Environmental Engineering Reference
In-Depth Information
lower part of Table 6: GHG and fossil energy savings are about 8% lower if heat is not used
to substitute oil-derived heat.
Comparison of the biorefinery with its fossil reference system shows that the biorefinery
can reduce GHG emissions of about 299.6 kt CO
2
-eq./a (0.66 t CO
2
-eq./t
dry wood
) and save
4527 TJ/a of fossil energy (10.05 GJ/t
dry wood
).
Even if the biorefinery system requires more than twice the primary energy demand of
the fossil reference system, it is mainly constituted (97%) by renewable energy, i.e. the
energy content of biomass feedstock, and the savings of fossil energy are relevant (almost
96%). As a consequence, the provision of biomass with sustainable practices is a crucial point
to ensure a renewable energy supply to biorefineries.
Table 8. Conversion yields and mass, carbon, energy and exergy
efficiencies of the biorefinery system.
Ethanol yield per t of feedstock (dry)
0.22
t/t
dry
Ethanol yield per ton of cellulose
0.49
t/t
cell
C conversion efficiency from wood to EtOH
22.45%
C conversion efficiency from cellulose to EtOH
56.98%
Furfural yield per t of feedstock (dry)
0.04
t/t
dry
Furfural yield per ton of xylan
0.68
t/t
xylan
C conversion efficiency from wood to furfural
5.35%
C conversion efficiency from hemicellulose to furfural
74.86%
Material products per t of feedstock (t
prod
/t
dry wood
)
a
2.96%
Energy products (net) per GJ of feedstock (GJ
prod
/GJ
wood
)
b
:
with heat
37.28%
without heat
35.12%
C conversion efficiency (from feedstock to products)
29.15%
Exergy conversion efficiency:
with heat
44.38%
without heat
42.95%
a
Only material products included (e.g. furan resins, FUMA, and O
2
)
b
Energy feedbacks to the plant subtracted (i.e. only final energy outputs included)
Biorefinery system performances in terms of product yields and mass, energy, exergy and
C conversion efficiencies are reported in Table 8. Bioethanol and furfural yields are
respectively 0.22 and 0.04 t per tonne of dry feedstock, but the yields increase up to 0.49 t of
bioethanol per tonne of cellulose and 0.78 t of furfural per tonne of xylan. The overall C
conversion efficiency of the plant from feedstock to products is 29.15% and the exergy
conversion efficiency is 44.38%. The material products of the biorefinery (i.e. furan resins,
FUMA and O
2
) constitute 2.96% of the total mass of the dry feedstock, while final net energy
products (bioethanol, MTHF, H
2
, biomethane, electricity and heat) store about 37% of the
raw material energy content. Without heat, this energy efficiency decreases to 35%. A
comparison can be done with other biomass conversion systems, which have energy
efficiencies higher than biorefinery. In fact, production of heat from wood combustion can
reach efficiencies higher than 85%, while in CHP application the overall energy efficiency is
around 80%. The reason is that a biorefinery system is made of several conversion steps and
biomass undergo chemical reactions and changes of state, while the aforementioned
