Emission and migration characteristics of mercury in a 0.3

Keywords: Mercury; Emission and migration; Pilot-scale CFB; NH4Br modified rice husk char;. 28 injection demecuration. 29. Page 1 of 20. ACS Paragon P...
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Emission and Migration Characteristics of Mercury in a 0.3 MWth CFB Boiler with Ammonium Bromide-Modified Rice Husk Char Injection into Flue Zhengkang Luo,† Yufeng Duan,*,† Tianfang Huang,† Shuai Liu,† Yaji Huang,† Lu Dong,† Shaojun Ren,† Jun Tao,‡ and Xiaobing Gu‡

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Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China ‡ Datang Environment Industry Group Co., Ltd., Beijing 100097, China ABSTRACT: The emission and migration characteristics of flue gas mercury in a pilot-scale 0.3 MWth circulating fluidized bed combustion boiler system by injecting the adsorbent of ammonium bromide (NH4Br)-modified rice husk char (NBr-RHC) into the flue duct before a fabric filter (FF) were investigated in this study. Mercury concentration in the flue gas was sampled by the Ontario Hydro Method. Meanwhile, the feeding coal, lime, process water, fly ashes both in the flue gas and FF hopper, bottom ash, gypsum, and wet flue gas desulfurization (WFGD) effluent were also collected during the flue gas mercury sampling. The temperature-programmed desorption (TPD) method was used to identify the mercury species in the sample of fly ashes. The results show that the combination system of the selective catalytic reduction (SCR) + adsorbent injection (AI) + FF + WFGD obtains a total flue gas mercury removal efficiency of 94.62%, which is 13.42% higher than that without the AI of NBr-RHC. A total of 93.83% of mercury exists in byproducts of the solid and liquid, and the amount of mercury collected by FF accounts for 97.28% due to the physisorption and chemisorption of gaseous mercury by NBr-RHC injection. The TPD analysis for mercury compound in the particulate matters from the AI device indicates that HgBr2 formation is the key reason for NBr-RHC having high mercury removal efficiency. Elemental mercury is found to oxidize across the SCR, with 70.23% converted to oxidized mercury. WFGD exhibits a good oxidized mercury absorption rate of 86.67%, but elemental mercury increases from 0.19 to 0.26 μg/m3 due to the oxidized mercury reduction, and more attention should be given on this increase. species determine their fate in the flue gas, resulting in migration, transformation, capture, and emission. Hg0 is particularly difficult to be removed by conventional air pollution control devices (APCDs) because of its high volatility, poor solubility, and chemical inertness.10 Compared to Hg0, Hg2+ and Hgp are more reactive and can be easily captured by an electrostatic precipitator (ESP) or fabric filter (FF) or wet scrubber like wet flue gas desulfurization (WFGD).11,12 The conventional APCDs like selective catalytic reduction (SCR), ESP or FF, and WFGD have been installed widely in power plants for NOx, particulates, and SOx control. Meanwhile, the APCDs can also synergistically remove mercury from the flue gas. Wang et al.1 conducted the characterization of mercury emission and their behavior in six typical coal-fired power plants in China and found that the average mercury removal efficiencies of ESP, ESP + wet FGD, and ESP + dry FGD + FF systems were 24%, 73%, and 66%, respectively. Tao et al.13 reported that the power plant with SCR, ESP, FF, and WFGD could obtain a mercury removal efficiency of 87.6%. Yokoyama et al.14 measured the mercury emissions at a 700 MW coal-fired power plant with a low-NOx burner + SCR + ESP + WFGD combination in Japan and

1. INTRODUCTION Mercury, a highly volatile and highly toxic trace element, with long-distance migration and long-term accumulation, can cause great damage to human beings and ecological environments.1−3 Over the past few decades, the risk of human exposure to this highly toxic substance has been significantly increased due to the massive popularization of industrial plants. Mercury has been regarded as a global pollutant by the United Nations Environment Programme (UNEP).4 At present, global anthropogenic emissions of mercury are mainly fossil fuel production and combustion, nonferrous metal smelting, cement production, etc.5,6 Coal combustion has been considered to be the largest source of anthropogenic mercury emission, accounting for 23% of the total mercury emissions into the atmosphere.7 On account of the Chinese huge coal combustion both in power plants and thermal plants, the total amount of mercury emitted into the environment contributes about 25%−40% of global mercury emission every year.8 Mercury in coal-fired flue gas has three forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulatebound mercury (Hgp).9 The combustion process releases Hg0 from the feeding coal into the exhaust gas at a high temperature. As the flue gas temperature decreases, Hg0 may be oxidized by acid gases like HCl and SO2 to form Hg2+. Hg2+ has a tendency to combine with particles to form Hgp. The distinguished physical and chemical properties of mercury © XXXX American Chemical Society

