Agriculture Reference
In-Depth Information
on the availability of data, which is someway a function of the time spent.
As the total consumption is the sum of the consumptions for the different operations of
the system, we have R
=
P
j
R
j
with R
j
the global rate of consumption for the operation
j, i.e. the contribution of j to R. Within the chain of operations the contribution R
conc
of concentrating sugar in the raw juice is given in J of energy per J of produced ethanol.
Comparison between R
j
permits to figure which operations deserve longer time of analysis
in order to refine R
j
. When only one or two data for j are available and we have a poor
understanding of its processes, an uncertainty of at least
±
50% must be considered.
To better study the consumption efficiency of each operation j, it is studied separately
using j own output. Hence we introduce the local rate for j, r
j
. We also work with opera-
tion direct consumptions to make r
j
process specific and closer to raw data. Consumption
efficiency for concentrating the raw juice, r
conc
, is measured in kg of required steam per kg
of removed water (
k
st
·
kg
−
w
).
This choice is guided in order to have r
j
nearly invariant over a large range of consump-
tion for a given process, or at least less variable than the operation global rate R
j
. It makes
r
j
less dependent on the industry where the process is used and data on j are retrieved.
Auxiliary operations must provide for the requirements of the system direct operations.
Their specific consumption is taken into account by a conversion factor, β
j
. A gas fueled
boiler can generate the steam used to concentrate the juice. Its factor is β
boiler
= 2
.
65
MJ
of gas LHV per kg of steam (assuming an efficient boiler at 97%LHV and steam energy
resulting mainly from water latent heat). Taking into account all the gas heat value (or high
heat value HHV) and the energy lost along processes to extract, refine and transport gas, we
obtain an overall factor of about β
boiler
= 3
.
15 MJ
tot
·
k
−
1
st
. Instead of a simple boiler, a
combined heat and power generator can also supply steam along with electricity, which is
what is considered in this study.
Under certain conditions (temperature, amount...), heat requirement of a process can be
provided by the waste heat from another one owing to process integration. In this case we
have for heat demand β
j,th
= 0
, except when r
j,th
already includes the saving. On the other
hand, the transfer increases electricity consumption.
Moreover, the separation between direct and auxiliary operations permits to study the
possibility of a self-reliant industry like the petroleum industry and, by necessity of energy
conservation, the global energy system (See chap. 5 of [9]). Ethanol can be used to produce
steam and other requirements of system direct operations with β
j
expressed in
MJ
OH
·
k
−
1
st
or
MJ
OH
·
MJ
−
e
.
The contribution R
j
to R from operation j is deduced from β
j
r
j
thanks to a variable
w
j
such as R
j
=
β
j
r
j
w
j
. w
j
represents the "weight" of β
j
r
j
in R. It is also the output
of j per unit of the system output. For instance w
conc
for juice concentration represents
the amount of water to remove from juice for one tonne of beet processed. It will depend
whether the raw juice is concentrated into a syrup at 61% of sugar or into a liquor to be
fermented at 16% of sugar.
We do not take into account energies to manufacture chemicals consumed at the fac-
tory and the equipment of the industry. We assume them negligible relative to the energy
consumptions in use, which is the case for equipment used at their capacity during their
normal life time (see the case of a tractor in Chap. 4 of [9]). However, it does not mean that
these contributions are small when looking at the industry financial balance. The difference
