Article Cite This: Energy Fuels 2019, 33, 6670−6677
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Study of Mercury-Removal Performance of Mechanical−ChemicalBrominated Coal-Fired Fly Ash Xinze Geng, Yufeng Duan,* Shilin Zhao, Yifan Xu, Tianfang Huang, Jiwei Hu, and Shaojun Ren
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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 ABSTRACT: The mechanical−chemical-modified fly ash (FA-MC) and mechanical−chemical-brominated fly ash (FA-MCBr) were prepared by omnidirectional planetary ball mill, and impregnated−brominated fly ash (FA-I-Br) was also prepared using the same mass ratio of fly ash/NH4Br as a comparison. The mercury-removal efficiency of raw fly ash (FA), FA-MC, FAMC-Br, and FA-I-Br was evaluated in a fixed-bed reactor. The physical and chemical properties of the four samples were investigated by the Brunauer−Emmet−Teller, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and mercury temperature-programmed desorption analysis. The results showed that the mercury-removal efficiency of the four samples followed the order of FA-MC-Br > FA-I-Br > FA-MC > FA, the value of which was 67, 30.98, 26.12, and 17.96%, respectively. The mercury-removal performance of the four samples was mainly reflected in the oxidation (>90%), while the adsorption only accounted for a small proportion ( Al2O3 > CaO > Fe2O3 > TiO2 > SO3 > K2O. Alumina silicates are the main component, which are inert to the adsorption and oxidation of mercury,14 whereas CaO, Fe2O3, TiO2, SO3, and K2O are the minor constituents of fly ash. The LOI of raw fly ash is 5.3%, and the mercury concentration is 0.34 μg/g measured by lumex RA-915M. 2.2. Fly Ash Modification. FA-MC and FA-MC-Br were prepared by omnidirectional planetary ball mill, as shown schematically in Figure 1. The omnidirectional planetary ball mill is equipped with four 250 mL agate ball mill tank, which contains 250 g of agate grinding balls with a diameter of 5 mm. Each ball mill tank can make a 360° rotation over the three-dimensional space while undergoing planetary motion. The mass ratio of the ball/fly ash was 10:1, and the mass ratio of FA/NH4Br was 100:1. The ball mill speed was 300 rpm, and the milling time was 1 h. The preparation process of FA-I-Br is as follows. Five grams of raw fly ash and 100 mL of NH4Br solution (mass fraction is 0.05%) were placed in a 250 mL beaker. The mixture was then stirred on a magnetic stirrer for 2 h. Finally, the filtered mixture was dried in a 45 °C oven for 8 h. It was worth noting that the FA/NH4Br mass ratio of FA-I-Br was 100:1, same as that of FA-MC-Br. 2.3. Sample Characterization. Specific surface area of the four samples was determined by N2 adsorption−desorption measurements at 77 K by employing the BET method (Gold App V-sorb 2008). The particle size distribution of FA and FA-MC-Br was measured by
η1 =
C2 × 100% C1
(R1)
60
η2 =
∫0 (C1 − C2)dt 60
× 100%
∫0 (C1)dt
(R2) 3
where C 1 and C 2 (μg/m ) is the inlet and the outlet Hg 0 concentrations at time t (min). 2.5. Mercury-Desorption Tests. To further understand the mercury-adsorption species and amount for each fresh and used sample, the Hg-TPD test was conducted in a temperatureprogrammed desorption system, as shown in Figure 3. It consists of a horizontal tube furnace, a PIC temperature control system, and lumex RA-915 online mercury analyzer. The carrier gas was N2, the desorption mass was 50 mg, the total gas flow rate was 200 mL/min, and the heating rate was controlled by PIC set to 10 °C/min. The pyrolyzed gaseous Hg in the effluent gas was detected by lumex RA915 mercury analyzer. The mercury-adsorption species can be derived from the peak position of the TPD desorption curve. The mercuryadsorption capacity and the adsorption and oxidation efficiencies of 6671
DOI: 10.1021/acs.energyfuels.9b01034 Energy Fuels 2019, 33, 6670−6677
Article
Energy & Fuels
Figure 2. Fixed-bed reactor.
