US3723047A

US3723047A - Control network for burning fuel oil and gases with reduced excess air

Info

Publication number: US3723047A
Application number: US00146669A
Authority: US
Inventors: De Livois G Baudelet
Original assignee: Controle Bailey SA
Current assignee: Controle Bailey SA
Priority date: 1970-05-26
Filing date: 1971-05-25
Publication date: 1973-03-27
Anticipated expiration: 1990-03-27
Also published as: FR2093025A5; CA984488A

Abstract

Control networks for combustion processes, utilizing liquid and gaseous fuels burned in either common or separate burners, which minimize heat losses through the flue by providing sufficient combustion air to allow substantially complete burning of both fuels at reduced excess air conditions. Unburned solids and gases sensed in the flue provide a signal causing the excess air to be automatically varied to maintain these unburned substances within acceptable limits. The excess air also may be automatically varied by a coordinated fuel flow to air flow signal which is corrected by the amount of unburned substances in the flue.

Description

United States Patent Baudelet de Livois 51 Mar. 27, 1973 De Livois ..431/76 X Sullivan ..431/76 [57] ABSTRACT Control networks for combustion processes, utilizing liquid and gaseous fuels burned in either common or separate burners, which minimize heat losses through the flue by providing sufficient combustion air to allow substantially complete burning of both fuels at reduced excess air conditions. Unburned solids and gases sensed in the flue provide a signal causing the excess air to be automatically varied to maintain these unburned substances within acceptable limits. The excess air also may be automatically varied by a coordinated fuel flow to air flow signal which is corrected by the amount of unburned substances in the flue.

l54| CONTROL NETWORK FOR BURNING FUEL OIL AND GASES WITH REDUCED EXCESS AIR {75] lnventor: Guy M. Baudelet de Livois, Paris,

France [73] Assignee: Controle Bailey (Societe Anonyme),

Clamart, France [22] Filed: May 25, 1971 [21] Appl. No.: 146,669

[30] Foreign Application Priority Data May 26, 1970 France ..7019186 [52] US. Cl. ..431/76, 236/15 E [51] Int. Cl ..F23n 5/08 [58] Field of Search ..431/76; 235/15 E [56] References Cited UNITED STATES PATENTS 3,216,661 11/1965 Sawyer ..431/76 X STEAM GASEOUS FUEL LlQUlD FUEL IN VEN'IOR.

ATTORNEY PATENWUHARZYIUYS SHEET 10F 2 EXCESS AIR ()UY M HAUDH F I do I IVOIS FIG. 3 p

CONTROL NETWORK FOR BURNING FUEL OIL AND GASES WITH REDUCED EXCESS AIR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention generally relates to improved control networks for operating a combustion process under reduced excess air conditions and particularly to a combustion process utilizing liquid and gaseous fuel simultaneously burned together in either separate or common burners with minimum heat loss from the flue of the combustion process.

2. Description of the Prior Art Heretofore single fuel combustion processes have been operated under reduced excess air conditions to achieve optimum operating efficiency and minimize corrosion problems As may be seen from an inspection of FIG. 2,. heat losses through the flue of the combustion process are a result of sensible heat loss due to excess air, indicated by curve X, and unburned substances, indicated by curve Y. It will be noted that heat losses from unburned substances appear only when excess air is reduced beyond a point A and these losses increase very rapidly as the excess air is even further reduced.

Optimum operating efficiency occurs at a point of minimum total heat losses from the flue. This point is located in the relatively narrow range of excess air values between points B and A. Since heat losses from the flue are excessive when excess air is reduced beyond the point B value, the operating excess air point is generally set at some value A around which a slight deviation will not result in excessive heat losses from the flue. This point-is beyond the value of excess air where unburned substancesare formed.

Control systems operating around this point maintain air and fuel flow rates in some constant ratio or vary air flow in relation to the amount of oxygen measured in the flue of the system. I Operating at increased excess air increases the possibility of corroding the low-temperature heat exchanger surface. In the presence of excess oxygen, the sulphur in the fuel causes sulphur trioxide to form. In the presence of condensation on the cold heat exchanger surface, sulphuric acid forms which corrodes the heat exchanger. When the controlling factor is to minimize corrosion, excess air is reduced to a bare minimum which will allow substantially all the fuel to burn. An example of such a point of reduced excess air is denoted by the point B of FIG. 2.

Control networks operating around this point use a sensor for detecting the unburned substances in the flue of the system.

