Theory And Application Of Sampling Method Based On Negative Pressure Pneumatic Conveying To Direct Determination Of Gas Content In Coal

Gas content is the key index for both the coal mine gas disasters prevention and the coal-bed methane exploitation.The shortcomings of the existing sampling methods were analyzed and the sampling method based on negative pressure pneumatic conveying was presented.The comprehensive research method,including method analysis,lab test,numerical simulation and the in-field test was adopted to study the sampling process and loss gas content calculation way.The sampling process was divided into four parts based on the pneumatic conveying theory.The prediction models of pressure drop and the critical velocity applied to the sampling methods were established,respectively.The system device of sampling based on negative pressure pneumatic conveying was developed.The pressure drop of drilling rod,the drilling pit and the coal sample collector were investigated under different drilling rod diameter and solid gas ratio,respectively.Models of coal particle additional pressure drop coefficient?_sin drilling rod and the local resistance coefficient?were presented based.The coal-gas flow characters in drilling pit and sample collector were investigated using FLUENT-EDEM coupling simulation method.The particle coal gas desorption pattern was studied by the self-developed particle coal gas desorption under negative pressure system.The experiments were carried out under adsorption equilibrium pressure were 500kPa,740kPa,1000kPa,and the negative pressure were-40kPa,-50kPa,-60kPa and the negative pressure effect time were 60s,90s,120,respectively.Agas loss calculation method was established.The precision and advantage of sampling method based on negative pressure pneumatic conveying was validated by the in-field test,comparing with the existing widely applied positive pressure pneumatic conveying.The experiment results of sampling method based on negative pressure pneumatic conveying showed that,1)The?_sdecreased with the Fr increasing at the same solid gas ratio and?_s decreased at the sameFr.?_s=am^bFr^c?a,b,c were the constant?;2)There was a gas velocity inducing minimum pressure drop and gas velocity increased with the drilling rod diameter increasing;3)The local resistance coefficient of drilling pit?could be described as?=487.05e^-0.102va and the pressure drop in pit played a main disadvantage role and should be optimized furtherly.The FLUENT-EDEM coupling simulation results about gas-coal flow in the drilling pit around showed that there were high velocity area and reverse velocity area for gas flow in the 0⁰.5m zone.High velocity area appeared above the axial line and became lager when the coal particle generation rate increasing.Reverse velocity area induced by the second flow in 0⁰.5m zone.The coal particle flow in 0⁰.5m zone had six characters with the gas velocity increasing:1)The particle number decreased;2)The thickness of coal particle layer in the bottom flow decreased;3)The highest thickness of coal particle layer moved further from the inlet;4)The low velocity?below1m/s?ratio decreased;5)Particle distribution along the flow direction became uniform;6)The character length of bottom-flow and the suspension acceleration decreased.The FLUENT-EDEM coupling simulation results about gas-coal flow in the sample collector?cyclone?showed that:1)The existence of particles was found to have a high impact on the conical separation and inlet regions;2)The particle flow pattern formed,and then remained steady when the gas velocity was lower than the particle velocity under the different solid gas ratios;3)The particle strand's descending distance became longer with the increased solid gas ratio;4)An ash top ring appeared,and became strong;5)The particle-wall interaction force was higher than the particle-particle interaction force at the same solid gas ratio,when the particle velocity higher was than gas velocity;6)The pressure drop first decreased with the low inlet solid gas ratio,and then remained independent of the inlet solid ratio when the inlet solid gas ratio was high.The particle coal gas desorption under negative pressure-atmospheric desorption after negative pressure desorption results showed that:1)Since increased negative pressure inducing gas cumulative desorption,the gas cumulative desorption amountincreased given a fixed effect time;2)analogously,under a fixed negative pressure,an increase of gas cumulative desorption amountcould be observed at a given effect time.The gas cumulative desorption amount had a negative relationship with the negative pressure and the effect time;3)Both the gas desorption pattern of gas desorption under negative pressure?0²mins?and in the first 10 minsatmospheric desorption after desorption under different negative pressure and effect time was well in agreement with the Barrel model.As a result,a gas loss calculation methodwas establishedbased on the negative pressure,and effect time,with the gas desorption pattern after pre-desorption under different negative pressure and effect time,withall the deviations were less than4.83%.The in-field test of sampling method based negative pressure pneumatic conveying showed that,The gas content measurement was enhanced using sampling method based negative pressure pneumatic conveying by 48.6%⁸0.3%when compared with thepositive pressure pneumatic conveying method for the coal sample purity was enhanced.