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以热解为先导的煤热解/燃烧多联产技术，可以将低阶煤炭转化过热蒸汽和清洁油气，实现了热、电、油、气的多联产和煤炭资源的梯级利用。含尘焦油是煤热解/燃烧工艺激冷工序产出的一种低品质重质煤焦油，存在固体粉尘和重质焦油含量高的问题。高含量的固体粉尘和焦油重质组分增加了含尘焦油后续精制处理的困难和成本。针对含尘焦油难以利用的问题，本文提出了一种含尘焦油循环热解的煤热解/燃烧新工艺。具体而言，将激冷工序得到的含尘焦油返回煤热解/燃烧系统中的热解反应器进行再热解，将含尘焦油转化为较为轻质的焦油、热解气和半焦。与煤热解/燃烧工艺相比，新工艺仅产出轻质的焦油、热解气和过热蒸汽，没有含尘焦油产物的产出。基于新工艺，围绕提高轻质焦油产物收率，本文首先对含尘焦油热解制油过程进行了实验研究，讨论了不同因素对焦油产物收率和品质的影响规律。研究结果表明：（1）重质组分中脂肪烃链或含杂原子脂肪烃链的分解会生成轻质焦油产物，该反应在500oC时基本完成；快速热解所得轻质焦油产物收率要优于慢速热解。（2）高加热速率有利于降低焦油产物中3环以上稠环芳烃和O/N/S元素含量，提高1-2环芳烃含量，焦油产物品质得到改善。（3）甲烷对焦油热解过程基本没有影响；氢气气氛可以提高轻质焦油产物收率，改善焦油产物品质；与低加热速率相比，高加热速率下，氢气气氛对轻质焦油产物收率的提高作用减弱。（4）固体粉尘不利于重质组分转化为轻质焦油产物。本文对含尘焦油高温热解制气过程进行了实验研究，讨论了热解温度、停留时间、水蒸气气氛和固体颗粒等因素对热解气收率、热值和含尘焦油热解制气效率的影响。研究结果表明：（1）当热解温度低于600oC，热解气主要来自于重质组分的分解；温度高于600oC后，热解气主要来自于轻质组分的分解。（2）热解温度提高有利于提高热解气收率和含尘焦油热解制气效率，热解气在400oC获得极大体积热值 (29.98 MJ/m3)。（3）水蒸气气氛下热解有利于含尘焦油转化为热解气，提高含尘焦油热解制气效率，但是会降低热解气热值。（4）固体颗粒有利于将含尘焦油转化为热解气，但会降低热值；除富含铁氧化物的灰颗粒外，固体粉尘、CaO和Al2O3等固体颗粒均可以提高含尘焦油热解制气效率。讨论了轻质组分和芳烃在重质组分热解过程中的作用，研究结果表明：（1）重质组分单独热解时，23%-23.6%重质组分可以转化为轻质焦油产物；轻质组分与重质组分共热解时，33.1%-38.9%重质组分转化为轻质焦油产物；轻质组分对于重质组分转化为轻质焦油产物具有关键的促进作用。（2）氢化蒽通过向重质组分热解碎片供氢，促进轻质焦油产物生成；蒽可以捕获重质组分热解所得H自由基转化为氢化蒽，随后参与重质组分热解；氢化蒽供氢的能力要优于蒽。（3）轻、重组分共热解时，轻质组分通过向重质组分热解碎片供氢，促使重质组分向轻质焦油产物转化。基于重质组分的元素分析和核磁分析结果，构建了重质组分分子结构模型，采用基于GPU的分子动力学程序系统 (GMD-Reax) 对重质组分热解反应进行了模拟，对热解碎片转化进行了能垒分析，获得了热解产物碎片的演变规律。（1）重质组分热解会生成自由基。（2）气相产物演变规律与实验所得气体产物收率基本吻合。（3）轻质焦油产物模拟收率存在极大值；重质组分热解碎片的进一步缩聚反应会影响轻质焦油产物模拟终值收率。（4）提高轻质焦油产物收率的关键在于抑制1-3环热解碎片的缩聚反应，强化4环以上热解碎片的加氢开环反应。最后，对含尘焦油循环热解的煤热解/燃烧工艺进行了衡算，结果表明：含尘焦油再热解可以提高轻质焦油产物和热解气收率，改善热解气的热值和煤热解制气效率，使热解效率略有下降。;The staged conversion technology based on coal pyrolysis/combustion process can take full advantage of low rank coal for co-production of gas, liquid fuel, and power, realizing the comprehensive utilization of low rank coal resources. Dusty tar, which is an undesired product from quench process of coal pyrolysis/combustion system, contains substantial dust particles and heavy tar. The high content of dust and heavy molecular weight components in dusty tar increases the difficulty in further refining and makes the post treatment very expensive. A new process, the coal pyrolysis/combustion coupled with recirculating pyrolysis of dusty tar is proposed. To be more specifically, dusty tar was recirculated back to the pyrolysis reactor of coal pyrolysis/combustion system and converted into lighter tar, fuel gas and char. This new process will only generates light tar, pyrolysis gas and char, significantly increasing the utilization efficiency of coal. Based on the new process, dusty tar pyrolysis was conducted to improve the yield of light tar product. The factors that have influences on light tar formation were investigated. The results suggest that (1): Decomposition of aliphatic chains in heavy tar which can be completed at 500oC can generate light tar product. Fast pyrolysis can favor light tar formation compared with slow pyrolysis. (2): High heating rate can decrease the content of 3, 4 ring aromatics and O/N/S element, and increase the content of 1, 2 ring aromatics than slow heating rate, improving the quality of tar product. (3): Compared with N2 atmosphere, CH4 has little effect on tar pyrolysis. The H2 atmosphere can increase the yield of light tar product and improve the quality of tar product. Compared with high heating rate, the ability of H2 atmosphere to increase the yield of light tar product is more obvious at slow heating rate. (4): Dust can inhibit conversion