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Performance Evaluation of an ESP Unit Using Low Grade Coal

A.Chandra, S.P.Sabberwal and A.K.Mukerjee
Centre for Energy Studies, Indian Institute of Technology
Hauz Khas, New Delhi-110016INTRODUCTION:1.

INTRODUCTION:

        There is a large gap between the demand and supply of electric power in India. In order to bridge the

gap, more than 28000 MWe of power capacity are going to be installed. Most of this power is going to be

generated by coal based thermal power plant. These plants generate large amounts of fly ash which has to be

collected effectively before being dispersed in atmosphere along with flue gases. The electrostatic precipitator

[ESP] is one of the most widely used device for controlling particulate matter consisting of fly ash. The ESPs are

found to extremely efficient ( >,99 %) for wide range of particle size (1-100 ) m. They can handle large volume

of gases (25-1000 m 3/s), have low pressure drop and can operate continuously with little maintenance 1, 2].

        The performance of ESP depends upon many factor the major are (i) characteristics of fly ash which depend upon

fuel and combustion parameters available in boiler (ii) charging of fly ash particles which is function of electric

field available, electrode spacing and configuration (iii) collection and dust removing, the particles are collected

at anode which are grounded, dust is removed through rapping and is collected in hoppers below the electrodes in ESP.

METHODOLOGY:

        We have carried out investigation on ESP at Badarpur Thermal Power Station (BTPS] operated by National

Thermal Power Corporation [NTPC] at Badarpur , New Delhi. The present investigations has been carried out at

unit I and III. The parameters of ESP units are given in table 1.

        For fly ash characterization the ash samples have been collected from the inlet of ESP unit, from the hoppers placed

below the electrodes in four field and from the out let. There are two paths for flue gases carrying through each unit

and each field again contains two Path. Thus the multiplicity of the paths for flue gas flow ensures uniformity and low

pressure drop through out the each ESP units. These samples have been used to determine ( i ) particle size distribution

at each stage (ii) chemical composition and (iii) Electrical resistivity. The particle size distribution measurements have

been done in laboratory. The chemical composition has been determined using Inductively Coupled Plasma Atomic

Emission Spectrometer.

        The resistivity is a complex function of temperature and composition of flue gas beside its own temperature. In

Laboratory it was not possible to simulate actual conditions existing in ESP . The fly ash resistivity was, however measured at

different temperature. For this a well shielded chamber was designed to house the fly ash pallets which were prepared by

applying high pressure (10 tons/cm2 )in a dye. Fine Aluminum coating was deposited on both sides of pallets for providing

ohmic contact. The resistivity of pallets was measured by knowing the field and current density through the pallets.

        The electrical measurements have been made at unit I and unit III. Continuous monitoring of voltage and current between

the electrodes at different field reveal the development of back Corona due to deposition of high resistivity dust on

collecting electrodes. The effect of rapping to remove the dust on electrodes on the current passing through the electrode

is being investigated. The voltage current characteristics have been obtained as to determine the optimum values for these

parameter as to check the development of sparks/ break down between the electrodes. The peak voltages have been measured

at different charge ratios at each field.

        The quality of coal is rather poor characterized by low calorific value and high ash contents. The quality of coal also

keeps on changing depending upon the availability of coal from different mines almost weekly . At Badarpur Thermal

Power Station where the present investigations have been carried out, had an average composition of coal as follows:

Total moisture -8.91%, Volatile matter- 17.3%, Fixed Carbon: 31.8%, Ash - 42% Calorific value-4000 KCal/Kg.

RESULTS AND DISCUSSIONS:

The particle size distribution varies at each stage and lie in the range (0-150) ? m. At the inlet of the unit

the sample is dominated by large particles (30-100) ? m having average diameter 40 ? m. on the other hand at

the outlet of the unit before going to stack small size particles dominate (0.5-20) ?m having average diameter

10 ? m. For rest of the places the size lie in between. Some large size particles are found at out let as well.

This may be due to leakage of gas with out going through the field, retrainment through hopper and below par

performance of the segments of fields due to field distortion, development of back corona etc. A significant

portion of outlet ash samples contains small size particle ( 5 m). These are not affected by electrostatic field

and escape into atmosphere.

        The chemical analysis reveals that it is dominated by Silicon oxide (55- 62) %. The electrically conducting

ion species like (K20) Na2O are in small 5 percent. Iron oxide, Magnesium oxide etc. are other notable compounds.

Some portion of fuel remains unburn (LOI) 4.00 percent.

        As expected from analysis the electrical resistivity of fly ash is very high (1011-1014) ohm cm. The variation of

electrical resistivity is shown in figure 1.

        The dust loading is measured at the inlet and at the outlet of ESPs for different units. These measurement determine

the collection efficiencies of the ESP units. The outlet dust loading is crucial as it shows the pollution level of dust

being dispersed in atmosphere. The collection efficiencies of the units are found in excess of 99 percent.

        The work is partially supported by NTPC Thermal Power Station Baldarpur. The cooperation extended by management

and technical staff is thankfully acknowledged. The authors appreciate the technical assistance provided by Mr. Ved

Pal, the research associate in the project.

References:

    1.    Oglesby S, Nichols G, Electrostatic Precipitation Marcel Dekker Inc New York (1978)

                    2.     Report on Design and operating Parameters of Electrostatic Precipitator Central Pollution Control Board

                            N. Delhi India (1992)

 

TABLE 1: Parameters of ESP units

 

1. Design conditions

                                                    (a) Gas flow rate                                                        320 Nm3 /Sec

                                                    (b) Temperature                                                         160 C

                                                    (c) Dust concentration                                              38.2gm/Nm3

                    2. Numbers of fields in series                                                                  4

                    3. Pressure drop across                                                                             75mm We
                        Precipitator for designed conditions

                    4. Collecting Electrodes 51

(a)  No. of rows of collecting
       electrodes pet field

                    (b) No. of collecting electrode                                  306
                          plates per field

(c) Total No. of collecting                                          2448
      plates per boiler

(d) Nominal height of collecting
      plates                                                                      13.5 meters

          (e) Nominal length of collecting
                plates                                                                      750 mm

                              (f) Specific collecting area                                       151.875 m2/m3/sec.

 

                    5. Emitting Electrode                                                                              Spiral with hooks

                                                 (a) No. of electrodes in each field                            2700

                   (b) Total length of electrodes per
                         field                                                                       15147 meter

                                       (c) Plate /wire spacing                                               225/300 mm

6. Electrical

    (i)     Voltage                                                                                        70 KV (peak)
    (ii)   Current                                                                                        1000 mA.

 

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Last updated: April 12, 1999.
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