Spinning sludge

After next year, Britain's sewage sludge can no longer be dumped at sea. But this good news for the fish could prove problematic on land, as a new disposal route must be found for the 5million tonnes of sewage that the UK currently sends to sea each year. Incineration is seen as the best option, but the current best technology - fluidised beds - would require more than 300 new facilities, each with a bed diameter of 3m. The answer, according to researchers from Sheffield University, could be to send the incinerators into a spin.

Rotating fluidised bed (RFB) incinerators, the subject of the team's research, are basically buckets that spin around their axis. Unlike a conventional bed, which is fluidised by a stream of air from its base, RFBs have perforated sides, and the air enters the bed through these perforations (see diagram).

The amount of waste that can be burned in a fluidised bed depends on the amount of fluidising air blown into it. Conventional incinerators fluidise when the speed of the fluidising gas overcomes the weight of the particles in the bed, and cannot exceed a maximum rate, which would form bubbles. In RFBs, the team explains, they have to overcome the centrifugal force imposed by the rotation. This minimum - and the corresponding maximum - can be increased by simply spinning the bucket faster.

There are still problems, the researchers concede, because of the complex mechanism of fluidisation in an RFB. The bed fluidises in layers, from the free surface outward, and must be entirely fluidised for the combustion to be uniform. If the bed is too thick, the material at the surface could fly out of the bucket while that near the walls was still packed solid. Ash build-up, which thickens the bed, is therefore important, and a method for removing ash must be built in, the team notes.