Title of Invention

A SYSTEM FOR PRECONDITIONING FLUE GAS

Abstract A system for preconditioning flue gas to be treated in an electrostatic precipitator having a set of electrostatic elements and a power supply, which supplies an intermittent power to the electrostatic elements, includes a source of a conditioning agent, and a current sensor and a voltage sensor for detecting the half-cycle current and voltage supplied to the electrostatic elements. The system also includes a power measuring circuit, responsive to the current sensor and to the voltage sensor, which develops an indication of the power supplied to the electrostatic elements and a controller, responsive to the power indication, which controls the amount of conditioning agent added to the flue gas in order to maintain the power at a substantially predetermined level. PRICE: THIRTY RUPEES
Full Text



FLUE GAS CONDITIONING SYSTEM FOR INTERMITTENTLY ENERGIZED PRECIPITATION
Field Of the Invention
The present invention relates to a system for conditioning flue gas with a conditioning agent such as SO3 in order to improve the efficiency of an electrostatic precipitator in collecting ash and other particulate matter from the flue gas, and more particularly, to such a system which is controlled by monitoring the power delivered to the electro¬static elements of an electrostatic precipitator.
BacXqround of the Invention
The flue gas of furnaces and boilers, such as those used in power generation plants, carries matter including ash and other particulates which pollute the atmosphere. Electrostatic precipitators are used to remove ash and other particulates carried in the flue gas. Electrostatic precipitators operate by causing the individual particles in the flue gas to accept an electrical charge and by attracting the charged particles to collector plates for disposal.
Electrostatic precipitation has been used pri¬marily in connection with the burning of coal. As coal burns, it produces HjO, COj, CO, SOj, SO3, ash and other particulate matter and products of combustion. The HjO and SO3 combine to form H2SO4 (sulfa-uric oci-:'} which coats the particulate iuatt-ei . The coating of H2SO4 reduces the resistance of the ash and other particulate matter and thereby facilitates the electrical charging of this par¬ticulate matter so that the charged particulate matter can be more easily attracted to the collector

plates of the electrostatic precipitator. If combustion produces insufficient HjSO’, however, the resistance of the particulate matter is high which reduces the efficiency of the electrostatic precipitator in charging the particulate matter suspended in the flue gas and, as a result, in collecting particulate matter from the flue gas.
When coal having a relatively high sulfur con¬tent is burned, sufficient SO3 is produced to form the proper amount of His. However, high sulfur coal also produces excess SO2 which, if exhausted to the atmosphere, is a pollutant that has been linked to acid rain.
In order to reduce SO2 emissions, the operators of coal fired boilers and furnaces have burned coal having a low sulfur content. However, low sulfur coal results in the production of less SO3 than that required to efficiently operate the electrostatic precipitators. Accordingly, one must balance the need for lower SOj emissions and the need for an adequate supply of SO3 to maintain the efficiency of electrostatic precipitators at a relatively high level. To provide this balance, the operators using lower sulfur content coal have injected a controlled
the inadequate amount of SO3 produced by combustion of the low sulfur coal. Thus, SOj emissions are held relatively low while electrostatic precipitator efficiency is increased.
As is apparent, the efficiency of some electrostatic precipitators is dependent upon the concentration of SO3 in the flue gas. That is, if the SO3 concentration in the flue gas is too low, an electrostatic precipitator may operate at less than optimal efficiency and an unacceptable plume of

particulates may result. Flue gas that has less than optimal SO3 concentrations as the flue gas enters the electrostatic precipitator constitutes an emissions problem. Coal fired power generation plants that are operating out of compliance with emission regulations can be forced to reduce their power output until the emissions are brought back into compliance. Accordingly, it is important to keep the emissions concentrations within an acceptable range while minimizing the power consumption of the electrostatic precipitator.
One prior art method of decreasing the power consumption of an electrostatic precipitator is to measure the opacity of the flue gas as it exits from a stack of the flue gas conditioning system and to control the amount of power supplied to the electrostatic precipitator accordingly. For example, Reese, et al., U.S. Patent No. 4,284,417 discloses a system for controlling the electric power supplied to an electrostatic precipitator having an opacity-sensitive transducer which produces an output signal proportional to the opacity of the flue gas exiting from the precip¬itator. The system also includes a comparator which compares the output signal preset upper and lower limits and a controller which controls the power supplied to the precipitator in order to restore the flue gas opacity to a permissible range when the output signal falls outside of the preset upper and lower limits. Krigmont, et al., U.S. Patent No. 4,987,839 discloses a system including a source of SO3 which adds SO3 to flue gas before it enters an electrostatic precipitator and a controller which controls the rate at which the SO3 is added to the flue gas. The controller is

responsive to the opacity of the flue gas exiting the electrostatic precipitator and to the power supplied to the electrostatic precipitator.
Measuring the opacity of the flue gas as it leaves the flue gas conditioning system, however, is not necessarily the best method of controlling the level of ash and other particulates in flues because the opacity of the flue gas is not a good indicator of the need for the addition of a particular additive, such as SO3.
Another method of increasing the efficiency of an electrostatic precipitator is to employ, as a control for the amount of SO3 delivered to the flue, the power delivered to the electrostatic precipitator. A system employing this method is disclosed in Woracek, et al., U.S. Patent No. 4,779,207, wherein a flue gas conditioning system includes automatic voltage controllers (AVCs) which supply power to transformer/rectifier sets which, in turn, provide a stepped-up and rectified voltage to elements or plates of an electrostatic precipitator. Power measuring elements produce signals indicative of the power delivered by each of the AVCs to each of the transformer/rectifier sets, and these signals are combined to produce an indication of the average power delivered to the electrostatic precipitator. The average power indication is used to control the amount of SO3 delivered to the flue so as to keep the average power delivered to the electrostatic precipitator within a predeterinined range.
It is also known in the prior art to inter¬mittently energize the elements of an electrostatic precipitator, which may include plates, electrodes and the like, in order to increase the electrostatic precipitator efficiency. A flue gas conditioning

