US 3760776 A
A Combustion Control System for a vapor generator in which flue gas is recirculated to the furnace thereof to control reheat temperature and a controlled portion of the recirculated gas is injected into the air supplied the burners to inhibit the formation of nitrogen oxides.
Description (OCR text may contain errors)
United States Patent 1191 Durrant Sept. 25, 1973 S Y S IEM EQB CONTROLLINGWTLLEH V  References Cited INJECTION OF AN INERT GAS INTO THE UNITED STATES PATENTS AIR SUPPLIED A BURNER TO'INHIBIT THE I I 3,335,782 8/1967 DeL1vo1s.. 431/9 FORMATION OF 1 3,146,821 9/1964 Wuetig 431/115 x 75] Inventor: Oliver w Durram Akron, Ohio 2,688,360 9/1954 Haynes et a1. 431/1 15  Assignee: The Babcock & Wilcox Company, primary Examine,. Edward Favms f' York I AttorneyJohn F. Luhrs  Filed: Dec.-l6, 1971 211 Appl. No.: 208,669  ABSTRACT T A Combustion Control System for a vapor generator in which flue gas is recirculated to the furnace thereof to [:2] :LS. Cl. 122/459, 431/115 control reheat temperature and a controlled portion of [5 lit. Cl.f F22g 1/02 the recirculated g is j t into the pp the 8] Field 0 Search 43 l5, 1 l6, 9; burners to the formation f nitrogen id 263/DIG. 2, 19 A; 122/459 10 Claims, 6 Drawing Figures i. J 1 [G9 SECSUL'R RE ET'R P1 5111 56% HE XT ER 8 [8A 11 I1 ElrmlEl 1115/ 1 M HUUUflUUbEUUUEU-UUE PATENTED SEPZSIBN AP- TNCHES OF WATER PER CENT TOTAL GAS RECIRCULATION SHLET 3 OF 3 GAS TO HOPPER so v L /GAS T0 AIR SUPPLY O 5.0 I I 7 I60 PER cam MAX. RATING FIG. 5
PER CENT MAX. RATING FIG. 6
v INVENTOR. OLTVER W. DURRANT Axa/A ATTORNEY A SYSTEM FOR CONTROLLING THE INJECTION OF AN INERT GAS INTO THE AIR SUPPLIED A BURNER TO INHIBIT THE FORMATION OF NO As a result of the reaction of the nitrogen and oxygen contained in the air supplied for combustion of a fossil fuel nitric oxide (NO) is formed at a rate dependent upon; (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and, (3) excess oxygen supply. The rate of formation of NO increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less NO than the same mixture at a lower temperature, but for a longer period. In vapor generators of the type hereunder discussion wherein the combustion of fuel and air may generate flame temperatures in the 'order of 3,700 F the time-temperature relationship governing' the reaction is such that at flame temperatures below 2,900 F. no appreciable NO is produced, whereas above 2,900 F. the rate of reaction increases rapidly. NO is an invisible, relatively harmless gas. However, as it passes through the vapor generator and comes in contact with oxygen it reacts to form N (nitrogen dioxide) or other oxides of nitrogen collectively referred to as NO,,. N0 is a yellow-brown gas which, in sufficient concentrations is toxic to plant and animal life. It is this gas which may create the visible haze at the stack discharge of a vapor generator.
It is apparent from the foregoing discussion that the formation of NO, can be reduced by' reducing flame temperature in any degree and that the formation of NO, will be minimized with a flame temperature at or below 2,900 F. Such a reduction in flame temperature can be accomplished by introducing into the combustion air supply an inert gas.
Further, as more or less conventional in the art with vapor generators of the type hereunder discussion, having'a super heater and reheater, the temperature of the vapor leaving the reheater is usually controlled wholly or in part by introducing recirculated flue gas into the furnace, thus altering the heat-absortpion pattern within the generator. Conveniently, part or all of the recirculated flue gas used for the control of reheat vapor temperature may also be used as a source of inert gas injected into the air supplied for combustion to reduce the flame temperature and consequently to reduce or eliminate the formation of N0 With the foregoing in mind it is a major object of this invention to provide a system for controlling the introduction of recirculated flue gas into the combustion air supply to the furnace of a vapor generator for the purpose of minimizing the formation of NO,,.
