Fires have always been a problem in wood pellet plants, particularly in those that handle dry material, including wood finishing plants, panel-board plants and wood pellet facilities. Historically, wood pellet production was a small industry with more than its share of fires and explosions. However with the emphasis on green energy, wood pellet production has skyrocketed and very large plants are being constructed.  There have been several recent major fires and explosions in wood pellet manufacturing, shipping, receiving, storage and power plant facilities.
Here are 2 accident cases occurring in wood pellet plants in the past 4 years.
Accident 1.
On October 20, 2011, a combustible dust fire began in the wood pellet cooler, most likely caused by a spark or ember from the pellet hammer mill. The fire then spread through the ductwork throughout the plant, eventually reaching the dust collector causing it to explode. When the collector exploded, the explosion vented through the baghouse’s explosion vents into adjacent storage silos setting them ablaze further spread the fire throughout the plant. More than 100 firefighters and emergency personnel from at least 14 towns worked for over 15 hours to put out the blaze.
The plant lacked adequate spark detection devices, fire suppression systems, and explosion venting/protection within the dust collection system.

Fire Prevention

Accident 2:
On August, 2013, a combustible-dust explosion and fire occurred at Inferno Wood Pellet Inc. factory in East Providence, R.I., in which one worker was injured and partially the building was destroyed. 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.
The lack of safeguards allowed the initial fire and explosion to occur and spread in a chain reaction to other equipment and through the plant. If this employer had adhered to applicable OSHA and National Fire Protection Association standards, that would have helped prevent the fire and explosion from occurring and spreading. In an investigation, it is found that plant workers were exposed to fire hazards due to inadequate or nonexistent preventive and protective measures in the wood-pellet processing system and its equipment.

fire hazard

Among the agency’s findings:
● The retention bin lacked spark detection, explosion suppression, fire/explosion isolation and explosion venting devices.
● Conveyor systems carrying combustible wood products lacked spark detection, fire suppression and/or fire isolation devices.
● Dust collection systems and dust segregation barriers were not maintained to minimize fire sources.
● An opening in the fire wall between the plant’s production room and chip room allowed a fireball to enter the chip room and spread the fire.
Actually, at each step of the wood pellet production process, there is potential for fire. Pellet production is the process of making fuel. To sum up, the source causing fire danger is the ignorance of fire and explosion hazard in the whole wood pellet line. For most investors, they care or know little about the insidious danger in the pellet line. Usually, people won`t care much about things that haven`t gone on about themselves. Now, let`s unlock the secrets of parlous sources of wood pellet line fire.

fire hazard hidden within the wood pellet plant

What fire hazards are hidden within the processing?
Let`s divide the processing into 4 parts: personnel pre-training, hammer mill, pelletizing process, and pellets storage.
Personnel pre-training:
Inadequate chemical hazard communication and training: It is critical that the management and staff of wood pellet plants gain knowledge in the area of hazard recognition, evaluation and analysis of potential explosion situations, and the steps required to eliminate the potential of a fire or explosion.
Hammer mill:
Frequent sources of fires in the hammer mill:
1.Pieces of metal/stones etc.
2. Overheating(overloading, material build-up etc.)
3. Failure
Pelletizing process:
1. Combustible wood pellet dust
Additional fire hazards includes the accumulation of combustible wood dust on various locations and surfaces within the plant. And, the dust created when handling wood pellets is very explosive, so explosion detection and suppression systems must be used. Storage in silos is appropriate but care must be taken to accommodate the explosive nature of wood pellet dust. Employers should take responsibility to ensure proper and effective safeguards at all times so that incidents like fire and explosion won`t occur.
2. Lack of dust hazard-control equipment where combustible wood dust accumulated
Combustible wood pellet dusts pose a risk of fire and explosion when specific conditions exist.  Awareness of these conditions is the first step towards establishing an effective combustible dust hazard control program.
Besides that, overheated roller/bearing assembly in a pelletizer, and insufficient experience, such as incomplete and inadequate fire prevention plan, incomplete respiratory protection program, lack of procedures and training to ensure that all equipment was properly deenergized to prevent unintended activation, also cause fire and explosion.
Pellets storage:
1. Physical and chemical changes of wood pellets during storage:
● Some of the challenges and hazards include significant temperature increases (as high as 100℃)
●A corresponding decrease in oxygen levels and increase carbon dioxide and carbon monoxide levels in and around large piles
● Emissions of volatile organic compounds(VOC) in and around large piles
●Airborne dust and attention of the pellets in high
2. Self-heating Fuel Piles
When wood pellets are stored in a pile, waiting for transport or use, they can spontaneously heat through oxidation. In order for this to happen, three conditions must sync: rate of heat generation, air supply, and insulation properties of the immediate surroundings. With most biomass material, there is a high moisture content combined with air and/or bacterial fermentation – both of which can cause spontaneous combustion through oxidation.
Clearly, the process of manufacturing wood pellets involves all the right ingredients for explosions and fires to occur with a concerning frequency the potential to cause serious injuries, damage to property, and interruption of production. Examining what went wrong in every incident highlights the need for diligence on the part of plant management and operators regarding the dangers of combustible dust.

