MONITORING COMBUSTIBLE MATTER IN A GASEOUS STREAM

Abstract

A method and device for monitoring combustible matter in a hot gaseous stream and generating a control signal, a controlled jet of an oxidant is injected into the gaseous stream with a lance extending between a window of a monitoring device and the flow path of the gaseous stream, the lance defining a line of sight between the window and the gaseous stream in the flow path, the combustible matter burns with the oxidant in a flame in the gaseous stream in front of the lance, one or more properties of the flame which are correlated with the concentration of combustible matter in the gaseous stream are detected by the monitoring device through the line of sight and the window and the monitoring device processes the one or more detected flame properties and generates a control signal on the basis of the one or more detected flame properties.

Claims

1.-15. (canceled)

16. A method for monitoring combustible matter in a gaseous stream by means of a monitoring device, whereby the gaseous stream flows at a temperature of at least 550? C. along a flow path extending from a gas inlet to a gas outlet, the method comprising: extending a lance between a window of the monitoring device and the flow path, said lance defining a line of sight between the window and the gaseous stream in the flow path, injecting a controlled jet of an oxidant having an oxygen content of 22 to 100% vol into the gaseous stream by means of the lance so that, in the presence of combustible matter in the gaseous stream, said combustible matter burns with the oxidant in a flame in the gaseous stream in front of the lance, detecting one or more properties of the flame which are correlated with the concentration of combustible matter in the gaseous stream by the monitoring device through the line of sight and the window, and processing the one or more detected flame properties in the monitoring device and generating a control signal on the basis of said one or more detected flame properties.

17. The method according to claim 16, whereby the controlled jet of oxidant is injected in the lance at a flow between 0.1 Nm.sup.3/h and 50.0 Nm.sup.3/h.

18. The method according to claim 16, whereby the lance is such that the line of sight forms an angle with main gaseous stream direction between 5? to 175?.

19. The method according to claim 15, whereby the lance extends into the flow path from 0% to 50% of the cross-section diameter of the flow path.

20. The method according to claim 15, whereby the control signal is communicated to an upstream installation generating the gaseous stream and used as a control parameter therein and/or is communicated to a downstream gas treatment installation and used as a control parameter for the control of said installation.

21. The method according to claim 15, whereby the monitoring device detects visible and/or non-visible radiation intensity of the flame.

22. The method according to claim 15, whereby the gaseous stream is a combustion flue gas stream from a combustion installation.

23. The method according to claim 22, whereby the combustion installation is selected from the group consisting of non-ferrous metal melting furnaces, iron and iron-alloy melting or remelting furnaces, furnaces for recovering metal from electronic waste, cement or lime kilns, waste incineration kilns or shaft furnaces, electric arc furnaces and boilers, non-ferrous metal melting furnaces, iron or iron alloy melting or remelting furnaces and furnaces for recovering metal from e-waste.

24. The method according to claim 22, whereby the control signal is communicated to the upstream combustion installation and used to adjust the combustion stoichiometry in the combustion installation.

25. The method according to claim 22, whereby an oxidant having an oxygen content of 22 to 100% vol is supplied from an oxidant source to the combustion installation as combustion oxidant, and whereby oxidant from the same oxidant source is injected by means of the lance into the gaseous stream as the controlled jet of oxidant.

26. A device for monitoring combustible matter in a gaseous stream, the device comprising: a lance having a first end near a window of the monitoring device and an open second end pointing away from said window, the lance defining a line of sight between the window and the open second end of the lance, the lance presenting an oxidant inlet at or on the side of the first end of the lance, a sensor located behind the window and capable of detecting, through the lance and the window, one or more properties of a flame located in front of the second end of the lance, and, a processing unit programmed to process the one or more detected flame properties and to generate a control signal on the basis of the one or more detected flame properties.

27. The device according to claim 26, further comprising a transmitter for transmitting the generated control signal.

28. The device according to claim 26, whereby the sensor is capable of detecting visible and/or non-visible radiation intensity.

29. The device according to claim 26, further comprising a thermocouple positioned to detect a temperature at or adjacent to the second end of the lance, the processing unit being in data communication with said thermocouple and receiving the temperatures detected by the thermocouple.

30. The device according to claim 29, whereby the thermocouple is positioned within the lance or within a tube adjacent the lance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The present invention and its advantages are described in more detail in the following non-limiting examples, reference being made to FIGS. 1 to 4, whereby:

[0056] FIG. 1 is a schematic representation of an installation comprising a combustion furnace and a monitoring device for use in accordance with the present invention,

[0057] FIG. 2 is a schematic representation of monitoring equipment suitable for use in accordance with the invention,

[0058] FIG. 3 is a schematic representation of monitoring equipment suitable for use in accordance with the invention, and

[0059] FIG. 4 is a schematic representation of monitoring equipment suitable for use in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0060] The melting process illustrated in FIG. 1 is carried out in a so-called Short Rotary Drum Furnace (SRF) 1.

