During normal operation, the base voltage Vdc is maintained about the corona
onset level, and the pulse amplitude and the pulse frequency are varied accord- ing to a certain preset control strategy dependent on the fly ash resistivity and electrical operating conditions. The results of using pulse energisation in a pre- cipitator can be better expressed by the V/I characteristics of the respective bus Figure 7.13 Idealised pulse voltage and current waveforms
section, as shown in Figure 7.14 [13]. Here, the specific current density, defined as the precipitator mean current divided by the collection area, is plotted as a function of the pulse amplitude for a constant base voltage Vdc and three
different pulse repetition frequencies.
This typical family of curves illustrates the following features of pulse energisation:
•
the precipitator current can be varied by changing the pulse frequency while the precipitator peak voltage (Vdc+ VP) is kept constant,•
the precipitator peak voltage is higher than the value for conventional energi- sation as a result of the very short duration of the pulses,•
the slope of the V/I curves is rather flat, being typical for high resistivity dust conditions.These features of pulse energisation result in the following operational advantages.
7.4.1 Current control capabilities
The V/I characteristic (Figure 7.14) shows that the precipitator current Ip can be
controlled independently of the precipitator voltage by varying the pulse repeti- tion frequency. This allows the current to be reduced towards the onset of back- corona, without decreasing the precipitator voltage, i.e. the precipitator can operate with a low current and high precipitator voltage. This results in a more stable and suitable electrical energisation system for high resistivity dust com- pared to traditional d.c. energisation, where current control cannot be achieved without reducing precipitator voltages and hence performance.
The dense ionic space charge initially produced by the pulse shields the dis- charge electrode and reduces the electrical field strength at its surface, which
Figure 7.14 Typical V/I characteristics obtained with pulse energisation (courtesy FLS Miljö a/s)
results in the suppression or limitation of the corona discharge during the remainder of the pulse period; this allows time for any collected charge to dissipate, thereby preventing voltage build up leading to positive ion emission.
7.4.2 Current distribution on collectors
With d.c. energisation, the corona discharge tends to be localised at discrete positions on the discharge electrode and consequently the current distribution on the collectors is far from uniform. With the application of pulsing, the nar- row high amplitude pulses superimposed on a base voltage around the corona onset voltage is such that the peak voltage significantly exceeds the corona onset level, which as indicated, produces an intense corona discharge and a correspondingly dense ionic space charge around the electrode. A discharge electrode with very spotty corona under traditional d.c. energisation can literally be made to glow under pulse energisation [11,14].
Investigations using d.c. energisation have shown that the collector current density at the beginning of any precipitator section is very low and increases along the section in the direction of the gas. With pulse energisation, however, the current density along the precipitator section is considerably more uniform. A good current distribution on the collecting plates is important in order to avoid the initiation of back-corona due to localised spots of high current dens- ity. This improvement in current distribution has been confirmed by measure- ments on a laboratory single duct precipitator and also in the field using larger pilot precipitators [15].
7.4.3 Electrical field strength in the inter-electrode area
With d.c. energisation, free electrons are constantly being generated, producing an ionic space charge density and a field strength that, in principle, does not vary with time, whereas with pulse energisation, the base voltage is kept just below the corona onset level, and free electrons and negative ions are only generated during the actual pulse period. During the pulsing, the ionic space charge rap- idly crosses the inter-electrode area because of the high voltage field, while during the time interval between pulses, the space charge migrates towards the collecting electrode driven only by the base voltage. As a consequence, the space charge and the field strength vary with time at each point in the inter-electrode space.
Measurements on a tubular laboratory precipitator have produced the results shown in Figure 7.15, where the relative field strength is plotted as a function of time. This shows that, following the pulse, the electrical field is determined by the d.c. voltage and the moving space charge, its strength decreasing until the actual front reaches the collecting electrode.
7.4.4 Particle charging
For particles greater than 1 μm in diameter, collision charging is the predomin- ant mechanism, and the saturation charge is determined by the maximum field strength created by the ionic space charge. With d.c. energisation, a particle at a certain position is surrounded by the ionic space charge, and its saturation charge depends on the electric field at that position. With pulse energisation, however, particle charging occurs only when the space charge passes the particle and its saturation charge is determined by the maximum field strength during the passage of that space charge. Because the maximum field strength with pulse energisation is much higher than for d.c. energisation, as indicated in Figure 7.15, this approach provides enhanced particle charging. Measurements have shown that the best results are obtained with a high pulse amplitude, which results in a higher ionic space charge density and field situation.
7.4.5 Power consumption
A precipitator section can be represented by a capacitance in parallel with a current generator accounting for the electronic, ionic and dust space charge currents. Each pulse has to raise the voltage across the capacitance CF, from the
base voltage Vdc to the peak voltage (Vdc+ VP), which involves a considerable
amount of energy usage. Supposing that in Figure 7.9 the coupling capacitor CC
is much larger than CF, the energy supplied by the pulsing system for charging
CF is
WC= ½CFVP2. (7.13)
Figure 7.15 Electric field as a function of time, following the initiation of the high voltage pulse (courtesy FLS Miljö a/s)
Using typical values of VP= 60 kV and CF= 100 nF, the energy dissipated WC is
therefore 180 J. If this has to be repeated 200 times per second, a large amount of power (36 kW) would be consumed. As the energy necessary for the corona generation is very small compared with the energy needed to charge CF, the
power consumption becomes excessive unless the pulse system includes means for energy recovery. The pulsing systems shown in Figures 7.9 and 7.10 include this energy saving feature in the form of anti-parallel diodes located in the series oscillating circuit. Here, when the voltage across CF is at its maximum, the pulse
current is zero and when the flow reverses in the negative half cycle the surplus energy from CF is passed back into the storage capacity CS.
The power consumed by a precipitator section energised by a pulse system with energy recovery [13] can be expressed by
P= IpVdc+ c × IpVP, (7.14) where the constant c has been found experimentally to be approximately equal to 0.5.
7.4.6 Worked example of energy recovery
If it is assumed that a pulse system is operating at Ip= 0.1 mA m−2, Vp= 60 kV,
Vdc= 40 kV and energises a 3000 m2 area bus section, the precipitator mean
current is therefore 0.1 × 3000= 300 mA, and if we apply these values to Equation (7.14) this gives the power consumed as
P= 0.3 × 40 + 0.5 × 0.3 × 60 = 12 + 9 = 21 kW.
This consumption corresponds to a power density of 7 W m−2, which is a typical value for medium resistivity dusts. For high resistivity dusts, the required power density is much lower, and by operating with a low pulse repetition frequency (2–20 p.p.s.) the precipitator mean current and total power is therefore correspondingly reduced.