Research PaperStudy on the anisotropy of mass transfer for oxygen in the ash layer of extremely low calorific oil shale semi-coke

- The anisotropy of mass transfer for oxygen in the ash layer of oil shale semi-coke is confirmed.
- The oxygen diffusion rate along the direction perpendicular to the bedding plane is slower than that parallel to the bedding plane.
- The ash layer diffusivities along the direction perpendicular and parallel to the bedding planes are respectively 0.25 × 10−6 m2/s and 2.36 × 10−6 m2/s.
- The ash layer diffusion resistance is the major control factor during the combustion of oil shale semi-coke.

Oil shale semi-coke produced from the oil shale retorting process causes great hazards to the environment due to its high phenol and polycyclic aromatic hydrocarbons content. A good solution of the current of semi-coke is to burn it as feedstock. In the present work, the anisotropy of mass transfer for oxygen in the ash layer of extremely low calorific oil shale semi-coke that is featured in laminated structure was investigated by using the one-dimensional unreacted model of unchanging size. Results indicated that the severe anisotropy of mass transfer for oxygen in the ash layer of semi-coke was confirmed, and the oxygen diffusion rate along the direction perpendicular to the bedding plane was much slower than that parallel to the bedding plane. This difference was attributed to different porous characters in the ash layer of semi-coke along different directions. When the burning temperature was 900 °C, the average ash layer diffusivities were 0.25 × 10−6 m2/s and 2.36 × 10−6 m2/s, respectively, along the direction perpendicular and parallel to the bedding planes. Furthermore, an unreacted shrinking sphere model of unchanging size was established to describe the combustion of extremely low calorific oil shale semi-coke, and it could be concluded that the ash layer diffusion resistance was the major control factor during the combustion of oil shale semi-coke, and the relative resistance of ash layer diffusion monotonously increased.