have always been a problem in wood processing plants, particularly in
those that handle dry material, including wood finishing plants,
panelboard plants and wood pellet facilities; less so in sawmills.
Historically, most sawmills in BC were accustomed to processing `green’
wood with moisture contents of 40 – 55%, wet basis. While not unknown,
fires were not a common experience with such wet wood, and explosions
were unheard of. So, the fires and explosions in northern BC in 2011 and
2012, which resulted in the total destruction of two large sawmills,
multiple deaths and injuries, is great cause for concern. Everyone is
asking, “What happened?” The relevant safety authorities are currently
investigating both incidents and at this point in time, firm causes have
yet to be identified.
There has been speculation that dust accumulations resulting from the
processing of dry (25%), Mountain Pine Beetle (MPB) killed wood were
contributors to the two explosions. The Mountain Pine Beetle has
always been present, but nature has kept them in check through long
periods of very cold weather. Over the last two decades the amount of
timber being killed by the MPB has grown and has reached astronomical
proportions in BC, with more than 60% of the pine resource being killed,
which has resulted in hundreds of thousands of hectares of forest being
decimated. After dying, the pine trees dry out to the ambient humidity
level. The longest-dead trees that are still standing are quite dry
with moisture contents 25%.
As the MPB plague spread through a region there was a rush to mill
the most recently killed trees before their value diminished too
drastically. In affected regions after 20 years, sawmills are now
processing pine that has been dead for a few years and is quite dry.
So, the amount of MPB killed wood being processed is increasing at the
same time as the dryness of the wood is decreasing. The dry MPB killed
wood is very brittle and tends to shatter and generates large quantities
of dust, which can overwhelm dust collection systems.
As are most BC interior mills, both destroyed mills were processing
large quantities of MPB wood and were reported to have been very dusty.
Since these two serious accidents, it has been learned that in recent
years there have been at least five other less calamitous explosions in
sawmills processing MPB killed wood. In these incidents, there were
varying amounts of property damage but no injuries.
Used to processing benign `green’ wood, the sawmilling industry has
been caught somewhat `flat-footed’ by the potential for disastrous
consequences from the handling of dry MPB killed wood. However, since
the calamitous fires and explosions, government and industry has
responded remarkably and has taken steps to avoid their reoccurrence.
One effect of these two tragedies has been to put a focus on wood
dust generation and accumulations in sawmills. As a consequence, the
provincial government has ordered all mills to conduct dust safety
surveys and WorkSafe BC and industry have compiled a document describing
the best industry standards for dust, fire and explosion control. (See
the reference at the end of this article)
Panelboard plants and wood finishing plants are very familiar with
the fire and explosion risks associated with handling dry wood
products. There is probably no dust so explosive as dry sander dust,
which is common in panelboard plants. Consequently, most panelboard
plants and wood finishing plants have long mastered the safe handling of
explosive dusts. Still, not many years ago, one particleboard plant in
Quebec was destroyed by dust explosions.
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. These new facilities are learning
that they have to employ safe handling practices for dry wood materials.
The purpose of this article is to describe to the newcomer to the
industry some of the safety issues associated with handling dry woody
biomass and provide general guidelines for dust control, spark, fire and
explosion detection and suppression. Relevant codes, regulations and
useful resources are listed at the end of the article.
Fire / Explosion Risks
Additionally, due to microbial / bacteriological action and oxidation in
certain storage conditions, wood can heat up to the point where it will
self-combust. The nature of the material being stored, moisture
content, particle size, air flow through the pile, compaction, size of
storage pile, and contaminants can all contribute to spontaneous
combustion. To read how these factors interact, see the article titled
“Biomass Storage Pile Basics” on my website www.advancedbiomass.com.
Wet biomass will also burn. Some of the fastest moving fires have
involved green sawdust soaked with hydraulic oil; all it takes is a
spark or flame to get the fire started. And, smouldering fires in large,
wet hog fuel piles are common even during very cold winters.
causes of biomass fires are numerous. All you need is fuel, oxygen and
a source of ignition. For a dust explosion / deflagration to occur,
two additional factors are required: mixing the explosive dust with air
in the necessary concentration and confining the mixture in a container
Some milling processes generate sparks. High speed hogs, grinders and
chippers are used to process biomass and while doing so, 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. So, it is important to contain and collect the dust,
decrease the metal and rock contamination at source and utilize metal
removing equipment before the milling process at the plant.
If not controlled, dust will build-up around belts, pulleys,
bearings, etc. Friction can result in enough heat build-up to start
No wood handling process is dust-free. With even the best designed,
constructed, operated and maintained system, fugitive dusting will
occur. Very fine, airborne dust can escape and float in the air for a
long time before settling out. And, whether it takes a day, a week or a
month, dust will build-up. Accumulations of fine dust are easily
disturbed and can become airborne, creating an explosive atmosphere. So,
proactive clean-up procedures are required to minimize dust
Accumulations of fine dust are easily ignited, particularly by `hot
work’ maintenance practices, so it is essential to wet areas down before
starting `hot work’ and have a fire watch posted during and after the
Even clean-up operations can result in fires and explosions. An
inexperienced worker utilizing compressed air or an air blower to
`blow-down’ an area can stir up the fine dust and re-entrain it in the
air stream; and if it drifts to an area where `hot work’ is underway,
the results can be explosive.
