Sludge Combustion

Fluidised bed furnaces are used for single-incineration plants. Fluidised bed technology is known for the combustion of waste characterised by a not excessively dispersed particle size spectrum, such as sewage sludge (case under consideration) and RDF (Refuse Derived Fuel). The material used for the bed in the start-up phase is sand, and fluidisation is generated by a current of air, which also acts as an oxidiser and is mainly blown into the lower part of the furnace and evenly distributed by special nozzles. As the air velocity increases, it passes from a fixed-bed state to a boiling fluidised bed and finally to a circulating fluidised bed. The incoming air can be pre-heated by recirculation from the flue gas exhaust.
The advantages of the fluidised bed are many, but the main one is the homogeneity of combustion. For this reason, there is no post-combustion chamber and no further post-treatment for VOC, SOx and NOx abatement.
Another important feature is the high versatility with respect to the type of waste to be incinerated, which in the present case is the variability of the waste and sludge that will be treated. The main limitations of fluidised bed furnaces can also be easily managed, in particular: I limiti principali dei forni a letto fluido inoltre possono essere facilmente gestiti, in particolare:

• They have high thermal inertia with consequent difficulty in following rapid load variations (in the case under consideration, a constant load and limited variability of the calorific value of the sludge is envisaged)

• The size of the components to be oxidised must be as small as possible and as regular as possible (in the case under consideration, a screening system at the dryer outlet is envisaged, which sends the size between 2 and 8 mm in diameter to combustion).

• The velocity of the fluidising gas (1 – 3 m/s) must be lower than the velocity at which the particles are dragged to the next drying stage. The particles are pushed upwards by the gaseous stream and fall back into the bed where they sink back down by gravity and begin the cycle again. The solid gas suspension, therefore, remains confined to the bottom of the combustion chamber.

The most important features of boiling fluid beds are related to the ease of temperature control, with the following advantages:

• No local overheating
• No ash melting
• Limited NOx formation within the limits specified in section 14.1
• High combustion efficiency