【作者】 鲍卫仁；

【导师】 谢克昌；

【作者基本信息】 太原理工大学 ， 化学工程与技术， 2010， 博士

【摘要】 乙炔是一种重要的化工产品,现有电石法生产工艺存在流程长、原料运输费用高、耗电量大,污染严重的缺点。利用等离子体热解技术直接生产乙炔的工艺具有流程短、成本低、洁净转化等特点。煤或富含甲烷的气体(焦炉煤气、煤热解气)作为可选的原料,资源相对丰富,是其高效清洁转化的路线之一。基于此,作者对国内外已有研究进行了归纳和总结,结合课题组多年的研究工作和构思,对煤及富含甲烷气等离子体热解制乙炔及副产纳米碳纤维做了进一步的深入探讨和研究,得到系列有意义的结论,主要包括以下几个方面:从煤的元素组成及等离子体热解煤的特点出发,采用Gibbs自由能函数最小法,对不同变质程度煤的平衡组成进行了计算并与实验研究结果比较认为,C-H-O-N-S多相热力学平衡体系的计算结果更具合理性,能更真实地为等离子体热解煤工艺过程提供指导:含氢气氛是优于氮气和氩气的等离子体热解煤制乙炔的工作气体;乙炔的浓度和产率均随煤中的H/C比和压力的增加而增大,H/C比大于2、压力为0.7MPa时,达到最大,随后变化趋缓;乙炔摩尔浓度最大值可达0.219,此时的乙炔产率为70.62%;C2H + C2H2最大值出现的温度区间为3650 ~ 4000K;煤中O含量对乙炔产率有不利的影响,但热解过程中主要消耗C源而并不影响H源。不同变质程度的煤的热解实验结果显示:H/C比大、O含量低、挥发份含量适中的煤种有相对高的乙炔收率;含25 ~ 40%挥发份的煤种在等离子热解过程中可以得到17 ~ 22%乙炔产率的最佳值;供煤速率太高或太低均不利于乙炔的生成,在0.5 ~ 4.0 g/s区间,乙炔的选择性高达95%以上;供粉速率增加,乙炔的比能耗降低、而煤的转化率和乙炔的收率也随之变小、甲烷等小分子烃类物质的收率和气相产物组分浓度则逐渐增大。煤中含氮物在温度高于1000K的等离子体热解条件下与常规加氢热解不同,主要气相含氮物是HCN,只有极少量氮转化为NH3;体系中氢含量的增加有利于HCN、CN和NH3的生成;煤中氧含量愈高,HCN和NH3的产率越低;煤进料量对HCN生成的影响与煤种有关,但进样量加大到一定值时,不同煤种的HCN产率趋于一致;等离子体煤热解生成的半焦中氮的形态变化与反应气氛有关,但不同于常规热解,Ar气氛中半焦的季氮型含氮基团的量较原煤高,含氢和CO2气氛中的吡咯型氮的含量则明显增大。煤中的硫在等离子体热解过程中主要转化成易被脱除的H2S气相产物,其产率随输煤速率的变化规律与煤种有关,最大转化率可达90%;原煤含有的无机硫可被完全脱除。反应后煤焦中的N/ C和S / C摩尔比都小于原煤,说明在等离子体热解条件下,含硫含氮化合物向气相中释放的速率高于含碳化合物。对富含甲烷气等离子体热解制乙炔的理论计算和系统的实验研究证实了C-H单相热力学平衡体系的合理性,H/C为4的单相体系可得到约98.6%的乙炔收率最大值;随甲烷流量的增加,产品气乙炔等碳氢化合物的浓度增大,而甲烷转化率、乙炔选择性和收率则减小,乙炔能耗出现一最低值后增大;4.0 Nm3?h-1是相对合理的甲烷流量,此时可得到最小的乙炔能耗9.68 kWh?kg-1,乙炔体积浓度和乙炔收率分别可达11.4%和86.2%。反应器结构的合理改善可以使甲烷的转化率、乙炔浓度和能耗达到最佳,改变淬冷时间最大可提高乙炔收率18%,缩短反应器长度减少停留时间可使乙炔收率最大增加55%。利用热等离子体进行甲烷的催化裂解可以制备出空心状和竹节状等形貌的纳米碳纤维。甲烷进料量对纳米纤维形貌的影响较大,进料流量为0.3 m3/h时的产物为空心套杯状,内壁平直、光滑,无金属填充物;流量增大制得的纳米碳纤维的纯度下降,产物掺杂有大量的竹节状纳米碳纤维和带有颗粒的空心管。和Ar工作气相比,N2气的热值高、高温持续时间长,有利于元素态金属原子参与完成碳结构的石墨化进程,所得纳米碳纤维较氩气气氛产物碳层连续、互相平行,纤维管内外表面平直、光滑、无沉积。Fe2O3为催化剂,纤维管直径分布范围小,杂质或无定形碳很少;而以Co2O3为催化剂,产物近似多壁碳纳米管结构,纯度较低;以Ni2O3为催化剂的管状产物是长度不到1μm的短截管,结晶度较差。更多还原

