CA1057910A - Automatic bed level control for furnaces - Google Patents

Abstract

Abstract An automatic bed level control system is provided for furnaces which burn a material on a bed, such as chemical recovery furnaces and the like. Optical sensing means sense light emitted by the burning of the bed material and provide a signal which varies as a function of the intensity of the sensed light. The sensing means are structured and oriented to respond to light emanating only from a particular, vertically limited zone in the furnace, such that variations in the intensity of the sensed light are indicative of variations in bed level. The signal provided by the sensing means is then used to regulate some bed level controlling parameter, as for instance primary air flow.

Description

- 10579~0 ~ackEround of the Disclosure The invention relstes to furnaces which burn fuel in a bed. More particularly it relates to a method and apparatus for controlling-the depth or level of the bed in a furnace. More particularly still it relates to a method and apparatus for utilizing the light associated with burning on the bed to regulate a variable uhich affects bed depth.
The control of the level or depth of the bed in furnaces such as chemical recovery furnaces, incinerators and the like may be quite important in optimizing the burning function of the furnace. The bed is the accumulation of various fuels and other materials which reside on a floor or Brate in the furnace and is burned, either to reduce its overall solid mass or to effect certain chemical changes or both. A particularly good example of the need to control bed depth is seen in chemical recovery furnaces.
The operation and control of chemical recovery furnaces for the recovery of chemicals from kraft black liquor is a difficult problem from the standpoint of both maximizing the efficiency of the process, and minimizing air pollution and the explosion safety hazards. One factor which makes the operation and control difficult is that the composition (particu-larly solids content) of the black liquor varies from time to time. Another factor is the variety of chemical reactions which take place in the furnace.
Several important changes in the black liquor take place in the chemical recovery furnace, the first of which is the vaporization of the water which has not been previously removed by direct and/or indirect evaporators upstream. The carbonaceous material in the black liquor then burns in a pile on the hearth while the inorganic materials fuse and form smelt in the furnace. Reducing conditions are maintained in the lower hearth region of the furnace while oxidizing conditions are maintained higher up in the furnace. Some of the primary reactions which take place in the chemical recovery furnace are as follows:

-2 1057~10 Na2S04 2C Na2S + 2C02 Na2C03 ~ Na20 (solid) + C02 Na20 (solid) + C ~ Na2 (gas) + C0 -Na2 (gas) + 1/202 ~ Na20 (fume) S + 2 ~ S2 S2 + 1/202 + S03 Na20 + S03 Na2S04 (fume) Na20 + C02 ' Na2C03 (fume) The control of the operating conditions in the lower furnace controls the reduction of the sodium sulfate to the sodium sulfide and conversion of sodium carbonate to sodium oxide and sodium oxide to sodium vapor.
The amount of sodium vapor formed by reduction in turn controls the concentration of sulfur oxides and the sodium sulfate and sodium carbonate fume emitted in flue gas from the furnace. Proper control of the burning, and particularly reducing, conditions is therefore necessary not only to regulate the efficiency of the production of sodium sulfide from sodium sulfate, but also to regulate furnace emissions. Increased steam production is an additional benefit of correct operation.
In general, a high bed burning temperature and a relatively deep char bed in the furnace are desirable to maximize reduction efficiency and to decrease sulfur oxide and particulate (fume) emission. However, furnace operators are often reluctant to operate with a deep char bed because it requires continuous attention and ad~ustment of the primary air dampers.
Another disadvantage of operating with an excessively deep char bed is that the air ports around the periphery of the furnace can become blocked, causing a blackout when an undercut char bed falls over toward a furnace wall. A further problem with deep bed operation is that it is often difficult to determine by mere visuA~ observation the exact height of the bed and, therefore, difficult to tell if the bed is getting too deep. For these reasons, many operators operate tXe furnace under conditions which will maintain a bed level which is excessively low, thereby impairing the -_ reduction efficiency and increasing the carry-over of char in the gas stream which forms low melting slag deposits on heat transfer surfaces downstream.
Operation uith a low bed requires little operator attention, but produces low chemical reduction efficiencies.
It is desirable, therefore, to operate the char bed at a depth ~ -~ufficient to obtain good reduction efficiency, but not so deep that the chance for collapse and blackout occur. This preferred bed level will tend to have an angle of repose of some 40 to 50 uith its base some 8-12 inches below the primary air ports. The angle of repose of an excessively high bed will exceed 50 and its base will be only an inch or two below the primary air port. When the angle of repose of a bed is less than 40 and its base more than 12 inches below the primary air ports, the bed is too low for the most efficient reduction.
Ir. Canadian Patent No. 1,013,904 entitled "Monitor~ng Chemical Reco~ery Furnace" by Hugh W. Nelson, which issued on July 19, 1977, there was disclosed a technique and means for monitoring the sodium vapor produced by the char bed as an indirect indication of the efficiency of the reduction of Na2 S04 to Na2S. An optical device sensitive to the photon energy emitted by - burning sodium and relatively insensitive to other photon energy from the ; 20 burning char bed was used to measure the rate of sodium vapor production.
The application further mentioned positioning the optical device above the char bed to look downward thereon and that a drop in the bed level would produce a change in output reading of the optical device.
Similarly, in other types of furnaces, such as incinerators and the like, which also burn a fuel on a bed to effect a desired chemical and/
or mass change, it is necessary to control the depth of the bed for optimum reduction or oxidation efficiency.
Summa of the Invention ry The present invention involves a technique and means for controlling the level or depth of the bed in a furnace, such as a chemical recovery furnacé. As several variables, such as primary air flow, and/or fuel --4_ . . .

