The increasing trend in global energy demand has led to an extensive use of fossil fuels and subsequently in a marked increase in atmospheric CO₂ content, which is the main culprit for the greenhouse effect. In order to successfully reverse this trend, many schemes for CO₂ mitigation have been proposed, taking into consideration that large-scale decarbonization is still infeasible. At the same time, the projected increase in the share of variable renewables in the future energy mix will necessitate large-scale curtailment of excess energy. Collectively, the above crucial problems can be addressed by the general scheme of CO₂ hydrogenation. This refers to the conversion of both captured CO₂ and green H₂ produced by RES-powered water electrolysis for the production of added-value chemicals and fuels, which are a great alternative to CO₂ sequestration and the use of green H₂ as a standalone fuel. Indeed, direct utilization of both CO₂ and H₂ via CO₂ hydrogenation offers, on the one hand, the advantage of CO₂ valorization instead of its permanent storage, and the direct transformation of otherwise curtailed excess electricity to stable and reliable carriers such as methane and methanol on the other, thereby bypassing the inherent complexities associated with the transformation towards a H₂-based economy. In light of the above, herein an overview of the two main CO₂ abatement schemes, Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU), is firstly presented, focusing on the route of CO₂ hydrogenation by green electrolytic hydrogen. Next, the integration of large-scale RES-based H₂ production with CO₂ capture units on-site industrial point sources for the production of added-value chemicals and energy carriers is contextualized and highlighted. In this regard, a specific reference is made to the so-called Power-to-X schemes, exemplified by the production of synthetic natural gas via the Power-to-Gas route. Lastly, several outlooks towards the future of CO₂ hydrogenation are presented.

In the present work, the individual or synergistic effect of Ni-based catalysts (Ni/CeO₂, Ni/Al₂O₃) and an eutectic carbonate salt mixture (MS) on the CO₂ gasification performance of olive kernels was investigated. It was found that the Ni/CeO₂ catalyst presented a relatively superior instant gasification reaction rate (Rco) compared to Ni/Al₂O₃, in line with the significant redox capability of CeO₂. On the other hand, the use of the binary eutectic carbonate salt mixture (MS) lowered the onset and maximum CO₂ gasification temperatures, resulting in a notably higher carbon conversion efficiency (81%) compared to the individual Ni-based catalysts and non-catalytic gasification tests (60%). Interestingly, a synergetic catalyst-carbonate salt mixture effect was revealed in the low and intermediate CO₂ gasification temperature regimes, boosting the instant gasification reaction rate (Rco). In fact, in the temperature range of 300 to 550 °C, the maximum Rco value for both MS-Ni/Al₂O₃ and MS-Ni/CeO₂ systems were four times higher (4 × 10⁻³ min⁻¹ at 460 °C) compared to the individual counterparts. The present results demonstrated for the first time the combined effect of two different Ni-based catalysts and an eutectic carbonate salt mixture towards enhancing the CO production rate during CO₂ gasification of olive kernel biomass fuel, especially in the devolatilization and tar cracking/reforming zones. On the basis of a systematic characterization study and lab-scale gasification experiments, the beneficial role of catalysts and molten carbonate salts on the gasification process was revealed, which can be ascribed to the catalytic activity as well as the improved mass and heat transport properties offered by the molten carbonate salts.

Gasification is a promising and attractive thermochemical method for biomass-to-energy conversion, with fluidized bed reactors being one of the best options for large-scale operations. Olive residues in particular are potentially excellent candidate biomass fuels in the Mediterranean area, due to the region’s increased capacity in olive oil production. Herein, the gasification experiments of olive kernels in a 2 MWth air-blown, bubbling fluidized bed reactor located at CENER’s facilities (BIO2C) in Navarra, Spain are presented. Even though technical issues were demonstrated due to the operation of the plant with a high-density biomass fuel and given the scale of the process, a quasi-steady-state and isothermal 12 h operation at an equivalence ratio of 0.25 ± 0.03 was attained. Given the satisfactory experimental results, an Aspen Plus simulation model of the process was also attempted. Notably, the proposed methodology agrees well with the experimental results and can be regarded as a starting point in future studies examining the gasification of relevant biomass in a MW-scale unit. Next, the effect of equivalence ratio and residual biomass moisture content were also evaluated, with the scope of designing future experiments that require minor modifications in the already existing apparatus. Finally, a syngas utilization route through the provision of energy for district heating purposes in the nearby village of Aoiz was proposed.

The presented work explores the structural properties, gasification reactivity, and syngas production of Greek lignite fuel (LG) and ex-situ produced chars during CO₂ gasification. Three different slow pyrolysis protocols were employed for char production involving torrefaction at 300 °C (LG300), mild-carbonization at 500 °C (LG500), and carbonization at 800 °C (LG800). Physicochemical characterization studies, including proximate and ultimate analysis, X-ray Diffraction (XRD), and Raman spectroscopy, revealed that the thermal treatment under inert atmospheres leads to chars with increased fixed carbon content and less ordered surface structures. The CO₂ gasification reactivity of pristine LG and as-produced chars was examined by thermogravimetric (TG) analysis and in batch mode gasification tests under both isothermal and non-isothermal conditions. The key parameters affecting the devolatilization and gasification steps in the overall process toward CO-rich gas mixtures were thoroughly explored. The gasification performance of the examined fuels in terms of carbon conversion, instant CO production rate, and syngas generation revealed an opposite reactivity order during each stage. TG analysis demonstrated that raw lignite (LG) was more reactive during the thermal devolatilization phase at low and intermediate temperatures (da/dt_max,devol. = 0.022 min⁻¹). By contrast, LG800 exhibited superior gasification reactivity at high temperatures (da/dt_max,gas. = 0.1 min⁻¹). The latter is additionally corroborated by the enhanced CO formation of LG800 samples under both non-isothermal (5.2 mmol) and isothermal (28 mmol) conditions, compared to 4.1 mmol and 13.8 mmol over the LG sample, respectively. The pronounced CO₂ gasification performance of LG800 was attributed to its higher fixed carbon content and disordered surface structure compared to LG, LG300, and LG500 samples.