Lecture_notes - Biofuel_handling.pdf
(
130 KB
)
Pobierz
Växjö university
02-12-06
Lesson 04: 1
Distance learning course
Bioenergy Technology
Biofuel production - Chapter 4, Collecting and Handling aspects
The scope of this chapter is to continue the description of biofuel production and to include
some handling aspects. The chapter will show that handling is not only a question of
collecting and transporting but that the fuel handling also has an impact not only on the fuel
quality but also on the sustainability of the whole system.
Energy balance expressed in m
2
From the previous chapters we know that the virgin biomass - generally speaking - has a low
energy density with respect to land area. It was shown that typical growth rates for low-
intensity farming fuels such as forest fuels is in the order of magnitude 0.35 W/m
2
in Sweden
while in tropical regions the values may be 10-20 times higher, i.e. order of magnitude 5
W/m
2
.
To use Sweden as an example it may be stated that a typical Swedish household uses heat at
an average rate of approximately 15 W/m
2
living area. The same generic household uses
electricity at a rate of approximately 4 W/m
2
living area. The wording "living area" is here
used to indicate indoor, heated area.
Thus, to provide the heat for 1 m
2
of living area - if virgin biofuel is to be used - the total
growth of
15
/
0.35
= 40-45 m
2
of land must be harvested. However, it is not desirable to extract
100 % of the biological material for ecological reasons. Neither is the conversion from
chemically bound energy in the fuel into heat 100 % efficient. Thus a value of 55
m
2
land
/
m
2
living area
for heating will be used throughout in the forthcoming text for an order-of-
magnitude. This value corresponds to 15 - 20 % of the total biological mass being left at the
felling site and to a conversion efficiency of 90 %. It must be stressed that this value is only
an example and that local conditions and technical status of the conversion plant - also in
Sweden - affects the ratio within wide limits.
If also the electricity consumption is to be provided from biofuels the conversion efficiency is
lower - order of magnitude 40 % (see the thermodynamic introduction) - and the ratio of
land/living area becomes 1.2
.
(
4
/
0.35
)
.
(
1
/
0.4
)
15
m
2
/
m
2
. Once again: These ratios are only
order-of-magnitude estimates based on Swedish conditions and must be regarded only as
such.
≈
The numbers above serve to illustrate that an efficient biofuel utilisation is highly dependent
on an efficient logistic system. This is especially true when more realistic systems are
considered, where only a portion of the harvested material is used for energy. If, for example,
the fuel is extracted from forests primarily used to supply pulp-and-paper industries and
sawmills, maybe only 5-10 % of the harvested material is available for primary use as fuel. In
this case the fuel necessary for the heating of 1 m
2
living area during one year must be
collected from 500 - 1000 m
2
and the fuel to provide the electricity for the household must be
collected from 150-300 m
2
. In co-generation plants the land needed to provide heat and
electricity thus becomes approximately 550 - 1150 m
2
/m
2
in case the fuel is a by-product or
1
/
20
’th to
1
/
10
’th thereof in case fuel production is the only purpose of the forest activities.
Now assume that the land where the fuel for a certain energy conversion plant is available can
Växjö university
02-12-06
Lesson 04: 2
Distance learning course
Bioenergy Technology
be described as a circle with the diameter
D
and further assume that the energy plant is
situated on the circumference of this circle. The geometry is outlined below.
If the diameter of the fuel uptake land area is
D
km, then the average transport distance
to the energy plant becomes 1.16
.
(
D
/
2
).
Assume that the energy plant is a co-
generation plant dimensioned to supply
heat and electricity for say 2 000
households within the municipality and
further assume that the average
household has a living area of 110 m
2
.
Fuel uptake land
If the forest is used solely for fuel production
a value of 61 m
2
/m
2
can be used to estimate the
land area and a total value of 13 420 000 m
2
is needed.
Municipality
Energy plant
This corresponds to a circular area with a diameter of slightly more than 4 km, which is the
land need in case of energy crop plantation and utilisation. In case the surrounding area hosts
intensive forest industry or agricultural production it may be wise to produce the fuel as a by-
product from the local base industries. In that case the fuel uptake will demand a land area of
maybe 134 200 000 - 268 400 000 m
2
corresponding to diameters 13 - 18.5 km. The
corresponding (mean) hauling distances become 2.3 km and 7.5- 10.7 km respectively.
