Rapid aqueous mineralisation of carbon dioxide in a scalable process with brines

Abstract

Aqueous mineralisation is a form of mineral carbonation process that reacts CO2 with reactants in aqueous phase to produce carbonates that fix CO2 permanently. A target to reach net-zero CO2 emissions by 2050 has been set, in a bid to control the global surface temperature rise to within 1.5 °C and slow down the rate of climate change. Carbon capture and utilisation (CCU) is a portfolio technology that converts captured CO2 into products (e.g. chemicals, fuels, polymers and construction materials) which can offset the capture and sequestration costs. CO2 mineralisation processes perform fundamental advantages with thermodynamically favoured and abundant availability of feedstock among these CCU techniques. The aim of this study is to develop a feasible technology to enable the scale-up design of aqueous mineralisation at optimal processing conditions, to achieve complete carbonation of the calcium content of the brine. An additional goal was to correlate processing conditions to morphological and mineralogical properties of the precipitated carbonates, to aid in scale-up of a process that produces highest value products for specific industries. Compared with the energy-intensive process of gas-solid accelerated mineralisation, aqueous carbonation remains an attractive option requiring milder process parameters, and such processes offer more opportunities for intensification through chemical additives, catalysts, process integration, use of alternative energy sources, reactor design, among other innovations. This thesis presents a strategy to sequester CO2 using alkaline desalination brines. Desalination brine is an attractive source of calcium and magnesium for mineral carbonation since the ions are dissolved in the solution which is ready to react with CO2. An alternative approach to conducting rapid and continuous CO2 mineralisation process in a tubular reactor is revealed which is favoured for scaling-up for industrial application. Different basic substances were utilised to synergistically boost CO2 solubility in the brine while neutralising the acidification following precipitation of alkaline earth metals, and blast furnace (BF) slags, sodium hydroxide and low concentration of MEA were implemented in this study. Nickel nanoparticle catalyst is introduced to accelerate the carbonation reaction. Moreover, a three-phase analytical model is established for the scale-up application. A one-dimensional time-dependent plug-flow model is developed to determine CO2 gas-to-liquid mass transfer rate, aqueous solution chemical speciation, fluid pressure, and carbonation efficiency within ii the tubular reactor, as a function of reactor length and residence time, under different process conditions. Moreover, a basic solid substance to replace the alkaline solution was developed, and more importantly, it can be recycled for several cycle usage. Hydrotalcite (HT) treated as a dechlorination agent rather than buffering the mineralisation reaction, and chloride ions were removed from the brine, increasing its pH, and then conducted the mineralisation reaction. It indicates that hydrotalcite can be easily regenerated, and its memory effect property guarantees high efficiency after several cycles. Gaseous CO2 and Na2CO3 solution were tested as the recharging agents, to replace the chloride anions from the spent HT interlayers and intercalate with HCO3 - or CO3 2- , followed by a calcination process to produce the reusable calcined-HT. Multiple reaction cycles were carried out to evaluate the regenerative and reusable property. The results showed that the tubular reactor could achieve full (100%) conversion of calcium from the brine and CO2 from the gas phase by adjusting the parameters to optimum conditions for both NaOH and MEA, thus proving to be an efficient process with high atom economy. Furthermore, calcium conversion efficiency is increased by about 10% in the present of 30 ppm nickel nanoparticles addition. The model explored the mass transfer of CO2 across the gas-liquid interface in a Taylor flow reactor that can predict the speciation, bubble size and other parameters of the outlet during the reaction in the tubular reactor. The overall mass transfer coefficients were obtained with various CO2 flowrates and MEA concentrations. Modelled results showed that the bubble length decreased with higher MEA concentration due to the rapid chemical reaction. The overall mass transfer coefficients increased with higher CO2 flowrates. Modelled data are also in agreement with the experimental data. With the benefit of the model, the ionised species distribution in the carbonation process can be predicted and that can be used to determine the optimal reactor size for scale up. Compared to previous studies on brine carbonation using NaOH and MEA as pHbuffering additives, the method of using the solid adsorbent calcined hydrotalcite can facilitate the regeneration process, likely reducing processing complexity, cost and energy demand. It was found that the chloride removal efficiency remains over 70% after five cycles, and calcium utilisation efficiency of the brine carbonation process can surpass 90%. In addition, the value-added carbonate produced from this process is pure CaCO3, which can be a commercial product that can offset the operational costs. iii This work indicates technical feasibility of the mineralisation process with alkaline brine solutions and represents a first step towards developing a scalable technology that can be used with a variety of brines to achieve sustainable CO2 mineralisation.