US3561895A

US3561895A - Control of fuel gas combustion properties in inspirating burners

Info

Publication number: US3561895A
Authority: US
Inventors: Herbert D Michelson
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1969-06-02
Filing date: 1969-06-02
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09
Also published as: FR2046437A5

Abstract

INSPIRATED AIR IN INSPIRATING TYPE BURNERS IS MAINTAINED CONSTANT AT A GIVEN LEVEL OF FIRING EVEN THOUGH ONE FUEL MAY BE CHANGED FOR ANOTHER BY MAINTAINING THE MOMENTUM FLOW OF FUEL THROUGH THE BURNER AT A SUBSTANTIALLY CONSTANT LEVEL, THE CONSTANT MOMENTUM FLOW BEING MAIN-

TAINED BY HEATING OR COOLING THE FUEL IN RESPONSE TO THE DENSITY VARIATION OF A NEW FUEL IN RELATION TO THE DENSITY OF THE ORIGINAL FUEL, AND SUPPLEMENTALLY BY ADDING ENRICHING OR DILUTING GASES TO THE NEW FUEL.

Description

Feb. 9; 1971 c so 3,561,895

CONTROL OF FUEL GAS COMBUSTION PROPERTIES IN INSPIRATING BURNERS.

Filed June 2, 1969 2 Sheets-Sheet 2 30 I 3| ADJUSTABLE COLLAR .AIR PORT THIMBLE STATIONARY 29 AIR DOOR AIR CONTROL HANDLE 28 BURNER NIPPLE 37 MIXER VENTURI 36 v MIXER ORIFICE 34 MIXER 33 FUEL GAS CONNECTION FIGURE 2 Hrberr D. Michelson Inventor By Attorney United States Patent Office Int. Cl. F23k /00 US. Cl. 431-11 1 19 Claims ABSTRACT OF THE DISCLOSURE inspirated air in inspirating type burners is maintained constant at a given level of firing even though one fuel may be changed for another by maintaining the momentum flow of fuel through the burner at a substantially constant level, the constant momentum flow being maintained by heating or cooling the fuel in response to the density variation of a new fuel inrelation to the density of the original fuel, and supplementally by adding enriching or diluting gases to the new fuel.

FIELD OF THE INVENTION This invention relates to a combustion control system. More particularly, this invention relates to a method and apparatus for maintaining both inspirated air and heat fired at relatively constant levels in inspirating type bumers when fuels,fhaving varying flow and combustion properties, i.e., density and heat release per pound of fuel are burned. Still more particularly, a method and apparatus are provided for maintaining both the heat release and the inspirating quality, i.e., amount of air inspirated by a fuel, of gaseous fuels through the burner at a substantially constant level, the inspirating quality for different gaseous fuels being maintained constant by heating, or cooling the fuel prior to its entry into the burner. In another embodiment of this invention, a diluting or enriching gas can be added to the gaseous fuel, in addition to temperature control of the fuel, to augment the effect of temperature changes.

PRIOR- ART Inspirating type burners, which burn a gaseous fuel, normally containing. mostly hydrocarbons, with air, are employed in high temperature furnaces (wherein temperatures, in a radiant tube metal section can range from about l200 -2000 R), such as steam cracking or reforming furnaces and thermal pyrolysis furnaces. Essentially, inspirating type burners operate by discharging a gaseous fuel through a burner nozzle or orifice into a venturi or mixing orifice and entraining air for combustion as the fuel enters the venturi. This entrained or inspirated air is known as primary 'air. The amount of primary air inspirated into the burner is generally determined by the momentum of the fuel through the burner nozzle. However, the amount of primary air inspirated is not normally sufficient to permit complete combustion of the fuel and, therefore, secondary air ports are provided. Thus, the total air requirements for a particular fuel is provided by primary and secondary air, i.e., air inspirated with the fuel and air drawn into the furnace through the secondary air ports, the air requirement being dictated by the amount necessary for the proper combustion of a given fuel. While it is desirable to burn the same fuel in such burners, this is not always practical. Thus,-Waste gases from a unit in, for example, a refinery, or from nearby units are generally employed as the fuel. However, the supply of such gases is not always constant and provision must be made for burning other fuels in the burners. But different fuels have different flow and 3,561,895 Patented Feb. 9, 1971 and heat release will occur, but since it is desirable to 5 maintain heat release constant, it" is necessary to change the fuel gas pressure. correspondingly, .the primary air inspirated will vary as the fuel gas pressure changes and resetting of the secondary air ports will be necessary in order to maintain total air rate to the burners at a substantially constant level for proper combustion. Changes in fuel properties, therefore, can have significant effects on the heat fired and/or combustion inside the furnace. In large furnaces, for example, many burners, e.g., hundreds, are employed. Obviously, resetting of the secondary airports for so many burners can be economically disadvantageous. This is particularly true where, as so often happens, the original fuel is only temporarily in short supply and the new fuel will be used only during the period of shortage of the original fueL'Such circumstances lead to a first setting of the secondary air ports for the original fuel, a second setting during the temporary shortage of the original fuel, and a third setting when the original fuel is again available. By the practice ofthis invention, however, primary air can be maintained constant. re-

