by Vibrafloor, France

 

The Drax power station in North Yorkshire, with 4,000 MW of generating capacity, provides seven percent of the UK's electricity needs. Drax is both the name of the village that houses the power plant and the name of the company that operates the electrical production site, Drax Power Limited.

Since 2012, this company has decided to progressively convert its supplies from being a coal-fired power station to biomass, and by 2020, its six boilers are expected to be fully pelletised. For the storage and distribution of pellets, on these industrial scale installations, their electricians have opted for Vibrafloor's vibrating floor dome and total drain silos, designed and manufactured in France.

 

A conversion under the guise of fighting climate change

The Drax plant, the largest in the UK, was also the second largest coal-fired power plant in Europe after Bełchatów in Poland. And just a few years ago, to produce 24 TWh a year on coal, it also emitted 22.8 million tonnes of carbon dioxide a year.

After projects of new biomass generating units in the 2000s, many tests in biomass and coal co-combustion were carried out in 2004 in the existing boilers, with willow blister, sunflower and peanut shells. In the view of the government, the solution is to modify existing boilers to consume crushed and pulverised wood pellets that have been retained for large-scale conversion of the plant.

In September 2012, the Drax Group announced the conversion of three of its six units into complete combustion pellets, each consuming about 2.3 million tonnes of granules per year. In 2013, the first of three planned units was successfully commissioned. In 2014, the second unit was, in turn, commissioned.

At this point, Drax is completing the construction of four storage domes, each of which can hold 75,000 tonnes, classified in the ATEX zone to store its pellets. At the same time the electrician invests in the United States in several pellet production plants to guarantee its supply.

In 2015, the Drax Group began converting its third unit and in 2016 announces that 70 percent of the electricity it generates comes from wood pellets, which is about 20 percent of the renewable energy produced in the United Kingdom.In 2018, Drax announced a fourth unit will switch to biomass.

 

The largest pellet silos in the world

At Drax, each dome is built on the basis of an inflatable polyurethane structure inside which concrete was sprayed, is 50 metres high and 63 metres in diameter. They are  equipped with the Vibrafloor system, which guarantees a total emptying of their contents without any external intervention in complete safety and reliability.

The bottom of each silo is covered with 370 Vibrafloor vibrating modules of dimensions 2 × 3.5m, independent of each other which, slowly but surely, convey the product towards the centre of the floor, under which two conveyors feed the boilers.

Each module is powered by its own 690W motor and ensures the transfer of biomass to the hoppers. Each dome thus has a capacity of supply to the power station of 4,600m³ of granules per hour. It is fed continuously at the top from full trains [IS6] to large cars that follow each half hour.

 

Technology that pushes storage limits

Drainage is gravitational, with no moving parts other than the vibrations of the plates, which represents a considerable advantage in view of the pressure exerted by the 75,000 tonnes of product per silo. Nothing can break, the modules are designed to support up to 220 tonnes per m², 10  times more than the actual pressure exerted in Drax silos. The fuel cannot vault or remain blocked, because propagating from the centre of the silo on the total surface, the vibrations erode, destabilise and dislocate the residual slope until complete emptying.

The operation is completed unlike any other extraction systems, without destruction of granules and without release of dust.

The Vibrafloor drain system is as energy efficient as other system. Thus, the total electrical power installed per dome, to operate the vibrating floor, does not exceed 255 kW. Sensors automatically trigger and stop pre-programmed vibration sequences, minimising up time and power consumption. The Vibrafloor technology meets the requirements of the Atex standard, essential in this type of work.

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