Electrospinning/Electrospray
ELECTROSTATIC PESTICIDES SPRAYING
2.4. ELECTROSTATIC PESTICIDES SPRAYING
DESCRIPTION
In order to protect food and fiber crops against insects, desease and weed pests used agricultural chemicals such as insecticides, fungicides and herbicides. With classical methods more chemical than theoretically nedded is often applied due to the variability in field conditions and the need to ensure complete coverage. In this case, 95% of the chemical applied can be wasted to the ground, for soil pollution, or at most 50% of mass transfer onto the desired plant. The subchapter shows that electrostatic spraying can offers a possible solution to those environmental problems by reducing spray drift and improving coverage of chemical to target plant. In the subchapter are presented principle of Electrostatic Spraying, the equippments, technological aspects and application.
CONTENT
2.4.1. Introduction
In order to protect food and fiber crops against insect, disease and weed pests, usage of agricultural chemicals such as insecticides , fungicides and herbicide is essential [1].
Entomological studies have established that in numerous cases, smaller droplets of pesticide spray provide greater biological efficacy per unit mass of pesticide than do the larger droplets for achieving insect control [2,3]. Thus, the recent concept of spraying is to spray the target pest more efficiently by selecting optimum droplet size and density for maximum retention and coverage. However, the droplet size requirement of many target pests are not always clear as there are conflicting requirements in relation to safety, coverage or cost. Customarily, more chemical than theoretically needed is often applied due to the variability in field conditions and the need to ensure complete coverage. Some cases in rather old data, 95% of the chemical applied can be wasted to the ground [4] or at most 50% of mass transfer onto the desired plant [5].
The optimum droplet size for maximum retention with an aqueous solution is reported to be 100 mm or less and such a reduction in droplet size would also improve coverage due to an increase in the number of droplets at the same volume application rate [6].
Thus, if drift is not a problem, a decrease in droplet size increases retention and coverage. The use of small droplets is, however, not so popular with the environmentalists as it is feared that non-target plants and organisms outside the treated area may be affected.
Electrostatic spraying would offers a possible solution to those environmental problems; by reducing spray drift and improving coverage of chemical to target plant. The idea of electrostatic spraying had been examined as early as in the 1940’s, and since then various prototype sprayers and commercial machines have been developed for years [7-18]. These application areas broadly include ground equipment for spraying plaints of row crops , orchards, and greenhouse, even aircraft spraying. By using the embedded electrode induction nozzle, which was developed by University of Georgia, the commercial greenhouse and row-crop electrostatic machines, which are marketed by Electrostatic Spraying Systems, Inc. are now in routine crop production use [19].
2.4.2. Principle of Electrostatic Spraying
One of the most important applications of electrostatics is the electrostatic spray coating that is now widely used in automobile industry. The principle of spry coating and the electrostatic crop praying is the same where a charged cloud of droplets is sprayed towards a grounded object driven by electrostatic force. Although there are many different methods of electrostatic spraying , the simplest model is as illustrated in Fig.2.4.1.
In this simplest case, the nozzle is raised in high potential. Thus the electric field lines departing from the nozzle terminate at the grounded object.
Although there are other electrostatic forces , the largest force in droplet dynamics would be Coulombic force.
(1)
where q: electric charge, E: electric field .
When the charged droplets are emitted at the nozzle, those charged particles are forced to move along the field lines according to above force. Since the electrostatic force is much stronger than gravitational or inertia forces for a small particle, the motion of a charged particle is easily controlled by the electrostatic force. The dynamic equation of a particle motion is expressed in the following equation where both electrostatic force and gravitational force exist:
(2)
where m: mass of particle, u: velocity, a: diameter of particle; h: viscosity of air and g: gravitational constant.
From this equation, terminal velocities can be obtained either external force is neglected.
only with the electrostatic force (3)
only with the gravitational force (4)
The ratio of these velocities becomes as follows:
(5)
Figure 2.4.1. - Trajectory of a charged particle with a simple object
The charge-to-mass ratio g/m is and important factor in electrostatics . The maximum charge-to-mass ratio of a particle could be estimated from many different point of views.
The largest value of the charge-to-mass ratio seems to be Rayleigh limit. If we assume a particle radius to be 100 mm, the electric field intensity 3 x 106 V/m and take a value of 10-2 C/kg for the charge-to-mass ratio, which approximately corresponds to Rayleigh limit, the terminal velocity ratio of Eq.5 becomes approximately 3000. This means that when a particle is highly charged, the electrostatic force is extremely strong. Compared with gravitational force. This can explain why the deposition efficiency to the target increases extensively with the electrostatic force.
