POWER May 2014 - 52

RENEWABLES
Biomass is typically delivered by truck, and
a typical biomass truck can hold from 25 to 35
tons of biomass. Trucks tend to operate on a
10-hour schedule for six days a week; therefore,
converting 10% of a 250-MW power
plant to biomass will require about two to
three biomass truck deliveries per hour. Local
residents may object to that much truck traffic
on the roads, and as a result the utility may be
asked to pay for upgrading bridges and roads.
Trucks will require a rapid dumping system
in order to avoid traffic congestion at the
plant and may require a " staging area " be
built by the plant entrance to hold trucks during
temporary outages (Figure 5).
Biomass Storage. Biomass storage is
often problematic at coal-fired power plants.
Because biomass fuels come in a variety of
types and sizes, plant owners must design
their storage and reclaim systems with care.
Covered biomass storage is recommended
or required for most fuels to reduce reclaim
efforts and to protect the biomass from degrading.
Pelletized biomass is often touted
as being " rain-hardy, " but experience has
shown that a single rainstorm can sometimes
transform a pile of biomass pellets into a
mountain of sludge. Thus, covered storage is
highly recommended.
Another concern with biomass storage is
the high levels of dust created during handling,
especially at transfer points. Biomass
dust is generally less harmful to humans and
the environment than coal dust, but it can be
susceptible to hazardous mold accumulation,
which can affect the health of both the fuel
yard staff and nearby residents. Compared to
coal, biomass dust is significantly more fireprone
due to its relatively low ignition temperatures.
This risk was exemplified by the 2008
biomass dust explosion at the Ontario Power
Generation Atikokan plant. Four years later, a
biomass fire among the wood pellets stored in
600-metric ton hoppers at the Tilbury B power
plant in the UK led to that plant being out of
service for nearly four months.
Although coal demand will decrease as it
is replaced with biomass, biomass fuel has
a much lower energy density than coal, and
more of it is required for the same net generation.
For a unit currently burning an 11,000
Btu/lb coal, cofiring just 5% straw biomass
containing 40% moisture will require a biomass
storage area 80% as large as the current
coal pile!
Comilling and Direct Injection. Many
technologies are available for introducing
biomass into a coal-fired boiler. Direct injection
of ground biomass into the furnace via
dedicated ports is one of the safest methods
for cofiring, although it is also one of the
most expensive.
For small quantities of biomass fuel (1%
to 5% by energy content) comilling can be
the best choice, provided your mills are up
to the task. Due to the slippery and plastic
nature of biomass materials, milling with a
roll-wheel mill or ball tube mill can be problematic.
Although the Hardgrove Grindability
(HGI) test is not intended to measure the
grindability of biomass materials, a rule of
thumb is to assume herbaceous biomass has
an HGI of 20 to 25.
As part of a biomass project that Black
& Veatch was involved with in conjunction
with Genesis Energy, New Zealand, at its
Huntly power station, the engineering and
corporate staff showed exceptional diligence
5. Rapid dumping. Parallel unloading systems may be required to avoid traffic congestion
from biomass trucks. Source: Black & Veatch
in planning for comilling of up to 20% biomass.
Here are some of the recommendations
of that study:
■ Biomass fuel can stick in bunker hoppers
and clog feeders, requiring vibratory
shakers or even redesign of the hoppers. In
some cases biomass fuel can jam feeders
and trip a mill.
■ Increased housekeeping should be implemented
to control dust and spilled fuel
from the tripper to the feeders. Improved
fire detection and suppression systems
may be needed for the coal conveyors,
tripper room, and bunkers.
■ Mill roll wheels can be modified with slots
cut into them, to give the wheels more
" bite " into the biomass fuel. Roller spring
adjustments may also be required.
■ Ball tube mills may find many biomass
fuels problematic and, therefore, may require
additional vigilance to ensure that
the fuel and balls don't conglomerate together
into a solid mass.
■ Classifier settings will need adjustment
for biomass use, with some plant engineers
opting to open up classifiers to avoid
accumulating stringy, oversized biomass.
■ Mill air flow must be checked, and air
gaps closed, to prevent under-bowl fires.
■ Primary air ducts should be checked to ensure
light biomass dust does not backflow
and accumulate in expansion joints, leading
to a potential fire or explosion risk.
■ Mill inerting and fire detection systems
are necessary for any long-term biomass
comilling program. Special care should be
taken to sweep mills clean of biomass material
during shutdown.
Direct injection of biomass can be into the
existing coal pipes downstream of the mill
or be introduced directly to the furnace via
ports or burners. In the former method, no
new furnace penetrations are required, the
unit maintains its full coal capacity, and the
capital cost is modest. In the case of direct injection
into the furnace, the greatest concerns
are triggering New Source Review by cutting
new fuel ports into the walls, and potential
space limitations that prevent optimum biomass
port or burner placement.
Whether biomass direct injection occurs
in the coal pipes or the furnace proper, it is
important to try to maintain a consistent biomass
quality and quantity entering the furnace,
so operators are able to keep the units
tuned for the best balance of unburned carbon,
CO, NOX
, and furnace deposition.
Boiler Concerns. Many biomass fuels can
reduce boiler maintenance activities by having
a low, nonabrasive ash content. Some biomass
ash has a lower melting point than coal
52
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POWER May 2014

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