Environmental Engineering Reference
In-Depth Information
conventional biomass system mainly involve one step (biomass combustion). In particular,
energy and C conversion efficiencies of the biorefinery are lowered by the fermentation step,
where C6 sugars are converted to bioethanol: C efficiency from cellulose to bioethanol is
about 57% while from hemicellulose to furfural (a chemical pathway, without
microorganisms) is about 75%. During fermentation almost half of the C in C6 sugars is
converted to the useless product CO
2
, thus lowering the overall efficiency of the process.
4.3.2. Allocation results
The allocation procedures previously described are applied in order to share the
environmental burdens of the biorefinery among the different products. The allocation criteria
are based on energy content, exergy content and economic value of products. An attempt to
avoid allocation through system expansion was also done. Results are reported in Table 9,
where the GHG emissions of the biorefinery system are allocated. Concerning the cumulated
primary energy demand, allocation can be performed considering the same shares of the total
GHG emissions of Table 9.
Table 9. Allocation of the GHG emissions to the biorefinery products using
different allocation methods.
Allocatio
n method
System
expa
n
sion
Unit
En
e
rgy
Exe
r
gy
Econo
mic value
main
produc
t
Transportation
(bioethanol)
kt CO
2
-eq./a
25.1
68.3%
25.7
69.8%
25.4
68.9%
-97.6
Transportation
(MTHF)
kt CO
2
-eq./a
2.82
7.68%
2.98
8.09%
2.36
6.41%
25.6
credits
Furan resins
kt CO
2
-eq./a
0.59
1.60%
0.62
1.69%
2.02
5.48%
4.31
credits
FUMA
kt CO
2
-eq./a
0.29
0.79%
0.33
0.90%
0.83
2.26%
17.6
credits
Electricity
kt CO
2
-eq./a
3.18
8.64%
2.60
7.07%
1.80
4.90%
39.8
credits
Heat
kt CO
2
-eq./a
2.14
5.83%
1.05
2.86%
0.41
1.11%
23.8
credits
Biomethane
kt CO
2
-eq./a
2.49
6.78%
3.22
8.75%
0.47
1.29%
21.8
credits
H
2
kt CO
2
-eq./a
0.13
0.36%
0.11
0.30%
0.09
0.25%
0.99
credits
O
2
kt CO
2
-eq./a
-
0.00%
0.01
0.02%
3.45
9.38%
0.51
credits
In Figure 19, the influence of the allocation methods on GHG emissions allocated to
biorefinery products is shown. It can be noticed that, besides system expansion which uses a
different approach, all the allocation criteria lead to similar results, especially concerning
transportation biofuels (bioethanol and MTHF). Allocation based on energy and exergy
content of products show similar results also for the other energy products and chemicals,
while allocation based on economic values increases the shares of chemical products, such as
furan resins, FUMA and mainly oxygen, while decreasing the environmental burdens
assigned to electricity, heat and biomethane.
The specific GHG emission factors for each product according to the allocation
procedure are listed in Table 10. In this table, GHG emissions per unit of product, i.e. km
driven for biofuels, GJ for electricity and heat, g for chemicals and gaseous biofuels, are
reported. For instance, these factors can be applied in a LCA if these products are used as
auxiliary materials in a future biobased society. Results of these biomass derived products and
services can be compared with those derived from oil refinery (reported in Table 3). For
