Detailed Kinetic Modeling of Gas Evolution and Energy Recovery during Chemical Quenching of Acetylene Production in Thermal Plasma

Detailed Kinetic Modeling of Gas Evolution and Energy Recovery during Chemical Quenching of Acetylene Production in Thermal Plasma

Yan Cheng, Tianyang Li, Yi Cheng ^*

Department of Chemical Engineering, Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Tsinghua University, Beijing 100084, PR China

Thermal plasma pyrolysis provides a convenient way to realize the one-step conversion from coal to acetylene. Taking advantage of the ultra-high temperature field provided by thermal plasma, the coal feedstock is heated up in milliseconds to give out volatiles. The volatiles undergo gas phase reactions afterwards to form acetylene owing to the fact that acetylene is thermodynamically stable at above 1500 K among small-molecule-hydrocarbons. However, acetylene will decompose into hydrogen and carbon black during cooling process. So efficient quenching is necessary to prevent acetylene decomposition so as to maintain the target products. It is noticed, on the other hand, that the high-temperature gas products still contain about 40% of the total energy input. So it is vital to make use of the latent energy during the quenching process.

In addition to the common physical quenching by water spray, chemical quenching method is an alternative solution to the process design. In chemical quenching, hydrocarbons are used as quenching media instead of water, which brings about co-products (e.g., ethylene) during the decomposition of hydrocarbons by the afterheat in gas products. The process profit can be accordingly improved as well.

In this work, we carried out modelling and simulation on the above-mentioned chemical quenching in thermal plasma technology. A gaseous detailed kinetics, coupled with a soot formation mechanism, was proposed to investigate the species evolutions during quenching. The model consisted of C1-C5 gas phase reactions, PAH (polycyclic aromatic hydrocarbons) growth and a simplified soot formation mechanism. The predicted acetylene decomposition and soot yield were well validated with published data. Then, numerical simulations were performed to reveal the process features, including acetylene decomposition evolution, chemical quenching behavior of different C1-C3 alkanes as well as the energy balance.

Finally, the model was used to theoretically propose an optimized chemical quenching design in a pilot-plant thermal plasma process. The results showed that under optimized operating conditions, propane quenching could recovery 35.4% of the total gas phase afterheat. At the same time, propane decomposed into smaller molecules in millisecond(s), which realized the co-production of ethylene at a ratio of 0.75 t C₂H₄/t C₂H₂. The optimal energy consumption was reduced by 43% per kilogram of C₂H₂+C₂H₄, which verified the feasibility of energy efficiency improvement via chemical quenching.

Keywords: Thermal plasma pyrolysis; Chemical quenching; Acetylene; Detailed kinetics

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* Corresponding author. Address: Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R. China. Tel: +86 10 62794468, Fax: +86 10 62772051. E-mail address: yicheng@tsinghua.edu.cn.