0 引 言

新疆地区煤炭种类丰富,以有高挥发分的低阶煤为主,尤其是哈密地区煤挥发分普遍高,甚至超过50%(干燥无灰基),适宜通过热解提取油气组分,进一步转化为高附加值产品[1],半焦是燃料、气化原料及碳材料等[2],此分级分质利用模式是双碳背景下煤高效清洁转化的重要途径。而热解焦油中重质组分含量高、黏度大、易带尘[3],给分离带来困难。

由于煤热解遵从自由基反应机理[4],要提高焦油产率/品质,引入活性富氢组分来稳定煤热解自由基[5,7],或通过富氢物质与煤共热解强化供氢过程[8-10]实现。与煤相比,通用塑料H/C原子比高,是理想供氢原料[11,13]。WU等[14]发现平朔煤与30%聚苯乙烯(PS)混合热解时,甲基苯酚和三甲基萘的相对含量比理论计算值分别减少33%和80%,但甲苯提高14.8倍,因为加入PS有利于苯酚与烯烃反应生成芳烃,增强苯酚脱水反应,促进多环芳烃分解产生苯和单环芳烃。此外,焦油中苯乙烯提高3.41倍,归因于共热解过程促进了聚苯乙烯主链上Cβ—Car键的断裂。当高密度聚乙烯(HDPE)与褐煤(1∶1)在500 ℃热解时,气体及焦油产率分别比理论值高8.59%和21.94%,而焦炭产率则减少30.53%,因为HDPE热解产生的·H等自由基可稳定煤热解自由基形成挥发物,抑制焦炭生成;此外,焦油组分中烃类约提高8倍,这与HDPE自身的结构特征有关[15]。虽然塑料是外在氢源,对焦油生成有利,但其加入过多会延长挥发物停留时间,加剧二次裂解[16]。聚苯乙烯因含芳烃结构,更易分解,与煤共热解的活化能比聚丙烯和聚乙烯低约40 kJ/mol,此外,聚苯乙烯在热解过程中抑制了煤的流动性,引起氧之间的交联反应,导致CO2产率增加[17]。ZHANG等[18]发现低阶煤与高密度聚乙烯(HDPE)分层放置时,焦油H/C原子比煤单独热解高0.19,因为HDPE产生的自由基是氢供体,促进煤中芳香族化合物的裂解。此外,焦油重质组分比二者混合时约低10%,分析认为分层放置诱发了半焦基体内催化反应,促进大分子挥发物裂解。

综上,塑料热解挥发物不仅与煤热解挥发物产生相互作用,还吸附于半焦表面或孔道中,在活性位点/矿物质作用下进一步反应,其效果与二者接触方式密切相关。为揭示该作用机制,基于聚丙烯热解过程中可产生丰富活性小分子自由基的特点,选择聚丙烯于煤上方分层放置,探究不同比例淖毛湖煤(NMH)与聚丙烯(PP)共热解过程中的失重,基于协同效应分析获取促进挥发物释出效果最显著的煤塑比,利用TG-FTIR-GC/MS在线监测该比例下热解产物逸出特征,揭示二者共热解过程中挥发物相互作用机制。

1 试 验

1.1 试验原料

试验原料为新疆淖毛湖煤(NMH)和东莞市樟木头华创塑胶原料商行提供的聚丙烯(PP)。将二者粉碎、筛分至0.15～0.18 mm,于105 ℃干燥3 h后取出密封保存。工业分析及元素分析见表1。NMH的H/C高达0.99,PP的H/C比为1.97,约为NMH的2倍。

当NMH与PP按质量比9∶1、8∶2、7∶3和6∶4分层放置时,记为9NMH-1PP、8NMH-2PP、7NMH-3PP和6NMH-4PP。

1.2 试验方法

采用TG-MS联用仪(日立,日本)在线监测PP热失重过程中挥发性物质逸出行为,样品质量(10±0.2) mg,以100 mL/min的高纯氮气为载气,10 ℃/min升温速率由室温升至终温800 ℃。

