Mineral carbonation is a naturally occurring process to capture CO2. Research at Abo Akademi University (AAU) has tried to speed up the process in order to find a suitable option for future CO2 cap ...
Mineral carbonation routes have been extensively studied for almost two decades at Åbo Akademi University, focusing on the extraction of magnesium from magnesium silicates using ammonium sulfate (AS) and/or ammonium bisulfate (ABS) flux salt followed by carbonation. There is, however, a need for proper recovery and recirculation of chemicals involved. This study focused on the separation of AS, ABS and aqueous ammonia using different setups of bipolar membrane electrodialysis using both synthetic and rock-derived solutions. Bipolar membranes offer the possibility to split water, which in turn makes it possible to regenerate chemicals like acids and bases needed in mineral carbonation without excess gas formation. Tests were run in batch, continuous, and recirculating mode, and exergy (electricity) input during the tests was calculated. The results show that separation of ions was achieved, even if the solutions obtained were still too weak for use in the downstream process to control pH. Energy demand for separating 1 kg of NH4+ varied in the range 1.7, 3.4, 302 and 340 MJ/kg NH4+, depending on setup chosen. More work must hence be done in order to make the separation more efficient, such as narrowing the cell width.
The separation of ammonium bisulfate (ABS) from ammonium sulfate (AS) in aqueous solutions by monovalent ion selective membranes was studied. Optimised usage of these chemicals is both an important and challenging step towards a more efficient CO2 mineralisation process route developed at Åbo Akademi University (ÅA). The membranes were placed in a three or five-compartment electrodialysis stack. Silver, stainless steel and platinum electrodes were tested, of which a combination of Pt (anode) and stainless steel (cathode) electrodes were found to be most suitable. Separation efficiencies close to 100% were reached based on ABS concentrations in the feed solution. The tests were performed with an initial voltage of either 10 V–20 V, but limitations in the electrical power supply equipment eventually resulted in a voltage drop as separation proceeded. Exergy calculations for energy efficiency assessment show that the input exergy (electrical power) is many times higher than the reversible mixing exergy, which indicates that design modifications must be made. Further work will focus on the possibilities to make the separation even more efficient and to develop the analysis methods, besides the use of another anode material.
Abstract Vast resources of serpenitinite rock available worldwide are capable of binding CO 2 amounts that diminish the capacity of methods based on geological storage of CO 2 . R&D has been ongoing in Finland for many years on developing large‐scale application of process routes for serpentinite carbonation. Several routes have been assessed in the laboratory, in all cases using ammonium salts to extract magnesium from rock followed by carbonation either in a gas/solid reactor at elevated temperatures and pressures or in an aqueous solution at ambient conditions. The choice for either route is motivated by the CO 2 ‐producing source, (waste) heat availability, the magnesium (hydro‐)carbonate product aimed at, and a preference for energy efficiency or simplicity. Rocks from several locations have been analysed. A special issue is the recovery of the ammonium flux salt, typically from an aqueous solution. As for application, several industry sectors are considered, such as a (natural gas fired) power plant, a lime kiln, or iron‐ and steelmaking, applying mineral carbonation (MC) to blast furnace top gas. The analysis includes life cycle assessment (LCA). Finally, the use of magnesium (hydro‐)carbonates for heat storage is addressed.
This paper reports on tests performed with the dual aim of minimizing the energy use (kilojoules per kilogram) and maximizing the conversion rate (kilograms per hour) of bipolar membrane electrodialysis (BPMED) for the regeneration of chemicals needed for the effective scale-up of the accelerated CO2 mineralization route developed at Åbo Akademi University (ÅA). The performance of two- and three-compartment stacks was compared with ammonium sulfate (AS) and ammonium bisulfate (ABS) as the input product feed, yielding sulfuric acid and aqueous ammonia, respectively, as the final products. It was shown that a two-compartment stack is more efficient with regard to energy use (i.e., electricity consumption), with values in the range of 3630–4844 kJ/kg of AS or ABS, compared to the three-compartment stack requiring 5102–7223 kJ/kg of AS or ABS. A maximum conversion rate of ∼0.13 kg/h was achieved with the two-compartment stack. We also concluded that approximately 25% of the energy needed for the process may give off heat, depending primarily on the voltage applied to the membrane stack. A two-compartment stack will require fewer membranes, which is an obvious benefit in terms of maintenance and cost. Furthermore, we concluded that AS provides a more efficient conversion than ABS, when considering both energy use and the amounts of the solution that need to be recycled in the BPMED step.
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