Factors Affecting Product Yields and Oil Quality during Oil Shale Retorting - a Screening Study

Seven process variables - retort temperature, solids recycle ratio, char content of recycle solids, recycle solids temperature, pre-treatment of recycle solids with ammonia, solids residence time and steam concentration - were selected for the screening study. The Plackett-Burman statistical experimental design, which has been known to be the most efficient at detecting main effects of large number of variables with a minimum amount of experimentation, was used to screen these process variables. Each variable was tested at two (low and high) levels: temperature at 510 and 560 C, recycle ratio at 1.3 and 3.0, char content at 0.0 and 5.0 wt%, recycle solids temperature at 650 and 750 C, pretreatment with and without ammonia, residence time at 10 and 16s and steam concentration at 25 and 70 vol%. The bench-scale integrated retorting/combustion oil shale (BIRCOS) facility at Lucas Heights was used to produce product yields and compositions under realistic recycle shale conditions. The variables can be ranked, in terms of the significance of their main effects on product yields and carbon conversion, in the following order: retort temperature, solids residence time, steam concentration, char content, solids recycle ratio, recycle solids temperature, and ammonia treatment. Retort temperature was the only variable that affects the atomic H/C ratio and nitrogen content of oil. Sulphur content in oil was independent of all variables.