Temperature Rise Characteristics And Pore Structure Modification Of Coals During Microwave Irradiation

Coalbed methane,an associated product of coals,is not only a major disaster and greenhouse gas source,but also a highly efficient clean resource.Because the coal seam in China has the characteristic of high gas and low permeability,it is very important to increase the understanding of the effect of pore structure modification on degassing coalbed methane.In this paper,based on the engineering background of degassing coalbed methane by microwave heating,the effect of microwave irradiation on temperature,pore structure and macroscopic damage was studied by laboratory experiments,numerical simulation and theoretical analysis.A microwave heating system was designed and set up to do experiments.According to this experiments system,a numerical model was developed and found microwave source,microwave frequency,material placement,dielectric constant and samples' size will influence microwave heating.Several experiments were carried out and achieved some results as following:The effect of various factors on temperature rise characteristic was discussed.Microwave power,particle size,moisture and mineral content can significantly influence temperature rise,but the oxygen concentration is not obvious.During intermittent heating,once the water is evaporated,the temperature rise rate is no longer change.The relation between temperature,mass and microwave energy was established,and relation between temperature and specific heat capacity was also taken.The effect of microwave irradiation on the pore structure was also discussed.Pore aperture,pore numbers and porosity of coals increase after microwave heating.However,irreducible water saturation increases at first then decreases with microwave energy.Seepage pore connectivity decreases at first then increases with heating temperature.But seepage pore connectivity monotonously increase with microwave power.However,the effect of microwave irradiation on pore connectivity will be not understandable when particle size was too small.The effect of oxygen contention on pore structure is not obvious.Connectivity of seepage pore maintain well during intermittent heating.In the experiment,it is found that the film saturation model is more suitable than the bimodal saturation model for describing the distribution of bound water in the coal.It is also found the porosity can be characterized as the integral area of T2 spectra.The relaxation time 1.5 ms is critical value between adsorption pore and seepage pore.The theoretical formula of irreducible water saturation and fractal dimension was also given.The effect of microwave irradiation on irreducible water saturation and fractal dimension is related to the microwave energy.With microwave heating time increasing,fractures on the surface of coal samples will be induced and increased becoming fracture network.After microwave heating,the ultrasonic wave velocity of the coal sample is attenuated.Furthermore,a nonlinear compaction section was obvious in the axial stress-strain curve.Finally,the potential application of degassing coalbed methane by microwave irradiation was proposed.Because microwave irradiation would decrease methane adsorption capacity and increase permeability,the potential is confirmed.The effect of microwave irradiation on spontaneous combustion and water blocking of coals was also discussed.The above conclusions contribute to understand temperature rise characteristic and pore structure modification during microwave irradiation.It is also provided the theory reference for the practical application.In the Ph D study phase,9 research papers were published,including 6 papers were collected by SCI database and 2 papers were collected by EI database.In addition,10 invention patents were authorized.