WO1984002403A1

WO1984002403A1 - Combustion control system

Info

Publication number: WO1984002403A1
Application number: PCT/US1982/001753
Authority: WO
Inventors: George C Broach
Original assignee: Broach Co G C
Priority date: 1982-12-13
Filing date: 1982-12-13
Publication date: 1984-06-21
Also published as: EP0128141A1

Abstract

A method and apparatus for controlling combustion in a process heater (10) in which the flow of fuel (20) and oxidant are controlled electronically includes variable resistor (38) means to modify a signal to either an air flow control damper (50) or a fuel control valve (58) to maintain maximum thermal efficiency. Typically, a fuel control valve (58) and an air flow control valve (50) are pre-set, and then, in response to a signal according to the demands of the system, move in parallel. The ratio controller (80) of this invention operates to allow an easy and efficient means of varying the ratio of fuel to oxidant.

Description

COMBUSTION CONTROL SYSTEM

This invention relates broadly to furnaces, boilers, incinerators, and like equipment wherein fuel is burned.

Most particularly, this invention relates to combustion equipment which regulates the flow of combustion air and fuel in response to a signal generated from a condition of a process. Still more particularly, this invention relates to combustion equipment which will provide a fuel- lean mixture of combustion components as process con¬ ditions modify the demand for process heating.

The efficient control of combustion relates to the efficient control of the fuel and combustion oxi- dant, usually air. Efficient control of combustion is essential in the power industry, chemical industry, and various other process industries.

The amount of heat furnished by the combustion process depends upon the amount of heat needed by the operation requiring the heat, whether for the generation of power, or for the chemical process, or for an oil refinery process. In addition, the amount of heat needed will continually vary, thus requiring continual control of the combustion.

In some industries, particularly the power in¬ dustry and the chemical industry, the relative ef¬ ficiency of the combustion process may be accomplished by the use of elaborate control systems, such as computers, various kinds of automatic valves, and electronic controls.

As a result, numerous control systems have been designed to control the combustion processes. And, with time, these systems tend to become more elab- orate. Then, as a system becomes more elaborate, the number of components, such as dials, gauges, recorders, computers, and the like, requiring the attention of the operator, increases. With increased complexity, the probability of someone making a mistake increases greatly. The operator has an in¬ creased number of recording devices to observe and maintain, dials to continually observe, and switches to be ready to operate, for example.

Disclosure of Invention

Therefore, the primary object of this invention is to provide a method and apparatus for controlling combustion in a process which is efficient, easy to operate, and economical. Another object of this invention is to provide a method and apparatus for controlling combustion in a process which will require only a minimum number of control components in order to regulate the combustion effectively. Still another object of this invention is to provide a method and apparatus for controlling com¬ bustion in a process in which fuel and combustion air are easily regulated for more efficient energy consumption. Still another object of this invention is to provide a method and apparatus for maintaining close control of the combustion in a process.

Another object of this invention is to provide a method and apparatus which can be easily adjusted to regulate a combustion temperature.

Another object of this invention is to provide a method and apparatus for controlling combustion in a process heater in which a combustion temperature may be easily maintained. Still another object of this invention is to provide a method and apparatus for maintaining close

OMPI \ control of the energy consumption in a process heater.

Still another object of this invention is to provide a method and apparatus to modify quickly and efficiently the ratio of air to fuel of a combustion system.

These and other objects of the invention will become apparent from the accompanying description and drawings and attached claims. This invention describes a greatly simplified and efficient method and apparatus for combustion control by eliminating many sophisticated and high¬ ly technical controlling devices commonly shown in other combustion control systems. By simplifying the combustion control system, this invention reduces the number of control devices which the operator must understand and closely ob¬ serve. Consequently, the risk of accident caused by inattention of the operator is greatly reduced. Further, this invention provides for a more efficient combustion control system by assuring a fuel-lean combustion during a change in combustion requirements.

All of these advantages are obtained by the application of very few, and simple, low-cost com¬ ponents.

The method and apparatus of this invention in¬ clude generating a signal from a condition of a process stream and adapting that signal to acuate various fuel and combustion oxidant control com¬ ponents. The type of signal generated includes an electrical, or electronic, signal. The condition of the process stream from which the signal is gen¬ erated is preferably a temperature of the process, or the pressure of a boiler.

In one typical application, a temperature-re¬ sponsive device measures a temperature of the process and emits an electrical signal which may be either an increasing voltage signal or a decreasing voltage signal.

The signal is communicated in parallel to a signal restrictive member, such as a resistance, or, more particularly, such as a variable resistance or potentiometer; to a first electro-responsive device, such as a first electronic relay; and to a second electro-responsive device, such as a second elec- tronic relay.

The first electro-responsive device is adapted to respond to a signal of a first characterization, and the second electro-responsive device is adapted to respond to a signal of a second characterization. The signal restrictive member is further adapt¬ able to communicate a signal to a signal collecting device, such as a capacitance, and the signal col¬ lecting device is adaptable to provide a signal of a first characterization and a signal of a second characterization.

