US3123295A - Means for analysing combustion products - Google Patents

Description

March 3, 1964 A. E. MARTIN 3,123,295

MEANS FoR ANALYSING COMBUSTION PRODUCTS AND VARYING AIR FUEL RATIO Filed Dec. 15, 1957 l l l I I l l a 28 2K tizia /5 /6 /4 /z /a a 6 4 Awe rd va/Way United States Patent O 3,123,295 MEANS FOR ANALYSING CGMBUSTION PROD- UCTS AND VARYING AIR-FUEL RATIO Albert E. Martin, Newcastle-upon-Tyne, England, assignor to Sir Howard Grubb Parsons & Company Limited, Newcastle-upon-Tyne, England Filed Dec. 13, 1957, Ser. No. 702,701 Claims priority, application Great Britain Jan. 2, 1957 1 Claim. (Cl. 236-15) It is well known that when any carbonaceous fuel is being burnt in a furnace or other equipment for producing heat or power, optimum performance is obtained by careful adjustment of the air/fuel ratio. Lf the air supply is limited complete oxidation of the combustible material is not attained and, on the other hand, if excess air is employed combustion is complete but the products of combustion -are diluted and heat is Wasted in heating the excess air.

Given a reliable CO2 meter, conditions can be varied until CO2 is obtained in the exhaust gas and this will correspond approximately to optimum working conditions.

An alternative is to determine CO, for which purpose `an infra red gas analyser is convenient, and then one operates close to zero CO. Yet another method is to mea-sure the temperature of the burning gas, but this is diicult since the temperature is generally very high and the value will only be useful if the rate of fuel consumption is kept constant.

All of these methods suifer from the disadvantage that the optimum working point tends to correspond to either a maximum (CO2 and temperature) or zero (C0). 'I'hus there is no continuous indication throughout a useful air-fuel ran-ge on both sides of the optimum value. If for example the CO2 meter indicates 12% CO2, one may be operating on either side of the optimum value and as the percent CO2 falls on both sides of the optimum value it is not obvious whether the air or the `fuel supply should be increased. Consequently it is not normally possible to use automatic control of air/fuel ratio to maintain optirnum combustion efficiency.

The object of the present invention is to provide means whereby this diiliculty may be overcome and a continuous indication obtained.

In the combustion control of the present invention the concentration of CO and CO2 in the products of combustion are added in certain proportions such that the summation increases as the rair/ fuel ratio decreases, said air/ fuel ratio being varied automatically in accordance with variation in the value of said summation.

The invention provides combustion control means comprising an infra-red gas analyser through which the products lof 'combustion are passed, the analyser being made sensitive to carbon monoxide and carbon dioxide and relatively insensitive to the other gases present, the output signal of the analyser being made proportional to a value (CO2),}-k(CO) where (CO2) and (CO) represent the concentrations of those gases in the combustion products and k is a constant appropriate to the combustion equipment in use, the value of k being chosen so that (CO2),-}-k(CO) increases as the air/fuel ratio decreases, thearrangement being that variation of the analyser output because of variation of the value (CO2)H-k(CO) from a predetermined value is used to vary the air/fuel ratio until such time as the original value is restored.

Referring to FIGURE 1 the curves shown are representative of those obtained when burning heavy fuel oil in a combustion chamber.

It will 'be seen that the optimum working point P tend-s to correspond to either maxi-mum CO2 and maximum 2 temperature, or minimum CO. Thus, if, for example, it were found by means of a CO2 meter that there is 12% CO2 present one may be operating at either point Q or point R and it is not obvious whether the air or the fuel should be increased.

The present invention overcomes this difficulty by arranging for the output of an infra-red gas analyser for analysing the products of combustion, to be marde proportion to a value (CO2);}(CO) where (CO2) and (CO) represent the concentration of those gases and k is a constant appropriate to the particular combustion equipment 1n use.

yBy way of example values `from FIGURE 1 will be considered and a value of (CO2);-ik(CO) -determined in each case using a value yfor k of 1.5.

Air to fuel, lb./lb 28 19 I 14 5 12 8.5 3.5

CO2 (percent).-. 8 12 15. 5 12 8 4 C O (percent) 0 0. 5 5. 5 10. 5 16. 5 COM-1.5 CO-- 8 l2 1GA 25 20 24 29 at this value and (CO2) at 15.5%. Il. (CO2)+1.5\(CO) drops below 16.25, air/fuel is automatically reduced while (CO2)+1.5(CO) rises above this value air/fuel is increased.

A preferred orm of apparatus vfor carrying out the invention is described with reference to FIGURE 2, in which infra-red radiations from heaters A, A pass through absorption tubes, B, C, D, into receiveing chambers E, E, said tubes and chambers being provided with windows suitable for the transmission of desired radiations. The chambers E, E communicate with a central compartment divided into two parts by an electrode of thin metal foil F and adjacent to F is a perforated insulated electrode G so arranged that a small deformation of the metal foil electrode lgives rise to a change .of capacitance between F and G. The chambers E, E and central compartment contain a mixture of CO2 and CO so that wavelengths close to 4.25 and 4.7;1. are absorbed from the incident radiations by these gases respectively. Assume at iirst that tubes B, C and D contain no absorbing gas so that the maximum amount of energy is absorbed by the CO/ CO2 mixture in chambers E, E.

A rotating shutter S allows radiation to pass intermittently but simultaneously through tubes B and C, finally to enter chambers E, E so that each time the shutter opens energy is absorbed by the gas in E, E and the pressure increases steadily so long as radiation is passing through the gas, falling away again when the radiation is shut off and the gas cools. Since, however, the pressure variationsl operate simultaneously on opposite sides of the flexible diaphragm, provided that by means of an adjustable shutter the pressure variations are made equal, the forces on opposite sides of the diaphragm will balance and no deformation of the diaphragm will occur.

