In today’s episode of the Dust Safety Science podcast, we’re covering fire and explosion safety for direct heated belt dryers. This topic is based on a project called “Safer Operation of Direct Heated Belt Dryers,” which is a collaborative effort between the Wood Pellet Association of Canada, the BC Forest Safety Council, Obex Risk, and the Biomass & Bioenergy Research Group (BBRG) at the University of British Columbia.
Why is This Project So Important?
Why does this project hold significance? And how did the concept of the belt dryer emerge? Two significant shifts in the Canadian wood pellet industry prompted the need for this project.
The Move Toward Feedstock With Greater Variations
Initially, the industry relied on a controlled feedstock obtained from connected sawmills. This feedstock consisted of planer shavings and dried residue that were relatively free from contaminants. It was then used to produce pellets, with the dryer system pre-drying the material before it reached the pelleting phase, resulting in fewer complications.
However, in recent years, there has been a transition towards a feedstock with greater variations. This feedstock is obtained from multiple external sources, not limited to connected sawmills, and is transported by trucks or trains from different locations. It consists of materials such as bush grind and hog fiber, along with numerous contaminants like metals, screws, batteries, man-made objects, lighters, and more.
The move to having more of this various different types of feedstock being included and being processed really caused issues in the drying process, and can cause issues in the pelting process as well. So that’s sort of shift number one – away from a very controlled feedstock in this industry to a feedstock that has a lot more variation.
Growing Use of Belt Dryers
The second shift involves a transition from the prevalent use of rotary drum dryers in pellet mills. The newer pellet plants, especially those in British Columbia, have adopted belt dryers as an alternative.
These belt dryers offer potential advantages such as reduced emissions and a decreased risk of fire incidents. However, the report states, “As direct heated belt dryers have become more common, the pellet industry has witnessed a rise in safety incidents in recent years.”
The increased frequency of fire incidents and related events associated with the use of these dryers prompted the Wood Pellet Association of Canada, in collaboration with the BC Forest Safety Council and media partner Canadian Biomass Magazine, to organize a belt dryer safety symposium. Thus, these two shifts, coupled with the occurrence of fire and even explosion incidents, have resulted in a heightened focus on examining belt dryer systems and addressing the associated safety concerns.
What Did the Actual Project Look Like?
In November 2020, a symposium on belt dryers was organized by the Canadian Wood Pellet Association, BC Forest Safety Council, and Canadian Biomass Magazine. The event saw a diverse range of participants, including pellet mill owners, operators, industry groups, belt dryer manufacturers, fire and explosion safety experts, WorkSafeBC, and various other industry and safety organizations. The symposium served as a platform for open discussions regarding the state of belt dryers and facilitated the sharing of challenges faced by the industry stakeholders.
A crucial aspect of the event was the open sharing of experiences regarding fires and explosion incidents. Participants discussed potential causes of these incidents while acknowledging that investigations were still ongoing and uncertainty surrounded the exact factors at play. They also openly discussed challenges related to different feedstocks and shared information about the equipment utilized to mitigate contamination issues within the dryer systems. This approach aligned with the shared motto often heard in British Columbia from these groups: “We don’t compete on safety.” It was truly remarkable to witness such commitment.
One significant outcome of the symposium was the establishment of a belt dryer working group. This group comprised 25 members representing the Wood Pellet Association of Canada, BC Forest Safety Council, two dryer manufacturers, pellet producers, safety equipment suppliers, consultants, academics, and technology providers. It was subsequently divided into four subgroups, each focusing on specific areas of expertise.
These subgroups were tasked with addressing key aspects, including feed product quality and contamination control, dryer operations, safety systems and controls, and procedures and maintenance practices. Operating independently, these subgroups worked diligently on their respective projects, pooling their findings together to form the foundation of the project report. This collaborative effort marked the starting point for creating the comprehensive project report.
What Were the Project Findings?
The major findings of this project can be categorized into three key areas.
Finding #1: Controlling The Input Feedstock To The Dryer Is Extremely Important
The first finding highlights the paramount importance of controlling the input feedstock to the dryer. This emphasizes the need for meticulous management and regulation of the materials entering the dryer system.
When biomass arrives at the site, it is typically stored either in open or closed storage piles. There is often a separation process involved, which will be further discussed shortly. To enhance dryer efficiency, an infeed hammer mill or similar size reduction equipment is commonly used. The material is then transferred to a bin equipped with an infeed screw conveyor, which feeds it into the dryer.
Following the heating process, the material often undergoes additional size reduction before entering the pelleting process. It is important to note that the project’s scope encompassed the journey of the material from the infeed collection and separation process, through any size reduction stages, and into the dryer system. The project specifically examined the behavior of the material within the dryer. To regulate the input feedstock, multiple samples were collected from various plants and sent to the BBRG group at the University of British Columbia for analysis. Detailed images of these samples can be found in Appendix A of the report.
The input materials can be broadly categorized into different groups, such as sawdust or planer shavings. These types of materials are typically homogenized and consistent, with minimal contamination. However, there is also hog grind, which consists of residual material after debarking and scalping processes in a sawmill. Hog grind often contains a high proportion of bark, soil, and various contaminants.
Another category is bush grind, which involves chipping forest residue in logging areas. This material can include oversized materials, branches, bark pieces, and chips, particularly from forestry operations or as a response to wildfires. The significance of bush grind has grown in recent years due to the prevalence of wildfires in British Columbia. It is commonly referred to as fire-kill bush grind.
