US3049300A

US3049300A - Combustion control for a furnace fired with fuels having different oxygenexcess air characteristics

Info

Publication number: US3049300A
Application number: US20719A
Authority: US
Inventors: Henry P Lewis; Joseph F Triolo
Original assignee: Bailey Meter Co
Current assignee: Elsag Bailey Inc
Priority date: 1960-04-07
Filing date: 1960-04-07
Publication date: 1962-08-14
Anticipated expiration: 1979-08-14

Description

Aug. 14, 1962 COMBUSTION CONTROL FOR A FURNACE FIRED WITH FUELS HAVING DIFFERENT OXYGEN-EXCESS Filed April 7, 1960 OXYGEN IN PRODUCTS OF COMBUSTION-PER CENT BY VOLUME H. P. LEWIS ETAL AIR CHARACTERISTICS 2 Sheets-Sheet l OIL Ay COAL S COKE EXCESS AIR PER CENT FIG. I

INVENTORS HENRY P. LEWIS AND BY JOSEPH F. TRIOLO- FLUECAS Aug. 14, 1962 H. P. LEWIS ETAL 3,049,300

COMBUSTION CONTROL FOR A FURNACE FIRED WITH A FUELS HAVING DIFFERENT OXYGEN-EXCESS AIR CHARACTERISTICS Filed April '7, 1960 2 Sheets-Sheet 2 STEAM GENERATOR INVENTORS K AND HENRY P. LEWIS JOSEPH F. TRIOLO 52K 1 xfvzw ATTORNEY United States Patent Office 3,049,300 Patented Aug. 14, 1962 3,049,300 COMBUS'IJIGN CUNTROL FDR A FURNACE FERED WITH FUELS HAVING DHFFERENT OXYGEN- EXCES AIR CHARACTERISTICS Henry E. Lewis, Cleveland, Ohio, and Joseph F. Trioio, Sharon Hill, Pa, assignors to Bailey Meter Company, a corporation of Delaware Filed Apr. 7, 1960, Ser. No. 20,719 12 Claims. (Cl. 23615) Our invention relates to combustion control and more particularly to combustion control for a furnace supplied with blast furnace gas, or other gas containing a large percentage of inerts, as one fuel and a supplementary fuel or fuels such as coke oven gas, natural gas, manufactured gas, oil, coal or coke.

A primary object of our invention is to maintain the excess air in such a furnace at a desired or predetermined value regardless of changes in the ratio between the rate of flow of blast furnace gas and the other fuel or fuels.

In accordance with our invention a predetermined ratio is maintained between the total fuel flow and air flow, the predetermined ratio is adjusted to maintain a desired oxygen content in the products of combustion and the desired oxygen content is determined from the ratio between the rate of fiow of blast furnace gas and the total fuel flow.

Further, in accordance with our invention there is provided a constant flow control of the total fuel and a similar control for the air, the set point of the latter being adjusted to maintain the desired heat release and that of the former being adjusted in accordance with the rate of air flow and readjusted to maintain a desired oxygen content in the products of combustion as determined from the ratio of blast furnace gas to total fuel.

Further, in accordance with our invention, the total fuel requirement is, if desired, obtained from blast furnace gas and the supplementary fuel or fuels supplied only as required to make up for a deficiency or lack of availability of the blast furnace gas.

In the drawings:

FIG. 1 is a graph illustrating the relationship between excess air and oxygen for various commercial fuels.

FIG. 2 is a schematic illustration of an embodiment of our invention.

To realize optimum combustion efficiency air and fuel should be supplied a furnace or other combustion zone in relative proportions to maintain a desired, or as it may be termed, predetermined excess air. By definition excess air is the amount of air supplied in excess of that theoretically required for perfect combustion. It is ordinarily expressed as the ratio between the air actually supplied to that theoretically required. Thus excess air means that 20% more air was supplied than theoretically required. Usually the optimum excess air will be the minimum excess air at which a furnace or other combustion zone can be operated with complete combustion of the fuel. Reducing the excess air below this point results in an unburned fuel loss, while increasing the excess air above this point results in an unnecessary heat loss in the products of combustion which are wasted to the atmosphere. These losses are usually spoken of as avoidable losses as they are eliminated by operating at the optimum excess air.

