Distribution and enrichment characteristics of arsenic in feed-coal and by-products of coal-fired power plants
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摘要: 采用微波消解法和氢化物发生-原子荧光光谱法，考察并比较了五台容量25-350 MW循环流化床(CFB)机组和五台容量300-600 MW煤粉炉(PC)机组中砷的分布和富集特性。通过比较常规湿法消解和三种混酸微波消解体系，确定了适宜消解方案为体积比6：2：2的HNO3-HCl-HF混酸溶液微波消解法。煤中砷燃烧后绝大部分挥发出来被飞灰捕获，底渣中砷含量仅为1.95-9.75 μg/g，烟气中砷主要被飞灰吸附后依次被除尘器和脱硫塔捕集，其中，飞灰砷含量为8.68-17.63 μg/g，脱硫石膏砷含量为1.71-4.0 μg/g。燃烧温度是决定砷迁移与富集的主要因素，PC机组更高的炉膛燃烧温度使得较多砷从煤中释放出来，导致残留在底渣中砷含量低于CFB机组，同时PC机组飞灰在高温下更易形成硅铝酸盐类型的玻璃质从而捕获烟气中挥发态砷，其飞灰中砷含量为12.08-17.63 μg/g，普遍高于CFB机组飞灰中砷含量8.68-13.84 μg/g；随着锅炉负荷增大，炉膛内温度升高，飞灰与入炉煤中砷含量比值呈增长趋势。CFB机组燃用煤中灰分含量为33.96%-59.63%，显著高于PC机组15.05%-41.67%，故其相对富集系数高于PC机组，同时CFB机组有更多除尘器尚未捕获的细颗粒进入脱硫系统，使其脱硫石膏中砷含量也高于PC机组。Abstract: The distribution and enrichment characteristics of arsenic in five circulating fluidized bed (CFB) units with capacity between 25 to 350 MW and five pulverized coal furnace (PC) units with capacity between 300 to 600 MW were investigated using microwave digestion and hydride generation-atomic fluorescence spectrometry. By comparing the conventional wet digestion method and three kinds of mixed-acid microwave digestion systems, the appropriate digestion method was determined to be HNO3-HCl-HF acid solution mixed in a volume ratio of 6:2:2 with microwave digestion. The majority of the arsenic in coal evaporates during combustion and captured by the fly ash, the arsenic content in the bottom slag is only 1.95-9.75 μg/g. Most arsenic in the flue gas is adsorbed by the fly ash, most of the adsorbed arsenic is successively captured by the dust collector and desulfurization system. The arsenic contents in the fly ash and gypsum is 8.68-17.63 μg/g and 1.71-4.0 μg/g, respectively. Combustion temperature is the key factor affecting the release of arsenic. PC has a higher furnace temperature than CFB and makes more arsenic volatilize from coal and less arsenic remain in bottom slag. Meanwhile, a higher combustion temperature in PC unit produces more glassiness as aluminosilicate in the fly ash, which can capture the arsenic from the flue gas. Therefore, the arsenic content in the fly ash from PC unit is 12.08-17.63 μg/g, which is significantly higher than that from CFB, 8.68-13.84 μg/g. Moreover, the furnace temperature increases with the boiler load, which makes the ratio of arsenic content in the fly ash to the feed coal show an increasing trend. The ash content of the coal used for CFB and PC units is 33.96%-59.63% and 15.05%-41.67%, which makes the relative enrichment factor of arsenic in CFB higher than that in PC. Furthermore, more fine particles escaped from the dust collector should be captured by the desulfurization system, resulting that the arsenic concentration in the desulfurization gypsum of CFB unit is higher than that in PC unit.
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图 1 混酸体系定量结果对比
Figure 1 Quantitative comparison of the digestion results among the four mixed-acid digestion systems
: HNO3-HCl-H2O2 microwave digestion; : HNO3-HCl-H2SO4 microwave digestion; : HNO3-HCl-HF microwave digcstion; : HNO3-HCl-HF wet digestion
图 2 入炉煤、飞灰与底渣中平均砷含量
Figure 2 Mean As contents in the feed coal, the fly ash and the bottom slag
图 3 飞灰和底渣与入炉煤中砷含量的比值
Figure 3 As content ratio of the fly ash/the feed coal and the bottom slag/the feed coal
表 1 所选燃煤机组容量、类型及取样点
Table 1 Capacity, boiler type and sampling points of the coal-fired power units
Capacity /MW Boiler type Sample Plant1(TS) 25 CFB feed coal + bottom slag + fly ash Plant2(GRT) 135 CFB feed coal + bottom slag + fly ash Plant3(WP) 200 CFB feed coal + fly ash + bottom slag + gypsum Plant4(PS) 300 CFB feed coal + fly ash + bottom slag + gypsum Plant5(YG) 300 CFB feed coal + fly ash + bottom slag + gypsum Plant6(CZ) 300 PC feed coal + fly ash + bottom slag + gypsum Plant7(HF) 330 PC feed coal + fly ash + bottom slag + gypsum Plant8(SH) 600 PC feed coal + fly ash + bottom slag + gypsum Plant9(DZ) 600 PC feed coal + fly ash + bottom slag + gypsum Plant10(ST) 600 PC feed coal + fly ash + bottom slag + gypsum 
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表 2 所选燃煤机组入炉煤的工业分析
Table 2 Proximate analysis of the feed coals from the coal-fired power units
Plant Proximate analysis wad/% M A V FC TS 1.94 59.63 15.81 22.62 GRT 2.23 41.50 29.72 26.55 WP 1.24 61.20 18.62 18.94 YG 2.36 33.96 24.94 38.74 PS 2.38 39.76 24.74 33.12 CZ 0.90 41.67 12.26 45.17 HF 1.25 29.32 10.67 58.76 SH 2.67 15.05 29.95 52.33 DZ 3.98 16.42 25.57 54.03 ST 2.83 29.67 27.32 40.18
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表 3 微波消解混酸体系配比表
Table 3 Composition of the acid solutions for the microwave digestion systems
Microwave digestion system Acid solution Ratio Mixed acid solution 1 HNO3-HCl-H2O2 6:2:2 Mixed acid solution 2 HNO3-HCl-H2SO4 6:2:2 Mixed acid solution 3 HNO3-HCl-HF 6:2:2
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表 4 混酸体系定性消解对比
Table 4 Qualitative comparison of digestion results among the four mixed-acid digestion systems
Digestion method Solution Residue HNO3-HCl-HF microwave digestion clear almost no residue HNO3-HCl-H2O2 microwave digestion clear almost no residue except the gypsum HNO3-HCl-H2SO4 microwave digestion clear and yellow almost no residue HNO3-HCl-HF wet digestion clear grey residue
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表 5 WP机组所采样品加标回收实验
Table 5 Recovery percentage and accuracy test of the samples from the WP unit
Sample Measuring result As content(μg/g，n=7) RSD /% recovery /% Feed coal 3.50±0.05 1.40 Feed coal + 2.0×10-8 As standard solution 22.54±0.85 3.76 95.20 Fly ash 10.78±0.35 3.21 Fly ash + 2.0×10-8 As standard solution 31.36±1.07 3.42 102.90 Bottom slag 3.79±0.14 3.77 Bottom slag + 2.0×10-8 As standard solution 24.68±1.30 5.29 104.45 Gypsum 3.99±0.20 4.91 Gypsum+ 2.0×10-8 As standard solution 22.22±0.34 1.54 94.40
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