Basu P. Biomass Gasification and Pyrolysis: Practical Design and Theory
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in the silo, the fuel outside of the channel remains stagnant for a long time. The
residence time of such fuels in the silo should be limited by emptying the silo
frequently or by using a first in–first out mass-flow pattern (Figure 8.6b), where
all of the material is in motion whenever the fuel is discharged. Biomass is
relatively free of this problem as most of it is not prone to spontaneous
ignition.
Achieving Mass Flow
To achieve mass flow, the following conditions are to be met:
The hopper wall must be sufficiently smooth for mass flow.
The hopper angle should be adequately steep to force solids to flow at the
walls.
The hopper outlet must be large enough to prevent arching.
The hopper outlet must be adequately large to achieve the maximum dis-
charge rate.
The required smoothness and sloping angle for mass flow in a hopper depends
on the friction between the particles and the hopper surface. This friction can
be measured in a laboratory using a standard test (ASTM, 2000).
Several factors can affect wall friction for a given fuel:
Wall material
Surface texture or roughness of the wall
Moisture content and variations in solids composition and particle size
Length of time solids remain unmoved
Corrosion of wall material due to reaction with solids
Scratching of wall material caused by abrasive materials
To enhance the smoothness of the surface, sometimes the hopper is coated or
a smooth lining is applied. Lining materials that can be used include polyure-
thane sheets, TIVAR-88, ultra-high-molecular-weight polyethylene plastic, and
krypton polyurethane.
Mass flow can be adversely affected by the narrowness of the hopper outlet.
A too-narrow outlet (compared to particle size) permits the particles to interlock
when exiting and form an arch over the outlet. The probability of this happening
increases when
The particles are large compared to the outlet width.
There is high moisture in the solids.
The particles are of a low shape factor and have a rough surface texture.
The particles are cohesive.
Wedge-shaped hoppers require a smaller width than conical hoppers do in order
to prevent bridging. Slotted outlets must be at least three times as long as they
are wide.
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8.3 Biomass-Handling System
Hopper Design for Mass Flow
The design of the hopper outlet significantly affects the flow of solids. When
solids flow through the hopper, air or gas enters, dilating the particles. It is
essential for powder solids to flow freely through the outlet. Air drag, which is
proportional to surface area, must be balanced by gravitational force that is
equal to the weight of the particle. Fine particles have a lower ratio of weight
to surface area compared to coarser particles. So, for fine particles, this force
balance becomes an important consideration. For such particles, the following
expression is used (Carleton, 1972):
4
15 500
0
2
2
0
4
1 3
5 3
V
B
V
d
g d
a
p p
p
sin
θ
ρ µ
ρ
+
( )
= <For mµ (8.1)
where
V
0
is the average solid velocity through the outlet, m/s
ρ
a
, ρ
p
is the density of the air and solids, respectively, kg/m
3
d
p
is the particle size, m
µ is the viscosity of the air, kg/m.s
θ is the semi-included angle of the hopper
g is the acceleration due to gravity, 9.81
m/s
2
B is the parameter
The mass-flow rate, m, is given in terms of the bulk solid density, ρ
b
, and
the outlet area, A:
m AV
b
=
ρ
0
(8.2)
For coarse particles (>500
µm), an alternative relation is used (Johanson, 1965):
m A
Bg
C
d
b p
=
+
( )
>
ρ
θ
2 1
500
tan
kg s for mµ (8.3)
Values of the parameters A, B, and C are given as
Parameter Conical outlet Symmetric slot
B
Outlet diameter, D Slot width, W
A
π
4
2
D
Width x breadth
C 1.0 0
Design Steps
Hopper design involves determining particle properties, such as interparticle
friction, particle-to-wall friction, and particle compressibility or permeability.
With these properties known, the outlet size, hopper angle, and discharge rate
are found.
Dedicated experiments like shear tests are carried out to determine the inter-
particle friction. A parameter, such as angle of repose, has little value in hopper
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design, as it simply gives the heap angle when solids are poured in. Particle–wall
friction should also be measured by purpose-designed experiments.
The stress distribution on the silo wall is important, especially for a tall unit.
Figure 8.7 compares the wall pressure in a biomass-filled silo with that of a
liquid-filled silo. As we can see, the wall pressure in a solid-filled silo does not
vary linearly with height, but it does in a liquid-filled silo. In the former case,
the pressure increases with depth, reaching an asymptotic value that depends
on the diameter of the hopper rather than on the height. Because there is no
further increase in wall stress with height, large silos are smaller in diameter
but taller.
