Electrostatic charging of water sprays by corona and induction for dust suppression

摘要

Dust control is a very significant issue in underground coal mining. The benefits ofreducing dust levels will be a lesser risk of lung disease to coal miners, improvedworking conditions and a reduced risk of dust explosions. Coal dust is commonlysuppressed by water sprays but suppression efficiency is not high because dust tendsto travel in the air flow round the water droplets rather than being captured by them. Ifwater sprays are electrostatically charged, then a significant improvement in dustsuppression efficiency may be achieved.Of the three principal droplet charging mechanisms, i.e. corona charging,induction charging and contact charging, corona charging is the most widely used inmany industrial fields including dust suppression, However, it requires a high appliedvoltage, ranging from thousands to more than a hundred thousand volts, depending onthe geometry of the charging equipment. Induction charging has been used inagricultural spraying since Law (1978) developed an embedded-electrode inductioncharging spraying nozzle. This nozzle provides a compact, inexpensively fabricateddroplet charger and, reduces design requirements on size and output voltage (of theorder of 1000 volts). It also reduces the potential for mechanical damage,misalignment and personnel hazard.In order to evaluate the effectiveness of dust suppression, either the charge onindividual droplets or the charge-to-mass ratio of water sprays needs to be known.However, the parameters which control the charge applied to water and the chargingrate have been unsolved theoretically for any charging mechanism. The existingtheories for the induction-charged and air-atomising a liquid jet have been found to beinadequate. And there is no theory available for corona charging of the dropletsproduced with a pneumatic nozzle in order to predict the spray charge level or thespray charge-to-mass ratio.In view of this situation, mathematical models have been developed in thisthesis for both the corona and induction charging mechanisms. During thedevelopment of the theories, it has been assumed that for corona charging, that the jetis disintegrated into droplets and the droplets are then charged; for induction charging,that the jet is first charged and the charged jet is then disintegrated into charged droplets. The Sauter mean diameter of the sprays, D32 , plays an important role inlinking the individual droplet charge to the spray charge-to-mass ratio for bothcharging mechanisms. The developed theories are general models suitable for anyliquid with both corona and induction charging.Theoretical calculations for the spray charge-to-mass ratio, individual dropletcharge and the ratio of droplet charge to the Rayleigh charge limit have been presentedfor almost all of the influencing electrical and mechanical parameters such as appliedvoltage, air flowrate, liquid flowrate, liquid conductivity, liquid dielectric constant,nozzle dimensions, cylindrical electrode dimensions, and fluid parameters, forexample, density, viscosity and surface tension. In the calculation for corona chargingof droplets, the effect of the droplets on corona current and corona-onset voltage hasbeen assessed for first time. The introduction of the Sauter mean diameter of thesprays, D32 , makes the assessment possible. Theoretical calculations for inductioncharging of liquid jets have shown that provided liquids have a conductivity valuehigher than the critical value, s = - 10 4 S/m, then they can be charged satisfactorily bythe induction charging method.Among all of the influencing parameters, the electrical and mechanicalparameters determine the charging rate and the water spray charge level. Thesuitability of employing these two charging mechanisms to dust suppression in coalmine has been evaluated based on the spray charge level, safety issues and thesimplicity or otherwise of the equipment. The induction charging method wasconsidered to have advantages over its corona charging counterpart, and has beenchosen for charging the water sprays in the experiment program. Water has aconductivity of s = - 10 2 S/m, higher than the critical value, s = - 10 4 S/m. Based upontheoretical considerations, it is concluded that water is an appropriate liquid for coronacharging, based on its dielectric constant, and for induction charging, based on itsconductivity.In order to facilitate the testing of electrostatically charged water spray cloudparameters, a spray charger/collector was designed and constructed by others and acomputerised data acquisition system has been employed. According to the theorydeveloped for induction charging, the optimum length of the charging electrode has been analysed based upon the assumption that water jet is first charged and then thecharged jet is disintegrated into charged droplets by the high pressure air.An experimental program examined the dependence of spray current uponfour parameters: air flowrate, water flowrate, applied voltage and jet diameter. Theexperiments have shown that the induction-charged air-atomising nozzle used in theexperiment is able to impart a significant charge into the water sprays. The spraycharge-to-mass ratio calculated based upon the measured spray current demonstratesthe same characteristics as predicted by theory: increasing with air flowrate, decreasingwith water flowrate, increasing with applied voltage to a peak value then decreasingwith further increase in the voltage, and increasing with jet diameter.A successful interpretation of an important phenomenon in the inductionchargedair-atomising a water jet, that spray charge-to-mass ratio and spray currentincrease with air pressure (or air flowrate) and decrease with increasing water flowrate,has been achieved based on the theories developed in this thesis. This phenomenonoccurs because increasing air flowrate and/or decreasing water flowrate leads to ahigher velocity of jet flowing through the induction electrode. However, when waterflowrate becomes very small, a decrease in spray current with increasing air pressure(or flowrate) may be caused both by jet breakup inside the electrode and bycontraction of the jet.In order to verify the theory, a preliminary comparison of experimental datawith theoretical predictions employing a constant kic in the air-jet interactioncoefficient a which was assumed to be 0.7 has shown a general agreement. However,the value of kic appears to be related to jet diameter, air/water mass ratio and appliedvoltage. An empirical equation for kic has then been formulated based on theexperimental data for the spray charge-to-mass ratio. Finally a comparison ofexperimental results with theoretical predictions using the formulated kic shows animproved agreement.It is concluded that the induction-charged air-atomising nozzle has a potentialapplication in dust suppression in coal mines, as the voltage required to charge watersprays is only 1000 ~ 1200 volts, and that the theory developed for induction chargingof water sprays can be used to guide laboratory investigations and design processes for dust suppression and other industrial applications which might employ theelectrostatic charging of liquid sprays.