If now, however, a sample gas from the combustion chamber is passed into tube D any CO or CO2 present will absorb energy and therefore diminish the quantity entering the corresponding chamber E. Every time shutter S opens a pressure pulse will therefore deform the diaphragm F which will consequently vibrate at the frequency of interruption of the shutter with an amplitude dependent on the energy absorbed in tube D, and the resulting variation in capacitance is caused by known means to produce a small alternating voltage which is ampli* ed by a conventional amplifier H. The rectied and smoothed signal from H is measured with a meter M which is provided with two contacts P and Q placed respectively a little below and a little above the desired operating point.

A minute gas leak is provided across diaphragm F between chambers E, E, so that although the leak is too small to cause any appreciable reduction in sensitivity of the detector, long term pressure dierences to temperature effects or unequal gas adsorption are equalised and do not deform the diaphragm.

Since CO2 absorbs more intensely than CO it is necessary to reduce the sensitivity of the instrument to CO2 relative to CO so that the desired sensitivity ratio can be obtained. This result can be achieved in various ways but a convenient method is to fill or partially fill tube C With CO2 and seal it oit so that the subsequent absorption by CO2 in the sample gas is reduced to the required level.

It is preferable also partially to ll tube B with CO2 to counter-balance the absorption in tube C, since the stability of the instrument is thereby improved.

supposing now that (CO2)}-k(CO) (where k=1.5) increases above the correct operating value, the output as shown by meter M will rise and contact with Q will be made, so completing the circuit from a battery through a solenoid-operated valve V1, which is arranged to increase the flow of air into the combustion chamber and so reduce (CO2){-k(CO). Conversely, if

(COQ-FMCG) falls below the correct operating value, contact P will be made and a solenoid-operated valve V2 will ettect an increase in the ilow of fuel.

This particular mode of operation is only given by way of example and other methods, e.g. those involving selfbalancing gas anaiysers and/or proportional controllers can be employed.

The essential feature of the form of invention described and illustrated in FIGURE 2 is the utilisation or a gas analyser sensitive to the presence of CO and CO2, but insensitive to all other gases present, and with provision for the ratio of sensitivities to CO and CO2 to be adjusted to the value necessary for the control of a particular piece of combustion equipment, a convenient means of providing said adjustment being through the inclusion in the optical path containing the sample Vgas of a lter tube containing a xed quantity of CO2.

While the use of an infra-red gas analyser sensitive to CO2 and CO obviously has attractive features, it is quite possible to use two separate infra-red gas analysers, one for each gas and electrically to combine the two outputs in the desired proportions, or even to use quite other forms of gas analyser.

An alternative form of infra-red gas analyser might depend on the radiations emitted from CO2 and CO in the iiame resulting from the combustion process, since it is well known that gases when heated emit radiations at wavelengths close to those capable of being absorbed at normal temperatures. To detect these high temperature radiations a detecting condenser of the type already described and iilled with CO2, CO or a mixture of the two gases, can be employed, while another possibility is to employ optical lters which can now be produced by known means to isolate narrow spectral regions appropriate to the gas to be determined, eg. 4.25 for CO2 and 4.7;@ for CO, and then to utilise a non-selective or partially selective detector such as a thermocouple, bolometer, semiconductive cell, photoelectric cell or magnetophotoelectric cell to measure finally the radiations falling on said detector.

A similar arrangement can be employed as an absorptiometer wherein radiations emitted by an infra-red source at, say, 700 C. are passed through an isolating lter, absorption cell containing the gas to be analysed and are then made to impinge, preferably after optical condensation, on to the detector. In place of an infra-red iter, it may be possible in certain cases to substitute inown radiation dispersive systems employing prisms, diraction gratings, or a combination of the two, to isolate a desired spectral region.

I claim:

In combustion control means for the analysis of combustion products and varying the air/fuel ratio in response to variation of the carbon monoxide and carbon dioxide content thereof, an infra-red gas analyser for the measurement of the carbon monoxide and carbon dioxide content, the output signal of said analyser being proportional to a value (CO2)-l- (CO) Where (CO2) and (CO) represent concentration of those gases in the combustion products and k is a constant appropriate to the combustion equipment in use, the value of k being chosen so that (CO2)-{k(CO) increases the air/fuel ratio decreases, the arrangement being that variation of the value (CO2)}k(CO) from a predetermined value is used to vary the air/fuel ratio until such time as the original value is restored, in which the gas analyser comprises an infra-red gas analyser having two beam paths one containing the sample to be analysed and the other a comparison substance the beams of radiation leaving each path falling on a detecting unit having a detection chamber in each path containing a mixture of carbon monoxide and carbon dioxide, the two detecting chambers being separated by a thin metal diaphragm adjacent an electrode such that any difference in pressure between the chambers produces an electric signal, a sealed tube being introduced into the sample path containing a predetermined quantity of carbon dioxide to reduce the absorption of carbon dioxide in the sample path to a level such that the output signal from the detecting unit is proportional to a value (CO2) +k(CO) References Cited in the le of this patent UNITED STATES PATENTS 1,591,444 Stein luly 6, 1926 1,645,350 Reineke Oct. 11, 1927 1,770,489 Loiiler July 15, 1930 2,269,674 Liddel et al. jan. 13, 11942 FOREGN PATENTS 496,436 Germany Apr. 23, 1930 500,700 Germany June 24, 1930 645,576 Great Britain Nov. 1, 1950 698,023 Great Britain Oct. 7, 1953

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