International Conference on Power Plants, 30.10-02.11.2012, Zlatibor, Serbia THE CONTROL OF ELECTROMAGNETIC VIBRATORY ACTUATOR FOR EFFICIENT FLOW OF DUST PARTICULATES FROM THE COLLECTING HOPPERS OF ELECTROSTATIC PRECIPITATORS Dr Željko V. Despotović, Dr Aleksandar I.Ribić Institute “Mihajlo Pupin”, University of Belgrade, Serbia, zeljko.despotovic@pupin.rs

INTRODUCTION The separation of coal dust particles and ash significantly reduces the negative impact of waste materials that are the products of combustion in thermal power plants and heating plants. Prevention of waste particles of coal dust and fly ash from the chimneys the mentioned plants, or their "collection" is achieved by electrostatic precipitator (ESP). World standards that are becoming more accepted in our country require emission limit values (ELV) less than 50mg/m3 , a tendency in the world is to reduce the ELVof the value of 25mg/m3 . In addition to the high voltage power supply and drive electrostatic precipitator rappers, very important is the system to remove precipitated dust from the collecting hoppers which are located directly below the electrode system. Vibration of collecting hoppers appropriate amplitude and frequency can significantly improve the removal of precipitated dust. The vibratory actuators having electromagnetic drive are commonly used in these systems as a source of vibration.

Typical distribution of ash in a percentage of a typical thermal power plant Most of the ash 75-80%, is allocated in hoppers which are placed directly below high voltage precipitation chambers of electrostatic precipitators (ESP's) A very small portion of ash, much less than 1% is extract at the bottom of the output chimney.

ESP Prevention of waste particles of coal dust and fly ash from the chimneys the mentioned plants, or their "collection" is achieved by electrostatic precipitator (ESP). The separation of the mentioned types of solid products is achieved by strong electrostatic field that forms in precipitation chamber of ESP. ESP

In the precipitation chamber, there are two types of electrodes: collecting and emissions, as well as devices for rapping of separated particles. Periodic shaking of deposited material in the mentioned electrodes to exercise its accumulation in host hoppers (bunkers) which are located just below the precipitation chamber

Typical types of ash and particulate material conveying-PNEUMATIC TRANSPORT PNEUMATIC CONVEYING

Typical types of ash and particulate material conveying-HYDRAULIC TRANSPORT HYDRAULIC CONVEYING

THE PHENOMENOLOGY OF VIBRATORY HOPPER DISCHARGE Although hoppers are common, the internal flow of the material is not well understood, relying heavily on empirical information to maintain operation. For example, when a batch of material is introduced into a hopper for the first time, the material at the exit may arch and prevent flow. To remedy the situation, vibration may be used, sometimes in the crude form of a hammer, to perturb the material and initiate the flow

HORIZONTAL VIBRATION OF HOPPER Horizontal vibration increased the mass discharge rate as compared with the discharge rate from a hopper without vibration and that the increase depended on the vibration velocity amplitude. In addition, the discharging granular material flows from alternating sides of the hopper producing an inverted funnel pattern.

VERTIKAL VIBRATION OF HOPPER For a hopper box that is vibrated sinusoidally in vertical direction, the bed of hopper exhibits several different flow patterns depending on the dimensionless acceleration amplitude Г Frequency of vibration For Г>1 , side- wall convection cells appear where particles move down along vertical hopper walls and up within the remainder of the hopper. The paper by Wassgren et al. [6] describes these phenomena in greater detail. [6].C.R.Wassgren, M.L.Hunt, P.J.Freese, J.Palamara and C.E.Brennen, Effect of vertical vibration on hopper flows of granular material, Physics of Fluids, Vol.14, No.10, pp.3439-3448, October 2002.

Without vibration, the mass discharge rate from a hopper, W, is proportional to the bulk density of the bed -ρb near the hopper exit, the square root of the acceleration acting on the bed, (the acceleration due to gravity)- g and the hydraulic diameter Dh of the hopper exit, raised to the 5/2 power. Since the hopper is oscillating, the effective gravity the bed experiences relative to the hopper walls, geff , will vary throughout an oscillation cycle as: IF ?????

