Renewable ‘wood chip’ fuel - from poo

Posted: Wednesday 4th January 2012

They look just like instant coffee granules but consuming them in a hot drink is not recommended. For these are in fact sewage flakes - highly-combustible new renewable fuel that burns like wood chip.

Thames Water has begun producing the flakes by drying sludge, the solids from sewage, in a futuristic, eco-friendly machine at Slough sewage works in Berkshire. The company then burns the flakes to generate electricity. The £1.5m sewage sludge dryer at Slough is the first in the UK to be used to create fuel. Until now they have been used simply to reduce down waste into order to get rid of it more conveniently.

The dryer promises to reduce the firm’s carbon emissions by more than 500 tonnes a year – the equivalent of taking 180 cars off the road for good, as well as bringing up to £300,000 a year of operational cost benefits. Rupert Kruger, Thames Water’s head of innovation, said: “This is the first time in Britain that a waste dryer has been used to create ready-to-burn fuel from sewage sludge, rather than simply being used as a waste-reducer. This innovative approach demonstrates our clear intent to help move Britain towards becoming a low-carbon economy by unlocking every ounce of renewable energy potential from waste.”

Five tonnes a day of sewage sludge, 20% of the solids left over from the treatment process at the Slough works, are being put through the new dryer, which works by heating the sludge to around 180?C and driving off the water using enclosed heated rotating paddles. The resulting sludge flakes are then transported to Crossness sewage works in Bexley, east London, where they are fed into a sludge-powered generator, which burns 160 tonnes a day of ‘sludge cake,’ the de-watered solids from sewage, to generate renewable electricity.

Because the sludge cake at Crossness is only about 25% dry, the process relies on non-renewable gas from the grid to keep the fire going in the sludge-powered generator. Using more readily-combustible sludge flakes, which are 95% dry, at the Crossness generator will reduce its reliance on gas, driving down the company’s fuel bills and reducing its carbon footprint. The cost saving resulting from the Slough sludge dryer comes from reducing use of non-renewable gas from the grid, additional Government ROC (Rewewable Obligation Certificate) feed-ins and reducing the volumes of leftover sludge that need to be trucked to spread on agricultural land as fertiliser.

Kruger added: “For decades we have generated £15m a year of electricity by burning biomethane from sewage. Last year we fed renewable gas, from sewage at Didcot in Oxfordshire, into the gas supply network for the first time in the UK, and agreed plans to build Europe’s first reactor to produce phosphate-based fertiliser from sewage, also at Slough. The new sludge dryer is the next chapter in our quest unlock the full energy potential of waste.”

The paddle-dryer at Slough sewage works burns biomethane gas from sewage to heat a closed-loop oil system, which heats two 2.5-metre-long hollow shafts to 180?C.

Sewage sludge that is 20% dry is fed into the shafts and interlocking paddles rotate, moving the sludge in the intense heat and drying 95% of the water content from it.

The dryer, developed by US firm Komline-Sanderson, is 98% ‘thermally efficient’, which means most of the energy used to heat the system is used effectively, with minimum wastage.

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