【Abstract】 Acetylene is one of important chemical products, which is produced by the calcium carbide process at present. This manufacturing route has the disadvantages of long process flows, high transportation cost of raw materials, big power consumption and serious pollution and so on. The technique, which acetylene is directly produced by the pyrolysis of coal or CH4-rich gas in the arc plasm jet, posseses the advantage of short process flows, low cost,clean conversion and byproduct carbon nanofibers. Coal and CH4-rich gas ( gases from coal pyrolysis or coking ) used as raw materials in this method have abundant resources and can be efficiently and reasonably utilized. The researches about acetylene and carbon nanofibers formation from pyrolysis of coal or CH4-rich gas in arc plasma jet are put up and carried out on the basis of the summarizing of the research status in this field and the preliminary studies in our group in this paper. The main works and results are shown as follows:The heterogeneous thermodynamics equilibrium system about a set of rank-ordered coals was calculated by the Gibbs minimization free energy method based on the elemental composition in coal and the characteristics of coal pyrolysis in arc plasma jet. The results from C-H-O-N-S system was confirmed as more reasonable by experiments. The ambient gas containing hydrogen is more suitable than the gases containing nitrogen and argon in this system. The concentration and yield of acetylene increase with the atomic ration of H/C in coal and pressure in reactor, and it reaches the maximum value of 0.219 and 70.62% when ration of H/C is 2 and pressure is 0.7MPa. The optimum temperature range to form C2H+ C2H2 is 3650 ~ 4000K. Oxygen element in coal is unfavorable for the formation of acetylene, but it mainly consumes carbon source and does not affect hydrogen.A set of rank-ordered ten kinds of coals were pyrolyzed in arc plasma jet under H2/Ar gases. The effect of coal characteristics and operating parameters on the acetylene formation were researched. Higher C/H mol ratio, lower O/C mol ratio and appropriate volatile content are favorable to form acetylene. Optimum acetylene yield of 17~22% is obtained from the pyrolysis of coal with volatile content range of 25~40%. When feeding rate of coal sample is increased, the specific energy consumption and concentration of acetylene increase, but the conversion of coal and the yield of acetylene decrease.The formations of NOx precursors (HCN and NH3) and SOx precursors (H2S and COS) from coal-N and coal-S were investigated during the pyrolysis of coal in arc plasma jet under different ambient gases. HCN is the predominant product of coal-N pyrolysis and only a little coal-N is converted to NH3, which is different with the gaseous products from coal conventional pyrolysis. The yields of HCN and NH3 increase with nitrogen content increasing and oxygen content decreasing in coal. Affect of coal feeding rate on the HCN formation is related with coal types and this action is weaken when the feeding rate is increased to a certain value. The forms of nitrogen in char from coal pyrolysis in arc plasma jet is affected by ambient gases, but they is different from that of conventional coal pyrolysis. H2S is mainly formed during the pyrolysis of coal-S in arc plasma jet and the maximum conversion of coal-S is up to 90%. Coal feeding rate affects the conversion of sulfur in coal and the release of inorganic-sulfur is very sufficient. The energy input in in a plasma coal pyrolysis processes significantly affects the formation of H2S.Acetylene from the pyrolysis of rich-CH4 gas in arc plasma jet was investigated through theoretical calculation and experimental methods. C-H homogeneous equilibrium system with H/C = 4 is regarded as reasonable and the maximum acetylene yield is 98.6%. The conversions of acetylene and other hydrocarbon product gases is increased with the increase of methane gas flow rate, but the conversion of methane, selectivity and yield of acetylene are decreased. There exist a minimum point of special energy consumption in the change of methane gas flow rate. It is relatively reasonable when the gas flow rate of methane is 4.0 Nm3?h-1 and the minimum special energy consumption of 9.68 kWh?kg-1, acetylene concentration of 11.4% and yield of 86.2% can be obtained under this condition. The yield of acetylene can be increased 18% and 55% respectively when the quenching and resident time is shortened by optimizing the structure of pyrolysis reactor.Carbon nanofibers (CNFs) from the decomposition of methane are prepared by arc plasma jet under atmospheric pressure conditions with N2 and Ar working gas. The effect of flow rate of methane, composition of working gas and type of catalysts on the morphology, structure and yield of carbon nanofibers were studied. The products of cup-stacked like hollow CNFs with high crystallization is obtained when the gas flow rate of methane is 0.3m3/h, a mixture of bamboo-shaped carbon nanofibers and hollow tube attached with a large number of nano-particles is formed and the purity of product decrease with increasing methane flow rate to 0.5m3/h. Compared to Ar gas atmosphere, carbon layer of the fiber tube are more continuous and parallel each other, both inner and outer surface of tube are more straight and smooth under N2 working gas. The CNFs product is entirely hollow carbon nanofiber which is at least 3μm in length, continuous carbon layer parallel each other and forms uniform angle with the fiber axis when Fe2O3 is used as catalysis. But the purity and crystallinity of CNFs decrease when Co2O3 and Ni2O3 are used as catalysts.更多还原