Hydrogen produced by the electrolysis of water using sustainable electricity will play an increasingly important role as an energy and a feedstock vector. Shifting from fossil to renewable resources means that new industrial platforms have to be set up to provide carbon-based fuels and bulk base chemicals to replace the current fossil resources based routes. The global demand cannot be met by indirect use of carbon dioxide via biomass necessitating the use from point sources or direct air capture, which changes the value of CO2 from waste to commodity chemicals. The production of chemicals by hydrogenation of CO2 is typically hampered by the thermodynamic conversion being far from 100% under currently viable reaction conditions. The equilibrium can, however, be shifted to increase conversion by removing one of the reaction products, namely water, from the reaction mixture with sorbents like zeolites. Prerequisite to conversion enhancement and process intensification is the close proximity of sorption and catalytic sites. This review presents the state of the art in synthesis and application of these, in fact, bifunctional materials.

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Sub-nanometer zeolite 13X-supported Ni-ceria catalysts were synthesized for CO₂ methanation. XRD and SEM results show the structure and morphology of 13X zeolite after impregnation and calcination. Ce loading affected the catalysts’ metal dispersion, reducibility, basicity and acidity, and thence their activity and selectivity. STEM-EDX elemental mappings showed that Ce and Ni are predominantly highly dispersed. Ce has a positive effect on the reduction of NiO and leads to a relatively high number of medium basic sites with a low Ce loading. Highly stable 5%Ni2.5%Ce13X had high activity and nearly 100% CH₄ selectivity in CO₂ methanation at 360 °C, which is mainly due to the high dispersion of metals and relatively high amount of medium basic sites. It can be inferred that this catalyst synthesis strategy has great potential for good catalyst dispersion, since metal uptake by the zeolite is selective for the metal citrate complexes in solution.

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Methanation is a potential large-scale option for CO₂ utilization, and it is one of the solutions for decreasing carbon emission and production of synthetic green fuels. However, the CO₂ conversion is limited by thermodynamics in conventional reaction conditions. However, around 100 % conversion can be obtained using sorption enhanced CO₂ methanation according to Le Chatelier's principle, where water is removed during the reaction using zeolite as a sorbent. In this work 5%Ni5A, 5%Ni13X, 5%NiL and 5%Ni2.5%Ce13X bifunctional materials with both catalytic and water adsorption properties were tested in a fixed bed reactor. The overall performance of the bifunctional materials decreased on going from 5%Ni2.5%Ce13X, 5%Ni13X, 5%Ni5A, to 5%NiL. The CO₂ conversion and CH₄ selectivity were approaching 100 % during prolonged stability testing in a 100 reactive adsorption – desorption cycles test for 5%Ni2.5%Ce13X, and only a slight decrease of the water uptake capacity was observed.

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Zeolite 13X and 5A supported Ni catalysts were synthesized for CO₂ methanation using the evaporation impregnation method. The influence of using different Ni precursors (nitrate, citrate, and acetate) as well as calcination temperatures on the catalyst properties and performance were investigated. XRD, SEM-EDX, TEM, STEM-EDX, N₂ physisorption, H_2-TPR, TPD-NH₃ and TG/DTA were used for detailed characterization of the catalysts. The parent structure of the zeolites did not change during catalyst synthesis. Using nickel citrate and acetate resulted in smaller NiO particle size compared to nitrate. STEM-EDX results showed that all the Ni-precursor complexes entered more efficiently the 13X zeolite structure, which is mainly due to steric hindrance resulting from the smaller pore size of 5A. Methanation experiments revealed that the 13X catalysts synthesized using nickel citrate (5% Ni) displayed clearly higher activity, compared to the catalysts synthesized using nickel nitrate or nickel acetate. A 79% conversion at 320 °C was obtained with 100% selectivity towards CH₄ and the catalyst showed excellent stability during 200 h testing. Overall, it can be concluded that the Ni precursor significantly influences the physico-chemical characteristics and catalytic properties of Ni 13X and Ni 5A zeolite catalysts in CO₂ methanation: complex size and pore size matter.

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Zeolites 13X and 5A were modified with nickel and/or ruthenium for CO₂ methanation. The catalysts were prepared by evaporation impregnation and XRD, SEM-EDX, TEM, STEM-EDX, nitrogen physisorption, H₂-TPR and NH₃-TPD were used to characterize the physico-chemical properties of the catalysts. The physico-chemical characterization results show that the zeolites structure did not change after the Ni, Ru modification, however. Ni was able to enter the pores of 13X, in the other case, 5A, an egg shell type structure was formed. Methanation experiments were performed in a lab scale fixed bed reactor system, the results showed that the mono-metallic catalysts out-performed the bi-metallic ones with Ni being the more active. One of the factors influencing the performance of the bi-metallic catalysts was the difficulty to obtain good dispersion when both metals were used. Also the morphology of the catalyst significantly influenced the selectivity. The catalysts with lower weak acidity benefit for getting a higher activity. The single metal catalysts 2.5 %Ru13X and 5%Ni13X showed good catalytic stability with around 97 % CH₄ selectivity at 360 °C, with no catalyst deactivation during the 200 h catalyst stability test.

