Using thermal plasma for coal pyrolysis to acetylene provides a direct route to make chemicals from coal resources. The possibility of this route was first proved in the 1960s. Since then, rapid pyrolysis of coal to gaseous products was ever paid much attention worldwide. As the coal particles are pyrolyzed in a plasma reactor under extreme conditions (e.g., the ultra-high temperature greater than 3000 K), complicated physical phase changes and chemical reactions occur simultaneously in milliseconds contact time. The rapid heating of coal particles to release volatile matters has significant impact on the overall reactor performance. It has been confirmed that thermal energy is the driving force for the coal devolatilization. Different heating rates and the residence time of particles in the high temperature zone will lead to different yields of light gases. Therefore, the reactor design is directed to the formation of desired temperature field inside the reactor to guarantee the sufficient heating of coal particles in milliseconds.
Plasma reactors with different configurations at the lab scale of ~10 kW have been built to study the millisecond pyrolysis performance for a variety of coal based feedstock. The influences of atmospheric environment, power input, coal injection position and feeding rate on the reactor performances were investigated. A comprehensive computational fluid dynamics with discrete phase model (CFD-DPM) has been established to describe rapid coal pyrolysis process in a reactor under ultra-high temperatures. The simulations based on this model helped to understand the complex gas-particle reaction behavior in the millisecond process of coal pyrolysis. The particle-scale physics such as the heat conduction inside solid materials, diffusion of released volatile gases, coal devolatilization, and tar cracking reactions were incorporated. The improved chemical percolation devolatilization (CPD) model was applied to describe the devolatilization behavior of rapidly heated coal based on the physical and chemical transformations of the coal structure. The CFD-DPM method was validated by comparing the predicted volume fractions of main species and yields of light gases to the experimental data under a set of typical operating conditions. Accordingly, the reactor design can be optimized with the guidance of the aforementioned simulations. In particular, the multi-stage design for coal pyrolysis using thermal plasma will be evaluated by both experiment and simulation results. At last, the recent progress of a 5-MW pilot-plant industrial unit is to be overviewed, showing the perspective of this novel, clean coal conversion technology.
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