A further object is to provide a control system proportioning the recirculated flue gas between the furnace and combustion air supply to both control the temperature of the vapor leaving the reheaterand formation of N0 Still another object is to provide a control system increasing the flow of recirculated gas above that required for the control of the temperature of the vapor leaving the reheater if required for the control of NO,
and thereafter controlling the reheat vapor temperature by supplemental means.
A further object of this invention is to provide a control system which maintains the requiredflow of recirculated gas with minimum power consumption and maximum efficiency.
IN THE DRAWINGS FIG. 1 is a fragmentary, diagrammatic view of a vapor generating and superheating unit from which the basic principles of my invention will be explained.
FIG. 2 is a fragmentary, sectional elevation of a portion of an airflow duct including an air foil.
FIG. 3 is a sectional elevation of the airflow duct taken in the direction of the arrows 3-3 in FIG. 2.
FIG. 4 is alogic diagram of a control system for the vapor generator shown in FIG. 1.
FIGS. 5 and 6 are graphs used in explaining the operation of the control system of FIG. 4.
DETAILED DESCRIPTION Referring to FIG. 1, I therein show in fragmentary and diagrammatic'form of vapor generator comprising a furnace 1 having fluid-cooled walls and a plurality of heating surfaces over which the flue gases pass. Fuel and air are admitted to the furnace 1 through one or more burners such as shown at 2. Fuel to the burners is admitted through a fuel pipe 3 from a common header 4. While I have shown, for convenience, fuel oil burners, it will be apparent as the description proceeeds that the invention is equally applicable to the combustion of any other type of fossil fuel such as gas or coal.
Air is supplied the burners 2 through a plenum chamber, or burner windbox 5 from a forced draft fan 6 to which it is connected by way of a duct 7 and air preheater 8. Disposed in a section of the duct 7 is an airflow measuring section 9 to be described in greater detail hereinafter.
The products of combustion, or flue gas as it is commonly called, after leaving the furnace 1, pass through a series of heating surfaces arranged in a conventional order such as a secondary superheater 10, a reheater 12, a primary superheater l4, and an economizer section 16. It is to be understood that by conventional is meant the order probably most frequently found in the art, but not the exclusive order. The order may also The temperature of the vapor leaving the reheater 12 i is controlled wholly or in part by introducing flue gas into the The temperature of the vapor leaving the reheater 12 is controlled wholly or in part by introducing flue gas into the furnace 1 thus altering the heat absorption pattern within the vapor generator. As shown, flue gas is drawn through a duct 20, connected to the duct 18 by means of a recirculating fan 22 and discharged therefrom into the furnace 1 through a duct 24. The recirculated gas is usually introduced into the furnace 1 in such a manner as to avoid interference with the combustion of the fuel.
The airflow measuring section 9, shown in detail in FIGS. 2 and 3, comprises one or more air foils 28 having a nose section 30 and a trailing recovery section 32. The air foil is hollow and extends from side to side of the duct 7 as shown in FIG. 3. The air foil produces a pressure differential varying in functional relation to the rate of airflow, which pressure differential is sensed at pressure taps P P connected to airflow controller 38 by tubes 34, 36 respectively.
The control signal generated by thecontroller 38 is ordinarily used in a combustion control system of one type or another. Representative of such a system I show in FIG. 4 an elementary and simplified form a combustion control system for a vapor generator such as shown in FIG. 1. In reference to FIG. 4 it should be noted that conventional control logic symbols have been used. The control components or hardware, as sometimes called, which such symbols represent are commercially available and their operation well understood by those familiar with the art. Furthermore I have used conventional logic symbols to avoid identification of the control system with any particular type of control such as pneumatic, hydraulic or electric, or a combination of these, as the invention may be incorporated in any one of these types. The primary controllers shown in FIG. 4 have been referenced into FIG. 1 as have the final control elements.
In the combustion control system shown in FIG. 4 a pressure controller 92 generates a master control signal proportional to the pressure of the vapor leaving the secondary superheater 10, which through a proportional plus integral unit 98 positions in parallel a damper drive unit 96 regulating the air supplied for combustion and a fuel control valve 94 regulating the fuel flow to burners 2 as required to maintain a predetermined vapor pressure.
