US 3865707 A
The sample of the gaseous fuel mixture to be analyzed is directed from the feed line leading to a combustion chamber and circulated through a flame arrestor and gas flow meter to a combustion mixture analyzer having an oxygen solid electrochemical cell contained therein. The fuel mixture is heated above its ignition temperature to obtain complete chemical equilibrium and the excess oxygen and/or fuel that remains, if any, mixed with the products of combustion circulate around the outer surface of the electrochemical cell. A suitable readout device measures the EMF produced across the electrodes of the cell when a gas of known partial pressure comes in contact with the inner surface of the electrochemical cell. The fraction of excess oxygen above that required for stoichiometric combustion that remains in the gaseous products of combustion is determined by the formula: X = D P0 +292 Sample/0.209 - P0 +292 Sample When unconsumed gaseous fuels are detected, the fraction of excess fuel in the combustible mixture of gaseous fuels and air in excess of the amount required for stoichiometric combustion is determined by the formula: A = log <->1 (937.5-EMF/107.5) (1 +b/2)/2 +b/2 + log-1 (937.5-EMF/107.
Description (OCR text may contain errors)
United States Patent 1 Sayles [451 Feb. 11, 1975 COMBUSTIBLE MIXTURE ANALYZER  Inventor: Donald A. Sayles, 1337 Beechwood Blvd., Pittsburgh, Pa.
 Filed: Dec. 27, 1972 [2|] Appl. N0.: 318,889
Primary ExaminerG. L. Kaplan Attorney, Agent, or Firm-Stanley J. Price, Jr.
 ABSTRACT The sample of the gaseous fuel mixture to be analyzed is directed from the feed line leading to a combustion chamber and circulated through a flame arrestor and gas flow meter to a combustion mixture. analyzer having an oxygen solid electrochemical cell contained therein. The fuel mixture is heated above its ignition temperature to obtain complete chemical equilibrium and the excess oxygen and/or fuel that remains, if any, mixed with the products of combustion circulate around the outer surface of the eliectrochemical cell. A suitable readout device measures the EMF produced across the electrodes of the cell when a gas of known partial pressure comes in contact with the inner surface of the electrochemical cell. The fraction of excess oxygen above that required for stoichiometric combustion that remains in the gaseous products of combustion is determined by the formula:
X D P 2 Sample/0.209 P 2 Sample A log (937.5EMF/107.5) (l +b/2)/2 +b/2 logl (937.5-EMF/107.5)
5 Claims, 2 Drawing Figures COMBUSTIBLE MIXTURE ANALYZER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and apparatus for measuring the ratio of fuel material to air in a combustible mixture and more particularly to a method and apparatus for measuring the fuel material to air ratio of a combustible mixture supplied premixed to burners for a combustion process.
2. Description of the Prior Art Electrochemical cells used to measure the partial pressure of oxygen in a gas to indicate the gas composition are well known, and a typical oxygen solid electrolyte cell is described in U.S. Pat. No. 3,597,345. The oxygen detection system described there includes an oxygen sensor having a differential oxygen pressure responsive electrolyte cell to control the ratio between the fuel and air supplied to a combustion chamber by measuring the products of combustion emitted from the furnace flue. The cell includes a closed end electrolyte tube which conducts oxygen ions in a gas entry tube extending axially into the electrolyte tube. Electrodes are provided on opposite sides of the electrolyte tube wall to provide a detection zone. Combustion products from the furnace are fed into the gas entry tube to the inner surface of the electrolyte tube. The outer surface of the tube is in contact with a gas, typically air, of known partial pressure. The gases remain separated on opposite sides of the tube walls to form a detection zone and a heater maintains the zone at a temperature from about 600C. to about l,000C.
When there is a difference between the partial pressures of oxygen on opposite sides of the wall, an EMF is produced across the electrodes. The potential difference produced by the cell is impressed on a control apparatus to control the ratio between the fuel and oxygen supplied to the system. Since the oxygen detection system is designated to monitor the oxygen content of the combustion products as they are discharged from the furnace flue, adjustments to be made to the ratio of fuel and oxygensupplied to the furnace cannot be detected until the comustion operation has taken place within the furnace chamber, having a resultant effect of lowering the operational efficiency of the combustion reaction of fuel with the oxygen in air. Furthermore, the detection system only teaches measurement of excess oxygen in the products of combustion and no provision is made to determine what percentage of excess fuel, if any, remains in the sample flue gas. In addition, a catalytic agent is disposed along the inner surfaces of the gas entry tube and the inner surface ofthe close end of the electrolyte tube to aid in providing complete combustion of the unburned fuel in the gas sample before it reaches the detection zone. Thus, the detector is limited to the analysis of oxygen excess as opposed to analysis of both oxygen and fuel excess in the sample.