Table 2. Pore Structure Parameters of the Four Samples
the four samples can be evaluated by the amount of mercury adsorbed (q, μg/g), adsorption efficiency (η2a, %), and oxidation efficiency (η2o, %), which are calculated by Q tpd 10mtpd
η2a =
∫0
700
(C tpd)dt
q Q fb m fb
60
∫0 (C1)dt
η2o = η2 − η2a
BET surface area (m2/g)
total pore volume (cm3/g)
average pore diameter (nm)
FA FA-MC FA-MC-Br FA-I-Br
2.66 2.46 2.60 3.83
0.007 0.006 0.006 0.010
9.67 8.44 8.57 9.75
milling. In contrast, the specific surface area, total pore volume, and average pore diameter of FA-I-Br increase because the water-soluble substance in fly ash dissolving in water during impregnated modification process causes more pores. Figure 4 shows that the volume percentage of large size particle is reduced and the particle size distribution moves from 0.4−110 to 0.3−50 μm after ball milling. Surface morphologies of the four samples were investigated with SEM, as shown in Figure 5 (magnification 10 000×). The FA has granular morphology, in which smooth glass spheres are dominant and a bit irregular-shaped unburned carbon can also be observed. Figure 5b and c shows that the large glassy
Figure 3. Temperature-programmed desorption system.
q=
sample
(R3)
× 100% (R4) (R5)
where Ctpd (μg/Nm3) is the outlet Hg0 concentrations at temperature T (°C), Qtpd (Nm3/min) is the gas flow rate in TPD, mtpd (g) is the mass of the sample in TPD, Qfb (Nm3/min) is the gas flow rate in the fixed-bed reactor, and mfb (g) is the mass of the sample in fixed-bed reactor.
3. RESULTS AND DISCUSSION 3.1. Physical Properties of Four Samples. As shown in Table 2, the specific surface area of FA is 2.66 m2/g, the total pore volume is 0.007 cm3/g, and the average pore diameter is 9.67 nm. It demonstrates that the pore structure of FA is not developed, which is not conducive for mercury adsorption. For both FA-MC and FA-MC-Br, the specific surface area, total pore volume, and average pore diameter all slightly decrease. This is because the small blind holes inside the fly ash are probably exposed due to the breakage, and the large holes of the fly ash are probably sealed due to agglomeration during ball
Figure 4. Particle size distribution of FA and FA-MC-Br. 6672
DOI: 10.1021/acs.energyfuels.9b01034 Energy Fuels 2019, 33, 6670−6677
Article
Energy & Fuels
curve of FA-MC-Br. It demonstrates that the part of Br in FAMC-Br existing in the form of NH4Br crystal, which is different from the Br-containing activated sites, has no effect on the oxidation and removal of mercury.20 Therefore, NH4Br is not fully loaded after mechanical−chemical bromination. After the impregnation process, there is no obvious difference between the XRD patterns of FA and FA-I-Br, demonstrating that the impregnated−brominated method has no effect on the fly ash crystal phase. 3.3. XPS Analysis. The atomic percentages of Br, Cl, and Ca in the four samples were analyzed using XPS; the results are shown in Table 3.29 The atomic percentage of Br on FA-MCTable 3. XPS Results for the Surface Atomic Percentages of Br, Cl, and Ca on Four Samples
Figure 5. SEM images of the four samples (magnification 10 000×).