Should a small fire occur, it is common practice for workers to use a
fire hose to put the fire out, but the stream of water can cause very
dry dust accumulations to become airborne and which can be ignited by
the fire resulting in a local primary explosion / deflagration.
A local primary explosion can shake a building causing more very fine
dust accumulations to fall off roof beams or wall girts, which if
ignited can result in a chain reaction of explosions propagating from
one area to another. These secondary explosions can destroy the entire
Electrostatic electricity can build-up in equipment and sudden
discharges are enough to ignite a dust cloud, so grounding / bonding
equipment is important.
Fire and explosions resulting from handling dry wood are a real risk,
consequently, it is necessary to assess the risks and utilize
appropriate asset protection measures.
- Risk Assessment
- Dust Control with spark detection and suppression
- Fire detection and suppression
- Explosion detection and suppression
- Gas detection / Monitoring
Risk AssessmentAsset protection is a complex topic and one could write a lengthy
manual to cover it completely. There are specialists available to help
you with assessing the risks and making recommendations; so when first
planning a project, engage an engineering company specializing in risk
assessment and asset protection early in your project definition. What
you don’t want to do is to underestimate the types and amounts of asset
protection systems required.
Plant areas fall into various area classifications, from those with
low risk of fire or explosions to those with a high risk. The asset
protection specialist will be able to assist you in determining the
classification that is appropriate for each area in your plant. Each
classification has certain requirements for equipment with respect to
bonding / grounding, and electrical classification.
Most areas in a wood pellet plant are in the highest risk
classification, Class II, Div. 1, Group G. For these areas, all
electrical equipment (lighting fixtures, switches, sensors, etc.) must
be designed and rated for the area and must be in CSA approved explosion
proof enclosures or connected to Intrinsically Safe (IS) Barriers
located in non-classified areas. All motors must be TEXP NEMA Premium
efficiency motors, CSA approved for Class II, Div. 1, Group G. Armoured
cables, rated for hazardous locations, are used for power, control and
PreventionThe processes involved in a dust explosion are extremely complex and
hard to predict. However, explosions can be prevented by eliminating the
fuel or ignition sources, by controlling dust concentrations or by
limiting the oxygen necessary to sustain combustion / deflagration.
Additionally, the potential effects of an explosion can be controlled by
the design of the enclosure. The best countermeasure against fire or
explosions is to prevent them from happening. Following are a few
- Reduce the generation of dust by keeping speeds low and designing chutes to minimize impacts and product degradation.
- Reduce the likelihood of fires by daily vacuuming up spills and
accumulated dust, particularly away from heat generators such as lights,
heaters, motors, etc.
- Provide deflectors (60°) over hard to reach places such as
purlins, roof beams and girts to prevent the build-up of dust on hard to
clean, horizontal surfaces.
- Reduce the explosive environment inside enclosed structures by:
- Maintaining a good airflow through structures / enclosures to reduce airborne dust concentrations below dangerous levels.
- Contain the dust inside equipment and evacuate airborne dust from the equipment; collect the dust and dispose of it.
- Note: Do not use
air blowers or compressed air to `blow down’ or clean equipment, as that
will create an airborne dust cloud that in the correct concentration
can be explosive.
- Reduce potential ignition sources by:
- Utilizing static conducting materials for belting, liners, grease, etc.
- Ground and bond all equipment and conveyors to drain away static electricity.
- Remove tramp metal from the product flow.
- Line chutes with non-sparking materials.
- Maintain equipment to reduce areas of friction.
- Don’t mix wet and dry biomass within the same storage pile, as that can promote biological action and self-heating.
- Measure the temperature of stored material and monitor temperature
trends. A sudden increase can be indicative of `runaway’ heating which
can quickly lead to self-ignition.
- Aeration can be used to keep the temperature level of stored biomass
from reaching dangerous levels; however, once temperatures reach the
`runaway’ level, aeration systems need to be turned off as they will
provide O2 to feed the fire.
- Monitor the gas levels inside biomass enclosures for CO, CO2, and O2, as changes in these can be indicative of fire.
Dust ControlThe design parameters for dust control systems should include:
- Totally enclosing the equipment to contain the product and to give heavier dust particles time to settle out.
- Keep the equipment under a negative pressure so that there is a net
inflow of air into the equipment, thereby reducing fugitive dusting.
- Remove dust laden air permanently from the system and collect the dust in containers for removal from the site.
- Do not reintroduce the dust back into the conveyor system where it would only be re-entrained at the next transfer point.