- lOS7910 temperature and fuel flow and/or fuel temperature are effective to control the bed level, the invention provides means for and a method of sensing the intensity of light emanating from the furnace, particularly light only from a zone of limited vertical extent, and using it to provide a control signal which is operative to vary one or more of the bed level controlling variables as a function of the sensed lights' intensity.
Opticai sensing means, preferably having a limited vertical sight angle within the furnace, is effective to control one or more of the bed level controlling variables, as for instance air flow. This may be done by controlling dampers in primary air ducts. The sensing means is preferably positioned to view the bed and furnace interior through the primary air ports.
The sensing means may be responsive to light of one or more wavelengths emanating from the furnace as a result of burning gases from the bed, though selective sensing of light of a single wavelength, such as emitted by excited -sodium atoms in a chemical recovery furnace, may be preferred.
Plural optical sensing means positioned to view the furnace interior at various positions around the bed, each associated with a different regional primary air supply and damper, provide a preferred means for controlling bed level.
For a better understanding of the invention, its operating advantages and the specific ob~ects obtained by its use, reference should be made to the accompanying drawings and description which relate to a preferred embodiment of the invention.
Brief Descri~tion of the Drawings FIGURE 1 is a schematic elevational view of a black liquor chemical recovery furnace incorporating the present invention.
FIGURE 2a is a somewhat diagrammatic elevational view of the furnace with the char bed at a preferred level.
FIGURE 2b is a somewhat diagrammatic elevational view of the furnace showing an excessively high char bed.

-10579~

FIGURE 2c is a somewhat diagrammatic elevational view of thefurnace showing an excesslvely low or shallow char bed.
FIGURE 3 is an enlarged view of a portion of FIGURE 1, schematically depicting the optical sensor, the primary air damper and the control circuitry and actuator connected therebetween.
FI&URE 4 is an elevational sectional view taken along line 4-4 of FIGURE 3 to approximate the view of and through the primary air ports by the optical sensor.
FIGURE 5 is a schematic cross-sectional view taken along ;ine 5-5 of FIGURE 1.
Description of the Preferred Embodiment While the present invention is generally applicable to furnaces of different types which burn a material on a bed, it finds particular utility in use in chemical recovery furnaces. Accordingly, the invention will be described as applied to 8 chemical recovery furnace, but finds applicability as well with other furnaces which maintain beds.
FIGURE 1 illustrates a chemical recovery furnace 10 which is typical of chemical recovery furnacés used for the processing of black liquor.
The walls of this furnace are lined with steam generating tubes 12 which form a part of the heat exchange surface of the chemical recovery unit with there being additional heat exchange surface identified generally at 14 in the upper region of the unit.
Black liquor obtained from the kraft pulping process and/or other sodium based pulping processes which has been processed by evaporation to the desired solids content is introduced into the furnace 10 through the nozzles 16. The liquor thus sprayed into the furnace descends downwardly towards the furnace bottom passing through rising combustion gas such that a ma~ority of the moisture in the liquor is immediately evaporated. The solid particles fall downwardly through this rising combustion gas stream and form a pile or char bed lô on the hearth or furnace bottom 20. A
portion of the combustibles are consumed during this descent through the i057910 furnace with remaining combustibles being consumed in the char bed 18. The noncombustibles, inorganic chemicals, are smelted and decanted from the furnace through the discharge spout 22.
Combustion-supporting air is introduced into the furnace at two locations. The primary air is introduced through nozzles or ports 24 spaced relatively close to the bottom while the secondary air is introduced through the nozzles or ports 26 located above the liquor nozzles 16.
In addition to the reduction of sodium sulfate in the bed according to reaction (1) (1) Na2S04 + 2C ) Na2S + 2C02 the sodium carbonate is thermally decomposed according to the reaction (2) (2) Na2C03 ~ Na20 (solid) + C02.
m e reverse reaction is depressed due to the escape of the C02 and also to the immediate use of Na20 in reaction (3)

