ELSEVIER Journal of Electrostatics 39 (1997) 65-81
Journal of
ELECTROSTATICS
Influence of plate spacing and ash resistivity on the efficiency of electrostatic precipitators
B. Nava r re t e , L. Caf iadas*, V. Cor t r s , L. Sa lvador , J. G a l i n d o
E.S,I.L, Dept. Ingenieria Quimica y Ambiental, Universidad de Sevilla, Avd. Reina Mercedes, s/n. 41012-Sevilla, Spain
Received 22 April 1996; accepted after revision 27 August 1996
Abstract
This paper contributes to the knowledge of the behaviour of electrostatic precipitators in pulverized-coal power plants. The information was obtained through real tests carried out on different types of coal with real flue gases in a pilot precipitator installed at a pulverized-coal power plant. In particular, the influence of different plate spacings, the characteristics of the ash generated by each type of coal and the mass load of ash in the gas flow are considered. The tests carried out lead to conclusions with respect to the most advantageous arrangement of the plates for each case. Conclusions with respect to other variables that affect the efficiency, such as gas velocity, can also be drawn.
The use of electrostatic precipitators for collecting fly ash from pulverized coal combustion is a conventional technology well established in power plant environ- mental control. In spite of this, the behaviour of these units continues to raise some uncertainties, especially because the origin and characteristics of the coal being used vary. These uncertainties affect the ash collection efficiency and therefore particle emissions from the facility. The characteristic property of the ash that seems to affect ESP efficiency the most is resistivity, although there is presently no direct correlation between resistivity and efficiency that would permit a prediction of the variation of design parameters or the specific operating condition adjustments for each resistivity value.
Also, other variables, including those related to the process (gas velocity, particle concentration, and particle size), those related to the design (type of electrodes, plate
66 B. Navarrete et al./Journal o f Electrostatics 39 (1997) 65-81
spacing), and those having to do with operation (field voltage and current, rapping frequency) can have a significant influence.
In order to draw conclusions about the behaviour of electrostatic precipitation of fly ash with different characteristics, an extensive program of tests was undertaken using a pilot precipitator (4 MWe) with real gases extracted from the flue gas of a pulverized coal power plant (550 MWe). The gas flow to be processed by the pilot precipitator is taken isokinetically from the flue that feeds the power plant ESP.
The main objective of the tests was to evaluate the performance of the pilot unit under different operating conditions and internal arrangements with fly ash from different sources and of different characteristics.
In particular, the performance of the pilot precipitator was evaluated by varying the plate spacing for two types of coal, type A and type B, which have high and low resistivity, respectively. The plate spacings under consideration are 300 and 400 mm. The 300 mm plate spacing coincides with the value most often used in the commercial design of electrostatic precipitators, while the 400 mm configuration is the most up-to-date tendency in the design of new precipitators.
2 . M e t h o d o l o g y
2.1. Description of the facility
The pilot precipitator (PESP), whose characteristics are described in Table 1, was conceived as a versatile unit that admits internal configuration changes to be able to test different plate spacing, electrode types, energization levels and systems, rapping levels and systems, etc., with which to analyze a conventional precipitator's different possibilities of operation [1].
Gases are extracted from the plant's ESP input flue by two fans. One fan is installed at the pilot precipitator input (forced draft), and the other is installed at the output (induced draft), which allows to regulate the flow of gases and the operating pressure (Fig. 1).
The plant is equipped with an automatic control of the most important operating variables (flow, pressures, and temperatures), and an automatic data-acquisition system that registers and stores the state of the units, gas flow, meter data, and electrical conditions of the PESP at all times. Thus, the variables of the tests to be carried out can be programmed automatically and the evolution of the results can be easily and exhaustively processed. The following automatic measurements were regis- tered and stored during each test: - Gas flow measured with a venturi at PESP outlet. - Gas temperature at the test facility inlet and at the PESP inlet and outlet (thermo-
couples). - Current, voltage, and sparking level of the three transformer-rectifiers (TRs). - Opacity measured with a laser opacimeter. - Precipitation chamber pressure. - Rapping activation. - Temperature in the insulator houses.