Received: May 6, 2019 Revised: June 12, 2019 Published: July 8, 2019 A

DOI: 10.1021/acs.energyfuels.9b01440 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. . Schematic diagram of a 0.3 MWth pilot-scale CFBCB system and the sampling point.

RHC AI system were also studied. Moreover, the mercury removal rates of each APCDs and the NBr-RHC AI system were discussed. The results will have a better understanding of mercury emission and migration characteristics for the CFBCB system with APCDs and AI, providing a significant reference for industrial applications of the NBr-RHC injection for mercury removal.

found that the relative distribution of mercury in ESP, WFGD, and stack ranged from 8.3% to 55.2%, 13.3% to 69.2%, and 12.2% to 44.4%, respectively. However, a few researches reported about the mercury species migration and transformation characteristics across the whole APCD. Also, the synergistic removal of mercury by conventional APCDs leads to a drawback where it causes difficulty for mercury centralized recycling and processing because of mercury uncertain distribution.15 Adsorbent injection (AI) technology has been shown to be a cost-effective, reliable operation to remove mercury effectively in power plants.16 For the purpose of largescale industrial applications, the development of AI technique has been focused on the two aspects: (i) regenerable adsorbent with low cost for cycle utilization of the adsorbent and (ii) efficient modification methods for adsorbents with low price and regenerable resources. Rice husk char (RHC) has abundant porous structures and highly diverse functional groups that lead to a high capacity for mercury physisorption and chemisorption. Meanwhile, the halogen element can form Cl-contained and Br-contained functional groups on the surface of the adsorbent that is beneficial to bind mercury and forms a stable mercury compound. The adsorption of mercury is significantly increased by halogen-modified chars.17−21 Former studies have shown that ammonium bromide (NH4Br)-modified rice husk char (NBr-RHC) possesses an excellent mercury removal performance in a fixed bed system and entrained flow reactor.15 However, the results of the adsorbent injection into the flue gas by the NBrRHC in the pilot-scale coal-fired combustion system equipped with the complete set of APCDs have not been reported, so is the dynamic mercury removal performance. In this study, the field sampling test about mercury emission was conducted in a pilot-scale 0.3 MWth circulating fluidized bed combustion boiler (CFBCB) equipped with SCR, FF, WFGD, and AI by use of the NBr-RHC adsorbent. The field sampling of mercury in the flue gas was achieved by the Ontario Hydro Method (OHM), which was considered as an internationally recognized standard method for vapor phase mercury speciation measurement and widely used to quantify the mercury concentration in coal-fired flue gas.22 The mercury mass balance and distribution in this pilot-scale CFBCB system as well as mercury migration across each APCDs and the NBr-

2. EXPERIMENTAL METHODOLOGY 2.1. Adsorbent Preparation. In this experiment, RHC was selected as the raw material, and chemical impregnation method was used to modify it for mercury removal. The detail process is shown as follows: (i) crush and screen the granular RHC into a particle diameter range of 200 to 400 mesh, designated as RHC; (ii) modify the weighed RHC by 1% NH4Br solution and mix uniformly. The mixture was stirred at a constant speed for 6 h using a magnetic stirrer and then immersed for 6 h; (iii) filter and dry at 60 °C for 8 h after the completion of impregnation to obtain NBr-RHC. In order to improve the minor feeding rate accuracy of the micro screw feeder for NBr-RHC feeding into the AI system, it is mixed with inertial quartz sand of the same particle size at a mass ratio of 1:9. 2.2. 0.3 MWth Pilot-Scale CFBCB System. The schematic diagram of a 0.3 MWth pilot-scale CFBCB test facility is shown in Figure 1. This comprises a CFB coal combustion system, coal and desulfurizer feeding systems, flue gas pollutant removal system of SCR + AI + FF + WFGD, and sampling and data acquisition system. The AI system is installed between the SCR and FF. The flue gas from the exit of the furnace enters the first-stage water-cooling heat exchanger, where the flue gas is cooled to the proper temperature for a working 141 SCR catalyst. After the SCR system, a secondary water-cooling heat exchanger is followed for cooling the flue gas again in order to reach a suitable temperature for the AI system. The AI system includes an adsorbent hopper, an adsorbent microspiral powder feeder with a feeding rate of 0.3−0.7 kg/h, and an adsorbent spraying device aimed at spreading the adsorbent into the flue gas evenly. The AI temperature and residence time are controlled by adjusting the two-stage water-cooling heat exchangers and the proper coal feeding rate. The used NBr-RHC adsorbent is captured by the FF and WFGD followed by SOx removal. 2.3. Sampling and Analysis. The flue gas mercury sampling sites are shown in Figure 1. During the stable operation of a 0.3 MWth pilot-scale CFBCB system, flue gas mercury sampling and collection of the feeding coal, lime, process water, fly ashes both in the flue gas and FF hopper, bottom ash, gypsum, and WFGD effluent were B

