Ashes to ashes, clumps to dumps
15 Jan 2000
The UK incinerates some 2.5million tonnes of municipal waste every year, generating around 750000t of fly-ash. Containing a significant amount of heavy metals, this ash is classified as hazardous waste, and has to be treated before disposal; if landfilled, the ash can leach metals into the water table. But current treatment methods are proving unsatisfactory, according to researchers from Sheffield; a new alternative could be both cheaper and more effective.
The process heats the ash at high temperature so that it sinters forms clumps which leach less than the original particles. The team says that this is more suitable than existing methods, which involve heating the ash to extreme high temperatures so that they form a glass; or using additives to stabilise and solidify the ash. The former method requires large amounts of electricity, they explain; the latter increases the volume of the waste which must be managed.
The process uses a pebble bed which pre-heats air before it enters a burner. The hot gases from the burner are fed into a cyclone, where it meets ash fed in via a screw-feeder. The gases at around 900 C sinter the ash particles in a fraction of a second, and the agglomerated lumps of ash fall to the bottom of the cyclone. The gases are then returned to another pebble bed, where their heat is recovered.