B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81
Configuration of the electrical sections Effective length (m) Effective height (m) No, of gas passages Plate spacing (ram) No. of electrodes per passage
Operating conditions Gas flow (m3/h) Precipitation area (m 2) Specific collection area - SCA (mZ/m3/s) Gas velocity (m/s)
Transformers-rectifiers Peak voltage (kV) Average maximum voltage (kV) Maximum effective current (mA)
12.6 2.5 2.6 3
2 2.2 3-7 200-500 4-12
9.000-20.000 79.2-184.8 14-74 0.8-1.8
120 78 42
GAS IHTAKI[ TRANSFORMER~R[CI~flER$ GAS OUTFLOW
Fig. 1. Pilot ESP instrumentation and control diagram.
2.2. Test methodology
The precipitator's behaviour was studied based on experimental tests programmed to find its sensitivity to each parameter. The testing program was designed with matrices of parametric tests obtained by applying factor analysis of experiments.
68 B. Navarrete et al./Journal o f Electrostatics 39 (1997) 65-81
The tests sought to establish the precipitator's efficiency under the different condi- tions reflected in the matrix of tests for each type of coal. Within the limitations imposed by working in series at an industrial plant, the operation variables were maintained as stable as possible, carrying out the tests during steady state boiler operation in the absence of interferences. Likewise, tests were made at full load conditions to maintain the parameters of the gases to be processed within narrow limits.
After the start-up of the pilot plant, and once all the parameters were adjusted to the specific level of each test, a stabilization time of around 2-3 h was allowed before beginning the evaluation and gathering of data and test samples. This stabilization time was enough to eliminate the transients in data gathering and avoid errors caused by the inertia of the start-up and the change of state.
Given the objective of these tests, accurate particle concentration values were required to find the unit's efficiency under different testing conditions. In all tests, the concentrations were evaluated by isokinetic sampling of particles using two units operating simultaneously at the precipitator's inlet and outlet. Samplings were carried out according to EPA method no. 17 [2]. Sampling time was set at 20 min, long enough to achieve a representative mean concentration and to compensate for the fluctuations due to normal boiler operation.
All relevant operational variables were registered automatically: opacity; flow; PESP inlet and outlet temperature; voltage, current, and sparking level of each electrical section; and plate and electrode rapping in order to characterize completely each of the tests. Therefore, the data gathered documents not only the efficiency analysis, sought as the main objective, but they also constitute a base for studying the energization parameters, rapping, and behaviour of the ash layer [3], and a valuable information to improve and validate ESP simulation models [4].
Ash samples have been taken at PESP inlet and outlet sections, as well as from the hoppers, to evaluate particle size and other significant physical properties.
2.3. Choice of parameters and variables
2.3.1. Types of coal The characteristics of coal A (high resistivity) and coal B (low resistivity) are as
follows: - Type A coal: Characterized by a medium ash content (12-14%) and a low sulphur
content. The composition, heat value, and resistivity, taken as medium values from different batches, are the following: • Ultimate analysis (d.b.):
B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81 69
- Type B coal: This type of coal is characterized by a lower ash content and lower resistivity. Its average characteristics were: • Ultimate analysis (d.b.):
2.3.2. Plate spacing Taking advantage of the versatility that the internal configuration of the precipita-
tor allows, (plate spacing can be set at between 200 and 500 ram), two test spacings have been selected according with usual design specifications for commercial Euro- pean precipitators:
At = 300 mm, A2 = 400 mm.
2.3.3. Gas flow rate In general, the velocity at which the gases flow through commercial ESPs is
designed around an optimum estimated between 1 and 1.2 m/s. To evaluate the incidence of this parameter on precipitation, tests were carried out for each of the following rates:
V1 = 0.8 m/s, V2 = 1.1 m/s, V3 = 1.4 m/s.
