Tools and Technology for Dust, Fire and Explosion Mitigation

Industry experts weigh in on one of the wood pellet industry’s most pressing matters.
By Luke Leroy | May 15, 2021

In the expansive scope of airborne combustibles, wood dust is typically a lower explosion severity dust as compared to others, particularly when compared to those produced in the pharmaceutical industry. Even though wood dust is considered moderate, the processes utilized in wood pellet production are intrinsic to creating a highly combustible fuel source. Unchecked, it could equate to a recipe for disaster.

However, today’s cutting-edge processes, technology and mitigation techniques are powerful tools to protect against and combat the inherent fire and explosion hazards associated with wood pellet production.
Through regulation standards and enforcement bodies, such as OSHA and the National Fire Protection Association, the challenges of managing these hazards are being met head on. But to truly mitigate combustible dust and fire/explosion potential, the responsibility falls on each individual facility.  

Assessing the Threat
Conducting a dust hazard analysis (DHA) has become the industry standard—and requirement—against which all wood pellet manufacturers are measured. The main goal of a DHA is to capture all the locations and equipment where a combustible dust hazard exists. By examining the ignitions source types to determine potential ignition sources within a given facility, specific fire and deflagration risk scenarios can be determined before an event occurs.

Once risk factors are identified, recommendations for additional safeguards and ignition control can then be implemented.

Existing facilities must have conducted a DHA by September 2020. For new building and facilities construction, and for processes and facilities undergoing modifications, DHAs must now be completed as part of the process. While the September deadline has come and gone, the requirements stands despite the COVID-19 pandemic and all its associated difficulties. Rumors of enforcement leniency on noncompliance exist, but it has been business for assessment conductors, with some companies providing flexible options during the coronavirus. “Remote DHAs are an option for some people,” says Jeff Davis, director of engineering at Conversion Technologies. “We’ve been working through a bunch of different on-site options as well, minimizing contact and minimizing time on site.”

“Hallam-ICS saw a lull as the pandemic first hit,” adds Chris Giusto, director of combustible dust safety. “But a large majority of our clients were considered essential businesses, and they all had to find ways to continue operating safely,” he says.

Equipment including bins, tanks and silos, hammermills, pulverizers and grinders, dryers and dust collectors, conveyors, screw augers and bucket elevators, sifters, screens and classifiers—are all pieces of equipment that must be evaluated both internally and externally for dust and particle buildup. In addition to equipment, general production areas must also be assessed. Combustible dust atmospheres in the rooms and buildings where equipment is housed, open processes, or from malfunctions from closed equipment also need to be risk assessed.

“Once risk areas are identified, evaluate each item for fire, deflagration and explosion hazards,” Davis adds. “Identify credible ignition sources, such as open flames. Examine all potential electrical hazards, as well as mechanical sparks ... those generated from grinding, impact or friction of processes. Also be aware of any hot surfaces, and locations where self-ignition can be an issue. Each of these threat types has a control measure to best mitigate the chances of fire or explosion.”

Once equipment and risk areas are evaluated, it’s pertinent to review existing safeguards and determine where additional controls are required. Consider how fires and deflagration can potentially move between pieces of equipment or buildings, under normal, abnormal and upset conditions.

“The final step is to assess the DHA and determine where additional safeguard and mitigation measures need implementation,” Davis says. “Remember, all DHAs must be performed by a qualified person, and each DHA must be reviewed and updated accordingly every five years.”

Professionally conducted DHAs aside, it is critical for facilities themselves to have a thorough understanding and ability to recognize and manage the hazards imposed by wood dust

Combustible Dust Safety Cycle
Chris Giusto serves as the director of combustible dust safety with engineering and controls frim Hallam-ICS, which directs its clients to follow what the firm calls the “combustible dust safety cycle.” It encompasses a DHA, mitigation, management of change (MOC) and—not to be overlooked—excellent housekeeping.  “Once a DHA has been completed, mitigation and controlling actions—such as explosion protection devices, equipment upgrades, new procedures or training—should be taken,” Giusto says. “This includes housekeeping.”

Ranked from most to least effective, controls include elimination, substitution, engineering controls, administrative controls and PPE. Eliminating a hazard is the most effective way to control risk; as the reliance on personnel increases to mitigate the controls, so does the risk factor. People are more susceptible to human error and, thus, statistically less effective.

The unfortunate situation with combustible dust is that elimination and substitution are generally not applicable as mitigation options, because that dust is an inherent part of the manufacturing of the finished product. For this reason, engineer controls are leaned upon, including explosion venting, suppression, isolation devices and fire protection systems.

For example, a dust collection system isolates people from the hazard by capturing the dust at the source and moving it to a safer location. This engineering control is then supplemented with the administrative control of housekeeping. It’s unwise to rely on housekeeping as a primary defense, but rather, it should be used to reinforce the engineering control of a properly functioning dust collection system.