通过珀金埃尔默的TG-FTIR-GC/MS分析NMH与PP共热解过程中气体与焦油组分逸出特征。该仪器由热重分析仪(TGA8000)、傅里叶红外光谱仪(Frontier)和GC/MS(Clarus680/SQ8T)组成。先将不同比例NMH和PP分层置于坩埚中(总质量(10±0.2) mg),下层为PP,中间用薄层石英棉隔开,使PP热解挥发物可随载气与NMH相互作用;接着以100 mL/min高纯N2吹扫整个热解体系20 min,确保空气被排尽,最后以10 ℃/min升温至终温800 ℃,测得NMH-PP失重曲线。采用在线联用监测时,将TGA温度升至40 ℃时热解气与载气一同泵入红外光谱仪中实时检测,泵速30 mL/min。根据失重特征,在选定数据采集温度下通过GC/MS分析产物成分。

条件与步骤同上,在TGA8000热重分析仪中探究不同升温速率(β=10、20和30 ℃/min)下NMH和PP及8NMH-2PP的失重行为。

1.3 协同效应分析

为量化NMH和PP共热解时相互作用程度[19],从热失重过程中质量变化角度分析二者协同效应,见式(1)和式(2):

Δw=wexp-wcal,

(1)

wcal=x1w1 x2w2,

(2)

式中,Δw为协同值;wexp、wcal分别为共热解样品的试验值和理论加和值,%;x1、x2分别为共热解中NMH和PP质量分数,%;w1、w2分别为NMH和PP单独热解的试验失质量,%。

根据Lambert-Beer定律,相同试验条件下,红外光谱测定的官能团吸收强度与物质的含量成正比,故气体产物FTIR光谱试验值和计算值[20]可用于描述相互作用,如式(3)所示:

ABScal=x1ABSexp x2ABSexp,

(3)

式中,ABScal、ABSexp分别为FTIR光谱的试验值与理论加和值。

1.4 动力学方法

采用Flynne-Walle-Ozawa(FWO)等转化率法计算NMH与PP共热解活化能[21]。根据Arrhenius方程,反应速率式(4)及变形式式(5)如下:

(4)

(5)

(6)

式中,α为原料转化率;t为时间;k(T)为反应速率常数;f(α)为反应模型函数;A为指前因子,min-1;E为表观活化能,kJ/mol;R为通用气体常数,8.314 J/(mol·K);T为热力学温度;β为升温速率,℃/min;m0、mt、m∞分别为初始、t时刻和最终时刻的样品质量,g。

(7)

(8)

式中,g(α)为f(α)积分;T0、T分别为初始温度和反应温度,K;P(u)为温度函数的指数积分形式。

基于式(8),FWO方程表达式如下:

(9)

其中,活化能E由ln β与1/T拟合求得。

2 结果与讨论

2.1 淖毛湖煤与聚丙烯的热失重特性与协同效应

图1给出了氮气气氛下NMH与PP的热失重曲线及PP热解挥发物TG-MS曲线。由图1(a)可知,NMH主要热解区间为340～600 ℃。PP从370 ℃开始分解,当温度高于500 ℃时几乎完全分解(图1(b)),说明二者主要热解温度区间存在较大范围重叠,为PP热解产物中富氢组分及活性自由基在共热解时参与到煤热解过程提供可能[22]。

由热解气体逸出曲线(图1(c))可知,PP热解过程中H2和·CH3初始释出温度略低,为394和377 ℃,于506和554 ℃释放完全。CH4和C2H6生成温度较高,分别升至407和398 ℃,释放完全的温度降至498和515 ℃,因为PP中含大量—CH3侧链,受热过程中易断裂,同时产生·H和·CH3,前者相互结合成H2,后者为C2H6。CH4由聚合物链随机断裂或二次反应生成[23]。综上,共热解过程中PP有稳定煤热解挥发物潜能。

