0 引 言

褐煤在全球范围内储量大,分布广,但因高水分、高挥发分、低热值等特性而在传统工业利用中多受限制[1-2]。以褐煤为原料,低温热解(<600 ℃)生产的半焦具有高固定碳、高反应活性等特点,在工业生产中被广泛用作高炉喷吹燃料、冶金还原剂、电石生产原料等[3-5]。另外,半焦因其燃烧反应性及燃尽率高于褐煤,且燃烧排放的颗粒物、气体污染物等显著降低,已被提倡并推广用作清洁民用燃料[6-8]。因此,在全球能源需求和节能减排要求逐渐提高的背景下,褐煤低温热解过程及产出的半焦受广泛关注[9-10]。

多环芳烃(PAHs)是环境介质中普遍存在的一类有机污染物,因致癌、致畸变的潜在风险被广泛关注[11-13]。煤作为一种有机可燃沉积岩,不仅本身含有丰富的可溶PAHs,在热解和不完全燃烧等过程中也生成大量PAHs并释放[14-15]。目前有关煤热解过程生成PAHs的研究,主要聚焦在气态、液态产物中PAHs含量及组成特征方面。DONG等[16-17]采用PY-GC/MS在线分析煤热解释放PAHs随热解温度的变化,结果表明PAHs释放量随温度的升高先增后降,800 ℃热解时释放量最高。YAN等[18]和GAO等[19]对不同煤阶煤热解释放PAHs的研究显示,烟煤热解释放量高于褐煤。LIU等[20]模拟低变质煤层不完全燃烧排放PAHs,发现PAHs排放量随粒径增大而增大。CHEN等[21]、XU等[22]和ZHANG等[23]通过分析低阶煤低温热解生成的气体、焦油中不同类型化合物的分布以及煤/半焦结构特征,探讨了煤热解过程中有机质的演化途径,表明煤结构是影响其热解特性的最重要因素之一,且半焦结构特征对揭示热解机制有重要意义。然而,半焦的结构特征研究主要是对芳香度、碳类型及官能团特征等进行表征[24-26],可溶PAHs作为其有机质组成的一部分尚未见研究报道。而从煤热解过程中生成PAHs的角度来说,不仅包括气态或液态产物中的PAHs,残留于半焦中的可溶PAHs也是不能忽视的一部分。

另外,现有对原煤中可溶PAHs和热解释放PAHs的研究多数局限于US EPA规定的16种优控母体PAHs(16PAHs),涉及烷基取代多环芳烃(aPAHs)、含杂原子PAHs的研究较少[16-17,19,27]。然而,PAHs是否有烷基侧链、侧链尺寸及取代位置都会导致PAHs具有不同的生物活性和反应性[28]。研究表明,aPAHs比母体PAHs的生物毒性更大,环境持久性更强,更应引起重视[29-30]。以3个不同来源褐煤及其550 ℃热解半焦为样本,对其可溶PAHs(包括16PAH、aPAHs和其他芳香烃)的含量及组成特征进行分析和比较,提高对半焦中可溶PAHs特征的认识,也为褐煤低温热解过程中PAHs的生成研究提供补充数据。

1 材料与方法

1.1 材料

3个褐煤样品分别来自内蒙古上海庙矿(SHM-0)、印度尼西亚(YN-0)、蒙古国(WM-0)。3个褐煤分别在550 ℃下热解产出的半焦为SHM-1、YN-1和WM-1。所有样品的工业分析和元素分析结果见表1。

热解半焦的制备:管式炉热解试验装置主要包括管式电阻炉(炉膛加热区尺寸φ 60 mm×600 mm)、不锈钢高温反应管(尺寸φ 50 cm×600 cm),温度控制仪及热电偶,冷凝与净化装置等。将100 g煤样(<0.2 mm)放入反应管并置于管式炉,插入热电偶后用法兰隔绝空气;以5 ℃/min升温速率加热至550 ℃,保温15 min后自然冷却至室温。

1.2 萃取

样品中PAHs的萃取以及GC/MS测试方法,参考文献[31]。步骤如下:称取粒径小于0.2 mm的样品1 g,与适量无水硫酸钠、活化铜片混合后放入10 mL螺口玻璃管中,加入萘-d8标样以及10 mL二氯甲烷,超声萃取5 h。取1.5 mL萃取液离心,然后取1 mL上清液过针式过滤器。滤液中加入适当浓度内标(苊-d10、菲-d10、-d10、苝-d12)后用正己烷定容至1 mL,待仪器分析。

