• Energy Research

Decarbonizing the air transportation sector remains one of the most challenging hurdles to mitigating climate change.

Author(s): Yang, M; Dahlberg, J; Baral, NR; Putnam, D; Scown, CD | Abstract: Forage sorghum is a promising feedstock for the production of biofuels and bioproducts because it is drought tolerant, high-yielding, and familiar to farmers across the world. However, sorghum spans a diverse range of phenotypes, and it is unclear which are most desirable as bioenergy feedstocks. This paper explores four forage sorghum types, including brown-midrib (bmr), non-bmr, photoperiod sensitive (PS), and photoperiod insensitive (non-PS), from the perspective of their impact on minimum bioethanol selling price (MESP) at an ionic liquid pretreatment-based biorefinery. Among these types, there are tradeoffs between biomass yield, lignin content, and starch and sugar contents. High biomass-yielding PS varieties have previously been considered preferable for bioenergy production, but, if most starch and sugars from the panicle are retained during storage, use of non-PS sorghum may result in lower-cost biofuels (MESP of $1.26/L-gasoline equivalent). If advances in lignin utilization increase its value such that it can be dried and sold for $0.50/kg, the MESP for each scenario is lowered and non-bmr varieties become the most attractive option (MESP of $1.08/L-gasoline equivalent). While bmr varieties have lower lignin content, their comparatively lower biomass yield results in higher transportation costs that negate its fuel-yield advantage.

Although ethanol remains the dominant liquid biofuel in the global market, there is a strong interest in high-energy density and low-hygroscopicity compounds that can be incorporated into gasoline at levels beyond the current ethanol blend wall. Isopentenol (3-methyl-3-buten-1-ol) is one of these promising advanced biofuels that is also an important precursor for isoprene (the main component of natural rubber). In this study, we model the production cost, greenhouse gas (GHG) emissions, and water footprint of biologically produced isopentenol, including the current state of the technology and the impact of potential improvements. We find that the minimum selling price of biobased isopentenol, given the current state of technology demonstrated at bench-scale, is $5.14/L-gasoline equivalent, and the GHG footprint exceeds that of gasoline. However, biobased isopentenol could reach a $0.62/L-gasoline equivalent [$2.4/gal-gasoline equivalent (gge), just 5% above the 10-year average gasoline price] in an optimized future case where yield and other process parameters are pushed to near their theoretical limits. In this future case, isopentenol could achieve a GHG reduction of 90% relative to gasoline and a carbon abatement cost of $9.3/metric ton CO2e. Reaching these goals will require dramatic improvements in isopentenol yield, near-100% recovery of ionic liquid used in pretreatment, and low-lignin and high-cellulose and-hemicellulose biomass feedstocks.

Scalable, low-cost biofuel and biochemical production can accelerate progress on the path to a more circular carbon economy and reduced dependence on crude oil. Rather than producing a single fuel product, lignocellulosic biorefineries have the potential to serve as hubs for the production of fuels, production of petrochemical replacements, and treatment of high-moisture organic waste. A detailed techno-economic analysis and life-cycle greenhouse gas assessment are developed to explore the cost and emission impacts of integrated corn stover-to-ethanol biorefineries that incorporate both codigestion of organic wastes and different strategies for utilizing biogas, including onsite energy generation, upgrading to bio-compressed natural gas (bioCNG), conversion to poly(3-hydroxybutyrate) (PHB) bioplastic, and conversion to single-cell protein (SCP). We find that codigesting manure or a combination of manure and food waste alongside process wastewater can reduce the biorefinery's total costs per metric ton of CO2 equivalent mitigated by half or more. Upgrading biogas to bioCNG is the most cost-effective climate mitigation strategy, while upgrading biogas to PHB or SCP is competitive with combusting biogas onsite.

Significance Cellulosic biofuels have not yet reached cost parity with conventional petroleum fuels. One strategy to address this challenge is to generate valuable coproducts alongside biofuels. Engineering bioenergy crops to generate value-added bioproducts in planta can reduce input requirements relative to microbial chassis and skip costly deconstruction and conversion steps. Although rapid progress has been made in plant metabolic engineering, there has been no systematic analysis devoted to quantifying the impact of such engineered bioenergy crops on biorefinery economics. Here, we provide new insights into how bioproduct accumulation in planta affects biofuel selling prices. We present the range of bioproduct selling prices and accumulation rates needed to compensate for additional extraction steps and reach a target $2.50/gal minimum biofuel selling price.

Technoeconomic analysis (TEA) is an approach for conducting process design and simulation, informed by empirical data, to estimate capital costs, operating costs, mass balances, and energy balances for a commercial scale biorefinery. TEA serves as a useful method to screen potential research priorities, identify cost bottlenecks at the earliest stages of research, and provide the mass and energy data needed to conduct life-cycle environmental assessments. Recent studies have produced new tools and methods to enable faster iteration on potential designs, more robust uncertainty analysis, and greater accessibility through the use of open-source platforms. There is also a trend toward more expansive system boundaries to incorporate the impact of policy incentives, use-phase performance differences, and potential impacts on global market supply.

The future bioeconomy promises drop-in or performance-advantaged biofuels and bioproducts derived from lignocellulosic biomass, substantial greenhouse gas emissions reductions in sectors with few or no alternatives, and increased domestic energy production in countries with sufficient biomass resources. Despite the slower than anticipated pace of commercializing next-generation biofuels, the research community continues to make dramatic improvements at every stage of production, from feedstock cultivation through conversion to final products. However, the interdisciplinary nature of bioenergy research, and the need for cross-coordination among biologists, chemists, agronomists, and engineers, make coordinating and optimizing these strategies challenging. This Perspective surveys recent advancements in bioenergy crop engineering, lignocellulosic biomass deconstruction and fractionation, catabolism of biomass-derived sugars and aromatics, and biological conversion to fuels and products. We organize major research approaches into broad categories and comment on which strategies offer synergies or trade-offs in the context of four approaches to improving the economics and carbon-efficiency of advanced biofuels and bioproducts: (1) maximize sugar conversion to a single product, (2) utilize diverse carbon sources for producing a single product, (3) convert lignin to high-value products, and (4) fractionate the hydrolysate to derive maximum value from each component.