Study On The Particle-scale Transport Phenomenon And Reaction Performance Of Coal Pyrolysis To Acetylene In Hydrogen Plasma

Coal pyrolysis to acetylene in thermal plasma provides a direct route to make chemicals from coal resources. This process is operated under high temperature and has contact time of milliseconds scale. Direct measurements inside the reactor are hardly implemented, which obstruct further studies of this kind of process. On the other hand, since the process has not been successfully industrialized in large scale, the economic analysis of it is also insufficient.In this work, the raw coal and residues after the decomposition and the plasma pyrolysis was analyzed. The results implied that ultrahigh temperature (>2000 K) broke more chemical bonds and formed more hydrocarbon than that of the decomposition process, and increased the ratio of C and H in the gas phase, which facilitated the generation of acetylene. The temperature of the coal and the depth of devolatilization would determine the yield of acetylene.A mechanism model incorporating the heat conduction in solid materials, diffusion of released volatile gases and reactions was proposed for a deep understanding of the heat transport inside a coal particle under the extreme environmental conditions, such as temperatures higher than 2,000 K and reaction time in milliseconds. Thermal balance between coal particles and the hot carrier gas was established. The predicted yield of volatiles considering the mechanism of the two resistances agreed reasonably with the experimental data under different operating conditions, and this effect became more evident for larger particles. It can be concluded that the proposed heat transport mechanism inside coal particles worked well in understanding the coal pyrolysis process at ultra-high temperatures. Limited by the milliseconds process of the coal pyrolysis, particles of size greater than 100μm or heating fluid of temperature lower than 2000 K is not recommended in the practical application due to the slow release of volatiles in coals.Furthermore, the materials balance and energy balance of the process was investigated systematically based on element balance. It is concluded that the overall plasma reactor performance was dominated by the gas-solid mixing efficiency, coal-hydrogen feeding rate and the residence time. Based on these constrains, the economic analysis of acetylene production indicated that the costs of raw coal and power were the main parts. The optimal utilization of energy and material-flows in the system can be of great significance to ensure the economic feasibility.