Other materials, such as wood chips from lumber and logging operations, as well as mixed pile materials, are also part of the input feedstock. The report includes illustrative images of example contaminants found in the samples analyzed by the University of British Columbia. These contaminants encompass a range of items such as rocks, screws, batteries, lighters, aluminum foil, and sandpaper. It is crucial to remove these contaminants before size reduction and entering the dryer to mitigate fire hazards.
The primary objective is to eliminate these contaminants prior to size reduction and entering the dryer system, as their presence can potentially lead to fire incidents.
The report provides a categorization of various in-feed controls that can be employed. The optimal combination of these controls depends on factors such as the type of material being received, the specific dryer system in use, and its operational parameters. The suggested controls include scalpers, density separators, rock drops, vibration screens, magnets, and visual inspection.
Each control functions differently. Scalpers, grizzly rolls, and disc screens are effective in removing large oversized materials. Density separators are utilized to eliminate heavy materials, while rock drops serve a similar purpose. Vibration screens are employed for size-based separation. Magnets are specifically designed to extract ferrous metal materials. Visual inspection involves screening the material at the infeed storage stage, allowing for the removal of larger pieces.
The report discusses these control options, emphasizing that there is no one-size-fits-all solution. The recommendations call for a review of other industries to identify equipment that could be adapted for in situ or in-flow assessment of contaminations and their removal while the material is in motion. The project advocates for exploring and adopting potential solutions from other industries as a means to enhance contaminant management in pellet mill operations.
Finding #2: Recommendation to Use Indirect Instead of Direct-Fired Dryers
Finding number two in the report recommends the use of indirect heated dryers as opposed to direct heated dryers. In an indirect heated dryer, a heating fluid such as oil is employed to heat the dryer system, whereas in direct heated dryers, the burner directly provides the heat.
The challenge with direct heated dryers lies in the potential entry of sparks generated in the burner and burner chamber, which can lead to fire or explosion incidents within the dryer. The presence of ignition sources within the system is inherent to direct heated dryers. To address this issue, the report offers several recommendations:
1. Utilizing passive spark capturing and elimination technology: This involves incorporating grates, altering flow directions, and implementing zigzagging patterns to reduce spark intensity and cool sparks before they reach the dryer.
2. Increasing the length of the burner feed channel: By extending the burner feed channel, the distance between the ignition source and the dryer can be increased, reducing the likelihood of sparks entering the dryer.
3. Installing dryer deluge systems: Deluge systems can be implemented to provide water or other fire-suppression agents in the event of a fire, mitigating its spread and minimizing potential damages.
4. Ensuring proper belt alignment control: Proper alignment of the belt within the dryer helps prevent material plugging and subsequent heat source build-up, reducing the risk of ignition.
These recommendations were discussed within the report as part of the suggestion to opt for indirect heated dryers as a safer alternative to direct heated ones.
Finding #3: Engineered Safety Controls and Technologies Are Likely Still Needed to Address Residual Risk
Finding number three, it focuses on engineered safety controls. The report highlights that even with an indirect heated belt dryer, precautions must be taken to address potential ignition sources. There is a risk of smoldering masses being carried into the dryer from the infeed system or generated within the dryer itself. Isolation measures are necessary to prevent fire and explosion events from propagating into the dryer. Varying material compositions can also contribute to smoldering risks. These hazards were analyzed through a bow tie analysis, as discussed in the report.
Several other comments and considerations are mentioned in the report, so we will highlight five of them. Firstly, it emphasizes the importance of proper housekeeping to remove dust from the surrounding areas. Although dryers are designed to heat materials, they can still be susceptible to fires, so maintaining cleanliness is crucial in connected equipment and surrounding spaces.
The second consideration is to review and appropriately design active engineered systems such as spark detection and infrared temperature monitoring. These systems should be thoughtfully integrated into the dryer setup, considering factors such as airflow patterns to ensure effective incident detection. The report shares an example of a temperature probe incident where the probe did not detect the fire due to either insufficient sensitivity or improper airflow in the dryer.
Maintenance is the third consideration, which recommends visual inspection of burner chambers and heat shields on a daily basis during commissioning and in the initial weeks of operation. Weekly inspections are advised during the first few months, followed by establishing an appropriate inspection frequency specific to the dryer. Additionally, proper cleaning and maintenance of the dryer belts is critical to prevent material from entering and causing fires.
The fourth consideration, raised by the manufacturers, emphasizes that temperature probes and other sensors that come with the dryers are not intended as safety devices. They operate on sampling frequencies of several seconds, making them inadequate for promptly detecting fires or deflagration events. It is essential to install a dedicated safety system in addition to these sensors.
Lastly, the report highlights that the risk assessments conducted by dryer manufacturers assume no contamination in the infeed, which is often not the case in real-world scenarios where contamination is prevalent. Therefore, when discussing risk assessments with manufacturers and providers, it is important to consider the level of contamination at your site and determine any additional safety controls required beyond the manufacturer’s assessment.
Conclusion
Overall, this project has made significant contributions to enhancing the safety of direct heated belt dryers in the wood pellet industry. The findings and recommendations outlined in the project report serve as valuable guidance for industry stakeholders, equipment manufacturers, and safety organizations. By addressing the identified challenges and implementing the recommended safety measures, the wood pellet industry can strive towards safer and more efficient operations in the future.
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Dust Safety Share
Organizations
Wood Pellet Association of Canada
BC Forest Safety Council
Obex Risk
Reports
Safer Operation of Direct Heated Belt Dryers (Final Technical Report)
Key takeaways from WPAC’s Belt Dryer Symposium – Canadian Biomass Magazine
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