【作者】 李美芬；

【导师】 曾凡桂；

【作者基本信息】 太原理工大学 ， 化学工程与技术， 2009， 博士

【摘要】 煤成气藏中气体的化合物组成以及煤的人工热降解产物的化合物组成均表明,甲烷、低碳烃类(C2-C4)、二氧化碳、水和氢气是煤化作用过程中的主要气态产物。在过去的几十年中,对这些气态产物的生成动力学已进行了大量的研究,并建立了三种脱挥发分模型,但由于煤结构的复杂性,对这些产物的生成机制还有待于深入系统的理解。由于系统的煤化作用理论必须建立在深刻理解煤热解产物生成机制的基础上,因此从分子水平上了解气态产物的生成机制以及热解产物生成动力学与煤结构之间的关系,是目前煤地质学、煤化学以及有机地球化学的重要研究内容之一。低煤级煤由于处于煤化作用的低级阶段,对低煤级煤热解特征的研究有利于对整个煤化作用阶段的了解。由沥青化作用所导致的第一次煤化作用跃变引起了煤的物理化学性质突变性变化,其实质与机制有待于从分子水平上来揭示,因此本文重点对镜质组反射率分别为0.33%、0.47%、0.51%、0.62%、0.65%、0.70%和0.81%的霍林河褐煤(HLH)、义马褐煤(YM)、神华长焰煤(SH)、兖州(YZ)、铁法(TF)及平朔气煤(PS)和大同不粘煤(DT)的大分子结构演化特征及其对热解气态产物生成的影响进行了系统的分析,期望能深入理解第一次煤化作用跃变的分子机制。同时,由于低煤级煤含有较为丰富的各种侧链和官能团结构,从煤分子工程的角度来说对其热解特征与机制的系统理解也是发展与深化煤分子工程的基础。本文应用傅立叶变换红外光谱(FTIR)法分析煤的大分子结构特征及其各种官能团的分布,而采用X-射线衍射(XRD)对煤的聚集态结构进行表征,同时,对部分煤还进行了核磁共振(NMR)实验和拉曼光谱(RAMAN)实验。在对煤结构进行系统分析的基础上,对煤进行了热重质谱联用(TG/MS)实验,研究了煤的热解特征和热解产物的生成动力学特征。主要结论如下:1.对煤的FTIR结构参数分析表明,A(1703+1745)/A1618、A(1703+1745)/A(2800-3000)、A1745/A1618以及Har/Hal均在第一次煤化作用跃变附近(镜质组反射率0.60%)最小;而A1618/A(1703+1618)在第一次煤化作用跃变附近最大。这就从分子水平上揭示了第一次煤化作用跃变的特征和实质,为深入认识第一次煤化作用跃变打下了基础。2.对不同方法得到的结构参数之间的相关关系进行分析发现,表征芳香层片的延展度La与表征脂肪链长程度的参数A2924/A2964线性正相关,而芳香层片的层间距d002与红外参数A(700-900)/A1460负相关。这说明在低煤级阶段脂肪侧链具有定向性,表现在煤微晶结构的延展度La不仅与芳香结构有关,而且与表征脂肪链长程度的结构参数有关。3.对煤的热解特征及热解产物的生成特征研究发现,煤热解的最大失重速率峰温在第一次煤化作用跃变附近最低,且热解失重率在跃变点附近发生转折,主热解阶段(300-600℃)的活化能在跃变点附近最大,热解CH4、CH3、C2-C4、C6H6和H2的生成峰温均在第一次煤化作用跃变附近最小,而CO2和H2O的开始生成温度在第一次煤化作用跃变附近最高。说明煤热解特征参数及热解产物的生成特征参数、动力学参数均在第一次煤化作用跃变附近发生了转折。这进一步证明在第一次煤化作用跃变点附近煤大分子结构发生了转折性变化,为全面系统认识第一次煤化作用跃变打下了基础。4.对热解CH4、CH3、C2-C4、C6H6以及H2的生成速率曲线进行分析发现,它们的生成不是一个反应的结果,而应该是多种反应综合作用的体现,本文用分峰拟合的方法对各生成速率曲线进行了分峰拟合,并结合其生成和动力学特征对各基元反应的生成机制进行了分析,结果如下:(1) CH4、C3H7及H2的生成为5个反应的结果,CH3的生成为4个反应的结果,C6H6的生成为4-5个反应的结果。(2) CH4生成机制:第一基元反应,对于HLH和YM煤,存在两种反应,一是吸附甲烷的脱附,二是甲氧基热解生成甲烷;对于其它5种煤,是以吸附或固溶体态存在的甲烷的析出;第二基元反应,含氧官能团脂肪侧链热解生成甲烷和乙基β位断裂产生甲基,进而生成甲烷;第三基元反应,主要发生的是长链烷烃类的二次热解生成甲基,并与甲苯热解生成的氢自由基结合形成甲烷,同时,亚甲基桥键断裂产生甲基和氢化芳香环的脱甲基反应也是该阶段的主要反应;第四基元反应,甲苯热解生成甲烷和脂肪链的环化和芳构化生成甲烷;第五基元反应为芳构化作用的结果。(3) C6H6生成机制:第一基元反应,可能是苯环上的甲氧基热解生成甲烷,同时导致苯的生成;第二基元反应,是大分子结构裂解生成苯;第三基元反应,为亚甲基桥键断裂和氢化芳香环的脱甲基反应,产生甲烷的同时生成苯;第四反应是链烷烃环化或环烷烃芳构化作用的结果;第五基元反应,是缩聚反应导致苯的生成。(4) CO2生成机制:低温时CO2的生成是羧基热解产生CO2和甲氧基热解生成甲烷和CO2两种反应的综合作用,较高温度时CO2的生成可能与煤中的含氧杂环有关,而700℃以后CO2的逸出与矿物质(主要是碳酸盐物质)的分解有关。(5) H2生成机制,第一基元反应,对于HLH, YM和DT煤而言,此反应主要是链烷烃环化脱氢,而对于TF,PS,YZ和SH煤而言,此反应H2的生成是两个反应的结果,一是甲苯热解生成苄自由基和氢自由基,氢自由基之间结合进而形成H2,二是长链脂肪烃二次热解会生成较为短链的脂肪类自由基,这些较短的脂肪类自由基进一步热解生成氢自由基,进而形成H2;第二基元反应,对于HLH, YM和DT煤而言,主要是环烷烃的芳构化形成H2,对于TF, PS, YZ和SH煤,主要是氢化芳香环脱氢;第三基元反应,主要发生的是芳环之间的缩聚产生H2;第四基元反应,此时各种热解气体基本结束生成,主要是H2大量生成的过程,是芳香体系脱氢的结果;第五基元反应,此时其它气体均已结束生成,只有H2仍在生成,是芳香体系增大的过程。5.通过低煤级煤结构演化及热解气态产物生成特征与动力学的研究,可以发现第一次煤化作用跃变在分子水平上的体现为含氧等杂原子官能团的脱除与缩聚反应相互之间竞争的结果。在镜质组反射率0.60%之前,以脱除杂原子官能团占优势,而在0.60%左右,则缩聚反应达到了第一次高峰,而随着煤化作用的进一步进行,则进入了烷基侧链的脱除阶段,缩聚反应被抑制,因此可以认为第一次煤化作用跃变的发生实质上是缩聚反应占优势的结果,但是这种缩聚反应不是芳香体系的缩聚,而是残余的含氧等杂原子官能团之间的相互作用,形成新的官能团,导致分子体系的增大。更多还原

【Abstract】 The gaseous compound composition of coal-bed gas reservior showed that CH4, C2-C4, CO2, H2O and H2 were the main gasous products of the coalification process. The generation kinetics of these gaseous products had been studied greatly for the past several decades, based on which, three devolatilisation model had been established. But as the complexity of coal structure, further comprehending of the generation mechanism of these products still needed. As deeply understanding of sysmatic coalification theory must taken profound understanding of the