Surveying Storage Solutions
Selecting the best system for storing woody biomass for power or heat
generation is not a new challenge, but as renewable energy
installations increase, so is interest in weighing different options.
The
effects wood material characteristics have on a pile in storage include
absorption, heat buildup, dissipation and more. The number of fines in a
respective fuel pile influences how much water is absorbed, how the
pile heats up and how air flows through a pile.
Isaac Slaven, applied engineering technology faculty in the
School of Technology at Eastern Illinois University, says the easiest
and best method to identify best storage practices is first-in,
first-out (FIFO) inventory management for the same particle size and
shape, material, and moisture content. “This way, the material loss from
fungal and bacterial infestation is reduced by not allowing as much
time for these infestations to occur,” he says. “Additionally, any
effect from contaminates is reduced. For a company that has a variety of
materials, moisture contents, and particle sizes and shapes, known (or
tested) rates of degradation based upon these factors may require a
combined schedule to reduce overall material and financial loss can be
optimally reduced. Furthermore, the seasonality of some materials (such
as corn stover or switchgrass) may complicate this management.”
Since there are so many identifiable variables that can cause
chip pile fire, it is difficult to single out only a few things, but
Slaven says it is safe to say: Keep it as dry as feasible, keep it as
free of contaminates as possible, and move it though inventory as
quickly as possible. In a paper he wrote at Purdue University,
“Properties of Wood Waste Stored for Energy Production,” Slaven notes
that how chips deteriorate in wood chip piles is not completely
understood, but the observed changes in the chips and chip piles are dry
matter weight loss, temperature changes, moisture content, heat of
combustion, and pH.
There is no single, easy formula for designing an appropriate
storage solution. While most fuel yards share many common elements, some
tailoring is almost always required because of differences in fuel
type, fuel quality, energy plant operating requirements and local
harvesting methods/practices.“Although most larger plants in North
America rely on wood chips for fuel, smaller heat or CHP projects in the
U.S. and a variety of plants in Europe rely on wood pellets,” says
Chris Lindsey, associate principal at the Antares Group, headquartered
in Lanham, Maryland. “Fuel yard considerations are very different
between these fuels. Some plants rely on very custom yards because of
specialty fuels, such as industrial processing residues, material
recovered from construction and demolition streams, that they may be
using in conjunction with clean wood chips.”
A Sound Strategy
Industry insiders note that having a fuel storage and inventory
management strategy can be a prerequisite to project financing, and
therefore must be carefully considered even prior to final design.
“Deciding how large the storage should be is primarily, but not
necessarily entirely, a factor of the size of the energy project being
supplied and local climate conditions,” Lindsey says. “Plants usually
shoot to have two to four weeks of fuel on hand to help get through
periods when suppliers may have trouble delivering fuel in harsh or
muddy weather. Smaller heating plants or smaller pellet fuelled
facilities may play their supplies tighter, it just depends on how
sensitive they are to interruptions due to temporary supply shortages.”
It is typical for larger plants to rely on piles that are
actively managed with loaders, dozers to minimize fire hazards and
maintain fuel quality. According to Lindsey, FIFO approaches are normal.
Pile reclaim may be done automatically, (under pad reclaim) or manually
using loaders and reclaim chutes and hoppers. “It is common for storage
to include day silos or bins (live storage) in conjunction with piles,”
he says. “The silos or bins allow for one to two days of live storage
that can be used to reliably feed the energy conversion unit regardless
of upstream issues and allows for yard operations to be shut down for
maintenance or operating personnel days off. At smaller, institutional
heating plants, conveyance may be simpler and include at-grade or
below-grade fuel bunkers, which may be primarily indoors.”
Typical elements to be designed into the system are yard truck
traffic control—sufficient area for truck delivery and turnaround, truck
receiving and unloading, either by tippers or walking floor truck, and
incorporating scales, initial conveyance to classification stand—sorting
for size and debris, resizing via milling, rejection—and conveyance to
pile storage or live storage. Systems will also include tramp metal
removal and real-time weighing at various points to determine flows to
storage or, at a minimum, the amount of material leaving live storage to
the energy conversion system, usually a boiler. Fuel sampling stations
may also be included.