1.3 GC/MS分析

分析仪器为气相色谱/质谱/质谱(GC/MS/MS,Waters Xevo TQGC)联用仪。DB-5MS(30.00 m×0.25 mm×0.25 μm)色谱柱,进样口温度280 ℃,进样体积1 μL,载气为氦气,不分流进样。升温程序:初始70 ℃,保留1 min,15 ℃/min升温至180 ℃,保留2 min,10 ℃/min升温至230 ℃,保留0.5 min,5 ℃/min升温至250 ℃,保留2 min,8 ℃/min升温至300 ℃,保留5 min。质谱条件:EI源,70 eV,离子源温度250 ℃,50～550 aum,全扫模式。

通过Masslynx V4.2软件处理采集数据,采用保留时间并且结合NIST17质谱图库比对方法对PAHs进行定性分析,内标法进行定量。烷基取代多环芳烃的定量方法参考其母体多环芳烃。空白试验中未检测出PAHs。萘-d8的回收率在67%～78%。

2 结果与讨论

不同褐煤及其热解半焦的可溶PAHs质量分数见表2。3个褐煤PAHs总量在4 362～125 635 ng/g,其中SHM-0含量最高,YN-0含量最低。3个热解半焦PAHs总量在4 679～10 289 ng/g,YN-1含量最高,SHM-1含量最低。WM-0和WM-1的PAHs总量接近,分别为5 773和5 440 ng/g。3个褐煤中的PAHs除常规母体16PAHs及其对应的aPAHs外,部分样品还检出联苯、苯基萘、二苯并呋喃、二苯并噻吩等化合物如图1所示。

2.1 16PAHs含量和组成

3个褐煤SHM-0、YN-0和WM-0的母体16PAHs质量分数(∑16PAHs)分别为35 039、1 871、2 583 ng/g(表2)。SHM-0中检出丰富的PAHs,而YN-0和WM-0中PAHs种类较少,苊烯、苊在3个褐煤中均未检出。3个褐煤的16PAHs组成均以2～3环小分子为主,4环次之,分别占∑16PAHs的66.2%～86.9%、13.1%～28.7%(图2)。5～6环大分子PAHs含量在SHM-0的∑16PAHs中约占10.0%,而在YN-0和WM-0中均未检出。前人研究表明,煤中16PAHs含量随煤变质程度的升高而先增后降,烟煤含量最高,褐煤次之,高变质无烟煤含量最低[32-33]。另外,煤源因素如成煤母质、沉积环境等对煤中PAHs含量也有影响[34]。YN-0和WM-0的w(H)/w(C)、w(O)/w(C)相近而高于SHM-0(表1),指示SHM-0的芳构化程度和变质程度稍高于前二者。因此,SHM-0的16PAHs含量高于YN-0和WM-0的结果与文献结论一致。但SHM-0的16PAHs含量相对高出1个数量级的巨大差距,表明煤源因素对PAHs含量的影响比变质程度更大。

热解半焦SHM-1、YN-1、和WM-1的∑16PAHs质量分数分别为1 831、6 354和3 981 ng/g,与WANG等[35]报道的岩浆侵入热变质无烟煤质量分数水平(2 500～5 500 ng/g)几乎相近。3个半焦的16PAHs仍然以2～3环小分子为主,4环次之,分别占∑16PAHs的82.5%～97.2%和2.8%～17.5%。

5～6环大分子PAHs在3个热解半焦中均检出(图2),这与WANG等[35]、LAUMANN等[34]报道的天然焦和受岩浆侵入热变质无烟煤中未检出高环PAHs的结果一致。与原煤的Σ16PAHs含量相比,SHM-1含量显著降低,仅为SHM-0质量分数的5.2%左右;而YN-1和WN-1的Σ16PAHs含量高于原煤,分别约为YN-0和WM-0质量分数的339.6%和154.1%,其中单体萘质量分数的增大是∑16PAHs质量分数增大的主要贡献者。另外,尽管半焦和原煤的16PAHs组成均以小分子为主,但半焦中2～3环PAHs占比高于原煤,4环占比则低于原煤。半焦中PAHs组成向更小分子化过渡,这可能是因为小分子化合物在结构更加致密的半焦中更易被保护。不同褐煤热解产出半焦的16PAHs含量相对于原煤呈不同变化,揭示褐煤低温热解时PAHs的生成与释放过程可能不同。

