Is aquatic bioenergy with carbon capture and storage a sustainable negative emission technology? Insights from a spatially explicit environmental life-cycle assessment

Abstract

It is anticipated that achievement of the Paris Climate Agreement goals will require widespread deployment of negative emission technologies (NETs). The most prominent NET is bioenergy with carbon capture and storage (BECCS), which is typically envisioned to use terrestrial crops as feedstock. Several recent studies have focused on aquatic BECCS (A-BECCS), making use of microalgae or macroalgae as feedstock, as possible means of reducing water and land use. However, the high logistical complexity of the A-BECCS supply chain makes it likely that regional biophysical and socio-technical factors will strongly influence its overall favorability. Therefore, this study applies life-cycle assessment (LCA) incorporating a geographic information system (GIS) framework to estimate the environmental impacts of A-BECCS over all stages of its life-cycle. Three candidate locations in the USA are evaluated based on seemingly good proximity to coastal regions and CO2 storage; namely, East Coast, West Coast, and the Gulf of Mexico. Monte Carlo simulation is used to characterize distributions of model outputs, including energy return on investment (EROI) and net global warming potential (GWP). Results reveal that only the Gulf of Mexico configuration has any likelihood of achieving both net energy production (probability of EROI > 1 = 29%) and net CO2 sequestration (probability of GWP < 0 = 6%), but the probability of achieving both together is very low (5%). The other two locations exhibit net positive energy production (EROI > 1), but not net negative carbon sequestration. These results call into question the feasibility of the modeled A-BECCS system as an energy-producing NET and offer insights into possible system reconfiguration. For example, anaerobic digestion offers very low EROI and creates multiple carbon-bearing waste streams, which strongly undercuts overall net CO2 sequestration. Finally, it is observed that enhanced oil recovery (EOR) strongly contributes to net-energy production (EROI > 1) in the modeled A-BECCS system, but also strongly undercuts net CO2 sequestration, which is arguably the main goal of any NET. This analysis showcases how geographically-explicit analysis can advance our understanding of biomass-based NETs.