PRODUCT : COMBUSTOR, THE CYCLONIC CHAMBER
A cyclonic chamber is a horizontal cylinder connected to the boiler through an inverted cone-shaped neck. Secondary combustion air is injected tangentially, inducing cyclonic effect (see figure 4). The gases produced in the gasifiers are burnt in this device where a very active and turbulent combustion zone is formed.
This system allows a very low oxygen excess leading to a lower flue gases flow. The particles of fuel (if any) are also injected tangentially to the cylinder. By introducing a restriction at the end of the cylinder (the cone-shaped neck) the pressure created in the annulus of the throat promotes axial forces. The dynamics of the system are such that particles entering at the closed end follow a helicoidally path at the periphery of the cylinder towards the throat, return to the close end following a smaller diameter helicoids, and finally leave the chamber, fully burned, through the throat. This flow system provides for the long residence time required for a complete combustion.
Measurements made in several installations reveal a particle residence time up to 60 times longer than the time the air takes to flow directly. The chamber is formed by tube rings connected to upper and lower headers. The whole tube system is connected to the natural circulation of the boiler by an independent system of risers and downcomers. The gap between two consecutive rings is filled by plates welded to the tubes, creating thus a complete water-cooled surface. Nozzles are placed along the gaps as needed to generate the aerodynamic fields.
The cyclonic chamber is appropriate for burning any liquid or solid biomass that can be transported in pneumatic suspension. The capability to be suspended depends on the density/cross sectional area ratio but, for many biomass types, this means roughly that the particles must have diameters ranging from 0, 1 to 30 mm. There is no limitation whatsoever in burning together particles with wildly different sizes or densities, as long as all of them can be maintained in suspension. The cyclonic chamber can also be used to burn gases generated in a gasifier, or a mixture of gases and solid particles.
Fuels with humidity below 20% can be injected into the cyclonic chamber without any additional treatment. If the humidity ranges from 20 to 25% the combustion air must be preheated to 100 to 150ºC. Finally, if the humidity is greater than 50%, the biomass must be dried before entering the cyclonic chamber.
The main advantages of the cyclonic chamber can be summarized as follows:
- It allows an efficient combustion of the celluloses residues, ie., with very little air excess and with high degree of completion.
- As a consequence, the gaseous emissions (particles, VOCs and CO) are within current and foreseeable regulations
- Multiple biomass combustion system.
- Flexible in operation range (40% to 100%)
- Its investment cost is lower to that of a grate and much lower than a fluidized bed.
- The cyclonic chamber technology is scalable over a wide range of required power. Many cyclonic chambers have been installed with powers ranging from 750 kW up to 40 MW.
- Most existing boilers can be retrofitted with cyclonic chambers.
- Cyclonic chambers can be applied to both watertube and firetube boilers.
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