Paula Teixeira

The potential of the supercritical antisolvent micronization (SAS) technique was evaluated for the production of CaO-based particles with a size and a physical structure that could enable high performance for CO₂ capture through the calcium looping process. Two sources of calcium derivative compounds were tested, waste marble powder (WMP) and dolomite. The SAS micronization of the derivate calcium acetate was carried out at 60 °C, 200 bar, a 0.5 mL min⁻¹ flow rate of liquid solution, and 20 mg mL⁻¹ concentration of solute, producing, with a yield of more than 70%, needle-like particles. Moreover, since dolomite presents with a mixture of calcium and magnesium carbonates, the influence of the magnesium fraction in the SAS micronization was also assessed. The micronized mixtures with lower magnesium content (higher calcium fraction) presented needle-like particles similar to WMP. On the other hand, for the higher magnesium fractions, the micronized material was similar to magnesium acetate micronization, presenting sphere-like particles. The use of the micronized material in the Ca-looping processes, considering 10 carbonation-calcination cycles under mild and realistic conditions, showed that under mild conditions, the micronized WMP improved CaO conversion. After 10 cycles the micronization, WMP presented a conversion 1.8 times greater than the unprocessed material. The micronized dolomite, under both mild and real conditions, maintained more stable conversion after 10 cycles.

CO₂ uptake by MgO-based sorbents at intermediate temperatures is attractive for pre- and post-combustion CO₂ capture applications. However, besides the high CO₂ uptake potential of these materials (1.1 g CO₂ g⁻¹ sorbent), in practice, the realistic CO₂ capture is far from that of the theorical values. In this work, the sol–gel method was used to synthetize unsupported and supported MgO sorbents (10% Ca⁻ or 10% Ce⁻ support, mol) that were impregnated with different fractions (15, 25, and 35; % mol) of a NaNO₃ single salt or a ternary alkali salt (NaNO₃, LiNO₃ and KNO₃ (18/30/52; % mol)). To understand the role of alkali metal salts (AMSs) in the MgO sorbents’ performance, the working and decomposition temperature ranges of AMS under different atmospheres (CO₂ and air) were evaluated. The findings show that the CO₂ uptake temperature range and maximum uptake (20–500 °C, CO₂ atmosphere) of sorbents are correlated. The cyclic CO₂ uptake of the most promising sorbents was tested along five carbonation–calcination cycles. For the first and fifth cycles, respectively, the 15 (Na, K, Li)-MgO sorbents showed the highest carrying capacity, i.e., 460–330 mg CO₂ g⁻¹ sorbent, while for the 15 (Na, K, Li)-MgO-Ca sorbents, it was 375–275 mg CO₂ g⁻¹. However, after the first cycle, the carbonation occurred faster for the 15 (Na, K, Li)-MgO-Ca sorbents, meaning that it can be a path to overpassing carbonation kinetics limitations of the MgO sorbent, making it viable for industrial applications.

Hydrogen is a versatile vector for heat and power, mobility, and stationary applications. Steam methane reforming and coal gasification have been, until now, the main technologies for H₂ production, and in the shorter term may remain due to the current costs of green H₂. To minimize the carbon footprint of these technologies, the capture of CO₂ emitted is a priority. The in situ capture of CO₂ during the reforming and gasification processes, or even during the syngas upgrade by water–gas shift (WGS) reaction, is especially profitable since it contributes to an additional production of H₂. This includes biomass gasification processes, where CO₂ capture can also contribute to negative emissions. In the sorption-enhanced processes, the WGS reaction and the CO₂ capture occur simultaneously, the selection of suitable CO₂ sorbents, i.e., with high activity and stability, being a crucial aspect for their success. This review identifies and describes the solid sorbents with more potential for in situ CO₂ capture at high and medium temperatures, i.e., Ca- or alkali-based sorbents, and Mg-based sorbents, respectively. The effects of temperature, steam and pressure on sorbents’ performance and H₂ production during the sorption-enhanced processes are discussed, as well as the influence of catalyst–sorbent arrangement, i.e., hybrid/mixed or sequential configuration.

