Pelletsare typically manufactured from wood chips and sawdust. The wet raw material in usually grinded in a hammer mill and then dried in a rotary dryer heated by a biomass furnace. The material is then collected in dryer cyclones to be pelletized.
The air conveying the wet grinded material is cleaned with a particulate filter before going to the atmosphere. A common type of filter used for this application is the cyclo-filter, which combines the operating principles of cyclones with a cartridge filter on top to prevent the fines from escaping. However, since these cartridges clog very often (due to the high moisture content of the product), some plants remove it to be able to operate. Despite solving the operational issue, this leads to a secondary problem: an emission increase. The total concentration escaping the cyclo-filters without the cartridge is between 400 to 600 mg/Nm3.
After the dryer the dryed material is carried over to the dryer cyclones. In addition to the wood particles a much smaller amount of fine ash from the biomass combustion is released and captured in the cyclones. The total concentration of wood dust and ash entering the cyclones is typically 200 000 to 300 000 mg/Nm3.
Usual cyclone dryers are capable of capturing the particles to a quite high degree (>99%), but often still have emissions of 200-350 mg/Nm3.
These levels of concentration of particles at the cyclone filter exit and at the dryer cyclones outlet are too high for most countries regulatory emission limits and therefore the plants have to be equipped with an end stage deduster to bring emissions under 50 mg/Nm3(or lower).