Where the fuel used is an industrial heating oil which normally contains sulphur, the detecting sensor may be an opacimeter which detects the opacity of the flue fumes when blackened by the solid unburned hydrocarbons from the fuel.-

These known control networks either aim to minimize the heat loss from the flue of the system or neutralize the corrosive effect to the fumes of the combustion process. Neither is able to produce a desirable compromise since each system is exclusive of the other.

Likewise, when a combination of fuels is used in the combustion process, such as fuel oil and fuel gas, the

previously known control networks are inadequate.

If the excess air is maintained at an optimum ratio relative to the fuel oil it may be completely unsatisfactory for the fuel gas and vice versa. An excess air value mutually satisfactory to both fuels must be maintained for minimum heat loss from the flue of the system.

An opacimeter may maintain the excess air level at a point where substantially all the fuel oil is burned but it would not detect to what extend the fuel gas was burned. Similarly an oxygen measurement of the flue would only indicate the extent to which the fuel gas was burned without regard for the fuel oil.

Mixed combustion of fuel oil and fuel gas is further complicated by the fact that both fuels may be burned in common or separate burners.

When common burners are used to burn both fuels, one excess air level must be provided which will allow substantially complete burning of both fuel oil and fuel gas. This type of control requires a control network utilizing OR logic.

SUMMARY OF THE INVENTION In accordance with the present invention, a control network for burning fuels with reduced excess air is provided comprising: a furnace, a servo-mechanism controlling the excess air for combustion, a first and second means for sensing the unburned solids and unburned gases exhausting from the flue of the furnace, a first and second controller means associated with the first and second sensing means and a selector circuit which transmits the higher controller means signal to the servo-mechanism which then provides sufficient excess air to burn the fuel.

Further in accordance with the invention the excess air may be controlled by a control network wherein the selector circuit means transmits the higher signal of the first and second sensing means to a controller means which directly controls the servo-mechanism means to provide sufficient excess air to burn the fuel.

Further in accordance with the invention the control network may utilize a third means for sensing and signalling fuel flow, a fourth means for sensing and signaling air flow and a summing circuit means for algebraically adding the third and fourth sensing and signaling means to the output of the controller means to produce a signal for controlling the servomechanism means.

Further in accordance with the invention a control network for burning liquid and gaseous fuel under reduced excess air conditions comprises: a furnace with liquid and gaseous fuel inlets and associated controlled air inlets, servo-mechanism means controlling the air flowing in these associated air inlets, first, second and third means signaling the unburned residue, combustion air flow and fuel flow, and summing circuit means for algebraically adding the first, second and third signaling means signals to control the servomechanism means.

This invention solves the problem of choosing an operating point value of excess air either for minimum heat loss through the flue of minimum corrosion of the heat exchanger by constantly providing a minimum excess air value which will satisfy both conditions. This is accomplished by the selector circuit means transmitting the higher signal from the first and second .sensing means.

This invention provides that when liquid and gaseous.

fuel is burned in a common burner the first and second sensing means in combination with the selector circuit means produce a control signal which assures the common excess air value point to be a minimum which will burn both fuels.

This invention also provides when a fuel flow and excess air flow are compared for proper burning of fuel, that the comparison take into account the signals of the first and second sensing means to provide a correction signal which maintains the excess air flow at a level for both conditions of minimum heat loss through the flue and minimum corrosion of the heat exchanger.

This invention also provides that when both liquid and gaseous fuels are burned together in a common burner, the sum of the two fuel flows is compared to the excess air flow and this comparison is corrected by the first and second sensing means to assure that minimum excess air is provided for complete combustion of both fuels.

This invention also provides that when a liquid and a gaseous fuel if burned in separate burners with separately controlled excess air inlets, that each fuel is provided a minimum excess air flow to burn that fuel.

The principal object of the invention, therefore, is to provide a control network which will allow a combustion process utilizing both liquid and gaseous fuel to burn both fuels, in separate or common burners, with minimum heat loss through the flue.

Another object of the invention is to provide a control network which will allow a combustion process utilizing one fuel to burn that fuel with minimum heat loss through the flue and minimum corrosion of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic general view of a boiler utilizing one embodiment of the control network of the invention.

FIG. 2 is a graph showing heat losses through the flue as affected by the combustion excess air.