If the acceleration amplitude of the oscillations is greater than one (Γ>1 ) the bed leaves the hopper walls during a portion of the oscillation cycle and contacts the walls at some later time. The equations originally derived by Suzuki et al. [8], also included an empirically derived expression for the bulk density of the bed as a function of Γ: when the bed rests on the hopper walls when the bed is flight when bed just impacts the hopper walls The acceleration presents the acceleration acting on the bed at impact ~ DISCHRGE RATE: [8]. A. Suzuki, H. Takahashi, and T. Tanaka, Behaviour of a particle bed in the field of vibration. II. Flow of particles through slits in the bottom of a vibrating vessel, Powder Technology, Vol.2, No.72, 1968.

MECHANICS of VIBRATORY HOPPER Resultant vertical vibration force ΣFv Resultant horizontal force ΣFh=0 Excitation forces F1,F2 and F3 Driving current i1(t),i2(t) and i3(t) EVA-Electromagnetic Vibratory Actuator

THE ELECTROMECHANICAL SYSTEM FOR EXCITING VIBRATORY HOPPER (a)-mechanical construction (b)-armature EVA on the active side (c)-armature EVA on the reactive side

EQUIVALENT ELECTRICAL MODEL OF VIBRATORY DISCHARGER EVA (i), i=1…3 THE MECHANICAL PART OF THE SYSTEM

PHASE CONTROLLED THYRISTOR CONVERTER for DRIVING VIBRATORY HOPPERS (a) unidirectional topology and (b) waveforms of the EVA current and voltage

PHASE CONTROLLED TRIAC CONVERTER for DRIVING VIBRATORY HOPPERS (a) bidirectional topology and (b) waveforms of the EVA current and voltage

PROBLEM: Degradation of elastic elements (springs) BREAKING OF SPRINGS!!!! SOLUTION: FIBERGLASS SPRINGS k1 k1 100Hz→6000 osc/min→360 000 osc/h→ →1∙106 osc/ day (mean value) PER YEAR 360∙106 cycles -drift of spring characteristics -decrease of stiffness k1 -change resonant frequency -decrease of resonant frequency -decreasing of amplitude oscillation

SWITCHING POWER CONVERTER for EVA EXCITATION two switch forward, symmetric half-bridge full-bridge

ELECTROMAGNETIC VIBRATORY ACTUATOR CURRENT CONTROL (amplitude and frequency) (a) principle circuit diagram (b) characteristics waveforms

EXPERIMENTAL PROTOTYPE OF IGBT CONVERTER

EXPERIMENTAL RESULTS Adjustment of vertical component acceleration amplitude of vibratory hopper means of current controlled IGBT converter; (a) duty cycle δ=15%, (b) duty cycle δ=30%

 EXPERIMENTAL RESULTS Frequency control of EVA for compensation of drift characteristic of springs; (a) stiffness of EVA springs (b) stiffness of EVA springs  (c) stiffness of EVA springs

MEASURED RELATIVE DISCHARGE RATE The discharge rate for frequencies below 50Hz decrease with increasing of relative acceleration of hopper. For frequencies 60Hz and 80Hz, the relative discharge rate is approximately equal or slightly greater than unity, but at the frequency 100Hz is observed a significant increase discharge rate of 20%.

CONCLUSIONS In presentation is present a possible solution for the amplitude and frequency control of electromagnetic vibratory actuator applied to vibratory hopper excitation. The solution of IGBT power converter represents a considerable improvement compared to the conventional In experiment are presented results relating to the compensation effect of changes in the resonant frequency of the EVA, which emerged as a result of the degradation of elastic elements (springs). In experimental investigations have shower that the relative discharge rate from vibrating hopper decrease for low frequency, with increasing of relative acceleration of hopper. For frequencies 60Hz and 80Hz, the relative discharge rate is approximately equal or slightly greater than unity, but at the frequency 100Hz is observed a significant increase discharge rate of 20%.

ACKNOWLEDGEMENTS The investigation has been carried out with the technical and financial supports of the Serbian Ministry of Science-Project of technological development grant No: TR33022.

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