The detection and measurement of combustible gases and vapors is also accomplished by measuring the temperature rise of a heated element having catalytic activity to produce combustion at relatively low temperatures. The calibration for the conventional catalytic detectors is empircal and varies greatly with different types of combustible materials. The catalytic activ ity is variable and diminishes, sometimes destroyed by many materials. Furthermore, the useful range of these devices is not continuous between the lower and upper explosive limits, and they cannot distinguish between fuel-rich and air-rich mixtures.
There is need for a method and apparatus to monitor and control the preparation ofa precise mixture of fuel gas and air prior to feeding the mixture to a burner for combustion therein. Also, there is a need for both a method and apparatus to produce combustion by heating a gaseous combustible mixture sample to above its ignition temperature to obtain complete chemical equilibrium and then to analyze the combustion products thus obtained in order to control the original mixture.
SUMMARY OF THE INVENTION In accordance with the present invention, a method and apparatus is provided to measure the ratio of cum bustible material to oxygen in a fuel mixture as the fuel mixture is being fed to a combustion chamber or burner. A sample of the gaseous fuel mixture is directed through a flame arrestor and a gas flow meter and conveyed therefrom at a preselected flow rate by a conduit to a combustion mixture analyzer. The combustion mixture analyzer includes a closed tubular housing having an analyzer chamber extending through the opposite ends thereof. Within the analyzer chamber and to an intermediate point therein is positioned an oxygen solid electrochemical cell. The gaseous fuel mixture passes through a diverging throat portion of the combustion mixture analyzer chamber and is heated above its ignition temperature to obtain com plete chemical equilibrium. The analyzer chamber has a suitable length to provide for sufficient admixing of the combustion reactants before they reach the electro chemical cell surface.
The solid electrochemical cell is constructed of a ceramic material that conducts oxygen ionically at an elevated temperature. The inner surface of the solid electrochemical cell is in contact with a gas of known. oxygen partial pressure, typically air. The outer surface of the electrochemical cell is in contact with a gaseous mixture of both the products of combustion and oxygen and/or fuel gas, if any, that remains unconsumed in the analyzer chamber. Heat generating means maintains the cell preferably at a temperature of 812C. Electrodes secured to the opposite surfaces of the ceramic member are connected to the terminals of a suitable readout device for measuring the EMF across the electrodes.
When, after comlete combustion in the analyzer chamber the fuel sample still contains oxygen, the EMF indicates the percentage of oxygen in the final mixture and also the ratio of combustible material to oxygen in the original mixture. In the case after complete combustion, when the fuel sample in the analyzer chamber contains unburned fuel, the EMF indicates the ratio of carbon monoxide plus hydrogen to the total amount of carbon dioxide plus water which, for any average value of hydrogen to carbon ratio in the fuel, will indicate the ratio of combustible material to oxygen in the original mixture.
Accordingly, the principal object of this invention is to provide a method and apparatus for completely burning a sample of a mixture of gaseous material and oxygen in air for the purpose of determining if an excess of either fuel or oxygen is present in the fuel mixture before being burned in a combustion chamber.
Another object of this invention is to provide a method and apparatus to completely burn a gaseous fuel sample for the purpose of determining the air demand of the fuel and of controlling the composition of the gaseous fuel mixture.
Another object of this invention is to burn a metered flow of gaseous combustible material with a constant flow of air so that changes which occur in the composition of the gaseous combustible material may be re corded by a suitable readout device.
A further object of this invention is to provide for both a method and apparatus to measure the concentration of combustible vapor in air and give a warning when the lower explosive limit is approached.
Still another object of this invention is to provide a method and apparatus for monitoring and controlling the preparation of a mixture of an organic chemical in vapor form with oxygen for reaction in a chemical reactOI'.
These and other objects and advantages of this invention will be more completely disclosed and described in the following specification, the accompanying drawing and the appended claims.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation, partially in section, of the combustion mixture analyzer according to the invention.