The signal collecting device is adaptable to communicate in parallel with the first electro-re¬ sponsive device and with the second electro-respon¬ sive device. The first signal electro-responsive device is further adaptable to communicate with an air control device which is adaptable to control a flow of combustion air in response to a signal from the first electro-responsive device.

The second electro-responsive device is further adaptable to communicate with a fuel control device which is adaptable to control a flow of fuel in response to a signal from the second electro-respon¬ sive device.

The invention teaches a method and apparatus for controlling combustion in which fuel-lean com¬ bustion is provided during an interval in which the firing rate is changing. For example, when the process system demands more heat, the flow of com¬ bustion air is increased prior to an increase in . the flow of fuel, thereby maintaining a fuel-lean mixture during the interval of change. Or, when the process system requires the heat to be reduced, the flow of fuel is decreased prior to a decrease in the flow of combustion air, thereby maintaining a fuel-lean mixture during this interval of change. In an alternate embodiment of the invention, the signal is communicated to a series resistor and a variable R-C ramping network. Voltage com¬ parators are utilized to compare the instantaneous input signal to the output of the ramp network. These voltage comparators control solid state switches which connect two (2) signals to the ap¬ propriate output amplifier. The output amplifiers control the signals to the final AIR/FUEL control devices so that during a signal change there will always be a fuel-lean mixture. When the instan¬ taneous input signal is decreasing, the fuel control signal amplifier receives the instant signal direct with the combustion air control amplifier receiving the ramping signal. When the instantaneous input signal is increasing, the fuel control signal am¬ plifier receives the ramping signal and the com¬ bustion air control amplifier receives the instan¬ taneous signal.

In a typical combustion operation, a fuel flow control valve and an air flow control valve, or damper, react in parallel, in proportion to the signal generated by the temperature controller. Thus, after being initially set, at chosen points, a fuel flow control valve and an air flow control valve maintain a given ratio of fuel to oxidant as they move in response to the signal generated by the temperature controller. A typical

O PI s. ^wιp° signal is within the range of four (4) to twenty (20) milliamperes.

I had become aware of the manner in which fuel and oxidant valves react proportionately in a typical combustion system, and described in my parent appli¬ cation, U. S. Serial Number 231,964, filed February 6, 1981, which was also filed August 25, 1981, as PCT Application PCT/US 81/01140, a method and appar¬ atus which provide for a fuel-lean mixture during an interval of change in the heat demand of a process system.

Now, in this application, I am describing a method and apparatus in which a variable, or poten¬ tiometer, is in the system in order to modify, or bias, the signal to either the fuel flow control valve or the air flow control valve. A potentiometer used in this manner, which I also refer to as a ratio controller can serve to: compensate for changes in the heating value of the fuel being utilized;

modify the fuel/air ratio at various furnace load conditions to maintain maximum thermal efficiency;

offset an undesirable fuel/air ratio that might exist at a particular time as a result of the relative fixed flow characteristics of the flow control devices.

Brief Description of Drawings:

In the accompanying drawings: FIGURE 1 is a schematic drawing of a typical process heating system.

FIGURE 2 is a diagrammatic drawing of a com¬ bustion control system according to this invention.

FIGURE 3 is a schematic drawing of a combustion control system according to this invention.

FIGURE 4 is a diagrammatic drawing of an alternate embodiment of a combustion control system according to this invention. FIGURE 5 is a schematic drawing of an alter¬ nate embodiment of a combustion control system ac¬ cording to this invention.

FIGURE 6 is a schematic drawing of a combus¬ tion control system according to this invention including a ratio control device.

Best Mode for Carrying Out the Invention

FIGURE 1 describes a typical combustion and heating system for a conventional process industry, which, in its basic arrangement, comprises a fur- nace or heater 10, generally, having a process in¬ let stream 12 entering therein, a typical heating coil 14, a process outlet stream 16, and a burner 18 fired by a fuel line 20 and air supplied as through opening 22. In one embodiment of my invention as described in FIGURE 3, line 24 provides communication between process outlet stream 16 and a temperature controller 26. Temperature controller 26 may be one of any conventional devices for measuring temperature and providing a signal to other equipment in response to that temperature. Thus, temperature controller 26 measures a temperature of the process stream and provides a signal through lines 28, 30, 32, and 34, in parallel to a first relay 36, a potentiometer 38, and a second relay 40.

First relay 36 is adapted to respond to an in¬ creasing voltage signal in lines 28 and 30. Relay 36 then transmits a signal through line 42 to an air flow control device 44, which, in this e bodi- ment of my invention, further communicates a signal through line 45 to an air control device 46, generally, which operates, through connection 48, louvers 50, positioned in air duct 52, to regulate the amount of combustion air.

Second relay 40 is adapted to respond to a decreasing voltage signal in lines 28 and 34. Relay 40 then transmits a signal through line 54 to a fuel control device 56, generally, which further transmits a signal through line 57 to fuel control valve 58, which operates to control the amount of fuel trans- mitted to burner 18.

Potentiometer 38, is, of course, an adjustable resistance, and, as such, may be adjusted to provide a selected potential through line 60 to condenser 62 which is adapted to transmit a delayed signal through lines 64, 66, and 68, to first relay 36 and second relay 40.

