Carbon capture and use may provide motivation for the global problem of mitigating global warming from substantial industrial emitters. Captured CO2 may be transformed into a range of products such as methanol as renewable energy sources. Polymers, cement, and heterogeneous catalysts for varying chemical synthesis are examples of commercial goods. Because some of these components may be converted into power, CO2 is a feedstock and excellent energy transporter. By employing collected CO2 from the atmosphere as the primary hydrocarbon source, a carbon-neutral fuel may be created. The fuel is subsequently burned, and CO2 is released into the atmosphere like a byproduct of the combustion process.
1. Introduction
Global carbon emissions fell 5% in the first quarter of 2020 compared to the first half of 2019, owing to lower coal consumption (8%), oil (4.5%), and natural gases (2.3 percent). Another paper provided the daily, monthly, and annual rhythms of CO
2 emissions and anticipated an 8.8 percent reduction in CO
2 in the first half of 2020
[1]. The COVID-19 epidemic caused the largest dramatic drop in worldwide CO
2 emissions since World War II
[2]. The Worldwide Atmospheric Research anticipated that global human fossil CO
2 emissions in 2020 will be 5.1 percent lower than in 2019, at 36.0 Gt CO
2, barely below the 36.2 Gt CO
2 emission level recorded in 2013
[3]. Global carbon emissions (fossil fuels) per unit of Gross Domestic Product (GDP) decreased in 2019, averaging 0.298 t CO
2/k USD/year, but per capita carbon emissions were steady at 4.93 t CO
2/capita/year, confirming a 15.9 percent increase since 1990
[4].
To minimize the magnitude of global warming, substantial reductions in CO
2 emissions from fossil fuel use are necessary. In view of the steadily increasing interest in preserving the environment and mitigating the consequences of climate change and global warming, hence, great attention is dedicated to finding alternative sustainable methods or eco-friendly economic materials to reduce the use of cement in various fields and consequently reduce the CO
2 emissions accompanying with intensive energy consumed in the cement industry. Various additives have been mixed with cement such as bitumen
[5][6], asphaltene
[7], glass
[8][9], polymers
[10][11][12], nanomaterials
[13][14], cement wastes
[15][16], bio-waste
[17][18][19] or natural clay
[20][21] to improve the material features and minimize the cement incorporation and environmental pollution.
However, carbon-based liquid fuels will continue to be key energy storage media for the foreseeable future
[22]. Researchers suggest using solar energy to recycle atmospheric CO
2 into liquid gasoline using a mix of mostly available technologies. The world population hit 7 billion in 2011 and is expected to exceed 9 billion by 2050, according to the United Nations, significantly increasing energy consumption. According to Hubbert’s peak theory, oil resources will plummet dramatically during the next 40 years
[23]. As a result, the years 2055–2060 may be the last in which oil output reaches zero. Furthermore, while taking current energy usage into account, the World Coal Institute estimates that current coal reserves will last another 130 years, oil reserves for 42 years, and natural gas reserves for 60 years. The need to replace old fossil fuels with sustainable alternatives and new energy scenarios is thus becoming critical. Another major environmental worry is the rising quantities of carbon dioxide (CO
2) in the atmosphere, which is caused by the combustion of fossil fuels and contributes to global warming
[24].
Carbon capture allows for the production of low-carbon power from fossil fuels as well as the reduction of CO
2 emissions from industrial operations, such as gas processing, cement production, and steel production, where alternative decarbonization options are restricted. CO
2 capture and usage is gaining popularity across the world
[24].
Human-caused CO
2 increases have disastrous repercussions, such as rising average temperatures, melting sea ice, flooding, and sea level rise, harming global life, health, and the economy. The staggering difference between the total amount of carbon emitted worldwide as CO
2 and the quantity of carbon present in five crucial carbon-containing commercial chemicals, not including derivative products such as ethylene oxide
[25], underscores the grave consequences of continued carbon emissions. Even with indirect effects, the greatest quantity of CO
2 that may be used remains quite little in comparison to total emissions. For example, if the whole worldwide yearly manufacturing of ethylene, the most extensively made commercial chemical containing carbon atoms, were carried out solely using carbon derived from CO
2, this would result in direct usage of less than 1.5 percent of total global CO
2 emissions
[26]. Even with additional commercial chemicals included and indirect mitigation assumptions applied, possible usage might constitute just a tiny fraction of overall emissions. Naturally, this ignores the reality that direct CO
2 conversion into hydrocarbons, particularly aromatics, is unlikely to become a commercial method in the near future.
