Lecture_notes - Fuel upgrading.pdf

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Växjö university
02-11-05
Lesson 03: 1
Distance learning course
Bioenergy Technology
Biofuel production - Chapter 3, Fuel upgrading aspects
The scope of this chapter is to continue the description of biofuel production and to include
such processing of the fuel that improves its quality. The chapter will show that biofuels are
not only the virgin biomass as such but that a wide variety of fuels are available from the raw
material.
The fundamental structure of biomass
The fundamental properties of biomass are mainly due to their cellular structure and to the
fact that they are a living material until cut. The main components in the solid material are
cellulose (the most common organic molecule on earth, very long chains of glucose molecular
units), hemicellulose (shorter chains composed not only of glucose but also of other
monosaccharides) and lignin (a phenolic polymer). These three are the ones that mainly build
up the cell walls and thus constitute the mechanical strength in the plant structure and do
usually add up to well above 90 % of the weight of the dry, ash-free substance. The remaining
part of the organic material is the extractives, such components that can be extracted from the
material using neutral solvents. Fats and hartz components belong to this group together with
terpenes and other lighter hydrocarbons.
The cell structure varies with the function of the cell but in softwoods (conifer trees) the main
part of the stem is made up by tracheids, tall cells making up a tubular structure well suited to
the vertical transport of water. In hardwoods (broadleaf trees) the cells are more diversified
due to the fact that the hardwood trees were developed about 100 million years after the first
softwood trees and that the evolution had taken some steps forward during the meantime.
Thus the conifer trees may be seen as more primitive in the respect that the number of
specialised cells is less than in the more developed hardwoods.
However, and in spite of the specialisation of the cell function, the cell wall itself has a similar
structure in softwoods and hardwoods. For simplicity, one may assume a cross-cut of woody
material perpendicular to the direction of main water transport to be a hexagonal structure of
pores separated by cell walls. If a typical c-c distance between the pores then is assumed to a ,
one finds the ratio circumference / cross-sectional area = 6 / a m -1 - or with a =0.01 mm a numerical
value of 6 . 10 5 m 2 /m 3 . Exercise : Do the calculations .
The cell wall is not flat but is in turn built up by bundles of microfibrils which are, in turn,
built up by bundles of cellulose molecules. Thus the above estimate of the internal area is
about three powers of ten too low and the real internal surface area in woody biomass is in the
order of magnitude 6 . 10 8 m 2 /m 3 .
The fibrous structure of typical biomass is also responsible for the very strong anisotropy
found in the mechanical properties of biomass. This affects the energy need for chipping
and/or milling biomass as well as it affects the shape of the particles resulting from these
operations. It is also obvious that since the water content of the biomass is enclosed within
cellulose based cell walls, the complete drying of biomass must involve either a time-
consuming diffusion process through the cell walls or a breaking of the cell walls due to
internal overpressure in the cells.
Växjö university
02-11-05
Lesson 03: 2
Distance learning course
Bioenergy Technology
Heat value of virgin biomass
The energy content of a biofuel may be computed from the e quation:
(
(
)
)
(
)
∆
H
≈
∆
H
daf
eff
⋅
1
−
f
−
f
−
f
⋅
Cp
⋅
ϑ
−
ϑ
+
∆
H
eff
ash
water
water
water
drying
in
Liquid
→
Steam
where
∆
H eff
is the lower heating value for the fuel as received, MJ/kg
daff
eff
∆
H
is the lower heating value for the dry, ash-free combustible substance (MJ/kg),
( may be estimated as f cellulose . 17 + f hemicellulose . 16 + f lignin . 28 + f extractives . 35,
from fundamental chemistry or by experiments)
f ash
is the weight fraction of ash in the dry substance
f water
is the weight fraction of water in the fuel
is the heat capacity of water (4191 J/kg . o C for air-saturated water, mean value
between 0 and 100 o C)
Cp water
drying is the temperature for fuel drying (usually set to 100 o C)
is the mean temperature for the fuel entering the boiler (usually 0 o C for
Swedish conditions)
ϑ
ϑ
in
drying (2.26 MJ/kg at 100 o C)
∆
H Liquid → Steam is the enthalpy for steam formation at
ϑ
daf
eff
∆
H
For estimates concerning forest fuels in Scandinavia, a value of 19.5 MJ/kg for
is
usually good enough and if the ash content is not known a value of 1.5 % may be used.
For normal estimates concerning forest fuel representative of Swedish conditions, the
following simplified equation yields sufficiently good values:
(
)
*
∆
H
≈
19
.
