Combustible Dust an Explosive Issue

Dust that accumulates at wood pellet plants during the production process can end up being a fuel source itself, fueling unwanted fires or explosions. This potential hazard makes effective dust management an integral part of wood pellet manufacturing. “Essentially we are taking a woody product, reducing its particle size and drying it into its most combustible form, and then densifying it afterward,” says Shawn Bells, general manager with Pacific BioEnergy. “So with regard to what we do, we’re probably doing the exact opposite of what you are supposed to be doing—we are creating a fuel—so then we have to deal with it.”

Producers recognize the nature of their operation can produce hazards. “We started really getting serious about dust about 10 years ago,” says Stephen Faehner, president and CEO of American Wood Fibers. “Then OSHA became focused on dust control and management. That’s, I think, when it heightened everyone’s awareness.”

It is the combustible nature of dust at pellet plants that makes it a hazard. Combustible dusts (CD) are defined by OSHA as fine particles that present an explosion hazard when suspended in air in certain conditions. Organizations like OSHA help the industry manage and enforce safety, and some organizations that cater to the pellet industry are beginning to step up compliance standards and standardize protocol. The International Standards Organization has launched an effort, under the direction of Working Group 4 of ISO/TC238, to develop global standards for numerous components of commercial, industrial and small-scale pellet production. Topics to be addressed include not only prevention, detection, suppression and management of fires and explosions, but also safe handling and storage, analysis of spontaneous heat generation and analysis of off-gassing products. Another example of the effort is WorkSafeBC’s new dust policies in British Columbia, Canada. Separate explosions in early 2012 that destroyed two sawmills and resulted in four deaths and 42 injured workers at Burns Lake and Prince George, British Columbia, spurred the organization to prepare a review and action plan that outlines plans for an agency restructuring and reviews mitigation of safety hazards from dust at sawmills and wood manufacturing facilities. “We crafted a policy that speaks to the responsibilities of employers, workers and supervisors with respect to the management and control of combustible dust,” says Al Johnson, vice president of prevention services with WorkSafeBC.

As part of the policy for employers, WorkSafeBC expects them to undertake a risk assessment to account for all of the potential risks and unique aspects  at  their particular mill. After the risk assessment, employers need to create a dust management and control program based on the assessment’s results. “That program then needs to be implemented and all of the workforce needs to be trained in that program,” Johnson says. “So that when we come knocking on their door doing an inspection, and we ask for their program, they can demonstrate that they have a written program.”

Additionally, the Wood Pellet Association of Canada and all of its British Columbia member mills have agreed to work with the BC Forest Safety Council to create a combustible dust audit tool customized for pellet mill operations. “This has basically helped mature the rest of the pellet industry by relying on the strengths of the larger producers to share the standards, the protocols, give them this inspection that they can use  to ensure that their systems are safe and up to the standard of the rest of the industry,” Bells says.

The pellet industry is evolving, and so are  dust management practices and the regulations and standards that accompany them. Equipment vendors are also onboard. “It is apparent that there is more focus on the dust collection and safety systems and devices within the pellet manufacturing facilities,” says Andy Clarke, vice president of sales at Clarke’s Sheet Metal Inc. “The basic concepts have remained the same, however, there have been advancements with regard to energy efficiency and advancements aimed at reducing downtime and increasing safety.”

Elements of Explosions

There are many important factors when considering the explosivity of CD including size, shape, moisture and environment. The industry refers to the dust explosion pentagon when illustrating the mix of components that need to be  present to cause explosions or fires. The five elements of the pentagon include fuel, ignition source, dispersion, confinement and oxygen. Removing any one of these elements can prevent an explosion, but not necessarily a fire.

Dust is present with other elements of explosions in every stage of the pellet-making process in both internal systems and external environments. Although producers can’t see what happens in the pipes and machines of the internal system, “what we do know is those internal systems have four elements of that explosion pentagon present,” says Scott Bax, senior vice president of operations with Pinnacle Renewable Energy Inc. “Really, the only thing you are missing at any given time is an ignition source. As a result, there is a tremendous amount of energy from all pellet producers to have very good systems.”

The external environment comes down to whether dust is present and can be dispersed. Few systems at plants are perfectly sealed, so dust can accumulate on places like ducts, shafts, cable trays or other places where a small amount of dust is leaking. “What is required is a management system; it should measure the rate of dust accumulation and link that to a rigorous clean-up schedule and program to minimize the presence of dust,” Bax says.

The National Fire Protection Association creates standards and guidelines for the pellet industry to follow. “As pellet producers, and, in general, the entire wood industry, we have a better understanding of the issue of combustible dust and that is paying off,” Bax says.

The NFPA provides the guideline of one-eighth of an inch over 5 percent of the area accumulating dust to meet the minimum explosible concentration. They define the size of deflagrable wood dust as .5 mm or less with a moisture content of less than 25 percent. “Mill practices on equipment operation and safety is continually influenced by the NFPA and OSHA, and we are continually working to comply with these changing standards,” says Bruce Livesay, vice president of marketing and owner of Western Pneumatics Inc. “Knowledge of continued process and safety regulations make it our responsibility to pass this knowledge to our customers.”