2.3.4. Number of fields in service To study the evolution of the precipitation efficiency with a progressive increase in
collection area, the performance in each case tested was evaluated, establishing three levels on the active fields in service. In this way the differential efficiency of each of the fields can also be obtained:
Nt = Field 3, active, N2 = Fields 2 and 3, active,
N3 = Fields 1, 2, and 3, active.
2.3.5. Rapping frequency The influence of rapping frequency on the average efficiency is relatively low, for the
flows and particle concentrations retained for PESP operation, given the short duration and uniformity of the testing periods. The duration of the rapping and intervals between rapping were kept constant for all the tests, within the typical range of commercial ESPs: a duration of 90 s at 5 min intervals (lst field), 10 min (2nd field), and 15 min (3rd field).
70 B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81
2.3.6. Other parameters Other variables affecting precipitation were kept constant during tests to simplify
the experiments and results processing: - Type o f electrodes. Barb wire type was chosen for all the tests because it is
commonly used in coal ESPs and coincides with the type used at the power plant. - Operatin9 temperature. All the tests were run with an adjustment for the gas
temperature coinciding with the mean value registered at full load at the preheater outlet, at around 130-135°C.
- Electrical f ield eneryization voltage. The microprocessor control system for each one of the TRs has the capacity for choosing automatically the voltage-current operation point when it is in continuous operation, optimizing the energization of each electrical section according to the sparking levels and average currents. Consequently, for each of the tests carried out, the TR control unit was operated on automatic, so the voltage and current during tests in each field could be evaluated.
3. Program and running of tests
According to the parameters and variables chosen, the Table 2 test matrix was prepared. The matrix for each type of coal is shown, so 36 tests were planned in all.
Before beginning the tests, the ESP's internal velocity profiles were measured on a 48-point grid at different flow rates to assure the uniformity of gas velocity inside the precipitation chamber. As a result of these measurements, typical deviations were obtained in the velocity at around 18% of the mean value. This outcome is within the adequate range of operation for ESPs, calculated at levels of less than 25% of the average [5].
At the beginning of each series of tests corresponding to a type of coal, the opacimeter was calibrated, and particle-extinction--concentration correlations were carried out to establish the correspondence between both magnitudes to continuously monitoring the levels of particle concentration in the exit gases.
Continuous-operation tests were also carried out prior to the tests to find the response time to the operating variables and the ways boiler operation could possibly distort the tests in the pilot plant. Similarly, fluctuations in inlet particle concentrations,
Table 2 Test matrix for each type of coal
Plate spacing Test no. Flow rate No. active (mm) fields
300 1-3 1.4 1-3 4~ 1.1 1-3. 7-9 0.8 1-3
400 10-12 1.4 1-3 13-15 1.1 1-3 16-18 0.8 1-3
B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81 71
moisture, and pressure were perceived to be produced by the preheater blowing and that could affect the outcome of the tests. To solve this problem, schedules were chosen for the tests that did not coincide with blowing periods.
4. Interpretation of results
The main test result is the precipitator collection efficiency, q, defined as the percentage by weight of the matter collected by the ESP based on the total input ash.
Cin - - Cou t q - • 100, (1)
Cin
where tin and Cout are the particle concentrations (mg/N m s) at the ESP inlet and outlet, respectively. This ESP efficiency is usually presented as penetration, P:
rout P = 100 - t /= • 100. (2)
Cin
The efficiency can be related to the geometric and operating parameters through different models [6, 7]. As a way to evaluate test results we have used three different single equation type precipitation models. The first one is the basic Deutsch-Ander- son model [8], expressed by
q = 1 -- exp - - ~ w D , (3)
where A is the total collection area of the ESP, Q is the total flow of processed gases, and WD is a parameter known as effective migration velocity of particles.
Another precipitation model widely used is represented by the Matts-Ohnfeldt formula [9], which is based on Deutsch's general equation, but it introduces a vari- ation in order to maintain the effective migration velocity (Wk) as a constant in relation with changes in the ESP area.
q = 1 - e x p --~Wk , (4)
where k is a parameter that depends on the nature of the material collected, and which usually takes values around 0.5 for coal ash.
