Page 165 - Materials Chemistry, Second Edition

150 A. Bjørn et al.

some flows are not monitored or reported. Errors in flow quantities may be caused
by errors in reported measurements (e.g. a technician writing ‘g’ instead of ‘mg’)or
errors in the calculation of flows and conversions of units (e.g. if one had forgotten
to convert the concentration unit in the example of Fig. 9.7). To avoid (critical)
incompleteness and quantitative errors, constructed unit processes should be
checked before they are used in an LCI model. Such a quality check can be
supported by calculation and interpretation of first iteration LCIA results, e.g.
through the identification of the most contributing process and substances.
Completeness of flows
There are three complementary approaches for validating the completeness of
flows.
1. Knowledge of similar processes can help identifying potentially missing flows.
For example, the LCA practitioner may suspect one or more missing flows, if a
unit process for a specific paper production process contains no chlorine con-
taining compounds in the wastewater to treatment and the practitioner knowns
from previous experience that chlorine compounds are typically present in the
effluent of paper production processes.
2. Knowledge of the nature of a physical transformation in a process can hint what
emissions or waste flows to treatment may be missing. For example, NO x gases
are known to be formed whenever a combustion process occurs in the presence
of nitrogen, the major constituent of atmospheric air. Filters can capture large
fractions of generated NO x before it becomes an emission, but usually not every
single molecule.
3. A qualitative comparison of input and output flows can show if there is dis-
agreement between the elements entering a process and the elements leaving a
process. For example, a process cannot emit large quantities of CO 2 , without
inputs of carbon sources in the form of fossil fuels (e.g. coal, natural gas or oil).
While using this validation technique it should be kept in mind that some flows
entering and leaving a process are elementarily heterogeneous. For example,
mercury is a common emission from the combustion of coal due to the mercury
content (typically in the order of 0.00001%) of the coal entering the process as a
heterogeneous material flow. In this case, the mercury input is ‘hidden’ in the
coal input and it would therefore be wrong to assume that a homogenous input
of mercury is missing on account of the emission of mercury.
Flow quantities

A unit process should obviously not only contain the right flows, but also the
right quantities of these flows. A number of validation approaches exist for
checking flow quantities.
A mass balance is a universal approach because the sum of flows entering a
process should amount to the same number as the sum of flows leaving a process
since no accumulation occurs inside the process. A mass balance is therefore an
efficient way of spotting errors, for example if the mass of outputs is on the order of