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Treatment of Hydrochloric Acid in Flue Gas from Municipal Solid Waste Incineration with Ca−Mg−Al Mixed Oxides at Medium−High Temperatures Jun Cao,† Wenqi Zhong,*,† Baosheng Jin,† Zhifei Wang,‡ and Kai Wang† †

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, and School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China

‡

ABSTRACT: A calcium−magnesium−aluminum (Ca−Mg−Al) mixed oxide sorbent was synthesized for the removal of HCl at medium−high temperatures. The operating conditions were specified in terms of the temperature of the gas (300−700 °C), the initial HCl concentration (500−1000 ppm), and the mass flow rate (0.5−1.3 L/min). X-ray powder diffraction (XRD) was applied to investigate the characteristics of Ca−Mg−Al mixed oxides. The results show that, because of the better properties of Ca−Mg−Al mixed oxides than traditional sorbents, which is related to the special structure in as-prepared, the reaction between HCl and Ca−Mg−Al mixed oxides is accelerated. The adsorption capacity of Ca−Mg−Al mixed oxides is the highest when compared to MgO, NaHCO3, and CaO. The removal efficiency of Ca−Mg−Al mixed oxides is more than 95% and even up to 99% under all of the operating conditions.

1. INTRODUCTION Until now, the amount of municipal solid waste (MSW) in China is up to 160 million tons per annum (t/a), increasing at a rate of 8−10% each year. Incineration is the environmentally soundest option for hazardous and municipal wastes. It has the advantages of volume reduction and the hygienic control, and the heat energy produced can be recovered to electrical power.1−4 However, incineration of MSW causes serious environmental problems, owning to producing large amounts of air pollutants (HCl, SOx, and NOx), solid wastes (e.g., heavy metals), toxic organic compounds (e.g., dioxins), etc.5−10 In recent years, more and more attention has been focused on dealing with pollutant emissions from MSW incinerators. Among these pollutants, hydrogen chloride (HCl) has been studied worldwide because it is a hazardous and corrosive gas, which harms the natural environment and human life. It is an important factor in the operation and design of the incinerator for causing hot corrosion to the body of the incinerator and piping.11,12 Shemwell et al.13 suggested that the capture of hydrogen chloride should take place at high temperatures, preferably above 500 °C, because the production of chlorine may contribute to subsequent formation of highly toxic chlorinated compounds, such as polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), in the municipal waste incineration process.14 The conventional technologies of dechlorination are wet, dry, and semi-dry mechanisms usually using limestone or lime.1,15,16 Wet dechlorination requires a large amount of space and high capital costs, and it is very complex to deal with the wastewater. The semi-dry system is simpler than wet; however, the dechlorination efficiency is limited on the basis of stoichiometry, and the equipment always wears seriously. The operation temperatures of both wet and semi-dry methods are usually below 300 °C, which cannot avoid the formation of PCDDs and PCDFs. The dry method has been widely employed recently because of the low capital cost and the high use rate of © 2014 American Chemical Society

the sorbents. In this paper, the dry method was used to capture HCl. Different sorbents were studied and used to adsorb HCl.5,6,16−27 The most often used solids for HCl capture are calcined limestone and CaO-based sorbents.16,22−24,27,28 Liu16 studied the removal efficiencies of HCl and SO2 in a spray dryer using Ca(OH)2 as the absorbent under 200−300 °C. Partanen et al.22−24 discussed the effect of the temperature, gas atmosphere, absorbent quality, and existence of calcium hydroxychloride on the adsorption of HCl by limestone in hot flue gases. It is found that the Ca-based sorbent has higher conversions at 650 °C. However, CaO was molten at the temperature, which would bind and block the piping. There are also other materials used as sorbents to remove HCl, such as sodium sorbents,5,26,29 MgO- and Mg-based sorbents,18,19,30 etc. Nunokawa et al.29 developed NaAlO2 as the HCl removal material at 400 °C. Verdone et al.26 used sodium carbonate to remove HCl conducted in a fixed-bed multi-layer reactor in the temperature range of 200−600 °C. The results showed that the collection efficiency of sodium carbonate reached the maximum values at 400−500 °C. However, the carbonate conversion did not ensure higher than 50% when the temperature was above or below this range. Kameda19 studied a new treatment method for gaseous HCl using Mg−Al-layered double hydroxide intercalated with carbonate ion. The degree of HCl removal could reach more than 99% at 1.75 times the stoichiometric quantity after 60 min for 10 000 ppm of HCl gas. However, the temperature is below 190 °C. Above all, the temperature of sorbents that are usually used to remove HCl is generally low, or the suitable temperature range is narrow. The hydrotalcite compound belongs to anionic clay minerals, also known as layered double hydroxides, and can Received: April 11, 2014 Revised: May 21, 2014 Published: May 22, 2014 4112