为阐明NMH与PP共热解过程中挥发分的释放,探究2种物料不同比例下的热失重。图2(a)给出失质量的试验值与理论计算值,差异大,说明NMH与PP共热解过程存在相互作用。40～536 ℃,实际失质量小于理论失质量,说明PP抑制挥发物生成,该效应于约500 ℃最强(图2(b))。因为PP热解过程中随温度升高会软化到塑性状态,使挥发物释放减缓[24],且该抑制效应随PP增加而增强(图2(b)),这与PP在此过程中膨胀有关[25]。温度继续升高,PP开始分解[26],短时期挥发分大量释放,产生的·CH3和·H等自由基(图1(c))可以抑制NMH热解生成的分子碎片相互结合[27],因而500～536 ℃区间抑制作用减弱,促进作用增强。温度升至536 ℃以上,实际失质量大于计算值,因为PP释放的大量挥发物促使煤热解半焦形成多孔结构,暴露出更多活性位点,利于被吸附的PP热解挥发物中大分子碎片裂解[28],产生的自由基亦可作为氢供体,促进煤中化学键断裂[17],NMH与PP质量比8∶2(8NMH-2PPexp)时,在552 ℃时协同效应最显著,协同值Δw为4.23%。

2.2 热解挥发产物释出规律

2.2.1 气体逸出行为

采用In-situ FTIR分析NMH和PP单独热解及二者最优比例下(8NMH-2PPexp)共热解中气体释出规律,结果如图3所示。热解气体产物和官能团对应的FTIR波数见表2。可知,产物种类随温度升高而增加[29]。8NMH-2PPexp产物峰与NMH和PP单独热解相似,但强度不同,说明共热解存在相互作用[30],CH4及1 460 ～1 510 cm-1烃类峰强度明显增加,这可能与共热解过程中PP自身热解产物释放有关[31]。

与NMH相比,整个热解过程8NMH-2PPexp挥发物释出温度区间更集中,CO2与H2O变化明显,因为热解前期PP软化作用阻碍NMH中小分子释放。共热解挥发物释放结束温度比PP单独热解高,因为PP与NMH间相互作用稳定NMH挥发物释出的同时也延缓PP自身分解[32]。结合TG曲线(图1(a)、(b))可知,室温至约400 ℃时NMH失重主要为吸附分子如CO2、H2O等的脱除和易分解产物的逸出,PP此时基本无产物特征峰,这与其C—C、C—H和CC键分解温度高有关[33]。为进一步阐释热解及共热解过程反应机制,分析4种主要释出小分子气体在热解过程中吸光度计算值与试验值,如图4所示。

由图4(a)可知,CO2有2个主要逸出峰。低温处源自煤中吸附CO2、脂肪键、醚键和羰基等的断裂,高温处归因于煤中矿物质分解[34],共热解峰强度试验值低于理论值,可能因为PP裂解时生成的·CH3、·H等活性组分改变NMH挥发物中C和O的释放形式,抑制二者结合形成CO2[29]。CO生成分为2阶段(图4(b)):温度≤579 ℃,主要来自甲氧基、醛和羧基的分解;温度≥600 ℃,源于羟基、醚和含氧杂环的断裂[35]。第一温度区间共热解实际释放量明显高于理论值,因为PP热解产生大量富氢组分促进NMH中甲氧基、醛等含氧基团断键[36]。

由图4(c)可知,PP单独热解释放大量CH4,但共热解试验峰强度(8NMH-2PPexp)远低于理论值(8NMH-2PPcal),表现为强抑制,因为本应形成CH4(·CHx ·HCH4)[37]的·CH3、·H等参与稳定煤热解挥发物过程。H2O有2个逸出峰(图4(d)),约100 ℃峰强度弱,与煤中较低自由水含量有关[38]。400～600 ℃,与CO逸出峰相似,H2O峰也明显增强,表现为显著促进作用,因为PP热解挥发物中富氢组分与NMH中甲氧基、羟基及羧基等官能团发生加氢反应,使其脱除[39]。

