POWER February 2011 - 62

PLANT DESIGN
Conversely, dry biomass can be stored
for long periods. Dry biomass has too little
moisture to support biological activity and,
without biological heating or another ignition
source, it is relatively stable.
Energy Density. For biomass products
such as wood chips, one of the limitations to
consider is its relatively low volumetric heating
value or energy density. This is a property
that is not normally of concern for other solid
fuels. The lower mass heating value and low
bulk density (kg/m3
) combine to significantly
reduce its volumetric heating value. As a result,
many more railcars, much larger stockpiles,
and wider belt conveyors are necessary
to deliver and store the energy equivalent of
other solid fuels.
Plants firing wood chips, for example, require
six times the volume of bituminous coal on a kJ/
m3
basis. The increased storage, processing, and
handling system space required can become significant
for large biomass-fired plants.
Fuel Blending as a Mitigation Strategy.
Cofiring biomass, coal, and possibly
other solid fuels enables plants to mitigate
fuel-usage concerns. Blending multiple fuels
into a designer fuel can help plants to best
meet boiler/combustion, emission, and economic
requirements.
Controlling fuel quality and cost by processing
and blending biomass products onsite
is a standard industry practice.
Not all biomass products are completely
suitable, and the amount of some, like straw
or miscanthus pellets, may be limited due to
their chlorine and potassium content, which
creates deposition/corrosion concerns. When
different, less-suitable products are used, accurate
control of the blending process is important
for combustion.
Managing Variable Flow Characteristics.
Flow characteristics are important to
the design of any solid fuel-handling systems,
but they are a particular concern for all
biomass project stakeholders.
Wood chips, chopped straw, and other agricultural
products have poor flow characteristics.
During storage in stockpiles and bins, biomass
will compress in volume, and particles can become
entwined, matted, and gain strength as a
mass, rather than behaving as unique individual
particles. Instead of having a sloping angle of
repose, the sides of a reclaim stockpile can be
vertical, which helps to address this problem.
Better yet, biomass storage bins are often designed
with negative wall angles; that is, the
bins have walls that diverge: The bins are wider
at the bottom than they are at the top, which is
very different from the hoppers typically used
for storing coal or other solid bulk products.
Chute angles and the choice of liner materials
should be determined based on the biomass's
poor flow properties.
62
As noted above, due to availability, economic,
combustion, and other reasons, different
biomass products are often purchased
and blended. The blended product can have
much poorer flow properties than if any single
product were used alone. The meshing of
different particle sizes/shapes and the compressibility
of the blend has a lot to do with
this physical attribute.
Pretreating Biomass
For the most part, large biomass power plants
cannot depend on a local industry to supply
fuel, just as most coal-fired power plants
cannot depend upon an adjacent mine. One
reason for the distance from a fuel source is
that siting generation facilities near power
consumption centers is usually a key consideration.
When biomass must be shipped
long distances, pretreatment offers several
advantages.
The down side of pretreatment options
described below is that pretreatment increases
fuel cost. For a small plant with a local
source of biomass and unique conditions,
there are good reasons to avoid pretreatment.
But for larger plants that depend upon large,
distant, multiple fuel sources, the savings in
transportation, handling, and operation can
easily offset fuel preparation costs.
A biomass pretreatment industry is developing
to address some of the following issues:
■ To help the wide variety of biomass products
better match up with the narrow fuel
specifications for most boilers.
■ To lower the relatively high costs of transportation,
handling, and storage.
■ To reduce plant investment, maintenance,
and labor costs by using a homogenous,
consistent fuel for combustion.
Pelletization. One example of pretreated
biomass is pellets. Biomass pellets are available
from a growing number of sources. A
variety of biomass feedstocks can be used
for pellets, including bagasse, corn cobs and
screenings, peanut shells, sawdust, switchgrass,
and wheat middlings.
The pelletization process dries the feedstock,
grinds oversized material, compacts
and extrudes the fine particles, and then cools
the product into a homogenous, high-energydensity
fuel. Conditioners and binders are
sometimes added. The pellets are cylindrical
in shape and 6 mm to 8 mm in diameter
and 15 mm to 25 mm in length. Pellets are a
convenient biomass product for cofiring in a
coal-fired plant.
Though pellets from different manufacturers
may appear to be similar, their fuel properties
are inherited from the source feedstock
material, and quality is dependent upon the
manufacturing process. Pellets manufactured
from sawdust might be directly suitable
as a fuel, but pellets manufactured from
switchgrass might be limited to no more than
10% of the blended biomass fuel. Likewise,
some manufacturers may use a binder such
as starch, while other processes will depend
upon the biomass's cellulosic lignin, which
with heat and moisture acts as a binder to
form a dense pellet.
The amount of handling and exposure can
also affect the quality of the delivered product.
Receiving a shipment of wood pellets
that has degraded into mostly sawdust can be
very disconcerting when you are expecting a
cargo of free-flowing, hard/clean, relatively
dust-free pellets.
Like grain and other agricultural products,
biomass pellets must be protected
from the weather. They will swell when
exposed to moist conditions and subsequently
degrade to their original particle
size and density. That is why pellets typically
are stored in silos (as shown in Figure
1), a dome, an A-frame building, or other
Free Webinar: Defining Your Biomass, Wastes, and
Low-Grade Fuels Firing Program
POWER sponsored an online webinar in December that discussed biomass and other
renewable waste products that are suitable as a fuel source for power generation. The
focus of the webinar was sharing users' experience with storing, handling, preparing,
and using biomass and waste products. Three industry experts discussed important
issues such as how to use low-grade fuels in your plant, how to implement a biomassfiring
program in your solid fuel plant, and the environmental advantages and compliance
costs of biomass and waste fuel combustion. The primary theme of the webinar
is user case studies where biomass and other wastes have been successfully used in
power plants.
To view the free webinar, please go to http://tinyurl.com/2dqf5d5, fill out a short
registration form, and enjoy the one-hour program courtesy of program sponsors AirCure
Inc., Bruks Rockwood, Arm & Hammer dry sorbents, and Martin Engineering.
www.powermag.com
POWER | February 2011
http://www.tinyurl.com/2dqf5d5 http://www.powermag.com
POWER February 2011

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