In a recent training, a question came up whether a common modelling and calculation is correct which is maybe interesting for others too; this post therefore tries to explain, using a very simple LCA model and system, which seems rather straightforward:

The picture shows a simple supply chain for aluminium, from recycled and first use aluminium. Process ‘aluminium scrap production’, bottom right, takes waste, aluminium scrap, and produces a product, aluminium scrap mix, which is again used in ingot casting to produce ingot. The aluminium scrap production process looks like this (amounts are of course simplified and meant for demonstration purposes only).

No allocation is applied:

openLCA calculates this system as follows (only inventory calculation, for 1000 kg ingot):


ingot casting inventory calculation results, no allocation

Outputs are e.g. 3 kg CO2, where aluminium scrap production contributes with 2 kg CO2, while the aluminium scrap production “itself” has 1 kg CO2 emissions (see above), thus this process is used twice in the system.

So, what is going on?

Aluminium scrap production has two functions, waste treatment and production of the product, the prepared aluminium scrap. There is no allocation specified. If no allocation is specified, in LCA, a process always fully contributes to a product that is used in a system; here, the process fully contributes with its two products, and thus, effectively, it contributes twice to the system. You can also say that the same process needs to fulfil and is “squeezed” between two conditions at the same time, which reminded me of a pincer.

In the calculated life cycle inventory, this also shows: there are 126 kg aluminium scrap mix output left, this occurs when the aluminium scrap production is used for the waste treatment, and there are 100 kg aluminium waste treatment input left, from when the aluminium scrap production is used for the production of aluminium scrap mix.


ingot casting calculation results, no allocation, showing contributing processes

The LCA calculation is thus not really intelligent; it does not see that in this special case here, the remaining product from the waste treatment can directly be used in the system, since the process already provides the correct amount, and does not require the same process a second time in this special case here; since the additionally produced product and the additionally treated waste are shown in the inventory result, the calculation is correct.

It calculates an unnecessary big system, however. If you add allocation, e.g. physical allocation..

The result is very different:


ingot casting calculation result, physical allocation

Now, we do not have any inputs, and we have less than 2 kg CO2 output. Reason for having even less than 2 kg is that the allocation considers three products of aluminium scrap production, the third being uptake, i.e. treatment, of aluminium from public waste collection, which however is not used in the system, and thus excluded. As consequence, the aluminium scrap production process is even not considered entirely, but only to 89.7%.

This small case maybe, and hopefully, is a somewhat instructive example of allocation; further, further, we see that a waste treatment process which produces a product from the waste treatment is of course a multifunctional process, and this multifunctionality needs to be considered in the modelling, in one way or other. Comments welcome!