By Ben Peetz
With today’s ongoing efforts to reduce dependence on fossil fuels, many areas are seeing facilities developed that produce fuels derived from biological materials, commonly called “biomass.” Primarily produced in a pelletized form, the product is made by taking the biomass material and compressing it into a small cylindrical extrusion that is broken into pellets. Recent statistics provided by a biomass industry organization indicate that as many as 200 of these facilities are in operation across the United States and at least 50 in Canada.
The raw material used for these fuel pellets could include anything from wood to corn, sugar cane, grasses, and other types of natural growth. The pellets are then used for heating in applications that range from small wood-burning stoves, used for supplemental residential heat, up to large high-efficiency boilers used in commercial power production. In some places in the United States, plants are producing hundreds of thousands of tons of biomass pellets, which are being shipped across the Atlantic to power areas of Europe.
How does that affect the fire service? Pellet fuel manufacturing inherently poses a high risk of fire and explosion because of the form of combustible material used, as well as the processes that produce large amounts of heat. The material is typically ground to a fine consistency and then dried before being compacted under tremendous pressure to form a pellet.
Let’s look more closely at the pellet-making process. Trucks or railcars bring raw materials into the pellet mills each day. In the case of wood pellets, the raw materials may be in a number of forms, including sawdust, wood chips, lumber scraps, trimmed treetops, or even full logs that have been deemed unsuitable for lumber. The raw materials may arrive in any moisture content ranging from fresh cut to kiln dried.
Some pellet mills start their process by taking large pieces of wood or logs and feeding them through a chipper or a shaving machine for processing into manageable particles. From there, the material enters a hammer mill, where the milling process begins. These machines break the resulting wood particles down into a smaller size; they are screened for consistency. It is necessary to break down materials to a very small, consistent particle to allow for drying and to ensure consistency in pressing through the pellet die. This is also the first major area for fire hazards, as any foreign materials that enter the hammer mill could produce sparks that could ignite the combustible dust produced during the milling process. Also, in high-speed chipping and milling equipment, bearings or other mechanical components that generate heat through friction can ignite the materials being processed.
After the milling process, it is critical that the particles be consistent in moisture content, commonly in the range of 12 percent to 15 percent overall moisture. If a mill uses green materials or if dried materials have been left exposed to precipitation or high humidity, then the material must undergo a drying process to meet the proper criteria. This is typically accomplished through the use of large drum dryers that may use gas, wood, or other fuels to provide heat. The material is fed into a large rotating drum, where the heat passes through the material. Fires often occur in this equipment when a power failure or other mechanical problem causes the drum to stop rotating or the material to stop feeding through the drum. When the material flow stops, the material has a chance to hold heat, often to the point of ignition. Drum dryers should be equipped with fire suppression equipment and have an emergency means of expelling material once ignition occurs. Ideally, these large drums dryers should be outside of a building.
Once the material reaches the desired moisture content, the dust is pressed through dies at a high rate of pressure. The friction and pressure in this process causes the wood particles to heat up and release natural lignins in the fiber that bind the pellet. Again, because excessive heating occurs in this process, the pellet must immediately be passed through a cooling tower process that allows for cooling and hardening of the pellet and to prevent ignition within the system. If some process failure that stops material flow occurs, the newly formed pellets can hold heat and cause a fire within the equipment.
After cooling, the pellets are often stored in silos or bins before they are bagged or loaded out for bulk distribution.
One common distribution method involves the metering of the pellets into 40- to 60-pound plastic bags. Once filled, the bags are typically sealed through a hot-press process and are stacked on pallets or skids, often in one-ton increments. In colder-climate areas, you can likely get a glimpse of this distribution method at the local hardware store. Bulk pellets are placed directly in trucks or railcars for delivery to bulk storage containers.
Of course, through the entire pellet process, there is at least one consistent fire and explosion hazard, combustible dust. These operations can be extremely dusty because of the various sizes of particulates generated and released from hammer mills and related equipment in the manufacturing process. This means that a formal dust management plan must be in place in these facilities; the plan must include excellent housekeeping procedures and a documented preventive maintenance program.
Because biomass fuel production is a growing technology and new facilities are being constructed, you may find yourself inspecting these facilities. If you are involved in the planning or inspection of new biomass operations, be sure that only noncombustible construction is recommended where pellet processes will be housed, and encourage the installation of automatic fire protection such as automatic sprinklers.
Beyond the basics of the pelleting processes, also look at all dust-collection, filtration, and management systems. A burning dust collector is a very dangerous situation because of the potential for a combustible dust explosion. Advocate that dust filtration units be located outdoors and ensure that automatic protective systems are installed so that firefighters are not placed in harm’s way if there is a fire within the system. This protection should include spark-detection and suppression systems on all ductwork and automatic sprinklers or manual deluge capabilities to the interiors of all dust-filtration units and storage silos and bins.
With a little bit of inspection and preincident planning, a fire in a biomass fuel pellet plant often can be kept to more manageable and safer incidents, or perhaps they can be prevented altogether.
BEN PEETZ, ASP, CFPS, is a senior loss prevention specialist and fire protection consultant for Lumbermen’s Underwriting Alliance, a commercial property insurance carrier specializing in forest products and other manufacturing industries. He is a second-generation fire service veteran with the Napoleon (IN) Volunteer Fire Department, where he has served for nearly 20 years. Peetz is an IFSAC-certified fire instructor II/III and a Fire Department Instructors Conference instructor. He is a National Fire Protection Association- certified fire protection specialist and a Board of Certified Safety Professionals associate safety professional. He has degrees from Purdue University.