Thermodynamical Response Of Coal And The Hydraulic-mechanical Coupling Mechanism Under Microwave Irradiation

Coal mine gas(coalbed methane,CBM)is not just a disaster resource but a clean energy.CBM exploitation can optimize energy structure and control gas accidents.Given the high content of methane and low permeability of coalbeds,methods for enhancing CBM recovery were proposed.These methods can promote gas desorption or enhance coal permeability by disturbing coal petrophysics.However,traditional hydraulic methods will induce water blocking damage.New permeability-enhancing methods have limitations such as complex process,high energy consumption or severe pollution.This thesis introduced a microwave irradiation system.Thermodynamical response of coal and the hydraulic-mechanical coupling mechanism under microwave irradiation were investigated through theoretical analyses,physical experiments and numerical simulations.The following conclusions were drown:First,a microwave irradiation system was introduced and a multiphase porous media model of coal in the microwave field was established.The thermodynamical response of coal to microwaves was simulated.It was found that thermal effects of microwaves on coal include heat production by dielectric loss,heat dissipation by water evaporation,heat convection in coal and surface heat dissipation.The water in coal evaporates under high temperature and the vapor will diffuse into the air.The uneven distribution of temperature in coal can induce the asynchronism of water evaporation,which in turn causes eruption of high pressure steam and coal damage.It is also found that the increase in the microwave power can enhance the asynchronism and heterogeneity of heating.Increasing the moisture content helps accurating the heating rate.Second,the effects of microwave irradiation on the molecular structure,pore morphology and distribution of coal were investigated by Fourier transform infrared spectroscopy,scanning electron microscope,mercury porosimetry and nuclear magnetic resonance(NMR),respectively.The results show that microwave can induce the decomposition of aliphatic hydrocarbon in coal.The impact of high temperature and pressure gas can cause the opening and interconnection of closed pores and the increase in the pore volume.Coal will experience aromatized reaction and condensation polymerization and,as a result,the aromatic degree is improved.Micropores decrease whereas mesopores and macropores increase,resulting in the decrease in the specific surface area of pore.In addition,microwave can also reduce hydrophilic oxygen-containing functional groups like hydroxyl,carbonyl and carboxyl,degrading water binding of coal and the gas adsorption capacity.A cyclic experiment was designed to study the effects of microwave irradiation on the macroscopic fracture,temperature and wave velocity.It is noted that microwave can induce fractures in coal.Increasing the microwave power can promote the development and connection of microcracks.As the irradiation time increases,the thermal damage accumulates,reducing the wave velocity.Furthermore,the larger the coal anisotropy,the greater microwave fracturing.Also,the increase in the water content helps microwave fracturing.Then,the effects of microwave irradiation on the dewatering,wettability,permeability and gas migration were investigated by NMR,contact angle tester,permeability tester and gas desorption apparatus,respectively.The results show that microwave irradiation can decrease coal wettability by reducing hydrophilic functional groups.Coal permeability decreases with the effective stress and increases with the water content.Microwave irradiation can also accurate gas desorption and increase the total desorption volume,indicating that microwave irradiation can facilitate gas flow by improve the pore structure.At last,a fully coupled electromagnetic-thermal-hydraulic-mechanical model was developed to study the effects of microwave irradiation on gas storage and transport.The results show that coal matrix shrinkage induced by gas desorption under microwave heating is the key factor that enhance coal porosity and permeability.The cumulative gas extraction quantity under microwave stimulation is 43.9% larger that that under regular extraction.Continuous microwave irradiation under lower powers can not only maintain high extraction rate but also prevent coalbed overheating.In addition,microwave stimulation is more applicable to gassy coal beds.During the postgraduate period,18 publications were made,in which 10 ones were the first author?identity(6 indexed by SCI-Expanded database and 2 indexed by EI database).Twenty-four patents were applied for,in which 12 were authorized.Four provincial/ministerial awards were obtained.