Patent Application
20090102103
The present invention relates to an apparatus for use in the analysis and control of operating parameters of industrial furnaces, and more particularly an apparatus which incorporates passive infrared sensor technology to detect the signatures of one or more off-gas stream components, and thereafter provide control signals to the furnace in response to the sensed data to better optimize furnace operating efficiencies.
Industrial furnaces are capital-intensive operations with major operating costs and environmental impact. To maximize energy efficiency, lower the conversion costs of raw materials, and reduce greenhouse gas (GHG) and other pollutant emissions, these large-scale combustion processes rely on various control measures which continuously alter furnace operating parameters such as oxygen content, fuel burn and flow rates, as well as the rate at which materials are introduced into the furnace itself.
Industrial combustion processes rely on control measures requiring a real-time knowledge of the furnace operating temperatures and by-product concentrations to maximize energy efficiency and pollution abatement. These measures are based on process data such as flue gas composition and temperature. As industrial furnaces are harsh environments, it is difficult to obtain process data in the vicinity of the combustion zone, such as gas temperatures and concentrations. When this data is available, however, processes may be optimized to maximize the use of energy and raw materials.
A wide range of active or extractive techniques exist to measure furnace off-gas composition where, for example, a sample of gas is removed under vacuum to a remote location for analysis. One such example is the EFSOP™ process optimization system developed by Techint-Goodfellow Technologies Inc. initially for electric arc furnaces (EAF) in the steel sector. This system relies on an extractive probe to continuously sample the off-gas for subsequent analysis to determine the concentrations of CO, CO2, O2, H2 and other constituents in the flue gas, and thereafter adjusting the furnace operating parameters to maximize energy savings and other process performance parameters. These extractive sampling methods can operate continuously however there is a delay of typically 30 seconds or less to allow for sample extraction, gas conditioning and chemical analysis. Extractive technologies are limited however since they can not measure the temperature of off-gasses leaving the furnace.
Tunable diode laser (TDL) spectroscopy is another option for obtaining industrial furnace process data with measurement response times of about one second. This optical technology can in theory detect the concentration of H2O, CO and other constituent gasses. Unlike extractive methods, insitu TDL technology can also measure flue gas temperatures.
In industrial furnace systems such as steel-making furnaces, reheat furnaces, cement kilns, glass manufacture, pulp and paper production and thermal power generation, the presence of high temperatures and sometimes corrosive gases plus excessive amounts of dust particles in flue gasses pose a severe challenge to TDL and extractive techniques. For example, dust particles block filters and sample lines and interfere with the effective transmission of laser beams. In addition, TDL laser sensor applications are also challenged by optical alignment issues, particularly as a result of thermal expansion and vibrations associated with industrial furnace operations.
As a result, the main challenge to extractive gas sampling techniques is the regular maintenance required to change particle filters on the sampling line and their inability to detect flue-gas temperatures. In spite of these limitations, conventional gas extraction sensing methods such as the EFSOP system have proven successful in measuring flue gas composition in many applications. The main challenge for TDL sensors relates to transmission difficulties and alignment issues. To date TDL sensors have proved largely impractical in the presence of variable, high particle loadings which are encountered in many industrial combustion applications including steelmaking, cement and thermal power generation.
Hence, since extractive systems can measure gas composition with a time delay and can not measure off-gas temperature and TDL technology which in theory can measure gas properties with minimal delay, but which is largely ineffective because of transmission limitations, there is a need for a sensing device that can withstand the harsh working environments associated with industrial furnaces and can effectively measure flue gas temperatures and composition in real-time.
The present invention seeks to provide a system and method which uses passive sensors, and more particularly passive light energy sensors in the analysis and/or control of industrial furnace operations. In a broadest embodiment, the passive sensors may advantageously be used to look for the signatures of carbon dioxide and carbon monoxide in the infrared region of the light emitted by the furnace, its exhaust gas and/or ash or dust particles of the furnace off-gas stream. The applicant has appreciated that remote operation and fast response times are the two features that set such passive sensors apart from conventional extractive and laser-based counterparts. Preferably, the present invention incorporates sensor assembly having one or more passive optical and/or IR sensors which are operable to retrieve an energy signature from visible or infrared light emitted by a furnace flame, off-gas stream component such as a furnace bi-product, a waste gas and/or ash, dust or other solid particles entrained therein. Most preferably, the sensors include pyroelectric or photoconductive detectors having short measurement times (e.g., as little as one reading per second), and wherein the analyzed data may be used to extrapolate furnace conditions such as the range of concentrations that may be retrieved for CO2.