It is well known in the art to maintain a desired excess air by maintaining a predetermined ratio between fuel flow and air flow and readjusting this ratio as required to maintain a predetermined oxygen content in the products of combustion, there being a constant relationship between the percent oxygen and excess air for most commercially available fuels. The control from oxygen is introduced as a readjustment of the primary fuel-air ratio control for the reason that it is a control having a relatively long time constant and subject to a transportation lag whereas the fuel-air ratio control is relatively fast acting so that a change in fuel flow can be made to immediately reflect a corresponding change in air flow or vice versa. Such ratio control cannot be relied upon however to give precise control of excess air as the air required per unit of fuel varies as the composition of the fuel changes.

That readjustment of the fuel-air ratio to maintain a predetermined oxygen content in the products of combustion will result in a constant excess air will be apparent from an inspection of FIG. 1 which shows the relationship between oxygen and excess air from most commercially available fuels. Thus, for example, readjusting to four percent by volume oxygen will maintain approximately 23% excess air regardless of whether natural gas, coke oven gas, oil, coal or coke is fired singly or in combination.

Quite frequently a furnace or other type of combustion zone is fired with multiple fuels and particularly in the iron and steel industry one of the fuels may be blast furnace gas and the other fuel or fuels may be coke oven gas, natural gas, manufactured gas, oil, coal or coke. Blast furnace gas as the name implies is a by-product of blast furnace operation. It is a low grade fuel having a low calorific value, which must ordinarily be utilized in the mill where produced or wasted. It is therefore desirable to satisfy the requirements of a furance with this fuel entirely or to the extent that it is available using a higher grade and more costly fuel or fuels only as required to supplement the blast furnace gas in order to satisfy the heat requirements.

As shown in FIG. 1 there is a material difference in the relationship between oxygen content in the products of combustion and excess air for blast furnace gas than for all other commercial fuels. This is due principally to the fact that blast furnace gas includes a large percentage of inerts in the form of free nitrogen. Accordingly it is apparent that where blast furnace gas is used in combination with other fuels readjustment to a predeter-mined oxygen content will not result in a desired excess air. Thus as previously stated for all fuels other than blast furnace gas readjustment to four percent 0xygen will maintain approximately 23% excess air. With blast furnace gas alone readjustment to 4% oxygen content will maintain approximately 48% excess air, and with blast furnace gas fired in combination with another fuel or fuels a readjustment to 4% oxygen will maintain an excess air anywhere between these limits depending upon the amount of blast furnace gas fired relative to the other fuel or fuels. Our invention is particularly directed to a method and apparatus whereby a desired or predetermined excess air may be maintained notvw'thstanding that blast furnace gas or other gas producing a discrete oxygen-excess air relationship is fired with one or more supplementary fuels in varying ratio. I

In FIG. 2 we show a combustion control system embodying our invention applied to a steam generator shown schematically at 1. The furnace or combustion zone of the generator is supplied with blast furnace gas, a supplementary fuel such as oil and air through conduits 2, 3 and 4 respectively. The steam generated is discharged through a pipe 5, and the products of combustion, commonly called flue gas are discharged through a duct 6 to a stack (not shown) and wasted to the atmosphere. In the term products of combustion or line gas we include, as is common in the art, not only the carbon dioxide, sulphur dioxide and water vapor formed in the usual combustion process but the nitrogen and free oxygen as well which does not enter into the combustion process.

The combustion control we have shown in FIG. 2 for the steam generator 1 is of the type commonly known as, pneumatically operated for the reason that compressed air is used as the operating medium. However, it will be apparent as the description proceeds that our invention may as readily be incorporated in an electric or hydraulic control. We have chosen to show a pneumatically operated control for the reason that the components making up the system are well known in the art and their operation readily understood.

Flow transmitters

7, 8 and 9 establish pneumatic loading pressures proportional to rate of flow of blast furnace gas, supplementary fuel and air respectively. We have shown the fiow transmitters schematically as these may be any one of the several types available. The loading pressure established by transmitter 7, proportional to the rate of flow of blast furnace gas, is introduced into the A chamber of a totalizing relay 1t) and the loading pressure established by transmitter 8 proportional to the rate of flow of the supplementary fuel is introduced into the C chamber of this relay. The relay 10 serves to produce an output pressure at D proportional to the sum of the loading pressures established by transmitters 7 and 3. Thus the output pressure at D is proportional to the total fuel fiow. The relay 1% may. for example, be of the type illustrated and described in United States Patent 2,805,678 issued to Michael Panich on Sept. 10, 1957.