DOI: 10.1021/acs.energyfuels.9b01440 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 2. Schematic of the OHM Hg sampling line. conducted simultaneously. The OHM was used for sampling the mercury in the flue gas. The sampling gas was sucked isokinetically from the flue gas via a heated probe maintaining a constant temperature of 120 °C in the case of mercury condensation. Hgp could be captured by the quartz fiber filter. Gaseous mercury speciation was partitioned in eight impingers in an ice bath. Hg2+ was absorbed by the first three impingers with 1.0 N potassium chloride (KCl) solution, and Hg0 was oxidized by both the fourth impinger with 5% (v/v) nitric acid (HNO3) + 10% (v/v) peroxide (H2O2) solution and the next three impingers with a solution of 4% (w/v) potassium permanganate (KMnO4) + 10% (v/v) sulfuric acid (H2SO4). The last impinger filled with a certain amount of silica gel was used for drying the sampling gas. The whole sampling process lasted for 2 h and ensured that the amount of flue gas was not less than 2 Nm3. The schematic of the OHM mercury sampling line is shown in Figure 2.23 The mercury content in the liquid samples, such as the dissolved solution in impingers, process water, and WFGD effluent, was detected by the Russia Lumex mercury analyzer RA-915M equipped with RP-92 based on the principle of the cold vapor atomic absorption spectrometry (CVAAS) complying with the EPA 1631, EN 1483, and EN 13806 method (detection limit = 1 ppt (1 ng/L)). Before mercury detection, the solution in the impingers should be recovered and digested according to the ASTM D6784-02 standard. The mercury content in the solid samples, such as coal, bottom ash, fly ashes both in the flue gas and FF hopper, and gypsum from WFGD, were detected by the Russia Lumex mercury analyzer RA915M equipped with PYRO-915+ based on the principle of the CVAAS complying with the EPA 7473 method (detection limit = 1 ng/g). The proximate and ultimate analysis of coal was analyzed according to the national standard of China GB/T212-2008 for the proximate of coal, GB/T19227-2008 for the determination of nitrogen in coal, GB/T214-2007 for the determination of total sulfur in coal, GB/T476-2008 for the determination of carbon and hydrogen in coal, GB/T3558-2014 for the determination of chlorine in coal, and GB/T213-2008 for the determination of the calorific value of coal. In order to reduce the impact from experimental error and uncertainties, parallel samples were set during sampling in the constant boiler operational conditions. Meanwhile, the pilot-scale CFBCB operational parameters were recorded during the flue gas mercury sampling. The oxygen concentration and temperature at the sampling sites are shown in Table 1. All the mercury concentrations in the flue gas were normalized to 6% O2 in order to better evaluate and compare the measurement data. 2.4. SEM and TPD Analysis. Scanning electron microscopy (SEM) analysis was used to characterize the physical structure of solid samples. A Hg temperature-programmed desorption (Hg-TPD) experiment was used for detecting mercury compounds on the adsorbent surface, which consisted of a nitrogen bottle, mass flow controller, temperature-programmed tube furnace, and continuous mercury analyzer, aiming at exploring the mercury species enrichment mechanism in solid samples. The sample was placed in a boat where

