251 Mercury Adsorption Characteristics of Sulphur-Impregnated Activated Carbon Pellets for the Flue Gas Condition of a Cement-Manufacturing Process Hyo-Ki Min 1, Tanveer Ahmad 1, Kwang-Yul Kim 1, Kwang-Joong Oh 2 and Sang-Sup Lee 1, * (1) Department of Environmental Engineering, Chungbuk National University, Chungbuk 362 763, Korea. (2) Department of Environmental Engineering, Pusan National University, Busan 609 735, Korea. (Received date: 23 August 2014; Accepted date: 18 January 2015) ABSTRACT: Powdered activated carbon is used for adsorption of mercury in flue gas. However, in the cement-manufacturing process, it is more suitable to use fixed-bed granular activated carbon than powdered activated carbon injection, so the cement can be reused. Because activated carbon is influenced by both adsorption temperature and composition of flue gas, its mercury adsorption capacity varies greatly. Therefore, it is necessary to evaluate the efficiency of activated carbon for mercury adsorption in advance under the operating conditions in which it is going to be used. We examined the mercury adsorption from exhaust gas in the cement-manufacturing process using sulphur-impregnated activated carbon pellets. The study results show that sulphur-impregnated activated carbon pellets had a high efficiency for mercury adsorption at a high exhaust gas temperature of 180 C. Experimental results also indicated that the activated carbon pellets had a high efficiency for mercury adsorption (>93%) at 90, 135 and 180 C from simulated gas without sulphur dioxide. These results indicate that activated carbon pellets have a high efficiency for mercury adsorption in the exhaust gas in the cement-manufacturing process, which is characterized by low levels of sulphur dioxide. 1. INTRODUCTION Anthropogenic activity is responsible for about 30% of the whole mercury emitted into the atmosphere, with the cement kilns being responsible for up to 9% of the whole anthropogenic emission sources (United Nations Environmental Programme 2013). The cement-manufacturing process includes the following stages: drying, calcining, sintering and cooling. The hightemperature heat that is required in the manufacturing process can be obtained from a fuel-burning kiln. The raw materials of cement are dried and calcined by the flue gas at high temperatures. These materials are finally sintered at very high temperatures of 1400 1450 C, and manufactured into cement. During this process involving high temperatures, mercury is generated from flue gases (Rezoni et al. 2010). The forms of mercury generated during the combustion of exhaust gas are elemental mercury (Hg 0 ), oxidized mercury (Hg 2+ ) and particulate mercury (Hg p ). Of these, large amounts of particulate mercury (Hg p ) can be readily removed by particulate-matter-control devices (e.g. electrostatic precipitator or fabric filter), and oxidized mercury is water soluble and thus can be relatively easily removed using a wet scrubber. However, elemental mercury is insoluble in water and stable, and thus, it is difficult to remove it using existing air-pollution-control devices *Author to whom all correspondence should be addressed. E-mail: slee@chungbuk.ac.kr (S.-S. Lee).
