Effect of Hydrogen Gas Addition on Acetylene Formation for the Plasma Treatment of Coal

Abstract

“Taiheiyo” coal was thermally decomposed by an argon thermal plasma to release the product gases. Hydrogen gas was added to the plasma flow with the coal particles to get high conversion. Acetylene, methane, hydrogen and a trace of carbon monoxide were found as the product gases. A high selectivity to acetylene was observed experimentally compared with a low selectivity to methane.
A one-dimensional model was us ed to investigate the mechanism of the decomposition and the way to increase the acetylene selectivity or conversion. We assumed ( a ) the rate determining step in the heat transfer which causes a uniform temperature of coal particles and ( b ) a fixed decomposition temperature. The addition of hydrogen causes ( 1 ) the rapid increase of temperature on the coal particle and ( 2 ) the rapid drop of temperature in plasma flow because of the increase in the thermal conductivity of the H₂-added plasma. The first means the short induction period prior to the decomposition, i. e. a large conversion, and the second means the small heat flow, i. e. a small conversion. These two factors have the same order influences, but the second has another aspect simultaneously, i. e. a long resident time in reaction zone, and this cancels the decreasing effect on the heat flow. The conversion increases with increasing the hydrogen gas amount mainly by the first reason.
The addition of gas to a plasma flow reduces the initial plasma temperat ure. The addition of argon gas was experimentally examined instead of hydrogen gas, and the conversion only decreases.