Table 1. Flue Gas O2 Concentration and the Temperature at the Sampling Location location O2 (%) temperature (°C)

inlet of SCR

inlet of AI

outlet of AI

outlet of FF

outlet of WFGD

9.00 363

9.50 204

10.00 182

9.62 149

8.24 51

the temperature-programmed furnace was heated from room temperature to 900 °C at a rate of 10 °C/min. The released Hg0 was conveyed by nitrogen gas with a flow rate of 1.5 L/min and then entered the VM-3000 mercury analyzer to read the concentration. Various mercury species were confirmed according to the definite desorption temperature for certain mercury compounds discussed by the literature.24 The schematic diagram of the Hg-TPD device is shown in Figure 3.23

3. RESULTS AND DISCUSSION 3.1. Coal Properties. Table 2 shows the proximate and ultimate analysis of the tested coal from Guizhou, China, with a particle size of 0−6 mm. Based on the National Coal Classification Standard of China (GB/T 5751-2009), the tested coal belongs to bituminous coal. Coal properties, especially the content of chlorine (Cl) and sulfur (S), exert a remarkable influence on mercury species in the combustion process and its migration and transformation downstream of the flue gas flowing process.13 The mercury content of this coal is 62.84 μg/kg, which is less than the average content in the coal from China of which the value is 190 μg/kg.25 The Cl content is 0.05% (500 mg/kg), close to the U.S. coal whose value is 614 mg/kg and higher than that in the Chinese coal.26 The S content belongs to the low-sulfur coal based on the range of 0.2%−8% of that in the Chinese coal.27 3.2. Mercury Mass Balance and Partitioning. The mercury mass balance of the whole system is calculated by the ratio of Hg output to Hg input, which is aimed at evaluating the accuracy of mercury sampling. Hg input is the mercury hourly income by the coal feeding into the CFB furnace and lime and process water entering the WFGD system. Hg output involves mercury in the bottom ash, fly ash, gypsum, WFGD effluent, and stack gas. Besides the mercury concentration of each substance mentioned above, the mass flow rate of all the input and output streams are also calculated for the mass balance of mercury, as formulated in the following equations: Hg input = Fc × Cc + Fl × C l + Fpw × Cpw C

(1)

DOI: 10.1021/acs.energyfuels.9b01440 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Schematic diagram of the Hg-TPD device.

Table 2. Proximate and Ultimate Analysis of the Feed Coal proximate analysis

LHV

ultimate analysis

Mar (%)

Aar (%)

Var (%)

FCar (%)

Qar,net (MJ/kg)

Car (%)

Har (%)

Oar (%)

Nar (%)

Sar (%)

Clar (%)

Hgar (ng/g)

4.27

25.73

27.75

42.83

21.26

56.03

3.63

8.06

0.83

0.96

0.05

62.84

Table 3. Hourly Amount of Hg Input and Output in the Pilot-Scale CFBCB System Hg input (mg/h)

Hg output (mg/h)

Out/in balance (%)

coal

lime

process water

fly ash

bottom ash

gypsum

WFGD effluent

stack gas

1.6880

0.0003

0.0134

1.4069

0.0021

0.0101

0.0264

0.0952

90.54

Hg output = Ffa × Cfa + Fba × C ba + Fg × Cg + FWFGDe × C WFGDe + Fsg × Csg

(2)

mass balance ratio = (Hg output)/(Hg input)

(3)

where Fc, Fl, and Fpw represent the feeding rate of coal, lime, and process water, respectively; Ffa, Fba, Fg, FWFGDe, and Fsg represent the flow rate of fly ash, bottom ash, gypsum, WFGD effluent, and stack gas, respectively; Cc, C1, and Cpw represent the mercury concentration in coal, lime, and process water, respectively; and Cfa, Cba, Cg, CWFGDe, and Csg represent the mercury concentration in fly ash, bottom ash, gypsum, WFGD effluent, and stack gas, respectively. The mercury mass balance measured in this system is tabulated in Table 3, displaying the hourly amount of Hg input and output. The mercury mass balance of the entire system is 90.54%. The source of error attributes to the fluctuation of operational parameters, such as the coal components, combustion products, variation of boiler load, the errors during sampling and sample analysis, and so on. This is acceptable for the mercury mass balance rate of 70%−130%.14 Figure 4 illustrates the partitioning of mercury in combustion products of the entire system in this filed test. Based on the data, mercury in fly ashes of the mixture composed of NBr-RHC and FF ash occupies the largest share of 91.32% among the entire Hg output. The dominant proportion is attributed to the adsorbent injection of NBrRHC, which demonstrates strong mercury adsorption capability and is then captured by the FF, indicating the adsorbent NBr-RHC being highly efficient for flue gas mercury removal. Moreover, the rest of the Hg output happens mainly in the stack gas (6.17%) followed by the WFGD effluent (1.71%), gypsum (0.66%) and bottom ash (0.14%). Except for the proportion of the bottom ash, the rest of mercury distributions are significantly different from former research results due to the existence of NBr-RHC injection.23 Generally,