⋅
1
−
f
−
f
−
f
⋅
2
679
eff
ash
water
water
where the asterisk on f ash indicates that this is the excess of ash above 1.5 %, i.e. if the ash
content is 3 % f * ash is input as 3-1.5=1.5 %.
The moisture content in wooden structures is of two kinds, namely the water bound in cell
walls, up to approximately 22 % by weight, and the water stored in pores, water content above
22 %. The maximum capacity to bind water in the cell walls is called the fibre saturation
point and the mechanism is that the water is bound by hydrogen bonds to the hydroxyl groups
in the cellulose molecules. Once the cellulose is saturated, excess water is stored in the pores
of the structure. The water bound to the cellulose requires some extra energy to be released
but for normal calculations of the heating value of the fuel this extra energy requirement is
negligible.
Density of virgin biofuels
The density of wooden fuels is mainly determined by their porous structure. The cell walls are
- in most species - similar, and are built up by cellulose, hemicellulose and lignin in
proportions that vary slightly between species. However, the density of the cell wall itself is
almost the same in most species and may be assumed to 1560 kg/m 3 . The porosity between
different species varies within wide limits, from balsa wood with porosities in the order of
magnitude 90 % down to ebony where the porosity may be as low as 20 %.
During water absorption in the fibres, i.e. at water contents below fibre saturation, the cell
 
Växjö university
02-11-05
Lesson 03: 3
Distance learning course
Bioenergy Technology
walls expand. The expansion is highly asymmetric with only very minor changes in the length
direction while the radius of a piece of wood may expand 5-10 % and hence its tangential
measures may change 10-30 % as the moisture content goes from 0 to fibre saturation. Thus
the introduction of moisture up to the point of fibre saturation into dry wood is accompanied
with a volume increase of 10 - 20 %, while additional moisture (above fibre saturation) in
practice implies only very minor changes in volume.
Since the water content f water is given by
m
f
≡
water
water
m
+
m
water
dry
subs
tan
ce
we may obtain the ratio m water / m dry substance as
m
f
water
= 1
water
m
−
f
dry
subst
water
The mass of a piece of wood containing 1 m 3 of cell wall material then becomes
f
1
m
(
f
)
1560
1
1560
=
⋅
+
water
=
⋅
wood
water
1
−
f
1
−
f
water
water
Now assume that the volume change for water contents below fibre saturation is linear to the
water content. Further assume these two to be related by a constant of proportionality, K , and
finally assume that the wood under consideration has the porosity P . Then the volume of this
piece of wood will be
K
(
)
V
(
f
)
1
f
=
⋅
+
while f water < f saturation
wood
water
water
1
−
P
Finally, it is assumed that the volume of the wood remains constant when the water content is
further increased above fibre saturation since the excess water is assumed to be contained in
the pores in the structure. Then the density may be estimated as
1
1560
⋅
m
1
−
f
1
−
P
ρ
(
f
,
P
)
wood
water
1560
while f water <
f saturation
≡
=
=
⋅
(
) (
)
wood
water
1
+
f
V
K
⋅
1
−
f
⋅
1
+
f
water
K
⋅
wood
water
water
1
−
P
and as
1
1560
⋅
m
1
−
f
1
−
P
ρ
(
f
,
P
)
≡
wood
=
water
=
1560
⋅
f w > f sat.
(
) (
)
wood
water
1
+
f
V
K
⋅
1
−
f
⋅
1
+
f
K
⋅
saturation
wood
water
saturation
1
−
P
The assumptions introduced above may not be used for anything but estimates concerning
fuel properties. They are not valid for any detailed calculations concerning the measures of
pieces of wood or anything like – but only to estimate fuel properties where round numbers
are sufficient.
964394753.060.png 964394753.066.png 964394753.067.png 964394753.001.png 964394753.002.png
 
Växjö university
02-11-05
Lesson 03: 4
Distance learning course
Bioenergy Technology
This yields the following densities for some typical species that might find use as fuels. The
densities are computed for the mean value of the porosity intervals and given in kg/m 3 with
K =1:
Specie
Poros
vol-%
Moist u re conte n t, % by weight
5 %
10 %
15 %
20 %
30 %
40 %
50 %
60 %
Alder
56-73
555
559
566
577
648
756
908
1135
Bamboo
75-80
352
354
359
366
411
480
575
719
Birch
51-67
641
646
654
666
749
874
1048
1310
Cedar
64-69
524
528
535
544
612
714
857
1071
Larch
64-68
532
536
543
552
621
724
870
1087
Maple
52-60
688
693
702
715
804
938
1125
1406
W. Pine
68-78
422
425
431
439
493
575
690
863
Spruce
55-69
594
599
606
618
694
810
972
1214
Willow
62-74
500
504
511
520
584
682
818
1023
MJ/kg, 1.5 % ash
18.1
17.0
15.9
14.8
12.6
10.4
8.3
6.0
The heat value (given in italics below the table) were computed using the simplified formula
(
)
*
∆
H
≈
19
.
