Cross‐Scale Modeling and Simulation of Coal Pyrolysis to Acetylene in Hydrogen Plasma Reactors

Abstract

Coal pyrolysis to acetylene in hydrogen plasma is carried out under ultrahigh temperature and milliseconds residence time. To better understand the complex gas-particle reaction behavior, a comprehensive computational fluid dynamics with discrete phase model has been established, with special consideration of the particle-scale physics such as the heat conduction inside particle. The improved chemical percolation devolatilization model that incorporates the tar cracking reactions is adopted. The model predictions are in good agreement with the performances of two pilot-plant reactors. The simulations reveal the detailed unmeasurable information of gas phase and particle-scale behaviors, then point out the facts that coal devolatilization almost finishes in the first 100 mm of the reaction chamber and the optimal particle diameter is suggested to be 20-50 lm. The different reactor performances during the scale-up from 2-to 5-MW unit are analyzed based on the detailed simulation results in combination with the operational experience.

Review of 30 Years of Research Using the Chemical Percolation Devolatilization Model

“…Good agreement was achieved with data from 29 coals at heating rates from 0.1 to 10 6 K/s by adjusting the kinetic parameters and the chemical structure parameters in the CPD model. Yan et al , used the CPD model to help optimize coal feed rates and particle size in the design of a plasma reactor to produce acetylene from coal. The rapid heating rates in the plasma reactor required solution of internal particle temperature gradients, even for small particles.…”

Production of Acetylene and Acetylene-based Chemicals from Coal

“…27,41 Particle size must be <200 μm, preferably ≤50 μm. 33,63,64 As particle size increases, coking becomes more of a problem; 44 the solid carbon is not reactive in hydrogen. 65 The outer surface area of the particles is recommended to be in the range 0.5−2.0 m 2 /cm 3 .…”

“…44 A higher coal feed rate corresponds to longer heating times and a lower final temperature of the coal particles, causing a lower yield of volatiles and longer time for complete devolatilization. 67,69 Residence times are on the order of milliseconds, 2,40,43,63,64,70,71 optimally <2.5 ms, 72 with a heat-up rate of about 1 ms to, 33 at most, a few milliseconds. 73 Short residence times help maximize yields of acetylene.…”

“…Work on plasma conversion of coal to acetylene is continuing in China. , The process has been scaled up to a 2-MW pilot-plant reactor and then to a 5-MW reactor, for which a useful model is available . The 5-MW reactor is thought to be the largest arc-plasma pyrolysis reactor in the world …”

Generalized model of heat transfer and volatiles evolution inside particles for coal devolatilization

“…Consequently, the detailed devolatilization information of pulverized coals is difficult to be grasped by experiments. 2 Therefore, it is important to establish the fundamental knowledge of coal pyrolysis under a wide range of operating conditions such as heating temperatures, heating rates, coal types, atmospheres, and so on. Because the practical measurements in thermal plasma are not realistic, it is expected to develop theoretic methods to enable reasonable predictions on coal pyrolysis behaviors in such a severe environment (e.g., ultrahigh temperature and ultrafast heating rate).…”