Figure 4. Partitioning of mercury in combustion products of the entire system.

the mercury forms existed in the coal are organic-bound mercury, sulfide-bound mercury, and silicate-bound mercury. Among them, silicate-bound mercury occupies the smallest ratio of the whole mercury but owns the highest mercury released temperature (>700 °C). When the coal is burning in the CFBCB, the high combustion temperature forces the majority of mercury in coal to form Hg0, in which a little amount of mercury enters into the bottom ash. When the flue gas exits from the furnace, its temperature will be lower and lower as it passes through the heat exchangers, leading to Hg0 conversion to Hg2+ and Hgp by homogeneous and heterogeneous reactions with the flue gas components and the fly ash constituted in the flue gas.28−30 The WFGD system captures Hg2+ efficiently due to the Hg2+ water solubility. Compared with gypsum, the WFGD effluent takes the relatively large portion of mercury, which indicates that Hg2+ dissolved in the desulfurization slurry is bigger than that in gypsum. It is important to notice that most of the mercury is not released into the atmosphere; most of them have been enriched in the fly ashes and the WFGD effluent. So, it must pay high attention to the mercury re-emission from the coal-fired power plant byproducts from the FF and WFGD.31 D

DOI: 10.1021/acs.energyfuels.9b01440 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 4. Concentrations of Hg Speciation across APCDs and the AI System inlet of SCR

outlet of SCR

out of AI

out of FF

out of WFGD

Hg species

μg/m3

%

μg/m3

%

μg/m3

%

μg/m3

%

μg/m3

%

Hg0 Hg2+ Hgp Hgt

2.62 0.26 2.33 5.21

50.29 4.99 44.72

0.78 1.98 2.35 5.11

15.26 38.75 45.99

0.22 0.18 3.62 4.02

5.47 4.48 90.05

0.19 0.15 0.00 0.34

55.88 44.12 0.00

0.26 0.02 0.00 0.28

92.86 7.14 0.00

Figure 5. SEM image of the NBr-RHC.

3.3.2. Transformation of Mercury Species in the AI System. As shown in Table 4, the concentrations of gaseous mercury at the outlet of AI (Hg0: 0.22 μg/m3; Hg2+: 0.18 μg/ m3) are much lower than those at the outlet of SCR (Hg0: 0.78 μg/m3; Hg2+: 1.98 μg/m3). Also, the majority of mercury detected in the flue gas is in the form of Hgp, occupying 90.05% of Hgt. From the SCR outlet to the AI outlet, this big change in mercury species is mainly caused by two aspects. First, the flue gas temperature decreased from 360 to 190 °C due to heat transfer occurring in the secondary heat exchanger that was installed between SCR and AI, which favored the conversion of Hg0 to Hg2+ and Hgp with the interaction of some acid gas and particulate adsorption. Second, when the flue gas temperature decreased to about 180 °C, the NBr-RHC adsorbent was applied to inject into the flue gas to capture gaseous mercury at its optimum temperature with a higher adsorption efficiency in a residence time of about 1.5 s. Zhang et al.34 measured the mercury concentration of power plants equipped with low temperature economizers (LTE), concluding that from the outlet of the SCR system to the LTE outlet with the flue gas temperature drop of 130 °C, the gaseous mercury concentration decreased by 10%. Zhao et al.35 conducted the mercury emission at a 660 MW ultralow power plant, finding that the gaseous mercury concentration decreased only about 9% from the SCR outlet to the ESP outlet. Therefore, it can be deduced that, in this experiment, it is the adsorbent injection of NBr-RHC that plays an important role to capture flue gas mercury effectively in optimal