Nonetheless, CO
2 activation is exceedingly difficult since CO
2 is a completely oxidized, thermodynamically stable, and chemically inactive molecule. Furthermore, hydrocarbon synthesis by CO
2 hydrogenation frequently results in the creation of undesirable short-chain hydrocarbons rather than the desired long-chain hydrocarbons
[27]. As a result, most research in this field has been on the sequential hydrogenation of CO
2 to CH
4, oxygenates, CH
3OH, HCOOH, and light olefins (C
2–C
4 olefins). There have been few experiments on creating liquid hydrocarbons with molecular weights of C
5+.
There are two methods of converting CO
2 to hydrocarbon gas liquids: an indirect pathway that transforms CO
2 to carbon monoxide (CO) or methanol and then to liquid hydrocarbons, or a direct CO
2 hydrogenation route that combines CO
2 reduction to CO through the use of the converse water-gas shift reaction shift (CWGS) and subsequent CO hydrogenation to long-chain hydrocarbons through the use of the Fischer-Tropsch synthesis (FTS)
[28][29]. Secondly, the more direct technique is typically viewed as cheaper and ecologically acceptable since it requires fewer chemical process stages and uses less energy overall. The following chemical reactions are relevant for the production of hydrocarbon fuel (Scheme 1).
Scheme 1. Chemical reactions used to produce hydrocarbon fuels.
Mitigation of greenhouse gases is one of the most pressing issues confronting society today. As a result, the best strategy to minimize greenhouse gas emissions is to employ carbon-free sources that do not emit more CO
2 into the environment
[30]. However, there is significant promise in CO
2 energy carriers and other materials, with several hurdles to overcome. Reduced CO
2 emissions and conversion to renewable fuels and valuable chemicals have been proposed as possible ways to reduce greenhouse gas emissions
[30].
2. Global Warming and Sustainability
Producing energy from CO
2 rather than absorbing and disposing of it is a sustainable energy technique. In reality, several thermochemical processes may transform CO
2 into value-added compounds such as hydrogen, methanol, and ethanol
[31]. These fuels may be utilized to power automobiles without requiring major modifications to existing internal combustion engines, paving the way for a more sustainable future. Discovering eco-friendly alternatives and new energy scenarios, primarily the conversion of CO
2 into value-added chemicals using sunlight, is one of the top-most research priorities to overcome today’s extreme reducing the use of fossil fuels and mitigate the global warming risks associated with high CO
2 concentrations in the atmosphere
[32].
Because of decreased prices, quick commercial adoption, and complete industrial development, the use of second-generation green fuels is one of the most promising approaches to achieving industrial and ecological aims in the short term. A green fuel, unlike fossil fuel, does not emit more CO
2 into the atmosphere since its burning, in the well-to-wheel cycle, creates a CO
2 release equivalent to that required for its manufacture, closing the CO
2 balance, and making it a carbon-neutral fuel
[33]. The practical application of hydrogenation catalysts in conjunction with commercially viable research and innovation technology. Indeed, catalytic conversion of atmospheric CO
2 into biofuels and fine chemicals would be one of the most economical and realistic solutions to the problem of greenhouse gas emissions, assuming that capture/sequestration and high-pressure storage technologies become commercially viable
[34][35]. Nonetheless, in a very short time, the most prosecutable actions and encouraging prospects look upon the use of CO
2-rich streams originating from industrial exhaust emissions, such as brick and cement work, despite the need for clean-up stages, as well as additional purification and concentration
[36]. Haldor-Topsoe is at the vanguard of using CO
2 as a carbon feedstock through various power-to-gas technologies, whereas ENI is redesigning its production lines with a greener vision, by developing new hydrogenation processes, called ENI Ecofining
TM, for green-fuels synthesis, advantageously depicting the production of ultra-pure CO
2 as industrial practice
[37]. Despite significant advances in fuel science, the commercial and economic viability of hydrogen-to-liquid-fuels such as methanol is still being debated. The discovery of more effective catalyst substances for the efficient hydrogenation of CO
2 looks to be of major importance in this scenario. CuO/ZnO-based catalysts, as is well known, offer lower costs and improved chemical stability when compared to other catalysts such as transition metal carbides (TMCs), bimetallic catalysts, or Au-supported catalysts
[38]. Although CuO/ZnO/Al
2O
3 is the most extensively investigated catalyst for methanol synthesis, it was shown that the inclusion of ZrO
2, CeO
2, and TiO
2 oxides improved both the activity and selectivity of CuO/ZnO-based catalysts in CO
2 hydrogenation processes
[39].