252 H.-K Min et al./adsorption Science & Technology Vol. 33 No. 3 2015 (Keener et al. 2012; Kilgroe et al. 2001). Among the various methods used for reducing the concentration of mercury in flue gas, the technology that injects powdered activated carbon into the flue gas to adsorb mercury has been widely studied (Rezoni et al. 2010). However, if activated carbon is injected for the purpose of removing the mercury that is emitted from the cement kilns, cement and activated carbon mix with each other, thereby restricting the recycling of cement; therefore, it is necessary to prevent the mixing of cement and activated carbon using fixed-bed granular activated carbon. Because granular activated carbon is less efficient in coming into contact with the mercury existing in the flue gas when compared with powdered activated carbon, it is necessary to use the impregnated activated carbon to enhance the efficiency of mercury removal. It is well-known that the mercury removal efficiency of activated carbon is largely influenced by the constituents and conditions of the flue gas, and therefore we first need to understand the conditions of the process in which activated carbon will be used. It is reported that during the combustion of flue gases, hydrogen chloride (HCl), oxygen (O 2 ), sulphur dioxide (SO 2 ) and nitrogen monoxide (NO) influence the mercury adsorption efficiency of activated carbon. Among these, HCl plays the most important role in increasing the mercury adsorption capacity of activated carbon (Yan et al. 2003; Yang et al. 2007). SO 2 forms a sulphurated functional group on the surface of carbon and improves elemental mercury adsorption, but it is known to reduce the mercury adsorption efficiency when co-existing with nitrogen oxides (NO x ; Diamantopoulou et al. 2010; Miller et al. 2000). Ochiai et al. (2009) examined the effects of HCl and SO 2 concentration on mercury removal using activated carbon made from coconut shells. The authors reported that as the concentration of HCl increased, the mercury adsorption efficiency also increased; by contrast, as the temperature increased, mercury adsorption efficiency decreased. Using backscattered electrons detector and scanning electron microscope with energy-dispersive spectrometer, Karatza et al. (2000) reported that a higher concentration of mercury was detected on the surface of sulphur-impregnated activated carbon powder (HGR; Calgon Carbon, USA) at the point where the concentration of sulphur is higher. Using nitrogen as background gas, Liu et al. (1998, 2000) reported that activated carbon with a high specific surface area and a high concentration of sulphur has a high adsorption capacity. Hsi and Chen (2012) compared the mercury adsorption capacity of sulphur-impregnated activated carbon treated with either HCl, O 2, SO 2 or NO with non-treated sample containing only the basic compositions (i.e. CO 2, H 2 O and N 2 ). When either of HCl, O 2, SO 2 or NO was injected into the activated carbon samples, mercury adsorption capacity was found to increase. By contrast, when two or more of the aforementioned components were injected, SO 2 decreased the mercury adsorption capacity, whereas HCl and NO increased the mercury adsorption capacity. The reaction between mercury and sulphur in sulphur-impregnated activated carbon is exothermic. As the temperature increases, the Gibbs energy change tends to increase gradually; therefore, as the adsorption temperature increases, the adsorption capacity of sulphur-impregnated activated carbon decreases (Liu et al. 2000). However, it is reported that when compared with raw activated carbon, the degree to which its adsorption capacity decreases is relatively small. The aforementioned results reported in the literature show that sulphur-impregnated activated carbon is influenced by sulphur content, adsorption temperature and the composition of the flue gas. Thus, the efficiency of mercury adsorption using sulphur-impregnated activated carbon is different from that of raw activated carbon. Therefore, in order to use sulphur-impregnated activated carbon to reduce the concentration of mercury in the flue gas, it is necessary to evaluate the efficiency of sulphur-impregnated activated carbon in advance under the operating conditions in which it is going to be used. As mentioned earlier, to reduce the concentration of mercury in the flue gas in the cement-manufacturing process, it is necessary to use fixed-bed granular activated carbon. However, because existing research has focussed only on powdered activated