mechanism of pyrolysis products as the basis, understanding mechanism of gaseous products and the relation of its kinetics and coal structure is one of the important research contents in coal geology,coal chemistry and organic geochemistry.As low rank coal situated at the lower stage of coalification, studying on the pyrolysis characteristics of low rank coal was benificial for understanding the whole coalification stage. The first coalification jump(TFCJ), which caused by bituminization, induced abrupt change of physical chemistry properties, but the essence and mechanism of TFCJ needs to be revealed on molecular scale, so HLH lignite, YM lignite, SH long flame coal, YZ, TF and PS gas coal and DT non-caking coal whose ROm ax were 0.33%, 0.47%, 0.51%, 0.62%, 0.65%, 0.70% and 0.81% were chosen as the research emphasis of this paper, the structural characteristics and its effect on the generation of gaseous products were analyzed systematically, and the molecular mechanism of TFCJ was expected to be deepen understood. Meanwhile, Because low rank coal has abundant side chains and various functional groups, systematically understanding of pyrolysis characteristics and mechanism on coal molecular project scale is the basis for developing and deepening coal molecular project.In this paper, the FTIR experiment was used to analyze coal macromolecular structure and various functional groups characteristics, and the XRD was used to obtain the aggregate structure. Meanwhile, NMR and RAMAN experiments were done for part of the samples. TG/MS experiment were done for the whole samples, and the generation kinetics of the pyrolysis products were analyzed. The main conclusions were as follows:1. Analysis of the FTIR structural parameters demonstrated that the relation of A(1703+1745)/A1618, A(1703+1745)/A(2800-3000), A1745/A1618 and Har/Hal with coal rank showed the minumin value at TFCJ, but the relation of A1618/A(1703+1618) and was opposite, the value of which showed the maximum at TFCJ. This revealed the characteristics and essence of the coalification jump and provided foundation for deeply realization of TFCJ.2. Analysis on the correlation between the different structural parameters showed that, La was positively linear related with A2924/A2964 and d002 was negative related with A(700-900)/A1460. The results demonstrated that La related not only with the aromatics, but also with the aliphatics, and the aliphatics in low rank coals presented directionality.3. Pyrolysis characteristics and generation kinetics of the gaseous products demonstrated that the maximum weight loss rate temperature was the lowest near TFCJ; the weight loss rate showed a turn near TFCJ; and the activation energy of the main pyrolysis stage(300-600℃) was the maximum near TFCJ; and the generation rate peak temperatures of CH4, CH3, C2-C4, C6H6 and H2 showed the minimum value and the initial generation temperatures of CO2 and H2O were the maximum value near TFCJ. So the pyrolysis characterisation and generation kinetic parameters all present transition near TFCJ, and this had proven further that transition change of the coal macromolecular structure near TFCJ, and this result provided foundation for fully and systematically realisation of TFCJ.4. The evolution curves of CH4, CH3, C3H6 ,C3H7, CO2 and C6H6 showed that they are not the results of one reaction but multi-reactions. Curve-fitting of these curves were done and the generation kinetics characteristics were analyzed, the mechanism of elementary reaction were analyzed. The main results are as below:(1) CH4, C3H7 and H2 is the result of 5 reactions, CH3 is the result of 4 reactions, and C6H6 is the result of 4-5 reactions.