Research PaperInfluence of CO2 gas on the rate and kinetics of HCl, SO2, and NO2 gas removal by Mg-Al layered double hydroxide intercalated with CO32−
Introduction
The emission of acidic gases such as HCl, SOx, and NOx from waste incineration facilities is subject to strict environmental regulation. Many facilities spray slaked lime on waste to remove HCl and SOx (Karlsson et al., 1981; Chu and Rochelle, 1989; Sakai et al., 2002; Kim et al., 2017; Pozzo et al., 2018). However, the slaked lime cannot be reused, and the reaction products are buried along with fly ash and increase the burden borne by landfills. Additionally, high-salinity effluents are discharged from landfill sites owing to the high water-solubility of CaCl2 as a reaction product. Instead of slaked lime, the reduction in the quantity of NOx is accomplished through combustion control, non-catalytic denitrification, and catalytic denitrification. In particular, catalytic denitrification is used more frequently owing to its high performance in NOx treatment (Shi et al., 2013; Bacher et al., 2015; Chen et al., 2016; Gao et al., 2017; Chen et al., 2018). However, the catalysts are often based on expensive transition metals. Moreover, a part of the generated heat is used to reheat exhaust gases for streamlining the catalytic denitrification process. Thus, when the incineration facilities are adjacent to power stations, the power-generation efficiency will also suffer. For these reasons, new methods to treat acidic exhaust gases are required.
In this study, we investigated the treatment of acidic exhaust gases using MgAl layered double hydroxide intercalated with CO32− (CO3·Mg-Al LDH) as the adsorbent. MgAl LDH has anion-exchange properties (Miyata, 1983), and its general formula is [Mg2+1−aAl3+a(OH)2][An−a/n・mH2O] (An−: anion, a = Al3+/(Mg2++ Al3+)) (Cavani et al., 1991; Mills et al., 2012). The LDH is comprised of host layers and guest layers. The host layers are formed by regular octahedral layers and carry positive charge, since a portion of Mg2+ in the Mg(OH)2 is replaced by Al3+. The guest layers, on the other hand, contain intercalated water and anions that neutralize the overall charge. Al3+ has a solubility limit ranging from 0.20–0.33 (Vegard's law). Various anions could be intercalated in the guest layer of MgAl LDH, and the intercalation is easier for anions with a higher charge density (Kameda et al., 2003). Extensive research has been recently conducted on wastewater treatment using MgAl LDH (Cao et al., 2019; Lee et al., 2019; Guo et al., 2020).
We have previously demonstrated that CO3·Mg-Al LDH can be used to treat single acidic gases of HCl, SO2, and NO2 (Kameda et al., 2019a; Kameda et al., 2019c; Kameda et al., 2020a), as well as their mixtures (Kameda et al., 2019b). CO3·Mg-Al LDH can also be used to treat NO in combination with MnO2 (Kameda et al., 2020b). Recently, calcined LDH has been examined as a catalyst for NOx reduction (Wang et al., 2019). The CoMnAl mixed metal oxides prepared by calcining LDHs at 500 °C exhibit excellent deNOx performance in low-temperature NH3-SCR (Wu et al., 2019). However, this method requires NH3 as a reactant, and cannot be applied to treat HCl and SOx. The method developed by us uses only CO3·Mg-Al LDH and MnO2, and does not require NH3, to treat HCl, SO2, and NOx simultaneously. It is expected that CO3·Mg-Al LDH can be used for treating acidic gases such as HCl, SOx, and NOx from waste incineration facilities. Since exhaust gases from incinerators also contain approximately 10% CO2, it is imperative to investigate their influence on the performance of CO3·Mg-Al LDH for practical applications. In addition, the reaction of CO3·Mg-Al LDH and CO2 gas should be considered. Furthermore, it is important to examine the reaction kinetics of the acidic gas removal by CO3·Mg-Al LDH to adopt it in industrial processes. In our previous report, we attributed the removal of HCl by CO3·Mg-Al LDH to the reaction between CO32− in the interlayer of MgAl LDH and HCl, and the intercalation of Cl− in the interlayer of MgAl LDH (Kameda et al., 2020a). This process is classified as chemical adsorption. We also reported that the removal of SO2 by CO3·Mg − Al LDH progresses by the reaction between intercalated CO32− and SO2, as well as SO2 adsorption onto the MgAl LDH surface. The former is classified as chemisorption, while the latter is classified as physisorption (Kameda et al., 2019a). We also reported that the removal of NO2 by CO3·Mg − Al LDH is driven by the reaction between NO2 and intercalated CO32− (chemisorption), and the adsorption of NO2 onto the MgAl LDH surface (physisorption) (Kameda et al., 2019c). The activation energy determined by the kinetic study can indicate whether the removal of HCl, SO2, or NO2 by CO3·Mg-Al LDH is dominated by chemisorption or physisorption. Thus, in this study, we analyzed the influence of coexisting CO2 in the exhaust gas on treating the single gas of HCl, SO2, or NO2 using CO3·Mg-Al LDH. The associated kinetics was investigated, and the related mechanisms are discussed.
Section snippets
Experimental
CO3·Mg-Al LDH specimens with Mg/Al = 2.0 and 4.0 were synthesized by a coprecipitation method in accordance with a previously published procedure (Kameda et al., 2016). The sample with Mg/Al = 2.0 contained 15.2 wt% Mg and 8.2 wt% Al, and the actual Mg/Al mole ratio was 2.1. The corresponding ratios for the sample with Mg/Al = 4.0 were 28.7 wt% Mg, 8.1 wt% Al, and Mg/Al = 3.9. Their respective BET specific surface areas are 54 and 84 m2/g. Eqs. (1), (2), (3) depict the theoretical reactions for
HCl removal
As shown in Fig. 1, the HCl removal rates by CO3·Mg − Al LDH with Mg/Al = 2.0 after 90 min are 86, 70, 72, and 70% at CO2 concentrations of 0, 5, 10, and 15%, respectively. The removal of HCl by CO3·Mg-Al LDH is attributed to the reaction between CO32− in the interlayer of MgAl LDH and HCl, according to Eq. (1), followed by the intercalation of Cl− in the interlayer of MgAl LDH (Kameda et al., 2020a, Kameda et al., 2020b). This is classified as chemisorption. While there was an obvious decrease
Conclusions
The CO2 gas coexisting with other acidic gases such as HCl, SO2, and NO2 in an exhaust gas decreases the removal rates of the acidic gases by CO3·Mg-Al LDH. This is attributed to the exchange reaction between MgAl LDH-intercalated CO32− and gaseous CO2, which inhibits the exchange between the intercalated CO32− and acidic gases. The HCl removal follows pseudo first order kinetics. The reaction mechanism was assigned to chemisorption based on the activation energies, suggesting that the HCl
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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