[0061] A metallic load 2 to which additives, such as for example coke, iron, fluor, etc., have been added, is charged into the furnace 1.

[0062] A naked flame 7 is produced inside the furnace 1 to heat the load 2 directly and to cause the melting-down of the metal or metals contained in the load 2. The obtained liquid metal phase is tapped and directed to subsequent metallurgical processing steps (not shown).

[0063] The SRF 1 is charged with metallic load 2 via front doors in the longitudinal axis of the furnace drum. Exhaust fumes leave the furnace through a rear opening of the furnace drum on the same axis. Fumes drawn from the SRF 1 are collected in a vertical off-gas channel 3 directly connected to the rear opening prior to being redirected to a cyclone (not shown) and a further fume filtration device (not shown). SRF 1 is fitted with one water-cooled oxy-gas burner 4 of 3MW nominal power. Natural gas 5 and oxygen 6 are injected from the burner tip to create flame 7 inside the furnace 1.

[0064] The illustrated process may, for example, be used for tin melting. In other words, the load which is charged into furnace 1 is a tin load.

[0065] Nominal natural gas flow varies between 250 and 300 Nm.sup.3/h. Nominal oxygen gas flow varies between 450 and 700 Nm.sup.3/h. Nominal off-gases flow is estimated to vary between 2300 and 2700 Nm.sup.3/h and the off-gases nominal temperature between 1100 and 1500? C. CO content in the off-gas varies dynamically during the melting process between 0 up to 30% by volume.

[0066] To detect and quantify the CO content in the off-gases, a monitoring device 9 is installed at the one end of a lance 8, which is mounted on off-gas channel 4 outside the furnace drum. The second extremity of lance 8 is open and located inside the stream of off-gas in such a way that the monitoring device 9 can observe the off-gases stream through lance 8. A controlled flow of substantially pure oxygen (at least 99% by volume oxygen) at between 1 to 30 Nm.sup.3/h at ambient temperature is injected through lance 8 towards and into the off-gas stream. Thereto lance 8 is connected to an oxygen source, in this case oxygen reservoir 12, though the oxygen source could also be an oxygen pipeline or an Air Separation Unit.

[0067] The oxygen thus injected into the off-gas stream causes the CO contained in the off-gas to burn and creates a further flame 10 proportional in dimension, temperature and radiation to the quantity of CO contained in the off-gases.

[0068] So as to avoid any misunderstanding, the term flame as used herein includes both visible flames and diluted flames, also referred to as flameless combustion.

[0069] The monitoring device 9, while observing the flame 10 through the lance 8 captures at least one of these flame properties (dimension, temperature and radiation) and generates a control signal proportional to the quantity of CO contained in the off-gas. The generated control signal is transmitted to a digital data processing unit 11, such as a programmable logic controller (PLC), for data processing. The processed data are used as a control signal to adjust the ratio of oxygen and natural gas injected through burner 4 in accordance with the method described in WO2010/022964. In this manner, the time-lapse between the detection of the one or more flame properties of flame 10 and the adjustment step in furnace 1 is kept very short (below 10 seconds, preferably below 4 seconds or even below 3 seconds).

[0070] By thus optimising the operation of furnace 1 and keeping the presence of combustible matter, such as CO, in the off-gas stream to a minimum, savings in specific natural gas consumption up to 20% can be achieved.

[0071] FIG. 2 shows a monitoring unit suitable for use in the present invention.

[0072] The monitoring unit comprises a monitoring device 9 with a window 91 through which one or more properties of a flame located in front of window 91 may be detected by monitoring device 9.

[0073] An injection lance 8 extends forward from window 91. At the end of lance 8 towards monitoring device 9 (the upstream end of lance 8), lance 8 is provided with an oxygen inlet 81 via which oxygen can be fed to lance 8 at a controlled rate. The opposite extremity 82 of lance 8 (downstream end) is an open end. Lance 8 is straight, with no internal obstructions, so that lance 8 defines a clear line of sight between window 91 of monitoring device 9 and the open end 82 of lance 8. Lance 8 is further provided with a flange 83 with which the monitoring unit can be mounted on a gas-flow duct, such as an off-gas channel of a combustion installation.

[0074] When flange 83 is mounted on a gas-flow duct through which a hot gas stream flows and a controlled flow of oxygen is supplied to lance 8 via oxygen inlet 81, said oxygen is injected into the hot gas stream as a controlled oxygen jet. When the hot gas stream contains combustible matter, a flame is generated where the combustible matter meets the oxygen of the controlled jet. Via the line of sight through lance 8 and window 91, monitoring device 9 detects one or more properties of said flame. For example, the monitoring device 9 may be constructed and programmed to detect the radiation intensity of said flame at the wavelength corresponding to CO combustion. The detected intensity is therefore an indication of the concentration of CO in the gas stream. Thereafter, monitoring device 9 generates a control signal based on the detected radiation intensity.