There is also another design formula proposed by Petersen [10] based on the Stearn Catalytic model, which provides a new equation for the migration velocity and efficiency:
A ,~-l/b ~1=1-- 1 + b-Q Wb) , (5)
where b is another parameter that is characteristic of the material collected, and which takes a value of 0.24 for coal ash.
72 B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81
For the PESP, we have evaluated the migration rates by means of those three models.
The specific collection area, SCA = A/Q, is the parameter that defines the dimen- sions of an ESP. In the precipitator tests, SCA was varied, to see the influence that it has on overall efficiency for the different types of ash and the different internal configurations. The SCA modifications were obtained on one hand by varying the gas flow rate (flow changes), and on the other hand, by deactivating the electric fields, which diminishes the total collection area.
The electrical conditions of the ESP are characterized by the voltage applied between the discharge electrodes and the collection plates and the current intensity registered in each electrical field. The measurement of the number of discharges (sparking) or arcs that occur in each section also supplies important information.
A very important parameter for precipitation is the electrical resistivity, p, of the layer of ash that is deposited on the collecting plates. The drop in potential caused by the layer is given by
Vr = pje, (6)
where j is the density of electric current that crosses the layer, and e is the layer's thickness. This voltage drop in the layer makes the corona discharge diminish on the emitting electrodes, and therefore an increase in resistivity tends to reduce the ESP collection efficiency. However, the greatest problem that high-resistivity ash can give is the appearance of a back corona on the ash layer. This circumstance greatly deteriorates the electrical ionization, load, and particle migration conditions, and causes significant efficiency losses.
The appearance of back corona shows up in the electrical operating conditions of the ESP with a strong increase in current in relation with the maintenance or even decrease of the applied voltage.
5. Results and discussion
5.1. Type-A coal
The data obtained in the tests carried out with high resistivity type-A coal (Figs. 2-4) show that the best results in efficiency are achieved for a plate spacing of 400 mm, so that all the 300 mm tests carried out under equal conditions give lower efficiencies, with differences of at least 4 points in some of the cases. Concerning the influence of the gas flow rate through the ESP, we see how, within the range tested, the efficiency always increases as the flow rate decreases. This is true for both plate spacings. The efficiency differences between the 300 and 400 mm configurations diminish as the flow rate increases.
The input concentration of the different tests underwent variations in the 6500 to 10600 mg/(Nm 3) range, caused by boiler operation. The representativeness of the tests in relation with the changes detected in the particle load was guaranteed after proving, by means of specific tests, that these changes had no significant effect on
B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81 7 3
I PLATE SPACING 300 mm I
PENETRATION (%)
. . . . ' - - ~v=os m/= ] I . v= , , m,, /
1 2 3
NUMBER OF ACTIVE FIELDS
PLATE SPACING 400 mm ]
PENETRATION (%)
j ' < - - L . . " " i " ~ - v = o . 8 . , , o
, ~LV=I.t m/s
2
NUMBER OF ACTIVE FIELDS
F i g . 2 . E f f i c i e n c y v s . no of active fields. Type A coal.
I PLATE SPACING ,300 mm I l PLATE SPACING 400 mm I
F i g . 3 . Migration velocity v s . S C A . T y p e A coal.
PESP efficiency. To do so, a series of samplings was carried out under constant operating conditions where the only variable parameter was the particle concentra- tion at the PESP inlet, sweeping the entire range of tests. The efficiencies obtained were all of the same order, and provide dispersion levels similar to those obtained in series of samples carried out with the same input concentrations.
