Bioenergy with or without carbon dioxide removal: influence of functional unit choice and parameter variability

PURPOSE. Bioenergy with carbon dioxide removal (CDR) is increasingly presented as an efficient way to mitigate climate change. This study set out to determine under which circumstances and methodological choices CDR bioenergy systems are preferable over a reference bioenergy system from a climate change mitigation perspective. In addition, the CDR systems investigated were compared to each other.METHODS. Three systems were modelled: two CDR systems (Biochar and Bioenergy with carbon capture and storage (BECCS)), with a combined heat and power (CHP) system as reference. A parametrised life cycle inventory model was developed and computed for all systems and four different functional units (FUs), resulting in distributions of climate impacts. Contribution analysis was performed, as well as a pairwise comparison of all scenarios to establish their ranking. First-order Sobol indices were computed to assess the contribution of each parameter to the variance. Whenever ranking of scenarios was largely dependent on parameter values, decision tree analysis was applied.RESULTS AND DISCUSSION. The CDR systems had a lower climate impact than CHP for most computations, across all FUs. However, when comparing the two CDR systems, the preferable system changed with FU; for heat or carbon sequestration as FU the Biochar system was preferable in general, while for electricity or biomass use as FU, the BECCS system had the lowest climate impact for most computations. For most system configurations, the contribution from energy substitutions were large and contributed to most of the variance in results. Furthermore, the ranking of systems depended on the reference activities of the background energy system.CONCLUSIONS. The results of this study indicate that the Biochar and BECCS systems are in general preferable over the reference CHP system from a climate mitigation perspective, particularly when the reference energy systems have a relatively low climate impact. However, FU and parameters affect the ranking of these three systems.RECOMMENDATIONS. When conducting a comparative LCA study for multifunctional systems, the LCA practitioner should be aware that choice of FU and specific system configurations, including reference activities, could potentially affect the climate impacts of the system, which in turn potentially affects the ranking among systems and conclusions communicated to decision makers. Conducting LCA for several FUs, with parametrised LCI, and contribution analysis, allows for a deeper analysis than traditional sensitivity analyses.