| 题名 | 复合流化床低阶碎煤分级热解研究 |
| 作者 | 钟梅 |
| 学位类别 | 博士 |
| 答辩日期 | 2012-05-24 |
| 授予单位 | 中国科学院研究生院 |
| 导师 | 高士秋 |
| 关键词 | 分级热解 流化床 输送床 反应气氛 半焦 |
| 其他题名 | Fractionate pyrolysis of low-rank powder coal in integrated fluidized bed |
| 学位专业 | 工业催化 |
| 中文摘要 | 由褐煤、次烟煤构成的低阶煤占我国煤炭储量的50 %以上，富含挥发分，其高效综合利用具有重要意义。现有利用低阶煤的技术包括直接燃烧、气化和干馏。本研究有关低阶煤干馏制备半焦和热解油气，具体针对现有基于移动床的干馏技术不能处理小粒径(如，<15 mm)碎煤的问题，提出利用耦合流化床气化和输送床热解的新型煤分级热解工艺，以制备含C量大于给定值的小粒径半焦(兰炭)为目标，针对工艺反应器特点，通过冷态实验研究优化煤分级操作参数，通过热态基础实验获得制备合格半焦产品的热解条件，并最终建立和运行了复合流化床低阶煤分级热解模式装置，验证碎煤分级热解技术的可行性。因此，本论文的主要研究内容和获得的重要结果如下： 1. 复合流化床碎煤气力分级与输送特性。以粒径≤10 mm的碎煤颗粒为对象，以尽量减少底部粗颗粒组分中小于某个粒径的细颗粒的含量为目的，在集成流化床与输送床的连续进料冷态复合床分级实验装置中，考察了底部流化床表观气速、进料口位置、进料速率以及复合床结构及构件对颗粒分级的影响。这里，上部输送床的气速足够大，确保了底部流化床气流夹带的全部细颗粒被带到复合床顶部。结果表明：对于不同分割粒径的煤颗粒分级，都存在一个实现最高牛顿分级效率的最优分级气速。在复合床中自上部输送床段进料有利于颗粒的带出和提高1-4 mm颗粒的分级效率，且低颗粒进料速率时的分级效果更佳。提高底部颗粒溢流的出料口位置而延长颗粒在底部流化床中的停留时间有利于细颗粒的夹带，从而使分级效率增加。在流化床中设置竖直挡板抑制了颗粒的带出，降低了颗粒分级效率。 2. 低阶碎煤热解工艺条件及产品分析。在间歇与连续流化床中研究了热解温度、O2浓度、原煤粒径及在床内停留时间对小粒径低阶碎煤制备兰炭 (半焦)的工艺条件，采用XRD、SEM、BET、Raman等表征评价了半焦品质，用模拟蒸馏及GC-MS/MS对焦油的组分及品质进行了分析。结果表明：(1) 工艺条件：针对粒径1-13 mm的新疆准东不粘煤在间歇流化床中热解温度大于850 ℃，O2浓度及热解时间分别不低于3 vol.%和120 s时可制得固定碳含量高于82 wt.%的兰炭。连续流化床中反应温度高于750 ℃，停留时间为180 s时，热解所得半焦中固定碳含量均高于82 wt.%。(2)温度影响：温度升高，半焦的比表面积与晶面层间距d002减小，H、O元素含量下降，反应活性呈降低趋势。(3) 高温下链烃及酚类化合物含量降低，苯类与PAHs成分增多。 3. 复杂气氛低阶碎煤热解与产物分析。在连续进出料的流化床中研究了含有不同浓度O2、H2、CO、CO2、CH4的反应气氛对热解产物分配、气体和焦油组成及半焦氧化活性的影响。结果表明：(1) 产物分布：气体产率随过量空气系数(ER)升高而增加；ER小于0.1时，O2的加入使CO含量明显增加，半焦及焦油产率降低；无O2气氛中，H2与CO2的存在降低了焦油产率，而CO与CH4的存在促进了焦油生成，CH4的裂解析碳使半焦产率上升。在有O2气氛中，反应气氛对产物分布和气体组成的影响与热解气体、半焦和焦油的氧化速率及气化反应速率紧密相关。(2) 半焦特性：ER增加，半焦的比表面积先增加后降低，ER=0.064时半焦的比表面积及反应活性均最高。CO2的气化作用促进了微孔的生成，使半焦的比表面积快速增加，半焦的反应活性趋于最高。CO歧化与CH4热裂解产生的析碳可堵塞部分孔道，因此降低了半焦的比表面积。H2与CH4所产生的氢自由基能渗透到半焦内部，引起半焦化学结构的缩聚，影响其氧化反应活性。(3) 焦油组成：ER增加，PAHs先降低后升高。CH4促进了烷基萘与苯类的生成，CO则趋于抑制酚类化合物的裂解生成苯类的反应。 4. 低阶碎煤复合流化床分级热解模式实验。基于上述实验室研究结果，设计建立了煤处理量6.0 kg/h、高7.5 m的碎煤分级热解热态模式实验装置，通过调试运行，考察了煤分级热解效果随输送床温度、加料口位置的变化。结果表明：输送床进料时，小颗粒煤热解不充分，半焦中残留的挥发分较多，半焦产品的品质不符合兰炭要求。流化床进料时，半焦总产率下降，然而小颗粒煤优先被氧化，有效降低了大颗粒半焦的气化速率，从输送床与流化床收集到的半焦产品中的固定碳含量均高于82wt.%，符合兰炭的质量要求，从而证明了复合流化床分级热解制备碎煤兰炭的技术可行性，并基本建立了对应的工艺条件。 |
| 英文摘要 | Low-rank coal including lignite and sub-bituminous coal accounts for more than 50 % of China’s total coal reserve. The coal is rich in volatiles and it is significant to use it comprehensively. Currently the low-rank coal is commonly combusted or gasified directly or pyrolyzed / carbonized to produce char (called Lantan in China). This study is regarding the pyrolysis or carboniuzation of low-rank coal to produce char product and meanwhile get pyrolysis tar and gas. By addressing the fact that the existing fixed bed carbonizer for making char product is applicable only to coal feedstock with sizes over, for example, 15 mm, a pyrolysis process in an integrated fluidized bed consiting of fluidized bottom and a transport top column was proposed to treat the granular coal in sizes below 15 mm to produce the granular char product with required C content. According to the technical features of newly proposed process, this study investigated first the coal particle classification in a cold-model apparatus of the intergrated bed to optimize the fluidization parameters and bed structure, and then the reaction fundamentals of pyrolysis were studied in fluidized bed to determine the suitable reaction conditions for producing qualified char product. On the basis of these studies, a hot model experimental appratus simulating the proposed process was built and tested to verify the technology feasibility and clarify the process characteristics. 