Unlike many agricultural products, biomass lacks consistent flow characteristics because its characteristics vary widely. The term biomass broadly refers to plant matter: Trees are biomass. Rice stover is biomass. Kelp is biomass. So, understand we’re speaking in generalities when addressing this material.
Variety is also why you should understand your application’s specific material. Even similar biomasses will behave differently and require a different approach. For example, a stream of pine shavings from a planer mill will behave very differently than a stream of shavings from a shavings machine. Although these materials have the same name, their shapes and bulk densities create very different handling characteristics.
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The above table includes characteristics of bulk materials that can affect the design of a bulk-handling system. Granted, not every characteristic will affect the stream flow (those that do are highlighted in green). For instance, how acidic the material is won’t affect whether your biomass will plug up a chute or not. But in general, these are the things that you should consider when designing a bulk-handling system.
Despite the many differences between biomasses, there are some traits that they all generally possess. The first is that biomass does not flow well. It’s like scrap paper: it’s irregularly shaped and has a high friction coefficient. Its particles are also cohesive—they nest and interlock, so it can be hard to get biomass moving once it stops flowing. If the biomass is wet, it can be even more challenging to move.
These characteristics can combine to create other issues. When the material stream includes large particles, a wide range of particle sizes, higher densities, and higher moisture levels, the system handling it will require more power. And those wide ranges of particles can limit your options for transporting material streams.
When biomass is added to other material streams, it also affects their characteristics. When added to coal for co-firing, even a small percentage can drastically affect the stream’s behavior—it increases friction, which increases the energy required to move it. Don’t underestimate how adding biomass to an existing material stream may affect it.
When optimizing flow, you must consider the material you’re working with and the material over which it slides because it affects the flow. Such was the conclusion of a team of scientists from Spain and the Netherlands who studied the flow behaviors of milled corn stover and poplar. In their research, they wanted to determine how a surface material affected the flowability of biomass. In doing so, they tested the materials at various moisture levels and particle sizes on different surfaces. Unsurprisingly, high molecular weight polyethylene (HMPE)—a low-friction plastic—proved a standout winner of the surface materials they tested in regards to how it affected wet biomass’s flowability.
Their conclusion makes sense: a surface with a higher coefficient of friction will “grab” at the material more than one with a lower coefficient of friction. Imagine pushing a small pile of woodchips over a wood floor. It’s not difficult—you can move the whole pile without difficulty. Now imagine shoving that same pile over a coarse carpet. You see what happens. The woodchips encounter much more resistance, so instead of sliding, they tumble.
The same thing occurs inside a bulk-flow system. For example, the interior surface of a bin can affect how easily the biomass flows downward to the discharge. One material may cause your biomass to collect inside a chute while another enables it to slide through freely.
Another study shows how other factors affect friction and flowability. For example, moisture makes materials more frictional (adversely affecting their handling); grain also affects friction. You may not be able to control all these factors, but you get the point: more than one thing affects biomass’s handling characteristics.
Still, while the nuances may change things from time to time, understanding the general characteristics will serve you well in most situations: biomass doesn’t flow well, it bridges easily, and it often contains contaminants you must remove.
 “Biomass Granular Screw Feeding: An Experimental Investigation.”
 “Study of Flow Characteristics of Biomass and Biomass-Coal Blends.”
 Handling Behavior of Two Milled Biomass: SFR Poplar and Corn Stover
 Friction of Wood on Steel