2.4.3. Charged Droplet Generation
Since the electrostatic force is expressed with a product of a charge and an electric field in Eq.1, highly charged particle is desirable to obtain strong electrostatic force. To achieve highly charged droplet, there are various means.
The first method is to use centrifugal force and simultaneously charge the droplets by electrical conduction. This technique to produce the droplets by a hydraulic nozzle (or pressurized nozzle) and electrify each droplet during its formation by induction charging, Liquid is forced through a small orifice under pressure to form a sheet which then breaks up to droplets. The third one is very similar to the second, but the difference is to use a twin-fluid atomizer, which utilizes mechanical force of airflow for liquid disintegration. Latter two techniques are also widely used in industrial paint spraying.
The fourth method of producing charged droplets is directly to use the electrostatic force applying to the liquid surface emerging from a nozzle or a narrow gap outlet. In this method, the disintegration of droplet and the charging are simultaneously performed by the electrostatic field. This method is sometimes called electrostatic spraying or electrohydrodynamic spraying. Supply of liquids achieved from a reservoir by gentle hydrostatic pressure .
The fifth method is to utilize corona discharge . In electrostatic paint spraying, corona charging is often used, especially in powder coating. However, even if other charging methods are used in pesticide spraying, this corona discharge might be occurring and do charge the disintegrated droplets.
For the case of powder pesticide, triboelectrification is another possible method of charging. Recently, the electrostatic dry powder coating became quite popular in metal finishing. The reason of using dry powder instead of wet paint is to eliminate solvent from environmental protection point of view, The triboelectrification is the main charging method of powder paint as well corona charging.
2.4.4. Driving and Deposition Force
Simplest model of electrostatic spraying is already illustrated in Fig.2.4.1, where charged droplets are ejected from the nozzle and the nozzle is raised to a high potential. Since the charge-to-mass ratio is extremely high as stated before, the charged droplets follow the electric field lines and reach even backside of the object. On the contrary, if there is only inertial force is utilized at the nozzle, droplets travel straightly outwards and no droplet could reach the backside of object. For realistic configuration of the object, such as live plant, the situation is the same as illustrated in Fig.2.4.2.
To obtain the high electric field , the potential of the nozzle must be as high as a few 10 s kilovolts to 100 kV, which are customary used in paint spraying. However, this high voltage is quite dangerous for agricultural use. Thus, many different approaches have been tried. On of the method is to utilize the airflow assisted method. The force air near to the crops conveys the charged droplets . When the charged droplets moved near the crops, there are other electrostatic forces become predominant, such as space charge force and image force.
When charged droplets are emitted at the nozzle, they form space charge cloud . Electric field due to this space charge cloud could be calculated by using Poisson’s equation.
Since the general solution is extremely difficult, a simple one-dimensional model with the spherical coordinates could be calculated.
(6)
where f: potential, r: charge density, e0: permitivity of free space.
By solving this equation as a function of r, potential f and electric field E can be obtained.
(7)
(8)
where C1 and C2 are integration constants.
Figure 2.4.2. - Particle trajectory to the plant
Figure 2.4.3. Surface charge integration scheme
Image force of a charged particle can be simple estimated from the Coulombic force between the real and the imaginary particles. However, more precise solution of force for surface charge distribution must be integrated as shown in fig.2.4.3 by using the following equation [12]:
(9)
where a: radius of a sphere, s: surface charge density,
x, x*: distances measured from the conducting surface.
2.4.5. Various Types of Pesticide Spraying
Since the dusting of agricultural crops was a standard practice for controlling plant pests and diseases in 1950’s or earlier, the attempt to use electrostatic force is concentrated on dust. In earlier works, the dust particles are carried in air stream through ionized air, which is formed by a needle or fine wire at the center of a grounded metal tube. The small radius of the needlepoint or wire allows a corona discharge to form.
The schematic configuration of the electrodes used by Bowen et al [8] is shown in Fig.2.4.4 where negative 12 kV DC high voltage is applied to the wire electrode. After the laboratory tests under various humidity conditions, field tests were performed. Power for charging the dust on the field model is obtained from a 6 V battery carried on the machine. This 6 V DC was used to drive a dynamotor to raise to 300 V DC. The electronic unit produced high voltage of 12 kV from this DC 300 V. They have tried dusting work on potatoes, onions, peas, celery, beans and clover. According to their results, significant improvement was recorded on some of the plots where comparisons were made between charged and uncharged dust. Another type of electrode is shown in Fig.2.4.5 [9].
