Basic Study On Conversion And Synthesis Of Coal-based Materials In Arc Plasma

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.