POWER February 2011 - 61

PLANT DESIGN
is an inappropriate size for a stoker boiler.
Particle size, the percentage of volatiles,
total ash and moisture content, ash constituents,
and heating value are all key parameters
considered by the boiler engineer.
CFB boilers are different than other
combustion technologies. The CFB boiler
has relatively low combustion temperatures,
has long combustion residence time,
and the injection of limestone into the furnace
allows the CFB boiler to use a wide
range of fuels while controlling emissions
using standard technologies.
In a CFB boiler, fuel is generally combusted
in a bed of material (typically sand)
that is expanded at a pressure/velocity that
is above the particle's saltation velocity,
but below the particle's transport velocity,
to sustain fuel particles in the fluidized
state. This enhanced turbulence allows
a longer residence time to fully combust
the fuel. The mass, volume, and shape of
particles are important to the efficiency of
this process.
The fuel feed to a CFB boiler will encompass
a range of particle sizes, from sand-sized
grains to small lumps. For any given fluidizing
velocity, smaller particles will transport
much easier than larger particles, but they
will also fully combust more quickly. CFB
boilers use cyclones to separate the smallest
(fly ash) particles from larger particles that
may be only partially combusted. Larger
particles are discharged to the bottom of the
cyclone's sealpot/loop seal and are reintroduced/recycled
to the combustion bed. The
combustion gases and fly ash are discharged
through the top of the cyclone to the superheater
and economizer. The particle removal
performance of the cyclones is dependent
upon particle size and flue gas velocity.
Boiler manufacturers and standards organizations
have developed particle size
curves to identify fuel requirements for
CFBs. Figure 5 illustrates the typical acceptable
size range of biomass fed to a
CFB, Austrian standard ÖNORM M 7133,
" Chipped Wood for Energetic Purposes-
Requirements and Test Specifications. "
Particle mass, size, shape, and volatile
content are key parameters considered in
developing the acceptable fuel supply size
curves. The fuel particle's fluidizing and
combustion characteristics establish the
shape and characteristics of the curve.
Some boiler manufacturers impose distinct
fuel requirements to meet contract
performance guarantees and warranties,
depending upon the design fuel and contract
terms. The cofiring of biomass fuel in
a PC boiler imposes similar requirements,
although the particles are larger, perhaps
crushed by an air-swept hammermill to
less than ¼ inch.
Controlling particle size to meet contract
terms and boiler guarantees is important.
Biomass-handling systems typically
include screens and hogs to control particle
size. However, fuel processing is typically
not the responsibility of the boiler manufacturer.
Nor does the boiler manufacturer
typically supply the fuel or furnish the fuel
5. Sizing wood fuel. A CFB boiler requires properly sized fuel to operate efficiently. Few
industry specifications for wood fuel exist; Austria's ÖNORM M 7133 provides acceptable sizing
for chipped wood used for boiler fuel. In this standard, acceptable particle sizes/quantities are
those between the orange and red lines. The curve can be read that no more than 4% of the fuel
supplied can be less than 1 mm. At 80%, the size distribution should have 80% of the particles between
5 mm and 63 mm; 95% of the particles should be no less than 100 mm. The cross-section
maximum is 10 square centimeters, and maximum length is 250 mm. Source: ÖNORM M 7133
Size range
100
90
80
70
60
50
40
30
20
10
1
February 2011 | POWER
10
Size (mm)
www.powermag.com
100
1,000
Cross section max. 10cm2 , length max. 250 mm
preparation system. Managing this project
interface often becomes a contentious issue
among the fuel supplier; engineering,
procurement, and construction contractor;
boiler supplier; and owner during commissioning
and the plant acceptance test.
Biomass Fuel Properties Must Be
Accounted For
A large number of component materials are
generically known as biomass. The choice of
one over the others is usually determined by
which option can provide the energy content
(kJ) desired rather than by the weight of fuel
purchased.
Some biomass fuels present unique utilization
issues. Wheat straw, for instance,
has very high levels of chlorine, and its
ash chemistry is dominated by silica from
the phytolith inorganic structures that have
significant potassium. For other forms of
biomass, fuel degradation and spontaneous
combustion are important concerns. The
amount of moisture in the fuel is a factor
in the purchase, storage, and usage of all
biomass fuels. Details of some of the more
common biomass properties, and strategies
for dealing with them, follow.
Temperature- and Moisture-Related
Problems. Biomass is subject to two different
natural processes, one a low-temperature
process and the other a high-temperature
process. The low-temperature process involves
the growth and respiration of microorganisms,
such as aerobic mold-fungi and
bacteria. The high-temperature process is
due to oxidation of the cellulosic materials.
Biological heating, under the influence of
water content or air humidity, can increase
biomass temperatures high enough to trigger
oxidation of the cellulose material, which
can start a fire.
Wet biomass does not pose a spontaneous
combustion problem because above a 60%
moisture level, too much energy is needed to
increase the water temperature to 100C and
then evaporate it. This high energy demand
drops the temperature of the biomass below
the level needed to sustain combustion. Biomass
will, however, continue to degrade due
to biological activity.
At moisture levels between 20% and 60%,
both degradation and spontaneous combustion
become a concern. This range of moisture
is what a plant will most often encounter.
The large surface area of particles like wood
chips-and their irregular shape, which traps
small air pockets-provides a near-ideal environment
for the breakdown of fibers. That, in
turn, increases surface temperatures and the
potential for spontaneous combustion. Smaller
chip sizes increase the total surface area
and the probability of biological heating.
61
Weight % through sieve

http://www.powermag.com

POWER February 2011

Table of Contents for the Digital Edition of POWER February 2011