Vertical silos are standard storage devices in many industries. But for biomass, they’re hardly ideal. Biomass tends to bridge, especially in round containers, and vertical silos have trouble reclaiming it. Because of these issues, companies that store biomass will want to consider the advantages of a horizontal system instead of a vertical one.
Without even considering biomass, there are disadvantages to storing material in a vertical silo. The first is that vertical silos are permanent structures. As such, they depreciate more slowly than machinery. Depreciation takes decades rather than years. The second is that they aren’t modular (stave silos being somewhat of an exception). Thus, a company with a silo cannot expand it to increase storage capacity. The third disadvantage is maintenance. If the reclaim arm breaks while the silo is filled, workers will not be able to perform maintenance without emptying the silo. The fourth disadvantage is that wheel loaders and truck dumps cannot directly fill the silos. Companies must utilize conveyors to transport material to the top of the silo—and pay for the maintenance and energy to do so.
Problems related to storing biomass in silos, specifically, include flow and maintenance. Biomass does not flow well and thus will bridge or form a rat hole. This problem is worse for long fiber biomass like veneer chips and bark, which weaves together when stored. When this occurs, reclaim arms strain as they try to tear apart the mat of woven biomass. Because of this, they commonly require more repairs and have a shorter life than promised.
Horizontal Storage Options
Store on a Slab
Many companies know about these issues, however, and store biomass in a horizontal system. The simplest way they do this is to place biomass on a slab. Slab-based storage has very low up-front costs, which is its main advantage. But the initial savings can dissipate with higher operational costs: companies need wheel loaders and operators to transfer the biomass on and off the slabs. Wheel loaders can cost hundreds of thousands of dollars a year when maintenance, fuel, purchase cost, and damage related to the wheel loader are taken into account—not to mention the cost of an operator (which are also not easy to find in today’s market).
It’s important to place biomass on a slab and not the ground because the biomass will absorb moisture from the ground, and the moisture in the chips will become uneven. Storing biomass on the ground will also mix dirt with the biomass as the wheel loader scoops up the chips. Dirt can foul boilers and will create more clinkers in the ash.
Store on a Moving Floor
If companies want the convenience of a slab but wish to eliminate the wheel loader, they may install SMART Floors (i.e. moving floors) on the slab. SMART Floors use push-pull strokers to automate the output of the biomass. The floors can receive biomass directly from trailers or truck dumps, and output from the floors is as complicated as pushing a button.
SMART Floors are inexpensive to install and operate. They can be installed onto existing slabs or with new construction.
We do not recommend you invest in moving floors with aluminum slats. Aluminum is soft and will degrade much more quickly than steel.
Another advantage to SMART Floors is that their strokers can run independently. This is advantageous to maintenance crews because they can replace individual hydraulics without having to shut off the entire system. Thus, downtime is avoided.
Crews can also reach the hydraulics without emptying the slab.
As a negative, storing material on a slab or SMART Floor does take more real estate. Also, if the slab is indoors, dust may prove a fire or explosive hazard and may harm workers’ lungs unless safety precautions are taken.
Store in a Horizontal Silo
Companies may also opt to store biomass in a horizontal silo, such as our SMART Container—a modified intermodal shipping container with SMART Floors.
Horizontal silos use strokers, slats, or chains to reclaim material. We prefer push-pull strokers over slat or chain reclaim systems. Slats and chains move the whole pile and put pressure on the exit wall. If a company has such a system, it may need to install a Drag-Back Conveyor to relieve the pressure. (As a side note, the exit wall may not be needed at all with a drag-back conveyor). You should also note that chains require very high torque, putting high stresses on the drive systems which adds to maintenance.
Just as with SMART Floors, strokers in SMART Containers are set up to run independently so maintenance personnel can repair them without downtime. Crews can access the hydraulics without having to empty the silo, as well.
An advantage that SMART Containers have over slabs is that it’s possible to stack them to gain vertical storage. This reduces the land required to store the material. SMART Containers also have an advantage over vertical silos in that they’re machines, not buildings, so they depreciate faster than vertical silos.
SMART Containers may be stationary or mobile. As mobile units, they attach to chassis, and companies can use them to receive biomass and feed it at a metered rate into their system. It’s such a setup that Lockheed Martin used to feed their gasification unit in upstate New York. Lockheed bought two mobile SMART Containers from us and uses them to feed the gasifier. That system has been repeated at several other sites since.
SMART Containers have the additional benefit of modular design. Companies can start with one container and add more when they need to increase their storage capacity. With such an option, companies don’t have to invest in a storage system with a capacity greater than their current need. On the other hand, if the company owns a storage system and needs to expand its capacity, they can install a SMART Container to increase their storage.
Much goes into deciding which storage system to buy: available land, budget, return. When land is at a premium, vertical storage might be the only viable solution. If space isn’t a problem, however, horizontal systems are a better choice for biomass in terms of performance and economics.