Oil Shale Pyrolysis Under Non-conventional Conditions

The utilization of oil shale as one of the alternative energy resources has become attractive nowadays due to the high-energy consumption.Pyrolysis is the main technology to utilize oil shale for generating high-value products in the form of shale oil and gas.The complex reactions in pyrolysis especially the secondary pyrolysis reactions of the generated vapors extremely influence the quality and quantity of pyrolysis products.This thesis intends to gain deep insights into the volatile release reactions to clarify the dominance of the retorting factors on the extent of the secondary pyrolysis reactions.The characteristic of pyrolysis in a small fixed bed reactor was firstly investigated under different operating conditions to evaluate their influences on secondary reactions.Particle bed thickness caused a significant effect to lower the shale oil yield,whereas reducing the reaction pressure evidently suppressed that effect.The shale oil yield decreased from 10.28 wt.%to 9.72 wt.%with increasing the particle thickness from 20 to 150 mm at a temperature of 550 ?.The produced shale oil was lighter for the pyrolysis with higher bed depth.Moreover,for a thicker particle bed there was a lower content of aliphatic hydrocarbons but an increase in the content of aromatic species.The loss of shale oil yield through the influence of particle bed thickness was more severe at higher pyrolysis temperatures.Reducing pyrolysis pressure evidently increased the shale oil yield under all tested particle bed depths.The shale oil yield slightly decreased from 10.75 wt.%to 10.44 wt.%with the thicker particle bed under reduced pressure.However,this shale oil yield was evidently higher.even at the thickest bed depth,in comparison with that from the pyrolysis under atmospheric pressure.This verified that the vacuum pyrolysis effectively suppressed the secondary reactions caused by particle bed thickness.The pyrolysis of oil shale under reduced pressures was further investigated in one-kg fixed bed reactor.The pyrolysis in the fixed bed reactor mounted with the particularly designed internals,a central gas collecting pipe and a few heat transfer plates,has shown an obvious improvement on pyrolysis performance in comparison with that in the fixed bed reactor without any internals.Thus,this thesis further worked to optimize the operating conditions through reducing the reaction pressure in this newly devised fixed bed reactor to get better pyrolysis performance in terms of shale oil production.Reducing the reaction pressure evidently increased the shale oil yield for the reactor with and without internals.Nevertheless,the effect from lowering operating pressure was more obvious in the reactor with internals than in the reactor without internals.The produced shale oil by vacuum pyrolysis was heavier and richer in aliphatic compounds but having smaller aromatic species yields,obviously indicating the suppression of secondary reactions.Moreover,the oil yield under reduced pressure condition(-40 kPa)in the reactor with internals reached 97.57%of the Fischer Assay yield(8.24 wt.%dry basis)at a furnace heating temperature of 1000 ?.Hence,there is certain synergistic effect between internals and reducing pressure to enhance the yield and quality of shale oil.The pyrolysis of oil shale was further carried out in an infrared heating fixed bed reactor.A newly shallow infrared heating fixed bed reactor was designed to investigate the characteristics of massive oil shale pyrolysis(20 g)at different heating rates and reaction temperatures under minimized secondary reactions towards the generated volatile.The maximum shale oil yield of 11.10 wt.%(almost 100%of the Fischer Assay yield)was obtained at the optimized operating conditions of a heating rate of 0.5 ?/s and a pyrolysis temperature of 550 ? under a reduced pressure(-40 kPa).Further increasing heating rate decreased the shale oil recovery but got higher production of pyrolysis gas.Nevertheless.increasing heating rate increased the total proportion of hydrogen in the volatile products at a specified temperature.The rapid pyrolysis of oil shale at a heating rate of 25 ?/s allowed higher production of total volatile in comparison with the Fischer Assay method.The recovered shale oil had more light oil components for the fast heating pyrolysis.The results indicate the existence of secondary cracking reactions with the tested rapid oil shale pyrolysis.Moreover,it has been found that pyrolysis of multilayer oil shale particles caused more serious secondary reactions than pyrolyzing oil shale in a single-layer.Therefore,the single-layer pyrolysis at a low heating rate and lowered operating pressure evidently suppressed the secondary reactions in this adopted infrared reactor.Through these studies,deeper understanding of the reactions for pyrolysis volatile of oil shale was obtained.There were inevitable secondary reactions to the released primary volatile even for the tested quick heating pyrolysis in the infrared heating reactor.Reducing reaction pressure effectively suppressed the secondary reactions for all tested conditions.Consequently,this thesis demonstrated that the pyrolysis of oil shale at a thin particle bed,low heating rate and reduced operating pressure had evidently suppressed secondary reactions,which thus caused the higher shale oil production.