' embodiment of the invention for burning liquid and gaseous fuels in separateburners.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings wherein the showings are for purposes of explaining the preferred embodiments and not for the purpose of limiting the invention thereto, FIG. 1 is a boiler 10 having a heat exchanger 12 heated by a flame 32. Water inside the heat exchanger 12 is converted to steam which then is exhausted through a steam outlet 14. Combustion products of the flame 32 are exhausted through a flue 34. Liquid and gaseous fuel to heat the boiler is pro-.

vided by a liquid fuel inlet 16 and a gaseous fuel inlet 18 leading to a common burner assembly 1). Combustion air is provided to the burner assembly 19 by a windbox 24 having a blower 26 supplying air flow and a damper 29 controlling the air flow. The damper 29 is operated by a servo-mechanism 28 which is controlled by the control circuit C. Liquid and gas fuel flows along with the combustion air flow are monitored by

flow meter assemblies

20, 22, 30 respectively which are well known to those familiar with the art.

The control circuit C comprises an opacimeter assembly 36, having a light source 38 which transmits a beam of light through-the flue 34 where it is detected by a detector 40, a carbon monoxide gas analyzer assembly 44 and a hydrogen gas analyzer assembly 46. Each

assembly

36, 44, 46 transmits a signal along associated

signal lines

42a, 42c, 42b respectively to

individual controllers

48a, 48c, 48b. Each controller has an associated

reference input line

50a, 50c, 50b for providing a reference signal to each controller 48a,

48c, 48b against which the input signal is compared. The controllers may use any combination of proportional, integral and derivative control modes as is well known to those familiar with the art. The output signals from controllers 48 are inputed to a selector circuit 54 along controller associated output lines 52a, 520, 5211. The selector circuit 54 compares the outputs of the controllers 48 and transmits the highest signal along an output line 56 to operate the servo-mechanism 28 accordingly.

In operation, the combustion products exhausted from the flue will result in sensible heat losses depending upon the amount of excess air provided forcombustion, as indicated by curve X of FIG. 2. If the excess air is restricted beyond point A, heat losses from unburned fuel will also appear, as indicated by curve Y.

If the liquid fuel is not burned completely, solid unburned hydrocarbons blacken the flue combustion products and cut down on the light transmitted by the opacimeter assembly 36 through the flue 34. If the gaseous fuel is not burned completely, the carbon monoxide content of the flue combustion products rises. Hydrogen content in the flue may also be used as an optional means of indicating the reducing-gas content of the flue. Thus the incomplete burning of the gaseous fuel may be sensed by the

gas analyzer assemblies

44, 46 either individually or together.

The controller references 50 are preset to levels where complete burning of the fuel sensed is indicated. The levels sensed by the opacimeter and

gas analyzer assemblies

36, 44, 46 are compared thereto and any variation between these signals results in an error signal being transmitted along the output lines 52 to the selector circuit 54. The highest error signal is transmitted by the selector circuit to the servo-mechanism 28 which adjusts the excess air flow. Since the excess air is adjusted according to the highest error signal, sufficient excess air is assured to burn both the liquid and gaseous fuel.

Referring now to FIG. 3, the opacimeter and

gas analyzer assemblies

36, 46, 44 are modified to transmit identical output signals, under conditions of complete fuel combustion with minimum excess air, directly to the selector circuit 54 through

signal lines

42a, 42b and 420 respectively. This is accomplished by amplifying each

assembly

36, 46, 44 signal i i,,, i, by a factor u u u respectively to form the following equation:

u i u i u i i where i is the amplified signal for the optimum excess air condition. Should any of the sensed conditions move away from optimum the new output signal is still amplified by the factor u but the signal is not them equal to i. The selector circuit 54 transmits the highest signal received along the output line 56 to servomechanism controller 58 which compares this signal to a common reference signal i received through a reference input line 60. The controller 58 compares the signal received to the common reference signal and transmits any variation in the form of an error signal to the servo-mechanism 28 through an output line 62. The controller may utilize any of the proportional, integral and derivative control modes well known to those familiar with the art. The gas analyzer assembly 44 and its output line 42c are shown in dotted lines to indicate its optional nature for the functioning of the circuit network.

Referring now to FIG. 4 the control network described with reference to FIG. 3 is combined with a summing circuit 64, a fuel flow meter assembly 66, an amplifier 76 and the air flow meter assembly 30 to burn a single fuel under reduced excess air conditions to minimize both heat loss through the flue and corrosion of the heat exchanger.

