Subscriber access provided by Kaohsiung Medical University
Environmental and Carbon Dioxide Issues
Low-Temperature Reactions of HCl with Metal-Doped Carbon Naoto Tsubouchi, Yuuki Mochizuki, Yuji Shinohara, Akiyuki Kawashima, and Yasuo Ohtsuka Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01165 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Low-Temperature Reactions of HCl with Metal-Doped Carbon Naoto Tsubouchi*, †, Yuuki Mochizuki†, Yuji Shinohara†, Akiyuki Kawashima‡, Yasuo Ohtsuka‡ †
Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan ‡
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan ABSTRACT: A model carbon prepared from phenolic resin was activated with O2 and then impregnated with Ca, Cu or Zn to examine the relationship between the amount of HCl desorbed from carbon samples containing various metals in this lower temperature range (100 and 300 °C), and clarified the effect of active sites on HCl adsorption. The results showed that interactions between HCl and the carbon were enhanced at temperatures in the range of 100 to 300 °C and that the amount of HCl reacted was increased by doping with these metals. The different HCl concentration profiles at 100 and 300 °C were obtained, with a greater overall decrease in HCl at the lower temperature (100 °C). Those are attributed to increased physical adsorption of the HCl at 100 °C. The mass of HCl reacted increased in the order of Cu < Ca < Zn at 300 °C and Ca < Cu < Zn at 100 °C, and the HCl reacted at 300 °C was evidently more stable than that reacted at 100 °C. Organochlorine compounds, chemisorbed HCl and inorganic chlorides were all identified on the carbon surfaces following exposure to a flow of HCl. The organochlorine species and chemisorbed HCl were desorbed, along with the generation of H2O, on heating the samples to 700 °C, while inorganic chlorides and additional H2O were desorbed above this temperature. Both the mass of organochlorines generated 1
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
and the amount of HCl absorbed were found to increase with the number of active sites at 300 °C; however, that was exceeded remarkably at 100 °C. 1. INTRODUCTION It is important to collect the generated chlorine (Cl) in high-temperature processes (such as pulverized coal firing, waste incineration and iron ore sintering) because these processes can generate volatile toxic compounds from Cl. As an example, heating coal results in pyrolysis and it has been reported that chlorine contained in the coal can be desorbed as HCl during this process.1, 2, 3 Herod studied six different British charcoals that included high concentrations of Cl (0.4 - 0.9 wt%), and found that heating to 300 °C at 2 °C/min over 24 h transitioned 40 - 60% of the Cl to HCl, based on mass spectrometric analysis.1 Other elements contained in coal are also discharged during combustion.4 Several potentially harmful elements (such as Hg, Se, As and Cd) will vaporize below 400 °C, and Hg is especially easy to release into the atmosphere. The Hg content in coal depends on the source from which it is mined, but is generally less than or equal to 0.5 ppm. This Hg moves through the electrostatic precipitator in conjunction with the flue gas at 90 - 300 °C and is subsequently released into the ambient air. The temperature of the precipitator is known to affect the extent to which Hg is released, and the Hg release rate also increases if the Cl content of the coal is high.5 HCl can also be produced by the incineration of waste, along with dioxins and chlorinated benzene.6 Mercury can occur in the exhaust gases from incineration, and is typically removed using activated carbon before disposing of bag filters. However, both HCl and organochlorines can still persist in the incineration effluent. In a blast furnace processing pig iron, coke is manufactured by carbonizing coking coal under an inert atmosphere in a coke oven. The Cl in the coal used to make the coke can desorb HCl during the carbonization process, and some of this Cl is recaptured in the solid-phase coke during treatment of the exhaust gas at low temperatures.7, 8, 9 The Cl that remains in the solid phase is released during the sintering of the ironstone under ambient air and can generate harmful dioxins and chlorinated aromatics.
