Special Issue on “CCUS: Paving the Way to Net Zero Emissions Technologies”

1. Introduction

Drastic measures must be taken in order to reduce carbon dioxide (CO₂) emissions due to the acceleration of climate change and global warming, and for which there was a value of 37.79 billion tons in 2023 [1]. Both phenomena are precipitating widespread environmental degradation. The aim specified in international agreements is to limit global temperature rise to well below 1.5/2 °C above the pre-industrial level [2].

Against this backdrop, carbon capture, utilization, and storage (CCUS) technologies have been recognised as critical tools for fighting the increase in CO₂ emissions. In a capture system, CO₂ can be captured directly from emission sources (e.g., power plants and industrial facilities), preventing its release into the atmosphere [3]. Alternatively, CO₂ can be captured directly from the air via direct air capture (DAC) technologies [4,5]. The captured CO₂ can either be stored or utilized in either chemical or fuel production. The CO₂ capture processes from flue gas that have been investigated are physical absorption, chemical absorption, adsorption, membrane, cryogenic distillation, calcium looping, and chemical looping combustion [6]. Absorption and adsorption are the most mature and investigated options for capturing CO₂ from the air [4]. Currently, CO₂ can be stored geologically, through mineral carbonation, or via ocean storage methods [7,8]. The CO₂ conversion routes that have been investigated in the literature are homogeneous catalytic conversion, heterogeneous catalytic conversion, photocatalytic conversion, electrocatalytic conversion, photo-thermal catalytic conversion, plasma–chemical/plasma–catalytic conversion, and bio-catalytic conversion [6,9,10]. CO₂ can be converted into methane, methanol, ethanol, polymers, urea, etc., in addition to cement and concrete, or it can be used directly (for yield boosting and enhanced oil recovery, as a solvent or heat transfer fluid, etc.) [11]. Recent advancements and challenges in carbon capture, utilization, and storage technologies are discussed in [6] and in the papers discussed in this Special Issue.

2. An Overview of the Published Articles

Recent developments in CO₂ capture, utilization, and storage technologies are discussed in this Special Issue.

Regarding CO₂ capture, Vedrtnam et al. (Contribution 1) found that integrating zeolitic imidazolate framework-8 (ZIF-8) into cellulose matrices such as a ZIF-8-based cellulose air filters (ZCAFs) can significantly improve CO₂ adsorption efficiency up to 22% compared to monoethanolamine (MEA) due to its large surface area, high porosity, and selective adsorption properties. ZIF-8 is thus a highly effective additive for enhancing the performance of traditional CO₂ capture materials like MEA. Alabid and Dinca (Contribution 2) advanced post-combustion capture with membranes. Their work addresses the impact of membrane selectivity with 99% purity of the CO₂ as a constraint. Their paper shows that membranes can allow 90% capture with high-purity CO₂ as the final product, but process configuration and the polymeric material used affect the costs and techno-economic feasibility of this process. In Arfelli et al.’s study (Contribution 3), the quantity of amine-based sorbents required to capture total CO₂ emissions (51.7 Mton in 2022 due to electricity consumption) for Bitcoin mining is evaluated and determined to be equal to almost 12 Bton. The Life Cycle Assessment (LCA) methodology was utilized toward this aim.

Regarding advancement in CO₂ storage, in Malik et al.’s study (Contribution 4), the reservoir capacity for effective CO₂ storage, considering porosity and permeability, was measured for Lithuanian deep saline aquifers (Syderiai and Vaskai), providing interesting experimental hitherto absent in the literature for the region analysed. Oliver et al. (Contribution 5) showed that although biological carbon fixation is an interesting topic, it currently has marginal relevance for application at scale owing to its low efficiency and ineffective rate of photosynthesis. In fact, through a small pilot study, they highlighted that this option is not actually a carbon removal technology due to the substantial emissions generated compared to the actual removal capacity. In the same context, Ughetti et al. (Contribution 6) investigated cyanobacteria grown in bicarbonate buffer solution for the bio-fixation of CO₂. The authors focused more on productivity. Despite observing the expected advantages associated with biological systems and, in general, the promising CO₂ storage capacity and CO₂ fixation efficiency of cyanobacteria, amounting to nearly 60%, they identified several potential limitations relating to scale-up and industrial capacity, such as the need for diluted conditions of the culture medium, light irradiation, and potential higher investment costs due to lower process intensification. Both studies highlighted the need for further research and disruptive enhancements in order to commercialize this kind of CCS option on an industrial scale.