This is accomplished by the opacimeter assembly 36 sensing the minimum corrosion point of excess air and the hydrogen gas analyzer assembly 46 along with the optional analyzer assembly 44 sensing the minimum heat loss point. The selector circuit 54 along with the controller 58 then provides a corrective signal to the summing circuit 64. The summing circuit 64 algebraically adds the fuel flow signal, transmitted from the flow meter assembly 66 along a signal line 70, to the air flow signal, transmitted from the flow meter assembly 30 along a signal line 72, and corrects this summation by adding the signal corrective signal from the controller 58. The summing circuit 64 then produces a signal from this summation to control the servo- This is accomplished by the liquid and gas fuel

flow meter assemblies

20, 22 transmitting their signals through

signal lines

80, 82, respectively, to the summing circuit 64. The opacimeter assembly 36 senses the amount of the unburned solids to indicate the combustion of the liquid fuel. The analyzer assembly 46 along with the optional analyzer assembly 44 senses the amount of unburned gas fuel. These analyzer signals are transmitted to the selector circuit 54 and from it to the controller 58 as described with reference to FIG. 4. The controller 58 transmits its signal to the summing circuit 64 where it is algebraically added to the liquid fuel flow, gas fuel flow and air flow signals to provide a control signal for adjusting the servomechanism 28. This control signal is amplified by the amplifier 76 and transmitted directly to the servomechanism to adjust it accordingly.

Referring now to FIG. 6 liquid fuel and gas fuel is burned in separate burners and the air flow rate is controlled by two separate networks.

The control network for the liquid fuel includes a summing circuit 96 which algebraically adds the liquid fuel flow, sensed by the flow meter 20, and the corrected air flow sensed by a flow meter 84. The summing circuit 96 transmits a control signal through an output line to an amplifier 104 which amplifies the control signal and transmits it, through an output line 108, to a servo-mechanism 112. The servo-mechanism 112 adjust the liquid fuel excess air flow accordingly.

The air flow signal is corrected before entering the summing circuit 96 by a summing circuit 92. The air flow signal is transmitted to the summing circuit 92 through a signal line 88. The circuit 92 also receives a signal from the opacimeter controller 48a through the output line 52a. These two signals are algebraically added and transmitted to the summing circuit 96 through an output line 93.

The control network for the gas fuel is analogous to the liquid fuel network and includes a summing circuit 98 which algebraically adds the gas fuel flow, sensed by the flow meter 22, and the corrected air flow, sensed by a flow meter 86. The summing circuit 98 transmits a control signal through an output line 102 to an amplifier 106 which amplifies the control signal and transmits it, through an output line 110, to a servo-mechanism 114. The servo-mechanism 114 adjusts the gas fuel excess air flow accordingly.

The air flow signal is corrected by a summing circuit 94. The air flow signal is transmitted to the summing circuit 94 through a signal line 90. The circuit 94 also receives 5 signal from the gas analyzer controller 58. These two signals are algebraically added and transmitted to the summing circuit 98 through an output line 95. This second control network operates independently of the first.

It will be noted that the summing

circuits

92, 94 could be replaced with multiplier circuits without imparing the function of the embodiment of the invention.

Various modifications will become obvious to persons skilled in the art upon reading this specification. It is intended that such various modifications also be incorporated.

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

l. A control network for simultaneous burning of liquid and gaseous fuels in a furnace under reduced excess air conditions comprising:

servo-mechanism means for controlling the combustion air supplied to the combustion air inlet of said furnace;

first means for sensing the amount of unburned solids exhausting from the flue of said furnace that characterize liquid fuel combustion quality; second means for sensing the amount of unburned gases exhausting from the flue of said furnace that characterize gaseous fuel combustion quality; first and second controller means having reference signals respectively reflecting complete burning of the liquid and the gaseous fuels associated with said first and second sensing means respectively for producing a signal output which is a function of the difference between said sensing means signal and said controller means reference signal; and,

selector circuit means for transmitting the higher signal output of either said first or second controller means to said servo-mechanism means to vary the combustion air according to the higher signal to assure burning of the liquid and gaseous fuels. 2. A control network as set forth in claim 1 wherein said first sensing means includes an opacimeter for producing a signal which is a function ofthe capacity of the substances exhausting from the flue of said furnace blackened by unburned hydrocarbons of the fuel burned.

3. A control network as set forth in claim 2 wherein said second sensing means includes a gas analyzer for producing a signal which is a function of the reducinggas content of the exhausting substances from said flue including unburned gas from the fuel burned.

4. A control network as set forth in claim 3 wherein said gas analyzer includes a carbon-monoxide analyzer and a hydrogen analyzer.

5. A control network as set forth in claim 4 wherein the fuel burned includes a fuel oil anda fuel gas burned simultaneously.

