Nov.07, 2018Keywords: wood pellets silo, silo fire
Smoke from smoldering wood pellets inside a PacBio silo containing 3,500 metric tons was first noticed on the evening of Aug. 23. Over seven days, the fire was controlled, extinguished, the silo was saved, and there were no injury or loss of life. Pellets worth over $500,000 were damaged, but millions of dollars of pellet plant infrastructure was not. Unfortunately, although the silo did not collapse and there were no explosions or fires that damaged the pellet mill or caused injury, the silo will be demolished because of uncertainty regarding its structural integrity.
Credit for this successful outcome belongs to the Pacific BioEnergy management and operations team, and the first responders from the Prince George fire department, all of whom followed a carefully crafted plan for controlling and extinguishing the fire. PacBio CEO Don Steele, and Vice President of Operations Shawn Bells and his operations team, along with vital guidance and support from the author of this paper, John Swaan, applied industry best practices to their tactical planning. They thought carefully before they acted, and used information developed from years of experience. They proceeded carefully, but deliberately, with a primary objective of keeping everybody safe.
What Not to Do
Historically, the industry has examples of not-so-successful outcomes of silo or dome fires. This has been due to the lack of knowledge about the characteristics of wood pellets, often by both pellet plant operators and first responders, and incorrect and self-defeating tactics. The following incorrect tactics have been the cause of loss of buildings and other assets, pellets, injuries and worst of all, in several incidents, loss of life.
Although water may help with controlling the flames of an out-of-control silo fire incident, deluging or spraying water on top of the pellets in a silo, dome, or flat storage will never aide in extinguishing a smoldering mass of wood pellets. The wood pellets on the top of the pile will absorb the water and swell, creating a blanket of material, restricting the ability of water to penetrate anywhere near the core of the smoldering pellets located somewhere within the center of the pile of pellets. Water contacting hot, pyrolyzed wood pellets will generate carbon monoxide (CO) and hydrogen, which add to the seriousness of the conflagration, and is not helpful for extinguishing a silo fire. Water may also create pinnacles and/or columns within the silo, which may become a problem when trying to remove the product.
To immediately begin removing the wood pellets from the silo, dome or flat storage pile before the pyrolysis activity within the core of the pile has been extinguished, is a recipe for disaster. The gases being released by the pyrolysis activity are nasty and dangerous, especially the methane, CO and other life-threatening gases. The ignition point of methane released from wood pellets is very low, and will ignite when it encounters the smoldering core, and the supply of oxygen from the open air. In other words, when removing the pellets down to the level where the pellets are exposed to both the smoldering core and atmospheric air, chances of an explosion and/or a rapidly spreading fire are very high.
The lesson learned is that inert gas injection significantly lowers the probability of negative outcomes. The danger of a gas and/or dust explosion causing serious injury, and extensive property damage is very possible. Nitrogen is most effective for minimizing these risks, and provides a low-risk pathway to gain control of the smoldering pyrolysis inside the pile while emptying the material.
Nitrogen injection is recognized as the better solution as an inert gas for mitigating silo fire incidents—it is more readily available in large quantities, is easier to vaporize, and is more economical than CO2. The use of nitrogen gas was a key part of the tactics used to control and extinguish the PacBio fire.
We recommend a review of the report published in 2013 by Henry Persson of SP Technical Research Institute of Sweden titled, “Silo Fires–Fire Extinguishing and Prevention, and Preparatory Measures.” This report should be a standard reference for every pellet plant, and for every fire department that may respond to a pellet silo or dome fire.
At PacBio, the report’s recommended nitrogen injection flow rates were referenced, and calculations for the size of the silo were made, and very quickly a call to the local gas supplier Praxair was made. A mobile nitrogen vaporizer and tank unit, along other with tankers to follow, were mobilized from Edmonton, Alberta. The gas and oil industry utilize this type equipment regularly. An engineer from Solid Industrial Solutions was also dispatched to provide on-site assistance with the setup of the nitrogen distribution system, and control the nitrogen injection of the flow rate.
Based on the needed flows and volumes of nitrogen, the PacBio team specified how to fabricate the lances to be driven into the side of the 80-foot (~24 meter) diameter silo. Within 24 hours of the call to mobilize the nitrogen; the vaporizing unit was set on-site, injection lances were in place, nitrogen distribution system connected, and the nitrogen injection began to flow.
Several attempts were made to foam the top of the silo, but regardless of the foam densities, the deluge system originally installed for water was not adequate for dispersing foam evenly over the top of the pellets to create an effective proper seal.
Emptying the silo commenced within 48 hours of nitrogen injection, after the oxygen level measures within the head space of the silo dropped below 10 percent. The PacBio team safely handled and evacuated the removed material. The first responders, equipped with respiration equipment, kept all personnel safe and out of harm’s way. Wood pellets and the carbonized clumps coming from the silo were conveyed safely, without incident, to a flat area away from the plant. Even when meeting atmospheric air, there were no issues.
It took approximately seven days to evacuate the 3,500 tons of aborted material, and each truckload was safely moved to a secure area away from other fiber residue stockpiles, and deluged with water as they were dumped, to ensure there were no residual hot spots.
The second lesson is to be prepared to detect and control silo/dome fires. Monitoring, detection and suppression systems must be installed and maintained in good working order. A properly installed and operating heat monitoring system will assist with discovering the location of a developing hot spot within the pellet silo or dome. Early warnings of an incident will be detected and alarmed when temperature monitors inside the silo are operating correctly. Early warning, before smoke is observed, will significantly lower the loss of product, and the likelihood of a much more serious incident.
Carbon monoxide and oxygen monitors installed on the top of the silo, providing constant measurement, will also assist with early incident detection. Once the nitrogen was being injected into the PacBio silo, obtaining readings without sampling equipment already installed at the top of the silo made it more challenging to determine the gas levels required for removing the material from the silo material safely. Testing and maintaining these systems must be part the weekly PM (preventive maintenance) program.
A permanent, properly sized and installed nitrogen injection system within the silo or dome, complete with a manifold in a safe location with a convenient hookup, is critical. If there are no nearby suppliers of nitrogen and evaporators, the plant should strongly consider having that equipment on-site. The quickly rigged manifold used at the PacBio plant was not optimal for controlling flow to the lances. Managing an even flow rate properly distributed into the silo would have been more effective, and may have controlled the pyrolyzing core quicker.
Silo or dome ventilation systems control are critical when managing a silo fire incident. The system should have the ability to shut down and seal off bottom asperation fans, as well as the ability to control the top ventilation of the silo. This is very beneficial to minimize the exhausting gas flow and improve nitrogen penetration, and to reduce the total volume of nitrogen required.
In most cases, it’s suspected that the ignition that sets off the pyrolysis activity is due to some foreign hot debris. This could be from failed pellet mill roller bearings, conveyor system roller and/or belt failure, or molten steel from hot maintenance work. All of the above have been the causes of incidents.
Because wood pellets are a biogenic product, self-heating can also be the cause of silo fire incidents. This may be due to microbiological activity, chemical oxidation processes, moisture migration, moisture absorption, or a combination of these. This process usually occurs within a temperature range up to 45 to 75 degrees Celsius, since microbes die at higher temperatures. Microbial activity primarily generates CO2, and may be detected by measuring the CO2 concentration in the silo headspace. At higher temperatures, self-heating is derived from chemical oxidation processes. In wood pellets, the cause is usually a chemical oxidation process, since the pellets are more or less sterilized during the production process. Practical experience shows that this oxidation process is especially likely in newly produced pellets, in part due to the oxidation of different resins contained in the wood material.
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