⋅
1
−
f
−
f
−
f
⋅
2
679
eff
ash
water
water
which is not really valid for several of the species in the list because their ash contents are
usually bigger than 1.5 %. However, the aim is only to give a rough feeling for how the
energy content varies with moisture content and to produce input for the table below.
This table shows - using the energy contents and the density values from the above table - the
number of m 3 of solid material needed to provide 1 TJ of energy, i.e. m 3 /TJ.
Moist u re conte n t, % by weight
Specie
Poros.
vol-%
5 %
10 %
15 %
20 %
30 %
40 %
50 %
60 %
Alder
56-73
100
105
111
117
122
127
133
147
Bamboo
75-80
157
166
175
185
193
200
210
232
Birch
51-67
86
91
96
101
106
110
115
127
Cedar
64-69
105
111
118
124
130
135
141
156
Larch
64-68
104
110
116
122
128
133
138
153
Maple
52-60
80
85
90
94
99
102
107
118
W. Pine
68-78
131
138
146
154
161
167
175
193
Spruce
55-69
93
98
104
109
114
119
124
137
Willow
62-74
110
117
123
130
136
141
147
163
Fuel quality and upgraded fuels
Virgin biomass is solid, has a low- and variable-density and also has a low- and variable heat-
value. On top of this the ash content and ash composition are notoriously unpredictable.
A high-quality fuel, in contrast, is a fuel with
•
A high heating value either by weight or by volume - depending on the demands set by the
plant where it is used
•
A constant - or at least predictable - composition
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Växjö university
02-11-05
Lesson 03: 5
Distance learning course
Bioenergy Technology
•
A well-defined ash content
•
Well defined ash properties
On top of this, we may wish that the high-quality fuel is easy to handle, either it is a gas or it
is a pumpable liquid or - at least - it is a well-defined powder with constant density particles
of a reasonably uniform shape.
Biomass is none of this.
Upgrading is the term for processes aiming to change the properties of the virgin biomass so
that it becomes a high-quality fuel in one or more respects.
Upgrading maintaining the physical and chemical structure
Drying is the simplest fuel upgrading process, especially if it is achieved by sun and wind in
open air and thus requires zero investment cost. The efficiency of sun and wind for drying is
highly dependent on the way the fuel is stored and the final moisture content is highly
dependent on how it is protected from rain during the drying period. The so-called “air-dried”
material will not attain moisture contents lower than 15-25 % when outdoor drying is used, no
matter how well protected it is for rainfall. The reason for this is that the cell walls below fibre
saturation settle in equilibrium with the surrounding air.
The costs associated with outdoor drying are generally very low and the main component is
often the capital cost for the fuel itself, the first term in the square bracket.
(
)
n
m
1
+
1
1
+
I
C
=
in
⋅
C
⋅
2
+
C
+
C
+
C
(
)
out
in
handling
site
losses
n
m
1
−
1
1
+
I
out
2
where
C out is the total cost per mass unit of outgoing material
m in is the amount of originally stored material
m out is the amount of material actually retrieved after storage
C in is the cost per mass unit for the material originally stored
I is the interest rate per time period
n is the number of interest time periods elapsed during the storage period
C handling is a fixed cost per unit of stored material related to loading/unloading etc
C site
is a fixed cost per unit of stored material for the site, manning, fees etc.
C losses
is the cost for material losses during storage per mass unit of stored material
Drying implies a change in heat value since the need of endothermic water vaporisation is
reduced. However, if the combustion plant is equipped with flue gas condensation, drying is
not altogether a positive thing for that plant.
In some cases it may be worthwhile to enhance the drying and to perform the drying process
in special processors - dryers. The evaporation energy is in these cases supplied either by
combustion of residue fuel or it is supplied by low-grade surplus process heat from a different
process. The dryer may be constructed for direct drying - in which case the drying goods is
brought into direct contact with the heat carrier or it may be indirect where the heat carrier is
physically separated from the drying goods.
964394753.065.png
 

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