The sensor assembly may be used in the monitoring and/or control of a variety of different types of industrial furnaces including, without restriction, those used in steel-making, cement production, thermal power generation, or other industrial furnace applications where process efficiency could be increased by the measurement of off-gas chemical and thermal properties and the production of green house gasses and other pollutants would be of a concern. The applicant has appreciated that by controlling operating parameters of such furnaces in response to the radiation energy signature of the furnace flame and/or off-gas stream components, it is possible to continuously adjust the furnace operations to maximize combustion efficiency and thereby minimize environmental impact. In particular, depending upon changes in detected radiation signatures, furnace operating parameters including, without restriction, burn or introduction rates of fuel, amounts of introduced oxygen or other reaction products, or furnace operating temperatures may be adjusted to maximize furnace efficiencies. Financial benefits come hand in hand with more control over process and environmental impacts. With forecasted energy efficiency increases of 1-3% or more resulting in lower energy costs, the financial sustainability of these industries will also improve making manufacturers more competitive and viable.
Another object of the invention is to develop a real-time industrial furnace control and optimization system using one or more infrared sensors, and wherein furnace operation performance is continuously monitored and/or adapted. More preferably, the furnace control systems incorporate one or more passive sensors that operate in conjunction with a system output to provide indications of real-time concentration and/or temperature data in exhaust gases with high dust content, either in conjunction with or without extraction systems for analysis of an extracted furnace gas sample.
Accordingly, in one aspect the present invention resides in a method of using an IR sensor assembly to control the operating parameters of a steelmaking furnace, the furnace having a vessel for heating an iron containing bath and a lance selectively operable to inject oxygen into the bath,
the IR sensor assembly being located proximate to said vessel and an off-gas stream from the bath, the sensor assembly including,
whereby when said furnace is operated to heat and refine said bath,
activating the sensor assembly to output sensed signature data correlated to a temperature of the off-gas stream component,
activating said lance to inject oxygen into the bath while the sensed signature data correlates at least to a first predetermined flue-gas temperature,
and upon said sensed signature data correlating to a second predetermined temperature which is different from said first temperature, deactivating said lance.
In another aspect the present invention resides in a method of using a passive sensor assembly to control operating parameters of an industrial furnace,
the sensor assembly being located remotely and in a visual line of sight with the furnace off-gas stream immediately adjacent the furnace and including,
whereby during operation of said furnace, activating the sensor assembly to collect said infrared signature;
outputting sensed signature data as data correlated to a temperature of the off-gas stream component;
comparing the component temperature data with predetermined data, and adjusting the furnace operating parameters in response to the comparison.
In a further aspect, the present invention resides in a furnace control system for controlling the operating parameters of an industrial furnace, the system including,
a housing having a window opening therethrough, the opening exposed to an off-gas stream of said furnace and allowing radiation energy into said housing,
a spectrometer positioned in said housing for receiving and dispersing radiation energy from said off-gas stream into a plurality different wavelengths,
an infrared sensor optically coupled to the spectrometer for detecting an infrared signature of an off-gas stream component in a range of said wavelengths selected at between about 3 and 6 microns, and outputting the detected infrared signature as sensed signature data,
an output for converting the sensed signature data as temperature output data indicative of the temperature of a furnace off-gas stream component.
In yet another aspect, the present invention resides in a method of controlling a basic oxygen furnace (BOF) for use in steelmaking, the furnace having a vessel for forging a molten iron bath, and a lance selectively operable to inject oxygen into the bath,
an IR sensor assembly being located proximate to an air gap adjacent said vessel,
the sensor assembly including a housing having a window positioned adjacent to view an off-gas stream from the bath,
whereby in operation of said furnace,
activating the sensor assembly to output data correlated to a temperature of the off-gas stream component,
activating said lance to inject oxygen into the bath while the sensed signature data correlates at least to a first predetermined temperature,
and upon said sensed signature data correlating to a second predetermined temperature which selected less than the first predetermined temperature by a threshold amount, deactivating said lance.
