Growing power at Stoke Bardolph
Severn Trent Water is now the proud owner of the plant which will generate over 15000MWh of electricity each year. Interserve’s build contract was valued at £12 million.
Use of the anaerobic digestion process is already widespread in continental Europe, where nearly 300 operational plants have already been provided by Schmack Biogas, our process contractor specialist.
Crop digestion plant: power generation from renewable sources
The Stoke Bardolph estate covers 750 hectares, the majority of which is now used for growing maize. This farmland has been fertilized with sludge from the sewage works for more than 120 years and cannot be used to grow crops for human consumption. In 2007, Severn Trent identified an opportunity to grow maize on this land for the purpose of generating renewable energy.
The footprint of the crop digestion plant is 22,000 sq m and consists of silage clamps, pascos (initial feed point), eucos (primary digesters), coccus tanks (secondary digesters), and a sula (tertiary digester).
Sustainability in action
Within the plant, maize silage is anaerobically digested over a period of around 90 days, during which time the majority of the organic matter is broken down to produce a methane-rich biogas. The gas is then used as a fuel in combined heat and power plants (CHP) to generate both electricity and heat. The electricity produced is used to help power the adjacent sewage works, reducing the amount of fossil fuel-derived electricity imports from the national grid to zero.
A second environmental benefit of the process is that during the cultivation of the maize crop, the plants naturally convert carbon dioxide into oxygen. The heat given off from the CHP engines is re-used in the process to maintain optimum operating temperature. And finally, after the maize silage has been digested it is recycled back to the fields as a fertilizer to help grow next year’s crop. This makes the plant highly efficient and represents an annual carbon saving of around 8,500 tonnes of CO2 equivalent.
The design and construction team overcame significant challenges including adopting European designs for UK legislation and water industry standards, poor ground conditions, bad weather, and a very tight programme. The site achieved an excellent safety record through a series of training and inductions and diligent supervision.
Due to the low bearing capacity of the ground, ground stabilisation was carried out as part of the earthworks. Rolling dynamic compaction took place, followed by the stabilisation of the ground by mixing cement into the formation by 600mm. This process increased the bearing capacity of the ground so as to be adequate for proposed structures, and avoided the import of some 30,000 tonnes of stone material, further reducing the carbon footprint of the project.
The first section was completed on time in September 2009. This required Interserve to complete the construction of five storage clamps (each measuring 25m x 75m x 5m high), access roads and site drainage to ensure that the crop could be stored safely on site. This delivery date was fixed as the first feed crop had already been planted before the construction was able to start.
Substantial completion was achieved on time despite starting four weeks later than planned. The CHP engines came on-line in August 2010, following the successful synchronisation with the National Grid, which was conducted by our supply chain partners and the regional electricity company.