Theoretical And Experimental Study On Thermal Initiation Process Of Autothermic Pyrolysis In-situ Conversion Technology For Oil Shale

In recent years,the increasing energy consumption and decreasing fossil energy resources have resulted in a growing stress on maintaining energy supply,and the development and transformation of the energy industry is facing new requirements and challenges.As a new type of unconventional fossil energy with huge reserves and wide distribution,the successful exploitation and utilization of oil shale is of great strategic significance to alleviate the energy tension in China.At present,autothermal in-situ conversion technology（ATS）is one of the most promising in-situ oil shale exploitation technologies,which mainly consists of two parts:the thermal initiation in the early stage and the reaction control in the later stage.Among them,thermal initiation includes the ignition of the oil shale reservoir in the initiation stage and the continuous generation and utilization of the heat donor in the chain reaction stage,which is the basis and key for the transformation of the ATS technology from the preheating stage to the chain reaction stage.Therefore,this paper focuses on the deficiencies in the current thermal initiation studies of oil shale and carries out theoretical and experimental studies of the thermal initiation process through various methods such as isothermal thermogravimetric experiments,indoor aluminum retort experiments,theoretical calculation analysis,and in-situ simulation experiments.The specific research contents of this paper and the main conclusions obtained are as follows:（1）The reaction characteristics,kinetic parameters,and gas product release properties of Huadian oil shale powder samples in O₂/N₂ mixture with different oxygen contents and at different constant final temperatures in the preheating and initiation stages were investigated by TG-FTIR technique using a conventional isothermal kinetic model.The results show that with the increase of reaction temperature,the oil shale show three different reaction behaviors in the initiation stage,which are oxidative weight gain,oxidative weight gain followed by reaction consumption,and reaction consumption.In addition,the activation energy of the oxidation weight gain process shows a trend of first decreasing and then increasing with the increase of oxygen content.The oxidation reaction process of the initiation stage is carried out in two main sub-reactions.With the increase of oxygen content,the activation energy of the first sub-reaction increases continuously and finally decreases abruptly at 100%O₂.In the second sub-reaction,the activation energy of the reaction increases slowly at first and then increases suddenly and significantly at100%O₂.It is assumed that the first sub-reaction is mainly the oxidation and pyrolysis of easily reactive components such as aliphatic hydrocarbons,while the second sub-reaction is mainly the oxidation of difficult components such as residual carbon and heavy oil.The products generated in the preheating stage of oil shale are mainly hydrocarbon gases,while the products in the initiation stage are mainly CO₂.As the preheating temperature continues to increase,the more intense the CO₂ output,the shorter the required reaction time and the more concentrated the release.（2）The oxidative assisted pyrolysis（OP）experiments were carried out on Huadian oil shale particle samples to systematically investigate the product composition and the OP reaction characteristics of oil shale under different preheating temperatures and oxygen flow rates,clarify the variations of oil and residue yields under different experimental conditions,and reveal the evolution of product components with preheating temperature and oxygen flow rate.The results show that the OP reaction process of oil shale is mainly controlled by the internal temperature,which is determined by the superposition of preheating temperature and self-heating effect.In addition,the self-heating effect is significantly influenced by the oxygen flow rate,and its intensity increases with the increase of oxygen flow rate.First,the OP reaction in oil shale shows a clear thermal initiation process at low preheating temperatures（～190°C）,indicating the existence of a first boundary temperature for the OP reaction,which is called the“initiation temperature”.Only when the preheating temperature exceeds the initiation temperature,the organic matter in the oil shale can react with oxygen and produce self-heating effect,which promotes the cracking and consumption of kerogen,residual organic matter,and residual carbon.When the internal temperature of oil shale exceeds the second boundary temperature of the OP reaction（called“ignition temperature”）,a large amount of residual organic matter and hydrocarbon products begin to react with oxygen for complete oxidation,resulting in a decrease in oil production.The effect of oxygen in the OP reaction is divided into three stages depending on the initiation and ignition temperatures.Oxygen has no effect on oil and gas output in the first stage,plays a boosting role in the second stage,and shows a significant consumption effect in the third stage.The self-heating effect in the OP reaction is mainly caused by the consumption of saturated aliphatic hydrocarbons,aromatic hydrocarbons,light components and residual carbon by-products at low temperatures,which promotes the pyrolysis of kerogen and increases the yield of resin,asphaltene,and heavy components.（3）The Huadian oil shale residues at different heat treatment temperatures were prepared,and the combustion activation energy,combustion characteristics parameters,combustion characteristics index,and combustion product release index of the residues were calculated and discussed respectively,and the calorific values of the residues was also measured.Moreover,the heat supply and demand relationship and the strategy of temperature parameter optimization at different stages of the ATS process were discussed.The results show that with the increase of pyrolysis temperature,the calorific value of residue gose through three different stages of increase,decrease and stability,and reaches the maximum value around 330°C.At330°C,a large amount of heat absorbed during the thermal treatment is stored in the organic matter inside the oil shale,resulting in substantial conversion of kerogen into asphalt.Moreover,the combustion reaction characteristics of the residue decrease significantly around 330°C due to severe pore blockage caused by the combination of massive bitumen generation and ignition coking.In the high temperature section（>350°C）,the combustion reaction characteristics of the residue continue to decrease due to the decreasing volatile content and the continuous generation of residual carbon and heavy fractions in the residue.（4）An oil shale in-situ thermal initiation apparatus was designed and produced,and a series of thermal initiation experiments were conducted on pressed Huadian oil shale cores using this apparatus,so as to investigate the effects of front-end preheating temperature,heating time,and air flow rate on the oil shale in-situ thermal initiation process.The results show that the oil shale cores exhibit two distinctly different situations of initiation success and initiation failure under different conditions.When the initiation is successful,the internal temperature of the core is continuously increased to above 500–600°C and maintained for a long time,forming a stable automatic thermal chain reaction inside.The organic matter inside the core is consumed in large quantities mainly by oxidation reactions.The residue,residual organic matter,TOC and residual heat are substantially reduced.The generated products are mainly water and gas,and only a small amount of oil products is generated.While the temperature inside the core keeps decreasing trend.At this time,the temperatures at different locations inside the core all keep decreasing,the organic matter is basically not consumed,and almost no oil and gas water products are generated.Based on the above results,it can be seen that the preheating temperature and heating time determine the amount of heat input to the core interior during the preheating stage,and the air flow rate determines how much heat is generated inside the core during the air injection stage.The heat input threshold for successful initiation is between 416–704 J/g at an air flow rate of 1000 ml/min.For a heat input of 704 J/g,the air flow threshold for a successful initiation is between 500–1000ml/min.Moreover,these two thresholds affect each other and a change in one may cause a change in the other.Only when both thresholds are satisfied can the oil shale core be successfully initiated and a stable automatic thermal chain reaction be formed.This study aims to enrich the basic theory of thermal initiation,clarify the influence mechanism of external factors on the initiation reaction process,elucidate the heat supply and demand relationship of the thermal initiation process,and explore the in-situ thermal initiation boundary conditions and control mechanism,so as to provide guidance and reference for the construction and regulation of the thermal initiation process in the ATS technology.