Free Radical Reactions In Thermal Conversion Of Heavy Organics

Under the background of global CO₂ emissions reduction,the utilization of traditional energy,especially heavy organic resources like coal,oil and biomass,should be more efficient and environmentally-friendly.Thermal conversion of heavy organics is the common approach to produce fuels and valuable chemicals,which follows the free radical reaction mechanism.Therefore,radical reactions of heavy organics in thermal conversion is the key point for the efficient and environmentally-friendly utilization of heavy organic resources.According to the free radical reaction mechanism,the radical reactions include 3 steps:（1）radical initiation reaction,heavy organics crack into radical fragments containing one or more unpaired electrons;（2）radical propagation reaction,free radicals crack into smaller fragments or isomerize into new radical molecules;（3）radial termination reaction,free radicals couple with each other to form stable molecules.However,the present study on free radicals during thermal conversion of heavy organics is restrained due to the difficulty in detecting free radicals,resulting the huge challenge to study radical reactions of heavy organics in thermal conversion.Therefore,this dissertation firstly studied the reactivity of different free radicals of heavy organics in thermal conversion.Secondly,the dissertation proposed 3 approaches to control the radical reactions of heavy organics in thermal conversion from the aspects of changing the radical initiation pathways,changing the radical propagation process and changing the radical termination pathways.The main results are described as follows:（1）Our group has revealed that reactive radicals generated in the thermal conversion of heavy organics are more fundamental than stable radicals detected by ESR since the quantity of reactive radicals is 3 magnitudes higher than the quantity of stable radicals.However,there is no method to distinguish the activity of reactive radicals.Besides,the behavior of reactive radicals of heavy organics in thermal conversion is still not clear.Therefore,the chapter 2quantified the reactive radicals with different reactivities of two heavy tar samples at 250–400°C using 9,10-dihydroanthracene（DHA）and tetrahydronaphthalene（THN）as hydrogen donor solvents.It was found that low-active radicals are generated faster in the first stage of thermal cracking（first 2–3 min at 300–400°C）while high-active radicals dominate in the second stage.The quantity of reactive radicals is related to the structure of heavy tar samples.Higher contents of resins and asphaltenes as well as longer average methylene chain,more reactive radicals will be generated in thermal cracking of heavy tar.（2）According to the radical reaction mechanism,the main energy barrier of naphtha cracking is the radical initiation step.If an active radical which could be formed at a lower temperature is introduced,it will possibly initiate the hydrocarbon cracking and change the subsequent radical reaction pathway,resulting in lower energy consumption of whole process.Therefore,the chapter3 investigated the conversion of naphtha model compound（n-pentane,n-hexane and n-heptane）with and without bibenzyl in 0.5–30 min at 400°C and 440°C.To reveal the radical induced cracking process,the gaseous products and liquid products were analyzed in detail.It was found that bibenzyl can increase the conversion of n-hexane and the yield of gas products.Benzyl radicals can abstract H atoms from n-hexane,which changes the initiation pathway of original radicals in n-hexane cracking.The inducing effect of benzyl radicals is related to the reaction temperature as well as the amount of bibenzyl.（3）Catalytic pyrolysis of waste plastics includes the pyrolysis of waste plastic to generate volatiles and the catalysis of volatiles over catalyst surface.However,the radical reactions of volatiles were neglected in most studies on plastic catalytic pyrolysis.Therefore,to control the distribution and yield of waste plastic catalytic pyrolysis,chapter 4 changed the radical reactions of waste plastic（polyethylene）by changing the volatile reaction conditions in thermal conversion.It was found that the liquid product was mainly C₇–C₄₂ at volatilization temperatures of 300–600°C with a volatilization time of 17.7–53.0 s.The yield of liquid products was 49.5–87.2 wt.%,32.8–44.6 wt.%of which was alkenes.The volatilization temperature and volatilization time influence the radical propagation reactions of volatiles in thermal conversion,resulting in the changes of both the yield and distribution of pyrolysis products.The carbon number of liquid products is related to the catalytic reaction of volatiles over HY zeolites while the contents of alkanes and alkenes are influenced by the radical reactions of volatiles.（4）Large volumes of waste petroleum coke stockpiled in open yard not only represent a huge loss of valuable material but also pose a significant risk to the environment.Carbon-based catalysts have been widely applied in catalytic removal of biomass gasification tar.However,the carbon-based catalysts will be deactivated by coke depositing on the catalyst surface.According to the mechanistic studies,the deposited coke is generated from coupling of radical fragments in thermal conversion.In other words,coke deposition is caused by the radical termination reactions over carbon-based catalysts.Therefore,chapter 5 proposed a strategy for waste petroleum coke valorization by exploring its catalytic performance of biomass gasification tar destruction.Waste petroleum coke was firstly activated by KOH to obtain high specific surface area as well as low sulfur and ash contents.In order to maintain the catalytic performance of petroleum coke derived catalyst,the radical termination reactions were changed by changing the gas agents（N₂,H₂ and steam）in catalytic removal of biomass gasification model compound（naphthalene）.It was found that the H·radicals from the dissociation of H₂ can quench the reactive radicals through radical termination reactions,leading to the good resistance to catalyst deactivation under H₂ atmosphere.During the reaction of carbonaceous support and H·radicals,the H-rich groups,O-rich groups and N-rich groups were removed,accompanied by the larger aromatic rings and higher graphite degree in carbonaceous support.Under steam atmosphere,H·and OH·free radicals generated from the dissociation of H₂O can regress into the structure of carbon support as well as the deposited coke through steam gasification,generating new micropores and O-containing groups（-Car-C=O and-C-O）in carbonaceous support.