spheres are replaced by small irregular block particles in FAMC and FA-MC-Br due to the mechanical force of ball milling, which is consistent with the result of particle size distribution (Figure 4). Different from FA-MC, flocculent and flaky materials can be observed in FA-MC-Br, which are most likely bromide compound generated after ball mill. Additionally, Figure 5d shows that the glassy spheres of FA-I-Br have a slightly smoother surface, and no irregular-shaped materials can be observed, indicating that impregnated bromination has no significant effect on surface morphology of fly ash. 3.2. XRD Analysis. XRD patterns of the four samples are listed in Figure 6. It shows that the main crystalline
sample
Br (%)
Cl (%)
Ca (%)
FA FA-MC FA-MC-Br FA-I-Br
0 0 1.34 0.17
1.44 1.18 0.77 0.52
2.03 5.64 5.34 1.73
Br is 1.34%, eight times that of FA-I-Br, indicating the bromine-loading rate of the mechanical-chemical method is better than that of the traditional impregnation method. It illustrates that the use of ball milling to induce mechanical− chemical bromination ensures best contact of bromine with the fly ash for the best access of bromine to solid surfaces and enhanced binding.30 The atomic percentage of Cl on FA is 1.44%. The ball-milling process can slightly reduce the chlorine content of FA, and it is further reduced with the addition of NH4Br, which is most likely because Br competes for the active site of Cl on the fly ash surface. Due to the crushing action of the ball-milling process, Ca in the fly ash is exposed. Also, the contents of Cl and Ca in FA-I-Br are the least among the four samples because of their water solubility. To identify the form of carbon and oxygen functional group of the four samples, Figure 7 shows the optimum fitting curves of the C 1s and O 1s spectra. The C 1s spectrum of FA is fitted to five characteristic peaks: C−C (graphitic carbon), C−O (hydroxyl), CO (carbonyl), COOH/C(O)−O−C (carboxyl/ester group), and π−π*, with the corresponding electronbonding energy (eV) of 284.5, 285.3, 286.5, 288.5, and 290.4, respectively.31 The amount of oxygen-containing functional groups (C−O, CO, COOH/C(O)−O−C) on the surface of the sample has a greater influence than inorganic carbon (C− C) on the performance of mercury removal by providing the activated sites for Hg0 binding.13,32 The surface atomic concentration of each functional group is given in Table 4. The results show that the proportion of inorganic carbon- and oxygen-containing functional groups of FA is 6.61 and 8.94%, respectively. The proportion of oxygen-containing functional groups of FA-MC and FA-I-Br is lower than that of FA, while FA-MC-Br has a higher content of oxygen-containing functional groups than FA. It demonstrates that under the process of ball milling, the addition of NH4Br is beneficial for increasing the oxygen-containing functional groups over the surface of fly ash. The inorganic carbon content of FA-I-Br is three times that of FA, illustrated by the fact that the carbon exposed by impregnation is mainly inorganic carbon. The O 1s spectrum of FA is fitted to two characteristic peaks: Oβ (weakly adsorbed oxygen) and −OH (hydroxyl), and the corresponding electron-bonding energy (eV) is 532.8
Figure 6. X-ray diffraction patterns of the four samples.
components in FA are mullite (dominant), silicon oxide, and calcite, which are always present as the major phases in coal combustion fly ash.28 In FA-MC and FA-MC-Br samples, mullite, silicon oxide, and calcite are also the main ingredients, while the corresponding peaks are slightly reduced, suggesting the transformation of the crystalline phase into an amorphous one under the effect of ball mill. Additionally, the obvious ammonium bromide crystal phase can be observed in the XRD 6673
DOI: 10.1021/acs.energyfuels.9b01034 Energy Fuels 2019, 33, 6670−6677
Article
Energy & Fuels
Figure 7. XPS spectra of the four samples over the spectral regions of C 1s (a) and O 1s (b).
Table 4. XPS Results for the Various Form of Carbon and Oxygen on Four Samples C
O
sample
C−C (%)
C−O (%)
CO (%)
COOH/C(O)−O−C (%)
π−π* (%)
Oβ (%)
hydroxyl (%)
Oα (%)
FA FA-MC FA-MC-Br FA-I-Br
6.61 9.20 7.57 19.49
4.00 3.36 4.37 2.81
3.77 3.36 4.76 2.67
1.17 1.37 3.01 2.23
1.51 1.37 0.97 0
26.05 29.18 33.50 28.85
23.41 16.36 11.53 12.79
0 0 0 9.67
and 531.7, respectively.33,34 There is new fitting peak at about 530.1 eV in the spectrum of FA-I-Br, which is expected for Oα (lattice oxygen).35 Based on the results of Table 4, it can be concluded that the Oβ content of the three modified fly ash is higher than that in FA, and that of FA-MC-Br is the most. Oβ is in favor of the redox reaction and plays an important role in oxidation.36 3.4. Result of Fixed-Bed Experiment. To investigate the effect of three modification methods on the mercury-removal performance of fly ash, four samples were tested in the fixedbed reactor; the result is shown in Figure 8. The mercury breakthrough rate of FA-MC is slightly lower than that of FA because the particle size decreases and the degree of amorphous phase increases after ball milling and the reaction contact area and the oxidative reactivity are increased.