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A combined experimental and modeling study was performed to evaluate the relation between sorbent characteristics and process performance for solid sorption postcombustion CO 2 capture. A pulverized coal (PC) and a natural gas combined cycle (NGCC) power plant were considered, addressing CO 2 and H 2 O sorption. The measured isotherms for PEI/silica sorbent were implemented in an equilibrium-based flow sheeting model. The PC regeneration heat demand is 3.9 GJ/ton CO 2 captured. This is lower than that of the NGCC and, though a direct comparison is not valid, similar to a literature MEA case. Solid sorption systems hold the promise to be energetically superior to MEA: a 2-fold increase in CO 2 adsorption capacity (to 4.4 mmol/g) yields a regeneration heat demand of 3.3 GJ/ton, even when accompanied by a similar increase in H 2 O adsorption capacity.@en

This study evaluates whether a transition of large ports facilities to biofuel production for mobility improves the environmental performance and satisfies the renewable energy directive (RED) and it is the first LCA study that considers biofuel production from torrefied wood. The systems studied are wood, torrefied wood, and straw pellets circulating fluidized bed gasification for H₂, synthetic natural gas, or Fischer–Tropsch (FT) diesel production and use. These systems are evaluated for their global warming, acidification, eutrophication and particulate matter potentials, as well as, for their aggregated environmental performance. The effects of the electricity mix selection and ecoinvent database’s economic allocation are also analyzed. All biomass systems result in a better aggregated environmental performance and benefits for the global warming potential. However, regarding the acidification, particulate matter, and eutrophication potentials, most biomass systems are inferior to the reference systems. Switching to a zero-emission electricity mix offers benefits for all the biomass and fossil-H₂ systems and researchers should use databases cautiously. The bio-H₂ and FT diesel of wood-based systems show the best environmental performance and satisfy the current and future RED targets. On one hand, the bio-H₂ systems result in the largest benefits regarding the global warming potential, and on the other hand, both wood-based FT diesel systems offer overall benefits which concern not only the sustainable target of CO₂ emissions reduction, but also the air quality improvement of the broader area as well.

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A study on integration of Power-to-Gas technology with bio-methane production from bio-syngas produced by biomass gasification shows that a significant amount of excess electricity can be accommodated in bio-SNG production. By adding hydrogen produced from intermittent renewable sources to a CO₂ methanation section, production capacity of methane can be doubled. The business case for Power-to-Gas for bio-methane has been evaluated using three future cumulative electricity prices curves. Results show that a positive business case exists only for price curves based on large amounts of intermittent electricity installed. The room for investment for the electrolyser will mainly and highly depend on future commodity prices and price curves, and will benefit significantly from a decrease in the cost price of the electrolyser. The projected room for investment available for a PEM electrolyser is lower than for a Solid Oxide Electrolyzer (SOE), because of its lower efficiency and resulting higher operating costs. In the case of large capacity of intermittent electricity, the projected room for investment of an SOE electrolyser is 650 €/kW and for a PEM electrolyser 350 €/kW, which corresponds to the projections of future electrolyser costs.

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Natural gas combined cycle (NGCC) power plants with pre-combustion CO2 capture called, integrated reforming combined cycle (IRCC) power plants could be operated in a flexible and environment friendly manner by incorporating hydrogen storage. The current article presents a thermodynamic model evaluation of such a flexible large scale power plant system with co-production of hydrogen and subsequent hydrogen storage with metal hydrides (MH). A comparative analysis has been presented using steady state ASPEN Plus models for a Reference NGCC system, an IRCC system and a Flexible IRCC system with MH (MgH2) based hydrogen storage. In addition, a detailed exergy analysis indicating irreversibilities in the system has been presented. The system study indicates that such Flexible IRCC systems could be operated with a time based average (considering charging and discharging) electrical efficiency above 45% depending on the amount of hydrogen stored, appropriate heat integration and choice of the metal hydride. Exergy analysis indicates that such flexible systems could still be operated at a relatively high average net exergy efficiency (considering charging and discharging). Addition of MH based H2 storage in an IRCC system does not lead to significant electrical/exergy efficiency penalty. Carbon capture and storage is a larger source of efficiency penalty than the MH based H2 storage unit. The largest exergy destruction is observed in the gas turbine combustor and reformer units, providing additional scope for efficiency enhancement. With a larger share of renewable power in the total electrical power production expected in future, Flexible IRCC systems as presented in this article could be used as an efficient solution to address the large fluctuations in power supply and demand. Additional investigations are however encouraged further toward optimizing the choice of the metal hydride, detailed design of a single heat integrated system (including part load behavior) and economic assessments based on the heat and mass balances developed in this study.@en