Minimization of Energy Consumption of CO2 Mineralization Using Recyclable Ammonium Salts

Aimaro Sanna*, Julie Gaubert, M. Mercedes Maroto-Valer

Abstract
Mineralization of CO2 using recyclable salts, namely Carbon dioxide Capture and Storage by Mineralization (CCSM,) is considered amongst the most reliable process to store CO2 permanently [1-2]. However, the main key challenge to deploy this technology at large scale is related to its high energy requirement [1].
CCSM involves the reaction of the CO2 with metal oxides to form carbonates. Since the carbonates obtained are stable thermodynamically at ambient conditions, it is not possible for the CO2 to be released under ambient conditions. Mineralization has the advantage to be an overall exothermic reaction and also, it can take place close to the CO2 emitters where inorganic wastes (e.g. steel slag in a steel work) can be used as in-situ carbonation feedstock. Otherwise mineralization can be advantageous if suitable mineral deposits are closely located [3].
However, at the current stage of development, this technique has some drawbacks such as its slow kinetics, low efficiency and large energy consumption. As natural carbonation reaction of silicate rocks is very slow because of its inherent low surface available and diffusion limitations, processes based on ex-situ carbonation reactions have been developed to accelerate its slow kinetics. It has been shown that the addition of a mineral dissolution step using chemicals enhances the reaction kinetics, and this rout is generally referred to as indirect mineral carbonation [2,4].
CCSM by pH swing using ammonium salts is a promising multistep process, which has been extensively described in previous works [4-7]. The ammonium salts are thermally regenerated at 300°C.  Wang and Maroto-Valer also optimized the CCSM technique to reduce the water used by increasing the serpentine to water ratio [5]. Despite this, high energy consumption required to separate the ammonium salt (to be regenerated) from the water solution, remains a problem towards the commercialization of this technology, due to the large energy penalty of water evaporation.  
To overcome this energy issue, the feasibility of an alternative water separation process was evaluated to replace the water evaporation step. The first process evaluated was a liquid-liquid extraction technique used commercially in the Somet process, which employs methanol as reagent. A set of different conditions were investigated, including temperature, reaction time, reagents concentration and, stirring rate.  An ammonium sulphate/water separation 0f 94% was achieved at 25°C, 10 minutes, 1bar, 100g/l S/L ratio, 115% methanol and, 350rpm. The associated energy consumption was calculated, resulting in an energy saving of 23% in comparison to water evaporation step. However, this energy saving is not enough to render the process economically viable. Therefore, further research is required to optimise this technique by evaluating different starting solid-liquid ratios and eliminating/reducing the excess of salts used to facilitate the dissolution of the rock resources.

References
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