costs and little effort.
The core element of biogas production is biomass fermentation by specialised fermentation bacteria. The better these can work, the higher the gas yield is – which means the plant concept should be designed to create the ideal habitat for the microbial aides. As far as possible, this includes good pre-maceration of the substrates to create more contact areas for fermentation, thorough mixing of the substrate to ensure full distribution of the bacteria and consistent temperature control of the fermenter. However, the usual biomass mix of silage, plant cuttings, dry components such as chicken droppings and manure offers a rather poor basis for this: the inhomogeneous composition not only limits fermentation, but also makes it more difficult to feed the fermenter. Screw conveyors are therefore often used. These can indeed transport any kind of material, but they are also susceptible to breakdowns, cumbersome and dirty.
Pulp feeding represents an alternative to dry feeding process described above. It involves the substrate already being homogenised, mixed and inoculated with fermentation bacteria before the fermenter. NETZSCH Pumpen & Systeme GmbH has developed the NEMO® B.Max® mixing pump specially for this application. This is based on proven progressing cavity pump technology which involves a helix rotor rotating in a stator that is geometrically adapted to it. In the process, conveying chambers with the same shape open up in which the medium is conveyed smoothly from the inlet to the discharge side, irrespective of its consistency. At the same time, the technology means there is no need for a separate mamixing tank and also enables conveyance of the biomass over longer distances and gradients allowing more flexibility in plant design. Where there is particularly lumpy, fibrous substrate with a high dry matter content, an aBP® bridge breaker can also be incorporated which prevents bridging on the sidewalls by means of rotating wheels. For even greater operational reliability – especially where it is intended to recycle very coarse basic materials – it is advisable to connect a macerator before the pump. This allows impurities such as stones to be removed and plant remains with long fibres to be chopped up before they can cause damage or clogging. NETZSCH developed the M-Ovas® cutting plate macerator for this, which comprises a knife head with hard metal knives and an edge made of hardened steel that can be used on both sides. The TORNADO® rotary lobe pump was for instance created for tasks involving high conveyance volumes, such as emptying manure barrels or substrate agitation. Its large conveying chamber and wide free ball passage mean it is to a large extent immune from clogging and can transport up to 1,000 m³/h, depending on the model.