system which is utilized for an electrostatic precipitator system with intermittent energization typically has an AVC which supplies an intermittent voltage, having a predetermined duty cycle, to a transformer/rectifier circuit which, in turn, provides a stepped-up, rectified, intermittent voltage to the electrostatic elements. Krigmont, et al., U.S. Patent No. 4,987,839 discloses an intermittently energized system having a control which is responsive to the duty cycle of the power delivered to an electrostatic precipitator and which uses this duty cycle to estimate the power delivered to the flue gas by the electrostatic elements.
Intermittent energization, however, creates a control problem in flue gas conditioning systems like that disclosed in the Krigmont, et al. patent and those which control the amount of SO3 delivered to the flue in accordance with a power signal developed from the duty cycle of the intermittent power supplied to the electrostatic precipitator. This problem occurs because neither (a) the duty cycle of the intermittent power nor (b) the actual power developed by the precipitator power source with which the flue gas conditioning system is used, are reliable indications of the power actually delivered to the flue gas by the electrostatic elements. As such, an average power signal developed from the duty cycle of the intermittent power is an inaccurate determination of the amount of SO3 which needs to be provided to the flue gas entering the electrostatic precipitator. This problem becomes particularly acute when the duty cycle of the power source is periodically changed during operation of the flue gas conditioning system. In such a system, therefore, other means

must be provided for measuring the power absorbed by the electrostatic elements of the electrostatic precipitator.
Summary of the Invention
The power absorbed by the electrostatic elements of an electrostatic precipitator is an indication of the resistively of the particulate matter within a flue and, therefore, the need for SO3. When a flue gas conditioning system is operated in a constant energization mode, one may normally use the power developed by a power source and delivered to the electrostatic precipitator as an accurate measure of the power absorbed by the electrostatic elements and, therefore, as a reliable indicator of the amount of flue gas conditioning agent (SO3) required to treat the particulates in the flue gas.
If the electrostatic elements of the electro¬static precipitator are intermittently energized, however, the power developed by the power source and the duty cycle of the intermittent energization are not reliable indications of the power absorbed by the electrostatic elements. Therefore, when intermittent energization is employed, other means must be provided for measuring the power absorbed by the electrostatic elements. According to one embodiment of the present invention, certain parameters of the power delivered directly to the electrostatic elements are measured, and these parameters are used to develop a power signal which, in turn, is used to regulate the amount of SO3 delivered to the flue gas.
Thus, the present invention relates to a system for preconditioning flue gas to be treated in an

intermittently energized electrostatic precipitator having a power source which supplies an intermittent power to electrostatic elements of an electrostatic precipitator. One aspect of this system includes a source of a conditioning agent, such as SO3, and a detection device which detects first and second parameters of the power supplied to the electrostatic elements. The system also includes a component which is responsive to the first and second parameters, and which develops an indication of the power supplied to the electrostatic elements. In addition, the system includes a controller, responsive to the power indication, which controls the amount of conditioning agent added to the flue gas in order to maintain the power at a substantially predetermined level.
The power source may include circuitry for delivering an intermittent voltage to a primary winding of a transformer having a transformer output coupled to the electrostatic elements. The power source may also include a circuit for selecting the intermittent duty cycle delivered to the transformer.
The detection device may include a currant sensor for measuring the current flowing through the transformer input or the transformer output and a voltage sensor for measuring the voltage at the transformer input or the transformer output. Specifically the current sensor may include circuitry for Treasurer. the half-cycle root mean squared (RMS) current, the peak current or the average current flowing through the transformer during one or more energized half-cycles of the intermittent power, while the Voltage sensor may include circuitry for measuring the average voltage.

the peak voltage, the half-cycle RMS voltage or the minimum voltage across transformer during one or more energized half-cycles of the intermittent power. Preferably, a multiplier multiplies the RMS, peak or average current with the average, peak, RMS or minimum voltage to produce the power indication.
Another aspect of the present invention is directed to an improvement in a flue gas conditioning system in which a source of a conditioning agent is added to flue gas, and in which an electrostatic precipitator, having a set of electrostatic elements which receive a power, treats the flue gas. Furthermore, a controller, responsive to an indication of the power, controls the amount of the conditioning agent added to the flue gas, and a power source, which operates in an intermittent energization mode, delivers a power having a duty cycle to the electrostatic elements. The improvement includes a measuring device which measures first and second -parameters of the power delivered to the electrostatic elements, and circuitry, responsive to the first and second parameters, which derives the indication of the power.
Preferably, the first and second parameters are current and voltage, respectively, and the measuring device includes a first sensor which detects current flowing into the electrostatic elements to produce a current signal and a second sensor which detects voltage developed across the electrostatic elements to produce a voltage signal. The circuitry combines the current signal and the voltage signal to produce the indication of the power.
Yet another aspect of the present invention is directed to a method of controlling a flue gas

conditioning system which includes a source of a conditioning agent, a component which adds the conditioning agent to flue gas, and an electrostatic precipitator having a set of electrostatic elements which receive a power, for treating the flue gas. The system fur flier includes a controller, responsive to a signal indicative of the power, which controls the amount of conditioning agent add’ to the flue gas. According to one embodiment of the method, a power source is operated in an intermittent energization mode to develop an input power having a duty cycle which delivers the power to the electrostatic elements. Furthermore, first and second parameters of the power are measured and are used to derive the signal indicative of the power.
Accordingly the present invention provides a system for preconditioning flue gas to be treated in an electrostatic precipitator having a set of electrostatic elements and a power supply which suppose intermittent power to the electrostatic elements, said system comprising : a source of a conditioning agent which suppose an amount of the conditioning agent to the flue gas; detecting means for detecting first and second parameters of the power supped to the electrostatic elements; developing means, responsive to the first and second parameters, for developing an indication of the power supplied to the electrostatic elements; and controlling means, responsive to the power indication, for controlling the amount of conditioning agent added to the flue gas.

