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DRY CLEANING OF PULVERIZED COAL USING A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR (RTS)
D. TAO 1,2 , A. SOBHY1 , Q. LI1, R. HONAKER 1 , AND Y. ZHAO2
1University of Kentucky, Lexington, KY, USA 2School of Chemical Engineering, China University of Mining Technology, Xuzhou, Jiangsu Province, China
Coal cleaning is often conducted using wet physical separation processes such as heavy medium vessels or spirals at coal prep- aration plants to remove impurities such as ash, sulfur, and mercury. However, the resultant clean coal product still contains a significant amount of impurity due to the fact that impurities are not well liber- ated from coal particles ranging from several millimeters to inches in size at which wet cleaning processes take place. A cleaner coal product can be obtained if a dry process is avaialble to further clean
pulverized and thus better liberated fine coal at the power plant prior to its combustion.
In this study, a novel rotary triboelectrostatic separator (RTS) was investigated for its application to dry cleaning of fine coal sam- ples acquired from the power plants in the state of Illinois. The pro- prietary RTS is characterized by an innovative high-efficiency rotary charger, charger electrification, laminar air flow, etc. Compared to existing triboelectrostatic separators, the RTS offers significant advantages in particle charging efficiency, solids throughput, The authors would like to sincerely acknowledge the funding of the Department of Commerce and Economic Opportunity of the state of Illinois through the Office of Coal Development and the Illinois Clean Coal Institute under the project Number of 08-1= 4.1A-1, which made this work possible. Technical assistance and professional advice from the project manager Joseph Hirschi is deeply appreciated. separation efficiency, applicable particle size range, etc. Important process parameters such as charger rotation speed, injection and co-flow rate, and feed rate were investigated for their effects on separation performance.
Keywords: Fine coal; Liberation; Particle charging; Rotary charger; Triboelectrostatic separation
INTRODUCTION
Coal is a major source of energy in the United States and more than 51% of the electricity used in the country is generated from coal. Numerous advanced coal-cleaning processes have been developed in recent years to reduce ash, sulfur, and mercury contents. However, most of the processes involve the use of water as a medium and thus the clean coal products must be dewatered before they can be transported and burned at power plants. The high cost associated with fine coal dewatering makes it difficult to deploy advanced coal-cleaning processes in commer- cial applications. Dry beneficiation technique is an alternate approach to solving this problem.
Recently, the literature on dry beneficiation methods for coals with specific reference to high-ash Indian coals has been summarized by Dwari and Rao [1]. Triboelectrostatic process is one of the key dry pro- cess techniques to separate the ash-forming inorganic minerals from coal. Electrostatic separator with tribo-charging technique has great potential for coal preparation in fine sizes.
The triboelectrostatic system can be divided into two major zones: a tribocharging zone to differentially charge a mixture of particles and a separation zone to physically separate charged particles. The coal- and ash-forming minerals are charged in the tribocharger based on their rela- tive work functions. After triboelectrification, the particles entering into the electric field get attracted towards a positive or negative electrode plate according to their charge polarity and magnitude. Several studies have shown that clean coal generally charges positively and ash-forming minerals or high-ash coals charge negatively [2–5].
The present study was conducted to investigate the novel rotary tri- boelectrostatic separator (RTS) for its application to dry cleaning of fine coal samples acquired from a power plant. The pulverized fine coal con- tains well liberated ash and pyrite minerals and is an ideal feed to the tri- boelectrostatic separator for further cleaning without the use of water or any chemical reagents. The coal particles are positively charged while ash particles are negatively charged as a result of differential charging in the charging chamber.
EXPERIMENTAL
Fine Coal Sample
The pulverized coal sample used in this study was acquired from a power plant that uses a wet pulverizer to reduce coal size feed to the boiler. The proximate analysis showed the coal sample had 16.32% moisture, 8.0% ash, 45.50% volatile matter, and 30.73% fixed carbon. Table 1 shows the size-by-size weight and ash distribution data of the coal sample. Approxi- mately 50.41% of the sample had a particle size less than 25 mm. In other words, this particular sample had a d50 of smaller than 25 mm.
The washability data shown in Table 2 for a t325 mesh particle size fraction was obtained using a lithium metatungstate (LMT) solution as heavy medium. The fact that there is a very small amount of t1.6 density fraction indicates this sample is very difficult for further cleaning by any gravity-based separation method. Flotation release analysis was also per- formed to produce the best possible flotation performance with this parti- cular coal sample and the results will be presented in the next section.
Triboelectrostatic Separation Tests
A 100-gram sample was fed to the rotary triboelectrostatic separator (RTS) in each experiment using a vibratory feeder. Three products were collected from the first stage. Each product from the first stage was further processed to generate nine final products. Process parameters investigated in this study included feed rate, charger rotation speed, charger material, charging voltage, injection flow rate, and co-flow rate. The feed was dried over night in an oven at a temperature slightly above 100F prior to the separation test.
