准东煤中碱金属的赋存形态及含量分析

新疆高碱煤具有低灰、低硫及反应活性高等特点,但在燃烧利用过程中,以气相形式释放的碱金属进入烟气后,将改变飞灰颗粒的物理化学特性,如降低灰熔融温度,并增强黏结性,进而对锅炉及换热设备造成严重的积灰、结渣、腐蚀等。碱金属的赋存形态决定其释放行为,因此深入研究碱金属具体存在形式及含量,可准确预测碱金属释放形式及含量,对准东煤的清洁、高效利用意义重大。以准东高钠煤为研究对象,采用扫描电镜能谱测试(SEM-EDS)分析样品表面元素分布,同时利用电感耦合等离子光谱(ICP-OES)分析碱金属Na赋存形态及含量,并通过X射线光电子能谱(XPS)及X射线衍射(XRD)研究样品表面元素含量及晶态结构。SEM-EDS结果表明:Ca、S在相同区域存在明显富集现象,表明CaSO4存在于该区域; Cl在扫描过程并未发现明亮区域,可认为Cl均匀分布在准东煤中;此外Na也存在少量明亮区域与S对应,表明Na2SO4晶体也存在于准东煤中。XRD结果表明:105℃干燥后样品中碱金属Na以NaCl及Na2SO4晶体结构存在,经去离子水萃取后,对应峰强度减弱,甚至消失。ICP-OES结果表明:准东煤中Na主要以水溶态形式存在,其次为醋酸铵溶态及酸溶态Na,其含量分别为78. 42%、11. 57%和6. 44%,并有少量以硅铝酸盐形式存在的不溶性Na,含量为3. 57%;水萃取液中阳离子主要以Na+、Ca2+、Mg2+为主,阴离子主要以Cl-、SO2-4以及HCO-3为主,经计算,阳离子总电荷数为5. 369 mmol/L,阴离子总电荷数为5. 385 mmol/L,可知溶液中阴阳离子电荷基本守恒,则水溶态Na可认为主要以NaCl、Na2SO4、NaHCO3等晶体或水合离子形式存在,在均匀分布条件下,可计算出其质量分数分别为5. 32%、85. 7%以及8. 98%。采用XPS对不同温度干燥后的样品进行表征分析,结果表明:随干燥温度升高,煤样中以水合离子形式存在的Na+也随着水分移动向样品表面富集,样品表面的Na/C质量比增加;干燥温度超过100℃后,水分蒸发并带走部分在表面富集的Na,因此样品表面Na/C比开始下降,该过程释放的Na可定义为水挥发性Na。 Zhundong coal,which is produced in Zhundong district,Xinjiang,is a kind of coal featured low ash and sulfur contents,and high reactivity.However,the alkali metals tend to release into the flue gas in the form of gas phase during combustion,which leads to changes in physicochemical properties of the fly ash particles,such as reducing their ash melting point and enhancing their cohesiveness and corrosivity.Furthermore,it will cause serious problems such as fouling,slagging,and corrosion on the surface of the boilers and heat exchange equipments.The occurrence modes of alkali metals determine the release behavior,thus in-depth study of occurrence forms and contents of alkali metals would be beneficial to accurately predict their release form and content,and it would be meaningful for clean and efficient use of Zhundong coal.Zhundong coal with high Na content was employed in this paper.The surface distribution characteristics of different elements were analyzed by scanning electron microscopy-energy dispersive spectrometry (SEM-EDS).Simultaneously,the occurrence modes and contents were investigated by inductively coupled plasma optical emission spectrometer (ICP-OES).X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were employed for calculating surface elemental content and crystalline structure of alkali metals,respectively.The SEM-EDS results show that Ca and S are obviously enriched in same regions,indicating that CaSO4 is presented in these regions;no highlight region for Cl was observed during the scanning process,suggesting that Cl is evenly distributed in Zhundong coal;In addition,a small number of highlight regions in Na