Adsorption of Mercury Using Sulphur-Impregnated Activated Carbon Pellets 253 carbon, it is necessary to study the mercury adsorption characteristics of granular activated carbon. In this study, fixed-bed granular activated carbon was used to reduce the concentration of mercury in flue gas in the cement-manufacturing process. For this purpose, we used simulated gas and evaluated the mercury adsorption characteristics of sulphur-impregnated activated carbon pellets. First, a small amount of sulphur-impregnated activated carbon sample (30 mg) was used to evaluate its inter-relationship with mercury. In addition, the compositions of the simulated flue gas were varied to understand their influence on mercury removal. A low concentration of HCl in the simulated flue gas was used, considering the fact that the concentration of acid gas in the flue gas is low due to the influence of cement particles, which are alkaline materials. By contrast, because SO 2 reduces the adsorption capacity of activated carbon, a relatively high concentration of SO 2 was maintained in the simulated flue gas in order to make the conditions unfavourable for mercury adsorption. Tests were also carried out using simulated flue gas without SO 2, so as to understand the influence of SO 2 on mercury adsorption. Based on these conditions, we tested the efficiency of granular sulphur-impregnated activated carbon under the full-scale process conditions, examined its mercury adsorption characteristics and evaluated possibilities for its application to control the concentration of mercury in the flue gas in the cement-manufacturing process. 2. EXPERIMENTAL APPARATUS AND METHODS 2.1. Powder Sample Test Setup In this study, we used sulphur-impregnated activated carbon as an adsorbent for mercury adsorption from flue gas in the cement-manufacturing process. This material was produced by Cabot Norit Activated Carbon (RBHg3; Amersfoort, Netherlands) and has been commercialized. The physical properties of the sulphur-impregnated activated carbon are summarized in Table 1. In this study, two types of adsorption test systems were used to examine the mercury control characteristics of the adsorbent (activated carbon) under the condition of the flue gas in the cement-manufacturing process. One of them was used to test a small amount of sulphurimpregnated activated carbon as shown in Figure 1. For this purpose, 30 mg of powdered sulphur-impregnated activated carbon was mixed with 6 g of SiO 2 granules (Fisher Scientific, USA) and placed in a fixed-bed reactor (inner diameter, 1.27 cm). The results of the preliminary test indicated that SiO 2 only had a minimal influence on mercury removal. The fixed-bed reactor was kept in a forced convection oven (OF-22GW, Jeio Tech, Korea) so that the temperature of the reactor remains constant. The temperature of the reactor was changed to 90, 135 and 180 C in order to test the influence of varying temperatures. Using a mass flow controller (Brooks Model 5850E; Brooks Instrument, USA), we adjusted the flow rate of each constituent in the simulated flue gas to be constant so that the flow rate of the whole simulated flue gas becomes 1 l/minute. Simulated flue gas had the following basic composition: 12% CO 2, 5% H 2 O, 5% O 2, 500 ppm SO 2, 400 ppm NO, 5 ppm HCl, 20 μg/m 3 Hg 0, with the remaining being nitrogen. We assumed that TABLE 1. Physical Properties of the Sulphur-Impregnated Activated Carbon Pellet BET surface area 782 m 2 /g Micropore area 767 m 2 /g Micropore volume 0.30 m 3 /g Sulphur content <10 wt%
254 H.-K Min et al./adsorption Science & Technology Vol. 33 No. 3 2015 N 2 Heat tape T Temperaturecontrolled oven H 2 O vapourizer Bypass 30 mg sample in 6 g SiO 2 N 2 HCI Fixed-bed reactor SO 2 NO CO 2 Air Hg 0 Bubble meter Hood Mercury analyzer 1 N KCI 1 N NaOH 4% KMnO 4 and 10% H 2 SO 4 Figure 1. Schematic of the fixed-bed reactor system used in the powder sample test. the concentration of acid gas is low in the flue gas in the cement-manufacturing process and in the simulated flue gas without SO 2. In addition, to examine the inter-relationship between sulphurimpregnated activated carbon and mercury, a test was carried out by injecting only 20 μg/m 3 Hg 0 (with nitrogen being the remaining composition) into the activated carbon. After an elemental mercury permeation tube (Dynacal permeation tubes, VICI Metronics, USA) was placed in the constant-temperature circulating water tank, the temperature of the water tank was adjusted so that the concentration of elemental mercury remained constant at 20 μg/m 3. Taking advantage of the variations in water vapour pressure with changing temperatures, moisture was injected into the flue gas mixture. Using a heat tape (DAIHAN Scientific, Korea) we supplied heat to the outer area of the tube in order to prevent moisture from condensing in the flowing gas. The fixed-bed reactor containing 30 mg of sulphur-impregnated activated carbon powder was kept inside the