(438b) The Role for Synthetic and Biofuel Pathways in an Integrated, Net-Zero Energy System

Integrated energy systems modeling is a key tool for identifying cost-effective, resilient, and equitable decarbonization pathways that account for various spatial, temporal, and technological interactions. While electrification of final energy is a key pillar for economy-wide decarbonization, the practical challenges associated with the electrification of certain end-uses as well as intermittency of variable renewable energy (VRE) electricity supply creates opportunities for a range of other vectors and technologies to achieve a net-zero emissions goal. These include the utilization of alternative energy carriers such as hydrogen (H2), biofuels and other synthetic fuels, as well as negative emission technologies (NETs) such as direct air capture (DAC) and bioenergy with carbon capture and sequestration (BECCS). The goal of this study is to quantify the drivers impacting the role for synthetic fuel and biofuel processes as part of the least-cost solution of a net-zero integrated energy system, while considering their interaction with supply chains for electricity, H2, and CO2, to satisfy specified final energy demands across end-uses in transportation, buildings, and industry.

Our approach relies on formulating and solving a capacity expansion model (CEM) that accounts for spatial and temporal variations in resources, supply and demand balances for each energy vector, as well as investment and operational cost characterization. The open-source modeling framework, EnX, builds on a previously developed open-source CEM for electricity and H2 infrastructure planning [1], and takes the perspective of a central planner to minimize operating and capital costs associated with the supply chain of each vector (electricity, CO2, H2, biomass, liquid fuels and methane). Uniquely, the EnX modeling framework allows for evaluating a range of technological options for meeting demand for liquid and gaseous fuels under net-zero emissions constraints while considering their interaction effects with the supply chain of other vectors. For liquid fuels and methane production, the model considers both synthetic fuel production from captured CO2 as well as biofuel production, along with the option of continuing to use fossil fuels in conjunction with NETs. For the biofuel and synthetic fuel processes, the model accounts for various liquid products (jet, gasoline, diesel) and their disposition along with the potential for sequestering or venting any unconverted CO2 in the process. In addition, the model emissions accounting boundary also incorporates energy needs and can incorporate non-CO2 greenhouse gases associated with production of primary energy sources like biomass and fossil fuels.

We apply the EnX model to explore net-zero energy system outcomes for the continental U.S. using a 9-zone spatial representation and considering 11 weeks of system operations at an hourly resolution to approximate temporal variability in energy supply and demand. We use the model to shed light on the value of alternative biofuel and other synthetic fuel processes under a net-zero emissions constraint, exogeneous final energy demands for H2, electricity, liquid and methane and varying assumptions about availability of biomass resources and CO2 sequestration sites. When CO2 sequestration and biomass resources are highly constrained, we find that synthetic fuels processes with high carbon conversion efficiency are favored for liquid fuel production, where the H2 is produced primarily from electrolysis and CO2 sourced from DAC. On the other hand, when both CO2 sequestration and biomass resources are less constrained, there is greater reliance on NETs, primarily via BECCs, to offset emissions from using fossil fuels to meet liquid demand.