Because fuel and oxygen sources are intrinsic to production processes, eliminating them is often not an option. In this case, minimizing ignition risks is the most effective means to mitigating the threats of fire, flash fire and explosion hazards. 

In short, MOC is what ensures all mitigations strategies remain effective, according to Giusto. Any housekeeping implications should be documented, noting accumulation levels. Changes here can highlight a potential problem elsewhere before it becomes a safety hazard.

“MOC is crucial, and is described as the best practices used to ensure that safety, health and environmental risks and hazards are properly controlled when an organization makes changes to their facilities, operations or personnel,” Giusto adds. “The overreaching goal of the MOC is to identify new potential hazards and mitigate them proactively.”

It’s also noteworthy that, although changes to facilities, operations, personnel or processes can eliminate or reduce hazards, they hold the potential to create new ones.

“Housekeeping is the hub of the combustible dust safety cycle,” Giusto continues. “Because good housekeeping is integral to all aspects of combustible dust safety, it must be considered and reviewed at every stage of the production cycle. In fact, good housekeeping is critical in the avoidance of secondary events, which are often the most destructive.”

A secondary event occurs when a primary explosion creates a pressure wave that travels faster than the actual flame front. This pressure wave disturbs fugitive dust accumulations throughout the facility. The flame front, which follows closely behind the pressure wave, ignites the newly airborne dust cloud, causing a chain reaction with the potential to destroy an entire operating facility.

As critical as housekeeping is, it must be considered a last line of defense.

So, how much dust is safe? According to NFPA guidelines, no more than a layered accumulation of 1/32-inch, which is approximately the thickness of a standard No. 1 paperclip. Note that this is a general guideline, and can vary by industry or material specifics. The NFPA also provides additional guidelines and formulas for calculating the maximum layer thickness that accounts for density variances of various dust types.
While all these science- and math-based formulas offer good guidelines, practical applications are often less black and white. “As a good rule of thumb, if you can’t see the color of the surface, or distinguish between two surface colors, then the dust layer is too thick,”  Giusto says. “You must remember that combustible dust is a fuel, and that no level of dust accumulation is 100 percent safe. Aim for zero … removing as much fugitive dust as possible. Get there as close as you can, as often as you can, within your means and resources.”

Hazard identification, protection strategies, training and awareness throughout the entire manufacturing cycle are most effective when plant personnel become involved at all levels. Management will become more aware of NFPA requirements, plant personnel will become involved with mitigation projects, and all employees will become more aware of hazards and how to best protect against them, Giusto says.
Formal training is the most effective means of speeding up this process, and NFPA 652 requires hazard awareness training for all affected employees, helping personnel identify hazards and conditions that can lead to hazards, and arming employees with the information needed to be proactive in preventing incidents.  

Evolving Industry Standards
In 2015, NFPA 652 was released, dubbed the Standard on Combustible Dust. It is designed to provide the basic principles and requirements for identifying and managing the fire and explosion hazards of combustible dusts and particle solids. This quickly became the gold standard, a format by which all other standards have been revised to match—creating a starting point for all industries to build from.

With biomass specifically, hazard identification becomes exceptionally important, and changes as the product goes through manufacturing processes. “On the front end of the process, we often have a green or recycled product coming in, which is typically being dried or milled into a much dryer product ... which is going to be much more dangerous from an ignition sensitivity and severity standpoint,” says Jason Krbec, sales engineering manager of CV Technology. “And then when we put it back into a pellet, we get a very different dust on the back end. As we go through the biomass process, that hazard identification changes.”
It’s noteworthy that NFPA 652 is now included in the International Fire Code as a standard to comply with when equipment, processes and operations involve dust explosion hazards. From there, industry- and commodity-specific standards must be references, all the way through state and local fire codes in order to maintain compliance. But despite interagency collaboration and attempted standardization of regulations, gaps still exist. The goal, of course, is to continue to consolidate standardization efforts because it improves combustible dust safety, and it can be challenging for companies with multiple industry functions to comply. In addition, this would maximize and streamline safety expertise.

In short, as Krbec describes it: A cleaner standard equals less combustible dust incidents—and that is the ultimate goal. This notion led to the infancy of NFPA 660: Combustible Dust Code (this proposed title and document number are still evolving). Within the next five years, the goal is to create a new, single standard resulting from the consolidation of fundamental standards with all the industry- and commodity-specific standards. It’s a process already underway, being led by a special task group amongst the combustible dust committees. As proposed, Chapters 1-9 would be fundamental to all industries; Chapter 10 dedicated to special fire protection requirements and 11-16 endemic to industry-specific requirements (NFPA 664 specific to the wood industry).

At this time, the new standard is slated to be issued effective sometime during 2024.

Identifying, Understanding Explosion Risk
When it comes to explosion risk and mitigation methods for critical process equipment in biomass facilities, knowing explosivity characteristics is critical to protection design in engineered controls. Explosivity parameters (dust cloud reactivity and concentration), ignition characteristics and minimum safe oxygen concentration are all key metrics to understand when planning protection efforts.

