CO2 valorization by mineral storage and abiotic hydrocarbons generation

This study examined hydrogen production, CO2 storage and abiotic hydrocarbon generation during gas-water-rock interactions by conducting hydrothermal experiments. The first part of this manuscript presents the simultaneous CO2 sequestration and hydrogen production by reacting New Caledonian mine tailings with CO2 saturated water at 473 K <T< 573 K and 15 MPa <PCO2< 30 MPa. The results showed that the best conditions for both these reactions were 523 K <T<540 K at 30 MPa, capturing 320.5 g of CO2 in the form of iron-rich magnesite ((Mg,Fe)CO3), and producing 0.57 g of H2 per 1 kg of mine tailings. In addition, considering the annual mine tailings production and the annual CO2 emission in New Caledonia, the proposed method could potentially capture ~90 % of New Caledonia’s CO2 emissions. In addition, the H2 produced by this method could offset ~10 % of New-Caledonia’s annual electrical consumption. Further investigation of secondary products and their mineral-water interfaces at nanometer scale indicated that the reactions were taken place by dissolving mine tailings followed by precipitation of iron rich magnesite, smectite group clay minerals (nontronite, vermiculite), traces of iron oxides and amorphous silica. Although, the phyllosilicates and amorphous silica could potentially act as passivating layers, slowing down the dissolution kinetics and consequently limiting the CO2 storage and H2 production capacities, our experiments demonstrated that the reactivity of New Caledonian mine tailings could also be lowered by the presence of glass. The second part of this manuscript presents the interaction of dissolved CO2 and H2 during the synthesis of “abiotic” hydrocarbons, via Fischer-Tropsch type (FTT) synthesis in the presence of two potential catalysts found in natural systems; sphalerite (ZnS) and marcasite (FeS2). The experiments were conducted at 573 K and 30 MPa in gold capsules heated and pressurize in autoclaves. Hydrogen necessary for the reaction was provided by Fe2+ oxidation of minerals such as olivine (Mg1.80Fe0.2SiO4), fayalite (Fe2SiO4) and Fe-rich chlorite or chamosite (6Fe5Al(AlSi3)O10(OH)8). Methane (CH4) produced in our experiments was one order of magnitude higher than those reported in previous studies when magnetite and iron oxide-chromite were used as catalysts for CH4 production, and in the same order of magnitude as pentlandite and Fe-Ni alloy catalysts. However, the small conversion rates of inorganic carbon into organic carbon as well as the Schulz-Flory distribution of C1-C4 alkanes demonstrated that sphalerite and marcasite do not explicitly catalyze the FTT synthesis of hydrocarbons under the conditions of these experiments.