Impact of Fly Ash Impurity on the Hydrate-Based Gas Separation Process for Carbon Dioxide Capture from a Flue Gas Mixture Asheesh Kumar, † Tushar Sakpal, † Praveen Linga, ‡ and Rajnish Kumar* ,† † Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India ‡ Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 ABSTRACT: The hydrate-based gas separation (HBGS) process for gas mixtures of CO 2 +N 2 (flue gas) and CO 2 +H 2 (fuel gas) has proven to be very efficient and highly selective for CO 2 capture. In addition to CO 2 and N 2 , flue gas from coal-based thermal power stations can contain impurities such as nitrogen oxides (NO x ), sulfur oxides (SO x ), and fly ash. In this work, the impact of fly ash on the HBGS process efficiency was investigated. Tetrahydrofuran (THF, 1 mol %) was used as a thermodynamic promoter, and sodium dodecyl sulfate (SDS) and sodium dodecyl benzenesulfonate (SDBS) (anionic surfactants) were used as kinetic promoters. Whereas the use of THF in the HBGS process reduces the operating pressure significantly, both SDS and SDBS were found to enhance the rate of hydrate formation. It was observed that the hydrate equilibrium conditions did not change in the presence of fly ash. However, the presence of fly ash enhanced the separation efficiency of the HBGS process by reducing the induction time and increasing the kinetics of hydrate formation. Therefore, the presence of fly ash in a flue gas mixture is not detrimental to the HBGS process, which is a positive factor for the capture and geological sequestration of CO 2 in the form of gas hydrates. 1. INTRODUCTION Postcombustion CO 2 capture involves the separation of CO 2 from the flue gas mixture. Along with gases like carbon dioxide (CO 2 ), nitrogen oxides (NO x ), and sulfur oxides (SO x ), fly ash as particulate matter are generated by coal-based thermal power plants. However, concerns have been raised that the application of CO 2 capture technologies might interfere with the current SO x , NO x , and fly ash removal technologies. 1-3 Large-scale implementation of carbon capture is not possible if the technology cannot withstand minor loads of fly ash and other acidic gases in the flue gas stream. It has been recognized that advanced/novel CO 2 capture technologies have not been optimized in the presence of other acidic gases or fly ash and thus exhibit a drop in performance in real-life applications. 3,4 CO 2 capture and separation from flue gas through the formation of clathrate hydrates is one approach that has attracted significant research interest for reducing carbon emissions and the greenhouse effect. 5-10 The hydrate-based gas separation (HBGS) process is a multistage hydrate formation and decomposition cycle, explained in detail elsewhere. 11 Gas hydrates (clathrates) are nonstoichiometric, crystalline, icelike solids formed by water and gas molecules. When a hydrate-forming gas and water are mixed together, the water molecules arrange themselves in a three-dimensional hydrogen-bonded network (cages) that is stabilized by entrapped gas molecules at an appropriate pressure and temperature. 12 It has also been shown that, upon hydrate formation from gas mixtures, a fractionation effect results in a particular component being preferentially captured in the solid hydrate phase compared to other components. 11 Thus, the HBGS process is quite efficient in separating mixtures of gases that form hydrates under different thermodynamic conditions. The flue gas mixture employed by most researchers for gas hydrate formation is a model flue gas (CO 2 /N 2 gas mixture) that is essentially free of any acidic gases and fly ash. 6,9,11,13-16 The effects of acidic gases and fly ash on gas-hydrate-based separation processes are not yet well understood. Recently, the impact of SO 2 (an acidic gas) on postcombustion carbon dioxide capture was investigated, and it was found that SO 2 shifts the equilibrium to milder conditions. 17 Beeskow-Strauch et al. also investigated the effects of SO 2 and NO 2 on hydrate formation and the stability of flue gas from a thermal power station. They found that, in the presence of 1% SO 2 , the mixed hydrate of CO 2 -SO 2 was more stable and the water-to-hydrate conversion increased, whereas the effect of NO 2 on the hydrate formation kinetics was not conclusive. 18 However, the impact of fly ash on the thermodynamics and kinetics of hydrate formation has not been studied. Therefore, in this work, the impact of fly ash in the flue gas stream was studied for the HBGS process. For an efficient HBGS process, three important conditions are necessary: (1) minimum operating pressure, (2) maximum water-to-hydrate conversion rate and ratio, and (3) maximum separation factor. The operating pressure in the HBGS process can be minimized by utilizing a small amount of additives, which decreases the minimum hydrate formation pressure by forming an alternate hydrate structure. Several liquid-phase and gas-phase additives such as tetrahydrofuran (THF), tetrabuty- lammonium bromide (TBAB), and propane (C 3 H 8 ) have been Received: January 15, 2014 Revised: May 16, 2014 Accepted: May 16, 2014 Published: May 16, 2014 Article pubs.acs.org/IECR © 2014 American Chemical Society 9849 dx.doi.org/10.1021/ie5001955 | Ind. Eng. Chem. Res. 2014, 53, 9849-9859