Reference may be had to the following detailed description taken together with the accompanying drawings in which:
a and 1b show schematically the experimental and modeling results for a premixed methane flame in accordance with the present invention;
In assessing the operability of passive infrared sensors as operable to collect infrared signatures of furnace operating parameters such as the furnace flame or off-gas parameters, preliminary testing has been undertaken. In particular, preliminary analysis of furnace operations of the Ontario Power Generation (OPG) Nanticoke Generating Station Unit 4 500 MW coal-fired boiler were undertaken to confirm the operability of the present invention. Nanticoke has eight generating units each consisting of two main cycles: a combustion cycle to produce steam in the coal-fired boiler and a steam cycle to generate electricity in a steam turbine, to a capacity of 3,920 MW of electric power.
A test installation was established in the Unit 4 coal-fired boiler. An infrared sensor having a pyroelectric array detector was placed at one of the corner view-ports on level 51/2, overlooking the fireball from the rows of burners on levels 3 and 31/2. The infrared sensor was set to look straight into the furnace. Safety devices like a metal conduit and a shield with a sapphire window were also adopted to minimize the intense radiant heat from the 38 cm×25 cm opening.
In a broadest embodiment, the present invention includes one or more remote infrared sensor arrays that may be used to capture the spectral signatures of CO and CO2 in the mid-infrared region and to retrieve gas temperatures. CO and CO2 are good indicators of the degree of completion of a combustion reaction. In industrial furnaces and the steelmaking industry, CO and CO2 concentrations can provide useful milestones in the operation of an EAF or a BOF.
Using an automatic data acquisition system from the pyroelectric array detector, two hundred scans were taken every three seconds over a ten minute period. Even though radiation from the water walls was assumed negligible, the presence of burning coal particles and fly ash added a blackbody-like background to the light emitted mostly by CO2 flue-gas stream. It was assumed that CO in the fireball would be below 1000 ppm and judging from the premixed burner experiments, CO's light emission would not be distinguishable from that of CO2. The concentration of CO2 above the fireball was estimated to be around 14% and the gas temperature about 1600° K. From the view port to a dividing wall inside the furnace there was a distance of 14 m.
RADCAL™ modelling determined CO2 to be “saturated”, i.e., emit like a blackbody in those conditions, with its spectral signature (between 4.2 μm and 4.7 μm) superimposed from the continuous radiation of solid particles in the gas only if the emissivity of the latter is less than one, since both gases and particles would be at the same temperature. For an emissivity of one a simple continuous blackbody curve is expected.
In preliminary testing, two hundred scans were taken over 10 minutes of steady operation of the furnace from a corner view-port overlooking three rows of burners. The acquired data confirmed that infrared sensors were sensitive enough to detect the infrared signature of small quantities of CO that fluctuate in time in the region above the burners.
Gas radiation occurs in bands over certain wavelengths. Spectral modeling shows signatures of CO and CO2 at room and high temperatures in the 3 μm to 5 μm region, and more preferably, the 4 μm to 5 μm region which are caused by their rotational-vibrational molecular transitions. Therefore the sensors are selected as an array of pyroelectric detectors which capture irradiance (light intensity) versus wavelength in the mid-infrared. The applicants have appreciated that array sensor by IR Microsystems consisting of 64 pixels (each pixel being a pyroelectric detector) is well suited to this application and is relatively inexpensive.
To match the modeled signatures of CO and CO2 a wavelength position is assigned to each of the 64 pixels on the detector array. Two narrow band pass filters with centre wavelengths 3.906 μm and 4.594 μm may be used to set the spectral position of two pixels. A transmission experiment (C/Co) involved recording the number of counts produced by an infrared light source with the narrow band pass filter (C) and without the filter (Co). From these two data points a linear relationship between pixel number and wavelength was derived in order to assign a wavelength position to all the pixels on the detector array. The measured wavelength range was from 3.711 μm to 4.987 μm, with a pixel width of 20.26 nm.
Contrary to laser spectroscopy where a fraction of the laser beam shone across a flame is absorbed by CO and CO2, light emission at signature wavelengths by the same high temperature gases may be used. The reason for the emission of radiation energy is that the flame is being observed through a gas layer at a lower temperature, i.e., the portion of the atmosphere at room temperature between the infrared detector and the flame. In this case, the source (B) and incident (I) radiance term in the 1-D Schzartchild equation become comparable.
