Carbon-Free Portland Cement

Fig. 1: Comparison of emissions from traditional and sulfate calcination, as described by Eqs. (1) and (2). (Source: A. Cruz)

A recent patent application by Brimstone Energy describes a method of producing Portland cement in a way that is "carbon-free". [1] Cement is traditionally made by calcining limestone (calcium carbonate) to form calcium oxide, with carbon dioxide produced as a byproduct. The new proposal is to treat calcium aluminosilicate with sulfuric acid to form kaolinite and calcium sulfate. [1] The calcium sulfate is then calcined to form calcium oxide, sulfur dioxide, and oxygen. The contact process is then used to regenerate sulfuric acid and prevent sulfur dioxide emissions to the atmosphere.

While the patent language claims that this series of reactions is enthalpically neutral, it will be very difficult to develop a process that is actually energy neutral and carbon free at commercial scale. The acid treatment and contact processes are energetically downhill. But recovering enough heat from these steps to calcine calcium sulfate is a significant engineering challenge because acid treatment and recovery both occur in water. Before the heat of the reaction can be used in calcination, it must raise the temperature of the water to 100°C and then vaporize it into steam. After these energy transfers, it is unlikely that the steam will be a high enough temperature to thermally treat the calcium sulfate. In all likelihood, it will be necessary for calcination to occur in a traditional fired kiln.

Limestone can be calcined into calcium oxide and CO₂ at 750°C [2] through the reaction

Using the enthalpies of formation in Table 1, we find the enthalpy of this calcination reaction to be 179.1 kJ/mol.

Calcium sulfate can similarly be calcined into calcium oxide, O₂ and SO₂ at 1,375°C through the reaction [3]

Again, using the enthalpies of formation in Table 1, the enthalpy of this calcination reaction is 502.8 kJ/mol.

Here we can see a clear tradeoff between proposed ne new process for making Portland cement and the traditional one. The calcination of calcium sulfate does indeed not produce carbon dioxide as a byproduct, but it has a much higher change in enthalpy. Calcination is an endothermic process, so heat must be delivered to the system from the outside in order to drive the reaction forward. The combustion of natural gas produces 802.5 kJ/mol and produces one mole of carbon dioxide per the reaction

We can now calculate the amount of carbon dioxide emitted for each mole of calcium oxide produced by calcination. The traditional process emits 0.223 mol CO₂ per mol CaO from combustion and 1 mol CO₂ per mol CaO from the reaction for a total emission of 1.223 mol CO₂/mol CaO. The proposed new process emits .627 mol CO₂/mol CaO from combustion. Both of these numbers assume 100% thermal efficiency in the kiln. We can calculate the normalized carbon dioxide emissions for traditional calcination E_T as a function of thermal efficiency η using the equation

We can calculate emissions from the sulfate calcination E_B as a function of thermal efficiency η using the equation

From Fig. 1, we can see that the sulfate process becomes more carbon intensive than the traditional process at thermal efficiencies below 40%.

Portland cement made by sulfate calcination can only be "carbon free" if the calcination step occurs in an electrically fired kiln. Because appliances are only as green as the grid they are connected to, industries should be conscious of their utilities if they choose to use an electric kiln. If one can engineer a kiln that has a very high thermal efficiency, it is indeed possible to deliver a product that significantly reduces carbon dioxide emissions, which would be quite useful. An alternative would be to sequester CO₂ from flue gas of a fired kiln. In order for this alternative process to be "carbon-free," the sequestration system must either have a parasitic load (emissions from generating the electricity required to operate the system) of 0% or be connected to a 100% renewable grid.

© Antone Cruz. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] C. E. Finke and H. F. Leandri, "Process to Make Calcium Oxide or Ordinary Portland Cement from Calcium Bearing Rocks and Minerals," U.S. Patent Application 2021/0070656 A1, 11 Mar 21.

[2] K. S. P. Karunadasa et al., "Thermal Decomposition of Calcium Carbonate (Calcite Polymorph) as Examined by In-Situ High-Temperature X-ray Powder Diffraction," J. Phys,. Chem. Solids. 134, 21 (2019).

[3] W. M. Swift et al., "Decomposition of Calcium Sulfate: A Review of the Literature," Argonne National Laboratory ANL-76-122, December 1976.

[4] N. J Tro, Principles of Chemistry, 4th Ed. (Pearson, 2019).