Since than, it is recognized that the application of agricultural chemicals in the form of sprays has more advantages than airborne dusts in handling and measuring. Thus, attempt to use electrostatic force for agricultural chemicals, is directed to spraying system.
Figure 2.4.4. - Wire electrode (Bowen)
Figure 2.4.5. - Needle electrode (Bowen)
In the electrostatic paint spraying, either corona charging or induction charging is commonly used to get highly charged droplets . Since the distance between the nozzle and the plant object to be sprayed is relatively wide, the high voltage of few 10 s of kilovolt must be applied to get enough induction charge. However, Law and Co-workers at the University of Georgia recognized that if the spacing between the nozzle and the induction electrode is very small, the applied voltage could be much lowered.
Thus, they developed the “embedded-electrode electrostatic-induction nozzle”.
The principle of this nozzle is illustrated in Fig.2.4.6 where air and liquid enter separately into the nozzle. The air moves at a high speed through the nozzle and disintegrates liquid into droplets. The droplets are charged by induction at the nozzle. High speed air flow through the annular area assures that the droplets are swept past the electrode and propelled outward from the orifice of the nozzle. Experimental results revealed that with 1 kV of applied voltage, 10-2 C/kg was obtained [11]. By using this nozzle, ESS Company produces various versions of the electrostatic sprayer in the worldwide market [19].
Coffee has again tried to develop and electrostatic dusting machine in 1970’s [12].
Although he tried to charge particles by bombardment charging method, main work was to develop the “triboelectrogasdynamic” generator system. Within a metal pipe, powders were recirculated by airflow and tribocharged. By using a principle of electrogas dynamics, high voltage is generated by these charged powders. Thus, the need for any external high-voltage source for particle charging and also driving charged particles in dusting process could be eliminated. Schematic diagram of TEGD machine is shown in Fig.2.4.7, with which the voltage as high 200 kV is obtained.
Figure 2.4.6. “Embedded induction nozzle” (Law)
Another approach to pesticide application was to use electrostatic force for disintegration of liquid, which is known as ‘electrostatic spraying ’ or ‘electrostatic atomization ’. Coffee developed the ‘Electrodyn’ by using this principle where no mechanical energy or no compressed air is required. Schematic diagram of ‘Electrodyn’ is shown in Fig.2.4.8. The atomizing nozzle consists of a pair of electrodes and narrow-gap outlet for liquid flow. It was shown that as low spray volumes as 0,5 l/ha was effective for cotton, which corresponds to a nozzle flow-rate of about 0.1 ml/sec.
Figure 2.4.7. - Schematic diagram of TEGD (Coffee)
Figure 2.4.8. - “Electrodyn” (Coffee)
To cover completely orchard tree with pesticide requires as large as 60 gals of mixture. Depending on their size, the pesticide particles do not impact direction the foliage and fall to the ground pr are entrained by air currents to eventually deposit developed by Inculet and Castle [14–6]. The sprayer developed was schematically illustrated in Fig.2.4.9.
Figure 2.4.9. - Electrostatic spray system (Inculet, Castle)
Figure 2.4.10. - Electrostatic forces
The low-volume mechanical sprayer manufactured by Kinkelder Co. was modifies for this purpose, which can atomize the liquid pesticides into a very dense fine spray (mmd 80 mm). The original cast aluminum heads were replaced by fiberglass-reinforced plastic. Opposite to each of the air shear nozzle, a petal electrode was embedded flush with the surface and energized from a 18 kV voltage power supply.
As stated before, generally Coulombic force is very effective for small plants.
However, extremely high velocity air (~300 km/h) could carry charged droplets directly near the trees. When these charged droplets approach the tree, other electrostatic forces become predominant. One is the image force and the other is space charge force as illustrated in Fig.2.4.10. They have tested this machine with McIntosh trees and marked improved deposition was found in the upper canopy of the trees and the uniformity.
Figure 2.4.11. - Electrostatic sprayer with rotary cup atomizer (Asano)
Asano’s approach was to apply high voltage directly to the nozzle not only for charging, but also for driving the charged particles toward the object. Although this principle is widely used in electrostatic paint spraying, it requires quite high voltage of power supply. Thus, the nozzle and liquid must be insulated from the ground. As shown in Fig.2.4.11, a rotary cup atomizer is used and driven by a high-speed motor up to 10,000 rpm. DC 50 kV is directly applied to the cup where liquid is fed through a pipe from the tank. Beans were chosen as sample plants because of their shape for from the tank, Bean were chosen as sample plants because of their suitable shape for spraying. The deposition rate on the leaves is measured as follows. The testing paper (mirror-coat paper) is attached on the upper and lower part of plant as shown in Fig.2.4.12. Then, the deposition rate on the papers is measured by using the image-processing system. The measured results shown in Fig.2.4.13 indicate that the deposition rate on the backside of the leaves is remarkably improved.
