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Emerging CO2 to SAF pathways facilitates diversification of fuel production with the potential for a near carbon neutral footprint.

The drive to reduce consumption of fossil resources, coupled with expanding capacity for renewable electricity, invites the exploration of new routes to utilize this energy for the sustainable prod...

The increasing availability of renewable electricity at costs competitive with, and even lower than, electricity from fossil sources along with growing interest and recent technological advancements in reducing carbon emissions through CO2 capture is challenging the status quo in the way that we produce and consume energy and products. Renewable electricity can be leveraged to produce fuels and chemicals from CO2, offering sustainable routes to reduce the carbon intensity of our energy and products-driven economy. A number of approaches have been developed for the electron-driven reduction of CO2 to products, including both direct and indirect (via an energy carrier such as H2) pathways and spanning from electrochemical to biological to thermocatalytic conversion. While these approaches are at various stages of development, there are technical barriers related to each core conversion technology that need to be addressed in order to accelerate commercialization and drive the transition towards a circular carbon economy. In this perspective, we assess and characterize the top technical barriers for utilizing renewable electricity for CO2 reduction across five different conversion approaches (direct electrochemical, direct bioelectrochemical, direct non-thermal plasma, indirect bioelectrochemical, and indirect thermochemical) under state-of-technology conditions, outline the R&D needs to overcome each barrier, and identify the most promising C1–C3 hydrocarbons and oxygenates based on their relative ease of formation, economic viability, CO2 utilization potential, and energy storage capacity. Our analysis suggests, based on current reported states of technology, that indirect pathways paired with the formation of C1 products offer the most technically feasible approach for electron driven CO2 reduction in the near term. However, as we strive for longer carbon chain molecules, and as technologies continue to advance, there are a multitude of advantages and limitations to be considered for all five approaches.

Five economically-advantaged biomass feedstocks identified in the United States Department of Energy’s 2016 Billion Ton Study were gasified and the syngas was reformed at the bench scale to study the feedstock price-performance relationship. The distribution of reformed syngas compositions, heating values, and yields were similar across the different feedstocks and blends thereof, which ranged from inexpensive residual wastes to more expensive and higher quality biomass, revealing that feedstock performance was mostly insensitive to its price. Custom blended feedstocks produced syngas with characteristics resembling linear combinations of syngas from single-component feedstocks, supporting the ability to customize and predict blended properties based on single-feedstock data. The experimental gasification data informed a techno-economic analysis of specific feedstock costs for producing high-octane gasoline, and the results showed that miscanthus and forest residues were the most cost-effective. A field-to-wheels life-cycle assessment of greenhouse gas emissions showed that forest residues was the most environmentally benign feedstock of those studied.

Synthesis gas (syngas), composed primarily of H2 and CO, can be produced from fossil resources, municipal solid waste, biogas, and terrestrial biomass and can be converted into oxygenated intermediates such as alcohols and aldehydes through both catalytic and biological routes. These oxygenates serve as precursors for the downstream production of fuels and chemicals. However, since these processes all proceed through syngas regardless of the feedstock, renewable resources do not offer any inherent chemical advantage over fossil resources, and the process economics is largely dictated by (i) the spread between feedstock cost and the cost of petroleum (the dominant existing feedstock for fuel and chemical production) and (ii) the conversion efficiency, in terms of both energy and carbon, normalized by capital costs. Thus, lower-cost renewable feedstocks and improved conversion efficiencies combined with policy incentives could enable increased incorporation of biocontent into fuels and chemicals through syn...

Global demand for methanol as both a chemical precursor and a fuel additive is rising. At the same time, numerous renewable methanol production pathways are under development, which, if commercialized, could provide significant environmental benefits over traditional methanol synthesis pathways. However, it is difficult to compare technologies at different maturity levels, with differing feedstocks, and with significant differences in overall process design. Thus, there is a need to harmonize the analyses of renewable pathways using a consistent techno-economic approach to evaluate the potential for commercialization of various pathways. This analysis uses a novel cross-comparison method to assess near-term and long-term viability of both low- and high-maturity level technologies. The techno-economic assessment considers cost factors critical to market acceptance combined with carbon- and energy-efficiency assessments of three renewable pathways compared with a commercial baseline. We find that biomass gasification to methanol represents a near-term viable pathway with a high technology readiness level and commercially competitive market price. If cost-reducing technological improvements can be realized and scaled up in the CO2 electrolysis pathways, the potential for higher carbon efficiencies may help drive market adoption of these more modular, direct conversion pathways in future markets as they present an opportunity to better support global decarbonization efforts through efficient waste carbon utilization.

The ability of degradable fibers to reduce the settling rate of ceramic proppant in a guar-based solution was studied as a function of fiber composition and shape. Various cellulose ester fibers were benchmarked against a PLA standard. Aqueous (2 wt% KCl in DI water) mixtures of degradable fiber, ceramic proppant, and guar were consistently mixed then allowed to settle. The position of the interface between the solids-rich bottom layer and solids-poor top layer was measured as a function of time for various fiber compositions, shapes, and loadings. Settling experiments were also repeated to determine measurement error. Viscosity measurements were performed both to determine appropriate guar loading and to investigate the effect of fibers on viscosity. Consistent with recent results published in the patent literature for PLA, fiber shape was found to have a strong effect on proppant suspension. However, these effects went well beyond those found for aspect ratio and degree of crimping reported previously. Indeed, fiber cross-sectional shape was found to have a very strong effect with "X" and trilobal shapes showing better proppant suspension capability than round fibers. Fiber loading with shaped fibers could be reduced up to half of that round fibers with the same proppant suspension performance. The degradable polymer composition may play some role in proppant suspension but it appears to be secondary relative to that of fiber shape. Degradable fibers with improved performance should enable broader consideration of their use for proppant transport, proppant suspension, and heterogeneous proppant placement in fiber-based hydraulic fracturing. Other stimulation applications may also value alternative degradable materials.

