Degradation of zirconate materials in ultra-high temperature reverse-flow pyrolysis reactors

Abstract

A reverse-flow reactor enables the pyrolysis of hydrocarbons at temperatures up to 2000 °C to high value petrochemical products. Materials in the reactor hot zone, exposed to such extreme temperatures, are also subject to rapid oxidative cycling over a period of several seconds between a regeneration (heat addition) step that is mildly oxidizing and a pyrolysis (cracking) step that is strongly carburizing. This paper addresses the performance of a class of zirconate ceramics such as BaZrO3, SrZrO3, and La2Zr2O7 that have been tested in a laboratory scale reverse-flow reactor and compares the results with thermodynamic predictions. It is observed that zirconate ceramics degrade by decomposition to zirconia and the second oxide, vaporization of the second oxide, and ceramic dusting corrosion of zirconia that involves carbide-oxide inter-conversion and carbon precipitation. The pyrolysis reaction promotes carbon deposition and carburization of zirconia leading to the formation of a non-protective porous carbide layer. The regeneration reaction converts the carbide layer partially back to oxide while also oxidizing away some of the deposited carbon. Repeated cycles of pyrolysis and regeneration steps eventually result in the breakup of the bulk zirconate ceramic structure into powder or dust.