of heavy tar in feed into tar product while favors gas and char formation.Fuel gas production from dusty tar pyrolysis is also studied. The effect of temperature, residence time of volatiles, steam and solid particles on pyrolysis gas yield, volumetric heating value of gas and the gas efficiency of dusty tar pyrolysis were discussed. (1): Below 600oC, the pyrolysis gas mainly derives from decomposition of chemicals with boiling point higher than 360oC. On the contrary, most of the pyrolysis gas is derived from decomposition of components with boiling point lower than 360oC when temperature exceeds 600oC. (2): As temperature elevates from 360oC to 950oC, yields of gas and gas efficiency of dusty tar pyrolysis simply increase while the volumetric Low Heating Value (LHV) of gas obtains a peak value of 29.98 MJ/m3 at 400oC. (3): Pyrolysis with steam can increase the gas yield and gas efficiency of dusty tar pyrolysis while the LHV of gas is reduced. (4): Solid particles favor conversion of dusty tar into pyrolysis gas and decrease the LHV. Ash, which is rich in Fe oxides, can decrease gas efficiency of dusty tar pyrolysis while dust, CaO and Al2O3 can increase it. Pyrolysis of heavy fraction (HF) of dusty tar was performed. The role of light fraction (LF) of dusty tar, anthracene (ANT) and hydrogenated anthracene (HAN) during fast pyrolysis of HF were evaluated. It is found that: (1): During pyrolysis of HF without LF, 23%-23.6% HF can be converted into light tar product. During co-pyrolysis of HF with LF, 33.1%-38.9% HF was converted into light tar product. LF play vital roles in converting HF into light tar product. (2): ANT and HAN both favor light tar product formation. HAN can provide H atom to fragments from HF pyrolysis, increasing yield of light tar product while ANT can capture H atom from HF pyrolysis and be transformed into HAN, and then take part into HF pyrolysis. HAN has higher H donation ability than ANT. (3): During co-pyrolysis of LF with HF, LF can provide H atom to cracked species from HF pyrolysis, favoring conversion of HF into light tar product. The molecular structure of HF was constructed based on the elemental analysis and NMR analysis of HF. The pyrolysis process of HF was simulated by the reactive forces field molecular dynamics based on GPU (GMD-Reax). The energy barrier for conversion of cracked species was analyzed. The evolution of pyrolysis product was obtained. (1): Radicals are formed during HF pyrolysis. Temperature increase will facilitate radical formation. (2): The evolution of gas yield coincides with experimental results. (3): There is a peak yield of light tar product during simulation of HF pyrolysis. The further polymerization of cracked species derived from HF pyrolysis can reduce the yield of light tar product obtained at the end of simulation. (4): The key step to increase the yield of light tar product is to suppress polymerization reactions of 1-3 ring fragments and enhance the hydrogenation and ring-opening reactions of large cracked species (larger than 4-ring).Finally, the mass and heat balance of the proposed process is analyzed. The results suggest that re-pyrolysis of dusty tar in the downer reactor can increase the yield of light tar product and pyrolysis gas, improve heating value of gas and enhance gas efficiency of coal pyrolysis while slightly reduce the pyrolysis efficiency.
|曹冬冬. 含尘焦油热解规律的基础研究[D]. 中国科学院大学,2018.|
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