Lines 1 and 2 utilise a fluidised bed mechanism A bed of heated inert sand particles is used to transfer heat to the waste that is to be incinerated in a fluidized bed incinerator. The bed rests on a perforated metal plate called a distributor plate and heated air is pumped from the underside of the plate, through the perforations, into the bed. The air bubbles through the particles causing it to act as a fluid, thus the name “fluidized” bed incinerator. Both the waste and the bed are enclosed in the combustion chamber which we refer to as the boiler. Line 3 utilises a moving grate technology Today, grate combustion is the most advanced technology regarding environmental friendliness, operating reliability, flexibility, and cost-effectiveness.
Residual waste is delivered to site in a variety of vehicles which pass over the weighbridge before being directed to the waste reception hall. Here the waste is discharged into a storage bunker. Overhead grab cranes then blend the waste to even out any variances in its calorific value (energy content). The same cranes then load the waste into a feed hopper, from where it is then fed onto the main combustion grate.
All combustion air is drawn from within the waste reception hall, which means that any odour arising from the waste is kept inside the plant.
Once the waste has been loaded into the feed hopper it moves down the feed chute and onto the feed table. Feed rams then push the waste onto the combustion grate. The grate is comprised of rows of inclined bars that move the waste material through the combustion zones whilst aerating it to provide optimum combustion conditions.
During the boiler start-up sequence, the process will use auxiliary diesel burners to bring the furnace up to a minimum permitted operating temperature of 850°C. Once this temperature is achieved, waste can be loaded onto the grate and the process becomes self-sustaining. At this point the diesel burners are no longer needed.
The waste passes through four distinct combustion zones as it moves down the grate i.e. drying, ignition, main combustion, and burnout.
At the end of the grate the ash drops into an ash extractor. This contains a quench bath which cools the hot ash and acts as an air seal for the furnace. The extractor slowly moves the cool ash onto a series of conveyors which take the ash to another storage bunker. On the way, the ash passes under a magnet which separates any ferrous metals contained within the ash.
The ash is called Incinerator Bottom Ash (IBA) and this is then taken off site for processing.