The loading pressure established by transmitter 9, proportional to the rate of flow of air to the combustion zone, is introduced into the A chamber of a relay 11 similar to the relay 10 but having a bellows 12 for automatic adjustment of the proportional band, illustrated and described more particularly in United States Patent 2,743,710 issued to Jack F. Shannon on May 1, 1956, and the function of which will be discussed more in detail hereinafter. The relay 11 serves to produce an output pressure at D proportional to the loading pressure introduced at A or in other words proportional to the rate of flow of air to the combustion zone. This loading pressure establishes the fuel demand and the actual total fuel flow is maintained equal to this demand by comparing the output loading pressure of relay 11 with the output loading pressure of relay 10 in a relay 13, having proportional plus automatic reset action, and having the output pressure thereof adjust total fuel flow by controlling the operation of final control elements shown as a control drive 14 for blast furnace gas and a control valve 15 for fuel oil. This adjustment is continuous and serves to maintain total fuel flow in desired proportion to air flow. Essentially the system so far described provides a constant flow control of total fuel, the set point of which is established in accordance with the existing rate of air flow.

We prefer to establish fuel demand from actual air flow rather than establishing air demand from actual fuel flow as a matter of safety and efliciency as it insures that actual fuel flow will never exceed the available air. A control arranged to establish air demand from fuel flow will operate satisfactorily under normal conditions, however, the available air supply may be limited by fan capacity, malfunctioning of the equipment and the like which may result in actual fuel flow exceeding the available air, causing a deficiency of air in the combustion zone which not only lowers efliciency but may lead to a hazardous condition at the instant when an excess of air is again available in the combustion zone.

The blast furnace gas and supplementary fuel are preferably totalized on an, air required basis rather than on a straight weight or volume basis. Thus, 0.6857 pound of air are required for perfect combustion of a pound of typical blast furnace gas; 15.9086 pounds are required for perfect combustion of a pound of typical natural gas and 14.031 pounds are required for perfect combustion of a pound of typical oil. Various means are available for having the output pressure of relay 10 represent total fuel flow on an air required basis. Thus the maximum capacity of

transmitters

7 and 8 may be selected so that a pound change in the loading pressures therefrom represent equal changes in air required; or the proportional band setting of relay 10 may be adjusted so that changes in loading pressure at A and C produce changes in output pressure at D proportional to changes in air required for the respective fuels.

As previously mentioned all of the heat requirements should be satisfied from blast furnace gas if available using the supplementary fuel only as a stand-by fuel to take over in the event of a failure in the supply of blast furnace gas or to assist in satisfying the heat requirements if sufficient blast furnace gas is not available. To accomplish this we show a stacked differential relay 16 into the A chamber of which the output pressure of relay 13 is introduced and which it will be recalled is the fuel demand signal. Normally this loading pressure, that is, with zero or some predetermined pressure in chamber B is reproduced in chamber D and transmitted to the A chamber of a relay 17 of the proportional plus automatic reset type. The loading pressure produced by transmitter 7 proportional to the flow of blast furnace gas is introduced into the B chamber of this relay. The output pressure of relay 17 at D is transmitted to and controls operation of control drive 14. Relay 17 thus compares the fuel demand signal with a signal proportional to the flow of blast furnace gas and effects adjustment of the flow of blast furnace gas until it is equal to the fuel demand signal or in other words until the entire fuel requirement is satisfied from blast furnace gas.

The loading pressure from relay 13, that is the fuel demand signal, is also introduced into the A chamber of a stacked differential relay 18 and compared with the loading pressure introduced into chamber B proportional to blast furnace gas flow. So long as these two loading pressures are equal, indicating that the fuel demand is being met by blast furnace gas, relay 18 is adjusted to produce in chamber D a loading pressure maintaining valve 15 closed or at some preselected minimum position. If the loading pressure introduced into chamber B of relay 18, indicative of actual blast furnace gas flow, is less than that introduced into chamber A of this relay indicating that the actual blast furnace gas flow is insufficient to meet fuel demand a change in the loading pressure produced in chamber D results which effects an opening of valve 15 to increase the supplementary fuel flow until the total actual fuel flow is equal to fuel demand.

The availability of blast furnace gas may be determined by one or more of several different ways. "It may for example be established by usage of the gas in more critical areas, rate of production, gas pressure, and for illustrative purposes we have chosen the latter as being representative. We show a pressure transmitter 19 arranged to establish a loading pressure proportional to blast furnace gas pressure and which is introduced into the A chamber of a relay 20 of the proportional plus automatic reset type. So long as the pressure of the blast furnace gas is at or above a predetermined value the output pressure at D of relay 20 will remain at zero or some fixed value. This output pressure is introduced into the B chamber of relay 16 and ordinarily produces no effect on the pressure established in the D chamber of this relay. However, upon the blast furnace gas pressure decreasing below the predetermined value the output pressure at D of relay 20 will increase causing a proportionate decrease in the pressure in chamber D of relay 16 effecting a cutback in the flow of blast furnace gas. Under such a condition the control will function to increase the how of supplementary fuel to compensate for the cutback in blast furnace gas flow.

