Mechanical dust collectors, often called cyclones or multiclones have been used extensively to separate large particles from a flue gas stream. Mechanical collectors are particulate control devices that use centrifugal force, gravity settling, and inertia to collect pollutants. Mechanical collectors can handle high dust loadings.
In cyclone separators, cyclonic separation method of removing particulates is used. Rotational effects and gravity are used to separate mixtures of solids and fluids.
Cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation method. The method can also be used to separate fine droplets of liquid from a gaseous stream.
A high speed rotating air flow is established within a cylindrical or conical container called a cyclone. Air flows in a helical pattern, starting from the top wide end of the cyclone and ending at the bottom narrow end then the air stream moves in a straight path through the center of the cyclone and comes out from the top. Larger particles in the rotating stream have too much inertia to follow the tight curve of the stream, due to this they strike the outside wall, and then fall to the bottom of the cyclone from where they can be removed easily.
In a conical system, as the rotating flow moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus separating the smaller particles. The cyclone geometry, together with flow rate, defines the cut point of the cyclone. Particles larger than the cut point will be removed with a greater efficiency and smaller particles with a lower efficiency.
There is an alternative cyclone design which uses a secondary air flow within the cyclone to avoid the collected particles from striking the walls, which helps to protect from abrasion. The primary air flow with the particulates enters from the bottom of the cyclone and is forced into spiral rotation by stationary spinner vanes.
The secondary air flow enters from the top of the cyclone and moves downward toward the bottom, intercepting the particulate from the primary air leading to separation of particulate. The secondary air flow gives the flexibility to the collector to optionally mount in horizontal position, because it pushes the particulate toward the collection area, and does not rely completely on gravity.
The mechanical collector is most effective on particles larger than 10 to 15 micron & for fine particles, the collection efficiency drops considerably below 80 %.
Mechanical collectors were adequate when the emissions regulations were less stringent and were used when firing techniques produced larger particles. These were frequently used for re injection to improve unit efficiency on stoker firing of coal and biomass. Due to strict emission regulations mechanical collectors can no longer be used as the primary control device.