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煤热解挥发分主要气体析出模型与Aspen Plus模拟应用
其他题名Major gases of Volatiles Evolution Model of Coal Pyrolysis and Its Application in Aspen Plus Simulation
蔡连国
学位类型博士
导师许光文
2012-05-24
学位授予单位中国科学院研究生院
学位专业化学工程
关键词煤热解 模型 Aspen Plus 循环流化床 过程模拟
摘要煤热解是燃烧、气化等主要煤热转化过程中首先发生的反应,也是可独立运行联产焦油、半焦/焦炭和热解气的重要转化技术。热解动力学是设计合适热解反应器的关键基础。煤热解反应与煤结构有直接关系,关联煤热解产物的产率与煤结构是获得煤种对热解产物分布影响的途径之一,并可建立热解模型,为热解工艺开发和技术经济评价提供有力的工具。本文对属于褐煤、次烟煤、烟煤和无烟煤的五种煤样,采用热天平和微型流化床反应器分别考察了程序升温挥发分和快速升温煤热解挥发分主要气体析出特性,获得了相应的动力学参数;利用13C核磁共振(13C NMR)和X射线光电子能谱(XPS)对五种煤结构进行表征,得到了煤中不同类型碳的含量,结合快速热解挥发分主要气体产率与不同类型碳含量的关系,建立了热解挥发分主要气体析出动力学模型。采用Aspen Plus软件对循环流化床(CFB)锅炉燃烧耦合热解系统进行了过程模拟和初步的经济评价。本论文研究获得了以下主要结果: (1) 煤热解挥发分主要气体析出动力学研究。程序升温实验揭示了热解挥发分主要气体开始析出的顺序依次为CO2、CO、CH4和H2,而等温热解实验也证明了CO2和CO的析出先于CH4和H2。针对考察的五种煤,程序升温热解的挥发分析出可分为三个阶段,第一阶段的活化能最低,为3.17~8.41 kJ/mol;第二阶段的活化能最高,为29.15~118.77 kJ/mol;第三阶段的活化能比第二阶段略有降低,为26.67~99.66 kJ/mol。而五种煤的微型流化床等温快速热解的挥发分主要气体总量析出活化能为17~35 kJ/mol,小于程序升温热解的挥发分气体总量析出活化能,反映了煤快速升温过程中各种脱挥发分反应的综合效果。同时,CO2和CO析出的反应级数为0.97~1.2,与1接近;CH4析出的反应级数为0.82~1.27,与1偏差稍大;而H2析出的反应级数为1.42~1.7,与1偏差较大,反映了CO2和CO与CH4和H2两类气体在生成机制上存在较大的差异。 (2) 煤热解机理及热解模型研究。通过对煤结构的表征得到了煤中不同类型的碳含量,关联了煤热解挥发分主要气体产率与煤中不同类型碳含量的关系,确定了CO2和CO的前驱体为煤中羧基碳和羰基碳,CH4和H2的前驱体为煤中甲基类组分。假定气体的生成速率与煤中剩余的该气体的前驱体成正比,建立了考虑煤结构的热解挥发分主要气体析出动力学模型,并对煤热解挥发分主要气体析出的转化率随时间的变化进行了计算。模型计算结果表明,CO2、CO和CH4的转化率计算值与实验值符合较好,说明一级反应可近似表达这三种气体的析出过程;H2的转化率计算值在反应刚开始时与实验值比较接近,但随着时间的延长,计算值开始大于实验值,说明H2的析出与一级反应模型有一定偏离,这与动力学实验得到的反应级数的变化情况一致,可能是由于热解过程中H2的生成机理比较复杂所致。 (3) 循环流化床燃烧耦合煤干燥和热解技术过程模拟。采用Aspen Plus软件对燃烧褐煤的1025 t/h CFB锅炉燃烧耦合煤干燥和热解系统进行了过程模拟。干燥煤量占总煤量的35%,采用锅炉烟气干燥,烟气温度从149 ℃降低到105 ℃时,煤含水量可从34.7%(收到基)干燥到11.0%(空干基),相应的锅炉热效率可提高2.4个百分点。耦合热解后的系统在全部煤先热解然后半焦燃烧的条件下,焦油和煤气作为产品输出,与CFB锅炉燃烧相比能源效率提高2.5个百分点。单纯CFB锅炉的有效能利用效率为47.3%,耦合上述热解系统后有效能利用效率增高至56.4%。如果35%的总煤量先干燥后热解,生成的半焦与剩余65%的煤去锅炉燃烧,能源效率为95.3%,有效能效率为50.2%,最适合对已有1025 t/h CFB锅炉的改造。模拟计算结果表明燃烧褐煤的CFB锅炉耦合干燥器和热解器能显著提高系统的综合能效。 (4) 循环流化床燃烧耦合热解技术的经济评价。对燃烧褐煤的1025 t/h CFB锅炉耦合煤处理量100 t/h的热解系统进行经济评价,得到的静态投资回收期为3.9年,小于煤炭行业基准投资回收期;投资利润率为29.4%,大于煤炭行业的平均投资收益率;基准折现率取12%时,与1025 t/h CFB锅炉耦合煤处理量100 t/h热解的联产系统的净现值约为5.2亿元。这些结果表明,CFB锅炉燃烧耦合热解技术具有良好的经济性。
其他摘要Coal pyrolysis is the reaction occurring first in the process of coal combustion and gasification, and can be also an independent coal conversion process to coproduce tar, char and gas through using the coal volatiles. Thus, the kinetics of coal pyrolysis represents the indispensable basis for designing coal conversion reactors. Coal pyrolysis reaction is related closely to coal structure, the clarification of the relationship between pyrolysis product yields and coal structure provides not only a good understanding the effect of coal type on pyrolysis but also an effective way to model the coal pyrolsyis. This study characterized the volatile evolution behaviors of five different coals belonging to lignite, subbituminous coal, bituminous coal and anthracite via both programmed heating in TG and isothermal rapid heating realized in micro fluidized bed (MFB). The corresponding kinetic parameters for volatile evolution were obtained. By correlating the evolved volatile yield and the coal structure determined by 13C Nuclear Magnetic Resonance (13C NMR) and X-ray photoelectron spectroscopy (XPS), the coal pyrolysis volatiles evolution model was developed. Process simulation using Aspen Plus and preliminary economic evaluation were conducted for circulating fluidized bed (CFB) boiler coupled with coal pyrolysis for co-production. (1) Volatile Evolution Kinetics in Coal Pyrolysis. The pyrolysis tests via programmed heating demonstrated that the release sequence of volatile gases was CO2, CO, CH4 and H2 from the first to the last, whereas the isothermal pyrolysis in MFBA showed also that the release of CO2 and CO was preceded the release CH4 and H2. For all the tested five coals, the evolution of volatiles in the pyrolysis via programmed heating can be divided into three stages from low to high temperatures. The activation energies for the first to the third stages were respectively 3.17~8.41, 29.15~118.77 and 26.67~99.66 kJ/mol, indicating that the activation energy was lowest for the first