FIG. 2 is a schematic representation of the apparatus for monitoring and controlling the burning characteristics of a fuel mixture according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, there is illustrated a combustion mixture analyzer generally designated by the numeral that includes a closed tubular housing 12 with a pair of openings 14 and 16 axially located in opposite ends of the housing 12 through which passes an analyzer chamber 18. The analyzer chamber 18 is made of a high-alloy tubing capable of resisting elevated temperatures. The analyzer chamber 18 extends through the interior of the housing 12 with a small diameter end portion 20 passing through the top opening 14 and a larger diameter end portion 22 passing through the opposite bottom opening 16. The smaller diameter end portion 20 projects above the housing 12 and terminates in an elbow joint 24 that joins in fluid communication with a conduit 26. An L- shaped adapter 28 seals in overlapping relationship the connection of the elbow joint 24 between the conduit 26 and the end portion 20. The larger diameter end portion 22 of the chamber 18 passing through the bottom opening 16 terminates in an exit chamber 30 located below the housing 12. An outlet tube 31 communicates with the interior of exit chamber 30 and extends outwardlytherefrom. A compression fitting 32 threadably engages an oxygen solid electrochemical cell 34 in a gas tight seal with exit chamber 30. The electrochemical cell 34 extends axially within the analyzer chamber 18 to a location intermediate of the larger diameter end portion 22 so that the closed end 36 of the cell 34 is displaced a substantial distance from the diverging throat portion 37.
The oxygen solid electrochemical cell 34 utilized with the combustion mixer analyzer 10 is of the type disclosed in my co-pending U.S. patent application Ser.
No. 261,017 filed Aug. 6, I972, now abandoned and manufactured and sold by Thermo Lab Instruments of Pittsburgh, Pa. The electrochemical cell described there includes a closed end ceramic tube. The tube is made of a ceramic material which conducts oxygen ionically when hot, as for example, stabilized zirco'nia (ZrO There the ceramic tube has one surface in contact with a gas of known oxygen partial pressure and the other surface in contact with a stream of gas that contains both the products of combustion and the gaseous combustible mixture of fuel materials and oxygen. The gases remain separated and in contact with opposite surfaces of the ceramic tube. The gas of known partial pressure in contact with the outer surface of the ceramic tube is typically air.
Heating coils 40 associated with the analyzer l0 maintain the surface of the electrochemical cell 34 at a constant temperature of preferably 812C. A temperature sensitive device 42 senses the temperature of the analyzer 10 to control the desired temperature of the heating coils 40. Electronically conductive electrode coatings (not shown) of inert material such as platinum or a wire wrapping are connected to the opposite sides of the ceramic tube in the electrochemical cell 34. The inner electrode covering the inner surface of the electrochemical cell 34 extends around the edge of the cell open end 35 into electrical contact with the surface of metal fitting 44. The outer electrode is electrically connected to the metal fitting 46 and is separated from the inner electrode by a gap between the two electrodes. Electrode lead wires 48 and 50 secured to each of the metal fittings 44 and 46 are connected to the terminals of a suitable readout device 52 such as a microammeter or a solid state millivoltmeter to measure the EMF across the electrodes when there is a difference between the partial pressures of oxygen on opposite sides of the cell ceramic tube.
With this arrangement, the combustible mixtures are directed from the feed line (not shown), leading to a combustion chamber, through a flame arrestor 54 into the conduit 56. The flame arrestor 54 is of conventional construction and includes a material that provides small diameter passages therethrough to increase the velocity of the gas flowing through the flame arrestor to an extent that the gas velocity exceeds the velocity of flame propagation. The flame arrestor 54 prevents explosions of the air/fuel mixture associated with high energy systems in the feed line that might otherwise occur when the combustibles are burned in the analyzer chamber 18. The flame arrestor 54 permits continuous flow of the combustible mixture from the lead line to the combustion mixture analyzer 10..The combustible gaseous mixture containing fuel materials and oxygen pass from the flame arrestor 54 through the conduit 56 to the gas flow meter 58 and through the throttle valve 60 in fluid communication therewith. The gas flow meter 58 measures the flow rate of the gas mixture as controlled by the throttle valve 60, preferably at a flow rate between 0.5 cfh and 0.8 cfh.
The combustible mixture passing from the gas flow meter 58 through the conduit 26 is admitted to the combustion mixture analyzer 10 through the analyzer chamber smaller diameter end portion 20 therein. The smaller diameter end portion 20 further aids to prevent flashback of the combustible fuels to the feed line. A flame is maintained at the throat portion 37 where the diameter of the analyzer chamber 18 diverges by heating the mixture of combustibles to a temperature above its ignition point. The combustibles are maintained at their ignition temperature for a sufficient length of time to obtain complete combustion of the reactants. Furthermore, the volume of chamber end portion 22 between throat 37 and cell closed end portion 36 aids to insure complete combustion of the fuel mixture before the products of combustion reach the closed end portion 36. The combustion of the fuel materials and oxygen reaches chemical equilibrium before the products of combustion pass over the electrochemical cell closed end portion 36.