gardless of the fuel being burned, thereby eliminating the necessity for resetting secondary air ports when the fuel is changed.

SUMMARY OF THE INVENTION In accordance with this invention, therefore, a method and apparatus are provided whereby both the primary air in the inspirating burners and the heat'fired are'maintained substantially constant when gaseous fuels of varying flow and combustion properties, i.e., density and heat release per pound, are substituted for one another, i.e., a first or original fuel and asecond or new fuel. This result is accomplished for fuels of essentially constant heat release per pound by adjusting the temperature of the new fuel so that it has the same density and, therefore, the same flow characteristics through the burner nozzle as the first fuel in accordance with the relationship M W M W, T1 T2 (1) wherein pg is the density of the gas in the burner, MW, is the molecular weight of the original or' first fuel, which is supplied at an absolute temperature of T and MW; is the molecular weight of the second.or new fuel which must be supplied at an absolute temperature T, in order to compensate for the molecular, weight change and maintain inspirated air and heat fired substantially constant. (Equation 1 is derived from the ideal gasflaw, PV=RT where P, V, R, and T are pressure, volume, an empirical constant, and absolute temperature respectively. The ideal gas law is simplified in this instance since the pressure on the fuel, original or new, through the burner will be constant and V, the volume of gas through the burner, remains constant so that the velocity and momentum flow also remain constant.) 60 Now, since the preferred fuels for inspirating burners are gaseous hydrocarbons, maintaining agconstant weight flow rate also results in the maintenance of constant 'heat fired. This result is due to the simple fact that the heat of combustion per pound for hydrocarbon fuels is virtually constant. For example, the heat of combustion of ethane is 22,304 B.t.u./pound while the heat of combustion for n-eicosane is 20,263 B.t.u./pound-, the variation over this wide range being only about 10%.

In inspirating type burners, the amount of primary air entrained with the fuel as the" fuel passes out of the burner nozzle into the mixer is essentially a function of the weight flow times velocity, or momentum of the fuel at constant pressure 7 its density accordingto therelationship for flow through (1 where M is the weight flow of gas through the orifice, P is the upstream pressure and p is the density of the fuel gas. Thus, as this invention speaks in terms of changing the density of the new fuel to the same density as the original fuel, it is the same as speaking in terms of changing the weight flow rate of the new fuel to the same weight flow rate of the original fuel. And, by virtue of the ideal gas law, density is inversely proportional to absolute temperature. Consequently, by adjusting the temperature of the new fuel, the density can also be.

adjusted and, therefore, the weight flow rate of the gas fuel through the burner can be maintained substantially constant regardless of the fuel employed. Additionally,

since the density and weight flow rate is substantially constant, the volume and velocity and hence the momentum flow required to inspirate the desired amount of air through the burner will also remain substantially constant.

In another embodiment of this invention and where simple temperature adjustment of the new fuel is not sufiicient to maintain control of the combustion system, e.g., when the required temperature, T of the new fuel would be so high as to cause cracking of the fuel or so low as to necessitate expensive cooling equipment, or where suflicient heating or cooling equipment is not available, or where the heat of combustion per pound of new fuel is not constant as when inerts, such as nitrogen, carbon dioxide, etc., are present in the new fuel, a diluent or enriching gas may be employed to augment temperature control. In most cases, the use of a diluent or enriching gas will change the weight flow rate and momentum in the burner (although such need not always be the case) and a slight adjustment to the driving force, e.g., fuel pressure, may be required to main tain inspirated primary air constant.