CO₂ methanation is typically carried out using Ni-supported catalysts containing promoters such as alkali or alkali-earth metals to improve their properties. In this work, bimetallic Ni-based USY zeolite catalysts containing alkali (Li, K and Cs) and alkali-earth (Mg, Ca) metal compounds were prepared using the same conditions (15 wt% of metals; co-impregnation), characterized by N₂ sorption, XRD, TGA, CO₂ adsorption–desorption, DRS UV-Vis and H₂-TPR, and finally applied in CO₂ methanation reaction (86,100 mL h⁻¹ g⁻¹, PCO₂ = 0.16 bar, H₂:CO₂ = 4:1). For each group, the effects of the second metal nature on the properties and performances were assessed. Alkali metals incorporation induced considerably low catalytic performances (CH₄ yields < 26%), attributed to their negative impact on zeolite structure preservation. On the contrary, alkali-earth metal-containing catalysts exhibited lower structural damage. However, the formation of Ni-Mg mixed oxides in Ni-Mg/USY catalyst and CaCO₃ during the reaction in Ni-Ca/USY sample could explain their performances, similar or lower than those obtained for Ni/USY catalyst. Among the studied metals, calcium was identified as the most interesting (CH₄ yield of 65% at 415 °C), which was ascribed to the slight improvement of the Ni⁰ dispersion.

The use of wastes of marble powder (WMP) and dolomite as sorbents for CO₂ capture is extremely promising to make the Ca-looping (CaL) process a more sustainable and eco-friendly technology. For the downstream utilization of CO₂, it is more realistic to produce a concentrated CO₂ stream in the calcination step of the CaL process, so more severe conditions are required in the calciner, such as an atmosphere with high concentration of CO₂ (>70%), which implies higher calcination temperatures (>900 °C). In this work, experimental CaL tests were carried out in a fixed bed reactor using natural CaO-based sorbent precursors, such as WMP, dolomite and their blend, under mild (800 °C, N₂) and realistic (930 °C, 80% CO₂) calcination conditions, and the sorbents CO₂ carrying capacity along the cycles was compared. A blend of WMP with dolomite was tested as an approach to improve the CO₂ carrying capacity of WMP. As regards the realistic calcination under high CO₂ concentration at high temperature, there is a strong synergetic effect of inert MgO grains of calcined dolomite in the blended WMP + dolomite sorbent that leads to an improved stability along the cycles when compared with WMP used separately. Hence, it is a promising approach to tailor cheap waste-based blended sorbents with improved carrying capacity and stability along the cycles under realistic calcination conditions.

CaO-based materials have a high commercialization potential to be used as Calcium-Looping (CaL) sorbents for CO2 capture, owing to their high CO2 sorption capacity and the lowcost raw materials. The present work focuses on the comparative study of two natural CaO-based sorbents (limestone and wastes of marble powder) with and without alumina incorporation, tested under CaL conditions for CO2 capture. The incorporation of alumina in CaO-based pellets was performed by extrusion followed by calcination at 800 ºC and granulation, using different CaO/Al2O3 ratios (50/50, 60/40 and 70/30) and MethocellTM as pore forming agent. The experiments were carried out in a laboratory scale fixed bed reactor unit along 10 carbonation-calcination cycles using a gas mixture with a CO2 concentration of 25% to mimic the real flue gases in the cement industry, and the calcination was performed under industrial realistic conditions (80% of CO2; 930 ºC) foreseen for the subsequent CO2 utilization. During the CaL tests a fraction of CaO was converted to calcium aluminate due to its exposure to the process high temperatures. Anyway, the use of modified natural CaO-based pellets containing Al2O3 as structural agent is attractive, since higher CaO conversion values were achieved. The enhanced performance of pellets can be justified by the higher specific surface area and stabilization of CaO crystallite sizes along the cycles promoted by the formation of calcium aluminates. The highest CaO conversion was achieved for the case of 60/40 CaO/Al2O3 pellets, allowing the reduction of the CaO make-up frequency along the time as well as the associated costs.