Thus, an increasing potential signal from temp¬ erature controller 26 through lines 28 and 32 will cause a delayed potential to develop on condenser 62 and result in a delayed potential drop between relay 36 and condenser 62 and between relay 40 and conden¬ ser 62.

The reaction of relay 36 to an increasing po¬ tential signal from temperature controller 26 will cause relay 36 to actuate air flow control device 44 and air control device 46 as explained above, assuring a flow of air to burner 18 before a flow of fuel begins.

Relay 40 will not react to the increasing po- tential through line 34 but will react to the de¬ layed increasing potential through line 68. In re¬ sponse to the increasing potential from condenser 62 to relay 40, relay 40 will actuate fuel control de¬ vice 56 and fuel control valve 58 to permit a flow of fuel to burner 18 after a flow of air has begun. A signal of decreasing potential from temper¬ ature controller 26 through line 34 will actuate relay 40 to cause relay 40 to actuate fuel control device 56 in a manner to close fuel control valve 58, shutting off the flow of fuel before the flow of air is shut off. Relay 36 will not react to a signal of de¬ creasing potential from temperature controller 26 through line 30, but will react to a delayed signal of decreasing potential through condenser 62, line 64, and line 66, to actuate air flow control device 44 to operate air control device 46 to shut off the supply of air to burner 18 after the flow of fuel to burner 18 has stopped.

FIGURE 2 provides a simple outline of a typical process industry embodiment of my invention. In an alternate embodiment of the invention as described above and in FIGURES 4 and 5, temperature controller 26 measures a temperature of the process stream and provides a milliampere' signal through lines 28 and 29 and resistor 31. Potentiometer 38 and capacitor 62 form a ramp signal. Switches 33 and 35 and voltage comparators 37 and 39 receive an instantaneous signal through line 30 and a ramp sig¬ nal through line 32. An output amplifier 41 is con¬ nected between switch 35 and air flow control device 44, and an output amplifier 43 is connected between switch 33 and fuel control device 56.

Voltage comparator 39 is set to switch line 30 to amplifier 41 through switch 35 when the signal on line 30 is greater than the signal on line 32. Voltage comparator 39 is set to switch line 32 through switch 35 to amplifier 41 when the signal on line 32 is greater than line 30. Amplifier 41 transmits a signal through 42 to air flow control device 44 as exemplified by current converter 44 which further communicates a signal through 45 to air control de¬ vice 46 which operates, through connection 48, louvers 50, positioned in air duct 52 to regulate the amount of combustion air.

Voltage comparator 37 is set to switch line 30 to amplifier 43 through switch 33 when the signal on line 30 is less than the signal on line 32. Volt- age comparator 37 is set to switch line 32 to ampli¬ fier 43 through switch 35 when the signal on line 32 is less than the signal on line 30. Amplifier 43 transmits a signal through 54 to fuel control device 56 as exemplified by current converter 56 which further communicates a signal through 57 to fuel control valve 58.

Potentiometer 38 in conjunction with capaci¬ tor 62 are used to determine the ramp signal.

A signal increase from the temperature con- troller 26 through line 28 will create an increase in voltage across resistor 31. This instantaneous voltage is fed to the ramp control 38 and 62 and through line 30 to comparators 37 and 39. The sig¬ nal on line 30 is greater than on line 32, therefore line 30 is switched to amplifier 41 by comparator 39 and switch 35, the ramp voltage on line 32 is switched to amplifier 43 by comparator 37 and switch 33.

Thus, an increased signal from the temperature controller will cause the air signal to increase immediately with the fuel signal ramping up to its desired point. A decrease in the temperature con¬ trol signal will cause the opposite to occur, i.e., the fuel signal will drop immediately and the air signal will ramp down to its desired point.

In the combustion control system described in FIGURE 6, I show a potentiometer 80, or ratio con¬ troller, connected with amplifier 41 and line 42, as it would be suitably placed for modifying the signal to the air flow control device 44.

As an alternative, I also show in FIGURE 6, a potentiometer 82 connected with amplifier 43 and line 54, as it would be suitably placed for modifying the signal to fuel control device 56. I have provided for the choice of a poten¬ tiometer in either air flow control line or fuel control line. It should be understood that I do not require the potentiometers 80 and 82 to be present in the system at the same time.

Since many different embodiments of this in¬ vention may be made without departing from the spirit and scope thereof, it is to be understood that the specific embodiments described in detail herein are not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

Claims

Clai s

1. In a combustion control system having means to control a flow of fuel and means to control a flow of oxidant in response to a signal gen- erated from a condition of a process, the im¬ provement in combination therewith comprising: means for controlling the ratio of fuel to oxidant.

2. A combustion control system as described in Claim 1, wherein: the means for controlling the ratio of fuel to oxidant includes a circuit having a potentiometer therein.

3. A combustion control system as described in Claim 2, wherein: the means for controlling the ratio of fuel to oxidant controls the flow of oxidant.

4. A combustion control system as described in Claim 2, wherein: the means for controlling the ratio of fuel to oxidant controls the flow of fuel.