Page 83 - Materials Chemistry, Second Edition

3.1 Basics 67

instance, via an operational input–output analysis of a factory in the context of
an environmental management system (see Section 1.1.5). In general, however, a
factory produces several products; inputs and outputs need to be attributed to these
products according to defined rules. For data acquisition in a factory in line with
the processes leading to a specific product, assignment is unnecessary because the
data are already available. This procedure is recommended by ISO but requires
much more data collection work.
Data acquisition is one of the most complex phases of LCA (see the Pellston
24)
Workshop on Global Guidance Principles for LCA Databases ), especially when
site-specific upstream and downstream data are required.

3.1.3.2 Balancing
Theoretically, a complete energy and mass assessment (input and output) should
be conducted for every unit process. In praxis this often fails due to the inadequacy
of the data: Usually the waste heat is not measured, the waste water output
is set equal to the fresh water input, the greenhouse gas CO formed during
2
combustion is usually not measured but calculated assuming an approximate
stoichiometry, for example, for long-chain aliphatic hydrocarbons. In the simplest
case, the combustion of methane (main part of natural gas), Equation 3.1a is
valid:

CH + 2O → CO + 2H O (3.1a)
2
2
2
4
For petrol, for example, Equation 3.1b is valid on the simplified assumption that it
contains pure Octane. 25)

2C H + 25O → 16CO + 18H O (3.1b)
8 18 2 2 2
−1
According to this equation, the combustion of 1 l petrol (average density 740 g l )
results in the release of 2.28 kg CO .
2
The principle conservation of mass cannot be applied in such cases as its validity
is a prerequisite of the equation. If the empirical basis of the chemical equation
is known, calculations as quoted are very precise. This, for instance, applies for
the formation of sulphur dioxide from the sulphur content of fuels, as it can very
securely be presumed that, via combustion, one molecule of SO is formed from
2
every single sulphur atom (Equation 3.1c).

S + O → SO 2 (3.1c)
2

24) Sonnemann and Vigon (2011).
25) Falbe and Reglitz (1995, p. 351).