Article pubs.acs.org/EF
Condensation Behavior of Heavy Metals during Oxy-fuel Combustion: Deposition, Species Distribution, and Their Particle Characteristics Wenjia Song,*,†,‡,§ Facun Jiao,§,∥ Naoomi Yamada,§ Yoshihiko Ninomiya,*,§ and Zibin Zhu‡ †
Department of Earth and Environmental Sciences, Ludwig-Maximilians University, Munich 80333, Germany The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China § Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan ∥ National Center of Coal Chemical Products Quality Supervision and Inspection, Huainan, Anhui 232007, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: This study aimed to characterize the condensation behavior of two heavy metals, namely, Pb and Zn, during oxyfuel combustion and to clarify and compare the differences in their behavior during oxy-fuel versus air-fired combustion. A labscale rotary quartz reactor with a multi-stage cooling zone was used to analyze the deposition content and species distribution of the condensed Pb and Zn vapors at different temperature ranges and/or points and to observe their particle characteristics in the simulated oxy-fuel flue gas (OFFG), air-fired flue gas (AFFG), oxy-fuel flue gas without steam (OFFGWS), and air-fired flue gas without steam (AFFGWS). The deposition content of the condensed Pb and Zn vapors in the AFFG was consistently higher than that of OFFG in the cooling zone from 800 to 100 °C. Moreover, the steam content had an obvious influence on the deposition content. The condensed Pb and Zn vapors were mostly deposited in the sulfates in OFFG at 600−300 °C, instead of in the chlorides in AFFG. The average diameter of particles that contain Pb and Zn increased as the temperature decreased, and their shape factor in both AFFG and AFFGWS was higher than that in OFFG and OFFGWS.
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INTRODUCTION Heavy metal emissions from coal-fired power plants have been a significant concern because of the damage that they cause to the environment as well as to human and animal health.1,2 The identification and quantification of the characteristics of heavy metals are one of the key steps for controlling their emission, which effectively reduces further environmental exposure to these heavy metals.3 Considerable efforts have been made in studying emission behavior,4 mechanisms and control,5 and nucleation in flue gas,6 governing the fate of these metals,7−9 and developing computer models for estimating the distributions and emissions10,11 of heavy metals in coal within an airfired combustion system. However, investigations on the deposition and species formation of heavy metals from coal as well as their particle characteristics during oxy-fuel combustion processes have not been able to establish an inventory of heavy metal emissions or are insufficient for assessing their potential environmental influence.12,13 Oxy-fuel combustion with recirculation flue gas, in which, instead of using air as an oxidizer, pure O2 or a mixture of O2 and recycled flue gas is used to generate a product gas with a high CO2 concentration.14,15 This process is considered as one promising solution to carbon capture and storage.16 During oxy-fuel combustion, the product fuel gas in the economizer section of the boiler containing mainly CO2 and steam, instead of N2, in air-fired combustion is used to keep the system cooler. The different gas properties of N2, CO2, and steam distinguish the flue gas in oxy-fuel combustion from that in conventional © 2013 American Chemical Society
air-fired combustion, which, in turn, could affect the behavior of heavy metals during oxy-fuel coal combustion. Lead (Pb) and zinc (Zn) are the major sources of air pollution in urban environments and two of the most abundant trace elements in the gas sample during coal combustion. These heavy metals have been used since the 1970s to monitor different pollution sources and survey anthropogenic emission, fallout, and the impact of heavy metals on the environment.17,18 These elements are highly toxic to humans, and excessive intake of these elements can cause anemia and diseases of the kidneys and the heart, as well as the immune, nervous, reproductive, and gastrointestinal systems.19,20 The physical and chemical properties of Pb and Zn, such as particle size and speciation, significantly affect their epidemiology and toxicology.21 For example, the toxicity of the chloride states of these heavy metals are higher than that of their respective sulfates according to the lethal dose 50 (LD50) test, which determines the dosage of a substance needed to kill 50% of a group of test animals. In their chloride states, these metals are highly water-soluble, which makes them mobile. In addition, these heavy metals are carcinogenic in their chloride state compared to their Special Issue: Impacts of Fuel Quality on Power Production and the Environment Received: March 19, 2013 Revised: July 7, 2013 Published: July 12, 2013 5640
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corresponding sulfate state.22 The differences in physical properties (such as heat capacity, thermal conductivity, and viscosity) of flue gases in oxy-fuel combustion and air-fired combustion could also result in Pb and Zn to have different condensation processes, which, in turn, may influence species formation, concentration, and particle morphology. The influence of these processes is due to the distinctive physical and chemical properties of each Pb and Zn species that govern their environmental impact. Therefore, the identification and quantification of the condensation behavior of Pb and Zn vapors in the oxy-fuel flue gas (OFFG) is imperative in addressing questions regarding toxicity, mobility, and transport mechanisms. The focus of the present study was to (a) characterize the condensation behavior of Pb and Zn vapors in OFFG and (b) clarify and compare the differences in the behavior of the condensed Pb and Zn vapors in OFFG versus those in air-fired flue gas (AFFG). Therefore, the temperature and gas composition dependence of deposition, species determination and their distribution, and the characteristics of the condensed particles (including average diameter, size distribution, and shape factor) of Pb and Zn vapors were studied by conducting experiments in a lab-scale, high-temperature rotary quartz reactor with a multi-stage cooling zone under simulated real OFFG, AFFG, oxy-fuel flue gas without steam (OFFGWS), and air-fired flue gas without steam (AFFGWS), including high partial pressure CO2/N2 and impurity gases, such as SO2, HCl, and steam. The outcome of this investigation is expected to provide an inventory of heavy metals that may be used for future research.