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Figure 1. Schematic of the sorbent dechlorination performance test system.

2. EXPERIMENTAL SECTION

be formed by the chemical process of byproducts, such as blast furnace slag, at low temperatures. The general molecular formula of hydrotalcite is given as [M 2+ 1−x M 3+ x (OH) 2 ] x+ [A n− x/n ] x− ·zH 2 O 31 (abbreviated as M2+−M3+−An−), where M2+ and M3+ are divalent and trivalent metal ions and often represent Mg2+ and Al3+, respectively, and An− is an intercalated anion, x varies between 0.2 to 0.33. The Mg−Al hydrotalcite that intercalates with CO32− can be transformed into Mg−Al oxide by calcinations, and the reaction is given as follows:19

2.1. Experimental Methods. Experiments to test the dechlorination characteristics were carried out in a fixed bed, as shown in Figure 1. The reactor employed a quartz glass tube of length 1300 mm and inside diameter of 16 mm, which was covered by thermal insulation materials. The tube was heated by an electrical heater. The length of the packed bed was 5 mm for the pellets of 1 g. The packed bed was placed in the center of the reactor. To purge all oxygen out of the tube, N2 was pumped in with a flow rate of 110 mL/min for 30 min after leak checking. Then, the reactor was heated, starting from room temperature to the target temperatures (300−700 °C). The concentration of 500−1000 ppm of HCl is similar to that of average composition, including exhaust gas of incinerators, in China. The gaseous mixture was obtained by mixing pure gases (HCl and N2) emitted from the gas cylinders using mass flow controllers. The flow rate of mixture gas ranged from 0.5 to 1.3 L/min. The residual HCl concentration of gaseous effluent was continuously analyzed by an online flue gas analyzer (model 7900FM HCL GFC). The accuracy in calibration is 0.02 ppm. The removal efficiency and average removal efficiency were calculated by the following equations:

Mg1 − xAl x(OH)2 (CO3)x /2 → Mg1 − xAlx O1 + x /2 + x /2CO2 + H 2O

(1)

When part of Mg2+ was replaced by other divalent metal ions, the hydrotalcite-like compound (HTlc) is formed. Structurally, the HTlc consists of two-dimensional brucite-like layers with a positive charge, where the cations are located in the center of an octahedron surrounded by six hydroxyl groups and anions together with water molecules are arranged in the interlayer to form neutral materials.32 Upon calcinations, hydrotalcites are decomposed into Mg/Al mixed oxides. The HTlcs are less used in catalytic reactions than the mixed oxides, which are easier to process and exhibit higher specific surface areas.33 In the present paper, the Ca−Mg−Al HTlcs were synthesized using Ca2+ to replaced part of Mg2+. Then, the predecessor was calcined at a certain temperature to obtain Ca−Mg−Al mixed oxides. The aim is to develop new emission control techniques having higher removal efficiencies of HCl gas in the temperature range of 300−700 °C using selfprepared Ca−Mg−Al mixed oxides under different conditions. A thermogravimetric analyzer (TG) and X-ray powder diffraction (XRD) were conducted to investigate the characteristics of Ca−Mg−Al mixed oxides.