3.3. Mercury Migration and Transformation. The concentrations of mercury speciation across APCDs and the AI system are shown in Table 4. As the flue gas flows, apparent changes happened in the concentration of different forms of mercury. The mercury behaviors occurring across the APCDs and AI system are discussed as follows. 3.3.1. Mercury Behavior in SCR System. As illustrated in Table 4, Hgp occupies a proportion of Hgt as high as 44.72% at the inlet of SCR, which is higher than the most pulverized coalfired combustion process. This is because the high unburned carbon (UBC) content at about 14 wt.% in fly ash of the flue gas in this combustion process. The Hgt concentration at the inlet of SCR is 5.21 μg/m3, while that at the SCR outlet is 5.11 μg/m3, indicating that SCR shows no obvious impact on the total mercury removal. However, the Hg0 concentration decreases from 2.62 to 0.78 μg/m3 through the SCR, and the Hg2+ concentration increases from 0.26 to 1.98 μg/m3. Hgp shows almost no change. This verifies that the SCR catalyst plays an oxidized role on Hg0. Previous studies have shown that HCl is considered as the key ingredient in the flue gas to oxidize Hg0 under the action of the SCR catalyst. HCl is absorbed on the SCR catalyst surface to generate active sites and subsequently oxidizes Hg0 to form Hg2+.32,33 The strong water solubility of Hg2+ makes it easier to be removed by WFGD. Also, Hg2+ is prone to be absorbed by the fly ash, which explains the slight increase in Hgp concentration in the SCR system. Therefore, SCR benefits the mercury removal by oxidizing Hg0 to Hg2+. E

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Energy & Fuels conditions of temperature range, resident time, and injection style. In order to further understand the role of NBr-RHC on the transformation of mercury speciation, SEM analysis of NBrRHC and TPD analysis of the fly ash obtained from the AI inlet and outlet were carried out as shown in Figure 5. It can be seen that the NBr-RHC possesses good channels for gaseous mercury passing from the outside into the inner surface of the adsorbent particle. From Figure 6, it shows that trigonal red

Figure 7. Schematic diagram of mercury removed by NBr-RHC.

dust particles are intercepted by the inertial force and collided by the fiber. The removal rate of FF for mercury largely depends on the amount of mercury in the fly ash, which is related to the component of flue gas and inner properties of fly ash. Fly ash plays a significant role in Hg0 oxidation and gaseous mercury adsorption, which leads to the formation of Hgp. The NO role for Hg0 oxidation is half downcast and half pleased. NO is in contact with O2 to form NO2 and then NO2 oxidizes Hg0 to Hg2+, which is relatively easier to be adsorbed by fly ash, while NO also consumes Cl2 and reacts with the OH group to hinder the Hg0 oxidation.39 The homogeneous reaction between Hg0 and HCl usually follows the Eley−Rideal mechanism, HCl is first adsorbed onto the surface of the fly ash, and mercury in the gaseous phase reacts with adsorbed Cl to produce chlorides.40 SO2 competes with Hg0 to seize the active sites, and SO3 exhibits a negative influence on mercury capture, while the absorbed SO2 and SO3 can be the oxidation site on the fly ash surface. UBC in fly ash can capture gaseous mercury. As the temperature decreases, the amount of mercury captured increases. As the UBC content increases, the amount of mercury captured increases.41 As Table 5 shows, inorganic components of the fly ash appear in decreasing proportion order as SiO2 > Al2O3 > Fe2O3 > CaO > TiO2 > K2O > MgO, which are basically consistent with a previous study.42 SiO2 accounts for the largest share (35.86%) of the fly ash. Al2O3 can be used as a carrier for the active component of Hg0 oxidation, and Fe2O3 is demonstrated to be a catalyst for Hg0 oxidation; the proportion is 26.48% and 4.32%, respectively.43,44 CaO (4.11%) is often regarded as a potential adsorbent for Hg2+ adsorption.45 Thus, Hgp is collected effectively in the FF, and there is no Hgp existing in the flue gas at the outlet of FF. 3.3.4. Mercury Removal in WFGD. WFGD is used to remove SO2, where alkaline slurry, such as limestone, lime slurry, or sodium carbonate, is used to react with SO2. SO2 is absorbed by the slurry to form sulfurous acid (H2SO3), which is ionized to generate H+ and HSO3− later. HSO3− reacts with CaCO3 in the circulating fluid to form calcium sulfite hemihydrate (CaSO3·1/2H2O). Under the impact of O2 in the air, CaSO3·1/2H2O is converted into CaSO4 ultimately. Due to the solubility of Hg2+ in water, Hg2+ is dissolved in the scrubber solution and retained in WFGD gypsum. As shown in Table 4, the Hg2+ concentration displays a sharp decrease (0.15 to 0.02 μg/m3) from the FF outlet to the WFGD outlet. However, during this process, the concentration of Hg0 increases relatively obviously, varying from 0.19 to 0.26 μg/ m3, which is attributed to the Hg2+ reduction. The specific reduction process can be described as follows: a water layer emerges around the gypsum, and the reaction between Hg2+ − and Hg0 occurs to generate Hg2+ 2 , which then reacts with OH

Figure 6. TPD curve of the fly ash at the AI inlet and outlet.

