Agriculture Reference
In-Depth Information
4.6.8.
Gains from ethanol by-products
Since ethanol save gasoline on LHV basis, its production is assessed in unit of its heat value.
Because its by-products are not used as transportation fuel but as animal feed and fertilizer,
the same equivalence can not be used.
On the other hand, stillage permits to save some synthetic fertilizer and, consequently,
the energy to produce it. Likewise beet pulp saves some feed and, as a result, the energy
to produce the feed. Hence an energy value can be attributed to them. As stillage is al-
ready included in the nutrient balances at the farm, no value is given. As far as the beet
is concerned, it must be compared to a common feed having the same caloric and protein
contents.
Dry matter of pulp has a quantity of digestible energy for ruminants close to that of
soft wheat grain at 86% DM (7.1 MJ
·
kg
−
1
for dairy cattle and 7.6 MJ
·
kg
−
1
for beef cattle).
Both contain a comparable amount of ruminant digestible proteins (90 g
·
kg
−
1
), although
the degradable part by micro-organisms in intestine is a little lower for pulp.
Subsequently, we assume that soft wheat grain is the equivalent feed. Its culture in
France requires a quantity of energy 2.070
GJ
tot
per tonne of harvest, of which about half
corresponds to petroleum products - mainly diesel for mechanized operations - and the other
half to gas - mostly to produce N fertilizers -, according to a study about ethanol production
from wheat in France [35]. The requirement is deduced from the consumption at the farm to
produce 1 J of ethanol LHV, 26.2%, and ethanol yield from wheat, Y
OH
= 7
.
9 GJ
OH
·
t
−
1
.
At 12% moisture wheat grains do not necessitate drying. For a pulp yield of 55
kg
DM
·
t
−
1
,
A
the energy gain amounts thus to of 95
MJ
LHV
·
t
−
1
or 115
MJ
tot
·
t
−
1
.
A
A
4.6.9.
Combined heat and power generator. Factory energy balance
Fig. 5 shows the direct requirements for the main operations to process 1 tonne of beet root
in the factory. It amounts to about 305
kg
st
·
t
−
1
of steam, 165
MJ
e
·
t
−
1
of electricity and
A
A
260
MJ
LHV
·
t
−
1
of gas.
A
Figure 5. Steam (
kg
st
), electricity (MJ
e
) and heat (MJ
LHV
) requirements for the main
factory operations to process 1 tonne of beet root.
Fig. 6 shows the energy flows to produce electricity and process steam at 3-4 bars
from a co-generator using a steam turbine. 4 bar steam is required for evaporator thermo-
compression. High pressure steam at 60 bars and 450
◦
C is generated at the gas fueled
boiler. This choice corresponds to existing or projected co-generation systems in ethanol
factories [11, 30]. In average during a year the system consumes close to 1.01
GJ
LHV
·
t
−
1
A