2.2 aPAHs含量和组成

褐煤SHM-0、YN-0和WM-0的aPAHs总量(∑aPAHs)分别为61 757、17 23和2 842 ng/g。3个褐煤中的aPAHs均以烷基萘为主,占∑aPAHs的71.0%～97.8%,而SHM-0中还有丰富的烷基菲/蒽、烷基荧蒽/芘。SHM-0和WM-0的∑aPAHs含量均高于对应母体∑16PAHs含量,尤其SHM-0的∑aPAHs是∑16PAHs的176.3%。YN-0的∑aPAHs稍低于∑16PAHs(图3)。可知不同来源褐煤中均有aPAHs存在,且含量可能比母体16PAHs更高。

半焦SHM-1、YN-1和WM-1的∑aPAHs质量分数分别为1 806、2 737和744 ng/g,均与褐煤一致,以烷基萘为主要组成。3个热解半焦的∑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。与对应褐煤的∑aPAHs 含量相比,SHM-1和WM-1的含量显著降低,分别为SHM-0和WM-0含量的2.9%和26.2%。但YN-1的∑aPAHs含量是YN-0含量的158.8%,其中C1-萘和C2-萘含量的增大是主要贡献。由此可见,aPAHs不仅在褐煤中含量丰富,在热解半焦中也普遍存在,且半焦中aPAHs含量可能比原煤中更高。

另一方面,对于褐煤和半焦中都丰富存在的萘系物(C0-5-NAP),其分布在褐煤SHM-0和WM-0均呈“钟形”成岩源模式,在YN-0褐煤中则呈“斜坡形”热解源分布模式(图4)[36-37]。这表明不同来源褐煤中低环PAHs烷基取代物的分布模式并不相同,且与前人报道的低变质煤中PAHs烷基取代同系物以成岩源模式分布的结论并不完全一致[34-36]。半焦中的萘系物分布均呈热解源模式,这符合PAHs的烷基侧链在热解过程中易断裂的行为,以及母体PAHs相对于aPAHs热力学更稳定的特征。

2.3 煤热解可溶PAHs变化机理

煤中有机质在热解过程中主要发生裂解和缩合两类化学反应[38-39]。研究表明,煤热解时通过大分子结构的裂解和析出小分子化合物的缩合反应都会形成PAHs,其中600～1 000 ℃是PAHs形成和释放的主要温度区间[19]。低于600 ℃热解时,大分子结构中一些弱桥键(如亚甲基、醚键等)的断裂也会释放少量PAHs[16-17,19-20]。另外,煤中原有的可溶PAHs在热解过程中可能直接挥发释放,也可能裂解为小分子化合物而转化[16]。因此,煤热解后残留于半焦中的可溶PAHs可能有3个来源:① 原煤可溶PAHs挥发或转化后的残留。② 原煤大分子结构中一些弱桥键的断裂产生新的PAHs,这些PAHs有部分被困于半焦而未能释放。这类新生成的PAHs与原煤大分子结构中弱桥键的类型和含量,以及芳香核的尺寸密切相关。③ 析出的一些小分子化合物或自由基碎片可能相互缩合形成PAHs并再次沉积在固体表面。

LIN等[40]对印尼褐煤结构特征的研究表明,其大分子结构的缩合程度较低,芳香核主要是通过脂肪环或桥键连接的苯和萘结构,无3环或以上芳香碳存在。因此,推测YN-0褐煤在550 ℃低温热解过程中因桥键的断裂而释放出大量苯环或萘环结构的芳香核,不稳定的芳香核与裂解产生的自由基碎片结合形成大量单环或双环芳香化合物。这可能是YN-1半焦中PAHs尤其萘和烷基萘含量显著高于原煤的主要原因。

