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汞是一种对人体和环境都具有严重毒性的重金属元素,对中枢神经系统有很强的毒性,被世界卫生组织列为第三类致癌物[1]。20世纪50年代日本发生的水俣病事件,使人们充分认识到汞(Hg),尤其是甲基汞(MeHg)对人类和动物的毒害[2]。
相关报道指出,每年全球人为排放至大气中的汞约2000吨[3-4],煤炭燃烧已被公认为世界上主要的人为汞排放源之一[5-7]。中国是世界上最大的煤炭生产国与消费国,2018年煤炭消耗量达到了27.38亿吨,约占全国能源消费比重的60%[8]。众多学者[9-17]对中国煤中汞含量进行了大量的研究,中国煤中汞含量为0.15—0.22 mg·kg−1,高于世界煤中汞含量(0.1 mg·kg−1)[18]。2015年中国燃煤汞排放量高达264.49 t[19],燃煤汞排放问题相当严峻[20-21]。排放进入大气中的汞可进行长距离的迁移并通过干湿沉降对土壤与水体造成汞污染,进一步威胁着人类健康与生态环境。
关于减少煤炭燃烧排放汞的措施主要分为燃烧前脱汞、燃烧中脱汞、燃烧后脱汞三类[22-23]。相对而言,燃烧前脱汞较为简单方便。煤炭洗选可作为燃烧前脱汞的一种有效手段[22]。 Zajusz-Zubek等[24]、Pan等[25]和冯立品等[26]认为,汞在煤中的赋存形态决定了汞的去除能力,煤中大部分与矿物结合的汞有被去除的可能。高硫煤中汞与硫分具有较强的亲和力,主要以黄铁矿或其他硫化物结合存在,与无机硫含量成正比[18, 27-29];低硫煤中汞多数以硫化物结合态和有机结合态为主,部分低硫煤可能因岩浆侵蚀作用使汞与硅酸盐等结合存在[30-31],故在理论上部分汞元素可通过煤炭洗选随灰分、硫分等被去除。Luttrel等[32]发现通过洗选,煤中汞最高脱除率约为80%,平均脱除率为46.71%,与煤中黄铁矿脱除率49.31%相似。另有研究表明,煤炭洗选后汞含量可降低10%—60%[33-36]。
目前研究多集中于高硫煤洗选过程中总汞的迁移与脱除,对于(特)低硫煤中汞元素的脱除行为及效果等方面相关研究报道较少,且未见煤中甲基汞相关方面研究。本文通过对煤中总汞与甲基汞含量的测定,针对其在低硫煤洗选过程中的迁移规律、脱除效果、质量平衡等进行综合研究,旨在为更好地理解煤炭洗选过程中汞元素的脱除行为提供科学依据。
甲基汞与总汞在低硫煤洗选过程中的脱除行为
Study on removal behavior of methylmercury and total mercury during the washing processes of low-sulfur coal
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摘要: 以焦作市演马洗煤厂进料原煤及洗选产品为研究对象,对低硫煤中总汞与甲基汞含量进行测定,初步探讨了洗选过程中总汞与甲基汞的迁移规律、脱除效果及质量平衡。研究发现,原煤及洗选产品中总汞含量0.128—0.392 mg·kg−1;值得注意的是,原煤及洗选产品中均检出了甲基汞,含量为33.58—56.75 ng·kg−1。经过洗选,总汞在精煤中得到脱除,脱除率为24.68%,在筛末煤、中煤、副产品中被富集;甲基汞在大部分洗选产品中含量均有所降低,整体呈脱除趋势,脱除率为-3.46%—38.78%。该低硫煤中总汞与灰分显著正相关,与硫分呈负相关,在洗选过程中受灰分影响较大;甲基汞与灰分、硫分均极低相关,无明显的迁移规律。煤炭洗选前后的总汞质量平衡为108.06%,甲基汞质量平衡为80.08%。该低硫煤通过洗选,约60.55%的总汞和35.37%的甲基汞随筛末煤、中煤与副产品的排出而被去除,故需留意部分洗煤产品和副产品的存放与使用,避免可能对环境造成的汞污染问题。Abstract: Taking raw coal and washing products of Yanma coal preparation plant in Jiaozuo City as the research object, the content of total mercury (THg) and methyl mercury (MeHg) in low sulfur coal was determined, and the migration rule, removal efficiency and mass balance of THg and MeHg in the washing processes were also studied. The results showed that the THg in the raw coal and washing products ranged from 0.128 mg·kg−1 to 0.392 mg·kg−1. It should be noted that MeHg was detected in all the raw coal and washing products, with contents between 33.58—56.75 ng·kg−1. After washing, THg in clean coal products dropped by 24.68%. On the contrary, THg in slack coal, middling coal, and by-products increased; MeHg in most of the products decreased with relative removal rates of - 3.46%—38.78%. Mercury in coal was significantly positively correlated with ash content and negatively correlated with sulfur, indicating that the migration of THg in coal was mainly affected by ash content. MeHg was weakly correlated with ash and sulfur contents, showing no obvious migration trend. The mass balances closure was 108.06% and 80.08% for THg and MeHg, respectively. By washing, about 60.55% of the total mercury and 35.37% of the MeHg in the low sulfur coal are removed with the discharge of the slack coal, middling coal, and by-products. Therefore, attention should be paid to the storage and use of some coal washing products and by-products to avoid possible mercury pollution to the environment.
