| Recirculation of hot solids is an efficient method of transferring heat in an oil shale retorting process, but it can also greatly lower the oil yield due to solids-induced coking and cracking reactions. Understanding these reactions is key to the development of solid-recycle pyrolysis processes for oil shale and other fuels. A two-stage fluidized-bed reactor was developed to investigate the secondary coking and cracking reactions that can occur within a fluidized bed. Processed oil shales (pyrolyzed, gasified and combusted) and other related solids were examined as coking and cracking substrates so that the importance of their porosity, chemistry and thermal history could be identified. Solid particles with the most meso- and macro-pore surface area and the lowest initial carbon content displayed the highest rates of carbon deposition. The substrates that were studied were small enough {dollar}(-841murm m + 250mu{dollar}m) and sufficiently porous that pore diffusion was not a limiting factor for coking. This was supported by the observation that coke formation was visually uniform throughout the interior of the particles. The rate of coking was characterized by two distinct periods. The first rate period was rapid and had an apparent activation energy of 10,000 J/gmol. The initial rate slowed exponentially to a final, constant rate period, which was reflective of coking onto carbon-covered surfaces, with an apparent mean activation energy of 67,000 J/gmol. The coke precursors were high in molecular weight, nitrogen content and aromaticity, and the rate of coke formation was found to be dependent on the oil vapor concentration. The gaseous products did not contain any species that were indicative of catalytic cracking, such as branched aliphatics or large quantities of C{dollar}sb3{dollar} to C{dollar}sb6{dollar} species. The decrease in coking rate with time was equated to a mono-layer coverage of carbon onto the mineral surfaces in the meso- and macro-pores. Empirical relationships based on added-carbon and time-on-stream were established to model this behavior. The time-on-stream deactivation function accurately reproduced the effects of solid-recycle ratio and coking activity of an integrated solid-recycle oil shale retorting system. |
