Page 115 - Materials Chemistry, Second Edition

3.3 Allocation 99

A very good example of system reduction is the NaOH production in the
context of chlorine alkali electrolysis according to the amalgam process, 86) a
typical coupled production (Equation 3.10):

2NaCl + E → 2Na + Cl (gas) (3.10a)
el 2
2Na + 2H O → 2NaOH + H (gas) (3.10b)
2
2
−−−−−−−−−−−−−−−−−−−−−−−−
2NaCl + 2H O + E → 2NaOH + Cl + H (3.10)
2 el 2 2
Commercially useful products are primarily chlorine gas (Cl ) and sodium
2
hydroxide (NaOH). Besides, hydrogen gas is formed which is mostly used
thermally in the factory. The sodium is not isolated as Na-metal but dissolves
to sodium amalgam in the liquid mercury electrode, which in a second
stage decomposes into caustic soda solution (Equation 3.10b). This solution
must be concentrated for transport and sale for which thermal energy is
necessary, which can be unambiguously attributed to this production step. As
the remaining energy demand is supplied by electricity, it is justified to assign
the entire thermal energy (primary energy carriers are usually fossil fuels)
to the production of the caustic soda solution alone. Electricity, however, must
be assigned to the three products (NaOH, Cl and H ). The allocation problem
2 2
is thus only partly solved by system reduction.
4. Physical causation: Scientific-technical arguments can be a reason for the alloca-
tion of environmental loads in defined sub processes. A frequent application is
the assessment of emissions of an incineration plant, which are to be allocated
to the assessed product to be burnt as waste. If the ingredients of the waste
are known by type and quantity, its oxidation products can be ideally calculated
according to chemical stoichiometry, and thereby a justified estimation of
emissions in the exhaust air can be made. Because of different conditions in
the incineration processes in reality compared to controlled oxidation processes
of single substances, the calculation of toxic emissions as traces can only with
87)
great difficulty be allocated to individual wastes. A detailed description of the
difficulties with allocation of emissions to single waste materials as well as
possible solutions can be found in (UBA, 2000, p. 81 ff., loc. cit).
Boundaries between (3) and (4) are fluent. The allocation of the thermal
energy to NaOH as illustrated above has physical causes; it can, however, only
be calculated following system reduction.
Only if steps (1) to (4) fail, a need for allocation rules is required according to
ISO 14044, which finally can only be provided by agreement (convention) and
whose application, case by case, must be justified and secured by sensitivity
analysis (see Chapter 5).

86) Boustead, 1994b.
87) Tiedemann, 2000.