Recently the influence of replacing Al
2O
3 with CeO
2 in the typical Cu-ZnO/Al
2O
3 catalytic discussed composition for syngas conversion, suggesting that the use of cerium oxide led to a remarkable positive effect on the catalyst stability, as a result of the preservation of the ratio of CuO/Cu
+ on the catalyst surface. As well as, on the particle growth, due to the strong electron interactions of the copper/ceria phase (Cu/Cu
2O/CuO, Ce
3+/Ce
4+) during hydrogenation reactions
[40]. Metal dispersion and CO
2 adsorption capacity are two key parameters affecting the CO
2 hydrogenation functionality of Cu-based systems. Similarly, researchers displayed that the use of several oxides (i.e., ZnO, ZrO
2, CeO
2, Al
2O
3, Gd
2O
3, Ce
2O
3, and Y
2O
3) and alkaline metals (i.e., Li, Cs, and K) could remarkably influence both catalyst structure and morphology, balancing the amount of the diverse copper species (i.e., Cu°/Cu
+/Cu
2+) and leading to a notable improvement of the catalytic performance. On this account, ZrO
2 has been shown to positively the affect morphology and texture of Cu-ZnO based catalysts, also favoring CO
2 adsorption / activation and methanol selectivity, while CeO
2 could act as both an electronic promoter and improver of surface functionality of Cu phase
[41]. As reported in the preliminary works, the scientists have proved a greater specific activity of CuZnO-CeO
2 catalyst with respect to that of similar catalytic systems containing ZrO
2 or other promoter oxides, facing several electronic and structural effects. Despite of a generally higher specific activity, researchers have evidenced that the CuZnO-Ceria catalyst suffers from several practical limitations, such as: a lower surface area, especially compared to that of ZrO
2 and Al
2O
3 promoted catalysts.
3. Green Hydrogen and Biofuel
Hydrogen has several benefits as an energy carrier and will play a key role in future energy production and circular economy
[42]. Because hydrogen can be created in a climate-neutral way and emits only water when used, it is regarded as a climate-friendly energy carrier
[43]. Hydrogen derived from renewable sources is commonly referred to as “green hydrogen.” It contrasts with “grey hydrogen,” which is defined as hydrogen derived from fossil, nonrenewable sources
[44]. Currently, grey hydrogen generated by steam reforming of fossil natural gas accounts for the majority of worldwide hydrogen production
[44]. Grey hydrogen may be converted to “blue hydrogen” by using carbon capture and storage technology, but it remains a non-renewable source
[45]. To distinguish between carbon-neutral green and blue hydrogen, researchers coined the term “carbon-negative hydrogen.” Hydrogen has a wide range of uses due to its high gravimetric energy density, as well as its ease of storage and transportation
[46]. Fuel cells may produce both electric power and heat from hydrogen. In the steel industry, hydrogen may also be utilized as a reducing agent
[46]. It is regarded as an unavoidable basic building element in difficult-to-electrify zones
[46]. Green hydrogen may be created using renewable power or renewable biomass, either through thermochemical processes or using microorganisms and biotechnological technologies. Biohydrogen, or hydrogen generated from biomass, can lead to more effective use of biogenic source materials by using residual and waste materials. Biohydrogen production technologies are classified as thermochemical or biotechnological. Biohydrogen may be generated on an industrial scale utilizing thermochemical methods from biomass (wood, straw, grass clippings, etc.) as well as other bioenergy sources (biogas, bioethanol, etc.)
[43]. Gasification and pyrolysis are two types of thermochemical reactions. The synthesis gas produced by gasification or pyrolysis contains varying proportions of H
2, CO
2, carbon monoxide (CO), methane (CH
4), and other components such as nitrogen, water vapor, light hydrocarbons, hydrochloric acid, alkali chlorides, Sulphur compounds, biochar, or tar
[47]. The following chemical processes occur as a result of subsequent steam reforming and/or water gas shift reactions, culminating in the creation of CO
2 and hydrogen
[48]. Minimal carbon-hydrogen (MCH) should be used initially in areas that are difficult to electrify directly
[49]. Researchers believe that generated bio-hydrogen is employed in difficult-to-electrify industrial sectors such as cement, steel, refining, ammonia, and glass. Refineries and ammonia plants are now the largest customers of hydrogen generated by steam methane reforming, a process that releases at least 10 kg CO
2 for every kilogram H
2 produced. Because many industries utilize carbon-intensive hydrogen, switching to MCH would be a near-term opportunity to increase hydrogen demand while lowering greenhouse gas emissions. Companies that are difficult to electrify may reduce their emissions and capture carbon dioxide by switching to alternative fuels and implementing carbon capture and storage technologies. This can help to enable their operations to contribute to the elimination of carbon dioxide from the atmosphere.
Country-specific forecasts of European nations’ ultimate energy consumption in 2050 are unavailable and difficult to determine. As a result, scientists evaluate country-specific final energy consumption in 2019 and anticipate that 5–30% of final energy consumption will be fulfilled with low-carbon hydrogen to minimize hard-to-electrify emissions. These scenarios enable researchers to set upper and lower bounds on the amount of hydrogen likely required by each European country to achieve net-zero emissions by 2050
[50].