Meanwhile, the increase of oxygen-containing functional groups and Oβ induced by mechanical force is beneficial to enhancing the adsorption and oxidation of elemental mercury.32,36 The mercury breakthrough rate of FA-MC-Br is significantly lower than that of the other three samples. Based on the analysis by Bisson et al.24 and XPS results in Table 3, it can be concluded that mechanical−chemical bromination ensures optimal contact of bromine with fly ash and reaction with calcium exposed from fly ash to form CaBr2. The fly ash/ NH4Br mass ratio of impregnated bromination is the same as with mechanical−chemical bromination. Therefore, the NH4Br mass fraction of the solvent used in the impregnated bromination is only 0.05%, which is far less than that in the previous research.20,26,27,37 Thus, large part of NH4Br and water-soluble active components can be carried away by the solvent during the filtration process. Due to the above reasons, the mercury-removal performance of FA-I-Br is far lower than that of FA-MC-Br. The trend of mercury breakthrough rate first decreases rapidly, then increases, and finally stabilizes. This is because the mercury removal at the beginning of the reaction (first 10 min) is a combination of adsorption and oxidation, and oxidation dominates after the adsorption saturation (after 10 min). 3.5. Mercury-Removal Mechanism. The mercuryremoval performance of the four samples follows the order of FA-MC-Br > FA-I-Br > FA-MC > FA. However, the details of mercury adsorption and oxidation by the four samples are still unclear. Hg-TPD was carried out to determine the mercuryadsorption species and amount for the fresh and used four samples; the corresponding Hg-TPD curve is shown in Figure 9. The mercury-adsorption species of the four fresh samples can be deduced according to the different mercury-adsorption species corresponding to different desorption peaks reported by Zhang;38 the XPS results are shown in Table 3. The
Figure 8. Mercury breakthrough rates of the four samples. 6674
DOI: 10.1021/acs.energyfuels.9b01034 Energy Fuels 2019, 33, 6670−6677
Article
Energy & Fuels
Figure 9. TPD curves of the fly ash samples before (a) and after reaction (b).
adsorption capacity of the four samples follows the order of FA-MC-Br > FA > FA-MC > FA-I-Br, whose value is 0.97, 0.95, 0.53, and 0.34 μg/g, respectively. The content of the oxygen-containing functional groups of fly ash is the core factor affecting the mercury-adsorption capacity.12,32 Due to the reduction of oxygen-containing functional groups on the surface of the sample (Table 4), the mercury-adsorption capacity of FA-MC and FA-I-Br is lower than that of FA. The mercury-adsorption capacity of FA-MC-Br is slightly higher than that of FA because oxygen-containing functional groups increase and CaBr2 is generated. The above results are well consistent with the mercury-adsorption efficiency, as shown in Figure 11.
mercury-adsorption species of fresh FA, FA-MC, and FA-I-Br is HgCl2 because the amount of Cl in them is higher than that of Br in them. Since the Br content in FA-MC-Br is higher than their Cl content and the ability of Br to bind to Hg is stronger than that of Cl,37 it is speculated that HgBr2 is the main form of mercury in FA-MC-Br. After the fixed-bed reaction, the mercury-adsorption species of the used FA and FA-MC remains the same because there is no Br introduction during the reaction. The main mercury-adsorption species of both FAMC-Br and FA-I-Br are HgBr2; furthermore, the desorption peak of 325 °C is observed on the curve of FA-MC-Br, which indicates that HgO is generated on FA-MC-Br along with 4% O2. Since FA-MC-Br has better bromine loading than FA-I-Br, O2 can be adsorbed by FA-MC-Br and then decomposed into oxygen atoms by chemisorption to react with Hg0 to generate HgO.20 The amount of mercury adsorbed by fresh and used samples was calculated by integrating the TPD curves; the results are shown in Figure 10. The mercury adsorption amount of FA-
Figure 11. Mercury-removal, -oxidation, and -adsorption efficiencies of the four samples.
Figure 11 shows that the mercury-removal performance of the four samples is mainly reflected in the oxidation (>90%), while adsorption only accounts for a small proportion ( FA-I-Br > FA-MC > FA. The mercury removal at the beginning of the reaction (first 10 min) is a combination of adsorption and oxidation, while oxidation dominates after adsorption saturation (after 10 min). The mercury-removal performance of the four samples is mainly oxidation (>90%), while adsorption only accounts for a small proportion (
