Diffusion Dynamics Of Rapid Desorption Of Gas From Pulverized Coal And Its Influence On Transporting Coal And Rock In Outbursts

The possibility of coal and gas outburst disasters increases with the increasing mining depth of coal resources in China.A considerable amount of accidents in literature have shown that the geological zones with broken and soft coal in coal seams are the places where outbursts easily occur.In addition,it is not difficult to find sorted coal powders in outburst sites.This kind of highly fragmented coal powder possesses a damaged pore system,which makes themselves easy for gas to transport through,consequently leading to a desorption with a high rate and a great mass.Based on the theories of molecular diffusion,fractal geometry,rock mechanics,seepage mechanics,geochemistry and adsorption dynamics,this paper established a Fickian diffusion model containing pore geometric parameters with the mathematical methods of limit approximation,variable substitution and variable separation.The results obtained benefit to explain the internal mechanism of the increasing desorption rate with decreasing particle sizes,and help to clarify the role and existent necessity of rapid desorption gas from coal powders in the development stage of outbursts.The main conclusions are:1)The change in the internal constituents and pore characteristics of coal controls the transport of gas through coal.During coal's initial fragmentation,intact coal body will be separated into several new particles with a size larger than the matrix size,whose desorption rate remains constant.Then further fragmentation may produce coal particles with a size equal to the matrix size,at which point the desorption rate begins to increase.When the fragmentation goes on,the matrix may be affected at last,consequently leading to an increasingly higher desorption rate.The metamorphic grade of coal stays stable,but the damage becomes more serious with the increase of pore width in the fragmentation process.2)The pore-fracture dual-porosity system dominates the flow behavior of gas transporting through itself.The partition pore diameter of the dual-porosity system,which may decrease with the decreasing particle size,is in the order of 10 nm approximately.When the flow mass in pore system is greater than that in fracture system,the flow will behave like being chocked by a throttle,which can be called as the throttling flow;otherwise,the flow from the pore system will not fill the entire fracture space,which we can call the unpressure flow.The critical point that the throttling flow becomes the unpressure flow is related to the permeability of the two systems,the gas flow direction,the pressure gradient etc.3)Coal particles of different particle sizes exhibit different gas sorption characteristics.For adsorption,the Langmuir constant “a” and “b” have no clear relationship with the particle size.However,for desorption,a clear partition property between the desorption rate and the particle size was obtained.If the particle size is smaller than the desorption limit size,the desorption rate will hold an increasingly proportional relation with the particle size;otherwise,the desorption rate will remain almost invariant.The desorption limit sizes for the experimental samples are about 0.5~1 mm and have little relationship with equilibrium pressure.4)The attenuation law of Fickian diffusion coefficients over time was obtained.There are two methods to obtain the time-dependent Fickian diffusion coefficients: one is to adopt the variable substitution method to derive the results based on the dualpermeability model;the other one is to use the limit approximation method by considering the average Fickian diffusion coefficients as the dynamic Fickian coefficients to estimate their values.Apparent Fick diffusion coefficients exhibit a similar attenuation law with that of self-diffusion coefficients.They all experienced a rapid decreasing period with the increasing of particle size and then tend to become constant.The fragmentation process increases the initial and limit value of apparent Fickian diffusion coefficients,making the attenuation curve to move up.Meantime,the fragmentation enlarges the gap between the initial and limit value,leading to a more obvious attenuation characteristic.5)The Fickian diffusion model containing pore geometric parameters was established.Based on the experimental results of time-dependent Fickian diffusion coefficients and the similarity between the attenuation law of the apparent diffusion coefficients and that of the self-diffusion coefficients,we obtained an attenuation model of Fickian diffusion coefficients containing pore geometric parameters,eg.,pore width,pore length and pore shape,which can illustrate the controlling effect of geometric parameters on the attenuation law.According to derivation of the classical unipore model of constant diffusion coefficients,we obtained the analytical solution to the new model with time-dependent diffusion coefficients.By introducing the attenuation model into the analytical solution,we finaly obtained the Fickian diffusion model containing pore geometric parameters whose fitting effect reaches up to 0.9944,much higher than that of the original unipore model.It can effectively describe the influence of geometric parameters on the desorption curves.6)The expansion energy formed by the rapidly desorbed gas from coal powders is the main source of the energy transporting the outburst coal.In the development stage of outbursts,the gas of free state in outburst coal itself cannot meet the high energy demand and needs extra energy from the gas to participate.By the pneumatic conveying theory,the critical blockage velocity that characterizes the flow state from the blockage to the plug flow was obtained,which can be used to calculate the effective gas content in outbursts.Then,based on the optimizational unipore diffusion model for short times,the mathematical relationship between the initial desorption rate and the particle size was obtained.In the end,with the historical data regarding the Zhongliangshan outburst,we concluded that if the transport of outburst coal was completed,part of coal needs to crushed the coal into powders of 100 ?m or smaller,which can be certified by statistical size distributions in other outbursts occurring in the Zhongliangshan mine.