(3) Ra20 + C ~ Na2 (vapor) + CO.
Sodium oxide is a thermally stable, relatively non-volatile solid with a boiling point of 2320F. This is well above bed temperatures which are normally about 1500F to 2000F. The elemental sodium produced in reaction (~) by contrast, has a low boiling point of 1618F and thus readily volatilizes from the bed. It is a very reactive substance and quickly burns according to reaction (4) ~ust above the bed giving off 3ubstantial heat and-a bright yellow light,

(4) Na2 (vapor) + 1/202 , Na20 (fume) + ~H + Light.
This yellow light comes from the thermal excitation of sodium atoms in the extremely fine, hot solid particles of ~a20 fume. It has sharp peaks of intensity in the 5890 Angstrom wavelength region of the visible light spectrum. This wavelength is characteri~tic of sodium either in the elemental or compound form. Other gases or vapors may also be evolved, contributing their characteristic spectral emissions to the light emitted on and above the char bed. This light may be both in and/or near the visual range and is typified by, though not limited to, the sodium spectral emissions. This light generally originates at the surface of and ~ust above char bed 18 and i9 represented in the drawings, such as FIGURES 2a, 2b and 2c, by the shimmering "waveform" 27. Light "waveform" 27 indicates the region in which the light originates and the amplitude of its oscillations provide a relative indication of the light intensity thereat, the light generally covering most of the bed and being of greatest intensity near the center and fading away from the center at the cooler side areas.
FIGURES 2a, 2b and 2c respectively, depict char bed 18 having a preferred level or depth, an excessively high level and an excessively low level. As previously described, the preferred bed will have an angle of repose between 40 and 50 and its base will be some 8-12 inches below the primary air ports 24; the excessively high bed of FIGURE 2b will have an angle of repose exceeding 50 and its base will be at or an inch or two below ports 24; and the low bed of FIGURE 2c will have an angle of repose less than 40 and its base will be 12-18 inches or more below ports 24.
The meter 28 for use in the present invention to receive and indicate the intensity of the photon energy emitted by burning gas or gases from bed 18 is an optical device substantially as described in the aforementioned applica-tion. Typically, meter 28 will include a photodetector of some well known type, such as a photoresistor, photodiode, phototransistor, photomultiplier or the like. The photodetector may be of a type respon3ive to a particular portion of the visible light spectrum, such as the portion including the sodium spectral emissions or it may be broader in range to include the full vi~ible spectrum and possibly beyond. Appropriate filters, prisms, diffraction gratings or the like might also be used to restrict the sensed light to a certain portion of the spectrum. Because most of the light in a chemical recovery furnace is in the sodium region of the spectrum, it is desirable that the meter 28 be responsive to such light.
Meter 28 is positioned and oriented such that it views a particular zone within furnace 10. The horizontai or azimuthal extent of the field of view of a meter 28 is not particularly critical, but should include a .
representative portion of char bed 18. At least one meter 28 should be positioned at each of the four sides of furnace lO, and preferably one with each of the primary air ducts 29, as seen in FIGURE 5 and described herein-&fter. The vertical field of view of a meter 28 is, however, somewhat more critical and should be restricted to a zone, indicated by dotted lines 30 in FIGURES 2a, 2b and 2c, which does not include the full vertical range of the light waveform 27 between the hieh bed level of FIGURE 2b and the lou bed level of FIGURE 2c. It is desirable that the positioning of meter 28 be such that changes in the bed level cause maximum changes in the intensity of the light sensed by it. This may be accomplished by positioning and orienting meter 28 such that it "sight3" in a substantially horizontal direction and its vertical angle or extent of view is limited to satisfy the requirement mentioned above.