When clathrate hydrates of xenon gas are formed deep within the stability field, anomalous melting behavior is readily observed in differential scanning calorimetry (DSC). In the DSC thermograms, multiple dissociation events may be observed, suggesting the presence of more than one solid phase. Following a suite of diffraction and NMR measurements, we are only able to detect the presence of simple structure I hydrate. Recognizing that hydrates are nonstoichiometric compounds, we look back to how the molar composition of a hydrate phase is determined. Making a mean-field improvement to current equilibrium models, we find that some conditions yield multiple solutions to the cage filling of the hydrate phase. Though the solutions are not truly stable, they would result in a kinetically trapped system. If such a case existed experimentally, this could explain the dissociation behavior observed for xenon hydrates. More importantly, this raises the question of how well defined the equilibrium condition is for a cell potential model, and whether or not multiple equilibrium solutions could exist.

Clathrate hydrates are crystalline materials made of water molecules forming host cages within which guest molecules are located. The hydrogen bond network ensuring the stability of the host substructure includes ionic defects, having an impact on the physicochemical properties of the systems. In this paper, a new way of introducing these ionic defects is proposed. Type II clathrate hydrates mixing tetrahydrofuran (THF) and perchloric acid guest molecules are synthesized and investigated by means of calorimetric, X-ray diffraction, and Raman scattering measurements together with a computational structure relaxation in the density functional theory approximation. The formation of the mixed clathrate hydrate with perchlorate anion included in the large cage of the cationic host-substructure of the THF type II clathrate hydrate requires the cooling of a (1-α) THF·αHClO4·17H2O solution with α less than 0.125. Above this inherent limitation, a multiphasic regime is observed in the formation of clathrate hydrat...

When hydrogen (H2) is mixed with small amounts of methane (CH4), the conditions required for clathrate hydrate formation can be significantly reduced when compared to that of simple H2 hydrate. With growing demand for CH4 as a commercially viable source of energy, H2 + CH4 binary hydrates may be more appealing than extensively studied H2 + tetrahydrofuran (THF) hydrates from an energy density standpoint. Using Raman spectroscopic and powder X-ray diffraction measurements, we show that hydrate structure and storage capacities of H2 + CH4 mixed hydrates are largely dependent on the composition of the initial gas mixture, total system pressure, and formation period. In some cases, H2 + CH4 hydrate kinetically forms structure I first, even though the thermodynamically stable phase is structure II.

Previously, large cage occupancy of H2 has only been confirmed in the structure II (sII) hydrate. Utilizing a hydrate synthesis pathway involving pressurizing preformed structure I (sI) hydrates, we now show H2 occupancy in both the small and the large cages of sI, as evidenced by powder X-ray diffraction and Raman spectroscopic measurements. The new H2 environments were determined to be singly and doubly occupied 51262 cages occurring at 4125–4131 and 4143–4149 cm–1, respectively. This work serves as proof-of-concept that, by altering the conventional hydrate synthesis procedure to incorporate preformed hydrates, it may be possible to promote the occupancy of H2 or possibly other guests in a desired structure through a “templating” effect by simply changing the initial hydrate structure.

The current knowledge and description of guest molecules within clathrate hydrates only accounts for occupancy within regular polyhedral water cages. Experimental measurements and simulations, examining the tert-butylamine + H2 + H2O hydrate system, now suggest that H2 can also be incorporated within hydrate crystal structures by occupying interstitial sites, that is, locations other than the interior of regular polyhedral water cages. Specifically, H2 is found within the shared heptagonal faces of the large (4(3)5(9)6(2)7(3)) cage and in cavities formed from the disruption of smaller (4(4)5(4)) water cages. The ability of H2 to occupy these interstitial sites and fluctuate position in the crystal lattice demonstrates the dynamic behavior of H2 in solids and reveals new insight into guest-guest and guest-host interactions in clathrate hydrates, with potential implications in increasing overall energy storage properties.

In this study the rapid growth of sII H(2) hydrate within 20 min of post formation quenching towards liquid nitrogen (LN(2)) temperature is presented. Initially at 72 MPa and 258 K, hydrate samples would cool to the conditions of ~60 MPa and ~90 K after quenching. Although within the stability region for H(2) hydrate, new hydrate growth only occurred under LN(2) quenching of the samples when preformed hydrate "seeds" of THF + H(2) were in the presence of unconverted ice. The characterization of hydrate seeds and the post-quenched samples was performed with confocal Raman spectroscopy. These results suggest that quenching to LN(2) temperature, a common preservation technique for ex situ hydrate analysis, can lead to rapid unintended hydrate growth. Specifically, guest such as H(2) that may otherwise need sufficiently long induction periods to nucleate, may still experience rapid growth through an increased kinetic effect from a preformed hydrate template.