Biomass Storage Overview

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As the demand for natural materials and alternative energy sources has grown, so has the number of investments in biomass facilities. We’ve witnessed a striking number of new biomass power plants and pellet mills that utilize wood in the past decade. We’ve also seen investments into processing nonwoody, cellulosic materials like hemp, miscanthus, stover, and rice straw, to name a few. The excitement has drawn investors from an array of backgrounds, but as a result, some poor choices related to storing the material. (Poor choices are already too often made by those who are familiar with these materials!) Owners will sign off on plans to hold and reclaim biomass in what are essentially grain silos, for example, because they don’t know biomass does not flow well and will bridge in the systems.

Storing biomass well requires much more than dumping material in a bin or building piles. If biomass processors want their operations to run without disruption and be as productive (i.e., profitable) as possible, they must understand what works and what does not work regarding biomass storage.

Biomass Storage Volume

Paul Kalil, principal of PBK Consulting Inc. and sales representative for BE&E, says that one of the first things processors need to consider regarding biomass storage is the volume required to protect downstream processes. For Canadian pulp mills, that volume is enough to feed between 30 and 60 days of production. 26,486,000 ft3 (750,000 m3) of material is not unheard of at these mills.[1] These mills store so much material because they anticipate disruptions in their supplies, especially during winter months when storms and seasonal ebbs in chip production occur. Other disruptions that biomass processors may encounter include:

  • Feedstock availability. Operations that depend on straw, for example, may only receive material once per year. Processors that only utilize straw must plan to keep a year’s worth of feedstock.
  • Weather-related events and disasters. Depending on where a processing plant is located, it will be at risk for various natural disasters and weather-related delays. Snowstorms can hamper deliveries in the North. Hurricanes can devastate infrastructure in the U.S. Southeast.

On top of storing enough material to keep operations running when supplies aren’t immediately accessible, processors also need to account for storage volumes during scheduled downtime. Depending on their contract, suppliers may continue to deliver when the processor has shut down during its maintenance cycles. Processors need to be able to accept the volume of material they receive in these periods of inactivity.

The individual material properties of whatever biomass processors utilize will also affect volume. Stover is more voluminous than wood chips, for example. For the same weight in tons, stover will require more space.


Properties of the Biomass

Other characteristics of biomass will affect storage. Average material size, the sizes of over and under, moisture percentage, and flow characteristics will all change how the material is stored and reclaimed.

“Moisture content and the tendency of the material to clump and bridge and prevent free-flowing of the material” are key among characteristics that affect storage, Kalil says. The inability for material to flow “can be a big issue, especially in colder climates,” he says.

Generally, the higher the moisture percentage in the material, the more difficult it is to handle that material, and the more likely it will not flow well. Green (undried) material, therefore, poses more difficulty than dry in terms of handling. Materials such as green sawdust, forest residuals, bark, and hog waste are more likely to clump, bridge, and form ratholes than materials like sawmill residuals. When temperatures drop below freezing, ice adds additional complexity as it freezes the material together. The ability to break up material thus becomes a necessary component of the reclaim system.

This isn’t to say that dry material flows well. Dry or green, biomass has a high coefficient of friction, which creates so much of an issue with the flow that it’s been known to strain and break reclaim systems that utilize rotating augers. Green material exaggerates these problems.

High moisture creates other problems. It also means a higher acidity, as water provides acids the means to leach outside the material. How acidic biomass is depends on the species and the area in which it grows. Even among a single species, acidity will vary. Western red cedar, for example, can have an average pH as low as 2.9 and as high as 4.0.[2] Because of this, biomass processors will want to handle biomass that is especially acidic with galvanized or stainless-steel equipment.

Another characteristic of biomass that processors need to consider is the material’s angle of inclination—the angle at which it forms a pile. As with acidity, different types of biomass will have a higher inclination angle than others. Corn stover has a near-vertical inclination angle, whereas woodchips may be at 60°. How the biomass is processed—its size and shape—likewise will affect the angle. Regardless, the angle will be high, meaning a biomass pile will naturally have steep sides.

The inclination angle affects processors’ storage in terms of how they utilize the space and spread material. Because the angle of inclination is so great, the material will not spread out much on its own. If the processor drops material in piles via conveyors, they will need multiple chutes to utilize the storage area better. If multiple chutes are not used or not spaced closely together, processors may need wheel loaders to spread material.