In our control, as previously described, the fuel demand is established by the rate of air flow. This latter How we adjust to maintain a desired heat release. r'Ihe particular index used to determine deviation of actual heat release from that desired depends upon the type of furnace. Thusif the control is applied to a heat treating furnace, temperature of the furnace or of the work therein may be used as the index, in another type of furnace it may be the flow of product therefrom, with a steam generator such as we have illustrated in FIG. 1 it is ordinarily the pressure of the steam generated; and we therefore show a pressure transmitter 21 arranged to establish a loading pressure proportional to steam pressure and which is introduced into the B chamber of a relay 22 having proportional plus reset action. So long as the pressure of the steam remains at the desired value the output pressure established in chamber D of relay 22 will remain at the then existing value. A decrease in steam pressure below the predeter mined value will immediately cause a proportionate increase in the loading pressure at D and thereafter a continuing increase until the pressure of the steam is restored to the desired value. An increase in steam pressure above the desired value causes a similar action except that the changes in the loading pressure established at D of relay 22 are in opposite sense. The loading pressure established at D of relay 22 is transmitted to a control drive 23 and serves to adjust the rate of air flow through conduit 4. Such adjustments cause corresponding changes in the rate of total fuel supply as heretofore described. It is apparent therefore that the rate of flow of fuel and air to the combustion zone of steam generator 1 will be adjusted as required to maintain the heat release therein equal to the desired or predetermined release.

We have explained that a fuel-air ratio control will not maintain a desired excess air in the combustion zone for the reason that the pounds of air required per pound of fuel change with changes in the composition of the fuel. Thus while we have given the pounds of air required for perfect combustion of a pound of typical blast furnace gas, natural gas and oil these fuels in the forms commercially available vary from time to time in composition. Hence a fuel-air ratio control may at times produce an excess air greater than that desired and at other times an excess air less than that desired or even a deficiency of air. We therefore readjust the primary fuel-air ratio control from a measure of the oxygen content in the products of combustion or flue gas and to eifect this show diagrammatically in FIG. 2 an oxygen analyzer 24 arranged to establish a loading pressure which is introduced into the B chamber of a proportional plus reset relay 25. The loading pressure introduced into the A chamber of this relay establishes the set point that is, the oxygen content in the products of combustion which the control will maintain. The output pressure generated in chamber D of relay 25 is transmitted to the bellows 12 of relay 11 and serves to adjust the proportionality constant between the input pressure to this relay and the output pressure therefrom. Thus, the fuel demand established by a given rate of air flow is adjusted depending upon the oxygen content, or in other words upon the excess air in the products of combustion. It will be noted that the loading pressure established by relay 25 adjusts the proportionality constant of relay 11 rather than merely adding or subtracting from the loading pressure proportional to the rate of air flow. This we prefer for the ma,

son that at low rates of air flow a given change in the loading pressure established by relay 25 should cause, for stable control operation, a proportionately smaller change in the rate of fuel flow than the same change in loading pressure established by relay 25 should generate at high rates of air flow.

