• Publication Date: 07/01/2021
  • Author(s):
    Dulin, Joel E.
  • Organization(s):
    Biomass Engineering & Equipment
  • Article Type: Technical Articles
  • Subjects: Mechanical conveying

Joel E. Dulin | Biomass Engineering & Equipment

Conveying systems are designed to handle specific types of materials. Determining which conveying system is well-suited for your material can be difficult if you don’t know much about the material involved. This article describes the importance of understanding your material — in this case, biomass — in order to determine the best material handling system for the application.

As demand for renewable and climate-friendly materials has grown, so has interest in cellulosic biomass — renewable plant materials such as forest or agricultural wastes. New industries that use biomass have thus emerged, and industries that used it on a small scale in the past have grown significantly. Examples of such industries include biochar, fuel pellets, and powdered lignin. These industries have grown so rapidly, however, that knowledge about what works and what doesn’t work in regard to handling large volumes of biomass hasn’t disseminated from industries familiar with biomass challenges, such as paper manufacturers and particle board makers. As a result, operations personnel in emerging industries must often determine through trial and error which systems work best for handling biomass materials.

To complicate matters, higher standards in safety and performance have outmoded many bulk handling systems on the market. Conveyors that were once good enough despite their drawbacks no longer meet the performance standards expected of them. This change in the market has created a need to identify better biomass handling systems than those that have traditionally been used.

Prioritizing material handling systems

Companies often de-emphasize bulk solids handling systems when designing a new plant because they often view these systems as ancillary equipment. As a result, operations that use biomass are often equipped with conveyors that aren’t robust enough to handle the material. In worst-case scenarios, biomass operations have failed at startup due to indifference about material handling systems. This is what happened at a massive pellet plant in Wawa, ON, when the engineering, procurement, and construction (EPC) firm overseeing the build equipped the plant with conveyors undersized for the application. The conveyors were continually overloaded with biomass and never functioned properly.

The firm likely did this because EPC companies typically design and procure production-area equipment first when designing a new manufacturing facility and then use the remaining budget to procure conveyors to tie the production areas together. This often results in the EPC companies sourcing cheap conveying equipment. EPC engineers, and many manufacturers, view material handling as an overhead expense — something that adds cost but isn’t as important as processing equipment because it doesn’t produce anything. The same attitude commonly accompanies waste handling systems (biomass is often process waste from lumber mills): because waste has little — if any — value, companies try to spend as little as possible on systems that handle it.

The importance of biomass handling systems comes to the forefront, however, when the system results in downtime or requires excessive attention from maintenance personnel. Regardless of whether these conveyors are handling waste or feedstock, when the conveyors stop, production halts, and any money the manufacturer saved by purchasing inexpensive handling systems quickly evaporates.

Understanding biomass characteristics

Biomass is highly variable and encompasses a great number of materials ranging from corn stover to wood chips, as shown in Figure 1. Despite its variability, biomass materials share characteristics that affect their handling.

Biomass materials, like these wood chips, share characteristics that affect their handling, such as combustibility, acidity, and abrasiveness.

Contamination. Unlike factory-made materials, biomass is harvested, the process of which inevitably picks up foreign materials such as rocks, dirt, sand, debris, scrap metal, and the occasional tool. Biomass contamination is so common that the National Fire Protection Agency (NFPA) stipulates in NFPA 664: Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities that manufacturers must inspect biomass for foreign materials prior to processing it (664.9.4.11.1-2).1 This standard applies to woody materials and all “wood-derived particulate and other cellulosic materials used as a substitute or supplement for wood” (664.9.3.1). Due to the volume of biomass being processed at most facilities, the standard all but requires manufacturers to mechanically screen their material. Should biomass not be screened or contaminants within go undetected, damage to the material handling equipment is possible, along with a fire or explosion risk, as shown in Figure 2.

Large foreign materials can destroy a conveyor’s internal components and lead to fires or explosions.

It should be noted that the NFPA is primarily concerned with foreign materials that may cause sparks as they enter processing equipment and ignite dust particles, thereby causing a fire or explosion. Contaminants such as metal or large rocks are therefore what the NFPA has in mind in Standard 664. Not all contaminants pose the same risk, of course. Sand, for example, is unlikely to cause a fire, though it will impact the quality of the final product and increase wear on processing and transfer equipment. Depending on how contaminants like sand and dirt will affect processes, these too may need to be screened out of the material stream.