6. A control network for simultaneous burning of liquid and gaseous fuels in a furnace under reduced excess air conditions comprising;

first means for sensing and signaling the amount of unburned solids exhausting from the flue of said furnace that characterize liquid fuel combustion quality;

second means for sensing and signaling the amount of unburned gases exhausting from the flue of said furnace that characterize gaseous fuel combustion quality;

selector circuit means for monitoring and transmitting the higher signal sensed by said first and second sensing means; and,

controller means having a reference signal that reflects a condition of complete fuels combustion with minimum excess air for producing a control signal to regulate said servo-mechanism means, said control signal being a function of the difference between the transmitted signal of said selector circuit means and the reference signal of said controller means.

7. A control network as set forth in claim 6 wherein said first and second sensing means are scaled to produce identical output signals under proper combustion conditions;

said controller means reference signal is set to be identical to the output signals of said first and second sensing means under proper combustion conditions.

8. A control network for simultaneous burning of liquid and gaseous fuels in a furnace under reduced excess air conditions comprising in order to minimize heat loss and heat exchanger corrosion:

servo-mechanism means for controlling the combustion air supplied to the combustion air inlet of said furnace; first means for signaling the amount of unburned solids exhausting from the flue of said furnace that characterize liquid fuel combustion quality;

second means for signaling the amount of unburned gases exhausting from said flue that characterize gaseous fuel combustion quality;

selector circuit means for transmitting the higher signal of said first and second signaling means; controller means having a reference signal that reflects a condition of complete fuels combustion with minimum excess air and heat exchanger corrosion for producing an output signal which is a function of the difference between the transmitted signal of said selector circuit means and the reference signal ofsaid controller means; third means for signaling the amount of at least one of the fuels flowing in the fuel inlet of said furnace; fourth means for signaling the amount of air flowing in the combustion air inlet of said furnace; and,

summing circuit means for algebraically adding said third signaling means signal to the output signal of said controller means and said fourth signaling means signal to produce an output signal forcontrolling said servo-mechanism.

9. A control network as set forth in claim 8 including an amplifying means for the output signal of said summing circuit means.

10. A control network as set forth in claim 9 wherein said summing circuit means adds said third signaling means signal to the output signal of said controller means and subtracts said fourth signaling means signal therefrom.

11. A control network as set forth in claim 10 wherein said third signaling means includes a gas flow signaling means and an oil flow signaling means.

12. A control network for burning liquid fuel and gaseous fuel in the same furnace under reduced excess air conditions comprising:

servo-mechanism means for controlling the amount of combustion air flowing in the liquid and gaseous fuel associated air inlets of said furnace; first means for signaling the amount of unburned residue exiting from the flue of said furnace; second means for signaling the amount of combustion air flowing in the air inlets of said furnace;

third means for signaling the amount of fuel flowing in the fuel inlets of said furnace;

summing circuit means for algebraically adding the signals from said first, second and third signaling means to produce output signals suitable for controlling said servo-mechanism means;

said first signaling means includes an opacimeter signaling the amount of unburned solids exiting from the flue of said furnace that characterize liquid fuel combustion and a gas analyzer signaling the amount of unburned gases exiting from the flue of said furnace that characterize gaseous fuel combustion;

said second signaling means includes a first and second air flow meter signaling the rate of air flow to the fuel associated air inlets of said furnace;

said third signaling means includes a liquid fuel flow meter and a gaseous fuel flow meter signaling the rate of flow of the respective fuels;

said servo-mechanism means includes a first servomechanism controlling the air flow in the liquid fuel associated air inlet and a second servomechanism controlling the air flow in the gaseous fuel associated air inlet of said furnace; and, v

said summing circuit means includes a first summing circuit adding the signals from the liquid fuel flow meter, the opacimeter and the first air flow meter to control the first servomechanism and adjust the liquid fuel excess air flow, and a second summing circuit adding the signals from the gaseous fuel flow meter, the gas analyzer and the second air flow meter to control the second servo-mechanism and adjust the gas fuel excess air flow thereby.

Claims

1. A control network for simultaneous burning of liquid and gaseous fuels in a furnace under reduced excess air conditions comprising: servo-mechanism means for controlling the combustion air supplied to the combustion air inlet of said furnace; first means for sensing the amount of unburned solids exhausting from the flue of said furnace that characterize liquid fuel combustion quality; second means for sensing the amount of unburned gases exhausting from the flue of said furnace that characterize gaseous fuel combustion quality; first and second controller means having reference signals respectively reflecting complete burning of the liquid and the gaseous fuels associated with said first and second sensing means respectively for producing a signal output which is a function of the difference between said sensing means signal and said controller means reference signal; and, selector circuit means for transmitting the higher signal output of either said first or second controller means to said servomechanism means to vary the combustion air according to the higher signal to assure burning of the liquid and gaseous fuels.

