Environmental Engineering Reference
In-Depth Information
Conventional fumaric acid is currently produced from oxygenation of benzene (C
6
H
6
+
4O
2
→ C
4
H
4
O
4
+ 2 CO
2
H
2
). The chemical properties of fumaric acid can be anticipated from
its component functional groups. This weak acid forms a diester, it undergoes additions
across the double bond, and it is an excellent dienophile.
The O
2
produced by the biorefinery plant (via alkaline water electrolysis) replaces the
molecular oxygen (O
2
) that is currently produced from processing of air, from which O
2
is
separated. About 100 million tonnes of O
2
are extracted from air for industrial uses annually.
The most common method to recover O
2
is to fractionally-distill liquefied air into its various
components, with nitrogen N
2
distilling as a vapor while oxygen O
2
is left as a liquid
(Emsley, 2001).
4.2.5. Functional unit
The functional unit is the foundation of biorefinery systems LCA: it sets the scale for
comparison of different technological routes which a biomass feedstock can undergo in order to be
converted to biofuels and chemicals. One of the main purposes of the functional unit is to provide
a reference to which the input and output data are normalized and the basis by which the final
results are shown. For instance, the results of a bioenergy system from dedicated biomass crops
should be expressed on a per hectare basis, since the available area for the production of biomass
raw materials is the biggest bottleneck for the production of biofuels (Schlamadinger et al., 2005).
However, in order to be comparable, biorefinery results have the need to be independent from the
kind of biomass feedstock (dedicated crops or residues) and from the conversion processes which
act on the biomass raw materials. As a consequence, they cannot be expressed per hectare basis or
per unit of output. The most suitable functional unit is then the unit of biomass input which, in this
study, is the amount of biomass treated per year by the biorefinery: 530 ktonnes/a. Therefore, all
the input flows reported in the following inventory list and the final results of GHG emissions and
cumulated primary energy demand are referred to this amount of biomass input.
4.2.6 Allocation
Allocation in LCA is carried out to attribute shares of the total environmental impact to
the different products of a system. This concept is extremely important for biorefinery
systems, where multiple energy and material products are produced. The question of the most
suitable allocation procedure is still an open issue. Scientific LCA publications show benefits
and disadvantages of several allocation methods (Curran 2007; Ekval and Finnveden, 2001;
Frischknecht 2000; Wang et al., 2004).
The ISO standards suggest to avoid allocation by expanding system boundaries, when
possible. This method relies on the expansion of the product system to include the additional
functions related to the co-products. This procedure (also called substitution method) has the
advantage to avoid allocation issues while has the disadvantage to make the system too
complex, especially if multiple co-products are present (like in biorefineries). In fact, this
method relies on identification of a main product and the environmental benefits of co-
products are assumed as credits, which are subtracted to the total GHG emissions and the
remaining emissions are completely assigned to the main product. Furthermore, the
identification of one of the output as main product is an arbitrary choice and can be a difficult
decision in biorefinery systems, where multiple useful and valuable outputs are produced.
Therefore, system expansion is not recommended when an elevated number of high quality
outputs is produced; this situation can even lead to a negative value (i.e. below zero) of the
