褐煤及其低温热解半焦中可溶多环芳烃特征

Characterization of soluble polycyclic aromatic hydrocarbons in ligniteand its low-temperature pyrolysis semi-coke

低温热解提质是实现褐煤清洁高效利用的重要途径之一。半焦作为提质后的固体产物在工业和民用清洁燃料方面得到广泛应用。以来自中国内蒙古(SHM)、印度尼西亚(YN)和蒙古国(WM)的3个褐煤及其550℃下热解半焦为样本,采用溶剂萃取结合GC/MS分析的方法研究其可溶多环芳烃(PAHs)含量及组成特征。结果显示,3个褐煤的PAHs总量(∑PAHs)、US EPA规定的优控16种PAHs质量分数(∑16PAHs)及对应烷基取代PAHs质量分数(∑aPAHs)分别在4 362~125 635、1 871~35 039和1 723~61 757 ng/g,揭示了不同来源褐煤的PAHs含量差异显著且aPAHs含量丰富的特征。3个褐煤对应热解半焦的∑PAHs、∑16PAHs和∑aPAHs质量分数分别在4 679~10 289、1 831~6 354和744~2 737 ng/g。不同来源褐煤产出半焦的PAHs含量变化不同,SHM半焦的PAHs含量显著低于原煤,YN半焦的PAHs含量高于原煤,WM半焦中PAHs总量与其原煤相近,但其16PAHs含量较原煤相比有所增加。半焦的∑aPAHs含量均低于对应母体∑16PAHs含量,表明存留于半焦的PAHs以热力学更稳定的母体形式为主。褐煤和半焦的PAHs组成均以2~3环为主,尤其YN半焦中萘及烷基萘含量显著高于原煤,是其PAHs总量增大的主要原因。YN褐煤热解过程中发生了煤分子内的激烈裂解反应并有大量小分子PAHs生成。不同褐煤及其半焦的可溶PAHs变化不同,不仅从分子层面反映不同煤在热解过程中的物理化学反应不同,也反映了煤大分子结构特征差异。

Low-temperature pyrolysis is one of the important ways to achieve clean and efficient utilization of lignite.Semi-coke as the solid product of pyrolysis is widely used in both industrial and civil clean fuels.In this study,three lignites from Inner Mongolia(SHM),Indonesia(YN)and Mongolia(WM),and their pyrolyzed semi-coke at 550℃were used as samples to investigate the concentration and composition changes of soluble polycyclic aromatic hydrocarbons(PAHs)by extraction combined with GC/MS analysis.The results showed that the concentrations of total PAHs(∑PAHs),16 US EPA priority PAHs(∑16PAHs)and their alkylated derivatives(∑aPAHs)of the three lignites were in the ranges of 4362-125635,1871-35039 and 1723-61757 ng/g,respectively.It revealed that the concentration of PAHs in lignite from different sources was significantly different,and aPAHs were abundant in all lignite.The concentrations of∑PAHs,∑16PAHs and∑aPAHs the three semi-cokes were in the ranges of 4679-10289,1831-6354 and 744-2737 ng/g,respectively.The relative abundance of PAHs in the semi-cokes with respect to the corresponding lignites was different.SHM semi-coke had a lower concentration of PAHs than SHM lignite,YN semi-coke had a higher concentration than YN lignite,and the PAHs content of WM semi-coke were similar to that of raw coal,but the parent 16PAHs content of WM semi-coke were relatively higher than that of WM raw coal.The∑aPAHs concentrations of three semi-cokes were all lower than the corresponding parent∑16PAHs concentration,indicating that the PAHs retained in semi-coke were dominated by the more thermodynamically stable parent form.The composition of PAHs in both lignite and semi-coke was dominated by 2-3 rings low molecular weight PAHs.Especially the concentration of naphthalene and alkylated naphthalenes in YN semi-coke was the main reason for the increase of the total concentration of PAHs.It indicated that cleavage reaction occurred within the coal molecule during pyrolysis and a large number of small molecules PAHs were generated.The different changes of soluble PAHs in different lignites and their semi-cokes not only reveal the different physical and chemical reactions of different coals during pyrolysis from the molecular level,but also reflect the differences in the macromolecular structural characteristics of coal.