Page 113 - Materials Chemistry, Second Edition

3.3 Allocation 97

10 kg raw material

2 kg co-product 1
100 MJ energy
Unit process 1 4 kg CO
2
1 kg ancillary 2 kg waste
material 1
7 kg intermediate product

1 kg co-product 2.1
50 MJ energy 2 kg co-product 2.2
Unit process 2
2 kg CO 2
2 kg ancillary
material 2 3 kg waste

3 kg end-product

System boundary
Figure 3.12 System expansion, compared to allocation per mass in Figure 3.11.

mass’ loads to be allocated to the final product were calculated. In the case of system
expansion, calculated loads are, however, allocated to a product-mix of the total
system including final product, co-product 1, co-product 2.1, and co-product 2.2.
With this method very large systems may be studied. This can make sense
for an overall representation of environmental loads of large industrial systems. 81)
Usually, however, in accordance with the definition of the goal and the specification
of an appropriate fU an allocation of load to specific products is required. Thus in
the case of system expansion the following additional problems have to be solved:

• The fU must be revised, since the performance of assessed co-products must be
included. This is an added value since it occurs in addition to the performance of
the examined product. If two products are compared with each other, identical
co-products rarely ever occur and thereby different added values have to be
considered. In order to compare the systems, the system symmetry has to be
restored again. In Section 2.2.5.3 (Figure 2.5) it has already been discussed in an
example of the waste industry as to how the added value by system expansion
can be included.
• System expansion can result in intransparent large systems, especially for a yield
of multiple co-products, in their turn used for multiple applications.
• Since all co-products must be analysed and downstream assessed, a substantially
increased effort at investigation is necessary.

81) Tiedemann, 2000.