These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the accompanying drawings.
Figure 1 comprises a block diagram of a flue gas exhaust and conditioning system according to an embodiment of the present invention;
Figure 2 comprises a combined block and simplified schematic diagram of a portion of the system of Figure 1;
Figure 3 comprises a set of waveform diagrams illustrating the operation of the system of Figure 1 in a full-cycle energization mode;
Figure 4 comprises a set of waveform diagrams illustrating the operation of the system of Figure 1 in an intermittent energization mode; and

Figure 5 comprises a flow chart illustrating a flue gas flow controller employed in the system of Figure 1.
Detailed Description of the Preferred Referring initially to Figure 1, a flue gas conditioning system is indicated generally at 8 and is used with a flue 9 connected to a boiler 10, such as a boiler in a coal burning power generation plant, which discharges by-products of combustion through flue 9. An electrostatic precipitator 12 is disposed in flue 9 and has multiple sets of electrostatic elements 14, which include electrodes and/or electrostatic plates, which are disposed parallel to the flow of the flue gas through the flue 9, for removing ash and other particulate material from the flue gas.
In order to increase the efficiency of elec¬trostatic precipitator 12 in precipitating out the ash and other particulate matter from -the flue gas in flue 9, a conditioning agent in the form of gaseous sulfur trioxide, SO3, is supplied to the flue gas in flue 9. This SO3 is produced by burning sulfur with oxygen to produce solver dioxide, SOj, and then converting the SOj, by the use of a catalytic converter, into SO3 which-then can be supplied to flue 9. Accordingly, a pump 16 delivers molten sulfur provided by a sulfur supply 18 to a sulfur burner 2 0 which burns the sulfur in the presence of oxygen in order to produce sulfur diox¬ide, SOj- Oxygen is supplied to the burner 20 in the form of air from an air blower 22. A gas mixture including SOj exits the sulfur burner 20 and is supplied to an SO3 generator 24, such as a cata¬lytic converter, which converts the sulfur dioxide,

SOj, into sulfur trioxide, SO3, typically by employing excess oxygen in the mixture exiting sulfur burner 20. SO3 generator 24 delivers SO3 to a set of injectors 25 which inject the SO3 into flue 9 where the SO3 combines with water vapor in the flue gas to form sulfuric acid vapor which condenses and reacts with the ash and other particulate material in flue 9. This process reduces the resistively of the particles in the flue gas and allows electrostatic elements 14 of electrostatic precipitator 12 to remove the particles more efficiently. Burning sulfur in the presence of oxygen to form sulfur dioxide and then converting that sulfur dioxide into sulfur trioxide may all be done in a conventional manner.
Pump far is driven cy a motor 2 6 under control of a speed controller 28. Speed controller 28 is regulated by a flue gas flow controller 30 which is responsive to a set of power signals developed by power measuring units 32 which provide an indication of the requirement of SO3 within the flue. Flow controller 30 may also be responsive to a flow signal produced by a flow rate sensor 33, connected between pump 16 and burner 20. Sensor 33 produces a signal indicative of the actual amount of sulfur being supplied to burner 20. Flow controller 30 may also be responsive to a boiler load signal from a sensor 34 which reflects the amount of combustion occurring in boiler 10.
A set of automatic voltage controllers (AVCs) 4 0 provide power in the form of AC voltage to a set of transformer/rectifier (T/R) sets 42, each of which includes a transformer and a rectifier and each of which is connected to one of the sets of electrostatic elements 14. Each of T/R sets 42

steps up the voltage supplied by the associated AVC 40 to produce a higher amplitude secondary voltage, rectifies the secondary voltage, and provides the rectified secondary voltage to the associated electrostatic elements 14, which remove ash and other particulate matter from the flue gas in electrostatic precipitator 12.
Power measuring units 3 2 are responsive to signals indicative of the voltage and current delivered to electrostatic elements 14 by T/R sets 42, and units 32 produce power signals indicative of the power absorbed by electrostatic elements 14. The power signals produced by power measuring units 32 are preferably indicative of the average power delivered to electrostatic elements 14 during each cycle or half-cycle of the AC voltage developed by AVCs 40, but could, alternatively, comprise instantaneous power signals, power signals indicative of the average power absorbed by the electrostatic elements 14 over longer periods of time, or any other type of power signal, if so desired. Flow controller 3 0 is responsive to the power signals developed by each of the power measuring units 3 2 and produces an electrostatic precipitator power signal indicative of the total average power absorbed by all of the sets of electrostatic elements 14 over a predetermined length of time.
Flow controller 30 uses the electrostatic precipitator power signal as a process value to control the speed controller 28. This control can be accomplished by comparing an electrostatic precipitator power signal process value to a power set point to produce a difference signal. If the electrostatic precipitator process value is greater