The source term is the light emitted by high temperature CO and CO2 and the incident radiation is either zero (e.g., in a flame-only experiment) or the blackbody-like radiance from an infrared light source placed behind the flame. In contrast, in laser spectroscopy, the source term is overpowered by the incident radiance of the laser beam and the result is an absorption-type measurement.
Light emitted by gases can become ‘saturated’ by radiating almost like a blackbody over their signature spectral bands. Saturation is a function of gas composition, gas column length and gas temperature. For instance RADCAL™ (a one-dimensional algorithm solver of the radiative heat transfer equation without scattering) predicts saturation of part of the 4.3 μm CO2 band for a path length of 14 m and a gas temperature of 1,600° K already at a 1% CO2 mole fraction. After total saturation of the core of the CO2 band has occurred, the modeled CO2 signal remains unchanged. Thus if a saturated signal for CO2 is observed, the only retrievable information from the flame or furnace light is the minimum mole fraction at which saturation would take place. However, the response of the wings (lines of weaker strength on both sides of the band core) to changes in the CO2 mole fraction may also be a factor. Saturation is not a problem in a setting with short path lengths (e.g., lab burners) and appears less critical for CO given the weaker emissivity of its 4.7 μm band compared to that of CO2 at 4.3 μm. Two types of laboratory experiments have been carried out with the infrared sensor prototype: transmission experiments to measure how room temperature CO2 in the atmosphere absorbs light from an infrared source and secondly, experiments with a premixed methane burner.
Room temperature transmission experiments showed the ability of a prototype sensor to record the spectral signature of both CO and CO2 at relevant concentrations (e.g., 9% by vol. of each at 1 atm). In particular, a premixed methane burner produces a one-inch square flame that contains high temperature CO2 and CO. Using a k-type thermocouple and NDIR (non-dispersive infrared absorption methods) to measure gas concentrations, the flame was characterized as having 9.4% CO2, 1.1% CO (by vol., dry basis) and an average temperature of 1363° K. The flame was imaged alone and with an infrared source at a temperature lower than the flame's placed behind it. The left shoulder in the flame-only peak between 4.2 μm and 4.33 μm and a result of the interaction of CO2 in the flame and in the atmosphere. It is the room temperature CO2 that absorbs light from the infrared source behind the flame at those same wavelengths (cf. right panel).
The preliminary testing supports the assumption that in comparing the raw signal between tube furnace and Nanticoke, there should be saturated CO2. Furthermore, looking at the time evolution of two wavelengths (averaged every five scans, i.e., every 15 seconds): one in the saturated CO2 region (4.6 μm) and another at the bottom of the room CO2 valley (4.2 μm), since the 4.2 μm signal stays fairly constant a change in the instrument response may be ruled out. As such, since the CO2 signal is saturated (meaning variations in CO2 concentrations should not change the number of counts), the only possibility is an increase in the gas temperature. In particular, an observed 18% increase in irradiance at 4.6 μm could be produced by a 9% increase in gas temperature (from 1600° K to 1750° K) as calculated by the blackbody emissive power equation. The inventors have thus appreciated that the passive approach as a sensible step in the evolution of combustion diagnostics, particularly in harsh industrial environment applications.
As shown in
The sensor assembly 10 is shown best in
Although a lens tube 44 integrating a 10 Hz mechanical chopper and a long pass filter (3.60 μm to 6.89 μm) is preferred, other lens tube 44 configurations and chopper devices may also be used. The filter 52 is preferably used to block wavelengths between 2 μm and 2.5 μm whose second order from the reflection on the spectrometer's grating would fall in the 3.7 μm to 5 μm region of interest. In this regard, a long pass filter along with a 75 line/mm diffraction grating blazed at 4.65 μm limits the overall wavelength range covered by the assembly 10 to 3.71 μm to 4.99 μm.
The spectrometer 46 most preferably is a defraction grating spectrometer having grating surfaces 54. The grating spectrometer 26 (such as the Oriel MS 125™) with a 120 mm focal length or such similar device is provided as the dispersive device for the infrared sensor 48. The spectrometer 46 is configured to achieve 75L/ml grating blazed at 4.65 microns, with light from the lens tube 44 split into its individual frequencies by grating surfaces 54.
The infrared sensor (proelectric detector) 48 is provided as an infrared sensor optically coupled to the spectrometer 46. The sensor 48 is most preferably but not necessarily a 64 pixel detector, as for example is sold by IR Microsystems. The pixel sensor array of the sensor 48 relays intensity of each frequency to the processor 12 for modelling to provide temperature measurement determination.
