The growing high demand of wood pellets makes the production of wood pellet under pressure constantly, wood pellet plant automation and pellet machine speeds are increased, and so the explosion risk caused by accumulated dust is increased too. In the wood pellet production process– woody materials are ground by wood hammer mill to reduce size, and dried in the sawdust drying machine into its most combustible form, and then densified in the wood pellet making machine into pellets.–dust will accumulates. If the dust can not be controlled and managed properly, it will be the potential hazard. Dust explosion in the wood pellet plant is not uncommon nowadays.
EHSToday reported on Mar 5, 2014–A combustible-dust explosion and fire at the plant injured one worker and partially destroyed the building. The ignition of wood dust in the plant’s production room migrated to a retention bin, resulting in an explosion that spread through the building, according to OSHA.
BURNS LAKE, B.C. reported on October 9, 2014– Three mill workers were injured Thursday in a fire and explosion at a northern British Columbia wood pellet plant, which was the site of a less serious explosion two years ago and was recently fined for “repeated” safety violations.
Dust explosion and fire cause serious injuries, damage to property, and interruption of production. However, there are strategies that can be implemented to reduce the risk of fire and dust explosion.
Several Possible Zones Where Fires or Explosion Can Occur in Pelleting Process
In wood pellets production plant the most common ignition source generators are places where friction generates overheated materials, such as wood grinding machine, sawdust dryer, dryer cyclones, intermediate storage, pellet mill, wood pellet cooler, pellet silo, and so on.
♦ Foreign metallic materials in the production process can cause sparks resulting in a fire. For example, when high speed wood grinding machine and wood chippers are used to process biomass materials, it can generate a lot of dust if the wood is dry; if a piece of metal or even a stone falls into the equipment, sparks can be produced, which can ignite the airborne dust.
♦ Bearing problems at one of the rollers of wood pellet machine can create friction and the friction can cause the overheated pellets. When overheated pellets exit the die of pellet machine, they maybe cause the fire or dust explosion.
♦ The accumulation of combustible wood dust in the pellet production process. The production of pellets from various woody materials can produce fines. When the concentration of wood dust reaches the limit, the fire or explosion will occur.
Safety Tips on Pellet Making Machinery to Prevent Fire or Explosion
♦ Remove foreign metallic materials in the raw materials by using Permanent Magnetic Drum or other magnets and this magnetic equipment should be located in duct just after the drying machine, prior to the wood hammer mill, prior to the wood pellet machine and inside the feed ductwork to all storage silos.
♦ Check the easy wearing parts of pellet machine such as roller shell, die, bearings and so on regularly in order to avoid the overheat caused by friction between worn parts.
♦ Pellets temperature should be automatically monitored with a high temperature warning at the discharge gate of wood pellet machines.
♦ Dust collection systems should be located through the whole pellet production line to reduce the dust accumulation. Horizontal and vertical surfaces should be cleaned of the accumulation of dust buildup on a scheduled basis. Surfaces should be cleaned in a manner that minimizes the potential generation of a dust cloud.
♦ Remove all elements that can cause fire or explosion.
Explosibility Characteristics of Dust from White Pellets and Bark Pellets
|Explosibility Characteristics of Dust from White Pellets and Bark Pellets|
|Test Mode||Test Parameter(dust <63μm)||Measure||White Dust||Bark Dust||Coal Dust||Lycopodium Spores||Testing Standards|
|Dust Cloud||Auto-ignition Temp(Godbert-Greenwald)||Tc||℃||450||450||585||430||ASTM E1491|
|Min Ignition Energy||MIE||mJoule||17||17||110||17||ASTM E2019|
|Max Explosion Pressure||Pmax||bar||8.1||8.4||7.3||7.4||ASTM E1226|
|Max Explosion Pressure Rate||dP/dtmax||bar/sec||537||595||426||511||ASTM E1226|
|Deflagration Index||KSt||bar.m/sec||146||162||124||139||ASTM E1226|
|Min Explosible Concentration||MEC||g/m3||70||70||65||30||ASTM E1515|
|Limiting Oxygen Concentration||LOC||%||10.5||10.5||12.5||14||ASTM E1515 mod|
|Dust Layer||Hot Surface Ignition Temp(5 mm)||Ts||℃||300||310||ASTM E2021|
|Hot Surface Ignition Temp(19 mm)||Ts||℃||260||250||ASTM E2021|
|Auto-ignition Temp||TL||℃||225||215||USBM (Bureau of Mines) RI 5624|
|Dust Class(>0 to 200 bar.m/sec)||St 1||St 1||St 1||St 1||ASTM E1226|
|Dust Class (Explosion Severity: ES>0.5||Class II||Class II||OSHA CPL 03-00-06|