Calculating the influence of alternative allocation scenarios in fossil fuel chains

Goal, Scope and Background

As part of an LCA study comparing an average Dutch passenger car running on petrol with a similar car running on bio-ethanol and comparing an average Dutch passenger car running on diesel with a similar car running on biodiesel, the question raised to get more insight into the allocations made in fossil fuel chains in existing data bases such as ecoinvent. Both biofuel and fossil fuel chains contain various allocation situations that have been approached differently by various authors leading to differing and incomparable results. For biofuel chains, stakeholders had obtained insight into the allocations in earlier studies, but for the allocations made for the fossil chains, this was not the case. Therefore, one part of the study, which is reported in this paper, focused on a quick scan of different allocation scenarios for fossil fuels chains using the Swiss ecoinvent v1.1 database.

Methods

The quick scan focused on three different allocation scenarios for fossil fuel chains: economic allocation, physical allocation and the ecoinvent default allocation. There appeared to be 54 multi-output (MO) processes linked to both the passenger car and the diesel system in the ecoinvent v1.1 database. Based on contribution analyses identifying which multi-output processes contribute most to one of the environmental impact categories of the characterisation, seven multi-output processes were selected that have been further analysed with the three allocation scenarios mentioned.

Results

The results show that although at the process level allocation factors may differ significantly (up to almost 250), the total results only differ modestly (1–1.5), at least for the present case.

Discussion

There is no general rule between these two. They depend on the scaling factor and the environmental impact related to the resource extractions and emissions of a particular multi-output process and its upstream processes in the total system analysed.

Conclusions

The results of this quick scan are mainly intended for illustrating and learning purposes focusing on the possible influence of different allocation scenarios for fossil fuel chains. Bearing these limitations in mind, it can be concluded that different allocation methods can generate large differences in allocation factors and thus also at the level of environmental impacts allocated to the derived single-output processes. Nevertheless, the aggregated results for the present case only differ modestly.