Investigation on mechanochemically modified calcium-based adsorbent for flue gas HCl removal

For purpose of reducing chlorine ions entering the wet flue gas desulfurazition (WFGD) tower to realize the WFGD waste water sequestration and the gypsum upgrading, a technical method of adsorbent injection into flue gas to remove HCl was proposed in this work. A new NaOH-modified CaO (CA-MC-NA) was prepared by the mechanochemical ball milling method as a superior adsorbent for HCl removal in flue gas. The influence of particle size and Na doping on the dechlorination efficiency and chlorine capacity was investigated in a fixed bed reaction system. The experimental results demonstrated that the HCl removal performance is improved significantly under proper particle size and Na doping, as manifested by the experiment results that the chlorine capacity increases from 27.18 to 38.36 mg/g by simple mechanical ball milling, while it surprisingly reaches 75.36 mg/g by doping 5 wt.% NaOH. The surface physicochemical characteristics was explored by combined sample characterization of BET, XRD, SEM, elemental mapping, and particle size distribution. Dechlorination mechanism of the adsorbents were deduced and discussed. It shows that the mechanical ball milling process reduces the particle sizes to the submicron scale that induces the break and reconstruction of molecular interfacial bonds. It also increases the oxygen defects on the adsorbent surface. The doping of NaOH does not change the crystal structure of the dechlorinator, and Na ions can be chemically loaded over CaO in the form of ionic bonds. Free oxygen atoms or adsorbed oxygen molecules could combine with dissociated hydrogen atoms thus forming hydroxide radicals, which enhances the alkalinity of the adsorbent and boosts the removal of hydrogen chloride.