An Integrated, Low Temperature Process to Capture and Sequester Carbon Dioxide from Industrial Emissions

Laboratory experiments show that it is possible to integrate (1) the chemistry of serpentine dissolution, (2) capture of CO₂ gas from the combustion of natural gas and coal-fired power plants using aqueous amine-based solvents, (3) long-term CO₂ sequestration via solid phase carbonate precipitation, and (4) capture solvent regeneration with acid recycling in a single, continuous process. In our process, magnesium is released from serpentine at 300°C via heat treatment with ammonium sulfate salts or at temperatures as low as 50°C via reaction with sulfuric acid. We have also demonstrated that various solid carbonate phases can be precipitated directly from aqueous amine-based (NH₃, MEA, DMEA) CO₂ capture solvent solutions at room temperature. Direct precipitation from the capture solvent enables regenerating CO₂ capture solvent without the need for heat and without the need to compress the CO₂ off gas. We propose that known low-temperature electrochemical methods can be integrated with this process to regenerate the aqueous amine capture solvent and recycle acid for dissolution of magnesium-bearing mineral feedstocks and magnesium release. Although the direct precipitation of magnesite at ambient conditions remains elusive, experimental results demonstrate that at temperatures ranging from 20°C to 60°C, either nesquehonite Mg(HCO₃)(OH)●2H₂O or a double salt with the formula [NH₄]₂Mg(CO₃)₂●4H₂O or an amorphous magnesium carbonate precipitate directly from the capture solvent. These phases are less desirable for CO₂ sequestration than magnesite because they potentially remove constituents (water, ammonia) from the reaction system, reducing the overall efficiency of the sequestration process. Accordingly, the integrated process can be accomplished with minimal energy consumption and loss of CO₂ capture and acid solvents, and a net generation of 1 to 4 moles of H₂O/6 moles of CO₂ sequestered (depending on the solid carbonate precipitate and amount of produced H₂ and O₂ gas reacted to produce heat and water). Features of the integrated process include the following: 1) the four separate processes have compatible chemistry, enabling design of an integrated, continuous process scheme for CO₂ capture and sequestration; 2) all 4 stages of the process can be conducted at ambient or slightly elevated temperatures; 3) precipitating carbonate directly from the capture solvent eliminates the need for costly CO₂ gas compression; and 4) recycling the acid used for serpentine dissolution and the solvent used for CO₂ capture reduces feed stock costs.