stage and highest for the second stage. For the isothermal fast pyrolysis in MFB, the estimated activation energes for the five coal samples was 17~35 kJ/mol, obviously lower than that for the pyrolysis via programmed heating. The reaction order of CO2 and CO release was found to be 0.97~1.2 and close to 1, while that of CH4 release was 0.82~1.27 with slightly large deviation from 1. The reaction order of H2 release was 1.42~1.7, obviously above 1. These results revealed essentially that the mechanism to form CO2 and CO is much different from forming CH4 and H2. (2) Coal Pyrolysis Mechanism and Model. Coal structure was analyzed with NMR and XPS to determine the contents of different types of carbons, and the release amount of different gas species was correlated with the carbon contents in the related functional groups. The precursors of CO2 and CO were the carboxyl carbon and carbonyl carbon, respectively. Both CH4 and H2 were found to have the same precursor of methyl functional group. On the basis of these, a coal volatile evolution model based on first order reaction was proposed by supposing that the evolution rate of a kind of volatile gas is proportional to the remained precursors that contribute to form the gas. The variation of gas formation rate with time was calculated using the model, showing that the calculated formation rates for CO2, CO and CH4 agreed well with experimental measurements and realing in fact that the release of these gases is subject to the proposed first order reaction model. The H2 release was well described at the beginning, but with the prolonging of reaction time the calculated formation rate was higher than experiment data. This was verified the kinetic study result that the reaction order for H2 evolution was deviated from 1 and there should be complicated formation mechanism for H2 in pyrolysis. (3) Process Simulation of CFB Combustion Coupled with Coal Drying and Pyrolysis. The performances of a 1025 t/h lignite-fired CFB boiler integrated with a dryer and a pyrolyzer were simulated using Aspen Plus. When 35% of the fed coal was dried using the sensible heat of flue gas from 149 to 105 °C, the moisture content of lignite was decreased from 34.7 wt.% (received basis) to 11.0 wt.% (air dry basis) and the consequent increase in thermal efficiency was 2.4 percentage points. For the system combining CFB boiler and coal pyrolyzer so that the coal is first pyrolyzed and the char is in turn burnt in the boiler, the energy efficiency can increase by 2.5 percentage points if tar and gas were as products. Correspondingly, the exergy efficiency for such case can reach 56.4%, whereas the exergy efficiency of the original CFB boiler is only 47.3%. It shows also that the optimal combination for an existing 1025 t/h lignite-burnt CFB boiler is to dry 35% of the consumed coal with flue gas and then pyrolyzed. The energy efficiency can be 95.3% and exergy efficiency is 50.2%. Consequently, for the lignite-fired CFB boiler its integration with a dryer and a pyrolyzer is technical feasibile in terms of energy efficiency and exergy efficiency of the system. (4) Economic Evaluation of CFB Combustion Coupled with Pyrolysis. The economic valuation was carried out for the simulated 1025 t/h lignite-burnt CFB boiler in coupling of a pyrolysis system with coal feeding rate of 100 t/h. The static investment payback period was found to be 3.9 year, less than the usually required reference investment payback period of coal industry. The investment profit ratio was 29.4%, higher than the average return rate of investment in coal industry. The net present value of the 100 t/h pyrolysis system in combination with the 1025 t/h CFB boiler is about 0.52 billion yuan (RMB) when standard discount rate is 12%. The evaluation shows in fact that the CFB combustion in coupling with coal pyrolysis would have good economic competitiveness.
语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/1828
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蔡连国. 煤热解挥发分主要气体析出模型与Aspen Plus模拟应用[D]. 中国科学院研究生院,2012.
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