The set poin of relay 25 is adjusted in accordance with the ratio between the rate of flow of blast furnace gas and total fuel flow to the combustion chamber. In the embodiment of our invention shown in FIG. 2. we determine the ratio between blast furnace gas flow and total fuel flow in a relay 26 into the B chamber of which the loading pressure proportional to blast furnace gas is introduced. Into the C chamber of this relay the loading pressure proportional to total fuel flow is introduced. The relay 26 is provided with an integral bellows 27 and is of the same type as the relay 11. As the loading pressure introduced into the bellows 27 is that generated in the output chamber D thereof the relay 26 operates to maintain this loading pressure proportional to the ratio between the loading pressures introduced at B and at C. The output loading pressure of relay 26 is modified in a calibrating relay 28 by setting of the proportional band therein so that the output pressure in chamber D thereof varies in desired proportion to changes in the output pressure at chamber D of relay 26. The output pressure in chamber D of relay 28 is transmitted to the A chamber of relay 25 and serves to adjust the set point of this relay as the proportion between blast furnace gas flow and total fuel flow varies. By proper setting of the proportional band adjustment of calibrating relay 28 the set point of relay 25 is automatically varied as required to maintain an oxygen content in the products of combustion resulting in the maintenance of the desired excess air. 'I hus referring to FIG. 1 if 20% is the desired excess air the set point of relay 25 will be automatically adjusted to maintain approximately 2% oxygen in the products of combustion when burning 100% blast furnace gas and approximately 3.8% oxygen in the products of combustion when burning a supplementary fuel entirely. In between these extremes the set point will be adjusted as required to maintain an excess air of 20% regardless of the ratio between blast furnace gas and total fuel.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of controlling combustion in a combustion zone supplied with multiple fuels and air, the combustion of one of said fuels with varying amounts of air producing a substantially different oxygen-excess air relationship than produced by any of the other fuels, which includes, adjusting the relationship between total fuel flow and air flow to maintain a desired oxygen content in the products of combustion and adjusting the desired oxygen content from the ratio between the rate of flow of said one fuel and the total rate of fuel flow to the furnace in a direction tending to maintain a desired excess air in the products of combustion regardless of the elalttive rates of supply of said one fuel and the other ue s.

2. The method of controlling combustion in a combustron zone supplied with blast furnace gas fuel and a supplementary fuel, the combustion of said blast furnace gas with varying amounts of air producing a different oxygenexcess air relationship than said supplementary fuel, which includes, adjusting the relationship between total fuel flow and air flow to maintain a desired oxygen content in the products of combustion and adjusting the desired oxygen content from the ratio between the rate of flow of blast furnace gas and the supplementary fuel in a direction tending to maintain a desired excess air in the products of combustion regardless of the relative rates of supply of blast furnace gas and supplementary fuel.

3. The method of controlling combustion in a combustion zone supplied with at least two fuels and air, the combustion of one of said fuels with varying amounts of air the other of said fuel or fuels, which includes, maintaining a predetermined ratio between total fuel flow and air flow to the combustion zone, adjusting the predetermined ratio to maintain a desired oxygen content in the products of combustion; and adjusting the desired oxygen content from the rate of flow of one fuel relative to the rate of flow of another fuel to the combustion zone in a direction to maintain a desired excess air in the products of combustion regardless of the relative rates of supply of said one fuel and said other fuel or fuels.

4. The method of controlling combustion in a combustion zone supplied with at least two fuels and air, the combustion of one of said fuels with varying amounts of air producing a different oxygen-excess air relationship than the other fuel or fuels which includes, maintaining a predetermined ratio between total fuel flow and air flow to the combustion zone, adjusting the predetermined ratio to maintain a desired oxygen content in the products of combustion; and adjusting the desired oxygen content in part from the rate of flow of one fuel relative to the total rate of fuel flow to the combustion zone in a direction tending to maintain a desired excess air in the products of combustion regardless of the relative rates of supply of said one fuel and the other fuel or fuels.

5. The method of controlling combustion in a combustion zone supplied with blast furnace gas, a supple mentary fuel and air, the combustion of said blast furnace gas with varying amounts of air producing a different oxygen-excess air relationship than said supplementary fuel, which includes, producing a first control effect proportional to the rate of air flow to the combustion zone, producing a second control effect varying in predetermined ratio with variations in said first effect, producing a third control effect proportional to the total of the rate of flow of blast furnace gas and supplementary fuel to the combustion zone, adjusting the rate of flow of blast furnace gas and the supplementary fuel to maintain a predetermined relation between the second and third control effects, producing a fourth control effect in accordance with the oxygen content in the products of combustion, adjusting said predetermined ratio to maintain the fourth control effect at a desired value, producing a fifth control effect proportional to the ratio between the flow of blast furnace gas and the sum of the rate of flow of blast furnace gas and supplementary fuel; and adjusting the fourth control effect in accordance with the fifth control effect in a direction tending to maintain a desired excess air in the products of combustion regardless of the relative rates of supply of blast furnacc gas and supplementary fuel.

6. The method of controlling combustion in a furnace supplied with multiple fuels and air, the combustion of one of said fuels with varying amounts of air producing a different oxygen-excess air relationship than the other fuels, which includes, maintaining a predetermined ratio between total fuel flow and air flow, adjusting the predetermined ratio to maintain a desired oxygen content in the products of combustion; and adjusting the desired oxygen content from the rate of flow of one fuel relative to the total fuel flow in a direction tending to maintain a substantially constant excess air in the products of combustion regardless of the relative rates of supply of said one fuel relative to the other fuels.