than the set point, the flow controller 30 causes speed controller 28 to decrease the amount of sulfur being provided by sulfur supply 18 to burner 20 and decrease the amount of SO3 injected at injectors 25, which increases the resistivity of the particles in the flue gas, and thereby decreases the power absorbed by the electrostatic elements. If, however, the electrostatic precipitator process value is below the set point value, flow controller 30 causes the speed controller to increase the amount of sulfur being provided by sulfur supply 18 to burner 2 0 and increase the amount of SO3 injected at injectors 25, which reduces the resistivity of the particles in the flue gas and thereby increases the power absorbed by the electrostatic elements. In this manner, flow controller 30 measures the power dissipated by electrostatic elements 14 and regulates the speed of motor 26 to ensure that the proper amount of sulfur trioxide, SO3, is supplied to flue 9 so as to maintain a substantially constant power usage within electrostatic elements 14 of the electrostatic precipitator 12.
Flow controller 30 shown in Figure 1 may be a programmable logic controller such as a Honeywell UDC 9000E which delivers a 4 to 20 Mililani signal to speed controller 28 which may be any suitable speed controller, such as a Westinghouse Acutrim Model 110, for accepting a 4 to 20 milliamp input and for providing a speed control output to motor 26. Each of AVCs 40 shown in Figure 1 may be, for example, an ABB Flak, Epic II automatic voltage controller system.
The amount of SO3 supplied to flue 9 is propor¬tional to the sulfur supplied to the burner 20 by pump 16 which is controlled by the speed of motor

26. In one embodiment, the amount of air from blower 22, which provides the oxygen for forming SOj and SO3, can be preset to comprise a constant flow of air sufficient to produce the required supply of oxygen at maximum SO3 demand. In another embodi¬ment, air flow from blower 22 can be varied in response to variations in the amount of sulfur delivered by sulfur pump 16. Typically, in both embodiments, there is always a surplus of air flow to burner 20 and to SO3 generator 24 to deliver to flue 9, a dilute mixture of gaseous SO3.
Figure 2 illustrates one AVC 40 in conjunction with one T/R set 42 and one power measuring unit 32. An AC input voltage, typically having an RMS voltage of 480, is produced by an external power source (ncit shown) and is supplied through a circuit breaker 50 to lines 52a and 52b. A current sensor 54 produces a primary current signal I , indicative of the current flowing through line 52a and into AVC 40, which is delivered to an AVC control unit 56. The power developed on line 52a is provided to silicon controlled rectifiers SCRl and SCR2 which are connected between line 52a and a line 57, in a reverse parallel configuration. AVC control unit 56 is coupled to the gate inputs of SCRl and SCR2 and controls the operation thereof.
A varistor V and a resistor Rl in conjunction with a capacitor CI are connected in parallel across silicon controlled rectifiers SCRl and SCR2, while a resistor R2 and a capacitor C2 are connected in series between the cathode of SCRl and line 52b. Varistor V, resistors Rl and R2 and capacitors CI and C2 operate as a protection circuit which filters out transients produced by SCRl and SCR2 when switching from an on state to an off state, or vice-

versa. This protection circuit prevents, for example, SCRl from turning SCR2 on when SCRl turns off. The values of varistor V, resistors and Rj and capacitors C’ and Cj are dependent upon the particular type of silicon controlled rectifier used and may be chosen in any conventional manner.
The cathode of SCRl and the anode of SCR2 are connected through line 57 to an input of a transformer 58 having a primary winding 60 and a secondary winding 62. Transformer 58 steps up the voltage appearing between lines 52b and 57 to a higher, secondary level to produce a stepped-up voltage across lines 63a and 63b, which may comprise a transformer output. Transformer 58 may, however, include a rectifier 64 connected in a full-bridge configuration having diodes D1-D4 connected as shown in Figure 2. Rectifier 64 is responsive to the stepped-up voltage appearing across lines 63a and 63b and produces a secondary voltage Vg across a positive transformer output 66 and a negative transformer output 68, Positive transformer output 66 is connected to an electrical ground while negative transformer ourput 68 is connected to a discharge electrode 14a comprising one or more of a plurality of electrodes associated with one of the sets of electrostatic elements 14. Electrostatic plates 14b, which comprise the other of the plurality of electrodes within one of the sets of electrostatic elements 14, are connected to electrical ground.
According to one embodiment of the present invention, power measuring unit 32 is responsive to secondary voltage Vg appearing across transformer outputs 66 and 68 and to a current signal I’, produced by a current sensor 70 connected to

positive transformer output 66. Power measuring unit 3 2 produces one signal, indicative of, for example, the root mean squared (RMS) current, the peak current or the average current flowing through the electrostatic elements 14 during one or more predetermined number of energized half-cycles of the input voltage. Power measuring unit 32 also produces a second signal indicative of, for example, the average voltage, the peak voltage, the RMS voltage or the minimum voltage appearing across electrostatic elements 14 during one or more pre¬determined number of energized half-cycles of the input voltage.
It should be noted, however, that power measur¬ing unit 3 2 can also be connected across the primary winding 60 of the transformer 58 to measure the current and voltage during one or more energized half-cycles of the input voltage. For example, the power measuring unit 32 can measure the RMS, peak or average current flowing through the primary winding 60 of the transformer 58 during each energized half-cycle of the input voltage and measure the average, peak, RMS or minimum voltage appearing across the primary winding 60 over one or more predetermined number of energized half-cycles of the input voltage in order to measure the power being delivered to the electrostatic elements 14.
Power measuring unit 32 combines the RMS, peak or average current signal and the average, peak, RMS or minimum voltage signal by multiplication, for example, to produce a signal indicative of the instantaneous, average or other power delivered to the electrostatic elements 14. Preferably, however, voltage values which are below a preset threshold