Figure 2.4.12. - Sampling method
It was found from the field test that the electrostatic sprayer of Fig.2.4.11 couldn’t be effectively used for tall and dense plants because the charged droplets cannot penetrate inside canopy.
Figure 2.4.13. - Deposition on leaves (Asano)
Figure 2.4.14. Electrostatic sprayer with air-assisted rotary atomizer (Asano)
Thus, air-assisted version of the sprayer was developed as shown in Fig.2.4.14. Although is showed good results, the improvement of deposition efficiency is not much as the machine of Fig.2.4.11, because the effect of blown air is another factor of improvement.
2.4.6. References
1. Law, S.E.(1995) Electrostatics technology for agricultural and biological applications status and trends, Inst.Phys.Conf.143, 1.
2. Law, S.E.(1995) Electrostatic Atomization and Spraying in Handbook of Electrostatic Processes Chap.20, Ed.by Chang, Kelly and Crowley, Marcel Dekker, New York
3. Fraser, R.P. (1958) The fuild kinetics of application of pesticidal chemicals, Advances in Pest Cont. Res. 2,1.
4. Graham-Bryce, I.J. (1977) Crop protection: a consideration of the effectivenes and disadvantages of current methods and of the scope for improvement, Phil.TransRoy.Soc.Lond.B281, 163.
5. Pimentel, D. and Levitan,L. (1986) Pesticides: amounts applied and amounts reaching pests, Bioscience 36 (2),86.
6. Heijne, C.G. (March 1980) A review of pesticide application system, Symp. On Spray.Sys. 1980’s, BCPC 75.
7. Hampe, P. (1947) Les poudreuses electriques a champ lonise, Review de Viticulture 93, 259.
8. Bowen,D.D., Hebblethwaite, P. and Carleton, W.M. (1952) Application of electrostatic charging to the deposition of insecticides and fungicides on plant surfaces, Agr.Eng. 33, 347.
9. Splinter, W.E. and Bowen, H.D. (1963) Electrostatic charging offers improved chemical deposition, Agr.Chem. 20.
10. Asano, K. Electrostatic Pesticide Spraying, in the Modern Problems of Electrostatics with Application in Environment Protection, NATO ASI Series.
11. Law, S.E. and Bowen, H.D. (1966) Charging liquid spray by electrostatic induction, trans. ASAE 9, 501.
12. Law, S.E. (1968) Embedded-electrode electrostatic-induction spray-charging nozzle: theoretical and engineering design, Trans. ASAE 21 (6), 1096.
13. Coffee, R.A. (1974) Depositional control of macroscopic particles by high-strength electric-field propulsion, IEEE Trans. Ind. Appl. IA-10, 511.
14. Coffee, R.A. (1979) Electrdynamic energy – a new approach to pesticide application, BCPC Conf. Pests and Disease, Brighton.
15. Inculet, I.I. Castle, G.S.P. and Kelly, C.B. (1976) Electrostatic application of pesticides in orchards by means of a narrow jet of particles, Conf.Rec. IEEE IAS, Chicago 174.
16. Inculet, I.I. Castle, G.S.P. Menzies, D.R. and Frank, R. (1981) Deposition studies with a novel form of electrostatic crop sprayer, J.Electrostat. 10, 65.
17. Castle, G.S.P. and Inculet,I.I. (1983) Space charge effects in orchard spraying, IEEE Trans.IA, IA-19 (3), 476.
18. Asano,K. (1983) Rep.Special Res.Project.Environ.Sci., B198-R34, Ministry of Education Japan, pp.167.
19. Asano, K.(1986) Electrostatic spraying of liquid pesticide, J.Electrostat. 18, 63.
20. ESS pamphlet (1992) “Air-Assisted Electrostatic Spraying” issued by Electrostatic Spraying Systems, Inc., P.O.Box 151, Watkinsville, Georgia 30677, USA.
21. SANO K, (19980, “Electrostatic Pesticide Spraying. in “The Modern Problems of Electrostatics with pplications in Environment Protection”, Ed. by I.I. Inculet, F.T. Tanasescu, R. Cramariuc. NATO ASI Series, vol.63, Kluver Academic Publisher, pp. 313-377.
22. Cramariuc, R., (1998), Tests for the electrostatic crop spraying as a method of treatment with ultra low volume, in The Modern Problems of Electrostatics with Application in Environment Protection, NATO ASI Series, vol.63, Kluver Academic Publisher, pp379 – 398.