Table 1. Particle size-by-size weight and ash distribution
Table 2. Density fractionation analysis of t325 mesh fine coal sample
The rotary triboelectrostatic separation (RTS) system shown in Figure 1 includes a vibratory sample feeder, a rotary charger or charge exchanger, a separation chamber, an injection gas unit, and two
high-voltage DC supplies. Samples are fed by the feeder into the rotary charger. A small amount of transport gas is injected with the particles. The gas-particle flow interacts with the rotary charger. Due toparticle-to-charger or particle-to-particle collisions, particles become charged negatively or positively, depending on their work functions. The charged particles then pass through the separation chamber and report to one of three cyclones attached to the system. More details about the apparatus can be found in a previous publication [5].
There are two high-voltage sources in the rotary separator system: one is for the particle charging, which is attached to the charger, and the other is for the separation of the charged particles, which is attached to the separation chamber. The most distinct feature of the rotary separ- ator is that particle charge density and polarity can be controlled by changing the applied voltage. This is an innovative concept that allows separation of multiple components at different stages with different applied voltages, which is analogous to adding different reagents at different stages in the flotation process.
Figure 1. Illustration of the rotary triboelectrostatic separator.
Description of Fundamentals of Triboelectrostatic Separation Triboelectrostatic separation is a dry separation process based on the fact that when two particles are rubbed against each other or a third object (referred to as charger), the particle with higher work function becomes negatively charged and the other positively charged. The work function / is defined as the minimum energy that must be supplied to extract an electron from a solid. It is a measure of how tightly electrons are bound to a material. The charged particles are subsequently sepa- rated in an external electric field as a result of their different motiontrajectories.
A separation zone shown in Figure 2 can be used to describe moving particle trajectories in an electric field where ‘‘x’’ represents the horizon- tal direction and ‘‘y’’the vertical direction. When a charged particle enters the electric field, its trajectory is governed by the momentum of the gas flow and gravity in addition to the electric force. It deflects to a positive or negative electrode, depending on its charge polarity.
Figure 2. Illustration of the separator chamber.
If the high voltage electrodes are mounted vertically as shown in Figure 2, the electrostatic force will accelerate the particles horizontally. The particle residence time, that is, the time for a particle
traveling through the separation chamber, is controlled by the particle vertical motion. However, the horizontal particle motion is controlled by electric field deflection. The law governing the horizontal displacement, xx, of the moving particle is:
where m is the mass of particle, xx the horizontal displacement vector, t the time, EE the electric field intensity, and q the charge of particle. The charge-to-mass ratio, q=m, is referred to as particle specific charge. It is a very important parameter for the motion of the particle in the electrostatic separation process. If the resistance of air with viscosity g is also considered, the hori- zontal motion of a moving spherical particle of radius r is given by:
A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR (RTS)
The solution to Equation 2 gives the speed of the particle as a function of time:
When or the horizontal terminal velocity of particle is:
Under these conditions, the horizontal terminal velocity is independent of the mass. However, since time t is in milliseconds with a practical sep- arator, the mass does play an important role in determining the horizon- tal motion of the particle and, therefore, the resultant trajectory that affects the separation performance.
Particle motion in the vertical direction is influenced by gravitational force and gas drag force. The governing equation is:
where g is the dynamic viscosity of gas and g is the gravitational acceler- ation.
For the initial conditions of t = 0, y(0) =0, and dy(0)/dt =V0 , Equation 5 can be solved as
where B =6πrη/m. The particle trajectories can be obtained from Equations 4 and 6 under given conditions. Figure 3 shows typical trajec- tories for negatively harged particles of different sizes. They deflect right to the positive electrode and can be readily separated from posi- tively charged metal particles that deflect left to the negative electrode.
The particle trajectory is affected by particle charge, mass of par- ticle, radius of particle, and electric field intensity, as indicated by Equa- tions 4 and 6. A larger difference in trajectories of different particles enhances separation efficiency. This may be achieved by the use of ahigher electric field intensity and greater particle charge density. How- ever, the electric field intensity is limited by airionization and is normally set at 300,000–500,000 V=m. A potentially huge improvement insepar- ation efficiency can be achieved by enhancing particle charge density, which can be achieved using the RTS that takes advantages of high rotation speed and controlled potential of the charger. However, higher charger rotation speed increases the wear rate of the roller and thus it was limited to 5000 rpm in the study. The maximum applied charger volt- age was imposed since too high a voltage (either negative or positive) will cause electric sparks as a result of ionization of air molecules.
Figure 3. Trajectories of particles of different sizes in an electric field.
RESULTS AND DISCUSSION
Several series of triboelectrostatic separation tests were conducted to
evaluate the dependence of process performance on operating para- meters. The separation performance data is presented in terms of com- bustible recovery versus normalized product ash that is defined as the percentage of product ash to feed ash. The normalized product ash is utilized in this study since the wet and sticky feed sample was very diffi- cult to homogenize completely and significant fluctuations in feed ash were observed in the tests. In addition, the separation efficiency (defined as the combustible recovery minus ash recovery) of the first stage of sep- aration is used to show the effect of individual process parameters. Unless otherwise specified, all separation tests were conducted using the copper charger under the following conditions: charger rotation speed: 3000 rpm; injection air velocity: 2.5 m/s; co-flow air velocity: 3.1 m/s; feed rate: 800 g/h; separation voltage: 22.5 kV; charger voltage: 2.5 kV; temperature: 24C.