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In the experiments carried out in this study, lead acetate and zinc acetate analytical reagents (0.1 mol) were first dissolved in 100 mL of deionized water and then loaded on porous titanium dioxide (TiO2) powders with an average diameter of 1.5 mm. Then, about 40 g of TiO2 powders were rinsed using 0.1 mol/L nitric acid (HNO3) for 48 h to remove surface contaminants, washed using deionized water, immersed in a metal-containing solution for 24 h, and dried at 50 °C in a rotary evaporator. Finally, the resulting heavy metal-laden TiO2 powders were calcined at 400 °C for 12 h in a muffle furnace to remove interfering organic substances and used as precursors for Pb and Zn. Experimental Equipment and Method. The experiments on the volatilization−condensation behavior of Pb and Zn vapors were carried out in a lab-scale, high-temperature rotary quartz reactor with a multi-stage cooling zone, which was used to generate Pb and Zn vapors at 1000 °C as well as to simulate a real flue gas atmosphere (Table 2) and the flue gas cooling process of the economizer in oxyfuel and air-fired coal utility boilers. A bench-scale high-temperature reactor was used to study the volatilization−condensation behavior of heavy metals under different gas compositions at high temperatures. As shown in Figure 1, the process consisted of three stages: (1) The volatilization stage (horizontal quartz tube furnace) at up to 1000 °C contains heavy metal vapors that were generated in a gas stream. The heavy metal vapor generator consists of an inner tube (26.53 mm inside diameter and 700 mm long) and an outer tube (53.34 mm inside diameter and 900 mm long). (2) The condensation stage (vertical quartz tube furnace) contains cooling zones I, II, and III composed of three quartz tubes (MeiJo, TE-32 glass), with a large temperature gradient from 800 to 25 °C in three steps and an average cooling rate of approximately 125 K/s to simulate the flue gas cooling process of the economizer in oxy-fuel and air-fired coal utility boilers and quartz filters (Advantec, 88RH) in the joint of the tubes that play an important role as seals to prevent heavy metal vapor from leakage, and monitored using three thermocouples installed on the three quartz filters that correspond to the location of the electrical furnace that has a temperature gradient range of 800−100 °C (Figure 2). (3) A scrubber (absorption device) is based on the United States Environmental Protection Agency (U.S. EPA) Method 29.23 Quartz wool was plugged into the joint between the cooling zone tube and absorption bottle to avoid blocking the flue gas flow in the exhausting tube in the cooling zone and to completely capture and impinge on the noncondensable Pb and Zn vapors in the three-stage cooling zones (5% HNO3 + 10% H2O2). Table 3 outlines the experimental plan. The first two sets of experiments (sets I and II) aimed to characterize the deposition content of condensed Pb and Zn vapors, namely, the condensation of heavy metals Pb and Zn on the inner surface of the quartz tube (Figure 2) and their various speciation distribution at three temperature ranges, namely, 800−600, 600−300, and 300−100 °C, in the simulated OFFG and AFFG. Parallel experiments were conducted for OFFGWS and AFFGWS in sets I and II because of the differences in the gas composition between OFFG and AFFG, excepted for the
EXPERIMENTAL SECTION
Heavy Metal Sampling. Two model compounds, namely, lead acetate [Pb(CH3COO)2] and zinc acetate [Zn(CH3COO)2] analytical reagents (0.1 mol), were selected as sources for each studied “trace” heavy metal. All compounds are available commercially and were chosen on the basis of their critical properties, such as volatilization temperature, the nature of the formed volatile species, and solubility in solvents, compatible with the analytical methods used, which are listed in Table 1.