In the preferred embodiment, the upper side of the field of view 30 i~ such that most of the light waveform 27 for a high bed, as in FIGURE 2b, is excluded from view; more of the light waveform 27 for an intermediate, and in this case, bed level as in FIGURE 2a, is included in the vertical field of view; and still more of the-light waveform 27 for a low bed level, as in FIGURE 2c, is included in the vertical zone or field of view. Stated another way, the field of view of meter 28 is such that an indication of low sensed light intensity is indicative of a high bed level; and indication of high sensed light intensity is indicative of a low bed level, and an indication of an intermediate sensed light intensity indicates a bed level in between. The indication may be calibrated against known or observed bed levels. While other conditions affect light intensity somewhat at any given bed level, the effects are minim~l and the control response to be discussed is in the correct direction.
The meter 28 is preferably mounted on primary air duct 29 such that it is outside observation window 32 and is oriented to "sight" through thé window and the air ports 24 at char bed 18. In this configuration the air ports 24 determine the zone viewed within furnace lO. Similarly, because of the spacing between meter 28 and air ports 24 and the vertical extent of the air ports in a typical furnace 10, the upper extent of the field of view in the furnace is held below most of light waveform 27 in the high bed condition of FIGURE 2b. Air duct 29 and air ports 24 serve as a form of collimator for meter 28, however it will be appreciated that other collimating mesns might be used and that the meter might be located elsewhere with similar limitations on the zone vïewed within the furnace. Light conductors, such as opticsl pipes or fibers, might be used to conduct light from the furnace wall to a remote sensor.
There are several parameters which affect the depth of bed 18, as - for instsnce, primary air flow and/or temperature, liquor flow and/or temper-ature, and/or percent of solids to water in the liquor. In an incinerator, the liquor would be replaced with refuse. While any or all of these parameters ms,v vsry somewhat, either inadvertantly or through human operator action, it 15 is desirable to select one, as for instance primary air flow, which may be sutomaticslly controlled by a signal from meter 28 to compensate for changes in the other parameters and maintain the bed at a desired level.
Primary air flow is a parameter which is largely responsible for controlIing the level of bed 18 and is relatively easy to vary. When the 20 primary sir flow ls increased, the bed level tends to decrease and when the primsry air is decreased, the bed level tends to increase. A damper 34 in each of the primary air ducts 29 i~ operative to control the rate of flow of primary air through its associated air duct. While such control has previously been of a manual nature and in response to occassional viewing of the bed 18 25 through window 32 by the operator, the invention utilizes meter 28 as means for providing a signal which serves to automatically control an actuator connected to damper 34. In this manner, accurate, continuous automatic control of the level of char bed 18 is effected~ -Referring to FIGURE 3, there is a somewhat diagrammatic schematical 30 view of furnace 10, optical meter 28, primary air damper 34, damper actuating means such as cylinder 54 and rod 58, and the control circuitry intermediate --10-- t ~0579~0 the meter and the actuating means. Meter 28 is shown as including a photo-detector represented by variable resistor 38. Typically, the photodetector will exhibit a characteristic, i.e. resistance change, which varies as a function of the intensity of light sensed. The detector might alternately