Combustibility. Biomass, especially when dry, produces fugitive dust. Fugitive dust is any dust generated simply from the act of moving material. Dust is a concern not only due to the mess it creates and the burden of cleanup but because dust is combustible. This characteristic is why the NFPA updated Standard 664 and NFPA 652: Standard on the Fundamentals of Combustible Dust with extensive requirements for systems that handle combustible dust. Among others, these standards’ requirements include:

  • Generally dust-tight equipment performance (664.9.3.6.1.2, 652.9.3.2.2, 652.9.3.15.1.1)
  • System-isolation capability in the event of a fire or explosion (664.9.7.2.1.1, 652 9.7.4.1)
  • Ability to withstand explosion pressure (664.9.7.1.1[1], 664.9.7.1.3.1, 652 9.7.4.3)
  • Incorporated explosion mitigation technology (664.9.7.1.1[2])
  • Incorporated explosion relief vents (664.9.7.1.1[3])

These rules apply only where an identified risk exists, as not all biomasses pose a fire or explosion risk. The rules do, however, require that manufacturers always evaluate their material for combustibility. In general, materials pose a greater risk of fire if they’re dry, and the explosion risk increases as the particle size decreases. The NFPA specifically considers a dust hazard to exist when the average dust size is less than 500 microns or when 10 percent of the dust mixture contains particles less than 80 microns in size (664.A.9.3.3.1).

Acidity. All biomasses contain acids. Moisture provides a path for these acids to corrode a conveyor’s steel components. Moisture may come from an external source, such as rain or dew, or an internal one, such as from the biomass itself. Internal moisture is why green (undried) materials are more corrosive than dry ones. Heat and salt make corrosion worse by speeding its formation. Corrosion also intensifies when biomass has the opportunity to sit in a conveyor pan for an extended time period, which happens routinely when a manufacturer shuts off production at the end of a shift. Acid corrodes the metal overnight, and when the conveyor gets turned on the next day, the material sweeps away the oxidized metal, exposing fresh steel beneath. What many manufacturers assume is wear from abrasion in mechanical drag systems is often corrosion-induced erosion, as shown in Figure 3.

Patches are used to cover holes from corrosion and mechanical wear, which have wreaked havoc on a poorly designed chain drag conveyor.

How acidic a specific type of biomass is depends on the species and the soil conditions in which it grows. Western red cedar, for example, is well-known for being extremely corrosive. The pH of undried red cedar has been measured as low as 2.9.2 The lower a biomass’ pH level, the more acidic the material is and the more corrosive it will be.

Galvanized conveyors do well for handling green, acidic biomass and material that has been exposed to salt. If the material is hot, the conveyors should be constructed of stainless steel. Opting to use mild steel for conveyors that handle these materials will shorten the conveyor’s life span to a point that won’t justify the cost of the equipment.

Abrasiveness. As with acidity, biomasses’ abrasiveness depends on several factors. Biomass that’s derived from bark, processed in the field, or otherwise contaminated with dirt or sand will be more abrasive than clean biomass, which is biomass that’s free of these contaminants. However, this isn’t to say that clean biomass is nonabrasive — it certainly is. This characteristic is one reason why pneumatic and tubular drag conveying systems typically aren’t, in our experience, the best choice for handling biomass, as the biomass and abrasive contaminants aggressively wear down these systems’ components.

For particularly abrasive materials, abrasion-­resistant (AR) plates will lengthen the life span of a conveyor. Ultrahigh-molecular-weight polyethylene — a low-friction plastic — can be used to protect conveyors handling material that’s both abrasive and acidic, since the polyethylene isn’t subject to corrosion, unlike steel.

Poor flowability. Biomasses routinely possess challenging flow characteristics, as biomass isn’t a free-flowing material. Rather, biomass particles tend to weave together, a trait that worsens as the material rests and in materials with flatter and wider particles. Because of this, reclaiming biomass from storage can become an arduous task: the material may bridge across the hopper opening or the hopper’s reclaim arm (the rotating screw that moves material toward the hopper’s discharge) may malfunction when the bin is only partially filled.

Considering biomass conveyor types

Biomass’ unique characteristics create challenges for most conveyor types, and the variety of biomass materials means that the best handling system depends on your specific application. As previously mentioned, companies have had to learn from experience which conveyor types work well and which don’t for handling biomass.

Belt conveyors. With all the issues related to abrasion and acidity, you may wonder why a manufacturer wouldn’t simply opt for a mechanical belt conveyor to transfer biomass. While in certain scenarios belts work well, there are various reasons why a manufacturer may not choose this type of conveyor. One reason is that belt conveyors can’t convey material at steep inclines unless fitted with sidewalls and cleats, features that increase upfront costs and the cost of ongoing maintenance. Thus, where manufacturers use flat belts to elevate material, they must significantly lengthen the conveyor system to create a gentle incline. This can prove impractical in terms of cost and space.

