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Frontiers in Energy

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ISSN 2095-1698(Online)

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Front. Energy    2021, Vol. 15 Issue (1) : 112-123    https://doi.org/10.1007/s11708-020-0682-3
RESEARCH ARTICLE
Mercury emission and adsorption characteristics of fly ash in PC and CFB boilers
Li JIA, Baoguo FAN, Xianrong ZHENG, Xiaolei QIAO, Yuxing YAO, Rui ZHAO, Jinrong GUO, Yan JIN()
College of Electrical and Power Engineering, Taiyuan University of Technology, Taiyuan 030024, China
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Abstract

The mercury emission was obtained by measuring the mercury contents in flue gas and solid samples in pulverized coal (PC) and circulating fluidized bed (CFB) utility boilers. The relationship was obtained between the mercury emission and adsorption characteristics of fly ash. The parameters included unburned carbon content, particle size, and pore structure of fly ash. The results showed that the majority of mercury released to the atmosphere with the flue gas in PC boiler, while the mercury was enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor was proposed to characterize the impact of coal property on mercury emissions in this paper. As the coal factor increased, the mercury emission to the atmosphere decreased. It was also found that the mercury content of fly ash in the CFB boiler was ten times higher than that in the PC boiler. As the unburned carbon content increased, the mercury adsorbed increased. The capacity of adsorbing mercury by fly ash was directly related to the particle size. The particle size corresponding to the highest content of mercury, which was about 560 ng/g, appeared in the range from 77.5 to 106 µm. The content of mesoporous (4–6 nm) of the fly ash in the particle size of 77.5–106 µm was the highest, which was beneficial to adsorbing the mercury. The specific surface area played a more significant role than specific pore volume in the mercury adsorption process.
Keywords mercury      combustion modes      coal property      fly ash      particle size     
Corresponding Author(s): Yan JIN   
Online First Date: 13 July 2020    Issue Date: 19 March 2021
 Cite this article:   
Li JIA,Baoguo FAN,Xianrong ZHENG, et al. Mercury emission and adsorption characteristics of fly ash in PC and CFB boilers[J]. Front. Energy, 2021, 15(1): 112-123.
 URL:  
https://academic.hep.com.cn/fie/EN/10.1007/s11708-020-0682-3
https://academic.hep.com.cn/fie/EN/Y2021/V15/I1/112
Boiler No. Type Capacity/MW Coals Pollutants control devices
1 Tangential firing, PC boiler 600 Bitumite SCR+ESP+WFGD
2 Tangential firing, PC boiler 600 Bitumite SCR+ESP+WFGD
3 Tangential firing, PC boiler 300 Anthracite SCR+ESP+WFGD
4 CFB boiler 200 Bitumite FF
5 CFB boiler 135 Lignite ESP+FF
Tab.1  Parameters of boilers
Coal
No.		 Proximate analysis/% Ultimate analysis
Mad Ad Vdaf FCd Cd/% Hd/% Od/% Nd/% Sd/% Cld/% Hg/(ng·g−1)
M1 5.34 14.86 37.20 61.30 67.70 4.33 11.66 0.95 0.47 0.054 122.70
M2 5.64 31.67 36.23 43.58 50.17 3.21 12.47 0.82 1.66 0.081 221.65
M3 3.44 31.42 33.28 45.75 55.82 1.06 7.42 2.61 1.68 0.064 136.65
M4 1.28 53.73 34.82 30.17 34.73 2.33 6.11 0.67 2.43 0.032 235.33
M5 0.86 42.87 41.47 33.44 43.49 3.14 9.34 0.79 2.90 0.037 103.00
Tab.2  Proximate and ultimate analysis of coal
Combustion modes Boiler No. Generation load/MW Input mercury/(g?h−1) Output mercury/(g?h−1) Demercuration efficiency/% Concentration of mercury in flue gas/(mg·m−3)
Coal Limestone Fly ash Slag Gypsum Flue gas