The View From Alaska
Amanda Byrd, biomass specialist for the Alaska Center for Energy
and Power at the University of Alaska, says the best storage systems
have to be covered out of the rain and snow, and have good ventilation
for drying, or a drying bin. Keeping the wood off the ground is also
important for air circulation and reducing rocks and other unwanted
debris. “Having a day bin is common practice, and in some places it is a
‘week bin’ holding enough fuel for a week. Sometimes the wood is
chipped at the plant site, and other times it is chipped where felled
and the chips transported,” she says. “We have had some of our biggest
wood storage issues happen in winter. For instance, a school in
Southeast Alaska received wood chips from a local mill, but the chips
were wet and when stored turned to blocks of ice. They since installed a
drying bunker for the chips to dry out before being augured into the
boiler.”
In Alaska, projects are often very diverse, and one solution is
not going to be the best fit for everyone, she adds. “We are still in
our development stages in Alaska. Often times to keep the costs down, we
must use what is available and affordable. So, using a rarely used
ice-hockey rink to store wood chips can be the easiest solution at the
time,” Byrd says. “We have economies of scale issue in Alaska—our needs
are quite small, and building large amounts of infrastructure often is
not economically feasible.”
Smaller Solutions
For smaller systems and institutional settings, where the owner
or the person who’s operating the system wants to spend as little time
as possible messing with the fuel storage, Timothy Maker, president and
CEO of Community Biomass Systems Inc. in Montpelier, Vermont, says
“unquestionably, a below-grade bin is the way to go. A concrete bin that
can hold at least one-and-a-third tractor-trailer loads of fuel. The
bigger the facility, the more storage you want to have,” he says. “The
person who is in charge of the system doesn’t want to be worrying about
running out of fuel when it’s cold. They want to know they’ve got a full
bin and they’ve got a few days of elbow room.”
He adds for indoor storage, the storage facility should not be
heated and it should have plenty of ventilation. “The reason why you
don’t want it heated is that in cold weather you don’t want to make it
more likely that you’re going to have moisture leaving the fuel and
filling up the air,” Maker says. “So if the space is well-ventilated
when the moisture goes off from the fuel, when it evaporates into the
air from the fuel, then it gets out instead of being stuck inside this
bin building.”
Challenges Abound
When dealing with storage solutions, climate is a very important
consideration, especially places prone to severe icing or snow. “Snow
removal must be a factor in the yard traffic design and icing must be
considered on any inclines. This includes truck ramps and even conveyor
angles,” Lindsey says. “In the case of the latter, even relatively
modest slopes can be problematic at times as conveyor belts will harden
up and material may slide back into unwanted places. Lower conveyor
angles mean more linear length and higher cost, so there is a balance to
be struck. Covered conveyors are also a good idea in places with lots
of annual precipitation.”
The polar vortex this past year wreaked havoc at many plants
simply because the cold weather pushed conditions beyond design and made
it a real challenge for some operations to maintain output. This was
not only true of the equipment design, but of maintenance.
For most biomass fuels, fuel dust is a nuisance factor that can
be mitigated, but usually not completely eliminated. Provisions in the
design for dealing with dust removal and cleanup of fines throughout the
fuel handling and processing system should be considered. This includes
clean-out ports in strategic locations and enclosures to protect
systems that are sensitive to dust.
Fuel consistency and quality is always a critical factor in
operating a plant. Pelletized fuels have an advantage in this regard.
Some plants put in multiple bins or silos to allow fuel blending to
maintain desired characteristics.
Final Thoughts
Plan on the system requiring fairly regular maintenance and be
sure that multiple redundancies are built into the system to deal with
unexpected failures in the reclaim/conveyor systems. “From my
experience, the best systems are the ones where a lot of care was taken
during the design process to involve the operator of the system,” Maker
says, “so the operator knows what they’re getting. They not only get a
chance to talk to the system supplier, but they’re actually working with
the system supplier during the installation of that equipment, so that
they know how to operate it, they know how to troubleshoot it. If all of
that is done carefully during the design and the early stages of
implementation, when it’s actually being constructed, then you get a
smooth project.”
Lindsey adds that fuel moisture in most large plants in North
America is managed by fuel spec and pile management. Volatile organic
compounds do evolve from piles and can change fuel quality over time.
“Preferably, wood residues stored outside and destined for sustainable
energy production should have adequate airflow and should be protected
from the rain and snow; however, the costs of protection may be
prohibitive,” he says. “In a world of increasing energy demands, proper
management and rapid inventory turnover can help make wood a part of a
renewable and sustainable energy portfolio.”