To find the stress in the barrel, or the vertical wall section, of a hopper, we
consider the equilibrium of forces on a differential element, dh, in a straight-
sided silo (Figure 8.8):
Vertical force due to pressure acting from above: P
v
A
Weight of material in element: ρAg dh
Vertical force due to pressure acting from below: (P
v
+ dP
v
) A
Solid friction on the wall acting upward: τπD dh
The force balance on the elemental solid cross-section gives
P dP A D dh P A Ag dh
v v v
+
( )
+ = +
τπ ρ
(8.4)
The wall friction is equal to the particle–wall friction coefficient, k
f
, times the
normal pressure on wall, P
w
:
τ
= k P
f w
(8.5)
Janssen (1895) assumed the lateral pressure to be proportional to the vertical
pressure, as shown in the following equation:
Pressure
Depth
Hydraulic pressure
Pressure of solid
Change in pressure
due to change in
cross-section
fIgure 8.7 Wall pressure distribution along the height of a hopper filled with solids. It is noted
that the pressure profile changes in the inclined section, which is not the case for the hydraulic
pressure that would be the wall pressure if the hopper was filled with water.
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8.3 Biomass-Handling System
P KP
w v
= (8.6)
where K is the Janssen coefficient. For liquids, the pressure is uniform in all
directions, so K is 1.0. This relation is not strictly valid for all solids, but for
engineering approximations we can start with this assumption.
Substituting Eqs. (8.5) and (8.6) in Eq. (8.4), we get
AdPv Ag dh k KP D dh
f v
= −
ρ π
(8.7)
Boundary conditions for this equation are h = 0, P
v
= 0; h = H, P
y
= P
0
. With
this, Eq. (8.7) is integrated from h = 0 to h = H to get the pressure at the base
of the silo’s vertical section, P
0
. Substituting
A
D
=
π
2
4
we get
P
Dg
k K
Hk K
D
f
f
0
4
1
4
= − −
ρ
exp (8.8)
This is known as the Janssen equation.
Figure 8.7 illustrated the pressure distribution along the height of a silo. The
straight line shows the pressure we expect if the stored substance is a liquid;
the discontinuous exponential curve is the one predicted for solids. There is a
sharp increase in pressure at the beginning of the inclined wall. The pressure
decreases with height (Figure 8.7).
The stress on the inclined section is different from that calculated from the
preceding. To calculate this, we use the Jenike equation, which states that the
radial pressure is proportional to the distance of the element from the hopper
apex, which is the point where inclined surfaces would meet if they were
D
P
v
A
(P
v
+ dp
v
)A
rA g dh
dh
tpD dh
fIgure 8.8 Force balance on an element of a storage silo.
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extended (Jenike, 1964). It can be seen that the magnitude of stress at the hopper
exit is the lowest, although this is the lowest point in the hopper.
example 8.1
Find the wall stress at the bottom of a large silo, 4.0 m in diameter and 20 m in
height, that uses a flat bottom for its discharge. Compare the stress when the silo
is filled with wood chips (bulk density 300
kg/m
3
) with that when it is filled with
water.
Given that the wall-to-wood chip friction coefficient, k
f
, is 0.37, assume the
Janssen coefficient, K, to be 0.4.
Solution
We use Eq. (8.8) to calculate the vertical pressure, P
0
, in the silo. Data given are
as follows:
The bulk density of the wood chips, ρ, is 300 kg/m
3
.
The wall–solid friction coefficient, k
f
, is 0.37.
The diameter, D, is 4.0 m.
The height, H, is 20 m.
The Janssen coefficient, K, is 0.4.
P
Dg
k K
Hk K
D
f
f
0
4
1
4
300 4 9 81
4 0 37 0 4
1
4 20
= − −
( )( )
=
× ×
× ×
− −
× ×
ρ
exp
.
. .
exp
00 37 0 4
4
18 854
. .
,
×
( )
= Pa
Since the lateral pressure, P
w
, is proportional to the vertical pressure, P
v
,
P KP
w
= = × =
0
0 4 18 854 7542. , Pa
For water, the vertical pressure is the weight of the liquid column:
P gH
0
=
ρ
Because the lateral and vertical pressures are the same (i.e., K = 1.0), we can
write
P P
w
= = × × =
0
1000 9 81 20 196 200. , Pa
The lateral pressure for water is therefore (196,200/7542) or 26 times greater
than that for wood chips.
