Fluidized bed

  • Photo 1: fluidized bed furnace
  • Photo 2: special "sand" used in fluidized bed incinerators

The fluidized bed incinerator

The combustion chamber is full of floating particles, like ash and kind of small stones, and in the floating area is the waste mix burning. At the bottom in the combustion chamber there is a gas/air mix injected. The amount of air/gas is maintaining the combustion process. This type of incinerator is made for waste or fuel which is able to float. For this the waste is like a homogeneous mixture. Like WWTP sludge, crushed coal or carbon, MBM (meat and bone meal), etc. If the waste would not be homogeneous it there would be part which are falling, parts which are floating and parts which are flying away by pneumatic transport. For this type of waste exist fluidized bed incinerators with a kind of basket. This basket fluidized bed applications are only in a few cases justified and correct working.

Fluidized bed principle

A normal combustion process requires air (oxygen). If the amount of air (oxygen) is to less, there is an incomplete combustion. If there is to much air, the air is cooling the combustion chamber. So there is an optimum of oxygen for good combustion. Extra, in a fluidized bed, the waste needs to float, so the combustion can be done from all sites. So the the first requirement is that we to make the waste floating, and secondly the correct amount of air is necessary for correct combustion. Because, if the correct amount of air makes the waste flying out of the combustion chamber it is also not suitable waste for fluidized bed combustion. Some fine tuning can be done by injecting gas with air, or adding sand particles for floating. But with limits, because injecting gas for example is agains the "waste-to-energy" principle.

Visualization of the waste requiremens and combustion process limitations for fluidized bed

 

Fluidized bed Heavy waste medium weight waste light waste
excess oxygen (a lot of air)
for the combustion process
Floating waste but air is cooling combustion chamber (gas needs to be added) pneumatic transport pneumatic transport
Optimum oxygen amount
for the combustion process
waste is falling, not floating ok, fluidized bed, floating waste and complete combustion.
This can be in combination with adding gas or sand for fine tuning.
waste is flying out, without complete combustion to the filters. Here it is possible to add sand to make more floating and heavy material in the combustion chamber
To low oxygen amount
(to less air)
for the combustion process
Waste falls by gravitation down incomplete combustion Floating with incomplete combustion

 

Table: simple summary of suitable waste structures for fluidized bed incineration

Conclusion: The homogeneous character and the calorific value of the waste mix are necessary for correct combustion in the floating principle, and the correct amount of air supply is necessary for the correct and efficient combustion.