Ca(OH)2-Based Calcium Looping Process Development at The Ohio State University

Tuning Reactive Crystallization Pathways for Integrated CO2 Capture, Conversion, and Storage via Mineralization

ConspectusAchieving carbon neutrality requires realizing scalable advances in energy- and material-efficient pathways to capture, convert, store, and remove anthropogenic CO2 emission in air and flue gas while cogenerating multiple high-value products. To this end, earth-abundant Ca- and Mg-bearing alkaline resources can be harnessed to cogenerate Ca- and Mg-hydroxide, silica, H2, O2, and a leachate bearing high-value metals in an electrochemical approach with the in situ generation of a pH gradient, which is a significant departure from existing pH-swing-based approaches. To accelerate CO2 capture and mineralization, CO2 in dilute sources is captured using solvents to produce CO2-loaded solvents. CO2-loaded solvents are reacted Ca- and Mg-bearing hydroxides to produce Ca- and Mg-carbonates while regenerating the solvents. These carbonates can be used as a temporary or permanent store of CO2 emissions. When carbonates are used as a temporary store of CO2 emissions, electrochemical sorbent regeneration pathways can be harnessed to produce high-purity CO2 while regenerating Ca- and Mg-hydroxide and coproducing H2 and O2. Figure 1 is a schematic representation of this integrated approach.Tuning the molecular-scale and nanoscale interactions underlying these reactive crystallization mechanisms for carbon transformations is crucial for achieving kinetic, chemical, and morphological controls over these pathways. To this end, the feasibility of (i) crystallizing Ca- and Mg-hydroxide during the electrochemical desilication of earth-abundant alkaline industrial residues, (ii) accelerating the conversion of Ca- and Mg-carbonates for temporary or permanent carbon storage by harnessing regenerable solvents, and (iii) regenerating Ca- and Mg-hydroxide while coproducing high-purity CO2, O2, and H2 electrochemically is established.Evidence of the fractionation of heterogeneous slag to coproduce silica, Ca- and Mg-hydroxide, and a leachate bearing metals during electrochemical desilication provides the basis for further tuning the physicochemical parameters to improve the energy and material efficiency of these pathways. To address the slow kinetics of CO2 capture and mineralization starting from ultradilute emissions, reactive capture pathways that harness solvents such as Na-glycinate are shown to be effective. The extents of carbon mineralization of Ca(OH)2 and Mg(OH)2 are 97% and 78% using CO2-loaded Na-glycinate upon reacting for 3 h at 90 °C. During the regeneration of Ca- and Mg-hydroxide and high-purity CO2 from carbonate sources, charge efficiencies of as high as 95% were observed for the dissolution of MgCO3 and CaCO3 while stirring at 100 rpm. Higher yields of Mg(OH)2 are observed compared to that for Ca(OH)2 during sorbent regeneration due to the lower solubility of Mg(OH)2. These findings provide the scientific basis for further tuning these reactive crystallization pathways for closing material and carbon cycles to advance a sustainable climate, energy, and environmental future.

Analysis of Operation Conditions of Ca(OH)2 Entrained Carbonator Reactors for CO2 Capture in Backup Power Plants

The share of renewables in the energy sector is increasing, and energy storage and backup power combustion systems to cover the periods of time with low renewable energy production are becoming increasingly needed. Flexible calcium looping configurations based on the storage of solids are a promising alternative to capture the CO₂ produced in such backup combustion systems. The use of Ca(OH)₂ instead of CaO is better suited to these applications due to the faster reaction kinetics and higher carbonation conversions as Ca(OH)₂ in powder form can achieve conversions of up to 0.7 in just a few seconds at temperatures of 550-650 °C. To take advantage of these fast reaction kinetics, compact carbonator reactors with short gas-solid contact times (i.e., a few seconds) can be designed. However, the low enthalpy of the carbonation reaction of Ca(OH)₂ makes it challenging to find the optimum conditions which maximize the CO₂ capture efficiency. In this work, a basic entrained reactor with recent experimental reaction kinetics has been used to determine suitable operational windows for this kind of carbonator. CO₂ capture efficiencies above 90% can be achieved for flue gases with low CO₂ concentrations (4%_v CO₂) when they are fed into the carbonator at temperatures of around 500-600 °C while maintaining low F _Ca/F _CO2 ratios (<2) and feeding the sorbent at ambient temperature. When capturing from a flue gas with a higher CO₂ concentration (14%_v CO₂), the sorbent needs to be fed at higher temperatures to effectively capture CO₂ in short contact times (i.e., 6 s).

Carbonation Kinetics of Ca(OH)2 Under Conditions of Entrained Reactors to Capture CO2

The use of Ca(OH)₂ as a CO₂ sorbent instead of CaO in calcium looping systems has the advantage of a much faster reaction rate of carbonation and a larger conversion degree to CaCO₃. This work investigates the carbonation kinetics of fine Ca(OH)₂ particles (<10 μm) in a range of reaction conditions (i.e., 350–650 °C and CO₂ concentrations up to 25%_v) that could be of interest for applications where a short contact time is expected between the solids and the gases (i.e., entrained bed carbonator reactors). For this purpose, experiments in a drop tube reactor with short reaction times (i.e., below 6 s) have been carried out. High carbonation conversions up to 0.7 have been measured under these conditions, supporting the viability of using entrained carbonator reactors. The experimental results have been fitted to a shirking core model, and the corresponding kinetic parameters for the carbonation reaction have been determined.

Calcium carbonate synthesis from waste concrete for carbon dioxide capture: From laboratory to pilot scale

This research article explains the synthesis and scale-up of calcium carbonate (CaCO₃) from waste concrete as calcium-rich material by an inorganic carbonation process. The operating parameters include S/L ratio, HCl concentration, contact time, and extraction pH were investigated. The calcium hydroxide (Ca(OH)₂) was synthesized by reaction between calcium chloride (CaCl₂) and sodium hydroxide (NaOH), which induced the spontaneous reaction of CaCO₃ without additional energy consumption. The productivity of CaCO₃ was 1 kg/d in the laboratory scale experiment, and the CaCO₃ productivity was scale-up to 20 kg/d through pilot scale process by same way as the laboratory scale. The approximately 4800 g of CaCO₃ was produced and 2112 g of CO₂ was captured per each cycle operation. Consequently, considered power consumption, the estimated amount of reduced CO₂ was 465 g of CO₂ in the pilot-scale reactor per cycle and produced CaCO₃ with a purity of 99.0 %.

Limestone calcination under calcium-looping conditions for CO2 capture and thermochemical energy storage in the presence of H2O: an in situ XRD analysis

The presence of H₂O at very low concentrations in the calciner significantly accelerates decomposition, while the resulting CaO crystal structure and reactivity are not modified.