are not used to derive this power signal. Also preferably, the power measuring unit 32 develops a power signal which represents the power delivered to electrostatic elements 14 during each energized half-cycle or full cycle of the input voltage appearing across lines 52a and 52b. However, the power measuring unit 3 2 may develop a power signal which represents the power delivered to the electrostatic elements over longer periods of time, if so desired. In any event, power measuring unit 32 delivers the power signal to flow controller 30 via a line 72.
In operation, the input voltage is delivered to lines 52a and 52b and thereby to silicon controlled rectifiers SCRl and SCR2. Control unit 56 responds to current signal Ip, to primary voltage Vp appearing across primary winding 60 of transformer 58, to secondary voltage Vg, and to secondary current signal Ig, and produces control signals at the gate inputs of SCRl and SCR2. These control signals turn SCRl and SCR2 on and off and thereby control the voltage delivered to transformer 58.
More specifically, control unit 56 provides a control signal to the gate input of SCRl which turns SCRl on during the positive half-cycles of the input voltage appearing between -lines 52a and 52b and which turns SCRl off during the negative half-cycles of the input voltage. Likewise, control unit 56 provides a control signal to the gate input of SCR2 which turns SCR2 on during the negative half-cycles of the input voltage and which turns SCR2 off during the positive half-cycles of the input voltage.
Control unit 56 controls the specific amount of power delivered to transformer 58 by controlling the exact turn-on time of SCRl and SCR2 during any

particular half-cycle of the input voltage. Control unit 56, for example, turns SCRl on at the beginning of a particular positive half-cycle of the input voltage in order to deliver maximum power to transformer 58 and to produce a maximum peak voltage across secondary winding 62 of transformer 58 during that particular half-cycle. Control unit 56, however, turns Scrim on later in a particular positive half-cycle of the input voltage in order to supply less power to transformer 58 and to produce a lower peak voltage across secondary winding 62 of transformer 58 which, in turn, results in less power being delivered to the electrostatic elements 14. Thus, turning SCRl on later during a particular half-cycle of the input voltage results in less power being delivered to transformer 58 which, in turn, results in less power being delivered to electrostatic elements 14. SCRl and SCR2 automatically unlatch or turn off at the end of each half-cycle of the input voltage, i.e., when the input voltage goes through the zero point. In this manner, AVCs 40, in conjunction with T/R sets 42, supply a controlled pulsating DC power to the electrostatic elements 14.
AVCs 40 in conjunction with T/R sets 42 may be operated in a full-cycle energization mode as is commonly known in the prior art. The waveform diagrams shown in Figure 3 illustrate a voltage signal Viper arid a current signal IpcE which represent the voltage and the current at the output of a T/R set 42 when an AVC 40 operates in a full-cycle energization mode, i.e., when electrostatic elements 14 are energized during all the positive half-cycles and the negative half-cycles of the input voltage. Voltage signal and the current signal IpcE»

therefore, represent parameters of power delivered to an electrostatic element 14 of electrostatic precipitator 12. As illustrated in Figure 3, AVC 40 gradually increases the peak voltage supplied to T/R set 42 over a plurality of input voltage half-cycles until the control unit 56 detects a spark within electrostatic precipitator 12. At this time, shown in Figure 3 as a time Tl, voltage signal KPMG drops to a value of zero while current signal Imp. increases until the end of that particular half-cycle and then drops to a value of zero. After a spark occurs in a particular set of electrostatic elements 14, the associated AVC 4 0 does not supply voltage to the associated T/R set 42 for a predetermined period of time. This delay allows the ionized path created by the spark to dissipate and prevents continuous sparking or sustained arcing within the particular set of electrostatic elements 14. At a time T2, AVC 40 begins to supply low amplitude voltage to the associated T/R set 42 and gradually increases the peak of the supplied voltage over a plurality of half-cycles until another spark occurs within the associated set of electrostatic elements 14. Each AVC 40 repeats this cycle so as to energize electrostatic elements 14 in a full-cycle energization mode.
It has been found, however, that the perform¬ance of electrostatic precipitator 12 can be improved and/or power can be saved, when AVCs 40 operate in an intermittent energization mode. During the intermittent energization mode, each AVC 40 provides voltage to associated T/R set 42 during some half-cycles (which may include both positive and negative half-cycles) of the input voltage while skipping other half-cycles. Figure 4 illustrates a

voltage signal Vigo and a current signal Jig which represent the voltage and current appearing at the output of T/R set 42 when the associated AVC 4 0 operates in an intermittent energization mode, having a duty cycle of 33% (i.e., one half-cycle on and two half-cycles off). During the intermittent energization mode, the AVC 40 increases the voltage delivered to associated T/R set 42 until a current or voltage limit is reached or until a spark occurs within electrostatic elements 14. At that time, illustrated in Figure 4 as a time T3, voltage signal Vijay, drops to a value of zero while current signal Jig increases until the end of that particular half-cycle and then drops to a value of zero. After the spark occurs, AVC 4 0 does not provide voltage to T/R set 4 2 for a predetermined period of time to allow the ionized path created by the spark to dissipate. After a predetermined delay, i.e., at a time T4, AVC 4 0 begins,to provide a low amplitude voltage to T/R set 42. AVC 40 may then operate in a full-cycle energization mode for a number of half-cycles of the input voltage in order to increase the amplitude of voltage signal VAT. to a useful level in a short period of time. At a time T5, AVC 40 switches back inn the intermittent energization mode, and once again, gradually increases the amplitude of voltage signal Vigo until another spark occurs within electrostatic elements 14. This cycle is repeated so as to energize electrostatic elements 14 in the intermittent energization mode.
In one embodiment of the present invention, AVCs 4 0 control the voltage delivered to T/R sets 42 in response to various input signals. These input signals may include, for example, a signal indica¬tive of the boiler load developed by boiler load