The heating coils 40 maintain the electrochemical cell 34 at the desired temperature of 812C. as the products of combustion contact the electrodes on the outer surface of the cell 34. The ratio of the oxygen partial pressure in the gaseous products of combustion in contact with the outer electrode and that of the known gas (air) in contact with the inner electrode products an EMF across the electrodes. After analysis, the combustion products pass from the analyzer chamber 18 through the outlet conduit 31 to the atmosphere.
The potential difference is recorded by the readout device 52 calibrated logarithmically to show to what extent the fuel sample drawn from the feed line leading to the combustion chamber contains an excess of oxygen and/or fuel. In addition, the readout device 52 may be calibrated to record a potential difference which indicates the percent oxygen and/or combustibles in the burned gas, percent excess air and/or excess fuel in the original mixture, the ratio of fuel to air in the original mixture, and the amount of combustible material in the original mixture as a percentage of the lower explosive limit. Further, the EMF produced across the electrochemical cell 34 may also express the amount of oxygen required to fully burn the fuel gas, thereby providing control of the fuel gas combustion potential. In keeping with the invention, a gaseous fuel mixture may be blended with air in the proportion adjusted initially to give stoichiometric combustion of oxygen required to completely burn the fuel mixture so that deviations from the initial composition of the fuel mixture may be measured.
When unconsumed oxygen remains mixed with the combustion products, the oxygen in the reference gas being known, the percentage of oxygen in the burned mixture is indicated directly by the EMF reading of the electrochemical cell 34 as expressed by the formula EMF 0.0538 log 0.209/P Sample where P sample is the fraction oxygen in burned air. The fraction of excess air can be mathematically determined from the ratio of the combustibles to air in the original mixture by the formula X DP pSampIe /0.209 P Sample where D, the expansion ratio of nonfuel gases, is l 0.209b/4 b where b is the ratio of hydrogen to carbon in the fuel.
When molecular oxygen is not present in the products of combustion, the electrochemical cell 34 responds to the small amount of oxygen produced by the dissociation of carbon dioxide and water at a high oper ating temperature to measure the gaseous fuel remaining in mixture with the combustion products. The amount of disassociated oxygen is determined by the amount of hydrogen and/or carbon monoxide present within the analyzer chamber 18 and will, therefore, indicate the ratio of water plus carbon dioxide to hydrogen plus carbon monoxide. The partial pressure of the disassociated oxygen is measured by the EMF produced across the electrodes and is mathematically determined by the following formula:
P02 l [PHZO l Fi th/[71 2+ p '0] l/log 18.1 (combustion products/unburned fuel gases) 2 Accordingly, the fraction of unburned combustible fuels that remains mixed with the combustion products in excess of the stoichiometric amount set in the original mixture of fuels fed to the combustion chamber is determined from the following mathematical relationships:
A N (1 b/2)/N 2 b/2 excess fractin of fuel where N log (E-0.9875/0.1075) unburned fuel gases/combustion products The percent of combustibles remaining in the com pletely burned fuel mixture can then be obtained once the fraction of unburned combustibles has been recorded from the following formula:
Further in accordance with the invention described above, FIG. 2 of the drawings illustrates a second em bodiment to be incorporated with operation of the combustion mixture analyzer in which the burning properties of the fuel mixture are continuously monitored and controlled for stoichiometric combustion with oxygen in the analyzer chamber. Initially, a fuel mixture having known combustion characteristics is supplied from a gas cylinder 62 for mixture with air delivered from a source 64 to produce stoichiometric combustion. The combustible mixture of gaseous fuel and air passes through conduit 66 and enters the flame arrestor 54 from which the mixture is conveyed by conduit 56 to the analyzer chamber 18, as illustrated in FIG. 1 and described hereinabove.
A sample fuel mixture having a variable composition in which the combustion characteristics thereof are unknown is drawn from the feed line (not shown) and is directed to a gas main 68. The sample mixture passes through the selector valve which is operable to provide for one way flow of fuel mixture from either conduit 68 or conduit 72 to conduit 74. The sample fuel mixtures of conduit 68 pass through the selector valve 70 closed to conduit 72 and are conveyed by conduit 74 to a suitable flow regulator 76 schematically represented in FIG. 2. The regulator 76 is calibrated to pro vide for the passage of fuel mixtures, having a known density, at a constant volumetric rate of fluid flow to produce stiochiometric combustion with air in the analyzer chamber. From the regulator 76, the fuel mixture is conveyed by conduit 78 to conduit 82 for eventual combination in conduit 66 with air fed through conduit 84.