Table 1. Properties of Heavy Metals heavy metal Pb Zn a
source lead acetate zinc acetate
content (mol) 0.1 0.1
carrier TiO2 powder TiO2 powder
gas phasea
condensed phasea
PbCl2
PbSO4
ZnCl2
ZnSO4
Predicted using FactSage thermodynamic software.
Table 2. Comparison between Flue Gas Compositions in Realistic Oxy-fuel and Air-Fired Conditions and Simulated Conditions in This Work industrial conditionsa
experimental conditions
flue gas constituents (vol %)
OFFGb
AFFGc
OFFG
AFFG
OFFGWSd
AFFGWSe
O2 N2 CO2 H2O SO2 (ppm) HCl (ppm)
3.0−4.0 0−10.0 60.0−70.0 20.0−25.0 1580 1000
3.0−4.0 70.0−75.0 12.0−14.0 10.0−15.0 1580 1000
3.50 14.70 70.0 22.5 2000 1000
3.50 71.50 13.0 12.5 2000 1000
3.50 83.00 13.00 0 2000 1000
3.50 14.70 81.80 0 2000 1000
Realistic condition means the realistic gases in the flue gas in the industrial-scale oxy-fuel and air-fired pilot plant. bOFFG = oxy-fuel combustion. AFFG = air combustion. dOFFGWS = oxy-fuel combustion without stream. eAFFGWS = air combustion without stream.
a c
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Figure 1. Schematic diagram of the lab-scale, high-temperature rotary quartz reactor with a multi-stage cooling zone.
Figure 2. Deposition of the condensed Pb and Zn vapors on the surface of the quartz tube located in the multi-stage flue gas cooling zone in OFFG, AFFG, OFFGWS, and AFFGWS.
Table 3. Summary of the Performed Experiments experimental conditions set
content
analysis
test (group/time)
gas composition
temperature
I II
deposition distribution
AAS IC and XRD
4/1 4/2
OFFG, AFFG, OFFGWS, and AFFGWS OFFG, AFFG, OFFGWS, and AFFGWS
III
morphology
SEM−EDS
4/2
OFFG, AFFG, OFFGWS, and AFFGWS
800−600, 600−300, and 300−100 °C 800−600, 600−300, and 300−100 °C TA = 800 °C, TB = 600 °C, and TC = 400 °C TA = 700 °C, TB = 500 °C, and TC = 300 °C
For Pb and Zn deposition and distribution, the temperature of the horizontal furnace and three quartz filters that correspond to the temperature in the vertical furnace were first heated at 1000 °C, 800 °C (TA), 600 °C (TB), and 300 °C (TC). Next, about 0.1 mol of lead acetate and zinc acetate absorbed on the surface of 40.00 g of TiO2 powder was placed and transferred to the center of the inner tube and then quickly moved into the outer tube. The quartz glass inner tube was continuously rotated to eliminate gas diffusion resistance. The four kinds of simulated flue gas, namely, OFFG, AFFG, OFFGWS, and
CO2/N2 ratio, including the partial pressure of steam (that is, the partial pressure of steam of 20−25 vol % in the real OFFG was approximately twice that of 10−15 vol % in the AFFG) (see Table 2) to study the effect of the CO2/N2 rate and stream concentrations separately. Set III experiments were designed to investigate the average diameter of the particles formed by the condensed Pb and Zn vapors and their morphologies at six temperature points in OFFG, AFFG, OFFGWS, and AFFGWS. 5642
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Figure 3. (a) Quartz filter and crucible used for carrying the deposition of condensed Pb and Zn vapors. (b) Scraped deposition of the condensed Pb and Zn vapors on the surface of the quartz tube after the experiment. (c) Schematic diagram of the analysis of the Pb and Zn species distribution at cooling II (600−300 °C) by IC.