forced convection oven and the temperature of the oven was increased. During this stage, we allowed the simulated flue gas to flow through the bypass line. Whether the concentration of mercury in simulated flue gas remained at approximately 20 μg/m 3 was verified using an elemental mercury vapour analyzer (RA-915M, Lumex, Cyprus). The simulated flue gas was injected into the sample in the fixed-bed reactor to begin the test. The gas that is emitted through the reactor goes through the impinger filled with a liquid absorbent and the elemental mercury vapour analyzer. A solution of potassium chloride (KCl; 0.1 N) was added to the first and second impingers to collect oxidized mercury. A solution of sodium hydroxide (NaOH; 0.1 N) was added to the third impinger in order to prevent the acid gas from influencing the elemental mercury
Adsorption of Mercury Using Sulphur-Impregnated Activated Carbon Pellets 255 vapour analyzer measurements. The fourth impinger was left empty to prevent the vapour from flowing into the elemental mercury vapour analyzer. The gas flowing through the impingers also goes through the elemental mercury vapour analyzer so that the concentration of the elemental mercury can be continuously measured. The gas then passes through two impingers filled with a solution containing 4% potassium permanganate (KMnO 4 ) and 10% sulphuric acid (H 2 SO 4 ) to collect elemental mercury. A bubble meter was used to measure the flow rate of the gas in the test. The bubble meter was connected to the hood (Figure 1). We measured the inlet elemental mercury concentration ([Hg 0 ] in ) by averaging the concentrations calculated when the gas that is discharged from the bypass line before and after the test goes through the elemental mercury vapour analyzer (RA-915M). We measured the outlet oxidized mercury concentration ([Hg 2+ ] out ) by analyzing the 0.1 N KCl solution that absorbed the oxidized mercury discharged during the test. We measured the outlet elemental mercury concentration ([Hg 0 ] out ) by averaging the total outlet elemental mercury concentrations calculated per second with the elemental mercury vapour analyzer (RA-915M) during the test. Based on the results from [Hg 0 ] out and [Hg 2+ ] out, Hg adsorption efficiency and Hg oxidation efficiency were calculated as follows: Hg adsorption efficiency (%) = 100 ([Hg 0 ] in [Hg 2+ ] out [Hg 0 ] out )/[Hg 0 ] in (1) Hg oxidation efficiency (%) = 100 ([Hg 2+ ] out )/[Hg 0 ] in (2) 2.2. Pellet Sample Test Setup As shown in Figure 2, the pellet sample test system used a fixed-bed reactor with a 4.5-cm inner diameter, which is wider than that of the fixed-bed reactor used for the powder sample test. The pellet sample test system was used to test the pellets as they are, that is, without pulverizing the sulphur-impregnated activated carbon. In addition, to simulate the space velocity in the full-scale process, only sulphur-impregnated activated carbon pellets were added to the fixed-bed reactor, increasing the height of the bed by 1 cm. The flow rate of the simulated flue gas was 3 l/minute and the test was performed under the condition of 11,000 hour 1, which is the space velocity used in the full-scale process. As with activated carbon powder sample test, simulated flue gas had the following basic composition: 12% CO 2, 5% H 2 O, 5% O 2, 500 ppm SO 2, 400 ppm NO, 5 ppm HCl, 20 μg/m 3 Hg, with nitrogen constituting the remaining amounts. The test was also carried out under the condition of the exhaust gas without HCl or SO 2. The temperatures of the reactor were similar to those in the activated carbon powder sample test (i.e. 90, 135 and 180 C). Elemental mercury vapour was supplied in the same way as that described for the activated carbon powder sample test. We placed the fixed-bed reactor with the sulphur-impregnated activated carbon pellet sample in an electric furnace with a temperature control. When raising the temperature of the furnace, the simulated flue gas was allowed to go through the bypass line. At this point, whether the concentration of mercury in simulated flue gas remained at approximately 20 μg/m 3 was verified with the elemental mercury vapour analyzer (VM-3000; Mercury Instruments Analytical Technologies, Germany). The simulated flue gas was then injected into the sample in the reactor and the test begins. When analyzing mercury concentration, 1.65 l/minute of the total exhaust gas was drawn and made to flow into the mercury analyzer. The concentration of mercury was analyzed in the same way as that in the powder sample, using an impinger filled with a liquid absorbent and an elemental mercury vapour analyzer (VM-3000). The conditions applied for the powder sample test and the pellet sample test are summarized in Table 2.