The apparatus required for accomplishing the'result described hereinabove comprises a burner, density changing means, which contains means for adjusting the temperature of the fuel which means are operatively connected to the burner, sensing means for sensing differences in density of the new fuel with relation to the original fuel and generating a signal (either through manual manipulation or automatic means) proportional to the change in density, and control means operatively connected to and responsive to the sensing means, and the density changing means. The'control means receiving the signal from the sensing means and actuates the density changing means by changingthe fuel temperature through heating or cooling means.-; In theembodiment where dilution or :enriching gas is also added to the gaseous fuel, the control means can also actuate the means for addition of this gas.

DRAWING DESCRIPTION control system at sensing means 11. When it is desired to substitute a new fuel for the original fuel, the original fuel supply is shut ofi by closing valve 9 and opening valve 7 to allow the new fuel, which may be propane, in line 6 to flow through valve 7 into line 10 and into the combustion control system at sensing means 11. The sensing means which may be a gas density meter and a weight flow meter for measuring weight flow and density generates a signal (manual or automatic) in proportion to the change in density of the new fuel with respect to the origin'alfuel. The fuel' 'then flows through line 12 to heat exchanger 13, e.g., the density changing means.

Sensing means .1 1 can employ electrical or magnetic means (as well as mechanical means, e.g., air signals) to transmit the density measurement by a signal in line 14 to control means 15, e.g., electrical switches for actuating the proper valves. Depending upon whether the new fuel requires heating or cooling, control means 15 can actuate heat exchanger 13 by a signal in line 81 opening valve 16 to allow a hot medium (e.g., steam or flue gas) from line 17 to heat the fuel in heat exchanger 13, or by a signal in line 71 opening valve 18 to allow cooling water or a refrigerant from line 91 to cool the fuel in heat exchanger 13. Also, depending upon whether a diluting or enriching gas is required to augment the temperature control system, such gas can be added by line 25 to the fuel stream. Such addition can also be controlled from control means 15 via a signal-in line 27 and valve 26. The addition of diluting or enriching gas can be either before or after heat exchanger =13. In FIG. 1, it is shown prior to the heat exchanger, this being preferred due to any heating or cooling elfect that the gas may possess. By adding the gas before the density changing means, the heating or cooling load therein can, in some instances, be minimized. In the case of using steam as a diluent gas, the steam would be added after the new gas was heated in density changing means 13 to prevent steam condensation. The hydrocarbon fuel gas of corrected density then flows to the burner and through burner nozzle 21 by way of line 20-and into the mixing orifice or venturi designated as 22. Primary air is inspirated circumferentially around the outlet of the burner nozzle and the path of the inspirated air is shown by arrows 23. Secondary air ports 24 provide they air needed for complete combustion.

FIG. 2 is a schematic representation of a typical burner assembly. The fuel gas connection 33 allows the gas of corrected temperature and density to flow into mixer 34. Primary air is inspirated, shown by arrows 35, and the air-fuel mixture is mixed in mixer orifice 36- and passes through venturi mixer 37 into the burner nipple 28 which is held in place by collar 31. Secondary air is provided through adjustable air port 30 which has an air control handle 29. The flame is ignited as it leaves the opening 32. In the instant invention, the air control handle 29 need be set only-once, 'at the start of burner operation.

The fuels employed herein can be either vapor or liquid, the only requirement being that the fuel be gaseous as it passes through the combustion control system, starting with the density sensing means. Thus, liquid fuels will require vaporizing prior to their entry into the combustion control system. Preferred fuels are hydrocarbons, particularly the light hydrocarbons, i.e., C -C hydrocarbons, most particularly C -C hydrocarbons.- Table I, below, lists various hydrocarbons that can be employed herein and "also shows the heat of combustion per pound for each fuel. It can be readily seen that the variation in heat ofcombustion is not significant and, therefore, heat fired will remain essentially constant regardless of the fuel employed.

