Towards Electrochemical Synthesis of Cement — an Electrolyzer-Based Process for Decarbonating CaCO3 while Producing Useful Gas Streams

Abstract

Cement production is currently the largest single industrial emitter of CO2, accounting for 8% (2.8 Gtons/year) of global CO2 emissions in 2015. Deep decarbonization of cement manufacturing will require remediation of both the CO2 emissions due to the decomposition of CaCO3 to CaO, and that due to combustion of fossil fuels (primarily coal) in the calcining (~900°C) and sintering (~1,450°C) processes. Here, we demonstrate an electrochemical process that uses neutral water electrolysis to produce a pH gradient in which CaCO3 is decarbonated at low pH and Ca(OH)2 is precipitated at neutral to high pH, concurrently producing a high purity O2/CO2 gas mixture (1:2 molar ratio at demonstrated stoichiometric operation) at the anode and H2 at the cathode, as shown in Figure 1. We show that the solid Ca(OH)2 product readily decomposes and reacts with SiO2 to form alite, the majority cementitious phase in Portland cement. Our electrochemical calcination approach produces concentrated gas streams from which the CO2 may be readily separated and sequestered, the H2 and/or O2 may be used to generate electric power via fuel cells or combustors, the O2 may be used as a component of oxyfuel in the cement kiln to further lower CO2 and NOx emissions, or the output gases may be used for other value-added processes including liquid fuel production. Analysis shows that in a scenario where the hydrogen produced by the reactor is combusted to heat the high temperature kiln, the electrochemical cement process can be powered solely by renewable electricity. Figure 1. Schematic of the electrolyzer-based decarbonation cell. Reactions 1a and 1b are the O2 evolution and H2 evolution half-cell reactions respectively, under near-neutral pH. Reactions 2a and 2b represent the decomposition of CaCO3 and release of CO2. Reaction 3 is the normal formation of water from its component ions. In Reaction 4, the hydroxide ions in Reaction 3 instead go towards the formation of Ca(OH)2, and the protons protonate carbonate ions (Reaction 2b). The overall reaction in which CaCO3 is converted to Ca(OH)2 with the attendant release of H2, O2 and CO2 is shown at the bottom. Figure 1