Biogas from biowaste: - Robust pumps enable a range of substrates to be conveyed, from liquid manure to food waste.

Biogas is in high demand as a source of energy for power and heat generation. Systems that not only process crops specifically grown for power and heat gen-eration, but also use otherwise unusable biowaste, are excellent for the envi-ronment and are highly economical.

A community of farmers in Alsace founded Agrivalor Énergie to exploit that potential - the company primarily feeds its co-generation plant with biogas from leftover food production and food that has passed its sell by date. As a significant range of different substrates has to be conveyed, depending the source material, the company uses the robust pro-gressing cavity pumps from NETZSCH Pumpen & Systeme GmbH. Maintenance has so far been unnecessary in the two years of operation since, and any ob-structions are also simple to clear thanks to the flexibility of the pump technology.

The aims of farmer Philippe Meinrad and his two colleagues when they founded Agrivalor Énergie included finding a solution to disposal of general biowaste, producing natural fertiliser and generating power and heat. The company has been operating a biogas plant with attached cogeneration plant in Ribeauvillé, Alsace, since 2012, mainly recycling typical basic substances such as maize silage, liquid manure, and grape marc, but also food waste from restaurants, schools and supermarkets, such as fats and dairy products. Instead of laborious and expensive waste disposal, the waste is used for a meaningful purpose.

The company collects some of the material in its own lorry from the source or a dealer and unpacks any food still in its packaging. The waste is sorted and shredded in a hammer mill, before being pressed, sanitised and conveyed into a mixing tank with liquid manure and marc. From there, the mixture travels on to the fermenter, where the biogas is formed. Substrate not consumed is used as fertiliser.

Reliable conveying thanks to resilient design

However, the varying mixtures of materials put a major strain on the plant's conveying systems. Some of the materials, such as the liquid manure, are corrosive and some are fibrous and likely to cause obstructions, such as grain waste. Materials of animal origin also have to be exposed to intense heat to prevent disease, so some media are still at 70 or 80°C when they are conveyed. Agrivalor Énergie ordered ten NETZSCH-made NEMO® progressing cavity pumps from the manufacturer of their biogas plant to achieve maximum possible reliability without significant maintenance or downtimes, despite the tough conditions.

The pumps follow the rotor/stator principle, so that they are very suited to varying media consistencies and higher proportions of dry substance. Within the pumps, a helix rotor rotates in a reverse-helix stator and in the process creates conveying chambers with the same shape, in which the substance is transported from the suction to the discharge side. The funnel-shaped stator inlet also facilitates filling. The system ensures continuous conveying at stable pressure regardless of the quality of the substrate and without strong pulsation or shear forces. The volume can be regulated easily via the rotation speed. The direction of rotation can also be reversed with ease. "Occasionally a pipe gets blocked. We reverse the pump for a bit and that does the job", reports Meinrad.

The pump chamber is designed throughout to be free of dead space to minimise the risk of obstructions and deposits. The rotor is made of stainless steel with hard chrome plating and the stator is made of particularly durable elastomer compound that can withstand, for example, the acidic ammonia in the liquid manure over the long term. Attention was also paid to ensuring that the materials would not be damaged by the hot substrates after sanitisation. The pin joints are enclosed in metal sleeves and very robust shaft seals were also installed. The design ensures long service life, despite tough media, and reduces the cost of maintenance and servicing. The ten units at Agrivalor Énergie have therefore been running completely without maintenance since first starting up in December 2011.

Ecological energy for the neighbourhood

Two of the larger pumps with capacities of around 40 m³/h are used to transport the mixture from the collecting tank (with around six to seven percent dry matter content) into the fermenter. Other NEMO® pumps with a capacity of 20 m³/h are used to convey the recirculated substance and introduce the sanitised hot liquid manure into the mixer. All the units are extremely compact and have a block design with flange-mounted motor, saving a lot of space on the premises of the privately owned biogas plant. The efficient pump design only requires minimal power, which - along with the long service life and long maintenance intervals - leads to low overall operating costs.

In total, 30,000 tonnes of different types of organic waste is processed by the company each year, two thirds of which is biowaste from retail, restaurants and the food industry. Around 1.4 MW of electricity is generated in the company's cogeneration plant and fed directly into the network. A further 1.3 MW of heat is sold primarily to a casino hotel to heat a spa. A dairy, three houses and a drying facility are also supplied with eco-friendly heat.

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