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Drax | Steady Improvement

A new biomass processing facility – claimed to be the first of its kind in the country – is being built to help Drax power station in north Yorkshire meet its carbon emissions reduction targets.

The modest Yorkshire landscape near Drax power station is set to get a new industrial facility. Although it might not look spectacular to its neighbours, its simple exterior will mask modern carbon emissions reduction technology.

Site owner Drax is the driving force behind the £50M new facility, which from next year will begin receiving, handling, storing and processing various biomass materials ready for direct injection into the power station’s coal-fired boilers.

Before construction could start contractor Topp & Holmes decided to bring in ground improvement firm Pennine to increase the bearing capacity of the ground beneath the planned building on the 18,000m2 site. This involved installing vibro stone columns into the ground. The columns descend into the firmer clays that lie below the softer made ground on the surface.

“The new facility needs to cope with quite high loadings in places. In particular there will be two silos and the ground needs [to be improved to have a load bearing capacity of] up to 150kN for these areas,” says Pennine contracts engineer Matthew Truelove.

Pennine’s rig operators have installed 1,179 columns, which amounts to a fairly substantial sized project for the firm and one which it hopes might be repeated if there is a boom in demand for biomass processing facilities elsewhere.

The columns are 450mm in diameter and are typically to a depth of 3.5m into the clays below the 1m of made ground. In the area where the silos will sit, extra load bearing capacity was needed so 6m deep columns were formed here.

Pennine used its own manufactured Terrafirmer rig and vibro-flot ground penetration tool to insert between 130 and 150 columns per day. This was done using the top-feed method. Rig operators vibrated the flot down to the required depth of between 4.5m and 6m, creating voids in the soil into which the stone could be introduced to form the columns. Excavators help to introduce the stone at ground level while the flot is in the ground. Repeatedly retracting and inserting the vibrating flot allows the stone to reach full depth and at the same time the tool allows compaction of the stone in 500mm layers.

In keeping with the environmental awareness of the project as a whole, Pennine sourced reclaimed material for the columns. In this case it was a 40mm to 75mm diameter recycled railway ballast stone.

In October, Drax awarded Doosan Babcock Energy a £10M contract to supply direct injection biomass co-firing systems to all six coal-fired generating units at its site. The direct injection firing systems are the main component of the new co-firing facility and are designed to receive and transport processed biomass materials to the power station’s coal-fired boilers. Installation of the direct injection systems is scheduled to be complete towards the end of next year, at which point the units are scheduled to be commissioned along with the biomass handling and processing plant. Alstom is delivering the biomass plant for Drax under an engineering, procurement and construction contract.

The new facility will help the power station meet its wider plan to help tackle climate change by cutting carbon dioxide emissions by more than 15%. This is a substantial figure when considering Drax is the UK’s largest coal-fired power station with an output of 4,000MW – enough to cater for 7% of the country’s electricity requirements. “At Drax, we are only too well aware of the need to tackle climate change and we firmly believe that we are part of the solution. “We have a role to play in the transition towards a low carbon economy while delivering reliable supplies of electricity,” said Drax chief executive Dorothy Thompson at the time the biomass project was announced.

The 3,000m2 building will handle 1.5Mt of organic matter such as straw each year. Contractor Topp & Holmes has a £3M contract to build the unit associated with the biomass main processing works.

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