In-situ reaction between arsenic/selenium and minerals in fly ash at high temperature during blended coal combustion
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摘要: 为了研究混煤燃烧过程中痕量元素与飞灰中矿物质的原位反应，选取烟煤（HLH）、无烟煤（ZW）及其混煤在1150 ℃时的沉降炉中进行燃烧，并分别收集和分析了高温段灰分（HTA）和低温段灰分（LTA）中砷和硒残留率。结果表明，砷在高温段灰分中的残留率低于低温段灰，说明在烟气冷却过程中砷会被灰重新吸附。ZW、Z3H1、Z1H1、Z1H3、HLH的高温段灰中砷的残留率分别为60.31%、26.85%、13.29%、20.23%、36.11%，说明混煤的高温段灰比原煤更难捕获砷。同时，硒在五种煤样的高温段灰中的残留率分别为24.68%、23.60%、20.58%、15.19%和38.13%，其残留规律与砷相同。此外，X射线衍射（XRD）分析结果表明，混煤燃烧过程中矿物形态发生了明显变化。与原煤不同的是，混煤的HTA中出现了莫来石，且莫来石的峰值随着混煤中ZW比例的增加而增强。这与HTA中砷和硒的残留趋势一致。说明在混煤燃烧过程中，矿物质种类的变化以及矿物质与痕量元素的原位反应对砷和硒的排放有显著影响。Abstract: Blended coal combustion technology was extensively used in coal-fired power plants in China. In order to investigate the in-situ reaction between trace elements and minerals in fly ash during blended coal combustion, a bituminous (HLH), anthracite (ZW) and the blended coal of these two parent coals were combusted in a drop tube furnace at 1150 ℃. The ash gathered at high temperature segment (HTA) and low temperature segment (LTA) of the furnace were analyzed, respectively. The results indicated that the retention rates of arsenic in HTA were lower than that in LTA, which suggested that arsenic would be re-absorbed by ash during cooling down of flue gas. For HTA the retention rates of arsenic in ash of ZW, Z3H1, Z1H1, Z1H3, HLH were 60.31%, 26.85%, 13.29%, 20.23% and 36.11%, respectively. The arsenic was more difficult to be captured by HTA of blended coal than that of parent coal. As for selenium, the retention rates in HTA of five coal samples were 24.68%, 23.60%, 20.58%, 15.19% and 38.13%, which had the same retention law as arsenic. The results of X-ray diffraction (XRD) demonstrated that the mineral morphology was changed obviously during blended coal combustion. Unlike parent coal, mullite appeared in HTA of blended coal, and peak of mullite was enhanced with proportion of ZW increased in blended coal. It was consistent with the trend of retention of As and Se in HTA. It illustrated that change of mineral species and in-situ reaction between minerals and trace elements significantly affected emission of arsenic and selenium during blended coal combustion.
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Key words:
- blended coal /
- arsenic /
- selenium /
- mineral /
- in-situ reaction
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Figure 1 Schematic diagram of the DTF used for the combustion tests
Schematic diagram of the DTF used for the combustion tests
Table 1 Ultimate/proximate analysis of the samples
Sample Ultimate analysis /% Proximate analysis wad /% Content /(μg·g-1) C H N S O* M A V FC As Se ZW 34.13 1.47 0.40 - 17.85 2.63 43.52 8.79 45.06 1.98 0.68 HLH 49.96 5.40 1.07 0.62 4.87 4.81 32.84 36.80 25.77 18.07 0.20 *: by different, ad: air dry, -: not detected 
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Table 2 Ash composition of the samples
Sample Composition w/% SiO2 Al2O3 CaO Fe2O3 Na2O K2O MgO SO3 TiO2 ZW 42.64 21.03 6.92 9.03 5.60 2.85 1.15 3.99 1.34 HLH 56.20 14.98 6.45 12.80 1.52 2.01 0.71 6.36 0.95
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Table 3 Sequential chemical extraction method
Step Solid phases Extraction(0.5 g of coal) Demonstration S1 exchangeable 15 mL of 1 mol/L MgCl2 oscillated 4 h at room temperature, then centrifuged
10 min at 3000 r/minS2 sulfide-bound 15 mL of 12.5% HNO3 (W/W) 0.5 h at 95 ℃ with intermittent oscillation, then centrifuged
10 min at 3000 r/minS3 organic-bound 10 mL of HNO3 (pH=2) and
5 mL of 30% H2O25 h at 80-85 ℃ with intermittent oscillation, then centrifuged
10 min at 3000 r/minS4 residual 8 mL of HNO3 and 2 mL of HF digested in the microwave digestion system
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