Figure 4. Speciation and dew point of (left) Pb and (right) Zn vapors with the predicted thermodynamic equilibrium in OFFG, AFFG, OFFGWS, and AFFGWS. AFFGWS, were introduced into the inner and outer quartz tubes. The gas compositions of the simulated flue gas systems are shown in Table 2. For the injected gases, aside from the main composition of OFFG and AFFG, CO2, N2, and O2, the gas impurities of HCl at 1500 ppmv were introduced into the inner tube to vaporize the heavy metal loaded into porous TiO2. A concentration of 2000 ppmv of SO2 was dosed either individually or together with steam at 0−22.5 vol % into the outer tube to simulate the practical flue gas composition in oxy-fuel and air-fired coal combustions. The Pb and Zn vapors and flue gas flowed into the second stage at a flow rate of 1.4 L min−1. The resulting gas velocity and residence time were 100 mm s−1 and about 6 s, respectively. The condensation of Pb and Zn upon flue gas cooling was then observed and measured (see Figure 2). The reaction time was 8 h for each run. For the morphological observation of the crystallized particle of Pb and Zn species, a small crucible (5 mm high and 4 mm in diameter) was attached to the inside wall of the quartz filter, and its bottom, which was positioned parallel to the ground, was used to carry the crystallized particles of Pb and Zn species (see Figure 3c). The number of quartz filters for each experiment was limited. Thus, the experiments were divided into two groups to observe the
morphology of the crystallized particles of Pb and Zn species at 800, 700, 600, 500, 400, 300, 200, and 100 °C. Thus, the temperatures of one group that correspond to the three crucibles were 800, 600, and 400 °C, whereas the temperatures that correspond to the other group were 700, 500, and 300 °C. Heavy Metal Chemical and Textural Analyses. An atomic absorption spectrophotometer (AAS, Shimadzu AA-6200) was used to determine the deposition content of Pb and Zn on the surface of quartz tubes and filters in the three cooling zones (Figure 3a), namely, cooling I at 800−600 °C, cooling II at 600−300 °C, and cooling III at 300−100 °C. The deposits were scraped out of the tube (Figure 3b), and X-ray diffraction (XRD) patterns of the deposits were recorded on a Rigaku/RINT diffractometer operated at 35 kV and 35 mA using Cu Kα radiation to analyze the crystalline species of the deposits on the surface of these quartz tubes in different cooling zones. The crystal phases were identified by comparing the diffraction lines to those of the powder diffraction files. The species distribution of the condensation of Pb and Zn deposited on the surface of the quartz tube in three cooling zones (I, II, and III) was quantified using ion chromatography (IC, Shimadzu) with a CDD-6A conductivity 5643
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Figure 5. Absolute deposition content of condensed Pb and Zn vapors in OFFG, AFFG, OFFGWS, and AFFGWS.
simulate the flue gas cooling process in a real combustion system. Figure 5 shows the absolute deposition content of Pb and Zn present in the simulated flue gas on the quartz tubes located in the three cooling zones, namely, cooling I zone at 800−600 °C, cooling II zone at 600−300 °C, and cooling III zone at 300− 100 °C, as well as the three quartz filters located at the three cooling points, namely, filter I at 800 °C, filter II at 600 °C, and filter III at 300 °C in OFFG and AFFG. This figure shows that, in most of the cooling zones and/or points, the absolute deposition content of Pb and Zn present in the AFFG was consistently higher than that in OFFG. The absolute deposition contents of Pb and Zn in the three quartz tubes and filters were determined in OFFGWS and AFFGWS to eliminate the influence of steam. The absolute deposition content of Pb and Zn in the three cooling zones and cooling points in AFFGWS was likewise consistently higher than that in OFFGWS. Moreover, for similar flue gas systems, such as OFFG and OFFGWS, steam can dramatically decrease the absolute deposition content of the condensed Pb and Zn vapors, which could also be visually observed from the experimental pictures of the deposition of Pb and Zn on the quartz tubes under different flue gas conditions, as shown in Figure 2. These phenomena indicate that concentrated CO2 and steam could lessen the shift of deposition of the condensed Pb and Zn vapors. One of the possible reasons is the difference between the heat capacity (Cp) of N2, CO2, and steam, which results in a significantly higher Cp value in OFFG and OFFGWS at these three cooling zones than that in AFFG and AFFGWS (see Table S1 of the Supporting Information).26 At the same cooling zones and cooling points, the flue gas with higher Cp will release large quantities of heat into the atmosphere and then possible lead to a higher surface temperature of the deposition and a
detector. The species distribution of the deposits on the surface of the quartz tube located in the cooling II zone was analyzed by dividing it into several ranges. Deposits on the crucible surface were analyzed using scanning electron microscopy (SEM, JEOL JSM-5600) coupled with energy-dispersive X-ray spectroscopy (EDX). The diameter distributions of the particles were measured using the National Institute of Health (NIH) Image program (freeware from NIH, http://rsb.info.nih.gov/nih-image/index.html). Thermodynamic Modeling. The physical properties, including heat capacity, thermal conductivity, and viscosity of pure CO2, N2, and steam, as well as OFFG, AFFG, OFFGWS, and AFFGWS were calculated using the FactSage software package at the different temperature ranges. 24 The polynomial model (Kohler/Toop) interpolation method is chosen as the solution model in the thermodynamic modeling because it is often used in ternary and high-order systems. The concrete values of the thermal properties of Pb and Zn vapors in different flue gas systems are supplied in Table S1 of the Supporting Information. The equilibrium composition of Pb and Zn was evaluated under OFFG, AFFG, OFFGWS, and AFFGWS at a pressure of 1 atm at different temperatures and is presented in Figure 4.