sensor 34 or a signal indicative of the sulfur flow rate developed by flow rate sensor 33 (Figure 1). Specifically, AVCs 40 may choose an intermittent duty cycle in response to these signals, or other desired control signals, which results in the most efficient operation of electrostatic precipitator 12. Preferably, AVCs 40 may automatically change the intermittent duty cycle during operation of flue gas conditioning system 8.
As illustrated in Figure 4, voltage signal Vjg, produced at transformer outputs 66 and 68, does not drop to a value of zero during the off half-cycles of the intermittent voltage supplied by AVC 40. This is a result of charge being stored in the electrostatic elements 14 during these off half-cycies. (The current signal Jig, however, does drop to a value of zero during the off half-cycles of the intermittent voltage.) It should also be noted that the peaks of the voltage signal, tend to be greater than those developed by the voltage signal Vice during the full-cycle energization mode shown in Figure 3. It is this increase in the peaks of the voltage signal Vjg, in conjunction with zero current flow during the off half-cycles of he input voltage, which increases the performance of electro¬static precipitator 12 during the intermittent energization mode of operation.
The performance of electrostatic precipitator 12 during the intermittent energization mode, however, is not directly correlated to the operating duty cycle of AVC 40, because the peak voltages produced at transformer outputs 66 and 68 during intermittent energization are not linearly related to the duty cycle of AVC 40. Thus, as the' ratio of energized half-cycles to deenergized half-cycles

(the duty cycle) is changed, the performance of electrostatic precipitator 12 changes without a 'irect correlation in the change of average power applied to electrostatic elements 14. For example, witching from the full-cycle energization mode to he intermittent energization mode with a duty cycle f 33% (i.e., one half-cycle on and two half-cycles ff) generally results in the same performance of lectrostatic precipitator 12 but also results in lectrostatic elements 14 absorbing an average power hat is approximately 4 0% of the average power bsorbed by the same electrostatic elements during he full-cycle energization mode.
Thus, the duty cycle used during the intermit-=nt energization mode is not a reliable indication L" the power being dissiparea by the electrostatic lements 14 and a change in the duty cycle by, for sample, 50%, does not necessarily change the i/erage power absorbed by electrostatic elements 14 —■ ■ jf .50%. As -a., result, flow controller 3a cannot rel?’i»-1 a measureiO-f, power developed by AVC 40, such as -’r’ le duty cycle, as an accurate indication of the i’erage power being delivered to electrostatic lements 14, but must, instead, measure the actual 3wer provided to eleritrostatic elements 14, as Lsclosed herein, in order to control the flow of D3 into the flue 9 in a precise manner.
It is, therefore, an important aspect of this -ivention to measure the precise voltage and the jrrent developed by each T/R set 42 (e,g., the altage appearing across and the current flowing irough electrostatic elements 14) to produce an :;curate indication of the power delivered to Lectrostatic elements 14. This power indication 3, preferably, developed from the half-cycle RMS,

peak or average value of current signal Ig during each of the energized half-cycles of the input voltage and from the average value of voltage Vg, the peak value of voltage Vg, the half-cycle RMS value of the voltage Vg or the minimum value of the voltage Vg over one or more energized half-cycles of the input voltage. This power indication could, however, also be developed from the half-cycle RMS, peak or average value of the current flowing through the primary of the transformer 58 and/or from the average, peak, half-cycle RMS or minimum value of the voltage across the primary of the transformer 58 over one or more energized half-cycles of the input voltage. Preferably, if a voltage measurement over a number of predetermined half-cycles is used, the power measuring unit 32 discards any voltage measurements from half-cycles which fall below a predetermined threshold because such measurements tend to occur during the half-cycles of the input voltage immediately following a spark within the electrostatic plates. A power signal, so developed, enables flow controller 3 0 to control flow of SO3 into flue 9 in a precise and accurate manner, regardless of the intermittent energization duty cycle chosen by AVCs 40= Furthermore, it should be noted that the duty cycle of the intermittent power supply can change during operation thereof without effecting the ability of the power measuring unit 32 to produce an accurate power signal, i.e., a signal which accurately indicates the power being delivered to the electrostatic elements.
Figure 5 shows a preferred embodiment of flow controller 3 0 although any other desired flow controller can be used instead. An electrostatic precipitator power signal 100 is developed, for

example, by averaging the outputs of power measuring units 32. Signal 100 is supplied to a process variable input of a proportional-integral-derivative (PID) controller 102. An electrostatic power set point 104 is supplied to a set point input of PID controller 102. PID controller 102 subtracts one of either electrostatic power set point 104 or electrostatic precipitator power signal 100 from the other to develop an error or difference signal. PID controller 102 applies any desired combination of propor¬tional, integral, and derivative control to this error signal to develop an electrostatic precipitator power control quantity for supply to a PID controller 106.
If desired, a comparator 108 tests the electro¬static precipitator power control quantity from PID controller 102 against a high threshold. If the electrostatic precipitator power control quantity is above the high threshold, the electrostatic precipi¬tator power control quantity is then set at a high limit 110. The electrostatic precipitator power control quantity is also compared to a low threshold by a comparator 112. If the electrostatic precipitator plover control quantity is below the low threshold, the electrostatic precipitator power control quantity is set at a low limit 114. Accordingly, the electrostatic precipitator power control quantity, or its high limit 110, or its low limit 114 is supplied to PID controller 106.
The output of the PID controller 102 or the output of the comparator 112, which may, for example, represent a percentage, can be converted into a signal representative of any other desired unit before being input into the PID controller 106.

