• Energy Research

Eggshell is a cheap and environmentally friendly waste stream from the food industry, which could be potentially used for different applications in a circular economy scenario. Carbonation of eggshell derived sorbents has been investigated for calcium looping high temperature applications. Nevertheless, the application of these sorbents for direct air capture is yet to be explored in detail. In this work, waste eggshell (ES) and three different ethanol/water treated eggshell samples (E70, E80 and E90) are assessed for direct air capture and compared to limestone. These samples are exposed to ambient air in two distinct conditions. Namely, i) ambient air at 25 °C in a laboratory and ii) refrigerator conditions at 4 °C in order to simulate how these sorbents might perform in different climatic scenarios for varying geographic areas. Carbonation and hydration conversions were calculated for times of ∼ 3000 h. It was found that treating the eggshells with ethanol was key in order to obtain a suitable material for this application. Two of the ethanol-treated samples obtained similar conversions in comparable amounts of time for ambient air conditions, while limestone still performed better in refrigerator conditions

This comprehensive review appraises the state-of-the-art in direct air capture materials, processes, economics, sustainability, and policy, to inform, challenge and inspire a broad audience of researchers, practitioners, and policymakers.

Start-up conditions largely dictate the performance longevity for solid oxide fuel cells (SOFCs). The SOFC anode is typically deposited as NiO-ceramic that is reduced to Ni-ceramic during start-up. Effective reduction is imperative to ensuring that the anode is electrochemically active and able to produce electronic and ionic current; the bi-polar plates (BPP) next to the anode allow the transport of current and gases, via land and channels, respectively. This study investigates a commercial SOFC stack that failed following a typical start-up procedure. The BPP design was found to substantially affect the spatiotemporal dynamics of the anode reduction; Raman spectroscopy detected electrochemically inactive NiO on the anode surface below the BPP land-contacts; X-ray computed tomography (CT) and scanning electron microscopy (SEM) identified associated contrasts in the electrode porosity, confirming the extension of heterogeneous features beyond the anode surface, towards the electrolyte-anode interface. Failure studies such as this are important for improving statistical confidence in commercial SOFCs and ultimately their competitiveness within the mass-market. Moreover, the spatiotemporal information presented here may aid in the development of novel BPP design and improved reduction protocol methods that minimize cell and stack strain, and thus maximize cell longevity.

In this article, an efficient and innovative coupled system that involves hydrogen generation from water and hydrogenation of a biomass derived compound is shown for the first time. We have demonstrated that it is possible to use hydrogen produced from photoelectrochemical water splitting for the hydrogenation of a widely available biomass-derived compound (levulinic acid) into a versatile fuel component and/or precursor (γ-valerolactone). This compound, conversely to hydrogen, can be easily stored and handled. The photoelectrocatalyst selected was a TiO2 nanostructure synthesized by electrochemical anodization. The hydrogen produced was simultaneously used to carry out a hydrogenation reaction to transform levulinic acid into γ-valerolactone. The generated hydrogen was transferred to a catalytic reactor containing an aqueous solution of levulinic acid (LA) in the presence of a Ru based catalyst. Remarkable formation of γ-valerolactone (GVL) was obtained at temperatures as low as 30-60 °C. Yields to GVL of ca. 95 % have been obtained with this novel coupled system at only 60 °C. These results confirm the potential utilization in the biomass valorization of in-situ hydrogen production by photoelectrochemical water splitting.

Synthetic biomass-templated cement-supported CaO-based sorbents were produced by granulation process for high-temperature post-combustion CO2 capture. Commercial flour was used as the biomass and served as a templating agent. The investigation of porosity showed that the pellets with biomass or cement resulted in enhancement of porosity. Four types of sorbents containing varying proportions of biomass and cement were subject to 20 cycles in a TGA under different calcination conditions. After first series of tests calcined at 850 °C in 100% N2, all composite sorbents clearly exhibited higher CO2 capture activity compared to untreated limestone with exception of sorbents doped by seawater. The biomass-templated cement-supported pellets exhibited the highest CO2 capture level of 46.5% relative to 20.8% for raw limestone after 20 cycles. However, the observed enhancement in performance was substantially reduced under 950 °C calcination condition. Considering the fact that both sorbents supported by cement exhibited relatively high conversion with a maximum value of 19.5%, cement promoted sorbents appear to be better at resisting of harsh calcination conditions. Although flour as biomass-templated material generated significantly enhancement in CO2 capture capacity, further exploration must be carried out to find the way of maintaining outstanding performance for CaO-based sorbents under severe reaction conditions. Keywords: Calcium looping; CO2 capture; Granulation; Limestone; Biomass; Cement

This study has demonstrated, for the first time, a simple, fast and flexible microwave processing method for the simultaneous preparation of bio-products (bio-oil, bio-gas and biochar) using a methodology that avoids any form of catalyst or chemical activation. The dielectric properties of biomass and physicochemical characterisation such as TGA, elemental and proximate analysis, XRD, SEM/EDX and textural properties, showed that 8 kJ g-1 of microwave energy can produce superior biochars for applications in CO2 capture. The maximum CO2 uptake capacity for biochar produced was 2.5 mmol g-1 and 2.0 mmol g-1 at 0 and 25 °C and 1 bar, which and also exhibited high gas selectivity compared with N2, fast kinetics of adsorption (95%) after 20 cycles. GC-MS analysis of generated bio-oil products revealed that higher microwave energies (>8 kJ g-1) significantly enhanced the amount of bio-oil produced (39%) and specifically the formation of levoglucosan, furfural and phenolics compounds, and bio-gas analysis identified trace levels of H2 and CH4. The results from this study confirm a green, inexpensive and efficient approach for biomass valorisation which can easily be embedded within bio-refinery process, and also demonstrates the potential of biochars for post-combustion CO2 uptake.

Calcium looping (CaL) is a promising technology for the decarbonation of power generation and carbon-intensive (cement, lime and steel) industries. Although CaL has been extensively researched, some issues need to be addressed before the deployment of this technology at commercial scale. One of the important challenges for CaL is decay of sorbent reactivity during capture/regeneration cycles. Numerous techniques have been explored to enhance natural sorbent performance, to create new synthetic sorbents, and to re-activate and re-use deactivated material. This review provides a critical analysis of natural and synthetic sorbents developed for use in CaL. Special attention is given to the suitability of modified materials for utilisation in fluidised-bed systems. Namely, besides requirements for a practical adsorption capacity; a mechanically strong material, resistant to attrition, is required for the fluidised bed CaL operating conditions. However, the main advantage of CaL is that it employs a widely available and inexpensive sorbent. Hence, a compromise must be made between improving the sorbent performance and increasing its cost, which means a relatively practical, scalable, and inexpensive method to enhance sorbent performance, should be found. This is often neglected when developing new materials focusing only on very high adsorption capacity.