256 H.-K Min et al./adsorption Science & Technology Vol. 33 No. 3 2015 N 2 Heat tape H 2 O vapourizer TC N 2 HCI Bypass Temperaturecontrolled furnace SO 2 NO Hg 0 TC Pellet sample CO 2 Air Mercury analyzer Hood Hood 1 N KCI 1 N NaOH Figure 2. Schematic of the fixed-bed reactor system used in the pellet sample test. 4% KMnO 4 and 10% H 2 SO 4 TABLE 2. Summary of the Experimental Conditions Simulated Powder sample test Pellet sample test flue gas Concentration Inner diameter of 1.27 cm Inner diameter of 4.5 cm N 2 Balance the reactor the reactor CO 2 12% Total flow rate 1 l/minute Total flow rate 3 l/minute H 2 O 5% O 2 5% Adsorbent amount 30 mg Space velocity 11,000 hour 1 SO 2 500 ppm NO 400 ppm Temperature 90, 135, 180 C Temperature 90, 135, 180 C HCl 5 ppm Test time 40 min Test time 40 min Hg 20 μg/m 3 We measured the inlet elemental mercury concentration ([Hg 0 ] in ) by averaging the concentrations analyzed when the gas that is discharged from the bypass line before and after the test goes through the elemental mercury vapour analyzer (VM-3000). The efficiency of Hg adsorption and its oxidation were calculated in the same way as described earlier for the powder sample test using equations (1) and (2).
Adsorption of Mercury Using Sulphur-Impregnated Activated Carbon Pellets 257 3. RESULTS AND DISCUSSION 3.1. Powder Sample Test Results 3.1.1. Effect of Temperature The mercury adsorption capacity of activated carbons is greatly influenced by adsorption temperature. In the cement-manufacturing process, depending on the location where the mercury adsorption equipment is installed and the conditions of the process, the temperature of the exhaust gas flowing in the mercury adsorption process system changes; therefore, the simulated flue gas was injected at different temperatures (90, 135 and 180 C) to test the efficiency of the sulphur-impregnated activated carbon powder sample and the influence of various temperatures on the adsorption process. Figure 3 shows the breakthrough curves obtained for the sulphur-impregnated activated carbon powder upon injecting the simulated flue gas. Table 3 summarizes Hg oxidation and adsorption efficiencies of the sulphur-impregnated activated carbon powder, respectively, for all gas conditions tested. As shown in the figure and table, the mercury adsorption capacity decreases at 180 C, whereas a high adsorption capacity is noted at both 90 and 135 C. These results are in accordance with the literature showing that as the temperature increases, the adsorption capacity of sulphur-impregnated activated carbon decreases (Liu et al. 2000), but the decrease in adsorption capacity only appeared at 180 C. 30 Simulated flue gas 90 C 135 C 180 C Outlet Hg 0 concentration (µg/m 3 ) 25 20 15 10 5 Inlet Test Inlet 0 0 10 20 30 40 50 Time (min) Figure 3. Breakthrough curves for the sulphur-impregnated activated carbon powder at 90, 135 and 180 C under simulated flue gas conditions. TABLE 3. Summary of the Mercury Oxidation and Adsorption Efficiencies of the Sulphur-Impregnated Activated Carbon Powder Sample Temperature ( C) Gas condition 90 135 180 Simulated flue gas Hg oxidation (%) 5.9 2.3 2.9 Hg adsorption (%) 91.8 94.6 72.2 Nitrogen gas Hg oxidation (%) 13.9 9.0 2.0 Hg adsorption (%) 77.0 46.9 23.7 Simulated gas without SO 2 Hg oxidation (%) 3.3 14.1 4.5 Hg adsorption (%) 93.0 83.9 92.3
258 H.-K Min et al./adsorption Science & Technology Vol. 33 No. 3 2015 3.1.2. Effect of Gas Composition To examine the influence of gas compositions on the process of mercury adsorption of sulphurimpregnated activated carbon, we performed the mercury adsorption test with nitrogen gas and then another test using simulated flue gas without SO 2. Except for the composition of simulated flue gas, the test was performed under the same conditions as stated earlier. After analyzing the concentration of elemental mercury and oxidized mercury emitted during the test, we calculated the efficiencies of mercury adsorption and oxidation using the same procedures described in the Powder Sample Test Setup section. Under the condition of nitrogen gas, the breakthrough curves obtained for the sulphurimpregnated activated carbon powder, mercury oxidation and adsorption efficiency are shown in Figure 4 and Table 3, respectively. Under the condition of simulated flue gas, the adsorption efficiency of mercury decreased only at 180 C, whereas under the condition of nitrogen gas, as the temperature increased, the