- A dust collector with sprinklers, explosion vents, rotary airlock and fan, preferably located outdoors.
- Ductwork has pick-up hoods, clean-outs, test ports, visual airflow indicators and blast gates to balance the airflows.
- An airflow transmitter is mounted on the duct between the dust collector and fan.
- IR spark / flame detectors and a quenching water spray and
fast-acting (milliseconds) abort gate are located in the main duct
before the collector. Dry chemical suppressors are utilized where water
cannot be used.
- A disposal bin is provided below the collector rotary airlock.
Fire Detection and SuppressionFire detection and suppression systems should comprise the following measures:
- Conveyors with heat sensing wires and sprinkler systems.
- Infra-red (IR) spark and heat sensors in particularly sensitive areas.
- Storage silos and bins with temperature and gas monitoring systems
and aeration systems, as well as inert gas injection nozzles for fire
- All structures and galleries must have sprinkler systems, and fire hose stations located near the entrances to structures.
- In warm climates, wet sprinkler systems can be used, but in cold climates, dry systems will be required.
- Fire alarm systems interlocked to shutdown process and HVAC equipment.
- Fire hydrants should be located outside the structures.
Deflagration / Explosion Detection and Suppression
- Wherever possible, equipment such as dust collectors that are
susceptible to dust explosions, should be physically isolated from other
equipment or structures. In order to prevent deflagrations / explosions
from propagating to upstream or downstream equipment, the following
isolating devices can be used:
- Rotary airlocks
- Isolation hoppers and gates
- Fast-acting isolation gates
- Material plugs
- Chemical isolation
- Blast walls around susceptible equipment
- Explosion flame-front detectors or pressure detectors trigger
fast-acting (milliseconds), dry chemical suppressors to reduce the
effects of and extinguish deflagrations. Whenever explosion detectors
are activated, the associated process equipment is immediately stopped.
- Chemical explosion suppression is provided to reduce the effects of
explosions occurring inside equipment, and chemical explosion isolation
is provided to prevent the propagation of explosions into upstream and
- Deflagration venting reduces the explosion pressures inside
structures by providing a pathway for the pressure to escape from
affected structures or equipment.
Gas Detection / Monitoring
- CO and CO2 gas levels can be indicative of smouldering biomass, so measure and trend these gas levels.
- Methane (CH4) is a product of biomass degradation, so measure and trend CH4.
- Ventilate enclosed spaces to evacuate harmful gasses. Where there is the possibility of CO, CO2 and CH4 generation, it is necessary to measure these and O2 levels in adjacent enclosed areas for personnel protection.
Reference Codes and Regulations
under many jurisdictions in Canada, including but not limited to:
- National Building Code (NBC)
- National Fire Code (NFC)
- Local building codes
- Canadian Electrical Code
- National Fire Protection Association (NFPA), particularly the following codes:
- NFPA 68, Standard on Explosion Protection by Venting
- NFPA 69, Standard on Explosion Prevention Systems
- NFPA 70, National Electrical Code, (particularly the sections on area classifications)
- NFPA 77, Standard on Static Electricity
- NFPA 499, Recommended Practice for the Classification of Combustible
Dusts and of Hazardous Locations for Electrical Installations
- NFPA 654, Standard for the Prevention of Fire and Dust Explosions
from the Manufacturing, Processing and Handling of Combustible
- NFPA 664, Standard for the Prevention of Fire and Explosions in Wood Processing and Woodworking Facilities
retained not only to provide help with the design of asset protection
systems, but also to provide assistance navigating the `regulatory
Other Helpful ResourcesPlant insurers such as FM Global have their own requirements. For
technical guidance on some of the issues, see the following FM Global
Property Loss Prevention Data Sheets:
- FM 7-10, Wood Processing and Woodworking Facilities
- FM 7-11, Belt Conveyors
- FM 7-17, Explosion Protection Systems
- FM 7-73, Dust Collectors and Collection Systems
- FM 7-76, Prevention and Mitigation of Combustible Dust Explosions and Fire
- FM 8-27, Storage of Wood Chips
After the two sawmill tragedies mentioned above, WorkSafe BC and
industry compiled a document titled “Wood Dust in Sawmills, Compilation
of Industry Best Practices”, published May 4,2012, which describes
industry best practices with regards to “wood dust clean-up, control and
associated fire prevention and protection measures”. This lengthy
document has a very good list of excellent references,
The BC Forest Safety Council has a Basic Audit and Safety Evaluation
(BASE) program for all wood handling operations and in 2011 extended the
program to include guidelines for the assessment and safety evaluation
of wood pellet mills. “Base Audit, Draft Guidelines Version 2.1, Pellet
The Wood Pellet Association of Canada (WPAC) has written a report
titled “Determination of Explosibility of Dust Layers in Pellet
Manufacturing Plants”, which provides guidelines for assessing the risk
posed by dust accumulations. While targeted at pellet plants, many of
the observations and recommendations are applicable to other wood