⎛ [HCl]import − [HCl]export ⎞ ⎟⎟ × 100% η = ⎜⎜ [HCl]import ⎝ ⎠ η̅ =

1 N

(2)

N

∑ ηi i=1

(3)

where η is the removal efficiency of the sorbent, η̅ is the average removal efficiency of the sorbent, [HCl]import and [HCl]export are the HCl concentrations of reactor import and export, respectively, N is the number of measurements, and ηi is one measurement of the removal efficiency. 2.2. Material. The Ca−Mg−Al HTlc material was synthesized by co-precipitation using analytical reagents. Mg(NO3)2·6H2O (Guangdong Xilong Chemical Co., Ltd.), Ca(NO3)2·4H2O (Guangdong Xilong Chemical Co., Ltd.), and Al(NO3)3·9H2O (Shanghai Sinpeuo Fine Chemical Co., Ltd.) were weighed by a certain molar ratio [(Ca + 4113

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Mg)/Al molar ratio = 4] and then dissolved in deionized water at 60 °C, and the solution of NaHCO3 or Na2CO3 (Shanghai Lingfeng Chemical Reagent Co., Ltd.) ([1 M]) was prepared as a precipitant. The two solutions were titrated in a beaker at the same time in 3 min while stirring; the pH was maintained at 8−10; and the mixture solution was stirred continuously at 60 °C for 30 min. Afterward, the sediment was isolated by filtering the resulting suspension, washed 3 or 4 times by deionized water until the pH was 7, and then dried at 80 °C for 24 h. The textural properties of the Ca−Mg−Al HTlcs are shown in Table 1. Finally, Ca−Mg−Al mixed oxides were obtained by

carbonate, and CaO. When the calcination temperature heated to 700 °C, the layered structure was almost destroyed. More CaO and CaCO3 were generated, especially CaO. The XRD also shows that, when Ca−Mg−Al mixed oxides obtained reacted with HCl at 550 °C, the peaks of CaO disappeared and the structure of the sample was almost recovered. It means that part of HCl reacted with CaO and part of HCl absorbed on the absorbing sites that formed by the removal of H2O and CO2, which recovered the structure of the sorbent. The adsorption process between Ca−Mg−Al HTlcs and HCl at high temperatures can be represented by eq 4.

Table 1. Textural Properties of the Ca−Mg−Al HTlcs suggested chemical formula tapped density(g/mL) paricle size (mm) surface area (m2/g) d spacing (nm) (003) crystallite size (nm) (003)

Ca0.608Mg0.152Al0.24(OH)2(CO3)0.12·0.765H2O

Ca 0.608Mg 0.152Al 0.24 (OH)2 (CO3)0.12 ·0.765H 2O

0.65 0.4−1 38.7 0.75 12.1

+ (y + 0.24)HCl → Ca 0.488Mg 0.152Al 0.24(OH)2 − y Cl y + 0.12CaCl 2 + 0.12CO2 + (0.885 + y)H 2O (y ≤ 2)

(4)

