How to avoid multiplying upstream process impacts when outputs go to different downstream processes?

Question

Hi,

I am modelling a biomass conversion process in openLCA (ELCD 3.2 database).

My system has the following structure:

• Process A (main conversion process) takes 1 kg of biomass as input and generates 4 outputs: product 1, product 2, product 3 and product 4
• Each output goes to a different downstream valorisation process (B, C, D and E respectively)
• A final aggregator process collects all valorised outputs and has Treated Biomass (1 kg) as its reference output

When I calculate the product system impact, Process A appears 4 times in the contribution tree, once for each downstream process, and its direct impacts are counted 4 times instead of once.

Additionally, in one scenario, one of the 4 outputs is partially fed back into Process A as an internal input for heating purposes. This creates a circular reference in the product system, which further multiplies the impacts of the recirculation process incorrectly.

How can I ensure that the direct impacts of Process A are only counted once in the final result, and not once per output product? And what is the correct approach to model internal recirculation of a by-product without creating circular references or incorrect impact multiplication?

Thank you!

Answer 1

In the result calculation, the process is contributing to each of the supply chain branches where it is used with the specific amount required in each branch. If you check the impact analysis, you will see the process only once, with all impacts combined. The contribution tree intentionally splits the entire system into different branches, so that you can see the impact of different branches of your life cycle on the result. You do not need to follow specific modeling tricks or so to make openLCA calculate this correctly.

Sometimes, in systems with many loops, the "linearisation" into branches that is done for the contribution tree is difficult and openLCA then uses an approximation. The entire result, as shown in the impact analysis, is not affected by this.

Hth, Andreas