5 exhibit a voltage or current change. While the photodetector might be part of a bridge circuit, it is shown here singly providing a light intensity signal for simplicity. The signal indicative of the intensity of the sensed light is applied, via conductor 39, as one input to a conventional motor operator control unit 40. Control unit 40 typically includes an 10 operational amplifier and a power switch. A set point signal is developed from potentiometer 42 and i9 applied as another input to control unit 40.
l~e set pcint signal and the light intensity signal wilI typically be of opposite polarity and the magnitude of the set point signal will be set at that value com~lensurate with the magnitude of the signal from rneter 28 15 during optimun or preferred char bed level conditions.
'rhe power switch portion of control unit 40 is operatively connected to the operationPl amplifier and includes a three position, three terminal switch, one terminal being connected by conductor 44 to the solenoid of valve 46 and another terminal being connected by conductor 48 to the solenoid of 20 valve 50. The common of the switch being connected to a source of power.
Both valves 46 and 50 are connected in a pneumatic circuit between air supply 52 and pneumatic cylinder 54. A pressure reducing valve 56 is also included ~ -in the pneumatic circuit. Valves 46 and 50 are normally closed when the switch is in its neutral third position and are opened only when the switch 25 closes the electrical circuit to the particular valve solenoid, only one valve being open at a time.
- Cylinder 54 is pivotably mounted to some support structure at one end and piston rod 58 extends from the other çnd. Piston rod 58 is connected to damper actuating linkage 60 which in turn is connected to damper 34.
30 Linkage 60 may, at least in part, comprise the existing manual control linkage for the damper. Movement of piston rod 58 results in rotation of damper 34, - ~05791U

to proportionally control the air between fully open (maximum air flow) and fully closed (no air flow). Means other than pneumatic cylinder 54, such as a motor or diaphragm, might be used to actuate damper 34.
Position indicating linkage 62 is rigidly connected at one end to piston rod 58 and at the other end to a rotary potentiometer 64. Potentiometer 64 has a voltage applied thereto and its wiper is connected through capacitor 66 by conductor 68 to another input of the operational amplifier of control unit 40. m e signal provided by potentiometer 64 and capacitor 66 is a form of derivative feedback. m e polarity of the voltage on potentiometer 64 and the direction of rotation of the potentiometer relative to the direction of tion of piston rod 58 are selected such that a feedback signal voltage is applied to the operational amplifier input during actuation of cylinder 54.
m is feedback signal is temporarily effective to cancel the difference between the set point signal and the control signal from meter 28, resulting in closure of both valves 46 and 50 and cessation of movement of piston rod 58. m e derivative feedback produces an integrating action for the damper position.
Considering now the operation of the automatic bed level control system with the described positioning of meter 28, when the level of char bed 18 is higher than preferred, as seen in FICURE 2b, the intensity of light from the zone of the bed sensed by meter 28 will be relatively low, causing an increase in resistance of photodetector 38 and a decrease in voltage applied by conductor 39 to control unit 40. This decreased voltage is less in magnitude than the set point and an error signal results, closing the switch to conductor 44 and opening valve 46. Pressure reduction valve 56 provides a lower pressure to the left side of cylinder 54 than the right and so piston rod 58 moves leftward to open damper 34 somewhat. The movement of rod 58 is effective, through linkage 62, to rotate potentiometer 64 a certain OEmount, changing the voltage applied to capacitor 66 and resulting in a dt voltage applied to control unit 40 to momentarily cancel the input error to the operational amplifier, thereby opening the circuit to the solenoid for valve 46 and thus closing the valve. This stops the movement of piston rod 58 and damper 34. However the dVt voltage is removed when movement of rod 58 ~tops, allowing any remaining error to once again open valve 46. This operation is repeated with attendant short steps of damper 34 until the error signal is removed by a change in bed level and sensed brightness or until the damper is full open, the latter ultimately producing the former.
Conversely, when the char bed level is below that preferred, as in FIGURE 2c, the increased intensity of the sensed light so decreases the resistance of photodetector 3O that the voltage applied by conductor 39 to control unit 40 exceed~ the set point signal voltage in the opposite direction, causing an error signal of opposite sense to actuate the power switch and close the circuit with the solenoid of valve 50 to open the valve. This vents the right side of cylinder 54, allowing piston rod 58 to move rightward to close damper 34. Again, but in a reverse sense, potentiometer 64 and capacitor 66 apply a ddt voltage to the control unit 40, such that the closing motion of the damper is accomplished in short steps. Again, this operation ~ -continues until the error signal is removed or the damper is fully closed.
- m e net result of this control during normal operation is to maintain bed 18 at or near the level desired, as dictated by the set point. This can usually be accomplished with but a minimum of displacement of the damper.
While the bed level control system ~ust described was associated with a single primary air duct 29, it will be appreciated that at least one such arrangement would exist at each of the four sides of the furnace 10.
If there is more than one duct 29 at each side of the furnace, one meter 28 per side might be used to control the several dampers 34 for that side.
Alternatively, as seen in FIGURE 5, a separate meter 28 might be associated with each duct 29 and its damper 34. Still further, if regional control of char bed depth is not required, the control signals from the several meters 28 may be averaged and the resultant used to control all of the dampers 34.

