Page 166 - Materials Chemistry, Second Edition

9 Life Cycle Inventory Analysis 151

1000 times the mass of inputs. Note, however, that flow quantities may be correct,
even if the law of conservation of mass seems to be violated. This is because most
of the constituents of atmospheric air, e.g. oxygen and nitrogen, are generally not
counted as resource inputs in unit processes, in which case the mass of outputs
appear larger than the mass of inputs (e.g. due to combustion products such as CO 2,
H 2 O and NO x ). A mass balance can also be applied at the level of individual
elements, but one should be aware of ‘hidden’ elements in heterogeneous flows, as
described above. Energy balances can in principle also be used as a validation
approach, but this would require calculations of the chemical energy stored in
inputs and outputs and quantification of heat lost to the environment, which is often
not reported as an emission in a unit process.
Following validation based on mass balance a complementary validation based
on stoichiometry can be carried out if the process to be validated involves one or
more chemical reactions. This serves to check if the ratio between inputs and
outputs involved in a chemical reaction is correct. For example, stoichiometry gives
us the correct ratio between inputs and outputs in the electrolysis of water in the
presence of sodium chloride: 2NaCl + 2H 2 O ! 2NaOH + H 2 +Cl 2 . The mass
(g) of each molecule can then be calculated by multiplying its stoichiometric
coefficient (mole) and its molar mass (g/mole).
Other validation approaches rely on comparisons to external information. This
could be information for similar processes that are expected to contain flows of
similar magnitudes as the process to be validated. The external information could
also be legal limits. For example, if an emission of nitrogen dioxide corresponds to
100 times a regulatory emission limit, it is a strong indication that there is an error
in the emission quantity (note however that many regulatory limits are given as
concentrations rather than mass flows, in which case a conversion is needed).
Yet another validation approach relies on the first iteration of LCIA results.
These are useful for identifying erroneously high flow quantities. For example, if
the contribution from a single elementary flow of a single unit process contributes
with 99.9% of the impact for an impact category, this is a strong indication that the
flow quantity is too high (e.g. due to a factor 1000 unit conversion mistake in a
calculation or data entry in the LCA software). This validation approach can also be
used to check for mistakes in the ID of an elementary flow, such as mistakenly
using the name ‘dioxin’ for an emission of ‘carbon dioxide’ (dioxin is a group of
extremely toxic chemicals).

9.4.2 Using Flow Names Compatible with LCA Software

To prepare a unit processes for use in an LCI model it is important that the LCA
software used ‘understands’ the identity of the flows of the unit process. If this is
not the case, a flow cannot be linked correctly to other processes or characterisation
factors (in the case of elementary flows). There have been attempts at harmonising
flow names across LCI databases and LCA software, but the LCA practitioner