The signal generated by the controller 38 operates as a feed back signal and acting through a difference unit 102 and proportional plus integral unit 106 establishes a fixed rate of air flow for each value of the control signal generated by the unit 98. A similar feed back signal is generated in a fuel flow controller 100 which through a proportional unit 103, difference unit 101, and proportional plus integral unit 105 acts to establish a fixed rate of fuel flow for each value of the control signal generated in unit 98. The system described is known as a parallel control system in that fuel and air are adjusted in parallel as required to maintain the vapor pressure at a predetermined value. As will be apparent to those skilled in the art various modifications, elaborations, and additions to the elementary control system I have described are usually required to adapt the control to a specific generating unit of the type hereunder discussion. For example, Manual-Automatic Transfer Stations such as shown at 107 and 108 are usually incorporated in the system to facilitate transfer of the control from Automatic to Manual during periods of upset, startup, and the like.
In addition to serving as a primary element generating a differential pressure varying in functional relation to the rate of airflow through duct 7, the air foil 28 also serves as a means for efficiently and economically injecting flue gas into the air supply. As shown the hollow interior of the air foil 28 is connected to a plenum chamber 44 into which the duct 26 discharges. Located in the recovery section 32 are a plurality of ports 40 (collectively forming an orifice) through which the flue gas is discharged into the combustion air supply. By arranging the ports 40 in the recovery section immediately following the plane of maximum velocity, or the plane of the vena contracta, as it is sometimes called,
mixing of the flue gas and air occurs and a homogeneous diluted air mixture is thus available at burners 2. The ports 40 are preferably located close to but following the plane of maximum velocity so that the airflow measurement as indicated by the pressure differential between pressure taps P and P is not adversely affected.
By so locating the ports 40 the flue gas is aspirated into the air stream in proportion to the rate of airflow and hence the proper ratio once having been established (usually to produce a diluted mixture comprised of approximately 10 20 percent flue gas) is thereafter to some extent self regulating. While I have suggested a diluted air mixture comprised of 10 20 percent flue or other inert gas, it is apparent that this percentage may be varied as required to depress the flame temperature sufficiently to inhibit or reduce the formation of NO, to an acceptable level.
While I have shown an air foil as a preferred form of primary element, my invention may be utilized with any other form of primary element such as a venturi, flow nozzle, or orifice. The technology for measuring flow with such devices is well established. With each such device the flow velocity is increased by means of a restriction in the flow path establishing a reduced pressure which is at a maximum at the plane of maximum velocity. Regardless of the type of primary element used in accordance with my invention, the ports 40 are preferably located just downstream of the downstream pressure tap P where a condition of minimum static pressure exists without interfering with the flow measuring device.
The control system shown in FIG. 4 operates to control the flow of recirculated gas to maintain both a desired reheat temperature and flame temperature. This requirement which the control system must meet will be appreciated by reference to FIG. 5. Typically, as shown by curve A the total gas recirculation required to maintain a desired reheat temperature varies inversely in non-linear relationship with generator rating. While the exact relationship varies from one type of generator to another, curve A may be taken as a generalization of the inverse, non-linear relationship between gas recirculation required and generator rating to maintain a desired or predetermined reheat temperature.
As air required for combustion varies in substantially straight line relation to generator rating it is evident, as shown by curve B, that the flow of inert gas injected into the air supply will also vary in straight line relation with generator rating. The slope of curve B will vary depending upon the composition of the air-gas mixture required to obtain a desired flame temperature.
Also as shown in FIG. 5, the recirculated gas required to maintain a desired air-gas mixture may, at high ratings, exceed that required for reheat temperature control. If such is the case the flow of recirculated gas is preferably adjusted as required to maintain the desired air-gas mixture and desired reheat temperature is maintained by supplemental means such as a spray attemperator.
The system for controlling the flow of recirculated gas to maintain a desired reheat temperature, as shown in FIG. 4, utilizes the signal generated by the controller 38 as a feed forward signal in that it is proportional to the rate of air supply for combustion and hence is essentially proportional to generator rating. This signal signal generated a deviceSO measuring the current or power consumed by fan 22 is compared to the feed forward signal in a difference unit 53. The current or power consumed by the fan is an index of the gas weight flow discharged by the fan. A similar control arrangement could be applied using any other index of gas flow, such' as the pressure drop across an orifice or venturi. The error signal generated in unit 53 is then applied to a proportional plus integral unit 55 and damper-drive unit 48 is positioned until the error signal is reduced to zero. e
A signal proportional to the temperature, of the vapor leaving the reheater 12 is generated in a temperature controller 52 and compared in a difference unit 110 with a signal corresponding to the desired reheat temperature as established by the adjustable set point signal generator 109. The error signal generated in unit 1 is then applied to a proportional plus integral unit 54 which generates a signal'biasing the feed forward signal in a summing unit'5l, so that during steady state conditions a correction to the feed forward signal from controller 38 is made tomaintain the reheat temperature at the desired value.-
As required, .various signal modifying and shaping components may be incorporated in the control. Thus, for example, the signal generated by the controller 38 may be biased upward or downward in unit' 56 and .scaled and inverted, if necessary, in a proportional unit 57. Hand-automatic Selector Station 60 may be incorporated in the control immediately preceding the damper-drive unit 48 whereby the control may be transferred to a manually generated signal duringperirange of the vapor. generator a desired fixed proportion between the rate of airflow and flue gas will be maintained.