wood pellet storage

What should be done to reduce the likelihood of a fire/explosion?
With the proper understanding of the various processes, and a properly designed and controlled process, all unnecessary hazards can be eliminated.
The key to preventing explosions and fires is preparation.The main causes of plant fires and explosions are dust, equipment failure, and human error. In order to reduce risk, plant owners should have a strict housekeeping regimen to reduce dust. Additional preventative measures include regularly inspecting equipment, training plant personnel on fire hazards, and installing and maintaining proper fire protection throughout the plant.
Without a stringent, documented housekeeping routine, even the most robust fire protection system will not prevent an explosion.
Dust explosions have a tendency to cause secondary explosions. The initial explosion frees more dust particles, sending them into the air, causing a secondary explosion. Secondary explosions have the highest death toll of any other type of plant conflagration.
Detection Devices
In order to protect the entire biomass facility, several different detectors are needed throughout the plant. Carbon monitors, infrared scanning, temperature scanning, or linear heat detectors are options that can be used throughout the plant. Linear heat detectors will detect heat along a length of space, working well along conveyor belts, which have a tendency to catch fire from idler/roller bearing failure.
Spark detection systems protect chutes from igniting. Spark detectors can sense a spark before it has the opportunity to create an explosion. The system allows the extinguishing equipment to suppress the spark before it grows into a larger problem.
Keepwood pellet dry
wood pellets, which absorb water easily, must be kept dry.  Fortunately wood pellets have great flow characteristics and are very suitable for storage in silos.
Do not add water to a wood pellet fire

do not add water to a wood pellet fire

The following tend to increase the risk of pellet breakdown:
● excessive clearance between auger flights and case
● excessively steep or long augers
● excessively long filling points causing further break-up of broken pellets
● aggressively angled flights
● varying inter-flight volume caused by shaft joints
● excessive auger speed (where the contents are starting to bounce and fly)
● low quality or waste wood pellets.
Homework key to safer pellet manufacturing:
● Consider the risk for fire and dust explosions already in the design of the process
● Protection of all main risk zones
● Detecting the right temperature
● Efficient extinguishing
● Proper housekeeping
● Proper service and maintenance of machines
● Proper service.
● Promote the improved safety of pellet production, storage and transportation through the interchange of technical knowledge, experience, and data.
The dust explosion risk can be minimized by:
● electrically earthing of all steelwork, especially delivery pipes
● removing all electric lights, sockets and switches from a fuel storage
● using appropriately rated electrical equipment
● ensuring that augers are designed for the fuel type
● filtered venting during blown delivery
● sourcing pellets from an organization which has suitable quality assurance (QA)
procedures in place and can assure consistent pellet quality/integrity.
Also, Great care should be taken to avoid any plastic component (for example, a pipe or screw conveyor case) coming into contact with moving dust.
Because the hazard from dust explosion is uncertain and unpredictable, a further layer of protection should be added to largely enclosed stores or silos. A ‘weak area’ in the silo can effectively provide directional explosion relief. This could be a marine plywood panel set in the bunker roof or wall but should be secured appropriately to prevent the panel becoming a missile.

The following is another article we shared from Industrial Fire Prevention, sincerely hope we will do you a great favor in the preventive measures of fire and explosion prevention of wood pellet processing line.
Fire and Explosion Protection in Wood Pellet Production
The key to preventing fires and explosions, and thus catastrophic secondary explosions is mitigating combustible dust, and ignition sources.
Hazard Recognition is Essential
The first step is hazard recognition. Recognizing that at each step of the process, in each part of the facility, there is potential for creating fires. From storing and loading wood chips, to milling and grinding, to pelletizing, cooling and storage, combustible dust is created, and ignition sources are present.
Every time the biomass product is moved or manipulated, it creates combustible dust, creating a possible fire and explosion hazard. Combustible dust from the process settles and migrates throughout the plant, creating a secondary explosion hazard.
Friction, Heat and Sparks are Created
Friction, heat and sparks are created in the processing of wood into pellets. The primary fire hazards are specifically in the drying, milling and grinding, and pelletizing of wood flour into pellets.
Combustible Dust Deflagration Hazards
The most dangerous parts of the process containing combustible dust in minimum explosible concentrations are found in the pellet cooler, dust collection, and storage bins and silos.
Risk Assessment
Risk can be quantified as potential for loss. Specifically the probability of occurrence, and potential magnitude of loss or consequences.
A risk assessment must be done to assess and quantify all risks associated with the production and storage of wood pellets, including production, business continuity and life safety. All risks are quantified, prioritized, and addressed by risk rating.
Principles of Prevention
The keys to prevention are controlling the combustible dust emissions, and controlling ignition sources.
Controlling Dust
Combustible dust emissions are primarily controlled through properly designed dust collection systems. Combustible dust migration from emissions can be controlled by utilizing oscillating fans to create an air barrier preventing migration of dust to high ceilings and rafters, or by using a misting system.
Hazardous Area Classification
Hazardous areas must be designated by class and division, or zone, depending on where, when and how dust emissions occur in production. Operators must understand these hazardous areas. Only electrical equipment classified for a specific classified area or zone can be used.
Process Design
Inherent safe design of the equipment and process is one key to process safety. Where possible, various combustible dust generating systems as well as ignition sources shall be segregated, separated and isolated from each other.
Engineering and Administrative Controls
Engineering and administrative controls are used to limit combustible dust emissions and migrations, as well as ignition sources. A hierarchy of controls are applied based on the risk assessment.
Engineering controls
Engineering controls consist of many various types of mitigation equipment and systems.
Dust collection, oscillating fans and water mist systems may be used to control dust migration.
Preventing Fires
To prevent fires, spark detection and extinguishing systems are a primary control. Bearing Ember, flame, temperature, CO and combustion gas, and emissions monitoring may also be used, as well as bearing temperature and bin temperature monitoring systems.
Preventing Explosions
Explosion Isolation, venting and suppression are used to control deflagrations.
Administrative Controls
Administrative controls such as housekeeping, change management, and preventative maintenance are used for hazard prevention and control.