HgS (peak at about 290 °C) and HgO (peaking at about 410 °C) are the dominant Hg species in fly ash at the AI inlet.24,36 The occurrence of trigonal red HgS is attributed to the reaction between Hg and sulfur remaining in the UBC in the fly ash. The appearance of HgO can explain that O2 in the flue gas contacts with fly ash to form oxygen-containing functional groups and reacts with gaseous mercury to form HgO.34 Sasmaz et al.37 used X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy to study the binding mechanism between Hg0 and brominated carbon, concluding that Hg0 is oxidized at the brominated carbon surfaces and coordinated to two Br atoms. Zhou et al.38 used TPD to analyze the mercury species of the reaction product between Hg0 and NH4Br-modified activated carbon (NH4Br-AC) in a fixed-bed reactor, finding that mercury adsorption on NH4Br-AC is majorly the chemisorption with the formation of HgBr2. Meanwhile, Figure 6 also shows that, except trigonal red HgS and HgO, HgBr2 (peaking about at 350 °C) is detected in fly ash at the AI outlet, which is consistent with the above mentioned discussion. It could be inferred that after RHC is chemically treated by NH4Br solution by impregnation method, a large number of Br groups are attached to the surface and pore structure of RHC. Hg2+ and Br groups are directly combined to form HgBr2. Hg0, free in the vicinity of NH4Br particles, is oxidized to Hg2+ by the Br group and combines with two Br atoms to form HgBr2, which is actually indirectly adsorbed on the NBr-RHC surface. The schematic diagram of mercury removal by NBr-RHC is shown in Figure 7. 3.3.3. Mercury Removal in FF. FF is a dust-removing device that collects solid particles in dust-contained gas by a bag filter made of fiber fabric. The working principle of FF is that the F

DOI: 10.1021/acs.energyfuels.9b01440 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 5. The UBC Content and Mineral Composition of Fly Ash SiO2

Al2O3

Fe2O3

CaO

TiO2

K2O

MgO

others

UBC

35.86%

26.48%

4.32%

4.11%

1.05%

1.04%

0.89%

26.25%

14%

Figure 8. The removal rate of mercury across the APCDs (a) with NBr-RHC injection and (b) without NBr-RHC injection.

in the slurry to form Hg0 and HgO, SO2 can reduce HgO to Hg0. The detailed reaction formulas are shown as follows:34 Hg 2 + + Hg 0 ↔ Hg 22 + Hg 22 +

−

+ 2OH ↔ H 2O + HgO + Hg

HgO + SO2 ↔ Hg 0 + SO3

above all factors. SCR holds the major effect on Hg0 oxidation efficiency defined as

(4) 0

Hg 0 oxidation rate =

(5)

Hgin0

SO2 could be dissolved in the scrubber solution to generate sulfite and/or sulfate and further reacts with Hg2+ to form mercury sulfite and mercury sulfate, which could be decomposed into Hg0. The detailed reaction formulas are shown as follows:34 (7)

Hg 2 + + SO24 − ↔ HgSO4 → Hg 0

(8)

Hg in 0

× 100% (9)

0

where and Hgout are the concentration at the SCR inlet and outlet, respectively. The Hg0, Hg2+, Hgp, and Hgt removal rates across the FF, WFGD, and the whole system (SCR + AI + FF + WFGD) are calculated as

(6)

