Article pubs.acs.org/IECR
Enhancement of CO2 Mineralization in Ca2+-/Mg2+-Rich Aqueous Solutions Using Insoluble Amine Wenlong Wang, Xin Liu, Peng Wang,* Yanli Zheng, and Man Wang National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, Jinan, China, 250061 ABSTRACT: The potential for using concentrated seawater to fix CO2 by adding insoluble amine extractant was tested and verified. The experimental results showed that over 90% of Ca2+ ions could be converted to precipitation. Ammonia was chosen as regenerant to regenerate the extracting agent; the regeneration rate can reach 95%. On the basis of the analysis of MgCO3 precipitation properties, a new CO2 mineralization process was proposed in which CaO is employed to react with Mg2+ in solution. Mg(OH)2 precipitation and Ca2+-rich aqueous solutions were produced, and both performed well in CO2 mineralization. This new process can produce different kinds of byproducts such as MgCO3, CaCO3, and NH4Cl. Since there is no energy consumption from phase separation, nor is there a heat requirement, it is therefore definitely less energy intensive. This approach has great application potential.
1. INTRODUCTION Carbon capture and storage (CCS) is the primary means to reduce carbon emissions at present. A variety of technological approaches, such as chemical absorption, physical adsorption, membrane absorption, etc.,1−6 have been studied extensively. However, there are still some disadvantages in almost all of the CCS processes, including the high cost, the required energy consumption that leads to new carbon emissions, the absence of byproduct output during the process, and the risk and uncertainty in geological7−9 and ocean storage,10,11etc. Therefore, the development of energy-saving, low-cost, and safe CCS processes are needed, and carbon capture, utilization, and storage (CCUS) approaches, which stress the utilization of CO2, represent an encouraging new technological strategy. Carbon mineralization is a method for CO2 storage, but it has more promising applications in the utilization of CO2. Initially, CO2 mineralization was developed by reactions between CO2 and silicates, such as serpentine.12 However, the costs and energy consumption are very high in these processes and the low reaction rate is not suitable for sequestration of CO2 from emission sources functioning on engineering time scales.13 A great deal of research has been done to improve mineralization processes.14−18 Using natural minerals and solid waste to fix CO2 is one of the most promising techniques because it is simultaneously a low-energy process and is capable of producing a high value-added product. For instance, Kodama et al.19 studied the mineralization process using steelmaking slag and ammonium chloride solution to immobilize CO2 while producing pure CaCO3. Nduagu et al.20 utilized ammonium sulfate and magnesium silicates to produce magnesium hydroxide, which was used to absorb CO2 and cogenerated valuable product such as MgCO3. Xie et al. studied phosphogypsum waste, magnesium chloride, and potash feldspar to achieve CO2 fixation and produce other valuable byproducts.21−24 Almost all of the mineralization reactions were realized by using magnesium and calcium in different minerals. Since some aqueous resources, such as seawater, subsurface brine, and industrial effluents, are also rich in magnesium and © XXXX American Chemical Society
calcium, they have potential applications in CO2 mineralization. Our research group has proved the feasibility of CO2 fixation by enhancing the formation of carbonate precipitation.25,26 In our work, theoretical analyses indicated that the carbonation reaction could be enhanced by raising the pH or the CO2 partial pressure, and experiments confirmed that over 90% of the Ca2+and Mg2+ ions in seawater could be converted by precipitation in the forms of MgCO3 and dolomite [Mg Ca(CO3)2]. NH3/NH4Cl buffer solution was used effectively as the pH regulator in the experiments. (However, NH3/NH4Cl buffer solution added to seawater is difficult to reclaim and reuse with low energy consumption, and future improvements in the method are needed.) Different soluble amines, such as MEA, MDEA, TEA, etc., were tested as pH regulators, and most of them performed well in enhancement of carbonate precipitation in seawater. Nanofiltration, which has fairly good separation effects on organic molecules (molecular mass >150) and multivalent ions, was used to reclaim the amines. However, results showed that the amines could not be completely separated from the mixed seawater solution. In addition, energy consumption was high. Subsequently, insoluble amine was tested by our working group. Tributylamine was adopted as the pH regulator. It was found that the tertiary amine could not only enhance the reactions between CO2 and calcium ions but also perform well in regeneration or separation from the aqueous solution. Therefore, CO2 mineralization in Ca2+-/Mg2+-rich aqueous solutions was proved feasible with the help of insoluble amine. This paper will provide a detailed introduction to this process. Received: January 24, 2013 Revised: April 15, 2013 Accepted: May 18, 2013
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dx.doi.org/10.1021/ie400284v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Table 1. Thermochemistry Calculation Results reaction equation
ΔrGθ, kJ/mol
ΔrHθ, kJ/mol
NaCl + CO2 + H2O + NR3 → NaHCO3 ↓+ NR3·HCl MgCl2 + CO2 + H2O +2NR3 → MgCO3 ↓+ 2NR3·HCl CaCl2 + CO2 + H2O +2NR3 → CaCO3 ↓+ 2NR3·HCl
