Sublime Systems ($40M for producing sustainable cement)

Sublime Systems uses electrolyzers, which are powered by renewable electricity and operate near ambient temperature, to produce acid and base solutions to generate reactants of the pozzolanic reaction for the formation of Portland cement with significantly reduced CO₂ emission.

Challenges: cement emissions

The production of cement is a major source of carbon dioxide (CO₂) emissions, accounting for around 8% of global CO₂ emissions. This is because the production of cement involves a chemical reaction called calcination, in which limestone (CaCO₃) is heated to over 1400 ºC in a kiln to produce lime (CaO). This process releases carbon dioxide according to the following reaction:

CaCO₃ + heat → CaO + CO₂

Additionally, the cement production also requires large amounts of energy to heat the kiln and grind the raw materials into the fine powder that is used to make cement. The energy required for cement production comes mainly from burning fossil fuels such as coal, oil, and natural gas, which also releases CO₂ into the atmosphere.

It is estimated that 75% of the infrastructure that will exist in 2050 has not yet been built, which will lead to a massive increase in demand for cement, which necessitates new approaches to producing low-carbon cement.

Efforts are being made to reduce its environmental impact, including using alternative fuels, such as hydrogen (H₂), biomass, and waste materials, to replace fossil fuels, using low-carbon raw materials, such as industrial byproducts and recycled materials, in cement production, and developing new technologies to capture and store CO₂ emissions from cement plants.

Sublime Systems Technology

Sublime Systems develops a new approach to producing Portland cement with lower CO₂ emission. The approach uses electrolyzers, which are powered by renewable electricity and operate near ambient temperature, to produce acid and base solutions. The produced acid and base solutions can process various raw materials to generate reactants of the pozzolanic reaction for the formation of the Portland cement.

The pozzolanic reaction is a slow reaction that involves calcium hydroxide or slaked lime (Ca(OH)₂, also known as portlandite) and silicic acid (H₄SiO₄ or Si(OH)₄) or silica (SiO₂) to form a strong cementation matrix according to the following reaction:

Ca(OH)₂ + H₄SiO₄ → CaH₂SiO₄∙2H₂O

The pozzolanic reaction can also be written in an ancient industrial silicate notations as:

Ca(OH)₂ + SiO₂ → CaSiO₃∙H₂O

The decarbonization reactor

The decarbonization reactor may be a neutral-water electrolyzer that can process input material of limestone (CaCO₃) into slaked lime (Ca(OH)₂). The electrolyzer produces acid and base at the anode and cathode, respectively, by the electrolysis of water with renewable electricity. CaCO₃ is dissolved at the acidic anode and Ca(OH)₂ is participated at the base cathode where the pH is greater than 12.5. The precipitated Ca(OH)₂ is used for the Portland cement production process.

The diagram below depicts the structure and working mechanism of the Sublime Systems’s decarbonization reactor.

The decarbonization reactor of Sublime Systems (ref. WO2022204059A1 and WO2022216741A1).

The decarbonization reactor comprises an anode, a cathode, and a porous separator.

Anode

The anode of the electrolyzer produces oxygen gas (O₂) and protons (H⁺) via the electrolysis of water according to the following reaction:

2H₂O → 4H⁺ + O₂(gas) + 4e⁻

The produced protons increase the acidity of the electrolyte near the anode. The input material of limestone (CaCO₃) in the anode chamber is dissolved by the acidic solution near the anode according to the reaction:

CaCO₃(solid) + 2H⁺ → Ca²⁺ + CO₂(gas) + H₂O

The O₂/CO₂ gas mixture from the anode chamber can be recirculated to the kiln for cleaner combustion in the cement sintering cycle. CO₂ can also be directly captured for carbon capture and sequestration (CCS).

Cahode

The cathode of the decarbonization reactor produces hydrogen gas (H₂) and hydroxide ions (OH⁻) via the electrolysis of water according to the reaction:

2H₂O + 2e⁻ → H₂(gas) + 2OH⁻

The produced hydroxide ions react with Ca²⁺ ions which diffuse from the anode chamber via the porous separator to the cathode chamber, resulting in the precipitation of Ca(OH)₂ according to the reaction:

Ca²⁺ + 2OH⁻ → Ca(OH)₂ (solid)

Ca(OH)₂ is used for the Portland cement production process.

H₂ can be used to generate electricity or eat by a fuel cell or a combustor.

Separator

The porous separator allows oxygen gas produced at the anode to diffuse to the cathode, such that the oxygen is reduced at the cathode to produce hydroxide ions (OH⁻), thereby increasing the overall amount of base produced at the cathode.

The separator also allows hydrogen gas produced at the cathode to diffuse to the anode, such that hydrogen is oxidized at the anode to produce protons (H⁺), thereby increasing the overall amount of acid produced at the anode.

The production of portland cement

The produced slaked lime, Ca(OH)₂, can react with silica or aluminosilicates to form Portland cement.

The diagram below depicts the production of aluminosilicates for Portland cement formation.

Sublime Systems’s process of producing aluminosilicate with an electrolyzer (ref. WO2022216741A1).

Sublime Systems may use a large-scale chlor-alkali electrolyzer, powered by renewable electricity and operating near ambient temperature, to produce strong acid and base solutions. The chlor-alkali electrolyzer uses sodium chloride (NaCl) electrolytes to produce sodium hydroxide (NaOH) and chloride (Cl₂) at the anode and hydrogen gas (H₂) at the cathode. The chloride and hydrogen react in an acid burner to create hydrochloric acid (HCl).

The HCl acidic solution is used to leach raw input materials, such as (Mg, Fe)₂SiO₄ olivines and coal ash. Alkaline earth and transition metals are leached from the olivines, leaving behind acid-insoluble silica and alumina, which are separated by filtration and used as components of pozzolanic reaction.

The soluble transition metals (Fe, Cu, Co, and Ni) are extracted from the acidic solution by electrowinning and sold as valuable products. The remaining electrowinning solution contains magnesium ions (Mg²⁺) and Ca²⁺ ions. Magnesium hydroxide (Mg(OH)₂) can be precipitated without Ca(OH)₂ any by controlling the pH within the range of 9 – 11. Ca(OH)₂ can be precipitated by controlling the pH over 11.35. Mg(OH)₂ can be sold as a value product. Ca(OH)₂ can be used as a component of pozzolanic reaction. The remaining salt brine is purified and recycled to the electrolyzer.

If extraction of olivine were increased to 10 Mt/y, it is estimated that in the US market the above processes could produce enough silica for > 25 Mt/y of pozzolanic cement and > 3 Mt/y magnesium hydroxide, thereby eliminating at least 25 Mt/y of CO₂ emissions.

Sublime Systems Patent

WO2022216741A1 Electrochemical materials production and processing
WO2022204059A1 Decarbonized cement blends
WO2022020572A1 Methods for extracting co2 from metal carbonates and use thereof

Sublime Systems Products

Sublime Systems plans to build a demonstration plant producing Portland cement with an annual capacity of 40,000 tonnes a year. The company will then be able to fill silos with its cement and supply the minimum quantity needed for ready-mix concrete suppliers to start using its cement commercially. The final phase is to scale production to one million tonnes a year by 2030.

Sublime Systems Funding

Sublime Systems has raised a total of $40M in funding over 2 rounds: a Seed round with unknown funding amount and a Series A round with $40M funding. Their latest funding was raised on Jan 17, 2023 from a Series A round.