SHM-0褐煤PAHs含量显著高于YN-0和WM-0,且在SHM-0中检出另2种褐煤中未检出的5～6环高分子PAHs(如苯并荧蒽、苯并[ghi]苝等),从分子层面指示SHM-0褐煤的芳香度相对较高。再结合SHM-0中w(H)/w(C)和w(O)/w(C)相对较低,推测SHM-0比YN-0和WM-0的大分子结构相对致密,芳香核的环尺寸相对偏大。因此,SHM-0在热解过程中发生的大分子结构裂解相对于YN-0可能非常有限。而半焦SHM-1的PAHs含量相对原煤显著降低的主要原因,可能是原有可溶PAHs大量挥发释放。WM-0煤的w(H)/w(C)和w(O)/w(C)与YN-0接近,但大分子结构中连接芳香核的氧醚键等弱桥键可能相对偏少(WM-0的氧含量低于YN-0)。且半焦WM-1中萘及烷基萘含量显著高于原煤,这与YN-1半焦的PAHs含量增大主要是萘系物的贡献一致,反映WM-0的大分子芳香核组成可能与YN-0相似,以低环为主,并在热解过程中通过裂解反应生成大量小分子PAHs。综上所述,煤大分子结构特征的差异可能是褐煤热解半焦PAHs含量相对原煤变化不同的主要原因。在对煤热解尤其是低温热解时PAHs生成及煤结构演化特征研究中,不能只以释放气态PAHs或液态焦油中的芳香化合物为目标,还应考虑存留于半焦中的可溶PAHs。

另外,从清洁燃料角度来说,不仅要考虑热解过程释放PAHs造成的环境污染,还应考虑煤/焦在运输、储存等过程中通过碎屑、颗粒等形态向环境介质逸散而传输的PAHs[13]。低温热解过程能明确降低褐煤的水分、挥发分等,但可溶PAHs含量变化不确定。如本研究YN-1半焦的PAHs含量显著高于原煤,半焦带来的环境PAHs污染可能比原煤严重。因此,建议半焦可溶PAHs含量也应纳入半焦质量的考察,并通过调整热解工艺条件优化。

3 结 论

1)对3个不同褐煤及其550 ℃热解半焦中可溶PAHs进行测定。不同来源褐煤中可溶PAHs含量差异显著,但都表现出aPAHs含量丰富的特征。半焦中的PAHs以母体形式为主,但aPAHs仍普遍存在。

2)不同半焦的PAHs含量相对于原煤呈不同变化,SHM-1的PAHs含量低于原煤,YN-1含量则高于原煤,而WM-1含量与原煤相近。褐煤和半焦中PAHs均以萘及烷基萘为主,且YN-1和WM-1半焦中萘和烷基萘含量相对原煤显著增大,揭示热解过程中通过裂解反应有大量小分子PAHs生成。

3)煤/焦中可溶PAHs含量及组成受煤变质程度、煤源因素影响,也是其结构特征的一种体现。半焦中可溶PAHs含量及组成是煤热解过程中生成PAHs的不可忽视的一部分,可为PAHs生成机制及煤结构演化途径研究提供有效信息。

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

LU Mengqian1,LI Shan1,SHI Yungu2,ZHANG Hui3,LIANG Handong1

(1.State Key Laboratory of Coal Resoures and Safe Mining,China University of Mining and Technology(Beijing),Beijing 100083,China;2.College of Geoscience and Surveying Engineering,China University of Mining and Technology(Beijing),Beijing 100083,China;3.School of Mechanics and Architectural Engineering,China University of Mining and Technology(Beijing),Beijing 100083,China)

Abstract：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 4 362-125 635, 1 871-35 039 and 1 723-61 757 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 4 679-10 289, 1 831-6 354 and 744-2 737 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.

Key words：lignite;semi-coke;low temperature pyrolysis;PAHs;alkylated PAHs

中图分类号：X830

文献标志码：A

文章编号：1006-6772(2023)11-0058-08

收稿日期：2022-05-21;

责任编辑:张 鑫

DOI：10.13226/j.issn.1006-6772.22052101

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基金项目：国家自然科学基金资助项目(41772157);煤炭资源与安全开采国家重点实验室自主研究课题资助项目(SKLCRSM19ZZ03)

作者简介：卢梦仟(1997—),女,广西河池人,硕士研究生。E-mail:lmq1870091117@163.com

通讯作者:梁汉东(1959—),男,山东东平人,教授,博士生导师,博士。E-mail:HDL6688@vip.sina.com

引用格式：卢梦仟,李珊,石运固,等.褐煤及其低温热解半焦中可溶多环芳烃特征[J].洁净煤技术,2023,29(11):58-65.

LU Mengqian,LI Shan,SHI Yungu,et al.Characterization of soluble polycyclic aromatic hydrocarbons in lignite and its low-temperature pyrolysis semi-coke[J].Clean Coal Technology,2023,29(11):58-65.