erated in series sequence, the damper operated by unit 68 remaining in an open position while the damper operated by unit 72 is positioned from a closed position to an open position to satisfy the requirements for flue gas injection as generator rating increases, and thereafter upon further increases in generator rating the damper operated by unit 68 is positioned in closing direction to satisfy the requirements for flue gas injectotal flow of recirculated gas is established regardless A differential pressure controller 61 generates a control signal which is fed into a difference unit 62 into which is also fed a control signal generated by the flow controller 38 as scaled in a proportional unit 64. The pressure taps for the differential controller 61 are preferably located so as not to be affected significally by varying velocity. To this end, as shown in FIG. 1, one pressure tap to the controller 61 is made into the burner windbox 5 where the velocity heads that exist at the location of the airflow measuring section 9 have been converted into static pressure and there is little or no effect from air velocity. The other pressure tap to the controller 61 is made into the duct 26 in an area of relatively low gas velocity with thus limited effect on static pressure.
The output signal (error signal) from difference unit 62 is applied to a proportional plus integral unit 66 and the output signal therefrom positions a damper-drive unit'68 located in duct 24 following the takeoff of recirculated gasinto duct 26 and a damper-drive unit 72 located in duct 26. The units 68 and 72 are preferably opof changes in system resistance occasioned by positioning of the dampers operated by damper-drive units 68 and 72. The operation of the control so far described maintains a substantially constant differential pressure of predetermined value as'established by adjustment of the proportional plus integral unit 66.
As shown in FIG. 6, the flow of recirculated gas into the air supply may, in certain instances, increase in proportion to generator rating, not withstanding the differential pressure is maintained constant, by virtue of the aspirating effect heretofor described. As an example, FIG. 6 shows that a substantially constant differential pressure of zero value maintained a diluted air mixture comprised of approximately 10 percent flue gas. Further, as shown by FIG. 6, the differential'pressure to maintain any other dilute air mixture is in substantially straight line relation to generator rating, the differential pressure decreasing with rating for low dilutions and increasing with rating for high dilutions. It should be understood that the curves shown in FIG. 6 have been arbitrarily drawn to illustrate the fact that the difintroducing-into the difference unit 62 the control signal generated by controller 38 after modification'by a proportional unit 64. Thus the control operates to maintain a differential pressure as established by the output signal from proportional unit 64. By adjustment of proportional unit 64 the differential pressure maintained may be made to fit any one of a family of curves such as shown in FIG. 6.
A feed forward signal from controller 38 is introduced into the control for damper-drive units 68 and 72 at a summing unit 70 after being modified in proportional unit 63. Thus during transient conditions an approximately correct adjustment is made to the injection of recirculated gas which is followed, during steady state conditions, by a precise correction initiated by the differential pressure controller 61.
Under exceptionally high loads, as previously mentioned, and as illustrated by curve A of FIG. 5, the rate .of flow of recirculated gas necessary to maintain desired reheat temperature may not be. sufficient to satisfy the requirements for gas injection into the air supply. When such a condition exists precedence is given to the requirement for gas injection and reheat temperature is maintained by supplementary means such as the spray attemperator 121 to which cooling fluid is supplied from any suitable source (not shown). The relative requirements of recirculated gas for reheat temperature control and gas injection are determined by comparing the values of the signals out of summing units 70 and 51 in a high signal selector 78. The signal from summing unit 70 is correlated with that from summing unit 51 by bias unit 74 and proportional unit 58. Normally the control signal from summing unit 51 will be greater than that from proportional unit 58 so that the total flow of recirculated gas is adjusted as required to maintain desired reheat temperature, upon this being exceeded by the requirement for flue gas injection the control signal from porportional unit 58 will be the greater and the total flow of flue gas will be adjusted as required for gas injection.