The high incline angles of biomass can be problematic even when the biomass is stored outdoors in piles. Often, when a wheel loader scoops material from the pile, the pile forms a sheer face—a 90° angle, greater than the material’s natural angle of inclination. As a result, the mound can collapse, burying the end loader or injuring personnel. Management must take care to ensure the safety of workers and equipment.

Operations deal with this problem in different ways. Smaller operations sometimes manually collapse the faces via rake-like attachments on their wheel loaders. Others drive onto the piles and reclaim from top to bottom or spread out the piles so they never grow tall enough to pose a severe risk. Still, others bypass the issue by reclaiming piles with stacker-reclaimers.

Biomass Storage Methods

Thus, the challenges that processors will face regarding issues like flow and safety have a great deal to do with how the biomass is stored and reclaimed. A processor that keeps its biomass in a live storage system, for instance, will not deal with the safety risks that do processors that utilize dead storage. Each method has its advantages and downsides, though, as we will discuss.

“Live” and “dead” are categories that refer to the level of automation in a storage system. A system that is 100 percent live will mechanically or pneumatically take material from where it is received to storage. It will also reclaim the material and feed the process systems upstream without manual labor. Consider a grain silo: a conveyor brings material from the receiving pit to the top of the silo, where the grain is stored. An auger at the bottom of the silo reclaims the grain and feeds it into another conveyor, delivering it to processing equipment, railcars, ships, or trucks. For non-flowing biomass, processors use chain floors, vibrating floors, slat-style moving floors, augers, and wedged strokers to reclaim material from storage and receiving areas.

Dead storage is the opposite: it refers to storage without automation. Personnel builds piles (usually outdoors) with wheel loaders and bulldozers, and personnel reclaim material and feed upstream processes with the same.

Live Storage

The advantage of live storage is that it costs less to operate than dead storage. By automating reclaim and infeed, manufacturers don’t need to pay to maintain heavy equipment to handle the biomass. They also avoid inevitable repairs to the structures and equipment from heavy equipment damage.

The ongoing costs of machines like wheel loaders are nothing to shrug off, as the cost of operating these vehicles has increased significantly over the past decades. One major particleboard manufacturer audited the cost of operating wheel loaders and discovered the actual cost was more than $400,000.00 per year per wheel loader.

Other advantages of live storage include reduced personnel. Workers add to the cost of dead storage, and in today’s market, they are difficult to find, especially in rural areas. Dependable workers are even harder to find. The trend away from blue-collar work is a decades-long trend, and it is unlikely to abate anytime soon. The problem has been so severe that companies are automating systems for this reason alone; they can’t find employees.

Another advantage is that automated systems provide first-in, first-out (FIFO) reclaim. In a FIFO system, the oldest product is reclaimed and processed first before newer material. Using older material first helps ensure the material a processor utilizes is consistent.

The biggest drawback of live storage is its up-front cost. Live storage is initially much more expensive than piles. In many cases, the cost is prohibitive, especially for large operations. If the processor can afford it, however, it will benefit the company in the long run.

How much of an advantage a live storage system provides, of course, depends on what kind of system it is and how it is designed. As mentioned, biomass does not flow well, so if the processor chooses the wrong type of storage and reclaim system, whether the system is “live” or not won’t matter: the difficulties they will face will cut into the system’s efficiency.

For example, traditional round silos are often the wrong choice for biomass because of the material’s flow characteristics. Round silos can be utilized, of course, but the processor or their supplier must carefully design the silo to ensure it will work properly. In some instances, input from an engineering firm specializing in biomass handling may be needed when working with round silos.

Another example is chain floor technology. While probably the most common feed and reclaim method for biomass, chain floors come with many challenges that can be avoided simply by choosing a different reclaim system. Problems associated with chain systems include:

  • More maintenance than other systems
  • Downtime due to damage to the chains and drive system
  • Tendency to pack material against the exit wall
  • Tendency to overflow material around the discharge area

Wedged, stroker-based systems like BE&E’s SMART Floors, have none of the above issues and serve as a better alternative. Engineers can even design these incorrectly, though. A power plant in Canada, for instance, utilized strokers to reclaim waste biomass from its receiving and storage area. But the processor fed the site with wheel loaders, which drove over the strokers to distribute the biomass. In doing so, the wheel loaders compacted the material so much that either the strokers could not function or the material would not flow from the area. As a result, the power plant shut down until another solution was in place. (Unfortunately, the management pinned the blame on the strokers and not the poorly engineered system as a whole.)

