(258a) Energy Utilization Strategies in Lignocellulosic Biorefineries

The adoption and development of second-generation lignocellulosic biorefineries can play a critical role towards a sustainable energy future¹. Lignocellulosic biorefineries not only offer a renewable and reliable source of bioenergy but also contribute to greenhouse gas (GHG) reduction, energy security, and can drive economic growth. The majority of system-level studies focus on converting excess chemical energy from side streams (e.g., lignin, biogas from anaerobic digestion, wastewater sludge, and conversion residue) into a single co-product, typically electricity^2,3. These studies have demonstrated that co-producing electricity can enhance overall energy efficiency and help displace fossil-based grid emissions. Although, this approach offers a straightforward means to utilize surplus energy, generate additional revenue, and integrate renewable energy into existing power infrastructure, it may not necessarily constitute the best way of utilizing these energy-rich side streams. Alternative strategies may lead to systems that are economically more attractive and more sustainable.

Upgrading biogas to biomethane is one such alternative ⁴. Biomethane, a renewable fuel, can be directly injected into natural gas pipelines. However, research on capturing and upgrading biogas to biomethane in second-generation biorefinery design remains limited. Lignin valorization is another promising alternative, with considerable research focused on studying various technologies and products, placing special emphasis on advancing catalytic approaches to enhance key economic drivers^5,6. Utilizing the surplus chemical energy in the side streams for heat generation further enables the integration of carbon capture and storage (CCS), which can significantly reduce GHG emissions^7–9.

While these strategies have been proposed, it remains unclear which one is the best in terms of economic viability, environmental sustainability, and overall system performance; as well as under what incentive structure. Furthermore, it is unclear how the use of natural gas for CO₂ capture impacts the economic potential and fossil-based emissions of biorefineries. Additionally, the impact of numerous factors (e.g., renewable energy and fuel incentives, CO₂ sequestration credits, energy demand of co-product technologies, and co-product selling prices) is not fully understood.

To address this gap, we assess and compare different lignocellulosic biorefinery systems to identify key drivers, evaluate trade-offs, and draw insights into system performance. We study three co-production baseline systems: (1) surplus electricity generation (B_EL), (2) upgrading biogas to biomethane (B_BM), and (3) valorizing lignin into high value bioproduct (B_LV). We examine carbon and energy flows, economic potential, carbon footprint (CF), as well as overall energy efficiency of these baseline systems. We then study a biorefinery with carbon capture (B^CCS) and subsequently explore the implications of natural gas usage. Finally, we extend the scope of our study to investigate the impacts of key parameters on system economics and CF, and further explore options for CO₂ mitigation by integrating carbon capture in the three baseline systems.

The attached figure shows the key metrics (energy efficiency, carbon efficiency, minimum fuel selling price, net carbon footprint) of the three co-production baseline systems, and the carbon capture system. We observe that from an energy perspective, the B_BM system has the highest efficiency (~58%) due to biomethane’s high specific energy. The B^CCS system has the highest carbon efficiency, but this is strongly linked to the efficiency of the capturing process. If we narrow down to the three co-production systems, the B_BM system achieves the highest carbon efficiency.

From the economic viewpoint, the B_BM system has the lowest minimum fuel selling price (MFSP) at US$3.94 GGE^-1 mainly due to numerous financial incentives associated with selling biomethane, while the B_LV system has the highest MFSP.

From a sustainability standpoint, the B^CCS system has the lowest net CF. If we narrow down to the three co-production baselines, the B_EL system exhibits the lowest CF, with a net value of 16.5 gCO₂e MJ^-1, followed by B_BM (36.9 gCO₂e MJ^-1) and B_LV systems (53.1 gCO₂e MJ^-1). The B_EL offsets approximately 23.2 gCO₂e MJ^-1 of emissions (based on the average emission factor of the US electricity grid). However, if electricity sales result in emission offset from different power sources (coal, natural gas, nuclear, and diesel power plants), the amount displaced, and consequently the net CF, would vary as indicated by the black vertical line in the B_EL net CF figure. Producing biomethane enables 2.8 gCO₂e MJ^-1 offset, while 11.6 gCO₂e MJ^-1 is displaced for acetic acid production via lignin valorization.

We also perform sensitivity analyses to determine how variations in key parameters impact the economics, carbon footprint, and overall system performance.

References

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3. Davis, R. E. et al. Process Design and Economics for the Conversion of Lignocellulosic Biomass to Hydrocarbon Fuels and Coproducts: 2018 Biochemical Design Case Update; Biochemical Deconstruction and Conversion of Biomass to Fuels and Products via Integrated Biorefinery Pathways. NREL/TP--5100-71949, 1483234 http://www.osti.gov/servlets/purl/1483234/ (2018) doi:10.2172/1483234.
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