adsorption efficiency decreased rapidly. These results show that the mercury adsorption efficiency of sulphur-impregnated activated carbon is greatly influenced by gas compositions as well as by the temperature. Considering that the concentration of acid vapour is low in the exhaust gas in the cement-manufacturing process, the test was carried out using simulated flue gas excluding SO 2. The purpose of this test was to examine the influence of SO 2 on mercury adsorption of sulphur-impregnated activated carbon. The results of this test help to understand the mercury adsorption characteristics of sulphur-impregnated activated carbon under the condition of the exhaust gas in the cement-manufacturing process in which the concentration of SO 2 is low. Table 3 also shows the results we obtained after injecting simulated gas without SO 2 to each sample under the temperature conditions of 90, 135 and 180 C. The results show that a very high adsorption efficiency is achieved at 180 C as well as at 90 and 135 C. Under the condition of simulated flue gas, a relatively reduced adsorption efficiency is shown at 180 C, whereas when SO 2 was excluded from the simulated flue gas, sulphur-impregnated activated carbon powder showed high mercury adsorption capacity even at 180 C. This shows that SO 2 has a negative influence on mercury adsorption, that is, it is adsorbed onto the adsorption sites of sulphur-impregnated activated carbon, which is in accordance with the results reported in the 30 N 2 90 C 135 C 180 C Outlet Hg 0 concentration (µg/m 3 ) 25 20 15 10 5 Inlet Test Inlet 0 0 10 20 30 40 50 Time (min) Figure 4. Breakthrough curves for the sulphur-impregnated activated carbon powder at 90, 135 and 180 C under nitrogen gas conditions.
Adsorption of Mercury Using Sulphur-Impregnated Activated Carbon Pellets 259 literature (Hsi and Chen 2012). In this test, however, the influence of SO 2 appeared only at 180 C; at 90 and 135 C, sulphur-impregnated activated carbon still showed a high efficiency for mercury adsorption. From these results, we were able to find that, under the condition of the exhaust gas in which SO 2 hardly exists, sulphur-impregnated activated carbon has a high efficiency for mercury adsorption even at a high temperature of 180 C. 3.2. Pellet Sample Test Results 3.2.1. Effect of Temperature From the aforementioned discussions, it is confirmed that the adsorption efficiency of mercury is influenced by both temperature and gas compositions. In particular, at 180 C, the composition of gas can make a huge difference in the adsorption efficiency of mercury. In this section, we will examine the use of fixed-bed granular sulphur-impregnated activated carbon for removing the mercury in the exhaust gas in the cement-manufacturing process. For this purpose, the test was performed under the condition of 11,000 hour 1, which is the space velocity used in the full-scale process. The temperatures and gas compositions were varied as in the powder sample test. Figure 5 shows the breakthrough curve for the mercury adsorption of sulphur-impregnated activated carbon pellet sample, which appears when simulated flue gas is injected. Table 4 summarizes the mercury oxidation and adsorption efficiencies of the sulphur-impregnated activated carbon pellet sample, respectively, for all the gas conditions tested. The methods for calculating the oxidation efficiency and adsorption efficiency of mercury under the condition of pellet sample test are shown in the Pellet Sample Test Setup section. At temperatures of 90 and 135 C, when the simulated flue gas flowing through the bypass line passes through the pellet sample, the concentration of mercury decreases rapidly, thus emitting only a low mercury concentration (0.1 0.2 μg/m 3 ). At a temperature of 180 C, when the simulated flue gas was passed through the pellet sample, the concentration of elemental mercury emitted decreased to the level of 2 μg/m 3, with further decrease in concentration with time. From Table 4, it can be seen that at temperatures of 90 and 135 C, a very high adsorption efficiency of 96% and 97.5% is achieved, whereas at 180 C a slightly reduced mercury adsorption efficiency of 90% was 30 Simulated flue gas 90 C 135 C 180 C Outlet Hg 0 concentration (µg/m 3 ) 25 20 15 10 5 Inlet Test Inlet 0 0 10 20 30 40 50 Time (min) Figure 5. Breakthrough curves for the sulphur-impregnated activated carbon pellets at 90, 135 and 180 C under simulated flue gas conditions.