7. The method of controlling combustion in a combustion zone supplied with blast furnace gas, at least one other fuel and air, the combustion of said blast furnace gas with varying amounts of air producing a different oxygen-excess air relationship than the other of said fuel or fuels, which includes, maintaining a predetermined ratio between the rate of air flow and the total of the rate of fiow of blast furnace gas and the other fuels, adjusting the predetermined ratio to maintain a desired oxygen content in the products of combustion and adjusting the desired oxygen content in part from the rate of flow of blast furnace gas relative to the rate of flow of oneof the other fuels in a direction tending to maintain a desired excess air in the products of combustion.

8. The method of controlling combustion in a combustion zone supplied with blast furnace gas, at least one other fuel and air, the combustion of said blast furnace gas with varying amounts of air producing a different oxygen-excess air relationship than the other of said fuel or fuels, which includes, maintaining a predetermined ratio between the rate of air flow and the total of the rate of flow of blast furnace gas and the other fuels, adjusting the predetermined ratio to maintain a desired oxygen content in the products of combustion; and adjusting the desired oxygen content in part from the rate of flow of blast furnace gas relative to the rate of total fuel flow to the combustion zone in a direction tending to maintain a desired excess air in the products of combustion.

9. The method of controlling combustion in a combustion zone supplied with blast furnace gas, a supplementary fuel and air, the combustion of said blast furnace gas with varying amounts of air producing a substantially different oxygen-excess air relationship than the supplementary fuel, which includes, adjusting the rate of flow of air to the combustion zone of maintain a desired heat release, adjusting the flow of blast furnace gas and supplementary fuel to the combustion zone to maintain a predetermined ratio between the total of the fuel fiows and air flow, adjusting said predetermined ratio to maintain a desired oxygen content in the products of combustion and adjusting the desired oxygen content from'the rate of flow of blast furnace gas relative to the total rate of fuel flow to the combustion zone in a direction tending to maintain a desired excess air in the prodnets of combustion.

10. In a combustion control system, a furnace, separate means for supplying blast furnace gas, a supplementary fuel and air to the furnace, the combustion of said blast furnace gas with varying amounts of air producing a substantially different oxygen-excess air relationship than the supplementary fuel, separate means for regulating the flow of blast furnace gas, the supplementary fuel and air to the furnace, means for adjusting the air regulating means to maintain a desired heat release in the furnace, means for maintaining the total of the flows of blast furnace gas and supplementary fuel constant at a predetermined value, means for adjusting said predetermined value in accordance with the rate of air flow to the furnace, means for readjusting said predetermined value to maintain a desired oxygen content in the products of combustion, and means for establishing the desired oxygen content responsive to the ratio between the rate of flow of blast furnace gas and the total of the rate of flow of blast furnace gas and supplemental fuel in a direction tending to maintain a desired excess air in the products of combustion regardless of the relative rfatels of supply of blast furnace gas and supplementary 11. In a combustion control system, a furnace, separate means for supplying blast furnace gas fuel, a supplementary fuel and air to the furnace, the combustion of said blast furnace gas fuel with varying amounts of air producing a substantially different oxygen-excess air relationship than the supplementary fuel, constant flow a control of the total fuel flow to the furnace having an adjustable set point, means for adjusting said set point in accordance with the rate of air flow to the furnace, means for readjusting said set point to maintain a desired oxygen content in the products of combustion, means for determining the ratio between the rate of flow of blast furnace gas and the total fuel flow to the furnace, and means responsive to said last named means for establishing the desired oxygen content in the fiue gas to maintain a desired excess air in the products of combustion regardless of the relative rates of supply of blast furnace gas fuel and supplementary fuel.

12. In a combustion control system for a furnace supplied With multiple fuels and air, the combustion of one of said fuels with varying amounts of air producing a ditferent oxygen-excess air relationship than the other of said fuels in combination, means for controlling the relative rates of fuel and air flow to maintain a desired oxygen content in the products of combustion, and means for adjusting the desired oxygen content responsive to 10 the ratio between the rate of flow of one fuel and the total fuel flow to the furnace to maintain a desired excess air in the products of combustion regardless of the relative rates of supply of said one fuel and the other fuels.

References Cited in the file of this patent UNITED STATES PATENTS 2,052,375 Wunsch Aug. 25, 1936 2,134,745 Ziebolz Nov. 1, 1938 2,143,820 Payn Jan. 10, 1939