2.2.2 挥发物组成

基于TGA分析,8NMH-2PPexp在552 ℃促进作用最佳(图2(b)),故采用气质联用仪识别样品在此温度下的热解挥发物(图5(a)),对其归类(图5(b)),分析PP与NMH共热解时协同效应Δw(图5(c))。NMH热解产物峰多,PP的相对少,但强度高,8NMH-2PPexp峰数量与PP的更接近。对于产物组成,NMH的以烷烃和CO2为主,相对含量分别为34.67%和14.84%(图5(b)),醚、醇、醛类含氧组分高于芳香化合物,这源于煤大分子结构中脂肪族及C—O桥键、醚键等含氧化合物裂解。PP产物主要为烯烃及烷烃类,碳数范围在C18～C35,出峰时间晚(图5(a)),与其自身含大量支链的结构特征和随机断链热解机制有关[40]。8NMH-2PPexp中CO2、烯烃及烷烃等产物强度与PP和NMH单独热解时差异大,且新生成2,3,3-三甲基-1-己烯等烯烃及3,7,11-三甲基-1-十二醇等醇类特征峰(图5(a))。保留时间≥45 min时出峰少,说明添加PP抑制重质组分生成。

与8NMH-2PPcal相比,共热解(8NMH-2PPexp)产物中烯烃和醇类相对含量显著增加(图5(b)),前者增幅30.58%(图5(c)),这与残留PP大分子碎片经二次反应与NMH热解挥发物相互作用形成含不同甲基侧链的烯烃有关。PP形成的短链分子与煤中羟基等含氧物质结合生成醇类[41],导致醇类增加16.18%。CO2和烷烃类分别减少8.89%和14.43%,酚及其他含氧等物质分别减少6.79%和7.12%。经分析,烷烃降低因为PP产生的自由基阻碍了NMH热解生成·CH3和·CH2等反应[42],PP热解挥发物也诱发挥发物与煤热解半焦的反应[43]。CO2减少说明PP和NMH相互作用影响CO转化[43],酚和醛类含量降低与PP热解挥发物中富氢组分促使NMH中甲氧基、羟基及羧基等含氧物质分解有关,与图4中CO和H2O红外变化一致[45]。综上,PP和NMH共热解过程中存在明显协同效应。

2.3 动力学分析

为进一步揭示共热解过程中相互作用,用FWO计算NMH、PP及8NMH-2PPexp不同转化率下动力学参数,见表3。R2均≥0.96,拟合效果好。NMH和PP单独热解,α=0.1～0.5时,二者活化能均升高,说明转化初期物料需吸收高能量断裂化学键,生成新物质[46]。α>0.5时,NMH活化能逐渐下降,PP的基本不变,归因于煤中挥发性物质随热解反应程度加深而减少和PP基本分解完全[22]。经对比,α<0.7时,共热解试验活化能值(8NMH-2PPexp)均低于理论值(8NMH-2PPcal),α>0.7则相反。因为转化前期主要为挥发物-挥发物之间的反应,PP是氢供体可及时稳定NMH热解自由基,使反应活化能降低;共热解后期,被吸附在半焦内的PP挥发物诱发的挥发物-半焦反应占主导,挥发物逸出需高能量。总体而言,8NMH-2PP平均活化能试验值比理论值低6.8 kJ/mol,说明PP与NMH之间相互作用降低了共热解活化能,促进挥发物释放。

3 结 论

1)NMH和PP以不同质量比分层放置时,挥发物释出作用规律存在差异,质量比8∶2(8NMH-2PPexp)时,促进效应最大,Δw为4.23%。

2)共热解过程中PP和NMH间相互作用促使氧元素以CO与H2O形式释出。烯烃及醇类物质比理论值分别增加30.58%和16.18%,CO2和烷烃类分别降低8.89%和14.43%,共热解产物较单独热解呈现轻质化趋势。

3)添加PP促进了NMH挥发物释放,共热解平均活化能试验值(8NMH-2PPexp)比理论值低(8NMH-2PPcal)6.8 kJ/mol。

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Synergetic effect during co-pyrolysis of low-rank coal and polypropylene

