Understanding Moisture’s Impact on Biomass Handling

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Woody biomass is inherently hygroscopic, meaning it absorbs moisture. We’re sure you know this (it’s not exactly news), but fewer people understand how moisture affects biomass. Moisture significantly alters biomass’s properties and flow behavior. It affects volume and mass, making moisture content a necessary metric to consider when designing process flows in industrial applications.

Weight and Volume

First, the obvious: moisture increases mass. Consequently, given two piles of material otherwise equal, the one with the higher moisture content will weigh more, and the power needed to convey it will increase—not only due to the mass but also due to an increase in friction (water increases the cohesive properties of biomass). Conversely, removing moisture reduces power requirements by decreasing material mass.

Moisture also affects volume. One way is through compression. Because biomass weighs more when it absorbs water, the material at the bottom of a pile will compress more than if the material above it was drier. A second way that moisture affects volume is through changes in particle shape. Imagine a bucket of dry, curly shavings. What happens when you put water in the bucket? The shavings go flat, reducing the volume.

A change in moisture affects flow in that the volumetric flow rate must change to accommodate a steady mass flow rate. For example, say your system is set up to handle 30 tons of woodchips per hour. A significantly higher volume of woodchips will need to flow through the system than at a low moisture content and a mass of 10 lbs./ft3 than at a high moisture content and a mass of 35 lbs./ft3. Therefore, volume affects the size of equipment you’ll need.

Flow Characteristics

The changes in mass and volume that moisture creates can also significantly affect a material’s flow behavior. As said, higher moisture levels lead to increased cohesion. The added cohesion causes woodchips and dust to adhere to the inside of conveyors, chutes, and bins. It also increases the material’s angle of inclination—the angle at which it naturally forms a pile. As a result, piles are more likely to form sheer walls, and material can more easily build up on ledges inside equipment. At the same time, you can elevate it at steeper angles without supports like paddles.

A higher cohesion significantly affects storage, exacerbating biomass’s tendency to bridge. As a result, with the same particle size, moist biomass is more likely to pack above screws or reclaim strokers, which hinders the bin or silo from discharging and bridge over discharge areas.

Considerations

While it’s evident that the moisture content of your biomass will change if it’s stored outdoors and exposed to precipitation, what’s less obvious is that the moisture percentage can still fluctuate even if it’s protected from the weather. If you receive material from an outside source, you cannot control the precise percentage at which it arrives. Factors like whether the trailer in which it was transported was covered or not affect moisture content, as does where it was stored at the vendor’s site. Whether the material is exposed to the weather while being conveyed outdoors will likewise affect its moisture content.

Therefore, accounting for moisture variability is crucial when designing your bulk handling systems. You should consider more than nominal or preferential material characteristics and what may affect the material. If the nominal moisture percentage of your biomass is 15 percent but can range between 12-20 percent, you should use the extremes when deciding on the equipment needed. You’ll want to design the equipment size to handle the volume at the lower end of that moisture range and the power to handle the upper end. The larger the extremes in material characteristics, the more significantly they’ll affect your system’s performance.

Understanding moisture’s multifaceted impact on biomass handling is essential for optimizing your industrial processes. Indeed, it’s necessary to avoid unscheduled downtime, excess maintenance, and safety hazards that accompany efforts to get material flowing.

Do you need to rethink your bulk-handling process? Contact us! Our years of specialized experience enable us to see problems you didn’t know existed and develop solutions no one else offers. Get in touch today!