2. A control network as set forth in claim 1 wherein said first sensing means includes an opacimeter for producing a signal which is a function of the capacity of the substances exhausting from the flue of said furnace blackened by unburned hydrocarbons of the fuel burned.

3. A control network as set forth in claim 2 wherein said second sensing means includes a gas analyzer for producing a signal which is a function of the reducing-gas content of the exhausting substances from said flue including unburned gas from the fuel burned.

5. A control network as set forth in claim 4 wherein the fuel burned includes a fuel oil and a fuel gas burned simultaneously.

6. A control network for simultaneous burning of liquid and gaseous fuels in a furnace under reduced excess air conditions comprising: servo-mechanism means for controlling the combustion air supplied to the combustion air inlet of said furnace; first means for sensing and signaling the amount of unburned solids exhausting from the flue of said furnace that characterize liquid fuel combustion quality; second means for sensing and signaling the amount of unburned gases exhausting from the flue of said furnace that characterize gaseous fuel combustion quality; selector circuit means for monitoring and transmitting the higher signal sensed by said first and second sensing means; and, controller means having a reference signal that reflects a condition of complete fuels combustion with minimum excess air for producing a control signal to regulate said servo-mechanism means, said control signal being a function of the difference between the transmitted signal of said selector circuit means and the reference signal of said controller means.

7. A control network as set forth in claim 6 wherein said first and second sensing means are scaled to produce identical output signals under proper combustion conditions; said controller means reference signal is set to be identical to the output signals of said first and second sensing means under proper combustion conditions.

8. A control network for simultaneoUs burning of liquid and gaseous fuels in a furnace under reduced excess air conditions comprising in order to minimize heat loss and heat exchanger corrosion: servo-mechanism means for controlling the combustion air supplied to the combustion air inlet of said furnace; first means for signaling the amount of unburned solids exhausting from the flue of said furnace that characterize liquid fuel combustion quality; second means for signaling the amount of unburned gases exhausting from said flue that characterize gaseous fuel combustion quality; selector circuit means for transmitting the higher signal of said first and second signaling means; controller means having a reference signal that reflects a condition of complete fuels combustion with minimum excess air and heat exchanger corrosion for producing an output signal which is a function of the difference between the transmitted signal of said selector circuit means and the reference signal of said controller means; third means for signaling the amount of at least one of the fuels flowing in the fuel inlet of said furnace; fourth means for signaling the amount of air flowing in the combustion air inlet of said furnace; and, summing circuit means for algebraically adding said third signaling means signal to the output signal of said controller means and said fourth signaling means signal to produce an output signal for controlling said servo-mechanism.

12. A control network for burning liquid fuel and gaseous fuel in the same furnace under reduced excess air conditions comprising: servo-mechanism means for controlling the amount of combustion air flowing in the liquid and gaseous fuel associated air inlets of said furnace; first means for signaling the amount of unburned residue exiting from the flue of said furnace; second means for signaling the amount of combustion air flowing in the air inlets of said furnace; third means for signaling the amount of fuel flowing in the fuel inlets of said furnace; summing circuit means for algebraically adding the signals from said first, second and third signaling means to produce output signals suitable for controlling said servo-mechanism means; said first signaling means includes an opacimeter signaling the amount of unburned solids exiting from the flue of said furnace that characterize liquid fuel combustion and a gas analyzer signaling the amount of unburned gases exiting from the flue of said furnace that characterize gaseous fuel combustion; said second signaling means includes a first and second air flow meter signaling the rate of air flow to the fuel associated air inlets of said furnace; said third signaling means includes a liquid fuel flow meter and a gaseous fuel flow meter signaling the rate of flow of the respective fuels; said servo-mechanism means includes a first servo-mechanism controlling the air flow in the liquid fuel associated air inlet and a second servo-mechanism controlling the air flow in the gaseous fuel associated air inlet of said furnace; and, said summing circuit means includes a first summing circuit adding the signals from the liquid fuel flow meter, the opacimeter and the first air flow meter to control the first servomechanism and adjust the liquid fuel excess air flow, and a second summing circuit adding the signals from the gaseous fuel flow meter, the gas analyzer and the second air flow meter to control the second servo-mechanism and adjust the gaS fuel excess air flow thereby.