[0075] Alternatively, or in combination with said radiation intensity, monitoring device 9 may detect, by way of flame property, IR (infrared) radiation from the flame and use the detected IR radiation to generate the control signal.

[0076] The monitoring unit of FIG. 3 differs from the monitoring unit of FIG. 2 in that it comprises, in addition to lance 8, a further oxygen tube 100 in which a thermocouple 101 is mounted. The thermocouple 101 is in data communication with monitoring device 9. The controlled flow of oxygen which is fed through oxygen inlet 81 is divided between lance 8 and tube 100 so that each injects a separate controlled jet of oxygen into the hot gas stream and when combustible matter is present in said gas stream, a flame is generated in the gas stream where the combustible matter comes into contact with the injected oxygen. Via the line of sight in lance 8 and window 91, monitoring device 9 detects one or more flame properties as described above. In addition, via thermocouple 101 in tube 100, monitoring device 9 further detects a flame temperature which may be used as a further input for monitoring device 9 to generate the control signal, as a possible backup input for generating the control signal or as a safety check to verify the prima facie correctness of the flame property or properties detected via the line of sight and window 91.

[0077] The monitoring unit of FIG. 4 differs from that shown in FIG. 3 in that tube 100 is not fluidly connected to oxygen inlet 81 of lance 8. In that case, tube 100, in which the thermocouple is positioned, may be separately supplied with a controlled jet of oxidant gas, which is subsequently injected into the gas stream. Alternatively, a relatively small amount of sweeping gas may be made to flow through tube 100 in which the thermocouple is positioned or no gas at all.

[0078] According to a non-illustrated embodiment, a thermocouple may be positioned within lance 8.

[0079] The method according to the present invention was used to monitor combustible matter in a stream of flue gas evacuated from a lab-scale furnace.

[0080] The flue gas temperature was greater than 1000? C. The carbon monoxide content in the flue gas varied from 0 to 12% vol.

[0081] The oxidant used was pure oxygen.

[0082] The monitoring unit was mounted onto the flue-gas duct of said furnace. The monitoring device was equipped with a photosensor located behind the window of the monitoring device of the unit. The photosensor was capable of measuring radiation intensity in the visible and IR range. Thus, when a flame was generated at the downstream end of the oxygen lance defining a line of sight between the window and said downstream end, monitoring device detected the intensity of the flame radiation in the visible and IR range via said line of sight and window.

[0083] In a first set of tests, the oxygen was injected at different flow rates into a gas stream in the temperature range of the furnace flue gas and at varying known concentrations of CO in the gas stream. In this manner, the oxygen flow rate through the lance, and thus also the oxygen injection velocity, providing the most pronounced temperature rise due to combustion of combustible matter with the oxygen in the gas stream was determined. Under the specific test conditions, an oxygen flow rate value of 0.25 Nm.sup.3/h was found to be optimal for every CO level. In other words, said oxygen flow rate provided the greatest flue-gas temperature increase detected by the thermocouple for every level of CO. The size of said maximal temperature increase itself increased with increasing CO level in the gas stream. In addition, it was found that said oxygen flow rate also provided the highest detected flame radiation intensities detected by the photosensor for every CO level, whereby the level of the detected radiation intensities also increased with increasing CO levels in the gas stream.

[0084] The first set of tests was used to calibrate the monitoring unit.

[0085] In a second set of tests, the calibrated monitoring device was used to monitor the CO in the hot flue-gas stream from the furnace operating at various power levels and combustion stoichiometries. When the photosensor detected radiation intensities corresponding to a CO level in the flue gas stream exceeding a predetermined acceptable upper limit, the monitoring device generated a control signal for the corresponding adjustment of the oxygen-to-fuel ratio of said burner.

[0086] The second set of tests proved it was possible, following calibration with test results, to use the monitoring device to reliably determine the CO content in an unknown flue gas stream with a monitoring device spaced apart from the flue gas stream and without interference from the flame in the furnace.

[0087] Comparison between the CO levels determined on the basis of flame radiation by the monitoring device with the CO levels determined on the basis of the temperature rise detected by the thermocouple confirmed the accuracy and reliability of the monitoring device of the invention.

[0088] The effectiveness of the method and device according to the present invention for monitoring combustible matter in a hot gaseous stream was confirmed during long-term testing on an industrial furnace for melting non-ferrous metal and in which a reducing atmosphere was maintained. The detected flame properties were pixelated multispectral radiation intensities in the visual and near infrared range. The control signal was generated on the basis of the principles described in WO-A-2010/022964. The method and device were found to be robust and reliable and considerable energy savings were achieved without increased oxidation of the charge.

[0089] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.