260 H.-K Min et al./adsorption Science & Technology Vol. 33 No. 3 2015 TABLE 4. Summary of the Mercury Oxidation and Adsorption Efficiencies of the Sulphur-Impregnated Activated Carbon Pellet Sample Temperature ( C) Gas condition 90 135 180 Simulated flue gas Hg oxidation (%) 1.6 1.6 1.5 Hg adsorption (%) 96.1 97.5 90.2 Nitrogen gas Hg oxidation (%) 1 1.3 0.6 Hg adsorption (%) 97.6 97.2 79.7 Simulated gas without SO 2 Hg oxidation (%) 3.8 2.8 2.1 Hg adsorption (%) 93.8 96.2 93.6 Simulated gas without HCl Hg oxidation (%) 2.2 1.1 0.5 Hg adsorption (%) 91.2 94.5 87.1 achieved. The oxidation efficiency of mercury was at the level of 1 2% for the temperatures tested. In both the pellet sample test and the powder sample test, relatively reduced adsorption efficiency was observed at 180 C; however, the adsorption efficiency of the pellet sample test (90%) was much higher than that achieved in the powder sample test (72%) at 180 C. 3.2.2. Effect of Gas Composition To examine the influence of gas compositions in the pellet sample test, we performed the mercury adsorption test with nitrogen gas as well as with the simulated flue gas without SO 2. In addition, we performed the test using various gas compositions. However, HCl, which is known to contribute the most to mercury oxidation, was not included for this purpose, and therefore, we evaluated the mercury adsorption efficiency of sulphur-impregnated activated carbon pellet sample under conditions unfavourable for mercury oxidation and adsorption. First, under the condition of nitrogen gas, mercury oxidation and adsorption efficiency of the adsorbent are shown in Table 4. As is the case with the results obtained using simulated flue gas conditions, a very high mercury adsorption efficiency was noted at 90 and 135 C, and a relatively low mercury adsorption efficiency was shown at 180 C. However, in the case of powder sample test, an adsorption efficiency of 47% and 24% was achieved at 135 and 180 C, respectively, whereas, in the case of pellet sample test, a higher adsorption efficiency of 97% and 80% was achieved. This is because, in the powder sample test, a very small amount of sulphur-impregnated activated carbon was used to closely evaluate the inter-relationship between sulphur-impregnated activated carbon and mercury, whereas in the pellet sample test, sulphur-impregnated activated carbon was evaluated under the full-scale process conditions, and thus relatively large amounts of activated sample was used compared with the powder sample test, which indicates that the influence of temperature and gas compositions was greatly reduced. Under the condition of simulated flue gas without SO 2 (Table 4), a high adsorption efficiency was achieved at the temperature conditions studied and, as in the case of powder sample test, when SO 2 is excluded, a high mercury adsorption efficiency was achieved even at 180 C. In addition, even when the test was performed under conditions unfavourable for mercury oxidation and adsorption (i.e. by excluding HCl from simulated flue gas; Table 4), there was only a little decrease in adsorption efficiency at all the temperature conditions studied; at temperatures of 90 and 135 C, an adsorption efficiency of over 90% was achieved and, at 180 C, an adsorption efficiency of 87%, which is still high, was achieved.