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RESULTS AND DISCUSSION
Deposition of the Condensed Pb and Zn Vapors. Oxyfuel coal combustion in utility power stations occurs in furnaces that operate at temperatures above 1400 °C. The flue gas then flows into the economizer section in the boiler, and its temperature decreases from the 700−800 °C range to the 300− 400 °C range.25 As the flue gas leaves the combustion chamber and begins to cool, the volatilized heavy metal begins to condense. To investigate the condensation behavior of Pb and Zn vapors during the flue gas cooling process, a multi-stage cooling system was installed downstream of the quartz reactor and had a large temperature gradient of 800−100 °C at three steps, with an average cooling rate of about 125 °C/s, to 5644
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Figure 6. Relative deposition content of condensed Pb and Zn vapors in OFFG, AFFG, OFFGWS, and AFFGWS.
Figure 7. XRD patterns of the deposition of the condensed (left) Pb and (right) Zn vapors on the surface of the quartz tube located in cooling I at 800−600 °C and cooling II at 600−300 °C, respectively, in OFFG, AFFG, OFFGWS, and AFFGWS.
slow condensation rate. This condition finally causes a decrease in the deposition content of Pb and Zn in OFFG and
OFFGWS. For example, Kiga27 reported that, during oxy-fuel combustion, a higher Cp of CO2 decreased the surface 5645
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Figure 8. Distribution of Pb and Zn species at different temperature ranges in OFFG, AFFG, OFFGWS, and AFFGWS.
However, the presence and increase of steam in the flue gas (e.g., steam of 22.5 vol % in OFFG and 12.5 vol % in AFFG, respectively) increased the relative condensation contents of Pb and Zn in cooling I zone and dramatically decreased the relative condensation contents in cooling II zone. These results could be partially due to the dew points of Pb and Zn vapors, which improved as the steam content increased, as discussed later in the paper, which promoted the nucleation and deposition of the condensed Pb and Zn vapors under AFFGWS and OFFGWS in cooling I zone and caused a relative decrease of the deposition content of these vapors in cooling II zone. The concrete dew points of Pb and Zn vapors in various simulated flue gases are shown in Figure 4. Pb and Zn Speciation Distribution. The influence of the impurity gas of SO2, HCl, and steam in the flue gas on the crystalline phases of deposition of the condensed Pb and Zn vapors and its relative content in the different flue gas systems were defined by detecting the deposition on the surface of quartz tubes located in cooling I zone at 800−600 °C and cooling II zone at 600−300 °C using XRD. The corresponding XRD patterns are illustrated in Figure 7. Condensed Pb and Zn were entirely deposited in the form of sulfates (PbSO4 and ZnSO4) in cooling I zone, whereas a mixture of sulfates and chlorides (PbCl2 and ZnCl2) of Pb and Zn were observed in cooling II zone. The relative sulfate and chloride contents of Pb and Zn deposited in the three cooling zones in OFFG, AFFG, OFFGWS, and AFFGWS were likewise measured using IC and are presented in Figure 8. Similar to the results analyzed using XRD, for these four simulated flue gas systems, the deposition of the condensed Pb and Zn vapors consisted entirely of sulfates in cooling I zone. This condition indicates the formation of part of Pb and Zn sulfates during the predominant phase, in which metallic chloride vapors that reacted with SO2 were condensed and deposited in cooling I zone as a result of the higher dew point of Pb and Zn sulfates compared to the corresponding chlorides.31 As flue gas flowed into cooling II zone, part of the metallic chloride vapors continued to react with SO2 and formed metallic sulfates, with the residual metallic chloride vapors possibly supersaturating and diffusing down quickly to form aerosols deposited on the surface of the quartz
temperature of the burning coal particles by approximately 400−500 °C compared to that in N2. The relative deposition content of Pb and Zn on the quartz tubes located in the three cooling zones as well as the three cooling points in OFFG, AFFG, OFFGWS, and AFFGWS is shown in Figure 6 to clarify the influence of the temperature on the condensation of heavy metal vapors. As illustrated in Figure 6, thermodynamic calculation (Figure 4) shows that, although the dew point of Pb and Zn vapors in these four simulated flue gas systems is above 650 °C, a large fraction of Pb and Zn vapors and their respective particles that formed in the simulated flue gas were mainly deposited on the quartz tube located in cooling I zone at 600−300 °C, instead of in cooling II zone at 800−600 °C, and in filter III at 300 °C, instead in filter II at 600 °C. This result could be due to the rapid flue gas cooling rate and short residence time of heavy metal vapors in the cooling section. The cooling rate is regarded as one of the main factors that determine nucleation and formation of particles of Pb and Zn during gas−solid conversion upon flue gas cooling, which causes the supercooling of the condensation of Pb and Zn vapors to move down through the high-temperature cooling zone.28 The influence of supercooling of flue gas during the cooling