Studies have been conducted on CO₂ utilization. In Bakratsa et al.’s work (Contribution 7), catalysts based on iron nanoparticles, unsupported and supported on zeolite, were synthesized and evaluated using physico–chemical techniques (e.g., BET, XRD, FT-IR, Raman, SEM/TEM, DLS, NH₃-TPD, and CO₂-TPD). The experimental results show the potential of this catalyst to produce C²⁺ deoxygenated products.

CO₂ capture, utilization, and storage are elements of a supply chain, and an overview of the LCA methodology applied to this system is presented in the study by Leonzio (Contribution 8).

This Special issue also covers the development of new renewable sources of energy that can compete with conventional sources based on fossil fuels as well as the study of non-conventional energy systems.

Berchiche et al. (Contribution 9) provided an extensive analysis on waste heat recovery in natural gas sweetening. The implementation of thermodynamic cycles and heat pumps could lower energy demands and contribute to the decarbonization of current industrial processes wherein additional energy input from fossils is required to supply heat for carbon capture. The study included a sensitivity analysis on flue gas composition, i.e., CO₂ content, and the fluid used in the thermodynamic cycles. It was found that the energy recovered can be significant, amounting to more than 20%.

Gómez-Coma et al. (Contribution 10) investigated H₂ storage technology. Sodium borohydride is a good candidate for storing H₂ in a solid form, but there is room for improvement. These authors’ experimental work shows that additives play a key role in the efficiency and rate of release of H₂. Among all the tested additives, polyacrylamide and sodium carboxymethyl cellulose were the most promising, and they exhibit a good compromise between H₂ storage capacity, release rate, and release efficiency. These materials can improve the state of the art and open new frontiers for this field.

Amelse et al. (Contribution 11) dived into the hot topic of carbon removal (CDR) from air. Their paper compares CDR options in terms of costs and efficiency as well as the advantages and disadvantages of the alternatives. Their analysis reveals that the terrestrial storage of biomass is an unexpensive and effective solution for permanently removing CO₂ without incurring a significant energy demand and impacting the environment by manipulating ecosystems.

3. Conclusions

The present Special Issue offers a valid overview of the existing research in the field of decarbonization, accounting for carbon capture, storage, utilization; more advanced removal applications, and H₂ storage. The published articles highlight several critical points before reliable and robust industrial deployment can be achieved.

• Carbon capture must be further enhanced to improve its profitability and energy integration, with recovery of waste heat being an opportunity we should exploit in order to mitigate energy demands. In addition, technology should be intensified, meaning that new solutions should exhibit good performance but have a lower energy demand, i.e., consume fewer utilities.
• Biological carbon capture and bio-fixation are potential options to pursue, but they have serious limitations that must be addressed before they can be scaled up owing to low intensification and the marginal rate of carbon removal compared to the actual emission volumes. For these reasons, this option currently seems quite far from feasible at scale.
• CDR constitutes a class of technologies with net negative CO₂ emissions. There are several options, and the most popular are DAC and BECCS. However, related analysis should also be extended to other alternatives, which could be profitable, with less of an energy demand and fewer concerns regarding the environment impact.
• A holistic approach should be considered when assessing technology to pave the way for net-zero emissions targets. LCA and energy footprints are fundamental tools in this framework, and they find applications even in contexts far away from environmental engineering, such as bitcoin and data mining, which consume huge amounts of energy, and this aspect is often neglected.