(563a) Coal Injection for Pyrolysis Process in A 2MW Hydrogen Plasma for Acetylene

Coal pyrolysis in hydrogen plasma opens up a direct means for producing acetylene, together with the valuable carbon materials in solid state and some ethylene, methane, hydrogen and carbon monoxide as the secondary products. The process essentially consists of several steps: (1) the design of a hydrogen plasma torch, (2) fast heating of coal powders in milliseconds to release the volatile matter in coal in a downer reactor, (3) fast quenching of the reacting stream to prevent the decomposition of acetylene into soot and hydrogen. Since the complex reaction process is operated at very high severity conditions, e.g., reaction temperature at 1500K to ~3000K, high gas velocity and residence time in milliseconds, the nozzle design for coal injection into the hot hydrogen stream becomes the first critical aspect in the reactor design to guarantee the rapid heating of coal powders, and accordingly the acetylene yield. A 2MW thermal plasma torch was built using hydrogen as the heat carrier. To work in a novel V-shaped plasma torch, coal powders must be injected to the hot stream around the downer reactor (with inside diameter of 100 mm) instead of using a center jet. Due to the high gas velocity inside the column, it is challenging to inject the coal efficiently into the hot medium with great stability to well control the reaction progress. The different nozzle designs are compared in terms of the visualizing results on the coal distribution across the injection plane in the pilot-plant reactor with gas volume flow rates of >2000 m3/hr at room temperature. Computational fluid dynamics (CFD) model for multiphase flow simulation is conducted to assist the design of nozzle shape and placement. Flat nozzle shape is adopted with comparison to the conventional round one. By tuning the angles of the flat nozzles around the downer wall, swirling flow during the gas-solid contact is implemented, which in turn strengthens the heat exchange between phases and therefore improves the coal conversion and acetylene yield.