PID controller 106 provides an SO3 control signal, which is based upon a calculated SO3 concen¬tration quantity, to a connection point 120. In order to calculate the SO3 concentration quantity, flow controller 30 receives a sulfur flow signal 122 from, for example, flow rate sensor 33 (Figure 1). A block 124 applies a proportionality constant K to sulfur flow signal 122.
Furthermore, a boiler load signal 126 is pro¬vided by boiler load sensor 34 (Figure 1), and the boiler load signal may be compared, if desired, by a comparator 128 to a low threshold. If the boiler load signal is below the low threshold, a block 130 initiates a standby condition, at the option of the operator. That is, if boiler 10 is operating at a substantially reduced boiler load, for example below 10% of its rated maximum capacity (i.e., the low threshold), the volume of the flue gas produced by boiler 10 is very low. Consequently, the amount of contaminants is sufficiently low at this volume of flue gas that the injection of SO3 is unnecessary. Thus, in standby, sulfur pump 16 is stopped so that no sulfur is unnecessarily burned by sulfur burner 20 (Figure 1).
On the other hand, if boiler lead signal 126 is not below the low threshold, the boiler load signal 126 is supplied to the block 124 which may comprise a microprocessor or other computing network. The Block 124 performs the following calculation:
where SO3PPM is the SO3 concentration quantity. Sulfur Flow is the sulfur flow signal 122 supplied by flow rate sensor 33, 460 is 460° Rankin which converts the Fahrenheit temperature scale to the

absolute temperature scale, Tag is the temperature in degrees Fahrenheit of the flue gas at injectors 25 at which SO3 conditioning agent is injected, Conver-terEff is the nominal efficiency of SO3 generator 24 (which may typically be 95%), 10’ converts the calculation to parts per million, 387 is the Ideal Gas Constant in cubic feet per pound mole, ACFM is the boiler load (representing the actual cubic feet per minute rate at which flue gas is produced), 60 converts the ACFM rate from cubic feet per minute to cubic feet per hour, 530 is a temperature reference equal to 460° Rankin plus the temperature base for the Ideal Gas Constant, i.e. 70° F, and 32 is the molecular weight of sulfur. Although the tem¬perature of the flue gas may be measured by a sensor (not shown) to produce a signal for Tg, the design output temperature of boiler 10 at full boiler load may instead be used for Tg.
Equation (1) can be rewritten in a simplified form as follows:
where C’’’ is the SO3 concentration quantity in parts
per million, Fg is the sulfur flew signal 122 supplied by flow rate sensor 33, i.e., Sulfur Flow of equation (1), Lg is boiler load signal 126 supplied by boiler load sensor 34, i.e., ACFM of equation (1), and K is the scaling factor applied to the sulfur flow signal 122 and is the collection of all terms on the right-hand side of equation (1) other than Sulfur Flow and ACFM.
Since other types of conditioning agents may be supplied to the flue gas produced by boiler 10, since any type of burner 20 can be used for boi-ler 10, and since it is possible to directly measure the rate at which the conditioning agent is supplied to

the flue gas, equation (2) can be further generalized according to the following equation:
where C’,;’ is the conditioning agent concentration quantity, F’,’ is a flow rate related to the rate at which the conditioning agent is supplied to the flue
gas, LB is boiler load (i.e., related to the rate at
which flue gas is produced), and K’ is a scaling factor appropriate to the sensors which measure F’’
and Lg and to the particular conditioning agent
which is selected for the treatment of the flue gas.
Referring again to Figure 5, the SO3 concentration quantity calculated by block 124 is supplied to a process variable input of PID controller 106. PID controller 106 produces an error signal by subtracting (a) the electrostatic precipitator power control quantity developed by the PID controller 102 from (b) the calculated SO3 concentration quantity supplied by block 124. PID controller 106 applies any desired combination of proportional, integral, and derivative control to this error signal to develop the 5O3 control signal delivered to connection point 12 0.
If desired, a comparator 134 tests the SO3 con¬trol signal against a high threshold. If the SO3 control signal is above the high threshold, the SO3 control signal is then set at a high limit 13 6. The SO3 control signal is also compared to a low thresh¬old by a comparator 138. If the SO3 control signal is below the low threshold, the SO3 control signal is set at a low limit 140. Accordingly, the SO3 control signal, or its high limit 136, or its low limit 140 is supplied to connection point 120.

The output of SO3 connection point 120 is supplied to speed controller 28 in order to control sulfur pump 16. Alternatively, the output of the connection point 120 may be supplied to a speed controller 144 in order to control a second pump 146.
As indicated in Figure 5, the electrostatic precipitator power control quantity developed by PID controller 102 is used as the set point for PID controller 106. As a result, the rate at which SO3 is supplied to the flue gas in flue 9 is controlled in order to achieve a balance between sulfur flow signal 122, boiler load signal 126, and the electro¬static precipitator power consumed by electrostatic precipitator 12 as sensed by power measuring units 32. Thus, if it is assumed, for example, that the power consumed by electrostatic precipitator 12 de¬creases, its efficiency in removing ash from the flue gas in flue 9 decreases. As the power consumed by electrostatic precipitator 12 decreases, elec¬trostatic precipitator power signal 100 decreases. PID controller 102 is arranged so that a decrease in electrostatic precipitator power signal 100 results in an increase in the electrostatic precipirator power control quantity prodnned by pjD controller 102. An increase in the electrostatic precipitator power control quantity results in an increase of the set point of PID controller 106. As the set point of PID controller 106 increases, the SO3 control signal changes in a direction to increase the speed of pump 16 thereby to increase sulfur flow, i.e., the SO3 control signal increases. As a result, SO3 is supplied to the flue gas at a faster rate in order to increase the efficiency of electrostatic precipitator 12 and to consequently increase the