In Fig. 2, penetration values are portrayed for tests with a 300 or 400 mm plate spacing as a function of the number of electrical sections in operation and the flow rates. We find a homogeneous tendency in the efficiency increase with the increase in active fields for different flow rates. This tendency becomes asymptotic as the active
74
1 O0
B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81
PENETRATION (%)
10
A
l PLATE SPACING
• 300 rnm A400 mm
0.1 0 10 20 30 40 50 60
SCA (m2/m3/s)
Fig. 4. SCA vs. penetration. Type A coal.
fields increase in the tests with a greater efficiency, which correspond to flow rates of 0.8 m/s.
Fig. 3 specifies the migration velocity values obtained according to Deutsch's [Eq. (3)], Matts-t)hnfeldt [Eq. (4)], and Petersen [Eq. (5)] formulas. Likewise, it shows the SCA value with which each test was carried out.
In general, higher migration velocities with 400 mm plate spacing are obtained, which imply, from the economic point of view, an appreciable savings in fixed costs by using this configuration instead of the 300 mm configuration.
The migration velocities calculated by Deutsch's formula and by the Petersen formula show a gradual decrease with SCA for both configurations. In Deutsch's curves, we can see a smaller dispersion of values and a greater independence between the migration velocity and the flow rate.
In the Matts-t)hnfeldt curves, we can see, for some flow rates, points of maximum migration velocity for certain SCA values. These points could be interpreted as the maximum SCA development, giving a measurement relative to the efficiency per SCA unit.
In Fig. 4, the penetration values, are shown measured by the SCA variable, of all the cases that correspond to type-A coal, considering the tests carried out with 300 and 400 mm plate spacing. In this way, it is shown the existence of two clear tendencies that will supply the SCA efficiency design curves for each plate spacing. Concerning
B. Navarrete et aL /Journal of Electrostatics 39 (1997) 65-8l 75
these tendencies, it can be visualized the better behaviour of the type-A coal ash with the 400 mm configuration, which is more marked at the points with the highest efficiency. Here, greater efficiencies than at 300 mm for the same SCA ranges are found. The difference between the two configurations is accentuated for SCA greater than 30-35 m z s/m 3, where the curve corresponding to the 300 mm configuration begins to reduce its slope, showing an apparently asymptotic tendency toward stabilization with the growth of the SCA.
Previously reported differences on PESP performance are linked to substantial changes in electrical operating conditions inside the filter when plate spacing is modified, as it is shown in Table 3 where the electrical data corresponding to tests with a gas velocity of 1.1 m/s are presented. A 400 mm plate spacing produces (as expected) a considerable increase in voltage in parallel with a decrease in current. However, in the case of type-A coal, the voltage increase associated to the 400 mm configuration supposes a net improvement on average electric field of about + 7% for the whole precipitator. This improvement is mainly due to the higher electrical stability achieved with the 400 mm plate spacing by reducing the incidence of back corona. In Fig. 5 the V - I curves corresponding to field 1 in operation for both filter configurations are shown, where it is clear the incidence of severe back corona conditions when a plate spacing of 300 mm is used with type-A coal, and the relatively smoother conditions obtained with 400 mm. Both effects, back corona suppression and electric field increase, observed with type-A coal when duct width is increased are a consequence of its very high resistivity fly ash, and the same effects are not observed when type-B coal is used. In fact, from electrical operating points presented in Table 3 it can be easily deducted that passing from 300 to 400 mm with type-B coal implies an average electric field reduction of - 7% rather than the increase observed with type-A coal. Also, comparing the V- I curves produced by both types of coal (Fig. 5), that from type-B coal presents a considerable reduction in slope with respect to that from type-A coal, due to the inexistence of back corona.