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Key words:
- low-sulfur coal /
- total mercury /
- methylmercury /
- coal washing /
- removal efficiency
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表 1 原煤及洗选产品中总汞和甲基汞的含量及其相关参数
Table 1. Content and related parameters of THg and MeHg in raw coal and washing products
总汞含量/ (mg·kg−1)
THgSTD 甲基汞含量/ (ng·kg−1)
MeHgSTD 灰分/%
Ash content硫分/%
Sulfur content总汞富集系数
THg CC原煤
Raw coal0.203 0.056 54.85 11.1 12.8 0.36 2.03 筛末煤
Slack coal0.251 0.049 52.61 9.16 22.6 0.34 2.51 精煤
Clean coal小块煤 0.155 0.038 49.33 18.05 9.6 0.36 1.55 粒煤 0.163 0.028 55.15 10.58 10 0.36 1.63 特优 0.128 0.019 33.62 8.77 10.7 0.35 1.28 二块煤 0.153 0.043 36.93 11.18 12.3 0.36 1.53 末精煤 0.165 0.012 38.06 8.23 11.3 0.36 1.65 中煤
Middlings块中煤 0.369 0.051 33.58 8.21 44.9 0.26 3.69 末中煤 0.386 0.049 56.75 7.01 47.8 0.28 3.86 副产品
By-product矸石 0.392 0.077 48.88 16.46 76.1 0.15 3.92 煤泥 0.293 0.033 40.54 10.51 44.6 0.27 2.93 表 2 总汞/甲基汞和灰分/硫分的相关性系数
Table 2. Correlation factors of total mercury, methylmercury, ash and sulfur
灰分 Ash content 硫分 Sulfur content 总汞 THg 甲基汞 MeHg 灰分 Ash content 1.000 −0.983** 0.931** 0.019 显著性 Sig. 6.54 10−8$ \times $ 3.06 10−5$ \times $ 0.839 硫分 Sulfur content −0.983** 1.000 −0.865** 0.037 显著性 Sig. 6.54 10−8$ \times $ 5.77 10−4$ \times $ 0.998 注:**表示在 0.01 级别(双尾),相关性显著。
** indicates significant correlation at 0.01 level (double tails).
相关系数|r|>0.8,高度相关;0.8>|r|>0.5,中度相关;0.5>|r|>0.3,低度相关;|r|<0.3,极低度相关;r>0,正相关;r<0,为负相关。
Correlation coefficient |r|>0.8, high correlation; 0.8>|r|>0.5, moderate correlation; 0.5>|r|>0.3, low correlation; |r|<0.3, very low correlation; r>0, positive correlation; r<0, negative correlation.表 3 煤中总汞线性回归分析
Table 3. Linear regression analysis of total mercury in coal
回归方程
Regression equation显著性
Sig.R2 标准化系数 Standardization coefficient 灰分 Ash content 硫分 Sulfur content 总汞 THg CTHg=10.83×(CA/%)+2221.83×(CS/%)−753.12 1.2×10−5 0.9409 2.372 1.466 CTHg: THg content; CA: Ash content; CS: Sulfur content. 表 4 煤炭洗选过程中总汞与甲基汞产量
Table 4. Production of THg and MeHg in coal washing process
煤炭年产量/t
Annual output of coal总汞年产量/kg
Annual output of THg甲基汞年产量/g
Annual output of MeHg原煤 Raw coal 61.03×104 123.9 33.48 筛末煤 Slack coal 11.64×104 29.16 6.12 精煤 Clean coal 小块煤 7.92×104 12.28 3.91 粒煤 1.08×104 1.76 0.6 特优 1.51×104 1.94 0.51 二块煤 4.74×104 7.26 1.75 末精煤 21.54×104 35.63 8.2 中煤 Middlings 块中煤 2.12×104 7.83 0.71 末中煤 4.30×103 1.66 0.24 副产品 By-product 矸石 4.93×104 19.33 2.41 煤泥 5.82×104 17.04 2.36 -
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