Managing the Mess

Information shared at a joint WPAC and WorkSafeBC combustible dust workshop held over the summer of 2014 discussed components of how to manage CD at pellet plants. Areas to improve a dust management program include a risk assessment process, implementation of controls, regularly scheduled inspections, thorough investigation of all incidents so as to prevent recurrence, education, training and supervision, program audit and review, a corrective action process and records and statistics to identify trends.

WorkSafeBC works with five companies operating 10 wood pellet mills to improve dust management programs, Pacific BioEnergy and Pinnacle are two. “WorkSafeBC’s done a very good job helping us with the education piece,” Bax says.

Employee training is regarded as essential moving forward. “A big one, probably our biggest one, is continuing to empower employees to understand combustible dust and report on any dust-related issues anywhere they believe there is a concentration of dust,” Bax says.

Equipment manufactures and vendors also take part in educating. “We train our customers how our equipment operates and how it should be maintained,” Livesay says. “Documentation is created to substantiate initial operating performance and periodic checks confirm the proper operation of our systems. We also perform inspections and critiques of systems which have been in operation for years and require upgrading.”

Designing for Dust

Building design and engineering controls are also impactful factors in dust management. Some pellet producers say this is the sustainable management practice. “The long-term solution to keeping your plant clean is not cleanup,” Bells says. “It’s meant to be an interim step until you have all the engineering, design, all of the steps necessary to keep the dust contained and keep it from spilling out of the process.”

Various equipment features can help fugitive dust from accumulating. Having round metal ducting, and monitoring overhead beams, ducting, electrical cable trays, lighting fixtures and more is important, as these are prime locations for fugitive dust to linger. Sonic air fans can help prevent dust from accumulating in elevated areas that can be difficult to reach, Bax says. Other equipment Pinnacle has for dust mitigation includes NFPA compliant vacuums that are antistatic, as well as wireless temperature sensors in pelletizers, among other equipment.

Western Pneumatics works with 12 large pellet plants, including dryer and pollution abatement systems for Green Circle, Georgia Biomass, German Pellets and Sega Biofuels, to name a few installations. “We developed new lines of dust collection filters and they have been received well for hammermill air-assist, cooler aspiration, dry fiber and pellet silo aspiration and truck and rail load-out aspiration,” Livesay says. “We also provide the high pressure pneumatic systems to move fines and dust from one end of the plant to the other.”

Dryers use wet precipitators and rapid thermal oxidizers to handle and clean large volumes of air before it is discharged into the atmosphere, according to Livesay.

Abort gates are one piece of equipment that exhaust hazardous air flow from the ducting. The gates are activated from spark detection system sensors. A spark detection system itself is primarily used as a fire prevention method in dust collectors by detecting and extinguishing sparks and embers. Sparks can be caused by various things such as a dull tool, damaged fan bearings or an overheated motor.

There is equipment to manage the fuel and ignition elements of the dust explosion pentagon, but elements like oxygen and confinement are inevitable. “So you try to focus on eliminating the fuel and minimizing ignition sources,” Bax says. “Those are the two items you have the most control over.”

The Combustible Challenge

“There’s a dust focus,” Faehner says. “The challenge is trying to change a lot of the culture in the wood products industry.”

He adds that almost everyone in the business is focused on manufacturing, and they’re certainly focused on safety, but at the end of the day they have to produce the goods. “With that in mind you have situations where people are driven to produce, produce, produce and they’re reluctant to either stop and fix a dust source—the dust is coming from somewhere—or to be mindful that you must have good housekeeping.”

Although the industry is taking dust seriously, organizations like WorkSafeBC, with a health and safety focus, hope it can become embedded into the overall day-to-day management of the facility. “We’re trying to integrate the safe management of wood dust into their overall operation culturally, if you will, so they just naturally see it as something they need to manage on a day-in-day-out basis,” Johnson says.

Johnson went on to say, stepping up the focus on health and safety is part and parcel of WorkSafeBC’s goal for pellet mill owners to maintain sustainable compliance in dust control management.

The standards take work to maintain for producers. “We have heightened our awareness and increased our focus to go from a very, very tidy facility to a spotless facility, and that’s a challenge because we are still at the stage where we’re still implementing more engineering to deal with some of the point source spillage issues,” Bells says.

Equipment design and system design is more technical than ever as dust control systems now involve many more safeguards, according to Livesay. “Existing mills will be challenged to improve dust control and once the dust is collected to install, equipment which will keep personnel safe and minimize the chance for fires or explosions,” he says.

Producers agree that one of the best paths forward is collaboration among industries dealing with CD issues. “The next evolution is a better understanding of where other organizations have been able to effectively manage dust and develop best practices, so focusing on improving collaboration within the pellet industry, within the forest products industry, and then even broader with payoff,” Bax says.

The work being done with CD standards in BC is just one example of how the industry is working to mitigate the fine particle byproduct of pellet production from becoming a source of fuel itself.  “As the regulator, we’re working closely with the pellet mills in the province and they’re working closely with us,” Johnson says. “A lot of work has been done, we see that, to effectively strengthen their programs around managing combustible dust in the pellet mill facilities. I think a little more work can be done, but the indications are positive that they’re moving in that direction.”

Author: Katie Fletcher
Staff Writer, Biomass Magazine
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