When the selector valve 70 is closed to conduit 72, a sample fuel mixture from the main 68 flows through the valve 70 to conduit 74 and the regulator 76. If the sample composition has a higher density than the fuel mixture supplied by cylinder 62 required to achieve stoichiometric combustion, the regulator 76 responds to the increase in density by directing the fuel mixture through conduit 86 to the throttle valve 88. The valve 88 then operably provides for a reduced volumetric rate of flow of gaseous fuel having a greater composition density for mixture with air in conduit 66. Accordingly, the valve 88 will increase the volumetric rate of flow for a fuel mixture having a lower composition density than the fuel mixture density initially calibrated to produce stoichiometric combustion.
When the products of combustion of the combustible mixture of fuel and air in the analyzer chamber 18 includes unconsumed oxygen supplied by the constant air supply, the excess oxygen will be quantitatively detected by the electrochemical cell 34. In cases where the air supply is not sufficient to completely burn the sample fuel mixture, excess fuel will be quantitatively detected in the cell 34. The apparatus then determines the following ratio:
K, air demand/unit volume/ V density K, represents the ratio of air required for stoichiometric combustion of the fuel mixture sample of variable composition in comparison with the fuel mixture of known composition and combustion characteristics. For hydrocarbon fuels, a similar index is provided by the Wobbe index which is the ratio of the gas heating valve to the square root of the gas density.
The constant flow regulator 90 is operably positioned in fluid communication with conduits 92 and 94 to provide for constant flow of air from the source 64 for mixture with the fuel mixture in conduit 66. Accordingly, a variation in the flow of air to produce either an excess or deficiency thereof in mixture with the fuel creates a deviation from stoichiometric combustion in the analyzer chamber 18. Therefore, the throttle valve 96 is inserted in conduit 98 to increase or decrease the air flow rate to maintain a consant rate.
With this arrangement the analyzer measures relative ratios of combustible materials to oxygen as compared to that ratio that produces stoichiometric combustion for both oxygen-rich and fuel-rich mixtures. The apparatus may be utilized in a wide variety of combustion applications where it is desirous to both monitor and control or detect combustion mixtures and, with accessory equipment, for metering fuel gas and air to produce a combustion mixture and to characterize a fuel gas. The readout devices may be provided with a range of zero to one thousand millivolts for instantaneously determining excess air, stoichiometric ratios and excess fuel for combustion processes before the combustible fuels and the oxygen in air mixtures are fed into the combustion chamber and also to determine dangerous levels of combustible vapors and gases in air.
According to the provisions of the patent statutes, the principal, preferred construction and mode of operation of the invention have been explained and what is now considered to represent its best embodiment has been illustrated and described. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than is illustrated and described.
1. A combustion mixture analyzer comprising,
an electrochemical cell having a closed end tube of solid electrolyte mounted in a tubular housing, a pair of electrodes secured to said electrochemical cell, one electrode secured to the inner surface and one electrode secured to the outer surface of said closed end tube,
said tubular housing having a chamber therein for analyzing separately the quantities of a fuel and oxygen admixed to form a combustible mixture, said electrochemical cell positioned axially in said chamber,
said electrochemical cell arranged within said cham her to extend from one chamber end portion to an intermediate position within said chamber,
said chamber having a reduced diameter end portion spaced from said electrochemical cell and extending through the said tubular housing,
conduit means having an inlet and outlet opening for supplying a continuous flow of the combustible mixture to said chamber,
flame arresting means located at the inlet to said conduit means for preventing explosions of the combustible mixture supplied to said conduit means,
metering means in said conduit means for measuring the volumetric rate of flow of the combustible mixture passing through said conduit means,
heating means for heating said electrochemical cell to a preselected temperature, and
means for controlling said heating means.
2. A combustion mixture analyzer as set forth in claim 1 in which,
said heating means is also arranged to heat said tubular housing chamber and thereby heat said combustible mixture above the ignition temperature in said chamber so that the combustion of the fuel and oxygen reaches complete chemical equilibrium.
3. A combustion mixture analyzer as set forth in claim 1 which includes,
a second conduit means having an inlet and outlet opening for supplying a continuous flow of air to said chamber,
said fuel and said air continuously transported-by said first and second conduit means respectively to said chamber to form a combustible mixture therein.
4. A combustion mixture analyzer as set forth in claim 3 in which said first and'said second conduit means includes,
a regulatingmeans for maintaining a substantially constant rate of fluid flow through said first and said second conduit means so that the quantity of the air required to be admixed with said fuel to produce combustion of said fuel in said chamber is continuously measured by said readout device,
said regulating means arranged in fluid communication with said first and said second conduit means.
5. A combustion mixture analyzer as set forth in claim 1 in which,
said outlet for said conduit means provides for the passage of said second gas from said chamber. l