With the very simple geometry outlined in the figure, the mean transport distances may - for
Swedish conditions - be estimated as
S
0. for the two cases of
energy crop plantation and for forest residue utilisation.
n
is the number of households
supplied and the mean distance is expressed in km. Thus the transport distance increases
roughly in proportion to the square root of the size of the plant.
≈⋅
005
.
n
or as
S
≈⋅
n
Obviously, the tonnage to be transported increases linearly with the size of the plant and the
total transport work (expressed in tonnes
.
km) needed to supply the plant with its fuel becomes
proportional to the size of the plant, as measured in households, raised to the power of
3
/
2
.
The specific transport work for forest-fuel-fired plants in Sweden can be estimated from
values in "Yearbook of Forest Statistics". It is then found that the mean hauling distance for
waste paper and for wood chips/waste wood (km/tonne) has changed successively according
to the table below:
1980
1985
1990
1995
1997
Wood pulp and waste paper
125
106
93
109
92
Wood chips and waste wood
86
60
82
89
70
The dominant mode for tranportation of these qualities is on lorry by road. Thus virgin
biofuel, such as waste wood, is generally transported shorter distances as compared to
recirculated biofuel such as waste paper.
Växjö university
02-12-06
Lesson 04: 3
Distance learning course
Bioenergy Technology
Transport efficiency
Previously (chapter 3) it was shown how the energy content of virgin biomass varied with
moisture content and to remind you the following table showing m
3
/TJ is reproduced again:
Specie
Poros
vol-%
Moist
u
re conte
n
t, % by weight
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
The bulk density, i.e. the effective density of the
material to be transported, is mainly determined by the
degree of packing of the material. For particulate matter
- such as wood chips or powder - the bulk density may
roughly be estimated as half the solid material density.
This means that half the volume is occupied bý solid
material and half the volume is occupied by the
interstitials between the particles.
The phenomenon may be illustrated by the two-
dimensional figure besides, where the packing of
identical cylinders in a square box is considered.
It is clear that the box sides are 4
.
d
where
d
is the diameter of the cylinders and hence that the
cross section of the box is 16
.
d
2
and it is also clear that the box contains 16 cylinders, each
with the area
.
(
d
/
2
)
2
. Thus the ratio of cylinder area to box area is
16
d
2
/
2
2
16
d
2
=
π
π
π
/
4
≈
0.78.
In this - two-dimensional - case, the degree of packing thus becomes
≈
78 %.
Now extend the reasoning by assuming the particles to be spheres and the box to extend one
particle diameter perpendicular to the paper. Then the box has the volume 16
.
d
2
.
d
or 16
.
d
3
.
Each spherical particle has the volume (
4
/
3
)
.
.
(
d
/
2
)
3
which finally becomes (
π
/
6
)
.
d
3
. The volume ratio (volume of spheres divided by volume of
box) now becomes
π
/
6
or about 52 %. This result corresponds well with what was said above:
A solid material may be assumed to fill a container halfway, while the rest of the container is
filled with air. Thus the effective volume-weight of a particulate material may be assumed
approximately half its fundamental density. Shaking, vibrating etc will cause finer particles to
fall down in the structure created by the coarser particles and – if there is sufficient fine
material – to ultimately fill the interstitials. But to completely fill up a given volume with
particles takes a very special size distribution with the particles and is never accomplished.
.
r
3
or – since the diameter d is 2
.
r
- (
4
/
3
)
.
π
π
Växjö university
02-12-06
Lesson 04: 4
Distance learning course
Bioenergy Technology
Also the time to vibrate or compact the material during loading is expensive and it is usually
not considered worthwile to spend all that work to reduce the transport cost. It simply does
not pay off.
“Just-in-time” fuel deliveries do not work ...
The so-called
JIT
philosophy (Just-In-Time, in its extreme no intermediate storages
whatsoever) is not applicable to a fuel supply chain. Anyway not to a fuel supply based on
household demand. The reason is that the demand in this case is extremely weather-dependant
and that the weather can not be predicted with any accuracy over periods longer than one
week. A planning based on
JIT
needs a longer planning horizon, since it is based on a
thorough scheduling of the transports of goods here-and-there.
