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The high increase in CO₂ emissions from the development of transport, industry, and other activities and their accumulation in the atmosphere, with levels above 32Gt CO₂ yearly (EPA, 2022), require to remove it, either by storing it in geological deposits and formations, or by using as a reagent to synthesize chemicals, i.e. methanol (Martin & Grossmann, 2017) or methane (David & Martin, 2014). Using renewable CO₂ and H₂ to produce methanol and methane is a potential and attractive option since they constitute the first C1 in the chain of organic chemicals, being valuable products in the market. Methanol constitutes one of the seven fundamental primary chemical products in the chemical industry (IEA, 2022). Methane can also be utilized in boilers, heating systems, generation energy in cogeneration and thermal power plants, and for domestic use, substituting natural gas. To produce both chemicals, CO₂ and renewable H₂ are needed. On the one hand, the capture of CO₂ can be carried out by employing solutions based on monoethylamine (MEA), which captures the CO₂ directly from concentrated gaseous effluents from thermal power plants, cement plants, etc. Alternatively, the technology based on direct air capture (DAC) using the conventional process based on basic solutions also constitutes a complementary option to remove CO₂ from the atmosphere compared to biomass (Galán et al., 2023). On the other hand, H₂ is produced by water electrolysis through the utilization of PV panels or wind turbines as a renewable source of energy (Martin & Grossmann, 2016). O₂ is generated as a by-product, which may be used in secondary processes or to obtain additional credit. Solar and wind can supply the energy needed by the water electrolysis and provide some or a total of the energy requirements for all the processes. This aspect is strongly influenced by the climatic conditions present in the location.

A facility location problem is formulated, which allows deciding which chemical to produce, the source of CO₂, the source of renewable energy, and the location of the capture and production facilities aiming at avoiding a certain level of emissions. To achieve this goal, the development of a multiscale optimization model based on the work by Heras & Martín (2021) is carried out. At the process scale, a techno-economic analysis is developed to determine the yield and the investment and product cost of MEA, conventional DAC, renewable H₂, methane, and methanol production. All the processes are modeled employing an equation-based approach. The conventional DAC process employs alkaline solutions based on KOH. The CO₂ is hydrogenized in each of the products. Water electrolysis, powered by renewable energy from PV panels or wind turbines, is considered the best alternative to produce renewable H₂. This is purified from traces of O₂ and H₂O. The O₂ stream could supply an extra chemical to employ in other processes or obtain earnings for the complete system. Methanation of CO₂ with renewable H₂ allows producing synthetic methane. Water is also generated in the process, and recycled to the electrolyzer, reducing freshwater makeup. Alternatively, CO₂ can be hydrogenized to methanol. The process is controlled by equilibria that require unreacted H₂ and CO₂ to be recycled. Referring to the macro scale, the KPIs of the different processes are used to formulate a network optimization problem to decide on installing the different technologies across the land/country for a given budget. A multi-period, multi-objective MINLP problem, is formulated, including social issues related to generating wealth and jobs in the region where the facilities are installed. A study of the required supply chain is carried out to determine the most appropriate area in which to locate the CO₂ capture, water electrolysis, and methane/methanol synthesis plants, as well as the type of technology necessary for the generation of solar energy and/or wind and the area or region and ground required in which to locate the technology. At the same time, the social and economic aspects of the use of CO₂ capture plants, the generation of methane and methanol and its distribution, and the use of PV panels and wind turbines are considered to mitigate the impact of climate change on the areas to be treated and developed the local and national economy. A sensitivity analysis is performed considering target prices and different budget allocations to assess the infrastructure to be installed. The years 2022, 2030, and 2050 are considered to evaluate the capture of some and/or the total CO₂ produced. The year 2022 is taken as a reference, achieving around 223.8 Mtons of CO₂ emitted.

As a case study, Spain is considered, except for the autonomous cities of Ceuta and Melilla, the Balearic and Canary Islands, and other Spanish enclaves in the Mediterranean Sea due to their small area. The target process for methanol and methane corresponds to market prices of 0.480 €/kg_Methane (Eurostat, 2022) and 0.400 €/kg_Methanol (Methanol Institute, 2022). The evaluation assumes 1%, 1.5%, and 2% of the total surface of Spain to install the facility processes, PV panels, and wind turbines, obtaining maximum volumes of CO₂ captured of 51.76%, 74.72%, and 96.57%, requiring investments above 800,000 MM€-1,600,000 MM€, and power supplies between 140 GW-280 GW in 2022. In the year 2030, there is a predicted reduction of 55% (European Commission, 2020) in GHG compared to 1990 in the European Union. In the year 2050, there is a predicted net zero emissions, considering a reduction of 90% of emissions, and the remaining 10% will be absorbed naturally (MITECO, 2020). Generally, methanol production is chosen over methane due to the more competitive production and investment costs, allowing obtain sell prices of 0.400 €/kg_Methanol, 0.390 €/kg_Methanol,185 €/kg_Methanol in 2022, 2030, and 2050 (Lloyd’s Register-UMAS, 2019). The same evaluation is carried out over methane production to determine the competitive production costs to produce it, resulting in 0.944 €/kg_Methane, 0.871 €/kg_Methane, and 0.424 €/kg_Methane. As is shown, these values can be reached through state subsidies and grants. Moreover, indirect aid will help to facilitate reaching these prices. The production of solar energy is placed in the central-southern peninsular locations since they have higher rates of solar radiation and the wind turbines in the coastal area, overall the northern third of the peninsula, some Mediterranean places, as provinces of Gerona, Alicante, and Murcia, and the Strait of Gibraltar, which have higher wind velocities. In the case of methane, regasification plants, and the pre-existing transportation network provide an adequate infrastructure for the national and international transportation levels.

References:

Davis W, Martín M. Optimal year-round operation for methane production from CO₂ and water using wind and/or solar energy. J Clean Prod. 2014;80:252-261.

European Commission. 2030 Climate Target Plan. Communication From the Commission to the European Parlament, the Council, the European Economic and Social Committee and the Committee of the Regions. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52020DC0562&from=EN; 2020.

Eurostat. Eurostat Statistics Explained. Natural Gas Price Statistics. https://ec.europa.eu/eurostat. https://ec.europa.eu/eurostat/statistics-explained/; 2022.

Galán G, Martín M, Grossmann IE. Systematic comparison of natural and engineering methods of capturing CO₂ from the air and its utilization. Sustainable Production and Consumption. 2023;37:78-95.

Heras J, Martin M. Multiscale analysis for power-to-gas-to-power facilities based on energy storage. Computers and Chemical Engineering. 2021;144:107147.