As will be described by analyzing the infrared signature emitted by furnace off-gas and dust particles in the off-gas stream 100 using the infrared sensor 48, the system 10 is operable to provide data which is indicative of current furnace operating parameters. A comparison of the sensed data against stored pre-determined optimal values may thus be used to provide control signals by way of CPU 12 to the furnace 20 to maximize operational efficiencies.
In a preferred mode of operation, the sensor assembly 10 is operable to detect the infrared signatures of fly-ash, dust and other particulate matter entrained in the off-gas stream 100. The IR sensor 48 is operable to measure gas and particle brightness temperatures in the off-gas stream 100 by analyzing the spectral radiance in the mid-infrared range. In particular, the sensor 48 is selected to measure radiation energy in the wavelength range of between about 3 and 6 μm and more preferably between 3.7 to 5.0 μm, including continuous radiation from solid particles and discrete radiation from CO and CO2 in the combustion gas.
The array of the sensor (pyroelectric detector) 48 operates on a principle whereby a pixel of ceramic material that is exposed to changes in temperature, experiences a transient electrical flow whose magnitude is proportional to the irradiance arriving at the pixel. By chopping the incoming light components emitted from the off-gas stream 100 at a fixed frequency, (most preferably about 10 Hz in the present case) one may expose the sensor (pyroelectric detector) 48 to the required temperature changes. The raw output of the sensor 48 is in number of counts versus pixel position. The former is a measure of irradiance and the latter of wavelength position. By dividing the collected radiation energy into different wavelengths, the irradiance at each of the 64 pixels of the sensor 48 will correspond to a small subset of wavelengths in the mid-infrared range.
In operation, the sensor assembly 10 is used to repeatedly scan the infrared profiles of the off-gas stream 100, with the lens tube 44 used to collect the radiation emitted from the off-gas stream 100 through window 42. The collected radiation is transmitted via lens tube 44 to the grating spectrometer 46 where it is dispersed into different wavelengths onto the linear array sensor 48. Each scan, i.e., the raw data is collected by the IR sensor 48 is undertaken at least every 10 second, and preferably every second, and consists of counts versus pixel information. As will be described, the scanned data is provided to the controller 12 where radiance and wavelength calibration procedures are used to convert counts into spectral radiance (W/m2/μ/sr) data and pixel position into spectral position data (i.e., wavelengths; measured in μm, where 1 μm=1×10−6 m). Thus, a calibrated scan may be plotted as x-y plot of spectral radiance versus wavelength in the mid-infrared as, for example, is shown in
The applicant has appreciated that there are three distinct spectral regions that fall between 3.7 μm and 5.0 μm. In order of increasing wavelength, these regions are characterized primarily by (a) particle-only radiation that is gray due to scattering and the gray nature of the particles [3.8-4.1 μm]; (b) a significant decrease in radiation due to absorption by room temperature CO2; and (c) saturated or blackbody radiation from CO, CO2 and particles at the off-gas stream [4.56-4.7 μm].
To determine the physical temperature of the gas and particle mixture and the brightness temperature of particles entrained in the off-gas stream 100, particles and combustion gases are assumed to be locally isothermal, and the gaseous radiation between 4.56 μm and 4.7 μm is presumed saturated over the expected range of temperatures in the combustion gap 32. In addition, the volume fraction of particles in the line of sight of the collecting tube 44, and the path length of the gas column are assumed to yield values of the optical depth well above unity in the particle region (i.e., optically thick medium).
Temperature calculation is effected by the CPU 12 based on a least-squared optimization method that compares the theoretical blackbody radiance from Planck's law, Bλ(T), and the measured raw data after radiance calibration, Iλ(T). An average over the gaseous spectral region [4.56-4.7 μm] of the regression variable
This is done for a series of plausible temperatures (e.g., between 500° K and 2000° K). The retrieved temperature (Tgas) is that for which R2 is smallest.