579~V
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Further, while the described embodiment of the invention was in use in a chemical recovery boiler, it will be similarly applicable to incinerators and other furnaces having beds for sensing light intensity as an indicator of bed depth and regulating a control variable accordingly.
In rurnaces in which the dominant source of light comes from burning carbon and hydrocarbons on the bed, the photodetector might be similarly oriented to view the bed with a vertically limited angle of view, but might respond to a broader range of optical wavelengths or else that or those characteristic Or burning carbon or hydrocarbons. Further, some other parameter or-- 10 parameters than ~ust primary air might be controlled to regulate bed depth, however the latter is generally preferred.
While the preferred embodiment of the present invention has been described herein, it should be understood that the description is merely illustrative and that variations and modifications can be made therein without departing from the spirit and scope of the invention as recited in the follow-ing claims.
What is claimed is:

.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination, a furnace in which a material is burned on a bed, emitting light, and in which the level of said bed is a function of a control variable, and means for controlling the level of said bed comprising: means for sensing said emitted light from a particular zone of limited vertical extent within said furnace such that the intensity of said sensed light is related to the level of the bed with respect to the point of measurement, and providing a signal which varies as a function of the intensity of said sensed light and is indicative of the level of said bed; and means responsive to said signal for controlling said control variable to control said bed depth.

2, The apparatus of claim 1 wherein primary air is supplied to said furnace for burning said material and comprises said control variable; and said signal responsive means include means for varying the flow of said primary air.

3. The apparatus of claim 2 wherein said primary air flow varying means include a damper and said signal responsive means further include an actuator operatively connected to said damper and responsive to said signal from said light sensor for varying the position of said damper in a direction to maintain said sensed light at a desired value commensurate with a de-sired bed level.

4. The apparatus of claim 3 wherein said primary air is supplied to said furnace through a plurality of air ducts communicating with said furnace, each duct providing air to a particular associated region of said furnace; each said duct includes a separate said damper therein; and a said sensing means is associated with each said air duct and associated damper to control air supply to the said particular associated region of said furnace.

5. The apparatus of claim 4 wherein each said air duct includes an air port at its downstream end and each said associated light sensing means views the interior of said furnace through said air port.

6. The apparatus of claim 5 wherein said furnace is a chemical recovery furnace; said material being burned includes a sodium compound and said light includes a wavelength characteristic of excited sodium atoms; and said sensing means is responsive only to said light wavelength characteristic of excited sodium atoms.

7. The apparatus of claim 2 wherein said furnace is a chemical recovery furnace; said material being burned includes a sodium compound and said light includes a wavelength characteristic of excited sodium atoms; and said sensing means is responsive only to said light wavelength characteristic of excited sodium atoms.

8. In a furnace in which a material is burned on a bed, emitting light, and in which the level of said bed is a function of the flow of primary air supplied thereto, a method for controlling the level of said char bed comprising the steps of: sensing only said light emanating from a particular zone of limited vertical extent such that the intensity of said sensed light varies as a function of the depth of said char bed in said furnace to provide a control signal which varies as a function of the intensity of said sensed light; and varying the flow of said primary air as a function of said control signal.

9. The method of claim 8 wherein said air flow is varied inversely of the intensity of said sensed light.

10. The method of claim 8 wherein said material being burned includes a sodium compound and said light includes a wavelength characteristic of excited sodium atoms and wherein said step of sensing said light includes sensing only that light having a wavelength characteristic of excited sodium atoms, whereby said control signal is indicative only of the intensity of said light wavelength characteristic of excited sodium atoms.