Secondary coking and cracking of shale oil vapours from pyrolysis or hydropyrolysis of a Kentucky Cleveland oil shale in a two-stage reactor

It is widely recognized that secondary reactions which are mainly associated with minerals during oil shale retorting have a marked influence on the product yields and compositions. To understand these phenomena more clearly, the secondary reactions of shale oil vapours from the pyrolysis (or hydropyrolysis) of Kentucky Cleveland oil shale were examined in a two-stage, fixed-bed reactor in flowing nitrogen or hydrogen at pressures of 0.1–15 MPa. The vapours from pyrolysis (first stage) were passed through a second stage containing combusted shale, upgrading catalyst or neither. Carbon conversion to volatile products in the first stage increased from 49% during thermal pyrolysis to 81% at 15 MPa H₂ partial pressure. During thermal pyrolysis, total pressure had only a slight effect on carbon removal from the raw shale and subsequent deposition on to the porous solids in the second stage. Carbon deposition on to the combusted shale in the second stage was reduced to zero at 15 MPa H₂ partial pressure. The n-alkane distributions of the oils as determined by gas chromatography clearly demonstrated that higher hydrogen pressure, contact with combusted shale, or both contributed to lower-molecular-weight products.