By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Oct 22 2024A recent article published in Sustainable Chemistry & Engineering proposed a ZeroCAL pathway to use CaCO3 as a Ca source to make hydrated lime (portlandite, Ca(OH)2) at ambient conditions while almost eliminating CO2 emissions. This method can help decarbonize cement as Ca(OH)2 is a zero-carbon precursor for cement and lime production.
Study: ZeroCAL: Eliminating Carbon Dioxide Emissions from Limestone’s Decomposition to Decarbonize Cement Production. Image Credit: Another77/Shutterstock.com
Background
Concrete, the second most used material globally, employs Portland cement (CM) as the binding agent. However, per ton of PC production emits ∼1 ton of CO2. These CO2 emissions result from the thermochemical decomposition of CaCO3 to produce CaO (process emissions, ∼60%) and the combustion of fossil fuels to heat the kiln to ∼1500 °C (combustion emissions, ∼40%) for clinkering reactions.
Using renewable heat sources is insufficient to decarbonize PC production. It also requires replacing, displacing, or transforming limestone as the CaO source. Accordingly, different methods have gained prominence to decarbonize PC production using non-carbonate Ca-containing feedstocks such as fly ashes, steel slags, basalts, etc., to avoid CO2 emissions.
However, they suffer multiple challenges, including a constrained feedstock supply and incapability to yield compositional equivalence to PC. Therefore, this study proposed a transformative approach that uses limestone within an aqueous electrochemical paradigm to produce portlandite (Ca(OH)2), a zero-carbon feedstock for PC production.
Methods
The ZeroCAL process involved dissolution, separation/recovery, and electrolysis stages. Firstly, CaCO3 was dissolved in a chelator (ethylenediaminetetraacetic acid (EDTA)). Subsequently, Ca enrichment and separation were performed using nanofiltration (NF), separating the Ca-EDTA complex from the accompanying bicarbonate (HCO3–) species.
A Ca-enriched stream was obtained through the decomplexation of Ca from EDTA, and Ca(OH)2 was rapidly precipitated from it using electrolytically produced alkalinity. These reactions were conducted in a seawater matrix yielding coproducts, including HCl and NaHCO3, resulting from electrolysis and limestone dissolution, respectively.
An analytical-grade calcite sample and a high-purity limestone rock were used as calcium precursors. Solid H4EDTA and NaOH powders were used to prepare EDTA-containing solutions. Eight commercial NF membranes were selected for separation steps based on their performance parameters.
Solid phase analysis was performed on the prepared powders. A light scattering analyzer was used to determine their particle size distribution. Additionally, they were characterized via X-ray fluorescence, thermogravimetric analysis, X-ray diffraction, and Fourier transform infrared spectroscopy.
In the solution-phase analysis, inductively coupled plasma optical emission spectrometry was performed for multi-elemental analysis to quantify Ca and Na concentrations. Volumetric titrations helped determine the concentration of dissolved EDTA, Ca-EDTA complex, and dissolved inorganic carbon (DIC). Alternatively, chloride concentrations in solution were determined using the iron(III) thiocyanate method with an ultraviolet-visible spectrophotometer.
Gas chromatography was used to quantify CO2 evolution during gas phase analysis. Furthermore, thermodynamic modeling of solid- and solution-phase equilibria was performed using Gibbs Energy Minimization-Selektor software.
Results and Discussion
The overall ZeroCAL involved Ca extraction (from limestone), its aqueous separation from CO2 while preventing CO2 degassing, and the electrolytic precipitation of Ca(OH)2. Analysis of the consumable reaction stoichiometries revealed that the production of 1 ton of Ca(OH)2 required 1.35 tons of CaCO3, 1.09 tons of water, and 0.79 tons of NaCl.
Notably, ∼75% of the NaCl electrolyte was recycled during the process. Additionally, ∼100% of the EDTA used was recovered and reused, minimizing additive demand. The process coproduced 0.54 tons of O2, 68 kg of H2, 0.49 tons of HCl, and 1.13 tons of NaHCO3 per ton of Ca(OH)2. The H2 could be co-combusted with oxygen to heat the cement kiln or produce electricity, offsetting significant energy demand.
Alternatively, (80%) of the HCl was used internally for EDTA recovery and pH conditioning within the process. The carbonate stream (NaHCO3) could be discharged into water streams for carbon storage or recovered as dilute soda.
Analysis of CO2 evolution during the process indicated a vanishingly small CO2 intensity. Specifically, process emissions were as low as 1.5% of the total mineralized CO2 contained in limestone when the dissolution was performed at a slightly alkaline pH (9.5), and two-stage NF was used for effective Ca/CO2 separation.
In total, the proposed ZeroCAL process ultimately resulted in over 98% process-CO2 emissions reductions while requiring ∼two times the total energy demand of conventional lime manufacturing or limestone decarbonization.
Conclusion
Overall, the researchers successfully established the proposed ZeroCAL approach as a process-emissions-free pathway to decarbonize PC production using limestone as a feedstock. In-situ electrolytic acid/base generation induced a self-contained pH-swing process, wherein EDTA-promoted CaCO3 dissolution while preventing CO2 exsolution.
Subsequent NF enabled effective Ca/CO2 separation, process water recycling, and scalable discharge of CO2 in the form of aqueous HCO3- at sufficient alkalinity so that CO2 could be stored durably and permanently in the near-surface environment.
Under optimized configurations, seawater, electricity, and limestone were the only inputs to the ZeroCAL process. Thus, electrification and hydrogen-based kiln heating of PC production are promising pathways to decarbonize cement.
Journal Reference
Leão, A. et al. (2024). ZeroCAL: Eliminating Carbon Dioxide Emissions from Limestone’s Decomposition to Decarbonize Cement Production. ACS Sustainable Chemistry & Engineering. DOI: 10.1021/acssuschemeng.4c03193, https://pubs.acs.org/doi/10.1021/acssuschemeng.4c03193
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