1. Pneumatic classification of coal particles in an experimental integrated bed. The pneumatic classification of coal particles in sizes of 0-10 mm was experimentally investigated in a transport column integrated a fluidized bottom. The classification aimed to reduce the frantion of fine particles below a given size in bottom product to the possible maxiamal degree, and for this the top transport bed had sufficiently high gas velocity to transport the elustrited fine particles from the fluidized bottom. The tested influential parameters thus included superficial gas velocity in the bottom fluidized bed, coal feeding position and rate, height of discharge exit and internal baffle mounted in the middle of the bottom fluidized bed. The Newton classification efficiency was employed to evaluate the classification performance. For any size expected to classify so that the fraction of the particles below this size is possiblly lowest in the particle product collected at the bottom, there was an optimal flusization gas velocity in bottom to achieve a highest classification efficiency. Feeding coal particles from the transport column and also at lower feeding rates facilitate the classification performances for classifying particles below 1mm to 4 mm. Elevating the overflow discharge position of particles in the bottom fluidized bed is equal to increase the residence time of particles in the fluidized bed, which facilitated fine particles to enter the top transport conlumn and resulted thus in higher classification efficiency. The addition of inner vertical plate into the partile bed of the fluidized bottom suppressed particle elutriation and decreased thus the particle classification efficiency for any classifying size tested. 2. Conditions and product analysis for low-rank coal pyrolysis. This chapter is devoted to investigating the necessary temperatures, O2 fraction in atmosphere and reaction time for making char product with fixed C above 82 wt.% from pyrolyzing low-rank powder coal in sizes of millimeters. The produced char was chsracterized using XRD, SEM, BET and Raman, and composition feature of tar was measured with simulating distillation and GC-MS/MS. The required conditions were first tested in batchwise pyrolysis of basketed coal by immersing it into fluidized sand particles at a preset temperature, finding that the produced char had the required carbon content above 82 wt.% if pyrolyzing the coal in sizes of 1-13 mm for at least 120 s at temperatures above 850 ℃and in atmospheres with O2 content not below 3 vol.%. The pyrolysis tests in a continuous fluidized bed designed according to the batch pyrolysis test clarified further that in order to ensure the C content of generated char to be over 82 wt.% the required conditions were reaction temperature above 750 ℃, an ER of 0.064 and pyrolysis time longer than 180 s. Increasing the temperature decreased the oxidation reactivity of the char, as a result of decreasing its specific surface area and H and O contents and reducing the interlayer spacing (d002) of the crystallite structures in the char. It was shown also that elevating the freeboard temperature of the fluidized bed pyrolyzer facilitated the formation of benzenes and PAHs but lowered that of chain hydrocarbons and phenols in the tar. 3. Pyrolysis behavor and product characterization in complex atmospheres. Coal pyrolysis with continuous coal feed and char discharge was conducted in a laboratoey fluidized bed reactor in N2-base