process29 could slow the nucleation rate of Pb and Zn in cooling I zone at 800−600 °C, then increase the partial pressure of Pb and Zn vapors in cooling II zone at 600−300 °C, which is the driving force for its initial nucleation and homogeneous and heterogeneous condensation,30 and finally, delay the condensation of volatile Pb and Zn species in cooling II zone. Moreover, panels c and d of Figure 6 show that the relative deposition content of Pb and Zn on the quartz filter at 300 °C (filter III) reached the maximum value, which could indirectly prove that condensation of Pb and Zn vapors was concentrated mainly in cooling II zone. Hence, the condensation of Pb and Zn vapors in the cooling flue gas process of full-scale power plants should be concentrated mainly in the temperature range of 600−300 °C. Experimental data in panels a and b of Figure 6 likewise indicate that, for the steam-free flue gas system, namely, OFFGWS and AFFGWS, the relative deposition content of Pb and Zn at three cooling zones has no significant difference. 5646
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Figure 9. Evolution of Pb and Zn sulfate and chloride distribution in cooling II zone at 600−300 °C in OFFG, AFFG, OFFGWS, and AFFGWS.
tube in cooling II zone. This reaction may be due to the coexistence of Pb and Zn sulfates and chlorides in the cooling zone. Finally, when the temperature of the flue gas was decreased to lower than 300 °C, the deposition consisted completely of Pb and Zn chlorides measured using IC. This reaction could be due to the fact that the lower temperature may promote the formation of nucleation of chloride vapors of Pb and Zn and eliminate the chemical reaction in the formation of Pb and Zn sulfates.32 Figure 8 also shows that gas composition in the various simulated flue gas systems had no significant influence on the species and their content in cooling I and cooling III zones. However, for cooling II zone, the relative content of Pb and Zn species (the mole ratio of Pb and Zn sulfates and chlorides) in the four simulated flue gas systems had obvious differences. Therefore, a change in flue gas composition will obviously influence the species of heavy metal (Pb and Zn) and their content in the cooling flue gas process of full-scale power plants. The continuous change trend of Pb and Zn sulfate and chloride distribution of the deposition in cooling II zone was measured by means of IC in OFFG, AFFG, OFFGWS, and AFFGWS, from 540 to 300 °C with a difference of 10 °C, to further clarify the effect of gas composition on the relative content of their species (see Figure 9). In all of the simulated flue gas systems, the proportion of Pb and Zn sulfates gradually decreased, whereas that of the Pb and Zn chlorides gradually increased as the temperature decreased because of the influence of control factors, such as supercooling and chemical reactions. Comparisons between the sulfate and chloride distributions of Pb and Zn in OFFG and OFFGWS to those in AFFG and AFFGWS in cooling II zone at 600−300 °C (Figure 9) showed that the relative Pb and Zn sulfate contents in OFFG and OFFGWS were consistently higher than the chloride contents at each small temperature range. However, the reverse was found in AFFG and AFFGWS, with the relative Pb and Zn chloride contents being higher than those of the corresponding sulfates, which could be ascribed to the relatively higher CO2
concentrations in OFFG and OFFGWS compared to those in AFFG and AFFGW (see Table 2). Generally, concentrated SO2 and O2 in the combustion zone can promote SO3 formation through the following reactions:33 SO2 (g) + 1/2O2 (g) → SO3(g)
(1)
SO2 (g) + OH(g) → HOSO2 (g)
(2)
HOSO2 (g) + O2 (g) → SO3(g) + HO2 (g)
(3)
In our experiment, increased CO2 could enhance OH− radical formation following the below reaction that subsequently facilitated SO3 formation through reactions 1 and 2:34 CO2 (g) + H+(g) → CO(g) + OH−(g)
(4)
The PbCl2- and ZnCl2-bearing vapors were then sulfated by reacting with SO3 following the below reaction:35 (Pb, Zn)Cl 2(g) + SO3(g) + 1/2O2 (g) + 2H+ → (Pb, Zn)SO4 (s) + 2HCl(g)
(5)
where the increase in SO3 concentrations increased the reaction rate of reaction 5 and, thus, facilitated the conversion of metallic chlorides into metallic sulfates. Moreover, for these simulated flue gas systems, Figure 9 shows that the relative Pb and Zn sulfate contents in OFFG and AFFG were consistently higher than those in OFFGWS and AFFGWS. In addition, the higher sulfate content can be attributed to the steam in the flue gas that promoted SO3 formation through reactions 2 and 3 and cause an increase in the sulfate content. In cooling II zone, the H2O-derived OH− radical could combine with SO2 to form thermally stable intermediate HOSO2, which subsequently reacted with chlorides to form sulfates.36 Particle Characteristics of the Condensed Pb and Zn Vapors. The particle characteristics of heavy metals, including particle size and morphology,37,38 are important microphysical traits that facilitate the analysis of particle aerodynamic 5647
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Figure 10. SEM images of particles formed by the condensed Pb vapors deposited on the surface of the crucible at eight temperature points in OFFG, AFFG, OFFGWS, and AFFGWS.