Effect of Feed Rate Figure 4 shows the changes in separation curves (left) and separation efficiency (right) with feed rate. As the feed rate doubled from 400 g/h to 800 g/h, the separation performance was essentially constant. As the feed rate increased further to 1500 and 2000 g/h, the separation curve shown in Figure 4 shifted away gradually from the upper left cor- ner and the value of separation efficiency decreased gradually. A more significant decrease in separation efficiency was observed when the feed rate increased from 2000 g/h to 3600 g/h, which suggests the maximum feed rate should be at about 2000 g/h. The lower separation efficiency at higher feed rate is mainly caused by fewer contacts with the charger sur- face and thus lower charge density on particle surface. Figure 4 (left) shows that a product 13% cleaner or of 5.5% ash can be obtained at 80% combustible recovery at a feed rate of up to 2000 g/h. A cleaner product can be produced at theexpense of combustible recovery. For example, a clean coal of about 4.5% ash, which represents 30% ash reduction, was achieved at 800 g=h feed rate with about 33% combustible recovery. It should be noted that during the tests the lower ash product moved toward the negative electrode and the higher ash product was deflected to the positive electrode, suggesting that carbon particles were positively charged and minerals were negatively charged.
Figure 4. Effect of feed rate on triboelectrostatic separation curves (left) and separation efficiency
(right).
Effect of Charger Rotation Speed The tribocharging is largely attributed to the relative speed between par- ticles and the rotary charger, with higher speed resulting in greater sur- face charge density. The easiest way to control tribocharging is perhaps to adjust the rotary charger rotation speed. The experimental results on the effects of rotation speed on fine coal separation are shown in Figure 5. Figure 5 indicates that the optimum rotation speed was 5000 rpm at which an ash reduction of more than 30% can be achieved at a combustible recovery of about 40%. The separation efficiency for the first stage of separation suggests that the maximum separation efficiency was achieved at 4000 rpm and a slightly lower efficiency was observed at 5000 rpm. It is clear from the data shown in Figure 5 that better separation was accomplished at a higher rotation speed of 4000 or 5000 rpm, which is consistent with the established theory that higher surface charge results from an increase in the relative motion speed between the charger and particles [6].
Figure 5. Effect of charger rotation speed on
triboelectrostatic separation curves (left) and separation efficiency (right).
Effect of Applied Charger Voltage.
One of the unique features of the rotary triboelectrostatic separator is the applied potential to the charger to enhance the particle-charging pro- cess. Figure 6 shows the separation curves and separation efficiency at different charging voltages ranging from 5 kV to t5 kV. It is quite clear from both figures that the separation performance was significantly increased as the charging voltage varied from t5 kV to 5 kV. It is inter- esting to point out that compared to 0 V or no charging voltage that is the case with the conventional triboelectrostatic separator, the separation at 5 kV was substantially more efficient. For example, a product ash reduction of 30% could hardly be obtained at 0 V charging voltage. How- ever, it can be easily obtained at a 5 kV charging potential with a com- bustible recovery of more than 55%. Comparing the separation curves at 0 V and 5 kV indicates that up to a 50% increase in combustible recov- ery was achieved if the charging voltage was changed from 0 V to 5 kV, which clearly illustrates the great importance of controlling the applied charger potential.
Effect of Injection Flow Rate
It is known that the injection flow rate or velocity affects the particle speed and the residency time in the charging and the separation cham- ber, and thus the particle charge density and separation efficiency. A higher injection flow rate results in a faster velocity at which particles
Figure 6. Effect of applied charger voltage on triboelectrostatic separation curves (left) and separation efficiency (right).
Figure 7. Effect of injection flow rate on triboelectrostatic separation curves (left) and separation efficiency (right).
struck the charger but it causes a shorter charging time and separation time. Figure 7 shows the separation curves and separation efficiency at different injection flow velocity. The best separation performance was obtained at about 2.5–3.7 m/s injection velocity. A velocity lower than 2.5 m/s or higher than 3.7 m/s resulted in poorer separation due to lower impact velocity and shorter residence time, respectively.
Effect of Co-Flow Rate
The gas flow that enters the separation zone on both sides of the connec- tor between the charging and separation chamber is referred to as co-flow. It is used to comb the misplaced particle and to force them to deflect to the desired product stream. Figure 8 shows the separation curves and efficiency, respectively at different co-flow rates. As the co-flow rate increased from 1.5 m/s to 2.5 m/s, the separation curve move considerably toward the upper left corner, indicating a better sep- aration. As the co-flow rate further increased to 3.1 m/s and 3.7 m/s, the separation shifted moderately lower toward the right side, suggesting a lower combustible recovery at a given product ash. However, the differ- ences in combustible recovery at 2.5, 3.1, and 3.7 m=s were quite small, especially at higher product ash values. The separation efficiency curve for the first stage of separation indicates that the optimum co-flow rate was at approximately 3 m=s, which is essentially the same as the optimum injection flow rate shown in Figure 7. This result is consistent with the
previous study with a fly ash sample that has
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