5.2. Type-B coal
The results of the tests for type-B coal are shown in Figs. 6-8. In this case, contrary to what happens with type-A coal, the efficiency is in general higher in the 300 mm
Table 3 Electrical operating points of the PESP
Type of Plate Field 1 Field 2 Field 3 coal spacing
(mm) Voltage Current Voltage Current Voltage Current (kV) (mA) (kV) (mA) (kV) (mA)
A 300 47 5 40 27 41 25 400 64 4 60 10 59 16
B 300 47 15 49 23 50 32 400 60 3 60 7 61 9
76 B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81
CURRENT (mA)
20 L I I r I
15 ~ "~-300 mm ~ _ _ ~ "ryp. A Coal
10/-=-4°°mm/ ~ ~ . , ~ 5 ~ - - ~ ~
0 10 20 30 40 50 60 70 80 VOLTAGE(k~
CURRENT (mA) 20 _..r I I
L ~ Type A Coal I I
15 "A-Type B C°al t J
105 "
0 10
400 m m j
20 30 40 50 60 70 80 VOLTAGE (kY)
Fig. 5. V-I curves f rom field 1.
I PLATE SPACING 300 mm I
PENETRATION (%)
1
lO
~v=o.8 m/s I v = l ~ mrs
[ " v = ' 4 m/s I
2
NUMBER OF ACTIVE FIELDS
PLATE SPACING 400 mm ]
PENETRATION (%)
2
NUMBER OF ACTIVE FIELDS
Fig. 6. Efficiency vs. no of active field. Type B coal.
B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81 77
I PLATE SPACING 300 mm
M I G R A T I O N V E L O C I T Y (cm/s) 7O 65 6O 55 - - - , 50
3 5
3 0
25
2O
15 lO! 5
0
I I P.AT SPAO,.G ,00 mm I MIGRATION VELOCITY (cm/s)
78 B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81
configuration than in the 400 mm configuration for the same flow rate conditions. At 400 m m , the differences are clear with low and medium flow rates (0.8 and 1 m/s), and are not very significant at the higher rate (1.4 m/s). At 300 mm, the higher efficiencies are achieved with a flow rate of 1.1 m/s, so for this configuration it is shown that velocity becomes not only the economic optimum but also a performance optimum. The efficiency drop at the higher rate is more marked than for 400 mm and increases with the number of active fields.
The inlet particle concentrations are significantly lower than those found for type-A coal because of the lower ash content of this coal. This parameter fluctuates depending on the boiler's operating conditions, although the differences are less than those seen with type-A coal. In relation with particle size, Laser-Coulter size measurements performed on ash samples isokinetically taken at PESP inlet indicate that both fly ashes present a similar size distribution although ash from type-B coal is slightly coarser, as can be seen in Fig. 9.
Fig. 6 shows, just as for type-A coal, penetration evolution by fields. With the 400 mm width, we can see some less-well-defined tendencies than for the 300 mm width. Even with two active fields, there is an overlapping between the efficiency of the 1.! and 1.4 m/s flow rate tests. At 300 mm, it is shown, just as for type-A coal, an asymptotic tendency of penetration with the number of active fields, which suggests a limitation of the efficiency increase in proportion to the increase in electrical
UNDERSIZE (Wt %) 1 O0
10
/ ' , i
/ 0.1,
0.1
i
lO
~ w B
r
i
j ,_+l TYPE A COAL ~ TYPE B COAL
I
100 1,000
PARTICLE DIAMETER (microns)
Fig. 9. Fly ash size distribution.
B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81 79
sections. This tendency is not seen at 400 mm, which would indicate the possibility of a greater efficiency rate with the sum of additional collection area. Nevertheless, it should be noted that at 300 ram, there are better efficiency levels to begin with. The existence of the optimum can also be observed at the 1.1 m/s rate with the 300 mm configuration.
Fig. 7 shows the values of the migration velocities evaluated for each of the tests of type-B coal by means of the three efficiency equations previously described.
The migration velocities are found within the same order for the same SCA values. The migration velocity values that appear with the 400 mm configuration, at the 1.4 m/s velocity and with the lowest SCA values ( < 10 (m 2 s)/m 3) stand out because of their negligible magnitude. These are more marked on the Matts-Ohnfeldt velocity curves. This result would denote the inefficiency of these SCA values at high velocities.
At 300 mm, the curves show a greater homogeneity, and the conservation of the migration rates as the SCA increases can be observed for some configurations, such as the Matts-Ohnfeldt configurations at 0.8 m/s.