power consumed by electrostatic precipitator 12. As the flow of sulfur increases, sulfur flow signal 122 increases to increase the calculated SO3 concentra¬tion quantity until a balance is achieved between sulfur flow signal 122, boiler load signal 126, and the electrostatic precipitator power consumed by electrostatic precipitator 12 as sensed by power measuring units 32.
Flow controller 30 as disclosed in Figure 5, may be implemented utilizing known analog hardware elements. Alternatively, flow controller 30 may be implemented by a microprocessor and a program which performs the functions of the elements disclosed in Figure 5.
The source of sulfur trioxide is shown in Fig¬ure 1 as comprising a source of sulfur 18, a burner 20 for converting the sulfur to sulfur dioxide in the presence of oxygen (from an air blower 22) and an SO3 generator 24, such as a catalytic converter, for converting the sulfur dioxide into sulfur triox¬ide. However, any suitable source of SO3 can be used; for example, a source of liquid SOj may be provided which can be vaporized and combined with air to be converted by a catalytic converter into sulfur trioxide, SO3; or sulfur trioxide (SOj) can be supplied directly by vaporizing liquid SO3 from a supply thereof. Electrostatic precipitator 12 may be any of the commercially available electrostatic precipitators. Also, an Allen-Bradley, Model 1771 controller may be used as the flow controller shown at 30 in Figure 1, instead of using the Honeywell UDC 9000E controller.
Furthermore, even though specific values for limits, constants, ranges and the like have been shown, it will be appreciated that any other desired

values may be used. These and other modifications will be apparent to one skilled in the art and lie within the present invention, the scope of which is determined only by the following claims.




WE CLAIM:
1. A system for preconditioning flue gas to be treated in an
electrostatic precipitator having a set of electrostatic elements and a power
supply which supplies intermittent power to the electrostatic elements, said
system comprising a source of a conditioning agent which supplies an
amount of the conditioning agent to the flue gas; detecting means for
detecting first and second parameters of the power supplied to flee
electrostatic elements; developing means, responsive to the first and second
parameters, for developing an indication of the power supplied to the
electrostatic elements; and controlling means, responsive to the power
indication, for controlling the amount of conditioning agent added to the flue
gas.
2. The system as claimed in claim 1, wherein the power supply has a
transformer having a primary winding, a secondary winding chilled to the
electrostatic elements and develops an intermittent voltage having energized
and non-energized half-cycles at the primary winding; the detecting means
has first means for measuring the current flowing through the transformer
during an energized half-cycle to develop a current measurement, and
second means for measuring the voltage across transformer during the
energized half-cycle to develop a voltage measurement; and the developing
means develops the power indication faro the current measurement and the
voltage measurement.

3. The system as claimed in claim 2, wherein the developing means has means for disregarding voltage measurements produced by the second measuring means which are below a predetermined threshold.
4. The system as claimed in claim 2, wherein the sirs measuring means has third means for measuring one of the half-cycle RMS current, the peak current or the average current flowing through the transformer.
5. The system as claimed in claim 2, wherein the second measuring means has fourth means for measuring one of the average voltage, the peak voltage, the half-cycle RMS voltage or minimum voltage across the transformer.
6. The system as claimed in claim 2, wherein the first measuring means has third means for measuring the current flowing through the transformer secondary winding during the energized half-cycle; and the second measuring means has fourth means for measuring the voltage across the transformer secondary winding during the energized half-cycle.
7. The system as claimed in claim 6, wherein the third measuring means has further current measuring means for measuring the half-cycle RMS current flowing through the transformer secondary winding during the energized half-cycle; and the fourth measuring means has further voltage measuring means for measuring the average voltage across the transformer secondary winding during the energized half-cycle.

8. The system as claimed in claim 2, wherein the first measuring means has third means for measuring the current flowing through the transformer primary winding and the second measuring means has fourth means for measuring the voltage across the transformer primary winding.
9. The system as claimed in claim 8, wherein the third measuring means has further current measuring means for measuring the half-cycle RMS current flowing through the transformer primary winding during the energized half-cycle; and the fourth measuring means has flirtier voltage measuring means for measuring of the average voltage across the transformer during the energized half-cycle.

10. The system as claimed in claim 1, wherein the detecting means comprises first means for producing a current signal representing the current flowing through the secondary winding of the transformer, second means for producing a voltage signal representing the voltage appearing at file secondary winding of the transformer, and wherein, the developing means has means for combining the current signal and the voltage signal to produce the power indication.
11. The system as claimed in claim 10, wherein the developing means has means for disregarding voltage signals produced by the second producing means which are below a predetermined threshold.

12. The system as claimed in claim 10, wherein the combining means
has a multiplier for multiplying the current signal with the voltage signal to
produce the power indication.
13. A system for preconditioning flue gas substantially as herein
described with reference to the accompanying drawings.


Documents:

620-mas-95 abstract.pdf

620-mas-95 claims.pdf

620-mas-95 correspondences-others.pdf

620-mas-95 correspondences-po.pdf

620-mas-95 description (complete).pdf

620-mas-95 drawings.pdf

620-mas-95 form-1.pdf

620-mas-95 form-26.pdf

620-mas-95 form-4.pdf

620-mas-95 pct.pdf


Patent Number 190286
Indian Patent Application Number 620/MAS/1995
PG Journal Number 31/2010
Publication Date 30-Jul-2010
Grant Date
Date of Filing 24-May-1995
Name of Patentee THE CHEMITHON CORPORATION
Applicant Address 5430 W. MARGINAL WAY, S.W., SEATTLE, WAHINGTON 98106-1598.
Inventors:
# Inventor's Name Inventor's Address
1 WILLIAM G. HANKINS 1610 242ND AVENUE S.W, ISSAGUAH, WASHINGTON 98027
PCT International Classification Number E04F17/02
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 08/254,937 1994-06-07 U.S.A.