2
ACS Paragon Plus Environment
Page 2 of 38
Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Activated carbon is a material having numerous highly reactive sites made from oxygen-containing functionals group on the carbon surface (such as carboxyl, phenol and hydroxyl groups) that are formed upon heating. This material is more reactive than the original aromatic network structure. It has been reported that activated sites are important in promoting the reaction between carbon material and gas.10, 11, 12 Our research group has previously studied the absorption of HCl by O2-activated metal-doped resins at 500 °C.13 However, similar studies have not been performed in the temperature range that is optimal for exhaust gas processing (100 - 300 °C). Therefore, the present work investigated the relationship between the amount of HCl desorbed from carbon samples containing various metals in this lower temperature range, and clarified the effect of active sites on HCl adsorption. Variations in the amounts of chemically-absorbed and physically-absorbed HCl were also assessed, using different temperatures and metals. The resulting data should be helpful in developing methods for processing exhaust gases. 2. EXPERIMENTAL SECTION 2.1 Carbon Samples. A pure model coal sample was fabricated from phenolic resin (Gun Ei Chemical Industry, Lot. No. 40311) by drying under high-purity N2 in a fixed bed quartz reactor for 1 h at 108 °C followed by heating for 1 h at 950 °C in a stream of high-purity He (99.99995%, Japan Fine Products Corp.). This material had a particle size range of 300 - 425 µm and was activated by heating at 500 °C under a 20% O2/He mixture. Herein, this coal is referred to as AC (Table 1). 2.2 Metal Addition. To examine the effect of metals on the reaction of HCl with carbon, 4.00 g of AC was added to solutions containing (CH3COO)2Ca·H2O, (CH3COO)2Cu or (CH3COO)2Zn·2H2O, using the concentrations and solvents in Table 2. The solvents were subsequently removed from each suspension using a rotary evaporator, to obtain specimens referred to herein as Ca /AC, Cu/AC and Zn/AC. The nominal metal concentration in each sample was 0.50 mass% and the required amount of the metal reagent in the impregnation solution was calculated using the following equation.
3
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
molar mass of the metal in the reagent g⁄mol molar mass of the reagent g⁄mol × 100 AC g dry + reagent mass g
reagent mass g × 0.50 mass% =
The surface areas and pore volumes of the various AC materials are summarized in Table 3, while Table 2 shows the actual concentrations of metals in the samples as determined by inductively coupled plasma (ICP) analysis. The surface areas and pore volumes of the various specimens were determined by N2 absorption using an instrument designed to measure specific surface area and pore distribution (QUANTACHROME). In each case, a 20 mg sample was prepared by drying under vacuum at 108 °C for 1 h, after which the dry weight of the sample was recorded at –196 °C. The surface area was determined using the Brunauer-Emmett-Teller (BET) method and the pore volume was obtained using density functional theory (DFT). These data demonstrate that the pore volumes and surface areas of the Ca/AC, Cu/AC and Zn/AC were all decreased relative to the values for the original AC. The quantitative analysis of Ca, Cu and Zn ions was performed by dispersing a 0.250 g sample in 25 ml of 10% HCl followed by shaking for 8 h in an incubator with heating at 40 °C. The metal ions were subsequently separated by a filtration/concentration process using a membrane filter (Toyo Roshi Kaisha, Ltd.), and their concentrations were determined by ICP analysis (Perkin Elmer). 2.3 HCl Treatment and subsequent TPD. The reaction of HCl with the AC and the assessment of desorption at a fixed temperature and via temperature programmed desorption (TPD) was performed using the procedure shown in Figure 1. A 0.250 g sample (with particle sizes of 250 – 425 µm) was transferred into a quartz tube and heated under a flow of high-purity He at a rate of 2 or 5 °C/min. After reaching 100 or 300 °C, the specimen was immediately exposed to 100 ppm HCl in N2, and the HCl concentration in the gas stream was analyzed at 1 min intervals using a non-dispersive infrared (IR) spectrometer (Thermo Electron). After two hours, the gas stream was changed to high-purity N2 and the concentration of HCl in the outgoing gas stream was measured while heating the sample at 5 °C/min. In addition, the concentrations of Cl2, CO and CO2 at the outlet of the reactor were also determined, using an electrochemical chlorine sensor (BW Technologies, Honeywell) at 2 s intervals and by micro gas chromatography (Agilent) at 3 min intervals. The difference between the reduction 4
ACS Paragon Plus Environment
Page 4 of 38
Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
in the level of HCl in the gas stream (Figure 1A) and the level of HCl in the subsequent N2 stream (Figure 1B) is considered to represent the amount of HCl reacted. The quantity of reacted HCl (as defined above) was confirmed by collecting the sample after treatment under N2 (Figure 1) followed by washing with water and analysis by TPD. Washing was performed by shaking a 0.20 g quantity of the sample in 50 cc of ultrapure water at 40 °C for 6 h using an incubator (Yamato Scientific). The concentration of Cl- ions in the wash water was subsequently determined by ion chromatography (Dionex). After water washing, each specimen was dried under vacuum overnight (Yamato Scientific) at 40 °C, and then assessed by TPD. 2.4 Determination of Active Sites. The concentration of active sites on a sample was determined by adsorbing O2 on the active sites after heating and then measuring the resulting quantities of CO and CO2 released during TPD.14,