Let us work a simple example to prove that there must be large stores included in the fuel
supply chain:
The need of heat, as well as the consumption of tap water, in houeseholds, follows a general
curve as a funciton of outdoor temperature, roughly described by the figure below. The
general characteristics are that the demand is constant above a certain temperature – based on
daily showers, needs for dishwater etc, and that the demand then increases more or less
linearly with decreasing temperature up to a limit set by the hardware. If you do a more
thorough analysis you will find that the dependance is not linear but for convenience it is
usually assumed to be. Typically, in Sweden, the upper limit for a single-family household
would be about 20 kW or 480 kWh/day whereas the lower limit – the need for tapwater on hot
summer days – would be about 20 % of that, 4 kW or 80 kWh/day. In Sweden the treshold
occurs at about +17
o
C – a temperature known as “the firing limit”.
100 %
50 %
0 %
Basic need of tap water for showers, washing etc.
-20
o
C
0
o
C
+20
o
C
The firing limit is the outside temperature above which the normal losses from kitchen stove,
light bulbs, people in the house, hot water pipes to the bathroom etc are sufficient to maintain
an indoor temperature at about 20 – 22
o
C without any extra energy. Thus: If a house is better
insulated, the firing limit for that house occurs at a lower temperature and the worse the
Växjö university
02-12-06
Lesson 04: 5
Distance learning course
Bioenergy Technology
insulation the higher the firing limit. The striking thing with this is that a house in a warmer
country – usually implying less well insulated – has a higher firing limit than a house in a
more severe climate.
The characteristics shown in the figure are often summarised in so-called “degree-days”.
Analogous names are found in several laguages – in Swedish it is “graddagar”. The definition
of degree-days is the temperature difference between actual outdoor temperature and firing
limit multiplied with the duration of that outdoor temperature in days.
Thus: Suppose the firing limit is 17
o
C and suppose the outside temperature is 13
o
C. The
difference is 17-13 = 4 degrees and if this temperature is constant for three days the total
number of degree-days for this period becomes 3
.
4=12. If the outside temperature instead is
+9 degrees and this temperature is constant for 3 days we get (17-9)
.
3=24 degree-days. If the
duration instead is 6 days we get 48 degree-days which is exactly what we get if the outside
temperature is only +1 degree during three days we get (17-1)
.
3=48. So from an energy
point of view 6 days at +9 degrees is equivalent to 3 days at +1 degree.
In case the outside temperature exceeds the firing limit the degree-day measure is set to 0.
Thus if the outside temperature is +25 the degree-days become zero regardless of the duration
of the heat-wave.
If the outside temperature falls below zero, the calculation proceeds just like before: Assume a
three day period with the temperatures +6, -7 and +4 respectively. These three days will then
sum up to (17-6)
+
(17-(-7))
+
(17-4)
=
(17-6)
+
(17+7)
+
(17-4) = 11+24+13 = 48 degree-
days. Once again: From an energy point of view, 3 days at +1 degree is equivalent to 6 days at
+9 degrees and to 3 days at +6, -7 and +4 degrees and to ...
So far we have only played around with the definition of degree-days to make the concept
comfortable – but let us now use it for a more serious purpose and then return to the example
we are working:
Assume a one-week period (7 days) at +15 degrees.
This equals 14 degree-days, OK?
Now assume the same period – one week – at +13.
This equals 28 degree-days.
One week at +9 degrees comes out as (17-9)
.
7
56 degree-days
Same period but only one degree above zero gives a total of
112 degree-days
And finally – winter time – minus 15 degrees for one week
224 degree-days
This shows that the energy need for heating is a very strong function of the outside
temperature. If we now assume that the power used for heating is 15 kW at –15
o
C, then that
means a weekly consumption of 2 520 kWh which (pellets having a heating value of
approximately 5 kWh/kg) corresponds to about 500 kg of pellets per week.
Say that there is a local wood-pellet retailer supplying a community with a thousand
households. Each household has – typically – a capacity to keep a maximum of about 1000 kg
of pellets in store at home, so on average at any moment we may assume that each household
has 600 kg of pellets at home. They all place an order for new pellets when their store is down
to 100 kg and delivery is the next day, seven days a week. So this is “just-in-time” on the
local level. Now comes a week when the temperature is usually aorund zero degrees but this
year it suddenly drops down to –15 and stays there.
Plik z chomika:
mmarmour
Inne pliki z tego folderu:
Exercise_ch_3-&-4.pdf
(46 KB)
Kapitel_4.pdf
(1035 KB)
Lecture_notes - Biofuel_handling.pdf
(130 KB)
Inne foldery tego chomika:
_1BT009-BioenergyTechnology_TEXTBOOK
01Introduction
02BiofuelProduction
03BiofuelQualityAndUpgrading
05BiofuelCombustion
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