The same as in the case of type-A coal, the lower dispersions with the flow rate are registered by the Deutsch velocities, with relative variations among the cases that are also lower than others.
Fig. 8 shows the variation of the penetration with the SCA value for each of the tests, divided into two series that correspond to each one of the plate spacing configurations. It can be observed, contrary to the case of type-A coal, how there is no such marked difference with the results of both configurations for the same SCA values. However, and for the low and medium flow rates, a better efficiency is achieved with 300 mm width as a consequence of the greater SCA of these tests.
6. Conclusions
By means of extensive parametric tests, the incidence of relevant operating para- meters on the overall efficiency of an ESP, has been evaluated trying to find the optimum combinations of parameters that maximize the efficiency for each of the tested fly-ashes.
For type-A (high resistivity) coal, the greater efficiency is obtained with a 400 mm plate spacing and with the lowest gas flow rate of those tested (0.8 m/s). For type-B (low resistivity) coal, the configuration that gives the greatest yields is that with a 300 mm plate spacing and, even though with 400 mm the efficiency increases as the flow rate decreases, it has an optimum yield at 300 mm with the 1.1 m/s rate, worsening somewhat for 0.8 and even more so for 1.4 m/s. The efficiency, in general, is greater for type-B coal ash than for type-A.
Ash concentrations at the ESP inlet vary depending on the type of coal, and are the main consequence of the greater or lesser ash content that each one has. In this way, the higher inlet concentrations are measured with type-A coal (some 7000 mg/(N m3)), with appreciably lower concentrations for type-B coal (3500-4500 mg/(N m3)), Type- A ash concentration also gives a greater dispersion because there are values of 6200 to 10 600 mg/(N m3). These differences, although they make the outlet emissions vary,
80 B. Navarrete et al./Journal o f Electrostatics 39 (1997) 65-81
have no effect on the efficiency in the range at which they appear; this was shown in a series of tests carried out to that effect. No substantial differences in size distribution have been found between the two fly ashes, although ash from type-B coal is slightly coarser.
From the test results, the ratio between efficiency and SCA for both types of coal, were obtained in each one of the ESP configurations, which supply the design curves for the SCA ranges tested and the tendencies toward greater SCA values.
The main conclusions of the present work clearly indicates the convenience to employ a wide plate spacing of 400 mm when high resistivity fly ash is processed. Whereas, an ESP with plate spacing of 300 mm will show a better performance for the collection of low resistivity fly ash. The type-A coal precipitation with 300 mm plate spacing is limited by the appearance of back corona. This problem is at least partially avoided by increasing plate spacing, driving to a substantial improvement in efficiency when 400 mm width is tested. In relation to type-B coal, the collection of a low resistivity ash tends to increase ESP efficiency with regard to type-A coal, and as it was said above, the 300 mm ESP configuration produces better results in efficiency for a same precipitator size. However, when dependence between efficiency and SCA is taken into account, a nearly neutral influence of plate spacing is observed, indicating that ESPs collecting low resistivity ash with the same SCA will achieve similar efficiency, independently of plate spacing. Nevertheless, this implies that an ESP using a 400 mm plate spacing will require a larger size than other using a 300 mm configuration.
In summary, an useful characterization and database for ESP design and operation has been developed. However, some very interesting questions about aspects which are not fully understood have arisen. These questions have to do with the develop- ment of electrostatic precipitation and deal with the significance of properties inherent to the ash layers, independent from their resistivity, and which can affect the perfor- mance and choice of the optimum ESP configuration.
Acknowledgements
This project was developed by the Department of Chemical and Environmental Engineering of the University of Seville in cooperation with Sevillana de Electricidad through the Electrotechnical Research and Development Program (PIE). The authors would like to thank Los Barrios Power Plant, property of Sevillana de Electricidad, for its cooperation and their contributions to the success of the project.
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B. Navarrete et al./Journal of Electrostatics 39 (1997) 65-81 81
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