Effect Of Nitrogen/Oxygen Substances On Pyrolysis Of Rich Alkane Gas To Acetylene By Thermal Plasma

Nowadays, fossil fuel still remains the dominant energy source in the world’s energy consumption. There are a great number of rich alkane gases which come from fossil fuel, e.g., coke oven gas, refinery dry gas, natural gas, unconventional natural gas (coalbed methane, shale gas, tight sandstone gas) and so on. But the utilizing ways of rich alkane gases are simple and extensive. Some are directly emitted, some are directly combusted, and only a little could be used as chemical raw materials for further processing, which not only causes huge energy waste, but also causes damage to the environment. Due to the high energy consumption, high pollution and high equipment requirement, traditional acetylene production is being phased out gradually. Because thermal plasma has advantages of high temperature, high enthalpy, high active particles, pyrolysis of alkane to acetylene by thermal plasma is a new technology which has obvious advantages of high utilization efficiency of raw materials, fast reaction rate, simple process, easy operation, low investment, low cost, no pollution, etc. Rich alkane gases contain different content of CO, CO2 or N2. They have great effects on the plasma pyrolysis process, e.g., energy requirement, yield of C2H2, gas separation, etc. Therefore, it is very necessary to further explore. The effect of nitrogen/oxygen substances on pyrolysis of rich alkane gases to acetylene by thermal plasma was investigated. C-H, C-H-O, C-H-N, C-H-O-N homogeneous and heterogeneous thermodynamic equilibria were calculated by the method of minimum Gibbs free energy. The results showed that oxygen element mainly existed in the form of CO, and nitrogen element existed in the form of N2 and HCN. CO2 and N2 would consume alkane carbon and reduce the yield of C2H2. The results of heterogeneous thermodynamic equilibria were more agreeable with the experimental results.In the experimental research, suitable processing conditions in C-H system were investigated at first. Then, the effect of different power and different concentration of CO or CO2 or N2 on CH4/H2/CO, CH4/H2/CO2, CH4/H2/N2 were investigated. Lastly, a research of choosing one kind of rich alkane gas, e.g., coke oven gas, was made on the complex system.The results showed that the relatively optimal process conditions in C-H system were H/C=6～8, input power=13～18 kW, flow rate=6～10 Nm3/h, magnetic induction=0.038～0.096 T. The input power and flow rate had a great effect on the pyrolysis of rich alkane gas to acetylene. The higher the input power was, the higher the specific energy requirement of C2H2 and conversion efficiency of alkane would become. The yield of C2H2 rose at first, and then it decreased. The larger the flow rate was, the lower the specific energy requirement of C2H2, yield of C2H2 and conversion efficiency of alkane would become. The minimum specific energy requirement of C2H2 was 15.8 kWh/kg C2H2, the maximum yield of C2H2 was 84.7%, the maximum conversion efficiency of alkane was 96.1%.The content of CO2 and N2 had a great effect on pyrolysis, while CO had little influence. The increase of content of CO2 and N2 would increase specific energy requirement of C2H2 and decrease yield of C2H2. CO2 mainly converted to CO. N2 partly converted to HCN, while a great number of N2 still existed in the form of N2. According to the cracking results of simulated coke oven gas,1 mol CO2 would consume 0.60～0.64 mol alkane carbon, and 1 mol N2 would consume 0.15～0.17 mol alkane carbon. In complex system with CO, CO2 and N2 at the same time, CO2 and N2 would compete for carbon from alkane. Besides, if rich alkane gas had high composition of CO or CO2 (>10%), it was possible to produce acetylene and syngas simultaneously. Therefore, if increasing the yield of C2H2 was the only purpose, CO2 and N2 in rich alkane gas should be separated in advance. Due to the influence degree of the nitrogen/oxygen substances was not the same, the comprehensive consideration should be made according to their content and ease of separation in practical production.