The brightness temperature of the entrained off-gas particles is not preset as equal to the particle physical temperature as, in general, particles are not perfect blackbodies in the mid-IR, i.e., εpar, λ<1. The physical temperature of the particles (Tmedium) is assumed to be the same as the gas at a specific location in the field of view (ie. at air gap 32). The particle brightness temperature (Tbri) is the temperature of a blackbody that would match the spectral radiance from the gray medium, as calculated by Formula (2) as follows:
I
λ(Tmedium)=εeffBλ(Tmedium)=Bλ(Tbri), Formula (2)
where Tmedium>Tbri and εeff is the effective emissivity of particles, which is a function of their physical emissivity. For a blackbody both its physical and brightness temperatures are the same. The particle brightness temperature is retrieved from an analogous regression variable R2par Formula 1) with Iλ and Bλ from the wavelength range λpar ε[3.8-4.1 μm].
It has been appreciated that the radiation measured by the IR sensor 48 in the gaseous spectral region will follow that of a blackbody at Tgas. Furthermore, the measured radiance in the particle region will be gray. Thus, two temperatures can be retrieved from the spectral data in each scan, Tgas (=Tmedium) and Tbri.
In a series of experimental batch or heat production heats represented by
After the processing, the molten steel may be transferred to a ladle furnace (not shown) where more materials are added to fine tune the final composition of the steel. In general, the temperature of the steel after the BOF furnace 20 has to be high enough so that the metal does not solidify as it goes through any subsequent manufacturing steps.
The box housing 40 is most preferably bolted on the furnace skirt or suitable alternative support, perpendicularly to the combustion gap 32 in order to capture the radiation energy at that location immediately where the off-gas stream 100 exits the vessel 22. The physical temperature of the gas and particles in the stream 100 (known as “Tgas”) and the brightness temperature of the particles entrained therein (known as “Tbri”) may then be measured using the sensor 48.
As the refining process proceeds, there is a point where the concentration of carbon in the molten steel reaches a threshold minimum, (at approximately 0.040 mass %, as dictated by equilibrium considerations). From then on the depletion in available carbon dissolved in the molten metal bath results in a growing proportion of the injected O2 reacting with the iron in the bath forming iron(II) oxide (FeO), which floats above the molten metal thereby reporting to the molten slag 26. Some of this FeO in the slag can be reduced back to iron by subsequent reduction by residual carbon still remaining in the molten metal bath. This refining process releases heat, since the oxidation of silicon, manganese, carbon iron and other impurities in the bath 27 is an exothermic reaction. Coincident with the change from carbon to iron as the preferential oxidation the reaction, the gaseous flow of CO and entrained solid particles in the off-gas stream 100 decreases dramatically as the carbon reaches its minimum concentration. Heat released by the formation of FeO raises the temperature of the molten bath 26, such that the bath temperature no longer correlates to either of the two temperatures that may be measured by the IR sensor 48 at the combustion gap 32.
In the context of steel furnaces 20, a heat may be unsuccessful for different reasons, as for example, if the aim final carbon concentration in the refined molten metal is not met or the molten metal temperature at the end of the heat is too low for the next step in the manufacturing of steel. In these cases, the oxygen lance 24 is often reinserted back into the bath and a second batch process, known as “reblow”, is started. Reblows add to the cost of producing the final steel product because they require additional process time and result in a reduced metal yield due to further oxidation of metallic iron to the discard slag 26.
The present system 10 and method permit the implementation of process controls to avoid having to perform reblows and to optimize successful heats. In particular, by sensing the furnace off-gas constituents and temperatures directly at the combustion gap 32 in real-time, the operation of the furnace 20 may be better controlled to keep the carbon content in the bath 26 near its equilibrium minimum and the bath temperature as high as possible. Both are desirable outcomes in terms of preventing wasteful reblows and optimizing the use of oxygen as well as the time that each heat takes. All these benefits relate directly to productivity and energy savings per ton of steel produced, and a reduction in CO2 emission per ton of steel produced.
After preliminary testing, a two-day measurement campaign for the sensor assembly 10 was undertaken. Information on eight heats was recorded, which became a secondary basis for analysis, integrating primary data from three sources:
The collected information is summarized for each heat as shown in
The aforementioned values are known as the first turn-down carbon content (measured in mass %) and the first turn-down bath temperature. As used herein, the term “first turn-down” refers to the first turndown of vessel 22 thereby providing the first test for temperature and carbon content of the molten metal 27 from a sample taken after the BOF vessel has been physically rotated or “turned down”.