(2) the mechanism of CH4, reaction type 1: it has two reactions for HLH and YM coals, the first is the desorption of adsorption CH4 and the second is the decomposition of–OCH3. Desorption of methane occurred as adsorption and solid solution is the main reaction for the other 5 coals; Reaction type 2: it’s the results of two reactions, one is decomposition of aliphatic side chain with oxygen-containing functional group, the other is decomposition ofβ-site of methylene; Reaction type 3: this reaction is formed through CH3 generated from secondary decomposition of long-chain aliphatics reacted with H generated from decomposition of toluene, meanwhile, rupture of Ar-C-C-Ar and hydroaromatic are also the main origin of this reaction; Reaction type 4: methane in this reaction is from decomposition of toluene and cyclization and aromatisation of aliphatic chain; Reaction type 5: methane in this reaction is the result of aromatisation.(3) the mechanism of benzene, Reaction type 1: decomposition of toluene; Reaction type 2: decomposition of macromolecular structure; Reaction type 3: rupture of Ar-C-C-Ar and hydroaromatic structure;Reaction type 4: cyclization of alkane and aromatisation of cycloalkane; Reaction type 5: condensation.(4) the mechanism of CO2: at low temperature, CO2 is generated from decomposition of–COOH and–OCH3,with the increase of temperature, CO2 is related with oxygen-heterocyclic, when the temperature higher than 700℃, CO2 is mainly from carbonate.(5) the mechanism of H2: Reaction type 1: for HLH, YM and DT coal, H2 is the result of cyclization of alkane, for TF,PS,YZ and SH coals, there are two reactions, one is from decomposition of toluene, the other is decomposition of aliphatic free adicals generated from secondary decomposition of long chain aliphatics; Reaction type 2: for HLH, YM and DT coal, H2 is mainly from aromatisation of cycloalkane, for TF, PS, YZ and SH coal, H2 is mainly from dehydrogenation of hydroaromatic cycle; Reaction type 3: H2 is from condensation of aromatic ring; Reaction type 4 and Reaction type 5: H2 is the result of condensation of aromatics.5. by the research of the structure evolution of low rank coal and the generation characteristic and kinetics of gaseous products during pyrolysis, the molecular scale of TFCJ reflects that it was the result of the competition between the decomposition and polymerization of hetero-atomic functional groups was found. Decomposition of heteroatom functional groups prevail before 0.60% of RmO ax, and polymerization reached the first peak near 0.60% of RO max, then with the progress of the coalification, aliphaitics side chains began to remove and polymerization was inhibited. So the essence of TFCJ was the result of the advantage of polymerization. However, this polymerization was not the polymerization of aromatics but the interaction between the rest oxygen-containing functional groups, then new functional groups formed and caused the increase of the molecular system.更多还原