Study On Flow Characteristic Evolution Of Single Crack In Lignite Under Thermo-mechanical Coupling

Lignite is a kind of fossil fuel with the lowest coalification degree.The lignite resources in China are very rich.According to the statistics,the proven reserves of lignite are 1303 million tons,accounting for about 13% of the country’s coal reserves.Lignite is the inferior coal,the degree of coalification is the lowest.In the air,it is easy to weathering,easy to break,and the burning point is low(about 270℃).The storage for two months is easy to spontaneous combustion,the water is large(generally 20%-50%),and the heating value is low(generally 5.45-14.65MJ/kg).Based on the characteristics of lignite,the unit energy transportation cost of lignite is relatively high during the long distance transportation process,so it is not conducive to long-distance transportation and long time storage.Because of the high water content,the water evaporation will take away a large amount of heat,which leads to the low thermal efficiency,which makes the use of lignite very limited.Therefore,the research and analysis of lignite dehydration and upgrading technology has important practical significance for the development and utilization of lignite.The new technology scheme of "lignite bed in-situ injection heat dehydration and upgrading" is put forward by Taiyuan University of Technology Coal Mining TechnologyResearch Institute.The technical scheme refers to the construction group wells in lignite bed to enter the lignite coal seam,using hydraulic fracturing to make the group well connected along the coal seam.A part of the well is used as the heat injection well,and a part of the well is used as the production well,and the hot water steam is injected into the coal seam from the heat injection well,so that the coal seam is heated by convection.To achieve the purpose of dewatering and upgrading of lignite coal seam.In view of this new subject,through experimental research and theoretical analysis,the evolution law of crack opening and permeability characteristics of single crack in lignite under the coupling of three axis stress and pyrolysis is studied by using a self-made three axis rock seepage test machine.The main conclusions of this paper are as follows:(1)according to the lignite pyrolysis weight loss curve,the three stages of lignite pyrolysis are obtained: at the stage of dehydration and degassing at25-350℃,the lignite has completed the drying and partial deoxidation process,and the first thermal decomposition stage of the lignite at 300-600 ℃,this stage is the active thermal decomposition stage of lignite,and the main depolymerization and decomposition reaction of lignite in this stage is in this stage.A large number of volatile substances are produced to produce the product of the tar liquid,and the second thermal decomposition stage of the lignite at600-900℃,the reaction of this stage is mainly the small molecule side chain of the polymerization reaction and the aromatic hydrocarbon edge joint or the exfoliate reaction of hydrogen;(2)the change trend of lignite conversion with temperature can be divided into three stages: the slow growth stage at 25-300℃,the rapid growth stage at300-600℃,and the deceleration and growth stage at 600-900℃;(3)the first order reaction kinetics model was used to analyze three kinds of lignite samples.It was found that the change law of activation energy of different lignite samples was the same,that the change law of activation energy of lignite was related to the heating rate;(4)under different temperature conditions,the permeability of the lignite sample decreases with the increase of the volume stress,but the change rule of the permeability of the lignite sample has a inflection point value with the increase of the volume stress.The inflection point value is 15 MPa at the room temperature to 200℃,and the inflection point value is 20 MPa at 300℃;(5)under different temperature conditions,the permeability of lignite sample decreases with the increase of pore pressure under different volume stress;(6)under the action of volume stress and pore pressure,the permeability of lignite increases first and then decreases with the increase of temperature.There is a peak permeability,and the same variation law is shown in the different volume stress.The rule of the permeability of lignite with the temperature changes basically conforms to the two parabolic type.(7)through the discussion of the engineering technical problems encountered in the process of in situ injection of lignite,the establishment of thefluid migration channel in the lignite coal seam,the formation and expansion mechanism of the lignite seam fracturing and the permeability characteristics of the fracture of the lignite coal seam are analyzed,and three problems are discussed,which provides technical support for the implementation of the technical scheme of in-situ thermal dewatering and upgrading of brown coal bed.