Hg 2 + + SO32 − ↔ HgSO3 → Hg 0

Hg in 0 − Hgout 0

ηpz =

Hg zp − in − Hg zp − out Hg zp − in

× 100% (10)

where z represents the types of mercury (Hg0, Hg2+, Hgp, and Hgt) and p refers to the APCDs (FF, WFGD, and the whole system). In this test, the SCR system holds a relatively higher synergistic mercury removal efficiency, the Hg0 oxidation rate of which reaches about 70.23%. This may be related to the chlorine content of the flue gas. The NBr-RHC injection system assists the mercury removal by absorbing gaseous mercury to generate Hgp, the efficiency of which achieves 85.51% due to the chemical and physical adsorption by NBrRHC. Figure 8a shows the removal rate of the APCDs with NBr-RHC injection. FF has an extraordinary efficiency of 100% for Hgp, and gaseous mercury gains a little drop about 15% through the FF. WFGD exhibits a high Hg2+ adsorption efficiency because of water solubility, up to 86.67%, while it has adverse an influence on Hg0 removal, meaning 36.84% of Hg2+ is reduced to Hg0. Figure 8b shows the removal rate of the APCDs without NBr-RHC injection. This indicates that the Hg0 and Hg2+ removal rates of increment of 19.91% and 61.63% are achieved due to the addition of the NBr-RHC injecting system, which benefits the conversion of Hg0/Hg2+ into Hgp. The addition of NBr-RHC injection contributes to the total mercury removal efficiency increase around 13.42% (from 81.20% to 94.62%).

0

Also, the Hg re-emission is determined by various factors. The pH, temperature, supersaturated sulfite and halogen ions (Cl−, Br−), and certain metal ions (Ca2+, Mg2+) are the main causes of Hg2+ reduction. Within a certain range, as the pH increases, the rate of reduction decreases, while the increase in temperature leads to the increase in Hg0 re-emission. Also, the increased concentration of supersaturated sulfite inhibits the reduction process of Hg2+ reduction. Cl− reacts with HgSO3 to form a complex (ClHgSO3−) to prevent the decomposition of HgSO3. Ca2+ and Mg2+ promote the formation of H2SO3 through a series of reactions to promote the reduction of Hg2+.46 3.4. Mercury Removal Rate of the APCDs and AI System. Many factors exert impact on the total mercury removal efficiency in this test, such as the concentration of oxygen and halogen, flue gas residence time, the mercury removal rate across the FF and WFGD, and the injected adsorbent of NBr-RHC capacity to adsorb mercury. High mercury removal rate is the result from the co-benefits of the G

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4. CONCLUSIONS In this paper, injecting the newly developed adsorbent of ammonium bromide (NH4Br)-modified rice husk char (NBrRHC) into the coal-fired flue duct cooperated with SCR, FF, and WFGD devices of a pilot-scale 0.3 MWth circulating fluidized bed combustion boiler (CFBCB) system was investigated. The emission, migration, and transformation characteristics of flue gas mercury across the APCDs were obtained. The conclusions can be drawn as follows. Conventional APCDs with adsorbent injection exhibits a significant role in the emission, migration, and transformation of flue gas mercury species. Hg0 is oxidized considerably across the SCR. The adsorbent injected effectively captures gaseous mercury and converts it into Hgp, which is then efficiently removed by FF. WFGD possesses a superior effect on the Hg2+ removal, while the Hg0 re-emission occurs in this process; thus, the stabilization of mercury in WFGD needs further study. The SCR catalyst converts 70.23% of Hg0 into Hg2+, and Hg2+ reaches 71.74% in the gaseous mercury. Approximately 85.51% of gaseous mercury is adsorbed to form Hgp with the NBr-RHC injection due to the strong adsorption of NBr-RHC, on which HgBr2 is detected as the major mercury species by the TPD analysis. The FF owns an extraordinary Hgp capture efficiency. The combination of NBr-RHC injection with FF shows remarkable mercury retention in fly ash accounting for 91.32%. WFGD illustrates Hg2+ removal efficiency of 86.67%. This indicates that the Hg0 and Hg2+ removal rates of increment of 19.91% and 61.63% are achieved due to the addition of NBr-RHC injecting system. The addition of NBrRHC injection contributes to the total mercury removal efficiency increase around 13.42% (from 81.20% to 94.62%). The concentrations of Hg0 and Hg2+ in the stack emitted to atmosphere are 0.26 and 0.02 μg/m3, respectively.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86-25-83795652. ORCID

Yufeng Duan: 0000-0002-9015-2619 Tianfang Huang: 0000-0002-3532-4270 Yaji Huang: 0000-0002-0176-4358 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2016YFB0600203), the National Natural Science Foundation of China (51576044, 51876039), and the project budget from the Datang Environment Industry Group Co., Ltd.

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DOI: 10.1021/acs.energyfuels.9b01440 Energy Fuels XXXX, XXX, XXX−XXX