Page 139 - Materials Chemistry, Second Edition

124 A. Bjørn et al.

Time 1 : 10 tonnes X þ 20 tonnes Y ¼ 10:000 kWh ð9:1Þ

Time 2 : 10 tonnes X þ 40 tonnes Y ¼ 12:000 kWh ð9:2Þ

Here, X and Y represent the electricity consumption of product x and
y (kWh/tonne) and by solving the equation system, one finds that X is
800 kWh/tonne and Y is 100 kWh/tonne. If time 1 is representative for the unit
process to be applied in the LCI model, then 80% (10 tonnes * 800 kWh/tonne
divided by 10.000 kWh) of the factory’s electricity consumption should be allo-
cated to product x. Note that this 80% allocation factor should not blindly be
applied to allocate the remaining flows (e.g. consumption of heat and emissions of
NO x ) between product x and y, for which the causal physical relationships may be
different. Note also that allocation according to a causal physical relationship is in
many cases not possible, because the ratio between co-products or co-services for
many processes cannot be changed. For example, it is not for practical purposes
possible to reduce or increase the production of straw, while keeping the production
of wheat constant.
The representative physical parameter approach is possible when co-products
provide a similar function. For example, in the case of a fractional distillation
process of crude oil, a similar function of many of the co-products (e.g. diesel,
petrol, kerosene, propane and bunker oil) is to serve as a fuel to drive a process
performing mechanical work, and therefore exergy, which can be interpreted as the
maximum useful work, is an appropriate representative physical parameter. The
parameter values of each co-product can typically be obtained from physical or
chemical compendiums (e.g. in the case of exergy values). Once the values have
been obtained, calculating the allocation factor is straightforward. For example, if
co-products x, y and z are produced in quantities 1, 3 and 6 kg and if their repre-
sentative physical parameter values are 10, 1, and 0.5 per kg, then the total
parameter value would be 16 i.e. (1 * 10 + 3 * 1 + 6 * 0.5) and the allocation
factor for product x would be 62.5% (1 * 10 divided by 16) and so on. Note that in
the distillation process case, the functions of the co-products are not entirely
identical. Airplanes cannot fly on bunker oil, and bitumen, one of the co-products,
cannot be used as a fuel. Allocating according to a representative physical
parameter is therefore not ideal, but may be the best solution, compared to other
allocation approaches. This example illustrates that there is often not a single
correct allocation approach and the choice of approach therefore depends on the
judgement of the LCA practitioner. The sensitivity of the LCA results to this
judgement may be investigated in a sensitivity analysis applying different possible
allocation factors, as explained in Sect. 9.6. Note that it is very important to choose
a representative parameter that is actually representative for the function of all
co-products. For example, mass is not a representative parameter for the
co-production of milk and meat from dairy cows because the functions of milk and
meat are not their mass. In this case, some measure of nutritional value would be a
more representative parameter.