Biogas solution saves on downtime and boosts environmental returns
29 Mar 2018
One of the largest French biogas plants, commissioned in 2017, is set to produce 2.2 million m³ of biomethane per annum, feeding the GRDF grid (Gaz Réseau Distribution France) and providing energy for 10% of the local population.
Almost half the digested organic waste mix processed comprises manure from cattle and poultry farming, with the remainder from local fish canneries and slaughterhouses.
Pre-treatment, the different waste streams are mixed into a homogeneous organic suspension, free of inert material.
The process (mesophilic digestion) deployed requires waste remains in the digester for 50 days rather than the typical 17-21 days – because of the lower processing temperatures and presence of vegetable matter, which takes longer to digest and accounts for 20% of content. Vegetables also reduce waste volume reduction after digestion to 20%, rather than the more usual 50%.
Digestate is reused as fertiliser for 20,000 hectares of surrounding farmland, reducing the need for 60 tonnes of chemical fertilisers and cutting carbon emissions by 4,222 tons of CO2
Three principal process transfer applications – buffer input to buffer tank via heat exchangers; buffer input to buffer tank through pasteurisers; buffer tank to digester via heat exchangers – present a challenge.
In the first part of the process, the slurry has a temperature of around 30°C and viscosity of 500 cPs, with 9-12% dry content (potentially up to 60%), creating very high abrasion.
This temperature rises to between 40-55°C in the second stage of digestion, but with a consequentially reduced viscosity. After pasteurisation, the temperature increases again to 70°C.
The pipework system involves considerable lengths and bends, from buffer input to the 5 metre high buffer tank (via heat exchangers), where the impact on back pressure could negatively affect many pump types.
Furthermore, low maintenance is paramount in biogas plants, which are highly automated with few human employees (the French plant has a staff of only three people).
With PC pumps, the potential for repeated maintenance is high: the stator is destroyed within seconds if the pump accidently runs dry. Factors such as pressure, abrasive content and temperature can all negatively affect performance.
Fit for process
To tackle the demands, the plant management opted for three Bredel 65 hose pumps from Watson-Marlow Fluid Technology Group (WMFTG).
These transfer viscous and abrasive slurries and sludge with up to 80% solids in suspension. Pump capacity is independent of suction and discharge conditions, and the choice of natural rubber hose means fluids with temperatures up to 80°C can be pumped, Hoses have a soft inner bore which cushions large and abrasive particles, whilst the multiple layers of reinforcing nylon and the tough outer rubber of the hose provide excellent performance and long hose life.
At the site, the Bredel 65 pumps run at relatively low speeds of 14 rpm to generate a flow of 4m3/h, on account of uptime and total cost of ownership requirements. Additionally, the pumps maintain constant flow against pressure, over the whole life of the hose, countering the issues created by long pipe runs.
The sole wear part – the hose –takes around 30 minutes to replace (in-situ) without the need for skilled personnel. Hose life is repeatable, ensuring that biogas plants can plan preventative maintenance accordingly.
The digestate produced ensures further cost and environmental gains: It is reused as a natural organic fertiliser for 20,000 hectares of surrounding farmland, reducing the need for 60 tonnes of chemical fertilisers, saving 17 adjacent farmers up to €15,000 each and cutting carbon emissions by 4,222 tons of CO2.