Adsorption of Mercury Using Sulphur-Impregnated Activated Carbon Pellets 261 4. CONCLUSIONS In this study, fixed-bed granular sulphur-impregnated activated carbon was used to remove the mercury from the exhaust gas in the cement-manufacturing process. The test was performed using two types of adsorption test systems. First, a small amount of powder sample was tested to accurately evaluate the inter-relationship between sulphur-impregnated activated carbon and mercury. The mercury adsorption capacity of the pellet sample was evaluated under the condition of the space velocity in the full-scale process. In the powder sample test, due to a small amount of sample, the mercury adsorption efficiency varied a lot depending on changes in the temperature and gas composition. However, although the powder sample showed that the mercury adsorption efficiency decreases with the increase of temperatures, under the condition of simulated flue gas without SO 2, a high mercury adsorption efficiency of 92% was achieved even at a high temperature of 180 C. Therefore, with regard to the exhaust gas in the cementmanufacturing process, which has a low concentration of SO 2, it was expected that sulphur-impregnated activated carbon would show a high mercury adsorption efficiency at high temperatures as well. Pellet sample test results indicated that there was only a minor influence of temperature and gas compositions on mercury adsorption efficiency of the activated carbon. In addition, under the condition of simulated flue gas without HCl, which is known to contribute the most to mercury oxidation, sulphur-impregnated activated carbon pellets showed a mercury adsorption efficiency of over 87%; under the condition of simulated flue gas without SO 2, a high mercury adsorption efficiency of over 93% was achieved for all the temperature conditions studied. Therefore, in the case of the exhaust gas in the cement-manufacturing process that is characterized by low levels of sulphur dioxide (SO 2 ), fixed-bed granular sulphur-impregnated activated carbon is expected to show a high mercury adsorption efficiency even at temperatures as high as 180 C. ACKNOWLEDGEMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0013242) and by the Korean Ministry of Environment as The Eco-Innovation Project. REFERENCES Diamantopoulou, Ir., Skodras, G. and Sakellaropoulos, G.P. (2010) Fuel Process. Technol. 91, 158. Hsi, H.C. and Chen, C.T. (2012) Fuel. 98, 229. Karatza, D., Lancia, A., Musmarra, D. and Zucchini, C. (2000) Exp. Therm. Fluid Sci. 21, 150. Keener, T.C., Oh, K.J. and Lee, S.S. (2012) Adsorpt. Sci. Technol. 30, 593. Kilgroe, J.D., Sedman, C.B., Srivastava, R.K., Ryan, J.V., Lee, C.W. and Thorneloe, S.A. (2001) Control of Mercury Emissions from Coal-Fired Electric Utility Boilers: Interim Report, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Washington, DC. Liu, W., Vidic, R.D. and Brown, T.D. (1998) Environ. Sci. Technol. 32, 531. Liu, W., Vidic, R.D. and Brown, T.D. (2000) Environ. Sci. Technol. 34, 154. Miller, S.J., Dunham, G.E., Olson, E.S. and Brown, T.D. (2000) Fuel Process. Technol. 65, 343. Ochiai, R., Uddin, M.A., Sasaoka, E. and Wu, S. (2009) Energy Fuel. 23, 4734.
262 H.-K Min et al./adsorption Science & Technology Vol. 33 No. 3 2015 Rezoni, R., Ullrich, C., Belboom, S. and Germain, A. (2010) Mercury in the Cement Industry, European Cement Association, Brussels, Belgium. United Nations Environmental Programme (UNEP) (2013) Global Mercury Assessment 2013: Sources, Emissions, Releases and Environmental Transport, UNEP Chemicals Branch, Geneva, Switzerland. Yan, R., Ng, Y.L., Liang, D.T., Lim, C.S. and Tay, J.H. (2003) Energy Fuel. 17, 1528. Yang, H., Xu, Z., Fan, M., Bland, A.E. and Judkins, R.R. (2007) J. Hazard. Mater. 146, 1