15
These trials were performed by transferring 0.250 g of each
sample (with a particle size range of 250 - 425 µm) into the reaction tube shown in Figure 2, followed by heating in a fixed-bed quartz reactor for 24 h at 500 °C under high-purity He. Any species attached to the oxygenated functional groups (such as carboxyl groups) that were present on the carbon material were desorbed. Following this, a stream of 2.00% O2 in He (Japan Fine Products Corp.) was absorbed for 30 min at 100 °C. Finally, the sample was cooled to room temperature under a flow of high-purity He and then heated at 2 °C/min from ambient temperature to 950 °C. The CO and CO2 generated by this TPD process were analyzed by gas chromatography, while the O2 concentration in the outlet of the reactor was determined at 2 s intervals using a zirconia-based oxygen analyzer (Toray Engineering). The H2O concentration was measured using an online infrared spectrometer (INNOVA) attached to reactor. For this measurement, a 0.250 g (250 - 425 µm) sample was transferred into the reaction tube and heated from room temperature to 950 °C at 5 °C/min under a stream of high-purity He. 3. RESULTS AND DISCUSSION 3.1 Concentration changes during HCl circulation. Figure 3 plots the HCl concentrations during the exposure of the samples to the HCl gas stream at 300 and 100 °C as functions of circulation time. Figure 4 plots the reductions in the absolute amounts of HCl as functions of circulation time. The 5
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
HCl concentrations over the Ca/AC, Cu/AC and Zn/AC were remarkably decreased compared to the decrease over the original AC at both temperatures. In addition, after this initial decrease, the HCl concentrations all gradually returned to 100 ppm. At 100 °C, the concentration change after 5 min was lower and more time was required to reach the lowest concentration relative to the results obtained at 300 °C. Therefore, even the low temperature (100 °C) caused an interaction between the carbon substrate and HCl. In addition, the reduction in the HCl concentration was enhanced by the presence of Ca, Cu and Zn. The different HCl concentration profiles at 100 and 300 °C, with a greater overall decrease in HCl at the lower temperature, are attributed to increased physical adsorption of the HCl at 100 °C. In all cases, the Cl2 concentration was constant at 0.05 ppm. 3.2 The desorption of physisorbed HCl. Figures 3 and 4 demonstrate that the HCl decrement at 100 °C was greater than at 300 °C, suggesting the physisorption of HCl on the AC at the lower temperature. To confirm this, samples were first exposed to a stream of 100 ppm HCl in N2, then to high-purity N2 for 1 h. Figure 5 plots the variations in the HCl concentration during this process, with zero on the abscissa equivalent to the time at which the new flow stream reached the HCl monitoring device. A trial with only quartz wool rather than a specimen (labeled “blank” in Figure 5) was used to examine the effects of residual HCl in the gas phase. The data show that there was some residual HCl, but that the concentration rapidly fell to zero. In a subsequent trial using the Ca/AC, the plot with the blank data subtracted (labeled “difference” in Figure 5) demonstrates surface interactions with HCl as a result of the physisorption of HCl. Based on these results, all subsequent data were corrected using the blank readings. Figure 6(a) shows the variations in the HCl concentration over time when maintaining a flow of high-purity N2 after HCl desorption at 300 °C (Figure 3(a)). All samples exhibited HCl desorption with the largest amounts generated by the metal-doped AC specimens, which required very long time spans to reach an HCl level of zero. These results are ascribed to the physical adsorption of HCl. Figure 7 (a) shows a relationship between the amount of desorbed HCl integrated from the plot of Figure 6 (a) and the time (up to 40 minutes). The HCl desorption quantities were 2.6, 32, 23 and 30 µmol/g in the trials using the AC, Ca/AC, Cu/AC and Zn/AC. Thus, the quantity of physisorbed HCl was increased by the presence of the metals, but each metal gave similar results. Figure 6(b) plots changes in the HCl concentration over time under high-purity N2 6
ACS Paragon Plus Environment
Page 6 of 38
Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
after HCl circulation at 100 °C (based on Figure 3 (b)). All samples evidently generated HCl at both temperatures. The amount of desorbed HCl was also greater when using the metal-doped AC specimens compared with the original AC and, in each case, the HCl amount slowly decreased after 2 - 3 min. The total amounts of desorbed HCl (35 - 60 µmol/g) obtained from the Ca/AC, Cu/AC and Zn/AC after 1 h were two to four times that generated by the AC (approximately 11 µmol/g) (Figure 7 (b)). Thus, the quantity of HCl that physically adhered was high at 100 °C in spite of the addition of the metals. This result demonstrates that HCl physisorption takes place on the AC at the lower temperature. In each case, the Cl2 concentration as simultaneously measured was below the detection limit of 0.05 ppm. 3.3 The reaction of HCl with the carbon substrate. The results obtained in the above trials are summarized in Table 4. The quantities of HCl reacted at both temperatures were in the order of AC
