FR2489521A1 - DEVICE COMPRISING AN OXYGEN DETECTOR - Google Patents

Abstract

DEVICE INCLUDING AN OXYGEN DETECTOR FOR COMBUSTION REGULATION. THE OXYGEN DETECTOR 15, EQUIPPED WITH A SOLID ELECTROLYTE CELL, IS PROVIDED IN THE BURNED GAS CURRENT FROM A RADIANT TUBE AT SLEEVE 1, IN A TEMPERATURE RANGE CORRESPONDING TO ITS OPERATING TEMPERATURE. MEANS ARE PROVIDED FOR DIRECTING A FLOW OF SAMPLE GAS THROUGH THE OXYGEN DETECTOR. APPLICATION TO THE REGULATION OF COMBUSTION, ESPECIALLY IN INDUSTRIAL FURNACES.

Description

The present invention relates to a device comprising a detection

  oxygen generator for controlling combustion in an industrial furnace

  triel. For reasons of economy of fuel and environmental protection, efforts are made to obtain as good use

  as possible fuel in industrial furnaces, including

  is produced by combustion, the fuel / air ratio (comburivore A power) exerting a decisive influence on the efficiency and the emission of pollutants. Variations in the calorific value, for example, can strongly influence the proportion of the mixture. To avoid operation with a rich mixture, the combustion is carried out with a relatively large excess of air, which, however, results in a reduction of the yield as a result of the increase in thermal losses by the nitrogen and oxygen ballast in the burnt gas. The yield also decreases in the case of a rich mixture, because the combustion is incomplete and produces soot and oxide of

carbon in certain conditions.

  The maximum yield and the minimum emission of pollutants are

  with slightly over-stoichiometric combustion,

  to say with a very small excess of air.

  The measurement of the oxygen content of the gases burned using

  appropriate oxygen sensors allow precise regulation of the

fuel / air port.

  Two different models of oxygen detectors are used.

  In a detector comprising a potentiometric cell, the sensibility

  High thermal sensitivity and decreasing sensitivity as the oxygen concentration increases exert a detrimental influence on the measurement method. However, an oxygen sensor comprising a galvanometric cell is more advantageous. Its precision

  measurement is virtually independent of temperature and

  oxygen concentration. Relatively simple construction means make it possible to obtain the continuous sample gas flow required for the measurement, whereas the thermal stabilization of the first oxygen detector cited imposes a relatively important apparatus.

in electronic control.

  The patent application of the Federal Republic of Germany

  published under No. 29 22 218 describes an oxygen / fuel

  tible, comprising an oxygen sensor operating with a

  potentiometric lule and therefore having the aforementioned drawbacks.

  The aim of the invention is to integrate in a simple manner an oxygen detector.

  a gene comprising a galvanometric measuring cell in a device

  control of combustion in industrial furnaces.

  According to an essential characteristic of the invention, the detection

  The oxygen generator, equipped with a solid electrolyte cell, is arranged in the flue gas stream of a sleeve-type radiant tube.

  a temperature range corresponding to its operating temperature.

  And means are provided for directing a stream of sample gas

in the oxygen sensor.

  According to another characteristic of the invention, the oxygen sensor is surrounded by a layer of defined porosity, through

  which a given flow of gas diffuses in the form of a current

tinu sample gas.

  According to another advantageous characteristic of the invention, a nozzle with a critical pressure ratio supplies the oxygen sensor.

  with a continuous stream of sample gas.

  The ratio of critical pressures is the ratio of the

  pressure p. at the final pressure p1 (p / P1), at the speed of sound.

  When the backpressure pC (downstream of the nozzle) decreases, the speed no longer increases in the weakest section, so that the flow

remains constant.

  According to another characteristic of the invention, the useful section of the sleeve radiating tube is divided so that the gas flow

  sample supplying the oxygen sensor is directly proportional

total gas flow.

  The oxygen sensor processes only a limited amount of oxygen and the corresponding flue gas stream must come into intimate contact with the sensor surface. Only part of the gas stream

  burned is then directed to the oxygen sensor.

  For a definite ratio of the offered sections of the detector and

  of the flue gas channel, the gas flow in the detector is directly

  proportional to the total gas flow. In order to obtain a signal corresponding to the percentage of oxygen, it is necessary to establish a proportion between the "gas flow in the detector" signal and the total gas flow, that is to say to divide the first signal by a signal proportional to the total gas flow, using a

microprocessor for example.

  The advantages of the invention lie in particular in the possibility

  to remove the separate electric heater, which has been necessary until now to establish the detector's operating temperature

  of oxygen, as well as the pump required for the production of a

  continuous stream of sample gas. The device thus presents a

  simpler constitution and less sensitivity to disturbances.

