Issue link: https://www.e-digitaleditions.com/i/1388570
32 / July 2021 powderbulk.com 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 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 stan- dard 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. It should be noted that the NFPA is primarily con- cerned with foreign materials that may cause sparks as they enter processing equipment and ignite dust particles, thereby causing a fire or explosion. Contam- inants 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 mate- rial stream. Combustibility. Biomass, espe- cially when dry, produces fugitive dust. Fugitive dust is any dust gener- ated 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 FIGURE 1 Biomass materials, like these wood chips, share characteristics that affect their handling, such as combustibility, acidity, and abrasiveness. FIGURE 2 Large foreign materials can destroy a conveyor's internal components and lead to fires or explosions. 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.