behavior, source identification, and possible health effects,39 such as epidemiology and toxicology. Figures 10 and11 show SEM images of particles formed by the condensed Pb and Zn vapors deposited on the surface of the crucible at eight temperature points in OFFG, AFFG, OFFGWS, and AFFGWS. The average diameter and shape factor of particles formed by the condensed Pb and Zn vapors at different temperatures in the simulated flue gas systems are summarized in Figure 12. The corresponding chemical species of these particles and their
crystal size distributions are illustrated in Figure 13 and Figure S1 of the Supporting Information, respectively. In coal combustion flue gas, more refractory elements will homogeneously nucleate at much higher temperatures than Pb and Zn (Si, Ca, and Fe). Thus, by the time the flue gas cools to the dew points of Pb and Zn compounds in a coal combustion system, there is already considerable surface area in the submicrometer aerosol. Aerosol formation during the combustion process takes place because some elements form volatile compounds, such as heavy metals (Zn and Pb). Then, these 5648
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Figure 11. SEM images of particles formed by the condensed Zn vapors deposited on the surface of the crucible at eight temperature points in OFFG, AFFG, OFFGWS, and AFFGWS.
demonstrated in Figure 12, in the hot zone in our experiment (i.e., the furnace heated to 1000 °C), volatilized Pb and Zn were thermodynamically expected to be Pb and Zn chlorides in gaseous form. Condensation of volatilized Pb and Zn occurs as the Pb and Zn vapors leave the furnace and began to cool. At 800−600 °C, the supersaturated chloride vapors of Pb and Zn first reacted with SO2 and then formed Pb and Zn sulfates as sub-micrometer particulate matter with a diameter less than 1.0 μm (PM1), which were created from vaporized compounds though the gas−particle conversion as a result of chemical
compounds may react and further become supersaturated because of a lowered vapor pressure of the new compound formed or simply because of the cooling of the flue gas. Once particles have formed, they collide with each other and may adhere or coalesce, which leads to a decrease of particles in the flue gas but an increase of the particle diameters.40 As Pb and Zn vapors leave the furnace and begin to cool, volatilized trace elements will condense. Particles can form through the nucleation of vaporized material and, subsequently, grow through coagulation and heterogeneous condensation. As 5649
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Figure 12. Average particle diameter and corresponding shape factors of the condensed (a−c) Pb and (d−f) Zn vapors in OFFG, AFFG, OFFGWS, and AFFGWS.
reaction and cooling. As the temperature of the flue gas, which contains Pb and Zn species particles formed by nucleation, continued to decrease to 500 °C, the crystal particle grew further from a combination of coagulation and surface deposition and became fine particulate matter, which is referred to as PM2.5 and has a diameter of less than 2.5 μm. Finally, when the temperature decreased from 400 to 300 °C, the supersaturated chloride vapors of Pb and Zn began to form nuclei, condensed on the surface of the existing nuclei, and finally, became particulate matter that was smaller than 10 μm and is termed PM10. Therefore, in the flue gas cooling process of the economizer in oxy-fuel and air-fired coal utility boilers, particles that were formed at the lower temperatures are more easily retained by electrostatic precipitators and fabric filters. Consequently, this prevents their emission to the atmosphere, in contrast with particles that were formed at higher temperatures. At 800−500 °C, the average diameter of the sulfate particles of Pb and Zn in AFFG and AFFGWS was higher than that in OFFG and OFFGWS. This result may be due to the higher thermal conductivity value in AFFG and AFFGWS than in OFFG and OFFGWS (see Table S1 of the Supporting Information), as well as the greater sensitivity of growth and deposition rates to temperature changes in AFFG and AFFGWS compared to those in OFFG and OFFGWS. This sensitivity could have resulted in the growth of particles from a combination of coagulation and surface deposition. Moreover, further agglomeration in AFFG and AFFGWS occurred more quickly than that in OFFG and OFFGWS. However, when the temperature decreased to 400 °C, the species of crystal particles changed from Pb and Zn sulfates into chlorides, whose EDX spectra are illustrated in Figure 13, which indicates that the chloride particles were produced by initial nucleation and