The secondary variables of interest include Tgas at removal of the lance 24, i.e., when the oxygen lance status goes from 1 (on) to 0 (off), and the time interval between a significant drop in Tbri and the lance removal time (ΔtT-lance). Tbri is shown to steadier than Tgas as the heat came to an end, and, therefore, provides a clearer indicator for the drop in temperature as the heat came to an end. Nonetheless Tgas is the variable that is tracked as it is the physical temperature of the gas and particle mixture (see Formula 2).
Table 1 summarizes the relevant primary and secondary variables that were obtained from the eight heats. The aim carbon variable, (provided by the Host test site), is the basis for qualitatively labelling heats either as high-carbon heats or low-carbon heats.
If Tbri did not drop before the oxygen flow to lance 24 was stopped, a value of 0 was assigned to ΔtT-lance. This happened in Heats 6 and 7.
The air pressure in the housing 40 that contained the IR sensor 48 was increased before Heat 7 started. Likely this started to cause more dust accumulation on the window in front of the IR Sensor. Because Tgas and Tbri for Heat 7 are very similar to those observed for the rest of the heats, there are solid grounds to believe that dust accumulation did not affect the results for Heat 7. This is, however, not the case for Heat 8. Significant dust accumulation will lower the magnitude of the retrieved temperatures, which is apparent in Tgas and Tbri for Heat 8. Since ΔtT-lance is based on a qualitative assessment of when Tbri starts to drop (regardless of its magnitude), its value for Heat 8 was taken as correct.
By correlating the two secondary variables to the data of Table 1, the graphs shown in
High Tgas at lance removal correlates with a zero value of ΔtT-lance. Hence the two same data points appear in the top-left quadrant of
Once the carbon content is almost depleted, the oxygen from the lance 24 starts to react preferentially with iron. This switch stops the formation of CO bubbles, and the associated gas flow at the combustion gap decreases sharply. Without gas as a carrier, fewer solid particles are present and the signal measured by the IR Sensor drops sharply (
Heat 5 was interesting because the lance 24 was removed only 11 seconds after Tbri started to drop. Even though this time was short, the carbon content had already reached its minimum. This fact makes the monitoring of Tbri from the IR sensor 48, a very sensitive indicator to determine accurately when the carbon content in the bath has been minimized as much as one may practically expect it to drop.
The dotted line in
On the other hand, for low-carbon heats, once gas and particles stop flowing upwards due to the switch in the combustion chemistry, there is a decoupling between Tgas at lance removal and TD1_T, since Tgas will be measuring ambient noise. Since for Heat 5, ΔtT-lance was very small, there was little time for the oxidation of iron. This explains why in Heat 5 Tgas at lance 24 removal was higher than for the other low-carbon heats while its TD1_T was the lowest.
The foregoing suggests a strategy which may be used for low- and high-carbon heats in steel production to prevent wasteful reblow processes (due to low temperature or high carbon content), and to optimize the operation of successful heats (those without reblow) which may be summarized as follows:
The applicant has appreciated that various benefits may be realized in steel furnace operations with the present invention, Productivity of the BOF furnace 20 may be improved since delays due to reblow can be minimized or eliminated entirely, or by using the oxygen lance 24 longer than necessary. In addition, energy usage per ton of steel produced is maximized since reblows are energy intensive and do not add to the production of more steel. Also high-purity oxygen and its injection at supersonic speeds require large energy inputs that are wasted when oxygen is not necessary any more. The tonnage of steel produced per heat can be increased since the oxidation of iron is minimized when the oxygen flow is stopped on time. As well, natural resources per ton of steel are optimized since less additives and oxygen are used when there are no reblows.
From the point of view of reducing greenhouse gasses, direct and indirect CO2 emissions per ton of steel are minimized since less CO2 is produced (via the combustion of CO) when there are no reblows (where carbon is added and then burned). In addition, the minimized use of the oxygen lance 24 also minimizes the indirect emission of CO2 to the atmosphere by the conservation of fossil fuels. As well, less steel is produced if iron is oxidized, thus increasing the ratio of [CO2 emissions/unit of steel produced].
Although the detailed description describes and illustrates various preferred embodiments, the invention is not so limited. Many modifications and variations will now occur to those skilled in the art. For a definition of the scope of the present invention, reference may be had to the appended claims.
As used herein, the following terms shall have the following meanings:
Subscripts
| Number | Date | Country | Kind |
|---|---|---|---|
| 2541092 | Mar 2006 | CA | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2007/000367 | 3/6/2007 | WO | 00 | 9/16/2008 |