A second reason why belt conveyors aren’t always a good choice for transferring biomass is because they don’t control dust well. While some belt conveyor manufacturers offer components such as hoods or tubular belts to contain dust, these features don’t address the primary source of fugitive dust from these systems: dragback. Dragback is when particles stick to the belt as it rounds the conveyor head and then come loose and pile up under the return side of the machine. In fully enclosed belt systems, the issue of dragback is worse, as material collects inside the conveyor and is difficult for personnel to remove. Overall, enclosed drag and pneumatic conveyors do better at containing dust than belt conveyors (provided there are no holes in the equipment from wear or corrosion, unlike what’s shown in Figure 4).

Sawdust, which escaped from a pneumatic conveying system (bottom right), blankets the pavement at a pallet recycler in 2019. Biomass can aggressively wear pneumatic conveying systems and form holes in the curved sections.

Screw conveyors. Unlike belt conveyors, which may work well in scenarios where dust isn’t a concern and where conveyor length isn’t an issue, screw conveyors should never be employed to handle biomass except to meter the material over short spans. The first reason companies should avoid these conveyors is maintenance. As the screw moves biomass through the system, its flights will grind the biomass and abrasive contaminants into the pan, quickly wearing through it.

The second reason is that biomass, if stringy, will wrap around the flights and catch on the screw’s hanger bearings. Maintenance personnel must then shut down the conveyor to clear the obstruction, a feat that can prove difficult due to the often-inaccessible location of these bearings. Biomass that gets caught in the bearings also wears these components, decreasing their life span.

The third reason companies should avoid transfer screws is that they degrade material. While this may not be a concern with biomass feedstock, it is with pellets, which are fragile. Screws will grind up the pellets, thereby decreasing overall system efficiency and lowering the quality of the final product.

Bucket elevators. Companies that use biomass should likewise avoid bucket elevators for handling biomass feedstock. Bucket elevators work well with free-flowing materials like pellets, but because biomass doesn’t flow well, the elevator will be strained as the buckets pull through the material. Bucket elevators are already notorious for their high-maintenance requirements, and biomass only exaggerates this tendency.

Chain drag conveyors. As for what does work well in handling biomass, there are two options: belt conveyors (in certain conditions, as previously discussed) and chain drag conveyors, also referred to as paddle conveyors. Chain drag conveyors have been around since the late 1800s, and basic models on the market today are built little differently than how they were a century ago, as shown in Figure 5.

This ad displayed in an 1881 edition of Canadian Forest Industries magazine shows how little mechanical drag conveyors on the market today have changed.

These systems are typically designed with one or two chains that drag material through the conveyor’s trough via paddles or chain flights. The chains and paddles themselves usually drag along the bottom pan. The conveyors may be designed with an open top or as an enclosed system, as shown in Figure 6. Due to their design, drag systems will commonly wear quickly, leak dust, and can require much maintenance. They can excel at elevating materials at steep angles, however, which is where facilities typically employ them.

Examples of enclosed, NFPA-compliant chain drag conveyors

Chain drag technology has made leaps in the past decade, however, which makes it more appealing than other types of conveyors. Premium models are now available that address the shortcomings of traditional chain systems. These models keep chains and paddles off the floor, which reduces wear and friction, and come fully enclosed, which allows them to capture dust in accordance with NFPA standards. These conveyors are also more efficient than other conveyance options, notably pneumatic systems.

Conclusion

To ensure a biomass operation runs as successfully as possible, those who source the material handling equipment must understand the material and must choose systems robust enough to handle it. Light-duty, agricultural-grade conveyors aren’t up to the task, and neither are bins or silos designed for free-­flowing materials. Just as biomass is being used in new ways and at larger scales, the way in which companies approach biomass handling needs to change. Decades-old technologies that create dusty messes and repurposed grain bins may work well enough to get by at a small mill, but they aren’t suitable for today’s high-volume operations.

PBE

References

  1. NFPA 664: Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA 02169-7471, 800-244-3555, (www.nfpa.org).
  2. Nick Umney, “Corrosion of Metals Associated with Wood,” Conservation Journal, no. 4, (July 1992): accessed May 7, 2021, http://www.vam.ac.uk/content/journals/conservation-journal/issue-04/corrosion-of-metals-­associated-with-wood. The pH was determined by soaking wood in distilled water.

For further reading

Find more information on this topic in articles listed under “Mechanical conveying” in our article archive.


Joel E. Dulin (317-522-0864) is the director of marketing and sales coordinator at Biomass Engineering & Equipment.

Biomass Engineering & Equipment
Indianapolis, IN
317-522-0864
www.biomassengineeringequipment.com

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