WANG Xueting1,ZHONG Mei1,DAI Zhenghua1,JIN Lijun1,3,ZHOU Qi2,YANG Xiao4

(1.School of Chemical Engineering and Technology Xinjiang University,State Key Laboratory of Chemistry and Utilization of Carbon Based Energy Resources,Xinjiang Key Laboratory of Coal Clean Conversion &Chemical Engineering,Urumqi 830046,China;2.National Energy Technology & Equipment Laboratory of Coal Utilization and Emission Control,CCTEG China Coal Research Institute,Beijing 100013,China;3.State Key Laboratory of Fine Chemicals,Institute of Coal Chemical Engineering,School of Chemical Engineering,Dalian University of Technology,Dalian 116024,China;4.Petro China Karamay Petrochemical Co.,Ltd.,Karamay 834003,China)

Abstract：The coal reserves in Xinjiang are rich,with a high proportion of low-rank coal. Among them, the coal in Hami region is generally characterized by high volatile content, which is suitable for the tar and gas by pyrolysis. However, there are many problems with pyrolysis tar, such as high content of heavy components, high viscosity and easy to carry dust. Therefore, how to reduce the content of heavy components in tar and improve the quality of tar is the key to realize high value utilization. Plastics and coal have similar pyrolysis intervals but the H/C atoms of plastics is much higher than that of coal. The hydrogen-rich components produced during pyrolysis could promote the primary pyrolysis reaction of coal or inhibit the cross-linking phenomenon during coal pyrolysis. Volatiles interaction exists during co-pyrolysis of coal and plastics, which is closely related to the contact mode of raw materials. The co-pyrolysis of polypropylene (PP) and Naomaohu coal (NMH) were investigated by combing TG, TG-MS and TG-FTIR-GC/MS. TG-MS results show main pyrolysis temperature range of the two rapid decomposition stages basically coincided, and the formation of ·CH3 during PP pyrolysis. When PP layer is placed on top of NMH layer with different mass ratios, the weight loss characteristics and the volatiles evolution of NMH exhibit an obvious difference. The strongest synergy appears on the mass ratio of 8∶2 for NMH and PP during the co-pyrolysis process. TG-FTIR-GC/MS experimental results reveal that the synergistic effect of NMH and PP changes the release form of oxygen because the radicals from PP pyrolysis facilitates the cleavage of covalent bonds of NMH, leading to the increase of CO and H2O yields but the decrease of CO2 yield. Compared with the theoretical calculated values, the relative content of alkene and alcohols increases by 30.58% and 16.18%, while those for CO2 and alkanes decreases by 8.89% and 14.43%, respectively. The activation energy of co-pyrolysis calculated by Flynne-Walle-Ozawa is 6.8 kJ/mol lower than the theoretical value, confirming that PP is conducive to the release of NMH volatiles.

Key words：Naomaohu coal and polypropylene;co-pyrolysis;synergetic effect;activation energy

中图分类号：TQ530.2

文献标志码：A

文章编号：1006-6772(2024)04-0187-09

收稿日期：2022-12-30;责任编辑：戴春雷

DOI：10.13226/j.issn.1006-6772.22123001

基金项目：新疆维吾尔自治区杰出人才资助项目(2020Q001);上海合作组织科技伙伴计划及国际科技合作计划资助项目(2020E01018);国家自然科学基金新疆联合基金青年培育资助项目(U1903130)

作者简介：王雪婷(1998—),女,河南开封人,硕士研究生。E-mail:1837821331@qq.com

通讯作者：钟 梅(1981—),女,四川中江人,教授,博士。E-mail:zhongmei0504@126.com

引用格式：王雪婷,钟梅,代正华,等.低阶煤与聚丙烯共热解协同效应分析[J].洁净煤技术,2024,30(4):187-195.
WANG Xueting,ZHONG Mei,DAI Zhenghua,et al.Synergetic effect during co-pyrolysis of low-rank coal and polypropylene[J].Clean Coal Technology,2024,30(4):187-195.