Dust is our business: mitigating risk of dust explosions in pellet plants

Wood pellets are designed to burn – they’re made from highly combustible dust that is pressed into pellet form and must be treated with care to prevent this fuel from lighting before it has been added to the boiler.

Dust on beams and rafters is usually not visible and will participate in the secondary explosions sometimes experienced in large manufacturing plants. The top of the beams are now gradually retrofitted with 45 degree brackets to avoid dust accumulation.

The business of making dust is challenging due to the risk of dust explosions. The dust is present in all stages of production and is also generated during transportation and storage due to attrition. The finished product is dropped dozens of times before reaching the destination and the drop height can be quite considerable, for example up to 25 metres during the initial stage of loading an ocean vessel.

Dust collection, dust suppression, explosion containment and explosion suppression are employed in facilities for manufacturing, handling and storage to minimize the risk of injuries and damage to equipment and facilities. The risk factors for initiating a fire or explosion includes hot particles or sparks landing on a dust layer setting the dust on fire, often with explosive force, which in turn may trigger a secondary massive explosion when dust in the rest of the facility becomes lofted. A rule of thumb to be used as a guideline for housekeeping is that a maximum thickness of 3.2 mm (1/8”) of dust should not be covering more than five per cent of the floor area. Also, dust on surfaces with temperatures higher than 250ºC will catch fire. If the burning dust is dispersed at high speed in what is referred to as deflagration, it will light everything in its path and continue to burn until the fuel is exhausted.

Surprisingly, dust lofted in the air catches fire if heated to +450ºC or higher. Small particles of dust may stay lofted in the air for a long period of time, particularly in turbulent air. In still air a dust particle with an equivalent diameter of 70 micron may stay in the air for several minutes and a one micron particle or smaller will stay constantly lofted in still air. If the concentration of lofted particles exceeds 70 gram/m3, a deflagration may be initiated by, for example, an electro-static discharge.

Static risk
Generation of tribo-electric charges due to fast moving equipment is a major concern. Wood dust has an extremely high electrical resistivity, which means that electrons are accumulated in the surface of dust particles and if materials in contact with the dust also have high resistivity, an electrostatic potential will build up and eventually discharge as a spark.

All materials in contact with wood dust have to be electrically conducting to bleed off the charge from the dust. All equipment has to be properly earthed, grounded and bonded to deflect any electrostatic voltage build-up. There is static paint available to minimize the dust sticking to walls and floors and there are also static dissipative hard toe shoes (CSA – SD certified) available to minimize discharge from workers using synthetic clothing.

Engineered for safety
Strict engineering design rules apply for mitigating the risk of fires and explosions in manufacturing, handling and storage facilities. In North America guidelines are published by NFPA (National Fire Protection Association), FM (Factory Mutual Insurance Company), OHS (Occupational Health and Safety), local fire codes, etc., some of which are voluntary and others are mandatory. In Europe, ATEX (Atmospheres Explosibles) is universally accepted as the guideline mitigation of risk of explosions. Equipment installed in a facility has to be rated in accordance with a risk zone classification.

In North America, feedstock is dried at high temperatures up to 450ºC, which is a critical ignition temperature for lofted material (as mentioned above). To prevent ignition, the oxygen in the dryer is kept below 10 per cent: the limiting oxygen concentration for wood dust. The thermal balance in the dryer is critical and needs special control strategies, particularly during shutdown or loss of electrical power to the drive system.

Hot particle detectors are installed along the material’s path as it makes its way to become the final product. Some brands of detectors have a temperature set-point capability as low as 250ºC to monitor hot particles destined to land on dust layers.  A set-point temperature of 400ºC is used to monitor lofted particles in areas of high dust concentrations. Besides the risk of fires and explosions caused by dust, there is also the risk of self-heating followed by fire when the feedstock and the final product is in storage. 

Lesson learned
After the severe and fatal accidents in the Babine and the Lakeland sawmills in British Columbia in 2012, WorkSafeBC and BC Forest Safety Council implemented a strict inspection regime in British Columbia for all woodworking facilities, including pellet plants. Safety audits are conducted based on a very strict protocol on a regular basis. Citations and even fines are issued for more serious or repeated non-compliance. Some surprise audits and work-stop orders can be issued. An inspection of a typical plant usually takes two days.

Employers are mandated to provide training regarding the risks and procedures for mitigating risk. Hot work guidelines, ignition source identification and housekeeping are at the heart of the training program. Workers have the right to initiate work-stop if conditions are considered risky. Special guidelines are stipulated for contractors temporarily doing work in a plant. Each plant needs to have a written procedure for managing change involving manpower, equipment and procedures to make sure safety is always paramount.

New safety guidelines across the country
The safety inspection scheme in British Columbia is likely to expand to other manufacturing facilities over time. A worksafe certification is likely to become a selling tool for the manufacturers since it minimizes the risk of non-performance by the suppliers and may in fact over time become a contractual obligation by key pellet consumers.

ISO Technical Committee 238 for Solid Biofuels is coming out with guidelines for safe handling and storage of pellets in residential as well as in large-scale industrial applications within the next two years. Also, fire prevention and firefighting guidelines for pellet silos are under development. In addition, protocols for laboratory testing of off-gassing and self-heating characteristics for pellets will be published by ISO. Safety is gradually becoming the overriding management tool for our industry – and dust will gradually become less of a menace.

The nitty gritty on wood pellets
Wood pellets and briquettes are produced from a variety of feedstock materials such as sawdust, planers shavings, cut blocks from lumber production, bush grind (harvest residue), bark and even higher quality hogfuel materials. The feedstock preparation involves separation of contaminants, grinding, drying, hammer milling to size before compression to a commercial products, which in turn are screened and cooled before storage and transportation to the end user.

The large majority of the pellets made in Canada are sold and transported in large bulk volumes and used for electrical power production or heat. Smaller volumes are packaged in bags and sold for residential space heaters or boilers. Briquettes are also produced in small volumes.

The quality of pellets and briquettes are specified by ISO Standards, currently being rolled out in Canada as a voluntary CSA Standard. These standards specify the mechanical and chemical composition as well as the acceptable feedstock materials. There are six different qualities for wood pellets and an additional five qualities for non-woody (agricultural) pellets. Woody briquettes are specified in three different qualities and non-woody briquettes are specified in five different qualities.

The common denominator for all these biofuel products is that the feedstock has to be reduced to particles before compression to a commercial product. For some of the products, such as industrial quality pellets, the maximum allowable particle size is stipulated in the ISO Standards. The pellets are crushed to a powder during the fuel preparation in the power plants before sprayed into the furnaces for combustion, much like coal powder or fuel oil.