Chute Design
In a silo, the solids are withdrawn through chutes at the bottom. Previous dis-
cussions examined solids flow through the silo. Now, we will look at the flow
out of the silo through the chute, which connects the silo to the feeder. A proper
chute design ensures uninterrupted flow from storage to feeder. Improper
283
8.3 Biomass-Handling System
design results in nonuniform flow. Figure 8.9 illustrates the problem, showing
partial solids flow with a uniform-area chute and full solids flow with a properly
designed chute. As the solids accumulate on the belt, their uniform flow through
the hopper prevents them from accumulating at the chute’s downstream section.
The chute’s expanded and lifted opening helps the solids spread well, allowing
uniform withdrawal. For this reason, the modified design of Figure 8.9(b)
shows the skirt on the chute to be lifted and expanded (in plan view) to facilitate
uniform solid discharge from the hopper. These angles (slope and discharge)
should be in the range of 3 to 5 degrees.
Figure 8.10 is another illustration of this phenomenon, this time with a
rotary feeder. Here the design on the left (Figure 8.10a) is without the short
vertical section like that on the right (Figure 8.10b). Solids are compressed in
the direction of rotation and pushed up through the hopper. The design on the
right uses a short vertical chute that limits this backflow only to the chute
height, giving a relatively steady flow.
The two key requirements for chute design are: (1) the entire cross-section
of the outlet must be active, permitting the flow of solids; and (2) the maximum
discharge rate of the chute must be higher than the maximum handling rate of
the feeder to which it is connected.
A restricted outlet, caused by a partially open slide gate, results in funnel
flow with a small active flow channel regardless of hopper design. A rectangular
outlet ensures that feeder capacity increases in the direction of the flow. With
a belt feeder, the increase in capacity is achieved by a tapered interface. The
Stagnant material
(a) (b)
Conveyor
Conveyor
Minimum
length
Minimum
width
Discharge angle of skir
t
Belt
Fuel
depth
Slope
angle
of skirt
f
Igure
8.9
Two feed chutes between a hopper and a belt conveyor: (a) a simple design that
causes partial flow; (b) a design that provides complete flow.
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capacity increase along the feeder length is achieved by the increase in height
and width of the interface above the belt.
Poor feeder design is a common cause of flow problems, as it prevents
smooth withdrawal of solids. If the discharge rate of the chute is lower than
the maximum designed feeding rate of the feeder, the feeder can be starved of
solids and its flow control will be affected.
8.3.3 feed Preparation
Biomass received from its source cannot be fed directly into the gasifier for the
following reasons:
Presence of foreign materials (e.g., rocks and metals)
Unacceptable level of moisture
Too large (or uneven in size)
Such undesirable conditions not only affect the flow of solids through the feeder,
but they also affect operation of the gasifier. It is thus necessary to eliminate them
and prepare the collected biomass appropriately for feeding. Foreign materials
pose a grave problem in biomass-fired plants. They jam feeders, form arches in
silos, and affect the gasifier operation, so it is vitally important to remove them
as much as possible. The three main foreign materials are: (1) stones, (2) ferrous
metals (e.g., iron), and (3) nonferrous metals (e.g., aluminum).
Some of the equipment used to remove foreign materials from the collected
biomass are as follows:
De-stoner. The basic purpose of a de-stoner is the separation of heavier-
than-biomass materials such as glass, stones, and metals. Typical de-stoners
Stagant
material
Inlet
height
Rotor
(a) (b)
Minimum
width
fIgure 8.10 (a) Feeder without vertical rise and (b) feeder with a vertical section.
285
8.3 Biomass-Handling System
use vibration in tandem with suitable air flow to stratify heavy materials
according to their specific gravity.
Nonferrous metal separators. Separation of nonferrous metals like alumi-
num has always been a challenge. One solution is an eddy current separa-
tor—essentially a rotor with magnet blocks—which, depending on the
application, is made of either standard ferrite ceramic or a more powerful
rare-earth magnet. The rotors are spun at high revolutions (more than
3000
rpm) to produce an “eddy current,” which reacts differently with dif-
ferent metals according to their specific mass and resistivity to create a
repelling force on the charged particle. If a metal is light yet conductive,
such as aluminum, it is easily levitated and ejected from the normal flow of
the product stream, making separation possible (Figure 8.11). Separation of
stainless steel is also possible depending on its grade. Particles from mate-
rial flows can be sorted down to a minimum size of 3/32 in. (2
mm) in
diameter. Eddy current separators are crucial in the recycling industry
because of their ability to separate nonmagnetic materials.
