Produzione e distribuzione di elettricità e calore
Technologies for CO2 Utilization
Autors: Marco Marchese, Massimo Santarelli, Andrea Lanzini, Rosanna Viscardi
Production and Distribution of Electricity and Heat
The utilization of carbon dioxide, often referred to as carbon capture and utilization (CCU) or carbon dioxide utilization (CDU), represents a viable solution to turn waste CO2 into a market commodity. Carbon dioxide can be captured from different point source emitters (i.e., flue gases from industrial processes or power generation technologies fed with fossil-based fuels), from distributed sources (e.g., CO2 concentrated in the air through the direct air capture technology), from biological sources (e.g., anaerobic digestion biogas undergoing an upgrading process often leaves a concentrated CO2 stream as by-product), or from bio-syngas coming from the gasification of biomass of mixed origin. Carbon dioxide can be then utilized as the carbon feedstock to produce services and end-products with potential market value through biological or chemical conversion. The ranges of CO2 utilization cover both direct and indirect applications. Direct uses include food and beverages production, metals fabrication, heat transfer medium in refrigeration and supercritical power systems, yield boosting for biological and injection into existing reservoirs as enhanced oil recovery (EOR) for oil and enhanced gas recovery (EGR) for natural gas. Within these processes, the CO2 molecule remains unaltered in its chemical form and is embedded in the service production process after impurities elimination. Within indirect applications, carbon dioxide is transformed through conversion processes into value-added products (thermo-/electro-chemical and biological conversion of CO2). This results in secondary compounds that can substitute their conventional counterparts, such as building materials, cement, CO2-cured concrete, fuels, and chemicals. The conversion of carbon dioxide into fuels and chemicals allows for the displacement of fossil-based compounds employed in the chemical, transport, power production, pharmaceutical sectors. For this type of conversion route, the carbon dioxide usually undergoes a one-step hydrogenation process (i.e., reaction of CO2 with H2) or a two-steps hydrogenation process. In the first case, CO2 directly reacts with H2. In the latter case, CO2 is firstly downgraded to CO by breaking the carbon dioxide C=O bonds, and subsequently into end-products by reaction between CO and H2. The hydrogenation of either CO2 or CO to the end-products usually takes place into chemical reactors, where the reaction is driven on a catalytic bed. Additionally, to make such a route environmentally competitive, the required hydrogen must be generated utilizing renewable energy sources (i.e., electrochemical conversion of steam to hydrogen via electrolysis technology fed with renewable energy electricity). The utilization of carbon dioxide can also be implemented into routes aiming at harvesting algae biomaterial, very promising and effective CO2 receivers thanks to their high photosynthesis capacity. The cultivation of algae can be done in raceway ponds (open system), or photobioreactors (closed system). Lastly, fixation of carbon dioxide into minerals and construction materials can follow in-situ and ex-situ processes. In-situ CO2 fixation processes inject carbon dioxide into geological storages rich in silicates and alkaline acquifers. By reaction of CO2 with minerals, calcium and magnesium silicates and carbonates can be obtained. With ex-situ applications, the carbonation process is chemically sustained in industrial plants, favouring the production of sodium, magnesium, calcium carbonates and sodium bicarbonate. Such materials can be further utilized in cement production and construction processes (e.g., utilization of CaO, CO2-cured concrete, building aggregates) Considering about 250/300 MtCO2/y currently employed in commercial-scale CCU routes, the food and beverage industry and the EOR/EGR applications account for 11 MtCO2/y and 6 MtCO2/y, respectively. For conversion application, the methanol production uses about 150 MtCO2/y for a market size of about 65 Mtmethanol/y. Lastly, urea represents the chemical with the highest carbon uptake (130 MtCO2/y) for a market potential in the range of 180 Mturea/y.