“There are three major types of explosion prevention measures, according to Rob Lade, chief technical officer for IEP Technologies. These are are containment, venting and suppression.

Containment involves isolating an entire facility to withstand an explosion. It’s effective at limiting a chain incident, but it’s largely not applicable with a large plant, according to Lade.

Venting consists of an engineered panel that’s programmed to open at a predetermined pressure, allowing the explosion to vent into a safe area. Venting does not extinguish flames, so a secondary flame suppression system also needs to be in place.

“It’s important to note that a vented explosion is a rather energetic process,” Lade says. “In an explosion venting occurrence, a fireball’s size on the exterior of the vessel is approximately eight times the vessel volume—all of which must be vented to a safe area. NFPA 68 details very descriptive standards for venting sizing.”

Suppression utilizes an explosion detector connected to a control panel, which releases an extinguishing agent into the vessel to engulf the fire, mitigate the pressure and contain the explosion. Deflagration isolation methods are also required for all prevention measures, to minimize the spread of an event from one process vessel to the next.

Types of applications typical to the biomass industry include dust collection units, which are by far the most common mitigation solution, followed closely by cyclone and cyclone separators.

Transfer points create an elevated opportunity for an incident to occur, especially on conveyor systems, where internal and external clutter can occur. Plus, it’s challenging to determine where the ignition can take place. “In these situations, optical protection is very nice because you can map pressures as a function of time,” Lade adds. “With optical detection, you know where the flame position is when it’s detected, and that gives much more information about the flame location and the propagation points of that flame.”

Understanding Fire Hazards
Expansion of the wood pellet and biomass energy sector has driven the need for large-scale storage capacity, and therefore, the number of fire-related incidents. “Between 2000 and 2018, 65 incidents have been reported,” says Vahid Ebadat, CEO of Stonehouse Public Safety. “Of those, nine occurred in 2017 alone, indicating that the frequency of these incidents is on the rise. Of these incidents, 59% involved fire, and more than a quarter of those incidents were confirmed to have been caused my self-heating as the ignition source.”

One of the largest incidents occurred in 2017, where a 500-ton pile of biomass fuels in Thailand caught fire, due to accumulated heat. When a material is bulked in mass volume, even a subtle ignition source can become a significant problem. Most often, these sources are associated with the inherent thermal instability of the material iself, overall bulk dimensions and heating processes. Once self-heating begins, the potential exists for a critical temperature to be reached, when the material smolders and temperatures continue to rise. “Without air, combustion cannot take place,” Ebadat says. “However, the bulk continues to heat, producing gasses hazardous to inhalation. At this point, if air is introduced, the trapped gasses are extremely susceptible to explosion.”

Operations and processes prone to self-heating hazards include material drying and heating, inadvertent sun heating, mechanical milling and grinding, fugitive dust on a hot surface, and bulk storage. Because exothermic reactions take time, it’s entirely possible for a hopper or bunker to catch fire days, or even weeks, after being filled. Self-heating measurements and identification within a given facility are often best identified through heating a sample under controlled conditions to determine the point at which temperature begins to increase, independent of the external heat source. Once this metric is identified, mitigation plans can be implemented.

“Keeping material temperatures at a safe margin below the determined onset temperature for self-heating is the best way to mitigate event risks,” Ebadat says. “Limiting storage time, facility and equipment design that avoids ledges, corners and dead zones, as well as limiting internal and external material deposits, are all steps that can be taken to avoid the rise of material temperatures.”

Like Ebadat, Timothy Heneks, director of engineering services at Dustcon Solutions Inc., recognizes the uptick in fire incidents within wood pellet manufacturing facilities. “According to, 108 major fires and explosions were reported within the wood industries between January 2019 and June 2020,” Heneks says. “Of those events, 16 percent directly involved biomass wood pellets.”

Surface layer fires involve material interaction with an abundance of air, occluding on conveyors, coolers, baghouses, milling areas and fugitive dust. On the other end of the spectrum, smoldering fires react in areas of low oxygen, and generally occur within a pile, such as in silos, storage heaps and domes. “Statistically speaking, silos and other storage areas represent 36 percent of fires and explosions taking place within wood pellet plants,” Heneks adds. “Dust collection is second on that list, followed closely by mechanical conveyors and then dryers.”

No Time like Now
While the official deadline for conducting and documenting an official DHA has come and gone, if not in compliance, the time to move forward is now. And though NFPA 660 is still a handful of years away from inception, it, too, will provide a clearer path forward in the mitigation of fire and combustible dust, specifically as it pertains to the wood pellet industry.

Until then—and beyond—the responsibility must be taken up by each manufacturing facility to address this ongoing cycle head on.

Author: Luke LeRoy
Pellet Mill Magazine Freelance Writer

Printed in Issue 2, 2021 of Pellet Mill Magazine