atmospheres containing O2, H2, CO, CH4 and CO2 at varied contents. (1) Pyrlysis product distribution. In O2-contaning atmospheres, elevating the oxygen equivalent ratio (ER) increased the pyrolysis gas yield but decreased the yields of char and tar. When ER was below about 0.1, the presence of O2 promoted the formation of CO greatly. Analysis shown that in the case with O2 the influence of the atmospheric gas composition on the pyrolysis product distribution is closely related to the competitive oxidation among the added gas component, char and tar. Without the presence of O2, adding H2 and CO2 into the atmosphere decreased the tar yield, whereas the inclution of CO and CH4 resulted in higher tar yield. The addition of CH4 into pure N2 atmosphere also increased the char yield, which was attributed to the cabon deposition from CH4 cracking. (2) Char characterization. The specific surface area and char reactivity reached their maximal values at an excessive air ration (ER) of 0.064. Rather higher ER in the atmosphere destroyed pore structure, decreasing thus the surface area. The presence of CO2 in the atmosphere created new micropores or opened some closed pores so that the char had high surface area and reactivity. The carbon deposition via the CO disproportionation and CH4 cracking blocked some pores of the char and lowered thus the surface area and reactivity of the char. The char produced in the atmosphere with H2 and CH4 exhbited more condensed crystallite structure and thus low oxidation reactivity, which would be related to the penetration of hydrogen radicals inside the char particles. (3) Tar composition. The content of PAHs in the generated tar first decreased and then increased with elevating ER. Increasing the atmospheric CH4 promoted the production of alkyl-substituted naphthalenes and benzene, but the presence of CO inhibited the cracking of phenols that led to the formation of benzene. 4. Pilot experiment of the fractionate pyrolysis technology. A model pilot plant treating coal of 6 kg/h was built and comissined for experimental running to verify the technology feasibility of the proposed fractionate pyrolysis technology for treating low-rank powder coal. In this study the tested influential parameters via this pilot apparatus were temperature in transport column and feeding position. The results shown that when coal was fed from the top transport section the prolysis of elutriated small coal particles was insufficient so that the produced fine char at the plant top had still high volatile content and low carbon content. Feeding the coal into the fluidized bottom decreased the char yield because large part of fine coal was preferentially oxidized in the fluidized bed section so that the produced char had increased fraction of large pariticles. In this case the fixed carbon content in the char collected at both the top exit of transport column and the overflow of the fluidized bed were above 82 wt.%. The results thus demonstrated fully the technical feasibility of the intergrated fluidized bed fractionate pyrolysis for producing qualified char with required carbon contents from powder low-rank coal. The tests obtained also the basic operating parameters and bed structure for the technology. |
| 语种 | 中文 |
| 公开日期 | 2013-09-25 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.ipe.ac.cn/handle/122111/1830]  |
| 专题 | 过程工程研究所_研究所（批量导入） |
| 推荐引用方式 GB/T 7714 | 钟梅. 复合流化床低阶碎煤分级热解研究[D]. 中国科学院研究生院. 2012. |
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