y is Cl− that absorbed on the absorbing sites that formed by the removal of H2O and CO2. 3.2. Reaction of HCl Gas with Ca−Mg−Al Mixed Oxides in a Fixed-Bed Reactor. 3.2.1. Adsorption Capacity of Various Sorbents. Of the four sorbents (MgO, NaHCO3, CaO, and Ca−Mg−Al mixed oxides self-prepared) tested at 550 °C, the flow rate was 0.9 L/min, the initial HCl concentration was 500 ppm, and the masses of various sorbents were 1 g. Each test lasted 200 min. The removal efficiencies of various sorbents and the breakthough curves for Ca−Mg−Al mixed oxides and CaO sorbents are shown in panels a and b of Figure 3, respectively. From Figure 3a, the adsorption capacities of MgO and NaHCO3 are worse, although at the first 74 min, the removal efficiency of NaHCO3 is high. The adsorption capacities of CaO and Ca−Mg−Al mixed oxides are relatively better at the first 200 min. After that, the removal efficiency of CaO dropped; however, the removal efficiencies of Ca−Mg−Al mixed oxide are more than 90% and last for 860 min, as shown in Figure 3b. The reason why Ca−Mg−Al mixed oxides were the best sorbent thus probably relates to the special structure of the material.34−36 The specific surface area of Ca−Mg−Al mixed oxides is 38.7 m2/g. It is 10 times more than that of CaO (3.11 m2/g). In comparison to CaO, when the temperature reaches about 350 °C, the interstices in the layer of Ca−Mg−Al mixed oxides that resulted from the removal of water and CO2 became the adsorbing sites for HCl molecules by the action of the polarity of the layer surface. At high temperatures, the HTlcs decomposed into CaCO3, Ca−Mg carbonate, and CaO, which have high activity to react with HCl. Moreover, the molecular radius of HCl is about 0.4 nm, which is much smaller than the interlayer spacing of 0.75 nm of HTlcs. Therefore, HCl can enter the interlayer easily. Above all, HCl removal of Ca−Mg−Al mixed oxides is a heterogeneous reaction by the following processes: (1) HCl reacted with CaO and CaCO3 generated by HTlcs decomposed, which is also called the chemical reaction. (2) HCl was absorbed on the surface, and interstices for the specific surface area and removal of H2O and CO2, which is called the diffusion reaction, include internal diffusion and external diffusion. (3) HCl was removed by the first and second processes simultaneously. The better properties of Ca−Mg−Al mixed oxides than traditional sorbents are related to the special structure, which accelerates the reaction between HCl and Ca−Mg−Al mixed oxides.

calcining the solid products at 350 °C for 2 h. The grain size of Ca− Mg−Al mixed oxides was sieved to obtain the diameter from 0.4 to 1 mm. 2.3. Characterization. XRD data of the solids were obtained with a D8 Advance diffractometer (Bruker Axs, Ltd., Germany) using Cu Kα radiation (40 kV and 100 mA) and a diffracted beam monochromator (D8 Venture). The diffraction pattern was collected at room temperature from 5° to 80° in 2θ using 0.02° 2θ step increments. Nitrogen adsorption/desorption analysis [Brunauer− Emmett−Teller (BET)] was carried out at −196 °C using an automated gas adsorption analyzer (ASAP 2020, Micromeritics).

3. RESULTS AND DISCUSSION 3.1. Characteristics of Ca−Mg−Al Mixed Oxides. The characteristic temperatures (Ca−Mg−Al HTlcs, calcined at 350 and 700 °C, after reacting with HCl at 550 °C) of sorbents were analyzed by XRD, as shown in Figure 2. The diffraction pattern of Ca−Mg−Al HTlcs before calcined shows planes of the HTlc phase with a pillared layered structure. In comparison to pristine Ca−Mg−Al HTlcs, the calcined samples were partly changed and mainly decomposed into CaCO3, Ca−Mg

Figure 2. XRD patterns of the prepared sample calcined at different temperatures. 4114

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Figure 4. Effect of the initial HCl concentration on the average removal efficiency at 550 °C, with 1 g of Ca−Mg−Al mixed oxides, flow rate of 0.9 L/min, and HCl concentrations of 500, 750, and 1000 ppm.

reaction between the sorbent and HCl. Simultaneously, H2O and CO2 were formed, and the diffusion rate of HCl was reduced. With the development of time, more and more H2O and CO2 were released from the particle, which means that there were more and more open pores for HCl to diffuse in the side of the layers of the sorbent. Therefore, the removal efficiencies of different inlet HCl concentrations increased. Then, with the consumption of the sorbent, the removal efficiencies were falling slowly. It can be obtained that removal efficiencies of the sorbent dropped slightly with the inlet HCl concentration increasing, but the removal efficiencies in the range of 500−1000 ppm are all more than 95%. 3.2.3. Effect of the Reaction Temperature. The average removal efficiencies of 1 g of Ca−Mg−Al mixed oxides after reacting with 500 ppm of HCl gas under the flow rate of 0.9 L/ min at 300−700 °C are shown in Figure 5. The removal efficiency of Ca−Mg−Al mixed oxides first increased with an increasing temperature from 300 to 440 °C. Then, the removal efficiency was decreased when the temperature continued to

Figure 3. Removal efficiencies and average removal efficiencies of various sorbents (Ca−Mg−Al mixed oxides, NaHCO3, CaO, and MgO) at 550 °C, with a sorbent mass of 1 g, HCl concentration of 500 ppm, and flow rate of 0.9 L/min: (a) removal efficiencies of various sorbents and (b) breakthough curves for Ca−Mg−Al mixed oxides and CaO sorbents.