Heat of combustion,

Compound: Batu/pound n-Butane (gas) 21,293 n-Butane (liquid) 21,134 Isobutane (gas) 21,242 Isobutane (liquid) 21,096 n-Pentane (gas) 21,072 n-Pentane (liquid) 20,914 Isopentane (gas) 21,025 Isopentane (liquid) 20,877 Neopentane (gas) 20,956 Neopentane (liquid) 20,824 n-Eicosane t 20,263

Enriching or dilution gases, i.e., gases extraneous to the process, can be employed to augment the temperature control system described above. An enriching gas is used when the new fuel is so much less dense in relation to the original fuel, e.g., a new fuel of methane and an original fuel of hexane, that it is not possible (for an economical refrigeration system) to cool the new fuel so that its density is increased to that of the original fuel. Without enrichment, the new, less dense fuel would require a high pressure to supply the same weight How to' the burner and then the momentum flow of the fuel would increase, resulting in a higher amount of air inspiration. Therefore, an enriching gas, e.g., a hydrocarbongas of higher molecular weight, suchas those previouslymentioned as being usable in this invention, is added to make the density of the mixture of new fuel and enriching gas the same as the density of the original fuel. By maintaining constant density in this way no change is needed in the fuel gas pressure to maintain a constant weight flow of fuel. This in turn maintains the momentum of the fuel and the amount of air inspirated constant.

A diluting gas is used when the-new fuel is so dense in relation to the original fuel, e.g., a new fuel of butane and an original fuel of methane, that it is not possible to heat the new fuel sufliciently (for practical reasons or because the new fuel may crack at the required temperature) to reduce its density to the same density as the original fuel. A high density fuel would require that the fuel gas pressure be lowered to keep the weight flow rate to the burner constant. In turn this would lower the momentum of the fuel and also lower the amount of air inspirated. As the density of a fuel increases at a constant fuel pressure, it is possible to maintain the weight flow rate of the hydrocarbon constant by adding a diluent gas of little or no heating value. Generally, diluent gases can be steam, inert gases such as nitrogen, argon, helium, carbon dioxide, etcl, or a mixture of an inert gas and a gas having a relatively low heat of combustion, e.g., carbon monoxide. Of the nonhydrocarbon gases mentioned, steam is normally preferred because of its ready availability, particularly in refineries.

Thus, enriching gases, i.e., gases used to increase the density of the new fuel to that of the original fuel,-, and dilution gases, i.e., gases used to decrease the density, of the new fuel to that of the original fuel, can be'utilized to augment or supplement the effects of density changing of the fuel by heating or cooling. Generally, the amount of diluting or enriching gas required for additionto the fuel stream is not great and, therefore, not critical to the combustion properties of the fuel. One skilled in the art will recognize that the object of the invention is to maintain not only the weight flow of fuel through the burner fairly constant but also the momentum flow constant and thereby maintaining the inspirated primary air fairly constant. Thus, the amount of enriching or diluting gas to be added for any" given operation can readily be determined by following the general directions given hereinabove and by conducting some very minor experiments.

Having now described the invention, the following example will serve to better illustrate the method disclosed herein. However, no limitations are to be implied from this example, since those skilled in the art will readily recognize that many variations and modifications of this example can exist.

At an existing refinery, amultitude of steam cracking furnaces are in existence which fire a total-of seven hundred million B.t.u./hour fuel gas. The fuel gas is a mixture of methane and ethane (mostly methane), having a molecular weight of 18 and is supplied to the burnersat 100 F. The fuel gas is generated by the steam cracking process and recovered in a light ends unit, i.e., demethanizer, de-ethanizer. In order to maintain onstream operation of the furnaces at all times and particularly during temporary shortages of the methane/ethane gas fuel,

'ratio 44/18 and the weight flow in the' burner increases by a factor of (44/18)- =1.56 (derived from the orifice equation, that the pressure drop across the burner is directlyproportional to the weight flow rate divided by the density of the gas through the burner squared, i.e.,

(W) AFN (P9) where AP is the pressure drop, W is the weight flow rate, and pg is the gas density through theburner). Now, since the heat of combustion per pound of fuel is nearly the same for both fuels, it would ordinarily be necessary to reduce the fuel gas pressure to keep heat fired constant. At reduced pressures, the momentum flow of fuel. gas would be reduced and less primary air would be inspirated. All of the secondary air ports would have to be reset. However, the increase in molecular weight of the fuel can be compensated for solely by a temperature increase using Equation 1.

ha A +460 Tz T,= (460-l-100) or T =1370 R. or 910 F.

Thus, bysimply increasing the temperature of the propane fuel gas to 910 F. weight flow rate of fuel gas through the burners will remain constant and the amount of inspirated primary air will not change as the fuel gas is changed.

