ZeroCAL: Eliminating Carbon Dioxide Emissions from Limestone's Decomposition to Decarbonize Cement Production

Area of Science:

• Materials Science
• Chemical Engineering
• Environmental Science

Background:

• Traditional lime production from limestone (calcite, CaCO3) involves high temperatures (∼800 °C), resulting in significant CO2 emissions (∼1 tonne CO2 per tonne CaO) and energy consumption (∼1.4 MWh per tonne CaO).
• The cement industry is a major contributor to global CO2 emissions, necessitating the development of low-carbon or zero-carbon production pathways.
• Hydrated lime (portlandite, Ca(OH)2) is a zero-carbon precursor for cement and lime production.

Purpose of the Study:

• To introduce a novel pathway for producing hydrated lime (Ca(OH)2) from limestone (CaCO3) under ambient conditions.
• To significantly reduce or nearly eliminate process CO2 emissions associated with lime production.
• To develop a sustainable method for producing zero-carbon precursors for cement manufacturing.

Main Methods:

• The Zero Carbon Lime (ZeroCAL) process involves three stages: dissolution, separation/recovery, and electrolysis.
• Step 1: Chelator-promoted dissolution of CaCO3 and Ca2+ complexation using ethylenediaminetetraacetic acid (EDTA) under basic conditions (pH > 9).
• Step 2: Nanofiltration (NF) for Ca-EDTA separation from bicarbonate, followed by acidity-promoted decomplexation for Ca recovery and chelator regeneration.
• Step 3: Rapid precipitation of Ca(OH)2 from the Ca-enriched stream using electrolytically produced alkalinity, with coproduction of H2 and O2.

Main Results:

• The ZeroCAL process achieves nearly complete elimination of process CO2 emissions, with emissions as low as 1.5 mol.% of the precursor CaCO3 (9 kg CO2 per tonne Ca(OH)2).
• Production of 1 tonne of Ca(OH)2 requires approximately 1.35 t CaCO3, 1.09 t water, 0.79 t NaCl, and ∼2 MWh of electrical energy.
• The process can be adapted for seawater, yielding coproducts like hydrochloric acid (HCl) and sodium bicarbonate (NaHCO3).
• Significant opportunities for process intensification exist.

Conclusions:

• The ZeroCAL process offers a significant advancement in producing zero-carbon cement precursors by utilizing limestone at ambient conditions with minimal CO2 footprint.
• This approach aligns with the electrification of industrial operations and can leverage existing cement plant infrastructure.
• The generated alkalinity facilitates cost-effective and scalable CO2 mineralization via Ca(OH)2 carbonation, further contributing to decarbonization efforts.