In addition, upon calcinations, hydrotalcites are decomposed into Mg/Al mixed oxides, exhibiting the periclase-like structure of MgO,33 which would reinforce the particle structure and prevent it from collapsing as the amount of molten material increases in the particle. 3.2.2. Effect of the Initial HCl Concentration. The removal efficiencies of 1 g of Ca−Mg−Al mixed oxides after reacting with HCl gas for 200 min under the flow rate of 0.9 L/min at 550 °C and the inlet HCl concentration of 500, 750, and 1000 ppm are shown in Figure 4. Figure 4 shows that, when the initial HCl concentration ranges from 500 to 1000 ppm, the removal efficiencies of Ca−Mg−Al mixed oxides were high, which were more than 90%. In Figure 4, during the tests, the initial HCl concentration had only a little effect on the removal efficiency. The removal efficiencies of the initial HCl concentrations of 500, 750, and 1000 ppm were almost the same and decreased with the react time prolonging until 37 min. One explanation for the phenomena is that, at the first stage of the adsorption reaction of HCl, the control kinetics of the adsorption reaction is the chemical reaction on the particle surfaces. The particle surfaces were covered by the product, which further hindered the

Figure 5. Effect of the temperature on the average removal efficiency, with 1 g of Ca−Mg−Al mixed oxides, HCl concentration of 500 ppm, flow rate of 0.9 L/min, and temperature of 300−700 °C. 4115

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4. CONCLUSION The Ca−Mg−Al mixed oxide sorbent was used to remove HCl at medium−high temperatures under different temperatures, initial HCl concentrations, and flow rates. The adsorption capacities of the self-prepared sorbent were also compared to the usual sorbents. The results show that the adsorption capacity of the Ca−Mg−Al mixed oxides is the best, more than 95% and even up to 99%. This is because the interstices in the layer that resulted from the removal of water and carbonate will become the adsorbing sites for HCl molecules by the action of the polarity of the layer surface. With the temperature increasing, more and more CaO and CaCO3 were produced, which have high efficiency with HCl. The better properties of Ca−Mg−Al mixed oxides accelerate the reaction between HCl and Ca−Mg−Al mixed oxides.

rise. The reason for that may be that, when the temperature was at 300−440 °C, H2O decomposed from the sorbent (carbonate was not decomposed until 500 °C) may result in the interstices in the layer becoming the adsorbing sites for HCl molecules. When the temperature was up to 550 °C, the carbonate was decomposed to form CO2, which would block the adsorption of HCl.37 However, with the temperature increasing, more and more CaO and CaCO3 were produced, which have high efficiency with HCl. The release of H2O and CO2 probably results in open pores and adsorbing sites, which would allow for reacting HCl to diffuse to the inner parts of the sorbent particles. Therefore, the removal efficiency of Ca−Mg−Al mixed oxides increased again with the further increase of the temperature. 3.2.4. Effect of the Flow Rate of the Gaseous Mixture. The average removal efficiencies and standard deviations of Ca− Mg−Al mixed oxides after reacting with 500 ppm of HCl gas for 200 min at 550 °C with the flow rate of the gaseous mixture in the range from 0.5 to 1.3 L/min are shown in Figure 6. The

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-25-83794744. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the National Basic Research Program of China (2011CB201505), the Distinguished Young Scholar Project of Jiangsu Province (BK20130022), the Scientific Research Foundation of the Graduate School of Southeast University (YBJJ1224), and the U.K. Engineering and Physical Sciences Research Council (EPSRC/China Project EP/G063176/1).

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Figure 6. Effect of the flow rate on the average removal efficiency, at 550 °C, with 1 g of Ca−Mg−Al mixed oxides, HCl concentration of 500 ppm, and flow rate of 0.5−1.3 L/min.

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