subsequent coagulation and that the average diameter of the Pb and Zn chloride particles in OFFG and OFFGWS was higher than that of the Pb and Zn chloride particles in AFFG and AFFGWS. These differences could be explained in part according to the nucleation theory,41 which indicates that the differences could be due to the partial pressure of chloride vapors of Pb and Zn and the higher concentration of these vapors in OFFG and OFFGWS than in AFFG and AFFGWS. For the morphology of the particles formed in the condensed Pb and Zn vapors, as seen in panels b and c and panels e and f of Figure 12, the shape factor is used to analyze the properties of particle shapes, which is a dimensionless quantity used in image analysis and microscopy that numerically describes the shape of a particle, independent of its size. In our experiment, the shape factor was defined as the ratio of the equivalent spherical diameter to the average screen size, as measured by the software Image J. The shape factor for perfect spheres will be equal to 1; however, for most bodies, it will be less than 1. Panels b and c and panels e and f of Figure 12 show that the shape factor of the particles of the condensed Pb and Zn vapors in AFFG and AFFGWS was consistently higher than that of OFFG and OFFGWS, which indicates that the particle shapes formed in AFFG and AFFGWS were close to spheres, unlike those formed in OFFG and OFFGWS. The final particle morphology at the exit of different temperatures is the result of multiple processes, such as nucleation and subsequent growth by coagulation. According to the coagulation theory,21 the consistently higher viscosity of OFFG and OFFGWS, which contain Pb and Zn vapors, compared to that of AFFG and AFFGWS (see Table S1 of the Supporting Information) could be part of the reason, because the theory assumes that particles coalesce on collision and that the flue gas has lower viscosity and, thus, a shorter coalescence period. Thus, the shape factor 5650
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Figure 13. EDX spectra indicated by the red cross shown in the SEM images of (a) Pb in Figure 10 and (b) Zn in Figure 11 in OFFG, AFFG, OFFGWS, and AFFGWS.
OFFG in the cooling zone from 800 to 100 °C. Moreover, for similar flue gas systems, such as OFFG and OFFGWS, steam can dramatically decrease the absolute deposition content of the condensed Pb and Zn vapors, which indicates that concentrated CO2 and steam could lessen the shift of deposition of the condensed Pb and Zn vapors. (2) The condensed Pb and Zn vapors were entirely deposited in the form of sulfates (PbSO4 and ZnSO4) in cooling I zone at 800− 600 °C, whereas a mixture of sulfates and chlorides (PbCl2 and ZnCl2) of Pb and Zn were observed in cooling II zone at 600− 300 °C. Particularly, the condensed Pb and Zn vapors were mostly deposited in the sulfates in OFFG at 600−300 °C, instead of in the chlorides in AFFG. (3) The average diameter
will be higher, given that the coalescence time is shorter than the time between particle collisions and the time during which the particles coalesce and maintain their sphericity.
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CONCLUSION A lab-scale rotary quartz reactor with a multi-stage cooling zone was used to analyze the deposition content and species distribution of the condensed Pb and Zn vapors at different temperature ranges and/or points and to observe their particle characteristics in the simulated OFFG, AFFG, OFFGWS, and AFFGWS. The main conclusions obtained were summarized as follows: (1) The deposition content of the condensed Pb and Zn vapors in the AFFG was consistently higher than that of 5651
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of particles that contain Pb and Zn increased as the temperature decreased, and the shape factor of the particles of the condensed Pb and Zn vapors in AFFG and AFFGWS was consistently higher than that of OFFG and OFFGWS, which indicates that the particle shapes formed in AFFG and AFFGWS were close to spheres, unlike those formed in OFFG and OFFGWS.
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ASSOCIATED CONTENT
S Supporting Information *
Comparison between the selected physical properties of CO2, N2, H2O, and four kinds of simulated flue gas systems at 1 atm (Table S1) and crystal size distribution of the particles formed by the condensed Pb and Zn vapors shown in SEM images of (a) Pb in Figure 10 and (b) Zn in Figure 11 in OFFG, AFFG, OFFGWS, and AFFGWS (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +49-0-89-2180-4293 (W.S.); 0568-51-9178 (Y.N.). Fax: +49-0-89-2180-4176 (W.S.); 0568-51-1499 (Y.N.). E-mail:
[email protected] (W.S.);
[email protected] (Y.N.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Wenjia Song acknowledges the support of the Great Nagoya Overseas Scholars Fellowship Program. REFERENCES
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