Environmental Engineering Reference
In-Depth Information
Energy input for the production of the fertilizers comes from data from a neighboring farm.
72 ha oats consume N-fertilizer corresponding to 7 tonne oil for 300 tonne oats. This corresponds
to 350
×
7/72
=
34 tonne
×
10MWh/tonne
=
340MWh for the 350 ha.
The total energy consumption for the 1750 tonne cereals would then be 108
+
45
+
230
+
340
=
723MWh. This gives 0.41MWh/tonne which corresponds to 0.41/(5.4MWh/tonne)
=
7.7% of
the heating value of the cereals.
If we produce ethanol we would get out approximately 250 liters per tonne cereals, which
corresponds to 0.25
×
7MWh/L
=
1.75MWh ethanol/tonne cereals.
The output as ethanol compared to input of primarily fossil fuels for the production then is
1.75/0.41
=
4.3 times. To this we have to add also evaporation of water to achieve pure ethanol,
but on the other hand we get a lot of other products out as well as 250 kg protein fodder, and also
heat and electricity corresponding to approximately 0.5
0.95MWh.
If we alsomake use of the straw, we could use approximately 3 tonne DS/ha
×
5.4
×
0.35
=
×
5.4/5
=
3.2MWh
per tonne cereals. The total
then would be 1.75MWh ethanol, 250 kg protein fodder
(0.25
×
5.4
=
1.35MWh)
+
0.95
+
3.2
=
4.15MWh heat
+
power. The total output then would
be 7.25MWh per tonne cereal grain. 7.25/0.41
17.7 times the fossil fuel input.
As seen the efficiency depends very much on what is included! If we should evaporate from
5% ethanol to 99% in one step this would mean an energy consumption of 20 liter water per
liter ethanol or 250
=
45,200 kJ/L ethanol or 11,300,000 kJ/tonne cereal. This
corresponds to 3.16MWh per tonne cereal. In reality, we can regain most of this energy as heat
for e.g. district heating and thus the real energy usage will depend on the economic optimum.
Capital
versus
energy!
This calculation was done to show that depending on what constraints we set we can achieve
very different results. If our purpose is to show that producing ethanol is bad, we just select the
conditions giving a low output, like only including the primary energy input but not using the
energy efficiently. If on the other hand you want to show that ethanol production is good you
show all the possibilities to reuse input energy and emphasize that protein is not destroyed but
used as fodder or even food for humans. It is sometimes very disturbing that facts can be used
for so different purposes and thereby affects the total energy system dramatically in the future,
depending on the part of the facts that wins!
In some countries, we have a very positive attitude to bioenergy use, like in Sweden, where
also the farmer organization has a very strong influence. In other countries or regions, we may
have a very different situation depending on other strong lobbying groups with other interests!
We therefore will have to both give facts to politicians and good arguments for the positive
aspects of biomass use for energy purposes. Only facts are not enough. Good examples are
also very important and these have to be presented in a convincing way. Then both regulatory
frameworks and interest from investors can be achieved, and thereby system development can
take place.
×
20
×
2260 kJ/kg
=
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