In some instances, refineries or. chemical plants may not have sufficient waste heat or spare. heating facilities to raise the propane temperature to 910 F. Consequently, an investment for an additional furnace would be necessary. However, many refineries can heat the propane to about 400LF. with readily available steam heaters. Thus, at 400 F; the weight flow would increase by a factor of creased, the heat fired decreased. When the ratio of 0.34 pound of steam per pound of ,propane was attained, the

original value of heat fired was obtained and the amount of air inspirated was essentially that of the original fuel.

What-is claimed is: 1. A method for combusting, in an inspirating type burner, a first fuel and a second fuel, said first fuel and density of said first'fuel.

"- 'equa.l izing the density of said'second fuel 'with the.

2. The method of claim 1 wherein said second fuel is i more dense than said first fuel and the density of said second fuel is equalized with the density of said first fuel by heating said second fuel. 1

3. The method of claim 1 wherein said second fuel is less dense than said first fuel and the density of said second fuel is equalized 'with the density of said first fuel by cooling said second fuel.

4. The method of claim 1 wherein said first fuel and said second fuel are both hydrocarbons.

5. The method of claim 4 wherein said hydrocarbons are in the gaseous phase when density variation of said second fuel with respect to said first fuel is determined.

6. The method of claim 4 wherein the temperature of said second fuel is adjusted in accordance with the relationship wherein MW and T are the molecular weight and absolute temperature, respectively, of said first fuel, and MW, and T are the molecular weight and absolute temperature, respectively.

7. The method of claim 4 wherein the pressure on said first fuel and the pressure on said second fuel are substantially the same, I

8. The method of claim 4 wherein an extraneous gas is employed to supplement temperature adjustment in equalizing the density of said second fuel and said first fuel.

9. The method of claim 8 wherein said second fuel is more dense than said first fuel and said extraneous gas dilutes said second fuel.

10. The method of claim 9 wherein said extraneous gas is selected from the group consisting of steam, inert gases, and mixtures thereof.

11. The method of claim 8 wherein said second fuel is less dense than said first fuel and said extraneous gas enriches said second fuel.

12. The method of claim 11 wherein said extraneous gas is a hydrocarbon having a higher molecular weight than said second fuel.

13. A method for combusting, in an inspirating type burner, a first gaseous hydrocarbon fuel and second gaseous hydrocarbon fuel, said first fuel and said second fuel having different molecular weights but having a substantially similar heat release per unit weight, whereby the amount of air inspirated into said burner for combustion of said first fuel and combustion of said second fuel is maintained substantially constant, and wherein the pressure of said first fuel and the pressure on said second fuel is also maintained substantially constant, which comprises:

variation in density of said second fuel with respect to said first fuel,

adjusting the temperature of said second fuel in accordance with the relationship wherein MW and T are the molecular weight and.

absolute temperature, respectively, of said first fuell and MW, and T are the molecular weight and absolute. temperature, respectively, of said second fuel,.

.- thereby changing the density of said second'fuel, and.

equalizing the density of said second fuel with thedensity of said first fuel.

14. Apparatus for combusting a first gaseous hydrocarbon fuel and a second gaseous hydrocarbon fuel, said firstfuel and said second fuel having different molecular weightsbut having a substantially similarheat release per unit weight, whereby the amount of. air required for the combustion of said first fuel and the combustion of said second fuel is maintained substantially constant, which comprises:

inspirating burner means, density changing means containing means for adjusting the temperature of said second fuel, means for sensing the variation in density of said second fuel with respect to saidfirst fuel, and control means operatively connected to both said sensing means and said density changing means, said control means being responsive to said sensing means and actuating said density changing means, thereby changing the temperatures of said second fuel in proportion to the variation in density of said second fuel with respect to said first fuel. 15. Apparatus of claim 14 wherein said sensing means contains means for generating a signal proportional to the density variation of said secondfuel'with respect to said first fuel. a

16. Apparatus of claim 15 which contains means for adding an extraneous gas to said second fuel.

17. Apparatus of claim 15 wherein-said means for sensing density variation comprises a gas density meter and a weight flow meter.

18. Apparatus of claim 15 wherein said second fuel is less dense than said first fuel and I said control means zctrllate said density changing means. to cool said second ue a 19. Apparatus of claim 15 wherein said second fuel is more dense than said first fuel and said control means gtrllate said density changing means to heat said second References Cited UNITED STATES PATENTS FREDERICK L. MATTESON, 1a., Primary Examiner R. A. DUA, Assistant Examiner I US. 01. X.R.