  Other features and advantages of the invention will be

  better understood using the detailed description below of

  some exemplary embodiments and the accompanying drawings in which Figure 1 is the longitudinal section of a sleeve radiant tube containing an oxygen sensor; FIG. 2 represents a detail of an oxygen detector comprising a body of defined porosity;

  FIG. 3 schematically represents the arrangement of the two sec-

  useful features of the oxygen sensor. radial tube with sleeve; Figure 4 shows a critical pressure ratio nozzle; and FIG. 5 represents a butterfly corresponding to the nozzle according to

figure 4.

  The radiating tube with a sleeve 1 of refractory steel contains at one end a coaxial gas burner 2, the end of the flame side of which penetrates into a tube-hearth 3 also coaxial inside the radiant tube 1. The flame burns in the 3, while the flue gases escape at the end of the latter, flow into an annular space 4 between the radiant tube 1 and the tube-hearth 3 and in an annular space 5 between the burner 2 and the radiant tube 1, then are discharged through a pipe 6. The sleeve radiating tube enters through an opening in the wall 7 of the oven, inside the latter, over a length of about 2 m maximum, and is connected by a seal 8 with flanges, with gas tightness, the envelope 9 of the oven. The burner head 2 carries the gas supply valve 10 and the air supply valve 11. 1 The annular space 5 between the burner 2 and the sleeve radiating tube 1 is slightly widened in the passage zone of said tube in the wall 7 of the oven, and filled in large part by an insulating lining 13, applied on the inner face 14 of the radiating tube turned outwardly, so that the flue gases flowing in the residual annular space 12 flow on the insulating lining 13 and not

  thus have more direct thermal contact with the radiant tube.

  The oxygen sensor 15 housed in the sleeve radiant tube may be surrounded by a porosity layer defined for supplying a continuous stream of sample gas. It is however possible

  also for the same purpose of embedding the oxygen sensor in a gar-

  insulating nissage 13 of defined porosity, as shown in Figure 1.

  The defined porosity layer can also be produced according to FIG. 2 in the form of a porous cylindrical body 16, inserted into the protective tube 17 of the oxygen sensor 15, in the region of the tube

solid electrolyte 18.

  Another exemplary embodiment for the production of a sample gas stream is described below with reference to FIGS. 4 and 5. The throttle valve 19 housed in the flue gas pipe 6 is adjusted so as to produce an upstream gas flow. The pressure is greater than 0. 2 MPa so that a critical pressure ratio for the nozzle 20 having sectional widening in the direction of flow is established with respect to the atmosphere. The flow is at the speed of sound. The speed no longer increases in the minimum section when the pressure decreases downstream of the nozzle, so that the flow remains constant. It is furthermore possible to arrange the oxygen sensor 15 according to FIG. 3 in the annular space 12 of the sleeve radiating tube, so that the useful section A1 of the annular space 12 and the useful section A2 of the detector oxygen have a ratio such (A1 / A2 = constant)

  that the oxygen sensor 15 is supplied with a flow of gas

  directly proportional to the total gas flow. The regulation requires in this case a signal proportional to the total flow and which provides, with the signal of the oxygen sensor and after formation of the ratio,

  a signal proportional to the percentage of oxygen.

  The regulation is not essential for the invention and is therefore not represented. The function is as follows: after connecting the device, for example by closing a signal contact "open gas valve", a delay element starts and blocks the measurement and control operation until that the combustion of the flame is stable and that the fumes reach the oxygen sensor (dead time). After the idle time, the signal

  delivered by the oxygen sensor (current proportional to the

  percentage of oxygen) is compared during a second time (measurement time) to a reference value, to determine the direction of the deviation (rich or poor) and its amplitude (between tolerance limits, fine or coarse). A relay corresponding to the direction of the deviation is activated - during the measuring time, for a duration corresponding to the amplitude of the deviation. At the end of the measurement time, the delay element is

  activated and the entire measurement operation is repeated.

  Of course, various modifications can be made by those skilled in the art to the principle and to the devices which have just been described as non-limiting examples, without departing from the

framework of the invention.

Claims (4)

claims   1. Device comprising an oxygen sensor for controlling the combustion in an industrial oven and characterized in that the oxygen sensor (15), provided with a solid electrolyte cell, is disposed in the flue gas stream. a radiant tube with   chon (1), in a temperature range corresponding to its temperature   service station; and means are provided for directing a current   of sample gas in the oxygen sensor.   2. Device according to claim 1, characterized in that the oxygen sensor is surrounded by a layer of defined porosity,   through which a given flow of gas diffuses in the form of a continuous stream of sample gas.   3. Device according to claim 1, characterized in that a nozzle (20) with a critical pressure ratio feeds the oxygen sensor   with a continuous stream of sample gas.   4. Device according to claim 1, characterized in that the sec-   of the sleeve-type radiant tube (1) is divided so that the stream of sample gas supplying the oxygen sensor is   directly proportional to the total gas flow.