During such periods when the flow of recirculated gas exceeds that required for reheat control, the output signal from proportional plus integral unit 54 will be of such value as to operate spray attemperator 121 as required to maintain the desired reheat temperature. As shown the output signal from unit 54 may be modified, if required, by a bias unit 120.
Hand-Automatic Stations as shown at 80 and 82 may be provided to transfer control of damper-drive units 68 and 72 respectively to manual control and vice versa. Also shown in FIG. 4 are transfer stations 84 and 86 which under emergency conditions transfer positioning of damper-drive units 68 and 72 to preset positions in accordance with signal generated in signal generators 88 and 90.
In the foregoing description mention has been made of various signal modifying units such as proportional and bias units primarily to indicate that such components as required to adapt the control to any specific application may be provided as will be understood by those familiar with the art. As further examples of such components there is shown at 104 an adjustable proportional unit whereby the signal generated in controller 38 may be properly correlated with the output signal from proportional unit 103 so that air to the burners 2 will be maintained in desired ratio to fuel flow. A I-land'Automatic Selector Station 99 may be provided for simultaneously transfering fuel and air to hand control and vice versa.
It will be apparent that the control arrangement shown is by way of example only and that various modifications can be made within the scope of the invention as defined in the appended claims.
1. In a combustion control system for a vapor generator having a furnace to which fuel and air supplied through a burner and wherein flue gas discharged from the vapor generator is injected into the air supplied the burner to depress the flame temperature and the formation of NO,, in combination, means establishing a first control signal corresponding to the rate of flow of flue gas into the air supply and means under the control of said first control signal maintaining a constant rate of flow of flue gas into the air supply.
2. A control system in accordance with claim 1 including means measuring the rate of air flow to the burner, means responsive to said last named means establishing a second control signal corresponding to the rate of air flow to the burner and means establishing the constant rate of flow maintained by said first control signal responsive to said second control signal whereby the rate of flow of flue gas injected into the air supply is maintained in functional relationship with the rate of air flow to the burner.
3. A control system in accordance with claim 2 wherein the vapor generator is provided with a first duct and fan means in said duct for recirculating flue gas discharged from the vapor generator into the furnace to alter the heat absorption pattern within the vapor generator, a second duct connected to said first duct on the discharge side of said fan for injecting flue gas into the air supply, and wherein the means under the control of said first signal is a first damper-drive unit located in said second duct.
4. A control system in accordance with claim 3 wherein the vapor generator is provided with a third duct and a fan connected to said duct for supplying air to the burners and wherein the means for measuring rate of air flow comprises an air foil located in said duct and means for measuring the differential pressure produced by the air foil.
5. A control system in accordance with claim 4 wherein the air foil extends from side to side of the air duct, the flue gas is injected into the air supply from the interior of the air foil through a plurality of ports located in the recovery section of the air foil and said means for measuring the rate of flue gas injection into the air supply comprises means for measuring the differential across said ports.
6. A control system according to claim 5 wherein the vapor generator is provided with a reheater and a second damper-drive unit is located in said first duct ahead of said fan means, means measuring the temperature of the vapor leaving the reheater, and means responsive to said measuring means generating a third control signal corresponding to the temperature of the vapor controlling the positioning of said second damper-drive unit as required to maintain the reheated vapor at a predetermined temperature.
7. A control system according to claim 6 further comprising means measuring the rate of flow of recirculated gas, means responsive to said last named means generating a feed back signal corresponding to the rate of flow of recirculated gas, a difference unit responsive to said third control signal and said feed back signal producing an error signal proportional to the difference between said third signal and said feed back signal, and a proportional plus integral unit responsive to said error signal generating a fourth control signal controlling the positioning of said second damper-drive unit.
8. A control system according to claim 7 further comprising a high signal selector responsive to said first and third control signals transferring the control of said second damper-drive unit from said third to said first control signal whenever said first control signal is greater than said third control signal.
9. A control system according to claim 3 further including a third damper-drive unit in said first duct between said furnace and the connection to said second duct, and wherein said first control signal first operates said first damper-drive unit to an open position as the requirement for flue gas injection into the air supply increases and thereafter operates said third damper-drive unit to a closed position upon further increases in the requirement for flue gas injection.
10. A control system according to claim 8 further including supplemental means for regulating reheat temperature responsive to said third control signal when said first control signal is greater than said third control signal.