PC
boiler		 1 600 31.32 0.02 1.13 0.340 3.60 32.54 13.48 0.015
2 600 68.05 0.38 3.78 0.010 24.95 36.27 44.21 0.022
3 300 19.95 0.03 2.26 0.001 6.38 13.34 39.30 0.013
CFB boiler 4 200 36.64 0.05 31.33 0.008 – 6.09 83.73 0.008
5 135 10.50 0.04 9.05 0.010 – 2.01 81.84 0.003
Tab.3  Distribution of mercury in various combustion modes
Fig.1  Hg distribution in combustion products of Nos. 1, 2, and 3 PC boilers.
Fig.2  Hg distribution in combustion products of Nos. 4 and 5 CFB boilers.
Coal No. Qar,net/(kJ?kg−1) Clred/% Ared/% Sred/% Hgred/%
M6 27356 0.004 2.41 0.112 2×10−6
M7 21666 0.007 3.31 0.107 2×10−6
M8 26608 0.003 2.76 0.082 2×10−6
M9 27199 0.009 2.35 0.074 2×10−6
M10 19874 0.017 6.66 0.349 5×10−6
M11 19629 0.014 6.69 0.358 3×10−6
Tab.4  Information of coal in PC boilers
Fig.3  Coal factor in PC boilers.
Coal No. Qar,net/(kJ?kg−1) Clred/% Ared/% Sred/% Hgred/%
M12 14315 0.015 14.99 1.075 7×10−6
M13 13206 0.009 18.44 0.675 7×10−6
M14 15116 0.008 14.86 0.672 6×10−6
M15 17729 0.008 10.11 0.684 2×10−6
Tab.5  Information of coal in CFB boilers
Fig.4  Coal factor in CFB boilers.
Boiler No. Fly ash No. Mercury content/(ng·g−1) Carbon content/%
1 F1 39.74 1.98
F2 49.18 2.19
F3 36.54 1.87
F4 33.13 1.79
2 F5 26.17 0.82
F6 35.99 0.81
F7 62.97 1.09
F8 99.11 1.63
F9 103.59 1.70
F10 115.35 1.71
F11 82.15 1.32
F12 43.22 1.30
4 F13 268.60 3.01
F14 560.20 6.31
F15 529.60 5.99
5 F16 706.25 7.06
F17 756.00 7.14
F18 681.00 6.41
Tab.6  Content of mercury and carbon in fly ash
Fig.5  N2 adsorption/desorption isotherms of (a) F5, (b) F10, and (c) F11.
Fig.6  Accumulated and differential pore volume of (a) F5, (b) F10, and (c) F11.
Sample Average pore size/nm Most probable pore size/nm BET specific surface area/(m2·g−1) Accumulated pore volume/(cm3·g−1) Accumulated pore surface area/(m2·g−1) Pore richness Z Relative specific pore volume/%
Micropore Mesopore Macropore
F5 13.57 4.05 0.3165 0.0071 1.06 44.58 3.50 58.25 38.25
F10 18.29 5.92 0.5333 0.0072 1.86 74.07 5.88 61.26 32.86
F11 12.69 4.54 0.4751 0.0067 1.76 70.91 5.09 57.85 37.06
Tab.7  Pore structure parameters of fly ash
Particle size/µm Mass fraction of fly ash/%
No. 4 boiler No. 5 boiler
<48 3.61 0.94
48–77.5 31.99 3.03
77.5–90 24.93 17.61
90–106 15.76 40.63
106–120 13.76 31.74
120–180 3.49 3.19
180–325 2.55 1.25
≥325 3.91 1.60
Tab.8  Mass fraction of fly ash (%) within the scope of different particle size
Boiler No. Items Particle size/µm
<48 48–77.5 77.5–90 90–106 106–120 120–180 180–325 ≥325
4 Mercury content/(ng·g−1) 491.30 510.40 529.60 558.20 513.70 275.30 83.77 35.85
Carbon content/% 5.23 5.77 5.99 6.23 5.86 3.16 1.55 1.30
5 Mercury content/(ng·g−1) 328.60 501.10 576.40 521.30 457.70 326.10 70.14 50.26
Carbon content/% 2.97 4.89 5.66 5.14 4.20 3.13 1.31 0.97
Tab.9  Content of mercury and carbon in fly ash within different size ranges
Fig.7  N2 adsorption/desorption isotherms of (a) FL1, (b) FL2, and (c) FL3.
Fig.8  Accumulated and differential pore area of (a) FL1, (b) FL2, (c) FL3.
Sample Particle size/μm Average
pore size/nm		 Most probable
pore size/nm		 BET specific surface area/(m2·g−1) Accumulated pore volume/(cm3·g−1) Accumulated pore surface area/(m2·g−1) Pore richness Z Specific pore volume/(cm3·g−1)
Micropore Mesopore Macropore Sum
FL1 48–77.5 26.41 3.67 2.233 0.0368 3.86 60.68 0 0.0202 0.0185 0.0387
FL2 90–106 35.78 5.94 1.319 0.0308 3.18 42.82 0 0.0186 0.0136 0.0322
FL3 120–180 23.89 4.04 1.134 0.0292 3.51 38.84 0 0.0174 0.0133 0.0307
Tab.10  Pore structure parameter of fly ash
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