An example of live storage that does work well is exemplified at Oaks Unlimited, a lumber mill in North Carolina that utilizes dry sawdust and woodchips as fuel for its boiler. Oaks receives the biomass from walking-floor trailers into a bin, which meters the material into one of BE&E’s SMART Conveyors™, a drag-chain conveyor system that excels in handling biomass. The SMART Conveyor™ transfers the material to a distribution conveyor (another SMART model) atop a horizontal silo—a two-tiered SMART Container system with a stroker-based reclaim system. The SMART Container ensures material is stored out of the elements and that it will not bridge. The SMART Container, in turn, discharges into a bin that meters material into the boiler-feed conveyor. The system works free of manual labor and does not have problems with flow.

Storing in Piles

While the biggest drawback of live storage is the installation cost, the most significant advantage of dead storage is its low bar of entry. Indeed, the highest cost associated with it is pouring concrete (storing biomass on a pad is much recommended). As mentioned, dead storage is the only feasible storage method for some operations like pulp mills. However, most processors choose this option because they lack or are unwilling to spend the capital necessary for live systems.

The disadvantages of dead storage are plentiful. There are higher ongoing costs due to labor and vehicle maintenance. There are fuel costs. There are costs for insurance and expenses for repair when the wheel loaders bang into things. There are also costs in terms of quality: wheel loaders will mix rocks and dirt into the biomass as they drive over and reclaim it—an issue that’s very much at the fore when the piles are not kept on concrete pads.

Material is more likely to degrade when stored in piles, too, especially if it’s outdoors. “[Biomass] will rot or potentially self-combust if stored in large piles and not turned frequently,” Kalil says. “But that’s in piles, not in a silo or receiving bin.”

Rot and heat come from the same source: bacteria and other microorganisms that feed on the biomass. When biomass is stored in a pile, heat concentrates inside it and can increase to the point of starting a fire. The fire risk worsens when the pile does not radiate heat well, which occurs when air does not flow sufficiently through the pile. Airflow worsens the higher the material density and moisture content and the smaller the particles.

These characteristics are why green sawdust is among the worst types of biomass to store outside. It is dense (so it will not radiate heat well), and it absorbs much water (then take a long time to dry). It’s necessary to store green sawdust and similar biomasses in live systems, where they can be kept dry.

When kept outside, turning the biomass and designing piles, so they shed as much rain as possible will help reduce heat and decay, as will covering the piles, thereby reducing microbes’ access to oxygen and water. Doing these things will not stop the rot, though; they can only slow it. If a processor expects to store its biomass for a long time and utilizes wood, it’s best they hold it in unprocessed logs and then chip the logs on-site as needed. Storing the fiber in logs is the best way to keep it from degrading.

How much fiber loss occurs ultimately depends on many factors, such as the biomass species and how personnel manage it. Generally speaking, the longer biomass is stored outdoors, the more fiber loss will occur. While some studies indicate that most fiber is lost the first few days after biomass (softwood) is chipped (logarithmic loss), others show that about one percent of chipped fiber is lost per month (linear loss).[3]

Despite its downsides, the cost of implementing dead storage means operations will continue to rely on this system, which is understandable given the price tag associated with live storage. The choice between live and dead storage is not one or the other, however. It is possible to build a hybrid system wherein live storage is sized to store only enough material to feed operations during normal periods of high in/out activity when trucks deliver feedstock. Dead storage otherwise stands ready to supply the operation.

For example, during the week when the material is arriving regularly, personnel can store enough material on a push-pull floor so it can run live at all times. During the weekend, when trucks arrive less often, dead storage can supplement the live system. For instance, we might design a system with a SMART Floor onto which dumps all incoming material. When the floor is full, we could divert excess incoming material to dead storage directly next to the live section. In periods when trucks are not often delivering material, personnel pushes the material in dead storage onto the live floor.

No matter how you plan on storing your biomass, we suggest you contact a professional for advice. Biomass, no matter the species or form, is difficult to handle, and inexperienced processors have faced extreme difficulty due to its challenges—sometimes to the point of failure. We’re here to help. We want you to succeed, and we know biomass. Contact us, and let us excel at what we do best so you can focus on what you do best.

[1] Source: Chip Storage and Handling for Pulp Mills

[2] Source: Corrosion of Metals Associated with Wood.

[3] Source: Biomass Storage Pile Basics.


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