Magnetic metal separation. The use of powerful magnets to separate iron
and other magnetic materials from the feed is a standard procedure in many
plants. Magnets are located at several places along the feed stream. They
are generally suspended above the belt to attract magnetic materials, which
are then discharged away.
Size Reducers
Biomass comes from different sources, so the presence of oversized solids or
trash is very common in the fuel delivered. Woody biomass may be sized and
classified at the source or at the plant. The list following the figure contains
some of the equipment used for its preliminary sizing along with the typical
sizes produced (Van Loo and Koppejan, 2008, p. 64):
Aluminum
scrap
Chute
separators
Biomass
Magnetic block
fIgure 8.11 Separation of nonferrous metals from biomass using eddy current separation.
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Chunker: 250 to 50 mm
Chipper: 50 to 5 mm
Grinder: <80 mm
Pulverizer: <100 µm (dust)
Different types of equipment are necessary for sizing biomass. One example
is a chunker with multiple blades. Another is a spiral chunker with a helical
cutter mounted on a shaft; as the wood is fed into the machine, the cutter draws
it in and slices it into chunks. The power consumption of a chunker is relatively
low.
Chippers are used to break wood into small pieces. Disc and drum chippers
are two common types. In a disc chipper, the wood is fed from the side, meeting
a large disc with several rotating knives. In a drum chipper, several knives are
embedded in grooves (Figure 8.12); as the drum rotates, wood fed at one end
is chipped. The chips, which are now uniform in size, are carried away and
thrown to the other end by the grooves.
Size-reducing machines consume energy in proportion to the reduction in
size. Chipping typically consumes energy equivalent to 1 to 3% of the energy
content of the wood (Van Loo and Koppejan, 2008, p. 65).
Grinding is needed when a finer size (<80
mm) of biomass particle is
needed. Hammer mills may be used for this purpose. The wood is thrown to
the wall of the mill and crushed by hammers. A conventional biomass combus-
tor or gasifier does not require biomass to be ground to such a fine size.
However, direct co-firing in pulverized-coal fired-boilers and the use of
entrained beds for gasification require the biomass to be ground extremely fine
so that the particles can be conveyed like pulverized coal.
Size Classification
Oversized materials often cause major problems in a biomass plant. They jam
belts, bunkers, and other components. Sometimes trummels are used in the fuel
yard to separate the oversize pieces before feeding to the plant. A trummel
(Figure 8.13) is a rotating drum, with holes of various sizes, that separates the
Ejection of
chips
Knife
Log feed
Rollers
f
Igure
8.12
A drum chipper uses a number of knife edges mounted in a drum as shown.
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8.3 Biomass-Handling System
smaller and larger feed. Several belt runs in the feed-processing stream divert
oversized feed from the gasifier.
Drying
Freshly cut biomass can contain up to 40 to 60% surface moisture when har-
vested, but thermal gasification typically requires a moisture content of less
than 10 to 15%—this moisture is inherent in the biomass. Furthermore, biomass
is hygroscopic, so even after dried it can still absorb moisture from the atmo-
sphere; only after torrefaction does the biomass stop absorbing moisture (see
Section 3.8). This could happen even when the dried biomass is stored in a
shed. Because biomass is bulky, with low energy density, a very large storage
space is necessary for the typical fuel inventories required in an energy conver-
sion (boiler or gasifier) plant. For this reason, the biomass is often stored out-
doors, though it could absorbs additional moisture from rain and snow. Leaving
freshly harvested biomass outdoors can at times have some positive effect. For
example, straw is sometimes left in the field for a few days or weeks to lose
moisture and then put in bales. Leaving wood logs outside over the summer
can reduce moisture by as much as 20% (Van Loo and Koppejan, 2008, p. 70).
The moisture in biomass must be reduced before use because it represents
a large drain on a plant’s deliverable energy. Every kilogram of moisture
needs about 2300
kJ of heat to vaporize and an additional 1500
kJ to be raised
to a typical gasifier temperature of 700 °C. This large amount of energy
(3800
kJ/kg) has to come from the energy released by the gasifier’s exothermic
reactions. Therefore, the lower the moisture, the higher the heat available in
the product gas.
Outdoor storage may not work well because of rain and snow, but precipita-
tion can have a beneficial effect on some herbaceous biomass, such as straw,
since it leaches water-soluble agglomerates and corrosion-causing elements
such as chlorine and potassium. The three types of moisture in a biomass gas-
ifier are: (1) surface moisture, (2) chemical moisture, and (3) moisture in air or
steam used for gasification.
fIgure 8.13 Portable trummel used in the fuel yard for size classification. (Source: Photograph
by the author).